Groundwater, the source of drinking water for nearly one-third of the world’s population, is under constant threat from contamination. Spills of petroleum products like toluene (TOL) and chlorinated solvents like tetrachloroethylene (PCE) can create toxic plumes that persist for decades, posing serious health risks. A promising cleanup technology called an Injectable Permeable Reactive Barrier (IPRB) involves injecting a filtering material into the ground to build a “wall” that intercepts and cleans the polluted water as it flows through. While effective, this method typically uses activated carbon, which can be expensive. A new study posted on Preprints.org by Damiano Feriaud and colleagues explores a more sustainable and low-cost alternative: biochar derived from pinewood.

Before the biochar could be used, the research team had to solve a major engineering challenge: how to inject a dry powder so that it spreads evenly within an aquifer. The solution was to create a stable liquid, or colloidal suspension, by mixing the biochar with sodium carboxymethylcellulose (CMC), a common biopolymer. This “BC@CMC” suspension first had to be optimized for real-world groundwater conditions. The team found that the suspension was very stable at the neutral-to-basic pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More levels (pH 6-9.4) common in many aquifers. However, it quickly clumped together and became unstable in water with high salinity (ionic strength > 50 mM) or at very acidic or alkaline pH levels, defining the chemical boundaries for its use.

The second challenge was physical. During initial tests injecting the suspension into a column packed with glass beads (simulating an aquifer), the column clogged almost immediately. The researchers hypothesized that a small fraction of large biochar aggregates in the suspension was blocking the pores at the entrance. The fix was a simple pre-filtration step. They found that filtering the BC@CMC suspension through a 64\mu sieve was the “sweet spot”. This removed the large, clogging-prone particles while allowing the effective, smaller colloidal particles to pass through and successfully form a “deep-bed” barrier. They also optimized the mixture, finding that a concentration of 3g\L-1 of biochar provided the maximum amount of injectable adsorbent while remaining stable.

With the injection method optimized, the team tested the biochar’s actual performance. First, batch tests confirmed that the pinewood biochar had a high affinity for both contaminants. Impressively, its adsorption capacity for PCE was “nearly identical” to that of expensive commercial activated carbon, making it a highly viable alternative. Finally, the team simulated the entire IPRB process in the lab. They injected their optimized BC@CMC suspension into the column to create the barrier, then flushed the column with water contaminated with TOL and PCE. The biochar barrier performed exceptionally well. It dramatically slowed the movement of the pollutants, demonstrating an experimental retardation factor of 144\pm for toluene and an even more impressive for tetrachloroethylene.

The researchers note that these retardation factors were significantly higher than predicted by the batch tests. They suggest this is because the CMC stabilization and injection process prevents the biochar from clumping in the aquifer, exposing far more of its surface area for adsorption. Based on these results, the team calculated that a real-world, 3-meter-thick barrier could theoretically last for 27 years against toluene and 82 years against tetrachloroethylene, proving that this sustainable biochar-based approach is a highly effective and viable strategy for long-term groundwater remediation.

Source: Feriaud, D., Cerra, S., Fratoddi, I., & Petrangeli Papini, M. (2025). Biochar-Carboxymethylcellulose Composite as an Injectable Colloidal Suspension for In-Situ Groundwater Remediation. Preprints.org