Efficacy and kinetics of adsorption of single and multiple heavy metal cations from aqueous solutions by fruit waste products
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Date
2016-12
Authors
Nthiga, Esther Wanja
Journal Title
Journal ISSN
Volume Title
Publisher
Kenyatta University
Abstract
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.
Description
A Research Thesis Submitted in Partial Fulfillment of the Requirements for the
Award of the Doctor of Philosophy Degree (Chemistry) in the School of Pure and
Applied Sciences of Kenyatta University. December, 2016