Facilitated transport of dibutylphosphate across fixed-site membrane

The transport of dibutylphosphate (DBP) across the fixed-site membrane was studied to develop a membrane-based process for regenerating tributylphosphate (TBP) solvent for metal ions extraction. DBP is a degradation product of TBP, which is used extensively in solvent extraction as an extractant. The fixed-site membrane was prepared by anchoring poly(vinyl benzyl chloride) (PVBCl) within the pores of a poly(propylene) microporous host membrane by in situ crosslinking of PVBCl with a diamine 1,4-diazabicyclo[2.2.2.]octane (DABCO). The resulting PVBCl-filled precursor membrane was converted to anion-exchange membrane by reacting it with excess of DABCO followed by alkylation with α,α′-dibromo-p-xylene. The comparison of scanning electron microscope micrographs of membrane samples before and after anchoring indicated that pores of poly(propylene) microporous host membrane were completely filled. The immobilization of highly crosslinked anionic microgel in the pores of membrane resulted in the formation of channels having closely spaced fixed carrier site. The transport of DBP across fixed-site membrane was studied from feed compartment containing 30% (v/v) TBP in n-dodecane to receiver compartment containing aqueous solution of 2% (w/v) Na2CO3. The transport of DBP to receiver compartment was found to be more than 90% within 100 min and 200 min from feed containing 1.8 g/L and 5.9 g/L DBP, respectively. This indicated that the driving force for transport of DBP was provided by the reaction of DBP with Na2CO3. H+ ions would also have transported to receiver compartment by hopping through water channels in the membrane to maintain the electrical neutrality in both the compartments. The permeation of the tritium tagged water across the membrane was found to be quite slow as compared to DBP. This indicated that diffusion transport of DBP as a neutral molecule is not possible because of the highly crosslinked chemical structure of the microgel anchored in the pores of the membrane. The experiments were also carried out with the actual organic waste solution (30% (v/v) TBP in n-dodecane) generated during plutonium and uranium reduction extraction (PUREX) process. Contrary to synthetic feed, only 60% of DBP could be recovered by using the membrane-based method developed in the present work. The lower recovery of DBP from actual organic waste was attributed to the complexation of DBP with metal ions during solvent extraction process. The sulphuric acid wash of organic waste solution was found to be effective for the quantitative recovery of DBP by the membrane-based method.