Sustainable Adsorbent Material Prepared by Soft Alkaline Activation of Spent Coffee Grounds: Characterisation and Adsorption Mechanism of Methylene Blue from Aqueous Solutions

Dyes are emerging as hazardous pollutants, which are the primary challenges for environmentalists. Dye removal from effluents is urgently needed. Adsorption technology has been widely employed in this context as an effective method for removing colours from the aqueous phase, and adsorption with the use of low-cost adsorbents has been shown to be more successful on a larger scale than other methods. In this study, spent coffee grounds (SCGs) were used as the precursor for the preparation of a low-cost activated carbon through the chemical activation with NaOH. The SCG sample was impregnated with NaOH and carbonised at 300 °C for three hours. Its morphological and physical-chemical properties were assessed using scanning electron microscopy (SEM), X-ray diffraction, and Raman spectroscopy analyses. The performance of the treated SCG as an adsorbent material for methylene blue (MB) was evaluated by analysing the effect of the initial pH ionic strength on the adsorption capacity and by evaluating the kinetics and the mechanisms of the process (using adsorption isotherms). The effect of the initial concentration (500 and 250 mg L −1 ) of MB on the kinetics of the process and the impact of the initial pH (7.5 and 6) on the adsorption isotherm were evaluated. The obtained results show that the pseudo-second order model controls the process for both the investigated initial concentration and the adsorption capacity, which are 142.8 and 113.6 mg L −1 , respectively. The results indicate that the pH value influences the adsorption isotherm model that regulates the process. Specifically, this process is regulated by the Temkin’s model with a pH of 7.5 and by the Langmuir’s model with a pH of 6. The thermodynamics of the process were also determined. The results show that SCG, treated and carbonised by soft alkaline activation, is a promising low-cost adsorbent material as its performance is comparable to that of conventional active carbon materials.