simultaneous photocatalytic reduction of silver and oxidation of cyanide from dicyanoargentate...
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Applied Catalysis B: Environmental 86 (2009) 5362
ecies was achieved. It is proposed that O2 plays a dual role in the reaction: it
supports the reduction of the metal acting as intermediate in the transfer of electrons and acts as reagent
in the oxidation of released cyanide to cyanate species. The inuence on the kinetics of the addition of
Contents lists availab
Applied Catalysis B
.e ls1. Introduction
Cyanides are widely used in electroplating, mining and photo-graphic processes due to their unique properties for complexingmetals such as silver, gold, copper or zinc. As a consequence,wastewaters coming out from these activities usually contain largeamounts of metal cyanide complexes with the general formula[M(CN)n]
x, whereM represents themetal cation, n is the number ofbound cyanide ions, and x is the total anionic charge of the complex[1,2]. Despite many of them are considered as weakly toxic becausethey are relatively stable, metal cyanide complexes are species ofenvironmental concern for their potential release of CN ions underuncontrolled ambient conditions. For that reason, it is important thetreatment of efuents containing these cyanocomplexes previouslyto their discharge into the environment . Common cyanideoxidation processes such as alkaline chlorination or advancedoxidation technologies based on the use of ozone or peroxides, donot achieve the complete removal ofmetal cyanidecomplexes . Incontrast, heterogeneous photocatalysis has shown a high efciency
in the removal not only of free cyanides [5,6], but also of iron [7,8],copper  and gold cyanocomplexes . To the best of ourknowledge, however, there are no references in open literature todate concerning the photocatalytic treatment of silvercyanidecomplexes, what constitutes the purpose of the present work.
To achieve an effective treatment of silver cyanide complexesnot only the oxidation of the CN groups but the recovery ofsilver would be also desirable in order to accomplish a doubleobjective: to avoid cyanide and metal pollution and to reusesilver in new processes. Previous studies have shown thefeasibility of removal and recovery of dissolved metal ions fromwastewater by heterogeneous photocatalysis with titaniumdioxide. The process involves the reduction and deposition ofmetals onto the semiconductor surface, followed by theirextraction through chemical or mechanical procedures . The photocatalytic deposition on TiO2 of a variety of metalssuch as gold [10,15], palladium , platinum [16,17], rhodium and silver  from their respective inorganic saltssolutions has been reported. Although it is thermodynamicallyfeasible to achieve the direct reduction of the metallic ions bythe photogenerated electrons as far as the potential of theconduction band of the semiconductor is more negative thanthe reduction potential of the Mn+/M couple, in practice the
methanol was studied. In anoxic conditions the rate of silver reduction was increased, what is attributed
to the effectiveness of methanol as hole scavenger and its ability to form reducing radicals, whereas the
oxidation of released CNwas inhibited. On the contrary, in aerobic medium the presence of the alcoholhad a detrimental effect on the metal reduction but no cyanide accumulation was produced. The
photocatalytic treatment of an industrial spent silver plating bath was carried out. In anoxic conditions,
the recovery of silver upon deposition on the catalyst as Ag0 was achieved. As the large amount of organic
matter in the solution inhibited the oxidation of cyanide ions, a two-step procedure is proposed for the
overall treatment of those wastewaters.
2008 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +34 91 6647464; fax: +34 91 4887068.
E-mail address: email@example.com (M.-J. Lopez-Munoz).
0926-3373/$ see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.apcatb.2008.07.022Simultaneous photocatalytic reductionfrom dicyanoargentate solutions
Mara-Jose Lopez-Munoz *, Jose Aguado, Rafael va
Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan
A R T I C L E I N F O
Received 8 May 2008
Received in revised form 11 July 2008
Accepted 19 July 2008
Available online 3 August 2008
A B S T R A C T
The feasibility of heterog
solution using titanium d
oxygen, as the simultane
cyanide ions to cyanate sp
journa l homepage: wwwsilver and oxidation of cyanide
rieken, Javier Marugan
os, C/Tulipan s/n, 28933 Mostoles, Madrid, Spain
us photocatalysis for the treatment of dicyanoargentate complexes in
de was investigated. The best results were obtained in the presence of
deposition of metallic silver on the catalyst and oxidation of released
le at ScienceDirect
evier .com/ locate /apcatb
lysiphotocatalytic reduction rate is governed by the kinetics .Therefore, it is essential to acquire the knowledge of whatoperational variables or added reactants can inuence on thephotocatalytic reduction efciency. In that respect, it has beenproved that metal deposition can be enhanced by the addition ofreducing organic species, what has been explained in terms of asynergism between the oxidation of the organic compound andthe simultaneous reduction of the metal cations . Forinstance, Szabo-Bardos et al. found that silver photoreductionon TiO2 was enhanced in the presence of oxalic acid ; Chenand Kay reported the promoting effect of a variety of organics onthe photocatalytic reduction of Hg(II) ; Kriek et al. observedno rhodium deposition on TiO2 from RhCl33H2O unless ethanolwas added to the system ; Schrank et al. investigated thesimultaneous photocatalytic Cr(VI) reduction and a dye oxida-tion with TiO2 . They found a benecial effect of the dye onthe metal photoreduction, explained by means of an efcientconsumption of the holes by the dye with the subsequentattenuation of charge carriers recombination.
On the other hand, negative results in terms ofmetal depositionare obtained when both the metal ions to be reduced and theoxidisable organic groups belong to the same chemical entity as itoccurs with metalEDTA complexes, species widely used inindustrial applications. The photocatalytic treatment of Cr(III),Cu(II), Ni(II), Pb(II), Zn(II), Cd(II), Hg(II), Fe(III), Co(II) and Mn(II)complexed with EDTA showed that whereas the oxidation of theEDTA organic ligands was achieved in most cases, the reduction ofmetal ions was not attained. The observed removal of metals fromthe solution was due to either a simultaneous adsorption of thefree metal cations over the titanium dioxide surface or theprecipitation of metal derivative species, with no reduction to themetallic state .
The present work is focused on the study of the photocatalyticperformance of TiO2 for achieving the simultaneous reduction ofsilver and oxidation of cyanide from aqueous dicyanoargentate(I)solutions and spent silver plating baths. Operating parameterssuch as presence or absence of dissolved oxygen and addition ofmethanol as hole scavenger at different concentration levels havebeen investigated. It should be noted that most studies inphotocatalysis dealing with silver and organics are mainly focusedon the metal inuence on the TiO2 activity for the degradation ofthe organic compounds rather than in the process of Ag recovering.On this basis, further investigations are required in order toestablish the feasibility of the whole process for the treatment ofwaste efuents. Moreover, in addition to the goals of cyanideremoval and silver recovery the photocatalytic treatment of silvercyanide complexes has a considerable interest for mechanisticreasons because of the fact that both target chemical groups to beoxidized and reduced belong to the same molecule and under-standing the mechanism of these type of processes can be decisivefor many practical applications.
DegussaP25TiO2wasusedas thephotocatalyst. This commercialtitanium dioxide, commonly used as standard material in photo-catalytic studies, consists of anatase and rutile crystallinephases in aratio of 4:1, respectively. It shows a B.E.T. specic surface area of50 m2 g1 and an average particle size of ca. 30 nm although insuspension larger polycrystalline aggregates are formed. All otherchemicals mentioned hereafter were of reagent grade and used asreceivedwithout further purication. Solutionswere preparedwith
M.-J. Lopez-Munoz et al. / Applied Cata54water from a Millipore Milli-Q water purication system.2.2. Experimental set-up and procedure
Photocatalytic reactions were carried out in a 1 l cylindricalPyrex reactor, provided with internal irradiation through a 150 Wmedium pressure mercury lamp (Heraeus TQ-150). The lightsource was surrounded by a water-cooled jacket to both lter UV(l < 300 nm) and IR radiations and tomaintain the temperature ofthe suspension at 25 1 8C. Withdrawing of samples and bubblingof gases were carried out through two openings in the upper part ofthe reactor.
Potassium dicyanoargentate(I) (Aldrich) was used for the pre-paration of the reacting solutions. The TiO2 powder (0.5 g l
1) wasadded to the silver cyanocomplex solution, for which the initialconcentration was xed at 0.385 mM. In order to facilitate totalmass balance calculations in all the gures and throughout theresults and discussion, the concentrations of dicyanoargentate(I)and its detected oxidatio