Toward sustainable activated carbon from peanut shells for efficient cationic dye removal: equilibrium, kinetics, and...

Transforming abundant agricultural residue into high-performance carbonaceous adsorbents has emerged as a green and cost-effective strategy for the efficient removal of dyes from contaminated effluents. In this study, activated carbon adsorbents (referred to as PNSAC-x) were synthesized from peanut shells (PNS) using phosphoric acid as a cost-effective activating agent. Activation was conducted at a relatively low temperature of 400 ℃, employing PNS: H₃PO₄ weight ratios of 1:1, 1:2, and 1:3. The developed carbons were evaluated for their adsorption performance in removing crystal violet (CV) as a model cationic dye. Among the synthesized samples, PNSAC-3 (1:3 ratio) exhibited superior performance, achieving 98.9% CV removal within only 20 min at natural pH, with an adsorbent dosage of 0.3 g L^- 1 at 25 °C. Comprehensive characterization confirmed that PNSAC-3 possesses an amorphous structure with a well-developed porous network, reaching a maximum adsorption capacity of 145 mg g^-1 under the investigated conditions. Cost analysis demonstrated the economic feasibility of PNSAC-3 production, with an estimated cost as low as 115 LE kg⁻¹ using commercial-grade phosphoric acid. The adsorption kinetics followed the pseudo-second-order model (R² = 0.998), while the equilibrium data were best described by the Langmuir isotherm model (R² = 0.999). Thermodynamic studies indicated a spontaneous and endothermic adsorption process with increased feasibility at elevated temperatures. pH-dependent studies showed that the adsorption process involved both electrostatic forces and non-electrostatic interactions, including hydrogen bonding, hydrophobic effects, and π-π interactions. PNSAC-3 also demonstrated excellent reusability, retaining more than 94.4% of its adsorption efficiency after five adsorption-desorption cycles. These findings underscore the potential of PNSAC-3 as a highly efficient and cost-competitive adsorbent for the large-scale removal of cationic dyes from industrial wastewater.