Extraction of arsenic-containing anions by supercritical CO₂ with ion-pairing

The extraction of arsenic-containing anions from solid matrices by supercritical CO₂ (sc-CO₂) extraction coupled with ion-pairing was investigated. Tetra-butylammonium bromide (Bu₄NBr), tetra-hexylammonium bromide (Hex₄NBr), tetra-octylammonium bromide (Oct₄NBr), dodecyltrimethylammonium bromide (Dod(Me)₃NBr), myristyltrimethylammonim bromide (Myr(Me)₃NBr) and hexadecyltrimethylammonium bromide (Hex(Me)₃NBr) were used as the ion-pairing reagents. A model of the extraction of the arsenic-containing anions by supercritical CO₂ extraction with pre-ion-pairing was proposed, and verified with experimental results. The extraction efficiency depends on two parameters, which are the theoretical extraction efficiency (E_max) and the rate constant (k). The effects of the molecular structure and size of the ion-pair reagent on E_max and k were discussed. The process parameter effects on the extraction efficiency, including the temperature, pressure, the mole flow rate of CO₂, the mole ratio of alcohol to CO₂ and the dosage of the ion-pair reagent were investigated using But₄NBr and Myr(Me)₃NBr as the ion-pairing reagents.