An increase in population initiating rapid industrialization has been found to consequently increase the effluents and domestic wastewater into the aquatic system. This has resulted in heavy metal pollution which has become one of the most serious environmental problems today. Similarly, excessive nutrients have an effect of increasing the accumulation of organic matter through biosynthesis. Therefore, there is need to assess the impact of these agricultural activities and urban runoff on heavy metal pollution and nutrient levels of the river. Since conventional methods for the removal of heavy metals from wastewater are often cost prohibitive, a search for cheap methods of removal of heavy metals has been on the increase and one such method is use of algae. The ability of algae to adsorb heavy metal ions (Ni2+, Pb2+, Zn2+ and Cu2+) by biosorption has been investigated in river Migori. Factors influencing the biosorption of the heavy metals, such as contact time at constant pH and temperature, were studied. Six sampling points, during wet and dry seasons, were established along the river, upstream and downstream of Migori town. The metal ion concentrations were determined at 15 minutes intervals using spectrophotometer. The results showed that Cu2+ varied between 0.02 and 0.29 mg/L in wet season and between 0.07 and 0.26 mg/L in dry season. Zn2+ ranged between 0.10 and 0.28 mg/L in wet season and between 0.09 and 0.22 mg/L in dry season. Lead ions and Ni2+ ranged between 0.00 to 0.07 mg/L and 0.02 to 0.08 mg/L, respectively in wet season, while in dry season their respective range was between 0.00 to 0.04 mg/L and 0.02 and 0.06 mg/L. The results obtained after biosorption showed that Cu2+ ions were most biosorbed (89%) while Pb2+ were least biosorbed (70%), and brown algae achieved the percent removal in the order of Cu2+ > Ni2+ > Zn2+ > Pb2+. The equilibrium biosorption data were analyzed using two isotherm models (Langmuir and Freundlich). The results indicate that Langmuir model provide best correlation of experimental data. From the results, brown algae have a potential for being used for decontamination of waters containing heavy metal ions. Samples were also analysed for nutrients (nitrates and phosphates), chemical oxygen demand (COD) and biochemical oxygen demand (BOD). Phosphates and NO3- were both determined by spectrophotometer while BOD was analyzed using Winkler method and COD by dichromate digestion method. Phosphates and NO3- ranged between 0.6 to 2.6 mg/L and 0.6 to 2.3, respectively. The respective BOD and COD varied between 7-25 mg/L and 1450 mg/L. Although the nutrient levels were within WHO standards, the river seemed contaminated by runoff from the town since the levels were higher downstream than upstream of town. The COD and BOD levels were higher than the WHO standards, but the levels were higher downstream of the town.



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