The feasibility of utilizing an abundant agricultural waste (desert date seed shell) as an alternative low-cost adsorbent for the removal of hazardous basic dyes [crystal violet (CV) and malachite green (MG)] and hexavalent chromium [Cr(VI)] from synthetic industrial effluent was investigated. Five different adsorbents including the raw, carbonized and chemically activated carbons were prepared and screened with respect to adsorption efficiency of the chosen adsorbates. The prepared adsorbents were characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and pH of zero point charge (pHzpc) analyses. The effects of operational variables such as solution pH, contact time and temperature on adsorption have been investigated. The removal of the adsorbates was found to be highly pH-dependent and the optimum pH was determined as 8.0 for the dyes and 2.0 for hexavalent chromium. The screening results revealed that the NaOH activated carbon (NAC) has the best adsorption characteristics with removal efficiencies of 91.10, 99.15 and 91.5 % for CV, MG and Cr(VI), respectively. The process dynamics was evaluated by pseudo-first-order and pseudo-second-order kinetic models. Experimental data have been found to be well in line with the pseudo-second-order model, suggesting therefore, a chemically-based sorption process. Negative Gibbs free energy change (∆G) values obtained from thermodynamic analysis indicated that the adsorption process was spontaneous and had a high feasibility. Positive values for enthalpy change (∆H) showed that the removal process was endothermic, implying that the amount of adsorbate adsorbed increased with increasing reaction temperatures. Additionally, positive values of entropy change (∆S) reflect the high affinity of the adsorbent material to the adsorbates. On the basis of results and their analyses, it has been established that adsorbent derived from desert date seed shell has a promising potential in environmental applications such as removing hazardous substances from industrial effluents. Through this work, it is believed that contributions are provided to the scientific investigations about the decontamination of precious water resources.

The removal of hexavalent chromium [Cr(VI)] is a challenging task due to its acute toxicity even at low concentration. In the present study, a low-cost activated carbon (LAC) was prepared from desert date seed shell by chemical activation with H3PO4 and utilized for the removal of hexavalent chromium from aqueous solution. Batch experiments were conducted to investigate the influence of operating variables such as pH, contact time, adsorbent dosage, initial concentration, co-existing ions and temperature. The amount of Cr(VI) adsorbed was found to vary with solution pH and maximum adsorption was observed at a pH value of 2.0. The extent of chromium uptake (mg g-1) was found to increase with increase in initial concentration and contact time. The applicability of the four isotherm models for the present equilibrium data follows the sequence: Freundlich > Temkin > Langmuir > Dubinin-Radushkevich. The mean free energy from the Duninin-Radushkevic isotherm model hinted that the adsorption of Cr(VI) onto the adsorbent surface follows physisorption mechanism. Thermodynamic parameters related to adsorption, Gibbs free energy change (∆G°), enthalpy change (∆H°), entropy change (∆S°), were also calculated and the negative value of ∆H° indicates the exothermic nature of the adsorption process. The considerable adsorption capacity of 99.09 mg g-1 is a signifier of the suitability of the prepared adsorbent for commercial application. The findings implicated that the adsorbent can be employed in the treatment of Cr-bearing water and wastewater.