Environ. Sci. Technol. 1993, 27, 1875-1879
Sunlight-Induced Photochemistry of Aqueous Solutions of II ) Ions Hexacyanoferrate(II)and -(I W. Scott Rader, Ljlljana Soiuji6, Emil 8. Miiosavijevl6,' and James L. Hendrlx
Department of Chemical and Metallurgical Engineering, Mackay School of Mines, University of Nevada, Reno, Nevada 89557 John H. Nelson
Department of Chemistry, University of Nevada, Reno, Nevada 89557 Photochemically induced processes in alkaline aqueous solutions of hexacyanoferrate(I1) and -(III) were investigated in detail. The studies were conducted in the presence or absence of a titanium(1V) oxide semiconductor photocatalyst utilizing sunlight as the irradiation source. It was established that the cyanide ion liberated by photodissociation from thermodynamically stable iron cyano species can be photocatalytically oxidized via cyanate and nitrite all the way to nitrate. In addition, all oxidation processes are enhanced when the photocatalyst was present in the irradiated solution. The results obtained may have ramifications for the possible uses of solar power for the efficient treatment of large quantities of precious metalsmill tailings wastes containing various cyanide species.
Introduction Cyanide metallurgy has been the basis for precious metals processing since the late 1880s (I). Not only has cyanide been extensively used during the past hundred years, but there is still no satisfactory replacement for cyanide gold metallurgy in the foreseeable future. Consequently, the safe and economical treatment of milling wastes containing different cyanide species is a current problem that should be addressed. Gold-mill tailing impoundments contain as the main complexed cyanide species hexacyanoferrate(I1) and sometimes hexacyanoferrate(II1) (2), both of which are known to undergo photochemical degradation (3). This suggests that solar power might be effectively used for the treatment of large quantities of tailings wastes containing these iron-cyano species. Much work has been done related to the photochemical reactions of hexacyanoferrate(I1) and -(I111 complex anions. It has been established that "free" cyanide ion is produced when these species are irradiated by UV light and that the main intermediary products of the photodecomposition of the hexacyano species are aquapentacyanoferrate(I1) and-(111)(4,5). On theotherhand,Frank and Bard (6, 7) established that cyanide ion is photocatalytically oxidized in the presence of a Ti02 semiconductor to cyanate. Our recent investigations (8,9)verified that cyanide is first oxidized to cyanate. However, contrary to earlier studies, we have established that cyanate is subsequently further photocatalytically oxidized via nitrite all the way to nitrate. To the best of our knowledge, there are only two studies dealing with the photoinduced reactions of iron-cyano complexes in the presence of a Ti02 photocatalyst (IO, 11). In the first report, Zaidi and Corey (10) found no evidence that UV irradiation, either by itself or in the 0013-938X/93/0927-1875$04.00/0
0 1993 American Chemical Society
presence of a Ti02 photocatalyst, was effective in removing cyanide from gold-mill effluent streams containing ironcyano complexes. Our preliminary results showed otherwise and were indirectly corroborated by a recent paper (II), which established photocatalytic degradation of hexacyanoferrate(II1) at the titania sol surface. However, in this paper no photodegradation products were identified. Also, it should be noted here that no change in hexacyanoferrate(II1) concentration was observed when the solution was UV irradiated in the absence of TiOz, which is contrary to previous research (5) and to our results. These obvious contradictions, as well as the lack of information related to the behavior of [Fe(CN)613-and [Fe(CN)6IP when irradiated by sunlight, prompted us to undertake the present work. The goal of this research was to better understand the sunlight-induced processes occurring in the aqueous solutions of hexacyanoferrate(11)and -(III) complex anions in the presence or absence of a photocatalyst. The research is a continuation of the feasibility studies designed to discern whether solar power can be used for efficient treatment of wastes containing various cyanide species. The importance of these investigations is further emphasized by the fact that none of the commercially available methods for removing or destroying cyanide (viz., alkaline chlorination, hydrogen peroxide treatment, INCO's SO2-air process, and FeSOr addition) are able to detoxify cyanide from thermodynamically stable iron-cyano complexes (12-14). Moreover, the addition of iron(I1) sulfate to the cyanide containing waste produces large amounts of [Fe(CN)61Pspecies.Even though this complex anion can be considered a$ nearly nontoxic itself, it is quite unstable in the presence of sunlight, dissociating the cyanide ion. This fact obviously sets limits on the amount of hexacyanoferrate(I1) which can be discharged to the environment. The TiOz/sunlight process can use the dissociation of CN- to its advantage and completely oxidize cyanide from iron-cyano complexes.
Experimental Section Materials. The mostly anatase form (70%) of TiOz, obtained from Degussa as material P25, was proven earlier to have high efficiency for cyanide oxidation (15). This material has a surface area of 50 f 15 m2/gwith an average particle size of only 30 nm and a compacted apparent density of 150 g/L (16). Potassium hexacyanoferrate(I1) (&[Fe(CN)6].3H20) and potassium hexacyanoferrate(II1) (&[Fe(CN)sl) were obtained from Aldrich as reagentgrade chemicals. All other chemicals were of reagent grade and were used as received. All solutions were prepared using distilled deionized water. Environ. Sci. Technol., Vol. 27, No. 9, 1993 1875
Experimental Procedure. The photocatalyzed oxidations of [Fe(CN)61Pand [Fe(CN)613-at Ti02 particles in the sunlight were performed in Reno, NV (39" N), from June 11to June 28,1992. In this experiment 1.0 L of 1.00 mM solutions of Kd[Fe(CN)&3H20 and &[Fe(CN)aI (pH adjusted to 10.5 with a 1.0 M NaOH solution) was placed into two separate large PVC dishes so that the solution depth was about 2.5 cm and the surface area was about 360 cm2. A total of 10.0g of titanium(1V) oxide was added (mass sufficient to cover the bottom of the dish with a thin layer of semiconductor powder). The PVC dishes and corresponding covers were UV transparent at wavelengths where Ti02 exhibits high absorbance (A
