Speciation of Mercury and Mode of Transport from Placer Gold Mine Tailings

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

Role of Organic Acids in Promoting Colloidal Transport of Mercury from Mine Tailings

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

Monitoring Tc Dynamics in a Bioreduced Sediment: An Investigation with Gamma Camera Imaging of ^99mTc-Pertechnetate and ^99mTc-DTPA

We demonstrate the utility of nuclear medical imaging technologies and a readily available radiotracer, [^99mTc]­TcO₄^–, 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 [^99mTc]­TcO₄^– 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 ^99mTc 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 ^99mTc association with the sediment. The rate and extent of ^99mTc association with the biostimulated sediment correlated well with the production of Fe­(II), and a mechanism of [^99mTc]­TcO₄^– 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 [^99mTc]­TcO₄^– 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 [^99mTc]­TcO₄^–, a second commercially available tracer, ^99mTc-DTPA (diethylenetriaminepentaacetic acid), had minimal association with sediment in both controls and biostimulated sediments. These experiments show the promise of [^99mTc]­TcO₄^– and ^99mTc-DTPA as noninvasive imaging probes for a redox-sensitive radiotracer and a conservative flow tracer, respectively