Two-step chromium photo-precipitation in the sequential UV/Sulfite/Manganese dioxide processes: Efficiency, kinetic, energy-economic evaluation, and sludge survey

In this study, removal of Chromium (Cr) in a novel process includes reduction, adsorption, oxidation and complexion in a UV/Sulfite/Manganese dioxide (USM) investigated. In our study, in the optimal condition was MnO2 = 1 mM, Na2SO3 = 0.4 mM, 6 min reaction time (synthetic sample) and at pH 7, and 10 mg L−1 Cr removed completely. In the first stage, the sulfite-sulfate radicals react with Cr, and then Cr-S are removed from the solution by forming a complex with MnOOH. Advantage of this method against other process include less time, higher efficiency, less use of reactive materials, and no need for large pH changes without release of sulfite or sulfate. At pH 7 about 60% and 40% of reaction species were reduction and oxidative species respectively. Considering the better efficiency at pH 7, it shows that reducing species have a more important and primary role in Cr removal. Also, the amount of energy consumed decreases from 16.14 to 3.25 kWh per cubic meter, Kobs (min−1) 0.533to 0.1837 and robs (mg/L.min) increase from 26.68 to 45.2 with change of Cr concentration from 50 to 250 mg L−1 respectively. The total cost of the USM process is much lower than other methods. In the UV, U Manganese dioxide, UV/Sulfite, and USM methods, the total cost were estimated 9.80–18.75, 5.74–8.75, 5.72–1.51, and 2.74–0.55 $ when the Cr concentration increase 100 to 250 mg L−1, respectively.