Minimizing Adsorbent and Time Requirements Using Multi-Stage Batch Adsorption for Malachite Green Removal Using Date Pit Derived Activated Carbons

Water scarcity is one of the main challenges faced in Qatar and around the world. Hard lignocellulose-containing and highly carbonaceous material such as date pits are excellent precursors to produce activated carbons with high adsorption capacities for water treatment activities. Several studies targeted the adsorption of different effluents onto date pits and date pit derived activated carbons. However, these adsorption studies did not consider all the factors affecting adsorption. The kinetics of adsorption was rarely reported, and the maximum adsorption capacities reported do not reflect the correct design adsorption capacity since real industrial discharge concentrations are likely to be on a much lower scale. The widely used toxic dye malachite green poses a significant risk to humans and the environment. In his study, waste date stones were characterized by particle size, SEM, TGA, pyrolysis and char yield, and elemental analysis to prove their suitability to be utilized to produce activated carbons. Drying applications and modeling onto 6 drying models showed negligible difference due to the shape and storage of date pits while the logarithmic drying model had the best fit to experimental data. A novel design selection criteria approach for selecting the correct design adsorption of malachite green onto four different types of date pit derived adsorbents was applied. Kinetic results were modeled with eight kinetic models by applying four error minimization methods. Kinetic models’ parameters were compared using AIC, AICc, BIC, and log-likelihood data. The Avrami model showed the best fit when it was applied with a best fit exponential order of reaction, but the model’s residuals showed bias. Other quality-of-fit contestants were the pseudo-first order and pseudo-second order models at lower concentrations. The pseudo-second order kinetic model was used to compare a single and two-stage adsorber for minimizing the adsorbent masses and the contact time. Other factors affecting adsorption were studied such as the pH and the solution temperature of the system. Thermodynamics studies showed that the reaction is spontaneous at lower temperatures and favorable. Adsorbent regeneration using hydrochloric acid proved to be possible with high percentage removal up to 5 cycles.