A novel method for removing arsenic (As) from intractable arsenopyrite (FeAsS)-bearing mine tailings was investigated that used a microwave technique. Although the tailings were poor microwave absorbers, absorption was indirectly assisted by adding various microwave-absorbing materials (MAMs), including MnO2 (oxidant), C (reductant), Fe3O4 and Fe2O3. The results revealed that the type of MAM and the microwave runtime significantly affected the efficiency of As-removal from the tailings and the magnetic effects of the reaction residuals. Compared with MnO2 and C, the best As-removal efficiencies of the tailings and the magnetic effects of the reaction residuals were obtained using Fe3O4 (90.12%, 24997.03 x 10(-8) m(3)/kg, in 4 min) and Fe2O3 (92.10%, 19774.73 x 10-8 m3/kg, in 14 min). In addition, their As-removal efficiencies positively contributed to the magnetic effects, which increased with the microwave runtime. The obtained products were characterized by using a variety of analytical techniques (e.g., SEM, EDAX and XRD). Overall, Fe2O3 and Fe3O4 underwent similar As-removal pathways and magnetic products. Arsenic was mainly removed in the form of As (elemental form) and AsS in the oxygen-free atmosphere, and As2O3 was removed in the oxidizing atmosphere. Meanwhile, the arsenopyrite (FeAsS) and pyrite (FeS2) in the tailings, mixed with Fe2O3 or Fe3O4, were mainly converted into valuable pyrrhotite (Fe1-xS and FeS) and magnetite (Fe3O4) in the oxygen-free atmosphere. This study provides a feasible and environmentally friendly method for removing arsenic from tailings (or minerals), regardless of their microwave-absorbing properties. (C) 2015 Elsevier B.V. All rights reserved.

A novel magnetic carrier with surface magnetic field of 4 mT was developed for studying the magnetic enhanced bio-effect on nitrification. The bio-effect on nitrificaton induced by the magnetic carrier was studied by comparing the performance of sequencing batch biofilm reactors filled with magnetic (MC) and non-magnetic (NMC) carriers. The result showed that the bioreactor with MC had better performance for nitrification than bioreactor with NMC. During the biofilm culturing period, the time required for nitrification formation in biofilm of the MC reactor was 25% less than that for the NMC reactor. The results also showed that the ammonium oxidation rate of the MC reactor was 1.6-fold faster than that in the NMC reactor at high influent NH4-N concentration, while nitrite oxidation rate was always accelerated regardless of influent NH4-N concentration. The specific oxygen uptake rate analysis revealed that ammonia and nitrite oxidation activities in biofilm of the MC reactor were 1.65 and 1.98 times greater than those of the NMC reactor, respectively.

One of the main obstacles to the agricultural use of the sludge produced in wastewater treatment plants are heavy metals that have accumulated. Electrokinetic treatment can be used to remove these heavy metals, but the process is time consuming, sometimes lasting several days. In this study, the effects of different potential gradients were investigated. Electrokinetic experiments were conducted under constant potential gradients (1 V/cm, 3 V/cm, and 5 V/cm) with a treatment time of 16 hours. Results showed that the most efficient removal of metals was achieved at a potential gradient of 5 V/cm. Average removal efficiencies of heavy metals were 30.29% for copper, 43.52% for nickel, and 33.38% for zinc. Accumulation of metals occurred at a distance of 7 cm from the anode at a potential gradient of 1 V/cm. Results of the sequential extraction and sludge pH profiles show that an acidic front generated at the anode reservoir flushed across the sludge chamber and that the higher potential gradient increased the migration velocity, which aided the dissociation and desorption of metals. Furthermore, the use of higher potential gradients made it possible to remove heavy metals from sludge cells in a shorter period of time.

The potential effects of potassium ferrate (K2FeO4) on sludge dewatering under different pH values (between 3 and 8) and the mechanism of its reaction were investigated in this study. Specific resistance of filtration (SRF) was used to evaluate sludge dewaterability. Sludge water distribution was measured by the drying test. Sludge floc structure was observed by microscopic examination. Soluble chemical oxygen demand (SCOD), extracellular polymeric substances (EPSs) content, sludge disintegration degree and sludge particle size were measured to explain the observed changes in sludge dewaterability. The results indicated that the potassium ferrate pretreatment at pH 3 enhanced sludge dewaterability, while potassium ferrate pretreatment caused deterioration of sludge dewaterability at pH values of 4–8. At pH 3, sludge dewaterability increased with the increase of potassium ferrate, then decreased slightly when the dosage of potassium ferrate was greater than 1200 mg/L. The results showed that a potassium ferrate dosage of 1200 mg/L at pH 3 was an ideal condition, yielding maximum sludge dewaterability characteristics by generating sludge with optimal disintegration, and EPSs concentration. However, particle size changed slightly after potassium ferrate pretreatment at pH 3.

BACKGROUND: To extend the working pH range of electro-Fenton, activated carbon fibre cathode (ACF) supported nano-Fe0 catalysts were prepared by electo-deposition and applied to treat a dye effluent in a neutral aqueous medium. The morphology of the Fe0/ACF cathode was characterized by X-ray diffraction and scanning electron microscopy. The experimental conditions, such as the initial pH, dye concentration and current density of the degradation of the dye effluent were investigated. A pseudo-first-order model was used to simulate the experimental results. RESULTS: Compared with Fe2+ and Fe3+, the degradation of Orange II solution by an Fe0/ACF cathode was shown to achieve much better results at neutral pH values. Additionally, the dissolved iron concentration in the solution was also evaluated. The results showed that 0.79 mg L−1 of total iron was present after 120 min reaction time at neutral pH, suggesting that the Fe0/ACF cathode can be used for a long time and is reusable under neutral pH conditions. A complete description of the possible degradation mechanism in solution is provided. CONCLUSION: The high catalytic activity of this cathode is probably due to the efficient electrotransfer at the Fe0/iron oxide interface. All this demonstrates that Fe0/ACF is a good and promising cathode for the neutral electro-Fenton process. Copyright © 2010 Society of Chemical Industry

This study investigates the physical and chemical characteristics of sludge treated with controlled levels of electric field. The results indicated that the potential gradient and contact time strongly influenced the physical and chemical characteristics of sludge. Based on the settling velocity measurements, a potential gradient of 6V/cm with a treatment time of 10 min is recommended as an optimal condition for improving sludge settling. For sludge disintegration, applying a higher potential gradient and a longer treatment time to the sludge are more efficient than applying lower levels. The results of the experiments presented here show that an electric field not only disintegrates sludge and destroys microbial cells but also removes and solubilizes organic substances. Possible mechanisms of electric field treatment are also discussed. Crown Copyright (C) 2011 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers. All rights reserved.

In this study, we investigate the physical and chemical properties of waste-activated sludge after treatment with microwave irradiation. The results indicate that microwave energy and contact time strongly influence the physical and chemical properties of sludge. According to the settling velocity and particle size measurements, the microwave energy of 900 W with a contact time of 60s may be the optimal condition for improving the ability of the sludge to settle. Results of the experiments have shown that supernatant turbidity, soluble chemical oxygen demand, volatile suspended solid solubilization, extracellular polymeric substances content, and inorganic nitrogen increase significantly with contact time. Based on these results, we find that the microwave irradiation treatment specified by the contact time not only improves settleability, but also disintegrates sludge and destroys microbial cells. Possible mechanisms of microwave treatment are also discussed.

We reported the study of the degradation of the azo dye cationic red X-GRL by the electro-Fenton process using an activated carbon fiber cathode. The electrogeneration of hydrogen peroxide in solution using different material cathodes fed with air was investigated, and the results revealed that the activated carbon fiber cathode was more effective compared to the graphite cathode. The decolorization and mineralization of cationic red X-GRL were also determined. The effect of the operating parameters, such as the initial Fe2+ concentration, temperature and initial dye concentration, was investigated. The optimum Fe2+ catalyst concentration values for the degradation of cationic red X-GRL was found to be 5mM. The rate of decolorization and mineralization of dye could be accelerated by increasing the temperature. In addition, the decolorization and total organic carbon (TOC) removal efficiency decreased with the increasing initial dye concentration, while the TOC removal increased. Two different transition metal ions (Cu2+ and Mn2+) were applied as substitutes for ferrous sulfate for evaluating catalytic effect. The results indicated that Cu2+ and Mn2+ were more effective than Fe2+ in catalyzing the degradation of the dye.