Aqueous Aggregation Behavior And Mechanism Of Iron(Hydr) Oxide Nanoparticles

Iron?hydr?oxide nanoparticles are widely used in the environmental engineering restoration fields as adsorbents and catalysts due to their high adsorption capacity for organics and heavy metals in the environment.Also in natural aquatic environments,iron?hydr?oxide nanoparticles can form easily through hydrolysis and precipitation processes.However,their aggregation behaviors,which determine their stability and the transport of the associated aqueous contaminants,are far from well understood.Here,the aggregation of magnetite nanoparticles?MNPs?and ferrihydrite nanoparticles?FNPs?,representing engineered and natural iron?hydr?oxide nanoparticles were systematically investigated.MNPs and FNPs are ubiquitous in many environments,which with characteristics of large specific surface area,high redox reactivity,high adsorption capacity,non-toxic,easy synthesis,and so on.Bare MNPs can easily aggregate in aquatic systems,thus previous researchers have tried to coat with organic stabilizing agents to improve their colloidal stability,while FNPs always contains various metal impurities during formation.Therefore,it is very important to study organic acid coated MNPs and heavy metal doped FNPs aggregation behaviors and mechanisms under varied aqueous environments,which has great significance for further understanding the behavioural process of environmental pollutants,and apply for engineering remediation.In this study,three organic acids?citric acid,poly?acrylic?acid,and acetic acid?were coated on MNPs surfaces to synthesize CA-MNPs,PAA-MNPs,and AA-MNPs,through co-precipitation under hydrothermal conditions;FNPs and heavy metal?Pb,Mn,Al and Cr?doped FNPs were synthesized by co-precipitation under room conditions.The size evolutions with time of synthesised nanoparticles were measured with dynamic light scattering?DLS?to represent their aggregation behaviors.We systematically investigated the aggregation behaviors of MNPs and FNPs under varied aqueous conditions,i.e.,varied pH?5.0?9.0?,ionic strength(0.03?1400 mmol·L^-1),in the presence of electrolytes?NaCl,CaCl₂,Na₂SO₄,and NaH₂PO₄/Na₂HPO₄?,and varied organics?humic acid,fulvic acid,and CH₃COONa?with different molecular weights.In addition,the aggregation mechanism was investigated by measuring zeta potential of nanoparticles,adsorption experiment and critical coagulation concentration?CCC?calculation with Derjaguin-Landau-Verwey-Overbeek?DLVO?theory.This study aims to investigate the stability of varied iron?hydr?oxide nanoparticles in many natural aqueous environments,and enables a better prediction of fate and transport of organic and inorganic contaminants in many natural environment.The main research results are as follows:1.In the natural and engineered aqueous environments with pH range of 5.0?9.0,the effects of citric acid?CA?,poly?acrylic?acid?PAA?,and acetic acid?AA?on the stability of magnetite nanoparticles?MNPs?were investigated.The zeta potential values of CA,PAA,and AA coated MNPs were more negatively charged over the pH range of 5.0?9.0 than pure MNPs,thus increasing the stability of MNPs in many natural aqueous environments.The effect of three organic acids on MNPs zeta potential values followed the order:PAA>CA>AA,which as the same order as the number of-COOH functional groups in organic acids.In addition,the pH isoelectric point(pH_IEP)of three organic acids coated MNPs all decreased compared with pure MNPs,and both their pH_IEP and zeta potential values from low to high followed the trends:PAA-MNPs<CA-MNPs<AA-MNPs.2.The effects of different electrolyte solutions and ionic strengths on the stability of CA,PAA and AA coated MNPs were investigated.In the presence of the same electrolyte solution,the aggregation behaviors of three organic acids coated MNPs were similar.With the increase of ionic strength,the stability of three organic acids coated MNPs decreased.As the electrical double layer was compressed at high ionic strength,and their zeta potential values became less negative,resulting in their fast aggregation.Under the same pH?5.0±0.2 and 7.0±0.2?and ionic strength,three organic acids coated MNPs were more stable in the presence of monovalent cation?Na⁺?compared with the presence of divalent cation(Ca²⁺).This was because that divalent Ca²⁺could neutralize the surface charge more significantly due to adsorption or bridge effect between Ca²⁺and carboxyl groups on CA-MNPs surface,thus promoted their aggregation.In addition,the ratios of CCC values in the presence of Ca²⁺over Na⁺were around z^-6 at pH 5.0±0.2 and 7.0±0.2,which agreed well with the Schulze-Hardy rule.At pH=7.0±0.2,the effects of different anions on the aggregation stability of organic acids coated MNPs were quite different under low IS,and the aggregation stability can be ranked as follows:phosphate>sulfate>chloride,which is consistent with the trends of CCC values in different anions.Because higher valence of anions have stronger the adsorption capacity on MNPs surface lead to stronger stability.Moreover,the aggregation behavior of three organic acides coated MNPs and their calculated CCC all agree well with DLVO theory.3.It was confirmed that the presence of natural organic matter?humic acid,HA?inhibited the aggregation behavior of CA-MNPs,PAA-MNPs,and AA-MNPs in the presence of electrolyte solutions.At pH=7.0±0.2,humic acid?HA?with the concentration of 2 mg C·L^-1,can significantly inhibit the aggregation of three organic acids coated MNPs in the presence of different electrolyte solutions.Here humic acid adsorbed onto MNPs surface increased the initial size of the nanoparticles and made the surface more negatively charged,thus increasing the electrostatic and steric repulsive forces between MNPs.Accordingly,humic acid improved MNPs stability in different electrolyte solutions.4.In the presence of organics,The effect of organic on the aggregation behavior and mechanism of ferrihydrite nanoparticles?FNPs?in the presence of different organic matters and their added amount were studied by dynamic light scattering?DLS?.In the presence of humic acid?HA?,fulvic acid?FA?,and CH₃COONa organics,the pH_IEP of ferrihydrite nanoparticles?FNPs?all shifted from neutral to acid pH conditions.Because these organics containing-COOH functional groups can be adsorbed onto FNPs surfaces,and these-COOH functional groups deprotonation under certain pH conditions could making the surface negatively charged.Besides,more organic adsorbed onto FNPs surface at higher C/Fe ratios,resulting in lower pH_IEP of FNPs.Compared with CH₃COONa,HA/FA showed stronger effects on shifting the pH_IEP of FNPs under the same C/Fe ratios.Moreover,at neutral pH conditions,HA/FA inhibited FNP aggregation significantly at concentrations higher than 0.5 mg C·L^-1,while CH₃COONa did not,due to different effects on steric and/or electrostatic interactions among FNPs by organics with varied pK_a values and molecular weights.5.The fast aggregation of FNPs in the presence of Na₂SO₄ was clarified by using FT-IR.Under both acidic and neutral pH conditions,in the presence of Na₂SO₄,at all pH conditions?2¹0?and ionic strength(0.01?100 mmol·L^-1),the zeta potentials of FNPs are always approximately neutral due to sulfate ion adsorption and FNPs phase transformation to schwertmannite.This phase transformation of FNPs to schwetmannite resulted in rapid aggregation in the presence of sulfate.In addition,under acidic and neutral pH conditions,their aggregation behavior of Pb,Mn,Al,and Cr doped FNPs in the presence of sulfate were similar as pure FNPs.6.Under acidic and neutral pH conditions,in the presence of various electrolyte solutions,the aggregation behavior and their difference of Pb,Mn,Al,and Cr heavy metal doped ferrihydrite?Mt-FNPs?were clarified.In the presence of NaCl and CaCl₂ solutions,Mt-FNPs were all stable at pH=5.0±0.1,and particle size remained²0 nm within 5 min.These Mt-FNPs aggregated rapidly under neutral pH conditions,and the particle sizes were in the range of 200⁶00 nm within 5 min.This is due to the pH_IEP of four Mt-FNPs in the range 7.6?8.8,and the neutral pH condition is close to their pH_IEP.In the presence of CH₃COONa,four varied Mt-FNPs all aggregated rapidly under both acidic and neutral pH conditions,and all the aggregation rates were relatively higher under neutral pH conditions.However,under different conditions,the aggregation behaviors of Pb-FNPs,Mn-FNPs,Al-FNPs,Cr-FNPs,and pure FNPs were similar.To sum up,the aggregation behavior of iron?hydr?oxide nanoparticles in natural and engineering water environments is affected by varied environmental factors,such as solution pH,ionic strength,electrolyte solutions,varied organic matters and amount,as well as the types of co-existing organic acids and metal ions.The stability of iron?hydr?oxide nanoparticles are related to deprotonation,adsorption,bond bridging and surface phase transformation,which can change the electrostatic and steric repulsive forces between iron?hydr?oxide nanoparticles to affect their aggregation.