A highly efficient reduction process of Cr (VI) with biochar was conducted in this paper. The results showed that nearly 100% Cr (VI) was reduced at selected reaction conditions: the dosage of biochar at m (C)/m(Cr) = 3.0, reaction temperature of 90 ℃, reaction time at 60 min and concentration of H2SO4 of 20 g/L, respectively. The reduction kinetics analysis demonstrated that the reduction of Cr (VI) fitted well with the pseudo-first-order model and the apparent activation energy was calculated to 40.24 kJ/mol. Response surface methodology confirmed that all the experimental parameters had positive effect on the reduction of Cr (VI). The influence of each parameter on the reduction process followed the order: dosage of biochar> Concentration of H2SO4 > Reaction Temperature > Reaction Time. This paper provided a versatile strategy for treatment of wastewater containing Cr (VI) and showed a bright tomorrow for wastewater treatment.

Removal of hexavalent chromium had attracted much more attention as it was a hazardous contaminant. Electrochemical reduction technology was applied to removal chromium (VI) from wastewater. The mechanism and parameters affect the reduction process were investigated. The results showed that the reduction efficiency was significantly affected by the concentration of H2SO4, current density and reaction temperature. And the reduction efficiency was up to 86.45% at concentration of H2SO4 of 100g/L, reaction temperature of 70 ℃, current density at 50 A/m2, reaction time at 180 min and stirring rate of 500 rpm. The reduction process of chromium (VI) was followed pseudo-first-order equation, and the reduction rate could be expressed as Kobs = k [H2SO4]1• [j] 4•exp-4170/RT.

The electrochemical oscillation was first observed in the charging process of anolyte in VRFBs. The chronopotentiometry with current ramp results could be used to judge the appearance of electrochemical oscillation. The electrochemical oscillation could be explained in terms of the competition between the growth and the chemical dissolution of V2O5 film in the H2SO4 solution. It was possible to regular the extra power consumption resulted by the electrochemical oscillation. This work might provide new focus on the charging process of the VRFBs and guide for new methods on energy saving.

This paper focused on the oxidative leaching process of vanadium from vanadium-chromium reducing residue in alkaline medium with MnO₂. The effect of experimental parameters including reaction time, reaction temperature, dosage of MnO₂, dosage of NaOH, and liquid-to-solid ratio on the leaching efficiency of vanadium had been studied. The results indicated that MnO₂ was an efficient oxidant for leaching out of vanadium. The leaching efficiency of vanadium was up to 97.25% under optimal reaction conditions: reaction temperature of 90 ℃, reaction time of 60 min, dosage of MnO₂ at 50 wt.%, concentration of NaOH at 30 wt.% and liquid-to-solid at 5:1 mL/g.

The direct Cr (VI) reduction process by oxalic acid was conducted and the results showed that the Cr (VI) was efficiently reduced by oxalic acid at high reaction temperature and high dosage of oxalic acid. The reduced product, Cr (III), was easily generated stable complex compounds (Cr(HC2O4)3) with oxalate, which displayed a negative effect on the reduction process. The high reaction temperature and high acidic medium could destroy the stable structure of a complex compound to release oxalate, and facilitate the reduction of Cr (VI). Generally, the present study provided a versatile strategy for Cr (VI) reduction, exhibiting a bright application future for real wastewater treatment.