Permanganate oxidation and ferric ion precipitation (KMnO4-Fe(III)) process for treating phenylarsenic compounds

Abstract

Once discharged into the environment, phenylarsenic feed additives, primarily p-arsanilic acid (p-ASA) and roxarsone (ROX), could pose potentially significant risk. A permanganate oxidation and ferric ion precipitation (KMnO4-Fe(III)) process was proposed to degrade these compounds leached from animal waste (by KMnO4) followed by sorptive removal of the arsenate released (with Fe(OH)3). The kinetics, mechanism, and pathways of p-ASA and ROX degradation in permanganate oxidation were investigated, and the effects of treatment conditions, including the dosages of reagents (KMnO4, Na2SO3, and FeCl3) and solution pH, on degradation of phenylarsenic compounds and removal of soluble manganese and inorganic arsenic were systematically evaluated. The apparent second-order rate constants for permanganate oxidation of p-ASA and ROX at pH 7.0 were determined to be 1.47 ± 0.07 and 0.13 ± 0.01 M−1·s−1, respectively. Under the optimum treatment conditions, >99% of p-ASA and ROX could be degraded (∼94% to As(V), the rest to self-coupling products) by 35 and 240 min of reaction, respectively. The overdosed KMnO4 was reduced by Na2SO3 and removed as MnO2 precipitate, followed by sorptive removal of the residual arsenic and manganese by Fe(OH)3 precipitate formed from FeCl3. The residual levels of manganese and arsenic in the solution phase could be kept below 25 and 80 μg·L−1, respectively. Both humic acid and the organic matter present in swine manure leachate greatly compromised the performance of KMnO4-Fe(III) process, while p-ASA/ROX degradation and residual manganese and arsenic control could still be achieved by substantially raising the reagent dosages. Compared to Fenton oxidation, KMnO4-Fe(III) process could obtain comparable treatment efficiency but with much less chemical consumption, and could thus serve as an economic and robust alternative method for treating p-ASA and ROX in manure leachate.