Silane coupling and mordanting as attachment techniques for pyridylazo and thiazolylazo ligands in the synthesis of adsorbents for uranium in seawater

Abstract

Activated carbon adsorbents modified with azo compounds (4-(2-pyridylazo)resorcinol, 1-(2-pyridylazo)-2-naphthol, 4-(2-thiazolylazo)resorcinol), or with allyl and vinylbenzyl derivatives of 4-(2-thiazolylazo)resorcinol, were observed to be highly effective in removing uranium from seawater and providing high loadings. Adsorbents consisting of azo compounds attached to fiber fabrics were prepared using silane coupling to attach azo reagents to silica fibers or mordanting agents such as tannic acid or aluminum acetate to attach them to cellulose fibers. Loadings of 15–45 mg U g⁻¹ adsorbent were obtained. Scanning electron microscopy/energy dispersive X-ray spectroscopy measurements confirmed the presence of high concentrations of uranium on the surface of the silica-based and cellulose-based fibers.

Introduction

In view of the limited resources of uranium available through mining operations on land and the environmental issues associated with uranium mining and ore processing, the feasibility of recovering uranium from the ocean by means of adsorbents or ion exchangers has been explored since the 1960s. The concentration of uranium in seawater is 3.3 µg l⁻¹, the pH is 8.1, and the dominant uranium-complexing species is carbonate. The major uranium-containing species in seawater is Ca₂UO₂(CO₃)₃ (Endrizzi and Rao, 2014).

Major surveys of potential adsorbents have been undertaken in order to identify sorbents suitable for removal of uranium from the seawater environment. This chemical environment poses a number of major difficulties. In low pH media, a large number of extracting agents have been known for many years to be highly effective in extracting U(VI), which is present in such media in the form of the UO₂²⁺ cation. Such agents include, for instance, tri-n-butylphosphate, bis-(2-ethylhexyl)-o-phosphoric acid, tri-n-octylphosphine oxide, and thenoyltrifluoroacetone (Korkisch, 1969). Far fewer extracting agents have been reported to be effective for the anionic complexes that U(VI) forms at high pH, and, in particular, for the neutral Ca₂UO₂(CO₃)₃ complex. Another major problem involves the very low concentration of uranium in seawater. Several solutes, such as Na and K, have low distribution coefficients relative to that of U with respect to common U adsorbents such as amidoxime-grafted polyethylene, but are present at much higher concentrations, while other solutes, such as Fe, Ni, and V are present at comparable concentration to that of U but have similar or higher distribution coefficients (Tamada, 2009). A third issue is the requirement that it should be possible to attach the extracting agent to a solid matrix with a high surface area suitable for immersion in the ocean, and that the bond between the active ligand and the matrix be strong, stable under seawater conditions, and unaffected by operation over a significant number of absorption/elution cycles. An extensive early survey of potential extracting agents (Schenk et al., 1982) indicated that very few of them exhibited the high effectiveness and selectivity for U justifying further study. The most promising of them has been amidoxime. Although other nitrogen-based ligands, such as glutarimidoxime (Tian et al., 2012) as well as phosphor-based compounds, such as bis(2-methacryloxyethyl) phosphate (Tissot, 2014), have been proposed as candidates, amidoxime is still the ligand on which most attention is focused.

The most effective technology for preparing high-quality amidoxime-based adsorbents was found to be grafting acrylonitrile onto polymeric fibers followed by amidoximation with hydroxylamine (Omichi et al., 1985). In most cases, grafting onto polymeric fibers (usually polyethylene) is accomplished by means of radiation-induced grafting polymerization (RIGP). Uranium loadings as high as 4.48 g U kg⁻¹ of adsorbent have been obtained upon testing such adsorbents in natural seawater (Das et al., 2016; Oyola et al., 2016). Uranium removal from seawater still faces several technical and economic problems, such as limited selectivity, in particular with respect to V and Fe, the need for irradiation, followed by amidoximation, during the preparation of the adsorbent, and the need for activation with an alkaline solution as a pretreatment as well as during regeneration, which causes degradation of the active group and thus severely limits the number of adsorption/elution cycles over which the adsorbent can be used. The use of chemical methods (Egawa et al., 1992) rather than RIGP in the preparation of adsorbents may offer significant advantages, such as applicability to a variety of adsorbent groups other than amidoxime, avoiding the costs of radiation processing, and improving the uniformity of the adsorbents when prepared on an industrial scale.

One approach toward the achievement of high selectivity toward uranium is based on the use of chelating agents used in the spectrophotometric or colorimetric analysis of uranium. Certain pyridylazo and thiazolylazo compounds, such as 1-(2-pyridylazo)-2-naphthol (PAN), 4-(2-pyridylazo)resorcinol (PAR), and 4-(2-thiazolylazo)resorcinol (TAR) have been shown to form uranium chelates in near neutral or weakly basic environments (Kalinich and McClain, 2007). In analytical use, these compounds have been shown to have a high selectivity for uranium. For instance, of the elements that are present in seawater at concentrations higher than or similar to the concentration of uranium, only pentavalent V was found to cause a major interference with the direct spectrophotometric determination of uranium at pH 8 using PAR (Florence and Farrar, 1963). The high affinity of these compounds toward uranium(VI) was demonstrated by the fact that PAR and (2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (Br-PADAP) were found to be suitable for determination of uranium(VI) in solutions of substances that form strong complexes with it, such as phosphoric acid (Brčić et al., 1985a) and sodium carbonate (Brčić et al., 1985b). Regardless of these early findings, the potential usefulness of pyridylazo- and thiazolylazo-based adsorbents in the recovery of uranium from seawater has not yet been investigated. Another advantage of these compounds is their high chemical stability, in particular in alkaline and oxidizing environments, relative to that of amidoxime.

Several methods may be used to attach pyridylazo and thiazolylazo reagents to nanostructured solids with very large surface area. Such solids include, for instance, silica with a highly ordered arrangement of pores in the mesoporous or microporous range, as well as nanotubes or nanorods of metal oxides such as TiO₂, Al₂O₃, ZnO, and NiO. Metal-adsorbing compounds, which are preferably nitrogen-containing aromatic compounds, are attached to the large surface solids by immersing the latter in solutions (e.g. ethanolic solutions) of the metal-adsorbent compounds, followed by heating under vacuum. The resulting adsorbents are contacted with aqueous solutions containing lanthanide or actinide elements using techniques such as immersion, passing through a column, or mixing and stirring (Elsafty and Halada, 2014). However, fiber fabrics deployed in stacks (Seko et al., 2003) or in the form of long braids (Seko et al., 2002; Tamada et al., 2006) are much more suitable for removal of uranium from large volumes of seawater.

The present study was intended to explore the possibility of using pyridylazo and thiazolylazo compounds for recovery of uranium from seawater using various types of solid adsorbents with attached azo compounds. In the screening stage, a number of pyridylazo and thiazolylazo compounds were adsorbed on activated carbon and tested to determine their effectiveness in removing dissolved uranium from seawater. In order to prepare the way for attaching such compounds to fiber fabrics to render them useful for extraction operations, several of these compounds were derivatized to contain C = C bonds, and the products were included in the screening tests.

The feasibility of using chemical methods, rather than RIGP, for the attachment of such compounds to fiber fabrics was investigated during the following stages of the study. Both silica and cellulose have been previously explored as candidate supports for functional groups, in particular amidoxime, which are potentially useful in removing uranium and other heavy metal ions from seawater (Basanir and Bayramgil, 2012; El-Khouly et al., 2011; Gunathilake et al., 2015). The use of chemical attachment rather than radiochemical methods can be expected to lead to the development of simpler, less costly manufacturing technologies, which will allow tighter control and volume uniformity, especially upon producing large quantities of the adsorbents. In this study, it was attempted to identify methods of obtaining strong chemical (rather than radiochemical) attachment of azo dyes to substrates consisting of silica and cellulose fibers. The two methods explored were silane coupling of pyridylazo compounds to silica fibers and attachment of such compounds to cellulose fibers pretreated with a mordant.

Thus, the two objectives of the study were testing the suitability of novel adsorbents based on highly selective and stable pyridylazo and thiazolylazo ligands for use in the extraction of uranium from seawater, and testing the feasibility of attaching these ligands, and potentially others, to high-surface substrates through novel uses of the techniques of silane coupling to inorganic fibers or utilizing mordants to provide bonding to cellulose fibers.

Materials and methods

The six pyridylazo and thiazolylazo complexing agents used in the present study were as follows:

PAN, ≥97.0%, Sigma Aldrich 82960, MW 249.27

2-(2-pyridylazo)-1-naphthol or ISOPAN, ≥95.0%, Sigma Aldrich 82963, MW 249.27

PAR, 96%, Sigma Aldrich 323209, MW 215.21

Br-PADAP or Bromo-PADAP, 97%, Sigma Aldrich 180017, MW 349.23

TAN, ≥99.0%, Sigma Aldrich 88413, MW 255.30

TAR, 97%, Sigma Aldrich 127345, MW 221.24

The full specifications of other materials (substrates, coupling agents or mordants, analytical reagents) are given in the text.

The activated carbon powder used in the first stage of the present study was Fisher Scientific Fisherbrand® Activated Carbon, 50–200 mesh (0.075–0.3 mm), Catalog No. 05–690A. Activated carbon media loaded with pyridylazo compounds such as PAR or PAN or thiazolylazo compounds such as TAR were prepared by weighing a quantity of 0.5–2.0 g of activated carbon and mixing it with a quantity of pyridylazo or thiazolylazo compound corresponding to 50 or 5% of the weight of the activated carbon. This was followed by adding 10 ml of methanol, rotating the mixture at 30 r min⁻¹ for seven days, and then discarding the methanol, air-drying the powder, and reweighing it. The weighing results showed that the pyridylazo and thiazolylazo compounds used in the present experiments were quantitatively or almost quantitatively adsorbed on the activated carbon. Furthermore, no dye was visually observed to remain in the methanol solution following the contact with the activated carbon. Accordingly, each adsorbent contained 33 or 4.8% of an azo compound, respectively, relative to its total weight.

Two thiazolylazo compounds containing C = C bonds were synthesized. One of these compounds was 5-(allyloxy)-2-(thiazol-2-yldiazenyl)phenol (A-TAR), which was prepared by adding sodium carbonate (2.2 g or 0.021 mol) and allyl bromide (2.00 g or 0.017 mol) to a stirred solution of 4-(thiazol-2-yldiazenyl)benzene-1,3-diol, also called 2-(2,4-dihydroxyphenylazo)thiazole, 97%, MW 221.24, Sigma-Aldrich 127345 (2.4 g or 0.011 mol) in acetone (150 ml) under nitrogen atmosphere. The mixture was refluxed under nitrogen for 48 h. The mixture was filtered in vacuum and the filtrate was concentrated in a rotary evaporator. The resulting residue was purified via column chromatography with 2:1 hexane/ethyl acetate as eluents. A-TAR (0.95 g) was obtained. The structure of the product was confirmed using NMR (Figure 1(a)). The schematic of the synthetic technique is as follows:

Figure 1.

(a)¹H NMR spectrum of A-TAR and (b) ¹H NMR spectrum of VB-TAR.

The other compound, 2-(thiazol-2-yldiazenyl)-5-(4-vinylbenzyloxy)phenol (VB-TAR), was synthesized by adding sodium carbonate (4.0 g or 0.038 mol) and 1-(chloromethyl)-4-vinylbenzene (4.0 g or 0.026 mol) to a stirred solution of 4-(thiazol-2-yldiazenyl)benzene-1,3-diol (4.60 g or 0.021 mol) in acetone (150 ml) under nitrogen atmosphere. The mixture was refluxed with stirring under nitrogen for 48 h. The mixture was filtered in vacuum and the filtrate was concentrated in a rotary evaporator. The resulting residue was purified via column chromatography with 2:1 hexane/ethyl acetate as eluents. The observed yield of VB-TAR was close to theoretical. The structure of the product was confirmed using NMR (Figure 1(b)). The schematic of the synthetic technique is as follows:

These two compounds were included in the screening tests following adsorption on activated carbon as described above.

The silica fiber fabrics used in the experiments consisted of silica mat insulation produced by LEWCO Specialty Products, Inc. The fibers had an approximate diameter of 10 µm, confirmed by SEM analysis. In order to prepare silanized silica fibers with PAR, a combined solution of 0.3 ml or 0.0017 mol of (3-aminopropyl)trimethoxysilane (APTMS), 97%, Sigma Aldrich 281778 and 0.0053 g or 0.00018 mol (as monomer) of paraformaldehyde, ≥95.0%, Sigma Aldrich 76240, in 1.0 ml of methanol was added to a solution of 0.027 g or 0.00013 mol of PAR in 6.0 ml of methanol. A quantity of 0.31 g of silica fibers was added. After standing for four days, the solution was decanted and the resulting red fibers washed with approximately 100 ml of methanol, followed by approximately 100 ml of deionized water, before being dried. In a control experiment, the APTMS–paraformaldehyde treatment was omitted and the silica fibers directly contacted with the PAR solution.

The cellulose fibers used in the experiments were highly purified Type 101 fibers or linters, Sigma-Aldrich Cat. No. S6970. According to the vendor, the cellulose linters had a dimensional distribution amounting to 25–30% below 10 µm, 4–10% above 20 µm, and the majority between 10 and 20 µm. These data were confirmed by SEM analysis. The cellulose fibers were pretreated with either tannic acid or aluminum acetate as a mordant, followed by treatment with PAR or PAN. The procedure using tannic acid as a mordant consisted of treating 1.00 g of cellulose fibers with a solution of 0.20 g of tannic acid (Sigma-Aldrich Cat. No. 403040) in 100 ml of water at 60°C for an hour. The solution was allowed to stand for a week and then the aqueous solution was discarded and the treated fibers were added to a solution of 0.10 g or 0.00040 mol PAR or 0.10 g or 0.00046 mol PAN in 40 ml of methanol. The procedure using aluminum acetate as a mordant consisted of treating 1.00 g of cellulose fibers with a solution of 0.048 g or 0.00030 mol of basic aluminum acetate (Sigma-Aldrich Cat. No. 289825) in 80 ml of water at 60°C for an hour. The solution was allowed to stand for a week, and then the aqueous solution was discarded and the treated fibers were added to a solution of 0.10 g PAR or PAN in 40 ml of methanol.

Test solutions of uranium in seawater were prepared by dissolving a suitable quantity of uranyl acetate dihydrate, Fisher Scientific Cat. No. U-4, in Atlantic Ocean seawater (34° 42′ N, 76° 43′ W). As a result of adding the uranyl ion in acetate form rather than as the nitrate salt, the introduction of up to 1 mg l⁻¹ of uranium did not produce a significant change in the pH of the seawater (8.2 ± 0.1). In tests with higher concentrations of uranium, the pH was readjusted to 8.2 using 1 M NaOH. In each test, a quantity of 15 mg of adsorbent (activated carbon granules, silanized silica fibers, or cellulose fibers coated with an azo dye according to the procedures described above) was added to a desired volume of uranium in seawater and the mixture was rotated together for a period of seven days, unless otherwise specified, at 30 r min⁻¹. At the end of this period, the solution was separated from the solid adsorbent and analyzed to determine the amount of uranium remaining in the solution by means of a spectrophotometric method based on the use of Arsenazo III as a color-forming reagent and triethylenetetraminehexaacetic acid (TTHA) as a masking agent (Strelow and Van Der Walt, 1979). The analytical procedure consisted of mixing together 2.0 ml of the test solution with 0.2 ml of a 5% solution of TTHA, ≥98.0%, Sigma-Aldrich T7633, in water, 0.2 ml of 0.05% solution of Arsenazo III, Sigma-Aldrich A92775, in water, and 0.1 ml of 1 M HCl to bring the pH to 1.0. The uranium concentration was obtained from the absorbance at 641 nm, which corresponds to the maximum peak wavelength associated with the uranyl–Arsenazo III complex. A blank consisting of seawater with the same volumes of reagents was used. Each adsorption test was carried out in quadruplicate.

Results and discussion

Results of measurements of uranium removal from seawater using activated carbon treated with azo dyes

The results of the measurements of adsorption of uranium from seawater spiked with UO₂(COO)₂·2H₂O to contain 0.2–1.0 mg l⁻¹ uranium(VI) (added in the form of UO₂(COO)₂·2H₂O) on a series of adsorbents are shown in Figure 2. The azo dyes included, in addition to PAR, PAN, and TAR, Br-PADAP, ISOPAN, and TAN. Each azo dye was adsorbed, at a loading level of 33%, on activated carbon. For comparison, results obtained with untreated activated carbon are included.

Figure 2.

Adsorption of uranium on 15 mg of activated carbon coated with 33% azo dye from 10 ml of uranium-spiked seawater. ♦ Br-PADAP; ▪ PAR; ▲ PAN; × ISOPAN; * TAR; • TAN; + untreated activated carbon.

The large extent of uranium removal using activated carbon samples coated with azo dyes at a concentration of 33% hinders comparison of the relative effectiveness of the various dyes. Accordingly, another set of measurements was performed on adsorbents containing the lower concentration of 4.8% of azo dye. The results are shown in Figure 3. These results show that the relative extent of uranium removal from the seawater was ISOPAN > PAR > PAN > TAR > TAN > Br-PADAP. (ISOPAN was not used in further stages of the study because this item was discontinued by the vendor.)

Figure 3.

Adsorption of uranium on 15 mg of activated carbon coated with 4.8% azo dye from 10 ml of uranium-spiked seawater. ♦ Br-PADAP; ▪ PAR; ▲ PAN; × ISOPAN; * TAR; • TAN; + untreated activated carbon.

In the case of activated carbon coated with 33% PAN, measurements with higher ratio of the quantity of uranium to the weight of adsorbent were also performed in order to find out if high loadings of uranium on the adsorbent could be obtained. Increasing the quantity of uranium in the aqueous phase was performed using two different methods. In one set of measurements, the initial concentration of uranium was held constant at 1 mg l⁻¹, and the volume was gradually increased from 10 to 100 ml. In the other set, the volume was held constant at 10 ml and the initial concentration of uranium was gradually increased from 1 to 40 mg l⁻¹. The loading of uranium on the adsorbent, in units of milligram adsorbed uranium per gram of adsorbent, was plotted against the initial amount of uranium in the solution. The results are shown in Figure 4. These results indicate that the loading increases linearly with increasing quantity of uranium in the seawater at least up to a quantity of 0.4 mg uranium in the solution, and that at that point the loading is 20 mg U g⁻¹ adsorbent. The results also show no difference between those belonging to the set of experiments in which the quantity of uranium was raised by increasing the volume at a constant initial concentration of uranium and the set of experiments in which the quantity of uranium was raised by increasing the initial concentration of uranium at constant volume.

Figure 4.

Adsorption of uranium on 15 mg of activated carbon coated with 33% PAN from uranium-spiked seawater. ♦ Volume variation, 10–100 ml, U concentration 1 mg l⁻¹; × concentration variation, 1–40 mg l⁻¹, volume 10 ml.

Results of measurements of uranium removal from seawater using activated carbon treated with derivatives of azo dyes containing C = C bonds

The testing procedure used to measure the effectiveness of uranium removal from seawater using azo dyes was applied to the A-TAR and VB-TAR derivatives which contain C = C bonds. The results obtained with activated carbon coated with these compounds at a level of 4.8% are shown in Figure 5. The results obtained with nonderivatized TAR under the same conditions are included for comparison.

Figure 5.

Adsorption of uranium on 15 mg of activated carbon coated with 4.8% azo dye from 10 ml of uranium-spiked seawater. ♦ TAR; ▪ A-TAR; ▲ VB-TAR.

Measurements of uranium removal from seawater using silanized silica fibers and mordant-pretreated cellulose fibers treated with PAR or PAN

Tests were run to quantify the removal of uranium from seawater using silanized silica fibers or mordant-pretreated cellulose fibers treated with PAR or PAN. In each case, 15 mg of adsorbent were rotated with 10 or 100 ml of the test solution for 21 days. The results are shown in Table 1. It should also be noted that no bleeding of the red color of the azo dyes into the seawater was observed at the end of continuous rotation for 21 days. In contrast, when PAR- or PAN-treated silica fibers or cellulose fibers were prepared without prior silanization or pretreatment with a mordant, the azo dyes were observed to be washed away quickly and completely into the seawater used in the test. Each adsorbent was tested in duplicate, and each test solution was analyzed in quadruplicate.

Table 1.

Results of 21-day tests on removal of uranium from seawater using 15 mg of adsorbent in each test.

Adsorbent	Initial U concentration in seawater (mg l−1)	Volume (ml)	%U removed	U loading (mg g−1)
Silanized silica fibers with PAR	0.2	10	>95	>0.13
	1	10	97 ± 1	0.65 ± 0.01
	40	100	5.9 ± 0.2	16 ± 1
Cellulose fibers with tannic acid and PAN	0.2	10	80 ± 4	0.11 ± 0.01
	1	10	67 ± 6	0.45 ± 0.04
Cellulose fibers with aluminum acetate and PAR	0.2	10	89 ± 1	0.12 ± 1
	1	10	81 ± 1	0.54 ± 0.01
	40	100	17 ± 1	46 ± 1
Cellulose fibers with aluminum acetate and PAN	1	10	51 ± 1	0.34 ± 0.01
	40	100	16 ± 1	43 ± 1

PAN: 1-(2-pyridylazo)-2-naphthol; PAR: 4-(2-pyridylazo)resorcinol.

The tests performed with 100 ml of 40.0 mg l⁻¹ of uranium in seawater were intended to evaluate the loading capacity of the adsorbent fibers for uranium, because the loadings obtained with smaller initial amounts of U in seawater, and corresponding to >50% removal of uranium from the solution, were clearly determined by the initial amount of uranium in the seawater rather than the capacity of the adsorbent. According to the results shown in Table 1, the loading capacities of silanized silica fibers/PAR, cellulose fibers/aluminum acetate/PAR, and cellulose fibers/aluminum acetate/PAN were (16 ± 1), (46 ± 1), and (43 ± 1) mg U g⁻¹ adsorbent, respectively, corresponding to 0.150, 0.140, and 0.064 atom% of uranium, respectively.

The samples of modified silica fibers or cellulose fibers contacted with 100 ml of 40.0 mg l⁻¹ U in seawater were subsequently separated from the test solutions by filtration, and the filter paper with the red fibers was washed three times, each time with 25 ml of deionized water. No red coloration was observed in the wash water, and no loss of red color was observed on the fibers. The silica fibers became largely embedded in the filter paper, while the cellulose fibers could be easily removed from the paper. After air-drying and coating with gold to suppress charging, samples of the two types of fibers were characterized by means of scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). As expected, EDS measurements with an energy of 10 keV on mats of cellulose fibers removed from the filter paper showed presence of uranium throughout the sample. In the case of the silica fibers on filter paper, a line scan for Si and U clearly indicated that both Si and U were present only on the fibers, as shown in Figure 6(a). The bottom of Figure 6(a) shows a part of an SEM image of a silica fiber, previously silanized, treated with PAR, contacted with 100 ml of seawater doped with 40 mg l⁻¹ U for 21 days, separated from the seawater by a piece of filter paper, washed with deionized water and allowed to dry, taken with the fiber lying on the filter paper. The top part of the figure shows line scans across the same area at the Kα X-ray emission wavelengths of Si (above) and U (below), respectively. An SEM image of a similarly treated cellulose fiber fabric is shown in Figure 6(b) and accompanied by elemental mapping for uranium at higher magnification in Figure 6(c). This material, unlike the distinct, straight silica fibers, consists of a mass of intertwined fibers and, as a result, this image shows the uranium to be distributed over the entire mass.

Figure 6.

(a) Micrograph of a 15 mg sample of PAR-treated silica fibers contacted with 100 ml of seawater containing 40 mg l⁻¹ for 21 days and corresponding line scans for Si and U, (b) SEM micrograph of a 15 mg sample of PAR-treated cellulose fibers contacted with 100 ml of seawater containing 40 mg l⁻¹ for 21 days—low magnification, and (c) SEM micrograph of a 15 mg sample of PAR-treated cellulose fibers contacted with 100 ml of seawater containing 40 mg l⁻¹ for 21 days and corresponding elemental mapping for U—high magnification (corresponding to the marked region in (b)).

The atom% loadings of U on cellulose fibers/aluminum acetate/PAR, cellulose fibers/aluminum acetate/PAN, and silica fibers/silane/PAR, subjected to the same treatment followed by EDS analysis, amounted to 0.98, 0.82, 1.03%, respectively.

Performance of pyridylazo- and thiazolylazo-based adsorbents

The results described above indicate that activated carbon coated with various pyridylazo or thiazolylazo reagent exhibits a high degree of effectiveness in removing uranium from seawater environments over a broad range of uranium concentrations. It is important to note that there is no decrease in the percentage of uranium removal as the initial uranium concentration in seawater is lowered from 1.0 to 0.2 mg l⁻¹. This is indicative of high selectivity of the adsorbents for uranium, since the ratios of the concentrations of other cations in natural seawater to the concentration of uranium increase as the concentration of introduced uranium is lowered. Another advantage of these adsorbents stems from the finding that as the quantity of uranium in the seawater is raised, such adsorbents (as demonstrated in the case of PAN) exhibit high loading levels, reaching at least 2% uranium relative to the weight of the adsorbent.

As mentioned above, activated carbon is less useful as a support material than fiber fabrics in cases where a metal such as uranium has to be extracted from very large volumes of seawater. The results described above indicate that C = C bonds can be attached to the thiazolylazo dye TAR without a large negative impact on the capacity of the dye, when coated on activated carbon, to remove uranium from seawater. When 25 mg of the coated activated carbon is contacted with 10 ml of seawater spiked with 1 mg l⁻¹ of uranium, the loading of uranium on the adsorbent is only decreased by 7% when TAR is replaced by A-TAR and only by 17% when TAR is replaced by VB-TAR. This finding indicates that the substitution of C = C groups, such as allyl and vinylbenzyl, into the structure of such azo dyes, leaves the capacity for uranium adsorption largely intact. The slight decrease in capacity can be attributed to the higher molecular weight of the A-TAR and VB-TAR derivatives relative to TAR, since the same weight of these derivatives contains a smaller number of moles of the active adsorbent groups than in the case of TAR. Accordingly, such substitution can be concluded to be a promising approach in attempting to attach such dyes to polymer fabrics, whether using radiation-induced grafting or chemical copolymerization. It should also be noted that A-TAR and VB-TAR, as well as TAR itself, do not exhibit gradually decreasing distribution coefficients for uranium as the concentrations of uranium added to the seawater decrease. This indicates that the introduction of C = C bonds does not have an adverse effect on the selectivity of these adsorbents as the concentration of uranium in seawater is decreased to levels at least as low as 0.2 mg l⁻¹.

The results obtained with fiber fabrics consisting of silanized silica or mordant-pretreated cellulose treated with PAR or PAN indicate that these fiber fabrics, produced through chemical treatments not requiring the use of ionizing radiation, can be potential candidates for use in uranium removal from seawater. Again, no decrease in performance is observed when the concentrations of uranium added to the seawater are lowered, and this finding is consistent with high selectivity. The fact that no bleeding of the dye into the seawater test solution was observed is indicative of stability of the bonding of the azo dyes to the silica or cellulose support. This shows that techniques such as silane coupling or the use of mordants can provide chemical alternatives to the use of radiation in the synthesis of effective adsorbents for uranium from seawater environments. The use of such techniques may be applicable to chelating reagents other than azo dyes and to separations involving dissolved metals other than uranium.

As detailed above, SEM surface analysis of cellulose fibers treated with PAR or PAN following pretreatment with aluminum acetate and of PAR-treated silanized silica fibers after exposure to seawater spiked with 40 mg l⁻¹ yielded high loadings (0.98, 0.82, and 1.03 atom%, respectively). These values were much higher than the corresponding values obtained by chemical analysis of the uranium removed from the solution relative to the entire weight of the adsorbent, which amounted to 0.150, 0.140, and 0.064 atom%, respectively, based on the data in Table 1. The difference between the two sets of values can be attributed to the fact that uranium is adsorbed on the surface of the fibers and that SEM/EDS measurements are limited to the near-surface region. The 10 keV electron beam used in the SEM/EDS measurements only has an effective range (penetration depth) of only about 0.2 µm in solids with densities in the range of 1.5–2.2 g cm⁻³ (Marković, 1981). On the other hand, the diameter of the silica fibers was approximately 10 µm and the dimensions of the cellulose linters were mostly between 10 and 20 µm, as confirmed by SEM microscopy (see “Materials and methods” section).

The same data show that the loading level of uranium as determined by solution analysis (but not by SEM surface analysis) in the case of the silica fibers was low compared with the corresponding level in the case of the cellulose fibers. This can be attributed to the fact that the silica fibers were much smoother and thus had a smaller surface area than the cellulose fibers. Higher loading levels of uranium on silica fibers may be obtained by using abraded fibers.

Further work is needed to characterize the performance of these adsorbents. Since many known adsorbents for uranium are only effective at low pH, the adsorbents explored in the present study may also be useful in removing uranium from contaminated seawater (Atun and Kilislioglu, 2002) and other near neutral or weakly alkaline media. Such media may include environmental streams contaminated with uranium as a result of mining, ore processing, and other industrial operations which generate uranium-containing wastes.

Further work is also needed in order to explore the operational characteristics of these adsorbents with respect to regeneration. The compounds included in the present study have been widely reported to be unaffected by highly acidic and basic environments as well as in the presence of various oxidizing agents (e.g. Hartenstein, 1990), and this suggests that they will be less susceptible than amidoxime compounds to degradation of performance upon exposure to aggressive reagents in the course of successive elution/regeneration cycles.

Conclusion

The study described above has two principal implications to the search for effective adsorbents to be used in the recovery of uranium from seawater. It shows that solid adsorbents which contain pyridylazo and thiazolylazo active groups attached to a solid support are highly effective in removing uranium from natural seawater doped with uranium, and that making the concentration of uranium in the seawater at least as low as 0.2 mg l⁻¹ does not affect the effectiveness of the removal regardless of the presence of other competing or otherwise interfering ions in seawater. Furthermore, it shows that compounds containing active ligands such as pyridylazo and thiazolylazo groups can be strongly bonded to silica fibers through silanization and to cellulose fibers through the use of mordants. These two conclusions have possible implications beyond the recovery of uranium from seawater, the first one in regard to removal of uranium from contaminated or industrial streams and the second one in regard to attachment of various ligands to solid supports with a view to removing desired metal ions from aqueous media. More extensive characterization of the adsorbents described in this study is needed with respect to their structure as well as in regard to operational characteristics such as the conditions required for elution of uranium and regeneration of the adsorbent.