Magnetic nanoparticles loaded hydrochar for effective Cr(VI) removal from water: Batch and column studies

Chromium considered as most toxic trace metal and is essential to remove it from aqueous using influential adsorbents. Hydrochar prepared from mango peels biomass by thermal activation process was applied to produce activated hydrochr (ACH) which further prepared magnetic nano-composite activated hydrochar (Fe₃O₄-ACH) material as an effectual sorbent and tested for adsorption of Cr(VI) ions. SEM/EDS, BET, FTIR, VSM, and XRD instruments were utilized to evaluate the characteristics of produced Fe₃O₄-ACH. The obtained results revealed that the nanoparticle modification increased the surface area and improved the adsorption ability of ACH. A vibrating sample magnetometer was applied for magnetic properties in which the external magnetic field responded by saturation magnetization leading to quickly adsorbed metal ions from water. The factorial design studies were achieved to optimize the removal conditions of metal ions. Equilibrium data for isotherms (Langmuir isotherm) and kinetic mechanisms (pseudo second order) well fitted with the models. The obtained adsorption capacity for Langmuir isotherm was 174.7 mg/g under optimized conditions (dose 0.03 g, contact time 120 min, Cr(VI) solution 50 mg/L). Practical applications of real water samples were studied by fixed-bed column adsorption. The obtained results revealed that the synthesized Fe₃O₄-ACH was promising for Cr(VI) removal with adsorption capacity of 60.24 mg/g at optimized conditions; bed height (1.3 cm), Cr(VI) ions concentration (50 mg/L) and flow rate (12 mL/min). The experimental breakthrough curve results were compared in a fixed-bed column and were successfully fitted with kinetic models. The given results of distribution and selectivity coefficients proven that Fe₃O₄-ACH composite has a greater adsorption selectivity than adsorption of Cr(VI). Furthermore, computational molecular modeling was performed to evaluate the interaction types between Cr(VI), Fe₃O₄, and ACH. The Cr(VI) adsorption mainly carried hydrogen bonding, π–π, hydrophobic and electrostatic interactions. Desorption of chromium ions from Fe₃O₄-ACH composite adsorbent was more effective with 94.1% desorption efficiency. An approximate cost analysis of 3.15 USD/Kg in a week was assumed for activated hydrochar which was cheaper than the commercially available activated carbon which cost 20 USD/Kg. The obtained results indicated that the produced Fe₃O₄-ACH composite is a capable adsorbent to decontaminate Cr(VI) from real water samples.