Impact of Redox Reactions on Colloid Transport in Saturated Porous

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Impact of Redox Reactions on Colloid Transport in Saturated Porous Media: An Example of Ferrihydrite Colloids Transport in the Presence of Sulfide Peng Liao, Songhu Yuan, and Dengjun Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02542 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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Impact of Redox Reactions on Colloid Transport in Saturated Porous

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Media: An Example of Ferrihydrite Colloids Transport in the

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Presence of Sulfide Peng Liao†, Songhu Yuan*,†, Dengjun Wang‡

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5

†

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of Geosciences, 388 Lumo Road, Wuhan 430074, P. R. China

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‡

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Science, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, P. R.

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China

State Key Laboratory of Biogeology and Environmental Geology, China University

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil

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* To whom correspondence should be addressed. E-mail: [email protected] (S.

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Yuan), Phone: +86-27-67848629, Fax: +86-27-67883456.

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ABSTRACT Transport of colloids in the subsurface is an important environmental

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process with most research interests centred on the transport in chemically stable

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conditions. While colloids can be formed under dynamic redox conditions, the impact

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of redox reactions on their transport is largely overlooked. Taking the redox reactions

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between ferrihydrite colloids and sulfide as an example, we investigated how and to

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what extent the redox reactions modulated the transport of ferrihydrite colloids in

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anoxic sand columns over a range of environmentally relevant conditions. Our results

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reveal that the presence of sulfide (7.8–46.9 µM) significantly decreased the

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breakthrough of ferrihydrite colloids in the sand column. The estimated travel

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distance of ferrihydrite colloids in the absence of sulfide was nearly 7-fold larger than

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that in the presence of 46.9 µM sulfide. The reduced breakthrough was primarily

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attributed to the reductive dissolution of ferrihydrite colloids by sulfide in parallel

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with formation of elemental sulfur (S(0)) particles from sulfide oxidation. Reductive

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dissolution decreased the total mass of ferrihydrite colloids, while the negatively

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charged S(0) decreased the overall zeta potential of ferrihydrite colloids by attaching

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onto their surfaces and thus enhanced their retention in the sand. Our findings provide

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novel insights into the critical role of redox reactions on the transport of

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redox-sensitive colloids in saturated porous media.

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INTRODUCTION

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The transport of naturally-occurring and engineered nanoparticles (NPs) and

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colloids in porous media has gained great attention over the past decade.1,2 Colloid

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transport is highly dependent on both the physicochemical conditions of aqueous

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phase and the properties of the solid surfaces.1‒4 Perturbations of physicochemical

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conditions (e.g., flow velocity, particle size, ionic strength and composition, pH, and

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ligands) have long been recognized to affect the aggregation, deposition, and

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remobilization of colloids in oxic porous media.1,5‒9 Potential mechanisms initiating

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colloid transport upon varying physicochemical perturbations primarily include the

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changes in Derjaguin-Landau-Verwey-Overbeek (DLVO) (i.e., electrostatic and van

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der Waals interactions) and non-DLVO (e.g., Born, hydration, and steric interactions)

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interactions.1,4,6,10,11

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Compared to the above perturbation conditions, transport of colloids also occurs

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in redox oscillation environments that occur naturally or are impacted by human

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acitivities.12‒14 Mounting evidences have documented that colloidal iron (e.g., goethite

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and hematite), silver nanoparticles (AgNPs), and carbon nanomaterials (e.g., graphene)

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could exist in either reducing or oxidizing conditions and play a central role in the

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mobility of contaminants loaded on them.15‒22 For example, due to the presence of

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electron donors and microbial population dynamics, transport of Fe(III) hydroxide

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colloids in anoxic environments may be accompanied by the reduction via biotic or

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abiotic reactions.12,13,19 Meanwhile, the contaminants that are adsorbed or precipitated

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onto the Fe(III) hydroxide colloids may be either released from or reduced on the 3 ACS Paragon Plus Environment

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surfaces, depending on the prevailing biogeochemical conditions.3,23 It is therefore

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logical to anticipate that the transport behavior of colloids under dynamic redox

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conditions could be different from that under chemically stable environments.4,10

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However, despite decades of research on the transport of colloids and the associated

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contaminants,1,3,4,11,24 the effect of redox reactions on the stability and transport of

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colloids has been largely neglected. It is thus of great importance to incorporate the

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knowledge of redox chemistry into colloid transport framework.

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Ferrihydrite colloids are ubiquitous in the subsurface environments and play a

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significant role in modulating the transport and transformation of nutrients (e.g.,

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phosphate) and contaminants (e.g., arsenic).3,25‒28 A number of laboratory and field

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studies have substantiated that the interplay of ferrihydrite (and other Fe(III)

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(hydro)oxides) and sulfide played a critical role in the redox dynamics of anoxic-oxic

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interface environments including aquifers, wetlands, and marine sediments, where

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sulfide was the predominant abiotic reductant for Fe(III) hydroxides.29‒34 While

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controlled experiments in well-mixed systems have established the mechanistic

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framework for the reactions between ferrihydrite and sulfide,31,32,35‒37 the effects of

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these reactions on the transport of ferrihydrite colloids in anoxic porous media remain

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obscure. It is noteworthy that in addition to the physical transport of ferrihydrite

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colloids in the presence of sulfide, the colloids could also be subjected to severe

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chemical alterations that may influence the mobility and stability during transport.17

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For instance, the transport of ferrihydrite colloids is likely to be accompanied by the

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reduction-induced transformation, thereby releasing Fe(II) and generating oxidized 4 ACS Paragon Plus Environment


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sulfur products simultaneously.31 The released Fe(II) is likely to accelerate the

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transformation of ferrihydrite colloids, forming more crystallized Fe oxides.38‒41 The

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products (e.g., sulfur) from ferrihydrite-sulfide reactions could adsorb or attach onto

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the ferrihydrite colloids, changing the particle size and surface charge of ferrihydrite

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colloids and eventually altering their transport. Although recent research advances on

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the cotransport of different types of colloids/NPs in saturated porous media

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demonstrated that the transport of primary colloids was strongly influenced by the

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coexisted secondary colloids,10,42-44 no redox reactions were involved in these

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processes.

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The objective of this study was to unravel the role of redox reactions between

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ferrihydrite and sulfide, as an example, on the transport of ferrihydrite colloids in

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saturated porous media. To this end, column experiments were designed to understand

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the transport and retention behavior of ferrihydrite colloids in anoxic quartz sand by

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monitoring their breakthrough curves (BTCs) and retention profiles (RPs) under

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environmentally relevant sulfide concentrations (0–46.9 µM) at pH 6.0. Knowledge

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generated from this study complements the traditional colloid transport research and

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provide novel insights into understanding the fate of colloid-associated contaminants

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and nutrients in redox dynamic environments.

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EXPERIMENTAL METHODS

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Ferrihydrite Colloids. Ferrihydrite colloids used in this study were synthesized

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following previously published procedures,45 with details provided in Section S1 in 5 ACS Paragon Plus Environment

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the Supporting Information (SI). The ferrihydrite stock suspensions were shown to be

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stable over a period of two months. Ferrihydrite colloid influent suspensions used for

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the column transport experiments were prepared by adding aliquots of ferrihydrite

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stock suspensions to 400 mL of 3 mM NaCl solution at pH 6.0 (described below),

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stirring for 2 min, and sonicating (100 W, 45k Hz) for 5 min at 25 °C to ensure a

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homogeneous suspension. The average hydrodynamic diameter and zeta potential of

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ferrihydrite colloids in the influent suspensions (0.375 mM ferrihydrite, 3 mM NaCl,

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and pH 6.0) were determined to be 112.5 ± 11.5 nm and 40.5 ± 1.9 mV, respectively,

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using the dynamic light scattering (DLS, Zetasizer Nano ZS90, Malvern Instruments

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Ltd., U.K.). The change in particle size of ferrihydrite colloids over the duration of 12

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h was insignificant (Figure S4), indicating a good stability throughout the column

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transport experiments.

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Porous Media. Quartz sand (ultrapure with 99.9% SiO2, Hebei Zhensheng

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Mining Ltd., China) with a mean diameter (d50) of 0.50 mm was used as granular

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porous media.6,7,10 Prior to use, the quartz sand was thoroughly cleaned to remove

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metal oxides, colloids, and clays adsorbed on the surface by soaking sequentially in

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12 M HCl, 1 M NaOH, and 30% H2O2 for 24 h each, rinsing with deionized (DI)

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water 20–30 times and drying at 105 °C for 24 h.

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Column Experiments. The column setup is depicted in Figure S5, and the

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corresponding experimental parameters are listed in Table S1. Cylindrical Plexiglas

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columns (2.5 cm inner diameter × 10 cm long) were dry-packed with quartz sand with

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a porosity of 0.43. The influent ferrihydrite colloid suspensions were continuously 6 ACS Paragon Plus Environment


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stirring on a shaker at 200 rpm. The influent pH was buffered at 6.0 ± 0.1 using 5 mM

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2-(N-morpholino)ethanesulfonic acid (MES). After packing, the columns were first

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flushed for 20 min with ultrapure N2 gas (99.999%) to replace O2 in the column. They

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were then pre-equilibrated with 10 pore volumes (PVs) of O2-free background

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solution (3 mM NaCl and 5 mM MES). Following pre-equilibration, 11.25 PVs of

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deoxygenated ferrihydrite colloid suspensions (0.714 mM) and 11.25 PVs of a

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solution containing different concentrations of sulfide were fed separately and mixed

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at a volume ratio of 1:1, resulting in an influent ferrihydrite colloids concentration of

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0.375 mM and the total injected PVs of 22.5. The mixing time of ferrihydrite colloids

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with sulfide before injecting into the column was less than 1.5 min. According to the

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reaction kinetics obtained in later batch experiments, approximately 50% of reactions

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occurred during the initial mixing period before injecting with the remaining reactions

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occurring during the first 3 cm of transport in the column. Fifteen PVs of background

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electrolyte solution (3 mM NaCl and 5 mM MES) were ultimately fed to flush the

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unattached ferrihydrite colloids in the column. Because H2S is the dominant form of

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sulfide at pH 6.0 (i.e., 91.2% H2S and 8.7% HS-), to ensure the actual concentrations

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of sulfide in the influent solution are equivalent to those designed for column

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transport experiments (i.e., 7.8, 15.6, 31.3, and 46.9 µM), the empirical values of

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~20% higher amount of initial sulfide concentrations than theoretically calculated

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ones were transferred into the column (Figure S6). Our measurements proved that the

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concentrations of sulfide in the influent suspension varied by less than 20% over the

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time frame of the column experiments (Figure S6). All column experiments were run 7 ACS Paragon Plus Environment

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at the room temperature (25 ± 1 °C) in an upward mode using a peristaltic pump (2

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mL/min or 5.87 m/d). All transport experiments were conducted at least in duplicate.

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To better understand the mechanisms controlling the transport of ferrihydrite

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colloids by sulfide, a series of separate experiments with respect to the effects of

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hydroquinone, dissolved Fe(II), and elemental sulfur (S(0)) particles, instead of

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sulfide, on ferrihydrite colloids transport, were performed (Table S1). The procedures

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were the same as those for the transport experiments described above. In addition to

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the transport of these separate experiments, a transient triple-pulse transport

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experiment was further performed to more clearly delineate the impacts of sulfide and

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S(0) particles on the transport of ferrihydrite colloids. This experiment involves the

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sequential injection of three influents: (i) ferrihydrite colloids alone (13 PVs), (ii)

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ferrihydrite colloids with sulfide (7 PVs), and (iii) ferrihydrite colloids with sulfide

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and S(0) (7 PVs) (Table S1).

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At predetermined time intervals, aliquots (5 mL each time) of suspensions from

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the column effluents were withdrawn from the sampling port using 5 mL vacuum

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syringes. The detailed protocol of sampling is provided in Section 2 in SI. Three mL

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of the sample was analyzed for total Fe, total Fe(II), and S(0) concentrations. The

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remaining 2 mL sample was filtered through a 0.22 µm nylon membrane for dissolved

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Fe(II) measurement. All the experimental operations were carefully conducted in the

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anoxic cabinet to minimize the potential Fe(II) oxidation because ferrihydrite colloids

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are likely to be reduced by sulfide. The parameters reflecting effluent redox properties

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including ORP, pH, and DO concentration were monitored with probes installed 8 ACS Paragon Plus Environment


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in-line at the exit of the column. Although the elaborative operations were taken to

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minimize potential interference of oxygen during the transport experiments, we

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recognize that a minute amount of oxygen (< 9.4 µM) could still occur in the column.

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After completion of the transport experiment, the sand was excavated from the

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column under gravity and dissected into 10 segments (1 cm long each) to obtain

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spatial distribution of the ferrihydrite colloids retained in the column. The column

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stayed saturated with background electrolyte solution during the course of dissection

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in an effort to avoid the remobilization of retained ferrihydrite colloids. This process

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was conducted in an argon-filled glovebox (O2: 0−10 ppmv). To dissolve the

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ferrihydrite colloids retained on the quartz sand, 10 mL of 1 M HCl solution was

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added into each segment (7.5 g sand), and the mixture was continuously shaken at 150

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rpm for 12 h. In an additional experiment (i.e., in the presence of 31.3 µM sulfide), 10

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mL of chloroform was transferred into each segment to extract and probe the retained

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S(0) in the column.

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Batch Experiments. To quantitatively identify the reaction intermediates of

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ferrihydrite upon reduction by sulfide, a series of batch experiments were conducted

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at conditions identical to those used in the transport experiments. All the experiments

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were operated at the room temperature with oxygen limited conditions (undetectable

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DO). For each test, 180 mL of 0.375 mM ferrihydrite suspension was transferred into

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a 190-mL reactor, and different initial sulfide concentrations (0‒46.9 µM) were

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obtained by diluting 31.3 mM stock Na2S solution. To counteract sulfide partitioning

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between liquid and gas phases, an empirical value of ~20 % higher initial sulfide 9 ACS Paragon Plus Environment

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concentrations, as described above, was spiked into the solution and the headspace of

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the reactor was kept at < 8 mL. Three mM NaCl was used as the background

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electrolyte and the solution pH was buffered at 6.0 ± 0.1 with 5 mM MES. The reactor

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was immediately sealed and stirred at 600 rpm using a Teflon-coated magnetic

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stirring bar. All batch experiments were conducted at least in duplicate.

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Analyses. The concentrations of ferrihydrite colloids collected in column

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experiments were quantified by measuring Fe(III) concentrations, which are expected

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to be proportional to the number concentrations of colloids since negligible

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differences in particle sizes were measured for the column influents and effluents

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using DLS. Dissolved Fe(II) after filtration and total Fe(II) after digestion by 6 M HCl

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were measured using a modified 1,10-phenanthroline method at a wavelength of 510

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nm via a UV-vis spectrophotometer.46 Total Fe was determined after the sample was

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digested by 6 M HCl and subsequently reduced by 10% hydroxylamine hydrochloride.

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Fe(III) concentration was calculated by subtracting Fe(II) concentration from the total

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Fe concentration.

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The concentration of dissolved sulfide in the stock solution was calibrated with

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the standard iodometric titration method.47 Dissolved sulfide in filtered (0.22 µm,

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Nylon) samples during the reaction was quantified by the methylene blue method.48

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Elemental sulfur (S(0)) was extracted by chloroform for 5 h and then analyzed by an

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LC-15C high performance liquid chromatography (HPLC, Shimadzu) equipped with a

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UV detector and an XDB-C18 column (4.6 × 50 mm) after derivatization with

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triphenyphosphine (TPP) to form triphenyphosphine sulfide (TPPS) in the presence of 10 ACS Paragon Plus Environment


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5% (w/v) phenol.49 The mobile phase consisted of a mixture of acetonitrile and water

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(80:20, v/v) at 0.6 mL/min, and the detection wavelength was 254 nm.

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The average hydrodynamic diameter and zeta potential of the ferrihydrite colloids

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before and after reaction with sulfide in batch experiments were determined by DLS.

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Solids for characterization were collected from the inlet of column (0–1 cm) after

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completion of transport experiment as well as from batch experiments after 30 min of

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reaction. The samples were prepared by placing wet pastes of solid materials onto

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glass slides and then argon-dried inside the glovebox chamber. The potential phase

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transformation of ferrihydrite upon reduction by sulfide was examined by X-ray

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diffraction (XRD) using a Bruker d8 Advance X-ray diffractometer equipped with a

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Cu Kα radiation source. X-ray photoelectron spectroscopy (XPS) spectra of the solid

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samples were collected on a Kratos Axis Ultra XPS using a monochromated Al-Ka

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X-ray source (1486.6 eV). Air exposure time during sample loading into the XPS

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chamber was less than 1 min, thus potential redox variations of iron and sulfur are

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regarded as negligible. Transmission electron microscope (TEM, Tecnai TM Spirit)

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was employed to unravel the interaction between ferrihydrite colloids and sulfide. The

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samples were prepared by placing a drop of suspension (~20 µL) collected from the

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column inlet (0–1 cm) after the transport experiments onto a 200-mesh carbon coated

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copper grid, followed by drying in the glovebox.

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Mass Recovery Calculation. The effluent mass recovery of ferrihydrite colloids

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(Meff) was obtained by dividing the mass of ferrihydrite colloids collected in the

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effluents (all 37.5 PVs) by that injected into the column. Details for the calculation are 11 ACS Paragon Plus Environment

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given in the Section S3 in SI. The mass recovery of ferrihydrite colloids retained in

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the column (Mret) was calculated by dividing the total mass of ferrihydrite colloids

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recovered from the dissection experiments by that injected into the column. Summing

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the percentages of effluent mass and retained mass resulted in the overall mass

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recoveries of ferrihydrite colloids in the transport experiments.

247 248

RESULTS AND DISCUSSION

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Effect of Sulfide on Ferrihydrite Colloids Transport. Since ferrihydrite colloids

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and quartz sands were positively and negatively charged, respectively (Table 1),

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electrostatic attractions predominate the deposition of ferrihydrite colloids in quartz

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sand initially. Regardless of sulfide concentrations examined, nearly all of the injected

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ferrihydrite colloids were retained (C/C0 ≈ 0) during the first 2 PVs (Figure 1a). Once

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these favorable retention sites of porous media (sand surfaces) are completely

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occupied, a substantive breakthrough of ferrihydrite colloids starts to occur. For

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example, in the absence of sulfide (0 µM), the value of C/C0 remained zero within the

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first 2 PVs, increased sharply with the flushing from 2 to 15 PVs, and reached a

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steady-state breakthrough (C/C0 = 0.95) at about 15 PVs. When the sand surfaces are

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occupied by the positively charged ferrihydrite colloids, electrostatic repulsions start

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to determine the transport of ferrihydrite colloids, resulting in a high breakthrough

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during later stages of transport (e.g., > 2 PV). Similar findings were reported by

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Kuhnen et al.6 who observed a nearly complete breakthrough of positively charged

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hematite colloids (C/C0 ≈ 1) in negatively charged quartz sands under the comparably 12 ACS Paragon Plus Environment


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experimental conditions (3 mM NaNO3 and pH 5.8) with our study.

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Interestingly, sulfide was found to have a significant impact on the breakthrough

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of ferrihydrite colloids (Figure 1a). The Meff of ferrihydrite colloids declined from

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94.7 to 81.6, to 68.5, and to 48.8% when the sulfide concentration was elevated from

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0 to 15.6, to 31.3, and to 46.9 µM, respectively (Table S2). This is due primarily to

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the less electrostatic repulsion interaction between positively charged ferrihydrite

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colloids at higher sulfide concentrations (Table 1). A negative linear correlation (R2 =

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0.985) existed between the Meff of ferrihydrite colloids and the initial sulfide

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concentration (Figure S7), suggesting that sulfide concentration controls the transport

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of ferrihydrite colloids. As expected, the Mret increased from 8.9 to 26.7 and to 37.4%

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as the concentration of sulfide elevated from 0 to 31.3 and to 46.9 µM, respectively

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(Figure 1b, Table S2).

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The presence of sulfide also substantially affected the shapes of BTCs and RPs of

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ferrihydrite colloids (Figure 1a,b). The BTCs of ferrihydrite colloids were almost

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symmetrical in shape and exhibited low tailing in the presence of low concentrations

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(0–7.8 µM) of sulfide. In contrast, the shapes of the BTCs became asymmetrical and

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the steady-state breakthrough started to disappear when the sulfide concentration was

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≥ 15.6 µM. In the presence of low concentrations of sulfide, the retention of

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ferrihydrite colloids in sand decreased linearly (R2 > 0.910, not shown) with the

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distance from inlet. In the presence of higher sulfide, however, the retention of

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ferrihydrite colloids exhibited a hyper-exponential decay with greater retention in the

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section adjacent to the column inlet (0−3 cm) (Figure 1b, Table S2). For example, 13 ACS Paragon Plus Environment

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with an initial 46.9 µM sulfide, the fraction of ferrihydrite colloids deposited in the

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column inlet (0−3 cm) was about 53.0%, compared to 32.6% in the absence of sulfide

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(Table S2). The similar type of hyper-exponential retention for colloids was

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previously

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conditions.7,24,50,51

encountered

under

both

favorable

and

unfavorable

attachment

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The overall mass recovery of ferrihydrite colloids, taking the Meff and Mret into

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account, decreased from 103.7% in the absence of sulfide to 86.2% in the presence of

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46.9 µM sulfide (Table S2). This unbalance triggered by sulfide reflects that the redox

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reactions (e.g., reductive dissolution) probably occurred between ferrihydrite colloids

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and sulfide, decreasing the total mass of ferrihydrite and thus affecting the fate of

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ferrihydrite colloids. In addition, the increase in Mret with increasing sulfide

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concentration suggests that sulfide enhanced the retention of ferrihydrite colloids in

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porous media. Possible mechanisms for these observations may be related to multiple

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factors such as ionic strength, pH, and the redox reactions along with the products

300

generated. As the ionic strength (3 mM NaCl and 5 mM MES) and pH (6.0) remained

301

constant over the course of the experiments, the influence of these two factors was

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precluded. Consequently, it is rational to assume that the redox reactions between

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ferrihydrite colloids and sulfide, as stated above, and the associated reaction products

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are most likely to be responsible for the reduced breakthrough of ferrihydrite in the

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column. More details will be discussed later.

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The effect of sulfide on the transport of ferrihydrite colloids was further evaluated

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with regard to the travel distance, which is defined as the distance with 99.9% of the 14 ACS Paragon Plus Environment


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particles retained in porous medium. The travel distance can be simply evaluated by4 L 0.01 = ln(0.01)

L Cout ln( ) C

(1)

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where L is the length of the packed bed (m), Cout is the effluent ferrihydrite colloids

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concentration obtained at 22.5 PVs, and C is the fraction of inlet ferrihydrite colloids

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concentration that was not suffered from redox reactions, which can be evaluated as C

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= C0 × (Meff + Mret), where C0 is the concentration of influent ferrihydrite colloids

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(0.375 mM) . Variation of travel distance exhibited an inverse correlation with sulfide

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concentration (Figure 1c), showing a remarkable decrease with increasing sulfide

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concentration. The estimated travel distance of ferrihydrite colloids in the absence of

317

sulfide was nearly 7-fold larger than that in the presence of 46.9 µM sulfide.

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Redox Reactions and Associated Products during the Transport. The redox

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reactions between ferrihydrite and sulfide have been proposed to occur through a

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sequence of reaction steps at the ferrihydrite surface, including surface complex

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formation, electron transfer, release of the oxidized product, and subsequent

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detachment of Fe(II).31,32,36 To explicitly decipher the reactions and the associated

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products in our study, batch experiments were first conducted at solution conditions

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identical to those used in the transport experiments. In all the cases investigated,

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dissolved sulfide concentration was completely consumed in the first 5 min (data not

326

shown). Total Fe(II) concentration increased rapidly within the first 5 min and

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subsequently leveled off during the rest of experiments (Figure 2a). These results

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suggest a fast kinetics upon reaction of ferrihydrite with sulfide, which agrees well


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with earlier findings in the same scenarios.31,32 The concentration of total Fe(II)

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rapidly increased with increasing initial sulfide concentration, with a predominance (>

331

80%) of dissolved Fe(II) (data not shown). An excellent linear correlation (R2 = 0.989)

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between Fe(II) generation and sulfide consumption was observed with a linear slope

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of 0.53 (Figure 2b). Given that only one electron was involved in the reduction of

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ferrihydrite to Fe(II), the slope of 0.53 reveals that each sulfide provided

335

approximately two electrons for ferrihydrite reduction. Thus, elemental sulfur (S(0))

336

was likely the sole sulfur product, consistent with the earlier observations.31,32 Indeed,

337

analysis of the solid phase shows that S(0) constituted most of the reaction products of

338

sulfide oxidation by ferrihydrite colloids (i.e., 92–96%, Table S3).

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FeS was demonstrated to be another essential sulfur product in addition to the S(0)

340

during ferrihydrite reduction by sulfide.32,33,52,53 However, the production of FeS in

341

our experiments is expected to be at least 10-fold lower compared to that of S(0)

342

(Table S3). It has been established that the production of FeS depended on the molar

343

ratio of initial sulfide concentration to the surface sites of Fe(III) oxides

344

(S(-II)/SS).33,53 Because the consumption of sulfide is much faster than the

345

detachment of Fe(II) from ferrihydrite surfaces, the higher S(-II)/SS ratio is conducive

346

to channel Fe(II) into FeS formation from the remaining sulfide while the lower

347

S(-II)/SS ratio may not be favorable for the production of significant amount of FeS

348

due to the low concentration of remaining sulfide in the solution.33 A recent study

349

showed that the build-up of substantial amount of FeS occurred when the S(-II)/SS

350

ratio was higher than a certain threshold (i.e., ca. 36).33 Assuming the concentration of 16 ACS Paragon Plus Environment


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351

surface sites of ferrihydrite was 6.3×10-6 mol/m2,54 the S(-II)/SS ratios obtained in our

352

study were 0.26–1.55, much lower than those reported.33 As a result, the formation of

353

FeS in our study is anticipated to be of minor importance.

354

In the column experiments, the generation of Fe(II) and S(0) as a function of

355

sulfide concentration was also observed (Figure 3). Consistent with the results

356

obtained in the batch experiments, Fe(II) concentration increased rapidly in response

357

to the increment in initial sulfide concentration (Figure 3a). The peak concentration of

358

S(0) accounted for ~80% of total sulfur when the initial concentration of sulfide was

359

31.3 µM (Figure 3b). Together with S(0) that was retained in the column (Figure S8),

360

the total of S(0) accounted for 92% of the initial sulfide, suggesting that S(0) was the

361

primary product of sulfide oxidation. To further identify the solid-phase sulfur

362

products, XPS analyses were performed for the samples collected from the column

363

inlet (0‒1 cm). Clearly, the high-resolution XPS spectra of S2s spectra proved that, in

364

the presence of 46.9 µM sulfide, the primary solid-phase sulfur species formed from

365

sulfide oxidation by ferrihydrite was elemental S(0) (Figure S9, Table S4).

366

To sum up, Fe(II) and S(0) are assigned to be the two main products for the

367

reactions between ferrihydrite colloids and sulfide in this study. Therefore, reductive

368

dissolution and formation of these two products are assumed to be accountable for the

369

reduced breakthrough of ferrihydrite colloids in the column in the presence of sulfide.

370

Role of Reductive Dissolution on the Reduced Breakthrough. The reductive

371

dissolution of ferrihydrite with production of Fe(II) unequivocally decreased the

372

overall mass of ferrihydrite, which apparently reduces the breakthrough of colloids in 17 ACS Paragon Plus Environment

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373

the column (see Mred, Table 1). To evaluate the impact of reductive dissolution on

374

ferrihydrite colloids transport, the transport of ferrihydrite colloids was examined in

375

the presence of hydroquinone, which has been proven to be effective in reducing

376

ferrihydrite colloids with Fe(II) as the sole inorganic product.45 Different from the

377

pronounced decrease caused by 31.3 µM sulfide, the transport of ferrihydrite colloids

378

was only slightly weakened by the same concentration of hydroquinone (Figure 4a).

379

The same extent of ferrihydrite reduction is reflected by the comparable BTCs of

380

Fe(II) in the presence of hydroquinone and sulfide (Figure 4b). As hydroquinone did

381

not result in any detectable alternation in the zeta potential of ferrihydrite colloids

382

(Table 1), the RPs of ferrihydrite colloids in the presence and absence of

383

hydroquinone were similar (Figure 4c). The direct contribution of ferrihydrite

384

reduction by sulfide can be approximately estimated from the difference of

385

ferrihydrite colloids transport with and without hydroquinone. Consequently, the

386

contribution of reduction on the reduced breakthrough of ferrihydrite colloids is

387

essentially attributed to the dissolution of ferrihydrite colloids by sulfide, which

388

decreased the total breakthrough concentration but did not alter the shapes of BTCs

389

and RPs of ferrihydrite colloids.

390

Role of Fe(II) on the Reduced Breakthrough. The dissolved Fe(II), which

391

makes up of a primary fraction of total Fe(II) produced, is expected to catalyze the

392

transformation of ferrihydrite colloids to either goethite or lepidocrocite,38‒41 and thus

393

likely alters the colloids transport in the column. To examine this influence, the sulfide

394

solution fed into the column was replaced by 53.6 µM dissolved Fe(II), which is close 18 ACS Paragon Plus Environment


Page 20 of 33

395

to that produced in the presence of 46.9 µM sulfide. Our XRD measurements

396

indicated that the transformation of ferrihydrite triggered by the dissolved Fe(II) was

397

negligible under the tested conditions (data not shown), largely due to the much

398

shorter contact time (~ 12 min) between ferrihydrite and dissolved Fe(II) in our study

399

than those in previous studies (i.e., days). The presence of 53.6 µM dissolved Fe(II)

400

did not significantly affect the transport and retention behaviors of ferrihydrite

401

colloids in quartz sand (Figure 4a), confirming that the influence of dissolved Fe(II)

402

on the transport of ferrihydrite colloids is insignificant.

403

Role of S(0) on the Reduced Breakthrough. The generated S(0) particles that

404

are negatively charged at ambient conditions55 likely affect the transport of positively

405

charged ferrihydrite colloids. Despite the observation of similar BTCs of Fe(II)

406

rendered by sulfide and hydroquinone due to the same number of electrons per mole

407

they donated, the transport and retention of ferrihydrite colloids in the presence of

408

31.3 µM hydroquinone was greater (Meff = 83.8 vs. 68.5%, Figure 4a, Table 1) and

409

lower (Mret = 6.6 vs. 26.7%, Figure 4c, Table 1), respectively, compared to the same

410

concentration (31.3 µM) of sulfide. Moreover, the zeta potential of ferrihydrite

411

colloids (initially 40.5 mV) in the presence of 31.3 µM hydroquinone (40.0 mV) was

412

much higher than that with 31.3 µM sulfide (32.1 mV) (Table 1). The differences in

413

the transport and zeta potential of ferrihydrite colloids collectively imply that, in

414

addition to the effect of direct reductive dissolution, the reaction products, i.e., S(0)

415

particles, play an appreciable role on the reduced breakthrough of ferrihydrite colloids

416

in the column. As the ferrihydrite colloids and sulfide were continuously injected into 19 ACS Paragon Plus Environment

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417

column, the formed S(0) particles during ferrihydrite colloids transport were likely to

418

adsorb onto ferrihydrite colloids and decreased their overall zeta potentials by

419

electrostatic neutralization effect, thus enhancing the retention of ferrihydrite colloids

420

in quartz sand.

421

To test the above speculation, additional experiments were performed by

422

measuring the transport of ferrihydrite colloids in the presence of 31.3 µM S(0)

423

particles (see preparation in Section S4) instead of sulfide. Considering that the

424

morphology of the synthesized S(0) particles was not identical to that of the S(0)

425

particles produced in the column transport experiments, the results presented here is

426

only for qualitative evaluation of the role of S(0) particles on ferrihydrite colloids

427

transport. Table 1 shows that the zeta potential of ferrihydrite colloids decreased from

428

40.5 to 34.5 mV when S(0) concentration was increased from 0 to 31.3 µM. A close

429

inspection of the BTCs of ferrihydrite colloids obtained with/without 31.3 µM S(0)

430

particles (Figure 4a) revealed that the Meff value was substantially lower in the

431

presence versus absence of 31.3 µM S(0) (73.7% vs. 94.7%). We further performed a

432

transient triple-pulse transport experiment to clearly compare the transport of

433

ferrihydrite colloids in the copresence of sulfide and S(0) particles with that in the

434

absence and presence of sulfide (Table S1, Figure 4d). Results showed that the

435

breakthrough C/C0 value of ferrihydrite colloids dropped from 0.94 in the absence of

436

sulfide to 0.70 in the presence of 31.3 µM sulfide, and further to 0.40 in the

437

concurrent presence of 31.3 µM sulfide and 31.3 µM S(0) particles (Figure 4d). These

438

observations strongly support the proposition that the presence of S(0) particles in the 20 ACS Paragon Plus Environment


439

Page 22 of 33

suspension does favor the retention of ferrihydrite colloids (see Mret-sulfide, Table 1).

440

The promotional effect of S(0) particles on retention accounted for the varying

441

shapes of RPs and thus the BTCs of ferrihydrite colloids. The hyper-exponential vs.

442

flat RPs in the presence of lower (0–7.8 µM) vs. higher (15.6–46.9 µM) sulfide

443

concentrations, respectively, may be attributed to the pronounced aggregation of

444

ferrihydrite NPs in the inlet triggered by the fast build-up of larger amount of negative

445

S(0) particles at higher sulfide concentrations. Comparison of the TEM micrographs

446

of suspensions collected from the column inlet (0–1 cm) clearly demonstrates that the

447

larger aggregates occurred at higher sulfide concentration (e.g., 46.9 µM, Figure S10).

448

These large aggregates could narrow down the pore throats of porous media and thus

449

aggravate the physical straining, producing the hyperexponential RPs for the

450

ferrihydrite colloids at high sulfide concentrations. Similarly, recent cotransport study

451

observed that the deposition of positively charged nTiO2 colloids in quartz sand was

452

largely expedited in the copresence of negatively charged nC60 particles.10 The

453

asymmetrical BTCs and lack of steady-state breakthrough for ferrihydrite colloids

454

transport occurring at higher sulfide concentration (Figure 1a) are likely due to the

455

fact that the adsorbed S(0) particles decreased the electrostatic repulsion attraction

456

between ferrihydrite colloids as well as between ferrihydrite colloids and sand (Table

457

1), thereby endowing more favorable sites for ferrihydrite colloids deposition onto

458

sand. This is ascertained by the batch adsorption experiments, demonstrating that the

459

adsorption of ferrihydrite colloids on quartz sand was substantially enhanced by the

460

addition of 31.3 µM S(0) particles (Figure S11). 21 ACS Paragon Plus Environment

Page 23 of 33


461

Relative Contributions of Reductive Dissolution and S(0) on the Reduced

462

Breakthrough. Given all the findings described above, we concluded that both the

463

reductive dissolution and the generation of elemental S(0) are responsible for the

464

reduced breakthrough of ferrihydrite colloids in the presence of sulfide. However, the

465

mechanisms for the influence are different. Reductive dissolution, as has already been

466

commented on above, only decreased the total mass of ferrihydrite colloids; whereas

467

elemental S(0) promoted the retention of ferrihydrite colloids in quartz sand, which

468

indirectly decreased their transport. A careful examination of the percentages of

469

ferrihydrite colloids decreased by reductive dissolution (Mred) and retained in column

470

as a result of the presence of sulfide concentration (Mret-sulfide) (Table 1) revealed that

471

the relative contribution of reductive dissolution to the reduced breakthrough of

472

ferrihydrite colloids in the column is similar to that of elemental S(0) over the tested

473

sulfide concentrations.

474

Implications. This work, to our knowledge, is the first study describing the role

475

of redox reactions between ferrihydrite colloids and sulfide on the transport of

476

ferrihydrite colloids in anoxic porous media. The most striking observation is that the

477

presence of sulfide at low concentrations significantly hinders the transport of

478

ferrihydrite colloids due to reductive dissolution and production of elemental S(0)

479

particles. The findings also highlight the distinct fate and transport of ferrihydrite

480

colloids in anoxic aquifers in the presence of low concentrations of sulfide.

481

Specifically, when the ferrihydrite colloids are injected into a contaminated aquifer to

482

elevate the efficiency of in situ bioremediation,56,57 the peculiar effect of sulfide 22 ACS Paragon Plus Environment


Page 24 of 33

483

produced from microbial sulfate reduction at organic-rich conditions is worthy of

484

attention because the presence of sulfide weakens the transport distance of ferrihydrite

485

colloids.

486

In contrast to the broad knowledge of colloids transport research,1,2,4,24,58 finding

487

in this study extends our basic understandings on colloid transport processes from

488

steady state redox conditions (i.e., in the absence of chemical reactions) to perturbed

489

redox conditions (i.e., in the presence of chemical reactions). While this study has

490

focused on ferrihydrite transport impacted by sulfide, our results may have a profound

491

implications to the transport of other redox sensitive colloids in other redox dynamic

492

environments, i.e., in the interface of groundwater and surface water interaction,

493

where steep chemical (e.g., sulfur and organic matter) gradients and microbial

494

population dynamics facilitate the redox shifts.59,60 The oxidation/reduction of these

495

colloids and the subsequent formation of secondary solid phase may also affect the

496

final mobility of colloids, which deserves further investigation. Classical DLVO

497

theory and transport models were used to model colloids stability and transport under

498

chemically stable conditions. However, the transport behavior in the presence of

499

redox reactions is more complicated than those previous studies. Thus, further model

500

development is required to gain a deeper understanding on the transport of colloids in

501

redox dynamic systems.

502 503 504

Supporting Information Available Additional information: Sections S1‒S4, Figure S1‒S11, Table S1‒S4. This 23 ACS Paragon Plus Environment

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505

material is available free of charge via the Internet at http://pubs.acs.org.

506 507

ACKNOWLEDGEMENTS

508

We sincerely thank Dr. Daniel Giammar at the Washington University in St. Louis

509

for his critical comments, constructive suggestion, and editing. Discussions with Dr.

510

Moli Wan at the University of Bayreuth, Dr. Bin Gao at the University of Florida and

511

Dr. Zimeng Wang at the Stanford University were instructive. This work was

512

supported by the Ministry of Education for New Century Excellent Talents Support

513

Plans (NCET-13-1014) and the Natural Science Foundation of China (NSFC, No.

514

41522208, 41521001). Peng Liao acknowledges financial support from Shanghai

515

Tongji Gao Tingyao Environmental Science and Technology Development

516

Foundation (STGEF).

517 518

REFERENCES

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doi: 10.1002/9783527691395.ch3. (54) Peiffer, S.; Gade, W. Reactivity of ferric oxides toward H2S at low pH. Environ. Sci. Technol. 2007, 41, 3159–3164. (55) Steudel, R. Aqueous sulfur sols. Top. Curr. Chem. 2003, 230, 153–166. (56) Tobler, N. B.; Hofstetter, T. B.; Straub, K. L.; Fontana, D.; Schwarzenbach, R. P. Iron-mediated microbial oxidation and abiotic reduction of organic contaminants under anoxic conditions. Environ. Sci. Technol. 2007, 41, 7765−7772. (57) Braunschweig, J.; Bosch, J.; Meckenstock, R. U. Iron oxide nanoparticles in geomicrobiology: From biogeochemistry to bioremediation. New Biotechnol. 2013, 30, 793−802. (58) Elimelech, M.; Gregory, J.; Jia, X.; Williams, R. Particle Deposition and Aggregation: Measurement, Modelling and Simulation; Butterworth-Heinemann: Woburn, MA, 1995. (59) Christensen, T.H.; Bjerg, P. L.; Banwart, S. A.; Jakobsen, R.; Heron, G.; Albrechtsen, H. J. Characterization of redox conditions in groundwater contaminant plumes. J. Contam. Hydrol. 2000, 45, 165–241. (60) Killeen, S. A handbook on the groundwater–surface water interface and hyporheic zone for environment managers. EPA Science reports, 2009.

681 682 683



(a)

1.0

Ferrihydrite (C/C0)

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0.8 0.6 0.4 0 µM 7.8 µM 15.6 µM 31.3 µM 46.9 µM

0.2 0.0 0

5

10

15

20

25

30

35

40

PV

Retention (µ M/g sand)

3.0

(b) 0 µM 7.8 µM 15.6 µM 31.3 µM 46.9 µM

2.5 2.0 1.5 1.0 0.5 0.0

1

2

3

4

5

6

7

8

9

10

Distance (cm)

(c)

10

L0.01 (m)

8 6 4 2 0 0

684 685 686 687 688 689 690 691 692

10

20

30

40

50

Sulfide (µM)

Figure 1 (a) Breakthrough curves, (b) retention profiles, and (c) predicted maximum travel distance (L0.01) of ferrihydrite colloids as a function of sulfide concentration (0, 7.8, 15.6, 31.3, and 46.9 µM, respectively). C/C0 of ferrihydrite colloids refers to the ratio of effluent concentration of ferrihydrite at a sampling time (C) to the influent concentration of ferrihydrite colloids (C0 = 0.375 mM). Lines in (a-c) are not models fits of data. They are only shown to guide the eye. Experimental conditions are: 0.375 mM initial ferrihydrite colloids concentration, 2 mL/min flow rate, pH 6.0, and 3 mM NaCl. Error bars indicate 95% confidence intervals based on replicate experiments. 29 ACS Paragon Plus Environment

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100

60 40 20 0

R2=0.989

40 30 20 10 0

0

693 694 695 696 697 698 699 700 701 702 703 704

(b) [S(-II)]consumed=0.53*[Fe(II)] + 0.512

80

Fe(II) (µ M)

50

(a)

15.6 µM 46.9 µM

S(-II)consumed (µ M)

7.8 µM 31.3 µM

5

10

15

20

25

0

30

20

40

60

80

Fe(II)generated (µM)

Time (min)

Figure 2 Identification of reaction products upon ferrihydrite reduction by sulfide in batch experiments. (a) Effect of sulfide concentration (7.8, 15.6, 31.3, and 46.9 µM, respectively) on Fe(II) generation. Fitted curves were derived from first-order equation (Ct = Ceq(1 − e-kt), where Ct and Ceq are the concentration of Fe(II) at time t (min) and equilibrium, respectively, k is a pseudo first-order rate constant). (b) The consumption of sulfide versus the generation of Fe(II) measured at the end of individual experiments (30 min). The initial sulfide concentration for each experiments were 7.8, 15.6, 23.4, 31.3, 39.1, and 46.9 µM, respectively. Batch experimental conditions are: 0.375 mM initial ferrihydrite colloids concentration, pH 6.0, and 3 mM NaCl.

705

0.15

Elemental sulfur (S(0)) (C/C0)

Fe(II)eff (C/C0)

(a)

7.8 µM 15.6 µM 31.3 µM 46.9 µM

0.20

0.10

0.05

0.00 0

706 707 708 709 710 711 712 713

5

10

15

20

25

30

35

40

1.0

(b)

0.8

31.3 µM

0.6 0.4 0.2 0.0 0

5

10

PV

15

20

25

30

35

40

PV

Figure 3 Identification of reaction products upon ferrihydrite reduction by sulfide in column experiments. (a) Production of Fe(II) at different sulfide concentrations in the effluent. (b) Production of elemental sulfur in the presence of 31.3 µM initial sulfide concentration in the effluent. The C/C0 of Fe(II) in (a) and S(0) in (b) refer to the ratio of effluent concentration of Fe(II) and S(0) at a sampling time, respectively, to the influent concentrations of ferrihydrite colloids (C0 = 0.375 mM) and sulfide (C0 = 31.3 µM). The experimental conditions are the same as Figure 1.

714



(a)

0.8 0.6 0.4 no sulfide 31.3 µM sulfide 31.3 µM hydroquinone 31.3 µM S(0) particles 53.6 µM dissolved Fe(II)

0.2 0.0 0

5

10

15

20

25

30

(b)

0.16

Fe(II)eff (C/C0)

Ferrihydrite (C/C0)

1.0

0.12 0.08 0.04 31.3 µM sulfide 31.3 µM hydroquinone

0.00

35

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0

40

5

10

15

PV

0.6

716 717 718 719 720 721 722 723 724 725

Ferrihydrite (C/C0)

Retention (µ M/g sand)

0.9

30

35

40

(d)

0.8 0.6

Step 1 no sulfide

0.4

Step 2 31.3 µM sulfide Step 3 31.3 µM sulfide + 31.3 µM S(0)

0.2

0.3 0.0

715

no sulfide 31.3 µM sulfide 31.3 µM hydroquinone 31.3 µM S(0) particles 53.6 µM dissolved Fe(II)

1.2

25

1.0

(c)

1.8 1.5

20

PV

0.0 1

2

3

4

5

6

7

8

9

10

0

5

10

Distance (cm)

15

20

25

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35

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PV

Figure 4 Effects of hydroquinone, dissolved Fe(II), S(0) particles, and sulfide on (a) breakthrough curves of ferrihydrite colloids, (b) breakthrough curves of Fe(II) production, and (c) retention profiles of ferrihydrite colloids. The concentration of hydroquinone and S(0) particles are the same as that of the sulfide concentration (31.3 µM). (d) Effect of multiple transport processes on ferrihydrite colloids transport in column. Step 1: 0.375 mM ferrihydrite colloids alone; Step 2: 0.375 mM ferrihydrite colloids + 31.3 µM sulfide; Step 3: 0.375 mM ferrihydrite colloids + 31.3 µM sulfide + 31.3 µM S(0) particles. Experimental conditions are: 0.375 mM initial ferrihydrite colloids concentration, 2 mL/min flow rate, pH 6.0, and 3 mM NaCl. Error bars indicate 95% confidence intervals based on replicate experiments.

726


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727 728


Table 1 Electrokinetic potentials of ferrihydrite colloids and quartz sands, average hydrodynamic diameters of ferrihydrite colloids, and mass balance percentages for ferrihydrite colloids transport in column experiments a Conditions

ζferrihydrite b (mV)

ζsand c (mV)

Sizeferrihydrite d (nm)

Meff e (%)

Mred f (%)

Mret g (%)

Mret-inlet h (%)

Mret-sulfidei (%)

Mtot j (%)

0 µM 40.5 ± 1.9 –45.5 ± 0.5 112.5 ± 11.5 94.7 ± 4.5 0.0 ± 0.0 8.9 ± 0.1 32.6 ± 1.1 N.A. 103.7 ± 4.4 sulfide 7.8 µM 38.1 ± 1.1 –45.3 ± 1.3 121.2 ± 2.1 90.0 ± 1.3 5.0 ± 0.6 14.9 ± 1.7 36.2 ± 1.2 6.0 ± 2.4 110.0 ± 5.1 sulfide 15.6 µM 35.6 ± 1.4 –45.5 ± 1.4 137.0 ± 10.5 81.6 ± 3.5 9.3 ± 0.8 19.1 ± 0.5 40.6 ± 2.4 10.2 ± 0.8 110.0 ± 6.7 sulfide 31.3 µM 32.1 ± 3.0 –44.7 ± 0.6 153.4 ± 21.6 68.5 ± 3.7 14.8 ± 0.8 26.7 ± 1.1 45.5 ± 1.4 17.8 ± 1.0 110.0 ± 4.1 sulfide 46.9 µM 29.8 ± 2.9 –43.9 ± 0.8 174.8 ± 13.9 48.8 ± 9.3 18.9 ± 0.7 37.4 ± 2.5 53.0 ± 0.9 28.5 ± 2.2 105.1 ± 11.9 sulfide 31.3 µM S(0) 34.5 ± 0.2 –46.7 ± 1.2 311.0 ± 36.1 73.7 ± 4.1 0.0±0.0 27.4 ± 7.2 45.6 ± 3.2 N.A. 101.1 ± 3.2 particles 31.3 µM 40.0 ± 0.3 –44.2 ± 1.9 108.5 ± 12.6 83.3 ± 0.3 12.8 ± 0.5 6.6 ± 0.7 30.9 ± 0.3 N.A. 102.6 ± 1.5 Hydroquinone 53.6 µM 40.9 ± 1.6 –45.5 ± 0.5 107.9 ± 6.6 95.1 ± 2.4 0.0 ± 0.0 5.2 ± 1.2 33.9 ± 4.9 N.A. 99.9 ± 1.2 dissolved Fe(II) a N.A.: not applicable. b,cζ-potentials of ferrihydrite colloids and quartz sand, respectively. dAverage hydrodynamic diameter of ferrihydrite colloids. b,dThe values were measured by DLS at the end of batch experiments (30 min reaction). eMeff and fMred refer to the mass percentage of ferrihydrite colloids passing through the columns and reduced by sulfide. Details calculation of eMeff and fMred can be found in the SI. gMret refers to the mass percentage of ferrihydrite colloids retained in the columns. hMret-inlet is the mass percentage of ferrihydrite colloids that are retained near the column inlet (0–3 cm). iMret-sulfide (= Mret-with sulfide – Mret-without sulfide (8.9%)), reflects to the mass percentage of retained ferrihydrite colloids caused by sulfide. jMtot (= Meff + Mred + Mret) denotes the total mass percentages of Fe recovered from the columns. 729

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Impact of Redox Reactions on Colloid Transport in Saturated Porous

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