Method for In-Situ Regeneration of Activated Carbon Loaded with Trihalomethanes Using Alkaline Hydrolysis

Abstract

The invention pertains to a process for in-situ regeneration of activated carbon loaded with trihalomethane (THM). Based on the invention, this is achieved with alkaline hydrolysis of the THM with increased temperatures within the activated carbon in halogen-free, good water-soluble, or gaseous compounds. After completion of the chemical hydrolysis treatment, the activated carbon is cleared of reagents and reaction products by rinsing with water and diluted acids, and is then available for reloading in the untreated water flow. During the entire cleaning process, the activated carbon bed must not be moved.

Claims

1. Process for in-situ regeneration of activated carbon loaded with trihalomethanes, being characterized in that the loaded activated carbon in the inactive fixed bed at increased temperature is brought into contact with aqueous alkaline solution and rinsed with a sequence of rinsing steps with water and/or diluted acids.

2. Process based on claim 1, characterized in that the trihalomethanes involve chloroform, fluoroform, bromoform, iodoform, bromodichoromethane, or dibromochloromethane, with chloroform preferred.

3. Process based on claim 1, characterized in that the aqueous alkaline solution has a pH value of 13 to 14.

4. Process based on claim 1, characterized in that sodium hydroxide or potassium hydroxide with a concentration of 0.1 to 10 molar, is used as the alkaline solution.

5. Process based on claim 1, characterized in that the reaction temperature in the inactive fixed bed is in the range of 40 to 80° C.

6. Process based on claim 1, characterized in that the treatment duration of the fixed bed with alkaline solution is in the range of 2 hours to 3 days, wherein the duration of treatment is optimized depending on the parameters of alkaline concentration, reaction temperature and required rate of THM elimination.

7. Process based on claim 1, characterized in that the alkaline solution is circulated through the fixed bed by pumping and a heat exchanger for heating and cooling the fixed bed is integrated into the alkaline circuit.

8. Process based on claim 1, characterized in that after the end of alkaline treatment, the fixed bed is rinsed with a sequence of rinsing steps with water and/or diluted acids.

9. Process based on claim 8, characterized in that after conclusion of the alkaline treatment, the fixed bed is then rinsed with one or more bed volumes of water, a higher concentration of the alkaline solution is maintained for additional treatment steps as a result, and the fixed bed is rinsed with a sequence of rinsing steps with diluted acids and/or water.

10. Process based on claim 9, characterized in that the sequence of rinsing steps consists of one to five rinsing steps each with one or more bed volumes of diluted acids at 0.05 to 0.5 molar hydrochloric acid or sulphuric acid, followed by an additional 5 to 100 rinsing steps each with one or more bed volumes of pure water.

11. Process based on claim 8 characterized in that the rinsing steps maintain a contact time between the rinsing medium and the fixed material of 10 minutes to 2 hours each, wherein the contact time is adjusted by varying the pump rate or by setting a rest phase between the pump phases.

12. Process for manufacturing ultrapure water including the process of claim 1.

13. Process based on claim 12 for extracting ultrapure water for the semiconductor industry.

14. Process based on claim 3, characterized in that sodium hydroxide or potassium hydroxide with a concentration of 0.1 to 10 molar is used as the alkaline solution.

15. Process based on claim 1, characterized in that sodium hydroxide or potassium hydroxide with a concentration of 0.5 to 2 molar, is used as the alkaline solution.

16. Process based on claim 5, characterized in that the reaction temperature in the inactive fixed bed is in the range of 55° to 70° C.

17. Process based on claim 6, characterized in that the treatment duration of the fixed bed with alkaline solution is in the range of 12 to 26 hours.

18. Process based on claim 3, characterized in that after the end of alkaline treatment, the fixed bed is rinsed with a sequence of rinsing steps with water and/or diluted acids.

19. Process based on claim 10, wherein the additional rinsing steps are in the range of 10 to 30.

20. Process based on claim 11, characterized in that said contact time between the rinsing medium and the fixed material is in the range of 15 to 30 minutes.

Description

DESIGN EXAMPLES

[0060] The examples are based on experiments at lab scale. In the process, a commercial GAC (particle fraction: 1 to 3 mm, specific surfaces based on the BET method: 1150 m.sup.2/g) from aqueous solution was loaded with approx. 0.05 percent by weight chloroform (or bromoform) and equilibrated for at least one week. Afterward 10 g of aliquot from the activated carbon were filled in thermostated columns and subjected to various treatments. Water samples were taken based on the specified treatment times and their chloride content determined. At the end of the treatment period the GAC columns were either

[0061] a) rigorously extracted with toluene as an extracting agent and the extract analyzed for existing CHC using gas chromatography, or

[0062] b) the columns were flushed with a sequence of rinsing steps. In so doing, the pH value and the electrical conductivity of the column eluate were measured.

EXAMPLE 1

[0063] One of the GAC columns prepared as described above was filled with 20 ml 1 M sodium hydroxide (pH=14) and loaded at ambient temperature (20±2° C.) for seven days. Afterwards the sodium hydroxide was removed and its chloride content determined. The amount was 1.5 mg/l. This value corresponds with a chloroform conversion rate of approx. 0.7%. The example shows that chloroform sorbed in GAC is hydrolyzed extremely slowly at ambient temperature.

EXAMPLE 2

[0064] Five GAC columns loaded with chloroform were subjected to the following treatment conditions in parallel trials (see Table 1):

TABLE-US-00001 TABLE 1 Chloroform conversion [%] t [hr] pH value T [° C.] from chloride balance Column 1 72 14 50 42 Column 2 48 14.3 60 91 Column 3 36 14.3 70 99 Column 4 .sup.1) 16 13.7 80 82 Column 5 .sup.2) 24 14 70 83 .sup.1) KOH instead of NaOH .sup.2) Initial loading of GAC 0.01 instead of 0.05 weight by percentage

[0065] Under suitable reaction conditions within 1 to 2 days of treatment time, a high rate of chloroform conversion could be achieved (>90%).

[0066] For confirmation of the chloroform conversions calculated from the chloride balance, the GAC charge from Column 3 was subjected to rigorous solvent extraction. The gas chromatography analysis of the extract showed a residual chloroform content in the GAC of approx. 2 μg/g which corresponds with a rate of elimination of 99.6%. This value also confirms that the hydrolysis of the chloroform results in complete chloride release and no chlorinated byproducts are formed.

[0067] The data from Column 5 show that very low loads can also be eliminated with the process.

EXAMPLE 3

[0068] In loading the GAC with 0.05 percent by weight chloroform, a commercial humic acid (from the Aldrich company) was also added with a concentration of 5 mg/l. This is used as a substitute for natural organic material (NOM) in untreated water. About half of this humic acid was adsorbed by the GAC at the same time as the chloroform and provided an additional load of approx. 0.5 percent by weight. This additional load was 10 times higher than the chloroform load. One of the columns filled with this co-loaded GAC was subject to hydrolysis similar to Example 2, Column 3. >97% chloride yield was measured in the sodium hydroxide, and 4 μg of unconverted chloroform per g of GAC was measured in the toluene extract, which corresponds with a chloroform residual of 0.8%.

[0069] The example shows that even higher loads with co-sorbates like humic compounds do not decisively inhibit the hydrolysis of chloroform in the GAC.

EXAMPLE 4

[0070] The GAC was loaded with 0.1 percent by weight bromoform (CHBr.sub.3) instead of chloroform and subjected to hydrolysis in accordance with Example 2, Column 3. The bromide determination in the sodium hydroxide indicated a recovery of >95%. The solvent extraction resulted in no unconverted bromoform whatsoever in the extract.

EXAMPLE 5

[0071] GAC was characterized before and after hydrolytic treatment per Example 2, Column 3 by measuring the inner surfaces with the BET method and by microscopic measurement of the average particle size. The BET surfaces remained unchanged within the method's measurement accuracy (±10%) at 1000 to 1200 m.sup.2/g. Particle size distribution and mechanical stability (abrasion resistance) also showed no significant changes. The chloroform adsorption of both GAC samples was likewise the same (coefficient of sorption K.sub.d≈2×10.sup.4 l/kg with C.sub.Chloroform≈50 μg/l).

EXAMPLE 6

[0072] A GAC column equilibrated in accordance with Example 1 with 1 M of sodium hydroxide was washed with a total of 50 bed volumes of deionized water (demineralized water) in intervals. In the process, the equilibrium time between two wash steps for every bed volume was 15 minutes. The last discharge water from the flushing showed a pH value of 11.2 and a conductivity of 350 μS/cm. This value corresponds with a NaOH residual concentration of 1.6 mM or an enrichment of the output concentration by a factor of 625. In the case of an “ideal” not-retarded wash of the GAC columns (with adverse complete backmixing) a enrichment factor of 2.sup.50 ≈10.sup.15 is calculated. The practically much less efficient flushing is essentially attributed to the acid-base buffer properties in the activated carbon.

[0073] The example shows that the neutral rinsing of a previously alkaline equilibrated GAC fixed bed with pure water is only attainable with great effort.

EXAMPLE 7

[0074] It was carried out analgous to Example 6, however with the difference that after two bed volumes of pure rinse water, it was washed with two additional bed volumes of 0.05M hydrochloric acid. The first eluate after the second column rinsing shows a pH value of 3. Afterwards it was washed at 15 minute intervals with an additional 29 bed volumes of demineralized water. The last discharge water from the flushing showed a pH value of 6.0±0.3 and a conductivity of 110 μS/cm. These values correspond to a neutral eluate with a NaCl residual concentration of approx. 0.9 mM. The reduction factor in the conductivity of the last eluate compared with the initial condition (1M NaOH) is calculated at about 2000. This result was achieved with 34 bed volumes of rinse water compared with 50 bed volumes in Example 6.

[0075] This example shows that an acid rinse in the flushing process can significantly improve the outcome. The wash process is always delayed by the buffer properties of the activated carbon; however the retarding of the NaCl is clearly less pronounced that that of the NaOH.