Assessment of the effectiveness of biosilica nanoparticles from wheatstraw in the removal of Per- and Polyfluoroalkyl substances from wastewater.

Abstract

Conventional wastewater treatment processes are ineffective for the complete removal of per- and polyfluoroalkyl substances (PFASs), particularly short-chain compounds, necessitating the development of sustainable and efficient adsorbents. Wheat straw, an abundant but underutilised agricultural by-product, is a rich source of silica and offers potential for value-added production of biosilica adsorbents. In this study, cetyltrimethylammonium bromide (CTAB) templated silica (CTS) was synthesized from wheat straw and evaluated for the removal of PFASs from wastewater. Biosilica synthesis involved controlled ash generation, acid leaching, alkaline extraction, and sol-gel processing. Powder X-ray diffraction analysis showed a broad diffraction peak at 22° 2θ, indicating an amorphous structure. SEM analysis revealed irregular, agglomerated particles with rough and non-uniform surfaces, while EDX confirmed silicon and oxygen as the dominant elements, with minor carbon from residual surfactant fragments. FTIR analysis exhibited a prominent band at 1039 cm⁻¹, characteristic of Si–O–Si stretching in the siloxane network. Batch adsorption experiments were carried out to determine the effects of pH, adsorbent dosage, contact time, and initial PFAS concentration. Optimal conditions were pH 4, adsorbent dosages of 40 mg/L for PFOS and 50 mg/L for PFBA, PFBS, and PFOA, an initial PFAS concentration of 25 ng/L, and a contact time of 30 min. PFBA fitted both Langmuir (R² = 0.9916) and Freundlich (R² = 0.9914) models, while PFBS, PFOA, and PFOS followed the Freundlich model (R² = 0.9921–0.9986), suggesting multilayer adsorption. Adsorption efficiency increased with PFAS chain length and functional group strength with sulfonates showing higher removal than carboxylates, and the long chain PFASs were adsorbed more effectively than short chain analogues. All PFASs followed pseudo-second-order kinetics (R² = 0.9888–0.9977), indicating that chemisorption dominated the removal. Application to wastewater samples showed 78–92 % removal efficiency of the individual PFASs. These results showed that CTS from wheat straw can remove PFASs from wastewater and has potential for future water treatment application.