The chromium(VI) biosorption onto guava seeds, as an alternative method for Cr
6+ removal from aqueous solutions, was investigated. The parameters affecting kinetics and equilibrium of Cr
6+ adsorption onto guava seeds were studied. An external mass-transfer diffusion coefficient
k and intra-particle diffusion coefficient
ki 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 Cr
6+ 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
R2=0.98. The maximum removal of Cr
6+ was obtained at pH 1 (about 100% for adsorbent dose of 15 g l
-1 and 25 mg l
-1 initial concentration of Cr
6+). The results indicated that the guava seeds exhibit acceptable sorption capacity.
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