Aerobic Bio-Precipitation of Heavy Metal Contaminated Dredged Materials from the Niger Delta
The abandonment of unconfined dredged materials in the Niger Delta following oil exploration is becoming a cause for concern because of the release of heavy metals during the natural weathering of sediments containing pyrites. During a laboratory scale bioremediation studies, Acidithiobacillus sp. was isolated from the spoil using 9 k media (10%) and used as inoculum in a controlled experiment for the leaching of two different abandoned dredged spoils (spoil 1 and 2) obtained from the mangrove forest of the Niger Delta. Results showed that the bacterium is able to catalyse the leaching of metals from the spoils into solution, with the following leaching efficiencies; Cu (92%), Cd (100%), Cr (50%), Ni (82%), Mn (80%) and Zn (92%) for spoil 1. Spoil 2 had similar leaching efficiencies. The mechanism of leaching involves the microbial oxidation of the pyrite in the dredged spoils resulting in pH decrease, which released the metals into solution due to changes in the redox chemistry of the leaching liquor. The study therefore have the potential of being applied in a large scale for the removal of heavy metals from dredged materials prior to their abandonment to prevent heavy metal pollution through natural weathering processes.
Oil exploration, which is the mainstay of the Nigerian economy accounted for over 90% of the countrys export and 80% of her earnings. Most of the Nigerias oil resources are located in the Niger Delta region. The Niger Delta is made up ecosystem that is dominated by the presence of wetlands and several creeks, creeklets and rivers that are relatively shallow, which made access to oil bearing location to be difficult by water craft. The coastal Niger Delta is also the route to Nigerias offshore oil locations. Logistic support and supply for the offshore facilities is threatened. Also, the high siltation rate is compounding the access challenges and reinforcing the need for dredging. In order to overcome these access limitations, dredging is carried out.
Dredging is one of the routine operations carried out by oil exploration companies
when operating in wetlands. Dredging is necessary to create navigable accesses
to facilitate routine operational activities such as drilling, pipeline construction
and installation production of facilities etc and for the support of production
logistics such as the transportation of oil production equipment and production
crew exchange and also the supply of food to field workers. During dredging,
waterway sediment, soil, creek banks and vegetation along the right of way are
typically removed and because of lack of better ways to manage the concomitant
dredged materials, they are deposited as spoils at the bank of the newly dredged
canal (Ohimain, 2004). A number of impacts are associated
with the abandonment of dredged materials especially in mangrove ecosystem in
addition to the direct impact of the dredging activity itself. For instance,
because the abandoned dredged materials are unconfined and uncapped, they are
exposed to the high torrential rainfall exceeding 3000 mm per annum. These rains
washes silt and sand from the spoil dump back into the canals and in addition
to the high sediment deposition in the Niger Delta, create the need for remedial
or maintenance dredging, which the oil industries called sweeping and the vicious
cycle of dredging and sedimentation continues to the detriment of the ecosystem.
The cycle of dredging and sedimentation often increase turbidity plumes causing
alterations in the population of phytoplankton and zooplankton communities (Toumazis,
1995; Ohimain et al., 2002, 2004,
Ohimain et al. (2008b) reported that though
the impact of dredging is short term on water quality, but leachates from dredged
materials containing pyrite have compounded and prolonged the impact arising
from the dredging. Mangrove soil and sediment are regarded as sulphidic because
of the presence of pyrite mineral. As long as pyrite remain in the sediment
in the absence of air, they are innocuous, but their disturbance through dredging
followed by oxidation, when they are disposed unconfined, leads to acidification
(Ohimain, 2004) because of the oxidation of pyrite to
sulphuric acid. Pyrite is common in the soil and sediments of the entire Gulf
of Guinea, with a high presence in the Niger Delta (Sylla
et al., 1996). Dredging has been reported to cause the re-suspension
of sedimentary pyrites, which is linked to the re-mobilization of contaminants
particularly heavy metals and increasing their bioavailability (Perin
et al., 1997). Re-suspension of sediment causes the oxidation of
sediment leading to the mobilization of metals into the water body (Saulnier
and Mucci, 2000). The weathering sulphidic spoils often results in acidification
and the release of heavy metals into the environment (Ohimain
et al., 2008b). Mangrove ecosystem are known to fix heavy metals
in their sediments (Ouyang et al., 2002; Stephens
et al., 2001a, b; Delaune
and Smith, 1985; Gambrell, 1994; Peterson
et al., 1997), but the disturbance of the sediment through dredging
and spoil disposal often change the redox chemistry of the ecosystem, which
triggers pyrite oxidation and acid production through the activities of an acidophilic
bacteria, Acidithiobacillus sp. using oxygen and ferric iron as catalysts
(Ohimain, 2008). Obiajunwa et
al. (2002) reported heavy metal pollution around hydrocarbon production
facilities in the Niger Delta.
The release of heavy metals from abandoned dredged spoil is a major threat
to environmental sustainability, thus militating against the realisation of
Millennium Development Goal No. 7. Heavy metals are toxic even at low concentration,
they cannot be biodegraded, but they bioaccumulate along the food chain. Kwon
and Lee (1998) reported that heavy metals are toxic and exert chronic and
lethal effects on aquatic animals and plants. Hence, the aim of this study is
to isolate the acidophilic iron oxidising bacteria from the despoiled sites
and use them as inoculum for the controlled leaching of contaminated spoils
for the bioremediation of heavy metals.
MATERIALS AND METHODS
This study was carried out in a dredged canal (5°31N, 5°31E)
leading off a tributary of the Warri River in the mangrove swamp of the Niger
Delta about 20 km from Warri in Delta State, Southern Nigeria. The vegetation
here is typical of mangrove swamp dominated by Rhizophora species. The
area is characterized by high relative humidity (80-92%) and annual average
rainfall exceeding 2800 mm. Although, there are two seasons (wet and dry), measurable
precipitation occurs in all the months of the year. Notwithstanding, the period
of April to October is often regarded as raining season, while November-March
is regarded as dry season. Atmospheric temperature ranged between 27 to 29°C
Two different composite dredged spoil samples were collected in June 2000 namely
spoil 1 (matured spoil i.e., about 5 years of abandonment) and spoil 2 (recent
spoil i.e., <4 months of abandonment). The samples were air-dried at ambient
conditions. They were pounded and sieved through 2 mm mesh and consequently
preserved for further analysis. Result of laboratory analysis (Ohimain
et al., 2008c) show that the spoils are contaminated by heavy metals
In order to isolate the mesophilic, chemolithotrophic, acidophilic bacteria
of the genus Acidithiobacillus, 1 g of spoil sample was suspended in
100 mL each of 10% 9 K medium in Erlenmeyer (shake) flask. The composition of
9 K medium used was according to Silverman and Lundgren
(1959) containing per litre: 3.0 g [NH4]2 SO4,
0.5 g K2HPO4, 0.5 g MgSO4. 7H2O,
0.01 g Ca[NO3]2 and 0.10 g KCl. Water was added and made
up to 700 mL. 1.0 mL of 10 N H2SO4 was added followed
by 300 mL of 14.74% FeSO4. 7H2O and was adjusted to pH
3.5 using concentrated sulphuric acid. Nine K medium is a specialized medium
for the isolation of iron oxidizing bacteria Acidithiobacillus ferrooxidans
(Silverman and Lundgren, 1959). Each shake flask was
plugged using cotton wool and incubated at 28±2°C for 4 weeks. The
cultures were shaken for 1 h each day at 150 rpm (semi-static conditions). Growth
was evident by increase in turbidity, color change of the media and microbial
population increase. Cultures (10 mL) were transferred to fresh medium containing
sterile spoils. This transfer continued until the population of active growing
Acidithiobacilli was in the order of 106 MPN 100 mL-1.
Ten grams each of spoils 1 and 2 were weighed into each of a 250 mL conical
flask containing 100 mL of 9 k medium. The samples were sterilized at 121°C
for 30 min. Each spoil was subjected to leaching processes using Acidithiobacilli
inocula. A control set of samples was prepared without the addition of the bacteria.
Eight pairs of media were prepared for the two spoil samples corresponding to
Day 0, 7, 14, 21, 28, 35, 42 and 49. The cultures were incubated at 28±2°C
in an incubator. Samples were collected weekly at each sampling day (0, 7, 14,
21, 28, 35, 42 and 49). Day 0 samples were collected after 30 min of incubation.
Samples were analysed for microbial species, redox potential, pH, turbidity,
optical density at 430 nm, ferrous iron, sulphate TDS and conductivity. The
remaining samples were digested using HNO3/HC1O4/ H2SO4
for heavy metal analysis. Heavy metals were analysed using Buck Scientific 200A
Atomic Absorption Spectrophotometer (AAS) for copper, cadmium, chromium, nickel,
manganese and zinc (APHA, 1995).
|| Heavy metal composition of abandoned dredged spoils
|Source: Oimain et al. (2008c)
Redox potential and pH were analysed using a combination platinum/reference
(Ag/Agcl) electrode (ATI Russel,) pH/Eh meter, turbidity was determined using
Hach turbidimeter 2100P, TDS and conductivity with conductivity/TDS meter (Hach
CO. 150), sulphate using turbidimetric method (Hach turbidimeter 2100P) and
optical density (λ = 430 nm) using Hach 4000 Spectrophotometer. A correlation
study was carried out among the parameters using SPSS version 11 and the significance
of the correlation was determined at 0.05 and 0.01 probability levels (two tailed).
Metal recovery/leaching efficiency were calculated using the following equation
(Seidel et al., 1998):
Earlier studies have shown that the abandoned dredged materials are rich in
heavy metals (Table 1), with the metal content of spoil 1
being lower due to longer period of natural weathering and leaching processes
(Ohimain et al., 2008c). During the controlled
leaching experiment, pH declined continuously from the start of the experiment
till the end (Fig. 1),indicating increasing acidity. Correlation
analysis revealed that pH had an inverse relationship with virtually all the
other parameters monitored (Table 2). The increasing acidity
coincided with increase in the levels of other parameters including optical
density, iron conductivity, sulphate and the population of Acidithiobacillus
|| Changes in selected microbial and physico-chemical parameters
|| Pearson correlations coefficient of the monitoring parameters
|*,**Correlation is significant at 0.05 and 0.01 level (2-tailed),
||Visual observation during bioleaching of dredged spoil inoculated
with Iron oxidizing bacteria using 10% 9 k medium
|1Increasing precipitation; -: None, +: Low, ++:
Moderate, +++: High. 2Increasing color intensity; SB: Slightly
brownish, GY: Greenish yellow, YY: Moderate yellow, B: Brownish. 3Increasing
turbidity; +: Low, ++: Moderate, +++: High; ++++: Very high. 4Increasing
slime production; -: None, +: Low, ++: Moderate, +++: High
The pH strongly correlated inversely with optical density, sulphate, conductivity
and bacteria population, which was significant at α<0.01. Soluble iron
increased from the beginning and attained peak concentration at day 28 and began
to decline till the end of the experiment. Iron had a strong inversely relationship
with optical density and bacteria at α<0.01. As the leaching experiment
progressed, the color of the leaching liquor changed from slight brown through
greenish yellow, yellow and became brownish towards the end of the experiment.
The precipitation of iron minerals (jarosites and other hydroxides of iron)
began on the 28th day (Table 3). Turbidity increased as the
population of the bacteria increased and microbial slimes became visible as
from day 14 upwards (Table 3). Beside iron, other metals in
the spoil matrix were leached into the liquor. At the end of the experiment,
the percentage leaching of heavy metals were; Cu (92%), Cd (100%), Cr (50%),
Ni (82%), Mn (80%) and Zn (92%) for spoil 1. Spoil 2 had similar leaching efficiencies
|| Heavy metal leaching efficiency
It has been variously reported that the leaching of sedimentary pyrite result
in the oxidation of pyrite to sulphuric acid (Rose and Cravotta,
1998; Ohimain et al., 2004; Ohimain,
2004, 2008). During this study, pyrite oxidation
coincided with increased concentration of sulphate and iron, while the population
of acidophilic bacteria also increased. Iron had a strong inversely relationship
with optical density and acidophilic bacteria at α = 0.01. Soluble iron
increased from the beginning and attained peak concentration at day 28 and began
to decline till the end of the experiment. This unusual behaviour of iron may
be as a result of precipitation of iron minerals (jarosites and other hydroxides
of iron), which began on the 28th day (Table 3). Acidic drainage
are usually formed by the oxidation of pyrite to release dissolved Fe2+,
SO42¯ and H+, followed by the further
oxidation of the Fe2+ to Fe3+ and the precipitation of
the iron as a hydroxide (Rose and Cravotta, 1998). Iron
are known to be soluble at low pH, but with increasing oxidation of pyrite,
ferric iron is formed, which precipitate out of solution, resulting in the decline
of soluble iron (ferrous iron). Physical observations of the leaching liquor
tend to support this assertion. As the leaching experiment progressed, the color
of the leaching liquor changed from slight brown through greenish yellow, yellow
and became brownish towards the end of the experiment. The various colors of
the media relates to the changes in the speciation of iron in the media.
The increased acidity resulted in the leaching of the sedimentary heavy metals
into solution. The percentage leaching of heavy metals were; Cu (92%), Cd (100%),
Cr (50%), Ni (82%), Mn (80%) and Zn (92%) for spoil 1. This result was slightly
higher than that of Seidel et al. (2003), who
obtained the following results; Zn, Cd, Ni, Co and Mn were leached by up to
80%. Cu was partially dissolved, while Cr and Pb proved nearly immobile. Overall,
they recorded 60-65% heavy metal removal within 6 weeks of leaching. Similarly,
Wong et al. (2002) recorded the following efficiencies
after 16 days of bioleaching; 50.2-78.4% of Cr, 63.7-74.1% of Cu, 74.9- 88.2%
of Zn and 15.5-38.6% of Ni. Reasons for the differences in leaching efficiencies
are due to the pH of the leaching media, redox changes, metal speciation, the
solubility constants of the different metal and the diversity, activities and
population of the acidophilic microorganisms in the leaching liquor and the
initial concentration of elemental sulphur in the leaching media (Lovley
and Coates, 1997).
The study was carried out on abandoned dredged spoils that are undergoing natural weathering and leaching heavy metals into the environment. The bacteria catalysing the oxidation of pyrite in nature, Acidithiobacillus sp. was isolated using 9 k media (10%) and used in a controlled laboratory experiment for the leaching of abandoned dredged spoils obtained from the mangrove forest of the Niger Delta. Results showed that the bacterium is able to catalyse the leaching of metals from the spoils into solution. The study therefore have the potential of being applied in a large scale for the removal of heavy metals from dredged materials prior to their abandonment to prevent heavy metal pollution through natural weathering process.
Author wish to express his gratitude to Dr. (Mrs) M.O. Benka-Coker for supervising his Ph.D research.
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