Article pubs.acs.org/est
Relative Importance of the Humic and Fulvic Fractions of Natural Organic Matter in the Aggregation and Deposition of Silver Nanoparticles Olha Furman,†,‡ Sascha Usenko,† and Boris L. T. Lau‡,* †
Department of Environmental Science, Baylor University, One Bear Place #97266, Waco, Texas 76798, United States Department of Geology, Baylor University, One Bear Place #97354, Waco, Texas 76798, United States
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S Supporting Information *
ABSTRACT: As engineered nanoparticles (NPs) are increasingly used, their entry into the environment has become an important topic for water sustainability. Recent investigations point to the critical role of natural organic matter (NOM) in altering the persistence of NPs by complexing with their surfaces. The NP-NOM complex, in turn, is the new entity that may potentially influence subsequent fate of NPs. To understand the relative impact of humic (HA) and fulvic fraction of NOM on the stability and mobility of silver nanoparticles (AgNPs), a combination of dynamic light scattering and quartz crystal microgravimetry with dissipation monitoring was used. In the absence of unbound NOM, (1) surface modification on either AgNP or silica substrate by different NOM fractions could lead to substantial changes in the extent and kinetics of AgNP aggregation and deposition, and (2) HA has a greater capability to enhance the transport of AgNPs by reducing their aggregation and deposition. With unbound NOM, HA seems to compete more successfully for binding sites on the substrate under electrostatically favorable conditions and formed a steric layer to prevent subsequent deposition of AgNPs. These findings highlighted the importance of NOM fraction in the overall environmental partitioning of AgNPs.
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INTRODUCTION As the field of nanotechnology continues to expand, increased entry of engineered nanoparticles (NPs) into the environment has resulted in growing environmental and human health concerns. Silver nanoparticles (AgNPs) were chosen for this study because of their growing production, occurrence,1,2 and potential toxic effects. Among the current list of more than 1300 consumer products that claim to include NPs, approximately 20% contain AgNPs.3 The most probable human exposure route of AgNPs is predicted to be through surface waters and wastewaters.4 In Europe alone, silver is being released to natural waters at a rate of 20−130 tons per year, resulting in a concentration that ranges from 40 to 320 ng L−1 of total Ag.5 Human exposure to ions and/or NPs of Ag has been shown to cause liver and kidney damage, eye and skin irritations, and changes to blood cells.6−8 As natural organic matter (NOM) is ubiquitous in natural water, a better understanding of the interaction between NP and NOM is essential in providing insight to the behaviors of NPs in the aqueous environment. It is widely recognized that NOM plays important roles in the fate and transport of organic and metallic contaminants.9−13 However, the implications of NP interactions with NOM are largely unknown. During the release of NPs into the aquatic environment, most NPs are prone to acquire a surface coating of NOM.14,15 It has been shown that the fate of NPs are altered as a result of NOM binding to NP surfaces.16−18 NP-NOM complexes, in turn, is the new entity that may potentially influence the overall fate © 2013 American Chemical Society
and transport of NPs. Once in the environment, AgNPs are likely to encounter, natural sediments/minerals, such as silica sand (SiO2) coated with a layer of NOM. Therefore, NOMcoated silica represents an environmentally relevant surface. Estimating the human and environmental health risk of emerging contaminants such as engineered NPs is hampered by inadequate knowledge of the relevant physical/chemical mechanisms (and associated kinetics) of environmental processes for their fate and transport. While aggregation (particle−particle interactions) of AgNPs has been increasingly studied,19−23 deposition (particle-substrate interactions) of AgNPs has not been investigated in the environmental context. Petosa et al. (2010) provided a comprehensive review on the theoretical and experimental approaches for evaluating NP aggregation and deposition in aquatic systems.24 Some recent studies pointed out the important role of solution chemistry in influencing metal NP deposition within porous media.25−27 The objective of this study was to understand the relative impact of humic acid (HA) and fulvic acid (FA) on the stability and mobility of AgNPs. With a combination of dynamic light scattering (DLS) and quartz crystal microgravimetry with dissipation monitoring (QCM-D), we quantified the aggregaReceived: Revised: Accepted: Published: 1349
August 13, 2012 December 19, 2012 January 8, 2013 January 8, 2013 dx.doi.org/10.1021/es303275g | Environ. Sci. Technol. 2013, 47, 1349−1356
Environmental Science & Technology
Article
Table 1. Characteristics and Aggregation Kinetics of AgNPs at 1 mM Ca(NO3)2 and pH = 8.0 ± 0.1 AgNPs with no NOM coated with HA coated with FA a
HA/FA DOC (mg L−1) 0
