Adsorption Of Soil Minerals On Non-humus Active Components And Its Stability Effects

Soil mineral carbon sequestration plays an irreplaceable role in improving the global carbon cycle,achieving "carbon neutrality" and enhancing the stability of soil organic matter.Therefore,the adsorption of soil minerals to organic matter,as the key to stabilizing soil organic matter,has always been a research hotspot in the field of environmental geochemistry.A large number of non-humus active components in soil organic matter and their rapid degradation rate greatly affect the overall stability of soil organic matter.However,the key role of soil non-humus active components in soil carbon sequestration has not been systematically understood.In this study,two typical minerals(kaolin,montmorillonite and iron oxide)and two representative non-humus active components(carbohydrate: fructose and water-soluble starch;small molecular organic acids: gallic acid and tannic acid),the adsorption of three kinds of soil minerals on two representative non-humus components was studied,and the adsorption mechanism was explained by employing relevant characterization methods and adsorption models.Further,the effect of mineral adsorption on the overall stability of non-humus active ingredients was evaluated through the chemical oxidation of the mineral adsorption system.The results showed that:(1)The adsorption rate of minerals to macromolecular organic matter is faster than that of small molecular organic matter(Small fructose and gallic acid were adsorbed within 5 h and 1 h,respectively,while large-molecular weight water-soluble starch and tannic acid were adsorbed at 10 h and 4 d,respectively).This can be explained by the fitting results of the internal diffusion kinetic model.For small molecular weight fructose and gallic acid,the diffusion process through the bound water is the adsorption rate-limiting step.On the other hand,compared with fructose and gallic acid,the molecular weight of water-soluble starch and tannic acid is higher,and the rapid adsorption occupy the mineral surface,resulting in greater steric hindrance,so that further adsorption is inhibited.(2)Three kinds of minerals can adsorb four kinds of organic matter,and the structural properties of organic matter(molecular weight,configuration,functional groups,etc.)and the physical and chemical properties of the mineral surface(specific surface area,functional groups,interlayer structure,etc.)all play an important role in the adsorption.The adsorption mechanism of fructose and water-soluble starch on three kinds of minerals includes hydrogen bonding,hydrophobic association and complex reaction.The adsorption mechanism of gallic acid and tannic acid on minerals is hydrogen bonding,surface coordination exchange and electrostatic attraction.In addition,the saturated adsorption capacity of minerals to organic matter is proportional to the molecular weight of organic matter.The saturated adsorption capacity of water-soluble starch(1.2-3.8 mg/g)and tannic acid(1.0-3.2 mg/g)with large molecular weight on minerals is higher than that of fructose(0.05-0.25 mg/g)and gallic acid(1.0-7.5 mg/g)with small molecular weight.The results also showed that the adsorption capacity of carbohydrates and small molecular organic acids are higher than that of small molecular organic acids with roughly the same molecular weight.(3)The adsorption of water-soluble starch and tannic acid with larger molecular weight by minerals can improve its stability and help resist the oxidation of potassium permanganate.However,the adsorption of low molecular weight fructose and gallic acid by minerals makes them more unstable and prone to oxidative degradation.This may be because the adsorption capacity and adsorption affinity of small molecular weight fructose and gallic acid on the mineral is low,and the addition of strong oxidant potassium permanganate may promote the desorption of gallic acid from the mineral.In addition,the concentration of gallic acid in the solution gradually decreases with the oxidation process,and the adsorption of gallic acid on the mineral leads to the increase of the concentration of gallic acid in the local range of the mineral,which stimulates the "concentration effect" of potassium permanganate oxidation of gallic acid and accelerates the oxidation rate.This also shows that mineral adsorption does not always make the organic matter more stable.The influence of mineral adsorption on the stability of organic matter is closely related to the structural properties of organic matter.This study explains the stability of soil organic matter and its environmental function from the perspective of active components of soil non-humus matter.This provides a theoretical basis for understanding the turnover of soil organic matter,using minerals to prioritize the protection of organic matter components,regulating the input of soil organic matter,and ultimately improving the overall stability of soil organic matter.