Evaluation of the photooxidation efficiency of As(III) applying the UVC/oxalate technique

Activation of periodate using ultrasonic waves and UV radiation for landfill leachate treatment

In the present work, saline leachate of the Bushehr coastal city (Iran) was purified using the ultraviolet/ultrasonication wave/periodate process. The initial TDS and TOC values of the leachate studied were 7390 mg/L and 975 mg/L, respectively. During the effect of various parameters on leachate purification, the experiments were optimized at pH 3, oxidizer concentration of 4 mM, and treatment time of 120 min. The initial BOD₅/COD ratio of 0.66 was reduced to 0.42 at the end of the purification time (120 min). After leachate treatment under optimal conditions, the amount of BOD₅, COD, and UV₂₅₄ were 451.5 mg/L, 1072 mg/L, and 12.69 cm^-1, respectively. Concentrations of heavy metals in crude leachate by ICP-OES were checked. Also, the concentration of organic compounds before and after purification was determined using GC-Mass. The leachate purification kinetics followed the first-order model using the designed method. Based on the COD factor, the system energy consumption for leachate treatment was calculated to be 11.4 kWh/m³. The results showed that the system explored (UV/US/IO₄^-) can effectively purify high salinity waste leachate.

Rapid degradation of p-arsanilic acid and simultaneous removal of the released arsenic species by Co-Fe@C activated peroxydisulfate process

In this study, a bimetallic composite catalyst (Co-Fe@C) was fabricated with calcination at high temperature (800 °C) by using Co-MIL-101 (Fe) as the precursor. The characterization results showed that the resulted Co-Fe@C composite mainly consisted of carbon, FeCo alloys, Fe₃O₄, Co₃O₄ and FeO, and owned evident magnetism. In addition, the Co-Fe@C was employed to activate the peroxydisulfate (PDS) to degrade a representative organic pollutant (p-arsanilic acid, p-ASA) and the main factors were optimized, which involved 0.2 g L^-1 of catalyst dosage, 1.0 g L^-1 of PDS dosage and 5.0 of initial pH. Under the optimal condition, Co-Fe@C/PDS system could completely degrade p-ASA (20 mg L^-1) in 5 min. In the Co-Fe@C/PDS system, SO₄^-·, Fe(IV) and ·OH were the main species during p-ASA degradation. Under the attack of these species, p-ASA was first decomposed into phenols and then transformed into the organics acids and finally mineralized into CO₂ and H₂O through a series of reactions like hydroxylation, dearsenification, deamination and benzene ring opening. Importantly, most of the released inorganic arsenic species (93.40%) could be efficiently adsorbed by the catalyst.

Enabling simultaneous redox transformation of toxic chromium(VI) and arsenic(III) in aqueous media-A review

Simultaneous conversion of most harmful As(III) and Cr(VI) to their less toxic counterparts is environmentally desirable and cost-effective. It has been confirmed that simultaneous oxidation of As(III) to As(V) and reduction of Cr(VI) to Cr(III) can occur via free radical or mediated electron transfer processes. While Cr(VI) is reduced by reacting with H^•, e_aq^-, photoelectron directly or undergoing ligand exchange with H₂O₂ and SO₃^2-, As(III) is oxidized by HO^•, SO₄^•-, O₂^•-, and holes (h⁺) in free radical process. The ability to concentrate Cr and As species on heterogeneous interface and conductivity determining the co-conversion efficiency in mediated electron transfer process. Acidity has positive effect on these co-conversion, while mediated electron transfer process is not much affected by dissolved oxygen (O₂). Organic compounds (e.g., oxalate, citrate and phenol) commonly favor Cr(VI) reduction and inhibit As(III) oxidation. To better understand the trends in the existing data and to identify the knowledge gaps, this review elaborates the complicated mechanisms for co-conversion of As(III) and Cr(VI) by various methods. Some challenges and prospects in this active field are also briefly discussed.

Abiotic oxidation of arsenite in natural and engineered systems: Mechanisms and related controversies over the last two decades (1999-2020)

Abiotic oxidation of toxic As(III) to As(V) is being deemed as a necessary step for the overall arsenic decontamination in both natural and engineered systems. Direct oxidation of As(III) by chemical oxidants, such as ozone, permanganate, ferrate, chlorine and chloramine, or naturally occurring minerals like Mn, Fe oxides, seems straightforward. Both O₂ and H₂O₂ are ineffective for arsenite oxidation, but they can be activated by reducing substances like Fe²⁺, Fe⁰ to increase the oxidation rates. Photo-induced oxidation of As(III) has been demonstrated effective in Fe complexes or minerals, NO₃^-/NO₂^-, dissolved organic matter (DOM), peroxygens and TiO₂ systems. Although a variety of oxidation methods have been developed over the past two decades, there remain many scientific and technical challenges that must be overcome before the rapid progress in basic knowledge can be translated into environmental benefits. To better understand the trends in the existing data and to identify the knowledge gaps, this review describes in detail the complicated mechanisms for As(III) oxidation by various methods and emphasizes on the conflicting data and explanation. Some prevailing concerns and challenges in the sphere of As(III) oxidation are also pointed out so as to appeal to researchers for further investigations.

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^-, SO₄^2-) 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.

Kinetic and mechanism study of UV/pre-magnetized-Fe0/oxalate for removing sulfamethazine

In this study, UV irradiated photochemical reactions of oxalate (Ox) with premagnetized-Fe⁰ (pre- Fe⁰) as the catalyst was used to degrade sulfamethazine (SMT). Magnetic field promoted the release of iron ion from Fe⁰ thus enhanced SMT and Ox removal in UV/pre- Fe⁰/Ox process. X-ray photoelectron spectroscopy demonstrated that the presence of UV and Ox promoted the transformation of Fe³⁺ to Fe²⁺ on Fe⁰, which enhanced the surface bound •OH (•OH_surf) generation. Ox inhibited the formation of iron (hydro)xides and enhanced the hydroxylation of Fe⁰ surface. •OH_surf was mainly responsible for SMT removal (44%), while UV direct photolysis and •OH in the solution both caused around 28% SMT removal. The process with Ox exhibited much higher efficiency in SMT degradation than that added with H₃PO₄, citric acid and ethylenediaminetetraacetic acid, which greatly expanded the chelate-modified Fenton processes and their treatment efficiency.