AQUEOUS FLUIDS FIELD OF THE INVENTION

The present invention relates to an adsorption method for treating a fluid containing undesired selenium contaminants in order to clean the fluid from the contaminant and to a process for the recovery of the adsorbent material. The method is suitable for the recovery of the adsorbent and removal of water dissolved selenium from the adsorbent for its recovery.

BACKGROUND OF THE INVENTION

Irrigation drainage and industrial wastewaters often contain selenium contaminants.

The presence of toxic selenium in waste and surface waters at elevated levels causes a severe environmental and health problem.

California's San Joaquin Valley is one of the world's most productive agricultural regions, supplying more than 70% of the United States' fruits and vegetables. Irrigation in these soils has produced high-salt drainage water containing selenium (Se) at levels ranging from 75 to 1,400 ppb. The main forms of selenium existing in these aqueous systems are selenite (SeO ₃ ^"2 ) and selenate (SeO ₄ ^"2 ), which is the more stable in aqueous solution and more difficult to remove. Selenium is found in effluents from thermal power stations, oil refineries and smelting plants, in addition to the industries of glass production, pigments, solar batteries and semiconductors. Selenium is an important nutrient for human and animal health in the range of 0.8-1.7 μmol/1, but toxic above this value.

No universal method exists for selenium removal from water. A large variety of possible methods for its removal are available, for example:

(i) Precipitation as elemental selenium. Zero-valent iron can be used for removing selenium from water by reducing selenium oxyanions to elemental selenium (Se ⁰ ).

However, this method is not effective for selenate removal and achieves only 87% removal effectivity (Zhang et al., 2005). Metallic zinc or aluminum at an acidic pH is also used to chemically reduce selenium ions to elemental selenium (US 3,933,635). US 4,806,264 discloses another technique in which ferrous iron is added to wastewater at a pH of about 9 to chemically reduce selenate and selenite to elemental selenium;

(ii) Co-precipitation with ferric hydroxide or aluminum hydroxide (Montgomery, 1985) is efficient at pH values of 7 or lower and is only effective for selenite; (iii) Adsorption of selenium by mineral adsorbents on ferrihydrite, activated alumina, ferric, manganese and aluminum oxy-hydroxides, and metal oxide-coated sand.

This method is effective only for Se ⁴⁺ . For this reason, adsorption has not been used directly to treat Se ⁶⁺ dominant in drainage water (Balistrieri and Chao, 1987, 1990; Kuan et al., 1998; Mavrov et al., 2006 Peak, 2006);

(iv) Oxidation of Se ⁰ to Se ⁴⁺ by adding a strong oxidant, such as aqueous permanganate solution, sulfuric acid and following selenite adsorption. Oxidation with permanganate solution produces manganese dioxide precipitate as a by product, which serves also as a selenite ions adsorbent (US 5,993,667).

(v) Reduction of Se ⁶⁺ to Se ⁴⁺ with sulfuric acid carried out at a temperature 80°C, followed by addition of a reducing agent comprising the powder of a metal or the ion of a metal. The time required for the reduction reaction is about 2 to 12 hr (JP10218611);

(vi) Ion-exchange process is effective for the removal of Se ⁶⁺ (Mavrov et al., 2006);

(vii) Biological processes: bacterial reduction of selenium species Se ⁴⁺ and Se ⁺ to elemental selenium (Se ⁰ ) requires long operating times and a large-sized apparatus (Zhang and Moore, 1997; Mavrov et al. 2006);

(viii) Methylation/volatilization to the atmosphere using bacteria, fungi and algae for methylating Se in aquatic systems. The rates of Se volatilization depend on the Se species present, microbial activity and various environmental conditions (Lin et al., 2002);

(ix) Solvent extraction is highly efficient and selective, but has some disadvantages including the complexity of the equipment and high costs; and

(x) Electrochemical method using dissolvable metal electrodes. Insoluble precipitates are obtained from the compounds of selenium with the ions of the dissolved metal electrode. The electrochemical method has high costs relating to the electrolyser construction and very high energy consumption, therefore, it is ineffective from the economic point of view.

In addition, all these traditional water treatment processes achieve removal of the undesired selenium contaminants by merely transferring the pollutants from one phase to another, producing concentrated sludge and leaving the problem of disposing of the transferred pollutants, recovering the removed adsorbent and producing concentrated selenium solution or crystals for secondary exploit.

Water treatment based on the adsorption of contaminants from solutions by using adsorbent material is useful and cost-effective for selenium removal by the purification of drinking water and groundwater, and for the cleaning of industrial wastewater (Balistrieri and Chao, 1987, 1990; Kuan et al., 1998; Zhang and Frankenberger, 2003; Mavrov et al., 2006; Peak, 2006; El-Shafey, 2007). Attempts have been made and reported here to exploit low-cost sorbent to remove selenium contaminants Se ⁴⁺ and Se ⁶⁺ simultaneously from water. Using adsorption processes for water treatment requires recovery of the adsorbent material. Application of an adsorbent depends on its cost and adsorptive capacity after several adsorption-recovery cycles. Therefore, novel materials and methods are needed for the treatment of waters contaminated with selenium.

Adsorption techniques for treatment of solutions containing undesired selenium contaminants may be found in US 6,599,429 and US 6,914,034. US2005/156136 describes a polymeric anion exchanger in which microparticles of hydrated Fe(III)oxides are irreversibly dispersed, for adsorption of selenite. WO2007/011770 describes a method for removing contaminants from solution using a surface-activated nanocrystalline TiO ₂ , optionally loaded onto a porous carbon. WO2006/032727 describes an adsorbent material containing iron oxyhydroxide in the form of granules with grain sizes between 0.5-4 mm that may be used for removing selenium, from an aqueous solution. WO 2008/001354 of the same applicant discloses active carbon loaded with iron oxide/hydroxide nanoadsorbent for treating a fluid containing contaminants selected from organic compounds, organisms, toxic substances, hazardous substances, ammonia, or mixtures thereof, and WO 2009/063456 of the same applicant discloses a method of phosphate removal from aqueous fluid using iron oxide/hydroxide nanoadsorbent optionally loaded on active carbon.

SUMMARY OF THE INVENTION

It has now been found that nanoadsorbents based on oxides or hydroxides of transition metals described as suitable for phosphate removal in WO 2009/063456, can be successfully used for removal of selenium contaminants from aqueous fluids.

Thus, in one aspect, the present invention provides a method for treating a polluted aqueous fluid containing undesired selenium contaminants, comprising adsorption of said selenium contaminants onto an adsorbent material by mixing with or passing the polluted aqueous fluid through said adsorbent material to yield aqueous fluid purified from selenium and the adsorbent loaded with undesired selenium contaminants, wherein said adsorbent material is selected from: (i) nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof, or

(ii) porous carbon, activated carbon, aluminum oxide or hydroxide, activated alumina, mineral clay, zeolite, or mixtures thereof in granular, particles or powder form, loaded with nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof.

In another aspect, the invention provides methods for recovering the adsorbent material for further use and recovering the selenium for its subsequent exploit.

The present invention provide an efficient and cost effective method for removal of selenium contaminants, particularly Se ⁴⁺ [selenite (SeO ₃ ^"2 )] and Se ⁶⁺ [selenate (SeO ₄ ^"2 )], or mixtures thereof; from aqueous fluids such as domestic water, surface water, groundwater, and industrial wastewater.

DETAILED DESCRIPTION OF THE INVENTION

In some embodiments of the present invention, the selenium adsorbents are nanoparticles of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof, also referred herein sometimes as "nanoadsorbents".

Examples of transition metals relevant for the present invention include, without being limited to, Fe, Ni, Co, Cu and Mn. In the most preferred embodiments, the transition metal is Fe, preferably Fe (III).

The term "oxides or hydroxides", as used herein, refers to oxides, hydroxides and oxides-hydroxides or oxy-hydroxides of transition metals or of aluminum and includes also mixed metal oxides, preferably comprised of iron and at least one other transition metal, or mixtures thereof. Thus, the nanoadsorbent material of the invention may be selected from Fe ₂ O ₃ , FeOOH, FeFe ₂ O ₃ , Fe(OH) ₃ , MnFe ₂ O ₃ ; CoFe ₂ O ₃ , CuFe ₂ O ₃ , NiFe ₂ O ₃ , FeO, Al ₂ O ₃ , AlOOH, Al(OH) ₃ , or mixtures thereof, in the form of nanoparticles or colloids.

In preferred embodiments, the nanoadsorbent is an iron (III) oxide or hydroxide that may be prepared in-situ from iron chloride hexahydrate (FeCl ₃ x6H ₂ O) by mixing with water at room temperature during 120 minutes. As shown in the examples, with this nanoadsorbent the concentration of Se ⁴⁺ and Se ⁶⁺ in contaminated water was reduced from 11.653 ppm and 1.075 ppm, respectively, to less than 0.02 ppm for nanoadsorbent concentrations of 120 ppm and 265 ppm Fe, respectively. The residual concentration of the iron oxide or hydroxide nanoadsorbent in the purified water was less than 0.02 ppm Fe, demonstrating its high adsorption activity.

In other embodiment of the present invention, the selenium adsorbents are not nanoparticles but adsorbent materials selected from porous carbon, activated carbon, granular activated carbon, aluminum oxide/hydroxide, activated alumina, mineral clay, zeolite, or mixtures thereof, in granular, particles or powder form loaded with nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum or mixtures thereof. In some preferred embodiments, the adsorbent material is composed of activated carbon loaded with nanoparticles of iron (III) oxide. As shown in Example 8 hereinafter, activated carbon loaded with iron oxide or hydroxide nanoparticles reduced selenite concentrations of contaminated water from 1.047 ppm and 10.22 ppm to 0.025 ppm and 0.021 ppm, respectively, for loaded activated carbon concentrations of 0.003 kg/kg. The residual Fe concentration in the purified water was less than 0.1-0.2 ppm Fe, demonstrating its high adsorption activity. The same experiment was repeated with activated carbon not loaded with the iron oxide or hydroxide nanoadsorbent and the selenite concentrations of contaminated water were reduced from 1.047 ppm and 10.22 ppm to 0.850 ppm and 8.214 ppm, respectively, showing the superiority of the activated carbon loaded with iron oxide or hydroxide nanoparticles as the adsorbent material vs. unloaded activated carbon.

The nanoparticles according to the invention may have a size within the range of about 5 to 400 nanometer, preferably about 50 to about 200, more preferably about 80 to about 150 or about 100 nm.

The method of the invention is suitable for removal of selenium contaminants from aqueous fluids, preferably water such as potable water, tap water, ground water, or industrial, agricultural or municipal wastewater. The method is suitable also for treatment of aqueous fluid obtained from sludge or other solid waste mixed with or adsorbed by soil contaminated with selenium, hi this case, the sludge, soil waste or soil is extracted with acidulated water to produce an aqueous fluid containing the undesired selenium contaminants, which is then treated according to the invention.

The adsorbent material for use in the method of the invention may be virgin or regenerated. It is indeed one of the advantages of the present invention that it allows the recovery/regeneration of the adsorbent material as well as of the selenium for further use concomitantly with the decontamination process. In the method of the invention, the adsorbent material, e.g. iron oxide or hydroxide nanoadsorbent loaded onto activated carbon, will gradually become saturated due to the adsorption of the selenium contaminants onto its surface. It is important economically and environmentally to recycle the spent adsorbent material and the selenium contaminants. The desorption process according to the method of the present invention allows efficient reactivation of the spent iron oxide or hydroxide and the selenium for further use.

The adsorption of the selenium contaminants is performed at pH conditions such as from pH of about 3 to about 8, preferably, from pH=5 to pH=6.0. The concentration of Se ⁴⁺ was reduced in these experiments from 11.653 ppm to less than 0.02 ppm for adsorption at pH range of 5-6, and to 1.48 ppm for pH value of about 8. The concentration of Se ⁶⁺ was reduced in these experiments from 1.075 ppm to less than 0.02 ppm for adsorption at pH range of 5-6.

The recovery/regeneration of the spent adsorbent material and of the pure Se contaminants for further exploitation is carried out by removal of the adsorbent loaded with selenium contaminants from water by producing a concentrated sludge or by secondary adsorption of this adsorbent loaded with selenium contaminants onto particles or granules of a secondary bulk adsorbent material.

In one embodiment, the recovery of the adsorbent material and of the selenium is carried out by a method comprising the following steps:

(i) separating the adsorbent material loaded with the undesired selenium contaminants from the purged water, thus producing a concentrated sludge, a bed of loaded adsorbent or a wet cake;

(ii) treating the produced concentrated sludge, bed of loaded adsorbent or wetcake by increasing the pH above 8;

(iii) washing with water or aqueous solution, thus recovering the adsorbent material free from selenium contaminants and producing a concentrated selenium solution or selenium slurry; and

(iv) separating the recovered/regenerated purified adsorbent from the selenium solution or slurry, thus obtaining purified adsorbent material and purified selenium solution or selenium slurry for further exploitation;

wherein said adsorbent material is selected from (i) nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof, or (H) porous carbon, activated carbon, aluminum oxide or hydroxide, activated alumina, mineral clay, zeolite, or mixtures thereof in granular, particles or powder form, loaded with nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof.

In some preferred embodiments the adsorbent material is nanoparticles of iron oxides or hydroxides. In other preferred embodiments the adsorbed material is activated carbon loaded with nanoparticles of iron oxides or hydroxides.

The separation of the adsorbent material loaded with selenium from the purified solution in step (i) can be carried out by means of separation techniques such as filtration, centrifugation, precipitation, etc. In the desorption step for recovering the adsorbent while producing a concentrated selenium solution or selenium slurry for repeated use, a water wash solution at pH above 8 is used for treating the adsorbent loaded with the selenium. The preferable pH range for desorption is from pH=8 to pH=12.5.

In another embodiment, the regeneration of the nanoadsorbent material and of the selenium for further use comprises the following steps:

(i) removing the nanoadsorbent material loaded with selenium contaminants from the water fluid by a secondary adsorption of the contaminated nanoadsorbent onto particles or granules of a secondary bulk adsorbent material selected from porous carbon, granular activated carbon, aluminum oxide, activated alumina, mineral clay, zeolite, or mixtures thereof;

(ii) washing with water or aqueous solution, thus recovering the secondary adsorbent material loaded with the selenium contaminated nanoadsorbent material and producing concentrated selenium solution or selenium slurry and recovery of the secondary adsorbent material for its subsequent exploit.

(iii) separating the recovered/regenerated secondary adsorbent loaded with the purified nanoadsorbent from the selenium solution or a selenium slurry.

In preferred embodiment the nanoadsorbent is iron (III) oxide or hydroxide and the secondary adsorbent material is granular activated carbon.

According to the invention, an adsorption/regeneration method for removal of undesired selenium contaminants from polluted aqueous fluid is provided comprising adsorption of said selenium contaminants onto an adsorbent material and recovery of the purified adsorbent material and of the purified selenium for further use, said method comprising:

(i) adsorption of the selenium contaminants onto an adsorbent material selected from: (/) nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof, or (H) activated carbon, activated alumina, aluminum oxide, mineral clay, or zeolite, or mixtures thereof, loaded with nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof, by mixing or passing the polluted aqueous fluid through said adsorbent material;

(ii) removal of the adsorbent material loaded with the undesired selenium contaminants from the purged water, thus producing a concentrated sludge, a bed of loaded adsorbent or a wet cake; and

(iii) recovery of the removed adsorbent to yield a purified adsorbent material free from selenium contaminants and producing a concentrated selenium solution or a selenium slurry for secondary exploit.

In another embodiment, the method comprises the steps:

(i) adsorption of the selenium contaminants onto an adsorbent material selected from nanoparticles or colloids of oxides or hydroxides of transition metals or of aluminum, or mixtures thereof, by mixing or passing the polluted aqueous fluid through said adsorbent material;

(ii) removal of the adsorbent material loaded with the undesired selenium contaminants from the purged water by secondary adsorption of the nanoadsorbent onto particles or granules of a secondary bulk adsorbent material selected from porous carbon, granular activated carbon, aluminum oxide, activated alumina, mineral clay, zeolite, or mixtures thereof; and

(iii) washing with water or aqueous solution, thus recovering the secondary adsorbent material loaded with the selenium contaminated nanoadsorbent material and producing concentrated selenium solution or selenium slurry for its subsequent exploit.

(iv) separating the recovered/regenerated secondary adsorbent loaded with the purified nanoadsorbent form the selenium solution or a selenium slurry.

hi preferred embodiments of the invention, the steps (i), (ii) and (iii) occur concomitantly, the adsorption of the contaminants in step (i) includes pH adjustment from about 3 to 8, preferably pH 5 to 6, and step (iii) includes pH adjustment from about 8 to 12.5.

The invention will now be illustrated by the following non-limiting examples. EXAMPLES

Materials and Methods

General. Iron-chloride hexahydrate, FeCl ₃ x6H ₂ O (analytical grade; Merck KGaA,

Germany), sodium selenate anhydrous, Na ₂ SeO ₄ and sodium selenite, Na ₂ SeO ₃ (analytical grade; Sigma- Aldrich, Israel Ltd), and activated carbon (Sigma- Aldrich Laborchemikalien

GmbH, Germany) were used as received.

The pH was determined using a Consort P-931 electrochemical analyzer. Iron and selenium concentrations were determined by Induced coupled plasma (ICP).

The starting material used for preparing the iron oxide/hydroxide was iron chloride hexa- hydrate, FeCl ₃ x6H ₂ O (analytical grade; Merck). Hydrolysis was used to prepare a 10% sol iron oxide/hydroxide nanoadsorbent. A series of iron oxide/hydroxide nanoadsorbent was then prepared by diluting the initial solution.

A series of experiments were conducted to investigate the adsorption-recovery properties of iron oxide/hydroxide and aluminum oxide/hydroxide nano-particles. All these experiments were carried at room temperature.

Example 1: Selenite (SeC> ₃ ^2" ) removal from water

Iron oxide/hydroxide nanoadsorbent was prepared as follows: 100 ml distillate water was mixed with 35 g iron chloride hexahydrate, FeCl ₃ x6H ₂ O (analytical grade; Merck) at room temperature during 120 min.

This 10% solution of iron oxide/hydroxide nanoadsorbent was used to purify a portion of polluted water: 1000 ml aqueous selenite (SeO ₃ ^2" ) solution containing 11.653 ppm Se ⁴⁺ with initial pH=7.2. The results of purification of polluted water experiments for different iron oxide/hydroxide nanoadsorbent concentrations are presented in Table 1.

Table: 1. Selenite (Seθ ₃ ^2" ) removal from water

In these experiments after adding iron oxide/hydroxide nanoadsorbent the pH of water was adjusted to pH values of5.0-6.0, by adding a solution of NaOH.

The adsorbent loaded with selenium contaminants was removed from water as a concentrated sludge by filtration using 0.45 μm filter paper (filter paper of pore size 0.45 μm). hi these experiments, the concentration of Se ⁴⁺ in contaminated water was reduced from 11.653 to < 0.02 ppm for nanoadsorbent concentrations of 120 ppm of Fe. In all these experiments residual concentration of the iron oxide/hydroxide nanoadsorbent in purified water was less than 0.02 ppm Fe. Therefore, the iron oxide/hydroxide nanoadsorbent demonstrated extremely high adsorption activity of Se ⁴⁺ . Example 2: Selenite (Seθ ₃ ^2~ ) removal from water using various iron oxide/hydroxide nanoadsorbent concentrations

The procedure described in Example 1 was repeated for the preparation of iron- oxide/hydroxide nanoadsorbent. This 10% solution of iron oxide/hydroxide nanoadsorbent was used to purify a portion of polluted water: 1000 ml aqueous selenite (SeO ₃ ^2" ) solution containing 1.047 ppm Se ⁴⁺ with initial pH=6.8. The results of purification of polluted water experiments for different iron oxide nanoadsorbent concentrations are presented in Table 2.

Table: 2. Selenite (Seθ ₃ ^2" ) removal from water

In these experiments after adding iron oxide/hydroxide nanoadsorbent the pH of the water was adjusted to pH=5.8-6.0 by adding solution of NaOH. The adsorbent loaded with selenium contaminants was removed from water as a concentrated sludge by filtration using 0.45 μm filter paper. In these experiments the concentration of Se ⁴⁺ in contaminated water was reduced from 1.047 to less than 0.02 ppm for nanoadsorbent concentrations of 32 ppm of Fe. In all these experiments residual concentration of the iron oxide/hydroxide nanoadsorbent in purified water was less than 0.02 ppm Fe. Therefore, the iron oxide/hydroxide nanoadsorbent demonstrated extremely high adsorption activity of Se ⁴⁺ .

Example 3: Selenate (SeO ₄ ^2" ) removal from water using various iron oxide/hydroxide nanoadsorbent concentrations

The procedure described in Example 1 was repeated for the preparation of iron- oxide/hydroxide nanoadsorbent. This 10% solution of iron oxide/hydroxide nanoadsorbent was used to purify a portion of polluted water: 1000 ml aqueous selenate (SeO ₄ ^2" ) solution containing 1.075 ppm Se ⁶⁺ with initial pH=6.9. The results of purification of polluted water experiments for different iron oxide/hydroxide nanoadsorbent concentrations are presented in Table 3.

Table: 3. Selenate (SeO ₄ ^2" ) removal from water

hi these experiments after adding iron oxide/hydroxide nanoadsorbent the pH of the water was adjusted to pH=5.5-6.1 by adding solution of NaOH. The adsorbent loaded with selenium contaminants was removed from water as a concentrated sludge by filtration using 0.45 μm filter paper. In these experiments the concentration of Se ⁶⁺ in contaminated water was reduced from 1.075 to less than 0.02 ppm for nanoadsorbent concentrations of 265 ppm of Fe. hi all these experiments residual concentration of the iron oxide/hydroxide nanoadsorbent in purified water was less than 0.02 ppm Fe. Therefore, the iron oxide/hydroxide nanoadsorbent demonstrated also high adsorption activity for Se ⁶⁺ . Example 4: Selenite (Seθ ₃ ^2" ) removal from water, as a function of pH

The procedure described in Example 1 was repeated for the preparation of iron- oxide/hydroxide nanoadsorbent. This 10% solution of iron oxide/hydroxide nanoadsorbent was used to purify a portion of polluted water: 1000 ml aqueous solution containing 11.653 ppm Se ⁴⁺ with initial pH=7.2. Before adding iron oxide/hydroxide nanoadsorbent the pH level of the water was adjusted to various values by adding solution of NaOH. As a result, selenium adsorption process onto nanoadsorbent was performed at different pH values of the solution. The adsorbent loaded with selenium contaminants was removed from water as a concentrated sludge by filtration using 0.45 μm filter paper. The initial iron- oxide/hydroxide nano-adsorbent concentration was 120 ppm Fe.

The results of purification of polluted water experiments for different pH values are presented in Table 4.

Table 4: Selenite (Seθ ₃ ^2~ ) removal from water as a function of pH

The concentration of Se was reduced in these experiments from 11.653 ppm Se ⁴⁺ to <0.02 for pH values of 3-5.5 (exp.4-1 to 4-3) during the adsorption process, to 0.339 ppm for pH values of 8.16 in experiment 4-6, and to 4.146 ppm for pH values above 9.5 in experiment 4-8. Example 5: Selenate (SeC> ₄ ^2" ) removal from water as a function of pH

The procedure described in Example 1 was repeated for the preparation of iron- oxide/hydroxide nanoadsorbent. This 10% solution of iron oxide/hydroxide nanoadsorbent was used to purify a portion of polluted water: 1000 ml aqueous solution containing 12.162 ppm Se ⁶⁺ with initial pH=6.7. Before adding iron oxide/hydroxide nanoadsorbent the pH level of the water was adjusted to various values by adding solution of NaOH. As a result, selenium adsorption process onto nanoadsorbent was performed at different pH values of the solution. The adsorbent loaded with selenium contaminants was removed from water as a concentrated sludge by filtration using 0.45 μm filter paper. The initial iron- oxide/hydroxide nanoadsorbent concentration was 400 ppm Fe.

The results of purification of polluted water experiments for different pH values are presented in Table 5.

Table 5: Selenate (SeO_} ^2" ) removal from water as a function of pH

The concentration of Se ⁶⁺ was reduced in these experiments from 12.162 to 0.089 ppm for pH values of 4.55 (experiment 5-1) during the adsorption process, to 0.379 ppm for pH of 5.24 in experiment 5-2, and to 8.708 ppm for pH of 7.198 in experiment 5-4.

Experiment 6: Se ⁶⁺ Recovery efficiency

The procedure described in Example 1 was repeated for the preparation of iron- oxide nanoadsorbent. This 10% solution of iron oxide nanoadsorbent was used to purify a portion of simulated polluted water: 1000 ml aqueous selenate (SeO ₄ ^2' ) solution containing 10.224 ppm Se ⁶⁺ with initial pH=6.8. The concentration of Se ⁶⁺ was reduced in these experiments from 10.224 to 0.2 ppm Se ⁶⁺ at pH values of 5-6. The adsorbent loaded with selenium contaminants was removed from the water solution as a concentrated sludge by filtration using 0.45 μm filter paper. The recovery at elevated pH removed the adsorbent and produced concentrated selenium solution. The pH of the slurry was adjusted to pH values of 7-12.5 in order to release the adsorbent from adsorbed selenium while producing concentrated selenium solution. The concentrated slurry was filtrated using 0.45 μm filter paper to yield iron-oxide nanoadsorbent free of selenium. The selenium recovery efficiency was calculated from the mass balancβj as follows:

Λ = -^-100(%)

m _n

where: mo -mass of selenium in the initial solution (10.224 ppm mj - mass of

Se ,6+ in concentrated selenium solution.

The selenium recovery results for different pH values are presented in Table 6.

Table: 6. Se _» 6+ recovery efficiency

The results show that at pH values of 9.5-10.5, 95-96% selenium (Se ⁶⁺ ) recovery was achieved concomitantly with adsorbent recovery. Experiment 7: Se ⁴⁺ Recovery efficiency

The procedure described in Example 1 was repeated for the preparation of iron- oxide nanoadsorbent. This 10% solution of iron oxide nanoadsorbent was used to purify a portion of simulated polluted water: 1000 ml aqueous selenite (SeO ₃ ^2" ) solution containing 1.0673 ppm Se ⁴⁺ with initial pH=7.12. The concentration of Se ⁴⁺ was reduced in these experiments from 1.0673 to 0.02 ppm Se ⁴⁺ at pH values of 5-6. The adsorbent loaded with selenium contaminants was removed from the water solution as a concentrated sludge by filtration using 0.45 μm filter paper. The recovery at elevated pH removed the adsorbent and produced concentrated selenium solution. The pH of the slurry was adjusted to pH values of 9-12.5 in order to release the adsorbent from adsorbed selenium while producing concentrated selenium solution. The concentrated solution was filtrated using 0.45 μm filter paper to yield iron-oxide nanoadsorbent free of selenium. The selenium recovery efficiency was calculated from the mass balance, as follows:

R = ^lOO( ⁰ Zo)

m ₀

where: mo -mass of selenium in the initial solution (1.0673 ppm Se ⁴⁺ ), ntj - mass of Se ⁴⁺ in concentrated selenium solution.

The selenium recovery results for different pH values are presented in Table 7.

Table: 7. Se -4+ recovery efficiency

The results show that at pH values of 11.5-12.5, 93-96% selenium (Se ⁴⁺ ) recovery was achieved concomitantly with adsorbent recovery. Example 8: Removal of Se ⁴⁺ from water: comparison of activated carbon and granular activated carbon loaded with iron oxide/hydroxide nanoparticles as adsorbents

The procedure described in Example 1 was repeated for preparation of iron-oxide nanoadsorbent. This 10% solution of iron oxide nanoadsorbent was used to prepare granular activated carbon loaded with iron oxide nanoparticles: 100 ml of aqueous solution containing 700 ppm of iron oxide/hydroxide nanoparticles was mixed with 1O g of virgin activated carbon. The concentration of iron oxide nanoparticles was reduced from 700 ppm to lower than 20 ppm. The activated carbon loaded with iron oxide nanoparticles was used to purify a portion of polluted water: 100 ml aqueous selenite (SeO ₃ ^2" ) solutions containing 1.047 ppm or 10.22 ppm selenium were mixed for 150 min with activated carbon which was either loaded or not loaded with iron oxide/hydroxide nanoparticles. The results of purification of polluted water for different activated carbon concentrations are presented in Table 8.

Table 8: Selenite (SeC^ ^2" ) removal from water

*- activated carbon without iron oxide/hydroxide nanoadsorbent

The selenium-loaded activated carbon was separated by filtration using 0.45 μm filter paper. The concentration of Se in water solution was reduced from 1 ppm to 0.850 ppm for activated carbon (AC) without iron oxide/hydroxide (0.003 kg/kg) and to 0.025 ppm for AC loaded with iron oxide/hydroxide (0.003 kg/kg). Similar performance was shown with higher initial concentration of Se. At the end of the process, the residual Fe concentration in the purified water was lower than 0.1-0.2 ppm. Thus, activated carbon loaded with iron oxide nanoparticles demonstrated high selenite (SeO ₃ ^2" ) adsorption ability versus the unloaded activated carbon.

REFERENCES:

Albert, M., Demesmay, C, Rocca, J.L. 1995. Analysis of organic and nonorganic arsenious or selenious compounds by capillary electrophoresis, Fresenius J. Anal. Chem. 351 (4-5), 426-432.

Amweg, E., Stuart, D., Weston, D. 2003. Comparative bioavailability of selenium to aquatic organisms after biological treatment of agricultural drainage water Aquatic Toxicology 63 (2003) 13-25.

Balistrieri, L.S., Chao, T.T. 1990. Adsorption of selenium by amorphous iron oxyhydroxides and manganese dioxide. Geochim Cosmochim Acta. 54, 739-751.

Balistrieri, L.S., Chao, T.T. 1987. Selenium adsorption by geothite. Soil Sci Soc

Am J 51, 1145-1151.

Cantafio, A., Hagen, K., Lewis, G., Bledsoe, T., Nunan, K., Macy, J. 1996. Pilot- scale selenium bioremediation of San Joaquin drainage water with Thauera selenatis. Appl. Environ. Microbiol. 62 (9), 3298-3303.

El-Shafey, E. 2007. Sorption of Cd(II) and Se(IV) from aqueous solution using modified rice husk. Journal of Hazardous Materials 147 (1-2), 546-555.

Engberg, R., Westcot, D., Delamore, M., HoIz, D. 1998. Federal and state perspectives on regulation and remediation of irrigation-induced selenium problems. In: Frankenberger W.T., Engberg R.A, editors, Environmental Chemistry of Selenium. New York: Marcel Dekker, pp. 1-25.

Glasauer, S, Doner, H., Gehring, A. 1995. Adsorption of selenite to goethite in a flow-through reaction chamber. Europ J Soil Sci. 46, 47-52.

Jacobs, L. 1989. Selenium in agriculture and the environment, American Society of Agronomy, Inc., Madison. WI.

Kashiwa, M., Nishimoto, S., Takahashi, K., Ike, M., Fujita, M. 2000. Factors affecting soluble selenium removal by a selenate reducing bacterium Bacillus sp. SF-I, J. Biosci. Bi89oeng. 89 (6), 528-533.

Kuan W., Lo, S., Wang, M., Lin, C. 1998. Removal of Se(IV) and Se(VI) from water by aluminum-oxide coated sand. Wat. Res. 32 (3), 915-923.

Lawson, S., Macy, J.M. 1995. Bioremediation of selenite in oil refinery wastewater. Appl. Microbiol. Biotechnol. 43 (4), 762-765. Lin, Z.-Q., Cervinka, V., Pickering, L, Zayed, A., Terry, N. 2002. Managing selenium-contaminated agricultural drainage water by the integrated on-farm drainage management system: role of selenium volatilization. Water Research 36, 3150-3160.

Losi, M., Frankenberger, W. 1997. Bioremediation of selenium in soil and water. Soil Sci. 162 (10), 692-702.

Mavrov, V., Stamenov, S., Todorova, E., Chmiel, H., Erwe, T. 2006. New hybrid electro-coagulation membrane process for removing selenium from industrial wastewater. Desalination 201, 290-296.

Mondal, K., Jegadeesan, G and Lalvani, Sh., B. 2004. Removal of selenate by Fe and NiFe nanosized particles. Ind. Eng. Chem. Res. 43, 4922-4934.

Montgomery, J. 1985. Water Treatment Principles & Design. Consulting Engineers, Inc., John Wiley & Sons, New York.

Ohlendorf, H. 1989. Bioaccumulation and effects of selenium in wildlife. In: Jacobs L. W., editor. Selenium in agriculture and the environment. Madison, WI: ASA and SSSA, 133-177.

Peak, D. 2006. Adsorption mechanisms of selenium oxy-anions at the aluminum oxide/water interface. Journal of Colloid and Interface Science 303, 337-345.

Presser, T., S., Ohlendorf, H., M. 1987. Biogeochemical cycling of selenium in the San Joaquin Valley, California, USA. Environ Manage.11, 805-821.

Zhang, Y., Moore, J.N. 1997. Environmental conditions controlling selenium volatilization from a wetland system, Environ Sci Technol. 31, 511-517.

Zhang, Y., Frankenberger, W.T. 2003. Factors affecting removal of selenate in agricultural drainage water utilizing rice straw. Science of the Total Environment 305, 207- 216.

Zhang, Y., Amrhein, Ch., Frankenberger, W.T. 2005. Effect of arsenate and molybdate on removal of selenate from an aqueous solution by zero-valent iron. Science of the Total Environment 350, 1-11.