Abstract
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.