Elemental copper nanoparticle toxicity to anaerobic ammonium oxidation and the influence of ethylene diamine-tetra acetic acid (EDTA) on copper toxicity
Graphical abstract
Introduction
Engineered nanoparticles (NP) are manufactured materials with at least one dimension smaller than or equal to 100 nm (Hansen et al., 2007). Copper-based NP are widely used for the production of wood preservatives, catalysts, conductive inks, printable electronics, and used in antimicrobial products (Youngil et al., 2008, Bondarenko et al., 2013, Wang et al., 2013). Elemental copper NP (Cu0 NP) are one of the most commonly used copper-based NP for technological purposes (Wang et al., 2013). Additionally, copper-based NP are a byproduct of chemical mechanical polishing (Golden et al., 2000).
The continuous use of NP in industrial and domestic activities has raised concerns regarding their fate and effect on the environment. Copper-based NP can potentially end up in industrial or municipal wastewater treatment plants. The discharge of effluent or land application of sludges from the treatment plants can lead to release of the NP to the environment in addition to unplanned inappropriate disposals (Clar et al., 2016). Life cycle assessment studies have shown that NP are likely to accumulate in sewage sludge, wastewater treatment plants, and incineration plants after being discarded (Gottschalk and Nowack, 2011). Particularly, accumulation of NP used in consumer products has been found in activated sludge waste water treatment plants (Kiser et al., 2009, Kiser et al., 2010, Ma et al., 2013).
Accumulation of NP in the activated sludge process may affect the performance of a variety of microorganisms involved with the treatment. The whole activated sludge system and additional biological treatment processes involve not only oxygen-utilizing microorganisms, but also a variety of cultures responsible for nutrient removal or sludge stabilization (Tchobanoglous et al., 2014). Thus, research has been conducted to investigate the toxic effect of NP on microorganisms involved in waste water treatment (Gonzalez-Estrella et al., 2015a).
Studies exploring copper-based NP toxicity to anaerobic wastewater treatment microorganisms have exhibited a strong inhibition to glucose fermentation, anaerobic propionate oxidation, methanogenesis, denitrification, and sulfate reduction (Luna-delRisco et al., 2011, Otero-González et al., 2014, Gonzalez-Estrella et al., 2015a, Gonzalez-Estrella et al., 2015b). Recent studies have also shown toxicity of Cu0 NP toxicity to aerobic microorganisms such as nitrifying bacteria (Clar et al., 2016). However, very limited information is available concerning the effect of copper-based NP on the anaerobic ammonium oxidation process (anammox) even though copper salts have shown to be toxic to anammox in other studies (Lotti et al., 2012, Yang et al., 2013, Li et al., 2014). Presence of copper in nitrogen rich waste water can be expected in supernatants from anaerobic digestion sludge, leachates from landfills, piggery and dairy slurries, streams from the production of nitrogenous fertilizers, and semiconductor manufacturing waste water effluents (Zhang et al., 2015).
Copper-based NP toxicity is mostly caused by the release of soluble ions as a result of dissolution due to corrosion (Clar et al., 2016, Gonzalez-Estrella et al., 2016). Recent studies have demonstrated that sulfide can attenuate Cu0 toxicity to methanogens by decreasing the concentration of free soluble Cu2+ ions released by Cu0 NPs (Gonzalez-Estrella et al., 2015b, Gonzalez-Estrella et al., 2016). This suggests that decreasing the availability of Cu2+ ions by other mechanisms may attenuate Cu0 NP toxicity as well. Chelating agents, for instance, are commonly applied to remediate soils contaminated with heavy metals (Dermont et al., 2008).
Chelating agents form a coordinate chemical bond with the metal (complexed metal) which facilitates and improves their solubility (Udovic and Lestan, 2010). Nonetheless, a complexed metal is not necessarily bioavailable (active); therefore, by complexing the metal and reducing its bioavailability the toxicity can be decreased (Rodea-Palomares et al., 2009). Ethylene diamine-tetra acetic acid (EDTA) is one of the most applied chelating agents to remove copper and other heavy metals from the soil (Udovic and Lestan, 2010). Thus, EDTA may decrease the toxicity of Cu0 NP by complexing ions released from Cu0 corrosion. The findings of Conway et al. (2015) indicated that copper ions released by Cu NP reached the maximum soluble concentration between 0 and 1 d after the NP were added to wastewater media. They also found that the ionic fraction represented up to 30% of total aqueous copper. Thus, EDTA may play an important role in controlling the concentration of the Cu2+ free ions release by Cu NP corrosion. It should be noted that the presence of high concentrations of EDTA, nitrogen and copper, and low concentrations of carbon sources can be expected in wastewater effluents from electroplating processes (Maketon et al., 2008). Soluble copper concentrations range from 5 to 460 mg L−1 (Maag et al., 2000, Wong et al., 2003) while the chelating agent concentration, mostly attributed to EDTA, ranged from 10 to 200 mg L−1 in semiconductor wastewater streams (Dakubo et al., 2012). Additionally, metal cleaning and printing inks industries, which use copper as part of their processes, represent 5 and 3% of the EDTA world market, respectively (Oviedo and Rodríguez, 2003). Such wastewater characteristics may be appropriate for the application of an anammox process if the toxic effect of copper is attenuated by the presence of EDTA already occurring in the wastewater.
This study evaluated the toxic effect of Cu0 NP on the anammox process and the influence of EDTA in attenuating copper toxicity.
Section snippets
Chemicals
Cu0-NPs (40–60 nm, 99%) were purchased from Sky-Spring Nanomaterials Inc. (Houston, TX, USA) CuCl2·H2O (99%) was acquired from Sigma Aldrich (St. Louis, MO, USA). He/CO2 (80/20, v/v) gas mix was acquired from Air Liquid America (Plumstedsville, PA, USA). CuCl2 was used in the experiments as a source of Cu2+ ions. EDTA (>99%) was purchased from Fisher Science (Waltham, MA, USA).
NP dispersions and metal solutions
Cu0-NP stock dispersions were sonicated (DEX® 130, 130 Watts, 20 kHz, Newtown, CT, USA) at 70% amplitude for 5 min in
Inhibition of anaerobic ammonium oxidizing (anammox) consortium in presence and absence of EDTA
The inhibitory effect of Cu0 and CuCl2 on an anammox consortium was explored in a series of batch experiments. Fig. 1 shows the normalized anammox activity as a function of increasing concentrations of added Cu0 NP and CuCl2 in EDTA-containing (0.07 mM) and EDTA-free conditions of the second substrate feed. The experiment showed two main findings; first, Cu0 NP and CuCl2 were toxic to anammox, and second, 0.07 mM EDTA decreased the toxicity effect from both forms of copper. Concentrations of
Main contributions
The findings of the present study indicated that Cu0 NP are toxic to anammox mainly by the release of Cu2+ ions (uncomplexed ions). Measurements of the soluble copper regardless the source indicated that similar soluble final concentrations imparted a similar inhibition to the anammox consortia at any given EDTA concentration. Results also indicated that EDTA decreased the toxicity effect of Cu0 NP and CuCl2 on anammox. Assays supplied with 0.07 mM EDTA increased the Ki constants (decreased
Conclusions
This study demonstrated that Cu0 NP are toxic to anammox. Likewise, CuCl2 were toxic to anammox activity. The results show that Cu0 NP toxicity was driven by the release of Cu2+ ions. Ki constants calculated with respect to the soluble concentration confirmed that Cu free soluble ions regardless their source (Cu0 NP or CuCl2) affect anammox in a similar fashion at any given EDTA concentration. Additionally, this study demonstrated that EDTA attenuates the toxicity effect of Cu0 NP on anammox.
Acknowledgements
This work was supported by the Semiconductor Research Corporation (SRC)/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing (426.036). This work was partly funded the National Institute of Environmental Health Sciences-supported Superfund Research Program (NIH ES-04940) and by the National Science Foundation (NSF CBET-1234211). Gonzalez-Estrella was also funded by CONACyT (308577).
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