## 1 Introduction  
Since China's reform and opening up, China's economy has been developing rapidly, and its industrial structure has been constantly upgraded. With the rapid development, environmental pollution has become more and more serious. Environmental pollution has seriously restricted the sustainable development of China's economy and society. Water pollution is becoming more and more serious, so it is urgent to control water pollution [1; 2; 3].  
Water pollution refers to the fact that the intervention of certain substances in water exceeds the self-purification capacity of water, resulting in the change of physical and chemical properties and biological properties of water and the destruction of the original ecosystem of water [4; 5; 6; 7]. The problem of water pollution not only reduces the use value of water, but also causes harm to the human body. Moreover, it will destroy the ecological environment and lead to the deterioration of water quality [8]. At present, the common water pollution treatment methods are divided into physical methods and chemical methods. Common physical methods include coagulation and flocculation, adsorption, and ultrasonic methods, while chemical methods often include chemical precipitation, solvent extraction, and separation, as well as active sludge dyeing and oxidation methods [9]. Among them, the adsorption method is widely used in water pollution treatment because of its advantages of simple operation, economic efficiency, reusability of adsorbents, and no secondary pollution. The commonly used adsorbents are activated carbon, alumina, and molecular sieves, but the high price of adsorbents limits their application in real life. Therefore, it is an urgent problem to develop a cheap and efficient adsorbent. Carbon-based materials have been widely used in various fields due to their good mechanical, chemical, physical, and thermal properties, and are of great significance to improving people's quality of life [10]. Carbon-based materials refer to carbon-rich nanostructures such as biochar, carbon nanotubes, graphene, Graphene oxide (GO), fullerene, carbon nano fiber (CNF), and carbon nanofibers [11]. Among them, biochar is a carbon-rich solid obtained from the pyrolysis of biochar at high temperature in the atmosphere of limited oxygen or no oxygen. It has the characteristics of abundant and cheap raw materials, high porosity, large specific surface area, and wide sources, and has great development potential in the removal of water pollutants. Although biochar has many advantages, it produces by-products in the pyrolysis process, resulting in the reduction of specific surface area and functional groups of biochar, which further weakens the adsorption capacity of biochar to substances. Therefore, the performance of biochar needs to be improved by means of doping and modification [12].  
To sum up, in order to improve the adsorption capacity of biochar, the biochar-graphene composite is prepared by one-step dip coating. In this research, a scanning electron microscope, X-ray photoelectron, spectrum analyzer, surface area meter, and other characterization equipment and techniques are used to analyze and characterize the morphology and element composition of biochar-graphene composites. Finally, the adsorption kinetics and adsorption isotherm results are analyzed. This research aims to provide a good idea for the application of BG composites in the field of environmental protection.