Simultaneous photocatalytic reduction of silver and oxidation of cyanide from dicyanoargentate solutions

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<ul><li><p>of</p><p>n G</p><p>Carl</p><p>Applied Catalysis B: Environmental 86 (2009) 5362</p><p>Keywords:</p><p>eneo</p><p>ioxi</p><p>ous</p><p>ecies was achieved. It is proposed that O2 plays a dual role in the reaction: it</p><p>supports the reduction of the metal acting as intermediate in the transfer of electrons and acts as reagent</p><p>in the oxidation of released cyanide to cyanate species. The inuence on the kinetics of the addition of</p><p>Contents lists availab</p><p>Applied Catalysis B</p><p>.e ls1. Introduction</p><p>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]</p><p>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 [3]. 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 [4]. Incontrast, heterogeneous photocatalysis has shown a high efciency</p><p>in the removal not only of free cyanides [5,6], but also of iron [7,8],copper [9] and gold cyanocomplexes [10]. 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.</p><p>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 [1125]. The photocatalytic deposition on TiO2 of a variety of metalssuch as gold [10,15], palladium [15], platinum [16,17], rhodium[18] and silver [1925] 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</p><p>Photocatalytic reduction</p><p>Photocatalytic oxidation</p><p>Silver recovery</p><p>Cyanide</p><p>Dicyanoargentate</p><p>Plating baths</p><p>methanol was studied. In anoxic conditions the rate of silver reduction was increased, what is attributed</p><p>to the effectiveness of methanol as hole scavenger and its ability to form reducing radicals, whereas the</p><p>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</p><p>photocatalytic treatment of an industrial spent silver plating bath was carried out. In anoxic conditions,</p><p>the recovery of silver upon deposition on the catalyst as Ag0 was achieved. As the large amount of organic</p><p>matter in the solution inhibited the oxidation of cyanide ions, a two-step procedure is proposed for the</p><p>overall treatment of those wastewaters.</p><p> 2008 Elsevier B.V. All rights reserved.</p><p>* Corresponding author. Tel.: +34 91 6647464; fax: +34 91 4887068.</p><p>E-mail address: mariajose.lopez@urjc.es (M.-J. Lopez-Munoz).</p><p>0926-3373/$ see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.apcatb.2008.07.022Simultaneous photocatalytic reductionfrom dicyanoargentate solutions</p><p>Mara-Jose Lopez-Munoz *, Jose Aguado, Rafael va</p><p>Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan</p><p>A R T I C L E I N F O</p><p>Article history:</p><p>Received 8 May 2008</p><p>Received in revised form 11 July 2008</p><p>Accepted 19 July 2008</p><p>Available online 3 August 2008</p><p>A B S T R A C T</p><p>The feasibility of heterog</p><p>solution using titanium d</p><p>oxygen, as the simultane</p><p>cyanide ions to cyanate sp</p><p>journa l homepage: wwwsilver and oxidation of cyanide</p><p>rieken, Javier Marugan</p><p>os, C/Tulipan s/n, 28933 Mostoles, Madrid, Spain</p><p>us photocatalysis for the treatment of dicyanoargentate complexes in</p><p>de was investigated. The best results were obtained in the presence of</p><p>deposition of metallic silver on the catalyst and oxidation of released</p><p>le at ScienceDirect</p><p>: Environmental</p><p>evier .com/ locate /apcatb</p></li><li><p>lysiphotocatalytic reduction rate is governed by the kinetics [12].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 [11]. Forinstance, Szabo-Bardos et al. found that silver photoreductionon TiO2 was enhanced in the presence of oxalic acid [22]; Chenand Kay reported the promoting effect of a variety of organics onthe photocatalytic reduction of Hg(II) [12]; Kriek et al. observedno rhodium deposition on TiO2 from RhCl33H2O unless ethanolwas added to the system [18]; Schrank et al. investigated thesimultaneous photocatalytic Cr(VI) reduction and a dye oxida-tion with TiO2 [14]. 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.</p><p>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 [25].</p><p>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.</p><p>2. Experimental</p><p>2.1. Materials</p><p>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</p><p>M.-J. Lopez-Munoz et al. / Applied Cata54water from a Millipore Milli-Q water purication system.2.2. Experimental set-up and procedure</p><p>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 &lt; 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.</p><p>Potassium dicyanoargentate(I) (Aldrich) was used for the pre-paration of the reacting solutions. The TiO2 powder (0.5 g l</p><p>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 oxidation products, mainly cyanate, have beenexpressed in terms of their equivalence to ppm of CN (i.e., a0.385 mM [Ag(CN)2]</p><p> solution contains 20 ppm of CN ions whoseoxidation would yield 32.3 ppm of CNO species).</p><p>The initial pH value was adjusted at 12 by addition of a NaOH(Scharlab) solution. Before starting the irradiation, oxygen orheliumwas bubbled for 30 min through the suspension in order toreach either the saturation or the elimination of oxygen respec-tively, depending on the purpose of the experiment. In themeantime, the lamp was switched on outside the reactor tostabilise the light emission.</p><p>When required, methanol (Scharlab) was added to the solutionas additional oxidizable compound. Reactions using the industrialspent plating bath solution provided by Metalor TechnologiesIberica S.A. were carried out without any previous treatment,except for dilution.</p><p>Along the reaction runs, that usually lasted 4 h, oxygen orhelium was continuously bubbled into the stirred suspension.Aliquots were taken at time intervals, following ltration through0.22mm Nylon membranes in order to remove the suspendedsolids before being analyzed.</p><p>The initial photonic efciencies were calculated as follows:</p><p>j0molAg Einstein1 r0molAg l1 s1I0Einstein l1 s1</p><p>where r0 is the initial slope of the dicyanoargentate degradationprole and I0 is the incident photon ow per liter, whose value(I0 = 1.37105 Einstein l1 s1) was determined by ferrioxalateactinometry according to the procedure described byHatchard andParker [26].</p><p>2.3. Analytical procedures</p><p>Total cyanide concentration (free cyanide plus weak aciddissociable cyanocomplexes such as dicyanoargentate(I)) wasdetermined through a standard colorimetric method using apyridine-barbituric acid reagent to form a highly colored complexwith maximum absorbance at 575 nm [27]. The absorptionmeasurements were performed with a Varian Cary 500 ScanUVvis-NIR spectrophotometer. Free cyanide concentration waspotentiometrically determined by using a CN-selective electrodein an expandable ion analyzer (Orion 720A). Cyanate concentrationwas measured by ion chromatography in a Metrohm equipment(Separation centre 733, IC detector 732, Pump Unit 752). Anaqueous solution of NaHCO3 (2.0 mM), and Na2CO3 (1.3 mM) wasused as eluent at a ow rate of 0.8 ml min1.</p><p>The concentrations of dissolved silver species were deter-</p><p>s B: Environmental 86 (2009) 5362mined with a Varian Vista AX inductively coupled plasma-atomic</p></li><li><p>emission spectrometer (ICP-AES). The spectral line at 328.068 nmwas used with quantication purposes after calibration withcertied standards. This analysis was also performed to evaluatethe silver content of the catalysts recovered after the reactionfollowing their treatment with nitric acid.</p><p>X-ray diffraction (XRD) patterns of the recovered solids wereacquired on a Philips XPERTMPD apparatus using Cu Ka radiation(l = 1.54059 A). Scans were made in the 2u range 20708 with astep size of 0.028 and a step time of 1 s, enough to obtain a goodsignal-to-noise ratio in all the studied reections. The morphologyof the solids was examined by transmission electron microscopy(TEM) using a JEOL JEM-2000 FX instrument operating at anaccelerating voltage of 200 kV. The powder was ultrasonicallydispersed in acetone and the suspension was deposited on acarbon-coated copper grid.</p><p>Total organic carbon (TOC) content of the spent plating bathswas determined using a combustion/non-dispersive infrared gas</p><p>M.-J. Lopez-Munoz et al. / Applied Catalysianalyzer model TOC-V Shimadzu.</p><p>3. Results and discussion</p><p>3.1. Blank and adsorption experiments</p><p>Blank experiments were performed without addition of TiO2 inorder to determine whether any photochemical reactions couldoccur in the absence of catalyst. No photolytic degradation ofdicyanoargentate was observed after 6 h of irradiation under thedifferent reaction atmospheres investigated, namely heliumpurged and oxygen saturated, thus assuring the photocatalyticnature of the subsequent results in the presence of titania. Also theadsorption of [Ag(CN)2]</p><p> on the titania surface was evaluated byanalyzing the dicyanoargentate(I) content of the supernatantsolution after stirring a suspension of the catalyst and the complexfor 1 h in the dark at pH 12. The results indicated negligible darkadsorption of [Ag(CN)2]</p><p> on the titania surface, what can beexplained by the electrostatical repulsion for the approach of theanionic complex to the negatively charged semiconductor surfaceat the studied pH value.</p><p>3.2. Photocatalytic degradation of Ag(CN)2 in the absence of oxygen</p><p>Fig. 1 shows the concentration proles of the photocatalyticdegradation of pure potassium dicyanoargentate(I) solutions withTiO2. The reaction was carried out in the absence of dissolvedoxygen by purging the solution with helium, as described in theexperimental. It may be observed that a slow degradation of theFig. 1. Photocatalytic degradation of Ag(CN)2 in the absence of oxygen.Ag(CN)2 complex was produced, together to the formation of</p><p>cyanate species derived from the oxidation of cyanide ligands. It isworth noting that the total CN mass balance was achievedbetween the silvercyanide complex and cyanate species and thatno free cyanide ions (CN) were detected in the medium along thereaction.</p><p>In the absence of oxygen, silver atoms of dicyanoargentate(I)are the acceptors of electrons generated by irradiation of thesemiconductor:</p><p>AgCN2 eCB!Ag0 2CN (1)</p><p>As...</p></li></ul>

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