Equilibrium and Kinetic Study on Chromium Adsorption onto Activated Carbon in Water

Adsorption on activated carbon is one of the most reliable and efficient technologies for the remediation of water polluted by heavy metals. In groundwater remediation, it actually found practical application in both pump-and-treat systems and permeable adsorbing barriers. One of the most significant limits to the practical adsorption of heavy metals in water streams is the absence of experimental data and modeling analysis coupling equilibrium and kinetic properties. In particular, kinetics and equilibrium results of hexavalent chromium adsorption onto activated carbon (AC) are significantly affected by ion speciation, which is made even more complex by the occurrence of reduction reactions on the AC surface. To the best of our knowledge, as at today reliable experimental data describing the mechanisms of chromium adsorption are still lacking as accurate supporting mathematical models for a proper design of industrial adsorbers are. In this work, the results of equilibrium and kinetic tests on the adsorption of chromium on a commercial granular AC are presented. Adsorption isotherms were determined in stirred batch reactors at different levels of pH, temperature and salinity. Kinetic tests were carried out in terms of breakthrough curves of a lab-scale fixed bed column at different levels of pH, inlet concentration and flow rate. Both the adsorption isotherms and the breakthrough curves showed non-linear and unconventional trends as a function of the main process parameters. The experimental results revealed that chromium speciation, in particular Cr(VI) reduction to Cr(III) reactions, played a key role in the adsorption process. Equilibrium tests were interpreted in light of a multi-component Langmuir model supported by the ions speciation analysis. Once the adsorption equilibrium assessed, it was possible to analyze the results of adsorption kinetics, so as to allow determining the mechanism that controls the adsorption rate in the investigated conditions.