GB2202862A - Electrochemically degrading organic contaminants in groundwater - Google Patents

Abstract

A process for electrochemically degrading organic contaminants in groundwater comprises embedding a grid work of rods in the ground in the region where groundwater flows which contain the organic contaminants. A voltage is applied to the plurality of space-apart oppositely charged rods to degrade electrochemically such organic contaminants in the groundwater. Sufficient voltage is applied across the plurality of oppositely charged rods to effect such electrochemical degradation. The process provides an effective relatively inexpensive form of minimizing or eliminating organic contaminants in groundwater, particularly contaminants such as phenols and trichloroethylene.

Description

ENHANCED IN-SITU ELECTRO-REDUCTIoN#OXIDAT1ON OF ORGANIC CONTAMINANTS IN GROUNDWATER SYSTEMS This invention relates to destruction of organic contaminants in groundwater systems by way of electro-chemical action.

The electro-chemical oxidation-reduction of various organic water contaminants, has been investigated. Such work has been directed primarily at the treatment of waste water effluent in a variety of flow-through reactor designs. Usually a bed of electrically chargeable material forms an electrode in the reactor, to establish the necessary redox conditions for the electro-chemical degradation of the contaminant.

Sharifian, H. and Kirk, D.(1985) Electrochemical Oxidation of Phenol, J. Electrochem.

Soc. 123, 921. discloses the electro chemical oxidation of phenol in a flow through reactor having a packed bed of lead oxide pellets. It was found that in this system, the electro-chemical oxidation of phenol produced hydroquinone, benzoquinone and other products including carbon dioxide. It was also found that the intermediate products were further electro-chemically degraded to simpler carbon containing compounds with further production of carbon dioxide gas. de Sucre, D.

Watkinson, A. (1981) "Anodic oxidation of phenol for waste water treatment", Can. Jour. Chem. Eng. 59;52, also addressed the flow through reactor design for degradation of phenols. The system provides for the removal of phenols from waste water effluent. The system entails the use of a lead oxide anode in the reactor.

It is also appreciated that electrical fields may be used to remove metal ion contaminants from water systems as disclosed in Runnells, D. and Larson, J.

(1986) "A laboratory study of electromigration as a possible field technique for removal of contamlnants from ground water", Groundwater Monitor. Rev. 6:85. The investigations were directed to the eloctromigration of copper contaminated solution. It was found that by properly positioned electrodes, copper ions readily migrated to the cathode for recovery and removal from the watersystem. It is suggested in this reference that a grid of electrodes may be implanted in the ground to achieve by electro-osmosis the removal of metal ion contaminants in groundwater systems. However, no thought has been given as to whether or not such a system could be similarly used to achieve the electro chemical oxidation-reduction of organic contaminants in groundwater systems.

According to this invention, a system for electrochemically degrading organic contaminants in groundwater, can be achieved by implanting a grid work of electrodes in the ground. Organic contaminants within the electrical field are either oxidized or reduced to yield less harmful organic constituents including carbon dioxide.

Figure 1 is a section of a flow through reactor used in investigating the electrochemical oxidation of phenol.

Figure 2 is a plot of the phenol degradation by electrochemical action; and Figure 3 is a plot of the trichloroethylene degradation by electrochemical action.

By way of in-situ groundwater treatment, using electrochemical methods, significant reductions in organic contaminants can be achieved by implanting the grid work of electrodes in ground aquafiers. The grid work is established by the sequential serial location of positive and negative electrodes, such that the flow of water through the aquifier is across the grid work of oppositely charged electrodes. Hence, a sequential electrochemical and/or oxidation of organic contaminants is achieved. For example, in the electro chemical reduction of phenols, benzoquinone and maleic acid, the benzoquinone and maleic acid are further electrochemically oxided to produce carbon dioxide, which can be accomplished sequentially as the groundwaters containing the organic contaminants flow through the grid work of oppositely charged electrodes.

It has also been established that electroreduction of trichloroethylene, another common groundwater contaminant can be achieved.

A flow through reactor shown in Figure 1 was provided to investigate the electro-oxidation of phenols. The reactor can be operated in either a down-flow or up-flow mode. In the up-flow mode, a pump is used to force the flow of liquid through the system.

The reactor 10 as shown in Figure 1, consists of a cylindrical housing 12 and end caps 14 and 16. An effluent/influent port 18 is provided at the base of the reactor extending through the end cap 16. In the upper region of the reactor is a head port 20 which provides constant level of liquid in the reactor. Material may be introduced in the over-flow and/or inlet port 22, depending upon the direction of flow desired.

The reactor comprises a media of crushed graphite 24, which is in contact with a graphite electrode 26. The electrode 26 is electrically connected, so as to be positively charged. The anode 28 is negatively charged by electrical connection 30 extending outwardly of the end cap 14. The carbon is contained in the reactor by way of perforated plate 32, which allows the liquid to flow through the carbon bed 24 in either direction. Furthermore, it is appreciated that the electrodes 26 and 28 can be charged in an opposite manner so that electrode 28 becomes the cathode in electrochemical reduction, rather than oxidation.

The bed of graphite may be formed from electrochemical grade graphite blocks. The blocks are crushed in a mechanical job crusher to achieve a textural range of .5mm to 2mm in diameter. The grannular graphite is packed between two 100 mesh stainless steel disks, and confined in place between the plexiglass plate 34 and the upper perforated plexiglass 32. The stainless steel isis are indicated as such at 36 and 38. The crushed graphite ..as chosen #n view of it being relatively inexpensive, and possessing good electrochemical properties.The graphite when crushed is ideal in providing a flow-through reactor, because the granular graphite increases the effect of the electrode surface area, For purposes of phenol oxidation, the organics are oxidized directly by electron transfer, and/or by reaction with the surface oxides that are produced in a charged transfer step from the constituents in the solution. A source of DC voltage was used, and operated at approximately 4 volts with a measured direct current in the range of 1-4 amps.

An alternative system which was used to establish electrochemical degradation of organics, is a system which simulates porous ground media common to an aquifier. Quartz sand of uniform texture was loaded into a container of approximately 4 litres in volume.

The quartz sand is inert with respect to ionic exchange and/or absorption reactions. The sand was saturated with a model wastewater, and optionally recycled with a pump. Electrodes were implanted in the sand, which were oppositely charged. The prefered type of electrodes are those made of titanium due to its resistance to oxidation.

Phenol solutions were passed through the reactor of Figure 1. The concentration of phenol in the effluent was measured by Chem Metrics colorimetric test kit. Further analyses was by Total Organic Carbon (TOC) analyses The TOC analyses was performed in a Beckman 915A Analyzer, which has separate organic and inorganic channels. Analyses were also conducted on a gas chromatograph Varian 3700, fitted with a 0.1% SP1000 on a Carbopack C column and flame ionization detection.

With reference to the following Table 1, reaction conditions of four continuous flow through four runs are quantified.

TABLE 1 Run 1 2 3 4 Feed Volume(b) 4.0 8.0 1 8.0 Initial Phenol(mgtL) 122.5 122.5 440 1750.0 6.0 6.0 6.0 5.0 Temperature (b) 240 250 250 250 Flow rate (ML/min) 200 200 25 100 Measured Potential(V-DC) 4.0 4.0 4.0 4.0 Measured Current(Amp) 1.3 1.3 1.4 3.5 Duration (Min) 40 80 40 2 hours Effluent Content (Phenol) mg/L (a) 88.0 93.0 246.0 625.0 (b) 92.0 87.5 238.0 650.0 (c) 90.5 90.5 225.0 575.0 (d) 89.5 91.0 215.0 600.0 Percent 27.3 26.6 57.0 65 concentration reduction The effluent was sampled at four different intervals as indicated by a, b, c and d.This information provided a better approximation of overall efficiency of the unit depending on the various parameters noted.

Additional runs similiar to that outlined in Table 1 were conducted, the results of which are shown in Figure 2. Runs number 1, 2, 3, 4 and 5 were analyzed on the basis of Chem Metrics colometric test system.

Run 3 was analyzed on the basis of Total Organic Carbon reduction.

The tests in the porous media were conducted using trichloroethylene water contaminent. The results are shown in Figure 3 as indicated by percent reduction in trichloroethylene, based on gas chromatograph analysis, and also based on total organic carbon analysis.

The results clearly indicate that substantial reductions in organic chemical content in water can be achieved by electrochemical methods. Up to 93E reduction in pheonal concentration can be achieved in 36 hours using a 12 volt DC 6 amps power supply.

Similarly, significant reductions in trichloroethylene are achieved by electroreductive dehalogenation.

Contaminated groundwater containing organics such as phenol or trichloroethylene can therefore be treated by electrochemical methods. By way of implanting, a multiple electrode array can be set up such that in knowing the direction of flow of groundwater through the aquifier, the groundwater can pass through sequentially positively and negatively charged electrodes. In so doing, the organics are degraded principally to carbon dioxide.

6 1/4" 20 x 1" HEX HEAD BOLTS ELECTRODE CONNECTION RUBBER O-RING CONSTANT HEAD PORT OVERFLOW/PORT INLET/PORT CARBON STEEL COUNTER ELECTRODE 2 1/4" x 20 x 6" LONG TREADED RODS PLEXIGLASS-PERFORATED PLATES GRAPHITE ELECTRODE EFFLUENT 1 INFLUENT PORT ELECTRODE CONNECTION FIGURE 1 - Porous Bed Flow Through Reactor The bench scale unit is 14 cm dia x 30 cm long.

The working electrode is a packed porous bed of granular graphite (median particle diameter of 1.0 cm).

The bed thickness can be varied up to 29 cm. Flow through the reactor is vertical and can be upward or downward. The unit has been operated primarily with an upward flow to reduce the potential effects of piping.

FIGURE 2 - Oxidation of Phenol A series of 24 experiments was completed using phenol as the reference compound. To simplify the interpretation, only five graphs are presented. For run 3, TOC was evaluated to allow for comparison with the data obtained by de Sucre and Watkinson (1981).

Phenol concentration was determined using a Chem Metrics colorometric test kit.

FIGURE 3 - Reduction of Trichloroethylene Two experiments were run using trichloroethylene (TCE) as a reference compound. TCE was selected as it is commonly encountered in wastewaters and groundwater and is a suitable model for other halogenated aliphatic compounds. Plot (a) above represents the results of one run monitored by TOC, while (b) displays the results of GC/MS analysis.

Claims (6)

1. A proofs for lectroabzically dqvadlnç organic

contaminants in grounowater, said process being characterized in applying a voltage to a ground quatier through a grid work of a plurality of spacedapart oppovitely charged and negative rods which are embedded in the ground, organic contaminants in the groundwator are sequentially degraded af the stroundwater flows through the grid work by applying sufficient voltage across sai plurality of oppositely charged rods to of foot Wid electroohemioal degradation,

2.A process according to clalt 1, oharacterized in that said plurality of rods are e*dded in the ground in a serial Beguential array, adjacent spac:ed-apart rods being oppositely charged.

3. A prooeie according to claim 1 or Z, characterized in that sufficient voltage is applied to oppoitaly charged rods to degrade eascstrochmically contaminznts selected from the group cons ng of phenol, banzbgUinone, maleic o acid, triohioroethylene and mixtures thereof.

4, A process according to olaim 1, 2 or 3, characterized in that said rods are embedded in sand through wnioh ~lid groundwaters flow.

5. A process according to any on of the preceeding clalv6, Characteri#.d in that sai rod are spaced-apart a distance which develops for a given applied voltage sufficient electrical Charge between adjacent rods to effect said eleotrochemicai degradation of said contaminants.

6. A process according to claim 1, substantially as hereinbefore described.