Degradation of roxarsone and simultaneous immobilization of inorganic arsenic by L-cys modified FeMnNi-LDH activated persulfate system

Abstract

A series of novel L-cysteine (L-cys) modified iron-manganic-nickel hydrotalcite (L-cys-LDH) were synthesized and used for heterogeneous catalytic activation of monopersulfate (PMS) to degrade roxarsone (ROX) and adsorption of released inorganic arsenic. The catalyst was characterized by TEM, XRD, FTIR, XPS, and other characterization techniques to analyze its structure and functional properties. The degradation experiment results showed that the activity of the catalyst was effectively improved after L-cys loading on LDH. In the pH range of 3–9, 0.2 g/L L-cys-LDH and 0.2 mM PMS could degrade more than 90 % of ROX in 20 min, three times higher than that of ordinary FeMnNi-LDH. Besides, the residual amount of inorganic arsenic in the solution was less than 0.01 mg/L. The electron paramagnetic resonance (EPR) test and quenching investigation identified surface-bound SO4-· and surface-bound ·OH as the main contributors to organic degradation. Mechanism studies have shown that cysteine embedded in LDH accelerated the conversion rate of Fe(III)/Mn(III) to Fe(II)/Mn(II) by self-oxidation to cystine, thereby improving the efficiency of catalyst activation of PMS. In addition, the main degradation products were obtained by LC-MS analysis, and the possible pathway of ROX degradation was given in L-cys-LDH/PMS system combined with DFT theoretical calculation. The work showed that L-cys-LDH exhibited excellent catalytic performance and inorganic arsenic adsorption capacity. It provides an idea for the improvement of hydrotalcite-based catalysts. It also sheds some light on the degrade of novel hydrotalcite-based catalysts for the one step degradation and adsorption of ROX.