Room Temperature Ionic Liquids for Separating Organics from Produced Water

Abstract

The distribution of polar organic compounds typical of water contaminants (organic acids, alcohols, and aromatic compounds) associated with oil and gas production was measured between water and nine hydrophobic, room‐temperature ionic liquids. The ionic liquids used in this study were 1‐butyl‐3‐methylimidazolium bistrifluoromethanesulfonylimide, 1‐hexyl‐3‐methylimidazolium bistrifluoromethanesulfonylimide, 1‐octyl‐3‐methylimidazolium bistrifluoromethanesulfonylimide, 1‐butyl‐3‐methylimidazolium hexafluorophosphate, trihexyltetradecylphosphonium bistrifluoromethanesulfonylimide, 1‐butyl‐1‐methyl‐pyrrolidinium bistrifluoromethanesulfonylimide, trihexyltetradecylphosphonium dodecylbenzenesulfonate, tributyltetradecylphosphonium dodecylbenzenesulfonate, and trihexyltetradecylphosphonium methanesulfonate. Sensitivity of the distribution coefficients to salinity, temperature, concentration, and pH was investigated. Partitioning into the ionic liquid varied considerably. Acetic acid did not significantly partition into the ionic liquid phase, except for the sulfonate‐anion ionic liquids. The solubility of hexanoic acid in the ionic liquids was significant, where uptake of the protonated form from aqueous solution was observed for all of the ionic liquids studied. Other organics also showed high distribution coefficients, up to several hundred in the case of toluene and 1‐nonanol. The distribution coefficients for toluene, 1‐nonanol, cyclohexanone, and hexanoic acid were independent of ionic liquid‐to‐water ratio over the range from 0.02 to 1.0. The ionic liquids showed a large capacity for some organics, with solubilities measured above 100 g·L−1. Regeneration of the ionic liquids by rinsing and heating was studied, with mixed success. These experiments show that certain hydrophobic ionic liquids do have an affinity for organic contaminants in aqueous solution. However, practical application of the ionic liquids tested for detection or removal of selected water‐soluble organics from the aqueous waste streams appears to be limited by the small, but significant, solubility of the ionic liquids in the aqueous phase and by difficulty in solvent regeneration. Further work aimed at determination of ionic liquids that dissolve target compounds and are nonhazardous and less soluble in aqueous solutions is recommended.