|Title of Invention||
"A METHOD FOR REMOVING ARSENIC FROM AN AQUEOUS FEED"
|Abstract||The present invention relates to a method for removing arsenic from an aqueous feed comprising the following steps: treating said aqueous feed in an oxidation zone with a compound containing cerium in the +4 oxidation state to oxidize said arsenic and thereby reduce said cerium to the +3 oxidation state; and removing said oxidized arsenic from said treated aqueous feed to form a purified aqueous liquid having a reduced arsenic concentration as compared to said aqueous feed the method characterized in that said oxidized arsenic is removed from said treated aqueous feed by contacting said aqueous feed with an arsenic precipitating agent containing ions that react with said oxidized arsenic to form insoluble arsenic compounds. 18|
|Full Text||CROSS-REFEREKfCES TO RBLATED APPLICATIQlflS This application claims Convention priority to and is a U.S. continuation-in-part (CIP) patent application of U.S. Patent Application No. 10/353,705 (filed 29 January 2003), the entire contents of which are hereby expressly iucorporated herein by this reference.
This invention relates generally to methods, compus i t, ious and devices for removing arsenic f rom aqueous st.reams and is particularly concerned with methods, compositions and devices for removing arsenic from grouudwater and drinking water using cerium in the +4 oxidation state to oxidize arsenic so it can be precipitated from the water.
Arsenic is a toxic element that naturally occurs in a variety of combined forms in the earth.' Its presence in natural waters may originate, for example, from geochemical reactions, industrial waste discharges and past agricultural uses of arsenic-containing pesticides. Decause the presence of high levels of arsenic may have carcinogenic and âther deleterious effects on living organisms, the U.S. Environmental Protection Agency (EPA) arid the World Health Organization have set the maximum contaminant level (MCD for arsenic in drinking water at 10 parts per billion (ppb). Arsenic concentrations in wastewaters, groundwaters, surface waters and geothermal watera frequently exceed this level. Thus, the current MCL and any future decreases, which may be to as low as 2.0 ppb, create the need for new techniques to economically and effectively remove arsenic from drinking water, well water and industrial waters.
Arsenic occurs in four oxidation or valence states, i.e., -3, O, +3, and +5. Under normal conditions arsenic is found dissolved in aqueous or aquatic systems in the +3 and +5 oxidation states, usually in the form of
arsenite (AsOa"1) and arsenate (As04~3) . The effective " removal of arsenic by coagulation techniques requires the arsenic to be in the arsenate form. Arsenite, in which the arsenic exists in the +3 oxidation state, is only partially removed by adsorption and coagulation techniques because its main form, arsenious acid (HAs02), is a weak acid and remains un-ionized at a pH between 5 and 8 where adsorption takes place most effectively.
Various technologies have been used in the past to remove arsenic from aqueous systems. Examples of such techniques include adsorption on high surface area materials, such as alumina and activated carbon, ion exchange with anion exchange resins, co-precipitation and electrodialysis. However, most technologies for arsenic removal are hindered by the difficulty of removing arsenite. The more successful techniques that have been used in large municipal water supplies are not practicai for residential applications because of space require-ments and the need to use danqerous chemicals, The two most coramon techniques for residential water treatment have been reverse osmosis and activated alumina. The former method produces arsenic-containing waste streams that must be disposed of, and the latter requires the use of caustic chemicals.
The above facts coupled with the potenţial for the decrease in MCL to between 2 and 10 ppb make it imperative that effective processes, compositions and devices for removing arsenic from water and other aqueous systems be deve.loped.
In accordance with the invention, it has now been found that arsenic can be efficiently and effectively removed from water and other aqueous feedstocks by treating the arsenic-containing aqueous feed with a compound containing cerium in the + 4 oxidation state, preferably cerium dioxide (Ce02), in order to oxidize the arsenic so that it can be more easily removed by precipitation from the treated aqueous feed to produce a purified aqueous liquid with a reduced concentration of arsenic. "Precipitation" as used herein
encompasses not only the removal of arsenic-containing ions in the form of insoluble species, but also includes the immobilization of arsenic-containing ions on or in insoluble particles. In one embodiment of the process of the invention, water or other aqueous liquid containing dissolved arsenic in the +3 and +5 oxidation states is contacted with cerium dioxide to oxidize arsenic in the +3 oxidation state to^arsenic in'the +5 oxidation state, and the arsenic in the +5 oxidation state is removed from the aqueous liguid by contacting the liquid with a precipitating agent that reacts with the arsenic in the +5 oxidation state to produce insoluble arsenic corapounds and an aqueous liquid of reduced arsenic content.
Typically, the oxidized arsenic is in the +5 oxidation state and dissolved in the water or other aqueous liquid in the form of arsenate • (As04~3) . The precipitating agent used to remove the oxidized arsenic from the aqueous liquid can be anything that reacts with the arsenate or other form of oxidized arsenic to produce insoluble arsenic compounds. For example, the precipitating agent can be cerium in the +3 oxidation state produced in the arsenic oxidation step when cerium in the +4 oxidation state is reduced. Alternatively, the precipitating agent can be any particulate solid containing cations in the +3 oxidation state, such as alumina, aluminosilicates, ion exchange resin and clays.
The oxidation and precipitation steps can be carried out in the same or separate zones. If the steps are carried out in the same zone, the compound containing cerium in the +4 oxidation state is usually mixed with the precipitating agent. Although this mixture can be made by supporting the cerium compound on the surface and/or in the pores of the precipitating solids, it is usually preferred that the cerium compound in particulate form be physically mixed with particles of the precipitating agent. A preferred composition of the invention
comprises a raixture of cerium dioxide and alumina. .
In a preferred embodiment of the process of the invention, an aqueous liquid containing dissolved arsenic in the f oria of arsenate and arsenite is contacted with a mixture of cerium dioxide particulates and alumina particulates in an oxidation zone such that the cerium dioxide oxidizes the arsenite to arsenate and the alumina reacts with the arsenate to form insoluble aluminum arsenate that sorbs onto the particles of alumina. The aqueous liquid exiting the oxidation zone contains a substantially reduced concentration of arsenic, usually ies s than about 2.0 ppb.
Although the process of the invention is primarily envisioned for removing dissolved arsenic from clrinking water and groundwater, it will be understood that the process can be used to treat any aqueous liquid feed that contains undesirable amounts of arsenic. Examples of such liquid feeds include, among others, well water, surface waters, such as water from lakes, ponds arid wetlands, agricultural waters, wastewater from industrial processes, and geothermal fluids. The arsenic-containing feed can also contain other inorganic contaminant s, such as seleniuru, cadmium, lead and raercury, and certain organic contaminants. Generally, the process of the invention can be used to treat any aqueous liquid feedstock containing more than 2.0 ppb arsenic and is effective for treating feeds containing ruore than 500 ppb arsenic. The process is effective in decreasing the arsenic levels in such feeds to below 5.0 ppb, usually to below 2.0 ppb.
The arsenic contaminating the aqueous feed is normally dissolved in the aqueous phase and usually exists in both the +3 and +5 oxidation states, respectively, as arsenite (AsCV1) and arsenate (As
Techniques for removing arsenate exist and are quite eftective, but removing the arsenite is a more difficult proposition because the present technologies for doing so are riot greatly effective. It has now been found that substantially all of the dissolved arsenite can be easily oxidized to arsenate by treating the aqueous feed with cerium in the +4 oxidation state and the resulting arsenate, along with the arsenate originally present in the aqueous feed, precipitated from the treated feed to produce an arsenic-depleted aqueous liquid.
In the process of the invention, the aqueous feed contaminated with arsenic is passed through an inlet into an oxidation vessel at a temperature and pressure, usually ambient conditions, such that the water in the feed remains in the liquid state. If the feed is contaminated with particulate solids, it is usually treated to reniove the solids before it is passed into the oxidation vessel. Any liquid-solids separation technique, such as filtration, centrifuging and hydrocycloning, can be used to remove the particulate solids.
In the oxidation vessel the aqueous feed is contacted with a compound containing cerium in the +4 oxidation state (hereinafter referred to as cerium +4), which Ce +4, is an extremely strong oxidizing agent and oxidizes any arsenite or other arsenic present in the +3 oxidation state to arsenate or other species containing arsenic in the +5 oxidation state. All of the arsenic species containing arsenic in the +5 oxidation state is then precipitated from the aqueous phase by contacting the oxidized aqueous feed with a precipitating, agent.
The oxidizing agent can be any solid or liquid containing cerium in the +4 oxidation state. Although it is generally preferred to use solid particles of cerium dioxide, which are insoluble in water and relatively attrition resistant as the oxidizing agent, water-soluble
cerium compounds can also be used. Examples of such compounds include ceric ammonium nitrate, ceric ammonium sulfate, ceric sulfate, and ceric nitrate.
The precipitating agent that reacts with the arsenate containing arsenic in the +5 oxidation state to form insoluble arsenic compounds can be present in the oxidation vessel with the cerium +4 compound so that the precipitation occurs essentially simultaneously with the oxidation. Alternatively, it can be in a separate vessel into which the treated liquid exiting the oxidation vessel passes. For simplicity purposes, it is normally preferred for both the cerium compound and precipitating agent to be present in the oxidation vessel. This embodiment of the invention eliminates the need for an extra vessel and thereby reducea the cost of installing and operating the process of the invention.
Although the precipitating agent can be any material, solid or liquid, that reacts with arsenate or other species containing arsenic in the +5 oxidation state to form insoluble arsenic compounds, it is usually a particulate solid that contains cations in 'the +3 oxidation state, which cations react with arsenate to form insoluble arsenate compounds. Examples of such solids containing cations in the +3 oxidation state include alumina, gamma-alumina, activated alumina, acidified alumina such as alumina treated with hydrochloric acid, metal oxides containing labile anions such as aluminum oxychloride, crystalline alumino-silicates such as zeolites, amorphous. silica-alumina, ion exchange resins, clays such as montmorillonite, ferric sulfate, porous ceramics, and cerium compounds containing cerium in the +3 oxidation state, such as cerous carbonate. Although lanthanum oxide and'other rare earth compounds caii be used as the precipitating agent, these materials are typically not employed (except of course for cerium compounds) in the process of the invention
because it is preferred to use a precipitating agent that has a rnuch smaller Ksp than that of the rare earth compounds.
As mentioned above it is normally preferable that the cerium +4 compound and precipitating agent both be presant in the oxidation vessel so that the arsenic is oxidized and precipitated essentially simultaneously in the same vessel. Although the cerium +4 compound and precipitating agent can both be water-soluble, it is normally preferred that the cerium +4 compound and precipitating agent both be water-insoluble particulate solids that are either slurried with the agueous feed in the oxidation vessel or physically mixed together in a fixed bed through which the aqueous feed is passed during the oxidation step. In an alternative embodiment of the invention, the cerium +4 compound can be dep'osited on the surface and/or in the pores of the solid precipitating agent. This embodiment is normally not preferred over a physical mixtura because supporting the cerium compound on or in the precipitating solids requires the cerium compound to be dissolved in a liquid, the resultant solution mixed with the support solids, and the wet solids dried. Such steps add significantly to the cost of practicing the process of the invention.
Normally, a sufficient amount of the cerium +4 compound is present in the oxidation vessel with the particulate precipitating agent so that the mixture of the two contains between about 8 and 60 weight percent of the cerium +4 compound calculated as the oxide. Preferably, the mixture will contain between about 10 and 50 weight percent, more preferably between about 20 and 30 weight percent, of the cerium +4 compound calculated as the oxide. However, in some instances, it may be desirable for the mixture to contain greater than 40 to 45 weight percent of the cerium +4 compound calculated as the oxide.
Regardless of whether the cerium +4 compound is present in the oxidation vessel in admixture with the particulate precipitating agent or supported on or in the pores of the precipitating agent, the solids will typically range in diameter between about.0.25 and 1.5, preferably from O.5 to 1.0, millimeters. When the cerium +4 compound and precipitating agent are present in the oxidation zone as a fixed bed, it is normally preferred that the particles be spherical in shape so the flow of the aqueous feed through the bed is evenly distributed. However, if desired, the particles may take other shapes including that of extrudates. Such extrudates would typically have a length between about 0.2 and about 3 . O millimeters.
During the oxidation step of the process of the invention, arsenite in the aqueous feed is oxidized to arsenate according to the following equation:
As the cerium +4 oxidizes the arsenite, it is reduced to cerium in the +3 oxidation state, which then reacts with the arsenate formed during the oxidation step to produce insoluble cerium arsenate as shown in the following
Ce+3 + As04"3 -> CeAs04 ,BoUd)
Although theoretically there is enough cerium +3 formed by reduction of cerium +4 to react with all of the arsenate formed in the oxidation reaction to precipitate the arsenate, it is normally preferred that an additional precipitating agent be present. This agent, which can be a compound containing cerium +3, reacts with any unreacted .arsenate to form an insoluble precipitate, which is removed from the aqueous feed to produce the desired arsenic-depleted aqueous liquid.
The oxidation step that takes place in the oxîdation vessel is normally carried out at ambient pressure, at a temperature from about 4° to 100° C., preferably from about 5° to 40 °C., and at a pH greater than about 3.0. The residence time of the aqueous feed in the oxidation vessel typically ranges f rom about 2.0 to about. 30 minutes. When the cerium +4 compound and arsenic precipitant are both solid .particulates and present together as a fixed bed in the oxidation vessel, the precipitated arsenic compounds will be sorbed by or otherwise associated with the solid particles of the precipitating agent so that the aqueous fluid exiting the oxidation vessel will contain essentially no solids and very little arsenic, usually less than about 10 ppb and guiţe frequently less than 2.0 ppb. If the precipitating agent is not in the oxidation vessel, the ef fluent f rom the vessel is passed to another vessel where it is treated separately with the arsenic precipitating agent. Finally, if the cerium 4-4 compound and precipitating agent are particulate solids that are slurried with the aqueous feed in the oxidation vessel, the effluent from the vessel is normally treated to separate the solids, including the insoluble arsenic compounds formed in the vessel, from the arsenic-depleted.liguid. Although the separation can be carried out in any type of device capable of removing particulates from liquids, a filtration system is typically employed.
I f the aqueous feed to the procesa of the invention contains other contaminanta that must be. removed in addition to arsenic to produce the desired purified aqueous product, the removal of these contamin-ants is typically carried either before or after the oxidation step. If the other contaminanta will interfere with the oxidation of the arsenic, they should be removed prior to the oxidation step.. In some cases the process of the invention is also effective for removing other
contaminants from the aqueous feed in addition to or to the exclusion of arsenic.
In a preferred embodiment of the invention, an arsenic purifying device contşining a cartridge or filter is used to treat residential drinking water. The treating device can be a free standing container with a filtering device containing the composition of the invention or a cartridge type device designed to fit under a sink. These devices are- situated so that the water entering the nome or business location passes through the filter or cartridge before it enters the sink faucet. The filter and cartridge devices are quite simple and comprise a inlet attached to the source of the drinking water, a filter or cartridge containing the cerium +4 oxidizing agent, usually in the fortn of a fixed bed and in admixture with an arsenic precipitant, and an outlet in communication with the sink faucet to direct the arsenic-depleted drinking water exiting the cartridge or filter to the entrance of the faucet. Alternatively, a cartridge or filter type device can be designed to fit onto the faucet so that water exiting the faucet passes through the cartridge or filter device before it is consumed.
In the filter or cartridge, arsenic in the +3 oxidation state is oxidized to arsenic in the +5 oxidation state, and substantially all of the dissolved arsenic +5 present reacts with cerium in the +3 oxidation state and the arsenic precipitating agent to form insoluble arsenic compounds that are sorbed onto the fixed bed solids. The precipitating agent is preferably alumina or an ion exchange resin. The effluent exiting the fixed bed and the outlet of the cartridge or filter device will typically have an arsenic concentration less than about 2.0 ppb. After the fixed bed in one of the cartridge or filter devices becomes saturated with arsenic, the cartridge or filter is replaced with a new
cartridge or filter of the same or similar design. The spent cartridge or filter is then disposed of in a legally approved manner.
In another embodiment, the process of the invention is used in community water treatment facilities to remove arsenic from drinking water before the water is distributed to local homes and businesses. For such use the cerium +4 oxidizing agent is typically present in large tanks in either slurry form or in a fixed bed so that relatively large amounts of arsenic-containing water can be treated either in a continuous or batch mode. The arsenic precipitant can be present either in the tank with the cerium +4 oxidizing agent or in a separate . vessel fed by the effluent from the tank. The water exiting the process typically has an arsenic concen-tration less than about 10 ppb, usually less than 5.0 ppb, and preferably less than 2.0 ppb.
The nature and objects of the invention are further illustrated by the following example, which is provided for illustrative purposes only and not to limit the invention as defined by the claims. The example shows that arsenic in the +3 and +5 oxidation state can be completely removed from water using cerium dioxide.
Test solutions were prepared to mimic arsenic-containing groundwater by mixing certified standard solutions of arsenic in the +3 and +5 oxidation states with ţap water containing no arsenic. Twenty grams of lanthanum oxide (La203) , 20 grams. of cerium dioxide (Ce02) , and a mixture of 10 grams of lanthanum oxide and 10 grams of cerium dioxide were separately placed in a sealed 100 milliliter glass container and slurried with about 96 milliliters of test solutions containing 100 ppb of arsenic +3, 100 ppb of arsenic +5, and 50 ppb of both arsenic +3 and arsenic +5. The resultant slurries were
agitated with a Teflon coated magnetic ştir bar for 15 minutes. After agitation, the ţap water was separated from the solids by filtration through Whatman #41 filter paper and sealed in 125 milliliter plastic sample bottles. The bottles were then sent to a certified drinking water analysis laboratory where the amount of arsenic in each sample was determined by graphite furnace atomic absorption spectroscopy. The results of these t.ests are set: forth below in Table 1.
The data for test 3 in the table show that, w'hen lantharmm oxide is used by itself, only 55 percent of the arsenic present in the arsenic-spiked ţap water is removed. Since the solubility of lanthanum arsenate, which contains arsenic +5, is very stnall, it was assumed that the arsenic remaining in solution was primarily arsenic +3 in the form of arsenite. The results of test 4, on the oth'er hand, show that ceriura dioxide can remove all of the arsenic f rom the-water. The disparity in these results is attributed to the fact that fcerium
exists in the +4 oxidation state in cerium dioxide and is a strong oxidizing agent, whereas the lanthanum in the lanthanum oxide, which is in the +3 oxidation state, is uot an oxidizing agent. Although the lanthanum +3 reacts with arsenic in the +5 oxidation state to precipitate it f r om the water, the lanthanum does not react with the arsenic in the +3 oxidation state. The cerium in the cerium dioxide oxidizes the arsenic +3 to arsenic +5, which then reacts with cerium +3 formed by the reduction of cerium +4 to precipitate all of the arsenic dissolved in the water, Tests 5-7 show that equal mixtures of cerium dioxide and lanthanum oxide are also effective in removing all of the arsenic from the ţap water.
This application discloses that arsenic is removed from water and other aqueous feeds by (1) treating the feed with a compound containing cerium in the +4 oxidation state, preferably cerium•dioxide, to oxidaze arsenic in the +3 oxidation state to arsenic in the +5 oxidation state and (2) removing the arsenic in the i-5 oxidation state from the aqueous phase, normally by contacting the treated feed with alumina or other precipitating agent containing cations in the +3 oxidation state.
Although this invention has been described by reference to several embodiments of the invention, it is evident that many alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace within the invention all such alternatives, modifications and variations that făli within the spirit and scope of the appended claims.
The Applicant reserves the right to claim or disclaim now or in the future any feature, combination of features, or sabcombination of features that is disclosed h ei: e i n .
All of the numerical and quantitative
rneasurements set forth in this application (including in the description, claims, abstract, drawings, and any appendices) are approximations.
The invention illustratively disclosed or
claimed herein suitably may be practiced in the absence of any element which is not specifically disclosed or claimed herein. Thus, the invention may comprise, consist of, or consist essentially of the elements io disclosed or claimed herein.
The following claims are entitled to the broadest possible scope consistent with this application. The claims shall not necessarily be limited to the preferred embodiments or to the embodiments shown in the 15 examples.
All patents, prior-filed patent applications, and any other documents and printed matter cited or referred to in this application are incorporated in their entirety herein by this reference.
1. A method for removing arsenic from an aqueous feed comprising the following steps:
(a) treating said aqueous feed in an oxidation zone with a compound containing cerium in the
+4 oxidation state to oxidize said arsenic and thereby reduce said cerium to the +3 oxidation
(b) removing said oxidized arsenic from said treated aqueous feed to form a purified aqueous
liquid having a reduced arsenic concentration as compared to said aqueous feed the method
characterized in that said oxidized arsenic is removed from said treated aqueous feed by
contacting said aqueous feed with an arsenic precipitating agent containing ions that react
with said oxidized arsenic to form insoluble arsenic compounds.
2. The method defined as claimed in claim 1 wherein arsenic in the +3 oxidation state in said
aqueous feed is oxidized to arsenic in the +5 oxidation state in said oxidation zone.
3. The method defined as claimed in claim 2 wherein arsenite (AsO2~1) in said aqueous feed is
oxidized to arsenate (AsO4~3) in said oxidation zone.
4. The method defined as claimed in claim 1 wherein said cerium-containing compound is a
5. The method defined as claimed in claim 4 wherein said cerium-containing compound
comprises cerium dioxide (CeO2).
6. The method defined as claimed in claim 1 wherein said steps (a) and (b) both take place in
said oxidation zone.
7. The method defined as claimed in claim 3 wherein said arsenate (AsO4"3) is removed from
said aqueous feed by reaction with said cerium in the +3 oxidation state to produce insoluble
8. The method defined as claimed in claim 1 wherein said oxidized arsenic is removed from said
aqueous feed by contacting said aqueous feed with an arsenic precipitating agent containing
ions that react with said oxidized arsenic to form insoluble arsenic compounds.
9. The method defined as claimed in claim 8 wherein said arsenic precipitating agent comprises
a particulate solid containing cations in the +3 oxidation state selected from the group
consisting of alumina, ion exchange resins, crystalline aluminosilicates, clays and porous
10. The method defined as claimed in claim 9 wherein said steps (a) and (b) both take place in
said oxidation zone.
11. The method defined as claimed in claim 10 wherein said arsenic precipitating agent
containing cations in the +3 oxidation state is present in said oxidation zone in combination
with said cerium compound containing cerium in the +4 oxidation state.
12. The method defined as claimed in claim 1 wherein said steps (a) and (b) are carried out in the
absence of lanthanum.
13. The method defined as claimed in claim 1 wherein step (a) is carried out in said oxidation
zone at a temperature between 5°C. and 40°C.
14. The method defined as claimed in claim 1 wherein said aqueous feed is selected from the
group consisting of groundwater, drinking water, industrial wastewater, agricultural water, lake
water, wetlands water and geothermal water.
15. The method defined as claimed in claim 1 wherein the concentration of arsenic in said purified
aqueous liquid is less than 10 ppb.
16. The method defined as claimed in claim 4 wherein said cerium-containing particulate solid is
present in said oxidation zone in the form of a fixed bed or a slurry.
|Indian Patent Application Number||3277/DELNP/2005|
|PG Journal Number||07/2009|
|Date of Filing||25-Jul-2005|
|Name of Patentee||UNION OIL COMPANY OF CALIFORNIA|
|Applicant Address||LEGAL DEPARTMENT, 2141 ROSECRANS AVENUE, SUITE 4000,EL SEGUNDO, CA 90245, UNITED STATES OF AMERICA.|
|PCT International Classification Number||C02F 1/28|
|PCT International Application Number||PCT/US2004/002430|
|PCT International Filing date||2004-01-28|