ABSTRACT
Heavy metal
concentrations and degradation efficiency of total petroleum hydrocarbons
(TPHs) on environment in Ibeno Local Government Area, Akwa Ibom State, Nigeria
was investigated. Experimental design method was adopted for this study.
Fifteen composite samples each of soil, leaves of Telfairia occidentalis, sediment and water were collected in December
2012 and June 2013. The sediment and water samples were collected using corer
and clean plastic bottles respectively. Soil and sediment samples were air
dried, mechanically ground using mortar and pestle, and 2 mm mesh size obtained
for further analysis. The soil and sediment samples (1.0 g) each were weighed
into Kjeldahl flasks. Aqua regia (15 cm3) was added, swirled to mix
and kept overnight. The flasks were heated on a hot plate to 50 oC
for 30 min; temperature was later adjusted to 120 oC and heated
continuously for 2 h. The mixture was cooled, and 0.2 M HNO3 (10 cm3)
added. The resulting mixture was filtered with a Whatman no. 541 filter paper.
The filtrate was transferred into a 50 cm3 standard flask and made
up to the mark with 0.2 M HNO3. The leavessamples were washed with de-ionized water, dried to constant
weight in an oven at 105 oC, pulverized and 2 mm mesh size obtained
for further analysis.The ground leaves were digested with 1.0 cm3
concentrated HClO4, 5 cm3 concentrated HNO3
and 0.5 cm3 concentrated H2SO4 in 50 cm3
Kjeldahl flask. Each water sample (10 cm3) was digested with 2 cm3
concentrated HNO3.Concentrations of the heavy metals
were determined using AAS Unicam 939 model. The soil samples (150 g) each were
transferred into four (4) plastic
buckets labeled A, B, C and D. Varying concentrations palm bunch ash (PBA) (0.0
g, 50.0 g), Tween 80 (50.0 g) and PBA +
Tween 80 (25.0 g) each were added to A, B, C, and D, where A served as control.
Portions (5 g each) of A, B, C and D were
weighed into standard flasks, 25 cm3 of xylene added and shaken,
NaCl (5 g) was added and left for 72 h. The liquid portion was decanted into a
separatory funnel, corked and shaken. The xylene layer was transferred into 100
cm3 centrifuge tube containing 5 g of Na2SO4and agitated for 15 min, the absorbance of the solution was measured at
425 nm and used for calculating concentrations of TPHs. Concentrations of TPHs
were determined at 20 days intervals for 60 days. The data were analyzed on the
basis of first order kinetic model InC = InCo– kt. Heavy metal concentrations
(mg kg-1) during dry season
were, soil: Fe (15.15 ± 5.91), Mn (10.36 ±3.18), Cd (0.23±0.31 ), V (0.17 ±
0.29), Ni (0.19 ± 0.05), leaves of Telfairia
occidentalis: Mn (7.73 ± 3.06), Fe (5.93±1.28), V (0.16±0.26), Cd (0.21 ± 0.16),
Ni (0.02 ± 0.01), sediment: Fe (22.18 ± 14.82), Mn (9.67±2.75), V
(3.39±3.30), Ni (2.18±0.78), Cd (0.48 ± 0.75),
and water: Mn (2.80±0.93), V (1.53±1.42), Ni (1.50 ± 1.53), Fe (0.86 ± 0.25),
Cd (0.27±0.21), During wet season, soil:
Fe (12.09±4.98), Mn (9.66 ± 2.18), Ni (0.05±0.03), V (0.04±0.01), Cd
(0.04±0.02); leaves of Telfairia
occidentalis: Mn (7.75±3.76), Fe (5.96±4.07), V (0.21±0.09), Cd
(0.19±0.06), Ni (0.03±0.06), sediment: Fe (23.28±0.24), Mn (9.45±2.63), V
(3.31±3.34), Ni (1.94±1.48), Cd (0.48±0.74), and water: Mn (3.13 ± 0.79),V
(1.88 ±1.45), Ni (1.45 ±1.04), Fe(1.05 ± 0.25), Cd (0.10 ± 0.13), were obtained.
The correlation coefficients were: V (0.556), Ni (0.376), Cd (-0.043), Pb (0.856),
Mn (0.813), Co (0.255), Zn (- 0.193), Fe (- 0.383), and V (-0.419), Ni (-
0.355), Cd (0.248), Pb (0.745), Mn (0.974), Co (- 0.022), Zn (0.886)
and Fe (-0.384) for dry and wet seasons respectively. The mean concentration of
TPHs in the soil was 14.55±0.01 mg kg1. Degradation efficiencies
obtained were PBA (86.69 %), PBA + Tween 80 (85.63 %), Tween 80 (76.70 %), and
control (5.40 %). The rates of degradation (mg kg‑1 day-1)ranged from 2.70×10-2 to 1.30×10-2; 5.00×10-1 to
2.18×101; 2.49×10-1 to 1.84×10-1 and 4.67×10-1
to 2.09×10-1 for A, B, C and D respectively. k ranged from 2.09 × 10-2
to 2.78 × 10-2, 3.79×10-2 to 5.81×10-2, 2.78×10-2
to 2.09×10-2, 5.13×10-2 to 3.23×10-2 for A, B,
C and D respectively. Concentrations of heavy metals in wet and dry seasons
were variables. The concentrations of all the investigated heavy metals in soil
were within permissible range as recommended by DPR, but higher than the
reference soil samples. Mean concentrations of some of the investigated heavy
metals (Ni, V, Pb, Zn and Co) inleaves of Telfairia occedentalis were
within the normal range of WHO and FME standards for vegetables and food stuff
except Cd, Fe and Mn. The concentrations of Ni, V, Cd, Pb, and Mn in water were
higher than WHO and DPR standards. Also, the concentrations of Mn, Ni, Pb, and
Zn in sediment were higher in dry season compared to wet season except Fe, V
and Co. Concentrations of Fe were the highest
in all the seasons; sediment retained the highest concentrations of heavy
metals. Telfairia occidentalis can be used as a resident indigenous
plant bio indicator for monitoring anthropogenic influenced V, Pb, Mn and Zn in
the soil of the study area. Degradation efficiency of TPHs were as follows: PBA
(86.69 %) > PBA + tween 80 (85.63 %) > tween 80 (76.70 %) > control
(5.40 %). The rate of degradation of TPHs decreased as the concentrations of
the surfactants decreased with time.
CHAPTER ONE
INTRODUCTION
Metal pollutants have been a part of human history since the dawn of civilization. However, toxic metals pollution of the biosphere has intensified rapidly since the onset of the industrial revolution, posing major environmental and health problems1. Recently, environmental scientists have raised concern on the increasing ecological and toxicological problems arising from pollution of the environment. Heavy metals represent an important source of the pollution 2. Heavy metals like As, Pb, Hg, Cd, Co, Cu, Ni, Zn, and Cr are phyto-toxic at all concentrations or above certain threshold levels3. Toxic metals are biologically magnified through the food chain. They infect the environment by affecting soil properties, its fertility, biomass, crops yield and human health3.
Heavy metals occur naturally in small quantities in soil though rarely at toxic level, but human activities have raised these to exceptionally high levels at many polluted land and water sites. Soil is a crucial component of rural and urban environments, and in both places, land management is the key to soil quality4. Human endeavours such as technology, industrialization, agriculture, transportation, education, construction, trade, commerce, as well as nutrition have rendered the whole environmental system a “throwing society”. This is true because indiscriminate disposal of wastewater coupled with increasing world population and urbanization have combined to worsen the situation. The use of synthetic products e.g. (pesticides, paints, batteries, industrial waste, and land application of industrial and domestic sludge) can result in heavy metal contamination of urban and agricultural soils.4
The extent of soil pollution by heavy metals and metal base ions, some of which are soil micronutrients is very alarming. Ademoroti 5, reported positive linear correlation between cadmium, lead, and nickel contents in the soil and vegetable.
Essein et al. 4, observed the trend of mean heavy metals concentrations in Mkpanak a community in the study area as Fe > Zn > Pb > Ni > V > Cd. The mean concentration of iron in the soil was quite high and exceeded the critical toxicity level. The result obtained also showed that the mean concentration of Cd was high and exceeded the lower limit of 0.01 mg kg-1. Also, Osuji et al.6, had earlier reported possible bio-magnification of Ni, V, Pb, Cu and Cd in the area. Industrial wastes are the major sources of soil pollution and originate from mining industries, chemical industries, metal processing and petroleum industries; the wastes include a variety of chemicals like heavy metals.6
While many heavy metals are essential elements at low levels, they can exert toxic effects at concentration higher. Soil receives heavy metals coming from different sources and at the same time acts as a buffer, which controls the movement of these heavy metals to other natural components2.
Increase in anthropogenic activities, heavy metals pollution of soil, water and atmosphere represent a growing environmental problem affecting food quality and human health 7 in the Niger Delta region of Nigeria. Nigeria as a major producer and exporter of crude petroleum oil continue to experience oil spill and this exposes the environment to hazards and its effects on agricultural lands as well as on plant growth8. Oil pollution of soil leads to the buildup of essential (Organic carbon, P, Ca, Cu) and non-essential (Mn, Pb, Zn, Fe, Co, Cu) elements in soil and the eventual translocation in plant tissues9. Industrialization coupled with an ever-increasing demand for petrochemicals have resulted in prospecting for more oil wells with consequent pollution of the environment. Causes of oil pollution in Nigeria include discharge from sludge, production test, drilling mud, and spills from pipelines, well blowouts, gas flaring and sabotage10. Oil spills have long effects on soil; an immediate effect of petroleum products in the soil is a depression in population of soil microorganisms. Besides the economic and aesthetic damages caused by oil spills, plants and animals life in both aquatic and terrestrial environment are affected as most life form die rapidly following spillage. Many unique plants and animals’ species have gone into extinction in the Niger Delta regions11.
Pollution of the ecosystem by toxic metals during man’s activities poses serious concerns because heavy metals are not biodegradable and are persistent in the ecosystem. Once metals are introduced and contaminate the environment, they will remain for a very long time.11
The presence of heavy metals in toxic concentrations can result in the formation of super oxide radicals, hydrogen peroxide (H2O2), hydroxide radicals (OH–), bio-molecules like lipids, protein and nucleic acid. Chromium, Copper and Zinc can induce the activity of various antioxidant enzymes and non-enzymes like ascorbate and glutathione3. Petroleum renders the soil infertile, burns vegetation and kills useful soil organism12.
In Nigeria, a study of heavy metals concentration near Warri refinery found three to seven times elevated levels of various heavy metals in the soil13. Although the petroleum industry is by far the largest industrial sub-sector in the Niger Delta, at least eight of the most polluting sub-sectors in Nigeria (steel work, metal fabrication, food processing, textile, refineries and paints manufacturing) operate in the Niger Delta13, 14.
Oil exploration and exploitation have uplifted Nigerian economy leading to rapid development but the impact on the environment is receiving less attention15. One of the major anthropogenic sources of heavy metals enrichment in terrestrial habitats of oil producing area of Nigeria is the frequent spills of crude oil on land and gas flaring 12. Nigerian crude oil is known to contain heavy metals such as Zn, As, Ba, Fe, Pb, Co, Cu, Cr, Ga, Mn, Ni and V.
