A cost-effective and simple configuration of birnessite–silicon solar cell (Bir–SSC) hybrid system is reported in this study. Birnessite, with a band gap of 2.1 eV as determined by UV-vis spectroscopy, was electrochemically deposited on a fluorine-doped tin oxide (FTO) for usage as the anode. It was thoroughly characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy, and its prompt response to visible light was further tested by linear sweep voltammetry (LSV). When birnessite electrode was connected with a silicon solar cell in a hybrid system, a remarkably enhanced methyl orange (MO) decolorization from 47.1% (with a bare SSC) to 95.8% was observed. The results indicated the synergistic effects of photoelectrochemical and electrochemical reactions in the hybrid system. In addition, the electron utilization efficiency was 15.29% and 8.54% with and without light irradiation on birnessite respectively. When applied with three different rated voltage SSC, 2.0 V SSC showed the best fit in the hybrid system. Cycling experiments exhibited the stable performance of birnessite electrode, where the MO color removal ratio in ten cycles remained stable at 90.1 ± 2.5%, which is close to the first cycle (95.8%). The hybrid system possesses the merits of cost-effectiveness, low-power consumption, and “green” fabrication strategy, which exbihits promising potential in solar energy utilization and wastewater treatment.

•A traditional MFC and a silicon solar cell (SSC) are combined to build a novel MFC–SSC.•Cell performances are significantly promoted by the SSC in MFC–SSC.•Anodic microbial oxidation of organic substrate is enhanced in MFC–SSC.•The SSC is compatible to promote the whole system without influencing anodic microbial reactions.•Cooperation of anodic microorganisms and SSC improves electron transfer efficiency in the MFC–SSC.This study focuses on the promotion of electron transfer in microbial fuel cells (MFCs) by equipping a silicon solar cell (SSC) into the circuit. As compared to a sole MFC, a significant improvement of power output is observed in the MFC–SSC, that the maximum power density increases from 7.5 W m−3–19 W m−3 by 2.53 times. A linear relationship between anodic potential and current has been observed when the current is below the limiting point of SSC. We estimate the electron transfer rate can be promoted in a MFC–SSC under the condition that the anodic microbial reactions are unaffected by the incorporation of a SSC. In this way, the anodic electrons are fully pumped and enter into the external circuit. This estimation is thereby demonstrated by the 24-h test, which shows the quantity of the electrons fluent in the circuit of a MFC–SSC is doubled and the microbial oxidation efficiency is improved to 341.6% as compared with a sole MFC.

An investigation aimed at checking the integration of cathodic pyrrhotite Fenton's reaction with anodic microbial respiration for the enhancement of MFC performance and treatment of a real landfill leachate was carried out. The MFC equipped with a pyrrhotite-coated graphite-cathode generated the maximum power density of 4.2 W/m3 that was 133% higher than graphite-cathode. Concomitantly, electrochemical impedance spectroscopy (EIS) showed that the polarization resistance of pyrrhotite-cathode (92 Ω) was much lower than the graphite-cathode (1057 Ω), indicating that the cathodic overpotential was significantly lowered, probably due to the occurrence of pyrrhotite Fenton's reaction. The in situ generation of Fenton's reagents (Fe2+ and H2O2) at the pyrrhotite-cathode was demonstrated by the cyclic voltammetry measurement. Besides, reactive oxygen species produced from the pyrrhotite Fenton's reaction were detected and demonstrated to be vital to the enhancement of MFC power output. Further, the effectiveness of this system was examined by treating an old-aged landfill leachate. 77% of color and 78% of COD were removed from the original leachate, indicating that the pyrrhotite not only acted as a cost-effective cathodic catalyst for MFCs in power generation, but also extended the practical merits of traditional MFCs towards advanced oxidation of biorefractory pollutants.

This study investigated semiconductor mineral (natural rutile) as a novel cathodic catalyst for microbial fuel cells (MFCs). Under the experimental conditions, MFCs installed with a natural rutile-coated cathode produced a maximal power density of 12.03 W/m3 under light irradiation, much higher than that of 7.64 W/m3 in the dark. The open circuit voltage (VOCV) of the MFC was 519 mV when irradiated with visible light, the short-circuit photocurrent density (ISC) was 50.91 A/m3, and the fill factor (FF) was 45.44%. In a separate unoptimized MFC device with a switchable cathode, the maximum power density achieved with natural rutile and platinum (Pt) as a cathode catalyst was comparably at 2.7 and 3.6 W/m3, respectively. The low “incident monochromatic photon to current conversion efficiency” (IPCE) (0.13% at 420 nm) of rutile used in this study indicates a potential for further material and system improvement. Results from this study demonstrate that natural rutile, and other expected semiconductor materials, may serve as a group of cost-effective alternative catalysts to noble metals in MFC applications.

Cathodic reduction of hexavalent chromium (Cr(VI)) and simultaneous power generation were successfully achieved in a microbial fuel cell (MFC) containing a novel rutile-coated cathode. The selected rutile was previously characterized to be sensitive to visible light and capable of both non-photo- and photocatalysis. In the MFCs containing rutile-coated cathode, Cr(VI) was rapidly reduced in the cathode chamber in presence and absence of light irradiation; and the rate of Cr(VI) reduction under light irradiation was substantially higher than that in the dark. Under light irradiation, 97% of Cr(VI) (initial concentration 26 mg/L) was reduced within 26 h, which was 1.6× faster than that in the dark controls in which only background non-photocatalysis occurred. The maximal potential generated under light irradiation was 0.80 vs. 0.55 V in the dark controls. These results indicate that photocatalysis at the rutile-coated cathode in the MFCs might have lowered the cathodic overpotential, and enhanced electron transfer from the cathode to Cr(VI) for its reduction. In addition, photoexcited electrons generated during the cathode photocatalysis might also have contributed to the higher Cr(VI) reduction rates when under light irradiation. This work assessed natural rutile as a novel cathodic catalyst for MFCs in power generation; particularly it extended the practical merits of conventional MFCs to cathodic reduction of environmental contaminants such as Cr(VI).

Industrial manganese sulfate from manganese mines has been utilized to synthesize cryptomelane in a simple and feasible route. K-birnessite precursor was prepared by air oxidation of the mixture of MnSO4 and KOH solutions under alkaline conditions, and then transformed to cryptomelane under a heating process. The effects of OH− concentration, airflow rate, liquid reaction temperature, stirring rate, liquid reaction time, washing condition, calcination time, and temperature were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) revealed that cryptomelane prepared under optimal conditions had a tetragonal symmetry and the particles were mostly in short lathy form with sizes of 20–30 nm. The average pore size and BET surface area of cryptomelane examined by N2 adsorption methods were 24.15 nm and 32.21 m2/g, respectively. X-ray photoelectron spectroscopy (XPS) studies demonstrated that the average oxidation state of manganese in cryptomelane was about 3.9 in comparison with prephase K-birnessite of 3.4. The synthesized cryptomelane sample showed improved catalytic activity for decomposition of hydrogen peroxide as compared with natural cryptomelane, but lower than those synthesized with hydrothermal and sol–gel methods. The results of this investigation will provide fundamental information for developing a large-scale production process for transforming manganese sulfate to cryptomelane.Synthetic cryptomelane with industrial manganese sulfate under optimal experimental conditions shows a slit-shaped mesopore character with average pore size of 24.15 nm.

Nano-fibriform silica was extracted from chrysotile by the acid-leaching method. The acid-leached residue of chrysotile has been studied by TEM, XRD, FT-IR, and thermal analysis techniques, etc. When the magnesium leaching degree (MLD) is over 90%, the nano-fibriform silica consists of hydrous silicon dioxide (above 90%) with small amount of magnesium trapped inside the SiO network. The amount of hydroxyl on surface of nano-fibriform silica is 6 unit nm−2. This value is between the values of fumed and precipitated silica. This study shows that nano-fibriform silica is a kind of amorphous matter with a high special surface area (368 m2/g), a high adsorption (330 cm3/g), and a larger pore volume (0.51 cm3/g). The diameter of a single silica fiber is 20–30 nm. The nitrogen adsorption isotherm is similar to Type IV curve. The nano-fibriform silica is one of mesopores materials.Nano-fibriform silica (MLD > 90%) was extracted from chrysolite by the acid-leaching method. Experimental results give evidence that nano-fibriform silica, one of mesopores material, is a kind of amorphous matter with a high special surface area, a high adsorption, and a larger pore bulk.

•Oxidative weathering of Jinchuan sulfide tailings releases heavy metals without AMD generation.•Upward migration of Ca2 + and SO42 − formed gypsum under strong evaporation.•Cu2 + and Ni2 + released from sulfides are immobilized by absorption or co-precipitation.•Heavy metals in poorly crystalline secondary minerals give rise to windblown dust pollution.A detailed mineralogical and geochemical study of a sulfide tailings impoundment was carried out in Jinchuan Cu-Ni sulfide deposit in western China. Since 1963, the tailings impoundment has been exposed to weathering in an extremely arid climate with dry-warm seasons. Samples from different depths of two boreholes, each located in the center and border of the impoundment, were analyzed to evaluate the oxidative weathering behaviors of the tailings. The obvious shift of Cu and Ni from the sulfide fraction to more mobile forms, increasing trend of SO42 − in the shallow of the tailings and decreasing trend of Fe content in sulfide fraction, show direct signs of sulfides oxidation, especially in the upper part of the tailings impoundment. Besides the upward migration of Ca2 + and SO42 − under the strong evaporation in hyper-arid climate, the heavy metals of Cu and Ni are retained by the in-situ adsorption and/or co-precipitation with secondary Fe3 + precipitates. These secondary minerals are poorly crystalline and unstable. So, for sulfide tailings with a simultaneous large production of carbonates and under extremely arid climate, acid pollutants can be avoided but heavy metal pollution from the windblown dust of secondary minerals is the key pollution source that should be taken seriously.

Birnessite films on fluorine-doped tin oxide (FTO) coated glass were prepared by cathodic reduction of aqueous KMnO4. The deposited birnessite films were characterized with X-ray diffraction, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. The photoelectrochemical activity of birnessite films was investigated and a remarkable photocurrent in response to visible light was observed in the presence of phenol, resulting from localized manganese d–d transitions. Based on this result, the photoelectrocatalytic oxidation of phenol was investigated. Compared with phenol degradation by the electrochemical oxidation process or photocatalysis separately, a synergetic photoelectrocatalytic degradation effect was observed in the presence of the birnessite film coated FTO electrode. Photoelectrocatalytic degradation ratios were influenced by film thickness and initial phenol concentrations. Phenol degradation with the thinnest birnessite film and initial phenol concentration of 10 mg/L showed the highest efficiency of 91.4% after 8 hr. Meanwhile, the kinetics of phenol removal was fit well by the pseudofirst-order kinetic model.Download high-res image (108KB)Download full-size image

This paper deals with treating high phenol-concentrated wastewater from coal gasification by manganese oxide method. Mixed-phase manganese oxides of manganite and hausmannite were synthesized with industrial MnSO4 and NaOH by air oxidation. The effects of sulfuric acid dosage, reaction time, temperature, manganese oxide grain size and concentration on removal efficiency of total phenols were studied with laboratory bench-scale experiments. The results indicated that the removal process was more effective under the experimental conditions, i.e. acidified media at pH < 4 and an excessive amount of fine particles with a long reaction time. Solution pH and manganese oxide concentration were two of the most important factors which should be well regulated to guarantee higher removal rate. The mixture of manganite and hausmannite showed improved activity for removal of total phenols, TOC and CODCr as compared with MnO2 (AR) but similar to cryptomelane and K-birnessite. Most of organic contaminants especially phenol which occupied absolute predominance in initial wastewater were removed to enhance the biodegradability for further biological treatment. This investigation will provide fundamental method for developing a pretreatment method of industrial phenolic wastewater with flexibility, simplicity and high activity.