EXPERIMENTS ON COMBINED ELECTRO AND BACTERIAL LEACHING (Short Communication)

EXPERIMENTS ON COMBINED ELECTRO AND BACTERIAL LEACHING (Short Communication) K.P. Tepper and R. Naveke Lehrstuhl fiir Mikrobiologie, Technische Universitat Braunschweig Braunschweig, FRG Abstract: The influence of direct current and of current with changing the poles two times per sec at intensities of 0,2...15 mA and at tensions of 12...170 Von leaching of pyrite dispersed in quartzite in percolators with Thiobacillus ferrooxidans was studied. The solution of iron was accelerated by bacteria without current and by current without bacteria, but in combination the acceleration did not exceed the sum of both single effects. The results suggest that there is no direct electrophysiological influence of current on bacteria under conditions of our experiments. At present we try to utilize indirect electrochemical effects of current on bacterial leaching of chalcopyrite. Introduction We report about our first experiments which were started to get answers on some questions related to a combination of electroleaching with bacterial leaching of ores. Do these processes influence each other or one another so that the effect is greater or smaller than the sum of the effects of the single processes? Is there any direct electrophysiological influence of electric field or current on the bacteria, or is there merely an indirect effect, e.g. by electrochemical processes in the medium stimulating or inhibiting the growth? May bacterial metabolism accelerate (or inhibit ) the electroleaching? To look for more direct electrophysiological influences in the first set of experiments we have chosen pyrite as the mineral to be leached for it is‘known that with low current intensities there is no formation of elemental sulfur from pyrite which would serve as energy source to Thtobaeilli. [ne for mation of S° would represent an indirect electrochemical effect. 212 K. P. Tepper and R. N&éveke Methods We used percolators as shown in fig. 1. The medium, 230 ml 9 K medium of Silverman and Lundgren /1/ without ferrous sulfate, was moved through the percolator by air lift. The calumn was filled with a mixture of acid washed quartzite (diameter of the pieces 5...10 mm) and 200 g pyrite (diameter 0,5...2,0 mm). A 1 cm? platinum sheet was inserted as electrode in the upper layer of the substrate. The other electrode - of the same shape - was inserted in the bottom part beneath a perforated teflon sheet. This part was filled with medium up to the quartzite. Voltage and current were delivered by an equipment by which a constant current intensity of 3, 8, 9, 12 or 15 mA (provided that the vaoltace of Up. to 230.) was. sufticient) or a constant voltage up to 230 V (provided that the current intensity did not exceed 15 mA) was kept constant. Iron concentration was determined by AAS, bacteria concentration by direct microscopic counting. UPPER ELECTRODE < QUARTZITE a + PYRITE } oe AIR LIFT PERFORATED f TEFLON SHEET 9K MEDIUM wtR LOWER ELECTRODE H SCREW TUBE CLAMP Fig. 1: Percolator for combination of bacterial and electro leaching of pyrite with Thiobacillus ferrooxidans Combined Electro and Bacterial Leaching Experiments with 9 mA 213 In most experiments we worked with direct current of an intensity of 9 mA, the upper electrode as anode, starting at the beginning or at the second day of the experiment. In some experiments we interrupted the current every 5 sec for 5 sec, and in some other experiments we the poles two times per sec. At the beginning the concentration teria was 2.<.14,5 x 107 ma and the pH was 3,0...3,2. Controls up without electric current, without bacteria and without both. 7? days run at 26°C in. the dark we tion in the medium. was measured and added to that of sults of some experiments of this are shown in table 1. : or The amount of Fe changed of bacwere set After measured pH and total iron concentraprecipitated on the electrodes the total iron in the medium. The ren group with current intensity of 9 mA 0 6 total iron leached from pyrite and pH 3 mA fe e after 7 days at 28°C in the dark current Be ne ao = 0 | bacFe| pH Fe| pH Fe| pH Fe| pH days | flow Ge os terial) mg, mg | mg, mg, o 0 0 112,78 112,60 1|2,90 112,778 113,15 oO o + 193 11,80 214 1,90 S50 (75 | 31911575 47614,821 212 11,90) 345 11,71 | 299)1,80 4.77 +| CONLIN «| ot = o (112525 30 12,25 PANN20 7 2.-7.{contin.| + - a 24 2,20| 5012.20 412,21 1.-7.{interr.| + - 0 38 |2,20 13512;29 4.7-7.\ contin. + = + 235 1,60) (00 11,95 9511,94] 269 11,80 45 12,00 63 ;2,00 972,01 2.57. COntin«l + = + 4621165 | 160 11,90 35631,70 156 /1,90 344-14 5.75 Ao 7 antennal = j 3241 11,75 7] 31214,80 | 354 11,721 169 11,96 | t 1.-7.| contin.|changing + i S541, 70 4 { i Table 1: Results of four series of experiments on pyrite leaching with Thiobacillus ferrooxidans in combination with electric current of 9 mA intensity. in percolators 214 K. P. Tepper and R. Naéveke The results show clearly that without bacteria and without current there is practically no solution of iron, with current alone without bacteria the leaching is only slowly. In experiments with bacteria one can not see an acceleration of bacterial leaching by current. It must be concluded from the great differences between the values of parallel resp. similar experiments, that there must be some unknown factors which influence the bacterial leaching process. But looking at all data it can be concluded, that it is unlikely that there are direct positive physiological effects of the current on the bacteria, at least under the conditions of our experiments. Experiments with 0,2 and 1,0 mA We had to take into account the possibility that a positive influence of the current on the bacteria was abolished by negative influences of electrochemical effects. So we started another series of experiments of similar type as described above but with current intensities of only 1,0 and 0,2 mA and over a period of 30 days. During this time we measured every day pH, total iron concentration and concentration of bacteria in the medium. Without bacteria there was almost no change at all in pH and no iron was dissolved. With bacteria there was a decrease of pH and a change in iron concentration with two maxima as it is seen in fig. 2. This shape of the curve was the same in several experiments, each with several pars allels. The concentration of bacteria decreased in all experiments in the first 3 days by about 10°, possibly by adhesion onto the surfaces of the solid; then it increased to 4 value slight above that at the beginning and then remained constant. It is seen in fig. 2 representing typical results of an experiment with 1,0 mA that there is practically no difference between percolators with current and without. This was the case as well with current intensity of 0,2 mA as with 1,0 mA. Conclusion It can be concluded from our results that - at least in our experiments - there is no direct physiological influence of current on the bacteria. An enhancement of bacterial leaching by low current intensity reported by other authors, e.g. /2/, must depend on indirect electrochemical effects. But obviously these effects were absent in our experiments with pyrite. Combined Electro and Bacterial Leaching 215 10+ ‘ 8a arm on [ 6r Oo = SN—WITH CURRENT Oo Se M9 SWITHOUT CURRENT Pp” oO os ~~ 20-4 10°+ ou _-WITHOUT CURRENT xb “WITH CURRENT , 19/4 << ye, ' io 10°4 \/! oo we © {| < x mM 10-4 3,07 tT 20+ _MWITHOUT CURRENT 10 : , —-WITH CURRENT 6 10 20 30 TIME [DAYS] Fig. 2: Leaching of pyrite with Thiobacillus ferrooxidans in percolators at 26°C, dark meme With direct current at constant intensity of 1,0 mA meme without current 216 K. P. Tepper and R. Naveke Outlook From reports of Ralph /3/ and of 1114 and Bertram /4/ it must be concluded that at least in the case of chalcopyrite one can expect positive influences of current on bacterial leaching. By anodic oxidation elemental sulfur is formed on the surface of chalcopyrite crystals so that an easily utilizable energy source for Thiobacilli, especially for T. thiooxidans, is delivered. This might be seen as an enhancement of electro leaching by bacteria rather than an enhancement of bacterial leaching by current: The formation of elemental sulfur decreases the current intensity by insulation the chalcopyrite pieces by a layer of sulfur on the surfaces and therefore inhibit: the leaching process. Thiobacilli oxidize this sulfur and thus keep the surfaces clean and moreover form sulfuric acid, which itself accelerates the leaching. We will follow this line of research. References /1/- M.P.Silverman and D.G.Lundgren, J. Bacteriol. by B42 (1 959)) /2/ §.8.Gajdarziev and F.N.Gencev, Chimija i Industriva, Sofija, 44. 18 (1972) cn 73/7 -B.9:Ralph, this monograph /4/ H.1I11i and R.Bertram, this monograph