应用XAFS研究草酸根和胡敏酸对As(V)在红壤中吸附的影响

结合吸附实验和X光吸收精细结构光谱(XAFS)分析,研究了草酸根和胡敏酸对As(V)在红壤中吸附的影响,分析了As(V)在红壤中的化学形态和微观结构以及草酸根、胡敏酸的影响特征。结果表明,当pH6.0时,红壤主要是通过基团交换反应吸附As(V),草酸根和胡敏酸可以通过竞争吸附位点抑制红壤中As(V)的吸附,其抑制作用随浓度增大而增强。XAFS光谱学数据表明,红壤中吸附的砷以+5价态存在,主要与铁铝矿物形成以约0.317 nm As-Al和0.328 nm As-Fe原子间距为特征的双齿双核结构的内层复合物,复合物结构类型不受砷浓度和草酸根、胡敏酸的影响。

Effect of oxalate and humic acid on arsenate sorption by a red soil using XAFS method

Arsenic sorption is the primary factor that governs bioavailability and mobility of arsenic in soils. However, studies on arsenate (As(V)) sorption have been mainly limited to soil minerals, and sorption reactions in soils as a whole in the presence of dissolved organic carbon are poorly understood. Mechanisms of As(V) sorption on red soil in the presence of oxalate or humic acid (HA) were studied using sorption isotherms and X-ray absorption fine structure (XAFS) spectroscopy. Arsenate, mainly as H₂AsO₄^- in soil solution at pH 6.0, was extensively sorbed by red soil mainly through ligand exchange. Both oxalate and HA could significantly decrease As(V) sorption through competing for sorption sites, and the competition was getting stronger with increasing concentrations of oxalate and HA. The XAFS data indicated that the sorbed As(V) formed inner-sphere complexes of bidentate binuclear configurations characterized by an As-Al bond distance of about 0.317 nm and an As-Fe bond distance of about 0.328 nm in red soil. The two As(V) complexes were stable regardless of As(V) loadings varying in the range studied. Their structures were free from the influence of oxalate and HA. This study illuminated the characteristics of As(V) sorption on red soil in the presence of oxalate and HA from molecular level and the findings might be of practical importance in controlling arsenate mobility and toxicity in soils.