Enhancing photo-precipitation of Cr (VI) with sulfur dioxide radical: Mechanism, kinetic and energy consumption and sludge survey

Abstract This study was aimed to appraisal the proficiency of sulfur dioxide radical anion produced from UV/sulfoxylate (US) process for the reduction of the Cr (VI) to Cr (III). Under the optimum conditions (sulfoxylate dosage, 0.12 g L−1; pH, 6.8 and reaction time, 60 min), 100 % of Cr (VI) is reduced using US, whereas the reduction proficiencies by UV alone and sulfoxylate alone were 2.1 % and 38 % under the same conditions. Also, the value of robs obtained from kinetic study was proved this phenomenon, as rate of Cr (VI) reduction in the US photoreactor was 760 and 15.2 times more than the UV alone and sulfoxylate alone, respectively. Also, during UV alone and sulfoxylate alone processes, the amounts of the consumed energy were calculated 10.94 kW h m-3 and 0.57 kW h m-3, respectively; whereas electrical energy per order is decreased to 0.06 kW h m-3 when photoreactor was operated in the presence of sulfoxylate (US). Investigation the influence of co-existing substances in aquatic systems indicated the positive impact of carbonate and the negative impact of sulfate and chloride on reduction process. Also in the presence of citrate and humic acid, the improvement of photo-precipitation was observed that can be due to their complex with Cr (III). The analysis of sludge of obtained at 60, 90 and 120 min of reaction by SEM/EDS analysis revealed an increase in the Cr (III) percentage as well as increase in the amount of clumps and coagulants over time.