Treatment of a platinum leachate by membrane distillation:Mechanism of combined silica scaling and organic fouling for distinct system performance decline

Platinum (Pt) is an important rare element and has been widely using in electronic industry, chemical industry and defense industry due to its unique physicochemical properties. Given the high cost of exploiting Pt from limited natural ores, it is desirable to recover Pt from secondary resources for sustainable development. In this study, direct contact membrane distillation (DCMD) was employed to treat a real acidic platinum leachate (PL) with high-salinity. Results of membrane autopsy and silica-membrane interaction revealed that DCMD system performance decline in varying degrees was due to combined silica scaling and organic fouling. Importantly, it was found that the mass-to-charge ratio (m/z) range change of organic compounds in PL had significant effect on system performance. The low molecular compounds in PL with m/z value distributed in a smaller range from 100 to 200 at pH of 1, 3 and 7 possibly resulted in the membrane pores blockage, which was the main reason for the permeate fluxes rapid decline during DCMD process. Despite high molecular compounds with m/z value ranging widely from 100 to 600 detected in the concentrated PL, there was negligible effect on permeate flux decline in the initial stage at initial pH of 5. Deterioration of DCMD performance at pH of 5 was mainly due to accumulated silica scaling caused by the lower concentration of silica.</p

Optimization of a continuous flow electrocoagulation as pretreatment for membrane distillation of the waste stream in vinyl ester resin production

Vinyl ester resin production wastewater (VERW) contains high concentrations of organics particularly, methacrylic acid and bisphenol A, which are hazardous chemicals and harmful to the aquatic environment. Therefore, there is an urgent need to properly treat the effluent before discharge into the aquatic system. In this work, direct contact membrane distillation (DCMD) was explored as an advanced treatment of the VERW pre-treated by a continuous flow electrocoagulation (EC) and peroxi-electrocoagulation (PEC) processes. Optimization of EC and PEC processes were investigated and the DCMD performance was evaluated. Results showed that the optimal value of current density and polyacrylamide (PAM) dosage was 15 mA/cm2 and 1 mg/L, respectively in the EC process. For the PEC process, the optimal addition of hydrogen peroxide (H2O2) dosage was four times of the chemical oxygen demand (COD) concentration of EC effluent. The COD of VERW was effectively removed via EC followed by PEC (EC-PEC), resulting in the significant alleviation of membrane fouling during DCMD filtration of VERW. The initial flux of DCMD filtration of VERW pre-treated via EC-PEC improved by 35%, compared that only pre-treated by EC. Moreover, the concentration factor (CF) of the DCMD system reached up to 8.1 and the conductivity of distillate was less than 33.2 μS/cm. Hence, the EC and membrane distillation hybrid process paves a new way for the effective treatment of waste steam from resin production.</p

Direct contact membrane distillation of refining waste stream from precious metal recovery:Chemistry of silica and chromium (III) in membrane scaling

Precious metals, such as platinum group metals (PGMs) with distinct catalytic activity, are widely used as active components in various industrial catalysts. It is, therefore, highly desirable to recover these valuable components from the end-of-life products. We explored treatment of refining wastewater from precious metals recovery using direct contact membrane distillation (DCMD). The role of various initial pH of refining wastewater on DCMD performance was assessed. Results suggested that hydrochloride acid (HCl) and high-quality water can be reclaimed from the real refining wastewater by adjusting initial pH. Furthermore, DCMD water flux decline was mainly caused by silica and chromium (III) scaling, which was dependent on initial pH of refining wastewater. Silica scaling was responsible for the decrease of DCMD performance when the initial pH of refining wastewater increased from original 0.03 to 5 and 7. Silica oligomers in the concentrated feed with various initial pH were identified using mass spectra. Dichlorotetraaquochromiun was identified by X-ray photoelectron spectroscopy and ultraviolet and visible absorbance spectra as the main species contributing to the green colour and scaling on the PTFE membrane surface. Our results suggest that DCMD can be used as a promising and feasible solution for resource recovery from acidic refining waste stream.</p

Optimization of a continuous flow electrocoagulation as pretreatment for membrane distillation of the waste stream in vinyl ester resin production

Vinyl ester resin production wastewater (VERW) contains high concentrations of organics particularly, methacrylic acid and bisphenol A, which are hazardous chemicals and harmful to the aquatic environment. Therefore, there is an urgent need to properly treat the effluent before discharge into the aquatic system. In this work, direct contact membrane distillation (DCMD) was explored as an advanced treatment of the VERW pre-treated by a continuous flow electrocoagulation (EC) and peroxi-electrocoagulation (PEC) processes. Optimization of EC and PEC processes were investigated and the DCMD performance was evaluated. Results showed that the optimal value of current density and polyacrylamide (PAM) dosage was 15 mA/cm2 and 1 mg/L, respectively in the EC process. For the PEC process, the optimal addition of hydrogen peroxide (H2O2) dosage was four times of the chemical oxygen demand (COD) concentration of EC effluent. The COD of VERW was effectively removed via EC followed by PEC (EC-PEC), resulting in the significant alleviation of membrane fouling during DCMD filtration of VERW. The initial flux of DCMD filtration of VERW pre-treated via EC-PEC improved by 35%, compared that only pre-treated by EC. Moreover, the concentration factor (CF) of the DCMD system reached up to 8.1 and the conductivity of distillate was less than 33.2 μS/cm. Hence, the EC and membrane distillation hybrid process paves a new way for the effective treatment of waste steam from resin production.</p

Rational design of high-performance continuous-flow microreactors based on gold nanoclusters and graphene for catalysis

In this work, we rationally designed a high-performance microreactor system for continuous-flow catalysis. The membrane consists of ultrasmall gold nanoclusters (AuNCs) and two-dimensional graphene. The Au cores of the NCs act as catalysts, while their ligands have two functions: (1) protecting the Au cores to avoid agglomeration and (2) providing a well-defined surfactant assembly to disperse graphene in aqueous solution. Hydrogenation of 4-nitrophenol (4-NP) was employed as model reaction to evaluate catalytic activity. The catalytic membrane microreactor demonstrated excellent catalytic activity and stability, where complete 4-NP conversion was readily achieved via a single pass through the membrane. This desirable performance was maintained over 12 h of continuous operation, although a certain amount of organic buildup on the membrane was observed. The catalytic membrane microreactor outperforms conventional batch reactors due to its improved mass transport. 4-NP-spiked real water samples were also completely converted. This study provides new insights for the rational design of membrane reactors for industrial applications

Nitrogen recovery from a palladium leachate via membrane distillation:System performance and ammonium chloride crystallization

Ammonia leaching process was often used to separate precious and rare-earth metals during their recovery via hydrometallurgy, resulting in the generation of nitrogen-rich leachate. Therefore, it is significantly important to recover the nitrogen from the leachate for reuse. In this work, we explored the treatment of palladium leachate (PL) via direct contact membrane distillation (DCMD) for nitrogen recovery. Effects of temperature and pH on system performance and nitrogen recovery mainly in the form of ammonium chloride (NH4Cl) were evaluated. Results showed that DCMD performance decline caused by membrane scaling was primarily impacted by feed temperature rather than pH. The higher temperature, the more severe membrane scaling occurrence. However, there was little to no effect of initial pH on distillate flux decline. Nitrogen in the form of NH4Cl was successfully recovered from PL, which was significantly influenced by initial pH. The weight of NH4Cl obtained ranged from 17.4 to 21.5 g/L under various operating conditions, and the purity of the NH4Cl was up to 97.4%, near the industrial grade, at feed temperature of 40°C and initial pH of 8.5. Additionally, results of gas chromatography-mass spectrometry (GC–MS) revealed that the primary dissolved organic matter (DOM) in the virgin PL were tetradecanoic acid and isosorbide. Our results suggest that DCMD can be used as a promising and feasible solution for nitrogen recovery from the PL.</p

The numerical simulation for coal gangue as roadbed material on groundwater nitrate concentration

Numerical simulation was used to elaborate temporal and spatial distribution of nitrate concentration in groundwater under one highway,which was constructed with coal gangue based on experiment.Experimental results indicated that the contaminated area spread around over time,but more obviously in horizontal direction,especially in groundwater flow direction.In addition,nitrate concentration decreased gradually in two-axis direction,and contaminated degree decreased with the increasing of distance from the contaminated source caused leaching solution.Numerical simulation suggests that the nitrate concentration (N) in the section will only meet the standard of class III (GB/T14848-93) for groundwater environmental quality after 10 years,although the concentration reaches the standard of class I currently

Treatment of a platinum leachate by membrane distillation:Mechanism of combined silica scaling and organic fouling for distinct system performance decline

Platinum (Pt) is an important rare element and has been widely using in electronic industry, chemical industry and defense industry due to its unique physicochemical properties. Given the high cost of exploiting Pt from limited natural ores, it is desirable to recover Pt from secondary resources for sustainable development. In this study, direct contact membrane distillation (DCMD) was employed to treat a real acidic platinum leachate (PL) with high-salinity. Results of membrane autopsy and silica-membrane interaction revealed that DCMD system performance decline in varying degrees was due to combined silica scaling and organic fouling. Importantly, it was found that the mass-to-charge ratio (m/z) range change of organic compounds in PL had significant effect on system performance. The low molecular compounds in PL with m/z value distributed in a smaller range from 100 to 200 at pH of 1, 3 and 7 possibly resulted in the membrane pores blockage, which was the main reason for the permeate fluxes rapid decline during DCMD process. Despite high molecular compounds with m/z value ranging widely from 100 to 600 detected in the concentrated PL, there was negligible effect on permeate flux decline in the initial stage at initial pH of 5. Deterioration of DCMD performance at pH of 5 was mainly due to accumulated silica scaling caused by the lower concentration of silica.</p

Study on carbon-fixing,oxygen-releasing,temperature-reducing and humidity-increasing effects of evergreen plants in south highway

Li-6400 portable photosynthesis system,was used to test the diurnal variations of photosynthetic rate and stomatal conductance of evergreen plants in Southern Highway,and to calculate their ability of absorbing carbon dioxide and releasing oxygen and to calculate the transpiring water volume and absorbing heat quantity of plants.Results showed that Euonymus fortunei Hand-Mazz,Hedera helix.Aucuba eriobotryaefolia had better carbon-fixing and oxygen-releasing effects,while Photinia serrulata,Trachycarpus fortunei,Radix Ophiopogonis had worse carbon-fixing and oxygen-releasing effects.Radix Ophiopogonis,Photinia glabra,Euonymus fortunei Hand.-Mazz had higher cooling and humidification ability,while Photinia serrulata,Trachycarpus fortunei did not act as well as them.Euonymus fortunei Hand.-Mazz and Hedera helix had higher leaf chlorophyll in per unit mass,values are 12.91、10.34、9.93 mg·g-1.Radix Ophiopogonis、Cinnamomum camphora(Linn.) Presl and Trachycarpus fortunei had lower leaf chlorophyll in per unit mass,value is 3.55、2.67、2.06 mg·g-1.Releasing oxygen,fixing carbon,net assimilation and chlorophyll content has good correlation(P＜0.05)