Study and identification of contaminant phases in commercial activated carbons

Journal Pre-proof Study and identification of contaminant phases in commercial activated carbons ´ M.E. C.F. Ramirez-Gutierrez, R. Arias-Niquepa, J.J. Pr´ıas-Barragan, Rodriguez-Garcia

PII:

S2213-3437(19)30759-6

DOI:

https://doi.org/10.1016/j.jece.2019.103636

Reference:

JECE 103636

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

5 November 2019

Revised Date:

12 December 2019

Accepted Date:

22 December 2019

Please cite this article as: C.F. Ramirez-Gutierrez, R. Arias-Niquepa, J.J. Pr\’ias-Barrag\’an, M.E. Rodriguez-Garcia, Study and identification of contaminant phases in commercial activated carbons, (2019), doi: https://doi.org/10.1016/j.jece.2019.103636

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Elsevier Editorial System(tm) for Journal of Environmental Chemical Engineering Manuscript Draft Manuscript Number: JECE-D-19-02202R1 Title: Study and identification of contaminant phases in commercial activated carbons Article Type: Research Paper Keywords: FTIR; BET; X-ray diffraction; methylene blue; charcoal Corresponding Author: Dr. Cristian Felipe Ramirez-Gutierrez, Ph.D.

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Corresponding Author's Institution: Universidad Nacional Autonoma de Mexico First Author: Cristian Felipe Ramirez-Gutierrez, Ph.D.

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Order of Authors: Cristian Felipe Ramirez-Gutierrez, Ph.D.; Rafael AriasNiquepa, M.Sc.; Jhon J Prías-Barragán, Ph.D; Mario E Rodriguez-Garcia, Ph.D

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Abstract: This work aims to identify and study contaminant phases present in commercial activated carbons (AC), also to determine the quality of the AC through methylene blue absorption. A straightforward methodology was implemented to carry out a complete characterization of AC that consists of the morphology determination by SEM, an elemental chemical composition by XRF, identification of crystalline phases using XRF, and vibrational analysis through FITR. The results showed that the primary contamination in AC is related to trace elements present in raw materials as well as residues from the activation process such as Mg, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, As, Sr, and Zr. It was observed that these elements could react during all steps of AC fabrication and form crystalline compounds, mainly calcium carbonates, calcium phosphates, and silicon oxides. Moreover, gas adsorption (BET) and methylene blue (MB) removal probes show that there is no correlation between the surface area and the MB removal capability.

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Research Data Related to this Submission -------------------------------------------------There are no linked research data sets for this submission. The following reason is given: Data will be made available on request

Cover Letter

UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO

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Editorial Board Journal of Environmental Chemical Engineering

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Querétaro, Qro., México, December 2019

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Dear editors, I am sending you the revised version of the manuscript titled: Study and identification of contaminant phases in commercial activated carbons, by C. F. Ramirez-Gutierrez, R. Arias-Niquepa, J.J. Prías-Barragán, and Mario E. Rodriguez-Garcia to be considered as an original paper to be published in Journal of Environmental Chemical Engineering

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This study is relevant because activated carbons are commonly used for water and air cleaning, as catalyst support, even in health and cosmetic application. So, the presence of other phases in commercial activated carbons can have critical effects on the above-mentioned applications.

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This version was revised and improved in accordance with the reefer’s comments. We very much appreciate that.

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On behalf of our team, my best regards

Cristian Felipe Ramirez-Gutierrez Centro de Física Aplicada y Tecnología Avanzada Universidad Nacional Autónoma de Mexico

*Conflict of Interest Form

Conflict of Interest and Authorship Conformation Form Please check the following as appropriate:

All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

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This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

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The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript

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Response to Reviewers

Queretaro, México. November 2019.

Guilherme Luiz Dotto, Ph.D Editor Journal of Environmental Chemical Engineering

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Concerning the manuscript entitled: Study and identification of contaminant phases in commercial activated carbons, it has been reviewed accordingly to the referee reports. We have worked on all the questions addressed by the referees. We are very grateful to the referees for their constructive criticisms and by making suggestions for improvements to our manuscript.

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Reviewer #2:

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Q1. More latest literature review can be added in introduction part including their proper references. Answer: We very much appreciate your comment. We included additional references to improve the discussion about manufacture, characterization and applications of activated carbon.

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Q2. The Paper can be accepted although some sentences can be shortened, and grammatical mistakes can be removed. Answer: Sections were rewritten and editing services revised and fix the language. Q3. More reference can be added to support the result and discussion part in every aspect of influencing factors. Answer: We add additional references to extend the discussion of contaminant phases in the AC quality.

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Detailed comments are attached in word file. 1. Referencing bracketing system needed to be in international style. Answer: All references were homogenized by using the latex template and bitext Elsevier citation style. 2. The document when analyzed on Plagiarisms software i.e turnitin, it is showing more than 28%.As per my view it must be lower down up to 15% or so for an article. Answer: All the paper was revised by using Plagiarism checker powered by Grammarly and the is no more than 1% of coinciding form sources on the web or in academic databases. On the other hand, here are coincidences in generic phrases related to the standard techniques and the procedures related to (i.e. XRF, XRD, FTIR, BET, and so on).

3. In reference section some references are de-shaped, may be due to formatting. They are also needed to be correction. Answer: All references were revised carefully and citation style was homogenized by using the latex template and bitext Elsevier citation style. 4. Pictures can be of more resolution in-order to make it more visible. Answer: Some images were improved in resolution and aspect ratio. 5. Introduction: The authors should describe the importance of their research more clearly. The references cited lacks articles on dye and phenol contaminants from last year. So, add more references (2014-2019) to support the author's points of view.

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Answer: In the introduction section we added additional references related to wastewater treatment and the issues related to “dirty” activated carbon. However, due to the bunch of types of activated carbon it is difficult to determine the real source of commercial activated carbon and activation process used. Also, it is well known that its physicochemical properties depend mainly on its origins. So, there are few works related specifically with quality tests of commercials AC. This point makes our work relevant.

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6. The author should have introduction not more than one and half pages including the recent studies. 1)Introduction: The author is needed to reduce down the introduction section to one and half page. The current length is too much lengthily. It is also suggested to write something about paper and its main highlight in the last paragraph of introduction section. Answer: Dear Editor, I believe that my introduction covers all fundamental aspects of the AC, for this reason I consider that it is not possible to shorten it.

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7. Samples selection and conditioning: In this section details of chemicals needed with proper grade needed to be mentioned.

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Answer: In the case of studied samples, six commercial activated carbons were used: charcoal activated by Merck-Germany, activated carbon by J.T. Baker, Netherlands, activated charcoal by Sigma Aldrich C9157, (cell culture tested) Germany, activated carbon (Vegetal), Sigma Aldrich, Mexico, activated carbon Hycel, Mexico, and activated carbon Golden Bell, Mexico. These were labeled: M1, B1, S1, S2, H1, and G1, respectively. All samples are powders and identification cards do not provide information about trace materials or common contaminants. So, they are considered as ACS Reagent grade. In the section Samples selection and conditioning, we clarify this fact. For MB removal test a Sigma-Aldrich Methylene Blue hydrate (≥97.0\%) was used.

8. Surface morphology: The authors have described in very good manner but if it was done in continuation then it would be more defining for this section. The Figure 1 and 2, needed to be separated from each other in-order to make them more readable. Some statements in

section 3.2, like SEM testing methods, pretty much unclear. It needed to be mentioned more specifically with proper reference. Answer: 9. XRD analysis: The authors also need to make a proper table showing the diffraction peaks and2theta values with their planes and removed typo errors. The authors can add a section before results defining the present study and past studies showing difference with proper references.

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Answer: We include the diffraction peaks and structural information for any phase found in supplementary materials as tables. The information comes from the ICDD PDF4 database. In fact, we apologies because some of the PDF numbers were wrong. All of there were fixed and I appreciate the comments. 10. Conclusion: Conclusion can be more precise and can be shortened by English improvement.

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Answer: We rewrite the conclusions section emphasizing the relevant findings. English was revised by native.

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Reviewer #3:

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- Specific issues 1. The "nanoscale" in the manuscript should be defined. What about the size of the nanoparticles?

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Answer: There are various categories to classify particle and porous size described in the literature. Despite the intensive discussion about the categorization. We include the pore size classification of IUPAC related to porous material calcification, and we removed along all the paper the categorization of particle size to avoid any confusion about size. Porous solids classified as either microporous (pore size up to 2 nm) or mesoporous (pore size 2 to 50 nm) and microporous (up to 50 nm). 2. I am confused that you describe your sequential extraction method, but I did not see your sequential extraction data. Answer: Dear reviewer, in fact all studied AC are commercial. 3. The styles of the reference citation and references listed are not in accordance with the journal requirements. Please check them and correct if necessary. Answer:

Answer: All references were fixed in accordance with the journal guideline. 4. Please send you paper to native. Answer: The paper was sent to a professional language editing service in order to improve the English.

EDITOR COMMENTS:

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1. English should be improved. Answer: The paper was sent to a professional language editing service in order to improve the English.

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2. For reviewers that ask you to cite some references, you only need to cite it if it is relevant to your work Answer: We very much appreciate your suggestion. In fact, We read the papers suggested by the referee and we found some useful information to cite.

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With best regards. On behalf of our team Cristian Felipe Ramirez-Gutierrez

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Dear Editor, we appreciate each one of the reviewer's comments and questions, which enabled us to improve the quality of the paper.

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Intensity

Graphical Abstract

S2 CaCO3 (04-007-0049) MgCO3 (01-080-3275) Graphite-2H (04-007-2081) Ca(PO3)2 (00-009-0363) Halite, potassian (01-075-0302) SiO2 (Coesite) (04-005-4734) SiO2 (Stishovite) (01-082-1646) Ca(OH)2 (00-044-1481) FeO (00-006-0615) Mg3As2 (01-073-1952) Ca(Fe,Mg)(CO3)2(00-041-0586)

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2q (°)

*Manuscript Click here to download Manuscript: VersionV1.tex

Study and identification of contaminant phases in commercial activated carbons C. F. Ramirez-Gutierreza,b,∗, R. Arias-Niquepac , J.J. Pr´ıas-Barrag´anc,d , M. E. Rodriguez-Garciad a

Universidad Polit´ecnica de Quer´etaro, El Marqu´es, Qro., M´exico Posgrado en Ciencia e Ingenier´ıa de Materiales, Centro de F´ısica Aplicada y Tecnolog´ıa Avanzada, Universidad Nacional Aut´ onoma de M´exico Campus Juriquilla, Qro., M´exico c Instituto Interdiciplinario de las Ciencias, Universidad del Quindio, Armenia, Colombia d Tecnolog´ıa e Instrumentaci´ on Electr´ onica, Facultad de Ciencias B´ asicas y Tecnolog´ıa, Universidad del Quind´ıo, Armenia, Colombia e Departamento de Nanotecnolog´ıa, Centro de F´ısica Aplicada y Tecnolog´ıa Avanzada, Universidad Nacional Aut´ onoma de M´exico Campus Juriquilla, Qro., M´exico

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Abstract

This work aims to identify and study contaminant phases present in com-

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mercial activated carbons (AC), also to determine the quality of the AC through methylene blue absorption. A straightforward methodology was im-

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plemented to carry out a complete characterization of AC that consists of the morphology determination by SEM, an elemental chemical composition by XRF, identification of crystalline phases using XRF, and vibrational analysis through FITR. The results showed that the primary contamination in AC is related to trace elements present in raw materials as well as residues from

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Click here to view linked References

the activation process such as Mg, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, As, Sr, and Zr. It was observed that these elements could react during all steps of ∗

Corresponding author Email address: [email protected] (C. F. Ramirez-Gutierrez)

Preprint submitted to Journal of Environmental Chemical EngineeringDecember 12, 2019

AC fabrication and form crystalline compounds, mainly calcium carbonates, calcium phosphates, and silicon oxides. Moreover, gas adsorption (BET) and methylene blue (MB) removal probes show that there is no correlation between the surface area and the MB removal capability.

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FTIR, BET, X-ray diffraction, methylene blue, charcoal

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Keywords:

1. Introduction

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Activated carbon (AC) is a porous material defined as porosity enclosed by carbon atoms [1]. The activation refers to the transformation of raw car-

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bon material into a porous one with a large functional surface area. AC production starts with a high contend carbon precursor like cellulose, and

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conventionally there are two main routes for this process: gasification through thermal activation (physical) and chemical activation. In the case of physical

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activation, the material is carbonized under an inert atmosphere and then activated at high temperatures using steam or carbon dioxide as the activating reagent [2].On the other hand, for chemical activation, the precursor is previously impregnated with proper chemicals such as salts (i.e., ZnCl2 , AlCl3 , MgCl2 , and FeCl3 ) or acids (i.e., H3 PO4 HNO3 , and H2 SO4 ) previous

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to co-carbonization. AC is obtained as a result of three-steps. The first one is carbonization or pyrolysis, which is used to remove some organic compounds such as fat and proteins of the precursors (coconuts shells, hardwoods, fruit stones, waste

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biomass, sludge, and another macromolecular system) [3]. The second step is the activation that can be triggered by using different chemical agents such as acids [4, 5] or basic treatments [6]. However, it is possible to have secondary chemical reactions between the raw material components and ac-

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tivation agents. Finally, a washing process is used to remove remaining compounds, ions, or salts in order to obtain an AC without impurities [7].

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Despite that, it is possible to obtain secondary phases due to thermal treatments, activation processes, residual mineral, and metallic contend intrinsi-

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cally present in the precursors [8, 9], or poor washing process.

Nowadays, AC has a high demand [10] because of its high porosity and surface

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area, surface reactivity, and variable characteristics of surface chemistry that make it useful in different applications such as water and wastewater treat-

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ments [11, 12], air purification, gas mixtures separation [13, 14], electronics such as supercapacitors, and sensors [15, 16, 17, 18, 19, 20], among others.

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However, due to their high manufacturing costs, highly pure AC turns out to be more expensive than other adsorbents; therefore, many studies have been addressed in order to develop low-cost ACs obtained from various agricultural byproducts or waste [21], or recycled AC by reactivation processes [22] producing a great variability of AC quality [20].

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As it can be seen in the works mentioned earlier, the use of AC is

widespread. However, for specific applications, the commercial AC product label does not contain the information to ensure that it satisfies the 3

required experimental conditions, and information about its quality is neglected. The possibility of contamination, chemical interactions, or direct toxicity with active compounds present in AC must be taken in to account. Therefore, before any process or experiment involving the use of AC, a prior

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physicochemical evaluation is required. The objective of this study was to characterize different commercial acti-

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vated carbons in order to determine their quality. X-ray diffraction was used

to determine the crystalline phases in the ACs, FTIR was used to determine

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the vibrational states, SEM was used to study the morphology, BET was used to determine the surface area, and blue methylene adsorption was used

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to determine the adsorption capability, in order to assess their quality and

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develop a methodology for easy characterization of commercial ACs.

2. Materials and methods

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2.1. Sample selection and conditioning

Six commercial activated carbons were used. These samples were bought

in companies that operate and sell chemical products for research in M´exico. Samples were labeled as M1, B1, S1, S2, H1, and G1. Each letter was used for identifying the company and the number for the product. This

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means that two samples from S company were tested, and only one from the others. Furthermore, it is essential to point out that AC labels do not provide information about AC origins and trace elements or contaminant contents, so they are considered as ACs reagent grade. All of them were obtained during 4

the year 2017, and they correspond to lots of the same years. Samples were homogenized through sieving using a 100 mesh (< 147µm), and a similar particle distribution was observed for all samples. 2.2. Elemental composition

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The elemental composition of AC samples was determined by energy-

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dispersive X-ray fluorescence (EDXRF) using PUMA 2 spectrometer by Bruker wit Pt target. The range of elements goes from sodium (Na) to uranium (U).

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All measurements were performed under vacuum conditions using a collima-

2.3. Structural properties (XRD)

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tion mask of 12 mm.

X-ray diffraction patterns of ACs M1, B1, S1, S2, H1, and G1 were used

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to determine the presence of crystalline phases. Powder samples (mesh 100) were densely packed in an Al holder. X-ray diffraction patterns of the samples

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were carried out on a Rigaku Ultima IV diffraction instrument operating at 40 kV, 30 mA with Cu Kα radiation wavelength of λ = 1.5406 ˚ A. Diffractograms were obtained from 5◦ to 80◦ on a 2θ scale with a 0.02◦ step size. 2.4. Surface microstructure (SEM) Morphologic analysis of all samples was carried out in a Jeol JSM 6060LV

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Scanning Electron Microscope. The analysis was performed using 20 kV electron acceleration voltages. Before the analysis, the samples were fixed on a copper specimen holder with carbon tape and covered with a gold thin film to make them conductive before testing. 5

2.5. Fourier transform infrared spectroscopy (FTIR) The chemical functionality of commercial activated carbons was qualitatively identified by Fourier transform infrared spectroscopy (FTIR). FTIR spectrum was recorded between 4000 to 400 cm−1 using a Bruker Vector 33

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in an ATR (diamond/ZnSe crystal) configuration.

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2.6. Brunauer-Emmett-Teller BET surface area

An Autosorb iQ Station instrument was used for the adsorption isotherms

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of N2 at 77.3 K to clarify the textural properties of produced ACs. Before measurement, the samples were degassed under vacuum at 473 K, and 6.58 ×

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10−5 Torr for 12 h. approximately 0.0218 g of the degassed samples were used in each adsorption experiment. By analyzing the N2 adsorption profile. The

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surface (SBET ) and pore diameter were obtained. 2.7. Removal of methylene blue (MB)

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This test was carried out following the methodology proposed by Deng

et al. [23]. A stock solution was prepared with 1.0 g/L of Sigma-Aldrich Methylene Blue hydrate (97.0%) in deionized water. A volume of 25 mL of MB stock solution was added to 0.1 g of AC in a glass-stoppered flask on a mechanical shaker at 130 rpm at 25±1◦ C for 2h in order to reach the adsorp-

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tion equilibrium conditions. Then the samples were filtered, and the residual concentration of MB in the filtrate was analyzed with a double beam UV-Vis spectrophotometer (Hitachi) at 663 nm. The amount of MB adsorbed (qe

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(mg/g)) per unit mass of adsorbent at equilibrium conditions was calculated by [24, 25]: V , m

(1)

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qe = (C0 − Ce )

where C0 (mg/L) is the initial concentration of the solution of MB, and Ce

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(mg/L) is the residual concentration in the equilibrium. V (L) is the volume

of solution, and m (g) is the mass of the AC sample. Finally, the adsorption

(2)

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3. Results and analysis

(Co − Ce ) × 100 C0

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adsorption(%) =

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percentage can be defined as:

3.1. Elemental composition

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Table 1 shows the elemental chemical composition of commercial ACs. Trace elements that are commonly present in raw materials ( hardwoods, coconut shells, among others) such as Si, Ca, Ti, Mn, and Fe were found. Furthermore, there are chemicals for activation processes such as P, Cl, and K.

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3.2. Surface morphology Scanning electron microscopy (SEM) was used to study the surface mor-

phology of the samples. Fig. 1 shows the SEM images of commercial active carbons taken at 200x. As it can seem in Fig. 1a for sample M1 shows that 7

Table 1: Chemical elemental composition of commercial activated carbons. All concentrations are expresent in wt%

H1 6.2 1.78 6.05 0.06 0.12 0.04 15.53 – – 0.11 – 0.03 –

G1 5.09 1.57 3.09 0.35 0.22 0.33 7.94 0.04 0.02 0.32 – – –

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S1 S2 7.92 4.94 2.9 2.08 – 10.12 0.98 – 0.71 0.08 – – 0.34 12.33 0.1 – 0.03 – 0.32 0.13 – 0.01 0.02 – 0.03 –

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B1 6.5 3.51 0.13 1.11 0.97 – 0.44 0.1 0.02 0.34 – – –

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M1 11.25 0.86 0.17 0.15 0.08 0.07 0.68 – 0.05 0.12 – – –

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Element Mg Si P S Cl K Ca Ti Mn Fe As Sr Zr

particles between 5 to 200 µm length and 5 to 30 µm thickness, some of these

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particles are longitudinal structures of carbon multilayers. Fig. 1b for B1 is formed by agglomerates about 200 µm diameter and particles from 5 to 200

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µm. Fig. 1c for S1 shows particles about 2 to 200 µm length and spherical agglomerates. Fig. 1d for S2 shows a more uniform distribution from 5 to 40 µm and some agglomerates. Fig. 1 e for H1 particle sizes vary from 3 to 80 µm, and some agglomerates exhibit semi-spherical shape, and Fig. 1f for G1 is mainly formed by particles from 5 to 130 µm, some of these particles have longitudinal structure. It is clear that the commercial ACs are composed

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of longitudinal and semi-spherical structures but do not present a uniform particle distribution.

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B1

S1

S2

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M1

G1

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H1

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Figure 1: SEM micrographs (200x) of the (a) M1, (b) B1, (c) S1, (d) S2, (e) H1, and (f) G1

Fig. 2a to f shows the SEM images taken at 15000x. The particle surface

has cracked due to the evaporation of the activation reagent, and the surface of the samples is not smooth. That effect contributes to the overall surface

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and porosity. Also, surface exhibit some agglomerates that can be polluting minerals.

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B1

S1

S2

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M1

G1

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H1

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Figure 2: SEM micrographs (15000x) of the (a) M1, (b) B1, (c) S1, (d) S2, (e) H1, and (f) G1

3.3. XRD analysis

Fig. 3 shows the X-ray diffraction patterns for the commercial activated

carbons. Table 2 summarizes all crystalline phases found. These crystalline

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phases were identified using the e PDF-4+ 2019. As it is well known, AC is usually obtained from hardwoods, coconut shells, and other macromolecular systems, this means that after pyrolysis and activation, it is possible to find crystalline compounds coming from raw ma10

terials and others that could be formed as a result of the physicochemical treatments. All samples exhibit a clear amorphous background of the carbon phase. Fig 3a shows the X-ray diffraction pattern of M1, which is characteristic of amorphous material and does not exhibit the presence of crystalline

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phases, although its chemical composition contains trace elements such as Mg and Si. Usually, crystalline phases like silicon oxides or graphite are formed

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at high temperatures. Therefore, the nonpresence of crystalline phases in M1

is an indication that the activation was obtained at relatively low tempera-

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tures, it means temperatures around decomposition of MgCO3 (470◦ C) [26] in atmospheres with low content of oxygen because there is no presence

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of crystalline phases of silicon oxides. Moreover, there is no formation of graphite.

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Sample B1 exhibits β-quartz (hexagonal) and graphite crystalline phases that are consistent with its chemical composition as indicative that pyrolysis

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and activation process was performed at temperatures higher than 573◦ C, that is the temperature of quartz inversion (α to β) [27]. Notwithstanding sample S1 has 7.92% of Mg, no crystalline phases related with this element were found, nor minority trace elements, S1 only holds graphite and SiO2 phases such as cristobalite and quartz. Even though cristobalite is

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thermodynamically stable only at temperatures above 1470◦ C, it can persist

metastable or even form at much lower temperatures [28]. Fig. 3(d) shows the XRD pattern of sample S2 which is the most contaminated with crystalline phases. The major trace elements are Ca, P, and 11

Table 2: Summary of crystalline compounds found within AC samples

– – – – – – 7 – – –

S2 – – 7 7 7 7 7 7 7 7 7 – 7 – – 7

H1 – – – 7 7 – – – 7 – – 7 7 7 – –

G1 7 – – – 7 – – 7 7 – – – – – 7 –

ICDD card# 01-075-8322 04-012-1126 04-005-4734 01-082-1646 04-007-0049 01-080-3275 00-009-0363 01-075-0302 00-044-1481 00-006-0615 01-073-1952 04-016-0227 04-007-2081 01-077-8800 01-070-2526 00-041-0586

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S1 7 7 – – –

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B1 7 – – – – – – – – – – – 7 – – –

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M1 – – – – – – – – – – – – – – – –

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Compound SiO2 -Quartz SiO2 -Cristobalite SiO2 -Coesite SiO2 -Stishovite CaCO3 MgCO3 Ca(PO3 )2 Halite, potassian Ca(OH)2 FeO Mg3 As2 (NH4 )Cl Graphite-2H KCl TiO2 Ca(Fe,Mg)(CO3 )2

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Mg, hence crystalline phases of calcium and magnesium carbonates, calcium phosphate, and calcium hydroxide were found. It is reported that this kind of

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compounds are formed about 250 to 500◦ C [29, 30] in the presence of steam. Elements such as Si, Fe, and As are usually present in the raw materials, and phases related with Si and Fe oxides which are formed in the presence of oxygen or steam at low temperatures (500◦ C) were found. Also, there was presence of halite-potassian in samples S2 and G1 that could be formed by

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the interaction between activation agents and washing processes. Samples H1 and G1 present calcium phases such as calcium carbonate and calcium hydroxide. H1 also has silicon oxide (stishovite), graphite, ammonium chloride, and potassium chloride. Finally, sample G1 presents quartz 12

and titanium oxide. (a)

M1

4000

2000

B1 Quartz (01-075-8322) Graphite-2H (04-007-2081)

(b)

8000

Intensity (counts)

Intensity (counts)

6000

6000 4000 2000

26.0

S1 Quartz (01-075-8322) SiO2 (Cristobalite) (04-012-1126) Graphite-2H (04-007-2081)

4000

2000

0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2500

H1 CaCO3 (01-075-6049) Ca(OH)2 (00-044-8141) SiO2 (Stishovite) (00-015-0026) NH4Cl (04-016-0227) Graphite-2H (04-007-2081) KCl (01-077-8800)

(e) 2000

lP

1500 1000 500

5

ur na

0

(d)

1000

5

2500

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2q (°)

S2 CaCO3 (04-007-0049) MgCO3 (01-080-3275) Graphite-2H (04-007-2081) Ca(PO3)2 (00-009-0363) Halite, potassian (01-075-0302) SiO2 (Coesite) (04-005-4734) SiO2 (Stishovite) (01-082-1646) Ca(OH)2 (00-044-1481) FeO (00-006-0615) Mg3As2 (01-073-1952)

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

re

2q (°)

Intensity (counts)

5

2q (°)

ro

2000

27.0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

-p

Intensity (counts)

2q (°)

(c)

6000

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Intensity (counts)

5

26.5

of

0

2q (°)

G1 CaCO3 (01-075-6049) Quartz (01-075-8322) Ca(OH)2 (00-044-1481) TiO2 (01-070-2556) Halite, potassian (01-075-0302)

(f)

2000

1500

1000 500

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2q (°)

Figure 3: X-ray diffraction patterns and phase identification of commercial activated carbons: (a) M1, (b) B1, (c) S1, (d) S2, (e) H1, and (f) G1.

3.4. Functional groups and vibrational analysis

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Intensity (counts)

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 61 62 63 64 65

Infrared characterization of AC provides information about functional

groups in the carbon materials, as well as the possible trace minerals. Activated carbon is an amorphous material, but it exhibits similar vibrational

13

properties of diamond-like carbon (DLC) materials with hydrogen, oxygen, and nitrogen contamination. This means that it is possible to find bands attributed to aromatic rings, olefinic chains, and hydrogen-carbon bonds (i.e., CH2 and CH3 ), C-O and C−O [31]. However, activated carbon as a definition

of

must not have absorption bands in this spectral region. Figure 4 shows the FTIR spectra of commercial AC where it is confirmed that less contaminated

ro

AC (M1, B1, and S1) only exhibits bands related with O-H groups, COO−1 , and silicon oxides, proving that these are highly pure.

-p

On the other hand, x-ray diffraction patterns show clearly that some of the commercial activated carbon have crystalline mineral related compounds,

re

and some of them can be verified by vibrational analysis. The most intense bands identified are related to calcium carbonates, calcium phosphates, and

ur na

lP

silicon oxides. These assignations are summarized in Table 3 and Figure 4.

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14

Table 3: Observed infrared bands positions in the ACs samples and its possible assignations

S2

ro n4-PO-3 4

n-Si-O

r-Si-O

-p n3-PO-3 4

n1-PO-3 4 n2-CO3-2

re n2-PO-3 4 n-Si-O-Si

n3-CO3-2

n-COO-

n-C=O

lP

n-O-H

n-O-H

H1

ur na

Transmittance (arb. units)

G1

Reference [32] [32] [31] [33] [34] [35, 36] [33, 37] [35, 38] [39] [34] [39] [40] [41, 33] [33]

of

Wavenumber (cm−1 ) 3734-3722 3700-3688 1700 1560-590 1409 1140 1070 1034 935 875 560 755 451 876, 727, 713

Functional group Structural ν-OH free ν-OH ν-C=O ν-COO− ν3as -CO−2 3 ν2 -PO−3 4 ν-Si-O-Si ν3 -PO−3 4 ν1 -PO−3 4 ν2as -CO−2 3 ν4 -PO−3 4 ν-Si-O ρSi-O Ankerite

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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 61 62 63 64 65

S1 B1

Ankerite

M1

3800

3750

3700

3650

1800 1650 1500 1350 1200 1050

900

750

600

450

Wavenumber (cm-1) Figure 4: FTIR spectra of commercial ACs: M1, B1, S1, S2, H1, and G1. This shows the main functional groups assignations of the contaminants and ACs.

15

3.5. Specific surface area and pore structure The nitrogen adsorption-desorption isotherms were obtained for the commercial ACs samples at 77.3 K in order to characterize the porous texture. The surface area was calculated by the BET model, which considers the Van

of

der Waals forces as responsible for the adsorption process, referring to physisorption phenomena.

ro

According to the isotherms obtained by N2 physisorption and by using the classification of the IUPAC [2, 42, 43], it can be seen that the AC samples

-p

M1, S2, and H1 have isotherms type I closely. However, these samples do not exhibit a clear saturation point. Of course, it is possible that the poros-

re

ity could be a combination of micropores (<2nm) and mesopores (2 to 50

lP

nm) [44] and multilayered formations. It means that the isotherms are a combination of type I and II. This behavior is typical in activated carbons.

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The samples B1, S1, and G1 exhibit type-IV Langmuir isotherm. The curves show the hysteresis in the adsorption isotherm of type H3 [45, 46], indicating that the AC particles are formed by the agglomeration of solid particles forming slit-shaped pores (plates or edged particles like cubes). Also, as it is shown in the isotherm, it is incomplete at low pressures, because

Jo

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the equipment senses only mesopores. For type IV isotherm, the low intake of nitrogen gas occurred at a starting point, and hysteresis loop appeared at the isotherm at relative pressures above 0.42 caused by capillary condensation [47, 48]. This isotherm proved that the AC is made up mostly of 16

mesopores. By using these curves and Langmuir model, it was calculated the geometrical parameters as the average diameter of the porous (Dp ) and

ro

Quantity Adsorbed (cm g

-1

STP )

of

specific surface area (SBET ). Table 4 summarizes these results.

B1

lP

re

-p

3

des G1

ur na

Relative pressure (P/P ) 0

Figure 5: BET (a) M1, (b) B1, (c) S1, (d) S2, (e) H1, and (f) G1.

Table 4: Porous structure parameters of the commercial ACs

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Sample M1 B1 S1 S2 H1 G1

SBET (m2 /g) 914.6 1563.2 1104.3 707.1 2453.4 750.2

17

Dp (nm) 3.8 3.4 3.4 3.4 3.4 3.8

3.6. Removal of methylene blue (MB) The MB is the most used dye to evaluate the decolorizing power or removal capability of AC, also gives evidence of the presence of macro and mesoporous structure. The MB adsorption was done by quantification using

of

a colorimetric probe [49]. Fig. 6a shows the spectrum of the MB stock solution and the spectra of the residual solutions after contact with the ACs.

ro

To carry out the quantification, the calibration curve was done at 663.2 nm,

that is the maximum wavelength of MB absorption. Fig. 6b shows the MB

-p

calibration curve ( R2 = 0.995) and the points where the residual solutions absorbance are present. Fig. 6c shows the results of UV-Vis absorbance as a

re

function of the surface area obtained by BET. Lowest values of absorbance

lP

imply higher retention of MB in the AC (adsorption), this means the AC with more removal capability. Finally, Fig. 6d summarizes the adsorption percentage calculated by the UV-Vis technique and the Eq. 2. Compar-

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ing Fig. 6c and d, it can be seen that the capability of removal of MB is not directly related to the surface area. For example, the sample B1 has the second higher surface area and the lowest percentage of adsorption of all samples measured. For H1 absorption percentage was 95% because it has the highest surface area and a porous structure, further macro- and mesoporous.

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However, it is one of the most polluted activated carbons.

18

Linear fit 2

R =0.995

B1

G1

M1

of

S2 H1

Absorbance (arb. units)

ro

100

B1

G1 S1

60

40

M1

re

20

H1 S2

-p

Adsortion %

80

0

2

-1

)

S1

Commercial ACs

lP

Surface area (m g

ur na

Figure 6: a) Absorption spectra of MB for commercial ACs, (b) calibration curve of MB UV-vis spectrum, with concentrations absorbed commercial ACs (c) Effects of absorbance on the surface area of activated commercial ACs, and (d) Absorption percentage for commercial ACs.

4. Conclusions

Activated carbon is usually produced from carbonaceous source materials

such as hardwoods that contain trace materials of their biological conforma-

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tion and metabolism such as Mg, Al, Si, K, Ca, Mn, and Fe, these elements remain and react during the AC activation process and can form other crystalline phases which are not necessarily removed during the pyrolysis, acti-

19

vation, or cleaning processes. According to the X-ray diffraction analysis of these samples, M1 does not exhibit the presence of any crystalline structure, and its XRD pattern is characteristic of amorphous carbon material. In the case of the other samples, other crystalline phases are present. Primary con-

of

taminant crystalline phases are formed during the thermal process (pyrolysis), and these were identified as common minerals that can be formed under

ro

the carbon activation conditions such as graphite, calcium carbonate, calcium phosphate, magnesium carbonate, silicon oxides, and iron oxide. Also,

-p

other crystalline phases such as KCl, Halite- potassian, and (NH4 ) Cl can be synthesized due to activation agents. On the other hand, absorption tests

re

(BET an MB absorption) show that there is not a correlation between surface area and MB absorption capability. This is shown in the case of sample H1,

lP

which exhibits the highest surface area but the lowest MB absorption. The contaminant phases in the commercial AC may imply a modification of its

ur na

capacity level to trap substances through adsorption. Also, deactivation is possible. As a consequence, AC can become reactive, leaching these elements to the environment. Finally, this work shows a straightforward methodology to follow in order to test the quality of an activated carbon sample.

Acknowledgments

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C.F. Ramirez-Gutierrez wants to thank CONACYT-M´exico for the fi-

nancial support of his Ph.D. Thesis. Authors thank Laboratorio Nacional de Caracterizaci´on de Materiales (LaNcaM) for all experimental support, and 20

Alicia de Real for operating the electron microscope, Beatriz Mill´an-Malo for her assistance in the XRD experiments, Carmen Peza-Ledesma for XRF measurements, Antonieta Mondrag´on Sosa for FTIR materials evaluation, and Rufino Nava-Mendoza and Abigail Moreno-Martell for BET and MB

of

removal measures. English editing by Kevin Maya-Martinez.

ro

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re

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Supplementary Material Click here to download Supplementary Material: PDFssuplementary.doc

*Declaration of Interest Statement

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

*Author Contributions Section

Author contributions Use this form to specify the contribution of each author of your manuscript. A distinction is made between five types of contributions: Conceived and designed the analysis; Collected the data; Contributed data or analysis tools; Performed the analysis; Wrote the paper.

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For each author of your manuscript, please indicate the types of contributions the author has made. An author may have made more than one type of contribution. Optionally, for each contribution type, you may specify the contribution of an author in more detail by providing a one-sentence statement in which the contribution is summarized. In the case of an author who contributed to performing the analysis, the author’s contribution for instance could be specified in more detail as ‘Performed the computer simulations’, ‘Performed the statistical analysis’, or ‘Performed the text mining analysis’. If an author has made a contribution that is not covered by the five pre-defined contribution types, then please choose ‘Other contribution’ and provide a one-sentence statement summarizing the author’s contribution.

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