Chitin and chitosan nanomaterials for efficient removal of heavy metals from waste water
Thesis (DSc)--Stellenbosch University, 2019.
ENGLISH ABSTRACT: Heavy metal contamination is a serious problem that is responsible for water pollution that can cause serious health issues to animals and humans. The problem is being addressed by various researchers through the attempt to manufacture low-cost adsorbents that are friendly to the environment and non-toxic. Several adsorbents show good adsorption capabilities for various type of heavy metal ions. One of these is chitosan, that has been investigated for the removal of lead, chromium, cadmium and mercury amongst other heavy metals. Some modifications have been implemented for chitosan to create a better surface area to improve the affinity for the above mentioned and other metal ions. In this study, chitosan has been modified into various new forms in an attempt to utilise the adsorption sites of this natural polymer effectively. Chitin nanowhiskers, which are (like chitosan) a derivative obtained from chitin, was investigated for the first time, to the best of our knowledge, for the removal of various type of heavy metal cations, while being subjected to varying pH conditions. The removal of hexavalent chromium, nickel, zinc, lead and copper were investigated during this study. Unlike other studies about low-cost adsorbents featuring chitosan that are grafted or crosslinked, which affects the bio-friendly characteristics of the chitosan (CTS) polymer, this project focuses using polymers that are bio-friendly and easy to manufacture. Characterisation of all five biosorbents in this study was done by using DSC, ATR-FTIR, TEM, SEM, TGA and confocal fluorescence microscopy. Some effects were seen in relation to the thermal stability of the biosorbents after the heavy metal ions were adsorbed. A horizontal electrospinning technique was used to synthesise nanofibers containing chitosan, chitosan nanoparticles and chitin nanowhiskers with poly(ethylene-co-vinyl alcohol) as a scafold, respectively. A chitosan sponge was synthesised using poly(ethylene-co-vinyl alcohol) as a matrix and the mechanical strength of this material was tested using a tensile tester. A zetasizer was used to determine the surface charge and behaviour of the chitin-derived material in an aqueous solution with pH varying from 3 to 11. Inductively coupled plasma spectroscopy (ICP-AES) was used to analyse the aqueous solution after each sorption process and to determine the amount of heavy metal ions present in the solution after specific time intervals. The effect of pH, initial sorbate (heavy metal ions) concentration and the contact time were investigated for each of the biosorbents. The initial sorbate concentration were investigated at 2, 5, 10, 20 and where required 50 and 100 mg.L-1 (ppm). The effect of pH was investigated at pH 2, 5 and 11. Samples were taken at time intervals of 10, 30, 60, 80, 100 and 120 minutes. The sorption process was investigated for longer than 2 hours but saturation occurred after 2 hours mainly because of the small amount of biosorbents used during the sorption process, which were 0.002 g.mL-1. Chitosan nanofibers (CTSNF) showed the highest adsorption capacity for all heavy metal ions, followed by chitosan nanoparticles with poly(ethylene-co-vinyl alcohol) nanofibers (CTS-NP/EVOH NF), with the exception of Cr(VI). Langmuir and Freundlich isotherms were used along with Pseudo- First- and Second-Order kinetic models to establish the mechanism of interaction between the metal ion and the biosorbent as well as the adsorption capacity and constants required to understand the behaviour of each biosorbent during the sorption processes. The Langmuirisotherm was favoured by all the biosorbents indicating monolayer adsorption. The theoretical and experimental adsorption capacities corresponded well using Langmuir. The Pseudo-Second-Order kinetic model was favoured for all biosorbents indicating that the sorption process followed a chemisorption mechanism. The theoretical and experimental adsorption capacities also corresponded well using Pseudo-Second-Order kinetic model. Desorption studies were also done, using ethylenediaminetetraacetic acid (EDTA), for each biosorbents at varying initial sorbate concentrations to establish the reusability of the biosorbents. The biosorbents, with the exception of CTS-NF and CTS powder, proved to be economic and reusable after 3 to 5 times. A decrease in adsorption capacity was after the first to 3rd cycle of re-use for CTS and CTS-NF.
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming