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Influence of UV radiation on organic matter transformation and subsequent alteration of leaching and transport processes of heavy metals
The Science of the Total Environment, 76 (1988) 41-48 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
41
INFLUENCE OF UV RADIATION ON ORGANIC MATTER TRANSFORMATION AND SUBSEQUENT ALTERATION OF LEACHING AND TRANSPORT PROCESSES OF HEAVY METALS
L. CAMPANELLA, E. CARDARELLI, T. FERRI, B.M. PETRONIO and A. PUPELLA Department of Chemistry, University of Rome "La Sapienza", Piazzale Aldo Moro, 5-00185 Rome (Italy)
(Received December 16th, 1987; accepted January 28th, 1988)
ABSTRACT This work deals with the effect of UV radiation on urban sludge. The effect on both organic matter and the related metal leaching processes was studied. The results show that UV radiation determines both the degradation of macrostructures and the re-polymerization of degradation products. Nevertheless, these reactions do not seem to strongly affect the leachability of metals. Experiments performed with and without temperature control showed that temperature also plays an important role in the fate of metals in the environment. INTRODUCTION T h e c o n t i n u o u s i n c r e a s e in t h e p r o d u c t i o n of sludge by u r b a n a n d i n d u s t r i a l t r e a t m e n t p l a n t s h a s led to the g r o w i n g p r o b l e m of disposal. T h e l e a c h i n g of toxic m e t a l s f r o m t h e s e sludges (deposited on soil) m u s t be t a k e n into a c c o u n t , as h e a v y m e t a l s f r o m sludge e n t e r i n g t h e e n v i r o n m e n t c a n c a u s e serious p r o b l e m s for m a n [1, 2]. A k n o w l e d g e of m e t a l s p e c i a t i o n [3-5] is v i t a l in e v a l u a t i n g t h e environm e n t a l i m p a c t of t h e s e metals, since t h e i r t o x i c i t y is s t r i c t l y r e l a t e d to t h e c h e m i c a l f o r m in w h i c h t h e y a r e p r e s e n t in the sludge. This also m e a n s t h a t t h e l e a c h i n g process of m e t a l s depends on t h e n a t u r e of t h e sludge. In p a r t i c u l a r , the f r a c t i o n of m e t a l b o u n d to n a t u r a l o r g a n i c ligands (humic a n d fulvic acids) a n d to s u b s t a n c e s of a n t h r o p o g e n i c origin, s u c h as a m i n o acids, fats, waxes, c a r b o h y d r a t e s , etc., is q u i t e h i g h in u r b a n sludge. F r o m this it follows t h a t c h a n g e s in t h e l e a c h i n g of m e t a l s m u s t be r e l a t e d to t h e m o d i f i c a t i o n of o r g a n i c m a t t e r . T h i s m o d i f i c a t i o n c a n be i n d u c e d by a g e n t s s u c h as m i c r o o r g a n i s m s or sunlight. T h e object of t h e p r e s e n t w o r k is to e v a l u a t e t h e effect of s u n l i g h t on t h e o r g a n i c m a t t e r in sludge a n d c o n s e q u e n t l y to a n a l y z e h o w s p e c i a t i o n a n d l e a c h i n g p r o c e s s e s of m e t a l s m a y be modified.
0048-9697/88/$03.50
© 1988 Elsevier Science Publishers B.V.
42 EXPERIMENTAL
The sludge employed in this work was the same as t h a t used in previous experiments [4, 5]. The preliminary treatment to which the sludge was submitted was also the same. Both the sludge composition and the characterization of the humic acids in it have been reported in a previous paper [6]. In the present case the experimental procedure was as follows: sludge samples (5g) were first irradiated by a mercury vapour lamp (125W) for different periods of time (from 30 to 180min), either under thermostated conditions or not, and later treated according to Scheme 1. This scheme has already been applied to non-irradiated sludge [6]. In addition to the experimental apparatus previously utilized, a Varian XL-300 NMR spectrometer was used to record 13C spectra. RESULTS AND DISCUSSION
In order to evaluate the effect of UV radiation on organic matter, the results of the application of Scheme 1 to non-irradiated sludge, reported in a previous paper [6], must be taken into consideration. These experiments led us to the following conclusions concerning organic matter: - - the fraction soluble in water is rich in nitrogen; - - nitrogen is absent in the fraction soluble in organic solvents but aliphatic compounds are present, as shown by both the H/C ratio and IR data; - - in humic complexes the metals may be bound not only by the usual carboxylic group but also by partially enolyzed fl-diketonic structures [7]; the fraction of sludge soluble in neither water nor organic solvents (70%) consists of acidic compounds (40%), and these are able to retain 36% of the Cu and 3% of the Zn. In addition, this acid fraction contains one-third of the total nitrogen of the raw sludge. What, then, happens if the sludge is exposed to UV radiation? Does the -
-
IIRRADIATEDSLUDGEl L
I WATER
Cu, Zn, ;'An and COD determination
RES)DUE I DIETHYLETHER~ RE$1 ' UE iD
I
BENZENE
RESIDUE Cu, Zn, 1,4n, organic ACETONE J ~ substances determination RESID ' UE IR I TETRAH!DROFURAN ~ ' ~ RESIDUE I ETHANOL ~ ' ~ i RESIDUE(TO,ELEMENTALANALYSIS) 1
Scheme 1.
43 r a d i a t i o n modify t h e o r g a n i c f r a c t i o n a n d c o n s e q u e n t l y affect t h e m e t a l l e a c h i n g process? L e t us first c o n s i d e r the effect of U V r a d i a t i o n in e x p e r i m e n t s p e r f o r m e d at a c o n s t a n t t e m p e r a t u r e . T a b l e 1 s h o w s t h e residue, COD v a l u e a n d p e r c e n t a g e of m e t a l s in t h e a q u e o u s e x t r a c t as a f u n c t i o n of i r r a d i a t i o n time. K n o w l e d g e of t h e influence of i r r a d i a t i o n on t h e f r a c t i o n soluble in w a t e r is v e r y i m p o r t a n t in e v a l u a t i n g the effect of r a i n a n d i r r i g a t i o n on sludge d e p o s i t e d on soil. I r r a d i a t i o n r e d u c e s t h e a m o u n t of o r g a n i c m a t t e r w h i c h is soluble in w a t e r , as s h o w n b o t h by COD v a l u e s a n d t h e r m o g r a v i m e t r i c d a t a (Fig. 1). T h i s c h a n g e in solubility m u s t be due to m o d i f i c a t i o n of t h e c h e m i c a l p r o p e r t i e s of t h e o r g a n i c m a t t e r i n d u c e d by U V r a d i a t i o n . In fact, U V r a d i a t i o n , by m e a n s of a r a d i c a l m e c h a n i s m , s h o u l d i n d u c e b o t h t h e m i n e r a l i z a t i o n of simple o r g a n i c m o l e c u l e s a n d t h e b r e a k i n g of m a c r o s t r u c t u r e s into s i m p l e r ones (with a l o w e r m o l e c u l a r weight). S o m e of these, t o g e t h e r w i t h o t h e r c o m p o u n d s a l r e a d y present, m a y u n d e r g o polym e r i z a t i o n , as in t h e o x i d a t i v e d e g r a d a t i o n of h u m i c s u s t a n c e s [8, 9]. T h i s effect is n o t t i m e d e p e n d e n t for t i m e s l o n g e r t h a n 30 min. TABLE 1 Residue, COD and metal contents (%) of aqueous extracts after different irradiation times Irradiation time (min)
COD (mg 0 2 g-t)
Residue (mg g-~)
Cu (%)
Zn (%)
Mn (%)
0 30 60 180
731 590 590 593
126 37 43 42
8.0 7.0 8.0 7.0
17.0 20.5 14.5 14.0
25.0 31.0 36.0 35.5
(a)
r i' F T--l F 200 400 600 800T/'C 20O 4OO 600 8OOT/'C Fig. 1. Direct and differential thermogravimetric curves for irradiated (a) and non-irradiated (b) sludge.
44 The above hypothesis seems to be confirmed by 13C NMR spectra of aqueous extracts of both non-irradiated and irradiated sludge. A comparison of them provides some useful information; in particular, irradiation gives rise to: - an increase in aliphatic compounds (peaks around 30 ppm); - the appearance of a well-defined and strong signal due to alcohol carbons such as in carbohydrates (70-75 ppm), whose origin also seems to be supported by the presence of a series of bands at 60 ppm; - - t h e presence of a signal due to anomeric carbon atoms (100-105ppm); - - and finally, peaks due to the carbons of both olefinic double bonds (130 ppm) and carboxylic groups (170 ppm) are clearly shown. UV radiation therefore causes the degradation of complex structures such as polysaccharides, humic substances, etc. Some of the simpler degradation products re-polymerize, while others remain unchanged. In fact, NMR spectroscopy shows that irradiation produces both simple saccharide structures (from polysaccharides) and aliphatic compounds; the latter, whose solubility in water is ensured by the carboxylic groups present, probably come from humus degradation. It is known that aliphatic dicarboxylic acids can be produced by humus degradation [10]. Moreover, monosaccharides could also be derived from the same source. The changing metal solubility may be explained by this hypothesis. In particular, the data show that even though irradiation does not significantly modify the solubility of copper in water it does affect the behaviour of other metals. For instance, while the solubility of manganese in water increases with irradiation time up to a constant value, that of zinc first increases and later decreases. Irradiation does not significantly affect the solubility of copper in water because the degradation of humic acids, to which copper is mainly bound, produces compounds of lower molecular weight (that are not soluble in water and may undergo further polymerization) which are still able to bond strongly to the copper. It is known t h a t the lower the molecular weight of the humic acid, the more stable the copper humic complex [11]. This also indicates that the copper in aqueous extracts should be present essentially as inorganic forms. The greater water solubility of zinc and manganese could perhaps best be explained in terms of their interaction with macromolecules. Whereas copper forms true complexes with humic or humic-like acids, zinc and manganese are bound up in the three-dimensional structure of the macromolecules and the structural rearrangement induced by UV irradiation frees the metals. To discover to what extent UV radiation affects the solubility of different fractions in organic solvents (if at all) requires that the degree of organic matter modification be determined. From an analysis of the data reported in Table 2, the following observations may be made: - - the effect of irradiation appears to vary from one solvent to another. In particular, irradiation does not affect the amount of organic matter soluble in benzene, acetone or ethanol, although some influence is observed in other
0 30 60 180 0 30 60 180
Copper (%)
Zinc (%)
ND = n o t detected.
0 30 60 180
O r g a n i c m a t t e r solubilized (mg g 2)
Irradiation time (rain)
0 ND a 0.5 0.5
0.5 2.0 9.5 8.5
20.0 8.5 7.5 8.0
Diethylether
ND 0.5 0.5 0.5
0.5 3.5 9.0 8.5
5.5 4.0 3.0 4.0
Benzene
R e s i d u e a n d m e t a l c o n t e n t s (%) of o r g a n i c e x t r a c t s a f t e r d i f f e r e n t i r r a d i a t i o n t i m e s
TABLE 2
0.5 1.0 0.5 1.0
2.5 6.0 9.5 9.5
17.0 14.0 16.5 17.0
Acetone
0 0.5 0.5 0.5
6.0 6.0 8.5 8.5
24.0 33.5 15.0 5.5
Tetrahydrofuran
0 0.5 0.5 1.0
1.0 4.5 9.5 8.5
6.0 4.0 6.0 11.0
Ethanol
46
-
-
-
solvents; in fact, both in tetrahydrofuran and in diethylether, the solubility decreases; - - the modifying effect of irradiation on organic substances, as shown by solubility trends, is confirmed by IR analysis. The spectra of irradiated samples do not show significant differences from those of non-irradiated samples, except in the case of tetrahydrofuran. In this solvent (Fig. 2), one can see both a series of new peaks at low frequencies, ascribed to aromatic structures, and a higher intensity of peaks usually present in other extracts; the effects of irradiation on organic matter, even if they are usually not strong enough to be detected in IR spectra, are however well evidenced by the trend of copper solubility in different organic solvents as a function of irradiation time; in fact, the percentage of solubilized copper increases with time of exposure up to a constant value. This trend is common to all solvent extracts; it should be noted that the total fraction of copper solubilized by all organic solvents is four times greater with than without irradiation. for zinc, a general increase in solubility is observed, but the trend does not seem significant; finally, no manganese was found in the organic extracts. Since sunlight also produces a thermal effect, experiments without temperature control were also performed. In Fig. 3 the percentage of metals -
-
-
4000
3000
2000
1600
1200 800 cm-1
Fig. 2. IR spectra of tetrahydrofuran extracts of non-irradiated (a) and irradiated (b) sludge.
%
Zn
Cu
3O
0 0.5 1
3
0 55
1
3
0 0.5
3 t/h
Fig. 3. Percentage of metal solubilized by water as a function of irradiation time in experiments carried out at constant ([]) or ambient (D) temperature.
47 % 15
1I I
~
10 5
0
1
2
t/h
Fig. 4. Percentage of copper solubilized by acetone as a function of irradiation time, with ( or without (. . . . ) temperature control during irradiation. solubilized by w a t e r is c o m p a r e d with the same d a t a o b t a i n e d w i t h o u t temp e r a t u r e control. As can be seen, the s i t u a t i o n is p r a c t i c a l l y the same for copper, b u t a g e n e r a l l y g r e a t e r availability is observed for zinc, while for m a n g a n e s e the solubility progressively decreases a f t e r an initial sharp increase. W i t h r e g a r d to e x t r a c t i o n in o r g a n i c solvents of b o t h o r g a n i c s u b s t a n c e s and metals, most of the t r e n d s as a f u n c t i o n of time do n o t differ significantly from those observed in the e x p e r i m e n t s p e r f o r m e d at c o n s t a n t t e m p e r a t u r e . The e x c e p t i o n is t h a t of copper, whose t r e n d is now m a r k e d l y different, a l t h o u g h the new t r e n d is the same for all solvents considered. In Fig. 4, as an example, the t r e n d in a c e t o n e is r e p o r t e d for b o t h e x p e r i m e n t a l conditions adopted. It can be seen t h a t w h e n the t e m p e r a t u r e is not controlled, m a x i m u m solubilization ( ~ 92% of t o t a l c o p p e r c o n s i d e r i n g the c o n t r i b u t i o n of all solvents) is a c h i e v e d w i t h i n a s h o r t e r time ( ~ 30 min). If i r r a d i a t i o n is prolonged, copper solubilization d e c r e a s e s g r e a t l y (to less t h a n one-half). This value, however, is still lower t h a n t h a t o b t a i n e d with t e m p e r a t u r e control. This suggests t h a t h i g h e r t e m p e r a t u r e s initially a c c e l e r a t e the d e g r a d a t i o n of m a c r o m o l e c u l e s into simpler ones able to complex the copper; b o t h the smaller size and the new d i s t r i b u t i o n of f u n c t i o n a l groups of these complexes m a k e t h e m more soluble in o r g a n i c solvents. S u b s e q u e n t l y , the h i g h e r t e m p e r a t u r e g r e a t l y favours the r e - p o l y m e r i z a t i o n processes of d e g r a d a t i o n compounds [12] into new, l a r g e r molecules no l o n g e r soluble in o r g a n i c solvents. ACKNOWLEDGEMENTS The a u t h o r s are g r e a t l y indebted to Professors M. T o m a s s e t t i and M. Delfini (of o u r d e p a r t m e n t ) for t h e r m o g r a v i m e t r i c and N M R analyses, respectively. REFERENCES 1 2 3 4
G. Sposito, Environ. Sci. Technol., 15 (1981) 396. R.W. Winner, Aquat. Toxicol., 5 (1984) 276. J. Slavek, J. Wold and W.F. Pickering, Talanta, 29 (1982) 743. L. Campanella, E. Cardarelli, T. Ferri, B.M. Petronio and A. Pupella, Sci. Total Environ., 61 (1987) 217.
48
5 6 7 8 9 10 11 12
L. Campanella, E. Cardarelli, T. Ferri, B.M. Petronio and A. Pupella, Sci. Total Environ., 61 (1987) 229. B.M. Petronio, L. Campanella, E. Cardarelli, T. Ferri and A. Pupella, Ann. Chim., 77 (1987) 721. F.J. Stevenson, Geochim. Cosmochim. Acta, 35 (1971) 471. E.R. Weiner and M.C. Goldberg, Toxicol. Environ. Chem., 9 (1985) 327. R.F. Christman, W.T. Liao, D.S. Millington and J.D. Johnson, Adv. Identif. Anal. Org. Pollut. Water, 2 (1984) 979; Chem. Abstr. 102:190485 g. C. Ogner, Geoderma, 35 (1985) 343. W. Haekel, Energy Res. Abstr., 2978 (1984). N.N. Bambalov and T.P. Smychnik, Vestsi Akad. Navuk BSSR, Ser. Khim. Navuk, 3 (1986) 75; Chem. Abstr. 105:155967 f.
41
INFLUENCE OF UV RADIATION ON ORGANIC MATTER TRANSFORMATION AND SUBSEQUENT ALTERATION OF LEACHING AND TRANSPORT PROCESSES OF HEAVY METALS
L. CAMPANELLA, E. CARDARELLI, T. FERRI, B.M. PETRONIO and A. PUPELLA Department of Chemistry, University of Rome "La Sapienza", Piazzale Aldo Moro, 5-00185 Rome (Italy)
(Received December 16th, 1987; accepted January 28th, 1988)
ABSTRACT This work deals with the effect of UV radiation on urban sludge. The effect on both organic matter and the related metal leaching processes was studied. The results show that UV radiation determines both the degradation of macrostructures and the re-polymerization of degradation products. Nevertheless, these reactions do not seem to strongly affect the leachability of metals. Experiments performed with and without temperature control showed that temperature also plays an important role in the fate of metals in the environment. INTRODUCTION T h e c o n t i n u o u s i n c r e a s e in t h e p r o d u c t i o n of sludge by u r b a n a n d i n d u s t r i a l t r e a t m e n t p l a n t s h a s led to the g r o w i n g p r o b l e m of disposal. T h e l e a c h i n g of toxic m e t a l s f r o m t h e s e sludges (deposited on soil) m u s t be t a k e n into a c c o u n t , as h e a v y m e t a l s f r o m sludge e n t e r i n g t h e e n v i r o n m e n t c a n c a u s e serious p r o b l e m s for m a n [1, 2]. A k n o w l e d g e of m e t a l s p e c i a t i o n [3-5] is v i t a l in e v a l u a t i n g t h e environm e n t a l i m p a c t of t h e s e metals, since t h e i r t o x i c i t y is s t r i c t l y r e l a t e d to t h e c h e m i c a l f o r m in w h i c h t h e y a r e p r e s e n t in the sludge. This also m e a n s t h a t t h e l e a c h i n g process of m e t a l s depends on t h e n a t u r e of t h e sludge. In p a r t i c u l a r , the f r a c t i o n of m e t a l b o u n d to n a t u r a l o r g a n i c ligands (humic a n d fulvic acids) a n d to s u b s t a n c e s of a n t h r o p o g e n i c origin, s u c h as a m i n o acids, fats, waxes, c a r b o h y d r a t e s , etc., is q u i t e h i g h in u r b a n sludge. F r o m this it follows t h a t c h a n g e s in t h e l e a c h i n g of m e t a l s m u s t be r e l a t e d to t h e m o d i f i c a t i o n of o r g a n i c m a t t e r . T h i s m o d i f i c a t i o n c a n be i n d u c e d by a g e n t s s u c h as m i c r o o r g a n i s m s or sunlight. T h e object of t h e p r e s e n t w o r k is to e v a l u a t e t h e effect of s u n l i g h t on t h e o r g a n i c m a t t e r in sludge a n d c o n s e q u e n t l y to a n a l y z e h o w s p e c i a t i o n a n d l e a c h i n g p r o c e s s e s of m e t a l s m a y be modified.
0048-9697/88/$03.50
© 1988 Elsevier Science Publishers B.V.
42 EXPERIMENTAL
The sludge employed in this work was the same as t h a t used in previous experiments [4, 5]. The preliminary treatment to which the sludge was submitted was also the same. Both the sludge composition and the characterization of the humic acids in it have been reported in a previous paper [6]. In the present case the experimental procedure was as follows: sludge samples (5g) were first irradiated by a mercury vapour lamp (125W) for different periods of time (from 30 to 180min), either under thermostated conditions or not, and later treated according to Scheme 1. This scheme has already been applied to non-irradiated sludge [6]. In addition to the experimental apparatus previously utilized, a Varian XL-300 NMR spectrometer was used to record 13C spectra. RESULTS AND DISCUSSION
In order to evaluate the effect of UV radiation on organic matter, the results of the application of Scheme 1 to non-irradiated sludge, reported in a previous paper [6], must be taken into consideration. These experiments led us to the following conclusions concerning organic matter: - - the fraction soluble in water is rich in nitrogen; - - nitrogen is absent in the fraction soluble in organic solvents but aliphatic compounds are present, as shown by both the H/C ratio and IR data; - - in humic complexes the metals may be bound not only by the usual carboxylic group but also by partially enolyzed fl-diketonic structures [7]; the fraction of sludge soluble in neither water nor organic solvents (70%) consists of acidic compounds (40%), and these are able to retain 36% of the Cu and 3% of the Zn. In addition, this acid fraction contains one-third of the total nitrogen of the raw sludge. What, then, happens if the sludge is exposed to UV radiation? Does the -
-
IIRRADIATEDSLUDGEl L
I WATER
Cu, Zn, ;'An and COD determination
RES)DUE I DIETHYLETHER~ RE$1 ' UE iD
I
BENZENE
RESIDUE Cu, Zn, 1,4n, organic ACETONE J ~ substances determination RESID ' UE IR I TETRAH!DROFURAN ~ ' ~ RESIDUE I ETHANOL ~ ' ~ i RESIDUE(TO,ELEMENTALANALYSIS) 1
Scheme 1.
43 r a d i a t i o n modify t h e o r g a n i c f r a c t i o n a n d c o n s e q u e n t l y affect t h e m e t a l l e a c h i n g process? L e t us first c o n s i d e r the effect of U V r a d i a t i o n in e x p e r i m e n t s p e r f o r m e d at a c o n s t a n t t e m p e r a t u r e . T a b l e 1 s h o w s t h e residue, COD v a l u e a n d p e r c e n t a g e of m e t a l s in t h e a q u e o u s e x t r a c t as a f u n c t i o n of i r r a d i a t i o n time. K n o w l e d g e of t h e influence of i r r a d i a t i o n on t h e f r a c t i o n soluble in w a t e r is v e r y i m p o r t a n t in e v a l u a t i n g the effect of r a i n a n d i r r i g a t i o n on sludge d e p o s i t e d on soil. I r r a d i a t i o n r e d u c e s t h e a m o u n t of o r g a n i c m a t t e r w h i c h is soluble in w a t e r , as s h o w n b o t h by COD v a l u e s a n d t h e r m o g r a v i m e t r i c d a t a (Fig. 1). T h i s c h a n g e in solubility m u s t be due to m o d i f i c a t i o n of t h e c h e m i c a l p r o p e r t i e s of t h e o r g a n i c m a t t e r i n d u c e d by U V r a d i a t i o n . In fact, U V r a d i a t i o n , by m e a n s of a r a d i c a l m e c h a n i s m , s h o u l d i n d u c e b o t h t h e m i n e r a l i z a t i o n of simple o r g a n i c m o l e c u l e s a n d t h e b r e a k i n g of m a c r o s t r u c t u r e s into s i m p l e r ones (with a l o w e r m o l e c u l a r weight). S o m e of these, t o g e t h e r w i t h o t h e r c o m p o u n d s a l r e a d y present, m a y u n d e r g o polym e r i z a t i o n , as in t h e o x i d a t i v e d e g r a d a t i o n of h u m i c s u s t a n c e s [8, 9]. T h i s effect is n o t t i m e d e p e n d e n t for t i m e s l o n g e r t h a n 30 min. TABLE 1 Residue, COD and metal contents (%) of aqueous extracts after different irradiation times Irradiation time (min)
COD (mg 0 2 g-t)
Residue (mg g-~)
Cu (%)
Zn (%)
Mn (%)
0 30 60 180
731 590 590 593
126 37 43 42
8.0 7.0 8.0 7.0
17.0 20.5 14.5 14.0
25.0 31.0 36.0 35.5
(a)
r i' F T--l F 200 400 600 800T/'C 20O 4OO 600 8OOT/'C Fig. 1. Direct and differential thermogravimetric curves for irradiated (a) and non-irradiated (b) sludge.
44 The above hypothesis seems to be confirmed by 13C NMR spectra of aqueous extracts of both non-irradiated and irradiated sludge. A comparison of them provides some useful information; in particular, irradiation gives rise to: - an increase in aliphatic compounds (peaks around 30 ppm); - the appearance of a well-defined and strong signal due to alcohol carbons such as in carbohydrates (70-75 ppm), whose origin also seems to be supported by the presence of a series of bands at 60 ppm; - - t h e presence of a signal due to anomeric carbon atoms (100-105ppm); - - and finally, peaks due to the carbons of both olefinic double bonds (130 ppm) and carboxylic groups (170 ppm) are clearly shown. UV radiation therefore causes the degradation of complex structures such as polysaccharides, humic substances, etc. Some of the simpler degradation products re-polymerize, while others remain unchanged. In fact, NMR spectroscopy shows that irradiation produces both simple saccharide structures (from polysaccharides) and aliphatic compounds; the latter, whose solubility in water is ensured by the carboxylic groups present, probably come from humus degradation. It is known that aliphatic dicarboxylic acids can be produced by humus degradation [10]. Moreover, monosaccharides could also be derived from the same source. The changing metal solubility may be explained by this hypothesis. In particular, the data show that even though irradiation does not significantly modify the solubility of copper in water it does affect the behaviour of other metals. For instance, while the solubility of manganese in water increases with irradiation time up to a constant value, that of zinc first increases and later decreases. Irradiation does not significantly affect the solubility of copper in water because the degradation of humic acids, to which copper is mainly bound, produces compounds of lower molecular weight (that are not soluble in water and may undergo further polymerization) which are still able to bond strongly to the copper. It is known t h a t the lower the molecular weight of the humic acid, the more stable the copper humic complex [11]. This also indicates that the copper in aqueous extracts should be present essentially as inorganic forms. The greater water solubility of zinc and manganese could perhaps best be explained in terms of their interaction with macromolecules. Whereas copper forms true complexes with humic or humic-like acids, zinc and manganese are bound up in the three-dimensional structure of the macromolecules and the structural rearrangement induced by UV irradiation frees the metals. To discover to what extent UV radiation affects the solubility of different fractions in organic solvents (if at all) requires that the degree of organic matter modification be determined. From an analysis of the data reported in Table 2, the following observations may be made: - - the effect of irradiation appears to vary from one solvent to another. In particular, irradiation does not affect the amount of organic matter soluble in benzene, acetone or ethanol, although some influence is observed in other
0 30 60 180 0 30 60 180
Copper (%)
Zinc (%)
ND = n o t detected.
0 30 60 180
O r g a n i c m a t t e r solubilized (mg g 2)
Irradiation time (rain)
0 ND a 0.5 0.5
0.5 2.0 9.5 8.5
20.0 8.5 7.5 8.0
Diethylether
ND 0.5 0.5 0.5
0.5 3.5 9.0 8.5
5.5 4.0 3.0 4.0
Benzene
R e s i d u e a n d m e t a l c o n t e n t s (%) of o r g a n i c e x t r a c t s a f t e r d i f f e r e n t i r r a d i a t i o n t i m e s
TABLE 2
0.5 1.0 0.5 1.0
2.5 6.0 9.5 9.5
17.0 14.0 16.5 17.0
Acetone
0 0.5 0.5 0.5
6.0 6.0 8.5 8.5
24.0 33.5 15.0 5.5
Tetrahydrofuran
0 0.5 0.5 1.0
1.0 4.5 9.5 8.5
6.0 4.0 6.0 11.0
Ethanol
46
-
-
-
solvents; in fact, both in tetrahydrofuran and in diethylether, the solubility decreases; - - the modifying effect of irradiation on organic substances, as shown by solubility trends, is confirmed by IR analysis. The spectra of irradiated samples do not show significant differences from those of non-irradiated samples, except in the case of tetrahydrofuran. In this solvent (Fig. 2), one can see both a series of new peaks at low frequencies, ascribed to aromatic structures, and a higher intensity of peaks usually present in other extracts; the effects of irradiation on organic matter, even if they are usually not strong enough to be detected in IR spectra, are however well evidenced by the trend of copper solubility in different organic solvents as a function of irradiation time; in fact, the percentage of solubilized copper increases with time of exposure up to a constant value. This trend is common to all solvent extracts; it should be noted that the total fraction of copper solubilized by all organic solvents is four times greater with than without irradiation. for zinc, a general increase in solubility is observed, but the trend does not seem significant; finally, no manganese was found in the organic extracts. Since sunlight also produces a thermal effect, experiments without temperature control were also performed. In Fig. 3 the percentage of metals -
-
-
4000
3000
2000
1600
1200 800 cm-1
Fig. 2. IR spectra of tetrahydrofuran extracts of non-irradiated (a) and irradiated (b) sludge.
%
Zn
Cu
3O
0 0.5 1
3
0 55
1
3
0 0.5
3 t/h
Fig. 3. Percentage of metal solubilized by water as a function of irradiation time in experiments carried out at constant ([]) or ambient (D) temperature.
47 % 15
1I I
~
10 5
0
1
2
t/h
Fig. 4. Percentage of copper solubilized by acetone as a function of irradiation time, with ( or without (. . . . ) temperature control during irradiation. solubilized by w a t e r is c o m p a r e d with the same d a t a o b t a i n e d w i t h o u t temp e r a t u r e control. As can be seen, the s i t u a t i o n is p r a c t i c a l l y the same for copper, b u t a g e n e r a l l y g r e a t e r availability is observed for zinc, while for m a n g a n e s e the solubility progressively decreases a f t e r an initial sharp increase. W i t h r e g a r d to e x t r a c t i o n in o r g a n i c solvents of b o t h o r g a n i c s u b s t a n c e s and metals, most of the t r e n d s as a f u n c t i o n of time do n o t differ significantly from those observed in the e x p e r i m e n t s p e r f o r m e d at c o n s t a n t t e m p e r a t u r e . The e x c e p t i o n is t h a t of copper, whose t r e n d is now m a r k e d l y different, a l t h o u g h the new t r e n d is the same for all solvents considered. In Fig. 4, as an example, the t r e n d in a c e t o n e is r e p o r t e d for b o t h e x p e r i m e n t a l conditions adopted. It can be seen t h a t w h e n the t e m p e r a t u r e is not controlled, m a x i m u m solubilization ( ~ 92% of t o t a l c o p p e r c o n s i d e r i n g the c o n t r i b u t i o n of all solvents) is a c h i e v e d w i t h i n a s h o r t e r time ( ~ 30 min). If i r r a d i a t i o n is prolonged, copper solubilization d e c r e a s e s g r e a t l y (to less t h a n one-half). This value, however, is still lower t h a n t h a t o b t a i n e d with t e m p e r a t u r e control. This suggests t h a t h i g h e r t e m p e r a t u r e s initially a c c e l e r a t e the d e g r a d a t i o n of m a c r o m o l e c u l e s into simpler ones able to complex the copper; b o t h the smaller size and the new d i s t r i b u t i o n of f u n c t i o n a l groups of these complexes m a k e t h e m more soluble in o r g a n i c solvents. S u b s e q u e n t l y , the h i g h e r t e m p e r a t u r e g r e a t l y favours the r e - p o l y m e r i z a t i o n processes of d e g r a d a t i o n compounds [12] into new, l a r g e r molecules no l o n g e r soluble in o r g a n i c solvents. ACKNOWLEDGEMENTS The a u t h o r s are g r e a t l y indebted to Professors M. T o m a s s e t t i and M. Delfini (of o u r d e p a r t m e n t ) for t h e r m o g r a v i m e t r i c and N M R analyses, respectively. REFERENCES 1 2 3 4
G. Sposito, Environ. Sci. Technol., 15 (1981) 396. R.W. Winner, Aquat. Toxicol., 5 (1984) 276. J. Slavek, J. Wold and W.F. Pickering, Talanta, 29 (1982) 743. L. Campanella, E. Cardarelli, T. Ferri, B.M. Petronio and A. Pupella, Sci. Total Environ., 61 (1987) 217.
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