Speciation of heavy metals in a tributary of Nairobi river using the joint expert speciation system (JESS) program

Abstract

his study assessed the quality of water in a tributary of the Nairobi River. The study covered six sampling points stretching from Lenana dam to Chiromo Kirichwa Kubwa. Sampling was done twice during the dry and the wet season between July and December 2 013. Physicochemical characteristics were analyzed using standard procedures, concentrations of some heavy metals using AAS and speciation analysis using the Joint Expert Speciation System (JESS).The range observed for pH, temperature, DO, COD, BOD, TSS, e lectrical conductivity, TDS, phosphate, sulphate, chloride, fluoride and nitrite was 6.89 - 7.77, 18.3 - 25.0 o C, 8.10 - 24.29 mg/L, 40 - 730 mg/L,10.93 - 25.26 mg/L, 131 - 414 mg/L, 348 - 881μS/cm 176 - 438 mg/L,0.03 - 5.96 mg/L, 0 - 68 mg/L, 0.0142 - 0.886 mg/L, 0.39 - 2.87 mg/L and 0.002 - 0.585 mg/L respectively. The range observed for heavy metals was 0.0974 - 3. 1438 mg/L, 0.0092 - 3.6843 mg/L, < 0.01 - 0. 1799 mg/L, <0.005 - 0.0197 mg/L, < 0.05 - 0.4415 mg/L and < 0.02 - 0.0846 mg/L for Fe, Mn, Cu, Zn, Pb and Cr respectively. Speciation an alysis indicated that 69.8% of total iron was in oxidation state III, and the dominant species were free Fe 2+ ions, sulphates, hydroxides, fluorides, and hydroxide - sulphate complexes. Manganese was observed to be in oxidation state II, and its major chemica l forms were Mn 2+ ions, fluoride and sulphate complexes. Copper was found mostly (>98%) in oxidation state II, and Cu 2+ ions, sulphate and hydroxide complexes were the major species. Zinc and l ead were divalent, and sulphates were their major species. Chrom ium was trivalent with hydroxide - sulphate and hydroxide complexes being the predominant chemical forms. The results show th at the concentrations of Zn, Cu, SO 4 2 - , Cl - , NO 2 - and TDS w ere below the KEBS limits . However, COD , EC, TSS, PO 4 3 - , F - Pb , Cr, Fe and Mn were above the KEBS limits for natural water in some sampling points . Cu, Fe and Mn were found to pose health risk since traces of their free ions were obtained in some sampling points. It was generally found that speciation varied with the total conce ntration of the species and the pH of the system