A New Method for the Deposition of Metallic Silver on Porous Ceramic Water Filters

A new method of silver application to a porous ceramic water filter used for point-of-use water treatment is developed. We evaluated filter performance for filters manufactured by the conventional method of painting an aqueous suspension of silver nanoparticles onto the filter and filters manufactured with a new method that applies silver nitrate to the clay-water-sawdust mixture prior to pressing and firing the filter. Filters were evaluated using miscible displacement flow-through experiments with pulse and continuous-feed injections of E. coli. Flow characteristics were quantified by tracer experiments using [H]H2O. Experiments using pulse injections of E. coli showed similar performance in breakthrough curves between the two application methods. Long-term challenge tests performed with a continuous feed of E. coli and growth medium resulted in similar log removal rates, but the removal rate by nanosilver filters decreased over time. Silver nitrate filters provided consistent removal with lower silver levels in the effluent and effective bacterial disinfection. Results from continued use with synthetic groundwater over 4 weeks, with a pulse injection of E. coli at 2 and 4 weeks, support similar conclusions—nanosilver filters perform better initially, but after 4 weeks of use, nanosilver filters suffer larger decreases in performance. Results show that including silver nitrate in the mixing step may effectively reduce costs, improve silver retention in the filter, increase effective lifespan, and maintain effective pathogen removal while also eliminating the risk of exposure to inhalation of silver nanoparticles by workers in developing-world filter production facilities.


Introduction
e World Health Organization (WHO) estimates that over 4 million deaths per year, of which more than 1.5 million involve children under the age of 5, are attributable to unsafe drinking water [1].Centralized water treatment facilities, like those found in cities and suburban areas in the developed world, are not feasible for many developing communities due to the large infrastructure investment.Alternatively, the WHO has suggested a decentralized approach of treatment in home immediately prior to consumption-commonly referred to as point-of-use (POU) water treatment [2,3].POU technologies have the potential to significantly improve microbial quality of drinking water and reduce the risk of diarrheal disease and death, particularly in children [4].A POU technology must be effective with respect to removal and/or deactivation of waterborne pathogens under a wide range of water chemistries and must be simple to use to ensure long-term effectiveness and reduce risk of recontamination [3,5].e technology much also be socially acceptable and affordable, commonly achieved by the use of local labor and materials [6][7][8].Ceramic water filters, produced with local labor and materials, are an appealing POU water treatment technology, and over 50 production facilities exist worldwide [9].Clay, sawdust, and water are mixed and then molded into a pot shape.e filter is then fired in a kiln, causing the clay to sinter into a ceramic and sawdust to combust. is creates pore channels that allow water flow.After quality testing, the filter is painted with a silver nanoparticle solution, where the silver acts as a wellstudied antimicrobial agent without changing the taste, color, or odor of treated water [10][11][12][13][14]. e ceramic filter is suspended inside a plastic bucket with a spigot on the bottom for personal use.Source water is poured into the ceramic filter, then the water percolates through to the lower reservoir, and clean water is dispensed through the spigot.
e relatively small pore size (mean around 10 µm) of the ceramic filter helps remove turbidity and larger particles [15].e silver release rate has generally been reported to produce silver concentrations below the secondary drinking-water standard of 0.1 mg/L set by the USEPA [16] and the World Health Organization [17].e "nano" size of the silver particles results in a high surface area to volume ratio, leading to better bactericidal activity [18].However, silver nanoparticles have poor retention in ceramic, shortening the effective lifespan of the filter and possibly causing silver levels in the effluent water above drinking-water standards [19].Mixing silver nanoparticles into the clay mixture prior to firing has been shown to result in higher retention of silver in the ceramic and a potentially longer lifespan than filters made with the silver nanoparticle method-which release silver at high levels quickly during early use [19]. is method has not been tested for bacterial disinfection nor has it been field tested, but it could be a promising alternative, despite the fact that it still relies on silver nanoparticles as a raw material.e method of silver application does not appear to be a factor affecting disinfection efficiency [15,20].Instead, the mass of colloidal silver in the ceramic determines effectiveness.
Ehdaie et al. [21] reported on the formation of silver nanopatches in a ceramic porous tablet.In this work, they mixed silver nitrate, a Redart clay, sawdust, and water together in different proportions.e mixture was pressed into the shape of a disk of varying thickness and diameter and fired in a kiln at a final temperature of 900 °C.Characterization of the resulting silver-ceramic tablet revealed patches of silver throughout the pore structure with mean diameters in the range of 2-3 nm.
Herein, we evaluated an alternative silver application method to ceramic water filters that adds silver nitrate to the clay-water-sawdust mix, similar to the methodology described by Ehdaie et al. [21] for ceramic tablets and building on results from Nunnelley et al. [22].During firing, we hypothesize the formation of silver nanopatches in the porous medium.Compared to silver nanoparticles, silver nitrate is less expensive, more accessible in developing regions of the world, and easier to apply to ceramic water filters.We hypothesize that our new application method will keep more silver in the filter compared to conventional methods. is may increase the effective performance lifespan of the filter and result in less ingestion of silver by the end users.We evaluate this method with a series of miscible flow experiments using [ 3 H]H 2 O as a conservative tracer and a nonpathogenic strain of E. coli as an indicator organism targeted for removal.Different formulations of the silver nitrate method are compared against the conventional silver nanoparticle application method.
For the silver nitrate method, 99.5% pure silver nitrate from Acros Organics was used for the ceramic filter disk fabrication.For bacterial tests, a nonpathogenic strain of E. coli was purchased from IDEXX Laboratories (cat.982900700, Lot 042313) and cultured, used, and stored in the same method outlined in Ehdaie et al. [21].A 10 mM phosphate buffer solution (PB) composed of 11.2 g/L of dipotassium phosphate, 4.8 g/L of potassium phosphate monobasic, 0.02 g/L of netetraacetic acid, and deionized, organic-free water was used to preserve viability of E. coli in solution while preventing growth.A 60 g/L solution of sodium thiosulfate, prepared by dissolving anhydrous sodium thiosulfate (Fisher Scientific) in deionized water, was used to treat samples at the collection time to inhibit continued disinfection during incubation.All materials and solutions used for microbial analyses were sterilized before use.4.3 μCi [ 3 H]H 2 O was used for conservative tracer tests.

Ceramic Filter Synthesis.
Two types of filters were studied in this investigation, and they will be referred to as the following throughout the manuscript: (i) silver nanoparticle filters and (ii) silver nitrate filters.Silver nanoparticle filters use a conventional synthesis method similar to that described by Oyandedel-Craver and Smith [15].Because of the results in Oyandedel-Craver and Smith [15] show that silver nanoparticles improve performance of ceramic water filters and that current production methods utilize silver nanoparticles, only filters with nanosilver were compared.168.75 g of Redart clay and 18.75 g of sawdust (total mass of 187.5 g) were mixed by hand.
en 57 mL of deionized, organic-free water was added and thoroughly mixed by hand.
is mix was then separated by hand into three portions of equal weight, placed in a 6.5 cm-diameter PVC mold, and compressed at 1000 psi for 1 minute.e resulting filter was an approximately 1 cm thick disk, providing a onedimensional simplified geometry for lab testing (Figure 1).After air-drying for 48 hr, the ceramic filters were fired in a kiln with the following temperature program: increase temperature from 20 °C at 150 °C/h to 600 °C, then increase at 300 °C/h to 900 °C, then isothermal for 3 h.
In order to produce the silver nanoparticle filter, a 496 mg/L silver nanoparticle suspension in deionized water was used.Silver amounts applied to the ceramic filter were chosen to be proportional to silver used in a full-size filter containing 0.3 g of silver. is ratio and silver amounts were chosen because of their current use at the PureMadi Mukondeni Production Facility in Mukondeni, Limpopo Province, South Africa.10 mL of this solution was painted with a brush on both sides and the edges of the filter [23]. is impregnates 4.96 mg of Ag per filter.
For the silver nitrate filters, the same dry mix described above was combined with either 117 mg AgNO 3 (5x filters) or 234 mg AgNO 3 (10x filters) dissolved in 57 mL deionized water.ese higher levels of silver were chosen since silver nitrate is much less expensive, and the removal of the painting 2 Journal of Nanotechnology step saves labor costs, also, due to concern that some silver nitrate would end up in dead-end pore channels, having no contact with water as it filters through.e silver nitrate filters were then pressed into the shape of a cylinder, air-dried, and fired as described above for the silver nanoparticle filters.After firing, silver nitrate filters do not require any additional fabrication steps, as the silver forms metallic silver nanopatches like those seen in the TEM micrograph in Figure 2.
e porosity of each filter was measured gravimetrically by weighing a dry filter, saturating in deaired, deionized water for 24 h and then reweighing.e difference in mass equates to the volume of water inside the filter.e porosity is then calculated as the volume of water divided by the volume of the filter.

Miscible Displacement Transport Experiments with E. coli.
e cylindrical ceramic filters were loaded into a flexible-wall permeameter, holding a 10 psi pressure on the cell to ensure flow through the filter, rather than around.A high-performance liquid chromatography (HPLC) pump (Acuflow series IV), a 1.0 mL syringe, and the inflow valve of the permeameter chamber were connected with a three-way stopcock.e HPLC pump maintained a constant flow rate of 0.6 mL/min to mimic the average flow of pot filters (1.5 L/h).For initial bacterial pulse testing, 10 mM phosphate buffer solution was used as the inflow solution.e effluent valve of the permeameter chamber was open to the atmosphere for collection of effluent water samples.Filters were saturated by pumping inflow solution through the filter for 24 hours prior to the experiment.During the saturation period, effluent water samples were collected for silver analysis.
After the saturation period, a 1.0 mL syringe was used for a pulse injection of approximately 10 10 MPN/100 mL E. coli.Effluent samples were collected and analyzed over time to define the breakthrough of the E. coli.Viable E. coli were quantified in each sample using the Colilert Defined-Substrate Technology System, a method approved by the U.S. EPA and recommended by the WHO for microbiological testing [24][25][26].Colilert media (cat.WP200I) was added to 100 mL of sample and mixed thoroughly, before being poured into IDEXX Quanti-trays (cat.WQT-2K) and incubated for 24 hours at 37 °C.A fluorescent UV lamp was used to count the number of fluorescing wells in the tray and correlated to E. coli concentrations using a most-probable-number table provided by the manufacturer.Samples were taken over time to measure nitrate in the effluent with Hach TNT835 Kit and the DR 3900 bench top spectrophotometer.Total silver was tested with a graphite furnace atomic adsorption spectrometer (PerkinElmer HGA 900).Upon collection, bacteria samples were treated with a 60 g/L solution of sodium thiosulfate, prepared by dissolving anhydrous sodium thiosulfate (Fisher Scientific) in deionized water, to deactivate silver from continuing to disinfect during sample incubation.Silver methods were tested in triplicates.

Long-Term Performance Evaluation with Constant
Exposure.To evaluate the performance of each filter type over an extended period of time, silver-ceramic filters were again loaded in to a flexible-wall permeameter with a 10 psi cell pressure.e saturation period was performed as described above, with one HPLC pump providing a constant feed of 10 mM phosphate buffer solution for 24 hr.After 24 hr of saturation, hydraulic conductivity was measured using a falling head analysis.en, as in Oyanedel-Craver and Smith [15], a 1.0 mL syringe was used to inject a 0.6 mL pulse of 4.3 μCi [ 3 H]H 2 O into the ceramic disk.Effluent samples were collected over time and measured by a liquid scintillation counter to define a conservative tracer breakthrough curve.Effluent tracer concentrations were simulated using a transient one-dimensional form of the advection-dispersion equation with first-order decay: Subject to the following initial and boundary conditions: (2)  R is the retardation coe cient, c is the concentration of [ 3 H]H 2 O in counts per minute per mL, t is time in minutes, t 0 is the tracer injection time, D is the dispersion coe cient in cm 2 /min, x is distance in cm, v is the linear velocity in cm/min, μ is the rst-order decay coe cient, and L is the thickness of the disk.CXTFIT [27] was used to provide the optimum t of the model to experimental data.D and v were determined with R 1 and μ 0 from the [ 3 H]H 2 O transport experiment.
After the completion, two high-performance liquid chromatography (HPLC) pumps (Acu ow series IV) were connected and mixed at a three-way push to connect tting immediately before the in ow valve of the permeameter chamber maintaining a constant ow rate of 0.6 mL/min to mimic the average ow of pot lters (1.5 L/h).For these experiments, one pump contained an in ow of EPA semihard synthetic groundwater solution plus acetate at a concentration of 6 mg/L (to allow bacterial growth similar to natural water) [28].e other pump contained ∼10 6 MPN/100 mL E. coli in synthetic groundwater without the added acetate.e growth medium was kept separate from the E. coli until immediately prior to entering the cell, which ensued a constant in uent concentration.e e uent valve of the permeameter chamber was open to the atmosphere for collection of e uent water samples.Pumping and sampling of e uent for silver levels and E. coli concentration continued for 2 weeks.A nal hydraulic conductivity was measured, and another [ 3 H]H 2 O breakthrough experiment was performed to compare initial and nal porous medium ow characteristics.For E. coli concentration, the same quanti cations were used as described above.A graphite furnace atomic adsorption spectrometer (PerkinElmer HGA 900) was used for total silver quanti cation.Silver methods were tested in duplicate.

Performance Evaluation after 2 and 4 Weeks of Flow.
To evaluate the long-term performance of the ceramic lters under conditions mimicking real-world use, we conducted experiments using 2-and 4-week pulse injections of E. coli without an added growth substrate.Ceramic lters were again in to a exible-wall permeameter with a 10-psi cell pressure.Like above, a HPLC pump, a 1.0 mL syringe, and the in ow valve of the permeameter chamber were Silver nanoparticle method Silver nitrate method, 5× Silver nitrate method, 10× Figure 3: Average log change of E. coli, log(C/C 0 ), versus pore volumes of ow after an one-min pulse injection of E. coli to ceramic lters manufactured using the silver nanoparticle method and the silver nitrate application method.Error bars show one standard error above and below the mean.For the silver nitrate application method, the mass of added silver was ve times (5x) or ten times (10x) the mass of silver applied to the ceramic lter fabricated using the silver nanoparticle method.Samples were taken at the same time for each disk and converted to pore volume.Since pore volumes varied minimally (3%) between disks, samples at the same time were averaged.
connected with a three-way stopcock.
e HPLC pump maintained a constant ow rate of 0.6 mL/min this time with an EPA semi-hard synthetic groundwater solution as the in ow solution [28].e e uent was collected the same as before for silver and E. coli sampling.After two weeks, a 0.6 mL pulse of ∼10 9 E. coli was injected through the lter via the syringe.E uent samples were collected to trace the bacterial breakthrough.e pump was left to continually pump synthetic groundwater for another two weeks with e uent sampling for silver levels.After a total of four weeks of pumping, another 0.6 mL pulse of ∼10 9 E. coli was injected through the lter via the syringe.E uent samples were collected to trace the bacterial breakthrough.For E. coli concentration, Colilert medium (cat.WP200I) was added to 100 mL of sample and mixed thoroughly, before being poured into the IDEXX Quanti-trays (cat.WQT-2K) and incubated for 24 h at 37 °C.A uorescent UV lamp was be used to count the number of uorescing wells in the tray and correlated to E. coli concentrations using a most-probablenumber table provided by the manufacturer.A graphite furnace atomic adsorption spectrometer (PerkinElmer HGA 900) was used for total silver quanti cation.Silver methods were tested in duplicate.

Miscible Displacement Transport Experiments with E. coli.
e results of the E. coli pulse injections into the ceramic lter disks are shown in Figures 3 and 4. Figure 3 shows the log removal of E. coli versus the pore volumes of ow.Both methods of silver application, silver nanoparticle and silver nitrate, performed similarly.Further, both levels of silver nitrate application (5x and 10x) performed similarly, showing the potential of the new method as a viable substitution.To calculate results, the e uent concentration (C) was divided by in uent concentration (C 0 ), followed by taking the log of C/C 0 .
Figure 4 shows the total silver measured in the e uent from the ceramic lters over time.e e uent from lters made with the silver nanoparticle method has the highest silver levels-even above the drinking-water standard for sampling times less than 5 hr-while the new silver nitrate method results in lower levels.
ese lower silver levels, while still an e ective antimicrobial agent, suggest a safer lter with a longer lifespan because silver is retained in the lter media rather than released into the treated water.ese graphic results suggest that, at least during shortterm experiments, the new silver nitrate method provides no bene t in regard to bacterial removal since the error bars overlap.However, in Figure 4, the error bars do not overlap between the silver nanoparticle and silver nitrate methods, showing a statistical di erence.Water chemistry will e ect the rate of silver release from the ceramic lter [29]; however, under the same conditions, the silver nitrate application method results in lower silver e uent levels, potentially improving long-term silver retention in the lter and the overall useful lter lifetime.is ts well with literature that a signi cant fraction of silver nanoparticles is being washed o over time, decreasing the long-term e ectiveness of ceramic water lters [19]. is is particularly notable since there is ve and ten times as much silver in the silver nitrate lters as the silver nanoparticle lters.New lters commonly require the rst few liters be discarded in case of high silver levels [30], and these silver levels are safely below the drinking-water standard quickly after rst use.

Long-Term Performance Evaluation with Constant
Exposure.In response to the bacterial tracer testing results, a longer-term experiment was performed in order to evaluate the hypothesis that ceramic lters made with the silver nanoparticle method have longer lifespans.Figure 5 shows the results of exposing the ceramic filter disks to a constant flow of E. coli with synthetic groundwater and an added carbon source.is ensures maximal bacterial growth and can potentially cause biofilm formation and bacteria growth.Each point represents a single data point due to the nature of sampling over such a large time scale with slight variance in pore volume between replicates.Replicates of the same silver method had data pooled together to present the trends seen in Figure 5.
Figure 5(a) shows the log removal of E. coli versus pore volumes of flow.e silver nanoparticle method of silver application shows better performance initially, but then inconsistent performance over time. is could potentially be explained by E. coli growth in the filter.e bacteria may cause clogging in the filter but the HPLC pump used to maintain constant influent flow rates might be increasing pressure in an attempt to keep a constant flow rate, dislodging the bacteria growth.is could cause bacteria to be released in clumps, creating low removal rates after continued use.
e silver nitrate filter performed more consistently, slightly increasing in performance over time and possessing a longer effective lifespan.Figure 5(b) shows the silver results over the same time period.As expected, the silver nanoparticle method resulted in much higher silver levels than the silver nitrate filters, with average effluent concentrations of 64 and 18 µg/L, respectively.Additionally, some of the early time points for the filters made with the silver nanoparticle method feature silver levels in the effluent above drinking-water standards.Silver concentrations in the effluent seem to vary some; this may be due to silver sloughing off close to a sampling time.Silver may not come out of the filter at a perfectly consistent rate; it may be releasing as pressure builds up and requires more pore channel space for flow.A mass balance of silver release over the course of the experiment shows that an average of 0.63 mg of silver have come out of the silver nanoparticle filters versus 0.29 mg of silver that have come out of the silver nitrate filters.e silver nanoparticle filters only started with 4.96 mg of silver per filter, while the silver nitrate filters had five times as much.e silver nitrate method shows up to a ten times longer lifespan with a silver loss rate about half that of the silver nanoparticle method and containing five times as much silver.
Hydraulic conductivity, K, was measured before and after the E. coli exposure to quantify clogging in the filters.Hydraulic conductivity decreased by an average of 38.89% and 75.15% for the silver nanoparticle and silver nitrate filters, respectively.is decrease was expected, as bacterial growth promoted by the influent would likely cause clogging of the filter.Silver nitrate filters have more silver and therefore might be causing more microbial death in the filter.However it is important to note that this is a relatively small change, but could affect performance in actual use.
Tritiated water was used as a conservative tracer in pulse injections both before and after the E. coli exposure to estimate the change in the linear velocity (v) and dispersion coefficient (D).Table 1 shows the model output values generated for v and D by CXTFIT from the experimental results.
e linear velocity and dispersion coefficient decreased for both silver methods.e silver nanoparticle filter caused decreases of 10.3% and 50.2% for v and D, respectively, while the silver nitrate disk saw a decrease of 17.73% and 46.01%for v and D, respectively.A decrease in diffusion coefficient, D, would be indicative of a less tortuous path for flow. is could be due to dislodging of loose ceramic particles and nanosilver along pore channels.e output model with these v and D values and an assumed R � 1 and μ � 0 can be seen in Figure 6. e changes of v and D between initial and final measurements for silver nanoparticle and silver nitrate filters are similar, showing the silver nitrate method maintains performance of flow over time equal to the current production protocol.

Performance Evaluation after 2 and 4 Weeks of Flow.
is experiment was performed to further evaluate the longerterm performance of the two different silver applications in the ceramic filters.Filters had synthetic groundwater flowing through for two weeks, and then an E. coli pulse was injected.
en, after two more weeks of synthetic groundwater flowing, another E. coli pulse was injected.e results of this experiment are found in Figure 7.
Again, using the graphed data and error bars for analysis, the filters with silver nanoparticle method perform better initially, allowing less E. coli to pass through the filter during the pulse after two weeks.But after four weeks of use, the same filter showed a large decrease if efficacy.
e silver nitrate filters were consistent and showed almost identical performance after two and four weeks of use.

Conclusions
e silver nitrate method presented here presents another potential benefit in that it reduces possible exposure of workers to silver nanoparticle inhalation.A recent article by Fewtrell et al. [31] assessed the safety of using silver nanoparticles in household water treatment and noted that one of the most significant silver exposure risks could be to workers fabricating the filters and inhaling the silver nanoparticles.
ey noted that occupational safety procedures in developing-world work environments may not be sufficient to protect the workers.ey based their assessment of a study that observed genotoxic damage to silver workers [32].
Furthermore, filters manufactured using the silver nitrate method release lower levels of silver into the treated water, and the form of the silver is Ag +1 (e.g., ionic silver) [21].Some studies show genotoxic effects caused by exposure to silver nanoparticles, albeit at silver doses that are orders of magnitude greater than what is found in drinking water treated with silver-ceramic water filters.If an adult weighing 50 kg were to consume 2 liters of water per day with 0.018 mg/L of silver (the mean silver concentration in water treated with filters manufactured using the silver nitrate method evaluated in this manuscript), 0.00072 mg/kg of body weight will be consumed per day.
is level is far  below the conservative Tolerable Daily Intake value of 0.0025 mg/kg of body weight per day suggested by Hadrup and Lam [33].e levels presented in Table 1 of Fewtrell et al. are 0.25 to 500 mg/kg, 125 to 250,000 times greater than the doses found in water treated by our lters [31].Studies with lower doses of silver show no genotoxic e ects, and studies with ionic silver show no genotoxic e ects at any reported concentration.Silver-ceramic water lters have been shown to release silver ions, not silver nanoparticles [21,34].Fewtrell et al. [31] and the response by Lantagne et al. [34] state that studies of silver ion toxicity have shown no adverse e ects.erefore, the silver nitrate method helps to insure the microbiological safety of the treated water while releasing only extremely low levels of ionic silver (more than 5 times below the EPA and WHO suggested permissible levels).At the same time, the method reduces the risks of exposure to workers in developing-world production facilities.
Each of the experiments points to a similar result when comparing the conventional method of painting on silver nanoparticles after ring and the new method of embedding silver nitrate prior to ring in ceramic water lters.e silver nanoparticle method may perform better initially, but decreases in antimicrobial e cacy over time and commonly releases silver at rates above the drinking-water standard.
e silver nitrate application method produced lters that release lower amounts of silver and performed more consistently in bacterial removal over time.ese results indicate that the silver nitrate application method is a viable substitute to painting on silver nanoparticles for the production of ceramic water lters.However, there are some concerns that this may not be ideal for all lter production facilities.Conventionally, lters that are manufactured will not receive the silver nanoparticle application until after passing quality control tests.With the new method of silver application prior to ring, lters that do not pass these pressure and ow rate tests will be wasting the silver applied to them.erefore, the new silver nitrate method of application may only be a good option to be incorporated at lter production facilities with relatively high pass rates for quality control tests.ese results from experiments in clean lab conditions are promising, but are not a perfect indicator of how silver nanoparticle and silver nitrate lters would perform in expected in home use.In homes, there will be varying ow rates, turbidities, ionic strengths, temperatures, and many more variables that could a ect performance.e next experiments to be done are to test the application of full-size lters and with real-world use, rather than simpli ed lab conditions.

Figure 2 :
Figure 2: Transmission electron microscopy image of silver nanopatches in the center of a ceramic tablet using scanning electron microscopy mode.e scale bar represents 20 nm in length [21].

Figure 1 :
Figure 1: Ceramic filter disks used for laboratory testing.

Figure 4 :
Figure 4: Total silver concentration as a function of time in e uent from silver-ceramic lter media.Data is for three lter types with varying silver application methods and amounts.Error bars show one standard error above and below the mean.

y−Figure 5 :
Figure 5: (a) Average log change of E. coli, Log (C/C 0 ), versus pore volume of flow with a long-term constant exposure to E. coli by silver application method.Trend lines show the change in performance of E. coli removal over time.e slope of the trend line for the silver nanoparticle filters 0.0027 shows a decrease in performance.While the silver nitrate filter trend line has a slope of −0.0005, meaning at least consistent performance.(b) Total silver concentration in effluent from silver-ceramic filter media as a function of pore volumes of flow with a long-term constant exposure to E. coli.Data are for two filter types with varying silver application methods and amounts.

Figure 6 :
Figure 6: Results of [ 3 H]H 2 O transport experiments and simulations through (a) silver nanoparticle ceramic filters and (b) silver nitrate ceramic filters (b).Tracer experiments and simulations are shown for experiments conducted at the start and end of a two-week period of constant flow of synthetic groundwater containing E. coli.
Silver nitrate method, 2 weeks Silver nanoparticle method, 4 weeks Silver nitrate method, 4 weeks

Figure 7 :
Figure 7: Average log change of E. coli, Log(C/C 0 ), versus pore volume after a pulse injection of E. coli over 1 min by silver application method and time of saturation.Error bars show one standard error above and below the mean.

Table 1 :
Linear velocity, dispersion coefficient, and hydraulic conductivity for silver nanoparticle and silver nitrate filters before and after 2 weeks of constant flow of E. coli.