Mechanisms And Models Of Cd Transformation Driven By The Biogeochemical Processes In Paddy Soils

Cadmium(Cd)is one of the most toxic elements in paddy soils,and humans are expose to Cd mainly via the consumption of Cd-contaminated rice.Most of the paddy fields are located in southern China,where the soil acidification has been enhanced in recent years and the Cd contamination became more serious because it is more available in acid condition.The periodically changing redox conditions during rice cultivation can significantly affect the Cd fractions,which may be due to the redox cycling of iron(Fe),nitrogen(N),sulfur(S)and the subsequent change of soil p H.Although the main factors affecting the Cd retention were known,the underlying mechanisms controlling Cd retention are still ambiguous,and the quantitative understanding of each mechanism is urgently needed.Moreover,the dynamics of Cd fractions are determined by the complicated redox conditions in the paddy field,and thereby it is difficult to evaluate the availability of Cd.Therefore,the following works were carried out.The acid-base buffering characterization was investigated by titration method and continuous potentiometric titrations.A surface complexation model(SCM)was applied to further evaluate the acid-base buffering properties of the soils by assuming the buffering system to be a protonation-deprotonation process.The results of the SCM showed that the Cd adsorption on soil surfaces was favored when p H was higher than p H_pzc.The Cd adsorption on paddy soil under different p H can be successfully modeled by the SCM based on the surface acid-base parameters.The dissolved Cd concentrations were calculated by SCM combined with Cd extraction method,and the result showed that the model-calculated dissolved Cd concentrations were significantly correlated with Cd contents in grain.It indicated that the estimates were better than those obtained using extraction methods,because the model is thermodynamically based.The microcosm experiments were conducted to simulate the occurrence of the typical flooded/drained paddy system.Cd-spiked paddy soil was incubated under alternating anaerobic and aerobic conditions.The available Cd contents were rapidly decreased during anaerobic stage,but when the oxygen re-introduced into the system,the availability of Cd increased again.Based on the redox cycling of iron(Fe),nitrogen(N)and sulfur(S),and the characterization of the soil colloids and suspension solids,the kinetic model of transformation of Cd fractions was extablished.The mechanisms controlling Cd retention under anaerobic and aerobic conditions were evaluated.Under anaerobic condition,the decrease of Cd availability may be attibuted to increasing p H induced retention of Cd with humic and fulvic acid(39.6%).Moreover,the new adsorption sites provided by the release of colloids and suspended solids during the reduction of Fe(III)oxides also contributed to immobilize Cd(14.6).In addition,the Cd²⁺may pricipitate with S^2–derived from the reduction of sulfate(11.6%).However,during the aerobic period,the rapid decrease of soil p H resulted in the release of Cd from surfaces of humic and fulvic acid,and moreover,the soil colloids and suspended solids re-aggregated and the adsorption site concentrations decreased,which may cause release of Cd from their surfaces.The Cd contaminated paddy soil was incubated under alternating anaerobic and aerobic stages.The dissolved,exchangeable and adsorbed Cd were regarded as potential available Cd,and other Cd fractions were considered as unavailable Cd.A kinetic model was established to quantify the Cd transformation between potential available Cd and unavailable Cd.Based on the redox cycling of major elements(Fe,N and S)and soil buffering system,a kinetic model of protons was also established.The dynamics of soil p H under anaerobic and aerobic conditions was obtained.Then the kinetic model of Cd transformation and SCM of Cd in the paddy soil were integrated by establishing the kinetic model of soil p H.The intergraged-model calculated available Cd contents were strongly correlated with Cd contents in brown rice(P<0.01).It suggested that the Cd fractions were not only determined by the redox cycling of Fe,N and S,but also the dynamic of soil p H affected the available Cd thermodynamical equilibrium at the soil–water interface.In conclusion,the transformation of Cd fractions at the soil–water interface was determined by the thermodynamic and kinetic processes.This work improved our understanding of the underlying mechanisms of Fe/C/N/S on Cd availability,and provided us new strateges for remediation of Cd polluted soils.Moreover,this work demonstrated the potential utility of integrated model in assessing and predicting the availability of Cd under periodically changing redox conditions during rice cultivation.