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 ion predominates over the initial pH ∼ 2–4, the CrO 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 (D1) is larger than that of the long-term stage (D2) for the initial pH 4 and pH 10. The diffusion rate of HCrO ion is faster than that of CrO ion for both processes. The D1 and D2 values are ∼ 1.38 × 10−7–10.1 × 10−7 and ∼ 0.41 × 10−7–1.60 × 10−7 cm2 s−1, respectively. The ion diffusion rate in both processes is concentration dependent and decreases with increasing initial concentration. The diffusion rate of HCrO ion is more concentration dependent than that of CrO ion for the external mass transport process. In the intraparticle diffusion process, the concentration dependence of the diffusion rate of HCrO and CrO 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 kd 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 ion is more temperature dependent than that of CrO ion for both processes. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2409–2418, 1999