Effect Of Organic Matter On The Suspension Stability Of Loess Nanoparticle And Its Mechanism

Loess is the most important parent material of loess plateau.The agglomeration and migration of nanoparticles in loess closely affect the transport of soil nutrients and pollutants,and have a great influence on the soil structure,soil fertility,soil erosion resistance,water and fertilizer retention ability.The organic matter is an important cementing material in loess soil,which is directly related to the stability of soil aggregates.In this paper,the effects of removal of organic matter and addition of humus substances on the stability of suspension of loess nanoparticles were studied.Meanwhile,the coagulation mechanism of colloids of humus substances under different environmental conditions was systematically studied,and the mechanism of organic matter affecting the stability of suspension of soil nanoparticles was systematically revealed from the perspective of soil electrochemistry.The main results are as follows:?1?The concentration kinetics of fulvic acid?FA?and humic acid colloid?HA?in K⁺,Na⁺,Ca²⁺and Mg²⁺electrolyte solutions with p H changes were compared and analyzed.The results show that both the fulvic acid and humic acid colloid show obvious colloidal properties and their surfaces contain a large number of functional groups such as hydroxyl and carboxyl groups.The deprotonization of these functional groups makes the colloidal surfaces carry a large number of negative charges.The absolute value of zeta potential on humic acid surface was greater than that on fulvic acid colloid under the corresponding p H.The two colloids exhibited typical coagulation behavior in the four cationic solutions,that is,slow coagulation occurred in the low electrolyte solution and fast coagulation occurred in the high electrolyte solution.The CCC values of colloidal condensation of humic substances induced by four cations were in the order of Ca²⁺>Mg²⁺>K⁺>Na⁺,indicating that the divalent cation has a stronger ability to accumulate and sink.Compared with humic acid colloid,the CCC value of fulic acid colloid is larger,which is because the surface negative charge of fulic acid colloid is more,the electrostatic repulsion between particles is greater,and the suspension stability is stronger.Humic colloids react differently to changes in the p H of the environment in both monovalent and divalent cation solutions.In Na⁺and K⁺solutions,the CCC value of fulvic acid and humic acid colloids increases with the p H of the solution increasing,and the suspension stability increases.This is because the ability of monovalent cation to initiate colloidal condensation is mainly due to charge neutralization.With the increase of p H,the amount of negative surface charge of humic colloid increases,and more cations are needed to neutralize the negative surface charge in order to induce rapid condensation.In the Ca²⁺and Mg²⁺solutions,with the increase of p H,the CCC value gradually decreased and the suspension stability gradually weakened.In addition to charge neutralization,the mechanism of colloidal coagulation induced by divalent ions is also the bridging bond between high ions and carboxyl groups on the colloidal surface of humic substances.With the increase of p H of the solution,the bridge bond is gradually strengthened,which promotes the agglomeration behavior between the colloidal particles of humic substances.It is of great significance to study the coagulation behavior and mechanism of fulvic acid and humic acid colloid in metal ionic solution for water purification,soil remediation of heavy metal pollution and environmental control of organic pollutants.?2?In this study soil nanoparticles were extracted,separately,from soil samples of Lou soil and cinnamon soil before and after soil organic matter was removed with the aid of the ultrasonic dispersion method and the Stokes'law based high-speed centrifugation method for analysis of particle size distribution,zeta potential,critical coagulation concentration?CCC?in Na Cl and Ca Cl₂ solutions and characterized with the dynamic light scattering technique.Furthermore,Hamaker constants of and interaction energies between the soil nanoparticles were calcuated in line with the DLVO?Derjauin-Landau-Verwey-Overbeek?theory,mechanism of organic matter removal affecting stability of soil nanoparticle suspensions explored and role of organic-mineral complexation in stabilizing soil aggregates explained.Results show that the Lou and cinnamon soil nanoparticles were 94.00 nm and 88.20 nm,respectively,in average diameter and the OMR?organic matter removed?loess nanoparticles slightly higher than 100 nm;Compared with the nanoparticles in the Lou and cinnamon soils,those in the OMR Lou and cinnamon were lower in absolute value of the zeta potential as they were lower in organic organic matter and the potential energy of electrostatic repulsion between the OMR Lou and cinnamon nanoparticles decreased.Based on DLVO-based simulation,the nanoparticles in the Lou and cinnamon soils in vacuum were 6.86×10^-20 J and 9.73×10^-20 J,in Hamaker constant and those in the OMR Lou and cinnamon soils were only 3.14×10^-20 J and 3.40×10^-20 J.Apparently,the Hamaker constants of the latters were lower than the formers and so were their van der Waals attractive potential energies.The total potential energy between the OMR loess nanoparticles was higher than that between the loess nanoparticles,and the CCC was larger and the dispersion ability was stronger.The higher the content of soil organic matter is,the higher the degree of organic and inorganic composite is,and the more the nanoparticles tend to condense,which may be one of the reasons why organic matter enhances the stability of aggregates.?3?The heterogeneous coagulation system of 5%fulvic acid or humic acid colloid combined with loess nanoparticles were studied,it's electrochemical properties were characterized and compared with those of loess nanoparticles.The results showed that:The negative charge content of loess nanoparticles increased with the addition of organic colloids;In the Na⁺solution,the addition of 5%fulic acid or humic acid increased the CCC value and the suspension stability of Lou nanoparticles and cinnamon nanoparticles.At this time,the heterogeneous coagulation process of the binary system of colloidal humus and loess nanoparticles is dominated by the neutralization of charge.In the solution of divalent ion Ca²⁺,the addition of 5%fulic acid increased the CCC value of Lou nanoparticles and cinnamon nanoparticles,but the addition of 5%humic acid promoted the agglomeration of loess nanoparticles.In the divalent ionic system,the heterogeneous coagulation process of humic colloid and loess nanoparticles is controlled by charge neutralization and bridging bond,and the bridging bond between humic acid and divalent ion is stronger than that between fulic acid and divalent ion.