Microwave-assisted synthesis of mesoporous silica nanoparticles from natural sand for efficient solar photocatalytic degradation of Rhodamine B

Photocatalysis is an emerging and sustainable wastewater treatment technology that utilizes light and semiconductor catalysts to degrade hazardous organic pollutants into harmless compounds. Among various photocatalysts, silica nanoparticles (SiNPs) have shown promising potential due to their chemical stability, ecofriendly nature, and tunable surface properties. In this study, SiNPs were sustainably synthesized from natural sand via an energy-efficient alkaline fusion method assisted by microwave irradiation. This approach eliminated the need for prior acid leaching while significantly reducing synthesis time and energy consumption compared to conventional thermal fusion. As a result, S i O 2 purity was upgraded from 7 7 . 5 2 % in the raw sand to 9 4 . 3 4 % in the S i N P s , leaving A l 2 ⁢ O 3 ( 3 . 8 2 % ) as the only detected metallic impurity. XRD and FTIR confirm that the as-prepared SiNPs retain a hybrid structure combining residual crystalline quartz and cristobalite domains within a predominant amorphous silica matrix (S BET = 65.48 m 2 /g), together with abundant surface silanol groups. The material exhibits a reduced optical band gap of approximately 3.55 eV, a value situated within the UV region and therefore mainly responsible for near-UV absorption; defect states associated with the amorphous fraction are proposed to extend light harvesting toward the near-visible edge. Under simulated solar irradiation (pH = 3), 20 mg of SiNPs achieved complete (100%) removal of 4.79 mg/L Rhodamine B within 30 min via a surface-mediated pseudo-first-order kinetic process (k 1 = 0.080 min −1 , R 2 = 0.931). Scavenger experiments confirmed photogenerated holes as the primary active species driving direct oxidation. The catalyst exhibited excellent stability and recyclability over five degradation cycles, underscoring its suitability for practical environmental remediation. These findings highlight the potential of microwave-assisted synthesized SiNPs as economically viable, environmentally friendly photocatalysts for wastewater treatment, with future work targeting effectiveness in real wastewater matrices and scale-up optimization.