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Supercritical fluid extraction of thorium from tissue paper matrix employing organophosphorus reagents

Summary Feasibility of supercritical CO2 extraction of thorium from tissue paper matrix was studied and various organophosphorus reagents (TBP, TOPO, TPP, TPPO, TBPO) were evaluated as co-solvent. Effects of pressure (100–300 atm), temperature (313–353 K), flow rate (0.5–3 mL min−1), static time (0–40 min) and extraction time (5–40 min) on thorium extraction efficiency were investigated. Pressure of 200 atm, temperature of 333 K, CO2 flow rate of 2 mL min−1, 20 min static period followed by 20 min of extraction were found to be optimum. Under optimum conditions employing various organophosphorus reagents as co-solvent, extraction efficiency indicated the trends: (i) Among phosphates, if aliphatic group attached to P was replaced by aromatic group, then extraction efficiency decreased e.g. TBP (53%±8%) and TPP (39%±5%). (ii) Among the phosphine oxides, higher the aliphatic chain length attached to P, higher was the extraction efficiency e.g. TOPO (68%±4%) and TBPO (50%±9%). (iii) In case of phosphine oxides, if phenyl group was attached to P i.e. TPPO (59%±15%) then extraction efficiency was intermediate to TOPO and TBPO. (iv) Between phosphates and phosphine oxides, for aliphatic hydrocarbons, the extraction efficiencies were comparable e.g. TBP and TBPO. For aromatic ring, extraction efficiency was higher in phosphine oxides e.g. TPP and TPPO. Both TBPO and TPP have shown higher extraction efficiency in combination with TBP. The solvents dissolving oraganophosphorus reagents were found to influence extraction efficiency. The extraction efficiency was found to decrease slightly with increasing aliphatic chain length of the alcohol. The maximum extraction efficiency was obtained with 0.2 M TOPO in methanol where thorium was extractable with 68%±4 efficiency.