Scheidegger AM, Lamble GM, Sparks DL
Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware, 19717-1303
J Colloid Interface Sci. 1997 Feb 1;186(1):118-28. doi: 10.1006/jcis.1996.4624.
Retention of heavy metal ions on soil mineral surfaces is an important process for maintaining environmental quality. A thorough understanding of the kinetics and mechanisms of heavy metal sorption on soil mineral surfaces is therefore of fundamental importance. The present study examines the kinetics and mechanisms of Ni(II) sorption onto pyrophyllite, kaolinite, gibbsite, and montmorillonite. Ni sorption reactions were initially fast (15-40% of the initial Ni was removed within the first hour). Thereafter, the rate of sorption decreased significantly. X-ray absorption fine structure (XAFS) spectroscopy was used to determine the local structural environment of Ni(II). Data analysis reveals the presence of polynuclear Ni surface complexes. Ni-Ni bond distances (3.00-3.03 A) were distinctly shorter than in Ni(OH)2(s) (3.09 A). We propose that the reduction of the Ni-Ni distances is caused by the formation of mixed Ni/Al hydroxide phases. The XAFS spectra and derived structural parameters are similar to those in takovite (Ni6Al2(OH)16CO3.H2O), thus suggesting the presence of a Ni phase of similar structure. Even though dissolved Al could not be detected in our samples, Al could have been released into solution and incorporated into mixed Ni/Al hydroxide-like phases. The formation of such phases can explain the finding that the dissolution rates (Si-release) are strongly enhanced (relative to the dissolution rates of the clays alone) as long as Ni sorption is pronounced. We suspect that the release of Al into solution is the rate-determining step for the formation of mixed Ni/Al hydroxide-like phases in our study. Our study demonstrates that mixed Ni/Al hydroxide-like compounds can form when Ni is introduced into a suitable environment in which there is a source of hydrolyzed species of Al. One can speculate that the formation of mixed-cation hydroxide compounds also represents a plausible "sorption mode" for other divalent metal ions when silicates or oxides are present. It has been shown that similar mixed-cation hydroxide compounds can be synthesized when Mg(II), Ni(II), Co(II), Zn(II), or Mn(II) is added to suspensions containing Al(III), Fe(III), and Cr(III). Thus, the formation of mixed-cation hydroxide compounds should be considered when conducting metal sorption experiments, modeling metal surface complexation, determining speciation, and assessing the risk of the migration of contaminants in polluted sites.
重金属离子在土壤矿物表面的吸附是维持环境质量的一个重要过程。因此，深入了解重金属在土壤矿物表面吸附的动力学和机制至关重要。本研究考察了镍（II）在叶蜡石、高岭石、三水铝石和蒙脱石上的吸附动力学和机制。镍的吸附反应最初很快（在最初一小时内去除了15%-40%的初始镍）。此后，吸附速率显著下降。利用X射线吸收精细结构（XAFS）光谱来确定镍（II）的局部结构环境。数据分析揭示了多核镍表面络合物的存在。镍-镍键距（3.00-3.03 Å）明显短于氢氧化镍（s）中的键距（3.09 Å）。我们认为镍-镍键距的缩短是由混合镍/铝氢氧化物相的形成引起的。XAFS光谱和推导的结构参数与水镍矿（Ni6Al2(OH)16CO3·H2O）中的相似，因此表明存在结构相似的镍相。尽管在我们的样品中未检测到溶解态的铝，但铝可能已释放到溶液中并结合到混合镍/铝氢氧化物类相中。只要镍的吸附显著，这些相的形成就能解释溶解速率（硅释放）相对于仅粘土的溶解速率大幅提高这一现象。我们怀疑铝释放到溶液中是我们研究中混合镍/铝氢氧化物类相形成的速率决定步骤。我们的研究表明，当镍被引入到有铝水解物种来源的合适环境中时，可以形成混合镍/铝氢氧化物类化合物。可以推测，当存在硅酸盐或氧化物时，混合阳离子氢氧化物化合物的形成对于其他二价金属离子来说也是一种合理的“吸附模式”。研究表明，当将镁（II）、镍（II）、钴（II）、锌（II）或锰（II）添加到含有铝（III）、铁（III）和铬（III）的悬浮液中时，可以合成类似的混合阳离子氢氧化物化合物。因此，在进行金属吸附实验、模拟金属表面络合、确定物种形态以及评估污染场地中污染物迁移风险时，应考虑混合阳离子氢氧化物化合物的形成。
