Eh-coupled release mechanisms of cadmium and arsenic in a contaminated
paddy soil: An microcosm an synchrontron-based spectroscopic study
Liangdong Zhangaand J. Yanga b
aInstitute of Environment and Sustainable Development in Agriculture,
Chinese Academy of Agricultural Sciences, China
bKey Laboratory of Agro-Environment, Ministry of Agriculture, China
liangdongz@qq.com
Cadmium (Cd) and arsenic (As) are identified as major contaminants in
paddy fields, which was characteristic of frequently soil redox
potential (Eh). However, Eh-coupled release mechanisms of Cd/As in
contaminated paddy soils remains unclear. In this experiment, the
combination of advanced microcosms and synchrotron-based X-ray
fluorescence spectroscopy (μ-XRF) were applied to investigate the
release mechanisms of As/Cd in the contaminated paddy soil at varied Eh
conditions (-250, -50, 170 mV). The results indicated the concentrations
of Cd, As, Al, S, P and Fe increased but DOC and pH decreased with the
Eh increase from -250 mV to 170 mV. Cd/S/Al exhibited similar increasing
pattern, with fast increase from -250 mV to -50 mV (Eh Range 1, EhR1),
but slow increase from -50 mV to 170 mV (Eh Range 2, EhR2). However,
As/Fe/P showed slow increase during EhR1 but fast increase during EhR2.
Those suggested Cd and As were probably mainly correlated with sulfide
oxidation and the reduction of Fe hydroxides during Eh changes,
respectively. Moreover, μ-XRF analysis found As tended to overlap with
Fe and S rather than K and Ca in the investigated soil for all Eh
values, and the correlation coefficients of As-Fe and As-S were much
higher at -250 mV and 170 mV compared to -50 mV. These results indicated
As was probably associated with iron sulfide at low Eh condition but Fe
hydroxides at high Eh condition. Collectively, iron sulfide was expected
to be as the predominant sorbents for Cd/As sequestration at the low Eh
condition, while the release of Cd was more sensitive to sulfide
oxidation but that of As mainly depended on the reduction of Fe
hydroxides with Eh changes from reducing to oxidation conditions. This
study provides new insights on the co-release mechanisms of Cd/As in
contaminated paddy soils, thus benefitting the control of rice
contamination in paddy soils.