Determining the ability of zeolitic imidazolate framework nanoparticles to remove copper from industrial wastewater
Various valuable metal elements (e.g., gold, silver, platinum, copper, and aluminium) are used in electronic devices. Recycling of electronic waste can extract valuable metal elements, which can be sold as raw materials for the manufacture of new products. Mint Innovation New Zealand is a company that has developed a novel process using microorganisms to recycle electronic waste. Although the technology is useful in generating value from electronic waste, it produces highly acidic wastewater streams containing highly concentrated dissolved heavy metal ions, which must be treated before safe disposal into the environment. Copper is one such dissolved heavy metal ion present in Mint Innovation’s wastewater streams, which must be reduced to below 10 mg/L in order to comply with Auckland Council Trade-Waste agreements. Suitable methods for selectively removing high concentrations of heavy metals from wastewater with low pH conditions are yet to be discovered. ZIF-8 nanoparticles were selected as a potential adsorbent material for removing copper from Mint Innovation’s wastewater due to their large surface area, high pore density, narrow pore sizes, good stability and abundance of possible binding sites. ZIF-8 was combined with chitosan and PSS separately to produce ZIF-8@chitosan and ZIF-8@PSS composites, respectively. Various reactant combinations were investigated for preparing ZIFs (ZIF-8, ZIF8@chitosan and ZIF-8@PSS). During adsorption in idealised solutions (pH 6.5, [Cu2+] = 60 mg/L), copper removal to below 10 mg/L limits was achieved by all ZIFs within 15 minutes. ZIFs also selectively adsorbed copper in idealised solutions containing co-existing calcium or sodium ions. However, ZIFs released large quantities of zinc (> 100 mg/L) ions into solution upon binding to copper ions. The release of Zn2+ from ZIFs was enhanced in idealised solutions with initial Cu2+ concentrations above 100 mg/L or initial pH less than 3. During adsorption in Mint Innovation’s wastewater, ZIFs were rapidly degraded by the low pH and strongly oxidising conditions. Upon raising the pH from 0.7 to 4 in one waste-stream, ZIF-8 successfully removed nearly all copper (89.8%), aluminium (93%) and calcium (89.3%) ions. However, the reliability of this experiment as a measure of metal removal by ZIF-8 was low because only a small number of wastewater samples (n = 3) were collected, which was due to both time constraints and having limited access to the analytical equipment required for detecting metal ions in wastewater solutions. Binding of Cu2+ to ZIFs was thought to have destabilised the framework structure, resulting in Zn2+ being released into solution. In attempt to stabilise ZIF particles and explore their compatibility with industry standard water filtration equipment, ZIF8@PSS was coated onto nylon ultrafiltration membranes via direct deposition or in situ self-assembly. The ZIF-8@PSS coated membranes were referred to as Mixed Matrix Membranes (MMMs). The chemical composition and physical characteristics of ZIFs and ZIF-8@PSS coated MMMs were analysed by Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), scanning electron microscopy (SEM). and water contact angle (CA) measurements. Direct deposition or in situ self-assembly both produced ZIF-8@PSS coated MMMs with non-uniform surfaces, resulting from ZIF-8@PSS aggregation in some regions of the membrane surface, but failing to attach to others (based on FTIR, SEM and CA analysis). The filtration performance of ZIF-8@PSS coated MMMs was tested using solutions of Congo red (CR) dye. ZIF-8@PSS coated MMMs failed to remove CR dye from solution during continuous filtration. It was believed that nanosized CR molecules could simply pass-through openings in the MMMs surface where ZIF-8@PSS failed to attach. Because copper ions are smaller than CR molecules, it was predicted that the ZIF-8@PSS coated MMMs will not remove copper ions during filtration. ZIFs successfully removed copper ions to below 10 mg/L in both idealised solutions and wastewater produced by Mint Innovation. However, Zn2+ release from ZIFs during ion exchange with copper means that one pollutant is simply replaced with another during wastewater treatment. Future research should focus on ZIF modifications that inhibit the ion-exchange mechanism of ZIFs without reducing the ability of ZIF to bind to copper ions.
Advisor: Ali, Azam; Agyei, Dominic
Degree Name: Master of Science
Degree Discipline: Food Science
Publisher: University of Otago
Keywords: ZIF-8; Adsorption; Copper; Zinc; Wastewater; Ion-Exchange; E-waste
Research Type: Thesis