Environ. Sci. Technol. 1998, 32, 670-675
Photoreduction of Mercuric Salt Solutions at High pH L I S A D . L A U , R E N EÄ R O D R I G U E Z , * SHANNON HENERY, AND DAVID MANUEL Department of Chemistry, Idaho State University, Pocatello, Idaho 83209 LYNN SCHWENDIMAN Lockheed Martin Idaho Technology Co., Idaho Falls, Idaho 83415
Titanium dioxide and a 100 W mercury spotlamp were used to photoreduce 100 ppm Hg aqueous mercuric chloride solutions. The solution’s basicity and temperature were varied. Two optimum photoreduction conditions were determined: pH 9, 0 °C and pH 11, 40 °C. TiO2-assisted photoreduction at these two conditions lowered the concentration of mercury left in the solution to below 200 ppb. Methodology was developed to perform an overall mercury mass balance on the process. The overall mercury balance revealed that more than 97% (average 103% ( 6%) of the mercury removed from solution was deposited as mercury metal on the surface of the TiO2 for the pH 9, 0 °C treatment conditions. This mercury could be driven off the TiO2 surface by heating to 100 °C for half of an hour under nitrogen flow. The pH and temperature information under light and dark conditions is consistent with a pH-dependent adsorption of a dissociated mercuric species by hydroxide ions on the TiO2 surface followed by nucleation of the reduced species. The TiO2 assisted photoreduction process shows promise for remediation of mercuric waste below the EPA 200 ppb mercury disposal limit as well as the potential for recycling the mercury and TiO2 catalyst.
Introduction Mercury is toxic and has no known biological function. Mercury and other metals cannot be degraded biologically or chemically in the environment as many organic pollutants often are. The mercury or its compounds can be changed to other forms in reactions. Often the reaction results in a more toxic species such as in the biological transformation of metallic mercury to methyl mercury. The potential hazards associated with mercury and compounds containing it have lead to the development of several methods for treatment of mercury waste. Chemical and physical waste treatment technologies for heavy metals transfer the metal from one form to another. Often, treatment will reduce the volume of pollutants, but as this happens, it is important to recognize that the pollutants in the secondary waste will be more concentrated. Treatment systems always have byproducts, and the cost of disposal of the resulting waste streams may be a major part of the treatment cost. A goal for mercury waste treatment is to remove the mercury without creating a more toxic secondary waste or allowing * Corresponding author e-mail address:
[email protected]. 670
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volatile mercury to escape to the atmosphere. Several remediation techniques exist for the removal of mercury compounds from waste water streams. A list of the most promising of the techniques include activated carbon absorption (1, 2), ion exchange (3, 4), precipitation (5, 6), liquid emulsion membranes (7, 8), and photoreduction. Oxidation-reduction techniques appear to be most promising for complete remediation since the mercuric ions are reduced to elemental mercury which may be recycled. Semiconductor-assisted photooxidation and photoreduction has been studied as a method for treatment of hazardous organic chemicals in wastewater, a method to split water to produce O2 and H2, and a method removing metals and inorganic contaminants in wastewater. Most of the research on photocatalytic reactions has been focused on the oxidation and mineralization of organic substances. However, the reaction has been studied, to a lesser extent, for the photoreductive processing of metallic wastes. In the photoreductive processing, the metal ions are removed from the solution by photoreduction to a lower oxidation state or a metallic state which is followed by chemisorption or physisorption on the TiO2 surface. In some cases, the metals can be recovered from the surface (9, 10). The photoreduction process begins when photons of light hit a semiconductor and are absorbed and scattered. The absorbed photons with energy greater than or equal to the semiconductor band gap energy excite electrons from the valence band to the conduction band. Electron (e-) hole (h+) pairs are formed that can either recombine and release heat or cause oxidation and reduction reactions by charge transfer to species adsorbed to the semiconductor (11). Serpone (9), Tanaka (10), and others (12-14) have studied the batch photoreduction technique using TiO2 to remove mercury from mercuric salt solutions. Aguado et al. (15) have studied mercuric nitrate in a flow system. In acidic solutions, less than 1% of the original mercury present in solution remained after treatment with TiO2. However, this was in the presence of an oxidizable agent such as ethanol. It was reported that less than 70% of the mercury attached to the surface came off upon placing the material in aqua regia (9). Presumably, this number was representative of the amount of adsorbed mercury present in metallic form. No quantitative mass balance was performed to match the amount of mercury present in the solution initially with the amount of mercury found in the products resulting from the reaction. A mercury mass balance would also show the amount of mercury lost to the atmosphere during the treatment process and serve as a check on the mercury amounts reported to be in solution and deposited on the TiO2 surface. Some of these studies on acidic mercury solutions added organic material as a reducing agent and also reported low amounts of mercury left in the solution. Photoreduction in basic solutions is expected to proceed at a higher rate with a given semiconductor. For oxide semiconductors, the fermi level depends on the pH of the solution and upon the light intensity (16). Presumably, less light intensity is sufficient for metal reduction at higher pH. TiO2-assisted batch photoreduction of mercuric species in basic aqueous solutions has been previously investigated at room temperature (13). The amount of mercury reported to be left in solution was greater than 10%. The presence of an organic reducing agent boosted the mercury remediation leaving less than 2% mercury in the solution. In flow studies of mercuric nitrate at 39.8 °C, the percent mercury left in solution ranged from 0 to 7 ( 1% (15). In these studies, the nature of the mercury deposited on the TiO2 was not S0013-936X(97)00424-0 CCC: $15.00
1998 American Chemical Society Published on Web 01/26/1998
FIGURE 1. Overview of HgCl2 photoreduction process. investigated, and no overall mercury mass balance was performed. In the absence of light, TiO2 acts as an adsorption surface for the removal of large amounts of mercury. At acidic pHs (at 35 °C), up to 20% of the mercury initially present in the solution may be removed (9). Under basic conditions, the amount of mercury attached at the surface is larger due to the presence of hydroxide groups present on the TiO2 surface or titanol groups. As much as 35 mg of Hg2+/g of TiO2 was found to be adsorbed in dark studies at pH 11 for mercuric nitrate (15). In order for the semiconductor-assisted photoreduction method to be viable as a mercury remediation method, the mercury left in solution after treatment would have to be at least less than the EPA-defined disposal limit of 200 ppb. In addition, the volume of mercury waste which is now concentrated on the TiO2 surface should be much less than the original solution. Also, all of the mercury initially present in the waste solution would have to be taken into account. The ideal situation is the case where all of the mercuric ion in solution is photoreduced on the surface of the TiO2 and subsequently released by mild heating. If all of the mercury were released from the surface as metallic mercury vapor and then captured and condensed, then both the mercury and the TiO2 material could conceivably be recycled. The process would not have any hazardous secondary waste. The purpose of this study was to more fully investigate the TiO2 photocatalytic process in basic conditions to determine if the process could effect a removal of mercuric ions below the EPA disposal limit of 0.2 ppm while changing the mercury ions in the waste into recyclable mercury metal at relatively low light wattage. This was accomplished by determining conditions under which mercuric solution with concentrations of about 100 ppm could be treated by a TiO2 semiconductor photoreduction process to yield treated waste streams with mercury concentrations below the EPA disposal limit of 200 ppb; developing a method to ascertain the amount of metallic mercury present on the TiO2 surface after treatment with the most promising conditions; and performing an overall mercury mass balance on the process.
Experimental Section Overview. Figure 1 shows an overview of the process. Starting with the mercuric waste solution and TiO2, the reaction takes place under a light and two products result: filtrate and mercury laden TiO2 agglomerate. The agglomerate is heated to separate the metallic mercury from the TiO2. Description of Chemicals and Equipment. The mercuric chloride used in the experiments was an Aldrich 99.99% reagent. The mercury present in the solutions was determined by cold vapor atomic absorption spectroscopy using
a Varian AA10 spectrometer. The standards were derived from a 1000 ppm mercury, mercuric nitrate solution obtained from Fisher Scientific Inc. Standards contained 40, 100, 200, and 300 ppb mercury. The water used for dilution was >1 MΩ reverse osmosis water unless specified otherwise. The TiO2 used in all experiments was Degussa Titanium Dioxide P-25 (anatase); chemical name Fumed Titania, CAS no. 13463-677-7. The amount of TiO2 used to treat a volume of mercury solution was a 10:1 weight ratio of TiO2 to Hg in the solutions unless otherwise noted. For example, 0.01 g of TiO2 was added to 10 mL of 100 ppm HgCl2 solution. The mercuric salt was weighed out to prepare the correct ppm Hg concentration, added to a 100 mL volumetric flask, and diluted to the mark with deionized water. Subsequently, the pH was adjusted by adding a few drops of dilute NaOH to the solution. Once the pH was at the correct level, an aliquot of the solution was added to the cylindrical quartz reaction cell, which contained a premeasured amount of Degussa TiO2. A Teflon-coated stir bar was added to the quartz cell to allow constant stirring with a magnetic stir plate. The solution was equilibrated to temperature in the dark. The temperatures were controlled to (1 °C using a Haake recirculator. Solutions were allowed to react for 35 min. The timing of the reaction was based upon the starting point coinciding with the placing of the reaction cell beneath the mercury spotlight. Timing for the dark studies was also started after allowing time for the solution to equilibrate to the correct temperature. The light source was a 100 W Sylvania Par 38 mercury spotlight bulb and a constant intensity of 0.5 W/cm2 light hit the exposed face of the cell. The area of the exposed face of the cell was 3.14 cm2. The amount of light absorbed was not measured directly. The light’s spectrum is typical of other mercury light sources. This wattage is much less than the 1500 W source used by other groups to perform the TiO2assisted photoreduction in acidic conditions (9). During the process, a film of mercury metal (or other mercury species) is deposited on the TiO2 semiconductor material. A filtration step is necessary to separate the mercury laden TiO2 from the solution making a TiO2 agglomerate. After reaction, the entire 10 mL mixture was filtered through a 0.5 m Teflon filter using a hand-operated vacuum pump. The cell was rinsed with a total of 1 mL of deionized water which was also filtered through the same filter. The filtrate was acidified with 2-3 drops of nitric acid and analyzed at a later date for mercury content. The mercury-laden TiO2 agglomerate left on the filter was placed into a glass storage vial for later analysis. In some cases the entire process, except for the exposure of the reaction mixture in the cell to the spotlamp, was carried out in a glovebag under a nitrogen atmosphere. Methodology did not exist to measure the amount of mercury present on the TiO2 agglomerates. Since mercury has a fairly high vapor pressure, analysis of it in the gas phase was attempted. Three instruments were used to accomplish the measurement of the volatile mercury substances adsorbed to the TiO2 agglomerate: a Shimadzu ultraviolet-visible spectrophotometer (UV-vis), a Varian AA-10 atomic absorption spectrophotometer (AA), and a Jerome mercury vapor analyzer. The UV-vis was used to determine the amount of mercury present based on the 254 nm absorbance peak. Mercuric chloride and mercuric oxides do not contribute to the observed peak. The AA reading is also based on the area of a 254 nm absorbance peak. The Jerome mercury vapor analyzer model 431-X by Arizona Instruments uses the conductivity change in a piece of gold foil caused by deposition of mercury from the vapor phase to determine the concentration of mercury. The mercury vapor analyzer does not detect mercuric chloride, but both elemental VOL. 32, NO. 5, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Equipment used to capture mercury from heated TiO2 agglomerates. mercury vapor and mercuric oxide vapor contribute to the overall reading. Figure 2 shows the equipment used to heat the TiO2/Hg agglomerates and collect the volatilized mercury. The TiO2 agglomerate was placed in a heating vial attached to an evacuated 20 L receiver bottle. The 20 L size was chosen to ensure that the mercury vapor density would not reach saturation levels even if all mercury present on the TiO2 were desorbed. The heating vial was wrapped with heating tape, and a thermocouple was attached so that the agglomerate could be heated to a desired temperature for a given amount of time. The heating vial was also attached to a regulated nitrogen flow. Pressure in the receiver-evacuated bottle was measured with a capacitance manometer. The nitrogen flow was regulated at a flow value which filled the 20 L vessel to approximately room pressure within 2.5 h. Heating times ranged from 15 to 140 min, and temperatures ranged from 50 to 150 °C. The optimum heating temperature and time was selected by experimentation. The concentration of mercury vapor in the receiver bottle was determined by transferring an aliquot of the gas in the bottle with a plastic syringe to a 10 cm evacuated quartz spectrophotometer cell. The cell had been modified by attaching a high vacuum valve to one of the 2 ports with a graded quartz to glass seal. The second port was sealed with a rubber septum. The AA, UV-vis, and mercury vapor analyzer were used to quantify the mercury vapor injected into the cell. The mercury vapor was then directly sampled from the receiver bottle with the mercury vapor analyzer. Calibration curves were prepared daily to convert the instrument readings obtained from the samples to micrograms of mercury. The calibration curves were made by withdrawing known amounts of saturated mercury vapor with a calibrated syringe, and injecting it into a stirred 20 L receiver bottle to obtain a calibration concentration. The vapor was allowed to mix for a minimum of 15 min before sampling. Calibration concentrations were selected to cover the expected concentration range for the mercury released from the TiO2. The temperature of the mercury calibration mixtures was always measured so that the correct value of the mercury vapor pressure was used in the mercury concentration calculations (17). The mercury mass balance involved comparing the amount of mercury initially present in the solution before the semiconductor-assisted photoreduction to the sum of the mercury present in the filtrate, the volatile metallic mercury heated off the surface, the amount of mercury released in the bag atmosphere during processing, and the amount of mercury released by dissolution of the TiO2 agglomerate in concentrated acid. To perform the mass balance, uniform agglomerates with predictable amounts of mercury were needed. The amount of mercury deposited on each agglomerate had to be within 672
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FIGURE 3. Parameters varied to determine lowest mercury levels in filtrate after treating 10 mL of 100 ppm HgCl2 solution for 35 min. the measurement range of the three instruments and also needed to be fairly consistent from agglomerate to agglomerate to facilitate comparisons of the mercury vapor released. This was accomplished by treating 25 mL of 40 ppm Hg2+ solution. The mercuric salt solution, a quartz reaction cell, filtration apparatus, and heating vial were placed in a deflated plastic glovebag Model X-17-17 from I2R Inc. The bag was filled and evacuated twice with N2 gas before filling a third time to a known volume. The bag volume was calculated based on the nitrogen gas flow rate and the inflation time. Larger 25 mL batches were prepared in the glovebag, put into the quartz cell with the preweighed TiO2, and then the quartz cell was sealed. The quartz cell was removed from the bag through a small feedthrough port and reacted in the constant temperature water bath under the spotlight. Next, the cell was passed back through the feedthrough into the glovebag and 1 mL portions of solution were withdrawn and filtered to form agglomerates. Each agglomerate’s “share” of mercury was approximately equal to the portion volume (1 mL) divided by the total volume of solution (25 mL). Keeping the solution well mixed while forming the TiO2 agglomerates was found to be an important factor in making them uniform. The filtrate was placed into a sample vial and prepared for cold vapor analysis as previously described. The small TiO2 agglomerates for the batch were placed in heating vials and sealed until they were heated. The third term in the mass balance analysis, the amount of mercury which vaporized into the bag atmosphere during the filtration process, was determined by measuring the mercury concentration in the bag after the filtration was finished. The bag atmosphere was sampled using the mercury vapor analyzer through a syringe septum connected to a port on the bag. The other potential mercury sink in the process, and the last term in the mass balance equation, is mercury chemisorbed to the TiO2 agglomerate surface as presumably the oxide or hydroxide. Assumedly, this is from the monolayers closest to the surface. To measure this mercury, after the agglomerates were heated to drive off the metallic mercury, the agglomerate was subsequently dissolved in a mixture of 3 mL of concentrated sulfuric acid and 2 mL of trace metal nitric acid. The mixture and a Teflon-coated stir bar were placed in a stoppered and secured glass flask. The mixture was heated to 80 °C in a water bath for a minimum of 8 h. Upon cooling the mixture was diluted and analyzed by cold vapor AA.
Results and Discussion Filtrate. The photoreduction process is known to be mechanistically different at various pH conditions (9), and
TABLE 1. Percent Initial Hg Remaining in Filtrate and Wash of Light-Treated HgCl2 Solutions after 35 min of Processing under the Lamp filtrate + wash
solution before treatment ID
mL
Hg (mg)
TABLE 2. Percent Initial Hg Measured after Heating TiO2 Agglomeration Cakes from HgCl2 Solutions Processed at pH 11, 40 °C
mL
Hg (ppm)
vapor vapor agglom no. of UVanalyzer analyzer avg % agglom vis AA (cell) (bottle) initial Hg
total Hg % initial (mg) Hg remaining
1 2 3 4 5 6 7 8 9
5 5 5 5 5 5 5 5 5
(pH ) 9, 0 °C and 20 ppm Hg) 99 12.2 0.051 1 99 13.4 0.079 1 99 14.0 0.804 11 99 14.6 0.756 11 99 14.5 0.119 2 99 14.6 0.602 9 99 14.3 0.055 1 99 14.4 0.101 1 99 14.4 0.060 1 avg 4 std dev 5
10 11 12 13 14 15 16 17
5 5 5 5 5 5 5 20
(pH ) 11, 40 °C and 20 ppm Hg) 98 14.0 0.055 1 98 14.3 0.044 1 98 14.2 0.062 1 98 14.3 0.060 1 98 14.1 0.057 1 98 14.2 0.092 1 98 13.9 0.224 3 200 19.3 0.165 3 avg 1 std dev 1
1 1 11 11 2 9 1 1 1 4 5 1 1 1 1 1 1 3 2 1 1
the kinetics of the process should depend on the temperature. Reactions were run under air conditions with varied pHs, temperatures, and lamp exposures to determine photoreduction levels in pure mercuric chloride solutions. Initial concentration was 100 ppm Hg; temperatures used were 0, 25, 40, and 55 °C; pHs used were 7.5, 9, 11, and 13; duration under lamp ranged 1-39 min. The solution was processed at each time and condition once. Figure 3 shows the lowest mercury concentration of the filtrate achieved by the different light exposure duration for each pH-temperature pair tested. For minimum exposure time, best results were typically obtained when solutions were exposed to 35 min of light. Several conditions were determined that resulted in mercury reduction below the EPA limit. They were 40 °C, pH 11, 120 ppb; 0 °C, pH 9, 130 ppb; 25 °C, pH 13, 190 ppb; 25 °C, pH 11, 300 ppb. All were processed at 35 min. Two light-reaction conditions were chosen for further study: pH 9 at 0 °C and pH 11 at 40 °C. Both removed mercury to below the EPA limit of 0.2 ppm, but the TiO2/Hg agglomerates appeared to be slightly different in color in each case. The agglomerates reacted at 0 °C were brownish gray and the agglomerates reacted at 40 °C were a bluish gray. The different colors may indicate different compounds of mercury, such as mercury metal or mercuric oxide deposited on the TiO2, or it could also indicate different crystalline sizes or phases due to a difference in the deposition temperature. Concentrations of the mercury in the filtrates treated at these two conditions proved to be somewhat inconsistent. Table 1 shows filtrate concentrations of mercury and the percent of mercury measured relative to the initial Hg concentration from reactions performed at both conditions. The standard deviation is rather large for these trials. In three of the trials at pH 9 and 0 °C, the mercury concentration in the leftover solution was greater than the EPA limit. The reactions processed at pH 11 and 40 °C reduced mercury levels in the solution more consistently below the EPA release levels. At pH 11, the concentration of mercury in the filtrate exceeded the EPA limit in only one out of eight trials. The
batch 1 avg std dev batch 2 avg std dev batch 3 avg std dev overall avg std dev
7 5 4
87 7 92 3 91 4 92 3
93 4 95 3 88 4 93 4
87 5 95 5 89 5 90 6
86 7 94 6 85 5 88 7
88 4 94 3 88 2 90 4
pH 9 treatment method is more desirable since the resulting waste solution would be at a lower pH and less caustic. Mercury levels of the filtrates from the 35 min control reactions performed in the dark for the two selected conditions were 71 ppm (pH 9, 0 °C) and 42 ppm (pH 11, 40 °C). The 40 °C data is in agreement with adsorption isotherm information presented by Aguado (15). The TiO2 concentration used in these studies was twice that of the Aguado’s studies. Initial mercury concentration of the solutions was 100 ppm. The results from the dark studies indicated that a negligible portion of the mercury on the TiO2 surface was reduced to metallic mercury because no color change of the TiO2 was observed. These observations are similar to those reported by other investigators. Presumably, the material on the agglomerate was mercuric oxide or mercuric chloride hydroxide. The presence of the TiO2 surface allowed these materials to form where significant concentrations would not normally form in solution. This is important because deposition on the surface is the first critical step in the process of the mercury reduction. Once the TiO2 was exposed to the light, electrons are excited into the conduction band and become available to reduce the mercury sitting on the surface of the TiO2. The converted metallic mercury then acts as a conductor to shuttle electrons to reach more of the adsorbed mercuric species. Presumably, Cl- or OH- acts as the hole scavenger forming hypochlorite or hydrogen peroxide since preliminary gas chromatographic studies indicate that O2 gas is not a product of the reaction. The chemistry of the reaction will be the subject of a subsequent paper. Previous investigators have suggested that oxygen present in the solution may play a role in the photoreduction process (13). It might be a product of the photoreaction (9), or it could be reduced by the TiO2 light process and compete with Hg for photoreduction sites (13). In either case, oxygen present could affect the reduction of Hg in the photoreduction process. Subsequently, the reduction repeatability experiment was performed again using N2 purged reactant mixtures. Nitrogen was bubbled into the pH-adjusted mercuric chloride solution for a few minutes before the solutions were mixed with the TiO2 and exposed to the light. The deviation was much less. For the reaction conditions of 40 °C, pH 11, and 35 min light, the average amount of mercury left in solution after eight trials of TiO2/light treatment was 0.2% with an average deviation of 0.2%. In no case did the final concentration of the filtrate solution exceed the 0.2 ppm disposal limit. Since the N2 seemed to increase the reproducibility of the results, all further samples were prepared under N2purged conditions. Volatile Mercury Present on the Treated TiO2 Semiconductor Material. The best feature of the semiconductor treatment method was the possibility to have zero-mercury containing waste leftover after the processing. Since the mercury in the mercuric solutions is pulled out of the solution by reduction of Hg2+ to Hg0 (metallic mercury), which VOL. 32, NO. 5, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Percent Initial Hg Desorbed from TiO2/Hg Agglomeratesa ID
processing conditions
first heating temp (°C)
time (min)
% initial Hg (bottle)
second heating temp (°C)
time (min)
% initial Hg (bottle)
total initial % Hg
18 19 20 22 23 24 25
pH 9 0 °C pH 11 40 °C pH 9 0 °C pH 9 0 °C pH 11 40 °C pH 11 40 °C pH 9 0 °C
25 50 50 100 100 100 150
120 30 45 15 30 30 60
40 60 98 91 76 81 95
150 100 150 150 150 150
60 60 30 60 30 60
61 20 9 3 0 0
101 80 107 94 76 81 95
a
Solutions were processed for 35 min under the light.
subsequently strongly physisorbs or chemisorbs to the TiO2 surface, there existed the potential to recycle the mercury. If the mercury on the TiO2 is really all metallic mercury, then it should be able to be recovered by driving the mercury off the surface and condensing the metallic mercury vapor. The next set of experiments were designed to determine how much of the mercury removed from the solution was reduced to volatile metallic mercury. Studies were performed to determine the best heating temperature and heating cycle times. Table 2 shows a comparison of the % mercury initially in the solution as Hg2+ ion which was released as Hg metal vapor from the TiO2 light treated agglomerates. It was found that a large portion of the mercury was very volatile and that the four measurement methods produced similar results. It was important to try to find a heating temperature and time that would be as low as possible while releasing a large percentage of mercury for the treated TiO2. Table 3 shows data from some experiments designed to estimate these parameters. The “% initial Hg” figures for this table were calculated from vapor analyzer measurements of the receiver bottle only. A large portion of mercury was removed from treated TiO2 agglomerates at 50 °C: 60% was detected in the receiver bottle after 30 min of heating a agglomerate and close to 100% was detected after 45 min of heating another agglomerate. The results for agglomerates heated to 100 °C were more satisfactory. Major portions of mercury were removed after heating for 30 min (agglomerates 23 and 24) and a second heating at 150 °C did not release any detectable mercury on either agglomerate. The heat treatment of agglomerate 18 was different from the other agglomerates shown in Table 3. Agglomerate 18 was placed in the heating vial but not heated. Nitrogen gas was allowed to flow through the vial for 2 h, and only 40% of the initial mercury in the solution was detected in the receiver bottle. The receiver bottle was evacuated, and the agglomerate was heated a second time to 150 °C for an hour. Of the initial mercury, 60% was measured in the solution with the Vapor Analyzer. The data in the table also indicates that the mercuric chloride solutions treated at pH 9 and 0 °C resulted in more mercury leaving the heated TiO2 surface. Average percent of initial mercury detected at 40 °C, pH 11, was 88% and at 0 °C, pH 9, was 100%. This could mean either that Hg 2+ is more completely converted to metallic Hg at pH 9 and 0 °C than it is at pH 11 and 40 °C or that the metallic mercury is more strongly bound to the TiO2 at pH 11 and 40 °C. If the second explanation were true, more mercury should come off at higher temperatures. From the measurements on agglomerates 23 and 24, no more mercury came off the TiO2 when the temperature was increased from 100 to 150 °C. Therefore, it is likely the mercury is chemically bonded to the TiO2 or has been converted to HgO. In any case, close to 90% of the mercury deposited was driven off of the treated TiO2 agglomerates and detected in the receiver bottle. This indicates a good potential for mercury recycling. 674
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TABLE 4. Mercury Mass Balancea % initial Hg % initial Hg % initial Hg % initial Hg total ID in glovebag in filtrate in bottle in cake mdigest % Hg 26 27 28 29 30 31 32 33 34 35 36 37 38
2 2 2 2 2 2 1 1 1 2 2 2 2
(HgCl2: pH 11, 40 °C) 0.6 89 0.6 78 0.6 78 0.6 85 0.6 96 0.6 91 0.3 100 0.3 89 0.3 94 0.1 90 0.1 85 0.1 78 0.1 85
39 22 18 25 21 20
2 2 2 2 2 2
(HgCl2: pH 9, 0 °C) 0.1 96 0.1 95 0.1 101 0.1 95 0.1 110 0.1 107
a
2.200 2.000 1.200 2.300 0.220 0.310 1.200 14.000 3.500 0.010 0.002 0.002 0.001 avg std dev
94 82 82 89 99 94 103 104 99 92 87 80 87 92 8
0.031 1.600 1.200 0.800 0.670 0.210 avg std dev
98 98 104 98 113 109 103 6
Solutions were processed for 35 min under the light.
Mercury Mass Balance. As stated previously, the overall mercury mass balance involved the sum of four terms: the mercury left in the filtrate solution (analyzed by cold vapor AA), the volatile metallic mercury present on the TiO2 surface after photoreduction (driven off the surface by heating and collected and measured in 20 L bottle), the mercury left on the TiO2 surface after heating (accounted for by dissolving the TiO2 in concentrated acids and analyzing by cold vapor AA), and the volatile mercury which escapes into the surroundings during the filtration steps (determined by sampling the bag atmosphere with a mercury vapor analyzer). The condition used to release the volatile mercury from the TiO2 surface was 100 °C for 1 h under constant nitrogen flow. The results for the overall mass balance studies are given in Table 4. Results for reactions carried out at both 40 °C, pH 11 and 0 °C, pH 9 are given in the table. The total Hg recovered at both conditions was within experimental error of 100%. However, the mass balance under the higher temperature conditions has more error and the average value is less than 100%. Also, the amount of mercury coming off the surface as metallic mercury was on the average lower for the 40 °C agglomerates than the volatile metallic mercury which came off the surface for the solutions treated at 0 °C. It is likely that, at 40 °C and pH 11, the initial layers of the mercury deposited on the TiO2 surface would be more
strongly bound to the surface and more difficult to release by heating or acid digestion. The results are consistent with a photoreduction mechanism in which the mercury is first adsorbed onto the TiO2 surface where it may then interact. At higher pH, more OH- is present at the TiO2 surface making it easy for the mercuric chloride species to chemisorb to the surface oxygen. At lower pH, the surface is less inundated with OH- species, but the lower temperature allows mercuric chloride species to bind as well. Thus, at the higher temperature, there is more chemisorption, at the lower more physisorption. Additionally, at pH 9 and 0 °C, the mercury which is photoreduced can easily nucleate and form larger metallic mercury crystals or islands of mercury metal. These mercury islands would be more readily vaporized and thus more volatile metallic mercury would be released from the TiO2 surface upon heating to 100 °C for a half of an hour. At 40 °C, the initial monolayer could conceivably be Hg, strongly bonded to O or OH at multiple sites. The percentage of mercury released in the agglomerate digest at pH 11, 40 °C was on average about the same or lower than the percentage mercury determined by the agglomerate digestion at 0 °C, pH 9. This seems contrary to the hypothesis that at pH 11, 40 °C, the mercury is more strongly bonded to the TiO2 surface than for processing under the 0 °C, pH 9 conditions. However, one must remember that the overall mass balance for the pH 11 samples was on average low, and conceivably all of the mercury was not released during the acid digestion, or the AA calibration matrix did not adequately account for the mercury present in solution after the filtrate digestion. The amount of mercury released in to the atmosphere during filtration and processing was about the same in both cases, as was the amount recovered in the filtrate. Many initial conditions may exist which would result in photoreduction of mercuric ions in mercuric aqueous solutions and recycle potential for the mercury removed appears to be good. More than 90% of the mercury was found to be deposited on the semiconductor and could be heated and removed. However, it appears that the conditions
used will affect the potential for the mercury and the TiO2 semiconductor to be reused.
Acknowledgments Funding for this project was received from Lockheed Martin Idaho Technology Co. Some of the cold vapor AA results were obtained by Kristi Boehm, Sabrina Morgan and Noelle Waugh, technicians in the research group.
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Received for review May 15, 1997. Accepted November 1, 1997. ES9704242
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