Chemical Extraction and Leaching

Chemical extraction or leaching in hazardous waste treatment is the removal of a hazardous constituent by chemical reaction with an extractant in solution. Poorly soluble heavy metal salts can be extracted by reaction of the salt anions with H , as illustrated by the following reaction  

Acids also dissolve basic organic compounds such as amines and aniline. Chelating agents, such as dissolved ethylenediaminetetraacetate (EDTA, HY ), dissolve insoluble metal salts by forming soluble species with metal ions  

Heavy metal ions in soil contaminated by hazardous wastes may be present in a coprecipitated form with insoluble iron(lll) and manganese(IV) oxides, Fc203 and Mn02, respectively. These oxides can be dissolved by reducing agents, such as solutions of sodium dithionate/citrate or hydrox-ylamine. This results in the production of soluble Fe and Mn and the release of heavy metal ions, such as Cd or Ni , which are removed with water. 

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This chapter gives an overview of decomposition methods and recent developments and applications of the decomposition of different materials. Other sample preparation methods, such as chemical extraction and leaching, solubilization with bases, enzymatic decomposition, thermal decomposition, and anodic oxidation, are beyond the scope of this contribution and are not discussed. 

Acid-base neutralization Chemical extraction and leaching Ion exchange... 

Soil extraction and leaching 250- 500 8-12 5,000-m minimum chemical recycle Residue disposal... 

James R. Fair, Ph.D. (Distillation and Absorption, Extraction and Leaching) (Co-editor), McKetta Chair Emeritus Professor, Department of Chemical Engineering, The University of Texas, Austin, TX Fellow, A.I.Ch.E., Registered Professional Engineer (Missouri and Texas)... 

Removal the contaminated soil is exposed to chemical extraction and/or thermal treatment with volatile elements or compounds, and to other leaching or immobilizing processes. It is a high-cost procedure. 

Other Metals. Because of the large number of chemical extractants available, virtually any metal can be extracted from its aqueous solution. In many cases extraction has been developed to form part of a viable process (275). A review of more recent developments in metal extraction including those for precious metals and rare earths is also available (262). In China a complex extraction process employing a cascade of 600 mixer—settlers has been developed to treat leach Hquor containing a mixture of rare earths (131). 

Recovery of Uranium from Leach Solutions. The uranium can be recovered from leach solutions using a variety of approaches including ion exchange (qv), solvent extraction, and chemical precipitation. The most common methods in practice are ion exchange and solvent extraction to purify and concentrate the uranium prior to final product precipitation. 

Chromium-containing wood preservatives and their chemical compositions are Hsted ia Table 13 (199). Chromium compounds have a triple function ia wood preservation (200). Most importantiy, after impregnation of the wood the Cr(VI) compounds used ia the formulations react with the wood extractives and the other preservative salts to produce relatively insoluble complexes from which preservative leaches only very slowly. This mechanism has been studied in the laboratory (201—206) and the field (207). Finally, although most of the chromium is reduced to chromium (ITT), there is probably some slight contribution of the chromium (VT) to the preservative value (208). 

The behavior of elements (toxicity, bioavailability, and distribution) in the environment depends strongly on their chemical forms and type of binding and cannot be reliably predicted on the basis of the total concentration. In order to assess the mobility and reactivity of heavy metal (HM) species in solid samples (soils and sediments), batch sequential extraction procedures are used. HM are fractionated into operationally defined forms under the action of selective leaching reagents. 

Several procedures have been reported for extraction of the suspected allelopathic agents from donor plants. Essentially all the procedures that were employed attempted to simulate the routes of entry of toxic substances into the natural environment. As shown previously, the allelopathic agents are released through leaves and roots, or escape into the environment as volatile materials. Table 3 suratBrizes the different extraction and bioassay procedures employed to isolate and detect the toxic chemicals (17). For extraction, the investigators used either the plant parts from the donor plants or the intact donor plants from which the suspected chemicals were leached through leaves, stems or roots. 

Distillation is probably the most widely used separation technique in the chemical process industries, and is covered in Chapter 11 of this volume, and Chapter 11 of Volume 2. Solvent extraction and the associated technique, leaching (solid-liquid extraction) are covered in Volume 2, Chapters 13 and 10. Adsorption, which can be used for the separation of liquid and gases mixtures, is covered in Chapter 17 of Volume 2. Adsorption is also covered in the books by Suziki (1990) and Crittenden and Thomas (1998). 

Finally, toxicity (defined in terms of a standard extraction procedure followed by chemical analysis for specific substances) is a characteristic of all chemicals, whether petroleum or nonpetroleum in origin. Toxic wastes are harmful or fatal when ingested or absorbed, and when such wastes are disposed of on land, the chemicals may drain (leach) from the waste and pollute groundwater. Leaching of such chemicals from contaminated soil may be particularly evident when the area is exposed to acid rain. The acidic nature of the water may impart mobility to the waste by changing the chemical character of the waste or the character of the minerals to which the waste species are adsorbed. 

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