Transport Of Soil Nanoparticles And Their Co-transport With Phenanthrene And Pentachlorephenol

Due to the unique physico-chemical properties of nanoparticles(NPs),the revolution in nanotechnology has triggered the widespread studies and applications of engineered NPs(e.g.,carbon nanotube and graphene).The large release of engineered NPs will undergo a series of environmental processes such as sorption,transport and accumulation.The related ecological and environmental risks have already attracted more and more concerns.Studies about environmental behaviors of micro-sized soil colloids are abundant in the past,while so far researches on NPs(below 100 nm)derived from natural soil are still in its infancy.Mobile colloids,especially NPs,are often considered to affect the transport and fate of various contaminants by serving as carriers.Therefore,it is believed to have importantly environmental significance in studying transport of soil NPs and their co-transport mechanisms with contaminants.We extracted NPs from typical zonal soils of China and investigated the effects of different environmental factors on tansport behaviors of soil NPs and their co-transport with two typical organic contaminants(phenanthrene,PHE and pentachlorophenol,PCP).The main results were as follows:(1)Differences in transport behaviors of soil NPs and clay colloids were revealed.Transport of NPs(<100 nm)and clay fractions(clay particles,<2 ^pm;coarse clay particles,1-2 ?m and fine clay particles,0.1-1?m)extracted from two natural soils(Inceptisol from Jilin and Oxisol from Hainan,China)were investigated in saturated sand columns at 1-30 mM NaCl and pH 5-9.Increasing NaCl concentrations decreased the mobility,while increasing pH increased the mobility of soil particles of various sizes.At pH 5 and 30 mM NaCl,NPs and clay fractions exhibited the different transport behaviors,and ripening was observed for Inceptisol NPs while blocking for Oxisol NPs in breakthrough curves(BTCs).The effluent mass recoveries(MRs)of NPs were much more than that of clay particles for both two soils(>1.9-fold)at all tested conditions,except for Inceptisol at pH 5 and 30 mM NaCl(with comparable MR).Based on Derj aguin-Landau-Verwey-Overbeek(DLVO)calculations and particle-collector size ratios,both secondary energy minimum and physical straining led to the retention of clay fractions at pH 5 and 30 mM NaCl,whereas primary energy minimum and straining induced by simultaneous aggregation caused the retention of NPs.The experimental attachment efficiency between soil particles of various sizes and sand collector for both two soils was in the order NPs<fine clay particles<clay particles<coarse clay particles,indicating that soil NPs had greater mobility than clay fractions.Consequently,NPs are suggested to have greater risks to transport contaminants to surface and groundwater,once they are released from soils.(2)We further elucidated the co-transport mechanisms of PHE and PCP by soil NPs through saturated sand columns.Soil NPs with high organic matter and particle concentration were the most effective in transporting PHE through columns.In addition,soil NPs significantly increased the transport of low-level PHE(0.2 mg L"')but there was no obvious increase at 1.0 mg L-1 PHE.This is attributed to a higher ratio of NP-associated PHE to total PHE at a low-level than at a high-level during transport.In contrast to PHE,the chemical speciation of PCP determined its mobility,which was highly dependent on solution pH.At pH 6.5,anionic PCP became dominant and soluble in the effluent.This could account for the negligible effect of soil NPs on PCP mobility.At pH 4.0,however,neutral molecular PCP dominated and,as expected,decreased mobility of PCP occurred.Soil NPs considerably enhanced the transport of neutral PCP in NP-associated forms compared to controls,due to the sorption of PCP to NPs.In addition,the mobility of soil NPs was little affected by small PHE and PCP molecules under tested conditions.This study indicated that highly mobile soil NPs may be effective carriers for organic contaminants and have potential risks in spreading them to adj acent water bodies.(3)The effects of humic acid(HA)and hematite on the co-transport of soil NPs and organic contaminants were investigated.The results showed that the addition of HA promoted the transport of PHE.The higher the concentration of HA,the better the transport efficiency,and the facilitated-transport effect was more significant at pH 6.5.The addition of hematite at pH 6.5 and 9.0 has the opposite effect on the transport of PHE.The former pH inhibited the transport of PHE and the latter one promoted the transport of PHE.The addition of hematite,the pH conditions of the solution and the properties of hematite(such as point of zero charge)were closely related to the transport of PHE.The effect of HA on PCP transport depended on the chemical speciation of PCP at different pH to a large extent.When pH<pKa,the addition of HA promoted the transport of PCP in the molecular state,and the higher the concentration of HA,the better the facilitated-transport effect;when pH>pKa,the addition of HA had no significant effect on the transport of PCP which exists mainly in the dissolved form.The addition of HA generally promoted the stabilization of the NPs and increased their mobility.The pH affected the transport of NPs by changing the surface charge of hematite.Hematite was added to inhibit the transport of NPs when the pH was below the point of zero charge.Conversely,the addition of hematite promoted the transport of NPs when the pH was above the point of zero charge.In conclusion,the results of this study are of great significance to further understand the transport mechanisms of soil NPs and co-transport with contaminants,so as to explore soil NPs as effective carriers of organic contaminants and predict their environmental fate and risk.