8176
Ind. Eng. Chem. Res. 2007, 46, 8176-8182
Removal of Hexavalent Chromium by Membrane-Based Hybrid Processes Prashant S. Kulkarni,*,† Vobbilisetty Kalyani, and Vijaykumar V. Mahajani Department of Chemical Engineering, UniVersity Institute of Chemical Technology (UICT), UniVersity of Mumbai, Mumbai 400 019, India
The presence of the toxic metal Cr(VI) in wastewater is a major concern from an environmental point of view, and its complete removal from waste solutions is a difficult task to accomplish. Membrane-based hybrid processes have been developed to maximize its efficient removal from aqueous wastes. One such process comprises emulsion liquid membrane (ELM) extraction and reduction-precipitation. ELM is a one-step process involving extraction and stripping, simultaneously. The liquid membrane constitutes of an extractant and surfactant dissolved in the organic diluent. The addition of strip phase into the organic phase results in the formation of an emulsion. The selection of surfactant is decisive in the ELM extraction process, as the concentration of the target metal inside the strip phase heavily relies on it. Aqueous waste containing Cr(VI) (300 mg/L) was treated with emulsion to gain higher concentrations of Cr(VI) inside the strip solution in one step. Important parameters affecting the ELM process of Cr(VI) concentration, such as feed phase pH, role of extractant, and surfactant and stripping concentration were studied. The preconcentrated Cr(VI) (>15 times) solutions obtained after breaking of emulsions were further reduced to nontoxic Cr(III) form using a FeSO4 catalytic method at pH 2. Effects of reactant ratio and temperature on the reduction process were examined. A complete reduction of Cr(VI) was achieved using a 10% stoichiometric excess of FeSO4 and 100 °C temperature. Further, the nontoxic metal Cr(III) is precipitated with alkaline solutions. The dependence on pH of the solubility of Cr(III) was identified. Finally, it was observed that the membrane-based hybrid processes minimized the concentration of Cr(VI) far below the level of the discharge limit. Introduction In recent years, the increased importance of natural environment has compelled industry to invent newer technologies to bring down the concentration of hazardous metal ions below the discharge standards. Chromium is one such hazardous metal ion predominantly present in aqueous wastes in two forms, namely, Cr(III) and Cr(VI). Cr(VI) is a toxic metal and is used or generated by a number of industrial processes including electroplating, tanning, pulp production, cooling with water, and steelmaking.1,2 Cr(VI) in the form of chromate (HCrO4-) and dichromate (Cr2O72-) compounds has been also released to the environment due to improper disposal and leakage of several industrial processes, such as the manufacture and usage of alloys, and ore processing, and in the galvanic, ceramic, and dye industries.3 Cr(III), the other stable oxidation state, is nontoxic or less toxic than Cr(VI) and is an essential trace element for man and animals.4,5 It is insoluble in water at near-neutral pH while Cr(VI) is soluble.6 The concentration of Cr(VI) in the effluents from the abovementioned sources varies from 50 to 500 mg/L.7 A safe disposal of large quantities of Cr(VI)-bearing toxic industrial wastewaters is one of the major environmental problems that many countries are facing. It is because the metal is very mobile in the environment and has a low, acute and chronic toxicity to humans at high doses. It causes diseases, such as epigastric pain nausea, vomiting, severe diarrhea, and hemorrhage by ingestion. Overexposure to chromium causes inflammation of the respiratory and gastrointestinal tracts. Acute poisoning leads to tubular necrosis of the kidney, leading to death. Cr(VI) plays a major role in lung cancer and bronchogenic cancer.8-10 A present limit * To whom correspondence should be addressed. Tel.: +351218417627. Fax: +351-218464455. E-mail:
[email protected]. † Present address: CQFM, Departamento de Engenharia Quı´mica e Biolo´gica, IST, 1049-001 Lisboa, Portugal.
put forth by various agencies such as WHO, USEPA, etc. for dissolved Cr(VI) in drinking water is 0.05 mg/L, and that for total chromium (all forms of chromium) is 2 mg/L.11 Hence, it is essential to treat Cr(VI) originating from aqueous waste effluents to meet stringent discharge standards. Several methods were practiced for its removal from wastewaters which involve solvent extraction, adsorption, chemical reduction followed by precipitation, ion exchange, bioremediation, and membrane technologies.7 However, most of these processes could not achieve the goal of pollution standards alone and also generated a large amount of secondary waste. Therefore, there is a need for the reorganization of present process designs and the development of new process schemes having high selectivity for the pollutant, low-energy consumption, moderate cost-toperformance ratio, and time effectiveness. Membrane-based hybrid processes are gaining importance worldwide in achieving an effective separation of the target component.12 Recently, Nataraj et al.13 and Thiruvenkatachari et al.14 have studied membrane-based hybrid processes for removal of organics from wastewater. In the present work, we have made an attempt to minimize the concentration of Cr(VI) in wastewater using a membrane-based hybrid technology, viz., the emulsion liquid membrane (ELM) extraction and reductionprecipitation process. The ELM process has numerous advantages over the conventional extraction process, such as extraction and stripping in one step, high surface area for mass transfer, potential for higher solute selectivity, lower inventory, and hence lower loss of solvent. Expensive and more selective solvents can be used in the membrane phase and higher solute concentration can be achieved in the receiving phase.15 An ELM pilot plant for zinc recovery from wastewaters was first reported in the late 1980s.16 A very few studies were reported on the extraction of Cr(VI) using the ELM process; however, to the best of our knowledge, they used a routine commercial surfactant, Span 80, and also did not report on the enrichment
10.1021/ie070592v CCC: $37.00 © 2007 American Chemical Society Published on Web 10/04/2007
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 8177
or concentration of Cr(VI) achieved in the strip phase.17-19 An enrichment or concentration of the target component inside the strip phase is an ultimate recovery in the ELM process. Therefore, the role of the surfactant is vital, as it affects the stability of the emulsion. In the present study, we have used a phthalic anhydride based surfactant, monesan, for the preparation of the emulsion.20 Emulsion containing surfactant, extractant, organic diluent, and stripping agent is further treated with aqueous waste containing Cr(VI). The strip phase containing Cr(VI) can be concentrated heavily in one step using the advantage of the ELM process. Additionally, the toxic metal is reduced to the nontoxic Cr(III) form and precipitated as a solid, before its discharge into the environment. Experimental Methods Materials. All chemicals were of analytical reagent grade and used as supplied. The sources of Cr(VI) and Fe(II) ions in the experiments were stock solutions made by dissolving measured amounts of K2Cr2O7 and FeSO4‚7H2O in deionized water. The main chemicals, surfactant monesan (phthalic anhydride based) and extractant Aliquat 336, were procured from Mohini Organics (P) Ltd. and Sisco Research Laboratories (P) Ltd., Mumbai, India, respectively. Dodecane was used as the organic solvent for liquid membrane preparation. The chemicals required for analysis, namely, acetone, 1,5-diphenyl carbazide (DPC), sulfuric acid, sodium iodide, lithium hydroxide, ammonium acetate, disodium hydrogen orthophosphate, and HPLC-grade methanol were obtained from S. D. Fine Chemicals. Pyridinedicarboxylic acid was obtained from Merck. Potassium permanganate was obtained from Glaxo Laboratories (India) Ltd. Concentration of Cr(VI) from Aqueous Waste by ELM Process. A synthetically simulated waste having 300 mg/L Cr(VI) concentration was used for the ELM study. The emulsion was prepared by emulsifying an aqueous solution of strip phase with an organic phase. The organic phase (membrane phase) consisted of a surfactant (monesan), a carrier reagent (Aliquat 336), and an organic diluent (dodecane). The internal strip phase (sodium hydroxide) was added dropwise into a glass reactor containing the organic phase. A volume ratio of 1:1 was maintained for the organic phase to the internal strip phase. The details of the experimental setup and procedure are described elsewhere.20 The emulsion obtained was dispersed into the feed phase containing simulated waste from which Cr(VI) was to be extracted. A treat ratio of 1:10 was kept for the emulsion to feed phase. Samples were withdrawn from the extractor at different intervals of time and filtered through a sintered glass plug to separate the emulsion from the aqueous feed phase. The internal droplet size (2-8 µm) was analyzed by a Coulter particle size analyzer, Model LS 230, and the emulsion globule size (2-6 mm) was analyzed using a high-speed camera with an Image Analyser, Benchmark Instrument. The emulsion was broken down by the application of heat at 80 °C for the analysis of strip phase. All experiments were carried out at 30 °C, and the reproducibility of experiments was checked at least twice. Reduction of Concentrated Cr(VI) to Cr(III) and Precipitation of Cr(III). A reduction process of Cr(VI) was carried out in a 500 mL glass reactor having four baffles and a flatblade turbine impeller. A reflux condenser was mounted on the top of the reactor. The temperature was measured with the help of a thermometer which was kept in a thermometer pocket. The entire assembly was kept at a constant temperature in the oil bath. The concentrations unless otherwise stated used were 1.47 g (0.05 M) of K2Cr2O7 and 100 mL of water in a 500 mL glass
Figure 1. Mechanism of transport of dichromate ions inside the strip (product) solution.
reactor. Subsequently, a stoichiometric amount, that is, 8.34 g (0.3 M) of ferrous sulfate acidified with H2SO4 was added to the reactor and the contents were stirred for 90 min. Reactions were carried out at 30, 50, and 100 °C using a temperature controller. After the reduction of Cr(VI) to Cr(III), Cr(III) was precipitated with alkali. Precipitation experiments were carried out in a 500 mL beaker with baffles. The alkaline solution for precipitation was added through a buret. The mixture was agitated with a flat six-blade turbine impeller. The mixture was stirred while alkali was added through the buret. Addition of alkali was continued until pH 8 was attained. The precipitated Cr(III) was filtered and then analyzed for Cr(VI) and Cr(III), as Cr(III) has a very low solubility in water. Analysis. (A) Chromim(III) and Chromium(VI). Analysis of trivalent and hexavalent chromium ions was carried out by using a DIONEX high performance ion chromatograph. A cation exchange column, HPIC-CS2, with a postcolumn UV detector was used for the determination of Cr(III) and Cr(VI) ions. The visible absorbencies of the Cr(III)-pyridinedicarboxylic acid (PDCA) complex and the Cr(VI)-diphenyl carbohydrazide (DPC) complex at 520 nm allow photometric detection of Cr(III) and Cr(VI). The eluent system was PDCA based, in which Cr(III) is separated as the Cr(PDCA)2 complex, while the Cr(VI) is separated as the chromate ion, CrO42-. The advantage of this method is that Cr(VI) does not form a complex with PDCA. For this scheme, eluent was prepared by dissolving pyridine-2,6-dicarboxylic acid (0.0019 M), disodium hydrogen phosphate (0.0014 M), sodium iodide (0.01 M), ammonium acetate (0.0049 M), and lithium hydroxide (0.0026 M) in 1000 mL. The postcolumn reagent was prepared by dissolving 0.5 g of diphenyl carbazide (DPC) in 100 mL of HPLC-grade methanol. After complete dissolution of DPC in methanol, 25 mL of H2SO4 was added and was diluted to 1000 mL. At nearly neutral pH of the eluent Cr(VI) exists as divalent chromate and Cr(III)-PDCA complex is a stable monovalent ion. The reagent from postcolumn chamber forms a stable complex of Cr(VI)DPC at 520 nm. This technique allows the simultaneous determination of Cr(III) and Cr(VI) to less than 50 ppb. Determination of Cr(VI) as chromate ion was also performed by using an anion column, HPIC-AS4, and conductivity detector. A detector with anion fiber suppresser (so solute ions could be recognized easily) provided a wide linear dynamic range, high selectivity, and ease of operation. For the analysis of chromate anions, a solution containing 0.0028 M NaHCO3 and 0.0021 M Na2CO3 was used as eluent and 0.01 M H2SO4 was used for anion suppression.21,22 (B) Total Chromium. Total chromium was determined after oxidizing Cr(III) to Cr(VI) by using an alkaline persulfate digestion. For this purpose, 25 mL of the sample was pipetted into a volumetric flask, to which 1 mL of 50% NaOH and 0.80 g of (NH4)2S2O8 were added. The sample was heated at 90 °C until it became colorless. Then the sample was cooled and diluted to 100 mL with deionized water. The dilutions of the
8178
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007
Figure 2. Effect of surfactant concentration on recovery of Cr(VI). Experimental conditions: feed phase pH ∼4.7; speed of agitation 200 rpm; contact time 5 min; 500 mol/m3 NaOH; 1% v/v extractant in dodecane.
Figure 3. Effect of feed phase pH on recovery of Cr(VI). Experimental conditions: speed of agitation 200 rpm; contact time 5 min; 500 mol/m3 NaOH; 1% v/v extractant; 4% w/v surfactant in dodecane.
Table 1. Dependence of Viscosity of the Organic Phase (1% v/v Aliquat 336, 50 mL of Dodecane and Monesan) Used for the Emulsion Preparation
internal water droplets of the emulsion globule. Figure 1 shows the schematics of the transport mechanism of Cr(VI) inside the liquid membrane. The ELM extraction of Cr(VI) is governed by several parameters. The major parameters include concentrations of surfactant and extractant, speed of agitation, treatment ratio of the aqueous to emulsion phase volumes, strip phase base concentration, and pH of the feed phase. Among all the parameters the contact time of the emulsion and the feed phase is very important in achieving the highest concentration of Cr(VI) inside the strip phase. A prolonged contact time for extraction results in more transfer of water inside the internal phase, which causes the membrane to swell and initiates the breakage of the emulsion phase.24 The breakage causes the depletion of product inside the strip phase. Therefore, considering these factors and with our prior experience, it was decided to use a contact time of 5 min between the emulsion and the feed phase. In the following section only the essential parameters are discussed along with the concentration factor (enrichment) attained inside the strip phase. (A) Surfactant Concentration. Proper selection of the surfactant and its concentration is vital in the success of the ELM process. We presume that the present inadequacy attached with the ELM process depends upon on the unavailability of a proper surfactant. More attention is required for the synthesis of surfactants that are used for the ELM purpose, as their poor selection leads to the formation of unstable emulsions which in turn affects their stability and makes the entire process less selective.25 The commercially available surfactant Span 80 is an ester of sorbitan monooleate, and hence, it becomes unstable in basic medium. Therefore, in the present study, a phthalic anhydride based surfactant, monesan, which can remain stable in basic medium, was chosen. Throughout this study the concentration of surfactant was varied and the other parameters, such as extractant and strip phase concentrations and speed of agitation, were kept constant. Figure 2 shows the effect of surfactant concentration on the concentration of Cr(VI) inside the strip phase. During the experiments, it was observed that the emulsion stability improves with an increase in the surfactant concentration. Table 1 shows the data on the viscosity of the organic phases with varying surfactant concentration. Figure 2 depicts that increasing the concentration of surfactant from 1 to 4% w/v increases the stability and viscosity of the membrane, which consecutively causes an increase in the concentration of
surfactant concn, % w/v
viscosity (µ) at 30 °C, cP
1 2 3 4 5
0.82 0.88 1.05 1.15 1.30
sample were accounted for in the calculation. The sample was then ready for injection, which was analyzed by using the HPICCS2 column and postcolumn UV detector (as described above for the Cr(VI) technique). The concentration of Cr(III) was calculated by subtracting the content of Cr(VI) from the total chromium content.22 Error in the analysis of Cr(III), Cr(VI), and total chromium was within (3%. Results and Discussion ELM Process for Preconcentration of Cr(VI). For ELM extraction studies, Aliquat 336 (extractant), monesan (surfactant), dodecane (organic diluent), and sodium hydroxide (strip phase) were used in emulsion preparations. The extraction chemistry of Cr(VI) with the quaternary ammonium extractant Aliquat 336 involves anionic exchange between the dichromate ions and halogen ions.23 The extraction and stripping are governed by the following equations.
Cr2O72- + 2[R3NR′-X]org T [(R3NR′)2Cr2O7]org + 2X- (1) where X ) Cl-. Stripping in caustic solution occurs as follows:
[(R3NR′)2Cr2O7]org + 2NaOH T 2[R3NR′OH]org + 2Na+ + Cr2O72- (2) Transport of dichromate ions inside the liquid membrane is determined by the following steps: diffusion of dichromate ions to the external interface of the emulsion globule, interfacial reaction between the dichromate ions and Aliquat 336 at the external interface, diffusion of the dichromate-extractant complex into the emulsion globule, and stripping reaction of the complex with the alkaline solutions at the interface of the
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 8179
Figure 6. Concentration profile vs time for Cr(VI) reduction. Experimental conditions: temperature 30 °C; pH 2; FeSO4 stoichiometric amount. Figure 4. Effect of strip phase NaOH concentration on recovery of Cr(VI). Experimental conditions: feed phase pH ∼4.7; speed of agitation 200 rpm; contact time 5 min; 1% v/v extractant; 4% w/v surfactant in dodecane.
Figure 5. Effect of extractant concentration on recovery of Cr(VI). Experimental conditions: feed phase pH ∼4.7; speed of agitation 200 rpm; contact time 5 min; 500 mol/m3 NaOH; 4% w/v surfactant in dodecane.
Cr(VI) inside the strip phase. Within this viscosity series (0.821.15 cP) the concentration of Cr(VI) was found to be increased from 1.2 to 5.6 g/L. However, the internal diffusion of Cr(VI)Aliquat 336 complex is also affected by the viscosity of the membrane phase. A further increase in surfactant concentration from 4 to 5% w/v increased the emulsion stability; however, mass transfer was adversely affected. In this range, the increase in viscosity was greater and resulted in a reduction of diffusivity of the metal complex inside the organic phase. Therefore, in order to get better mass transfer, it was decided to maintain 4% w/v surfactant concentration in all the future experiments. (B) Feed Phase pH. Aqueous feed phase pH played an important role in the preconcentration of Cr(VI) inside the product solution. The acidic pH adjustment was made with the help of hydrochloric acid, and the basic adjustment was made with sodium hydroxide. Figure 3 shows that higher concentration of Cr(VI) inside the strip phase occurred as pH moved toward the acidic range. This is because the acidic nature of the feed phase favors the formation of dichromate ions, which results in stronger complexation of Cr(VI)-Aliquat 336 complex; hence the concentration of Cr(VI) was also high at the stripping side. Therefore, at feed phase pH 2 the concentration of Cr(VI) inside
Figure 7. Concentration profile vs time for Cr(VI) reduction using excess FeSO4, where A ) Cr(VI) with 5% excess FeSO4 and B ) Cr(VI) with 10% excess FeSO4. Experimental conditions: temperature 30 °C; pH 2.
the strip phase was found to be 5.5 g/L while the extraction of Cr(VI) by Aliquat 336 was 95%. However, with increase in the feed phase pH the concentration of Cr(VI) was found to be decreased: only 2 g/L Cr(VI) was achieved at feed phase pH 10. This was mainly attributed to the change in osmotic pressure difference of the feed and strip phases, which makes the emulsion unstable, leading to swelling and breakage. Thus, Cr(VI) waste solutions having high alkaline pH may not favor the present ELM scheme and for better recovery they must be in the acidic range. (C) Concentration of Stripping Agent. Experiments were carried out to investigate the influence of strip phase concentration on transfer of Cr(VI) inside the membrane. The strip phase concentration of sodium hydroxide was varied from 100 to 2000 mol/m3. Figure 4 shows the effect of strip concentration on the concentration of Cr(VI). Initially, it was observed that the concentration of Cr(VI) was increased when the base concentration was varied from 100 to 500 mol/m3. However, further increase in the concentration of caustic in the strip phase from 500 to 2000 mol/m3 reduced the extent of Cr(VI) concentration inside the strip solution. It is again due to the osmotic imbalance of strip phase and external feed phase. The cotransport of water along with Cr(VI) at higher concentration of strip phase resulted in swelling of the emulsion. The swelling dilutes the internal
8180
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007
Figure 8. Effect of temperature on Cr(VI) reduction, where A ) Cr(VI) at temperature 30 °C, B ) Cr(VI) at temperature 50 °C, and C ) Cr(VI) at temperature 100 °C. Experimental conditions: pH 2; FeSO4 5% molar excess.
Figure 9. Effect of temperature on Cr(VI) reduction, where A ) Cr(VI) at temperature 30 °C, B ) Cr(VI) at temperature 50 °C, and C ) Cr(VI) at temperature 100 °C. Experimental conditions: pH 2; FeSO4 10% molar excess.
strip phase, and hence, reduction in the concentration factor was observed. Therefore, a strip concentration of 500 mol/m3 caustic inside the strip phase is recommended for the present ELM process where the highest value of 5.4 g/L of Cr(VI) was found. (D) Extractant Concentration. For the ELM extraction process an expensive and selective extractant can also be used, because the extractant acts merely as a shuttle between the external feed phase and internal strip phase. The selection of the minimum concentration of extractant is significant in the recovery process. Figure 5 demonstrates that by increasing the concentration of the extractant Aliquat 336 from 0.1 to 1.0% v/v, the final stripping of Cr(VI) was increased. The extraction of Cr(VI) at this concentration was observed to be 93%, and 5.5 g/L Cr(VI) concentration inside the strip was achieved. With further increase in the concentration of extractant from 1 to 3% v/v, 100% extraction of Cr(VI) was observed, but the stripping of Cr(VI) inside the strip phase was badly affected. This was because of the governance of the extraction process over the stripping process at an excessive concentration of Aliquat 336 at given contact times and parameters. It affected the final recovery of Cr(VI) by the ELM process. Therefore, extractant
Figure 10. Effect of pH and type of alkali on solubility of Cr(III).
should only acts as a shuttle in the ELM process and is not to be saturated with Cr(VI). Reduction of Concentrated Cr(VI) to Cr(III) Using FeSO4 and Precipitation of Cr(III). The use of the membrane process has resulted in the impressive enrichment of Cr(VI) concentrations from aqueous solutions in one step. Remediation of Cr(VI)-containing waste solutions involves the reduction of Cr(VI) to the less mobile and less toxic Cr(III). Reduction of Cr(VI) to Cr(III) took place if a strong chemical reductant was added, and Cr(III) was then removed through precipitation by adding sodium hydroxide or lime. Several reducing agents were reported for reduction of Cr(VI) to Cr(III) including iron sulfides, magnetized Fe particles, H2O2, ferrous salt, Fe electrode, FeSO4, biomass, etc.26-31 In the present membrane-based hybrid process of Cr(VI) removal, the next method proposed is the reduction of toxic Cr(VI) to the nontoxic Cr(III) form. A reducing agent, FeSO4, was added in the presence of H2SO4. The length of time required to remove aqueous Cr(VI) by various treatment methods is determined by the reactivity of the Fe(II) source and the composition of the feed phase. Eary and Rai26 reported that, at higher pH, the molar ratios of Cr(VI) and Fe(II) deviated from the predicted ratio of 3.0 to significantly higher values, indicating a highly nonstoichiometric amount under strong alkaline conditions. In the present case, the concentrated solutions of Cr(VI) obtained after ELM experiments contained a very high amount of alkali; further, they may contain a large amount of dissolved oxygen, too. Therefore, it was thought desirable to use pH 2 to achieve complete reduction of Cr(VI) to Cr(III). The reduction reaction can be described by the overall reaction as given below.26
Cr(VI)(aq) + 3Fe(II)(aq) f Cr(III)(aq) + 3Fe(III)(aq)
(3)
Products obtained after the reduction process will be precipitated as hydroxide solids in slightly alkaline solutions. Precipitation was carried out using caustic and lime at pH 8. At this pH, Cr(III) has minimum solubility in water (
