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Photoinductive efficiency of soil extracted humic and fulvic acids
Chemosphere 49 (2002) 259–262 www.elsevier.com/locate/chemosphere
Photoinductive efficiency of soil extracted humic and fulvic acids J.-P. Aguer a, C. Richard a,*, O. Trubetskaya b, O. Trubetskoj c, J. Leveque d, F. Andreux d a
d
Laboratoire de Photochimie Mol eculaire et Macromol eculaire, UMR CNRS-Universit e Blaise Pascal no. 6505, Ensemble Universitaire des C ezeaux, F-63177 Aubi ere Cedex, France b Branch of Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, 142290 Pushchino, Moscow, Russia c Institute of Basic Biological Problems, Russian Academy of Sciences, 142290 Pushchino, Moscow, Russia Unit e Mixte de Recherche A 111 INRA-Universit e de Bourgogne, Microbiologie des Sols-G eoSol, Centre des Sciences de la Terre, 6, Boulevard Gabriel, F-21000 Dijon, France Received 23 January 2002; received in revised form 21 May 2002; accepted 24 May 2002
Abstract Humic and fulvic acids extracted from soils of different genesis were investigated for their ability to photoinduce the transformation of fenuron (2 104 mol l1 ) at 365 nm. The ratio of the initial rate of fenuron consumption over the rate of light absorption by humic substances was found to be higher for fulvic acids (range 2:0 103 to 9:0 105 ) than for humic acids (range 1:7 104 to 3:6 105 ). Within the FAs population, this ratio decreased as the specific absorption coefficient at 365 nm increased. It seems therefore that most of 365-nm absorbing components have no photoinductive activity and even reduce that of photoinductive chromophores. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Phototransformation; Humic substances; Fenuron; Chromophores; Colored components
1. Introduction Humic substances (HS) are the most representative part of stable organic carbon in the biosphere, comprising approximately 60–70% of the total organic carbon in soils and 60–90% of dissolved organic carbon in natural waters. Despite the different origins responsible for their outer appearance and structural characteristics, they all constitute refractory products of chemical and biological degradation and condensation reactions of
* Corresponding author. Tel.: +33-473-407142; fax: +33-473407700. E-mail address: [email protected] (C. Richard).
plant and animal residues, and play a crucial role in many biogeochemical processes. During the last 20 years it has been found that HS enhance the photodegradation of pollutants in soils and aquatic environments (pesticides, herbicides and other organic pollutants) acting as sensitizors or precursors of reactive species. A lot of works have been devoted to the identification of the photoreactants produced upon excitation of HS and some of them were firmly identified (solvated electrons, hydroxyl radicals and singlet oxygen) while others (oxidizing triplet states, long-lived photoreactants) remain to be characterized (Zepp et al., 1985; Frimmel et al., 1987; Cooper et al., 1989; Hoigne, 1989; Canonica et al., 1995; Aguer et al., 1997; Vaughan and Blough, 1998; Thomas-Smith and Blough, 2001; Aguer et al., 2001).
0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 2 8 2 - 5
260
J.-P. Aguer et al. / Chemosphere 49 (2002) 259–262
Humic and fulvic acids (HAs and FAs) are a subclass of the HS, which are operationally divided in function of their solubilities at pH 6 1 (HAs insoluble in acid and FAs soluble in acid). They are the most abundant in the upper 30–60 cm of the Earth’s crust, where they interact with air and water. However, comparative studies of HAs and FAs from various soil sources differing by genesis, bioclimatic, and geographic regions in relation to their ability to promote the degradation of pesticides on light excitation (photoinductive activity) in identical and well-defined conditions are still lacking. The aim of the present work was the comparative study of several FA/HA couples from soils of different sources in relation of their ability to photoinduce the transformation of an oxidable substrate 3phenyl-1,1-dimethyl urea (fenuron). Fenuron does not absorb light at k > 290 nm and irradiations were performed at 365 nm to ensure the selective excitation of HSs.
2. Results and discussion FA/HA couples were extracted from various soils (see Table 1) using the International Humic Substances Society (IHSS) procedure or by 0:1 N Na4 P2 O7 þ 0:1 N NaOH, pH 13, as previously described (Trubetskoj et al., 1997, 1998). Soil standard samples were purchased from IHSS and Laurentian HA/FA were obtained from
Fredricks Research Products (Netherlands). Ash content of HSs was generally low (less than 3%) except in the case of Ferralsol HA where it reached 9.5% (Trubetskoj et al., 1999). The solutions of HSs (100 mg l1 ) were prepared by dissolving 2 mg of HA or FA in 20 ml of pure water (Milli-Q device, Millipore) buffered at pH 6.5 with phosphates. The solutions were stirred until the complete dissolution of the humic material and filtered on 0.45 lm Millipore cellulose filters prior to use. Filtration led only small losses of material. Specific absorption coefficients at 365 nm, a365 , were computed by dividing the absorbance of solutions at 365 nm by the cell pathlength (0.01 m) and by the dissolved amount of HS (100 mg l1 ). As shown in Table 1, they were within the range 0.12–0.62 l mg1 m1 for FAs and between 0.68 and 1.6 l mg1 m1 for HAs. Ratios ða365 ÞFA =ða365 ÞHA for FA/HA couples were within the range 0.31–0.76 except for Ranker and Chernozem samples that exhibited much smaller values (0.14 and 0.08 respectively). The stronger absorption of HAs compared to FAs and the similarity in absorptivity of HAs extracted from soils in various parts of the world are in good accordance with data reported by Zepp and Schlotzhauer (1981). The solutions containing HAs or FAs (100 mg l1 ) and fenuron (2 104 mol l1 ) were prepared by dissolving 2 mg of HS in 2 ml of fenuron aqueous solution (2 103 mol l1 ) and 18 ml of pH 6.5 buffered water. As previously described, solutions were stirred and fil-
Table 1 Origin of soils, specific absorption coefficients at 365 nm a365 (l mg1 m1 ), ratio of specific absorption coefficients at 365 nm of FA and FA HA HA (aFA =PEHA ) 365 =a365 ) and ratio of photoinductive efficiencies of FA and HA (PE Soil (origin)
HS
a365 (l mg1 m1 )
(a365 ÞFA =ða365 ÞHA
PEFA =PEHA
Chernozem (cultivated soil, Kursk region, Russia)
FA HA
0.12 1.5
0.08
16
IHSS soil standard
FA HA
0.50 1.6
0.31
14
Ranker (Mountain Ranker soil under grassland, Sierra de la Demanda, Spain)
FA HA
0.17 1.2
0.14
12
Douglas Morvan (Mountain brown acid soil under Douglas fir, Morvan, France)
FA HA
0.48 0.91
0.53
9.4
Brown soil (cultivated brown leached soil, C^ ote-Saint-Andre, France)
FA HA
0.45 0.85
0.53
6.0
Douglas Beaujolais (Mountain brown acid soil under Douglas fir Beaujolais, France)
FA HA
0.62 0.73
0.76
5.1
Ferralsol (Batumy region, Georgia, former USSR)
FA HA
0.25 0.80
0.31
4.0
Hetraie Morvan (Mountain brown acid soil under beech tree, Morvan, France)
FA HA
0.47 0.79
0.59
4.5
Laurentian (Fredricks Research Products)
FA HA
0.50 0.68
0.73
1.6
J.-P. Aguer et al. / Chemosphere 49 (2002) 259–262
tered prior to irradiation. Five ml of solutions were irradiated in a water-cooled cylindrical reactor using a device equipped with three black lamps MAZDA MAW 125 emitting 93% of light at 365 nm. The rate of light absorption by humic substances (Ia ) was computed using the relationship: Ia ðEinstein s1 Þ ¼5 105 ð1–10A10 Þ, with A the absorbance of solutions at 365 nm for a cell pathlength equal to 1 cm. This relationship was established by irradiating 5 ml of several chemical actinometer solutions exhibiting various absorbances at 365 nm. The fenuron consumption was monitored by HPLC-UV detection. Fenuron disappeared under irradiation only. Plots of ln (c0 =c) vs t, where c0 and c are the fenuron concentrations at t ¼ 0 and t respectively, were linear until a conversion extent of 30% showing that the consumption of fenuron followed a pseudo-first order kinetics in the first stages of the reaction. The initial rates of fenuron consumption (r) were calculated using the slope of the straight lines (k) and the relationship: r ¼ k c0 . Values are given in Fig. 1. In all cases, FA was more efficient as photoinductor than the corresponding HA. In order to take into account the differences of light absorption by humic substances, we divided the initial rate of fenuron consumption by the rate of light absorption by humic substances. This ratio is noted PE. It corresponds to the photoinductive efficiencies of HSs in our experimental conditions and has no absolute meaning. In particular it may depend on the substrate concentration. PE values are shown in Fig. 2. As a general rule, FAs were more efficient as photoinductors than HAs, PE values ranging from 2:0 103 to 9:0 105 for the formers and from 1:7 104 to 3:6 105 for the latters. Considering the diversity of soil origins, the differences in PE values in the HAs population appeared very small; FAs showed more contrasting results. For each FA/HA couple, we computed the ratio PEFA =PEHA . Values laid within the range
Fig. 1. Initial rates of fenuron consumption in the presence of FA/HA couples.
261
Fig. 2. Plot of PE vs the specific absorption coefficient at 365 nm; () data of FAs and () data of HAs.
1.6–16 (see Table 1). Chernozem and Ranker couples HA that showed the lowest aFA 365 =a365 ratios exhibited high FA HA PE =PE values (16 and 12, respectively). In contrast, HA Laurentian couple that showed a high aFA 365 =a365 ratio FA HA exhibited the lowest PE =PE value (1.6). It seems therefore that PE was connected to the absorbance at 365 nm. The lower absorbance, the higher PE value. Plot of PE vs the absorbance at 365 nm confirms the relationship between absorption and photoinductive efficiency in the FAs population (see Fig. 2): PE shows a tendency to decrease as the absorbance of FA increased. In contrast, in the HAs population, no relationship appears. The weak photoinductive efficiency of highly absorbing samples demonstrates that absorbing components, in their great majority, have no photoinductive activity. They may or not photogenerate reactive species able to transform fenuron; however if they do, they probably scavenge them preventing degradation of fenuron. In any case, they reduce the photoinductive activity of photoinductive chromophores through inner filter effect, i.e., by the fact that they absorb incident light in competition with them. In this hypothesis, FAs may be better photoinductors than corresponding HAs because their content in absorbing but non-photoinductive chromophores is lower. As well, FAs exhibiting the highest photoinductive properties among the FAs population may be those containing the smallest concentrations of non-photoinductive chromophores. Differences in photoinductive efficiency may have also other grounds. Indeed, HA and FA extracted from the Douglas Beaujolais soil absorbed similarly at 365 nm, but FA was found to be about fivefold more efficient as photoinductor than HA. The differences may be connected to the nature and/or the concentration of photoinductive chromophores. In this respect, the extraction procedure and the soil origin may have an
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J.-P. Aguer et al. / Chemosphere 49 (2002) 259–262
influence. Regarding to the extraction method, all HA/ FA couples were extracted using the IHSS procedure, except Chernozem and Ferralsol FAs, which were obtained after HA precipitation from HS solution at pH 1 with further dialization and lyophilisation of the supernatant. As seen in Fig. 2, PE values of Chernozem and Ferralsol FAs were smaller than expected on the basis of their absorbance at 365 nm. This low efficiency may be due to the extraction method that could have left some admixture of non-humic components in FAs. Regarding to the origin of the HS, the local climate under which they were produced is probably not a crucial factor: mountain soils from the same area had different values whereas similar values were obtained for materials extracted from very different places. On the other hand, the vegetation which contributes to humus formation seems to be a leading factor: HA and FA under Douglas fir showed similar values, even though the Morvan soil was on granit and the Beaujolais soil on volcanic tuf. Conversely, the H^etraie Morvan and Douglas Morvan HAs and FAs, which were developed on the same granit rock, but under beech tree and Douglas fir, respectively, showed very different values. In this particular case, the coniferous vegetation probably was responsible for the production of more active substances than the deciduous vegetation. Nevertheless, the possibility to generalize such results is still very restricted, due to the lack of structural information about the studied HS, as well as about HS in general.
3. Conclusion In conclusion, whatever the soil origin FAs were shown to photoinduce more efficiently than HAs the phototransformation of fenuron at 365 nm. This better photoinductive activity may be due in part to a lower content in absorbing but non-photoinductive components acting as inner filters. The content and reactivity of photoinductive chromophores is likely to be also an important parameters. Further investigation is needed in order to establish precise relationship between photoreactivity of humic and fulvic acids and their respective structural characteristics.
Acknowledgements This research has been supported with funds provided by INTAS (project no 2001-0186).
References Aguer, J.-P., Richard, C., Andreux, F., 1997. Comparison of the photoinductive properties of commercial, synthetic and soil-extracted humic substances. J. Photochem. Photobiol. A: Chemistry 103, 163–168. Aguer, J.-P., Trubetskaya, O.E., Trubetskoj, O.A., Richard, C., 2001. Photoinductive properties of soil humic acids and their fractions obtained by tandem size exclusion chromatography-polyacrylamide gel electrophoresis. Chemosphere 44, 205–209. Canonica, S., Jans, U., Stemmler, K., Hoigne, J., 1995. Transformation kinetics of phenols in water: photosensitization by dissolved organic material and aromatic ketones. Environ. Sci. Technol. 29, 1822–1831. Cooper, W.J., Zika, R.G., Petasne, R.G., Fisher, A.-M., 1989. Sunlight-induced photochemistry of humic substances in natural waters:major reactive species. Adv. Chem. Ser. 219, 349–362. Frimmel, F.H., Bauer, H., Putzien, J., Murasecco, P., Braun, A.-M., 1987. Laser flash photolysis of dissolved aquatic humic material and the sensitized production of singlet oxygen. Environ. Sci. Technol. 21, 541–545. Hoigne, J., Faust, B.C., Haag, W.R., Scully, F.E., Zepp, R., 1989. Aquatic humic substances as sources and sinks of photochemically produced transients reactants. G. Adv. Chem. Ser. 219, 363–381. Thomas-Smith, T.E., Blough, N.V., 2001. Photoproduction of solvated electron from constituents of natural waters. Environ. Sci. Technol. 35, 2721–2726. Trubetskoj, O.A., Trubetskaya, O.E., Afanas’eva, G.V., Reznikova, O.I., Saiz-Jimenez, C., 1997. Polyacrylamide gel electrophoresis of soil humic acid fractionated by sizeexclusion chromatography and ultrafiltration. J. Chromatogr. A 767, 285–292. Trubetskoj, O.A., Trubetskaya, O.E., Afanas’eva, G.V., Reznikova, O.I., Hermosin, B., Saiz-Jimenez, C., 1998. Tandem size exclusion chromatography-polyacrylamide gel electrophoresis of humic acids. Z. Pflanzenern€ahr. Bodenk. 161, 619–625. Trubetskoj, O.A., Trubetskaya, O.E., Reznikova, O.I., Afanas’eva, G.V., 1999. Weight and optical differences between soil humic acids fractions obtained by coupling SEC-PAGE. Geoderma 93, 277–287. Vaughan, P.P., Blough, N.V., 1998. Photochemical formation of hydroxyl radical from constituents of natural waters. Environ. Sci. Technol. 32, 2947–2953. Zepp, R.G., Schlotzhauer, P.F., 1981. Comparison of photochemical behavior of various humic substances in water: Spectroscopic properties of humic substances. Chemosphere 10, 479–486. Zepp, R.G., Scholtzhauer, P.F., Sink, R.M., 1985. Photosensitized transformation involving electronic energy transfer in natural waters: role of humic substances. Environ. Sci. Technol. 19, 74–81.
Photoinductive efficiency of soil extracted humic and fulvic acids J.-P. Aguer a, C. Richard a,*, O. Trubetskaya b, O. Trubetskoj c, J. Leveque d, F. Andreux d a
d
Laboratoire de Photochimie Mol eculaire et Macromol eculaire, UMR CNRS-Universit e Blaise Pascal no. 6505, Ensemble Universitaire des C ezeaux, F-63177 Aubi ere Cedex, France b Branch of Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, 142290 Pushchino, Moscow, Russia c Institute of Basic Biological Problems, Russian Academy of Sciences, 142290 Pushchino, Moscow, Russia Unit e Mixte de Recherche A 111 INRA-Universit e de Bourgogne, Microbiologie des Sols-G eoSol, Centre des Sciences de la Terre, 6, Boulevard Gabriel, F-21000 Dijon, France Received 23 January 2002; received in revised form 21 May 2002; accepted 24 May 2002
Abstract Humic and fulvic acids extracted from soils of different genesis were investigated for their ability to photoinduce the transformation of fenuron (2 104 mol l1 ) at 365 nm. The ratio of the initial rate of fenuron consumption over the rate of light absorption by humic substances was found to be higher for fulvic acids (range 2:0 103 to 9:0 105 ) than for humic acids (range 1:7 104 to 3:6 105 ). Within the FAs population, this ratio decreased as the specific absorption coefficient at 365 nm increased. It seems therefore that most of 365-nm absorbing components have no photoinductive activity and even reduce that of photoinductive chromophores. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Phototransformation; Humic substances; Fenuron; Chromophores; Colored components
1. Introduction Humic substances (HS) are the most representative part of stable organic carbon in the biosphere, comprising approximately 60–70% of the total organic carbon in soils and 60–90% of dissolved organic carbon in natural waters. Despite the different origins responsible for their outer appearance and structural characteristics, they all constitute refractory products of chemical and biological degradation and condensation reactions of
* Corresponding author. Tel.: +33-473-407142; fax: +33-473407700. E-mail address: [email protected] (C. Richard).
plant and animal residues, and play a crucial role in many biogeochemical processes. During the last 20 years it has been found that HS enhance the photodegradation of pollutants in soils and aquatic environments (pesticides, herbicides and other organic pollutants) acting as sensitizors or precursors of reactive species. A lot of works have been devoted to the identification of the photoreactants produced upon excitation of HS and some of them were firmly identified (solvated electrons, hydroxyl radicals and singlet oxygen) while others (oxidizing triplet states, long-lived photoreactants) remain to be characterized (Zepp et al., 1985; Frimmel et al., 1987; Cooper et al., 1989; Hoigne, 1989; Canonica et al., 1995; Aguer et al., 1997; Vaughan and Blough, 1998; Thomas-Smith and Blough, 2001; Aguer et al., 2001).
0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 2 8 2 - 5
260
J.-P. Aguer et al. / Chemosphere 49 (2002) 259–262
Humic and fulvic acids (HAs and FAs) are a subclass of the HS, which are operationally divided in function of their solubilities at pH 6 1 (HAs insoluble in acid and FAs soluble in acid). They are the most abundant in the upper 30–60 cm of the Earth’s crust, where they interact with air and water. However, comparative studies of HAs and FAs from various soil sources differing by genesis, bioclimatic, and geographic regions in relation to their ability to promote the degradation of pesticides on light excitation (photoinductive activity) in identical and well-defined conditions are still lacking. The aim of the present work was the comparative study of several FA/HA couples from soils of different sources in relation of their ability to photoinduce the transformation of an oxidable substrate 3phenyl-1,1-dimethyl urea (fenuron). Fenuron does not absorb light at k > 290 nm and irradiations were performed at 365 nm to ensure the selective excitation of HSs.
2. Results and discussion FA/HA couples were extracted from various soils (see Table 1) using the International Humic Substances Society (IHSS) procedure or by 0:1 N Na4 P2 O7 þ 0:1 N NaOH, pH 13, as previously described (Trubetskoj et al., 1997, 1998). Soil standard samples were purchased from IHSS and Laurentian HA/FA were obtained from
Fredricks Research Products (Netherlands). Ash content of HSs was generally low (less than 3%) except in the case of Ferralsol HA where it reached 9.5% (Trubetskoj et al., 1999). The solutions of HSs (100 mg l1 ) were prepared by dissolving 2 mg of HA or FA in 20 ml of pure water (Milli-Q device, Millipore) buffered at pH 6.5 with phosphates. The solutions were stirred until the complete dissolution of the humic material and filtered on 0.45 lm Millipore cellulose filters prior to use. Filtration led only small losses of material. Specific absorption coefficients at 365 nm, a365 , were computed by dividing the absorbance of solutions at 365 nm by the cell pathlength (0.01 m) and by the dissolved amount of HS (100 mg l1 ). As shown in Table 1, they were within the range 0.12–0.62 l mg1 m1 for FAs and between 0.68 and 1.6 l mg1 m1 for HAs. Ratios ða365 ÞFA =ða365 ÞHA for FA/HA couples were within the range 0.31–0.76 except for Ranker and Chernozem samples that exhibited much smaller values (0.14 and 0.08 respectively). The stronger absorption of HAs compared to FAs and the similarity in absorptivity of HAs extracted from soils in various parts of the world are in good accordance with data reported by Zepp and Schlotzhauer (1981). The solutions containing HAs or FAs (100 mg l1 ) and fenuron (2 104 mol l1 ) were prepared by dissolving 2 mg of HS in 2 ml of fenuron aqueous solution (2 103 mol l1 ) and 18 ml of pH 6.5 buffered water. As previously described, solutions were stirred and fil-
Table 1 Origin of soils, specific absorption coefficients at 365 nm a365 (l mg1 m1 ), ratio of specific absorption coefficients at 365 nm of FA and FA HA HA (aFA =PEHA ) 365 =a365 ) and ratio of photoinductive efficiencies of FA and HA (PE Soil (origin)
HS
a365 (l mg1 m1 )
(a365 ÞFA =ða365 ÞHA
PEFA =PEHA
Chernozem (cultivated soil, Kursk region, Russia)
FA HA
0.12 1.5
0.08
16
IHSS soil standard
FA HA
0.50 1.6
0.31
14
Ranker (Mountain Ranker soil under grassland, Sierra de la Demanda, Spain)
FA HA
0.17 1.2
0.14
12
Douglas Morvan (Mountain brown acid soil under Douglas fir, Morvan, France)
FA HA
0.48 0.91
0.53
9.4
Brown soil (cultivated brown leached soil, C^ ote-Saint-Andre, France)
FA HA
0.45 0.85
0.53
6.0
Douglas Beaujolais (Mountain brown acid soil under Douglas fir Beaujolais, France)
FA HA
0.62 0.73
0.76
5.1
Ferralsol (Batumy region, Georgia, former USSR)
FA HA
0.25 0.80
0.31
4.0
Hetraie Morvan (Mountain brown acid soil under beech tree, Morvan, France)
FA HA
0.47 0.79
0.59
4.5
Laurentian (Fredricks Research Products)
FA HA
0.50 0.68
0.73
1.6
J.-P. Aguer et al. / Chemosphere 49 (2002) 259–262
tered prior to irradiation. Five ml of solutions were irradiated in a water-cooled cylindrical reactor using a device equipped with three black lamps MAZDA MAW 125 emitting 93% of light at 365 nm. The rate of light absorption by humic substances (Ia ) was computed using the relationship: Ia ðEinstein s1 Þ ¼5 105 ð1–10A10 Þ, with A the absorbance of solutions at 365 nm for a cell pathlength equal to 1 cm. This relationship was established by irradiating 5 ml of several chemical actinometer solutions exhibiting various absorbances at 365 nm. The fenuron consumption was monitored by HPLC-UV detection. Fenuron disappeared under irradiation only. Plots of ln (c0 =c) vs t, where c0 and c are the fenuron concentrations at t ¼ 0 and t respectively, were linear until a conversion extent of 30% showing that the consumption of fenuron followed a pseudo-first order kinetics in the first stages of the reaction. The initial rates of fenuron consumption (r) were calculated using the slope of the straight lines (k) and the relationship: r ¼ k c0 . Values are given in Fig. 1. In all cases, FA was more efficient as photoinductor than the corresponding HA. In order to take into account the differences of light absorption by humic substances, we divided the initial rate of fenuron consumption by the rate of light absorption by humic substances. This ratio is noted PE. It corresponds to the photoinductive efficiencies of HSs in our experimental conditions and has no absolute meaning. In particular it may depend on the substrate concentration. PE values are shown in Fig. 2. As a general rule, FAs were more efficient as photoinductors than HAs, PE values ranging from 2:0 103 to 9:0 105 for the formers and from 1:7 104 to 3:6 105 for the latters. Considering the diversity of soil origins, the differences in PE values in the HAs population appeared very small; FAs showed more contrasting results. For each FA/HA couple, we computed the ratio PEFA =PEHA . Values laid within the range
Fig. 1. Initial rates of fenuron consumption in the presence of FA/HA couples.
261
Fig. 2. Plot of PE vs the specific absorption coefficient at 365 nm; () data of FAs and () data of HAs.
1.6–16 (see Table 1). Chernozem and Ranker couples HA that showed the lowest aFA 365 =a365 ratios exhibited high FA HA PE =PE values (16 and 12, respectively). In contrast, HA Laurentian couple that showed a high aFA 365 =a365 ratio FA HA exhibited the lowest PE =PE value (1.6). It seems therefore that PE was connected to the absorbance at 365 nm. The lower absorbance, the higher PE value. Plot of PE vs the absorbance at 365 nm confirms the relationship between absorption and photoinductive efficiency in the FAs population (see Fig. 2): PE shows a tendency to decrease as the absorbance of FA increased. In contrast, in the HAs population, no relationship appears. The weak photoinductive efficiency of highly absorbing samples demonstrates that absorbing components, in their great majority, have no photoinductive activity. They may or not photogenerate reactive species able to transform fenuron; however if they do, they probably scavenge them preventing degradation of fenuron. In any case, they reduce the photoinductive activity of photoinductive chromophores through inner filter effect, i.e., by the fact that they absorb incident light in competition with them. In this hypothesis, FAs may be better photoinductors than corresponding HAs because their content in absorbing but non-photoinductive chromophores is lower. As well, FAs exhibiting the highest photoinductive properties among the FAs population may be those containing the smallest concentrations of non-photoinductive chromophores. Differences in photoinductive efficiency may have also other grounds. Indeed, HA and FA extracted from the Douglas Beaujolais soil absorbed similarly at 365 nm, but FA was found to be about fivefold more efficient as photoinductor than HA. The differences may be connected to the nature and/or the concentration of photoinductive chromophores. In this respect, the extraction procedure and the soil origin may have an
262
J.-P. Aguer et al. / Chemosphere 49 (2002) 259–262
influence. Regarding to the extraction method, all HA/ FA couples were extracted using the IHSS procedure, except Chernozem and Ferralsol FAs, which were obtained after HA precipitation from HS solution at pH 1 with further dialization and lyophilisation of the supernatant. As seen in Fig. 2, PE values of Chernozem and Ferralsol FAs were smaller than expected on the basis of their absorbance at 365 nm. This low efficiency may be due to the extraction method that could have left some admixture of non-humic components in FAs. Regarding to the origin of the HS, the local climate under which they were produced is probably not a crucial factor: mountain soils from the same area had different values whereas similar values were obtained for materials extracted from very different places. On the other hand, the vegetation which contributes to humus formation seems to be a leading factor: HA and FA under Douglas fir showed similar values, even though the Morvan soil was on granit and the Beaujolais soil on volcanic tuf. Conversely, the H^etraie Morvan and Douglas Morvan HAs and FAs, which were developed on the same granit rock, but under beech tree and Douglas fir, respectively, showed very different values. In this particular case, the coniferous vegetation probably was responsible for the production of more active substances than the deciduous vegetation. Nevertheless, the possibility to generalize such results is still very restricted, due to the lack of structural information about the studied HS, as well as about HS in general.
3. Conclusion In conclusion, whatever the soil origin FAs were shown to photoinduce more efficiently than HAs the phototransformation of fenuron at 365 nm. This better photoinductive activity may be due in part to a lower content in absorbing but non-photoinductive components acting as inner filters. The content and reactivity of photoinductive chromophores is likely to be also an important parameters. Further investigation is needed in order to establish precise relationship between photoreactivity of humic and fulvic acids and their respective structural characteristics.
Acknowledgements This research has been supported with funds provided by INTAS (project no 2001-0186).
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