A mechanistic study of stable dispersion of titanium oxide nanoparticles by humic acid

Highlights

• Dispersion of TNPs is critical for their efficiency as photocatalysts and sorbents.

• HA can disperse TNPs at a very low concentration with the help of ultra-sonication.

• Electrostatic repulsion is the main mechanism controlling dispersion of TNPs by HA.

• Optimal methods for TNPs dispersion can be developed based on this study.

Abstract

Stable dispersion of nanoparticles with environmentally-friendly materials is important for their various applications including environmental remediation. In this study, we systematically examined the mechanisms of stable dispersion of two types of TiO₂ nanoparticles (TNPs) with anatase and rutile crystalline structures by naturally occurring dissolved organic matter (humic acid) at different pHs, including at, below and above the Point of Zero Charge (PZC). The results showed that stable dispersion of TNPs by humic acid (HA) at all pHs tested can only be achieved with the assistance of ultra-sonication. The dispersion of TNPs by HA differed at the three pHs tested. Generally, HA greatly decreased the hydrodynamic diameters of TNPs at a very low concentration. The dispersion of TNPs became relatively stable when the HA concentration exceeded 5 mg/L, indicating that this HA concentration is required for stable dispersion of TNPs. The mechanisms involved in dispersion of TNPs by HA included electrostatic repulsion, steric hindrance and hydrophobic interaction. Electrostatic repulsion was identified to be the dominant mechanism. The dispersion of TNPs was enhanced when HA was added before ultra-sonication to avoid the partly irreversible re-aggregation of TNPs after sonication. The crystalline phases and concentrations of TNPs were also found to influence their stable dispersion. The findings from this work enhance understanding of the combined effects of HA, pH, ultra-sonication and crystalline structures of TNPs on their stable dispersion. The mechanisms identified can improve applications of TNPs in environmental water pollution control.

Introduction

Nanoparticles are promising novel materials due to their unique physicochemical properties including small size, large surface area, special morphology, catalytic reactivity, and optical properties (Ju-Nam and Lead, 2008, Liu et al., 2011). Among all nanoparticles, titanium dioxide nanoparticles (TNPs) have large production volumes with extensive applications in photo catalysis, as UV protection agent, for sensing and energy storage (Chen and Mao, 2007). Although environmental risks of nanoparticles such as toxic effects on aquatic organisms have been widely reported (Hou et al., 2017a, Hou et al., 2017b), TNPs also have promising potential for environmental remediation because of their high photocatalytic activity (Varshney et al., 2016). As low cost and effective sorbents, they have been applied in the removal of a variety of heavy metals including arsenic, lead and mercury (Qu et al., 2013). They also have a high sorption capacity for organic pollutants (Wang et al., 2008). However, the strong aggregation tendency of TNPs in the aqueous phase is an obstacle to their applications because aggregation may decrease their exposed surface area and alter their surface properties, resulting in decreased sorption capacity and catalytic efficiency (Ju-Nam and Lead, 2008). Modification of TNPs to prevent their agglomeration has been shown to be effective in improving their photocatalytic performance (Li et al., 2010). Enhanced sorption of sulfamethoxazole to carbon nanotubes suspended by HA was also reported (Pan et al., 2013). Thus, to utilize TNPs, it is imperative to improve their dispersion in the aqueous phase.

Ultra-sonication is more effective than other dispersing techniques in breaking up aggregated TNP clusters. It can refine the NPs structure by cavitation and erosion, thus dispersing them (Taurozzi et al., 2011). However, re-agglomeration after sonication is often observed, indicating that the nanoparticle suspension was not sufficiently stabilized (Nguyen et al., 2011, Zhang et al., 2009). As previously reported, the aggregate size of TNPs in tap water increased from 500 nm to 2000 nm within 8 h after sonication for 10 min (Zhang et al., 2008). To prepare stable nanoparticle suspensions, various methods were adopted in addition to sonication, including optimization of the aqueous chemistry (especially pH condition), surface modification of nanoparticles, and addition of dispersant including polymers and surfactants (He and Zhao, 2007, Singh et al., 2012). Although nanoparticle suspensions can be well-stabilized with some dispersants, it should be noted that introduction of these synthetic dispersants may bring new potential threats to the aquatic environment (Dickson et al., 2012). The surface properties of the nanoparticles would also be altered by these dispersants, thus affecting the interactions between nanoparticles and other pollutants (Phenrat et al., 2009). Therefore, one of the critical criteria for selecting dispersants is that they should be natural. Natural organic matter (NOM), which is ubiquitous in aquatic systems, can be an environmentally friendly dispersant option. Humic acid (HA), a major component of NOM and a typical representative of humic substances, may interact with TNPs via electrostatic attraction and ligand exchange (Yang et al., 2009). Its adsorption would increase the electrostatic repulsion and steric hindrance between individual nanoparticles, thereby enhancing their dispersion and stability (Chen et al., 2012). A few studies investigated the influence of humic acid on the stability of TNPs through studying variations of the hydrodynamic diameters of TNPs aggregates. It was observed that TNPs can be disaggregated into small clusters with sizes ranging from 50 to 250 nm by HA at a low concentration representative of environmental levels (Loosli et al., 2013, Zhang et al., 2009). However, these studies mainly focused on the aggregation behavior of TNPs instead of their dispersion. TNPs aggregate size was generally adopted as an indicator of their stability at various HA concentrations. While aggregate size cannot fully characterize the dispersion of TNPs; the aggregated large clusters of TNPs would settle well after standing or centrifugation. The optimal HA concentration for stable TNPs dispersion merits further exploration, and the connection between aggregate size and dispersion of TNPs deserves careful investigation. Besides, the combined effects of sonication and HA on dispersion of TNPs has rarely been discussed, since TNPs were generally pre-dispersed and stabilized before use or were purchased as suspension in previous studies (Loosli et al., 2013, Zhang et al., 2009). The possible difference in stable dispersion of TNPs by HA added before and after sonication and the underlying mechanisms have not yet been investigated. Furthermore, impacts of the intrinsic properties of TNPs and HA such as the concentration and crystalline structure of TNPs, and the effects of extrinsic conditions including pH and ultra-sonication on the role of HA in dispersing TNPs have not yet been elucidated.

To help systematically understand the underlying mechanisms controlling stable dispersion of TNPs, the roles of naturally occurring dissolved organic matter (HA), pH, and ultra-sonication in the dispersion of TNPs with different crystalline structures (anatase and rutile) were tested and compared. The findings of the present work will facilitate the application of stably dispersed TNPs for sustainable water pollution control.