A novel strategy for arsenic removal from acid wastewater via strong reduction processing

Due to the high-acidic arsenic-containing wastewater pollution greatly threatening human health and ecological safety, a simple and efficient method for reducing arsenic was proposed in this paper to solve this problem. By using potassium borohydride (KBH₄) as a reducing agent, the soluble arsenic was converted into the gaseous arsine (AsH₃) or solid arsenic (As⁰) to achieve the purpose of removing arsenic in wastewater. By exploring the reaction kinetics of the arsenic removal process, it was found that the fast reaction stage (0-2 min) conformed to pseudo-first-order kinetics. The removal rate of arsenic increased to over 73% in 0.5 min, and reaction equilibrium was reached after 30 min. Various influence factors including arsenic valence, aeration, addition method, concentrations of reducing agent, and hydrogen ion (H⁺) were investigated. The results showed that As(III) was easier to be removed by reduction than As(V), while adding KBH₄ in multiples and aeration were both favorable to the removal of arsenic. Increased concentration of KBH₄ also enhanced the removal of arsenic. Appropriate H⁺ concentration contributed to the arsenic removal, but excessive H⁺ concentration conversely has an inhibitory effect. The maximum removal rate of arsenic was 95.87%, with the maximum removal capacity of 45.50 mg/g. Based on the XRD and SEM-EDS analysis of residue, amorphous arsenic (As⁰) with a mass ratio of more than 94.52% was generated after the reduction of soluble arsenic. Our study demonstrated that the reaction mechanism of reductive degradation is soluble arsenic with hydrogen radicals (H•) to form arsenic (As⁰) and arsine (AsH₃) (in the molar ratio of 6:1). Although the generated solid arsenic (As⁰) is convenient for the soluble arsenic removal from wastewater, attention must be paid to the formation of AsH₃, and strategies for AsH₃ treatment should be considered.