Efficient Congo red removal using rice husk-derived biochar/NiMn LDH nanohybrid: Synthesis, characterization, and adsorption performance

Congo red (CR), a representative azo dye with carcinogenic and mutagenic potential, poses serious environmental risks due to its high stability and persistence in wastewater. In this study, a sustainable biochar-based NiMn layered double hydroxide (BC/NiMn-LDH) nanohybrid was synthesized via a facile in situ hydrothermal growth strategy using KOH-activated rice husk biochar as a functional support. The activated biochar provides a porous, defect-rich, and oxygen-functionalized surface, enabling uniform deposition and strong interfacial coupling of LDH nanosheets. Comprehensive characterization (FT-IR, Raman, XRD, BET, FE-SEM, TEM, and TGA) confirms the successful formation of a well-integrated hierarchical structure in which NiMn-LDH nanosheets are uniformly anchored onto the biochar matrix, resulting in enhanced structural stability and abundant accessible active sites. The prepared nanohybrid exhibits excellent adsorption performance toward CR, achieving a removal efficiency of ~97% under optimized conditions (20 mg L⁻¹ initial concentration, 8 mg adsorbent dose, and 8 min contact time). Equilibrium adsorption data were well described by the Langmuir isotherm model, with a maximum adsorption capacity of 106.4 mg g⁻¹, indicating monolayer adsorption on energetically homogeneous active sites. The enhanced adsorption performance is attributed to the synergistic effects of hierarchical porosity, surface functional groups, and well-dispersed NiMn-LDH active phases, which promote electrostatic attraction, hydrogen bonding, and π–π interactions with CR molecules. This work demonstrates an efficient and sustainable approach for the development of biochar–LDH nanohybrids as high-performance adsorbents for dye-contaminated wastewater treatment, while simultaneously valorizing agricultural waste into value-added functional materials. • KOH-activated rice husk biochar was used as a conductive support for NiMn-LDH growth. • Biochar prevented aggregation of LDH nanosheets and enabled a hierarchical structure. • The nanohybrid achieved ~97% Congo red removal within 8 min. • A maximum adsorption capacity of 106.4 mg g⁻¹ was obtained, fitting the Langmuir model. • The study provides a sustainable route for wastewater treatment via agricultural waste valorization.