EP1742883A1

EP1742883A1 - Method for the elimination of uranium(vi) species in the form of uranyl complexes from waters

Info

Publication number: EP1742883A1
Application number: EP05736327A
Authority: EP
Inventors: Wolfgang HÖLL; Günther Mann
Original assignee: ATC Dr Mann eK; Rohm and Haas Co
Current assignee: ATC ADVANCED TECHNOLOGIES DR. MANN GMBH; Rohm and Haas Co
Priority date: 2004-05-05
Filing date: 2005-05-04
Publication date: 2007-01-17
Also published as: DE202004021710U1; DE102004022705B4; CA2565637A1; US8137644B2; CA2565637C; DE102004022705A1; US20080112863A1; WO2005108303A1

Abstract

The invention relates to a method for eliminating uranium(VI) species from waters by means of slightly basic polyacryl-based anion exchangers, said uranium(VI) species being provided in the form of uranyl complexes as dissolved uranyl.

Description

Process for removing uranium (VI) species in the form of uranyl complexes from water

description

The invention relates to a process for the separation of uranium (VI) species from water by means of weakly basic anion exchangers based on polyacrylics, the uranium (VI) species being in the form of uranyl complexes as dissolved uranyl.

Uranium is a common, radioactive and reactive heavy metal on Earth. Because of its responsiveness, it is not found in nature as a pure metal. Uranium compounds can be a natural component of rocks and minerals as well as water, soil and air. Uranium enters the natural hydrological cycle e.g. through weathering of rock. It also reaches rivers, lakes and the ocean via water. How high the concentrations in natural waters are depends on various factors, e.g. Duration of contact between water and the rock, uranium content in the rock itself, redox conditions, availability of complexing ions in the water, etc.

In addition to the natural input, uranium also enters the environment through human activities. Sources are, for example, old tailings piles from uranium mining and the processing industry, but also the combustion of fuels and coal as well as the spreading of uranium-containing phosphate fertilizers and emissions from the nuclear industry. Uranium occurs in nature in various valences (+2, +3, +4, +5 and +6), but usually in its hexavalent form, bound to oxygen as uranyl ion (UO ₂ ²⁺ ), especially in Form of uranyl complex species, in particular they are present as carbonato or sulfato complex species.

Uranium complex species of hexavalent uranium can be very effectively eliminated with conventional strongly basic anion exchangers (usually based on polystyrene with conditioning with chloride or sulfate ions) because, apart from iron, most impurities do not form anionic complex species (YJ Song, Y. Wang, LH Wang, CX Song, ZZ Yang, A. Zhao, Recovery of Uranium from carbonate Solutions using strongly basic anion exchanger. 4. Column Operation and quantitative analysis, Reactive & Functional Polymers 39 (1999), 245-252). In natural waters, which usually contain species of carbonic acid, there are practically predominantly carbonato complex species, since they have greater stability than, for example, sulfato complexes. The experiments described in the literature (e.g. in T. Sorg, Methods for removing uranium from drinking water, J. AWWA 1988, 105-111) for the elimination of uranyl complex species from natural waters have shown that strongly basic anion exchangers in chloride form are very have a large absorption capacity for uranium carbonato complex species. With raw water concentrations of 22 - 104 μg / L, 9,000 to 60,000 bed volumes of water could be passed through appropriate filters before the outlet concentration exceeded 1 μg / L. With raw water contents of 300 μg / L, the throughput was 9,000 bed volumes until the 1 μg / L limit was exceeded.

In accordance with the general principles of ion exchange, the uranium species preferentially accumulate at the filter inlet. As corresponding studies have shown, the mean loading of the exchange material was about 35.7 g / L (as U ₃ O ₈ ). The corresponding activity was 7.8 × 10 ⁴ pCi / g dry resin.

Studies in the USA (SW Hanson, DB Wilson, NN Gunaji, SW Hathaway, Removal of uranium from drinking water by ion exchange and chemical clarification, US-EPA Report EPA / 600 / S2-87 / 076, 1987) also focus on the use of strong, basic exchange resins based on polystyrene in the chloride form. Sorption can be represented formally using the example of carbonato complexes as follows: «* R-UO ₂ (CO ₃ ) 2- + 2C1 R- (Cr) + UO ₂ (CO ₃ ) ₃ * - ** R-Up ₂ (GO ₃ ) J ^* + 4Cl

with R as the exchange matrix with the functional group Chorid. The symbols overlined here indicate the exchange phase. The symbol (CI ^" ) in parentheses illustrates the stoichiometric amount of chloride ions.

Analogous experiences were gained in uranium extraction, where practically only the uranyl carbonate species were sorbed and the exchanger after Exhaustion was practically completely loaded with it. The loads reached were approx. 80 g / L dry resin (as U ₃ O ₈ ).

For the removal of uranyl sulfato complexes, strongly basic anion exchangers, preferably based on polystyrene in sulfate form, are also used: R-UO ₂ (SO ₄ ) 2- + S0 | R- (Sθ5-) ₂ + UO ₂ (SO ₄ ) t- <»R-UO ₂ (SO ₄ ) ^ ^" + 2Sθt ^~ R- (Sθf-) + UO ₂ SO ₄ «» R-UO ₂ (SO ₄ ) ^_

Loads are achieved which are comparable to those when ingesting carbonato complex species. The high loadings and long running times "in the sorption of both carbonato and sulfato complex species result from the extremely high selectivity of the exchangers, especially for the tetravalent negative uranyl species.

As is known, the regeneration or elution of the uranium species from the strongly basic anion exchangers must be carried out with solutions of NaCl, NaNO ₃ or (NH ₄ ) ₂ CO ₃ . Depending on the concentration and added volumes, removal rates of 40 to 90% are achieved. Uranium concentrations of up to 5 g / L (as U) are reached in the eluates. The uranium is then precipitated from the regenerates. For this, the solution must either be mixed with strong acids or bases, or the uranium compounds must be reduced with hydrogen or precipitated by stripping with steam.

Strongly basic anion exchangers allow a very effective elimination of uranyl complex species in exchange for chloride ions, and in exceptional cases also for sulfate ions. However, the disadvantage of using these strongly basic exchangers is that they reversibly change the water composition and absorb and release both sulfate and (hydrogen) carbonate ions, so that the product water composition does not remain constant. These fluctuations have a negative impact on waterworks. Waterworks with large storage tanks may allow buffering, but this causes additional effort and costs. Smaller plants, however, cannot compensate for the fluctuations. Another disadvantage of the strongly basic exchangers is regeneration. NaCI, NaNO ₃ or (NH ₄ ) ₂ CO ₃ solutions are required for this, of which only NaCI can be used for cost reasons. However, concentrated solutions in a substantially stoichiometric excess must be added, which results in larger volumes of concentrated salt solutions as waste, which are difficult to dispose of. This is especially true for smaller waterworks.

The invention is therefore based on the object of developing a process for separating uranium (VI) species from water which does not have the disadvantages and limitations of the prior art mentioned. In particular, a method is to be provided with which only the uranyl complex species are separated without thereby changing the composition of the rest of the water.

The invention is solved by the method described in claim 1. Preferred embodiments of the method are specified in the dependent claims.

The method according to the invention is based on the fact that uranium occurs predominantly in its hexavalent oxidation state and thus in the form of negatively charged uranyl complex species. The uranyl is complexed in aqueous solution depending on the pH and the presence of appropriate ligands. In CO ₃ ^2- containing water, dominant species are uranyl carbonate complexes. Accordingly, it is preferred to remove uranyl carbonate complex species with the structures UO ₂ (CO ₃ ) ₂ ^{2 "} and UO ₂ (CO ₃ ) ₃ ⁴ \ in the water.

In addition, other ligands are contained in natural waters, so that the uranyl complexes, for example, also as sulfato complex species such as UO ₂ (SO ₄ ) ₂ ^{2 "} and UO ₂ (SO ₄ ) ₃ ^4" or as phosphate complexes such as UO ₂ (HPO ₄ ) ₂ ^{2 "} are also available as chlorides or fluorides.

Cationic uranium species can only be found in reduced water. However, since such waters generally contain iron, which must be removed by ventilation or oxidation and filtration before contact with ion exchangers, it can be assumed that the cationic uranium species will also be converted to the anionic uranyl complex species. The method according to the invention is aimed in particular at removing uranyl complex species with the general formula [UO ₂ (X)] ^y_ , where X

Represent anions of natural water, preferably Cl ^" , F ^" , CO ₃ ^{2 "} , (HCO ₃ ) ^" , (SO ₄ ) ^{2 "} , (HPO ₄ ) ^2" , and y is preferably 1 to 4.

The object of the invention is achieved in that weakly basic anion exchangers based on polyacrylics are used in the free base form to separate uranium (VI) species in the form of uranyl complexes from water.

A weakly basic anion exchanger based on polyacrylamide is preferably used for the uranyl complex elimination. According to the invention, it is preferably a weakly basic polyacrylamide-based exchanger of the general formula

R- (NR ' ₂ .H ₂ O) where R acts as a polyacrylic exchange matrix and R' represents hydrogen, substituted or unsubstituted alkyl (C, - C ₈ ) or substituted or unsubstituted aryl, preferably tertiary amines act as exchange-active groups. In a preferred embodiment of the invention, the exchanger is a modified tert-amine-acrylic copolymer, particularly preferably a tert-amine-acrylic-divinylbenzene copolymer.

The weakly basic anion exchanger is particularly preferably in the form of a gel.

The exchangers used according to the invention are further preferably of a total capacity of> 1.6 mol / L (free base form), a moisture content of 56 to 65% (free base form), and a density of 1, 030 to 1, 090 (free base form ) and have a preferred bulk density of 700 g / L.

Preferred grain sizes of the weakly basic anion exchanger are characterized by the following features: harmonic means 500-750 μm, coefficient of equality <1.8; Fine particle fraction <0.300 mm: 3.0% max; Large balls> 1, 180 mm: 5.0% max; or harmonics 700-950 μm, equality coefficient £ 1.7; Fine particle content <0.355 mm: 0.5% max; Large balls> 1.180 mm: 5.0-25.0% max.

Weakly basic ion exchange resins from Rohm and Haas Company, which are sold under the name Amberlite® IRA67 and IRA67RF, are particularly preferably used. With the preferred anion exchangers, which can be used in the free base form without additional conditioning, the uranyl complex species are sorbed without releasing e.g. Chloride or sulfate ions, as is the case when using conditioned, strongly basic exchangers of the prior art. Surprisingly, the uranyl complex species are almost exclusively taken up, the carbonato complexes predominating due to the greater stability. The reaction taking place is therefore illustrated using uranyl carbonate complexes:

R- (R ₂ -H ₂ O) + UO ₂ (CO ₃ ) i- «* R-UO ₂ (CO ₃ ) | - + 2H ₂ 0 R- (R ₂ H ₂ O) + Ü0 ₂ (CO ₃ ) ₃ ^ <=> R -UO ₂ (CO ₃ ) 3 * - + 4H ₂ 0

where R and R 'are the above. Have meanings.

The use of the weakly basic anion exchangers according to the invention has the advantage that separation takes place on one side and there is no exchange except for water constituents. For practical implementation of the method, the conventional filter arrangement is preferred for practical reasons. Other arrangements, such as a stirred tank with a flow through it, are also conceivable in principle.

The use of weakly basic anion exchangers in pure base form without additional conditioning has the further great advantage that the regeneration of the Exchanger can only be carried out using NaOH. This takes advantage of the fact that weakly basic exchangers are not protonated at high pH values and therefore cannot absorb anions. Because of the strong deprotonation at high pH values, only small volumes of regenerates are obtained, which are easy to dispose of.

The regeneration using the example of carbonato complex can be formally represented as

R -UO ₂ (C0 ₃ ) + (NaOH) «* R- (NR ₂ • H ₂ O) + (a ⁺ ) + UO ₂ (CO ₃ ) i- R -UO ₂ (CO ₃ ) r + (NaOH ) ^' <=> R- (NR ₂ • H ₂ O) + (Na ⁺ ) + UO ₂ (CO ₃ ) ₃ -

Technically, NaOH is easy to use and there is no conditioning of the exchanger. Instead of regeneration, the loaded filter material can also be disposed of directly.

The present process is used to remove uranium (VI) species, especially in waters that are used for drinking water production. This is preferably groundwater or surface water.

With the weakly basic anion exchangers based on polyacrylics used according to the invention, which can be used in free base form without any conditioning, high loads and long running times for the sorption, in particular of carbonato and sulfato complex species, are achieved. However, phosphato complexes and chloride and fluoride compounds can also be successfully eliminated.

The results result from the extremely high selectivity of the exchangers used according to the invention, in particular for the uranium (VI) species. The uranyl complex species can be removed effectively at pH values between 5.8 and 8.0. The efficiency corresponds to the elimination with conditionedπ strongly basic exchangers based on polystyrene in this area, which is particularly important for drinking water treatment, or shows improved values in comparison to the strongly basic polystyrene ion exchangers, which are known to be used. However, the successful possibility of using weakly basic exchangers based on polyacrylics in free base form has further advantages in addition to those already mentioned. The Regeneration or elution of the uranium species from the chess-based anion exchangers is carried out by adding NaOH; uranium can be eluted almost completely. Uranium concentrations of up to A of the concentrations on the adsorber material were reached in the eluates.

The invention is explained in more detail below using an exemplary embodiment.

example

A filter with an inner diameter of 12.5 cm was filled with 9.7 liters (L) of a modified tertiary amine-acrylic copolymer (Amberlite® IRA67 from Rohm and Haas Company), which is a weakly basic exchange material according to the invention based on polyacrylamide , filled up to a dumping height of 79 cm and flowed through with natural groundwater containing uranium at a throughput of 60 L / h. The uranium concentration of the natural groundwater was about 9 to 17 μg / L. After four months of operation and a total throughput of 17,600 bed volumes, which corresponds to a volume of approx. 169 m ³ , the drainage concentration of uranium was still below 0.1 μg / L.

Claims

claims:

1. A process for removing uranium (VI) species in the form of uranyl complexes from water, characterized in that the water containing uranyl complexes are brought into contact with at least one weakly basic anion exchanger based on polyacrylic in free base form, the uranyl complexes being adsorbed.

2. The method according to claim 1, characterized in that the uranyl complexes to be removed are carbonato complexes, sulfato complexes, phosphato complexes, chloride and fluoride complexes.

3. The method according to claim 1 or 2, characterized in that a weakly basic anion exchanger based on polyacrylamide is used, preferably with the formula R- (NR ' ₂ .H ₂ O) where R acts as a polyacrylic exchange matrix and R' is hydrogen, substituted or not substituted alkyl (C, - C ₈ ) or substituted or unsubstituted aryl.

4. The method according to claim 3, characterized in that the anion exchanger based on polyacrylamide is a modified tert-amine-acrylic copolymer, preferably a tert-amine-acrylic-divinylbenzene copolymer.

5. The method according to any one of claims 1 to 4, characterized in that the weakly basic anion exchanger is present as a gel type.

6. The method according to any one of claims 3 to 5, characterized in that the weakly basic anion exchanger has a total capacity of> 1.6 mol / L (free base form), a moisture content of 56 to 65% (free base form), and a density from 1,030 to 1,090 (free base form).

7. The method according to any one of claims 3 to 6, characterized in that the weakly basic anion exchanger has a bulk density of 700 g / L.

8. The method according to any one of claims 3 to 7, characterized in that the grain size of the weakly basic anion exchanger is characterized by the following features: harmonic means 500-750 microns, equality coefficient <1, 8; Fine particle content <0.300 mm: 3.0% max; Large balls> 1, 180 mm: 5.0% max.

9. The method according to any one of claims 3 to 7, characterized in that the grain size of the weakly basic anion exchanger is characterized by the following features: harmonic mean 700-950 microns, equality coefficient 1, 7; Fine particle content <0.355 mm: 0.5% max; Large balls> 1, 180 mm: 5.0-25.0% max.

10. The method according to any one of claims 1 or 9, characterized in that water is groundwater or surface water, which are used for drinking water production.

11. The method according to any one of claims 1 or 10, characterized in that the weakly basic anion exchanger is used in a filter arrangement and is flowed through by the water which contains the uranyl complex species to be removed.