pH、钙和磷对砷在水稻土中溶解度的影响及表面络合机制解析

Effects of pH,Calcium,and Phosphate on the Solubility of Arsenic in Paddy Soil Based on Surface Complexation Modeling

为深入理解稻田土壤砷(As)的环境行为,以及不同环境因素对土壤溶液中As有效性的影响机制,通过吸附实验结合天然有机质-电荷分配(NOM-CD)模型分析了pH、 Ca^(2+)和PO_(4)^(3-)对As在湖南株洲稻田土壤中溶解度和化学形态分布的影响,并揭示其影响机制.结果表明,土壤总As溶解度在pH 6.0左右最低,主要受Ca^(2+)、 PO_(4)^(3-)、 As(Ⅲ)和As(Ⅴ)的静电作用及位点竞争作用控制;土壤溶液中Ca^(2+)浓度的增加会促进As的吸附,降低土壤溶液中As的浓度,尤其在碱性条件下由于Ca^(2+)的吸附增加了土壤氧化物表面正电荷,因而土壤溶液中As(Ⅴ)浓度随Ca^(2+)浓度的升高明显降低;pH低于5.5时,添加PO_(4)^(3-)抑制了土壤As(Ⅴ)的还原,但pH高于5.5后,添加PO_(4)^(3-)促进了土壤As(Ⅴ)的还原,并由于PO_(4)^(3-)与As之间的竞争作用导致土壤溶液中As(Ⅲ)和As(Ⅴ)浓度显著提高;但无论是否添加PO_(4)^(3-),土壤中还原态As均在较低浓度Ca^(2+)时占比更高,这可能主要是由于土壤溶液中磷的有效性更高,有利于土壤中As或铁的异化还原.NOM-CD模型能够预测pH、 Ca^(2+)和PO_(4)^(3-)对稻田土壤As溶解度及化学形态分布的影响并揭示其主要机制,可为土壤或其他固-液体系中As风险评估提供科学定量的分析方法.

This study aimed to obtain a better understanding on the environmental behavior of As in paddy soil and to reveal the influence mechanisms of different environmental factors on the availability of As in the soil solution.The effects of pH,calcium,and phosphate on the solubility and speciation distribution of As in the paddy soil collected from Zhuzhou of Hunan province were studied by combining the adsorption experiments with the NOM-CD model.The results showed that the minimum concentration of soluble As in the soil was at approximately a pH of 6.0,which was mainly affected by both electrical interactions and site competition between Ca^(2+),PO_(4)^(3-),As(Ⅲ),and As(Ⅴ).The adsorption of As onto soil particles could be increased by an increase in Ca^(2+) in the soil system,leading to the decrease in soluble As concentration.This effect became significant at a higher pH,because adsorbed Ca^(2+) increased the positive charge on(hydr)oxide surfaces.With phosphate addition,the reduction in As(Ⅴ) in the soil was inhibited at pH5.5.Moreover,the concentration of soluble As(Ⅲ) and As(Ⅴ) in the soil solution was dramatically increased with the addition of phosphate owing to the competitive adsorption between As and phosphate.At a lower background of Ca^(2+),there was a higher fraction of As(Ⅲ) in the soil either with or without phosphate addition.This phenomenon might be caused by the higher bioavailability of phosphorus in soil at a lower concentration of Ca^(2+),which favors the dissimilatory reduction of As or iron(hydr)oxides.The results indicated that the NOM-CD model could predict the influence of pH,calcium,and phosphate on the solubility and speciation distribution of As in paddy soil and reveal its main mechanisms.Therefore,the NOM-CD model would provide the quantitative and scientific method for evaluating the risk of As in soils or other solid-water systems.