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Indian Journal of Experimental Biology Vol. 41, September 2003, pp. 967-971

Biorecovery of gold

Ronald Eisler*

U.S. Geological Survey, Patuxent Wildlife Research Center, Laurel, Maryland 20708·4019, USA

Recovery of ionic and metallic gold (Au) from a wide variety of solutions by selected species of bacteria, yeasts. fungi. algae. and higher plants is documented. Gold accumulations were up to 7.0 g/kg dry weight (OW) in various species of bacteria, 25.0 g/kg OW in freshwater algae, 84.0 g/kg OW in peat. and 100.0 g/kg OW in dried fungus mixed with kerati­nOlls material. Mechanisms of accumulation include oxidation, dissolution, reduction. leaching, and sorption. Uptake patterns are significantly modified by the physicochemical milieu. Crab exoskeletons accumulate up to 4.9 g Au/kg OW; however, gold accumulations in various tissues of living teleosts, decapod crustaceans, and bivalve molluscs are negligible.

Keywords : Biorecovery of gold, Gold uptake

Extraction of gold from solutions is under active investigation using a variety of physical, chemical, and biological processes. Recovery of ionic gold from dilute solutions usually involves either precipitation by zinc dust, carbon adsorption, solvent extraction, or ion exchange resins. All of these are of low selectivity and comparatively expensive I.2. Chemical methods for the recovery of gold from ores include cyanidation and thiourea leaching, which present environmental and health risks3.4. Biorecovery of dissolved gold from solution presents fewer environmental risks than chemical methods, and is documented for

. . 25-20 I 1421 l' 22 ?3 mIcroorganisms ' ,a gae . , water lerns , peat- , alfalfa4, seaweeds2. 12. 1 8.24.25, fungi 2. 18.26.29, yeasts l4 , and crab exoskeletons3o. This account briefly reviews the potential of living and dead plants and animals to accumulate gold from solution, and some of the processes involved - including biooxidation, dissolution, bioreduction, bacterial leaching, and biosorption.

Gold uptake by microorganisms, fungi, and higher plants

Biomining processes are llsed successfully on a commercial scale for the recovery of gold and other metals, and are based on the activity of obligate chemoautolithotropHic bacteria that use iron or sulfur as their energy source and grow in highly acidic media 13. Biooxidation of difficult to treat gold-bearing arsenopyrite ores occurs in aerated, stirred tanks and

*For correspondence: E·mail: ronald.cisler@usgs.gov Fax : 301-497-5744 Phone : 301·497-5724

rapidly-growing, arsenic-resistant bacterial strains of Thiobacillus ferrooxidans (Beijerinck), LeptospiriLlium ferrooxidans (Beijerinck), and Thiobacillus ferrooxidans (Beijerinck). These bacterial species obtain their energy through the oxidation of ferrous to ferric iron (T. ferrooxidans, L. ferrooxidans) or through the reduction of inorganic sulfur compounds to sulfate (Thiobacillus spp.). Monetary costs of biooxidation are reported to be about 50% lower than roasting or pressure oxidation 13. Adding Thiobacillus ferrooxidans into the thiourea leaching solution produces a 20% increase in the extraction of gold. The reaction describing gold dissolution in an acidic solution of thiourea in the presence of ferric ion IS

described by Kai et a/.f:> as :

Au· + Fe3+ + 2CS(NH2h ~ Au[CS(NH2hf+ + Fe2

+

The use of bacteria in pretreatment processes to degrade recalcitrant gold-bearing arsenopyrite ores and concentrates is well established7

• 19. Recalcitrant ores are those in which the gold is enclosed in a matrix of pyrite and arsenopyrite, and cannot be solubilized by direct cyanidation. Bacterial decomposition of arsenopyrite assists in opening the molecular mineral structure, permitting access of the gold to cyanide. However, greater quantities of cyanide are required to solubilize gold after bacterial treatment when ores contain high quantities of gold. A possible cause of this excessive cyanide is the presence of the enzyme rhodanese, produced by Thiobacillus caldus (Beijerinck), a common species of bacterium encountered in biooxidation facilities l9

.

Optimum microbiological leaching by Thiobacillus spp. and Sulfolobus spp. of refractory sulfide ores for

968 INDIAN J EXP BIOL, SEPTEMBER 2003

recovery of gold in tanks is poss ible under controlled conditions of pH, dissolved oxygen, carbon diox ide, sul fur balance, redox potenti al, tox ic metal concentrations, and rate of leaching7.

Several species of Fe3+ -reducing bacteri a (Bacteria

spp., Archaea spp.) can precipitate gold by reducing Au' + to Au' with hydrogen as the electron donor"o. Rate of bacterial ox idati on by ThiobacilLus fe rrooxidans and Leptospirillium fe rrooxidans of three South Afri can refractory gold ores of varying gold-arsenopyrite composition was dependent mainl y on crystal structureR

• These go ld ores were c1 ass i fi ed as refractory due to the presence of gold inclusions in arsenopyrite and pyrite, and submi croscopic gold mainly in arsenopyrite. Refractory gold occurs at sites whi ch are preferenti all y leached by the bacteri a. The rate of go ld liberation fro m sul fides is enhanced during the earl y stages of bac teri al ox idation. Defects in crys tal structure influence the rate of bioox idati on and is directly related to the crystal structure of the sulfide mineral, the crystall ograpHic ori entati on of the exposed surfaces, and diffe rences In chemical composIti on and mechanica l dev iat ions in the crys talsx. Pretreatment of refractory gold concentrates with the bacterium Thiobacilllls fe rrooxidans ul timately results in sulpHur and sulpHide ox idati on by fe rric ions from bacteri al ox idati on of ferrous ions. The max imum concentration of attached Thiobacillus increases with increasing concentrati on of Fe2+ and decreases with increasing size of the refrac tory gold concentrate particles '6 . In Chile, which produced 30,000 kg of gold in 1990, Thiobacillus ferrooxidans was used to recover gold from a complex ore under laboratory conditions9

. The ore contained 8.2% Fe, 0.78% Cu, 0.88% As, and 3.5 g Au/ton, with pyrite, hematite, arsenopyrite, and chalcopyrite as the mai n metal-bearing minerals. Initi al gold recovery by conventional cyanidation on a crushed ore sample was 54%; concentration by fl otation improved recovery to 56%. Concentrated samples (17 .0 g Au/ton) were leached in reactors at p H 1.8. In the presence of bacteri a, all di ssolved iron was present as ferri c ion; gold recovery by cyanidati on increased from 13% fo r the initi al concentrate to 97% after 10 days of bacterial leaching. To furth er increase gold recovery, fl otation tailings were submitted to cyanidation9

.

Some mi croorganisms isolated from go ld-bearing depos its are capabl,e of di ssolving gold ; di ssolution was aided by the presence of aspartic acid, histidine, serine, alanine, glyc ine, and metal ox idants5

. Bacleri f­ann go ld is well know n, with uptake of Au3+ from

chloride solutions documented fo r at least seven genera of freshwater cyanobacteria3 1

. Some bacteriform gold is biogenic - the result of precIpitati on by bac teria - and may be usefu l indicators of gold deposits and of processes of gold accumul ati on. PLectonema terebrans Bornet, a spec ies of fil amentous marine cyanobacteria, accumul ates gold in its sheath fro m an aqueous solu tion of AuCl 3.

Sheaths are among the few structures likely to be preserved in some form in microfossi ls of ancient bacteri a. In marine medi a, it is ex pected that AuCI3

(2.0 g Au/l ) will fo rm AuCI ~ , Au' ; , and AuCi ;31. Biosorption of Au3+, as AuCi ~ , by dried Pseudomonas

strains of bac teri a was inhibited by palladium, as Pd2+, and possibl y other metal ions 10 .

Gold adsorption from cyanide solutions by dead biomass of bacteri a (Bacillus subtilis), fun gus (Penicillium chrysogenum Thom), or seaweed (Sargassum flu itans Linnaeus) at pH 2.0 were 1.8 g Au/kg OW fo r bacteri a, 1.4 g/kg OW for fun gus, and

0.6 g Au/kg OW fo r seaweed. Ani oni c AuC ;

adsorpti on was the major mechani sm in go ld biosor­ption fro m cyanide so lutions, being most efficient at lower p H values2

. L-cysteine increased gold-cyanide bi osorpti on of Bacillus, PenicilliulIl, and Sa rgassum l R

•

At pH 2, the max imum gold uptakes were 4.0 g Au/kg OW fo r bac teri a, 2.8 g/kg fo r fungus, and 0.9 g/kg fo r seaweed, or 150-250% greater than in the absence of cysteine. The anion ic gold cyanide species were adsorbed by ioni zable fun cti onal groups on cysteine­loaded biomass; depos ited gold could be eluted fro m gold-loaded biomass at p H 5.018

•

Gold-resistant strains of bac teri a that also accumul ate gold are documented, although the fundamenta l mechani sm of resistance to gold in mi croorgani sms is not known or understood 15. One strain of Burkholderia (Pseudomonas) cepacia Burkholder contained millimolar concentrations of Au+ thiolates. Burkholderia cells were large, accumul ated polyhyroxybutyrate and gold, and excreted thi orin , a low molecul ar weight protein into the cultu re medium. This effect was not observed with the Au3+ complexes tes ted, which were reduced to metallic gold in the medium. Gold-res istant strains of fun gi and heterotrophic bacteri a are also known' 5

.

Rapid recovery of gold from gold-thiourea solutions was documented fo r waste biomass of yeasts (Saccharomyces cerevisiae), cyanobacteri a (Sp iruLina platensis [Nordst]), and bac teri a (Streptomyces erythralus [Waksmanl)' 4 The process is p H-

EISLER : BIORECOVERY OF GOLD 969

dependent for yeast and bacteri a, and pH-independent fo r Spirulina. Of all stra ins of microorgani sms examined, Spirulina platensis has the hi ghest affinity and capac ity for gold, even at low pH values. Gold uptake by Spirulina was 7 .0 g Au/kg bio mass OW in 1-2 hr at p H 2.0, and about 3.0 g Au/~g OW in 15 min atpH 2 through 7 14.

Metabolically acti ve fungal cells of Aspergillus f umigatus Fresen and A. niger Tiegh removed gold from cyanide leach liquor of a Brazilian gold extraction plant more effi ciently than did dried fungal biomass or other species of Aspergillus tested . These two species of fungi removed 35 to 37% of gold from solutio ns containing 2.8 mg Au/I in 84 hr26. Gold removal fro m cyanide-containing solutio ns is documented for a strain of Aspergillus niger, a fungus iso lated from the gold extraction plant at Nova Linda, BraziI26

-28

. The leach liquor contained, in mg/I , 18 1.0 cyanide, 1.3 gold, 0 .4 silver, 7. 1 copper, 5.2 iron, and 4 .5 zinc. After 60-72 hr of incubation, A. niger removed from solution, probably by adsorption, 64% o f the gold, I 00% of the si lver, 59% of the copper, 80% of the iron, and 74% of the zinc; all gold was removed after 120 hr. Use of this fungus to develop a bioprocess to reduce metal and cyanide levels as well as recovery of va luable metals shows promjse26-28. Uptake patterns of gold from Au3+ solutions by dead fungal biomass fo llowed mathematical uptake models of Langmuir and Freu­ndlich; biomass was prepared from the fruiting body of a mushroom co llected fro m the forests of Kerala, India29

. Dried fungus, Cladosporium cladosporoides Fresen, mi xed with keratinous materi a l of natural origin to form a bead, proved effecti ve in absorbing gold fro m solution l 2

. The biosorpent beads adsorbed 100.0 g Au/kg beads fro m a solution containing 100 .0 mg Au/ I. Max imum biosorptio n of 80% occurred at acid pH ( 1-5) in less than 20 min . The biosorpent beads degraded in so il in about 140 days . The beads also removed 55% of the gold from e lectroplating solutions containing 46.0 mg Au/I , w ith observed gold loading capacity of 36.0 g/kg beads 12. Dried biosorpents encapsul ated in polysul fo ne were prepared fro m microorganisms isolated from pri stine or ac id mine drainage environments " . Biosorpent materi a l ri ch in exopolysaccharides fro m the ac id mine drainage site bound Au3+ three times more effecti vely than did other materi als, and removed 100% of the Au3

+ from solutions containing 1.0 mg Au/ I within 16 hr at 23°C andpH 3.0.

Algal ce ll s, ali ve or dead , rapidl y accumulate Au3+ and begin to reduce it to Au· and Au+ within 2 days33 .

Uptake of Au3+ by Chlorella vulgaris Beij erinck, a unicellular green alga, from solutio ns conta ining 10.0 or 20.0 mg Au3+/l is documented21. Chlorella accumulated up to 16.5 g Au/kg OW. Inacti vating the a lgal cell s by various treatments resulted in some enhancement in uptake capac ity o ver the pri stine cell s. Inacti vati on by heat treatment yie lded up to 18.8 g/kg DW; for alkali treatment, thi s was 20.2 g/kg OW ; for formaldehyde treatment, 25.5 g/kg OW ; and for acid treatment, 25 .4 g/kg OW. E lementa l go ld (Au") was measured by X-ray photoelectron spectroscopy on the cell surface, indi cating that a reducti on had occurred21. Studies with li ving Chlor­ella vulgaris suggest that accumulated Au 3

+ is rap id ly reduced to Au+, fo ll owed by a slow reducti on to AU· 15. With dead a lgae, Au· ini tiates a seeding process which results in the formation of e lemental gold .

Sequeste ring meta l ions using li ving or dead pl ants is a proposed economical means o f remov ing gold and other meta ls via intracellul ar accumul ation or surface adsorption. However, in the case of li ve pl ants, thi s is freq uentl y a re lati vely slow and time­consuming process. Nonli ving pl ant materi a l fo r surface adsorptio n offers several advantages over live plants, incl uding reduced cost , greate r availabil ity , eas ie r regenerati on, and higher meta l spec ific ity4. In South Afri can mining effluents, go ld usually ranges between I and 10 mg/I. In studi es of 180-min durati on, dried red water fe rns, Azalla .filiculoides Lamarck, removed 86 to 100% of Au3+ fro m ini tial so lutio n of 2 to 10 mg Au3+/l , increas ing with increased initi a l concentration o f Au3+ (ref. 22). The bio mass gave >95 % removal e ffic iency at all bio mass concentrations measured . Optimum (99.9%) removal of gold occUlTed within 20 min at pH 2, 42% removal at pH 3 and 4, 63 % at p H 5, and 73% removal at p H 6; removal effic iency seemed independent of temperature22 . Similar results were observed with fo ur species of ground dried seaweeds (Sargassum sp., Gracilaria sp., Eisenia sp., and Ulva Sp. )25 Treated seawecds removed 75-90% of the go ld within 60 min at pH 2 fro m solutio ns co ntaining 5.0 mg A u3+/l . Gold (A u3+) can be sequestered f rom ac id solutio ns by dead bio mass of a brown alga, Sargassum na(ans (Linnaeus), and depos ited in its e lemental fo rm, AU·24

.

The ce ll wall of Sargassum was the major locale fo r go ld depositio n, w ith carbony l groups (C = 0 ) pl ay ing a major ro le in binding, and N-conta ining groups a lesser ro le. Like activated carbon, the biomass of Sa rgassum natans is ex treme ly porous, reportedly more than most bio materi als, and accounts, in part ,

970 INDIAN J EXP BIOL, SEPTEMBER 2003

fo r its ability to accumulate gold24. Dried ground

shoots of alfalfa, Medicago sa/iva Linnaeus, were effective in removing gold from solution4.The accumulation process in volved the reduction of Au 3+ to colloidal Au', and was most effici ent at elevated temperatures and ac id pH. In so lutions containing 60 mg Au)' /I, about 90% of the Au 3+ was bound to dri ed alfalfa shoots in about 2 hI' at pH 2 and 55°C. The mechanism to account for thi s phenomenon is unknown , but may involve reduction of Au 3+ to Au +, the latter being unstabl e in water to form Au' and Au 3+ (ref. 4). Dried peat from a Brazilian bog accumulated up to 84.0 g Au/kg OW within 60 min from so lutions conta ining 30.0 mg Au3+/1 (ref. 23).

Gold uptake by aquatic macrofauna Except for crab exoskeletons, gold recovery from

the medium by various species of li ving molluscs, crustaceans, and fi shes is neg li gible.

Certain chitinous materials , such as exoskeletons of the swamp ghost crab, Ucides corda /u s (Linnaeus), can remove and concentrate go ld from ani oni c gold cyanide so lutions over a wide range of pH values:10

The max imum AuCN ; uptake occurred at pH 3.7,

corresponding to a fin al va lue of 4.9 g Au/k g OW ; exoskeletons burnt in a non-oxid izing atmosphere removed 90% of the gold at pH 10. phenolic groups created during the heat treatment seemed to be the

main functional group respons ible for AueN ;

bind ing by burnt ac id-washed crab shells3o.

Bioconcentration factors (BCFs) were recorded for carrier-free 19XAu+ (ph ys ical half-life of 2.7 days) in freshwater organi sms after immersion for 2 1 days in a med ium containing 25,000 pCiIl =675 ,700 Bq1l 32 . In goldfi sh, Ca rassills a llra/us (Linnaeus), the hi ghest BCFs measured were < I in mu cle (i.e., less than 675.700 Bq/kg FW musc le), 10 in viscera, and 9 in whole fish. In the freshwater winged tloater clam, Anodonw /lIma/liana Lea, the maximum BCF was 7 in soft parts; for crayfish (A slacus sp. ), BCFs were < I ill mu scle and 14 in viscera. For marine organ isms immersed for 26 days in sy ntheti c seawater cont aining 33.000 pCi/1 = 89 1 ,900 Bqi l. max imum BCFs measured were tl in musc le and 16 in viscera of the red crab . Callcer produ c/ll s Randall , 11 in so ft parts of the buller ciJm, Saxidol1l11s gigan /ells (Deshayes), 12 in soft parts of the common mussel, MYlilus edlllis Linnaeus, and < I in muscle and I in a whole gobiid fish. the longjaw mudsucker, C illiclilhys l1Iimbilis Coope1}2. Max imum stable go ld concentrati ons recorded in soft ti ssues of marine molluscs and

crustaceans ranged from 0.3 to 38.0 J1. g Au/kg OW ; for fish musc le, the mean concentrations were O. I J1. g/k g OW and 2.6 J1.g/kg ash weight35 . In studi es with the eastern oyster, Crassos/rea virgill ica (Gmelin ), the blue crab, Callinectes sapidus Rathbun , and the mummichog Fundulus heleroclitus (Linnaeus), an estuarine cyprinodontiform fi sh, all species were exposed in cages under field conditi ons to sediment­sorbed, carrier-free, 198Au+ :14. The max imum level of rad iogold in the caged organi sms was detected in oysters 17 hr after contact with 198Au-spiked sed iments. Indigenous orga ni sms collected 4 1 hr after contact with the 198 Au-labeled sed iments contai ned no detectable rad ioactivit/ 4

. In a 25-day study with blue crab, northern quahog clam Mercenaria mercenaria (Linnaeus), and the sheepshead minnow Cyprinodon variegatus Lacepede, all species were maintained in a 1000-1 aquarium containing bentonite clay and seawater spiked with carrier-free 199 Au (physical half-life of 3.2 days), as AuCi); crabs accumulated the most radioacti vi­ty, followed by clams, clay, and fi sh, in that order:l~ .

Bioconcentration factors (BCF's) for metal s and aquatic organi sms derived from carrier-free radio­tracers in the medium are probably artificiall y hi gh, and should be interpreted with cauti on35.36. For metals it is a general observati on that high BCF's are associated with low concentrations in the medium. and th at BCF's are especiall y hi gh when they are derived from carrier-free radioi sotopes. Typica ll y, BCF's for metals and other chemicals studi ed reach a plateau before declining with increasing concent ra­ti ons in solution35

.36

. The maximum concentration of stable gold measured in ti ssues of living marine organi sms was 38.0 J1.g/k g FW35.

Conclusion Gold recovery by selected species of bacteria, algae,

fungi, yeasts, and hi gher plants from dilute solutions under contro lled physicochemical conditi ons seems economica lly viable, with concentrat ion: up to 100 g Au/kg OW documented . Dried crab exos keletons also show promise for commercial gold recovery from solut ion (4.9 g Au/kg OW); however. gold uptake by li ving species of aquatic macrofauna seems negligible. The mechani sms of accumulat ion wh ich include ox idation, reduction, dissolution, leaching and sorption are not knovin with certainty and merit additiona l resea rch effort .

Acknowledgement The author thanks Drs. Peter H. Albers and Barnett

A. Rattner for their constructi ve co mments on an earl y draft.

EISLER: BIORECOVER Y OF GOLD 97 1

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