Enhanced Cr(VI) adsorption on nitrogen and sulfur functionalized granular activated carbon

The effect of functionalizing a granular activated carbon (GAC) adsorbent with nitrogen and sulfur groups was investigated, with the aim of enhancing the adsorption capacity for hexavalent chromium (Cr(VI)) from aqueous solutions. The modification of GAC with nitrogen and sulfur groups was achieved in a one pot hydrothermal process using thiourea as the precursor. The amount of nitrogen and sulfur incorporated onto the GAC using this process was 1.6 at% and 1.0 at% based on XPS analysis, and 0.72 at% and 1.22 at% based on CHNS analysis, respectively. The modified GAC exhibited higher adsorption capacity compared with the unmodified GAC across all tested pH levels and ionic strengths with the highest value of 23.2 mg g^-1 at pH 3, compared with 20.5 mg g^-1 for the unmodified GAC. The N/S functionalized GAC favored the reduction of Cr(VI) to Cr(III) during the adsorption process, which is considered to have lower toxicity. XPS and FTIR analyses, together with desorption experiments, suggest that adsorption primarily occurs through hydrogen bonding and reduction-adsorption mechanisms for N/S functionalized GAC. Isotherm analysis revealed that Cr(VI) adsorbs predominantly as a monolayer of HCrO₄^- on the heterogeneous GAC surface (Sips model), whereas N/S-functionalized GAC follows the Freundlich model, indicating heterogeneous adsorption through the formation of Cr(III) hydroxides near functional groups. Kinetic studies indicated that the adsorption follows a revised pseudo-second order kinetic model on both surfaces, where chemisorption is the rate-controlling step. The rate of adsorption is similar for both types of GAC in the initial regime with fast kinetics. However, after 200 min, N/S functionalized GAC exhibits a higher uptake, likely associated with the additional Cr(VI) adsorption through a reduction process. Regeneration experiments revealed that NaOH and KOH solutions achieved significantly higher desorption efficiencies (>90% for unmodified GAC and >70% for N/S functionalized GAC) compared to acidic regeneration using HCl solution (<40% for GAC and <25% for N/S-GAC).