FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
1. Title of the invention
A WATER PURIFICATION COMPOSITION
2. Applicant(s)
Name Nationality Address
TATA CHEMICALS LIMITED INDIA BOMBAY HOUSE, 24 HOMI MODI STREET, MUMBAI,
INDIA
3. Preamble to the description
COMPLETE SPECIFICATION
The following specification particularly describes the invention and the manner in which it is
to be performed.

The present disclosure provides a water purification composition for the treatment of water. The present disclosure also provides a method of making a water purification composition for treatment of water.
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 counties do not have access to clean potable water.
Growing population, lack of sanitary condition, 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.
Many water purification devices like in-line (electricity operated) devices, terminal end devices including counter top and faucet mounted filtration and self-contained batch system including gravity fed systems have been introduced into the market. However, many communities of the developing countries do not have access to piped water or reliable electricity connection to make use of inline or electrically driven purification units. They also do not have access to central water purification units installed near the water bodies from where they source their water. Moreover, the people in these communities cannot afford the point-of-use water purifiers currently available in the market. The maintenance requirements and the high cost of consumables of these purifiers pose further hurdles for the adoption of these devices.
Rice husk ash has been used as a water purification composition for treatment of water, as a low cost alternative. However, rice husk ash when used alone as a water

purification composition is not able to remove all bacteria. Further the filter that use rice husk ash alone bulky as a large amount of rice husk ash is required to achieve satisfactory performance.
In order to use rice husk ash in water purification devices rice husk ash has been coated with bactericidal agents, specifically rice husk ash has been coated with silver nanoparticles as disclosed in Indian patent application No.l576/MUM/2008. Since, rice husk ash is brittle, continuous flow of water through the silver nanoparticle coated rice husk ash bed creates compaction and channel formation in the bed. This leads to inconsistent flow of water and instances of choking of the filter bed. This also results in inconsistencies in leaching of silver ions due to which variation is observed in anti microbial activity during water purification.
Thus there is a need for a water purification composition that is inexpensive, easy to use and shows consistency in removing microbial contamination from drinking water.
SUMMARY
In one aspect of the present disclosure, a water purification composition is provided. The water purification composition includes porous granules of rice husk ash-silver nanoparticles-chitosan matrix, obtained by adding rice husk ash bonded to silver nanoparticles or rice husk ash and silver nanoparticles to a chitosan gel, the chitosan gel formed by dissolving chitosan in an aqueous solution of organic acid, such that the ratio of chitosan to rice husk ash is in a range of 3:100 to 15: 100 weight by weight wherein the porous granules are obtained in a range of 0.15 mm to 1 mm.
In another aspect, a method for preparing a water purification composition is also provided. The method includes dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel, adding rice husk ash bonded to silver nanoparticles to the

chitosan gel, the concentration of chitosan being in a range of 3% to 15% weight by weight of rice husk ash bonded to silver nanoparticles, carrying out a size reduction step of the mixture of chitosan gel and rice husk ash bonded to silver nanoparticles, drying the mixture to reduce moisture content to at least 5% by weight of the mixture; and sieving the dried mixture to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.
In yet another aspect, the method includes dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel, adding rice husk ash and silver nanoparticles to the chitosan gel, the concentration of chitosan being in the range of 3% to 15% weight by weight of rice husk ash, carrying out a size reduction step of the mixture of chitosan gel, rice husk ash and silver nanoparticles, drying the mixture to reduce moisture content to at least 5% by weight of the mixture, and sieving the dried mixture to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.
BRIEF DESCRIPTION OF ACCOMPANYING FIGURES
The accompanying figure illustrates the preferred embodiments of the invention and together with the following detailed description serves to explain the principles of the invention.
Figure 1 illustrates a water purification composition for treatment of water.
DETAILED DESCRIPTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to embodiments 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 disclosed 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.
Reference throughout this specification to "one embodiment" "an embodiment" or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase "in one embodiment", "in an embodiment" and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
The present disclosure provides a water purification composition for treatment of water. Particularly, the present disclosure provides a water purification composition comprising porous granules of rice husk ash- silver nanoparticles-chitosan matrix, obtained by adding rice husk ash bonded to silver nanoparticles or rice husk ash and silver nanoparticles to a chitosan gel, the chitosan gel formed by dissolving chitosan in an aqueous solution of an organic acid, such that the ratio of chitosan to rice husk ash is in a range of 3:100 to 15:100 weight by weight wherein the porous granules are obtained in a range of 0.15 mm to 1 mm.
In accordance with an embodiment, the concentration of silver nanoparticles is in a range of 0.5% to 5% weight by weight of rice husk ash.
The present disclosure also provides a method of making a water purification composition. The method comprises of dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel and adding rice husk ash bonded to silver

nanoparticles to the chitosan gel to obtain a mixture, the concentration of chitosan being in a range of 3% to 15% weight by weight of the rice husk ash bonded to silver nanoparticles.
The method further comprises of carrying out a size reduction step of the mixture of chitosan gel and rice husk ash bonded to silver nanoparticles and drying the mixture to reduce moisture content to at least 5% by weight of the mixture. The dried mixture is then sieved to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.
The rice husk ash bonded with silver nanoparticles may be obtained by any known method that allows for bonding of silver nanoparticles to the rice husk ash. Indian patent application nos. 1570/MUM/2008 and 1571/MUM/2008 disclose methods of bonding silver nanoparticles to rice husk ash and are hereby incorporated by reference in their entirety.
In accordance with an embodiment, the rice husk ash bonded to silver nanoparticles has a particle size in the range of 1 micron to 1mm. In accordance with an embodiment, the rice husk ash is ground to obtain a particle size in the range of 1 micron to 1mm prior to binding with silver nanoparticles.
In accordance with an alternative embodiment, the method comprises of dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel and adding rice husk ash and silver nanoparticles to the chitosan gel to obtain a mixture, the concentration of chitosan being in the range of 3% to 15% weight by weight of rice husk ash.
The method further comprises of carrying out a size reduction step of the mixture of chitosan gel, rice husk ash and silver nanoparticles and drying the mixture to reduce moisture content to at least 5% by weight of the mixture. The dried mixture is then sieved to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.

In accordance with an embodiment, the concentration of silver nanoparticles added to the chitosan gel is in a range of 0.5% to 5% weight by weight of rice husk ash.
In accordance with an embodiment, the silver nanoparticles added are encapsulated by chitosan. The silver nanoparticles encapsulated by chitosan may be prepared by any known method. By way of a specific example the process for preparing silver nanoparticles encapsulated by chitosan comprises of adding citric acid to an aqueous solution of silver nitrate; adding to this solution chitosan powder to obtain a silver nitrate chitosan mixture; adding to the silver nitrate chitosan mixture a reducing agent to reduce silver nitrate and adding a base to precipitate silver nanoparticles encapsulated by chitosan. The reducing agent may include but is not limited to sodium citrate and sodium borohydrate.
In accordance with an aspect, in order to achieve the concentration of chitosan to 3% to 15% weight by weight of rice husk ash, the concentration of chitosan in the chitosan gel is in a range of 3% to 15%. In accordance with an embodiment, chitosan gel is obtained by dissolving chitosan in a 10% aqueous solution of organic acid. In accordance with an embodiment, organic acid is citric acid.
In accordance with an aspect, the size reduction step may be carried by any known method such as grinding and milling, that facilitates breaking up of the mixture.
In accordance with an embodiment, the mixture is dried between 70°C to 100°C and specifically the mixture is dried at 90°C in an oven.
The dried mixture is subjected to sieving through a filter having a mesh size of approximate 1 mm and the dried mixture that passes through the filter is collected. The mixture collected after passing through the filter having a mesh size of 1mm is subjected to sieving through filter having a mesh size of 150 urn to obtain porous granules of rice husk ash-silver nanoparticles- chitosan matrix. In accordance with an aspect, the porous

granules of rice husk ash-silver nanoparticles- chitosan matrix should be at least 150 μrn in size. The porous granules are collected on top of the filter.
A method of treatment of water for removal of microbial content is disclosed. The method includes passing the water over a water purification composition at a specified flow rate wherein the water purification composition comprises porous granules of rice husk ash-silver nanoparticles-chitosan matrix, obtained by adding rice husk ash bonded to silver nanoparticles or rice husk ash and silver nanoparticles to a chitosan gel, the chitosan gel formed by dissolving chitosan in an aqueous solution of organic acid, such that the ratio of chitosan to rice husk ash is in a range of 3:100 to 15:100 weight by weight wherein the porous granules are obtained in a range of 0.15 mm to 1 mm. By way of specific example, the method of treatment of water includes passing the water over a composition for treatment of water at a flow rate that ensures that the water remains in contact with the composition for a time period sufficient to allow removal of microbial content from water.
The following example(s) are exemplary and should not be understood to be in any way limiting.
Example 1:
50 grams citric acid is added to 1 liter of 0.1 M Silver Nitrate (AgNO3) solution and stirred well for mixing till the solution is clear. 20 grams of chitosan powder is added to the above solution while stirring for 15 minutes. The solution mixture is then heated up to boiling (-90°C). Then 500 milliliters of 5% tri sodium citrate solution is added slowly to it and stirred for 20 minutes at 90 °C. This step will induce the synthesis of silver nanoparticles with the development of yellow coloration of the solution mixture. Thereafter, 300 milliliters of 10% Sodium hydroxide (NaOH) solution is added to above solution at room temp while stirring is continued. This will lead to the formation of dark brown precipitate and slowly

settles down at the bottom of the reaction vessel.The precipitate is filtered and washed thoroughly with ample amount of distilled water before dried in an oven at 80°C. The dried lumps were grounded in a grinder to produce silver nanoparticles encapsulated with chitosan.
Example 2:
50grams of citric acid is added to 1 liter of 0.1 M Silver Nitrate (AgNO3) solution and stirred well for mixing till the solution is clear. 20 grams of chitosan powder is added to the above solution while stirring for 15 minutes to obtain a silver nitrate-chitosan solution. 4 grams of sodium borohydride is dissolved in 500 ml of ice cold water and added slowly to the silver nitrate-chitosan solution. This step will induce the synthesis of silver nanoparticles with the development of yellow coloration. Thereafter, 300ml of 10% sodium hydroxide (NaOH) solution is added at room temp while stirring is continued. This will lead to the formation of dark brown precipitate and slowly settles down at the bottom of the reaction vessel. The precipitate is filtered and washed thoroughly with ample amount of distilled water before dried in an oven at 80°C. The dried lumps were grounded in a grinder to produce silver nanoparticles encapsulated with chitosan.
Example 3:
A citric acid solution is prepared by dissolving 10 grams of citric acid in 100 milliliter of distilled water. 8 grams of chitosan is dissolved slowly in the citric acid solution by stirring it for 1 hour to obtain a chitosan gel. Thereafter, 80 milliliters of the chitosan gel is mixed with 100 grams of rice husk ash and 2 to 5 grams of silver nanoparticles is added to obtain a mixture. This mixture is added to a kitchen grinder container and ground for approximately 1 minute. The ground mixture is then taken out and dried at 80°C in an oven. The dried mixture is sieved through a 1 mm and a 150 urn filter consecutively. The granules obtained between 150 μm to 1 mm are used for further experiments.

Example 4:
A citric acid solution is prepared by dissolving 10 grams of citric acid in 100 milliliters of distilled water. 8 grams of chitosan is dissolved slowly in the citric acid solution by stirring it for 1 hour to obtain a chitosan gel. Thereafter, 80 milliliters of chitosan gel is mixed with 100 grams of rice husk ash bonded to silver nanoparticles obtained by in-situ process as disclosed in Indian Patent application number 1570/MUM/2008. This mixture is added to a kitchen grinder container, and is ground for approximately 1 minute. The ground mixture is then taken out and dried at 80 °C in an oven. The dried mixture is sieved through a 1 mm and a 150 μm filter consecutively. The granules obtained between 150 μm to 1 mm are used for further experiments.
Example 5:
A citric acid solution is prepared by dissolving 10 grams of citric acid in 100 milliliter of distilled water. 8 grams of chitosan is dissolved slowly in the citric acid solution by stirring it for 1 hour. 100 grams of rice husk ash is ground to fine powder. The ground rice husk ash is bonded to silver nanoparticles obtained by in-situ process disclosed in Indian Patent application number 1570/MUM/2008 to obtain rice husk ash bonded to silver nanoparticles. Thereafter, 80 milliliters of chitosan gel is mixed with 100 grams of the rice husk ash bonded to silver nanoparticles. This mixture is added to a kitchen grinder container and is ground for approximately 1 minute. The ground mixture is then taken out and dried at 80 °C in an oven. The dried mixture is sieved through a 1 mm and a 150 μm filters. The granules obtained between 150 μm to 1 mm are used for further experiments.
Example 6:
Filter blocks are prepared by packing 100 grams of the water purification composition. To test the efficiency of the water purification composition water having

different levels of Total Dissolved solids (TDS) and different concentrations of the bacteria Escherichia coli ATCC 11229 is passed through the filter blocks at a specified flow rate. Water after being passed through the water purification composition was allowed a residence time of 3 hours. Residence time herein refers to the time for which the water is held before conducting further studies. Each test is conducted in triplicate and is depicted below as Granule block 1, Granule block 2 and Granule block 3. Results of each test conducted are depicted in Table 1 below.

Filter Name Residence time pH Conductivity (μ mho) TDS
(ppm) Input-
CFU/ml
of E.
Coli Output-CFU/ml
of E.
Coli Log Reduction Flow Rate (1/hr) Water Passed
(I)
Granule block-1 3hr 7.61 591 387 380000 0 5.58 5 18
Granule block-2 3hr 7.61 591 387 380000 0 5.58 5.6 18
Granule block-3 3hr 7.61 591 387 380000 0 5.58 5.1 18
Granule block-1 3hr 7.49 592 389 1110000 16 4.84 4.4 375
Granule block-2 3hr 7.49 592 389 1110000 172 3.81 4.8 380
Granule block-3 3hr 7.54 616 406 1140000 18.5 4.79 4.5 484
Granule block-1 3hr 7.54 616 406 1140000 2.5 5.66 4.9 477
Granule block-2 3hr 7.54 616 406 1140000 3 5.58 4.8 480
Granule block-3 3hr 7.56 591 388 3080000 1 6.49 5 782

Granule block-1 3hr 7.56 591 388 3080000 9.5 5.51 5.2 799
Granule block-2 3hr 7.56 591 388 3080000 6 5.71 5 795
Granule block-3 3hr 7.64 599 396 3620000 0 6.56 4.2 859
Granule block-1 3hr 7.64 599 396 3620000 3 6.08 4.9 873
Granule block-2 3hr 7.64 599 396 3620000 2 6.26 4.5 869
Granule block-3 3hr 7.63 616 404 1690000 2.5 5.83 4.4 984
Granule block-1 3hr 7.58 589 386 1510000 3 5.70 4.4 970
Granule block-2 3hr 7.67 622 408 2000000 0 6.30 5 990
Granule block-3 3hr 7.66 597 393 5030000 31 5.21 5.2 1082
Granule block-1 3hr 7.66 597 393 5030000 25 5.30 5 1092
Granule block-2 3hr 7.51 583 380 3050000 20 5.18 5 1069
Granule block-3 3hr 7.61 587 384 1910000 12 5.20 4.2 1316
Granule block-1 3hr 7.66 590 388 2270000 4.5 5.70 4.65 1372
Granule block-2 3hr 7.66 590 388 2270000 6 5.58 4.5 1397
Granule block-3 3hr 7.66 580 378 1250000 2.5 5.70 4.3 1368
Granule block-1 3hr 7.66 580 378 1250000 0 6.10 4.6 1416

Granule block-2 3hr 7.66 580 378 1250000 1 6.10 4.3 1433
Granule block-3 3hr 7.88 590 388 2000000 2.5 5.90 3.9 1386
Granule block-1 3hr 7.88 590 388 2000000 7 5.46 4.6 1434
Granule block-2 3hr 7.88 590 388 2000000 14 5.15 4.6 1463
Granule block-3 3hr 7.73 589 386 1840000 1 6.26 3.7 1464
Granule block-1 3hr 7.73 589 386 1840000 0 6.26 4 1519
Granule block-2 3hr 7.73 589 386 1840000 0 6.26 3.9 1546
Granule block-3 3hr 7.68 578 375 2200000 65 4.53 4.4 1574
Granule block-1 3hr 7.75 585 383 1330000 2 5.82 4.25 1728
Granule block-2 3hr 7.75 585 383 1330000 3 5.65 4.6 1843
Granule block-3 3hr 7.75 585 383 1330000 0 6.12 4 1858
Granule block-1 3hr 7.68 624 410 2120000 2 6.03 4.2 1867
Granule block-2 3hr 7.68 624 410 2120000 3.5 5.78 4.9 1954
Granule block-3 3hr 7.59 592 388 2020000 3.5 5.76 4.4 1961
Granule block-1 3hr 7.61 585 382 1610000 69 4.37 3.6 1924
Granule block-2 3hr 7.69 593 391 1760000 88.5 4.30 4.1 2026

Granule block-3 3hr 7.69 593 391 1760000 43.5 4.61 3.9 2024
Granule block-1 3hr 7.71 592 389 1270000 4 5.50 3.7 2005
Granule block-2 3hr 7.71 592 389 1270000 4 5.50 4.5 2113
Granule block-3 3hr 7.71 592 389 1270000 3 5.63 3.6 2106
Granule block-1 3hr 7.78 624 412 1900000 149 4.11 4.4 2212
Granule block-2 3hr 7.58 584 382 2510000 27.5 4.96 3.5 2177
Granule block-3 3hr 7.81 593 390 650000 4.5 5.16 3.2 2177
Granule block-1 3hr 7.81 593 390 650000 1.5 5.64 5.1 2307
Granule block-2 3hr 7.81 593 390 650000 2 5.51 2.9 2272
Granule block-3 3hr 7.84 622 408 1480000 21 4.85 3.7 2271
Granule block-1 3hr 7.84 622 408 1480000 118.5 4.10 4.6 2416
Granule block-2 3hr 7.84 622 408 1480000 42 4.55 2 2365
Granule block-3 3hr 7.69 584 381 2070000 9 5.36 3.1 2295
Granule block-1 3hr 7.69 584 381 2070000 110 4.27 4.2 2443
Granule block-2 3hr 7.69 584 381 2070000 0 6.32 0.7 2381
Granule block-3 3hr 7.76 606 399 3460000 18 5.28 2.75 2367

Granule block-1 3hr 7.66 622 408 1220000 26 4.67 3.25 2437
Granule block-2 3hr 7.66 622 408 1220000 63 4.29 4.4 2583
Granule block-3 3hr 7.72 618 404 1480000 8.5 5.24 3 2683
Granule block-1 3hr 7.72 618 404 1480000 4.5 5.52 4.1 2866
Granule block-2 3hr 7.64 618 408 1770000 242.5 3.86 i j 2987
Granule block-3 3hr 7.64 618 408 1770000 150.5 4.07 2.3 2789
Granule block-1 3hr 7.68 624 410 1370000 68 4.30 3.9 3053
Granule block-2 3hr 7.68 622 408 1670000 54 4.49 2 2908
Granule block-3 3hr 5.58 592 388 1580000 8.5 5.27 2.1 2971
SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW
A water purification composition comprising porous granules of rice husk ash-silver nanoparticles-chitosan matrix, obtained by adding rice husk ash bonded to silver nanoparticles or rice husk ash and silver nanoparticles to a chitosan gel, the chitosan gel formed by dissolving chitosan in an aqueous solution of an organic acid, such that the ratio of chitosan to rice husk ash is in a range of 3:100 to 15:100 weight by weight wherein the porous granules are obtained in a range of 0.15 mm to 1 mm.
Such water purification composition(s), wherein the concentration of silver nanoparticles is in a range of 0.5 % to 5% weight by weight of rice husk ash.

FURTHER SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW
A method of making a water purification composition comprising dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel, adding rice husk ash bonded to silver nanoparticles to the chitosan gel, the concentration of chitosan being in a range of 3% to 15% weight by weight of rice husk ash bonded to silver nanoparticles, carrying out a size reduction step of the mixture of chitosan gel and rice husk ash bonded to silver nanoparticles, drying the mixture to reduce moisture content to at least 5% by weight of the mixture; and sieving the dried mixture to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.
Such method(s), wherein the rice husk ash bonded to silver nanoparticles has a particle size in the range of 1 micron to 1mm.
FURTHER SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW
A method of making a water purification composition comprising dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel, adding rice husk ash and silver nanoparticles to the chitosan gel, the concentration of chitosan being in the range of 3% to 15% weight by weight of rice husk ash, carrying out a size reduction step of the mixture of chitosan gel and rice husk ash bonded to silver nanoparticles, drying the mixture to reduce moisture content to at least 5% by weight of the mixture, and sieving the dried mixture to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.
Such method(s), wherein the concentration of silver nanoparticles added to the chitosan gel is in the range of 0.5% to 5% weight by weight of rice husk ash.

Such method(s), wherein chitosan gel is obtained by dissolving chitosan in a 10% aqueous solution of organic acid.
Such method(s), wherein the organic acid is citric acid.
INDUSTRIAL APPLICABILITY
The water purification composition comprising of rice husk ash- silver nanoparticles-chitosan matrix effectively removes the microbial content from water. As both silver nanoparticles and chitosan have antimicrobial activity, they work in a synergistic manner. Additionally, chitosan acts as binder for the granules and being a high molecular weight biopolymer, it holds silver nanoparticles strongly thereby allowing controlled release of silver from the porous granules. Further as the water purification composition is in the form of granules that are highly porous and swell upon contact with water, no choking or channeling is observed when the water purification composition is packed in a bed. The method of making a water purification composition described above is easy to perform and cost effective.

WE CLAIM:
1. A water purification composition comprising porous granules of rice husk ash- silver nanoparticles-chitosan matrix, obtained by adding rice husk ash bonded to silver nanoparticles or rice husk ash and silver nanoparticles to a chitosan gel, the chitosan gel formed by dissolving chitosan in an aqueous solution of an organic acid, such that the ratio of chitosan to rice husk ash is in a range of 3:100 to 15:100 weight by weight wherein the porous granules are obtained in a range of 0.15 mm to 1 mm.
2. A water purification composition as claimed in claim 1, wherein the concentration of silver nanoparticles is in a range of 0.5 % to 5% weight by weight of rice husk ash.
3. A method of making a water purification composition comprising:
dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel,
adding rice husk ash bonded to silver nanoparticles to the chitosan gel, the concentration of chitosan being in a range of 3% to 15% weight by weight of rice husk ash bonded to silver nanoparticles;
carrying out a size reduction step of the mixture of chitosan gel and rice husk ash bonded to silver nanoparticles;
drying the mixture to reduce moisture content to at least 5% by weight of the mixture; and
sieving the dried mixture to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.

4. A method as claimed in claim 2, wherein the rice husk ash bonded to silver nanoparticles has a particle size in the range of 1 micron to 1mm.
5. A method of making a water purification composition comprising:
dissolving chitosan in an aqueous solution of an organic acid to obtain a chitosan gel,
adding rice husk ash and silver nanoparticles to the chitosan gel, the concentration of chitosan being in the range of 3% to 15% weight by weight of rice husk ash;
carrying out a size reduction step of the mixture of chitosan gel and rice husk ash bonded to silver nanoparticles;
drying the mixture to reduce moisture content to at least 5% by weight of the mixture; and
sieving the dried mixture to obtain porous granules of rice husk ash-silver nanoparticles-chitosan matrix having a size in the range of 0.15 mm to 1 mm.
6. A method as claimed in claim 5, wherein the concentration of silver nanoparticles added to the chitosan gel is in the range of 0.5% to 5% weight by weight of rice husk ash.
7. A method as claimed in claim 5, wherein the silver nanoparticles are encapsulated in chitosan.
8. A method as claimed in claim 3 or 5, wherein chitosan gel is obtained by dissolving chitosan in a 10% aqueous solution of organic acid.

9. A method as claimed in claim 1 or 3, wherein the organic acid is citric acid.