Efficiency and Sorption Capacity of Unmodified and Modified Onion Skins (Allium Cepa) to Adsorb Selected Heavy Metals from Water
Waweru, Beatrice Wanjiku
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The contamination of water by toxic heavy metals is a worldwide environmental problem. Global environmental changes have dramatically increased the environmental “load” of heavy metals. Therefore, there is need to find an economical method to remove pollutants from water. Onion skins are known to contain flavonoids which are polyphenolic. Quercetin is an example of flavonols, which is mainly in onions. Therefore, onion skins can bind metal ions enabling adsorption. This study evaluated the efficiency and capacity of onion skins in their unmodified and modified forms on sorption of lead, copper and cadmium in aqueous media. Modification process was done by anchoring ethylene-1, 2-diamine (EDA) within the structure of the onionskins. It was envisioned that this would improve the quality of this adsorbent material. Sorption study on lead, copper and cadmium ions in parameters such as pH, contact time and dosage were optimised. It was found that the best sorption pH range for the metals was between 4-6; the optimum pH for copper was 5.9 and 4.2 that of cadmium was 6 and 5 for modified and raw sorbent respectively while that of lead was 5 for both modified and raw sorbent. It was found out that the sorbent could remove 90 % of the metals within the first 20 minutes of contact. The data obtained was analysed using Freudlich and Langmuir adsorption isotherms. The three metal ions data fitted well in Langmuir isotherm model with R2= 0.95 for Pb2+, R2=0.89 for Cu2+ and R2=0.92 for Cd2+ by the modified material. For the unmodified material, the R2=0.95, R2=0.94 and R2= 0.95 for lead, copper and cadmium ions respectively. It was found that the unmodified onion skins leached soluble organic substances in the form of colour and aroma thus causing a secondary pollution in the water whereas the modified material did not. The sorption capacity for lead, cadmium and copper were found to be 71.85, 68.03 and 79.36 mg g-1 in the modified sorbent respectively and 87.49, 96.99 and 90.8 mg g-1 in the same order for the unmodified sorbent. This shows that sorption capacity did not improve, but minimized secondary pollution from 15.1 mg L-1 to 7.9 mg L-1. The developed method was then applied on real 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±0.01% for lead, cadmium and copper ions respectively. This indicated that the adsorbent has the potential application for remediation of metal laden waters. As a result, this will reduce the financial burden on medical expense due to consumption of metal laden drinking water.