Speciation and sorption mechanisms of metals in soils using bulk and micro-focused spectroscopic and microscopic techniques
Due to anthropogenic processes, including metal smelting and refining, land application of biosolids, and other industrial processes, heavy metals are often found in very high concentrations in soils. Elevated levels of heavy metals in soils can cause phytotoxicity and can ultimately result in pronounced environmental degradation. The mobility and ultimate fate of metals in the environment is greatly dependent on reactions occurring at the interface between solid surfaces found in soils soil solutions. Understanding these reactions is of crucial importance for predicting the long term fate of metals in surface and subsurface environments. In this study, metal sorption mechanisms to several solid phases was investigated using an array of analytical techniques so that direct identification of mechanisms could be achieved.
The kinetics and mechanisms of Ni sorption on a Delaware soil and soil clay mineral fraction were monitored using X-ray absorption fine structure (XAFS) spectroscopy. Above a pH value of 6.8, formation of Ni-Al layer double hydroxide (Ni-Al LDH) phases was a major mechanisms for Ni removal from solution. This is one of the first studies to have identified LDH phases in soils. Ni in precipitated LDH phases was more resistant to desorption compared to systems in which no precipitation occurred. Moreover, an increased aging time had a stabilizing effect on Ni-Al LDH phases.
Zinc may also forms Zn-Al LDH phases upon sorption to clay minerals and oxides. To assess this, Zn sorption kinetics and mechanisms on amorphous silica and gibbsite were monitored using XAFS spectroscopy. In the presence of amorphous silica, Zn did not favor precipitate formation and was present mostly as inner-sphere sorption complexes bound to silica tetrahedra in a tetrahderal geometry. Only at increased values of Zn on the silica surface did evidence of a precipitate phase occur. In the case of high surface area gibbsite, Zn formed inner-sphere sorption complexes to alumina octahedra in a distorted octahedral geometry. In the case of low surface area gibbsite a Zn-Al LDH phase formed at pH 7.5. The results indicate Zn has variable speciation when adsorbed to the two metal oxide phases.
Using the spectroscopic information gleaned for Ni and Zn sorption to laboratory-contaminated systems, direct identification of Zn species in a contaminated soil was investigated. The soils of the Blue Mountain operable unit near the former Zn smelter at Palmerton, PA are devegetated due to elevated levels of heavy metals. In the surface soil XAFS was used to directly identify ZnFe2O4 (franklinite) and ZnS (sphalerite), mineral phases introduced to the soils as a result of the smelting operations. Results indicated that upon their dissolution, Zn re mobilized and repartitioned in the subsurface soil, binding to Al, Fe, and Mn oxide minerals. Micro-XAFS was employed to determine the micro-scale heterogeneity in this complicated system. Zn was found to be more bound to Al-bearing minerals relative to Mn and Fe minerals. Zinc was more labile in the subsurface soil where it was found to occur in both outer- and inner-sphere sorption complexes.
0768: Environmental science
0481: Soil sciences