Sorption Kinetics of Trace Elements in Soils and Soil Materials
Daniel G. Strawn and Donald L. Sparks
Environmental contamination resulting from the extensive use of metals and semimetals in industry, agriculture, and in manufactured products has magnified the threat of toxicity for plants, animals, and society. Since soils and sediments have a large capacity for sorbing trace elements, an understanding of metal reaction mechanisms with natural materials is critical. Many studies have appeared in the literature on various aspects of metal sorption. Results from these studies have been used to develop government regulations, devise cleanup strategies, and develop models that predict the fate of trace elements in the environment. However, in conducting these studies researchers often overlook two important aspects: (1) the length of time soils are exposed to a contaminant (residence time) in the laboratory is relatively short compared with the much longer residence times that exist in field contaminated soils, and (2) the kinetics of metal sorption and desorption are often slow. These oversights lead to improper evaluation of contaminant behavior in the environment, resulting in regulations that may be improper, and models and remediation strategies that may be unsuccessful. This chapter will investigate the effects of residence time (aging) and slow kinetics on sorption and desorption reaction mechanisms of metals with soils and soil materials (e.g., clay minerals, metal oxides, and organic matter). Such information is important, and can be used in combination with transport models to predict the fate of trace metals through the vadose zone, and can provide information on metal bioavailability and speciation.
Trace elements exist in the soil as either aqueous species, as structural elements in solids, or sorbed onto the surfaces of soil materials. While many of these trace elements are present naturally in the environment, their indigenous levels are usually nonthreatening. The buildup of these elements to dangerous levels is a result of commercial use and disposal practices. The following are a few examples of common sources of contamination: disposal of batteries that contain Pb, Cd, and Hg; exhaust from automobiles that burn gasoline with Pb additives; application of pesticides that contain Pb and As, e.g., Pb3AsO4)2; the use of Pb in paint; trace elements which are used in manufacturing that end up in waste disposal and the environment from either discarding the product or as a by-product of the manufacturing process; desiccation of agricultural runoff water in ponds which results in Se and As concentrating to dangerous levels; disposal of sewage which contains several trace elements, in particular heavy metals; and mine drainage which is often acidic and can increase the mobility of metals.
Scientific studies have clearly shown that exposure to metal contaminants at higher than natural levels is toxic. As a result, many past uses and disposal practices of metals are now illegal, and trace element contamination of the environment is now regulated more closely. However, due to the relatively low solubility of many trace metals, and often strong sorption to soils, environmental contamination persists, and the threat from contaminants remains a problem that merits continued scientific investigation.
While toxicity from trace elements, and their presence in the environment at dangerous levels are well-established facts, the questions remain: how does one remediate contaminated soils effectively, and how can significant risks be accurately evaluated? Finding effective answers to these questions hinges on a clear understanding of the behavior and interactions of trace elements with soils. In particular, an understanding of slow desorption and release kinetics from environmental settings which have been contaminated for long periods is critical. For example, Smith and Comans (1996) conducted sorption and desorption experiments on Cs contaminated sediments. They found that failure to include slow reactions in their model gave much lower estimates of the remobilization potential of the Cs. They concluded from model fits that sorption half-lives were between 50 and 125 days, and desorption half-lives were on the order of 10 years.
Many studies rely on an equilibrium approach to predict the retention of contaminants on natural materials and subsequent migration through the vadose zone. Researchers often focus on determining parameters such as distribution coefficients, and the maximum amount of sorption possible. These studies are often based on the contaminant-solid interactions over a short period (24 hours or less) because it is assumed that the reaction has reached completion (Griffin et al., 1986). However, field soils are seldom, if ever, at equilibrium, often laboratory studies are also far from equilibrium, and slow sorption may change the distribution between solid and solution over a period of time (Smith and Comans, 1996; Sparks, 1998). This is primarily due to slow metal sorption and desorption kinetics. The failure to account for the slow kinetics results in either underpredictions of the amount of contaminants retained by soils and minerals, or overpredictions of contaminant availability in the environment. A better approach is to base mobility estimates, remediation strategies, and risk assessments on the true availability of the contaminant, which is often controlled by a rate-limited sorption reaction.
Most soils are heterogeneous media that contain a host of different minerals, solids, and organic materials. Thus, the interaction of trace elements with soils is a heterogeneous process. Several possible sorption mechanisms have been proposed (Figure 1.1): diffusion into micropores and solids followed by subsequent sorption onto interior surfaces; sorption to sites of variable reactivity, including sites which involve different bonding mechanisms, i.e., inner-sphere vs. outer-sphere and monodentate vs. bidentate; and surface precipitation (Fuller et al., 1993; Loehr and Webster, 1996; Scheidegger and Sparks, 1996). Due to the heterogeneity of soil, these processes can occur simultaneously. A measured sorption or desorption rate often reflects a combination of all of the sorption mechanisms. However, it is possible that one mechanism may dominate at a particular time in the sorption reaction and the measured rate is primarily an expression of that reaction rate. For example, outer-sphere complexation can precede inner-sphere complexation, which can precede surface precipitation. The significance of this continuum in sorption is that while many sorption and desorption reactions may appear to have reached equilibrium, in fact the reaction can be continuous, and the slow process will not be measured if the experimentalist studies a short reaction time. In such cases, important secondary processes which are slower than the primary process may be completely overlooked. Thus, predictions on the fate of the contaminant may be inaccurate. This can cause increased threats of toxic exposure, improper evaluation of risks, and/or misappropriation of valuable cleanup and public safety funds.
To protect human health and the environment from overexposure there must exist effective cleanup strategies, accurate risk assessment technologies, and models that correctly predict the fate of trace elements. For these tasks to be accomplished, time dependent reactions of trace elements with soils must be taken into consideration. Thus, the goals of this chapter are to discuss the kinetics of trace element interactions with soil and soil components, including the importance of slow reactions and possible sorption mechanisms.