Speciation of heavy metals in plants, water and sediments along Msimbazi river in Dar es Salaam, Tanzania

This dissertation reports on the speciation of five heavy metals: Pb, Zn, Cu, Cr and Cd in plants, water and sediments along Msimbazi River in Dar es Salaam, Tanzania. The concentrations of the different forms of the selected heavy metals were determined using Atomic Absorption Spectrophotometric (AAS) method. All measurements were carried out in triplicate. Digestion procedures were used to obtain four forms of the five heavy metal species in both water and sediments and two forms of the five heavy metals in the plant samples. In terms of heavy metal speciation forms in water samples, the results indicated that the heavy metal contents (mg/L) in the form of total labile trace metals ranged from 0.31 to 0.70 for Pb, 0.04 to 0.41 for Zn, 0.19 to 0.29 for Cu, 0.005 to 0.098 for Cr and 0.0026 to 0.0073 for Cd. In the form of total trace metals, the heavy metal contents (mg/L) ranged from 0.65 to 1.101 for Pb, 0.06 to 0.54 for Zn, 0.22 to 0.36 for Cu, 0.008 to 0.101 for Cr and 0.004 to 0.014 for Cd. The heavy metal contents (mg/L) in the form of soluble trace metals ranged from 0.03 to 0.49 for Pb, Below Detection Limit (BDL) to 0.26 for Zn, BDL to 0.03 for Cu, BDL to 0.012 for Cr and 0.0006 to 0.0024 for Cd. In the form of labile trace metals, the heavy metal contents (mg/L) ranged from 0.16 to 0.60 for Pb, 0.02 to 0.32 for Zn, 0.02 to 0.17 for Cu, 0.003 to 0.025 for Cr and 0.0013 to 0.0048 for cadmium. At the same time, the results of heavy metal speciation forms in sediment samples showed that the heavy metal contents (mg/kg) in the form of total trace metals ranged from 50.34 to 161.63 for Pb, 68.38 to 501.85 for Zn, 18.43 to 66.61 for Cu, 20.93 to 76.46 for Cr and BDL to 0.23 for Cd. In exchangeable trace metals, the heavy metal contents (mg/kg) ranged from 5.04 to 5.79 for Pb, 2.72 to 4.37 for Zn, 0.15 to 0.46 for Cu, 0.19 to 1.02 and Cd level were below detection limit. The contents (mg/kg) of heavy metals in the form of metals bound to organic matter ranged from 18.24 to 46.58 for Pb, 27.64 to 228.62 for Zn, 3.55 to 16.76 for Cu, 1.37 to 4.10 for Cr and Cd levels were below detection limit. In the form of residual trace metals, the heavy metal contents ranged from 9.65 to 53.80 for Pb, 21.69 to 204.14 for Zn, 9.98 to 67.47 for Cu, 19.03 to 86.44 for Cr and BDL to 0.09 for Cd. Likewise, in plant samples, the results indicated that the heavy metal contents (mg/kg) in the form of total labile heavy metals ranged from 0.86 to 1.46 for Pb, 2.68 to 4.59 for Zn, BDL to 0.41 for copper, BDL to 0.21 for Cr and levels of Cd were below detection limit. In the form of total heavy metals, the contents of heavy metals (mg/kg) ranged from 19.62 to 36.68 for Pb, 33.56 to 68.80 for Zn, 3.33 to 13.06 for Cu and the levels of both Cr and Cd were below detection limits. The levels of lead in water samples were found to be above the permissible levels recommended by WHO and TBS while the levels of most forms of Zn, Cu, Cr, and Cd were below the permissible levels except in a few cases for cadmium and chromium the levels were above the permissible levels. The levels of lead in plant samples from all sites along the river were found to be above the permissible levels recommended by both WHO and TBS while the levels for the remaining heavy metals were found to be below WHO and TBS levels except for zinc at Kajima which exhibited the level 0f 68.80 mg/kg. It is concluded, therefore, that the Msimbazi River is potentially hazardous to many living organisms since it contains high concentrations of lead in water, sediment and plants grown alongside the river.