Controlling silver release from antibacterial surface coatings on stainless steel for biofouling control.

This study focuses on the in-situ nucleation of silver nanoparticles (AgNPs) on stainless steel (SS) to provide a localized antibacterial action for biofouling control during space missions. Since AgNPs rapidly dissolve in water, partial passivation of AgNPs was provided to slow down silver release and extend the lifetime of the antibacterial coating. Two different passivation approaches, based on the formation of low solubility silver sulfide (Ag2S) or silver bromide (AgBr) shells, were compared to identify the optimal passivation for biofouling control. Highest bacterial inactivation (up to 75%) occurred with sulfidized AgNPs as opposed to bromidized (up to 50%) NPs. The optimal passivation treatment for biofouling control was found at 10-5 M Na2S (for Ag2S) and 10-3 M NaBr (for AgBr) concentrations. Scanning Electron Microscopy (SEM) analyses confirmed the presence of AgNPs on AgBr and Ag2S-coated samples. Further investigation revealed that compared to pristine AgNPs, Ag release from both sulfidized and bromidized NPs was significantly lower (16% vs 6% or less). Overall, both sulfidized and bromidized AgNPs were effective at controlling biofilm formation; however, sulfidized NPs exhibited the maximum antibacterial activity, making it the preferable passivation strategy for AgNPs on SS surfaces.

Impact of hydraulic and carbon loading rates of constructed wetlands on contaminants of emerging concern (CECs) removal.

Constructed wetlands remove trace organic contaminants via synergistic processes involving plant biomass that include hydrolysis, volatilization, sorption, biodegradation, and photolysis. Wetland design conditions, such as hydraulic loading rates (HLRs) and carbon loading rates (CLRs), influence these processes. Contaminant of emerging concern (CEC) removal by wetland plants was investigated at varying HLRs and CLRs. Rate constants and parameters obtained from batch-scale studies were used in a mechanistic model to evaluate the effect of these two loading rates on CEC removal. CLR significantly influenced CEC removal when wetlands were operated at HLR >5 cm/d. High values of CLR increased removal of estradiol and carbamazepine but lowered that of testosterone and atrazine. Without increasing the cumulative HLR, operating two wetlands in series with varying CLRs could be a way to improve CEC removal.

Sorption of trace organics and engineered nanomaterials onto wetland plant material.

Wastewater treatment plant (WWTP) effluents are sources for emerging pollutants, including organic compounds and engineered nanomaterials (ENMs), which then flow into aquatic systems. In this article, natural attenuation of pollutants by constructed wetland plants was investigated using lab-scale microcosm and batch sorption studies. The microcosms were operated at varying hydraulic residence times (HRTs) and contained decaying plant materials. Representative organic compounds and ENMs were simultaneously spiked into the microcosm influent, along with a conservative tracer (bromide), and then monitored in the effluent over time. It was observed that a more hydrophobic compound-natural estrogen achieved better removal than a polar organic compound ­ para-chlorobenzoic acid (pCBA), which mimics the behaviour of the tracer. Batch sorption experiments showed that estrogen has higher sorption affinity than pCBA, highlighting the importance of sorption to the plant materials as a removal process for the organic contaminants in the microcosms. Wetland plants were also found a potential sorbent for ENMs. Two different ENMs (nano-silver and aqueous fullerenes) were included in this study, both of which experienced comparable removal in the microcosms. Relative to the tracer, the highest removal of ENMs and trace organics was 60% and 70%, respectively. A more than two-fold increase in HRT increased the removal efficiency of the contaminants in the range of 20­60%. The outcome of this study supports that plant materials of wetlands can play an important role in removing emerging pollutants from WWTP effluent.