Enhancing waste management and nutrient recovery: Preparation of adsorption-type sludge-biochar value-added fertilizer from sewage sludge and Pistia stratiotes

Sustainable and eco-friendly sewage sludge treatment and disposal are crucial for pollution prevention, resource utilization, and control. This study used solid sewage sludge and Pistia stratiotes to produce high-performance biochar through modification and high-temperature pyrolysis. The resulting biochar was used to adsorb ammonia nitrogen and phosphate in wastewater, leading to the development of a green and safe value-added fertilizer. Results showed that the combination of Pistia stratiotes and chemical modification optimized the biochar's pore structure, enhanced chemical functional groups, and immobilized heavy metals. The pseudo-second-order kinetic model and Langmuir isotherm model provided better fitting for the adsorption process. The combination of magnesium chloride and Pistia stratiotes treatment exhibited the highest adsorption performance for ammonia nitrogen and phosphate, with maximum theoretical saturation capacities of 62.47 mg g−1 and 33.56 mg g−1. BET, FTIR, and SEM-EDS analysis revealed that surface functional groups, ion exchange, chemical co-precipitation, electrostatic adsorption, and physical adsorption were the main adsorption mechanisms. Regarding desorption, the ammonia nitrogen and phosphate release rates of the value-added fertilizer were remarkably low during the rapid release stage (0–40 min), with only 20.34% and 33.97%, respectively. Moreover, in a pot experiment, the showed greater potential for lettuce growth compared to raw biochar, especially when modified with magnesium chloride, resulting in a 53.78% increase in lettuce biomass. In conclusion, the proposed approach for sewage sludge treatment and disposal offers significant environmental and agricultural benefits through solid waste resource utilization ammonia nitrogen-phosphate recovery from wastewater.