| Nanomaterials exhibit strong stability and transport in environmental media due to their small size and large specific surface area.They can fix pollutants in soil and groundwater in situ and are widely used in soil remediation.Soil micro-agglomeration affects the stability and mobility of nanomaterials in the environment,thus affecting their removal efficiency of pollutants.However,there are few systematic studies on the suspension stability of nanomaterials in soil microaggregates,especially the long-term stability of functional nanomaterials synthesized in the presence of dispersants in porous media.In this study,nano-ferrihydrite(HFO)and nano-magnetite(MNPs)were used as the research objects.The stability of two nanomaterials in different soil microaggregates was preliminarily investigated.The suspension mechanism of nano-HFO and nano-MNPs in different soil microaggregates was discussed by kinetic model and DLVO theory,which provided a basis for evaluating the migration behavior and rational utilization of nanomaterials in soil.The results showed that:In soil,the sedimentation performance of nano-HFO increased significantly.The stability of nano-HFO in black soil and loess microaggregates is basically the same,and the order is:50～250μm>5～50μm>2～5μm>0.25～2 mm>(<2μm);Nano-HFO has good sedimentation performance and poor stability in soil large particle aggregates(0.25～2 mm).It is easily adsorbed on the surface of soil large particles and captured by the rough structure of soil surface.For soil clay(<2μm)with relatively high content of organic matter(SOM),free iron oxide(Fe_d),cation exchange capacity(CEC),etc.nano-HFO deposition is obvious,and unstable HFO does not show a significant gap,and the stability is poor;in5～50μm and 50～250μm soil aggregates,the suspension stability of CMC-stabilized nano-HFO was better,which was significantly higher than that of unstable HFO.After removing SOM and Fe_d,the stability of nano-HFO in the soil was stronger than that of the original soil,and the stability was enhanced.The Zeta potential of nano-HFO is-38.9 m V,and the stability is good.However,after entering the soil,with the increase of electrolyte concentration,the absolute value of Zeta potential decreases to varying degrees,the negative charge of nano-HFO decreases,and the electrostatic repulsion between particles weakens,which in turn affects the agglomeration behavior of nanomaterials.Among them,the Zeta potential of(<2μm)particle suspension in black soil changed the most,and the negative charge was the weakest,with a value of-5.29 m V,and the stability was the worst.The negative charge in 5～50μm and50～250μm suspensions is strong,which is-20.2 m V and-21.6 m V,respectively.The Zeta potential of 0.25～2 mm particle suspension in loess is-14.2 m V,and the negative charge is the weakest.The negative charge of 5～50μm and 50～250μm particle suspension is the strongest,which is-17.2 m V and-19.8 m V respectively.The energy calculation results of Derjaguin-Landau-Verwey-Overbeek(DLVO)show that the energy barrier between nano-HFO synthesized with CMC as stabilizer is 1.1×10~3KT,and the second minimum value is positive,which is not easy to agglomerate.In the range of different particle sizes of the two tested soils,the energy barrier between nano-HFO-soil microaggregates also decreased with the decrease of particle size.In coarse sand(0.25～2 mm)and clay(<2μm),the energy barrier between nano-HFO and soil microaggregates is between1.3×10~2～2.9×10~2k T,and their second minimum appears significantly negative.Therefore,nano-HFO is very prone to heterogeneous agglomeration or deposition on its surface.Compared with nano-HFO,starch-stabilized MNPs had poor suspension stability,with particle size less than 30 nm and Zeta potential of-28.7 m V.The negative charge of nano-MNPs in 5～50μm and 50～250μm microaggregates of black soil was the strongest compared with other particle sizes,which were-16.3 m V and-18.4 m V,respectively,while the Zeta potential values in other particle size mixtures were greater than-15 m V;The Zeta potential of nano-MNPs in loess suspensions was less negative than that in black soil,most of which were concentrated above-10 m V,and the negative charge was the weakest in 0.25～2 mm and(<2μm)suspensions.The stability of MNPs in the two soils was in the order of 50～250μm>5～50μm>2～5μm>(<2μm)>0.25～2 mm.After removing SOM and Fe_d,the stability of starch-stabilized nano-MNPs was stronger than that of raw soil.Through DLVO calculation,the energy barrier between MNPs is 8.07×10~2 KT,and the second minimum value is positive.When particles collide,it is difficult to agglomerate due to the existence of energy barrier.The energy barriers between nano-MNPs and coarse sand(0.25～2 mm)and clay(<2μm)were the lowest,ranging from 0.75×10~2～1.75×10~2 k T,and their second minimum values appeared and were significantly negative. |
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