Enhanced chromium recovery from tanning wastewater

A large volume of tanning wastewater was generated with a Cr3+ concentration of 3000–6000 mg/L. The prevailing method for Cr3+ recycle is NaOH precipitation, which is severely limited by the poor sedimentation of the sludge formed. Therefore, bridging cations, Ca2+ and Mg2+, and sonication were used to enhance the sedimentation of the precipitate. Microwave irradiation was employed to improve the re-dissolution of Cr-sludge for reuse. All alkalis effectively removed Cr3+ from the aqueous phase with a removal of higher than 99% and a recovery of ∼60%. The substitution of NaOH with CaO or MgO resulted in much less sludge and shorter sedimentation time. MgO also enhanced the purity and dewatering capability of the sludge. The best alkali was a mixture of CaO and MgO (4:1, by weight) to balance the cost and performance. COD and SS were removed in the process via sweeping by the precipitate with removal percentages of 47.6% and 86.3%, respectively. Two minutes sonication at 0.12 W/cm3 greatly accelerated the sludge sedimentation, cutting the settling time from 3 h to 1 h. Sonication did not alter the particle size or purity of the sludge. Microwave irradiation of 5 min increased the Cr recovery ratio from 60% to 80%.