Comparative study of solvothermal and catalytic solvothermal carbonization of rice husk for Fe(iii), Zn(ii), Cu(ii), Pb(ii) and Mn(ii) adsorption, kinetics, surface chemistry and reaction mechanism

Abstract

Over the past few decades, there has been extensive research into the development of waste-biomass-derived adsorbents due to their sustainable nature, cost-effectiveness, and ability to manage waste effectively. Therefore, this study focused on improving the adsorption properties of rice husk, an agricultural residue, using a novel solvothermal and catalytic solvothermal carbonization technique. Water and alcoholic solvents (ethanol and isopropanol) were used in solvothermal carbonization, and potassium hydroxide (KOH) was used as the catalyst. The maximum yield of solvochar was obtained without catalytic effect at approximately 34–41%. The characterization results presented a maximum surface area of 63.69 m² g⁻¹ followed by 33.16 m² g⁻¹ by the solvochars synthesized with ethanol (RE) and isopropanol (RI) without catalytic effect, respectively. On the other hand, solvochar with ethanol and catalyst presented the highest negative surface charge of −40.2 mV. CHNS, FTIR, XRD, XRF and XPS analyses presented abundant amorphous carbon contents, oxygen-containing functional groups, and inorganic compounds. Further, a continuous adsorption study of multi-heavy metal ions Fe³⁺, Mn²⁺, Pb²⁺, Zn²⁺, and Cu²⁺ was performed using a fixed-bed flow column. The maximum adsorption capacity of multi-heavy metal ions was obtained by RE (13.24 mg g⁻¹ Cu²⁺, 40.16 mg g⁻¹ Fe³⁺, 7.72 mg g⁻¹ Mn²⁺, 57.22 mg g⁻¹ Pb²⁺, and 41.88 mg g⁻¹ Zn²⁺) followed by RI (25.59 mg g⁻¹ Cu²⁺, 27.32 mg g⁻¹ Fe³⁺, 21.46 mg g⁻¹ Mn²⁺, 47.20 mg g⁻¹ Pb²⁺, and 26.98 mg g⁻¹ Zn²⁺). On the other hand, catalytic solvochars synthesized by ethanol (REK) and isopropanol (RIK) have a lower metal ion uptake, which is 16.74 mg g⁻¹ Cu²⁺, 0.6 mg g⁻¹ Fe³⁺, 13.84 mg g⁻¹ Mn²⁺, 22.19 mg g⁻¹ Pb²⁺, 19.42 mg g⁻¹ Zn²⁺ and 16.92 mg g⁻¹ Cu²⁺, 10.05 mg g⁻¹ Fe³⁺, 12.13 mg g⁻¹ Mn²⁺, 21.3 mg g⁻¹ Pb²⁺, 20.43 mg g⁻¹ Zn²⁺, respectively. The kinetics of multi-heavy metal ion adsorption was analyzed using intra-particle diffusion modeling. Post-adsorption characterization results showed a significant drop in surface area and porosity in the catalytic solvochar, while the other solvochar remains almost unchanged. Therefore, it is recommended that the metal ions be recovered through a desorption process and the solvochars regenerated without catalysts for further study.