Remediation of lead, cadmium and copper polluted waters by onion skins (Allium Cepa)
Waweru, Beatrice W.
Mwangi, Isaac W.
Wanjau, Ruth N.
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The chemical structure of water makes it have special properties due to lopsided electrical charge that enable water to be a solvent for ionic compounds. That dissolving power is very important for life and has contributed to 97 % of the world’s water being salty. As water interacts with the environmental matrix, it dissolves ionic material at different concentrations which may render this vital commodity polluted with dissolved matter of both organic and inorganic origin. Presence of heavy metals in water is a worldwide environmental concern due to pollution. Therefore, there is need to find a sustainable and affordable method to remove such pollutants from water to render it safe. This study investigated the sorption efficiency and mechanism of onionskin in both their raw and modified forms on the removal of lead, copper and cadmium in aqueous media. The modification of the study material was done by anchoring ethylene-1, 2-diamine (EDA) within the structure of the onionskins. This was anticipated to improve the quality of the adsorbent material for application on the removal process. Sorption parameters were optimized and the respective optimal values applied. Experimental results confirmed that best sorption pH range for the metals was ranging between 4.0 -6.0 which is within the physiological pH of water. It was also realized that the sorbent material could remove 90 % of the metals within the first 20 minutes of contact. The experimental data was analysed using Freundlich and Langmuir adsorption isotherms and found out that the three metal ions fitted best in Langmuir model. However, the unmodified onionskin leached soluble organic substances in the form of colour and smell thus causing a secondary pollution. This was overcome by modification of the material. The sorption capacities for lead, cadmium and copper were found to be 71.85, 68.03 and 79.36 mg g-1 in the modified sorbent material respectively and 87.49, 96.99 and 90.8 mg g-1 in the same order for the unmodified form. Despite the sorption capacity not improving, the effect of secondary pollution was minimized from 15.1 mg L-1 to 7.9 mg L-1 thus improving the quality of the sorbent material. The resulting material was then applied on environmental water samples whose concentration was determined and thereafter spiked with lead, cadmium and copper ions of varying concentration. The maximum percent recoveries were determined as 87.8±0.01%, 80.6±0.01% and 77.1±% for lead, cadmium and copper ions respectively. This indicated that the material which initially had no useful purpose with disposal problems has potential application for remediation of metal laden waters