REMOVAL OF LEAD AND CADMIUM IONS FROM SYNTHETICALLY CONTAMINATED WATER USING MORINGA OLEIFERA AND M. STENOPETALA SEED POWDERS

Abstract

The discharge of heavy metals into aquatic ecosystems has become a matter of concern in Malawi recently. These pollutants are introduced into the aquatic systems such as rivers and streams as a result of various industrial operations, agricultural and domestic activities. The pollutants of concern include lead and cadmium, which may be derived from sludge disposal, vehicle emissions, paints, batteries, fertilisers, pesticides or preservatives. A recent study of streams and rivers in the city of Blantyre, Malawi revealed lead and cadmium levels exceeding the World Health Organisation acceptable levels for drinking water. This study was carried out to investigate the possibility of using Moringa oleifera and Moringa stenopetala seed powders for lead and cadmium ion removal from aqueous solutions by means of batch experiments using jar tests. The following parameters, pH, stirring time, initial metal ion concentration, ionic strength, water hardness and temperature were investigated for lead and cadmium ion removal from aqueous solutions. Equilibrium sorption capacities were evaluated by fitting the sorption data to Langmuir, Freundlich and Dubinin Radushkevich models. The residual metal ion concentrations were determined by atomic absorption spectroscopy.M. oleifera and M. stenopetala whole seed powders (at a dose of 2.5 g/100 mL) reduced the concentrations of lead ions from aqueous solutions by 78 and 96% respectively. The cadmium ion removals were 53 and 54 % respectively. Generally M. stenopetala was more effective than M. oleifera in the removal of these cations. Percentage removal increased with an increase in pH and optimum sorption for lead ions was obtained at pH ≥ 3 and that for cadmium at pH ≥ 5. Lead removal increased with time until equilibrium was reached at 4 h for both powders. However, cadmium removal decreased with stirring time. Results for lead sorption at 30oC were better fitted to Lagergren first order equation and the k value was 2.40 x 10-4 /s for both seed powders. For cadmium it was mostly desorption taking place with time and hence the zero order rate constants obtained were negative. Pb2+ and Cd2+ uptake increased with a raise in initial metal ion concentration and decreased with an increase in ionic strength. Magnesium/calcium water hardness did not show any general trend in percentage viremoval for lead ion sorption. A high decrease in percentage removal was observed for cadmium sorption. This was ascribed to the competition between magnesium/calcium ions and cadmium ions for binding sites. Carbonates/bicarbonates hardness enhanced metal ion sorption for both metal ions. This was attributed to the increase in pH and formation of insoluble metal carbonates. Temperature increase enhanced lead ion sorption and reduced cadmium ion sorption for both powders. Equilibrium studies using whole seed powders showed that lead ion sorption followed the Langmuir sorption isotherm better than Freundlich isotherms and cadmium sorption followed both Langmuir and Freundlich sorption models. Using the Dubinin-Radushkevich sorption model the energies of adsorption for lead ion sorption were 20.41 and 11.95 kJ/mol for M. oleifera and M. stenopetala respectively. The adsorption energies for cadmium were 3.68 and 4.58 kJ/mol for M. oleifera and M. stenopetala treatments respectively. Thus, lead ion sorption involved chemisorption, while cadmium ion sorption involved physisorption. Desorption studies showed that metal ions could be desorbed to varying extents from metal loaded powders and the optimum nitric acid concentration for metal desorption was 0.06 mol/L.