Use of Pyrolysed Almond and Walnut Shells (PAS and PWS) for the Adsorption of Cationic Dye: Reusing Agro-Waste for Sustainable Development

Agro-wastes are recognised as a carbon-rich source, which can be converted into value-added products in sustainable development. In this study, the effect of pH, contact time, initial concentration, and ionic strength were evaluated in Methylene Blue (MB) adsorption by using an activated carbon obtained from pyrolysed almond (PAS) and walnut shells (PWS). The characterisation of PAS and PWS was conducted by SEM-EDX, FT-IR and BET analysis. The removal efficiency of 6 mg/L initial MB concentration improved from 10.6% to 50.42% for PAS, when the adsorbent dose was increased from 0.5 g to 3.5 g in 1 L dye solution. It also improved from 14.8% to 48.7% for PWS, when the adsorbent dose was increased from 0.5 g to 3.5 g. The adsorption fits well with the Freundlich isotherm model and the second-order kinetic model is more favourable. In the adsorption experiments using PWS, 48% removal efficiency was obtained in the absence of NaCl. Depending on the increasing NaCl concentration, the removal efficiencies showed a decrease. 36% removal efficiency was obtained for PWS when 2500 mg/L NaCl was used. In the adsorption experiments using PAS, 40% removal efficiency was obtained in the absence of NaCl. When 500 mg/L NaCl was used, the maximum removal efficiency improved to 48%. However, with the increase in ionic strength, removal efficiencies decreased to approximately 39%. This study revealed that PAS and PWS could be used effectively instead of commercial activated carbon, which also provides an advantageous option from an economic point of view.