Treatment Strategy for Cr(VI)-Bearing Wastes

The effects of reductants and alkali on the sludge quantity of chromium wastes and the teachability and physical properties of solidified chromium matrix are investigated in this study. FeSO₄ and NaHSO₃ are model reductants, and NaOH and Ca(OH)₂ serve as bases, respectively. Portland cement was used to solidify Cr sludges generated from real wastes. Results from potentiometric redox titration showed that a 99.9% destruction of Cr(VI) at equivalence point is not achievable unless additional dosage is imposed. The extra amounts of reducing agent necessary for 99.9% conversion of Cr(VI) to Cr(III) is dependent on system parameters such as pH and type of reductants applied. An increase in operating pH where redox proceeds may acquire much more reducing chemicals than lower pH does. The optimum pH range for maximum precipitation is from 8.5 to 9.0. Chromium sludges generated from synthetic wastewater have a specific resistance from 8.7×l0¹¹ to 1.28×l0¹³ m/kg. The use of Ca(OH)₂ as base to precipitate Cr(III) produces sludge with lower specific resistance and larger particle size (near 20 to 60 µm). When examined with TCLP test, the chromium leachated out to the solution from matrix is generally less than 1 mg/L. Compressive strength of solidified sludge can reach near 10 kg/cm² in 3 days based on 1:1 sludge/cement ratio. The compressive strength is affected by the type of reductants as well as the bases employed in the wastewater treatment processes. To producc a matrix with better compressive strength, it is recommended to use FeSO₄ along with Ca(OH)₂; however, if NaOH is used to control system pH, NaHSO₃ is suggested to serve as reductant.