Oxidative dissolution of Fe sulfides is an important geochemical process mediating As mobility in aqueous environments. However, little is known regarding the presence of As(III) on the oxidative dissolution rate of FeS and on speciation of secondary Fe minerals under pH-neutral and alkaline conditions. In this study, oxidative dissolution of FeS under different concentrations of As(III) and pHs were carried out. The enhanced dissolution rates of Fe(II) from FeS with increasing As(III) concentrations (≤ 4.7 mM) indicate that strong interactions between As(III) and S(− II) in FeS enhance the dissolution rates of structural Fe(II), whereas such an enhancement cannot be observed in the As(III)-FeS system under anoxic conditions at pHs 7.0 and 9.0, suggesting the importance of dissolved oxygen in the enhanced dissolution of FeS by As(III). On the other hand, > 98.5% of 6.7 and 4.7 mM As(III) can be immobilized by 2 g L− 1 FeS within 1.5 h at pHs 7.0 and 9.0, respectively, demonstrating the high efficiency of amorphous FeS in the immobilization of concentrated As(III) under oxic conditions. XPS analysis further reveals that, irrespective of pH and initial As(III) concentrations, > 60% As(III) was oxidized to As(V) before it was immobilization in secondary Fe minerals, and no As-S compound could be observed after 48 h of oxidation. As(III) also influences speciation of secondary Fe minerals during oxidative dissolution of FeS. Analyses of XRD, FTIR, Raman, SEM and TEM demonstrate that the plate lepidocrocite is the primary oxidation product in the absence of As(III), but shows degenerations with increasing As(III) concentrations, and completely changes to ferric arsenate in the presence of 6.7 mM As(III), revealing the inhibition of As(III) and/or As(V) on the development of lepidocrocite at pHs 7.0 and 9.0. This also suggests that, apart from adsorption, coprecipitation of ferric arsenate is another important pathway to immobilize concentrated As(III) during oxidative dissolution of FeS.Download high-res image (109KB)Download full-size image

•γ-MnOOH was an efficient catalyst for persulfate activation in phenol oxidation.•Phenol can be effectively oxidized in the full pH range.•Mechanisms involved in phenol oxidation were pH-dependent in PS-γ-MnOOH system.In this study, three trivalent Mn (oxyhydr)oxides, bixbyite (α-Mn2O3), hausmannite (Mn3O4) and manganite (γ-MnOOH) were synthesized and used as persulfate (PS) activators in phenol oxidation. Of these three Mn (oxyhydr)oxides, only γ-MnOOH shows a good catalytic activity in PS activation with more than 80% phenol oxidized within 360 min in the full pH range. With increasing concentrations of PS and γ-MnOOH, phenol removal efficiency increased. TOC results also suggest that phenol oxidation at pH 11 is more complete than those at pH 7 and 3. We further confirm that mechanism involved in phenol oxidation is pH dependent in γ-MnOOH/PS system. Although SO4− and OH were both found under acidic and alkaline conditions, they did not contribute equally under different pHs. At pH 11, SO4− and OH generated from PS activation efficiently oxidize phenol. At pH 3, the strong oxidation activity of γ-MnOOH and an oxidative intermediate formed between S2O82− and γ-MnOOH primarily contribute to phenol oxidation. At pH 7, an oxidative intermediate formed between γ-MnOOH and S2O82− is the predominant oxidative species in phenol oxidation.

•Ag2O/Ag2CO3/g-C3N4 was firstly prepared by a facile phase transformation route.•Ag2O/Ag2CO3/g-C3N4 exhibited good activity in phenol photodegradation.•Holes radicals were the main active species.Ag2O/Ag2CO3/g-C3N4 composite has been prepared for the first time by a facile phase transformation method. XRD and TEM analysis demonstrated that Ag2O and Ag2CO3 particles were well distributed on the surface of g-C3N4, and Ag2O/Ag2CO3/g-C3N4 heterojunction was formed. Phenol photodegradation by Ag2O/Ag2CO3/g-C3N4 was 11.0 and 19.1 times faster than that of g-C3N4 under UV and visible light irradiation, respectively. The enhanced photocatalytic activity resulted from the high separation efficiency of photogenerated electrons and holes. Moreover, holes radicals were the main active species. This study provides an approach to fabricate other novel heterostructured catalysts via the phase transformation of thermally unstable materials.

Removal of toxic Cr(VI) by activated sludge and DOM derived from activated sludge was investigated in this study. A rapid increase in TOC concentration from 50.93 to 127.40 mg L−1 is observed during the Cr(VI) removal process by activated sludge in the pH range of 2–9. Removal efficiencies of Cr(VI) by either activated sludge or DOM greatly decreased with the increasing initial pH. Kinetics of Cr(VI) removal by activated sludge indicate that both biosorption and bioreduction are involved in the Cr(VI) removal. Cr(VI) removal by DOM is slow in dark, but it is greatly enhanced when UV light is applied. The first-order constant increases from 0.0033 (in dark) to 0.079 min−1 (UV illumination) at pH 2.0 and 1068 mg L−1 DOM. The enhancement of Cr(VI) reduction is due to the generation of the reactive intermediates such as O2●_ and DOM* as DOM absorbed light energy, which plays important roles in the reduction of Cr(VI).

•Abiotic oxidation of Mn(II) to Mn(III, IV) (hydr)oxides (MnOx) occurred onto lepidocrocite.•MnOx formed on lepidocrocite contributes to oxidation of soluble and adsorbed As(III) to As(V).•A fraction of the oxidized As(V) was released back into solution.Manganese (oxy)hydroxides (MnOX) play important roles in the oxidation and mobilization of toxic As(III) in natural environments. Abiotic oxidation of Mn(II) to MnOX in the presence of Fe minerals has been proved to be an important pathway in the formation of Mn(III, IV) (oxy)hydroxides. However, interactions between Mn(II) and As(III) in the presence of Fe minerals are still poorly understood. In this study, abiotic oxidation of Mn(II) on lepidocrocite, and its effect on the oxidation and mobilization of As(III) were investigated. The results show that MnOX species are detected on lepidocrocite and their contents increase with increasing pH values ranging from 7.5 to 8.4. After 10 days, an MnOx component, groutite (α-MnOOH) was found on lepidocrocite. During the simultaneous oxidation of Mn(II) and As(III), and the As(III) pre-adsorbed processes, the presence and oxidation of Mn(II) significantly promotes the removal of soluble As(III). In addition, MnOx formed on lepidocrocite also contributes to the oxidation of soluble and adsorbed As(III) to As(V), the latter being subsequently released into solution. In the process where Mn(II) is pre-adsorbed on lepidocrocite, less As(III) is removed, given that the active sites occupied by MnOx inhibit the adsorption of As(III). In all experiments, the removal percentages of As(III) and the release of As(V) are correlated positively with pH values and initial concentrations of Mn(II), although they are not apparent in the Mn(II) pre-adsorbed system.

•S(-II)diss could cause the reductive dissolution of As(V)-ferrihydrite.•Mobilization of As(V) could be caused by S(-II)diss under anoxic conditions.•High S(-II):Fe ratio caused a large fraction of As(V) released into solution.•As(V) loading showed a negative effect on the release of arsenate.In this study, reductive dissolution of As(V)-ferrihydrite and the mobilization of As(V) in the presence of S(-II) were investigated under anoxic conditions. Mobilization of As(V) strongly depended on the S(-II):Fe ratio and the amount of As(V) loading on ferrihydrite. High S(-II):Fe ratio caused a more complete dissolution of ferrihydrite and a large fraction of As(V) could be released into solution. The percentages of the released As(V) were 2.5% and 7.5% at S(-II):Fe ratios of 0.240 and 24.0, respectively, at pH 6.1, while the released As(V) were 5.5%, 16.3% at pH 8.0 under similar conditions. As(V) loading showed a negative effect on the release of arsenate, with smaller fraction of arsenate released into solution when more As (V) adsorbed on ferrihydrite. After 43 h, 14.1%, 5.5%, 1.6% and 0.7% of As(V) were released as for 10, 20, 50 and 100 mg L−1 of As(V) loading, respectively, at pH 8.0. During the dissolution, secondary minerals such as goethite, magnetite and FeS were detected and played different roles in the mobilization of As(V). The released As(V) was mainly repartitioned on the residual ferrihydrite, the newly-formed goethite and magnetite but not FeS.Download full-size image