Development of strategies for the detection, extraction, and recovery of aqueous anions using highly selective redox-recyclable materials

Abstract

Highly selective redox-recyclable extractant materials were prepared by physisorbing the redox active lipophilic extractant HEP+NO3- [HEP = 1,1',3,3'- tetrakis(2-methyl-2-hexyl)ferrocene] onto various polymeric supports. Simple adsorption of the tetraalkylated ferrocenium salts onto polymeric supports was preferred to the more difficult and more expensive alternative of covalently grafting redox active moieties onto a solid support. The new materials were evaluated for their effectiveness towards the extraction of ReO.4- and CnF2n+1SO3- (PFS) ions from a variety of aqueous solutions. The effects of coverage and size of the tetraalkylated ferrocenium cation (size of the tetraalkylated ferrocene was adjusted by varying the length of the alkyl substituents attached to the ferrocene from four to ten carbon atoms in length) were evaluated as well as the selectivity over several redox cycles. The materials were also evaluated for their effectiveness under a variety of conditions including contact time, pH. and concentration of a competing ion. The materials were found to be highly selective for NO3-/ReO4- ion exchange even in the presence of 1.0 M NaNO3. The polarity, pore size, and surface area of the polymer used as a support had a dramatic effect on the effectiveness of the resulting anion-exchange materials. More polar polymers achieved faster anion-exchange kinetics compared to non-polar polymers. The polym er beads with larger pores and lower surface areas were found to give higher distribution ratios. It was also found that as the size of the cation increased, the material became more selective, indicating a size bias towards ReO4- for larger cations. The new materials were stable over several extraction-deactivation/recovery-reactivation cycles, with no loss in effectiveness over seven redox cycles, compared to previous studies in which silica gel supported materials lost 20% of their capacity over five cycles. The new redox-recyclable material HEP+NO3-/XAD-7 (XAD-7 = acrylic ester polymer) was found to efficiently remove PFS anions from aqueous solutions, but because of the low solubility of longer-chain PFS surfactants in water at room temperature, deactivation of the loaded extractant material resulted in immediate precipitation of longer-chain PFS salts. The precipitation of the PFS salts was initially perceived as a problem for the recovery of the pollutant in a small volume, however, this disadvantage was turned into an advantage. The solubility of PFS salts dramatically increased when the aqueous solution was heated above the Krafft point of the PFS salts. A hot-water/cold-water recovery process was developed in which extracted PFS salts were efficiently washed from a deactivated column by heating the recovery solution to temperatures above the Krafft point of the surfactant and recovering them as a solid in a cold trap. This technique allowed the recovery of the PFS pollutant as a crystalline solid, resulting in a volume reduction of 99.99%. Without the ability to easily activate and deactivate the extractant material, this type of recovery and subsequent volume reduction would not be possible. Several methods for the detection, identification, and quantification of fluorinated surfactants have been investigated. A colorimetric assay method was developed that allowed a one-step detection scheme using spectrophotometry. The method involved exchanging the colorless surfactant in the aqueous solution quantitatively with a colored anion that had been immobilized onto a solid support, and subsequent spectrophotometric detection. The assay provided a general test for the presence of anionic surfactants, and was quantitative between 50 and 275 μg L-1 for one formulation of aqueous film forming foam (AFFF) that contained a mixture of fluorinated and non-fluorinated surfactants. A negative ion electrospray mass spectrometric method ((-)ES-M S) was developed for the detection and identification of fluorinated surfactants in aqueous solution. The method was able to achieve a much lower detection limit compared to the colorimetric method (below 6 μg L-1), and had the added advantage of identification through mass analysis. Reliable quantification information was achieved by using an internal standard with a similar sensitivity coefficient to the anions of interest. To determine capacity and extraction performance of a column packed with HEP+NO3-/XAD-7, an ATR-FTIR technique was developed to monitor the breakthrough of an IR-active analyte in real time. This technique, although limited by the strong absorbance of water in the infrared region, was able to detect 10 mM triflate within one minute of breakthrough. Enhancing the sensitivity of an ATR-FTIR spectrometer for IR-active anions in aqueous solutions by physisorbing highly selective water insoluble organometallic extractants to the surface of the probe was investigated. A thin film of the organometallic extractant 1,3-bis(diphenylphosphino)propanedichloronickel(II) (NiCl2(dppp)) applied to the tip of the ATR probe was used to detect CN- in the low microgram per liter (ppb) concentration range. The detection limit for an unmodified probe was found to be 130,000 μg L-1. By simply applying a thin film of NiCl2(dppp), the detection limit for aqueous cyanide dropped to 5 μg L-1. thereby increasing the sensitivity by a factor of 26,000. Applying a thin film of 1.1',3,3'-tetrakis(2-methyl-2-nonyl)ferrocenium nitrate (DEC+NO3-) afforded a sensitivity increase of 96,000 for CIO4- and 4.000 for C8F17SO3-. The added advantage of using infrared spectroscopy was positive identification of the target analytes through the unique IR absorbances produced in the resulting spectra. The detection limits reported here are unprecedented for infrared detection of these ions in aqueous solutions.