Effective reduction and recovery of As(III) and As(V) from alkaline wastewater by thiourea dioxide: Efficiency and mechanism

Abstract

For alkaline wastewater with high arsenic concentration, the traditional lime precipitation inevitably produces large amounts of hazardous waste. Herein, a heat-activated reduction method employing thiourea dioxide (TDO) as the reductant was proposed to efficiently remove and recover As(III)/As(V) from alkaline wastewater in the form of valuable As(0). More than 99.9% of As(III)/As(V) (2-400 mM) were reduced to As(0) with a high purity of more than 99.5 wt% by TDO within 30 min. The highly reductive e_aq^- and SO₂^- radical generated during TDO decomposition contribute to the arsenic reduction, and the contribution ratios of e_aq^- and SO₂^- radical were estimated to be approximately 57.6% and 42.4% for As(III) removal and 62.2% and 37.8% for As(V) removal, respectively. The arsenic reduction was greatly improved by increasing pH and temperature, which could accelerate the cleavage of C-S bond in TDO for the e_aq^- and SO₂^- formation. The presence of dissolved oxygen, which can not only scavenge e_aq^-/SO₂^- but also directly oxidize SO₂^2-, had a negative effect on the arsenic removal. The presence of CO₃^2- slightly suppressed the arsenic removal due to the e_aq^- scavenging effect while SiO₃^2-, PO₄^3-, Cl^-, SO₄^2- and NH₄⁺ had negligible effects. The proposed method was a potential technology for the efficient removal and reduction of arsenic in alkaline wastewater.

Keywords: Arsenic reduction; Hydrated electron; Sulfur dioxide anion radical; Thiourea dioxide.