KOH-mediated structural modification of activated charcoal by heat treatment for the efficient adsorption of organic dyes

Modification of the structural features of activated charcoal by KOH-mediated heat treatment is reported. Improved adsorption of rhodamine B (RhB) and methyl orange (MO) is observed, which is dependent on the KOH/precursor ratio. At the optimum ratio of 2:1, the removal of RhB is (92 ± 2)% and that of MO is (82 ± 1)% in 15 min. The influence of contact time, adsorbent dose, adsorbate concentration and temperature on the adsorption are analyzed. The kinetics and equilibrium studies are also carried out. The process follows pseudo-second-order kinetics for both the dyes. The Langmuir isotherm results in the best linear fit with a monolayer adsorption capacity of 312.5 mg g^-1 in the case of RhB adsorption and 188.7 mg g^-1 for MO adsorption. Thermodynamic studies reveal the predominance of chemisorption in the removal of RhB, whereas in the case of MO, physisorption is favoured. The versatility in removing various organic dyes is evident from the ∼100% removal of methylene blue (MB), malachite green (MG), crystal violet (CV) dyes as well as a 1:1 mixture of RhB and MO. The structure and chemical composition before and after activation is analyzed. The activation results in a significant improvement in specific surface area from 787.9 to 975.4 m² g^-1 and an enhancement in the number of nanopores. The mechanism of adsorption is discussed in terms of the improved specific surface area, surface functionalization and the resultant adsorbent-adsorbate interactions.