Recovery of zinc from electroplating wastewater using green emulsion liquid membrane Open Access

Pollution of water bodies due to metal contamination has gained significant attention recently due to the volume of the contaminated wastewater generated and recalcitrant nature of the metals. Use of metals has increased due to their increased use in automobile, infrastructure, insulation manufacturing and other specialty industries. Zinc, a metal used extensively galvanization applications, is a potential candidate used in corrosion protection through plating techniques. Zinc emanated from electroplating industries contributes a huge amount in the wastewater and is required to be treated or recovered. Due to the toxic nature of zinc, the disposal of untreated wastewater contaminated with zinc is unacceptable and leads to bioaccumulation of the metal in the ecosystem (Wang et al. 2019). Also, presence of zinc in water causes heath disorders like bone disorders, pancreatic dysfunctional, vertigo and other neurological damage when present in high concentrations (Hafshejani et al. 2015; Fernandez-Gonzalez et al. 2019).

Treatment of metal contaminated water has been investigated using several physico-chemical methods like adsorption, coagulation, precipitation and ion exchange. Many of these methods are considered to be inefficient in terms of low recovery rates at low metal concentrations, excessive chemical usage leading to secondary pollution, lack of eco-friendliness and disposal difficulties (Ma et al. 2018). Emulsion liquid membrane (ELM) is identified as a suitable recovery method for metals found in wastewater and other organic and inorganic components (Kislik 2010; Jusoh & Othman 2017). ELM, proposed as an advanced membrane process, operates through creation of a three phase system and enhanced mass transfer areas achieved by the water–oil emulsion globules. Emulsion stability is reported as one of the key factors contributing to extraction efficiency (Liu et al. 2017; Albaraka 2020). Design of experiments, a methodical approach for studying the effect of operating variables, is a key requirement for effective statistical analysis and development of a new approach. Response surface methodology (RSM) is a popular technique which helps identify the optimal conditions through a constructive set of desired experiments (Benyahia et al. 2014; Seifollahi & Rahbar-Kelishami 2019). Studies on removal of phenol using ELM has been proved to yield 83% removal efficiency under optimal conditions of treat ratio, agitation speed, extraction time and ionic liquid concentration (Rosly et al. 2019). Extraction of lead using ELM has reported a maximum recovery of 82.61% by Aliquat 336 at an acidic pH (5.5) from a feed phase of lead (II) nitrate of 207.2 ppm under 30 min of stirring at 210 rpm (Mesli & Belkhouche 2018). Application of ELM for recovery of amoxicillin was reported using RSM optimization technique and the extraction yield achieved was 99.8%. Removal of zinc has been studied using modified carbon nanotube membranes (Ali et al. 2019), cedar ash biosorption (Hafshejani et al. 2015), coconut shell-based activated carbon fiber adsorption (Shrestha et al. 2013), nano materials (Kocanova et al. 2017) and acid leaching (Wang et al. 2019). The novelty of this study is achieved by the utilization of used cooking oil as a carrier for the recovery of zinc using a green emulsion liquid membrane (GELM). The effects of operating parameters, namely, external pH, surfactant concentration, internal phase concentration, zinc concentration, external phase to emulsion volume ratio and carrier concentration, on the extraction of zinc were investigated and RSM was employed to optimize the operating parameters and study the interaction effects.

The waste cooking oil used in this work was procured from a nearby restaurant. It is filtered through a white cotton cloth to remove solid impurities. Sulfuric acid (40%), the internal/stripping phase, and Span 80, the surfactant, were procured from Merck, Germany. Distilled water was used to perform the experiments. Wastewater was collected from a small scale electroplating industry and used.

The extraction of zinc was carried out in 250 mL Erlenmeyer flask at room temperature in an agitator. GELM used in this work is water-in-oil emulsion. The internal phase solution was prepared using sulfuric acid in distilled water. The liquid membrane phase was prepared by dissolving a suitable amount of Span 80 and waste cooking oil in a 100 mL beaker. To form the GELM, the internal phase solution was added dropwise to the oil with the aid of a burette and stirred at 3,000 rpm in a magnetic stirrer for 12 min (Bjorkegren & Karimi 2012; Laki & Kargari 2016). A fresh emulsion was prepared for each experiment using the above said procedure.

β_i,β_ii,β_ij are coefficients estimated from regression and represent the linear, quadratic and interaction effect, respectively: β_{0 is} the intercept term: X_i, X_j … X_k are the input variables in coded form.

Figure 1 shows a good match between the experimental and predicted zinc extraction (R² = 0.9364). Hence the above Equation (1) predicted the percentage of zinc extraction effectively. The ANOVA results obtained were tabulated in Table 2. The model F-value of 14.18 and p-value of less than 0.05 implies that the model was absolutely significant. From Table 2, it was observed that the terms A, B, D, F, AB, AC, BE, EF, A², C², D², E², F² were the influential parameters of the model.

The external phase pH was reported to have significant effect on the extraction of metals in ELM. The extraction of zinc was studied in the pH range of 3.0–7.0. Figure 2 shows the effect of initial external phase pH on zinc extraction using emulsions. From this figure, it was inferred that, at a low pH 3, the extraction of zinc was found to be in the range of 80–85%. At pH around 3.8, maximum zinc recovery was obtained. From this figure, it was also found that the zinc extraction efficiency decreases with increase in pH above 3.8. In the pH range 3–4, effective extraction of zinc was observed and, above pH 5, there was no effect on zinc extraction from electroplating wastewater. In the pH range 3–4, extraction was high due to cation exchange reaction in which protons were released. At pH above 5, an increasing pH leads to formation of other species and similar observations were reported (Mellah & Benachour 2006; Fouad 2008). The optimal pH for maximum separation of zinc from wastewater was found to be pH 3.8.

The span 80 (surfactant) concentration sustains the stability of ELM. The effect of span 80 concentration on zinc extraction was depicted in Figure 2. When the surfactant concentration was increased from 2 to 4% (vol.), the extraction of zinc metal increased. At increasing surfactant concentration, viscosity of the emulsion membrane and emulsion stability increased. Mass transfer coefficient increase could have contributed to better removal of zinc. Further increasing the surfactant concentration resulted in high interfacial resistance to mass transfer and lower extraction efficiency was reported (Sulaiman et al. 2020). At lower span 80 concentration (2%), no emulsion was formed owing to a lack of the surfactant adsorbing the organic/aqueous interface (Chiha et al. 2010). The optimum surfactant concentration for the maximum extraction of zinc from electroplating wastewater was 4% (vol.).

The internal phase concentration affects the capacity of the emulsion to extract solute. The increase in acid concentration increases the emulsion capacity. The effect of internal phase stripping acid concentration on extraction of zinc was shown in Figure 3. The figure shows that there was an increase in extraction with an increase in internal phase acid concentration from 0.5 N to 1.61 N. The driving force in the ELM was difference in hydrogen ion chemical potentials between two aqueous phases. With increasing H₂SO₄ concentration from 0.5–1.61 N, increase in the extraction of zinc was observed as the capacity of receiving phase increased. It was expected that emulsion viscosity increases by increasing amount of H₂SO₄ in the internal phase thus decreasing the difference of densities. The increasing viscosity of membrane resulted in increasing droplet size (Chaouchi & Hamdaoui 2014).

The effect of zinc concentration was studied between 500 mg/L to 2,500 mg/L. At low zinc concentration, most of the solute diffusing within the emulsion globule was stripped by the internal phase droplets that were situated in the peripheral regions of the emulsion globule. When the zinc concentration increases, the peripheral droplets get rapidly exhausted, necessitating the solute to permeate deeper within the globule prior to get stripped. Therefore, an increase in zinc concentration, upto 742 mg/L, corresponds to an increase in diffusional path lengths. Increased path lengths lead to a decline in the extraction of zinc. This was clearly observed in Figure 4. Further increase in the feed concentration of zinc led to decline in the extraction. This could be due to the increase in mass transfer resistance in the emulsion globule (Daas & Hamdaoui 2010).

The ratio of emulsion phase to the feed phase, known as treat ratio (TR), was a measure of emulsion holdup in the system. Increase in treat ratio results in an increase in the extraction capacity of the emulsion. It also results in an increase in the amount of carrier and overall surface area for mass transfer in the system. An increase in emulsion phase to the feed phase was expected to increase both the rate and extent of extraction (Sengupta et al. 2006). Figure 5 shows the effect of emulsion phase to the feed phase on extraction behavior. When the emulsion phase holdup was low, the zinc extraction was low. Increase in the ratio up to 0.94 substantially increased the extraction of zinc but further increase in the ratio leads to decrease the zinc extraction. Increase in emulsion holdup results in a plethora of complex effects by increasing the amount of extractant in the system and resulted in formation of larger globules. It shifts the globule size distribution to the higher end of the spectrum. Larger globule sizes lead to a decrease in external mass transfer areas that, in some cases, become rate-limiting. Large globules also cause an increase the effective diffusion path lengths within the globule that results in a decline in extraction rates. Increase in emulsion holdup also enhances globule interactions causing globule breakage, globule coalescence, redispersion, etc., which result in release of the encapsulated zinc back to the feed phase (Sengupta et al. 2006). Hence, increase in treat ratio brings forward a net effect which was a complex interaction of all these effects and it was not easy to decipher any single cause as dominant. In this case, apparently it appears that increase in extractant concentration was the prime reason for the enhancement of extraction rates.

In ELM, the carrier concentration is one of the important variables to be optimized. Figure 6 shows the effect of carrier concentration on extraction behavior. Increase in the carrier concentration in the oil membrane phase of the emulsion from 4% (v/v) to 8.9% (v/v) led to enhancement of zinc removal from feed phase. Increase in the carrier concentration in the range under investigation also augments the rate of extraction in another fashion, utilizing the surface active nature of the extractant. Decline in carrier concentration causes an increase in interfacial tension between the emulsion and feed phase, which leads to an increase in emulsion globule size, thereby lowering the external surface area and, as a consequence, lowering the extraction rates as well. Some extractant have a tendency to aggregate at high concentrations that tends to reduce its metal binding capacity. Decline in extraction rates were observed at these carrier concentrations deviating from the usual trend of increase in extraction rates with increase in carrier concentrations.

In Figures 2–6, the elliptical nature of the contours and bending degree of the axes show the interaction between individual variables in affecting zinc extraction. Figure 7 is the perturbation plot which shows the comparative effect of the variables on the response. A steep curvature in curves D and B shows that the zinc extraction was very sensitive to surfactant concentration, and zinc concentration. The comparatively semi-flat F and A curves show less sensitivity of carrier concentration and pH on the zinc extraction when compared to surfactant concentration, and zinc concentration. The internal phase concentration and external phase to emulsion volume ratio has less impact in the extraction process when compared to other variables. The perturbation plot (Figure 7) shows that the extraction of zinc was affected by variables in the following order of effect: zinc concentration > surfactant concentration > carrier concentration > external pH > external phase to emulsion volume ratio > internal phase concentration.

The RSM analysis identified that the optimal variable values for the maximum extraction of zinc were: external pH – 3.8, surfactant concentration 4% (vol.), internal phase concentration – 1.61 N, zinc concentration – 742 mg/L, external phase to emulsion volume ratio – 0.94 and carrier concentration – 8.9%. At the optimized conditions experiment was carried out and the maximum extraction was found to be 97.4%. From Table 3, at unoptimized conditions, the maximum zinc extraction 94% (Run no. 36). But after finding the exact optimum conditions using RSM, the percentage of zinc extraction was increased by 3.5%. At these conditions, the recovery of zinc from the stripping phase was found to be 78%. The outcome of this work proved that efficient recovery of zinc from electroplating wastewater was feasible using GELM technology. In addition, this study has identified the optimal conditions required for the recovery of metal from electroplating wastewater.

The zinc extraction from electroplating wastewater was investigated using waste cooking oil based emulsion liquid membrane as green technology and appreciable zinc extraction efficiency was achieved. The process parameters including external pH, surfactant concentration, internal phase concentration, zinc concentration, external phase to emulsion volume ratio and carrier concentration were optimized using RSM method. The interaction effects between the variables were well represented by the contour plots. In the order of effect, zinc concentration is identified to influence the most. The optimum condition for maximum removal is: external pH – 3.8, surfactant concentration 4% (vol.), internal phase concentration – 1.61 N, zinc concentration – 742 mg/L, external phase to emulsion volume ratio – 0.94 and carrier concentration – 8.9%. Under optimal conditions, the maximum extraction was found to be 97.4% and the recovery of zinc from the stripping phase was 78%. The successful recovery of zinc using a waste resource based ELM confirmed the sustainable application.

The authors declare that they have no known competing financial interests in relation to this published work.

All relevant data are included in the paper or its Supplementary Information.