Studies on the levels of heavy metals in clams, mussels and sediments from the lower Volta River in Alabonu using neutron activation analysis

The concentration of 22 toxic and potentially toxic elements (i.e. Al, As, Br, Ca, Cd, Cl, Co, Cr, Fe, Hg, La, K, Mg, Mn, Ni, Sb, Sc, Sr, V and Zn) were studied in sediments and two bivalve species Galatea paradoxa and Aspataria wahlbergi. The study was conducted in the lower Volta River at Alabonu, a small farming village in the Volta Region of Ghana. Samples were analysed using Instrumental Neutron Activation Analysis (INAA) and Preconcentration Neutron Activation Analysis (PNAA). The elements in the detrical sediments of the river had average concentrations (in μg/g unless otherwise stated), in decreasing order of 2.25±0.21%(Fe), 1.67±0.04%(Mn), 0.84±0.29%(Ca), 0.44±0.07%(Na), 0.30±0.03%(K), 0.24±0.04%(Mg), 0.18±0.01%(Al), 92.55±13.88(Sr), 37.19±5.58(Cr), 36.51±6.22(V), 29.82±4.48(Ni), 12.67±1.35(Cu), 12.56±1.88(La), 12.27±1.84(Zn), 10.25±2.22(Co), 7.54±1.13(Br), 2.48±0.16(Sc), 1.16±0.27(As), and 0.75±0.11(Sb), Hg, Cd and Cl were below detection limit. The average concentrations of these elements in Galatea paradoxa (in μg/g unless otherwise indicated) in decreasing order were Fe(0.94±0.26%) > Ca(0.44±0.04%) > K(0.27±0.04%) > Mg(0.16±0.03%) > Cl(0.13±0.02%) > Na(0.12±0.001%) > Mn(491.18±7.53μg/g) > Zn(92.29±13.84μg/g) > Cu(56.42±11.20μg/g) > Al(54.93±2.69μg/g) > Sr(35.92±5.39μg/g) > Br(22.35±3.35μg/g) > La(5.33±1.02μg/g) > As(3.67±0.54μg/g) > V(2.87±0.52μg/g) > Cr(1.62±0.25 μg/g) > Sc(0.72± 0.16μg/g) > Sb(0.066±0.01μg/g) using INAA. The concentration of Cd and Hg were below detection limits of INAA. The average concentration of some toxic and potentially toxic elements in the Aspataria wahlbergi tissue were Mg(4.3±0.66%)>Mn(4.3±0.66%)>Cl(0.43±0.07μg/g)>Ca(31.64±1.79%)>Fe(0.33±0.003 %) >K(0.12±0.04%)>Na(0.08±0.01%)>Al(217.45±11.55 μg/g)>Br(37.98±5.69μg/g)>La(19.07±2.86μg/g)>Zn(14.75±2.21μg/g)>Sr(10.93.18± 1.63μg/g)>As(2.72±0.41μg/g) > Co(2.0±0.39μg/g) > Cr(1.28±0.19μg/g) > Sc(0.77±0.11μg/g) > Cu(0.48±0.07μg/g) > Sb(0.05±0.008μg/g). Ni, Cd, Hg and V were below the detection limits of INAA. The determination of Cd and Hg by PNAA using APDC in chloroform was studied. APDC was used as a complexing agent. Experimental conditions, such as pH of the solution, were optimized. The Cd(PDC)2 and Hg(PDC)2 complexes were very stable at pH < 7.0 and could be quantitatively chelated in this pH range than pH > 7.0. The concentrations of Cd and Hg obtained in Galatea paradoxa (at pH < 7.0) were 0.27±0.04 μg/g and 0.08±0.007 μg/g respectively whiles that in Aspataria wahlbergi were 0.55±0.13 μg/g and 0.07±0.006 μg/g respectively using PNAA. The biosediment accumulation factor studies revealed the following decreasing order; Mg>Br>Mn>As>La>Zn>K>Ca>Sc>Co>Na>Fe>Al>Sr>Sb>Cr, Cu (in Aspataria wahlbergi) and Zn>Cu>As>Br>K>Mg>Ca>Fe>Sr>Sc>La, Na>V, Sb>Cr>Mn, Al>Co (in Galatea paradoxa). Given the relatively high levels of metals in the sedimentary habitat of Galatea paradoxa and Aspataria wahlbergi, the river sediments are likely to be the main source of toxic and potentially toxic elements to these organisms either directly or indirectly following desorption.