The Authors are research scientists at Geochem Research BV in De Bilt, the Netherlands. This organisation has over 15 years experience in environmental management, and applies their knowledge of geochemistry and mineralogy to find cost-effective solutions for environmental problems. A selection of projects comprises: Treatment of Jarosite waste from the Zinc industry; Removal of Phosphate from manure by means of Struvite precipitation; Removal of Heavy Metals (Cu, Zn, As,...) and neutralization of Acid Mine Water by means of sorption on pelletized Red Mud filters and; Neutralization of AMW with Olivine.
Geochemical engineering is a rather new discipline in environmental management. It recently evolved from geochemistry, a fundamental science concerned with the chemistry of the earth. Geochemical engineers study the properties of minerals, soils, rocks, waters and natural chemical processes, in order to find cost-effective solutions for environmental problems. Although geochemical engineers use the same instruments as chemical engineers and also perform tests and experiments in the laboratory, it is mainly nature that inspires them, when conceiving innovative environmental technologies.
Geochemical engineers realize that any harmful substance produced by industry will eventually enter the environment and become part of the geochemical cycle. In nature, numerous substances are found that have the same toxic properties as anthropogenic pollutants and yet these often pose no serious problem to the health of human beings and life in general. This is so, because these substances occur in low concentrations or, if they are highly concentrated, they are not mobile and not available to organisms.
We do not yet fully understand the precise mechanisms that control the distribution of toxic substances in nature. Too often government and environmental protection agencies impose environmental legislation and use environmental management technologies, that are not very effective in the best case, or even dangerous in the worst cases. Often this is a consequence of our lack of understanding concerning the natural chemical processes that characterize the geochemical cycle. As an example of the importance of geochemical investigation and the problems that may arise if one does not take the natural chemical processes into consideration, we present a case that occurred recently in the small town of Baarlo (The Netherlands).
Several decades ago, the chemical industry used to dump its toxic waste in the ground and simply cover it up. As the population density rose in the Netherlands, municipalities claimed the same land for habitation. After a while, however, the new inhabitants started to complain about strange smells and curious diseases. Billions of dollars had to be spent to clean up these toxic sites. One can imagine the panic that arose when the municipality of Baarlo discovered concentrations of the very toxic arsenic on their land, which were in the order of 1400 mg/kg, about 30 times as high as the sanitation norm of 50 mg/kg imposed by the government. First it was suggested that farmers had illegally dumped large quantities of arsenic-bearing fungicides in the past, and preparations were made to remove this toxic waste at great cost. Fortunately, however, the municipality was wise enough to ask also the advice from a geochemist. A site investigation showed that the high arsenic values occurred in an iron-rich soil horizon, and were due to natural causes.
At this site, reducing groundwater with perfectly normal iron and arsenic concentrations, a few mg and microgram per liter, respectively, rises to the surface and is oxidized in contact with the air. The arsenic becomes insoluble and is rendered completely harmless as it is immobilized in a stable FeAsO4 complex. Removal of the soil would be of no use, because a new layer with high arsenic concentration would have started to form immediately afterwards, as the flow of groundwater could not be stopped. The town council of Baarlo wisely decided not to remediate the site, and to proceed with the expansion of the town, notwithstanding the fact that the arsenic content of the soil was above the legally admissible norm.
A good understanding of the geochemical environment is important, not only for the distinction between natural and anthropogenic pollution, but also for the recognition of natural processes that may serve as an example for the development of innovative environmental technologies. For instance, one may consider the acid lake at Armyansk (Crimea, Ukraine), where waste sulfuric acid is discharged in a ponded shallow bay. Due to equilibrium between the volume of water evaporated in a dry climate and volume of acid discharged, acidity increases continuously. However, no acid leaks to the groundwater as it reacts with carbonate in underlying clays. As a consequence of this reaction, an impermeable hardpan of gypsum and iron hydroxide is formed. gypsum has a molar volume that is twice as large as that of the dissolved carbonate, and thus the pores of the clay are effectively sealed. This observation of a 'natural' self-sealing process gave rise to the development of new types of liners for landfills. Additionally, it inspired the development of a method to neutralize waste sulfuric acid and raise the earth surface artificially, by means of sulfuric acid injection into subsurface limestone reservoirs. During the injection of several thousand liters of sulfuric acid in a field experiment, it was shown that the newly formed gypsum is able to expand the subsurface limestone reservoir, and to lift the earth surface above. Simultaneously, toxic metals can be incorporated in the gypsum, or precipitated as hydroxides at the reaction front where the pH steeply rises. Likewise, polluted Acid Mine Drainage, that is sulfuric acid with high concentrations of metals which is produced during the oxidation of metal sulfide ore, can be injected into subsurface limestone. The principle of rock expansion can also be used in the prevention of the seepage of AMD from mine shafts and galleries, simply by filling these with crushed limestone which will form an expanding and self-sealing plug on reaction with AMD.
In this contribution we shall elucidate the principles of geochemical engineering with the help of several other examples that have a worldwide application. Most of these examples concern geochemical engineering methods, that have been conceived by emeritus Prof. Dr. R.D. Schuiling, and that are developed in cooperation with his colleagues at the Utrecht University (Utrecht, The Netherlands), at the International Institute for Hydraulic and Environmental Engineering (Delft, The Netherlands)(1), and at Geochem Research B.V. (De Bilt, The Netherlands)(2). Part of this text has been published before and the examples and related subjects have been described more elaborately elsewhere (Vriend & Zijlstra, 1998)(3).
Environmental problems are encountered if the concentrations of mobile substances, available to organisms, are either too low or too high. Many elements play an essential role in the physiology of plants, animals and men. Apart from the major elements H, C, N, O, P, S, Ca and Fe, these include Li, B, F, Na, Mg, Cl, K, V, Mn, Co, Ni, Cu, Zn, Se, I, Mo and probably several others. Most organisms are adapted to the range of concentrations commonly encountered in nature. Geo-diversity is the natural counterpart of bio-diversity and one may consider it an important condition for and cause of bio-diversity. At the extreme ends of the concentration range we find environments that are lethal to many organisms, but in which certain organisms find their natural niche. The occurrence of extreme concentrations of hazardous components in certain parts of our environment has not exclusively originated by the activities of men, but is as old as the earth. Poisonous gases associated with volcanic eruptions, sulfuric acid from volcanoes or from the oxidation of sulfides, high concentrations of heavy metals in ore deposits and toxic levels of fluorine or arsenic in ground waters are all generated by common geologic processes. Many of the concentrations forming in nature would be considered as environmental hazards if originating from human activities.
Even so, there are many sites that are polluted by human activities to such a degree, that they constitute a real threat to the local ecosystem. If we cannot prevent anthropogenic pollution, remediation measures have to be taken to restore the natural conditions. These measures should mainly be concerned with ways to reduce pollutant levels in the bio-available, mobile phase, and they fall under the following five categories:
By studying the ways in which these processes take place in nature, we can learn how to devise efficient, inexpensive and environmentally safe technologies.
A commonly discussed example of natural breakdown is microbially mediated breakdown of organic components and reduction and oxidation reactions e.g., nitrification/denitrification, sulfate reduction and sulfide oxidation. Apart from biologically mediated breakdown, chemical weathering plays also an important role, which is essentially the neutralization of acids by minerals. Weathering of silicate rocks produces clay-type residues and cations in solution. Weathering of carbonate rocks is by congruent solution.
An interesting example of a geochemical engineering method, that uses a weathering process for the breakdown of a harmful industrial waste acid, is the process of neutralization by the mineral olivine. Sulfuric acid is produced by many chemical industries and also forms during the oxidation of metal sulfides produced by the mining industry. Olivine is a cheap magnesium nesosilicate of which the individual SiO4 groups are not linked by strong oxygen bonds, and therefore it weathers rather easily. Contrary to the neutralization with carbonate, neutralization with olivine does not produce undesirable carbon dioxide gas and contaminated gypsum. Additionally, it yields commercially interesting products, such as silica for structural filling of rubber and concrete, silica for catalysts, iron oxidized and precipitated as magnetite or hematite for the preparation of dense liquids or pigments, and magnesium sulfate for the paper industry.
(Mg,Fe)SiO4 + 2H2SO4 > Mg2+ + Fe2+ + 2SO42- + H4SiO4
With finely ground olivine, the reaction is essentially complete within several hours if carried out in a stirred reactor at temperatures between 70 and 100 degrees Celsius.
A similar approach is investigated to sequester CO2 from power generation by reaction with ground olivine, as a means to combat the greenhouse effect. Because carbonic acid is a weak acid and the reaction must go rapidly to completion, the reaction is carried out in an autoclave under pressure and at temperatures well above 100oC. It is expected that by this process of enhanced weathering large volumes of carbon dioxide can be fixed into environmentally friendly materials like magnesium carbonate.
The most extreme examples of concentration are ore deposits. They have in common that their metals were once dispersed in large volumes, from which they were mobilized in solution, and subsequently concentrated locally. Concentration often occurs at geological discontinuities, which act as a geochemical trap, whenever there is a large gradient of pH, Eh, rock composition, temperature or pressure.
Geochemical engineers have devised several methods that use the principle of concentration, in order to decrease the quantity of a harmful compound in solution. One of them concerns the removal of fluorine from drinking water, which locally occurs in calcium-poor ground waters at concentrations well above the WHO norm of 1.5 ppm. An excess of fluorine can cause deformation of bones and, in its final stages, this fluorosis may lead to immobilization of a patient.
It is estimated that in the order of 80 million people suffer from fluorosis worldwide. Countries with a high incidence of fluorosis include India, Pakistan, Ethiopia, Kenya, Tanzania, Senegal and NE-Brazil, but signs of it have also been found in a.o. the Ukraine and in Gaza. Fluorosis is particularly widespread in poor rural communities, where people depend for their drinking water almost exclusively on the local groundwater. Methods for fluorine removal must be cheap and should not demand technological skills or daily supervision.
It appears that the fluorine can be easily removed from ground water if the calcium concentration is raised and fluorite (CaF2) can precipitate. The concentration of fluorine in the mineral fluorite is achieved by passing water over a bed of gypsum grains. Gypsum has sufficient solubility and the following reaction takes place:
CaSO4.2H2O + 2F- > CaF2 + SO42- + 2H2O
Although, in this way, the fluorine concentration can be lowered to about 3 ppm, this is still twice the WHO norm. Nevertheless, as ground waters often carry fluorine in excess of 3 ppm, the application of this cheap method can significantly reduce the incidence or the severity of fluorosis.
Quite a different method of concentration concerns the capture of water by hygroscopic salts. It was observed that patches of the desert become wet shortly after sunrise and dry up later in the day. Close inspection showed that these patches are characterized by high concentrations of calcium chloride in the pores of the sand close to the desert surface. It appears that during the night, as the air gets cooler and the relative humidity increases, the CaCl2 salt attracts water from the humid air and forms a hydrate. At sunrise, as the temperature rises again, the salt-hydrates dissociate and dissolve in their own water of crystallization. The water evaporates during the day and anhydrous salt remains, after which the cycle is repeated the next day. The principle of this natural process has been used to devise a geochemical engineering method that allows the concentration of water from humid air in the desert, with the help of a small evaporator/condenser system, driven by solar energy.
Finally, we like to discuss a concentration method that is also based on the observation of a natural geochemical process. On New Year´s eve of 1979, a cinema burned down in the center of Amsterdam. The Town Archeological Service used the opportunity to excavate the remains of 13th and 14th century buildings and found among various archeological objects, strange, large, brownish colored crystals. These were determined as the mineral struvite, an ammonium magnesium phosphate (NH4MgPO4.6H2O). It was concluded that this struvite was a mineralization from urine and feces, and that the same process of mineralization could be used to tackle part of the manure problem in the Netherlands, which is caused by intensive farming.
In some areas, the produced manure can no longer be discharged because of the high phosphate concentration, while in other areas phosphate has to be added as a fertilizer. Consequently, it was decided to build a small pilot plant to remove the phosphate by means of K-struvite precipitation, and use the K-struvite as a slow-releasing fertilizer. The process is simple and only demands the addition of MgO slurry to denitrified, potassium-rich calf´s manure, while stirring. The reaction is as follows:
K+ + MgO + H2PO4- + 5H2O > KMgPO4.6H2O
As can be seen from this reaction, the addition of MgO serves two purposes. It provides the required magnesium, and it shifts the pH to alkaline conditions, thereby converting the phosphoric acid to supersaturated phosphate. The method operates full-scale in a very satisfactory manner in a calf manure treatment plant at Putten since 1998.
Struvite had already been successfully tested as a fertilizer in the USA and Germany several decades ago, but its production from commercially available chemicals appeared to be too expensive. This demonstrates that the use of geochemical engineering to design an environmental technology has its economic advantages. Besides, the proposed recovery of phosphate from waste streams contributes to a decrease of the un-natural import of nutrients, and it preserves sedimentary phosphorus ores, avoiding the release of associated toxic compounds such as fluorine, cadmium and uranium, commonly contained in sedimentary phosphate ores.
Dilution in geology is ubiquitous. High concentrations of elements in rocks decrease by dispersion due to weathering and erosion. In solutions, the dispersion occurs by mixing with other solutions, and mixing with the atmosphere dilutes gases. Dilution is often considered an improper environmental technology: 'dilution is no solution to pollution'. However, dilution is a natural process at the dynamic earth surface and even deep subsurface isolation can only delay the inevitable dilution. If the pollutant is not a persistent compound that accumulates in the food chain, dilution is a viable option as long as it occurs in such a way, that at no time the concentration of the pollutant in the mobile phase exceeds certain critical limits.
Considering again the problem of the accumulation of manure with a high phosphate content, it should be noted that the spreading of manure over farmland is a nice example of an old and perfectly sound method of dilution. Only when certain limits are surpassed, an environmental problem is created. As proper dilution is considered acceptable, an interesting geochemical engineering method can be conceived. Most of the ocean is known as biological desert, because there is a deficit of essential nutrients, in particular phosphate. If the ocean is fertilized with phosphate, then algae start to grow and biomass is increased. Not only will this method of dilution increase the amount of fish, but it also contributes to a decrease of the carbon-dioxide content of the atmosphere, and to a reduction of the greenhouse effect. Living matter in the ocean contains carbon, nitrogen and phosphorus in a particular ratio, the so-called Redfield ratio (C:N:P = 106:16:1). For each atom of phosphorus added to the ocean and incorporated by algae, 106 atoms of carbon are removed from seawater and consequently also 106 molecules of carbon dioxide are removed from the atmosphere. In Norway this method is used to increase fish production, while in the same country a penalty of US$50 has to be paid for each tonne of CO2 produced by means of combustion. The handling of manure on land costs around US$20 per cubic meter, The dilution at sea, at the other hand, will cost only a few US$ per cubic meter, if after digestion of the manure only a P-rich concentrate is transported to sea.
As a last example of dilution, we consider the problem of fly ash that is generated during the combustion of coal. This fine-grained material may contain elevated concentrations of toxic metals and is often diluted by mixing with Portland cement. However, large amounts remain to be disposed in landfills. This is a poor form of isolation and it is suggested to dilute the fly ash by means of mixing with fertilizers. The fly ash breaks down in the soil like during the weathering of volcanic ash. Natural clay minerals, carbonates and moderate amounts of trace elements remain, which are micronutrients that enhance the productivity of soils.
These examples show that dilution is a proper method, in order to handle certain environmental problems efficiently and cost-effectively.
Common examples of isolation in nature are trapped oil and gas in permeable reservoirs, which are isolated by impermeable seals of evaporites or clays. Probably the clearest example of a waste that has to be isolated is the radioactive waste of nuclear reactors. Part of this waste can be reprocessed. This requires transport through the public sector, as well as rather complicated remote handling and reprocessing technology, including the dissolution of the material in hot nitric acid. The remaining material has still to be disposed of. As a first step, soluble nitrates and oxides are converted to a stable form, usually a glass. However, preferably, this should be a sintered ceramic product that is stable over geologic periods of time. The ceramic product is best packed in canisters of copper, titanium or Ni3Fe, which can be safely stored in the subsurface, with a minimal risk of rapid corrosion.
The storage of nuclear waste in the subsurface requires considerable knowledge of the geochemical conditions and it offers an opportunity for geochemical engineers to contribute to a proper method of disposal. If we exclude the dumping on the seabed of the abyssal plain, at great water depth, various options remain on land. Relatively safe environments can be found in non-fractured igneous rocks of stable shields and in thick non-fractured clay layers. Even though disposal in less favorable geological repositories may constitute a transnational hazard, countries are not allowed to export their nuclear waste. Therefore in some countries, even less stable environments, such as deep mine galleries in salt diapirs, have to be considered. This latter option shall be discussed, as it is the best investigated choice for the Netherlands. The Dutch government requires that the waste is retrievable, in case problems arise with the disposal method, or when better disposal methods come available. However, it is doubtful whether the ability of retrieval contributes to the long-term safety of the disposal, as the isolation is less optimal if access routes for retrieval must be left open.
The canisters will probably be placed in vertical holes, spaced along a horizontal gallery, and the hole will be back filled with crushed salt. Rock salt flows fairly easily, and so the walls of the holes will converge. Risk analysis carried out for salt diapir disposal shows that the major geological risk is from the intrusion of brines from the bottom of the disposal shafts, and their subsequent escape to the surface. It may be worthwhile to add the swelling clay mineral bentonite as an additional immobilization step at very low cost, although there is some concern about radiolysis of the water contained in the bentonite. On the other hand, clays, as can be concluded from (geo-) experimental evidence effectively adsorb fission products. It was observed that clays surrounding 2 billion-year-old natural nuclear reactors in uranium-rich deposits at Oklo, Gabon (Africa) have largely immobilized these fission products.
In order to further diminish the risk of the migration of radioactive brines towards the surface, it is proposed to fill the remainder of the disposal holes with dry CaCl2, instead of crushed rock salt. Calcium chloride is highly hygroscopic; thus any water moving towards the canisters will be taken up, thereby converting the calcium chloride to CaCl2.6H2O. Additionally, the hydration of calcium chloride causes sealing of fluid pathways, due to a volume increase of 148%. If the water flow becomes excessive, then CaCl2 will dissolve and form very dense brines that do not rise to the surface, but that sink.
Immobilization is probably the most widespread mechanism in nature by which the risks to life of high concentrations of potentially hazardous substances are reduced. Immobilization can take the form of precipitation of an insoluble mineral, capture of an element in the lattice of insoluble minerals, or its adsorption on clays or zeolites. Physical forms of immobilization are the cementation of rocks, their recrystallization to a dense rock with low permeability, or in unusual cases the transformation into a glass. In these last cases immobilization is achieved because solutions can no longer enter and leach the rock under consideration.
Immobilization as an environmental option has met with considerable opposition from environmentalists and environmental agencies. The main reason is that the pollutant is still present after immobilization, and it is feared that it will be set free again when there is a change in conditions at the disposal site. It is formally true that the pollutant is still present, and that it will be set free eventually over a long period of time, as no substance is perfectly insoluble. Immobilization does serve, however, the ultimate goal of environmental action: it guarantees that the concentrations of the pollutants in the mobile phase will not exceed certain safety limits, and that detoxification will proceed in a slow and orderly manner.
Recently, a promising waste to waste technology has been introduced, which uses immobilization by means of adsorption, in order to purify waste water or soil pore water, contaminated by heavy metals. Fine-grained, caustic aluminum- and iron-oxide waste sludge (red mud), produced during the refinement of bauxite earth for the aluminum industry, was neutralized and transformed into a powerful adsorbent by means of treatment with sea water. This material (Bauxsol) has been successfully used by Virotec (Australia), in order to clean millions of cubic meters of acid mine water from tailing ponds, with high metal concentrations, to drinking water standards, within just a few weeks. The same material has been transformed into cemented granules by Geochem Research (The Netherlands), and is currently tested for the use in filters and permeable drainage dams around tailing ponds, in order to serve for cost-effective, passive water purification.
Another interesting new concept that illustrates the principles of immobilization as a proper environmental technology, is the storage of hazardous waste in what we call an 'environmental pyramid'. In the Netherlands, hazardous industrial waste and harbor sludge, which cannot be treated properly, are disposed in isolated landfills, with the intention to separate this waste from the environment for eternity. As this option is not realistic, such waste dumps will eventually start to leak pollutants to the ground water. Therefore, we have proposed to immobilize the waste by means of cementation into blocks, which are then piled up in the form of a pyramid, resting on an impermeable foundation, which is surrounded by a ditch. The stable form of the pyramid guarantees its sustainability, and only its exterior is exposed to rainwater during periods of precipitation. Hazardous pollutants will be released very slowly and are fed in low concentrations to rain water that runs off into the ditch. This method confirms the notion that pollutants may be released into the environment, if this occurs gradually and with concentrations in the mobile phase that do not exceed critical values.
The different methods that have been discussed above and that are used by geochemical engineers to handle waste in a more cost-effective, natural way, can also be combined. As an example we present a combination of breakdown, concentration and immobilization that is used to recycle waste. In the Nikopol area of the Dnjepropetrovsk District, Ukraine, manganese ore is mined at Marganetz and Ordzonikidze. After ore dressing, the physical treatment of sieving, gravitational separation, etc., a large volume of tailings is still left, which contains no less than 13% manganese. With current technology this manganese cannot be recovered economically, and it is therefore discharged into large tailing ponds, which contain already 165 million tons of dry weight, in the Nikopol district alone. The amount of manganese in these tailings is equivalent to 2-3 times the world production! Manganese is highly insoluble under oxidizing conditions, but under reducing and moderately acid conditions, manganese dissolves readily and significant concentrations in solution can be reached.
The rapid breakdown of insoluble manganese oxide at elevated temperatures of 100-200oC is achieved with sulfuric waste acid and a reducing agent. Concentration of the reduced manganese in the solution can reach values as high as 30 g/l. The pH of the concentrated fluid is raised and the fluid is aerated to oxidize the manganese again. The manganese then precipitates and this immobilization step does yield a commercially attractive manganese hydroxide concentrate.
A major concern in the design of any technological process is the rate at which it proceeds. Slow reactions require large reactors to attain a certain production. In environmental technology this can also be problematic, as public pressure demands the fast clean up of a polluted site. Geochemical processes may be very slow and this disadvantage has to be compensated. There are essentially two ways to handle this problem. The first is to speed up the reaction rate by increasing the temperature, increasing the reactant surface by grinding, increasing the strength of solutions and/or adding a catalyst. The second approach is to accept the slow reaction and reduce the costs of the environmental technology. Nature provides its own reactor and pollution is treated in situ. Space and time constraints become less severe and personnel costs are limited, as the process is a self-remediation, requiring only minimal supervision and monitoring.
Experience with current environmental technologies shows that, while the applied technology itself may be fast, unexpected side effects may require additional costly and time-consuming measures. Only a fraction of the technologies that are effective on a laboratory scale, perform equally well under field conditions. These problems are often related to the fact that the applied process turns out to be incompatible with the inherent properties and local conditions of the treated natural system, a disadvantage that may be overcome by adhering to geochemical engineering principles.
Geochemical processes act on many different scales, from the surface of a mineral to global processes affecting the whole hydrosphere or atmosphere. In order to device an efficient strategy to combat environmental pollution on any spatial and temporal scale, it is necessary to understand the geochemical cycle, and to design methods that fit into the natural cycle.
The fate of polluting elements is defined on the scale of atoms and mineral surfaces. At the level of the individual minerals, ways to remove a compound from solution are precipitation, adsorption, co-precipitation, or isomorphic substitution. Examples of these are the precipitation of phosphate as struvite, the adsorption of ammonia from waste waters by zeolites, the co-precipitation of arsenate with ferrihydroxides, or the isomorphic substitution of nickel in the crystal lattice of magnetite.
The importance of mineralogy is demonstrated by various cases. This starts with recognition of the sources and sinks of the pollutants. The source of lead pollution in soil, for instance, shows up through the mineralogy of the lead compounds. Small spheres of native lead are from hunting, lead sulfides (galena) point to pollution by lead ore and white flakes of lead oxide are produced by the paint industry. Also the toxicity of certain compounds is dependent on the mineralogy. As an example one may consider asbestos that has been widely used for fire prevention. Nowadays, billions of dollars are spent to remove asbestos from buildings, because asbestos is thought to be carcinogenic. However, although some varieties of asbestos, like crocidolite and amosite are carcinogenic, the most common one, chrysotile, is not. The hazard of these fibers is also a function of their concentration, because even in the natural environment, low concentrations of fibrous minerals released by the weathering and erosion of asbestos-bearing rocks occur everywhere. These low concentrations are inhaled and apparently handled effectively by organisms.
At the scale of polluted sites, geochemical information on the local hydro-geology, hydrochemistry, and properties of the underlying rock may help to design the best strategy of remediation for that particular environment, and to prevent unwanted side effects. A case in point is the acid lake at Armyansk (Crimea, Ukraine), where waste sulfuric acid is discharged in a ponded shallow bay, and that has been discussed already in the introduction.
Apparently, geochemical information on the local rock formations and pore waters may help us to produce self-sealing liners or plugs, and to break down harmful chemicals and immobilize pollutants. As every man-made isolation will give way in a, geologically speaking, short period, such 'natural' self-sealing isolations are much cheaper and safer. It is possible to engineer self-sealing liners by the juxtaposition of two reacting waste types. As an example, a self-sealing layer forms within days if alkaline coal fly ash is brought in contact with acidic jarosite waste from the zinc industry.
This draws attention to another option that may greatly improve the environmental management of hazardous waste. This is the co-deposition of two or more types of hazardous waste that can react to a harmless substance. Although co-disposal is often prohibited as it is considered equal to indiscriminate dumping, it should certainly not be ruled out as a viable option in environmental management. As an example, one may consider the co-disposal of iron or manganese (hydr) oxides and toxic organic sludge. Bacteria that use the oxygen from the metal (hydr) oxides can break down the organics into harmless compounds. Also the co-disposal of organic sludge with soluble and toxic metal hydroxides may be a proper method, as reducing conditions are generated that lead to the immobilization of the metals as sulfides.
Concluding, a proper understanding of climatic conditions and the geodynamics of the environment are of importance, in order to identify proper sites for waste disposal, and to take appropriate measures to prevent hazardous conditions and catastrophic accidents.
Geochemical engineering also finds its application on a regional scale, where small-scale environmental remediation technologies are no longer applicable. Diffuse pollution sources, like fertilizers, require measures to diminish the production of pollutants or counteract their effects. Attention should go to the discrimination between natural concentrations of polluting substances and anthropogenically elevated concentrations of these substances. Often, it is observed that particular plants show a certain tolerance to toxic compounds or even accumulate these in their organic tissue. Identification of the geochemistry of soils and of the proper accumulator plants allows the use of effective phyto-remediation technologies. Accumulator plants can be harvested and incinerated, which serves to concentrate pollutants in manageable volumes and, additionally, provides a sustainable energy source. Although phyto-remediation is strictly spoken a biochemical engineering method, it is clear that plants are part of the geochemical environment.
Another example of the importance of geochemistry on a regional scale is the emission of carcinogenic radon gas from uranium-rich geologic formations or from sand deposits with high concentrations of radioactive heavy minerals, e.g. zircon or monazite. Where these conditions have been properly defined and related to elevated cancer incidence, simple measures are often sufficient to remediate the problem. Often it suffices to ventilate homes or retard the radon flow, because radon decays in a relatively short period and its concentration is halved each 3.8 days.
Certain environmental problems have global dimensions. The most obvious are the warming of the earth by increased carbon-dioxide emission (greenhouse effect), the depletion of ozone, acid rain, desertification, deforestation, erosion, and the exhaustion of non-renewable resources. The underlying cause to these problems is the population pressure and the increase in standard of living, both leading to higher consumption and a larger waste production.
As an example of the complexities encountered, in case remediation of pollution on a very large scale is required, we like to discuss the problem of acid rain. The burning of fossil fuels of poor quality and with a high concentration of sulfur leads to an atmosphere that is enriched in sulfur oxides, which form acids with rain water. Acid rain leaches aluminium from aluminosilicates in soils and leads to the poisoning of forests and aquatic life in lakes. New power plants with costly but effective installations to de-sulfurize emissions have been introduced in the developed world, but it is observed that the problem of acid rain shifts towards the developing world.
Besides the prevention of acid rain, remediation of forest soil may be necessary. There is a fierce discussion going on, whether existing forests which suffer from the effects of acid rain should be provided with missing nutrients and pH buffers. Proponents point out that several essential nutrients are in short supply due to leaching, and that such deficits should be counterbalanced in order to restore the health of the forest. Opponents to restoration argue that as a consequence of acid rain, the litter layer in a forest has increased in thickness, due to the fact that bacterial and fungal degradation has slowed down under these circumstances. The litter layer has stored a large amount of nitrates, sulfates and heavy metals. These will be set free again once a healthy soil life is restored and the sudden release of pollutants will find its way to the groundwater, causing widespread pollution. Another group of opponents against interference is formed by 'conservative' environmentalists, who consider a dying forest as a distress signal, which should stay as a permanent warning against the dire consequences of our industrial activities.
However, it has been pointed out that a proper understanding of the natural environment allows us to apply natural geochemical engineering methods and that sound remediation techniques can be developed. For instance, there is the possibility to restore the pH, nutrient balance and soil activity in a gradual way. We propose to use finely ground olivine as a slow pH buffer in quantities of 1 to 1.5 tonnes per hectare to maintain a more or less neutral pH for several tens of years. Slow-release fertilizers such as potassium struvite (KMgPO4.6H2O) can gradually diminish nutrient deficits. It was demonstrated that this mineral can be easily produced from denitrified manure or other waste streams, high in phosphorus and potassium, as encountered in dairy, potato and sugar industries.
The 'Western' countries, including USA, Canada, Western Europe and Japan, have a strong and diversified industry. Per capita income is higher than in the rest of the world, and this leads to a consumption society. Because there is a high production and consumption, including a high turnover rate of (not so) durable goods, waste production and energy consumption are also high. Environmental regulation and the enforcement of environmental laws are quite advanced, and there is a large amount of money available for the environment. Salaries are high, which means that environmental technologies must be automated to a high degree and should lead to rapid results. A major factor in the compliance with environmental regulations is the fact that industries in the West do not like to be known as polluters. This stems at least as much from economic considerations as from a moral sense of responsibility. Thus, more and more it can be seen that industries solve their environmental problems in close cooperation with government agencies.
The economy of the 'Eastern' countries, in particular Russia and former Soviet states, depended strongly on heavy industry, and the associated environmental problems are enormous. In the old system, emphasis was on production, and not on waste management or environment. Environmental awareness and environmental regulation have certainly improved in recent years, but enforcement is still weak. The authorities realize that a strict environmental policy would lead to the closure of many industries, and to further economic collapse. Even so, a better discipline by the plants would already go a long way to improve the environmental situation, and may even in the short run improve the economy of the industry at no extra cost. A major problem, of course, is that many industries are old, use outdated and polluting technologies, and have no capital for renovation.
In the emerging economies, such as India, Korea, Taiwan, Indonesia and Brazil, faced with high birth rates, governments pay much attention to rapid industrialization to improve the standard of living. Too often it is considered that any money spent on the environment is lost for industrialization. The lesson that the Western countries have learned the hard way, that it is much more expensive to clean up a mess, than to avoid it in the first place, goes unheeded in these quarters. The industries here realize that cleaner production leads to more cost-effective operations, which require fewer raw materials and less energy. However, cleaner production requires investment, which is not always available. The People´s Republic of China sets, to a certain extent, a positive example, as it is the official policy of China´s government that economy and environment should be developed simultaneously and in harmony.
The environmental problems of the last category, the have-nots, are again different. Very often these countries, like Bolivia, Columbia, and many African States rely heavily on one or two commodities only (e.g. oil, coffee, cacao, copper, tin). In a year that such commodities fetch a reasonable price on the world market, their economies can barely keep up, in poor years they are a disaster. These countries cannot afford strict environmental regulations, or are not capable to impose them properly. Polluting mining practices, including the uncontrolled release of mining wastes, as well as the non-expert use in agriculture of dangerous and persistent pesticides, which are banned in the West, are common. It is exactly this lack of environmental control, as well as the low wages, which attract some heavily polluting industries. Some of the worst practices have alarmed world opinion and provoke courageous battles waged by worried responsible citizens.
Geochemical engineering solutions, as compared to other environmental technologies, are natural solutions that can be applied in any country. The local climatic and geologic conditions define the best solutions to environmental problems, and not so much gross per capita income, the degree of industrialization, or the sophistication of government.
Many geochemical engineering solutions do not require high-tech equipment, which may not function well under conditions of large voltage fluctuations, frequent power failures, and in absence of facilities for repair and maintenance. Geochemical engineering solutions require little energy, they make use of locally available raw materials, and make optimum use of the prevailing climatic conditions.
Geochemistry concerns the investigation of natural chemical materials and reactions in and on the Earth. A geochemical engineer applies this fundamental understanding by designing methods that aim at the most efficient transformation of an undesirable into a desirable chemical environment. Although geochemical engineering is closely related to chemical and civil engineering, it is distinguished by its use of natural minerals in addition to industrial chemicals, and by the development of large-scale, long-term processes in the natural environment, in addition to the small-scale, short-term industrial processes. Geochemical engineering is an attitude of geochemists that recognize the need of the efficient use of limited resources; the development of alternatives and the design of closed production and recycling, or open production that fits in the natural geochemical cycle. Geochemical engineering requires close cooperation between the geochemist who understands the implication of natural processes for the industry, and the chemical engineer who wishes to improve the design of industrial processes, using nature as an example.
It has been shown that good solutions to waste problems have to fit into the natural environment and the geochemical cycle. Some important principles have been discussed and they can be summarized as follows: toxic chemical substances are only dangerous if they occur in the bio-available mobile phase, and at concentrations that exceed certain critical values. It is not possible to isolate toxic waste forever in the dynamic geologic environment, and efforts to do so are costly and bound to fail. A proper sequence of measures is required, when dealing with environmental pollution. These measures are dictated by natural, technological and economic constraints. Most important, the amount of raw material that is used and the amount of waste that is produced, have to be minimized. Maximum effort should be undertaken to prevent waste from polluting the environment, by adhering to the method of concentration and waste recycling. Preferentially, the waste from one industry should be used as a raw material in another industry. By mixing waste, applying a waste to waste technology, it can be transformed into a useful raw material, or else rendered harmless. If the above mentioned options have been exhausted, then one starts with the break down of waste into harmless natural components. Finally, if any waste is left, one tries to keep the concentration below the critical concentration in the bio-available mobile phase. This is done by means of dilution, immobilization, and, at last, by semi-isolation with an inevitable, but controlled release of pollutants.
By discussion of several examples, it was shown that cost-effective and sustainable handling of waste requires a good understanding of the geodynamics of the Earth. This involves knowledge of the catalysis of chemical reactions by bacteria and plants in phyto-remediation, and knowledge of the chemical reactions between minerals and mobile pollutants in water and air. It is recognized that a long-term solution of environmental problems concerns the involvement of thermodynamic stable minerals as part of the geochemical cycle.
It is hoped that the presented examples do elucidate the principles of geochemical engineering and inspire young scientists to develop new methods of their own. Methods that will find their way, not only in the western society, but also in the developing world. It should be realized that the larger part of the environmental problems is yet to be expected, as developing countries strive to reach living standards of the modern post-industrial society. It seems impossible to return to a pristine environment, like it existed before the arrival of Mankind. Nowadays, we are influencing the course of events on a worldwide scale. Therefore, we also have to learn to control environmental processes on a worldwide scale. It is inevitable that we will make mistakes, however, this cannot be an argument to lean back and remain idle, to refrain from drastic measures out of fear, or to neglect our responsibility for the well-being of future generations. We are confident that geochemical engineering will play an important role in our future and that it shall contribute to a sophisticated management of our natural environment on a global scale.
The authors thank Rein van Enk and Huig Bergsma for critically reading earlier drafts of this contribution and for their constructive advise.