A mechanistic study of stable dispersion of titanium oxide nanoparticles by humic acid
Graphical abstract
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.
Section snippets
TiO2 nanoparticles
Both the pristine anatase and rutile TiO2 nanoparticles (ATNPs and RTNPs) were used in this study. They were purchased from Nanjing Guanye Chemical Industry Co., Ltd. XRD spectra of ATNPs and RTNPs are available in our previous publications (Wang et al., 2014). The TNP suspension at concentrations of 100 and 200 mg/L was prepared by adding TNP powder to the background solution containing 3 mM NaCl (prepared with Milli-Q water). The pH of the suspension was adjusted with 0.1 M HCl and NaOH. To
Effect of pH on the dispersion of TNPs
Aggregation behaviors of TNPs under different pH conditions have been discussed in previous studies (Loosli et al., 2013, Loosli et al., 2015, Zhang et al., 2008). These studies highlighted the influence of pH on TNPs stability, which was mainly through changing their ζ potential values. The ζ potential of TNPs is generally positive at low pH, and gradually decreases to negative values as pH is increased. The closeness of pH to PZC determines the electrostatic repulsive force between
Conclusions
The impacts of HA and ultra-sonication on the dispersion of TNPs with two crystalline structures at different pHs and TNP concentrations were systematically investigated in this work. The main conclusions can be summarized as follows:
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Stable dispersion of TNPs requires both HA and the assistance of ultra-sonication. Ultra-sonication was effective in breaking up TNPs agglomerations, but re-aggregation of TNPs after ultra-sonication was inevitable in the absence of HA, especially at the PZC of
Acknowledgments
This study was supported by the National Science Fund for Distinguished Young Scientist (41525005), the 973 Program (2014CB441104), National Natural Science Foundation of China (41390240, and 41629101), and the 111 Program (B14001).
References (37)
Aggregation and disaggregation of iron oxide nanoparticles: influence of particle concentration, pH and natural organic matter
Sci. Total Environ.
(2009)- et al.
Dispersion and stability of bare hematite nanoparticles: effect of dispersion tools, nanoparticle concentration, humic acid and ionic strength
Sci. Total Environ.
(2012) - et al.
Ecotoxicological effects and mechanism of CuO nanoparticles to individual organisms
Environ. Pollut.
(2017) - et al.
Manufactured nanoparticles: an overview of their chemistry, interactions and potential environmental implications
Sci. Total Environ.
(2008) - et al.
Effect of the agglomeration of TiO2 nanoparticles on their photocatalytic performance in the aqueous phase
J. Colloid Interface Sci.
(2010) - et al.
Effects of material properties on sedimentation and aggregation of titanium dioxide nanoparticles of anatase and rutile in the aqueous phase
J. Colloid Interface Sci.
(2011) - et al.
TiO2 nanoparticles aggregation and disaggregation in presence of alginate and Suwannee River humic acids. pH and concentration effects on nanoparticle stability
Water Res.
(2013) - et al.
Effect of electrolyte valency, alginate concentration and pH on engineered TiO2 nanoparticle stability in aqueous solution
Sci. Total Environ.
(2015) - et al.
Environmental implications of aggregation phenomena: current understanding
Curr. Opin. Colloid In.
(2006) - et al.
Effect of ultrasonication and dispersion stability on the cluster size of alumina nanoscale particles in aqueous solutions
Ultrason. Sonochem.
(2011)
Applications of nanotechnology in water and wastewater treatment
Water Res.
Further insights into the universality of colloidal aggregation
Adv. Colloid Interfac
The role of poly(methacrylic acid) conformation on dispersion behavior of nano TiO2 powder
Appl. Surf. Sci.
Nanoscale TiO2 films and their application in remediation of organic pollutants
Coord. Chem. Rev.
Effect of model dissolved organic matter coating on sorption of phenanthrene by TiO2 nanoparticles
Environ. Pollut.
Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles
Water Res.
Stability of commercial metal oxide nanoparticles in water
Water Res.
Distinct effects of humic acid on transport and retention of TiO2 rutile nanoparticles in saturated sand columns
Environ. Sci. Technol.
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