•The combination of ozone with Zn(0) is an effective approach to the degradation of p-chloronitrobenzene.•The degradation efficiency of p-chloronitrobenzene markedly increased with an increase of Zn(0) dosage.•O2− is dominant active species responsible for p-chloronitrobenzene degradation by ozone induced with Zn(0).•p-Chloronitrobenzene could be effectively degraded in an initial pH range of 4–10.The degradation of p-chloronitrobenzene (pCNB) in different systems including ozone alone, ozone together with zero-valent zinc (Zn(0)) and Zn(0) bubbled with air instead of ozone was investigated. The results demonstrated that the degradation of pCNB by ozone alone was weak. However, the coexistence of Zn(0) and ozone exhibited a significantly synergistic role in pCNB degradation and approximately 98% of the initial pCNB (10 mg/L) was removed within 18 min. The degradation efficiency of pCNB was related to Zn(0) dosage and ozone flow amount, and the optimum conditions were 0.5 g/L and 4.8 mg/min, respectively. pCNB could be effectively degraded in the initial pH range of 4–10, but a higher or a lower pH was not beneficial to the removal of pCNB. Accompanying anions also affected pCNB degradation. Chloride and sulfate slightly improved pCNB degradation, whereas bicarbonate and phosphate markedly suppressed the degradation of pCNB. The reuse of Zn(0) (4 cycles) for pCNB ozonation indicated that the catalytic activity of Zn(0) maintained high and stable. The experiments of free radical scavengers (tert-butyl alcohol and p-benzoquinone) corroborated the dominant active species responsible for pCNB degradation by a combination of ozone with Zn(0) was O2− rather than OH. Based on the results obtained in this study, it can be concluded that ozone activated by Zn(0) is an effective and promising approach to degrade organic contaminants.

•Schwertmannite was proved to be a novel photocatalyst for the degradation of azo dyes.•The coexistence of schwertmannite and citric acid greatly enhanced the removal of AO7.•A possible mechanism for AO7 degradation in schwertmannite/citric acid system is proposed.Schwertmannite was synthesized through the oxidation of FeSO4 by Acidithiobacillus ferrooxidans LX5 cell suspension and characterized using X-ray diffraction spectroscopy (XRD) and scanning electron microscope (SEM). Schwertmannite photocatalytic degradation of acid orange 7 (AO7) assisted by citric acid was further investigated at different initial pH values and different concentrations of schwertmannite or citric acid. The results showed that it was difficult for AO7 to be discomposed by schwertmannite or citric acid alone under UV irradiation. However, the removal of AO7 was significantly enhanced when biogenic schwertmannite and citric acid coexisted in the reaction system. Low pH and high initial concentrations of citric acid and schwertmannite were beneficial to the degradation of AO7. Hydroxyl radical (OH) and Fe(II), the intermediates, were investigated during the reactions to reveal their correlation with the degradation of AO7. A possible mechanism for biogenic schwertmannite photocatalytic decomposition of AO7 in the presence of citric acid was proposed.

•Zn0-activated persulfate is a novel advanced oxidation technology.•MO could be effectively degraded by Zn0-activated persulfate with a high removal of TOC.•Both OH and SO4− contributed to MO degradation.•The optimum concentrations of PS and Zn0 were 71 mg/L and 1.3 g/L, respectively.•The highest removal of MO by Zn0-activated PS was realized at pH 5.Zn0-activated persulfate (PS) is a novel advanced oxidation technology for the degradation of organic pollutants in aqueous solution. The effects of the initial solution pH, the dosages of PS and Zn0, and the temperature were investigated through a series of batch experiments using methyl orange (MO), an azo dye, as a model organic pollutant. The results demonstrated that MO could be effectively degraded by Zn0-activated PS. The chemical oxygen demand and the total organic carbon decreased by approximately 85% and 58%, respectively, in the solution containing 98 mg/L MO at the initial pH 5 and 25 °C within 3 h. The optimum dosages of PS and Zn0 were 71 mg/L and 1.3 g/L, respectively. The highest removal of MO was realized at an initial pH 5. Tertiary butyl alcohol, an OH-specific radical scavenger, and l-histidine, a universal radical scavenger, corroborated that both OH and SO4− contributed to MO degradation. Three stages were observed during the degradation of MO at 15 and 25 °C, a rapid removal of MO in the initial stage of the reaction, followed by a very slow one and then a relatively quick degradation process.

The nanoscale zero-valent iron supported on biochar (B-nZVI) was prepared by liquid-phase reduction method and used for the removal of acid orange 7 (AO7). The structure of composited B-nZVI was characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller surface area analysis. nZVI was well dispersed on the surface of biochar with a specific surface area 52.21 m2/g, and no obvious aggregation was observed. Batch experiments demonstrated that the degradation of AO7 (20 mg/L) by B-nZVI (2 g/L) at initial pH 2 reached 98.3 % within 10 min. There was a good linearity (r2 = 0.99) between kobs and B-nZVI dosage. The reductive cleavage of the azo group in AO7 to amino group may be the dominant stage. This study demonstrated that B-nZVI has the potential to be a promising material for the removal of azo dyes.

Low-cost activated carbons (AC) were prepared from peanut shell (PS) and rice bran (RB) by microwave heating with ZnCl2 as the activating agent and characterized by solid-state 13C-NMR, FT-IR, Boehm titration, and mass titration methods. The solid-state 13C-NMR spectra showed that aliphatic structures of the raw materials were almost completely transformed to condensed aromatic rings, a typical characteristic of AC. The potentials of ACs produced from peanut shell (PACs) and from rice bran (RACs) for Cr(VI) removal were examined by using iodine and methylene blue (MB) as adsorbates, respectively. The effects of solution pH, the dosage of AC, and the initial Cr(VI) concentration on Cr(VI) removal were investigated. The desorption experiments were also performed to examine the regeneration of AC. The results demonstrated that both PAC and RAC were excellent adsorbents for Cr(VI) removal. The maximum removal amounts of Cr(VI) by PAC and RAC reached to 96.27 and 76.04 mg/g, respectively. The adsorption capacity of PAC was higher than that of RAC because PAC held a larger surface area and a more microporous pore structure than RAC. The removal of Cr(VI) by PAC and RAC greatly depended on solution pH, with low pH (in a pH range of 2 to 6) being more conducive to the removal of Cr(VI). The adsorption-coupled reduction of Cr(VI) contributed to the removal of Cr(VI) by PAC and RAC.

Zn0-activated persulfate as a novel and potential approach to the degradation of azo dyes has hardly been reported. In this study, the effects of initial pH, persulfate concentration, Zn0 dosage, and temperature on the decomposition of Congo red (CR), an azo dye, were investigated. The results demonstrated that Zn0-activated persulfate could effectively mineralize CR. At the initial pH 5.5 and 25 °C, chemical oxygen demand (COD) and total organic carbon (TOC) in the solution with 95 mg/L CR decreased by approximately 87 and 60 %, respectively, within 3 h. The optimum dosages of persulfate and Zn0 were approximately 95 mg/L and 2 g/L, respectively. The highest decolorization efficiency of CR was realized at the initial pH 5.5. Both ·OH and SO4−· contributed to the degradation of CR, and the spectra of free radicals showed that SO4−· was gradually converted to ·OH with pH increasing from weak acidic to neutral condition.

We developed an effective method for degradation of carbon tetrachloride (CT) in contaminated water. Zinc metal as a reducing agent for CT in aqueous solutions has been previously studied in some detail, but the rapid corrosion of zinc surface usually reduces its efficiency in removing CT. We assumed that citric acid could enhance the degradation of CT by zinc powder due to the elimination of a passivation layer of Zn(II) (hydr)oxides on the surface of zinc powder through chelating of organic ligands with Zn(II) produced from the reaction and keeping the exposure of active sites to targets. Here the influence of citric acid on the decomposing of CT by commercial micro-scale zinc powder was investigated in a pH range of 3.5–7.5 at 25°C in batch experiments. Reaction mixtures were analysed by gas chromatography/headspace analysis, and Cl− concentration was determined by turbidimetry. The results demonstrate that the degradation of CT by zinc metal alone is very weak, but the addition of citric acid can assist zinc powder to decompose CT more completely and rapidly at all pHs. Degradation of CT took place mainly in the first 10 min of reaction, coupled with 75–95% of CT removal. Maximum dechlorination percentage (82.4%) of CT was obtained at pH 5.5. In that case, chloroform and dichloromethane, as main intermediates, were found at low levels during the whole reaction, suggesting that CT may be sequentially and multiply degraded so quickly that methane is yielded before the intermediates can be desorbed and released to aqueous solution. When compared with the current methods of nano-scale zinc and bimetallic systems, the application of commercial micro-scale zinc particles assisted by organic ligands is of environmental significance since it allows decontamination of aqueous chlorinated organic compounds at low cost and with high efficiency.

Photo-oxidation is a potential pathway for the transformation of Cr(III) to Cr(VI) in natural environments. In this study, the Cr(III)−citrate complex (Cr(III)−cit) was prepared and its speciation was determined by high performance liquid chromatography (HPLC). Results showed that Cr(III)−cit existed in [Cr(III)−H−cit]+ and [Cr(III)−cit] species in a pH range of 3−5, in [Cr(III)−cit] only from pH 6−8, in [Cr(III)−cit] and [Cr(III)−OH−cit]− from pH 9−11, and only in [Cr(III)−OH−cit]− at pH 12. Additional experiments were conducted in batch systems with pHs of 5 to 12 at 25 °C, where aqueous Cr(III) and Cr(III)−cit were fully exposed to light from medium pressure mercury lamps and a xenon lamp mimicking solar light irradiation. Results demonstrated that oxidation of Cr(III) in Cr(III)−cit was much faster than that in aqueous Cr(III). Rates of Cr(III) photo-oxidation were not sensitive to pH in the range from 7 to 9 but increased significantly with further increases in pH, which was consistent with the distribution of Cr(III) forms. It appeared that [Cr(III)−cit−OH]− was the most photochemically active form and Cr(II), resulting from a ligand-to-metal charge-transfer (LMCT) pathway after light absorption, was a precursor of the oxidation of Cr(III) to Cr(VI). Both dissolved oxygen and the hydroxyl radical (•OH), an intermediate, served as oxidants and facilitated the oxidation of Cr(II) to Cr(VI) via a multiple step pathway. The photoproduction of •OH was detected by HPLC using benzene as a probe, supporting the proposed reaction mechanism.

•Cu(II) can markedly improve the reduction of Cr(VI) by citric acid with simulated solar light.•The optimal removal of Cr(VI) by citric acid with simulated solar light is realized at pH 4.•The catalytic role of Cu(II) in the reduction of Cr(VI) is ascribed to the formation of Cu(II)–citric acid complex.•α-OH group in organic acids was the key factor for the Cu(II) photocatalytic reduction of Cr(VI).The catalytic role of Cu(II) in the reduction of Cr(VI) by citric acid with simulated solar light was investigated. The results demonstrated that Cu(II) could significantly accelerate Cr(VI) reduction and the reaction obeyed to pseudo zero-order kinetics with respect to Cr(VI). The removal of Cr(VI) was related to the initial concentrations of Cu(II), citric acid, and the types of organic acids. The optimal removal of Cr(VI) was achieved at pH 4, and the rates of Cu(II) photocatalytic reduction of Cr(VI) by organic acids were in the order: tartaric acid (two α-OH groups, two –COOH groups) > citric acid (one α-OH group, three –COOH groups) > malic acid (one α-OH group, two –COOH groups) > lactic acid (one α-OH group, one –COOH group) ≫ succinic acid (two –COOH groups), suggesting that the number of α-OH was the key factor for the reaction, followed by the number of –COOH. The formation of Cu(II)–citric acid complex could generate Cu(I) and radicals through a pathway of metal–ligand–electron transfer, promoting the reduction of Cr(VI). This study is helpful to fully understanding the conversion of Cr(VI) in the existence of both organic acids and Cu(II) with solar light in aquatic environments.

The catalysis of manganese(II) (Mn2+) on chromium(VI) (Cr6+) reduction by citrate was studied through batch experiments with the concentration of citrate greatly in excess of Cr6+ at 25°C and in pH ranges of 4.0 to 5.0. Results showed that at pH 4.5 within 22 h direct reduction of Cr6+ by citrate was not observed, but for the same time when Mn2+ (50 to 200 μmol L−1) was added, nearly all Cr6+ was reduced, with the higher initial Mn2+ concentration having faster Cr6+ reduction. In the initial stage of the reaction, the Cr6+ reduction could be described with a pseudo-first-order kinetics equation. In the later stage of the reaction, plots of Inc(Cr6+) versus t, where c(Cr6+) is the Cr6+ concentration in the reaction and t is the reaction time, deviated from the initial linear trend. The deviations suggested that the pseudo-first-order kinetics did not apply to the whole experimental period and that some reaction intermediates could have greatly accelerated Cr6+ reduction by citrate. The catalysis of the intermediates increased with the reaction time and gradually reached stability. Then, the plot of Inc(Cr6+) versus t in the presence of Mn2+ was linear again, with the rate constant increasing by 102 times compared with the absence of Mn2+. Complexation between Mn2+ and citrate was likely a prerequisite for the catalysis of Mn2+ on the reaction. Additional experiments showed that introducing ethylenediaminetetraacetic acid (EDTA) into the reaction system strongly suppressed the catalysis of Mn2+.

•Cr(III)-cit/tar was synthesized, and then purified with cation exchange resin.•The oxidation of Cr(III)-cit/tar by δ-MnO2 was much lower than aqueous Cr(III).•The oxidation of Cr(III)-cit/tar over δ-MnO2 can be divided into two phases.•NH4+ significantly improved the oxidation of Cr(III)-cit/tar by δ-MnO2.Cr(III)-cit and Cr(III)-tar were synthesized and purified, and then their stability in the presence of δ-MnO2 was further investigated in batch experiments under different conditions to predict the potential oxidation behaviors of Cr(III)–organic acid complexes in the environment. The results indicated that although the rates and extents of Cr(III)-cit and Cr(III)-tar oxidation by δ-MnO2 were much lower than those of aqueous Cr(III), Cr(VI) could be gradually released through the whole reaction. The oxidations of Cr(III)-cit and Cr(III)-tar were affected by the initial concentrations of δ-MnO2, pH and co-existing ions. Lower pH and higher concentrations of δ-MnO2 markedly enhanced the production of Cr(VI). Ammonium ions significantly improved the oxidation of Cr(III)-cit and Cr(III)-tar, but phosphate ions demonstrated an opposite effect due to the formation of more stable CrPO4. The oxidation process of Cr(III)-cit and Cr(III)-tar by δ-MnO2 can be divided into two phases. At the initial phase, a relatively rapid reaction obeyed the first-order model, and then a zero-order one was followed. It was observed that in all the cases the extent of Cr(III)-cit oxidation was lower than that of Cr(III)-tar. Thus, it was concluded that the stability of Cr(III)-cit was higher than that of Cr(III)-tar.

Degradation of carbon tetrachloride (CT) by microscale zero-valent iron (ZVI) was investigated in batch systems with or without organic ligands (ethylenediaminetetraacetic acid (EDTA), citric acid, tartaric acid, malic acid and oxalic acid) at pHs from 3.5 to 7.5. The results demonstrated that at 25 °C, the dechlorination of CT by microscale ZVI is slow in the absence of organic ligands, with a pseudo-first-order rate constant of 0.0217 h−1 at pH 3.5 and being further dropped to 0.0052 h−1 at pH 7.5. However, addition of organic ligands significantly enhanced the rates and the extents of CT removal, as indicated by the rate constant increases of 39, 31, 32, 28 and 18 times in the presence of EDTA, citric acid, tartaric acid, malic acid and oxalic acid, respectively, at pH 3.5 and 25 °C. The effect of EDTA was most significant; the dechlorination of CT at an initial concentration of 20 mg l−1 increased from 16.3% (no ligands) to 89.1% (with EDTA) at the end of 8 h reaction. The enhanced CT degradation in the presence of organic ligands was primarily attributed to the elimination of a surface passivation layer of Fe(III) (hydr)oxides on the microscale ZVI through chelating of organic ligands with Fe(III), which maintained the exposure of active sites on ZVI surface to CT.