WO2010003267A1

WO2010003267A1 - Water treatment system with adsorbent material based on mineral grains for removal of arsenic and methods of production, recycling and use

Info

Publication number: WO2010003267A1
Application number: PCT/CH2009/000243
Authority: WO
Inventors: Con Hong Tran
Original assignee: Bibus Ag; Hcth Technology Inc.
Priority date: 2008-07-10
Filing date: 2009-07-08
Publication date: 2010-01-14

Abstract

The invention relates to the field of water treatment and more specifically to systems for water purification to remove arsenic and possibly other heavy metals, microbes and other pollutants from drinking water and it relates to adsorbent materials based on surface activated mineral grains, methods of producing and recycling the adsorbent materials and the use of the water treatment systems. The water treatment system according to the invention comprises adsorbent material, of which at least a fraction is based on mineral grains and comprises manganese dioxide in a surface layer. The method of producing the adsorbent material for water purification according to the invention is based on calcinated mineral grains which are coated to comprise manganese dioxide on the grain surfaces.

Description

WATER TREATMENT SYSTEM WITH ADSORBENT

MATERIAL BASED ON MINERAL GRAINS FOR REMOVAL

OF ARSENIC AND METHODS OF PRODUCTION,

RECYCLING AND USE

FIELD OF THE INVENTION

The invention relates to the field of water treatment and more specifically to systems for water purification to remove arsenic and possibly other heavy metals, microbes and other pollutants from drinking water and it relates to adsorbent materials based on surface activated mineral grains, methods of producing and recycling them and to the use of the system.

BACKGROUND OF THE INVENTION

Arsenic contamination in drinking water has been recognized as a severe health hazard in many countries across the globe including the United States, Argentina, Hungary, India, Bangladesh, China and Vietnam. Particularly in rural areas where people depend on private wells such as in many parts of Vietnam there is an urgent demand for efficient arsenic removal from drinking water for application at a small scale such as household level, where arsenic and often other pollutants in the drinking water including microbes are a health risk for the population. It is known from experience in Bangladesh that natural minerals such as clay or laterite have a natural adsorbing capacity for arsenic. In order to improve the physical properties and remove naturally bound arsenic the exploited mineral is generally calcinated and ground. Furthermore it is known from prior art e.g. as described in the Vietnamese Patent Application number 1-2006-01842 that the capacity of laterite grains to adsorbe arsenic is lower in calcinated than in uncalcinated laterite grains and can be increased by treatment with acid to restore a hydroxide layer on the grains' surface. However, currently known materials based on calcinated mineral grains such as clay or laterite grains have a relatively low adsorbing capacity, thus a large amount of adsorbent material is necessary to remove arsenic and other pollutants to meet the standards required for drinking water. The permitted level for arsenic is very low; it is required to be less than 50 μg/L in Vietnam and the World Health Organization recommends less than 10 μg/L. Furthermore, disposal of fully saturated adsorbent material is an environmental problem. Thus, the currently available adsorbent materials are not adequate for decentralized household applications.

Additionally, there are teachings which are not yet used for water treatment based on mineral grains as described in US application 2007/0033603 for the antimicrobial effect of silver and as described in US application 2007/0086935 using nano structured material for binding arsenic.

DESCRIPTION OF THE INVENTION

It is therefore an object of the invention to provide systems for water purification and to produce adsorbent materials for it based on mineral grains, with improved adsorption capacities for arsenic and which are recyclable to allow for save deposit of adsorbed arsenic. This object is achieved by systems for water treatment and methods of production and recycling of adsorbent material as defined in the claims. The water treatment system according to the invention comprises adsorbent material, of which at least a fraction is based on mineral grains and comprises manganese dioxide in a surface layer. Preferred methods of producing the adsorbent material for water purification according to embodiments of the invention are based on calcinated mineral grains which are treated to comprise manganese dioxide on the grain surfaces.

The mineral grains can be produced from a variety of minerals, and the described embodiments described with laterite are not limiting the invention to the use of laterite but the invention extends also to the use of other minerals such as limonite or clay etc. It is generally advantageous to use a mineral which at least after calcination has good physical stability in water and is highly porous and exhibiting a large surface area per volume and weight of adsorbing material comprising inner and outer adsorbing surfaces.

Arsenic is present in water in a variety of forms mainly in the form of arsenate and arsenite. The term arsenic used within the context of this specification means arsenic in at least one chemical form depending on the chemical context.

A first aspect of the invention concerns water treatment systems including highly efficient adsorbent material for arsenic based on mineral grains comprising manganese dioxide or both iron(III) oxyhydroxide and manganese dioxide on the surface. This results in a surprisingly high adsorption capacity for arsenic, compared to the adsorption capacity of calcinated mineral grains treated to comprise iron oxyhydroxide not in combination with manganese dioxide on the surface as known from prior art. In preferred embodiments of the first aspect of the invention, the water treatment systems comprises several fractions of adsorbent material for adsorption of multiple pollutants.

In one embodiment of a water treatment system according to the invention a water treatment system comprises calcinated laterite grains with iron oxyhydroxide and manganese dioxide on the surface for removal of arsenic.

In one variant the water treatment system additionally comprises a fraction of calcinated mineral grains in the filtration apparatus for removal of heavy metals in cationic form. This fraction of mineral grains is treated with acid (then alkaline to neutralize) creating a hydroxide layer on the surface comprising iron and aluminum hydroxides. It adsorbs heavy metals in cationic form such as lead, copper, nickel and mercury. According to preferred embodiments of the water treatment system such a filtration apparatus comprises fractions of laterite grains which are treated differently in separate compartments separated from each other by a permeable barrier. In further variants of the water treatment system one or more additional fractions of adsorbent material for other pollutants e.g. for removal of bacteria based on mineral grains coated with nano-silver particles are included.

It is certainly also within the spirit of the invention to use mineral grains derived from minerals other than laterite treated such to comprise iron(III) oxyhydroxide and manganese dioxide on the surface for filtration systems or to combine mineral grains sourced from different minerals in a single water treatment system or furthermore to include grains which are not derived from minerals such as silica gel granules for removal of other pollutants than arsenic.

However, water treatment systems for removal of multiple pollutants are particularly efficiently produced when the adsorbent material is produced from grains of one mineral with different fractions along the production process yielding grains with different adsorptive properties. This permits not only efficient production processes but also less complexity for the production facilities. A second aspect of the invention concerns methods of production of differently treated fractions of adsorbent material for various embodiments of the water treatment system according to the invention.

Embodiments of the second aspect of the invention are described which concern the production of the highly efficient arsenic adsorption material based on mineral grains which are treated to comprise manganese dioxide or a combination of iron oxyhydroxide and manganese dioxide on the surface. Further embodiments concern methods of production of adsorption material for removal of other pollutants based on calcinated mineral grains, which in special cases can be substituted by non- mineral grains such as silica granules.

In one embodiment of the method of producing the adsorbent material comprising an iron(III) hydroxide on the surface, the exploited mineral naturally has a high iron content, such as laterite. The iron oxyhydroxide on the surface of the mineral grains is produced by hydrolyzing iron oxide present in the calcinated mineral by treatment with acid. For higher adsorption capacity for arsenic manganese dioxide may subsequently be introduced additionally to the surface of the acid treated grains. It is important that the surface layer is thin.

In further embodiments of the method of producing adsorbent material comprising iron oxyhydroxide and / or manganese dioxide on the surface, they are both introduced to the mineral grains' surface from an external source. In a preferred embodiment dried calcinated laterite grains are contacted with colloidal solutions comprising nano particles of iron(III) oxyhydroxide and / or manganese dioxide.

According to yet a further embodiment of methods of production of different fractions of adsorbent material based on mineral grains, laterite grains are coated with silver nano particles for removal of bacteria. Described methods include treating mineral grains such as calcinated laterite grains or silica gel granules with a solution comprising nano silver particles, or in other variants soaking the grains in a solution containing silver ions and then the nano silver particles are formed on the grains surface by reductive reaction. It is well known that silver is a generally safe and effective anti-microbial metal. Thus, when writing removal of bacteria, the removal of other microbes is included in the context of this invention.

In a third aspect of the invention the recycling of adsorbent materials which is saturated with arsenic is disclosed. Besides recycling of the saturated laterite grains the arsenic can be precipitated from the washing solution providing for reutilization of the basic washing solution and for a safe method of disposal of calcium arsenate in concentrated form or reuse of calcium arsenate for other purposes.

A fourth aspect of the invention includes the use of water treatment systems with adsorbent material based on mineral grains at least a fraction of which contains manganese dioxide or manganese dioxide and iron oxyhydroxide on the surface for removal of arsenic in a centralized water treatment system e.g. in a commercial or communal setting for the removal of arsenic and other pollutants or for industrial or communal drinking water production.

BRIEF DESCRIPTION OF THE FIGURES

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:

Figure 1 shows an SEM picture of the surface of calcinated laterite grains.

Figure 2 shows an SEM picture of calcinated laterite surface after acid treatment according to example 1. Figure 3 shows a time equilibrium curve of arsenic adsorption in example 1.

Figure 4 shows the Langmuir isothermal curve of example 1.

Figure 5 shows the Langmuir isothermal curve of example 3.

Figure 6 shows the break-through curve for example 3.

Figure 7 shows an EM of the laterite surface after calcination of example 1.

Figure 8 shows an EM of the laterite surface after acid treatment and coating with MnO₂ according of example 3.

Figure 9 shows a TEM image OfMnO₂ nano particles.

Figure 10 shows a SEM image of calcinated laterite surface prior to coating with MnO₂ nano particles.

Figure 11 shows a SEM image of calcinated laterite surface after coating with MnO₂ nano particles.

Figure 12 Equilibrium time curve for the adsorption of arsenic by laterite grains coated with MnO2 nano particles.

Figure 13 The Langmuir adsorption isothermal curve for arsenic adsorption by laterite grains coated with MnO₂ nano particles,

Figure 14 shows a TEM image of FeOOH nano particles.

Figure 15 Influence of the ethanol concentration on the formation of FeOOH nano particles.

Figure 16 shows the surface of calcinated laterite grains before (a) and after (b) coating with nano FeOOH.

Figure 17 Equilibrium time curve for the adsorption of arsenic by laterite grains coated with FeOOH nano particles.

Figure 18 The Langmuir adsorption isothermal curve for arsenic adsorption by laterite grains coated with FeOOH nano particles. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Following is a detailed description of an exemplary embodiment of the first aspect of the invention, a water treatment system for the removal of arsenic and other pollutants from water:

The water treatment system comprises a combination of at least two fractions of adsorbent materials based on laterite in a column of a filtration apparatus with several compartments comprising at least one compartment for the adsorption of arsenic and other pollutants present in anionic form and another compartment for the adsorption of other heavy metals in cationic form. The compartments are stacked in a column within the filtration apparatus and are separated from each other by a grid or by a porous membrane e.g. a cotton cloth. A receiving compartment can be at the top or at the bottom of the column and is connected with the water inlet. Downstream of the receiving compartment is a compartment containing laterite grains of a larger size of approximately 3-4 mm treated with acid (and then alkaline to neutralize) to exhibit a hydroxide layer including iron hydroxide and aluminium hydroxide on the laterite grain surface. In this compartment cationic adsorption removes heavy metal ions. A subsequent compartment further downstream contains laterite grains of a smaller size e.g. of about 0.5 - 3 mm grain size treated to exhibit a surface layer comprising a combination of iron oxhydroxide and manganese dioxide efficiently removing arsenic. A discharge compartment comprises the water outlet. The arrangement of compartments is not limited to a vertical arrangement and in other embodiments the water flow can be in reverse direction.

In another favored embodiment of a column of a filtration apparatus the laterite grain size in the compartment for adsorption of cationic heavy metal ions is approximately 1-3 mm and the compartment for the adsorption of arsenic and other anionic pollutants contains laterite grains of about 0.5-1 mm grain size. Self-evidently, in other embodiments of the first aspect of the invention additional compartments are included, or different fractions of adsorption material are not separated in different compartments but may be layered or mixed or there are no separate compartments for water inlet and outlet or there is only one fraction of adsorption material for the removal of arsenic with mineral grains comprising either manganese dioxide or a combination of iron oxyhydroxide and manganese dioxide in the surface layer.

In preferred embodiments with additional fractions of adsorbent materials there is also a fraction of adsorbent material for removal of microbes included with mineral grains or silica gel granules comprising nano silver on the surface.

In further embodiments the inlet compartment comprises material which may be based on ceramic material or sand for the filtration of the incoming water for removal of turbidities.

Such filtering columns can eliminate arsenic from drinking water at least to below 10 μg/L, which is the level that should not be exceeded in drinking water according to WHO.

Following are detailed descriptions of variants of the second aspect of the invention i.e. methods of production for adsorbent material. The described methods mainly concern implementations based on the use of laterite, however the invention is not limited to the use of laterite. Equivalent methods of production can be applied to clay, limonite and other minerals.

A first preferred variant of a production method yields both a fraction of adsorbent material for arsenic and a fraction of adsorbent material for heavy metals in cationic form and comprises the following 2 or 3-steps, respectively: In step 1 , calcinated laterite grains are produced: For some implementations relying on the natural iron content of laterite, it ideally has an iron content of 25-35%, with the rest being clay and other components. Blocks of naturally dried laterite are calcinated at 900° to 1000°C for several hours, preferably at 950° +/- 10 °C for 4 hours in order to remove naturally adsorbed arsenic and to enhance the physical stability of the laterite by turning it into a ceramic form which is stable in water. The calcinated laterite is ground to grains of a particle size between 0.5 mm and 5 mm or up to 50 mm or up to 200 mm. According to some implementations fractions of variable grain size are separated e.g. a smaller sized fraction with a grain size between 0.5 mm and 4 mm or preferably between 1 mm and 3 mm and a larger sized fraction between 2 mm or 3 mm and 4 or 5 or 6 mm, preferably between 3 mm and 4 mm. In an alternative embodiment of the first variant of production method the grains are separated into a fraction of smaller sized grains between 0.5mm and lmm and a fraction of larger sized grains with a lower limit of 1 mm and an upper limit of 3 or 4 mm. Next the grains are washed to remove dust, and dried in preparation for the subsequent treatment. Calcination of laterite converts iron and aluminum ions originally present in laterite into uncharged oxides which diminishes the Arsenic adsorption capacity. The calcinated laterite grains are porous mineral grains comprising inner and outer adsorbing surfaces.

In step 2, the dried calcinated laterite grains are treated with acid to restore the arsenic adsorption capacity of the laterite surface. According to a preferred acid treatment procedure the concentration of iron (III) ions in the acidic solution is controlled to optimize the adsorption capacity of the laterite grains. Accordingly, the grains are soaked with acid e.g. with 1.0 M HCl for a controlled time (between 10 and 30 minutes) to obtain a predetermined Fe³⁺ concentration e.g. between 10^"3 and 10^'2 M e.g. by photometric measurement with sulfosalisilic acid as color indicator. After draining off the HCl, the laterite grains are neutralized with base, e.g. with 0.5 M NaOH to obtain a pH of 6 to 7 or 6 to 9 The laterite grains are washed with deionized water to remove all chloride ions and dried to reduce the water content to less than 10 %, preferably to less than 5% by mass. This creates a thin layer of iron oxyhydroxide and aluminum hydroxide on the outer surface and the inner surface of the pores of the laterite grains. Laterite grains produced by steps 1 and 2 is applicable as adsorbent material for removal of heavy metals in cationic form.

In step 3, manganese dioxide is additionally introduced into the hydroxide surface layer of the porous laterite grains. There are various methods to introduce manganese dioxide, hi a preferred method the grains treated with step 1 and 2 are soaked in a potassium permanganate solution with a concentration of preferably 0.1 M for 30 minutes with a 1 :1 volume ratio grains to liquid. The concentration of the potassium permanganate solution may be varied e.g. within 0.01 M and 0.1 M with the volume ratio and the incubation time adjusted accordingly. The potassium permanganate diffuses into the iron oxyhydroxide - aluminum hydroxide layer created in step 2. After draining and drying to reduce the water content to less than 10 %, preferably to less than 5% by mass, rinsing the grains with sodium sulfite solution of a concentration preferably in the range of 0.2 M to 2 M until the rinsing solution is colorless and Mn(VII) is reduced to Mn(IV). Then the grains are washed with deionized water until sulfate ions are undetectable by a barium chloride test solution and again dried to a water content of less than 10 %, preferably to less than 5% by mass. Laterite grains produced by steps 1, step 2 and step 3 have manganese dioxide intercalated with the iron oxyhydroxide — aluminum hydroxide layer covering the outer surface and the inner surface of the laterite grains. This is the adsorptive surface responsible for highly efficient arsenic removal from water. The adsorption capacity of laterite grains treated according to steps 1 to 3 is approx. 75-140 mg, often 120 -140 mg arsenic per g adsorption material. Alternative methods of step 3, which is the additional introduction of manganese dioxide into the hydroxide surface layer, include treatment of laterite grains obtained in step 2 with manganese(II) sulfate and oxygen from air in neutral or slightly alkaline media or with manganese sulfate and hydrogen peroxide. Thus, in this first preferred variant of a production method, the yield of adsorbing material after step 1 and step 2 is applicable as adsorbing material for heavy metals in cationic form and it also exhibits some arsenic adsorbing capacity. The additional treatment of step 3 produces material with much enhanced arsenic adsorbing capacity which however has lost its adsorbing capacity for cationic heavy metal ions. For water treatment systems according to embodiments of the first aspect of the invention both adsorbent material fractions treated with step 1 and 2 only and adsorbent material fractions produced with steps 1 , 2 and 3 are applicable.

In a second preferred variant of a production method step 2 and step 3 of the first variant are replaced with a different treatment. This treatment is an alternative method to create a manganese dioxide/iron hydroxide surface layer on mineral grains. It yields adsorbent material which is highly adsorptive for arsenic(III) and arsenic(V) anions. In a preferred embodiment calcinated laterite grains are produced according to step 1 of the first variant described above but alternatively other mineral grains can be used. Subsequently the mineral grains, such as calcinated laterite grains are soaked in 1-3 M HCl together with manganese(II) sulfate for a suitable time. Then the wet mass is dropped into a solution containing an equivalent amount of base to neutralize and an equivalent amount of permanganate salt to oxidize the Mn(II) sulfate to MnO₂. Then sulfate ions are removed by washing and the grains are dried yielding the adsorbent material.

In yet a third variant of production methods of arsenic adsorbent material, the porous surface of mineral grains is coated with nano dimensional MnO₂ and FeOOH. In one embodiment calcinated laterite grains are produced according to step 1 of the first variant described above prior to the coating. Subsequently, these calcinated laterite grains are contacted with colloidal solutions containing iron oxyhydroxide and / or manganese dioxide nano particles. Production methods of nano dimensional MnO₂ and FeOOH in ethanol - water media and the fact that the concentration of ethanol in the solution plays an important role for the formation of nano-particles and anticoagulation are known from the state of the art. The size of the MnO₂ particles obtained is approximately 10 run to 80 nm as determined by Transmission Electron Microscopy method; the size ofthe FeOOH particles is about 10 nm and 30 nm.

These production methods of adsorbent material according to the third variant of the second aspect of the invention create high performance adsorption material for arsenic removal from groundwater for drinking water production by coating calcinated mineral grains with MnO₂ and FeOOH nano particles present in colloidal solution. . The surface of the grains is changed as revealed by Scanning Electron Microscopy images. The arsenic adsorption capacity of these adsorbent materials produced according to the third variant is even higher than the arsenic adsorption capacity of the material produced according to the first variant or the second variant described above.

The maximum adsorption capacity of adsorbent material coated with manganese dioxide nano particles is 195 mg arsenic per gram adsorbent material and for adsorbent material coated with iron oxyhydroxide nano particles it is 135 mg per gram. There are also good results with the combination of the two coatings to even increase the adsorption capacity further.

For the coating of mineral grains with nano dimensional manganese dioxide and iron oxyhydroxide in a first step colloidal solutions of nano particles of metals' oxide or hydroxide were prepared. Water or organic solvents in water media were used for creation of nano dimensional MnO₂ and FeOOH from their inorganic salts. The pH of the solution, the concentration of the salts and the reaction temperature all influence the quality of the product as shown in the detailed description of the examples. In a second step these colloidal solutions comprising manganese dioxide and/or iron oxyhydroxide nano particles are used for coating calcinated mineral grains such as laterite grains. Further embodiments of the second aspect of the invention include methods of production of mineral grains or other granules with nano silver particles on the surface for removal of microbes.

In one variant mineral grains such as calcinated laterite grains or silica gel granules are soaked in a solution comprising nano particles of metallic silver particles. The solution is prepared e.g. by reductive reaction of silver ions in water or water-organic solvents with formaldehyde. Thus, the silver nano particles attach to the granules surface. In another variant mineral grains such as laterite granules are soaked in a solution comprising silver ions and ammonia. Silver ions which are adsorbed to the grains surface are subsequently reduced to silver metal e.g. by adding formaldehyde. A third variant involves soaking silica gel granules in a solution comprising silver ions, then hydrolysing silver ions to form silver oxide, which by heating e.g. at 700 °C for 30 min forms silver metal.

It is within the spirit of the invention to provide water treatment systems including highly efficient adsorbent material for arsenic based on mineral grains comprising both iron oxyhydroxide and manganese dioxide or nano dimensional manganese dioxide on the surface and additionally fractions of other adsorbent materials based on mineral or other granules to remove other pollutants in addition to arsenic.

Following is a detailed description of the third aspect of the invention i.e. methods of recycling adsorbent material, which is saturated with arsenic:

Adsorption material for water treatment based on mineral grains such as laterite which is fully saturated with arsenic is recyclable by washing such adsorbent material with a basic washing solution in order to release the adsorbed arsenate from the laterite grains into the basic washing solution. The base used is generally NaOH or KOH, e.g. 0.2 M NaOH. Subsequently the basic washing solution may be treated for precipitating the released arsenate in form of an insoluble arsenate salt. As precipitant hydroxides of calcium, magnesium of barium can be used, preferably calcium hydroxide is used and arsenic is collected as Ca₃(As0₄)₂ precipitate, ready for safe disposal.

It is within the scope of the invention to combine features differently from the disclosure in this specification.

E X A M P L E S Example 1 :

Production of calcinated laterite grains and treatment of these grains with acid (and base to neutralize) yielding an adsorbent material comprising iron oxyhydroxide and aluminum hydroxide on the surface.

This adsorbant material adsorbs heavy metal ions in cationic form also serves as starting material for example 3 and it furthermore, this material exhibits arsenic binding at a relatively low level. 68,5 kg raw natural laterite after 30 days of drying at ambient temperature yielded 50 kg dried laterite. Dried laterite was then calcinated at 950° ± 10⁰C for 4 hours and cooled to ambient temperature for 24 h. This calcinated laterite was ground and sieved to collect the 1 - 3 mm fraction which was washed by deionized water to remove dust and then dried. Dried laterite grains were soaked in 1 M hydrochloric acid solution for 30 min with the iron(III) concentration controlled to reach a concentration of 0.001 - 0.01 M.After discharging the acid the grains were neutralized by 0.5 M sodium hydroxide solution. These laterite grains were washed with deionized water until chloride ions were undetectable by a silver nitrate test. The resulting yield of dried laterite grains coated by a hydroxide layer was 19.7 kg. The arsenic adsorption capacity was 6.5 g arsenic per 1 kg material which is the level of the state of the art. Depending on the use of this material either as starting material for treatment according to example 3 or as adsorbent material for heavy metals in cationic form the selection of the size of the calcinated grains can be adjusted.

Example 2:

Production of calcinated laterite grains and treatment of these grains with potassium permanganate and a reductant to introduce manganese dioxide to the surface of the calcinated laterite grains.

Of 50 kg raw laterite remained 41.4 kg after calcination at 950⁰C ± 10⁰C for 4 hours and cooling to room temperature for 24 hours as in example 1. The mentioned quantity of calcinated laterite was ground and sieved, the 1 - 3 mm fraction was collected, washed and dried as in example 1. Next dried laterite grains were soaked in 0.1 N potassium permanganate solution for 30 min, removed and dried again. These laterite grains were subsequently soaked in 0.2 N sodium sulfite solution for 20 min with softly mixing, removed and dried at 90°C to obtain a water content of less than 5% by mass. Finally, the grains were washed with deionized water until SO₄ ^2" ions were undetectable by a barium chloride test and dried again. The yield was about 41.5 kg. The arsenic adsorption capacity was 75.5 g to 80.8 g arsenic per 1 kg adsorbent material. Approximately the same yield and adsorption quality can be achieved if the drying temperature in the above procedure is lowered to a temperature between 50°-90°, e.g. to 60°.

Example 3 a:

Treatment of acid treated calcinated laterite grains prepared according to example 1 with potassium permanganate and a reductant to introduce in addition to the iron oxyhydroxide manganese dioxide to the surface of the calcinated laterite grains:

25 kg of material from example 1 was taken for example 3. The mass of the material was soaked 0.1 N potassium permanganate solution for 30 min, removed and then dried until the water content of the grains is less than 5% by mass. This laterite material was soaked in a 0.2 N sodium sulfite solution for 20 min with softly mixing, taken out and then dried at 90⁰C to obtain a water content of less than 5% by mass. Finally the grains were washed by deionized water until SO₄ ^2' ions are undetectable by a barium chloride test and dried again. The yield was about 24.5 kg. The arsenic adsorption capacity was 120.5g to 138.5 g arsenic per 1 kg adsorbent material. Approximately the same yield and adsorption quality can be achieved if the drying temperature in the above procedure is lowered to a temperature between 50°-90°, e.g. to 60°.

Example 3b:

Treatment of acid treated calcinated laterite grains prepared according to example 1 first with a solution containing iron cations and manganese(II) cations by soaking laterite grains in acid containing manganese(II) sulfate and then this wet mass is added to a basic solution containing permanganate which results in concurrent formation of an iron hydroxide and manganese dioxide comprising surface layer on the laterite grains:

2 kg grains fraction with diameter 0.5 — 1.0 mm is soaked in 2 M HCl solution containing 6 - 12 g of MnSO4.H2O for 10 - 24 hours in room temperature. The solid and liquid ratio is 2kg per IL. The wet mass is then dropped into IL of a solution of

3 - 5 M NaOH and 0.2 - 0.4 M KMnO4. and let soaking for 10 - 30 hours. The excess liquid is several times drained off and filled back during the soaking time. Then, the material is washed with deionized water to remove sulfate anions and dried. This material prepared in optimal conditions has high efficiency of arsenic removal with an arsenic adsorption capacity of 75 - 120 mg arsenic per kg adsorbent material. Methods of producing silver coated calcinated laterite grains or silica gel granules by soaking them in a nano silver solution to yield Bacterial Removal Material (= BRM).

Example 4a:

First the solution comprising silver nano particles was prepared: To 100 ml of 0.1 — 0.01 M silver nitrate solution, 5 ml of concentrated (30-35%) ammonium hydroxide solution, 5 ml of 99% ethanol and 3 ml of 5% polyvinyl alcohol in water solution were added and stirred for 30 min at room temperature. Then 5 ml of concentrated formaldehyde concentrated solution were added and stirred continuously until the solution changes to yellow color (about 60 min). The yellow solution is the nano silver solution i.e. comprising nano-silver particles and it is used to coat mineral grains or other granules: e.g. 100 g of dried calcinated laterite grains or 200 g of silica gel granules were dropped into 50 ml of the nano-silver solution. After soaking them for 30 min they were dried 90⁰C until all liquid had evaporated. After cooling to room temperature the grains were washed with deionized water until nitrate ions were undetectable. Finally the washed grains coated with nano silver particles were dried ready to use as BRM.

Example 4b

First the solution comprising silver nano particles was prepared: To 100 ml of 0.001 — 0.01 M silver nitrate solution, 5 ml of concentrated (30-35%) ammonium hydroxide solution, 5 ml of 99% ethanol and 3 ml of 5% polyvinyl alcohol in water solution were added and stirred for 30 min at room temperature. Then 100 ml of a 1 M glucose solution were added and stirred continuously until the solution changes to dark yellow color (about 60 min). The dark yellow solution is the nano silver solution i.e. comprising nano-silver particles and it is used to coat mineral grains or other granules e.g. according to the procedure described in Example 4a. Example 5:

Method of preparing silver coated BRM from laterite

According to this method the calcinated laterite grains are soaked in a solution comprising silver ions and ammonia which are adsorbed to the grains surface followed by a reduction step to silver metal by adding formaldehyde.

First 100 ml of a silver nitrate solution with a concentration in the range of 0.01- 0.1 M is mixed with 100 ml 1.0 M ammonia. The solution was shaken and allowed to cool. The solution was then poured onto 500 g of dried laterite grains which were soaked for 30 min. to adsorb silver ions to the surface. 100 ml of a 1 M formaldehyde in deionized water solution was added to the mixture, softly mixed and let sit at room temperature for 90 min. The mixture was then completely dried at 90⁰C. The dried material was washed with deionized water until ammonium ions were undetectable. Finally the washed grains coated with nano silver particles were dried again and ready to use as BRM.

Example 6:

Method of preparing silver coated BRM from silica gel granules:

500 g of silica gel granules were exposed to open air for 90 min. and then 100 ml of a silver nitrate solution with a concentration in the range of 0.01 - 0.1 M was added, mixed up and down to submerge all granules and set aside for 30 min. Next, 100 ml of a 1.0 M sodium hydroxide solution were added to the mixture, mixed up and down to submerge all granules and set aside for 90 min at room temperature. The mixture was completely dried at 90°C. The mixture was washed with deionized water until nitrate ions were undetectable. The washed granules were then calcinated at 700°C for 30 min. After cooling they are ready as BRM for use.

Example 7:

Efficiency of removal of bacteria with adsorbent material based on mineral grains or other granules coated with nano silver particles (=Bacterial Removal Material BRM) in Various Filtration Columns 1. The influence of the thickness of the BRM layer

The influence of the thickness of the BRM layer on bacterial removal in the eluent is measured. Nano silver coated grains are filled in a column with a BRM layer thickness of 0.5; 1.0; 1.5; 2.0 and 3.0 cm. A water sample prepared in the laboratory containing bacteria at an initial concentration of 280 MPN/100 ml (MPN = most probable number) is loaded. The water flow rate is 1.3 ml per min and cm2.

The result is shown in table 1. With the above conditions, a minimum thickness of the BRM layer of 1 cm is necessary.

Table 1.

2. Influence of water flow rate

The flow rate is measured in the range of 1.3 to 13.6 ml / min cm² on the same column as before with a thickness of the BRM layer of 1.0 cm. The results are shown in table 2. With these conditions the column has the capacity to efficiently remove bacteria up to a flow rate of 10 ml / min cm².

Table 2. Thus, columns with a layer of at least 1 cm thickness with adsorbent material based on mineral grains or other granules coated with nano silver particles (=BRM) efficiently remove microbes from water flowing through the layer at a rate of 1.3 to 10.2 ml / min cm².

Example 8 : Preparation of colloidal solutions containing MnO₂ nano particles and their use for the coating of calcinated laterite grains. Preparation of colloidal MnO₂ nano particles solution:

Organic solvents ethanol, methanol, acetone and sodium stearate are generally used and they all proved their effect on nano dimensional MnO₂ formation. In this application ethanol is taken as an example:

MnSO₄ at a concentration of 3 x 10^"2 M and KMnO₄ at a concentration of 2 x 10^"2 M in solutions of different ethanol concentrations from 0% to 100% for both solutions were prepared and then the MnSO₄ and KMnO₄ solutions were combined with each other. The MnO₂ nano particles were produced by the following procedure: slowly adding KMnO₄ solutions one by one with ethanol concentrations of 0, 5, 10, 25, 50, 75 and 100% into the series OfMnSO₄ solutions having the same volume and ethanol concentrations from 0 to 100%. The adding rate was 2.5 ml solution per min. During the reaction time, the mixture was intensively stirred. The dark brown colloidal solution of MnO₂ nano particles was taken for particle size analysis and for coating of denaturated laterite material.

The yield of nano dimensional MnO₂ formation was calculated as percentage of mass ratio between amount of nano dimensional MnO₂ and the theoretical amount based on reaction stoichiometry. Table 1. The effect of ethanol concentration in reagent solutions on nanodimensional MnO₂ formation (%)

EPl: Percentage concentration of ethanol in MnSO4 solution and EP2: Percentage concentration of ethanol in KMnO4 solution

Table 1 shows the strong effect of the ethanol concentration in the solution of the reagents on the formation OfMnO₂ nano particles.

Figure 9 shows a TEM image of MnO₂ nano particles. Most of them have approximately the same size with a length of 60 nm and a width of 20 nm.

Coating of calcinated laterite grains using a colloidal solution comprising MnO₂ nano particles :

A suitable amount of dried calcinated laterite with size of 0.5 — 1.0 mm diameter was dropped into a colloidal solution of MnO₂ nano particles and softly shaken for 60 min. When almost all of the MnO₂ particles were adsorbed on the laterite surface, the solution became colorless and was discharged. The coated grains were washed by an aqueous solution with the same ethanol concentration as in the colloidal solution and then dried at 105⁰C for 4 hours. The maximum adsorption capacity was determined to be 138.89 mg arsenic per 1 gram of adsorbent. In comparison, the maximum adsorption capacity of calcinated laterite without coating was only 0.48 mg arsenic per 1 gram adsorbent and for MnO₂ particles it reached a value of approximately 20 mg arsenic per gram adsorbent.

Figures 10 and 11 show the surface of denaturated laterite before and after coating of MnO₂ particles in SEM images and the difference is obvious. Before coating, the surface of laterite is quite smooth; but after coating there are nano crystals Of MnO₂ with a needle shape distributed all over the laterite surface.

Figure 12 shows a time equilibrium curve of arsenic adsorption by calcinated laterite grains coated with MnO₂ nano particles. 1 gram adsorbent was dropped into 250 ml arsenic solution of 1000 ppb concentration. The solution was stirred continuously. Periodically the arsenic concentration was determined and the results are shown in the figure. The equilibrium adsorption time was 8 hours.

Figure 13 shows the Langmuir adsorption isothermal curve for calcinated laterite grains coated with MnO₂ nano particles which was established with the initial concentrations of arsenic from 0.00 to 100 ppm and the results are shown in the figure.

Example 9:

Preparation of colloidal solutions containing FeOOH nano particles and their use for the coating of calcinated laterite grains: Iron(III) of a 0.5 M FeCl₃ solution was hydrolyzed by water or an aqueous solution containing ethanol from 5, 10, 20, 50, 75 and 100 %. The FeOOH nano particles were formed by adding 10 ml iron chloride solution into 500 ml hydrolyzing solution at a rate of 2 ml per min while stirring well. Then 5 M ammonia solution was slowly added into the hydrolyzing mixture to reach pH 4. The reaction temperature was kept at 80⁰C and the mixture was continuously stirred. The obtained dark brown colloidal solution of FeOOH nano particles was used for coating nano dimensional FeOOH on laterite grains according to the same procedure as the coating with MnO₂ nanoparticles.

Figure 14 shows a TEM image of FeOOH nano particles which were prepared particles as described above. The TEM image reveals uniform needle shaped nano crystals of FeOOH with a size of about 40 x 10 nm.

Figure 15 shows the influence of ethanol concentration on the formation of FeOOH nano particles.

Figure 16 shows the SEM image of adsorbent surface before and after coating with FeOOH nano particles.

Figure 17 shows a time equilibrium curve of arsenic adsorption by calcinated laterite grains coated with FeOOH nano particles. Upon the adsorption time in for 0 to 10 hours, arsenic concentration in liquid phase was measured and 6 hours were determined to be the adsorption equilibrium time.

Figure 18 shows the Langmuir adsorption isothermal curve for calcinated laterite grains coated with FeOOH nano particles which was established with the initial concentrations of arsenic from 0.00 to 100 ppm and the results are shown in the figure. The adsorption capacity was determined to be 92 mg arsenic per 1 g adsorbent material. Example 10:

Recycling Adsorbent Material based on Laterite:

A filtration column with 4.2 kg laterite treated according to example 1 and 4.2 kg laterite grains treated according to example 3 is saturated with adsorbed arsenic after about 10'0OO 1 of water with an arsenic concentration in the range of 198.4 μg/L. The adsorbent material can be recycled by washing with 0.25 M NaOH followed by washing with deionized water until a value of pH 6-7 of the outlet water is reached The column can be refreshed at least ten times with the adsorption capacity decreasing about 20% and loss of mass of about 5%. The concentrated arsenate in washing solution was precipitated to form insoluble arsenate salt such as calcium arsenate. The calcium arsenate can be used for other industrial purposes or disposed of safely and the NaOH can be recycled as washing solution.

While the invention has been described in present preferred embodiments of the invention, it is distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the claims.

Claims

P A T E N T C L A I M S

1. A system for water treatment comprising adsorbent material at least a fraction of which is based on mineral grains and comprises manganese dioxide in a surface layer.

2. The system for water treatment according to claim 1, wherein the fraction comprising manganese dioxide additionally comprises iron oxyhydroxide in the surface layer.

3. The system according to claim 1 or 2 in which the mineral grains are derived from laterite, limonite or clay.

4. The system according to anyone of claims 1 to 3 in which the mineral grains are calcinated and porous comprising inner and outer adsorbing surfaces.

5. The system for water treatment according to anyone of claims 1 to 4, in which the manganese dioxide is comprised in nano particles.

6. The system for water treatment according to anyone of claims 2 or 5, in which the iron oxyhydroxide is comprised in nano particles.

7. The system according to anyone of claims 1 to 6, comprising at least one additional fraction of a further adsorbent material.

8. The system according to claim 7, wherein at least one of the additional fractions of further adsorbent materials comprises iron oxyhydroxide but no manganese dioxide in the surface layer.

9. The system according to claim 7, wherein at least one of the additional fractions of the further adsorbent materials comprises nano particles of silver on the surface.

10. The system according to any one of claims 7 to 9, wherein at least one of the additional fractions of adsorbent material is based on mineral grains.

11. The system according to any one of claims 9, wherein the additional fraction of adsorbent material comprising nano particles of silver on the surface is based on silica granules or another synthetic carrier.

12. The system according to anyone of claims 1 to 11 comprising at least fractions 3 and 4 of the following four fractions of adsorbent material: a fraction 1 based on minerals for removal of turbidities in the inlet water, a fraction 2 comprising nano particles of silver on the surface of the adsorbent material for the removal of microbes, a fraction 3 based on minerals comprising manganese dioxide in the form of nano particles or the combination of manganese dioxide and iron oxyhydroxide in a surface layer for the removal of arsenic and a fraction 4 based on minerals comprising iron oxyhydroxide but no manganese dioxide in a surface layer for the removal of heavy metals in cationic form.

13. A method of producing an adsorbent material for water purification by providing a mineral and calcinating and grinding it to yield calcinated mineral grains, the method further comprising coating the grain surfaces to comprise manganese dioxide.

14. The method according to claim 13 wherein the grain surfaces are reacted or coated to further comprise iron oxyhydroxide.

15. A method according to 13 or 14, wherein the calcinated mineral grains are produced from laterite, limonite or clay.

16. The method according to claim 14 or 15, wherein the provided mineral comprises iron and the calcinated mineral grains produced from it comprise iron oxides and wherein the method comprises hydrolysis with acidic solution of the iron oxides and a neutralization step with base.

17. The method according to claim 16, wherein the hydrolysis of iron(III) dissolved from iron oxides present in the calcinated mineral grains is adjusted to form an iron oxyhydroxide surface layer of a predetermined thickness by controlling an iron(III) concentration in the acidic solution to reach a predetermined value.

18. The method according to claim 17, wherein the iron concentration is between 10^"3 and l0^'2 M.

19. The method according to anyone of claims 13 - 18, wherein the calcinated mineral grains after the hydrolysis step are exposed to a manganese containing compound which is reacted to introduce the manganese dioxide forming a mixed surface layer comprising iron oxyhydroxide and manganese dioxide.

20. The method according to any one of claim 13 -19, wherein manganese dioxide is introduced into the surface layer comprising the following steps: 1. soaking the mineral grains in a potassium permanganate solution, 2. draining and drying the grains, 3. reducing the permanganate with a reductant such as sodium disulfite to manganese dioxide and 4. washing and drying the grains.

21. The method according to claim 14 or 15, wherein the coating of the calcinated mineral grains with manganese dioxide and iron oxyhydroxide comprises the following steps: 1. soaking the calcinated mineral grains with an acidic solution containing a manganese(II) salt , 2. adding the soaked grains of step 1 to a basic solution containing a permanganate salt and 3. washing and drying the grains.

22. The method according to anyone of claims 13-15, wherein the coating of the mineral grains comprises contacting the mineral grains with a colloidal solution of manganese dioxide nano particles.

23. The method according to claim 22 wherein the coating of the mineral grains comprises contacting the mineral grains with a colloidal solution of iron oxyhydroxide nano particles.

24. The method according to claim 23 wherein the coating of the grain surfaces to comprise manganese dioxide and iron oxyhydroxides involves contacting the mineral grains with a colloidal solution of manganese dioxide and iron oxyhydroxide nano particles in one or two steps.

25. A method of producing an adsorbent material for water purification by coating adsorbent material to comprise nano particles of silver on the surface.

26. The method of producing an adsorbent material according to claim 25 providing a mineral and calcinating and grinding it to yield calcinated mineral grains, the method further comprising coating the grain surfaces to comprise nano silver particles.

27. A method of producing various fractions of adsorbent materials according to anyone of claims 13 to 26, wherein one or more of the fractions for removal of one type of pollutant is also an intermediate of the method of producing another fraction for adsorption of another type of pollutant.

28. A method of recycling adsorbent material produced according to anyone of claims 13 - 24 saturated with adsorbed arsenic by washing the saturated adsorbent material with a basic washing solution.

29. The method according to claim 28 wherein the basic washing solution is NaOH.

30. The method according to claim 28 or 29 further comprising recycling of the washing solution by precipitating the arsenate as an insoluble arsenate salt.

31. Use of the adsorbent material of the water treatment systems according to anyone of claim 1-12 for removing arsenic and possibly other pollutants for communal or industrial drinking water production.

32. Use of the adsorbent material of the water treatment systems according to claim 1-12 for communal or industrial waste treatment.