Bacterial leachings of sample of covellite using a strain of Thiobacillus ferrooxidans in 9 K medium with and without iron were made at several pulp densities. Rates of solubilization with and without iron correspond to total and to direct mechanism respectively. Orders of reaction (at each pulp density) for each mechanism at two temperatures were calculated.Facultad de Ciencias Exacta

Donati, Edgardo Rubén

Porro, Silvia

Tedesco, Pascual Héctor

Biotechnology Letters

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Biotechnology Letters Vol i0 No 12 889-894 (1988) Received as revised October ii DIRECT AND INDIRECT MECHANISMS IN THE BACTERIAL LEACHING OF COVELLITE E.R.Donati,S.Porro I and P.H.Tedesco 2 Facultad de Ciencias Exactas de la Universidad Nacional de La Plata 47 y l15,1900,La Plata,Argentina. 1 Under a scholarship granted by CIC(Comisi6n de Investigaciones Cientfficas de la Provincia de Buenos Aires). 2 Principal Researcher of CIC SUMMARY Bacterial leachings of sample of covellite using a strain of Thiobacillus ferrooxidans in 9 K medium with and without iron were made at several pulp densities.Rates of solubilization with and without iron correspond to total and to direct mechanism respectively.Orders of reaction(at each pulp density) for each mechanism at two temperatures were calculated. INTRODUCTION In the last twenty years many authors(Duncan et al.,1964;Sakaguchi et al., 1978;Brierley, 1978;Kelly, 1976;Lundgren et al.,1980;Torma et al.,1984)have studied the bioleaching of low grade sulphide minerals,which can be represented by the equation: MS(s ) + 2 02(g ) -> MSO4(aq ) (I) This represents the conversion of an insoluble compound MS into a soluble product MSO&(with the exception of PbSO&).The process is catalyzed by the microorganiw Thiobacillus ferrooxidans~(T.f.), which has been characterised in numerous papers(Colmer et al.,1947;Silverman et al.,1964;Holt et ai.,1974; Tuovinen et a!.,1972).This bacteria is able to obtain energy from the oxidation of Fe2+ and reduced sulphur compounds mainly $2-. Metallic sulphides in ores are frequently together with reduced iron as pyrite or chalcopyrite and two bioleaching mechanisms have been proposed: l)direct attack of metallic sulphide with bacterial catalysis,which is represented by equation (i). 2)indirect attack represented by these equations: Fe 2+ ....... -~ FEB++ e- (2) O MS + 2 Fe 3+ -) 2 Fe 2+ + S + M 2+ (3) which means that the microorganisms catalyze the oxidation of Fe 2+ to Fe 3+ and this ion oxidizes the S 2- to S~ are able to catalyze the oxidation of S~ S~ + 02 + H20 ........ --> H2SO 4 (4) There is no agreement on the way by which these mechanisms occur:some authors claim that it requires the previous dissolution of the sulphide according to the equation: MS(s ) ........ ~ M 2§ S 2- (aq) + (aq) (5) and then the dissolved S2- is captured and oxidized by bacterial action, shifting the equilibrium of the equation(5) to the right(Torma et ai.,1978; Torma,1982).Others propose models which imply a direct action on the substrate surface(Bruynesteyn, 1980;Tributsch et al.,1981;Tributsch et al., 1981). 889 In this paper we try to arrive to a better understanding of these mechanisms by studying the bioleaching of a simple system as covellite (CuS) with and without Fe2+ in the medium.Rates of bioleaching and its variation with the temperature were measured and analysed. MATERIALS AND METHODS The used substrate was pure CuS Fluka A.G. Strains of Thiobacillus ferrooxidans identified as T.f. 18 isolated at the University of Cayetano Heredia(Per~)and provided by Dr. Carlos Aguirre,were used.The bacteria were initially adapted to the substrate in modified 9 K medium(Silverman et. al.,1959)in which CuS was used as energy source instead of FeSO4.Final cultures were prepared using pulp densities(grams of solid in i00 ml solution)between 0.5 and 2.5(%w/v) in 95 ml of 9 K medium with and without Fe2+ (iron concentration,when used,was 0.16 M )and adding inocule with the bacteria in the log-phase to 100 ml. Sterile samples were prepared using 5 ml of a 2% solution of thymol in methanol instead of the inoculum. Erlenmeyers flasks with the cultures were agitated at 200 rpm at 3! or 35~ adjusting the pH to 2~ were the optimum reported conditions(Torma et al.,1970;Sakaguchi et ai.,1976). Periodically samples were taken for analysis of the Cu2+ by atomic absortion spectrophotometry and the volume replaced by 9 K medium. RESULTS AND DISCUSSION In figure 1 the curves correspond to pulp density 0.5 %.Similar curves were obtained for other pulp densities(not shown in the figure).From the slopes of the curves concentration vs. time(in the zone of rapid increasing of the metallic ion concentration corresponding to the first six hours)the rates of disolution were determined for three different times.These rates were plotted against time and extrapolated to zero time and the obtained rate was used in the following calculations.When the 9 K medium was used with Fe2+ the obtained rate was considered as the total rate(both mechanisms included)and when Fe 2+ was not added the direct rate was obtained(which corresponds to direct mechanism).These data are shown in table i. In a first and rough approach the indirect rate(corresponding to indirect mechanism)was calculated as the difference between the total and the direct rate. From the relationship V= K.d n (Torma et al~ V is the rate of solubilization(% w/v .h-1),d the pulp density(% w/v),K the specific rate and n the order of the reaction~K and n were calculated from the intercept and the slope respectively of the plot log V vs. log d. The data at two temperatures are shown in table 2. The influence of temperature on each mechanism and-for each pulp density- on the relationships of rates between the two different mechanisms are shown in tables 3 and 4 ,respectively. Moreover the activation energy for each mechanism was calculated using the Arrhenius equation~values are indicated in table 5. The orders of reaction can be considered as the exponents which show the dependence of rates of reaction and surface of the substratum,using pulp densities.These orders of reaction at both temperatures shown in the tables are lower for the total mechanism than the observed orders when Fe2+ is not present,i.e.,when only the direct mechanism operates.This means that the surface is more important in this last case.The reasons supporting this point of view are: l)the direct process occurs predominantly at the surface and the indirect process probably in Solution. 890 w~ 8OO = 600 ._~ = 400 4~ ZOO o Inoculated (with Fe 2+) A Inoculated (without Fe z* ) 9 Sterile (with Fe 2§ A Sterile (without Fe 2.) o.__....RF_._o--- "/ ~ ~-. ---'- A''''R~'" A'--- / -- -----~:-- ~1 = ~-- & ~--~'~ &--iF----k-- 0 6 24 40 75 150 Time (hrs) Figure 1 Bioleaching of CuS 2)both processes occurs principally near the surface but the need of surface is more important for the bacterial action than for the chemical action of the Fe3+;so the surface is not the limiting factor for the indirect rate. In any case,if it is accepted that the bacteria and the Fe3+ are fundamentally electron acceptors,the higher rate in presence of Fe3+ (observed for all pulp densities at two temperatures)can be explained considering that the number of acceptors(Fe3+ and bacteria)is higher than in absence of Fe3+ (only bacteria). Moreover ferric ion appears very quickly in the leaching medium because bacteria are able to act much more quickly on soluble substrate(Torma, 1977). According to this,it is reasonable to consider the first period of leaching as due almost exclusively to the Fe3+ action because the bacterial action on the sulphide will be more retarded than in absence of Fe3+. Whether this lower rate for the attack of an insoluble substrate is conected with the bacterial sorption on the surface it is not yet known. It has to be stated again that the assumption according to which the indirect rate can be calculated as the difference between the total and the direct rate is no more than a rough approach,because it implies that the direct mechanism occurs in the same quantitative way with and without Fe3+, which is very unlikely.Nevertheless,if that approach is correct it would explain the strange relationship between indirect rate and surface,according to which an increase of surface produces a decrease of rate. Another interesting fact is that after a variable period of leaching (depending on the pulp density)the direct and total rates tend to the same value,as can be observed in figure l(same slopes).This suggest a similar action in both cases,possibly the direct mechanism,which is the only one that occurs both with and without Fe3+. A better understanding of these facts would require an analysis of the causes of the decrease of rates through the processes.The most important reason in the presence of iron is the precipitation of basic iron salts on the surface.Others reasons-perhaps the only causes in the absence of iron- are: l)the rather abundant precipitation of sulphur. 2)a progressive sorption of bacteria on the surface,which would delay the dissolution of sulphide and,as a consequence,its oxidation. 3)a decrease of bacterial action with time,if they act directly on the surface. If this can be proved it would clarify this point. In tables 3 and 4 it is possible to observe that the influence of temperature is higher at higher pulp density,probably due to the strong dependency of the action associated to the surface with the temperature, although the indirect mechanism is the more affected by temperature changes. We think that is necessary to solve some other points,like the magnitude of the indirect mechanism in absence of the direct one,because it has been only proved so far with the action of increasing amounts of Fe3+ but without possibility of its constant regeneration.This is important to a better understanding of bacterial leaching of metallic sulphides or natural ores. REFERENCES Brierley,C.L.(1978) CRC.Crit.Rev.Microbiol.,6,207. Bruynesteyn,A.(1980) presented at the Conference"Use of microorganism in Hidrometallurgy".Pecs,Hungary. Colmer,A.R.,Hinkle,M.E.(1947) Science,106,253. Duncan,D.W.,Trussell,P.C.,Walden,C.C.(1964) Appl.Microbiol.,12,122. Holt,S.C.,Shiveley,J.M.,Greenawalt,J.W.(1974) Can.J.Micro~iol.,20,1347 KelIy,D.P.(1976) in Microbial Energy Conversion,Schlegal,H.G. and Barnea,J. Eds.(F.Cotze KG,Gottinger). Lundgren,D.G.,Silver,M.(1980) Ann.Rev.Microbiol.,34,263. 892 Sakaguchi,H.,Silver,M.(1978) Bioteehnol.Bioeng.,18,1091. Silverman,M.P.,Lundgren,D.G.(1959) J.Bacteriol.,77,642. $ilverman,M.P.,Ehrlich,H.L.(1964) Adv.Appl.Microbiol.,6,153. Torma,A.E.,Walden,C.C.,Branion,R.M.Ro(1970) Biotechnol.Bioeng.,12,501. Torma,A.E.(1977) Advances in Biochemical Engineering,6,16. Torma,A.E.,Sakaguchi,H.(1978) J.Ferm.Technol.,56,173. Torma,A.E.,Rozgonyi,T.(1980) in Biogeochemistry of Ancient and Modern Environments,Trudinger,P.A.,Walter,M.R. and Ralph,J.Eds. Springer- Verlag,Berlin. Torma,A.E.(1982) Progress in Industrial Microbiology, 16,77. Torma,A.E.,Banhegyi,l.G.(1984) Trends Biotechnol.,2(1),13o Tributseh,H.,Bennett,J.(1981) J.Chem.Tech.Biotechnol.,31,565o Tributsch,H.,Bennett,J.(1981) J.Chem.Tech.Biotechnol.,31,627~ Tuovinen,O.H.,Kelly,O.P.(1972) Z.Allg.Mikrobiol.,12,311. Table l:Rates of Cu 2+ dissolution(in % w/v .h-l).Direct and Indirect mechanism. Pulp Density(% w/v) Temperature(C) Total Rate .~4 Direct Rate.10~ 0.5 31 125 80 1.0 31 150 123 1.5 31 180 160 2.0 31 190 176 2.5 31 205 200 0.5 35 130 85 1.0 35 186 144 1.5 35 240 200 2,0 35 260 236 2.5 35 300 277 Table 2:Order of reaction and rates constants for the total,direct and indirect processes. Temp. K tot.f04 n tot. - K dir.104 n dir K indir.104 n indir, 31C 149 0.32 123 0.54 24.5 -0.74 35 C 186 0.48 143 0.72 41.6 -0.26 893 Table 3:Influence of the temperature on each mechanism. r = V a/ V b , a = 35 C , b = 31C c c c rl= ratio of total rates. r2= ratio of direct rates. r3= ratio of indirect rates. Pulp Densities(% w/v) r I r 2 r 3 0.5 I.ii 1.03 1.22 1.0 1.24 1.16 1.70 1.5 1.33 1.25 2.06 2.0 1.36 1.34 2.40 2.5 1.46 1.39 2.63 Table 4: Ratio of rates of the different processes a Val / V a r I/2 = 2 b Vbl / V b r 1/2 = 2 V 1 = total rate V 2 = direct rate a va2/ V a , r 2/3 = 3 b vb2 / V b , r 2/3 = 3 V 3 = indirect rate 9 a= 35 C , b--31 C Pulp Density(% w/v) a b a b r 1/2 r 1/2 r 2/3 r 2/3 0.5 1.42 1.54 2.07 1.74 1.0 1.21 1.30 5.02 3.44 1.5 i.ii 1.18 8.43 5.11 2.0 1.08 i.i0 12.10 6.80 2.5 1.02 1.08 16.10 8.50 Table 5:Activation energies for the total,direct and indirect mechanisms. Act.Energy Total Act.Energy Direct Act.Energy Indirect 42.6 KJ/mol 29.3 KJ/mol 103.0 KJ/mol 894 

Direct and indirect mechanisms in the bacterial leaching of covellite

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Direct and indirect mechanisms in the bacterial leaching of covellite

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