Evaluating Phosphorus Sorption and Desorption in Agricultural Wastewater Using Designer Biochar Pellets

Tile drains enhance crop productivity but also increase phosphorus (P) runoff into nearby water bodies, contributing to harmful algal blooms. This study examines the effectiveness of designer biochar pellets (DBPs) in removing or releasing P from agricultural effluents, soils, or deionized water, respectively. The DBPs are composed of pine sawdust biomass and bentonite clay, pretreated with lime sludge prior to pyrolysis, and subsequently exposed to various wastewater effluents and field conditions. DBP treatment in P removal varied across effluent types, ranging from 18 to 155 mg kg^-1. In contrast, P desorption in deionized water ranged from 0.1 to 8.9 mg L^-1. DBP extracted from the field after the trial showed contrasting soil phosphorus extraction results, ranging from 0.45 to 0.6 mg L^-1 for new and 0.3 to 1.2 mg L^-1 for spent, respectively. Furthermore, P extracted from soil before planting (1 to 5 mg L^-1), no lime sludge and DPB exposure to soil, after planting (3 to 15 mg L^-1), after manure waste, lime sludge, and DBP exposure to soil, and after harvesting on plots treated as new, spent, and control was found to range from 10 to 55, 5 to 30, and 5 to 35 mg L^-1, respectively, indicating that DBPs may serve as a P-removal agent and an amendment. Scanning electron microscopy (SEM) confirmed phosphorus sorption in the pellets, ranging from 0% to 0.2%, and ICP analyses identified other elements such as iron and silicon. The sorption and desorption experiment in this study is governed by four primary components: pH, salts (Ca, Mg, and K), P, and dissolved organic carbon (DOC) concentrations. Among these factors, pH plays a central role in regulating sorption behavior by influencing surface charge, ion speciation, and mineral reactivity. Additionally, lime sludge in DBPs enhances phosphorus removal by promoting P precipitation, further strengthening the system's sorption capacity. This underscores the importance of tailoring effluent treatment based on the specific characteristics of the source.