Adsorbent for removing arsenic from an aqueous solution The present invention relates to an adsorbent for reducing arsenic content of an aqueous solution and a method for manu ^¬ facturing the adsorbent.

Arsenic contamination in potable water is considered to be one of the worst natural disasters of the century. The pres ^¬ ence of arsenic in natural water is well known in geographic locations, such as Bangladesh, India, Argentina, and China. At certain geographic locations, the concentrations levels of arsenic in natural water are very much higher than the per- missible limit prescribed by WHO. Medical problems linked to arsenic ingestion, include skin cancer, liver cancer, and bladder cancer.

In water, arsenic is usually found in two valence states, as trivalent As (III) and pentavalent As (V) , the former being 25- 50 times more toxic than the later. Techniques developed for removal of arsenic from water have been found to remove As (V) effectively. Accordingly, to remove arsenic present in the form As (III), oxidation of As (III) to As (V) has been sug- gested. Under natural ambient conditions, As (III) is oxidized to As (V) due to the presence of dissolved oxygen. However, the oxidation of As (III) to As (V) under natural conditions proceeds at an extremely slow rate. It is an object of the embodiments of the invention to pro ^¬ vide an adsorbent for reducing arsenic content of an aqueous solution .

The above object is achieved by an adsorbent for reducing ar- senic content of an aqueous solution according to claim 1.

The mixture comprising the first metal oxide and the second metal oxide may be coated onto the adsorbent media. The first metal oxide enables in oxidizing As (III) to As (V) . Typically, As (V) is more efficiently removed than As (III) . The second metal oxide enhances binding of arsenic. Thus, arsenic may be efficiently removed using the adsorbent.

According to an embodiment, the first metal oxide and the second metal oxide are selected from the group consisting of Μηθχ, LaO _x , CeO _x , FeO _x , ZrO _x , and TiO _x .

According to yet another embodiment, the first metal oxide is selected from the group consisting of MnO _x , LaO _x , CeO _x , and FeO _x , and the second metal oxide is selected from the group consisting of ZrO _x , and TiO _x .

According to yet another embodiment, the first metal oxide is CeO _x and the second metal oxide is FeO _x . The first metal ox ^¬ ide is CeO _x as the latter is an oxidant. The second metal ox ^¬ ide is FeO _x as the latter comprises hydroxyl groups of the surface . According to yet another embodiment, the adsorbent media is selected from the group consisting of a diatomite, calcium silicate, sepiolite, and titanium oxide. Diatomite, calcium silicate, sepiolite, and titanium oxide being inexpensive en ^¬ able in reducing the cost of the adsorbent.

According to yet another embodiment, the first metal oxide is to oxidize As (III) to As (V) . The mixture comprises the first metal oxide as the latter is an oxidant and may oxidize

As (III) to As (V) .

Another embodiment includes, a method of manufacturing an ad ^¬ sorbent for reducing arsenic content of an aqueous solution, the method comprising providing an adsorbent media, and coat ^¬ ing the adsorbent media with a mixture comprising a first metal oxide and a second metal oxide, wherein the first metal oxide being an oxidant and the second metal oxide comprises hydroxyl groups on the surface to bind arsenic. The adsorbent media may be coated with a mixture comprising the first metal oxide and the second metal oxide. The first metal oxide enables in oxidizing As (III) to As (V) . Typically, As (V) is more efficiently removed than As (III) . The second metal oxide enhances binding of arsenic. Thus, arsenic may be efficiently removed using the adsorbent.

According to another embodiment, the first metal oxide and the second metal oxide are selected from the group consisting of Μηθχ, LaO _x , CeO _x , FeO _x , ZrO _x , and TiO _x .

According to yet another embodiment, the first metal oxide is selected from the group consisting of MnO _x , LaO _x , CeO _x , and FeO _x , and the second metal oxide is selected from the group consisting of ZrO _x , and TiO _x .

According to yet another embodiment, the first metal oxide is CeOx and the second metal oxide is FeO _x . According to yet another embodiment, the first metal oxide is to oxidize As (III) to As (V) .

According to yet another embodiment, the adsorbent media is selected from the group consisting of a diatomite, calcium silicate, sepiolite, and titanium oxide.

According to yet another embodiment, the coating of the ad ^¬ sorbent media with the mixture comprises dissolving a first precursor compound for the first metal oxide into a first distilled water solution and a second precursor compound for the second metal oxide into a second distilled water solu ^¬ tion, mixing the first distilled water solution and the sec ^¬ ond distilled water solution to obtain a mixed solution, contacting the adsorbent media with the mixed solution, wherein the adsorbent media in maintained in contact with the mixed solution for more than a pre-determined time period, adjust ^¬ ing a pH of the mixed solution to an alkaline pH, thereby, depositing the mixture onto the adsorbent media, and extract ^¬ ing the adsorbent media from the mixed solution.

The first precursor compound eventually produces the first metal oxide in the first distilled water solution and the second precursor eventually produces the second metal oxide in the second distilled water. Contacting the adsorbent media with the mixed solution may include soaking or immersing the adsorbent media into the mixed solution. Alkaline pH is typi- cally a pH of the range 7 - 14. The mixture deposited onto the adsorbent media typically is coated onto the adsorbent media .

According to yet another embodiment, the method may further comprise separating the adsorbent media having a pre-defined particle size from the extracted adsorbent media. The adsorb ^¬ ent media having a pre-defined particle size may be separated by passing the extracted adsorbent media though a sieve. According to yet another embodiment, the pH of the mixed so ^¬ lution is adjusted using a solution comprising ammonium of a predetermined concentration. Different precursor compounds may produce different metal oxides in the distilled water. Thus, the pH of distilled water depends on the type of pre- cursor compound dissolved in the same. Accordingly, the ph may be adjusted by using varying concentration of ammonium solution based on the precursor compound dissolved.

Embodiments of the present invention are further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG 1 is a flow diagram illustrating a method of manufacturing an adsorbent according to an embodiment herein,

FIG 2 is a flow diagram illustrating a method of coating an adsorbent media with a mixture according to an embodi ^¬ ment herein, and FIG 3 illustrates Inductively Coupled Plasmon (ICP) analysis results of the column test and the breakthrough curve of FeOx-CeO _x -LaO _x coated sepiolite for arsenic remov- able.

Various embodiments are described with reference to the draw ^¬ ings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practiced without these specific details. The embodiments of the present invention provide an adsorbent for reducing arsenic contamination in an aqueous solution and a method of manufacturing the adsorbent. Experiments herein, demonstrate that an adsorbent media coated with a mixture of metal oxides enhances the removable of arsenic from the aque- ous solution. In an aspect, the aqueous solution may be wa ^¬ ter, such as, ground water, surface water and waste water. Arsenic from the aqueous solution may be removed by contact ^¬ ing the aqueous solution with the adsorbent. FIG 1 illustrates a method of manufacturing the adsorbent ac ^¬ cording to an embodiment herein. At block 10, an adsorbent media is provided. The adsorbent media includes, but is not limited to sepiolote, diatomite, calcium silicate, or tita ^¬ nium oxide. Next, at block 15, a mixture comprising a first metal oxide and a second metal oxide may be coated onto the adsorbent media. For example, the mixture may be coated onto the surface of the adsorbent media. In an aspect, the first metal oxide and the second metal oxide may be selected from Μηθχ, LaO _x , CeO _x , FeO _x , ZrO _x , and TiO _x . For example, the first metal oxide may comprise an oxidant, and the second metal ox ^¬ ide may comprise hydroxyl groups on the surface to bind arse ^¬ nic. The first metal oxide being an oxidant enables in oxi ^¬ dizing As (III) to As (V) . Conversion of As (III) to As (V) en- ables efficient removable of arsenic as As (V) is more effi ^¬ ciently adsorbed by the adsorbent. In an aspect, the first metal oxide may comprise MnO _x , LaO _x , CeO _x , FeO _x and the like and thus enable oxidation of As (III) to As (v) . The second metal oxide may comprise ZrO _x , TiO _x and the like, and thus, may bind arsenic to the adsorbent. This enables in enhancing the removable of arsenic from the aqueous solution. In an ^¬ other aspect, the first metal oxide may comprise CeO _x and the second metal oxide may comprise FeO _x . Additionally, the mix- ture may comprise a third metal oxide, such as, LaO _x , ZrO _x and TiO _x . For example, if the third metal oxide is LaO _x , the oxi ^¬ dation of As (III) to As (V) may be enhanced. If the third metal oxide is ZrO _x or TiO _x , the binding of arsenic to the ad ^¬ sorbent may be further enhanced.

The examples of adsorbent media mentioned above are rela ^¬ tively of lower cost. Thus, using these as the adsorbent me ^¬ dia for manufacturing the adsorbent enables in obtaining a cost efficient adsorbent with relatively equal or increased arsenic removal efficiency.

FIG 2 illustrates a method of coating the adsorbent media with a mixture comprising the first metal oxide and the sec ^¬ ond metal oxide according to an embodiment herein. At block 22, a first precursor compound for the first metal oxide is dissolved into a first distilled water solution and a second precursor compound for the second metal oxide is dissolved into a second distilled water solution. Next, at block 24, the first distilled water solution and the second distilled water solution are mixed to obtain a mixed solution. Moving next to block 26, the adsorbent media is contacted with the mixed solution. For example, the adsorbent media may be soaked or immersed into the mixed solution. In an aspect, the adsorbent media may be maintained in contact with the mixed solution for more than a pre-determined time period. The pre ^¬ determined time period may vary depending on the precursor compounds for the first metal oxide and the second metal ox ^¬ ide . Referring still to FIG 2, next, at block 28, a pH of the mixed solution may be adjusted to an alkaline pH. The adjust ^¬ ment of pH may thereby deposit the mixture onto the adsorbent media. In an aspect, the pH of the mixed solution may be ad ^¬ justed using a solution comprising ammonium of a predetermined concentration. The pre-determined concentration may vary depending on the precursor compounds for the first metal oxide and the second metal oxide. At block 30, the ad- sorbent media is extracted from the mixed solution. The ex ^¬ tracted adsorbent media may be washed using water and dried thereafter. In an aspect, the extracted adsorbent media may be dried in an oven. From the dried adsorbent media, the ad ^¬ sorbent media having a pre-defined particle size may be sepa- rated. For example, the dried adsorbent media may be passed though a sieve to separate the pre-defined particle size of the adsorbent media.

Example 1 : The present example describes preparation of an adsorbent having diatomite as the adsorbent media and a mix ^¬ ture of LaO _x -ZrO _x coated onto diatomite and its performance for arsenic removal.

The precursor compounds, 8.5 g lanthanum (III) nitrate hexa- hydrate and 6.5 g zirconium ( IV) oxychloride octahydrate were dissolved in 50 ml distilled water separately. The two pre ^¬ cursor solutions were mixed together. Diatomite (Huaying Technology Co. Ltd, China) was soaked in the mixed solution for several hours. Thereafter, ammonium solution (5~6 ) was used to adjust pH value to 7-8. The solid obtained was fil ^¬ trated, washed using water for several times and dried in an oven at 105 C overnight. The dry particles were sieved to ob ^¬ tain a fraction having a particle size of 0.25 to 0.5 mm. The arsenic removal performance of LaO _x -ZrO _x coated diatomite was tested using simulated ground water, containing 1 ppm of Arsenic (80:20 of As (V) & As (III)). The simulated ground wa ^¬ ter contains 100 ppm each of bicarbonate, sulphate; 2 ppm of phosphate and 50 ppm of silica. The adsorption test was car ^¬ ried out in batch by agitating 0.5 g of the prepared adsorb ^¬ ent in 500 ml of test solution. The arsenic content of the test solution was analyzed by Inductively Coupled Plasmon (ICP) before and after adsorption. The modified diatomite showed about 69% removal of arsenic under this condition.

The total arsenic uptake capacity of LaO _x -ZrO _x coated diato ^¬ mite was determined by agitating 0.5 g adsorbent with 500 ml of different concentrations of arsenic test solutions (1 ppm, 10 ppm and 25 ppm) . The adsorption performance of LaO _x -ZrO _x coated diatomite towards arsenic is shown in table 1. Based on equilibrium concentration, the adsorption capacity Qmax was calculated to be 12 mg/g (Langmuir Isotherm Analysis) .

Example 2 : The present example describes preparation of an adsorbent having sepiolite as the adsorbent media and a mix ^¬ ture of LaO _x -TiO _x coated onto sepiolite and its performance for arsenic removal.

The procedure for preparation of LaO _x -TiO _x coated sepiolite was the same as described in example 1. However, the precur ^¬ sor compound used for TiO _x was titanium oxysulfate and the adsorbent media used was sepiolite (Sepiolita, Tolsa group) . The adsorption performance of LaOx-TiO _x coated sepiolite to ^¬ wards arsenic is shown in Table 1. The adsorption capacity Q _max was calculated to be 18 mg/g.

Example 3: The present example describes preparation of an adsorbent having diatomite as the adsorbent media and a mix ^¬ ture of FeO _x -CeO _x coated onto diatomite and its performance for arsenic removal.

The precursor compounds, 27.8 g iron (II) sulfate heptahy- drate and 5 g cerium chloride heptahydrate were dissolved in 25 ml distilled water separately. The two precursor solutions were mixed together. Diatomite (Huaying Technology Co. Ltd, China) was soaked in this solution overnight. After that, am- monium solution (25%) was used to adjust pH value to 8-9. The solid obtained was filtrated, washed by water for several times and dried in an oven overnight at 105°C. The dry parti ^¬ cles were then sieved to obtain a fraction having a particle size of 0.25 to 0.5 mm. The adsorption performance of FeO _x - CeO _x coated diatomite towards arsenic is shown in table 1. The adsorption capacity Q _ma x was calculated to be 17 mg/g.

Example 4 : The present example describes preparation of an adsorbent having sepiolite as the adsorbent media and a mix ^¬ ture of FeO _x -CeO _x -LaO _x coated onto sepiolite and its perform ^¬ ance for arsenic removal.

The procedure of preparation and testing of FeOx-CeOx-LaOx coated sepiolite was the same as described in example 1. How ^¬ ever, the precursor compound used for LaO _x was lanthanum (111) chloride hexahydrate and the adsorbent media used was sepiolite (Sepiolita, Tolsa group) . The performance of FeOx- CeOx-LaOx coated sepiolite towards arsenic is shown in table 1. The adsorption capacity Q _max was calculated to be 9 mg/g.

Table 1 below shows arsenic removal performance of LaO _x -ZrO _x coated diatomite, LaO _x -TiO _x coated sepiolite, FeO _x -CeO _x coated diatomite and FeO _x -CeO _x -La0 _x coated sepiolite.

Example 5: Column test of FeO _x -CeO _x -LaO _x coated sepiolite for arsenic removal

Column test experiments (column dimensions: cp2.5 cm x 40 cm) were conducted on FeOx-CeO _x -La0 _x coated sepiolite by using an arsenic containing liquor. The chemical composition of the liquor is shown in table 2. A quantity of 30 ml of the ad ^¬ sorbent was placed into the column. The arsenic containing liquor was passed through the column at a flow rate of 6 ml/min. The empty bed contact time was 5 minutes. Samples were collected at regular time intervals. Inductively Coupled Plasmon (ICP) analysis results of the column test and the breakthrough curve of FeOx-CeO _x -La0 _x coated sepiolite for ar ^¬ senic removable are shown in FIG 3. The breakthrough point (10 ppb arsenic concentration) was obtained for a bed volume (BV) or run length of 2600. Commercially available granular ferric hydroxide showed a lower breakthrough point under the same working conditions.

The bed volume (BV) or run length is defined as:

BV = (Flow rate x time) / (volume of adsorbent)

Table 2 below shows chemical composition of test liquor for column test.

The embodiments described herein provide an adsorbent for ef ^¬ ficiently reducing the content of arsenic of an aqueous solu ^¬ tion. Additionally, the adsorbent is affordable as the same is cost efficient due to the type of adsorbent media used, and thus, may be implemented in rural areas and developing countries. The adsorbent enables in efficient removable of arsenic as As (III) is oxidized to As (V) and also the binding of arsenic is enhanced. As the adsorbent enables in oxidation and As (III) and enhances the binding of arsenic, a pre- oxidation step required for oxidizing As (III) is eliminated.

While this invention has been described in detail with refer ^¬ ence to certain preferred embodiments, it should be appreci ^¬ ated that the present invention is not limited to those pre ^¬ cise embodiments. Rather, in view of the present disclosure which describes the current best mode for practicing the in ^¬ vention, many modifications and variations would present themselves, to those of skill in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.