Effects of Soil Organic Matter on the Kinetics and Mechanisms of Pb(II) Sorption and Desorption in Soil

make better predictions about the mobility of contaminants, it is critical that time-dependent metal sorption To improve predictions of the toxicity and threat from Pb contamiand desorption behavior on soil be understood, as well nated soil, it is critical that time-dependent sorption and desorption behavior be understood. In this paper, the sorption and desorption as the mechanism of the sorption–desorption reactions. behavior (pH 5 5.50, I 5 0.05 M ) of Pb in a Matapeake silt loam Lead sorption behavior is often initially fast, followed soil (Typic Hapludult) were studied by stirred-flow and batch experiby a slow reaction (Benjamin et al., 1981; Hayes et al., ments. In addition, we studied the effects of soil organic matter (SOM) 1986; Strawn et al., 1998). The fast reaction is most likely on sorption and desorption behavior by treating the soil with sodium adsorption via electrostatic attraction, and/or innerhypochlorite to remove the SOM fraction, and using a soil with six sphere complexation with functional groups present on times as much SOM (St. Johns loamy sand [Typic Haplaquods]) as the soil components. There are several possible reasons the Matapeake soil. Lead sorption consisted of a fast initial reaction for the slow sorption steps, such as: slow interparticle in which all of the Pb added to the stirred-flow chamber was sorbed. diffusion in porous minerals and organic matter, formaFollowing this initial fast reaction, sorption continued and appears to tion of precipitates on surfaces which can sometimes be be rate limited (indicated by a decrease in the outflow concentration when the flow rate was decreased, or when the flow was stopped). slower than typical sorption, and adsorption onto sites The total amount of Pb sorbed was 102, 44, and 27 mmol kg21 for that have relatively large activation energies (Fuller et the St. Johns soil and the untreated and treated Matapeake soils, al., 1993; Papelis et al., 1995; Loehr et al., 1996; Scheidegrespectively. Desorption experiments were conducted on the soils ger et al., 1998). It is possible that multiple slow reaction with the background electrolyte as the eluent in the stirred-flow chammechanisms are responsible for the slow sorption reacber. In the St. Johns soil only, 32% of the total sorbed Pb was desorbed, tions in soils. while 47 and 76% of the sorbed Pb was released from the untreated There are three main processes that control the fate and treated Matapeake soil, respectively. The correlation between and bioavailability of metals in soils: (i) removal of metSOM in the soils, and the percentage Pb desorbed from the soils als from the soil solution by sorption onto soil particles, suggests that SOM plays an important role in slow desorption reactions (ii) release of the metal from the soil particle to the soil of Pb from soil materials. Aging experiments in which sorbed Pb was incubated for 1, 10, and 32 d showed that sorption incubation time solution (desorption), and (iii) precipitation–dissolution had no effect on Pb desorption behavior. Analysis of the treated and of the metal as an independent phase in the soil matrix. untreated Matapeake soils by x-ray absorption fine structure (XAFS) Less is known about the desorption behavior of metals spectroscopy revealed that the local atomic structure of sorbed Pb is from soils than the other two processes. This is unfortudistinctly different in the two samples. In the soil treated to remove nate since once a soil is contaminated desorption is an SOM, the data were well represented by theoretical models using O, important process that controls the bioavailability of Si, and Pb backscattering atoms. In the untreated soil, the XAFS data the contaminant. If accurate assessments of the fate of were best described by O and C backscatterers. These XAFS results metals in soils are to be gained, it is critical that desorpconfirm that the sorption mechanisms in the two systems are different. tion behavior be studied as well as sorption behavior. It is often observed that desorption reactions are slower than sorption reactions. Smith and Comans T predict the fate and transport of contaminants (1996) predicted that adsorption half-lives for Cs sorpsuch as Pb in the environment, scientists and engition on sediments were between 50 and 125 d, while neers commonly rely on distribution coefficients and desorption half-lives were on the order of 10 yr. Failure maximum adsorption levels that are obtained from exto include the slow desorption reaction in transport periments in which it is assumed that the reaction is at models severely underestimated the remobilization poequilibrium. However, reactions in the environment are tential of Cs. Slow desorption reactions have also been rarely at equilibrium, but instead are in a state of continobserved for metals (Kuo and Mikkelsen, 1980; McKenuous change because of the dynamic processes occurring zie, 1980; Schultz et al., 1987; Ainsworth et al., 1994; (Sposito, 1989; Sparks, 1995). Another discrepancy that Scheidegger et al., 1996). A possible reason that desorpcauses errors in predicting the potential toxicity of a tion reactions are slower than sorption reactions is becontaminant is that researchers often neglect desorption cause the sorbate undergoes a transformation from one behavior, which is an important process controlling state to another, for example: conversion from an admetal bioavailability in the environment. Thus, in order sorbed species to a surface precipitate. In addition, deto improve remediation strategies, risk assessments, and sorption reactions often require large activation energies, which cause the reaction to proceed slowly D.G. Strawn, Dep. of Plant, Soil, and Entomological Sci., Univ. of (McBride, 1994). Idaho, P.O. Box 442339, Moscow, ID 83844-2339; D.L. Sparks, Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717The rates of Pb sorption and desorption on mineral 1303. Received 14 Jan. 1999. *Corresponding author (dgstrawn@ uidaho.edu). Abbreviations: CV, chamber volume; SOM, soil organic matter; XAFS, x-ray absorption fine structure. Published in Soil Sci. Soc. Am. J. 64:144–156 (2000).