Impact of Pore Size on Fenton Oxidation of Methyl Orange Adsorbed

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Impact of pore size on Fenton oxidation of methyl orange adsorbed on magnetic carbon materials: Trade-off between capacity and regenerability Ye Xiao, and Josephine M. Hill Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00089 • Publication Date (Web): 29 Mar 2017 Downloaded from http://pubs.acs.org on March 30, 2017

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Environmental Science & Technology

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Impact of pore size on Fenton oxidation of methyl orange adsorbed on magnetic

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carbon materials: Trade-off between capacity and regenerability

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Ye Xiao, Josephine M. Hill*

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Department of Chemical & Petroleum Engineering, Schulich School of Engineering, University

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of Calgary, 2500 University Dr NW, Calgary, AB, Canada, T2N 1N4

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*Corresponding Author Tel: +1 403 210 9488; fax: 1 403 284 4852; e-mail: [email protected]

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1 ACS Paragon Plus Environment


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Abstract: The economic clean-up of wastewater continues to be an active area of research. In

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this study, the influence of pore size on regeneration by Fenton oxidation for carbon materials

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with adsorbed methyl orange (MO) was investigated. More specifically three carbon supports,

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with pore sizes ranging from mainly microporous to half microporous-half mesoporous to mainly

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mesoporous, were impregnated with γ-Fe2O3 to make them magnetic and easy to separate from

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solution. The carbon samples were characterized before adsorption and after regeneration with

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hydrogen peroxide at 20 °C. In addition, adsorption kinetics and isotherms were collected, and

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the Weber-Morris intraparticle diffusion model and Freundlich isotherm model fit to the data.

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The adsorption capacity increased with increasing microporosity while the regeneration

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efficiency increased with increasing mesoporosity. Further experiments with varying

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regeneration and adsorption conditions suggested that the regeneration process may be diffusion

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limited. The MO adsorbed in the micropores was strongly adsorbed and difficult to remove

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unlike the MO adsorbed in the mesopores, which could be reacted under relatively mild

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conditions. Thus, there was a trade-off between adsorption capacity and regeneration.

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Keywords: regeneration; Fenton oxidation; carbon; methyl orange; pore size


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1. Introduction

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Adsorption technologies, especially those using carbon materials such as activated carbon (AC)

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1

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been widely investigated for the treatment of water contaminated by organic pollutants. The

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adsorption process concentrates the organic pollutants on the carbon matrix, which can then be

34

disposed or regenerated. Because of the potential for the leaching of pollutants during the

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disposal process and high price of activated carbon (>$1500/ton 5), it may be more

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environmentally and economically beneficial to regenerate the used carbon materials. Thus, this

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study focuses on improving the regeneration process by studying the relationship between pore

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size and regeneration efficiency of three carbon-based adsorbents with different physical

39

structures.

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The current commercial thermal regeneration processes require high temperatures (800-900 °C)

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and suffer from loss of carbon, high energy intensity, and high cost 6-7. Desorption-based

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technologies may alleviate these problems 7-9; the adsorbates are desorbed intact by manipulating

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pH, solvent polarity, pressure, and temperature (lower than 300 °C). Many adsorbates, however,

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will not desorb at the lower temperatures and so degradation-based advanced oxidation

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technologies, of which Fenton oxidation is the most common, are used to mineralize the

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adsorbates. Although some novel electrochemical advanced oxidation methods reported over

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90% regeneration efficiencies 10-16, the regeneration efficiencies of Fenton oxidation (without

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electrochemistry) are generally less than ~60% 17-21, with exceptions for the treatment of volatile

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organic compounds 19, 22.

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Some improvements in regeneration efficiency for the Fenton oxidation process have been

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achieved by introducing UV light 18, increasing the Fe content 21, using an acid pretreatment 19,

, carbon nanotubes 2, three-dimensional graphene-based macrostructures 3, and biochar 4, have



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increasing temperature 19, and reducing particle size 20. Despite the improvements, the

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regeneration efficiencies were still less than 70% after one cycle. The loss of adsorption capacity

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was attributed to (i) damage of the carbon matrix by the strongly oxidizing hydroxyl radicals

55

(•OH), (ii) blockage of pores by the oxidation products, and (iii) incomplete removal of the

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adsorbates 8. The first two reasons are inconsistent with the often observed stable adsorption

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capacity after the first adsorption-regeneration cycle 23. That is, carbon deterioration and pore

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blockage should continuously decrease the adsorption capacity with each subsequent

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regeneration cycle. The third reason has been related to the slow diffusion of H2O2 within the

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activated carbon 20. Most adsorbents have high (>1000 m2/g) surface areas to maximize

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adsorption capacity, but these high surface areas can only be achieved with microporous

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materials, which will have diffusion limitations. Indeed, the highest regeneration efficiencies (>

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90%) were reported for mesoporous carbon materials 24-25 but a direct study of the role of pore

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structure has not been done.

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In this paper, we propose that the pore structure of the carbon materials play a critical role during

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the Fenton oxidation regeneration process. To test this hypothesis, three samples with different

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sized pores were cycled through adsorption and regeneration steps. The samples were powders

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with particle sizes below 0.1 mm that were impregnated with iron oxide to facilitate separation

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from solution. Methyl orange (MO) was chosen as the adsorbate because it is a typical organic

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pollutant likely to be irreversibly adsorbed at the testing temperatures (20 °C), removing the

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complication of desorption 26-28, and easily detected by ultraviolet/visible (UV/Vis)

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spectrophotometry. Adsorption isotherms and kinetics were collected, the regeneration and

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adsorption conditions varied, and the samples characterized at various stages throughout the

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experiments.


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2. Materials and methods

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2.1 Materials

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Mesoporous carbon (>99.95%, denoted as SMC) and activated charcoal (denoted as NORIT)

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were purchased from Sigma-Aldrich (St Louis, MO, USA), while activated carbon Colorsorb G5

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(denoted as G5) was obtained from Jacobi Carbon company (Columbus, OH, USA). These three

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carbon samples were modified using ferrous sulfate (FeSO4•7H2O, ≥ 99.0%, Anachemia,

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Montreal, QC, Canada), iron (III) chloride (FeCl3, 97%, Sigma-Aldrich), and sodium carbonate

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(Na2CO3, ≥ 99.5%, EM Science, Gibbstown, NJ, USA). Methyl orange (MO, C14H14N3NaO3S,

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ACS reagent grade, Ricca Chemical Company, Arlington, TX, USA) was the model organic

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pollutant. Regeneration was done with a hydrogen peroxide solution (H2O2, 30 wt% in water,

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Sigma-Aldrich) diluted to different concentrations.

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2.2 Preparation and characterization of magnetic samples

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The magnetic samples were prepared according to a method reported previously 29-30 with slight

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modifications. First, ~0.19 g of FeCl3 and ~0.66 g of FeSO4•7H2O were dissolved in 20 mL of

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water at 70 °C. The solution was maintained at this temperature with constant stirring while ~1.0

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g of carbon (SMC, G5, or NORIT) was added to form a suspension, which was subsequently

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stirred for 30 min, before the drop-wise addition of 60 mL of Na2CO3 solution (13.5 g/L). After 1

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h, the suspension was cooled to room temperature and aged for 24 h. The suspension was filtered

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and washed with deionized water. The solid on the filter paper was collected and dried at 50 °C

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overnight before storing. Iron oxide without carbon addition was also prepared by the same

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procedure. The solids were named as follows: Fe2O3, Fe2O3/SMC, Fe2O3/G5 and Fe2O3/NORIT

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and each contained approximately 20 wt% Fe, which was a sufficient loading to be able to easily

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magnetically separate them from solution. 5 ACS Paragon Plus Environment


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The crystal structures of the prepared samples were determined by an X-ray diffractometer

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(Multiflex, Rigaku, USA) with a scan rate of 2 °/min at 40 kV and 30 mA from 10° to 90° 2-

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theta. The particle size of the deposited iron oxide was estimated with the Scherrer equation. The

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physical properties of the samples were determined by N2 adsorption at -196 °C. The BET

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surface area and total pore volume were determined from the N2 adsorption isotherms within

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partial pressure ranges of 0.02-0.3 and 0.96-0.97, respectively. The pore size distributions and

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micropore volumes were derived from the N2 adsorption isotherms using the two-dimensional

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non-local density functional theory (2D-NLDFT) method. The metal loadings were determined

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with thermogravimetric analysis (SDT Q600, TA Instruments-Waters LLC, USA) by heating the

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samples (~ 6 mg) at 10 °C/min to 800 °C and holding at this temperature for 10 min in flowing

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air (50 mL/min) while the mass was monitored.

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2.3 Adsorption isotherms

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Methyl orange (MO) adsorption isotherms were collected using batch adsorption with an

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incubator shaker (VWR symphony 5000I, Henry Troemner LLC, USA). Several magnetic

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samples (8 mg) were weighed and placed in separate glass vials. Then 20 mL of MO solutions

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with different concentrations were added to the vials, which were then placed in the shaker and

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shaken at 250 rpm and 25 °C for 24 h. After adsorption, the magnetic samples were separated

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from the solution with a magnet, and the MO concentration in the solution determined by a UV-

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Vis spectrophotometer (EVOLUTION 220, Thermo scientific, USA). The experimental errors

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were less than 3.0% at a 95% confidence level based on duplicate experiments.

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2.4 Adsorption and Fenton oxidation regeneration


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In a typical experiment, the first step consisted of mixing 0.1 g of fresh magnetic sample and 100

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mL of MO solution in a beaker, and stirring the resulting suspension at 20 °C on a stir plate. To

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obtain the adsorption kinetics, 0.5 mL of solution was withdrawn at various times, and its MO

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concentration determined with the UV/Vis spectrophotometer. After 24 h of stirring, the

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suspension was separated with a magnet. The upper solution layer was decanted from the beaker

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leaving the saturated sample behind. Then 40 mL of deionized water was added to the beaker,

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and the pH of the suspension was adjusted to 3.0 by adding a dilute HNO3 solution. Next, 0.4 mL

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of 30 wt% H2O2 solution, which was sufficient as indicated by the small changes in H2O2

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concentration after regeneration (Figure S1), was added and the new suspension stirred at 20 °C

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for 8 h. The suspension was then magnetically separated, and the upper layer decanted. The

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remaining solid was the regenerated magnetic sample. The second cycle involved returning to

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the first step but using the regenerated sample rather than fresh sample. After regeneration, the

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prefix “R-” was added to the sample name (e.g. R-Fe2O3/SMC).

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3. Results and discussion

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3.1 Characterization of samples

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The XRD patterns (Figure S2 in Supporting Information) of the prepared samples had six peaks

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at 30.4°, 35.7°, 43.4°, 53.8°, 57.3°, and 62.9° 2-theta, corresponding to the (220), (311), (400),

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(422), (511), and (440), respectively, planes of maghemite (γ-Fe2O3). Maghemite is magnetic

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and the average particle sizes were 23 nm, 20 nm, and 18 nm on Fe2O3/SMC, Fe2O3/G5, and

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Fe2O3/NORIT, respectively, as calculated by the Scherrer equation using the MDI Jade 5.0

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software. The particle size was similar to that obtained by Oliveira et al. 29 for Fe2O3 supported

140

on Aldrich Darco G60 AC. The peak at 26.2° 2-theta in the XRD pattern of Fe2O3/SMC



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corresponds to the (002) plane of graphite (#08-0415). The patterns of the samples supported on

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G5 and NORIT have a peak at 21° 2-theta that may be associated with silica (#74-0201).

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Table 1 contains the physical properties of the samples and their metal oxide loadings. The BET

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surface areas, total pore volumes, and micropore volumes consistently increased in the following

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order: Fe2O3/SMC < Fe2O3/G5 < Fe2O3/NORIT. After the addition of Fe2O3, the surface areas

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and pore volumes decreased (except for the pore volume of the Fe2O3/SMC sample). The

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majority of this decrease was a result of the change in composition of the samples – the metal

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loadings were approximately 20 wt% and, thus, the specific surface areas were expected to

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decrease to ~80% of the values for the bare carbon supports if the added Fe2O3 only increased

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the mass and did not contribute to the porosity. The fact that the total pore volumes of the SMC-

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supported samples were identical (0.22 cm3/g) suggests that the added Fe2O3 was porous.

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Comparing the pore size distributions of Fe2O3/SMC and SMC (Figure S3), there were

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additional pores in the size ranges of 10-20 nm and 30-40 nm for the magnetic sample. As the

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Fe2O3 crystal sizes (18-23 nm) and metal loadings were similar (~20-22%) on all magnetic

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samples (Table 1 and Figure S4), the decreased pore volumes on the G5- and NORIT-supported

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samples may have partially been a result of pores being blocked by the added Fe2O3. Analysis of

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the XRD peaks only provides an average crystal size and not the distribution. Figure 1 illustrates

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the pore size differences of the magnetic samples. Fe2O3/SMC has mesopores with pores ranging

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in diameter from 3-20 nm and 28-50 nm, while Fe2O3/NORIT has mainly micropores 1-2 nm in

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diameter. Fe2O3/G5 had a mixture of micropores and mesopores up to ~20 nm. Both NORIT and

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G5 had some pores of diameter 28-50 nm but considerably fewer than SMC.

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The surface areas and pore volumes were lower after regeneration (again with the exception of

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the pore volume of the SMC-supported sample, Table 1 and Figure 1). The decreases for the 8 ACS Paragon Plus Environment

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Fe2O3/SMC sample were relatively small compared to the decreases for the other two samples.

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Blockage of the smallest pores is visible in the pore size distributions in Figure 1 (see insets in

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each graph). In addition, the measured ash content decreased after regeneration for the G5- and

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NORIT-supported samples. These results are consistent with MO being trapped in the

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micropores and increasing the combustible carbon content of the regenerated samples R-

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Fe2O3/G5 and R-Fe2O3/NORIT (Figure S4).

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3.2 Adsorption kinetics

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The rate of MO adsorption was determined on the fresh and regenerated magnetic samples as

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shown in Figure 2. Within ~30 min, 80% of the total MO adsorption capacity was reached on

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the fresh Fe2O3/NORIT and Fe2O3/G5 samples, and these adsorption capacities were 362 mg/g

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and 178 mg/g, respectively. After regeneration, 460 min and 230 min were required to reach

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80% of the total capacities and these capacities were reduced to 30.7 mg/g and 64.0 mg/g,

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respectively. In contrast, the adsorption on the Fe2O3/SMC sample was much faster, with almost

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complete adsorption in 30 min on both the fresh and regenerated samples but the total capacity

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was lower at 26.5 mg/g. These results confirmed that a mixing time of 1 day (1440 min) was

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sufficient to reach equilibrium and obtain the adsorption isotherms.

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The Weber-Morris intraparticle diffusion 31-32 model was applied to the data in Figure 2 and the

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resulting model parameters are given in Table 2. The model fit was good in terms of correlation

182

coefficients (R2 >0.92). According to the Weber-Morris adsorption kinetics model, if the

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adsorption process is only controlled by an intraparticle diffusion process, a straight line through

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the origin will be obtained by plotting versus . 33. For MO adsorption on all samples, these

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plots were not linear (Figure S5). When the collected data was divided into three steps, governed

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by film diffusion, mesopore diffusion, and micropore diffusion, consistent with previous dye 9 ACS Paragon Plus Environment


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adsorption studies 32, 34-35, the fits were linear in each step (Table 2 and Figure S5). The

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diffusion rate constants increased after regeneration for Fe2O3/SMC, while they decreased by 31-

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74% for Fe2O3/G5 and 64-97% for Fe2O3/NORIT, consistent with the previously discussed data

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(Table 1, Figures 1 and 2).

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3.3 Adsorption isotherms

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The adsorption isotherms for MO on the magnetic samples before and after regeneration are

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shown in Figure 3 (note the change in the y-axis range between Figures 3a and b). Consistent

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with the characterization data described above, the adsorption capacities of Fe2O3/NORIT and

195

Fe2O3/G5 reduced significantly after regeneration (Figure 3b). In contrast, the isotherms for

196

Fe2O3/SMC as prepared and after regeneration were essentially the same.

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As used in many research publications 10, 17, 23, 36, the regeneration efficiency was calculated with

198

the following equation:

199

% = × 100%

(1)

200

where is the adsorption capacity of fresh carbon (mg/g), is the adsorption capacity of

201

regenerated carbon (mg/g) and the conditions for adsorption are kept constant. As seen in Figure

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3, the isotherm shape can change after regeneration, which complicates the use of Eqn. (1) and

203

may lead to an overestimation of the regeneration efficiency. To avoid this issue, high initial MO

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concentrations of 100 mg/L, 400 mg/L, and 600 mg/L were used with Fe2O3/SMC, Fe2O3/G5,

205

and Fe2O3/NORIT, respectively, according to the isotherms (Figure 3) 36. Under these

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adsorption conditions (Figure 2), the calculated regeneration efficiencies were 97%, 36%, and

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8.5% for Fe2O3/SMC, Fe2O3/G5, and Fe2O3/NORIT, respectively. The regeneration efficiencies


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decreased in the order of Fe2O3/SMC > Fe2O3/G5 > Fe2O3/NORIT - the reverse order of their

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micropore volumes (Table 1).

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To quantify the change in isotherm shape after regeneration, the Freundlich and Langmuir

211

models were applied to the data in Figure 3 and the resulting parameters are given in Table 2

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and Table S1. The isotherm data was better fit by the Freundlich model according to the values

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of R2, which were between 0.86 and 1.0 for the Freundlich model and between 0.71 and 0.94 for

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the Langmuir model. The fits were significantly poorer for the R-Fe2O3/NORIT sample than for

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the other samples. The values of the surface heterogeneity indicator (7.7 < n < 17, Table 2)

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indicated the non-uniform nature of the adsorption sites on both fresh and regenerated magnetic

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samples 37, which is common for adsorption on carbon substrates 38. After regeneration, the value

218

of n decreased for the Fe2O3/SMC and Fe2O3/G5 samples, and increased for the Fe2O3/NORIT

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sample. Although the fit was poorer with the Langmuir model, the change in the separation

220

factor (RL) calculated for this model 37 was consistent with adsorption being less favourable after

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regeneration for all samples but more so for the G5- and NORIT-supported samples than for the

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SMC-supported samples. The similar adsorption capacities and isotherm parameters of SMC

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samples before and after regeneration indicated that the Fenton oxidation process had little effect

224

on the carbon surface properties. There were no visible changes in sample morphology indicated

225

by SEM (Figure S6), which is consistent with the literature that states little or no reduction

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adsorption capacity resulted from oxidation of the carbon surface 20, 39. Thus, the sites on which

227

adsorption was the strongest on the G5 and NORIT materials were not regenerated. Based on the

228

characterization data, these sites are located in the micropores in which the molecules would

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have more interaction with the carbon substrate, and hence, a stronger adsorption.

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3.4 Effect of adsorption and regeneration conditions 11 ACS Paragon Plus Environment


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The regeneration and adsorption conditions were varied to study their influence on the MO

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capacity and regeneration efficiencies and the results are shown in Figure 4 (note the different y-

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axis ranges in Figures 4a, b, and c). As the adsorption was fastest on the Fe2O3/SMC sample, the

234

regeneration time was reduced from 8 h to 4 h and to 1 h. As shown in Figure 4a, reducing the

235

time by a factor of two had a relatively small impact on the adsorption capacity while reducing

236

the time by a factor of eight had a much larger impact – capacities of 25.8 mg/g, 24.1 mg/g and

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13.6 mg/g, respectively. To confirm that the impact of desorption was minor, a regeneration

238

experiment with only deionized water (no H2O2) was performed (Figure S7). After desorption,

239

less than 5% of adsorbed MO was desorbed into the water for all the carbon samples.

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The regeneration time, initial H2O2 concentration, and volume of H2O2 solution (reported as a

241

ratio of H2O2 to MO) were increased separately for Fe2O3/G5 regeneration and all these factors

242

improved the regeneration efficiencies as shown in Figure 4b. A similar increase in adsorption

243

capacity (56 mg/g to ~76 mg/g) was achieved by increasing the regeneration time from 8 to 16 h

244

or the initial H2O2 concentration from 0.3 to 1.5 wt.%. Increasing the amount of available H2O2

245

(by increasing the volume of solution while maintaining the initial H2O2 concentration) resulted

246

in a smaller increase in adsorption capacity - 55.7 mg/g to 64.0 mg/g. In addition, the influence

247

of shaking speed on the regeneration efficiency was investigated (Figure S8). There were no

248

differences in the adsorption capacities of regenerated Fe2O3/G5 with (250 rpm) or without

249

shaking suggesting that the Fenton oxidation regeneration of MO saturated carbon was not

250

controlled by diffusion in the bulk solution.

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Finally, the MO concentration was varied with the Fe2O3/NORIT sample, which had the highest

252

initial (i.e., before regeneration) adsorption capacity. Increasing the initial MO concentrations

253

from 500 mg/L to 520 mg/L to 600 mg/L increased the adsorption capacities from 342 mg/g to 12 ACS Paragon Plus Environment

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351 mg/g to 362 mg/g, respectively, but also decreased the adsorption capacities after

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regeneration from 63.7 mg/g to 50.7 mg/g to 30.7 mg/g, respectively (Figure 4c). Note, the

256

regeneration conditions were varied within this set of experiments. A longer regeneration time

257

(20 h) was used for the second experiment (MO initial concentration of 520 mg/L), and a larger

258

volume of H2O2 solution (540 mL) was used for the third experiment (MO initial concentration

259

of 600 mg/L). The application of more rigorous regeneration conditions, which were effective

260

for Fe2O3/SMC and Fe2O3/G5, were not able to compensate for the additional MO adsorption on

261

the Fe2O3/NORIT sample. Extrapolating the adsorption isotherm for Fe2O3/NORIT (Figure 3) to

262

higher equilibrium concentrations, this sample should have additional adsorption capacities of

263

~40 mg/g, 39 mg/g, and 22 mg/g (i.e., equilibrium capacities at 440 mg/L, 470 mg/L, and 570

264

mg/L). Thus, even without regeneration, the sample would have adsorbed more MO on a second

265

exposure to the MO solutions, complicating the calculation of a regeneration efficiency and

266

possibly explaining some of the discrepancies in the literature.

267

3.5 Multiple adsorption-regeneration cycles

268

The Fe2O3/SMC and Fe2O3/G5 samples were tested for stability over multiple

269

adsorption/regeneration cycles and the results are shown in Figure 5. The adsorption capacity of

270

Fe2O3/SMC was 27 mg/g initially and then 24 mg/g and 23 mg/g in cycles 2 and 3, respectively

271

(Figure 5a). The regeneration between the third and fourth cycles was improved by leaving the

272

sample overnight after the 4 h regeneration and solution decanting. The solution remaining in the

273

pores continued the Fenton oxidation overnight removing more of the MO and resulting in

274

higher adsorption capacities of 32 mg/g after the fourth and fifth cycles. After the fifth cycle, the

275

original regeneration method was employed and the adsorption capacity in the sixth cycle was 25

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mg/g, which corresponds to a regeneration efficiency of 93%. The Fe2O3/G5 sample, which had 13 ACS Paragon Plus Environment


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more micropores than the Fe2O3/SMC sample, had a more significant decrease in adsorption

278

capacity in the first two cycles from 97 mg/g to 78 mg/g, with a more gradual decrease in

279

capacity between cycles 3 (73 mg/g) and 6 (65 mg/g) (Figure 5b).

280

3.6 Relationship between regeneration efficiency and pore structure

281

To better understand the effect of pore structure on the Fenton oxidation process, regeneration

282

efficiency was plotted versus microporosity (%) and the Weber-Morris intraparticle diffusion

283

rate constants from Tables 1 and S2 (Figure 6). The regeneration efficiency was inversely

284

proportional to microporosity (Figure 6a) - the higher the fresh sample microporosity, the less

285

effective the regeneration. The relationship between regeneration efficiency and the parameters

286

of the Weber-Morris interparticle diffusion model were varied (Figures 6b, c, and d). A linear

287

relationship between regeneration efficiency and

288

of 0.982), and there appeared to be a positive trend (R2 of 0.75) with

289

no trend with

290

strongly in the micropores and not being removed during the regeneration – an explanation that

291

is in line with the studies that report regeneration efficiencies of less than 60% for microporous

292

AC regenerated with Fenton oxidation 17-21, 40. As shown in Figure 1, the micropores were

293

blocked after regeneration.

294

Several studies state that electrostatic interactions govern the adsorption process for MO

295

especially when metals or metal oxides were involved 23, 41-42, but these interactions could not

296

explain the high MO adsorption capacity at high pH conditions when the adsorbent surface was

297

negatively charged 23, 43. Other mechanisms including hydrogen bonding, pore filling, and van

298

der Waals interactions were also proposed for MO adsorption 43-44. Dispersion forces could

!,%

!,#

(mesopore diffusion) was observed (R2 !,$ ,

(film diffusion) but

(micropore diffusion). These results are consistent with the MO binding


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299

dominate the interaction as MO and the surface groups on AC are dissimilar 45. Due to the

300

anisotropy of dispersion force as well as repulsive steric forces, the MO molecule would adsorb

301

with the orientation of closest distance to the carbon surface 45. In addition, for solution

302

concentrations below ~300 mg/L, MO anions tend to generate dimers 46. Adsorption of a MO

303

dimer could result in complete blockage of a micropore of ~1.6 nm in diameter according to the

304

MO molecular size (1.61 nm × 0.61 nm × 0.52 nm) 44, and leave the benzenesulfonate (-SO3-) or

305

dimethylanilinium (-N+(CH3)2) side exposed. This orientation is illustrated in Figure 7.

306

In the Fenton oxidation process, no ferric ions were detected in the solutions after 8 h

307

regeneration of the samples, which was consistent with literature reports that only a small

308

amount of Fe (

Impact of Pore Size on Fenton Oxidation of Methyl Orange Adsorbed

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