Geochemical Engineering


Introduction

Geochemical engineering is a rather new discipline in environmental management that recently evolved from geochemistry, a fundamental science concerned with the chemistry of the earth. Geochemists study the properties of minerals, soils, rocks, waters and natural chemical processes. A geochemical engineer applies fundamental understanding of geochemistry 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 alternative industrial production and of environmental management that fits within the natural geochemical cycle. Geochemical engineering requires close cooperation between the geochemist who understands the importance of natural processes for modern industry and the chemical engineer who wishes to improve process design, using nature as an example. 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, then 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. This is a consequence of our lack of understanding of the natural chemical processes that characterize the geochemical cycle.


Environmental problems

Environmental problems are encountered if the concentrations of mobile bio-available substances are either too low or too high. Many chemical elements play an essential role in the physiology of plants and animals, including humans. 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. In a bio-diverse ecosystem, the organisms are adapted to the range of concentrations commonly encountered in nature. Geo-diversity of chemical elements is an important condition for and cause of biodiversity. 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 itself. 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 environmental hazards if originating from human activities.


Geochemical engineering solutions

Apart from natural toxic environments, there are many sites that are polluted by human activities to such a degree that they constitute a real threat to the 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:

  • Breakdown
  • Concentration
  • Dilution
  • Isolation
  • Immobilization

By studying the ways in which these processes take place in nature, we can learn how to devise efficient, cost-effective and environmentally safe technologies.


Breakdown

A well known example of natural breakdown is microbially mediated degradation 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 produces clay-type residues and cations in solution.


Concentration

The most extreme examples of concentration are ore deposits. They have in common that their chemical elements 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.


Dilution

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 gases are diluted by mixing with the atmosphere. 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.


Isolation

Common examples of isolation in nature are trapped oil and gas in permeable reservoirs, which are isolated by impermeable seals of evaporites or clays. Isolation is one of the most popular ways of waste management. Most commonly municipal waste, but also dredged sludge, incineration ashes and even radioactive waste is concentrated and isolated from the environment. Geochemical engineering teaches that no chemical element can be kept indefinitely from again entering the geochemical cycle. Therefore, complete isolation is not possible and hiding waste in covered storage removes it from sight and mind but does not make it disappear. Whenever possible, geochemical engineering advocates that it is better to keep waste in the open, exposed to natural degradation processes, accompanied by a gradual controlled return of chemical elements into the geochemical cycle, without trespassing toxicity limits for bio-available substances.


Immobilization

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. Physical forms of immobilization are the cementation of rocks, their recrystallization into 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 potentially toxic substances are still present after immobilization and will be set free again when conditions at the disposal site change. However, also in this case, potentially harmful chemical elements need ultimately to return to the geochemical cycle and it should only be guaranteed that the remobilization is slow and that the concentrations of mobile phases will not exceed certain safety limits.


Rate of reaction

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 because time is money and public pressure may demand 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, pollution is treated in situ and slowly formed reaction products are of better quality. Space and time constraints become less severe and personnel costs are limited because 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 more closely adhering to geochemical engineering principles.


Scales of geochemical engineering

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 develop 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 in.


Scale of minerals

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. Geochemical engineering examples of these are the precipitation of phosphate as struvite, the adsorption of ammonia from waste waters by zeolite, the co-precipitation of arsenate with ferrihydroxide, 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.


Scale of sites

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, preventing unwanted side effects. Not only the relation between waste and natural environment needs to be taken into account, but also the relation between two or more types of hazardous waste that can react to a harmless substance, in case of co-occurrence or co-deposition. Although co-disposal is often prohibited as it is considered equal to indiscriminate dumping or a form of often unjustly opposed dilution, it should certainly not be ruled out as a viable option in environmental management.


Scale of regions

When developing environmental remediation methods, attention should go to the discrimination between natural back-ground concentrations of potentially toxic substances on a regional scale and the locally anthropogenically elevated concentrations of these substances. Adherence to (inter)national standards for concentration limits is not always justified when dealing with local anomalic geochemical environments. The variation of climatic, economic and cultural conditions on a global scale requires diverse environmental remediation technologies, adapted to specific national conditions. Geochemical engineering, using natural mineral reagents, applicable at low costs in natural open reactors, providing self-sustaining processes independent of complex installations and reliable energy supply, requiring little supervision by highly qualified personnel, promises to provide the effective solutions for environmental problems that are needed also in remote and underdeveloped areas.


Conclusion

It has been argued that good solutions to environmental 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. Indefinite isolation measures are costly, bound to fail and can better be replaced by scenarios of controlled release of potentially harmful chemical substances. All production and proper environmental management is governed by technological and economic constraints. The consumption of raw material and the production of waste need to be minimized. If waste cannot be recycled, then it should be broken-down into harmless natural compounds. If any waste is left, release of potentially toxic compounds should be minimized and concentration kept below critical limits for bio-available mobile phases. This is done by means of breakdown, concentration, dilution, immobilization, or semi-isolation with an inevitable but controlled release of potentially toxic compounds. It is understood that the dynamics of chemical processes in the biosphere is to a large extend controlled by bacteria and plants. Therefore, microbial- and phyto-remediation technologies are indispensible in modern environmental management as far as concerns the brake-down and concentration of potentially harmful compounds. However, long-term thermodynamic stability of waste within the geochemical cycle depends mainly on chemical reactions between minerals and mobile pollutants in water and air. 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 is impossible to return to a pristine environment like it existed before the arrival of Mankind. Nowadays, we even influence the course of events on a worldwide scale and we also need to learn how to control environmental processes on a that scale. Students of geochemical engineering shall be particularly well equipped to contribute to more sophisticated management of our natural environment on a all scales and in sync with emerging principles of a more sustainable society, characterized by production that fits within the natural geochemical cycle.