碱性土壤对Pb的吸附特性:内圈吸附和形成沉淀的XAFS证据

摘要

采用批平衡法和X射线吸收精细结构光谱(XAFS)研究了碱性土壤吸附Pb的特性和机理.土壤对Pb的吸附过程表现出明显的非线性,符合Langmiur模型.XAFS分析表明土壤对Pb的吸附机制有三种:内圈吸附、形成含Pb碳酸钙(PbCaCO3)沉淀和外圈吸附.内圈吸附和形成PbCaCO3沉淀说明土壤中的碳酸钙对Pb具有强烈吸附作用.土壤对Pb的吸附随初始Pb浓度的增大(500～1 000 mg·L-1),第一配位层(Pb-O)半径减小(0.169 2～0.166 8 nm),说明内圈吸附比重加大.%Adsorption mechanisms of Pb on soil with high CaCO3 content were investigated by combined batch sorption and X-ray absorption fine structure (XAFS). Date from the batch equilibrium studies showed that Pb sorption was nonlinear and was well fitted to Langmiur isotherm. The XAFS data indicated that Pb could be adsorbed via the inner-sphere complex, the precipitation of calcium carbonate containing Pb (PbCaCO3), and outer-sphere Pb sorption complex. The formations of inner-sphere complexes and PbCaCO3 implied strong metal interactions with the surfaces the mechanistic reason for the affinity of Pb for CaCO3 as observed in macroscopic studies. At low metal concentration, 500 mg · L-1 of initial Pb, radial distance of the first-shell Pb-O (R1) was 0.169 2 nm, however, at 1 000 rmg · L-1 of initial Pb, the R1 was 0. 166 8 nm. These revealed that the percentage of inner-sphere complexes increased when the initial Pb was increased from 500 to 1 000 mg · L-1.