A thermal-responsive quaternary ammonium-functionalized composite SiO2@QA-PNIPAM for efficient removal of Cr(VI) from wastewater.

Hexavalent chromium (Cr(VI)) is a highly toxic, mobile heavy metal that threatens water safety and human health. Although the adsorption method is widely used for Cr(VI) removal, the regeneration of conventional adsorbents often relies on chemical elution by toxic compounds, risking secondary pollution. To address this, we developed a thermal-responsive composite adsorbent, SiO2@QA-PNIPAM, for the environment-friendly and energy-efficient Cr(VI) removal. This intelligent material was fabricated by grafting temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) and positively charged quaternary ammonium groups onto silica. Batch experiments demonstrated that SiO2@QA-PNIPAM can effectively remove hexavalent chromium with rapid adsorption kinetics (<13 min), observed at room temperature (∼25 °C). The adsorption process followed Langmuir model and pseudo-second-order kinetics. Notably, SiO2@QA-PNIPAM can be efficiently regenerated through a simple ion exchange process when heated to 60 °C. In the fixed-bed column tests for treating low-concentration Cr(VI) wastewater, the breakthrough time was 250 min, and 2 g of the adsorbent could treat approximately 250 mL of wastewater. Stable performance was maintained across multiple regeneration cycles, indicating its potential for practical, continuous applications. Mechanistic studies revealed that the effective removal of Cr(VI) by SiO2@QA-PNIPAM involves a synergistic process: initial capture via ion exchange and electrostatic interaction with quaternary ammonium groups, followed by the reduction of highly toxic Cr(VI) to less toxic Cr(III) by amino groups. This work presents a temperature-controlled regeneration method for adsorbents, enabling a green treatment strategy for chromium-containing wastewater.