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Article Author: Angela R. Bielefeldt, Michael W. Stewart, Elisabeth Mansfield, et al
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Article Title: Effects of chlorine and other water quality parameters on the release of silver nanoparticles from a ceramic surface
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Effects of chlorine and other water quality parameters on the release of silver nanoparticles from a ceramic surface Angela R. Bielefeldt b,*, Michael W. Stewart a,b, Elisabeth Mansfield a,2, R. Scott Summers b, Joseph N. Ryan b a
National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USA1 University of Colorado Boulder, Dept. Civil, Environmental, & Architectural Engineering, 428 UCB, Boulder, CO 80309-0428, USA b
article info
abstract
Article history:
A quartz crystal microbalance was used to determine the effects of different water quality
Received 1 June 2012
parameters on the detachment of silver nanoparticles from surfaces representative of
Received in revised form
ceramic pot filters (CPFs). Silver nanoparticles stabilized with casein were used in the ex-
14 December 2012
periments. The average hydrodynamic diameter of the nanoparticles ranged from 20 nm to
Accepted 28 January 2013
100 nm over a pH range of 6.5e10.5. The isoelectric point was about 3.5 and the zeta po-
Available online 22 March 2013
tential was 45 mV from pH 4.5 to 9.5. The silver nanoparticles were deposited onto silica surfaces and a quartz crystal microbalance was used to monitor silver release from the
Keywords:
surface. At environmentally relevant ranges of pH (4.8e9.3), ionic strength (0 and 150 mol/
Silver release
m3 NaNO3 or 150 mol/m3 Ca(NO3)2), and turbidity (0 and 51.5 NTU kaolin clay), the rates of
Quartz crystal microbalance
silver release were similar. A high concentration of sodium chloride and bacteria (Escher-
pH
ichia coli in 10% tryptic soy broth) caused rapid silver release. Water containing sodium
Ionic strength
hypochlorite removed 85% of the silver from the silica surface within 3 h. The results
Chlorine
suggest that contact between CPFs and prechlorinated water or bleach CPF cleaning should
Ceramic water filter
be avoided. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Silver nanoparticles (AgNPs) have found an increasing number of uses in recent decades, all of which require an improved understanding of the lifecycle of the silver. AgNPs have been employed in over 250 commercial products, making silver the most prevalent commercial nanoparticle (PEN, 2009). AgNPs, added to improve the disinfection attributes, are now present in clothing, air purifiers, and water filters (PEN, 2009).
Ceramic water filters are used in many developing countries as a household water treatment technology. Currently, there are over 30 operational factories in 18 different countries with a total production capacity of nearly 80,000 filters per year (Rayner, 2009; Clasen, 2009; CMWG, 2011). A common production technique for these filters involves the application of AgNPs to the surfaces of the ceramic filter. There are many examples of ceramic water filtration in both developed (e.g., ceramic membranes) and developing regions (e.g., ceramic
* Corresponding author. Tel.: þ1 303 492 8433; fax: þ1 303 492 7317. E-mail addresses:
[email protected] (A.R. Bielefeldt),
[email protected] (E. Mansfield). 1 Contribution of the US government; not subject to copyright. 2 Tel.: þ1 303 497 6405; fax: þ1 303 497 5030. 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.01.058
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 4 0 3 2 e4 0 3 9
candle filters) that employ silver for its known antimicrobial properties (Doulton Ceramic Water Filters, 1997). In the ceramic pot filter (CPF) application, an aqueous solution of nanoparticle, or colloidal, silver is applied to the entire surface area of the filter. Potters for Peace promotes application of about 45 mg of silver per filter, either by brushing a colloidal solution onto the filter or by dipping the entire filter into the silver solution (Nardo, 2005). Other production models exist where silver nitrate is applied to the filter instead of nanoparticulate silver, or where AgNPs are actually fired into the ceramic matrix (CMWG, 2011; Hagan et al., 2009). There is a general lack of understanding about the factors that impact both deposition and release of AgNPs from ceramic surfaces. In CPFs it has been observed that the silver releases from the surface over time leading to degradation of the disinfection attributes of the filter. Studies that have monitored the release of silver have traditionally focused on the microbial disinfection efficiency of the overall filter rather than the behavior of the silver itself (Oyanedel-Craver and Smith, 2008; Kohler, 2009; Lantagne, 2001). This approach is valuable in that the end goal of these filters is to produce safe drinking water by reducing the microbial contamination. However, silver release as a function of different manufacturing processes and water quality is unknown. A deeper examination of the behavior of the silver may allow for improvements in the coating process, better prediction of the lifetime of the coating, and recommendations for filter handling procedures once the coating has been applied. The quartz crystal microbalance (QCM) technique is used to study surface interactions where high mass sensitivity is required. QCM can measure the mass, viscosity, and density properties of a sample at the surface of the sensor. This provides insight that may be otherwise lost by simply analyzing bulk properties of the specimen. The fundamental piezoelectric behavior that is integral to QCM measurements was described as early as the 1880’s (Ward and Buttry, 1990; Mould, 2007). Sauerbrey (1959) adapted the piezoelectric resonance behavior of quartz crystals into a measurement showing that the change in resonance frequency is directly proportional to the quantity of mass added to the sensor’s surface, provided that the added mass has the form of a thin, uniform film that is rigidly attached to the quartz crystal. The QCM method can reliably detect a 1 Hz shift, up to 2% change, in the baseline resonant frequency of the sensor, which corresponds to a mass of about 1 nge150 mg on the surface of the typical 0.24 cm2 working electrode (SRS, 2005). Through measurement of the dissipation of the crystal resonance at multiple harmonics, the QCM can also provide information about the viscosity and elasticity of the material being analyzed (Rodahl, 1995; Hook, 1997). QCM methods to evaluate aqueous nanoparticle-surface interactions are well established. Nanoparticles such as quantum dots have been studied to understand interaction and deposition on silica surfaces using the QCM (Quevedo and Tufenkji, 2009). Quantum dot studies have shown that surface interactions can be greatly influenced by ionic strength and monovalent or divalent ions. The QCM platform has also been used to detect Cryptosporidium parvum in water samples containing natural interferents, such as dissolved organic matter (DOM), other microbes, microbial secretions, and other broadly categorized natural colloidal particles (Poitras et al.,
4033
2009). In the work by Poitras et al. (2009), the dissipation was determined to be a better data source as a biosensor than the frequency shifts. Microbes have a detectable viscosity that may affect the dissipation readings greater than the frequency. For example, Escherichia coli O157:H7 at a concentration of 5 106 cells/mL flowing across the surface of the antiC. parvum antibody functionalized QCM-D sensor (QSX301, gold-coated) was not sufficient to cause detectable shifts in either frequency or dissipation (Poitras et al., 2009). In this study, the QCM was utilized to gain insight into the effects of pH, turbidity, ionic strength, DOM, and chlorine on the release of silver from a ceramic surface. The conditions of the study were chosen to represent environmental exposures of the CPFs in the field. Source water may contain a range of these attributes, while chlorine may be encountered due to intermittent presence in piped water or from user-adopted cleaning methods.
2.
Materials and methods3
2.1.
Materials
All silver used in these experiments was the commercially available product Collargol Colloidal Silver French IX Edition, a powdered form of silver nanoparticles produced by Laboratorios Argenol of Spain and used by many CPF factories. The AgNPs were provided mixed with casein (C47H48N3O7S2Na), a dairy milk-derived protein, to aid in their suspension. According to the manufacturer specifications, the mixture contains 70e75 percent silver by weight, leaving 25e30 wt% casein. For coating CPFs, Potters for Peace recommends mixing 2 mL of a stock 3.2 wt% solution (prepared earlier from the powdered AgNPs) with 250 mL of filtered water, creating a solution of 0.018 wt% silver, assuming a silver content of 70 percent of the product supplied by Laboratorios Argenol (Lantagne, 2001; Nardo, 2005). This 0.018 wt% AgNP solution was prepared with ultrapure water and is termed the “application solution”. Ultrapure water was generated with a Direct-Q3 UV system (Millipore). pH adjustment utilized 103 mol/m3 nitric acid or sodium hydroxide. Turbidity was increased to a nominal value of 50 NTU using kaolin powder (Fisher Scientific) and measurements were taken on a Hach Ratio/XR Turbidimeter. Sodium nitrate (NaNO3) and calcium nitrate (Ca(NO3)2) were selected as the representative monovalent and divalent ions. Sodium hypochlorite solutions at concentrations of 8.8 and 525 mg/L, were used to represent chlorinated drinking water and a household bleach (1%) cleaning solution, respectively. At a pH of 8.5, the free chlorine was a mix of HOCl and mostly OCl (pKa ¼ 7.6). Dissolved organic matter extracted from Mirror Lake (CO) was added to yield 15 mg/L total organic carbon (TOC) with an ionic strength of 0.93 mol/m3 based on specific conductivity. 3 Certain commercial equipment, instruments, or materials are identified in this document. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products identified are necessarily the best available for the purpose.
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The nanoparticles and nanoparticle solutions were characterized using transmission electron microscopy (TEM; JEOL 2000FX). A 10 mL drop of the application solution was applied to a standard 300 mesh copper TEM grid were used to image the silver nanoparticles at 200 kV with LaB6 filament using a 1024 1024 pixel CCD camera. The particle size distribution (PSD) and zeta potential were determined by use of a Zetasizer Nano instrument (Malvern Instruments) by means of dynamic light scattering (DLS). Measurements were taken across a range of pH values from 3 to 10, adjusted using 103 mol/m3 nitric acid or sodium hydroxide, and repeated five times at each pH value. Inductively coupled plasma mass spectroscopy (ICP-MS) was employed to determine the elemental composition of the colloidal silver solution and effluent silver concentrations. The mixing and timing of these solution measurements were consistent with the solution applied to the QCM sensors.
2.2.
Quartz crystal microbalance studies
A set of initial experiments were conducted using the Q-sense E4 QCM and commercially available standard sensors with gold electrodes. Different amounts of silver were drop deposited onto the sensors. Two different sets of aqueous solutions were flowed over the sensors at 0.2 mL/min (Table 1). First, ultrapure water followed by 10% tryptic soy broth (TSB), followed by the addition of E. coli DH5-a to the TSB, and finally a rinse with 10.5 g/L NaOCl at pH 7.1, which yields a mix of OCl and mostly HOCl. TSB is comprised of 57% enzymatic digest of casein, 10% enzymatic digest of soybean meal, 17% sodium chloride, 8% dipotassium phosphate, and 8% dextrose. In the second set of initial experiments, the flow solution was 1% phosphate buffered saline (PBS) followed by the addition of E. coli to the PBS, and finally the 10.5 g/L NaOCl rinse at pH 7.1. The results of these experiments informed the subsequent experiments using different sensor surfaces to better represent alumino-silicates.
The impact of pH, turbidity, ionic strength, and DOM were measured using the E4 QCM and QSX-303 quartz sensors (Q-sense). The QCM allows replication by running four sensors in parallel using the exact same influent water. The fundamental resonance frequency is 4.95 MHz (0.05 MHz) with an electrode surface area of 0.24 cm2. The sensors have a 50 nm layer of silicon dioxide, or silica, coating that is sputtered onto the working surface of the sensor, on top of the gold electrode. This crystalline SiO2 material (isoelectric point w1.5e3) is the closest commercially available material to represent the alumino-silicates typical of CPFs (isoelectric point 4e5) (Jara et al., 2005). The clay used in CPFs are generally illite, kaolinite, or bentonite and these clays often contain some SiO2 (Adamis et al., 2005). Each sensor was cleaned following the procedure recommended by the manufacturer. The baseline resonance was determined under dry conditions and then with flow of the sample water over the sensor. Data collection was carried out for a minimum of 10 min, seeking a stable baseline measurement. During that time, if any of the sensor’s frequency readings drifted more than 2 Hz, the measurement was started over (per manufacturer’s recommendation and Cho et al., 2010). The types of waters tested are summarized in Table 1. Next, each sensor had AgNPs applied. First, the sensors were cleaned using the multi-step process recommended by the manufacturer. Each sensor was placed onto the cleaned surface of a hot plate maintained between 65 and 75 C and allowed to equilibrate by resting on the hot plate for 30e60 min. Then 5 mL of the application solution (prepared as described above by adding colloidal silver to Millipore ultrapure water) was slowly drop cast onto the center of the electrode area using a 5 mL mechanical pipette. A fraction of the application solution was applied and after the majority of the water had evaporated from that droplet, another small fraction of the application solution was released onto the same location, and the procedure repeated a total of about 3e4 times until the entire 5 mL had been added to the sensor. The sensor then remained on the hot plate
Table 1 e Experimental conditions. Sensor Standard quartz with gold electrode
Quartz with silica coated gold electrode
Silver, mg/sensor
Aqueous solution
Replicate sensors
pH
0 1 5 10 0 1 5 10
10% tryptic soy broth (TSB), 1.2 g/L TOC followed by 109 CFU/mL E. coli in 10% TSB followed by 10.5 g/L NaOCl
1 1 1 1 1 1 1 1
7.0 7.0 7.0 7.0 7.1 7.1 7.1 7.1
21 21 21 21 2 2 2 2
7 3 4 4 4 4 4 4 4
5.8 4.8 9.3 5.6 5.4 5.7 8.5 8.5 7.1
8.8E-4 5.0E-3 0.015 8.8E-4 150 150 0.12 7.0 0.9
1 1 1 1 1 1 1 1 1
1% phosphate buffered saline (PBS) followed by 105 CFU/mL E. coli in 1% PBS followed by 10.5 g/L NaOCl
Millipore ultrapure Low pH (HNO3) High pH (NaOH) Turbidity 51.5 NTU kaolin Ionic strength NaNO3 Ionic strength Ca(NO3)2 NaOCl 8.8 mg/L NaOCl cleaning 525 mg/L DOM e TOC 15 mg/L
TOC of aqueous solutions
