1. Chemical functionalization: Involves treating biochar with chemical agents to introduce functional groups onto its surface. For example, biochar can be treated with acids, bases, oxidizing agents, or alkylating agents to attach specific functional groups such as hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), or sulfonic (-SO3H) groups. These functional groups can enhance the biochar's reactivity, sorption capacity, and interaction with target compounds .