Environmental Fate and Behavior of Arsenate and Phosphate in Soil and Water based on the Surface Complexation at Iron (Hydr)oxides by DFT Calculation and ATR-FTIR and XAFS Spectroscopy

Abstract

Oxyanions are key components of organism, and mutual interaction among oxyanions, organic acids, and metal (hydr)oxides determines the bioavailability and leacheability of the oxyanion in the soil environment. The structural configuration, competition, and preferential sorption control the behaviors of oxyanions but it has not fully understood under various environmental fluctuations yet. In this study, previous studies were reviewed on the iron (hydr)oxides transformation and structural configuration, competitive sorption, and preferential sorption of oxyanions. The structural configuration of oxyanions by changing the pH and surface loading condition was studied, and the competitive and preferential sorption were examined by employing bi-sorbate system or bi-sorbent system. As a result, the EXAFS study identified tridentate complex of arsenate on a goethite surface at low pH and surface loading condition, and a bidentate and monodentate complex were observed at high pH and surface loading condition. Langmuir isotherm with batch experiment examined that the arsenate is more preferentially adsorbed at low pH on four synthetic iron (hydr)oxides but vice versa in the phosphate (bi-sorbate system). The sequential development of promotive effect and competition was observed by increasing the surface loading in the bi-sorbate system. In the bisorbent experiment, the preferential adsorption of phosphate on the maghemite over the goethite was firstly identified using ATR-FTIR and FE-SEM-EDS. Based on our observation, it was concluded that the environmental conditions such as pH, surface loading, sample phase, competitor concentration and type of available surface determines not only the structural configuration, competitive and preferential sorption of oxyanions, but also the transformation of iron (hydr)oxides, and the complexly inter-connected mechanism determines the behavior of oxyanions in the soil environment. I believe that the bottom-up approach from simplest system to the more complex system would eventually explain the adsorption characteristics of all types of soils, and the environmental problems would be diminished by controlling the bioavailability and leachability.