The Influence of Small Molecule Organic Acids on the Adsorption of Carbamazepine by Straw Biochar/Montmorillonite Complex and the Threshold Effect

Crop straw biochar, as a green and efficient pollution remediation material, demonstrates significant environmental benefits and economic advantages in removing pharmaceutical active compounds (PhACs) from soil. Carbamazepine (CBZ), a persistent environmental risk pollutant within PhACs, represents a crucial target substance for removal from soil. However, the impact of low molecular weight organic acids (LMWOAs) produced by plant root exudates and organic matter degradation on the CBZ adsorption capacity of straw biochar remains poorly understood. This study investigates the adsorption capacity of biochar derived from corn, wheat, and rice straws on CBZ in montmorillonite through laboratory batch experiments and chromatographic analysis, as well as the influence of two LMWOAs (citric acid and oxalic acid) on the adsorption process. The results indicate that the adsorption capacity of montmorillonite for CBZ progressively increases with higher CBZ concentrations. Significant correlations between O/C, H/C, and (O+N)/C ratios with the adsorption affinity coefficient K_F suggest that CBZ adsorption by straw biochar results from the combined effects of physical and chemical adsorption mechanisms, including pore filling, hydrophobic interactions, and π–π interactions. Notably, rice straw biochar exhibits the highest maximum adsorption capacity for CBZ at 115.6mg/g, significantly surpassing corn straw biochar (13.1mg/g) and wheat straw biochar (19.5mg/g), attributed to its stronger surface aromaticity and non-polarity. Although the addition of citric acid and oxalic acid reduces the adsorption coefficient (K_d) of CBZ on montmorillonite by 99%, it has minimal impact on the CBZ adsorption capacity of straw biochar. The threshold effect of LMWOAs in the montmorillonite-biochar composite system, characterized by “low concentration promotion and high concentration inhibition” in CBZ adsorption, indicates their influence on mineral-biochar interface interactions. These findings demonstrate that the incorporation of straw biochar into soil significantly enhances CBZ adsorption capacity, suggesting its potential as a soil remediation agent with robust resistance to environmental interference and promising application prospects. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202505160124.