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Activation of Persulfate by Nanosized Zero-valent Iron (NZVI): Mechanisms and Transformation Products of NZVI Cheolyong Kim, Jun-Young Ahn, Tae Yoo Kim, Won Sik Shin, and Inseong Hwang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05847 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018
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Environmental Science & Technology
Activation of Persulfate by Nanosized Zero-valent Iron (NZVI): Mechanisms and Transformation Products of NZVI
Cheolyong Kim1, Jun-Young Ahn1, Tae Yoo Kim1, Won Sik Shin2, and Inseong Hwang1* 1
Department of Civil and Environmental Engineering, Pusan National University, Busan 46241,
Republic of Korea 2
School of Architecture, Civil, Environmental and Energy Engineering, Kyungpook National
University, Daegu 41566, Republic of Korea
E-mail addresses:
[email protected] (Cheolyong Kim);
[email protected] (Junyoung Ahn);
[email protected] (Tae Yoo Kim);
[email protected] (Won Sik Shin)
[email protected] (Inseong Hwang)
* Corresponding author: Inseong Hwang, Department of Civil and Environmental Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea E-mail address:
[email protected]; phone: +82-51-510-3523; fax: +82-51-514-9574
Manuscript submitted to Environmental Science & Technology February 6, 2018 1
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Abstract
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The mechanisms involved in the activation of persulfate by nanosized zero-valent iron (NZVI)
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were elucidated and the NZVI transformation products identified. Two distinct reaction stages,
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in terms of the kinetics and radical formation mechanism, were found when phenol was
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oxidized by the persulfate/NZVI system. In the initial stage, lasting 10 min, Fe0(s) was
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consumed rapidly and sulfate radicals were produced through activation by aqueous Fe2+. The
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second stage was governed by Fe catalyzed activation in the presence of aqueous Fe3+ and iron
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(oxyhydr)oxides in the NZVI shells. The second stage was three orders of magnitude slower
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than the initial stage. An electron balance showed that the sulfate radical yield per mole of
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persulfate was more than two times higher in the persulfate/NZVI system than in the
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persulfate/Fe2+ system. Radicals were believed to be produced more efficiently in the
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persulfate/NZVI system because aqueous Fe2+ was supplied slowly, preventing sulfate radicals
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being scavenged by excess aqueous Fe2+. In the second stage, the multi-layered shell conducted
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electrons, and magnetite in the shell provided electrons for the activation of persulfate. Iron
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speciation analysis (including X-ray absorption spectroscopy) results indicated that a shrinking
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core/growing shell model explained NZVI transformation during the persulfate/NZVI process.
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TOC Art
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1. Introduction
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Chemical oxidation using activated persulfate (S2O82−) is an emerging method of
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remediating contaminated groundwater. Zero-valent iron (ZVI) has attracted interest as a
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persulfate activator because it is an efficient activator and can perform heterogeneous
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catalysis.1-5 ZVI corrodes to form dissolved Fe2+ (Fe2+(aq)) in the presence of dissolved oxygen
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(eq. 1) and persulfate ions (eq. 2). Fe2+(aq) then activates persulfate ions through electron
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transfer (eq. 3).4 ZVI can regenerate Fe2+ from the Fe3+ (eq. 4) formed through persulfate
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activation.6
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2+ 2𝐹𝑒 0 + 𝑂2 + 2𝐻2 𝑂 → 2𝐹𝑒(𝑎𝑞) + 4𝑂𝐻 −
(1)
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2+ 𝐹𝑒 0 + 𝑆2 𝑂82− → 𝐹𝑒(𝑎𝑞) + 2𝑆𝑂42−
(2)
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2+ 3+ 𝑆2 𝑂82− + 𝐹𝑒(𝑎𝑞) → 𝑆𝑂4•− + 𝑆𝑂42− + 𝐹𝑒(𝑎𝑞)
(3)
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𝐹𝑒 0 + 2𝐹𝑒 3+ → 3𝐹𝑒 2+
(4)
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In addition to being activated by Fe2+(aq) formed through the corrosion of Fe0(s), persulfate can
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be activated through catalysis by Fe2+/Fe3+ redox couple. This Fe catalyzed persulfate
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activation occurs homogeneously by aqueous Fe species and heterogeneously by the surface
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(oxyhydr)oxide layer of ZVI (eq. 5 and 6).7,8 It has previously been found that iron
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(oxyhydr)oxide such as magnetite (Fe3O4(s)) and goethite (α-FeOOH(s)) may be present on ZVI 4
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surfaces and can activate persulfate.8-10
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𝐹𝑒 3+ + 𝑆2 𝑂82− →
𝐹𝑒 2+ + 𝑆2 𝑂8•−
(5)
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𝐹𝑒 2+ + 𝑆2 𝑂82− →
𝐹𝑒 3+ + 𝑆𝑂4•− + 𝑆𝑂42−
(6)
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Nanosized ZVI (NZVI) is one of the most frequently studied materials for remediating
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groundwater because it is more reactive than larger ZVI particles and is mobile in subsurface.11
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Better pollutant removal efficiencies have been found using NZVI as a persulfate activator than
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using Fe2+(aq) or ZVI in several studies.5,12 However, the mechanisms of persulfate activation
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by NZVI in consideration of the NZVI core-shell structure and its transformation have not been
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elucidated so far, because the evolution of the NZVI core-shell structure in the presence of
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persulfate has not been adequately characterized. Previous studies found that the efficiency at
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which persulfate was activated by granular or microscale ZVI was controlled by the rate of iron
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corrosion (eq. 2), and scavenging of sulfate radicals by excess Fe2+(aq) (eq. 7) was suppressed
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because corrosion occurred relatively slowly.1,13,14
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2+ 3+ 𝑆𝑂4•− + 𝐹𝑒(𝑎𝑞) → 𝑆𝑂42− + 𝐹𝑒(𝑎𝑞)
(7)
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The activation mechanisms, which were observed in the granular/microscale ZVI system, may 5
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not occur in NZVI/persulfate systems because specific surface area of NZVI is several orders
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of magnitude larger than that of granular/microscale ZVI.
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Various ZVI transformation products in persulfate-based oxidation systems have been
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identified. Li et al.15 found magnetite, goethite, and hematite on ZVI surfaces in a persulfate-
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based oxidation system. Iron sulfate salts such as FeSO4(s) and Fe2(SO4)3(s) have also been
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found to be ZVI transformation products.16,17 These observations have been limited in that iron
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(oxyhydr)oxide minerals were identified without taking the sulfate-rich conditions into account,
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so predicting the formation of iron sulfate minerals was speculative and not quantitative. The
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effects of ZVI core-shell structure evolution have not been fully investigated, and the shell has
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been assumed only to inhibit persulfate activation because of the formation of passivated
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surface layers.2,15 Identifying the mechanisms involved in persulfate activation by NZVI and
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predicting sulfate radical formation require electron transfers between the iron and persulfate
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to be quantified. Furthermore, persulfate activation by pristine NZVI (mainly Fe0(s)) in the early
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stages of the persulfate activation process and persulfate activation by Fe2+ and Fe3+ (NZVI
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transformation products) in the later stages need to be studied.
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In this study we aimed to elucidate the mechanisms involved in persulfate
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activation by NZVI using kinetic and stoichiometric approaches. The roles of NZVI
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transformation products on persulfate activation were investigated by studying the kinetics of
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phenol removal in activation systems using aqueous Fe, NZVI, and NZVI transformation
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products. The evolution of the NZVI core-shell structure was studied by quantitatively
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determining iron species during the persulfate/NZVI reaction process. The formation and
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scavenging of sulfate radicals were quantitatively predicted from the electron balance on iron 6
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species/persulfate/phenol reactions.
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2. Materials and methods
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2.1. Batch experiments
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Deionized water (18.2 MΩ·cm), produced using a Barnstead Nanopure Diamond
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system (Thermo Fisher Scientific, USA), was used in all the experiments. Stock solutions of
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persulfate (1200 mM) and phenol (15 mM) were prepared by dissolving sodium persulfate
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(98%, Sigma-Aldrich, USA) and crystalline phenol (99%, Sigma-Aldrich) in water,
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respectively. Commercial NZVI, Nanofer25, was purchased from Nano Iron (Czech Republic).
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The typical characteristics of Nanofer25 are described elsewhere.18,19 The NZVI particles were
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suspended in deoxygenated water and dispersed by ultrasonicating the mixture for 30 min
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before use. Iron(II) sulfate (99%, Junsei Chemical Co., Japan) and Iron(III) sulfate (60-80%,
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Junsei Chemical Co., Japan) were used for preparing Fe2+(aq) and Fe3+(aq) solutions, respectively.
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Magnetite (97%, Sigma-Aldrich) was purchased and schwertmannite was synthesized after
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Reinsch et al.20
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Batch experiments with persulfate and NZVI were conducted using the method
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described next. Water, phenol stock solution, and dispersed NZVI were added to a 125 mL
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borosilicate amber glass bottle to give 100 mL of a mixture containing 1.5 mM phenol and 6
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mM Fe. The mixture was allowed to equilibrate for 5 min, then an aliquot of the persulfate
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stock solution was added to give a persulfate concentration of 12 mM. The persulfate to Fe
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molar ratio was 2, previously found to be a relatively effective reactant ratio.21 Tests in which 7
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persulfate was activated by Fe2+(aq) or Fe3+(aq) were also performed using a persulfate to Fe
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molar ratio of 2. Each test was performed in triplicate unless otherwise specified.
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Preliminary tests applying persulfate/NZVI process to buffered solutions, a
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contaminated groundwater, and a contaminated soil slurry indicated that the pH decreased
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almost instantly to
