Mechanisms of nC60 removal by the alum coagulation–flocculation–sedimentation process

Accepted Manuscript Mechanisms of nC60 Removal by The Alum Coagulation-Flocculation-Sedimentation Process Chao WANG, Chii SHANG, Guanghao CHEN, Xiaoshan ZHU PII: DOI: Reference:

S0021-9797(13)00769-8 http://dx.doi.org/10.1016/j.jcis.2013.08.023 YJCIS 19037

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Journal of Colloid and Interface Science

Received Date: Accepted Date:

2 July 2013 15 August 2013

Please cite this article as: C. WANG, C. SHANG, G. CHEN, X. ZHU, Mechanisms of nC60 Removal by The Alum Coagulation-Flocculation-Sedimentation Process, Journal of Colloid and Interface Science (2013), doi: http:// dx.doi.org/10.1016/j.jcis.2013.08.023

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Mechanisms of nC60 Removal by The Alum Coagulation-FlocculationSedimentation Process Chao WANGa,b, Chii SHANGa,*, Guanghao CHENa, and Xiaoshan ZHUc a

Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China

b

Faculty of Science and Technology, Technological and Higher Education Institute of Hong Kong, Hong Kong, China c

Graduate School at Shenzhen, Tsinghua University, Shenzhen, China

* Corresponding author. Phone: (852)2358 7885; fax: (852)2358 1534; e-mail: [email protected]. E-mail addresses: [email protected] (Chao WANG), [email protected] (Guanghao CHEN), [email protected] (Xiaoshan ZHU)

ABSTRACT

This study explored the mechanisms for nC60 removal in pure water and filtered saline wastewater by the alum coagulation-flocculation-sedimentation process through analyzing the hydrolyzed aluminum species and exploring the complexation of nC60 with aluminum hydroxide precipitates. Sweep flocculation (enmeshment and adsorption) with Alc is the most dominant mechanism contributing to the nC60 removal in pure water. In filtered saline wastewater, hetero-precipitation of Alb with nC60, colloids and dissolved solids also contributes to the nC60 removal. Alkalinity affected the nC60 removal by changing the hydrolyzed aluminum species distributions. XPS, FTIR and SEM evidences

suggest that the enmeshment and adsorption of nC60 onto the aluminum hydroxide precipitates can be described as the inner-sphere complexation. Based on the above observations, conceptual models for nC60 removal by the alum coagulation-flocculationsedimentation process in the different water matrices are proposed. Keywords: Adsorption, Complexation, Ferron method, Hydrolyzed aluminum species, Nanomaterial

1．Introduction C60, one of the most common fullerene-based nanomaterials, is used in many fields, such as biomedical, electronic, environmental and optics industries [1]. Mass production and extensive use of C60 in the coming years, therefore, are highly expected. The production, use and final disposal of C60-containing products increase the likelihood of releasing C60 into our environment. The solubility of C60 in water is low and estimated at 2.63 ng/L [2], but it forms stable, nanoscaled colloidal aggregates in water (usually referred to as nC60) by a number of synthetic methods [3]. The water-soluble nC60 is receiving attentions of many water scientists in understanding its fate, transformation and toxicity in our aqueous environment. Adverse effects, such as oxidative damage to human cell membranes, DNA damage, and human cell death, have been reported after exposure to nC60 suspensions [4 6]. The toxicity effects are associated with the production of reactive oxygen species (ROS) and nC60 uptake by cells [6]. C60 is thus listed as an emerging contaminant. Limited studies have reported the removal of nC60 in engineered water and wastewater treatment processes [7 10]. nC60 can be effectively removed by the conventional 2

coagulation-flocculation-sedimentation process that is used for water and wastewater treatment at practically-relevant alum dosages and pH ranges [7,10]. The pH- and alum dosage-dependent trends of nC60 removal in water and saline wastewater were found to be different, likely owing to the different water characteristics [10]. However, the mechanisms for nC60 removal by the alum coagulation-flocculation-sedimentation process were less understood. It has been proposed that nC60 adsorption to the aluminum hydroxide precipitates likely contribute to the removal [7,10]. Hydrolyzed aluminum species are important for the success of the alum coagulationflocculation-sedimentation process, because contaminants are commonly removed through the interactions with different hydrolyzed aluminum species [11]. Once added into water, alum experiences a series of hydrolysis processes and forms a number of categories of hydrolyzed products depending on solution pH [12]. A few methods have been used in characterizing the hydrolyzed aluminum species [13], among which the Ferron method has been widely used to distinguish the hydrolyzed aluminum species in pure water and filtered wastewater [12,14]. The Ferron method operationally defines hydrolyzed aluminum species, based on different reaction rates with the Ferron reagent (8-hydroxy-7-iodo-5-quinoline sulfonic acid), into three categories: mononuclear species (Ala), polynuclear species (Alb) and colloidal/precipitated species (Alc) [14,15]. Ala is mainly composed of monomeric species such as Al3+, Al(OH)2+ and Al(OH)2+, dimer Al2(OH)24+, trimer Al3(OH)45+, some small polymers, and rapid reactive surface species; Alb is the intermediate polymer species with apparent molecular weight in the range of 500 to 3,000 Da, such as Al13O4(OH)247+; Alc is the inert large polymers or precipitates with molecular weight normally larger than 3,000 Da [14,15].

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As for characterization of surface interactions, numerous spectroscopic techniques have been used to study the surface complexation of hydrolyzed aluminum species with other aqueous constituents. For example, Fourier transform infrared (FTIR) spectrometry has been used to characterize the complexation of hydrolyzed aluminum species with acetate, humic acid, and bicarbonate and carbonate ions [16]. X-ray photoelectron spectroscopy (XPS) has been used to explore the mechanisms of fluoride ions removal by aluminum hydroxide flocs [17] and the mechanisms of Pb(II) adsorption onto γ-Al2O3 [18]. Scanning electron microscopy (SEM) was used to supply visual observation of the adsorption of arsenate onto alum-impregnated activated alumina [19]. The study explores the mechanisms for nC60 removal in pure water and filtered saline wastewater by the alum coagulation-flocculation-sedimentation process. To achieve this aim, hydrolyzed aluminum species formed under different conditions were analyzed using the Ferron method, and the complexation of nC60 with aluminum hydroxide precipitates was explored using several spectroscopic techniques including XPS, FTIR and SEM.

2．Experimental 2.1. Materials and chemicals C60 (purity > 99.9%) was purchased from the MER Corporation (Tucson, AZ, USA). The Ferron reagent (8-hydroxy-7-iodo-5-quinoline sulfonic acid) and aluminum plates (purity > 99.9%) were obtained from Sigma (UK). Toluene and ethanol (of HPLC grade) were obtained from Mallinckrodt Baker (USA) and Merck KGaA (Germany), respectively. All other chemicals used in this study, such as NaCl, NaAc and

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Al2SO4·18H2O were of reagent grade or higher and were used without further purification. All solutions were prepared with pure water (18.2 MΩ/cm) purified by a NANOpure system (Barnstead, IA, USA) with an organic free cartridge. The raw saline wastewater sample used in this study was collected from the Stonecutters Island Sewage Treatment Works (SCISTWs), Hong Kong. The characteristics of the wastewater sample are shown in Table S1 of Supporting Information. As suspended solids in the wastewater sample significantly influenced the accuracy of the Ferron method, filtration of the wastewater sample was necessary and was achieved by filtering the raw wastewater sample through GC50 membranes with an average pore size of 1.2 μm (Advantec, Japan). It was assumed that the filtration would not significantly affect the distribution of the hydrolyzed aluminum species in wastewater, because the makeup of the dissolved constituents remained the same [12].

2.2. Aqueous nC60 suspension preparation An aqueous nC60 stock suspension was prepared following a solvent exchange procedure using toluene [20]. The mean hydrodynamic diameter of the aqueous nC60 in the stock solution was ~92 nm and the zeta potential was approximately

40 mV.

2.3. Alum coagulation-flocculation-sedimentation process simulation The alum coagulation-flocculation-sedimentation process, including rapid mixing (coagulation), slow mixing (flocculation) and sedimentation steps, was simulated by jar tests, which were conducted with a jar testing apparatus (Stuart Scientific, UK). For nC60 removal experiments, each sample (0.8 L) spiked with 200 µg/L of nC60, after pH adjustment with NaOH or HCl, was rapidly mixed at 130 rpm for 1 min, slowly mixed at 5

30 rpm for 10 min and at 20 rpm for another 10 min, consecutively, and then settled for 30 min. With rapid mixing, alum at a predetermined concentration was added to initiate the simulation. After settling, the supernatant was collected for analysis of the remaining nC60 concentration. Two duplicate jar tests were performed under each condition. For spectroscopic investigation, the same jar test procedure was used to collect the settled flocs, with or without adsorbed nC60. The testing conditions remained similar except that the nC60 concentration, the alum dosage, and pH were 15 mg/L (if spiked), 50 mg/L, and 7, respectively. The high concentration, 15 mg/L, of nC60 was used to collect sufficient quantities of nC60 for the spectroscopic analyses.

2.4. Analytical methods Size distribution and zeta potential of the obtained aqueous nC60 were determined by a dynamic light scattering (DLS) analyzer and a zeta potential analyzer (ZetaPlus Brookhaven Instrument Corporation, USA), respectively. The nC60 concentration in the stock solution was standardized [3] and the remaining nC60 concentration in the supernatant collected from jar test simulation was measured by the method of liquidliquid extraction followed by HPLC/UV-vis spectroscopy [20]. Solution pH was measured using a pH meter (Orion Model 420 A, Boston, MA, USA) and salinity was measured using a refractometer (P&R Labpak Limited, UK). The hydrolyzed aluminum species distribution was determined using the Ferron method following Ref. 21. The settled flocs obtained from jar tests were collected and deposited on silicate plates to dry for SEM analysis. A thin film of gold was coated on the sample under vacuum prior to the analysis to avoid charging of the solid sample. The morphologies of the flocs

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were recorded with a SEM (JEOL, JSM-6300F) at a dispersive energy of 15 kV. The EDAX mappings of the flocs were obtained with an XFlash Detector 4010 (Bruker AXS) using another SEM (JEOL, JSM-6390), and 2D mapping signals were recorded at a dispersive energy of 20 kV. The flocs collected above were also freeze-dried for FTIR and XPS analysis. The FTIR specimens were prepared by mixing the freeze-dried flocs with an aliquot amount of spectroscopic grade KBr, grinding to fine powders by hand with a mortar and pestle, and pressing to thin pellets using a pressing machine. FTIR spectra of the samples were recorded by an FTIR spectrometer (Bio Rad FTS 6000) equipped with a DTGS detector scanning from 4,000 to 400 cm-1. Sixteen scans with a resolution of 4 cm-1 were averaged to obtain each spectrum. XPS analysis of the freeze-dried flocs was performed on a PHI 5600 Multi techniques photoelectron spectrometer (Physical electronics) and the core level spectra were measured using a monochromatic Al Kα X-ray source (hν = 1,486.6 eV). The analyzer was operated at 23.5 eV pass energy and the analyzed area was 800 µm in diameter.

3. Results and discussion 3.1. Analysis of hydrolyzed aluminum species using the Ferron method The removal of nC60 in pure water and filtered saline wastewater by the alum coagulation-flocculation-sedimentation process was explained from the viewpoint of the hydrolyzed aluminum species distribution categorized by the Ferron method. Figures 1A and 1B show the removal of nC60 and the distribution of hydrolyzed aluminum species in pure water, respectively, at different pHs and 50-mg/L alum. In this

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water matrix, the nC60 removal could be attributable to the hydrolyzed aluminum species only. As shown, little or no nC60 was removed at around pH 3.0, in which nearly all Al existed as Ala. Ala could promote flocculation by forming nC60-Ala complexes to reduce negative charges on the surface of nC60 through charge neutralization and thus reduce interparticle repulsion. However, the DLS measurement revealed that the nC60-Ala complexes remained in suspension (data not shown). Alb, the intermediate polymer species with positive charges, not only exhibited a high neutralization ability to destabilize nC60 by complexation/charge neutralization, but also acted as nuclei [15], in this study, to interact with nC60 aggregates or nC60-Ala complexes to form flocs in the colloidal form. However, the flocs were too small to settle, because at around pH 8.7, only 3% of nC60 was removed with the fraction of Alb up to 9% and the negligible fraction of Alc. Alb was not stable and could undergo rearrangement to transform into Alc [14]. Alc, the settleable amorphous aluminum hydroxide precipitates, could effectively remove nC60, nC60-Ala and nC60-Alb complexes through sweep flocculation (enmeshment and adsorption onto the precipitates). The maximum removal of nC60 was thus correspondingly achieved at a pH range of 6.8 8.2. The pH range was slightly higher than that for the minimum solubility of alum (pH 5.1 7.5), because at a pH slightly higher than the minimum solubility pH, the hydrolyzed aluminum species were in the forms of high molecular weight polymers or sols [22]. Consequently, the mechanism of enmeshment and adsorption onto the precipitates by Alc was suggested as the most dominant mechanism for nC60 removal in pure water. The hydrolyzed aluminum species distribution in filtered saline wastewater at different pHs and 50-mg/L alum, as shown in Figure 2A, was different from that in pure water. At

8

pH 3.0, Alb was formed in the filtered wastewater, while only Ala was formed in pure water. The fraction of Alb was higher than that of Alc at all evaluated pHs from 3.0–9.2 in the filtered wastewater; however, in pure water, the fraction of Alb was less than that of Alc at pH 4.6–8.5. It has been reported that some constituents in wastewater, such as humic substances, organic complexes, phosphate species, and other inorganic ligands, could slow down both the rate of generating aluminum hydroxide precipitates and the rate or extent of aging of aluminum hydroxide precipitates [12,23,24]. Thus, compared with that in pure water, a higher fraction of Alb existed in the filtered wastewater at a wide pH range. As for nC60 removal, at around pH 7.0 and 50-mg/L alum, similar removal of nC60 in pure water (Figure 1A) and the filtered wastewater (Figure S1 in Supporting Information) was achieved, although the Alc fraction in the filtered wastewater (Figure 2A) was much lower than that in pure water (Figure 1B). At around pH 5.0 where the fraction of Alc only occupied 6%, up to 75% nC60 removal (Figure S1 in Supporting Information) was also achieved in the filtered wastewater. At around pH 9.0, the sum of fractions of Alb and Alc in the filtered wastewater (Figure 2A) was about 63%, which contributed to the 73% nC60 removal (Figure S1 in Supporting Information). These results indicated that the mechanisms for nC60 removal in the filtered wastewater were different from that in pure water. In the filtered wastewater, it is suggested that Alb not only neutralized the negatively-charged nC60 and colloids, but could also promote surface precipitation/adsorption on the dissolved solids existing in the filtered wastewater, which resulted in forming a layer of amorphous aluminum hydroxide precipitates on the solid surfaces and subsequent aggregation of the solids to form flocs [14,15]. Consequently, in addition to the mechanism of nC60 removal by enmeshment and

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adsorption onto the precipitates of Alc, the hetero-precipitation by Alb with nC60, colloids and dissolved solids was also important for nC60 removal in the filtered wastewater. In unfiltered wastewater, though the speciation of hydrolyzed aluminum species cannot be assessed by the Ferron method directly, this hetero-precipitation mechanism is expected to be more important, because more particles are available to interact with Alb and nC60. One major factor that has been found affecting the nC60 removal in pure water and wastewater is alkalinity [7,10]. Alkalinity has also been reported as an important factor to affect the hydrolysis of alum [25]. The effect of alkalinity on the different hydrolyzed aluminum species distributions was evaluated by spiking a certain amount of NaHCO3 into pure water (hereafter referred to as high-alkalinity pure water) to represent the alkalinity concentration of the wastewater used in this study. Figure 2B shows the aluminum species distribution after flocculation in high-alkalinity pure water at different pHs and 50-mg/L alum. As shown in Figure 2B, the addition of alkalinity changed the hydrolyzed aluminum species distribution greatly. In fact, the trend of hydrolyzed aluminum species distribution in the high-alkalinity pure water displayed the mixed feature of the trends in pure water and the filtered wastewater shown in Figures 1B and 2A, respectively. For example, at pH 3.0, Al existed as Ala in pure water with or without bicarbonate addition. At around pH 4.6, the fraction of Alc was 20% larger than Alb in the high-alkalinity pure water, which was however much smaller than the difference (35%) between the fractions of Alb and Alc in the pure water without bicarbonate addition (Figure 1B). At pH 5.1–7.4, the fraction of Alb was larger than Alc in the high-alkalinity pure water, which was the same case as that in the filtered wastewater. Alb in the highalkalinity pure water and filtered wastewater may also include sodium aluminum

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hydroxycarbonate species [26], which are more effective in removing nC60. At pH 7.9– 8.5, the hydrolyzed aluminum species distribution of the high-alkalinity pure water displayed a mixed figure of the pure water (without bicarbonate addition) and the filtered wastewater. The fractions of both Alb and Alc dropped in the high-alkalinity pure water down to minimum at pH 9.0, resulting in little or no removal of nC60 (Figure S2 in Supporting Information). The effect of salinity on the hydrolyzed aluminum species distribution was also evaluated at different pHs and 50-mg/L alum by spiking an aliquot of NaCl into the pure water to represent the salinity of the saline wastewater used in this study. As shown in Figure 2C, although the addition of salts changed the hydrolyzed aluminum species distribution to some extent, the relative dominancy of different hydrolyzed aluminum species at different pHs remained similar. This may partially explain the minor effect of salinity on the nC60 removal in the alum coagulation of wastewater reported in Ref. 10.

3.2. Characterization of enmeshment and adsorption of nC60 onto aluminum hydroxide precipitates using spectroscopic techniques Because nC60 enmeshment and adsorption onto amorphous aluminum hydroxide precipitates, as discussed in Section 3.1, is one important mechanism for nC60 removal, spectroscopic techniques, including XPS, FTIR, and SEM were used to evidence the surface interactions. The investigation was conducted with pure water only, because the presence of sewage organic matters, salts and other constituents in wastewater masked the spectroscopic signal to make the investigation impossible. The amorphous aluminum hydroxide precipitates were produced at pH 7 and 50-mg/L alum, with or without the

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presence of nC60. At the pH and the alum dosage, as shown in Figure 1B, Alc was the predominant fraction and occupied about 80% of total aluminum species. XPS XPS analysis was carried out to observe the evolution of the oxygen and aluminum of the amorphous aluminum hydroxide precipitates before and after enmeshment and adsorption of nC60. As shown in Figure 3, the Al2p orbital XPS spectrum of the pure precipitates displays a single peak at 74.30 eV [27] and the O1s orbital XPS spectrum of the pure precipitates presents a single peak at 531.30 eV. After nC60 was enmeshed and adsorbed onto the precipitates, the peaks of O1s and Al2p both shifted toward the lower binding energy, from 531.30 to 529.61 eV and 74.30 to 72.36 eV, respectively. In general, a chemical shift in XPS analysis is caused by changes in the electrostatic potential field experienced by the core electrons [28]. The shifts observed in the current study suggested that the enmeshment and adsorption of nC60 onto the precipitates could change the electrostatic potential field of the core electrons of Al and O, likely attributable to the charging effects [29] of the enmeshed and adsorbed nC60. FTIR Figure 4 displays the FTIR spectra of nC60 and the amorphous aluminum hydroxide precipitates alone and that of the precipitates with enmeshed and adsorbed nC60. The strong bands at 3,435, 1,645, 1,125, and 610 cm-1 and the shoulder at 982 cm-1 characterized and demonstrated typical pure amorphous aluminum hydroxide precipitates (the dot line in Figure 4). The 3,435 cm-1 has been attributed to the stretching vibration of the –OH group [30], and the 1,645 cm-1 has been assigned to the bending mode of the 12

adsorbed water molecular [31]. The intense band at 1,125 cm-1 and the shoulder at 982 cm-1 have been respectively assigned to the symmetric bending mode of Al–OH and the asymmetric bending mode of Al–OH [32]. The strong band at 610 cm-1 has been assigned to the stretching vibration of Al–O [30]. Peaks of pure nC60 which are located at 1,429, 1,182, 577 and 527 cm-1 are in consistent with those reported in the literature [33]. The precipitates with enmeshed and adsorbed nC60 also displayed the same four characteristic bands of nC60, which did not show any shift comparing with those of pure nC60. On the other hand, shifts of the characteristic bands of the amorphous aluminum hydroxide precipitates were observed after the enmeshment and adsorption of nC60, namely +12 cm1

of the stretching vibration of the –OH group,

adsorbed water molecule,

6 cm-1 of the bending mode of the

12 cm-1 of the symmetric bending mode of Al–OH,

4 cm-1

of the asymmetric bending mode of Al–OH, and +9 cm-1 of the stretching vibration of Al–O. The large shifts of the symmetric bending mode of Al–OH and the stretching vibration of Al–O indicated that the nC60 with negative charges was coordinated directly to the Al(III) ion through ionic bonding in an inner-sphere mode [31]. The coordination of nC60 to the Al(III) ion also caused the shifts of the stretching vibration of the –OH group and the bending mode of adsorbed water molecule in the amorphous aluminum hydroxide precipitates because of the relatively large size and the surface negative charge of nC60. SEM Figure 5 displays the different micrographs of the amorphous aluminum hydroxide precipitates, alone and with enmeshed and adsorbed nC60. The SEM resolution is insufficient to unambiguously characterize individual particle, but it provides a view of 13

the general population [19]. For the pure precipitates, the surface morphology was characterized as agglomerates of porous and irregular structures with uniform sizes in 2030 nm. The surface morphology of the precipitates with enmeshed and adsorbed nC60 was characterized as agglomerates of porous and irregular structures with non-uniform sizes. The large and lumpy particles observed in the micrograph of the nC60-enmeshed and adsorbed precipitates were speculated to be nC60, because their sizes (around 100 150 nm) were in consistent with that of the prepared nC60 in suspension (with a mean hydrodynamic diameter ~92 nm). The EDAX mapping of the nC60-enmeshed and adsorbed precipitates provided information on the distribution and frequency of nC60 (measured as C) in the amorphous precipitates (measured as Al and O) (Figure 6). As demonstrated, nC60 existed in large density on the surface and the sections of the precipitates.

3.3. Conceptual models for nC60 removal by the alum coagulation-flocculationsedimentation process in pure water and wastewater Based on the results obtained in this study and the parallel removal study [10], two conceptual models for nC60 removal by the alum coagulation-flocculation-sedimentation process in pure water (Figure S3) and wastewater (Figure S4) are proposed. In pure water, nC60 exists as aggregates of around 100 nm, with its surface negatively-charged. When alum is added into the pure water, it hydrolyzes into different fractions of Ala, Alb and Alc depending on solution pH; meanwhile, transformation of “Ala to Alb to Alc” also occurred with time [14]. Ala aggregates nC60 by charge neutralization but the nC60-Ala complexes are still soluble. Alb neutralizes the negative charge of nC60 and adsorbs nC60 and nC60Ala complexes. However, the flocs of Alb that are enmeshed with nC60 or nC60-Ala 14

complexes are likely in the unsettleable colloidal form. Alc enmeshes and adsorbs nC60 and nC60-Ala and nC60-Alb complexes to form settleable flocs. The removal of nC60 in saline wastewater is complicated by the diverse wastewater constituents. When alum is added into saline wastewater, it hydrolyzes into different fractions of Ala, Alb and Alc depending on solution pH, alkalinity and salinity in the wastewater. pH significantly affects the distribution and dominancy of Ala, Alb and Alc; however, the relative dominancy of different hydrolyzed aluminum species remains at the range of salinity tested. Parameters including pH, alkalinity, suspended solids, sewage organic matter and salinity in the wastewater may also affect the interactions of nC60 with hydrolyzed aluminum precipitates [10]. The nC60 removal in wastewater thus can be achieved by several ways: nC60 can be removed by adsorption onto gravitationally settleable suspended solids; Alb removes nC60 by the hetero-precipitation mechanism with suspended solids and colloids; and Alc can effectively remove nC60 by the sweep flocculation (enmeshment and adsorption) mechanism. In addition, the physicochemical properties of nC60 such as aggregation size and surface charge are altered in saline wastewater by salts and sewage organic matters [10]. However, the effects of salts and sewage organic matters on the nC60 removal are not significant [10].

4. Conclusions The mechanisms for nC60 removal by the alum coagulation-flocculation-sedimentation process in pure water and filtered saline wastewater were explained from the viewpoint of hydrolyzed aluminum species distribution differentiated by the Ferron method. These conclusions are obtained:

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In pure water, the sweep flocculation (enmeshment and adsorption) mechanism with Alc was the most important mechanism for the nC60 removal.



In wastewater, the removal of nC60 was more complicated by the constituents in wastewater, which not only affected the physicochemical properties of nC60, but also the hydrolysis of alum and the interactions of nC60 with hydrolyzed aluminum species.



In wastewater, the nC60 removal would be achieved by adsorption of nC60 onto settleable suspended solids; hetero-precipitation of nC60 with suspended solids, colloids and Alb; and sweep flocculation (enmeshment and adsorption) with Alc.



The enmeshment and adsorption mechanism of nC60 onto the amorphous aluminum hydroxide precipitates was explored using spectroscopic techniques and the results suggested the importance of inner-sphere complexation.

Acknowledgements The work was supported partially by The Hong Kong University of Science and Technology under grant number RPC06/07.EG03. The authors also would like to thank the partial financial support from the Technological and Higher Education Institute of Hong Kong.

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[26] Serna, C.J., White, J.L., Hem, S.L., 1977. Hydrolysis of aluminum-tri-(sec-butoxide) in ionic and nonionic media. Clays and Clay Minerals 25, 384 391. [27] Novosselov, A., Pajaczkowska, A., Talik, E., 2001. X-ray photoelectron spectroscopic studies on coloration of SrLaAlO4 single crystals. Crystal Research and Technology 36, 859 864. [28] Kloprogge, J.T., Duong, L.V., Wood, B.J., Frost, R.L., 2006. XPS study of the major minerals in bauxite: gibbsite, bayerite and (pseudo-)boehmite. Journal of Colloid and Interface Science 296, 572 576. [29] Barnier, V., Heintz, O., Roberts, D.E., Oltra, R., Costil, S., 2006. XPS and SIMS study of aluminium native oxide modifications induced by Q-switched Nd: YAG laser treatment. Surface and Interface Analysis 38, 406 409. [30] Ma, M.G., Zhu, J.F., 2009. A facile solvothermal route to synthesis of γ-alumina with bundle-like and flower-like morphologies. Materials Letters 63, 881 883. [31] Clausén, M., Öhman, L.O., Persson, P., 2005. Spectroscopic studies of aqueous gallium (III) and aluminum (III) citrate complexes. Journal of Inorganic Biochemistry 99, 716 726. [32] Nakakuki, M., Shiono, A., Kobayashi, I., Tajima, N., Yamakami, T., Hayashibe, R., Abe, K., Kamimura, K., Obata, M., Miyamoto, M., 2008. Characterization of Al-based insulating films fabricated by physical vapor deposition. Japanese Journal of Applied Physics 47, 609 611. [33] Andrievsky, G.V., Klochkov, V.K., Bordyuh, A.B., Dovbeshko, G.I., 2002. Comparative analysis of two aqueous-colloidal solutions of C60 fullerene with help of FTIR reflectance and UV-Vis spectroscopy. Chemical Physics Letters 364, 8 17.

20

Fig. 1 (A) Removal of nC60 in pure water at different pHs and 50-mg/L alum and (B) hydrolyzed aluminum species distribution after flocculation in pure water at different pHs and 50-mg/L alum. Fig. 2 (A) Hydrolyzed aluminum species distribution after flocculation in filtered saline wastewater at different pHs and 50-mg/L alum, (B) hydrolyzed aluminum species distribution after flocculation in high-alkalinity pure water at different pHs and 50-mg/L alum, and (C) hydrolyzed aluminum species distribution after flocculation in highsalinity pure water at different pHs and 50-mg/L alum. Fig. 3 (A) Al2p orbital XPS spectra of pure aluminum hydroxide precipitates and nC60 enmeshed and adsorbed aluminum hydroxide precipitates and (B) O1s orbital XPS spectra of pure aluminum hydroxide precipitates and nC60 enmeshed and adsorbed aluminum hydroxide precipitates. Fig. 4 FTIR spectra of nC60, pure aluminum hydroxide precipitates and nC60 enmeshed and adsorbed aluminum hydroxide precipitates. Fig. 5 SEM micrographs of (A) pure aluminum hydroxide precipitates and (B) nC60 enmeshed and adsorbed aluminum hydroxide precipitates. Fig. 6 EDAX mappings of (A) the surface of and (B) the section of nC60 enmeshed and adsorbed aluminum hydroxide precipitates.

21

100

Removal Efficiency (%)

A 80

60

40

20

0 2

3

4

5

6

7

8

9

10

7

8

9

10

pH

100

B

Fraction (%)

80

60

40 Ala Alb Alc

20

0 2

3

4

5

6

pH

Fig. 1

22

100

A

Fraction (%)

80

60 Ala Alb

40

Alc

20

0 2

3

4

5

6

7

8

9

10

7

8

9

10

7

8

9

10

pH 100

B

Fraction (%)

80

60

40

20

0 2

3

4

5

6

pH 100

C

Fraction (%)

80

60

40

20

0 2

3

4

5

6

pH

Fig. 2

23

2500 Pure aluminum hydroxide precipitates nC60 enmeshed and adsorbed aluminum hydroxide precipitates

A

2000

C/s

1500

1000

500

0 85

83

81

79

77

75

73

71

69

67

Binding Energy (eV) 12000 Pure aluminum hydroxide precipitates nC60 enmeshed and adsorbed aluminum hydroxide precipitates

B

10000

C/s

8000

6000

4000

2000

0 542

540

538

536

534

532

530

Binding Energy (eV) Fig. 3

24

528

526

524

610

3435

1125

3447 619 577 527

Absorbance

982 1113

1645 1182

978 527

1639 1429 577

1429 1182

4000

3600

3200

2800

2400

2000

1600

1200

800

-1

Wavenumber (cm ) nC60 enmeshed and adsorbed aluminum hydroxide precipitates Pure aluminum hydroxide precipitates nC60

Fig. 4

25

400

Fig. 5

26

Fig. 6 (A)

Fig. 6 (B)

27

Hydrolyzed Aluminum Species +

+

+

+ +

+ +

+

+

+

+

+

+

+

+ +

+ +

+

+

+

+

+

+

+

+ +

+ +

-

+ +

+

+ +

-

- --

-

+

+

+

+

-

+

+ +

+

+

+

+

nC60

-

Ala

Alb

Alc

Coagulation-Flocculation-Sedimentation

-+ +

+

-

+ +

+ + +

+

+

-

+

+

+

+

+

+ +

--

+

+

+

--

-

--

Colloidal

-

+

-

+

--

+

-

+

- -

+ +

+

-

--

+ +

+ +

+

+

+

+

+

+

+ +

+

+ +

+

+

+

-

+

+

+

-

-

Dissolved

+

+

+

+

+

+

+

+

-

- -

- -

+

+

Adsorption

Charge Neutralization

+

+

+

+

-

Precipitated

Conceptual model for nC60 removal in pure water by the alum coagulation-flocculationsedimentation process

28

Highlights •

In pure water, sweep flocculation with Alc is an important nC60 removal mechanism.

•

Wastewater constituents complicate nC60 removal by coagulation and settling.

•

The constituents affect alum hydrolysis and nC60’s interactions with Al species.

•

The adsorption of nC60 onto Alc is in the form of inner-sphere complexation.

29