NATIVE AND MODIFIED STARCHES OF AFRICAN YAM BEANS (Sphenostylissternocarpa) AND COCOYAM (Xanthosomasagittifollium) AS ADOSORBENTS FOR COPPER REMOVAL FROM AQUEOUS SOLUTION

Abstract

Native African yam beans (NAY) (Sphenostylisstenocarpa) and Native Cocoyam, (NCY) (Xanthosomasagittifollium) starches were isolated and subjected to chemical modifications using NaOCland acetic anhydride to obtain oxidized (OAY,OCY) and acetylated (AAY,ACY) starches. The native African yam bean, Cocoyam and various modified starches were characterized in terms of pasting properties, functional properties. African yam bean, Cocoyam and their modified starches were investigated for their as adsorbent ability for the removal of copper (II) ions from aqueous solutions. Results of pasting property showed that peak viscosity ranged between 157.3 - 399.6 BU with NAY having the highest value and ACY having the least value. The set back value ranges from 33.5 – 94.0BU with NAY having the highest value and ACY having least value. The trough value ranged from 90.1 -206.5 with NAY beans having highest value and ACY having least value. The peak time value ranged from 4.4 -4.8 mins with NAY having the highest value and OAY having the least value. The functional properties(%) of samples ranged between 118.7 and 339.3% for water absorption capacity (WAC) with ACY having highest value and NCY having least value. The oil absorption capacity (OAC) ranged between 119.3 and 178.7% with ACY having highest value and NCY having least value. The swelling power ranged between 7.3 and 27.4 with NCY having highest value and ACY least value. Bulk density ranged between 0.6 and 0.7 with OAY having highest value and OAY having least value. The least gelation capacity ranged between 6-10%. The batch studies conducted for the adsorption of Cu (II) ions on modified and unmodified starches showed that the optimum pH and time for the removal of copper ion was 7 and 2 hours respectively for the native and modified starches. Increase in initial adsorbate concentration decreased the percentage adsorption from 91.5 to 29.1%, 95.8 to 31.7%, and 99.9 to 5.6% for NCY, ACY and OCY respectively, and also 99.0 to 29.4%, 99.4 to 30.0% and 99.8 to 34.0% for NAY,AAY and OAY respectively. The effect of adsorbent dosage on the adsorption of metals showed that the percentage of metal removed increased with increase in adsorbent dose. Adsorption data for wide ranges of adsorbate concentration were treated using Langmuir and Freundlich isotherms; Metal adsorption of native and modified African yam beans were fitted into Lagmuir adsorption isotherm with correlation coefficient ranged between 0.8121 and 0.8885 with AAY having the highest value and OCY having the least value. Metal adsorption of native and modified cocoyamwerewell fitted into freundlich adsorption isotherm with correlation coefficient ranged between 0.81 and 0.91 with NCY having the highest value and OCY having the least value. Considering the kinetic model of the adsorbents using estimated correlation coefficient associated to the Lagergren pseudo first order and second order kinetic model, metal adsorption of cocoyam followed Pseudo second order kinetic model with correlation coefficient ranged between 0.98-0.99 with OCY having the highest value and ACY having the least value. The adsorption of African yam beans also followed Pseudo second order Results of Pseudo second order with correlation coefficient ranged between 0.85 -0.99 with OCY having the highest value and NAY having the least value.