Theoretically chemical treatment involves the use of any kind of chemical (s) that can react with the hazardous wastes or soil, and convert them to either nonhazardous or less hazardous compounds. Chemical treatment is important when the soil or groundwater is contaminated by acidic chemicals, alkaline chemicals, toxic heavy metals , or toxic organics that cannot be treated by biological processes .
The most common chemical treatment processes for site remediation include:
• pH adjustment. Acidic chemicals are used to neutralize basic hazardous wastes or soils, while alkaline chemicals are used to neutralize acidic hazardous wastes or soils.
• KPEG chemical treatment. Chemical reagents prepared from polyethylene glycols and potassium hydroxide (KPEG) have been demonstrated under mild conditions (25-140°C) to dehalogenate PCBs, PCDDs, and PCDFs with destruction efficiencies exceeding 99.9999%. The reaction mechanism is nucleophilic substitution at an aromatic carbon.
• Chemical precipitation and stabilization. Chemical precipitation agents (such as hydroxides and sulfides) are used to precipitate, fix, and separate heavy metals, in turn, to purify the soil and groundwater .
The pH adjustment is one of the most common site remediation technologies, while KPEG chemical treatment of one of the five best demonstrated available technologies (BDAT).
In the KPEG reagent preparation, KOH reacts with HO-PEG (poly ethylene glycol; with a molecular weight approximately=400) to form KO-PEG (alkoxide) and water. The KO-PEG (alkoxide) in turn reacts initially with one or more of the chlorine atoms on the aryl ring (aryl-Cl) to produce aryl-O-PEG (ether) and KCl (potassium chloride) salt.
In 1986, a 2700 gallon KPEG reactor was used in Montana, on a wood-preserving site, and in Washington, on a waste disposal site, to successfully detoxify PCDDs and PCDFs (120 ppb to 200 ppm) in 17,000 gallons of liquid waste to nondetectable levels. A reactor designed to treat both liquids and solids was tested by US Department of Defense sites. These field studies validated conditions for destruction of PCBs, PCDDs, and PCDFs, to acceptable levels required by the regulations .
Wang etal. [23-25,41,45,68,69] have developed a physicochemical sequencing batch reactor (PC-SBR) process, which is identical to a conventional biological sequencing batch reactor (SBR), except that the PC-SBR is a 100% physicochemical process. The PC-SBR has been adopted in full scale for recovering toxic chromium from tannery wastewater for reuse at Germanakos SA Tannery near Athens in Greece . Naturally, PC-SBR can be adopted for chemical treatment of contaminated soil slurry or contaminated groundwater. Since the process equipment of conventional biological SBR can be adopted for the PC-SBR process operation, the PC-SBR process equipment is considered to be commercially available .
Figure 10 shows a typical chemical treatment system for removal of toxic chromium from a contaminated groundwater. The process is developed by Geochem of Terra Vac Co., Lakewood, CO. In operation, the contaminated groundwater is brought to the surface and treated using conventional treatment systems, such as ferrous ion (Fig. 10). Next, a reductant is added to the treated water, which is reinjected around the plume margin. Here it reacts with and reduces residual levels of chromium, forming a precipitate. Such reinjection creates a "barrier" of elevated water levels around the plume, enhancing the gradient and associated hydraulic control.
The reinjection also allows for in situ reduction and subsequent fixation of residual chromium. The process is capable of treating dissolved hexavalent chromium in groundwater at concentrations ranging from the detection limit to several hundred ppm.
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