Kaolinite-Composited Biochar and Hydrochar as Low-Cost Adsorbents for the Removal of Cadmium, Copper, Lead, and Zinc from Aqueous Solutions

During the last decade, due to an increase in anthropogenic activities, a higher environmental accumulation of heavy metals has been found, which has resulted in disturbed biogeochemical balance. Many kinds of remediation techniques have been practiced to mitigate heavy metal toxicity in the aqueous phase; however, adsorption is the most commonly accepted technique for efficient heavy metal removal. In this study, conocarpus waste was pretreated with 0%, 10%, and 20% kaolinite and pyrolyzed at 600 °C for 1 h to synthesize biochars (BC, BCK10, and BCK20, respectively), while hydrothermalized at 200 °C for 6 h to synthesize hydrochars (HC, HCK10, and HCK20, respectively). After characterization, synthesized materials were employed for the removal of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) from contaminated water. Experimental data was further subjected to isotherm and kinetic models to estimate the adsorption mechanism. Among all the tested adsorbents, kaolinite-synthesized materials revealed comparatively higher adsorption compared to pristine materials. It was found that pH 7 was optimum for the maximum removal of tested heavy metals. Adsorption of tested heavy metals was well explained by Langmuir and Freundlich isotherms, while pseudo-second order and Elovich kinetics models fitted well for adsorption kinetics. The maximum adsorption capacity, as predicted by the Langmuir isotherm, was the highest for BCK20 (63.19 mg g−1 for Cd, 228.05 mg g−1 for Cu, 248.33 mg g−1 for Pb, and 45.79 mg g−1 for Zn) compared to the other tested materials, and for HCK20 (31.93 mg g−1 for Cd, 181.78 mg g−1 for Cu, 231.85 mg g−1 for Pb, and 45.72 mg g−1), it was higher than pristine HC. Isotherm and kinetics modeling data indicated that multiple mechanisms were involved in Cd, Cu, Pb, and Zn removal, such as chemisorption and electrostatic interactions. The amount of oxygen-containing surface functional groups and SiO2 particles could be responsible for the maximum adsorption of heavy metals by BCK20 and HCK20. Our findings suggest that biochar, hydrochar, and their kaolinite-modified composites possess the excellent potential to remove heavy metals from contaminated aqueous media, and could be further applied to treat wastewater to mitigate heavy metal toxicity for a sustainable environment.