Tiny carbon particles from soil amendment could ferry pollutants into groundwater

Researchers find that nanoparticles from biochar, a popular soil additive, can transport harmful organic chemicals, with effects that change as the material ages in the environment

The Unseen Journey of Soil Contaminants

Biochar, a charcoal-like substance produced from plant matter, is widely applied to agricultural lands to improve soil health and sequester carbon. Over time, this material can break down into microscopic fragments known as biochar nanoparticles, or BCNPs. A study by researchers from Nankai University, Henan University, and Nanjing University examined how these tiny particles interact with common organic pollutants in the soil and their potential to move these pollutants toward groundwater sources.

A Microscopic Taxi Service

The research team, led by Wei Chen of Nankai University, conducted laboratory experiments showing that BCNPs can act as effective carriers for certain types of contaminants. In simulated sandy soil environments, the presence of BCNPs at concentrations of just 20 milligrams per liter caused a substantial increase in the movement of water-repelling nonpolar compounds, such as naphthalene and pyrene, and positively charged polar contaminants like the antibiotics trimethoprim and ciprofloxacin. These pollutants bind strongly to the BCNPs, which then transport them through the soil.

Not All Passengers Are Equal

The interaction between BCNPs and contaminants is complex and depends heavily on the chemical properties of the pollutant. While the nanoparticles enhanced the transport of some chemicals, they slightly inhibited the movement of others. For mobile compounds that are negatively charged or neutral, such as sulfamethoxazole and bisphenol A, associating with the BCNPs meant they were more likely to be retained in the soil as the BCNPs themselves were deposited.

The Influence of the Environment

The researchers also found that solution chemistry plays a significant role in this transport process. For contaminants whose electric charge changes with acidity, such as certain antibiotics, altering the pH of the water affected how strongly they attached to the BCNPs. At higher pH levels, where electrostatic attraction was weaker, the ability of the BCNPs to mobilize these contaminants was reduced, showing the sensitivity of these interactions to environmental conditions.

Aging Changes the Game

In the natural environment, biochar is not static; it undergoes aging processes through chemical and biological interactions. The team simulated two common, mild aging processes—sulfide reduction and the leaching of organic carbon—to see how they would affect the BCNPs' ability to transport pollutants. They found that nanoparticles from the aged biochar were better at moving contaminants through the soil column.

A More Mobile Carrier

Further investigation showed that this increased transport was not because the pollutants were sticking more tightly to the aged nanoparticles. Instead, the aging process primarily made the BCNPs themselves more mobile in the porous soil. The loss of certain surface groups on the nanoparticles reduced their tendency to get stuck to soil grains, allowing them to travel farther and carry their contaminant cargo with them. This finding separates the mobility of the carrier from the strength of its bond with the passenger.

Implications for Environmental Management

The work by authors Pengkun Ma, Zhichong Qi, Xuan Wu, Rong Ji, and Wei Chen provides important information about the long-term environmental fate of biochar. The findings suggest that the potential for biochar to act as a vehicle for co-existing pollutants needs to be considered in environmental risk assessments. Understanding these nanoparticle-mediated transport mechanisms is essential for ensuring that the application of biochar as a soil amendment does not lead to unintended contamination of subsurface water resources.

Corresponding Author:

Zhichong Qi

Original Source:

https://doi.org/10.1007/s44246-023-00036-6

Contributions:

Pengkun Ma carried out the experiments, processed the data, and drafted the manuscript. Xuan Wu synthesized 14C-labeled SMX. Rong Ji provided 14C labeled chemicals and measurement instruments. Zhichong Qi edited the manuscript. Wei Chen conceptualized and supervised the work, acquired funding, and edited the manuscript. The authors read and approved the final manuscript.