Sequestering Of Selected Heavy Metal Ions In Wastewater From Industrial Area In Nairobi Using Water Hyacinth As A Low Cost Adsorbent

Abstract

The adsorption of heavy metals from wastewater and aqueous solution by water hyacinth powder was investigated under batch experiments. The factors such as pH, contact time, particle size and adsorbent dose that influence adsorption efficiency were evaluated. The water hyacinth plant was collected from a point on Lake Victoria, Kisumu County. The stems were washed to remove any dirt and air dried until sufficiently dry and then dried in an oven at 110 0C overnight to remove all the moisture. The dried stems were ground using pestle and mortar and sieved through standardized sieves of 300-2800μm to obtain the powder for the adsorption studies. The heavy metals in the wastewater samples were determined using Atomic Absorption Spectrophotometer (Model AA-6300 Shimadzu AAS). The levels of heavy metals in wastewater were in the range of: 1.2-75.3 ppm for lead, 0.4-87.6 ppm for chromium, 0.1-63.5 ppm for nickel, 0.5-95.5 ppm for zinc and 0.8-52.7 ppm for cadmium. The levels of zinc, lead and cadmium were above the limits set by the National Environment Management Authority (NEMA) for discharge into the environment (0.01 ppm for cadmium and lead, 0.5 ppm for zinc). The study showed that the adsorption efficiency of water hyacinth powder was higher in aqueous solution than in wastewater while at low metal concentrations (0.1-3.2 ppm), the adsorption efficiency was 100% in both wastewater and aqueous solution. The effect of particle size on adsorption showed that adsorption efficiency increased with decrease in particle size and followed the order: >425<2800, >300<425 and <300 μm for each of the concentrations. The adsorption also increased with pH however the optimum adsorption varied with the metal. The pH range for adsorption of Pb, Cr and Cd was 2-5 and 2-6 for Ni and Zn after which a plateau was reached and then the adsorption efficiency dropped. The adsorption also increased with contact time until a steady state was attained however the optimum adsorption time varied with the metal. The study revealed that optimum time for adsorption of Pb was 30 minutes, 40 minutes for Cr, 80 minutes for Ni and 60 minutes for both Cd and Zn. The adsorbent dosage increased until an optimum adsorption was obtained and it was observed that adsorbent dosage for Pb and Zn was 2.5g; Cd was 3.0g; Cr was 4.0g and 4.5g for Ni. The adsorption isotherm according to Langmuir model fitted well the adsorption of Zn 2+, Cd2+, Ni2+ and Pb2+ in aqueous solution while the adsorption of Cr3+ fitted well the Freundlich model. Assessment of kinetics studies showed that the removal of Cd2+, Ni2+, Pb2+, Zn 2+ and Cr3+ followed pseudo first order rate equations according to R2 values. The characterization of water hyacinth powder by Fourier Transform Infrared Spectroscopy (FTIR) showed that the adsorption of heavy metals could be attributed to the existence of functional groups such as carboxyl, hydroxyl and carbonyl in the water hyacinth fibre. The study showed that water hyacinth powder is a low cost adsorbent which could be used to remove heavy metals from wastewater and aqueous solution.