Sustainable Valorization of Ornamental Rock Processing Waste for Porous Adsorbent Development: Experimental and Density Functional Theory Analysis

Sandstone and white gravel residues were converted into adsorbents through alkaline fusion using KOH. Successful amorphization increased the porosity and specific surface area of sandstone from 7.07 to 36.74 m 2 g⁻ 1 , yielding an adsorbent, ASA, with markedly enhanced adsorption performance. Four probe cationic/anionic molecules were initially evaluated, with ASA exhibiting the highest adsorption capacity towards Crystal Violet (78.34 mg g⁻ 1 ; 81.24% removal). Response surface methodology optimized adsorption at an adsorbent dosage of 0.8 g L⁻ 1 and pH 9. Equilibrium was reached within 60 min, with a maximum adsorption capacity of 129.11 mg g⁻ 1 at 298 K. Thermodynamic analysis indicated that adsorption is spontaneous, exothermic, and predominantly driven by electrostatic interactions. At the optimum adsorption condition, only Crystal Violet and Methylene Blue remain in their cationic forms, favoring electrostatic attraction toward the negatively charged ASA surface (point of zero charge = 6.4). To rationalize the experimental adsorption behavior, quantum chemical calculations were performed using the predominant ionic forms of the investigated dyes. Although both dyes exhibited favorable electronic properties, Crystal Violet presented a broader and more accessible positive electrostatic potential distribution, intermediate electrophilicity (ω = 15.6 eV), and low chemical hardness (η = 1.48 eV), favoring multiple interactions and better accommodation on the ASA surface, consistent with its superior experimental results. At last, ASA maintained satisfactory performance over four adsorption–desorption cycles. These results demonstrate that alkaline fusion is an effective strategy for converting ornamental stone residues into a promising and reusable adsorbent for dye-contaminated wastewater.