Mechanisms for solubilization of cobalt, copper and nickel from Indian Ocean nodules at near neutral pH by a marine isolate.

Abstract

Polymetallic ocean nodules offer an alternative source for extracting valuable strategic metals like Cu, Co and Ni. A novel biodissolution process was carried out, employing the cell-free spent growth medium from a marine organism ( Bacillus M1) isolated from nodules; and Cu, Co and Ni solubilization from the nodules was observed to be beyond the theoretical solubility limits at near neutral pH. Different characterization techniques revealed the presence of phenolic substances in the spent growth medium, which might have formed soluble complexes with the transition metals. The low prevailing E(h) redox value in the medium suggested a strong reducing environment, favoring the reductive dissolution of the oxides. A correlation study of dissolution of Cu, Co and Ni with that of Mn and Fe in the nodules was made to investigate the mechanisms of metal solubilization by the marine isolate. Under the influence of a strong reducing environment coupled with complexation by a phenolic substance present in the spent growth medium, Mn and Fe oxides were solubilized from the nodules, resulting in concomitant dissolution of Cu, Co and Ni associated with them in the nodules.

Full text

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ORIGINAL PAPER
Amitava Mukherjee Æ Ashok M. Raichur
Jayant M. Modak Æ K. A. Natarajan
Mechanisms for solubilization of cobalt, copper
and nickel from Indian Ocean nodules at near neutral pH
by a marine isolate
Received: 30 June 2004/ Accepted: 3 September 2004/Published online: 22 September 2004
? Society for Industrial Microbiology 2004
Abstract Polymetallic ocean nodules offer an alternative
source for extracting valuable strategic metals like Cu,
Co and Ni. A novel biodissolution process was carried
out, employing the cell-free spent growth medium from
a marine organism (Bacillus M1) isolated from nodules;
and Cu, Co and Ni solubilization from the nodules was
observed to be beyond the theoretical solubility limits at
near neutral pH. Different characterization techniques
revealed the presence of phenolic substances in the spent
growth medium, which might have formed soluble
complexes with the transition metals. The low prevailing
Eh redox value in the medium suggested a strong
reducing environment, favoring the reductive dissolution
of the oxides. A correlation study of dissolution of Cu,
Co and Ni with that of Mn and Fe in the nodules was
made to investigate the mechanisms of metal solubili-
zation by the marine isolate. Under the influence of a
strong reducing environment coupled with complexation
by a phenolic substance present in the spent growth
medium, Mn and Fe oxides were solubilized from the
nodules, resulting in concomitant dissolution of Cu, Co
and Ni associated with them in the nodules.
Introduction
Bioleaching of ocean nodules using a marine isolate, to
recover strategic metals like Cu, Ni and Co, is relevant
with respect to ongoing research for different environ-
ment friendly methods to exploit unconventional min-
eral resources.
Manganese and ferromanganese deposits are found
extensively as nodules, crusts and pavements in present-
day oceans. These ocean nodules consist of oxides of
manganese and iron, together with various valuable
metals such as copper, nickel and cobalt. One of the
important prerequisites for extracting metals from these
nodules is the solubilization of the desired metals. Since
the valuable transition metals are often mineralogically
associated with insoluble manganese and/or ferric oxide
phases, prior dissolution of oxides is necessary to solu-
bilize them.
The biological reduction of Mn (IV) oxide by marine
isolates is a well studied area. All the Mn (IV)-reducing
bacteria isolated so far from seawater, marine sediments
and ferromanganese concretions have proved to be
heterotrophs. Most of the isolates are aerobes which can
reduce Mn (IV) aerobically or anaerobically. Ehrlich [4]
successfully isolated and characterized a Mn (IV)-
reducing organism (Bacillus 29) from Atlantic Ocean
nodules. Growing cultures of Bacillus 29 were able to
reduce MnO2both aerobically and anaerobically, using
glucose as electron donor. However, a component of the
electron transport system involved in MnO2reduction in
this culture was only inducible under aerobic conditions.
Both Mn2+and MnO2can serve as inducers [5, 16, 17].
MnO2reducibility has been found to be inducible in all
marine cultures tested so far by Ehrlich [6]. However, the
microbial flora associated with Indian Ocean nodules
has not been studied extensively, nor has the ability of
members of this flora to extract metals from nodules
been tested until now.
Recently, Mukherjee et al. [9, 10] studied biologically
mediated metal extraction from Indian Ocean nodules
A. Mukherjee Æ A. M. Raichur (&) Æ K. A. Natarajan
Department of Metallurgy, Indian Institute of Science,
Bangalore, 560012, India
J. M. Modak
Department of Chemical Engineering,
Indian Institute of Science,
Bangalore, 560012, India
Present address: A. M. Raichur
Max Planck Institute for Colloids and Interfaces,
Golm, Germany
E-mail: amr@met.iisc.ernet.in
Tel.: +91-80-22933238
Fax: +91-80-23600472
J Ind Microbiol Biotechnol (2004) 31: 462–468
DOI 10.1007/s10295-004-0170-5

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using a marine isolate, Bacillus M1. The spent growth
medium from the cultures of the organism solubilized
metals from the nodules at near neutral pH and room
temperature. Almost 45% Co and 25% Cu and Ni were
obtained in about 4 h at an operating pulp density of
10 g/l [9]. The major objective of our present work was
to study the mechanism of Cu, Co and Ni solubilization
from the nodules by spent growth medium of cultures of
the marine isolate.
Materials and methods
Ocean nodule
Ocean nodule samples were collected from the bed of the
Indian Ocean by the National Institute of Oceanogra-
phy, Goa, India. These were ground in a pre-sterilized
mortar and pestle and sieved to obtain appropriate size
fractions. All the leaching experiments were performed
with the 50–75 lm size fraction of nodules, containing
around (wt%): 21.83% Mn, 6.4% Fe, 1.14% Cu, 1.15%
Ni and 0.17% Co. One gram of nodules per 100 ml of
leach pulp (1% pulp density) contained 44.64 mM Mn,
11.42 mM Fe, 1.79 mM Cu, 1.96 mM Ni and 0.3 mM
Co.
Microorganism
Bacillus M1, isolated from Indian Ocean nodules [9] was
grown in 250-ml baffled Erlenmeyer flasks at 30?C on a
rotary shaker (200 rpm), using sterilized artificial sea-
water nutrient broth (ASWNB) which had the following
components (per liter of distilled water): 28.13 g NaCl,
0.77 g KCl, 1.60 g CaCl2Æ6H2O, 4.8 g MgCl2Æ6H2O,
0.11 g NaHCO3, 3.5 g MgCl2Æ7H2O, 5 g peptone and
3 g beef extract.
Bioleaching studies
Ten percent of an active inoculum (from the late expo-
nential phase) containing at least 109cells/ml was added
to the sterilized ASWNB medium. To obtain the cell-
free spent growth medium, a fully grown batch culture
(after 10 h of growth) of Bacillus M1was centrifuged at
17,700 g for 10 min at 5?C followed by pressure filtra-
tion through a sterile 0.2-lm Millipore filter. The ab-
sence of any cells in the resultant spent growth medium
was confirmed by examination under a phase contrast
microscope.
After completion of the leaching treatment, the pulp
residue was separated from the spent medium by filtra-
tion through Whatman 42 paper and digested in 1:1 HCl
at 60–70?C. The resultant filtrate was analyzed for Cu,
Co, Ni, Mn and Fe by inductively coupled plasma
spectrometry.
UV spectroscopic studies
Cell-free spent medium was prepared from growing
cultures of the marine isolate. UV difference spectra
were recorded for the spent growth medium at pH 10, 11
and 12 (obtained by addition of 0.01 N KOH solution),
using a Shimadzu model UV-260 UV-visible spectro-
photometer. Each spectrum was run with sterilized
ASWNB at the respective pH value as the reference.
A total of 20 ppm of Cu, Ni and Co and 400 ppm of
Mn and Fe were added to the spent growth medium
using a standard AAS solution of the metal ions (sup-
plied by Thomas Baker, Bombay). After 4 h of incuba-
tion at 27?C, spent medium with the added metal ions
was centrifuged at 17,700 g and UV spectra were run
with spent medium at pH 8.2 as a reference.
The leached solutions collected after specified time
intervals during a bioleaching experiment with ocean
nodules were centrifuged at 17,700 g. UV spectra were
recorded for the supernatant solutions using spent
medium at pH 8.2 as a reference.
Mass spectroscopy
A total of 10 ml of spent medium was evaporated to
dryness by heating. A volume of 2–3 ml of chloroform
was mixed with the residue. The portion of the residue
not dissolved by the chloroform was dried and rinsed
several times with 2–3 ml of methanol. It formed a yel-
low-orange colored solution. This solution was evapo-
rated to dryness and the dry sample was used for mass
spectral analysis by matrix-assisted laser desorption/
ionization.
Redox potential measurements
Redox potentials of liquid samples were measured using
a saturated calomel electrode as reference.
Results
Cu, Co, Ni, Fe and Mn solubilization from the nodules
in spent growth medium of Bacillus M1
Table 1 shows the extent of dissolution of Cu, Co, Ni,
Mn and Fe in the spent growth medium at pH 8.2 for
different pulp densities after 4 h. Maximum dissolution
for all the metals occurred at 10% pulp density. The
extent of dissolved Fe (49.0 mM) was greater than
that of Mn (44.6 mM) at 10% pulp density. The ex-
tent of Cu, Ni and Co dissolution at 10% pulp density
was 3.5, 4.5 and 1.2 mM, respectively. Taking into
account the low concentration of Co (3.9 mM) com-
pared with Cu (18.0 mM) and Ni (19.6 mM) in the
nodules, the fraction of Co that was dissolved was
greater.
463

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Following the basic thermodynamic principles of
solubility, equilibrium solubility diagrams for Cu, Co,
Ni, Fe and Mn were prepared with the help of Envi-
ronmental Research MINEQL+ver. 2.22 software. The
initial concentration of metal ions was chosen to corre-
spond to the typical chemical composition of nodules at
1% pulp density. Figure 1 sums up the theoretical
aqueous solubility of the five metals. At the operating
pH range of nodule leaching, the theoretical solubility of
metalionstendsto
Mn2+>Co2+>Fe2+>Ni2+>Cu2+. Next, we super-
imposed the metal dissolution values obtained in a
typical bioleaching experiment of ocean nodules with the
spent growth medium of the isolate (1% pulp density,
50–75 lm size fraction) in the same figure. The open
symbols signify the dissolved metal concentrations in the
spent growth medium. In the case of Cu, Ni, Co and Fe,
followasequence:
the dissolved metal concentration was observed to be
above the respective metal solubility line. The dissolved
Mn concentration was within the thermodynamic solu-
bility limit.
Spectral study of the spent growth medium
A kinetic study with the spent liquor to observe changes
in the UV spectra during the growth of the marine isolate
was reported [9]. The spent growth medium of cultures of
the isolate had a p Kavalue in the range 11.5–12.5, sug-
gesting a change in the structure of metabolite(s) present
therein. To confirm the structural changes, UV-visible
spectra of the spent growth medium were recorded at pH
values above 10.0. The results are summarized in Ta-
ble 2, which clearly shows the emergence of a peak near
298 nm. The spent growth medium at pH 8.0 had two
peaks, one near 230 nm and another near 310 nm. These
spectral results indicate a structural change in the
metabolites with increasing pH in the spent growth
medium. The UV band near 298 nm in the highly alka-
line pH range suggests the presence of phenolic sub-
stances having attached carboxylic acid groups, like
catechol and di-hydroxybenzoic acid (DHBA). Standard
phenolic compounds, catechols and para-hydroxyben-
zoic acid in aqueous solution and spent growth medium
reveal a strong band at 280–300 nm at the same pH.
The mass spectra of partially purified spent growth
medium gave a distinct peak at 377.8 nm.
Spectral analysis during leaching
The UV-visible spectra of spent growth medium from
cultures of marine isolate collected after 10 h of growth
revealed two bands, one near 225 nm and another near
310 nm. To investigate the structural changes taking
place during the leaching experiment using crushed
nodules in the spent medium, leaching flasks were
examined under UV after specified time intervals. The
results are summarized in Table 3. It can be seen that the
band near 310 nm showed an up-shift in peak position
with an increase in absorbance for 3 h, followed by a
further slight up-shift in peak position, but a small drop
in absorbance over the next hour. A band appeared near
Table 1 Metal solubility in the spent growth medium at different
pulp densities
MetalPulp
density (%)
Starting
concentration
(mM)
Dissolution
(%)
Soluble
concentration
(mM)
Mn 1
5
10
1
5
10
1
5
10
1
5
10
1
5
10
44.64
223.3
446.4
11.42
57.14
114.2
0.3
1.5
3
1.79
8.97
17.9
1.96
9.81
19.6
25
24
10
45
43
43
45
40
40
22
23
20
25
24
23
11.15
53.60
44.64
5.13
24.60
49.10
0.14
0.60
1.20
0.40
2.06
3.58
0.49
2.35
4.50
Fe
Co
Cu
Ni
Table 2 Changes in UV spectra of spent growth medium with
change in pH. Absorbance values are given in parentheses. ND No
peaks detected
pH of spent
growth medium
Peaks recorded (nm)
220–230250–290 290–300
8.0
10.5
11
11.5
12
233.8
ND
ND
ND
ND
ND
ND
ND
ND
ND
308.2
298.8 (0.551)
297.4 (0.449)
298.7 (0.642)
299.0 (0.849)
46810 12
0
20
40
60
80
100
% total concentration
pH
Initial conc. (mM)
Cu 1.79
Ni1.96
Co 0.30
Mn44.64
Fe 11.42
Fig. 1 Solubility of Cu, Co, Ni, Mn and Fe as a function of pH.
Open symbols Typical metal dissolution in spent growth medium at
1% pulp density.
464

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425 nm in the second hour of the experiment, followed
by a down-shift in position during the third hour and
disappearance in the fourth hour. The absorbance of
this band increased from the second to the third hour.
The band near 425 nm signifies a Fe (III) siderophore
complex [11]. Depending on the ligand–metal ratio, the
wavelength value may vary within a range.
Standard solutions of Cu, Co, Ni, Mn and Fe were
added individually and together to the spent growth
medium, to observe changes in the structure of metab-
olites therein; and the results are summarized in Table 4.
From Table 1, we can observe that the added metal ion
concentration in the spent growth medium approxi-
mated that of the dissolved metal concentration after 4 h
at a 1% pulp density of nodule substance. The added
Cu, Ni, and Co each produced two peaks, one near
295 nm with strong absorbance and one in the range
245–260 nm with weak absorbance.
The absorbance increased in the order Niabs<-
Cuabs<Coabs. Dissolved Mn in spent growth medium
gave a very weak band near 260 nm and a stronger peak
at 312.2 nm. Two bands, one at 248.6 nm and another
at 342.2 nm, were resolved in the case of Fe in the spent
growth medium. The absorbance of the Fe peak at
342.0 nm was almost double that of the Mn peak at
312.2 nm. Strong Fe complexation in the spent growth
medium may have caused this difference.
Changes in redox value during leaching
The changes in Ehredox values during leaching by spent
growth medium are shown in Fig. 2. In the first hour,
the Ehof the leach pulp increased from 37 mV to 46 mV.
Around 25% of the Mn from nodules was dissolved in
that time period. After 1 h, the Ehin the system again
decreased, before stabilizing at 32 mV.
Discussion
From Table 1 and Fig. 1, it can be concluded that the
metal dissolution in spent growth medium at pH values
near 8 is enhanced in the presence of certain complexing
agents in the spent medium of Bacillus M1. To look into
this phenomenon, characterization studies were under-
taken using the spent growth medium of Bacillus M1
and the leached liquor collected after leaching.
Two absorption peaks near 220 nm and 308 nm were
noted in the spent growth medium from fully grown
cultures [9]. Chakravarty et al. [3] isolated and partially
characterized a catechol-type siderophore compound
produced by Pseudomonas stutzeri grown under iron-
depleted conditions. The purified siderophore gave a UV
spectrum (bands at 250 nm, 330 nm) which was found
to be identical with that of 2,3-DHBA. In another
characterization study with siderophores, O‘Brien and
Gibson [12] isolated five compounds from culture media
of Escherichia coli and Aerobacter aerogenes under
conditions of iron deficiency. The compound (2,3-di-
hydroxy-N-benzoylserine) gave two distinct UV peaks in
water, one near 246 nm and the other near 308 nm, as
shown in Table 5. The absorption peaks in the UV
spectra of spent growth medium that were obtained in
the present work occurred at similar locations in the
above-cited work, suggesting that they were caused by
compounds resembling dihydroxybenzoate. However,
the spectra in the cited studies were obtained with
solutions of purified siderophores. The shift of the 310-
nm band in the UV spectrum of spent medium at high
alkaline pH (moving to 298 nm) can be explained as an
ionization effect.
Table 3 Changes in UV peaks of spent growth medium during
leaching with nodules
Time
(h)
Peaks recorded (nm)
220–230 250–275300–315 >315
1
2
3
4
ND
ND
ND
ND
263.9 (0.928)
256.0 (0.447)
261.4 (0.861)
262.6 (0.083)
307.2 (0.577)
ND
309.0 (0.783)
310.4 (0.614)
ND
433.6 (0.432)
423.0 (0.744)
ND
Table 4 UV spectra of spent growth medium containing dissolved
pure metal ions
Ionic species
in spent growth
medium
Peaks recorded (nm)
250–275 250–300300–350
Cu (20 ppm)
Co (20 ppm)
Ni (20 ppm)
Mn (400 ppm)
Fe (300 ppm)
Cu+Co+Mn
+Fe+Ni
246.0 (0.175)
261.0 (0.195)
255.8 (0.175)
260.0 (0.062)
248.6 (0.207)
256.2 (0.560)
294.8 (0.880)
295.8 (1.007)
295.2 (0.717)
ND
ND
ND
ND
ND
ND
312.2 (1.299)
342.2 (3.978)
302.8 (1.758)
012345
0
5
10
15
20
25
30
35
40
% dissolution of Mn, Cu and Ni
Time (h)
Initial conc. (mM)
Ni 1.96
Cu 1.79
Mn 44.64
28
32
36
40
44
48
52
Eh
Fig. 2 Change in Eh(Eh) during leaching of Cu, Ni and Mn from
ocean nodules by the spent growth medium
465

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The solution infrared (IR) spectra of the spent
growth medium revealed the following bands (in
decreasing order): 3,099.0, 2,923.0, 2,853.17, 1,639.2,
1,456.96, 1,376.93, 1,336.43 and 1,108.87 [9]. These
bands are suggestive of a phenolic compound with an
attached carboxyl group. IR spectra of fresh ASWNB
medium (control) did not reveal any of these specific
bands.
Table 6 compares the mass spectrum of spent AS-
WNB medium with the mass spectra of known sidero-
phores cited in the literature. The catecholate-type
siderophores have a dihydroxybenzoic acid structure
with amino acid fragments attached. Persmark et al. [13]
isolated and purified a catechol-type siderophore (chry-
sobactin) from the phytopathogenic bacterium Erwinia
chrysanthemi. Mass spectral analysis of the triethylam-
monium salt of chrysobactin revealed a m/z peak at
370.1614 in the cationic detection mode, which was as-
signed to [MH+]. In anionic detection mode, a peak
was observed at m/z=368 and was assigned to [M]?1.
Chrysobactin therefore had a Mrof 369. They concluded
that chrysobactin consisted of one DHBA, one lysine
and one serine molecules. Comparing the mass spectra
of the spent growth medium of marine isolate with these
results, a difference of 8.8 in m/z values is noted. This
difference may arise for two reasons. Either, the exact
structure of the siderophore(s) may not be identical in
both cases, or the difference in amino acid fragments
may give the difference in mass. Persmark et al. [13]
isolated chrysobactin and purified the compound before
taking mass spectra. In our case, the spent growth
medium of the marine isolate may have a number of
metabolites. So, the mass spectra of the partially purified
spent growth medium of the marine isolate resembled
that of standard siderophores from literature, confirm-
ing the presence of complex-forming phenolic sub-
stances.
The bioleaching system of the nodules was a complex
one, where the spent growth medium containing a
number of metabolites interacted with a mixture of
transition metal oxides.
Comparing the UV spectra of the leached liquor from
Table 3 with that of synthetic solutions of metal ions in
the spent growth medium as shown in Table 4, we can
infer that the spent growth medium of cultures of marine
isolate was able to solubilize Cu, Co, Ni, Mn and Fe
ions. The spent growth medium might contain some
complexing ligands capable of forming soluble transi-
tion metal complexes. These complexes absorbed UV
rays and revealed characteristic peaks.
A correlation of Co dissolution with Fe and Mn
dissolution by spent culture medium produced by the
marine isolate is shown in Fig. 3. The data collected
from the bioleaching tests of nodules with the marine
isolate at different pulp densities, as shown in the Ta-
ble 1, are plotted in the diagram. The straight line sig-
nifies the linear fit of the corresponding data in both
cases. In the figure, the linear regression analysis of Fe
and Co dissolution data showed r=0.97, while that of
Mn and Co dissolution revealed r=0.89. From Fig. 3, it
is observed that the correlation between Co and Fe
dissolution is better than that between Co and Mn dis-
solution.
The results from a mineralogical study of nodules by
Glasby [7] revealed a stable iron phase consisting of
limonites (Fe oxyhydroxides), in which the valence state
Table 5 Comparison of the UV bands of the spent growth medium with those of siderophores from previous work. ND Not determined
Microbial
strain
Type of
siderophores
Characterization UV
peaks (nm)
References
Pseudomonas stutzeri
Escherichia Coli
Bacillus M1
Catecholate
Catecholate
ND
2,3 DHBA and arginine
2,3 DHBA-N-benzoylseine
ND
250 and 330
246 and 308
224.8 and 308.2
[3]
[12]
Current
study
Table 6 Comparison with mass values of standard compounds
MassSuggested structure Reference
154.0266
105.0426
146.1056
369
377.8
DHBA (C7H6O4)
Serine (C7H6NO3)
Lysine (C6H14N2O2)
M-H2O, Chrysobactin
DHBA with amino acid fragments
[14]
[14]
[14]
[13]
Current study
010 20
% Fe and Mn dissolution
30 4050 6070
0
10
20
30
40
50
60
70
80
Fe
Mn
% Co dissolution
Fig. 3 Correlation of Co dissolution with Fe and Mn dissolution
by the spent growth medium
466

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of Fe was +3. Burns and Brown [1] found that the Fe in
nodules had an oxidation state of +3, using Mossbaur
spectroscopy. At near neutral pH, the thermodynamic
solubility of Fe (III) is very low (?log Ks= 38 [15]). In
the presence of atmospheric oxygen, Fe (II) is rapidly
oxidized to Fe (III) and the latter forms an insoluble
ferric hydroxide precipitate.
We inferred the presence of phenolic compounds in
spent medium from cultures of Bacillus M1 in the
present work. These compounds may have complexed
the Fe(III) and thereby enhanced its solubility. A com-
parison of the experimental results shown in Figs. 3, 4
suggests that such a complexing mechanism occurred in
this case.
Figures 4, 5 show the correlation of Cu and Ni dis-
solution with Mn and Fe, respectively, in the leaching
experiments with the spent growth medium. The dotted
line in both figures represents the line of best fit for Ni,
and the solid line in both figures represents Cu. A linear
regression analysis shows that the r-value for the Cu-Mn
line was 0.96 while that for Ni-Mn was found to be 0.85.
So, Cu dissolution was more dependent on Mn solubi-
lization, compared with that of Ni. The linear regression
analysis (Fig. 5) shows that the Fe-Cu r-value was 0.84
and Fe-Ni gave a value of 0.79. A comparison between
these two figures suggests that both Cu and Ni dissolu-
tion had a better correlation with Mn solubilization.
This dependence on Mn dissolution can be related to the
reducing environment in the spent growth medium.
In a 6-h kinetic study of leaching with the spent
growth medium, the pH did not change greatly, con-
firming that dissolution by acid was not responsible for
leaching. The low prevailing Ehduring leaching (Fig. 2)
signifies a reducing environment in the spent growth
medium. In the nodules, we had two major oxide phases:
Mn (IV) oxide and Fe (III) oxide. Mn (IV) would be
more affected by the reducing environment due to a
greater abundance of Mn (IV) in the nodules compared
with Fe (III). From our chemical analysis, we observe
that the ratio of Mn (IV):Fe (III) was 4:1.
aÞ1
2dMnO2ðsÞ þ 2Hþþ 2e?!1
E0¼ 1:23V
bÞ1
E0¼ 0:67V
From the above two equations, it can be noted that
MnO2 is a more powerful oxidant than Fe (III).
Therefore, Mn (IV) oxide would be more reducible by
the spent growth medium than the Fe phase.
Godtfredsen and Stone [8] studied the solubilization
of Cu bound by Mn dioxide by naturally occurring low-
molecular-weight organics, which included hydroxy
aromatic moieties. They found that natural organic
compounds with only reducing characteristics dissolved
Cu by reductive dissolution of Mn, while compounds
capable of forming stable Cu complexes in solution
caused additional Cu release that could surpass the re-
lease by reduction processes alone. One of the formation
processes of the ferromanganese nodules is inorganic
scavenging of metals from the basinal water on active
surfaces through selective chemisorption and autocata-
lytic oxidation [1]. So, a part of Cu and Ni may be
chemisorbed onto the Mn (IV) oxide surface in the
nodule.
The spent growth medium of cultures of the isolate
was a strong reducing agent; and, in addition, it con-
tained phenolic complexing agents, as we observed from
the characterization studies. Therefore, we can extend
Godtfredsen and Stone’s findings in the case of nodule
leaching by spent growth medium.
Reduction of MnO2to Mn (II) by metabolites pres-
ent in the spent growth medium may be written as:
2Mn2þþ 2H2O
ð1Þ
2FeOOHðsÞ þ 3Hþþ e?! Fe2þþ 2H2O
ð2Þ
0102030 40 506070
0
10
20
30
40
50
60
70
% Cu and Ni dissolution
% Fe dissolution
Cu
Ni
Fig. 5 Correlation between Fe, Cu and Ni dissolution in leaching
of ocean nodules by the spent growth medium
01020304050
0
10
20
30
40
50
% Cu and Ni dissolution
% Mn dissolution
Cu
Ni
Fig. 4 Correlation between Mn, Cu and Ni dissolution in leaching
of ocean nodules by the spent growth medium
467

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MnO2ðsÞ þ PH2ðaqÞ þ 2Hþ! Mn2þþ P þ 2H2O
where PH2 is the reducing metabolite present in the
spent liquor and P is the oxidized form of PH2
According to this reaction, metabolites may bring
about Cu and Ni release indirectly by generating MnII
(which competes with Cu and Ni for metal-sorbing sites
on residual MnO2) and directly by consuming metal-
sorbing sites as a result of the reductive dissolution of
MnO2.
An additional mechanism of Cu and Ni dissolution
may have been operating in our experiments, namely the
formation of Cu, Ni complexes in solution. Butler [2]
discussed the solubilization of transition metals other
than Fe by microorganisms. In sea water, many of the
transition metals are partially or fully complexed by yet-
undefined organic ligands. The initial evidence pointed
to metal specific organic ligands, distinct from organic
ligand or ligand classes complexing Fe (III) in the ocean.
So, ligands for Cu and Ni might be different from Fe
(III) complexants. In Table 4, showing changes in UV
bands during leaching, we observed that the presence of
Cu and Ni ions in the spent growth medium gave dis-
tinct bands. Complexation of Cu and Ni by the metab-
olites might be responsible for these specific bands.
ð3Þ
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