of Surfactants and Shaking

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1 APPLIED MICROBIOLOGY Vol. 12, No. 2, p March, 1964 Copyright 1964 American Society for Microbiology Printed in U.S.A. Leaching of Chalcopyrite with Thiobacillus ferrooxidans: Effect of Surfactants and Shaking British Columbia Research Council, ABSTRACT DUNCAN, D. W. (British Columbia Research Council, University of British Columbia, Vancouver, B.C., Canada), P. C. TRUSSELL, AND C. C. WALDEN. Leaching of chalcopyrite with Thiobacillus ferrooxidans: effect of surfactants and shaking. Appl. Microbiol. 12: The rate of leaching of chalcopyrite by Thiobacillus ferrooxidans has been greatly accelerated by using shaken flasks in place of stationary bottles or percolators. A further increase in rate and extent of leaching was obtained by the use of Tween 2, 4, 6, and 8, Triton X-1, Quaker TT 5386, and Hyamine Tween 2 was the most effective surfactant. No individual component of the Tween molecule was responsible for the improved leaching. The Tween-to-chalcopyrite ratio is more important than the Tween-to-medium ratio. The effect of the surfactants is probably due to increased contact between the mineral surface and the organism, and shaking provides the necessary oxygen. Rates and yields obtained by use of surfactants and shaking as aids to microbiological leaching approach those obtained with acidified erric sulfate leaching. The microbiological release of copper and other metals from their sulfides is relatively slow, compared with the rate obtained with acidified ferric sulfate solutions. Using acidified ferric sulfate in a batch system, Colombo and Frommer (1962) released 58 % of the copper from a Michigan copper ore in 4 hr and over 9% in 2 to 3 weeks. Using percolators and microorganisms, Bryner et al. (1954) released only 2.8 % of the copper from one chalcopyrite sample in 7 days and 6.6 % from another sample in 56 days. Also using percolators, Malouf and Prater (1961) leached 5% of the copper present in their sample of chalcopyrite in about 17 days and a total of 6% in 47 days. Raell (1962) released 25 % of the copper from chalcopyrite in 6 days with percolators and 35 % in 1 days in stationary flint-glass bottles. After a 7-month leach, using stationary bottles, Raell claimed that "8% of the copper in chalcopyrite is converted to acid-soluble copper but only 45 % of this is soluble at ph 2.5." Later, Raell and Trussell (1963a) improved this yield to 4% of the copper in 55 days. These excessively long leach times and usually poor yields limit the practical application of microbiological leaching. Factors demonstrated to affect the rate and extent of biological leaching are particle sie, tenmperature, ph, aeration, inoculum sie, and exposure to ultraviolet light D. W. DUNCAN, P. C. TRUSSELL, AND C. C. WALDEN University of British Columbia, Vancouver, British Columbia, Canada Received for publication 29 October (Bryner et al., 1954; Bryner and Anderson, 1957; Silverman and Lundgren, 1959b; Malouf and Prater, 1961; Silverman, Rogoff, and Wender, 1961; Raell and Trussell, 1963a, b; Sutton and Corrick, 1963). Optimal conditions include a particle sie of -325-mesh, a temperature near 35 C, a ph between 2 and 3, rapid aeration, a large inoculum, and protection from sunlight. MIicrobiological leaching of sulfide ores is most commonly studied with air-lift percolators (Bryner et al., 1954), Warburg respirometers (Beck, 196), and stationary leach bottles (Raell and Trussell, 1963b). Percolators and stationary leach bottles normally meet all the requirements for optimal leaching, with the possible exception of aeration. Anaerobic conditions do not occur in the Warburg apparatus, but its sie and principle of operation render it unsuitable for evaluating many practical leaching variables. A practical method which produces rapid aeration and permits evaluation of the experimental variables is the use of Erlenmeyer flasks on a shaking apparatus. All the conditions for optimal leaching can be encompassed with such an arrangement, except possibly the necessity for a finite contact time between the ore particle and the bacterium. If Thiobacillus ferr ooxidans attacks the crystal lattice of sulfide minerals directly, then the rate of leaching miiay be increased by providing for intimate contact between the organisms and the surface of the mineral. Starkey, Jones, and Frederick (1956) showed that the rate of oxidation of sulfur by T. thiooxidans in a liquid medium was increased by shaking with surface-active agents. More recently, Schaeffer and Umbreit (1963) showed that T. thiooxidans produces a surface-active agent that wets the sulfur to be oxidied. The present paper presents the effect of surfaceactive agents and shaking on the biological leaching of chalcopyrite by T. ferrooxidans. MATERIALS AND MIETHODS The culture of T. fenrooxidans used in this study was isolated by Raell and Trussell (1963a) from the minewater of the Britannia IMine of the Howe Sound i\lining Co. The organism was grown at 35 C in medium 9K (Silverman and Lundgren, 1959a) on a gyratory shaker. The sample of museum-grade chalcopyrite was crushed and ground to pass through a 325-mesh sieve. Chemical

2 VOL. 12) 1964 LEACHING OF CHALCOPYRITE 123 analysis showed the chalcopyrite (CuFeS2) to be % pure on the basis of copper and % pure on the basis of iron. The leaches were carried out in 25-nd Erlenmeyer flasks containing 1. g of -325-mesh chalcopyrite and 75 ml of medium 9K (ph 2.5) in which the iron solution was replaced by distilled water. The required amount of surfactant was added on a per cent by volume basis. The uninoculated control flasks contained Hg++ as a growth inhibitor. The flasks were inoculated with three drops of an 8- to 1-day-old culture of T. ferrooxidans which had been harvested according to the procedure of Laaroff (1963), washed twice with ph 2 sulfuric acid, and resuspended in ph 2.5 sulfuric acid at 1 times the original concentration. The flasks were incubated in the dark at 35 C on a gyratory shaker (New Brunswick Scientific Co., Inc., New Brunswich, N.J.) at 18 rev/min. Periodically, the flasks were analyed for soluble copper by adding 4 ml of 1.9 N ammonium hydroxide to 1 ml of sample, centrifuging at 5 X g for 1 min to precipitate the iron, and reading the optical density at 62 m,u (Mehlig, 1941). When necessary, the residue from the copper determinations was dissolved in.5 ml of 6 N HCl, and iron determinations were carried out with thiocyanate (Sandell, 1959). The surface active agents used are listed in Table 1. Their properties and manufacturers are listed in Detergents and Emulsifiers Annual (Anonymous, 1963). RESULTS The surfactants listed in Table 1 were initially evaluated at the.1,.2,.5, and.1 % levels for their effect on the oxidation of the iron in medium 9K by T. fenrooxidans. Geigy Amine T, Naccanol NR, and Triton X-4 inhibited iron oxidation at all concentrations, whereas the others permitted oxidation at some concentrations. Certain of those anionic and nonionic surfactants which were noninhibitory to iron oxidation were evaluated at their maximal noninhibitory concentration for their effect on the biological leaching of chalcopyrite, in both stationary bottles and shake flasks. The results (Table 2) TABLE 1. Surface-active agents Anionic Cationic Nonionic Duponol 8 Armour 461 Igepal CO-63 Petrowet R Hyamine 2389 Span 2 Petrowet WN Geigy Amine T Span 8 Naccanol NR Triton X-1 Nopco CVT Tween 2 Quaker TT 5386 Tween 4 Quaker TT 5518 Tween 6 Triton X-4 Tween 8 Ultrawet 4 Tween 81 Tween 85 indicate that shake flasks produce higher yields than stationary bottles, and thus are preferred for studying microbiological leaching. The control shake flask, containing T. ferrooxidans but no surfactant, leached 35 % more copper in 33 days than did the corresponding stationary bottle. The results (Table 2) show also that Tween 2 was the only surfactant in the anionic nonionic group which improved the biological leaching of copper. The improvement was particularly pronounced in the shake flasks, where the release of copper was doubled. Triton X-1 gave a final yield similar to the inoculated control, but initial leaching rates were greater. Table 3 presents the results with cationic surfactants in shake flasks. Armour 461, Nopco CVT, and Ultrawet 4 TABLE 2. Effect of various anionic and nonionic surfactants on the release of copper from museum-grade chalcopyrite by Thiobacillus ferrooxidans Leach Surfactant Concn method Copper concn (ppm) days 28 days 33 days Inoculated control Stationary Shake 24 1,1 1,85 Uninoculated control Stationary Shake Duponol 8.1 Stationary Shake Igepal CO-63.1 Stationary Shake Petrowet R.5 Stationary Shake 2 95 Petrowet WN.1 Stationary Shake Triton X-1.5 Stationary Shake Tween 2.1 Stationary Shake 197 2,32 TABLE 3. Effect of various cationic surfactants on the release of copper from museum-grade chalcopyrite by Thiobacillus ferrooxidans in shake flasks Surfactant ConcnSuranInocu- Copper concn (ppm) days 19 days 26 days Inoculated control Uninoculated control Armour Hyamine , Nopco CVT Quaker TT , Quaker TT Ultrawet

3 DUNCAN, TRUSSELL, AND WALDEN had no effect on leaching, whereas Quaker TT 5518 was inhibitory. The only systems to release more copper thani the controls were those containing Hyamine 2389 and Quaker TT The uninoculated surfactant flasks in Table 3 showed that the surfactants themselves had no ability to leach chalcopyrite. Of the four surfactants that accelerated the leaching of chalcopyrite, Tween 2 was the most effective and Triton X-1 was considered to have potential value because of the rapid initial rate of leaching it produced. A more detailed study of the optimal concentration of these two surfactants was made (Table 4). The results indicate that in 18 days Tween 2 released more copper than did Triton X-1 and that the optimal concentrations were.2 to.3 %c for Triton X-1 and.3 to.4's, for Tween 2. After 18 days of leaching,.2cj Triton X-1 released twice as much copper as the inoculated control, and.3 %. Tween 2 released three times as much. In working with stationary bottles, Raell and Trussell (1963a) postulated that an insoluble copper-iron complex interfered with the estimation of copper released from chalcopyrite. Before the addition of 2 volumles of 6 x HCI, they recovered only 65 % of the released copper. Upon termination of the leach reported in Table 2, certain flasks were examiinled for their soluble copper content. Results given in Table 5, for analyses made before and after the solutions are adjusted to a concentration of 1 % HCl, show that at least 95 %O of the copper was in solution before the HCl was added. The increase in soluble copper in 1 % HCl solutionis (Table 5) is an anialytical artifact. Determiiinationi of known amounts of copper in 1 % HCl gave recoveries of 15 "/'. When the Tween 2 sample was miiade to 1 /c HCl, stirred, filtered, and the ph adjusted to 2, the increase in determiined copper was negligible (Table 5). Therefore, no insoluble copper-iron complex formls under the conditions employed here, and the addition of HCl in the final determination of copper was henceforth omitted. TABLE 4. Surfactant Inoculated control Uninoculated control Triton X-1 Tween 2 Since Tween 2 (polyoxyethylene sorbitan moniolaurate) was the most effective surfactant investigated, other related compounds were examiiined. Laurie acid, stearic acid, oleic acid, and sorbitol at the.1 %G level had no effect on the rate or extent of leaching. Palimiitic acid inhibited the release of copper completely for 27 days anid then slow leaching began. At the.3 %/ level, Span 2, Span 8, Tween 81, and Tween 85 did not imlprove leaching, although Span 8 caused leaching to start at 3 days instead of 6. Inoculated flasks containing Tween 4, 6, and 8 all started to leach rapidly and gave inmproved yields over control flasks. These results indicate that Ino inidividual coimlponieint of the Tweens is responsible for the improved leaching. The poor results with the Spans indicate that the polyoxyethylene side chain is necessary, and that it canniot be shortened was indicated by the lack of effect of Tween 81. Three fatty acid groups, as in the trioleate, Tween 85, were as undesirable as the lack of the polyoxyethylene side chain in Span 8. Improved leaching is associated only with the molecular constituents as comiibined in Tween 2, 4, 6, and 8. Results obtained in evaluating the effect of Tween 2, 4, 6, and 8 at four different concentrations are given in Table 6. 1\Iaxinmal effect is displayed by Tween 2 at the.1 %O level, which gave 2.25 timies as iiiuch copper as the inoculated conitrol. In uninoculated leaches,.5 %, of the Tween surfactants leached no miiore copper than the control, and about one-tenth as miiuch as the inioculated control. Wheni the rate of leachinig declined in the Tween 6 flasks, the 9K imiediumn and Tween 6 were replaced in the.3 and.5 71 flasks. Instead of restoring the initial leach rate, leachinig stopped coiimpletely (Table 6). Leaching rates for all concentrations of Tweeni 2 listed in Table 6 are shown graphically in Fig 1. For.3 and.1 % Tween 2, rates and final yields are alnmost identical. The rate of leaching with the.57% Tween 2 flask was appreciably slower than that with the higher concentrations of Tween 2. Effect of concentration of T'riton X-1 and 7'ween 2 on the mictobiological release of copper from museum-grade chalcopyrite Concn 7, Inoculated Concn of copper (ppm) days 4 days 7 days days 14 days 18 days ,575 1,685 1,645 1,625 1,765 1,765 1,98 1, ,9 2,3 1,925 1, ,31 2,41 2,535 2, ,485 2,41 2,29 2, ,84 2,94 3,9 3,25 1, ,33 2,33 2,45 2, ,72 3,4 3,33 3,35 APPL. AIICROBIOL. 1, ,51 2,61 2,65 2, ,95 3,2 3,67 3,67 Yield of copper (7,

4 VOL. 12) 1964 LEACHING OF CHALCOPYRITE 125 The previous experiments had shown that.1 to.3 % Tween 2 gave optimal leaching of chalcopyrite under the conditions employed. Table 7 presents the results of an experiment set up to determine whether this optimum involved the ratio of Tween to medium or Tween to chalcopyrite. With.3 % Tween 2, the percentage release of copper from 1 and 2 g of chalcopyrite was the same and from 4 g only slightly less. At a Tween 2 concentration of.6 %, the yield of copper from 2 g of chalcopyrite was the same as for.3 % Tween 2. With 4 g of chalcopyrite, the yields were decreased slightly, but the percentage of copper released was slightly greater with.6 % Tween 2 than with.3 or.12 %o. TABLE 5. Soluble copper before and after making the solution up to 1% HCI Flask Soluble copper Soluble copper Soluble in 1% HCI before HCI mg mg % Duponol Petrowet R Petrowet WN Tween * 1 * The ph of the filtrate was adjusted to 2 before copper determination. TABLE 6. Percentage yield of copper in 24 days from -325-mesh museum-grade chalcopyrite with various Tweens and Thiobacillus ferrooxidans 2. (L il-l 2.4 cr I.- w u 2 u x w a. a. u Concn Inoculated Tween 2 Tween 4 Tween 6 Tween 8 Control * t t 66 * Maximum after 19 days. t Media replacement experiments. TIME IN DAYS FIG. 1. Effect of various concentrations of Tween 2 on the release of copper from chalcopyrite. The overall low yields with 4 g of chalcopyrite were caused probably by a suboptimal chalcopyrite-to-medium ratio, or possibly by an inhibitory effect of copper concentrations in excess of 1, ppm. A high chalcopyriteto-medium ratio may reduce access of bacteria to ore surfaces. Data in Tables 4 and 6 indicate that with 1 g of chalcopyrite in 75 ml of medium concentrations of Tween 2 most effective in releasing copper were.3 % or less. Above this concentration, copper release was inhibited. On the other hand, in Table 7,.6 % Tween 2 does not depress copper release from 2- and 4-g samples of chalcopyrite. These results indicate that the Tween-to-chalcopyrite ratio is more significant than the Tween-to-medium ratio. DIsCUSSION Beck (196) showed that, during the oxidation of iron by resting-cell suspensions of T. ferrooxidans, the rate of oxygen consumption is high. Thus, during active metabolism the oxygen in solution may be depleted faster than it is replaced. This danger is particularly great with stationary leach flasks where the only mechanism for oxygen transfer is diffusion. In the time intervals involved in these experiments, the stationary flasks consistently gave lower yields than did the shake flasks. Slow diffusion of oxygen into the solution probably was responsible for depressing the leaching rates. In view of this, shake flasks have replaced the use of stationary bottles for studying microbiological leaching in this laboratory. The possibility of oxygen deficiency also exists with percolators. From the descriptions and illustrations of percolators given in the literature (Bryner et al., 1954; Bryner and Anderson, 1957; Malouf and Prater, 1961; Sutton and Corrick, 1963), the columns are flooded with nutrient solution to a height of 3 to 5 cm above the surface of the sand and ore. The liquid that is entering the top of this column of ore may be saturated with oxygen, but with the rapid rate at which oxygen is consumed, at least during iron oxidation, anaerobic conditions probably occur TABLE 7. Effect of variation in the ratio of the concentration of Tween 2 to amount of -325-mesh museum-grade chalcopyrite on the release of copper by Thiobacillus ferrooxidans Concn of Tween 2 Amt of.3%*.6%*.12%* chalcopyrite Amt of Yield Amt of Yield Amt of Yield Amt of Yield Cu Cu Cu Cu g mg % mg % mg % mg % t 5t * On basis of 75 ml of liquid volume. t IlJninoculated control

5 126 DUNCAN, TRUSSELL, AND WALDEN APPL. MICROBIOL. in the lower reaches of the columiin. The improved release of copper from chalcopyrite by T. ferrooxidans by use of shake flasks in the absence of a surfactant (Table 6) supports this hypothesis. These conclusions are in agreement with the results obtained by Audsley and Daborn (1962) during the leaching of uranium ores in percolators. They found that, in a deep column of uranium ore plus pyrite, over 83 % of the uranium had been released from the top two-fifths and only 32 % from the bottom one-fifth. When the frequency at which the mineral surfaces of another deep column were wetted was decreased from daily to weekly intervals, an increased rate of leaching was obtained. They concluded that this was due to a greater volume of air in the voids. The addition of Hyamine 2389, Quaker TT 5386, Triton X-1, and Tween 2, 4, 6, and 8 to the leach solutions aided the microbiological leaching of chalcopyrite. The surfactants probably wetted the chalcopyrite surface, permitting more rapid contact and more intimate association by the bacteria. The rapid aeration of the systenm provided the necessary oxygen. Shaking might be expected to mrinimie residence time of the organism on the mineral surface. Starkey et al. (1956) obtained poorer oxidation of sulfur on a reciprocal shaker than on a rotary shaker, which they attributed to sulfur accumulating on the necks of the flasks above the liquid surface. Another possibility may be that the snaplike motion of a reciprocating shaker limits the contact time of the bacteria on the sulfur particles. In the studies reported here, a rotary shaker was used, and the motion it imparts to the system may not hinder bacterial contact with the mineral surfaces. The increased rate and extent of biological leaching of chalcopyrite, by use of surfactants and shaking, increases the commercial attractiveness of this process. Yields of copper from chalcopyrite as high as 85 % in 24 days have been obtained with T. ferrooxidans, Tween 2, and aeration. Initial rates are much higher than these figures indicate; release of copper over the first 5 to 7 days has been 1% per day. These rates and yields are much higher than the previously reported maximums of 6% copper in 47 days by 1\Ialouf and Prater (1961) and 4% copper in 6 days by Raell and Trussell (1963b). They approach the results obtained by Colombo and Frommer (1962) during chemical leaching of chalcocite. The shake-leach process as applied to other minerals and ores was presented elsewhere (Duncan and Trussell, in press). These authors showed that vastly improved leach rates and yields result with improved aeration but that only museum-grade chalcopyrite, museum-grade millerite, and two copper ores responded to the presence of Tween 2. ACKNOWLEDGMENT The authors wish to acknowledge the technical assistance of H. Kurt. LITERATURE CITED ANONYMOUS Detergents and emulsifiers annual. John W. McCutcheon, Inc., Morristown, N.J. AUDSLEY, A., AND G. It. DABORN Natural leachiing of uranium ores. 2. A study of the experimental variables. Trans. Inst. Mining Met. 72: BECK, J. V A ferrous ion-oxidiing bacterium. I. Isolation and some general physiological characteristics. J. Bacteriol. 79: BRYNER, L. C., AND R. ANDERSON Microorganismis in leaching sulfide minerals. Ind. Eng. Chem. 49: BRYNER, L. C., J. V. BECK, D. B. DAVIS, AND D. G. WILSON Microorganisms in leaching sulphide minerals. Ind. Eng. Chem. 46: COLOMBO, A. F., AND D. W. FROMMER Leaching Michigan copper ore and mill tailings with acidified ferric sulphate. U.S. Bur. Mines Rept. Invest LAZAROFF, N.' Sulfate requirements for iron oxidation by Thiobacillus ferrooxidans. J. Bacteriol. 85: MALOUF, E. E., AND J. D. PRATER Role of bacteria in the alteration of sulphide minerals. J. Metals 13: MEHLIG, J. P Colorimetric determination of copper with ammnonia. Ind. Eng. Chem. Anal. Ed. 13: RAZZELL, W. E Bacterial leaching of metallic sulphides. Trans. Can. Inst. Mining Met. 65: RAZZELL, W. E., AND P. C. TRUSSELL. 1963a. Isolation and properties of an iron-oxidiing Thiobacillus. J. Bacteriol. 85: RAZZELL, W. E., AND P. C. TRUSSELL. 1963b. Microbiological leaching of metallic sulfides. Appl. Microbiol. 11: SANDELL, E. B Colorimetric determination of traces of metals, 3rd ed. Interscience Publishers, Inc., New York. SCHAEFFER, W. I., AND W. W. UMBREIT Phosphotidylinositol as a wetting agent in sulfur oxidation by Thiobacillus thiooxidans. J. Bacteriol. 85: SILVERMAN, M. P., AND D. G. LUNDGREN. 1959a. Studies on the chemoautotrophic iron bacterium Ferrobacillus ferrooxidans. I. An improved mediuni and a harvesting procedure for securing high cell yields. J. Bacteriol. 77: SILVERMAN, M. P., AND D. G. LUNDGREN. 1959b. Studies on the chemoautotrophic iron bacterium Ferrobacillus ferrooxidans. II. Manometric studies. J. Bacteriol. 78: SILVERMAN, M. P., M. H. ROGOFF, AND I. WENDER Bacterial oxidation of pyritic materials in coal. Appl. Microbiol. 9: STARKEY, R. L., G. E. JONES, AND L. R. FREDERICK Effects of medium agitation and wetting agents on oxidation of sulphur by Thiobacillus thiooxidans. J. Gen. Microbiol. 15: SUTTON, J. A., AND J. D. CORRICK Microbiological leaching of copper minerals. Mining Eng. 15:37-4. TEMPLE, K. L., AND E. W. DELCHAMPS Autotrophic bacteria and the formation of acid in bituminous coal mines. Appl. Microbiol. 1: