Constructing novel semiconductor heterojunctions is one of the most significant approaches to improving the photocatalytic performance of a photocatalyst. Herein, the Ag3VO4/Bi2WO6 heterojunction was prepared through in-situ anchoring Ag3VO4 nanoparticles (size: ∼21 nm) on the surface of Bi2WO6 microflowers (diameter: 2.5–4.5 μm) by a facile deposition route. The photocatalytic activity of these heterojunctions were studied by decomposing cationic dye rhodamine B (RhB), anionic dye methyl orange (MO) and neutral para-chlorophenol (4-CP) under visible light irradiation (λ > 400 nm). Among all the tested catalysts, the heterojunction with a Ag3VO4/Bi2WO6 molar ratio of 0.15/1 displays the maximum activity with the RhB degradation rate constant of up to 0.0392 min−1, a 6.7 or 1.7 times more enhancement compared with the pure Bi2WO6 or Ag3VO4. It is found that the introduction of Ag3VO4 is in favor of suppressing the electron-hole pair recombination of Bi2WO6, leading to an enhanced photocatalytic activity with good stability. The photogenerated holes (h+) and superoxide radicals (O2-) play critical roles during the photocatalytic process. Ag3VO4/Bi2WO6 will have great potential in applications for environmental remediation due to the facile preparation method and superior photocatalytic activity.Download high-res image (288KB)Download full-size image

•Effects of ammonium and chloride ions on Co/PMS process were firstly reported.•An inhibitory effect of ammonium on the rate of AO7 degradation was observed.•It is recommended to monitor NH4+/Cl− in wastewater when Co/PMS is used.A large amount of chloride and ammonium ions were produced and released from industrial processes with non-biodegradable organic pollutants to affect efficiencies of advanced oxidation processes (AOPs). Here, the influences of chloride and ammonium ions on Co/peroxymonosulfate (Co/PMS) reaction system, a widely used AOPs to produce sulfate radicals, were investigated by examining the degradation efficiency of an azo dye (Acid Orange 7, AO7). The experimental results showed that a significant decrease in the degradation rate of AO7 was observed in the presence of NH4+, while a dual effect of chloride on AO7 bleaching appeared. The presence of NH4Cl was unfavorable for AO7 degradation at low concentration (<20 mM), whereas further addition of NH4Cl (>20 mM) apparently accelerated AO7 discoloration rate. The apparent effects of the two co-existing inorganic ions were determined by roles of the dominating ions at varied molar ratio of [NH4+]/[Cl−]. The present study may have technical implications for the treatment of industrial wastewater containing diverse ions in practice.

•Reaction of trace Br− with PMS results in chlorophenol degradation.•Chlorobromoaromatic compounds are identified for the first time.•The possible reaction pathways of TCP and its derivatives are explained.Trace bromide (Br−) released from industrial effluents or brominated compounds is able to directly react with peroxymonosulfate (PMS) to generate a series of reactive oxidants which can oxidize and also halogenate organics. We report the identification and evolution of by-products during 2,4,6-trichlorophenol (TCP) degradation in the presence of PMS and trace Br−. The influencing factors, including Br− concentration and pH, were investigated. The depletion of TCP was accelerated with increasing trace Br− concentration (0–0.2 mM) and was affected by the initial pH (3.0–7.0). The chlorinated and brominated compounds were identified in simulated wastewater during treatment with PMS. Notably, the potential formation of chlorobromoaromatic by-products was demonstrated for the first time in the presence of PMS and trace Br−. The possible reaction pathways of TCP and its derivatives are discussed. These findings have important implications for the future applications of PMS-based oxidation processes.Download high-res image (159KB)Download full-size image

•Effects of Cl− on degradation of dyes and their intermediate (PA) were different.•Less chlorinated products were formed during PA oxidation at high chloride level.•A possible degradation pathway of PA in Co/PMS process was proposed.A considerable effort has been devoted to elucidating the roles of chloride in oxidative degradation and chlorination of dyes. However, few investigations are available on kinetic analysis and transformation pathways of secondary degradation byproducts of dyes in saline wastewater treatment. Here the impact of chlorine on the degradation rate of phthalic acid, a typical dye degradation intermediate, by the Co2+/peroxymonosulfate (PMS) process was examined. Degradation efficiency, intermediate products, AOX (adsorbable organic halogen) formation and mineralization were considered. An overall negative impact was observed within the concentration of Cl− up to 100 mM, differing from the dual effect of chloride on dye degradation process as previously observed. The presence of high levels of Cl− led to a low production of AOX and a reduction of the formation of chlorinated by-products. The mineralization was also restrained when the Cl− concentration was increased. Degradation pathways for these processes are proposed. These findings provide valuable information about the degradation pathways of dyes and about the formation mechanism of chlorinated by-products in industrial saline wastewater treatment.

•No significant effects of chloride on MCPs degradation kinetics by UV/PS.•Undesirable chlorinated byproducts were identified.•An accumulation and relative increase of AOX was observed.The efficiency and, accordingly, the success of the advanced oxidation processes (AOPs) has generally been evaluated on the basis of degradation kinetics. In practice, chloride in saline wastewater is often found to inhibit degradation processes. Therefore its highly desirable to develop more effective processes which are not affected by chloride. In this study, no significant interference of chloride with monochlorophenols (MCPs, e.g. 2-CP, 3-CP and 4-CP) degradation by the UV photo-activated persulfate (UV/PS) process has been observed. This indicated the “illusion” that the UV/PS process might have been an appropriate technology working under saline conditions. To further evaluate its applicability, the generation of reaction intermediates, of adsorbable organic halogen (AOX) accumulation and of acute toxicity of MCPs in the UV/PS system were examined. In reality, several aromatic chlorinated compounds (number of chlorine atoms ≥2), such as dichlorophenols and 2,3,5,3′,5′-pentachloro-biphenyl, were identified and quantified. An accumulation and relative increase of AOX with reaction time was observed in the UV/PS/Cl system. The acute toxicity tests with Photobacterium phosphoreum indicated that the inhibition effect of UV/PS reactions increased with reaction time regardless of the presence of chloride or not. The results of this study might be helpful for assessing the PS-based technologies for saline wastewater treatment.Download high-res image (84KB)Download full-size image

The rapidly increasing and widespread use of graphene oxide (GO) as catalyst supports, requires further understanding of its chemical stability in advanced oxidation processes (AOPs). In this study, UV/H2O2 and UV/persulfate (UV/PS) processes were selected to test the chemical instability of GO in terms of their performance in producing highly reactive hydroxyl radicals (OH) and sulfate radicals (SO4−), respectively. The degradation intermediates were characterized using UV–visible absorption spectra (UV–vis), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, and matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Experimental data indicate that UV/PS process was more effective in enhancing GO degradation than the UV/H2O2 system. The overall oxygen-containing functionalities (e.g. CO, CO and OCO groups) dramatically declined. After radical attack, sheet-like GO was destructed into lots of flakes and some low-molecular-weight molecules were detected. The results suggest GO is most vulnerable against SO4− radical attack, which deserves special attention while GO acts as a catalyst support or even as a catalyst itself. Therefore, stability of GO and its derivatives should be carefully assessed before they are applied to SO4−-based AOPs.Download high-res image (71KB)Download full-size image

The presence of external chloride can lead to a 47-fold increment in degradation rates of 4-chlorophenol than those in the absence of chloride in UV/peroxymonosulfate process. The other side of the same coin is an undesirable accumulation and increase in absorbable organic halogen (AOX) was observed in the presence of chloride, with formation of some more toxic tetrachlorinated byproducts.

Degradation of acid orange 7 (AO7) by Fe0-based Advance Oxidation Process (AOPs) with common peroxygens, persulfate (PS), peroxymonosulfate (PMS) and hydrogen peroxide (H2O2), was investigated, in which sulfate radicals (SO4˙−) and/or hydroxyl radicals (˙OH) are powerful oxidizing species. The effects of Fe0 dosage, peroxygen concentration, initial pH and the presence of chloride on the degradation of AO7 were examined. The AO7 degradation efficiencies by four systems, including Fe0, Fe0/H2O2, Fe0/PMS and Fe0/PS were compared. AO7 degradation rate by Fe0 activated AOPs in descending order is H2O2 ≧ PS > PMS. Increasing acidity and iron dosage favored a rapid degradation of AO7. The presence of chloride greatly inhibited dye removal in Fe0/H2O2 and Fe0/PS systems, whilst accelerated dye degradation was observed in the Fe0/PMS system. In contrast, mineralization of AO7 in the Fe0/PMS/Cl− system was minimal, because of formation of lots of refractory chlorinated phenols as identified by GC-MS. These findings are useful for selecting the most appropriate technology for textile wastewater treatment, depending on the wastewater constituents and pH.

•Enhanced base activation of PMS with polyphosphate was firstly reported.•PMS and polyphosphate are indispensable for the oxidative degradation of pollutants.•Polyphosphate/PMS process is much more favorable than PDS/base process.Base activation of peroxydisulfate (PDS) is a common process aiming for water treatment, but requires high doses of PDS and strongly basic solutions. Peroxymonosulfate (PMS), another peroxygen of sulfurate derived from PDS, may also be activated by a less basic solution. However, enhancing the base-PMS reactivity is still challenging. Here it is reported that pyrophosphate (PA) and tripolyphosphate (PB) can efficiently enhance PMS activation under weakly alkaline conditions (pH 9.5) via the formation of superoxide anion radical (O2•−) and singlet oxygen (1O2). The rate constant of Acid Orange 7 (AO7) degradation in PA/PMS system (kPA/PMS) was nearly 4.4–15.9 fold higher than that in PMS/base system (kPMS/base) without any polyphosphates. Increases in PA (or PB) concentration, PMS dose and pH favored the rapid dye degradation. Gas chromatograph-mass spectrometer (GC-MS) data confirmed AO7 and 2,4,6-trichlorophenol (2,4,6-TCP) were decomposed to a series of organic intermediates. The radical quenching and probe oxidation experiments indicate the degradation of organic compounds in the PA/PMS and PB/PMS processes was not reliant on sulfate radical (SO4•−) and hydroxyl radical (OH) species but on O2− and 1O2 reactive species. Comparison experiments show that the polyphosphate/PMS process was much more favorable than PDS/base process. The present work provides a novel way to activate PMS for contaminant removal using industrial polyphosphate wastewaters.Download high-res image (176KB)Download full-size image

Addition order of reagents in the Acid Orange 7 (AO7) degradation process was investigated by varying the concentration of Fe(II), the Fe(II)/peroxymonosulfate (PMS) molar ratio and stepwise addition of Fe(II) and PMS. The importance of addition order of reagents was confirmed and an order of Fe(II)–PMS to improve the oxidation efficiency was recommended.

Reduction of Cr(VI) is often deemed necessary to detoxify chromium contaminants; however, few investigations utilized this reaction for the purpose of treating other industrial wastewaters. Here a widely used Cr(VI)–sulfite reaction system was upgraded to simultaneously transform multiple pollutants, namely, the reduction of Cr(VI) and oxidation of sulfite and other organic/inorganic pollutants in an acidic solution. As(III) was selected as a probe pollutant to examine the oxidation capacity of a Cr(VI)–sulfite system. Both •OH and SO4•– were considered as the primary oxidants for As(III) oxidation, based on the results of electron spin resonance, fluorescence spectroscopy, and specific radicals quenching. As(III)-scavenging, oxidative radicals greatly accelerated Cr(VI) reduction and simultaneously consumed less sulfite. In comparison with a Cr(VI)–H2O2 system with 50 μM Cr(VI), Cr(VI), the sulfite system had excellent performance for both As(III) oxidation and Cr(VI) reduction at pH 3.5. Moreover, in this escalated process, less sulfite was required to reduce Cr(VI) than the traditional Cr(VI) reduction by sulfite process. This effectively improves the environmental compatibility of this Cr(VI) detoxification process, alleviating the potential for SO2 release and sulfate ion production in water. Generally, this study provides an excellent example of a “waste control by waste” strategy for the detoxification of multiple industrial pollutants.

The conventional photo-Fenton reaction often suffers from the constraints of operation pH, low iron loading, ultraviolet availability in solar light and instability of iron-based catalysts. Here we report a novel heterogeneous Fenton reaction which works with a dye-photosensitized structural Fe(III)/Fe(II) redox cycling mechanism. The synthesized nontronite catalyst (NAU) was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) analysis, and thermal gravimetric analysis (TG). NAU exhibited excellent catalytic activity over a wide pH range (3.0–8.0) for highly efficient degradation of Rhodamine B by hydrogen peroxide (H2O2) under visible light irradiation (λ > 420 nm). The excited dye molecule donates electrons to structural iron sandwiched in NAU which further catalyzes H2O2 to generate highly reactive ˙OH radicals. This iron-rich clay mineral (total Fe, 24.4 wt%) is chemically and mechanically stable. There are no measurable iron leaching, nor any noticeable loss of activity and damage to the clay structure observed after 6 recycles. Therefore, NAU clay has outstanding merits for the practical treatment of organic dye pollutants at large scale.

The diverse redox processes of the photo/ferrioxalate system (PFS) were investigated by varying the concentrations of Fe(III), oxalate and oxygen. Photoreactivity of PFS is determined by the prevalence of the most photolabile Fe(III) and abundance of Fe(III) and oxalate, which is critical for the operation optimization of PFS in wastewater treatment.

We demonstrated the importance of basal planes and crystal edge for electron transfer within montmorillonite (MK10) and nontronite (NAu-2) by a facile dye-sensitized photoreduction method. It was found that not all structural Fe in the clay matrix was redox-active. The results are vital to the utilization of naturally abundant clays in environmental redox chemistry.

An efficient and green advanced oxidation process (i.e., photo-sulfite reaction) for the simultaneous oxidation of sulfite and organic pollutants in water is reported. The photo-sulfite system (UV–Fe(III)–sulfite) is based on the Fe-catalyzed sulfite oxidation and photochemistry of Fe(III) species. SO4•– and •OH radicals were identified in the photo-sulfite system with radical scavenging experiments using specific alcohols. This novel technology was consistently proven to be more favorable than the alternative Fe(III)–sulfite systems for the degradation of 2,4,6-trichlorophenol (2,4,6-TCP) and other organic pollutants at all conditions tested. The reactivity of photo-sulfite system was sustained due to the spontaneous switch of photoactive species from Fe(III)–sulfito to Fe(III)–hydroxo complexes with the depletion of sulfite and the decrease in pH. In contrast, in the absence of light the performance of the Fe(III)–sulfite system was greatly diminished after the consumption of sulfite. The formation of the Fe(III)–sulfito complex is a necessary step for initiating the photo-sulfite reaction. Inhibition of the oxidation of 2,4,6-TCP and methyl orange (MO) was observed in the presence of ligands that can stabilize one or more of the reactants: Fe(III), Fe(II), or sulfite. Our study provides a new facile route for the generation of SO4•– and simultaneous removal of organic and inorganic pollutants.

Arsenic and chromium are often abundant constituents of acid mine drainage (AMD) and are most harmful as arsenite (As(III)) and hexavalent (Cr(VI)). To simultaneously change their oxidation state from As(III) to As(V), and Cr(VI) to Cr(III), is a potentially effective and attractive strategy for environmental remediation. The coabundance of As(III) and Cr(VI) in natural environments indicates their negligible direct interaction. The addition of H2O2 enables and greatly accelerates the simultaneous oxidation of As(III) and reduction of Cr(VI). These reactions are further enhanced at acidic pH and higher concentrations of Cr(VI). However, the presence of ligands (i.e., oxalate, citrate, pyrophosphate) greatly retards the oxidation of As(III), even though it enhances the reduction of Cr(VI). To explain these results we propose a reaction mechanism where Cr(VI) is primarily reduced to Cr(III) by H2O2, via the intermediate tetraperoxochromate Cr(V). Cr(V) is then involved in the formation of •OH radicals. In the presence of ligands, the capacity of Cr(V) to form •OH radicals, which are primarily responsible for As(III) oxidation, is practically inhibited. Our findings demonstrate the feasibility for the coconversion of As(III) and Cr(VI) in AMD and real-world constraints to this strategy for environmental remediation.

The plasma generated around the anode during contact glow discharge electrolysis (CGDE) is a rich source of hydroxyl (OH) radicals that can efficiently degrade organic contaminants in aqueous solutions. The degradation of textile azo dyestuffs, Reactive Yellow 176 (Y3RS), Reactive Red 239 (R3BS) and Reactive Black 5 (B5), by anodic CGDE was investigated in the presence of chloride (Cl−) ions. The degradation kinetics of the dyes was dependent on the concentration of Cl− ions and on the respective dye being treated. R3BS degradation was inhibited by Cl− ions in the range of 0–0.01 M. When the Cl− ion concentration was less than 0.02 M, the dyes followed pseudo first-order degradation kinetics. For concentrations greater than 0.02 M, the degradation of Y3RS and B5 was significantly enhanced compared to the degradation of R3BS and deviated from first-order reaction kinetics. The presence of Cl− ions (0.03 M) did not appear to improve dye mineralization but resulted in the formation of adsorbable organic halogens (AOX). The results indicated that the AOX could be abated with prolonged electrolytic treatment. This observation is significant for the assessment of the environmental impact of this technology for wastewater treatment.

•Effect of chloride ions on electrochemical degradation kinetics of dye was examined.•MO degradation was significantly enhanced by higher concentrations of chloride.•32 chlorinated organic byproducts of MO were identified by GC–MS method.Formation of toxic chlorinated organic byproducts is of great concern when selecting electrochemical oxidation (EO) as decontamination technology for saline dye wastewater, but still not verified. To test the applicability of EO, methyl orange (MO) was used as a model dye for anodic contact glow discharge electrolysis (CGDE) and conventional electrolysis (CE) in the presence of chloride. The degradation kinetics and organic intermediates were analyzed. In the presence of chloride, the rates of dye degradation were significantly increased as CGDE and CE were applied. CE resulted in higher mineralization efficiency than CGDE which needs much energy input. Several refractory chlorinated aromatic and even aliphatic compounds were identified during MO degradation, as well as the other anthraquinone dye, alizarin red S (AR). Therefore, the issues of toxic chlorinated byproducts and energy cost should be preferentially evaluated prior to the selection of EO technologies.

•Thermal transformation of arsenate-coprecipitated ferrihydrite was examined.•The transformation products were well characterized by a series of techniques.•Arsenate was found to retard the thermal transformation of ferrihydrite.•The results have implications for predicting long-term stability of ferrihydrite.2-line ferrihydrite, a ubiquitous iron oxy-hydroxide found in natural and engineered systems, is an efficient sink for the toxic metalloids such as arsenic. While much is known of the excellent capacity of ferrihydrite to coprecipitate arsenate, there is little information concerning the long-term stability of arsenate-accumulated ferrihydrite. By thermal treatment methodology, the expedited transformation of ferrihydrite in the presence of coprecipitated arsenate was studied at varying As/Fe ratios (0–0.5) and different heating temperature (40, 300, 450, 600 °C). Pure and transformed minerals were characterized by thermogravimetry (TG), X-ray diffraction (XRD), Electron Spin Resonance (ESR), Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy (SEM–EDX) and Fourier Transform Infrared Spectroscopy (FTIR). Arsenate was found to retard the thermal transformation of ferrihydrite. The extents of ferrihydrite transformation to hematite decreased with increasing As/Fe ratios, but increased at a higher heating temperature. It is predicted that the coprecipitated arsenate can stabilize the amorphous iron oxides against the transformation to more crystalline solids. Arsenate concentration appears to play an important role in this predicted long-term stability.