Environ. Sci. Technol. 2000, 34, 291-296
Speciation and Complexation of Cadmium in Extracted Soil Solutions S EÄ B A S T I E N S A U V EÄ , * , † , ‡ WENDELL A. NORVELL,§ MURRAY MCBRIDE,† AND WILLIAM HENDERSHOT| Soil, Crop, and Atmospheric Sciences Department, Cornell University, Bradfield Hall, Ithaca, New York 14853, Institut national de 1a recherche scientifique-Institut Armand Frappier, 245 Hymus, Pointe-Claire, H9R 1G6, Quebec, Canada, U.S. Plant, Soil, and Nutrition Laboratory, Tower Road, Ithaca, New York 14853, Department of Natural Resources Sciences, McGill UniversitysMacdonald Campus, Ste-Anne-de-Bellevue, H9X 3V9, Quebec, Canada
We report the soil solution speciation of Cd in 64 fieldcollected contaminated soils containing between 0.1 and 38 mg Cd kg-1. The soils were analyzed for pH (3.5-8.1), soil organic matter (8.0-108 g C kg-1), total dissolved Cd (0.03182 µg Cd L-1), dissolved organic carbon (1.5-12 mg C L-1), and free Cd2+ (10-10-2 × 10-7 M). Free Cd2+ activity in solution was determined using differential pulse anodic stripping voltammetry (DPASV), assuming DPASV is sensitive to easily dissociated inorganic ion-pairs and free Cd2+ while excluding organic complexes. The solid/liquid partition coefficient (Kd) varied over a range from 10 to 100 000, and the fraction of the dissolved Cd present in solution as the estimated free Cd2+ species varied between 0 and 60% but averaged about 20%. The dissolved concentrations of Cd and the free Cd2+ activity in the soil solutions of contaminated soils of different origins can be predicted with reasonable accuracy using a simple competitive adsorption model dependent on pH and total metal loading.
Introduction Cadmium is a toxic trace element which may accumulate in soils from various human activities. Widespread and generally low-level inputs come from phosphate fertilizers and atmospheric deposition. They have caused a gradual increase in the average soil Cd concentrations in many agricultural regions of the world. More localized and high-level inputs can arise from land application of sewage sludge products and industrial wastes. “Pristine” soils average ∼0.2 mg Cd kg-1 (1), but “naturally” occurring levels of up to 4.3 mg Cd kg-1 have been reported in some soils of particular geochemical origin (2). There is a consensus in the literature that total soil metal content alone is not a good measure of short-term bioavailability and not a very useful tool to determine potential risks from soil contamination (3-5). Unfortunately, there is no consensus on the “availability index” that should be used as * Corresponding author phone: (514)847-1714; fax: (514)845-2073; e-mail:
[email protected]. Present address: QSAR Risk Assessment Service, 360 St-Jacques (suite 800), Montre´al (QC), Canada, H2Y 1P5. † Cornell University. ‡ Institut national de 1a recherche scientifique-Institut Armand Frappier. § U.S. Plant, Soil, and Nutrition Laboratory. | McGill University. 10.1021/es990202z CCC: $19.00 Published on Web 12/07/1999
2000 American Chemical Society
an alternative to total content. Chemical extractions in various forms have been used to try to identify a soil metal pool that is well correlated to plant uptake or to availability to soilexposed organisms (6). No single extractant has been found universally useful for soil testing purposes. Correlations between chemically extracted quantities and availability are often inconsistent for different species of plants or soil organisms. Correlations vary also among soils and are influenced by many interactions with the physicochemical properties of the soils, e.g. ref 7. The approach we are proposing is based on the free ion activity model, which assumes that metal bioavailability and toxicity are controlled by the buffered activity of the free ions in solution, not the total dissolved concentration. This model is generally accepted in aquatic toxicological studies (8), although there are cases where it is unsatisfactory (9). In soils, for example, there are situations where the bioavailability of Cd may depend also on factors such as Cl- or salinity (10, 11). It is not clear whether the increased uptake of Cd in the presence of chloride is due to an increased ease of diffusion of Cd to roots (through the solution complexation of Cd by chloride) or if the Cd-chloride complexes are themselves bioavailable. Others have argued that empirical conformity to the free ion activity model does not preclude biological uptake of metals as metal complexes (12). Soil solutions, unlike water bodies, are in intimate contact with the soil solid phase and are therefore much more influenced by mineral equilibria, cation exchange, and sorption reactions as well as complexation by organic matter both on the solid phase and dissolved in the solution. Although it is difficult to evaluate free metal activity, its importance with regards to soil metal bioavailability is a stimulus to further investigation. Few studies report actual measurements of free Cd2+ species in soil solution extracts. Workman and Lindsay (13) used a DTPA competitive chelation technique involving reaction with an added solid phase, PbCO3, to measure pCd2+ activities of 9.5-6.5 in eight uncontaminated, alkaline soils. The pCd2+ represents the negative log molar quantity of free Cd2+ ions in solutions, by analogy to pH, pCd2+ of 8 represent 10-8 M Cd2+ ions. The same chelation approach was used by Ma and Lindsay (14) and El-Falaky et al. (15). These estimated activities encompass the range of pCd2+ values (8.5-7.2) reported by Holm et al. (16, 17) for exchange resin speciation of soil solutions from 11 contaminated soils (0.8-17 mg Cd kg-1) of pH 6.1-7.8. Salam and Helmke (18) used an ion exchange membrane technique to determine free Cd2+ activity in two slightly contaminated soils containing 1.5 and 0.5 mg of Cd kg-1. Jing and Logan (19) have used exchange resins to measure pCd2+ activities of 6-9 in sewage sludge extracts. Holm et al. (20) have also compared various techniques for Cd speciation in solutions of varied origins. Many authors report Cd2+ “speciation” based on measured total dissolved Cd and computation of free Cd2+ using chemical equilibrium models (11, 20, 21). This “speciation” provides a computed estimate of free Cd2+ whose accuracy depends on the appropriateness of the models and the associated stability constants (22). The stability constants for Cd-humics are particularly uncertain, and proper evaluation of these constants is difficult. Estimation of the stability of humic complexes can be used to calculate Cd2+ activities, but the results cannot usually be validated. Studies using this approach have found the pCd2+ to vary between 8 and 5 (23) or 6.4-4.6 (24). These are relatively high values of free Cd2+, which may suggest that the assumed Cd-humic stability constants were too low. On the basis of commonly used VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
291
stability constants, organic complexing of Cd is often interpreted as being negligible, e.g. refs 23 and 25-27. The objectives of this study were to (1) measure the dissolved concentration of Cd and the free Cd2+ activity in the soil solutions from a variety of Cd-contaminated soils and (2) determine the relationships between simple soil properties (such as pH, organic matter, and total metal content) and the solubility and speciation of Cd2+.
Materials and Methods Soils Samples. This study reports the solubility and speciation of Cd2+ in 64 field-collected soils originating from 18 different locations (multiple distinct samples were collected on certain sites). This dataset is representative of soil contamination as it occurs in the field. These soils have been subjected to natural biogeochemical and pedogenetic processes over many years, and hence the Cd has had time to reach a steadystate or quasi-equilibrium with respect to metal solubility and adsorption on the soil’s solid phase. Agricultural soils samples were collected from the orchards at Cornell University (Ithaca, NY), at the Macdonald Campus of McGill University (Ste-Anne-de-Bellevue, QC) and also from three Colorado soil series. Urban soils from various contaminated sites in the city of Montre´al (QC) were sampled along with uncontaminated controls from acidic forest soils around Montre´al and Ithaca. The soils were air-dried and sieved to < 2 mm, except samples nos. 1, 26-35, and 51-56 which were sieved moist to 100fold span of free Cd2+ activities over a pH range of almost three units. The comparison presented in Figure 4 is indirect (not comparing true measurements but using a regression) and must be interpreted carefully. It would be valuable to more rigorously compare the various speciation techniques (Donnan membrane, exchange resin, electrochemistry) for the soil solution speciation of Cd2+ as well as other trace metals such as Pb2+, Cu2+, Ni2+, and Zn2+ using the same set of soils and similar extraction protocols.
Acknowledgments This project was made possible through grants from a National Research Initiative competitive Grants Program/ USDA award no. 95-37107-1620 to M.M., an operating grant to W.H., and a postdoctoral fellowship to S.S. from the Natural Sciences and Engineering Research Council of Canada.
Literature Cited (1) Holmgren, G. G. S.; Meyer, M. W.; Chaney, R. L.; Daniels, R. B. J. Environ. Qual. 1993, 22, 335-348. (2) Baize, D.; Chre´tien, J. EÄ tude et gestion des sols 1994, 2, 7-27. (3) Tack, F. M. G.; Verloo, M. G. Intern. J. Environ. Anal. Chem. 1995, 59, 225-238. (4) Peijnenburg, W. J. G. M.; Posthuma, L.; Eijsackers, H. J. P.; Allen, H. E. Ecotoxicol. Environ. Saf. 1997, 37, 163-172. (5) Sauve´, S.; Dumestre, A.; McBride, M.; Hendershot, W. Environ. Toxicol. Chem. 1998, 17, 1481-1489. (6) Beckett, P. H. T. Adv. Soil Sci. 1989, 9, 143-176. (7) Qian, J.; Wang, Z.-j.; Shan, X.-q.; Tu, Q.; Wen, B.; Chen, B. Environ. Pollut. 1996, 91, 309-315. (8) Allen, H. E.; Hansen, D. J. Water Environ. Res. 1996, 68, 42-53. (9) Campbell, P. G. C. In Metal speciation and bioavailability in aquatic systems; Tessier, A., Turner, D. R., Eds.; John Wiley & Sons Ltd: Chichester/New York, 1995; pp 45-102. VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
295
(10) Smolders, E.; Lambregts, R. M.; Mclaughlin, M. J.; Tiller, K. G. J. Environ. Qual. 1998, 27, 426-431. (11) Bingham, F. T.; Strong, J. E.; Sposito, G. Soil Sci. 1983, 135, 160-165. (12) Parker, D. R.; Pedler, J. F. Plant Soil 1997, 196, 223-228. (13) Workman, S. M.; Lindsay, W. L. Soil Sci. Soc. Am. J. 1990, 54, 987-993. (14) Ma, Q. Y.; Lindsay, W. L. Geoderma 1995, 68, 123-133. (15) El-Falaky, A. A.; Aboulroos, S. A.; Lindsay, W. L. Soil Sci. Soc. Am. J. 1991, 55, 974-979. (16) Holm, P. E.; Christensen, T. H.; Tjell, J. C.; McGrath, S. P. J. Environ. Qual. 1995, 24, 183-190. (17) Holm, P. E.; Christensen, T. H.; Lorenz, S. E.; Hamon, R. E.; Domingues, H. C.; Sequeira, E. M.; McGrath, S. P. Water Air Soil Pollut. 1998, 102, 105-115. (18) Salam, A. K.; Helmke, P. A. Geoderma 1998, 83, 281-291. (19) Jing, J.; Logan, T. J. J. Environ. Qual. 1992, 21, 73-81. (20) Holm, P. E.; Andersen, S.; Christensen, T. H. Water Res. 1995, 29, 803-809. (21) McGrath, S. P.; Sanders, J. R.; Laurie, S. H.; Tancock, N. P. Analyst 1986, 111, 459-465. (22) Turner, D. R. In Metal speciation and bioavailability in aquatic systems; Tessier, A., Turner, D. R., Eds.; John Wiley & Sons Ltd: Chichester/New York, 1995; pp 149-203. (23) Jopony, M.; Young, S. D. Eur. J. Soil Sci. 1994, 45, 59-70. (24) Candelaria, L. M.; Chang, A. C. Soil Sci. 1997, 162, 722-731. (25) Temminghoff, E. J. M.; Van der Zee, S. E. A. T. M.; de Haan, F. A. M. Eur. J. Soil Sci. 1995, 46, 649-655. (26) Elzinga, E. J.; van Grinsven, J. J. M.; Swartjes, F. A. Eur. J. Soil Sci. 1999, 50, 139-149. (27) Hirsch, D.; Banin, A. J. Environ. Qual. 1990, 19, 366-372. (28) Dumestre, A.; Sauve´, S.; McBride, M.; Baveye, P.; Berthelin, J. Arch. Environ. Contam. Toxicol. 1999, 36, 124-131. (29) Sauve´, S.; McBride, M. B.; Hendershot, W. H. Environ. Pollut. 1997, 98, 149-155. (30) Sauve´, S.; McBride, M.; Norvell, W. A.; Hendershot, W. Water, Air, Soil Pollut. 1997, 100, 133-149. (31) Florence, T. M. Analyst 1986, 111, 489-505. (32) Ro¨mkens, P. F. A. M.; Dolfing, J. Environ. Sci. Technol. 1998, 32, 363-369. (33) Nelson, L. A.; Mantoura, R. F. C. In Complexation of trace metals in natural waters; Kramer, C. J. M., Duinker, J. C., Eds.; Martinus Nijhoff/W. Junk Publishers: Chelsea, MI, 1984; pp 119-130. (34) Gray, C. W.; McLaren, R. G.; Roberts, A. H. C.; Condron, L. M. Aust. J. Soil Res. 1998, 36, 199-216. (35) Sauve´, S.; McBride, M. B.; Hendershot, W. H. Arch. Environ. Contam. Toxicol. 1995, 29, 373-379.
296
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 2, 2000
(36) Nelson, D. W.; Sommers, L. E. In Methods of soil analysis, Part 2. Chemical and microbiological properties-Agronomy monograph no. 9; Page, A. L., Ed.; Soil Science Society of America: Madison, WI, 1982; pp 539-579. (37) Moore, T. R. Soil Sci. Soc. Am. J. 1985, 49, 1590-1592. (38) Waller, P. A.; Pickering, W. F. Chem. Speciation Bioavailability 1993, 5, 11-22. (39) Figura, P.; McDuffie, B. Anal. Chem. 1980, 52, 1433-1439. (40) Saar, R. A.; Weber, J. H. Geochim. Cosmochim. Acta 1980, 44, 1381. (41) Lindsay, W. L. 1979. Chemical equilibria in soils; John Wiley and Sons: New York. (42) Sauve´, S. Ph.D. Dissertation, Cornell University, Ithaca, NY, 1999. (43) McBride, M. B.; Sauve´, S.; Hendershot, W. Eur. J. Soil Sci. 1997, 48, 337-346. (44) Gooddy, D. C.; Shand, P.; Kinniburgh, D. G.; van Riemdsjik, W. H. Eur. J. Soil Sci. 1995, 46, 265-285. (45) Christensen, T. H. Water, Air, Soil Pollut. 1985, 26, 265-274. (46) Christensen, T. H. Water, Air, Soil Pollut. 1987, 34, 305-314. (47) Moolenar, S. W.; Beltrami, P. J. Environ. Qual. 1998, 27, 828835. (48) Anderson, P. R.; Christensen, T. H. J. Soil Sci. 1988, 39, 15-22. (49) Christensen, T. H. Water, Air, Soil Pollut. 1984, 21, 105-114. (50) Christensen, T. H. Water, Air, Soil Pollut. 1987, 34, 293. (51) Gerritse, R. G.; van Driel, W. J. Environ. Qual. 1984, 13, 197204. (52) Pardo, M. T. Soil Sci. 1997, 162, 733-740. (53) Escrig, I.; Morell, I. Water, Air, Soil Pollut. 1998, 105, 507-520. (54) Doner, H. E. Soil Sci. Soc. Am. J. 1978, 42, 882-885. (55) McLaughlin, M. J.; Maier, N. A.; Correl, R. L.; Smart, M. K.; Sparrow, L. A.; McKay, A. Aust. J. Soil Res. 1999, 37, 191-207. (56) Janssen, R. P. T.; Peijnenburg, W. J. G. M.; Postuma, R.; van den Hoop, M. A. G. T. Environ. Toxicol. Chem. 1997, 16, 2470-2478. (57) Lee, S.-Z.; Allen, H. E.; Huang, C. P.; Sparks, D. L.; Sanders, P. F.; Peijnenburg, W. J. G. M. Environ. Sci. Technol. 1996, 30, 3418-3424. (58) Kalbasi, M.; Peryea, F. J.; Lindsay, W. L.; Drake, S. R. Soil Sci. Soc. Am. J. 1995, 59, 1274-1280. (59) Wilkinson, L. SYSTAT for Windows: Statistics Version 5; SYSTAT Inc.: Evanston, IL, 1992.
Received for review February 19, 1999. Revised manuscript received August 31, 1999. Accepted November 1, 1999. ES990202Z
