Conclusions
The study successfully functionalised green seaweeds with ethylenediamine. The FT-IR analysis confirmed the presence of the functional groups that acted as the binding sites for adsorption of metals. The presence of carbons from ethylenediamine in the modified adsorbent was confirmed by solid-state CP-MAS 13C NMR. The functionalised material showed an adsorption capacity of 5.27 mg g−1, 2.12 mg g−1 and 2.16 mg g−1 for Cu, Cd and Pb with modified seaweed, respectively, and 3.29 mg g−1, 4.57 mg g−1 and 1.05 mg g−1 with the unmodified adsorbent in the same order. However, adsorption of cadmium did not show improvement on the modified seaweed due to the absence of the carbonyl functional group which became derivatised with ethylenediamine in the modified sorbent. Moreover, modification provided a significant improvement on the sorption for copper and lead ions in water. The copper and cadmium data fitted the Langmuir model well (R2 values for Cu and Cd by the modified material were 0.866 and 0.964, respectively and for the unmodified adsorbent 0.999, and 0.950 in the same order), indicating a monolayer binding mechanism. On the other hand, lead prescribed to the Freundlich model using the modified sorbent (R2 = 0.947). The adsorption of lead obeyed the pseudo-second-order model confirming a multisite interaction, whereas the pseudo-first-order equation provided the best correlation for copper and cadmium adsorption for single-site adsorption. The thermal stability of the modified adsorbent was improved by the modification. The rate of adsorption was very fast as more >90% of the metal was adsorbed in less than 10 min. It was observed that the adsorbent in the SPE column achieved a pre-concentration factor of 20 and could be regenerated by stripping the attached metal ions with dilute nitric acid. Results of this work illustrate the potential application of the SPE technique using the modified seaweed for adsorption of copper, cadmium and lead ions in water. Modification minimised the leaching of organic matter hence alleviating the risk of secondary pollution during water treatment. In general the study offers a solution to addressing the problem of water pollution through treatment by removing metal pollutants. However, more work ought to be done particularly using the sorbent for removal of organics as well as wider range of metals.
Acknowledgements
Isaac W. Mwangi wishes to thank OPCW for financial support, the University of Johannesburg for sponsorship to attend the WATERNET conference, ANEMAS research group at UKZN Westville, School of Chemistry, and his employer Kenyatta University for granting the study leave to undertake a Ph.D. programme. The authors acknowledge the contribution of Prof. Amália Uamusse of Eduado Modlane University for her help with seaweed collection.
References
Agrawal and Sahu, 2006
A. Agrawal, K.K. Sahu