# Efficient Phosphate Adsorption by Ball-Milled Fe3O4–Modified Biochar Derived from Agricultural Waste

## TL;DR

This paper introduces a modified biochar made from agricultural waste that efficiently removes phosphate from water, offering a sustainable solution to eutrophication.

## Contribution

The novel use of ball-milled Fe3O4 modification significantly enhances biochar's phosphate adsorption capacity and reusability.

## Key findings

- Modified biochar achieved a maximum phosphate adsorption capacity of 125.38 mg·g–1.
- The material retained over 89% removal efficiency after five regeneration cycles.
- Calcium ions had the strongest inhibitory effect on phosphate adsorption.

## Abstract

Phosphorus is a critical factor contributing to eutrophication
in aquatic environments, with agricultural nonpoint source pollution
identified as its primary source. The development of efficient, eco-friendly,
and renewable adsorbents is of significant importance for controlling
phosphorus pollution in rural aquatic bodies. Biochar, a porous carbonaceous
material, has demonstrated considerable adsorption potential; yet
it exhibits limited affinity for phosphate, necessitating performance
enhancements through modification. In this study, biochar was prepared
from pig manure and wheat husk via high-temperature pyrolysis, and
modified by loading nano-Fe3O4 using a ball
milling process, which offers a greener alternative to chemical treatments.
The material’s structure was characterized using Brunauer–Emmett–Teller
analysis and scanning electron microscopy. Adsorption experiments,
including kinetic modeling, isotherm fitting, pH variation, ion interference,
and regeneration assessments, were conducted to investigate phosphate
removal performance. The wheat husk-derived biochar modified with
iron oxide at 800 °C exhibited a specific surface area of 181.71
m2·g–1, approximately 420 times
greater than that of its unmodified counterpart. It followed a pseudo-second-order
kinetic model and Langmuir isotherm, with a maximum phosphate adsorption
capacity of 125.38 mg·g–1 under an initial
concentration of (50 mg·L–1, pH 7.0, and 8
h equilibrium time), indicating that chemisorption may play a role
in the adsorption process. The material demonstrated optimal performance
at neutral pH, with calcium ions showing the greatest inhibitory effect
on adsorption. After five adsorption–desorption cycles, the
removal efficiency remained above 89%, confirming the material’s
robust regeneration capacity. Overall, the ball-milled iron oxide
modification provides a sustainable and effective means to enhance
biochar functionality. The resulting magnetic biochar exhibits high
adsorption efficiency, rapid kinetics, pH sensitivity, and excellent
reusability, making it a promising candidate for addressing nonpoint
source phosphorus pollution in aquatic systems.