Investigation of Environmental Colloids
What are Colloids?
In physics lesson and chemistry lessons at school one learns about the three states of matter - solid, liquid and gaseous. The laws of phase transition are also taught - melting, sublimation, and condensation First, pure substances are discussed, then solutions are gone into. Solutions are homogeneous mixtures of chemical substances that are dispersed on a molecular scale. The existence of an intermediate state of matter, laying between the macroscopic volume phase and molecularly-dispersed systems, had remained unknown until about one hundred years ago. In systems showing this state, a component is dispersed in another one. However, the degree of dispersion is lower than in simple molecular solutions. Systems of this kind, colloids possess special properties. These special properties make investigating colloidal systems difficult. They have limited the success of colloid investigations for long periods of time.
Although already the alchemists had experimented on colloidal suspensions and the first systematic scientific investigations on colloids have been performed as early as in 1856 (Faraday), the English physicist Hedges wrote as late as in 1913: 'For some the word colloid evokes the imagination of things which are poorly defined with regard to their shape, their chemical composition and their physical properties and which are unsteady concerning their chemical behavior, i.e., things which are mysterious and not controllable'. The German physico-chemist Ostwald (1853-1932) regretted the lack of knowledge in the field of colloid research and called the world of colloids 'the world of the neglected dimensions'.
It was not until the second half of the 20th century that a certain helplessness concerning the colloidal state was overcome. However, the experimental difficulties that hampered the elucidation of the behavior of colloids in the past still cause a certain tendency to avoid, if possible, the formation of colloids in chemical experiments or, if this is impossible, to neglect the occurrence of colloids.
The special properties of the colloids result primarily from their large specific surface area which is due to the fine-grained dispersion of the colloidal particles in the dispersing medium. The size of colloidal particles lies in the range of 1 nanometer (1 millionth of a millimeter) to 1 micron (1 thousandth of a millimeter). The diameters of atoms and molecules are usually in the range of several tenth of nanometers. Thus the percentage of atoms/molecules located at a phase boundary is significantly higher in colloidal systems than in compact macroscopic solids. It is this high fraction of atoms/molecules sitting close to a phase boundary what causes the special properties of colloids. One of these properties is, for instance, the instability of many colloids (their tendency to coagulate). This instability makes the experimental investigation of colloidal systems difficult. It causes one of the most significant problems of colloid experiments: the unintentional change of the colloids during taking the samples and during the measurements.
What well-known examples are there?
Examples of colloidal systems 'solid in a liquid' are paints, oozes of clay, latex suspensions or blood. Milk is a 'liquid-liquid' colloid. Examples of macromolecular colloids are jellies, solutions of polysaccharides, and glues. The expression 'colloid' results from the Greek word for glue. It was coined by the Scottish chemist Graham in 1861. Graham is regarded as the founder of colloid chemistry.
What are environmental colloids?
One of the most striking properties of colloidal solutions is their ability to scatter the light (Tyndall effect), that they often are turbid or even entirely opaque. The latter refers, for instance, to milk. Visibly turbid colloids play also a part in environmental research. However, more typical are different colloidal solutions in environmental research: solutions that are not looked their particle content with the naked eye. Many 'classic' colloid chemists would perhaps hesitate to call, for example, a clear groundwater a 'colloidal solution'. Nevertheless, instruments that are more sensitive than the human eye proof that any natural water contains particles of the colloidal size range (1 nm to 1 µm). Clear groundwaters show typically colloid concentrations of 0.01 to 1 mg/L. Typical particle number concentrations of such waters are 1012 to 1014 particles per liter. The colloid concentrations of mining waters or turbid river waters can be significantly higher (however, we do not refer to the so-called 'suspended matter' which has particle sizes of over 1 µm as colloids). The very low colloid concentrations of many natural waters add further difficulties to the above-mentioned difficulties of colloid research in environmental studies.
There are primarily two ways on which the inorganic colloid particles get into natural waters: stone fragments are rinsed off by the weathering of the rock that is washed by the water and particles can be formed within the water from truly dissolved substances by precipitation (formation of secondary minerals). One can differentiate between several groups of colloids according to the generation mechanisms of the particles. The first group include above all silicate colloids (layer silicates, quartz etc.). Oxides, hydroxides or carbonates of iron, aluminum, manganese, or calcium belong to the second group, the group of secondary mineral colloids. A further group of environmental colloids are organic colloids (fulvic acids, humic acids, humin, polysaccharides). Their mechanism of generation is the degradation of the remains of plants and animals. Finally, biogenic particles (bacteria, viruses, fungi) form a fourth class of colloids in natural waters.
Why do we investigate environmental colloids?
The environment is usually treated as a diphase system in analyses of contaminant transport by ground waters, mining waters, or surface waters and in calculational models for the prediction of this contaminant transport. The contaminants (radionuclides, toxic heavy metals, organic poisons) are distributed among the mobile aqueous phase and the immobile solid phase according to this approach. The migration of the contaminants with a strong tendency to adsorb onto the rock of the solid phase is delayed in comparison to the water velocity. Substances with very strong tendencies to attach to the walls, for instance, should virtually not move at all in an aquifer. However, there are increasingly observations and experimental results which indicate that the fraction of the solid phase existing in suspended form as colloidal particles is not neglectable under certain conditions. It is obvious that the contaminants that tend to be adsorbed onto solids are also adsorbed onto these particles.
The total mass of the colloid particles in natural waters is usually small. However, the particles possess a large specific surface area. Therefore, they offer a lot of sorption sites to the sorbing contaminants. The association of contaminants with this additional mobile phase can increase the velocity of contaminant transport. Vice versa, filtration effects or coagulation and precipitation of colloids can delay the contaminant transport. The assessment of contamination incidents, the prediction of migration scenarios in the vicinity of contaminated areas and of potential hazards as well as the development of effective strategies of remediation for contaminated areas - they all require a profound understanding of the migration-enhancing and the migration-delaying influences of the colloidal particles on the contaminants. Background for our research is the problem of contaminant migration in the surroundings of mines as e.g. the abandoned uranium mines in Saxony or the assessment of the long-term behavior of radioactive waste disposal sites.
The typical difficulties of colloid research play also a role in the case of environmental colloids. They explain why the question of colloids is still often neglected in estimations of contaminant transport. The colloid-facilitated contaminant transport is regarded as elusive, the results of colloid experiments are often poorly reproducible, deriving quantitative relationships is often not successful. However, it is increasingly realized now that simple ignorance is the most unfavorable way of dealing with the colloid problem. The American geochemist B. D. Honeyman stated 1999 in Nature that the colloid- facilitated transport of contaminants has become a sort of Gordian knot for environmental scientists. We share this point of view and we also share Honeyman's opinion that this Gordian knot has not yet been cut.
How do we investigate environmental colloids?
The following techniques are available at Rossendorf:
- Dynamic light scattering - photon correlation spectroscopy (Fig. of the device, 105 kB)
- Static light scattering
- Asymmetric flow field-flow fractionation (Fig. of the device, 101 kB)
- Filtration and ultrafiltration
- High-speed centrifugation (up to 70 000 g)
- Scanning electron microscopy
- Transmission electron microscopy
Based on these techniques, we developed a separation and detection scheme for colloids which is applicable to very different environmental samples.
- Zänker, H., Richter, W., Brendler, V.
Colloid-Borne Uranium and Other Heavy Metals in the Water of a Mine Drainage Gallery.
'MIGRATION 99' (Chemistry and Migration Behavior of Actinides and Fission Products in the Geosphere). Incline Village (Lake Tahoe), USA. Sep 26 - Oct 1, 1999 (for publication in Radiochimica Acta).
- Zänker, H., Richter, W., Brendler, V., Nitsche, H.
Colloids in the Main Drainage Gallery of the Freiberg Mining Area (Rothschönberger Stolln).
Annual Meeting of GDCh-Fachgruppe Wasserchemie, Regensburg, May 10 -12, 1999 (for publication in Acta Hydrochim. Hydrobiol.).
- Mertig, M., Klemm, D., Pompe, W., Zänker, H., Böttger, M.
Scanning Force Microscopy of Spin-Coated Humic Acid.
Surf. Interf. Analysis 27(1999)426-432.
- Schmeide, K., Zänker, H., Hüttig, G., Heise, K. H., Bernhard, G.
Complexation of Aquatic Humic Substances from the Bog "Kleiner Kranichsee" with Uranium(VI).
In: 2nd Technical Progress Report of the EC Project 'Effects of Humic Substances on the Migration of Radionuclides: Complexation and Transport of Actinides'. Project Nr. FI4W-CT96-0027 (ed. G. Buckau). Report FZKA 6124. Forschungszentrum Karlsruhe, June 1999, p. 179.
- Schmeide, K., Zänker, H., Heise, K. H., Nitsche H.
Isolation and Characterization of Humic Substances from the Bog "Kleiner Kranichsee".
In: 1st Technical Progress Report of the EC Project 'Effects of Humic Substances on the Migration of Radionuclides: Complexation and Transport of Actinides'. Project Nr. FI4W-CT96-0027 (ed. G. Buckau). Report FZKA 6124. Forschungszentrum Karlsruhe, August 1998, p. 161.
- Zänker, H., Mertig, M., Böttger, M., Hüttig G., Pompe, S., Pompe, W., Nitsche, H.
Photon Correlation Spectroscopy and Atomic Force Microscopy of Humic Acid.
Submitted to Geochim. Cosmochim. Acta.
- Zänker, H., Hüttig, G., Arnold, T., Zorn, T., Nitsche,H.
Detection of Iron and Aluminum Hydroxide Colloids in a Suspension of Ground Phyllite.
Submitted to Aquatic Geochemistry.