•The solubility of Na2MoO4·2H2O and Na2WO4·2H2O in Na2MoO4–Na2WO4–Na2SO4–H2O were performed.•The solubility of sodium tungstate dihydrate in Na2WO4–Na2SO4–H2O was determined.•The new model was established via regressing the published and the determined data.•The Pitzer parameter and the solubility product constant of the salt in solution were calculated.•The model was used to estimate the solubility of the sodium molybdate and sodium tungstate.The solubility of sodium tungstate dihydrate and sodium molybdate dihydrate in the (Na2MoO4 + Na2WO4 + Na2SO4 + H2O) system was studied using experimental and calculated methods. The osmotic coefficient of sodium tungstate was fitted to calculate the thermodynamics parameters of (Na2WO4 + H2O) system. The solubility of sodium tungstate dihydrate was determined using the dynamic method in Na2WO4–Na2SO4–H2O to establish the new model which can provide an estimate the solubility of sodium tungstate dihydrate in various conditions, combined with the data published, the solubility of sodium tungstate dihydrate and the sodium molybdate dihydrate in quaternary system of (Na2MoO4 + Na2WO4 + Na2SO4 + H2O) was estimated using the parameters of the two ternary systems of (Na2WO4 + Na2SO4 + H2O) and (Na2MoO4 + Na2SO4 + H2O). The results show that the AARD is always small and the calculated value is basically consistent with the experimental values for the system studied.

•The solubility of Na2MoO4·2H2O in Na+ + MoO42− + SO42− system was performed.•The new model was established via regressing the published and the determined data.•The Pitzer parameter and the solubility product constant of the salts in solution were calculated.•The model was applied to estimate the solubility of the sodium molybdate in various conditions.The solubility of Na2MoO4·2H2O in (Na+ + MoO42− + SO42−) system was carried out using a dynamic method within the temperature range from 293.15 K to 343.15 K. The new model was established via regression of the published and the determined values to predict the solubility. From the results, the solubility of sodium molybdate increases with the temperature increase, however, it decreases with the increasing concentration of sodium sulfate. The Pitzer parameters and the solubility product constant of sodium sulfate and sodium molybdate in aqueous solution were obtained using the literature data. The solubilities of the sodium molybdate in the sodium sulfate solution as well as the thermodynamic parameters were calculated based on the experimental values obtained. The new model was also applied to estimate the solubility of the sodium molybdate under various conditions. The calculated values agree well with the experiment results.

A series of reduced graphene oxide (RGO) colloids with different amounts of surface charges was prepared. The change in surface charge at different pH values was detected by zeta potential measurement, and the evolution of oxygen-containing functional groups attached to the RGOs was analysed by Fourier-transform infrared spectroscopy and ultraviolet-visible absorption spectroscopy. Results showed that the edge phenolic hydroxyl and carboxyl groups made more contributions to the negative surface charge compared with the basal-plane hydroxyl and epoxy groups. Electrical impedance spectroscopy results proved that the surface charge of RGOs significantly affected their electrochemical properties. Furthermore, GO and RGOs (graphene oxide materials) were also used to construct electrochemical sensors for quantitive measurement of Cu2+ by differential pulse anodic stripping voltammetry. The results revealed that the increase in negative surface charge on RGO enhanced its electrocatalytic activity for Cu2+ reduction. Thus, considering that studies on the properties of graphene oxide materials can be simplified by the surface charge, its analysis is an important means of material characterisation.

The photocatalytic hydrogen (H2) evolution with transition-metal substituted Keggin-type titanium tungstates Na5(6)[MTiW11O39]·xH2O (abbreviated as TiW11M (M = Fe, Co, Zn)) as photocatalyst was reported. They showed good photocatalytic activities toward H2 evolution in homogeneous systems under simulated solar light irradiation. The order of photocatalytic activity was TiW11Co ≈ TiW11Zn > TiW11Fe ≫ H3PW12O40 (PW12). The selected POMs showed good stability during the reactions. Karl Fischer analysis, ICPE (Inductive Coupled Plasma Emission) and FT-IR (Fourier Transform Infrared Spectroscopy) were used to confirm the composition and structure. UV-Vis absorption spectra, kinetics investigation and cyclic voltammetric characterizations were used to explain the photocatalytic results. Finally, an analogous “Z-scheme” mechanism was proposed for the homogeneous systems toward photocatalytic H2 evolution under simulated solar light irradiation.

One of the key steps of coking wastewater treatment is phenolic and tar removal via extraction. However, the high loss of the extractant, i.e., methyl isobutyl ketone (MIBK), leads to the high cost of the process. The adoption of a novel solvent or solvent blend is considered as an efficient way to address this problem. In this paper, seven solvents (benzene, toluene, m-xylene, ethylbenzene, 1, 3, 5-trimethylbenze, cyclohexane, and octanol), selected as candidate diluters for MIBK according to operating requirements, are studied with a nonlinear programming (NLP) model based on ideal counter-current extraction. The results, verified with experiments, suggest toluene is the most promising candidate. Further investigation of this solvent blend reveals that both Dblend (the distribution coefficient of phenol between solvent blend and water) and mMIBK (the MIBK concentration in raffinate) increase with xMIBK (the molar fraction of MIBK in blend). The trade-off between the extraction performance and MIBK loss recommends the blend with xMIBK = 0.05 as extractant for coking wastewater treatment. An industrial process consisting of extraction, back stripping, distillation, and mixer is presented. A corresponding NLP model is established for its operating optimization. To improve the accuracy, the representatives of typical phenolics and tar in wastewater (2,4 dimethyl phenol, m-xylene, and quinolone) are also considered in addition to phenol. The case study indicates that the blend exhibits economic advantage over pure MIBK with a makeup cost of 11.15 ¥/t, much less than the 185.15 ¥/t in the case of MIBK.

Given that the consumption of organic substances entails costly biodesulfurization, the characteristics of the bacterial community in a reactor should be determined to increase the desulfurizing rate under low organic loading condition. In this study, the bacterial community distribution in the expanded granular sludge bed reactor used to treat sulfate-containing wastewater with low organic loading rate was determined by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and 16S rDNA clone library analyses. DGGE results showed that the predominant bacteria were stable and accounted for ~90 % sulfate removal efficiency. Differences in band positions and intensities indicated that the distribution and abundance of bacteria were affected by their positions in the reactor. Typical bands were identified in the bacterial community comprising Desulfovibrio, Desulfomicrobium, Thiomonas, Acinetobacter, Bacteroidetes, and Chloroflexi. Their functions in the reactor were also discussed. The possible links between the functional and microbial responses were also investigated based on the characteristic and spatial distribution of each bacterium in consortium.

•Solubility of CO2 was measured in aqueous MDA up to 1.97 CO2 loading.•KE model was used to correlate VLE data in α < 1 and α > 1 regions separately.•Four chemical equilibrium constants were determined.•Sterically hindering effect for MDA in CO2 absorption was demonstrated.•MDA absorption efficiency was compared with MEA, MDEA and PZ.The solubility of CO2 in aqueous 1,8-p-menthane-diamine (MDA) solution with substance concentrations of 0.625 and 1.25 mol · L−1 was measured at temperatures (313.15, 333.15 and 353.15) K with CO2 partial pressures ranging from (0.55 to 776.0) kPa and CO2 loading ranging from (0.120 to 1.97) mol CO2 per mol MDA. The gas solubility results are expressed as the partial pressure of CO2 (PCO2PCO2) against its mole ratio, i.e.  αCO2αCO2 (mol CO2 per mol MDA). The chemical absorption reaction and thermodynamic model have been proposed. The physicochemical Kent–Eisenberg model was used to correlate all the experimental results of the solubility of CO2 in the aqueous MDA solutions under investigation. The chemical equilibrium constants and model parameters were determined by fitting the VLE data.

•Multielectron redox capabilities of the high nuclearity CoPOM.•Excellent photocatalytic H2 generation activities of the high nuclearity CoPOM.•Analogous “Z-scheme” and “dye” sensitized mechanisms for CoPOM and TiO2/CoPOM.The photocatalytic hydrogen (H2) generation by the high nuclearity Co substituted polyoxometalates (POMs), K10Na12[{Co3(B-β-SiW9O33(OH))(B-β-SiW8O29 (OH)2)}2]·49H2O (abbreviated as CoPOM) was reported. Owing to the multielectron redox capabilities of the high nuclearity CoPOM, the POM showed excellent photocatalytic activities toward H2 evolution in both molecule scale (homogeneous) and composite (heterogeneous) systems. The photocatalytic activities of CoPOM were much better than H3PW12O40. UV–Vis–NIR absorption spectral, Raman spectral, cyclic voltammetric behavior and intermittent photoelectrochemical current response were used to characterize the structure of the TiO2/CoPOM composite and interaction between TiO2 and CoPOM. Analogous “Z-scheme” and “dye” sensitized mechanisms were proposed for the homogeneous and heterogeneous systems toward photocatalytic H2 evolution under solar light irradiation, respectively.The high nuclearity K10Na12[{Co3(B-β-SiW9O33(OH))(B-β-SiW8O29(OH)2)}2]·49H2O (abbreviated as CoPOM) formed by the combination of several polyanion units shows excellent photocatalytic activities toward H2 evolution from molecule scale to composite systems. Analogous “Z-scheme” and “dye” sensitized catalytic mechanisms have been proposed for molecule scale and composite systems, respectively.

A novel technique for internal structure and elemental distribution analyses of granular sludge is presented. Sludge samples were freeze-dried and embedded in epoxy resin to form a module, which were then ground and polished to obtain sequential cross-sections. The cross-sections were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). SEM observations showed that one granule was formed having several cores with different inorganic minerals, rather than a single core. EDX results indicate that the main elements of the granules are O, Ca, Mg, and P. In addition, the distribution areas of calcium and magnesium in the granule do not coincide.

In this paper, composite coagulants (PFS, PFSC05, PFSC1 and PFSC5), prepared by mixing polyferric sulfate (PFS) and cationic polyelectrolyte (CP) coagulants with different weight percent (Wp) of CP (Wp = 0%, 0.5%, 1% and 5%, respectively), were adopted to treat cyanide-containing wastewater. PFSC5 exhibited superior coagulation performances at optimal conditions: the removal of total cyanide (TCN) and chemical oxygen demand (COD) was 95%–97% and 50%–55%, respectively. The effects of CP on the properties and structure of flocs were investigated by laser diffraction instrument and small-angle laser light scattering (SALLS), respectively. The results show that the flocs of PFSC5 have higher growth rate, higher strength factor and lower recovery factor than other flocs. They are also much denser and more uniform owing to the higher fractal dimension (Df) and less microflocs (10–100 μm). Furthermore, the dense structure of the PFSC5 flocs can be restored after shear and is more resistant to hydraulic conditions. Particularly, detailed morphology evolution of the flocs was in-situ detected by on-line particle imaging. Due to strong ionic strength in wastewater, the CP in PFSC5 plays a significant role of adsorption, while the main mechanism of CP is electrostatic patch aggregation during the PFSC05 systems.

A double layered, one-pot hydrothermal method was adopted in this work to prepare transition metal ions (Fe3+, Ni2+, Cu2+ and Co2+) doped TiO2. The morphology and chemical properties of TiO2 and the status of metal ions were characterized with XRD, TEM, BET, UV-Vis and XPS analysis. TEM images show that the obtained TiO2 was very uniform with an average particle size of 10.4 nm. XPS, TEM and XRD results show that transitional metals were doped onto TiO2 in the form of ions. Photocatalytic decomposition of oxalic acid under UV illumination and methylene blue degradation under visible light on these materials were conducted, respectively. The results reveal that Cu2+-TiO2 and Co2+-TiO2 showed a highest activity under UV and visible light illumination, respectively, and they were both more active than commercial P25 TiO2. With this special design of double layers, the hydrolysis of titanium precursor in the system with water can be easily controlled and metal ions are simply doped. This strategy can be further applied to synthesize metal ion doped TiO2 using various metal precursors with controllable amounts, and thus lead to better optimization of highly active photocatalyst.

The solubility of ammonium metavanadate (NH4VO3) in ammonium salt solution is significant for the precipitation of NH4VO3from the vanadium-containing solution. The solubilities of NH4VO3 in the (NH4)2SO4–H2O, NH4Cl–H2O, and (NH4)2SO4–NH3–H2O systems were measured using dynamic method in the temperature range from (298 to 343) K. The investigated concentration is up to 3.0 mol·kg–1, 2.5 mol·kg–1, and 5.2 mol·kg–1 for (NH4)2SO4, NH4Cl, and NH3, respectively. The solubility of NH4VO3 decreases sharply with the addition of (NH4)2SO4 or NH4Cl while the solubility of NH4VO3 in the mixed NH3 and (NH4)2SO4 (0.10 mol·kg–1) solution increases first and then levels off with increasing NH3 concentration. The solubility product constant (KSP) of NH4VO3 was also determined using the standard-state thermodynamic data. The Bromley–Zemaitis model was applied to model the solubility of NH4VO3 in the systems mentioned above, and five pairs of new model parameters were obtained via the regression of the experimental solubilities. A chemical model was established to successfully calculate the solubility of NH4VO3 in all systems. All of the work done in this research will provide a thermodynamic basis for industrial development in the precipitation of NH4VO3.

In this study, a novel horizontal rotating soil washing process and equipment were developed and tested for pilot-scale remediation of soils from a site polluted by chromium ore process residue. Operating parameters, including cylinder rotational velocity, cylinder tilt angle, heating temperature and liquid/soil ratio, were investigated. The Taguchi method was used for the experiment design, and the standard L16 orthogonal array with four parameters and four levels was selected for optimising the operating parameters. Optimal removal efficiency was achieved at cylinder rotational velocity of 2.5 rpm, cylinder tilt angle of 2.6°, heating temperature of 200 °C and liquid/soil ratio of 8. The efficiency of citric acid as an extractant in the novel process was compared with that of water. The analysis of the residual Cr(VI) concentration of the soil shows that citric acid could efficiently remove 22.89 % more Cr(VI) than water in one-stage washing. The residual Cr(VI) concentration in the soil after the three-stage washing is as low as 26.16 mg/kg, which meets the screening levels for soil environmental risk assessment of sites in Beijing City (30 mg/kg). Further study is currently underway to optimise the novel process and equipment for commercial-scale use.

This paper aims to make an overview on the current status and new tendency for recycling cathodic active materials from spent lithium-ion batteries. Firstly, it introduces several kinds of pretreatment technologies, followed by the summary of all kinds of single recycling processes mainly focusing on organic acid leaching and synergistic extraction. Then, several examples of typical combined processes and industrial recycling processes are presented in detail. Meanwhile, the advantages, disadvantages and prospect of each single process, combined process, as well as industrial recycling processes, are discussed. Finally, based on a novel acidic organic solvent, the authors briefly introduce an environmental friendly process to directly recycle and resynthesize cathodic active material LiNi1/3Co1/3Mn1/3O2 from spent lithium-ion batteries. The preliminary experimental results demonstrated the advantages of low reaction temperature, high separation efficiency and organic solvent cycling and preventing secondary pollution to the environment. This process may be used for large-scale recycling of spent lithium-ion batteries after further study.

The solubility of gypsum (CaSO4·2H2O) in ammonium solutions plays a significant role to prevent gypsum scaling on the heater and tower in the treatment of ammonium-N wastewater bearing sulfate ions by the steam stripping process. In this work solubilities of calcium sulfate dihydrate in NH4Cl, NH4NO3, and mixed NH4Cl and (NH4)2SO4 solutions up to 343.15 K were measured using the classic isothermal dissolution method. The investigated concentration (at ambient temperature) is up to 1.50 mol·dm–3 for both NH4Cl and NH4NO3. The solubility of CaSO4·2H2O was found to increase sharply with either NH4Cl or NH4NO3 concentration, whereas the temperature has a limited effect. The XRD analysis of equilibrated solids for these systems shows that CaSO4·2H2O is stable in all cases over the temperature range (298.15 to 343.15) K. The electrolyte nonrandom two-liquid (the electrolyte NRTL) model embedded in AspenPlus was applied to model the solubility of CaSO4·2H2O in the above systems. The newly obtained model parameters were used to well estimate the solubility of CaSO4·2H2O in all cases with a relative deviation of 1.52 %.

Catalytic nitrate reduction is a promising technology in groundwater purification. In this study, PdCu bimetallic catalysts supported on an industrial amorphous silica−alumina (ASA) were synthesized and used to simulate catalytic removal of nitrate in groundwater. The catalysts exhibited very high activity and the highest catalytic selectivity toward N2O and N2 was 90.2%. The optimal Pd/Cu weight ratio was four. Relatively low reduction temperature was found benefit the catalytic stability and 300 °C was the appropriate reduction temperature during catalyst preparation. With an average particle size 5.4 nm, the metal particles were very uniformly distributed on the catalyst surface prepared with the codeposition method. This kept the catalyst more stable than the PdCu/Al2O3 catalyst with larger metal particles. According to XRD, TEM, and XPS results, the metals maintained zero-valence but aggregated by about 2 nm during the denitration reaction, which caused gradual deactivation of the catalysts. Little leaching of Cu and Pd from the catalyst might also have a slightly negative impact to the stability of the catalysts. A simple treatment was found to redistribute the particles on the deactivated catalysts, and high catalytic activity was recovered after this process.

Sulfate reducing bacteria (SRB) is identified as the primary organisms responsible for the treatment of heavy metal wastewater. However, most heavy metals can inhibit the growth of SRB during heavy metal treatment processes. Sulfide is a metabolic product of SRB and it can precipitate or reduce heavy metals. This study focused on the effects of sulfide on SRB resistance to Cu(II), Hg(I) and Cr(VI) toxicity. First, we considered the existence style of various heavy metals with and without sulfide addition by sequential extraction experiments. Second, the particle size distribution was evaluated and the cell structure during the metabolism of a SRB culture, containing different heavy metals, was analyzed by particle size distribution and TEM analyses. Third, the evolution of sulfate under the influence of different concentrations of heavy metals with and without sulfide addition was investigated to evaluate SRB activity. The results indicated that sulfide played an important role in alleviating and even eliminating the toxicity of Cu(II), Hg(II) and Cr(VI). We also discuss the mechanism of sulfide on SRB resistance to Cu(II), Hg(I) and Cr(VI) toxicity.

Controlling the morphology of transition-metal nanoparticles (TMNPs) can be an effective way to produce nanomaterials with favourable properties (activity and selectivity, etc.). Here we reported the shape control syntheses of TMNPs including Ru, Rh, Pd, Os, Ir, Pt, alloy PtRu, PtOs, and PtRuOs by co-reducing their metal precursors and Ag(I) ions in an organic medium. In this approach, Ag(I) ions were reduced first for their higher reduction potential, leading to the formation of spherical Ag nanoparticles. Then the subsequently reduced atoms of other transition-metals grew on the pre-existing Ag nanoparticles, resulting in the TMNPs with spherical or quasi-spherical shapes. In the absence of Ag precursors, the TMNPs obtained under same experimental conditions were dominated by irregular shapes, for example, rods, multi-pods, worm-like, or star-like, etc. The dependence of the property on the shape of the particles was also demonstrated using the catalytic oxidation of methanol as an example.Graphical abstractAn Ag facilitated approach for the shape control of transition-metal nanoparticles was developed, which is actually built on the formation of core–shell Ag-metal structures although the syntheses were carried out in a one-pot way.Highlights► We developed a Ag-facilitated approach to control the shape of transition-metal nanoparticles (TMNPs). ► Quasi-spherically shaped TMNPs were prepared by co-reducing the metal precursors and Ag(I) ions in an organic medium. ► The shape control of TMNPs was actually based on the formation of core-shell Ag-metal structures. ► The Ag facilitated alloy TMNPs showed superior catalytic activities towards methanol oxidation.

Selectively extracting tungsten to purify sodium molybdate solution was studied using primary amine N1923, including various conditions, such as equilibrium pH values, concentrations of extractant in organic phase and metals in feed solution, contact time of extraction and stripping, types of acidic solutions added, temperature of extraction and the dosage of stripping agent. Graphical method using McCabe-Thiele diagram and counter-current extraction simulation (CCES) were used to determine the required stages of deep removal of tungsten. The results show that appropriate concentrations of tungsten and molybdenum in feed solution have great influence on the difficult degree of the separation between the two metals. Organic phase including 0.051 M of N1923 modified by LK-N21X diluted in kerosene can extract 92.7% of tungsten in sulfuric acid solution at equilibrium pH value of 7.07 for 10 min in one-stage extraction at 15.5 °C, and the separation factor reaches 329 under those optimum conditions. A two-stage CCES removes 99.83% of tungsten and obtains the purified sodium molybdate solution where W/Mo (mass ratio) reaches 2.9 × 10−5. Stripping can be accomplished by NaOH solution completely and quickly. Furthermore, the extraction and stripping reactions of tungsten and molybdenum in equilibrium were proposed.

A novel quantitative electroanalysis method, triple potential step amperometry (TPSA), was developed and explained with an example of nitrobenzene analyzing in water. The selectivity of TPSA was improved by controlling the potential step within a narrow interval and using enzyme-modified electrode, the narrow potential step makes the method avoid most interferents, and enzyme-modified electrode can enhance the response of target substance selectively. The peak area was investigated for quantitative calibration, such as nitrobenzene concentration showing a linear relation with the peak area, with the correlation coefficients being 0.9995. The t-test and F-test were applied to evaluating the reliability of TPSA, the results showed that there was no evidence of systematic error for TPSA, and the method was of no significant difference from CV. The merit of fast detecting and few potential changing times make the TPSA suitably applicable to low-cost automatic monitoring equipments.

Multi-walled carbon nanotubes (MWNTs) were electrochemically oxidized by a constant-potential electrolysis method and then investigated in detail using scanning electron microscope, transmission electron microscope, FT-IR, electrical impedance spectroscopy, and cyclic voltammetry. The FT-IR spectra showed that the amount of hydroxyl generated on the surface of MWNTs increased with increasing the electrochemical oxidation time of MWNTs. The CV results, being conducted in nitrobenzene solution, showed that the nitrobenzene reduction current increased with the increase in oxidation time of the MWNTs within the first 60 min of electrolysis. An electrical equivalent circuit model for electrical impedance spectroscopy was further established to analyze the surface capacitance and resistance of the MWNTs, and the model results showed that the capacitance of the oxidized MWNTs increased greatly while the charge transfer resistance decreased, suggesting electrochemical oxidized MWNTs modified pyrolytic carbon electrode being an effective electrochemical sensor for nitrobenzene determination.