Abstract
Water is one of the most essential resources on earth and the most utilized resource by most living things on earth. However, water pollution is still a great challenge that needs urgent attention. The presence of organic compounds such as benzene, toluene, ethylbenzene, and xylenes (BTEX) in industrial and municipal wastewaters, has been recognized as a serious environmental problem. BTEX compounds are common groundwater and potable water pollutants, which are introduced into the environment through leakages from underground storage tanks, accidental spillages, and improper waste disposal practices, causing the contamination of surface soils, seawater, and groundwater. The United States Environmental Protection Agency (EPA) has classified BTEX as priority pollutants that pose a significant threat to human health and the environment due to their carcinogenic properties. Health concerns of BTEX compounds include irritation of mucosal membranes, irritation of organs, and respiratory problems. Despite the negative health effects, BTEX pollutants remain untreated or undetected in municipal water treatment systems. This makes their removal from water essential.
Treatment techniques used currently for the removal of BTEX compounds include phytoremediation, air stripping, natural attenuation, bioremediation, and adsorption. For this study, adsorption was selected as a method of treatment as it has been described in studies as a cost-effective treatment technique that delivers high performance and is simple to use. This study investigated the adsorptive performance of multiwalled carbon nanotubes (MWCNTs) and macadamia nutshell-derived activated carbon (MACs) for the removal of BTEX compounds from synthetic industrial wastewater. The performance evaluations of the MWCNTs and MACs were carried out in batch adsorption mode. The synthetic wastewater was prepared by dissolving 100 mg/L of each of the BTEX compounds (Benzene, Toluene, Ethylbenzene and Xylene) in distilled water in a 250 mL volumetric flask to prepare the stock solution. Effect of initial concentration of BTEX compounds, contact time, and mass of the adsorbent for the removal of BTEX compounds from the synthetic wastewater was investigated. Fourier Transform Infrared Spectra (FTIR) evaluated surface functional groups of the adsorbents. The specific surface area, pore size, and pore volume at 77 K nitrogen adsorption were evaluated using Brunauer-Emmett-Teller (BET). Surface morphology and the crystalline structure of the adsorbents were assessed using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD), respectively.
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The adsorbents showed high porosity in their structures with the presence of C-O, C=C, C=O, and OH groups on their surface and later confirmed through FTIR results. The batch experiments conducted proved that the adsorbents could adsorb BTEX compounds. Among all the components of the BTEX group, benzene was the least adsorbed. This is due to the increasing water solubility and the decreased molecular weight of the compounds. Macadamia nutshell activated carbon adsorption capacity was calculated to be 17.59 mg/g, 51.59 mg/g, 55.59 mg/g, and 57.59 mg/g for benzene, toluene, ethylbenzene, and xylene, respectively. The adsorption capacity of MWCNTs was calculated at 17.46 mg/g, 35.23 mg/g, 37.98 mg/g and 41.63 mg/g for benzene, toluene, ethylbenzene, and xylene, respectively. As observed, activated carbon performed better than carbon nanotubes. This is due to the high surface area of activated carbon (605 m2) compared to the 175 m2 observed for carbon nanotubes. MACs had removal percentage of 99.03%, 84.57%, 78.00% and 68.57% for xylene, ethylbenzene, toluene and benzene, respectively, while MWCNTs have 45.03%, 39.57%, 35.05% and 27.57% for xylene, ethylbenzene, toluene and benzene, respectively. The equilibrium adsorption was better represented by Langmuir isotherms, which indicates a monolayer coverage onto the surface of the MAC and MWCNTs. The kinetic study indicated that the pseudo second-order described the data better. These results may have been affected also by the solubility of the adsorbates and the competition for adsorption sites.