Coconut shell hookah charcoal ash for methylene blue and malachite green adsorption

Introduction: Coconut shell-derived hookah charcoal ash (CSHC-ash) has emerged as a waste product with promise as an inexpensive adsorbent for removing cationic dyes from water. This research assessed the physicochemical properties and adsorption efficiency of CSHC-ash specifically for eliminating methylene blue (MB) and malachite green (MG).

Materials and methods: Coconut shell-derived hookah charcoal was utilized after its use in a hookah, and the resultant ash was collected and sieved with a 125 µm sieve, avoiding any washing or chemical activation. Batch adsorption experiments were conducted with 10 mL dye solutions across pH levels from 3 to 10. Initial dye concentrations ranged from 20 to 200 mg L ⁻¹ , with adsorbent dosages varying between 0.01 and 0.15 g. Contact times were set between 15 and 270 min, and temperatures were maintained between 25 and 55 °C. To evaluate the adsorption behavior, Langmuir, Freundlich, and Temkin isotherms were applied, alongside pseudo-first-order and pseudo-second-order kinetic models and thermodynamic parameters. The adsorbent underwent characterization using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, and determination of the point of zero charge (pHpzc).

Results: The greatest removal efficiencies were achieved at pH 8.0, with 79.96% for MB and 95.11% for MG. An optimal adsorbent dosage of 0.01 g per 10 mL, equating to 1.0 g L ⁻¹ , was determined. Equilibrium times were approximately 180 min for MB and 120 min for MG. The Langmuir model was the most suitable among the evaluated isotherm models, showing maximum adsorption capacities of 243.31 mg g ⁻¹ for MB and 239.81 mg g ⁻¹ for MG. The kinetic data aligned more accurately with the pseudo-second-order model than with the pseudo-first-order model. Thermodynamic analysis revealed negative standard Gibbs free energy (ΔG°) values along with positive standard enthalpy (ΔH°) and standard entropy (ΔS°) over the tested temperature range. Characterization results showed a heterogeneous and irregular surface, featuring both crystalline and less-ordered elements, alongside a limited BET surface area of 5.414 m ² g ⁻¹ .

Conclusions: CSHC-ash displayed notable adsorption capacities for MB and MG under the tested experimental conditions. The equilibrium, kinetic, and thermodynamic analyses suggest that the adsorption process is both favorable and endothermic. Despite this, the characterization and adsorption results do not point to a singular adsorption mechanism. These findings highlight the potential of using post-consumption hookah charcoal ash as an adsorbent and lay the groundwork for its application in removing dyes from water.