Bioadsorbents for the Removal of Pollutants in Wastewater: Adsorption Kinetics, Validation Test Using Methylene Blue and Methyl Orange

The presence of emerging contaminants in water has led to a need for the development of new materials and treatments. Four low-cost adsorbents derived from lignocellulosic biomass waste (pine nut shells and olive stones) were prepared via chemical treatment (with H3PO4 or NaOH) followed by thermal activation (at 550 °C under N2). Characterization of the bioadsorbents was carried out using N2 adsorption–desorption isotherms, FTIR and Raman spectroscopic analyses, and pHpzc determination. The electrostatic interactions between the adsorbent surface and the dyes were determined, and it was found that the interactions in both adsorbents were attractive for the methylene blue and repulsive for methyl orange, at pH basic or neutral. The performance of the obtained activated carbons was evaluated at lab scale with two dyes (methylene blue and methyl orange), and a comparison was made between both adsorbents and with commercial charcoal. The H3PO4-activated adsorbents exhibited higher adsorption capacities (up to 300 mg/g for methylene blue and 285 mg/g for methyl orange), with adsorption efficiencies close to 100%. More than 10 adsorption–desorption cycles were performed, with efficiencies exceeding 85%. The good reusability shown by the H3PO4-activated adsorbents suggests significant potential for industrial application; namely, in the removal emerging contaminants from urban wastewater. It should be noted that the adsorption efficiency decreased after the fifth cycle, indicating a gradual reduction in performance over time (although it remained above 85% in the performed experiments). This study aims to achieve the goal of zero waste and contribute to the circular economy through the sustainable use of residual biomass.