Recently, visible-light-driven photocatalysis is of great interest in the environmental pollutant remediation. In the present study, a novel heterostructured photocatalyst AgI/BiVO4 was synthesized by an in situ precipitation procedure. The AgI/BiVO4 heterojunctions exhibited excellent photoactivity for the refractory pollutant (tetracycline (TC), a typical antibiotic) decomposition under visible light illumination. The synthetic sample with 1:4 mass ratio of AgI:BiVO4 possessed the highest photocatalytic performance in all of the as-prepared catalysts. The TC molecules were substantially eliminated (94.91%) within 60 min, and degradation efficiency was considerably better than those of bare BiVO4 (62.68%) and AgI (75.43%) under identical conditions. Simultaneously, 90.46% of TOC removal was also achieved within 120 min, suggesting that the mineralization was superior and further confirmed by three-dimensional excitation–emission matrix fluorescence spectroscopy (3D EEMs). The XRD, XPS, DRS, and PL measurements revealed that a small amount of Ag nanoparticles was produced at the early photodegradation process. The structure transformation from AgI/BiVO4 (double-type) to AgI/Ag/BiVO4 (sandwich-like) improved the corresponding visible-light absorption performance. The self-assembly Z-scheme heterojunction that consisted of AgI, Ag, and BiVO4 also efficiently accelerated photoinduced electron–hole pairs’ separation and ultimately improved the efficiency of TC degradation. The responsible photocatalytic mechanism was discussed in detail on the basis of the reactive species capturing tests and ESR analysis, and the experimental results had been validated that superoxide radicals and holes played a vital role during the photocatalytic process. Furthermore, TC degradation efficiency was not of significant loss after four consecutive cycles, suggesting the excellent photostability of AgI/BiVO4 nanocomposite. These features demonstrate that the AgI/BiVO4 heterojunction has great application potential for refractory pollutants’ removal from wastewater.Keywords: AgI/BiVO4; degradation mechanism; mineralization; photocatalysis; tetracycline;

•AI nanoparticle-sensitized BOI microspheres were successfully synthesized.•The prepared photocatalysts exhibited excellent visible-light photocatalytic performance.•Higher antibiotic adaptability and superior photostability were confirmed.•Enhanced visible light absorption and photoinduced charge separation were achieved.•A possible Z-scheme heterojunction degradation mechanism was proposed.The development of efficient visible light photocatalysts for refractory organic pollutant degradation has gained considerable attention in wastewater treatment. Here, close-connected AgI/Bi5O7I (AI/BOI) heterojunctions were successfully synthesized by a facile deposition–precipitation approach. Multiple antibiotics, including tetracycline, deoxytetracycline, oxytetracycline, and ciprofloxacin, were employed as target pollutants to evaluate the visible light photoactivity of the prepared samples. The obtained AI/BOI-5 exhibited optimal photocatalytic activity and photoelectric property, which was 8.62 times (tetracycline degradation rate) and 12.44-fold (photocurrent intensity) than those of bare BOI, respectively. The strengthened visible light absorption and effective separation and transfer of the photoinduced electrons and holes should be responsible for the improvement of photocatalytic performance. The mineralization ability comparison was explored by total organic carbon and three-dimensional excitation–emission matrix fluorescence spectra measurements. The AI/BOI-5 also revealed good adaptability to higher initial contaminant concentrations and desired photodegradation stability in practical applications. By the studies of reactive species trapping, electron spin resonance and nitroblue tetrazolium agent of O2− transformation experiments verified that O2−, h+, and OH were all produced in AI/BOI photocatalytic systems, while only O2− and h+ worked during BOI photolysis. A possible Z-scheme heterojunction mechanism can be ascribed to the enhanced photocatalytic degradation of multiple antibiotics induced by AI/BOI. This work gives deep insight into heterostructured photocatalysis and provides a novel way to construct and design highly efficient photocatalysts for water purification.Download high-res image (95KB)Download full-size image

•Perchlorate and nitrate was bioreduced on autotrophic denitrifying biocathode.•Substrate bioreduction and electricity generation reached optimum at NO3−/ClO4− 1:1.•High concentration perchlorate inhibited the biological activity of biocathode.•Thauera related to β-Proteobacteria was the major genus on biocathodes.In this study, an autotrophic denitrifying biocathode was investigated to couple the reduction of nitrate or/and perchlorate with electricity generation. Results showed that when the current density in microbial fuel cell (MFC) with sole perchlorate and sole nitrate as the substrate stabilized at 3.00 and 1.52 mA/m3 respectively, the perchlorate and nitrate removal efficiency achieved 53.14% and 87.05%. As influent molar ratio of NO3−/ClO4− was 1:1, the stable current density reached the a peak value (3.10 A/m3) accompanied by the maximum integral mixed substrate removal (40.97% for ClO4− and 86.03% for NO3−). Open and close circuit experiments demonstrated that the nitrate and perchlorate removal should be ascribed to the bioreduction of autotrophic denitrifying biocathode. Cyclic voltammetry (CV) curves showed that all of biocathodes had strong electrochemical activity and there were few clear distinctions of redox potential between the biocathodes fed with different substrates. Results of 16S rRNA sequencing revealed a predominance of β-Proteobacteria in the autotrophic denitrifying biocathode, which is a well-known environmental nitrogen cycler.Download high-res image (60KB)Download full-size image

•The photochemical technologies for removing PFOX from water were summarized.•Characteristics of PFOX photo-oxidation and photo-reduction processes were explored.•Factors affecting the PFOX degradation and defluorination procedure were discussed.•The PFOX photochemical degradation mechanisms were elucidated in details.•The application prospects for PFOX photodegradation were suggested.Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are persistent organic pollutants in the environment and have serious health risks, including endocrine disrupting properties, immunotoxicity and developmental effects etc. The photochemical degradation has been proven to be a low-cost, efficient and sustainable technology for the removal of PFOX (X = A or S) in water. At present, most of the investigations have been conducted in ultrapure water and at concentrations much higher comparing to those detected in the real wastewaters. Furthermore, there are few studies about the toxicity of treated water. In this paper, the state of knowledge on the photocatalytic degradation of PFOX, including photo-oxidative and photo-reductive degradation, is reviewed comprehensively. Compared with photo-oxidation, photo-reduction appears to be more suitable for the PFOX removal since it is more favorable for the defluorination of PFOX and further complete mineralization. The effects of key parameters on the photocatalytic degradation and defluorination process of PFOX are commendably accessed, such as light wavelength, photocatalyst concentration, initial PFOX concentration, pH, reaction atmosphere, temperature, and coexisting organic or inorganic matters. The mechanisms of PFOX photodegradation process are also elucidated in detail. This paper will help to deeply understand PFOX decomposition process and put forward better perspectives in the future for researchers who work in this field.Download high-res image (95KB)Download full-size image

•High concentration of Ni(II) has an adverse effect on the EBPR system.•The transformations of PHAs were suppressed by short-term exposure of Ni(II).•Impact of Ni(II) on the surface integrity of activated sludge was negligible.•Inhibition of PPX activity was likely attributed to high ROS production.•Long-term exposure to Ni(II) changed the microbial community of EBPR system.Nickel (Ni(II)) is commonly present in municipal and industrial wastewaters, and thus its potential toxicity to activated sludge in wastewater treatment plants attracts increasing concerns. Although considerable efforts have been paid to this topic, the potential effect of Ni(II) on biological phosphorus removal has not been reported. In this work, short-term and long-term effects of Ni(II) in the range of 0.1–10 mg·L−1 on enhanced biological phosphorus removal (EBPR) were therefore investigated. Compared with the control, short-term exposure to 1 and 10 mg·L−1 of Ni(II) resulted in the decrease of phosphorus removal efficiency from 99.7% to 38.3% and 0, respectively. The phosphorus removal was unaffected after short-term exposure to 0.1 mg·L−1 of Ni(II), but it was completely collapsed after 30-day exposure. The mechanism studies revealed that the cell membrane of microorganisms in activated sludge was not damaged, though the production of reactive oxygen species increased with the increase of Ni(II) exposure concentration. However, the presence of Ni(II) inhibited the anaerobic release of polyphosphate and the activity of exopolyphosphatase but enhanced the transformations of poly-3-hydroxyvalerate and glycogen. Microbial community investigation with high-throughput sequencing analysis showed that alphaproteobacterial glycogen accumulating organisms instead of polyphosphate accumulating organisms became the predominant microorganisms in EBPR systems after long-term exposure to Ni(II).Download high-res image (176KB)Download full-size image

•Zn/Fe-MMO was prepared by facile co-precipitation and high temperature calcination.•Zn/Fe-MMO exhibited better photocatalytic activity to dye (MB) and antibiotic (TC) degradation.•High photostability and repeatability was achieved under visible-light irradiation.•O2− and OH acted as the predominant active species.•Reaction mechanism for Pollutant degradation over Zn/Fe-MMO was illustrated.In this study, two typical refractory organic pollutants, methylene blue (MB) and tetracycline (TC) was effectively degraded under visible-light irradiation catalyzed by Zn/Fe mixed metal oxide (Zn/Fe-MMO) nanocomposite, which derived from the high-temperature calcination of Zn/Fe layered double hydroxide (Zn/Fe-LDH). The Zn/Fe-MMO was composed of wurzite ZnO phases and spinel-structured ZnFe2O4. The Zn/Fe-MMO exhibited superior photocatalytic activity than Zn/Fe-LDH precursor owing to the better crystallinity, broader visible light adsorption, and higher photo-generated electron-hole pairs separation efficiency. Simple assembling method, high photocatalytic activity to the degradation of organic pollutants and good reusability make Zn/Fe-MMO has great potential in practical wastewater treatment.Simple assembling method, high photocatalytic activity typical refractory organic pollutants removal and good reusability.Download high-res image (97KB)Download full-size image

•Novel graphene-bridged Ag3PO4/Ag/BiVO4 nanocomposite was prepared by a hierarchical assembly method.•The nanocomposite exhibited a greatly improved photoactivity towards TC degradation.•Synergistic effects resulted in faster charge carriers separation, higher surface area and photo-stability.•Excellent photostability and photoelectric properties were achieved for the greater application potentials.•Perferable applications potentials for real wastewater treatment.A novel graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme heterojunction with excellent visible-light-driven photocatalytic performance was fabricated using a facile in situ deposition method followed by photo-reduction. The as-obtained nanocomposite was employed to degrade tetracycline (TC) in water under visible light irradiation. Compared to pure BiVO4, Ag3PO4 and other nanocomposites, Ag/Ag3PO4/BiVO4/RGO displayed more superior photodegradation efficiency with 94.96% removal of TC (10 mg/L) in 60 min, where the optimal conditions was catalysis dosage 0.50 g/L and initial pH at ca. 6.75. The influences of TC concentrations, light irradiation condition, coexistence ions and water sources were also investigated in details. The enhanced photocatalytic activities could be attributed to the suppression of charge recombination, high specific surface area and desirable absorption capability of Ag/Ag3PO4/BiVO4/RGO, which were in sequence confirmed by PL, PC, EIS, BET and DRS tests. The synergistic effects of RGO and Ag/Ag3PO4 in the hybrid could also contribute to the improved photo-stability and recyclability towards TC decomposition. In addition, radical trapping experiments and ESR measurement revealed that the photo-induced active species superoxide radical (O2−) and holes (h+) were the predominant active species in the photocatalytic system. The Ag/Ag3PO4/BiVO4/RGO nanocomposite also possessed desirable photocatalytic performance on the degradation of TC from real wastewater, further verifying its potential in practical industries. This work provides a promising approach to construct visible-light response and more stabilized nanocomposite photocatalysts applied in efficient treatment of persistent pollutants in wastewater.Graphene-bridged Ag3PO4/Ag/BiVO4 Z-scheme heterojunction possessed superior photocatalytic activity towards TC degradation, effective suppression of charge recombination, high specific surface area, desirable absorption capability and photo-stability was achieved. Preferable application potentials were presented for the real wastewater treatment.Download high-res image (202KB)Download full-size image

•GO/Ag2CrO4/g-C3N4 ternary photocatalyst was synthesized by one-step precipitation method.•Higher photocatalytic activities towards multiple refractory pollutants degradation.•Superior photostability was achieved and confirmed in detail.•Synergistic effect facilitated the separation and transfer photo-induced electron-hole pairs.•Double Z-scheme photocatalytic mechanism was proposed.In this work, a ternary composite photocatalyst consisted of graphitic carbon (g-C3N4), graphene oxide (GO) and Ag2CrO4 was successfully synthesized through one-step chemical precipitation route. The GO/Ag2CrO4/g-C3N4 (GO/ACR/CN) nanocomposite exhibited superior photocatalytic performance towards dyes (rhodamine (RhB) and methylene blue (MB)) and two other refractory pollutants (phenol and oxytetracycline) degradation under visible light irradiation. The efficient photo-induced electron-hole pairs separation, multi-step charge transfer and enhanced visible light absorption should be concluded as the synergistic effects among three components, resulting in the improved photoactivity. The decreased degradation efficiency of RhB (MB) over bare ACR was about 25.74% (43.22%) after four times cycles, while insignificant loss was perceived over GO/ACR/CN. The corresponding anti-photocorrosion property was further confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). For in-depth insight into practical applications, the effects of initial concentration and different water sources were also taken into discussions. This work demonstrated that rational and design of ternary nanocomposites could provide a new approach for the development of more efficient visible-light photocatalysts for wastewater treatment and environmental remediation.Download high-res image (182KB)Download full-size image

•Ternary Ag@g-C3N4@BiVO4 heterojunction was prepared via wet-impregnation and photo-deposition method.•The sample exhibited superior photocatalytic performance towards TC degradation under wider spectrum.•The coexisting ions in practical wastewater were discussed.•O2−, h+ and OH are the main active species.•The Z-scheme mechanism was systematically investigated.A novel and highly efficient three-component Ag@g-C3N4@BiVO4 heterojunction was successfully synthesized and characterized in terms of structure, porosity, chemical composition and optical properties. The photocatalytic activities of as-prepared samples were evaluated by the photocatalytic decomposition of tetracycline (TC) in the aqueous phase. Compared with single semiconductor BiVO4 and g-C3N4, binary composites Ag@BiVO4 and g-C3N4@BiVO4, the ternary Ag@g-C3N4@BiVO4 heterojunction exhibited the higher photocatalytic activity under wider light spectrum irradiation. Furthermore, we also investigated the effects of initial TC concentrations and coexisting ions were in simulated practical wastewater. The mechanism research showed that matching of band structure between BiVO4 and g-C3N4 induced an efficient photogenerated electrons and holes transfer from the CB of BiVO4 and VB of g-C3N4, respectively. As a charge transfer center, Ag nanoparticles were well photodeposited onto the surface of BiVO4 and g-C3N4 and increased the visible light absorption, even near-infrared via the surface plasmon resonance. The synergy effects between Ag, g-C3N4 and BiVO4 with the aid of Z-scheme mechanism were the main reason for improved photocatalytic performance. The trapping experiments and ESR tests confirmed that the O2−, h+ and OH were main active species in photocatalytic degradation of TC. From the viewpoint of practical application, Ag@g-C3N4@BiVO4 ternary structure displayed superior photostability after four times recycle.Download high-res image (105KB)Download full-size image

•Triclocarban affected SCFA production from anaerobic fermentation of sludge.•A significant level of triclocarban was degraded in the fermentation process.•Triclocarban facilitated solubilization, acidogenesis, acetogenesis, and homoacetogenesis processes.•Triclocarban inhibited methanogenesis process.Triclocarban (TCC), one typical antibacterial agent being widely used in various applications, was found to be present in waste activated sludge at significant levels. To date, however, its effect on anaerobic fermentation of sludge has not been investigated. This work therefore aims to fill this knowledge gap. Experimental results showed that when TCC content in sludge increased from 26.7 ± 5.3 to 520.5 ± 12.6 mg per kilogram total suspended solids, the maximum concentration of short-chain fatty acids (SCFA) increased from 32.6 ± 2.5 to 228.2 ± 3.6 (without pH control) and from 211.7 ± 2.4 to 378.3 ± 3.2 mg COD/g VSS (initial pH 10), respectively. The large promotion of acetic acid was found to be the major reason for the enhancement of total SCFA production. Although a significant level of TCC was degraded in the fermentation process, SCFA was neither produced from TCC nor affected by its major intermediates at the relevant levels. It was found that TCC facilitated solubilization, acidogenesis, acetogenesis, and homoacetogenesis processes but inhibited methanogenesis process. Microbial analysis revealed that the increase of TCC increased the microbial community diversity, the abundances of SCFA (especially acetic acid) producers, and the activities of key enzymes relevant to acetic acid production.Download high-res image (252KB)Download full-size image

•Details of how NaCl affects methane production from food waste were clarified.•An efficient approach to mitigate the impact of NaCl on the production of methane was reported.•The mechanisms for co-digestion of food waste and WAS exhibiting higher methane production were investigated.Previous studies have demonstrated that the presence of sodium chloride (NaCl) inhibited the production of methane from food waste anaerobic digestion. However, the details of how NaCl affects methane production from food waste remain unknown by now and the efficient approach to mitigate the impact of NaCl on methane production was seldom reported. In this paper, the details of how NaCl affects methane production was first investigated via a series of batch experiments. Experimental results showed the effect of NaCl on methane production was dosage dependent. Low level of NaCl improved the hydrolysis and acidification but inhibited the process of methanogenesis whereas high level of NaCl inhibit both steps of acidification and methanogenesis. Then an efficient approach, i.e. co-digestion of food waste and waste activated sludge, to mitigate the impact of NaCl on methane production was reported. Finally, the mechanisms of how co-digestion mitigates the effect on methane production caused by NaCl in co-digestion system were revealed. These findings obtained in this work might be of great importance for the operation of methane recovery from food waste in the presence of NaCl.

•Aged refuse accelerated sludge solubilization, hydrolysis, and acidogenesis.•Enzymes and anaerobes in aged refuse were the primary reason.•Humic substances in aged refuse benefited hydrolysis and acidogenesis.•The effect of heavy metals in aged refuse on hydrolysis and acidogenesis was dosage dependent.In this work, a low-cost alternative approach (i.e., adding aged refuse (AR) into waste activated sludge) to significantly enhance anaerobic digestion of sludge was reported. Experimental results showed that with the addition dosage of AR increasing from 0 to 400 mg/g dry sludge soluble chemical oxygen demand (COD) increased from 1150 to 5240 mg/L at the digestion time of 5 d, while the maximal production of volatile fatty acids (VFA) increased from 82.6 to 183.9 mg COD/g volatile suspended solids. Although further increase of AR addition decreased the concentrations of both soluble COD and VFA, their contents in these systems with AR addition at any concentration investigated were still higher than those in the blank, which resulted in higher methane yields in these systems. Mechanism studies revealed that pertinent addition of AR promoted solubilization, hydrolysis, and acidogenesis processes and did not affect methanogenesis significantly. It was found that varieties of enzymes and anaerobes in AR were primary reason for the enhancement of anaerobic digestion. Humic substances in AR benefited hydrolysis and acidogenesis but inhibited methanogenesis. The effect of heavy metals in AR on sludge anaerobic digestion was dosage dependent. Sludge anaerobic digestion was enhanced by appropriate amounts of heavy metals but inhibited by excessive amounts of heavy metals. The relative abundances of microorganisms responsible for sludge hydrolysis and acidogenesis were also observed to be improved in the system with AR addition, which was consistent with the performance of anaerobic digestion.Download high-res image (254KB)Download full-size image

•Solubilization was enhanced by low Cd levels but inhibited by high Cd level.•0.1 mg/g VSS Cd benefited both the hydrolysis and acidification.•10 mg/g VSS Cd inhibited all the hydrolysis, acidification, and methanogenesis.•High level Cd decreased the activity of enzyme and microbial diversity.•An effective strategy to mitigate the adverse impact of high Cd levels was proposed.Cadmium (Cd) is present in significant levels in waste activated sludge, but its potential toxicities on anaerobic fermentation of sludge remain largely unknown. This work therefore aims to provide such support. Experimental results showed that the impact of Cd on short-chain fatty acids (SCFA) production from sludge anaerobic fermentation was dose-dependent. The presence of environmentally relevant level of Cd (e.g., 0.1 mg/g VSS) enhanced SCFA production by 10.6%, but 10 mg/g VSS of Cd caused 68.1% of inhibition. Mechanism exploration revealed that although all levels of Cd did not cause extra leakage of intracellular substrates, 0.1 mg/g VSS Cd increased the contents of both soluble and loosely-bound extracellular polymeric substances (EPS), thereby benefitting sludge solubilization. On the contrary, 10 mg/g VSS Cd decreased the levels of all EPS layers, which reduced the content of soluble substrates. It was also found that 0.1 mg/g VSS Cd benefited both the hydrolysis and acidogenesis but 10 mg/g VSS Cd inhibited all the hydrolysis, acidogenesis, and methanogenesis processes. Further investigations with microbial community and enzyme analysis showed that the pertinent presence of Cd enhanced the activities of protease, acetate kinase, and oxaloacetate transcarboxylase whereas 10 mg/g VSS Cd decreased the microbial diversity, the abundances of functional microbes, and the activities of key enzymes. Finally, one strategy that could effectively mitigate the adverse impact of high Cd levels on SCFA production was proposed and examined. This work provides insights into Cd-present sludge fermentation systems, and the findings obtained may guide engineers to manipulate sludge treatment systems in the future.Download high-res image (200KB)Download full-size image

•The hydrochar shortened the time to achieve the maximal production of SCFA.•The production of SCFA and acetic acid kept stable in the presence of hydrochar.•Hydrochar produced in 220 °C contained more organic matters.•Humic substances in hydrochar improved the SCFA production.In this study, waste activated sludge (WAS) was used as feedstock to generate hydrochars at different temperatures (220 °C and 260 °C) and their effect on sludge anaerobic digestion was evaluated. Experimental results showed that the maximum yield of short-chain fatty acid (SCFA) enhanced by hydrochar (220 °C) and hydrochar (260 °C) were 507.33 and 270.80 mg chemical oxygen demand (COD)/L respectively, which were much higher than that in blank (141.49 mg COD/L). Mechanism investigation confirmed that hydrochar remarkably accelerated the solubilization and hydrolysis of organic matters, enhanced the acidification of hydrolyzed products, and inhibited the activity of methanogenic bacteria as well as promoted the activities of key enzymes. Meanwhile, the organic matters especially humic substances existed in the hydrochar played an important role during anaerobic digestion.Download high-res image (121KB)Download full-size image

Denitrifying anaerobic methane oxidation (DAMO) can concurrently reduce methane emissions and nitrogen levels in aquatic environments, but how useful is this process? We propose the use of DAMO-based technology as a tool for sustainably operating wastewater treatment plants (WWTPs).

•The biological nutrient removal efficiency was inhibited by ciprofloxacin.•The transformations of PHAs and glycogen were suppressed by ciprofloxacin addition.•Ciprofloxacin inhibited NIR and PPK activities.•Chronic-exposure ciprofloxacin affected the microbial community.In this work, both short-term and long-term experiments were therefore conducted to assess the effects of ciprofloxacin (0.2 and 2 mg·L− 1) on wastewater nutrient removal. The results showed that both levels of ciprofloxacin had no acute and chronic adverse effects on the surface integrity and viability of activated sludge. Short-term exposure to all the ciprofloxacin levels induced negligible influences on wastewater nutrient removal. However, the prolonged exposure to ciprofloxacin decreased total phosphorus and nitrogen removal efficiencies from 96.8, 95.8% (control) to 91.7, 84.9% (0.2 mg·L− 1) and 90.5%, 80.2% (2 mg·L− 1), respectively. The mechanism study showed that ciprofloxacin exposure suppressed denitrification and phosphorus uptake processes. It was also found that ciprofloxacin affected the transformations of intracellular polyhydroxyalkanoates and glycogen in the oxic and anoxic stages. Moreover the activities of nitrite reductase and polyphosphate kinase were inhibited by the presence of ciprofloxacin. Further analysis with high-throughput sequencing revealed that compared with the control, the abundances of polyphosphate accumulating organisms, glycogen accumulating organisms and denitrifying bacteria in ciprofloxacin exposure reactors reduced, which were consistent with the decreased nutrient removal performance measured in these reactors.Download high-res image (163KB)Download full-size image

Recently, anaerobic cofermentation of waste activated sludge (WAS) and food waste for short chain fatty acid (SCFA) production has drawn growing attention. However, the details of how sodium chloride (NaCl) in food waste affects SCFA generation from the cofermentation remain largely unknown, which provides limited understanding of the cofermentation process. This work therefore aims to provide such support. Experimental results showed that the effect of NaCl on SCFA production was dosage dependent. With the increase of NaCl level from 0 to 8 g/L SCFA production increased from 367.6 to 638.5 mg chemical oxygen demand (COD)/g of volatile suspended solids (VSS). However, further increase of NaCl caused severe inhibition of SCFA production. The presence of NaCl not only accelerated the release of soluble substances from food waste and disruption of both extracellular polymers and cell envelopes in sludge but also promoted the conversion of protein released from the disintegration process, thereby causing more substrates for SCFA production. It was also found that low NaCl levels improved hydrolysis and acidification processes but inhibited methanogenesis while both acidification and methanogenesis processes were seriously inhibited by high NaCl levels. Further investigation with enzyme analysis showed that the activities of protease and α-glucosidase were in the order of high NaCl > low NaCl > blank while the activities of oxaloacetate transcarboxylase and CoA transferase were in the sequence of low NaCl > blank > high NaCl. However, the activity of coenzyme F420 decreased with increasing NaCl level. This work reveals the underlying mechanism of how NaCl affects SCFA production from the cofermentation process and might be of significance for the operation of the cofermentation system.Keywords: Anaerobic cofermentation; Food waste; Short-chain fatty acid production; Sodium chloride; Waste activated sludge

In this study, Fe(II)-loaded granular activated carbon (GAC) was used as a heterogeneous persulfate catalyst for the pretreatment of mature landfill leachate. The effects of Fe2+ dosage, persulfate concentration and initial pH on the degradation of the organic pollutants in the landfill leachate were investigated. In single factor experiments, the maximum Chemical Oxygen Demand (COD) removal rate reached 66.8, 66.2 and 76.3% at an Fe2+ dosage of 127 mg L−1 (Fe2+/S2O82− = 254 mg mol−1), a persulfate concentration of 0.5 mol L−1 (i.e. S2O82−:12COD0 = 1.08) and an initial pH of 3, respectively. Obviously, pH played a more important role in the persulfate oxidation treatment than the other two factors. The synthetic experimental results showed that the COD removal rate exceeded 87.8% when the reaction conditions had a controlled Fe2+ dosage of 127 mg L−1, persulfate concentration of 0.5 mol L−1 and initial pH of 3.0, simultaneously. The recycle experiments displayed that the catalytic ability of reused Fe(II)-GAC declined considerably and the COD removal rate dropped by approximately half after reusing three times. But the catalytic ability of the catalyst used could be well recovered after regeneration at 550 °C in a N2 atmosphere. Finally, fluorescence excitation–emission matrix (EEM) spectroscopy preliminarily explained the degradation mechanism of the landfill leachate.

Novel Ag2O@Ag-modified BiVO4 composites with highly efficient visible light photocatalytic activity were synthesized by a facile pH-mediated chemical precipitation method accompanied by a self-built Z-scheme heterojunction under visible light irradiation in this study. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) analysis and electrochemical measurements demonstrated that an acicular sheaf-like heterojunction was formed. Rhodamine B (RhB), methylene blue (MB) and their mixture were selected as the target pollutants to evaluate the enhanced photocatalytic activities of the as-prepared composites. Compared with pure BiVO4 and Ag2O, Ag2O@Ag@BiVO4 had a high photocatalytic activity because of the efficient separation of electron–hole pairs in the heterojunction construction. The composites with an initial Ag2O:BiVO4 mass ratio of 1:10 exhibited the highest degradation efficiency for dye. Experimental results indicated that single RhB or MB could be degraded completely within a short irradiation time (15 min for 10 mg L−1 RhB, 48 min for 20 mg L−1 MB), but there was obviously competition with the active species, resulting in a decrease of the degradation rate for both dyes. Regardless of the suppression effect, a relatively higher photocatalytic performance was also achieved for the dye mixture. Several parameters, including initial dye concentration, calcination temperature, initial pH and mineralization ability are discussed in detail. The kinetics study suggested that photocatalytic degradation processes for single or complex dye were well described by first-order kinetics. Due to the generated Ag–Ag2O self-stability structure, these acicular sheaf-like three-dimensional heterojunction photocatalysts were fairly stable and there was no obvious loss of photocatalytic activity after four consecutive cycles.

Commercial strongly basic resin D201-Cl was impregnated with hydrous ferric oxide (HFO) in FeCl3–HCl–NaCl solution for absorptive removal of bromate from aqueous solution. The influences of initial bromate concentration, contact time, initial pH, temperature, and competing anions on the uptake of bromate by HFO-impregnated resin D201-Cl (HFO-201) were investigated by batch adsorption experiments. Experimental results showed that the adsorption process could be described by pseudo-first-order kinetics and be divided into three stages according to the intraparticle diffusion parameter. The maximum adsorption potential of HFO-impregnated resin for bromate was 292.81 mg·g–1 at 298 K, which is much greater than those reported in previous studies. The experimental data fitted well to the Redlich–Peterson isotherm model (R2 > 0.99). Thermodynamic parameters such as free energy, enthalpy, and entropy demonstrated that bromate adsorption on HFO-201 was spontaneous and exothermic and occurred by physisorption. Meanwhile, HFO-201 maintained greater adsorption capacity for bromate in salt coexisting solution. Results of the present study suggest that the cheap and facile HFO-impregnated resin possesses great potential for bromate removal from drinking water.

An efficient visible-light-driven photocatalyst Ag@AgCl/ZnSn(OH)6 (Ag@AgCl/ZSH) was successfully fabricated by an ultrasonic assisted precipitation-photoreduction method at room temperature. The photophysical properties of the as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and photoluminescence emission spectra (PL) analysis. The photocatalytic activities of the as-prepared samples were evaluated by the photodegradation of rhodamine B (RhB), crystal violet (CV) and phenol aqueous solution. The Ag@AgCl (8 wt%)/ZSH-20 composite exhibited the optimal photocatalytic performance, and the corresponding degradation rates for RhB, CV and phenol solution were as high as 22/3.6, 15/4 and 16/3.6 times those of pure ZSH and the conventional visible-light photocatalyst N–TiO2, respectively. The effect of photo-reduction time on the photocatalytic properties of the Ag@AgCl/ZSH composites was systematically investigated. Moreover, a possible degradation mechanism was proposed based on reaction equations and a simulated scheme on the basis of active species trapping experiments and band energy analysis. The dramatically enhanced photocatalytic performance of Ag@AgCl/ZSH should be ascribed to the surface plasmon resonance (SPR) effect from Ag@AgCl nanoparticles and high separation of photogenerated electron–hole pairs in the photocatalytic process, leading to low recombination rates of the photoinduced electron–hole pairs. High degradation efficiencies and physicochemical features were maintained after five recycling experiments, indicating that the photocatalysts were relatively durable and stable. It is expected that the plasmonic photocatalyst Ag@AgCl/ZSH is a promising candidate material for the photodegradation of organic pollutants in wastewater.

Nanoscale zerovalent iron (nZVI) was reported as an effective material for the removal of bromate. However, its reactivity may be weakened due to its aggregation. In this study, nZVI was dispersed onto modified activated carbon (pretreated by nitric acid or/and ammonia) by impregnating carbon in ferrous sulfate with NaBH4 as reducing agent. The nZVI supported on modified activated carbon (nZVI/MAC) was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). Good distribution of nZVI particles (about 50 nm) on the MAC was observed. The removal efficiencies of bromate by AC, nZVI, and nZVI/MAC were respectively evaluated. Experimental results indicated that nZVI/MAC showed the highest removal efficiency to bromate. In addition, the effects of initial bromate concentration (0.78–3.91 μmol/L) and pH (2.0–10.0) were investigated by batch experiments. Kinetic studies showed that the simultaneous adsorption and reduction of bromate by nZVI/MAC followed pseudo-first-order kinetics. Finally, bromine mass balance demonstrated that bromide was the only product for bromate reduction by nZVI/MAC, suggesting that bromate in aqueous solution was first adsorbed onto nZVI/MAC and subsequently reduced to innocuous bromide by nZVI.

The effect of biosurfactant rhamnolipid (RL) on hydrolysis and acidification of waste activated sludge (WAS) was investigated. The results indicated that RL could greatly reduce the surface tension of sludge, resulting in stimulating the hydrolysis rate of WAS and enhancing the production of short-chain fatty acids (SCFAs). With the increase of RL dosage from 0.2 to 0.5 g/g DS, the maximum soluble chemical oxygen demand (SCOD), protein and carbohydrate concentration increased correspondingly. After 6 h of hydrolysis, SCOD, protein and carbohydrate concentration increased from 371.9, 93.3 and 9.0 mg/l to 3,994.5, 800.0 and 401.4 mg/l at RL 0.3 g/g DS, respectively. Furthermore, the release of NH4+-N, PO43−-P and the accumulation of SCFAs also improved in the presence of RL. The maximum SCFAs was 1,829.9 mg COD/l at RL 0.3 g/g DS, while it was only 377.7 mg COD/l for the blank test. The propionic acid and acetic acid were the mainly SCFAs produced, accounting for 50–60% of total SCFAs.

•Mature landfill leachate was effectively pretreated by ultrasonic activated persulfate oxidation.•Integrated Taguchi and RSM method well optimized the operating variables.•The TOC removal efficiency was 77.32% at optimal conditions.•The process exhibits the synergistic effect of sono-catalysis and persulfate oxidation.A novel advanced oxidation process (AOP) using ultrasonic activated persulfate oxidation was used to pretreat mature landfill leachate. The effects of different operating variables (e.g., the initial S2O82− concentration, pH, temperature, ultrasonic power and reaction time) on the oxidation performance were investigated regarding the total organic carbon (TOC) removal efficiency, and the variables were optimized using the integrated Taguchi method and response surface methodology (RSM). Based on the Taguchi method under L16 (45) arrays and a grey relational analysis, the most significant variables included the initial S2O82− concentration, temperature and reaction time. The concentrations of these variables were further optimized using RSM. Using the integrated optimization method, the optimal conditions included an initial S2O82− concentration of 8.5 mM, a reaction temperature of 70 °C and a reaction time of 2.46 h, which resulted in a TOC removal efficiency of 77.32%. The experimental results showed that the enhanced TOC removal from mature landfill leachate by sono-activated persulfate oxidation could be attributed to the combined effects of ultrasonic catalysis and sulfate radical-AOP. Overall, ultrasonic activated persulfate oxidation is a promising method for the pretreatment of landfill leachate.

The individual alkaline or microwave pretreatment has been proved to be effective in disintegration and acidification of waste activated sludge (WAS). In this study, the effects of combined alkaline and microwave pretreatment at different pH and specific energy input (Es) on WAS disintegration were investigated using response surface methodology (RSM). Combined pretreatment achieved disintegration degree (DD) of 65.87% at Es of 38,400 kJ/kg TS and pH 11.0. The ANOVA further demonstrated that pH showed more significant effect on DD than Es. Anaerobic batch experiment results showed that combined pretreatment not only significantly improved volatile fatty acids (VFAs) accumulation but also shortened the time for the highest VFAs accumulation. The maximal VFAs accumulation (1500 mg COD/L) obtained at Es of 28,800 kJ/kg TS and fermentation time of 72 h, which was about two times that of the treatment without microwave (850 mg COD/L) at 96 h. The analysis of VFAs composition showed that the VFAs mainly consisted of acetic and iso-valeric acids, accounting for 57.3–70.1% of total VFAs.

•Long-term exposure to Ni(II) exhibited adverse effects on the sludge flocculability.•The content of PN in EPS decreased after long-term exposure to Ni(II).•EEM and FTIR showed the change of structures and functional groups in EPS.•The hydrophobicity of sludge decreased with the increasing Ni(II) level.•The deleterious effect of Ni(II) could be mitigated by adding the EDTA or citrate.Short-term and long-term effects of nickel (Ni) (0.1–10 mg/L) on the physicochemical properties of activated sludge, including the flocculability, settleability, and dewaterability, were investigated. It was found that these properties were unaffected after short-term exposure (1 day) to Ni(II) even at the level of 10 mg/L. After long-term exposure (60 days) to 1 and 10 mg/L of Ni(II), however, the sludge flocculability has seriously deteriorated, while the settleability, and dewaterability became gradually better than the control. The mechanism studies revealed that long-term exposure to Ni(II) resulted in the decrease of protein content in extracellular polymeric substances (EPS) and the damage to EPS structures. Although Ni(II) did not bring any adverse effect on the cell membrane, the relative hydrophobicity of activated sludge was significantly decreased. The negative effects on the flocculability and phosphorus removal performance of activated sludge could be completely eliminated by adding the chelator such as EDTA and citrate.Download high-res image (154KB)Download full-size image

•SCFAs production from food waste was greatly enhanced by the APG addition into anaerobic fermentation system.•0.2 g/g TS was the optimal APG dosage for SCFAs production.•APG favored solubilization, hydrolysis and acidification but inhibited methanogenesis.•The activities of key enzymes related with SCFAs production were improved by APG.Short-chain fatty acids (SCFAs) are the valuable products derived from the anaerobic fermentation of organic solid waste. However, SCFAs yield was limited by the worse solubilization and hydrolysis of particulate organic matter, and rapid consumption of organic acid by methanogens. In this study, an efficient and green strategy, i.e. adding biosurfactant alkyl polyglycosides (APG) into anaerobic fermentation system, was applied to enhance SCFAs production from food waste. Experimental results showed that APG not only greatly improved SCFAs production but also shortened the fermentation time for the maximum SCFAs accumulation. The SCFAs yield at optimal APG dosage 0.2 g/g TS (total solid) reached 37.2 g/L, which was 3.1-fold of that in blank. Meanwhile, the time to accumulate the maximum SCFAs in the presence of APG was shortened from day 14 to day 6. The activities of key enzymes such as hydrolytic and acid-forming enzymes were greatly promoted due to the presence of APG. These results demonstrated that the enhanced mechanism of SCFAs production should be attributed to the acceleration of solubilization and hydrolysis, enhancement of acidification and inhibition of methanogenesis by APG.

•AgI/ZSH was synthesized by a facile deposition–precipitation method at room temperature for the first time.•The composite exhibited much superior photocatalytic activity.•AgI loading, catalysts dosage and initial dye concentration were investigated in detail.•High mineralization ability and photostability were achieved.Novel visible-light-driven photocatalyst AgI/ZnSn(OH)6 (AgI/ZSH) was synthesized by a facile deposition–precipitation method. The effect of several parameters such as the AgI loading, catalysts dosage and initial dye concentration on the photodegradation of rhodamine B (RhB) was investigated. The degradation efficiency of RhB could be almost 100% within 40 min and the removal rate of TOC reached 91% within 60 min at 20% AgI/ZSH dosage 1 g/L, which was 34 and 4.6 times of ZSH and AgI, respectively. The enhanced photoactivity of AgI/ZSH should be contributed to the faster separation of electron–hole pairs with ZSH as an electron trap.Download high-res image (115KB)Download full-size image

•D201-Cl resin exhibited greater bromate adsorption capacity.•The adsorption kinetics agreed with the pseudo-first-order model.•The equilibrium data can be well described by Freundlich and Redlich–Peterson model.•The resin showed excellent regeneration performance and reusability.Bromate, a cancerigenic disinfection by-product (DBP), has been increasingly concerned in recent years. In this study, macroporous Cl-type strong base anion exchange resin (D201-Cl) was used to remove trace levels of bromate from aqueous solution. Batch sorption experiments were performed to evaluate the influence of various factors such as initial bromate concentration, contact time, solution initial pH and temperature on the uptake of bromate. D201-Cl resin exhibited higher bromate removal efficiency (residual concentration of bromate was under the maximum contaminant level (MCL) of 10 μg·L− 1 at all conditions investigated) and broader pH scope of application (4.50–9.92). The maximum sorption capacity reached to 105.5 mg·g− 1 at 298 K. The kinetics data were well described by pseudo-first-order kinetic model, and Freundlich isotherm model and Redlich–Peterson isotherm model fitted the sorption isotherms (R2 > 0.99). Thermodynamic analysis showed that the sorption process was spontaneous and endothermic. In addition, D201-Cl resin still maintained high bromate removal efficiency after regenerated by 0.1 M NaCl solution for five cycles. The results indicate that D201-Cl resin is a low-cost and efficient sorbent for bromate removal from drinking water, especially for trace levels of bromate.