Amphoteric Polyelectrolyte Biosilica Scaffolds: Effective Adsorbents for Pollutant Removal

Rice husk-derived silica nanoparticles (RSN-50) possess an inherently negative surface charge over a wide pH range, which limits their effectiveness for the removal of chemically diverse pollutants. To overcome this dominant charge limitation, an amphoteric polyelectrolyte scaffold (ACS-4B) composed of chitosan (CS), alginate (Alg), and biosilica was designed to enable pH-responsive surface charge regulation using complementary functional groups (-COOH, -NH₃⁺, and -SiOH). ACS-4B demonstrated efficient adsorption of cationic dye (methylene blue, MB), anionic dye (Congo red, CR), and toxic metal ions (Pb²⁺ and Cr⁶⁺) across a wide pH range without pH adjustment. Mesoporous ACS-4B scaffolds with a specific surface area (233.2 m²/g) could achieve maximum adsorption capacity for MB (630 mg/g), CR (387 mg/g), Pb²⁺ (539 mg/g), and Cr⁶⁺ (585 mg/g) within 150 min at the inherent solution pH, indicating the pH-responsive amphoteric behavior of ACS-4B (CR (6.7), MB (5.9), Pb²⁺ (2.9), and Cr⁶⁺ (4.9)). The selective adsorption trends under mixed-pollutant conditions further reflect charge-adaptive interactions. ACS-4B retained its adsorption functionality over four consecutive regeneration cycles, indicating preliminary reusability. The combined amphoteric surface chemistry, inherent postadsorption pH-buffering ability, and biowaste-derived design highlight ACS-4B as a promising adsorbent for textile dye and metal-containing wastewater treatment.