Doum fiber-derived activated carbon via H₃PO₄-assisted pyrolysis for sulfamethoxazole removal: Box-Behnken Design optimization and mechanistic approach

The widespread presence of pharmaceutical pollutants such as sulfamethoxazole (SMX) in water bodies has become an increasingly pressing environmental issue, largely due to their persistence and potential risks to human and ecological health. Traditional water treatment technologies often fall short in removing these contaminants efficiently, underscoring the need for innovative, sustainable solutions. In this study, we explore the use of an eco-friendly mesoporous activated carbon derived from Doum fiber waste (Chamaerops humilis) for the effective removal of SMX from aqueous environments. The material was produced through chemical activation using phosphoric acid (H₃PO₄) and pyrolysis, resulting in a porous adsorbent with a high surface area of 768 m²/g. Characterization of the raw Doum fiber revealed it to be rich in lignocellulosic components, with cellulose, hemicellulose, and lignin contents of 31 wt%, 25 wt%, and 17.8 wt%, respectively. The structural and surface features of the synthesized carbon were thoroughly examined using techniques such as FTIR, XRD, SEM, TGA, and BET analysis. To systematically optimize the adsorption conditions, we employed Response Surface Methodology (RSM) coupled with Box-Behnken Design (BBD), investigating the effects of four key factors: adsorbent dose (5–35 mg), SMX concentration (5–45 mg/L), solution pH (2−12), and contact time (10–120 min). Moreover, the adsorption mechanism was investigated at the molecular level through Density Functional Theory (DFT) calculations, offering detailed insights into the interaction between SMX molecules and the activated carbon surface. Experimental results indicated that the adsorption followed the Langmuir isotherm model, suggesting monolayer adsorption, with a maximum capacity of 77.02 mg/g. Kinetic data best fit the pseudo-second-order model (R² = 0.99, χ² = 0.22), implying that chemisorption is the rate-limiting step. Thermodynamic parameters confirmed that the process is spontaneous and exothermic. Under optimized conditions (25 mg adsorbent, 29 mg/L SMX, pH 9, and 107 min of contact time), SMX removal reached 96.12 %. The adsorption was primarily governed by electrostatic attractions, π–π interactions, hydrogen bonding, and pore-filling mechanisms, particularly effective under acidic to neutral pH levels. To the best of our knowledge, this work is the first to demonstrate the potential of phosphoric acid-activated Doum fiber carbon for SMX removal, highlighting its promise as a sustainable, high-efficiency material for water purification applications.