Effects of Short-Term Acidification on the Adsorption of Dissolved Organic Matter by Soil Minerals and Its Mechanism of Action

Abstract

In order to investigate the impact of soil acidification on the adsorption of dissolved organic matter by soil minerals and understand its mechanism, this study selected commonly found minerals in soils, namely illite, kaolin, and hematite, as the research objects. Glucose and tannic acid were considered as the representative compounds for studying the adsorption of dissolved organic matter in soils. By analyzing the effects of the three minerals on the adsorption characteristics of glucose and tannic acid after a short-term acidification treatment, this study aimed to explore the underlying mechanism. To achieve this, scanning electron microscopy and a specific surface area analyzer were utilized. The results of this study indicate that the adsorption modes of the minerals studied were unaffected by short-term acidification. Chemisorption, as well as surface and mesopore diffusion, were found to dominate the adsorption process. In terms of adsorption behavior, the minerals exhibited multilayer inhomogeneous adsorption with glucose and kaolin, while tannic acid showed monolayer adsorption with illite and hematite. When exposed to the same acidification conditions, the saturated adsorption of glucose and tannic acid was found to be illite ≥ hematite > kaolin. The kinetic adsorption processes exhibited three stages: fast adsorption, slow adsorption, and adsorption equilibrium. Interestingly, as the intensity of the acidification increased, the saturated adsorption capacity generally followed the trend of S3 (test minerals with pH adjusted to 3 value) > S5 (test minerals with pH adjusted to 5 value) > CK (the control group). The acidification-induced solvation led to an increase in the specific surface area and the number of active adsorption sites on the minerals. Additionally, the protonation reaction triggered a change in the surface charge, which in turn affected the hydrogen bonding, ligand exchange, and charge transfer between the minerals and glucose and tannic acids. These interactions ultimately enhanced the adsorption capacity.