Thermal-chemical modified rice husk-based porous adsorbents for Cu (II), Pb (II), Zn (II), Mn (II) and Fe (III) adsorption

Heavy metal ions contamination, a threat to ecological balance, aquatic life, and human health, co-exists in water bodies and soil systems such as mining sites, tanneries, petrochemical refineries, pesticide, fertilizer, construction, chemical industries, and others. In this study, raw rice husk and potassium hydroxide (KOH) activated novel rice husk (RH)-based adsorbents: potassium hydroxide (KOH) activated RH (KOH-RH), KOH activated RH hydrochar (KOH-RHH), and KOH activated silver (Ag) nanoparticle modified RH (AgNP-KOH-RH) were synthesized and applied on synthetic wastewater to treat hazardous heavy metal ions, Cu2+, Fe3+, Mn2+, Pb2+, and Zn2+ through fixed-bed flow column adsorption. Maximum Cu2+, Pb2+, Zn2+ uptake was obtained 7.14 mg/g, 9.02 mg/g and 8.53 mg/g, respectively by KOH-RHH. The maximum Fe3+ and Mn2+ uptake was 1.66 mg/g and 0.98 mg/g, respectively by AgNP-KOH-RH. After 1st run of adsorption, the adsorbents were recovered and re-used for 2nd adsorption run. Maximum Cu2+, Fe3+, Mn2+, Pb2+, and Zn2+ uptake was 11.98 mg/g, 9.46 mg/g, 1.29 mg/g, 19.55 mg/g and 14.58 mg/g, respectively obtained by KOH-RHH at 2nd adsorption run. AgNP-KOH-RH obtained the minimum retention time for adsorption and maximum color removal for both runs. Compared with commercial adsorbents such as coconut shell activated carbon, higher heavy metal ions adsorption capacity (>90%) and color removal have been obtained with both KOH-RHH and AgNP-KOH-RH. The adsorption kinetics was well fitted into the intra-particle diffusion (IPD) kinetics model. The detailed characterization of the adsorbents such as surface area, porosity, and surface morphology was conducted for deriving the mechanism. It can be stated that the significant adsorption capacity of heavy metals and color by KOH-RHH and AgNP-KOH-RH with repetitive adsorption runs will provide new insight into adsorption mechanisms and the development of next-generation commercial adsorbents for industrial wastewater treatment.