DE4407057A1 - Electrochemical degradation of halogenated hydrocarbon(s) in polluted water - Google Patents

Abstract

Electrochemical process for degradation of halogenated hydrocarbons in contaminated water, which is carried out without an external current, is new.

Description

The pollution of all types of water by halogenated hydrocarbons used in many areas of technology is a common one Problem / 1 /. There are a number of methods for cleaning such contaminated water Method:

• - Physical processes based on extraction, adsorption, separation by stripping and evaporating etc.
• - Thermal processes, which in principle are based on a concentration z. B. by Evaporation with subsequent combustion of the pollutants are based.
• - Chemical processes that involve an oxidative or reductive reaction Make use.
• - Microbiological processes that regulate the metabolism of specific microorganisms use for dismantling.

The methods listed have some characteristics that are necessary for the evaluation are important. The physical processes only separate the pollutants, without solving the problem of elimination. The separated products must either recycled or dismantled in separate processes / 2 /. Ther Mixing processes are very expensive and can e.g. B. in the combustion again cause additional environmental problems / 3 /. Microbiological degradation is one characterized by slow reactions, the other lacking in Nature of specific microorganisms, so that they first have to be adapted sen / 4 /.

The chemical-oxidative degradation by oxygen (wet oxidation), hydrogen per Oxide or ozone usually sets a complex process and reac tion technology ahead / 5, 6 /.

Reductive degradation requires the use of strong reducing agents such as lithium or sodium, which because of their high reactivity not in protic or aq media can be used / 7, 8 /. With gas phase dehalogenations is usually in the presence of reducing atmospheres at elevated pressure and temperature conditions worked, which thereby with a high energy may be connected / 9, 10 /.  

The use of electrochemical processes is despite promising approaches have not been successful on a technical scale so far. They are based on Re Production processes on the cathode, using a variety of electrode materials and Electrode constructions were used / 11-13 /. The disadvantages of electroche mixing process and thus the lack of a breakthrough with this technique can be attributed to the following causes:

• - The reaction and process engineering of electrochemical processes is com often complicates and fails due to the lack of acceptance in companies.
• - The reduction on the cathode avoids the addition of chemicals but in aqueous or protic solutions always with the development of Hydrogen connected, so that undesirable stripping effects occur. In particular their volatile halogenated hydrocarbons thus get into the Katho dengas and must be separated again.
• - The cathode process is inevitably coupled with anode processes. Contains the electrolyte chloride, then anodic chlorine can form, which in turn gives rise to chlorination reactions with new, undesirable products. The result of this is that divided cells have to be operated on in turn lead to more complicated process engineering.
• - The current yields are usually very low because they are undesirable Parallel reactions dominate, e.g. B. the already mentioned hydrogen ent winding.
• - The achievable final concentrations of halogenated hydrocarbons are right relatively high and usually do not meet the requirements for ge legal limits.

In principle, the new method presented here is also based on electrochemical processes, but it works in contrast to electrolysis without external electricity. Driving force is a corrosion process that occurs in a number of metals in aqueous solutions can get going.

The following requirements must be met when using the de-energized dismantling process halogen hydrocarbons:

• - The reducing agent (corrosive metal) must have sufficient reduction possess strength.
• - The reducing agent should not contain gaseous water during the corrosion process Develop fabric (avoid stripping).  
• - The rate should be fast and long for halocarbon breakdown sam for the corrosion reaction. This can be done through targeted control of oxide and cover layers.

The basic advantage of the method proposed here over the previous electrochemical process is that of the electrochemical cell is effective in the micro range and on the construction of macroscopic electrolysis cells can be dispensed with. This makes the process simple Fixed bed or fluidized bed reactor with metal particles of appropriate size (Granules, metal shot, etc.).

As the following examples show, is from the range of reductive effects Metals such as B. iron, zinc, aluminum, magnesium, the zinc particularly suitable net. The process is for the degradation of chlorinated hydrocarbons, their Chlorine substituents are attached to saturated carbon atoms (e.g. hexa chlorcyclohexane, trichloroethane, dichloromethane, etc.) very effective. In contrast to recent investigations, is an activation of the metals with copper not necessary / 14 /.  

literature

/ 1 / Franzius, V.
in "State of Research on Contaminated Sites", Federal Ministry of Research and Technology 13 (1986)
/ 2 / Thome-Kozmiensky, KJ
Chem. Ing. Techn. 62 (1990), 286
/ 3 / Rordorf, BE
Thermochimica Acta 85 (1985), 439
/ 4 / Cook, AM, Scholtz, R., Leisinger, Th.
gwf Wasser Abwasser 129 (1988), 61
/ 5 / Leitzke, O.
WLB-Wasser, Luft und Boden, 7-8 (1990), 24
/ 6 / Gehringer, P., Proksch, E., Szinovatz, W., Eschweiler, H.
Water Res. 22 (1988), 57
/ 7 / DEGUSSA
wlb 10 (1987), 63
/ 8 / Hawari, JA, Samson, R.
U.S. Patent 4,950,833 August 21, 1990 Appl. No .: 413.942
/ 9 / Elderman, M., Mees, PAJ
PT-Procestech. 45 (1), (1990), 32
/ 10 / Dockner, T., Krug, H.
Open DE 35 10 034 (Cl. C07C1 / 26) September 25, 1986 Appl .: March 20, 1986
/ 11 / Gemmler, A., Bolch, T., Keller, W., Müller, KJ
Metal surface 44 (1990), 109
/ 12 / Beck, F., Schulz, H., Wermeckes, B.
Chem. Eng. Technol. 3: 371, 1990
/ 13 / Criddle, CS, McCarty, PL
Environ. Sci. Technol. 25: 973, 1991
/ 14 / Bachmann, T., Vermes, l., Heitz, E.
DECHEMA Monographies, Volume 124 (1991), 221.

example 1 Degradation of γ-hexachlorocyclohexane (lindane) with zinc

In the reservoir of the micro-pilot apparatus according to Fig. 1, 2 l of an aqueous Linda solution (pH = 5) were charged and rinsed h with a gentle stream of nitrogen against degradation beginning 1. The fixed bed is filled with 200 g of zinc particles in the particle size range of 0.2-0.4 mm and the solution is passed through the fixed bed at a flow rate of 0.8 l / h. The solution leaving the reactor is worked up with solid phase extraction and analyzed with a combined gas chromatography / mass spectrometry and an ion-selective chloride electrode.

An aqueous lindane solution with a concentration of 4.06 mg / l is introduced into the reactor. after about 7 min. stationary end conditions have been reached. The measured lindane concentration at the reactor outlet is then 50 µg / l, corresponding to a conversion of approx. 98.8% of lindane ( Fig. 2). In addition to chloride, benzene is formed in the reaction as the main and in small amounts of chlorobenzene in a ratio of 20: 1, which can then be recovered or sent to a degradation process.

In the anodic sub-step of the reaction, zinc digests in solution

• (1) Zn → Zn ²⁺ + 2e⁻ In the cathodic sub-step of the reaction, lindane is treated in
• (2) C₆H₆Cl₆ + 6e⁻ → C₆H₆ + 6 Cl⁻ or (3) C₆H₆Cl₆ + 4e⁻ → C₆H₅Cl⁻ + 5 Cl⁻ + H⁺reductively dechlorinated.

Example 2 Comparative investigation of the degradation effect of different metals in lindane containing aqueous solutions

If instead of zinc according to Example 1 iron, aluminum or magnesium is used as the reducing agent, the rate of the degradation reaction decreases considerably. If the reaction rates are converted into space-time yields (STA), the values shown in Fig. 3 are obtained. It should be noted that the RZA are logarithmic and that zinc is by far the highest value.

Example 3 Degradation of 1,1,1-trichloroethane and other chlorinated hydrocarbons with zinc

The breakdown of 1,1,1-trichloroethane gives a result which almost corresponds to Example 1: Fig. 4. The main reduction products detected are ethane, small amounts of ethene and chloride. The breakdown takes place according to the equations:

anodic Zn → Zn ²⁺ + 2e⁻
cathodic C₂H₃Cl₃ + 3 H⁺ + 6e⁻ → C₂H₆ + 3 Cl⁻

Other chlorinated hydrocarbons with chlorine substitutes on saturated carbon lenstoffatomen such as. B. dichloromethane, chloroform and trichloropropane be degraded in the same way.

Claims (6)

1. Electrochemical process for the degradation of halogenated hydrocarbons in contaminated waters, characterized in that it is used without external electricity. 2. The method according to claim 1, characterized in that it with a Kor rosion reaction is coupled. 3. The method according to claim 1, characterized in that as a reduction Mit tel metals such as B. zinc, iron, aluminum, magnesium and all others in aqueous solution corroding electronegative metals, their alloy or their combinations with electropositive metals (opposite Hydrogen) can be selected. 4. The method according to claim 1, characterized in that the reduction be conditions by choice of the reducing agent, the aqueous solution and the Operating conditions are set so that the degradation reactions with maximum and corrosion processes run at minimum speed. 5. The method according to claim 1, characterized in that it is preferably for the breakdown of chlorinated hydrocarbons from saturated coals Contain chlorine bonded atoms, different in aqueous solutions of all kinds in the vicinity of the neutral range (pH = 4 to pH = 10) becomes. 6. The method according to claim 1, characterized in that it is easier Way continuously in a tubular reactor or in a stationary set reactor or batchwise in a batch reactor.