Magnetic Nanoparticles Interaction with Humic Acid: In the Presence

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Magnetic Nanoparticles Interaction with Humic Acid: In the presence of Surfactants Zhi Tang, Xiaoli Zhao, Tianhui Zhao, Wang Hao, Peifang Wang, Fengchang Wu, and John P. Giesy Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01749 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 13, 2016

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Environmental Science & Technology

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Magnetic Nanoparticles Interaction with Humic Acid: In the

2

presence of Surfactants

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Zhi Tang†,‡, Xiaoli Zhao†,‡,*, Tianhui Zhao‡, Wang Hao‡, Peifang Wang§, Fengchang

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Wu ‡*,and John P. Giesy£

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‡

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Research Academy of Environmental Sciences, Beijing 100012, China;

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§

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Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing

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210098, People’s Republic of China

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Key Laboratory of Integrated Regulation and Resources Development on Shallow

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£

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Saskatchewan, Saskatoon, Saskatchewan, Canada.

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Author Contributions：†These authors contributed equally to this work

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* To whom correspondence may be addressed

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Telephone number: +86-10-84915312

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Fax number: +86-10-84931804

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E-mail: [email protected], [email protected].

Department of Veterinary Biomedical Sciences and Toxicology Centre, University of

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1

ACS Paragon Plus Environment


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Abstract

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Adsorbed Humic acid (HA) on surfaces of nanoparticles （NPs）will affect their

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transport, transfer, and fate in the aquatic environment, especially in the presence of

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surfactants，and thereby potentially alter exposures and bioavailable fractions of NPs

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and surfactants. This study investigated adsorption of HA on Fe3O4 NPs in the

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presence or absence of surfactant. Surfactant established a bridge connecting HA and

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Fe3O4 NPs, and significantly changed adsorption behavior of HA on NPs. Adsorption

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of HA in the absence of surfactant was 120.3 mg/g, but 350.0 mg/g and 146.5 mg/g in

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the present of CTAB (Hexadecyl trimethyl ammonium bromide) and SDS (sodium

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dodecyl sulfate), respectively. Surfactants can form different stages (hemimicelles,

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mixed hemimicelles and admicelles) on Fe3O4 NPs by electrostatic and hydrophobic

29

interactions, adsorption of HA was different for each of those stages. Adsorption of

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HA on surface of Fe3O4 NPs/CTAB was co-determined by hydrophobic, electrostatic

31

interactions and ligand exchange. The presence of CTAB or SDS changed

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mechanisms for adsorption and effects of functional groups. Results of Fourier

33

transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) indicated that

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carbohydrate carbon was important in adsorption of HA on Fe3O4 NPs in the presence

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of surfactants.

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Introduction

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In the last decade, there have been applications of nanoparticles (NPs) in

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commerce and increasing concerns about their potential effects on aquatic organisms

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and humans since NPs have been shown to accumulate in and be toxic to plants and

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animals1-4. More and more attention has been drawn to environmental behavior of

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NPs made of various metals, such as Ag5, copper6, titanium oxide7, alumina8.

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Magnetic nanoparticles (MNPs) have the unique property that they can easily be

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separated from water by use of a magnetic field due to their superparamagnetic

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properties9-13. For this reason their behavior in the environment might be more

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complex. In fact，in addition to synthetic MNPs engineered by humans, there are a

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number of natural nanoscale iron and iron oxide particles that exist in nature,

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especially during mining operations. A key issue in assessing fated of NPs in the

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environment and potential exposure of aquatic organisms to NPs and subsequent toxic

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effects are dependent on their bioavailable fraction14-16.

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NPs have been perceived as powerful adsorbents for a wide variety of

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hydrophobic organic compounds (HOCs) and heavy metals10,17. Therefore, sorption of

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toxic chemicals by NPs may not only make NPs more toxic, but also affect

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environmental transport, fate, and bioavailability of chemicals18. Surfactants are the

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most discharged synthetic chemicals to the environment and can have adverse effects

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on membranes of organisms19. Furthermore, due to their special properties of low

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molar concentrations, and large surface activities, surfactants have been more and

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more used in synthesize and modify various nanomaterials20-24. Effects of mixtures of

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surfactants and NPs are of concern, since they can occur together, especially in

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wastewater effluents. 3



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Natural organic matter (NOM), a key component of both soils and sediments, is

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ubiquitous in the environment and known to be a predominant factor in determining

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the stability of NPs in suspension in the aqueous phase25,26. This can have a large

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effect on aggregation of MNPs and influence fate and transport of MNPs and

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determine their bioavailable fractions. In the aqueous environment, HA typically

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represent an important portion of NOM27,28, and have significant reactivities with

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other contaminants through various types of functional group such as carboxylate,

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phenolic and carbonyl functional group29-32. Environmental behavior of MNPs is

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strongly influenced by HA, especially in certain ranges of pH and ionic strength33-35.

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Given the widespread presence of HA and surfactants in aquatic environments, NPs

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will inevitably interact with them, which could greatly influence environmental

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behavior of NPs. For instance, carbon nanotubes were stabilized in the aqueous phase

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by NOM, which might provide sterically and electrostatically stable surfaces on

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carbon nanotubes7. Surfactants might provide similar effects after adsorption on NPs,

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so effects of surfactants on behaviors of NPs in aquatic environments cannot be

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ignored4. In fact, to accurately describe behaviors of MNPs in the environment,

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mechanism of interactions between NOM and MNPs, and effects of surfactants on

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this interaction need to be elucidated. However, there is limited information about

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strengths of interactions of MNPs with HA in the presence of surfactants.

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Interactions of MNPs with HA in the presence of surfactants are complex.

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Surfactants can self-assemble on surfaces of MNPs by hydrophobic interaction and

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electrostatic attraction36,37. Due to changes in properties of surfaces of MNPs such as

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charge, hydrophobic and isoelectric point (iep), their stabilization (suspension) can

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change38,39. Thus, HA and surfactants affect dispersion or aggregations of NPs. Ionic

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surfactants can bind to HA in aquatic environments

19,40,41

4


, which can change

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physicochemical characteristics of HA, such as charge density, hydrophobicity and

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internal structures, thereby affecting forms and environmental dynamics of both HA

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and surfactants42. Understanding mechanisms of adsorption of each constituent is

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important for determining environmental behavior in complex environments36,37. The

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self-assembly of surfactants on surfaces of MNPs can be divided into three stages:

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hemimicelle, mixed hemimicelle and admicelle37. At each stage properties of surfaces

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on MNPs change, which provides a powerful tool for studying mechanisms of

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adsorption.

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To the best of our knowledge, previous studies focused primarily on adsorption

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of HA to NPs, and interaction of HA and MNPs in the presence of surfactants had not

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been investigated. In this study, Fe3O4 NPs with diameter of approximately 10 nm was

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synthetized, and effects of cationic surfactant, cetyltrimethylammonium bromide

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(CTAB) and anionic surfactant, sodium dodecyl sulfate (SDS) on HA adsorbed on

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surfaces of Fe3O4 NPs during three stages of adsorption were investigated. Fourier

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transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and

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zeta-potential were used to explore the mechanisms of interaction.

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Materials and methods

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2.1 Chemicals and Materials

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Sodium hydroxide and hydrochloric acid used in this study were purchased from

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Beijing Chemicals Corporation. Ferric chloride (FeCl3·6H2O), ferrous chloride

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(FeCl2·4H2O), Cetyltrimethylammonium bromide (CTAB) and Sodium dodecyl

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sulfate (SDS) were purchased from Sigma-Aldrich Co. Ultrapure water used in all of

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the experiments was prepared by using Milli-Q SP reagent water system (Millipore,

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Bedford, MA, USA). 5



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Soil-derived HA （SHA）was extracted, based on the procedure recommended by

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the International Humic Substances Society (IHSS)43, from the soils from Jiu

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Mountain in Beijing, China. Contents of ash (3.01%), C (51.32%), H (3.29%), N

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(1.16%) and S (1.65%) of HA were determined by use of elemental analysis

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(Elementar vario macro EL, Germany). The SUVA254 and SUVA280 (the UV

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absorbance of a given sample determined at 254, 280 nm and divided by the organic

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carbon content of the solution) were determined by UV/visible absorbance (Agilent

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8453, USA). In general, SHA exhibited greater aromaticity, and a greater degree of

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humification compared to the HA of International Humic Substances Society (IHSS)

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(the SUVA254 and SUVA280 are 3.34 , 2.60 L/mg﹒m, respectively).

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Fe3O4 NPs were prepared by coprecipitation method44. The concentration of

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Fe3O4 NPs in suspension was 10 mg/mL. HA were dissolved with 0.5 mol/L NaOH to

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prepare solutions containing 500 mg/L with the pH adjusted to 10.0. Surfactants were

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dissolved with deionized water to prepare solutions containing 10 mg/mL. The

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isoelectric point (iep) for Fe3O4 NPs was approximately pH 6.0 ( Fig S1), which was

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consistent

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photomicrographs (TEM) of Fe3O4 NPs, Fe3O4 NPs /CTAB, Fe3O4 NPs/HA and

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Fe3O4 NPs/CTAB/HA were shown in Fig. S2. Fe3O4 NPs were nearly uniform with

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a size of approximately 10 nm.

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2.2 Sorption Experiments

with

previously

reported

values

36,45 .

Transmission

electron

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Adsorption of HA on surfaces of Fe3O4 NPs was conducted in batch mode by

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adding 10 mg Fe3O4 NPs to various concentrations of HA solution, with the pH

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adjusted to 5.0, the container was 120 mL polycarbonate bottle and was brought to a

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final volume to 100 mL. 6


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10.0 mg Fe3O4 NPs was added to surfactants (0-300 mg/L) in a 120 mL

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polycarbonate bottle, and pH was adjusted from 4.0 to 12.0 with 0.1 M NaOH and

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HCl. The mixture was sonicated for 5 min and placed for 30 min. Various amounts of

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HA were then added to the 120 mL polycarbonate bottle containing suspensions of

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surfactant-coated Fe3O4 NPs, and was brought to a final volume to 100 mL. Finally,

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Fe3O4 NPs that adsorbed surfactant and HA from the suspension, were isolated by use

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of a strong, rectangular (150×130×50 mm) Nd-Fe-B magnet. After approximately 5

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min, the suspension clarified and concentrations of HA were determined. The

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precipitate was lyophilized and analyzed using TEM, FTIR and XPS.

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Adsorption of HA on Fe3O4 NPs was also studied in natural surface water and

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influences of CTAB and SDS determined. Natural water was collected from the Qing

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River in Beijing. pH of river water was 8.6, and temperature was 27.6 ℃. The study

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in the absence of surfactants was conducted in batch mode by adding 10.0 mg Fe3O4

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NPs, 2, 5 or 10 mg CTAB or 5 mg SDS with 10 mg Fe3O4 NPs added to natural water.

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The container was a 120 mL polycarbonate bottle and was brought to a final volume

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to 100 mL.

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2.3 Characterization

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The amount of HA adsorbed on Fe3O4 NPs was determined from differences in

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UV-vis absorbance at 254 nm. UV-vis absorbance measurements were performed by

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using a 1 cm optical path length quartz cuvette with an Agilent Technology 8453

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spectrophotometer46,47. Concentrations of HA in nature water were characterized by

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total organic C (TOC) (Multi N/C 3100, Germany). ζ-potential of particles were

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measured by a Zetasizer Nano ZS (Malvern Instruments, Malvern, UK). Transmission 7



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electron microscopy (TEM; H-7500, Hitachi, Tokyo, Japan) was used to observe the

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morphology and sizes of particles of Fe3O4 NPs, and changes before and after

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adsorption with surfactant and HA. Fourier transform IR (FTIR) spectra of Fe3O4 NPs,

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surfactant coated Fe3O4 NPs and HA-surfactant-coated Fe3O4 NPs were measured by

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using a Nicolet Magna-IR 750 FTIR spectrometer (Nicolet Magna-IR 750, Nicolet,

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USA). The KBr powder was used as a background. FT-IR spectra were recorded from

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400 to 4000 cm-1 at a resolution of 4 cm

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characteristics of samples were determined by X-ray photoelectron spectroscopy

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(XPS) by use of a PHI Quantera SXMTM spectrometer equipped with a

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monochromatic AlKa X-ray source (1486.6 eV, 600 W and 15 kV). Binding energies

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were corrected by the adventitious C 1s calibration peak at 284.8 eV. A charge

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neutralizer filament was used during all measurements to compensate for charging of

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the samples. High-resolution XPS was performed using a 0.1 eV step size and pass

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energy of 23.5 eV (the depth of samples analyzed ranged from 8 to 10 nm). Data

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processing and peak fitting were performed using XPS Peak Processing.

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Results and Discussion

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3.1 Adsorption of HA on Fe3O4 NPs

-1

averaged over 200 scans. Surface

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For comparison, adsorption of HA on Fe3O4 NPs was measured in the absence of

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surfactant. Adsorption of HA was inversely proportional to pH, and when the solution

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pH reached 10.0, the HA was hardly adsorbed on surfaces of Fe3O4 NPs (Fig. 1a).

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This result indicates that pH strongly influenced adsorption, as has been commonly

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observed for HA adsorbed on oxide surfaces. The mechanism might be electrostatic

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interaction. At lesser pH, surfaces of Fe3O4 NPs were more positively charged,

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whereas HA was less negatively charged48. However, pH of the solution was not the 8


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only mechanism affecting adsorption of HA on surfaces of Fe3O4 NPs. Adsorption of

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HA might also be controlled by other mechanisms such as ligand exchange and

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inner-sphere complexing7,48.49. Hydrophobic effects might not a mechanism for

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adsorption of HA on surfaces of Fe3O4 NPs, because surfaces of metal oxides are

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hydrophilic, and exhibited poor adsorption for hydrophobic organic compounds50. The

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adsorption isotherm (Fig.1b) was fit according with to the Langmuir equation and

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maximum adsorption capacity of HA on Fe3O4 NPs surface was 120.3 mg/g (Table

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S1).

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The ζ-potential isotherm was an useful for understanding surface-charges of

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Fe3O4 NPs37. The ζ-potential of Fe3O4 NPs after adsorption to HA is given (Fig. 1c).

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At pH 5.0, surfaces of Fe3O4 NPs were positively charged, and HA can be adsorbed

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onto its surfaces by electrostatic interactions and ζ-potential of Fe3O4 NPs/HA rapidly

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shifted from positive to negative after HA was adsorbed on the surface of Fe3O4 NPs,

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(Fig. 1c). This might be due to the fact that HA had almost fully occupied surfaces of

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Fe3O4 NPs, such that positive charges on surfaces of Fe3O4 NPs were neutralized by

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HA. This also demonstrates that electrostatic interaction is one of the mechanisms. At

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greater concentrations of HA, ζ-potentials of Fe3O4 NPs/HA decreased then remained

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constant. This observation indicates that HA in solution can continue to be adsorbed

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on surfaces of Fe3O4 NPs /HA by hydrophobic interactions of HA molecules.

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3.2 Adsorption of Surfactants on Fe3O4 NPs.

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The three stages of adsorption of surfactants on Fe3O4 NPs can be easily inferred

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from the ζ-potential isotherm37. ζ-potential isotherms of cationic surfactants (CTAB)

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and anionic surfactants (SDS) can be divided into three stages: hemimicelles, mixed

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hemimicelles, and admicelles (Fig. 2). In the first stages, ζ-potential of Fe3O4 NPs was 9



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negative at pH 10.0, which is greater than the isoelectric point (iep), and cationic

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surfactants were sparsely adsorbed onto negatively charged Fe3O4 NPs surface. The

207

ζ-potential shifted from negative to zero due to gradual formation of hemimicelles on

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surfaces of Fe3O4 NPs. The range of 20.0-100.0 mg/L for CTAB are in the region of

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mixed hemimicelles (Fig. 2a). The mechanism of adsorption in this region is via both

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ionic, surfactant-oxide electrostatic interaction and lateral hydrophobic interactions

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between adsorbed surfactant monomers. In the second stage, upon increasing the

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cationic surfactant concentration, surfactant bilayers called admicelles were formed

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on surfaces of Fe3O4 NPs. This was caused by hydrophobic interactions between

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hydrophobic chains of surfactant molecules. Results of previous studies51-53 indicated

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that admicelles are discrete surface aggregates, which cannot be defined as a

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monolayer or bilayers, because the outer surface of admicelles is positive, the

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ζ-potential increases from zero to positive. The amount of surfactants that should be

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added to samples can be inferred from the corresponding ζ-potential isotherm. In the

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last region, the concentration of CTAB was greater than the critical concentrations of

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micelles (CMC), aqueous surfactant micelles were in equilibrium with admicelles and

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the ζ-potential remained constant.

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When the pH was less than the iep, Fe3O4 NPs were positively charged. The

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process and mechanism of adsorption of the anionic surfactant SDS on surfaces of

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Fe3O4 NPs is similar to that of cationic surfactants. The ζ-potential isotherm can be

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divided into three regions (Fig. 2b), hemimicelles, mixed hemimicelles and admicelles

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in the range of 0 – 28.0, 28.0 – 60.0 and greater than 60.0 mg/L for SDS.

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3.3 HA adsorbed on Fe3O4 NPs surface in the present of surfactants

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Adsorption of HA on surfaces of Fe3O4 NPs was significantly different in the 10


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presence of surfactants. Effects of type and concentration of surfactants, pH of the

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solution and kinetics of adsorption were investigated. Surfactants established a bridge

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connecting HA and Fe3O4 NPs, and adsorption of HA increased with the

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concentration of surfactants adsorbed on Fe3O4 NPs and was significantly affected by

233

hydrophobic, electrostatic interactions and ligand exchange.

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Effect of pH on adsorption of HA on Fe3O4 NPs/surfactant. In the presence of

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cationic surfactant, adsorption of HA on surfaces of Fe3O4 NPs/CTAB was controlled

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by hydrophobic and electrostatic interactions. The influence of pH on adsorption of

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HA on Fe3O4 NPs/CTAB was explored by adjusting the pH from 4.0 to 12.0.

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Adsorption of HA at various pHs in the presence of CTAB are given (Fig. 3a).

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Adsorption of HA was significantly increased in the presence of CTAB. When the pH

240

was 10.0, adsorption of HA reached a maximum of 350.0 mg/g. This trend is

241

consistent with changes in charges on surfaces of Fe3O4 NPs, as a function of pH.

242

Surface charges of Fe3O4 NPs shifted from positive to negative, and charge density

243

increased. When pH was greater than 10.0, adsorption of HA was less and inversely

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proportional to pH. This was likely due to increasing ionization, solubility and

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hydrophilic of HA52. Due to electrostatic repulsion between molecules of HA, it is

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difficult for the chain of HA to coil so that it would take up greater volumes on

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surfaces of Fe3O4 NPs/CTAB and thus hinder adsorption of HA on surfaces of Fe3O4

248

NPs/CTAB.

249

To further investigate factors influencing adsorption, HA adsorbed on Fe3O4 NPs

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surface in the presence of SDS, which was typical anionic surfactant was studied at

251

several pHs in the presence of 10.0 mg Fe3O4 NPs and 40.0 mg/L SDS. Adsorption of 11



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HA was significantly greater at lesser (Fig. 3b). When pH approached 10.0, HA could

253

not be adsorbed on surfaces of Fe3O4 NPs. Under acidic conditions, SDS and HA

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could also be adsorbed on surfaces of Fe3O4 NPs through electrostatic interactions.

255

HA could also be adsorbed on surfaces of Fe3O4 NPs/SDS via hydrophobic

256

interactions, so adsorption of HA reached a maximum (146.5 mg/g). Under alkaline

257

conditions, the surface charge of Fe3O4 NPs shifted from positive to negative, such

258

that SDS and HA, both of which had negative charges cannot adsorbed on surface of

259

Fe3O4 NPs through electrostatic interaction. Therefore, adsorption of HA was

260

inversely proportional to pH.

261

Effects of concentration of surfactants. Effect of type and concentration of

262

surfactants on adsorption of HA on surfaces of Fe3O4 NPs was studied in batch mode.

263

Influences of CTAB and SDS on adsorption of HA on surfaces of Fe3O4 NPs

264

various pH were investigated (Fig. 4a, b). In the absence of CTAB, at pH10.0,

265

ζ-potential of Fe3O4 NPs was negative, and little HA was adsorbed on surfaces of

266

Fe3O4 NPs. In the presence of CTAB, adsorption of HA was significantly greater.

267

This result was primarily due to the surface charge of Fe3O4 NPs/CTAB gradually

268

being shifted from negative to positive after CTAB was adsorbed on surfaces of Fe3O4

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NPs. Adsorption of HA on Fe3O4 NPs/CTAB can be divided into three stages. During

270

the stage of hemimicelles (range 10.0 – 30.0 mg/L CTAB), with increasing

271

concentration of CTAB, the adsorption of HA increased significantly. This result

272

indicates that adsorption of HA on surfaces of Fe3O4 NPs /CTAB is strongly

273

influenced by concentration of CTAB. During the hemimicelles stage, monolayers of

274

surfactants adsorbing with their head group down on an oppositely charged Fe3O4

275

NPs surface, while the hydrocarbon tail-groups protrude into the solution, HA could

276

be adsorbed on surfaces of Fe3O4 NPs/ by hydrophobic interaction. In summary, 12


at

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during the mixed hemimicelles stage, after saturation of the surfactant on surfaces of

278

Fe3O4 NPs, hydrophobic interactions between tails of surfactants hydrocarbon chains

279

result in formation of admicelles. During that stage HA might bind to head groups of

280

admicelles by electrostatic attraction. However, in the range of 45.0-65.0 mg/L CTAB

281

also has mixed hemimicelles regions (Fig. 4a). In theory, adsorption of HA should

282

increase as a function of the concentration of CTAB during the mixed hemimicelles

283

stage, but the actual experimental result demonstrates that adsorption of HA was

284

significantly inversely related to concentrations of CTAB during the later stage of

285

mixed hemimicelles when concentrations of CTAB were greater than 45.0 mg/L.

286

There are two possibilities for this trend. The first possibility is that greater

287

concentrations of CTAB or SDS in water can result in greater solubility of HA in

288

water

289

at greater concentration of surfactant, the surfactants forms micelles, that could bind

290

to HA and minimize adsorption of HA on surfaces of Fe3O4 NPs, This result is

291

consistent with previous report 31,55.

55,56

, which then resulted in lesser adsorption of HA. The another possibility is

292

The effect of the concentration of SDS on HA adsorbed on Fe3O4 NPs was

293

studied at pH 5.0. Adsorption of HA slightly decreased as a function of increasing

294

SDS, which indicates that under acidic conditions, SDS can block HA from adsorbing

295

to surfaces of Fe3O4 NPs/SDS (Fig. 4b). This phenomenon might be caused by a

296

combination of hydrophobic and electrostatic interactions. The ζ-potential of the

297

surface of Fe3O4 NPs was positive in acidic solution. When the concentrations of SDS

298

was insufficient to occupying the entire surface of Fe3O4 NPs, HA could still be

299

adsorbed on surface of Fe3O4 NPs by electrostatic interactions. When the surface of

300

Fe3O4 NPs was completely occupied by SDS, HA could still be adsorbed on surfaces

13



301

of Fe3O4 NPs /SDS due to hydrophobic interactions between hydrophobic chains of

302

SDS and HA. Therefore, adsorption of HA was not significantly less.

303

ζ-potential Isotherm. The ζ-potential changed as a function of HA adsorbed on

304

Fe3O4 NPs /CTAB during phases of formation of micelles at pH 10.0 (Fig. 5a).

305

During the phase of formation of hemimicelles (10 mg/L CTAB), the monolayers of

306

CTAB forming on surfaces of Fe3O4 NPs due to the ζ-potential of Fe3O4 NPs was

307

negative at pH 10.0 (greater than the iep) , HA could adsorbed on the negatively

308

charged surfaces of Fe3O4 NPs/CTAB, mostly due to hydrophobic interactions. When

309

adsorption of HA on surface of Fe3O4 NPs/CTAB reached 20.0 mg/g, the ζ-potential

310

decreased and remained constant at amounts of HA greater 20.0 mg/g. When the

311

concentration of CTAB above 50.0 mg/L, although the ζ-potential of Fe3O4 NPs at pH

312

10.0 (greater than the iep) was still negative, but the amount of CTAB was enough to

313

form bilayers on surfaces of Fe3O4 NPs. The charge on surfaces of Fe3O4 NPs/CTAB

314

was positive, such that negatively charged HA could bind with positively charged

315

head-groups of CTAB, which on the outer surface of admicelles and the hydrocarbon

316

tail-group which on the surface of Fe3O4 NPs. Binding would have been due to

317

electrostatic attraction and hydrophobic interactions with these two moieties,

318

respectively. Previous results have indicated that the adsorption of HA was strongly

319

influenced by concentration of cationic surfactants, and in the concentration ranges

320

where hemimicelles or mixed-hemimicelles would form, the primary mechanisms of

321

adsorption were hydrophobic interaction and electrostatic interaction, respectively.

322

Changes of ζ-potential due to HA adsorbed on Fe3O4 NPs /SDS was investigated

323

at solution pH 5.0 (Fig. 5b). When the concentration of SDS was 10 mg/L

324

(hemimicelles phase), the ζ-potential of Fe3O4 NPs (less than the iep) is positive, but 14


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325

the amount of SDS was not sufficient to form monolayers on surfaces of Fe3O4 NPs,

326

so HA could not only adsorbed on negatively charged surfaces of Fe3O4 NPs/SDS

327

through hydrophobic interactions, but also the positively charged surfaces of Fe3O4

328

NPs through electrostatic interactions. When concentrations of SDS reached 50 mg/L

329

(mixed hemimicelles phase), SDS occupied the entire surface of the Fe3O4 NPs, so

330

that HA adsorbed on the surface of the Fe3O4 NPs/SDS only by hydrophobic

331

interaction, and

332

adsorption of HA was greater when electrostatic interactions occurred, in the range of

333

concentrations where hemimicelles would be formed, and the amount of SDS

334

influenced adsorption of HA on surfaces of Fe3O4 NPs/SDS.

adsorption of HA decreased. This result indicates that the

335

Adsorption kinetics. Kinetics of adsorption of HA on Fe3O4 NPs/CTAB and

336

Fe3O4 NPs/SDS was shown in Fig. S3. Due to faster kinetics of adsorption for smaller

337

particles, adsorption of HA was initially rapid, but after approximately 10 min it

338

decreased. The time required to reach equilibrium was 60 min. Kinetics of adsorption

339

of HA onto surfaces of Fe3O4 NPs/CTAB fit the linear pseudo-second-order kinetic

340

model (r2=0.9990-1.0000)57. Adsorption capacity (qe) at equilibrium, evaluated from

341

the pseudo-second-order plot, the constant k2, the initial sorption rate ho and

342

equilibrium adsorption capacity (qe) obtained from the slope and intercept of plots are

343

presented (Table S2). The results showed greater k2, ho and qe values at greater

344

concentrations of CTAB. Adsorption of HA increased from 61.7 to 153.9 mg/g as

345

concentrations of CTAB was increased from 20 to 40 mg/L. The initial sorption rate

346

ho increased from 42.2 to 714.3 mg/g/min, which suggested that CTAB would 15



347

accelerate adsorption of HA on Fe3O4 NPs/CTAB in water. Rapid, initial adsorption

348

was presumably due to co-determined by hydrophobic and electrostatic interactions,

349

and slower adsorption in the later stage represented a gradual uptake of HA at the

350

inner surface by complexation. The equilibrium adsorption capacity (qe) evaluated of

351

adsorption of HA on the surface of Fe3O4 NPs/SDS, from the linear

352

pseudo-second-order plot, decreased relative to Fe3O4 NPs/CTAB (Table S2).

353

Adsorption of HA on surfaces of Fe3O4 NPs in river water. Interactions of

354

magnetic nanoparticles with HA and surfactants in natural water is important and

355

might provide basic information for potential toxicity of nanoparticles. Adsorption of

356

HA on surfaces of Fe3O4 NPs in natural water, in the presence and absence of CTAB

357

and SDS was investigated (Fig. 6). Adsorption of HA in natural, river water slightly

358

decreased when Fe3O4 NPs was added to water. When Fe3O4 NPs/CTAB or Fe3O4

359

NPs/SDS was added to natural water, adsorption was less. Adsorption of HA followed

360

the order: 100 mg/L CTAB > 50 mg/L CTAB > 20 mg/L CTAB > 50 mg/L SDS >

361

absence of surfactant. In the presence and absence of CTAB or SDS, the phenomenon

362

and mechanism was similar to those of laboratory studies. The slight decrease of

363

adsorption efficiency might be due to natural water containing various chemical

364

compositions, influencing interactions of Fe3O4 NPs with surfactants and HA.

365

3.4 Mechanisms of Adsorption

366

FTIR Spectra. Surface chemistry of adsorption was studied using FTIR, which

367

could provide information about the mechanisms of adsorption. In the spectrum of

368

Fe3O4 NPs/HA, the hydroxy group on the Fe3O4 NPs shifted from the 3401 cm-1 band 16


Page 16 of 36

Page 17 of 36


369

to 3240 cm-1 band, and the intensity also significantly decreased, indicates that HA

370

had coated the surface of Fe3O4 NPs （Fig. S4）. In the spectrum of Fe3O4 NPs/HA the

371

peak at 1702 cm-1 was shifted to 1681cm-1, which is indicative of sorption of

372

carboxylate moieties. The adsorption of aromatic C=C or COOH in carboxylate can

373

be further distinguished from the shifting of the 1615 cm-1 band in HA to the

374

1570cm-1 band in Fe3O4 NPs/HA. Similar behavior of carboxylate in HA was

375

observed on goethite and Fe2O3 surface33,59. Adsorption of methyl or methylene on

376

surfaces of Fe3O4 NPs is represented by shifting of the 1423 and 1382 cm-1 bands of

377

HA to the 1420 and 1382 cm-1 bands of Fe3O4 NPs/HA. The above results indicate

378

successful adsorption of HA on surfaces of Fe3O4 NPs.

379

Comparison of spectra in the presence and absence of surfactant could elucidate

380

the role of surfactant in adsorption of HA. Surface properties of Fe3O4

381

NPs/CTAB/HA are shown (Fig. S5a). There were significant differences in the FTIR

382

spectrua of Fe3O4 NPs and that of Fe3O4 NPs/CTAB. The peaks at 2925, 2849 and

383

1467 cm-1 were characteristic absorption peaks of stretching vibrations of -CH2 and

384

rocking vibration in plane of the long-chain alkyl of CTAB. The peak at 1621 cm-1,

385

attributed to the stretching vibration of the primary amide, indicates that successful

386

adsorption of CTAB on surfaces of Fe3O4 NPs, and that the carboxyl on the Fe3O4

387

NPs surface had reacted with CTAB, and formed the primary amide. The change in

388

the Fe3O4 NPs/CTAB/HA spectrum was significantly different from that of Fe3O4

389

NPs/HA. Peaks at 2917 and 2849 cm-1 observed in the spectrum of Fe3O4

390

NPs/CTAB/HA were attributed to the dissymmetrical and symmetrical stretching

391

vibrations of aliphatic -CH2-. Intensity significantly increased compared to Fe3O4

392

NPs/CTAB, especially the dissymmetrical stretching vibrations which were stronger. 17



393

Since these two peaks did not appear the spectrum of Fe3O4 NPs/HA, -CH2 in HA

394

cannot adsorbed directly by the surface of Fe3O4 NPs, but it can be adsorbed on the

395

surface of Fe3O4 NPs/CTAB through interactions with CTAB. The peaks at 1465 and

396

1378 cm-1 in the spectrum of Fe3O4 NPs/CTAB/HA were attributed to stretching

397

vibrations of methyl or methylene. The same as the HA adsorbed on the surface of

398

Fe3O4 NPs, the stretching band of carboxylate COO- of the adsorbed HA fractions

399

onto Fe3O4 NPs shifted from 1615 to 1570 cm-1, which indicated that aromatic C=C

400

functional groups of HA might have been adsorbed on the surface of Fe3O4

401

NPs/CTAB. However, the peak of the stretching band of carboxylate COOH of HA

402

fractions was not observed in the spectrum of Fe3O4 NPs/CTAB/HA, which indicates

403

that carboxylate COOH in HA fractions might not be adsorbed on the surface of

404

Fe3O4 NPs/CTAB surface.

405

The FTIR spectrum of HA adsorbed on Fe3O4 NPs in the present of SDS is

406

shown(Fig. S5b). The FTIR spectra of Fe3O4 NPs and Fe3O4 NPs/SDS were similar

407

with a subtle difference. Only the peak at 1385 cm-1 in the spectrum of Fe3O4

408

NPs/SDS/HA was different. This difference was attributed to stretching vibrations of

409

methyl or methylene. This difference indicates that HA adsorbed on Fe3O4 NPs/SDS

410

is not bound through chemical action, but might be through hydrophobic effects.

411

XPS Spectrum. The XPS spectrum is a powerful tool to study the chemistry

412

characterization of surfaces of nanomaterials. In this study, XPS spectra were used to

413

study mechanisms of adsorption of HA on surfaces of Fe3O4 NPs in the presence and

414

absence of surfactant. High resolution Fe 2p XPS spectra of HA adsorbed on Fe3O4

415

NPs in the presence and absence of CTAB are shown in Fig. 7. The spectra of Fe3O4

416

NPs present two distinct peaks at binding energy of 709.0 eV for Fe 2p3/2 and 722.9 18


Page 18 of 36

Page 19 of 36


417

eV for Fe 2p1/2, which can be attributed to the Fe3+ and Fe2+ in Fe3O4 NPs. In other

418

samples, XPS spectra are characterized by two Fe 2p main lines, and the change of

419

binding energy of Fe 2p1/2 and Fe 2p2/3 of Fe3O4 NPs was regular prior to and after

420

interaction with CTAB or HA. By comparing the distance between the energetic

421

positions of Fe 2p2/3 and Fe 2p1/2 , for Fe3O4 NPs, the energetic positions of Fe 2p2/3 at

422

722.9 and Fe 2p1/2 at 709.0 eV, with a peak separation of 13.9 eV between the two

423

peaks. The distance of other samples changed slightly compared to that of Fe3O4 NPs.

424

It was 14.0 eV, 13.6 eV and 13.7 eV for Fe3O4 NPs/CTAB, Fe3O4 NPs/HA and Fe3O4

425

NPs/CTAB/HA, respectively. The change in spectrum of Fe3O4 NPs/CTAB was more

426

obvious than that of others, indicating that interactions of Fe3O4 NPs with CTAB were

427

stronger than interactions of HA with Fe3O4 NPs or Fe3O4 NPs/CTAB. Second, in the

428

absence of CTAB, not only did the Fe3O4 NPs/HA XPS spectrum show slightly

429

upshifted binding energy at 709.4 eV and 723.9eV for Fe3O4 NPs/HA, but the

430

intensity was also significantly reduced, indicating the HA was successful coated on

431

surfaces of Fe3O4 NPs and that the process of adsorption on Fe3O4 NPs was via

432

chemical binding.

433

The O1s, N1s and C1s survey scans are further conducted to identify the change

434

of surface elements states (Fig. S6). The O1s region shifted from binding energy

435

528.3 to 528.4 eV for Fe3O4 NPs prior to and after reaction with HA. The O1s region

436

can be deconvolved into three peaks at 528.3, 529.1 and 530 eV, corresponding to the

437

lattice O2- from metallic oxides. However, the deconvoluted results of Fe3O4 NPs/HA

438

demonstrated not only the peaks at binding energies 528.8 and 530.1 eV which

439

corresponding to the lattice O2- from metallic oxides, but also a peak at binding

440

energy 531.6 which corresponding to the oxygen in the organic acid or other organic

441

matter, indicates that the oxygen in HA was adsorbed on the surface of Fe3O4 NPs. 19



442

After CTAB was adsorbed on the surface of Fe3O4 NPs, the O1s peak was observed to

443

shifted to a lower binding energy of 527.8 eV, and the O1s region can be

444

deconvoluted into three peaks at 527.6, 528.2, and 529.5 eV, the two higher peaks are

445

consistent with the Fe3O4 NPs, but the lower peak at 527.6 eV indicates adsorption of

446

CTAB on Fe3O4 NPs consisted of a reduction reaction. Then, after HA was adsorbed

447

on the surface of Fe3O4 NPs/CTAB, the energetic position of O1s was not obviously

448

different, but the shape was slightly changed. The O1s region of Fe3O4 NPs

449

/CTAB/HA can be deconvolved into four peaks at 527.5, 528.0, 528.8 and 530.4 eV,

450

those peaks were not only similar with peaks of Fe3O4 NPs /CTAB, but also with

451

Fe3O4 NPs /HA, indicating that the oxygen in the HA successful absorbed on Fe3O4

452

NPs /CTAB surface.

453

The N1s spectrum changed more obviously. There was no evident N1s region on

454

the surface of Fe3O4 NPs, but that the surface of Fe3O4 NPs/HA exhibited obvious

455

peaks at binding energy 398.7 eV, and the region can be deconvoluted into three

456

peaks at 397.89, 398.8, and 399.8 eV, which correspond with Fe-N and N-H in the

457

HA, respectively (Fig. S6). Hence, appearance of nitrogen on the surface of Fe3O4

458

NPs/HA, the changed shape of the peak and, the shifted of energetic position of C 1s

459

and O 1s XPS spectra all indicate chemical binding of oxygen and nitrogen in HA on

460

the surface of Fe3O4 NPs. The N1s spectra of Fe3O4 NPs/CTAB and Fe3O4

461

NPs/CTAB/HA demonstrated more differences. The Fe3O4 NPs/CTAB spectrum

462

demonstrates only one significant peak at 400.5 eV, which is attributed to the nitrogen

463

in C-N, indicating that the nitrogen in the CTAB had absorbed on the surface of Fe3O4

464

NPs through the carbon chain. The N1s spectrum of Fe3O4NPs/CTAB/HA two peaks

465

were observed at binding energies of 397.8 and 400.8 eV, which were attributed to

466

nitrogen in HA and CTAB, respectively. Meanwhile, compared with Fe3O4 20


Page 20 of 36

Page 21 of 36


467

NPs/CTAB and Fe3O4 NPs /HA, the binding energy of N1s didn’t change, which

468

indicated that nitrogen in the HA adsorbed on Fe3O4 NPs surface might not be

469

adsorbed by direct chemical binding, but through linking with carbon chain.

470

Given the widespread presence of surfactants and HA in natural waters, NPs will

471

inevitably interact with them, thus influencing their behavior in the environment. In

472

this study, adsorption of HA on Fe3O4 NPs was investigated in the presence and

473

absence of surfactants. CTAB and SDS were found to strongly influence adsorption of

474

HA under appropriate conditions. In the absence of surfactants, adsorption of HA was

475

inversely proportional to pH and maximum adsorption capacity of HA was 120.3

476

mg/g. In the presence of surfactant, CTAB and SDS can form three different stages on

477

Fe3O4 NPs by electrostatic and hydrophobic interactions, and adsorption of HA was

478

significantly different in those three stages. Adsorption of HA on surfaces of Fe3O4

479

NPs/CTAB was controlled by ligand exchange, hydrophobic and electrostatic

480

interactions. Adsorption of HA significantly increased in the presence of CTAB.

481

When solution pH greater than 10.0, adsorption of HA slowly decreased by increasing

482

ionization, solubility and hydrophilic of HA. Results of FTIR and XPS indicate that

483

CTAB and SDS established a bridge connecting HA and Fe3O4 NPs. Functional

484

groups of HA played a significant role in adsorption. Therefore, adsorption of HA on

485

surfaces of Fe3O4 NPs would significantly changes in the presence CTAB and SDS,

486

and those results would be helpful for the understanding of the transport, transfer, and

487

fate of NPs in the aquatic environment.

488

ASSOCIATED CONTENT 21



489

Supporting Information

490

Parameters of isotherm and kinetics of adsorption. Characterization of HA

491

adsorbed on surface of Fe3O4 NPs in the presence of surfactants including adsorption

492

kinetic, morphology of the nanoparticles, isoelectric point. Fourier transform infrared

493

and X-ray photoelectron spectroscopy of HA adsorbed on surfaces of Fe3O4 NPs and

494

Fe3O4 NPs/surfactants. This material is available free of charge via the Internet at

495

http://pubs.acs.org.

496

22


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497

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27



678

Fig. 1 Effect of the pH on the HA adsorbed on Fe3O4 NPs (a); adsorption

679

isotherm of HA on Fe3O4 NPs at pH 5.0. (b); ζ-potential of Fe3O4 NPs by

680

adsorption of HA at pH 5.0 (c). Conditions: Amount of Fe3O4 NPs: 10.0 mg,

681

Concentration of HA: 30.0 mg/L. Solution volume: 100 mL.

682

Fig.2 ζ-potential of Fe3O4 NPs by adsorption of CTAB (a) at pH 10.0 and SDS (b)

683

at pH 5.0. Amount of Fe3O4 NPs: 10.0 mg; Solution volume: 100 mL.

684

Fig.3 Effect of the pH on the HA adsorbed on Fe3O4 NPs/CTAB (a), Fe3O4

685

NPs/SDS (b). Amount of Fe3O4 NPs: 10.0 mg; Concentration of CTAB and SDS:

686

50 .0mg; Concentration of HA: 50.0 mg/L Solution volume: 100 mL.

687

Fig.4 Effect of the amount of CTAB (pH: 10.0) (a) and SDS (pH: 5.0) (b) on the

688

Fe3O4 NPs adsorption to HA. Operated in batch mode. Amount of Fe3O4 NPs:

689

10.0 mg; Concentration of HA: 30.0 mg/L; Solution volume: 100 mL.

690

Fig.5 ζ-potential of Fe3O4 NPs by adsorption of HA in presence of CTAB (a) at

691

pH 10.0 and SDS at pH 5.0 (b). Amount of Fe3O4 NPs: 10.0 mg; Concentration of

692

HA: 30.0 mg/L; Solution volume: 100 mL.

693

Fig. 6 Adsorption of HA on Fe3O4 NPs in the absence and presence of CTAB and

694

SDS in natural water. Amount of Fe3O4 NPs: 10.0 mg; Concentration of CTAB:

695

20.0 mg/L, 50.0 mg/L, 100.0 mg/L. Concentration of SDS: 50.0 mg/L; Solution

696

volume: 100 mL.

697

Fig.7 XPS spectra of iron for HA adsorbed on Fe3O4 NPs in the presence of

698

CTAB.

699

28


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700

701 702

Fig.1

703

29



704 705 706 707

Fig.2

708 709

30


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710

711 712 713 714

Fig.3

715 716

31



717

718

719 720

Fig.4

721

32


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Page 33 of 36


722 723

Fig.5

724

33



725 726

727 728

Fig.6

729

34


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Page 35 of 36


730 731

Fig.7

732

35



733

SYNOPSIS TOC

734 735

36


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Magnetic Nanoparticles Interaction with Humic Acid: In the Presence

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