Lanthanum-modified biochar for dual removal of particulate/dissolved phosphorus in agricultural runoff: Performance and reuse as slow-release fertilizer

Phosphorus is one of the main contributors to the eutrophication of water bodies; therefore, phosphorus recycling from agricultural wastewater represents a synergistic approach to achieve both phosphorus management and effective resource use. However, the specific removal pathway remains to be optimized. This study investigated the adsorption effect of lanthanum-modified biochar (La-BC) made from agricultural waste on dissolved phosphorus (DP). The maximum unit adsorption capacity of La-BC reached 41.09 mg P/g, representing a 215.35 % increase compared to the original biochar (BC) (C₀: 500 mg/L; pH: 7). Further, the removal effect of La-BC on agricultural non-point source pollutants with different particulate phosphorus (PP) percentages was investigated through dynamic interception and adsorption simulation experiments. The results showed that PP was mostly intercepted by quartz sand while DP was mainly adsorbed by La-BC. Thus, La-BC could effectively remove DP and PP from agricultural wastewater and delay the adsorption depletion point of the dynamic adsorption device, subsequently prolonging its service life. Finally, the agricultural availability of La-BC after adsorption and filtration (La-BC-P) was further explored by soil and plant cultivation. The results indicated that La-BC-P with adsorption interception of fertilizer distribution water had higher plant availability. A higher DP percentage increased the beneficial effects on soil nutrients (available phosphorus increased by 12-16-fold) and plant growth (plant fresh weight increased by 33 %). This study innovatively verified the technical feasibility of the "agricultural waste-agricultural non-point source pollution treatment-carbon-based fertilizer" recycling model based on the characteristics of agricultural source wastewater.