Polyester waste activated carbon assisted micropollutant methylene blue and Congo red dyes removal from wastewater: RSM modeling, kinetics, mechanism, and reusability study

Abstract

This study explores the synthesis of activated carbon (AC) from polyester waste using zinc chloride (ZnCl2) as a chemical activator, designed to efficiently remove both positive methylene blue (MB) and negatively charged Congo red (CR) dyes from wastewater. Response Surface Methodology integrated with Central Composite Design was employed to optimize critical parameters, including adsorbent dosage, solution pH, dye concentration, and temperature, for enhanced dye removal. The results showed remarkable removal efficiencies of 100 % for MB and 99 % for CR under optimal conditions. The kinetic analysis showed that the pseudo-second-order model best described the adsorption process because of chemisorption, which includes ion exchange, complexation, π–π stacking, electrostatic interaction, and hydrogen bonding, while intra-particle diffusion was a key factor in the rate-limiting step. The results suggest MB followed the Langmuir isotherm, while the CR adsorption favored both Langmuir and Freundlich models. Thermodynamic studies revealed distinct adsorption behaviors: MB adsorption was exothermic, while CR adsorption was endothermic. Additionally, reusability tests confirmed the material's ability to maintain high performance over five adsorption–desorption cycles with no significant structural changes. These findings underscore the potential of polyester waste-derived AC as a sustainable and effective material for wastewater treatment.