Optimisation of chromium adsorption by the Box-Behnken design method: Isothermal and kinetic study on biochar and activated carbon

The contamination of water systems by heavy metals, particularly hexavalent chromium (Cr(VI)), represents a major environmental and public health challenge.Chromium is extensively used in various industries, making its removal from wastewater crucial.This study investigates the potential of biochar (ArS BC) and activated carbon (ArS AC) derived from argan shell biomass (ArS) as sustainable adsorbents for Cr(VI) removal.ArS BC was prepared by pyrolyzing ArS at 500 C for one hour, while ArS AC was produced through chemical activation with KOH (1:4 ratio) and thermal activation at 700 C for one hour.The materials were characterized using FTIR, XRD, pHpzc, TGA, and BET techniques to evaluate their physicochemical properties and recycling potential.The Box-Behnken Design (BBD) within the response surface methodology (RSM) framework was applied to optimize operational parameters, including pH, adsorbent dosage, and initial Cr(VI) concentration.The optimal conditions for ArS BC were found to be pH 3, a dosage of 1 g/L, and an initial Cr(VI) concentration of 10 mg/L, resulting in a maximum adsorption capacity of 48.77%.For ArS AC, the optimal conditions were pH 3, a dosage of 0.6 g/L, and an initial Cr(VI) concentration of 50 mg/L, yielding a significantly higher maximum adsorption capacity of 91.34%.Adsorption followed the Langmuir isotherm model, with R values of 0.99 for ArS BC and 0.999 for ArS AC.The maximum adsorption capacities were 47.62 mg/g for ArS BC and 333.33 mg/g for ArS AC.Adsorption efficiency was highly influenced by pH, and desorption of Cr(VI) using acidic reagents proved challenging.ArS AC demonstrated superior stability across multiple adsorption cycles compared to ArS BC.This study highlights the promising potential of utilizing locally available argan shell waste as an efficient, cost-effective, and environmentally sustainable material for the removal of hexavalent chromium from contaminated water, offering a viable solution for water purification in regions affected by industrial pollution.