Chromium (Ⅵ) and zinc (Ⅱ) waste water co-treatment by forming layered double hydroxides: Mechanism discussion via two different processes and application in real plating water

摘要

Two processes, adsorption after synthesis (AAS) and adsorption during synthesis (ADS) were compared in CrO_4~(2-) and Zn~(2+)/CrO_4~(2-) removal. Kinetic results showed that ADS was a better method than AAS, since Cr content was 0.65/0.81 mmol/g in Cr-ADS/ZnCr-ADS, but it was only 0.37/0.56 mmol/g in Cr-AAS/ZnCr-AAS. Then, a low-cost mixture was proposed to function as ADS raw materials in treating real plating waters. This mixture first got an isothermal saturation of 1.1 mmol/g in simulated CrO_4~(2-) water. When Zn~(2+) was co-treated, it was increased to 1.3 mmol/g. At the same time, a Zn~(2+) removal of 1.5 mmol/g was attained. Furthermore, real plating water co-treatment reached equilibrium in 6 h and obtained 1.4/0.9 mmol/g for Zn~(2+)/CrO_4~(2-), respectively. According to XRD analysis, this co-treatment enhancement resulted from the formation of Zn and Cr contained layered double hydroxide.