Lead immobilization processes in soils subjected to freeze-thaw cycles

Lina Dua, b, M. Dyckb, W. Shotykb, H. L. Hea, J. L. Lva, T. Leia, J. Y. Biea

a College of Natural Resources and Environment, Northwest A& F University, China

b Department of Renewable Resources, University of Alberta, Canada

lina.du@nwafu.edu.cn

Soil freeze-thaw cycles (FTCs) change soil physical and chemical properties, however information regarding their variations resulted from FTCs in response to sorption and desorption of heavy metal in soil is limited. Sorption isotherms and successive desorption tests of lead (Pb) were measured for three soils from North China (Chestnut soil, Lou soil and Black soil) following multiple freeze-thaw cycles (0, 1, 3, 6 and 9 FTCs) of -5 ℃ for 12 h and then +5 ℃ for 12 h. The results showed that lead adsorption dominated the sorption process in our soils and its capacity increased by repeated FTCs. The estimated sorption capacity represented by Freundlich affinity parameter, A (Lβ mmol1-β kg-1), of soils for Pb with multiple FTCs was linearly correlated with carbonate content in soils. The effects of FTCs on lead adsorption was more dependent on carbonate and clay contents than organic matter (OM), cation exchange capacity (CEC) and amorphous iron in soils. Carbonate-rich soil had the highest sorption capacity for Pb among three soils. Repeated FTCs increased pH values of soil solutions at applied Pb concentrations > 1.4 mmol L-1, which could facilitate specific adsorption for Pb in studied soils. Cation exchange could occupy specific adsorption sites with increasing doses in soils and it can also be facilitated by FTCs. Our results have demonstrated the great potential for immobilization of Pb by repeated FTCs through facilitating both formations of inner-sphere and outer-sphere complexes. Hence, these findings provide useful information on remediating Pb-contaminated soils in environments that undergo frequent FTCs.

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