Mechanisms Of Cr Transformation Driven By The Biogeochemical Processes In Paddy Soils

Arable soil in China is seriously polluted by heavy metals,and chromium is one of the heavy metals in soil risk control.Rice is the main food crop in China.High Cr content in soil will increase Cr absorption by rice and lead to increased risk of Cr pollution in rice.The alternation of drying and wetting in paddy fields affected the content of oxygen in soil,promoted the redox cycle of iron,nitrogen,sulfur and other elements,and caused the release and accumulation of soil particles,which jointly affected the occurrence of Cr on the soil-water interface,and then determined the accumulation of chromium in rice.Although the influencing factors have been gradually clarified,the key components and key processes under different sources of chromium pollution are still unclear,and there is a lack of quantitative models to determine the contributions of the main mechanisms,especially in the Cr naturally enriched acidic rice soils.Therefore,the following research work is carried out for the above problems.The available of available state of exogenous chromium in paddy soil under anaerobic-aerobic alternation was studied by microcosmic experiment.In the system of exogenous Cr(VI),the availability of Cr(VI)decreased under the condition of anaerobic-aerobic alternation.Through the establishment of kinetic model and thermodynamic model,combined with the qualitative and quantitative methods of the process,it is confirmed that the bonded state of iron and manganese oxides and organic matter are the main transformation process in the anaerobic stage.In the aerobic stage,the residue form is the main form,and the transformation from iron and manganese oxide to residue form is the key process.The main fixation mechanism is as follows:under anaerobic conditions,iron oxide is reduced and dissolved,colloidal suspended particles are released,surface active sites increase,and available chromium is adsorbed and fixed by iron and manganese oxides and organic matter in soil.Under aerobic conditions,new iron oxides generated by ferrous oxidation adsorbed chromium back to the soil,reducing the availability of chromium.In the system of exogenous Cr(III),Cr(III)entered the paddy field and was fixed in the anaerobic stage,while Cr(III)was released in the aerobic stage.The kinetic model shows that Cr(III)aging process is dominant in anaerobic stage,and activation reaction occurs in aerobic stage.Typical basalt-derived paddy soils were selected and incubation under alternating redox conditions.The Cr fractions and the dynamics of Fe/N/S/C were examined.The HCl-extractable Cr increased under anaerobic condition and then decreased during aerobic stage.The UV-vis spectra of the supernatant showed that amounts of colloids were released under anaerobic condition,and then re-aggregated during aerobic phase.The scanning transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy(XPS)revealed that Fe oxides were reduced and became dispersed during anaerobic stage,whereas Fe(II)was oxidized and recrystallized under aerobic condition.Based on these results,a kinetic model was established to further distinguish the relationship between the transformation of Cr and Fe.During anaerobic phase,the reduction of Fe(III)oxides not only directly released the structurally bound Cr,but also enhanced the breakdown of soil aggregation and dissolution of organic matter causing indirect mobilization of Cr.During aerobic phase,the oxidation of Fe(II)and further recrystallization of newly formed Fe(III)oxides might induce the re-aggregation of soil colloids and further incorporation of Cr.In addition,the kinetic model of Cr and Fe transformation was further verified in the pot experiment.The model-based findings demonstrated that the Cr transformation in the basalt-derived paddy soil with high geological background values was highly driven by redox sensitive iron cycling.The availability and morphology changes of chromium in microcosmic experiments were verified by pot experiments in soil contaminated with high geological background.The results show that the change of chromium availability and morphology is consistent with that of microcosmic experiment,but the difference lies in the increase of organic bound chromium in the flooding stage,which can be attributed to the soil-water ratio of microcosmic experiment is much higher than that of pot experiment.Rice samples were collected regularly and the kinetic model of chromium migration in rice plants was combined.The results showed that the key processes were chromium transport from root to stem in the early stage of flooding,from stem to grain in the middle stage of flooding,and from root to stem in the drainage stage.At the same time,basalt weathering from the soil,its food security is worth noting.In this paper,we studied different sources of chromium in soil,including how the availability of chromium in different valence states changes from anthropogenic sources,how the form of chromium changes from natural sources,and how these processes were verified in pot experiments,and explored the kinetics of chromium migration in rice.The key processes of chromium fixation and release at soil-water interface and chromium migration in rice were clarified,and the overall mechanism of chromium migration and transformation in soil-rice system was analyzed.This study provides a theoretical basis for the remediation of heavy metal Cr pollution in paddy fields based on key soil processes.This study covers the change of Cr available state and morphological transformation in paddy soil,as well as verifying these processes in pot experiments and studying the whole process of Cr from soil to grain.The influence mechanism of iron,carbon,nitrogen and sulfur element cycling on Cr species was revealed,and the whole process of Cr uptake and transformation in rice was clarified.This provides a theoretical basis for the treatment of heavy metal Cr pollution in paddy fields based on key soil processes.