Ind. Eng. Chem. Res. 1993,32, 1186-1195
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GENERALRESEARCH Hollow Fiber Solvent Extraction Removal of Toxic Heavy Metals from Aqueous Waste Streams Chang H. Yun,t Ravi PrasadJ Asim K. Guha) and Kamalesh K. Sirkar'a Department of Chemistry and Chemical Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, New Jersey 07030
This study is concerned with the applications of the immobilized interface-based techniques to reversible chemical complexation-based solvent extraction of toxic heavy metals from industrial wastewaters using microporous hydrophobic hollow fiber (MHF) modules. Toxic heavy metals studied were copper and chromium(V1). Each metal was individually removed in separate oncethrough experiments from a synthetic wastewater by organic extractants flowing in the shell-side countercurrent to wastewater flowing in the fiber bore. The organic extractant used for copper extraction was 5-20% v/v LIX 84 diluted in n-heptane, and that for chromium extraction was 30% v/v TOA (tri-n-octylamine) diluted in xylene. A mathematical model was developed to predict the extent of copper extraction from the aqueous synthetic wastewater by the MHF module. The equilibrium constant for copper was determined to be 1.7 from experimental partitioning data. The experimental data on copper extraction in the MHF module are described well by the model if the forward interfacial chemical reaction rate constant is 9.0 X 10-6 cm/s.
Introduction Solvent extraction may be used to remove efficiently toxic heavy metals, e.g., Zn, Cu, Cr, Ni, Cd, and Hg, from effluents to environmentally acceptable levels and recycle these metals to the original processes (Ritcey and Ashbrook, 1984;Lo et al., 1983). Pilot plant studies have been made (Lo et al., 19831, but no economic information is available. This work explores the feasibility of nondispersive solvent extraction (Kiani et al., 1984; Prasad and Sirkar, 1988; Prasad and Sirkar, 1989) of heavy metals, e.g., copper and chromium (Cr6+), from a synthetic wastewater stream using microporous hydrophobic hollow fiber membrane modules and modeling the solvent extraction of copper. The first phase involved determination of the distribution coefficient/equilibrium constant of each metal between water and an organic diluent having a suitable extracting agent. This was followed by the extraction of each heavy metal using a microporous hydrophobic hollow fiber module in separate experiments with the aqueous solution flowing on the tube side and the extracting solvent phase flowing countercurrently on the shell side. The aqueous phase was maintained at a pressure higher than the organic-phase pressure (Kiani et al., 1984;Prasad and Sirkar, 1988) to keep the reaction interface at the inside wall of the lumen, since the membrane was hydrophobic. The module employed here was used earlier to extract priority organic pollutants successfully (Yun et al., 1992). Alexander and Callahan (1987)used such a hydrophobic hollow fiber membrane-based solvent extraction technique
* To whom correspondence should be addressed. t Current address: Korea Explosives Group, Daejon, South Korea. Current address: Separations Products Division, Hoechst Celanese, Charlotte, NC 28273. Current address: Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102.
for gold extraction. They demonstrated that nondispersive metal extraction is feasible by using the batch recirculation mode which, however, led to a rather limited extent of metal removal or limited extent of water purification. Continuous countercurrent contact in the once-through mode is needed for efficient water purification and larger scale use of such a technique. Kim (1984)and Haan et al. (1989)have used membranebased solvent extraction and back extraction for copper removal from a synthetic wastewater. They used two microporous hollow fiber (MHF) modules, one for extraction and the other for back extraction; the two modules were connected to each other by an organic extractant recirculating line. Haan et al. (1989)used batch operation. They achieved only a limited extent of metal removal like Alexander and Callahan (1987). Haan et al. (1989) observed aqueous-phase leakage through the hollow fiber membrane after only 83 min. This leakage may have come about from using a membrane with large pores and inappropriate pressure balance. Thus, Haan et al. (1989) had to use an aqueous-organic separator in the organic recirculation line. Kim (1984) used once-through mode and tested one MHF module (single-pass flow) or two MHF modules (recirculating flow) system with various membrane materials. He had to use an aqueous-organic separator in the organic recirculation line to remove the aqueous phase which accumulated in the organic phase by bulk leakage. A mathematical model to predict the performance of membrane solvent extraction based copper removal has been developed here for the MHF module without assuming any controlling step and using the once-through operating mode. A cubic equation has been developed for the local copper flux. The analysis incorporates variation in the axial direction of the module unlike Haan et al. (19891, who used a lumped module analysis. The model results have been compared with the experimental data. For the model predictions, binary diffusivities between
0888-588519312632-1186$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1187
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