Aging Induced Changes in Biochar's Functionality and Adsorption Behavior for Phosphate and Ammonium

Area of Science:

• Soil Science
• Environmental Chemistry
• Materials Science

Background:

• Biochar, a pyrogenic carbon material, enhances soil properties and agricultural sustainability.
• Biochar's surface chemistry evolves over time, impacting its nutrient adsorption capabilities.
• The aging process and its effect on ionic nutrient (ammonium, phosphate) adsorption by biochar are not fully understood.

Purpose of the Study:

• To investigate the impact of chemical oxidation on wood biochar's surface chemistry.
• To determine how biochar aging affects ammonium and phosphate adsorption.
• To evaluate the role of biochar-derived organic matter (BDOM) in nutrient retention by aged biochar.

Main Methods:

• Wood biochar (550 °C) was aged via chemical oxidation using 5-15% H2O2.
• Changes in biochar surface functionality (carboxylic, phenolic, oxonium groups, pyridine/pyridone) were analyzed.
• Ammonium and phosphate adsorption capacities were measured, with and without BDOM.

Main Results:

• Oxidation introduced carboxylic and phenolic groups, reduced oxonium groups, and transformed pyridine to pyridone.
• Ammonium adsorption increased significantly with oxidation (r² = 0.94 with surface charge density).
• Phosphate adsorption decreased post-oxidation, attributed to electrostatic repulsion and loss of positive charge.
• Inclusion of BDOM substantially increased both ammonium and phosphate adsorption by oxidized biochar.

Conclusions:

• Biochar aging via oxidation can reverse its affinity for cationic and anionic nutrients.
• Surface charge density is a key factor in ammonium adsorption by aged biochar.
• BDOM plays a crucial role in enhancing the ionic nutrient and contaminant retention capacity of aged biochar, highlighting its potential for soil amendment.