Efficacy and kinetics of adsorption of single and multiple heavy metal cations from aqueous solutions by fruit waste products
Nthiga, Esther Wanja
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Removal of toxic heavy metals from water has been a major challenge, especially in rural areas. Various methods have been used for this purpose; among them is biosorption based technology. The technology has been recognized as an economical and eco-friendly method for removal of toxic anions and cations from wastewater. Moreover, the efficacy of biomass in adsorption of cations and anions can be improved by different modifications, including treatment with sulphuric acid. Numerous approaches have been developed for adsorption of single ions in solution. However, toxic metallic or non-metallic ions rarely occur singly in wastewater. The presence of multiple ions in solution may often have agonistic or antagonistic effect on the efficiency of an adsorbent, and there is insufficient information on the efficacy of different methods for removing multiple ions. This study assessed the efficacy and mechanism of acid treated (modified) and raw (unmodified) biosorbents (derived from lemon, sweet yellow passion, banana, watermelon peels, and avocado seeds) for removal of toxic cations (Cd, Pb and Cu) from water. Functional groups of adsorbents were identified by mid-infrared spectroscopy (MIR) and their surface morphology was probed by scanning electron microscopy (SEM). the efficacy of each adsorbent was evaluated by quantifying the kinetics and levels of cations adsorbed at different pH of the solution, initial concentrations, contact time and adsorbent dose. Desorption experiments were conducted to determine the possibility of recovering ions and reusing the sorbents for next cycle of deployment. Experimental data of each metal ion was described by either Freundlich isotherm or Langmuir isotherm. The acid treated fruit peels and avocado seeds recorded higher efficacy as compared to raw adsorbents. Generally, acid treated watermelon peels demonstrated the highest uptake of 130.23 mg/g of Pb (II), followed by 114.234 mg/g of Cu (II) and 97.14 mg/g of Cd (II) ions. Further adsorption trials with binary and ternary metal blends on showed significant reductions in metal uptake capacities of evaluated adsorbents as compared to single metal systems. On account of metal preference, the selectivity order for metal ions towards the all the studied biomass was Pb (II) > Cu (II) > Cd (II). Time-course measurements indicated involvement of pseudo-second-order kinetics in adsorptions. Desorption efficacies were high on acid-treated adsorbents: 99.97 % ofPb(II) ions from acid treated avocado seeds; and 99.79 % of Cu(II) and 99.23 % of Cd (II) from acid treated watermelon peels. The results show good performance of the fruit peels and avocado seeds in adsorbing single and multiple metal ions, and the potential of using such wastes for purifying drinking and cooking water at household level.
- PHD-Chemistry