Chromium is a metal, occurring in nature in a bound form and is widely used in many industries. Hexavalent Chromium being the most prevalent form. At many locations it has been released into the environment via leakages, poor storages or improper disposal practices. Groundwater contamination is one of the major impacts. Groundwater can become contaminated with metals directly by infiltration of leachate from land disposal of wastes. Chromium exists in several oxidation states, of which Hexavalent Chromium is a priority toxic, mutagenic and carcinogenic chemical whereas the trivalent form is much less toxic and insoluble. Hexavalent chromium causes various chronic health disorders including organ damage, dermatitis, and respiratory impairments.
It is necessary to understand the risks associated with the contaminated site on sensitive receptors such as Flora and Fauna, soil, groundwater and surface water resources, human health and regional economic and livelihood impacts. The associated risks will decide the quantum of intervention required and the level of remediation.
Remediation activities and detailed project reports will have to take into consideration the extent of contamination and the transport mechanism. This will involve detailed monitoring and modeling.
Hexavalent chromium is extremely toxic and bioccumulative. It is very mobile in ground water. Ground water extraction and treatment has been traditionally used to remediate chromium containing plumes. This method, while providing interception and hydraulic containment of the plume, may require long term application to meet the stipulated standards. The pump and treat System is also associated with tailing and rebound effects. It may also not be effective in remediating the chromium accumulated in the soil and other source zone sites. Other alternatives systems, including bioremediation are available. A number of technologies use chemical reduction and fixation for chromium remediation – necessarily conversion of Cr+6 to Cr+3 and fixing the same in soil where it is rendered immobile. They may include geochemical fixation, permeable reactive barriers and reactive zones. Other types of advanced treatment technologies include enhanced extraction, electro kinetics, biological processes that can be used with permeable reactive barriers and reactive zones, natural attenuation and phytoremediation.
The objective of this technology is to reduce Cr+6 in groundwater and contaminated soils to the more thermodynamically stable Cr+3. The reduced chromium geochemically fixes itself to aquifer solids. Contaminated water is dosed with the reductant to reduce any residual Cr+6 contaminations remaining. The total chromium contamination in the acquifer system is not decreased but chromium is precipitated and fixed so that it is not available in the ground water.
Permeable Reactive Barriers (Treatment Walls)
PRB’S provide in situ treatment of groundwater emitting from source zones. These reactive barriers differ from the highly impermeable barriers like grouts, slurries or sheet pilings that are used to restrict the movement of chromium bearing plumes. PRB’S can be installed as permanent, semi permanent or replaceable units across the flow path of a contaminated plume and act as a treatment wall. The contaminated groundwater flows by natural gradient through this reactive barrier and the contaminants immobilized or chemically transformed to the less toxic forms. In the case of chromium, it is immobilized by precipitation onto reactive media or acquifer solids. Zero valent iron (Fe°) has been utilised to reduce Cr+6 to Cr+3. The Fe° donates the electrons necessary to reduce the Chromate and becomes oxidized to Fe+2 or Fe+3. When iron is present, the Cr+3 precipitates as mixed chromium – iron hydroxide solid solution and both the toxicity and mobility of Chromium is considerably removed. Granular Iron filings are used as a source of Fe°. Absorbents have to be removed from the ground after they have become finally loaded with the contaminant. The E.P.A. recognizes PRB’S as a technology capable of more effective remediation at significant cost saving compared to other traditional technologies.
In situ reactive zones are based on intercepting the migrating plumes down gradient of the source of contamination by injecting reagents and solution in predetermined locations within the contaminated groundwater plume and allowing them to react with the contaminants. This allows the groundwater to flow freely and it is not directed to any subsurface barrier as in the case of PRB technology. Reagent based reactive zones accomplish reduction and precipitation in the reactive zones by using chemicals. Injection of a carbohydrate solution such as dilute molasses can promote the in situ reduction of Cr+6 to Cr+3. Degradation of molasses leads to anaerobic (reducing) conditions which bring about the transformation. Unicellular yeast and other micro processes have the capacity to bio accumulate metals and many bacteria reduce Cr+6 to Cr+3.
Saccharomyces cerevisiae did not result in any unpleasant or side effects such as odour, taste or pathogenecity. Dissimilatory metal reducing bacteria may also bring about a reduction in metals. Biomineralisation involves use of specific microbial strains to form geologically stable minerals which can incorporate and immobilize metal contaminants. Soil flushing and chromium extraction could be utilized for contaminated soil and source remediation.
Electrokinetic remediation is the application of electric current to ground water for remediation. This treatment concentrates contaminants in the solution around electrodes. Contaminants can be removed from this solution by electroplating or precipitation at the electrodes or by pumping the contaminant, recovering the extracted metal and reusing the processing fluid.
Natural Attenuation refers to a reduction in contaminant concentration and toxicity without human intervention and through natural processes like bio-degradation, dispersion sorption, dilution, volatilization, chemical or biological stabilization, transformation or destruction of contaminants.
Phytoremediation utilizes plants to remediate contaminated soil and ground water by taking advantage of the plants natural abilities to take up, accumulate and/or degrade inorganic and organic constituents. Selection of appropriate technologies is dependent on a number of features like the source contaminants destruction and concentration, the ground water chemistry, the lithology of soil, porosity, grain size, soil moisture, the total organic carbon and dissolved organic carbon in groundwater and soil, the iron and manganese content, the ground water and soil micro flora and fauna, Cr+6, Cr+3, electrical conductivity, pH of groundwater. It is also important to estimate the contaminant distribution, transport, geologic setting, geologic structure, stratigraphy and ground water hydrogeology. Laboratory and pilot scale tests may have to be done on technology options.