Adsorption of Anionic and Cationic Dyes Using Functionalized ZnCl₂-Activated Charcoal Derived from Coconut Shells

Abstract

Synthetic dyes, both cationic and anionic, are among the primary pollutants impacting aquatic systems. This work explores the adsorption performance of nitric acid–modified ZnCl₂-activated carbon obtained from coconut shells for the elimination of Methylene Blue (MB) and Congo Red (CR) dyes from water. The adsorbent was characterized using Fourier Transform Infrared (FTIR) spectroscopy and Brunauer–Emmett–Teller (BET) surface area analysis. Batch adsorption tests were carried out to determine the effects of contact time, initial dye concentration, adsorbent dosage, and pH on dye removal efficiency. Equilibrium was reached within 30 minutes, and the adsorption data fit both Langmuir and Freundlich models, with the Langmuir isotherm providing a slightly better correlation. Activation of the ordinary carbonised coconut shell (OCCNS) with phosphoric acid increases its specific surface area from 135.5 to 435.98m²/g and pore volume from 0.055 to 0.154cm/g. Post treatment of the activated charcoal with nitric acid further increases its specific surface area to 810.44m²/g and pore volume of 0.296cm/g which improves the adsorption capacity of the adsorbents. Surface modification significantly enhanced both the surface area and adsorption capacity; the highest monolayer adsorption capacities for MB and CR were 6.33 mg/g and 7.04 mg/g, correspondingly. The functionalized activated carbon showed high potential as a cost-effective and environmentally sustainable adsorbent for the purification of dye-laden wastewater due to its amphoteric surface behavior.