Lotus-leaf biochar modified with metal oxide nanoparticles: synthesise, characterisation and application to the photocatalytic removal of Cr6+, Cr3+ and Co2+ ions

Different modified biochars were prepared by impregnating with different metal oxide nanoparticles including AgO, CuO and ZnO onto leaf of lotus tree. The prepared nanocomposites were characterised using X-ray diffraction (XRD) and field emission-scanning electron microscopy (FESEM). The results revealed that the metal oxide affected morphology of the biochars. Graphene sheets were evident especially in the case of Zn-modified biochar. Particle size of biochar, Ag-biochar, Cu-biochar and Zn-biochar revealed to be in the range of 33.5–214.5 nm, 105.3–231.9 nm, 33.5–142.9 nm and 37.2–66.9 nm, respectively. Textural properties of the biochars were determined by using Brunauer–Emmett–Teller (BET) and BJH models based on N2-adsorption-desorption isotherms. Box-Behnken design (BBD) was used to optimise the different effective parameters and to compare the efficiency of the different biochars towards the removal of Cr and Co ions. Isotherms of the biochars were also studied. The adsorption isotherms were fitted to Langmuir and Freundlich models. The biochars obeyed Langmuir adsorption model and their maximum adsorption capacities were 107.5 mg/g for Co2+ ion and 113.6 mg/g for Cr3+. The modified biochar was capable of removing Cr3+ and Co2+ ions up to 78% and 98%, respectively. The modified biochar has also shown excellent photocatalytic performance for the degradation of chromate ion. 81.6% of chromate ion degradation has been achieved with the incorporation of Zn-biochar under sunlight irradiation. Adsorption characteristics of the modified lotus leaf biochar were compared with the previously reported biochars. The results confirmed the superiority of the proposed biochar.