Hofmeister Effects In Different Single/Multi-Components Soil Colloids Aggregation

Hofmeister effects（or specific ion effects）significantly affect the interaction between soil colloidal particles,which further affect the macroscopic processes of soil.However,the soil colloids are complex mixture systems composed of a variety of colloids,and different colloids（both single component colloids and multi-components colloids）exhibit different Hofmeister effects.This thesis employs the dynamic light scattering（DLS）technique to characterize the Hofmeister effects in the aggregation of montmorillonite（Mont）,kaolinite（Kaol）,goetite（Goet）,humic acid（HA）)and their mixture colloids.On this basis,the Hofmeister effects three real soil colloids aggregation（purple soil and chernozite soil with permanent charges,yellow soil with variable charges）were studied.Meanwhile,a DLS method was established to determine the Hamaker constant of organic colloids.The Hofmeister effects and its mechanism of different types of single colloids,mixture colloids and three real soil colloids aggregation were elucidated.The main results obtained were as follows:（1）The Hofmeister effects and its mechanism in different single-component colloids aggregation were different.It was found that the critical concentration of coagulation（CCC）of permanently charged Mont with followed the Hofmeister sequence Na⁺>K⁺>Cs⁺,Mg²⁺≈Ca²⁺>Zn²⁺≈Cd²⁺>Cu²⁺≈Pb²⁺;the CCC of Kaol containing permanent and variable charges followed the Hofmeister sequence Na⁺>K⁺>Cs⁺,Mg²⁺>Ca²⁺≈Zn²⁺≈Cd²⁺>Cu²⁺≈Pb²⁺;the CCC of variably charged inorganic colloid-Goet followed Cu²⁺>Cd²⁺>Zn²⁺>Pb²⁺>Ca²⁺>Mg²⁺;and the CCC of variably charged organic colloid HA followed the Hofmeister sequence Mg²⁺>Ca²⁺>Zn²⁺>Cd²⁺>Cu²⁺>Pb²⁺.By analyzing the aggregation mechanism Mont,Kaol and HA with net negative charges,we found that the different surface charge density/electric field intensity of Mont,Kaol and HA causing that the net DLVO repulsion between different particles followed HA>Mont>Kaol,which further causing that the CCCs for different colloids aggregation followed the sequence of HA>Mont>Kaol.Meanwhile,due to the different surface electric field intensity and surface atomic composition/surface structure of different colloids,the adsorption mode and strength of metal cations on the surface of these colloidal particles were very different.The interfacial reaction of alkali metal ions,alkaline earth metals ions and heavy metals ions on Mont（with permanent charges）,Kaol（with permanent and variable charges）and HA（with variable charges）surface exhibited strong non-electrostatic adsorption energies,and the non-electrostatic adsorption energies followed HA>Mont>Kaol.For Goet with positive charges,the aggregation processes were promoted by the electrostatic adsorption of anions.The difference of non-electrostatic adsorption energy of different divalent cations on the surface of Goet lead to different positive charge density on Goet surface,which finally lead to the Hofmeister sequence in Goet aggregation.（2）In the study of mixture colloids aggregation,Mont,Kaol and HA were mixed in different proportion,and the CCC of different mixture colloids aggregation induced by KNO₃ were determined.The results showed that there was only one CCC value observed for the mixture components.Meanwhile,the CCCs of the two-mixture components aggregation were always between the CCC values of each single component in the mixture,and the CCC values of the three-mixture components aggregation were always between that of the two-components aggregation.These results indicated that in the mixture colloid system,all types of colloids aggregated simultaneously in a random mixing way.For example,the CCC values of different mixture colloids followed Mont(7.88 mmol L^-1)>“80%Mont+20%Kaol”(7.09 mmol L^-1)>“50%Mont+50%Kaol”(6.44mmol L^-1)>“20%Mont+80%Kaol”(4.71 mmol L^-1)>Kaol(0.560 mmol L^-1);HA(∞mmol L^-1)>“98%Mont+2%HA”(79.9 mmol L^-1)>“99%Mont+1%HA”(36.0 mmol L^-1)>Mont;HA(∞mmol L^-1)>"98%Kaol+2%HA”(311 mmol L^-1)>“99%Kaol+1%HA”(89.8 mmol L^-1)>Kaol(0.569 mmol L^-1);“49%Mont+49%Kaol+2%HA”(109 mmol L^-1)>“49.5%Mont+49.5%Kaol+1%HA”(47.4 mmol L^-1).Hence,for mixture colloids aggregation,all the components would participate in the aggregation with equal probability,and the aggregation should be driven by the average DLVO forces of the mixture components.Based on those experimental results we speculated that,the aggregation processes of natural soil colloids with multi-components would be co-aggregation process of different components.Meanwhile,HA would decrease soil particle aggregation by increasing the DLVO/XDLVO repulsive forces（electrostatic repulsion and spatial repulsion）,and the inhibition effect of HA on Kaol aggregation was stronger than that on Mont,resulting in the lower CCC values of“Mont+HA”aggregation than that of“Kaol+HA”aggregation.（3）By mixing two different types of colloids with negative charges,the aggregation processes of mixture colloids containing two negatively charged components were determined,and Hofmeister effects were observed in mixture colloids aggregation.The CCCs of“50%Mont+50%Kaol”followed the Hofmeister sequence of Na⁺>K⁺>Cs⁺,Mg²⁺>Ca²⁺>Zn²⁺≈Cd²⁺>Cu²⁺≈Pb²⁺,while the CCCs of“99%Mont/Kaol+1%HA”followed Na⁺>K⁺>Cs⁺,Mg²⁺>Ca²⁺>Zn²⁺>Cd²⁺>Cu²⁺>Pb²⁺.The non-electrostatic adsorption energy of monovalent cations in“99%Mont+1%HA”was close to that in“99%Kaol+1%HA”,while the non-electrostatic adsorption energy of divalent cations in“99%Kaol+1%HA”was significantly higher than that in“99%Mont+1%HA”.This might relate to the covalent interaction between divalent cations and the hydroxyl groups on the variable charge surface of Kaol.Also,the non-electrostatic adsorption energy of cations in the negatively charged mixture colloids was different from that in the single colloid system.The non-electrostatic adsorption energy of ions in“50%Mont+50%Kaol”was slightly higher than that in Mont and Kaol.The non-electrostatic adsorption energies of cations（except K⁺）in“99%Mont+1%HA”and“99%Kaol+1%HA”were higher than those in Mont and Kaol.The possible reason was that compared with the single colloid system,the ionic interface reaction modes of the mixture colloids system were increased.（4）Strong Hofmeister effect has also been observed in the aggregation of the mixture colloids composed of both positively and negatively charged components.However,the Hofmeister effects were obviously different in different mixture colloids.The CCCs of different cations in“90%Mont+10%Goet”aggregation followed the Hofmeister sequence Na⁺>K⁺>Cs⁺,Mg²⁺≈Ca²⁺>Zn²⁺≈Cd²⁺>Cu²⁺≈Pb²⁺,the CCCs of“10%Mont+90%Goet”followed Zn²⁺>Cd²⁺≈Pb²⁺>Mg²⁺≈Ca²⁺≈Cu²⁺,the CCCs of“10%Kaol+90%Goet”followed Cd²⁺>Zn²⁺≈Cu²⁺>Pb²⁺>Ca²⁺>Mg²⁺,while the CCCs of“99%Goet+1%HA”followed Cu²⁺>Cd²⁺>Zn²⁺>Ca²⁺≈Mg²⁺.It was found that the CCC values of the mixture colloids aggregation were lower than that of the single colloids aggregation.This phenomenon was related to the surface charge properties/the amount of charge of different colloids and the interaction between different types of colloids.Compared with Mont,the addition of 10%Goet reduced the negative charges in“90%Mont+10%Goet”,resulting in the lower CCCs in“90%Mont+10%Goet”aggregation.However,when a small amount of Mont,Kaol and HA were added into Goet colloids,the number of positive charges decreased,leading to the result that it was easier for“10%Mont+90%Goet”,“10%Kaol+90%Goet”,“99%Goet+1%HA”to aggregate than Goet.（5）Hofmeister effects existed in the aggregation of“49.5%Mont+49.5%Kaol+1%HA”and“89.1%Kaol+9.9%Goet+1%HA”,and the CCCs of the two mixture colloids contained three components followed a similar Hofmeister sequence Na⁺>K⁺>Cs⁺,Mg²⁺>Ca²⁺>Zn²⁺≈Cd²⁺>Cu²⁺>Pb²⁺.By comparing the different Hofmeister effects in different mixture colloids aggregation,we found that the mixture colloids with net negative charges had similar Hofmeister sequences,but the intensities of Hofmeister effects were significantly different.For monovalent cations K⁺and Cs⁺,the non-electrostatic adsorption energy in the“49.5%Mont+49.5%Kaol+1%HA”were lower than that in“99%Mont+1%HA”and“99%Kaol+1%HA”.However,when it comes to bivalent cations,the non-electrostatic adsorption energy followed“99%Mont+1%HA”<“49.5%Mont+49.5%Kaol+1%HA”<“99%Kaol+1%HA”.The results indicated that the higher the proportion of variably charged components,the higher the non-electrostatic adsorption energy of divalent cations.This once again indicated that divalent cations tend to covalently interact with hydroxyl functional groups on variably-charged surfaces.（6）Hofmeister effects were observed in three real soil colloids（yellow earth soil,purple soil and chernozem soil）aggregation,and the intensity of Hofmeister effects were significantly different in different soil colloids aggregation.For monovalent cations,the CCCs of three soil colloids followed the same Hofmeister sequence of Na⁺>K⁺>Cs⁺.However,for bivalent cations,the Hofmeister sequence of CCC values of different soil colloids were different.The CCCs in yellow earth soil aggregation followed Mg²⁺≈Ca²>Zn²⁺≈Cd²⁺≈Cu²⁺≈Pb²⁺,the CCCs in purple soil aggregation followed Mg²⁺>Ca²⁺>Zn²⁺≈Cd²⁺>Cu²⁺>Pb²⁺,while the CCCs in chernozem soil aggregation followed Mg²⁺>Ca²⁺>Zn²⁺>Cd²⁺>Cu²⁺>Pb²⁺.In addition,the non-electrostatic adsorption energy of monovalent cations in the three soil colloid systems followed yellow earth soil<chernozem soil<purple soil,while the non-electrostatic adsorption energy of bivalent cations followed yellow earth soil<chernozem soil<purple soil.Since the mineral composition,charge density and organic matter contents were significantly different in these three soils,the interaction modes between cations and surface would be significantly different,which further affected the interaction forces between particles and ultimately lead to the different Hofmeister effects in three soil colloids aggregation.In addition,because of the difference of ion interface reaction energy and soil surface charge density of the three soil colloids,the DLVO repulsive force between the colloidal particles of the three soils was yellow earth soil<purple soil<chernozem soil,which lead to the result that the CCC of different soil colloids aggregation followed yellow earth soil<purple soil<chernozem soil.（7）Based on the experimental results that the aggregation process of mixture colloids was determined by average effect,we proposed a method to determine the Hamaker constant of colloid which was hard to aggregate by DLS technique.The reliability of the proposed method has been tested.Based on the above results,we draw the following conclusions:（1）Strong Hofmeister effects were existed in the aggregation processes of single-component colloids,mixture colloids and different types of real soil colloids induced by the interfacial reaction of alkali metal cations,alkali earth metal cations and heavy metal cations.（2）The strong Hofmeister effects were result from the non-electrostatic adsorption energy of ion interface reactions.However,the colloidal aggregation theory in classical colloidal chemistry was based on the electrostatic adsorption in ion interface reaction,and the famous“Schultz-Hardy”rule in colloidal aggregation was established based on electrostatic adsorption.The strong Hofmeister effects in soil colloid aggregation indicated that soil colloids aggregation no longer obeyed the“Schultz-Hardy”rule.It has been pointed out that not only divalent alkaline earth metal ions and heavy metal ions have strong non-electrostatic adsorption energy on the variably charged colloids surface,but also univalent alkaline metal ions generally have strong non-electrostatic adsorption energy on the permanently charges colloids surface.Therefore,it is of great significance in soil science to reveal the mechanism of non-electrostatic adsorption on different soil colloids surfaces and to clarify the influence of non-electrostatic adsorption on soil particle interaction.（3）In the aggregation process of mixture colloids or real soil colloids,all the components would participate in the aggregation with equal probability,and the average DLVO force of colloid particles of each component drive the aggregation of soil colloids.In other words:the aggregation process of soil colloids with complex components was actually the“co-aggregation”process of all the components.This scientific discovery provided an important methodological basis for studying the interaction of colloidal particles in complex soil systems on the formation of soil“mineral-organic-microbial”complexes and the formation/stability of soil aggregates.