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Interactions of atrazine with humic substances of different origins and their hydrolysed products
The Science o f the Total Environment, 117/118 (1992) 403-412 Elsevier Science Publishers B.V., Amsterdam
403
Interactions of atrazine with humic substances of different origins and their hydrolysed products A. Piccolo, G. C e l a n o a n d C. D e S i m o n e Istituto Sperimentale per lo Studio e la Difesa del Suolo, Piazza M. D'Azeglio 30. 50121 Florence, Italy
ABSTRACT Three humic acids (HA) extracted, respectively, from a volcanic soil, a North Dakota Leonardite, and an oxidised coal and their hydrolysed products obtained after acidic hydrolysis, were studied for their interaction with atrazine. Adsorption kinetics showed that HA from oxidised coal had the highest capacity of adsorbing atrazine (84%) followed in order by HAs from Leonardite (45.3%) and volcanic soil (31.7%). A reverse order is shown by the hydrolysed products, HA from volcanic soil being the highest adsorber (69.2%) followed by Leonardite HA (45.6%) and by oxidised coal HA (27%). Desorption kinetics, mass balance extractions and adsorption isotherms confirmed these results. Characterization by ~3C NMR indicated that humic acid aromaticity decreased and aliphaticity increased in the order: oxidised coal, Leonardite, and volcanic soil. Chemical characterization showed that total, carboxylic, and phenolic acidities increased after hydrolysis for all HAs and failed to explain the observed differences in atrazine adsorption. Molecular weight distribution by gel permeation chromatography showed that the structural modifications induced by hydrolysis are able to explain the different adsorbing capacities of HA samples. Molecular dimension and concentration of chromophore groups seemed to govern atrazine adsorption. These results indicate that (1) atrazine is mainly adsorbed through a charge-transfer mechanism between electronpoor groups of HAs and electron-rich atoms in atrazine and (2) more atrazine is adsorbed the higher the aromaticity, the polycondensation, and the molecular size of HAs. Key words: humic substances; pesticides; soil; atrazine
INTRODUCTION It h a s b e e n well d o c u m e n t e d t h a t o r g a n i c m a t t e r p l a y s a m a j o r role in the a d s o r p t i o n o f herbicides in soils a n d t h a t o r g a n i c m a t t e r c o n t e n t is usually the soil f a c t o r m o s t directly related to h e r b i c i d a l d y n a m i c s [1]. D e s p i t e this widely a c c e p t e d principle, o n l y few studies h a v e a t t e m p t e d to c o r r e l a t e the c h e m i c a l a n d p h y s i c o - c h e m i c a l c h a r a c t e r i s t i c s o f h u m i c s u b s t a n c e s , the b u l k o f soil o r g a n i c m a t t e r , with h e r b i c i d a l b e h a v i o u r in soils [2]. M o s t o f the literature is c o n c e r n e d w i t h a d s o r p t i o n o f herbicides o n w h o l e soils a n d fails
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A. PICCOLO ET AL.
to appreciate the variability introduced by the different nature of humic substances. The objective of this work was to study the absorption and desorption of atrazine on three different purified humic substances before and after acidic hydrolysis. MATERIALS AND METHODS
Humic substances Humic substances were extracted from: (1) a volcanic soil (Typic Dystrandept); (2) a North Dakota Leonardite (Mammoth, Chem. Co.); (3) an oxidised coal (Eniricerche, SPA). Extraction was by NaOH solution under N2 as outlined by Stevenson [1]. The resulting humic acids were purified by a HCIHF treatment [3], dialysed against water and freeze-dried. Part of these samples underwent acidic hydrolysis with 6 N HC1 overnight and were again dialysed and freeze-dried. Total, carboxylic, and phenolic acidities were determined according to the procedures of Schnitzer [4]. Molecular weight distribution was achieved by eluting at constant flow a 20-mg sample with 1 M 2-amino-2-hydroxy-methyl-1,3-propandiol (TRIS) hydrochloride buffer at pH = 9 on a Sephadex G150 column (70 x 1.6 cm). The absorbance of eluates (470 nm) was automatically recorded with a continuous flow spectrophotometer ISCO UA-5. 13C-NMR spectra of the purified humic acids were obtained in a quantitative mode using a Varian XL 300 spectrometer with inverse gated decoupling, 45 ° pulse, acquisition time of 0.1 s, and a relaxation delay (decoupler off) of 1.9 s [5].
Atrazine interaction with humic substances Adsorption and desorption kinetics A 20-ml solution of 25 p p m of pure atrazine (99%) in 0.01 M CaCI2 was shaken with 100 mg of humic sample at 25°C for four different times of contact (1, 2, 4 and 16 h), centrifuged and the amount of atrazine left in solution determined. Most of the supernatant was removed, the volume measured and 20 ml of 0.01 M CaC12 was added to the humic residue. The suspension was shaken (1, 2, 4 and 16 h), centrifuged and the amount of the atrazine in solution determined (referred to as first desorption). This procedure was repeated once (referred to as second desorption). Mass extraction and adsorption isotherms A 100-mg sample was shaken for 16 h with 20 ml o f a 25-ppm solution of atrazine in 0.01 M CaC12, centrifuged and the supernatant measured and
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INTERACTIONS OF ATRAZINE WITH HUMIC SUBSTANCES
discarded. To the residue 20 ml of a mixture of CH3CN-H20 (9:1) was added and shaken for 16 h. The suspension was then centrifuged and the atrazine in the supernatant determined (first desorption). The procedure was repeated again (second desorption). Adsorption isotherms were constructed from the results of experiments where 100 mg of humic sample was in contact with 20 ml of atrazine solution in CaC12 of increasing concentrations (1, 5, 10 and 25 ppm) at the different contact times (1, 2, 4 and 16 h). The logarithm of the amount of atrazine left in solution and that adsorbed per unit weight of humic sample were fitted into a linear regression and the adsorption constant (Kf) and the slope of the Freundlich isotherms (l/n) were obtained.
Atrazine determination Atrazine was determined by HPLC using a Spheri5 Brownlee 220 × 4.6 mm column, with Propachlor as internal standard, isocratic elution of 60% CH3CN in H20 at 2 ml/min, and a UV (220 mm) detector. RESULTS AND DISCUSSION Kinetics of atrazine adsorption (Table 1) show that the percent adsorption on soil (HA1) and Leonardite (HA2) humic materials did not differ within 2 h of contact but did increase more rapidly for HA2 than for HA1 approaching 16 h of contact. Much higher adsorption is shown by the humic extract of oxidised coal (HA3) which retains almost 49% of atrazine after only 1 h of contact and reaches 84% adsorption after 16 h of contact time.
TABLE 1 Percent adsorption of atrazine per unit weight of humus Contact (h)
HA1 HAI-H HA2 HA2-H HA3 HA3-H
1
2
4
16
24.98 61.22 27.31 26.72 48.68 5.23
26.78 69.68 24.25 28.21 62.20 11.93
28.33 75.40 35.00 30.74 68.52 17.20
31.76 69.17 45.26 45.64 84.06 26.97
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A. PICCOLOET AL.
TABLE 2 Atrazine percent desorption at four contact times a n d mass extraction (ME) at 16 h per unit weight of h u m u s C o n t a c t (h)
HA 1 H A 1-H HA2 HA2-H HA3 HA3-H
1
2
4
16
ME
71.09 44.72 74.97 44.72 21.04 42.22
71.90 25.83 91.75 27.19 22.60 29.32
73.81 21.55 81.81 28.45 25.14 38.96
65.23 34.07 64.25 38.38 14.23 37.55
80.16 77.17 87.27 85.19 69.91 95.62
Acidic hydrolysis brings about a rather drastic change of adsorption on HAl and HA3, whereas it does not change significantly the behaviour of HA2. Hydrolysed HA1 becomes the strongest adsorber with almost three times more atrazine than the original sample and an adsorption maximum at 4 h of contact time. Conversely, the hydrolysed extract from oxidised coal (HA3-H) lost most of its retaining capacity reaching only about 5% adsorption after 1 h of contact and a maximum of no more than 27% after 16 h. Kinetics of atrazine desorption seemed to follow the same pattern as shown by the adsorption kinetics (Table 2). Percent desorption was very high for HA1 and HA2, showing that not only was adsorption limited on these materials but also that the adsorbed atrazine was not strongly bound. Desorption decreased significantly for HA1-H and HA2-H. Surprisingly, though HA1 had shown a percent adsorption similar to both HA2 and HA2-H, desorption was significantly less for HA1-H suggesting that, in this material, adsorption sites of higher energy are available after hydrolysis. Desorption for HA3 reflects the adsorption behaviour since less atrazine is desorbed from the high adsorbing original extract whereas desorption is higher (though comparable to HA1-H and HA2-2) from the hydrolysed product. Mass extraction also confirms the differences in atrazine adsorption capacity of the HAs. Extractability from HA1 and HA2 did not change significantly between the extract and its hydrolysed product, whereas for HA3, mass extraction was about 25% more efficient than for HA3-H, substantiating the strength of atrazine adsorption on this material. Adsorption isotherms constructed by the Freundlich equation allow the
INTERACT[ONSOF ATRAZ1NEWITHHUMICSUBSTANCES
407
TABLE 3 Values for Kf and l/n from adsorption isotherms calculated from the logarithmic form of Freundlich equation at different contact times Contact (h) 1
HAl HA1-H HA2 HA2-H HA3 HA3-H
2
16
4
K!
1/n
Ky
1/n
K!
1/n
K/
1/n
2.16 2.87 2.15 2.69 2.51 2.01
0.81 0.70 0.94 0.71 0.81 0.72
2.16 2.96 2.21 2.66 2.69 2.81
0.83 0.66 0.90 0.59 0.73 0.63
2.18 2.93 2.29 2.54 2.79 2.89
0.89 0.70 0.92 0.73 0.76 0.63
2.39 3.13 2.63 2.59 3.02 3.02
0.89 0.66 0.85 0.81 0.69 0.57
c a l c u l a t i o n o f the a d s o r p t i o n c o n s t a n t , Kj~ a useful index o f the degree o f the a d s o r p t i o n , a n d l/n, w h i c h is related to the n u m b e r o f a d s o r p t i o n sites o f different surface e n e r g y ( T a b l e 3). Kf i n c r e a s e d with c o n t a c t time for all original h u m i c extract, a n d t h o s e o f H A 3 h a d the highest values. C o n v e r s e l y , the h y d r o l y s e d p r o d u c t s s h o w e d a d e c r e a s e o f Kf for H A 3 - H at 1 h o f c o n tact time a n d an e n h a n c e m e n t for H A 1 - H a n d H A 2 - H f o r all c o n t a c t times. Values f o r l/n, w h i c h were c o m p a r a b l e f o r H A 1 a n d H A 2 , d e c r e a s e d s o m e w h a t for H A 3 . A sensible r e d u c t i o n o f 1/n was o b s e r v e d in all h y d r o l y s ed p r o d u c t s , t h o u g h to a l a r g e r extent for H A l a n d H A 2 . F u r t h e r increases in KU were c o r r e l a t e d with decreases in l/n. A regression line c a l c u l a t e d for Ks a n d 1/n values p r o d u c e d a c o r r e l a t i o n coefficient (r) o f 0.878. This
TABLE 4 Percent distribution of carbon 13 groups in humic materials as measured by NMR (ppm)
HA1 HA2 HA3
0 - 1 l0
110-160
Aliphatic
Aromatic
160-190 Carboxylic
53.6 29.8 18.0
37.0 55.7 72.0
8.4 14.5 10.0
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A. PICCOLO
ET AL.
TABLE 5 Content (meuiv./g) of acidic functional groups of humic materials
HAl HAl-HI HA2 HA2-H HA3 HA3-H
Total
Carboxylic
Phenolic
10.73 13.35 9.30 10.09 9.02 11.63
4.68 4.11 4.39 3.72 3.58 3.39
6.05 9.24 4.92 6.37 5.44 8.24
generally suggests that a high degree of adsorption (Kf) corresponds to a flattening of the slope of the Freundlich isotherms, which indicates an increase of bonding surface energies. 13C-NMR spectra were obtained for the three humic extracts (Table 4). The aliphatic carbon region (0-110 ppm) was the largest in HA1 followed by HA2 and HA3, in that order, whereas the oxidised coal extract revealed the largest content of aromatic carbon (110-160 ppm). Carboxylic carbon was of the same order of magnitude in all samples though slightly higher in HA2. Chemical analysis showed (Table 5) that for all samples acidic hydrolysis produced a higher content of total and phenolic acidity, whereas carboxylic acidity appeared reduced. Phenols may have originated from the hydrolysis
1.0 ¢::
/ ~ 0. 7
Z
/
0.5
HA 1 //
/
",
~-=~°< '
/
i2.o
,,
V
40
80
\ \\
,
',
120
ELUTION V O L U M E (ml)
Fig. 1. Molecular weight distribution by gel permeation chromatography of humic acid extracted from soil (solid line) and its hydrolysed product (dashed line).
409
I N T E R A C T I O N S OF A T R A Z I N E W I T H H U M I C S U B S T A N C E S
1.0 HA 2 t-
0.7
O
I/
w 0.5
/// ~///
U Z < II1 r'r
~k \, 4
~ 0.2 II] < Vo 50
90
130
160
ELUTION VOLUME (ml)
Fig. 2. Molecular weight distribution by gel permeation chromatography of humic acid extracted from Leonardite (solid line) and its hydrolysed product (dashed line).
of ester bonds whereas carboxyl groups may have been lost through decarboxylation. Total acidity is related to one of the adsorption mechanisms for atrazine and humic substances because the herbicide, being a weak base, can be protonated by the acidic functional groups of humic materials and thus adsorbed [6]. The variations found for the acidic groups (Table 5) before and after hydrolysis are generally similar for all three humic materials and do not explain the differences observed in adsorption and desorption of atrazine among the humic samples. Conversely, variations between original and hydrolysed materials are observed in MW distribution (Figs 1-3). HA1 from soil showed, after hydrolysis, a large increase in absorbance for both eluted peaks suggesting
2.0
rop. ,~
HA 3
1.2
w
z,< 0.8 n-" O
0.4
II1
<
/1
L
V o 40
I
i
80
120
160
ELUTION VOLUME (ml)
Fig. 3. Molecular weight distribution by gel permeation chromatography of humic acid extracted from oxidised coal (solid line) and its hydrolysed product (dashed line).
410
A. PICCOLO ET AL.
an enhanced concentration of light absorbing groups (chromophores) in the macromolecular structure. Furthermore, a distinct shift of both peaks towards lower eluting volumes was observed, thereby indicating that acidic hydrolysis had condensed this HA and increased its molecular weight. Less variation was observed for HA2, for which absorbance increased only for the high MW peak whereas the diffused peak was just slightly shifted towards higher MW. The chromatograms of HA3 show that the material is mainly composed of a high MW fraction eluting at the V0 and hydrolysis only decreased the molecular size, shifting the eluting peak towards lower elution volumes, without changing peak absorbance. These results may partly explain the different behaviour of the HAs in adsorbing and desorbing atrazine. Concentration of chromophores and molecular dimension seemed to control interactions with atrazine. HA 1 and HA2, which both showed a small peak for the high MW fraction with an absorbance never exceeding 0.36, gave comparable atrazine adsorption (Table 1) up to 2 h of contact time. HA2, which revealed a diffused peak with absorbance about twice as much (0.7) as HA1 (0.4) was able to adsorb more than HA1 at higher contact times. The highest atrazine adsorber, HA3, was composed mainly of a high MW fraction whose peak absorbance reached 1.18. Its atrazine adsorbing capacity increased steadily with contact times up to 84%. Acidic hydrolysis changed drastically the chromatogram of HA 1 by increasing the first peak absorbance (0.92) to almost the HA3 level and by enhancing the percent of high MW in the material. HA2 showed also an increase of the first peak absorbance, though to a much lower extent (0.46), whereas the diffused peak was just slightly shifted to higher MW. The change for HA3 was limited to a distinct shift of the first peak towards lower MW. The strong absorbance and MW increase of HAl corresponded to the observed large enhancement of atrazine adsorption (Table 1). For HA2, the relatively small changes introduced by hydrolysis in MW distribution did not influence adsorption significantly. Conversely, in HA3 the reduction in molecular dimension, probably caused by the depolymerization following the hydrolysis of ester bonds, must be related to the drastic decrease in atrazine adsorption. Desorption kinetics can be explained by a similar logic. The low molecular dimension and chromophore-poor HA1 and HA2 gave the largest desorption (Table 2), whereas HA3, the highest adsorber, showed the lowest desorption, indicating the presence of high energy adsorbing sites. Furthermore, desorption percent decreased substantially for the hydrolysed HA1, reflecting the acquired adsorbing capacity of this material. Desorption decreased also for HA2 which, contrary to HA1, did not show a significant increase of atrazine adsorption after hydrolysis.
INTERACTIONS OF ATRAZ1NE WITH HUMIC SUBSTANCES
41 ]
CONCLUSIONS
13C-NMR spectra of humic extracts showed that the degree of aromaticity increases in the order: HA1 < HA2 < HA3. Previous results [5] have shown that acid-treated humic substances loose preferentially aliphatic carbon components while they generally gain in aromaticity. The absorbance increase, after hydrolysis, of the gel permeation peak of HA1, a material originally rich in aliphatic components, may well be interpreted as a loss of aliphatic carbon. The concomitant expected gain of aromaticity was also accompanied by an increase in molecular dimension. A similar behaviour was shown by HA2, though to a lesser extent as one might have expected considering the lower content of aliphatic components. Conversely, for HA3, which was mainly aromatic in nature, the GPC did not show variation in peak absorbance with hydrolysis but a decrease in molecular dimension. These results support a charge-transfer mechanism for atrazine adsorption on humic substances. This mechanism seems more effective at greater degrees of molecular complexity of humic materials. A charge-transfer interaction may arise between electron-poor groups in HAs such as quinones and electron-rich groups in the atrazine molecule such as the various nitrogen atoms [61. Ion exchange and hydrogen bonding mechanisms, though they may play a role, do not appear to dominate the adsorbing capacity of humic substances. Despite the increase for all samples in total, carboxyl and phenolic acidities observed after hydrolysis, which should have driven towards higher atrazine adsorption for all samples had protonation been the predominant mechanism, only HA1 showed an increase in adsorption. Our results are rather more appropriately explained by a chargetransfer mechanism whose efficiency is strengthened the higher the concentration of donor-acceptor groups and the larger the molecular complexity of humic materials. Senesi and Testini [7] based on ESR results had already expressed doubts that ionic exchange is the leading mechanism of interaction between humic substances and s-triazines. Muller-Wegener [8] confirmed the charge-transfer mechanism directly between quinoid compounds and organic nitrogen heterocycles and indirectly between the latter and humic acids. Our results show that the electron-donor processes that generate charge transfer bonds are then favoured in humic extracts of high molecular dimensions. In such complex humic aggregates which have a high degree of aromaticity, large conjugated systems are formed in which electrons are shared and easily delocalized. In these aggregates the bonding surface energy of humic molecules is enhanced and the adsorption capacity towards atrazine is also strongly increased.
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A PICCOLO El" AL.
REFERENCES 1 F.J. Stevenson, Humus Chemistry: Genesis, Composition, Reactions, Wiley, New York, 1982. 2 S.U. Khan, Pesticides in the Soil Environment, Elsevier, Amsterdam, 1980. 3 M. Schnitzer, in M. Schnitzer and S.U. Khan (Eds), Humic Substances: Chemistry and Reactions in Soil Organic Matter, Elsevier, Amsterdam, 1978 pp. 1-58. 4 M. Schnitzer, in Page et al. (Eds), Organic Matter Characterization, Methods of Soil Analysis, Part. 2 (2nd edn.), American Society of Agronomy, Madison, WI, 1982, pp. 581-594. 5 A. Piccolo, L. Campanella and B.M. Petronio, Carbon-13 NMR spectra of soil humic substances extracted by different mechanisms. Soil Sci. Soc. Am. J., 54 (1990) 750-755. 6 F.J. Stevenson, Organic matter reactions involving herbicides analysis in soil. J. Environ. Qual., 1 (1972) 333-343. 7 N. Senesi and C. Testini, Physical chemical investigations of interaction mechanism between s-triazines herbicides and soil humic acids. Geoderma, 28 (1982) 129-146. 8 U. Muller-Wegener, Electron donor acceptor complexes between organic nitrogen heterocycles and humic acid. Sci. Total Environ., 62 (1987) 297-304.
403
Interactions of atrazine with humic substances of different origins and their hydrolysed products A. Piccolo, G. C e l a n o a n d C. D e S i m o n e Istituto Sperimentale per lo Studio e la Difesa del Suolo, Piazza M. D'Azeglio 30. 50121 Florence, Italy
ABSTRACT Three humic acids (HA) extracted, respectively, from a volcanic soil, a North Dakota Leonardite, and an oxidised coal and their hydrolysed products obtained after acidic hydrolysis, were studied for their interaction with atrazine. Adsorption kinetics showed that HA from oxidised coal had the highest capacity of adsorbing atrazine (84%) followed in order by HAs from Leonardite (45.3%) and volcanic soil (31.7%). A reverse order is shown by the hydrolysed products, HA from volcanic soil being the highest adsorber (69.2%) followed by Leonardite HA (45.6%) and by oxidised coal HA (27%). Desorption kinetics, mass balance extractions and adsorption isotherms confirmed these results. Characterization by ~3C NMR indicated that humic acid aromaticity decreased and aliphaticity increased in the order: oxidised coal, Leonardite, and volcanic soil. Chemical characterization showed that total, carboxylic, and phenolic acidities increased after hydrolysis for all HAs and failed to explain the observed differences in atrazine adsorption. Molecular weight distribution by gel permeation chromatography showed that the structural modifications induced by hydrolysis are able to explain the different adsorbing capacities of HA samples. Molecular dimension and concentration of chromophore groups seemed to govern atrazine adsorption. These results indicate that (1) atrazine is mainly adsorbed through a charge-transfer mechanism between electronpoor groups of HAs and electron-rich atoms in atrazine and (2) more atrazine is adsorbed the higher the aromaticity, the polycondensation, and the molecular size of HAs. Key words: humic substances; pesticides; soil; atrazine
INTRODUCTION It h a s b e e n well d o c u m e n t e d t h a t o r g a n i c m a t t e r p l a y s a m a j o r role in the a d s o r p t i o n o f herbicides in soils a n d t h a t o r g a n i c m a t t e r c o n t e n t is usually the soil f a c t o r m o s t directly related to h e r b i c i d a l d y n a m i c s [1]. D e s p i t e this widely a c c e p t e d principle, o n l y few studies h a v e a t t e m p t e d to c o r r e l a t e the c h e m i c a l a n d p h y s i c o - c h e m i c a l c h a r a c t e r i s t i c s o f h u m i c s u b s t a n c e s , the b u l k o f soil o r g a n i c m a t t e r , with h e r b i c i d a l b e h a v i o u r in soils [2]. M o s t o f the literature is c o n c e r n e d w i t h a d s o r p t i o n o f herbicides o n w h o l e soils a n d fails
404
A. PICCOLO ET AL.
to appreciate the variability introduced by the different nature of humic substances. The objective of this work was to study the absorption and desorption of atrazine on three different purified humic substances before and after acidic hydrolysis. MATERIALS AND METHODS
Humic substances Humic substances were extracted from: (1) a volcanic soil (Typic Dystrandept); (2) a North Dakota Leonardite (Mammoth, Chem. Co.); (3) an oxidised coal (Eniricerche, SPA). Extraction was by NaOH solution under N2 as outlined by Stevenson [1]. The resulting humic acids were purified by a HCIHF treatment [3], dialysed against water and freeze-dried. Part of these samples underwent acidic hydrolysis with 6 N HC1 overnight and were again dialysed and freeze-dried. Total, carboxylic, and phenolic acidities were determined according to the procedures of Schnitzer [4]. Molecular weight distribution was achieved by eluting at constant flow a 20-mg sample with 1 M 2-amino-2-hydroxy-methyl-1,3-propandiol (TRIS) hydrochloride buffer at pH = 9 on a Sephadex G150 column (70 x 1.6 cm). The absorbance of eluates (470 nm) was automatically recorded with a continuous flow spectrophotometer ISCO UA-5. 13C-NMR spectra of the purified humic acids were obtained in a quantitative mode using a Varian XL 300 spectrometer with inverse gated decoupling, 45 ° pulse, acquisition time of 0.1 s, and a relaxation delay (decoupler off) of 1.9 s [5].
Atrazine interaction with humic substances Adsorption and desorption kinetics A 20-ml solution of 25 p p m of pure atrazine (99%) in 0.01 M CaCI2 was shaken with 100 mg of humic sample at 25°C for four different times of contact (1, 2, 4 and 16 h), centrifuged and the amount of atrazine left in solution determined. Most of the supernatant was removed, the volume measured and 20 ml of 0.01 M CaC12 was added to the humic residue. The suspension was shaken (1, 2, 4 and 16 h), centrifuged and the amount of the atrazine in solution determined (referred to as first desorption). This procedure was repeated once (referred to as second desorption). Mass extraction and adsorption isotherms A 100-mg sample was shaken for 16 h with 20 ml o f a 25-ppm solution of atrazine in 0.01 M CaC12, centrifuged and the supernatant measured and
405
INTERACTIONS OF ATRAZINE WITH HUMIC SUBSTANCES
discarded. To the residue 20 ml of a mixture of CH3CN-H20 (9:1) was added and shaken for 16 h. The suspension was then centrifuged and the atrazine in the supernatant determined (first desorption). The procedure was repeated again (second desorption). Adsorption isotherms were constructed from the results of experiments where 100 mg of humic sample was in contact with 20 ml of atrazine solution in CaC12 of increasing concentrations (1, 5, 10 and 25 ppm) at the different contact times (1, 2, 4 and 16 h). The logarithm of the amount of atrazine left in solution and that adsorbed per unit weight of humic sample were fitted into a linear regression and the adsorption constant (Kf) and the slope of the Freundlich isotherms (l/n) were obtained.
Atrazine determination Atrazine was determined by HPLC using a Spheri5 Brownlee 220 × 4.6 mm column, with Propachlor as internal standard, isocratic elution of 60% CH3CN in H20 at 2 ml/min, and a UV (220 mm) detector. RESULTS AND DISCUSSION Kinetics of atrazine adsorption (Table 1) show that the percent adsorption on soil (HA1) and Leonardite (HA2) humic materials did not differ within 2 h of contact but did increase more rapidly for HA2 than for HA1 approaching 16 h of contact. Much higher adsorption is shown by the humic extract of oxidised coal (HA3) which retains almost 49% of atrazine after only 1 h of contact and reaches 84% adsorption after 16 h of contact time.
TABLE 1 Percent adsorption of atrazine per unit weight of humus Contact (h)
HA1 HAI-H HA2 HA2-H HA3 HA3-H
1
2
4
16
24.98 61.22 27.31 26.72 48.68 5.23
26.78 69.68 24.25 28.21 62.20 11.93
28.33 75.40 35.00 30.74 68.52 17.20
31.76 69.17 45.26 45.64 84.06 26.97
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A. PICCOLOET AL.
TABLE 2 Atrazine percent desorption at four contact times a n d mass extraction (ME) at 16 h per unit weight of h u m u s C o n t a c t (h)
HA 1 H A 1-H HA2 HA2-H HA3 HA3-H
1
2
4
16
ME
71.09 44.72 74.97 44.72 21.04 42.22
71.90 25.83 91.75 27.19 22.60 29.32
73.81 21.55 81.81 28.45 25.14 38.96
65.23 34.07 64.25 38.38 14.23 37.55
80.16 77.17 87.27 85.19 69.91 95.62
Acidic hydrolysis brings about a rather drastic change of adsorption on HAl and HA3, whereas it does not change significantly the behaviour of HA2. Hydrolysed HA1 becomes the strongest adsorber with almost three times more atrazine than the original sample and an adsorption maximum at 4 h of contact time. Conversely, the hydrolysed extract from oxidised coal (HA3-H) lost most of its retaining capacity reaching only about 5% adsorption after 1 h of contact and a maximum of no more than 27% after 16 h. Kinetics of atrazine desorption seemed to follow the same pattern as shown by the adsorption kinetics (Table 2). Percent desorption was very high for HA1 and HA2, showing that not only was adsorption limited on these materials but also that the adsorbed atrazine was not strongly bound. Desorption decreased significantly for HA1-H and HA2-H. Surprisingly, though HA1 had shown a percent adsorption similar to both HA2 and HA2-H, desorption was significantly less for HA1-H suggesting that, in this material, adsorption sites of higher energy are available after hydrolysis. Desorption for HA3 reflects the adsorption behaviour since less atrazine is desorbed from the high adsorbing original extract whereas desorption is higher (though comparable to HA1-H and HA2-2) from the hydrolysed product. Mass extraction also confirms the differences in atrazine adsorption capacity of the HAs. Extractability from HA1 and HA2 did not change significantly between the extract and its hydrolysed product, whereas for HA3, mass extraction was about 25% more efficient than for HA3-H, substantiating the strength of atrazine adsorption on this material. Adsorption isotherms constructed by the Freundlich equation allow the
INTERACT[ONSOF ATRAZ1NEWITHHUMICSUBSTANCES
407
TABLE 3 Values for Kf and l/n from adsorption isotherms calculated from the logarithmic form of Freundlich equation at different contact times Contact (h) 1
HAl HA1-H HA2 HA2-H HA3 HA3-H
2
16
4
K!
1/n
Ky
1/n
K!
1/n
K/
1/n
2.16 2.87 2.15 2.69 2.51 2.01
0.81 0.70 0.94 0.71 0.81 0.72
2.16 2.96 2.21 2.66 2.69 2.81
0.83 0.66 0.90 0.59 0.73 0.63
2.18 2.93 2.29 2.54 2.79 2.89
0.89 0.70 0.92 0.73 0.76 0.63
2.39 3.13 2.63 2.59 3.02 3.02
0.89 0.66 0.85 0.81 0.69 0.57
c a l c u l a t i o n o f the a d s o r p t i o n c o n s t a n t , Kj~ a useful index o f the degree o f the a d s o r p t i o n , a n d l/n, w h i c h is related to the n u m b e r o f a d s o r p t i o n sites o f different surface e n e r g y ( T a b l e 3). Kf i n c r e a s e d with c o n t a c t time for all original h u m i c extract, a n d t h o s e o f H A 3 h a d the highest values. C o n v e r s e l y , the h y d r o l y s e d p r o d u c t s s h o w e d a d e c r e a s e o f Kf for H A 3 - H at 1 h o f c o n tact time a n d an e n h a n c e m e n t for H A 1 - H a n d H A 2 - H f o r all c o n t a c t times. Values f o r l/n, w h i c h were c o m p a r a b l e f o r H A 1 a n d H A 2 , d e c r e a s e d s o m e w h a t for H A 3 . A sensible r e d u c t i o n o f 1/n was o b s e r v e d in all h y d r o l y s ed p r o d u c t s , t h o u g h to a l a r g e r extent for H A l a n d H A 2 . F u r t h e r increases in KU were c o r r e l a t e d with decreases in l/n. A regression line c a l c u l a t e d for Ks a n d 1/n values p r o d u c e d a c o r r e l a t i o n coefficient (r) o f 0.878. This
TABLE 4 Percent distribution of carbon 13 groups in humic materials as measured by NMR (ppm)
HA1 HA2 HA3
0 - 1 l0
110-160
Aliphatic
Aromatic
160-190 Carboxylic
53.6 29.8 18.0
37.0 55.7 72.0
8.4 14.5 10.0
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A. PICCOLO
ET AL.
TABLE 5 Content (meuiv./g) of acidic functional groups of humic materials
HAl HAl-HI HA2 HA2-H HA3 HA3-H
Total
Carboxylic
Phenolic
10.73 13.35 9.30 10.09 9.02 11.63
4.68 4.11 4.39 3.72 3.58 3.39
6.05 9.24 4.92 6.37 5.44 8.24
generally suggests that a high degree of adsorption (Kf) corresponds to a flattening of the slope of the Freundlich isotherms, which indicates an increase of bonding surface energies. 13C-NMR spectra were obtained for the three humic extracts (Table 4). The aliphatic carbon region (0-110 ppm) was the largest in HA1 followed by HA2 and HA3, in that order, whereas the oxidised coal extract revealed the largest content of aromatic carbon (110-160 ppm). Carboxylic carbon was of the same order of magnitude in all samples though slightly higher in HA2. Chemical analysis showed (Table 5) that for all samples acidic hydrolysis produced a higher content of total and phenolic acidity, whereas carboxylic acidity appeared reduced. Phenols may have originated from the hydrolysis
1.0 ¢::
/ ~ 0. 7
Z
/
0.5
HA 1 //
/
",
~-=~°< '
/
i2.o
,,
V
40
80
\ \\
,
',
120
ELUTION V O L U M E (ml)
Fig. 1. Molecular weight distribution by gel permeation chromatography of humic acid extracted from soil (solid line) and its hydrolysed product (dashed line).
409
I N T E R A C T I O N S OF A T R A Z I N E W I T H H U M I C S U B S T A N C E S
1.0 HA 2 t-
0.7
O
I/
w 0.5
/// ~///
U Z < II1 r'r
~k \, 4
~ 0.2 II] < Vo 50
90
130
160
ELUTION VOLUME (ml)
Fig. 2. Molecular weight distribution by gel permeation chromatography of humic acid extracted from Leonardite (solid line) and its hydrolysed product (dashed line).
of ester bonds whereas carboxyl groups may have been lost through decarboxylation. Total acidity is related to one of the adsorption mechanisms for atrazine and humic substances because the herbicide, being a weak base, can be protonated by the acidic functional groups of humic materials and thus adsorbed [6]. The variations found for the acidic groups (Table 5) before and after hydrolysis are generally similar for all three humic materials and do not explain the differences observed in adsorption and desorption of atrazine among the humic samples. Conversely, variations between original and hydrolysed materials are observed in MW distribution (Figs 1-3). HA1 from soil showed, after hydrolysis, a large increase in absorbance for both eluted peaks suggesting
2.0
rop. ,~
HA 3
1.2
w
z,< 0.8 n-" O
0.4
II1
<
/1
L
V o 40
I
i
80
120
160
ELUTION VOLUME (ml)
Fig. 3. Molecular weight distribution by gel permeation chromatography of humic acid extracted from oxidised coal (solid line) and its hydrolysed product (dashed line).
410
A. PICCOLO ET AL.
an enhanced concentration of light absorbing groups (chromophores) in the macromolecular structure. Furthermore, a distinct shift of both peaks towards lower eluting volumes was observed, thereby indicating that acidic hydrolysis had condensed this HA and increased its molecular weight. Less variation was observed for HA2, for which absorbance increased only for the high MW peak whereas the diffused peak was just slightly shifted towards higher MW. The chromatograms of HA3 show that the material is mainly composed of a high MW fraction eluting at the V0 and hydrolysis only decreased the molecular size, shifting the eluting peak towards lower elution volumes, without changing peak absorbance. These results may partly explain the different behaviour of the HAs in adsorbing and desorbing atrazine. Concentration of chromophores and molecular dimension seemed to control interactions with atrazine. HA 1 and HA2, which both showed a small peak for the high MW fraction with an absorbance never exceeding 0.36, gave comparable atrazine adsorption (Table 1) up to 2 h of contact time. HA2, which revealed a diffused peak with absorbance about twice as much (0.7) as HA1 (0.4) was able to adsorb more than HA1 at higher contact times. The highest atrazine adsorber, HA3, was composed mainly of a high MW fraction whose peak absorbance reached 1.18. Its atrazine adsorbing capacity increased steadily with contact times up to 84%. Acidic hydrolysis changed drastically the chromatogram of HA 1 by increasing the first peak absorbance (0.92) to almost the HA3 level and by enhancing the percent of high MW in the material. HA2 showed also an increase of the first peak absorbance, though to a much lower extent (0.46), whereas the diffused peak was just slightly shifted to higher MW. The change for HA3 was limited to a distinct shift of the first peak towards lower MW. The strong absorbance and MW increase of HAl corresponded to the observed large enhancement of atrazine adsorption (Table 1). For HA2, the relatively small changes introduced by hydrolysis in MW distribution did not influence adsorption significantly. Conversely, in HA3 the reduction in molecular dimension, probably caused by the depolymerization following the hydrolysis of ester bonds, must be related to the drastic decrease in atrazine adsorption. Desorption kinetics can be explained by a similar logic. The low molecular dimension and chromophore-poor HA1 and HA2 gave the largest desorption (Table 2), whereas HA3, the highest adsorber, showed the lowest desorption, indicating the presence of high energy adsorbing sites. Furthermore, desorption percent decreased substantially for the hydrolysed HA1, reflecting the acquired adsorbing capacity of this material. Desorption decreased also for HA2 which, contrary to HA1, did not show a significant increase of atrazine adsorption after hydrolysis.
INTERACTIONS OF ATRAZ1NE WITH HUMIC SUBSTANCES
41 ]
CONCLUSIONS
13C-NMR spectra of humic extracts showed that the degree of aromaticity increases in the order: HA1 < HA2 < HA3. Previous results [5] have shown that acid-treated humic substances loose preferentially aliphatic carbon components while they generally gain in aromaticity. The absorbance increase, after hydrolysis, of the gel permeation peak of HA1, a material originally rich in aliphatic components, may well be interpreted as a loss of aliphatic carbon. The concomitant expected gain of aromaticity was also accompanied by an increase in molecular dimension. A similar behaviour was shown by HA2, though to a lesser extent as one might have expected considering the lower content of aliphatic components. Conversely, for HA3, which was mainly aromatic in nature, the GPC did not show variation in peak absorbance with hydrolysis but a decrease in molecular dimension. These results support a charge-transfer mechanism for atrazine adsorption on humic substances. This mechanism seems more effective at greater degrees of molecular complexity of humic materials. A charge-transfer interaction may arise between electron-poor groups in HAs such as quinones and electron-rich groups in the atrazine molecule such as the various nitrogen atoms [61. Ion exchange and hydrogen bonding mechanisms, though they may play a role, do not appear to dominate the adsorbing capacity of humic substances. Despite the increase for all samples in total, carboxyl and phenolic acidities observed after hydrolysis, which should have driven towards higher atrazine adsorption for all samples had protonation been the predominant mechanism, only HA1 showed an increase in adsorption. Our results are rather more appropriately explained by a chargetransfer mechanism whose efficiency is strengthened the higher the concentration of donor-acceptor groups and the larger the molecular complexity of humic materials. Senesi and Testini [7] based on ESR results had already expressed doubts that ionic exchange is the leading mechanism of interaction between humic substances and s-triazines. Muller-Wegener [8] confirmed the charge-transfer mechanism directly between quinoid compounds and organic nitrogen heterocycles and indirectly between the latter and humic acids. Our results show that the electron-donor processes that generate charge transfer bonds are then favoured in humic extracts of high molecular dimensions. In such complex humic aggregates which have a high degree of aromaticity, large conjugated systems are formed in which electrons are shared and easily delocalized. In these aggregates the bonding surface energy of humic molecules is enhanced and the adsorption capacity towards atrazine is also strongly increased.
412
A PICCOLO El" AL.
REFERENCES 1 F.J. Stevenson, Humus Chemistry: Genesis, Composition, Reactions, Wiley, New York, 1982. 2 S.U. Khan, Pesticides in the Soil Environment, Elsevier, Amsterdam, 1980. 3 M. Schnitzer, in M. Schnitzer and S.U. Khan (Eds), Humic Substances: Chemistry and Reactions in Soil Organic Matter, Elsevier, Amsterdam, 1978 pp. 1-58. 4 M. Schnitzer, in Page et al. (Eds), Organic Matter Characterization, Methods of Soil Analysis, Part. 2 (2nd edn.), American Society of Agronomy, Madison, WI, 1982, pp. 581-594. 5 A. Piccolo, L. Campanella and B.M. Petronio, Carbon-13 NMR spectra of soil humic substances extracted by different mechanisms. Soil Sci. Soc. Am. J., 54 (1990) 750-755. 6 F.J. Stevenson, Organic matter reactions involving herbicides analysis in soil. J. Environ. Qual., 1 (1972) 333-343. 7 N. Senesi and C. Testini, Physical chemical investigations of interaction mechanism between s-triazines herbicides and soil humic acids. Geoderma, 28 (1982) 129-146. 8 U. Muller-Wegener, Electron donor acceptor complexes between organic nitrogen heterocycles and humic acid. Sci. Total Environ., 62 (1987) 297-304.