Structure-Dependent Adsorption of Synthetic and Natural Dyes onto Commercial Activated Carbon: Kinetic, Diffusion, and Isotherm Modelling

This study systematically investigates the structure-dependent adsorption of three structurally distinct dyes, Curcumin, Allura red, and Congo red onto commercial activated carbon. Batch adsorption experiments were performed to determine the effects of pH, contact time, initial dye concentration, and adsorbent dosage. The kinetic data obtained were fitted to Pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion models, while equilibrium data were studied using Langmuir, Freundlich, Temkin, and linear isotherm models. PSO model best described the adsorption of CUR and CR (R² = 0.9994 and 0.9995, respectively), indicating chemisorption-controlled processes, while AR followed PFO kinetics (R² = 0.9737), suggesting a predominantly physisorption-driven mechanism. Intraparticle diffusion studies proved that adsorption occurred through a multi-step process involving both boundary layer diffusion and pore diffusion, with diffusion not being the sole rate-limiting step. CUR adsorption conformed to the Langmuir isotherm (R² = 0.9972), indicating monolayer adsorption on relatively homogeneous sites. Conversely, AR and CR were better described by the Freundlich model with R² = 0.9915 and 0.9960, respectively, reflecting heterogeneous surface adsorption and multilayer formation. Freundlich intensity parameters (0.91 ≤ n ≤ 1.31) and Langmuir separation factors (0 < RL < 1) confirmed favorable adsorption under the conditions investigated. This comparative study demonstrates that the adsorption performance of CAC was strongly influenced by dye molecular structure, with larger and more functionally complex molecules exhibiting stronger surface interactions. Findings provide mechanistic insight into multi-dye adsorption systems and highlight the suitability of CAC in designing targeted removal strategies in wastewater treatment and food processing applications.