Photocatalytic Reduction and Recovery of Mercury by

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Environ. Sci. Technol. 2005, 39, 4242-4248

Photocatalytic Reduction and Recovery of Mercury by Polyoxometalates E. GKIKA, A. TROUPIS, A. HISKIA, AND E. PAPACONSTANTINOU* Institute of Physical Chemistry, NCSR Demokritos, 153 10 Athens, Greece

Photocatalytic reduction of mercury in aqueous solutions using PW12O403- or SiW12O404- as photocatalysts has been studied as a function of irradiation time, concentration of Hg(II), polyoxometalate, and organic substrate in the presence or absence of dioxygen. The photocatalytic cycle starts with irradiation of polyoxometalate, goes through the oxidation of, for instance, propan-2-ol (used as sacrificial reagent), and closes with the reoxidation of reduced polyoxometalate by Hg2+ ions. Mercury(II) is reduced to mercury(I) and finally to Hg0 giving a dark-gray deposit, following a staged one-by-one electron process and a firstorder kinetics in [Hg2+]. The process is slightly more efficient in the absence of dioxygen, while the increase of either catalyst or propan-2-ol concentration results in the augmentation of the rate of reduction till a certain point where it reaches a plateau. The results show that this method is suitable for a great range of mercury concentration from 20 to 800 ppm achieving almost complete recovery of mercury up to nondetected traces (99.95%) were used for deaeration or oxygenation of solutions. A typical experiment was as follows: 4 mL of aqueous Hg(CH3COO)2 solution containing propan-2-ol and H3PW12O40 or H4SiW12O40 catalyst was added to a spectrophotometer cell (1-cm path length), deaerated, and covered with a cerum cap. The pH was adjusted at pH 1 with HClO4 whenever necessary. Photolysis was performed with an Oriel 1000 W Xe arc lamp equipped with a cool water circulating filter to absorb the near-IR radiation and a 320-nm cutoff filter to avoid direct photolysis of substrates. The total photonic flux (320-345 nm) determined by ferrioxalate actinometer was 7.9 × 10-6 einstein min-1. As a matter of comparison, for typical experiments (PW12O403- 0.7 mM, propan-2-ol 0.2 M at pH 1) the quantum yield of formation of the 1-equiv reduced POM at 254 nm is ca. 12% for PW12O404as has been reported previously (27). The quantum yield is independent of wavelength below ca. 350 nm, as has been recently verified (37). The degree of reduction of POM in photolyzed deaerated solutions was calculated from the known extinction coefficient of reduced catalyst at ca. 750 nm (for the one-electron reduced 12-tungstophosphate, PW12O404-, 752nm ) 2000 M-1 cm-1 and for the one-electron-reduced 12-tungstosilicate, SiW12O405-, 730nm ) 2100 M-1 cm-1) using a Perkin-Elmer Lambda19 Spectrometer. The concentration of mercury ions was determined with a GBC flame atomic absorption spectrometer monitored at 253.7 nm after filtration of the photolyzed solutions with a 0.45-µm Millipore filter. The initial rate of mercury recovery was determined by monitoring the concentration of Hg2+ in the photolyzed filtered solutions and calculating the slope of the curve obtained, after the induction period of mercury deposition and until almost 30% of mercury had been removed. Thermal reduction of Hg2+ by reduced POM was performed as follows: 1-equiv reduced tungstate, PW12O404-, ca. 0.17 mM is produced upon illumination of 4 mL aqueous solutions of propan-2-ol 0.5 M, POM 0.7 mM, and HClO4 0.1 M. To this solution, 100 µL of deaerated solution of Hg2+ (final Hg2+ concentration 0-0.025 mM) is added, and the drop of the absorbance is monitored at 752 nm (the characteristic absorbance of the 1-equiv reduced tungstate). Characterization of the photodeposited mercury species was executed with a D-500 Siemens powder X-rays diffractometer using CuKa radiation.

Results To examine whether a direct photoreaction between propan2-ol and Hg2+ ions takes place, blank experiments were performed in the absence of POM. The obtained results showed that no precipitation happens after 4 h of photolysis of a deaerated solution containing propan-2-ol 0.5 M and Hg2+ 2 mM (pH 1); furthermore, measurements of mercury concentration by atomic absorption spectrometry during the irradiation time showed no change. Further experiments were performed to investigate the existence of a POM-Hg2+ complex interfering in the photocatalytic process. After 3 h of irradiation of a deaerated solution of POM 0.7 mM and Hg2+ 2 mM (pH 1), no effect on the concentration of mercury ions was noted, while no change in the absorption spectra of POM was observed when mixed with mercury ions, suggesting no complexation of mercury and POM ions. Photocatalytic Reduction and Recovery of Mercury. The examination of the role of POM in photocatalytic reduction and the recovery of mercury was performed by irratiation

FIGURE 1. Variation of Hg2+ concentration upon irradiation of deaerated aqueous solutions containing 0.5 M propan-2-ol, 0.7 mM H3PW12O40, and 2mM Hg2+ at pH 1 (λ > 320 nm; T, 18 °C), with illumination time.

FIGURE 2. UV-vis spectra of irradiated solutions for various irradiation times, showing the formation of the 1-equiv reduced tungstate, PW12O404-, which appears after the removal-reduction of mercury. Illumination time is indicated on spectra. Experimental conditions as in Figure 1. (λ > 320 nm) of deaerated solutions containing mercury acetate 2 mM (400 ppm), propan-2-ol 0.5 M, and PW12O4037 × 10-4 M. It can be seen (Figure 1) that in the first 10 min of illumination no change in the concentration of dissolved mercury is observed, in parallel with no marked deposition. After 10 min of photolysis, a white precipitate appears with concomitant fast decrease in dissolved mercury. This deposit gradually turns gray and finally, upon further photolysis, dark gray. At this time, within 30 min of photolysis, the concentration of mercury has been driven to undetectable limits. Absorption spectra during photolysis are exhibited in Figure 2. No absorbance at 752 nm (characteristic peak for 1-equiv reduced catalyst, PW12O404-) is observed in the presence of Hg2+, during the first 30 min of photolysis. Only after almost 30 min of photolysis, when practically no Hg2+ is left in the solution, the blue color appears because of the formation of the one-electron reduced catalyst, PW12O404-. Effect of POM and pH. SiW12O404- is stable at pH values up to 5.5, while PW12O403- is stable at pH values ∼1. However, since under these conditions PW12O403- is ca. 3 times faster than SiW12O404- in the recovery of mercury, PW12O403- is the photocatalyst examined in detail herein. The pH value was always maintained at 1 to avoid hydrolysis of the parent POM. For practical reasons, experiments with PW12O403- were also VOL. 39, NO. 11, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effect of dioxygen in the rate of mercury recovery. Irradiation of deaerated (9) and oxygenated (b) aqueous solution containing 0.5 M propan-2-ol, 0.7 mM PW12O403-, and 2 mM Hg2+ at pH 1 (λ > 320 nm; T, 18 °C).

FIGURE 4. Influence of PW12O403- concentration on the initial rates, R0, of mercury recovery (9), or formation of 1-equiv reduced 12tungstophosphate (PW12O404-) in the absence of Hg2+ (b). Deaerated solutions of 0.5 M propan-2-ol and 2 mM Hg2+ at pH 1 (λ > 320 nm; T, 18 °C). executed at pH ∼4 and the process, though 3 times slower, was still effective, indicating that the hydrolysis species, that is, PW11O397- that predominates at these pH values (26), can also account for the photocatalytic reductive recovery of mercury, under environmental friendly pH values. Effect of O2. Figure 3 shows the influence of dioxygen in the induction period before the commencement of mercury deposition. A 12-min retardation of the induction period is observed in the oxygenated solutions, whereas the initial rate of mercury precipitation is practically unaffected. Effect of POM Concentration. Figure 4 demonstrates the effect of catalyst concentration on the initial rate of mercury recovery from dearated aqueous solutions (lower curve) and the initial rate of formation of the 1-equiv reduced tungstophosphate (PW12O404-) in the absence of Hg2+ (upper curve). A linear dependence is observed for both rates when the concentrations of PW12O403- are smaller than ca. 0.5 mM, while zero order rates (with respect to catalyst) is noted for concentrations of PW12O403- greater than 1 mM because of the saturation in photon absorption of the system. Figure 5 illustrates the influence of catalyst concentration on the induction period before mercury starts precipitating. The increase of PW12O403- concentration lowers the induction time of mercury deposition. 4244

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FIGURE 5. Influence of PW12O403- concentration on the retention time of mercury precipitation in deaerated solutions of 0.5 M propan2-ol and 2 mM Hg2+ at pH 1 (λ > 320 nm; T, 18 °C).

FIGURE 6. Influence of propan-2-ol concentration on the initial rates, R0, of mercury recovery (9), or formation of 1-equiv reduced 12-tungstophosphate (PW12O404-) in the absence of Hg2+ (b). Deaerated solutions of 0.7 mM PW12O403- and 2 mM Hg2+ at pH 1 (λ > 320 nm; T, 18 °C). Effect of Substrate Concentration. Figure 6 depicts the initial rate of photocatalytic mercury recovery as a function of the concentration of organic substrate, in the absence of oxygen. A linear dependence on [propan-2-ol] is observed for concentration below ca. 0.25 M while there is a zero order dependence for concentration greater than ca. 2 M. It must be kept in mind that the recovery of mercury begins after an induction period. High concentrations of propan-2-ol were used to speed the process. However, lower concentrations of propan-2-ol are also effective but the process is slower (Figure 6). Figure 7 shows how the induction period is affected from the concentration of propan-2-ol in deaerated solutions. Increase of propan-2-ol concentration lowers the induction time of mercury precipitation. Effect of Mercury Ion Concentration. Figure 8 shows a plot of the initial rate of mercury recovery as a function of [Hg2+], while Figure 9 exhibits the increase of induction time of mercury precipitation with increasing mercury ion concentration. Characterization of Mercury Particles. The dark-gray deposit obtained after prolonged photolysis of PW12O403-/ propan-2-ol/Hg2+ solutions was soluble in concentrated nitric acid. This property, together with the fact that the deposit

FIGURE 10. X-ray pattern of calomel particles obtained after 15 min of illumination. Experimental conditions as in Figure 1. FIGURE 7. Influence of propan-2-olconcentration on the retention time of mercury precipitation in deaerated solutions of 0.7 mM PW12O403- and 2 mM Hg2+ at pH 1 (λ > 320 nm; T, 18 °C).

FIGURE 11. Thermal reoxidation of PW12O404- by Hg2+ for various Hg2+ initial concentrations, after mixing of the corresponding solutions in the absence of light.

FIGURE 8. Influence of mercury concentration on the initial rates, R0, of mercury recovery. Deaerated solutions of 0.5 M propan-2-ol and 0.7 mM PW12O403- at pH 1 (λ > 320 nm; T, 18 °C).

FIGURE 9. Influence of mercury concentration on the retention time of mercury precipitation in deaerated solutions of 0.5 M propan2-ol and 0.7 mM PW12O403- at pH 1 (λ > 320 nm; T, 18 °C). is a 2e- reduced product, on the basis of the 2:1 stoichiometry (PW12O403-: Hg2+) of the thermal experiments (Figure 11), suggests that the deposit is elemental Hg; see below. On the contrary, the white-to-pale gray precipitate obtained in the

initial stages of photolysis was indicative of a mixture of Hg and calomel. Although the former could not be identified by powder XRD measurements because of its low crystallinity, the formation of Hg2Cl2 was suggested by XRD data, see Figure 10. Thermal Reduction of Hg2+ by Reduced POM. Upon mixing of photochemically produced PW12O404- 0.18 mM with 100 µL of deaerated solution of Hg2+ (final Hg2+ concentration 0-0.025 mM), a rapid decrease of the absorbance is observed reaching a plateau within less than 10 s, Figure 11. The rate of thermal reoxidation of PW12O404- by Hg2+ increases with increasing concentration of added Hg2+ ions. This reaction is very fast to measure under pseudo-firstorder conditions. Thus, using initial rates, the second-order rate constants obtained for (PW12O404- 0.18 mM and Hg2+ 0.0006-0.025 mM) is k ) (1.23 ( 0.1) × 103 M-1 s-1. The role of O2 as electron acceptor from reduced POM has already been reported (38). The corresponding rate constant with O2 measured under similar conditions for comparison is (0.94 ( 0.1) × 102 M-1 s-1.The last is in good agreement with literature values obtained with different methods (39, 40).

Discussion The following reactions summarize the mechanistic scheme suggested to take place during photocatalytic recovery of mercury in the presence of POM:

POM + hv f POM*

(1)

POM* f POM + Q

(2)

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POM* + propan-2-ol f POM (e-) + oxidized products (3) POM (e-) + Hg2+ f POM + 1/2 Hg22+

(4)

POM (e-) + 1/2 Hg22+ f POM + Hg0 V

(5)

Hg22+ a Hg2+ + Hg0 V

(6)

Reactions 1-3 provide an overall account of the photocatalytic oxidation of propan-2-ol and the concomitant formation of reduced POM. Details, that is, formation of radicals involved in the process and so forth, have been reported elsewhere (33, 41). The reducing role of hydroxylalkyl radicals in the Hg2+ reduction system seems to be minor. Other organic species, for instance, 2,4 dichlorophenol used as e-donors, have had the same result. When mercury ions are in excess, a fast reoxidation of the reduced catalyst occurs according to reactions 4 and 5 and no blue color of the 1-equiv reduced tungstate is observed (see initial photolysis time in Figure 2). When the solution is depleted of Hg2+ after prolonged irradiation time, the bluecolored PW12O404- is feasibly developed (see Figure 2 for photolysis times greater than 30 min). Since 1-equiv reduced tungstophosphate (PW12O404-) is a one-electron donor, two, staged, one-electron-transfer processes should take place (reactions 4 and 5). At first, Hg2+ ions are reduced to Hg22+ according to reaction 4 (42). Both reactions are thermodynamically favored since the redox potential of the PW12O403-/4- pair [E° PW12O403-/4- ) 0.221 V (43)] is more negative than those of mercury species [E° (Hg2+/ Hg22+) ) 0.911 V and E° (Hg22+/Hg0) ) 0.796 V (44)]. In turn, the formation of elemental Hg can be achieved via further reduction of Hg22+ by the reduced catalyst (reaction 5), which is also thermodynamically favored. Elemental Hg could also be formed through the disproportionation of Hg22+ according to equilibrium 6, which is a well-known rapid process with an equilibrium constant 1.14 × 10-2 (45). The two-staged, one-electron reduction of Hg2+ ions is also suggested by the traces of calomel detected by XRD measurements. The presence of chloride ions is, somehow, strange and it can be attributed to the traces of chlorides that remain during the synthetic procedure of POM catalyst. Anyway, the final product is elemental Hg. The net reaction of Hg2+ reduction to Hg0 is derived upon summation of reactions 4 and 5 (reaction 7)

2PW12O404- + Hg2+ f 2PW12O403- + Hg0

(7)

and reflects an overall stoichiometry 2:1. This is in accordance to the experimental data, where the ratio of initial rates of PW12O404- formation and mercury precipitation is 2:1 (see Figures 4, 6). Moreover, a 2:1 stoichiometry is also noticed (Figure 11) upon mixing deaerated solutions of PW12O403and Hg2+ (reaction 7). Now,

R0 ) k[PW12O404-]0 [Hg2+]0x

(8)

and, taking the neperian logarithm,

ln(R0/[PW12O404-]0) ) ln k + x ln[Hg2+]0

(9)

Figure 12 depicts the linear dependence of eq 9. A slope (∼0.9) is indicative of first-order kinetics with respect to Hg2+ (reaction 7). The first-order kinetics is in agreement with the addressed order for photocatalytic reduction of Hg2+ on semiconductor photocatalysts, such as TiO2 (16, 17, 46) and doped WO3 (25). However, different kinetic regimes, such as 0.5 kinetic order 4246

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FIGURE 12. Logarithmic plot of the initial rate of reoxidation of PW12O404- vs the initial concentration of Hg2+ in the absence of light, showing first-order dependence on Hg2+ according to eq 9. Deaerated solution containing PW12O404- ca. 0.18 mM and propan2-ol 0.5 M (pH 1). for TiO2 (19, 20) or 7/5 for ZnO (23), have also been reported by other researchers. Induction Period in Mercury Precipitation. The precipitation of mercury does not start immediately after illumination of the solution. The existence of an induction period in mercury removal denotes that Hg2+ reduction is not a one-step process suggesting that some intermediate, that is, Hg+ ion, is involved. This induction period in mercury photodeposition depends on mercury, catalyst, and propan-2-ol concentrations. As shown in Figures 5 and 7, an increase in the concentration of the organic substrate or the catalyst accelerates the formation of white precipitate. On the contrary, when the mercury ion concentration increases, the deposition starts later (Figure 9). The following provides a qualitative explanation: Higher concentration of Hg2+ ions require larger concentration of reduced catalyst (i.e., longer illumination time) to reach the Hg0 stage. On the contrary, higher concentrations of the catalyst or propan-2-ol accelerate the formation of POM-, eq 3, shortening the recovery of mercury, via eqs 4 and 5. In the reduction of Hg2+, the addition of the second electron (that is, formation of elemental Hg) prevails to the addition of the first electron only when about 50% of Hg2+ has been consumed (47). The initiation of mercury deposition in our case (Figure 1) takes place after ca. 10 min of photolysis, that is, when ca. 1.25 × 10-3 M of the 1-equiv reduced POM, PW12O404-, has been produced and reacted with Hg2+ which amounts to ca. half of the initial concentration of Hg2+. Litter has also reported that in another system, where TiO2 was used as the photocatalyst, gray deposits of mercury were visually observed only at conversions higher than 50% (17). An induction period in mercury precipitation has also been observed in other cases of the TiO2-based photocatalytic recovery of mercury (48). Effect of Oxygen. Simple calculations indicate that mercury started precipitating out after more than 35% of dioxygen of the solution had been consumed by the photogenerated 1-equiv reduced tungstophosphate (PW12O404-). Thus, dioxygen retards the start of photodeposition of mercury until the concentration of the former is effectively reduced. Then, the rate of mercury recovery is approximately the same as in the absence of dioxygen, Figure 3. This result is promising for practical purposes, as thorough exclusion of O2 is not needed.

Oxygen reacts fast with many reduced POM (reaction 10) (38).

POM(e-) + O2 f POM + O2-

(10)

However, thermal experiments have indicated that Hg2+ ions reoxidize PW12O404- an order of magnitude faster than dioxygen, as shown earlier, and therefore, dissolved oxygen is not expected to compete efficiently with mercury ions in abstracting an electron from the reduced POM through reaction 10. This is not, though, the case as mentioned earlier, Figure 3. As has been shown by Henglein, O2 has the ability to oxidize mercury particles when they are still in nanoparticle dimension (47, 49). Therefore, in our case, the retardation of Hg recovery in O2 saturated solutions could be assigned to oxidation of mercury particles (Hg0)x at the initial stages of aggregation, when x is still small. Other potential oxidizing reagents of Hg nanoparticles are the peroxy and superoxy radicals, obtained through eq 10 (38). However, although such oxidizing reagents could oxidize mercury nanoparticles, their presence in the photolyzed solutions must be minor, comparing the lower rate of eq 10 with the rate of eq 7. Practical Aspects. Some aspects of practical concern in the POM-based photocatalytic removal-recovery of mercury are as follows: • The process is effective in a wide range of mercury concentration, varying from 20 to 800 ppm, whereas prolonged irradiation leads to complete decontamination from Hg2+ ions up to nondetected traces (detection limit 50 ppb), Figure 8. The wide range of mercury concentration at which the POM method is applied as well as the low final concentration of Hg2+ ions left in solution can account for the environmental assessment of the process. • The process is also effective at environmental friendly pH values (ca. 5.5). • This method remains effective even in the presence of air, a data that simplifies the whole procedure since a predeaeration step is not required. • As depicted in XRD pattern in Figure 10, the metal product is free of the POM catalyst. The POM photocatalyst remains soluble and intact in the photolyzed solution, ready for a new photocatalytic action. The final metal product is obtained in pure form and no further treatment is required. • When the solution is depleted of Hg2+ ions, the blue color of reduced POM appears promptly. This is an interesting aspect since the process is rendered self-indicating and could be automated. • A variety of toxic reagents can be used even in traces in the place of propan-2-ol as e-donors, such as phenol, chlorophenols, s-triazines, pesticides, haloaliphatic acids, and so forth (33), so that the POM-photocatalytic process can, in principle, be a detoxification process for both organic compounds and Hg2+.

Acknowledgments We thank the Ministry of Development, General Secretariat of Research and Technology of Greece for supporting part of this work. A.T is grateful to Institute of Physical Chemistry, NCSR Demokritos, for a postdoctoral fellowship. We thank Dr. K. Ochsenku ¨ hn for his kind help with atomic absorption spectrometer and A. Bakandritsos for taking the XRD-pattern.

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Received for review May 6, 2004. Revised manuscript received March 10, 2005. Accepted March 10, 2005. ES0493143