Mass transport process for the adsorption of Cr(VI) onto water-insoluble cationic starch synthetic polymers in aqueous systems

Abstract

Dynamic adsorption behaviors between Cr(VI) ion and water-insoluble amphoteric starches was investigated. It was found that the HCrO 4 - ion predominates over the initial pH ∼ 2-4, the CrO 4 -2 ion predominates over the initial pH ∼ 10-12, and both ions coexist over the initial pH ∼ 6-8. The sorption process occurs in two stages: the external mass transport process occurs in the early stage and the intraparticle diffusion process occurs in the long-term stage. The diffusion coefficient of the early stage (D 1 ) is larger than that of the long-term stage (D 2 ) for the initial pH 4 and pH 10. The diffusion rate of HCrO 4 ion is faster than that of CrO -2 4 ion for both processes. The D 1 and D 2 values are ∼ 1.38 × 10 7 -10.1 X 10 -7 and ∼ 0.41 × 10 7 -1.60 × 10 -7 cm 2 s -1 , respectively. The ion diffusion rate in both processes is concentration dependent and decreases with increasing initial concentration. The diffusion rate of HCrO 4 ion is more concentration dependent than that of CrO 4 -2 ion for the external mass transport process. In the intraparticle diffusion process, the concentration dependence of the diffusion rate of HCrO 4 and CrO 4 2 ions is about the same. The external mass transport and intraparticle diffusion processes are endothermic and exothermic, respectively, for the initial pH 4 and pH 10. The k d values of the external mass transport and intraparticle diffusion processes are ∼ 15.20-30.45 and ∼ -3.53 to -12.67 kJ mol -1 , respectively. The diffusion rate of HCrO - 4 ion is more temperature dependent than that of CrO -2 4 ion for both processes.