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Effects of Sulfidation, Magnetization, and Oxygenation on Azo Dye Reduction by Zerovalent Iron Chunhua Xu, Bingliang Zhang, Yahao Wang, Qianqian Shao, Wei-zhi Zhou, Dimin Fan, Joel Z Bandstra, Zhenqing Shi, and Paul G. Tratnyek Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03184 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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Environmental Science & Technology

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Effects of Sulfidation, Magnetization, and Oxygenation

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on Azo Dye Reduction by Zerovalent Iron

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Chunhua Xu1*, Bingliang Zhang1, Yahao Wang1, Qianqian Shao1, Weizhi Zhou1 Dimin Fan2, Joel Z. Bandstra3, Zhenqing Shi4, and Paul G. Tratnyek5* 1

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School of Environmental Science and Engineering

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Shandong University, Jinan,

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Shandong 250100, P.R. China

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2

U.S. Environmental Protection Agency, Office of Superfund Remediation and Technology Innovation, Arlington, VA 22202 USA 3

School of Sciences, Saint Francis University 117 Evergreen Drive, Loretto, PA 15940 USA 4

School of Environment and Energy South China University of Technology Guangzhou, Guangdong 510006, P.R. China 5

Institute of Environmental Health Oregon Health & Science University 3181 SW Sam Jackson Park Road, Portland, OR 97239 USA *Corresponding authors: Email: [email protected], Phone: 86-531-88362586, Fax: 86-531-88364513 Email: [email protected], Phone: 503-346-3431, Fax: 503-346-3427 Prepared for Publication in Environ. Sci. Technol. 3 September 2016

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ACS Paragon Plus Environment


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Abstract

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Applications of zerovalent iron (ZVI) for water treatment under aerobic conditions include

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sequestration of metals (e.g., in acid mine drainage) and decolorization of dyes (in wastewaters

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from textile manufacturing). The processes responsible for contaminant removal can be a

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complex mixture of reduction, oxidation, sorption, and co-precipitation processes, which are

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further complicated by the dynamics of oxygen intrusion, mixing, and oxide precipitation. To

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better understand such systems, the removal of an azo dye (Orange I) by micron-sized granular

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ZVI at neutral pH was studied in open (aerobic) stirred batch reactors, by measuring the kinetics

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of Orange I decolorization and changes in “geochemical” properties (DO, Fe(II), and Eh), with

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and without two treatments that might improve the long-term performance of this system:

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sulfidation by pretreatment with sulfide, and magnetization by application of a weak magnetic

12

field (WMF). The results show that the changes in solution chemistry are coupled to the

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dynamics of oxygen intrusion, which was modeled as analogous to dissolved oxygen sag curves.

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Both sulfidation and magnetization increased

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Orange I removal rates 2.4-71.8 fold, but there

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was little synergistic benefit to applying both

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enhancements together. Respike experiments

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showed that the enhancement from

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magnetization carries over from magnetization

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to sulfidation, but not the reverse.

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Keywords:

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Azo dyes, Zerovalent iron, Decolorization,

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Reduction, Sulfidation, Magnetization,

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Oxygenation, Respike, Modeling, Oxygen sag

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curves

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Introduction

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Zero valent iron (ZVI) is applicable to treatment of contaminants in a wide range of media;

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including groundwater, sediment, soil, and wastewater. Most early research on these processes

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focused on anaerobic conditions, and contaminants that are degraded by reduction, in order to

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model in situ remediation of groundwater.1-3 More recently, there has been a shift toward

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research on aerobic conditions, and contaminants that are removed by oxidation and

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(co)precipitation—as well as reduction—in order to model ex situ water treatment processes.

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The contaminants that are most amenable to treatment by ZVI under aerobic conditions are

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metals and metalloids such as the oxyanions of As(III),4 As(V),5 Se(IV),6, 7 Sb(III),8 and Sb(V).9

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Several prominent applications of this process are in constructed wetlands to treat acid mine

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drainage,10-12 storm water filters for urban and roof runoff,13-15 and point-of-use filters to remove

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arsenic from well water in Southeast Asia.16, 17

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Another application of ZVI to treatment of aerobic wastewater is to remove textile dyes,

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especially azo dyes, which are decolorized by ZVI, mainly by reduction of the chromophoric azo

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moieties.18-21Early research on this reaction was mostly done under anaerobic conditions, to

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simplify the system and address various fundamental questions regarding ZVI reactions with

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contaminants.22-25However, in the last 10 years, there has been a notable shift toward research on

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treatment of dyes by ZVI under aerobic conditions (Figure S1). Some of these studies have

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reported that dissolved O2 can enhance the removal of azo dyes, but the mechanism of this

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enhancement is likely to be complex and possibly variable. For example, one study concluded

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that enhanced decolorization of Acid Orange 7 was due to reduction of the dye coupled to rapid

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aerobic oxidation and corrosion of ZVI,26 whereas another study concluded that enhanced

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degradation of Acid Orange II by aerobic ZVI was due to Fenton-like oxidation processes.27

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Sorption and coprecipitation may also contribute to removal of dyes, especially for (non-azo)

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cationic dyes like methylene blue under aerobic conditions during active corrosion of ZVI and

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precipitation of fresh iron oxides.28, 29

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The kinetics of azo dye removal by ZVI under anaerobic conditions are relatively fast and

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generally conform to pseudo-first order kinetics over most of the length of typical laboratory

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experiments,23, 30, 31 but more complex kinetics are expected under aerobic conditions. In

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particular, aerobic corrosion of ZVI—and the resulting accumulation of iron oxides—are likely



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to alter the kinetics and capacity for dye removal. The implications of these oxygenation effects

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on the long-term sustainability of treatment processes involving aerobic ZVI have received little

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attention. Even in studies performed with well-stirred batch reactors, aerobic ZVI systems

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exhibit challenging characteristics such as the coupled dynamics of interaction between

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(de)passivation and (re)oxygenation, as we recently described for the sequestration of Sb(III)8

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and Se(IV).6 Additional complications are expected in systems composed of packed porous

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media, such as decreased and non-uniform flow due to the occlusion of pore spaces by

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accumulation of iron oxides.29, 32 The anticipated challenges to sustaining long-term performance of aerobic water

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treatment processes involving ZVI make this system an even more compelling context for

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application of the various methods that have been developed to enhance, preserve, and/or

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maintain the performance of anaerobic ZVI-based treatment processes.33 Two of the most

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promising of these methods are included in this study: sulfidation and magnetization. Sulfidation

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involves addition of reduced sulfur (e.g., as sulfide or dithionite) to ZVI in such a way that the

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passive layer on the particles becomes predominantly iron sulfides, instead of the iron oxide

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passive film that controls the reactivity of (not sulfidated) ZVI. To distinguish these two types of

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materials, we designated the former as Fe/FeS and the latter as Fe/FeO in several recent studies34,

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35

and will continue to do so in this study. The benefits of ZVI sulfidation under anaerobic

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conditions include (i) increased rates of contaminant removal,34-37 (ii) more favorable (benign or

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irreversible) products from contaminant removal,38, 39 and (iii) decreased rates of water reduction

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(and hydrogen formation) and/or improved selectivity for reduction of contaminants over

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water.37, 40-42 To date, the effect of sulfidation on aerobic ZVI systems has only been investigated

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using nano ZVI,43 not micro-sized ZVI, as used in this study.

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Magnetization of ZVI at relatively high field strength causes heating—by

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electromagnetic induction—which can enhance rates of contaminant mobilization or

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degradation,44, 45 but the application of a weak magnetic field (WMF) to ZVI has also been found

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to consistently increase contaminant removal rates. The WMF enhancement appears to be mainly

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due to increased flux of paramagnetic ions (Fe(II)) under the Lorentz forces and gradient forces

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induced at the interface of ZVI.7, 8, 46 The WMF effect has been observed mostly with metal

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contaminants,4, 5, 7-9, 47 but also with several organic contaminants, including one azo dye (Orange


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II48). The effect is somewhat persistent even after removal of the WMF (due to remanance49), but

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it is very sensitive to the changes in the composition of the oxide film on the ZVI.50, 51

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Presumably, the WMF effect will also be influenced by sulfidation, but this has not been

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addressed in any studies to date.

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The defining features of this study—azo dye decolorization by sulfidated and/or

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magnetized ZVI under aerobic conditions—constitute an especially complex and dynamic

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system that can only be fully understood using a complementary suite of innovative and state-of-

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the-art approaches. Three novel approaches included in this study are (i) the “time series

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correlation” analysis that we introduced in a recent study,8 (ii) respike experiments with

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alternating sequences of treatments, and (iii) global fitting of a geochemical model that is

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analogous to the Streeter-Phelps model for dissolved oxygen sag curves. By selecting an azo dye

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as the probe/reactant compound, we expanded the range of contaminants that have been studied

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under conditions of sulfidation and magnetization. By including both sulfidation and

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magnetization, under aerobic conditions, we were able to characterize the coupled effects of

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these three factors. The fundamental implications of these results are mainly to expand our

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understanding of how layers of oxides and sulfides control the reactivity of contaminants with

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ZVI, and the practical implications are that sulfidation and/or magnetization are promising

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approaches to managing the reactivity of ZVI for applications to treatment of aerobic wastewater,

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such as those produced by textile dye operations.

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Experimental Section

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Chemical Reagents. The ZVI used in this study was obtained from the Beijing Enviro-Chem

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Environmental Technology Co., Ltd. (Beijing, P.R. China), stored in a desiccator, and used as

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received. This material was chosen because it currently is used in one of the leading commercial

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products for full-scale remediation applications in China. Additional background on this material

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and a summary of its properties are given in the Supporting Information (SI). Orange I (4-((4-

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hydroxy-1-naphthyl)diazenyl) benzenesulfonate sodium; CAS RN 523-44-4; >90% purity) and

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other reagents were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, P.R.

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China), and used without further purification. Buffers were prepared with acetic acid (HAc),

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sodium acetate (NaAc), and 4-(2-hydroxyethyl) piperazine-1-ethanesulfonic acid (HEPES)

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supplied by Sigma-Aldrich Co. LLC. (China). Standard of sulfanilic acid and 4-amino-1-



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naphthol hydrochloride were purchased from Sigma-Aldrich Co. LLC. All solutions were

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prepared with ultrapure deionized water (UPW-1-90T, Ulupure company, China).

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Sulfidation. Fe/FeS was synthesized in 250 mL glass serum bottles. Each bottle was filled with

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250 mL of HAc-NaAc buffer solution (0.2 M, pH 6.0) and then deoxygenated by bubbling with

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N2 for 30 min. The ZVI (1 g) was added to the deoxygenated medium and the bottle was

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immediately crimp-sealed with polyethylene septa. The bottles were mixed in a constant

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temperature rotary mixer (Jintan Boke Instrument Co., China) at 120 rpm and 25 ± 0.2 °C. After

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10 min, 1.5 mL Na2S (1 M) was added to each bottle by injection through the septum, and the

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bottles were then returned to the mixer for another 12 hr of pre-equilibration. After pre-

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equilibration, the solids were collected with membrane filters (0.22 µm polyvinylidene fluoride

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(PVDF)) under a N2 atmosphere. The filtered particles were freeze-dried (0.15 torr vacuum) for 2

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hr, and then stored in 1.5 mL plastic centrifuge tubes.

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Batch Experiments. All experiments were done open to air in 1 L beakers, by adding 2.0 g

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(most experiments) Fe/FeO or Fe/FeS to 1 L of 0.285 mM (100 mg/L) Orange I buffered with 10

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mM HEPES at pH 7.0, and then mixing at 400 rpm in a water bath at 25 ± 0.2 °C. Samples (5

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mL) were taken periodically, and filtered with 0.22 µm membrane filters for further analysis.

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Some experiments were done with permanent ring magnets (with inner diameter 4 cm, outer

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diameter 7.9 cm, and thickness of 1.5 cm) located underneath the beaker to provide constant

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weak magnetic field (with maximum magnetic field intensity of ~20 mT). In some cases, up to

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four respikes of Orange I were performed on one batch of (sulfidated) ZVI suspension. The

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volume of Orange I stock solution (2.5 g/L) used in each respike was adjusted to achieve the

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same Orange I concentration (~100 mg/L) at the beginning of each cycle.

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Liquid Phase Characterization. The absorbance of the reaction solution for Orange I at the λmax

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(476 nm) was measured by a UV-Vis spectrophotometer (UV 1901 PC, Shanghai Aoxi Scientific

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Instrument Co., China). The solution was diluted 10 times before scanning to record spectra in

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most appropriate absorbance range. Dissolved oxygen (DO), oxidation-reduction potential (Eh),

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and pH were monitored with commercial combination electrodes (DO-957, 501 ORP, E-201-C

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pH, INESA Scientific Instrument Co., Ltd, China). Ferrous iron (Fe(II)) was determined

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according to the phenanthroline spectrophotometric method.52 Orange I and its degradation


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products were analyzed through High Performance Liquid Chromatography-Mass Spectrometry

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(HPLC-MS, Thermo Ultimate 3000).

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Solid Phase Characterization. The surface compositions of Fe/FeS samples at reaction time 0

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min, 30 min, and 120 min and Fe/FeO (0 min) samples were analyzed by X-ray photoelectron

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spectroscopy (XPS). It was conducted with monochromatic Al Kα radiation (1486.6 eV) and the

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power at 150 W. The binding energies were calibrated to the C 1s peak at 284.8 eV. Surface

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morphologies and elemental mapping of the Fe/FeS and Fe/FeO samples were characterized by a

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scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The

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reaction products were further investigated by a Fourier Transform Infrared Spectrometer (FTIR).

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The results of these solid phase characterizations are given in SI. All the samples of solids were

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collected with membrane filters (0.22 µm polyvinylidene fluoride (PVDF)), and the filtered

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particles were freeze-dried for 2 hr.

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Data Analysis. Model fitting was performed by numerical solution computed using the adaptive

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step size Runge-Kutta-Fehlberg technique with an error scaling factor of 10−6. Selected

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simulations performed with an error scaling factor of 10−7 showed no difference from the results

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with the larger scaling factor indicating adequate error control. Minimum χ2 model fits to data

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were obtained using the Levenberg-Marquardt algorithm, as implemented in Igor Pro

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(Wavemetrics, Lake Oswego, OR).

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Results and Discussion

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Characterization of Pristine and Sulfidation of ZVI. Before treatment, the ZVI used in this

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study consisted of irregular but roughly spherical, d50 ≈ 30 µm particles with relatively smooth

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surfaces (Figure S2A), which is typical of ZVI produced by water atomization.49, 53 Pretreatment

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of the ZVI in HAc-NaAc buffer before sulfidation caused breakdown and dissolution of the

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surface oxide layer. Then, upon addition of Na2S, most of the Fe(II) precipitates as FeS, forming

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Fe/FeS. Sulfidation does not appear to have caused changes in particle size, but surface

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roughness increased (Figure S2C). Evidence that the Fe/FeS was coated with iron sulfides can

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be seen in the EDS (Figure S3C) and XPS (Figure S4) data. The XPS suggests that the surface

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layer was iron (hydr)oxides (Fe2O3 and FeOOH) on untreated ZVI and was FeSx and Fe(II)-O on



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Fe/FeS. The latter is similar to the results reported in previous studies of Fe/FeS.35, 37, 38 In

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Figure S4E, the three XPS peaks for S2−, S22− (surface), and S22− (bulk) in the S 2p3/2 spectrum

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are consistent with FeS and FeS2,37, 54 so the x in FeSx can be 1 and/or 2.

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Time Series Analysis of Changes in Solution Chemistry. To investigate the effects of

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sulfidation and magnetization on the reactivity of ZVI, Orange I was used as a model

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contaminant. Full UV-vis absorbance spectra for a comparable set of the four treatment

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combinations (with and without sulfidation and/or magnetization) are shown in Figure S5. The

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decrease in absorbance at the λmax for Orange I (476 nm) was used to monitor decolorization of

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the dye, and the first ~30 min were fit to pseudo-first-order kinetics (Figure 1A), after which the

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absorbance showed significant tailing or even rebound (which is discussed below). Synchronous

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measurements of the three most important “geochemical” properties of the system (DO, Fe(II),

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and Eh) were determined for every experiment, and the time series for these properties are shown

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in Figures 1B-1D. The data in Figure 1 have many features in common with the data in Figure 1

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of our recent paper on Sb(III) sequestration by aerobic ZVI,8 except that pH was buffered in this

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study, so it changed little during the experiments and therefore is not shown (the measured pH’s

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were between 7.15 and 6.92 in all experiments).

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In general, the time series changes in the four measured properties shown in Figure 1 are

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consistent with the interpretation given to the data in our previous study: rapid corrosion at the

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beginning generates corrosion products that gradually accumulate and passivate the ZVI to

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further reaction, resulting in a transition that creates a peak in the time series data.8 The biphasic

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character of this system is even more clearly seen using the time series correlation analysis that

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we have developed over several recent studies.7, 8 In this case (Figure S6), the time series

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correlations generally show well-defined curvature that further resolves the trends responsible

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for the peaks in Figure 1. This curvature is the same behavior that was seen with Se(IV) under

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“Group II” conditions7 and with Sb(III),8 if the ±WMF data are considered together.

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The major treatment effect seen in the time series data (Figures 1 and S6) is that the

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changes in all four measured properties were greater (both faster and further) with sulfidation

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and/or magnetization. The relative reactivities following the trend Fe/FeO –WMF 1 indicates

7

enhancement and R < 1 indicates inhibition in the rate of the measured process.8 kobs (Fe/FeO +WMF), kobs (Fe/FeS −WMF), kobs (Fe/FeS +WMF)

1

8

1 R = 100 50

20

10

5

2.5

1

0.1

Fe/FeS, −WMF, C0 Dye Fe/FeS, −WMF, [Fe/FeO or Fe/FeS] Fe/FeS, −WMF, Fe(II)aq Fe/FeS, −WMF, IPA Fe/FeS, −WMF, Na2SO4 Fe/FeS, +WMF, C0 Dye Fe/FeS, +WMF, [Fe/FeO or Fe/FeS] Fe/FeO, +WMF, C0 Dye Fe/FeO, +WMF, [Fe/FeO or Fe/FeS] Orange II, +Fe(II), Xiao et al. (48)

0.01

0.001

0.001

0.01

0.1

1

kobs (Fe/FeO, −WMF)

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Figure 2. Comparison of kobs of Fe/FeO with (+) WMF and kobs of Fe/FeS with (+) and without (–) WMF versus the kobs of Fe/FeO without (–) WMF, under a variety of experimental conditions. All kobs values are in min−1. Data and experimental details are given in Table S1.

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The data in the upper-left corner of Figure 2, at R ≈ 75, are all data obtained in this study with

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magnetization (+WMF), regardless whether the ZVI was sulfidated or any other experimental

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factors. Just below that range, at R ≈ 30 are most of the data obtained with sulfidation (without

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magnetization, but regardless of other factors). The only exceptional result among the data from

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this study with Orange I is for −WMF with added sulfate (asterisks in Figure 2). In this case, the

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sulfate alone appears to have depassivated the Fe/FeO sufficiently to cause a roughly 10-fold

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increase in kobs, leaving little room for enhancement from sulfidation or magnetization. Only

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minor effects of electrolyte have been seen in most previous studies,56 so the large effect of

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sulfate in these data will be investigated further in future work. Some of the other experimental

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factors tested did have their expected effects—e.g., kobs increased with dose of ZVI and

4

decreased with initial concentration of dye—but these effects are negligible in Figure 2. The

5

lack of a notable effect of IPA suggests that •OH generated by Fenton-like reactions does not

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play a significant role in Orange I decolorization under the conditions tested in this study.

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To expand the perspective provided by Figure 2, we added data from a recent study48

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that used Orange II as the probe compound, but with most other experimental conditions similar

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to those used in this study, including aerobic well mixed batch reactors, micron-sized and high-

10

purity ZVI, and permanent magnets to apply a WMF during the reaction. All of the Orange II

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data follow the contour for R ≈ 5 (even though the corresponding data for kobs vary over several

12

orders of magnitude), which is significantly less enhancement from magnetization than was

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obtained in this study. Differences in properties of the two azo dyes might play a role in this, but

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the main factor is likely to be the pH of the experiments. Most of the Orange II data were

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obtained at pH ≈ 3.3, which favors dissolution of iron oxides, so there should be a thinner

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passive film for the WMF act on, whereas the Orange I data were obtained at pH ≈ 7, which

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favors a thicker oxide film, and therefore there is a greater barrier to reactions that can be

18

decreased by the interfacial forces induced by the WMF.46, 57

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Mechanism of Orange I Decolorization. The decolorization of azo dyes by reduction of their

20

chromophoric azo groups has been described for many dyes and reducing systems, including

21

Orange I and ZVI.21, 22, 58 The expected pathway leads through the hydrazo intermediate to

22

sulfanilic acid and 1-amino-4-naphthol as the primary products (Figure S7). HPLC-MS analysis

23

of the aqueous phase after 10, 30, 60, and 120 min of reaction confirmed sulfanilic acid was

24

formed and reached a peak concentration at about 30 min, while the concentration of Orange I

25

decreased gradually to nearly zero at 120 min (Figure S8 and Table S2). Due to the instability of

26

1-amino-4-naphthol (it is readily transformed to the corresponding quinone-imine21), it was not

27

detected by HPLC-MS. Increased absorbance from the quinone-imine is likely to be responsible

28

for the apparent rebound in Orange I (Figure 1A), because detection in these experiments was by

29

spectrophotometry without seperation. Analysis of the solid from experiments sacrificed at 30

30

and 120 min gave evidence for the N-H bonding at the interface (Figure S9), which is consistent



1

with azo bond cleavage and adsorption of the products. There was appearance of one product

2

peak at 251 nm in UV-vis spectra obtained on the sample solutions (Figure S5 B-D), which is

3

attributable to 1-amino-4-naphthol.21, 58 However, another possible product, sulfanilic acid, has a

4

λmax = 249 nm, so it cannot be excluded by UV-vis. The same products were formed by ZVI with

5

magnetization and/or sulfidation. Therefore, we concluded that the effect of these treatments was

6

to increase the rate of Orange I decolorization, but not to change the reaction pathway.

7

Persistence of Sulfidation and Magnetization Effects. To investigate the persistence and

8

compatibility of the effects of magnetization with sulfidation, sequential sets of four batch

9

experiments were performed by respiking with Orange I while varying the order of

10

magnetization. Four sequences of magnetization were tested: presence at each respike (+ + + +),

11

absence throughout (− − − −), alternating beginning with presence (+ − + −), and alternating

12

beginning with absence (− + − +). The resulting changes in Orange I concentration, DO, and Eh

13

are shown in Figure 3 for Fe/FeO and Figure S10 for Fe/FeS.

14

For the dye decolorization kinetics obtained with Fe/FeO (Figure 3A), comparison of the

15

respike experiments obtained with WMF throughout (blue circles) to without WMF throughout

16

(red circles) shows that the increase in contaminant removal rates due to magnetization is

17

persistent over all four cycles. The WMF effect can also be seen in the DO and Eh data (Figures

18

3B and 3C, respectively) by noting that the initial drop in both of these variables is much more

19

pronounced in the presence of the WMF than in the absence, while the subsequent rebound to

20

less reduced conditions is much more prevalent in the absence of the WMF. These same trends

21

are seen in the respike experiments conducted with Fe/FeS (Figure S10), but with less

22

pronounced differences between the + + + + WMF and − − − − WMF treatments because the

23

latter was also enhanced by sulfidation.


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A

Orange I (mg/L)

100 80 60 40 20 0

B

8

DO (mg/L)

6 Fe/FeO, + + + + WMF Fe/FeO, − − − − WMF Fe/FeO, + − + − WMF Fe/FeO, − + − + WMF

4

2

0 400

C

Eh (mV vs SHE)

300 200 100 0 -100 0

1 2 3 4 5 6

20

40

60

80

100

120

Time (min)

Figure 3. Time-series plots of (A) dye concentration, (B) dissolved oxygen concentration and (C) redox potential for re-spike batch experiments conducted with Fe/FeO and in the presence (or absence) of a WMF during each cycle (indicated in the legend by + or −). The legend in B applies to A and C. The corresponding set of respike data obtained with Fe/FeS re-spike are presented in Figure S10.



The data for the alternating sequences of magnetization (hour glass symbols in Figures 3

1 2

and S10) show additional effects. During the first cycle, these data follow the same pattern as the

3

corresponding data for +WMF or –WMF throughout (i.e., + − + − starts the same as + + + + and

4

− + − + starts the same as − − − −). In the second cycle, however, + − + − continues the trend

5

seen in the + + + + sequence, but − + − + deviates from − − − − and shows the trend seen in + +

6

+ +. These results demonstrate that while the WMF effect can be induced by adding the WMF at

7

any time, the WMF effect persists even after the WMF is removed. The latter is due to the

8

persistence of magnetization (i.e., “remanence”) that is a characteristic of iron and some related

9

metals and metal oxides4, 59-61, and which has recently been shown to significantly enhance the

10

removal of a variety of contaminants by ZVI (R = 1.2 – 12.2 after premagnetization for 2 min4,

11 12

13

49

). No evidence for temporal decay in remanence is evident in our respike data, but cycles

longer than 30 min were not investigated. To clarify the relationship between the magnetization and sulfidation effects on Orange I

14

removal in the respike experiments, we fit the dye decolorization data in Figures 3A and S10A

15

to pseudo-first order kinetics. The fits are not shown in the figures for clarity, but the good

16

agreement between model and data is reflected in the small uncertainties in kobs, which are

17

recorded in Table S3. Summarizing these data in Figure 4 confirms that the conclusions made

18

above hold regardless of position in the treatment sequence: e.g., (i) that the overall trend in

19

reactivity is Fe/FeO –WMF

Effects of Sulfidation, Magnetization, and ... - ACS Publications

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