Biorecovery of gold

\

~

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

Biorecovery of gold

Ronald

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 kerati- nOlls 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 expensi ve

Chemical methods for the recovery of gold from ores include cyanidation and thiourea leaching, which present environmental and health ri sks3.4. Biorecovery of dissolved gold from solution presents fewer environmental risks than chemical methods, and is documented for

. . 25

20 I 1421

22

mIcroorganisms

,a gae

, water lerns , peat-

alfalfa4, seaweeds2. 12. 1 8.24.25, fungi

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 1

. 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

(Beijerinck),

(Beijerinck), and

(Beijerinck). These bacterial species obtain their energy through the oxidation of ferrous to ferric iron

or through the reduction of inorganic sulfur compounds to sulfate

spp.). Monetary costs of biooxidation are reported to be about 50% lower than roasting or pressure oxidation 13. Adding

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

described by Kai

as

Au·

h

Au[CS(NH2 hf+

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

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

(Beijerinck), a common species of bacterium encountered in biooxidation facilities

Optimum microbiological leaching by

spp

spp. of refractory sulfide ores for

968 ^{INDIAN J} EXP BIOL, SEPTEMBER 2003

recovery of gold in tanks is possible under controlled conditions of pH, dissolved oxygen, carbon dioxide, sulfur balance, redox potential, toxic metal concentrations, and rate of leaching7.

Several species of Fe³⁺ -reducing bacteria (Bacteria spp., Archaea spp.) can precipitate gold by reducing Au'+ to Au' with hydrogen as the electron donor"o.

Rate of bacterial oxidation by ThiobacilLus ferrooxidans and Leptospirillium ferrooxidans of three South African refractory gold ores of varying gold-arsenopyrite composition was dependent mainly on crystal structure^R• These gold ores were c1assi fied as refractory due to the presence of gold inclusions in arsenopyrite and pyrite, and submicroscopic gold mainly in arsenopyrite. Refractory gold occurs at sites which are preferentially leached by the bacteria. The rate of gold liberation from sulfides is enhanced during the early stages of bacterial oxidation. Defects in crystal structure influence the rate of biooxidation and is directly related to the crystal structure of the sulfide mineral, the crystallograpHic orientation of the exposed surfaces, and differences In chemical composItion and mechanical deviations in the crystals^x. Pretreatment of refractory gold concentrates with the bacterium Thiobacilllls ferrooxidans ultimately results in sulpHur and sulpHide oxidation by ferric ions from bacterial oxidation of ferrous ions.

The maximum concentration of attached Thiobacillus increases with increasing concentration of Fe²+ and decreases with increasing size of the refractory 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 main metal-bearing minerals. Initial gold recovery by conventional cyanidation on a crushed ore sample was 54%; concentration by flotation improved recovery to 56%. Concentrated samples (17.0 g Au/ton) were leached in reactors at pH 1.8. In the presence of bacteria, all dissolved iron was present as ferric ion;

gold recovery by cyanidation increased from 13% for the initial concentrate to 97% after 10 days of bacterial leaching. To further increase gold recovery, flotation tailings were submitted to cyanidation⁹.

Some microorganisms isolated from gold-bearing deposits are capabl,e of dissolving gold; dissolution was aided by the presence of aspartic acid, histidine, serine, alanine, glycine, and metal oxidants⁵. Baclerif- ann gold is well known, with uptake of Au³+ from

chloride solutions documented for at least seven genera of freshwater cyanobacteria³¹. Some bacteriform gold is biogenic - the result of precIpitation by bacteria - and may be useful indicators of gold deposits and of processes of gold accumulation. PLectonema terebrans Bornet, a species of filamentous marine cyanobacteria, accumulates gold in its sheath from an aqueous solution of AuCl₃.

Sheaths are among the few structures likely to be preserved in some form in microfossils of ancient bacteria. In marine media, it is expected that AuCI₃ (2.0 g Au/l) will form AuCI ~, Au';, and AuCi

;31 .

Biosorption of Au³+, as AuCi ~ , by dried Pseudomonas strains of bacteria was inhibited by palladium, as Pd²+, and possibly other metal ions 10.

Gold adsorption from cyanide solutions by dead biomass of bacteria (Bacillus subtilis), fungus (Penicillium chrysogenum Thom), or seaweed (Sargassum fluitans Linnaeus) at pH 2.0 were 1.8 g Au/kg OW for bacteria, 1.4 g/kg OW for fungus, and 0.6 g Au/kg OW for seaweed. Anionic AuC ; adsorption was the major mechanism in gold biosor- ption from cyanide solutions, being most efficient at lower pH values^2. L-cysteine increased gold-cyanide biosorption of Bacillus, PenicilliulIl, and Sargassum^lR• At pH 2, the maximum gold uptakes were 4.0 g Au/kg OW for bacteria, 2.8 g/kg for fungus, and 0.9 g/kg for seaweed, or 150-250% greater than in the absence of cysteine. The anionic gold cyanide species were adsorbed by ionizable functional groups on cysteine- loaded biomass; deposited gold could be eluted from gold-loaded biomass at pH 5.0¹⁸•

Gold-resistant strains of bacteria that also accumulate gold are documented, although the fundamental mechanism of resistance to gold in microorganisms is not known or understood 15. One strain of Burkholderia (Pseudomonas) cepacia Burkholder contained millimolar concentrations of Au+ thiolates. Burkholderia cells were large, accumulated polyhyroxybutyrate and gold, and excreted thiorin, a low molecular weight protein into the culture medium. This effect was not observed with the Au³+ complexes tested, which were reduced to metallic gold in the medium. Gold-resistant strains of fungi and heterotrophic bacteria are also known^'5.

Rapid recovery of gold from gold-thiourea solutions was documented for waste biomass of yeasts (Saccharomyces cerevisiae), cyanobacteria (SpiruLina platensis [Nordst]), and bacteria (Streptomyces erythralus [Waksmanl)'4 The process is pH-

EISLER: BIORECOVERY OF GOLD 969

dependent for yeast and bacteria, and pH-independent for Spirulina. Of all strains of microorganisms examined, Spirulina platensis has the highest affinity and capacity for gold, even at low pH values. Gold uptake by Spirulina was 7.0 g Au/kg biomass OW in 1-2 hr at pH 2.0, and about 3.0 g Au/~g OW in 15 min atpH 2 through 714.

Metabolically active fungal cells of Aspergillus fumigatus Fresen and A. niger Tiegh removed gold from cyanide leach liquor of a Brazilian gold extraction plant more efficiently than did dried fungal biomass or other species of Aspergillus tested. These two species of fungi removed 35 to 37% of gold from solutions containing 2.8 mg Au/I in 84 hr26. Gold removal from cyanide-containing solutions is documented for a strain of Aspergillus niger, a fungus isolated from the gold extraction plant at Nova Linda, BraziI²⁶-28

. The leach liquor contained, in mg/I, 181.0 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% of the gold, I 00% of the silver, 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 valuable metals shows promjse26-28. Uptake patterns of gold from Au³+ solutions by dead fungal biomass followed mathematical uptake models of Langmuir and Freu- ndlich; biomass was prepared from the fruiting body of a mushroom collected from the forests of Kerala, India^29. Dried fungus, Cladosporium cladosporoides Fresen, mixed with keratinous material of natural origin to form a bead, proved effective in absorbing gold from solution^l2. The biosorpent beads adsorbed 100.0 g Au/kg beads from a solution containing 100.0 mg Au/I. Maximum biosorption of 80% occurred at acid pH (1-5) in less than 20 min. The biosorpent beads degraded in soil in about 140 days. The beads also removed 55% of the gold from electroplating solutions containing 46.0 mg Au/I, with observed gold loading capacity of 36.0 g/kg beads 12. Dried biosorpents encapsulated in polysulfone were prepared from microorganisms isolated from pristine or acid mine drainage environments". Biosorpent material rich in exopolysaccharides from the acid mine drainage site bound Au³+ three times more effectively than did other materials, and removed 100% of the Au³+ from solutions containing 1.0 mg Au/I within 16 hr at 23°C andpH 3.0.

Algal cells, alive or dead, rapidly accumulate Au³+ and begin to reduce it to Au· and Au+ within 2 days^33.

Uptake of Au³+ by Chlorella vulgaris Beijerinck, a unicellular green alga, from solutions containing 10.0 or 20.0 mg Au³+/l is documented21. Chlorella accumulated up to 16.5 g Au/kg OW. Inactivating the algal cells by various treatments resulted in some enhancement in uptake capacity over the pristine cells. Inactivation by heat treatment yielded up to 18.8 g/kg DW; for alkali treatment, this was 20.2 g/kg OW; for formaldehyde treatment, 25.5 g/kg OW; and for acid treatment, 25.4 g/kg OW. Elemental gold (Au") was measured by X-ray photoelectron spectroscopy on the cell surface, indicating that a reduction had occurred21. Studies with living Chlor- ella vulgaris suggest that accumulated Au³+ is rapidly reduced to Au+, followed by a slow reduction to AU·15

. With dead algae, Au· initiates a seeding process which results in the formation of elemental gold.

Sequestering metal ions using living or dead plants is a proposed economical means of removing gold and other metals via intracellular accumulation or surface adsorption. However, in the case of live plants, this is frequently a relatively slow and time- consuming process. Nonliving plant material for surface adsorption offers several advantages over live plants, including reduced cost, greater availability, easier regeneration, and higher metal specificity4. In South African mining effluents, gold usually ranges between I and 10 mg/I. In studies of 180-min duration, dried red water ferns, Azalla .filiculoides Lamarck, removed 86 to 100% of Au³+ from initial solution of 2 to 10 mg Au³+/l, increasing with increased initial concentration of Au³+ (ref. 22). The biomass gave >95% removal efficiency at all biomass concentrations measured. Optimum (99.9%) removal of gold occUlTed within 20 min at pH 2, 42% removal at pH 3 and 4, 63% at pH 5, and 73% removal at pH 6; removal efficiency seemed independent of temperature22. Similar results were observed with four species of ground dried seaweeds (Sargassum sp., Gracilaria sp., Eisenia sp., and Ulva Sp.)25 Treated seawecds removed 75-90% of the gold within 60 min at pH 2 from solutions containing 5.0 mg Au³+/l. Gold (Au³+) can be sequestered from acid solutions by dead biomass of a brown alga, Sargassum na(ans (Linnaeus), and deposited in its elemental form, AU·²⁴.

The cell wall of Sargassum was the major locale for gold deposition, with carbonyl groups (C = 0) playing a major role in binding, and N-containing groups a lesser role. Like activated carbon, the biomass of Sargassum natans is extremely porous, reportedly more than most biomaterials, and accounts, in part,

970 ^{INDIAN J} EXP BIOL, SEPTEMBER 2003

for its ability to accumulate gold²⁴. Dried ground shoots of alfalfa, Medicago sa/iva Linnaeus, were effective in removing gold from solution⁴.The accumulation process involved the reduction of Au³+ to colloidal Au', and was most efficient at elevated temperatures and acid pH. In solutions containing 60 mg Au)'/I, about 90% of the Au³+ was bound to dried alfalfa shoots in about 2 hI' at pH 2 and 55°C.

The mechanism to account for this phenomenon is unknown, but may involve reduction of Au³+ to Au+, the latter being unstable in water to form Au' and Au³+ (ref. 4). Dried peat from a Brazilian bog accumulated up to 84.0 g Au/kg OW within 60 min from solutions containing 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 living molluscs, crustaceans, and fishes is negligible.

Certain chitinous materials, such as exoskeletons of the swamp ghost crab, Ucides corda/us (Linnaeus), can remove and concentrate gold from anionic gold cyanide solutions over a wide range of pH values:10 The maximum AuCN; uptake occurred at pH 3.7, corresponding to a final value of 4.9 g Au/kg OW; exoskeletons burnt in a non-oxidizing atmosphere removed 90% of the gold at pH 10. phenolic groups created during the heat treatment seemed to be the main functional group responsible for AueN; binding by burnt acid-washed crab shells^3o.

Bioconcentration factors (BCFs) were recorded for carrier-free 19XAu+ (physical half-life of 2.7 days) in freshwater organisms after immersion for 21 days in a medium containing 25,000 pCiIl =675,700 Bq1l32. In goldfish, Carassills allra/us (Linnaeus), the highest BCFs measured were <I in mu cle (i.e., less than 675.700 Bq/kg FW muscle), 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 (Aslacus sp.), BCFs were < I ill muscle and 14 in viscera. For marine organisms immersed for 26 days in synthetic seawater containing 33.000 pCi/1 = 891 ,900 Bqil. maximum BCFs measured were tl in muscle and 16 in viscera of the red crab. Callcer produc/lls Randall, 11 in soft 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, Cilliclilhys l1Iimbilis Coope1}2. Maximum stable gold concentrations recorded in soft tissues of marine molluscs and

crustaceans ranged from 0.3 to 38.0 J1.g Au/kg OW; for fish muscle, the mean concentrations were O. I J1.g/kg OW and 2.6 J1.g/kg ash weight35

. In studies with the eastern oyster, Crassos/rea virgillica (Gmelin), the blue crab, Callinectes sapidus Rathbun, and the mummichog Fundulus heleroclitus (Linnaeus), an estuarine cyprinodontiform fish, all species were exposed in cages under field conditions to sediment- sorbed, carrier-free, 198Au+ :14. The maximum level of radiogold in the caged organisms was detected in oysters 17 hr after contact with 198Au-spiked sediments.

Indigenous organisms collected 41 hr after contact with the 198 Au-labeled sediments contained no detectable radioactivit/ ^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 radioactivi- ty, followed by clams, clay, and fish, in that order:l~.

Bioconcentration factors (BCF's) for metals and aquatic organisms derived from carrier-free radio- tracers in the medium are probably artificially high, and should be interpreted with caution35.36. For metals it is a general observation that high BCF's are associated with low concentrations in the medium.

and that BCF's are especially high when they are derived from carrier-free radioisotopes. Typically, BCF's for metals and other chemicals studied reach a plateau before declining with increasing concentra- tions in solution^35.36. The maximum concentration of stable gold measured in tissues of living marine organisms was 38.0 J1.g/kg FW35.

Conclusion

Gold recovery by selected species of bacteria, algae, fungi, yeasts, and higher plants from dilute solutions under controlled physicochemical conditions seems economically viable, with concentration: up to 100 g Au/kg OW documented. Dried crab exoskeletons also show promise for commercial gold recovery from solution (4.9 g Au/kg OW); however. gold uptake by living species of aquatic macrofauna seems negligible.

The mechanisms of accumulation which include oxidation, reduction, dissolution, leaching and sorption are not knovin with certainty and merit additional research effort.

Acknowledgement

The author thanks Drs. Peter H. Albers and Barnett A. Rattner for their constructive comments on an early draft.

EISLER: BIORECOVERY OF GOLD 971

References

Suyama K, Fukazawa Y & Suzumura H, Biosorption of precious metal ions by chicken feather, Appl Biochem Biotechnol, 57/58 (1996) 67.

2 Niu H & Volesky B. Characteristics of gold biosorption from cyanide solution, J Chem Technol Biotechnol, 74. (1999) 778.

3 Eisler R, Clark D R Jr, Wiemeyer S N & Henny C J, Sodium cyanide hazards to fish and other wildlife from gold mining operations, in Ellvirollmenlal impacts of mining activities:

Emphasis on mitigation and remedial measures, edited by J M Azcue, (Springer-Verlag, Berlin) 1999,55.

4 Gardea-Ton'esdey J L, Tiemann K J, Gamez G, Dokken K, Cano-Aguilera I, Furenlid L R & Renner M W, Reduction and accumulation of gold(lII) by Medicago sativa alfalfa biomass:

X-ray absorption, spectroscopy, pH, and temperature dependence, Environ Sci Tecl1l1ol, 34 (2000) 4392.

5 Puddephatt R J, The chemistry of gold (Elsevier, Amsterdam) 1978, I.

6 Kai T, Yamasaki K & Takahashi T, Application of iron oxidizing bacteria in thiourea leaching of gold bearing ores.

Biorecovery,2 (1992) 83.

7 Lindstrom E B, Gunneriusson E & Tuovinen 0 H, Bacterial oxidation of refractory sulfide ores for gold recovery, Br Rev Biotechnol, 12 (1992) 133.

8 Claassen R, Mineralogical controls on the bacterial oxidation of refractory Barberton gold ores. Feder Eur Microbiol Soc Microbiol Rev, II (1993) 197.

9 Maturana H, Lagos U, Flores V, Gaete M. Cornejo L &

Wiertz J V, Integrated biological process for the treatment of a Chilean complex gold ore, Fed Eur Microbiol Soc Microbiol Rev, II (1993) 2l5.

10 Tsezos M, Remoudaki E & Angelatou V, A study of the effects of competing ions on the biosorption of metals, Inter Bioderer Biodegrad, 38 (1996) 19.

Il Xie J Z, Chang H L & Kilbane J J II, Removal and recovery of metal ions from wastewater using biosorbents and chemically modified biosorbents. Bioresour Teclmol, 57 (1996) 127.

12 Pethkar A V & Paknikar K M, Recovery of gold from solutions using Cladosporiul1l c/adosporoides biomass beads, J Biotechllol. 63 (1998) 121.

13 Rawlings D E, Industrial practice and the biology of leaching of metals from ores, J Indus Microbiol Biotechnol, 20 (1998) 268.

14 Savvaidis I, Recovery of gold from thiourea solutions using microorganisms, Biol1'letals, II (1998) 145.

15 Savvaidis Y, Karamushka V I, Lee H & Trevors J T, Microorganism-gold interactions, Biometals, II (1998) 69.

16 Gonzalez R, Gentina J C & Acevedo F, Attachment behaviour of Thiobacillus !errooxidans cells to refractory gold conccntrate particles, Biotechllol Lell; 21 (1999) 715.

17 Karamushka V I & Gadd G M, Interaction of Saccharomyces caevisiae with gold: Toxicity and accumulation, Biometals,

12 (1999) 289.

18 Niu H & Volesky B, Gold-cyanide biosorption with L·

cysteine, J Chem Technol Biotechnol, 75 (2000) 436.

19 Gardner M N & Rawlings D E, Production of rhodanese by bacteria present in bio-oxidation plants used to recover gold from arsenopyrite concentrates, J Appl Microbiol, 89 (2000) 185.

20 Kashefi K, Tor J M, Nevin K P & Lovley D R, Reductive precipitation of gold by dissimilatory Fe (ILI)-reducing Bacteria and Archaea, Appl Enviroll Microbiol, 67 (2001) 3275.

21 Ting Y P, Teo W K & Soh C Y, Gold uptake by Chlorella vulgaris, J Appl Phycol, 7 (1995) 97.

22 Antunes A P M, Watkins G M & Duncan J R. Batch studies on the removal of gold (Ill) from aqueous solution by Azolla j1liculoides, Biotechnol Lell, 23 (200 I) 249.

23 Wagener A D L R & Andrade W D 0, On the binding capacity of peat to several metal ions of environmental and economic concern, Cien Cult J Brazil Assoc Adv Sci. 49 (1997) 48.

24 Kuyucak N & Volesky B, The mechanism of gold biosorption, Biorecovery, I (1989) 219.

25 Zhao Y, Hao Y & Ramelow G J, Evaluation of treatment techniques for increasing the uptake of metal ions from solution by nonliving seaweed algal biomass, Ellviron Monitor Assess, 33 (1994) 61.

26 Gomes N C M & Linardi V R, Removal of gold, silver and copper by living and nonliving fungi from leach liquor obtained from the gold mining industry, Revista Microbiol.

27 (1996) 218.

27 Gomes N C M, Camargos E R S, Dias J C T & Linardi V R, Gold and silver accumulation by Aspergillus niger from cyanide-containing solution obtained from the gold mining industry. World J Microbiol Biotechllol, 14 (1998) 149.

28 Gomes N C M, Figueira M M, Camargos E R S, Mendonca- Hagler L C S, Dias J C T & Linardi V R, Cyano-metal complexes uptake by Aspergillus niger, Biotechnol Lell, 21 (1999) 487.

29 Ting Y P & Mitlal A K, An evaluation of equilibrium and kinetic models for gold biosorption, Resour Ellviroll Biotechnol, 2 (1999) 311.

30 Niu H & Volesky B, Gold adsorption from cyanide solution

by chitinous materials, J Chem Technol Biotechnol, 76 (2001) 291.

31 Dyer B D. Krumbein W E & Mossman D J, Accumulation of gold in the sheath of Plectonema terebrans (filamentous marine cyanobacteria), Geomicrobiol J, 12 (1994) 91.

32 Harrison F L, Availability to aquatic animals of short-lived radionuclides from a Plowshare cratering event, Health physics, 24 (1973) 331.

33 Robinson M G, Brown L N & Hall B D, Effect of gold (Ill) on the fouling diatom Amphora coffeaefonnis: Uptake, toxicity and interactions with copper, Biofouling, II (1997) 59.

34 Duke T W, Baptist J P & Hoss 0 E, BioacculTIulation of radioactive gold used as a sediment tracer, US Fish Bul!. 65 (1966) 427.

35 Eisler R, Trace metal concentrations ill marine organisms (Pergamon, Elmsford, New York) 1981, I .

36 Eisler R, Handbook of chemical risk assessment: health hazards to humans. plal1ls. and animals. Vol. 1-3 (Lewis Publishers, Boca Raton, Florida) 2000, I.