Thioleaching of heavy metal contaminated sediments using matin's medium.
Petroleum exploration in the Niger Delta has brought much wealth to the Nigerian state. But it has also contributed to environmental degradation of the Niger Delta oil province. Environmental impacts of oil exploration in the Niger Delta includes air quality impacts arising from gas flaring, water quality/soil/sediment impacts caused by the discharge of improperly treated production effluents and oil spills. While the oil industries have procedures and processes in place to manage these negative externalities, they are at loss on how to manage the impacts arising from dredging activities and the concomitant heavy metal pollution arising from the poor management of contaminated sediments.
During oil exploration in a predominant wetland ecosystem, access is required for marine transportation, rig movement, drilling activities, pipeline construction, flowstation/ gas plants/ compressor stations installation etc. Because of the shallow meandering creeks/ creeklets dominating the landscape and the large drought required for the transportation of oil exploration construction facilities, barges and rigs, dredging becomes necessary. Dredging which is carried out in order to increase access to these wetlands, involves deepening/widening of existing channel or cutting new access channels . During dredging, variable amount of waterway bed and/or bank sediments are removed, transported, disposed and abandoned as dredged spoils. Dredging in the Niger Delta have been reported to cause environmental impacts including disturbance of algal communities , impairment of benthic invertebrates  and destruction of zooplankton , alteration of water physico-chemistry  and heavy metal pollution . While most of the other direct impacts of dredging are short to medium term , the impact of heavy metals is long term .
The abandonment of dredged spoils arising from oil exploration dredging activities is of major concern . Estuarine sediments are known to act as sinks for heavy metals , these metals are often released or remobilized during dredging [9-16]. It has been reported that dredged spoils in the Niger Delta are rich in pyrite (Fe[S.sub.2]) and principally contaminated by heavy metals [17, 18]. The exposure and oxidation of abandoned spoils through natural weathering processes causes the release of heavy metals [19, 20], which tend to prolong the impacts associated with dredging. High levels of heavy metals have been variously reported to be associated with oil development activities in the Niger Delta [5, 6, 21-28] and there are indications of heavy metals bioaccumulation [29-34]. The uncontrolled leaching of heavy metals from abandoned spoils poses a health risk to the people and the environment. In this study, we therefore consider using sulphur oxidising bacteria in a controlled leaching using Matin's medium to recover heavy metals prior to spoil abandonment in a bioremediation perspective.
Materials and methods
This study was carried out in and near a dredged canal (5[degrees]31"N, 5[degrees]31"E) leading off a tributary of the Warri River in the mangrove swamp of the Niger Delta about 20km 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 2800mm. 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[degrees]C to 29[degrees]C .
Two different composite dredged spoil samples were collected during sampling 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 2mm mesh and consequently preserved for further analysis. Result of laboratory analysis show that the spoils are contaminated by heavy metals (Table 1) .
In order to isolate the mesophilic, chemolithotrophic, acidophilic bacteria of the genus Acidithiobacillus, 1 g of spoil sample was suspended in 100ml each of Matin's medium (see Table 2 for media composition) in Erlenmeyer (shake) flask. Each shake flask was plugged using cotton wool and incubated at 28 [+ or -] 2[degrees]C for 4 weeks. The growth of sulphur oxidising bacteria is evident by increased turbidity, colour change of the media, microbial population increase and turning of 0.01% bromophenol blue indicator to light green . 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 [10.sup.6] MPN/100ml.
Ten grams each of spoils 1 and 2 were weighed into each of a 250 ml conical flask containing 100 ml of the leaching medium. The samples were sterilized at 121[degrees]C for 30 minutes. 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[degrees] [+ or -] 2[degrees]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 minutes of incubation. Samples were analysed for microbial species, redox potential, pH, turbidity, optical density (at 430, and 660nm), ferrous iron, sulphate and conductivity. The remaining samples were digested using HN[O.sub.3]/HC1[O.sub.4]/ [H.sub.2]S[O.sub.4] for heavy metal analysis. Heavy metals were analysed using Buck Scientific 200A Atomic Absorption Spectrophotometer (AAS) for copper, cadmium, chromium, nickel, manganese and zinc . 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, conductivity with conductivity/TDS meter (Hach CO. 150), sulphate using turbidimetric method (Hach turbidimeter 2100P) and optical density ([delta] = 430nm and 660nm) using Hach 4000 Spectrophotometer. A correlation study was carried out among the parameters using SPSS version 11 (Lead Technologies Inc, 2001) and the significance of the correlation was determined at 0.01 and 0.05 probability levels (two tailed).
Metal recovery/leaching efficiency were calculated using the following equation :
Metal recovery, % = metal content in leachate X 100/ Initial metal content in dredged spoil
Results and discussion
Figure 1 present the growth pattern of Acidithiobacillus species in the shake flasks containing the two different dredged spoils in Matin's medium. The growth pattern of the mesophilic acidophile in both dredged spoil samples is similar, though the bacteria population in spoil 1 was generally higher. Visual observations (Table 3) made during the study and the physico-chemical parameters monitored (Fig. 2) tend to support this pattern. The colour of the leaching medium, which was initially slightly brownish, changed to yellow of increasing intensities within 14 days and became brownish till the end of the experiment. Turbidity measurements made using Hach 2100P turbidimeter increased throughout the duration of the experiment (Fig. 2). Microbial slime production also increased as the experiment progressed (Table 3). The result of correlation analysis revealed that turbidity is highly positively correlated with Acidithiobacillus population, which is significant even at the 0.01 probability level (Table 4). Other measurements such as optical density (@ 430 and 660nm) also increased as the experiment progresses.
Redox potential did not exhibit any discernable pattern; it was generally high, ranging from 200--400 mV. All the other parameters measured increased as the population of actively growing acidithiobacilli increased including acidity, sulphate, conductivity and soluble iron. These parameters had direct relationship, many of which are significant at least at the 0.05 probability level. After seven weeks of bioleaching, the heavy metal recoveries from spoil 1 are copper (81%), cadmium (86%), chromium (34%), nickel (61%), manganese (63%) and zinc (82%), hence the pattern of heavy metal recovery from the spoil is as follows: Cd > Zn > Cu > Ni > Mn > Cr. Spoil 2 had a similar trend (Fig. 3).
It therefore appears that the pattern of leaching of the dredged spoil involves the growth of Acidithiobacillus species (suspected to be A. thioxidans) using pyrite and thiosulphate as energy sources leading to the formation of sulphate, which resulted in the reduction of pH and the release of iron and other metals. The low pH (<3) arising from the oxidation of insoluble metal sulphide to soluble metal sulphates releases heavy metals into solution. Erhlich  and Gadd  had earlier reported that the process of biological oxidation and reduction of sulphur compounds in the biosphere are closely related to the mobilization and immobilization of metals in biogeochemical cycles. Ohimain  provided options for the management of acidification and heavy metal pollution arising from dredged spoil abandonment. Controlled bioleaching processes can therefore be used to recover heavy metals from the spoils prior to their disposal/abandonment to prevent heavy metal pollution.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Matin's medium containing thiosulphate as energy source, can be used as a leaching medium for the isolation and enrichment of indigenous Acidithiobacillus species for the leaching of heavy metals from contaminated dredged spoils. These spoils are typically abandoned following dredging operations in the Niger Delta, where through weathering they release heavy metals into the environment. This study demonstrates that heavy metal could be recovered from dredged spoil prior to their abandonment to prevent heavy metal pollution.
We wish to express our gratitude to Dr (Mrs) M. O. Benka-Coker for supervising the PhD research of the first author
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Elijah I. Ohimain *, Ebisomu C. Agedah and Frank O. Briyai Biological Sciences Department, Niger Delta University, Wilberforce Island, Amassoma, Bayelsa State, Nigeria *Corresponding author: email@example.com
Table 1: Heavy metal composition of abandoned dredged spoils  Metal (mg/kg) Spoil 1 Spoil 2 Copper 93.3 [+ or -] 0.26 121.1 [+ or -] 0.30 Cadmium 122 [+ or -] 2.00 130.8 [+ or -] 0.53 Chromium 95.2 [+ or -] 0.20 143 [+ or -] 1.00 Nickel 93 [+ or -] 0.20 138.5 [+ or -] 0.36 Manganese 251.4 [+ or -] 0.80 296 [+ or -] 2.00 Zinc 118.1 [+ or -] 0.78 131.5 [+ or -] 0.50 Table 2: Composition of Matins medium used for the study  [Na.sub.2[[S.sub.2][0.sub.3] x 5[H.sub.2]0 5.0g MgS[0.sub.4] x 7[H.sub.2]0 1.02g [K.sub.2]HP[O.sub.4] 0.60g K[H.sub.2]P[O.sub.4] 0.40g N[H.sub.4]Cl 1.0g Ca[Cl.sub.2] x 2[H.sub.2]0 0.13g Biotin 0.244g Trace metal solution 5.0ml Fe--EDTA 5.00ml Water 1000ml Table 3: Visual observations during leaching of dredged spoil using Matin's medium inoculated with culture of sulphur oxidizing bacteria Trace Metal Solution EDTA 50.0g Zn S[0.sub.4] x 7[H.sub.2]0 2.2g Ca[Cl.sub.2] x 2[H.sub.2]0 7.34g Ma[Cl.sub.2] x 4[H.sub.2]0 5.06g FeS[0.sub.4] x 7[H.sub.2]0 5.35g [N[H.sub.4]6]M[0.sub.7][0.sub.24] x 4[H.sub.2]0 1.17 CuS[0.sub.4] x 5[H.sub.2]0 1.57 Co[Cl.sub.2] x 6[H.sub.2]0 1.61 Water 1000ml Sampling Day 0 7 14 21 OBSERVATION SPOIL SPOIL SPOIL SPOIL SPOIL SPOIL SPOIL I II I II I II I Turbidity * + + + + +++ ++ +++ Microbial slime - - - - + + ++ * Colour of the SB SB YY Y YYY YY B leaching medium ** Sampling Day 28 35 OBSERVATION SPOIL SPOIL SPOIL SPOIL II I II I Turbidity * ++ +++ +++ +++ Microbial slime + ++ + ++ * Colour of the YY B B B leaching medium ** Increasing turbidity and microbial slime production: -none/clear; +low; ++moderate; +++high; ++++very high ** Increasing intensity: SB--Slightly brownish; B--Brownish; Y- Yellow of increasing intensity (GY-Greenish yellow; Y-low; YY-moderate; YYY-high; YYYY-very high) Table 4: Correlation coefficients of monitoring parameters Spoil 1 Acidithiobacillus Turbidity pH OD @ 430 Acidithiobacillus 1 Turbidity 0.956 ** 1 pH 0.718 * 0.826 * 1 OD @ 430 0.763 * 0.890 ** 0.891 ** 1 OD @ 660 0.743 * 0.896 ** 0.849 ** 0.968 ** Iron 0.835 ** 0.766 * 0.624 0.731 * Sulphate 0.764 * 0.882 ** 0.871 ** 0.996 ** Redox -0.022 -0.131 -0.033 -0.330 Conductivity 0.548 0.694 0.970 ** 0.858 Spoil 1 OD @ 660 Iron Sulphate Redox Acidithiobacillus Turbidity pH OD @ 430 OD @ 660 1 Iron 0.642 1 Sulphate 0.960 0.744 ** 1 Redox -0.335 -0.053 -0.369 1 Conductivity 0.814 * 0.535 0.839 ** -0.090 Spoil 1 Conductivity Acidithiobacillus Turbidity pH OD @ 430 OD @ 660 Iron Sulphate Redox Conductivity 1 Spoil 2 Acidithiobacillus Turbidity pH OD @ 430 Acidithiobacillus 1 Turbidity 0.850 ** 1 pH 0.878 ** 0.697 1 OD @ 430 0.695 0.543 0.911 ** 1 OD @ 660 0.929 ** 0.945 ** 0.874 ** 0.729 * Iron 0.246 -0.059 0.428 0.479 Sulphate 0.923 ** 0.889 ** 0.924 ** 0.809 * Redox 0.609 0.895 ** 0.503 0.352 Conductivity 0.577 0.397 0.884 ** 0.935 ** Spoil 2 OD @ 660 Iron Sulphate Redox Acidithiobacillus Turbidity pH OD @ 430 OD @ 660 1 Iron 0.056 1 Sulphate 0.988 ** 0.134 1 Redox 0.775 * -0.089 0.694 1 Conductivity 0.626 0.512 0.718 * 0.307 Spoil 2 Conductivity Acidithiobacillus Turbidity pH OD @ 430 OD @ 660 Iron Sulphate Redox Conductivity 1 ** Correlation is significant at the 0.01 level (2-tailed). * Correlation is significant at the 0.05 level (2-tailed).
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|Author:||Ohimain, Elijah I.; Agedah, Ebisomu C.; Briyai, Frank O.|
|Publication:||International Journal of Biotechnology & Biochemistry|
|Date:||Sep 1, 2008|
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