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

## TL;DR

This study shows that designer biochar pellets can effectively remove phosphorus from agricultural wastewater, reducing runoff that causes algal blooms.

## Contribution

The novelty lies in the development and evaluation of designer biochar pellets for phosphorus removal in agricultural effluents.

## Key findings

- DBPs removed 18 to 155 mg kg−1 of phosphorus from different effluents.
- SEM and ICP confirmed phosphorus sorption and the role of elements like iron and calcium.
- DBPs can lead to soil pore clogging due to phosphorus precipitation.

## Abstract

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.

Designer biochar pellets (DBPs) remove phosphorus via sorption and precipitation processes.The chemical makeup of agricultural effluents and microbial loads greatly influences DBP performance.SEM, ICP, FTIR, and XRD confirmed modulated phosphorus adsorption by Ca, Mg, K, Al, and Fe concentrations in the effluent.DBP composition promotes alkaline conditions in solutions, decreasing P sorption.On soil, DBPs promoting precipitation can lead to soil pore clogging.

Designer biochar pellets (DBPs) remove phosphorus via sorption and precipitation processes.

The chemical makeup of agricultural effluents and microbial loads greatly influences DBP performance.

SEM, ICP, FTIR, and XRD confirmed modulated phosphorus adsorption by Ca, Mg, K, Al, and Fe concentrations in the effluent.

DBP composition promotes alkaline conditions in solutions, decreasing P sorption.

On soil, DBPs promoting precipitation can lead to soil pore clogging.

This study evaluated the effectiveness of designer biochar pellets (DBPs) in adsorbing phosphorus from tile drain outflow and agricultural wastewater effluents, with performance varying depending on the effluent source. Results highlight the potential of biochar‐based filters for targeted phosphorus removal, providing a promising approach to reduce phosphorus runoff and prevent eutrophication in downstream water bodies.