Heavy metals are carcinogenic and not biodegradable. Among the various heavy metals hexavalent chromium (Cr(VI)) is considered one of the most deleterious pollutant. Tanning, electroplating, dyeing, paints/pigments and mining are the major industries that are responsible for chromium (Cr) contamination to natural water body (Chakravarti et. al., 1995; Chiha et. al., 2006). Separation or removal of hexavalent chromium (Cr(VI)) through liquid membrane (LM) is very promising technology. Among the various LM techniques supported liquid membrane (SLM) technique can be used as an effective substitute to traditional solvent extraction methods (Chakrabarty et. al., 2010; Bhatluri et. al., 2014). Some of the noted advantages of SLM are the low volume of solvent required, low operating cost and energy consumed. It can be seen that most of the researchers, dealing with Cr(VI) transport through SLM, used conventional hazardous solvent. Djane et al. (1999) have reported separation of total chromium, Cr (III) & Cr (VI), through SLM using two serially connected membrane units with kerosene as a solvent. They have collected samples from the drainage path of the tannery industry to extract total chromium. Venkateswaran et al. (2005) have used tri-n-butyl phosphate (TBP) as a carrier to separate Cr(VI) through SLM where polytetrafluoroethylene (PTFE) membrane used as a membrane support. They have optimized parameters to maximize the Cr(VI) separation and tried applicability of those parameters with real plating effluent. There are very few researchers who have explored the environmentally benign solvent in supported liquid membrane (SLM) to separate the heavy metal. The present study explores the potential of environmentally benign solvent with SLM technique for simultaneous extraction and recovery of Cr(VI). With the context of environmentally benign solvent, sunflower oil used as a green solvent and n-methyl-n,n,n-trioctylammonium chloride (a.k.a Aliquat 336) as an extractant/carrier, where Aliquat 336 showed very good extraction capabilities for transport of Cr(VI). Various physico-chemical parameters are tuned to optimize the process, which would be more relevant for commercial operation. Di-sodium ethylene-di-amine-tetraacetic acid (Na2-EDTA) was selected as stripping agent for its affinity towards metal. SLM method has proved very effective method over the other traditional solvent extraction methods. Polyvinylidene fluoride (PVDF) was used as a support in order to hold and contain the membrane liquid in the SLM structure. The size of the pores in the support is 0.22 μm. The transport performance of Cr(VI) was affected by various parameters such as pH of feed phase, pH of strip phase, concentration of strip phase, concentration of carrier, stirring speed and period of operation (i.e. run time). The role of Na2-EDTA as a stripping agent is very crucial for recovery of Cr from membrane phase. Various strip concentration ranges from 0.005 - 0.04 M had been experimented to check the effect of concentration of Na2-EDTA on the recovery efficiency. Both extraction and recovery are maximum at 0.03 M Na2-EDTA. However, beyond 0.03 M concentration both extraction and recovery start declining. This is due to crowding effect of excess Na2-EDTA in strip phase which inhibits the transport of Cr(VI) from strip-membrane interface to bulk of strip phase. The pH of strip phase plays a vital role on extraction and recovery of Cr(VI). The kinetics of extraction and recovery were examined in the pH range of 4 to 9. The recovery of Cr(VI) was increased with increase in pH and reached its optimum value at pH 6.5. Again beyond pH 6.5 or at higher pH the recovery slowly decreased. Aqueous solution of Na2-EDTA is stable at lower acidic condition, but very low pH, Cr ions compete with H+ ions and create problem to form Cr-EDTA complex. On the other hand, at very high pH metal ions are prone to form insoluble hydroxide and it makes metal ions less accessible to EDTA. The period of operation or time is an important parameter for any chemical process viability in case of industrial scenario. The experiment was performed for 48 h and samples were periodically taken. Maximum extraction and recovery of Cr(VI) after 48 h are over 80% and 70% respectively. The effect of carrier (i.e. Aliquat 336) concentration in organic phase on extraction and recovery were studied at concentration ranging from 0-3%. The extraction and recovery of Cr(VI) increased with increase in carrier concentration and reached its maximum at 1 volume% Aliquat 336 concentration. With further increase in carrier concentration the extraction and recovery of Cr(VI) are decreased. Carrier helped to form solute-carrier complex at the feed-membrane interface which enhance the diffusive mass transfer rate, but beyond 1 volume% carrier concentration the extraction rate of Cr(VI) decreased because of increase in viscosity of membrane phase which resist the diffusion rate of solute-carrier complex. Over 80 % extraction and over 70 % recovery were obtained with following operating conditions: concentration of carrier = 1% (v/v) aliquat 336, pH of feed phase = 4.56, pH of strip phase = 6.5, concentration of strip phase = 0.03 M, stirring speed =120 rpm and run time = 48 h. The results of SLM study can further be used to design the three phase hollow fibre membrane (HFM) based separation process.
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