During the production of chromium-iron complex, the separation of Cr(VI) from the feed phase and its reduction to Cr(III) are also carried out simultaneously. The optimal parameters found for SLM investigation are: concentration of Cr(VI) in feed phase 100 mg L−1, pH of feed phase 4.5, concentration of NaCl in strip phase 0.15 M, pH of strip phase 8, concentration of.
Introduction
- Liquid membrane (LM)
- Mechanism of transport of solute in LM
- Carrier
- Types of LM
- Bulk liquid membrane (BLM)
- Supported liquid membrane (SLM)
- Pollutant studied
- Chromium (VI)
- Nickel (II)
- Zinc (II)
There are two types of transport mechanisms involved in the carrier-mediated transport: (1) part of A transports freely due to the concentration gradient, which is called the solution-diffusion mechanism, and (2) A forms a complex with the carrier, which increases the solubility of A in the wearer improves. membrane phase to increase the transport rate of solute A. The selection of the carrier depends mainly on the nature of the solute present in the aqueous phase.
Literature review
LM for general application
They used BLM and SLM technology with coconut oil as solvent for the transport of Cd(II) and Pb(II) from wastewater. They prepared ELM using n-heptane by mixing iso-octyl-phenoxypolyethoxyethanol as surfactant and aliquat336 as carrier.
LM based separation of Cr(VI), Ni(II) and Zn(II)
They have used Alamine 308 as a carrier and tributyl phosphate (TBP) as a modifier to improve membrane performance. They have used tri-ethanolamine (TEA) as carrier with cyclohexanone as solvent to prepare organic phase.
Gap areas
Importance and objective of the research work
Working solutions for pollutants
The base solution (1000 mg L−1) of pollutants was prepared by dissolving the required amount of salt (K2Cr2O7 for Cr(VI), NiCl2.6H2O for Ni(II) and ZnSO4.7H2O for Zn(II)) in Milli-QR deionized water in 100 mL volumetric flask and then transfer to a 1 L volumetric flask, fill to the 1 L mark with Milli-QR deionized water and store for further use as and when required. In a similar manner, the strip phase was prepared by dissolving the required amount of stripping agent in Milli-QR deionized water.
Analytical instruments
Atomic Absorption Spectroscopy (AAS)
Other analytical instruments
Physical properties of vegetable oils
Two phase equilibrium set-up and procedure
Three phase experimental studies with BLM
Three phase experimental studies with FS-SLM
FS-SLM setup and experimental procedure
3 – FESEM analysis of membrane support (PVDF): (a) before impregnation with LM on the support, (b) after impregnation with LM on the support, (c) support with LM in the pores after 48 hours and (d) cross-sectional view of the support with LM in the pores after 48 hours of operation. The excess amount of organic liquid on the surface of the membrane was gently removed with a tissue. The prepared organic impregnated membrane support would act as the desired SLM. The empty pores are clearly visible in Figure 2.6a, while the solvent-filled pores are in Figure 2.6b.
Three phase experimental studies with FS-SLM 43. completion of 48 h experimental run and this reflects the loss of some solvent from the surface of the membrane support.
FS-SLM setup with electrochemical and electrodeposition module 43
The actual configuration of FS-SLM with in situ electrochemical reaction or electrodeposition module used in this research work for three-phase experiments is shown in Figure 2.8. For the electrochemical reaction two electrodes (one iron plate as anode, the other graphite rod as cathode) were kept inside the strip phase solution. Measuring the efficiency of precipitation/solute deposition in the strip phase is difficult with the weighing method.
This was therefore done indirectly from the feed phase and strip phase concentrations, assuming that at the time of the experiment no metal was trapped in the membrane phase.
Experimental optimization through RSM
Reaction mechanism
EDTA inhibits the normal behavior of metal ions in combination with added materials, especially the formation of coordination compounds or chelates of metal ions.
Results and discussion
Two-phase equilibrium study
- Selection of solvent
- Effect of concentration of extractant
- Effect of period of extraction
- Detection of appropriate stirring condition
- Effect of pH of aqueous phase
Furthermore, the efficiency of extraction of Cr(VI) is slightly better with sunflower oil than coconut oil. It reveals that for solutions with initial feed concentrations of 100 mg L−1 or less, the extraction efficiency reaches almost its maximum value (above 99%) at 150 rpm, and no appreciable increase in efficiency is observed beyond the speed of 200 rpm. The reaction mechanism, Equations reveal that the role of the proton is very important in the extraction process, and therefore the extraction efficiency is highly dependent on the pH value of the aqueous solution.
We observe that the extraction efficiency is unchanged (98.7%) up to an initial concentration of 100 mg L-1 Cr(VI) in the aqueous phase.
Three-phase BLM study
- Effect of concentration of strippant in strip phase
- Effect of pH of strip phase
- Effect of speed of stirring
- Effect of period of operation
- Effect of concentration of carrier
- Fed batch system
The effect of pH in the strip phase (i.e. 0.03 M Na2-EDTA solution) on the extraction and recovery of Cr(VI) was investigated by varying the pH within the range 3–9. Effects of agitation speed on transport of Cr(VI) need to be re-evaluated in three-phase operation. Therefore, the effects of carrier concentration on simultaneous extraction and recovery of Cr(VI) are investigated.
On the other hand, Cr(VI) recovery is almost constant beyond 1% (vol/vol) aliquat 336 due to the formation of a stable solution–carrier complex.
Three-phase SLM study
- Effects of concentration of strippant and pH in strip
- Effect of speed of stirring
- Effect of period of operation
- Effect of concentration of carrier
- Fed batch system
As expected, the increase in stirring speed from 60 to 120 rpm manifested in higher % extraction and % recovery of Cr(VI), reaching over 80% extraction and over 70% recovery of Cr(VI), much higher recovery than in BLM. Interestingly, it is also observed that % extraction and % recovery show decreasing trend after 68 hours of operation. It is also noted that %extraction and %recovery of Cr(VI) do not increase significantly after
The effect of concentration of aliquat 336 in organic phase on extraction and recovery was studied at concentrations in the range 0-3%.
Experimental optimization of operating parameters in SLM
In both cases, the quadratic models, F-values are quite high and p-values<0.0001 (see Table 3.6) indicate strong significance. From Table 3.6 it can be seen that sufficient precision value for both models is quite high. According to these results, the empirical model obtained from CCD (Table 3.5) can be used as an analyst for the optimization of the selected variables.
Based on the analysis of the experimental data, the relationship between the process factors (i.e. band phase concentration, pH and band phase concentration) and the desired responses (i.e. % extraction and % Cr(VI) recovery) is graphically shown in Figure 3.21. .
Summary of studies on the separation of Cr(VI) through BLM and SLM 82
Reaction mechanism and transport methodology
However, in this work, NaCl is used as the scavenging agent instead of Na2-EDTA, because the electrochemical reaction requires the presence of an electrolyte. The solute-carrier complex diffuses from the feed membrane to the strip membrane interface due to the concentration gradient. The solvent is re-extracted from the solute carrier complex with a scavenger (NaCl) and binds with sodium ion to form HCrO−4Na+ at the strip membrane interface and diffuses back into the strip phase in the bulk.
Since the Fe2+ ion is unstable, it tends to oxidize by reducing Cr6+ to Cr3+ (see.
Model calculation
Then the total flux J (mg cm−2 s−1) can be calculated according to the following equation:. 4.10) where V is the volume of aqueous solution and S is the effective area of strip-membrane interface (cm2).
Results and discussion
Three-phase SLM study
- Selection of strippant
- Effect of pH of feed phase
- Effect of concentration of strip phase
- Effect of pH in strip phase
- Effect of concentration of carrier
- Required period of operation
- Effect of stirring in aqueous phase
- Fed batch system
Therefore, the concentration of Na+ ions in the stripping phase solution affects the transport rate of chromium. While the chromium concentration in the feed phase decreased rapidly when an electric field was applied, it reached ~3 mg L−1 after 8 h. The concentration of chromium in the stripping phase never increased above 2.5 mg L−1 upon application of an electric field and eventually reached below the detection limit.
The confirmation of presence of chromium in the precipitates in strip phase was discussed in Section 4.2.2.3.
Synthesis and characterization of chromium-iron oxide
- X-ray diffraction (XRD) spectroscopy
- FESEM
- TEM
- FTIR
The characteristic peaks (see Figure 4.11) at 2θ exactly match the crystalline plane reported elsewhere [204]. The TEM image in Figure 4.14 shows that the range of particle size is within 100 nm. An energy dispersive X-ray (EDX) in TEM (refer Figure 4.15) was also done for elemental analysis.
Fourier transform infrared (FTIR) spectroscopy (manufacturer: Shimadzu, model: IRAffinity-1) of chromium iron oxide (see Figure 4.16) was performed by mixing the calcined complex with an excess of dried potassium bromide (KBr).
Experimental optimization of operating parameters in SLM
The scatter points of predicted values across the horizontal line of residuals (see Figure 4.17a) are in the ±3% range, suggesting a good fit of the model. These predicted values were compared with experimental results to verify the consistency and acceptability of the empirical model (see Figure 4.17b). To understand the interaction between parameters or the relationship between input factors (i.e. concentration of NaCl in strip phase, pH of strip phase and concentration of aliquat in sunflower oil) required for optimum condition for % precipitation of chromium is illustrated in Figure 4.18.
The experiment was carried out according to the optimized parameters and the precipitation of chromium was found to be 88.48%.
Summary of studies on electrochemical separation of Cr(VI)
Reaction mechanism and transport methodology
The complex of metal ion binds with carrier to form a metal-carrier complex (R3NH+MCl2−4), which diffuses over the organic phase to the strip-membrane interface due to a concentration gradient. The metal carrier complex releases metal ion which binds with ammonium chloride in the strip phase to form a metal-ammonium complex, viz. M(NH3)2Cl2. The carrier diffuses back to the feeder-membrane interface and rebinds with another metal ion.
When an electric potential is applied to the strip phase, the metal ions present in the strip phase are deposited on the cathode surface.
Results and discussion
Two-phase equilibrium study
- Selection of solvent-extractant combination
- Effect of initial concentration of solute in aqueous phase114
- Effect of concentration of extractant
- Effect of period of extraction
- Detection of appropriate stirring conditions
Corresponding experiments were performed with initial aqueous phase solute concentrations in the range of 50-500 mg L-1. Therefore, it is fair to maintain 50-100 mg L-1 initial solute concentration in the aqueous phase. The presence of HCl in the aqueous phase helps the metal ion to form a metal chloride complex.
An increase in the stirring speed increases the extraction of Ni(II) up to 200 rpm.
Three-Phase SLM Study
- Selection of strippant
- Selection of suitable cathode
- Importance of surface area of cathode
- Saturation point in period of operation
- Effect of pH in feed phase
- Effect of pH in strip phase
- Effect of concentration of carrier
- Effect of concentration of strip phase
- Role of electric field for transportation of metal ion . 128
- Effect of stirring in aqueous phase
When the electric potential is applied in the stripping stage, metal is deposited on the cathode plate. The concentration of NH4Cl in the stripping phase solution will influence the transport of the metal ion. An increase in the stripping phase concentration leads to an increase in the deposition of metal ions on the cathode plate.
The change in the concentration of the metal ion in the feeding and stripping phases with and without electric field in the stripping phase is shown in Figure 5.12.
Simultaneous separation of metals
- Simultaneous transportation and electrodeposition of
- Simultaneous transportation and electrodeposition of
- Experimental optimization of proportions of solute
The kinetics of the transport and deposition of metals at a solute ratio of Zn(II)/Ni(II) = 1:4 was measured. The higher the ratio of Zn(II)/Ni(II), the higher will be the preference of Zn(II) over Ni(II). Chapter 5. Electrodeposition of Ni(II) and Zn(II) Table 5.5: Three-phase SLM-EP study: % extraction and % electrodeposition of Zn(II) and Ni(II) from their mixed feed in various proportions using 5% (v /v) TOA and 3% (v/v) D2EHPA as carriers.
Residuals versus predicted graphs for % deposition of Zn(II) and Ni(II) are plotted in Figures 5.19a and 5.19b.
Summary of studies on electrodeposition of Ni(II) and Zn(II)
The experiments were carried out using two-phase equilibrium study, three-phase BLM study and three-phase FS-SLM study. X In FS-SLM study for separation of Cr(VI) combination of PVDF-sunflower oil-aliquat 336-Na2-EDTA was found suitable. X Na2-EDTA was found to be the best stripping agent and the yield of % extraction and % recovery of Cr(VI) was 80% and 73% respectively in FS-SLM study.
X The electrochemical reaction was successfully carried out for the synthesis of chromium-iron oxide using FS-SLM technology where the Cr separation efficiency reached about 97%.
Recommendations for future work
Effect of different anions on supported liquid membrane separation of copper and zinc using TOPS-99 as a mobile carrier. Separation of Cd(II) from its aqueous solution using environmentally friendly vegetable oil as a liquid membrane. Extraction of hexavalent chromium by an environmentally friendly green emulsion liquid membrane using tridodeciamine as an extractant.
Selective transport of mercury as HgCl2−4 across a liquid membrane using K+-dicyclohexyl-18-crown-6 as a carrier.
Procedure for analysis of Cr(VI), Ni(II) and Zn(II) by AAS
Preparation of the sample
Preparation of standards
