Sorption Studies of Synthetically Modified Carbon Nanomaterials

Abstract

The level of risk originating from toxic (heavy) metals in the environment and ecological systems is continuously escalating due to our imprudent development of mineral resources such as coal and gold. For example, selenium as one of the major components in coal has contaminated surface and groundwater sources, and represents a threat to human and ecosystem health accumulation in organisms known as selenosis. Arsenic, like selenium, has also a negative effect to human beings, so called "arsenicosis" if it is accumulated in an organism through dietary pathways. Therefore, these elements have threatened waterways by contaminating surface and groundwater sources, and the WHO has established the drinking water quality guideline as 10 ppb for selenium and arsenic. The development of surface modified carbon nano-materials was motivated by considering how toxic metal species such as selenium and arsenic can be effectively removed from aquatic environments such as mineral tailings ponds found at mine sites. The materials design strategy employed herein hypothesizes of the incorporation of Lewis acid-base sites by the preparation of surface modified carbon nano-material with magnetite (magnetite composite). The resulting composite materials were anticipated to have variable π-π interactions and H-bonding between (non-)metals and ligands. These novel composite sorbents were evaluated for sorptive removal of selenium and arsenic species in aqueous solution at variable conditions. Selenium and arsenic have variable adsorption affinity onto the surface of magnetite (iron oxide) and its composites and goethite (iron oxyhydrate) in aqueous solution. The sorptive properties of these materials were correlated to the synthetic strategy as evidenced by the characterization of these minerals and their adsorbent properties. The adsorptive properties were evaluated by comparing the adsorption of inorganic selenium species with various adsorbents (magnetite, magnetite composites, activated carbon, and goethite) through adsorption kinetics and at equilibrium conditions. A novel “in situ” kinetic set-up for this experiment was developed using a non-magnetic stirrer device with a semi-permeable filtration barrier. The analytical measurement of selenium uptake was achieved using hydride generation atomic absorption spectroscopy. An arylarsenical (roxarsone) in aqueous solution was removed by using the same adsorbents used for selenium sorption and using a novel one-pot kinetic experiment with a non-magnetic stirrer and a dialysis-based tubing filter. Determination of roxarsone uptake was evaluated with UV-vis spectroscopy. This study showed the prepared magnetite composites might be excellent adsorbents for removing organic (aryl) and inorganic forms of Se and As chemical species in aqueous solution. The composite nature of the composite adsorbents suggests their potential as dual function sorbents due to their affinity toward organic (aryl) and inorganic anion species. In the occurrence of iron leaching, it was attenuated at low temperatures for the composite materials; whereas, greater leaching occurred above room temperature due to the increased thermal breakdown of magnetite particles in the pores or on the surface of activated carbon. In addition to the aforementioned tunable surface reactivity and surface area, magnetite composites have magnetic susceptibility properties that enable physical separation of adsorbents in water treatment processes by employing an electro-magnet to induce phase separation.