- Email: [email protected]
Photochemical behaviour of copper(II) nitrilotriacetate in aqueous solution
J. Photochem. Phorobiol. A: Chem., 73 (1993) 213-216
213
Photochemical behaviour of copper solution Samoela
L. Andrianirinaharivelo
Laboratoim de Photochimie Mokulatie
and
nitrilotriacetate
in aqueous
M. Bolte+
et Macmmolkulaire,
URA CNRS 433, Universitk Blaise Pascal 63177 Aubiere Ceder (France)
(Received December 4, 1992; accepted March 11, 1993)
Abstract The phototransformation of copper(I1) nitrilotriacetate (CuNTA) was studied at 20 “C using monochromatic excitation at various pH values. Regardless of irradiation wavelength and pH, the observed phenomenon is a redox process giving rise to &(I), HCHO, CO, and iminodiacetic acid. The quantum yield is low and wavelength dependent with only a slight influence of oxygen. The I:1 ratio between copper(I) and HCHO formation supports a redox process between Cu(II) and the carboxylate group. The quasi-immediate re-oxidation of copper(I) into copper confers a catalytic effect of interest to the system with regard to the fate of nitrilotriacetate acid (NTA) in the environment.
1. Introduction The degradation reactions of nitrilotriacetic acid (NTA) are of importance due to its use as a substitute for phosphate in detergents and its eventual accumulation in wastewater [l]. The role of NTA in detergents involves the complexing of metals present in natural waters. NTA strongly complexes with iron and copper and stabilizes the higher valence states of the metals [2]. The complexation of NTA with a transition metal, such as iron(III) or copper( provokes the onset of absorption in the near-UV-visible region and, as a result, iron(II1) or copper(H) nitrilotriacetate can undergo phototransformation on excitation by solar light. General studies on NTA degradation photoinduced by complexation with a metal, iron(II1) [3] or copper [4] have been described. The major photoproducts are the reduced metal, iminodiacetic acid (IDA), CO, and formaldehyde. However, no systematic or quantitative studies have been reported on NTA photodegradation in terms of excitation wavelength and the stoichiometry of the photoproducts. We have previously reported the photochemical behaviour of iron(II1) nitrilotriacetate (FeNTA) [5] and in this paper the phototransformation of the copper salt (CuNTA) is studied. In order ‘Author to whom correspondence should be addressed.
101W5030/93/$6.00
to elucidate the complete mechanism of the photoreaction, solution media more acidic than those present in natural waters and deaerated solutions were investigated and a11wavelengths corresponding to the absorption spectrum of the complex were used for excitation, not just the wavelengths of the solar spectrum. The work was performed at very low concentrations (approximately 10 ppm) of copper( i.e. similar to concentrations in natural waters.
2. Experimental
details
2.1. Materials NTA (Fluka) was used without further pur& cation. CuCIZ (anhydrous) was obtained from Merck. Chromotropic acid disodium salt hydrate (Fluka) was purified by recrystallization from H,O. Solutions of CuCl&TA were degassed by bubbling with Ar for 30 min at 22 “C. The ionic strength was not controlled and the pH was adjusted with NaOH to f0.02 pH unit. 2.2. Apparatus and procedure A low-pressure Hg lamp was used for irradiation at 254 nm and a high-pressure Hg lamp with a Bausch and Lomb monochromator was used for irradiation at 296 and 313 nm. The beam was parallel and the reactor was a cylindrical quartz cell with a path length of 1 cm. The light intensity
0 1993 - Elsevier Sequoia. AU rights resewed
214
S. L. Andrianirinaharivelo. M. Bolte i Phototransformation of Cu(I1) nitrilotriacetatein water
was measured by ferrioxalate actinometry. UV-visible spectra were recorded on a Cary 118 C double-beam spectrophotometer. High performance liquid chromatography (HPLC) experiments were carried out using a Waters 40 chromatograph equipped with a diode array detector (Waters 990), with a mixture of 500 ml sodium acetate (0.03 M) and 100 ml CH,CN as eluent. The flow rate was 1 ml min-l and the column was a Lichrosphere 100 RP 18 (length, 25 cm). 2.3. Photoproduct anarysis Attempts to detect copper(I) were made by complexometry with o-phenanthroline. CO2 evolution was detected by Fourier transform IR (FTIR) spectroscopy at v=2350 cm-l. HCHO formation was measured by calorimetry with chromotropic acid as described by Lappin and Clark [6].
3. Results 3.1. Characterization of CuNTA in solution The UV-visible spectrum of a stoichiometric mixture of CuCl, and NTA presents a maximum at 238 nm with a tail up to 380 nm and is pH independent in the pH domain 3-8. To assess the stoichiometry of the copper(H) salt in aqueous solution, we measured the absorbance at 238 nm as a function of the ligand to metal ratio (pH 3.5). As shown in Fig. 1, the 1:l complex is present under our experimental conditions. 3.2. Photochemical behaviour The changes in the UV-visible spectrum of a stoichiometric C&l,-NTA mixture (so-called CuNTA) are independent of the excitation wave-
t
.
.
I
length (254, 296 or 313 nm) and pH in the range 3-8. In contrast, oxygen strongly affects the evolution of the UV-visible spectrum. In garefully degassed solutions, continuous absorption is observed, reflecting the presence of a colloidal suspension (Fig. 2). When opened to the air, the suspension quickly disappears and the optical density at 238 nm increases again almost to the starting value. Figure 3 shows the changes in optical density at 238 nm and the decrease in CuNTA concentration determined by HPLC. In the early stages of the reaction, there is good agreement between the two methods. Continuous absorption is not observed. On further reaction the CuNTA concentration continues to decrease (HPLC), whereas the optical density at 238 nm reaches a plateau at the same time as the onset of continuous absorption. Finally, both the optical density and the concentration of CuNTA decrease. In aerated solution, no traces of suspension are detected, but an isosbestic point is observed at 257 nm. The quantum yield of CuNTA transformation determined from HPLC measurements is low and wavelength dependent (Table 1). All experiments were performed at two pH values (3.5 and 7). No significant differences exist between the values. Oxygen does not strongly affect the values of the initial quantum yields. The curing by chromotropic acid of a CuNTA solution irradiated in the presence or absence of oxygen, in acidic or neutral conditions, reveals the formation of formaldehyde. The formation of HCHO is directly related to CuNTA transformation: one HCHO molecule is formed for every one molecule of CuNTA which disappears. The results are listed in Table 2. Since the photoredox process requires H+ ions [5], H + consumption was also measured. The ratio of H’ ions consumed to Cu(II) species reduced decreases as a function of the percentage of CuNTA conversion: it tends towards two at high degrees of conversion (Fig. 4). This is true regardless of the excitation wavelength and the presence or absence of oxygen. As previously noted [7], attempts to illustrate Cu(1) formation failed.
4. Discussion L/M
Fig. 1. Evolution of the optical density (OD) at 238 nm of a CuCl&TIA mixture as a function of the NTA to Cu(I1) ratio [Cu(II)] = lo-’ M).
The 1:l CuNTA complex is the thermodynamically stable species in dilute aqueous solution. On irradiation at different monochromatic excitation wavelengths (254, 296 or 313 nm), an aqueous
S. L. Andrianirinaharivelo, M. Bolte I Phototransformattin of Cu(II) ninilotn’aceiate in water
200 Fig. 2. Spectral
250 changes
300
of a CuNTA
solution
350
X (nm) (A,, =2.54 nm; [CuNTA]=lO-’
M; pH 3.5; in the absence
TABLE 1. Quantum by HPLC)
I
2 t,,,( hrI
215
1
of O*).
yield of CuNTA transformation
(determined
Lc (nm)
PH
Aerated
Deaerated
254
3s 7
6.6~10-~ 9.4x 10-3
.5.7x 10-j 11x10-”
296
3.5 7
1.6~ lo-” 1.3x 1o-3
313
3.5 7
1.2x 10-3 1.4x 10-s
3
Fig. 3. CuNTA disappearance determined by HPLC and by UV measurements (A=238 nm) as a function of irradiation time. 0, OD (238 nm); 0, HPLC.
solution of CuNTA undergoes photo-oxidoreduction giving rise to the reduced metal and the oxidized ligand. It is worth noting that the redox process is observed in acidic and neutral solutions. The quantum yields are low, wavelength dependent, unaffected by the presence or absence of oxygen and independent of the pH of the solution in the range 3-7. The 1:l stoichiometry between the formation of HCHO and the phototransformation of CuNTA
TABLE =3x10-’
2. Formation M)
of HCHO
(A,=254
nm;
[CuNTA],
CuNTA disappeared (mol I-‘)
PH
HCHO formed (mol I-‘)
0.46x lo-’ 0.47x 1o-a 0.8 x lo-’ 0.9 x lo-’ 1.0x10-’ 1.2x lo-”
3.5 3.5 7.0 7.0 7.0 3.5
0.54x 10-4 0.52x 10-4 0.7 x lo-’ 09x 1o-4 1.1 x lo-’ 1.2x 10-a
is in agreement with a mechanism involving charge transfer between Cu(II) and the carboxylate group (R, = CH,COOH, R2 =CH,COO-)
S. L. Andrianirinahorivelo, M. Bolte I Phofotransformation of Cu(iI) nittilotriacetate in water
216
according to the equation [S] In 2 ~lR=
k
s A1
0
IO
5 dis. Cu U11.10-5
M
ratio as a function of CttNTA conversion: Fig. 4. H&JcuNTA,,,,r n , 313 nm (02); 4, 296 nm (Or); 0, 254 nm (03; 0, 254 nm
@WOW@)
dA
where r,/2 is the half-life (s), @ is the quantum yield of disappearance of CuNTA on excitation at h, E is the molar extinction coefficient at A (1 mol-’ cm-l) and Z, is the intensity of sunlight at wavelength A (einsteins cm-‘). A half-life of approximately 6 h was calculated using the solar irradiation intensities reported by Franck and Klijpfer [9].
(02).
5. Conclusions R
Cub-C-
H,
c jN\ R1 R,
cur + ‘o-
hv,
ff
Despite the absorption at short wavelength and only a limited overlap of the sunlight spectrum and the CuNTA spectrum, the complexation with copper(I1) present in natural waters leads to a relatively efficient process of NTA photodegradation.
F
F: n” R,
R2
I
References
R2
+
Cd + HCHO + HN= Rl
cut
T
cd
+ &x2 + co, r?, RI
+ ..
R2
Cut1
Oxygen is an important parameter, although it does not affect either the nature of the process or the initial rate. It permits the very fast reoxidation of copper(I) present as a colloidal suspension into copper(I1). The complexation of IDA with copper(R) accounts for the quasi-reintroduction of the starting optical density. The easy re-oxidation of copper(I) to copper provides an interesting catalytic aspect to the degradation of NTA photosensitized by copper(I1). The half-life of NTA when irradiated by solar light in the presence of copper(R) can be evaluated
M. E. Bender, W. R. Matson and R. A. Jordan, Environ. Sci. Technol., 4 (1971) 520. L. G. Sillen and A. E. Mat-tell, Stability Cons~unts ofMetal-Ion Compkres, Special Publication 17, Chemical Society, London, 1964. T. Trott, R. Hanwnod and C. H. Langford, Environ. Sci Technol, 6 (1972) 367. R. Stolzberg and D. Hume, Environ. Sci. Technol., 9 (1975) 654. A. Svenson, L. Kas and H. Bjorndal, Chemosphen, 18 (1989) 1805. C. H. Langford, M. Wingham and V. S. Sastri, Environ. Sci TechnoL, 7 (1973) 820. S. I_ Andrianirinaharivelo, J. F. Philichowski and M. Bolte, Transition Met. Chem., I8 (1993) 37. G. Lappin and L. Clark, Anal. Chem., 23 (1951) 541. J. Y. Morimoto and B. A. de Graf, J. Phys. Chem., 76 (1972) 1387. European Chemical Industry Ecology and Toxicology Centre, B. 63, The Phototransformnttin of Chemicals in Water. Results of (I Ring Test, Technical Report 12, Brussels, 1984. R. Franck and W. Ktopffer, Chemosphenr, 17 (1988) 985.
213
Photochemical behaviour of copper solution Samoela
L. Andrianirinaharivelo
Laboratoim de Photochimie Mokulatie
and
nitrilotriacetate
in aqueous
M. Bolte+
et Macmmolkulaire,
URA CNRS 433, Universitk Blaise Pascal 63177 Aubiere Ceder (France)
(Received December 4, 1992; accepted March 11, 1993)
Abstract The phototransformation of copper(I1) nitrilotriacetate (CuNTA) was studied at 20 “C using monochromatic excitation at various pH values. Regardless of irradiation wavelength and pH, the observed phenomenon is a redox process giving rise to &(I), HCHO, CO, and iminodiacetic acid. The quantum yield is low and wavelength dependent with only a slight influence of oxygen. The I:1 ratio between copper(I) and HCHO formation supports a redox process between Cu(II) and the carboxylate group. The quasi-immediate re-oxidation of copper(I) into copper confers a catalytic effect of interest to the system with regard to the fate of nitrilotriacetate acid (NTA) in the environment.
1. Introduction The degradation reactions of nitrilotriacetic acid (NTA) are of importance due to its use as a substitute for phosphate in detergents and its eventual accumulation in wastewater [l]. The role of NTA in detergents involves the complexing of metals present in natural waters. NTA strongly complexes with iron and copper and stabilizes the higher valence states of the metals [2]. The complexation of NTA with a transition metal, such as iron(III) or copper( provokes the onset of absorption in the near-UV-visible region and, as a result, iron(II1) or copper(H) nitrilotriacetate can undergo phototransformation on excitation by solar light. General studies on NTA degradation photoinduced by complexation with a metal, iron(II1) [3] or copper [4] have been described. The major photoproducts are the reduced metal, iminodiacetic acid (IDA), CO, and formaldehyde. However, no systematic or quantitative studies have been reported on NTA photodegradation in terms of excitation wavelength and the stoichiometry of the photoproducts. We have previously reported the photochemical behaviour of iron(II1) nitrilotriacetate (FeNTA) [5] and in this paper the phototransformation of the copper salt (CuNTA) is studied. In order ‘Author to whom correspondence should be addressed.
101W5030/93/$6.00
to elucidate the complete mechanism of the photoreaction, solution media more acidic than those present in natural waters and deaerated solutions were investigated and a11wavelengths corresponding to the absorption spectrum of the complex were used for excitation, not just the wavelengths of the solar spectrum. The work was performed at very low concentrations (approximately 10 ppm) of copper( i.e. similar to concentrations in natural waters.
2. Experimental
details
2.1. Materials NTA (Fluka) was used without further pur& cation. CuCIZ (anhydrous) was obtained from Merck. Chromotropic acid disodium salt hydrate (Fluka) was purified by recrystallization from H,O. Solutions of CuCl&TA were degassed by bubbling with Ar for 30 min at 22 “C. The ionic strength was not controlled and the pH was adjusted with NaOH to f0.02 pH unit. 2.2. Apparatus and procedure A low-pressure Hg lamp was used for irradiation at 254 nm and a high-pressure Hg lamp with a Bausch and Lomb monochromator was used for irradiation at 296 and 313 nm. The beam was parallel and the reactor was a cylindrical quartz cell with a path length of 1 cm. The light intensity
0 1993 - Elsevier Sequoia. AU rights resewed
214
S. L. Andrianirinaharivelo. M. Bolte i Phototransformation of Cu(I1) nitrilotriacetatein water
was measured by ferrioxalate actinometry. UV-visible spectra were recorded on a Cary 118 C double-beam spectrophotometer. High performance liquid chromatography (HPLC) experiments were carried out using a Waters 40 chromatograph equipped with a diode array detector (Waters 990), with a mixture of 500 ml sodium acetate (0.03 M) and 100 ml CH,CN as eluent. The flow rate was 1 ml min-l and the column was a Lichrosphere 100 RP 18 (length, 25 cm). 2.3. Photoproduct anarysis Attempts to detect copper(I) were made by complexometry with o-phenanthroline. CO2 evolution was detected by Fourier transform IR (FTIR) spectroscopy at v=2350 cm-l. HCHO formation was measured by calorimetry with chromotropic acid as described by Lappin and Clark [6].
3. Results 3.1. Characterization of CuNTA in solution The UV-visible spectrum of a stoichiometric mixture of CuCl, and NTA presents a maximum at 238 nm with a tail up to 380 nm and is pH independent in the pH domain 3-8. To assess the stoichiometry of the copper(H) salt in aqueous solution, we measured the absorbance at 238 nm as a function of the ligand to metal ratio (pH 3.5). As shown in Fig. 1, the 1:l complex is present under our experimental conditions. 3.2. Photochemical behaviour The changes in the UV-visible spectrum of a stoichiometric C&l,-NTA mixture (so-called CuNTA) are independent of the excitation wave-
t
.
.
I
length (254, 296 or 313 nm) and pH in the range 3-8. In contrast, oxygen strongly affects the evolution of the UV-visible spectrum. In garefully degassed solutions, continuous absorption is observed, reflecting the presence of a colloidal suspension (Fig. 2). When opened to the air, the suspension quickly disappears and the optical density at 238 nm increases again almost to the starting value. Figure 3 shows the changes in optical density at 238 nm and the decrease in CuNTA concentration determined by HPLC. In the early stages of the reaction, there is good agreement between the two methods. Continuous absorption is not observed. On further reaction the CuNTA concentration continues to decrease (HPLC), whereas the optical density at 238 nm reaches a plateau at the same time as the onset of continuous absorption. Finally, both the optical density and the concentration of CuNTA decrease. In aerated solution, no traces of suspension are detected, but an isosbestic point is observed at 257 nm. The quantum yield of CuNTA transformation determined from HPLC measurements is low and wavelength dependent (Table 1). All experiments were performed at two pH values (3.5 and 7). No significant differences exist between the values. Oxygen does not strongly affect the values of the initial quantum yields. The curing by chromotropic acid of a CuNTA solution irradiated in the presence or absence of oxygen, in acidic or neutral conditions, reveals the formation of formaldehyde. The formation of HCHO is directly related to CuNTA transformation: one HCHO molecule is formed for every one molecule of CuNTA which disappears. The results are listed in Table 2. Since the photoredox process requires H+ ions [5], H + consumption was also measured. The ratio of H’ ions consumed to Cu(II) species reduced decreases as a function of the percentage of CuNTA conversion: it tends towards two at high degrees of conversion (Fig. 4). This is true regardless of the excitation wavelength and the presence or absence of oxygen. As previously noted [7], attempts to illustrate Cu(1) formation failed.
4. Discussion L/M
Fig. 1. Evolution of the optical density (OD) at 238 nm of a CuCl&TIA mixture as a function of the NTA to Cu(I1) ratio [Cu(II)] = lo-’ M).
The 1:l CuNTA complex is the thermodynamically stable species in dilute aqueous solution. On irradiation at different monochromatic excitation wavelengths (254, 296 or 313 nm), an aqueous
S. L. Andrianirinaharivelo, M. Bolte I Phototransformattin of Cu(II) ninilotn’aceiate in water
200 Fig. 2. Spectral
250 changes
300
of a CuNTA
solution
350
X (nm) (A,, =2.54 nm; [CuNTA]=lO-’
M; pH 3.5; in the absence
TABLE 1. Quantum by HPLC)
I
2 t,,,( hrI
215
1
of O*).
yield of CuNTA transformation
(determined
Lc (nm)
PH
Aerated
Deaerated
254
3s 7
6.6~10-~ 9.4x 10-3
.5.7x 10-j 11x10-”
296
3.5 7
1.6~ lo-” 1.3x 1o-3
313
3.5 7
1.2x 10-3 1.4x 10-s
3
Fig. 3. CuNTA disappearance determined by HPLC and by UV measurements (A=238 nm) as a function of irradiation time. 0, OD (238 nm); 0, HPLC.
solution of CuNTA undergoes photo-oxidoreduction giving rise to the reduced metal and the oxidized ligand. It is worth noting that the redox process is observed in acidic and neutral solutions. The quantum yields are low, wavelength dependent, unaffected by the presence or absence of oxygen and independent of the pH of the solution in the range 3-7. The 1:l stoichiometry between the formation of HCHO and the phototransformation of CuNTA
TABLE =3x10-’
2. Formation M)
of HCHO
(A,=254
nm;
[CuNTA],
CuNTA disappeared (mol I-‘)
PH
HCHO formed (mol I-‘)
0.46x lo-’ 0.47x 1o-a 0.8 x lo-’ 0.9 x lo-’ 1.0x10-’ 1.2x lo-”
3.5 3.5 7.0 7.0 7.0 3.5
0.54x 10-4 0.52x 10-4 0.7 x lo-’ 09x 1o-4 1.1 x lo-’ 1.2x 10-a
is in agreement with a mechanism involving charge transfer between Cu(II) and the carboxylate group (R, = CH,COOH, R2 =CH,COO-)
S. L. Andrianirinahorivelo, M. Bolte I Phofotransformation of Cu(iI) nittilotriacetate in water
216
according to the equation [S] In 2 ~lR=
k
s A1
0
IO
5 dis. Cu U11.10-5
M
ratio as a function of CttNTA conversion: Fig. 4. H&JcuNTA,,,,r n , 313 nm (02); 4, 296 nm (Or); 0, 254 nm (03; 0, 254 nm
@WOW@)
dA
where r,/2 is the half-life (s), @ is the quantum yield of disappearance of CuNTA on excitation at h, E is the molar extinction coefficient at A (1 mol-’ cm-l) and Z, is the intensity of sunlight at wavelength A (einsteins cm-‘). A half-life of approximately 6 h was calculated using the solar irradiation intensities reported by Franck and Klijpfer [9].
(02).
5. Conclusions R
Cub-C-
H,
c jN\ R1 R,
cur + ‘o-
hv,
ff
Despite the absorption at short wavelength and only a limited overlap of the sunlight spectrum and the CuNTA spectrum, the complexation with copper(I1) present in natural waters leads to a relatively efficient process of NTA photodegradation.
F
F: n” R,
R2
I
References
R2
+
Cd + HCHO + HN= Rl
cut
T
cd
+ &x2 + co, r?, RI
+ ..
R2
Cut1
Oxygen is an important parameter, although it does not affect either the nature of the process or the initial rate. It permits the very fast reoxidation of copper(I) present as a colloidal suspension into copper(I1). The complexation of IDA with copper(R) accounts for the quasi-reintroduction of the starting optical density. The easy re-oxidation of copper(I) to copper provides an interesting catalytic aspect to the degradation of NTA photosensitized by copper(I1). The half-life of NTA when irradiated by solar light in the presence of copper(R) can be evaluated
M. E. Bender, W. R. Matson and R. A. Jordan, Environ. Sci. Technol., 4 (1971) 520. L. G. Sillen and A. E. Mat-tell, Stability Cons~unts ofMetal-Ion Compkres, Special Publication 17, Chemical Society, London, 1964. T. Trott, R. Hanwnod and C. H. Langford, Environ. Sci Technol, 6 (1972) 367. R. Stolzberg and D. Hume, Environ. Sci. Technol., 9 (1975) 654. A. Svenson, L. Kas and H. Bjorndal, Chemosphen, 18 (1989) 1805. C. H. Langford, M. Wingham and V. S. Sastri, Environ. Sci TechnoL, 7 (1973) 820. S. I_ Andrianirinaharivelo, J. F. Philichowski and M. Bolte, Transition Met. Chem., I8 (1993) 37. G. Lappin and L. Clark, Anal. Chem., 23 (1951) 541. J. Y. Morimoto and B. A. de Graf, J. Phys. Chem., 76 (1972) 1387. European Chemical Industry Ecology and Toxicology Centre, B. 63, The Phototransformnttin of Chemicals in Water. Results of (I Ring Test, Technical Report 12, Brussels, 1984. R. Franck and W. Ktopffer, Chemosphenr, 17 (1988) 985.