<i>Casuarina equisetifolia</i>-Derived Mesoporous Carbon for Adsorptive Removal of Phenolic Pollutants: Insights into Optimization, Isotherm, Kinetics, Thermodynamics, and Mechanism

Herein, <i>Casuarina equisetifolia</i>-derived activated carbon was developed by pyrolysis at a low temperature (773 K). The derived adsorbent was characterized as an extremely porous and amorphous carbon with a surface area of 1007 m²/g. The point of zero charge (pH_PZC) of activated carbon was obtained as 3.89. The reported carbon was used for the remediation of phenolic pollutants [phenol and 2,4-dichlorophenol (2,4-DCP)]. To begin with, the optimization of parameters demonstrated that a pH of 2 and a temperature of 293 K had significant impacts on the adsorption of both pollutants. The isotherm studies revealed that the Freundlich (<i>R</i> ² = 0.9956) and Langmuir (<i>R</i> ² = 0.9866) models better fitted the adsorption of phenol and 2,4-DCP, respectively. The study also reported an exceptionally elevated monolayer adsorption efficiency of 364.62 and 382.03 mg/g for phenol and 2,4-DCP, respectively. This significantly greater adsorption would aid in the elimination of phenolic pollutants, especially in coal processing industries, which are major contributors to phenolic discharge. Furthermore, kinetic studies revealed that the chemisorption mechanism dominated with <i>R</i> ² > 0.999 for all concentrations ranging between 25 and 200 mg/L. In addition, the thermodynamic behavior of phenol and 2,4-DCP revealed exothermic (Δ<i>H</i> < -26.70 kJ/mol) and feasible type of adsorption. The value of isosteric heat of adsorption corroborated physisorption dominating the entire process with Δ<i>H</i> _x < 25 kJ/mol. The adsorption mechanism aspect of phenol/2,4-DCP suggests that the removal of pollutants followed a combination of physical and chemical adsorption, accompanied by pore diffusion and electrostatic attraction coupled with π-π interaction and hydrogen bonding. The <i>Casuarina equisetifolia</i> activated carbon (CEAC) performed significantly well for four cycles of adsorption-desorption. The broader significance of these findings would yield sustainable production of <i>Casuarina equisetifolia</i>-derived carbon, which is an easily available carbon-rich source, additionally offering a cost-effective alternative solution/replacement for the remediation of phenol and chlorinated phenols from synthetic wastewater.