Mercury Release Characteristics from Broken CFLs
Figure shows time-resolved mercury release data from two CFL models. The release is initially rapid producing vapor concentrations from 200−800 μg/m3
during the first hour, which far exceed the OSHA occupational limits. The release decays on a time scale of hours and continues at significant rate for at least four days (data beyond 24 h not shown). The total Hg released after 24 h is 504 (13 W model) and 113 μg (for 9 W) by integration, which are 11.1% and 1.9% of the total Hg content specified by the vendors, respectively. Over 4 days (extended data not shown), the 13 W bulb released 1.34 mg or 30% of the total Hg. In general, Hg° evaporation is known to be slow under ambient conditions, and our data suggest that much of the original mercury remains in the bulb debris after 96 h and will continue to evaporate slowly. Saturated Hg° vapor (15
) in a typical lamp volume (50 mL) corresponds to only 0.65 μg of vapor phase Hg°, which is much less than the actual mercury release during the first hour, 12−43 μg. The majority of Hg in a CFL must therefore be in a condensed phase originally, and the mercury release we observe must be primarily caused by desorption/evaporation phenomena. Figure also compares the actual CFL release with the evaporation of a free Hg° droplet under the same set of conditions. The actual CFL release exceeds the release from a free Hg° droplet of equal mass (see Figure ), which likely reflects the much larger surface area of the adsorbed phase (on the phosphor, end caps, or glass) relative to the single drop. Similar release patterns but lower amounts were seen for spent bulbs (example result 90 μg in 24 h) or from the fracture site of a new bulb after glass removal to simulate cleanup. Removing large glass shards by hand after breakage on a carpet did not eliminate Hg release, but reduced it by 67% relative to the data in Figure . The remaining (33%) release from the fracture site is believed to be primarily associated with spilled phosphor powder, which is known to be the primary site for adsorbed Hg partitioning in fresh bulbs (1
Figure 1 Mercury vapor release characteristics for two brands of compact fluorescent lamps following catastrophic fracture at room temperature. A: Hg-vapor concentrations and release rates in a 2 L PTFE enclosure purged with a 1 L/min flow. For comparison, the (more ...)
Sorbent Synthesis, Characterization, and Testing
Because mercury vapor capture on solids occurs by adsorption or gas−solid reaction where kinetics or capacities typically depend on surface area (in addition to other factors such as composition), we hypothesized that high-area, nanoscale formulations of common mercury sorbents will show enhanced performance. This section evaluates a large set of new nanomaterial sorbents for ambient temperature Hg0
vapor capture and compares their performance to conventional microscale formulations of the same materials. Manchester et al. (18
) shows an example breakthrough curve that is the raw output of the fixed-bed sorbent tests. Integrating the area between the baseline inlet (60 μg/m3
) and the outlet concentration curve and dividing by sorbent mass yields a capacity reported in μg-Hg/g-sorbent (18
). Table is a complete list of the sorbents and their Hg capacities under our standard conditions (60 μg/m3
inlet stream), and the following sections discuss the results by sorbent class.
Sulfur-containing materials are widely used for mercury capture (19
). Zero-valent sulfur reacts with mercury to form stable mercuric sulfide in one of two crystal forms: red cinnabar (ΔHf
° = −58 kJ/mol, ΔGf
° = −49 kJ/mol) or black metacinnabar (ΔHf
° = −54 kJ/mol, ΔGf
° = −46 kJ/mol) and is thus attractive for waste or stockpile stabilization (20
). Oji (20
) discusses the advantages of HgS relative to Zn amalgam for the stabilization and disposal of Hg-containing mixed wastes, and Svensson et al. (21
) discuss favorable conditions for HgS formation from Hg or HgO in geological repositories. Surprisingly, there are few reports of nano-sulfur synthesis (22
) and to our knowledge no studies of nano-sulfur as a mercury sorbent.
Here we choose a convenient templating route to obtain small quantities of nanostructured sulfur for sorbent testing. Figure shows the morphology and sorption behavior of sulfur nanotubes fabricated by spontaneous infiltration of CS2/S solutions into nanochannel alumina templates followed by solvent evaporation and chemical etching of the template. The sulfur nanotubes show a 90-fold increase in surface area and a 24-fold increase in Hg capacity over conventional powdered sulfur. The total captured Hg is much less than the HgS stoichiometric limit and much less than even surface monolayer capacity, and the capacities increase with increasing temperature. These results indicate a kinetically limited chemisorption/reaction on active sites that represent a small fraction of the nanotube surfaces.
Standard Hg adsorption capacities for elemental sulfur nanotubes and conventional sulfur powder as a function of adsorption reaction temperature. Image is SEM micrograph of template S-nanotubes.
Metals and Metal Sulfides
There is an extensive literature on Hg interaction with metals (25
), much of it focused on elevated temperatures using conventional film or microparticle formulations. Here we investigate newly available nanoparticles as room-temperature Hg sorbents and compare them to conventional microscale powders. Table shows that mercury capacities vary greatly with chemistry (Ag > Cu > Ni > Zn) and, for each metal, are significantly enhanced by nanosynthesis. The rank order parallels the standard free energies for metal oxidation, n
M + 1
° = −9.3 kJ/mol; CuO, ΔGf
° = −133.5 kJ/mol; NiO, ΔGf
° = −216 kJ/mol; ZnO, ΔGf
° = −318.5 kJ/mol), and (complete) oxidation of copper is shown to greatly reduce its sorption activity (31.8 to 4.3 μg/g). Interestingly, copper metal activity is observed to increase modestly as the fresh metal nanoparticles age in the atmosphere, which may suggest elevated activity for partially
oxidized surfaces. The nanometal capacities represent from about 10−6
(Zn) to 35% (Ag) of theoretical monolayer coverage on the nominal outer surfaces indicating that the process is far from reaching stoichiometric alloy formation, even in an outer shell, and the reactions are limited to specific active surface sites under these low temperature conditions. Among these metal sorbents, nano-silver is potentially attractive as a high-capacity sorbent (capacities up to 8510 μg/g) for room temperature applications like CFL capture. Annealing nano-silver reduces both its surface area and Hg capture capacity (Table ).
Granite et al. (28
) investigated metal sulfides MoS2
as Hg sorbents at elevated temperature and report a high capacity for MoS2
. In preliminary experiments, we found WS2
to be significantly more reactive than MoS2
(both conventional powders) and therefore were motivated to test WS2
nanoparticles as potential high-capacity sorbents. In this case, nanosynthesis offered no significant advantage, and none of the metal sulfides appear among the most active and useful low-temperature sorbents in Table .
Activated carbons are widely used to capture mercury vapor, and their performance can be enhanced by surface modification with sulfur, halogen, or oxygen-containing functional groups (18
). Because carbons are capable of developing extensive internal
surface area, there is little motivation to enhance the external
surface area through nanosynthesis methods. Here we evaluate carbons as readily available reference materials that are market-relevant benchmarks for the new nanosorbents. Table shows low to modest capacities on carbons (0.45−115 μg/g) with the exception of the S-impregnated material (2600 μg/g), which is one of best commercially available sorbents in this study.
Selenium has an extremely high affinity for mercury. In the body, it sequesters mercury into insoluble and metabolically inactive mercury selenides and by this mechanism is protective against mercury neurotoxicity (9
). Its antioxidant nature helps to protect against mercury-induced DNA damage (35
). In the environment the stable sequestration of mercury by selenium may reduce its mobility, bioavailability, and ecotoxicity (9
). Strong Hg/Se binding may be key to understanding the biological and environmental behavior of both mercury and selenium (38
). There are few published studies of selenium-based mercury vapor capture, although selenium has been used in Hg removal from off gases in sulfide ore processing (41
) and is being considered for Hg stockpile stabilization and long-term storage (42
). The presumed capture mechanism is reaction to HgSe (ΔGf
° = −38.1 kJ/mol) (43
Here we focus on amorphous nanoselenium, which has received recent attention in chemoprevention (17
) but has not to our knowledge been used for low-temperature Hg vapor capture. Figure shows the colloidal synthesis of nanoselenium, the particle size distributions, and the mercury capture behavior of competing Se forms. The original synthesis method uses glutathione (GSH) as a reductant and bovine serum albumin (BSA) as a surface stabilizing agent to achieve very small particles in colloidal suspension (17
) as shown in Figure A, left. Surprisingly the BSA-stabilized nano-Se has a lower capacity than conventional Se powder despite the much
smaller particle size (6−60 nm vs 10−200 μm). We hypothesized that the protein stabilizer (BSA) either blocked Hg access to the Se surfaces or chemically passivated the surfaces through Se−thiol interactions. We therefore removed the BSA, as shown in Figure A, right, to make “unstabilized nano-Se”, which Figure C shows to have a remarkably high Hg sorption capacity and much faster kinetics than conventional micro-Se. Mercury uptake continues over very long times, and a 184 h experiment was necessary to approach the end state, at which point the unstabilized nano-Se had adsorbed 188
000 μg Hg/g or approximately 20% Hg/Se mass ratio. X-ray diffraction analysis shows both the micro-Se and unstabilized nano-Se are amorphous, as is the stabilized nano-Se (45
Figure 3 Synthesis, particle size distributions, and Hg-uptake kinetics of competing forms of selenium. A: Colloidal synthesis of BSA-stabilized (left) and unstabilized (right) nano-Se. B: Particle-size distributions in aqueous media by dynamic light scattering (more ...)
Comparison of Sorbents
Figure shows a comparison of the new and reference sorbents in this study. The right-hand axis gives the amount of sorbent required to capture 1 mg of Hg vapor, typical of CFL release. Surprisingly, some common sorbents such as powdered S or Zn require enormous amounts of material (>10 kg) to treat the vapor released from a single CFL and most of the sorbents require amounts that are not attractive for incorporation into consumer packaging (>10 g). A small number of sorbents (nano-Ag, S-impregnated activated carbon, and two selenium forms) have capacities that should allow <1 g of sorbent to be used. The most effective sorbent is unstabilized nano-Se, which can capture the contents of a CFL with amounts less than 10 mg. This capacity corresponds to about five monolayer equivalents indicating significant subsurface penetration of mercury into selenium nanoparticles (unlike the other sorbents). The capacity is still only about 7% of the bulk stoichiometric conversion to HgSe, however, indicating the potential for further capacity improvement by sorbent optimization.
Comparison of the sorbents in this study. Left axis: Standard Hg adsorption capacity. Right axis: Amount of sorbent required for capture of 1 mg of Hg vapor typical of the total release from a single CFL over a three-day period.
In Situ Capture of CFL Mercury
Although the amount of Hg released from CFLs on fracture is small (typically <1 mg), only a few sorbents have sufficient capacity to sequester it all at room temperature for practical application (see Figure ). For in situ capture, where the sorbent is supplied to consumers in the form of a safe disposal bag, impregnated cloth, or modified retail package, only nano-Ag, selenium forms, or sulfur-impregnated activated carbon could be used in reasonable quantities. The concept of in situ capture is demonstrated below, here “treatment” is defined as sealing the fractured CFL and sorbent in a confined space for 24 h, then removing the sorbent and measuring the residual vapor release.
Figure 5 Effect of sorbents applied in situ on mercury vapor release following catastrophic fracture of a CFL at room temperature. Top curve: No sorbent. Bottom curves: Same CFL broken in presence of sulfur-impregnated activated carbon (1 g of HgR) and unstabilized (more ...)
The commercial sulfur-impregnated activated carbon reduced the mercury release by 83% over the untreated bulb, making it a viable candidate for in situ capture of mercury vapor. Moreover, the low cost and low toxicity of this material make it an attractive option for consumer use. Even better performance was exhibited by the unstabilized nano-selenium, which decreased the mercury release by 99% over an untreated bulb, regardless of the application method, and with 100-fold less sorbent mass. Nearly complete suppression of mercury vapor from fractured lamps can be achieved by sealing the lamp in a confined space with 10 mg of unstabilized nano-selenium for 24 h, either as an impregnated cloth draped over the fractured bulb or as a loose powder in vials.
The present article provides sufficient motivation to pursue further development of sorbent-based technologies for suppressing mercury vapor release from broken fluorescent lamps. Work is underway to engineer (i) sorbent-impregnated reactive barrier cloths for remediation of porous substrates such as carpets at break sites and (ii) sorbent-containing disposal bags or recycle boxes to allow safe handling and stable disposition in the environment. Important issues in this development include reaction kinetics, landfill stability, impregnated cloth design, bag design, and management of secondary risks to both human health and the environment associated with possible release or and exposure to the nanomaterial sorbents themselves.