The invention relates to an antimicrobial composition and the method for preparing the antimicrobial composition. More particularly the invention relates to an antimicrobial composition comprising of rice husk ash with an antimicrobial compound and process for preparing the same.

DESCRIPTION OF RELATED ART

Rice husk ash (RHA) as filtration media is fairly known for a long time. The small pore size of rice husk ash enables the entrapment of microorganisms when fluids are passed through it. Thus, microbes are filtered out from fluid. Nearly 90% bacterial colony reduction has been obtained in laboratory conditions. However, the trapped bacteria may survive on the rice husk ash and may even seep into the water that is filtered through the rice husk ash.

Hence there is a need to modify the filter so that the filtration can be made effective. There is a need for a process that would impart antimicrobial properties to rice husk ash, so that the microbes trapped in pores of rice husk ash are destroyed and hence render the water safe for drinking purposes. There is also a need for water purification composition that is inexpensive, easy to use and effective in removing bacterial contamination from drinking water.

SUMMARY

The invention relates to an antimicrobial composition comprising rice husk ash with an antimicrobial compound coated thereon. The said antimicrobial compound consists of copper nanoparticles bonded to silver nanoparticles with an intermetallic bond. The invention also relates to method of preparing the antimicrobial composition, the process comprises adding silver precursor to the rice husk with copper nanoparticles coated thereon, wherein the antimicrobial compound is formed by transmetallation reaction between the silver precursor and copper nanoparticles.

DESCRIPTION OF ACCOMPANYING DRAWINGS

The accompanying drawings illustrate the preferred embodiments of the invention and together with the following detailed description serve to explain the principles of the invention.

Figure 1 illustrates the viable bacterial colonies obtained before and after addition of coated RHA (Cu/Ag in 2: 1 ratio) in shake flask experiment.

Figure 2 illustrates the viable bacterial colonies obtained before and after addition of coated RHA (Cu/Ag in 1 : 1 ratio) in shake flask experiment.

Table 1 illustrates the X-ray Fluorescence Spectroscopy measurement of rice husk ash coated with copper and silver nanoparticles.

Table 2 illustrates the E. coli colony count observed under various treatments in shake flask experiment.

Table 3 illustrates the bacterial log reduction data obtained when bacteria spiked water was passed through a filtration bed made up of rice husk ash coated with Cu-Ag (1 : 1) nanoparticles .

DETAILED DESCRIPTION

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment described and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the process, and such further applications of the principles of the invention therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

The following description also discusses certain specific compounds such as stabilizing agents and reducing agents to explain the principles of the invention. The invention however is not restricted to such compounds as equivalent chemical compounds may be utilized to achieve the desired end result as taught by the invention. Reference throughout the specification to an embodiment, an aspect or similar language means that a particular feature, characteristic or step in connection with the embodiment is included in at least one embodiment of the present invention. References to an aspect or an embodiment may, but do not necessarily all refer to the same embodiment.

Rice husk is a perennially renewable agro-waste available at virtually no cost wherever rice paddy is grown. On combustion, the rice husk ash residue contains 85-95% silica, 4-12% carbon and the rest comprises of various metal oxides such as alkali, alkali earth metal and ion oxides. On account of its crypto-crystalline or amorphous and highly porous structure, the BET(Brunauer Emmett Teller) surface area of rice husk ash can be as high as 80-100 square meters per gram, depending on the conditions employed for the combustion of rice husk. Its high surface area and porosity make rice husk ash an effective filtration medium that removes particulate matter as well as color and odor from water.

The rice husk ash used for the process may be any rice husk ash that is produced by burning rice husk. The rice husk ash may be produced by burning rice husk in heaps, in a step grate furnace, fluidized bed furnace or tube-in-basket (TiB) burner. The rice husk ash may also be obtained from boilers and brick kiln, provided it is free of unburned husk and wood tar, grit, stone, and fused lumps of silica. In accordance with an aspect the rice husk ash should have high silica content. Preferably the rice husk ash should have a silica content of 60 to 90%.

Silver is safe and effective anti-microbial agent and is lethal to single cell microorganism but harmless to human cells. Silver ions bind to the cellular components of microorganisms, disrupting the normal reproduction and growth cycle resulting in the death of the microbial cell. Silver's antimicrobial property stems from its extremely slow release of silver ions. Silver ions denature proteins (enzymes) of the target cell or organism by binding to reactive groups resulting in their precipitation and inactivation. Silver inactivates enzymes by reacting with the sulfhydryl groups to form silver sulfides. Silver also reacts with the amino-, carboxyl-, phosphate-, and imidazole-groups and diminishes the activities of lactate dehydrogenase and glutathione peroxidase. Moreover unlike silver ions, elemental silver (Ag°) is not easily deactivated by chloride or organic matter that may be present in water.

Copper is harmless to humans and an inherently antimicrobial agent. Copper has the ability to exhibit antimicrobial properties when it comes in contact with a microorganism and this property makes copper an effective touch surface antimicrobial material. Copper is also proven to continuously kill the infection causing bacteria and remains effective even after wet/dry abrasion and re-contamination. In addition to the antibacterial properties of copper, it can kill and/or inactivate fungi, algae and viruses, thus it provides protection against wider range of micro organisms.

These metals when prepared in nanoparticle form, due to their small size and high surface to volume ratio provide higher surface area and enable greater release of ions and therefore become an even stronger antimicrobial agent. These qualities make copper and silver nanoparticles a suitable antimicrobial agent that may be bound to rice husk ash to impart antimicrobial properties to the rice husk ash.

An antimicrobial composition is disclosed. The antimicrobial composition in accordance with the invention comprises rice husk ash having an anti-microbial compound coated thereon, the anti-microbial compound including copper nanoparticles and silver nanoparticles having an inter metallic bond, the antimicrobial compound formed on the rice husk ash by a transmetallation reaction.

In accordance with an aspect, the antimicrobial composition may comprise of silver nanoparticles and copper nanoparticles in the ratio ranging substantially from 1 : 1 to 1 :3.

In accordance with an aspect, at least 0.4 percent by weight of the antimicrobial compound is present in the antimicrobial composition.

The invention also resides in a method for preparing the antimicrobial composition, said method comprising adding a silver precursor to the rice husk ash having copper nanoparticles coated thereon to cause an in situ reduction of silver ions in the silver precursor by a transmetallation reaction with the copper nanoparticles to form an antimicrobial compound coated on the rice husk ash, the anti-microbial compound including copper nanoparticles and silver nanoparticles having an inter metallic bond.

In accordance with an aspect, the method involves providing the copper nanoparticles on surface of rice husk ash, followed by addition of silver precursor to the copper nanoparticle coated rice husk ash. A compound including copper nanoparticles and silver nanoparticles having an inter metallic bond is thus formed on the rice husk ash by the transmetallation reaction between the copper nanoparticles and the silver precursor.

In accordance with an aspect, the copper nanoparticles are formed in situ on the surface of rice husk ash by adding a copper precursor to the rice husk ash in the presence of a stabilizing agent, followed by addition of a reducing agent to cause the reduction of the copper precursor to form the copper nanoparticles.

In accordance with an aspect, the stabilizing agent is completely dissolved in the copper precursor by heating to 60-80°C, before adding the copper precursor to the rice husk ash.

In accordance with an aspect, the rice husk ash is soaked with copper precursor and stabilizing agent for two to six hours with mild stirring. For example, rice husk ash is soaked with copper precursor and stabilizing agent for four hours with mild stirring.

In accordance with an aspect, the reducing agent is added to the rice husk ash soaked with the copper precursor and the stabilizing agent with stirring and heating to 60- 80°C.For example, the reducing agent may be added to the rice husk ash soaked with the copper precursor and the stabilizing agent with stirring and heating to 80°C.

After adding the reducing agent to the rice husk ash soaked with the copper precursor and the stabilizing agent, the final mixture is allowed to soak overnight at room temperature. The rice husk ash bonded to the copper nanoparticles is filtered and washed with water. The rice husk ash bonded to copper nanoparticles may be separated by any means including but not limited to filtration or centrifugation, and preferably filtration. The separated rice husk ash to which the copper nanoparticles are bonded is washed with copious amount of water. After washing, the separated rice husk ash bound with the copper nanoparticles, are dried by any method including but not limited to air drying or drying in a vacuum oven.

The copper precursor may be any base metal salts including but not limited to CuSC-4, CuCl ₂ , Cu (HCOO) ₂ , Cu(CH ₃ COO) ₂₃ CuC03, Cu(N03)2.Preferably the copper precursor is CuS04. In accordance with an aspect, the concentration of copper precursor may range from .001 to 1 M. By way of specific example, the concentration of copper precursor is 0.0145 M.

Any known stabilizing agent can be used in the process. Preferably, the stabilizing agent is gelatin.

In accordance with an aspect, the amount of stabilizing agent is atleast 0.01 %. By way of specific example, the amount of stabilizing agent is 0.15%.

^' Any reducing agent capable of reducing Cu to Cu , including but not limited to Sodium Formaldehyde Sulfoxylate (SFS), hydrazine hydrate, sodium borohydride, ascorbic acid and preferably the reducing agent is Sodium Formaldehyde Sulfoxylate (SFS). The concentration of reducing agent may range from 0.01- 2 M. For example, the concentration of reducing agent is 0.5 M.

In accordance with an aspect, the silver precursor is added to the rice husk ash bonded to the copper nanoparticles at room temperature, with mild stirring for a predetermined time period.

In accordance with an embodiment, the silver precursor is added to the rice husk ash bonded to copper nanoparticles in the presence of a stabilizing agent. Particularly, the process involves forming a solution of the silver precursor and the stabilizing agent, followed by addition of the solution of the silver precursor and the stabilizing agent to the rice husk ash bonded to the copper nanoparticles.

In accordance with an aspect, the silver precursor is added to the rice husk ash bonded to copper nanoparticle at room temperature with mild stirring for a predetermined time period.

In the presence of Copper nanoparticles, Ag ⁺ is reduced to Ag° by transmetallation reaction, resulting in the formation of compound of copper nanoparticles bonded to silver nanoparticles with an intermetallic bond. This reaction takes place due to the difference in reduction potential values of copper and silver. The reduction potential value of silver is higher than copper, hence Ag ⁺ reduces to Ag° when Ag ⁺ is reacted with Cu°.

The silver precursor may be any base metal salts . including but not limited to AgN0 ₃ , AgBF ₄ , AgPF ₆ , Ag ₂ 0, CH ₃ COOAg, AgCF ₃ S0 ₃ , AgC10 ₄ , AgCl, Ag ₂ S0 ₄ . Preferably the silver precursor is silver nitrate (AgN0 ₃ ).

In accordance with an aspect the concentration of silver precursor may range between 0.001 M and 1M. By way of a specific example, the concentration of silver precursor may be 0.00925 M.

In accordance with an aspect the stabilizing agent may be any compound that prevents the aggregation of the silver nanoparticles that are formed. The stabilizing agent includes but is not limited to chitosan, tri-sodium citrate dihydrate, L-lysine, tyrosine, sodium bis(2-ethylhexyl) sulfosuccinate, sodium dodecyl sulphate, cetyl trimethyl ammonium bromide, polyvinyl pyrrolidone, polyvinyl alcohol or oleylamine used alone or in combination with one another. The preferred stabilizing agent is chitosan. The amount of stabilizing agent ranges from 0.1 to 10 wt %. By way of a specific example, the amount of stabilizing agent may be 1 wt %.

By way of a specific example chitosan is dissolved in 5% citric acid. The citric acid is required to dissolve the chitosan and it also helps in maintaining mild acidic conditions during reduction.

In accordance with an aspect, X-ray fluorescence measurement of rice husk ash coated with copper and silver nanoparticles was carried out to measure the copper and silver content of the antimicrobial composition obtained in Example 1 and 2(given below). Table 1 is result of X-ray fluorescence, showing the amount of silica, copper and silver in the rice husk ash coated with copper and silver nanoparticles. As may be seen, the antimicrobial composition can include 3971 ppm of silver nanoparticles and 4819 ppm of copper nanoparticles. The antimicrobial composition can also include 2049 ppm of silver nanoparticles and 4985 ppm of copper nanoparticles.

In accordance with an aspect, an experiment was conducted to evaluate the antibacterial activity of rice husk ash coated with copper and silver nanoparticles. Culture of E. coli was inoculated to obtain bacteria contaminated water samples. Further, two samples of rice husk ash coated with copper and silver nanoparticles, one containing copper: silver nanoparticles in ratio 1 : 1 and the other in ratio 2: 1 were added to the bacteria contaminated water samples. Table 2 tabulates the results of the number of bacterial colonies obtained after incubating the samples of rice husk ash coated with copper and silver nanoparticles in bacteria contaminated water sample for 1 hour. Figure 1 and 2 shows the number of bacterial colonies in bacteria contaminated water sample before and after the treatment with rice husk ash coated with copper and silver nanoparticles.

In accordance with an aspect, another experiment was conducted to evaluate the anti bacterial activity of rice husk ash coated with copper and silver nanoparticles, when used as filter bed. Bacteria spiked water was made to pass through filter bed consisting of rice husk coated with copper and silver nanoparticles (copper: silver in ratio 1 : 1). Table 3 tabulates the result of bacterial colonies in bacteria spiked water before and after passing the water through the said filter bed.

In accordance with an aspect, the copper and silver leaching was measured. Bacteria contaminated water was passed though a bed of rice husk ash coated with copper and silver nanoparticles and amount of copper and silver present in the output water was measured. Table 4 tabulates the results of the leaching test conducted on random samples of output water. According to an aspect, the antimicrobial composition can be used in a water filter. It is however believed that the composition can also be used in air filter.

The following examples are provided to explain and illustrate certain preferred embodiments of the process of the invention:

Example 1

0.0145 M of Copper Sulfate (CuS04.5H20; 2.5439 grams) is dissolved in 700 milliliters of distilled water. In the first step, 1.015 grams of gelatin ( 0.15 %) is added to CuS0 ₄ solution followed by heating the CuS0 ₄ solution at 80 C for 20 minutes with stirring. Once gelatin is completely dissolved, 100 grams of rice husk ash is added to the CuSCVgelatin solution and is allowed to soak for 4 hours with mild stirring. 0.5 M sodium formaldehyde sulfoxylate (SFS) is prepared in an aqueous medium by stirring it for 20 minutes till it dissolves completely. SFS solution is then added drop wise (10 milliliters/minute) to the rice husk ash-CuS0 ₄ -gelatin mixture with stirring and heating at o 2-}- Q

80 C. The in-situ reduction of Cu ions to Cu occurs during the addition of SFS to the rice husk ash-copper sulfate -gelatin mixture. Heating is continued for 30 minutes after addition of SFS. The final mixture is soaked overnight at room temperature. The copper nanoparticle coated rice husk ash was filtered followed by washing with copious amount of distilled water. Copper nanoparticles coated rice husk ash is air dried or dried in an oven.

In the second step, 0.00925 M of silver nitrate (AgN0 ₃ ; 0.7864 grams) is dissolved in 500 milliliters of distilled water. 20 milliliters of (5% citric acid+1% chitosan) solution is added to AgN0 ₃ solution with stirring. The copper nanoparticle coated RHA is added to the silver nitrate-chitosan solution at room temperature with mild stirring for a period of 30 minutes. The in-situ reduction of Ag ⁺ ions to Ag° occurs due to the transmetallation reaction between Cu and Ag ⁺ ions. The resultant mixture is soaked overnight followed by filtration and washing with copious amount of distilled water. The copper and silver nanoparticles coated RHA is then dried in an oven at 100° C before further use.

Example 2

0.0145 M of copper sulfate (CuS04.5H20; 2.5439 grams) is dissolved in 700 milliliters of distilled water. In the first step 1.015 grams of gelatin ( 0.15 %) is added to CuS0 ₄ solution, followed by heating the CuS0 ₄ solution at 80 C for 20 minutes with stirring. Once the gelatin is completely dissolved, 100 grams of rice husk ash is added to the CuS04-gelatin solution and is allowed to soak for 4 hours with mild stirring. 0.5 M sodium formaldehyde sulfoxylate (SFS) is prepared in an aqueous medium by stirring it for 20 minutes till it dissolves completely. SFS solution is then added drop wise (10 milliliters/minute) to the rice husk ash-CuS04-gelatin mixture while stirring and heating at 80 C. The in-situ reduction of Cu ²⁺ ions to Cu° occurs during the addition of SFS to the rice husk ash-copper sulfate -gelatin mixture. Heating is continued for 30 minutes after addition of SFS. The final mixture is soaked overnight at room temperature. The copper nanoparticle coated rice husk ash is filtered followed by washing with copious amount of distilled water. Copper nanoparticle coated rice husk ash is air dried or dried in an oven.

In the second step, 0.00462 M of silver nitrate (AgN03; 0.3932 grams) is dissolved in 500 milliliters of distilled water. 20 milliliters of (5% citric acid+1% chitosan) solution is added to AgN0 ₃ solution while stirring. The copper nanoparticle coated RHA is added to the silver nitrate-chitosan solution at room temperature with mild stirring for a period of 30 minutes. The in-situ reduction of Ag ⁺ ions to Ag° occurs due to the transmetallation reaction between Cu and Ag ⁺ ions. The final mixture is soaked overnight followed by filtration and washing with copious amount of distilled water. The copper and silver nanoparticle coated RHA is then dried in an oven at 100°C before further use. Example 3

0.0145 M of copper sulfate (CuS04.5H20; 2.5439 grams) is dissolved in 700 milliliters of distilled water. In the first step 1.015 grams of gelatin ( 0.15%) is added to CuS0 ₄ solution followed by heating the solution at 80 °C for 20 minutes with stirring. Once gelatin is completely dissolved, 100 grams of rice husk ash is added to the CuS04- gelatin solution and is allowed to soak for 4 hours with mild stirring. 0.5 M sodium formaldehyde sulfoxylate (SFS) is prepared in an aqueous medium by stirring it for 20 minutes till it dissolves completely. SFS solution is then added drop wise (10 milliliters/minute) to the rice husk ash-CuS0 ₄ -gelatin mixture with stirring and heating at 80 °C. The in-situ reduction of Cu ²⁺ ions to Cu° occurs during the addition of SFS to the rice husk ash-copper sulfate -gelatin mixture. Heating is continued for 30 minutes after addition of SFS. The final mixture is soaked overnight at room temperature. The copper nanoparticle coated rice husk ash was filtered followed by washing with copious amount of distilled water. Copper nanoparticle coated rice husk ash is air dried or dried in an oven.

In the second step, 0.00925 M of silver nitrate (AgN03; 0.7864 grams) is dissolved in 500 milliliters of distilled water. The copper nanoparticle coated RHA is added to the AgNC"3 solution at room temperature with mild stirring for a period of 30 minutes. The in- situ reduction of Ag ⁺ ions to Ag° occurs due to the transmetallation reaction between Cu and Ag ⁺ ions. The resultant mixture is soaked overnight followed by filtration and washing with copious amount of distilled water. The coated RHA is then dried in an oven at 100°C before further use.

SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW

An anti-microbial composition comprising rice husk ash having an anti-microbial compound coated thereon, the anti-microbial compound including copper nano-particles and silver nano-particles having an inter metallic bond, the anti-microbial compound formed on the rice husk ash by a transmetallation reaction.

Such an anti-microbial composition, wherein the ratio of silver nano particles to copper nano particles in the anti-microbial compound is in the range from 1 : 1 to 1 :3.

Such an anti-microbial composition, wherein at least 0.4 percent by weight of the antimicrobial compound is present in the antimicrobial composition.

Such an anti-microbial composition, wherein the anti-microbial compound is formed and coated on the rice husk ash by an in situ transmetallation reaction, the transmetallation reaction between the copper nano-particles and a silver precursor.

A method of preparing an anti-microbial composition comprising: adding a silver precursor to rice husk ash having copper nano particles coated thereon to cause an in situ reduction of silver ions in the silver precursor by a transmetallation reaction with the copper nano particles to form an anti-microbial compound coated on the rice husk ash, the anti-microbial compound including copper nano-particles and silver nano-particles having an inter metallic bond.

Such a method of preparing an anti-microbial composition comprising forming the copper nano particles on the rice husk ash by an in situ process of reacting rice husk ash with a copper precursor, and a reducing agent in the presence of a stabilizing agent.

Such a method of preparing an anti-microbial composition, wherein the stabilizing agent is first completely dissolved in the copper precursor by heating between 60 to 80 degrees.

Such a method of preparing an anti-microbial composition, wherein the rice husk ash is soaked with the copper precursor and the stabilizing agent for two to six hours prior to reaction with the reducing agent. Such a method of preparing an anti-microbial composition, wherein the reaction with the reducing agent is accompanied by heating between 60 to 80 degrees.

Such a method of preparing an anti-microbial composition,comprising adding a stabilizing agent to the silver precursor.

Such a method of preparing an anti-microbial composition, wherein the silver precursor is added to the rice husk ash having copper nano particles coated thereon at room temperature.

Such a method of preparing an anti-microbial composition, wherein the ratio of silver nano particles to copper nano particles in the anti-microbial compound is in the range from 1 : 1 to 1 :3.

Such a method of preparing an anti-microbial composition, wherein at least 0.4 percent by weight of the antimicrobial compound is present in the antimicrobial composition.

INDUSTRIAL APPLICABILITY

The antimicrobial composition as described above is a simple composition that is inexpensive and effective in removing various microbial contaminations from potable water. The antimicrobial composition can be used in a water purification system. It is also believed that the antimicrobial composition can be used in air filter. The process to prepare the said antimicrobial composition as described above is easy to perform and is inexpensive.