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•Cr(VI) was efficiently reduced to Cr(III) at alkaline pHs by UV/sulfite process.•Common ions and organic matters interfered with Cr(VI) reduction slightly.•Spontaneous precipitation ...of Cr(III) occurred in the presence of Ca2+ (>2ppm).•eaq− was identified as the dominant reactive species in UV/sulfite process.
Chemical reduction of Cr(VI) to Cr(III) followed by Cr(III) precipitation is a widely employed strategy to mitigate Cr(VI) pollution from industrial effluents. Nevertheless, most of the available reduction processes are feasible at acidic pHs only, and very few technologies are capable of reducing Cr(VI) at alkaline pHs. Herein, we demonstrated that the UV/sulfite process is very promising for alkaline Cr(VI) remediation, including the Cr(VI) reduction to Cr(III) and simultaneous Cr(III) precipitation. In this process Cr(VI) reduction followed near zero-order kinetics, declining with an increase of pH (5–10) but boosting with increasing sulfite concentration. The co-existing Cl− and SO42− in water exerted negligible effect on Cr(VI) reduction, whereas the reduction kinetics was improved in the presence of citrate, EDTA or humic acid possibly due to their complexation with Cr(III). Similarly, the presence of borate buffer would significantly inhibit Cr(VI) reduction to Cr(III) as well as the final Cr(III) removal during precipitation. Fortunately, the presence of calcium ions even at trace level would favor Cr(III) precipitation and result in one-step removal of the total Cr at alkaline pH. The mechanism of Cr(VI) reduction was probed through irradiation manipulation and N2O addition, and the results suggested that excitation of sulfite is essential for alkaline Cr(VI) reduction, and eaq− is the dominant reactive species in the UV/sulfite process.
Halogenated aromatic compounds have attracted increasing concerns due to their toxicity and persistency in the environment, and dehalogenation is one of the promising treatment and detoxification ...methods. Herein, we systematically studied the debromination efficiency and mechanism of para-bromophenol（4-BP） by a recently developed UV/sulfite process. 4-BP underwent rapid degradation with the kinetics accelerated with the increasing sulfite concentration, pH（6.1–10） and temperature, whereas inhibited by dissolved oxygen and organic solvents. The apparent activation energy was estimated to be 27.8 kJ/mol. The degradation mechanism and pathways of 4-BP were explored by employing N2O and nitrate as the electron scavengers and liquid chromatography/mass spectrometry to identify the intermediates. 4-BP degradation proceeded via at least two pathways including direct photolysis and hydrated electron-induced debromination. The contributions of both pathways were distinguished by quantifying the quantum yields of 4-BP via direct photolysis and hydrated electron production in the system. 4-BP could be readily completely debrominated with all the substituted Br released as Br-, and the degradation pathways were also proposed. This study would shed new light on the efficient dehalogenation of brominated aromatics by using the UV/sulfite process.
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•EDTA enhanced HO production during ozonation at acidic pHs.•The O3/EDTA process is more efficient and less temperature dependent than either O3 or O3/H2O2 processes.•Rct(∫OHdt/∫O3dt) ...of the O3/EDTA process was enhanced by 8–50times.•A mechanism was proposed for the enhanced HO production.
Activation of ozone for water treatment in acidic solution is challenging due to the lack of efficient initiators (e.g., HO−). Here, we report a marked enhancement of HO production from ozone at acidic pHs by EDTA. EDTA significantly accelerated the degradation of benzoic acid (BA), an HO probe, by ozonation in the pH range of 2.3–5.5, with higher efficiency than the O3/H2O2 process. Such enhancement was less dependent on temperature compared to the O3 or O3/H2O2 processes. The O3 decay and BA degradation kinetics were both highly dependent on the EDTA concentration. At an O3 dosage of 1.0mg/L and pH 2.3, Rct(∫OHdt/∫O3dt) was significantly increased by 8–50times with increasing the EDTA concentration from 1.0 to 40μM. The enhanced HO production was further confirmed by electron paramagnetic resonance, with the HO yield at ∼14.4%. Continuous batch runs were performed with the initial addition of EDTA only, and a similar enhancement effect was observed during cyclic runs. A plausible mechanism involving the formation of organonitrogen intermediates and nitrogen- or carbon- centered radicals is proposed to interpret the HO generation in the system. This study sheds new light on the activation of ozone at acidic pHs, and is believed to broaden the scope of ozonation in water treatment.
Ozonation of recalcitrant contaminants under acidic conditions is inefficient due to the lack of initiator (e.g., OH−) for ozone to produce hydroxyl radicals (HO). In this study, we reported that ...benzoic acid (BA), which is inert to ozone attack, underwent efficient degradation by ozone at acidic pH (2.3). The kinetics of BA degradation and ozone decomposition were both enhanced by increasing BA concentrations. Essentially, it is a HO-mediated reaction. Based on the exclusion of possible contributions of H2O2 and phenol-like intermediates for HO production, the reaction mechanism involved the formation of ozone ion (O3·−), which is an effective precursor of HO, was thus proposed. The hydroxycyclohexadienyl-type radicals generated during the attack of BA by HO may lead to the formation of O3·−. Meanwhile, O3·− could also be possibly formed from the reaction between ozone and organic (e.g., ROO∙) or inorganic peroxyl radicals (e.g., HO2). In addition, the hydroxylated products like phenol-like intermediates also played a positive role in HO production. Consequently, HO was produced efficiently under acidic conditions, resulting in rapid degradation of BA. This study provides a new approach for ozone activation even at acidic pHs, and broadens the knowledge of ozonation in removal of micropollutants from water.
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•Efficient ozonation of benzoic acid at pH = 2.3 was observed.•BA degradation and ozone decomposition was enhanced by increasing BA concentration.•Intermediates formed during BA ozonation served as an initiator to produce OH.•Possible mechanism of ozone activation by BA was proposed.
Activation of ozone for water treatment in acidic solution is challenging due to the lack of efficient initiators (e.g., HO super(-)). Here, we report a marked enhancement of HO super(radical dot) ...production from ozone at acidic pHs by EDTA. EDTA significantly accelerated the degradation of benzoic acid (BA), an HO super(radical dot) probe, by ozonation in the pH range of 2.3-5.5, with higher efficiency than the O sub(3)/H sub(2)O sub(2) process. Such enhancement was less dependent on temperature compared to the O sub(3) or O sub(3)/H sub(2)O sub(2) processes. The O sub(3) decay and BA degradation kinetics were both highly dependent on the EDTA concentration. At an O sub(3) dosage of 1.0 mg/L and pH 2.3, R sub(ct) was significantly increased by 8-50 times with increasing the EDTA concentration from 1.0 to 40 mu M. The enhanced HO super(radical dot) production was further confirmed by electron paramagnetic resonance, with the HO super(radical dot) yield at similar to 14.4%. Continuous batch runs were performed with the initial addition of EDTA only, and a similar enhancement effect was observed during cyclic runs. A plausible mechanism involving the formation of organonitrogen intermediates and nitrogen- or carbon- centered radicals is proposed to interpret the HO super(radical dot) generation in the system. This study sheds new light on the activation of ozone at acidic pHs, and is believed to broaden the scope of ozonation in water treatment. Abbreviations * AAS, atomic absorption spectrometry * BA, benzoic acid * DMPO, 5-dimethyl-1-pyrolin-N-oxide * DOC, dissolved organic carbon * E sub(a) super(app), apparent activation energy * ED2A, ethylenediaminediacetic acid * ED3A, ethylenediaminetriacetic * EDA, ethylenediamine * EDMA, N-aminoethyleneglycine * EDTA, ethylenediaminetetraacetic acid * EfOM, effluent organic matter * EPR, electron paramagnetic resonance * ERMs, electron-rich moieties * IMDA, iminodiacetic acid * HO super(radical dot), hydroxyl radicals * NOM, natural organic matter * NTA, nitrilotriacetic acid * R sub(ct), * TBA, tert-butyl alcohol * TN, total nitrogen * TOC, total organic carbon
