WO2010010569A1

WO2010010569A1 - A process for forming a rice husk ash composition

Info

Publication number: WO2010010569A1
Application number: PCT/IN2008/000811
Authority: WO
Inventors: Debabrata Rautaray; Murali Sastry
Original assignee: Tata Chemicals Ltd
Current assignee: Tata Chemicals Ltd
Priority date: 2008-07-24
Filing date: 2008-12-05
Publication date: 2010-01-28
Anticipated expiration: 2011-01-24
Also published as: AP2011005588A0; ZA201101305B

Abstract

The invention relates to process comprising preparing silver nanoparticles in the presence of a stabilizing agent and adding the silver nanoparticles to rice husk ash to obtain rice husk ash with bonded silver nanoparticles.

Description

The invention relates to a process for binding metal nanoparticles to rice husk ash. More particularly the invention relates to a process for binding silver nanoparticles to rice husk ash. DESCRIPTION OF RELATED ART

Clean potable water is a basic human requirement. However, a large portion of the world's population, especially those living in developing countries do not have access to clean potable water.

Growing population, lack of sanitary conditions, poverty, poor planning, industrial pollution, over exploitation of natural water and national disasters are the main reasons of contamination of water. This contaminated water is the source of many diseases such as diarrhea, dysentery, fever, abdominal pain, and constipation, caused due to bacterial contamination transmitted through water. In India for example, as per the data collected by the Ministry of Health and Family Welfare, in 2003 there were 10.5 million cases of diarrhea with 4709 deaths resulting majorly due to consumption of contaminated water. According to the World Health Organization, the provision of safe water alone can reduce diarrheal and enteric disease by up to 50%, even in the absence of improved sanitation and other hygiene measures.

Rice husk ash has been used as a water purifier but the results obtained are not consistent over time, the rice husk ash is not able to remove all bacteria, and the filter devices required for adequate filtration tend to be bulky. Moreover as the rice husk ash has a potential to trap water born bacteria due to the small size of its pores, trapped bacteria may survive on the rice husk ash and may even seep into the water that is filtered through the rice husk ash. In order to use rice husk ash as an effective water filtration medium, there is a need for a process that would impart antimicrobial properties to rice husk ash, so that the rice husk ash is also able to destroy microbes that are present in water. There is also a need for a water purification composition that is inexpensive, easy to use and effective in removing bacterial contamination from drinking water.

SUMMARY

The invention relates to a process comprising binding silver nanoparticles to rice husk ash. Silver nanoparticles are synthesized by the reduction of silver precursor using a reducing agent in the presence of a stabilizing agent. Thereafter, rice husk ash is soaked in the as synthesized silver nanoparticles solution for suitable time to obtain rice husk ash bonded to silver nanoparticles. BRIEF 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.

Fig 1(A) Transmission electron micrograph of silver nanoparticles synthesized using chitosan as a stabilizing agent and ascorbic acid as a reducing agent.

Fig 1 (B) Selected area electron diffraction pattern of silver nanoparticles synthesized using chitosan as a stabilizing agent and ascorbic acid as a reducing agent.

Fig l(C) UV- Vis spectra of silver nanoparticles synthesized using chitosan as a stabilizing agent and ascorbic acid as a reducing agent.

Fig 2 UV-Vis spectra of silver nanoparticles synthesized using sodium bis(2-ethylhexyl) sulfosuccinate as a stabilizing agent and sodium borohydride as a reducing agent. 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.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.

The following description explains the principles of the invention as applied to the bonding of silver nanoparticles to rice husk ash. It is however believed that the teachings of the document may be equally applied to the other metals such as copper, gold, zinc, magnesium, platinum or titanium. In particular, a process for treating rice husk ash with a bactericidal agent is described. The following description also discusses certain specific compounds such as stabilizing agents, reducing agents and surfactants 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 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 uriburned 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 silica content of 60 to 90%.

Silver is a safe and effective antimicrobial agent that is lethal to single cell microorganism but is harmless to human cells. Silver's antimicrobial property stems from its extremely slow release of silver ions. These silver ions bind to the cellular components of microorganisms, disrupting the normal reproduction and growth cycle resulting in the death of the microbial cell. When made into particles only a few nanometers in size silver releases a lot more ions and therefore becomes an even stronger antimicrobial agent. Moreover unlike silver ions, elemental silver (Ag⁰) is not deactivated by chloride or organic matter that may be present in water. These qualities make nano silver a suitable anti microbial agent that may be bound to rice husk ash to impart antimicrobial properties to the rice husk ash.

A process for binding silver nanoparticles to rice husk ash is described. The process effectively binds silver nanoparticles to rice husk ash such that minimum leaching of silver occurs when water is passed through the rice husk ash. The rice husk ash with bonded silver nanoparticles is a useful composition for water purification and the bactericidal qualities of rice husk ash and silver work together in making the composition very effective.

The process involves preparing silver nanoparticles in the presence of a stabilising agent and adding to the rice husk ash the silver nanoparticles to obtain rice husk ash with bonded silver nanoparticles. The process involves the preparation of silver nanoparticles in the presence of a stabilising agent. The silver nanoparticles may be prepared in the presence of a stabilising agent by any known method. The stabilizing agent prevents the aggregation of nanoparticles during their formation by capping the silver nanoparticles that are formed. The stabilizing agent also facilitates in binding of silver nanoparticles to rice husk ash due to associated charge. The silver nanoparticles formed are then added to rice husk ash to obtain rice husk ash with bonded silver nanoparticles. In accordance with an aspect the silver nanoparticles are prepared in an aqueous medium.

In accordance with an aspect the silver nanoparticles are prepared in the presence of a stabilizing agent and stored to avoid aggregation of the prepared nanoparticles. The rice husk ash is added to the solution of capped silver nanoparticles to obtain rice husk ash with bonded nanoparticles. hi accordance with an aspect the rice husk ash is soaked with the silver nanoparticles for a predetermined period of time. The rice husk ash is soaked with the silver nanoparticles for a period ranging from 10 minutes to 24 hours and preferably for a period of 12 hours.

The rice husk ash to which silver nanoparticles are bonded is separated and washed with water. The rice husk ash with bound silver 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 nano silver is bonded is washed with copious amount of water and the water used for washing the rice husk ash with bound nano silver may be removed by any method including but not limited to filtration or vacuum filtration, preferably filtration. The rice husk ash to which silver nanoparticles are bound is dried by any method including but not limited to air drying or drying in a vacuum oven. The amount of silver nanoparticles should be at least 0.001% by weight for bonding with the rice husk ash. In accordance with an aspect the size of silver nanoparticles used are in the range of 5 nm to 100 nm.

In accordance with an aspect the process comprises, preparing silver nanoparticles comprising adding to a silver precursor a stabilizing agent and reducing the silver precursor to obtain silver nanoparticles, and adding the silver nanoparticles to rice husk ash to obtain rice husk ash with bonded silver nanoparticles.

By way of a specific example, the process comprises, preparing silver nanoparticles by adding to a solution of silver nitrate, a solution of chitosan to obtain a silver nitrate chitosan solution and reducing the silver nitrate by adding tri-sodium citrate dihydrate to obtain capped silver nanoparticles; and adding to the rice husk ash the silver nanoparticles to obtain a rice husk ash bonded with silver nanoparticles.

In accordance with an embodiment the silver nitrate chitosan solution is heated to a boiling temperature while stirring. The solution of tri-sodium citrate is added to the boiling silver nitrate chitosan solution. In accordance with an aspect the solution of tri-sodium citrate is not added as a dumping action but is added slowly. In accordance with a preferred embodiment the silver nitrate chitosan solution is boiled further after the addition of the reducing agent. Boiling assists in reducing the silver nitrate to silver nanoparticles.

In accordance with an aspect the process comprises preparing silver nanoparticles by forming a silver precursor solution, mixing a stabilising agent with a reducing agent and adding the stabilising agent and reducing agent solution to the silver precursor solution to obtain a solution of capped silver nanoparticles; and adding to rice husk ash the silver nanoparticle solution to obtain rice husk ash with bonded silver nanoparticles.

By way of a specific example the process comprises, preparing silver nanoparticles by adding to a solution of ascorbic acid a solution of chitosan to obtain an ascorbic acid-chitosan solution and adding the ascorbic acid chitosan solution a silver nitrate solution to obtain capped silver nanoparticles; and adding to the rice husk ash the silver nanoparticles to obtain a rice husk ash bonded with silver nanoparticles. The silver nanoparticles obtained by this process are triangular in shape.

In accordance with an aspect transmission electron microscopy, selected area electron diffraction and UV- Visible spectroscopy of the silver nanoparticles was carried out. Figure 1(A) shows a transmission electron micrograph of silver nanoparticles synthesized using chitosan as a stabilizing agent and ascorbic acid as a reducing agent. Figure 1(B) shows a selected area electron diffraction pattern of silver nanoparticles synthesized using chitosan as a stabilizing agent and ascorbic acid as a reducing agent. Figure l(C) shows UV- Vis spectra of silver nanoparticles synthesized using chitosan as a stabilizing agent and ascorbic acid as a reducing agent.

In accordance with an embodiment the silver precursor may be any base metal salts including but not limited to AgNO₃, AgBF₄, AgPF₆, Ag₂O, CH₃COOAg₅ AgCF₃SO₃, AgClO₄, AgCl, Ag₂SO₄,. Preferably the silver precursor is silver nitrate (AgNO₃). The concentration of silver salt may range between 0.001M and IM.

In accordance with an aspect the stabilising 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, hi accordance with an embodiment the stabilizing agent is chitosan. In accordance with an aspect 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.

The reducing agent used may be any reducing agent that is capable of converting Ag⁺ ions to Ag⁰ including but not limited to tri-sodium citrate, ascorbic acid, sodium borohydride, hydrazine hydrate, tyrosine or D-Glucose and preferably the reducing agent is tri-sodium citrate dihydrate. The amount of reducing agents ranges from 0.1 to 10 wt % and preferably the amount of reducing agent is 2 wt %. hi accordance with an aspect the nanoparticles obtained by this process are of spherical or triangular or flat structure.

In accordance with an aspect the process comprises, preparing silver nanoparticles by adding to a solution of silver nitrate, a solution of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) to obtain a silver nitrate AOT solution and reducing the silver nitrate by adding sodium borohydride to obtain capped silver nanoparticles; and adding to the rice husk ash the silver nanoparticles to obtain rice husk ash bonded with silver nanoparticles. hi accordance with an aspect transmission electron microscopy, selected area electron diffraction and UV- Visible spectroscopy of silver nanoparticles was carried out. Figure 2 shows the UV- Vis spectra of silver nanoparticles synthesized using sodium bis(2-ethylhexyl) sulfosuccinate as a stabilizing agent and sodium borohydride as a reducing agent.

La accordance with an embodiment, the silver nanoparticles are prepared in an aqueous medium by dissolving a surfactant in ethanol or an ethanol- water mixture to obtain a first solution, adding the first solution to a solution of a silver precursor in water and reducing the solution so obtained by adding a reducing agent in a predetermined manner to form silver nanoparticles. The surfactant in this process stabilizes the silver nanoparticles by preventing their aggregation in the aqueous medium. Rice husk ash is then added to the silver nanoparticle solution to obtain rice husk ash with bound silver nanoparticles.

In accordance with an embodiment, silver nanoparticles are prepared by mixing an aqueous silver sulfate solution with aqueous solution of tyrosine to obtain a silver sulfate tyrosine solution, adding to the silver sulfate tyrosine solution a solution of KOH and boiling the said solution to form silver nanoparticles. Silver nanoparticles obtained by this method are highly stable by the capping nature of tyrosine molecules on the nanoparticle surface δ preventing any aggregation. Rice husk ash is then added to the silver nanoparticle solution to obtain rice husk ash with bound silver nanoparticles.

In accordance with an aspect the process as described may also be used to bind other metal nanoparticles to rice husk ash. In accordance with an embodiment the process comprises of preparing metal nanoparticles in the presence of a stabilising agent and adding to the rice husk ash the metal nanoparticles to obtain rice husk ash with bonded metal nanoparticles.

The metal nanoparticles may be nanoparticles of any metal including copper, zinc, magnesium, titanium or alloys. Preferably the metal nanoparticle is a bactericidal agent.

In accordance with an aspect the rice husk ash with bonded silver nanoparticles obtained by this process may be used as a composition for removal of bacterial content from water in a water purification system.

The following examples are provided to explain and illustrate certain preferred embodiments of the process of the invention. Example 1

0.008 M silver nitrate (AgNO3) and 1% Chitosan [dissolved in 5% citric acid (C₆H₈O₇)] aqueous solutions are prepared. In the first step, 14 ml of (1%) chitosan solution is added to 1386 ml of AgNCβ solution and stirred for 5mins. Thereafter, the silver nitrate- chitosan solution is heated up to boiling temperature while stirring. 56 ml of 5% sodium citrate solution is added slowly to the above solution while stirring. The boiling is continued for another 15 mins after addition of sodium citrate solution. The reduction of Ag⁺ ions to Ag⁰ occurs during the boiling of silver nitrate-chitosan solution in the presence of tri-sodium citrate followed by instantaneous capping of silver nanoparticles with chitosan. Then, the solution was cooled to room temperature. 200 g of rice husk ash is added to the silver nanoparticles solution and allowed to soak for 12 hrs for bonding silver nanoparticles to rice husk ash. The rice husk ash with bonded silver nanoparticles was filtered followed by washing with copious amount of water. Example 2

0.008 M silver nitrate (AgNO3) and 0.001 M sodium bis(2-ethylhexyl) sulfosuccinate (AOT; C20H37NaO7S) aqueous solutions are prepared. In the first step, 0.001 M AOT is dissolved in 1400 ml of distilled water by stirring for 15 minutes. 1.9357 g Of AgNO₃ is added to the above solution and stirred for uniform mixing. Thereafter, 51.2 ml of freshly prepared 0.01 M sodium borohydride (NaBH4) solution is added slowly to the above mixture while stirring. The reduction of Ag⁺ ions to Ag⁰ occurs during the reaction of silver nitrate with sodium borohydride in the presence of AOT as a stabilizing agent. 200 g of rice husk ash is added to the silver nanoparticles solution and allowed to soak for 12 hrs for bonding silver nanoparticles to rice husk ash. The rice husk ash with bonded silver nanoparticles was filtered followed by washing with copious amount of water. Example 3

In a typical experiment, 0.008 M silver nitrate (AgNO₃), 1% Chitosan and 1% Ascorbic acid (C₆H₈Oe) aqueous solutions are prepared. In the first step, 1.9357 g OfAgNO₃ is dissolved in 1200 ml of distilled water. Thereafter, 200 ml of distilled water containing 1% ascorbic acid & 1% chitosan was added slowly to above solution while stirring. Stirring continued for 15 minutes. The reduction of Ag⁺ ions to Ag⁰ occurs during the reaction of silver nitrate with ascorbic acid in the presence of chitosan as a stabilizing agent. 200 g of rice husk ash is added to the silver nanoparticles solution and allowed to soak for 12 hrs for bonding silver nanoparticles to rice husk ash. The rice husk ash with bonded silver nanoparticles was filtered followed by washing with copious amount of water.

Claims

We claim:

1. A process comprising preparing silver nanoparticles in the presence of a stabilizing agent and adding the silver nanoparticles to rice husk ash to obtain rice husk ash with bonded silver nanoparticles.

2. A process as claimed in claim 1 wherein the rice husk ash is soaked with the silver nanoparticles for a period ranging from 10 minutes to 24 hours and preferably for a period of 12 hours.

3. A process as claimed in claim 1 wherein the process of preparing silver nanoparticles comprises reducing a silver precursor in the presence of a stabilising agent to obtain capped silver nanoparticles.

4. A process as claimed in claim 3 wherein the stabilizing agent is added to the silver precursor.

5. A process as claimed in claim 3 wherein the stabilizing agent is added along with the reducing agent.

6. A process as claimed in claim 3, 4 or 5 wherein the silver precursor is a silver base salt including AgNO₃, AgBF₄, AgPF₆, Ag₂O, CH₃COOAg₅ AgCF₃SO₃, AgClO₄, AgCl, Ag₂SO₄, and preferably the silver precursor is AgNO₃.

7. A process as claimed in claim 3, 4 or 5 wherein the molar concentration of silver precursor ranges from 0.001M to IM.

8. A process as claimed in claim 3, 4 or 5 wherein the stabilizing agent is one of any chitosan, trisodium citrate dihydrate, L-lysine, tyrosine, sodium bis(2-ethylhexyl) sulfosuccinate, sodium dodecyl sulphate, cetyl trimethyl ammonium bromide, polyvinyl pyrrolidone, polyvinyl alcohol, or oleylamine.

9. A process as claimed in claim 3, 4 or 5 wherein the stabilizing agent ranges from 0.1 to 10 wt % and preferably the stabilizing agent is 1 wt %.

10. A process as claimed in claim 3 wherein the reducing agent is any one of tri-sodium citrate dihydrate, ascorbic acid, tyrosine, hydrazine hydrate, D-Glucose or sodium borohydride and preferably the reducing agent is tri-sodium citrate dihydrate.

11. A process as claimed in claim 3 wherein the reducing agent ranges from 0.1 to 10 wt % and preferably the amount of reducing agent is 2 wt %.

12. A process comprising preparing metal nanoparticles in the presence of a stabilizing agent and adding the metal nanoparticles to rice husk ash to obtain rice husk ash with bonded metal nanoparticles.

13. A process as claimed in claim 12 wherein the metal nanoparticles are nanoparticles of copper, gold, silver, platinum, zinc, magnesium, titanium or alloys.

14. A process as claimed in claim 12 wherein the metal nanoparticle is a bactericidal agent.

15. A process comprising preparing silver nanoparticles by dissolving a surfactant in ethanol to obtain a first solution; dissolving a silver precursor in water to obtain a second solution; adding the second solution to the first solution to obtain a third solution; dissolving a reducing agent in water to obtain a reducing agent solution and adding the reducing agent solution to the third solution to obtain silver nanoparticles; and adding rice husk ash to the silver nanoparticles in solution to obtain rice husk ash with bound silver nanoparticles.

16. A process as claimed in any of the preceding claims wherein the size of nanoparticles obtained is in the range of 5nm to 300 nm, more particularly 10 to 50 nm.

17. A process as claimed in any of the preceding claims wherein the nanoparticles obtained are of spherical or triangular or flat structure.

18. A rice husk ash with silver nanoparticles bonded to it as claimed in any of the preceding claims used in a water purification system.

19. A rice husk ash composition having silver nanoparticles bonded to it by a process as claimed in any of the preceding claims.

20. A process of adding a coating to rice husk ash substantially as herein described with reference to and as illustrated by the accompanying figures.