Effects of soil organic matter on the kinetics and mechanisms of Pb(II) sorption and desorption in soil

To improve predictions of the toxicity and threat from Pb contaminated soil , it is critical that time-dependent sorption and desorption behavior be un derstood, In this paper, the sorption and desorption behavior (pH = 5.50, I = 0.05 M) of Pb in a Matapeake silt loam soil (Typic Hapludult) were studi ed by stirred-now and batch experiments. In addition, we studied the effect s of soil organic matter (SOM) on sorption and desorption behavior by treat ing the soil with sodium hypochlorite to remove the SOM fraction, and using a soil with six times as much SOM (St, Johns loamy sand [Typic Haplaquods] ) as the Matapeake soil. Lead sorption consisted of a fast initial reaction in which all of the Pb added to the stirred-flow chamber was sorbed, Follo wing this initial fast reaction, sorption continued and appears to be rate limited (indicated by a decrease in the outflow concentration when the flow rate was decreased, or when the flow was stopped). The total amount of Pb sorbed was 102, 44, and 27 mmol kg(-1) for the St. Johns soil and the untre ated and treated Matapeake soils, respectively, Desorption experiments were conducted on the soils with the background electrolyte as the eluent in th e stirred-flow chamber. In the St. Johns soil only, 32% of the total sorbed Pb was desorbed, while 47 and 76% of the sorbed Pb was released from the u ntreated and treated Matapeake soil, respectively. The correlation between SOM in the soils, and the percentage Pb desorbed from the soils suggests th at SOM plays an important role in slow desorption reactions of Pb from soil materials. Aging experiments in which sorbed Pb was incubated for 1, 10, a nd 32 d showed that sorption incubation time had no effect on Pb desorption behavior. Analysis of the treated and untreated Matapeake soils by x-ray a bsorption fine structure (XAFS) spectroscopy revealed that the local atomic structure of sorbed Pb is distinctly different in the two samples, In the soil treated to remove SOM, the data were well represented by theoretical m odels using O, Si, and Pb backscattering atoms. In the untreated soil, the XAFS data were best described by O and C backscatterers. These XAFS results confirm that the sorption mechanisms in the two systems are different.