Efficiency And Mechanism Of As(Ⅲ) Catalytic Oxidation In Alkaline System

Arsenic is a ubiquitous associated element in nonferrous metallic minerals, As³⁺ is more toxic than As⁵⁺, thus the oxidation of As³⁺ is an absolutely necessary process in normal arsenic-removing process. Former researches on arsenic oxidation were mainly dealing with acidic system, and few achievements had been made in alkaline system. In this article, the arsenic air oxidation process was systematically studied in basic system. Analyzed and compared the catalytic effects of different catalysts, potassium permanganate was confirmed the most effective catalyst in this system. Influences of experimental conditions on arsenic oxidation rates were also discussed, such as solution pH value, arsenic concentration, air flow and reaction temperature. Reaction products were characterized by x-ray diffraction analysis, SEM paragraph analysis and chemical phase analysis.The results showed that potassium permanganate could oxidize far more arsenic than simply calculated from chemical reaction equations, As³⁺ were effectively oxidized when system As/Mn molar ratio exceeded 21:1. Potassium permanganate could promote As³⁺ oxidation by the following mechanisms: potassium permanganate directly oxidized As³⁺; manganese ions or manganese compounds could catalyze the air oxidation process; potassium permanganate reacted product K_0.27MnO₂(H₂O)_0.54 could adsorb As³⁺, thus promoting the catalytic oxidation process. There existed two different reaction rates in Mn-As reaction process, the fast reaction rate in the beginning would slow down when reaction progressed for about 30min, controlled by variations in reaction mechanism and diffusion process. The system pH values could significantly influence oxidation effects by changing product properties, K_0.27MnO₂(H₂O)_0.54 which belongs to potassium-richδ-MnO₂ would be produced when initial system pH values were 12.5～13, however liquid manganese product would be produced when initial pH changed to 12. The increase in arsenic concentration would influence product morphology and decelerate reaction rates, the optimum As concentrations were 0.0667～0.2002mol/L. It took 2h for the complete oxidation of As³⁺ when C_As=0.0667mol/L. The increase in air flow or temperature would accelerate As³⁺ oxidation, and the optimum conditions were 0.2m³/h and 80℃.Taking potassium permanganate as catalyst, the air catalization oxidation process of arsenic has the advantages as little reagent dosage, easy operation and high oxidation efficiency, thus it has wide application prospect in industry.