DE19820320C2 - Process for raising the pH in waters with a high sulfate and iron content - Google Patents

Description

The invention relates to a method for raising the pH in water with high sulfate and iron content, especially opencast lakes.

The causes of a strong acidification of water can be diverse his. Open-cast mining lakes, which after the termination of mining activities due to the Fill natural groundwater rise with water, for example, often show has a pH of approximately 2, the cause of which is iron sulfide weathering. Since this weathering process in mining areas is very complex, only the basic geochemical reactions are described below.

The degradation and the associated aeration of deep-lying tertiary layers during lignite mining leads to the weathering, i.e. oxidation, of iron sulfides (pyrite (FeS ₂ ), marcasite). Sulfide sulfur (oxidation numbers: 0 and 2) is oxidized to sulfate sulfur (oxidation number: +6) and iron (II) to iron (III):

Since iron (III) is only sparingly soluble, it is usually used as Iron hydroxide or in a more chemically complex composition, e.g. B. in the form iron sulfates (coquimbit, melanterite) or iron hydroxosulfates (jarosite, Copiapit), failed.

When the weathered waste material is covered with further fillings and when the groundwater rises, anoxic, ie oxygen-depleted, conditions occur, so that oxygen is no longer available for pyrite weathering. In the previously ventilated tip areas, however, there is abundant iron (III) hydroxide, jarosite and other iron (III) compounds, which have arisen, among others, according to equation (I) and now act as oxidizing agents for further pyrite (FeS ₂ ) weathering . Here the iron (III) is reduced to iron (II):

FeS ₂ + 14Fe ³⁺ + 8H ₂ O → 15Fe ²⁺ + 2SO ₄ ^2- + 16H ⁺ (2)

Since iron (II) is soluble in much higher concentrations than iron (III), it contains the tipping groundwater mostly very high iron (II) contents, the concentrations of can reach up to several hundred milligrams per liter.

When the anoxic tipper groundwater passes into the residual lakes, which are aerated by limnological processes - for example during full circulation in spring and summer - or when the tipper groundwater escapes from the tipper embankments, iron (II) comes into contact with oxygen from the air or dissolved oxygen and is oxidized to iron (III):

which is then hydrolyzed:

Since iron (III) is only slightly soluble in oxidized surface water, finally iron (III) hydroxide precipitates in the residual lake as sediment. Thereby form rust-brown sediment layers and sometimes thin, floating iron hydroxide crusts.

The gross reaction of iron oxidation, iron hydrolysis and iron precipitation can be written as follows:

With the oxidation and precipitation of the iron falls for the previously anoxic Tipping groundwater away the most important acid buffer. The stoichiometric Equation shows that when the reaction is complete, an iron (II) ion is replaced by two Hydrogen ions is replaced. That is why iron oxidation, hydrolysis and -recipitation with a significant decrease in pH, d. H. an acidification, hand in hand.

Lime is often used to remediate sulphate and iron-rich, over-acidified waters in order to neutralize them. However, depending on the size of the water, very high amounts of lye, e.g. B. lime milk (suspension of hydrated lime Ca (OH) ₂ in lime water) into the water, which can also hardly be distributed evenly. It is also known to flood the open-pit lakes or to connect them to the natural receiving water, in order to achieve an increase in the pH value by mixing the acidic water with the largely neutral flood water. However, this is often complex or not possible in all cases due to local conditions.

It is an object of the present invention, a method of the aforementioned Kind in such a way that a sustainable, ecologically justifiable and enables cost-effective remediation of sulfate-rich acidotrophic waters becomes.

This object is achieved in the method of the type mentioned at the outset by  that compacts from carbon-containing substrate and / or with carbon-containing Substrate filled, preferably rotten, containers on the water or in floating a short distance below the surface of the water in the water be arranged within which anaerobic conditions occur, so that there are largely anaerobic zones for Desulfuri in the area of the substrate edges, especially sulfate-reducing bacteria.

The structure of the compacts from the carbon-containing substrate and / or the containers, preferably rotted, filled with the carbon-containing substrate is such that the compacts or containers do not dissolve over a longer period of time. Water can penetrate into the compacts or containers and come into contact with the carbon-containing substrate enclosed therein, so that bacterial desulfurization can occur with the result of a gradual increase in the pH. Within the compacts or containers, protons are accordingly taken from the water, which can not only diffuse into the compacts or containers, but also out of them, and according to the reaction for the formation of H described above using equation (9) ₂ O and H ₂ S used with the consequence already mentioned that there is a gradual increase in the pH.

The compacts and / or containers are according to the invention on the water or floating in the water at a short distance below the surface of the water arranged, for example, attached to floating lines and preferred be provided with floats. Leave the compacts or containers then hang up in a row on the floating line and then swim - similar to tea bags - in the water. Through the waves the mass transfer and thus the neutralizing effect is also increased.  

The advantages of the invention are in particular that the bacterial sulfide bil dung - the so-called bacterial desulfurization - is used to develop a medium-term d. H. neutralization of acidotrophic waters that can be carried out in a few months to reach. By creating anaerobic zones for desulfuricants according to the invention these bacteria can undergo an oxidative, anaerobic metabolism Operate the use of the carbon-containing substrate as an energy source. At This metabolism uses the sulfate in the water as a terminal electron acceptor used to oxidize the carbon. As a result, this leads to anaerobic Oxidation on the part of the bacteria through proton consumption leads to an increase in the pH as explained below.

Another advantage of the method according to the invention is that in Heavy metals dissolved in water are precipitated as almost insoluble sulfides. Since the solubility of iron (III) decreases with increasing pH, this sets in the course of the process, primarily on the water bed with binding from cloudy substances. This process of iron (III) deposition is also called Mud aging.

As long as iron is sufficiently dissolved in the water, the sulfur is determined by the desulfurants, especially the sulfate-reducing bacteria, in the form of monosulfide:

CH ₃ COOH + FeSO ₄ → 2CO ₂ + 2H ₂ O + FeS. (8th)

The monosulfide (FeS) is converted back to iron pyrite (FeS ₂ ) via a lengthy diagenesis. Furthermore, protons that are in the form of dissolved sulfuric acid H ₂ SO ₄ are consumed by the sulfate-reducing bacteria:

CH ₃ COOH + H ₂ SO ₄ → 2CO ₂ + 2H ₂ O + H ₂ S. (9)

Since the sulfate reduction by the bacteria is strictly oxygen-free, i.e. H. anaerobic Conditions required may not be solved in the first phase if possible Oxygen, nitrites and nitrates are also present. Because also an excessive Eutrophication of the water should be avoided, the brought in In addition to a high carbon content, substrate contains little nitrogen and phosphorus Have connections.

From DE 36 22 721 A1 a method is already known with which chem raising the pH of water is achieved; however treated this prior art denitrification of a nitrate-containing water not carbon-containing substrates are introduced into the water, but the nitrate-rich water with oxygen and / or oxygen-releasing agents is enriched and also not anaerobic treatment, but denitrification takes place under aerobic conditions. In contrast, the differs Invention of this prior art in that no denitrification, son desulfurization is used, the water is not acidic Enriched substance and / or oxygen-releasing agents, but carbon containing substrate is introduced into the water and this treatment is not under aerobic conditions, but instead takes place in such a way that Form substrates largely anaerobic zones for the desulfuricants.

Preferred developments of the method according to the invention are in the ab pending claims marked.  

Very different materials can be used as the carbon-containing substrate become. For example, straw, exhausted and chopped paper Labels from the beverage industry, molasses, spent grain, liquid manure, process water from silos or use dairies and mash. These materials can either individually or in combination and with the addition of clay marl, lime marl etc. be used. The substrates mentioned mainly fall into local ones Operated or on nearby agricultural land, so that long trans port routes are avoided. For their part, paper labels from the beverage industry are usually either disposed of or composted, but can be continue to use advantageously by means of the inventive method. Likewise can process water with carbon-containing residues, which in the Cleaning of silos or dairies with NaOH occurs, for example integrate the method according to the invention into a natural process and needs not to be disposed of.  

The compacts can preferably be paper labels, chopped straw and / or Show lime marl, all of which are relatively easy to compact.

Has the pH of the water due to the appropriate introduction of carbon-containing substrate reaches a value of approximately 3.5 - this takes time For example, a few months - the rotting containers, for example consist of jute, and / or the compacts preferably in the bank and embankment area spent and preferably planted with acidophilic reeds or similar. This procedure has the advantage that it is usually also overacidified Embankment area through which containers or compacts can be covered To keep out atmospheric oxygen. The containers / compacts spent on the bank area as well the plants prevent that after extensive neutralization of the Acidic water is washed away from the bank area and that Acidified waters again. Because the plant or reed belt is cyclical renewed, it provides a carbon source of energy in the form of dying Biomass available, which is deposited on the bank floor. This will form  further anaerobic zones within the plant belt, in which Desulfuri edges can settle. Another advantage is that the former heaped up embankment area before erosion and settlement phenomena is protected and therefore easier to recultivate.

It has proven advantageous to build acidophilic microalgae To inoculate primary production of plants and thus biomass into the water. These microalgae can develop in the early stages of remediation. H. still at Increase pH values below 4 and thus deliver biomass. The acidophiles Microalgae thus act as a starter culture for those who start at higher pH values Plant growth.

Through one or more of the measures described above - which may also apply can be repeated several times - the pH can be increased from 2 to, for example 4, 5 increase. From this pH, the natural primary production of Use aquatic plants that after dying in deeper areas of the Sinking water and the soil as a carbon source for desulfurants cover. The low oxygen content in these deep water layers promotes in addition, bacterial desulfurization in the anoxic biomass areas. Thus, the water to be rehabilitated gradually approaches the state of naturally occurring waters.

After neutralization, food chains are created that entail further acid-binding processes and thus support the sustainability of water restoration. These processes also include denitrification, which binds protons according to the following reaction equation:

Advantageous further developments are characterized in the subclaims.

The following is a method example of the invention with reference to the drawings explained in more detail. Show it:

Fig. 1 shows a cross section through a schematically illustrated therein waters with a floating containers arranged;

FIG. 2 shows a cross section through the water according to FIG. 1 with the containers of FIG. 1 that have been placed in the bank area;

3 shows a cross section through the body of water as shown in FIGS 1 and 2 again in a floating containers arranged..; and

Fig. 4 shows a cross section through the water according to FIGS. 1 to 3 in the rehabilitated final state.

In Fig. 1, a body of water 1 , for example a lake, is shown schematically. The body of water 1 contains high concentrations of iron oxide and sulfuric acid due to pyrite oxidation, which is the cause of a pH value of the body of water 1 of approximately 2. Plants can no longer grow in such acidified waters, so that no dead biomass is available for natural acidification processes. Such natural processes are, in particular in the case of sulfuric waters with relatively high sulfate and iron concentrations, anaerobic bacterial desulfurization processes which take place in and below the dead biomass layers. However, if there is no biomass on the water bed due to a lack of plant growth, the necessary anaerobic zones cannot form.

In the water 1 to be rehabilitated, pressed articles or containers 10 are introduced at a distance from one another, in which carbon-containing substrate is enclosed. The substrate consists, for example, of vacuumed and chopped paper labels from the beverage industry and / or a straw / lime marl mixture. The two free ends of a floating line 12 are attached to remotely located support rods 16 which are anchored in the water bottom 5 near the shore. Buoyancy bodies 14 are arranged at predetermined intervals along the floating line 12 , and a container 10 is fastened between each two adjacent buoyancy bodies 14 . Here, the container 10 swim a short distance below the water surface.

Anaerobic conditions arise over time in the containers 10 , in which desulfuricants carry out an oxidative, anaerobic metabolism using carbon, which originates from the carbon-containing substrate. This anaerobic oxidation causes the pH to increase over time (see equation (9)), the associated decreasing solubility contributing to the precipitation of heavy metals dissolved in water 1 . During this phase of the remediation, there is usually a severe clouding of the water 1 , which is due to the formation of iron oxides and sulfur compounds and the beginning of biological substance sales (see dotted area 9 in Fig. 1).

Since the containers 10 are arranged just below the surface of the water, they are moved back and forth by the waves (see double arrows in FIG. 1), so that the mass transfer between water 1 and containers 10 increases.

In a second phase of water restoration ( FIG. 2), the substrate-filled containers 10 , which consist, for example, of rotting jute, are brought in and brought to the bank and embankment area 3 of the water 1 . In addition, acidophilic reed plants 7 or other suitable plants are preferably planted on and between the containers 10 . In this way, on the one hand, the bank area 3 can be sustainably secured and, on the other hand, its re-acidification can be prevented, since anaerobic zones for desulfurizing agents form in and below the containers 10 . Likewise, 3 additional anaerobic zones are created in the bank area due to the deposition of carbon-containing biomass, which arises from the death of the acid-tolerant reed plants 7 that have been planted.

During this second phase, the amount of turbidity (iron oxides, sulfur compounds, first biomass products) lies on the bottom 5 of the water as a thin sediment 8 . In particular, with increasing pH, increasingly less soluble Fe (III) is deposited, to which turbidities also bind.

As (FIG. 1) are just placed below the water surface for further increasing the pH value in a third reorganization phase (Fig. 3) nochmalig container 10 with carbon containing substrate at a floating line 12 and floats 14 in the first phase. The reactions induced by the bacteria are essentially the same as those in the first phase, with the pH of the water now rising to over 4.5. This increase in pH activates an increased primary production of natural organisms. This is a prerequisite that later - ie after these plants have died - sufficient biomass can sink to the bottom 5 of the body of water. As indicated in Fig. 3, the biomass is deposited on the bottom 5 of the body of water as sediment 8 , which creates further anaerobic zones which support the desulfurization processes on the part of the bacteria. This is a prerequisite that natural water neutralization can take place without external intervention.

To promote the primary production of plants or other carbon-containing organisms, specially adapted acidophilic microalgae can be inoculated into the water 1 . These microalgae grow even at lower pH values, so that after they have died, they also provide biomass at these pH values, the existence of which is a prerequisite for a microbially caused increase in pH value and thus for increased plant growth.

In FIG. 4, the reconstructed final state of the body of water 1 is shown, incurred in the natural food chains. Since the pH is above 6, plants can grow and die naturally and thus generate enough carbon for the acid-binding bacterial processes of desulfurization. These processes of the fourth phase - like those of the first to third phases - take place within oxygen-depleted sediment layers 8 and in the bank area 3 . In addition, the neutralization is supported by denitrification processes, in which protons are bound and hydroxide ions are formed (see equation (10)).

Claims (7)

1. A method for raising the pH in water ( 1 ) with a high sulfate and iron content, in particular open-cast lakes, characterized in that compacts made of carbon-containing substrate and / or with carbon-containing substrate filled, preferably rotten, container ( 10 ) on the Waters or at a short distance below the surface of the water are arranged to float in the water, within which anaerobic conditions occur, so that largely anaerobic zones for desulfurizing agents, in particular sulfate-reducing bacteria, form in the area of the substrate. 2. The method according to claim 1, characterized in that straw, exhausted and chopped as substrate Paper labels from the beverage industry, molasses, spent grain, liquid manure, process water from silos or dairies, mash and / or other substrates with high coal substance content are used. 3. The method according to claim 2, characterized in that the one or more substrates with clay marl or lime marl be transferred. 4. The method according to at least one of the preceding claims, characterized in that the compacts paper labels, chopped straw and / or lime marl.   5. The method according to at least one of the preceding claims, characterized in that the compacts or containers ( 10 ) are attached to floating lines ( 12 ) and are preferably provided with buoyancy bodies ( 14 ). 6. The method according to at least one of the preceding claims, characterized in that the container ( 10 ) and / or the compacts after contacting with the water ( 1 ) in the bank and embankment area ( 3 ) and optionally reed plants ( 7 ) be planted in this area ( 3 ) to create anaerobic buffer zones. 7. The method according to at least one of the preceding claims, characterized in that acidophilic microalgae as a starter culture for production be inoculated by carbonaceous biomass.