Abstract
Over the course of the past decade, there has been an increasing interest in the development of nanocomposite materials for purification of water. This is very essential because lots of people in South Africa experience shortage of clean water due to water contamination. Water pollutants such as heavy metals are known to have extensive effect on the environment and its removal from wastewater and industrial waste has become a very important issue. These compounds have been noted to be found in surface water, groundwater, and even in drinking water due to various activities. Lead ions have been identified as causes of illnesses such as blood disorders, kidneys failure, and loss of appetite. Conventional methods including reduction, oxidation, ion exchange, reverse osmosis, membrane filtration and adsorption have been widely used for the removal of lead ions from aqueous solutions. However, limitations such as operational cost, production of larger volume of rejected residuals, low adsorption capacities hindered their application and make them unsuitable for lead removal. As a result, many researchers have focused on adsorption using green synthesized adsorbents. In this work magnetic composite based on carbon nanotubes (CNTs) was proposed. CNTs have a relatively large surface area which makes them the most effective option for organic and inorganic contaminants removal from water. They can also be used as supports for adsorption materials. The gold-iron oxide nanoparticles were synthesized by reacting dried green tea leaves extract with iron chloride (FeCl2) and gold (III) chloride (HAuCl4) precursors while CNTs were commercially obtained and functionalized with a mixture of concentrated sulphuric and nitric acids under ultra-sonication for 3 hours to remove residual metal impurities from the tubes and produce oxidized carbon nanotubes (CNT-COOH). The adsorbents were then cross-linked to form a composite using formaldehyde. The adsorbents were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to assess their surface morphology, Fourier transform infrared (FTIR) spectroscopy to identify the functional groups present, X-ray diffraction (XRD) to ascertain the crystallographic structure of the green adsorbent and Raman spectroscopy to determine the sample purity. SEM results showed highly agglomerated and poly-dispersed nanoparticles, owing to the presence of phytochemicals in the tea extract and magnetic interaction between the individual particles indicating the successful synthesis of Au/Fe3O4 adsorbent. Furthermore, an increase in the amount of Pb2+ removed per unit mass (qe) of adsorbent from 1.233 to 7.266 mg‧g-1 at 298 K was observed. A high sorption capacity was noticed for MWCNT-Au/Fe3O4 as compared to the MWCNT-
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COOH. The Pb2+ removal percentage increased from 50% to 78% with an increase in MWCNT-Au/Fe3O4 dosage from 0.02 g to 0.1 g. Adsorption isotherm data fitted well to the Freundlich and Langmuir isotherm models for MWCNT-COOH and MWCNT-Au/Fe3O4 adsorbents and the rate of Pb(II) adsorption by MWCNT-Au/Fe3O4 encountered an increase with increasing solution temperature and followed the pseudo-second-order model. The outcome of this project could contribute towards assisting industries such as recycling wastewater industries, mining, and municipalities in solving a problem of water contamination and alleviate health hazards.
Keywords: MWCNT-Au/Fe3O4 nanocomposite, wastewater treatment, heavy metals, lead ions.