Leveraging Liquid-Liquid Extraction for Contaminant Removal

This article presents case studies on how to design efficient, circular industrial wastewater treatment processes for the removal of emerging contaminants.

Liquid-liquid extraction (LLE) has historically been an effective and economical technology for removing high-boiling or non-volatile components from an aqueous stream (1–3). Thus, this technology has been used to treat various wastewater streams containing compounds such as phenol, carboxylic acids, ammonium salts, dimethylformamide (DMF), aniline, nitrated organics (such as dinitrotoluene [DNT]), and fluorochemical compounds. In recent years, the U.S. Environmental Protection Agency (EPA) has identified compounds that are classified as emerging contaminants. These chemicals have a perceived, potential, or real threat to human health or the environment. Some of these chemicals are steroids and hormones, endocrine-disrupting compounds (EDCs), pharmaceuticals and personal care products, and the aforementioned fluorochemical compounds. LLE solutions should be considered for many of these contaminants because they are generally compounds that have higher boiling temperatures than water.

Two types of LLE processes should be considered for removing and recovering any such contaminants from a wastewater stream: solvation extraction and solvation with reaction extraction. Solvation extraction most often uses a solvent with a lower boiling temperature than water, while solvation with reaction extraction uses a solvent with a higher boiling temperature than water. A brief description of both technologies follows.

Solvation extraction

Solvation extraction is based on the solute’s (i.e., contaminant to be removed) liquid-liquid equilibrium relationship between the aqueous feed and the solvent. This is typically measured by the distribution coefficient (m), which is defined as the concentration of the solute in the solvent phase divided by the concentration of the solute in the feed phase at equilibrium. Thus, a key factor in employing this technology effectively and economically is to select a solvent that has a lower boiling point than water and has the highest possible distribution coefficient for the contaminant to be removed while having the lowest possible distribution coefficient for the components that are not to be removed. This is defined as selectivity.

A typical extraction process using a low-boiling solvent is shown in Figure 1. The process includes a minimum of three separation steps. In the extraction column, the low-boiling solvent is contacted with the feed counter-currently, producing a raffinate stream (i.e., the aqueous phase after removing the contaminant) and an extract phase (i.e., the organic phase consisting of the solvent with the contaminant). The raffinate phase leaving the extraction column will contain the amount of solvent that is soluble in the water. Typically, a stripping column removes and recovers this solvent while generating a clean wastewater stream that can be recycled to the plant or sent to a wastewater treatment facility. The extract phase leaving the extraction column is sent to a distillation column where the lower-boiling solvent is recovered overhead, and a concentrated form of the contaminant is the bottoms stream...