Synthesis, performance and reaction mechanisms of Ag-modified multi-functional rice husk solvochar for removal of multi-heavy metals and water-borne bacteria from wastewater

Abstract

This study introduces a new thermo-chemical conversion technique, solvothermal carbonization on rice husk with alcoholic solvents, ethanol, and isopropanol (IPA) at a low temperature, 280 ℃ with 2 h reaction time to produce innovative char, solvochar. Solvochars were hierarchically activated by potassium hydroxide (KOH), impregnated with silver nanoparticles (AgNPs), and stabilized through the calcination process at high temperatures and residence time. The functionalized solvochar was comprehensively characterized by FTIR, BET-BJH, XRD, XRF and XPS analyses. The maximum surface area, pore diameter and pore volume were obtained, 77.38 m2/g, 18.44 nm and 0.32 cm3/g by IPA-AgNPs. The active units in functionalized solvochars were enhanced surface area-porosity, sp2, and sp3-hybridized aromatic carbon compounds and oxygen-containing alcoholic groups for physio-chemisorption of heavy metal ions. Further, the functionalized solvochars were applied for the adsorption of multi-heavy metal ions, Cu2+, Fe3+, Pb2+, Zn2+, and Mn2+ through fixed-bed flow column adsorption till reaching the saturation stage of two consecutive cycles. The heavy metal ion adsorption uptake for experimented solvochars were ⁓100–230 mg/g Cu2+, ⁓513–569 mg/g Fe3+, ⁓3–9 mg/g Mn2+, ⁓164–190 mg/g Pb2+, ⁓64–100 mg/g Zn2+ for 1st cycle and ⁓2–57 mg/g Cu2+, ⁓6–132 mg/g Fe3+, ⁓3–5 mg/g Mn2+, ⁓3–30 mg/g Pb2+, ⁓2–19 mg/g Zn2+ for 2nd cycle. The adsorption kinetics have been analyzed by intra-particle diffusion (IPD) modelling. Among multi-heavy metal ions, Fe3+ was adsorbed continuously till the saturation stage and R2 was obtained, 0.67–80 through IPD modelling. The post-adsorption characterization of solvochars presented the possibility of recycling. The significant presence of sp3d-hybridized AgNPs in all adsorbents acted as a potent antimicrobial agent against the most common wastewater-relevant bacteria, Escherichia coli, and Staphylococcus aureus. 0.03 wt% loading rate of Ag-functionalized solvochars inhibited both bacteria growths completely. This phenomenon reinforces the potential of the Ag-modified solvochar composites to be applied as potential multi-functional adsorbents for realistic approach.