TECHNICAL FIELD OF THE INVENTION The present invention relates to a particulate composition containing oligodynamic silver coated sand particles. The invention further relates to a water treatment device comprising such a particulate composition and to the use of such a particulate composition for treating contaminated water. The oligodynamic silver coated sand particles of the present invention can be used in water filters and other equipment for the treatment of contaminated water.

BACKGROUND OF THE INVENTION The oligodynamic effect is a biocidal effect of metals, especially heavy metals. Brass doorknobs and silverware both exhibit this effect.

The metabolism of bacteria is adversely affected by silver ions at concentrations of 0.01 to 0.1 mg/L. Therefore, even lesser soluble silver compounds, such as silver chloride, also act as bactericides or germicides. In the presence of atmospheric oxygen, metallic silver also has a bactericidal effect due to the formation of silver oxide, which is soluble enough to cause it. Even objects with a solid silver surface (e.g., table silver, silver coins, or silver foil) have bactericidal effect. The oligodynamic effect of silver can be used to treat contaminated water.

Various filter media are used for purification of water. These include particulate media like powdered or granular activated carbon, diatomaceous earth, activated alumina, sand and zeolites. Such particulate media are generally used in free state (unbound state) but bound filter blocks are more common. When in unbound state, a filter medium is usually packed in a flow-through container.

Most particulate filters perform on the principle of size exclusion. Such filters are generally good at removal of suspended particles of dirt. However contaminated water also contains microbes such as cysts, bacteria and virus. Most filter media perse do not remove or deactivate microorganisms, especially some bacteria and virus. For this reason, it is customary to include or to in situ generate in the filter media a particulate metal or metallic compound having antimicrobial activity.

Although it is well known that silver is a good antibacterial agent, the nature of silver present in the water determines its antibacterial efficiency. Recently, silver has been extensively used in the form of metallic nanoparticles. The antibacterial property of silver nanoparticles emerges either from nanoparticle-bacteria surface interaction or silver ions released from the nanoparticles or both.

Although a number of methods have been developed for synthesis of silver nanoparticles, keeping reactive particles in nanometer size for a long time in water is very difficult. This is due to ion induced aggregation, surface modification, salt deposition and other factors. Therefore, an important requirement while employing reactive silver nanoparticles in water purification is size stabilization and prevention of surface modification over extended periods. Furthermore, for toxicological reasons release of silver nanoparticles into purified water must be controlled and moderated. An important aspect of use of silver nanoparticles for anti-bacterial performance is the fraction of silver ions released (quantity of silver ions released/quantities of silver nanoparticle used). It is known that although significant quantities of silver

nanoparticles are used, a small amount of silver ions are released into the

contaminated water.

Constant release of silver ions for longer time is essential for consistent anti-microbial performance and release of silver ions below permissible limits prescribed by the World Health Organization (WHO). The rate of silver ion release has been discussed in the literature. For example, Epple et al. (Chem. Mater. 2010, 22, 4548 and Hurt et al. Environ. Sci. Technol. 2010, 44, 2169) demonstrated that the release of silver ions from silver nanoparticles in distilled water depends on temperature, incubation days, and species present in the water such as dissolved oxygen level, salt, and organic matter. The rate of dissolution is not constant but attains saturation in a short period of time.

US 2013/0084339 AA (FUNDACION INSTITUTO TECNOLOGICO DE MATERIALES DE ASTURIAS ITM) describes a composition that comprises an aluminium silicate with silver nanoparticles with a size of less than 50 nm distributed on the surface thereof.. The US patent application describes a process of preparing such a composition, the process comprising the steps of:

• providing an aqueous suspension of aluminium silicate with a surfactant agent, · adding AgNC to the suspension, and

• reducing the silver.

Kaolin, metakaolin, montmorillonite and mica are mentioned as examples of aluminium silicates that can be used Ngwenya et al. (Transport and viability of Escherichia coli cells in clean and iron oxide coated sand following coating with silver nanoparticles, Journal of Contaminant Hydrology, (2015) 179, 35 to 46) describes the coating of sand with silver

nanoparticles. General-purpose silica sand was sieved to collect the 120 to 350 μηη fraction and first heated in an oven at 450 °C for 4 hours to remove organic matter, followed by soaking in 20% v/v nitric acid to desorb trace metals. The sand was then rinsed in deionised water repeatedly to remove any fine sediment and raise the pH back to neutral. The sand was dried overnight at 60 °C. A portion of the cleaned sand was coated with silver nanoparticles (CS-NP) by first soaking in 1 M ammonia solution to raise the pH above 9 and hence deprotonate silanol functional groups on the surface. The deprotonated sand was soaked in 5 mM silver nitrate solution at a mass to volume ratio of 1 to 8 overnight, which led to silver ions being adsorbed by the deprotonated sand. The silver adsorbed to the sand was then reduced to

nanoparticulate silver by overnight exposure to UV light while still soaked in silver nitrate solution. Subsequently, the solution was decanted and unadsorbed

nanoparticles were removed by repeated washing in deionised water until the rinses were clear before drying the coated sand overnight at 60 °C. There is a need for a composition for water purification that achieves controlled constant release of silver in a sustained manner.

SUMMARY OF THE INVENTION

The present inventors have prepared oligodynamic silver coated sand particles that exhibit prolonged, constant anti-microbial activity and that do not suffer from

uncontrolled leaching. The oligodynamic silver coated sand particles of the present invention have diameter in the range of 50 to 10,000 μηη and are composed of etched sand particles carrying elemental silver deposited on it. The total silver content of the oligodynamic sand particles is in the range of 10 to 5,000 mg/kg and not more than 30 % by weight of the silver contained in the oligodynamic sand particles is present in the form of nanoparticles having diameter of 100 nm or more. Although the inventors do not wish to be bound by theory, it is believed that in the silver coated sand particles according to the invention the elemental silver is evenly distributed across the surface of the etched sand particles. Due to the etching, the surface area of the sand particles increases which enhances binding of silver nanoparticles onto sand.

Excessive leaching of silver from the silver coated sand particles of the present invention is minimized effectively as nanoparticles of elemental silver that can detach from the coated sand particles are virtually absent. Furthermore, the silver deposit is strongly attached to the surface of the sand particles and is only released only into water by slow solubilisation resulting from oxidation. The silver ion leach from the oligodynamic sand particles as per the invention is in the range of 50 to 200 ppb (parts per billion by weight), preferably in the range of 75 to 125 ppb.

The invention further relates to a water treatment device comprising a particulate composition that contains the oligodynamic silver coated sand particles of the present invention. The invention also relates to the use of a particulate composition containing the aforementioned oligodynamic silver coated sand particles for treating contaminated water.

Finally, the invention provides a process of preparing oligodynamic silver coated sand particles, said process comprising the steps of:

(i) providing a particulate sand substrate comprising 30 to 100 % by weight of etched sand particles having diameter in the range of 50 to 10,000 μηη; and,

(ii) depositing elemental silver onto the etched sand particles by contacting the sand substrate with a silver precipitation solution containing 10 to 5,000 mg/l dissolved silver and a reducing agent.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the present invention concerns a particulate composition comprising at least 10 % by weight oligodynamic silver coated sand particles, said oligodynamic sand particles having a diameter in the range of 50 to 10,000 μηη and being composed of etched sand particles carrying elemental silver deposit, wherein the oligodynamic sand particles have a total silver content of 10 to 5,000 mg/kg and wherein not more than 30 % by weight of the silver contained in the oligodynamic sand particles is present in the form of nanoparticles having diameter of 100 nm or more.

The term "sand" as used herein, refers to naturally occurring granular material composed of finely divided rock and mineral particles. As per this invention sand is preferably substantially composed on silica, more preferably the sand comprises higher than 90 wt% silica.

The diameter of sand particles, coated or uncoated, etched or non-etched, can suitably be determined by using a set of sieves having a different mesh sizes. The presence of silver nanoparticles can suitably be determined by means of scanning electron microscopy and energy dispersive x-ray spectroscopy (EDAX). The silver content of the silver coated sand particles can be analysed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).

The particulate composition of the present invention preferably contains at least 20 % by weight, more preferably a least 40 % by weight and most preferably at least 80 % by weight of the oligodynamic silver coated sand particles.

Besides the oligodynamic silver coated sand particles the particulate composition can contain other particulate components, such as uncoated sand particles, carbon, silica and combinations thereof.

The oligodynamic sand particles of the present invention preferably have a diameter in the range of 60 to 5,000 μηη, more preferably in the range of 70 to 1 ,000 μηη, most preferably in the range of 75 to 500 μηη. The total silver content preferably is in the range of 20 to 2,000 mg/kg, more preferably in the range of 40 to 800 mg/kg, most preferably in the range of 50 to 250 mg/kg.

The etched sand particles employed in the oligodynamic sand particles of the present invention preferably have been acid etched and/or caustic etched. Most preferably, these etched sand particles have both been acid etched and caustic etched. By acid etched is meant that the particles are etched with an acidic material i.e with a material having a pH below 7 and includes mineral acids like hydrochloric acid, sulphuric acid, nitric acid or organic acids like carboxylic acids. By caustic etched is meant that the particles are etched with a caustic material which are also known as alkalis or bases. Alkalis or bases are materials which have a pH higher than 7 and include alkali or alkaline earth metal hydroxides like sodium or potassium hydroxides.

The oligodynamic sand particles typically have an average surface roughness (R _a ) of 10 to 1000 μηη. More preferably, the average surface roughness is in the range of 20 to 400 μηη, most preferably in the range of 30 to 100 μηη. The average surface roughness of the sand particles can be determined by confocal microscopy. The oligodynamic particles of the present invention preferably do not contain substantial quantities of silver nanoparticles that may detach from the oligodynamic particles when used to treat water, and that may end up in the purified water.

Accordingly, in a particularly preferred embodiment, not more than 10 % by weight, more preferably not more than 5 % by weight of the silver contained in the

oligodynamic sand particles is present in the form of nanoparticles having diameter of 100 nm or more.

In accordance with another preferred embodiment not more than 30 % by weight, more preferably not more than 20 % by weight and most preferably not more than 10 % by weight of the silver contained in the oligodynamic sand particles is present in the form of nanoparticles having diameter of 50 nm or more.

An important characterizing feature of the invention is the silver ion leach from the oligodynamic sand particles. Silver leaching is determined in the treated water. For preparing treated water, 5 g of the desired sand particle are treated with 25 ml water for 20 minutes. The silver content of the treated water is determined using ICP analysis. Standard calibration plot was made by measuring the silver ion concentration using silver standard solution (1000 ppm from Sigma). 20 ml of treated water solution was taken and acidified by adding 0.5 ml of Suprapur nitric acid for preparing the ICP sample. The samples were filtered by passing through a 0.45 μηη syringe filter in order to remove any particulate impurities from the solution after the acidification process. Based on the calibration plot, the concentrations of the treated water samples were subsequently measured.

Another aspect of the invention relates to a water treatment device comprising the particulate composition according to the present invention. More preferably, the water treatment device contains a filter bed comprising the particulate composition of the present invention. Typically, such a filter bed contains at least 5 % by weight, more preferably at least 20 % by weight , most preferably at least 50 % by weight of the particulate composition of the present invention. The water treatment device of the present invention preferably comprises a housing; water purification means comprising a filter unit accommodated in the housing, containing the particulate composition of the present invention; a piping comprising a water inlet for contaminated water, which inlet is in fluid communication with the water purification means and is connectable to a water supply system, and comprising a water outlet for dispensing purified water, which outlet is in fluid communication with the water purification means, wherein contaminated water entering the device through the water inlet for contaminated water is forced through the filter element towards the outlet for dispensing purified water.

A further aspect of the present invention relates to the use of a particulate composition according to the present invention for disinfecting contaminated water. Preferably, this use comprises the application of a filter bed comprising the particulate composition of the present invention.

Yet another aspect of the present invention relates to a process of preparing oligodynamic silver coated sand particles, said process comprising the steps of:

(i) providing a particulate sand substrate comprising 30 to 100 % by weight of etched sand particles having a diameter in the range of 50 to 10,000 μηη; and,

(ii) depositing elemental silver onto the etched sand particles by contacting the sand substrate with a silver precipitation solution containing 10 to 5,000 mg/l dissolved silver and a reducing agent.

The particulate sand substrate employed in the present process preferably comprises at least 50 % by weight, more preferably at least 80 % by weight and most preferably at least 95 % by weight of etched sand particles having diameter in the range of 50 to 10,000 m.

According to a particularly preferred embodiment, the silver precipitation solution is a solution of silver nitrate and/or silver acetate. The silver precipitation solution that is used in the present process preferably contains 0.001 to 1 M, more preferably 0.01 to 0.5 M and most preferably 0.05 to 0.2 M of the reducing agent. The reducing agent employed in the present process causes silver ions in the silver precipitation solution to be reduced to a zero valent state. Preferably, the reducing agent is selected from citrate, borohydride, hydrazine and combinations thereof. Here the term "citrate" encompasses citric acid as well as salts of citric acid. Most preferably, the reducing agent is citrate.

In the present process, the sand substrate is preferably contacted with the silver precipitation solution for at least 10 minutes, preferably for at least 20 minutes at a temperature of at least 50 °C, more preferably of at least 60 °C In a preferred embodiment of the present process, the sand substrate is first combined with an aqueous solution of reducing agent, followed by the addition of an aqueous silver solution.

Typically, in the present process the silver precipitation solution is combined with the sand substrate to produce a slurry containing 0.01 to 100 mg silver per gram of sand substrate. More preferably the slurry produced contains 0.1 to 50 mg silver per gram of sand substrate, most preferably 0.5 to 25 mg silver per gram of sand substrate.

The slurry that is produced by combining the silver precipitation solution with the acid etched sand typically contains 0.04 to 400 mg reducing agent per gram of sand substrate. More preferably, said slurry contains 0.4 to 200 mg reducing agent per gram of sand substrate, most preferably 2 to 100 mg reducing agent per gram of sand substrate. The etched sand particles employed in the present process preferably are acid etched or caustic etched sand particles. Most preferably, the etched sand particles employed have been both acid etched and caustic etched. The process of the present invention preferably yields oligodynamic sand particles as defined herein before.

The invention is further illustrated by the following non-limiting examples.

EXAMPLES Example 1 Etching of sand particles

200 g of river sand particles (at least 90 % by weight less than 500 μηη) were treated with 50 ml of concentrated HCI (10 N) and aged for one day. Next, the acid treated sand particles were washed with water and subsequently treated with concentrated NaOH solution (20 M) for one day. After this the caustic treated sand particles were washed and again treated with concentrated HCI (10 N) for one day. Finally, the sand was thoroughly washed with water and dried in oven at 90°C. The roughness of the etched sand particles was 47 ± 7 μηη. Coating of etched sand particles

50 g of the acid and caustic etched sand particles were dispersed in 50 ml water. The suspension was heated in a water bath maintained at 80 °C. Subsequently, 20 ml of freshly prepared 10 % sodium citrate dihydrate was added. After 10 minutes, 5 ml of freshly prepared 10 % silver nitrate solution was added and the reduction reaction was continued for 15 minutes. The suspension was filtered and the retentate was dried at 60 °C overnight. The colour of the sand particle had changed to brown. The dried silver coated sand was thoroughly washed with at least 2 litres of water, followed by drying at 90 °C for 2 hours. The silver content of the sand was measured to be 40 ppm.

Figure 1 shows a representative electron microscopic image of a silver coated particle that was thus obtained. Spherical silver nanoparticles having a diameter in the range of 5-50 nm can be seen on the surface. The presence of silver is confirmed through Energy-dispersive X-ray spectroscopic analysis carried out using a FESEM (ZEISS) operated at 10 kV electron voltage. The analyzed weight% of different elements on the surface is shown in Table 1 :

Table 1

Example 2

The antimicrobial activity of silver coated sand particles prepared as in Example 1 was tested using gram positive bacteria Staphylococcus aureus and gram negative bacteria Escherichia coli bacteria by the Time kill assay.

5 g of sand (control and silver coated) was weighed in a sterile container. 25 ml_ bacterial suspension of Staphylococcus aureus (10 ⁸ cells/mL) and E. coli (10 ⁸ cells/mL), respectively, was added to the tubes containing sand. The sample container was kept shaking in a shaker incubator at 30 °C for 12 hours. During this period samples were taken after 20 minutes, 1 hour and 2 hours. The samples were diluted with Dey-Engley Neutralizing Broth to inactivate the bacteria. The neutralized samples were then plated after serial dilution in Trypticase Soy Agar (nutrient medium) to enumerate the residual bacteria. The results of the tests are summarized in Table 2.

Table 2 Test Culture 1 .E+07 -

20 Minutes 9.E+06 ~0

Control 1 Hour 7.E+06 -0.5

2 Hours 5.E+06 0.5

20 Minutes 0 ≥ 7

Silver coated sand 1 Hour 0 ≥ 7

2 Hours 0 ≥ 7

Note: Log reduction = ¹⁰ log (input concentration/output concentration)

Example 3

Example 1 was repeated, but without the etching pre-treatment. Next, the antimicrobial activity of the treated sand was measured using the methodology described in Example 2. It was found that a reduction of not even 1.5 log was achieved, even after 24 hours. Example 4

Example 1 was repeated, except that the sand was not subjected to caustic treatment. Next, the antimicrobial activity of the treated sand was measured using the

methodology described in Example 2. The results are shown in Table 3.

Table 3

Assay Details Log(cfu/ml) log reduction

Test Culture 5.64 -

20 min 5.57 ~0

Control

1 Hour 5.59 ~0 2 Hours 5.59 ~0

20 Minutes 1.65 ~4

Treated Sand Sample 1 Hour <1 >4.6

2 Hours <1 >4.6

Examples 5 and 6: Comparison of silver ion leach of Example 1 as compared to silver ion leach of untreated sand particle (sand particle not subiected to etching)

Example 5: The release rate of silver from the silver coated sand of Example 1 was determined.

Example 6: 100 g of river sand (at least 90 wt% < 500 μηη) was washed with water. This was followed by addition of 300 mg of tannic acid (dissolved in 60 ml water) and 6 ml of 1 % KOH. The above suspension was heated at 75 °C and dried well. This was followed by washing with fresh Dl water three times. For silver deposition on sand, 1 g of silver nitrate (dissolved in 20 ml water) was added. The color changed immediately to brownish. The sample was dried well and washed thrice with Dl water to remove any unadsorbed silver from sand.

The leach rate of silver coated etched sand particles (Example 5) and that of silver coated unetched sand particle (Example 6) were determinged after dispersing 5 gram of the sand in 25 ml water and measuring the silver concentration in the water after 20 minutes. The data is summarised in Table - 4 below:

Table - 4

Silver ion leach (ppb by weight)

Control Not detected

Example 5 108

Example 6 47 The data in Table - 4 above indicates that samples of oligodynamic sand particles prepared using etched sand particles (Example 5) gave vastly superior silver ion leach rate ( within the desired range of 50 to 200 ppb) whereas that obtained from unetched sand particles are outside the desired range.