Heavy Metals in Epiphytic Lichens and Mosses of Oil–Producing Communities of Eket and Ibeno, Akwa Ibom State – Nigeria

Abstract

Epiphytic lichen (Parmelia carperata) and moss (Polytrichum juniperinum, Calymperes erosum and Racopilum africanum) samples were used as bioindicators and bioaccumulators of atmospheric heavy metals deposition in oil–producing host communities of Eket and Ibeno Local Government Areas of Akwa Ibom State – Nigeria. Sampling of lichen and moss species that are found to grow extensively and abundantly on the stems and branches of several plants was performed during September 2014 at 25 sampling location sevenly distributed over the two oil–producing host communities studied. Unwashed, oven dried and homogenized powdered lichen and moss samples were mineralized using wet digestion with 3:1 mixture of concentrated nitric acid and perchloric acid in Teflon beakers on a Gerhardt digestion hot plate. The concentrations of heavy metals and/or trace elements were determined by atomic absorption spectrometry (AAS) equipped with flame and/or graphite furnace systems. The concentration of heavy metals in lichen and moss samples ranged from 0.003 – 0.009 μg g−1 for Cadmium (Cd); 0.006 – 7.654 μg g−1 for Chromium (Cr); 1.120 – 1.999 μg g−1 for Cobalt (Co); 8.954 – 116.760 μg g−1 for Copper; 25.980 – 193.260 μg g−1 for Manganese (Mn); 2.268 – 23.783 μg g−1 for Nickel (Ni); 0.034 – 14.880 μg g−1 for Lead (Pb), and 26.230 – 98.780μg g−1 for Zinc (Zn).The mean concentration of heavy metals in the lichen and moss samples can be arranged in the decreasing order as follows: Mn > Zn > Cu > Ni > Cr > Pb > Co > Cd and the statistical analyses revealed that strong correlations exist between Cu–Pb, Cu–Zn, Pb–Ni and Mn–Zn concentrations. Some of the target heavy metals such as Cd, Cr, Mn, Ni and Zn were accumulated at higher concentrations in mosses compared to lichens from the same sampling location. There is some evidence that different site–specific characteristics affect the spatial distributions patterns and temporal trends of atmospheric deposition of heavy metals in the two oil–producing communities of Eket and Ibeno, Akwa Ibom State – Nigeria. However, a comparison with the previous study conducted in 2004by Ite et al. showed a slightly decreasing trend of atmospheric heavy metal deposition and these results confirmed that air quality has not further deteriorated in the two oil–producing communities studied over the last 10 years.