Synthesis of biochar-ZnAl-layered double hydroxide composite for effective heavy metal adsorption: Exploring mechanisms and structural transformations

Rapid industrialization has increased the extent of heavy metal contamination in water sources, demanding effective wastewater treatment methods. To mitigate this issue, adsorption techniques play a crucial role in wastewater treatment. Layered double hydroxides (LDHs) are commonly employed in such processes; however, they exhibit limitations in effectively adsorbing contaminants. Here, biochar (BC) was incorporated into LDHs using a simple hydrothermal method and subsequently used for Cd2+ and Pb2+ adsorption. Additionally, the physicochemical characteristics of BC and ZnAl-LDH-BC were assessed utilizing various instruments. The obtained ZnAl-LDH-BC400 and ZnAl-LDH-BC600 had surface areas of 52.55 and 102.56 m2/g, respectively, several times greater than those of the original BC. Adsorption performance was investigated using different kinetic and isothermal models. The Langmuir-Freundlich and pseudo-second-order kinetic models demonstrated the adsorption of single-layer chemicals. Furthermore, the highest observed adsorption capacities for Cd2+ and Pb2+ were 99.8 mg g-1 and 128.4 mg g-1, respectively, when using ZnAl-LDH-BC600. In contrast, BC600 demonstrated adsorption capacities of 42.1 mg g-1 for Cd2+ and 51.7 mg g-1 for Pb2+. Cd2+ and Pb2+ adsorption by ZnAl-LDHBC was primarily governed by surface precipitation and cation exchange processes. These findings highlight the mechanisms underlying the removal of heavy metals using ZnAl-LDH-BC and provide valuable theoretical and applied insights into the management of agricultural waste and control of water pollution.