METHOD FOR IN-SITU IMMOBILISATION OF HEAVY METALS AND/OR SULPHATE IN WATER-CONTAINING ZONES

Abstract: The invention relates to a method for in-situ immobilisation of heavy metals and/or sulphate in water-containing zones by microbiological preparation of sulphide using sulphate-reducing bacteria, a substrate for the sulphate-reducing bacteria being introduced into the zone and the density of the substrate at the temperature prevailing in the zones differing by at least 10 % from the density of the water present in the zones.

Description: Method for in-situ immobilisation of heavy metals and/or sulphate in water-containing zones The invention relates to a method for in-situ immobilisation of heavy metals and/or sulphate in water-containing zones by microbiological preparation of sulphide using sulphate-reducing bacteria, an organic substrate for the sulphate-reducing bacteria being introduced into the zone. A method of this type is disclosed in US Patent 5 155 042, chromium being immobilised in calcium- and chromium-containing alkaline residual solids or residues which are produced on roasting chromium-containing ores and which are present underground. This is achieved by injecting a first aqueous solution into a location in the ground which is contaminated with said residues, the first solution having a pH of 6.5 to 9.5. A second aqueous solution which contains Cr(VI) and calcium in dissolved form and has a high salt content and a pH of 6.5 to 9.5 is then withdrawn ftom the ground in a different location, the rate at which the second solution is withdrawn being equal to the rate at which the first solution is injected into the ground. In a subsequent step the second solution is brought into contact with sulphate-reducing bacteria, which are able to withstand a high salt content, in the presence of a sulphate source, for a length of time such that essentially all the Cr(VI) is converted into insoluble Cr(III), the quantity of the sulphate source being such that said source supplies at least 10 millimole sulphate per litre to the second solution. The insoluble Cr(lII) is then separated off from the second solution, with the formation of a solution which contains active, sulphate-reducing anaerobic bacteria. In the final step acid is added to said latter solution until the solution has a pH of 6.5 to 9.5 and said solution is fed to the First solution for recycling into the ground, which leads to the in-situ reduction of CrfNI) to insoluble Cr(III), which is immobilised in the residual solids or residues. However, the method according to US Patent 5 155 042 has the disadvantage that it demands circulation of an aqueous stream and therefore is unfavourable from the process standpoint because maintaining such an aqueous stream requires a high pump capacity and therefore a great deal of energy. US Patent 5 263 795 relates to a method for in-situ treatment of groundwater and soil which are contaminated with metals, a sulphate- containing liquid being introduced into the contaminated zone. By this means sulphate-reducing bacteria are stimulated to produce sulphide, as a result of which the metals present in the soil and in groundwater are converted to non-toxic and stable metal sulphides. US Patent 5 587 079 relates to a biological method for the removal of sulphate and metal ions from solutions, sulphate-reducing bacteria being used. This method comprises feeding the solution to anaerobic bacteria which are able to convert sulphate ions into hydrogen sulphide and supplying gaseous nutrients, such as hydrogen and a carbon oxide, the metal ions being converted to metal sulphides. Furthermore, US Patent 4 519 912 discloses a method for reducing the solubility of water-soluble ionic selenium and sulphate compounds and water-soluble ionic compounds of heavy metals. According to this method an aqueous solution, which contains the selenium and sulphate compounds and the heavy metal compounds and which has a pH of about 6, is introduced into a matrix, such as soil, the matrix containing anaerobic bacteria. The bacteria comprise at least bacteria of the genus Clostridium and those which are chosen from the genera Desulfovibrio and Desulfotomac lum and these bacteria are capable of converting the ionic selenium compounds into metallic selenium and the sulphate into hydrogen sulphide. The solution is then fed at a certain temperature through the matrix in the presence of nutrients and an aqueous effluent is obtained which has a lower content of water-soluble ionic selenium and sulphate compounds and water-soluble ionic compounds of heavy metals than the original solution. The method according to US Patent 4 519 912 also has the disadvantage that an aqueous stream has to be maintained, in particular for a continuous process. European Patent Application 0 692 458 describes a method for the continuous treatment of liquid or solid waste or mud which contains sulphate and heavy metals. According to this method, in the case of -solid waste, water is first added to the waste in a quantity such that a salt content is obtained which is suitable for bacterial growth. A neutralisation step is then carried out using a strong acid, so that a pH of 6 to 7.5 is obtained. The waste stream is then brought into contact with a consortium of sulphate- reducing bacteria and lactobacilli in a biological reactor, whey being fed to the reactor as carbon source and as nitrogen source for the bacteria. However, the method according to the European Patent Application is not suitable for the treatment of large quantities of waste or contaminated soil because the treatment of the waste or the soil has to take place in a reactor and large-scale process control is therefore technically very difficult or even impossible and in any event requires a great deal of energy. Russian Patent Application 1 838 598 discloses a method for the treatment of contaminated groundwater, in which a mixture containing sulphate-reducing bacteria and, as substrate, a biopolymer in the form of plant residues is injected underground at the boundaries of the contaminated areas. The method is suitable for the removal of sulphate, nitrate and heavy metals. A disadvantage of this method is that bacteria have to be injected, which is not efficient when it is desired to treat large underground areas which are contaminated. This also applies to the substrate, which cannot be transported through the soil because it is not soluble. Furthermore, European Patent Application 0 436 254 relates to a method for the treatment of aqueous waste streams which contain sulphate and possibly heavy metals, said waste streams being brought into contact with sulphate-reducing bacteria in the presence of a small amount of alcohol as carbon and energy source and sulphide being formed from the sulphate and, if heavy metals are present in the waste stream, metal sulphides also being formed. Said treatment takes place in a reactor, so that large-scale process control is technically very difficult or even impossible and will require a great deal of energy. In general, locations in which the groundwater is contaminated with heavy metals and or sulphate are usually cleaned up by pumping up the contaminated groundwater and treating it above ground. This leads to relatively high costs for withdrawal and treatment oi Jie groundwater. Moreover, it is found that a method of this type virtually always takes longer than predicted because there is "non-linear" desorption behaviour and metals continue to be released from poorly flushed zones (for example clay lenses). It has been found that for locations contaminated with heavy metals a treatment period of ten years or more is not unusual. The duration of groundwater control (where groundwater is withdrawn only in a quantity such that no further dissemination of the contamination occurs) often varies from 100 years to "eternity". "Such long periods result in high costs and the large quantities of groundwater which have to be withdrawn constitute an assault on groundwater reserves, which are becoming increasingly scarcer. The present invention provides a solution to the above entioned problems. The method according to the invention is suitable for the treatment of large underground areas which are contaminated with heavy metals and/or sulphate, requires little energy and results in an effective and simple immobilisation of heavy metals and sulphate. The invention therefore relates to a method as specified in the preamble, wherein the density of the substrate at the temperature prevailing in the zones differs by at least 10 % from the density of the water present in the zones. Said difference in density is utilised to disperse the substrate in situ in the vertical direction in the contaminated zone(s). For horizontal dispersion, use is made of die natural flow of groundwater. In some cases the horizontal dispersion can, if desired, be accelerated by pumping up water and reinfiltrating. With regard to the literature discussed above it is pointed out that the densities of the flushed solutions and substrate solutions used will as a rule differ by no more than 1 % from the density of water present in the zones. At high salt concentrations, such as those which are used in the method according to US Patent 5 155 042, the density of the recycled liquid is about 5 % higher than the density of the water to be treated. However, differences in density are not used as transport mechanism. The use of very high substrate concentrations is not only effective for passive vertical dispersion, but also prevents clogging of infiltration means. A frequently occurring problem is that on infiltration of a substrate into the zone or soil to be treated the infiltration well becomes blocked because of superfluous growth of bacteria. This is now prevented because the substrate concentration at the location of the injection point is so high that no growth of bacteria can take place at the injection point. In this description the term "water-containing zones" is used to define water- containing pi/ous media, for example the saturated zone of a soil or deposits of waste substances in or on a soil, the pores in the waste substances being completely or partially filled with water. A further advantage of the present invention is that in the case of withdrawal and treatment of contaminated groundwater, in situ (underground) immobilisation of heavy metals is virtually always possible without this resulting in contamination of the zone or the soil (the solid phase). If, for example, the zinc concentration in the groundwater is 8 mg l (this is ten times the Dutch intervention level, which means that there is question of serious contamination) and this amount is immobilised, the content in the solid phase rises by about 2 mg/kg solids, which is only a slight rise with respect to the target value of 50 mg/kg solids. A further advantage of the invention is that the method can be combined with the degradation of organic impurities, in particular in those situations in which reducing conditions are favourable. An example of degradation of organic impurities which is suitable for combination with the method according to the invention is the reductive dechlorination of substances, such as of perchloroethene. The conditions for the method according to the present invention and, for example, in the case of water treatment differ appreciably. For instance, mixing of the reagents with water present in the zones is frequently incomplete, the residence time of the water is.of the order of weeks and years and it is possible for metals to be released later from the solid phase. It is therefore desirable to form a reservoir of available sulphides in the zones. According to the invention it is therefore preferable that more organic substrate is fed to die zones than is strictly required for immobilisation of the heavy metals and or sulphate currently present. Because the preparation of sulphide from sulphate requires 8 mole electrons per mole sulphate, according to the present invention a quantity of the organic substrate is preferably introduced into the zones such that, on a mole basis, at least a twofold excess of sulphide can be prepared therewith, with, respect to the quantity of mobile heavy metals present. The substrate can contain one or more sulphate compounds, preferably inorganic sulphate compounds, such as sodium sulphate or sulphuric acid. Furthermore a quantity of substrate such that the molar concentration of sulphate in the zones is at least twice as high as the molar concentration of heavy metals is preferably introduced into the zones. This has the advantage that a reservoir of sulphide is present in the zones, so that if further heavy metals are released thes can still be immobilised. The substrate can contain iron, manganese and/or calcium. It is known that sulphides of heavy metals such as mercury, copper, lead and cadmium are sparingly soluble in water, whereas the sulphides of the metals iron, manganese and calcium are more readily soluble. Therefore, a reservoir of iron, manganese and/or calcium sulphide present in the zones will drive the heavy metals, such as mercury, copper, lead and cadmium, present in the water, out of solution, iron, manganese and/or calcium going into solution and the heavy metals, such as mercury, copper, lead and cadmium, being immobilised in the form of sulphides in the zones. The substrate preferably contains iron, more preferably iron(II). According to the invention, the quantity of the substrate introduced into the zones is preferably such that the molar concentration of iron is higher than the molar concentration of heavy metals. The iron will then be precipitated as iron sulphide in the zone, so that if heavy metals are subsequently released these can still be immobilised. According to the invention, the iron is preferably iron sulphate and in particular iron(II) sulphate. If the zones to be treated contain few nutrients which are suitable for the sulphate- reducing bacteria, a substrate which contains these nutrients can be introduced into the zones. Preferably, the substrate then contains one or more ammonium and/or phosphate compounds. Suitable ammonium compounds are inorganic ammonium compounds, such as, for example, ammonium sulphate and ammonium phosphate. Suitable phosphate compounds are inorganic phosphate salts, such as, for example, ammonium phosphate and sodium phosphate. In particular, according to the invention the substrate preferably contains a mixture of ammonium phosphate and ammonium sulphate. It can be desirable or necessaiy for the pH of the zones or of the water present in the zones to be controlled such that said pH promotes the growth of the sulphate-reducing bacteria. The pH is preferably 5 to 9 and in particular 5.5 to 8. Therefore, the substrate can contain a base or an acid, preferably an inorganic base or an inorganic acid. A suitable inorganic base is sodium hydroxide. The inorganic acid is preferably sulphuric acid. The sulphate-reducing bacteria can belong to the genera Desulfovibrio, Desulfomonas, Desulfobulbus, Desulfococc s, Desulfobacter, Desulfosarcina, Desulfonema and/or Desulfotomac l m . One embodiment of the present invention is shown diagrammatically in Figures 1 and 2. Figure 1 is a side view of this embodiment and comprises a reservoir (11) for chemicals to be injected, a pump (12) and an injection well or drain (13). The reservoir is located at ground level (14). The chemicals a :■: fed via the drain into the zone, which is delimited by top layer (15). The direction of flow of the groundwater is indicated by arrows (16). Figure 2 is a top view of the area to be treated, which is delimited by boundary (21 ). A number of injection wells or drains (22) have been made in the area and arrow (23) indicates the direction of flow of the groundwater. The number of injection wells or drains depends on the size of the zone(s) or the terrain and the desired duration of the treatment. It is possible to position the wells only around the boundary of the contaminated zones ("bioscreen"). The duration is then determined by the rate at which the contaminated water moves. Mixing of water and chemicals takes place essentially via density flow of the substrate, the natural flow of the water and by dispersion. Treatment of the water is accelerated by installing more than one row of injection wells or drains. Another embodiment of the invention is shown in Figures 3 and 4. The side view of this embodiment according to Figure 3 comprises a reservoir (31) for chemicals to be injected, a metering pump (32), a extraction well (33), an infiltration well for the groundwater (34), an injection well for the substrate (39) and a suction pump (35). The reservoir is located at ground level (36) and the zone is delimited by top layer (37). Arrows (38) indicate die direction of flow of the groundwater. Figure 4 shows a top view, in which the contaminated area is delimited by boundary (41). In this area there are one or more reservoirs (42) with a metering pump, one or more suction pumps (43), extraction wells (44), infiltration wells for groundwater (45) and injection wells for substrate (46). Arrow (47) indicates the direction of flow of the groundwater. In the embodiment according to Figures 3 and 4, water is actively pumped up and then fed back into the zones. Substrate etering takes place separately, as a result of which the risk of the injection well becoming clogged is reduced. Sometimes it is possible to combine metering of chemicals with the infiltration of the water to be recycled. The number of embodiments needed depends on die extent of the contamination and on the size of die zones. As a result of the withdrawal of groundwater, flow through the contaminated zone is accelerated and the duration will be shorter flian in die case of the embodiment according to Figures 1 and 2. A preferred embodiment is shown, in Figures 5 and 6. The side view of this embodiment shown in Figure 5 comprises a reservoir (51 containing chemicals to be injected, a pump (52), a spray installation (53) and/or a horizontal drain (54). The reservoir is located at ground level (55). The zone is> delimited by top layer (56). Arrows (57) indicate the direction of flow of the substrate to be injected. Figure 6 shows a top view of this embodiment, the area being delimited by boundary (61), within which a number of spray installations (62) connected to one another have been installed. The range of each spray installation is indicated by circle (63). The reservoir (64), which is provided with a metering pump, is preferably located outside the contaminated area but, of course, the reservoir can optionally also be placed within the area. Instead of spray installations it is also possible to install, in the area, one or more horizontal drains (65) which are connected to one another, boundary (66) indicating die range of each drain. The direction of flow of the groundwater is indicated by arrow (67). Of course, care is taken to ensure that the ranges of either the spray installations or the drains overlap one another somewhat, so that the entire area is effectively treated. In this embodiment use is made of differences in density, the density of the substrate being at least 10 %, preferably 20 % and in particular 30 % higher than the density of the water. In this embodiment the substrate is, for example, a concentrated solution of a salt of an organic acid in water. Preferably, the salt of the organic acid is lactate or acetate and the solution preferably contains 3 to 6 mole of the salt of the organic acid per litre. The substrate is preferably introduced at or just below ground level by means of spraying or irrigation, or infiltration thereof is achieved in some other way. Infiltration can also take place via horizontal drains. A second preferred embodiment according to the invention is shown in Figures 7 and 8. Figure 7 shows a side view and Figure 8 a top view of this embodiment. In Figure 7 die embodiment comprises a reservoir (71) containing chemicals, a pump (72), an injection well or drain (73) and, optionally, a heat exchanger (74). The reservoir is located at ground level and the contaminated zone is indicated by boundaries (75). Arrows (76) indicate the direction of flow of the substrate to be injected- In Figure 8 one or more injection wells which are fed by means of one or more reservoirs and metering pumps are installed in an area (81). The range of the injection wells is also indicated, in which context it is preferably taken into account when siting die injection wells that the ranges overlap one another somewhat. Arrow (83) indicates the direction of flow of the groundwater. With this embodiment use is made of differences in density between die infiltration liquid and the water present in the zones, as a result of which upward flow is produced. In addition, the chemicals are disseminated by the natural flow of the water and by dispersion. The difference in density is produced by injecting a conci -trated substrate having a density which is preferably 10 % and in particular 20 % lower tiian the density of die water. Said substrate preferably contains one or more alkanols, in particular alkanols having 1 to 5 carbon atoms. Preferably, die alkanols are methanol or ethanol and in particular meuianoi. An advantage of a substrate containing methanol is good reaction kinetics. It has been found that sulphate reduction by methanol proceeds more slowly than with other substrates, in particular lactate- and ethanol-ccmtaining substrates, with the result that the residence time of methanol is longer. According to the invention, die substrate is advantageously a concentrated solution of methanol and in particular pure methanol (density 0.79 kg/dm 3 ). It is possible to heat the substrate prior to injection, as a result of which a difference in density is also obtained or the difference between the density of the substrate and that of the water is increased. A maximum difference in density is obtained by injection of gaseous substrates, such as methanol in die gas phase. As in the case of the other embodiments, the number of injection wells or horizontal injection drains is dependent on the size of the zones. According to the invention the second preferred embodiment, that is to say the embodiment which is shown in Figure 7, is the most preferred. The invention will be explained in more detail with the aid of a few examples. Example 1 Mixtures of soil and groundwater obtained from the Budel-Dorplein industrial estate were used. The soil was sampled under anoxic conditions at a depth of about 20 m below ground level. By means of enzymatic testing it was demonstrated that die soil contained sulphate-reducing bacteria in an amount of about 10* - 10 s cells per gram of soil. The soil was mixed in a ratio of 1:10 with contaminated groundwater and shaken. Only various organic substrates, specifically lactate, ethanol and methanol, were added to the mixture. Black coloration, the odour of H 2 S, a decrease in the sulphate concentration and a fall in the redox potential show that sulphate reduction gets underway (Table 1). Table 1 In this context it is pointed out that the redox potential is a criterion for the stability of the various sulphides. A list of d e solubilities of various metal sulphides is given in Table 2 [Dissociation constants of metal sulphides (25 °C) at pH 7, 10 '3 M sulphate and 0.0003 atm C0 2 (g) and values of die redox potential (E h ) at which metal sulphides are formed; .L. Lindsay, "Chemical equilibria in soils", J. Wiley & Sons (1979)]. Table 2 It can also be seen that the heavy metals cadmium and zinc are precipitated (see Figures 9 and 10, respectively; Figure 9 shows the change in the cadmium concentration in μg/1 as a function of time in days and Figure 10 shows the change in the zinc concentration in mg l as a function of time in days). Substrates a, b, c, and d are, successively, the blank, methanol, ethanol and lactate. It is clear that the rate at which the reaction proceeds differs per substi ;te, lactate giving d e highest and methanol the lowest rate. In particular, the process gets underway slowly with methanol. Nevertheless, good precipitation of the heavy metals is also obtained with methanol. This illustrates the high affinity of sulphide for heavy metals. Example 2 The samples and experimental set-up were the same as in Example 1. For this experiment ethanol was used as the substrate and phosphorus was also added. The results are summarised in Table 3 [Change in the redox potential (E h , in mV) and the sulphate, zinc, iron and cadmium concentrations (mg/l, Cd in g/1) after the addition of ethanol and phosphate to a mixture of soil and groundwater from the Budel Dorplein industrial estate]. It can be seen from this that not only cadmium and zinc but also iron precipitate. Initially the iron concentration rises, which is a consequence of reduction of iron(III) to iron(II). The selectivity of the sulphide precipitation can be seen from the fact that first cadmium and zinc are essentially removed and iron is removed only thereafter. Table 4.

Claims: Claims 1. Method for in-situ immobilisation of heavy metals and/or sulphate in water- containing zones by microbiological preparation of sulphide using sulphate-reducing bacteria, an organic substrate for the sulphate-reducing bacteria being introduced into the zone, characterised in that the density of the substrate at the temperature prevailing in the zones differs by at least 10 % from die density of d e water present in the zones. 2. Metfiod according to Claim 1, wherein a quantity of the organic substrate is introduced into the zones such that, on a mole basis, at least a two-fold excess of sulphide can be prepared therewith, with respect to the quantity of mobile heavy metals present. 3. Method according to Claim 1 or 2, wherein the substrate contains one or more sulphate compounds and a quantity of substrate is introduced into die zones such that the molar concentration of sulphate in die zones is at least twice as high as the molar concentration of mobile heavy metals. 4. Method according to one of die preceding claims, wherein die substrate contains iron and a quantity of substrate is introduced into the zones such mat the molar concentration of iron is higher dian the molar concentration of mobile heavy metals. 5. Method according to one of the preceding claims, wherein the substrate contains iron sulphate. 6. Method according to one of the preceding claims, wherein the substrate contains one or more ammonium and/or phosphate compounds. 7. Method according to one of die preceding claims, wherein the substrate contains a base or an acid. 8. Metiiod according to one of the preceding claims, wherein the density of the substrate is 10 % higher than the density of the water. 9. Method according to Claim 8, wherein the substrate is a solution of a salt of an organic acid in water. 10. Method according to Claim 8 or 9, wherein die substrate is introduced at or just below ground level by means of spraying or irrigation. 11. Method according to Claims 1-7, wherein d e density of the substrate is 10 % lower than the density of the water. 12. Method according to Claim 11, wherein the substrate contains one or more alkanols. 13. Method according to Claim 11 or 12, wherein die substrate is methanol. 14. Method according to one of Claims 11-13, wherein the substrate is introduced into or below the zones by injection. 15. Mediod according to one of d e preceding claims, wherein the substrate is introduced into the zones in one or more locations. 16. Method according to one of the preceding claims, wherein the water present in the zones is pumped up and reinfiltrated. 17. Method according to one of the preceding claims, wherein heat is supplied to the zones, for example by heating the groundwater to be reinfiltrated.

Inventors: ALPHENAAR PIETER ARNE, STEKETEE JACOB JACOBUS, ALPHENAAR, PIETER, ARNE, STEKETEE, JACOB, JACOBUS
Applicants: ALPHENAAR PIETER ARNE, BUDEL ZINK B V, STEKETEE JACOB JACOBUS, TAUW MILIEU BV, ALPHENAAR, PIETER, ARNE, BUDEL ZINK B.V., STEKETEE, JACOB, JACOBUS, TAUW MILIEU B.V.
Application year: 1998
Main classification: C02F1
IPC codes: A62D3, B09C1, C02F1, C02F3