Ind. Eng. Chem. Res. 2009, 48, 6805–6810
6805
Nafion-117 Behavior during Cation Separation from Spent Chromium Plating Solutions M. Elmuntasir I. Ahmed,† Kuo-Lin Huang,*,‡ and Thomas M. Holsen§ Department of CiVil Engineering, UniVersity of Khartoum, Khartoum 11111, Sudan, Department of EnVironmental Engineering and Science, National Pingtung UniVersity of Science and Technology, Pingtung 91201, Taiwan, and Department of CiVil and EnVironmental Engineering, Clarkson UniVersity, Potsdam, New York 13699
This study investigated the behavior of Nafion-117 ion-exchange membranes that are commonly used as the separator in electrolytic regeneration of spent chromium plating solutions. In the co-presence of Cu2+, Ni2+, and Fe3+ impurities (∼0.01-0.1 M) in ∼0.5-2.0 M H2CrO4, the exchange equivalents of Ni2+ by Nafion117 was double that of Cu2+ (0.02 mmol of H+/(g of Nafion)) but smaller than that (∼0.03-0.15 mmol of H+/(g of Nafion)) of Fe3+, exhibiting the order Fe3+ g Ni2+ g Cu2+ in Nafion affinity. The water content (∼41%) of a Nafion-117 membrane soaked in 2 M H2CrO4 was greater than those (∼35-38%) in the contaminated 2 M chromic acid solutions, whereas the porosity (0.24) of the former was smaller than those (0.29-0.37) of the latter. For the Nafion-117 in 2 M chromic acid solutions with/without cation impurities, (1) the tortuosity coefficient of Fe3+ was about 1.8 times that of Cu2+ or Ni2+, (2) the calculated pore radius was 3.2 nm for a multiimpurity case (0.1 M Cu(II), Fe(III), Ni(II), and Cr(III) each) based on the Gierke’s model, and (3) the cation impurities did not significantly influence the numbers of water molecules in Nafion clusters. The Nafion-117 withstood the pH < 0 environment with only minor rotation in the fluorocarbon backbone due to accommodation of anionic Cr(VI) within clusters. 1. Introduction Nafion perfluoronated membranes are made from a precursor copolymer of tetrafluoroethylene and sulfonyl vinyl ether.1 This copolymer has the following general formula:
where the value of m can be as low as 1 and the value of n ranges between 6 and 13. The sulfonyl fluoride group is easily hydrolyzed to form a strongly acidic sulfonic acid site for cation exchange. This polymer possesses exceptional thermal, chemical, and mechanical stability. It has been employed as a membrane separator in electrochemical applications and as an acid catalyst in synthetic applications.1 A laboratory-scale cell utilizing a Nafion-117 membrane as a separator and as the fuel cell oxygen cathode/electrolyte interface has been used to test the feasibility of in situ electrochemical removal of metal contaminants from spent chromic acid via electrodialysis.2-7 The process removes contaminants from spent chromium electroplating baths, thereby extending their lives and hence reducing the amount of hazardous chromium waste discharged into the environment. This was achieved at very low energy consumption in a * To whom correspondence should be addressed. Tel.: +886-8770-3202ext. 7092. Fax: +886-8-774-0256. E-mail: huangkL@ mail.npust.edu.tw. † University of Khartoum. ‡ National Pingtung University of Science and Technology. § Clarkson University.
laboratory-scale cell.2-6 Whether the removal of metal ions is via their diffusion/migration from anolyte to catholyte or plating on the cathode surface or both, the (Nafion) separator is the main barrier for controlling cation transport in the process because of its selectivity toward cations.8-10 This selectivity is primarily due to the affinity of the exchange sites for cations and partially due to favorable cation membrane-phase transport parameters including diffusion coefficients and mobilities. Understanding Nafion-117 selectivity is needed for a better understanding of the separator properties and its role in separation processes. In addition, adequate chemical inertness and mechanical strength are needed to maintain the Nafion117 cation-exchange properties such as anion (chromate) rejection. Understanding the behavior of Nafion-117 in contaminated chromic acid allows simplifying assumptions to be made and parameters to be evaluated for the development of a realistic mathematical model of the process.2,6 The Nafion-117 selectivity or water absorption is important for Nafion separators used in transport-oriented removal of impurities from spent chromium solutions11 and electrochemical regeneration of chromium containing solutions in divided cells.12 It is also useful for other applications of Nafion such as fuel cells13 or the use of fuel cell electrodes to regenerate spent chromium plating solutions.14,15 Cr3+, Fe3+, Cu2+, and Ni2+ are impurities commonly found in spent chromium plating solutions that must be removed to required levels if the solution is to be regenerated. This study, therefore, investigated uptake characteristics of these cations by Nafion-117 which is commonly used as a separator for electrolytic regeneration of spent chromium plating solutions. In simulated spent chromium plating solutions, the selective uptake of these cations by Nafion-117 were measured and compared. The porosity and water content parameters were also measured for membrane pore radius calculation. The Nafion structure before and after use were examined by X-ray diffrac-
10.1021/ie900149a CCC: $40.75 2009 American Chemical Society Published on Web 06/04/2009
6806
Ind. Eng. Chem. Res., Vol. 48, No. 14, 2009
tion analysis, and the membrane’s ion clustering parameters were estimated on the basis of Gierke’s model.
Table 1. Initial Concentrations (mmol/L) of Cation Impurities in Prepared Chromic Acid Solutions (∼0.48-2.02 M as CrO3, pH < 0) test
2. Experimental Procedure
ion species
2.1. Nafion Preparation and Conditioning. Before use, Nafion-117 (DuPont) obtained from Electrochem Inc. was boiled for an hour and a half in 6 M HNO3 followed by boiling in distilled deionized water (DDW) for another hour and a half.16 2.2. Contaminated Chromium Plating Solution. The concentrations of chromic acid are typically 200-400 g/L (2-4 M) (as CrO3) for chromium plating.17 Therefore, this concentration range was used to prepare chromium plating solutions containing impurities for tests. To measure membrane porosity, the Cu2+, Fe3+, Ni2+, and Cr3+ impurities were added into 2 M chromic acid in the form of sulfates (CuSO4, Fe2(SO4)3 · 7H2O, NiSO4 · 6H2O, and Cr2(SO4)3 · 12H2O, respectively (Fisher Scientific)) to achieve impurity concentrations in the desired range.3,7 Barium carbonate (BaCO3) was used to eliminate excess sulfate anion by the formation of a water-insoluble precipitate (BaSO4). Filters were used to separate BaSO4 precipitate from the prepared solutions. 2.3. Ion Uptake. Chromic acid solutions (25-105 g/L as Cr) (∼0.48-2.02 M as CrO3) containing different impurity concentrations (∼0.01-0.05 M Cu2+, ∼0.02-0.10 M Fe3+, and ∼0.02-0.07 M Ni2+) were used for cation uptake experiments. The ranges of impurity concentrations are similar to those reported in literature for chromium plating baths;18 in general, the total concentration of impurity should be kept less than 5% of chromic acid concentration during chromium plating.19 A known amount of Nafion was soaked in contaminated chromic acid solutions and stirred overnight. Nafion was then removed from the solution and blotted dry. A grade A volumetric flask (250 mL) was then filled with nitric acid (3.0 N, pH < 0.0 based on both the pH calculation and the measurements in diluted samples), and for complete metal leaching, the Nafion was soaked, stirred, and heated at 70 °C for at least 6 h.16 The flask was then cooled and filled to the mark, and the concentration (C in milligrams per liter) of each stripped impurity was determined using atomic absorption spectrometry (AAS; PerkinElmer Model 680). The cation uptake (CU in millimoles per gram of Nafion) is then CU )
0.25C MW × W
(1)
where MW is the molecular weight of cations and W is the weight of Nafion. 2.4. Porosity and Water Content Measurement. The dimensions and weight of a circular piece of Nafion-117 were measured prior to equilibration with the desired solutions for a period over 24 h.16 Then the pieces were blotted dry, and their weights and diameters/thicknesses were measured using an electronic five-digit balance and a caliber of 0.001 mm accuracy, respectively. Afterward, the samples were heated in an oven at 150 °C for over 4 h and the weight, diameter, and thickness were measured again. The effective porosity (p) of Nafion-117 membrane is defined as p)
Vwet - Vdry Vwet
(2)
where Vwet is the volume occupied by the membrane when it is equilibrated with the electrolyte solution and Vdry is the volume of the dry membrane. The term “effective porosity” has been
2+
Cu Fe3+ Ni2+ Cr(VI)
C1
C2
C3
C4
C5
11 21 18 481
19 34 28 769
31 64 49 1250
42 82 60 1538
52 103 72 2019
used here since the poly(tetrafluoroethylene) backbone of the Nafion films is probably not rigid, and it may not have the same density in the dry state as in the fully hydrated state. In actuality, the effective porosity is the relative, fractional volume change upon hydration. 3. Results and Discussion 3.1. Ion Uptake. It is important to characterize the equilibrium ion exchange selectivity for ion exchange polymers in order to understand their dynamic (chain rotation and rearrangement) properties when used in a membrane form. The (theoretical) ion exchange capacity of Nafion-117 is 0.91 mequiv/g,8,20 and all sulfonate groups are available for exchange with different cations;21 however, in concentrated solutions more ion equivalents are possibly accommodated within the Nafion-117.21 Thus, in this study, cation uptake is defined as the total Nafion-117 retained cations (at equilibrium) including those exchanged, within the pores, and accommodated in the backbone of the Nafion-117. The ion uptakes at different initial impurity concentrations in prepared solutions (Table 1) are provided in Figure 1. In the co-presence of three cation impurities, the uptakes of Cu2+ in Nafion-117 were close to 0.01 mmol/g-Nafion, while those of Ni2+ were double that amount. With increasing initial concentration, the uptakes of these two divalent cations varied slightly with increasing chromic acid concentration. On the other hand, the uptake of Fe3+ in Nafion-117 decreased with the increase of chromic acid concentration, although the Fe3+ concentration increased. This finding is attributed to the fact that more protons entered the Nafion domain and they hindered the Fe3+ uptake by Nafion. Nevertheless, the uptake of Fe3+ was significantly greater than those of Cu2+ and Ni2+ in e1.25 M (as CrO3) chromic acid solutions. In the 2 M chromic acid solution, although the uptake of Fe3+ was
