The Application of Fiber Ion Exchange Sorbents for Wastewater Treatment and Purification of Gas Mixtures

We developed a method of modifying the polyester material to produce ion exchange fiber sorbents. The production of cation exchange sorbents involved the treatment of polyester fibers with a 20–25% solution of NH2NH2H2O at 70–90°C and a 5% solution of NaOH at 40°C. Anion exchange sorbents were prepared by the treatment of cation exchange sorbents with a 1– 5% solution of polyethylenimine at ambient temperature. These new types of sorbents can be used to remove radionuclides, heavy metal ions and organic contaminants from wastewater and drinking water. We studied main properties of these sorbents and their ability to remove 57 Co, 60 Co, 65 Zn, 89 Sr, 90 Sr, 134 Cs, 137 Cs and other radionuclides, heavy metal ions (Zn, Ni, Cu, Sb, Pb, Cd, Cr, U, etc.), organic molecules M (pesticides, phenols, dioxins, benzene, toluene, etc.), radio-labeled organic molecules M32 P, M131 I, M99 Mo+ 99m Tc, M14 C, etc. The static exchange capacity is 1–2 meq/g for cationic sorbents and 0.5–1 meq/g for anionic sorbents. The developed sorbents have been effective in removing low concentrations of contaminants from water (lower than 100–200 mg/L) as well as in purifying the gas mixtures from toxic and aggressive gases: SO2, SO3, NH3, H2S, etc.


Introduction
Drinking water and wastewater purification from radionuclides, heavy metal ions and organic contaminants is one of the most important problems of the modern world. To solve it, the ion exchange method utilizing different types of resins and fiber sorbents [1] is widely used.
The advantage of fiber ion-exchange sorbents over resin is their high sorption rate, effective regeneration and a low pressure drop value of the sorbent layer for purified water [2][3][4]. The specific surface of fiber sorbents is (2)(3).10 4 m 2 /kg, i.e. about 10 2 times greater than that of resin (10 2 m 2 /kg). Due to that, fiber sorbents have a much higher sorption rate than resin.
There are many fiber filters produced in various plants, but most of them are intended only for air purification from dust and aerosols, and they do not have ion-exchange properties. The purpose of this work is to develop a technology for producing dust fiber filters with ion exchange properties. In many cases, this approach is economically more effective because it does not require construction of new plants. One of the most produced dust filters is a polyester fiber filter. Its production process includes acrylonitrilic emulsion treatment to improve mechanical characteristics. This feature can be used in the technology for producing cation and anion exchange sorbents.

Materials and Methods
A polyester fiber filter with surface density of 0.270 kg/m 2 and thickness of 10 mm was utilized as a raw material for making ion exchange sorbents. The mass of the polyacrylonitrilic layer on the fibers' surface was 15% of the total mass of the filter.
The 0.001M CuCl 2 solution labeled by 64 Cu and the K 2 Cr 2 O 7 solution (pH 2) labeled by 51 Cr were used to find out the best technology of making cation and anion exchange sorbents, respectively. The radionuclides 64 Cu and 51 Cr were produced by irradiating CuCl 2 and K 2 Cr 2 O 7 in the nuclear reactor of the Institute of Nuclear Physics (Tashkent, Uzbekistan). A Ge(Li) detector with a resolution of about 1.9 keV at 1.33 MeV and a 4096-channel multichannel analyzer were used to detect a γ-quantum from radionuclides. Areas under γ-peaks of radionuclides 64 Cu (the half-life, T 1/2 , is equal to 12.8 h, the energy of the γ-peak, E γ , is equal to 0.511 MeV) and 51 Cr (the half-life, T 1/2 , is equal to 27.72 d, the energy of the γ-peak, E γ , is equal to 0.320 MeV) were measured to calculate the amount of Cu and Cr, respectively.
Other radionuclides used in investigations of sorbents' characteristics as labels of ions and organic substances are given in Table 1. The exchange capacity Q, meq/g, was calculated by Equation (1): where B is the carrier amount, meq; W is the exchanger weight, g; A 0 is a count rate of the original solution; A e is a count rate of the solution at equilibrium; A B is a background count. The distribution coefficient K d and the percent adsorption P were calculated by Equations (2), (3): where V is a total volume of the solution, ml. The sorption processes of ions from water in dynamic conditions were studied by using columns with a diameter of 12 mm; the weight of sorbents was 1 g.

Results and Discussion
The kinetics of saponification of the fibers and the alteration of linkage quality between the polyester fiber and the polyacrylonitrile layer after chemical treatment of filters were studied in the range of NaOH solution concentration from 1 to 10%. For example, the results for concentration of 5% are given in Figures 1 and 2.
An increase in solution treatment temperature and treatment duration causes filter capacity increase, but linkage between the polyester fiber and the polyacrylonitrile layer is attenuated and the layer begins scattering. Thus, the treatment with a 5% solution of NaOH at 45-50°C for 1 hour was selected as the optimal condition for cation exchange sorbents production. The exchange capacity (Cu 2+ ) of the sorbents is 0.25 meq/g.

Figure 1. Kinetics of saponification of the fibers in a 5% NaOH solution at 25°C (1), 30°C (2), 40°C (3), 50°C (4), 70°C (5) and 90°C (6).
Anion exchange sorbents are made by treatment of cation exchange filters in an H-form with a water solution of polyethylenimine. Amine groups attach to carboxy groups by electrostatic forces. The kinetics of anion exchange groups' formation at concentrations of polyethylenimine from 0.5 to 5% and temperature from 20°C to 70°C were studied. Figure 3 demonstrates a kinetics curve at 40°C and a 1% concentration of polyethylenimine. Figure 4 shows dependence of exchange capacities on the concentration of polyethylenimine at 40°C and the treatment time of 8 hours.
The treatment of the cation exchange sorbents with a 1% solution of polyethylenimine at 40°C for 8-10 hours was

Conclusion
The above described results of investigations show that chemically modified polyester fiber filters have satisfactory adsorption characteristics. These cation and anion exchange filters can be used for removing metal ions (Zn, Ni, Cu, Sb, Co, Cd, Cr, etc.) and organic compounds (M-32 P, M-131 I, M-99 Mo+ 99m Tc, etc.) from wastewater. The cation exchange sorbents' capacity is 0.25 meq/g (Cu 2+ ) while the anion exchange sorbents' capacity is 0.45 meq/g (Cr 6+ ).