Primary study on the treatment of hexavalent chromium wastewater of leather

The research on wastewater treatment of hexavalent chromium is very active in China.  According to the basic line, one is to reduce hexavalent chromium to low-toxicity trivalent chromium and then remove by chemical precipitation, the methods including chemical reduction and electrolytic reduction condensation;  the other is recovery method, such as ion exchange, activated carbon adsorption, reverse osmosis, etc., which have been reported in many studies.   Chemical reduction, ion exchange, electrolytic reduction and other methods are widely used.  The treatment of wastewater containing hexavalent chromium by chemical reduction includes reagent reduction, ferrite reduction, iron dust reduction and so on.  Its basic principle is to use chemical reducing agent to reduce hexavalent chromium to trivalent chromium under acidic conditions, and then use alkali precipitation to generate chromium hydroxide precipitation and remove.  

Hexavalent chromium in wastewater mainly exists in two forms, trivalent chromium and hexavalent chromium, and there is a conversion balance between the two forms with different pH.  The reduction of hexavalent chromium reaction is faster under acidic conditions, generally requires pH<4, usually control PH 2.5-3, the reducing agents commonly used are: sodium pyrosulfite, sodium sulfite, sodium bisulfite, sodium thiosulfate, ferrous sulfate, sulfur dioxide, hydration trap, iron dust iron powder, etc..  The best pH for Cr(OH)3 precipitation of Gr3+ after reduction is 7-9, generally controlled at pH 8, so the wastewater after chromium reduction should be neutralized.  The neutralizer commonly used are NaOH and lime.  Some small enterprises use fly ash and calcium carbide paste and other waste as neutralization.   

When we consider to select chemical reduction precipitation method, reducing agent and precipitant for chromium-containing wastewater, we should not only consider the reduction and removal efficiency of chromium, but also should consider the source and cost of reagents.  Meanwhile, we should consider the possibility of settling sludge disposal and utilization.