|CERES No. 135 (FAO Ceres, 1992, 50 p.)|
Where heavy metals are concerned the best cure is
by Winfried E.H. Blum
Our civilization's excessive use of fossil fuels and other raw materials mined from deep within the earth has broken a natural equilibrium that lasted for millions of years, and raised the threat of global pollution of soils and even entire ecosystems by heavy metals. The danger the world now faces cannot be overemphasized. Once contaminated, many soils will not be usable to produce food or fodder for generations to come.
The problem is so enormous that it can only be tackled through international cooperation. The present state of soil pollution must be assessed worldwide and its causes and impacts analysed. Ongoing pollution must be supervised and monitored. Heavy metal emissions must be reduced or prevented - and contaminated soils treated to the limited extent possible with present technology.
We are faced with a true emergency.
It is important to understand how this situation arose.
For millions of years, rocks at the earth's surface were the only source from which heavy metals were released into the soil. This happened largely through weathering by rain, wind and other processes. Organisms and plant roots transferred the metals from the soil to the above-ground biomass of terrestrial and partially aquatic ecosystems. From there, the elements were recycled back to the soil through the food chain or other turnover processes. Having evolved in this setting, organisms living on the base of the biomass, including people, were genetically adapted to the natural heavy metal concentrations through the steady state equilibriums between uptake and recycling.
This natural cycle was severely disturbed in the last half of the 19th century when heavy metals bound in fossil energy, such as coal, and non-renewable raw materials, such as mining ores, were extracted in increasing amounts from deep and inert positions of the inner earth and distributed on the land surface directly or following processing. The situation worsened dramatically when consumption of raw materials and fossil energy, including oil and gas, began skyrocketing in the 20th century, especially since the 1950s. If this continues, the inevitable result will be global pollution of most terrestrial ecosystems and soils by heavy metals - deposited from the atmosphere, through local contamination of the water cycle, from the dumping of waste and sewage sludge, from contaminated manure and from the use of fertilizers, phytosanitary and other products in agriculture and forestry.
Because soils act as a final repository for heavy metals - and this is a non-reversible process - it seems only a matter of time until protective soil functions, such as filter and buffer capacities, are overcharged and heavy metals are released into the soil solution. From there, they will be taken up by soil organisms and plant roots and leached into the groundwater. The result will be poisoning of soil organisms, pollution of the food chain and deterioration of groundwater quality.
Defining heavy metals
Heavy metals are usually defined as metals with densities larger than five grams per cubic centimetre. This group comprises about 70 elements, including the familiar lead, silver and mercury, but only some 20 species are important to ecology. They are essential micronutrients for animals and plants but can also be toxic, and both toxicity and necessity vary greatly from metal to metal and from organism to organism. (For example, Figure page 44 shows the wide range of effects between predominant growth promotion or growth inhibition of plants by four different heavy metals.)
Moreover, positive or negative (toxic) effects depend not only on the type of element and its reactive concentration but also on the genetically based physiological behavior of different organisms. This has to be taken into account when considering problems caused by heavy metals in soils and terrestrial ecosystems.
The content of heavy metals in soils derives from natural sources as well as from anthropogenic (man-made) pollution.
The natural contents vary widely and can be very high in soils that developed on special rock forms. Other natural sources are forest fires, soil transport and deposition by wind, water and volcanic eruptions.
Anthropogenic heavy metal pollution of soils happens along three general pathways: the air, the water cycle and mechanical transport.
Global and regional atmospheric pollution is caused by emissions from industry, traffic, power stations, waste and refuse incineration and is more difficult to monitor and control than pollution in the water cycle and through mechanical transport. The amount of heavy metals deposited in the soil, both locally and regionally, depends on how far the soil is from the emission source and the type of its vegetation cover. Wooded areas receive two to five times larger depositions than agricultural ones, because forests filter solid and gaseous emissions and aerosols. Man-made acidification of forest soils also leads to increased mobility of heavy metals, through leaching into the soil solution. This is not as serious in agricultural soils, which have generally higher pH values and benefit from liming, as well as applications of fertilizer and manure.
Industry and agriculture
Specific and locally controlled depositions of heavy metals are caused by industrial and other contaminated effluents as well as by dumping or recycling of refuse and sewage sludge from industry, urban utilities and other sources. The high quantity of heavy metals used in industrial processes is the most important source of heavy metal contamination of soils. In agricultural land use, the worst offenders are sewage sludge, phosphate fertilizers and contaminated manure, but plant protection products may also contribute. Because the most severe pollution by heavy metals is usually caused by contaminated refuse material, especially sewage sludge, many countries now ban their spreading on agricultural land.
Fertilizers produced from rock phosphates contain on average five to 40 parts per million of cadmium. Therefore, the average annual input of cadmium in agricultural soils amounts to two to six grams per hectare. Special P-fertilizers like Thomas phosphate contain nearly no cadmium, but large amounts of vanadium and chromium.
Pig manure is usually contaminated by copper and zinc from copper supplements to improve food conversion. Application of liquid pig manure can cause severe soil pollution in 20 to 30 years, especially in areas with high livestock densities.
Heavy metal pollution by plant protection products is decreasing because organic products are replacing most of the inorganic pesticides like cadmium-containing bordeaux mixtures (a fungicide made by reaction of copper, sulphate, lime and water) and Pb-arsenides.
Reactions in soils
The binding capacity of soils for heavy metals in general is very high. The heavy metal reactions in soils include mechanical, biological and physicochemical processes between pollutants and the solid and liquid phases of the soil. These processes are extremely complex because soils consist of heterogeneous mixtures of solid organic and inorganic constituents, such as humic substances, clay minerals, oxides of aluminum, iron and manganese as well as soluble components. Soils also vary considerably in pH and redox (oxidation reduction) conditions, which have a major effect on the reaction process. Therefore, a metal may form different species with specific soil components, depending on the type of bonds and the bonding energy.
Within the main functional parameters and reaction processes of heavy metals in soils, the three processes work this way:
mechanical filtration of liquid and solid heavy metal compounds in the porous space of the soil;
uptake of heavy metals by soil organisms, especially plant roots and microorganisms, as a process of biological binding. Uptake occurs from the liquid phase and depends on the quantity and quality of the soil biomass as well as on soil pH, redox and other parameters;
most importantly, the physico-chemical processes of: adsorption and desorption by ion-exchange at the surface of humic substances, clay minerals or oxides of iron, aluminum, manganese and others;
complexation by humic substances (strong complex bonds);
occlusion in oxides of iron, aluminum, manganese and others, mainly through co-precipitation;
structural binding in clay minerals and oxides through diffusion of heavy metals into the crystal structure;
precipitation and dissolution of defined compounds, such as carbonates, phosphates and sulphites.
A program of action
Action to save the soil has to start with an assessment of the actual state of pollution and an analysis of its causes and impacts. The assessment should systematically cover the total surface of entire states or regions, using methods that will later allow data to be compared. The need to standardize methods is urgent because, even where efforts have been made to assess the situation at regional, national and international levels, generally accepted, comprehensive concepts are lacking.
There are both direct and indirect approaches to the analysis of soil pollution. The direct approach is the chemical analysis of heavy metals in the soil. Indirect approaches are input-output analyses or analyses of the impacts of soil contamination on soil organisms, plants or groundwater. These give indications but no data on the exact concentration of heavy metals in the soil at a given time.
The causes of soil pollution can be determined, or at least assessed, by spatial evaluation (vertical and horizontal) of pollution data and sources of pollution. By establishing a network of permanent sampling plots, pollution could be monitored continuously by repeated soil analysis and comparison of results.
Additional data are needed to assess contamination risks, especially physical data on the texture, structure, water absorption capacity of the soil and biological and climatic information. Time series of chemical analyses from permanent sampling networks will make it possible to assess future impacts.
The analytical data, especially when based on time series, allow the supervision, monitoring and prediction of pollution and impacts.
Supervision and monitoring, the second part of a program of action, are urgently needed to convince public officials, decision-makers and politicians to enact laws and regulations to reduce heavy metal emissions. It will take international cooperation to harmonize methods, exchange data and define common threshold values.
Preventive action, the third part of the program, should focus on reducing or preventing heavy metal emissions, especially from sources of widespread, diffuse contamination on an international scale. National measures should be enforced to reduce or ban specific and locally controlled pollution in the water cycle or spread through mechanical transport. Because heavy metal pollution is not reversible by present methods, the only way to prevent it is to reduce or eliminate heavy metals at the source. An outstanding example is given by Sweden, which has prohibited the use of cadmium, replacing the element by other less polluting products.
Treatment options limited
Possibilities for treating contaminated soils, the fourth and final part of the program, are still very limited, and the treatments are extremely expensive. Large-scale decontamination is not presently feasible because of the intensive binding of heavy metals in soils and expense involved. Only two practical alternatives exist:
dilution of heavily contaminated soils by mixing them with non-contaminated matter that has high sorption capacity, such as oxides and clays. Mixing with organic matter is less efficient because of its biodegradability; adjustment of the soil pH and of the redox conditions. This can be done by liming or aeration in the case of low redox potentials. The heavy metal concentration remains the same, but the binding capacity is increased and the mobility of the heavy metals in the soil is reduced.
Such newly developed methods as extracting heavy metals through plant species with high root uptake capacity are not yet sufficiently cleared or are not applicable under all conditions.
Prevention remains the best cure - and the sooner the better if we want to preserve the capacity to feed the burgeoning population of the 21 st century.