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Metals from sediments
War. Res. Vol. 23, No. 2, pp. 247-252, 1989 Printed in Great Britain. All rights reserved
0043-1354/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc
METALS FROM SEDIMENTS WOLFGANG LIETZ and GOTTFRIEDGALLING Botanisches Institut der TU Braunschweig, Humboldtstra~ 1, 3300 Braunschweig, F.R.G.
(First received March 1988; accepted in revised form September 1988) Abstract--River sediments were investigated relative to their desorption characteristics of zinc, cadmium and lead. We examined naturally contaminated sediments of the Oker river in the Federal Republic of Germany. Using dialysis pipes for separating sediments from the medium, a direct influenceof complexes is prevented. The only possibility of this is from diffusion through the membrane. Diffusion is dependent upon the uptake capacity of the surrounding medium. Without any metal adsorbing substances in solution, values of desorption reached a maximum of 1.96% lead, 0.31% cadmium and 0.02% zinc. The addition of 1/~M NTA or EDTA to the medium increased metal desorption. Incubation, together with these chelating agents, raised zinc values 28-fold: lead and cadmium are enhanced by a maximum 5-fold. The addition of algae to the medium has no effect on desorption. All three metals have only slightly varied concentrations when compared with the medium alone. Uptake only reaches 10-15% of the total desorbed cadmium and lead concentrations in the medium.
Key words--sediment, zinc, cadmium, lead, desorption
INTRODUCTION Today heavy metals are of enhanced importance in industry. In the last decades pollution of the environment therefore has been studied with interest. Many rivers in densely populated countries contain large amounts of metals. For example the Oker has substantially increased concentrations of zinc, lead and cadmium, compared with unpolluted streams (Baumann et aL, 1977; Stumm, 1972). Metal concentrations in rivers are able to oscillate in wide ranges (Ahlf, 1982; F6rstner and Stiefel, 1978). One important fact in varying the concentrations in water is the influence of metals bound to sediments. The heavy metal load of sediments often reaches such high levels that a sudden resorption would be a great danger (Davies, 1970; Salomons and F6rstner, 1984). Therefore sorption and desorption processes influence water quality. Occasionally transport of sediments-rich water results in increased heavy metal concentration in rivers (F6rstner and Patchineelam, 1976). Another important factor in increasing metal concentration is the existence of complexess such as humides or other organic substances. With regard to this, the influence of the complexes EDTA and NTA (both produced by humans) are of interest. Dependent from other elements and from the concentrations of complexes, they enrich metals in water. In fertilizers EDTA is able to reach concentrations of up to 100/ag/l (Gardiner, 1975), normal values of wastewaters reach 3 mg/l (F6rstner et al., 1983). Our experiments investigated the heavy metals bound to the sediments in the Oker river. We measured desorption of zinc, cadmium and lead deWR 23 '2--1
pending on the incubation time and the composition of the medium. The difference between direct and indirect reactions of extraction solution to sediments will be discussed. By using dialysis pipes for incubation, this examination gives several new points of view. MATERIALSAND METHODS Sediments from the Oker river, directly next to the Botanical Institute of the University in Braunsehweig, were used and examined in our experiments. The sediment was apparently anaerobic in the river. It contained mostly components of the silt fraction and, to a high degree, organic components. The sediment was dried at 60°C for about 24 h. Afterwards every I0 g were mixed with 5 ml of distilled water and incubated in dialysis pipes (Visking 20/32). The filled dialysis pipes were incubated in aerated glass tubes together with I00 ml of medium. This deficiency medium was prepared according to Inner (1982) without zinc, cobalt and copper ions. Desorption was detected by measuring the metal concentrations in the medium. Experiments were carried out during 44 h. At the beginning and after 4, 8, 20, 30 and at 44 h, the zinc, cadmium and lead contents of 2 ml samples were determined. We examined different desorption solutions: medium only, medium with 1/~M EDTA (ethyldiaminnitrilotetraacid) or with 1/tM NTA (nitrilotriacetic acid) and medium containing algae. Culture of Chlorella saccharophila (21 l-la) and Clorella vulgaris (2 ! 1-11b) occurred under dark: light cycles( 10:14 h at 5000 lux/Osram L40W/15-1) for 8-10 days in 100 ml glass tubes. Their medium was prepared according to Kuhl (1964) with the one exception that the zinc concentration was lowered to 0.002 g/l. Afterwards the cells were cultured for 48 h in 250 ml glass tubes. The culture took place under continued light conditions at 4500 lux in deficiencymedium. During the latter period, the algae were continuously aerated. Prior to the measurements, the algae material was washed once, incubated in fresh deficiency medium and
247
248
WOLFGANG LI£TZ and GOTTFRIED GALLING
(a) II
600
/
~300
0
I
I
4
8
,I
24 Time (h)
I
48
3000 I" (b)
_
/
° ~ locx)
o
0
4
8
24 Time (h)
48
Fig. I. Velocity of dialysis. Incubation of zinc, cadmium and lead in dialysis pipes. The pipes were incubated in 100 ml
medium. After complete diffusion of the added metals, metal concentrations of 15 I~M must result. We show the development of (a) zinc and (b) cadmium ( - - - ) and lead ( ), Cycles give results of desorption with an additional I~M EDTA in the medium.
exposed to light. Each experiment was carried out with identical dry weights of 6.7mg/10ml. The strains were grown and uptake experiments carried out at 23°C. We used a pH of 6.2. Each sample was evaporated and afterwards treated with 0.5 ml HPO 4 and heated for 20 rain at 90°C, 60 min at 150°C and for a minimum of 8 h at 190°C. The metals were measured with an ion selective electrode (Neeb, 1978). Sediment concentrations were determined with flameless atomic absorption. RESULTS
In our experiments dialysis pipes were used. The first experiment examined their permeability to metal ions (Fig. I). The heavy metals zinc, cadmium and lead in the form of their chlorides were incubated in the dialysis pipes and their diffussion through the membrane and the appearance of the surrounding medium were measured. After only l h all three metals were measurable. The intensity of diffusion decreased in time because of the decreased differences in concentration between the dialysis pipes and the medium. With all three metals, an equalization of 80% minimum occurred, at the latest, at 4 h. EDTA enhanced the activity of diffusion. After 1 h, no more increased values were measured.
Desorption of zinc is shown in Fig. 2. Varied composition of the medium led to differentcharacteristics.Addition of the medium only resulted in almost linearly increased zinc concentrations. At the end of the experiment, no more than doubled values were reached. E D T A and N T A treatment caused completely different behaviour when compared with medium only. A considerable increase of up to 28-fold was measured. At the end of the experiment metal concentrations with N T A exceeded that of E D T A . The presence of algae in the medium did not enhance the total concentrations. Desorption was equal when compared with the addition of the medium only. During the experiment only a slight increase in metals absorbed by the algae occurred. With consideration of the waving values, both algae behaved similarly. Like zinc, as in the case of cadmium in the medium without any other substances, only a small amount of desorption occurred (Fig. 3). The values increased slowly and reached 18 mg/ml. CMorella vulgaris, as well as Chlorella saccharophila, had no influence on the diffusion of heavy metal desorption. The concentrations with or without algae were equal. Uptake of cadmium by algae was extraordinarily small. The influence of complexes on desorption of metals enhanced cadmium concentrations in the medium. Comparing both complexes, EDTA possessed higher desorption capacities than N T A at the end of the experiment. With EDTA and N T A 3- and 2-fold higher values, respectively, were detected than with the medium only (Fig. 1). The values of the experiments with lead are shown in Fig. 4. During the incubation time the lead concentrations in the medium were increased. Medium only, or medium with algae, resulted in comparable curves and concentrations. Uptake of algae was very small and the increase in time was negligible. The complexes EDTA and NTA enhanced desorption characteristics up to 5-fold with E D T A and 2.5-fold with NTA. NTA showed only slightly varied values compared with EDTA. Also medium only or medium with both strains of algae did not differ significantly in desorption characteristics. Therefore, because of their typical character, the results of desorption with EDTA and medium were chosen in Table 2. It was possible to calculate the uptake, in percent of total sediment concentration, by using the values from Figs 1-3. The comparison of these two typical types of behaviour showed, in all cases, increased desorption in the
Table I. Heavy metal content of the sediment
Metal Zinc
Cadmium
Lead
Concentration (ppm) 2296.9 78.0
293.0
Standard deviation (ppm) _+328.8 + 12.9
_+66.7
Metals from sediments
1200-
249
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=
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/
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10 ¸
o.
!
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i
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Fig. 2. Desorption of zinc. In the dialysis pipes, 10g sediment together with 5 ml medium were resuspended. Afterwards the pipes were incubated in I00 ml solution. This solution contained (a) EDTA (---) or NTA ( ), (b) deficiencymedium only or (e) the algae Chlorellasaceharophila and (d) Chlorella rulgaris. The straight lines show concentrations of algae and medium, the dotted line gives the result of uptake by the strains.
sequence Zn > Cd > Pb. For that reason the strength of binding to the sediments decreased in the same sequence. If one looks only at the absolute concentrations of desorption, the high values of zinc probably displace the relations. DISCUSSION
The experiments were obtained under specific conditions. The sediment was isolated from the medium by the walls of the dialysis pipes and therefore an influence on free exchange of ions existed. The test of permeability showed sufficient diffusion velocity using the chloride salts of zinc, cadmium and lead (Fig. 1). In the sediments most of the metals were
bound to the surfaces of organic components or to more or less large inorganic complexes. The already slightly decreased diffusion rate of ions, measuring the test of permeability, indicated that chelated metals were unable to permeate through the dialysis pipes. Prior to diffusion, a change in their chemical form was therefore necessary. The higher the capacity of metal sorption of the sediments, the less often the exchange of metal into ion form, and even less often was the occurrence of diffusion through the dialysis pipes. Desorption from the sediment and also diffusion was dependent upon the composition of the medium. Normal medium contained neither sulfates phosphates nor any complexes. For this reason the possi-
250
WOLFGANG LIETZ and GOTTFRIr=D GALLING
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120 w
~e 80
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"~X
/
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ii \
/ /
4(
St
I
°t time
( h )
time
25.
( h )
2S
J 15" Ck
5.
:I time
( h )
time
(h )
Fig. 3. Desorption of cadmium. In the dialysis pipes, 10g sediments together with 5 ml medium were resuspended. Afterwards the pipes were incubated in 100 ml solution. This solution contained (a) EDTA ( - - - ) or NTA ( ), (b) deficiency medium only or (c) the algae Chlorella saccharophila and (d) Chlorella vulgaris. The straight lines show concentrations of algae and medium, the dotted line gives the result of uptake by the strains.
change in the equilibrium occurred. Ions of the bilities of uptake were restricted. Under these condimedium rapidly became bound by this complex and tions concentrations in the medium increased by differences between the concentrations remained small amounts. high. The weaker the binding of this metal to the EDTA and NTA are both chemical compounds sediments, the greater the possibilities of exchanges which form complexes with metals. In the case of through the membrane and therefore higher concenEDTA no binding to surfaces exists (Gardiner, 1975). trations were detectable in the medium. Medium which contains complexes results in enorTaking into account these considerations, our remously desorbed concentrations of metals. Desorpsults indicated that more than 90% of all three metals tion increased depending on incubation time and with was firmly bound to the sediments. Nevertheless large enhance concentrations of complexes (Laxen, 1985; differences in desorption between zinc, cadmium and Garnet et aL, 1985). In contrast to the experiments which we carried out, the membrane of the dialysis lead existed. The binding of lead and cadmium to sediments was much more stable than that of zinc. pipes prevented direct contact of complexes with Free ions in the sediments seldom existed. Therefore sediments. The only effect of the complexes was to increase the sorption possibilities of the medium. A -large concentration differences between medium and
Metals from sediments
2400'
251
600"
6
==
i
s
-- 1600, p i /I
800.
~/ ~00"
J
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01
time
( h )
600~
time
( h ]
600'
~
E
~' 4 0 o
400.
200-
200.
O-
O"
time
{ h )
time
( h )
Fig. 4. Dcsorption of lead. In the dialysis pipes, 10 g sediment together with 5 ml medium were resuspended. Afterwards the pipes were incubated in 100 ml solution. This solution contained (a) EDTA ( - - - ) or NTA ( ), (b) deficiency medium only or (c) the algae Chlorella saccharophila and (d) Chlorella vulgaris. The straight lines show concentrations of algae and medium, the dotted line gives the result of uptake by the strains. sediment resulted in only small increases of metals in the medium. Uptake of heavy metals by algae has often been investigated. While active uptake of zinc (Higham and Saler, 1984) and possibly cadmium (Mang and Tromballa, 1978) occurred, lead is adsorbed passively on cell surfaces (Inner, 1982). Enormous uptake of lead by this passive uptake is potentially possible (Schulz-Baides, 1976). Our idea was an uptake of metals by the algae which influence the equilibrium concentrations between medium and sediment. Therefore, successive depletion of metals by algae and a behaviour comparable to the effects of EDTA and NTA was possible. The experiments showed exactly the contrary [Figs 2(c), (d), 3(c), (d) and 4(c), (d)]. Both strains of algae had no influence on
desorption characteristics. In medium only and medium with algae, equal concentrations were detected. The uptake by the algae was very low and the highest values reached were less than 10% of the available amounts in the medium. Over wide ranges of concentrations Clorella vulgaris and Chlorella saccharophila are able to absorb up to 80% of the available lead in solution Table 2. Highest desorbed metal concentrations in mediumand EDTA. Valuesare expressedas percent of total concentrations of the sediment within the dialysispipes Metal IrDTA Medium Zinc 0.72 0.02 Cadmium 1.15 0.31 Lead 6.66 1.96
252
WOLFGANGLIETZ and GOTTFRIEDGALLING
(Lietz, 1987). The difference from earlier experiments was the altered ion composition of the medium. In the case of sediments many other ions, for example potassium, calcium, magnesium and the measured cadmium and zinc ions, diffused through the membrane and influenced uptake. Turbulence of water was another important factor which influenced metals bound to sediments. Frequent fluctuation of water contents in rivers enhance mobility of sediments. At the same time increasing possibilities of exchange of heavy metals into water exists. A short comparison of the results with the processes which occur under natural conditions follows. Caused by aeration of the solution, the experiments simulated a high turbulence in the dialysis pipes. Although the concentrations of the heavy metals in the sediment were very high, desorption only occurred in a maximum o f up to 2% o f the total concentrations. As described, the influence of the medium in laboratory experiments, was an important factor. Comparable conditions exist in rivers. Many organic substances and complexes are dissolved. They are able to enhance concentrations of metals in water (Ravera, 1984). In simulation of this the chelating agents EDTA and NTA were used. Depending upon the metal examined, both substances resulted in more or less increased desorption. The effect on zinc was enormous [28-fold increase, Fig. 2(a)]. Cadmium and lead only showed at a maximum 5-fold increase in concentration [Figs 3(a) and 4(a)]. However, one has to notice that increased heavy metal contents did not automatically lead to enhanced uptake by the algae, even though at the same time increased concentrations of complexes are available. This was measured for zinc (Ernst, 1977), cadmium (Laddaga et al,, 1985) and lead (Schulz-Baldes and Levin, 1976). Metals became desorbed and concentrations in water increased, but uptake from metals by algae which were bound to complexes was prevented. In a bay in West Australia, one strain of algae was measured which could only exist with high metal concentration levels because of increased EDTA concentrations caused by industrialization in the surrounding area (Fisher and Frood, 1980). We think we have pointed out with our results that influences of human-produced complexes on heavy metal desorption has possibly fewer harmful effects on plants than is often asserted. Herms and Brfimmer (1978) even expressed the opinion that increased concentrations of EDTA, and consequently increased heavy metal concentration, did not increase uptake by algae. REFERENCES
Ahtf W. (! 982) Untersuchungen zur Schwermetallbelastung
der Elbe unter Verwendung einer neuen BiomonitoringTechnik. Dissertation, Hamburg. Baumann A., Best G. und Kaufmann, R. (1977) Hohe Schwermetali.Gehalte in Hochflutsedimenten der Oker (Niedersachsen). DGM 21, 113-118. Davies A. G. (1970) Iron chelation and growth of marine phytoplankton. I. Growth kinetics and chlorophyll production in cultures of the euryhaline flagellate Dunaliella tertiolecta under iron-limiting conditions. J. mar. biol. Ass. U.K. 50, 65-86. Ernst W. H. O. (1977) Physiology of heavy metal resistance in plants. In Proc. int. Conf. on Heavy Metals in the Environment, Vol. 2, pp. 121-136. Fisher N. S. and Frood D. (1980) Heavy metals and marine diatoms: Influence of dissolved organic compounds on toxicity and selection for metal tolerance among four species. Mar. Biol. 59, 85-93, F6rstner U. und Stiefel R. (1978) Umweltprobleme dutch Metallanreicherungen in kommunalen Abw/issern. Chem. Z. 102, 161-168. F6rstner U., Ahlf W,, Ca|mano W., Schuman C. und S¢llhorn C. (1983) Einflu8 yon Nitriloessigs//ure auf die Sorption yon Schwerrnetallen an definierten Feststoffphasen (Calcit, Illit, Montmorillionit, Algenz¢llw~inde). Vom Wass. 61, 155-168. Gardiner J. (1975) The complexation of trace metals in natural waters. In Int. Conf. on Heavy Metals in the Envir. Syrup. Proc., Vol. 1, pp. 303-318. Garnett K., Kirk P. W. W., Lester J. N. and Perry g. (1985) The effect of nitrilotriacetic on the solubilities of metals in soil-sludge mixtures. J. envir. Qual. 14, 549-553. Herms B. und Brfimmer G. (1978) EinfluB organishcher Substanzen auf die L6slichkeit yon Schwermetallen. Mitt. dr. bodenk. Ges. 27, 18l - 192. Higham D. P. and Sadler P. J. (1984) Cadmium-resistant Pseudomonas putida synthesizes novel cadmium proteins. Science 225, 1043-1046. lrmer U. (1982) Die Wirkung der Schwermetalle Blei, Cadmium und Mangan auf die SiiBwassergrfinalge Chlamydomonas rheinhardii und Chlorella fusca. Dissertation, Hamburg. Kuhl A. (1964) Zur Physiologie der Speicherung kondensierter anorganischer Phospate in Chlorella. In Beitr~ge zur Physiologie und Morphologw der Algen Vortr. dr. Bot. Ges., pp. 157-166. Fischer, Stuttgart.
Laddaga R. A. Bessen R. and Silver S. (1985) A cadmiumresistant mutant of Bacillus subtilis 168 with reduced cadmium transport. J. Bact. 162, 1106-1110. Laxen D. P. H. (1985) Trace metal adsorption/coprecipitation on hydrous ferric oxide under realistic conditions. The role of humic substances. War. Res. 19, 1229-1236. Lietz W. (1987) Aufnahme der Schwermetalle Zink, Cadmium und Blei und deren synergistische Wirkung auf die Griinalge Chlorella saccharophila und die aus der Oker isolierte Alge Chlorella vulgaris. Dissertation, Braunschweig. Mang S. und Tromballa H. W. (1978) Aufnahme yon Cadmium durch Chlorella fusca. Z. PflPhysiol. 90, 293-302. Neeb R. (1978) lm,erse Voltametrie. Metrohm AG, Schweiz. Ravera O. (1984) Cadmium in freshwater ecosystems. Experientia 40, 2-14.
Schulz-Baldes M. and Lewin R. A. (1976) Lead uptake in two marine phytoplankton organisms. Biol. Bull. 150, !18-127. Stumm W. (1972) Die Rolle der Komplexbildung in natfirlichen Gewissern und allf~illige Beziehungen zur Eutrophierung. Gewiissers. Wass. Abwaxs. 8, 57-87.
0043-1354/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc
METALS FROM SEDIMENTS WOLFGANG LIETZ and GOTTFRIEDGALLING Botanisches Institut der TU Braunschweig, Humboldtstra~ 1, 3300 Braunschweig, F.R.G.
(First received March 1988; accepted in revised form September 1988) Abstract--River sediments were investigated relative to their desorption characteristics of zinc, cadmium and lead. We examined naturally contaminated sediments of the Oker river in the Federal Republic of Germany. Using dialysis pipes for separating sediments from the medium, a direct influenceof complexes is prevented. The only possibility of this is from diffusion through the membrane. Diffusion is dependent upon the uptake capacity of the surrounding medium. Without any metal adsorbing substances in solution, values of desorption reached a maximum of 1.96% lead, 0.31% cadmium and 0.02% zinc. The addition of 1/~M NTA or EDTA to the medium increased metal desorption. Incubation, together with these chelating agents, raised zinc values 28-fold: lead and cadmium are enhanced by a maximum 5-fold. The addition of algae to the medium has no effect on desorption. All three metals have only slightly varied concentrations when compared with the medium alone. Uptake only reaches 10-15% of the total desorbed cadmium and lead concentrations in the medium.
Key words--sediment, zinc, cadmium, lead, desorption
INTRODUCTION Today heavy metals are of enhanced importance in industry. In the last decades pollution of the environment therefore has been studied with interest. Many rivers in densely populated countries contain large amounts of metals. For example the Oker has substantially increased concentrations of zinc, lead and cadmium, compared with unpolluted streams (Baumann et aL, 1977; Stumm, 1972). Metal concentrations in rivers are able to oscillate in wide ranges (Ahlf, 1982; F6rstner and Stiefel, 1978). One important fact in varying the concentrations in water is the influence of metals bound to sediments. The heavy metal load of sediments often reaches such high levels that a sudden resorption would be a great danger (Davies, 1970; Salomons and F6rstner, 1984). Therefore sorption and desorption processes influence water quality. Occasionally transport of sediments-rich water results in increased heavy metal concentration in rivers (F6rstner and Patchineelam, 1976). Another important factor in increasing metal concentration is the existence of complexess such as humides or other organic substances. With regard to this, the influence of the complexes EDTA and NTA (both produced by humans) are of interest. Dependent from other elements and from the concentrations of complexes, they enrich metals in water. In fertilizers EDTA is able to reach concentrations of up to 100/ag/l (Gardiner, 1975), normal values of wastewaters reach 3 mg/l (F6rstner et al., 1983). Our experiments investigated the heavy metals bound to the sediments in the Oker river. We measured desorption of zinc, cadmium and lead deWR 23 '2--1
pending on the incubation time and the composition of the medium. The difference between direct and indirect reactions of extraction solution to sediments will be discussed. By using dialysis pipes for incubation, this examination gives several new points of view. MATERIALSAND METHODS Sediments from the Oker river, directly next to the Botanical Institute of the University in Braunsehweig, were used and examined in our experiments. The sediment was apparently anaerobic in the river. It contained mostly components of the silt fraction and, to a high degree, organic components. The sediment was dried at 60°C for about 24 h. Afterwards every I0 g were mixed with 5 ml of distilled water and incubated in dialysis pipes (Visking 20/32). The filled dialysis pipes were incubated in aerated glass tubes together with I00 ml of medium. This deficiency medium was prepared according to Inner (1982) without zinc, cobalt and copper ions. Desorption was detected by measuring the metal concentrations in the medium. Experiments were carried out during 44 h. At the beginning and after 4, 8, 20, 30 and at 44 h, the zinc, cadmium and lead contents of 2 ml samples were determined. We examined different desorption solutions: medium only, medium with 1/~M EDTA (ethyldiaminnitrilotetraacid) or with 1/tM NTA (nitrilotriacetic acid) and medium containing algae. Culture of Chlorella saccharophila (21 l-la) and Clorella vulgaris (2 ! 1-11b) occurred under dark: light cycles( 10:14 h at 5000 lux/Osram L40W/15-1) for 8-10 days in 100 ml glass tubes. Their medium was prepared according to Kuhl (1964) with the one exception that the zinc concentration was lowered to 0.002 g/l. Afterwards the cells were cultured for 48 h in 250 ml glass tubes. The culture took place under continued light conditions at 4500 lux in deficiencymedium. During the latter period, the algae were continuously aerated. Prior to the measurements, the algae material was washed once, incubated in fresh deficiency medium and
247
248
WOLFGANG LI£TZ and GOTTFRIED GALLING
(a) II
600
/
~300
0
I
I
4
8
,I
24 Time (h)
I
48
3000 I" (b)
_
/
° ~ locx)
o
0
4
8
24 Time (h)
48
Fig. I. Velocity of dialysis. Incubation of zinc, cadmium and lead in dialysis pipes. The pipes were incubated in 100 ml
medium. After complete diffusion of the added metals, metal concentrations of 15 I~M must result. We show the development of (a) zinc and (b) cadmium ( - - - ) and lead ( ), Cycles give results of desorption with an additional I~M EDTA in the medium.
exposed to light. Each experiment was carried out with identical dry weights of 6.7mg/10ml. The strains were grown and uptake experiments carried out at 23°C. We used a pH of 6.2. Each sample was evaporated and afterwards treated with 0.5 ml HPO 4 and heated for 20 rain at 90°C, 60 min at 150°C and for a minimum of 8 h at 190°C. The metals were measured with an ion selective electrode (Neeb, 1978). Sediment concentrations were determined with flameless atomic absorption. RESULTS
In our experiments dialysis pipes were used. The first experiment examined their permeability to metal ions (Fig. I). The heavy metals zinc, cadmium and lead in the form of their chlorides were incubated in the dialysis pipes and their diffussion through the membrane and the appearance of the surrounding medium were measured. After only l h all three metals were measurable. The intensity of diffusion decreased in time because of the decreased differences in concentration between the dialysis pipes and the medium. With all three metals, an equalization of 80% minimum occurred, at the latest, at 4 h. EDTA enhanced the activity of diffusion. After 1 h, no more increased values were measured.
Desorption of zinc is shown in Fig. 2. Varied composition of the medium led to differentcharacteristics.Addition of the medium only resulted in almost linearly increased zinc concentrations. At the end of the experiment, no more than doubled values were reached. E D T A and N T A treatment caused completely different behaviour when compared with medium only. A considerable increase of up to 28-fold was measured. At the end of the experiment metal concentrations with N T A exceeded that of E D T A . The presence of algae in the medium did not enhance the total concentrations. Desorption was equal when compared with the addition of the medium only. During the experiment only a slight increase in metals absorbed by the algae occurred. With consideration of the waving values, both algae behaved similarly. Like zinc, as in the case of cadmium in the medium without any other substances, only a small amount of desorption occurred (Fig. 3). The values increased slowly and reached 18 mg/ml. CMorella vulgaris, as well as Chlorella saccharophila, had no influence on the diffusion of heavy metal desorption. The concentrations with or without algae were equal. Uptake of cadmium by algae was extraordinarily small. The influence of complexes on desorption of metals enhanced cadmium concentrations in the medium. Comparing both complexes, EDTA possessed higher desorption capacities than N T A at the end of the experiment. With EDTA and N T A 3- and 2-fold higher values, respectively, were detected than with the medium only (Fig. 1). The values of the experiments with lead are shown in Fig. 4. During the incubation time the lead concentrations in the medium were increased. Medium only, or medium with algae, resulted in comparable curves and concentrations. Uptake of algae was very small and the increase in time was negligible. The complexes EDTA and NTA enhanced desorption characteristics up to 5-fold with E D T A and 2.5-fold with NTA. NTA showed only slightly varied values compared with EDTA. Also medium only or medium with both strains of algae did not differ significantly in desorption characteristics. Therefore, because of their typical character, the results of desorption with EDTA and medium were chosen in Table 2. It was possible to calculate the uptake, in percent of total sediment concentration, by using the values from Figs 1-3. The comparison of these two typical types of behaviour showed, in all cases, increased desorption in the
Table I. Heavy metal content of the sediment
Metal Zinc
Cadmium
Lead
Concentration (ppm) 2296.9 78.0
293.0
Standard deviation (ppm) _+328.8 + 12.9
_+66.7
Metals from sediments
1200-
249
60-
E
=
,.X//~~
fl00-
400-
40-
/
20"
°l
O-
time
/
( h )
time
l+0.
40
30-
3o
( h }
E
20"
10 ¸
o.
!
\x
1
"'x"
20
/ X/
// 04
h
E
i
time
( h )
time
( h )
Fig. 2. Desorption of zinc. In the dialysis pipes, 10g sediment together with 5 ml medium were resuspended. Afterwards the pipes were incubated in I00 ml solution. This solution contained (a) EDTA (---) or NTA ( ), (b) deficiencymedium only or (e) the algae Chlorellasaceharophila and (d) Chlorella rulgaris. The straight lines show concentrations of algae and medium, the dotted line gives the result of uptake by the strains.
sequence Zn > Cd > Pb. For that reason the strength of binding to the sediments decreased in the same sequence. If one looks only at the absolute concentrations of desorption, the high values of zinc probably displace the relations. DISCUSSION
The experiments were obtained under specific conditions. The sediment was isolated from the medium by the walls of the dialysis pipes and therefore an influence on free exchange of ions existed. The test of permeability showed sufficient diffusion velocity using the chloride salts of zinc, cadmium and lead (Fig. 1). In the sediments most of the metals were
bound to the surfaces of organic components or to more or less large inorganic complexes. The already slightly decreased diffusion rate of ions, measuring the test of permeability, indicated that chelated metals were unable to permeate through the dialysis pipes. Prior to diffusion, a change in their chemical form was therefore necessary. The higher the capacity of metal sorption of the sediments, the less often the exchange of metal into ion form, and even less often was the occurrence of diffusion through the dialysis pipes. Desorption from the sediment and also diffusion was dependent upon the composition of the medium. Normal medium contained neither sulfates phosphates nor any complexes. For this reason the possi-
250
WOLFGANG LIETZ and GOTTFRIr=D GALLING
2st
120 w
~e 80
//
"~X
/
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Fig. 3. Desorption of cadmium. In the dialysis pipes, 10g sediments together with 5 ml medium were resuspended. Afterwards the pipes were incubated in 100 ml solution. This solution contained (a) EDTA ( - - - ) or NTA ( ), (b) deficiency medium only or (c) the algae Chlorella saccharophila and (d) Chlorella vulgaris. The straight lines show concentrations of algae and medium, the dotted line gives the result of uptake by the strains.
change in the equilibrium occurred. Ions of the bilities of uptake were restricted. Under these condimedium rapidly became bound by this complex and tions concentrations in the medium increased by differences between the concentrations remained small amounts. high. The weaker the binding of this metal to the EDTA and NTA are both chemical compounds sediments, the greater the possibilities of exchanges which form complexes with metals. In the case of through the membrane and therefore higher concenEDTA no binding to surfaces exists (Gardiner, 1975). trations were detectable in the medium. Medium which contains complexes results in enorTaking into account these considerations, our remously desorbed concentrations of metals. Desorpsults indicated that more than 90% of all three metals tion increased depending on incubation time and with was firmly bound to the sediments. Nevertheless large enhance concentrations of complexes (Laxen, 1985; differences in desorption between zinc, cadmium and Garnet et aL, 1985). In contrast to the experiments which we carried out, the membrane of the dialysis lead existed. The binding of lead and cadmium to sediments was much more stable than that of zinc. pipes prevented direct contact of complexes with Free ions in the sediments seldom existed. Therefore sediments. The only effect of the complexes was to increase the sorption possibilities of the medium. A -large concentration differences between medium and
Metals from sediments
2400'
251
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6
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i
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Fig. 4. Dcsorption of lead. In the dialysis pipes, 10 g sediment together with 5 ml medium were resuspended. Afterwards the pipes were incubated in 100 ml solution. This solution contained (a) EDTA ( - - - ) or NTA ( ), (b) deficiency medium only or (c) the algae Chlorella saccharophila and (d) Chlorella vulgaris. The straight lines show concentrations of algae and medium, the dotted line gives the result of uptake by the strains. sediment resulted in only small increases of metals in the medium. Uptake of heavy metals by algae has often been investigated. While active uptake of zinc (Higham and Saler, 1984) and possibly cadmium (Mang and Tromballa, 1978) occurred, lead is adsorbed passively on cell surfaces (Inner, 1982). Enormous uptake of lead by this passive uptake is potentially possible (Schulz-Baides, 1976). Our idea was an uptake of metals by the algae which influence the equilibrium concentrations between medium and sediment. Therefore, successive depletion of metals by algae and a behaviour comparable to the effects of EDTA and NTA was possible. The experiments showed exactly the contrary [Figs 2(c), (d), 3(c), (d) and 4(c), (d)]. Both strains of algae had no influence on
desorption characteristics. In medium only and medium with algae, equal concentrations were detected. The uptake by the algae was very low and the highest values reached were less than 10% of the available amounts in the medium. Over wide ranges of concentrations Clorella vulgaris and Chlorella saccharophila are able to absorb up to 80% of the available lead in solution Table 2. Highest desorbed metal concentrations in mediumand EDTA. Valuesare expressedas percent of total concentrations of the sediment within the dialysispipes Metal IrDTA Medium Zinc 0.72 0.02 Cadmium 1.15 0.31 Lead 6.66 1.96
252
WOLFGANGLIETZ and GOTTFRIEDGALLING
(Lietz, 1987). The difference from earlier experiments was the altered ion composition of the medium. In the case of sediments many other ions, for example potassium, calcium, magnesium and the measured cadmium and zinc ions, diffused through the membrane and influenced uptake. Turbulence of water was another important factor which influenced metals bound to sediments. Frequent fluctuation of water contents in rivers enhance mobility of sediments. At the same time increasing possibilities of exchange of heavy metals into water exists. A short comparison of the results with the processes which occur under natural conditions follows. Caused by aeration of the solution, the experiments simulated a high turbulence in the dialysis pipes. Although the concentrations of the heavy metals in the sediment were very high, desorption only occurred in a maximum o f up to 2% o f the total concentrations. As described, the influence of the medium in laboratory experiments, was an important factor. Comparable conditions exist in rivers. Many organic substances and complexes are dissolved. They are able to enhance concentrations of metals in water (Ravera, 1984). In simulation of this the chelating agents EDTA and NTA were used. Depending upon the metal examined, both substances resulted in more or less increased desorption. The effect on zinc was enormous [28-fold increase, Fig. 2(a)]. Cadmium and lead only showed at a maximum 5-fold increase in concentration [Figs 3(a) and 4(a)]. However, one has to notice that increased heavy metal contents did not automatically lead to enhanced uptake by the algae, even though at the same time increased concentrations of complexes are available. This was measured for zinc (Ernst, 1977), cadmium (Laddaga et al,, 1985) and lead (Schulz-Baldes and Levin, 1976). Metals became desorbed and concentrations in water increased, but uptake from metals by algae which were bound to complexes was prevented. In a bay in West Australia, one strain of algae was measured which could only exist with high metal concentration levels because of increased EDTA concentrations caused by industrialization in the surrounding area (Fisher and Frood, 1980). We think we have pointed out with our results that influences of human-produced complexes on heavy metal desorption has possibly fewer harmful effects on plants than is often asserted. Herms and Brfimmer (1978) even expressed the opinion that increased concentrations of EDTA, and consequently increased heavy metal concentration, did not increase uptake by algae. REFERENCES
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