Environmental Science
A solvent used in the extraction process. It is required to possess the ability to selectively dissolve specific components within a liquid or solid mixture. In the case of liquid-liquid extraction, the solvent must also be insoluble or only sparingly soluble in the solution being treated. Extraction is a common physicochemical method for wastewater treatment. When an extractant is introduced into wastewater, a mixing and mass-transfer process causes the solutes (pollutants) in the water to dissolve into the extractant. The extractant is then separated from the wastewater based on density differences; the wastewater is purified to a certain degree, while the solutes (pollutants) are recovered from the extractant, allowing the extractant itself to be regenerated and reused. Examples include the recovery of phenols from phenolic wastewater and lanolin from wool-washing wastewater using extractants. Common extractants include benzene, xylene, light tar, butyl acetate, tricresyl phosphate, and isopropyl ether.
Civil Engineering & Construction
A solvent used to achieve extractive separation. It may consist of a single component or a mixture of multiple components. Requirements for extractants used in wastewater treatment include: a high partition coefficient for the target substance to minimize extractant usage and maximize efficiency; insolubility or low solubility in water to reduce extractant loss; physical and chemical properties significantly different from those of water-such as a distinct density difference allowing for gravity-based separation, or a significant difference in boiling points among the solvent, water, and solute to facilitate solvent recovery via distillation or evaporation; ease of recovery and regeneration; good chemical stability; and non-toxicity, ensuring that any small amount of lost extractant does not create new toxic wastewater.
Chemical Engineering Processes
Refers to a chemical reagent capable of binding with the substance to be extracted, thereby causing that substance to transfer into the reagent. In general, an extractant is required to possess high extraction capacity (i.e., a high distribution coefficient) for the target component in the mixture and good extraction selectivity (i.e., a high separation factor). Additionally, it should be insoluble or sparingly soluble in the solution being extracted, have a high boiling point and flash point, exhibit high thermal and chemical stability, possess low corrosivity and toxicity, and be inexpensive and readily available. Extractants may consist of a single-component solvent or a multi-component solvent mixture. Examples include the use of tetraethylene glycol ether, N-methylpyrrolidone, or sulfolane to extract aromatics from catalytic reformate or hydrorefined cracked gasoline; the use of di(2-ethylhexyl) phosphoric acid (with kerosene as a diluent and tributyl phosphate as a modifier) to extract uranium from sulfuric acid leach liquors of uranium ore; and the use of trichloroethylene to decaffeinate coffee. Based on their properties, extractants can be classified into: neutral extractants (e.g., alcohols, ketones, ethers, esters, aldehydes, and hydrocarbons); acidic extractants (e.g., carboxylic acids and acidic phosphate esters); chelating extractants (which are also acidic extractants that form chelate compounds with the target ions while releasing hydrogen ions); and amine-based extractants (e.g., tertiary amines and quaternary ammonium salts). The solvent used for back-extraction is known as a stripping agent; pure water or aqueous solutions of acids, bases, or salts are typically used to strip solutes from the organic phase.