Recycling Spent Cr Adsorbents as Catalyst for Eliminating

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Remediation and Control Technologies

Recycling Spent Cr-Adsorbents as Catalyst for Eliminating Methylmercaptan Dedong He, Liming Zhang, Yutong Zhao, Yi Mei, Dingkai Chen, Sufang He, and Yongming Luo Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06357 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Environmental Science & Technology

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Recycling Spent Cr-Adsorbents as Catalyst for Eliminating Methylmercaptan

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Dedong He,†,‡ Liming Zhang,† Yutong Zhao,† Yi Mei,‡ Dingkai Chen,† Sufang He,§ and

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Yongming Luo,*,†

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†

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Technology, Kunming 650500, P. R. China.

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‡

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Kunming 650500, P. R. China.

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§

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Technology, Kunming 650093, P. R. China.

Faculty of Environmental Science and Engineering, Kunming University of Science and

Faculty of Chemical Engineering, Kunming University of Science and Technology,

Research Center for Analysis and Measurement, Kunming University of Science and

10 11 12

* Corresponding author

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Tel: +86-871-65103845

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Fax: +86-871-65103845

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E-mail: [email protected] (Y. Luo)

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Table of Contents (TOC)

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Abstract

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Waste adsorbents generated from treating Cr(VI) containing wastewater are hazardous

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materials and generally landfilled or treated by acid or base desorption, with concomitant

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high cost and toxic effects. The present work shows that these Cr-adsorbents can be reused

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as highly efficient catalysts for treating sulfur-containing VOCs (CH3SH), not only

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avoiding the economic and environmental impact from the conventional approaches, but

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also achieving the efficient treatment of sulfur-containing waste gas. Importantly, these

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reused Cr-adsorbents exhibit enhanced activity and stability compared with the catalysts

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reported elsewhere, indicating a new avenue of green chemistry. The highly toxic adsorbed

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Cr(VI) species are reduced to a Cr2O3 crystalline phase by calcination and finally

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immobilized as a Cr2S3 solid phase while converting and eliminating CH3SH. Still, the

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presence of Cr(VI) species on the reused Cr adsorbent provides enough reactive sites for

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reaction, but high concentration of Cr(VI) species causes serious accumulation of coke

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deposit on the catalyst, leading to fast deactivation of the catalyst.

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Keywords: Waste Cr Adsorbents; Reuse and Recycle; Highly Efficient Catalyst; CH3SH

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Catalytic Elimination.



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1. INTRODUCTION

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Hexavalent chromium (Cr(VI)) combines high toxicity, potential carcinogenicity and

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mutagenicity and poses a significant threat to human health.1,2 Various treatment strategies,

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such as adsorption, have been extensively employed for Cr(VI) removal from wastewater

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effluents, due to its simplicity and low cost.3,4,5,6,7 One of the most promising adsorbents for

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Cr(VI) treatment are an organic-inorganic modified silicon material, MCM-41 with good

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mechanical stability and high adsorption capacity.8,9,10 However, after adsorption, the

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question of how to dispose of adsorbents after usage cannot be avoided.

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Present disposal strategies proposed for the adsorbents after usage are either landfilling11,12

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or desorption by acid and base treatment.6,13,14,15 Landfill is often the preferred option and

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the most direct and easy method to tackle the problem of the generated adsorbents. Yet,

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these chromium-loaded adsorbents may generally exhibit toxic effects on human health and

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environment.16 Complete desorption of chromium from theadsorbents, therefore, is a

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necessity before these are released. An acid and base treatment is suggested as a feasible

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method.6,13,14,15 However, the required disposal of the spent acid and base solution, with a

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high concentration of Cr(VI) is a major disadvantage. It is therefore important to develop

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an alternative strategy to dispose Cr-adsorbents.

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In recent decades, the use of Cr-MCM-41 has attracted interest as a catalyst.17,18,19,20

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Generally, the presence of Cr(VI) species introduces favorable redox properties through the

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system of Cr(VI)/Cr(III).19 Meanwhile, the Cr(VI) species are related to the development of

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Brönsted centers, while the Cr(III) centers contribute to Lewis acidity.21 Our recent

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work22,23,24,25 and other reports26,27 have shown that redox and acidity properties may play


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an important role in any methyl mercaptan (CH3SH) catalytic elimination system. This

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compound is representative for sulfur-containing volatile organic compounds (SVOCs),

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which is harmful to human health and causing serious smell problems.28 The treatment of

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VOCs has gained considerable attention in environmental protection areas.29

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Based on this background, an alternative way of disposing and reusing of Cr-adsorbents is

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proposed in this paper. A Cr-MCM-41 sample with high catalytic performance for CH3SH

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elimination can be obtained directly by calcinating Cr-adsorbents in air. Hence, the

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environmental impact of landfilling or desorption by using acid and base treatment are

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avoided, and a high value catalyst and reactant is created. To the best of our knowledge,

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such recycling of Cr-adsorbents as high performance catalyst has not been reported in

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previous studies. Actually, also so far there has been no report proposing Cr-MCM-41

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catalysts for CH3SH elimination. To our surprise, the Cr-MCM-41 samples obtained from

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Cr-adsorbents exhibit enhanced activity and stability, compared with all other

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catalysts22,23,24,25,26,27,30,31 reported for CH3SH treatment. Therefore, we analyze the physical

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and chemical properties of the obtained Cr-MCM-41 catalyst and appreciate the reason

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why this reused Cr-adsorbent demonstrates such high catalytic performances for CH3SH

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elimination, on the basis of characterization results.

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2. MATERIALS AND METHODS

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2.1 Chemicals and Materials

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Potassium

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tetraethylorthosilicate, sodium hydroxide, and other chemical reagents were purchased

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from Shanghai Chemical Reagent Company of China. All chemicals were of at least

dichromate,

ammonium

chromate,

cetyltrimethylammonium


bromide,


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analytical grade and used without further treatment. Methyl mercaptan (1 vol. % CH3SH in

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N2) was obtained from Dalian Special Gases Co. Ltd., China.

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2.2 Reusing Cr-Adsorbents

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Details on the synthesis of adsorbents and the chromium column adsorption experiment are

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recorded in the Supporting Information. The Cr-adsorbents were directly calcined in air at

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550 oC for 6 h to obtain the Cr/MCM-41 catalyst ready for use. A schematic pathway for

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preparing and reusing the Cr-adsorbents is suggested in Scheme 1.

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For comparison purposes, the fresh adsorbents (without any chromium species) were

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calcined at 550 oC for 6 h as blank catalyst supports (MCM-41). Then, K2Cr2O7 and

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(NH4)2CrO4 were loaded on the calcined fresh adsorbent by the conventional incipient

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wetness impregnation method. Details are recorded in Supporting Information.

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2.3 Characterization

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Details on catalyst characterization are shown in Supporting Information.

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2.4 CH3SH Catalytic Elimination Experiment

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The CH3SH catalytic elimination experiment was carried out in a fixed-bed micro-reactor

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at atmospheric pressure. An amount of 0.2 g of catalyst was loaded into a quartz tube

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reactor. Then 1 vol. % CH3SH in N2 was introduced into the reaction system with flow rate

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of 30 mL/min. The reaction temperature was increased from 350 to 600 oC in 25 oC

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increments (from 350 to 400 oC) and 50 oC increments (from 400 to 600 oC), and held for

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0.5 h for stabilization. The stability test was carried out at 500 oC with 0.2 g of CeO2 or

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reused Cr adsorbent and 0.4 g of HZSM-5. The reactants and products were analyzed by

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online GC (gas chromatograph) equipped with FID, TCD and FPD detectors. CH3SH 6 ACS Paragon Plus Environment

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conversion was calculated by the following equation:

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CH3SH Conversion =

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C[CH3SH]in represents the initial concentration of CH3SH before the reaction, while C[CH3SH]out

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is the remaining ones after the reaction.

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2.5 Catalyst Regeneration

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Regeneration of the spent catalyst (after reaction) was carried out at 500 oC for 2 h, by

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introducing 30 mL/min air flow, and then catalytic reactivity of the regenerated

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Cr-adsorbent was measured.

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3. RESULTS AND DISCUSSION

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3.1 Catalyst Performance

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The evaluation of catalytic activity and stability on the elimination of CH3SH is shown in

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Figure 1(A) and (B), respectively. Figure 1(A) shows that the reused Cr adsorbent is more

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active than reported for CeO222,23,30,31 and HZSM-524,25,27,28 catalysts. At 400 oC, reused Cr

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adsorbent exhibits the highest activity and its CH3SH conversion is almost 100%, while

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about 93% and 64% are obtained over CeO2 and HZSM-5 catalysts.

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Moreover, a remarkable stability of the reused Cr adsorbent is also observed. Figure 1(B)

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shows that the reused Cr adsorbent maintains nearly 100% of its initial activity for 90 h

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when the reaction is carried out at 500 oC, while the corresponding time for the reported

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CeO2 and HZSM-5 catalysts is only 10 and 15 h, respectively. Thus, the reused Cr

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adsorbent demonstrates higher stability than both CeO2 and HZSM-5 that are reported

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previously22,23,24,25,27,28,30,31. The above findings indicate that for the same reaction

C[ CH 3SH ]in − C[ CH 3SH ]out × 100% C[ CH 3SH ]in



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conditions, our reused Cr adsorbent presents an enhanced activity and stability for CH3SH

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catalytic elimination.

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The reason for the reused Cr adsorbent shows much higher stability in comparison with

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CeO2 and HZSM-5 is stated as follows: on the one hand, active Cr sites can be well

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dispersed on the MCM-41 support with high surface area. On the other hand, due to

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weak/no acidity of MCM-41, small amount of coke deposit can be formed and accumulated

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on catalyst (Figure S13 in Supporting Information). All these should be responsible for

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high stability of the reused Cr adsorbent.

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3.2 Catalyst Characterization

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The low-angle XRD pattern of the reused Cr adsorbent displays three diffraction peaks in

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the 2θ range of 1-5o (Figure S1 in Supporting Information), which can be indexed to the

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(100), (110) and (200) reflexes and are characteristic of hexagonal mesostructures.32

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Therefore, XRD results suggest that the ordered structure of MCM-41 is retained after the

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reuse. To some extent, however, Cr-incorporation may destroy the ordered arrangement of

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the MCM-41 structure,33 as confirmed by the TEM image in Figure S2 (Supporting

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Information). The above results illustrate that the reused Cr adsorbent basically preserves

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its original structural features.

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The Cr 2p XPS spectra for the reused Cr adsorbent and the calcined fresh adsorbents

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(MCM-41) are shown in Figure 2(A). The Cr 2p signals at binding energy (BE) values of

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577 and 586 eV are assigned to trivalent chromium Cr(III) species, whereas those at BE

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values of 579 and 588 eV are attributed to hexavalent chromium Cr(VI) species.34,35

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Considering the oxidation state of the introduced chromium (K2Cr2O7, see Supporting 8 ACS Paragon Plus Environment

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Information) from the adsorption process is Cr(VI), the presence of both Cr(VI) and Cr(III)

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herein obviously indicates the reduction of Cr(VI) to Cr(III) occurs. To analyze this

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reduction process, a comparative FT-IR and XPS (N 1s) study of the reused Cr adsorbent

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before and after calcination is presented (note that the adsorbents are not calcined before

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conducting Cr adsorption experiments, which is different from the calcined fresh

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adsorbents, see Scheme S1 in Supporting Information). Figure S3A (Supporting

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Information) displays the FT-IR spectra. It is found that three peaks at about 2923, 2854

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and 1473 cm-1, respectively corresponding to the stretching and bending vibrations of -CH3,

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-CH2 and N-C, are observed in the spectrum of the Cr adsorbent before calcination, but not

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in the spectrum of the calcined Cr adsorbent. Furthermore, as demonstrated from the N1s

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spectra (Figure S3B in Supporting Information), the total nitrogen amount is substantially

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reduced after calcination, suggesting interaction of the positively charged ammonium group

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with the electron-withdrawing Cr(VI) species (e.g. HCrO4-, Cr2O72-) due to the electrostatic

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interaction.36,37 After that, when a proton of the ammonium group is released, the reduction

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of Cr(VI) to Cr(III) occurs.38 This assumption is consistent with the result exhibited in

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Figure S3B (Supporting Information), where the peak intensity for the group of (-NH3)+

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decreases while it for the group of (-NH2) increases. This conclusion is well in accordance

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with the reported results.38 Therefore, the above results can generally be applied to explain

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the existence of Cr(III) species on the reused Cr adsorbent, although the introduced

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chromium source is Cr(VI) from K2Cr2O7 (See Scheme S2 in Supporting Information).

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In order to verify the above conclusions and also for purposes of comparison, K2Cr2O7 and

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(NH4)2Cr2O7 were directly impregnated on the calcined fresh adsorbents (MCM-41). An 9 ACS Paragon Plus Environment


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examination of Cr 2p spectra (Figure S4A in Supporting Information) reveals that the major

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oxidation state of the chromium species formed on the K2Cr2O7 impregnated sample is

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Cr(VI). This is probably due to the absence of -CH3 and -NH3 etc. species on the calcined

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fresh adsorbents, which cannot provide the proton for Cr(VI) reduction. In contrast, as for

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the (NH4)2Cr2O7 impregnated sample, the peaks at 577 and 586 eV, which signify the

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presence of Cr(III), suggesting that the ammonium group causes Cr(VI) to be reduced,

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which is in conformity with the foregoing analysis. Besides, XRD measurements (Figure

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S4B in Supporting Information) confirm that both (NH4)2Cr2O7 impregnated sample and

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the reused Cr adsorbent show the development of Cr2O3 crystalline phase,32 whereas in the

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pattern corresponding to the K2Cr2O7 impregnated sample, one sharp diffraction peak

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centered around 2θ = 26o indicates the presence of the CrO3 phase.21 These XRD

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observations are consistent with the XPS results and also verified by the UV-vis DR

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analysis. Figure S4C (Supporting Information) shows the (NH4)2Cr2O7 impregnated sample

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as well as the reused Cr adsorbent displays absorption above 550 nm due mainly to Cr(III)

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species in Cr2O3 particles,38 which is not detected in the K2Cr2O7 impregnated sample.

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Hence, the above XPS, XRD and UV-vis DR investigations evidence the reduction process

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of Cr(VI) to Cr(III) towards the reused Cr adsorbent.

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Figure 2(B) illustrates the O 1s spectra of the reused Cr adsorbent and the calcined fresh

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adsorbents (MCM-41). For the MCM-41 sample, symmetric O 1s peak is seen at a BE

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value of 533 eV, likely resulting from the contribution of oxygen from the silica

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support.34,39 Over the reused Cr adsorbent sample, there is additional signal with BE value

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of 530 eV attributable to the surface lattice oxygen of Cr2O3.34,39 Thus O 1s measurement 10 ACS Paragon Plus Environment

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confirms the presence of agglomerated metal chromium oxides on the reused Cr adsorbent.

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Figure 2(C) shows the high-angle XRD patterns obtained for the reused Cr adsorbent and

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the fresh MCM-41sample. From Figure 2(C), a broad band located at 2θ ≈ 25o is

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characteristic of amorphous silica.21 Furthermore, for the sample of reused Cr adsorbent,

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XRD exhibits nine sharp diffraction peaks, which are ascribed to the presence of Cr2O3

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crystalline phase.32 These XRD investigations concur with the results of XPS. Figure 2(D)

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exhibits the UV-vis DR spectra of the reused Cr adsorbent and the fresh MCM-41sample.

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One can observe three broad absorption peaks for the reused Cr adsorbent sample. In

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general, the regions at 200-400 and 400-550 nm are attributed to the Cr(VI) ions in the

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monochromate and di/polychromates species, respectively.32,38 Meanwhile, the region at

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higher wavelengths (above 550 nm) could be assigned to the formation of crystalline

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Cr2O3.32,38 Therefore, the UV-vis DR results indicate the existence of crystalline Cr2O3 on

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the reused Cr adsorbent sample, again in agreement with the mentioned XPS and XRD

200

investigations.

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H2-TPR profiles of the reused Cr adsorbent and the fresh MCM-41 catalyst samples in the

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temperature range of 100-800 oC are given in Figure 2(E). In the case of the reused Cr

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adsorbent sample, two reduction peaks appear at about 320 and 460 oC as a result of the

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reduction of Cr(VI) species (2CrO3 + 3H2 = Cr2O3 + 3H2O) interacting differently with the

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support.19,32,38 The first peak could correspond to the reduction of di/polychromates Cr(VI)

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species, and the second one could be due to the reduction of monochromates Cr(VI)

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species.32,40 As shown in Figure S4D (Supporting Information), the K2Cr2O7 impregnated

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sample shows an obvious reduction peak since the Cr species present on this sample is



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mainly Cr(VI), as proved by the mentioned characterization results of XRD, XPS and

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UV-vis. Moreover, from Figure S4D (Supporting Information), it is noted that (NH4)2Cr2O7

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impregnated sample demonstrates increased intensity for the first reduction peak than that

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of the reused Cr adsorbent, suggesting that more monochromates Cr(VI) species are formed

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on the reused Cr adsorbent. In fact, it is accepted that the distribution of the adsorbed Cr

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species from the column adsorption experiment would be more uniform than that from the

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traditional impregnation process, thus contributing to the more monochromates Cr(VI)

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species on the adsorbents.

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The acidic properties of the fresh MCM-41 and the reused Cr adsorbent samples were

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investigated by using the NH3-TPD method, and the corresponding results are collected in

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Figure 2(F). The NH3-TPD profile for the reused Cr adsorbent exhibits a low-temperature

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peak at about 220 oC attributed to weak Lewis type acid sites, and a high-temperature peak

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above 600 oC, ascribed to the presence of Brönsted acid sites.41,42 This location in acidity is

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in good agreement with the reported results,43 whereas the K2Cr2O7 impregnated sample

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(mainly containing Cr(VI) species) shows the presence of the high-temperature peak only

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(Figure S4E in Supporting Information). Previous reports have revealed that the

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development of Lewis acidic centers is directly related to Cr(III) species, while the

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presence of Cr(VI) induces the appearance of Brönsted acidic centers.21,44 Hence, our

227

NH3-TPD results confirm this conclusion.

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Meanwhile, catalytic stability tests over the synthesized catalysts (K2Cr2O7/(NH4)2Cr2O7

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impregnated MCM-41 samples and the reused Cr adsorbent) are shown in Figure S5

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(Supporting Information). Clearly, the K2Cr2O7 impregnated MCM-41 sample exhibits 12 ACS Paragon Plus Environment

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lower catalytic stability, since CH3SH conversion after 30 h time on stream decreases to

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less than 50%. Conversely, a (NH4)2Cr2O7 impregnated MCM-41 sample reveals enhanced

233

catalytic stability, and CH3SH conversion can maintain 100% after 50 h of exposure time,

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though still lower than that of our reused Cr adsorbent. As discussed before, the distribution

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of adsorbed Cr(VI) species on reused Cr adsorbent is more uniform than that of the

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traditional impregnation samples. As expected, our reused Cr adsorbent exhibits better

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catalytic performance than the impregnated one. However, K2Cr2O7 impregnated MCM-41

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sample with high concentration of Cr(VI) species (Table S1 and Figure S4A in Supporting

239

Information) shows very poor catalytic performance. As known, this sample contains high

240

amount of Brönsted acidic centers contributed by the Cr(VI) species (as proved by the XPS,

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H2-TPR and NH3-TPD investigations). As a matter of fact, Brönsted acid sites may result in

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the formation of coke and catalyst deactivation during the dehydrogenation and/or cracking

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of hydrocarbons.45,46 Therefore, it is deduced that the poor catalytic stability of the K2Cr2O7

244

impregnated MCM-41 sample is to be attributed to the severe accumulation of coke

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deposits during the reaction. As exhibited in Figure S6 (Supporting Information), the band

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intensity corresponding to the presence of coke deposit46,47 over the spent K2Cr2O7

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impregnated MCM-41 sample is higher than that on other spent samples after the same

248

exposure time (30 h), providing evidence for the deactivation of K2Cr2O7 impregnated

249

MCM-41 catalyst and verify the above conclusions.

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On the whole, the results from the aforementioned analysis indicate that the presence of

251

Cr(VI) on reused Cr adsorbent can provide enough reactive sites for CH3SH catalytic

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elimination (as indicated and analyzed in Figure S7 and Figure S8 in Supporting



253

Information). Nevertheless, a high concentration of Cr(VI) species may cause fast

254

deactivation of the catalyst (e.g. K2Cr2O7 impregnated MCM-41 sample), and as a result,

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serious accumulation of coke deposit is observed on this catalyst.

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3.3 Analysis of the Spent Catalyst and Catalyst Regeneration

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The XRD patterns and Cr 2p XPS spectra of the fresh (the reused Cr adsorbent) and the

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spent (the reused Cr adsorbent after reaction) catalyst samples are presented in Figure 3A

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and Figure 3B, respectively. From Figure 3A, several new diffraction peaks assigned as the

260

formation of Cr2S3 phase (JCPDS 82-1545) are found in the XRD patterns over the spent

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catalyst. Furthermore, as obtained from Figure 3B, the relative percent content of Cr(III) on

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the spent catalyst sample is increased to 89.33% compared to the fresh one (62.27%). To

263

our surprise, the formation of Cr2S3 phase on the spent catalyst illustrates that the present

264

reaction system can achieve the goal for the immobilization of both chromium and sulfur

265

species, which reflects important environmental significance. Moreover, to analyze how did

266

CH3SH interact with active Cr species on adsorbent surface, in situ FTIR technique was

267

used to study adsorption and transformation behaviors of CH3SH over the reused Cr

268

adsorbent, and FTIR spectra is shown in Figure S9 (as analyzed in Supporting Information).

269

Besides, the mechanism involving immobilization of Cr2O3 by thiol groups onto Cr2S3 is

270

suggested in Figure S10 (Supporting Information).

271

Furthermore, this spent catalyst is regenerated and the catalytic reactivity of the regenerated

272

Cr-adsorbent is displayed in Figure S11. It indicates that the regenerated sample can be

273

recovered to its original reactivity. Meanwhile, from Figure 3C and 3D, it is seen that the

274

regenerated Cr-adsorbent shows characteristic diffraction peaks of Cr2O3, and the relative 14 ACS Paragon Plus Environment

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percent content of Cr(VI) on the regenerated sample is increased to 48.12%. Besides, no

276

accumulation of coke deposit and sulfur species is observed on regenerated Cr-adsorbent.

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Therefore, it is deduced that spent reused adsorbent is to be regenerated for the case that

278

they are intended for recycle.

279

3.4 Effects of Other Chemical Species in Wastewaters

280

In order that the results might be applicable for real spent Cr-adsorbents, different initial

281

concentrations (20, 50, 100, 200 and 300 mg/L) of Cr(VI) have been applied for the

282

synthesis of Cr-adsorbents, and the influences of various chemical species (coexisted

283

anion/cation ions) present in real wastewaters are considered. From Figure 4A, the reused

284

Cr-adsorbents synthesized from different concentrations of Cr(VI) show similar reactivity

285

for CH3SH elimination, revealing that initial concentration of Cr(VI) has no significant

286

influence on catalytic activity of the reused Cr-adsorbents. From Figure 4B, it is seen that

287

coexisted cation ions has no influence on activity while sample synthesized from coexisted

288

anion ions exhibits slightly decreased reactivity, but it still higher than the reported

289

HZSM-5 catalyst. Hence, the above investigations indicate that it can be applicable for real

290

spent Cr-adsorbents, which highlights its application. Besides, the effects of these other

291

chemical species on physicochemical properties of the reused Cr-adsorbent are also

292

investigated, and results are shown in Figure S12 (as analyzed in Supporting Information).

293

3.5 Environmental Implications

294

Large amounts of waste Cr adsorbents are generated during the removal of Cr(VI) from

295

wastewater. Disposal options include landfilling and acid and base desorption treatment

296

generally show toxic effects on environment. Therefore, the exploration of the reuse of the



297

spent Cr-adsorbents as a highly efficient catalyst for sulfur-containing VOCs (CH3SH)

298

catalytic elimination has important environmental implications. Our results suggest that the

299

highly toxic Cr(VI) contamination could finally be immobilized as Cr2S3 solid phase (as

300

reported4, Cr(III) is 100 times less toxic than Cr(VI)). Furthermore, CH3SH can be

301

selectively transformed into value added hydrocarbons (including CH4, olefins and BTX),

302

hydrogen sulfide (the produced H2S can be commercially converted to elemental sulfur

303

from Claus technique) and CS2 (the produced CS2 can be used for the production of

304

sulfuric acid and applied as a raw material for fine petrochemicals48) over the reused Cr

305

adsorbents (as shown in Figure S14 in Supporting Information). More importantly, the

306

obtained catalysts from the spent Cr-adsorbents show better performance than the reported

307

catalysts. Meanwhile, the study of effects of other chemical species indicates that it can be

308

applicable for real spent Cr-adsorbents. Besides, the spent reused adsorbent can be

309

regenerated for the case of recycle. All these indicated that the recycle of the Cr-adsorbents

310

as a highly efficient catalyst for CH3SH catalytic elimination has great potential in treating

311

waste Cr adsorbents and is therefore of great environmental importance. Furthermore, the

312

reused Cr adsorbents of the present work can also be applied to other reactive systems (e.g.

313

catalytic dehydrogenation of propane to propene).

314

Acknowledgements

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The National Natural Science Foundation of China (U1402233, 21667016 and 21367015)

316

is gratefully acknowledged for financial support to this research work.

317

Supporting Information Available

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Details about synthesis of materials (adsorbents, K2Cr2O7/MCM-41 and (NH4)2CrO4


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impregnated MCM-41), and chromium column adsorption experiment are available as

320

Supporting Information. Characterization results of the fresh and spent samples and

321

catalytic stability tests are shown. The schematic mechanism of CH3SH removal by spent

322

Cr-adsorbents is illustrated.

323

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Scheme 1. Schematic pathway for preparing and recycling the Cr-adsorbents.

501 502 503 504 505 506 507 508 509 510 511


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513 514 515 516

Figure 1. The evaluation of (A) activity and (B) stability on the catalytic elimination of CH3SH.



517

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Figure 2. (A) and (B) Cr 2p and O 1s XPS spectra, (C) high-angle XRD patterns, (D) UV-vis DR spectra, (E) H2-TPR profiles and (F) NH3-TPD profiles of the reused Cr adsorbents and the fresh MCM-41sample.



525 526

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Figure 3. (A) The XRD patterns, (B) Cr 2p XPS spectra of the fresh (the reused Cr adsorbent) and the spent (the reused Cr adsorbent after reaction) catalyst samples; (C) The XRD patterns, (D) Raman spectra, S 2p and Cr 2p XPS spectra of the spent and regenerated catalyst samples. 29 ACS Paragon Plus Environment


537 538

539 540 541 542

Figure 4. Effect of (A) initial concentrations of Cr(VI), (B) coexisted ions (initial concentrations of Cr(VI) is 20 mg/L) on the catalytic activity of the reused Cr adsorbents. 30 ACS Paragon Plus Environment

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Recycling Spent Cr Adsorbents as Catalyst for Eliminating

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