Environmental Pollution 194 (2014) 31e37
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Effect of model dissolved organic matter coating on sorption of phenanthrene by TiO2 nanoparticles Xilong Wang a, *, Enxing Ma a, Xiaofang Shen a, Xiaoying Guo a, Meng Zhang a, Haiyun Zhang a, Ye Liu a, Fei Cai a, Shu Tao a, Baoshan Xing b, * a b
Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2014 Received in revised form 24 June 2014 Accepted 26 June 2014 Available online
Dissolved organic matter (DOM) may alter the sorption of hydrophobic organic contaminants (HOC) to metal oxide nanoparticles (NPs), but the role of DOM and NP types is poorly understood. Here, phenanthrene sorption was quantified on four types of nano-TiO2 (three rutile, one anatase), and a bulk, raw TiO2 powder. Prior to the sorption experiments, these nanoparticles were coated using four different organic materials: Lignin (LIG), tannic acid (TAN), Congo red (CON), and capsorubin (CAP). Lignin, tannic acid, congo red and capsorubin coating substantially enhanced phenanthrene sorption to various TiO2 particles. After coating with a specific DOM, Kd values by the DOM-coated TiO2 particles on percent organic carbon content and surface area (SA) basis (Koc/SA) generally followed the order: TiO2 NPs with hydrophobic surfaces > bulk TiO2 particles > other TiO2 NPs. Different Koc/SA values of various DOM-TiO2 complexes resulted from distinct conformation of the coated DOM and aggregation. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles Coating Sorption Phenanthrene
1. Introduction Nanoparticles have widely been used in commercial products. Metal oxide nanoparticles such as TiO2, SiO2, Fe2O3, Al2O3 and ZnO are the most extensively applied nanomaterials. In recent decades, the engineered TiO2 nanoparticles (NPs) have been produced in a large volume and widely used in various fields. The annual production of TiO2 NPs in USA reached 7800e38,000 t during 2007 and 2010 (Hendren et al., 2011; Piccinno et al., 2012). Therefore, it is imminent that the higher rate of production and usage of oxide NPs (e.g., TiO2) would lead to their environmental release in substantial amount (Benn and Westerhoff, 2008; Nowack and Bucheli, 2007). Sorption of hydrophobic organic contaminants (HOCs) to soils is an important process that affects their environmental behavior. Organic matter content in soils is generally far below than that of minerals, but it plays a critical role in HOC sorption (Schwarzenbach and Westall, 1981). In contrast, oxides and minerals are less important for HOC sorption mainly because water molecules reduce its accessibility to the polar surfaces (Kile et al.,
Abbreviation: dissolved organic matter, DOM; nanoparticles, NPs. * Corresponding author. E-mail addresses: [email protected], [email protected] (X. Wang), bx@ umass.edu (B. Xing). http://dx.doi.org/10.1016/j.envpol.2014.06.039 0269-7491/© 2014 Elsevier Ltd. All rights reserved.
1995). Once oxide NPs are released into the environment, they may interact with dissolved organic matter (DOM) in soils resulting in surface modifications. Organic matter in soils is generally present in the form of organic coatings on minerals and oxides (Murphy et al., 1994). DOM is a mixture of organic materials containing humic, fulvic and tannic acids, as well as lignin with considerably different molecular weights (Ferro-García et al., 1998). Therefore, influence of DOM coating on the physicochemical properties of oxides could be dependent on its composition. As reported, removal of organic materials from the surface coated natural samples and surficial sediments dramatically reduced sorption of bisphenol A, revealing that a considerable amount of BPA was sorbed on the coated organic materials of these two sorbents (Li et al., 2009). Organic matter coating enhanced phenanthrene sorption to kaolinite at all ionic strengths (Brunk et al., 1997), and both solution chemistry and mineral characteristics influenced phenanthrene sorption to the humic acid (HA)coated minerals (Laor et al., 1998). Composition of the coated DOM affected HOC sorption towards hematite and kaolinite (Murphy et al., 1994). Furthermore, sorption of aromatics on montmorillonite and kaolinite was affected by conformation of the coated DOM (Chen and Xing, 2005; Feng et al., 2006). As for the studies on the impact of organic matter coating on HOC sorption to NPs, Yang and Xing (2009) found higher nonlinearity of phenanthrene sorption isotherms to HA-coated TiO2 and ZnO NPs compared to the
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X. Wang et al. / Environmental Pollution 194 (2014) 31e37
original HA. After coating HA on Al2O3 NPs, the organic carbon content (foc)-normalized sorption coefficients (Koc) of phenanthrene by the HA-coated Al2O3 NPs were lower than those by the original HAs, and the Koc difference was more pronounced at relatively high sorbate concentration (Yang et al., 2010). Li et al. (2008) found that sodium dodecyl sulfate (SDS)-coated nanosized alumina had higher sorption affinity, capacity and rate for di-ethylphthalate than the SDS-coated micrometer-sized alumina. Sorption of formaldehyde to g-Al2O3 NPs was enhanced considerably after they were grafted with the functional groups of 2,4dinitrophenylhydrazine (Afkhami et al., 2011). Similarly, Jing et al. (2013) also reported that sorption of naphthalene and p-nitrophenol to nano-SiO2 was enhanced substantially after coating with a cationic surfactant (e.g., cetylpyridinium chloride, CPC). To date, difference in conformation of DOM that was coated on oxide particles with dissimilar particle size and surface properties, and the associated impact on HOC sorption still remain unclear. Also, information regarding the effect of DOM coating on aggregation of oxide NPs and associated influence on HOC sorption is scarce. As a first attempt to address these scientific issues, the key objectives of this study are to: 1) compare sorption of phenanthrene to bulk and various TiO2 NPs with different particle size and surface properties; and 2) examine the difference in conformation of the coated DOM on various TiO2 particles and influence of DOM coating on aggregation of various TiO2 particles, and the resulting impact on phenanthrene sorption along with the responsible mechanisms. 2. Materials and methods 2.1. Sorbates and sorbents Four types of TiO2 NPs including three types of rutile with distinct particle size and SA and only one anatase that was commercially available were purchased from Nanjing Guanye Chemical Industry Co., Ltd. Among these three rutile TiO2 NPs, one was the pristine NPs, and the other two samples were respectively treated by the supplier to have hydrophobic and hydrophilic surfaces. The bulk TiO2 powder was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Lignin (LIG) is a polyphenolic material, accounting for a large part of natural organic matter in the environment. Tannic acid (TAN) is widely present in plants, and it has been used extensively as a surrogate of DOM for environmentally relevant studies (Yamamoto spedes et al., 2006). Congo red (CON) is a benzidine-based and et al., 2004; Flores-Ce water-soluble dye, widely used in many industries (Ghaedi et al., 2012). The model DOMs including LIG, TAN and CON have aromatic structure (Fig. S1 in the Supplementary Data). Capsorubin (CAP) is a kind of carotenoid and polyene pigment extracted from paprika without aromatic structure, which has widely been used as colorant and food antioxidant (Requena et al., 2008). TAN and CON were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. LIG (alkali) was obtained from J&K Chemical Co., Ltd., USA. The concentrated CAP solution was kindly donated by Shenzhen Yicheng Trading Company Ltd. The basic chemical structures of these model DOMs are presented in Fig. S1, and they were used to coat on TiO2 particles to form DOM-TiO2 complexes. The details for DOM-coating on various TiO2 particles are presented in the Supplementary Data. The original bulk TiO2, four types of TiO2 NPs including one anatase, one pristine rutile and two rutiles with hydrophilic and hydrophobic surfaces were labeled as BT, NAnT, NRuT, NLRuT and NBRuT, respectively. The TiO2 particles after coating with LIG were marked as LIG-BT, LIG-NAnT, LIG-NRuT, LIG-NLRuT and LIG-NBRuT, and those after coating with TAN, CON, and CAP were correspondingly labeled as TAN-BT, TANNAnT, TAN-NRuT, TAN-NLRuT and TAN-NBRuT; CON-BT, CON-NAnT, CON-NRuT, CON-NLRuT and CON-NBRuT as well as CAP-BT, CAP-NAnT, CAP-NRuT, CAP-NLRuT and CAP-NBRuT, respectively. Phenanthrene was used as the sorbate, and both 14Clabled and nonlabeled forms of this compound were purchased from SigmaeAldrich Chemical Co. 2.2. Sorbent characterization Carbon, hydrogen, and nitrogen content of DOM-TiO2 complexes were measured using a Vario EL CHN elemental analyzer (Germany). Since the coated amount of DOM on various TiO2 particles estimated from carbon content of the DOMeTiO2 complex was low (0.2e6.0%), it was not possible to obtain the bulk oxygen content of DOMeTiO2 complex by mass balance. Surface elemental composition and carbonbased functionalities of all DOM-coated TiO2 particles were determined using an AXIS-Ultra X-ray imaging photoelectron spectrometer (Kratos Analytical Ltd. UK) with a monochromatic Al Ka radiation source operated at 225 W, 15 mA, and 15 kV.
Approximately 2 mg of powdered solid samples were used for both elemental analysis and XPS scanning. Since SA of sorbents is an important factor that may affect HOC sorption, sorptionedesorption isotherms of N2 to all samples at 77 K were obtained to get their SAs using an Autosorb-1-MP surface area analyzer (Quantachrome Instruments, USA) after outgassing at 105 C for 16 h. SAs of all samples were calculated from N2 sorption isotherm data using multipoint BET method with relative pressure P/P0 ranging from 0.05 to 0.3. X-ray powder diffraction (XRD) patterns of all samples were collected using a X’Pert Pro MPD diffractometer (Cu Ka radiation) coupled with the X'celerator detector at a scanning rate of 6 /min in the 2q range of 10e70 . The XRD patterns were refined with Rietveld method and the mean crystallite size of samples was calculated from peak broadening using X'Pert Highscore Plus and Fullprof softwares. To evaluate aggregation of all sorbents in the sorption systems, their size distribution was determined with an LS13 320 multi-wave length laser diffraction particle size analyzer (Beckman Coulter Inc., Miami, FL), and the zeta potential of sorbents was measured with a Nano-ZS90 Zetasizer (Malvern Instruments Technical Ltd., UK). Details of the analytical procedures are described in the Supplementary Data. It is possible that surface properties of the sorbents in aqueous phase are different from those in the solid state. Contact angles of all sorbents were detected using a video contact angle system with a sessile drop method (OCA20, Dataphysics, Germany), to get information on surface hydrophobicity of the tested samples. Details of sample preparation and contact angle measurement are presented in the Supplementary Data. 2.3. Sorption experiment Sorption isotherms of phenanthrene to all sorbents were obtained using a batch equilibration technique in screw cap vials with aluminum foil-Teflon liners. Stock solutions of 14C labeled and non-labled phenanthrene were dissolved in methanol. The background solution contained 0.01 M CaCl2 to maintain a stable ionic strength and 200 mg/L of NaN3 to inhabit bioactivity, and the pH was adjusted to neutral using 0.1 M NaOH and HCl. The 14C-labeled and nonlabled stock solutions of phenanthrene were added to the background solution to prepare the test solutions with different sorbate concentrations. After shaking for 1 h, test solutions were added to the screw cap vials which contained appropriate amount of weighed sorbent, until a minimal headspace was reached. The solute-to-sorbent ratio was adjusted to have 20e80% phenanthrene uptake by various sorbents. Methanol concentration in the test solutions was kept below 0.1% (v/v) to minimize the cosolvent effect. Our preliminary test indicated that apparent sorption equilibrium was reached within 8 d (Fig. S2). Therefore all vials were mixed on a rotary shaker for 9 d at room temperature. After mixing, all vials were centrifuged at 4000 rpm for 20 min, and then 1.8 ml supernatant was sampled and added to ScintiVerse cocktail (4 ml) (Fisher Scientific Co.) for scintillation counting with a liquid scintillation counter (Beckman Coulter, LS 6500). All samples and blanks were run in duplicate. Because of the negligible mass loss of phenanthrene as indicated in the blanks ( Congo red > Lignin > Tannic acid. Such an order was consistent with the Koc values of phenanthrene by TiO2 particles correspondingly coated with these DOMs (Table 1), indicating that sorption of this compound to the coated DOM on unit carbon basis controlled its sorption to the DOMeTiO2 complex. Different Koc values of phenanthrene by a given TiO2 sample coated with various DOMs seem to be a result of their hydrophobicity difference. However, the Koc values of phenanthrene by various DOMs were not measured, because these DOMs are readily dissolved and previous study
Table 2 Organic carbon content and surface area of the original and DOM-coated bulk and TiO2 NPs. Sorbents BT NAnT NRuT NLRuT NBRuT
foc (%)
SA (m2/g)
Sorbents
foc (%)
SA (m2/g)
CT
Sorbents
foc (%)
SA (m2/g)
CT
Sorbents
foc (%)
SA (m2/g)
CT
Sorbents
foc (%)
SA (m2/g)
CT
0.1 1.1
7.4 82.7 36.9 29.5 22.8
LIG-BT LIG-NAnT LIG-NRuT LIG-NLRuT LIG-NBRuT
0.2 0.4 0.3 0.2 1.2
11.5 70.7 42.2 23.4 18.5
0.17 0.06 0.07 0.09 0.65
TAN-BT TAN-NAnT TAN-NRuT TAN-NLRuT TAN-NBRuT
0.8 2.8 2.8 0.8 2.8
10.4 50.1 29.4 55.7 32.2
0.77 0.56 0.95 0.14 0.87
CON-BT CON-NAnT CON-NRuT CON-NLRuT CON-NBRuT
0.2 1.3 0.4 0.2 1.3
13.0 60.7 51.8 30.9 27.5
0.15 0.21 0.08 0.06 0.47
CAP-BT CAP-NAnT CAP-NRuT CAP-NLRuT CAP-NBRuT
1.2 4.4 3.8 1.1 6.0
10.7 30.6 46.7 22.5 20.9
1.12 1.44 0.81 0.49 2.87
OC: organic carbon content (%); SA: surface area (m2/g); CT: coated carbon thickness from DOM to TiO2 particles estimated by foc/SA (mg/m2).
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Fig. 1. Agglomerate size distribution of the original and DOM-coated TiO2 NPs. Original and DOM-coated BT (A); original and DOM-coated NAnT (B); original and DOM-coated NRuT (C); original and DOM-coated NLRuT (D); original and DOM-coated NBRuT (E). Here, DOM includes lignin (LIG); tannic acid (TAN); congo red (CON) and capsorubin (CAP). The TiO2 particles include the bulk particles (BT); the pristine anatase-type TiO2 NP (NAnT); the pristine rutile-type TiO2 NP (NRuT); and two rutile-type TiO2 NPs with hydrophilic (NLRuT) and hydrophobic surfaces (NBRuT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 3 Agglomerate size of the original and DOM-coated bulk and TiO2 NPs in the sorption systems. Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
BT NAnT NRuT NLRuT NBRuT
3.63 21.29 25.74 43.58 46.15
1.67 4.82 15.58 19.78 38.20
LIG-BT LIG-NAnT LIG-NRuT LIG-NLRuT LIG-NBRuT
8.28 26.79 117.50 36.29 138.80
1.93 5.87 43.80 13.18 110.40
TAN-BT TAN-NAnT TAN-NRuT TAN-NLRuT TAN-NBRuT
14.48 20.31 83.27 40.16 54.71
2.76 7.39 33.11 15.06 12.28
CON-BT CON-NAnT CON-NRuT CON-NLRuT CON-NBRuT
10.28 99.55 163.10 27.90 68.49
2.04 34.53 82.76 4.61 55.12
CAP-BT CAP-NAnT CAP-NRuT CAP-NLRuT CAP-NBRuT
18.93 188.00 169.00 18.94 188.9
2.93 86.33 97.37 3.24 130.30
showed that a same DOM (i.e., HA) in dissolved form had significantly higher sorption for this compound than that in the solid state (Pan et al., 2007). Further, it was reported that organic matter was the dominant sorption medium for HOCs in soil if its content was over 0.1% by mass (Schwarzenbach and Westall, 1981). Such a phenomenon was proved by the huge difference in Kd values of phenanthrene by the original and DOM-coated TiO2 particles (Table 1). Hence, Koc values of phenanthrene by the DOMeTiO2 complex can be viewed as that by the DOM in solid state. Further analysis showed that the difference in sorption enhancement of various coated DOMs was not due to the surface hydrophobicity difference of DOM-TiO2 complexes. This is because the sum of the percentages of polar functionalities CeO and C]O expressed as (fCO þ fC ¼ O) at the surfaces of the LIG- and CAP-coated TiO2 was comparable (Table S2), but sorption enhancement of phenanthrene to a given TiO2 sample induced by separately coating with unit carbon mass of these two DOMs was very different (Fig. 2). It was
interesting to find that relative abundance of the hydrophobic carbon domains at the DOMeTiO2 complex surfaces expressed as surface foc fCC/(fCO þ fC ¼ O) followed the same order as sorption enhancement of the corresponding DOMs, suggesting that sorption enhancement of phenanthrene to a given TiO2 sample resulting from DOM coating was regulated by the relative abundance of hydrophobic carbon domains in the coated DOM. The relative increase in Kd/SA values of phenanthrene by NAnT, NRuT and NLRuT resulting from DOM coating derived from (Kd, coated/SAcoated-Kd, original/SAoriginal)/(Kd, original/SAoriginal) reached 5.36.4, 9.4-23.3, 1.2-19.0 and 335.0-734.7 for LIG, TAN, CON and CAP coating, respectively, followed by BT, and that of NBRuT was the lowest with the corresponding values being 0.81, 0.76, 0.79 and 13.3. Such a trend was reverse to the coated carbon thickness on these sorbents estimated from foc/SA. This can be due to the fact that, when the coated carbon thickness on TiO2 particles was low, the coated DOM dramatically enhanced hydrophobicity of the oxide surfaces, thereby substantially increasing phenanthrene sorption. Surface properties of the TiO2 particles may not be altered too much with further carbon coating, so the relative increase in Kd/SA values would slightly change (Fig. S6). Such an explanation was reasonable because NBRuT originally had 1.1% of foc, and the coated carbon thickness from DOM to NBRuT surfaces was the highest among all TiO2 particles (Table 2). The relative increase in Kd/SA value of this sorbent caused by DOM coating was the lowest (Fig. S6).
3.3. Influence of the aggregation of DOM-coated TiO2 particles on phenanthrene sorption
Fig. 2. Sorption enhancement index of phenanthrene by the bulk and TiO2 NPs as induced by unit organic carbon loading mass. The index was calculated from (KcomKm)/(Km,foc). Km and Kcom are distribution coefficients of phenanthrene by the original and DOM-coated TiO2 particles, respectively (L/kg). Here, foc is the organic carbon content of the DOMeTiO2 complex, and each column corresponds to the complex of Y (DOM)-coated X (TiO2 particle). Here, DOM includes lignin (LIG); tannic acid (TAN); congo red (CON) and capsorubin (CAP). TiO2 particle includes the original bulk TiO2 (BT); anatase-type TiO2 NP (NAnT); the pristine rutile-type TiO2 NP (NRuT); the rutile-type TiO2 NP with hydrophilic (NLRuT) and hydrophobic surfaces (NBRuT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
To probe the influence of DOM-coated TiO2 NPs aggregation on phenanthrene sorption, the number of phenanthrene molecule adsorbed on unit SA was normalized with foc of the DOMeTiO2 complex and plotted against sorbate molecule number in aqueous phase at equilibrium (Fig. S7). After normalization, Koc/SA values of phenanthrene by the DOM-coated TiO2 particles generally followed the order: NBRuT > BT > NAnT, NRuT and NLRuT. This implied that Koc values of phenanthrene by a given DOM-coated TiO2 particle on unit SA were different, possibly due to distinct aggregation of various DOM-TiO2 complexes.
X. Wang et al. / Environmental Pollution 194 (2014) 31e37
It was documented that surface charge and surface properties of oxide NPs determined their aggregation behaviors in aqueous phase (Dachs and Bayona, 1997; Bian et al., 2011). The zeta potential data showed that LIG-BT, LIG-NAnT, LIG-NRuT, CON-BT, CON-NAnT, CON-NRuT, CAP-NAnT and CAP-NRuT were positively charged, whereas the rest of the DOM-coated TiO2 particles were negatively charged in the background solution identical to the sorption experiments (Table 4). The DOM-coated TiO2 particles would repel each other when they were positively or negatively charged, thus inhibiting aggregation. However, all the DOM-coated TiO2 particles were highly aggregated in aqueous phase (Fig. 1, Table 3). Strong aggregation of negatively-charged DOM-TiO2 complexes can be a result of Ca2þ-induced neutralization of the negative surface charges, thus facilitating aggregation. The same phenomenon was reported in previous studies (Ghosh et al., 2011), where HA coated Al2O3 and gFe2O3 NPs displayed aggregation in the presence of Ca2þ. However, such a mechanism cannot interpret aggregation of the DOM-TiO2 complexes with positive charges. Possibly hydrophobic moieties in the coated DOM were exposed outside during the coating process as hydrophilic functional groups tend to anchor the oxide surface (Gu et al., 1995). The aromatic components in the coated LIG and CON had both pep and hydrophobic interactions and the alkyl carbon moieties in the coated CAP were able to interact through hydrophobic interactions, thereby serving as the dominant driving force to attract the complex particles to each other, followed by aggregation. The attracting force between DOM-coated TiO2 particles should be stronger than the repelling one, so that they all strongly aggregated. Another point was that the zeta potential values of all DOMcoated TiO2 particles generally ranged from 30 to 30 mV (Table 4), and all suspensions would be more or less unstable a et al., 2012). under such condition (Csapo Aggregation enhancement of TiO2 particles induced by DOM coating was evident as indicated by the smaller mean agglomerate size of the uncoated TiO2 particles relative to the DOM-coated ones. Differently, aggregation of sodium dodecyl sulfate (SDS)-coated silver NPs was quite similar to that of the uncoated ones (Li et al., 2012). The mean particle size of the LIG-, TAN-, CON- and CAPcoated TiO2 NPs was 3.2e16.8, 2.7e12.0, 2.3e40.6 and 1.1e44.5 times of BT after correspondingly coating with these DOMs (Table 3), demonstrating that as DOM was coated on TiO2 particles, aggregation of TiO2 NPs was stronger than BT. Due to aggregation of the DOM-coated TiO2 particles, accessibility of a portion of sorption domains in the coated DOM would be reduced, thereby decreasing their effectiveness for phenanthrene sorption. Such an effect was more evident for NAnT, NRuT and NLRuT than BT (Fig. 1). Hence, Koc/SA values of phenanthrene by a given DOM-coated BT were higher than those by NAnT, NRuT and NLRuT after coating with the same DOM (Fig. S7, Table 1). Aggregation of DOM-coated NBRuT was comparable to that of other TiO2 NPs coated with the same DOM, but Koc/SA values of phenanthrene by DOM-coated NBRuT were generally much higher than those by DOM-coated BT and other TiO2 NPs (Fig. S7, Table 1). This was likely due to the fact that the estimated coated carbon thickness from DOM to NBRuT was generally much higher than the bulk and other
35
TiO2 NPs (Table 2). As the DOMeTiO2 complex aggregated, a portion of sorption sites possibly in glassy phase in the coated DOM were wrapped inside. However, a larger quantity of sorption domains in the coated DOM in DOM-NBRuT complex were exposed outside for phenanthrene sorption as indicated by their much larger contact angles relative to the others (Table S2). Moreover, NBRuT particles with thicker carbon coating in comparison with others could have a thicker rubbery phase, thus leading to enhanced diffusion of sorbate molecules. Therefore, the Koc/SA values of DOM-coated NBRuT were generally much higher than other TiO2 particles coated with the same DOM. 3.4. Effect of sorption domain conformation in the coated DOM on phenanthrene sorption It was proposed that the mineral-bound organic matter contained rubbery and glassy domains. The difference between rubbery and glassy domains is that the latter one had rigid hydrophobic nano-scaled microvoids in its matrix (Johnson et al., 1999). Sorption of HOCs to the rubbery domains is a dissolutionlike partitioning process and that to the glassy ones is dominantly a surface-adsorption one. Sorption isotherms of phenanthrene to the TAN- and CAP-coated TiO2 particles were practically linear (Table 1, Freundlich n ranging from 0.952 to 1.050), suggesting a partitioning-dominant mechanism. However, its sorption to the LIG- and CON-coated TiO2 particles was slightly nonlinear (Fig. S5, Table 1). Nonlinear sorption isotherms of LIG-coated TiO2 particles can be attributed to the adsorption in hydrophobic nanoavoids in glassy domains (Young and LeBoeuf, 2000) because LIG is a glassy biopolymer (LeBoeuf and Weber, 2000), whereas those for the CON-coated TiO2 particles could be a result of the formation of glassy domains at CONeTiO2 interfaces. Difference in interactions between TAN and CAP, as well as CON with TiO2 particles was most likely due to their distinct chemical composition. Chen and Xing (2005) examined impact of wax-coating on sorption of pyrene, phenanthrene and naphthalene to montmorillonite. They observed that sorption of phenanthrene and naphthalene to the wax-coated montmorillonite with different coating levels was practically linear. However, it was reported in the same work that, sorption isotherms of pyrene to the wax-coated montmorillonite with low wax loading was slightly nonlinear, which was ascribed to the creation of glassy domains at montmorilloniteewax interfaces (Gunasekara and Xing, 2003). Wang and Xing (2005) reported that sorption isotherms of phenanthrene to HA-coated montmorillonite and kaolinite tend to be more linear with increasing HA loading amount. In this case, the coated organic carbon content of HA to montmorillonite increased from 0.34 to 0.97% and that to kaolinite increased from 0.26 to 0.47%. An increase in foc of montmorillonite and kaolinite from HA coating was comparable to most of the TiO2 samples coated with various DOMs (Table 2). Difference in sorption isotherm nonlinearity of phenanthrene to the HA-coated montmorillonite and kaolinite as well as the TiO2 particles coated with various DOMs (in our case) has suggested conformational variation of the organic carbon domains formed at these interfaces.
Table 4 Zeta potential values of the original and DOM-coated bulk and TiO2 NPs in the sorption systems. Sorbents
ZP (mV)
Sorbents
ZP (mV)
Sorbents
ZP (mV)
Sorbents
ZP (mV)
Sorbents
ZP (mV)
BT NAnT NRuT NLRuT NBRuT
7.6 25.0 38.1 14.6 14.1
LIG-BT LIG-NAnT LIG-NRuT LIG-NLRuT LIG-NBRuT
11.1 9.1 17.9 14.3 12.5
TAN-BT TAN-NAnT TAN-NRuT TAN-NLRuT TAN-NBRuT
19.1 14.4 17.1 20.1 9.6
CON-BT CON-NAnT CON-NRuT CON-NLRuT CON-NBRuT
6.3 17.6 5.5 13.1 12.2
CAP-BT CAP-NAnT CAP-NRuT CAP-NLRuT CAP-NBRuT
8.2 17.4 3.2 10.8 0.8
ZP: Zeta potential. Each sample was measured twice and the averaged data are presented here and used for discussion.
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X. Wang et al. / Environmental Pollution 194 (2014) 31e37
In addition, different Koc/SA values of a given DOM preloaded on various TiO2 particles can also be a result of the dissimilar conformation of DOM adsorbed on the particles. Since the coated carbon thickness from DOM on NAnT, NRuT and NLRuT was generally the lowest among all TiO2 particles (Table 2), a greater portion of the coated DOM would be tightly bound to these TiO2 NPs and possibly form glassy domains in contrast to other sorbents. It is highly difficult for phenanthrene molecules to penetrate into the sorption domains in the coated DOM on these TiO2 NPs relative to NBRuT and BT, thereby giving the lowest Koc/SA values (23e1383 L/kg) for this compound among all TiO2 particles (Fig. S7, Table 1). As for DOM coating on both BT and NBRuT, the first layer would be tightly bound to the particle surfaces due to their strong interactions. The coated DOM layers beyond the first layer would be more loosely bound to BT and NBRuT as the attractive forces tend to be weaker after the limited high-energy sites at their surfaces were occupied (Chen and Xing, 2005; Wang and Xing, 2005), thereby possessing the more flexible conformation as the distance from surfaces of these particles increased (Gunasekara and Xing, 2003). The more loosely bound DOM to BT and NBRuT relative to that bound to NAnT, NRuT and NLRuT should have higher sorption affinity for phenanthrene due to higher mobility and accessibility of domains, thereby facilitating diffusion of the sorbate molecules (Schreiber and Schonherr, 1993). Such a phenomenon should be more evident for NBRuT as compared to BT, because of the higher coated carbon thickness on its surfaces and higher hydrophobicity of the DOM coated NBRuT surfaces relative to the other TiO2 particles, as indicated by much larger contact angles of the DOM-NBRuT complexes (Table 2 and Table S2). However, it is noted that the glassy and rubbery sorption domains in both tightly bound and flexible regions in the coated organic matter on bulk and various TiO2 NPs contributed to phenanthrene sorption (Wang and Xing, 2005). The crystalline phase in organic matter is generally assembled in a regular lattice, which can exclude the sorbing molecules that may otherwise diffuse across the barrier, thereby decreasing their sorption (Chen and Xing, 2005). To test if the crystalline domains were created at the DOM-oxide interfaces or in the coated DOM matrix, XRD patterns of the original and DOM-coated bulk and TiO2 NPs were obtained and shown in Fig. S8. The anatase and rutile structures of the original TiO2 NPs were clearly differentiated as they had relatively dominant diffraction peaks at different positions, e.g., 2q ¼ 25, 38 and 48 for anatase structure and 2q ¼ 28, 36 and 55 for rutile structure. Clearly, positions of all the diffraction peaks of original bulk and TiO2 NPs did not shift and their intensities also did not change after coating with DOMs, and DOM coating did not alter the crystallite size of the original bulk and various TiO2 NPs (Table 5). In addition, no clear changes in lattice parameter on both a- and c-axis of the original bulk and TiO2 NPs after coating with DOM were observed (Table 5). No difference in the XRD pattern of the original and DOM-coated TiO2 particles was probably due to the very low mass ratio of the coated DOM to the TiO2 particles. 4. Conclusions This study revealed that although SA of TiO2 NPs was 3.1e11.2 times of the bulk ones, they had comparable sorption for phenanthrene, mainly due to much stronger aggregation of the former. DOM coating substantially increased Kd/SA values of phenanthrene by all TiO2 particles, and the sorption enhancement was more pronounced when the coated carbon thickness was low. After normalizing Kd/SA values of DOMeTiO2 complex with its foc, the resulting Koc/SA values by DOM-NBRuT complex were highest among all TiO2 particles coated with DOM. This suggested that after DOM coating, the TiO2 NPs previously with hydrophobic surfaces
Table 5 Crystalline structure, lattice parameter, and crystallite size of the original and DOMcoated bulk and TiO2 NPs. Sorbents
BT LIG-BT TAN-BT CON-BT CAP-BT NAnT LIG-NAnT TAN-NAnT CON-NAnT CAP-NAnT NruT LIG-NRuT TAN-NRuT CON-NRuT CAP-NRuT NLRuT LIG-NLRuT TAN-NLRuT CON-NLRuT CAP-NLRuT NBRuT LIG-NBRuT TAN-NBRuT CON-NBRuT CAP-NBRuT
Crystallite structure Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile
Lattice parameter (Å) a
c
3.783 3.783 3.785 3.783 3.783 3.785 3.785 3.785 3.786 3.785 4.595 4.595 4.595 4.594 4.595 4.596 4.594 4.594 4.594 4.595 4.595 4.593 4.594 4.594 4.594
9.516 9.517 9.520 9.515 9.517 9.506 9.506 9.505 9.505 9.507 2.958 2.958 2.958 2.958 2.958 2.959 2.958 2.958 2.958 2.959 2.959 2.958 2.959 2.958 2.959
Mean crystallite size (Å) 783.8 778.0 746.6 777.7 770.9 252.2 227.7 227.0 232.1 235.0 330.1 330.4 330.0 329.1 328.1 408.9 393.0 382.3 389.7 398.8 396.1 406.3 420.0 415.6 414.2
would more strongly reduce the mobility of phenanthrene in the environment than the bulk and other TiO2 NPs. Acknowledgments This study was supported by the National Natural Science Foundation of China (41271461, 41328003, 41390240 and 41130754), 973 Program (2014CB441101), 111 Program (B14001) and the Startup Fund for the Peking University 100-Talent Program. We thank Dr. Saikat Ghosh for editing and improving the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2014.06.039. References Afkhami, A., Bagheri, H., Madrakian, T., 2011. Alumina nanoparticles grafted with functional groups as a new adsorbent in efficient removal of formaldehyde from water samples. Desalination 281, 151e158. Benn, T., Westerhoff, P., 2008. Nanoparticle silver released into water from commercially available sock fabrics. Environ. Sci. Technol. 42, 4133e4139. Bian, S.W., Mudunkotuwa, I.A., Rupasinghe, T., Grassian, V.H., 2011. Aggregation and dissolution of 4 nm ZnO nanoparticles in aqueous environments: influence of pH, ionic strength, size, and adsorption of humic acid. Langmuir 27, 6059e6068. Brunk, B.K., Jirka, G.H., Lion, L.W., 1997. Effects of salinity changes and the formation of dissolved organic matter coatings on the sorption of phenanthrene: implications for pollutant trapping in estuaries. Environ. Sci. Technol. 31, 119e125. Chen, B.L., Xing, B.S., 2005. Sorption and conformational characteristics of reconstituted plant cuticular waxes on montmorillonite. Environ. Sci. Technol. 39, 8315e8323. Chiou, C.T., McGroddy, S.E., Kile, D.E., 1998. Partition characteristics of polycyclic aromatic hydrocarbons on soils and sediments. Environ. Sci. Technol. 32, 264e269. Szalai, A., Csete, M., a, E., Patakfalvi, R., Hornok, V., To th, L.T., Sipos, A., Csapo k De any, I., 2012. Effect of pH on stability and plasmonic properties of cysteinefunctionalized silver nanoparticle dispersion. Colloid Surf. B 98, 43e49. Dachs, J., Bayona, J.M., 1997. Langmuir-derived model for diffusion- and reactionlimited adsorption of organic compounds on fractal aggregates. Environ. Sci. Technol. 31, 2754e2760.
X. Wang et al. / Environmental Pollution 194 (2014) 31e37 Feng, X.J., Simpson, A.J., Simpson, M.J., 2006. Investigating the role of mineralbound humic acid in phenanthrene sorption. Environ. Sci. Technol. 40, 3260e3266. Ferro-García, M.A., Rivera-Utrilla, J., Bautista-Toledo, I., Moreno-Castilla, C., 1998. Adsorption of humic substances on activated carbon from aqueous solutions and their effect on the removal of Cr(III) ions. Langmuir 14, 1880e1886. spedes, F., Ferna ndez-Pe rez, M., Villafranca-S lezFlores-Ce anchez, M., Gonza Pradas, E., 2006. Cosorption study of organic pollutants and dissolved organic matter in a soil. Environ. Pollut. 142, 449e456. Ghaedi, M., Biyareh, M.N., Kokhdan, S.N., Shamsaldini, S., Sahraei, R., Daneshfar, A., Shahriyar, S., 2012. Comparison of the efficiency of palladium and silver nanoparticles loaded on activated carbon and zinc oxide nanorods loaded on activated carbon as new adsorbents for removal of Congo red from aqueous solution: kinetic and isotherm study. Mat. Sci. Eng. 32, 725e734. Ghosh, S., Jiang, W., McClements, J.D., Xing, B.S., 2011. Colloidal stability of magnetic iron oxide nanoparticles: influence of natural organic matter and synthetic polyelectrolytes. Langmuir 27, 8036e8043. Gu, B., Schmitt, J., Chen, Z., Liang, L., McCarthy, J.F., 1995. Adsorption and desorption of different organic matter fractions on iron oxide. Geochim. Cosmochim. Acta 59, 219e229. Gunasekara, A., Xing, B.S., 2003. Sorption and desorption of naphthalene by soil organic matter: importance of aromatic and aliphatic components. J. Environ. Qual. 32, 240e246. € ge, J., Wiesner, M.R., 2011. Estimating production Hendren, C.O., Mesnard, X., Dro data for five engineered nanomaterials as a basis for exposure assessment. Environ. Sci. Technol. 45, 2562e2569. Huang, W.L., Schlautman, M.A., Weber Jr., W.J., 1996. A distributed reactivity model for sorption by soils and sediments. 5. The influence of near-surface characteristics in mineral domains. Environ. Sci. Technol. 30, 2993e3000. Jing, Q.F., Yi, Z.L., Lin, D.H., Zhu, L.Z., Yang, K., 2013. Enhanced sorption of naphthalene and p-nitrophenol by Nano-SiO2 modified with a cationic surfactant. Water Res. 47, 4006e4012. Johnson, M.D., Huang, W., Dang, Z., Weber Jr., W.J., 1999. A distributed reactivity model for sorption by soils and sediments. 12. Effects of subcritical water extraction and alterations of soil organic matter on sorption equilibria. Environ. Sci. Technol. 33, 1657e1663. Kile, D.E., Chiou, C.T., Zhou, H., Li, H., Xu, O., 1995. Partition of nonpolar organic pollutants from water to soil and sediment organic matters. Environ. Sci. Technol. 29, 1401e1406. Laor, Y., Farmer, W.J., Aochi, Y., Strom, P.F., 1998. Phenanthrene binding and sorption to dissolved and to mineral-associated humic acid. Water Res. 32, 1923e1931. LeBoeuf, E.J., Weber Jr., W.J., 2000. Macromolecular characteristics of natural organic matter. 1. Insights from glass transition and enthalpic relaxation behavior. Environ. Sci. Technol. 34, 3623e3631.
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Li, J.D., Shi, Y.L., Cai, Y.Q., Mou, S.F., Jiang, G.B., 2008. Adsorption of di-ethylphthalate from aqueous solutions with surfactant-coated nano/microsized alumina. Chem. Eng. J. 140, 214e220. Li, X., Lenhart, J.J., Walker, H.W., 2012. Aggregation kinetics and dissolution of coated silver nanoparticles. Langmuir 28, 1095e1104. Li, Y., Li, N., Chen, D., Wang, X.L., Xu, Z.L., Dong, D.M., 2009. Bisphenol a adsorption onto metals oxides and organic materials in the natural surface coatings samples (NSCSs) and surficial sediments (SSs): inhibition for the importance of Mn oxides. Water Air Soil. Pollut. 196, 41e49. Murphy, E.M., Zachara, J.M., Smith, S.C., Philips, J.L., Wietsma, T.W., 1994. Interaction of hydrophobic organic compounds with mineral-bound humic substances. Environ. Sci. Technol. 28, 1291e1299. Nowack, B., Bucheli, T.D., 2007. Occurrence, behavior and effects of nanoparticles in the environment. Environ. Pollut. 150, 5e22. Pan, B., Xing, B.S., Tao, S., Liu, W.X., Lin, X.M., Xiao, Y., Dai, H.C., Zhang, X.M., Zhang, Y.X., Yuan, H.S., 2007. Effect of physical forms of soil organic matter on phenanthrene sorption. Chemosphere 68, 1262e1269. Piccinno, F., Gottschalk, F., Seeger, S., Nowack, B., 2012. Industrial production quantities and uses of ten engineered nanomaterials in Europe and the world. J. Nanopart. Res. 14 http://dx.doi.org/10.1007/s11051-012-1109-9. Requena, A., Ceron-Carrasco, J.P., Bastida, A., Miguel, B.A., 2008. Density functional theory study of the structure and vibrational spectra of b-carotene, capsanthin, and capsorubin. J. Phys. Chem. 112, 4815e4825. Schreiber, L., Schonherr, J., 1993. Mobilities of organic compounds in reconstituted cuticular wax of barley leaves: determination of diffusion coefficients. Pestic. Sci. 38, 353e361. Schwarzenbach, R.P., Westall, J., 1981. Transport of nonpolar organic compounds from surface water to groundwater. Laboratory sorption studies. Environ. Sci. Technol. 15, 1360e1367. Wang, K.J., Xing, B.S., 2005. Structural and sorption characteristics of adsorbed humic acid on clay minerals. J. Environ. Qual. 34, 342e349. Yamamoto, H., Liljestrand, H.M., Shimizu, Y., 2004. Effects of dissolved organic matter surrogates on the partitioning of 17b-estradiol and p-nonylphenol between synthetic membrane vesicles and water. Environ. Sci. Technol. 38, 2351e2358. Yang, K., Xing, B.S., 2009. Sorption of phenanthrene by humic acid-coated nanosized TiO2 and ZnO Environ. Sci. Technol 43, 1845e1851. Yang, K., Zhu, L.Z., Xing, B.S., 2010. Sorption of phenanthrene by nanosized alumina coated with sequentially extracted humic acids. Environ. Sci. Pollut. Res. 17, 410e419. Young, K.D., LeBoeuf, E.J., 2000. Glass transition behavior in a peat humic acid and an aquatic fulvic acid. Environ. Sci. Technol. 34, 4549e4553.
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Effect of model dissolved organic matter coating on sorption of phenanthrene by TiO2 nanoparticles Xilong Wang a, *, Enxing Ma a, Xiaofang Shen a, Xiaoying Guo a, Meng Zhang a, Haiyun Zhang a, Ye Liu a, Fei Cai a, Shu Tao a, Baoshan Xing b, * a b
Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2014 Received in revised form 24 June 2014 Accepted 26 June 2014 Available online
Dissolved organic matter (DOM) may alter the sorption of hydrophobic organic contaminants (HOC) to metal oxide nanoparticles (NPs), but the role of DOM and NP types is poorly understood. Here, phenanthrene sorption was quantified on four types of nano-TiO2 (three rutile, one anatase), and a bulk, raw TiO2 powder. Prior to the sorption experiments, these nanoparticles were coated using four different organic materials: Lignin (LIG), tannic acid (TAN), Congo red (CON), and capsorubin (CAP). Lignin, tannic acid, congo red and capsorubin coating substantially enhanced phenanthrene sorption to various TiO2 particles. After coating with a specific DOM, Kd values by the DOM-coated TiO2 particles on percent organic carbon content and surface area (SA) basis (Koc/SA) generally followed the order: TiO2 NPs with hydrophobic surfaces > bulk TiO2 particles > other TiO2 NPs. Different Koc/SA values of various DOM-TiO2 complexes resulted from distinct conformation of the coated DOM and aggregation. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles Coating Sorption Phenanthrene
1. Introduction Nanoparticles have widely been used in commercial products. Metal oxide nanoparticles such as TiO2, SiO2, Fe2O3, Al2O3 and ZnO are the most extensively applied nanomaterials. In recent decades, the engineered TiO2 nanoparticles (NPs) have been produced in a large volume and widely used in various fields. The annual production of TiO2 NPs in USA reached 7800e38,000 t during 2007 and 2010 (Hendren et al., 2011; Piccinno et al., 2012). Therefore, it is imminent that the higher rate of production and usage of oxide NPs (e.g., TiO2) would lead to their environmental release in substantial amount (Benn and Westerhoff, 2008; Nowack and Bucheli, 2007). Sorption of hydrophobic organic contaminants (HOCs) to soils is an important process that affects their environmental behavior. Organic matter content in soils is generally far below than that of minerals, but it plays a critical role in HOC sorption (Schwarzenbach and Westall, 1981). In contrast, oxides and minerals are less important for HOC sorption mainly because water molecules reduce its accessibility to the polar surfaces (Kile et al.,
Abbreviation: dissolved organic matter, DOM; nanoparticles, NPs. * Corresponding author. E-mail addresses: [email protected], [email protected] (X. Wang), bx@ umass.edu (B. Xing). http://dx.doi.org/10.1016/j.envpol.2014.06.039 0269-7491/© 2014 Elsevier Ltd. All rights reserved.
1995). Once oxide NPs are released into the environment, they may interact with dissolved organic matter (DOM) in soils resulting in surface modifications. Organic matter in soils is generally present in the form of organic coatings on minerals and oxides (Murphy et al., 1994). DOM is a mixture of organic materials containing humic, fulvic and tannic acids, as well as lignin with considerably different molecular weights (Ferro-García et al., 1998). Therefore, influence of DOM coating on the physicochemical properties of oxides could be dependent on its composition. As reported, removal of organic materials from the surface coated natural samples and surficial sediments dramatically reduced sorption of bisphenol A, revealing that a considerable amount of BPA was sorbed on the coated organic materials of these two sorbents (Li et al., 2009). Organic matter coating enhanced phenanthrene sorption to kaolinite at all ionic strengths (Brunk et al., 1997), and both solution chemistry and mineral characteristics influenced phenanthrene sorption to the humic acid (HA)coated minerals (Laor et al., 1998). Composition of the coated DOM affected HOC sorption towards hematite and kaolinite (Murphy et al., 1994). Furthermore, sorption of aromatics on montmorillonite and kaolinite was affected by conformation of the coated DOM (Chen and Xing, 2005; Feng et al., 2006). As for the studies on the impact of organic matter coating on HOC sorption to NPs, Yang and Xing (2009) found higher nonlinearity of phenanthrene sorption isotherms to HA-coated TiO2 and ZnO NPs compared to the
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X. Wang et al. / Environmental Pollution 194 (2014) 31e37
original HA. After coating HA on Al2O3 NPs, the organic carbon content (foc)-normalized sorption coefficients (Koc) of phenanthrene by the HA-coated Al2O3 NPs were lower than those by the original HAs, and the Koc difference was more pronounced at relatively high sorbate concentration (Yang et al., 2010). Li et al. (2008) found that sodium dodecyl sulfate (SDS)-coated nanosized alumina had higher sorption affinity, capacity and rate for di-ethylphthalate than the SDS-coated micrometer-sized alumina. Sorption of formaldehyde to g-Al2O3 NPs was enhanced considerably after they were grafted with the functional groups of 2,4dinitrophenylhydrazine (Afkhami et al., 2011). Similarly, Jing et al. (2013) also reported that sorption of naphthalene and p-nitrophenol to nano-SiO2 was enhanced substantially after coating with a cationic surfactant (e.g., cetylpyridinium chloride, CPC). To date, difference in conformation of DOM that was coated on oxide particles with dissimilar particle size and surface properties, and the associated impact on HOC sorption still remain unclear. Also, information regarding the effect of DOM coating on aggregation of oxide NPs and associated influence on HOC sorption is scarce. As a first attempt to address these scientific issues, the key objectives of this study are to: 1) compare sorption of phenanthrene to bulk and various TiO2 NPs with different particle size and surface properties; and 2) examine the difference in conformation of the coated DOM on various TiO2 particles and influence of DOM coating on aggregation of various TiO2 particles, and the resulting impact on phenanthrene sorption along with the responsible mechanisms. 2. Materials and methods 2.1. Sorbates and sorbents Four types of TiO2 NPs including three types of rutile with distinct particle size and SA and only one anatase that was commercially available were purchased from Nanjing Guanye Chemical Industry Co., Ltd. Among these three rutile TiO2 NPs, one was the pristine NPs, and the other two samples were respectively treated by the supplier to have hydrophobic and hydrophilic surfaces. The bulk TiO2 powder was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Lignin (LIG) is a polyphenolic material, accounting for a large part of natural organic matter in the environment. Tannic acid (TAN) is widely present in plants, and it has been used extensively as a surrogate of DOM for environmentally relevant studies (Yamamoto spedes et al., 2006). Congo red (CON) is a benzidine-based and et al., 2004; Flores-Ce water-soluble dye, widely used in many industries (Ghaedi et al., 2012). The model DOMs including LIG, TAN and CON have aromatic structure (Fig. S1 in the Supplementary Data). Capsorubin (CAP) is a kind of carotenoid and polyene pigment extracted from paprika without aromatic structure, which has widely been used as colorant and food antioxidant (Requena et al., 2008). TAN and CON were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. LIG (alkali) was obtained from J&K Chemical Co., Ltd., USA. The concentrated CAP solution was kindly donated by Shenzhen Yicheng Trading Company Ltd. The basic chemical structures of these model DOMs are presented in Fig. S1, and they were used to coat on TiO2 particles to form DOM-TiO2 complexes. The details for DOM-coating on various TiO2 particles are presented in the Supplementary Data. The original bulk TiO2, four types of TiO2 NPs including one anatase, one pristine rutile and two rutiles with hydrophilic and hydrophobic surfaces were labeled as BT, NAnT, NRuT, NLRuT and NBRuT, respectively. The TiO2 particles after coating with LIG were marked as LIG-BT, LIG-NAnT, LIG-NRuT, LIG-NLRuT and LIG-NBRuT, and those after coating with TAN, CON, and CAP were correspondingly labeled as TAN-BT, TANNAnT, TAN-NRuT, TAN-NLRuT and TAN-NBRuT; CON-BT, CON-NAnT, CON-NRuT, CON-NLRuT and CON-NBRuT as well as CAP-BT, CAP-NAnT, CAP-NRuT, CAP-NLRuT and CAP-NBRuT, respectively. Phenanthrene was used as the sorbate, and both 14Clabled and nonlabeled forms of this compound were purchased from SigmaeAldrich Chemical Co. 2.2. Sorbent characterization Carbon, hydrogen, and nitrogen content of DOM-TiO2 complexes were measured using a Vario EL CHN elemental analyzer (Germany). Since the coated amount of DOM on various TiO2 particles estimated from carbon content of the DOMeTiO2 complex was low (0.2e6.0%), it was not possible to obtain the bulk oxygen content of DOMeTiO2 complex by mass balance. Surface elemental composition and carbonbased functionalities of all DOM-coated TiO2 particles were determined using an AXIS-Ultra X-ray imaging photoelectron spectrometer (Kratos Analytical Ltd. UK) with a monochromatic Al Ka radiation source operated at 225 W, 15 mA, and 15 kV.
Approximately 2 mg of powdered solid samples were used for both elemental analysis and XPS scanning. Since SA of sorbents is an important factor that may affect HOC sorption, sorptionedesorption isotherms of N2 to all samples at 77 K were obtained to get their SAs using an Autosorb-1-MP surface area analyzer (Quantachrome Instruments, USA) after outgassing at 105 C for 16 h. SAs of all samples were calculated from N2 sorption isotherm data using multipoint BET method with relative pressure P/P0 ranging from 0.05 to 0.3. X-ray powder diffraction (XRD) patterns of all samples were collected using a X’Pert Pro MPD diffractometer (Cu Ka radiation) coupled with the X'celerator detector at a scanning rate of 6 /min in the 2q range of 10e70 . The XRD patterns were refined with Rietveld method and the mean crystallite size of samples was calculated from peak broadening using X'Pert Highscore Plus and Fullprof softwares. To evaluate aggregation of all sorbents in the sorption systems, their size distribution was determined with an LS13 320 multi-wave length laser diffraction particle size analyzer (Beckman Coulter Inc., Miami, FL), and the zeta potential of sorbents was measured with a Nano-ZS90 Zetasizer (Malvern Instruments Technical Ltd., UK). Details of the analytical procedures are described in the Supplementary Data. It is possible that surface properties of the sorbents in aqueous phase are different from those in the solid state. Contact angles of all sorbents were detected using a video contact angle system with a sessile drop method (OCA20, Dataphysics, Germany), to get information on surface hydrophobicity of the tested samples. Details of sample preparation and contact angle measurement are presented in the Supplementary Data. 2.3. Sorption experiment Sorption isotherms of phenanthrene to all sorbents were obtained using a batch equilibration technique in screw cap vials with aluminum foil-Teflon liners. Stock solutions of 14C labeled and non-labled phenanthrene were dissolved in methanol. The background solution contained 0.01 M CaCl2 to maintain a stable ionic strength and 200 mg/L of NaN3 to inhabit bioactivity, and the pH was adjusted to neutral using 0.1 M NaOH and HCl. The 14C-labeled and nonlabled stock solutions of phenanthrene were added to the background solution to prepare the test solutions with different sorbate concentrations. After shaking for 1 h, test solutions were added to the screw cap vials which contained appropriate amount of weighed sorbent, until a minimal headspace was reached. The solute-to-sorbent ratio was adjusted to have 20e80% phenanthrene uptake by various sorbents. Methanol concentration in the test solutions was kept below 0.1% (v/v) to minimize the cosolvent effect. Our preliminary test indicated that apparent sorption equilibrium was reached within 8 d (Fig. S2). Therefore all vials were mixed on a rotary shaker for 9 d at room temperature. After mixing, all vials were centrifuged at 4000 rpm for 20 min, and then 1.8 ml supernatant was sampled and added to ScintiVerse cocktail (4 ml) (Fisher Scientific Co.) for scintillation counting with a liquid scintillation counter (Beckman Coulter, LS 6500). All samples and blanks were run in duplicate. Because of the negligible mass loss of phenanthrene as indicated in the blanks ( Congo red > Lignin > Tannic acid. Such an order was consistent with the Koc values of phenanthrene by TiO2 particles correspondingly coated with these DOMs (Table 1), indicating that sorption of this compound to the coated DOM on unit carbon basis controlled its sorption to the DOMeTiO2 complex. Different Koc values of phenanthrene by a given TiO2 sample coated with various DOMs seem to be a result of their hydrophobicity difference. However, the Koc values of phenanthrene by various DOMs were not measured, because these DOMs are readily dissolved and previous study
Table 2 Organic carbon content and surface area of the original and DOM-coated bulk and TiO2 NPs. Sorbents BT NAnT NRuT NLRuT NBRuT
foc (%)
SA (m2/g)
Sorbents
foc (%)
SA (m2/g)
CT
Sorbents
foc (%)
SA (m2/g)
CT
Sorbents
foc (%)
SA (m2/g)
CT
Sorbents
foc (%)
SA (m2/g)
CT
0.1 1.1
7.4 82.7 36.9 29.5 22.8
LIG-BT LIG-NAnT LIG-NRuT LIG-NLRuT LIG-NBRuT
0.2 0.4 0.3 0.2 1.2
11.5 70.7 42.2 23.4 18.5
0.17 0.06 0.07 0.09 0.65
TAN-BT TAN-NAnT TAN-NRuT TAN-NLRuT TAN-NBRuT
0.8 2.8 2.8 0.8 2.8
10.4 50.1 29.4 55.7 32.2
0.77 0.56 0.95 0.14 0.87
CON-BT CON-NAnT CON-NRuT CON-NLRuT CON-NBRuT
0.2 1.3 0.4 0.2 1.3
13.0 60.7 51.8 30.9 27.5
0.15 0.21 0.08 0.06 0.47
CAP-BT CAP-NAnT CAP-NRuT CAP-NLRuT CAP-NBRuT
1.2 4.4 3.8 1.1 6.0
10.7 30.6 46.7 22.5 20.9
1.12 1.44 0.81 0.49 2.87
OC: organic carbon content (%); SA: surface area (m2/g); CT: coated carbon thickness from DOM to TiO2 particles estimated by foc/SA (mg/m2).
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X. Wang et al. / Environmental Pollution 194 (2014) 31e37
Fig. 1. Agglomerate size distribution of the original and DOM-coated TiO2 NPs. Original and DOM-coated BT (A); original and DOM-coated NAnT (B); original and DOM-coated NRuT (C); original and DOM-coated NLRuT (D); original and DOM-coated NBRuT (E). Here, DOM includes lignin (LIG); tannic acid (TAN); congo red (CON) and capsorubin (CAP). The TiO2 particles include the bulk particles (BT); the pristine anatase-type TiO2 NP (NAnT); the pristine rutile-type TiO2 NP (NRuT); and two rutile-type TiO2 NPs with hydrophilic (NLRuT) and hydrophobic surfaces (NBRuT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 3 Agglomerate size of the original and DOM-coated bulk and TiO2 NPs in the sorption systems. Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
Sorbents
Mean (mm)
Median (mm)
BT NAnT NRuT NLRuT NBRuT
3.63 21.29 25.74 43.58 46.15
1.67 4.82 15.58 19.78 38.20
LIG-BT LIG-NAnT LIG-NRuT LIG-NLRuT LIG-NBRuT
8.28 26.79 117.50 36.29 138.80
1.93 5.87 43.80 13.18 110.40
TAN-BT TAN-NAnT TAN-NRuT TAN-NLRuT TAN-NBRuT
14.48 20.31 83.27 40.16 54.71
2.76 7.39 33.11 15.06 12.28
CON-BT CON-NAnT CON-NRuT CON-NLRuT CON-NBRuT
10.28 99.55 163.10 27.90 68.49
2.04 34.53 82.76 4.61 55.12
CAP-BT CAP-NAnT CAP-NRuT CAP-NLRuT CAP-NBRuT
18.93 188.00 169.00 18.94 188.9
2.93 86.33 97.37 3.24 130.30
showed that a same DOM (i.e., HA) in dissolved form had significantly higher sorption for this compound than that in the solid state (Pan et al., 2007). Further, it was reported that organic matter was the dominant sorption medium for HOCs in soil if its content was over 0.1% by mass (Schwarzenbach and Westall, 1981). Such a phenomenon was proved by the huge difference in Kd values of phenanthrene by the original and DOM-coated TiO2 particles (Table 1). Hence, Koc values of phenanthrene by the DOMeTiO2 complex can be viewed as that by the DOM in solid state. Further analysis showed that the difference in sorption enhancement of various coated DOMs was not due to the surface hydrophobicity difference of DOM-TiO2 complexes. This is because the sum of the percentages of polar functionalities CeO and C]O expressed as (fCO þ fC ¼ O) at the surfaces of the LIG- and CAP-coated TiO2 was comparable (Table S2), but sorption enhancement of phenanthrene to a given TiO2 sample induced by separately coating with unit carbon mass of these two DOMs was very different (Fig. 2). It was
interesting to find that relative abundance of the hydrophobic carbon domains at the DOMeTiO2 complex surfaces expressed as surface foc fCC/(fCO þ fC ¼ O) followed the same order as sorption enhancement of the corresponding DOMs, suggesting that sorption enhancement of phenanthrene to a given TiO2 sample resulting from DOM coating was regulated by the relative abundance of hydrophobic carbon domains in the coated DOM. The relative increase in Kd/SA values of phenanthrene by NAnT, NRuT and NLRuT resulting from DOM coating derived from (Kd, coated/SAcoated-Kd, original/SAoriginal)/(Kd, original/SAoriginal) reached 5.36.4, 9.4-23.3, 1.2-19.0 and 335.0-734.7 for LIG, TAN, CON and CAP coating, respectively, followed by BT, and that of NBRuT was the lowest with the corresponding values being 0.81, 0.76, 0.79 and 13.3. Such a trend was reverse to the coated carbon thickness on these sorbents estimated from foc/SA. This can be due to the fact that, when the coated carbon thickness on TiO2 particles was low, the coated DOM dramatically enhanced hydrophobicity of the oxide surfaces, thereby substantially increasing phenanthrene sorption. Surface properties of the TiO2 particles may not be altered too much with further carbon coating, so the relative increase in Kd/SA values would slightly change (Fig. S6). Such an explanation was reasonable because NBRuT originally had 1.1% of foc, and the coated carbon thickness from DOM to NBRuT surfaces was the highest among all TiO2 particles (Table 2). The relative increase in Kd/SA value of this sorbent caused by DOM coating was the lowest (Fig. S6).
3.3. Influence of the aggregation of DOM-coated TiO2 particles on phenanthrene sorption
Fig. 2. Sorption enhancement index of phenanthrene by the bulk and TiO2 NPs as induced by unit organic carbon loading mass. The index was calculated from (KcomKm)/(Km,foc). Km and Kcom are distribution coefficients of phenanthrene by the original and DOM-coated TiO2 particles, respectively (L/kg). Here, foc is the organic carbon content of the DOMeTiO2 complex, and each column corresponds to the complex of Y (DOM)-coated X (TiO2 particle). Here, DOM includes lignin (LIG); tannic acid (TAN); congo red (CON) and capsorubin (CAP). TiO2 particle includes the original bulk TiO2 (BT); anatase-type TiO2 NP (NAnT); the pristine rutile-type TiO2 NP (NRuT); the rutile-type TiO2 NP with hydrophilic (NLRuT) and hydrophobic surfaces (NBRuT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
To probe the influence of DOM-coated TiO2 NPs aggregation on phenanthrene sorption, the number of phenanthrene molecule adsorbed on unit SA was normalized with foc of the DOMeTiO2 complex and plotted against sorbate molecule number in aqueous phase at equilibrium (Fig. S7). After normalization, Koc/SA values of phenanthrene by the DOM-coated TiO2 particles generally followed the order: NBRuT > BT > NAnT, NRuT and NLRuT. This implied that Koc values of phenanthrene by a given DOM-coated TiO2 particle on unit SA were different, possibly due to distinct aggregation of various DOM-TiO2 complexes.
X. Wang et al. / Environmental Pollution 194 (2014) 31e37
It was documented that surface charge and surface properties of oxide NPs determined their aggregation behaviors in aqueous phase (Dachs and Bayona, 1997; Bian et al., 2011). The zeta potential data showed that LIG-BT, LIG-NAnT, LIG-NRuT, CON-BT, CON-NAnT, CON-NRuT, CAP-NAnT and CAP-NRuT were positively charged, whereas the rest of the DOM-coated TiO2 particles were negatively charged in the background solution identical to the sorption experiments (Table 4). The DOM-coated TiO2 particles would repel each other when they were positively or negatively charged, thus inhibiting aggregation. However, all the DOM-coated TiO2 particles were highly aggregated in aqueous phase (Fig. 1, Table 3). Strong aggregation of negatively-charged DOM-TiO2 complexes can be a result of Ca2þ-induced neutralization of the negative surface charges, thus facilitating aggregation. The same phenomenon was reported in previous studies (Ghosh et al., 2011), where HA coated Al2O3 and gFe2O3 NPs displayed aggregation in the presence of Ca2þ. However, such a mechanism cannot interpret aggregation of the DOM-TiO2 complexes with positive charges. Possibly hydrophobic moieties in the coated DOM were exposed outside during the coating process as hydrophilic functional groups tend to anchor the oxide surface (Gu et al., 1995). The aromatic components in the coated LIG and CON had both pep and hydrophobic interactions and the alkyl carbon moieties in the coated CAP were able to interact through hydrophobic interactions, thereby serving as the dominant driving force to attract the complex particles to each other, followed by aggregation. The attracting force between DOM-coated TiO2 particles should be stronger than the repelling one, so that they all strongly aggregated. Another point was that the zeta potential values of all DOMcoated TiO2 particles generally ranged from 30 to 30 mV (Table 4), and all suspensions would be more or less unstable a et al., 2012). under such condition (Csapo Aggregation enhancement of TiO2 particles induced by DOM coating was evident as indicated by the smaller mean agglomerate size of the uncoated TiO2 particles relative to the DOM-coated ones. Differently, aggregation of sodium dodecyl sulfate (SDS)-coated silver NPs was quite similar to that of the uncoated ones (Li et al., 2012). The mean particle size of the LIG-, TAN-, CON- and CAPcoated TiO2 NPs was 3.2e16.8, 2.7e12.0, 2.3e40.6 and 1.1e44.5 times of BT after correspondingly coating with these DOMs (Table 3), demonstrating that as DOM was coated on TiO2 particles, aggregation of TiO2 NPs was stronger than BT. Due to aggregation of the DOM-coated TiO2 particles, accessibility of a portion of sorption domains in the coated DOM would be reduced, thereby decreasing their effectiveness for phenanthrene sorption. Such an effect was more evident for NAnT, NRuT and NLRuT than BT (Fig. 1). Hence, Koc/SA values of phenanthrene by a given DOM-coated BT were higher than those by NAnT, NRuT and NLRuT after coating with the same DOM (Fig. S7, Table 1). Aggregation of DOM-coated NBRuT was comparable to that of other TiO2 NPs coated with the same DOM, but Koc/SA values of phenanthrene by DOM-coated NBRuT were generally much higher than those by DOM-coated BT and other TiO2 NPs (Fig. S7, Table 1). This was likely due to the fact that the estimated coated carbon thickness from DOM to NBRuT was generally much higher than the bulk and other
35
TiO2 NPs (Table 2). As the DOMeTiO2 complex aggregated, a portion of sorption sites possibly in glassy phase in the coated DOM were wrapped inside. However, a larger quantity of sorption domains in the coated DOM in DOM-NBRuT complex were exposed outside for phenanthrene sorption as indicated by their much larger contact angles relative to the others (Table S2). Moreover, NBRuT particles with thicker carbon coating in comparison with others could have a thicker rubbery phase, thus leading to enhanced diffusion of sorbate molecules. Therefore, the Koc/SA values of DOM-coated NBRuT were generally much higher than other TiO2 particles coated with the same DOM. 3.4. Effect of sorption domain conformation in the coated DOM on phenanthrene sorption It was proposed that the mineral-bound organic matter contained rubbery and glassy domains. The difference between rubbery and glassy domains is that the latter one had rigid hydrophobic nano-scaled microvoids in its matrix (Johnson et al., 1999). Sorption of HOCs to the rubbery domains is a dissolutionlike partitioning process and that to the glassy ones is dominantly a surface-adsorption one. Sorption isotherms of phenanthrene to the TAN- and CAP-coated TiO2 particles were practically linear (Table 1, Freundlich n ranging from 0.952 to 1.050), suggesting a partitioning-dominant mechanism. However, its sorption to the LIG- and CON-coated TiO2 particles was slightly nonlinear (Fig. S5, Table 1). Nonlinear sorption isotherms of LIG-coated TiO2 particles can be attributed to the adsorption in hydrophobic nanoavoids in glassy domains (Young and LeBoeuf, 2000) because LIG is a glassy biopolymer (LeBoeuf and Weber, 2000), whereas those for the CON-coated TiO2 particles could be a result of the formation of glassy domains at CONeTiO2 interfaces. Difference in interactions between TAN and CAP, as well as CON with TiO2 particles was most likely due to their distinct chemical composition. Chen and Xing (2005) examined impact of wax-coating on sorption of pyrene, phenanthrene and naphthalene to montmorillonite. They observed that sorption of phenanthrene and naphthalene to the wax-coated montmorillonite with different coating levels was practically linear. However, it was reported in the same work that, sorption isotherms of pyrene to the wax-coated montmorillonite with low wax loading was slightly nonlinear, which was ascribed to the creation of glassy domains at montmorilloniteewax interfaces (Gunasekara and Xing, 2003). Wang and Xing (2005) reported that sorption isotherms of phenanthrene to HA-coated montmorillonite and kaolinite tend to be more linear with increasing HA loading amount. In this case, the coated organic carbon content of HA to montmorillonite increased from 0.34 to 0.97% and that to kaolinite increased from 0.26 to 0.47%. An increase in foc of montmorillonite and kaolinite from HA coating was comparable to most of the TiO2 samples coated with various DOMs (Table 2). Difference in sorption isotherm nonlinearity of phenanthrene to the HA-coated montmorillonite and kaolinite as well as the TiO2 particles coated with various DOMs (in our case) has suggested conformational variation of the organic carbon domains formed at these interfaces.
Table 4 Zeta potential values of the original and DOM-coated bulk and TiO2 NPs in the sorption systems. Sorbents
ZP (mV)
Sorbents
ZP (mV)
Sorbents
ZP (mV)
Sorbents
ZP (mV)
Sorbents
ZP (mV)
BT NAnT NRuT NLRuT NBRuT
7.6 25.0 38.1 14.6 14.1
LIG-BT LIG-NAnT LIG-NRuT LIG-NLRuT LIG-NBRuT
11.1 9.1 17.9 14.3 12.5
TAN-BT TAN-NAnT TAN-NRuT TAN-NLRuT TAN-NBRuT
19.1 14.4 17.1 20.1 9.6
CON-BT CON-NAnT CON-NRuT CON-NLRuT CON-NBRuT
6.3 17.6 5.5 13.1 12.2
CAP-BT CAP-NAnT CAP-NRuT CAP-NLRuT CAP-NBRuT
8.2 17.4 3.2 10.8 0.8
ZP: Zeta potential. Each sample was measured twice and the averaged data are presented here and used for discussion.
36
X. Wang et al. / Environmental Pollution 194 (2014) 31e37
In addition, different Koc/SA values of a given DOM preloaded on various TiO2 particles can also be a result of the dissimilar conformation of DOM adsorbed on the particles. Since the coated carbon thickness from DOM on NAnT, NRuT and NLRuT was generally the lowest among all TiO2 particles (Table 2), a greater portion of the coated DOM would be tightly bound to these TiO2 NPs and possibly form glassy domains in contrast to other sorbents. It is highly difficult for phenanthrene molecules to penetrate into the sorption domains in the coated DOM on these TiO2 NPs relative to NBRuT and BT, thereby giving the lowest Koc/SA values (23e1383 L/kg) for this compound among all TiO2 particles (Fig. S7, Table 1). As for DOM coating on both BT and NBRuT, the first layer would be tightly bound to the particle surfaces due to their strong interactions. The coated DOM layers beyond the first layer would be more loosely bound to BT and NBRuT as the attractive forces tend to be weaker after the limited high-energy sites at their surfaces were occupied (Chen and Xing, 2005; Wang and Xing, 2005), thereby possessing the more flexible conformation as the distance from surfaces of these particles increased (Gunasekara and Xing, 2003). The more loosely bound DOM to BT and NBRuT relative to that bound to NAnT, NRuT and NLRuT should have higher sorption affinity for phenanthrene due to higher mobility and accessibility of domains, thereby facilitating diffusion of the sorbate molecules (Schreiber and Schonherr, 1993). Such a phenomenon should be more evident for NBRuT as compared to BT, because of the higher coated carbon thickness on its surfaces and higher hydrophobicity of the DOM coated NBRuT surfaces relative to the other TiO2 particles, as indicated by much larger contact angles of the DOM-NBRuT complexes (Table 2 and Table S2). However, it is noted that the glassy and rubbery sorption domains in both tightly bound and flexible regions in the coated organic matter on bulk and various TiO2 NPs contributed to phenanthrene sorption (Wang and Xing, 2005). The crystalline phase in organic matter is generally assembled in a regular lattice, which can exclude the sorbing molecules that may otherwise diffuse across the barrier, thereby decreasing their sorption (Chen and Xing, 2005). To test if the crystalline domains were created at the DOM-oxide interfaces or in the coated DOM matrix, XRD patterns of the original and DOM-coated bulk and TiO2 NPs were obtained and shown in Fig. S8. The anatase and rutile structures of the original TiO2 NPs were clearly differentiated as they had relatively dominant diffraction peaks at different positions, e.g., 2q ¼ 25, 38 and 48 for anatase structure and 2q ¼ 28, 36 and 55 for rutile structure. Clearly, positions of all the diffraction peaks of original bulk and TiO2 NPs did not shift and their intensities also did not change after coating with DOMs, and DOM coating did not alter the crystallite size of the original bulk and various TiO2 NPs (Table 5). In addition, no clear changes in lattice parameter on both a- and c-axis of the original bulk and TiO2 NPs after coating with DOM were observed (Table 5). No difference in the XRD pattern of the original and DOM-coated TiO2 particles was probably due to the very low mass ratio of the coated DOM to the TiO2 particles. 4. Conclusions This study revealed that although SA of TiO2 NPs was 3.1e11.2 times of the bulk ones, they had comparable sorption for phenanthrene, mainly due to much stronger aggregation of the former. DOM coating substantially increased Kd/SA values of phenanthrene by all TiO2 particles, and the sorption enhancement was more pronounced when the coated carbon thickness was low. After normalizing Kd/SA values of DOMeTiO2 complex with its foc, the resulting Koc/SA values by DOM-NBRuT complex were highest among all TiO2 particles coated with DOM. This suggested that after DOM coating, the TiO2 NPs previously with hydrophobic surfaces
Table 5 Crystalline structure, lattice parameter, and crystallite size of the original and DOMcoated bulk and TiO2 NPs. Sorbents
BT LIG-BT TAN-BT CON-BT CAP-BT NAnT LIG-NAnT TAN-NAnT CON-NAnT CAP-NAnT NruT LIG-NRuT TAN-NRuT CON-NRuT CAP-NRuT NLRuT LIG-NLRuT TAN-NLRuT CON-NLRuT CAP-NLRuT NBRuT LIG-NBRuT TAN-NBRuT CON-NBRuT CAP-NBRuT
Crystallite structure Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile
Lattice parameter (Å) a
c
3.783 3.783 3.785 3.783 3.783 3.785 3.785 3.785 3.786 3.785 4.595 4.595 4.595 4.594 4.595 4.596 4.594 4.594 4.594 4.595 4.595 4.593 4.594 4.594 4.594
9.516 9.517 9.520 9.515 9.517 9.506 9.506 9.505 9.505 9.507 2.958 2.958 2.958 2.958 2.958 2.959 2.958 2.958 2.958 2.959 2.959 2.958 2.959 2.958 2.959
Mean crystallite size (Å) 783.8 778.0 746.6 777.7 770.9 252.2 227.7 227.0 232.1 235.0 330.1 330.4 330.0 329.1 328.1 408.9 393.0 382.3 389.7 398.8 396.1 406.3 420.0 415.6 414.2
would more strongly reduce the mobility of phenanthrene in the environment than the bulk and other TiO2 NPs. Acknowledgments This study was supported by the National Natural Science Foundation of China (41271461, 41328003, 41390240 and 41130754), 973 Program (2014CB441101), 111 Program (B14001) and the Startup Fund for the Peking University 100-Talent Program. We thank Dr. Saikat Ghosh for editing and improving the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2014.06.039. References Afkhami, A., Bagheri, H., Madrakian, T., 2011. Alumina nanoparticles grafted with functional groups as a new adsorbent in efficient removal of formaldehyde from water samples. Desalination 281, 151e158. Benn, T., Westerhoff, P., 2008. Nanoparticle silver released into water from commercially available sock fabrics. Environ. Sci. Technol. 42, 4133e4139. Bian, S.W., Mudunkotuwa, I.A., Rupasinghe, T., Grassian, V.H., 2011. Aggregation and dissolution of 4 nm ZnO nanoparticles in aqueous environments: influence of pH, ionic strength, size, and adsorption of humic acid. Langmuir 27, 6059e6068. Brunk, B.K., Jirka, G.H., Lion, L.W., 1997. Effects of salinity changes and the formation of dissolved organic matter coatings on the sorption of phenanthrene: implications for pollutant trapping in estuaries. Environ. Sci. Technol. 31, 119e125. Chen, B.L., Xing, B.S., 2005. Sorption and conformational characteristics of reconstituted plant cuticular waxes on montmorillonite. Environ. Sci. Technol. 39, 8315e8323. Chiou, C.T., McGroddy, S.E., Kile, D.E., 1998. Partition characteristics of polycyclic aromatic hydrocarbons on soils and sediments. Environ. Sci. Technol. 32, 264e269. Szalai, A., Csete, M., a, E., Patakfalvi, R., Hornok, V., To th, L.T., Sipos, A., Csapo k De any, I., 2012. Effect of pH on stability and plasmonic properties of cysteinefunctionalized silver nanoparticle dispersion. Colloid Surf. B 98, 43e49. Dachs, J., Bayona, J.M., 1997. Langmuir-derived model for diffusion- and reactionlimited adsorption of organic compounds on fractal aggregates. Environ. Sci. Technol. 31, 2754e2760.
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