Enhanced removal of the ammonium, nitrate and phosphate by biochars derived from apple tree branches via different modification methods

Chemical fertilizer products have greatly enhanced the yield and quality of crops, but excessive application has led to harmful accumulation of nitrate (NO₃^–) and phosphate (PO₄^3-), causing contamination and exacerbating water eutrophication. Developing efficient adsorbent materials is an effective approach to remove excessive nutritional elements in water bodies. This study investigated the adsorption capacities of apple tree branch biochar (AB) for ammonia ion (NH₄⁺), NO₃^–, and PO₄^3- and also its modified products via the mechanical modification of ball milling (MAB), physical modification of KOH pretreatment (K-MAB), chemical modification of MgCl₂/AlCl₃ immersion (0.1 Mg-MAB, 0.3 Mg-MAB, 0.1Al-MAB, and 0.3Al-MAB). Results showed that the average diameters of modified biochars ranged from 152 to 438 nm. MAB exhibited an increase in specific surface area and porosity. AB and MAB were rich in calcium carbonate, whereas K-MAB contained potassium-rich carbonate and potassium chloride. Mg-MAB included magnesium nitrate, magnesium oxide, and magnesium hydroxide, while 0.3Al-MAB produced a new amorphous aluminum oxide. The AB, MAB and K-MAB were negatively charged, while the points of zero charge for 0.1 Mg-MAB, 0.3 Mg-MAB, 0.1Al-MAB, and 0.3Al-MAB were 4.0, 5.7, 9.2, and 9.0, respectively. K-MAB demonstrated the highest adsorption capacity for NH₄⁺. NH₄⁺ adsorption on the K-MAB surface was characterized by heterogeneous multilayer adsorption, primarily driven by electrostatic interactions and chemisorption. At pH 10 and 298 K, K-MAB reached a maximum adsorption capacity of 9.24 mg/g. 0.1Al-MAB had the highest adsorption capacity for NO₃^–. NO₃^– adsorption on the surface of 0.1Al-MAB primarily involved single-molecule adsorption dominated by chemical interactions. At 298 K, the maximum adsorption capacity of 0.1Al-MAB for NO₃^– was 43.70 mg/g. 0.3 Mg-MAB showed the best adsorption effect for PO₄^3-. The adsorption mechanism of 0.3 Mg-MAB to PO₄^3- was mainly single-layer chemisorption precipitation, regulated by electrostatic interactions. At pH 7 and 298 K, the maximum adsorption capacity of 0.3 Mg-MAB for PO₄^3- reached 349.29 mg/g. The newly-developed modified apple branch biochar showed promising potential in removing contaminants from degraded water bodies. The results provide valuable insights for the preparation of low-cost, high-efficiency biochar-based adsorbent materials derived from agriculture and forestry-sourced wastes.