Most sorption technologies act like a sponge or a filter, soaking up contaminants until they run out of surface area. Sorption in remediation processes as an add-on to Pump and Treat systems, in spill containment and cleanup, and in the subsurface.
Sorption is the common term used for both absorption and adsorption. These terms are often confused. Absorption is the incorporation of a substance in one state into another of a different state (e.g., liquids being absorbed by a solid or gases being absorbed by water). Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another molecule. It is the most common form of sorption used in cleanup. Unless it is clear which process is operative, sorption is the preferred term.
In above-ground sorption, the most common adsorbent is granulated activated carbon (GAC) (see descriptions of Vapor-Phase GAC and Liquid-Phase GAC). Other natural and synthetic adsorbents include activated alumina, forager sponge, lignin clays, sorption clays, and synthetic resins.
Activated alumina is a filter medium that is porous and highly adsorptive. Activated alumina filters a variety of contaminants, including fluoride, arsenic, and selenium. The alumina can be regenerated.
The forager sponge is a cellulose sponge that incorporates a polymer that selectively sorbs dissolved heavy metals. Lignin adsorptive/sorptive clays are used to treat aqueous waste streams with organic, inorganic and heavy metal contamination. Synthetic resins are more expensive than GAC, but they can be designed to achieve higher degrees of selectivity and adsorption capacity for certain compounds than activated carbon. Resins are typically regenerated.
Sorption in the subsurface often refers to amendments that are placed in the subsurface to enhance the sorption rate. For example, apatite (bone char) is considered for use based on its reactivity with uranium. Apatite is soluble in groundwater and slowly releases phosphate, which can sorb with dissolved hexavalent uranium (U (VI)). The relatively low solubility of this combination makes it an inert host for in-situ sequestration and immobilization of U (VI). This general strategy (phosphate mineral formation) has previously been shown to have a high capacity for attenuating lead and cadmium contamination. Apatite is often placed in a permeable reactive barrier wall to filter out metal- and radionuclide-contaminated groundwater, or it is mixed with soil.
Sorption in the subsurface is typically thought of as a primary mechanism in Monitored Natural Attenuation. In this technology, biological degradation, dilution, and sorption can all be used in combination to reduce contaminant concentrations in groundwater.
Limitations and Concerns
Small molecules are not adsorbed well. Therefore, when GAC is used for liquid or off-gas treatment, it may have difficulty adsorbing some organic by-products, such as vinyl chloride.
Synthetic resins are better than GAC for treating unstable compounds such as explosives, due to the resinsŐ non-thermal regeneration requirements.
Sorption is not applicable for waste streams that have high levels of oily substances combined with other wastes, as the oily wastes rapidly adsorb onto the medium.
Most sorption technology merely transfers the contaminant from one medium to another.
When sorption is used as the major mechanism for natural attenuation, communities often object because, among other things, there is no degradation of the contaminant. There is always the possibility that environmental conditions will change and remobilize the trapped contaminant.
The target contaminants for adsorption/absorption processes are most organic contaminants and selected inorganic contaminants from liquid and gas streams. GAC is used most often for the liquid and off-gas capture of organic materials. Activated alumina can remove fluoride and heavy metals. The forager sponge is specifically used to remove heavy metals. Lignin adsorption/sorptive clays treat organic, inorganic and heavy metal contamination within aqueous waste streams. In the subsurface, soil or amendments to soil are often used to sorb contaminants.
Technology Development Status
Sorption technologies are well developed and commercial.
Other Resources and Demonstrations
See http://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminants-on-Ranges/Protecting-Groundwater-Resources/ER-1229/ER-1229 for a description of the role of sorption in immobilization of energetics.
See http://www.osti.gov/bridge/servlets/purl/463590-3QuiaS/webviewable/ for sorption of radioactive fission products strontium-90 (90Sr) and cesium-137 (137Cs) at Hanford, Savannah River, Idaho, and Oak Ridge with Colloidal Silica (CS).
See http://www-ssrl.slac.stanford.edu/research/highlights_archive/u_ha_prb.pdf and http://www.pimsnw.com/papers/apatite2/ for a description of use of apatite.