FIELD OF INVENTION

The present disclosure relates to reactivation of exhausted silver metal particle-based antimicrobial composition for its use in water purification. The process of reactivation has been found to be applicable for a broad range of silver metal particle-based anti-microbial compositions; the process of reactivation has been demonstrated for silver metal particle loaded on different support matrices.

BACKGROUND OF THE INVENTION

Antimicrobial property of silver is well-known for centuries. In ancient days, Phoenicians stored water, wine, vinegar, etc., in silver vessels to keep it fresh during long sea voyages. The medicinal use of silver can be dated to around 400 B.C. where Hippocrates, the "Father of Medicine," taught that silver healed wounds and controlled spread of diseases and he listed it as a singular treatment for ulcers (Hippocrates, Edited and translated by Paul Potter, Loeb Classical Library 482, 1995, The President and Fellows Of Harvard College, VIII, 359) In modern days, silver has been employed as an antimicrobial agent in the form of ionic solution, complex/colloid of insoluble salts and zerovalent particles; it has been used in textiles, refrigerators, air conditioners, washing machines, vacuum cleaners, wound dressing pads, water filters, etc. In the area of water purification, numerous brands of silver metal particle-based water filters are already available in the market and the use in water purification continues to grow increasingly.

Antimicrobial property of silver nanoparticles has been covered in a number of patent applications, covering a broad range of subject areas. Some recent examples are:

(i) Improvement in method of synthesis: Pal et. al. in Appl Environ Microbiol., 2007, 73(6), 1712; De Windt et. al. in United States patent application 20100272770; Sastry et. al. in Indian patent application 936/MUM/2008

(ii) Synthesis in media other than water: Chang et. al. in United States patent 7329301

(iii) Anchored on various substrates: Rautaray and Sastry in Indian patent application 1571/MUM/2008; Nangmenyi et. al. in Mater. Lett., 2011, 65, 1191; Zhang and Chen in Environ. Sci. Technol. 2009, 43, 2905 and Shankar et al. in J Chem Technol Biotechnol. 2008, 83, 1177).

Although silver nanoparticles are well-known antimicrobial agents, the capability of nanoparticles to serve as antimicrobial agents for repeated times determines the lifetime of the product. Silver's antimicrobial activity is primarily due to the release of Ag+ ion in water. As nanoparticles have very reactive surfaces, they tend to saturate rapidly due to the presence of various ions in water which have high affinity to silver surface. This leads to a decline in the antimicrobial activity as Ag+ ion release from saturated surface is weakened. Even though quantity of Ag dissolved to total Ag is quite less, the use of composition for water purification is to be stopped and results in incomplete usage of antimicrobial composition. This is due to the fact that surface experiences a number of changes in real water due to factors such as ion induced aggregation, surface modification and salt deposition, etc.

This has been reported previously through several research reports. For example, Hoek et al. J Nanopart Res. 2010, 12, 1531, Hoek et al. Environ. Sci. Technol. 2010, 44, 7321, Bonzongo et al. Environ. Sci. Technol. 2009, 43, 3322 and Lead et al. Environ. Sci. Technol. 2009, 43, 7285 demonstrated that the antibacterial property of silver nanoparticles depends on intrinsic features such as particle size, stability and shape of the nanoparticle as well as extrinsic features such as hardness of the water, soluble ligands in the water, ionic composition, organic matter and pH. Epple et al. Chem. Mater. 2010, 22, 4548 and Hurt et al. Environ. Sci. Technol. 2010, 44, 2169 demonstrated that release of silver ion from silver nanoparticles depends on various parameters such as temperature, incubation days, dissolved oxygen level, salt and organic matters. They showed that the rate of dissolution is not constant with time and can reach a minimum level rapidly. Hoek et al. Environ. Sci. Technol. 2010, 44, 7321 reported that the fraction of dissolved silver is less than 0.1% of the total mass of silver added, regardless of the initial source, i.e., AgNC>3 or silver nanoparticles.

In the view of the above highlighted problems, we have demonstrated in our previous patent applications (947/CHE/2011, 1522/CHE/2011) that it is possible to achieve constant release of silver ions (concentration well below the permissible limits for drinking water, prescribed by WHO) sufficient for ensuring microbiologically safe water, over prolonged period of use, by using organic-templated-metal oxide/oxyhydroxide/hydroxide- nanoarchitecture as a support and stabilizing agent for silver nanoparticles. This leads to a significant increase in the scope of the silver based composition to bring clean drinking water to masses.

It is also of significant commercial interest to improve the lifecycle for use of silver metal based water filters (stated in volume of water purified) by reactivating the saturated silver metal particles.

Hence there is a compelling need to develop methods to reactivate the saturated silver nanoparticles-based filters. This invention aims to develop various reactivation methods which are crucial in furthering the widespread use of silver nanoparticle based antimicrobial composition for water purification.

PRIOR ART

The concept of reactivating/regenerating various adsorbent compositions used for water purification is already in practice. For example, exhausted porous carbon filters can be reactivated by steam activation. The exhausted fluoride (or arsenic) removal media such as activated alumina can be regenerated by alkali wash followed by acid wash or alum wash.

Generally the method involves removing the saturation coverage of the adsorbate for which the adsorbent was designed, such as the removal of arsenate or fluoride. The reactivation method varies from one catalyst to another. For example, Kolbel and Ackermann in U.S. Patent 2,775,607 reported the ineffectiveness of the common regeneration methods based on oxidation, reduction and solvent extraction on deactivated iron catalyst suspensions. They stated that methods which result in a substantial change of the physical or chemical condition of the catalyst are more effective.

Thermal reactivation combined with/without different reactivation methods have also been considered. For example, Qi and George in U.S. Patent application number 20,100,152,022 demonstrated a regeneration method for a noble and transition metal catalyst supported on an inorganic carrier. The method includes calcining a deactivated catalyst in an oxygen-containing gas to produce a calcined catalyst and then contacting the calcined catalyst in a hydrogen-containing gas at a temperature higher than 430 °C. In another example, Qu et al. J. Phys. Chem. B 2005, 109, 15842-15848 demonstrated a reactivation method associated with physical changes in the silver crystallites. They demonstrated that pretreatment at 500 °C in the presence of oxygen gas followed by pretreatment at <300 °C in presence of hydrogen gas resulted in surface restructuring and crystallite re-dispersion of silver nanoparticles loaded SiC>2 and showed a marked influence on the activity of the catalyst.

Reactivation method involving oxidation is commonly tried for silver based catalysts. Bon et al. U.S. Patent 4,755,266 reported that silver cathodes can be activated and reactivated by contacting the cathode with an alkaline aqueous solution comprising an essentially heavy metal-free oxidizing agent capable of oxidizing silver to silver oxide. M. Jung et al. U.S. Patent 3,253,961 reported that silver containing fuel cell electrodes can be reactivated using soluble, complex forming compounds such as ammonia and alkali metal cyanides.

Promoters such as metals and metal oxides have been used to reactivate the silver surface. For example, Cambron, A & Alexander, W. A. Can. J. Chem. 1956, 34, 665 demonstrated the action of calcium oxide as a promoter for silver surfaces. They suggested that reduction of calcium oxide by silver might take place locally on the surface of the catalyst, resulting in the formation of a calcium silver compound at the surface of the catalyst, followed by regeneration by the action of oxygen gas.

As silver nanoparticles are sensitive to light, different light energies have been used to activate the silver nanoparticles. Ohko, Y. et al. Nature Mater, 2003, 29-31 demonstrated a photochrome material based on Ag/TiO2 which changes its color reversibly in response to light. They showed that under UV irradiation, Ag+ is self reduced to Ag° and under visible light, Ag° is oxidized to Ag+ and the process is reversible. Zhang, L and Yu, J. C. Catalysis Communications, 2005, 6, 684-687 reported a simple regeneration method wherein ambient light illumination was used to reactivate the deactivated silver-coated TiO2 photocatalyst. The process converts Ag° to Ag+ in the form of Ag2O. When the activated catalyst is used in photocatalysis, Ag+ would be eventually reduced to Ag° under UV irradiation forming fresh non-aggregated silver nanoparticles on TiO2.

It is learnt from prior art that the concept of reactivation of silver-based materials specifically silver nanoparticles based antimicrobial compositions used for water purification applications has not been reported earlier. Additionally, it is also important to develop a method of reactivation for silver based antimicrobial composition such that it can be easily practiced by consumers at the household level, requiring practically zero technical skills, equipment or consumables.

Therefore, the main object of the present invention is to develop an effective, straightforward and cost-effective procedure for reactivation of the silver metal particles based antimicrobial composition.

Another object of the present invention is to develop a reactivation procedure that can be universally applied for any silver metal particle based antimicrobial composition, i.e., the method should be independent of synthetic route used for the nanoparticle synthesis, shape/size of the nanoparticle, surface protecting agent used on the nanoparticle surface and the substrate used for anchoring silver metal particles.

Another object of the present invention is to develop a reactivation procedure that can be repeatedly done on the composition, for at least 5 cycles, while maintaining the antimicrobial activity in the successive cycles.

Another object of the present invention is to utilize the reactivation procedure for metal particle based antimicrobial composition wherein the antimicrobial property is due to dissolution of metal ions in water.

SUMMARY OF THE INVENTION

It is well-understood from prior art that quantity of silver ion leached from a silver nanoparticle- based antimicrobial composition during the lifecycle of the product use is small.

Therefore, a large portion of silver in the antimicrobial composition remains unutilized when the composition is exhausted. This leads to high cost for the consumer. In the view of the need, the present invention demonstrates a simple reactivation procedure to regain the performance of silver particle-based antimicrobial composition.

The present invention describes the art of reactivating the silver metal particle based antimicrobial composition by a very simple method wherein it is contacted with hot water having a temperature of about 50-70 °C for 3 hours. There are a number of advantages of this approach: it applies on wide variants of silver particles based compositions, it is very easy to understand/practice and it results in no damage to the environment.

The present invention also describes a few methods of reactivating the silver metal particles based antimicrobial composition by reaction-based approaches wherein the composition is contacted with either mild etching agent or anti-scaling agent.

DESCRIPTION

DESCRIPTION OF THE INVENTION

Experimental methods Material characterization

X-ray Photoelectron Spectroscopic (XPS) analysis was done using ESCA Probe TPD of Omicron Nanotechnology. Polychromatic Mg Ka was used as the X-ray source (hv = 1253.6 eV). Spectra in the required binding energy range were collected and an average was taken. Beam induced damage of the sample was reduced by adjusting the X-ray flux. Binding energy was calibrated with respect to C 1s at 285.0 eV. Silver ion concentration in the water was detected using inductively coupled plasma mass spectrometry (Agilent 7700 x ICP-MS).

Examples 1A to F are provided to illustrate the synthetic methods used for the preparation of silver metal particles based antimicrobial composition. The examples should not be construed as limiting the scope of the invention. In this present invention silver nanoparticles are loaded either homogeneously in the support matrix or loaded at the surface of the support matrix. It should be noted that the percentage of silver nanoparticles loaded on any matrix was kept constant throughout the study.

Example 1A

In-situ impregnation of silver nanoparticles on the organic-templated-metal oxide/hydroxide/oxyhydroxide-nanoarchitecture (OTMN) gel:

This example describes the in-situ impregnation of silver nanoparticles on OTMN gel. Ag loaded in OTMN was prepared as reported in the previous patent applications (947/CHE/2011, 1522/CHE/2011). Preparation of OTMN is further described in a previous patent application (1529/CHE/2010). The metal precursor can be Fe(ll), Fe(lll), Al(lll), Si(IV), Ti(IV), Ce(IV), Zn(ll), La(lll), Mn(ll), Mn(lll), Mn(IV), Cu(ll) or a combination thereof. The OTMN gel obtained after washing the salt content was used for the formation of silver nanoparticles. The OTMN gel was again re-dispersed in water, to which 1 mM silver precursor (silver nitrate, silver fluoride, silver acetate, silver permanganate, silver sulfate, silver nitrite, silver salicylate or any combination of the above) was added drop-wise. The weight ratio of Ag to OTMN can be varied anywhere between 0.1-1.5%. After stirring the solution overnight, 10 mM sodium borohydride was added to the solution drop wise (in ice- cold condition, temperature < 5 °C). Then, the solution was allowed to stir for half an hour, filtered and washed with copious amount of water. The obtained gel was then dried for further studies.

Example 1B

In-situ impregnation of silver nanoparticles on OTMN particle:

The dried OTMN powder was crushed to a particle size of 100-150 micron. The powder is stirred in water, using a shaker. 1 mM silver precursor solution was then slowly added. The weight ratio of Ag to OTMN can be varied anywhere between 0.1-1.5%. After stirring the mixture overnight, 10 mM sodium borohydride was added to the mixture drop wise (in ice-cold condition, temperature < 5 °C). Then, the mixture was allowed to stir for half an hour, filtered and washed with copious amounts of water. The obtained powder was then dried at room temperature for further studies.

Example 1C

Ex-situ impregnation of silver nanoparticles on OTMN gel:

The OTMN gel obtained after washing the salt content was used for the impregnation of silver nanoparticles. The OTMN gel was again re-dispersed in water, to which 1 mM silver nanoparticles solution (prepared by methods as reported in the literature, e.g., Turkevich method for preparation of citrate capped silver nanoparticles) was added drop-wise. The weight ratio of Ag to OTMN can be varied anywhere between 0.1-1.5%. After stirring the solution overnight, it was filtered and washed with copious amounts of water. The obtained gel was then dried for further studies.

Example 1D

Ex-situ impregnation of silver nanoparticles on OTMN particle:

The dried OTMN powder was crushed to a particle size of 100-150 pm. The powder was stirred in water, using a shaker. 1 mM silver nanoparticles solution (prepared by methods as reported in the literature, e.g., Turkevich method for preparation of citrate capped silver nanoparticles) was added drop-wise. The weight ratio of Ag to OTMN can be varied anywhere between 0.1-1.5%. After stirring the solution overnight, it was filtered and washed with copious amounts of water. The obtained powder was then dried at room temperature for further studies.

Example 1E

In-situ impregnation of silver nanoparticles on adsorption media commonly used for water purification
The adsorption media can be chosen amongst activated carbon, natural or synthetic polymer and metal oxyhydroxide/oxide/hydroxide such as boehmite, activated alumina, aluminium hydroxide, titania, ferric oxyhydroxide, ferric oxide, ceria, manganese dioxide or silica. 1 mM silver precursor solution was then slowly added. The weight ratio of Ag to adsorption medium can be varied anywhere between 0.1-1.5%. After incubating the mixture overnight, 10 mM sodium borohydride was added to the mixture drop wise (in ice-cold condition, temperature < 5 °C). Then, the mixture was allowed to stir for half an hour, filtered and washed with copious amounts of water. The obtained particles were then dried for further use.

Example 1F

Ex-situ impregnation of silver nanoparticles on adsorption media commonly used for water purification
The adsorption media can be chosen amongst activated carbon, natural or synthetic polymer and metal oxyhydroxide/oxide/hydroxide such as boehmite, activated alumina, aluminium hydroxide, titania, ferric oxyhydroxide, ferric oxide, ceria, manganese dioxide or silica. 1 mM silver nanoparticles solution (prepared by any route reported in the literature) was added drop-wise. The weight ratio of Ag to adsorption medium can be varied anywhere between 0.1-1.5%. After stirring the solution overnight, it was filtered and washed with copious amounts of water. The obtained particles were then dried for further use.
The size of the silver nanoparticle in in-situ and ex-situ impregnated silver nanoparticles on metal oxide/hydroxide/oxyhydroxide gel/particle varies from 1 nm to 1000 nm, preferably from 5 to 500 nm.

Example 2A

Antibacterial activity of silver metal particles based composition was tested in the following way: 100 ml_ of water was shaken with the material and 1 x 105 CFU/mL of bacterial load was added to the water. Tap water having a TDS value between 300-500 ppm was used in the study. After one hour of shaking, 1 mL of the sample along with nutrient agar was plated on a sterile petridish using the pour plate method. After 48 hrs of incubation at 37 °C, the colonies were counted and recorded. This procedure was repeated for minimum 400 trials. After every 10 trials, sample was collected and plated as described above.

Example 2B

Antiviral activity of silver metal particles based composition was tested in the following way: 100 mL of water was shaken with the material and 1 x 103 PFU/mL of MS2 coliphage load was added to the water. The tap water having the TDS between 300-500 ppm was used in the study. After one hour of shaking, virus count was done by plaque assay method. After 24 hrs of incubation at 37 °C, the plaques were counted and recorded. This procedure was repeated for minimum 400 trials. After every 10 trials, sample was collected and plated as described above.

Reactivation of a material was done as described in the examples 3A to 3G, wherein 3A to 3E belong to physical treatment and 3F & 3G belong to reaction-based treatment. The examples should not be construed as limiting the scope of the invention. The general scheme of the reactivation method corresponding to 3A to 3E is shown in figure 1.

Example 3A

This example describes the procedure for reactivating the spent antimicrobial composition. The antimicrobial composition (50 g) was contacted with ~250 mL of hot water (temperature 70-100 °C), occasionally shaken and kept for 1 h. The procedure may be repeated. After reactivation, material was washed thoroughly and used.

Example 3B

This example describes the procedure for reactivating the spent antimicrobial composition using a microwave oven. The wet antimicrobial composition was taken and subjected to household microwave heating for 30 minutes. The metal-selective heating property of microwave was utilized for reactivating Ag-OTMN. After reactivation, material was washed and used. The same procedure can be applied for reactivating dry antimicrobial composition.

Example 3C

This example describes the procedure for reactivating the spent antimicrobial composition using an induction stove. The antimicrobial composition (50 g) was contacted with -250 mL of hot water (temperature 70-100 °C), occasionally shaken and kept for 1 h. The procedure may be repeated. After regeneration, material was washed thoroughly and used.

Example 3D

This example describes the procedure for reactivating the spent antimicrobial composition using a sand bath. The antimicrobial composition (50 g) was contacted with -250 mL of hot water (temperature 70-100 °C). Occasionally shaken and kept for 1 h. The procedure may be repeated. After regeneration, material was washed thoroughly and used.

Example 3E

This example describes the procedure for reactivating the spent antimicrobial composition. The antimicrobial composition (50 g) was shaken with -250 mL of nearly boiling water for 15-30 sec. The procedure may be repeated. After regeneration, material was washed thoroughly and used.

Example 3F

This example describes the procedure for reactivating the spent antimicrobial composition. In this procedure, citric acid, lactic acid, tartaric acid, vinegar, juice of ripened lemon, juice of ripened tamarind or a combination thereof was used. The antimicrobial composition (50 g) was contacted with -100 mL of water having lemon's juice, occasionally shaken and kept for 1 h. The procedure may be repeated. After regeneration, material was washed thoroughly using drinking water and used. The same procedure can be done using 0.02% citric acid.

Example 3G

This example describes the reaction-based treatment procedure for reactivating the antimicrobial composition. The antimicrobial composition (50 g) was taken with nearly 250 mL of 0.01% H202 and shaken for 1 h. The procedure may be repeated. After regeneration, material was washed thoroughly using warm water.

DESCRIPTION WITH REFERENCE TO DRAWINGS:

Figure 1. General scheme of the reactivation method corresponding to example 3A to E This scheme describes the procedure for reactivating the spent antimicrobial composition. The saturated antimicrobial composition (50 g) is contacted with -250 mL of hot water (temperature 70-100 °C), occasionally shaken and kept for 1 h. The procedure may be repeated.

This reactivated composition is washed repeatedly using water and repacked in a filter cartridge.

Figure 2. Effect of reactivation on the antibacterial activity of Ag-OTMN (Metal ion precursor =AI3+) studied over 5 cycles (a cycle = in use → performance drop → reactivation).

Figure 3. Effect of reactivation on the antibacterial activity of Ag-OTMN (Metal ion precursor = Fe3+) studied over 5 cycles (a cycle = in use → performance drop → reactivation).

Figure 4. Effect of reactivation on the antibacterial activity of Ag on activated Al203 studied over 5 cycles (a cycle = in use → performance drop → reactivation).

The freshly prepared silver based composition as explained in examples 1A, 1C and 1E were used for batch study. In these examples, silver nanoparticles loaded on chitosan stabilized AIOOH (example 1A), silver nanoparticles loaded on chitosan stabilized FeOOH (example 1C) and silver nanoparticles loaded on activated Al203 (example 1E) were taken. As explained in example 2A, the antibacterial activity was tested in the batch mode. Figure 2 shows the antibacterial efficiency of Ag-OTMN (Metal ion precursor =AI3+) with number of trials. Figure 3 and 4 show the corresponding antibacterial efficiency of Ag-OTMN (Metal ion precursor =Fe3+) and Ag-AI203, respectively.

Figures 2 to 4 show the number of surviving E.coli colonies after an hour of standing time. Each data point shown in the curve of Figures 2 to 4 corresponds to the number of surviving E.coli colonies at every 10th trial. It is clear from the curves that all the compositions completely kill the E.coli present in the water and maintain its activity for certain number of trials; thereafter biocidal performance drops. When the output E.coli colonies count >1000 CFU/mL was seen, the batch trials were continued for a few more trials to ensure drop in the performance and then reactivation was done as described in any one of the examples of 3A to E. The series of vertical lines in Figures 2 to 4 show the number of reactivations done on that particular material. After every reactivations (denoted by series of vertical lines), all the materials showed nearly 100% antibacterial activity for minimum 30 trials.

A typical cycle (comprising of, in use → performance drop → reactivation) was repeated for a minimum of 5 times. All the materials were tested for a minimum of 400 trials.

In order to obtain healthy bacteria in log phase, the culture is revived on a day to day basis. The concentration of E.coli in input water varies typically between 105 - 106 CFU/mL. The only fixed numbers of batch trials are carried out every day. On days when the input is higher than 1x106 CFU/mL, there is a slight increase in the bacterial count in the output water. This explains the minor fluctuations in the output bacterial count as shown in the curves of Figure 2 to 4.

It may be noted that the number of reactivation possible depends on the nature of material. It may also be noted that the trials were stopped after 400 trials and materials were not reactivated further. Materials were used for microwave-assisted acid digestion studies, in order to determine the quantity of silver ion leached during 400 trials. The trials may further be extended.

It should also be noted that the effect of reactivation on the antiviral activity of Ag- OTMN was also studied. The freshly prepared Ag-OTMN (Metal ion precursor=Fe(lll), Zn(ll) and Cu(ll)) composition as explained in examples 1A, was used for batch study. As explained in example 2B, the antiviral activity was tested in the batch mode for minimum 400 trials and the data similar to the curves shown in Figure 2 to 4 was obtained (Data not shown here).

Figure 5. Silver ion concentration in tap water at 30 °C measured by ICP-MS and corresponding bacterial output count in CFU/mL from batch measurements.

At regular intervals, output water samples were collected from the study described in Figure 2. Silver ion concentration released from the Ag-OTMN material was measured using Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Figure 5 shows the relationship between the drop in antibacterial activity and concentration of silver ion released into water. With increasing number of cycles, concentration of silver ion released decreased. And output E.coli colonies above 1000 CFU/mL were seen when silver ion concentration decreased below the critical concentration. At this stage, materials were reactivated. The vertical line in Figure 5 shows the point at which regeneration was done. After reactivation, the rate of silver ion dissolution again increased to original level and continued to decrease with increasing cycles.

Percentage of silver ion released from Ag-OTMN before and after batch trials.

To estimate the total quantity of silver released from the composition over 450 trials, a measured quantity of both initial and final samples were acid digested for silver metal analysis. Silver ion concentration for acid digested samples was measured using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). It was found that nearly 30% of original silver content has been released into water as silver ion, over a period of 450 trials including 5 times reactivation.

Figure 6. Effect of alternative methods of reactivation on the antimicrobial activity of Ag-OTMN studied over one cycle.

Chemical treatment as an alternative method of reactivation of Ag-OTMN is demonstrated using citric acid and hydrogen peroxide. Citric acid has been used to remove hard water stains from kitchen wares and used to dissolve rust from steel. The method of reactivation using citric acid is described in example 3F. Citric acid was chosen as it is widely available owing to its use as a food preservative. A saturated Ag-OTMN material which is showing an output water E.coli count of 1000 CFU/mL was taken and treated with 0.02% citric acid. As described in example 2A, the antibacterial performance was tested for batch mode. Figure 6A shows the antibacterial efficiency of Ag-OTMN with number of trials after reactivation and the vertical line shows the trial number at which reactivation was done.

Similarly 0.01% hydrogen peroxide was used to reactivate Ag-OTMN. Hydrogen peroxide is a mild etching agent for nanoparticles. 0.01% hydrogen peroxide was taken as it is widely available in medical stores, is safe to handle and can be disposed anywhere since it is known to treat wastewater. A saturated Ag-OTMN material which is showing 1000 CFU/Ml output E.coli count was taken and treated with 0.01% hydrogen peroxide. As described in example 2A, the antibacterial performance was tested for batch mode. Figure 6B shows the antibacterial efficiency of Ag-OTMN with number of trials after chemical treatment with hydrogen peroxide and the vertical line shows the point at which chemical treatment was done.

Figure 7. Effect of quality of water used in the reactivation of Ag-OTMN studied over 3 cycles.

Demineralized water and tap water were used to reactivate the Ag-OTMN by method as described in example 3A. Demineralized water having conductivity between 2 to 20 (jS/cm and pH between 6.3 to 6.9 and tap water having conductivity 600±50|jS/cm and pH 8±0.2 were used for this study. Ag-OTMN material (method of preparation described in example 1B) was taken for this study.

The chosen material was divided into two portions and batch trials were conducted separately using tap water. The antibacterial performance of the material was tested as described in example 2A. When the performance of the materials dropped, they were reactivated using demineralized and tap water separately. Figure 7 shows the antibacterial efficiency of Ag-OTMN with number of trials. The series of vertical lines in figure 7 show the number of reactivation.

Curve A (dark line) and B (dotted line) in figure 7 corresponds to antibacterial activity of Ag-OTMN after reactivation in demineralized water and tap water, respectively. Unlike reactivation in tap water, reactivation of Ag-OTMN in demineralized water leads to nearly complete recovery of anti-bacterial performance. The composition exhibits only 2 cycles of reactivation before getting exhausted in tap water. On the contrary, the composition can be reactivated repeatedly 5 cycles, if reactivation is conducted in demineralised water. It is clear from curve A that the demineralized water is a useful medium to reactivate the material repeatedly.

Figure 8. XPS analysis of initial, saturated and reactivated Ag-AI203 tested in batch mode.

Figure 9. XPS analysis of initial, saturated and reactivated Ag-OTMN (metal ion precursor =AI3+) tested in batch mode.

XPS analysis was done for initial, saturated and reactivated silver loaded on alumina (Ag-alumina). Corresponding survey spectra are shown in Figure 8. These survey spectra clearly confirm the existence of silver along with the key elements present in metal oxide support, such as Al and O. In order to find out the chemical form of the initial, saturated and reactivated silver nanoparticles loaded alumina, detailed scans of specific regions of key elements (Al 2p, O 1s, Ag 3d) were carried out and are shown in Figure 8. The XPS spectrum of Al 2p shows peak at 75.1 eV, which is in agreement with Al in Al203. The presence of metallic silver (Ag 3d5/2 at 367.8 eV) is shown in Figure 8B. In the case of Ag on Al203, XPS analyses show the expected decrease in intensity of silver from initial to saturated sample to reactivated sample. Al to Ag intensity ratio of initial, saturated and reactivated samples are 1:1.65, 1:0.6 and 1:0.41, respectively. In the case of Ag@citrate on alumina, silver nanoparticles are loaded on the surface of the activated alumina particles. The concentration of silver at the surface of the activated alumina decreases progressively, as silver ions are continuously released into the water, which is reflected in the progressive decrease in Al/Ag intensity for initial, saturated and reactivated material.

Silver loaded on OTMN (Metal ion precursor =AI3+) at initial, saturated and reactivated state were taken. Corresponding XPS survey spectra are shown in Figure 9A. The survey spectra in Figure 9 show the presence of silver along with the key elements present in metal oxyhydroxide support such as Al and O in AIOOH. In order to find out the chemical form of the elements in three states (initial, saturated and reactivated) detailed scans of specific regions of key elements (Al 2p, O 1s and Ag 3d) were carried out and are shown in Figure 9. The XPS spectra of Al 2p3/2 shows peak at 75.2 eV, which is in agreement with Al in AIOOH. The presence of metallic silver (Ag 3d5/2 at 368.2 eV) is shown in Figure 9. The XPS of the material in saturated state shows Ag 3d5/2 peak with reduced intensity in comparison to the material in initial state. We find that factors such as silicate deposition, principally in the form of CaSi03 are responsible for this partial filling of the support matrix with sealants, reducing silver release. As a result of scaling, Ca 2p and Si 2p peaks appear in XPS scan. Effect of scaling on performance is confirmed by conducting performance trials in tap water and ultrapure water (conductivity: 18 MO-cm) upon constant exposure to tap water, Ag-OTMN experiences silicate and organic deposition in the matrix, which limits its efficacy after a period of time. On the contrary, when the trials were done in ultrapure water, composition exhibits excellent antimicrobial activity continuously over 400 trials, without requiring any reactivation.

We Claim:

1. A method of reactivating an exhausted silver metal particle based antimicrobial composition used for drinking water applications, consisting essentially of a physical treatment of the composition by treating the composition with hot water.

2. A method as claimed in claim 1 wherein the temperature of the hot water is below 100 °C.

3. A method as claimed in claim 1 wherein contact time of the composition with hot water is for a duration of about 5 minutes to about 3 hours.

4. A method as claimed in claim 1 wherein the silver metal particle based antimicrobial composition consists of silver metal particles anchored on a solid surface.

5. A method as claimed in claim 4 wherein the size of silver metal particle is less than about 1000 nm.

6. A method as claimed in claim 4 wherein the solid surface comprises metal oxide, metal hydroxide, metal oxyhydroxide, organic polymer, activated carbon powder, activated carbon granules or ceramic materials derived from biological sources.

7. A method of reactivating an exhausted silver metal particle based antimicrobial composition, consisting essentially of a reaction-based treatment of the composition by treating the composition with acidic water.

8. A method as claimed in claim 7 wherein the pH of the acidic water is about 3.5 to about 6.

9. A method as claimed in claim 7 wherein the acidic water consists of citric acid, lemon juice, lactic acid, tartaric acid, acetic acid or a combination thereof.

10. A method as claimed in claim 7 wherein contact time of the composition with acidic water is for a duration of about 5 minutes to about 3 hours.

11. A method as claimed in claim 7 wherein the silver metal particle based antimicrobial composition consists of silver metal particles anchored on a solid surface.

12. A method as claimed in claim 11 wherein the size of silver metal particle is less than about 1000 nm.

13. A method as claimed in claim 11 wherein the solid surface comprises metal oxide, metal hydroxide, metal oxyhydroxide, organic polymer, activated carbon powder, activated carbon granules or ceramic materials derived from biological sources.

14. A method of reactivating an exhausted silver metal particle based antimicrobial composition, consisting essentially of a reaction-based treatment of the composition by treating the composition with oxidizing water.

15. A method as claimed in claim 14 wherein the oxidizing component in water is hydrogen peroxide.

16. A method as claimed in claim 14 wherein contact time of the composition with oxidizing water is for a duration of about 5 minutes to about 3 hours.

17. A method as claimed in claim 14 wherein the silver metal particle based antimicrobial composition consists of silver metal particles anchored on a solid surface.

18. A method as claimed in claim 17 wherein the size of silver metal particle is less than about 1000 nm.

19. A method as claimed in claim 17 wherein the solid surface comprises metal oxide, metal hydroxide, metal oxyhydroxide, organic polymer, activated carbon powder, activated carbon granules or ceramic materials derived from biological sources.

20. A method as claimed in claim 1, 7 & 14 wherein the silver metal particle based antimicrobial composition consists of alloys of silver or compositions in which silver is one of the antimicrobial agents anchored on a solid surface.