Acid-base synergistic activation of coal gasification fine slag into hierarchical porous carbon for enhanced Cr(vi) adsorption and reduction

Abstract

A sequential acid–alkali activation strategy was developed to convert coal gasification fine slag (CGFS) into a hierarchical porous carbon (FC) for efficient Cr(VI) removal. During the process, mineral ash was removed by acid leaching to expose encapsulated carbon domains, while subsequent alkali activation disrupted silicate frameworks and reconstructed interconnected micro–meso–macroporous networks, producing a porous carbon with a high specific surface area of 630.3 m² g⁻¹. Benefiting from the optimized pore architecture and heteroatom-enriched surface chemistry, FC exhibited excellent affinity toward Cr(VI), achieving a maximum adsorption capacity of 179.9 mg g⁻¹ at 318 K and pH 2. Adsorption kinetics and isotherm analyses indicated monolayer adsorption behavior, with chemisorption serving as the rate-limiting step, while thermodynamic parameters suggested that the process was spontaneous and endothermic. Coexisting anions imposed varying degrees of competitive inhibition on Cr(VI) adsorption in the order of SO₄²⁻ > HPO₄²⁻ > CO₃²⁻ > NO₃⁻ > Cl⁻; nevertheless, considerable removal efficiency was maintained in multi-ion systems, and 63.32% of the initial performance was retained after five regeneration cycles. Mechanistic analyses revealed that Cr(VI) removal was governed by synergistic processes involving pore-filling enrichment, electrostatic attraction and hydrogen bonding, as well as surface redox reactions and complexation. These findings demonstrate the feasibility of valorizing CGFS as a low-cost precursor for advanced adsorbents in heavy-metal wastewater remediation.