Enhanced Cr(VI) Adsorption on Nitrogen and Sulfur Functionalized Granular Activated Carbon

Abstract

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 were 1.6% and 1.0% based on XPS analysis, and 0.8% and 3.2% based on CHNS analysis, respectively which indicates a significant difference in surface vs. bulk incorporation of N- and S- groups into GAC. The modified GAC exhibited higher adsorption capacity compared with the unmodified GAC across all tested pH levels and ionic strengths with highest value of 23.2 mg/g at pH 3, compared with 20.5 mg/g for the unmodified GAC. From analysis of the isotherm, Cr was found to adsorb as a monolayer on a heterogenous GAC surface, whereas on the N/S functionalized GAC, Cr adsorption follows a hybrid mechanism combining sorption and surface chemical reactions. 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. 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 faster 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 an acidic regeneration using HCl solution (<40% for GAC and <25% for N/S-GAC).