The Influence of Natural Organic Matter Rigidity on the Sorption,Desorption,and Competitive Displacement Rates of 1,2-Dichlorobenzene

摘要

The influence of natural organic matter (NOM) rigidity on the sorption,desorption,and competitive displacement rates of 1,2-Dichlorobenzene (1,2-DCB) was evaluated using batch reactor experiments with two surface soils (Yolo and Forbes) and a shale (Ohio).Previous characterization suggests that the shale NOM is the most reduced and condensed,the Yolo soil is the most oxidized and amorphous,and Forbes soil has an intermediate NOM structure.The rate study for each sorbent was conducted under the same reactor parameters,and 1,2-DCB mass-transfer rates were determined using the distributed first-order mass-transfer rate model based on the y probability density function.To measure competitive displacement rates,1,2,4-trichloro-benzene (1,2,4-TCB) was delivered as a competitor after 34 days pre-equilibration.Higher fractions of contaminant subject to instantaneous mass transfer and much faster rates of approach to apparent sorption equilibrium are found in Yolo soil when compared with Forbes soil and the shale.The size of the instantaneously desorbing fraction thus appears inversely related to the hard carbon fraction.In the NOM compartment where mass transfer is rate-limited,rate coefficient distributions are shifted toward lower rates for desorption and competitive displacement of 1,2-DCB in Ohio shale,followed by Forbes soil.Sorption and desorption rate distributions are almost the same for the shale,while desorption rates are a few times greater than sorption rates in Yolo and Forbes soils.Mass-transfer coefficients for competitive displacement are considerably slower than those for desorption in Forbes soil and the shale.However,the mass-transfer rates for the two processes seem to be similar in Yolo soil,which has a NOM matrix comprising a relatively larger soft organic carbon fraction.The concept of "solute induced softening" is discussed as a mechanistic rationale forthe experimental observations.