Hexavalent chromium (Cr(VI)) in industrial effluents remains a critical environmental issue owing to its high toxicity, mobility, and persistence. In this work, a sustainable adsorbent was developed by encapsulating wheat straw-derived activated carbon (ACW), obtained via chemical activation and vacuum pyrolysis, within a Ca 2+ -crosslinked sodium alginate matrix to produce alginate-activated carbon composite beads (BAA). SEM-EDS, BET, FTIR, XRD, and Raman characterizations confirmed a porous, defect-rich structure with abundant oxygen-containing functional groups. Process optimization using response surface methodology identified optimal conditions at pH = 2, initial Cr(VI) concentration of 5.78 mg L −1 , and adsorbent dose of 0.75 g, with a predicted removal efficiency of 93%, in good agreement with the experimentally obtained value of 95%. Non-linear regression of the original kinetic and equilibrium data showed that pseudo-second-order kinetics ( R 2 = 0.9949) and the Langmuir isotherm ( Qₘ ≈ 10.9 mg g −1 , R 2 = 0.9947) were the best-fitting models. The concurrent satisfactory fit of the Freundlich model suggests that surface heterogeneity may also contribute to the adsorption process. The strong pH dependence of Cr(VI) removal (95% at pH 2 vs. 42% at pH 10), combined with pHpzc determination (6.73) and Cr(VI) speciation, suggests that electrostatic interactions between protonated surface groups and HCrO₄ − species play a dominant role in the adsorption process, while ion exchange and possible redox interactions may also contribute. Thermodynamic parameters indicated a spontaneous and exothermic adsorption process. Furthermore, BAA beads retained more than 81% of their initial removal efficiency over five regeneration cycles, highlighting their potential as reusable and bio-based adsorbents for Cr(VI)-contaminated wastewater treatment.
Benhadj et al. (Fri,) studied this question.