Comparative insight into chemical and nanomaterial functionalization of date stone to enhance adsorption of azo dyes

This work presents a comparative investigation of an environmentally sustainable strategy for the surface functionalization of date stones (DS), evaluating the effects of chemical modification using the diazonium salt 4-aminobenzenediazonium (SD) compared to various nanomaterials, including Fe, Ag, CuO, and Fe 2 O 3 . The aim was to compare their adsorption efficiency toward two representative azo dyes, Azorubine (Az) and Amaranth (Am). The resulting functionalized materials (DS, DS-SD, DS-Ag, DS-Fe, DS-CuO, and DS-Fe 2 O 3 ) were thoroughly characterized by fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric Analysis (TG) and differential scanning calorimetry (DSC), confirming successful surface modification, enhanced thermal behavior, and preserved structural integrity. Adsorption experiments were systematically optimized by assessing the influence of dye concentration, contact time, pH, and temperature. Among all modifications, DS-SD exhibited the highest adsorption performance, achieving up to ∼95% removal of Az and Am within only 10 minutes. To further elucidate the adsorption mechanism, zeta potential analysis, dynamic light scattering (DLS), and Brunauer–Emmett–Teller (BET) measurements were conducted, indicating that the process is mainly driven by electrostatic interactions, hydrogen bonding, and π–π stacking interactions. Kinetic analysis demonstrated that the adsorption process follows the pseudo-second-order model, with adsorption capacities of approximately 16.25 mg·g⁻¹ for Az and 18.13 mg·g⁻¹ for Am. Isotherm modeling revealed that the Langmuir model (R² = 0.98–0.99) provides the best fit for both dyes, indicating monolayer adsorption onto a homogeneous surface. These findings demonstrate that diazonium-based functionalization significantly enhances the surface reactivity of DS and introduces new active adsorption sites, enabling highly efficient and rapid removal of hazardous dyes. This environmentally friendly, low-cost strategy offers strong potential for the development of advanced adsorbents for wastewater treatment.