Jiang Jun, Dai Zhaoxia, Sun Rui, Zhao Zhenjie, Dong Ying, Hong Zhineng, Xu Renkou
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P. O. Box 821, Nanjing, China.
The City Vocational College of Jiangsu, Nanjing Open University, Nanjing, 210002, China.
Chemosphere. 2017 Jul;179:232-241. doi: 10.1016/j.chemosphere.2017.03.115. Epub 2017 Apr 2.
Iron oxides are dominant effective adsorbents for arsenate in iron oxide-rich variable charge soils. Oxisol-derived paddy soils undergo intensive ferrolysis, which results in high leaching and transformation of iron oxides. However, little information is available concerning the effect of ferrolysis on arsenate adsorption by paddy soil and parent Oxisol. In the present study, we examined the arsenate affinity of soils using arsenate adsorption/desorption isotherms, zeta potential, adsorption kinetics, pH effect and phosphate competition experiments. Results showed that ferrolysis in an alternating flooding-drying Oxisol-derived paddy soil resulted in a significant decrease of free iron oxides and increase of amorphous iron oxides in the surface and subsurface layers. There were more reactive sites exposed on amorphous than on crystalline iron oxides. Therefore, disproportionate ratios of arsenate adsorption capacities and contents of free iron oxides were observed in the studied Oxisols compared with paddy soils. The Gibbs free energy values corroborated that both electrostatic and non-electrostatic adsorption mechanisms contributed to the arsenate adsorption by bulk soils, and the kinetic adsorption data further suggested that the rate-limiting step was chemisorption. The zeta potential of soil colloids decreased after arsenate was adsorbed on the surfaces, forming inner-sphere complexes and thus transferring their negative charges to the soil particle surfaces. The adsorption/desorption isotherms showed that non-electrostatic adsorption was the main mechanism responsible for arsenate binding to the Oxisol and derived paddy soils, representing 91.42-94.65% of the adsorption capacities. Further studies revealed that arsenate adsorption was greatly inhibited by increasing suspension pH and incorporation of phosphate.
在富含氧化铁的可变电荷土壤中，氧化铁是砷酸盐的主要有效吸附剂。源自氧化土的水稻土经历强烈的铁解作用，导致氧化铁的大量淋溶和转化。然而，关于铁解作用对水稻土及其母质氧化土吸附砷酸盐的影响，目前所知甚少。在本研究中，我们通过砷酸盐吸附/解吸等温线、zeta电位、吸附动力学、pH效应和磷酸盐竞争实验，研究了土壤对砷酸盐的亲和力。结果表明，在交替淹水-干燥的源自氧化土的水稻土中，铁解作用导致表层和亚表层游离氧化铁显著减少，无定形氧化铁增加。无定形氧化铁比结晶氧化铁暴露更多的活性位点。因此，与水稻土相比，在所研究的氧化土中观察到砷酸盐吸附容量与游离氧化铁含量的比例失调。吉布斯自由能值证实，静电和非静电吸附机制均对土壤整体吸附砷酸盐有贡献，动力学吸附数据进一步表明限速步骤是化学吸附。砷酸盐吸附在土壤胶体表面后，土壤胶体的zeta电位降低，形成内球络合物，从而将其负电荷转移到土壤颗粒表面。吸附/解吸等温线表明，非静电吸附是砷酸盐与氧化土及其衍生水稻土结合的主要机制，占吸附容量的91.42-94.65%。进一步研究表明，增加悬浮液pH值和添加磷酸盐会极大地抑制砷酸盐的吸附。
