Effect of natural organic matter on cerium dioxide nanoparticles settling in model fresh water

Abstract

The ecological risk assessment of chemicals including nanoparticles is based on the determination of adverse effects on organisms and on the environmental concentrations to which biota are exposed. The aim of this work was to better understand the behavior of nanoparticles in the environment, with the ultimate goal of predicting future exposure concentrations in water. We measured the concentrations and particle size distributions of CeO₂ nanoparticles in algae growth medium and deionized water in the presence of various concentrations and two types of natural organic matter (NOM). The presence of natural organic matter stabilizes the CeO₂ nanoparticles in suspension. In presence of NOM, up to 88% of the initially added CeO₂ nanoparticles remained suspended in deionized water and 41% in algae growth medium after 12 d of settling. The adsorbed organic matter decreases the zeta potential from about −15 mV to −55 mV. This reduces aggregation by increased electrostatic repulsion. The particle diameter, pH, electric conductivity and NOM content shows significant correlation with the fraction of CeO₂ nanoparticles remaining in suspension.

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.