Introduction To Chemical Fixation
There are several approaches to treatment of chromium and other toxic metals such as lead arsenic, and cadmium in soil and groundwater, including isolation, immobilization, toxicity reduction, physical separation, and extraction (U.S. EPA. 2002).
Brief Overview Of Metals Remediation Methods:
Conventional approaches to ex-situ remediation include excavation and disposal of the impacted soil. Case studies are not necessary in this type of remediation, since the problem has only been moved from the source property to another, the landfill. Ex-situ electrokinetics of metals contaminated soil is another, less common ex-situ method, where-in the metals are pulled to the cathode. The soil moisture content must be near saturation to allow for electromigration of the metals. Various acids, such as acetic or citric can be used for the electrolyte solution to enhance the migration of the metals to the cathodes.
For conventional groundwater remediation of toxic metals, pump and treat methods have been used for decades with varying degrees of success. Pump and treat is effective at controlling hydraulic movement, but mass removal tends to be difficult with this technology. A chemical enhancement of pump and treat system may add a reductant to the remedial program. The reductant is useful to overcome the tailing effect and reduce the overall time required for the in-situ pump and treat remedial program.
There are generally three main remediation zones: 1) the source zone, whereby the concentrated chromium compound is leaked into the ground and down through the vadose zone to the top of the groundwater table. This area has high levels. 2) The concentrated zone is the main and highest part of the dissolved chromium groundwater plume. This is the zone where pump and treat technology helps to maintain hydraulic control of the area. 3) The zone outside the concentrated zone, the dilute zone, which contains lowrer levels. This zone is less likely to be successful with a pump and treat system, as levels are fairly low and significant mass reduction is unlikely in a short period of time.
Less Conventional Methods
In-situ remediation of chromium, for example, uses chemical reduction or fixation. The aoal of chemical reduction or fixation of hexavalent chromium is to reduce it to the more thermodynamically stable trivalent chromium, which will precipitate or fix onto aquifer solids. There are primarily four main technologies or approaches:
- Geochemical Fixation
- Permeable Reactive Barriers (PRBs)
- Reactive Zones
- Natural Attenuation
In addition to the above mentioned methods, there are several other methods available for in-situ chromium remediation:
Soil Flushing and Enhanced Extraction
- Biological Processes: