Mass-transfer processes of chromium(VI) adsorption onto guava seeds

Abstract

The chromium(VI) biosorption onto guava seeds, as an alternative method for Cr6+ removal from aqueous solutions, was investigated. The parameters affecting kinetics and equilibrium of Cr6+ adsorption onto guava seeds were studied. An external mass-transfer diffusion coefficient k and intra-particle diffusion coefficient k i were determined to measure the rate-limiting step of adsorption. A single external mass-transfer diffusion model and intra-particle diffusion models were used. The effects of initial pH, sorbent mass, and initial Cr6+ concentrations on mass-transfer coefficients were investigated. The external mass-transfer coefficient has an average value of 7.2×10−3 cm s−1, while the intra-particle mass-transfer diffusion coefficient was 0.34 mg g−1 min−0.5. This indicates that external diffusion to the guava seeds surface and intra-particle diffusion are both involved in the sorption process. The isotherm equilibrium data were well fitted by the Langmuir and Freundlich models with an average correlation coefficient R 2=0.98. The maximum removal of Cr6+ was obtained at pH 1 (about 100% for adsorbent dose of 15 g l−1 and 25 mg l−1 initial concentration of Cr6+). The results indicated that the guava seeds exhibit acceptable sorption capacity.