The effect of humic acid on the aggregation of titanium dioxide nanoparticles under different pH and ionic strengths
Graphical abstract
Introduction
The application of nanotechnology in consumer and industrial products has increased exponentially over the past several years. Nanomaterials are widely used in herbicides, cosmetics, printers, electronics, groundwater remediation, waste water treatment, and many other applications (Keller et al., 2013, Theron et al., 2008). The toxicity of nanomaterials is determined by their size, shape, chemical structure and surface properties. It is imperative to evaluate the potential risks that these novel materials pose to the environment and human health, and the first step is to assess their mobility in the environment (Wiesner et al., 2006).
During the recent years, the potential risks of nanoparticles have raised concerns. The cytotoxicity of TiO2 NPs has been discussed (Jin et al., 2008). It has been reported that TiO2 NPs can cause respiratory toxicity and disturbances in metabolism (Federici et al., 2007). A single intratracheal injection of 0.1Ā mg nanoTiO2 can induce severe pulmonary inflammation and emphysema (Chen et al., 2006). TiO2 NPs can induce clastogenicity, genotoxicity, oxidative DNA damage, and inflammation (Trouiller et al., 2009). In addition, under low intensity UVR, ROS in seawater increases with increasing nano-TiO2 concentration, which leads to increased overall oxidative stress in seawater, and causes decreased resiliency of marine ecosystems (Miller et al., 2012). Nano-TiO2 had positive effects on root elongation in some species (Song et al., 2013). TiO2 NPs have cytotoxic, genotoxic and hemolytic effects on human erythrocyte and lymphocyte cells in vitro and can induce a significant reduction in mitochondrial dehydrogenase activity in human lymphocyte cells (Ghosh et al., 2013). Nanoparticles can be transported in the human body, deposited in organs, transferred across cell membranes and accumulated in mitochondria, causing damage to humans (Colvin, 2003).
To some extent, the size range of nanoparticles in natural water determines their stability, toxicity, transport and ultimate fate (Zhang et al., 2008, Zhang et al., 2012). As reported, the size of nanoparticles is related to solution chemistry, typically including natural organic matter (NOM), ions and pH, and it was also found that three nanoparticles (TiO2, ZnO, CeO2) can aggregate easily when the IS is high and the total organic carbon (TOC) is low; conversely, their stability is high when the TOC is high for a wide range of IS, but not at very high IS (Keller et al., 2010). NOM in aquatic systems has a significant effect on the transport and transformation of the nanoparticles, and the nanoparticles' adsorption capacity toward hydrophobic organic compounds (HOCs) will change their stability and toxicity (Adegboyega et al., 2013, Baalousha et al., 2013, Louie et al., 2013, Wirth et al., 2012, Yang et al., 2009, Zhang et al., 2013). NOM is also commonly present in ground water and surface water, although at different concentrations (Wang et al., 2010, Wang et al., 2011). These macromolecules can be adsorbed onto the surfaces of the nanoparticles, enhancing their stability and mobility in flowing waters (Chappell et al., 2009, Hyung et al., 2007). The adsorption capacity of the nanoparticles is related to the pH, and NOM is absorbed by the nanoparticles' surface via electrostatic adsorption and ligand exchange (mainly hydroxyl and carboxyl groups). The adsorption of NOM reduces the surface zeta potential and changes the electrostatic repulsion, which affects the nanoparticle stability (Yang et al., 2009).
Humic substances (HS) represent an active and important fraction of natural organic matter (NOM), and they play important roles in the fate and transport of pollutants (Aiken, 1985, Buffle et al., 1998, Zheng et al., 2008). HS can be categorized into three groups: fulvic acids (FAs), which are the major component and the smallest structures of HS, and are soluble at any pH; HA, which represents bigger structures that are insoluble at pH <Ā 2; and humin, which is insoluble at any pH (Jones and Bryan, 1998, Piccolo, 2001). The role of HA in nanoparticle transport is of particular interest. The fate of fullerenes (nC60) and their toxic implications in natural freshwaters are affected by humic acid (Meng et al., 2013, Pakarinen et al., 2013), and humic acid is more effective than fulvic acid at stabilizing nC60 nanoparticles (Zhang et al., 2013). Humic acid affects the adsorption of other pollutants on the surface of multiwalled carbon nanotubes (MWCNTs) (Hou et al., 2013). Humic acid also reduces the removal of TiO2 NPs via coagulation (Wang et al., 2013).
TiO2 NPs are some of the most widely used nanoparticles. Most TiO2 NPs are used for coatings, paints, and pigments as well as cosmetics (Keller et al., 2013). In 2010, over 34,000Ā t of TiO2 NPs were used in coatings, paints, and pigments, and TiO2 is by far the most significant engineered nanoparticle materials in terms of exposure, based on the estimated releases and use in the dominant applications (Keller and Lazareva, 2013). At the nanoscale, the refractive index of TiO2 NPs is even higher; therefore, it is also used in sunscreens and cosmetics as well as paints, varnishes, and coatings (Hendren et al., 2011). The adsorption of HA increased TiO2 NP stability in suspension, and TiO2 NPs were more toxic in the presence of HA (Yang et al., 2013). It was demonstrated that the aggregation of TiO2 NPs is dependent on the presence and copresence of NOM (0.2ā41.1Ā mg/L DOC), the pH of the solution, as well as other properties, such as the IS of the solution and the presence of relevant monovalent and divalent ions (Ottofuelling et al., 2011). Zhang et al. tested the stability of TiO2 at various DOC concentrations (1ā10Ā mg/L NOM, equivalent to 0.406ā4.06Ā mg/L DOC) and observed the stability in the presence of 0.4Ā mg/L DOC (Zhang et al., 2009). However, it is not clear if a very low concentration of NOM (e.g., <Ā 0.2Ā mg/L DOC) has the same effect on the stability of TiO2 NPs; a low concentration of DOC is possible in some industrial wastewater.
The goal of this study was to study the effect of HA on the aggregation or stabilization of TiO2 NPs under different pH and ISs. We chose three typical pH: pHĀ =Ā 4 (pHĀ <Ā pHpzc); pHĀ =Ā 5.8 (pHĀ =Ā pHpzc); and pHĀ =Ā 8 (pHĀ >Ā pHpzc). Because TiO2 NPs are very stable at pHĀ =Ā 8, we also measured the critical coagulation concentration (CCC) to further investigate the role of HA in the stabilization of TiO2 NPs at high pH. Finally, to compare the different CCCs at pHĀ =Ā 4 and pHĀ =Ā 8, we studied the aggregation and sedimentation performances under high IS (>Ā CCC).
Section snippets
Titanium dioxide nanoparticles
NanoTiO2 (rutile) was purchased from Sigma-Aldrich Trading Co., Ltd. (Shanghai, China). As reported by the manufacturer, these nanoparticles have a diameterĀ ĆĀ length of 10Ā nmĀ ĆĀ 40Ā nm; specific surface areaĀ =Ā 130ā190Ā m2/g; purityĀ =Ā 99.5%; and may contain up to 5Ā wt.% silicon dioxide as a surface coating. In our recent publication (Qi et al., 2013), we used scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images to characterize the size and shape of the TiO2 NPs. The
TiO2-NP stability as a function of HA concentration
At pHĀ =Ā 4 (pHĀ <Ā pHpzc) and in the absence of HA, the zeta potential of the TiO2 NPs is 37Ā mV (Fig. S1). With the addition of 210Ā Ī¼g/L HA, the zeta potential decreases to āĀ 20.6Ā mV (Fig.Ā 1 inset). Fig. S2 shows that HA carries negative charge when pH ranges from 4 to 8. The charge of the hydrophobic fraction of NOM is associated mainly with the carboxylic and phenolic groups (Duffy, 2010, Edwards et al., 1996). The former have pKa values in the range between 2.5 and 5, while the phenolic hydrogens have
Conclusions
Our results indicated that HA plays different roles under different pH and IS conditions. When the IS is very low, the aggregation of the TiO2 NPs is promoted by the adsorption of HA only when the TiO2 NP surface is positively charged. When the pH is below the pHpzc, the addition of HA reduces the zeta potential of the TiO2 NPs and reverses the surface charge of the TiO2 NPs. When the pH is close to the pHpzc, a low concentration of HA (31.5Ā Ī¼g/L) can stabilize 20Ā mg/L of TiO2 NPs by altering
Conflict of interest statement
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgments
This work was supported in part by the National Natural Science Foundation of China Fund (No. 51108328). The research was also partially supported by 111 Project and the Fundamental Research Funds for the Central Universities (0400219184) and State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRY11011).
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