Equilibrium, kinetic, and diffusion models of chromium(VI) removal usingPhragmites australis and Ziziphus spina-christi biomass

In this study, we investigated the modeling of chromium (Cr(VI)) removal using globally available plant biomass:Phragmites australisandZiziphus spina-christi. Biosorption parameters were initial Cr(VI) concentration (50-800 mg L-1), contact time (1-180 min), adsorbent dose (0.25-2.0 g L-1), and pH (2-8) at agitation speed of 100 rpm. Based on the results of batch experiments and modeling, pseudo-second-order model was fitted to the experimental data whereR(2) = 0.99; besides, diffusion model played a significant role in the rate-determining step. Isotherm models were fitted in the order of Langmuir > Freundlich > Temkin models. Maximum adsorption capacities were recorded 21.32 mg g(-1)and 15.55 mg g(-1)forPhragmites australisandZiziphus spina-christi, respectively. Insights into biosorption behavior were determined using Fourier-transform infrared spectra (FT-IR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). SEM-EDX revealed the chromium presence and its accumulation on both biosorbents after the biosorption process. Cr(VI) biosorption mechanism is illustrated and can be related to electrostatic interactions, reduction and chelation/complexation with the functional groups of both adsorbents.