Results and Discussion

Adsorption characteristics of silicas with covalently bound phenolic-type analytical reagents (8-hydroxyquinoline, PAN, PAR). Silicas with covalently anchored (via the one-step Mannich reaction) phenolic-type analytical reagents were applied for separation of toxic metals ions (Table 1). In accordance with the obtained data, in the pH range 6-8 silica with grafted 8-hydroxy-quinoline extracts toxic metal ions, such as Pb(II), Zn(II), Mo(VI) (full removal), Cu(II), Fe(III) (90-98% removal) and Cd(II) and Al(III) (partial removal, from 60 to 80%). In the neutral and low-alkaline media silica with covalently bound PAR quantitatively removes ions of such high-toxic metals as Pb(II), Cd(II), Hg(II), Ni(II) and partially adsorbs Zn(II), Co(II), Cu(II), and Fe(III). In the acidic media (at pH = 1-4) this adsorbent did not extract quantitatively any metals studied. At pH = 6-9 silica with grafted PAN removes 92-95% of Zn(II), Pb(II), Cu(II) ions and adsorbs partly Cd(II), Ni(II), Fe(III) ions. Obtained results correlate well with the stability constants of complexes of the studied metals with 8-hydroxyquinoline, PAR and PAN in solutions.1'9

TABLE 1. Adsorption properties of silica gels with covalently bound analytical reagents towards toxic metal ions.

Adsorbent

pH of solutions

Toxic metals adsorbed with a separation degree of 95 ± 5%

Silica with bound oxine

4-9

Zn2+, Pb2+, Al3+, Mo6+, Cu2+

Silica with bound PAR

6-9

Zn2+, Pb2+, Cd2+, Hg2+, Ni2+, Cu2+

Silica with bound PAN

7-9

Zn2+, Pb2+, Cd2+, Ni2+, Cu2+

Sorption capacities of the synthesized modified silicas for toxic metals studied are sufficiently high (about 4 mmol/g), and such adsorbents can be applied for extraction of micro amounts of these metals from water solutions within a concentration interval of 0.1-100 ng/mL. This circumstance can be applied for development of the adsorption water-purification process.10

All synthesized adsorbents are characterized by a high rate of extraction of heavy metals: the maximum degree of adsorption can be achieved for 2-5 min of contact (Figure 1 and Table 2). Therefore, the obtained materials

40 20

40 20

0 20 40 60 80 100 120

Figure 1. Dependence of degree of extraction (R) of Pb2+ (1), Zn2+ (2) and Cd2+ (3) ions by silica with covalently bound 8-hydroxyquinoline on adsorption time at pH = 6.8-7.

TABLE 2. Kinetics of adsorption of toxic metals ions by silica gel with covalently bound PAR.

Metal ion

pH of

Sorption time (min)

solutions

5

10

20

60

Adsorption degree (%)

Zn2+

6.8

88.8

89.3

89.3

89.3

Cd2+

7.0

99.9

99.9

99.9

99.9

Pb2+

6.8-7.0

99.9

99.9

99.9

99.9

Cu2+

6.8

83.7

75.0

83.0

80.0

Fe3+

6.8

94.2

95.1

95.4

95.8

Co2+

6.8

98.8

99.9

99.9

99.9

Ni2+

6.8

51.3

73.4

79.8

99.9

can be employed to remove heavy metal ions from water solutions in the dynamic adsorption mode.

Data about structure of the surface complexes of toxic metals with grafted phenolic-type analytical reagents (8-hydroxyquinoline, PAN, PAR). Structures of the complexes of toxic metal with phenolic-type analytical reagents anchored on the silica surface were studied by IR, ESR and electronic diffuse reflectance spectroscopies.

ESR spectra of the complexes of copper with 8-hydroxyquinoline, PAN and PAR, covalently bound with silica surface are characterized by anisotropy and hyperfine structure of the gII signal (Tables 3 and 4). These data testify about divalent state of copper on the surface of the chemically modified silicas.

TABLE 3. Characteristics of ESR spectra of Cu(II) complexes with PAN chemically bound with silica surface.

Sample [Cu2+]ads

gII

g-

An-10-4

Structure of the coordination

(mmol/g)

(cm-1)

unit

1 0.01

2.254

2.030

147

Cu [2N2O]

2 0.05

2.276

2.025

149

Cu [2N2O] with deformation

3 0.20

-

2.047

-

planar structure

TABLE 4. Parameters of ESR spectra of Cu(II) complexes with PAR chemically bound with

silica surface.

Sample [Cu2+]adi

gII

g-

Aii-10-4

Structure of the coordination

(mmol/g)

(cm-1)

unit

1 0.013

2.38

2.039

179

Cu [2N2O]

2 0.062

2.31

2.028

173

Cu [4O2O]

similar to CuSO4-5H2O

In electronic diffuse reflectance spectra the absorption bands of grafted PAN molecules and bands d-d transitions for complexes with Zn(II) - at 580 nm, with Pb(II) - at 620 nm, with Cu(II) and Cd(II) - at 550 nm were observed. These bands correlate well with values of absorption bands for d-d transitions these metals in complexes with PAN in solutions.1'9 It shows that coordination of these metal ions with PAN molecules bound with silica surface is similar to their interactions in solutions.

According to calculations based on ESR spectra of complexes of copper(II) ions with bound 8-hydroxyquinoline, the value of the gy-factor is 2.372, = 2.064, constant of hyperfine coupling A = 201 x 10-4 cm-1. This allows one to assume the nearest coordination surrounding of Cu2+ ions in the surface complex. Most probable, 4[0] and 2[N] distorted octahedron structures are formed with O-atoms of hydroxyl groups in the equatorial position and N-atoms in the axial position, or with N-atom and two O-atoms in the equatorial position and two O-atoms in the axial position.1112

Form and calculated parameters of ESR spectra of the complexes of copper(II) ions with covalently bound PAN (Table 3) allow to conclude that:

- At copper(II) concentrations on the adsorbent surface ([Cu2+]ads) less than concentration of grafted PAN molecules (sample 1), environment of copper(II) in complex corresponds to four-coordinated Cu(II) with [2N2O] coordination12,13

- At an increase of [Cu2+]ads value (sample 2) signals from some types of complexes with similar parameters were observed, it can be connected with distortion of the planar structure of complexes14

- At [Cu2+]ads more than concentration of grafted PAN molecules (sample 3), signal of hyperfine splitting is indistinct, but asymmetry and anisotropy of the spectrum are retained

The g-factor and AII values calculated from ESR spectra of complexes of copper(II) ions with bound PAR for sample 1 (Table 4), where concentration of adsorbed copper is less in some times than concentration of grafted PAR molecules, are characteristic for the octahedron oxygen-containing complexes such as Cu[4020]-adducts of bischelates of copper with strong axial coordination into 5 and 6 place or octahedron complexes such as realized in CuSO^^O.13,14 Such complexes may be formed at coordination copper(II) with atoms of oxygen of two hydroxyl groups of two PAR molecules and two water molecules or two hydroxyl groups of one PAR molecule and four water molecules. As a result of this unusual coordination (different from pathway of coordination in solutions) of Cu(II) with PAR molecules, covalently bound with silica surface, incomplete adsorption of Cu(II) ions (82-85% at pH = 6-9) can take place, though in water solutions the stability constant of the complex of Cu(II) with PAR has the highest value among metals studied.12

ESR spectrum of sample 2 (a quantity of adsorbed copper is compare with concentration of grafted PAR molecules) showed that gII and AII values are decreasing (Table 4). Calculated parameters of spectrum of sample 2 (concentration of coordinated metal was 0.062 mmol/g) are close to complexes of four-coordinated copper(II) with coordination unit [2N2O]1112 as it takes place at coordination PAR molecules with Cu(II) ions in the solutions.

Electronic diffuse reflectance spectra of metal complexes with chemically bound PAR are characterized by following absorption bands in the visible region of spectrum: for Cu(II) - at Xmax = 525 nm; for Cd(II) - at Xmax = 510 nm; for Fe(III) - at Xmax = 700 nm; for Pb(II) - at Xmax = 520 and 700 nm; for Zn(II) - at A,max = 500 nm. These dates testify that pathway of coordination of PAR molecules chemically bound with silica surface for Cd(II) and Pb(II) is the same as in solutions. Absorption band at 500 nm for Zn(II) correlates well with data of paper,15 where Zn(II) complexes with PAR-containing adsorbent obtained by sol-gel method are also characterized by absorption at 500 nm.

Peculiarities of adsorption-x-ray fluorescence determination of toxic metals after their preconcentration on the silica with covalently bound

8-hydroxyquinoline and PAR. For ascertainment of a possibility of simultaneous x-ray fluorescence determination of heavy toxic metals, which quantitatively adsorbed onto synthesized chemically modified silica (Table 1), model water solutions (25-1,000 ml) contained mixtures of metal ions of different concentrations were prepared. These solutions were transmitted through a cone-shaped plastic column filled with 0.2 g adsorbent. The samples of modified silica with adsorbed metals were dried in air, then were ground in an agate mortar and studied by x-ray fluorescence spectroscopy. The content of every metal in the adsorbent phase has been calculated by a comparison of the absolute characteristic x-ray intensities of metals adsorbed on silica from the model solution with the calibration curves for the individual metals.

According to the obtained data, micro amounts of lead and cadmium ions can be quantitatively detected by x-ray fluorescence method after pre-concentration these metals upon silica with covalently bound 8-hydroxy-quinoline. At the same time zinc and molybdenum can be detected in these conditions only at 50-60%, it is caused their non-complete extraction in the lead(II) presence. Therefore adsorption-x-ray fluorescence analysis with application of the silica with covalently bound 8-hydroxyquinoline can be used only for the qualitative detection of these elements.

Micro amounts of lead, cadmium and mercury adsorbed upon silica with covalently bound PAR from neutral water solutions (pH = 6.8-7) can be quantitatively detected by x-ray fluorescence method using intensity of the characteristic La-line of lead, Á^-line of cadmium and La-line of mercury. Influence of the other metals presence on an intensity of characteristic x-ray lines of the detecting element was studied at simultaneous adsorption-x-ray fluorescence analysis of samples after preconcentration of metal ions by the silicas with covalently bound 8-hydroxyquinoline and PAR. A decrease of intensity of the La Pb-line was shown to take place at the presence of micro amounts of cadmium and zinc in the adsorbent phase (Figure 2). An increase of intensities of the .^-line of cadmium and La-line of zinc was detected in the presence of lead, mercury and other more heavy metals on the silica surface (Figure 3). Effect of intensity decrease for La Pb-line in the presence of micro amounts of cadmium and zinc in the adsorbent phase can be explained by higher absorption coefficients of La-line of lead in the presence of zinc and cadmium in comparison with adsorbent containing only lead (^ of Pb La-line is 111 cm2/g ). Introducing zinc and cadmium into samples results in decrease of the lead signal.16'17

Figure 2. Dependence of intensity of characteristic x-ray fluorescence Pb La-line on surface lead contents without (1) and in the presence of cadmium (3), zinc (4), cadmium and zinc simultaneously (2) in the adsorbent phase.

Pb contents, |ag

Figure 2. Dependence of intensity of characteristic x-ray fluorescence Pb La-line on surface lead contents without (1) and in the presence of cadmium (3), zinc (4), cadmium and zinc simultaneously (2) in the adsorbent phase.

Cd contents, |g

Figure 3. Comparison of calibration curves for adsorption-x-ray fluorescence determination Cd after solid-phase extraction by silica chemically modified with PAR: 1 - only Cd, 2 - Cd with the same amount Pb and Hg.

Cd contents, |g

Figure 3. Comparison of calibration curves for adsorption-x-ray fluorescence determination Cd after solid-phase extraction by silica chemically modified with PAR: 1 - only Cd, 2 - Cd with the same amount Pb and Hg.

Elaborated adsorption-x-ray fluorescence method was applied for determination of micro amounts of Pb(II), Cd(II) and Hg(II) in the Kyiv's rivers Dnipro and Lybid during the winter period after metals preconcen-tration upon the silica with chemically bound PAR. The column adsorption method was used for the preconcentration studies. With this aim 1 L of water from the river was flowed through a column with the adsorbent and intensities of the characteristic Pb, Cd and Hg lines were measured in the adsorbent. Results of these measurements were confronted with the calibration curves of the appropriate metals mixtures. As it follows from the data of Table 5, results of adsorption-x-ray fluorescence determination of micro amounts of Pb(II), Cd(II) and Hg(II) after their preconcentration upon silica with chemically bound PAR correlate well with values of concentration of these metals, obtained by flame atomic absorption analysis after an evaporation of the solution volume up to 20 times smaller.

TABLE 5. Pb(II), Cd(II) and Hg(II) contents in water from river Lybid (samples I and II) and river Dnipro detected by atomic absorption analysis and x-ray fluorescence method (after extraction by silica modified with PAR).

River Dnipro River Lybid River Lybid sample I sample II

TABLE 5. Pb(II), Cd(II) and Hg(II) contents in water from river Lybid (samples I and II) and river Dnipro detected by atomic absorption analysis and x-ray fluorescence method (after extraction by silica modified with PAR).

River Dnipro River Lybid River Lybid sample I sample II

Atomic

Adsorption-

Atomic Adsorption-x-

Atomic

Adsorption-

absorption

x-ray

absorption ray

absorption

x-ray

analysis

fluorescence

analysis fluorescence

analysis

fluorescence

analysis

analysis Pb(II) concentration (^g/L)

analysis

100

125 ± 5

<5 20 ± 2 Cd(II) concentration, ^g/L

5

30 ± 2

<10

8 ± 2

<10 <2 Hg(II) concentration (^g/L)

2

<2

-

5 ± 2

- 4 ± 2

-

<2

It is necessary to take into consideration that the adsorption-x-ray fluorescence method is faster, more accurate and simpler than usual atomic absorption analysis. Moreover, apparently quantities of lead in the river Lybid (samples I and II) obtained by flame atomic absorption method were incorrect. Really, the amounts of lead, determined by adsorption-x-ray fluorescence method, manifest a gradual increase of the metal concentration from the sample I to the sample II. It seems more probable, because water probe of the sample I was taken up-stream and probe of the sample II was drawn down-stream from urban area.

Hereby, the developed adsorption-x-ray fluorescence method of determination of micro amounts of Pb(II), Cd(II) and Hg(II) after their precon-centration upon silica with chemically bound PAR can be effectively applied for analysis of natural and artificial multicomponent water solutions.

Adsorption characteristics of silicas with immobilizedpolyionene towards metal-containing anions. Adsorption of 1,4-MePh polyionene, synthesized in the laboratory of Prof. M. Burmistr18 and having the following structure (n < 6):

CH3 CH3

N—CH2—CH2—CH2—CH2—CH2—CH2—N—CH2 CH2

CH3 CH3

was carried out on silica gel with specific surface area about 256 m2/g (Merck, fraction with a diameter of particles of 0.06-0.16 mm). In water solutions because of partial dissociation of silanol groups the silica surface has negative charge. Adsorption of polyionene on silica surface would thus be expected to result in formation of adsorption complexes due to electrostatic interactions:

The concentrations of polymer were determined by spectrophotometric analysis using bromphenol blue (A, = 600 nm). Silica with adsorbed polymer was applied for preconcentration of molybdenum-, chromium- and tungsten-containing anions from water solutions. Photometric determination of MoO42-, WO42-, Cr2O72- ions concentrations based on their ability to form color complexes. Mo-containing anions form a red complex in the presence of SCN-anions (A = 470 nm).19 In water solutions W(VI) can be identified in the form of blue heteropolyacid (A = 610 nm)20 formed after reaction with molybdate ions. Cr-containing anions were detected as a violet complex with 1,5-diphenylcarbazide (A = 540 nm).19

Adsorption properties of the silica-supported polyionene in respect to metal-containing anions, in particular dependence of separation degree (%) on the medium pH are represented in Table 6.

TABLE 6. Dependence of separation degree (%) of metal-containing anions by silica-supported polyionene on pH medium at the static adsorption mode.

Anions

Separation degree (%)

pH = 1.0

pH = 1.7

pH = 4.0 pH = 6.8 pH = 7.0

pH = 8.0

pH = 9.2

Mo(VI)

88.0

95.2

92.7 88.0 88.0

67.8

67.0

W(VI)

67.0

74.0

94.0 0 0

0

55.6

Cr(VI)

8.9

64.9

35.3 11.2 10.2

25.0

0

As it follows from these data, amounts of W(VI) and Cr(VI) adsorbed upon silica gel with immobilized polyionene were rather larger (up to 94% and 65% accordingly) in the acidic media at pH = 1.7-4, when polytungstate-and polychromate-ions are usually formed.21 The molybdate-ions are preferably extracted in the acidic medium (up to 95%), where they present as anions of the isopolyacid. At the same time, a separation degree is 67% for adsorption from the basic medium, where Mo(VI) prevails as MoO42- ions.21

Data on kinetics of adsorption of metal-containing anions on silica gel with immobilized polyionene at the optimum for each metal value of the medium pH are presented in Figure 4. It was established that all studied anions could be mostly extracted from solutions for several minutes. Thus, silica gels with anchored in the surface layer polyionene conserve kinetic properties of silica matrices and are characterized by a high rate of an establishment of the adsorption equilibrium. Such type adsorbents can be used for extraction of metal-containing anions from water solutions in the dynamic adsorption mode.

0 20 40 60 80 100

0 20 40 60 80 100

Figure 4. Kinetics of adsorption of metal-containing anions on silica-supported polyionene: 1 - Cr(VI), pH 2; 2 - W(VI), pH 4; 3 - Mo(VI), pH 2.

Comparison of isotherms of adsorption for anions of molybdenum (Figure 5), tungsten (Figure 6) and chromium (Figure 7) upon nonmodified silica gels (curves 1) and silica gels with immobilized polyionene (curves 2) testifies that modified adsorbents exhibit expressed specificity towards the studied anions.

Figure 5. Adsorption isotherms of MoO42- ions on initial silica gel (1) and silica-supported polyionene (2).

Figure 5. Adsorption isotherms of MoO42- ions on initial silica gel (1) and silica-supported polyionene (2).

Figure 6. Adsorption isotherms of WO42 ions on initial silica gel (1) and silica-supported polyionene (2).

Figure 6. Adsorption isotherms of WO42 ions on initial silica gel (1) and silica-supported polyionene (2).

Figure 7. Adsorption isotherms of Cr2O72 ions on initial silica gel (1) and silica-supported polyionene (2).

From obtained isotherms it is possible to estimate limiting adsorption values for the anions studied. In particular, modified adsorbent adsorbs (estimation on pure metal): molybdenum up to 38 mg/g, chromium up to 110 mg/g and tungsten up to 158 mg/g. All studied anions was shown are adsorbed upon the silica gel with immobilized polyionene in 60-100 times better in comparison with their adsorption on the initial nonmodified silica

0 0

Post a comment