Influence of extracellular polymeric substances on the aggregation kinetics of TiO₂ nanoparticles

Highlights

• TiO₂ particles were significantly aggregated in the tested electrolyte solutions.

• Ca²⁺ ions were more efficient in destabilizing TiO₂ NP than Na⁺ ions.

• The aggregation of TiO₂ NP in solutions was agreement with DLVO theory.

• In NaCl and low CaCl₂, EPS stabilized the NP suspension due to steric repulsion.

• At high CaCl₂, EPS increased aggregation rate of NPs by intermolecular bridging.

Abstract

The early stage of aggregation of titanium oxide (TiO₂) nanoparticles was investigated in the presence of extracellular polymeric substance (EPS) constituents and common monovalent and divalent electrolytes through time-resolved dynamic light scattering (DLS). The hydrodynamic diameter was measured and the subsequent aggregation kinetics and attachment efficiencies were calculated across a range of 1–500 mM NaCl and 0.05–40 mM CaCl₂ solutions. TiO₂ particles were significantly aggregated in the tested range of monovalent and divalent electrolyte concentrations. The aggregation behavior of TiO₂ particles in electrolyte solutions was in excellent agreement with the predictions based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Divalent electrolytes were more efficient in destabilizing TiO₂ particles, as indicated by the considerably lower critical coagulation concentrations (CCC) (1.3 mM CaCl₂ vs 11 mM NaCl). The addition of EPS to the NaCl and low concentration CaCl₂ (0.05–10 mM) solutions resulted in a dramatic decrease in the aggregation rate and an increase in the CCC values. For solutions of 11 mM NaCl (the CCC values of TiO₂ in the absence of EPS) and above, the resulting attachment efficiency was less than one, suggesting that the adsorbed EPS on the TiO₂ nanoparticles led to steric repulsion, which effectively stabilized the nanoparticle suspension. At high CaCl₂ concentrations (10–40 mM), however, the presence of EPS increased the aggregation rate. This is attributed to the aggregation of the dissolved extracellular polymeric macromolecules via intermolecular bridging, which in turn linked the TiO₂ nanoparticles and aggregates together, resulting in enhanced aggregate growth. These results have important implications for assessing the fate and transport of TiO₂ nanomaterials released in aquatic environments.

Introduction

With the rapid growth of the nanotechnology industry, large amounts of manufactured nanoparticles (NPs) have been released into the environment, especially into surface water and soil (Batley et al., 2013, Dwivedi et al., 2015, Keller et al., 2013, Wiesner et al., 2006). Because of their small size and high capacity to convey toxic substances, nanoparticles can have adverse effects on bacterial communities and human health (Christian et al., 2008, Ge et al., 2011, Jeng and Swanson, 2006). Interactions of manufactured NPs with natural colloids including organic matter (Chowdhury et al., 2012) and clay minerals (Batley et al., 2013) will dramatically change their fate and potential toxicity in the environment due to the impact of surface coating, surface charge modifications, aggregation, or stabilization and dispersion.

Among oxide based NPs, titanium dioxide (TiO₂) is one of the most heavily produced nanomaterials due to its wide use in sunscreens, cosmetics, catalysts, bottle coatings, energy storage, plastic, fibers, foods, pharmaceuticals, and antimicrobial materials (Chen et al., 2015, Chen and Mao, 2007, Loosli et al., 2013). According to Keller et al. (2013), over 34,000 tons of TiO₂ is produced all over the world per year. It has been reported that TiO₂ can enter the aquatic systems through multiple pathways, and the wastewater treatment plants (WWTPs) are thought to be the most important distribution pathway (Fang et al., 2009, Gottschalk et al., 2010, Kaegi et al., 2008). Such exposures can adversely impact the aquatic environment, and this water when used in agriculture, can have deleterious effects on the soil systems (Batley et al., 2013, Seitz et al., 2012, Weir et al., 2012). Much attention has been paid to understanding the factors controlling the fate and transport behavior of TiO₂ in aqueous systems including solution pH, ionic strength, electrolyte valence, and primary particle size (Erhayem and Sohn, 2014, French et al., 2009, Guzman et al., 2006, Loosli et al., 2013). For example, TiO₂ NP aggregation occurred near the point of zero charge but particles were observed to be stable at other pH values (Guzman et al., 2006, Loosli et al., 2013). At any given pH, an increase in ionic strength generally results in increased aggregation, and divalent cations are more effective than monovalent cations in facilitating TiO₂ aggregate formation (Erhayem and Sohn, 2014, French et al., 2009). By using static light scattering, Chowdhury et al. (2013) found a difference in the aggregate morphology of TiO₂ NPs as a function of particle size, pH, ionic strength, and ion valence.

When discharged into natural environments, TiO₂ NPs interact with natural organic matter (NOM), which is composed of humic substances (HS) and non-humic substances (Huangfu et al., 2013, Li et al., 2015, Loosli et al., 2013, Saleh et al., 2010). HS mainly consist of fulvic acids (FA) and humic acids (HA), and non-humic substances mainly consist of biological macromolecules such as proteins and polysaccharides (Stevenson, 1994). Sorbed NOM can change particle surface charge, thereby altering stability (Chowdhury et al., 2012, Domingos et al., 2009, Sheng et al., 2016). As previously reported, the adsorption of the Suwannee River Fulvic Acid resulted in less aggregation of TiO₂ NPs due to increase steric repulsion and electrostatic force (Domingos et al., 2009). Different structures and constituents of NOM can also lead to distinct effects on nanoparticles aggregation. Several studies have shown that bovine serum albumin (globular protein molecules) is effective in reducing the rate of aggregation, whereas alginate (linear polysaccharide block copolymer) accelerates TiO₂ aggregation obviously in CaCl₂ electrolyte solution (Hu et al., 2014, Liu et al., 2008, Romanello and Fidalgo de Cortalezzi, 2013). In addition, aromatic-rich HA has been found to be more capable of stabilizing TiO₂ NPs than aliphatic-rich HA (Li et al., 2015). The findings from these studies indicate that organic molecules do impact the aggregation of TiO₂ NPs. In surface water and soil environments, extracellular polymeric substance (EPS) is a heterogeneous mixture continuously secreted by microorganisms during growth and metabolism (Beveridge et al., 1997, Wingender et al., 1999). EPS consists mainly of polysaccharides and proteins with carboxyl, phosphoryl, amide, amino, and hydroxyl functional groups (Hoffman and Decho, 1999, Omoike and Chorover, 2006). Once TiO₂ NPs are released into the environment, their stability is assumed to be affected by EPS. Although these biological macromolecules are ubiquitous in the environment, to our knowledge, few studies have been conducted on the role of EPS in the aggregation of particles (Koukal et al., 2007, Labille et al., 2005).

The objective of the present study was to investigate the effect of EPS constituents on the aggregation kinetics of TiO₂ NPs in the presence of NaCl and CaCl₂ through time-resolved dynamic light scattering (DLS). In addition, Fourier transform infrared (FTIR) spectroscopy was used to qualitatively describe the constituents and functional groups of EPS, protease-treated EPS (PT-EPS) and cellulose-treated EPS (CT-EPS). The results of this study will provide fundamental information on the stability of TiO₂ NPs in natural environments.