Abstract
M.Sc. (Chemistry)
In this study the stability as well as the fate and behavior of ZnO engineered
nanoparticles (ENPs) in municipal wastewater systems were investigated. The first
part of the study examined the influence of pH and ionic strength on the stability of
ZnO ENPs in domestic wastewater to elucidate the dynamic changes on the ENPs
physicochemical characteristics (e.g. aggregation or dissolution). The ZnO ENPs
obtained commercially were characterized using transmission electron microscopy
(TEM), X-ray diffraction spectroscopy (XRD), dynamic light scattering (DLS), BET
surface area determination, and energy dispersive X-ray spectrometry (EDS).
Results derived from inductively coupled plasma optical emission spectrometry
(ICP-OES) for zinc analysis indicated a decrease on the released zinc
concentration from wastewater as the pH and ionic strength increased.
Conversely, an increase on zinc concentration from the sludge was observed. The
findings suggest the removal of ZnO ENPs from the influent wastewater as the
sludge settled out, and the removal efficiency was directly proportional to ionic
strength and pH. In addition, the ZnO ENPs suspension in the wastewater was
used to monitor the particle size distribution using the dynamic light scattering
analysis (DLS). The formation of agglomerates was observed which the TEM and
EDS analysis confirmed to be ZnO aggregates. The distribution of zinc in the
sludge was investigated using XRD analysis and the findings indicated partial
sedimentation of ZnO ENPs as the sludge settled out.
The second part of the study assessed the fate and behavior of ZnO ENPs in
wastewater treatment systems. This study was carried out in a simulated activated
sludge wastewater treatment plant (AS WWTP), constructed in accordance to the
Organization for Economic Co-operation and Development (OECD 303 A)
guidelines. Results from the ICP-OES analysis for zinc indicated 50 – 200 μgL-1
and about 3 000 mgkg-1 were released into the effluent and sludge, respectively,
after spiking the influent wastewater with 5 mgL-1 ZnO ENPs. Moreover, we noted
that increasing the ZnO ENPs concentration up to 20 mgL-1 resulted in a linear
increase in the zinc releases into effluent wastewater. However, the increase was
insignificant in comparison to the zinc found in the control unit. Therefore, the
findings indicated that ZnO ENPs had stronger affinity for the suspended bio-solids
during wastewater treatment, and postulated that the ENPs removal from the
influent wastewater was due to bio-sorption, and bio-solid settling mechanisms.
These were confirmed by results from XRD and diffuse reflectance spectroscopy
(DRS) analysis of the sludge as they showed the presence of ZnO in the sludge.
The dissolved organic carbon (DOC) and chemical oxygen demand (COD) were
concurrently monitored during the ENPs exposure studies to indirectly assess the
ZnO ENPs impact to the bacterial degradation of the organic matter. An average
of 43 and 91 % for DOC and COD removal efficiencies, respectively, were
observed throughout the study.
Overall, from results obtained indicated the suitability of the OECD 303 A method
to assess the fate and behavior of ZnO ENPs in WWTPs. Secondly, in light of low
concentrations of ZnO ENPs found in the treated effluent due to their removal with
the waste activated sludge (WAS), suggest low likelihood of ZnO ENPs release
and dispersion into the aquatic systems from WWTPs as point sources. Finally,
the elevated concentrations of ZnO ENPs in the sludge therefore necessitates
additional treatment steps to ensure mitigation of possible dispersion of ENPs from
various disposal mechanisms such as landfilling, incineration, and agricultural
applications.