Reaction mechanism and kinetics for the treatment of hexavalent chromium laden mines wastewater by LD slag and simultaneous volume stabilization of LD Slag

Feasibility study to assess the removal of Cr (VI) from mines wastewater using LD slag as reducing agent was conducted. Under the set treatment of slag size 0-6 mm, stirring speed of 150 rpm and reaction time of 60 min and with different solid liquid ratios the maximum 99.71% removal of Cr (VI) was achieved, meeting the CPCB norms (Central Pollution Control Board India). Mass balance of Cr (VI) in solid and liquid proves that the removal of Cr (VI) is due to its reduction into Cr (III) not because of its adsorption in slag. Redox reaction between Fe (II) present in LD slag and Cr (VI) present in wastewater was found to be the reason for removal of Cr (VI). Also 80.49% reduction in free lime content of LD slag was measured. Stabilization of LD slag was evident by conversion of free lime into bound lime as merwinite and gypsum through FTIR and XRD study. A significant increase in TDS of water was also observed. The mechanism of removal of Cr(VI) was described by using several successive steps. Removal of Cr(VI) by LD slag followed pseudo-second order kinetic model.