Ann. occup. Hyg., Vol. 47, No. 6, pp. 441-459, 2003
© 2003 British Occupational Hygiene Society
Published by Oxford University Press
Review |
Technical, Occupational Health and Environmental Aspects of Metal Degreasing with Aqueous Cleaners
Groupe de Recherche en Toxicologie Humaine (TOXHUM), Département de Santé Environnementale et Santé au Travail, Faculté de Médecine, Université de Montréal, PO Box 6128, Main Station, Montreal, Quebec H3C 3J7, Canada
Received 20 August 2002; in final form 17 March 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSPHYSICOCHEMISTRY OF AQUEOUS...TOXICOLOGY OF AQUEOUS CLEANERSENVIRONMENTAL IMPACT OF AQUEOUS...RECYCLING AND WASTE DISPOSAL...DISCUSSIONCONCLUSIONSREFERENCES |
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Aqueous cleaners used for metal degreasing are detergent formulations containing surfactants (such as linear alkylbenzene sulphonates, alkylphenol ethoxylates or alcohol ethoxylates), builders (such as hydroxides, phosphates or silicates), sequestrants (such as EDTA or NTA), anti-corrosive agents (such as ethanolamines), solvents (such as glycol ethers or d-limonene) and other specialty additives. Generally sold as concentrates, they are typically diluted between 3 and 20 times in water, leading to solutions containing only a few per cent active products. The cleaning efficiency depends on physicochemical phenomena such as wetting, solubilization, emulsification, dispersion, sequestration and saponification, and is enhanced by thermal and mechanical energy. Cleaning equipment is based on spraying or immersion of the parts and may include drying and rinsing steps. Because of the complexity and variability of the mixtures, the occupational health and environmental evaluation of aqueous cleaners is based on the study of their components. Aqueous cleaners are generally believed to present a low risk to workers’ health and to the environment. However, some anionic surfactants and strong alkalis are skin and eye irritants, ethanolamines are allergenic and several glycol ethers of the ethylene glycol family are proven systemic toxicants that are easily absorbed through the skin. Although most components of aqueous cleaners are biodegradable and of low ecotoxicity, alkylphenol ethoxylates degrade into persistent and toxic compounds. Phosphates, if released directly into the environment, may cause eutrophication of rivers and lakes. Waste recycling or treatment by specialized facilities is usually required for spent solutions containing contaminants such as oils and heavy metals. From a technical, toxicological and environmental standpoint, aqueous cleaners can be used successfully to replace traditional organic solvents used in metal degreasing.
Keywords: aqueous cleaners; detergency; environment; metal degreasing; occupational health; surfactants; toxicology
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSPHYSICOCHEMISTRY OF AQUEOUS...TOXICOLOGY OF AQUEOUS CLEANERSENVIRONMENTAL IMPACT OF AQUEOUS...RECYCLING AND WASTE DISPOSAL...DISCUSSIONCONCLUSIONSREFERENCES |
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Substitution, defined as the elimination of a dangerous substance from the workplace by its replacement with a less dangerous one or by the implementation of a new process, is an important tool for prevention in occupational health and safety and in environmental protection (Goldschmidt, 1993; Gérin et al., 1996). In the field of solvent substitution, metal degreasing constitutes a key industrial sector. Indeed, industrial metal degreasing has traditionally been carried out using petroleum distillates and chlorinated solvents such as trichloroethylene. These substances have been shown to exert or are suspected of exerting adverse effects on human health or the environment, including tropospheric ozone formation and fire hazards for petroleum distillates and neurotoxicity and carcinogenicity for chlorinated solvents (Burgess, 1995; Callahan and Green, 1995). Owing to the growing legal restrictions concerning the aforementioned solvents and the associated costs of their use and disposal, water-based cleaners, containing only a few per cent active substances once diluted in water, are becoming more and more desirable (IRTA, 1994; Monroe, 1994; Pirrota and Roberts, 1994; US EPA, 1994a,b; Wolf, 1994; Antonsson, 1995; Goris, 1995; Hunt and Linton, 1996; Knipe, 1997; Wolf and Morris, 1997; Averill et al., 1998; Sherman et al., 1998; Anonymous, 1999; Tetra Tech EM Inc., 1999a,b). Aqueous cleaning is not a new technique: caustic aqueous cleaners have been widespread in industrial cleaning for a long time (Spring, 1974). However, aqueous cleaning formulations have evolved greatly, especially with the development of surfactant technology. Present water-based formulations are far less alkaline than those used previously, and more and more are neutral pH, leading to an increased potential for use in the traditional sink-on-drum cleaning apparatus and in other specialized systems. The number of water-based products is growing rapidly and there is often little information available from manufacturers. This is of real concern to industrial hygienists, especially since not much scientific literature exists about the toxicity and exposure potential of aqueous cleaners (Sørensen and Styhr Petersen, 1994; Wolkoff et al., 1998; Welsh et al., 2000). This review aims to summarize relevant information on water-based cleaners in all aspects of solvent substitution (i.e. what constitutes today’s aqueous cleaners, how they work and what are the health, safety and environmental issues related to their use).
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSPHYSICOCHEMISTRY OF AQUEOUS...TOXICOLOGY OF AQUEOUS CLEANERSENVIRONMENTAL IMPACT OF AQUEOUS...RECYCLING AND WASTE DISPOSAL...DISCUSSIONCONCLUSIONSREFERENCES |
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A search of bibliographical databases (Medline, Toxline, PolTox, Current Contents) identified most scientific articles, review monographs and ‘grey literature’ reports. The use of the Internet also identified manufacturers and retailers of water-based cleaners, in order to obtain the material safety data sheets (MSDSs) and technical data sheets for their products. A telephone survey in Quebec, Canada, led to the collection of about 70 MSDSs of aqueous cleaners available in this province. The MSDSs were collected in order to help identify the different substances used in existing formulations and to assess the extent to which relevant information was provided on these sheets. A chemical specialist from a Canadian company manufacturing water-based cleaners was also consulted.
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PHYSICOCHEMISTRY OF AQUEOUS CLEANERS |
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TOPABSTRACTINTRODUCTIONMETHODSPHYSICOCHEMISTRY OF AQUEOUS...TOXICOLOGY OF AQUEOUS CLEANERSENVIRONMENTAL IMPACT OF AQUEOUS...RECYCLING AND WASTE DISPOSAL...DISCUSSIONCONCLUSIONSREFERENCES |
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Aqueous cleaners can be defined as detergent solutions containing no more than a few per cent active chemical components, dissolved or suspended during use. They are most often sold as concentrates that have to be diluted to between about 3 and 20 times. The mechanisms of aqueous cleaning, as opposed to the rather straightforward chemistry of solvent degreasing, involve a range of physicochemical phenomena. These phenomena include wetting, solubilization, emulsification, dispersion, sequestration and saponification (see Fig. 1) (Jakobi et al., 1985). The chemical theory underlying detergency, comprising both solid, liquid and mixed soil removal, is quite complex and its complete description is not within the scope of this review. Only a brief overview of the principles underlying soil removal is presented here.
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MECHANISMS OF DETERGENCY
Liquid soil
Liquid soil removal in aqueous cleaning is mainly based on the chemistry of surface tension reduction (Lynn, 1993). The thermodynamic equilibrium in the cleaning bath is defined by the surface tensions existing between the different phases (i.e. substrate, soil, cleaning solution). ‘Roll up’ (illustrated in Fig. 1), which is a major determinant of oil and grease removal, results from a reduction in the surface tensions at the substrate/solution and soil/solution interfaces by active chemical components. This reduction leads to a new thermodynamic equilibrium in which soil is separated from the substrate, which becomes wetted by the cleaning solution. The chemical components responsible for this phenomenon are called surfactants (contraction of ‘surface active agent’) or wetting agents (Lange, 1994). Other phenomena occur during liquid soil cleaning (Fig. 1).
Under the stabilizing action of surfactants and agitation, macro- and micro-emulsions of oily soil in water can occur, preventing its redeposition on the substrate (Lange, 1994).
When their concentration increases in the cleaning bath, molecules of surfactant tend to gather to form globule-like microstructures called micelles, which are able to ‘store’ a certain amount of insoluble liquid soil (such as fatty acids and alcohols and hydrocarbons), preventing redeposition on the substrate (Lynn and Bory, 1997).
At alkaline pH, fatty acids and triglycerides (glycerol esters of fatty acids) react with hydroxides to form soaps (metallic salts of fatty acids), which are then dispersed or solubilized in the cleaning bath. This reaction is called saponification.
Solid soil
Two distinct theories explain the mechanism of removal of solid soil, and especially particulate soil, during detergency (Ho Tan Tai, 1999). The electric theory is based on the amount of energy needed to separate a particle from the substrate, determined by the combination of van der Waals forces (attraction) and electrostatic forces (repulsion) between the particle and the substrate. The presence of anionic surfactants at the interface greatly increases the repulsion forces and favours the separated (or clean) state. According to thermodynamic theory, surfactants act as surface tension reducers between bath and soil and substrate and bath, to lower the free energy of the clean state of the system (Lynn, 1993).
FACTORS AFFECTING DETERGENCY
The substrate itself, either because of a complex geometry (e.g. blind holes) or a rough surface (reduced wettability), can impede the detergency of an otherwise efficient cleaning solution (Spring, 1974). Some metals and alloys are sensitive to specific cleaning solution properties, thus restricting the types of formulations that can be used for these metals (Spring, 1974; Peterson, 1997). Knowledge of the characteristics of the soil to be cleaned is an important determinant of the cleaning conditions and formulation in any cleaning process. This knowledge becomes critical in aqueous cleaning, where formulations have to be adapted to a rather narrow spectrum of soils to ensure optimal cleaning efficiency (Knipe, 1997; Kanegsberg, 1998). Elevated water hardness (i.e. high concentrations of magnesium and calcium salts) can inactivate the action of surfactants, precipitate other cleaning components and alter the cleaning equipment (by the formation of calcareous scale). This phenomenon requires the use of soft water in cleaning systems or the addition of chemicals able to deactivate metallic cations (Jakobi et al., 1985; Lynn, 1993). Foam, an air-in-water emulsion produced by some surfactants, may increase the contact time between the substrate and the solution and improve the cleaning efficiency in the case of spray systems. However, in closed systems using agitation, foam can build up and cause spills or cavitation (Lynn, 1993; Pirrota and Roberts, 1994; McLaughlin and Zisman, 1998). Mechanical energy helps active cleaning agents to separate the soil from the substrate and to keep it dispersed. High energy equipment, such as ultrasonics and power sprays, also improve cleaning by direct soil abrasion (McLaughlin and Zisman, 1998). Raising the temperature has a kinetic impact on chemical reactions taking place in the cleaning solution, e.g. by accelerating emulsion, solubilization and saponification (Lynn, 1993; Ho Tan Tai, 1999).
COMPONENTS OF AQUEOUS CLEANERS
Detergency has been improved with the use of a vast array of active chemical agents including surfactants, builders and specialty additives such as anti-corrosive agents, solvents and dispersants.
Chemicals used in aqueous cleaners
Surfactants: Surfactants are defined as amphiphilic organic molecules, i.e. having both a hydrophilic (water-soluble) and a hydrophobic (non-water-soluble) part. They are classified according to the chemical nature of their hydrophilic part as anionic (ionized and negatively charged), cationic (ionized and positively charged), non-ionic (polar non-ionized) or amphoteric (having a positive or negative charge depending on the solution pH). The hydrophobic part of a surfactant is usually formed by an aliphatic hydrocarbon chain, either linear or branched. Some molecules may also have an aromatic hydrophobic tail. While anionic surfactants have historically been the most common surfactants, non-ionic surfactants are becoming popular today, especially because these chemicals provide good detergency at low temperature and in neutral solutions. Amphoteric surfactants, added to formulations in small quantities to enhance the detersive properties of other surfactants, reduce their irritation potential or enhance their solubility in the cleaning solution and are usually used in neutral formulations (Jakobi et al., 1985; Lynn and Bory, 1997; Oldenhove de Guertechin, 1999). Cationic surfactants are rarely present in metal degreasing formulations because of their affinity for metallic (negatively charged) substrates and their incompatibility with anionic surfactants (McLaughlin and Zisman, 1998).
Builders: Builders, which may include a variety of compounds from alkaline metallic salts to water softeners and alkali reserve providers, are not homogeneously described in the literature (Peterson, 1997; McLaughlin and Zisman, 1998). In our paper, we consider alkaline agents, which in the cleaning solution neutralize acid soil, enhance anionic surfactant solubility and efficiency, stabilize emulsions and disperse soil, and multifunctional phosphates as builders, while more specific softening agents like sodium ethylenediaminetetraacetate (EDTA) or sodium nitrilotriacetate (NTA) are reviewed in a separate category (i.e. sequestrants).
Other additives: Anti-corrosive agents are added to most aqueous cleaning formulations and rinsing solutions to prevent corrosion of metallic substrates (Peterson, 1997; McLaughlin and Zisman, 1998). Aqueous cleaners can also contain solvents (either water-soluble or solubilized by hydrotropes) in a proportion usually between 5 and 10% of the concentrate (Peterson, 1997; McLaughlin and Zisman, 1998). Semi-aqueous cleaners, not specifically presented here, may contain up to 50% solvents. Numerous other specialty additives can be found in aqueous cleaning formulations, such as foam stabilizers or inhibitors, dispersants, hydrotropes, bactericides, perfumes and colourants (Lynn, 1993). Table 1 lists the most commonly encountered components in aqueous cleaners.
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Composition of aqueous cleaners
The number of parameters required to formulate an aqueous cleaner (e.g. cleaning apparatus, substrate type, cleaning temperature or cleaning solution pH) is too large to permit descriptions of all possible formulations. Thus, based upon the available MSDSs and consultation with an expert, we restricted the number of classes according to the pH of the solution as shown in Table 2. Since the irritation potential is largely determined by pH, this simplification makes toxicological sense.
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EQUIPMENT USED IN AQUEOUS CLEANING
Equipment performance is a more influential determinant of cleaning efficiency for aqueous cleaners than for solvents, because thermal and mechanical energy are less critical factors with solvents (Peterson, 1997). Aqueous cleaning machines have to be adapted to the cleaning conditions required by the substrate, soil or industrial process characteristics. Aqueous cleaning equipment can be divided into two types, i.e. that with low to moderate parts throughput, such as used in maintenance shops, and that with high parts throughput, which is generally automated.
Low parts throughput equipment
Fountain cleaning, use of microbes: This type of equipment is based on the same principle as the traditional solvent sink-on-drum system. Parts are put in the sink and cleaned manually by a worker applying the cleaning solution, usually with a brush-like pipe nozzle. The cleaning solution tank is often heated to between 40 and 50°C. Fountain aqueous cleaning formulations are diluted to around 1 in 3 with water before use (Wolf and Morris, 1997; Tetra Tech EM Inc., 1999a,b). Enzymatic (or bioremediation) cleaners are modified fountain cleaners in which microbial colonies are kept alive in the system in order to digest soil dissolved or dispersed in the cleaning solution (McNally, 1999).
Immersion cleaning: Immersion cleaning systems are mainly tanks in which the parts are immersed in the cleaning solution for a certain amount of time. They operate at the same temperature and dilution range as fountain cleaning systems. In order to enhance cleaning performance, immersion systems can be equipped with mechanical oscillation, submerged spray nozzles or ultrasonics (Koepfer, 1995; Peterson, 1997; Tetra Tech EM Inc., 1999a,b).
Spray cleaning: Spray cabinets function on the same principle as dishwashers. The parts are placed in a steel tank and undergo a cleaning cycle of variable length. During the cycle, the parts are sprayed with the cleaning and rinsing solutions. Since there is no human contact, these systems operate at higher temperatures than immersion or fountain equipment (usually between 50 and 90°C). Operating pressures range from 200 to 700 kPa. Manually operated power washer systems may also be used to clean very large parts (McLaughlin and Zisman, 1998). Spray cleaning formulations are diluted to between 1/10 and 1/15 with water (IRTA, 1994; Peterson, 1997).
High parts throughput equipment
Most industrial aqueous cleaning systems require relatively little human intervention (Peterson, 1997). Systems based on immersion usually consist of a series of tanks in which the parts are sequentially dipped, often automatically. The tanks contain pre-cleaning, cleaning and rinsing solutions. In spraying-based equipment, the different solutions are sprayed on the parts moving continuously through the system. Since human intervention is minimal, cleaning conditions are often more aggressive than in low parts throughput equipment (Fuchs, 1997; McLaughlin and Zisman, 1998).
RINSING AND DRYING
Rinsing and drying are important issues in aqueous cleaning since aqueous solutions do not evaporate rapidly from the substrate after cleaning. The need for rinsing depends on the industrial process and the objective of the cleaning (e.g. degree of cleanliness) (Peterson, 1997; McLaughlin and Zisman, 1998). Rinsing solutions are generally applied using the same principles as in the cleaning step, i.e. immersion or spraying. They can contain chemicals such as surfactants or anti-corrosive agents. Drying is often critical in aqueous cleaning since most metals can be corroded and slow evaporation can leave tarnished spots on the surface of the substrates (Thomas et al., 1997). The different methods of drying used in metal cleaning include compressed air, air knife, vibration, evaporation, centrifugation, chemical absorption and solvent displacement (Peterson, 1997; McLaughlin and Zisman, 1998).
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TOXICOLOGY OF AQUEOUS CLEANERS |
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TOPABSTRACTINTRODUCTIONMETHODSPHYSICOCHEMISTRY OF AQUEOUS...TOXICOLOGY OF AQUEOUS CLEANERSENVIRONMENTAL IMPACT OF AQUEOUS...RECYCLING AND WASTE DISPOSAL...DISCUSSIONCONCLUSIONSREFERENCES |
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There are no available occupational health data on aqueous cleaning formulations. Furthermore, given the complex composition of aqueous cleaners, it would be unrealistic to try to present an exhaustive toxicological review of all potential components. This review is thus restricted to the health effects of the major surfactant families and of the most representative additives found in aqueous cleaners. Moreover, since exposure to aqueous cleaners seems most likely to occur by skin or eye contact and to a lesser extent by inhalation, more focus has been put on these routes.
Toxicology of surfactants
The toxicology of the most common surfactants is well described in the scientific literature. Furthermore, since some commercial surfactants are often mixtures of homologous components (e.g. of differing chain length), conclusions have to be drawn based on the assumption of similarity of biological activity.
Biological activity of surfactants
The biological properties of surfactants come from their chemical interactions with fundamental structures such as proteins, enzymes and membranes. Hence, they can form adsorption complexes with proteins, rendering them water soluble, or denature proteins in some cases. This phenomenon has been correlated with the cutaneous irritation potential in the case of common anionic surfactants (Bartnick, 1992). The denaturing effect of surfactants on proteins generally leads to the inactivation of enzymes. However, at low doses or with less aggressive products (such as non-ionic or amphoteric surfactants), activation can occur, caused by a conformational change in the enzyme, without denaturation (Cserhati, 1995). Through their denaturing and solubilizing properties towards biological membranes, surfactants also exercise a cytolytic activity. Cytolytic effects, although highly variable among and within different chemical families, tend to decrease in the following order: cationic (used as bactericides), anionic, non-ionic and amphoteric surfactants (Bartnick, 1992; Cserhati, 1995).
Biological disposition of surfactants
The cutaneous absorption potential of anionic surfactants has been shown to be almost null in vitro and in vivo (Black and Howes, 1992). Although they are also very poorly absorbed through skin, non-ionic surfactants tend to be better absorbed than anionics, especially for the most hydrophobic homologues. A proportion of percutaneous absorption as high as 15% of alcohol ethoxylate surfactant has been reported in humans (Talmage, 1994). Anionic and non-ionic surfactants are easily and rapidly absorbed through the gastrointestinal tract (Black and Howes, 1992; Rodriguez and Singer, 1996). No studies were found on the absorption potential of surfactants by inhalation. Following gastrointestinal absorption, animal studies show that most surfactants are distributed mainly in the liver and kidneys, where they are metabolized, and are excreted in various proportions in urine and faeces with half-lives in the range of hours (Black and Howes, 1992; Talmage, 1994; Rodriguez and Singer, 1996). The extent of biotransformation depends on the specific structure of the molecule.
Effects of surfactants on skin
In California, detergent use represents the third main cause of contact dermatitis, after solvents and poison ivy. This has been attributed to excessive use of formulations intended for skin contact, to skin contact with formulations not intended for such contact or to skin contact with formulations intended for industrial use (Mathias, 1999). Irritation caused by detergent formulations (except for strong alkaline formulations) has largely been attributed to the presence of surfactants (Effendy and Maibach, 1995; Barany et al., 1999; Mathias, 1999).
Mechanisms of skin irritation: The irritation potential of surfactants is linked to their adverse action on proteins and membranes and their ability to solubilize lipids. Hence, surfactants can solubilize glycolipids of the skin horny layer and phospholipids of the skin cell membranes and alter the properties of keratin. The cell damage in the deeper layers of derma can then lead to inflammation (Effendy and Maibach, 1995; Barany et al., 1999; Warren et al., 1996).
Skin irritation potential: The observed irritation potential of surfactants decreases from anionic to cationic (although cationic compounds are more cytolytic than anionics), non-ionic and amphoteric compounds (Mathias, 1999). The anionic alkyl sulphates and alkylether sulphates are considered the most irritating surfactants of their chemical families, causing reactions at concentrations as low as 0.1% (Matthies, 1992). Linear alkylbenzene sulphonates (LAS) and olefin sulphonates are considered to have the same irritation properties, with threshold concentrations for light to moderate irritation at ~1% (IPCS, 1996). The non-ionic alcohol ethoxylates have a variable irritation potential, decreasing with increasing length of the polyethoxy chain. While 1% solutions of several compounds have caused moderate irritation, no effect has been reported for solutions under 0.1%. Alkylphenol ethoxylates have shown no irritation potential in human studies at concentrations as high as 50% (Talmage, 1994). Alkylpolyglucosides tend to cause skin irritation at concentrations between 30 and 50% (Kocher and Wiegand, 2000). According to Rodriguez and Singer (1996), ethylene oxide/propylene oxide (EO/PO) block copolymers have only a negligible irritation potential. In a study in which 12 volunteers were exposed for 24 h to different surfactants on the forearm, no significant reaction was observed compared with controls for amphoteric cocoamphoacetate and cocoamidiopropylbetaine (Barany et al., 1999). The variability in irritation potential within families of surfactants prevents prediction of irritation for a specific molecule. Moreover, the use of surfactant mixtures, especially with non-ionic or amphoteric compounds, seems to reduce the irritation potential of a formulation (Effendy and Maibach, 1995).
Effects on skin as a barrier: Surfactants modify skin permeability, either by loss of biological integrity or by facilitating the percutaneous absorption of other substances. In vivo and in vitro studies have shown significant increases in the transport of water, ions and organic compounds through the intact skin after simultaneous exposure to surfactants (Priborsky et al., 1992; Nielsen, 2000; Nielsen et al., 2000).
Allergic properties: According to Mathias, all common families of surfactants, except cationics, have had at least one of their members considered as the cause of allergic dermatitis (Mathias, 1999). The author nevertheless emphasized that the diagnosis was often uncertain since the reaction was attributed to one component of a complex mixture including other surfactants and chemicals (Mathias, 1999). Allergic properties of alkyl sulphates, alkyl sulphonates, alkylether sulphates and olefin sulphonates have been attributed to impurities of synthesis (sultones and chlorosultones), the concentrations of which were later reduced to safe levels (IPCS, 1996). There is no evidence of allergenic activity of LAS compounds (IPCS, 1996). Human and animal studies conducted by manufacturers show no allergic properties of alcohol ethoxylates and alkylphenol ethoxylates (Talmage, 1994). Recent reports, though, indicate that alcohol ethoxylates tend to auto-oxidize during storage, leading to the formation of allergenic compounds (Bergh et al., 1997, 1998; Karlberg and Bergh, 1999). There is no evidence that alkylpolyglucosides and alkanolamides have allergic properties (Knaak et al., 1997; Kocher and Wiegand, 2000).
Ocular irritancy of surfactants
The ocular irritancy of surfactants was mainly studied using the Draize test on the eyes of albino rabbits. Common anionic surfactants (sulphates and sulphonates) cause irritation at levels between 0.01 and 0.1% in aqueous solutions (IPCS, 1996), while alcohol ethoxylates and alkylphenol ethoxylates usually do not cause irritation at concentrations below 0.1% (Talmage, 1994). Block copolymers showed only weak irritation potential when instilled pure into rabbit eyes (Rodriguez and Singer, 1996). The ocular irritancy of alkylpolyglucosides varies from severe to nil, decreasing with an increasing hydrocarbon chain length and mean number of glucoside units (Kocher and Wiegand, 2000). Aqueous solutions of four alkanolamide-type non-ionic surfactants (10 and 30% concentration) showed minor to moderate irritation in the Draize test (Knaak et al., 1997).
Acute toxicity of surfactants
Apart from ocular and cutaneous irritation, acute toxicity studies involving surfactants are available only on animals. Surfactants generally have low acute toxicity in all species after oral or cutaneous exposure (LD50 values ranging from 400 to >25 000 mg/kg) (Potokar, 1992; Talmage, 1994; IPCS, 1996). Lower values for the i.p. and i.v. routes (ranging from 60 to 1000 mg/kg) are partially explained by lower absorption rates by cutaneous and oral administration (Potokar, 1992). In one inhalation study Guinea pigs were exposed 8 h a day for 6 days to liquid aerosols containing up to 1% different anionic surfactants (LAS, alkyl sulphate, sulphosuccinate). Dyspnea and lethargy were reported at concentrations >0.5% (Potokar, 1992). Talmage described inhalation studies in rats in which exposure to alcohol ethoxylates caused no adverse effect at concentrations below the saturated vapour concentrations of the compounds (Talmage, 1994). Inhalation LC50 values in rats for alcohol ethoxylates are between 1.5 and 20.7 g/m3 (exposure to mists), with the exposure duration ranging from 1 to 4 h. Talmage also reports inhalation no observed effect concentration (NOEC) for alkylphenol ethoxylates for 8 h exposures to mists varying from 20 to 25 ml/m3 (
20–25 g/m3) (Talmage, 1994).
Chronic toxicity of surfactants
Systemic and local effects: Systemic chronic toxicity of surfactants is well documented, the literature being more abundant for anionic compounds than for non-ionic or amphoteric ones (Potokar, 1992; Talmage, 1994). Most of the available data deal with oral administration of water-diluted surfactants to rats and the observed effects are generally non-specific, with most reported no observed effect level (NOEL) values between 100 and 1000 mg/kg/day (Potokar, 1992; Talmage, 1994; Rodriguez and Singer, 1996; IPCS, 1996; Knaak et al., 1997; Kocher and Wiegand, 2000). Only two subchronic inhalation studies of surfactants were found in the literature. Coate et al. observed respiratory distress, bronchoconstriction and reversible pulmonary damage in monkeys exposed for 6 months to the respirable fraction of an aerosol of a powder detergent formulation (containing LAS, sodium tripolyphosphate, sodium sulphate and a silicate). The animals were exposed to 100 mg/m3 for 6 h a day, 5 days per week. One animal died during the study (Coate et al., 1978). Exposure of rats for 1 h or 4 h a day for 10 weeks to 6.7 or 22.8 mg/m3 of a liquid aerosol containing a non-ionic surfactant caused reversible damage to type II pneumocytes (Kissler and Morgenroth, 1981).
Mutagenic effects: On the basis of limited studies, LAS and alkyl sulphates are considered non-mutagenic in vitro and in vivo. Olefin sulphonate compounds have only been shown to be non-mutagenic in in vitro systems (IPCS, 1996). According to Oba and Takei (1992), alkylether sulphates, alkyl sulphoacetates and ethoxylated alkyl sulphates can be considered to be non-genotoxic. Other major surfactants (alcohol ethoxylates, alkylphenol ethoxylates, alkylpolyglucosides and alkanolamides) generally did not show mutagenic activity in different test systems (Talmage, 1994; Rodriguez and Singer, 1996; Knaak et al., 1997; Kocher and Wiegand et al., 2000).
Carcinogenicity: Several chronic studies in animals have reported no carcinogenic potential for many surfactants (LAS, olefin sulphonates, alkyl sulphates, alkylether sulphates, alcohol ethoxylates, alkylphenol ethoxylates and EO/PO block copolymers), but the small numbers of animals and absence of maximal tolerated dose leave open the possibility of carcinogenic effects (Oba and Takei, 1992; Talmage, 1994; IPCS, 1996; Rodriguez and Singer, 1996). No studies on the carcinogenicity of alkylpolyglucosides and alkanolamides have been found.
Reproductive toxicity: Only the major surfactant families have been tested for reproductive toxicity and only in a limited number of studies. Adverse effects have mostly been observed at doses toxic for the mother. Table 3 summarizes the results found in the literature for the major surfactants present in aqueous formulations.
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Toxicology of builders
As seen in Table 1, most of the builders used in aqueous formulations are alkaline substances, with potential adverse effects on human health due to tissue irritation and/or corrosion (Young et al., 1988). Young et al. classified a substance as irritant or corrosive based on its solution pH and alkali reserve. Alkali reserve of solution A is the quantity in grams of sodium hydroxide added to a neutral solution that would require the subsequent addition of the same volume of 2 N sulphuric acid as added to solution A to bring the pH to 10. Hence, compounds with lower pH values but a high alkali reserve can be more irritating than those with higher pH values, as illustrated by sodium metasilicate. Table 4 shows alkali reserve values for some builders commonly found in aqueous formulations, along with their classification as irritant or corrosive. This classification allows comparison of the different builders presented in Table 1. It was validated with the official European Community classification when available, based on 4 h skin tests on rabbits (EEC, 1967, 1984).
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Sodium and potassium hydroxide are strong alkalis, which are corrosive in concentrated solution (Table 4). Both of these compounds have threshold limit value (TLV) ceiling exposure limits of 2 mg/m3 based on the prevention of irritation and/or corrosion (ACGIH, 2003). Unlike sodium metasilicate, which is considered to be corrosive, sodium sesquisilicate is considered only as an irritant (Pierce, 2001). Sodium carbonate is a weak alkali, with 1% solutions causing no irritation when applied to human skin (Pierce, 2001). Sodium orthophosphate and sodium pyrophosphate have recommended exposure limits, namely an AIHA workplace environmental exposure level (WEEL) (15 min) of 5 mg/m3 and an ACGIH time-weighted average TLV (TLV-TWA) of 5 mg/m3, respectively, both based on irritation (AIHA, 2002; ACGIH, 2003). Sodium tripolyphosphate and sodium hexametaphosphate do not have exposure limits, probably because they are less likely to cause irritation (Table 4). Borates are poorly absorbed through the skin and have weak irritating properties (Hubbard, 1998). Although human epidemiological studies are inconclusive, adverse effects have been observed for borates in animals where male fertility, testicles and embryo development were affected at oral doses between 8.8 and 27 mg/kg depending on the species (Culver et al., 2001). Borax (sodium tetraborate decahydrate, CAS no. 1303-96-4) has a TLV-TWA of 5 mg/m3 based on irritation (ACGIH, 2003).
Toxicology of sequestering agents
EDTA (C10H12N2O8.4Na, CAS no. 64-02-8) is a corrosive substance, concentrated solutions of which can cause skin, eye and respiratory tract irritation. EDTA is poorly absorbed through skin. Thus, inhalation of aerosols is the main potential absorption route for EDTA in aqueous cleaners (REPTOX, 1994). The oral LD50 of EDTA is reported to be >2000 mg/kg (REPTOX, 1994; HSDB, 2000b). Available data on EDTA reproductive toxicology are inconclusive. Fetotoxicity and teratogenic effects have been observed in rats at doses >500 mg/kg, with toxic effects for the mothers. Other reproductive studies have reported no effects at doses as high as 1000 mg/kg (Kimmel, 1977; Schardein et al., 1981).
NTA (C6H6NO6Na3, CAS no. 5064-31-3) is also a corrosive substance, which, in concentrated solutions, can cause skin, eye and respiratory tract irritation. Inhalation of aerosols is the main potential absorption route for NTA in aqueous cleaners (IARC, 1999). Oral LD50 values of 1100 and 750 mg/kg are reported in rats and monkeys, respectively (Lewis, 1992). NTA is not suspected of causing reproductive effects by the experts at IARC, based on in vivo and in vitro studies. Although it has not shown any mutagenic potential in several in vivo and in vitro tests, NTA is considered possibly carcinogenic to humans (group 2B) by IARC, on the basis of animal studies involving bladder, kidney and urethral tumours (IARC, 1999).
Sodium gluconate (C6H11NaO7, CAS no. 527-07-1) is widely used in food additives and in pharmaceutical and cosmetic formulations (e.g. toothpaste). The lowest i.v. dose that has caused a toxic response in rabbits is reported to be 7630 mg/kg (Lewis, 1992).
Toxicology of anti-corrosive agents
Monoethanolamine (MEA; CAS no. 141-43-5) and triethanolamine (TEA; CAS no. 102-71-6) are found in aqueous formulations because of their anti-corrosive properties. Diethanolamine (DEA; CAS no. 111-42-2), on the other hand, can be found in formulations containing alkanolamides as an impurity (Lynn and Bory, 1997; Oldenhove de Guertechin, 1999). The main absorption route for ethanolamines is skin, but inhalation can be of concern when aerosols are generated. MEA, DEA and TEA are irritating to the skin, eyes and respiratory tract, with the irritancy decreasing from MEA to TEA (Knaak et al., 1997). They have been shown to be causally related to allergic dermatitis and bronchitis following exposure to cutting fluids, cosmetics and detergents (Savonius et al., 1994; IARC, 2000a,c). MEA, DEA and TEA have TLV-TWA values of 7, 2 and 5 mg/m3, respectively; MEA and DEA also have a 15 mg/m3 short-term exposure limit (TLV-STEL) and a skin notation, respectively. These limits are based on irritation for MEA, irritation and systemic effects for TEA and solely on systemic effects for DEA (ACGIH, 2003). A MEA 6 h TWA concentration of 0.25 mg/m3 was measured near an aqueous cleaning system, which contained 20–30% d-limonene, 1–5% MEA and 5–10% DEA (DEA wasn’t detected) (Kiefer et al., 1993). DEA and TEA are able to react with nitrosating agents (the reaction is 10–20 times slower in the case of TEA because it needs to be dealkylated first) in the environment or the body, to form N-nitrosodiethanolamine, a substance classified as possibly carcinogenic to humans by the IARC (IARC, 2000b).
Sodium nitrite (NaNO2, CAS no. 7632-00-0) is used in aqueous formulations because of its anti-corrosive and bactericidal properties. It is also found in the food industry as a preservative and colour fixing agent. Water solutions of sodium nitrite typically have a pH of ~9 and are not irritating to the skin or eyes (HSDB, 2000a). The main occupational absorption route for sodium nitrite is the inhalation of aerosols. A 4 h LC50 of 5.5 mg/m3 has been reported in rats (REPTOX, 1992). The principal reported mechanism of sodium nitrite toxicity is methaemoglobinaemia (Vittozzi, 1992). Sodium nitrite is not suspected of exerting teratogenic effects, although some authors consider this compound as capable of causing adverse effects on development (Vittozzi, 1992; Sørensen and Styhr Petersen, 1994; Frazier and Hage, 1998).
Toxicology of solvents
d-Limonene (CAS no. 5989-27-5), a combustible liquid hydrocarbon, is widely used in current aqueous formulations. Recent reviews are available about the toxicology of this substance (Karlberg and Lindell, 1993; Falk-Filipsson et al., 1998; Bégin and Gérin, 1999). Thus, d-limonene is considered to be irritating to skin and eyes and seems to possess an allergenic potential linked to its auto-oxidation products. This compound is easily absorbed through the lungs and gastrointestinal (GI) tract and to a lesser extent through skin. The AIHA recommends an 8 h WEEL of 166 mg/m3 based on long-term hepatic effects in rats (AIHA, 1993, 2002). d-Limonene was measured at levels between 0.9 and 6 mg/m3 in the vicinity of an opened immersion degreasing system in which an aqueous formulation containing 6% d-limonene was used (Karlberg and Lindell, 1993).
Glycol ethers are amphiphilic solvents of low volatility. They can be divided into two categories based on their derivation from ethylene glycol or propylene glycol. The toxicology of glycol ethers was recently the subject of several reviews (Guillemin, 1994; ECETOC, 1995; INSERM, 1999; Boatman and Knaak, 2001; Cragg and Boatman, 2001). They are, in varying degrees, irritants of the eye, skin and respiratory tract and are easily absorbed through the skin, lungs and GI tract (INSERM, 1999). Haemolysis and adverse effects on fertility and development by several ethylene glycol-based ethers have been observed in animals. In humans, leucopenia and anaemia have been observed following occupational exposure to three ethylene glycol-based ethers. Furthermore, epidemiological evidence strongly suggests an association between some glycol ethers in this series and male infertility. These compounds share a similar elimination pathway that involves the formation of toxic metabolites. Homologues of higher molecular weight in this series (e.g. 2-butoxyethanol) seem to present no reproductive hazard and less haemolytic hazard towards humans; however, there is uncertainty as to their carcinogenic potential (INSERM, 1999; ACGIH, 2003). Glycol ethers based on propylene glycol and whose alcohol function is secondary are not suspected of affecting reproduction or development because of a distinct metabolic pathway. No adverse effect on blood and the haemopoietic system has been reported for propylene glycol-based compounds (INSERM, 1999). TLV-TWA and STEL values for several glycol ethers are based on irritation or systemic effects (ACGIH, 2003). Regarding exposures, diethylene glycol butyl ether (DGBE; CAS no. 112-34-5) was not detected in the vicinity of an immersion aqueous cleaning system containing water, DGBE and soda flake (Decker and Driscoll, 1991). Another study reported a significant increase in a DGBE metabolite in the urine of printing workers exposed to a water-based cleaner containing between 10 and 15% of this compound (Göen et al., 2001). Average concentrations of 2 and 4.2 mg/m3 DGBE were measured in a poorly ventilated room during washing of the walls with two undiluted detergent products containing 4 and 8% of that compound, respectively (Gibson et al., 1991). Full-shift personal TWA concentrations of 2-butoxyethanol were measured in the breathing zone of car cleaning workers using aqueous window cleaning formulations. The concentrations were between <0.48 and 0.87 mg/m3 (median <0.48) for the formulation containing 5.7% 2-butoxyethanol, but they ranged between <0.48 and 35.43 mg/m3 (median 0.70) for two other formulations containing 21.2% of this compound. Significant increases in urinary concentrations of a 2-butoxyethanol metabolite were observed between the beginning and the end of the working shift for all formulations (Vincent et al., 1993).
Toxicology of sodium xylene sulphonate
Sodium xylene sulphonate [(CH3)2C6H3SO3–Na+, CAS no. 1300-72-7] is used in a number of neutral aqueous formulations for its dispersant and hydro-tropic properties. It has very low acute toxicity. No information about the absorption, distribution and metabolism of sodium xylene sulphonate has been found in the literature (HSDB, 1994). The few available data about the toxicity of this substance come from the US National Toxicology Program’s studies on its carcinogenicity (NTP Working Group, 1998). Chronic cutaneous administration of doses as high as 240 mg/kg/day in rats and 727 mg/kg/day in mice caused no observed clinical effect or carcinogenic activity. Cutaneous NOELs for subchronic exposure reached 800 and 2000 mg/kg/day in rats and mice, respectively.
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Water pollution is cited as a potential weakness of aqueous cleaners compared with traditional solvents in terms of environmental impact (Anonymous, 1997). Indeed, liquid waste is the major effluent of aqueous cleaning (Morris and Wolf, 1998a,b; Russo, 2001). The following sections deal with the environmental effects of aqueous cleaners and with avenues for minimizing wastes.
Composition of aqueous cleaning effluents
As seen previously, the cleaning process involves a range of interactions between metal parts, liquid or solid soil, the cleaning solution and the cleaning system, which provides thermal and mechanical energy. This leads to the generation of mainly liquid waste that will contain, depending on the conditions of operation, excess thermal energy, alkalinity caused by builders or sequestrants, free or chemically bound active agents and a variable amount of liquid and/or solid soil, either dissolved or suspended. Heavy metals such as lead or cadmium, either as particles or in solution, may also be found as a result of chemical or mechanical abrasion of the parts (Morris and Wolf, 1998a,b; Russo, 2001).
Most soil in metal degreasing is composed of lubricants with specific characteristics depending on their function. Film anti-corrosion fluids and lubricating oils can be non-polar mineral oils or polar compounds such as fatty acids, greases (typically composed of mineral oil thickened with a gel-forming substance such as soaps of lithium, calcium or magnesium) or synthetic oils (aqueous micellar solutions containing components such as polyisobutylene or polyglycols). Metal forming and metal machining fluids are mainly composed of soluble oils (macro-emulsions of oil in water), emulsions and fatty acids. Engine and transmission lubricants are mostly mineral oils and greases. Forging fluids, composed of mineral oils and greases, often have a high graphite content and are subjected during use to extreme conditions, leading to cooked-on soil. Several metal working compounds (such as fluids for buffing, lapping, chipping and burnishing), in addition to their water-soluble liquid phase (light emulsions or soaps), often contain fine particles (Spring, 1974; Peterson, 1997; Booser, 1999). Numerous additives that are potentially harmful to the environment may also be present in lubricants. Most solid soil encountered in metal cleaning is comprised of metallic oxides of silica or alumina or other mineral colloids, not soluble in water or organic compounds (Spring, 1974).
Non-aqueous waste, including residual sludge, spent filters and recovered oil, is also generated by aqueous cleaning, mostly when recycling is implemented to reduce liquid effluents (Tetra Tech EM Inc., 1999a,b). Solid waste generally has to be treated as hazardous material but is often in much smaller quantities than liquid waste and is, therefore, more easily disposed of (Wolf, 1994; Wolf and Morris, 1997; McLaughlin and Zisman, 1998).
Specific environmental properties of some aqueous cleaner components
It is not within the scope of this paper to review the environmental properties of all the aqueous components previously described, especially since most of them are only present in small quantities in cleaning effluents (see sections Components of aqueous cleaners and Equipment used in aqueous cleaning). Therefore, more attention has been put on compounds that appear to present a potentially greater environmental hazard than that posed by the biological oxygen demand.
Surfactants
Most of the surfactants used in detergents are ubiquitous in the environment as a result of their widespread industrial and domestic use and subsequent discharge into public wastewater or into surface waters. In fact, industrial aqueous cleaning accounts for a minor proportion of the total surfactant consumption (<5% in Germany) (BUA, 1998). Surfactants have mostly been observed in surface water and, to a smaller extent, in sediments (Talmage, 1994; IPCS, 1996). Surfactants have mostly been measured in river water at levels below 0.01 mg/l, while concentrations >1 mg/l have been found in highly contaminated areas (for example upstream from sewage treatment plants) (Talmage, 1994; IPCS, 1996). These concentrations are generally below acute toxicity thresholds for aquatic flora and fauna. However, the implications of these discharges for chronic toxicity are not well understood (Lewis, 1991). Based upon the results of biodegradation tests, the non-ionic EO/PO block copolymers and alkylphenol ethoxylates are not readily biodegraded in sewage treatment plants or the environment. Nonetheless, EO/PO block copolymers seem to have a very low ecotoxic potency (Bailey, 1996). Likewise, metabolites of alkylphenol ethoxylates (alkylphenols and mono- and di-ethoxylated alkylphenols) also seem to be resistant to biodegradation. However, the degradation is variable for these metabolites in sewage treatment, with between 5 and 96% removed (Talmage, 1994). These metabolites, which also have industrial uses (such as nonylphenol), are not only more persistent in the environment but have also been shown to be 25- to 250-fold more toxic than their mother substances, depending on the toxic end point and type of study (acute or chronic) (Miles-Richardson et al., 1999). Moreover, nonylphenol and its mono- and di-ethoxylated derivatives are considered to be endocrine disruptors and have caused sublethal and behavioural effects at environmentally relevant levels (Talmage, 1994; Miles-Richardson et al., 1999).
Builders
Once in the environment, the different types of phosphates are quickly converted into orthophosphates. Although not particularly toxic to fauna or flora, the continuous discharge of orthophosphates into the environment over many years has caused the eutrophication of rivers; this led to the banning of these compounds from a range of household products (such as laundry detergents) during the 1970s. Nonetheless, phosphates are still widely used in industrial formulations. Wastewater treatment plants equipped with tertiary treatment completely remove phosphates by a precipitation process (Jakobi et al., 1985; Sørensen and Styhr Petersen, 1994).
Sequestrants
Detergents are the main source of NTA and EDTA emissions into the environment (HSDB, 2000b, 2001). Both of these compounds are readily biodegradable by aerobic and anaerobic microorganisms and have only low ecotoxic properties (HSDB, 2000b, 2001). According to Sørensen and Styhr Petersen (1994), citing a Danish government document, the affinities of EDTA and NTA for heavy metals are sufficient to allow their discharge from wastewater treatment plants in the form of metal chelates and their subsequent environmental release. Moreover, free NTA is likely to form complexes with metals stabilized in sediments, subsequently releasing them in water (Lo et al., 1994). However, these phenomena, observed to a significant extent for metals such as copper, zinc, lead and cadmium at elevated NTA concentrations, are minimal at current environmental levels (Anderson et al., 1985).
Solvents
Glycol ethers and d-limonene are considered as volatile organic compounds (VOCs) by US and European regulations, i.e. capable of contributing to the formation of tropospheric ozone (Council of the European Union, 1999; US EPA, undated).
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RECYCLING AND WASTE DISPOSAL OF AQUEOUS CLEANERS |
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Low parts throughput equipment
This type of equipment generally operates on a batch basis (i.e. once in the system, the cleaning solution is recirculated until it loses efficiency). It is then disposed of and replaced. A feasibility study in Los Angeles County showed lifetimes of the solutions varying between 4 and 6 months, even reaching 1.5 yr in the case of enzymatic digestion systems (Tetra Tech EM Inc., 1999a). Available means for increasing the cleaning lifetime include the use of filters retaining gross particulate matter, the addition of supplementary concentrate and the use of oil skimmers that collect oil and grease accumulated at the solution surface. The recovered oil can then be transferred to the plant’s used oil collector system or sent to a waste treatment facility (Morris and Wolf, 1998b; Tetra Tech EM Inc., 1999a). Morris and Wolf recommend the use of formulations based on oil dispersion rather than solubilization or emulsification. The former favours the accumulation of oil at the surface of the solution, facilitating its subsequent recycling. Enzymatic cleaning systems are an exception to this recommendation since the solubilized or emulsified soils enhance enzymatic digestion (Morris and Wolf, 1998b).
Several feasibility studies in the US report that none of the tested solutions were in compliance with the local standards for discharge into sewers, mostly because of oil content (Morris and Wolf, 1998b; Tetra Tech EM Inc., 1999a). Moreover, 75% of the solutions analysed during the study of Morris and Wolf were designated as hazardous materials under Californian laws, because of heavy metal contamination (such as cadmium and lead) (Morris and Wolf, 1998b). As a consequence, a variety of vendors provide customer services that include used solution, used filter and residual sludge disposal (Tetra Tech EM Inc., 1999a,b).
High parts throughput equipment
Integrated systems, often including washing and rinsing stages, use large quantities of water. Effluent decontamination helps to minimize liquid waste quantities and active agent recycling serves to enhance cleaning efficiency (Endres and Bolkan, 2000). The creation of a recycling system implies a rigorous technical and economic analysis, especially since such systems often represent significant investments (Lindsey et al., 1994; McLaughlin and Zisman, 1998; Russo, 2001). According to Russo (2001), investing in a recycling system is probably not viable in the case of cleaning systems generating less than 300 l of effluent per week. However, if recycling is justified, several types of techniques can be combined.
Pretreatment methods
MacLaughlin and Zisman describe an effluent pretreatment method that includes a primary flocculation step, followed by acidification, which breaks up oil emulsions. The solution is thereafter alkalinized in order to precipitate metal ions. This procedure allows a significant amount of the contamination to be removed when the solution passes through clarifiers, settling tanks or skimming systems (McLaughlin and Zisman, 1998).
Filtration methods
Metal grids of various dimensions (50–500 µm) allow for the separation of gross solids from the solution. Disposable paper filters allow insoluble oil to be trapped, but constitute an added source of solid waste. Activated carbon cartridges retain organic matter, but their capacity is limited and they also remove active agents that cannot then be recycled. McLaughlin and Zisman (1998) recommend the use of activated carbon as a last step before discharge, to minimize effluent contaminant levels. Membrane filtration methods, including microfiltration, ultrafiltration and reverse osmosis, are highly efficient recycling methods, but are costly. Microfiltration, which retains oil emulsions but generally allows free surfactants and other active agents to cross the membrane, is best suited for recycling cleaning solutions. Ultrafiltration and reverse osmosis, being more stringent methods, are better adapted to rinsing solutions and effluent treatment before discharge (Heller, 1999). Membrane filtration methods are efficient but require added process control because of their sensitivity to varying filtration conditions (Underwood and Thomas, 1995). The use of these methods to recycle cleaning solutions is estimated to expand their lifetime 3- to 7-fold (Underwood and Thomas, 1995; Heller, 1999).
Evaporation
According to Isaacs (2000), evaporation would allow a 90–99% reduction in the amount of treated effluents. Evaporation involves the heating of effluents to their boiling point, with regular addition of solution, until the liquid phase is too concentrated to allow an efficient separation. The author estimates that the cost of this method varies between US$0.03/gal and 0.30/gal, compared with 0.2–1/gal for treatment by a specialized company (Isaacs, 2000). Evaporation requires special maintenance that is not always readily available in a facility (Isaacs, 2000) and can require permits for atmospheric emission, depending on local regulations (Russo, 2001).
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Technical aspects
Aqueous cleaners for metal degreasing are nowadays proposed as substitutes for chlorinated solvents and petroleum fractions. The market for these products has undergone significant development since 1996, motivated by Californian legislation that outlawed the use of products containing >5% VOCs. This led to the availability on the current US market of a number of systems and formulations, allowing for more flexibility in the choice of a particular cleaning solution (Thomas et al., 1997).
Aqueous cleaner components
The identification of the different possible components of these complex mixtures is a critical aspect of the study of water-based cleaning formulations. In this review, the bulk of the consulted literature dealt with household detergents, which could have led to a possible bias in determining the composition of current industrial parts cleaning formulations. Moreover, the vast array of potential constituents rendered any exhaustive treatment unrealistic. However, the collection of current product MSDSs, the consultation of formulation monographs (Flick, 1989, 1994, 1995, 1996) and expert review allowed for a better appraisal of the composition of actual products and guided the choice of the compounds to be studied.
MSDS documentation was often very poor, especially regarding surfactants. Hence, quotations such as ‘neutral detergent’ in place of the surfactant type was common, even in the formulation monographs. This phenomenon was particularly significant for the most recent formulations, which tend to be neutral and to rely mostly on non-ionic surfactants and dispersants and for which the hazardous components section of the MSDS was almost always blank. With reference to this issue, Welsch et al. reported that several Canadian and American studies showed that a significant proportion of MSDS for chemical mixtures did not conform to regulations requiring disclosure of hazardous substances. The authors analysed a water-based formulation containing no hazardous compounds according to the MSDS and found that the product actually contained 8% 2-butoxyethanol and significant amounts of silicates, which should have been mentioned (Welsh et al., 2000).
Setting up an aqueous cleaning system
Compared with traditional organic solvents, aqueous cleaners are characterized by narrower spectra of soil removed by a given combination of chemicals, conditions and equipment, requiring extra care in making a substitution (Wolf, 1994; Ellenbecker, 1996; Knipe, 1997; Kanegsberg, 1998). The following parameters are critical determinants of efficiency in aqueous cleaning: the formulation, its dilution, the temperature of operation, the length of cleaning, the mechanical energy brought to the system and the efficiency of auxiliary components that expose the substrate to the cleaning and rinsing solutions and to the drying system (Wolf, 1994). The optimization of these parameters requires careful consideration of the following points.
Type of substrate to be cleaned. The detergent formulation should be selected according to the substrate’s chemical compatibilities and corrosion potential. This also applies when choosing the cleaning apparatus (for example, immersion seems more appropriate for parts with complex geometry).
Type of soil to be removed. The importance of the soil’s characteristics is illustrated by the following general rules: mineral lubricants are solubilized or dispersed by concentrated non-ionic surfactants; solid particles are efficiently removed by anionic surfactants, while greases are disrupted by solubilization of their gel-forming components in alkaline media. Baked-on or polymerized soil usually requires the use of supplementary energy, such as higher temperatures or the use of spraying or ultrasonic systems.
Functional requirements following cleaning. Since these steps imply additional costs, the need for drying and/or rinsing has to be carefully assessed in the light of the requirements of the process steps following cleaning.
Flexibility in work organization surrounding the cleaning task. The wide range of available aqueous cleaning equipment (such as immersion and automatic spraying or ultrasonic systems) may allow for significant labour reduction (Wolf and Morris, 1997; Tetra Tech EM Inc., 1999a). Its use, however, requires some work reorganization since, for example, workers may have to manage the time during which parts are automatically cleaned.
Once the equipment and formulation have been selected, the implementation of small scale tests is recommended to verify and optimize the cleaning efficiency (Knipe, 1997; McLaughlin and Zisman, 1998). McLaughlin and Zisman (1998) have described simple methods to simulate different aqueous cleaning systems.
The importance of the preceding steps in implementing aqueous cleaning can be illustrated by the case of the City of Los Angeles public facilities (Tetra Tech EM Inc., 1999a). Following legislation limiting VOC emissions, the City replaced all the solvent sink-on-drum systems with aqueous sink-on-drum cleaning. This ‘drop in’ replacement partially failed in many cases because the cleaning often took more time and was less efficient than with solvents. Subsequent re-analysis of the cleaning needs of each facility, with the implementation of optimal aqueous cleaning systems, led to significant reductions in cleaning cost at most facilities. Numerous case studies of aqueous cleaning implementation have been reported. All have achieved satisfactory cleaning efficiency and, for the majority, cost reduction, often due to reduced labour time. Most of the authors emphasize that, despite their advantages, aqueous cleaning systems generally require higher initial investment than traditional solvent systems (Randall and Kranz, 1994; Goris, 1995; Hunt and Linton, 1996; Wolf and Morris, 1997; Morris and Wolf, 1998a,b; Tetra Tech EM Inc., 1999a,b). The SAGE (Solvent Alternatives Guide) software, published by the Research Triangle Institute (Research Triangle Park, NC) and available on the Web, is a useful tool for the selection of a cleaning formulation (Anonymous, 2001).
Occupational health aspects
Workplace exposure
Very few data are available regarding workplace exposure to aqueous cleaners, even though some components such as ethanolamines, glycol ethers, d-limonene and some builders have occupational exposure limits. The studies reported by Göen et al. (2001), Gibson et al. (1991) and Vincent et al. (1993), although not directly relevant to metal parts degreasing, illustrate the potential for exposure to glycol ethers diluted in water. Actual exposure should be lower in most parts cleaning operations because of the higher dilutions used (except when using mist generating equipment). The airborne levels of d-limonene, glycol ethers and ethanolamines reported by Karlberg and Lindell (1993), Kiefer et al. (1993) and Decker and Driscoll (1991) point towards low concentrations of these contaminants but are insufficient to make definitive conclusions.
Looking at the theoretical potential of exposure to aqueous cleaners, two different exposure scenarios can be distinguished.
Exposure to the concentrate (powder or liquid). Aqueous formulation concentrates may contain up to 15% surfactants, 60–70% builders and 10% solvents, with other additives generally being present at levels below 5%. Exposure to the concentrate can occur during cleaning solution replacement or during the feeding of a continuous system. This scenario seems to involve relatively little potential for exposure since aqueous cleaning solutions have lifetimes of several months and continuous systems are often automatically fed with concentrate.
Exposure to the cleaning solution. The various dilutions of concentrates used in aqueous cleaning are highly variable, with values found in the literature ranging from 2 to 100%. Median values would be between 3 and 20%, with lower dilutions in sink-on-drum systems. Exposure to the cleaning solution is likely to occur according to a variety of scenarios depending on the cleaning equipment. Hence, with sink-on-drum or immersion systems, skin contact appears to be the most likely exposure route. However, eye contact can also occur during accidental splashes and VOCs can be inhaled, especially since the cleaning solutions are often heated to 40°C. In spraying systems such as spray cabinets, which are generally operated at higher temperature (typically 90°C), exposure to cleaning solution mists or vapours is likely to occur during opening of the system after the cleaning cycle. Exposure to mists will also occur when power washer systems with spray wands are used.
Health effects of water-based formulations
As a general rule, surfactants used in aqueous cleaning can be considered as having low acute and chronic systemic toxicity which, together with their low potential for absorption, should prevent the occurrence of toxic effects. Most of them are irritating to the skin and/or eyes. Anionic surfactants from the sulphate and sulphonate families are most likely to be irritating at dilutions encountered in cleaning solutions. The allergic properties of surfactants are poorly documented in the literature and recent studies describe the potential of ethoxylated compounds to oxidize into allergenic products (Bergh et al., 1997, 1998; Karlberg and Bergh, 1999).
Irritation and/or corrosion caused at high pH by builders represent their principal toxic potential. Borates have been shown to exert reproductive and developmental toxicity in animals. However, borates are very poorly absorbed via the skin and their potential for becoming airborne is quite low; thus the potential for toxic effects from borates is considered small.
Among the different sequestering agents described in this review, sodium gluconate does not present any toxic potential. NTA is classified as a possible carcinogen. It is impossible to determine the reproductive toxicity of EDTA since the reported effects are contradictory and have been mostly observed at doses toxic for the mother. However, again, any significant absorption of EDTA and NTA during aqueous cleaning is unlikely.
Ethanolamines are absorbed through the skin and several allergic cutaneous or bronchial reactions have been attributed to their presence in detergents or cutting oils. Diethanolamine and triethanolamine, moreover, can react with nitrogenous compounds (such as nitrites) to produce N-nitrosodiethanolamine, a possible carcinogen. Nitrites are not likely to be absorbed in harmful doses during aqueous cleaning.
Glycol ethers, although present in limited quantities in most aqueous formulations (i.e. <10% in the concentrate), are generally well absorbed through the skin and lungs. Most ethylene glycol-based compounds are toxic to blood, reproduction and development. On the other hand, glycol ethers derived from propylene glycol with a secondary alcohol function have no significant systemic toxic potential. The different MSDSs collected by the authors showed a trend toward the use of these glycol ethers, but compounds derived from ethylene glycol such as 2-butoxyethanol still represent a large part of the market. The relevance to humans of the confirmed animal carcinogenicity of this compound is unknown (ACGIH, 2003). Considering the conditions of use of aqueous cleaners, d-limonene is believed to present only a low risk of toxic effects, while sodium xylene sulphonate should present no risk at all.
Synthesis regarding occupational health aspects
The lack of occupational health data and the fact that aqueous cleaners are complex mixtures of active agents do not permit any definitive health risk assessment of these products. However, taking into account the low concentrations of the various components and the low workplace exposure potential (mainly skin contact and accidental eye splashes), aqueous cleaners should be considered as generally posing no significant health risk to workers. There are some exceptions, however. Alkaline formulations or formulations containing surfactants of the sulphate or sulphonate families may cause acute or chronic skin irritation, ethanolamines are allergenic and some glycol ethers may enter the body through the skin to cause systemic toxic effects. Pulmonary irritation may also result from exposure to alkaline mists when spraying-based equipment is opened after operation at high temperatures or power spray wands are used. Other toxic compounds, not described here, may be present in aqueous formulations. Furthermore, the various soil components (e.g. oils and metals) should also be taken into account. Surprisingly, no discussion of this specific issue was found in the literature.
Environmental aspects
Aqueous cleaning generates effluents with variable characteristics depending on the formulation, the type of soil, the substrate and the equipment used for cleaning. Certain compounds are potentially toxic to the environment, namely alkaline compounds, oils and greases, solid suspended matter, dissolved organic compounds and heavy metals. Alkylphenol ethoxylates can biodegrade to persistent metabolites suspected of endocrine disruption activity towards aquatic organisms. Regulations already exist in Canada to reduce the use of these surfactants and similar regulations are currently being drafted in Europe (Renner, 1997; Senior, 2000; Government of Canada, 2003). Phosphates cause the eutrophication of rivers and lakes and are eliminated only in wastewater treatment plants having tertiary chemical treatment. Also, EDTA and NTA, because of their strong chelating ability, may permit heavy metals to bypass water treatment processes. Solvents such as glycol ethers or d-limonene can be emitted into the atmosphere. However, these chemicals are too diluted in aqueous cleaning solutions to present a significant source of VOCs in the atmosphere (median concentration of 5% in concentrates, divided by 3–20 after dilution).
In most cases, direct aqueous cleaning effluents do not conform to laws regulating emission into rivers or public sewers (Morris and Wolf, 1998b; Tetra Tech EM Inc., 1999a,b). Compliance with these laws requires pollution prevention measures, including neutralization of alkalinity, oil recovery (filtration, skimming, ultra- and micro-filtration), solid matter recovery (filtration, sedimentation) and dissolved metal recovery (ion exchange). These measures not only prevent entry into the environment of toxic compounds but also allow for the optimization of water-based cleaner systems. Several vendors of aqueous cleaners provide an effluent recovery service, thus taking the legislative weight off the user’s shoulders.
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Considering technical, health and environmental aspects, aqueous cleaners are acceptable substitutes for traditional solvents for metal parts degreasing. The health risk posed by aqueous cleaners should be small in most cases. However, the use of highly alkaline solutions or solutions containing high concentrations of alkyl sulphates or alkyl sulphonates should be minimized. Ethylene glycol-based glycol ethers and ethanolamines should also be avoided. Since workplace exposure is generally limited to skin contact and accidental eye splashes, the use of protective gloves and glasses is recommended. This is true for all aqueous cleaners because of the potential effects of soil present in used cleaning solutions. The composition of aqueous cleaning formulations used in spray equipment operated at hot temperatures should be carefully examined because of potential exposure to mists. Effluents generated by aqueous cleaners are likely to cause toxic effects if emitted directly into surface waters, mainly because of their possible alkalinity and their content of dissolved or suspended organic matter and metals. Contamination can be reduced by reducing effluent quantities (e.g. longer solution lifetime), by effluent treatment (e.g. recycling) and by using formulations containing compounds with low environmental impacts. Thus, the use of phosphates, EDTA, NTA and alkylphenol ethoxylates should be reduced.
Because of the diversity of available water-based formulations and ingredients, it is possible to choose products containing compounds with minimal environmental and health effects. However, to arrive at these choices, industrial hygienists should obtain detailed compositions from manufacturers, beyond what is available in the MSDSs. This should help in integrating environmental and health concerns with the implementation of water-based cleaning. Solvent substitution often leads to a compromise between technical, health and safety and environmental aspects, and aqueous cleaners are no exception. Systematic substitution procedures proposed by various authors allow the integration of all critical issues (Goldschmidt, 1993; Callahan and Green, 1995; Gérin et al., 1996).
Acknowledgements—The authors would like to thank the Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail (IRSST) for its financial support (grant no. 099-070).
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* Author to whom correspondence should be addressed. Tel: +1-514-343-6134; fax: +1-514-343-2200; e-mail: michel.gerin{at}umontreal.ca
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TOPABSTRACTINTRODUCTIONMETHODSPHYSICOCHEMISTRY OF AQUEOUS...TOXICOLOGY OF AQUEOUS CLEANERSENVIRONMENTAL IMPACT OF AQUEOUS...RECYCLING AND WASTE DISPOSAL...DISCUSSIONCONCLUSIONSREFERENCES |
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