Effects of humic acid and Mg2+ on morphology and aggregation behavior of silica aerogels

Accepted Manuscript Effects of humic acid and Mg2+ on morphology and aggregation behavior of silica aerogels

Liqiang Tan, Xiaoli Tan, Ming Fang, Zhimin Yu, Xiangke Wang PII: DOI: Reference:

S0167-7322(18)31812-9 doi:10.1016/j.molliq.2018.05.064 MOLLIQ 9119

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

5 April 2018 11 May 2018 14 May 2018

Please cite this article as: Liqiang Tan, Xiaoli Tan, Ming Fang, Zhimin Yu, Xiangke Wang , Effects of humic acid and Mg2+ on morphology and aggregation behavior of silica aerogels. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2018.05.064

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ACCEPTED MANUSCRIPT Effects of Humic Acid and Mg2+ on Morphology and Aggregation Behavior of Silica Aerogels Liqiang Tana, Xiaoli Tana*, Ming Fanga*, Zhimin Yub, Xiangke Wanga,b* a

College of Environmental Science and Engineering, North China Electric Power

Department of Biology and Environmental Engineering, Hefei University, Hefei

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b

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University, Beijing 102206, China

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230000, P.R. China

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* Corresponding author. Email: [email protected] (X. Tan); [email protected]

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(M. Fang); [email protected] (X. Wang). Tel (Fax): 86-10-61772890.

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ACCEPTED MANUSCRIPT Abstract With the widespread use of nanomaterials, the impacts of nanoparticles on environment have attracted great concern. In this study, the effects of HA and Mg2+ on the aggregation of silica aerogels were investigated for understanding the fate and

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transport of silica aerogels in aqueous environments. Dynamic light scattering (DLS)

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was used to monitor the size distribution of the silica aerogels. Zeta potentials of HA

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and silica aerogels both exhibit strong negative surface charge at pH 2 to 12 due to their surface functional groups. The reunition of silica aerogels is due to the destabilization

which

could

be

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electrostatic

explained

by

the

classic

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Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The combination of HA endows silica aerogels with a high critical coagulation concentration (CCC), which means that

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HA reduces the aggregation of silica aerogels at low Mg2+ concentration due to the

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electrostatic repulsion and steric hindrance effects. Whereas, HA increases the aggregation of silica aerogels when the concentration of Mg2+ ions is higher than

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CCC, which is because the HA-Mg2+ chemical bonds, formed in the reaction process,

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act as bridge among silica aerogel aggregates. The colloid interface morphology of silica aerogel aggregates was characterized by transmission electron microscopy (TEM). The complexation between Mg2+ ions and HA plays a critical role in silica aerogels’ aggregation and is the main mechanism to increase the aggregation. These findings of the significant effects of HA and metal ions on the aggregation of silica aerogels are important for better understanding the transport and subsequent transformation of silica aerogels in aquatic environments. 2

ACCEPTED MANUSCRIPT Key words: silica aerogels; humic acid; Mg2+ ions; aggregation; bridging effect 1. Introduction Nanomaterials have gone into the daily life of human today in various fields, including food, everyday chemical products, communication, transportation,

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environment treatment, etc. [1]. Among them, silica nanoparticles may be the earliest

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and most widely applied ones in these fields [2,3]. However, as the increasing

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utilization of nanomaterials, they are released into environments through various pathways, especially the aqueous systems, which could lead to severe threaten to the

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organisms [4-6]. Even silica nanoparticles (NPs), which were thought to be harmless,

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have recently been found to have impact on organisms. 112 mg/L silica nanoparticles could damage the membrane of Hela cells and reduce the viability of hepatocytes [7].

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The enrichment of silica NPs in the environment would lead to unpredictable threats

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to wildlife and human consumers. Therefore, the investigation and understanding of the behavior of nanomaterials in environment have attracted great interests, which lay

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the foundation for the removal of contaminant including by NPs. Early researches

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revealed that the aggregation of NPs has great effect on their behaviors in the environment, such as the particle sizes, size distributions, surface charge characteristics, migration, transformation and toxicity in aqueous environments [8,9]. It is crucially important to understand the aggregation process of silica NPs to assess their fates in soil and water environments. Natural organic matter (NOM) is the organic compound produced mainly from the decomposition of plant and animal residues during the microbial metabolic 3

ACCEPTED MANUSCRIPT processes, and different functional groups such as carboxyl, phenol and amino, etc., can be found on its surface [10-14]. The widespread NOM can severely impact the chemical transformation, morphology, and potential toxicity of various NPs such as TiO2, C60, ZnO, CeO2 in water environment [15-18]. Humic acid (HA) is one of the

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most important NOMs, which usually plays important role in the fate and

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transportation of NPs [19]. A substantial amount of work has been reported to obtain

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deeper understanding of the effect of HA on the NPs’ stability and bioavailability in the environment [20-22]. When HA is adsorbed on the surface, the aggregation of

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NPs is usually influenced by its electrostatic repulsion and steric effects. A detailed

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study on the interaction between NPs and HA allows us to better understand their fate and transport in the environment. However, limited effects had been paid on the

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bonding mechanism of the interactions between HA and silica NPs at a molecular

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level, although silica frequently accompanies with HA in natural environments [23]. Therefore, a systematic research on the interactions between HA and silica NPs may

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level.

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reveal new structural characters that are relevant to silica coagulation at molecular

Divalent cations, such as Mg2+ and Ca2+, are usually found in soil and water environments and can strongly interact with HA and some NPs. It was reported that the transport of NPs became very complicated due to the introduction of divalent cations in the presence of HA. For example, the fullerene and boron NPs aggregated seriously at high CaCl2 concentrations due to the presence of HA [22]. HA and Ca2+ were responsible for the micro-aggregates formation of montmorillonite particles in 4

ACCEPTED MANUSCRIPT soil [24]. Therefore, this study is aimed to reveal the morphology and surface chemistry changes of silica aerogels due to the presence of Mg2+ and HA in the solution. Characterizations of HA with fluorescence excitation-emission matrix (EEM) 13

C nuclear

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spectroscopy, Fourier transforms infrared (FT-IR) and solid-state

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magnetic resonance (NMR) spectroscopy offer comprehensive insight into the

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bonding properties of HA. Dynamic light scattering (DLS) were applied to study the effect of HA and Mg2+ on the particle size distribution of SiO2 NPs, whereas

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transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS)

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analyses were used to investigate the microstructure of SiO2 aggregates and delineate the aggregation mechanism.

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2. Experimental Section

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2.1 Materials

The silica aerogels used in the work were synthesized via a sol-gel technique.

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The detailed information about the synthesis of silica aerogels was well-documented

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in previous reports [25]. The HA sample is a commercial product provided by Sinopharm Chemical Reagent Co., China. To reduce potential interference from undissolved materials, the sample was purified prior to use. Detailed purification process and the main element analysis of HA were illustrated in Table S1. AR-grade NaOH, HCl and Electrolyte (MgCl2) stock solutions were prepared using analytical reagents and were filtered through 0.2 μm filters before use. 2.2 Methods 5

ACCEPTED MANUSCRIPT The stock solutions of HA and MgCl2 were first prepared by using deionized water at room temperature. Both of them were then spiked to the silica aerogels suspensions (100 mg/L) to obtained needed concentration of HA (1, 3, 5 and 10 mg/L) and Mg2+ ions (concentrations ranging from 10 to 180 mM). All experiments and

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measurements were conducted at pH 6.0 ± 0.1 unless otherwise stated. The purified

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HA was dissolved in deionized water to produce concentration of 100 mg/L with pH

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value of 6.0 ± 0.1 adjusting by adding negligible 0.1 M NaOH and 0.1 M HCl. After 12 h, the solution was analyzed by the fluorescence EEM; and the remnant solution 13

C NMR analysis. The zeta potential

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was freeze-dried for the FT-IR and

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measurements were carried out on a Malvern Zetasizer Nano ZS90 system. The attachment efficiency, known as the inverse stability ratio, is used to

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quantitatively describe the aggregation behavior of silica aerogels using dynamic light

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scattering (DLS). The concentration of silica aerogels was 100 mg/L and HA was 10 mg/L. The influence of Mg2+ was investigated by adding MgCl2 stock solutions.

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[26]:

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Attachment efficiency was obtained by normalizing the aggregation rate constant



ka  k fast

1 N0 1 N 0, fast

d Dh (t ))t 0 dt d ( Dh (t ))t 0, fast dt (

(1)

Where N0 is the initial nanoparticle concentration in the suspension; Dh is the hydrodynamic diameter; ka is the aggregation rate constant. Here, kfast is equal to (dDh/dt)t→0,fast which reflects aggregation rate constant obtained under favorable (non-repulsive, fast) conditions. The critical coagulation concentration (CCC) is 6

ACCEPTED MANUSCRIPT defined as the minimum concentration of counterions to induce coagulation of colloidal particles [27,28]. 2.3 Characterization Fluorescence measurement of HA solutions at pH 6.0 was carried out on a

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HITACHI F-7000 spectrofluorimeter. To measure EEM, the scan speed was set at

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2400 nm/min. Scanning emission (Em) spectra from 300 to 600 nm were obtained in

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5 nm increments by varying the excitation (Ex) wavelength from 200 to 600 nm with increment of 5 nm. The purified HA were analyzed by FT-IR (Perkin-Elmer 1725) 13

C CP/MAS NMR experiments were performed with a 4 mm HX

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spectrometer.

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double-resonance MAS probe on a Bruker AVANCE III 600 spectrometer at a resonance frequency of 150.9 MHz. A contact time of 2 ms, a recycle delay of 5 s, a

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CP/MAS measurement.

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MAS spinning speed of 14 kHz and 4000 accumulations were used for the 1H-13C

The zeta potential, electrophoretic mobility (EPM) and the hydrodynamic

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diameter were obtained by Zetasizer Nano ZS90 instrument (Malvern Instruments,

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UK). The morphology investigation was performed on a 200 kV Hitachi A7650 Transmission Electron Microscope (TEM). 3. Results and discussion 3.1 Characterization of silica aerogels and HA materials The diameter of the silica aerogel particles is approximately 20-30 nm as indicated by the TEM image shown in Fig. 1A, and some particles aggregated together. However, the hydrodynamic diameter () of silica aerogels measured by 7

ACCEPTED MANUSCRIPT the DLS measurements (Fig. 1B) is mainly at 250 nm, which is much larger than the TEM results, indicating an obvious aggregation of the colloidal particles. The FT-IR spectrum is shown in Fig. 1C. Strong absorption band in the range between 1250 and 1000 cm-1 is assigned to Si-O-Si asymmetric stretching vibration, and the band in the

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range between 900 and 750 cm-1 is assigned to Si-OH bending vibration from the

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SiO2 nanocrystal. The broad band centered at 3500 cm-1 can be assigned to the Si-OH

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stretching vibration and the band at 1640 cm-1 is possibly due to the OH bending vibration from adsorbed water [29,30].

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The TEM image of solid HA particles are shown in Fig. 1D. It can be seen that

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HA particles gathered together and a single HA solid particle is approximately 200 nm in diameter. The average measured by DLS is approximately 275 nm (Fig. 1E),

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which is a little larger than the result of TEM. In addition, the particle size distribution

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of HA (Fig. 1E) is much broader than that of silica aerogels. HA substances are complex mixtures of high to low molecular weight species, so they are polydisperse

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systems with a broader size distribution in solution [31]. Fluorescence spectroscopy is

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usually used to provide information about the molecular structure, functional groups and conformation. The fluorescence excitation-emission matrix (EEM) spectra of HA at pH 6.0 are shown in Fig. 1F, where two obvious peaks with Ex/Em centered at 260-300/420-480 nm, and 350-400/445-500 nm are observed. The emission intensity results show two independent components with Ex/Em located at 269/458 (peak A) and 372/490 (peak B) (Fig. S1), which are attributed to the intramolecular charge transfer of carboxylic-like and phenolic-like fluorophores in HA [31,32]. Further, 8

ACCEPTED MANUSCRIPT FT-IR and 13C NMR analyses are illustrated in SI (Fig. S2). The aqueous HA was found to carry negative charge at pH 6.0 and various chemically reactive functional groups (carboxylic, phenolic –OH groups, etc.) are found on its surface, which can generate a significant particle electric field and influence the dynamics and extent of

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its interactions with oppositely charged ions.

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3.2 Effect of surface charge of silica aerogels

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The zeta potential of silica aerogels is measured at various pH values (Fig. 2A), which becomes more negative with increasing the pH from 2.0 to 7.0. When pH

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values are higher than 7.0, the zeta potential does not change significantly and is more

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negative comparing with other NPs, such as Al2O3 and TiO2 NPs [33,34]. The negative surface charge is mainly due to the deprotonation of the surface Si-OH

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species in water at pH > 2 [35,36], which can also be confirmed by the FT-IR analysis

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presented in Fig. 1C. Based on the highly negatively charged surface, silica aerogels are more likely to form stable dispersions in aqueous environment.

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To explore the effect of HA on the surface charge of silica aerogels, we also

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studied the zeta potential of HA in solution as a function of pH (Fig. 2B). The corresponding titration curves are determined with the measurement of the zeta potential. HA exhibits a strongly negatively charged surface from -47.3 mV (pH 12.0) to -14.0 mV (pH 2.0) mainly due to the presence of carboxylic acid and phenolic functional groups, which is in good agreement with previous works [37]. As shown in Fig. 2C, the surface charge of silica aerogels decreases slightly with increasing the HA concentration. The surface charge changes significantly in different 9

ACCEPTED MANUSCRIPT Mg2+ concentrations. For example, at low Mg2+ concentration (MgCl2 < 30 mM), HA (10, 5 and 3 mg/L) impacts on the zeta potential of silica aerogels greatly; at high Mg2+ concentration (MgCl2 > 30 mM), the zeta potential of silica aerogels has no significant change with increasing the content of HA. The presence of HA increases

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the absolute value of zeta potential of silica aerogels and thus leads to higher repulsive

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force between them, which makes the particles more difficult to aggregate and

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therefore exist steadily in the system. However, with increasing the concentration of MgCl2, the positive charge on the surface of silica aerogels increased and gradually

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destabilized the system. These results indicate that high ionic strength may weaken

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the negative surface charge of HA and the influence of HA on the repulsive force between silica aerogels [38]. The sorption behaviors of HA on the surface of silica

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aerogels was studied in Fig. 2D. Experiment results indicate that over 60% of HA is

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adsorbed at pH 6.0 when HA concentration is smaller than 15 mg/L, which confirms the important influence of HA on the surface charge of silica aerogels (Fig. 2C).

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3.3 Influence of Mg2+ ions and HA on the aggregation of silica aerogels

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Figure 3A presents the effect of Mg2+ concentration on changes of silica aerogels. With increasing the concentration of Mg2+ over 120 mM, the of silica aerogels increases rapidly and finally reaches the maximum of approximately 1800 nm. In order to understand the effect of time on the aggregates size of silica aerogels, kinetic curves obtained at different Mg2+ concentrations are shown in Fig. S3. At the concentration of 10 mM Mg2+, the aggregation is insignificant and the aggregates size is approximately 350 nm. However, at higher Mg2+ concentrations, the of silica 10

ACCEPTED MANUSCRIPT aerogels increases rapidly over time and reaches a maximum at about 4000 s, then, the aggregation curves maintain almost constant levels. Therefore, 4500 s is adequate for determining the aggregation kinetics. The presence of Mg2+ ions plays important role in the aggregation of silica aerogels. However, in the solution, HA (1-50 mg/L) has

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little influence on the of silica aerogels (Fig. 3B). The silica aerogels are stable

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and the maintains the initial value of approximately 310 nm, which is a litter

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larger than that of the pure silica aerogels in solution due to the adsorbed HA on its surface.

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3.4 Effect of HA on the aggregation and attachment efficiency of silica aerogels.

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The effect of HA on the aggregation of silica aerogels in the presence of different concentrations of Mg2+ ions are presented in Fig. 4. With increasing the concentration

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of HA, higher Mg2+ concentration is needed to induce coagulation of silica aerogels.

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For example, to coagulate silica aerogels, 10 mM Mg2+ ions are needed in the absence of HA (Fig. S3), while at least 20, 30 and 40 mM Mg2+ ions can work in the presence

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of 1, 3 and 5 mg/L HA, respectively, which indicates that HA hindered the

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aggregation of silica aerogels. Moreover, a distinct difference in the maximum aggregates size can be observed at different HA concentration, which increases from approximately 1800 nm at 1 mg/L HA to 2400 nm at 5 mg/L HA (Fig. 4A, B and C). This is because with increasing the concentration of HA, more HA in solution can bond with Mg2+ ions and form complexation between silica aerogel aggregates, therefore, leads to larger aggregates size. Kinetic data were analyzed in terms of attachment efficiency in Fig. 4. In the 11

ACCEPTED MANUSCRIPT case of 0 mg/L HA and 10 mM MgCl2 (Fig. S3), the attachment efficiency (Fig. 4D) increases rapidly with the increase of MgCl2 concentration from 10 to 60 mM, which is caused by decreasing the interaction with the repulsive double layer and hence decreases the energy barrier [39], and the regime is defined as the reaction-controlled

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regime. With the increase of MgCl2 concentration from 60 to 100 mM, the attachment

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efficiency keeps constant indicating a diffusion-controlled regime. The critical

curve

of

silica

aerogels

at

various

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coagulation concentration (CCC) of Mg2+ is estimated to be 60 mM. The aggregation MgCl2

concentrations

follows

the

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Derjaguin-Landau-Verwey-Overbeel (DLVO) model [40]. With 5 mg/L HA, silica

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aerogels start to aggregate at 25 mM MgCl2, the aggregation process requires a much higher Mg2+ content than that without HA (10 mM MgCl2), and also the aggregation

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curve exhibited a typical DLVO profile in this case. Moreover, in the

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reaction-controlled regime, the attachment efficiency obtained at high HA concentrations (1, 3, 5 mg/L) is lower than that obtained at the same Mg2+

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concentrations in the absence of HA; whereas, the attachment efficiency maintains

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constant and is about the same to that in the absence of HA in the diffusion-controlled regime. This is due to the ionic strength dominates the aggregation process and compresses electrical double layer, while the effect of HA is weakened. In addition, CCC value obtained in the presence of HA (1, 3 and 5 mg/L) is smaller than that in the absence of HA (0 mg/L) at a fixed Mg2+ concentration in the reaction-controlled regime. As shown in Fig. 4D, the concentration of HA influences the CCC of silica aerogels, a higher CCC value can be obtained at a higher HA concentration. This 12

ACCEPTED MANUSCRIPT indicates that HA greatly hindered the aggregation of silica aerogels. Such inhibitory effect is also reported for other NPs, such as Ag NPs [41]. To investigate the impacts of MgCl2 concentration on the aggregation of silica aerogels, 20 mM and 150 mM MgCl2 solutions were selected as representative

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concentrations in the system to make sure that the ionic strength was below or above

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the CCC values of the system. When the ionic strength (20 mM) is lower than CCC,

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decreases obviously from ~540 nm in the absence of HA to ~390 nm in the presence of 5 mg/L HA (Fig. 5A). HA shows obvious negative influence on the

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aggregation size of silica aerogels. However, HA promote the aggregation of silica

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aerogels when ionic strength (150 mM) is higher than CCC. From Fig. 5B, larger aggregates can be obtained at lower HA concentration (1 mg/L) comparing to that in

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the absence of HA. At pH 6.0, silica aerogel surface is possessed of negative charge.

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When the concentration of Mg2+ is lower than CCC, the surface of silica aerogels was covered with HA molecules, electrostatic repulsion between HA and silica aerogels

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would result in steric hindrance and therefore inhibit the aggregation among silica

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aerogels [39]. As Mg2+ concentration increased, electrostatic stabilization of silica aerogels would be broken due to the compression of the electric double layer and charge screening [42]. Additionally, HA promoted the adsorption of Mg2+ onto the surface of silica aerogels through the formation of HA-Mg2+ chemical bridging bonds, which would enhance SiO2 nanoparticles’ aggregation [42]. 3.5 TEM imaging analysis TEM image in Fig. 6 is used to delineate the aggregation morphology of the 13

ACCEPTED MANUSCRIPT silica aerogels with and without HA, and to provide evidence that the formation of gel cluster was due to the complexation between Mg2+ (140 mM) and HA (10 mg/L). The inset in Fig. 6A-a of the selected area electron diffraction (SAED) pattern with diffused rings indicates the amorphous phase in the corresponding area. Fig. 6A-b is

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the TEM image of silica aerogels. The amorphous HA directed by blue line (a) (Fig.

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6A-a) combined with the silica aerogel aggregates together. This is due to the HA

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acting as a bridge among silica aerogels, however, which is not observed in the absence of HA as shown (Fig. 6B). Therefore, the formation of the gel clusters of

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silica aerogel aggregates in Fig. 6A-a is attributed to the complexation between HA

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Mg2+ and silica aerogels [40]. Similar experiment results can be observed for the aggregation of fullerene NPs due to the presence of HA and Ca2+ ions [43].

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Furthermore, TEM element mapping is applied to offer direct evidence on the

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interaction mechanism. In the presence of HA (Fig. 7A), the profile of silicon element agrees well with the TEM image, while carbon element covers mainly including the

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silica aerogel aggregates and the bridge areas. However, the distribution of the oxygen

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element is different and can be graded into different levels. The weakest lightness of oxygen element (back ground noise) over the whole area is caused by the adsorbed oxygen on the surface of the carbon film. Oxygen element with middle lightness among the silica aerogel aggregates can be attributed to the carboxylic or phenolic OH groups of HA, and the brightest region of the oxygen elemental profile is originated from SiO2 [41]. The intensity distribution of the oxygen element indicates that HA molecules played a significant role in the aggregation of silica aerogels. 14

ACCEPTED MANUSCRIPT Importantly, Mg2+ ions are found in the bridge area, further proving Mg2+ ions also play important role in the aggregation process [19]. The TEM and elemental mapping images of silica aerogels in MgCl2 solutions in the absence of HA are shown in Fig. 7B. The silicon and oxygen elemental

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distributions are corresponded with the TEM image as expected. The distribution of

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Mg element is similar to that in the presence of HA (Fig. 7A). Mg2+ ions can be

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adsorbed to the surface of the particles, which may weaken the negative surface charge and suppress the double layer repulsion between silica aerogels, therefore,

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inducing the aggregation. Mg2+ ions are significant in inducing the aggregation in the

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absence of HA. On the contrary, although Mg2+ ions at the same ionic strength would also suppress the electrical double layer between silica aerogels in the presence of HA

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as indicated by the zeta potential measurements (Fig. 2C), the bridging effect of HA

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with Mg2+ was the main mechanism during the aggregation process. This impact can be proven directly by the element mapping of C and O, which was more associated

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with the aggregate positions than Mg2+ ions [44].

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3.6 Proposed mechanisms of aggregation between HA, Mg2+ ion, and silica aerogels Aggregation kinetics and TEM image analysis indicate that the presence of HA and Mg2+ ions play an important role in inducing the silica aerogels aggregation. The interaction mechanism among HA, Mg2+ and silica aerogels are proposed, and the aggregation process of silica aerogels are illustrated by the diagrammatic sketch in Fig. 8. HA doesn’t induce silica aerogels aggregation due to the electrostatic repulsive force between them. However, when there are only silica aerogels and Mg2+ ions, the 15

ACCEPTED MANUSCRIPT interaction between them can be expressed as follows: ≡Si-O- + Mg2+ + ≡Si-O-→ ≡Si-O-Mg-O-Si≡

(Fig. 8A)

where ≡Si represents silica aerogels. Mg2+ ions are strongly adsorbed on the surface of the particles and thus compress the electric double layers. The process lowers the

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repulsive force among silica aerogels and leads to an accelerated aggregation [45,46].

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Similarly, as HA is added together with Mg2+ ions, the interactions can be described

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as:

HA-COO- + Mg2+ + ≡Si-O- → HA-COO-Mg-O-Si≡

(Fig. 8B)

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It suggests HA is adsorbed onto the surface of silica aerogels via Mg2+ ion bridging

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effect. 4. Conclusions

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The interaction processes and coagulation behaviors of silica aerogels in the

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presence of HA and Mg2+ ions were investigated. Our results clearly indicated that HA and Mg2+ ions played an important role in the aggregation behaviors and transport

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of silica aerogels in aqueous environments. The absolute zeta potential value of silica

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aerogels becomes larger due to the presence of HA in the solution, and which facilitates the stability of the particles in the system. As Mg2+ ions were together adding into the solution, higher ionic strength weakened the influence of HA and destabilized the system. The aggregation curve of silica aerogels at various MgCl2 concentrations agreed well with the DLVO model. A negative impact of HA on the aggregation of silica aerogels could be observed when Mg2+ ion concentration is very low (< CCC) due to the steric hindrance and electrostatic repulsion effect between HA 16

ACCEPTED MANUSCRIPT and silica aerogels; whereas, at higher Mg2+ ion concentration (> CCC), the presence of HA promoted the aggregation efficiently due to the formation of HA-Mg2+-SiO2. Complexation between Mg2+ and HA played an important role in enhancing silica aerogels aggregation. The work provides a deep understand on the aggregation

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mechanism at molecular level and predicts the transport and fate of silica aerogels in

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the environment.

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Associated Content Supplement information

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Purification of HA and the main elements of HA; 13C NMR and FT-IR spectral of HA

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samples at pH 6.0; Hydrodynamic diameter changes of silica aerogels and HA aggregates.

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Acknowledgment

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The authors gratefully acknowledge the financial support of National Key Research and Development Program of China (2017YFA0207002), the Science Challenge

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Project (TZ2016004), the National Natural Science Foundation of China (U1607102,

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21577032), the Fundamental Research Funds for the Central Universities (2018ZD11, 2018MS114) are acknowledged. References

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Figures

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Fig.1. Characterization of silica aerogels：TEM imaging (A), particle size distributions

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(B) and FT-IR spectra (C); characterization of HA: TEM imaging (D), particle size

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distributions (E), and fluorescence EEM spectra (F).

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Fig.2. Zeta potential changes of silica aerogels (100 mg/L) (A) and HA (50 mg/L) (B)

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as a function of solution pH; the effect of HA on the zeta potential changes of silica aerogels (100 mg/L) under different Mg2+ concentrations at pH 6.0 (C); the sorption

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of HA (5-50 mg/L) onto silica aerogels (100mg/L) at pH 6.0 (D).

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Fig.3. Effects of Mg2+ (A) and HA (B) concentrations on the aggregation size of silica

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aerogels at pH 6.0. Csilica aerogels = 100 mg/L.

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Fig.4. Aggregation kinetics (A, B and C) and attachment efficiency (D) of silica

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6.0. Csilica aerogels = 100 mg/L

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Fig.5. Aggregation kinetics of silica aerogels in the presence of (A) lower (CMg2+ = 20

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Fig.6. TEM image of silica aerogels aggregates in the presence of HA and Mg2+ ions

= 100 mg/L, CHA = 50 mg/L, CMg2+ = 140 mM.

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(A); and only Mg2+ ions (B) at pH 6.0 after a long aggregation time (~ 4500 s). Csilica

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Fig.7. TEM-EDS images of silica aerogels in MgCl2 solutions in the presence (A) and absence (B) of HA at pH 6.0. Csilica aerogels = 100 mg/L, CHA = 50 mg/L, CMg2+ = 140 mM. 30

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Fig.8. Mechanisms of the aggregation of silica aerogels: (A) ion induced aggregation;

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(B) The bridging effect of HA and Mg2+ ions on aggregation.

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ACCEPTED MANUSCRIPT Highlights 1. Electrostatic repulsion between HA and silica aerogels inhibits the aggregation of silica aerogels. 2. Mg2+ ions can weaken the negative surface charge of silica aerogels and enhance

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3. Complexation between Mg2+ and HA promotes the aggregation of silica aerogels.

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4. The aggregation curve of silica aerogels exhibits a typical DLVO profile.

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