Adsorptive Remediation of Cr(Ⅵ) Contaminated Groundwater with Chemically Synthesized Schwertmannite

Abstract

Schwertmannite is usually naturally found in acidic mining wastewater and frequently used in the adsorption of heavy metal anions from water and wastewater. Schwertmannite was synthesized through a facile chemical method and utilized to remove Cr(Ⅵ) from contaminated groundwater. The kinetics, thermodynamics and isotherms, as well as the effects of environmental factors on the Schwertmannite adsorption processes were investigated. The experimental results showed that the synthesized Schwertmannite had a strong adsorption capability of Cr(Ⅵ) from aqueous solution. At the pre-set initial concentrations of Cr(Ⅵ), the Schwertmannite adsorption of Cr(Ⅵ) achieved equilibrium within 24 h, and the Lagergren's second-order model fitted the adsorption process better compared to Lagergren's first-order model and intraparticle diffusion model. Langmiur equation fitted the adsorption isotherms better than Freundlich equation. The Cr(Ⅵ) adsorption on Schwertmannite mainly involved ion exchange reaction between Cr(Ⅵ) and anions such as OH- and SO42- and surface complexation reactions. The ΔHθ and ΔGθ were 6.368 kJ·mol-1 and -1.215 kJ·mol-1, respectively, therefore the adsorption of Cr(Ⅵ) was a spontaneous and endothermic process. The removal of Cr(Ⅵ) from aqueous solution increased with increasing Schwertmannite dosage at pH=4.5. Acidic pH in the range of 4.5-6 favored Cr(Ⅵ) removal with Schwertmannite compared to that under basic conditions. Under the conditions of 5 mg·L-1of initial Cr(Ⅵ) concentration, 0.5 g·L-1 of Schwertmannite dosage, pH=6, maximum Cr(Ⅵ) removal of 93.1% was achieved and the adsorption capacity of Cr(Ⅵ) with Schwertmannite reached up to 40.4 mg·g-1. Batch tests showed that the presence of HCO3- and SO42- inhibited the adsorption of Cr(Ⅵ) while Cl- had no significant impact. Cations and natural organic matter had a pH-dependent impact on Cr(Ⅵ) removal:at pH=8 natural organic matter and cations would significantly inhibit the Cr(Ⅵ) sorption, while the impact could be neglected at weak acidic conditions (pH=6).