Treatment of Cr(VI) Polluted Wastewater with Scrap Iron. Part 2: Removal of Species Resulted from Cr(VI) Reduction

摘要

Hexavalent chromium, a widespread industrial waste, is a strong toxic, mutagenic and carcinogenic agent. It is highly soluble and easily spreads in the environment. Therefore, it should be removed from wastewaters before their disposal to natural aquatic environments. One way of detoxifying waters polluted with Cr(VI) is by reduction of Cr(VI) to the less dangerous Cr(III), using chemical reducing agents. Scrap iron is a non-conventional Cr(VI) reducing agent that can replace commonly used reducing agents such as ferrous sulphate, sulphur dioxide and sodium sulphites. The effluent that results from the total reduction of Cr(VI) with scrap iron still contains Cr(III), Fe(II), and Fe(III) species, which must be removed from the aqueous solution in a final treatment step. The aim of this study was to explore the simultaneous removal of Cr(III), Fe(II) and Fe(III) from aqueous solutions by precipitation with Na_2CO_3. The effects of Na_2CO_3 dose, mixing time, mixing intensity, settling time and solution temperature were investigated. The optimum conditions of the precipitation process, which resulted from this study, are: Na_2CO_3 dose: 1000 mg/L, mixing time: 5 minutes, mixing intensity: 50 rpm, settling time: 45 minutes, solution temperature: 33 ℃.