Surface Chemistry of Nanosized Hydrated Ferric Oxide Encapsulated

Subscriber access provided by EASTERN KENTUCKY UNIVERSITY

Article

Surface Chemistry of Hydrated Ferric Oxide Encapsulated Inside Porous Polymer: Modeling and Experimental Studies Guangze Nie, Bingcai Pan, Shujuan Zhang, and Bingjun Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3119154 • Publication Date (Web): 07 Mar 2013 Downloaded from http://pubs.acs.org on March 9, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Surface Chemistry of Hydrated Ferric Oxide Encapsulated Inside Porous Polymer: Modeling and Experimental Studies

Guangze Niea, Bingcai Pana*, Shujuan Zhanga, Bingjun Panb

a. State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR China b. College of Engineering, Temple University, PA 19122 USA

*Corresponding author (Bingcai Pan). Tel: +86-25-8968-0390 E-mail address: [email protected]

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 35

ABSTRACT Elucidating the effect of porous host on the intrinsic physicochemical properties and reactivity of the encapsulated hydrous ferric oxides (HFOs) is believed important to better understand how HFOs interact with ionic pollutants inside the pore region. Here we prepared a hybrid adsorbent (HFO-CMPS) by dispersing nanosized HFOs onto an inactive porous polymeric support (i.e., chloromethylated polystyrene, CMPS). Surface complexation model (SCM) was employed to quantitatively evaluate the acid-base reactions and adsorption behaviors of HFO-CMPS as compared to the bare HFOs. Results demonstrated that the intrinsic equilibrium constants for acid-base reactions of surface sites of HFOs distinctly changed upon loading, that is, the log K (+) values decreased while its log K(-) values increased, resulting in its pHpzc value shifting from ~8.2 to ~6.3. Meanwhile, the titration curves of HFO-CMPS showed a markedly weaker dependence upon ionic strength. The results of model fittings of Cu(II) and As(V) adsorption indicate that the change of Coulombic term, reflecting the effect of the electrical potential on the adsorption activities, played an important role in the difference in pH-dependent adsorption of Cu(II) and As(V) between HFO-CMPS and bare HFOs. Additionally, the greater tendency of the encapsulated HFOs to dissolve in acidic solution was observed and may be due to its weaker pH buffering capacity, which possibly result from size-dependency of surface charges. All the results indicated that porous hosts play a significant role in the properties of the attached metal oxides for their application in water treatment. KEYWORDS: Nancomposite, host effect, acid-base properties, adsorption, surface charge, Environmental Nanotechnology.

2


Page 3 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Hydrated iron oxides (HFOs) have been proven to exhibit excellent adsorption for arsenic and heavy metals.1-4 Because of such important physicochemical properties as well as their low processing cost, environmental compatibility and chemical stability over a wide pH range, HFOs exhibit great potential in the treatment of natural waters or industrial effluents containing arsenic or toxic metals.5, 6 To gain insights into how HFOs interact with ionic pollutants, great efforts have been made to reveal their surface chemistry and adsorption properties with the aid of modern spectroscopic techniques such as extended X-ray absorption of fine structure (EXAFS) and X-ray absorption near edge structures (XANES), and formation of inner-sphere complexes is proved to be the main mechanism for arsenic or toxic metals removal by HFOs.7, 8 In addition, surface complexation models (SCMs), designed to calculate values of thermodynamic properties mathematically, have been widely used as an effective approach to describe the interfacial reactions of oxide-solution interface and to further probe the adsorption mechanisms involved.4, 9, 10 Armed with the equilibrium constants of interfacial reactions, SCMs can provide a quantitative reaction-based framework to interpret and predict the interfacial processes by accounting for the electrostatic potentials at the charged surface. The sizes of freshly precipitated amorphous HFO particles usually vary between 20 and 100 nm.11 Obviously, such ultrafine particles cannot be directly applied in fixed beds or any flow-through systems because of the excessive pressure drops and poor mechanical strength. An effective approach to overcoming the foregoing bottlenecks is to

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 35

prepare hybrid materials by impregnating or coating the fine submicron particles onto porous solid hosts of larger sizes. The resultant hybrid adsorbents retain the inherent properties of metal oxide particles, while the support materials provide satisfactory hydrodynamic performance and excellent mechanical strength for long-term use. In the late 1980s and early 1990s, Benjamin and coworkers12, 13 coated HFOs onto sand and used the resultant composite as the filtration medium for toxic metals removal from water. More recently, various solid hosts of porous nature such as activated carbon,14 zeolite,15 diatomite,16 and synthetic polymers11,

17-20

have been extensively examined, and the

resultant hybrid adsorbents could be readily employed in fixed-bed column or other flow-through systems for removal of ionic pollutants. Among the reported host materials, porous polymeric beads are a particularly attractive option partly because of their adjustable pore size and surface chemistry. Cumbal and SenGupta11 found that polymeric hosts containing non-diffusible charged functional groups would reject or enhance permeation of targeted ionic solutes because of the Donnan membrane effect and could serve as excellent hosts for HFO loadings. In our laboratory, several newly synthesized hybrid adsorbents, which were fabricated by impregnating HFO nanoparticles into porous polystyrene cation/anion exchanger resins, have demonstrated excellent removal of heavy metals and phosphate from aqueous solution.21, 22 To the best of our knowledge, almost all the available work concerning HFO-loaded hybrids focused on the pathways to obtaining the materials as well as their performance in pollutant sequestration under different solution chemistry and operating parameters.

4


Page 5 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Comparatively, little is known on how the porous hosts affect the properties of the encapsulated HFO nanoparticles. Recent studies based on molecular simulation demonstrated that the structure of water clusters would change significantly in nanopores,23-25 implying different solution chemistry inside the nanoporous region as compared to the bulk solution. Additionally, Molochnikov et al.26 used nitroxide spin probes to directly measure H+ activity inside polystyrene resins and found it was significantly different from that in bulk solution. For example, the pH values of the external solution are lower than the corresponding values inside the phase of phosphonic cation-exchange resin (KRF-2p), while the reverse is true for the carboxylic cation-exchanger (KB-2×3) and the anion-exchange resin (AN-31). Considering that the solution chemistry inside the nanopores of the hosts is quite different from the surrounding solution, we supposed that the encapsulated HFOs exhibit different properties from bulky HFOs for pollutant removal. Based on the above discussion, the main objective of the current work is to reveal the different properties of HFO nanoparticles before and after encapsulation within the nanoporous host. Here we employed a commercial chloromethylated polystyrene (CMPS) resin as the host to prepare a HFO-loaded hybrid material (HFO-CMPS) with a chemical precipitation method. The bare HFOs synthesized in a similar way without CMPS addition were also employed for comparison purpose. CMPS is an inert macroreticular adsorbent without any active functional groups for ion adsorption in aqueous solutions. Surface complexation model was employed to quantitatively evaluate the changes of

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 35

surface acid-base chemistry as well as to investigate the solid-liquid interfacial behaviors of HFOs upon loading. Moreover, Cu(II) and As(V) adsorption onto both materials were performed to further validate the results from SCM.

2. EXPERIMENTAL AND THEORETIACL METHODS 2.1. Materials. All chemicals used in this study were of analytical grade and were used without further purification. Copper nitrate and sodium arsenate were employed as the Cu(II) and As(V) sources, and dissolved in deionized water as stock solutions for further use. The CMPS resin was purchased from Zhenguang Resin Co. (Hangzhou, China) and present as spherical beads. It was sieved to keep sizes ranging from 0.55 to 0.63 mm. Prior to use, they were subjected to extraction with ethanol for 6 h in a Soxhlet apparatus to remove residue impurities and then vacuum desiccated at 333 K for 12 h. The synthesis procedure of HFO-CMPS has been described in our previous work.27 First, ~30.0 g of CMPS resin were soaked in 500 mL of 1:2 (V/V) ethanol-water solution containing ~150 g Fe(NO3)2·9H2O. The mixture was stirred for 24 h to allow the Fe(III) ions to penetrate into the nanopores of the resin. Second, the Fe(III)-containing polymer beads were filtered, vacuum desiccated, and transferred to 500 mL of 0.1 M NaOH solution, and then stirred for another 24 h. Fe(III) preloaded on CMPS resin was thus precipitated as Fe(III) hydroxides onto the inner surface of the polymer. Then, the resultant particles were washed with the deionized water until the filtrate reached a pH of ~7, followed by rinsing with 50:50 (v/v) ethanol-water solution. Finally, the solid

6


Page 7 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


particles were thermally treated at 328 K until reaching a constant weight to obtain the hybrid adsorbent HFO-CMPS. The amorphous HFO particles employed in the study were obtained by direct liquid precipitation and thermal treatment under the same conditions as for HFO-CMPS, based on procedures proposed by Schwertmann and Cornell.28 The final dried solids were gently ground using a mortar and pestle to break up large aggregates. 2.2. Adsorbents Characterization. The specific surface areas of the adsorbents were determined by N2 adsorption at 77 K using a Quantachrome Nova 3000 surface and pore analyzer. Prior to measurement, the samples were outgassed at 80°C (HFOs) or 60°C (HFO-CMPS) for 24 h. The microscopic features of the resulting hybrids, bare HFOs, and the host CMPS were observed with high-solution transmission electron microscopy (TEM) (JEOL JEM-100S electron microscope). Mineralogy of the samples was investigated by powder X-ray diffraction patterns (XTRA, Switzerland) with a Cu Kα radiation (40 kV, 25 mA). The amount of HFOs loaded onto CMPS was determined by acid digestion of the hybrid particles, followed by atomic absorption spectroscopy analysis (TAS-990 PGENERAL). 2.3. Potentiometric Titrations. The acid-base surface chemistry of HFOs and HFO-CMPS was investigated by conducting potentiometric titration experiments, using 0.1, 0.01 and 0.001 M NaNO3 solution as the background electrolyte. A computer-controlled automatic titration system (T50, Mettler Toledo) with a combined glass electrode (DGi115-SC) was employed for the titration experiments. All titrations were conducted in 100-ml vessels with a polyethylene lid at a temperature of 25±0.2°C.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 35

Before beginning the titrations, dried HFOs (0.08 g) and HFO-CMPS (1.52 g, containing same Fe amount of 0.08g HFOs) were suspended in 40 ml NaNO3 solution and purged with high purity N2 gas for at least two hours. The pH values were quickly lowered to 3.0 by addition of 0.20 M HNO3 solution. After one hour of equilibrium, the suspensions were slowly back-titrated at a constant pH increment with 0.030 M NaOH standard solution till pH 11, where the volume of NaOH solution added (0.005~0.1 ml) was automatically adjusted to obtain the desired pH value. Each step was allowed to stabilize until the pH drift was less than 0.005 pH unit per minute. To avoid the disturbance of carbonate species, all solutions were prepared using preboiled deionized water and then kept under a nitrogen atmosphere. Moreover, high purity N2 gas was bubbled into solutions to exclude CO2 (g) throughout the titration. 2.4. Adsorption and Acidic Leaching Experiments. The adsorption behavior of Cu(II) or As(V) on the two adsorbents was investigated in 50-mL polyethylene centrifuge tubes with screw caps in a nitrogen atmosphere containing 0.01 M NaNO3 as the background electrolyte. In order to conveniently compare the adsorption capacities of those two materials, 0.010 g HFOs and 0.19 g HFO-CMPS samples, which both contained the same amount of Fe, were weighed for all the batch experiments, respectively. Then, the Cu(II) or As(V) stock solution and NaNO3 stock solution were added to achieve the desired concentration of variable components. The total volume of the liquid mixture was 20 mL. Finally, a negligible volume of HNO3 or NaOH was added to obtain the desired pH of the aqueous solutions. Blank tests with adsorbent-free samples

8


Page 9 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were conducted to determine any possible loss of solute concentration over the duration of the experiment. The sealed tubes were then placed in a thermostatic shaker under 150 rpm for three days at 25°C. Kinetic measurement proved that adsorption equilibrium was reached under such experimental conditions. After equilibrium, final adsorbent/liquid separation was achieved by centrifugation at 3000 rpm for 30min, and the supernatant solutions were further filtered through 0.22 µm membranes and the pH was measured immediately. The concentrations of Cu(II) and As(V) in solution were determined by a flame atomic absorption spectroscopy (TAS-990, Beijing Purkinje, China) and an atomic fluorescence spectrophotometer (PF6-1, Beijing Purkinje, China), respectively. Leaching experiments were conducted in 150-mL polyethylene bottles to evaluate the solubility of HFOs and HFO-CMPS in acidic aqueous solutions. HFOs (0.010 g) or HFO-CMPS (0.19 g) was added into 50 mL solution containing 0.01 M of NaNO3. The solution pH was adjusted by the addition of concentrated HNO3. After 5 days of equilibrium in a shaker at 25°C, the pH of the solution and the concentrations of Fe(III) were measured by using the same analysis method for Cu measurement. 2.5. Modeling Scheme. SCMs describe the adsorption of ions at the solid-liquid interface as complexation reactions with specific surface sites (acid-base reactions and surface complexation reaction) analogous to complex formation in solution that is governed by mass law equations. They account for the electrostatic potentials at the charged surface and the charge of the adsorbate. The effect of pH and ionic strength on the surface charge can be explained by the Coulombic term (CT), which is practically an

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 35

activity coefficient for the surface species that takes into account the effect of the electrical potential at the surface (see more discussion later). The mass law equations containing the Coulombic term with equilibrium constants provide a quantitative framework to describe the adsorption process. Evidently, spectroscopic studies have confirmed the formation of inner-sphere complexes as the primary adsorption mechanism of heavy metal and oxyanions adsorption on the HFO surfaces.7, 8, 29 The constant capacitance model (CCM) has been recognized as one of the SCMs consistent with spectroscopic evidence, which assumes that all surface species are inner-sphere complexes and no surface complexes are formed with ions from the background electrolyte. Because of this, Cu(II) and As(V) adsorption onto various of solids have been investigated using CCM.30, 31 The previous CCM studies can be helpful guides and constraints to determine model parameters and equilibrium constants. In addition, CCM assumes that one plane of charge represents the surface and has a simple linear relationship between the surface potential,ψ , and the surface charge density, σ , through the capacitance, κ ,32: σ =κψ . The simple format makes it convenient to describe the interfacial reactions of HFOs. Dzombak and Morel4 have shown that the active sites of HFOs were amphoteric and the surface ionization reactions can be expressed as +

+

>FeOH + H = >FeOH2 +

K( + ) =

[>FeOH2 ] +

[>FeOH][H ]

(1)

ψF

exp(

RT

)

10


Page 11 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


−

>FeOH = >FeO + H −

K( − ) =

+

[>FeO ][H ] [>FeOH]

+

exp(−

(2) ψF RT

)

Where K(+) and K(-) are the intrinsic equilibrium constants, square brackets [ ] represent activities of the species, the term exp( ±ψ F / RT ) is derived from the Boltzmann equation and used to account for the electrostatic properties of the charged surfaces, ψ is the surface potential (V), F is the Faraday constant (96,485 C mol-1), R is the gas constant (8.314 J mol-1 K-1), and T is the absolute temperature (K). Surface complexation of metal ions with HFOs involves the formation of bonds with surface oxygen atoms and the release of protons from the surface, while specific adsorption of anions occurs via ligand exchange reactions between surface hydroxyl ions and the adsorptive ions. Based on the previous studies,4, 9, 10 we assumed Cu(II) and As(V) formed monodentate complexes on the surfaces of HFOs. The adsorption reactions and mass law equations are listed in Table 1. The least-square fitting program FITEQL 4.033 was used to optimize the intrinsic surface ionization reaction constants, reactive surface site densities, and surface complexation constants. The goodness of fit is given by the overall variance, weighted sum of squares divided by the degrees of freedom (WSOS/DF). Experience has shown that WSOS/DF values in the range of 0~20 are common for reasonably good fits.33 Constants for the protonation of aqueous copper and arsenic species were obtained from Visual MINTEQ (ver. 3.0).

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 35

Table 1. List of adsorption reactions and the surface complexation constants fitted by model. Adsorption reactions >FeOH + Cu2+ = >FeOCu+ + H+

KintCu = [>FeOCu+] [H+] / [>FeOH] [ Cu2+] · exp(FΨ / RT)

>FeOH + H2AsO4- + H+ = >FeH2AsO4 + H2O

KintAs-1 = [>FeH2AsO4] / [>FeOH] [H2AsO4-] [ H+]

>FeOH + HAsO42- + H+ = >FeHAsO4- + H2O

KintAs-2 = [>FeHAsO4-] / [>FeOH] [HAsO42-] [ H+] · exp(-FΨ / RT)

>FeOH + AsO43- + H+ = >FeAsO42- + H2O

KintAs-3 = [>FeAsO42-] / [>FeOH] [AsO43-] [ H+] · exp(-2FΨ / RT)

Surface complexation constants log KintCu

CTCua

log Kapp Cu

log KintAs-1

log KintAs-2

CTAs-2

log Kapp As-2

log KintAs-3

CTAs-3

log Kapp As-3

CTAs-3

log Kapp As-3

HFOs

-0.06

0.036b

-1.50b

28.95

24.03

0.031c

22.52c

17.71

0.001c

14.71c

0.00023d

14.07d

HFO-CMPS

-0.25

0.42b

-0.63b

30.42

26.20

0.35c

25.74c

17.81

0.12c

16.89c

0.14d

16.95d

a

CT = Coulombic term, Kapp = Kint·CT; b calculated by model at pH = 5, c at pH=8 and d pH=11.

3. RESULTS AND DISCUSSION 3.1. Characteristics of adsorbents. Measured specific surface areas and other properties of bare HFO particles and HFO-CMPS bead are presented in Table 2. As compared to CMPS, an increase in BET surface area of HFO-CMPS was observed due to the presence of HFOs. Dzombak and Morel4 indicated that N2 adsorption with BET analysis significantly underestimates the surface area available for sorbates on HFOs due to surface decomposition during the drying step required in the BET method and prevention of relatively large N2 molecules from entering micropores that are available for smaller molecules or ions. Therefore, an estimated value of 600 m2 g-1, recommended by Davis and co-workers34 for typical HFO particles, was employed for fitting the acid-base titration and adsorption data of HFOs. TEM images of samples are depicted in

12


Page 13 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. As shown, the bare HFOs are present as nanoparticle aggregates, while HFOs loaded within the reticular structure of the host polymers are well dispersed as nanoparticles of sizes 10 ~ 50 nm. The X-ray diffraction spectra indicate both the bare HFOs and the loaded HFO nanoparticles in CMPS are amorphous in nature (Figure S1 in supporting information).

Table 2. Main proprieties of the adsorbents. Proprieties

HFO

Matrix structure

—

BET surface area (m2/g)

278

38

45

Average pore diameter (nm)

—

22.41

12.96

58.15%

0

3.05%

8.02b/52.36c

—

10.64b/66.67c

HFO content (Fe mass %)

Sorption capacity (mg/gFe)a a

CMPS

HFO-CMPS

Polystyrene-divinylbenzene

Sorption capacity was expressed based on the mass of iron (details are in the supporting information); b sorption

capacity for Cu(II); c sorption capacity for As(V).

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 35

Figure 1. TEM images of the samples. (a) HFOs, (b) HFO-CMPS and (c) CMPS.

3.2. Potentiometric Titrations of HFOs and HFO-CMPS. The potentiometric titration data of HFOs and HFO-CMPS are showed in Figure 2. The total proton concentration is defined as TOTH=[H+]-[OH-]+[>FeOH2+]-[>FeO-] and calculated from the following equation:

14


Page 15 of 35

TOTH = (Ca Va -Cb Vb )/(V0 +Va +Vb )

(3)

where Ca and Cb are the molar concentrations of the HNO3 and NaOH used, V0 is the initial volume of the suspension and Va and Vb are the volumes of HNO3 and NaOH added. Usually, the surface charge density of metal oxides is affected by ionic strength,35 and titration curves display strong ionic strength dependence. Such characteristic is observed for bare HFOs in this study (Figure 2a) and consistent with other report.36 In contrast, the encapsulated HFOs in porous CMPS support exhibited distinct acid-base properties. The acid-base titration curves of HFO-CMPS display obviously weaker dependence on the ionic strength of bulk solution (Figure 2b). Note that the titration curve of mere CMPS completely overlapped that of pure DI water (Figure S3), indicating that CMPS have no functional group to consume protons, and the results exclusively reflect the acid-base properties of the encapsulated HFOs. Similar phenomenon was also mentioned by Lim et al. for the encapsulated iron oxides into alginate shell.19

-3

3.0x10

HFO -3

0.1M 0.01M 0.001M

2.0x10

-3

1.0x10

TOTH (M)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

-3

-1.0x10

(a)

-3

-2.0x10

2

4

6

8

10

12

pH

15



-3

3.0x10

HFO-CMPS 0.1M 0.01M 0.001M

-3

2.0x10

-3

1.0x10

TOTH (M)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

0.0

-3

-1.0x10

-3

-2.0x10

(b) -3

-3.0x10

2

4

6

8

10

12

pH

Figure 2. The acid-base titration results for HFOs (a) and HFO-CMPS (b) at different ionic strengths. The dots represent experimental data while the lines represent model calculations. Each of titration systems contains the same amount of Fe. (HFO=0.08 g, HFO-CMPS=1.52 g, V=40 mL)

Bare HFOs and HFO-CMPS have the same active compositions (i.e., HFO particles). Thus, the different acid-base behaviors between them should be attributed to the impact of porous support (i.e., CMPS) on the encapsulated HFOs. The effect of ionic strength on the titration curve of bare HFOs can be explained by that, generally, increasing the concentration or valence of the counter-ions would compress the electrical double layer (EDL) and consequently increase the electrical potential gradient, because the double layer thickness (Debye length) is inversely proportional to the ionic strength. Thus, ionic strength results in the changes of the acid-base properties of HFOs. To sum up, the markedly weaker dependence of acid-base properties of the encapsulated HFOs on ionic strength implied that, the presence of porous CMPS support would substantially abate the

16


Page 17 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


impact of ionic strength of bulk solution on the surface electrical potential of the encapsulated HFOs, possibly due to well-defined water clusters inside the porous support.25 Additionally, the impact of porous support was further exhibited by the different pHpzc values (i.e., the point of zero charge) of bare HFOs and HFO-CMPS, which can be estimated from Figure 2 through the location of titration curve at TOTH equal to zero. As shown, the pHpzc of HFOs is ~8.2, which is close to that reported elsewhere,4, 36 while that of HFO-CMPS was shifted to lower value (i.e., ~6.3). 3.3. Acid-base Surface Chemistry by Modeling. One-site CCM model was employed to describe the acid-base behavior of bare HFOs and the encapsulated HFOs. The fitting titration curves are also displayed in Figure 2 (solid line). Based on the model assumption, the site density and acid-base reaction constants are optimized by the potentiometric titration data sets with the aid of FITEQL program and are summarized in Table 3.

Table 3. The intrinsic acid-base equilibrium constants and site concentration of HFOs and HFO-CMPS. HFOs

HFO-CMPS

Ionic strength

0.001M

0.01M

0.1M

0.001M

0.01M

0.1M

logK(+) logK(-) site density (mol/gFe) pHPZC WSOS/DF

5.94 -10.38 8.86E-4 8.16 40

6.44 -10.26 1.05E-3 8.35 26

7.03 -9.34 1.25E-3 8.19 12

5.08 -7.39 5.79E-4 6.23 10

5.23 -7.07 6.84E-4 6.15 17

5.51 -7.10 7.86E-4 6.31 18

As the results show, variations in surface properties are observed for HFOs after

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

loading. The values of K(+) decreased by about one order of magnitude, while those of K(-) increased by more than two orders of magnitude at different ionic strength, indicating a decrease in the affinity between H+ and the surface sites of HFOs and an increase for OHaffinity. As aforementioned, different solution chemistry inside the nanopore space was observed as compared with the external solution in terms of water cluster structure23, 24 and H+ activity,26 and we suppose that such affinity variation be associated with the variation in solution chemistry as the solution surrounds the HFO particles and greatly affects its surface chemistry. However, further direct evidence is needed to validate the assumption. On the other hand, the active site density (mol/g Fe) decreased by about thirty five percent at different ionic strength. Generally, part of HFOs dispersed in the porous supports is located in the micropore region where the pore entrances may be occupied by other HFO particles, thus is inaccessible due to pore blockage. Similar observation was reported for the encapsulation of nanosized titanium phosphate within polystyrene beads,37 and the accessible ion exchange sites are far below the theoretical values. + − At the pHpzc, an oxide has zero surface charge, i.e. [>FeOH2 ] = [>FeO ] . Substituting

+

−

[>FeOH2 ] and [>FeO ] with equations (1) and (2), a relationship between pHpzc and K(+), K (-)

was obtained:

pH pzc = [ log K ( + ) + log K ( − ) ] / 2

(4)

Expectably, the pHpzc values of the two adsorbents calculated by Eq 4 (Table 3) are consistent with the values obtained by the location of titration curve at TOTH equal to

18


Page 19 of 35

zero. The equilibrium constants and site density based on CCM model varied with ionic strength,38 and the corresponding values at ionic strength equal to zero can be obtained based on Davies equation.4

1.0

>FeOH

0.8 +

>FeOH2

0.6 0.4

Relative species

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-

>FeO

0.2

(a)

0.0 1.0

>FeOH

0.8

-

+

>FeO

>FeOH2

0.6 0.4 0.2

(b)

0.0 2

4

6

8

10

12

pH

Figure 3. The surface species distribution of HFOs (a) and HFO-CMPS (b) in 0.01 M NaNO3 solution predicted by the model.

The distributions of surface species at different ionic strengths calculated with the FITEQL program are displayed in Figure 3 (0.01 M NaNO3) and Figure S5 (0.1 M and 0.001 M NaNO3). As shown, the relative proportions of surface species change as a function of pH. As compared with the bare HFOs, the relative fraction of >FeO

−

on

HFO-CMPS increases obviously throughout the studied pH range and thus results in a lower pHpzc; the peak curve of >FeOH becomes much sharper and shifts to the lower pH value. In all, the relative proportion of neutral species on HFO-CMPS is smaller than that on HFOs alone. Meanwhile, HFO-CMPS has a larger proportion of charged species (i.e.,

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

+

Page 20 of 35

−

>FeOH2 and >FeO ) than the bare HFOs. Given the Coulombic term in the mass law

equations, the surface reactions on HFO-CMPS may be affected by the surface charge at greater degree. 3.4. Adsorption Properties. Cu(II) and As(V) adsorption curves on HFOs and HFO-CMPS as a function of pH, with 0.01 M NaNO3 as the background electrolyte, are illustrated in Figure 4 a-b. Note that the adsorption experiments on mere CMPS (Figure S4), under otherwise same experimental condition, demonstrate that negligible adsorption of Cu(II) and As(V) adsorption on mere CMPS occurred. For both adsorbents, Cu(II) adsorption increase with increasing pH, and HFO-CMPS exhibits more favorable Cu(II) adsorption over the investigated pH range. For example, the adsorption capacity of HFO-CMPS is about twice of much of bare HFOs at pH 5. Additionally, the shapes of the adsorption edges of Cu(II) for HFOs and HFO-CMPS are almost the same and are similar to those previously reported.4, 9 Figure S6 (a-b) show the relative distribution of Cu(II) species calculated from the copper hydrolysis constants. It is clear that the main species is Cu2+ and the amount of precipitate Cu(OH)2 is negligible at pH < 7.0. As for As(V) adsorption, the removal efficiencies of two adsorbents decreased with increasing pH, and HFO-CMPS had higher adsorption capacities than bare HFOs in the pH range of 3~9.5, while an opposite trend was observed above pH 9.5.

20


Page 21 of 35

100

-5 initial Cu(II) = 5×10 M HFOs = 0.01 g

Cu(II) adsorption (%)

80

HFO-CMPS = 0.19 g V = 20 mL I = 0.01 M

60

40 HFO HFO model HFO-CMPS HFO-CMPS model

20

(a)

0

2

3

4

5

6

7

8

pH -4 initial As(V) = 2.5×10 M HFOs = 0.01 g

100

HFO-CMPS = 0.19 g V = 20 mL I = 0.01 M

As(V) adsorption (%)

80

60

40

HFO HFO model HFO-CMPS HFO-CMPS model

20

(b)

0

2

4

6

8

10

12

pH 100

HFO

80 -

>FeH2AsO4

60

As(V) adsorption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


>FeHAsO4

40 20

2-

>FeAsO4

0 100

HFO-CMPS

80 >FeH2AsO4

60

-

>FeHAsO4

40 2-

20

>FeAsO4

(c)

0 2

4

6

8

10

12

pH

Figure 4. Experimental data and fitting curves for (a) Cu(II) and (b) As(V) adsorption onto HFOs and HFO-CMPS (both adsorbents in the adsorption system were weighed containing the same amount of Fe), (c)surface complex speciation repartition of As(V) adsorption on HFO and HFO-CMPS.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

The magnitudes of intrinsic equilibrium constants of complexation reactions for adsorption were used as a measure of the intrinsic adsorption affinity between the adsorbent and ions. Values of these constants were determined by fitting the adsorption data using CCM with the aid of FITEQL. The adsorption affinity in specific solution condition can be measured by apparent equilibrium constant with a relationship of K

app

= K exp(−ΔZFψ / RT ) int

(5)

Then, the surface adsorption species are given by: [>FeOCu ] = K Cu [>FeOH][Cu ]/[H ] +

app

2+

+

(6)

[>FeH2 AsO4 ] = K As -1[>FeOH][H2 AsO4 ][H ] +

(7)

[>FeHAsO4 ] = K As -2 [>FeOH][HAsO4 ][H ]

(8)

[>FeAsO4 ] = K As -3 [>FeOH][AsO4 ][H ]

(9)

app

-

2-

-

app

app

2-

3-

+

+

where Kapp is the apparent equilibrium constant; Kint is an intrinsic equilibrium constant independent upon surface charge; ΔZ is the change in surface charge due to the adsorption reaction and exp(−ΔZFψ / RT ) , defined as the Coulombic term (CT), is an activity coefficient for the long-range electrical effects of charged surface groups. The best fitting results of the Cu(II) and As(V) adsorption, based on the lowest WSOS/DF, are illustrated in Figure 4a and 4b, and the fitting parameters are listed in Table 1. The Coulombic terms for Cu(II) adsorption (pH=5) and As(V) adsorption (pH=8 or 11) were also calculated (Table 1). Clearly, the CCM model simulates the experimental data well since the values of WSOS/DF are much lower than 20 (2.4 and 4.9 for Cu(II) adsorption on bare HFOs and HFO-CMPS, respectively; 4.5 and 8.3 for

22


Page 23 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As(V) adsorption on bare HFOs and HFO-CMPS, respectively). As shown, HFO-CMPS has smaller Cu(II) adsorption equilibrium constants (Kint Cu ) and larger As(V) adsorption int int equilibrium constants (Kint As-1, KAs-2 and KAs-3) than the bare HFOs, suggesting a decrease of

intrinsic adsorption affinity for Cu(II) and an increase for As(V) after HFOs were dispersed in CMPS. For Cu(II) adsorption, although the intrinsic equilibrium constant of HFO-CMPS is smaller than that of bare HFOs, the values of Coulombic terms for HFO-CMPS are much larger due to the stronger surface potential. For example, the value of Coulombic term for Cu(II) adsorption on HFO-CMPS is an order of magnitude larger than that on HFOs at pH 5, which ultimately results in larger value of apparent equilibrium constant of HFO-CMPS (Table 1). Considering HFO-CMPS has fewer site density (>FeOH) than HFOs, it can be inferred that Coulombic term plays an important role in Cu(II) adsorption. Also, larger Coulombic term were observed for As(V) adsorption. For the anionic As(V) (the acid dissociation constants pKa1 = 2.24, pKa2 = 6.96 and pKa3 = 11.50 39), the predominant species in the NaNO3 solution are H2AsO4- and HAsO42- at acid and neutral pH (Figure S6-c). As shown in Figure 4c, HFO-CMPS has better As(V) adsorption in such pH range, since the apparent equilibrium constants for >FeH2AsO4 and >FeHAsO4of HFO-CMPS are larger than those of HFOs, and, both adsorbents have comparable amount of >FeOH site in this pH range (Figure S8). On the contrary, at high pH (>9.5), although HFO-CMPS still has a higher adsorption affinity than HFOs for the dominant species AsO43-, a larger adsorption capacity of HFOs than HFO-CMPS was observed

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

because the former have many more >FeOH sites for AsO43- (as illustrated in Figure 3). 3.5. Leaching of HFOs from adsorbents. The leaching of Fe(III) from HFO-based adsorbents is an area of interest for their practical application in the treatment of acidic wastewater. Therefore, leaching experiments were conducted to evaluate the solubility of HFOs and HFO-CMPS at different solution pH. As shown in Figure 5, negligible amounts of Fe(III) were dissolved in the solution at pH above 2.8 for both adsorbents. This result suggests that both adsorbents have a strong acid-proof property and probably could work effectively under acidic wastewater with a pH range above 2.8. A noteworthy observation is that the percentage of Fe dissolved from HFO-CMPS was enhanced as compared to the bare HFO particles at pH smaller than 2.8. Recent studies have investigated the size-dependent dissolution of nanoparticles and dissolution enhancement were found with decreasing particle size.40-42 This size effect can explain the variation of solubility between the bare HFOs and HFO-CMPS since HFO particles were well dispersed in the CMPS host and formed smaller nanoparticles (Figure 1). However, what causes the size effect on solubility is still an open question. It is well known that dissolution of iron oxides is closely related to H+ activity of solution. Generally, interfacial water molecules have weaker hydrogen bonding ability than their bulk liquid counterparts.43 The H+ activity difference between the external solution and the surface of medium is a function of the surface electrical potential (ψ ). The surface pH (pHi) can be calculated by its relationship with the solution pH:4

24


Page 25 of 35

pH i = pH + Fψ / 2.3RT

(10)

Researchers have revealed that surface charge density of nanomaterials exhibited size-dependent behavior, that is, the smaller the particle size, the greater the surface charge44,

45

. Therefore, the size effect on the dissolution may be attributed to the

size-dependency of the surface charge.

100

HFO HFO-CMPS 80

Fe dissolved (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

pH

Figure 5. Solubility of HFOs and HFO-CMPS in acidic aqueous solution with 0.01M NaNO3 as the background electrolyte.

Here we calculated the surface electrical potential (ψ ) at different solution pH using CCM by fitting the titration data sets with the aid of FITEQL, and then the pHi values were obtained. The values of surface pHi of the adsorbents employed in this study versus the corresponding values of bulk pH are illustrated in Figure 6. According to the thermodynamic theory, the difference between the bulk and the surface pH could be used to express the (non-dimensional) free energy required to bring an H+ ion from the bulk

25



solution to the surface since there exist a linear relationship between pH difference and the free energy.4 As shown in Figure 6, the absolute value of the pH difference, ΔpH (i.e., │pHi–pH│), increased with the increase of ionic strength and ΔpH equaled zero at pHpzc where the surface electrical potential was zero. In acidic conditions (pH9.5). Modeling of the adsorption process suggested that a change of adsorption affinity after HFOs were dispersed in CMPS and the removal efficiency for Cu(II) and As(V) were strongly influenced by surface properties. In addition, HFO particles loaded on host can be more easily dissolved in acidic solution than the bare HFO particles. The calculated surface H+ activity for these two sorbents by CCM showed that the loaded HFO particles with smaller size have stronger buffering capacity for the acid-base change of external solution than bare HFO particles. The size effect on the dissolution may be caused by the

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

size-dependent of surface charge. The modeling results are in good accordance with the experimental ones, further demonstrating the suitability of the model to describe adsorption onto both adsorbents. Thus, we believe that the above findings of this study may provide an insight into the preparation of hybrid iron oxide sorbents.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (21177059/51078179), Changjiang Scholars Innovative Research Team in University (IRT1019), and Jiangsu NSF (BK2012017/2011016).

Supporting Information Available The calculated Coulombic terms for Cu(II) and As(V) adsorption at different pH; XRD spectra of the sorbents; pore size distribution of HFO-CMPS; acid-base titration results for CMPS; adsorption of Cu(II) and As(V) on the bare CMPS; modeling simulation of surface species distribution in 0.1M and 0.001M NaNO3 solution; relative proportion of species of Cu(II) and As(V); the precipitation curve of Cu(II); adsorption isotherms of Cu(II) and As(V) on the bare HFO and HFO-CMPS; surface species distribution of Cu(II) and As(V) adsorption on HFO and HFO-CMPS. This information is available free of charge via the Internet at http://pubs.acs.org

28


Page 29 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Pierce, M. L.; Moore, C. B. Adsorption of Arsenite and Arsenate on Amorphous Iron Hydroxide. Water Res. 1982, 16, 1247-1253. (2) Mercer, K. L.; Tobiason, J. E. Removal of Arsenic from High Ionic Strength Solutions: Effects of Ionic Strength, pH, and Preformed Versus in Situ Formed HFO. Environ. Sci. Technol. 2008, 42, 3797-3802. (3) Gadde, R. R.; Laitinen, H. A. Heavy Metal Adsorption by Hydrous Iron and Manganese Oxides. Anal. Chem. 1974, 46, 2022-2026. (4) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide. John Wiley & Sons Inc.: New York, 1990. (5) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrences, and Uses. Wiley-VCH: Weinheim, 2003. (6) Filip, J.; Zboril, R.; Schneeweiss, O.; Zeman, J.; Cernik, M.; Kvapil, P.; Otyepka, M. Environmental Applications of Chemically Pure Natural Ferrihydrite. Environ. Sci. Technol. 2007, 41, 4367-4374. (7) Scheinost, A. C.; Abend, S.; Pandya, K. I.; Sparks, D. L. Kinetic Controls on Cu and Pb Sorption by Ferrihydrite. Environ. Sci. Technol. 2001, 35, 1090-1096. (8) Manning, B. A.; Fendorf, S. E.; Goldberg, S. Surface Structures and Stability of Arsenic(III) on Goethite: Spectroscopic Evidence for Inner-Sphere Complexes. Environ. Sci. Technol. 1998, 32, 2383-2388. (9) Lund, T. J.; Koretsky, C. M.; Landry, C. J.; Schaller, M. S.; Das, S. Surface

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

Complexation Modeling of Cu(II) Adsorption on Mixtures of Hydrous Ferric Oxide and Kaolinite. Geochem. Trans. 2008, 9, 9. (10) Karthikeyan, K. G.; Elliott, H. A. Surface Complexation Modeling of Copper Sorption by Hydrous Oxides of Iron and Aluminum. J. Colloid Interface Sci. 1999, 220, 88-95. (11) Cumbal, L.; SenGupta, A. K. Arsenic Removal Using Polymer-Supported Hydrated Iron (III) Oxide Nanoparticles: Role of Donnan Membrane Effect. Environ. Sci. Technol. 2005, 39, 6508-6515. (12) Edwards, M.; Benjamin, M. M. Adsorptive Filtration Using Coated Sand: A New Approach for Treatment of Metal-Bearing Wastes. J. Water Pollut. Control Fed. 1989, 61, 1523-1533. (13) Bailey, R.; Bennett, T.; Benjamin, M. Sorption onto and Recovery of Cr (VI) Using Iron-Oxide-Coated Sand. Water Sci. Technol. 1992, 26, 1239-1244. (14) Jang, M.; Chen, W.; Cannon, F. S. Preloading Hydrous Ferric Oxide into Granular Activated Carbon for Arsenic Removal. Environ. Sci. Technol. 2008, 42, 3369-3374. (15) Oliveira, L. C. A.; Petkowicz, D. I.; Smaniotto, A.; Pergher, S. B. C. Magnetic Zeolites: A New Adsorbent for Removal of Metallic Contaminants from Water. Water Res. 2004, 38, 3699-3704. (16) Jang, M.; Min, S. H.; Park, J. K.; Tlachac, E. J. Hydrous Ferric Oxide Incorporated Diatomite for Remediation of Arsenic Contaminated Groundwater. Environ. Sci. Technol. 2007, 41, 3322-3328.

30


Page 31 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Gupta, A.; Chauhan, V. S.; Sankararamakrishnan, N. Preparation and Evaluation of Iron-Chitosan Composites for Removal of As (III) and As (V) from Arsenic Contaminated Real Life Groundwater. Water Res. 2009, 43, 3862-3870. (18) Guo, X. J.; Chen, F. H. Removal of Arsenic by Bead Cellulose Loaded with Iron Oxyhydroxide from Groundwater. Environ. Sci. Technol. 2005, 39, 6808-6818. (19) Lim, S. F.; Zheng, Y. M.; Zou, S. W.; Chen, J. P. Characterization of Copper Adsorption onto an Alginate Encapsulated Magnetic Sorbent by a Combined FT-IR, XPS, and Mathematical Modeling Study. Environ. Sci. Technol. 2008, 42, 2551-2556. (20) Iesan, C. M.; Capat, C.; Ruta, F.; Udrea, I. Evaluation of a Novel Hybrid Inorganic/Organic Polymer Type Material in the Arsenic Removal Process from Drinking Water. Water Res. 2008, 42, 4327-4333. (21) Pan, B. J.; Wu, J.; Pan, B. C.; Lv, L.; Zhang, W. M.; Xiao, L. L.; Wang, X. S.; Tao, X. C.; Zheng, S. R. Development of Polymer-Based Nanosized Hydrated Ferric Oxides (HFOs) for Enhanced Phosphate Removal from Waste Effluents. Water Res. 2009, 43, 4421-4429. (22) Pan, B. J.; Qiu, H.; Pan, B. C.; Nie, G. Z.; Xiao, L. L.; Lv, L.; Zhang, W. M.; Zhang, Q. X.; Zheng, S. R. Highly Efficient Removal of Heavy Metals by Polymer-Supported Nanosized Hydrated Fe(III) Oxides: Behavior and XPS Study. Water Res. 2010, 44, 815-824. (23) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water Conduction through the Hydrophobic Channel of a Carbon Nanotube. Nature 2001, 414, 188-190.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

(24) Cailliez, F.; Stirnemann, G.; Boutin, A.; Demachy, I.; Fuchs, A. H. Does Water Condense in Hydrophobic Cavities? A Molecular Simulation Study of Hydration in Heterogeneous Nanopores. J. Phys. Chem. C 2008, 112, 10435-10445. (25) Febles, M.; Pérez-Hernández, N.; Pérez, C.; Rodríguez, M. L.; Foces-Foces, C.; Roux, M. V.; Morales, E. Q.; Buntkowsky, G.; Limbach, H. H.; Martín, J. D. Distinct Dynamic Behaviors of Water Molecules in Hydrated Pores. J. Am. Chem. Soc. 2006, 128, 10008-10009. (26) Molochnikov, L. S.; Kovalyova, E. G.; Grigor'ev, I. A.; Zagorodni, A. A. Direct Measurement of H+ Activity inside Cross-Linked Functional Polymers Using Nitroxide Spin Probes. J. Phys. Chem. B 2004, 108, 1302-1313. (27) Qiu, H.; Zhang, S.; Pan, B.; Zhang, W.; Lv, L. Effect of Sulfate on Cu(II) Sorption to Polymer-Supported Nano-Iron Oxides: Behavior and XPS Study. J. Colloid Interface Sci. 2012, 366, 37-43. (28) Schwertmann, U.; Cornell, R. M., Iron Oxides in the Laboratory: Preparation and Characterization. Wiley-VCH: New York, 2000. (29) Fendorf, S.; Eick, M. J.; Grossl, P.; Sparks, D. L. Arsenate and Chromate Retention Mechanisms on Goethite. 1. Surface Structure. Environ. Sci. Technol. 1997, 31, 315-320. (30) Jang, M.; Min, S. H.; Kim, T. H.; Park, J. K. Removal of Arsenite and Arsenate Using Hydrous Ferric Oxide Incorporated into Naturally Occurring Porous Diatomite. Environ. Sci. Technol. 2006, 40, 1636-1643. (31) Gu, X.; Evans, L., Surface Complexation Modelling of Cd(II), Cu(II), Ni(II), Pb(II)

32


Page 33 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Zn(II) Adsorption onto Kaolinite. Geochim. Cosmochim. Acta 2008, 72, 267-276. (32) Goldberg, S. Use of Surface Complexation Models in Soil Chemical Systems. Adv. Agron. 1992, 47, 233-329. (33) Herbelin, A. L.; Westall, J. C. FITEQL, a Computer Program for Determination of Chemical Equilibrium Constants from Experimental Data. Version 4.0. Chemistry Dept.,Oregon State University: 1999. (34) Luoma, S. N.; Davis, J. A. Requirements for Modeling Trace Metal Partitioning in Oxidized Estuarine Sediments. Mar. Chem. 1983, 12, 159-181. (35) Stumm, W. Chemistry of the Solid-Water Interface: Processes at the Mineral-Water and Particle-Water Interface in Natural Systems. John Wiley & Son Inc.: 1992. (36) Kraepiel, A. M. L.; Keller, K.; Morel, F. M. M. On the Acid-Base Chemistry of Permanently Charged Minerals. Environ. Sci. Technol. 1998, 32, 2829-2838. (37) Jia, K.; Pan, B. C.; Lv, L.; Zhang, Q. R.; Wang, X. S.; Pan, B. J.; Zhang, W. M. Impregnating Titanium Phosphate Nanoparticles onto a Porous Cation Exchanger for Enhanced Lead Removal from Waters. J. Colloid Interface Sci. 2009, 331, 453-457. (38) Chen, C. L.; Hu, J.; Xu, D.; Tan, X. L.; Meng, Y. D.; Wang, X. K. Surface Complexation Modeling of Sr(II) and Eu(III) Adsorption onto Oxidized Multiwall Carbon Nanotubes. J. Colloid Interface Sci. 2008, 323, 33-41. (39) Ngantcha, T. A.; Vaughan, R.; Reed, B. E. Modeling As(III) and As(V) Removal by an Iron Oxide Impregnated Activated Carbon in a Binary Adsorbate System. Sep. Sci. Technol. 2011, 46, 1419-1429.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

(40) Mudunkotuwa, I. A.; Rupasinghe, T.; Wu, C.-M.; Grassian, V. H. Dissolution of ZnO Nanoparticles at Circumneutral pH: A Study of Size Effects in the Presence and Absence of Citric Acid. Langmuir 2012, 28, 396-403. (41) Liu, J.; Aruguete, D. M.; Murayama, M.; Hochella Jr, M. F. Influence of Size and Aggregation on the Reactivity of an Environmentally and Industrially Relevant Nanomaterial (PbS). Environ. Sci. Technol. 2009, 43, 8178-8183. (42) Bian, S. W.; Mudunkotuwa, I. A.; Rupasinghe, T.; Grassian, V. H. Aggregation and Dissolution of 4 nm ZnO Nanoparticles in Aqueous Environments: Influence of pH, Ionic Strength, Size, and Adsorption of Humic Acid. Langmuir 2011, 27, 6059-6068. (43) Boily, J. F. Water Structure and Hydrogen Bonding at Goethite/Water Interfaces: Implications for Proton Affinities. J. Phys. Chem. C 2012, 116, 4714-4724. (44) Abbas, Z.; Labbez, C.; Nordholm, S.; Ahlberg, E. Size-Dependent Surface Charging of Nanoparticles. J. Phys. Chem. C 2008, 112, 5715-5723. (45) Vayssières, L.; Chanéac, C.; Tronc, E.; Jolivet, J. P. Size Tailoring of Magnetite Particles Formed by Aqueous Precipitation: An Example of Thermodynamic Stability of Nanometric Oxide Particles. J. Colloid Interface Sci. 1998, 205, 205-212.

34


Page 35 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of contents (TOC):

35


Surface Chemistry of Nanosized Hydrated Ferric Oxide Encapsulated

Recommend Documents