Dissociation and removal of complex chromium ions containing in dye wastewaters

Heating and acidification as pretreatment were used to dissociate chromium-azo complexes into simple ions Cr3+, which then were precipitated as Cr(OH)3 using neutralization-flocculation method. The combined method significantly increased the treatment efficiency for dye wastewater containing chromium complexes. Temperature and acidity have obvious influence on the thermodynamic stability of the complexes. The results showed that yellow or orange chromium complex containing aromatic carboxyl group could be easily destroyed by either heating or acidification. However, for more stable black chromium complex, the destruction of complex is accomplished by both heating and acidification. The residual chromium in the wastewater was dropped below 1.0 mg/L after acidification, preheating, and neutralization-flocculation.