Tiberg Charlotta, Sjöstedt Carin, Gustafsson Jon Petter
Department of Soil and Environment, Swedish University of Agricultural Sciences, Box 7014, SE-750 07, Uppsala, Sweden; Swedish Geotechnical Institute, Kornhamnstorg 61, SE-111 27, Stockholm, Sweden.
Department of Soil and Environment, Swedish University of Agricultural Sciences, Box 7014, SE-750 07, Uppsala, Sweden.
Chemosphere. 2018 Apr;196:556-565. doi: 10.1016/j.chemosphere.2018.01.004. Epub 2018 Jan 4.
While metal sorption mechanisms have been studied extensively for soil surface horizons, little information exists for subsoils, for example Spodosol Bs horizons. Here the sorption of cadmium(II), copper(II) and lead(II) to seven Bs horizons from five sites was studied. Extended X-ray absorption fine structure (EXAFS) spectroscopy showed that cadmium(II) and lead(II) were bound as inner-sphere complexes to organic matter. Addition of o-phosphate (to 1 μmol l) did not result in any significant enhancement of metal sorption, nor did it influence EXAFS speciation. An assemblage model using the SHM and CD-MUSIC models overestimated metal sorption for six out of seven soil samples. To agree with experimental results, substantial decreases (up to 8-fold) had to be made for the fraction 'active organic matter', f, while the point-of-zero charge (PZC) of ferrihydrite had to be increased. The largest decreases of f were found for the soils with the lowest ratio of pyrophosphate-to oxalate-extractable Al (Al/Al), suggesting that in these soils, humic and fulvic acids were to a large extent inaccessible for metal sorption. The low reactivity of ferrihydrite towards lead(II) can be explained by potential spillover effects from co-existing allophane, but other factors such as ferrihydrite crystallisation could not be ruled out. In conclusion, organic matter was the predominant sorbent for cadmium(II), copper(II) and lead(II). However, for lead(II) the optimised model suggests additional, but minor, contributions from Fe (hydr)oxide surface complexes. These results will be important to correctly model metal sorption in spodic materials.
虽然金属吸附机制在土壤表层已得到广泛研究，但关于底土（例如灰化土的Bs层）的相关信息却很少。本文研究了镉（II）、铜（II）和铅（II）在来自五个地点的七个Bs层中的吸附情况。扩展X射线吸收精细结构（EXAFS）光谱表明，镉（II）和铅（II）以内层配合物的形式与有机物结合。添加邻磷酸（至1μmol·l）并未导致金属吸附有任何显著增强，也未影响EXAFS形态。使用SHM和CD - MUSIC模型的组合模型高估了七个土壤样品中六个的金属吸附量。为了与实验结果相符，必须大幅降低（高达8倍）“活性有机物”分数f，同时必须提高水铁矿的零电荷点（PZC）。焦磷酸盐与草酸盐可提取铝的比率（Al/Al）最低的土壤中f的降幅最大，这表明在这些土壤中，腐殖酸和富里酸在很大程度上无法用于金属吸附。水铁矿对铅（II）的低反应性可以用共存的水铝英石的潜在溢出效应来解释，但其他因素如水铁矿结晶也不能排除。总之，有机物是镉（II）、铜（II）和铅（II）的主要吸附剂。然而，对于铅（II），优化后的模型表明铁（氢）氧化物表面配合物有额外但较小的贡献。这些结果对于正确模拟灰化物质中的金属吸附将具有重要意义。
