Multifunctional capacity of CoMnFe-LDH/LDO activated peroxymonosulfate for p-arsanilic acid removal and inorganic arsenic immobilization: Performance and surface-bound radical mechanism.

Organoarsenic contaminants existing in water body threat human health and ecological environment due to insufficient bifunctional treatment technologies for organoarsenic degradation and inorganic arsenic immobilization. In order to safely and efficiently treat organoarsenic contaminants discharged into the aquatic environment, Co-Mn-Fe layered double hydroxide (CoMnFe-LDH) and Co-Mn-Fe layered double oxide (CoMnFe-LDO) were fabricated and employed as peroxymonosulfate (PMS) activator for organoarsenic degradation and inorganic arsenic immobilization, and p-arsanilic acid (p-ASA) was selected as target pollutant. Results demonstrated that the satisfactory removal of p-ASA (100.0%) in both CoMnFe-LDH/PMS and CoMnFe-LDO/PMS systems was obtained within 30 min, and substantial inorganic arsenic adsorption could be achieved (below 0.5 mg/L) in two systems with converting major inorganic arsenic species to arsenate. As XPS, ESR and quenching experiment revealed, the existence and generation of surface-bound radicals in two systems were identified. Based on density functional theory calculation and XPS analysis, the catalytic mechanism of CoMnFe-LDO/PMS system that PMS could be activated via direct electron transfer from adsorbed p-ASA was clarified, which differed from PMS activation via coupling with surface hydroxyl groups in CoMnFe-LDH/PMS system. Catalytic performance assessment under various critical operation parameters indicated that CoMnFe-LDH presented more stable ability of p-ASA removal in a wide pH range and complex aquatic environment. The recycle experiment demonstrated the excellent stability and reusability of CoMnFe-LDH(LDO). Besides, seven degradation products of p-ASA in CoMnFe-LDH/PMS system including phenolic compounds, azophenylarsonic acid, nitrobenzene and benzoquinne were identified by UV-Vis spectra and LC-TOF-MS analysis, and the corresponding degradation pathway was proposed. In summary, compared to CoMnFe-LDO/PMS, CoMnFe-LDH/PMS holds great promise for the development of an oxidation-adsorption process for efficient control of organoarsenic pollutant.

Simultaneous removal of para-arsanilic acid and the released inorganic arsenic species by CuFe2O4 activated peroxymonosulfate process.

para-arsanilic acid (p-ASA), as a major phenylarsonic feed additive, was used annually in many countries. Once it enters the water environment, p-ASA would be transformed into hypertoxic inorganic arsenic species, causing severe arsenic pollution. In this study, magnetic copper ferrite (CuFe2O4) was applied to activate peroxymonosulfate (PMS) for p-ASA removal and synchronous control of the released inorganic arsenic species. Results showed that CuFe2O4/PMS system presented favorable oxidation ability and close to 85% of 10 mg/L p-ASA was eliminated under the condition of simultaneous dosing 0.2 g/L CuFe2O4 and 1 mM PMS. The rapid decomposition of p-ASA resulted from homogeneous PMS oxidation and the attack of reactive oxygen species (i.e., SO4-, HO and O2-), which was involved the heterogeneous PMS activation through the cycles between Fe(II)/Fe(III) and Cu(II)/Cu(I). Meanwhile, the released inorganic arsenic species during p-ASA degradation were found to be controllable via the adsorption on CuFe2O4 surface and metal hydroxyl groups played the crucial role. CuFe2O4/PMS system exhibited the stable and efficient performance within the broad range of pH 3.0-11.0. The existence of common anions (Cl-, NO3-, HCO3-, SO42-) and humic acid presented the slight inhibition for p-ASA degradation. The reduction of initial p-ASA concentration favored the p-ASA removal. Besides, the catalyst retained a favorable reactivity and stability even after four successive cycles and almost no metal leaching was observed. The rational degradation pathway was mainly involved in the cleavage of AsC bond, oxidation of amino group, substitution and oxidation of hydroxyl group. The transformation of arsenic species could be divided into the release of inorganic arsenic species, the oxidation of As(III) into As(V) and the adsorption of As(V) by CuFe2O4.