ABSTRACT
Rapid industrialization and
urbanization have resulted in the generation of large quantities of aqueous
effluents, many of which contain high levels of toxic heavy metals and
xenobiotics that pollute groundwater and soil of affected farmlands. Heavy metals
are not biodegradable and as such not removed from the soil but rather
accumulate and persist in soil reservoirs, consequently entering the food chain
and exerting toxic effects on living organisms. Copper and lead which exert
toxic effects even at very low concentrations are common constituents of the
Nigerian crude oil and consequently are found in its effluent. Research has
shown that removal/recovery of these metals (through
bioaccumulation/biosorption by bacteria) is an attractive alternative to traditional
physicochemical techniques. Microorganisms tolerant to metals are often
isolated from areas of high metal loading, suggesting that metal tolerance or
resistance is an adaptive response to excessive metal exposure. In this study,
crude oil effluent was analyzed for copper and lead contents and both metals
were found to show concentrations higher than the U.S Environmental Protection
Agency (EPA) and the Compendium of Environmental Laws for African Countries
(CELAC) recommended environmentally accepted standards. Microorganisms were
isolated from the effluent and from the effluent-contaminated soil from the
site. The largest/most successful colony was subsequently characterized.
Through morphological and biochemical tests, it was identified as Bacillus subtilis.
Four test groups of mineral salt media containing copper only (Group A), lead
only (group B), copper + lead (Group C) and no lead or copper (Group D,
control) set at different pHs of 5.0, 5.5, 6.0, 6.5, 7.0, 7.5 and 8.0 for each
group were used. The organism was standardized and found to contain 6.0 x 108
Bacillus subtilis cells per ml of suspension. Five ml of the organism
was inoculated into each experimental medium. The absorbance change (turbidity)
of the mineral salt media were measured at 540nm on the 10th, 17th
and the 24th days - the evaluation criteria for microorganism growth
and adaptation in the used media. The experimental media showing the highest
growths for each group was analyzed for residual copper and lead. Also, the
bacterial biomass from these media were harvested and analyzed for recovered
lead and copper. Results showed that Group D had the highest growth, followed
by Group B, Group A and lastly Group C. The organism grew most at pH 7.5 - 8.0.
The experimental media that showed the highest growths for each group, when
analyzed for residual copper and lead had no trace of metals, implying complete
biosorption by the B.subtilis. B.subtilis is therefore
recommended for removal of lead at pH 7.5 - 8.0 in crude oil pollution.
CHAPTER ONE
1.1 INTRODUCTION
Toxic heavy metals in air,
soil and water are growing threats to humanity. A number of these heavy metal
compounds represent an ongoing eco-toxicological threat (Sag, 2000). Heavy
metals have a tendency to bioaccumulate and end up as permanent additions to
the environment. For many of the heavy metals, the amounts contributed globally
from anthropogenic sources, such as industrial wastes, now exceed those from
natural sources (Deans and Dixon, 1992). The disposal of effluent on land has
become a regular practice for some industries leading to subsequent pollution
of groundwater and farmlands. Copper (Cu), lead (Pb), mercury (Hg), cadmium
(Cd) are common heavy metal pollutants at sites in which industrial waste
effluents are discharged. One good example of such effluents includes crude oil
waste effluent. Crude oil effluent is the water that is mixed with crude oil
when it is mined or during refining/processing. Crude oil effluent have been
associated with increased concentrations of some heavy metals. Disposal of such
effluents over time in the environment may lead to eco-toxicological hazards.
This is common where mining and manufacturing operations take place,
particularly those established a number of years ago. Copper and lead, which
are common constituents of Nigerian crude oil are known to exert toxic effects
at low concentrations (Pandey et al 2007). However at very low
concentrations, some of these heavy metals such as copper, zinc and boron have
been found to be essential in all higher plants and animals. In slightly
elevated concentrations, these metals may be taken up by plants and
concentrated in certain parts of the plant such as the leaf, stem, and root.
When these are consumed by animals, they are further concentrated in them
resulting in biomagnification. Consumption of the animal parts in which these
metals are concentrated may lead to their significant concentration in human
beings (Alloway, 1995) which could be toxic. Consequently, there is a pressing
need to remove/recover these.
1.2 COPPER
Copper
is the first element of group 1B of the periodic table and displays four
oxidation states: Cu(0), Cu(I), Cu(II) and Cu(III). Cu(II) or cupric ion is the
most important oxidation state of copper generally encountered in water (Cotton
and Wilkinson, 1988). Copper does not break down in the environment and when
introduced into the environment as Cu2+, it typically binds to
inorganic and organic materials contained within water, soil and sediments with
varying affinities. As in water, the binding affinities of Cu(II) with
inorganic and organic matter in sediments and soil is dependent on pH, the
oxidation-reduction potential in the local environment and the presence of
competing metal ions and inorganic anions.
1.2.1 Biological
Role of Copper
Copper
is a trace metal which is essential in all higher plants and animals. A wide
range of enzymes exploit copper chemistry to catalyze reactions which include
cytochrome oxidase, superoxide dismutase, dopamine ß-hydroxylase, lysyl oxidase
and ceruloplasmin. Thus copper ions are essential in cellular respiration,
antioxidant defence, neurotransmitter function, connective tissue biosynthesis
and cellular iron metabolism. Cytosolic superoxide dismutase (SOD) is an
important copper metalloenzyme that protects lipid cell membrane structures
from oxidation. This enzyme catalyzes the transformation of free oxygen
radicals into hydrogen peroxide, which is later converted to water and
molecular oxygen by a cytosolic catalase. In addition to enzymatic roles,
proteins take advantage of the redox nature of copper to achieve facile
electron transfer reactions and to bind reactive intermediates and avoid their
reactivity. Nevertheless, the chemical properties that make copper biologically
useful are also potentially toxic.
1.2.2 Environmentally
Acceptable Limits for Copper
•
The U.S
Environmental Protection Agency (EPA) requires that levels of copper in
drinking water be less than 1.3mg/l.
•
The U.S
Maximum Contaminant Level Goal (MCLG) for copper in water is 1.3mg/l.
•
Permissible
limit for copper in waste water is less than 1mg/l given by Compendium of
Environmental Laws for African Countries (CELAC).
•
The U.S
Department of Agriculture has set the recommended daily allowance for copper at
900µg of copper per day for people above the age of 8.
1.2.3 Molecular
Mechanism of Copper Toxicity
At
very low concentrations (1- 1.5µg), copper improves the efficiency of
Photosystem II (PS II) apparatus. On the other hand, high concentrations could
be toxic, hence the extensive use of copper as fungicide in agricultural
practice. Results from other researchers suggest that copper inhibits either
the donor or the acceptor side in the PS II. Copper ions oxidize directly the
Cyt b559 LP (Low Potential) and HP (High Potential) forms. Using
Mossbaner spectroscopy, Cu2+ was shown to influence the valence and
spin states of the non-haem iron and the haem iron of Cyt b559.
Copper ions oxidized the heme iron into a high spin Fe3+state and
enhance the covalency of the bound non-haem iron, keeping the iron in a low
spin ferrous state. The new valence and spin states of the non-haem and haem
iron reveal the important roles of the quinine-iron complex and cytochrome b559
as regulatory components of the electron transport in PS II.....
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