Aquatic Contaminants in Colloidal Phases and Humic ComplexesIntroduction
Strategies for remediation of contaminants in ground and surface water depend on knowledge of the chemical form of the contaminants. Some trace metal and organic contaminants are present in very small particles (colloids) or are chemically bound (complexed) by naturally occurring, yellow-colored organic acids (humic substances). The U.S. Geological Survey (USGS) has developed sampling and isolation techniques to determine the size and chemical form of trace metal and organic contaminants. These techniques prevent chemical alteration of contaminants and avoid sampling artifacts. Also, these techniques produce preparative quantities of the particulate, colloidal, and humic fractions sufficient for chemical and physical characterization. Characterization of these fractions provides an understanding of the chemical reactions responsible for the contaminant associations.
The sampling and isolation techniques developed by the USGS can be used to remove small particles or colloids without causing aggregation of the particles or clogging of the pores in the filter membranes. This is achieved by using cross-flow ultra-filtration, where the water sample flows across the membrane under pressure. The USGS has developed procedures for sequential ultrafiltration, which results in the isolation of particles in a number of size ranges. The contaminant association is determined in two ways: (1) by a difference calculation between the contaminant concentration in the sample water and the concentration in the filtrate (sample water that has passed through the filter) and (2) by direct measurement of the concentration of the contaminant in the isolated particles.
Identifying the carrier for the contaminant can be critical for designing remediation techniques for a particular site. Conventional filtration procedures can erroneously indicate that contaminants are large (>.45 mm) because the filters become plugged and the effective pore size is much smaller than the nominal filter pore size. Using these incorrect data can result in poor performance of a remediation process because colloid-borne contaminants are not retained.
Humic substances are ubiquitous in natural waters. They are macromolecular organic acids that can react with both trace metals and some organic contaminants. For trace metals, complexes form between the metal ion and the carboxylic acid functional groups of the humic macro-molecule. For hydrophobic organic contaminants, the association occurs through preferential interaction with hydrophobic portions of the humic macromolecule.
Humic substances can be recovered from natural waters using column chromatography methodologies developed by the USGS. These methods are based on the use of macroreticular resins. The humic substances sorb on the resin when the acidic groups are protonated (at low pH) and desorb when the acidic groups are ionized (at high pH). As in colloids, contaminant association with humic substances can be determined under various pH conditions by differences between column influent and effluent concentration and by direct measurement of contaminants in isolated humic fractions. The humic material can also be characterized in laboratory experiments to evaluate the chemical interactions under a range of contaminant concentrations and environmental conditions.
Some remediation techniques remove humic substances from natural waters by various methods, such as coagulation with alum and sorption on resins. Knowledge of contaminant association with humic substances can allow remedial designs to incorporate removal of humic substances and associated contaminants.
In the collection of colloid and humic samples, it is critical that environmental conditions be maintained during sample collection and processing. Another issue is avoiding the introduction of particles or colloids into the sample by the disruption of either aquifer material or the streambed. The USGS has developed inline sample monitoring technologies for ascertaining that sample conditions are stable. For example, dissolved gases are maintained at ambient concentrations and conductivity, pH, and redox state are monitored using flow-through, inline instrumentation.
These technologies can be applied to assess the contaminant form in systems that have very low concentrations of particles and humic substances, such as ground waters, or in systems that have very high concentrations of particles and humic substances, such as some surface waters. These technologies can also be applied in a wide range of contaminants studies, such as radionuclides in ground and surface water.
Radionuclides in Ground and Surface Water
The USGS has determined that the actinides Pu, Am, and U are associated with colloidal phases and that Pu is complexed by dissolved humic substances at a nuclear facility in Colorado (Rocky Flats). This information was useful in designing remediation procedures for the facility. The investment in making these detailed assessments of the chemical form of contaminants has the potential to greatly enhance the outcome of remediation at this and other nuclear sites. This information is also critical for designing monitoring programs for such sites. For example, concentrations of humic substances vary seasonally, and greater sampling frequency for periods with high humic concentrations may be warranted.
Trace Metals in Surface Water
There are typically many sources of water flowing to large reservoirs and rivers. Some of the inflows may contain metal contaminants in dissolved, colloidal, or particulate forms. These forms can undergo transformations within the reservoir or river. Colloidal phases and humic complexes of trace metals can be more stable because they do not settle or sorb to settling particles. Measurement of the chemical form can be very useful in assessing the importance of various contaminant sources to a system and prioritizing remediation strategies.
The chemical characteristics of particulate organic carbon in Pueblo Reservoir, Colorado, were examined. The Arkansas River is the major inflow to this reservoir. Four particulate organic carbon fractions were isolated by sedimentation, crossflow ultrafiltration, and centrifugation. A silt fraction, two organic colloid fractions, and a mineral colloid fraction were obtained. The results indicate that algal materials may be the dominant sources of the organic colloidal fractions. Iron, manganese, and zinc were enriched in the organic and mineral colloids, demonstrating the importance of these fractions in transport of trace elements.
These characterization methodologies can be useful in environmental assessment and in remediation of contaminated sites.
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