Characterization of Acid-Aged Biochar and its Ammonium Adsorption in an Aqueous Solution

According to its characteristics, biochar originating originating from biomass is accepted as a multifunctional carbon material that supports a wide range of applications. With the successfully used in reducing nitrate and adsorbing ammonium, the mechanism of biochar for nitrogen fixation in long-term brought increasing attention. However, there is a lack of analysis of the NH₄⁺-N adsorption capacity of biochar after aging treatments. In this study, four kinds of acid and oxidation treatments were used to simulate biochar aging conditions to determine the adsorption of NH₄⁺-N by biochar under acidic aging conditions. According to the results, acid-aged biochar demonstrated an enhanced maximum NH₄⁺-N adsorption capacity of peanut shell biochar (PBC) from 24.58 to 123.28 mg·g^-1 after a H₂O₂ modification. After the characteristic analysis, the acid aging treatments, unlike normal chemical modification methods, did not significantly change the chemical properties of the biochar, and the functional groups and chemical bonds on the biochar surface were quite similar before and after the acid aging process. The increased NH₄⁺-N sorption ability was mainly related to physical property changes, such as increasing surface area and porosity. During the NH₄⁺ sorption process, the N-containing functional groups on the biochar surface changed from pyrrolic nitrogen to pyridinic nitrogen, which showed that the adsorption on the surface of the aged biochar was mainly chemical adsorption due to the combination of π-π bonds in the sp² hybrid orbital and a hydrogen bonding effect. Therefore, this research establishes a theoretical basis for the agricultural use of aged biochar.