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Behavior of cadmium, lead and zinc at the sediment–water interface by electrochemically initiated processes
Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 261 /271 www.elsevier.com/locate/colsurfa
Behavior of cadmium, lead and zinc at the sediment water interface by electrochemically initiated processes /
Reena Shrestha a,*, R. Fischer a, D. Rahner b a
b
Institute of Water Chemistry, Dresden University of Technology, D-01062 Dresden, Germany Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, D-01062 Dresden, Germany
Abstract Nowadays, electrokinetic remediation is one of the popular and cheapest in situ remediation techniques for contaminated soils. This method uses a low-level electrical energy and is known for removal of heavy metals like, cadmium, chromium, copper, iron, lead, mercury, nickel, zinc and the metalloid arsenic from polluted and spiked soils. The driving force is the migration of those ions in the soil body by the applied electric field. In most cases the heavy metals are concentrated near the cathode depending on the actual pH profile and the solubility products of formed nonsoluble compounds (oxides, oxihydroxides, carbonates etc.). Sometimes it is also possible to deposit heavy metals directly at the cathode surface (e.g. copper, cadmium). Often it is necessary to add an acid to prevent the precipitation during such an electrochemical process. Therefore, the mobilization and accumulation of cadmium, lead and zinc at the sediment /water interface with different positions and conditions of the electrode arrangement was studied. The tests were carried out with a natural heavy metal containing sediment from the river Weisse Elster of Germany. The results showed that they were mobilized by the effects of the anodic polarization and transported by migration from sediment into the water /sediment interface. By constructing a pH-barrier at the sediment /water interface, those metals were precipitated at the steep pH-gradient. The metals were accumulated at the sediment /water interface. In the opposite situation, where the cathode is at the sediment surface, the alkaline front penetrates into the sediment and a fixation of amphoteric metals at weak alkaline and a mobilization at strong alkaline conditions occurs. In our view, the electrode arrangement anode in the sediment /cathode in the water body with a relatively small distance between both electrodes to create a steep pH- and Eh-gradient is the best arrangement for the predicted mobilization and accumulation of those heavy metals. After this it should be possible to mobilize them in a relative short time and to remove the concentrated heavy metals from the solid phase by reversing the polarity of the electrodes. This will be a possibility to support the insitu-cleaning of sediments by constructing a pH- and Eh-barrier to eliminate the heavy metals. # 2003 Published by Elsevier B.V. Keywords: Mobilization/accumulation; Heavy metals; pH; Redox barrier; Sediment
1. Introduction * Corresponding author. E-mail address: [email protected] Shrestha).
(R.
0927-7757/03/$ - see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0927-7757(03)00231-0
The study of the mobilization and accumulation of cadmium, lead and zinc at the sediment /water interface was carried out by using electrochemi-
262
R. Shrestha et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 261 /271
cally initiated processes and reactions at or nearby the electrodes. In these processes, pH- and redoxbarrier were established. The water in the immediate vicinity of the electrodes is electrolyzed by applying a constant voltage, which generates H ions at the anode and OH -ions at the cathode by the following primary electrolysis reactions: 2H2 O4e 0 O2 4H (anode) (cathode) 4H2 O4e 0 2H2 4OH
(1) (2)
Depending on the electrode material, the hydrogen evolution is dominated in the process. Secondary chemical and electrochemical reactions may exist depending upon the concentration of available species, e.g. 4Men 4H O2 0 4Me(n1) 2H2 O (redox barrier) Men ne 0 Me(s) Men nOH 0 Me(OH)n (s) (pHbarrier) Me2 HCO 0 MeCO3 (s)H2 O 3 OH
(3)
2. Experimental section
(4)
2.1. Experimental setup
(5)
The experimental assembly is shown schematically in Fig. 2. PVC-tubes (length: 0.6 m, width: 0.14 m) were filled up of 1700 g sediment without compression (volume: 1.36 l, about 20% of the column volume) from the river Weisse Elster, near Kleindalzig, Germany, which is well known as former mining area and tap water (volume: 5.43 l, about 80% of the column volume). Volume: 1.36 l, about 20% of the volume). The initial physicochemical properties of the sediment samples are shown in Table 1, the quality of the overlying water in Table 2 and the binding forms of these metals in Table 3 [11]. After filling the columns, two electrodes according to the experimental variants were installed (Fig. 3). The cathode material should have a high hydrogen overvoltage as opposed to the anode material, which should have small oxygen overvoltage. The anode should remain stable under conditions of current flow. A further important criterion is, that the material should not have ecotoxicological nature. Nothing that may be released by anodic dissolution during the process should produce environmentally harmful reaction products. Therefore, the electrode was taken of conductive polymer (polyethylene with carbon black) with a specific resistance of about 20 V cm 1. The middle coverage of the electrode in relation to the sediment surface amount was to
(6)
Me refers to a metal. The secondary reactions (Eqs. (4) /(6)) might be favored at the cathode. In the case of the positioning */the anode in the sediment and the cathode in the water */due to the acidic solution near the anode, the displacement of the metal from the sediment/soil occurs as follows: nH (Sediment)n Men 0 (Sediment)n nH Men
soils [1 /7]. Extraction rates of over 90%, when both electrodes are positioned in the soil are reported with species seemed to precipitate with OH-ions near the cathode [5]. However, it could also be shown that conditioning the catholyte by adding sulfuric acid avoided too high hydroxide concentrations and lead to high removal rates (98.5%) of cadmium, from the clayey material [8]. As the behavior of Cd, Zn and Pb at the interface sediment /water was not well understood in concerning the mobilization and accumulation of these heavy metals at sediment /water interface with different positions of electrodes, this study was carried out.
(7)
At later stages, when the basic front reaches the sediment /water interface the mobilized metal ion or complexes are precipitated as hydroxides or carbonates. Consequently, an enrichment of the heavy metals occurs (Fig. 1). Another important aspect of electrochemically initiated processes is the application of a low level current density in the order of magnitude mA cm 2 of the electrode area to extract the metals from the sediment. These processes had a great potential to extract heavy metals like cadmium, chromium, copper, iron, lead, mercury, nickel, zinc, some organic compounds, radioactive nuclides and arsenic from the polluted and spiked
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263
Fig. 1. A sketch scheme of the electrokinetic migration processes.
control instruments. There were two fixed measurement points for the determination of pH-value and O2-concentration in the aqueous phase. One point was due directly to the cathode, the other directly at the sediment /water interface. Furthermore for the taking of water samples, two sampling pipes have been installed. At the end of the pipes a Pt-wire for the measuring of the redox potential was positioned. A small fermenting tube was installed to guarantee pressure balance. A second similar column, but without power supply, served as a reference system. All the experiments were carried out at room temperature without light to prevent from unnecessary development of oxygen by autotrophic organisms. 2.2. Sampling, analysis Fig. 2. Experimental assembly.
about 20%. It was found that even only 12% coverage did not affect significantly in the migration of iron and manganese [12]. The voltage between the electrodes was U/3 V and the electric field strength 37.5 V m 1. Voltage and distances of the electrodes from the sediment surface could be adjusted by external
As the migration and accumulation of the metals’ phenomenon in the sediment needs long time about years to complete the process, one could not make a perfect picture from initial and final characteristics of the sediment in the short time. Even if the initial concentration was taken as the account, it could always see the continuous migration of metals towards the interface. It is difficult to determine the concentration of metals by taken the sediment from the column periodi-
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Table 1 Physico-chemical parameters of the used sediment Parameters
Methods
pH
DIN 38404C5 Redox potential DIN 38404C5 Conductivity DIN 38404C5 Total organic compounds TOC analyzera Not or small biologically degradable BOD/TOC organic substances, especially humic substances Total sulfur content RFAa Sulfidic bound sulfur As Co Cr Cu Fe (predominantly as FeOOH) Mn (predominantly as MnO2) Ni Cd Pb Zn a
Table 2 Characteristics of the used tap water Values
Qualitative parameters
Values
5.85
pH Oxygen content (mg l 1) Conductivity (mS cm 1) Total hardness (8dH) Ba2 (mg l 1) K (mg l 1) Na (mg l 1) Ca2 (mg l 1) Mg2 (mg l 1) Total iron (mg l 1) Mn2 (mg l 1) NO2 (mg l 1) NO3 (mg l 1) Cl (mg l 1) SO2 (mg l 1) 4 PO3 (mg l 1) 4 2 (mg l 1) Pb Cd2 (mg l1) Zn2 (mg l 1)
8.1 10.0 272 6.6 B/0.1 2 5.4 40.4 4.0 B/0.05 0.02 B/0.01 14.7 7.4 40.8 0.01 B/2.0 0.83 0.04
166 mV 2.64 mS cm 1 9.4% 5.7%
14.9 g kg 1 DIN 51724- 9.65 g 2 kg 1 AAS 28.6 mg kg 1 AAS 40.4 mg kg 1 AAS 235.9 mg kg 1 AAS 295 mg kg 1 AAS 68.5 g kg 1 AAS 600 mg kg 1 AAS 275.9 mg kg 1 AAS 30 mg kg 1 AAS 237.5 mg kg 1 AAS 2.72 g kg 1
UFZ-Report, No. 13/2002, 1993 [9].
cally, which could affect the total mass of the sediment in the column. The trend of accumulation and mobilization can be observed clearly by measured the water samples from the interface. Therefore, the volume of the taken samples was each 10 ml, that is about 0.1% of the total volume. The water sample did not affect the total conditions of the system [13]. The taken volume was refilled with the same volume of tap-water after sampling. The metal concentrations in the water, in the interstitial and in the sediment were measured by using atomic absorption spectro-
metry (AAS), atomic absorption spectrometry with induced couple plasma (AAS-ICP) and with atomic emission spectrometry (ICP-AES). For pH- and O2-measurements, ion sensitive electrodes (WTW, Weilheim, Germany) were used. The oxygen concentrations were measured approximately 35 cm above the sediment surface near the cathode.
3. Results and discussion 3.1. Determination of the required minimal working voltage The working voltage required for the electrode system depends on the water quality, sediment composition, redox reactions at/nearby the electrodes, distance between the electrodes and the electrode material. The minimum voltage was determined by establishing a current /voltage curve in the pre-operational test. It was carried out by measuring the current with stepwise increment of voltage in the range of U/0/10 V in the sediment /water system where the cathode is 4 cm above and the anode 4 cm below the sediment surface. The electrodes were first polarized in the
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Table 3 Percentage of Cd, Pb and Zn in the sediment determined through sequential extraction after Fo¨rstner and Calmano (1982) [10] Binding forms of the metals in the examined sediment
Cd (%)
Pb (%)
Zn (%)
Exchangeable Carbonates Oxides or hydroxides Sulfide or organometals Residual Rest
0.5 1 44 23 29 2.5
1 10 21 2 50 16
5 18 65 2 10 0
anodic and then in the cathodic direction. Depending on inhibition of anodic and cathodic refluxes, a shift of the anodic and cathodic curve (hysteresis) was measured. From the graph (Fig. 4) it follows that the intersection point of the approximately linear rise of the U/I curve in the anodic direction with the abscissa at I/0 A represents the minimum necessary voltage. In this case the minimum voltage was at about 2.9 V. Only during exceeding of these values the electrode polarization and overvoltage would be compensated and redox processes at the electrodes were also observed. Therefore, at our tests a voltage of U/3 V is used. 3.2. Electrode arrangement 3.2.1. Anode at the sediment At this experimental series, the anode was positioned at the sediment and the cathode 8 cm above it. There are six phases, as shown in Fig. 5. In the currentless phase I nearly neutral conditions at the sediment /water interface are observed. The heavy metal concentrations at the sediment /water interface are relatively low and conducted about 25 mg l 1 Cd, 5 mg l 1 Pb and 2.500 mg l 1 Zn. In the phase II (U /3 V) a strong increase of Cd-, Pb- and Zn-concentration is measured, because of the anodic H -production at the sediment /water interface. The pH was lowed up to pH :/2.0. At these conditions Cd is mobilized from 50 to 240 mg l1, Pb from 5 to 85 mg l 1 and Zn from 5200 to 42 000 mg l 1 (Fig. 5). After about 7 days the pH increases slowly again, because of the impact of the cathode. Cathodically produced OH -ions migrate towards the anode and neutralize the protons partially. The next currentless phase III is characterized by the further neutralization of the
protons. It results nearly neutral conditions at the sediment /water interface and relatively small heavy metal concentrations. The phases IV and VI with repeated current influence (I /3 mA) show a pH-decrease again, with a similar concentration development as in phase II. However, the pH decrease is by far not so great as in the initial phase II. The exact reason is still unclear. It is presumed, that the precipitated heavy metal hydroxides and carbonates at the sediment /water interface have a raised buffer capacity. The anodically produced protons are buffered through this layer. The rate of mobilization depends on individual ions. Cd: Starting from the small soluble Cd-substances CdS, Cd(OH)2, CdCO3 the Cd-solubility rises with decreasing pH value in the pH-range B/
Fig. 3. Different positions and conditions of electrodes. A: reference column without current influence. B: anode at the sediment surface, cathode 8 cm above the sediment surface. C: anode 4 cm below and cathode 4 cm above the sediment surface at various voltages. D: anode 8 cm below the sediment surface, cathode at the sediment surface.
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Fig. 4. Current /voltage curve for determination of the least voltage (electrode material: polyethylene with carbon black, distance between the electrodes. 8 cm, anode 4 cm below and cathode 4 cm above the sediment).
10. Through the anodic influence the pH decrease up to 2 and the Cd-concentration is increasing. The solubility minimum is at about pH 10 for CdCO3 and pH 11 /12 at Cd(OH)2. At pH-values more than 10 and 12, respectively, the solubility increases again, caused by the formation of hydroxo and humic complexes. Pb: The entered lead is transformed into substances strongly bonding with sediment and/or just in combination forms. Pb is bounded most strongly from all heavy metals as PbS under anaerobic conditions and by specific adsorption processes, particularly, Fe-, Al- and Mn-oxides. So a high bonding capacity for Pb in sediments was seen increasing with the pH. The Pb solubility is determined from the pH-value and the redox conditions. Unlike Pb shows normally a very small solubility at pH-values /5.0 and is relatively immobile in sediments. The solubility, shift and availability of Pb increases first with pH-values B/ 4.0. Zn: It is well-known, that at pH-values B/7 the solubility of zinc sulfides, hydroxides and carbonates increases with decreasing pH. In addition the Zn affinity to humic substances and Mn- and Fe-
hydroxides decreases very strongly at pH values B/5.0 [14]. Therefore, the Zn content of the soil solution rises with decreasing pH. In phase III, there was an pH increase up to /6.0. Here the heavy metals were only slight or not mobilized. With a rise in the pH values /7, it accepts that the Zn affinity to Mn- and Fe-oxides increases strongly too. If the pH at the sediment surface decreases not under pH 3 (phase IV and VI) no significant Zn-mobilization occurs.
3.2.2. Cathode at sediment When the cathode was at the sediment surface and the anode 8 cm below inside the sediment the sediment /water interface was at first alkaline during application of voltage. Due to the production of OH -ions, the pH-value increased from / 5.5 to 12.0 in phase II, to 10 in phase IV and to 9.0 in phase VI (Fig. 6). This is a favorable range for Cd, Pb and Zn to be absorbed and/or precipitated as explained before. Cd: In the reference column in the first 70 days an increase in Cd-concentration by microbial sulfide oxidation was observed. The microbial
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267
Fig. 5. pH-values and metal concentrations of Cd, Pb and Zn at the sediment /water interface [Conc.(RC) and pH(RC): metal concentration and pH in the reference column without voltage supply, Conc.(EC) and pH(EC): metal concentration and pH in the experimental column with anode at the sediment without/with voltage, (U/3 V)].
activity was high due to high organic content in the phases I and II. After these phases, microorganism were not so active, having less oxygen in the semi closed system.
CdS2O2 (chemoautotrophic microorganisms) 0 Cd2 SO2 4
(8)
Then the Cd-concentration is slowly decreasing,
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Fig. 6. pH-values and metal concentrations of Cd, Pb and Zn at the sediment /water interface [Conc.(RC) and pH(RC): metal concentration and pH of reference column without voltage supply, Conc.(EC) and pH(EC): metal concentration and pH of experimental column with cathode at the sediment without/with voltage, (U/3 V)].
because at near neutral pH-conditions Cd is precipitated as Cd(OH)2. In the phases with current, through the OH-ions made cathodically, there was a fast Cd-fixation in the form of Cd(OH)2, or CdCO3 [15] after carrying out following reactions:
Cd2 HCO 0 CdCO3 (s)H2 O 3 OH
Cd2 2OH 0 Cd(OH)2 (s)
(9) (10)
In this case, pH-values around 8 are sufficient in order to fix almost completely the cadmium.
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Pb: Relatively low initial Pb-values in the water were obtained at the equilibrium stage in the original sediment because of the strongly bonding of Pb as PbS at the sediment lattice. The reason of the increasing Pb-concentration in the reference and the currentless phases III and V in the experimental column was also the microbial sulfide oxidation, analogous to Eq. (8). When the pH-value decreases in the currentless phase after the current influenced phase, one can observed Pb-concentration up to 20/30 mg l 1 (phase III). In phase V, it can be observed, that the Pb-mobilization is not so much, although the pH also increase to values of about 7. The reason is the microbially formed sulfate reacted with the Pb2-ions to relatively insoluble PbSO4. In the current phases a strong decrease of the Pbconcentration as a result of the Pb(OH)2 precipitation occurs. At pH 11, no Pb can be measured in the solution. If the critical pH-value of about 11 is exceeded the formation of the soluble Pb-trihydroxocomplexes occurs (Eq. (12)). Pb2 2OH 0 Pb(OH)2 (s) Pb(OH)2 (s)OH H2 O
(11)
0 [Pb(OH)3 H2 O]
(12)
They are to be recognized first tendencies at the end of the phase II. Zn: The small increase of Zn-concentration in the first phase reduced to the microbial ZnS-oxidation with entered oxygen after ZnS2O2 (chemoautotrophic microorganisms) 0 Zn2 SO2 4
(13)
2
In addition to Zn and ZnOH , at pH values / 6.5 Zn(OH)02, ZnCO03 and other Zn species are available. With increasing pH-value oxides of Mn and Fe can adsorb particularly Zn2, ZnOH and Zn-complexes. Therefore, the higher the pH value the smaller is the Zn-concentration because of the formation of solid Zn(OH)2 (Eqs. (14) / (16)). Zn2 OH 0 ZnOH
ZnOH OH 0 Zn(OH)02
Zn(OH)02
0 Zn(OH)2 (s)
(14) (15) (16)
269
At high pH-values pH /11 significant amounts of the soluble Zn-complex [Zn(OH)3] (l) are produced. Zn(OH)2 (s)OH 0 [Zn(OH)3 ] (l):
(17)
At a small scale, this effect was measured at the end of the phase II. In the currentless phases the pH-value decreased slowly because of the buffering effect of the precipitated solids. 3.2.3. Cathode 4 cm above and anode 4 cm below the sediment In this experiment the cathode was placed 4 cm above and the anode 4 cm below the sediment (Fig. 7). The current flow through the system during phase IV was negligible in comparison to phase II (the figure is not shown here). The reason is, that in the phase IV the cathode was covered by a small insulating layer of carbonates and therefore, no current flow was observed in the system. One can handle phase IV as passing without current. In phase II the pH value increases slowly to values of 12. At the same way the Cd- and Znconcentrations sink to values near zero. Pb is only available at very small amounts at the sediment/ water interface and hardly changes in its concentration. In the phases III, IV and V the heavy metal concentration increases to values, measured that in the reference column.
4. Conclusions The trend of mobilization and accumulation of cadmium, lead and zinc was studied in a small scale. The first results show that the examined heavy metals are mobilized by the effects of the anodic polarization and transported by migration from the sediment into the water /sediment interface. By constructing of a pH-barrier at the sediment /water interface, those metals were precipitated at the steep pH-gradient. The metals were accumulated at the sediment /water interface. With increasing voltage in the range of 3 /4 V the mobilization of the heavy metals from the sediment and the immobilization at the sediment/ water interface took place faster. In the case where
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Fig. 7. pH-values and metal concentrations of Cd, Pb and Zn at the sediment /water interface [Conc.(RC) and pH(RC): metal concentration and pH in the reference column without voltage supply, Conc.(EC) and pH(EC): metal concentration and pH in the experimental column with anode 4 cm below and cathode 4 cm above at the sediment with/without voltage, (U/3 V)].
R. Shrestha et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 261 /271
the cathode is positioned at the sediment surface this arrangement did not allow those metals to migrate to the water /sediment interface. In our view, the electrode arrangement anode in the sediment /cathode in the water body with a relatively small distance between both electrodes to create a steep pH- and Eh-gradient is the best arrangement for the calculated mobilizing and enrichment of those heavy metals in a relative small layer. If necessary the electrodes can be combined with a counter anode to neutralize OH -ions, that migrate in the water body. This will be a possibility to support the in-situ-cleaning of sediments by constructing a pH- and Eh-barrier to accumulate the heavy metals. After this it should be possible to mobilize them in a relative short time and to remove the concentrated heavy metals containing solution from the solid phase by reversing the polarity of the electrodes.
Acknowledgements Authors acknowledge the support from DFG / GK scholarship council and all our friends who helped during this work.
References [1] S. Pamukcu, J.K. Wittle, Environ. Prog. 11 (3) (1992) 241 / 250.
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[2] Y.B. Acar, R.J. Gale, A.N. Alshawabkeh, R.E. Marks, S. Puppala, M. Bricka, R. Parker, J. Hazard. Mater. 42 (2) (1995) 117 /137. [3] C.D. Cox, A. Shoesmith, M.M. Ghosh, Environ. Sci. Technol. 30 (6) (1996) 1933 /1938. [4] B.A. Reed, M.T. Berg, J.C. Thompson, J.H. Hatfield, J. Environ. Eng. */Asce 121 (11) (1995) 805 /815. [5] Z. Li, J. Yu, W. Neretnieks, J. Contam. Hydrol. 22 (1996) 241 /256. [6] Y.B. Acar, A.N. Alshawabkeh, J. Geotech. Eng. 122 (3) (1996) 173 /185. [7] P. Marceau, P. Broquet, Bull. Eng. Geol. Environ. 57 (1999) 377 /385. [8] P. Marceau, P. Broquet, P. Baticle, C. R. Acad. Sci. Paris, Earth Planet. Sci. 328 (1999) 37 /43. [9] C. Lo¨ser, A. Zehnsdorf, P. Hoffmann, H. Seidel, UFZBericht, No. 13/2002 (2002) ISSN 0948-9452. [10] U. Fo¨rstner, W. Calmano, Bindungsformen von Schwermetallen in Baggerschla¨mmen, Vom Wasser 59 (1982) 83 / 92. [11] R. Shrestha, R. Fischer, Proceedings of des DFG */ Graduiertenkolleg, Geowissenschaftliche und Geotechnische Umweltforschung, an der TU Bergakademie Freiberg, 18 (2001) 179 /185. [12] R. Fischer, C. Wolf, D. Rahner, L. Paul, T. Deppe, C.E.W. Steinberg, Aquat. Sci. (in press). [13] Cyfluthrin, Emulsion, Oil in Water, WHO/IS/98.2.3.R1, 1999. [14] P. Schachtschabel, H.P. Blume, G. Bru¨mmer, K.H. Hartge, U. Schwertmann, Lehrbuch der Bodenkunde, Ferdinand Enke Verlag, Stuttgart, 1998 (Kap. 7). [15] U. Herms, G.W. Bru¨mmer, Landw. Forsch. 33 (1980) 408 /423.
Behavior of cadmium, lead and zinc at the sediment water interface by electrochemically initiated processes /
Reena Shrestha a,*, R. Fischer a, D. Rahner b a
b
Institute of Water Chemistry, Dresden University of Technology, D-01062 Dresden, Germany Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, D-01062 Dresden, Germany
Abstract Nowadays, electrokinetic remediation is one of the popular and cheapest in situ remediation techniques for contaminated soils. This method uses a low-level electrical energy and is known for removal of heavy metals like, cadmium, chromium, copper, iron, lead, mercury, nickel, zinc and the metalloid arsenic from polluted and spiked soils. The driving force is the migration of those ions in the soil body by the applied electric field. In most cases the heavy metals are concentrated near the cathode depending on the actual pH profile and the solubility products of formed nonsoluble compounds (oxides, oxihydroxides, carbonates etc.). Sometimes it is also possible to deposit heavy metals directly at the cathode surface (e.g. copper, cadmium). Often it is necessary to add an acid to prevent the precipitation during such an electrochemical process. Therefore, the mobilization and accumulation of cadmium, lead and zinc at the sediment /water interface with different positions and conditions of the electrode arrangement was studied. The tests were carried out with a natural heavy metal containing sediment from the river Weisse Elster of Germany. The results showed that they were mobilized by the effects of the anodic polarization and transported by migration from sediment into the water /sediment interface. By constructing a pH-barrier at the sediment /water interface, those metals were precipitated at the steep pH-gradient. The metals were accumulated at the sediment /water interface. In the opposite situation, where the cathode is at the sediment surface, the alkaline front penetrates into the sediment and a fixation of amphoteric metals at weak alkaline and a mobilization at strong alkaline conditions occurs. In our view, the electrode arrangement anode in the sediment /cathode in the water body with a relatively small distance between both electrodes to create a steep pH- and Eh-gradient is the best arrangement for the predicted mobilization and accumulation of those heavy metals. After this it should be possible to mobilize them in a relative short time and to remove the concentrated heavy metals from the solid phase by reversing the polarity of the electrodes. This will be a possibility to support the insitu-cleaning of sediments by constructing a pH- and Eh-barrier to eliminate the heavy metals. # 2003 Published by Elsevier B.V. Keywords: Mobilization/accumulation; Heavy metals; pH; Redox barrier; Sediment
1. Introduction * Corresponding author. E-mail address: [email protected] Shrestha).
(R.
0927-7757/03/$ - see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0927-7757(03)00231-0
The study of the mobilization and accumulation of cadmium, lead and zinc at the sediment /water interface was carried out by using electrochemi-
262
R. Shrestha et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 261 /271
cally initiated processes and reactions at or nearby the electrodes. In these processes, pH- and redoxbarrier were established. The water in the immediate vicinity of the electrodes is electrolyzed by applying a constant voltage, which generates H ions at the anode and OH -ions at the cathode by the following primary electrolysis reactions: 2H2 O4e 0 O2 4H (anode) (cathode) 4H2 O4e 0 2H2 4OH
(1) (2)
Depending on the electrode material, the hydrogen evolution is dominated in the process. Secondary chemical and electrochemical reactions may exist depending upon the concentration of available species, e.g. 4Men 4H O2 0 4Me(n1) 2H2 O (redox barrier) Men ne 0 Me(s) Men nOH 0 Me(OH)n (s) (pHbarrier) Me2 HCO 0 MeCO3 (s)H2 O 3 OH
(3)
2. Experimental section
(4)
2.1. Experimental setup
(5)
The experimental assembly is shown schematically in Fig. 2. PVC-tubes (length: 0.6 m, width: 0.14 m) were filled up of 1700 g sediment without compression (volume: 1.36 l, about 20% of the column volume) from the river Weisse Elster, near Kleindalzig, Germany, which is well known as former mining area and tap water (volume: 5.43 l, about 80% of the column volume). Volume: 1.36 l, about 20% of the volume). The initial physicochemical properties of the sediment samples are shown in Table 1, the quality of the overlying water in Table 2 and the binding forms of these metals in Table 3 [11]. After filling the columns, two electrodes according to the experimental variants were installed (Fig. 3). The cathode material should have a high hydrogen overvoltage as opposed to the anode material, which should have small oxygen overvoltage. The anode should remain stable under conditions of current flow. A further important criterion is, that the material should not have ecotoxicological nature. Nothing that may be released by anodic dissolution during the process should produce environmentally harmful reaction products. Therefore, the electrode was taken of conductive polymer (polyethylene with carbon black) with a specific resistance of about 20 V cm 1. The middle coverage of the electrode in relation to the sediment surface amount was to
(6)
Me refers to a metal. The secondary reactions (Eqs. (4) /(6)) might be favored at the cathode. In the case of the positioning */the anode in the sediment and the cathode in the water */due to the acidic solution near the anode, the displacement of the metal from the sediment/soil occurs as follows: nH (Sediment)n Men 0 (Sediment)n nH Men
soils [1 /7]. Extraction rates of over 90%, when both electrodes are positioned in the soil are reported with species seemed to precipitate with OH-ions near the cathode [5]. However, it could also be shown that conditioning the catholyte by adding sulfuric acid avoided too high hydroxide concentrations and lead to high removal rates (98.5%) of cadmium, from the clayey material [8]. As the behavior of Cd, Zn and Pb at the interface sediment /water was not well understood in concerning the mobilization and accumulation of these heavy metals at sediment /water interface with different positions of electrodes, this study was carried out.
(7)
At later stages, when the basic front reaches the sediment /water interface the mobilized metal ion or complexes are precipitated as hydroxides or carbonates. Consequently, an enrichment of the heavy metals occurs (Fig. 1). Another important aspect of electrochemically initiated processes is the application of a low level current density in the order of magnitude mA cm 2 of the electrode area to extract the metals from the sediment. These processes had a great potential to extract heavy metals like cadmium, chromium, copper, iron, lead, mercury, nickel, zinc, some organic compounds, radioactive nuclides and arsenic from the polluted and spiked
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Fig. 1. A sketch scheme of the electrokinetic migration processes.
control instruments. There were two fixed measurement points for the determination of pH-value and O2-concentration in the aqueous phase. One point was due directly to the cathode, the other directly at the sediment /water interface. Furthermore for the taking of water samples, two sampling pipes have been installed. At the end of the pipes a Pt-wire for the measuring of the redox potential was positioned. A small fermenting tube was installed to guarantee pressure balance. A second similar column, but without power supply, served as a reference system. All the experiments were carried out at room temperature without light to prevent from unnecessary development of oxygen by autotrophic organisms. 2.2. Sampling, analysis Fig. 2. Experimental assembly.
about 20%. It was found that even only 12% coverage did not affect significantly in the migration of iron and manganese [12]. The voltage between the electrodes was U/3 V and the electric field strength 37.5 V m 1. Voltage and distances of the electrodes from the sediment surface could be adjusted by external
As the migration and accumulation of the metals’ phenomenon in the sediment needs long time about years to complete the process, one could not make a perfect picture from initial and final characteristics of the sediment in the short time. Even if the initial concentration was taken as the account, it could always see the continuous migration of metals towards the interface. It is difficult to determine the concentration of metals by taken the sediment from the column periodi-
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Table 1 Physico-chemical parameters of the used sediment Parameters
Methods
pH
DIN 38404C5 Redox potential DIN 38404C5 Conductivity DIN 38404C5 Total organic compounds TOC analyzera Not or small biologically degradable BOD/TOC organic substances, especially humic substances Total sulfur content RFAa Sulfidic bound sulfur As Co Cr Cu Fe (predominantly as FeOOH) Mn (predominantly as MnO2) Ni Cd Pb Zn a
Table 2 Characteristics of the used tap water Values
Qualitative parameters
Values
5.85
pH Oxygen content (mg l 1) Conductivity (mS cm 1) Total hardness (8dH) Ba2 (mg l 1) K (mg l 1) Na (mg l 1) Ca2 (mg l 1) Mg2 (mg l 1) Total iron (mg l 1) Mn2 (mg l 1) NO2 (mg l 1) NO3 (mg l 1) Cl (mg l 1) SO2 (mg l 1) 4 PO3 (mg l 1) 4 2 (mg l 1) Pb Cd2 (mg l1) Zn2 (mg l 1)
8.1 10.0 272 6.6 B/0.1 2 5.4 40.4 4.0 B/0.05 0.02 B/0.01 14.7 7.4 40.8 0.01 B/2.0 0.83 0.04
166 mV 2.64 mS cm 1 9.4% 5.7%
14.9 g kg 1 DIN 51724- 9.65 g 2 kg 1 AAS 28.6 mg kg 1 AAS 40.4 mg kg 1 AAS 235.9 mg kg 1 AAS 295 mg kg 1 AAS 68.5 g kg 1 AAS 600 mg kg 1 AAS 275.9 mg kg 1 AAS 30 mg kg 1 AAS 237.5 mg kg 1 AAS 2.72 g kg 1
UFZ-Report, No. 13/2002, 1993 [9].
cally, which could affect the total mass of the sediment in the column. The trend of accumulation and mobilization can be observed clearly by measured the water samples from the interface. Therefore, the volume of the taken samples was each 10 ml, that is about 0.1% of the total volume. The water sample did not affect the total conditions of the system [13]. The taken volume was refilled with the same volume of tap-water after sampling. The metal concentrations in the water, in the interstitial and in the sediment were measured by using atomic absorption spectro-
metry (AAS), atomic absorption spectrometry with induced couple plasma (AAS-ICP) and with atomic emission spectrometry (ICP-AES). For pH- and O2-measurements, ion sensitive electrodes (WTW, Weilheim, Germany) were used. The oxygen concentrations were measured approximately 35 cm above the sediment surface near the cathode.
3. Results and discussion 3.1. Determination of the required minimal working voltage The working voltage required for the electrode system depends on the water quality, sediment composition, redox reactions at/nearby the electrodes, distance between the electrodes and the electrode material. The minimum voltage was determined by establishing a current /voltage curve in the pre-operational test. It was carried out by measuring the current with stepwise increment of voltage in the range of U/0/10 V in the sediment /water system where the cathode is 4 cm above and the anode 4 cm below the sediment surface. The electrodes were first polarized in the
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Table 3 Percentage of Cd, Pb and Zn in the sediment determined through sequential extraction after Fo¨rstner and Calmano (1982) [10] Binding forms of the metals in the examined sediment
Cd (%)
Pb (%)
Zn (%)
Exchangeable Carbonates Oxides or hydroxides Sulfide or organometals Residual Rest
0.5 1 44 23 29 2.5
1 10 21 2 50 16
5 18 65 2 10 0
anodic and then in the cathodic direction. Depending on inhibition of anodic and cathodic refluxes, a shift of the anodic and cathodic curve (hysteresis) was measured. From the graph (Fig. 4) it follows that the intersection point of the approximately linear rise of the U/I curve in the anodic direction with the abscissa at I/0 A represents the minimum necessary voltage. In this case the minimum voltage was at about 2.9 V. Only during exceeding of these values the electrode polarization and overvoltage would be compensated and redox processes at the electrodes were also observed. Therefore, at our tests a voltage of U/3 V is used. 3.2. Electrode arrangement 3.2.1. Anode at the sediment At this experimental series, the anode was positioned at the sediment and the cathode 8 cm above it. There are six phases, as shown in Fig. 5. In the currentless phase I nearly neutral conditions at the sediment /water interface are observed. The heavy metal concentrations at the sediment /water interface are relatively low and conducted about 25 mg l 1 Cd, 5 mg l 1 Pb and 2.500 mg l 1 Zn. In the phase II (U /3 V) a strong increase of Cd-, Pb- and Zn-concentration is measured, because of the anodic H -production at the sediment /water interface. The pH was lowed up to pH :/2.0. At these conditions Cd is mobilized from 50 to 240 mg l1, Pb from 5 to 85 mg l 1 and Zn from 5200 to 42 000 mg l 1 (Fig. 5). After about 7 days the pH increases slowly again, because of the impact of the cathode. Cathodically produced OH -ions migrate towards the anode and neutralize the protons partially. The next currentless phase III is characterized by the further neutralization of the
protons. It results nearly neutral conditions at the sediment /water interface and relatively small heavy metal concentrations. The phases IV and VI with repeated current influence (I /3 mA) show a pH-decrease again, with a similar concentration development as in phase II. However, the pH decrease is by far not so great as in the initial phase II. The exact reason is still unclear. It is presumed, that the precipitated heavy metal hydroxides and carbonates at the sediment /water interface have a raised buffer capacity. The anodically produced protons are buffered through this layer. The rate of mobilization depends on individual ions. Cd: Starting from the small soluble Cd-substances CdS, Cd(OH)2, CdCO3 the Cd-solubility rises with decreasing pH value in the pH-range B/
Fig. 3. Different positions and conditions of electrodes. A: reference column without current influence. B: anode at the sediment surface, cathode 8 cm above the sediment surface. C: anode 4 cm below and cathode 4 cm above the sediment surface at various voltages. D: anode 8 cm below the sediment surface, cathode at the sediment surface.
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Fig. 4. Current /voltage curve for determination of the least voltage (electrode material: polyethylene with carbon black, distance between the electrodes. 8 cm, anode 4 cm below and cathode 4 cm above the sediment).
10. Through the anodic influence the pH decrease up to 2 and the Cd-concentration is increasing. The solubility minimum is at about pH 10 for CdCO3 and pH 11 /12 at Cd(OH)2. At pH-values more than 10 and 12, respectively, the solubility increases again, caused by the formation of hydroxo and humic complexes. Pb: The entered lead is transformed into substances strongly bonding with sediment and/or just in combination forms. Pb is bounded most strongly from all heavy metals as PbS under anaerobic conditions and by specific adsorption processes, particularly, Fe-, Al- and Mn-oxides. So a high bonding capacity for Pb in sediments was seen increasing with the pH. The Pb solubility is determined from the pH-value and the redox conditions. Unlike Pb shows normally a very small solubility at pH-values /5.0 and is relatively immobile in sediments. The solubility, shift and availability of Pb increases first with pH-values B/ 4.0. Zn: It is well-known, that at pH-values B/7 the solubility of zinc sulfides, hydroxides and carbonates increases with decreasing pH. In addition the Zn affinity to humic substances and Mn- and Fe-
hydroxides decreases very strongly at pH values B/5.0 [14]. Therefore, the Zn content of the soil solution rises with decreasing pH. In phase III, there was an pH increase up to /6.0. Here the heavy metals were only slight or not mobilized. With a rise in the pH values /7, it accepts that the Zn affinity to Mn- and Fe-oxides increases strongly too. If the pH at the sediment surface decreases not under pH 3 (phase IV and VI) no significant Zn-mobilization occurs.
3.2.2. Cathode at sediment When the cathode was at the sediment surface and the anode 8 cm below inside the sediment the sediment /water interface was at first alkaline during application of voltage. Due to the production of OH -ions, the pH-value increased from / 5.5 to 12.0 in phase II, to 10 in phase IV and to 9.0 in phase VI (Fig. 6). This is a favorable range for Cd, Pb and Zn to be absorbed and/or precipitated as explained before. Cd: In the reference column in the first 70 days an increase in Cd-concentration by microbial sulfide oxidation was observed. The microbial
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Fig. 5. pH-values and metal concentrations of Cd, Pb and Zn at the sediment /water interface [Conc.(RC) and pH(RC): metal concentration and pH in the reference column without voltage supply, Conc.(EC) and pH(EC): metal concentration and pH in the experimental column with anode at the sediment without/with voltage, (U/3 V)].
activity was high due to high organic content in the phases I and II. After these phases, microorganism were not so active, having less oxygen in the semi closed system.
CdS2O2 (chemoautotrophic microorganisms) 0 Cd2 SO2 4
(8)
Then the Cd-concentration is slowly decreasing,
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Fig. 6. pH-values and metal concentrations of Cd, Pb and Zn at the sediment /water interface [Conc.(RC) and pH(RC): metal concentration and pH of reference column without voltage supply, Conc.(EC) and pH(EC): metal concentration and pH of experimental column with cathode at the sediment without/with voltage, (U/3 V)].
because at near neutral pH-conditions Cd is precipitated as Cd(OH)2. In the phases with current, through the OH-ions made cathodically, there was a fast Cd-fixation in the form of Cd(OH)2, or CdCO3 [15] after carrying out following reactions:
Cd2 HCO 0 CdCO3 (s)H2 O 3 OH
Cd2 2OH 0 Cd(OH)2 (s)
(9) (10)
In this case, pH-values around 8 are sufficient in order to fix almost completely the cadmium.
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Pb: Relatively low initial Pb-values in the water were obtained at the equilibrium stage in the original sediment because of the strongly bonding of Pb as PbS at the sediment lattice. The reason of the increasing Pb-concentration in the reference and the currentless phases III and V in the experimental column was also the microbial sulfide oxidation, analogous to Eq. (8). When the pH-value decreases in the currentless phase after the current influenced phase, one can observed Pb-concentration up to 20/30 mg l 1 (phase III). In phase V, it can be observed, that the Pb-mobilization is not so much, although the pH also increase to values of about 7. The reason is the microbially formed sulfate reacted with the Pb2-ions to relatively insoluble PbSO4. In the current phases a strong decrease of the Pbconcentration as a result of the Pb(OH)2 precipitation occurs. At pH 11, no Pb can be measured in the solution. If the critical pH-value of about 11 is exceeded the formation of the soluble Pb-trihydroxocomplexes occurs (Eq. (12)). Pb2 2OH 0 Pb(OH)2 (s) Pb(OH)2 (s)OH H2 O
(11)
0 [Pb(OH)3 H2 O]
(12)
They are to be recognized first tendencies at the end of the phase II. Zn: The small increase of Zn-concentration in the first phase reduced to the microbial ZnS-oxidation with entered oxygen after ZnS2O2 (chemoautotrophic microorganisms) 0 Zn2 SO2 4
(13)
2
In addition to Zn and ZnOH , at pH values / 6.5 Zn(OH)02, ZnCO03 and other Zn species are available. With increasing pH-value oxides of Mn and Fe can adsorb particularly Zn2, ZnOH and Zn-complexes. Therefore, the higher the pH value the smaller is the Zn-concentration because of the formation of solid Zn(OH)2 (Eqs. (14) / (16)). Zn2 OH 0 ZnOH
ZnOH OH 0 Zn(OH)02
Zn(OH)02
0 Zn(OH)2 (s)
(14) (15) (16)
269
At high pH-values pH /11 significant amounts of the soluble Zn-complex [Zn(OH)3] (l) are produced. Zn(OH)2 (s)OH 0 [Zn(OH)3 ] (l):
(17)
At a small scale, this effect was measured at the end of the phase II. In the currentless phases the pH-value decreased slowly because of the buffering effect of the precipitated solids. 3.2.3. Cathode 4 cm above and anode 4 cm below the sediment In this experiment the cathode was placed 4 cm above and the anode 4 cm below the sediment (Fig. 7). The current flow through the system during phase IV was negligible in comparison to phase II (the figure is not shown here). The reason is, that in the phase IV the cathode was covered by a small insulating layer of carbonates and therefore, no current flow was observed in the system. One can handle phase IV as passing without current. In phase II the pH value increases slowly to values of 12. At the same way the Cd- and Znconcentrations sink to values near zero. Pb is only available at very small amounts at the sediment/ water interface and hardly changes in its concentration. In the phases III, IV and V the heavy metal concentration increases to values, measured that in the reference column.
4. Conclusions The trend of mobilization and accumulation of cadmium, lead and zinc was studied in a small scale. The first results show that the examined heavy metals are mobilized by the effects of the anodic polarization and transported by migration from the sediment into the water /sediment interface. By constructing of a pH-barrier at the sediment /water interface, those metals were precipitated at the steep pH-gradient. The metals were accumulated at the sediment /water interface. With increasing voltage in the range of 3 /4 V the mobilization of the heavy metals from the sediment and the immobilization at the sediment/ water interface took place faster. In the case where
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Fig. 7. pH-values and metal concentrations of Cd, Pb and Zn at the sediment /water interface [Conc.(RC) and pH(RC): metal concentration and pH in the reference column without voltage supply, Conc.(EC) and pH(EC): metal concentration and pH in the experimental column with anode 4 cm below and cathode 4 cm above at the sediment with/without voltage, (U/3 V)].
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the cathode is positioned at the sediment surface this arrangement did not allow those metals to migrate to the water /sediment interface. In our view, the electrode arrangement anode in the sediment /cathode in the water body with a relatively small distance between both electrodes to create a steep pH- and Eh-gradient is the best arrangement for the calculated mobilizing and enrichment of those heavy metals in a relative small layer. If necessary the electrodes can be combined with a counter anode to neutralize OH -ions, that migrate in the water body. This will be a possibility to support the in-situ-cleaning of sediments by constructing a pH- and Eh-barrier to accumulate the heavy metals. After this it should be possible to mobilize them in a relative short time and to remove the concentrated heavy metals containing solution from the solid phase by reversing the polarity of the electrodes.
Acknowledgements Authors acknowledge the support from DFG / GK scholarship council and all our friends who helped during this work.
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