Oxidation of Roxarsone Coupled with Sorptive Removal of the Inorganic Arsenic Released by Iron–Carbon (Fe–C) Microelectrolysis

Abstract

Iron–carbon (Fe–C) microelectrolysis was proposed as a novel method for simultaneous degradation of a model phenylarsenic compound, roxarsone (ROX), in manure leachate and removal of the inorganic arsenic released. The initial rate equation for ROX degradation was determined as r init = 1.55­[ROX]1.0[Fe–C]0.9[H+]0.5 (μM/min), at the initial solution pH of 2.0–5.0 and in the presence of overdosed Fe–C filler. A unique advantage of Fe–C microelectrolysis process is that ROX was first reduced to 3-amino-4-hydroxyphenylarsenic acid by the atomic hydrogen generated by cathodic reduction under acidic conditions, which facilitated subsequent oxidative attack by HO•. The iron (hydro)­oxide precipitate formed from iron corrosion effectively captured the inorganic arsenic released, reducing the residual arsenic level in the treated water and swine manure leachate to <100 μg/L (standard for irrigation water in China), while the treatment performance was barely affected by the major constitutes of water matrix. The residual level of arsenic in the effluent of a Fe–C microelectrolysis column could be kept consistently below 100 μg/L for 5 months with proper replenishment of the Fe–C filler. The concurrent occurrence of reduction and oxidation reactions and the in situ generation of iron (hydro)­oxides make Fe–C microelectrolysis process particularly efficient for treating nitroaromatic compounds and organoarsenicals.

Photolysis of Roxarsone in Aqueous Solutions Containing Goethite under Simulated Sunlight Irradiation: Kinetics, Mechanism, and Degradation Pathways

“…Similar intermediates were identified during the oxidative degradation of ROX by HO • , ferrate, and Fe-C microelectrolysis. 7,29,48 Inorganic arsenic species released adsorbed on the α-FeOOH surface, 49 which was further confirmed by XPS analysis (Figure 5b). The appearance of peaks at approximately 45.0 and 44.0 eV in the As 3d XPS spectra suggests the adsorption of As(V) and As(III) on the α-FeOOH surface, respectively.…”

“…Similar intermediates were identified during the oxidative degradation of ROX by HO • , ferrate, and Fe-C micro-electrolysis. ,, Inorganic arsenic species released adsorbed on the α-FeOOH surface, which was further confirmed by XPS analysis (Figure b). The appearance of peaks at approximately 45.0 and 44.0 eV in the As 3d XPS spectra suggests the adsorption of As­(V) and As­(III) on the α-FeOOH surface, respectively. , These transformation intermediates can competitively consume the ROS formed and occupy the adsorption sites on the α-FeOOH surface, resulting in the retarded degradation of ROX, as illustrated in Section .…”

“…That is, approximately 6.2% of ROX degradation was attributed to direct photolysis ([ROX] 0 = 5.0 μM, [α-FeOOH] 0 = 2.2 mM), and the rest could be ascribed to α-FeOOH-mediated indirect photolysis. To quantify the role of α-FeOOH in ROX photodegradation, the overall reactions for ROX degradation were empirically simplified as , The initial rate of ROX degradation could be estimated through the well-accepted initial rate method , The initial rates of ROX photodegradation were obtained by fitting the initial degradation data before a significant deviation from pseudo-first-order kinetics occurred (i.e., 0–2.0 h; fitting results are shown in Tables S6–S10, R 2 > 0.92) . Reaction orders with respect to ROX, α-FeOOH, and H + were determined by varying these parameters.…”

“…34 A total of 38.2% of ROX was converted into As(V) at the end of 6.0 h reaction (Figure 5a). Oxidative substitution of the arsenic group by ROS resulted in the release of As(III) and the corresponding hydroxylation product, 7,29,48 i.e., 2-nitro-hydroquinone (Figure S16a). Oxidation of 2-nitro-hydroquinone by ROS generated nitro-1,4-benzoquinone (Figure S16b), which would be further oxidized to release NO 3 − (Figure S16c).…”

“…The appearance of peaks at approximately 45.0 and 44.0 eV in the As 3d XPS spectra suggests the adsorption of As(V) and As(III) on the α-FeOOH surface, respectively. 29,50 These transformation intermediates can competitively consume the ROS formed and occupy the adsorption sites on the α-FeOOH surface, resulting in the retarded degradation of ROX, as illustrated in Section 3.1.…”