Liwei Chen

Influence of pH on the Formation of Sulfate and Hydroxyl Radicals in the UV/Peroxymonosulfate System

The influence of pH on the degradation of refractory organics (benzoic acid, BA) in UV(254 nm)/Peroxymonosulfate (UV/PMS) system was investigated. The degradation of BA was significantly enhanced at the pH range of 8-11, which could not be explained only by the generally accepted theory that SO(4)(•-) was converted to HO(•) at higher pH. A hypothesis was proposed that the rate of PMS photolysis into HO(•) and SO(4)(•-) increased with pH. The hypothesis was evidenced by the measured increase of apparent-molar absorption coefficient of PMS (ε(PMS), 13.8-149.5 M(-1)·cm(-1)) and photolysis rate of PMS with pH, and further proved by the increased quasi-stationary concentrations of both HO(•) and SO(4)(•-) at the pH range of 8-10. The formation of HO(•) and SO(4)(•-) in the UV/PMS system was confirmed mainly from the cooperation of the photolysis of PMS, the decay of peroxomonosulfate radical (SO(5)(•-)) and the conversion of SO(4)(•-) to HO(•) by simulation and experimental results. Additionally, the apparent quantum yield for SO(4)(•-) in the UV/PMS system was calculated as 0.52 ± 0.01 at pH 7. The conclusions above as well as the general kinetic expressions given might provide some references for the UV/PMS applications.

Rapid Acceleration of Ferrous Iron/Peroxymonosulfate Oxidation of Organic Pollutants by Promoting Fe(III)/Fe(II) Cycle with Hydroxylamine

The reaction between ferrous iron (Fe(II)) with peroxymonosulfate (PMS) generates reactive oxidants capable of degrading refractory organic contaminants. However, the slow transformation from ferric iron (Fe(III)) back to Fe(II) limits its widespread application. Here, we added hydroxylamine (HA), a common reducing agent, into Fe(II)/PMS process to accelerate the transformation from Fe(III) to Fe(II). With benzoic acid (BA) as probe compound, the addition of HA into Fe(II)/PMS process accelerated the degradation of BA rapidly in the pH range of 2.0-6.0 by accelerating the key reactions, including the redox cycle of Fe(III)/Fe(II) and the generation of reactive oxidants. Both sulfate radicals and hydroxyl radicals were considered as the primary reactive oxidants for the degradation of BA in HA/Fe(II)/PMS process with the experiments of electron spin resonance and alcohols quenching. Moreover, HA was gradually degraded to N2, N2O, NO2 (−), and NO3 (−), while the environmentally friendly gas of N2 was considered as its major end product in the process. The present study might provide a promising idea based on Fe(II)/PMS process for the rapid degradation of refractory organic contaminants in water treatment.

Strong Enhancement on Fenton Oxidation by Addition of Hydroxylamine to Accelerate the Ferric and Ferrous Iron Cycles

The Fenton system generates reactive species with high oxidation potential such as hydroxyl radicals (HO(•)) or ferryl via the reaction between Fe (II) and H₂O₂. However, a number of drawbacks limit its widespread application including the accumulation of Fe (III) and the narrow pH range limits, etc. The aim of this study is to propose a much more efficient Fenton-HA system which is characterized by combining Fenton system with hydroxylamine (NH₂OH), a common reducing agent, to relieve the aforementioned drawbacks, with benzoic acid (BA) as the probe reagent. The presence of NH₂OH in Fentons reagent accelerated the Fe (III)/Fe (II) redox cycles, leading to relatively steady Fe (II) recovery, thus, increased the pseudo first-order reaction rates and expanded the effective pH range up to 5.7. The HO(•) mechanism was confirmed to be dominating in the Fenton-HA system, and the generation of HO(•) was much faster and the amount of HO(•) formed was higher than that in the classical Fenton system. Furthermore, the major end products of NH₂OH in Fenton-HA system were supposed to be NO₃(-) and N₂O.

Efficient reductive dechlorination of monochloroacetic acid by sulfite/UV process.

Most halogenated organic compounds (HOCs) are toxic and persistent, and their efficient destruction is currently a challenge. Here, we proposed a sulfite/UV (253.7 nm) process to eliminate HOCs. Monochloroacetic acid (MCAA) was selected as the target compound and was degraded rapidly in the sulfite/UV process. The degradation kinetics were accelerated proportionally to the increased sulfite concentration, while the significant enhancement by increasing pH only occurred in a pH range of 6.0-8.7. The degradation proceeded via a reductive dechlorination mechanism induced by hydrated electron (e(aq)(-)), and complete dechlorination was readily achieved with almost all the chlorine atoms in MCAA released as chloride ions. Mass balance (C and Cl) studies showed that acetate, succinate, sulfoacetate, and chloride ions were the major products, and a degradation pathway was proposed. The dual roles of pH were not only to regulate the S(IV) species distribution but also to control the interconversion between e(aq)(-) and H(•). Effective quantum efficiency (Φ) for the formation of e(aq)(-) in the process was determined to be 0.116 ± 0.002 mol/einstein. The present study may provide a promising alternative for complete dehalogenation of most HOCs and reductive detoxification of numerous toxicants.

Production of Hydroxyl Radical via the Activation of Hydrogen Peroxide by Hydroxylamine

The production of the hydroxyl radical (HO·) is important in environmental chemistry. This study reports a new source of HO· generated solely from hydrogen peroxide (H2O2) activated by hydroxylamine (HA). Electron paramagnetic resonance analysis and the oxidation of a HO· probe, benzoic acid, were used to confirm the production of HO·. The production of HO· increased with increasing concentrations of either HA or H2O2 as well as decreasing pH. The second-order rate constant for the reaction was (2.2 ± 0.2) × 10(-4) M(-1) s(-1). HO· was probably produced in two steps: the activation of H2O2 by protonated HA and then reaction between the H2O2 and the intermediate protonated aminoxyl radical generated in the first step. Such a two-step oxidation can possibly be ascribed to the ionizable hydroxyl moiety in the molecular structure of HA, as is suggested by comparing the reactivity of a series of HA derivatives in HO· production. The results shed light on a previously unknown source of HO· formation, which broadens the understanding of its role in environmental processes.