贵州砂页岩母质黄壤镉吸附及表面络合模型研究

Cadmium Adsorption and Surface Complexation Model of Sand Shale Yellow Soil in Guizhou Province

重金属在土壤中的吸附行为受土壤表面酸碱性质的影响.贵州酸性黄壤镉(Cd)污染形势严峻,研究黄壤中Cd的吸附特征及形态变化对镉的防控和治理具有重要意义.本文以贵州省百花湖镇不同深度的砂页岩母质黄壤为供试土壤,基于连续酸碱电位滴定试验,利用1-site/2-pKa表面络合模型(SCM),探讨土壤表面酸碱性质;基于不同pH下黄壤Cd吸附试验,运用SCM模拟Cd的吸附行为及其形态分布;同时解析黄壤组成与表面酸碱性质的相关性.结果表明:(1)SCM计算所得腐质层(0~45 cm)、淋溶层(>45~57 cm)、淀积层(>57~81 cm)、风化层(>81~110 cm)、母质层(>110~183 cm)土壤表面位点浓度(Hs)随深度的增加大致呈下降趋势,各层H_(s)分别为0.120、0.090、0.097、0.056、0.055 mol/L,而电荷零点(pH_(pzc,SCM))变化趋势与之相反,分别为4.83、4.71、4.61、4.96、4.98,pH_(pzc,SCM)与试验测定数据(pH_(pzc,M))接近且变化趋势一致,说明1-site/2-pK_(a)SCM适用于黄壤的表面酸碱性质研究.(2)Cd吸附试验表明,随pH的增加,Cd从溶解态逐渐转向吸附态,并在pH=7附近达到完全吸附;SCM拟合得到的不同pH下Cd的溶解态和吸附态浓度与吸附试验数据吻合良好(相关系数R≥0.995),且Cd的吸附行为与不同pH下黄壤表面位点3种形态(≡SOH_(2)^(+)、≡SOH、≡SO^(−))分布的模拟结果一致,≡SOH_(2)^(+)、≡SO^(−)分别为pHpH_(pzc)时的主要形态,揭示了黄壤表面酸碱性质对Cd吸附行为的影响机制,进一步说明SCM适用于描述Cd随土壤pH变化的吸附特征.(3)相关性分析表明,有机质、游离氧化铁对Hs起主要作用,不同深度土壤的电荷零点(pHpzc)则主要由土壤中的黏土矿物、石英等无机胶体控制.研究显示,利用1-site/2-pK_(a)SCM可以准确描述贵州省酸性黄壤的表面酸碱性质以及Cd的吸附特征及其形态分布,可为重金属在土壤中的迁移转化行为研究提供新思路.

The adsorption behavior of heavy metals in soil is affected by the acid-base properties of soil surface.The acid yellow soil in Guizhou Province is severely polluted by cadmium.Studying the adsorption characteristics and morphological changes of cadmium in yellow soil is thus of great importance for preventing and remediating cadmium contamination.Sandy shale yellow soil samples at different depths in Baihuahu Town,Guizhou Province were selected in this research.Based on the results of continuous acid-base potentiometric titration experiments,the 1-site/2-pKa surface complexation model(SCM)was used to explore the surface acid-base properties of these soils.The SCM was used to simulate the adsorption behavior and speciation distribution of Cd in the yellow soil under different pH conditions.Besides,the correlation between the yellow soil composition and surface acid-base properties was analyzed.Firstly,SCM calculations indicated that the soil surface sites(Hs)concentrations in humus layer(0-45 cm),leaching layer(>45-57 cm),deposition layer(>57-81 cm),regolith(>81-110 cm)and parent material layer(>110-183 cm)had a general decreasing trend with depth,which were 0.120,0.090,0.097,0.056 and 0.055 mol/L,respectively.While the point of zero charge(pH_(pzc,SCM))showed an opposite trend,which were 4.83,4.71,4.61,4.96 and 4.98,respectively.The pH_(pzc,SCM) data were close to the experimental data(pH_(pzc,M)),and their change trends were consistent,indicating that it was feasible to apply 1-site/2-pKa SCM to study the acid-base properties of the yellow soil.Secondly,Cd was gradually adsorbed with the increasing of pH and reached complete adsorption at pH=7.The concentrations of dissolved and adsorbed of Cd at different pH calculated by SCM were in good agreement with the experimental adsorption data(correlation coefficient R≥0.995).In addition,the experimental adsorption behavior of Cd was consistent with the simulation results of the three species(≡SOH_(2)^(+),≡SOH,≡SO^(−))distribution in yellow soil surface sites at different pH.The≡SOH_(2)^(+)and≡SO^(−)were the main surface species of the soils at pH<pHpzc and pH>pHpzc,respectively.This revealed the influence mechanism of the acid-base properties of the yellow soil surface on the adsorption behavior of Cd,which further showed that SCM was suitable for describing the adsorption characteristics of Cd with soil pH.Thirdly,correlation analysis showed that organic matter and free iron oxide played important roles in Hs,and the pHpzc of soil at different depths was mainly controlled by inorganic colloids such as clay minerals and quartz.These results showed that 1-site/2-pKa SCM could accurately describe the surface acid-base properties,Cd adsorption characteristics and morphology distribution of acidic yellow soil in Guizhou Province,which provide a new idea for studying the migration and transformation behavior of heavy metals in soil.