Heavy Metal Concentrations And Degradation Efficiency Of Total Petroleum Hydrocarbons On Environment In Ibeno Local Government Area, Akwa Ibom State, Nigeria

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 leaves samples 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

Na2SO4 and 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) in leaves 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.