| The aging effects of heavy metals in the soil affect their migration,transformation and bioavailability in the environment.Studying the effect of aging of heavy metal will provide a theoretical basis and practical guidance for the whole process of contaminated site management.To better understanding the aging process and influencing factors of heavy metals in the soil.In this study,cadmium and chromium,which were often found in contaminated soils in an industrial cluster in Pearl River delta,were selected as research pollutants.The transformation and distribution of fractions of cadmium and chromium in soils with exogenous single cadmium,chromium pollution and exogenous combined cadmium and chromium pollution were analyzed during the simulated aging incubation and four kinetic models were used to fit the aging processes.In order to further understand the effects of different components of the soil on the aging process,cadmium and chromium in kaolinite and montmorillonite,and the effect of fulvic acid and hematite on aging process were studied.The transformation of distribution of heavy metals in the mineral system was analyzed.The mineral system was characterized by X-ray diffraction technique and X-ray photoelectron spectroscopy technique.The results showed that during the aging incubation for 90 days,the whole-soil redistribution index of cadmium and chromium with single pollution decreased 5.414.8%and45.753.8%.The reduced partition index of cadmium and chromium increased 10.838.6%and 27.449.8%.This indicated that the fractions of cadmium and chromium in soil transform in the direction of increased stability and the binding strength of heavy metals increased and the availability of heavy metal decreased gradually.So the aging effect of heavy metals in the soil should be considered in the contaminated site management,including risk assessment and remediation plan.Under the condition of combined external cadmium and chromium pollution,the fractions transformation trend of cadmium and chromium in soil is consistent with the condition of single exogenous cadmium and chromium,but the stability of distribution between frations of hevay metal and the binding strength to soil are greater than those single exogenous cadmium and chromium.This indicated that the aging of cadmium and chromium in soil has a synergistic effect.It is suggested that the combined pollution effects of various heavy metals should be considered in the heavy metal contaminated site management.The Elovich equation can better fit the changes of acid-extractable fractions and residual fractions of cadmium and chromium during aging incubation,indicating that the aging processes of cadmium and chromium in soil are a heterogeneous diffusion effect controlled by various mechanisms.The transformation trends of fractions of cadmium and chromium in different clay minerals are slightly different,but they all shift toward the direction in which the acid-extractable fractions decreases and the residual fractions increases.The XRD results showed that the main phase of clay minerals did not change during the aging process of 360 days.The XPS results showed that Cd and Cr have a tendency to lose electrons,and O and Si have an tendency to accept electrons during the 30 days of aging processes.The bingding state of cadmium on montmorillonite and kaolinite after 360 days incubation may be CdO and Cd2SiO4.The bingding state of chromium on montmorillonite may be CrO3,and on kaolinite may be Cr5O12.After the addition of fulvic acid to montmorillonite and kaolinite,the binding strength of heavy metal in the clay mineral system decreased gradually during the 30 days of aging process,indicating that fulvic acid can slowly activate the heavy metals adsorbed in clay mineral systems and inhibits the aging process of cadmium and chromium in clay minerals in short term and can be an activator for phytoremediation of heavy metal contaminated soils.The addition of hematite to the montmorillonite-fulvic acid system and the kaolinite-fulvic acid system partially offsets the inhibition of the aging reaction by fulvic acid,and the hematite undergoes a redox reaction during the aging reaction. |
