Physico-electrochemical assessment of pollutants in Ngong river

Abstract

In this thesis, the quality of water in Nairobi River is assessed for pollutants. Heavy metals assessed include lead, copper, cadmium, zinc, iron, nickel and chromium. Biochemical analyses of the water which entailed assessment of dissolved oxygen (through BOD and COD) and coliforms was also undertaken. The main method used to assess these pollutants was cyclic voltammetry, a versatile electrochemical method. In .. . this electrochemical technique, surface modified electrodes were used as the working electrodes. Other complimenting techniques used were Atomic Absorption Spectrometry, X-Ray Spectroscopy and titrimetric methods. Physical parameters assessed included density, viscosity, transmittance, pH, temperature, surface tension and conductance. The electrochemical method was also used to assess adsorption capacities of the heavy metals on bentonite, zeolite, kisii soapstone, carbon, and amberlite whereby the latter were used as host matrices in surface modified electrodes. Those host matrices had been allowed contact with the metal ions to allow adsorption prior to use in the modification of the working electrode. These adsorbates which served as host matrices in the surface modified electrodes were also placed in a unit and its capacity for water purification assessed. Physical appearance of the water from Nairobi River indicated high levels of pollution as the water in most sampling points appeared brackish (transmittance values of upto 84%) and had a foul smell. Viscosity, surface tension and conductivity values recorded were a lot higher than those of pure water, as average values of 0.9 centi poise, 74dynes/cm3 and 0.5milli Siemens respectively were recorded. The pH values recorded averaged about 7.4 forthe first, second, fourth, fifth and sixth sampling points. The third sampling point gave high values of about 8.4, suggesting alkaline inputs into the water system from the environment. Tempenrture of the water increased from 18.2°C at sampling point number oneto 21.3°C at sampling point number six. Heavy metal analysis using Electrochemical methods and AAS indicated varying profiles across the river, with average values for lead, zinc, nickel, iron, manganese, cadmium, chromium and copper recorded being approximately 0.05, 0.60, 0.03, 3.00, 2.50, 0.02, 0.02 and ,0.05 parts per million respectively. Most of these values are higher than those allowed by the World Health Organisation for potable water. Biochemical assessment indicated high levels of pollution with BOD, COD and Coliform values averageing 250 mg/l, 150mg/1 and 600,000 mpn respectively. These values do not comply with the requirements of World Health Organization. In the electrochemical experiments, calibration curve~ were obtai~ed for the vanous metals, and cyclic voltammograms for the various samples obtained using bentonitemodified electrodes. These indicated the various electro active materials present in the samples. In the experiments with different host matrices, the matrices adsorbed the heavy metals with the minimum amount of metal ion adsorbed recorded as 80%, and the maximum being 97%. A new method of getting the cationic exchange capacity (CEC) of an ion exchange matrix using solely electrochemical methods was developed. These host matrices were used in a purification unit modelled after the zero B tap Attachment (stainless' steel) ion exchange (India) template. Volume of solution of known metal ion concentration flowing into .the unit was controlled (flow rate) as different matrices were placed in the unit. The metal ion concentration remaining in the solution was then determined by spectrophotometric methods. Generally, the amount adsorbed decreased with increase in flowrate.