Mercury is a global contaminant of concern due to its transformation by microorganisms to form methylmercury, a toxic species that accumulates in biological tissues. The effect of dissolved organic ...matter (DOM) isolated from natural waters on reactions between mercury(II) (Hg) and sulfide (S(-II)) to form HgS
(s) nanoparticles across a range of Hg and S(-II) concentrations was investigated. Hg was equilibrated with DOM, after which S(-II) was added. Dissolved Hg (Hg
aq) was periodically quantified using ultracentrifugation and chemical analysis following the addition of S(-II). Particle size and identity were determined using dynamic light scattering and X-ray absorption spectroscopy. S(-II) reacts with Hg to form 20 to 200
nm aggregates consisting of 1–2
nm HgS
(s) subunits that are more structurally disordered than metacinnabar in the presence of 2
×
10
−9 to 8
×
10
−6
M Hg and 10 (mg C)
L
−1 DOM. Some of the HgS
(s) nanoparticle aggregates are subsequently dissolved by DOM and (re)precipitated by S(-II) over periods of hours to days. At least three fractions of Hg–DOM species were observed with respect to reactivity toward S(-II): 0.3
μmol reactive Hg per mmol C (60 percent), 0.1
μmol per mmol C (20 percent) that are kinetically hindered, and another 0.1
μmol Hg per mmol C (20 percent) that are inert to reaction with S(-II). Following an initial S(-II)-driven precipitation of HgS
(s), HgS
(s) was dissolved by DOM or organic sulfur compounds. HgS
(s) formation during this second phase was counterintuitively favored by lower S(-II) concentrations, suggesting surface association of DOM moieties that are less capable of dissolving HgS
(s). DOM partially inhibits HgS
(s) formation and mediates reactions between Hg and S(-II) such that HgS
(s) is susceptible to dissolution. These findings indicate that Hg accessibility to microorganisms could be controlled by kinetic (intermediate) species in the presence of S(-II) and DOM, undermining the premise that equilibrium Hg species distributions should correlate to the extent or rate of Hg methylation in soils and sediments.
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Historic placer gold mining in the Clear Creek tributary to the Sacramento River (Redding, CA) has highly impacted the hydrology and ecology of an important salmonid spawning stream. Restoration of ...the watershed utilized dredge tailings contaminated with mercury (Hg) introduced during gold mining, posing the possibility of persistent Hg release to the surrounding environment, including the San Francisco Bay Delta. Column experiments have been performed to evaluate the extent of Hg transport under chemical conditions potentially similar to those in river restoration projects utilizing dredge tailings such as at Clear Creek. Physicochemical perturbations, in the form of shifts in column influent ionic strength and the presence of a low molecular weight organic acid, were applied to coarse and fine sand placer tailings containing 109−194 and 69−90 ng of Hg/g, respectively. Significant concentrations of mercury, up to 16 μg/L, leach from these sediments in dissolved and particle-associated forms. Sequential chemical extractions (SCE) of these tailings indicate that elemental Hg initially introduced during gold mining has been transformed to readily soluble species, such as mercury oxides and chlorides (3−4%), intermediately extractable phases that likely include (in)organic sorption complexes and amalgams (75−87%), and fractions of highly insoluble forms such as mercury sulfides (6−20%; e.g., cinnabar and metacinnabar). Extended X-ray absorption fine structure (EXAFS) spectroscopic analysis of colloids obtained from column effluent identified cinnabar particles as the dominant mobile mercury-bearing phase. The fraction of intermediately extractable Hg phases also likely includes mobile colloids to which Hg is adsorbed.
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Background
Although uniquely capable of measuring multiple redox constituents nearly simultaneously with no or minimal sample pretreatment, voltammetry is currently underutilized in characterizing ...redox conditions in aquatic and terrestrial systems. Investigation of undisturbed media such as pore water requires a solid-state electrode, and such electrodes can be difficult to fabricate reproducibly. An approach to determine the concentrations of electroactive constituents using indirectly calibrated electrodes has been developed, but the protocol for and accuracy of this approach—the pilot ion method—has not been documented in detail.
Results
A detailed procedure for testing electrode quality is provided, and the application and limitations of the pilot ion method have been documented. To quantify Fe(II) and Mn(II), subtraction of non-linear baseline functions from voltammetric signals produced better calibration curves than did linear baselines, enabled lower detection limits and reliable deconvolution of overlapping signals, and was successfully applied to sediment pore water signals. We observed that electrode sensitivities often vary by tens of percent, and that the sensitivity declines over time. The ratio of calibration slopes of Mn(II) to Fe(II) varied by no more than 11% from one Hg/Au electrode to another and Fe(II) concentrations predicted by the Mn(II) pilot ion were, on average, 13% different from their actual values. However, concentration predictions by the pilot ion method were worse for less than 15 μM Fe(II) (46% different on average). The ratio of calibration slopes of Mn(II) to S(−II) varied by almost 20% from one Hg/Au electrode to another, and S(−II) predicted concentrations were as much as 58% different from their actual values. These predictions of Fe(II) and S(−II) concentrations indicate that the accuracy of the pilot ion method depends on how independent calibration slope ratios are from the electrode used. At medium-to-high concentration for the ocean, naturally derived dissolved organic carbon did not significantly affect the baseline-corrected electrode response of Mn(II) and Fe(II), but did significantly affect the response of S(−II).
Conclusions
Despite their intrinsic variability, Hg/Au electrodes fabricated by hand can be used to quantify O
2
, S(−II), Fe(II), and Mn(II) without calibrating every electrode for every constituent of interest. The pilot ion method can achieve accuracies to within 20% or less, provided that the underlying principle—the independence of slope ratios—is demonstrated for all voltammetric techniques used, and effects of the physicochemical properties of the system on voltammetric signals are addressed through baseline subtraction.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
In addition to affecting biogeochemical transformations, the speciation of As also influences its transport from tailings at inoperative mines. The speciation of As in tailings from the Sulfur Bank ...Mercury Mine site in Clear Lake, California (USA) (a hot-spring Hg deposit) and particles mobilized from these tailings have been examined during laboratory-column experiments. Solutions containing two common, plant-derived organic acids (oxalic and citric acid) were pumped at 13 pore volumes d
−1 through 25 by 500
mm columns of calcined Hg ore, analogous to the pedogenesis of tailings. Chemical analysis of column effluent indicated that all of the As mobilized was particulate (1.5
mg, or 6% of the total As in the column through 255 pore volumes of leaching). Arsenic speciation was evaluated using X-ray absorption spectroscopy (XAS), indicating the dominance of arsenate As(V) sorbed to poorly crystalline Fe(III)-(hydr)oxides and coprecipitated with jarosite KFe
3(SO
4,
AsO
4)
2(OH)
6 with no detectable primary or secondary minerals in the tailings and mobilized particles. Sequential chemical extractions (SCE) of <45
μm mine tailings fractions also suggest that As occurs adsorbed to Fe (hydr)oxides (35%) and coprecipitated within poorly crystalline phases (45%). In addition, SCEs suggest that As is associated with 1
N acid-soluble phases such as carbonate minerals (20%) and within crystalline Fe-(hydr)oxides (10%). The finding that As is transported from these mine tailings dominantly as As(V) adsorbed to Fe (hydr)oxides or coprecipitated within hydroxysulfates such as jarosite suggests that As release from soils and sediments contaminated with tailings will be controlled by either organic acid-promoted dissolution or reductive dissolution of host phases.
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A number of factors affect the transport of dissolved and particulate mercury (Hg) from inoperative Hg mines, including the presence of organic acids in the rooting zone of vegetated mine waste. We ...examined the role of the two most common organic acids in soils (oxalic and citric acid) on Hg transport from such waste by pumping a mixed organic acid solution (pH 5.7) at 1 mL/min through Hg mine tailings columns. For the two total organic acid concentrations investigated (20 μM and 1 mM), particle-associated Hg was mobilized, with the onset of particulate Hg transport occurring later for the lower organic acid concentration. Chemical analyses of column effluent indicate that 98 wt % of Hg mobilized from the column was particulate. Hg speciation was determined using extended X-ray absorption fine structure spectroscopy and transmission electron microscopy, showing that HgS minerals are dominant in the mobilized particles. Hg adsorbed to colloids is another likely mode of transport due to the abundance of Fe−(oxyhydr)oxides, Fe−sulfides, alunite, and jarosite in the tailings to which Hg(II) adsorbs. Organic acids produced by plants are likely to enhance the transport of colloid-associated Hg from vegetated Hg mine tailings by dissolving cements to enable colloid release.
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The adsorption of oxalate on a model aluminum oxide, corundum (α-Al2O3), has been examined over a broad range of oxalate concentrations (0.125−25.0 mM) and pH conditions (2−10). In situ attenuated ...total reflectance Fourier transform infrared (ATR-FTIR) measurements indicate that at low to intermediate concentrations (oxalate ≤ 2.50 mM), oxalate adsorbs to corundum predominantly as a bidentate, mononuclear, inner-sphere complex involving both carboxyl groups. Significant contributions from outer-spherically bound oxalate and aqueous Ox2- are additionally observed at higher oxalate concentrations. Consistent with the ATR-FTIR findings, macroscopic adsorption data measured for oxalate concentrations of 0.125−2.50 mM can be generally well modeled with a single bidentate, inner-sphere oxalate complex using the charge distribution multisite complexation (CD-MUSIC) model. However, at intermediate oxalate concentrations (0.50 and 1.25 mM) and pH < 5, the extent of oxalate adsorption measured experimentally is found to fall significantly below that predicted by CD-MUSIC simulations. The latter finding is interpreted in terms of competition for oxalate from dissolved Al(III), the formation of which is promoted by the dissolution-enhancing properties of the adsorbed oxalate anion. In accordance with this expectation, increasing concentrations of dissolved Al(III) in solution are found to significantly decrease the extent of oxalate adsorption on corundum under acidic pH conditions, presumably through promoting the formation of Al(III)−oxalate complexes with reduced affinities for the corundum surface compared with the uncomplexed oxalate anion.
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We demonstrate the utility of nuclear medical imaging technologies and a readily available radiotracer, (99m)TcTcO(4)(-), for the noninvasive monitoring of Fe(II) production in acetate-stimulated ...sediments from Old Rifle, CO, USA. Microcosms consisting of sediment in artificial groundwater media amended with acetate were probed by repeated injection of radiotracer over three weeks. Gamma camera imaging was used to noninvasively quantify the rate and extent of (99m)TcTcO(4)(-) partitioning from solution to sediment. Aqueous Fe(II) and sediment-associated Fe(II) were also measured and correlated with the observed tracer behavior. For each injection of tracer, curves of (99m)Tc concentration in solution vs time were fitted to an analytic function that accounts for both the observed rate of sedimentation as well as the rate of (99m)Tc association with the sediment. The rate and extent of (99m)Tc association with the biostimulated sediment correlated well with the production of Fe(II), and a mechanism of (99m)TcTcO(4)(-) reduction via reaction with surface-bound Fe(II) to form an immobile Tc(IV) species was inferred. After three weeks of bioreduction, a subset of microcosms was aerated in order to reoxidize the Fe(II) to Fe(III), which also destroyed the affinity of the (99m)TcTcO(4)(-) for the sediments. However, within 3 days postoxidation, the rate of Tc(VII) reduction was faster than immediately before oxidation implying a rapid return to more extensive bioreduction. Furthermore, aeration soon after a tracer injection showed that sediment-bound Tc(IV) is rapidly resolubilized to Tc(VII). In contrast to the (99m)TcTcO(4)(-), a second commercially available tracer, (99m)Tc-DTPA (diethylenetriaminepentaacetic acid), had minimal association with sediment in both controls and biostimulated sediments. These experiments show the promise of (99m)TcTcO(4)(-) and (99m)Tc-DTPA as noninvasive imaging probes for a redox-sensitive radiotracer and a conservative flow tracer, respectively.
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(99m)Tc-DTPA has been used as a conservative tracer to quantify water transport through porous media. However, more information on the reactivity of this (99m)Tc compound under varying geochemical ...conditions is desirable to better understand its potential uses. We measured the speciation of Tc following amendment of (99m)Tc-DTPA to batch systems spanning a range of controlled biogeochemical conditions. Our results suggest that (99m)Tc-DTPA is stable under the reducing conditions tested. However, freshly precipitated Al-ferrihydrite may displace Tc(IV) from DTPA in the absence of Fe(III)-reducing conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Formation of Fe(II)‐As(III) solids is suspected to limit dissolved As concentrations in anaerobic environments. Iron(II) precipitates enriched in As(III) have been observed after microbial reduction ...of As(V)‐loaded lepidocrocite (γ‐FeOOH) and symplesite (Fe(II)3(As(V)O4)2·8H2O) and upon abiotic reaction of Fe(II) with As(III). However, the conditions favorable for Fe(II)‐As(III) precipitation and the long‐term stability (relative to dissolution) of this phase are unknown. Here we examine the composition, local structure, and solubility of an Fe(II)‐As(III) precipitate to determine environments where such a solid may form and persist. We reveal that the Fe(II)‐As(III) precipitate has a composition of H7Fe4(AsO3)5 and a log Kso of 34 for the dissolution reaction defined as: H7Fe4(AsO3)5 + 8H+ = 4Fe2+ + 5H3AsO3. Extended X‐ray absorption fine structure spectroscopic analysis of H7Fe4(AsO3)5 shows that the molecular environment of Fe is dominated by edge‐sharing octahedra within an Fe(OH)2 sheet and that As is dominated by corner‐sharing AsIIIO3 pyramids, which are consistent with previously published structures of As(III)‐rich Fe(II) solids. The H7Fe4(AsO3)5 solid has a pH‐dependent solubility and requires millimolar concentrations of dissolved Fe(II) and As(III) to precipitate at pH <7.5. By contrast, alkaline conditions are more conducive to formation of H7Fe4(AsO3)5; however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.
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Formation of Fe(II)-As(III) solids is suspected to limit dissolved As concentrations in anaerobic environments. Iron(II) precipitates enriched in As(III) have been observed after microbial reduction ...of As(V)-loaded lepidocrocite ( gamma -FeOOH) and symplesite (Fe(II) sub(3)(As(V)O sub(4)) sub(2) .8H sub(2)O) and upon abiotic reaction of Fe(II) with As(III). However, the conditions favorable for Fe(II)-As(III) precipitation and the long-term stability (relative to dissolution) of this phase are unknown. Here we examine the composition, local structure, and solubility of an Fe(II)-As(III) precipitate to determine environments where such a solid may form and persist. We reveal that the Fe(II)-As(III) precipitate has a composition of H sub(7)Fe sub(4)(AsO sub(3)) sub(5) and a log K sub(so) of 34 for the dissolution reaction defined as: H sub( 7)Fe sub(4)(AsO sub(3)) sub(5) + 8H super(+) = 4Fe super(2+) + 5H sub(3)AsO sub(3). Extended X-ray absorption fine structure spectroscopic analysis of H sub(7)Fe sub(4)(AsO sub(3)) sub(5) shows that the molecular environment of Fe is dominated by edge-sharing octahedra within an Fe(OH) sub(2) sheet and that As is dominated by corner-sharing As super( III)O sub(3) pyramids, which are consistent with previously published structures of As(III)-rich Fe(II) solids. The H sub(7)Fe sub(4)(AsO sub(3)) sub(5) solid has a pH-dependent solubility and requires millimolar concentrations of dissolved Fe(II) and As(III) to precipitate at pH <7.5. By contrast, alkaline conditions are more conducive to formation of H sub(7)Fe sub(4)(AsO sub(3)) sub(5); however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.
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