Specific Ion Effects On Biochar And Montmorillonite Colloid Aggregation

The interaction between mineral,organic and microbial particles in soil affects the formation,stability and dispersion of soil aggregates,and then affects the occurrence of a series of macro processes in soil.Interaction between soil colloidal particles is not only affected by ion concentration,valence,pH and other factors,but also by ion species,showing ion-specific effects or Hofmeister effects.Ion-specific effects are prevalent in soil systems.New studies have shown that the strong polarization effect derived from ions near the interface may be the intrinsic nature of the occurrence of ion-specific effects.Because soil particles carry a large amount of negative charge,a strong electrostatic field can be generated near their surface,and the adsorbed cation in this electric field will undergo a strong nonclassical polarization.The applied force on the surface of the polarized cation will be much stronger than the electrostatic force on the surface,thus enhancing the ability of cations to shield negative electric fields around soil particles.Then the electrostatic repulsive force,hydration repulsive force and molecular gravity between soil particles are affected and finally affecting the interaction between soil particles and the agglomeration of soil particles.Biochar is a solid particle with a particle size of nanometers to micrometers after thermal pyrolysis of biomass materials in an anoxic or anaerobic environment,and its particle surface is usually negatively charged.Biochar is widely used in carbon sequestration emission reduction,water purification,heavy metal adsorption and soil improvement,which can provide solutions to hot issues of global concern such as climate change,environmental pollution and soil functional degradation to some extent.Biochar produced by biomass pyrolysis has attracted more and more attention from all walks of life.With the increasing application of biochar in soil,more nanoscale biochar will be formed and accumulated,thus making its impact on the environment more significant.Once biochar particles are exposed to soil,it is inevitable to contact with soil minerals.Biochar interactions with minerals may allow soil organic matter to enter the biochar-mineral complex,thereby immobilizing organic carbon in biochar-modified soils.The combination of minerals and biochar not only improves the oxidation resistance of biochar,but also improves the structural stability of soil aggregates.Because both biochar particles and soil mineral particles have negative charge,electric field exists on the surface of biochar particles and in the medium near the surface.Therefore,the interaction between biochar and mineral particles may have ion-specific effects triggered by ion nonclassical polarization.So it is of great significance to study the ion-specific effect of the interaction between biochar and clay mineral particles,to reveal the interaction mechanism between biochar and soil particles and then promote the rational utilization of biochar.In this paper,based on the dynamic light scattering study of the interaction between biochar and montmorillonite colloidal particles,the coagulation kinetics process of the mixed system of biochar and montmorillonite colloidal particles under different cationic conditions was determined to elucidate the ion-specific effects of"biochar-clay mineral"agglomeration.This study obtained the following results:（1）Ion-specific effects affected the agglomeration rate,critical concentration and activation energy during single mineral colloidal montmorillonite agglomeration.The Hofmeister sequence of agglomeration rate of montmorillonite particles was Li⁺<Na⁺<K⁺<Cs⁺<<Ca²⁺.And the Hofmeister sequence of condensed critical electrolyte concentration and condensed activation energy were Li⁺>Na⁺>K⁺>Cs⁺>>Ca²⁺.（2）Ion-specific effects affected biochar colloid of NBC500 agglomeration,.The Hofmeister sequence of agglomeration rate was Li⁺<Na⁺<K⁺<Cs⁺<<Ca²⁺.Biochar colloid of NBC500 did not agglomerate in the 1:1 electrolyte solution,but significantly in the 2:1 electrolyte solution.The Hofmeister sequence of agglomeration rate was Li⁺/Na⁺/K⁺/Cs⁺<<Ca²⁺.（3）Adding biochar can enhance the stability of montmorillonite colloidal suspension.And as the proportion of biochar increased,the particle size of colloid aggregates and the agglomeration rate decreased,the critical coagulation concentration increased,which meant the stability of the system was enhanced.The results showed that with the increase of the proportion of biochar,the growth rate of condensate slowed down and the critical concentration increased,so the stability of suspension increased.（4）Biochar and montmorillonite mixed colloids showed strong ion-specific effects during colloid agglomeration under the action of LiNO₃,NaNO_3,KNO₃,CsNO₃ and Ca（NO₃）₂ electrolytes.The specific performances were as follows.The Hofmeister sequence of the growth rate of the mixed colloidal condensate during agglomeration was Li⁺<Na⁺<K⁺<Cs⁺<<Ca²⁺.And the Hofmeister sequence of the critical concentration and activation energy of the mixed colloid agglomeration were Li⁺>Na⁺>K⁺>Cs⁺>>Ca²⁺.（5）The surface of"biochar-montmorillonite"mixed colloidal particles were negatively charged.The Hofmeister sequence of the averageζpotential of mixed colloidal particles in the presence of different ions was Li⁺>Na⁺>K⁺>Cs⁺>>Ca²⁺.（6）The Homfeister sequence of colloid agglomeration was consistent with the Hofmeister sequence of ion nonclassical polarization.Based on the above results,the following conclusions could be drawn:（1）Adding biochar can enhance the stability of montmorillonite colloidal suspension.（2）The process of"biochar-montmorillonite"agglomeration was dominated by ion-specific effects.（3）The larger the ionic radius,the higher the nonclassical polarizability,and the stronger the adsorption force of ions on the particle surface,the stronger the ability of the ion to shield the electric field.Therefore,the ion-specific effect in the ion interface reaction was the reason for the ion-specific effect of colloid agglomeration.