Dynamic adsorption and regeneration of gaseous ammonia on oxalic acid-modified corn-straw biochar

Ammonia is a gas that often goes unnoticed until it becomes a problem. It escapes into the air from farmyards, livestock waste, and the breakdown of fertilizers, adding to the mix of pollutants that form fine particles in the atmosphere. These tiny particles can travel deep into the lungs and contribute to smog, while the nitrogen they carry can acidify soil and choke waterways. Removing this gas from the air is difficult because it is often present in low concentrations, yet it remains a significant environmental challenge. Scientists have long looked for materials that can act like sponges, trapping ammonia molecules as air passes over them. One promising candidate is biochar, a charcoal-like substance made by heating plant matter in the absence of oxygen. While biochar is abundant and cheap, plain versions often lack the specific chemical features needed to grab onto ammonia effectively. To solve this, researchers have begun exploring ways to tweak the surface of these materials, making them more chemically attractive to the gas they need to catch.

A team of researchers at Harbin University of Science and Technology set out to improve the ability of corn-straw biochar to capture gaseous ammonia. They focused on a simple chemical treatment using oxalic acid, a common organic acid found in many plants, to modify the surface of the charcoal. The goal was to see if this treatment could create a material that not only trapped ammonia well but could also be cleaned and used again. The researchers started by taking dried corn stalks, grinding them into a fine powder, and heating them in a furnace under a flow of nitrogen gas to prevent them from burning. They tested different heating temperatures and durations to find the best base material. Once they had a solid starting point, they soaked the charcoal in solutions of oxalic acid with varying strengths, stirring the mixture for hours before drying it out. This process was designed to coat the charcoal with new chemical groups that might interact more strongly with ammonia.

To find the perfect recipe, the team used a systematic approach to test sixteen different combinations of heating temperature, heating time, and acid concentration. They passed a stream of air containing ammonia through a column filled with each of these sixteen modified materials and measured how long it took for the gas to break through the other side. The results pointed to a clear winner: a sample made by heating the corn straw at 500 degrees Celsius for four hours and then treating it with a specific concentration of oxalic acid. This particular material, which the researchers labeled CC-0.4-500-4, held onto the ammonia far longer than any of the others, capturing a total of 27.56 milligrams of ammonia for every gram of material. This performance was significantly better than the untreated charcoal, which struggled to hold the gas for even ten minutes.

When the scientists looked closely at this winning material, they found that the oxalic acid treatment had changed its physical and chemical makeup in helpful ways. The charcoal was not a solid block but a porous network, much like a sponge, with a large number of medium-sized pores that allowed the gas to flow easily inside. The surface of the material was rich in oxygen-based chemical groups, specifically carboxyl and carbonyl groups, which act like tiny magnets for the ammonia molecules. These groups allowed the ammonia to stick to the surface through chemical attraction, while the pores provided a pathway for the gas to reach those sticky spots deep inside the material. The researchers also detected the presence of persistent free radicals, which are unstable atoms that can influence how the surface reacts with other chemicals, potentially helping the capture process.

A crucial part of the study was determining if this material could be used more than once. Adsorbents are only practical if they can be cleaned and reused without losing their effectiveness. The team tried washing the ammonia-saturated charcoal with water and ethanol to see if they could release the trapped gas and restore the material's ability to catch more. They found that a brief wash with ethanol for ten minutes was the most effective method, recovering nearly two-thirds of the material's original capacity. Even after two cycles of capturing and cleaning, the material still retained almost half of its initial ability to trap ammonia. This suggests that the chemical bonds holding the ammonia were not so strong that they could not be broken by a gentle solvent, nor so weak that the material fell apart.

The study concluded that the success of this material came from a combination of factors working together. The pores helped transport the gas to the active sites, while the oxygen-rich surface chemistry provided the necessary grip to hold it. The presence of free radicals and the formation of specific nitrogen-containing compounds on the surface further supported the idea that the interaction was complex and chemical in nature. While the material did lose some capacity over repeated use, likely due to changes in the surface chemistry and pore structure, the results showed that oxalic acid-modified corn-straw biochar is a viable option for capturing ammonia from the air. It offers a way to turn agricultural waste into a useful tool for pollution control, one that can be cleaned and reused with simple solvents, providing a practical step toward managing a gas that is otherwise difficult to remove from the atmosphere.