Development of low-cost adsorbents from coconut shell for energy-efficient dye removal from laboratory effluent discharge

Abstract

Laboratory effluents containing dyes, particularly methylene blue (MB) impact human health negatively and the need to eliminate it from wastewaters using low-cost adsorbent such as coconut shell activated carbon (CS-AC) is paramount. Sodium hydroxide (NaOH) was used to activate the conversion of CS to AC, which was subsequently employed to adsorb MB from laboratory effluent. Preliminary optimization study led to the selection of 0.2g CS-AC dosage, pH of 6, 303K temperature, 45 min contact time, 100 mg/L initial MB concentration and a 0.5 L solution, which gives a maximum adsorption capacity ($q_{\max }$) of 153.765 mg/g and 62.285 % removal efficiency (RE). A suite of isotherm (Langmuir, Freundlich, Temkin, Halsey, and Dubinin–Radushkevich) and kinetic (Pseudo-First Order, Pseudo-Second Order, Elovich, Intraparticle Diffusion, Boyd, Bhattacharya-Venkobachar, and Natarajan–Khalaf) models were applied using both graphical (GT) and nonlinear regression techniques (NRT) using LAB Fit V7.2.50 software. The Langmuir model, using NRT, gave a valid separation factor ($R_L$ = 0.215) and a high maximum capacity ($q_e$ = 172.12 mg/g), confirming favorable monolayer adsorption. Freundlich model yielded a realistic adsorption intensity (n = 2.7) with $K_f$ = 56.48 L/g, suggesting heterogeneous adsorption, although GT estimates were physically inconsistent. Temkin and Halsey models also showed good physical reliability under NRT with adsorption energies of 1.485 kJ/mol and exponent $n_h$ = 3.06, respectively. Dubinin–Radushkevich model supported a physisorption mechanism with a low energy (E = 0.047 kJ/mol) from GT and a consistent $q_e$ of 161.03 mg/g from NRT. Pseudo-Second Order is the best kinetic model fit – reflecting chemisorption at high $q_e$ and moderate ‘h’. FTIR analysis confirmed the presence of oxygen-containing functional groups and the absence of nitrogen functionalities, which may explain the moderate adsorption performance. SEM images revealed significant morphological changes post-adsorption, including pore blockage and surface smoothing. A general assessment show that NRT estimates proved more reliable, indicating that CS-AC exhibits a predominantly physical, favorable, and heterogeneous adsorption behavior toward MB. This study confirms CS-AC's potential as a cost-effective adsorbent for dye-contaminated laboratory wastewater and recommends further surface engineering to enhance adsorption performance. Water is in constant use in the laboratory, whose recycle or treated discharge will safeguard the environment.