Effect of natural organic matter on cerium dioxide nanoparticles settling in model fresh water
Introduction
Manufactured nanomaterials are used in a wide variety of applications such as cosmetics, medicine, engineering, electronics, and environmental protection, which will inherently result in their emission into the environment and thereby lead to the exposure of organisms. The ecological risk assessment of chemicals and more recently of nanomaterials is based on the determination of adverse effects on organisms and on evaluation of the environmental concentrations to which biota are exposed (Moore, 2006). Currently, the awareness of the potential adverse effects of manufactured nanomaterials on organisms is increasing. However, knowledge about the fate of nanomaterials in the environment is developing only slowly (Klaine, 2009). In particular, the exposure concentration of manufactured nanoparticles (NPs) in the environment is largely unknown and not easily measured in situ. Cerium dioxide (CeO2) is one of the manufactured NPs focused on by the Organization for Economic Co-operation and Development (OECD) as being a priority NP due to its current use (OECD, 2008). CeO2 has several applications, such as a fuel additive in the automotive industry (Fall et al., 2007) and a UV blocking agent in the cosmetic industry (Masui et al., 2000). Studies on the solubility of CeO2 nanoparticles are scarce, but they are generally considered to be insoluble. In a study by Van Hoecke et al. (2009) solubility was below the detection limit of approximately 1 μg L−1 in deionized water and algae growth medium.
Recently, much attention has been given to the important role that NOM has in the stability of particle suspensions in which NOM generally decreases aggregation. NOM originates from the breakdown of plant and animal tissue in the environment. As such, it varies in composition and concentration depending on the source and location of a water system (Dawson et al., 2009). Generally, the main constituents of NOM are humic acids, fulvic acids, and a hydrophilic fraction (Frimmel, 1998, Harbour et al., 2007). NOM has long been known to adsorb onto colloidal particles and influence their colloidal stability (Tipping and Higgins, 1982, Au et al., 1999). In waste water treatment, NOM is known to reduce the coagulation of particles (Walker and Bob, 2001). It has been shown more recently that carbon nanotubes and fullerenes are suspended in water in the presence of NOM (Chen and Elimelech, 2007, Hyung et al., 2007). Several metal (Cumberland and Lead, 2009) and metal oxide (Domingos et al., 2009, Yang et al., 2009, Zhang et al., 2009) NPs are stabilized by the adsorption of NOM. Most studies have shown increased electrostatic repulsion due to adsorption of NOM fractions to the particle surface (Mosley et al., 2003, Harbour et al., 2007). This causes stabilization at moderate ionic strengths due to an absolute increase in the particle charge. Additionally, the adsorbed fraction of NOM is thought to cause steric hindrance, which reduces aggregation irrespective of particle charge and ionic strength (Mosley et al., 2003, Harbour et al., 2007, Domingos et al., 2009). The increase in colloidal stability is generally thought to affect the exposure of organisms in the aquatic environment to NPs. To predict the particle concentrations in water, we need to quantitatively understand the relationship between the particle concentration and the physicochemical properties of the particles and environment. Unfortunately, little quantitative information exists on the influence of physical and chemical properties on particle concentrations, which seriously hampers our ability to describe and predict the concentrations of nanomaterials in suspension. The known stabilizing potential of NOM for NPs is based on measuring several particle characteristics, like the zeta potential to indicate an increased electrostatic repulsion, and the particle size to see if the addition of NOM reduces aggregation. The aggregation size is related to the suspension concentrations in water.
With the present study, we aim to contribute to a better understanding of the behavior of NPs in the aquatic environment. This will ultimately serve to predict future exposure concentrations of CeO2 NPs suspended in natural waters. We hypothesize that NOM content greatly influences the particle concentration in suspension due to its known stabilizing effect. In order to test this, we measured the concentration and particle diameter of CeO2 NPs in suspension during 12 d to be able to relate the effect of NOM stabilization to a concentration of CeO2 NPs in water. In our experiments we used a well known algae growth medium (OECD, 2006) as the model fresh water, with the addition of NOM to mimic environmental conditions. Most of the recent studies on the interaction of NOM and NPs have focused on NOM from the Suwannee River in Georgia, USA, which is seen as a reference material. In our experiments, we additionally used NOM from Bihain, Belgium to test whether the origin of the NOM matters. The stabilizing effect of NOM on CeO2 NPs has not been reported yet.
Section snippets
Nanoparticles and suspensions
CeO2 NPs were obtained as 100 g L−1 suspensions at pH 4 (kindly supplied by Umicore Ltd. as part of the NanoInteract project). The manufacturer reported a BET surface area of 42 m2 g−1 and calculated a BET surface based particle diameter of 20 nm. The CeO2 particles have an isoelectric point at pH 8.0 (Fig. 1), which is similar to the values reported earlier (De Faria and Trasatti, 1994, Limbach et al., 2008, Van Hoecke et al., 2009). The algae growth medium (pH 8.0) was prepared according to the
Characterization
In deionized water and algae medium almost all CeO2 NPs had settled out of suspension by day 12. Less than 0.07 mg L−1 of CeO2 (0.8% of the initially added NPs) remained in suspension. The particle size distribution of the CeO2 NPs in deionized water and algae medium showed the formation of aggregates, with an average diameter of 301 and 417 nm in deionized water and algae medium, respectively, after 1 d of settling. This is larger than the average particle diameter of 169 nm found for the stock
Discussion
The addition of NOM to the particle suspensions had a clear effect on the concentration of CeO2 NPs remaining suspended after a prolonged period of settling. Natural organic matter clearly increases the stability of CeO2 NPs in water as previously indicated for other types of NPs (Yang et al., 2009; Zhang et al., 2009). The main mechanism explaining the increased stability is the adsorption of NOM to the particle surface. The strong adsorption of NOM to iron oxide, titanium dioxide, aluminum
Acknowledgements
This work was carried out as part of the European Union Sixth Framework Programme NanoInteract (NMP4-CT-2006-033231). We thank Alex de Haan from the Research and Development department of the Netherlands Vaccine Institute for help with the zeta potential measurements. We thank Rob Comans and André van Zomeren from ECN for the characterization of the natural organic matter.
References (27)
- et al.
Natural organic matter at oxide/water interfaces: complexation and conformation
Geochim. Cosmochim. Acta
(1999) - et al.
Influence of humic acid on the aggregation kinetics of fullerene (c-60) nanoparticles in monovalent and divalent electrolyte solutions
J. Colloid Interface Sci.
(2007) - et al.
Particle size distributions of silver nanoparticles at environmentally relevant conditions
J. Chromatogr.
(2009) - et al.
The point of zero charge of CeO2
J. Colloid Interface Sci.
(1994) Characterization of natural organic matter as major constituents in aquatic systems
J. Contam. Hydrol.
(1998)- et al.
The role of natural organic matter in suspension stability – 1. Electrokinetic–rheology relationships
Colloids Surf., A
(2007) Do nanoparticles present ecotoxicological risks for the health of the aquatic environment?
Environ. Int.
(2006)- et al.
Stability of particle flocs upon addition of natural organic matter under quiescent conditions
Water Res.
(2001) - et al.
Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles
Water Res.
(2009) - et al.
Aggregation and surface properties of iron oxide nanoparticles: influence of pH and natural organic matter
Environ. Toxicol. Chem.
(2008)
Characterization of aquatic colloids and macromolecules. 1. Structure and behavior of colloidal material
Environ. Sci. Technol.
A generalized description of aquatic colloidal interactions: the three-colloidal component approach
Environ. Sci. Technol.
Is the composition of dissolved organic carbon changing in upland acidic streams?
Environ. Sci. Technol.
Cited by (158)
Nanomaterials: A comprehensive review of applications, toxicity, impact, and fate to environment
2023, Journal of Molecular LiquidsNatural organic matter (NOM), an underexplored resource for environmental conservation and remediation
2022, Materials Today SustainabilityNanotechnology in agriculture: Comparison of the toxicity between conventional and nano-based agrochemicals on non-target aquatic species
2022, Journal of Hazardous MaterialsNanocarriers for the topical treatment of psoriasis - pathophysiology, conventional treatments, nanotechnology, regulatory and toxicology
2022, European Journal of Pharmaceutics and BiopharmaceuticsNovel multimethod approach for the determination of the colloidal stability of nanomaterials in complex environmental mixtures using a global stability index: TiO<inf>2</inf> as case study
2021, Science of the Total EnvironmentCitation Excerpt :The interactions between components of natural origin and ENMs have been extensively studied, focusing separately on SPM-ENM (Praetorius et al., 2014; Quik et al., 2014, 2012) or NOM-ENM interactions (Baalousha et al., 2009; Liu et al., 2018; Loosli et al., 2015, 2013; Oriekhova and Stoll, 2016; Philippe and Schaumann, 2014; Saleh et al., 2008; Slowey, 2010; Yu et al., 2018), and recently the more complex system of SPM, NOM and ENMs together (Feng et al., 2019; Gallego-Urrea et al., 2016; Labille et al., 2015; Li et al., 2020; Slomberg et al., 2019; Smith et al., 2015). Depending on its composition, NOM can induce several and contrasting effects, such as electrostatic stabilization/destabilization, cation bridging, polymer bridging and disaggregation of ENMs (Li and Chen, 2012; Philippe and Schaumann, 2014; Quik et al., 2010). Similarly, different types of SPM exhibit different affinities for ENM-SPM heteroaggregation and affect mainly ENMs transport and transformation processes, since aggregation results in differences in sedimentation, shifts the particle size distribution and surface charge (Praetorius et al., 2014; Slomberg et al., 2019).
Behavior of engineered nanoparticles in aquatic environmental samples: Current status and challenges
2021, Science of the Total Environment