With the recognition of perchlorate as a groundwater contaminant, a lot of effort has been made in developing a variety of technologies capable of perchlorate remediation.
The current technologies available for perchlorate remediation can be classified into destruction or removal technologies.
Major Destruction Technologies: Biological Treatment, Chemical and Electrochemical reduction.
Major Removal Technologies: Physical Treatment (Adsorption with Activated Carbon & Ion Exchange) ; Membrane Filtration.
2.1 Biological Treatment :
Bioremediation is the use of organisms to destroy or transform contaminants.Several microorganisms have been identified to reduce perchlorate to chloride. The basic principle involved is that perchlorate reducing bacteria once stimulated, in the absence of oxygen, can utilize the anion as a respiratory terminal electron acceptor, reducing it to chloride. This process occurs sequentially i.e first perchlorate is reduced to chlorite and then chlorite can be further disproportioned to O2 and Chloride by chlorite dismutase (a special enzyme that catalyzes the chemical reaction ClO2- → Cl− + O2)[2]. The oxygen is further reduced by DPRB to water.The large reduction potential of perchlorate (ClO4-/Cl- Eo = 1.287 V) makes it an ideal electron acceptor for microbial metabolism.
2.1.1 Dissimilatory perchlorate-reducing bacteria (DPRB)
• Using both culture-based and culture-independent methods, two novel genera, Dechloromonas species and Azopira species have been identified. These DPRB are considered to represent the most environmentally relevant perchlorate-reducing bacteria.
2.1.2 Perchlorate Degradation Pathway
• DPRB’S follow a three step mechanism of perchlorate reduction in which chlorate, chlorite and dissolved oxygen are sequentially produced. In the presence of an electron donor such as acetate,the pathway is :
ClO4- ClO3- ClO2-O2 +Cl-.
A two-enzyme system appears to catalyze the reaction. The initial enzyme, a reductase, reduces perchlorate to chlorate and then to chlorite . A second, highly conserved, chlorite dismutase then disproportionate the chlorite into chloride and ox- yen.
2.1.3 Factors Affecting DPRB action
• Presence of Oxygen : Perchlorate reducers are facultative anaerobes (anaerobic bacteria that are able to use oxygen) or microaerophilic bacteria, and the presence of oxygen inhibits perchlorate reduction.
• Presence of Nitrate : Most perchlorate reducers are also able to respire nitrate and, in fact, prefer nitrate over perchlorate as an electron acceptor, so the presence of nitrate will generally inhibit perchlorate reduction.
• pH : Perchlorate reduction is most rapid near neutral pH values, though perchlorate reducers are able to tolerate slightly acidic conditions (pH 5.0)
• Nutritional requirements for DPRB : Iron , Molybdenum and selenium are important for growth and also for degradation of perchlorate.
2.1.4 Advantages & disadvantages of microbial reduction.
• Biological remediation is a cost effective method for water containing high concentrations of perchlorate.
• Biological remediation results in complete destruction of perchlorate, thus eliminating disposal problems.
• Scientifically intensive procedure with must be tailored to the site specifications.
• Lack of public acceptance for the introduction of microorganisms into drinking water and also using subsequent technologies to remoe possible pathogens and nutrients supplied during the treatment process.
2.2 Physical Treatment
2.2.1 Ion Exchange
Ion exchange is one of the most effective and commonly used technology in for perchlorate removal. It is a physio-chemical process based on exchanging an anion with perchlorate ion. Strong Basic anionic exchange resins, that have a strong positively charged functional group ( typically quaternary amine R4N+) are employed to target the perchlorate ion.
R4N+Cl- + ClO4- R4N+ ClO4-+ Cl-
2.2.1.1 Types of Resins
Ion exchange resins can be classified into non-selective & selective resins.
a) Non Selective resins : These resins sorb variety of ions and have affinities for ions with multiple charges and ions at higher concentrations. Non selective resins have minimal use for perchlorate removal unless perchlorate concentration is extremely high or concentration of other ions is extremely low.
b) Selective Resins : Selective resins are resins that have high selectivity for the target compound. Selective resins for perchlorate are highly specific for perchlorate irrespective of the concentrations of other ions. The specificity is because of the attachment of the quaternary ammonium group to the polystyrene divnylbenzene matrix. The matrix is inherently hydrophobic and selects for poorly hydrated anions.
2.2.1.2 Regeneration of Resins
• Non selective resins can be regenerated with simple brine solutions. Though regeneration of these resins is easier, the low sorption potential leads to increase in frequency of regeneration and thus producing large quantities of contaminated brine solution.
• Selective resins cannot be regenerated using brine solution. They require specialized regeneration solutions for regeneration. For eg. Using FeCl4- on the spent resin in a concentrated hydrochloric acid. FeCl4- replaces perchlorate from exchange sites on the resins, and when the bound FeCl4- is exposed to a netural pH solution, it dechlorinates, producing a net positive or neutral charge, and then is quickly desorbs. Only the acid stabilized eluent contain perchlorate.
2.2.2 Activated Carbon Adsorption
Another method for perchlorate removal is using activated carbon by using its natural exchange sites or sites added to the carbon by modification.
Virgin Granular Activated Carbon is not very effective to remove perchlorate, but modifying it gives efficiency comparable to ion-exchange and microbial treatment. Modifications include preloading the carbon with iron and oxalic acid, cationic polymer and cationic surfactants. A research Chen et Al,(3) showed that modifying the AC surface with ammonia resulted in 4 fold increase in the perchlorate adsorption capacity of the GAC increased. This was due to the fact that the increase in the nitrogen content on the carbon resulted in an increase of positive charge on its surface hence increasing the adsorption capacity.
2.2.2.1 Regeneration
Activated carbon can be regenerated thermally by heating at 800oC, thus burning off perchlorate. This eliminates the need for brine and the problems related to the disposal of regenerate stream as discussed in ion exchange resins.
2.2.3 Advantages & Disadvantages of Physical Treatment
• Physical Adsorption particularly ion exchange can rapidly remove perchlorate from water, even when it is at low concentrations
• Activated Carbon is already used in water treatment, thus it would be easier to retrofit it to target perchlorate removal. The ability to remove both organic compounds and perchlorate simultaneously would be advantageous.
• Physical adsorption doesn’t lead to contaminant destruction , thus disposal technologies need to be developed.
• Physical treatment method are quite expensive. Developing specific resins & regeneration solution or modifying the activated carbon surface drive up the process costs.