Influence of pH and ionic strength on the deposition of silver nanoparticles on microfiltration membranes
Introduction
The rapid development of nanotechnology will inevitably result in the discharge of nanosized particles into the environment as a result of the production, use, and disposal of nanomaterials [1]. Nanosilver is currently one of the most promising and widely used nanomaterials and can be found in a myriad of products, such as cosmetics, detergent, clothing, personal care products, and medical devices [2], [3]. Because of the growing use of nanosilver in commercial products, there is concern about the possible release of silver nanoparticles (AgNPs) into the natural environment [4] and their fate in natural and engineered systems.
Silver ions (Ag+) are well known to be toxic to microorganisms and some higher aquatic animals, and act by deactivating the thiol groups of membrane-bound enzymes and proteins [5], [6]. Many commercial products incorporating AgNPs exhibit bactericidal properties. It is thought that the bactericidal activity of these products results from the release of Ag+ during particle dissolution [7], [8]. Some studies, however, suggest the AgNPs themselves may be toxic to microorganisms and animals (for a review, see [9]). The generation of reactive oxygen species (ROS) is one proposed mechanism by which AgNPs inhibit microbial growth and cause organ injury in animals [10], [11]. A recent study, however, suggests that dissolved silver ions are the primary toxicant associated with AgNPs and that the morphologic properties of AgNPs known to affect toxicity do so by influencing the solubility of AgNPs [12].
Microfiltration is a promising water treatment technique for supplying high quality drinking water with low energy demand [13], [14] and is seeing increasing application worldwide. Little is known, however, about the fate of AgNPs during the microfiltration process, especially under different solution conditions. During the filtration process, suspended colloids and particles are either rejected, transported from the bulk solution and deposited onto the membrane surface or within membrane pores, or they pass through the membrane into the permeate. The relative importance of these different pathways will determine the fate of AgNPs in microfiltration systems and the extent to which these particles impact the treated water (i.e., permeate) or waste (i.e., concentrate).
The transport of nanosized particles to the membrane surface is governed by Brownian diffusion at low to moderate cross-flow velocities [15] and advective transport at higher permeate flow rates. Upon close approach to the membrane surface, deposition is controlled by the net attractive or repulsive surface chemical forces that develop between the particles and the membrane or between particles and other deposited particles. The deposition of colloidal particles has traditionally been described by DLVO (Derjaguin, Landau, Verwey and Overbeek) theory in which the total interaction energy is the sum of van der Waals and double layer interactions energies [16], [17]. DLVO theory predicts, and experimental studies have shown, that solution chemistry (e.g., pH and ionic strength) plays an important role in controlling particle–particle and particle–membrane interactions by affecting the surface charge of the colloidal particles and the membrane or influencing the electrical double layer or both [18], [19], [20]. For example, Singh and Song showed that fouling of ceramic membranes by colloidal silica significantly decreased with increasing pH as a result of the increasingly negative surface charge of the particles and the development of greater colloid–colloid repulsive forces [19]. Alternately, increasing ionic strength compresses the electric double layer, thereby, reducing electrostatic repulsive forces and allowing deposition and fouling to increase [19], [21], [22]. The presence of chloride ion (Cl−) complicates the mechanisms of AgNP removal during membrane filtration. Silver ions (Ag+) released or adsorbed onto the surface of AgNPs can combine with the chloride ion to form AgCl particles and to dissolve as AgCl2−, AgCl32−, which facilitates the dissolution of AgNPs and changes the particle surface charge [23], [24].
The objective of this study was to investigate the effects of pH and ionic strength on the deposition of AgNPs on microfiltration membranes. Laboratory-synthesized AgNPs and a commercial microfiltration membrane were used in a dead-end filtration system under different pH and ionic strength conditions. Sodium chloride was introduced into the system to adjust ionic strength. To quantitatively evaluate colloidal deposition and membrane fouling, the concentration of total silver in the permeate was measured and permeate flux decline was evaluated. The effect of chloride ion on AgNPs removal was also examined. The results presented here provide important new information regarding the fate of AgNPs in membrane filtration systems.
Section snippets
Chemicals
AgNO3 (99.8%) and d-maltose (99%) were purchased from Sigma-Aldrich. Ammonium hydroxide (trace metal grade) was purchased from Fisher Scientific. Other chemicals such as sodium chloride, sodium bicarbonate, and sodium hydroxide were analytical grade or better. Silver analytical standards for ICP-AES calibration were purchased from Fisher Scientific. All solutions were prepared with deionized water (Milli-Q, Millipore) with a resistivity of 18.2 MΩ cm. The background electrolyte consisted of 3.0×10
Characterization of AgNPs and the membrane
The hydrodynamic diameter of the AgNPs as measured by DLS was 73.3±1.9 nm. The particles were considered roughly spherical and the size distribution monodisperse based on our previous studies using this same synthesis technique [25]. The UV–vis absorption spectrum (Fig. 2) of the AgNPs showed peak absorption at a wavelength of 440.5 nm indicating the existence of an oxidized surface layer, consistent with our previous study [25]. Based on zeta potential measurements (Fig. 3), the particles
Conclusions
Results from this study show that pH, ionic strength and surface complexation play an important role in controlling the fate of AgNPs during microfiltration. Increasing pH enhanced repulsive particle–particle and particle–membrane electrostatic interactions resulting in greater particle breakthrough. Increasing ionic strength, on the other hand, reduced electrostatic repulsive forces thereby leading to greater AgNPs deposition on the membrane and lower concentrations of silver in the permeate
Acknowledgments
This work was funded through a grant from the Ohio Water Development Authority. Support for T. Yin from the Ohio Water Resources Center is also gratefully acknowledged.
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