Removal of Cr(VI) from aqueous solutions by low-cost biosorbents: marine macroalgae and agricultural by-products

In this study, adsorption of Cr(VI) onto the four low-cost biosorbents (Laminaria japonica, P. yezoensis Ueda, rice bran and wheat bran) was investigated depending on solution pH, contact time, adsorbent concentration and adsorption isotherms by employing batch adsorption technique. The adsorption capacities were significantly influenced by solution pH, with lower pH favoring higher Cr(VI) removal for various biosorbents. The ionic strength of NaCl was also observed to have a significant impact on the Cr(VI) adsorption due to the competition of Cl(-) in the aqueous solutions. The batch equilibrium data were correlated to Langmuir and Freundlich isotherms and the data fitted better to the Freundlich isotherm equation. The apparent thermodynamic parameters were calculated for each of the four biosorbents and the obtained numerical values showed that the Cr(VI) adsorption onto the various low-cost biosorbents is spontaneous, entropy-driven and endothermic processes. The batch kinetic data were correlated to the pseudo-first order and pseudo-second order models and the data fitted better to the pseudo-second order equation. An intraparticle diffusion model was applied to investigate the adsorption mechanisms. The adsorption capacities for various biosorbents studied in this work were inversely proportional to the adsorbent concentrations.