Mechanisms Of Soil Mineral Adsorption And Protection On Low Molecular Weight Organic Acids

Global warming has affected economic and social development worldwide.The international community has reached a consensus and made a series of efforts to control the greenhouse effect.China has also been an important contributor and participant in tackling global warming.Fulfilling the current carbon reduction target ahead of schedule,China has proposed a target that have CO₂ emissions peak and achieve carbon neutrality before 2030 and 2060,respectively.On the one hand,policies had been adopted to make the development use less energy,increase the proportion of non-fossil energy and reduce greenhouse gas emissions.On the other hand,carbon sequestration measures such as increasing ecological carbon sink and carbon capture were used to reduce the existing atmospheric CO₂.As the largest land carbon sink,soil has great potential for ecological carbon sequestration,which can be achieved by increasing the input of soil organic matter and improving the stability of soil organic matter,and the later is more effective for soil carbon sequestration.The adsorption and protection of soil minerals on organic matter is considered to be the main mechanism to improve the stability and retention of organic matter in soil.The higher the adsorption amount of minerals on organic matter,the stronger the adsorption and protection effect was supposed to be.At the same time,soil organic matter has diversity and complexity,and the contribution of labile organic matters such as organic acids,sugars and proteins to carbon emissions is far greater than that of stable components due to its larger input and faster degradation.Improving the stability of labile organic matters is the key to improving the overall stability of soil.As an important component of labile organic matter,low molecule weight organic acids have representative environmental behaviors.Therefore,this study focused on the mechanism of soil mineral adsorption and protection of low molecule weight organic acids.The adsorption characteristics and stability of low molecule weight organic acids on soil minerals are vital when studying the mechanism of adsorption protection.In this study,tannic acid(TA)and gallic acid(GA)were selected as model compounds of low molecule weight organic acid with different molecular sizes but similar basic structures.Montmorillonite(Mon)and hematite(Hem)were selected as typical model soil minerals.The adsorption capacity,adsorption force and adsorption kinetics of the low molecule weight organic acids on minerals were investigated.In combination with the degradation of low molecule weight organic acids under the action of different minerals,the relationship between the above adsorption characteristics and the improvement of the stability of labile organic matter was analyzed.The main results of this study are as follows:(1)The accuracy of permanganate method was improved to determin the degradation kinetics of low molecule weight organic acids.The kinetics of organic matter oxidation by potassium permanganate(KMn O₄)was used as the evaluation basis for the stability of organic matter.The influence of manganese dioxide(Mn O₂)colloid,which formed during the reaction,on the quantification of KMn O₄ was observed,and the synchronicity of KMn O₄ consumption and Mn O₂ formation was analyzed by two-dimensional correlation spectroscopy.Dual-wavelength method was used to eliminate the interference of Mn O₂ in the determination of KMn O₄ consumption by visible spectroscopy and improve the accuracy of KMn O₄ oxidation method in the determination of degradation kinetics of low molecule weight organic acids.The substrate degradation kinetics,total organic carbon degradation kinetics and KMn O₄consumption kinetics were compared,and it was found that KMn O₄ consumption could better reflect the overall degradation of low molecule weight organic acids in the system.This method has the advantages of controllable reaction process and easy determination of degradation index;hence it is more suitable for determining the degradation kinetics of low molecule weight organic acids in the presence of minerals.The method is also conducive to analyzing the relationship between adsorption characteristics and adsorption protection ability at the molecular level.(2)It was found that the adsorption capacity could not be used to reflect the adsorption protection ability of minerals to low molecule weight organic acids.In this study,the adsorption isotherms of TA and GA on Mon and Hem were analyzed.It was found that Hem had higher adsorption capacity for TA and GA at lower concentration than Mon due to strong electrostatic attraction.The larger specific surface area and more adsorption sites of Mon lead to the adsorption capacity of TA and GA of high concentration exceeding that of Hem.Because TA has a dendritic structure,the adsorption capacity of TA on Mon and Hem at low concentration is higher than that of GA with small molecular weight.At high concentration,the outer surface of Mon is occupied,and GA has a higher adsorption capacity than TA because it could enter the inner layer of Mon.The degradation kinetics of organic matter adsorption equilibrium system was constructed.It was found that the two minerals had stronger protective effect on TA than GA.Even if the adsorption capacity of Mon to TA and GA was higher than that of Hem,the adsorption protection capacity of Mon was still weaker.It indicates that the conflict between adsorption capacity and adsorption protection capacity observed in previous studies may be caused by ignoring the difference of organic matter concentration.Moreover,adsorption capacity of minerals to labile organic matter cannot be used as the evaluation basis of mineral protection ability.(3)The adsorption affinity between minerals and low molecule weight organic acids was more suitable to describe the adsorption protection ability of minerals.By calculating the adsorption coefficient and the energy of adsorption site of adsorbed organic matter on mineral surface,it is found that there were more TA adsorption sites on Mon,but the average energy of adsorption site on Hem was higher overall.Organic mineral complexes with the same content of adsorbed organic matter but different adsorption energy were prepared and degraded.A significant positive correlation between the adsorption protection ability of the same mineral and the energy of the adsorption site was found,indicating that the stronger adsorption force might facilitate the stronger the adsorption protection ability of the mineral to organic matter.Adsorption energy could also describe the adsorption protection ability of different minerals to small organic acids.To explain the relationship between adsorption energy and adsorption protection,Fourier transform infrared spectroscopy(FTIR)of the complexes at different adsorption and degradation stages was measured.Proved by two-dimensional infrared correlation spectroscopy,the hydrogen bond,electron acceptor and ligand exchange successively played an important role in the adsorption of TA on Mon with the increase of TA initial concentration.The Hem surface mainly absorbed TA through electrostatic attraction and hydrogen bonding simultaneously.In the process of complex degradation,the TA adsorbed by electron donor and acceptor mechanism on Mon surface was degraded first,and the that by ligand exchange mechanism and hydrogen bond wass degraded successively,which is contrary to the order of adsorption mechanism in the adsorption process.Only TA adsorbed by hydrogen bonding on Hem was degraded.Finally,it is proved that low molecule weight organic acids adsorbed by strong adsorption mechanism have stronger resistance to degradation,and the adsorption energy of organic matter can well judge and predict the adsorption protection ability of minerals to low molecule weight organic acids.(4)The adsorption rate of low molecule weight organic acids was controlled by the diffusion of the boundary water layer and affects the adsorption protection of minerals.The adsorption kinetics experiments showed that the adsorption rates of TA on Mon and Hem were two orders of magnitude higher than that of GA with smaller molecular size.Kinetic model fitting confirmed that the diffusion process through boundary water layer was the decisive step of GA adsorption kinetics.By increasing the ionic strength of the adsorption system,the diffusion of GA in the boundary water layer was accelerated;however,the adsorption kinetics of TA was not affected.The phenomenon that the relaxation rate of ¹H NMR decreased with the increase of ionic strength confirmed the hypothesis that the thickness of boundary water layer became thinner,thus accelerating GA adsorption.A system in which adsorption and degradation occurred simultaneously was constructed.It was found that with the increase of ionic strength,the adsorption protection ability of minerals to organic matter was significantly enhanced due to the acceleration of adsorption kinetics.These results confirmed that the adsorption affinity and kinetics of low molecule weight organic acids on minerals play a decisive role in their stability.The differences of adsorption mechanisms of clay minerals and metal oxides on active components of soil organic matter in typical soils were pointed out.The theory of soil carbon sequestration has been enriched from the understanding of the adsorption process at the micro interface from the perspective of the strength and speed of adsorption,helping to guide the research and development of soil carbon sequestration technology,and improving the ecosystem service function of soil.