Biogeochemistry of Chelating Agents

Chapter 13

Distribution and Fate of Chelating Agents in the Environment Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 19, 2017 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0910.ch013

M i k a E. T. Sillanpää Department of Environmental Sciences, University of Kuopio, P.O. Box 181, 50101 Mikkeli, Finland

This chapter discusses the behaviour of the chelating agents EDTA and DTPA in the receiving aquatic environment. EDTA and DTPA are used in high amounts in different industrial and household applications and this has raised concern about their ultimate fate in the environment. These compounds are not expected to cause direct ecotoxicological effects at the levels typically found in natural waters. However, they do contain nitrogen and have the capability to affect metal balance in aquatic ecosystems.

Parts of this chapter are adapted with permission from Environmental Fate of EDTA and DTPA 1997,152, 89, 95, 98-104. Copyright 1997 Springs-Derlag. Nowack and VanBriesen; Biogeochemistry of Chelating Agents ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 19, 2017 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0910.ch013

227 Ethylenediarainetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) are synthetic complexing agents that have been utilized extensively as metal séquestrants by a wide variety of industries. EDTA has been used mainly as a chelating agent, e.g., in the metal, rubber, leather, photographic, textile, pulp and paper, pharmaceutic, cosmetic, and food industries. Applications of DTPA include the inaction of metal ions in the pulp and paper industry and use as a drug in heavy metal poisoning. The huge amounts of these compounds used in industrial applications as well as in households and agriculture have raised concern about their ultimate release to the aquatic environment. The production of EDTA in Europe was about 32,740 metric tons in 1998 (1). The sales quantity of DTPA was 14,000 tons in 1997 (2). EDTA and DTPA are expected to be released to aquatic environments due to their relatively low degradability. This chapter discusses the possible environmental impacts that these compounds may have in receiving aquatic environments.

Environmental Occurrence EDTA is commonly found in natural waters, typical concentrations range from few μg/L up to 100 pg/L (3-5). Thus, EDTA is classified as one of the major organic anthropogenic pollutants in Central Europe. DTPA has received much less attention. However, DTPA is found at concentrations between 2 and 15 pg/L in the river Rhine, Germany (6) and between 9 and 18 pg/L in a Finnish lake near a pulp mill (7). Chemical equilibrium calculations are complicated by the presence of competing natural and anthropogenic ligands, concentrations of different cations, and suspended particles and sediments. The initial input of EDTA and DTPA also plays a significant role. EDTA is known to react with metal cations at a molar ration of 1:1. Because of slow dissociation, the proportion of Fe(III)-EDTA cannot be assessed by means of calculated equilibrium speciation (8). It has been proven that exchange reactions do occur, in the presence of excess of Ca over trace metal, under natural aquatic conditions (9). Fe-EDTA also converts slowly into Zn thermodynamicaily favorable species under investigated conditions (8). However, it was also reported that, under anoxic conditions, the reduction of Fe could lead to increased exchanged reaction. Ca and Mg chelates were found to be the dominant EDTA species in sea water (10). Metal-DTPA interactions in natural waters has recently been studied (11). The detailed study on the speciation of complexing agents is presented elsewhere in this book. Adsorption of free EDTA onto metal oxides has been demonstrated (12,13). EDTA was negligibly sorbed to clay minerals and sediments. Adsorption of various metal-EDTA complexes onto aluminum oxides and crystalline and amorphous iron oxides as well as onto goethite has also been investigated (14,15). It was reported that the maximum adsorption capacity of Fe-EDTA chelated and that several adsorption mechanisms were revealed, depending on the structure of divalent and tri valent metal-EDTA chelates (14). The adsorption behavior of DTPA and its metal complexes has received little research attention.

Nowack and VanBriesen; Biogeochemistry of Chelating Agents ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

228


Degradation Knowledge of the rate of degradation of pollutants by both biotic and abiotic processes when released to natural water is a key issue when estimating their fate. From the ecological point of view, one of the most important properties of environmentally compatible organic chemicals is their biological degradability. Biodégradation is a primary means of organic compound removal in the environment (16). The biological degradation has received recent attention and is discussed extensively elsewhere in this book. However, it can be concluded here that EDTA was long considered non-biodegradable, but recent studies show its ability to be oxidized by microorganisms (2). Photochemical transformation is strongly dependent on natural conditions. Thus, the results obtained at maximum conditions in the laboratory cannot directly be applied to natural waters, where several factors may have an impact, when estimating the half-lives of pollutants. In natural waters, sunlight intensity is attenuated through adsorption and scattering. The photolysis rates of pollutants in pure water, seawater, and inland water and their dependence on conditions such as season, latitude, time of day, depth, ozone layer thickness and light attenuation are convincingly discussed in Reference (17). There exsists wide-spread unanimity concerning the photolability of the FeEDTA complex (18-22). Also, Fe-DTPA was found to be photolabile (). Free DTPA is shown to be much more photodegradable than free EDTA (23). The possible interactions of EDTA, DTPA, and their metal complexes with sediments and, perhaps more importantly, suspended matter were not considered in the foregoing studies; therefore, the results apply only when in solution. It has been reported that Fe-EDTA is the only environmentally relevant EDTA species that undergoes direct photolysis (24,25). It was concluded that in shallow rivers Fe-EDTA is rapidly photodegraded in summer, but the rate may be decreased in lakes because of additional light attenuation. It was also sugguested that the EDTA found in the North and Black Seas consists of species that are resistant to photochemical and biological decomposition. As indicated, photolytic degradation may be a remarkable pathway to prevent the environmental accumulation of EDTA and DTPA. These compounds are already released to receiving waters as iron chelates to some extent. Also natural waters contain ferric ions and thus other metal complexes might be converted into Fe complexes in due course. On the other hand, iron in natural waters is present in its colloidal and amorphous iron hydroxides and thus may not be complexed by EDTA (18). Also the slow dissociation of Fe-EDTA in natural waters has been convincingly reported (8). Laboratory conditions do not directly apply to natural aquatic environment. At a minimum, the absorption of U V light by solids, plants, and macrophytes, and the daily and annual periodicity of light, cloudiness, ice cover, and shadowing effects, should be taken into consideration. Also, the intensity of sunlight decreases with angular height of the sun, from the tropics to higher latitudes. Thus, it is extremely difficult to estimate the amount of available light within the waters column. Actually, photodegradation might be a significant factor in EDTA decomposition, because, in the case of water environments, little or no light is received except for immediate surfaces (22,26).


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Other abiotic processes man photodegradation are not suggested to contribute significantly to EDTA degradation (18). EDTA and DTPA are rather stable in industrial processes (27). Environmentally relevant degradation processes of EDTA have been discussed (24). It was concluded that hydrolysis, reaction with solvated electrons, organic peroxyradicais, singlet oxygen, and hydroxyi radicals are irrelevant as transformation pathways for EDTA and its metal complexes in river water that have short residence time. However, they reported that chemical degradation, excluding direct photolysis of the Fe-EDTA, may have some significance in the long-term behavior of EDTA and its complexes. It is expected that these conclusions apply to DTPA as well.

Ecological Risks Eutrophic Effects Algal growth in natural waters is dependent on the availability of nitrogen and phosphorus. Nitrogen is often the limiting factor in the marine environment, while phosphorus usually limits growth in limnic systems (28). When these nutrients are present in excess, algal growth can be inhibited also by the lack of essential trace elements such as Cu, Zn, and Fe mat are indispensable for photosynthetic production. The importance of iron as a growth-limiting factor in offshore areas has been emphasized (29). EDTA and DTPA contain about 10% nitrogen, and thus have, particularly once mineralized, the potential to contribute to eutrophication. As significant amounts of these compounds are used in several industries as well as in agriculture and households and cannot effectively be removed in waste water treatment plants, the role of EDTA and DTPA as a nitrogen source for algae growth is one of the main concerns in assessing their environmental impact. For example, EDTA has been found to be necessary for the growth of Microcystis aeuginosa (30). The addition of EDTA also enhanced the growth of Dunadiella and Amphidinium (28).The nitrogen in EDTA and DTPA cannot likely be used directly by algae; I, e., degradation of the molecule is required. As indicated, under certain circumstances either biological or photochemical degradation might occur, causing nitrogen to be present in bioavailable form. Inorganic phosphates are present in sediments mainly in their Fe- and Ca- bound forms (31). EDTA and DTPA are known to form the most stable complexes with trivatent iron. Therefore, EDTA and DTPA may be able to desorb Fe form sediments, remobilizing phosphates into natural waters and thus, indirectly, enhancing eutrophication. In summary, data are scarce and the knowledge of the direct influence of EDTA and DTPA on algal growth is rather limited.

Direct Ecotoxicological Effects Daphnia, being an important food source for vertebrate and invertebrate predators, was used in many of the toxic studies. EDTA and DTPA seem to be



230 relatively nontoxic, as shown by acute exposure (32-37). However, in general, little is known of their chronic toxicity, which is of importance because these agents are likely to be rather persistent in the aquatic environment and also because acute toxicity does not take into account the impact of sustained exposure during the entire life cycle. The toxicity of EDTA and DTPA is strongly dependent on their chemical speciation, by many orders of magnitude in some cases (34). Thus the cations present and their concentrations might partially explain the differences in toxicological response. Naturally, species also have different sensitivities toward chemicals. It was reported that mortality of Daphnia carinata increased significantly at 100 mg/L of DTPA in 6 d, while significant decreases were observed of 50 mg/ L and 10 mg/L, respectively (36). On the other hand, it is reported that deinking plant effluent containing 113 mg/L of DTPA caused no mortality in Melanotaenia fluviatilis and had no impact on egg hatchability, growth or larval mortality after 14 d (38,39). The mechanism of action for toxicity may be the depletion of essential trace metals by DTPA, thus possibly leading to trace metal imbalances and nutritional deficiencies (36). It was found that chronic exposure to 10 mg/L DTPA significantly decreased brood size and the cumulative number of offsping per adult in Daphnia carinata (37). It was also observed that the toxicity of Fe-DTPA was noticeably lower than that of DTPA and concluded that direct toxicity of DTPA can occur only under extreme conditions. It can be summarized that the concentrations of EDTA and DTPA in receiving waters are generally at least two orders of magnitude below the levels for direct ecotoxic effects. Therefore, it is not expected that direct, acute toxic effects occur in aquatic organisms. However, the possible long-term ecotoxicity remains unknown.

Desorption of Heavy Metals The ultimate release of EDTA and DTPA, being hydrophilic and extremely strong chelating agents for metals, to the aquatic environment raises concern about their capability to remobilize heavy metal from soils and sediments, thus causing secondary pollution. There are several studies concerning the influence of EDTA on heavy metal balance in natural waters (40-46). DTPA has received less attention man EDTA as a metal extractant. However, it is reported that both EDTA and DTPA were able to desorb all metals investigated from diverse soil materials (46). DTPA was found to have more effective remobilizing capacity than EDTA, which is logical as DTPA forms stronger complexes with metals. Heavy metal remobilization capacity of complexing agents is discussed elsewhere in this book, but EDTA and DTPA are expected to be able to remobilize heavy metals in soils and sediments to some extent, depending on metal and chelate concentrations, pH, and the presence of other competing complexing agents and cations, whether anthropogenic or natural.

Heavy Metal Toxicity Effects The impact of

EDTA and DTPA on heavy metal toxicity has received



231 considerable attention (34, 47-51). According to an early hypothesis, it has been suggested that by increasing metal solubility EDTA also enhances the availability of metals to phytoplankton (52). However, later articles reveal that it is mainly the concentration of free metal ions and not the total metal concentration that determines metal toxicity (49,53). The effect of copper on the photosynthetic activity of Selenastrum capricornutum in the presence and absence of different concentrations of EDTA has been investigated (51). It was found that inhibition of bacterial growth could not be explained by free Cu ions only; thus, the compiexed copper is also bioavailable to a significant extent. In all, there are controversial data concerning the effect of chelation on the toxicity of trace metals. Both decreasing and negligible effects of EDTA and DTPA have been reported on the toxicity of Cu, Cd, Zn, and EDTA and DTPA enhanced, decreased, and or were insignificant to the toxicity of Fe, depending on the organism. On the other hand, EDTA and DTPA have consistently decreased the toxicities of Mn and Pb and, in the cases of Cr and As, EDTA has no effect. It is inferred mat the results depend on the organism, intial speciation, exposure time, chelate and metal concentrations, and aquatic media, amoung other factors. Thus, definitive conclusions cannot be drown, but generally the metal complexes of EDTA and DTPA are expected to be less toxic than free metals.

Conclusions EDTA, and in some cases also DTPA, are generally found in the receiving waters of many industiai areas, thus being classified as one of the major organic pollutant discharged in waters. The photochemical degredation of Fe complexes of these compounds is documented, but the extent to which these results can be applied to natural waters is not clear. There exist still some uncertainties in the chemical speciation, adsorption, overall degradation, and ultimately the eutrophication effect of EDTA and especially those of DTPA. It can be inferred that EDTA can effect the essential and nonessential metal balance in natural waters as well as in aquatic organisms. The estimation of the chemical speciation of EDTA and DTPA is a challenging task because of the complexicity of the system and should be based not only on equilibrium calculations but also on direct analytical determinations of diverse metal species. EDTA and DTPA are not expected to be acutely toxic to aquatic organisms. On the other hand, in natural waters, several compounds effect organisms simultaneously. Therefore, EDTA and DTPA may contribute to the aquatic toxicity at significantly lower concentrations than those determined by short-term toxicity tests. Also, more studies should be directed to estimating chronic effects, including the possible imbalance of body calcium in animals and other organisms. EDTA and DTPA may desorb heavy metals bound to sediments and also prevent heavy metal sedimentation, thus increasing their cycle in water. However, these metal complexes are not expected to be as bioavailable as free metal ions.


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Biogeochemistry of Chelating Agents

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