Method of removing chlorate and bromate compounds from water by catalytic reduction

The invention relates to a method for removing water constituents, in particular compounds and by-products of oxidative water treatment.

The known methods for water treatment are often multi-stage processes, z. B. a thermal-alkaline, an oxidative, a biological, or an adsorptive treatment can be linked together in different ways.

It is known that surface water z. B. be used by bank filtration. For hygienic reasons and to comply with the limits of the Drinking Water Ordinance, the germs and organic substances in particular must be removed.

Physical processes such as B. membrane separation processes or filtration processes are known and are also used.

In the oxidative water treatment, e.g. B. by disinfection with chlorine, hypochlorite, chlorine dioxide and ozone, in the presence of oxidizable substances also by-products arise, for. B. halogen-oxygen compounds such as chlorine oxygen compounds or bromates, halogenated hydrocarbons such as trihalomethanes, which must be removed so that the water can be used for drinking water supply, or as purified wastewater z. B. can be drained into a receiving water or can be fed into the production process as process water.

Through the oxidative treatment z. B. with ozone halide, for example the chloride or bromide, is oxidized according to the following reaction mechanism.<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>O</mtext></mrow><mrow><mtext>3</mtext></mrow></msub><msup><mrow><mtext> + Hal</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msub><mrow><mtext> → 0</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msup><mrow><mtext> + OHal</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><mspace linebreak="newline" /><msub><mrow><mtext>2O</mtext></mrow><mrow><mtext>3</mtext></mrow></msub><msup><mrow><mtext> + OHal</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msub><mrow><mtext> → 2O</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> + HalO</mtext></mrow><mrow><mtext>3</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>-</mtext></mrow></msup></mrow></math><img file="EP0779880B1_D0001.tif" /></maths>

This reaction depends on the pH, the amount of oxidizing agent and the time.

As bromine ions have been shown to be carcinogenic, they must be removed from the water. The World Health Organization (WHO) demands that the bromate content in drinking water should not exceed 25 µg / l, with a limit of 3µg / l being targeted in the future.

Since chlorate ions are undesirable in drinking water, they must be removed from the water. Disinfection with chlorine dioxide also produces undesirable by-products, such as chlorites and chlorates, which have been shown to cause hemolytic anemia and should therefore not be present in drinking water.

However, the ingredients mentioned must also be removed from water that is not to be used as drinking water, since their oxidation potential can be disruptive in many cases.

EP 0 586 998 describes a process for waste water treatment with which adsorbable organic halogen compounds (AOX) can be removed. The wastewater is first treated thermally alkaline and then catalytically.

The object of the invention is to provide a method for water treatment with which halogen-oxygen compounds can be removed economically or whose residual substance concentrations can be minimized.

Halogen-oxygen compounds in the sense of the invention are chlorate and bromate compounds.

According to the invention, these compounds are reduced on a supported noble metal catalyst using hydrogen.

The reductive decomposition of bromate takes place according to the following equation:<chemistry id="chem0001" num="0001"><img file="EP0779880B1_D0002.tif" /></chemistry>

The reductive degradation of chlorate takes place according to the following equation:<chemistry id="chem0002" num="0002"><img file="EP0779880B1_D0003.tif" /></chemistry>

Precious metal catalysts which contain metals of the eighth subgroup of the PSE as active substance are catalysts. B. platinum, palladium, iridium, rhodium, preferably palladium or a combination thereof with a metal from the copper group, preferably copper or silver, in particular copper.

In a preferred embodiment of the invention, supported palladium / copper or palladium catalysts are used.

Inorganic oxides, for example Al₂O₃ preferably γ-Al₂O₃, SiO₂, ZrO₂, MgO or TiO₂ used. Combinations of these materials or other carrier materials such as aluminum silicates, magnesium aluminum silicates or activated carbon are also suitable. Inorganic materials that are water and abrasion resistant are preferably used.

According to the invention, hydrogen gas is introduced into the water to be treated and the water loaded with hydrogen is contacted with the catalyst.

In a preferred variant, a catalyst is used which preferably contains palladium and / or rhodium or palladium and a metal of the copper group, in particular copper, as the metal component. The catalyst support material is impregnated with the metal component in a known manner.

For example, an impregnated support can be used which has an inhomogeneous distribution of the metal or metals with a concentration in the surface area.

Also suitable are materials that have a particle diameter in the range from 10 to 5,000 μm, preferably 50 to 600 μm.

In the context of the present invention, water and aqueous solutions of any origin can be treated, provided that they are free of substances which are known to act as poisons for catalysts containing palladium, rhodium or copper or which attack the carrier material. In the present invention, the term "water" refers to such waters and aqueous solutions.

On the one hand, the method can be used to treat water which, in its degree of purity, corresponds to water which has undergone natural filtration. Such water can contain water-soluble substances, e.g. B. inorganic salts, in the order of magnitude as they are found in groundwater, so z. B. up to a few grams per liter.

Such waters are e.g. B. groundwater, well water, spring water, surface water or bank filtrates or pre-cleaned wastewater, z. B. industrial waste water, for example from flue gas washers, but also drinks such as mineral water, lemonades and fruit juices.

The method is therefore suitable for use in the context of drinking water treatment and the treatment of industrial water, for example for the food or beverage industry.

On the other hand, this process is also suitable for reducing halogen oxygen compounds in highly contaminated wastewater (eg chlorate, bromate concentrations&gt; 3 g / l).

According to the invention, the hydrogen can be introduced into the water either by direct introduction of hydrogen gas or by means of suitable saturation systems, such as static mixers, bubble column reactors or via membranes. Other known methods are also suitable for introducing hydrogen.

In a preferred variant of the method, the water is gassed with hydrogen in a manner known per se, e.g. B. via gas saturator, but it is essential that the hydrogen is introduced as fine as possible and without gas bubbles and is evenly distributed in the water. The known permeation gasification has proven to be particularly suitable. The gas is introduced into the water via a solid membrane, for example an unreinforced or fabric-reinforced silicone rubber membrane or a solid supporting membrane with a 5 to 20 µm thin silicone layer, as a composite membrane. An essential feature of the permeation gas is the bubble-free gas entry due to the mass transport in the pore-free membrane material, which is based exclusively on diffusion and solubility processes. A further advantage of the permeation gas is that the gas input is increased by simply increasing the gas partial pressure in the membrane system up to the pressure-dependent saturation limit of the hydrogen in the water, or by increasing the flow rate of the water, thereby reducing the boundary layer at the membrane-water phase boundary can. This proves to be advantageous if larger amounts of hydrogen are required.

The hydrogen input can either take place simultaneously with the contacting of the water with the catalyst or separately. In a preferred variant, the hydrogen is introduced before the actual catalytic conversion.

For the catalytic breakdown of high halogen oxygen ion concentrations, the hydrogen is advantageously brought into contact with the catalyst at the same time as the water in a three-phase reactor (for example a trickle bed reactor).

It has proven to be expedient to carry out the treatment of the water in the presence of a quantity of hydrogen which corresponds at least to the equivalent quantities of the substances to be removed, and these are also reduced in the presence of further reducible substances.

The process according to the invention can be carried out at normal pressure or slightly elevated pressure, e.g. B. up to 10 atmospheres, work. The solubility of the hydrogen gas in the water is below 2 mg / l at normal pressure and temperatures between 10 and 25 ° C. and is also doubled when the pressure is doubled. Where correspondingly larger amounts of hydrogen are required to reduce larger amounts of oxygen compounds, the use of three-phase reactors has proven itself.

If the gassing of the water with hydrogen and the contact with the catalyst take place simultaneously, the water is brought into contact with the catalyst for such a period of time which is necessary in order to reduce the substances to be removed according to the invention. The catalytic treatment can be carried out either in a fixed bed or in a fluidized bed or fluidized bed reactor.

The treatment can be carried out either continuously or batchwise.

In one embodiment, water, e.g. B. has already undergone an oxidative treatment stage, but in which there are still halogen-oxygen compounds and z. B. residual traces of ozone, with a pH of 4 to 12, preferably 5 to 11, in particular 6.5 to 9.0, are introduced into at least one reactor in which the supported noble metal catalyst is located and at 0 to 100 ° C. , preferably 5 to 40 ° C, in particular 10 to 25 ° C and 1 to 10 bar treated with hydrogen.

It is also an object of the invention to feed water containing chlorate or bromine ions directly to the catalytic reduction without having been oxidatively pretreated.

For example, it is possible to treat wastewater with a pH of 3 to 14 and a bromate concentration of, for example, 4 g / l at temperatures of 5 to 90 ° C and a hydrogen pressure of 1 to 10 bar using this method. Here, bromate sales of over 90% can be achieved.

If desired, the water can pass through several reaction units connected in series, each containing a gassing device and a reactor, one after the other. Here, the pH of the water can be readjusted when it is passed on from one reaction unit to the subsequent reaction unit. In another embodiment of the process, a water whose content of substances to be removed according to the invention was not completely removed when the gassing device and the reactor were run through for the first time can be returned to the reaction cycle.

Metal catalysts which are formed from a porous support material impregnated with the metal component are used for the process according to the invention. Palladium and / or rhodium are used as the metal component. It is also an object of the invention to use palladium in combination with a metal of the copper group or to use rhodium. Copper and silver are particularly suitable as metals of the copper group. Copper is preferably used.

The proportion of metal components in the total catalyst can be between 0.1 and 10% by weight, preferably between 0.1 and 5, in particular between 0.2 and 2.0% by weight.

A palladium content of 0.1 to 2.0% by weight, in particular 0.1 to 1.0% by weight, based on the total weight of the catalyst, has proven advantageous.

In a preferred variant, a combination of palladium with copper is used as the metal component of the catalyst. The weight ratio of palladium to copper can be between 1: 1 and 8: 1, in particular 1: 1 and 4: 1. In one embodiment of the invention, catalysts can be used, the supports of which either consist of porous material which has a bimodal pore radius distribution with at least 20% of the total pore volume of macropores with a minimum radius of 2,000 Å, or which has an inhomogeneous distribution of the Metal with a concentration in the surface area with a layer thickness of 20 to 100 µm depending on the particle diameter, in particular with a particle diameter of 50 to 1,000 μm or those that are present as powder with a particle diameter of less than 50 μm.

Suitable porous carrier materials with a bimodal pore radius distribution are materials with a maximum of the pore radius distribution in the range of small pores with a radius up to about 400 Å, for example between about 50 and 350 Å, and a second maximum of the pore radius distribution in the range of macropores with a radius of at least about 2,000 Å. A carrier material with a maximum of the pore radius distribution in the region of small pores with a radius of 50 to 300 Å, in particular 50 to 200 Å, has proven to be favorable. For the macro-pore range, pore radii in the range from approximately 5,000 to approximately 20,000 Å are favorable. The proportion of macropores in the bimodal carrier materials should be sufficiently high to ensure rapid diffusion and can vary depending on the type and size of the carrier particles. Appropriate prove to be z. B. bimodal carrier materials with a macropore fraction between 20 and 80%, for example 20 and 60%, preferably 40 and 60%, in particular 40 and 50%, based on the total pore volume. For particles with a homogeneous pore distribution, most pores should have a radius of 30 to 100 Å. The same applies in principle to powdery particles.

When performing the procedure, the reacting agents should be quickly removed from the active area.

Rapid diffusion of the reacting agents out of the catalytically active region of the catalyst can also be promoted by using catalysts in which there is an inhomogeneous distribution of the metal on the support with a concentration in the surface area. It proves useful. B. an inhomogeneous metal distribution, in which the metal is concentrated on the surface with a penetration depth between 20 and 100 microns.

Rapid diffusion of the reacting agents from the catalyst can also be achieved by using powdered catalysts, for example catalyst powders, the particles of which have particle diameters of less than 50 μm, in particular less than 20 μm.

The BET surfaces of support materials, or of the catalysts with the structures described above, can range from about 20 to 360, in particular 60 to 300, m²/ g vary. For carrier materials with a bimodal pore distribution, the BET surface areas are typically in the range from 20-30 to 200 m²/ g, in the case of powdered catalysts or catalysts with homogeneous metal distribution in the range from 50 to 200 m²/G.

It is also possible to use support materials whose porosities differ significantly from those previously described. It has been found, for example, that the application of a porous oxidic layer to a practically non-porous core, such as magnesium alumosilicate, leads to a suitable carrier material. It has also been found that magnesium aluminosilicate is also suitable without this porous layer.

These carrier materials naturally have a BET surface area in the range from &lt;1 to 30 m²/G.

Carrier particles of various shapes can be used. So the carrier z. B. in the form of powders, granules, spheres, beads, cylinders, hollow cylinders or hollow spheres.

Smaller particle sizes are also suitable for fluidized bed applications, e.g. B. powdered catalysts.

Support materials with a bimodal pore radius distribution can be produced in a manner known per se. For example, for the production of porous ceramic materials with a bimodal pore radius distribution, substances can be added to the support masses during manufacture that can be washed out or burned out again in the course of the manufacturing process and thereby lead to the formation of macropores. As so-called burn-out substances, combustible organic substances such as, for example, wood flour, starch, sucrose or an ammonium salt of an organic acid such as ammonium acetate, or also carbon black, can be added, which burn out from the material during the subsequent burning of the carrier particles and leave macropores. This process is particularly suitable for the production of bimodal aluminum oxide supports. For example, spherical aluminum oxide supports can be obtained by the process described in DE-OS 25 04 463 and 25 46 318 by using an aluminum oxide hydrosol with a base that is hydrolyzable in heat, for example Hexamethylenetetramine, mixed and admixing the mixture with water-insoluble combustible organic substances or carbon black and possibly also alumina and / or hydrated alumina, the mixture is then dropped or sprayed into a water-immiscible liquid at elevated temperature, for example temperatures between 60 and 100 ° C. the gel particles formed in the water-immiscible liquid age at the falling temperature, then washes and dries and then calcines.

A bimodal pore radius distribution can also be obtained in a manner known per se through a subsequent targeted tempering of the carrier materials at temperatures in the range from approximately 600 to approximately 1000 ° C. This method is particularly suitable for pore expansion in SiO₂Carriers. So z. B. SiO₂- Transfer carrier materials with pore radii between 50 and 350 Å into bimodal carriers by subsequent tempering. For example, in SiO₂-Beads with pore radii around 215 Å by a 5-hour heat treatment at 700 ° C and subsequent annealing at 800 ° C for a 20% share of macropores in the range of 5,000 to 50,000 Å.

The impregnation of the carrier particles with the metal component can be carried out according to conventional methods for the production of catalysts. For example, metal salts or complex metal compounds can be applied to the carrier material in the impregnation process, spraying process or precipitation process and can be reduced in a manner known per se after drying and subsequent calcination. For example, the carrier particles can be soaked or sprayed with a solution or suspensions of metal salts or complex metal compounds in water or an organic solvent, for example a lower alcohol such as ethanol or ketone, or their mixtures, after drying, if appropriate at temperatures up to 600 ° C, for example between 500 and 600 ° C, and then calcined with a metal-free reducing agent, preferably hydrogen or, if appropriate, with thermal treatment at temperatures in the range up to 550 ° C., for example between about 200 and 550 ° C., or in the aqueous phase with sodium borohydride or sodium formate at temperatures between 10 and 50 ° C.

The metal distribution on the carrier material can be varied in a manner known per se by the type of impregnation. So z. B. when impregnating the carrier material with a solution of a soluble metal compound, the depth of penetration of the metal into the carrier material can be controlled by varying the impregnation time, e.g. B. between 1 and 30 min and the solvent, for example water or a faster evaporating organic solvent, e.g. B. a lower alcohol such as ethanol, or mixtures thereof, or by the type of metal compound to be soaked with, or by changing the pH.

The depth of penetration of the metal depends on the time, the pH and the metal compound. Short impregnation times means that the metal is mainly spread only in the surface area of ​​the carrier material. Extensive concentration of the metal on the surface area of ​​the carrier material can also be achieved in the precipitation process, by spraying on a solution or suspension of the metal compound or by coating the carrier material with a liquid containing the metal compound. In the case of catalysts with an inhomogeneous metal distribution with a concentration of the metal in the surface area, so-called shell catalysts, the course of the reaction is significantly more independent of diffusion processes than with catalysts with a homogeneous metal distribution.

In one embodiment, the process for drinking water treatment is carried out continuously. This preferred embodiment is characterized in that the water is continuously introduced into a metering container in which the pH is checked and, if necessary, by adding acid to a value of at most pH 12, preferably between pH 4 and pH 11, in particular pH 6. 5 and pH 9, and then via a pump regulating the flow rate with variable delivery through one or more reaction units, which each contain a gassing unit and a reactor, in which the water is first passed through the gassing unit and gassed with hydrogen gas, if necessary under pressure, and then passed through the reactor containing a catalyst bed with the metal catalyst, the water passing through so many reaction units in total may be necessary for reducing the substances to be removed according to the invention.

The water obtained can be processed further in a manner known per se. The water is practically free of oxygen, chlorine, chlorine oxygen and bromine oxygen compounds. It can be used directly for such purposes where oxygen-free water is required, e.g. B. as brewery water. If desired, it can also be aerated in a manner known per se for the re-uptake of oxygen in the course of drinking water treatment, with any small amounts of residual gases that may still have been removed being removed.

When treating water with high concentrations of halogen oxygen compounds in a three-phase reactor, it is not necessary to adjust the pH beforehand.

The following examples are intended to illustrate the invention but not to limit it.

<u>Example 1:</u>

360 ml of water containing 2 mg / l BrO₃^--Ions contained submitted. 5 g of Pd catalyst (1% Pd content) were in the stirred reactor. With a hydrogen input of 1 l / h, no bromate could be detected after a dwell time of 12 minutes. Furthermore, an amount of 1.2 mg / l bromide ion was formed as a reaction product.

<u>Example 2:</u>

Reaction conditions analogous to example 1:

A Pd / Cu catalyst (1% Pd, 0.25% Cu) was used. The water to be treated contained 2 mg / l BrO₃^--Ions. After a dwell time of 12 minutes, bromate could no longer be detected. 1.25 mg / l bromide ions were formed as the reaction product.

<u>Example 3:</u>

The bromate was reacted on a catalyst fluid bed. 100 g of a Pd-Cu carrier catalyst (1% by weight Pd, 0.25% by weight Cu) were used.

Water with a BrO₃^-Ion content of 0.5 mg / l and a pH of 6.3 was passed through the fluid bed at a flow rate of 11 l / h and at 5 bar. 20 ml / l of hydrogen gas were dissolved in the water. The BrO in the water removed was₃^--Ion concentration less than 0.01 mg / l, the Br^--Ion concentration could be determined with 0.3 to 0.32 mg / l.

<u>Example 4:</u>

In a stirred reactor, 360 ml of water containing 5 mg / 1 ClO₃^--Ions contained, submitted. 5 g of Pd catalyst (1% Pd content) were in the stirred reactor; with a hydrogen input of 1 l / h, the ClO was able to remain after a residence time of 15 minutes₃^--Ion concentration can be reduced to a value below 0.1 mg / l. Furthermore, an amount of 2.1 mg / l of chloride ions was formed.

<u>Examples 5 - 8:</u>

In a fluidized bed reactor, both demineralized water (DI water) and tap water were treated in the presence of hydrogen on different Pd-containing Alugel supported catalysts.

The following operating conditions have been set: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Water flow</entry><entry namest="col2" nameend="col2" align="left">71 / h</entry></row><row><entry namest="col1" nameend="col1" align="left">Hydrogen entry</entry><entry namest="col2" nameend="col2" align="left">0.3 l / h (at 20 ° C, 1bar)</entry></row><row><entry namest="col1" nameend="col1" align="left">Water temperature</entry><entry namest="col2" nameend="col2" align="left">10 ° C</entry></row><row><entry namest="col1" nameend="col1" align="left">PH value</entry><entry namest="col2" nameend="col2" align="left">7</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">pressure</entry><entry namest="col2" nameend="col2" align="left">5.8 bar (abs)</entry></row></tbody></tgroup></table></tables>

The bromate reduction achieved is shown in Table 1.

The analysis values ​​take into account a detection limit of 20 µg / l and a standard deviation of ± 20 µg / l for bromide and a detection limit of 2 µg / l and a standard deviation ± 2 µg / l for bromate.

<u>Example 9:</u>

300 ml of water containing 972 mg / l of chlorate ions were placed in a stirred reactor. The stirred reactor contained 5 g of a supported Pd / Cu catalyst (0.23% Pd, 0.28% Cu on alumina). There were 2 l H₂/ h entered. At a reaction temperature of 90 ° C, after a reaction time of 3 hours only 35 mg / l chlorine ions but 406 mg / l chloride ions could be detected.

<u>Example 10:</u>

A trickle bed reactor (three-phase fixed bed reactor) contained 1021.6 g of a Pd catalyst [0.89% Pd on ZrO₂/ Cordierite (magnesium aluminum silicate)]. In addition to the hydrogen, a continuous volume flow of 3.7 l / h of highly polluted waste water (pH 11.7) with a bromine ion concentration of 3000 mg / l was passed through this reactor. The reaction temperature was 60 ° C; a pressure of 4 bar was measured in the reactor. In the run-off, bromate concentrations below 50 mg / l were measured, ie catalytic bromate breakdown of over 98% was achieved.<tables id="tabl0002" num="0002"><img file="EP0779880B1_D0004.tif" /></tables>