Extraction and Recovery of Various Wastewater Contaminants Using Liquid Membrane Techniques

Abstract

This work demonstrates a systematic investigation on extraction and recovery performances of characteristically different species having environmental and industrial importance in Liquid Membrane (LM) transport. The investigated species were: lignosulfonate (LS), congo red (CR) and methylene blue (MB) dyes, hexavalent chromium and curcumin. One major objective of this work was to use renewable vegetable oils as the LM diluent or solvent (green solvents). In a typical bulk liquid membrane (BLM) experiment, detailed 2 and 3 phase studies were carried out for selection of suitable stripping agent, LM solvent, carrier along with estimation of optimum process conditions like carrier concentration, feed phase pH, feed and strip phase concentrations, phase ratios, temperature and stirring rate to maximize the extraction and recovery percentages of the targeted species. Additionally, kinetic estimation of data for individual species in the BLM transport was conducted. Highest extraction (E) and recovery (R) percentages of LS were measured at 92.4 and 75.2 respectively in sunflower oil/tri-n-octylamine (TOA) system using Na2SO4 of solution strength of 0.2 (M) as the stripping medium. 4 vol.% TOA concentration, feed phase pH~2 at 300 rpm stirring speed and 40oC temperature were the optimum manifold conditions with higher rate constants for stripping ( ) than extraction ( ) process, explaining extraction to be the rate determining step. Between %E and %R of two contrastingly different dyes viz. CR (anionic) and MB (cationic), it was observed that the transport of CR was favored using sunflower oil/TOA (4%) system with Na2CO3 (1 M) as effective stripping medium, leading to 99.9 %E and 75.6 %R. Meanwhile, MB reported a 95.2 %E and 92.4 %R using sunflower oil as solvent, phenol as carrier (1 M) and H2SO4 (1.25 M) as stripping medium. Interestingly, CR preferred a highly acidic feed medium (pH~2) whereas MB transport was effective in an alkaline feed condition (pH~12). BLM separation of hexavalent Cr showed maximum 99.2 %E and 94.8 %R using sunflower oil/TOA (2%) as LM medium and CH3COONa (0.2 M) as stripping medium. curcumin’s (or, Curcuma Longa) transport inside the BLM ensemble was most effective using sunflower oil as the diluent, TOA (4%) as carrier and NaOH (0.75 M) as the stripping agent. The overall %E and %R for curcumin was reportedly 99.7 and 96.5 respectively. Equilibrium distribution time for various investigated species were LS (3 h), CR (8 h), MB (8 h), Cr (VI) (2 h) and curcumin (2 h) 2 k 1 k respectively. The estimated values of activation energies for various species during extraction and recovery processes were: (36.85, 54.79 kJ mol-1) for LS, (45.92, 74.05 kJ mol-1) for CR, (25.87, 36.94 kJ mol-1) for MB, (118.71, 90.68 kJ mol-1) for Cr (VI) and (118.83, 109.54 kJ mol-1) for curcumin respectively. Subsequently, transport of LS and Cr (VI) via supported liquid membrane (SLM) was investigated where sunflower oil/TOA was immobilized on the porous network of hydrophobic polytetrafluoroethylene (PTFE) to be used as the membrane medium. LS reported E and R percentages of 90.7 and 64.2 respectively whereas Cr (VI) reported values of 97.6 (%E) and 94.4 (%R). The results were reproducible even after 12th cycle for Cr (VI) with each cycle lasting of 8 h. However, with LS, the results were reproducible upto 2 cycles (each cycle lasting for 12 h). The SLM were found to be stable for 8 (LS) and 15 (hexavalent Cr) experimental runs with 24 and 16 h between each run respectively. The emulsion liquid membrane (ELM) study with Cr (VI) yielded %E of 99.2 with very short (~ 5 min) equilibrium time. MIL-101 (Cr) metal organic framework (MOF) synthesized microwave route reported exceptionally high specific surface area (ca. 3075 m2g-1) and aqueous phase stability and high adsorptive loading for the targeted species viz. LS and CR. Adsorption sweep experiments showed an overall improvement in extraction percentage of LS from 92% (regular BLM study) to ca. 97%. The recovery percentage of LS was decreased from ca. 75% (regular BLM study) to <10% because of adsorption of LS from bulk stripping medium, leading to an increase in concentration gradient across LM-strip interphase. For CR, adsorption sweep had minimal influence in the overall transport vis-à-vis LS. Favorable chemistry of the stripping medium was found to be important for maintaining suitable pH conditions for the success of adsorption sweep process.