Novel nanocomposites synthesis and modelling for hexavalent chromium reduction

Chromium (Cr) is a heavy metal pollutant prevalent in freshwater resources. It enters the human circulatory system through drinking water or food chain enrichment and have harmful effects. Existing materials (e.g., bimetallic materials, adsorbent materials, etc.) for Cr (VI) removal lack sufficient adsorption capacity.
In this work, a new Cr (VI) removal material was designed and produced: comprising reduced graphene oxide (RGO) as a support with a high specific surface area, and a combination of Fe and Ni nanoparticles (NPs) as catalytic reducing agents. Such a design permits the composite particle with three integrated functions: adsorption, catalysis, and reduction, with RGO enhancing Cr (VI) adsorption and Fe/Ni NPs enhancing catalytic reducing efficiency. The use of a microchip mixer enhanced the mixing of GO and subsequent decorating of RGO with Fe and Ni NPs. Ni-Fe/RGO exhibited an adsorption capacity of 150.45 mg/g at pH=7 for Cr (VI), which is about 2 times those reported for other materials under similar conditions. The capacity is even higher at lower pH, e.g., 197.43 mg/g at pH 5
Taking into account the cost and reusability of RGO, a ternary system consisting of iron, nickel and multi-walled carbon nanotubes (MWCNTs) were synthesised; Fe and Ni perform the same function as Fe-Ni/RGO. Due to the unusual one-dimensional structure of MWCNTs, the Cr (VI) adsorption capacity is about 20% greater than Fe-Ni/RGO, and the reaction rate is much faster. Fe-Ni/MWCNTs have a degree of reusability, with 91% of adsorption capacity retained for reuse, and MWCNT is an inexpensive material for industrial applications.
The ratio of constituents has a significant impact on its absorption capability. The amount of work required to determine the optimal ratio rises exponentially as additional components are added. Based on the reaction mechanism analysis, a model has been developed in this thesis, and shown to accurately predict the reaction curve and adsorption capacity for various Fe/MWCNTs ratios. This (modelling) approach can significantly reduce the effort for future study and has the potential to be applied to other comparable substances.