Investigation On The Anaerobic Biological Degradation And Fenton Oxidation Of Arsanilic Acid

Arsanilic acid (4-aminophenylarsonic acid) is widely used in the poultry and animal industries as a feed additive in the diets. Most of the added arsanilic acid is excreted unchanged in manure and wastewater, resulting in the risk of arsenic contamination. Anaerobic biological treatment was conventional treatment process for for wastewater and organic waste, but the degradation of arsanilic acid in anaerobic process and Fenton process was investigated. The following conclusions were obtained.Firstly, the effects of arsanilic acid on the kinetics, inhibition of methanogenic process and its biotransformation were investigated. The methane yield was not affected by arsanilic acid at concentration< 0.46 mM (mmol/L, millimoles per liter), while the methane production was completely inhibited at the concentration of 0.92 mM. The IC50 of arsanilic acid in this study was 0.47 mM. After 115 days of incubation,37-59% of the added arsanilic acid was degraded. The species analysis indicated that at lower initial arsanilic acid concentration, the soluble inorganic arsenic mainly existed in the species of arsenate (As(Ⅴ)), while at higher initial arsanilic acid concentration (>0.460 mM), the soluble inorganic arsenic mainly existed in the species of arsenite (As(Ⅲ)), which explains why higher arsanilic acid concentration has severe inhibition to methanogens.Secondly, the degradation kinetics, biotransformation of arsanilic acid under thermophilic anaerobic conditions was studied. Compared with mesophilic methanogenic inhibition, thermophilic methanogenic inhibition of arsanilic acid is stronger. The removal of arsanilic acid under thermophilic conditions can be divided into two stages. In the first stage, adsorption is the main mechanism for the removal of arsanilic acid from water phase which could be described by pseudo-second-order model. In the second stage, biodegradation is the main mechanism for the removal of arsanilic acid from water phase which could be described by pseudo-first-order model. After thermophilic degradation of arsanilic acid, the ratios of As(Ⅲ) to inorganic arsenic is bigger than that in mesophilic conditions, which can explain the severe inhibition of arsanilic acid to methanogens. According to species analysis of arsenic in water phase and sludge phase, arsanilic acid was completely degraded in two month, when the concentration of arsanilic acid is less than 0.2 mM.Finally, the degradation of arsanilic acid by Fenton reaction and its mechanism were investigated. The influence of SiO32-, Ca2+, Cl-, H2O2 and Fe2+ doses, degradation kinetic and pathway of arsanilic acid were studied. High efficiency of arsanilic acid degradation by Fenton was obtained at pH 4.5. The degradation rate constant is 0.04951×(Carsaniiic adid0.2934)×(Cfenton0.8374). The influence of Ca2+ and SiO32- on the degradation of arsanilic acid by Fenton is negligible. Cl- showed severe inhibition on the degradation. The degradation pathway was proposed according to the detected intermediate products.