by Martina Vítková, Associate Professor, Czech University of Life Sciences Prague
Contamination of soil caused by human activity is a long-term problem that is difficult to solve. Metals and other potentially hazardous elements remain in the soil for many years and their excess levels are a threat to all living organisms, including humans. Therefore, researchers are studying ways to not only accurately identify unwanted risk elements in the soil, but also to minimize their hazards. Electron microscopy is critical to this research.
Determining the Source of Hazardous Elements in the Soil
Lead, arsenic, cadmium and chromium: these are just some of the most frequently monitored risk elements in the pedosphere. Some of them are toxic even in small quantities, and therefore we talk about contamination when exceeding limits in the order of units up to tens of milligrams per 1 kg of soil mass. On the other hand, some metals and other risk elements occur naturally in the environment; for example, copper and zinc in small amounts are necessary for the life of soil organisms. Therefore, researchers must not only accurately measure the amount of risk elements in the soil, but also determine whether they are caused by anthropogenic activity.
The Limits Are Exceeded Many Times
The problem occurs when the content of risk elements exceeds the safe limits. One of the biggest sources of pollution is the mining and processing of minerals. Specific sources of pollution include mining waste, old sludge ponds, mine water, heaps of waste slag or metallurgical ash from smelters. Very heavily polluted soil can be found in many developed industrial countries in Europe, including the Czech Republic, Germany, France, Spain and Norway.
Whether it is the fall of flue gases from chimneys or the leakage of mine water or mining and metallurgical waste into surface water, risk elements enter the soil within a perimeter of many kilometers from the original source.
The Degree of Risk Elements’ Hazard Is Influenced by Their Form
The first step in research is to identify the source of the contamination and to map out the ways in which contamination occurs. Because it is a complex process, researchers use complementary tools and a variety of techniques. Soil chemical analysis provides information on the elemental composition, but does not reflect the potential mobility of particles in the environment. Using electron microscopy, researchers are then able to determine whether the contaminant occurs in readily soluble or insoluble forms, which has an impact on the risk of further spread of contamination, whether to the surrounding soil, plants, or groundwater, for example (Figure 1).
Figure 1: TESCAN SEM image of organic sorbent with iron particles (left: bright particles of iron “shining” in dark organic matter, right: surface topography).
Electron Microscopes Help Czech Scientists Study Soil Pollution
Our work at the Department of Environmental Geosciences of the Czech University of Life Sciences in Prague involves finding micrometer particles in soil samples.
First, we take a soil sample directly in the field. We dry it and obtain particles of precisely defined fractions by sieving. Hazardous metals are usually concentrated in the finest fraction. However, it is often necessary to use other separation methods. One of them is separation in the so-called heavy liquid (liquid with a very high density), thanks to which we can separate relatively heavier metal particles from relatively lighter soil particles by centrifugation. This provides the so-called heavy mineral fraction, which we further analyze.
This is where electron microscopy comes in. We place the sample in a scanning electron microscope (SEM) where heavy elements, i.e., the target hazardous metals, are highlighted using chemical contrast mode. In SEM images, heavier particles appear lighter than a dark background. Therefore, scientists most often work in the backscattered electron mode (using a BSE detector), which is the most suitable for displaying chemical contrast. The SEMs are also equipped with EDS detectors, which provide accurate information about the elemental composition of the displayed particle (Figure 2).
Figure 2: TESCAN SEM image of metallic particles (left: porous structure of biochar – metal sorbent; right: chemical contrast reveals relatively heavier metal-containing particles).
Researchers Are Looking for Solutions to Extract Risk Elements from the Soil, or at Least Prevent Their Spread
However, the research does not end with the identification of risk elements. Electron microscopy is used not only in the detection of heavy metals in the soil, but also in the application of solutions for its decontamination.
Because metals do not decompose in the soil, the traditional solution was to extract and remove the contaminated soil, which is very expensive and even impossible in the case of larger areas of pollution. The current solution consists of the use of active sorbents, which are incorporated into the soil directly at the site of contamination and can capture metals in the soil and thus prevent their migration through the subsoil. Often the very presence of elements in the soil does not matter, but the problem arises if they get into groundwater, plants or watercourses.
Special Sorbents Can Help
The main principle of using sorbents is based on the assumption that the reactivity of anthropogenic particles depends on the characteristics of the solid phase. It is necessary to understand in which compounds the risk elements occur in the soil and how these compounds react with their environment and sorbent. By combining various research techniques led by electron microscopy, it is possible to understand these processes in detail and to perform a very accurate analysis of soil chemical reactions.
One of the main advantages of electron microscopy is the ability to visualize the used sorbent in detail and thus verify its functionality and the efficiency of capturing contaminating metals.
Figure 3: TESCAN SEM image of different types of soil pollution sorbents: a) iron oxides, b) manganese oxides, c) newly-formed crystals on the sorbent surface, d) biochar (here: charred wood).
Thanks to the study of contaminated sorbents, scientists are able to accurately interpret the mechanisms by which risk elements can be captured in the soil. Iron oxides, manganese oxides, zeolites or carbonized organic matter are used as sorption materials. In order for the sorbent to work effectively, a relatively large amount needs to be added to the soil. Therefore, natural materials or unused waste products are generally sought that can be processed into a functional sorbent while being affordable (Figure 3).
Soil Pollution Is a Cumulative Problem That Nature Cannot Deal with on Its Own
Remediation of soil pollution is an increasing challenge for humanity, as the amount of heavy metals, metalloids and other hazardous elements in the soil due to human influence is constantly increasing. For example, it is estimated that the cadmium content of soils in the Czech Republic has increased by 30-55% over the last 150 years. Contamination has consequences that we are often unable to realize in full. What is certain, however, is that heavy metals in the soil are a long-term problem in terms of their constant accumulation. Our research, of which the TESCAN electron microscopes are an important part, can not only help to map their presence more accurately, but also contribute to their effective capture. Even if we fail to extract heavy metals from the soil, we can at least prevent their further spread through the environment.
About the Author: Martina Vítková, Ph.D., is an associate professor in the Department of Geoenvironmental Sciences at Czech University of Life Sciences Prague. She received her Ph.D. in environmental geochemistry from Charles University in Prague. Her main areas of research include environmental geochemstriy and mineralogy, focused on the behavior and solid speciation of metals and metalloids. Her current research deals with chemical stabilization in contaminated soils with a principal interest in localities impacted by mining and smelting.