A study into the interaction of gold nanoparticles released into drinking water and wastewater system

Abstract

This research involves the investigation of the interaction of different sized Nano Gold particles released into municipal drinking water and municipal waste water. Waste water was collected from Malamulele waste water treatment plant and the municipal water was collected at Mintek in Johannesburg, Randburg, South Africa. The waste water was analysed using ICP-MS to detect the metals and anions in it. The results showed the abundance of Sulphur (464 ppm), Calcium (28 ppm), Chloride (27.8 ppm), Iron (20 ppm), Magnesium (8.2 ppm), silicon (6.192 ppm) in descending order and other trace elements, including gold, that were immeasurable (<0.1). The simulated situation was created by adding 20nm gold and 40nm gold nanoparticles into municipal drinking water and waste water and kept at different environmental conditions (light, light and agitation, dark, dark and agitation) under aerobic and anaerobic conditions over a period of two months. Physico-chemical properties (pH and chemical oxygen demand) of the solutions were checked once in a month. The pH fluctuated between the acceptable ranges (5.5 – 9.5) for the two month period. Both municipal water and waste water, with and without gold nanoparticles, under aerobic condition showed an increase in chemical oxygen demand. The gold content in waste water under anaerobic condition showed an increase while under aerobic condition the decline in gold content was evident. The zeta potential of gold nanoparticles in waste water in light and agitation showed (-30 mV) while waste water on other environmental condition (light, dark and dark with agitation) presenting unstable (-18 mV) charge, but the charge shifted positively on the second month rendering them also unstable. Dynamic light scattering and TEM were used to check any possible aggregation or agglomeration of nanoparticles in the waste water. There were some few discrepancies where TEM and DLS contradict, but overall there was no significant probability of any aggregation of gold nanoparticles. The EDX was used to confirm the presence of Au0 in the waste water (with added gold nanoparticles). The research did show that the gold nanoparticles would exist as Au0 in the waste water and thus the discharge of Au-NPs to the sewer system is not recommended, but rather recycle them.