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

Abstract

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-LDH-BC 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.