Surface Adsorption of Suwannee River Humic Acid on TiO2

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Surface Adsorption of Suwannee River Humic Acid on TiO2 Nanoparticles: A Study of pH and Particle Size Sanjaya Jayalath, Haibin Wu, Sarah C Larsen, and Vicki H. Grassian Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00300 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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Surface Adsorption of Suwannee River Humic Acid on TiO2 Nanoparticles: A Study of pH and Particle Size Sanjaya Jayalath,1 Haibin Wu,2 Sarah C. Larsen,1* Vicki H. Grassian2,3,4* 1 2

Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States

Department of Chemistry & Biochemistry, University of California San Diego, La Jolla, CA 92093, United States 3

Department of Nanoengineering, University of California San Diego, La Jolla, CA 92093, United States

4

Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, United States Email: [email protected] and [email protected]

Abstract TiO2 nanoparticles are some of the most widely used metal oxide nanomaterials mainly due to their diverse industrial applications. Increasing usage of these nanoparticles raises concerns about the potential adverse effects on the environment. Humic acid is a ubiquitous component of natural organic matter in the environment that is known to get adsorbed onto nanoparticle surfaces. In this study, the adsorption of humic acid on TiO2 nanoparticles of two different sizes (5 nm and 22 nm) is studied at different environmentally relevant pH values using Attenuated Total Reflectance Fourier Transformation Infrared (ATR-FTIR) spectroscopy. These vibrational spectra provide insights into the nature of the adsorption process (extent of adsorption and reversibility) as a function of pH as well as information about the bonding to the surface. Additionally, the impact of humic acid adsorption on surface charge and aggregation has been investigated. Interestingly, the results show that the humic acid adsorption is strongly pH dependent and adsorption of humic acid on TiO2 nanoparticles alters the extent of aggregation and modifies the zeta potential and surface charges depending on the pH thus potentially increasing the bioavailabity of TiO2 nanoparticles in the environment.

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! Introduction TiO2 nanoparticles (NPs) are a desirable additive in many products such as paints, cosmetics and sunscreens.1-5 TiO2 NPs are also used in wastewater treatment plants to remove organic pollutants from contaminated water.6 During the past few decades, the use of TiO2 NPs in various industries has increased and there are some concerns that these small particles can cause harm to the environment.7 In fact some studies have shown that TiO2 can be toxic to organisms such as animals8-10, plants11-12, bacteria13-14 and algae.15 However, the toxicity of TiO2 NPs depends on many factors including size, surface area and surface functionalization.16-18 Transformations of TiO2 NPs in aqueous environments, such as aggregation, and surface adsorption, are greatly affected by the details of the environmental milieu including acidity (pH) and natural organic matter.19 In particular, TiO2 NPs can interact with different organic components such as humic acid, fulvic acid and other biomolecules, such as proteins, forming a so-called “ecocorona” around the nanoparticle surface.20 Humic acid (HA) is the fraction of humic substances that is soluble in water above pH 2.21-22 The molecular weight of HA ranges from 1-5 kDa.23 HA molecules are complex containing a variety of different functional groups such as carboxylic, phenolic, quinonyl, ester, ketone, hydroxyl and amino groups. Figure 1 shows a representative structure of HA that identifies the most abundant functional groups present and the range of pKa values that are found for these functional groups.21 Carboxylic and phenolic groups are the most abundant functional groups that are responsible for the charge of the HA molecules in solution.2324

HA can bind with cations via these functional groups and this ability to chelate ions plays a major

role in soil fertility.25 HA is known to interact with different metal oxides, metal hydroxides and metal ions.26-30 Eita (2011) studied the adsorption of HA on Al2O3 quantitatively by using QCM-D (Quartz crystal microbalance with dissipation) and determined that HA adsorption is highest at pH 3.0 compared to other pH values (5.0 and 6.8). At pH 3.0, HA undergoes a two-step adsorption process, whereby 2 ! ACS Paragon Plus Environment

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! Several studies have investigated the interaction of TiO2 NPs with HA. Many of these studies are focused on quantitation of the adsorption of HA on TiO2 NPs. Few studies have focused on the mechanism of adsorption of HA on TiO2 NPs.24, 34-35 A study on photocatalytic degradation of HA on TiO2 NPs has suggested that carboxylate groups play a major role in the adsorption process based on diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic evidence.34 Chen and co-workers have also found that carboxylic and phenolic groups play key roles in HA adsorption on certain types of TiO2 NPs using 2D-IR analysis.24 In another study, HA adsorption on TiO2 NPs is pH dependent and suggested phenolic functional groups as being important in HA adsorption.35 Overall, there seems to be some disagreement over the mechanisms of HA adsorption on TiO2 NPs and roles of different functional groups on the adsorption process. Therefore, to further probe the surface adsorption of HA on TiO2 NPs, we have investigated the mechanism of adsorption of HA onto TiO2 NPs using spectroscopic probes. It is also necessary to study the influence of pH on the mechanisms of adsorption since the pH can potentially affect these mechanisms, given the fact that both surface properties and speciation of HA functional groups largely depend on pH. These studies will probe different mechanisms of HA adsorption in order to better understand the transformations of TiO2 NPs in the environment. Adsorption of HA on TiO2 NPs is studied using attenuated total reflectance Fourier transformation infrared spectroscopy (ATR-FTIR). This technique has previously been used to study the adsorption of different species on many different materials including NPs.36-38 There are several key advantages of the ATR-FTIR spectroscopy including, the high sensitivity to adsorbed species and the ability to provide molecular scale information on adsorption processes under different solvent conditions. Moreover, adsorption and kinetics can be studied concurrently using this technique. In the ATRFTIR study, the HA adsorption is studied on a thin film of NPs under different pH conditions in order to understand the pH dependence of the HA adsorption. Three environmentally relevant pH values (3.7, 6.0 and 8.0) were selected in order to represent the broad range of soil pH (see Figure 4 ! ACS Paragon Plus Environment

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! 2). Furthermore, the impact of NP size on the adsorption behavior of HA on TiO2 NPs will be looked at in these studies although it should be noted that individual nanoparticles are not being interrogated. Instead, a thin film is used. These thin films best can better represent a large aggregate of nanoparticles in the environment. However, in other experiments using dynamic light scattering, the impact of HA adsorption on colloidal stability of TiO2 NPs is also studied in order to better understand the impact of HA adsorption on TiO2 NP transformation in the environment.

Figure 2. Schematic representation of the pH range used in the study and its environmental relevance as well as the range of pKas for different functional groups within SRHA Experimental Section Materials. Two TiO2 NP samples were purchased from Nanostructured and Amorphous Materials, Inc. (Houston, TX) and Sigma Aldrich ((SKU-718467). These vendors report primary particle sizes of 5 nm and 25 nm, respectively. Suwannee river humic acid standard (SRHA) was purchased from International humic substances society (Minneapolis, MN). pH adjustments were done using NaOH (Fisher Scientific ACS plus) or HCl (Fisher Scientific ACS plus). NaCl (Fisher Scientific ACS plus) and KNO3 (Sigmal Aldrich, purity > 99%) was used to adjust the ionic strength of the reaction media. All the solutions were prepared using optima water (Fisher Scientific). Nanoparticle characterization. The size of individual primary nanoparticles was determined using transmission electron microscopy (TEM). Microscopic images were obtained from JEOL JEM5 ! ACS Paragon Plus Environment

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! 1230 transmission electron microscope and size analyses were carried out using ImageJ software program. Crystallinity of TiO2 NPs was analyzed using powder X-ray diffraction (XRD). Analyses were carried out using Bruker Advance D8 diffractometer with Cu K! radiation. The specific surface area of the particles was determined using a Quantachrome 4200e surface area analyzer. Seven-point N2-BET adsorption isotherms were used for the analyses. All the samples were degassed for 24 h at 393 K prior to the analysis. ATR-FTIR Spectroscopy. Attenuated total reflectance Fourier transformation infrared (ATR-FTIR) spectroscopic studies were carried out using a Thermo-Nicolet 670 FTIR Spectrometer (MCT/A detector) equipped with a horizontal ATR element (Pike Technologies, Inc.). Adsorption of SRHA on TiO2 was studied by flowing 50 mg/L SRHA solution (pH 3.7, 6.0 and 8.0) over a thin layer of TiO2 (5 nm and 22 nm, 1 mg of powdered sample) deposited on an amorphous material transmitting infrared radiation (AMTIR) crystal. Meanwhile, IR spectra were collected every 5 min for 4 hours. All the spectra were collected in the mid IR region ranging from 750 to 4000 cm-1 with 4 cm-1 spectral resolution. Desorption spectra were also collected following the adsorption spectra using the same experimental parameters while flowing optima water at the given pH over the SRHA adsorbed NP layer. Solution phase ATR-FTIR spectroscopy measurements were also collected using 50 mg/L SRHA solutions prepared at pH 3.7, 6.0 and 8.0. Hydrodynamic diameter and zeta potential of TiO2 NPs. Dynamic light scattering (DLS) was used to study the hydrodynamic diameter (dH) and zeta potential as a function of SRHA concentration at pH 3.7, 6.0 and 8.0. pH adjusted solutions of TiO2 NPs (100 mg/L, 10 mL) in the presence of varying concentrations of SRHA (0, 10, 20, 50 mg/L) were prepared in scintillation vials. NaCl (0.02 M) was used to maintain the ionic strength of the prepared solutions in a constant level. Zeta potential measurements were done to determine changes in surface charge of TiO2 NPs. Prepared solutions were sonicated for 1 hour and allowed to stand for 24 hours. These measurements, dH and zeta potential, were done using a Delsa nano C DLS instrument. 6 ! ACS Paragon Plus Environment

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! and 1036 cm-1. Many of these peaks are quite broad and in this spectral region can be assigned to the different types of carboxylic groups present in SRHA. Table 1. Vibrational mode frequencies (cm-1) and assignments for SRHA in solution phase and adsorbed phase on TiO2 nanoparticle surfaces. mode frequencyliterature23, 39-41

Vibrational mode

mode frequencythis study

"(C=O)

1720 -1700

1721-1690

"as (COO-)

1590 -1570

1590-1550

"s (COO-)

1412 -1380

1412-1380

# (OCOCH)/"(OC$OH), "(C$O$H) phenolic

1270 -1250

1274-1261

"s (C-O-C) ether

1120 -1080

1115-1094

"s (C$OH) alcohol

1050-1020

1036

Table 1 gives the assignments used in this study for the different absorption bands for SRHA. The band at 1721 cm-1 is assigned to the stretching vibration of the C=O within protonated –COOH groups. At higher pH (6.0 and 8.0), this band disappears due to the deprotonation of these carboxylic acid groups. The bands present between 1550 and 1590 cm-1 and those between 1380 and 1412 cm-1 region are associated with the asymmetric and symmetric vibrations of deprotonated COO- groups respectively. Based on this, it is clear that at pH 3.7 there are both protonated and deprotonated carboxylic moieties present in SRHA. Due to the complex structure of SRHA, as already discussed, the carboxylic acid groups are in different chemical environments within SRHA and therefore there is a large range of pKa values that results in a broad range of speciation associated with the protonated and deprotonated forms depending on pH. The intensity of the C=O peak of COOH groups is decreasing rapidly with increasing pH; however, the peak is still visible even at 6.0 and completely disappears at pH 8.0. Additionally, the intense peak at 1261 cm-1 that 8 ! ACS Paragon Plus Environment

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! Figure 4 shows the time-dependence of the infrared spectra following the adsorption of SRHA on 5 nm and 22 nm TiO2 NPs under different pH conditions. The spectra of adsorbed SRHA show a similar pH dependence to that of solution phase SRHA in that the carboxylic acid groups become protonated at lower pH and there are slight shifts for the more intense band around 1580 cm-1 that can be assigned to asymmetric vibrations of COO- groups. However, the adsorbed SRHA at each pH shows very different spectra in terms of relative band intensities and frequencies from that of SRHA in solution at the same corresponding pH, thus indicating structural changes in SRHA molecules upon adsorption on the TiO2 NP surface. These changes can be ascribed to the interactions between the TiO2 surfaces and SRHA molecules during the adsorption of SRHA on the TiO2 NP surface. Figure 4 (left panel) shows the adsorption of SRHA on 5 nm TiO2 NPs at pH 3.7, 6.0 and 8.0. At pH 3.7, the spectra of initial adsorption stages are considerably different from that of the free SRHA. There are several differences in the spectra of initial adsorption stages when compared to free SRHA. The vibrational frequencies for surface-bound species depend on the coordination to the surface. Therefore, different modes of coordination on the surface yield different vibrational frequencies. For example, the difference in the frequency of the asymmetric and symmetric stretching motions (!" = "asym–"sym) for coordinated and ionic salt environments for carboxylate follows the order; %" (monodentate/unidentate) > %" (ionic) > %" (bidentate). As shown in Figure 5, the monodentate coordination mode is for carboxylate surface bonded to a single surface Ti4+ ion via one of the oxygen atoms, whereas bidentate bridging is for carboxylate surface bonded to two neighboring surface Ti ions via both two oxygen atoms. Both of these two binding motifs result in the formation of inner sphere complexes. For outer sphere complexes, a water molecule is present between surface functional group and the adsorbed molecule.


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! The frequency difference, !", for monodentate binding is typically between 300 and 500 cm-1. For the bidentate bridging coordination, !" is between 150 to 260 cm-1.42-43 Two peaks at 1292 cm-1 and 1274 cm-1 in the adsorption spectra at initial stages of adsorption can be tentatively assigned to "s (COO$) of RCO2–Ti vibrations of surface complexed carboxylate groups, via monodentate complexation between a carboxylate group and Ti on the nanoparticle surface, respectively. Due to the presence of a number of different carboxylic acid functional groups, surface adsorption of SRHA on the TiO2 nanoparticle surface via multiple inner sphere complexation mechanisms between carboxylic functional groups and surface Ti atoms is possible. In fact, theoretical calculations for adsorption of formic acid and acetic acid on TiO2 surface show that the bidentate bridging and monodentate binding modes are energetically favorable on TiO2 surface.44-45 At the beginning surface adsorption, the main asymmetric stretching frequency is around 1561 cm-1 on 5 nm TiO2 NPs (1558 cm-1 on 22 nm TiO2 NPs), and the symmetric vibration is around 1411 cm-1 (1410 cm-1 for 22 nm TiO2 NPs), these bands correlate with bidentate bridging mode adsorption (%" ~ 150 cm-1). This is also observed for oxalate adsorption on TiO2 NPs.46 The combinations of 1582 cm-1 and 1397 cm-1 on 5 nm TiO2 NPs and 1581 cm-1 and 1401 cm-1 on 22 nm TiO2 NPs indicate various bridging binding modes in different chemical environments. Therefore, from these data, it is suggested that monodentate and bridging bidendate binding coexist on the surface at the initial stages of adsorption for both 5 and 22 nm TiO2 NPs.47 Apart from the surface bounded carboxylic groups, protonated carboxylic groups are also present in adsorbed SRHA, as seen by the absorption band at 1710 cm-1 that is assigned to the stretching vibration of C=O of protonated SRHA. However, a certain degree of deprotonation of COOH groups upon complexation is also observed. Reduced relative peak intensity of protonated C=O groups, and the disappearance of the sharp peak assigned to OC-OH vibrations when compared to free SRHA clearly indicates the surface induced deprotonation of carboxylic groups. Deprotonation of COOH moieties upon surface complexation is reported in the literature in 11 ! ACS Paragon Plus Environment

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! different adsorption studies.25, 37 Yoon et. al has reported that fulvic acid adsorbed on boehmite undergoes deprotonation at acidic pH.25 Deprotonation of citric acid adsorbed on TiO2 NPs at low pH has also been reported.37 Because the adsorption process involves deprotonation of carboxylic groups at low pH, the results of forming chemical bonds with Ti4+ on the surface are very similar to free carboxylate groups at high pH. In outer-sphere adsorption, the water layer plays an important role. However, the difference of %" between ionic and bidentate bridging binding is too similar to distinguish the two states. Furthermore, water molecules contribute to stabilizing ionic bonds at the interface.48 and the outer sphere complex may be considered an initial step in forming inner sphere complexes on the nanoparticle surface, especially at low pH. Additionally, the infrared spectra at later adsorption stages, when the coverage is higher, are more similar to that of the free SRHA than for the low coverage spectra seen initially. At pH 3.7, only a small part of carboxylic groups deprotonates; therefore, the carbonyl stretching vibration shows up around 1720 cm-1 in solution phase. The frequency of C=O displays a 10 cm-1 downshift when adsorbed on the TiO2 NP surface. For polycarboxylic acids, a red shift in the carbonyl stretching vibration from 1720 cm-1 to a doublet of 1710 cm-1 and 1690 cm-1 is suggestive of coupling of carboxylic groups.49. At this stage, due to the increasing coverage of humic acid on the TiO2 NP surface, the intermolecular interaction becomes stronger, reflecting the increasing absorption bands associated with hydrogen bonding of carboxylic acid groups on adjacent humic acid molecules. These data indicate that at higher coverages SRHA adsorbs via relatively different mechanisms than at lower coverages. Relatively weaker attractive interactions between adsorbed and free SRHA molecules, such as H-bonding, is suggested as the binding mode at higher coverages. These infrared data support previous reports that showed for low pH, below pH 5, humic acid tended to form multilayers.27 Therefore, in agreement the increase in intensity over time in the spectra shown in Figure 5 may be due to the formation of multilayers.


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! type of asymmetric and symmetric vibrations of carboxylate. The large %" (~ 300 cm-1) suggests the existence of the monodentate binding. The appearance of the band at 1637 cm-1 on 5 nm TiO2 (1632 cm-1 on 22 nm TiO2) is also possibly caused by dangling carboxylic groups coupling, such as -COO- ••• H+ ••• -OOC- and -COO- Na+••• Na+ -OOC-.47, 50-51 The absorption at 1750 cm-1 is assigned to the "(C=O), carbonyl stretch, in protonated -CO-OH, and means the presence of dangling protonated carboxylic groups at pH 6. Because the bands that appear in the adsorbed SRHA spectra on 5 nm TiO2 after 4 hours (1571 and 1393 cm-1 ) are approximately similar to those of free SRHA (1574 and 1390 cm-1) with %" is 178 cm-1 compared to that of the free SRHA (184 cm-1), outer sphere complexation can be suggested as another mechanism of surface adsorption of SRHA on 5 nm TiO2 NPs at pH 6.0.25, 39 However, the outer-sphere complex is thermodynamically less favorable on the surface.52 Thus, the inner sphere complexation (monodentate and bridge binding) is the more probable adsorption model. For 22 nm TiO2 NPs, the adsorption of SRHA (Figure 4-right panel middle) is similar. Initially, several prominent absorptions are seen in these spectra at 1632, 1574, 1488, 1392 and 1327 cm-1 with a large %" (~ 300 cm-1). After 4 hours of adsorption, some of the bands (1574 cm-1 and 1392 cm-1) grow in intensity whereas others (1632 cm-1 and 1327 cm-1) do not and instead become less distinct in the spectra. The %" of adsorbed SRHA after 4 hours is 182 cm-1, this is similar to the %" value of 184 cm-1 for solution phase, uncomplexed SRHA. Overall, based on spectroscopic data and thermodynamic considerations, inner sphere complexation of carboxylic acid groups away from the surface can be suggested as a mechanism of SRHA adsorption on 22 nm TiO2 NPs at pH 6.0. Similarly, for SRHA adsorption at pH 8.0 the formation of inner sphere complexes can be suggested as the mechanism of SRHA adsorption on both 5nm (figure 4-left panel top) and 22 nm TiO2 NPs. (figure 4-right panel top). Some previous reports proposed outer sphere complex for aromatic molecules.25,

40

However, due to similarity of band splitting of

bidentate bridging and the free ionic state, the bridging binding is possibly mistaken for the outer 14 ! ACS Paragon Plus Environment

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! sphere complexation. It is in a good agreement with the theoretical result that monodentate and bridging binding are energetically favorable.44 In addition, the adsorption band shown around ~ 1292 cm-1 is thought to be a shift of symmetric vibration of COO- to symmetric vibration O=C-OTi. The vibration of O=C-O-Ti is a direct evidence that humic acid is able to directly bound to the TiO2 surface. Therefore, at all pH the adsorption of humic acid on TiO2 nanoparticle surface takes place mainly through bridging and monodentate binding. The reversibility of the adsorption process was also investigated and it was determined that the adsorption of SRHA on both 5 and 22 nm TiO2 NPs at all the pH values studied is irreversible in nature. This can be seen by monitoring the intensity of the bands in the spectra that grow in upon adsorption. When the SRHA is removed from solution and water at the appropriate pH, it can be seen that very little SRHA is removed (as seen in Supporting Information Figure SI-2). Therefore, it is concluded that a majority of the adsorbed SRHA remains on the surface even after 4 hours of desorption indicating the irreversible nature of the adsorption process. However, it should be noted that the desorption study was limited to 4 hours in is this study. Additionally from the data shown in Supporting Information Figure SI-2, there are some differences in the shape of the kinetic curves between the two different sized NPs. The intensity of the bands in these spectra indicate the amount of adsorption differs between 5 and 22 nm TiO2 NPs. By comparing adsorbed spectra in Figure 4, it can be clearly seen that the adsorption of SRHA is higher at 3.7 compared to pH 6.0 and 8.0. A decreasing trend in the adsorption with increasing pH can be seen in the range of the pH studied. This has been observed in several other adsorption studies and has been related to the availability of the surface adsorption sites that depends on the pH of the point zero charge (pzc) of the surface. When pH is below the pzc, adsorption seems to be higher than when it is close to the pzc or higher. As reported in the literature for bulk TiO2 pzc is 5.9, however for NPs it is in the range of 4.8 to 8.1 depending on the exact surface structure and properties of the NPs.50 Experimentally determined pzc values, 5.8 and 15 ! ACS Paragon Plus Environment

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! nm TiO2 NPs at pH 3.7 and thus higher stability. The higher the value of the zeta potential (irrespective of the sign) the greater the stability of NPs due to electrostatic repulsions. The zeta potential values of bare TiO2 NPs (both 5 nm and 22 nm) are negative at pH 6.0 and 8.0. The negatively charged surface is due to the higher degree of deprotonation of surface hydroxyl groups. Zeta potentials at higher pH were also different between the two types of NPs studied. The observed difference might be due to the differences in composition for the two types of TiO2 NPs where, the 5 nm TiO2 is entirely composed of anatase and the 22 nm TiO2 is composed of both anatase (88 %) and rutile (12 %). The difference in the phase composition of the two NP types can be considered a possible reason for the difference in the experimentally determined pzc values of the two NP types, measured at 5.8 and 6.7 for 5 nm TiO2 and 22 nm TiO2 NPs, respectively. The pzc is dependent on NP properties such as size, crystal phase and surface orientation.53 As shown in Figure 6, the zeta potential of the SRHA adsorbed TiO2 NPs is negative at all pH values studied, regardless the SRHA concentration. Moreover, the magnitude of the zeta potential, at a given pH, is also higher in SRHA adsorbed TiO2 NPs than the corresponding bare TiO2 NPs regardless of the SRHA concentration. Therefore, the adsorption of SRHA increases the stability of both 5 nm and 22 nm TiO2 NPs due to the increased electrostatic repulsion between SRHA coated NPs. At pH 3.7, the SRHA coating transforms the positive zeta potential of both types of TiO2 NPs to negative zeta potential values. As discussed above, the observed SRHA signal in ATR-FTIR spectroscopy was highest at pH 3.7 indicating the highest adsorption capacity. This is attributed to the electrostatic attraction between positively charged TiO2 surface and the negatively charged SRHA molecules. Previous studies have also suggested that electrostatic interactions could play a major role in adsorption of natural organic matter on NP surfaces. Therefore, the higher adsorption of SRHA at pH 3.7 leads to the large change in the zeta potential value between the bare and SRHA coated TiO2 NPs. In addition, as noted above, the zeta potentials of SRHA adsorbed TiO2 NPs at 17 ! ACS Paragon Plus Environment

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! Conclusions Several conclusions can be drawn from the overall results of this study on the mechanism of SRHA adsorption on TiO2 NPs for two different sized nanoparticles and over a range of pH values. The adsorption of SRHA on TiO2 NPs is dependent on the pH such that the degree of adsorption decreases with increasing pH in the range of pH studied (3.7 to 8.0). A small difference in the degree of SRHA adsorption between 5 nm and 22 nm TiO2 NPs was observed, and the adsorption was relatively higher in 5 nm TiO2 NPs compared to 22 nm TiO2 NPs at all the pH values studied most likely due to the increased surface area of these smaller particles. SRHA exhibited irreversible adsorption on TiO2 surfaces at all pH values studied and different adsorption mechanisms were suggested based on the spectral data. When the pH is low, (3.7) the adsorption process initially takes place via inner-sphere complexation mechanisms yielding monodentate and bidentate binding to the surface with respect to the carboxylate moieties. Water in the outer sphere complexation may be a precursor and contribute to deprotonating humic acid that results in the formation of inner sphere complexation. Further adsorption of SRHA also takes place via other mechanisms such as H bonding between adsorbed SRHA and SRHA in solution. At higher pH (6.0 and 8.0) the surface adsorption could be better described as the inner-sphere complexation of SRHA with the NPs surface. The surface charge (as measured by zeta potential) of NPs is changed upon adsorption of SRHA depending on the pH. At pH 3.7, the impact of SRHA adsorption on surface charge was relatively higher than at other pH values studied. The effect of SRHA adsorption on surface charge is greater for 22 nm TiO2 NPs than 5 nm TiO2 NPs. At low pH, the surface charge is increased (and reversed) by the surface adsorbed SRHA, leading to increased electrostatic repulsions between TiO2 NPs. Increased electrostatic repulsions increases stability of the NPs by minimizing the aggregation. Hence, the adsorption of SRHA can increase the presence of TiO2 NPs in the soil solution, and thus will increase the probability of being absorbed by living organism. Furthermore, the results indicate that the SRHA adsorption decreases agglomerate sizes at all the pH values 21 ! ACS Paragon Plus Environment

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! studied and the effect of SRHA adsorption is higher at lower pH. Smaller agglomerates are more stable in suspensions than larger aggregates that are prone to sedimentation due to the larger size. Therefore, SRHA adsorption can increase the bioavailability of TiO2 NPs in soil water by decreasing the agglomerate sizes.

Acknowledgements.

This material is based on the work supported by the National Science

Foundation under grant number CHE1606607. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

Supporting Information.

Supporting information includes the following figures: (SI-1)

characterization data for TiO2 nanoparticles used in this study are shown; (SI-2) adsorption and desorption kinetics data for SRHA on TiO2 nanoparticles; and (SI-3) zeta potential variation of TiO2 nanoparticles as a function of pH are provided.

Graphical TOC.


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Surface Adsorption of Suwannee River Humic Acid on TiO2

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