Sorption model identification for chromium transport in unconsolidated sediments

The adsorption of hexavalent chromium Cr(VI) onto six different natural unconsolidated sediments (two loamy sand, two sandy loam, loam, silty loam clay) was studied using batch and column experiments. Equilibrium adsorption capacities and kinetic rate were calibrated using batch experimental data. Elovich, pseudo first- and second-order models were used to fit the kinetic adsorption data, respectively. Henry’s, Freundlich, and Langmuir isotherms were used to fit the equilibrium adsorption data. Four model selection criteria, Akaike information criterion (AIC), modified Akaike information criterion (AICc), Bayesian information criterion (BIC), and Hannan information criterion (HIC) were used to discriminate kinetic and equilibrium models. These criteria suggest that the selected optimal model depends on the sediment type. Specifically, we studied effects of different factors including pH, solid/solution ratio, particle size, and clay mineral content on adsorption capacities. Column experiments were performed and a deterministic equilibrium model as well as a chemical non-equilibrium model were applied to fit the breakthrough curves. Results revealed a high retention of Cr(VI) in sandy loam, loam and silty loam clay, and a high mobility in loamy sand. It was found that particle size and clay minerals played an important role in adsorption process. The results from this study provide important insight for us to understand the transport behaviors of Cr(VI) in porous media.