Study On The Mechanisms Of Interactions Between Nanoparticles And Interfaces Of Aquatic Environments And The Application Of Nano MnO₂ In Enhanced Treatment For Trace Heavy Metal Removal

Although the huge marketing potential of nanotechnology has aroused much attention at both home and abroad to the development prospect of nanomaterials,the release of nanoproducts based on nanoparticles into aquatic environment also aroused great concern to their potential threat to ecological security.Nanoparticles can inevitably enter the aquatic environment during the process of their manufacturing,transportation,usage and eventual disposal.Due to their unique physic-chemical properties,the massive applications may pose toxic effects on organisms at different degrees,which depended on their distribution and transport distance.Therefore,a clear understanding of the transport and deposition behavior of nanoparticles in aquatic environment and the mechanisms for the interaction of nanoparticles with the environmentally relevant surfaces can provide an effective way to predict the possible exposure behavior and exposure pathway of nanomaterials to aquatic organisms.In return,we can better monitor and control their environmental fate and thus reduce their exposure risk in aqueous environment.This study selects typical manganese oxide nanoparticle（nMnO₂）and typical engineering nanoparticle(i.e.,fullerene(C₆₀)and cerium dioxide（CeO₂）)to investigate the transport and deposition behaviors on environmentally relevant surfaces they commonly encountered in aquatic systems.The influence of solution chemistries,types of interfaces,macromolecular organics and ionic specificity on their stability and deposition behaviors in aquatic environment was investigated.The classical colloid model and modified colloid model were employed for further understand the interaction mechanisms.Furthermore,the influence of nMnO₂ on the removal efficiency of heavy metals in surface water in the process of strengthening coagulation was explored as well.The initial deposition kinetics of colloidal MnO₂ on three representative surfaces in aquatic systems（i.e.,silica,magnetite,and alumina）in NaNO₃ solution were investigated in the presence of model constituents,including humic acid（HA）,a polysaccharide（alginate）,and a protein(bovine serum albumin（BSA）,using laboratory quartz crystal microbalance with dissipation monitoring equipment（QCM-D）.The results indicated that the deposition behaviors of MnO₂ colloids on three surfaces were in good agreement with classical Derjaguin-Landau-Verwey-Overbeek（DLVO）theory.Critical deposition concentrations（CDC）were determined to be 15.5 mM NaNO₃ and9.0 mM NaNO₃ when colloidal MnO₂ was deposited onto silica and magnetite,respectively.Both HA and alginate could largely retard the deposition of MnO₂ colloids onto three selected surfaces due to steric repulsion,and HA was more effective in decreasing the deposition rate relative to alginate.However,the presence of BSA can provide more attractive deposition site and thus lead to greater deposition behavior of MnO₂ colloids onto surfaces.The dissipative properties of the deposited layer were also influenced by surface type,electrolyte concentration,and organic matter characteristics.The“softest”deposited MnO₂ colloidal layer was formed on the silica surface while more rigid layer formed from the deposition of MnO₂ colloids onto magnetite and alumina.The effects of common environmental factors such as ionic strength,surface type,pH value and natural organic matters on the release behavior of MnO₂ colloids form above three representative surfaces in aquatic systems were also discussed.The results indicated that the decrease in background electrolyte concentration can lead to the release of MnO₂ colloids.The type of surface was of a significance on the release of MnO₂ colloids.Specifically,the release rate and friction were highest at the SiO₂surface while the lowest in the case of Al₂O₃ surface,which depends on the electrostatic interaction between surface and particles.The change of the solution pH will also lead to the release of MnO₂ colloids.The release rate at pH 9.85 was significantly higher than that at the pH value of 4.5,indicating that the alkaline conditions were more conducive to the transport of MnO₂ colloids in the water environment.In the presence of macromolecular organics,due to the adsorption of BSA on surface can produce an attractive effect,BSA showed an inhibiting effect on the release of particles from the surface.Conversely,in the presence of humic acid,MnO₂ colloids was more tend to be released from SiO₂ surface.For engineered nanoparticles,the ion specific effects are of great significance to their transport behavior on the environmental surfaces.Therefore,fullerene(C₆₀)and cerium dioxide（CeO₂）nanoparticles with different electrical properties were selected as representative engineered nanoparticles in the present study.The influence of the ion specific effects of added ions and natural organic matter and organic matter on the deposition behavior of the two nanoparticles were explored.The results demonstrated that the cations can decrease the deposition rates of different charged NPs with the decreasing hydration degree of additive cations in the order of Li⁺>Na⁺>K⁺>Rb⁺.The variance in the deposition behaviors of NPs at fixed electrolyte concentration can be quantitatively explained by a modified DLVO theory introduced the hydration forces,which arose a shortrange repulsion within the model that dominated the colloidal interactions of NPs with surface and resulted in these specific effects.Moreover,two biomacromolecules impacted the mobility of both NPs in the opposite way:BSA could enhance the deposition of NPs on silica surface while alginate can decrease the deposition and enhance their mobility in aqueous solutions,dependent on the molecular complexity.Due to the remarkable removal efficiency of nanoparticles in heavy metals,MnO₂nanoparticles was selected as the representatively to examine the effect of nanosized manganese oxide on the removal efficiency of trace Pb²⁺,Cd²⁺and Ni²⁺in surface water during the coagulation process in the environmentally relevant conditions,using the traditional coagulants Al₂（SO₄）₃ and FeCl₃.Generally,coagulant types did not impact removal efficiency of trace heavy metals.Nanosized MnO₂ could efficiently strengthen the trace heavy metal removal from mimic water.Water pH,competitive Ca²⁺,and humic acid affected heavy metal removal to a certain extent.Low pH lead to a higher elemental residual,while higher removal efficiencies were observed under alkaline conditions.Ca²⁺additive resulted in a lower removal of all three selected heavy metals.Similarly,the introduction of humic acid led to a significant increase in residual heavy metals.Overall,these results provide insights into the deposition behavior of nanoparticles on environmental surfaces and have significant implications for predicting the transport potential of common engineered nanoparticles in natural environments and engineered systems.Besides,Coagulation enhanced by MnO₂ may offer some useful suggestions to develop the conventional drinking water treatment processes for the removal of trace harmful heavy metals from contaminated surface source waters.