Biochar–Microbial Synergistic Systems for Nitrogen and Phosphorus Removal in Polluted Waters: Mechanisms, Studies, and Insights

Eutrophication caused by increasing non-point source pollution has resulted in the accumulation of nitrogen and phosphorus in water bodies, leading to severe algal blooms, oxygen depletion, and ecosystem degradation. However, conventional remediation technologies often face limitations in efficiency and sustainability. Biochar, derived from organic waste, has gained attention for its strong adsorption capacity. Recent advances show that combining biochar with functional microorganisms—forming biochar-microbial synergistic systems—can significantly enhance the removal of nitrogen and phosphorus from polluted water. For example, laboratory studies have reported that microbial-immobilised biochar can achieve over 90% removal of ammonium and phosphate, far exceeding the performance of single biochar or free bacteria. Such ecological and environmental benefits are due to the physical and chemical properties of biochar and its ability as a multifunctional carrier for microorganisms, which therefore form a comprehensive purification mechanism of physical adsorption, biological transformation and chemical precipitation. This review systematically summarises the mechanisms, advantages, and recent progress of biochar-microbial synergistic systems, including key factors influencing removal efficiency and the prospects for in-situ application. Additionally, comparative analyses between biochar-microbial synergistic technology and traditional technologies are presented, discussing economic feasibility and current challenges. This review highlights the unique potential of biochar-microbial composites to achieve high efficiency, environmental friendliness, and practical scalability in water pollution remediation, providing a reference for future research and large-scale applications.