In situ carbonation mediated immobilization of arsenic oxyanions

Abstract

Arsenic oxyanions in water can pose severe environmental and health concerns due to associated toxic and carcinogen characteristics. In calcareous environment, calcium carbonate polymorphs (PCC) may have a dominant contribution in arsenic oxyanions immobilization and transmission in water. Arsenic oxyanions interaction with calcium of PCC is still controversial with claims of adsorption, incorporation, and precipitation. In present study, co-precipitation and sorption experiments of arsenic oxyanions (V and III) integrated with in situ carbonation of calcium oxide were investigated. Following a thermodynamic investigation to evaluate the probable chemical interactions between Ca2+, As(V/III), and CO32– ions, experiments were undertaken to further investigate various experimental factors impacting arsenic oxyanions solubility. Maximum As(V) and As(III) oxyanions immobilization efficiencies i.e., 97.3 % (0.4 mg) and 86.0 % (2.1 mg) respectively, were observed with CaO dosage (5 g), CO2 flowrate (100 mL Min−1), and solution temperature (60 °C). K-edge X-ray Near Edge Absorption Spectroscopy and Field Emission Scanning Electron Microscopy confirmed the occurrence of adsorbed and incorporated As(V) and As(III) on and within post As(V/III) oxyanions treated calcite crystals. When As(V) oxyanions were present in high concentrations, calcite crystals distorted by rounding the symmetric corners of the crystals, as well as the genesis of vaterite phase due to nucleation hindrance was observed, whereas no such distortions were observed with As(III) oxyanions treated precipitates. In addition, based on the corresponding stable energies, molecular dynamic simulations were used to evaluate the potential Ca2+ - As (V/III) interactions. The mechanism of arsenic oxyanions immobilization was discovered to be the synergistic effect of surface adsorption and incorporation of arsenic oxyanions within calcium carbonate polymorph. Theoretical and experimental results indicate that As(V) has considerably higher interaction propensity with calcite/vaterite formed during in situ carbonation than As(III), implying that calcite might be an important reservoir for arsenic in a variety of geological contexts.