Study On The Aggregation Kinetics Of Typicle Engineering Nanoparticles (CeO₂ NPs,Cit-Ag NPs,Ag₂S NPs) And Effects Of Extracellular Polymers Substance Extracted From Activated Sludge

There is increasing concern regarding engineered nanoparticles(ENPs)due to the rapid growth of the nanotechnology industry.Thus,ENPs can be released into the environment in multiple ways,and wastewater treatment plants(WWTPs)are considered to be the first distribution route.The aggregation of NPs plays a vital part in their stability and environmental risks by affecting their transport,fate,bioavailability and biological behaviors.Previous researches have proved that due to their small size and high capacity to convey toxic substances,nanoparticles might have unfavorable influences on bacterial communities.In wastewater treatment reactors,EPS covers the surface or fills in the interior of cells of microbial aggregates.Therefore,the EPS of sludge could be an important factor in the aggregation of ENPs.Considering there is a great chance that ENPs will enter WWTPs,where EPS is abundant,the presence of EPS is assumed to affect the aggregation of ENPs and thereby influence their performance in wastewater treatment.This study takes typical positively charged engineering CeO₂nanoparticles and typical negatively charged engineering Cit-Ag nanoparticles and its transforming particles Ag₂S nanoparticles as research objects,to explore the aggregation behavior of nanoparticles in common salt solutions.The effects of common environmental factors(ionic strength,ionic valence,and the rich sludge extracellular polymers(EPS)in wastewater treatment systems)are mainly investigated,and the agglomeration characteristics and interaction mechanism between EPS.The theoretical model was used to explore the aggregation characteristics of different nanoparticles and the interaction mechanism between EPS and nanoparticles.The early stage of aggregation of cerium oxide nanoparticles(CeO₂ NPs)in anion solutions was inspected in the absence and presence of extracellular polymeric substance(EPS)with a help of time-resolved dynamic light scattering(DLS).The aggregation kinetics and attachment efficiencies were calculated according to measured hydrodynamic diameter across a range of 1-500 m M Na NO₃ and 0.01-100m M Na2SO4.The aggregation of CeO2 NPs in both Na NO3 and Na₂SO₄solution conformed with the Derjaguin-Landau-Verwey-Overbeek(DLVO)theory.In Na NO₃solution,the critical coagulation concentrations(CCC)of CeO₂NPs was calculated to be about 47 m M;in Na₂SO₄solution,CeO2 NPs showed a re-stabilization process and thus there was no CCC value.SO4^2-had intenser effects on CeO2 NPs aggregation than NO₃^-might because of the distinction between their polarization,consisting in Hofmeister series.The presence of bound EPS(B-EPS),tightly bound EPS(TB-EPS)and loosely bound EPS(LB-EPS)in Na NO₃ solutions all lead to significant decrease in CeO₂ NPs aggregation.Steric repulsive force produced by absorbed EPS on CeO₂ NPs might take main responsibility in stabilizing CeO₂ NPs.Besides,Extended Derjaguin-Landau-Verwey-Overbeek(EDLVO)model successfully predicted the energy barrier between CeO₂ NPs with B-EPS,TB-EPS and LB-EPS as a function of Na NO₃concentration.Furthermore,the difference in impeding the CeO₂ NPs aggregation with B-EPS,TB-EPS and LB-EPS may be caused by the divergence in molecular weight and component mass fraction especially protein content.These results might subserve the assessment on the fate and transport behaviors of CeO₂ NPs released in wastewater treatment plants.This article also takes the negatively-charged common engineering nanoparticles Cit-Ag and Ag₂S nanoparticles as research objects to explores the effcts of cation strength,ionic valence,the extracellular polymers of sludge and their components(extracellular polysaccharides and extracellular proteins)on Cit-Ag NP and Ag₂S NP aggregation behavior.The results indicated that aggregation of Cit-Ag nanoparticles and Ag₂S nanoparticles were facilitated as the ionic strength incereased during a certain concentration range.In addition,in salt solutions containing monovalent or divalent cations,the agglomeration behavior of Cit-Ag NPs and Ag₂S NPs basically conforms to the classic DLVO theory.The critical coagulation concentrations in Na NO₃ solution are30m M and 275m M,respectively and the critical agglomeration concentrations in Ca(NO₃)₂ solution were 1.6 m M and 2.5 m M,respectively,indicating that the divalent cations promoted the aggregation of the nanoparticles more strongly than the monovalent cations.Besides,Ag₂S nanoparticles are more stable than Cit-Ag nanoparticles in all texted salt solutions,which can be attributed to the difference in van der Waals forces between the respective particles.The calculation of EDLVO action energy confirmed that steric hindrance makes a critical impact in the effect of EPS on the aggregation of Cit-Ag NPs and Ag₂S NPs in the presence of monovalent cations,which might because EPS obsorbed onto the surfaceof nanoparticles.In Na NO₃ solution and low-concentration Ca(NO₃)₂ solution,the addition of EPS greatly increased the stability and inhibited the aggregation of Cit-Ag NPs and Ag₂S NPs.The protein-rich AT-EPS inhibited most strongly,followed by B-EPS,and finally PT-EPS,which is rich in extracellular polysaccharides,indicating that extracellular proteins play an important role in the process of suppressing nanoparticle aggregation.Moreover,the stability of Ag₂S NPs were stronger than that of Cit-Ag NPs in the presence of B-EPS,AT-EPS and PT-EPS,that might be due to the difference in the adsorption capacity of B-EPS,AT-EPS and PT-EPS on the surfaces of the two kinds of nanoparticles.However,at high Ca(NO₃)₂concentrations,the presence of B-EPS or PT-EPS acccelarated aggregation of Cit-Ag NPs and Ag₂S NPs,which can be attributed to the aggregation of the dissolved extracellular polysaccharide macromolecules to connect the nanoparticles with the macromolecular aggregates through intermolecular bridges.The macromolecular aggregates are connected together,thereby increasing the growth of the aggregates,reducing the toxicity to the organism and improving the removal efficiency.