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Effects of Sulfide Concentration and Dissolved Organic Matter Characteristics on the Structure of Nanocolloidal Metacinnabar Brett A. Poulin, Chase A. Gerbig, Christopher Kim, John P Stegemeier, Joseph N. Ryan, and George R. Aiken Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02687 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Environmental Science & Technology

Effects of Sulfide Concentration and Dissolved Organic Matter Characteristics on the Structure of Nanocolloidal Metacinnabar

Brett A. Poulin1*, Chase A. Gerbig2, Christopher S. Kim3, John P. Stegemeier3, Joseph N. Ryan2, George R. Aiken1

1

U.S. Geological Survey, 3215 Marine St., Suite E127, Boulder, CO 80303, United States

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Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, UCB 607, Boulder, CO 80309, United States

3

Schmid College of Science and Technology, Chapman University, One University Drive, Orange, CA 92866, United States

*

Corresponding Authors: Tel: +1 303 541 3050. Fax: +1 303 541 3084. Email: [email protected]

TOC Art

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Abstract Understanding the speciation of divalent mercury (Hg(II)) in aquatic systems containing

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dissolved organic matter (DOM) and sulfide is necessary to predict the conversion of Hg(II) to

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bioavailable methylmercury. We used X-ray absorption spectroscopy to characterize the

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structural order of mercury in Hg(II)-DOM-sulfide systems for a range of sulfide concentration

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(1-100 µM), DOM aromaticity (specific ultraviolet absorbance (SUVA254)), and Hg(II)-DOM and

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Hg(II)-DOM-sulfide equilibration times (4-142 h). In all systems, Hg(II) was present as

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structurally-disordered nanocolloidal metacinnabar (β-HgS). β-HgS nanocolloids were

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significantly smaller or less ordered at lower sulfide concentration, as indicated by under-

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coordination of Hg(II) in β-HgS. The size or structural order of β-HgS nanocolloids increased with

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increasing sulfide abundance and decreased with increasing SUVA254 of the DOM. The Hg(II)-

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DOM or Hg(II)-DOM-sulfide equilibration times did not significantly influence the extent of

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structural order in nanocolloidal β-HgS. Geochemical factors that control the structural order of

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nanocolloidal β-HgS, which are expected to influence nanocolloid surface reactivity and

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solubility, should be considered in the context of mercury bioavailability.

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Introduction

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The methylation of divalent mercury (Hg(II)) in the environment, which governs mercury

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bioaccumulation in aquatic biota,1 is controlled by the geochemical form of Hg(II) and anaerobic

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metabolic pathways that facilitate Hg(II) methylation.2–4 Under suboxic-to-anoxic conditions,

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Hg(II) speciation is controlled by interactions with dissolved organic matter (DOM) and sulfide,4 2 ACS Paragon Plus Environment

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the latter from dissimilatory sulfate reduction. Thiol groups in DOM strongly bind Hg(II),5–8 but

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they can be outcompeted by sulfide, which results in the formation of nanocolloidal

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metacinnabar (β-HgS).9–11 β-HgS nanocolloids, observed as small as 3-5 nm in diameter in

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laboratory10 and contaminated natural systems,12,13 are also expected to form in

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uncontaminated aquatic systems with low mercury burden.9 Hg(II) associated with

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nanocolloidal β-HgS can be methylated by anaerobic microorganisms;10,14–18 additionally, rates

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of Hg(II) methylation are influenced by the sulfide concentration,16 composition of DOM

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(e.g., aromaticity, reduced sulfur content),16,18 ratio of Hg(II) to dissolved organic carbon

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(Hg(II):DOC),15 and kinetics of Hg(II)-DOM-sulfide interactions.10,17 However, geochemical

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explanations for differences in methylation of Hg(II) associated with nanocolloidal β-HgS are

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lacking under conditions typical of aquatic systems where methylation is prevalent.

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The influence of aqueous chemistry on the structural order of nanocolloidal β-HgS may

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explain observed differences in Hg(II) methylation in Hg(II)-DOM-sulfide systems.10,14–18 For

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example, nanocolloidal β-HgS is smaller or less structurally ordered (i.e., sulfur coordination

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number < ideal β-HgS)9 and more bioavailable to methylation15 with decreasing Hg(II):DOC ratio

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in solutions. Generally, nanocolloids less than 30 nm in diameter have a higher percentage of

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metal atoms at or near the colloid surface (i.e., under-coordinated) and exhibit enhanced

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surface reactivity and faster rates of dissolution and renucleation.19,20 Effects of other

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important conditions on the structural order of nanocolloidal β-HgS (e.g., sulfide concentration,

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DOM composition, Hg(II)-DOM-sulfide equilibration time) either have not been evaluated or

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have been surveyed under Hg(II):DOC conditions (e.g., ≥ 103 nmol Hg(II) (mg DOC)-1)10,11,14 that

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greatly exceed environmental conditions (10-4-10-3 nmol Hg(II) (mg DOC)-1)21,22 and exceed the 3 ACS Paragon Plus Environment


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strong Hg(II) binding site capacity of DOM (~10 nmol Hg(II) (mg DOC)-1).5 High Hg(II):DOC in

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laboratory studies on nanocolloidal β-HgS structure are, in part, necessary because of analytical

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limitations of X-ray absorption spectroscopy that can be overcome by concentrating mercury

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species with solid phase extraction (SPE).9

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Studies on nanocolloidal10,11,14 and bulk β-HgS formation,23 and those on interactions

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between DOM and bulk mercuric sulfide24,25 and other natural nanocolloids,26 demonstrate the

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importance of DOM aromaticity, sulfide concentration, and kinetics of Hg(II)-DOM-sulfide

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interactions. Increasing DOM aromaticity and decreasing sulfide concentration results in slower

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nanocolloidal β-HgS formation.11,27 In the environment, the specific ultraviolet absorbance

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(SUVA254) of DOM, a proxy for aromaticity,28 can vary considerably (1.0 ≤ SUVA254 ≤ 5.0)

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between and within aquatic systems.29,30 Furthermore, sulfide concentration between 1-

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100 µM are optimal for mercury methylation, above which levels of mercury methylation

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decline.31–34 Kinetic factors may also influence nanocolloidal β-HgS structure10,11 and therefore

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bioavailability.10,17 In environments pertinent for methylation, Hg(II) is deposited in oxic waters

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from atmospheric sources or delivered from up-gradient terrestrial pools, slowly forms strong

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Hg(II)-DOM complexes (> 24 h),35–37 and then encounters sulfide in reduced sediments where

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methylation occurs. It is unclear if the formation of strong Hg(II)-DOM complexes prior to

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sulfide exposure influence nanocolloidal β-HgS structure. Studies suggest that nanocolloidal β-

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HgS becomes more structured with increased Hg(II)-DOM-sulfide equilibration time.10,11

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Ultimately, improved ability to predict Hg(II) methylation in the environment requires better

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understanding of geochemical constraints on nanocolloidal β-HgS structure under conditions

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that resemble natural systems. 4 ACS Paragon Plus Environment

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To this end, we quantified the effects of sulfide concentration, DOM aromaticity, and

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kinetics of mercury binding with DOM and sulfide on the size or structural order of

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nanocolloidal β-HgS. We prepared Hg(II)-DOM-sulfide solutions at low Hg(II):DOC with varied

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experimental conditions (sulfide concentration, DOM composition, equilibration times),

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concentrated mercury species by SPE, and analyzed samples by extended X-ray absorption fine

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structure (EXAFS) spectroscopy. These experiments built on our previous investigation that

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demonstrated the importance of the Hg(II):DOC ratio on nanocolloidal β-HgS structure.9

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Materials and Methods

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Materials

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Ultrahigh purity water (≥18 MΩ cm resistivity) and trace-metal grade acids were used.

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Stock solutions were prepared in fluorinated ethylene propylene bottles cleaned with a solution

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of 10% HNO3 and 10% HCl for 24 h. Experimental solutions were prepared in borosilicate glass

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vessels with Teflon®-lined caps (I-Chem 200 Series, Fisher Scientific) cleaned with a solution of

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10% HNO3 and 10% HCl for 24 h and baked at 450 °C for 4 h. Inorganic reagents were purchased

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from Fisher Scientific (Na2S∙9H2O, Na2HPO4, NaOH, KBr, KBrO3), Acros Organics (NaClO4), the

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National Institute of Standards and Technology (NIST; Mercury Standard Reference Material

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3133), and Alfa Aesar (cinnabar (α-HgS); β-HgS).

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DOM samples used in this study, selected to span a range in source material and

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composition and used previously to study mercury-DOM interactions,5,9,15–17,23–25,38 included

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Florida Everglades F1 Site hydrophobic organic acid (F1-HPOA), Florida Everglades 2B South

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HPOA (2BS-HPOA), Suwannee River fulvic acid (SR-FA), Williams Lake HPOA (WL-HPOA), and 5 ACS Paragon Plus Environment


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Pacific Ocean FA (PO-FA). DOM HPOA and FA fractions were isolated according to Aiken et al.

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(1992).39 Site descriptions and elemental compositions of DOM samples are provided in

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Table S1 (Supporting Information (SI)).

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Experimental Solutions We quantified short-range structural changes in nanocolloidal β-HgS in response to

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Hg(II)-DOM equilibration time (t1) and Hg(II)-DOM-sulfide equilibration time (t2) (Experiment 1),

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sulfide concentration (Experiment 2), and DOM composition (Experiment 3) (Table 1). Stock and

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experimental solutions were prepared in an anoxic glovebox (95% N2, 5% H2; < 1 ppm O2;

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296±1 K) with high-purity water de-aerated by purging with ultra-high purity nitrogen for 1 h at

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373 K. Inorganic stock solutions were prepared daily (sulfide, washed Na2S∙9H2O salt) or

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monthly (Hg(NO3)2 in 10% HNO3, NIST 3133; phosphate buffer, Na2HPO4; NaClO4) and filtered

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prior to use (0.45 µm Supor® polyethersulfone membrane, Pall Life Sciences). DOM stock

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solutions were prepared daily by reconstituting DOM isolates in high-purity water, adjusting to

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pH 7 with 0.1 M NaOH, and passing through a 0.45 µm Supor® polyethersulfone membrane

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(Pall Life Sciences).

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Solution compositions and equilibration times (t1, t2) for Experiments 1-3 are provided in

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Table 1. t1 specifies the Hg(II)-DOM pre-equilibration time prior to sulfide addition. t2 specifies

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the time allowed for Hg(II)-DOM-sulfide solutions to equilibrate prior to mercury concentration

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by SPE. t1 and t2 are ≥ 4 h due to the time required to concentrate mercury by SPE.9 All

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experimental solutions (Experiments 1-3) were prepared from stock solutions in 1 L volumes at

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pH 6.5±0.1 (0.01 M phosphate buffer adjusted with 0.1 M NaOH), ionic strength of 0.1 M 6 ACS Paragon Plus Environment

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(adjusted with NaClO4 as calculated by Visual MINTEQ),40 and 22-25 mg L-1 DOC (Table 1). The

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oxidation of sulfide (100 µM) by NaClO4 (0.09 M) was negligible over the time frame of

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experiments (≤ 142 h). Experiment 1 varied t1 and t2 equilibration times of solutions contained

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approximately 150 nM Hg(II) and 100 µM sulfide; the Hg(II):DOC ratios did not exceed the

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strong Hg(II) binding site capacity of the DOM (Table 1).5 t1 was varied (12-142 h) to identify the

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influence of strong Hg(II)-DOM complex formation35–37 on nanocolloidal β-HgS structure, and t2

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was varied (4-121 h) to identify the influence of aging of Hg(II)-DOM-sulfide solutions on

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nanocolloidal β-HgS structure. Experiment 2 solutions varied the concentration of sulfide (1-

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100 µM) at low (92-110 nM) and high (400-750 nM) Hg(II) concentrations, which represent

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Hg(II):DOC conditions below and above the strong Hg(II) binding site capacity of the DOM,

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respectively. The range in sulfide concentration tested (1-100 µM) is consistent with levels

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observed of optimal mercury methylation in the environment31–34 and pure-culture laboratory

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incubations.16 Equilibration times of Experiment 2 solutions were uniform (t1 = 24±1 h;

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t2 = 24±1 h). Experiment 3 solutions varied the SUVA254 of the DOM at approximately 150 nM

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Hg(II) and 100 µM sulfide, which represent Hg(II):DOC conditions below the strong Hg(II)

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binding site capacity of the DOM.5 Equilibration times of Experiment 3 solutions were uniform

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(t1 = 24±1 h; t2 = 24±1 h). All solutions were supersaturated with respect to β-HgS as calculated

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by Visual MINTEQ.9,40 Immediately following solution preparation, vessels were capped with a

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nitrogen head space, covered in aluminum foil, and mixed on an orbital shaker table rotating at

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150 rpm at 296±1 K (Thermo Scientific, Max Q 2000). The uniform background composition and

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handling of solutions allowed for direct evaluation of the tested variables in Experiments 1-3.



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For comparison, a solution was prepared that exceeded the solubility of β-HgS (150 nM

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Hg(II) and 100 µM sulfide; Visual MINTEQ)9,40 to which no DOM was added (referred to as

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“DOM-free”). The inorganic composition of the DOM-free solution was identical to those from

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Experiments 1-3. The EXAFS spectrum of the DOM-free sample was compared with mercury

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species formed in DOM-containing solutions (Experiments 1-3) and bulk α-HgS and β-HgS solids.

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Solid Phase Extraction Solid phase extraction of mercury from aqueous solutions for EXAFS spectroscopy

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analysis has been described by Gerbig et al. (2011).9 The evaluation of the chromatography of

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the SPE method9 confirm that mercury-mercury interactions are not responsible for Hg(II)

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retention on the C18 resin, and support that mercury species isolated on the C18 resin are

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present in aqueous solution. Briefly, 1 L of experimental solution was passed through a

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chromatography column containing C18 resin (Supelclean ENVI-18, Spectrapor) in an anoxic

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glovebox (95% N2, 5% H2). The majority of the mercury was retained in the upper one-third of

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the loaded C18 resin,9 which was removed from the column and immediately stored at 273 K

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under a nitrogen atmosphere until EXAFS analysis. In all cases, ≥ 98% of the total mercury in

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experimental solutions was retained on the C18 resin as quantified by cold vapor atomic

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fluorescence spectroscopy (CVAFS).

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Chemical Analyses

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DOC concentration was determined by persulfate oxidation (OI Analytical, model 700).

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Ultraviolet and visible light (UV-vis) absorption spectra were measured from 190-800 nm using 8 ACS Paragon Plus Environment

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a spectrophotometer (Agilent Technologies, model 8453) and a quartz cuvette; sample spectra

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were measured with respect to a blank spectrum of a cuvette containing high purity water. The

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SUVA254 of DOM samples, a proxy for aromaticity,28 is defined as the decadic UV absorbance at

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254 nm divided by the DOC concentration. Total aqueous mercury concentration on initial and

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effluent samples from the SPE was determined on oxidized samples (2% volume/volume 0.2 M

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BrCl for > 24 h) by CVAFS (Millennium Merlin Mercury analyzer, EPA Method 245.7). Calibration

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standards (0.01-0.5 nM), prepared from NIST Standard Reference Material 3133, showed an

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average recovery of 90-100%. The average daily detection limit for total aqueous mercury was

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≤ 0.013 nM determined as three times the standard deviation of the 0.01 nM standard (n = 7).

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The relative deviation in duplicate measurements of samples was ≤ 10%.

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Mercury L3-Edge EXAFS Spectroscopy

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Mercury L3-edge EXAFS spectra were collected on beamline 11-2 at the Stanford

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Synchrotron Radiation Lightsource using a Si(220) monochromator crystal (φ = 90° orientation).

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In an oxygen-free atmosphere, mercury-containing C18 resin was loaded in 2 mm-thick

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aluminum holders, sealed with Kapton® tape, and slowly cooled to 77 K in liquid nitrogen.

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Spectra were collected at 77 K, which minimizes noise from thermal vibration and possible

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beam damage of the sample, in fluorescence yield mode using a 32-element germanium

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detector. Gallium filters were used to minimize inelastic scattering. X-ray energy was calibrated

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using HgCl2 as a constant internal standard throughout data collection, with the maximum of

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the first peak in the first derivative calibrated to a value of 12282.0 eV. The number of scans

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collected per sample varied from 9-32 depending on the concentration of mercury in the C18 9 ACS Paragon Plus Environment


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resin (35-250 µg g-1 total mercury) and quality of spectra. No energy drift was detected during

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the course of a single sample’s data collection. In addition, EXAFS spectra (3 scans per sample)

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were collected on two commercially-available mercuric sulfide solids (α-HgS, β-HgS); solids

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were diluted as fine powders into boron nitride to a total mercury concentration of 300 µg g-1

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to minimize self-absorption.41 We recognize the low purity of commercially-available β-HgS.42

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For each experimental and reference sample, EXAFS scans were energy-corrected based on the

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calibration standard, deadtime-corrected, averaged, converted to k-space with k3-weighting

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(the E0 value was set at 12302 eV, 20 eV above the edge position), and Fourier-transformed. All

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data generated or analyzed during this study are included in the main text of this publication.

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Sample spectra were fit over an EXAFS k-range of 2.0-9.5 Å-1 and a Fourier transform

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range of 1.5-2.5 Å; accordingly, only first-shell fitting was performed. Using SixPACK43 and

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Feff6l,44 phase and amplitude functions were calculated ab initio for Hg-C, Hg-O, and Hg-S

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bonds. The usage of theoretical phase and amplitude functions to simulate single-shell

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scattering interactions, which is consistent with past efforts,10,11,42 allows flexibility and

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accounts for a wide range of variability in the samples during the fitting process. EXAFS fitting of

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experimental samples (Experiments 1-3) involved two steps. First, spectra were fit allowing the

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coordination number (CN), interatomic Hg-S distance (R), and Debye-Waller factor (σ2; a

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measure of static disorder) to float, while the scale factor (S0) was fixed at 0.9. The average

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Debye-Waller factor was calculated for each set of experimental samples

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(i.e., Experiment 1, 2, and 3; standard errors varied by 9%, 4%, and 10% of the averages,

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respectively) (Table 1) and differed at most by 0.0018 Å2 between experiments; values uses

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here are comparable to those used in previous studies on nanocolloidal β-HgS structure.10,11 10 ACS Paragon Plus Environment

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Spectra were then re-fit allowing the coordination number and Hg-S bond distance to float, and

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using a fixed S0 (0.9) and a fixed Debye-Waller factor (the average for each experiment). Due to

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the co-dependence of the coordination number and Debye-Waller factor, fixing the Debye-

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Waller factor and fitting the coordination number decreases uncertainty in fit parameters and

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allows for direct comparison of fitting results for each set of experimental samples for which

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the Debye-Waller factor was uniform. Thus, the structural order of nanocolloidal β-HgS was

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evaluated by the Hg-S coordination number and interatomic Hg-S distance.9,45,46 For the DOM-

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free sample, used as reference, the spectrum was fit allowing the coordination number and Hg-

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S bond distance to float, and using a fixed S0 (0.9) and a fixed Debye-Waller factor (the average

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fit value from Experiments 1-3; σ2 = 0.0090 Å2; Table S2). For the fitting of EXAFS spectra of bulk

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α-HgS and β-HgS solids, the Debye-Waller factor, coordination number, and Hg-S bond distance

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were allowed to float and S0 was fixed (0.9). Error in fit parameters (±) was determined at the

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95% confidence level. In all cases, goodness-of-fit was assessed through the R-factor of the fit.

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Results

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EXAFS Spectra of DOM-Free and Bulk Mercuric Sulfide Samples

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EXAFS fitting results of the DOM-free sample (CN = 3.9±0.3, R = 2.51±0.02 Å) are

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consistent with the theoretical (CN = 4, R = 2.53 Å)47 and measured coordination environment

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of bulk β-HgS (CN = 3.8±1.1, R = 2.51±0.02 Å) (Figure S1; Table S2). In comparison to EXAFS

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spectra of the DOM-free sample and bulk β-HgS, the EXAFS spectrum of bulk α-HgS is out of

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phase (Figure S1) and the primary Hg-S coordination environment (CN = 2.1±0.8,

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R = 2.38±0.02 Å; Table S2) is consistent with previous measurement of α-HgS.47 Thus, under the 11 ACS Paragon Plus Environment


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experimental conditions and in the absence of DOM, EXAFS results confirm that mercuric

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sulfide forms with a short-range structure consistent with bulk β-HgS.

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Effect of Equilibration Times on Short-Range Structure of Nanocolloidal Metacinnabar

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The Hg(II)-DOM (t1) and Hg(II)-DOM-sulfide equilibration times (t2) proved to have

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minimal influence on the structural order of nanocolloidal β-HgS (Experiment 1). EXAFS spectra,

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spectral fits, Fourier transforms, and fitting results of samples are presented in Figure S2 and

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Table 1. The k3-weighted spectra of all six samples are in phase with one another. The Hg-S

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interatomic distances of samples (2.50-2.53 Å, Table 1) did not differ significantly in response to

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variation in t1 or t2, and were comparable with that of the DOM-free sample (Table S2) and

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therefore crystalline β-HgS.47 Similarly, the coordination number did not differ significantly in

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response to manipulation of t1 or t2 (Table 1, Figure S3).

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Effect of Sulfide Concentration on Short-Range Structure of Nanocolloidal Metacinnabar The effect of sulfide concentration on the structure of nanocolloidal β-HgS was

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evaluated at low Hg(II) and high Hg(II) concentration, which represent Hg(II):DOC conditions

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below and above the strong Hg(II) binding site capacity of the DOM (Table 1; Experiment 2).

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EXAFS spectra, spectral fits, Fourier transforms, and fitting results for samples at low Hg(II)

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(Figure 1, spectra i-iii) and high Hg(II) (Figure 1, spectra iv-vi) concentration show significant

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differences as a result of varied sulfide concentration (Figure 1, Table 1). For the low sulfide and

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low Hg(II) sample (Figure 1, spectrum i), the EXAFS spectrum was fit with an average Hg-S

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scattering interaction at 2.48±0.02 Å, which is slightly shorter than the bond distance of the 12 ACS Paragon Plus Environment

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DOM-free sample (2.51±0.02 Å) but considerably longer than interatomic distances of known

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Hg(II) inorganic complexes (2.30-2.40 Å),48,49 Hg(II) organic complexes (2.34±0.01 Å),9,42,50 and

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α-HgS (2.30 Å).47 All other samples in Experiment 2 (Figure 1, spectra ii-vi) display EXAFS spectra

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which, when fitted, yield Hg-S interatomic distances ranging from 2.51-2.53 Å that agree, within

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uncertainties, with the Hg-S bond distance of the DOM-free sample (2.51±0.02 Å).

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Two notable trends were observed in these EXAFS spectra. First, for samples prepared

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at both low Hg(II) (Figure 1a, spectra i-iii) and high Hg(II) concentration (Figure 1a, spectra iv-vi),

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the amplitude of oscillation frequencies of k3-weighted spectra increased with increasing sulfide

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concentration. Shifts in k3-weighted spectra as a result of increased sulfide concentration

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correspond with an increase in (1) the amplitudes of the first-neighbor Fourier transform

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feature at approximately 2 Å (corresponding to the Hg-S bond distance of β-HgS corrected for

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phase shift (Δ)), and (2) the coordination number of spectral fits (Table 1). As shown in

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Figure 1c, low Hg(II) samples with 1, 10, and 100 µM sulfide exhibited coordination numbers of

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EXAFS spectral fits of 2.2±0.2, 2.7±0.2, and 3.3±0.3, respectively. Similar relative increases in

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the coordination number of high Hg(II) samples were observed with increasing sulfide

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concentration: from 3.1±0.2, 3.8±0.3, and 4.5±0.4 at 1, 10, and 100 µM sulfide, respectively

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(Figure 1c). Second, at comparable sulfide concentration, a greater coordination number was

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observed in samples prepared at high versus low Hg(II) concentration (Figure 1c). When

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Experiment 2 data are evaluated as a function of the ratio of Hg(II) to sulfide (Hg(II):S(-II)) of

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solutions, an increase in the coordination number is observed with decreasing Hg(II):S(-II) for

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samples at low Hg(II) and high Hg(II) concentration (Figure S4). In summary, at the same DOC

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concentration, increasing the concentration of either Hg(II) or sulfide, while holding the 13 ACS Paragon Plus Environment


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concentration of the other constant, resulted in an increase in the coordination number of

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EXAFS spectral fits.

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Effect of DOM Composition on Short-Range Structure of Nanocolloidal Metacinnabar

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EXAFS spectra, spectral fits, Fourier transforms, and fitting results show that the

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structure of mercuric sulfide in samples differs when formed in the presence of DOM of varying

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SUVA254 (Figure 2, Table 1). All spectra of samples from Experiment 3 were fitted with Hg-S

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interatomic distances of 2.49-2.53 Å (±0.01-0.02 Å), which is consistent with samples from

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Experiment 1 and 2 with sulfide ≥ 10 µM. In the presence of DOM with the lowest SUVA254

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(PO-FA), the EXAFS spectrum (Figure 2, spectra v) and fitting results (CN = 4.4±0.4,

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R = 2.49±0.02 Å) are consistent with that of the DOM-free sample (Figure S1, Table S2). With

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increasing SUVA254 of the DOM, a decrease was observed in (1) the amplitudes of the first-

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neighbor Fourier transform feature at approximately 2 Å (corresponding to the Hg-S bond

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distance of β-HgS corrected for phase shift (Δ)) and (2) the coordination number (Figure 2,

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Table 1). A negative correlation was observed between the SUVA254 of the DOM and the

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coordination number (p = 0.042, R2 = 0.80; Figure 2c). Correlations between the coordination

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number of samples and other properties of the DOM, for example the total sulfur content

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(Table S1), were not observed.

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Discussion

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Geochemical Factors Controlling the Short-Range Structure of Nanocolloidal Metacinnabar


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The short-range structure of nanocolloidal β-HgS, known to be influenced by the

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Hg(II):DOC ratio in solution,9 was examined here for the first time at low Hg(II):DOC (4-

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7 nmol Hg(II) (mg DOC)-1) under conditions of varying sulfide concentration, DOM composition,

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and Hg(II)-DOM and Hg(II)-DOM-sulfide equilibration times. In all Hg(II)-DOM-sulfide solutions,

314

EXAFS spectra were fit solely by first-shell Hg-S scattering interactions (2.48-2.53 Å; Table 1) and

315

commonly exhibited coordination numbers lower than that of bulk β-HgS (i.e., CN < 4). These

316

observations suggest that the predominant form of mercury in Hg(II)-DOM-sulfide solutions

317

was structurally-disordered nanocolloidal β-HgS, which is consistent with previous

318

observations.9–11 Here, observed differences in the coordination number of nanocolloidal β-

319

HgS, and to a lesser extent the Hg-S interatomic distance, as a result of varied experimental

320

conditions are interpreted to signify changes in the size or structural order of nanocolloidal β-

321

HgS.45,46 Therefore, results indicate that the size or structural order of nanocolloidal β-HgS

322

increased with increasing sulfide and Hg(II) concentrations, and decreased with increasing DOM

323

aromaticity (i.e., SUVA254). Results further suggest that the size or structural order of

324

nanocolloidal β-HgS was not influenced significantly by the Hg(II)-DOM or Hg(II)-DOM-sulfide

325

equilibration time (Figure S3, Table 1).

326

The most prominent shifts in EXAFS spectra and fitting results were observed in

327

response to changes in sulfide and Hg(II) concentrations (Figure 1c, Table 1). At low Hg(II) and

328

low sulfide concentrations (Figure 1, spectrum i) the coordination number (CN = 2.2±0.2) and

329

Hg-S interatomic distance (R = 2.48±0.02 Å) were significantly lower than that of crystalline β-

330

HgS (CN = 4, R = 2.53 Å)47 and the DOM-free sample (CN = 3.9±0.3, R = 2.51±0.02 Å). A less-

331

than-ideal coordination number and shorter average Hg-S interatomic distance could signify 15 ACS Paragon Plus Environment


332

highly disordered nanocolloidal β-HgS, where Hg(II) atoms at the surface of nanocolloids are

333

under-coordinated.9,11,19,46 However, this observation could also arise from the sample

334

containing a mixture of Hg(II) linearly coordinated with DOM thiols (R = 2.34±0.01 Å)9,42,50 and

335

nanocolloidal β-HgS (R = 2.53 Å), similar to Hg(II)-amended peats that contained Hg-S

336

interactions of different lengths (two sulfur atoms at 2.34 Å and one sulfur atom at 2.53 Å).42 At

337

low Hg(II) concentration, a novel observation here is that increasing sulfide concentration

338

(1-100 µM) resulted in an increase in the coordination number and Hg-S interatomic distance

339

(Figure 1c, Table 1), reflecting an increase in structural order of nanocolloidal β-HgS or a shift in

340

Hg(II) speciation to predominantly nanocolloidal β-HgS. At high Hg(II) concentration, the

341

structural order of nanocolloidal β-HgS increased with increasing sulfide concentration

342

(Figure 1c) and decreasing Hg(II):S(-II) (Figure S4). The observation of greater nanocolloidal β-

343

HgS structure at higher sulfide concentration contradicts equilibrium speciation models that

344

predict a decrease in the saturation index of β-HgS with increasing sulfide concentration due to

345

formation of charged aqueous Hg-S species;15,27,51,52 though it may not be appropriate to apply

346

such models here because of uncertain assumptions (e.g., equilibrium conditions)11 and model

347

inputs (e.g., β-HgS solubility does not account for particle size),4,20 and the lack of information

348

on the nucleation pathway of nanocolloidal β-HgS in the presence of DOM and free sulfide. At

349

the same sulfide concentration, nanocolloidal β-HgS was of higher structural order when

350

formed in solutions with higher Hg(II) concentration (Figure 1c, Figure S4). These results, which

351

agree with our previous observation,9 are likely due to saturation of strong Hg(II) binding sites

352

of the DOM at high Hg(II) concentration that hinder that effectiveness of DOM to decreasing

353

nanocolloidal β-HgS size or crystalline order (Figure 1c, Figure S4).9,11 We conclude that higher 16 ACS Paragon Plus Environment

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sulfide concentration favors the formation of larger or more ordered nanocolloidal β-HgS, and

355

the size or order of nanocolloidal β-HgS is ultimately a function of relative concentrations of

356

Hg(II), DOM, and sulfide.

357

DOM composition also exerts significant influence on the structural order of

358

nanocolloidal β-HgS. The SUVA254 of the DOM was negatively correlated with the coordination

359

number of nanocolloidal β-HgS (Figure 2c), which suggests that more aromatic fractions of

360

DOM28 are more effective at inhibiting the formation of crystalline nanocolloidal β-HgS

361

compared to DOM of lower aromaticity. This finding, the first at low Hg(II):DOC across a range

362

in DOM SUVA254, is consistent with the observation by Slowey (2010)11 of greater order of

363

nanocolloidal β-HgS formed in the presence of low versus high aromatic DOM. The mechanism

364

by which DOM limits the structural order of nanocolloidal β-HgS may be through sorption of

365

hydrophobic DOM molecules to surfaces of nucleating β-HgS nanocolloids that (1) inhibits

366

nanocolloid polymerization through steric hindrance and (2) stabilizes nanocolloids by

367

increasing electrostatic repulsion.26,53,54 This interpretation, which does not account for the

368

unknown influence of DOM on the initial nucleation of β-HgS, is supported by observation of

369

preferential association between β-HgS nanocolloids and the hydrophobic fraction of DOM

370

under the experimental conditions described here.9 Moreover, other laboratory efforts have

371

observed that DOM aromaticity positively relates to the ability of DOM to inhibit β-HgS

372

formation at higher Hg(II):DOC23,27 and nanocolloidal ZnS formation,53 and noted that more

373

hydrophobic DOM exhibits enhanced surface reactivity with bulk α-HgS.24,25 Alternatively,

374

complexation of Hg(II) by DOM thiol groups could hinder nanocolloidal β-HgS growth, as

375

inferred by experiments with model thiol ligands,27 though this mechanism is less likely given 17 ACS Paragon Plus Environment


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that the DOM sulfur content did not correlate with the structural order of nanocolloidal β-HgS.

377

Thus, we assert that the aromaticity of DOM in Hg(II)-DOM-sulfide solutions also exerts

378

influence on the structural order of nanocolloidal β-HgS.

379

To support the discussion of these results in the context of previous studies on the

380

formation9–11,27 and bioavailability of nanocolloidal β-HgS,10,14–18 Figure 3 presents a continuum

381

in nanocolloidal β-HgS formation with respect to (1) governing geochemical conditions,

382

(2) relative kinetics of transformations, and (3) the bioavailability of Hg(II) to methylation. In the

383

absence of free sulfide, representative of oxic systems where Hg(II) is atmospherically

384

deposited, Hg(II) forms strong complexes with thiol groups in DOM5–7 that take upwards of 24 h

385

to form via competitive ligand exchange;35–37 the strength of the Hg(II)-DOM complex is

386

dependent on the Hg(II):DOC ratio (i.e., Hg(II):DOC less or greater than the strong binding site

387

capacity of the DOM)5 and independent of DOM composition at low Hg(II):DOC where thiol

388

groups are abundant.38 When exposed to free sulfide (≥ 1 µM), Hg(II) is scavenged from Hg(II)-

389

DOM complexes initiating the formation of disordered nanocolloidal β-HgS. Formation of strong

390

Hg(II)-DOM complexes prior to sulfide exposure, evaluated here by manipulation of the Hg(II)-

391

DOM equilibration time (12 ≤ t1 ≤ 142 h), did not significantly influence nanocolloidal β-HgS

392

structure (Figure S3a). Thus, the release of Hg(II) from strong Hg(II)-DOM complexes does not

393

appear to be a rate-limiting step in nanocolloidal β-HgS formation. It is worth noting that over

394

week-to-month time scales, thiol-bound Hg(II) can also undergo conversion to nanoparticulate

395

β-HgS in the absence of inorganic sulfide.12 In the presence of free sulfide, disordered

396

nanocolloidal β-HgS forms quickly, as observed here (≤ 4 h; Figure S3a) and previously (≤ 1 h),10

397

with particle size estimates of approximately 1-2 nm in diameter.9,11 18 ACS Paragon Plus Environment

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The size or order of nanocolloidal β-HgS increases with increasing Hg(II):DOC ratio

399

(Figure 3)9 and sulfide concentration (Figure 1c), and decreases with increasing SUVA254 of the

400

DOM (Figure 2c). The age of Hg(II)-DOM-sulfide solutions over the time scale of hours to days,

401

shown previously to increase the structural order of β-HgS nanocolloids of size 3-5 nm in

402

diameter,10 did not significantly influence the size or structural order of nanocolloidal β-HgS

403

tested here (4 ≤ t2 ≤ 121 h, Figure S3b). Differences in experimental conditions may explain this

404

discrepancy, including lower Hg(II):DOC or Hg(II):S(-II) ratios here (6-7 nmol Hg(II) (mg DOC)-1

405

and 1-2 nmol Hg(II) (µmol S(-II))-1, respectively) versus previous studies

406

(≥ 103 nmol Hg(II) (mg DOC)-1 and 6-103 nmol Hg(II) (µmol S(-II))-1, respectively),10,11 or the

407

continuous mixing of solutions in this study versus quiescent conditions in previous works.10,11

408

Specifically, nanocolloidal β-HgS formation and aggregation in Hg(II)-DOM-sulfide solutions is

409

slower under conditions of lower Hg(II):DOC11,27 and DOM exhibits enhanced ability to slow

410

nanocolloidal HgS formation at lower Hg(II):S(-II).11 Here, measurable changes in the size or

411

order of nanocolloidal β-HgS may occur over time frames shorter or longer than that evaluated

412

(4 ≤ t2 ≤ 121 h).11 Nevertheless, concurrent with the aging of materials, interfacial processes can

413

facilitate Hg(II) release from β-HgS nanocolloids and agglomerates of individual nanocolloids

414

form (20-200 nm in diameter).10,11,27 Higher concentrations of DOM (i.e., lower Hg(II):DOC) and

415

DOM of greater aromaticity slow aggregation rates of nanocolloidal β-HgS.11,27

416

In the environment, variance in DOM quantity, DOM composition, and sulfide

417

concentration will likely dictate the structure of nanocolloidal β-HgS. First, small or disordered

418

nanocolloidal β-HgS formed under the conditions here (≥ 4 nmol Hg(II) (mg DOC)-1) and are

419

anticipated to form at lower Hg(II):DOC conditions found in aquatic environments (10-4-1019 ACS Paragon Plus Environment


420

3

nmol Hg(II) (mg DOC)-1).21,22 Furthermore, DOM aromaticity varies between and within aquatic

421

systems (1.0 ≤ SUVA254 ≤ 5.0)29,30 comparable to the range evaluated here (Table S1). We

422

anticipate more disordered β-HgS nanocolloids to form in systems with more aromatic DOM.

423

Lastly, sulfide concentration can vary spatially and temporally by 4 orders of magnitude in

424

sediment pore water of freshwater wetlands,31,32,55 estuaries,56 and the marine systems,33 and

425

is expected to positively correlate to nanocolloidal β-HgS size or structure.

426 427 428

Implications of Nanocolloidal Metacinnabar Structure on Hg(II) Bioavailability These findings provide a basis to interpret geochemical controls on Hg(II) methylation in

429

Hg(II)-DOM-sulfide systems, which are relevant to sulfidic environments where Hg(II)

430

methylation occurs via anaerobic bacteria.2,3,32 The mechanism explaining the methylation of

431

Hg(II) in solutions containing nanocolloidal β-HgS is unknown, but is hypothesized to be limited

432

by Hg(II) dissolution and ligand exchange at the surface of β-HgS nanocolloids15,16,18 that

433

facilitated Hg(II) uptake across the cell wall or exchange with cell wall components.4,57 Under

434

this premise, the methylation potential of Hg(II) associated with nanocolloidal β-HgS will likely

435

depend on the (1) size or order of nanocolloidal β-HgS and (2) concentrations of aqueous

436

ligands (e.g., DOM thiols, sulfide). Regarding the size or order of nanocolloidal β-HgS, Hg(II) in

437

Hg(II)-DOM-sulfide systems is more bioavailable at lower Hg(II):DOC15 and in the presence of

438

DOM of greater aromaticity16,17 likely due to the formation of smaller or more disordered β-HgS

439

nanocolloids (Figure 3). Smaller or more disordered β-HgS nanocolloids may be more

440

bioavailable for Hg(II) methylation because they exhibit enhanced surface reactivity and faster

441

rates of dissolution and renucleation.19,20 Efforts to quantify Hg(II) dissolution potential of β-HgS 20 ACS Paragon Plus Environment

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nanocolloids of various size, however, have not directly aligned with bioavailability

443

measurements10 perhaps due to analytical challenges in separating dissolved Hg(II) from β-HgS

444

nanocolloids. Furthermore, the ability of DOM to enhance Hg(II) methylation decreases with

445

increasing sulfide concentration (1-100 µM) in Hg(II)-DOM-sulfide systems,16 which may be

446

explained by formation of larger of more ordered β-HgS nanocolloids with increasing sulfide

447

concentration (Figure 1c). High concentrations of aqueous ligands including sulfide and DOM

448

thiols also influence Hg(II) methylation presumably due to formation of charged aqueous Hg-S

449

species15,27,51,52 and Hg(II)-DOM complexes,15,16,18 respectively. Ultimately, Hg(II) methylation in

450

Hg(II)-DOM-sulfide systems exhibits a time dependence10,14,17 that likely reflects non-

451

equilibrium geochemical conditions as a result of formation of aqueous Hg(II) complexes and

452

simultaneous nanocolloidal β-HgS formation and dissolution. The effects of sulfide

453

concentration and DOM composition on the structure of nanocolloidal β-HgS presented here

454

are critical considerations in the context of mercury bioavailability in the environment.

455

456

Supporting Information

457

Properties of dissolved organic matter isolates, and EXAFS spectra and fitting results. This

458

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

459

460 461 462

Acknowledgements The authors would like to thank Kathryn Nagy (UIC) and three anonomous reviewers for constructive comments on the manuscript and Chapman University researchers Connor Reilly 21 ACS Paragon Plus Environment


463

and Manny Vejar for assistance with EXAFS fitting. Research was supported by the National

464

Science Foundation (EAR-0447386) and the U.S. Geological Survey National Research, Greater

465

Everglades Priority Ecosystems Science (GEPES), and Toxic Substances Hydrology Programs. Use

466

of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is

467

supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences

468

under Contract No. DE-AC02-76SF00515. Any use of trade, firm, or product names is for

469

descriptive purposes only and does not imply endorsement by the U.S. Government.

470

471

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Manceau, A.; Marcus, M. A.; Tamura, N. Quantitative speciation of heavy metals in soils and sediments by synchrotron X-ray techniques. In Applications of Synchrotron Radiation in LowTemperature Geochemistry and Environmental Science; Fenter, P. A., Rivers, M. L., Sturchio, N. C., Sutton, S. R., Eds.; Mineralogical Society of America: Washington, D.C., 2002; pp 341–428. DOI: 10.2138/gsrmg.49.1.341. Nagy, K. L.; Manceau, A.; Gasper, J. D.; Ryan, J. N.; Aiken, G. R. Metallothionein-like multinuclear clusters of mercury(II) and sulfur in peat. Environ. Sci. Technol. 2011, 45 (17), 7298–7306; DOI: 10.1021/es201025v. Webb, S. M. SIXpack: A graphical user interface for XAS analysis using IFEFFIT. Phys. Scr. T 2005, T115, 1011–1014; DOI: 10.1238/Physica.Topical.115a01011. Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. Theoretical X-ray absorption fine structure standards. J. Am. Chem. Soc. 1991, 113 (14), 5135–5140; DOI: 10.1021/ja00014a001. Combes, J. M.; Manceau, A.; Calas, G.; Bottero, J. Y. Formation of ferric oxides from aqueous solutions: A polyhedral approach by X-ray absorption spectroscopy: I. Hydrolysis and formation of ferric gels. Geochim. Cosmochim. Acta 1989, 53 (3), 583–594; DOI: 10.1016/00167037(89)90001-X. Frenkel, A. I.; Hills, C. W.; Nuzzo, R. G. A view from the inside: Complexity in the atomic scale ordering of supported metal nanoparticles. J. Phys. Chem. B 2001, 105 (51), 12689–12703; DOI: 10.1021/jp012769j. Charnock, J. M.; Moyes, L. N.; Pattrick, R. A. D.; Mosselmans, J. F. W.; Vaughan, D. J.; Livens, F. R. The structural evolution of mercury sulfide precipitate: An XAS and XRD study. Am. Mineral. 2003, 88 (8-9), 1197–1203; DOI: 10.2138/am-2003-8-903. Bell, A. M. T.; Charnock, J. M.; Helz, G. R.; Lennie, A. R.; Livens, F. R.; Mosselmans, J. F. W.; Pattrick, R. A. D.; Vaughan, D. J. Evidence for dissolved polymeric mercury(II)-sulfur complexes? Chem. Geol. 2007, 243 (1–2), 122–127; DOI: 10.1016/j.chemgeo.2007.05.013. Tossell, J. A. Calculation of the structures, stabilities, and properties of mercury sulfide species in aqueous solution. J. Phys. Chem. A 2001, 105 (5), 935–941; DOI: 10.1021/jp003550s. Skyllberg, U.; Bloom, P. R.; Qian, J.; Lin, C.-M.; Bleam, W. F. Complexation of mercury(II) in soil organic matter: EXAFS evidence for linear two-coordination with reduced sulfur groups. Environ. Sci. Technol. 2006, 40 (13), 4174–4180; DOI: 10.1021/es0600577. Schwarzenbach, G.; Widmer, M. Die Löslichkeit von Metallsulfiden I. Schwarzes Quecksilbersulfid. Helv. Chim. Acta 1963, 46 (7), 2613–2628; DOI: 10.1002/hlca.19630460719 Paquette, K. E.; Helz, G. R. Inorganic speciation of mercury in sulfidic waters: The importance of zero-valent sulfur. Environ. Sci. Technol. 1997, 31 (7), 2148–2153; DOI: 10.1021/es961001n. Deonarine, A.; Lau, B. L. T.; Aiken, G. R.; Ryan, J. N.; Hsu-Kim, H. Effects of humic substances on precipitation and aggregation of zinc sulfide nanoparticles. Environ. Sci. Technol. 2011, 45 (8), 3217–3223; DOI: 10.1021/es1029798. Horzempa, L. M.; Helz, G. R. Controls on the stability of sulfide sols: Colloidal covellite as an example. Geochim. Cosmochim. Acta 1979, 43 (10), 1645–1650; DOI: 10.1016/00167037(79)90183-2. Poulin, B. A.; Ryan, J. N.; Nagy, K. L.; Stubbins, A.; Dittmar, T.; Orem, W.; Krabbenhoft, D. P.; Aiken, G. R. Spatial dependence of reduced sulfur in Everglades dissolved organic matter controlled by sulfate enrichment. Environ. Sci. Technol. 2017, 51 (7), 3630–3639; DOI: 10.1021/acs.est.6b04142. Marvin-DiPasquale, M. C.; Boynton, W. R.; Capone, D. G. Benthic sulfate reduction along the Chesapeake Bay central channel. II. Temporal controls. Mar. Ecol. Prog. Ser. 2003, 260, 55–70; DOI: 10.3354/meps168213. Szczuka, A.; Morel, F. M. M.; Schaefer, J. K. Effect of thiols, zinc, and redox conditions on Hg



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uptake in Shewanella oneidensis. Environ. Sci. Technol. 2015, 49 (12), 7432–7438; DOI: 10.1021/acs.est.5b00676.


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Table 1. Solution compositions, equilibration times, and mercury L3-edge EXAFS first-shell fitting results for experiments that varied the Hg(II)DOM (t1) and Hg(II)-DOM-sulfide (t2) equilibration times (Experiment 1), sulfide concentration (Experiment 2), and DOM composition (Experiment 3). --------------------------- Solution Compositions ------------------------DOM Isolate a

-- Equilibration Times --

DOC

DOM SUVA254

Hg(II)

S(-II)

Hg(II):DOC Ratio b

Hg(II)-DOM t1

Hg(II)-DOMS(-II) t2

(mg L-1)

(L (mg m)-1)

(nM)

(µM)

(nmol Hg(II) (mg DOC)-1)

(h)

(h)

----------------------------- L3-Edge EXAFS Fitting Results ---------------------------Figure Spectra

Hg-S Coord. Number c CN

Hg-S Bond Distance c R

Debye-Waller factor d σ2

(Å)

(Å2)

E0 Shift (eV)

R-factor

Experiment 1: Varying Hg(II)-DOM (t1) and Hg(II)-DOM-Sulfide Equilibration times (t2) F1-HPOA 24.9 4.2 169 100 6.8 12 F1-HPOA 24.2 4.2 171 100 7.1 24 F1-HPOA 24.9 4.2 169 100 6.8 142 F1-HPOA 23.1 4.2 142 100 6.1 24 F1-HPOA 22.8 4.2 158 100 6.9 24 F1-HPOA 23.0 4.2 161 100 7.0 24

24 24 24 4 12 121

Fig. S2.i Fig. S2.ii Fig. S2.iii Fig. S2.iv Fig. S2.v Fig. S2.vi

3.6±0.2 3.3±0.4 3.8±0.3 3.8±0.2 4.0±0.4 4.0±0.3

2.51±0.01 2.53±0.02 2.50±0.02 2.52±0.01 2.50±0.02 2.52±0.02

0.0086 0.0086 0.0086 0.0086 0.0086 0.0086

-10.6±2.1 -7.7±3.4 -11.9±2.9 -9.6±1.7 -12.8±3.3 -10.1±2.2

0.0067 0.0192 0.0125 0.0048 0.0154 0.0078

Experiment 2: Varying Sulfide Concentration at Low and High Hg(II) Concentration F1-HPOA 24.0 4.2 110 1 4.6 24 F1-HPOA 24.3 4.2 115 10 4.7 24 F1-HPOA 23.5 4.2 95 100 4.0 24 F1-HPOA 24.4 4.2 750 1 30.7 24 F1-HPOA 22.5 4.2 424 10 18.8 24 F1-HPOA 23.1 4.2 400 100 17.3 24

24 24 24 24 24 24

Fig. 1.i Fig. 1.ii Fig. 1.iii Fig. 1.iv Fig. 1.v Fig. 1.vi

2.2±0.2 2.7±0.2 3.3±0.3 3.1±0.2 3.8±0.3 4.5±0.4

2.48±0.02 2.51±0.02 2.52±0.02 2.52±0.02 2.53±0.02 2.53±0.02

0.0104 0.0104 0.0104 0.0104 0.0104 0.0104

-10.6±3.1 -8.3±2.6 -9.6±2.6 -8.1±2.2 -8.4±2.4 -8.5±2.9

0.0136 0.0109 0.0112 0.0079 0.0094 0.0143

24 24 24 24 24

Fig. 2.i Fig. 2.ii Fig. 2.iii Fig. 2.iv Fig. 2.v

3.1±0.2 3.6±0.3 3.9±0.2 4.0±0.2 4.4±0.4

2.53±0.01 2.53±0.02 2.49±0.01 2.50±0.01 2.49±0.02

0.0096 0.0096 0.0096 0.0096 0.0096

-8.2±1.9 -7.8±2.7 -12.8±2.0 -12.4±2.1 -10.7±2.9

0.0062 0.0123 0.0059 0.0061 0.0124

Experiment 3: Varying DOM Composition SR-FA F1-HPOA 2BS-HPOA WL-HPOA PO-FA

25.7 24.2 25.0 24.7 24.7

4.1 4.2 3.2 2.1 0.7

154 171 163 158 142

100 100 100 100 100

6.0 7.1 6.5 6.4 5.7

24 24 24 24 24

a Abbreviations denote Florida Everglades F1 Site hydrophobic organic acid (F1-HPOA), Florida Everglades 2B South HPOA (2BS-HPOA), Suwannee River fulvic acid (SR-FA), Williams Lake HPOA (WL-HPOA), and Pacific Ocean FA (PO-FA); site descriptions and chemical properties of DOM isolates are provided in Table S1 in the Supporting Information. b Strong Hg(II) binding site capacity exceeded when the Hg(II):DOC ratio exceeds 10 nmol Hg(II) (mg DOC)-1.5 c Error values represent 95% confidence intervals of fit parameters. d Debye-Waller factors (σ2) are average values for each set of experimental samples. See the Materials and Methods section for details on the determination on average σ2 values for each experiment.

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Figure 1. (a) k3-weighted mercury L3-edge EXAFS and (b) Fourier transforms of collected spectra (solid black lines) and spectral fits (dashed blue lines) of mercury-DOM-sulfide samples that varied the sulfide concentration (S(-II); 1-100 µM) at low Hg(II) (spectra i-iii) and high Hg(II) (spectra iv-vi) concentration (Experiment 2); and (c) the sulfur coordination number (CN) of samples as a function of sulfide concentration at low and high Hg(II) concentration; error bars represent 95% confidence intervals and dashed lines are provided to guide the eye. The vertical red dashed line in (b) corresponds to a Hg-S bond distance of metacinnabar (β-HgS, 2.53 Å)47 after accounting for the phase shift (Δ). The DebyeWaller factor was fixed at 0.0104 Å2 for all fits. Solution compositions are provided in (a). EXAFS fitting results are provided in Table 1.

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Figure 2. (a) k3-weighted mercury L3-edge EXAFS and (b) Fourier transforms of collected spectra (solid black lines) and spectral fits (dashed blue lines) of mercury-DOM-sulfide samples that varied the DOM composition (Experiment 3); and (c) the negative correlation between the coordination number (CN) of samples and the DOM specific ultraviolet absorbance at 254 nm (SUVA254);28 error bars represent 95% confidence intervals of CN values, the dashed gray line is the linear fit to experimental data, and the dotted black lines correspond to the 95% confidence intervals of the linear fit. The vertical dashed line in (b) corresponds to a Hg-S bond distance of metacinnabar (β-HgS, 2.53 Å)47 after accounting for the phase shift (Δ). The Debye-Waller factor was fixed at 0.0096 Å2 for all fits. Solution compositions and EXAFS fitting results are provided in Table 1.

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Figure 3. Schematic describing the progressive formation of nanocolloidal metacinnabar (nano β-HgS) including (1) complexation of divalent mercury (Hg(II)) by reduced sulfur groups in dissolved organic matter (DOM),5–8 (2) the formation of amorphous nano β-HgS in the presence of free sulfide (HS-/H2S) and DOM, (3) the aging of amorphous nano β-HgS to suspended crystalline nano β-HgS, and (4) aggregation of crystalline nano β-HgS.9–11,27 Size ranges of structural units are based on measurements (Hg(II)-DOM complex,9,42,50 and suspended and aggregated nano β-HgS)10,27 or estimates (denoted by asterisks).9,11 Horizontal bars indicate governing conditions on nano β-HgS formation (sulfide concentration (this study), Hg(II):DOC ratio,5,9 and DOM SUVA254 (this study)),38 the time of transformations,10,11,27,35–37 and bioavailability of species;10,14–17 the shading of horizontal bars corresponds qualitatively with the relative importance of the given variable across this continuum. The Hg(II)-DOM complex depicts Hg(II) coordinated to two proximal thiolates (yellow atoms) and two distant thioethers (orange atoms).8

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