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Conditions and rates of biotic and abiotic iron precipitation in selected Danish freshwater plants and microscopic analysis of precipitate morphology
PII: S0043-1354(00)00002-6
Wat. Res. Vol. 34, No. 10, pp. 2675±2682, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
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CONDITIONS AND RATES OF BIOTIC AND ABIOTIC IRON PRECIPITATION IN SELECTED DANISH FRESHWATER PLANTS AND MICROSCOPIC ANALYSIS OF PRECIPITATE MORPHOLOGY ERIK G. SéGAARD1*, ROBIN MEDENWALDT2 and JOANNA V. ABRAHAM-PESKIR2 1
Department of Chemistry, University of Aalborg, Institute for Chemistry and Applied Engineering Science, Niels Bohrs Vej 8, DK-6700, Esbjerg, Denmark and 2Institute for Storage Ring Facilities, University of Aarhus, Ny Munkegade, DK-8000, Aarhus C, Denmark (First received 1 March 1999; accepted in revised form 1 January 2000)
AbstractÐThis study compares the biotic precipitation of iron in the sand ®lters of a new freshwater plant, Astrup, with the abiotic precipitation of iron in the sand ®lters of a traditional freshwater plant, Forum, in the same area of Denmark. We have observed that a third freshwater plant, Grindsted, which was planned to precipitate iron in the traditional abiotic way is in fact precipitating iron biotically because of poor aeration and very low oxygen content of the raw water. The dominant ironprecipitating bacteria was Gallionella ferruginea in both Astrup and Grindsted. The morphology of the iron precipitates were investigated using light, X-ray, scanning electron and transmission electron microscopy. The physicochemical conditions governing precipitation and the precipitated iron sludge were also investigated. The biotically precipitated iron was shown to be oxidised and precipitated with a rate about 60 times faster than the traditional abiotic process in spite of the much poorer physicochemical conditions for the process. The faster kinetics indicate a catalytic activity due to the presence of exopolymers from Gallionella ferruginea. A model is proposed for the relationship between the dierences in characteristics of the iron precipitate and the kinetics of the precipitation. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐiron ®ltration, Gallionella ferruginea, morphology, kinetics, physicochemical conditions
INTRODUCTION
Groundwater normally has an iron and manganese content which necessitates precipitation before the water can be released into the freshwater supply systems. This especially applies to the Ribe formation in the south-western parts of Jutland, Denmark, which has a high content of iron (1± 19 mg/l) in the groundwater. The normal chemical way to decrease the iron content is by aeration, which initiates iron precipitation. The waters are almost saturated with oxygen to about 8 mg/l before ®ltering. This strips o excess carbon dioxide, methane and some ammonia. The ®lter systems are usually composed of two sections. In the ®rst ®lter, only iron is precipitated, whereas in the second ®lter, manganese is precipitated together with residual iron. An alternative method for iron precipitation is biological, where bacteria are actively involved in *Author to whom all correspondence should be addressed. Tel.: +45-7912-7666; fax: +45-7545-3643; e-mail: [email protected]
the process (Frischherz et al., 1985; Badjo and Mouchet, 1989; Bourgine et al., 1994). The eciency of the method depends on the chemical and physical properties of the water and the additives, which have to be adjusted for optimal performance. According to Mouchet (1992), a shift from abiotic to biotic precipitation can increase the water plant capacity substantially and reduce operation costs by up to 80%. Redox potential Eh and pH are the main factors that determine whether the precipitation is going to take place biotically or abiotically. Iron precipitating bacteria such as Gallionella, Leptothrix, and Siderocapsa, demand a lower pH and a lower Eh than normally used for abiotic precipitation (Hem, 1961; Hanert, 1992; Mouchet 1992). Biological iron removal is best at an rH2 greater than 14, however the boundary between physico-chemical and biological iron precipitation is not well de®ned (DegremoÂnt, 1991). rH2 ÿlog pH2 Eh =0:0296V 2pH corresponding to a rewriting of the Nernst equation for the reduction potential of H+ (see e.g. Ehrlich, 1990). G. ferruginea is an autotroph able to derive
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energy from the reduction of HCOÿ 3 /CO2 while oxidising another source possibly Fe(II) (Hanert, 1992; Ghiorse, 1984; Madigan et al., 1997). It is an obligate autotroph according LuÈtters-Czekalla (1990). The carbon dioxide used by the bacteria is expected to come from oxidising reactions of organic compounds in the ground waters (Stumm, 1992). Chemical autooxidation is important above pH 5 (Ghiorse and Ehrlich, 1992). However, G. ferruginea is only reported in slightly acidic environments with relatively low concentrations of Fe(II), up to 25 mg/ l (LuÈtters and Hanert, 1989). Gradient-loving bacteria such as Gallionella are reported to depend on sulphate reducing and other anaerobic bacteria to maintain the micro-aerophilic conditions in their environments (Ghiorse, 1984). Some of the Fe(II) in the anoxic zone of the ground water is chelated with the organic compounds from the destruction of humic substances (Stumm and Lee, 1961; Stumm 1992). Through reduction involving organic ligands as an electron bridge, the Fe(II)-organic chelates are expected to increase the rate of solvating for iron(III) hydroxides in the anoxic zone of the groundwater, therefore, contributing in part to the iron content in the groundwater (Wherli et al., 1989). In the boundary between oxic and anoxic waters, the amorphous iron(III)hydroxides in this way have become more soluble through autocatalysis from Fe(II) and ligand formation (Hering and Stumm, 1990). Fe(II) only creates weak complexes with all ligands except the bisulphide ion, while Fe(III) forms strong (innersphere) complexes with most ligands (Langmuir, 1997). Therefore, the resulting Fe(III) product of the Fe(II)/Fe(III) oxidation process is the more stable part of the couple. In this study, we compared iron sludge from three ®lter systems belonging to three dierent Danish freshwater plants. In Denmark, the ®rst of two freshwater plants, Astrup, was designed to precipitate iron biologically. Forum is operated using the traditional method, where iron is precipitated abiotically. The third of these plants, Grindsted, was originally designed to precipitate iron abiotically. However, this study showed that the iron in the ®lters of Grindsted is precipitated biotically. This research aimed to characterise the precipitated iron sludge, mainly in its original wet condition and to investigate the correlation between the conditions for precipitation and the rate of precipitation. The backwash water iron precipitates were analysed by a variety of microscopic techniques e.g. visible light microscopy, X-ray microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) together with energy dispersive Xray analysis (EDX). For comparison we have investigated the conditions for abiotic and biotic precipitation by means of selected analytical chemical and colloidal physical methods.
METHODS AND MATERIALS
Iron sludge samples were collected from three freshwater plants Astrup, Forum, and Grindsted (south-west Jutland, Denmark). Containers were ®lled with backwashing water directly from the ®lters at the beginning of the backwashing period, at dierent times during and at the end of the period. Samples were obtained from more than one backwashing period and from dierent ®lters. On the macroscopic and microscopic level no dierences were observed between precipitates collected from a given ®lter system. Samples were stored in the containers at 38C until analysis which was completed within 24 h. No ageing eect of the samples was observed after several months of storage. All samples from all three freshwater plants were analysed by the following methods. Visible light microscopy A Leica visible light microscope (LM) with phase contrast and dierential interference contrast (DIC) was used. Images were recorded electronically with a video camera or with a camera using photo®lms. X-ray microscopy A full ®eld imaging X-ray microscope (XRM) located at ISA, Aarhus, Denmark (Medenwaldt and Uggerhùj, 1998) was used to obtain the presented X-ray micrographs. The microscope was equipped with circular diraction gratings, so called zone plates, as optical elements, thereby providing a resolution of around 30 nm. The high resolution could be combined with high contrast in the micrographs by choosing an X-ray wavelength of 2.4 nm, where the absorption in organic (carbon and nitrogen containing) material was much higher than in water. The optical setup of the X-ray microscope was equivalent to visible light transmission microscopes. Radiation from a synchrotron source was focused by a condenser lens onto the sample. An objective lens behind the sample formed a transmission image on a CCD detector which was stored digitally for subsequent analysis. During imaging, samples were kept in a sealed chamber between two thin silicon foils at room temperature and atmospheric pressure. Typical imaging times were of the order of 30 s. Transmission electron microscopy Samples were air-dried onto copper grids and viewed with a Philips CM20 transmission electron microscope. The electron diraction option, SAED, was used to determine whether a sample was crystalline or amorphous. Scanning electron microscopy An aliquot of sample was air-dried onto aluminium stubs and imaged with a CamScan MaXim scanning electron microscope (SEM) equipped with an EDX system. This was used to determine the elemental composition of the samples. z-potential measurements and particle size distributions The z-potentials were measured by means of Malvern ZetaMasterS Version PCS: v1.27. The particle size distributions were analysed with a Dantec/Invent's Particle Analyser system based on the phase Doppler principle. Only distributions of particles smaller than 30 mm were measured and only spherical particles were accepted. Kinetic calculations The dierential equation described by Davison and Seed (1983) was used to calculate the speed of precipitation in kinetic calculations ÿr kFe II pO2 OH ÿ 2 ,
1
Iron precipitation in Danish freshwater plants where r is the rate of the oxidation of Fe(II) to Fe(III), [Fe(II)] is the molar concentration of Fe(II), [OHÿ] is the molar concentration of the hydroxide, and pO2 is the partial pressure of O2 above the water which is in equilibrium with its gas. For natural and synthetic waters, they found a rate constant of k 2 1013 Mÿ2 atmÿ1 minÿ1. This value describes the maximum possible rate of iron precipitation if the mechanism is abiotic without added catalysts.
RESULTS
Table 1 shows the analytical data collected from each of the three ®lter systems. Figures 1±4 show representative images of sludge from the three freshwater plants. Light microscopy images gave a general overview of the sludge structure. X-ray microscopy allowed imaging of wet samples at atmospheric pressure with a resolution an order of magnitude better than that achieved by light microscopy, whilst retaining the gentle sample preparation methods. X-ray microscopy revealed details of the fully-hydrated sludge on the submicron level. Electron microscopy provided an even higher resolution, but this came at a cost of drying the samples. However, it was seen by comparing XRM, SEM and TEM that drying gently at room temperature in a desiccator did not alter the structure. In Fig. 1(a), showing sludge from Astrup, all precipitated iron is connected to exopolymers of ironprecipitating bacteria. The characteristic shape with the twisted stalks makes it most likely to be G. ferruginia. The exopolymers are relatively small and some of them have a less de®ned structure, which could be fractions of former intact polymers. In contrast, the sludge from Forum (Fig. 1(b)) generally had a homogeneous appearance consisting mainly of calcite (determined by EDX) resulting
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from added calcium hydroxide. Single bacteria sheaths, usually of the Gallionella type, could be found only rarely in the sludge. In Grindsted (Fig. 1(c)), the exopolymers were generally smaller than in Astrup, but the appearance of the sludge was very similar. Though Gallionella was probably the dominating bacteria, other iron-precipitating bacteria types were observed. The sludge from Astrup (Fig. 2(a)) appears as a network of exopolymers linking the smaller particles of the iron precipitate together. It is possible to see that the stalks are composed of smaller parallel threads that seem to be twisted, which is characteristic for the bacteria G. ferruginea (Ghiorse and Ehrlich, 1992). XRM images of the Forum sludge (Fig. 2(b)) resembled that of the LM images. However, at this scale the sludge does not look totally homogeneous. No distinct structure of this colloidal substance could be seen. Two examples of structures found in the sludge from Grindsted are shown in Fig. 2(c) and (d). In this sludge, the iron was found in lumps connected to exopolymers. The twisted structure in Fig. 2(c) is typical for G. ferruginea, but in contrast to the Astrup exopolymers, the single threads are not observed, which might be due to a higher iron content in the exopolymers. Figure 2(d) shows an exopolymer, which most likely belongs to a dierent bacteria species. The transmission electron micrograph in Fig. 3(a), is an image from the Astrup sludge showing threads from the stalks of G. ferruginea. The iron precipitate is localised within the threads and is also found as big lumps in connection with the sheaths. By electron diraction techniques (SAED), we found that the iron precipitated in the threads is amor-
Table 1.
Number of borings Treated raw water Number of 1st ®lters Filter height and area Filter material and grain size Max. Filtering velocity Backwashing frequency Aeration before 1st ®lter Typical raw water pH and Eh Typical Eh after 1st ®lter Typical backwash water pH z-potential Mean particle size of iron precipitate from ®lter Typical iron content in raw water Iron content of dry precipitate Density of wet precipitate Typical MnOÿ 4 consumption for raw water Typical MnOÿ 4 consumption for potable water Typical CO2 content in raw water Typical CO2 content in pot. Water
Astrup (biological)
Forum (chemical)
Grindsted (biological)
7 250, max 325 m3/h 2 1.5 m, 15 m2 Quarts 2.5±3 mm (top) and 3±5 mm (bottom) 11.6 m/h 3500 m3/®lter 2±3 mg/L O2 by free fall
5 250, max 310 m3/h 4 0.9 m, 14.2 m2 Anthracite (top 30 cm) and quarts 2±5 mm 5.5 m/h 1750 m3/®lter 8 mg/L by aeration with compressed air pH=5.5 Eh=160 mV Eh=155 mV pH=8.2 ÿ23.827.0 mV 2.90 mm 2 mg/L
4±5 300, max 400 m3/h 6 0.9 m, 17 m2 Quarts 2.4±4 mm
pH=7.3 Eh=ÿ60 mV Eh=195 mV pH=7.7±7.8 ÿ24.726.8 mV 2.09 mm 3 mg/L
15%22% 0.01120.001 g/ml 2±3 mg/L 1.5 mg/L 60 mg/L Not measurable
40%25% 0.09820.005 g/ml 2±6 mg/L 1 mg/L 15 mg/L 2 mg/L
pH=7 Eh=ÿ60 mV Eh=100 mV pH=7.2±7.3 ÿ24.526.9 mV 1.43 mm 11±13 mg/L max: 20 mg/L 45%25% 0.06820.005 g/ml 10 mg/L 1.5 mg/L 20 mg/L 4 mg/L
8 m/h 1250 m3/®lter 1±2 mg O2 by free fall
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Fig. 1. Light microscopy images of iron sludge from (a) Astrup, )b) Forum, and (c) Grindsted.
phous, while the iron in the lumps is microcrystalline. The parallel appearance of the threads resembles the impression of the stalks from the X-ray micrograph (Fig. 2(a)). The Forum sludge did not seem to have any de®nite structure on the smaller scale in the TEM images, however, the iron precipitate and the calcite shown in Fig. 3(b) were found to be microcrystalline as could be concluded from the electron diraction pattern. One of the exopolymers in the sludge from Grindsted can be seen in Fig. 3(c). Again, we see the typical stalks of Gallionella. In the twisting area, the precipitation seems to be more condensed. In the enlarged part of Fig. 3(c), shown in Fig. 3(d), we observe an area with a relatively large amount of precipitated iron. In comparison to Fig. 3(a), the threads are smaller and more compact, which probably is due to their higher iron content. This com-
pactness is also re¯ected in the X-ray micrograph of Fig. 2(c) where the single threads are not as distinct as in Fig. 2(a). The twisted structures in the centre of Fig. 4(a), again, are typical for G. ferruginea. EDX measurements showed a relatively large iron content in the precipitates that are connected to the exopolymers. Very little calcium was found neither in the granular precipitate nor in the exopolymers. In contrast, the composition of the chemically precipitated sludge of Forum was rich in calcium; Fig. 4(b) shows the dried calcite precipitate with the collected iron oxide. In the sludge from Grindsted (Fig. 4(c)), similar to the Astrup sludge, exopolymers and iron oxide particles were connected, however, the exopolymers were typically smaller than in the Astrup sludge. Using SEM-EDX all samples were found to contain Ca, Cl, Fe, Na, P, S, and Si. The mean size of the particles, see Table 1, was substantially larger for the abiotic precipitation. To investigate the in¯uence of the calcite on the Forum particle distribution, a second sample of raw water from Forum (before addition of calcium hydroxide) was aerated to saturation in the laboratory before the particle size was measured for comparison. The mean size of the particles in this case (4.97 mm) was even larger. The measured z-potentials were all negative with mean values about ÿ24 mV disregarding precipitation method and dierences in pH. The biological iron precipitate was much more dense than the abiotic iron precipitate by a factor of 7±9 times. A pH of 7 was measured in raw water decreasing to about 6.7 during ®ltration. The value of pO2 was calculated for an oxygen content of 3.4 mg/l, (measured at the entrance of the ®lter). The oxygen content decreases through the ®lter to 2.3 mg/l, corresponding to pO2 0:058 atm. By keeping [OHÿ] and pO2 constant in equation (1), we get a pseudo-®rst order reaction with the solution ln Fe II 0 =Fe II k 0 t, where k 0 kOH ÿ pO2 : From the average treated amount of water (250 m3/h) and the ®lter capacity in Astrup of 45 m3, we get residence time t 45=250 h 10:8 min. Inserting Fe II 0 10 mg/l and Fe II 0:05 mg/l in the above expression yields k 0 0:49 minÿ1. With OH ÿ 10 ÿ7 and pO2 0:084 atm T 8:5 ^ C we end up with kAstrup 58 1013 atmÿ1 Mÿ2 minÿ1. In Grindsted, where the pH is 7 as in Astrup, but where the oxygen content is particularly low, 0.037 atm, the calculated k-value is about the same as in Astrup, kGrindsted 60 1013 atmÿ1 Mÿ2 minÿ1. In contrast, the kinetic of precipitation of iron oxides in Forum with pH 7.6 after aeration and pO2 0:21 atm gives a much lower k-value of kForum 0:9 1013 atmÿ1 Mÿ2 minÿ1. This estimate shows that bio®ltration is expected to be much more ecient (60 times or more) than purely abiotic precipitation.
Iron precipitation in Danish freshwater plants
Fig. 2. X-ray micrographs of iron sludge from (a) Astrup, (b) Forum, and (c) and d) Grindsted.
Fig. 3. TEM images of iron sludge from (a) Astrup, (b) Forum, and (c) Grindsted.
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Fig. 4. SEM images of iron sludge from (a) Astrup, (b) Forum, and (c) Grindsted. DISCUSSION
In Astrup, the conditions for biotic iron precipitation are excellent despite variations in water quality from dierent water boring sites. Raw water at the inlet has a Eh of about ÿ60 mV to ÿ100 mV, a pH of about 7, and a negligible oxygen content. To obtain the best conditions for biotic iron precipitation, raw water from two borings are usually mixed during on-line measurements of Eh and pH. The sludge resulting from biotic precipitation is much denser than after abiotic precipitation (Table 1), having a number of advantages: The voluminous amount of sludge is smaller, so that ®lters do not clog as frequently and the backwashing frequency of the ®lters is reduced. The required amount of backwash water per kg precipitated iron is smaller than in normal sand®lters and the iron sludge in the backwash water is easy to reprecipitate. This gives a unique possibility at
Astrup, that the backwash water can be ®ltered again in a special closed sand ®lter built for this purpose and radiated with UV-light, aerated and sent back to join the raw water between the ®rst and second ®lter. Furthermore, slurry of calcium hydroxide and a computer-regulated amount of carbon dioxide can be added to the raw water before its entrance to the second ®lter for manganese precipitation. Why is the sludge so much denser after biological treatment? In Forum, calcium hydroxide is added to the raw water in order to reduce the amount of aggressive CO2. The resulting calcium carbonate is then precipitated together with the iron oxides as a low-density colloid sludge in the ®rst ®lter. The negative z-potential keeps the iron precipitate in colloidal solution for large portions of the iron precipitate. Conversely, in Astrup, calcium hydroxide is not added to the raw water until after the ®rst ®lter. The biotic precipitation in the ®rst ®lter then results in a very dense structure of the iron sludge constituents. The reason for the interaction of bacteria with iron oxides and the mechanism involved is not well understood. However, a number of factors in¯uence the precipitation process when the bacteria compete with abiotic precipitation. The highest content of HCOÿ 3 /CO2 was found at Forum, where aeration is done before ®ltering, so most of the carbon dioxide is already stripped o when the water reaches the ®lters. In Astrup and Grindsted, on the other hand, the aeration and addition of calcium hydroxide is done between the ®lters, leaving the CO2 for bacterial growth in the ®rst ®lter. At Astrup and Grindsted the pH is 7 and the iron concentration 11 and 3 mg/l, respectively. How can these iron-precipitating bacteria thrive so successfully in an environment where they have to compete with autooxidation? An intracytoplasmic membrane system in G. ferruginea is thought to be a protection mechanism against chemical autooxidation of the limited amount of Fe(II) (LuÈtters and Hanert, 1989). Hallbeck and Pedersen (1990), however, think that stalk formation will only occur under increased levels of oxygen and also has a protective function. The negative z-potentials of the biotic precipitate implied that Fe(II) was easily bound within the stalks. Iron precipitate in exopolymers results in a mineralised cellular matrix containing iron oxides that are not easily re-dissolved (Beveridge and Fyfe, 1985). In Forum, it was the colloid precipitated autooxidised sludge with its calcite matrix that carried a negative z-potential. The Eh is another important factor governing iron precipitation and bacterial growth. G. ferruginea does not form stalks at an Eh less than ÿ40 mV (Hallbeck and Pedersen, 1990) and the preferred Eh-conditions are above +200 mV (Ehrlich, 1990). Hanert (1992) reports the optimal conditions to be
Iron precipitation in Danish freshwater plants
between +200 mV to +320 mV with an oxygen content of 0.1±1 mg/l O2. However, an oxygen content of 0.1 mg/l corresponds to an Eh of 370 mV at pH 7 when oxygen is the sole oxidation component and therefore Eh-dominating. rH2 26:5: In Astrup, the Eh rises from about ÿ60 mV to about 100 mV after the iron-precipitating ®lter and in Grindsted from ÿ60 mV to about 200 mV, which would correspond to very low oxygen contents. However, the measured oxygen content in the ®lters was higher. Therefore oxygen alone cannot be the Eh-dominating factor in the ®lters. At the measured pH of 7 these values correspond to rH2-values between 12 and 21. The sulphate content of the raw water at Astrup was about 40 mg/l and the sulphide content was below the detection limit for the analytical procedure used (0.1 mg/l). A calculation based on these numbers gives an Eh of ÿ35 mV, provided sulphate were the only oxidation component. This shows that the Eh-dominating species in the raw water was sulphate because of its higher amount relative to iron. As a conclusion, in the sand ®lter, the dominating role changes gradually from sulphur to oxygen with an increase in Eh as a result. Some of the sulphate in the water from Astrup was used during the ®ltering, because the initial measured content (40 mg/l) decreased. In one case it fell to about 20 mg/l during the ®rst ®lter without any detectable sulphide release into the water. Some of the sulphate could have been reduced to HSÿ and re-oxidised to non-soluble compounds, So or metallic polysulphides (Langmuir, 1997). Indeed, by EDS we found sulphur in the sludge, but in small amounts. So, it is still an open question as to where the majority of the sulphur ends up, as it is neither found in the exopolymers, nor in the bacteria at such high levels. However, total biomass was not determined. Freshwater treatment plants use permanganate consumption as an indication of the level of organic substances in the water. In this study, the permanganate consumption was higher in the two biologically treated water plants than in the chemically treated plant, which could indicate the presence of more organics in the Astrup and Grindsted cases. Purely physical±chemical oxidation of Fe(II) has been observed to be retarded in synthetic and natural waters when Fe(II) is chelated with organics and the O2 partial pressure is kept at 0.5 atm (Theis and Singer, 1974). Furthermore, Sung and Morgan (1980) investigated the eect of Na2SO4 (0.165 M) on the rate of oxidative precipitation of Fe(II). They found that Na2SO4 reduces the rate by a factor of 100 compared to solutions of a freshwaterlike composition. Our sulphate contents are much lower (about 0.4 mM) so this direct physical±chemical eect is expected to be almost negligible. A tenfold increase in ionic strength reduces the rate constant to about half the value (Sung and Morgan,
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1980). This indicates that the speed of biological precipitation relative to the physicochemical case may be even higher than expected from our kinetics calculation if the raw waters from the borings of the three freshwater plants were identical with respect to organics, complexing cations, and ionic strength. The increased precipitation rate in the sand ®lter seen in the bacteriological case could be congruent with the fact that when iron is bound to organic compounds the redox potential of the Fe(III)/Fe(II) couple normally decreases substantially relatively to the O2/H2O couple (Stumm, 1992). However, chelating organics normally reduce the abiotic precipitation rate (Theis and Singer 1974), indicating another reason for the increased rate of oxidationprecipitation. It is known that complexation of Fe(II) to OH± or the adsorption of Fe(II) to surface hydroxyl groups of the precipitated hydrous oxide, e.g. ferrihydrite, lowers the redox potential of the iron couple relatively to the O2/H2O couple. This gives a relative increase in reaction rate constants of several orders of magnitude (Wherli, 1990). In fact, the rate determining step of the O2 reduction is thought to be the ®rst step, O2 e ÿ 4 O2ÿ , E o ÿ0:16 V. The next step, H HO2 e ÿ 4 H2 O2 , has a slightly higher Eo than the O2/H2O couple (Wherli, 1990). In the biotic case, this eect of complexation between Fe(II) and OHÿ is much lower than in the abiotic case due to the lower pH. Despite the fact that the pH is lower the precipitation rate is higher in the biotic case. An explanation of the increased rate of oxidationprecipitation is likely to be a chelate creation between Fe(II) and the exopolymers of Gallionella. The exopolymers are composed mainly of carbohydrates, which contain rows of hydroxyl groups with electron lone pairs for chelating with Fe(II). The Fe(II) will be kept in a ®xed position by the exopolymers in readiness for oxidation. After oxidation the amorphous iron(III)hydroxides are partly kept in the stalks of the exopolymer and partly released to start crystallisation as micro crystallites. In this way, the exopolymers act as a catalyst in the biotic oxidation of Fe(II) contrary to the inhibiting eect of Fe(II)-organic chelates in aqueous solution during abiotic oxidation. The enhanced precipitation speed could be a side eect of the concentration and incorporation of the precipitates in the exopolymers. It could, however, also be such that the bacteria actively participate in the oxidation process as a part of their metabolism. CONCLUSION
In this work, we have analysed iron precipitates from the three freshwater plants in order to evaluate biotic iron hydroxide precipitation as compared to abiotic precipitation. We propose that a probable explanation for the increased precipitation rate in
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the biological ®lters relative to the physicochemical ®lters could be that precipitation of iron hydroxides takes place in contact with the exopolymers of the bacterium G. ferruginea. The exopolymers act as a catalyst for the oxidation-precipitation process of iron and prevent re-dissolution of the iron hydroxides in spite of the oxic/anoxic conditions found in the biological ®lters. The precipitated iron hydroxides, e.g. ferrihydrite, autocatalyse the oxidation/ precipitation of Fe(II) by reducing the redox potential of the Fe(OH)3 (amorphous)/Fe(II) couple relative to the Fe(III)/Fe(II) couple in aqueous solution. That the iron-precipitating bacterium G. ferruginea is chemolithotropic with respect to the oxidation of Fe(II) is not proven, but it seems plausible. Though many arguments and interpretations have been presented here to elucidate the functioning of bacteria in the ®lter systems, there are still a number of open questions that need further investigation. However, there are clear advantages in using bacteria for iron precipitation in freshwater plants. AcknowledgementsÐThe authors express their thanks to the personnel at Astrup, Forum and Grindsted freshwater plants, especially thanks to Svend Laursen and Holger Brinckmann for their help in the collecting of samples. Thanks to engineer J. C. Blandford, KruÈger A/S, Horsens, Denmark for discussions about Astrup. Finally, thanks to Jacques Chevallier, Institute for Physics and Astronomy, University of Aarhus, Denmark for his help with TEM and SEM. REFERENCES
Badjo Y. and Mouchet P. (1989) Appropriate technologiesÐexample of large biological iron removal plant in Togo. Aqua 38, 197±206. Beveridge T. J. and Fyfe W. S. (1985) Metal ®xation by bacterial cell walls. Can. Jour. Earth Sci. 22, 1893±1898. Bourgine F. P., et al. (1994) Biological processes at Saint Hill water-treatment plant, Kent. J. JWEM 8, 379±392. Davison W. and Seed G. (1983) The kinetics of the oxidation of ferrous iron in synthetic and natural waters. Geochim. Cosmochimi. Acta 47, 67±79. DegremoÂnt. (1991) Water treatment Handbook. Ehrlich H. L. (1990) Geomicrobiology, 2nd ed. Marcel Dekker, New York. Frischherz H., et al. (1985) Biological elimination of iron and manganese. Supply, pply 3, 125±136.
Ghiorse W. C. (1984) Biology of iron- and manganesedepositing bacteria. Ann. Rev. Microbiol. 38, 515±550. Ghiorse W. C. and Ehrlich H. L. (1992) Microbial biomineralization of iron and manganese. In Biomineralization, eds H. C. W. Skinner and R. W. Fitzpatrick. Catena Verlag, Germany. Hallbeck L. and Pedersen K. (1990) Culture parameters regulating stalk formation and growth rate of Gallionella ferruginea. Journ. Gen. Microbiol. 136, 1675±1680. Hanert H. H. (1992) The genus Gallionella. In The Prokaryotes2nd ed., Vol. IV, eds A. Balows, et al., pp. 4082± 4088. Springer Verlag, Berlin, New York. Hem J. D. (1961) Stability ®elds diagrams as aids in iron chemistry studies. J. AWWA 53, 211±232. Hering J. G. and Stumm W. (1990) Oxidative and reductive dissolution of minerals. In Mineral-Water Interface Geochemistry, eds M. F. Hochella Jr and Art F. White, Reviews in Mineralogy, Vol. 23, pp. 427±459. Langmuir D. (1997) Aqueous Environmental Geochemistry. Prentice Hall. LuÈtters S. and Hanert H. H. (1989) The ultrastructure of chemolithoautotrophic Gallionella ferruginea and Thiobacillus ferrooxidans as revealed by chemical ®xation and freeze-etching. Arch. Microbiol. 151, 245±251. LuÈtters-Czekalla S. (1990) Lithoautotropic growth of the iron bacterium Gallionella ferruginea with thiosulfate or sulphide as energy sources. Arch. Microbiol. 154, 417± 421. Madigan M. T., et al. (1997) Brock Biology of Microorganisms. Prentice Hall. Medenwaldt R. and Uggerhùj E. (1998) Description of an X-ray microscope with 30 nm resolution. Rev. Scient. Instrum. 69(8), 2974±2977. Mouchet P. (1992) From conventional to biological removal of iron and manganese in France. Jour. AWWA 84(4), 158±167. Stumm W. (1992) Chemistry of the Solid-Water Interface. J. Wiley & Sons. Stumm W. and Lee G. F. (1961) Oxygenation of ferrous iron. Industrial Engr. Chem. 53, 143±146. Sung W. and Morgan J. J. (1980) Kinetics and product of ferrous iron oxygenation in aqueous systems. Environmental Science & Technology 14(5). Theis T. L. and Singer P. C. (1974) Complexation of iron(II) by organic matter and its eect on iron(II) oxygenation. Environmental Science & Technology 8(6), 569± 573. Wherli B., et al. (1989) Redox processes catalyzed by hydrous oxide surfaces. Chem. Geology 78, 167±179. Wherli B. (1990) Redox reactions of metal ions at mineral surfaces. In Aquatic Chemical Kinetics, Reaction Rates of Processes in Natural Waters, ed. W. Stumm. J. Wiley & Sons.
Wat. Res. Vol. 34, No. 10, pp. 2675±2682, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
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CONDITIONS AND RATES OF BIOTIC AND ABIOTIC IRON PRECIPITATION IN SELECTED DANISH FRESHWATER PLANTS AND MICROSCOPIC ANALYSIS OF PRECIPITATE MORPHOLOGY ERIK G. SéGAARD1*, ROBIN MEDENWALDT2 and JOANNA V. ABRAHAM-PESKIR2 1
Department of Chemistry, University of Aalborg, Institute for Chemistry and Applied Engineering Science, Niels Bohrs Vej 8, DK-6700, Esbjerg, Denmark and 2Institute for Storage Ring Facilities, University of Aarhus, Ny Munkegade, DK-8000, Aarhus C, Denmark (First received 1 March 1999; accepted in revised form 1 January 2000)
AbstractÐThis study compares the biotic precipitation of iron in the sand ®lters of a new freshwater plant, Astrup, with the abiotic precipitation of iron in the sand ®lters of a traditional freshwater plant, Forum, in the same area of Denmark. We have observed that a third freshwater plant, Grindsted, which was planned to precipitate iron in the traditional abiotic way is in fact precipitating iron biotically because of poor aeration and very low oxygen content of the raw water. The dominant ironprecipitating bacteria was Gallionella ferruginea in both Astrup and Grindsted. The morphology of the iron precipitates were investigated using light, X-ray, scanning electron and transmission electron microscopy. The physicochemical conditions governing precipitation and the precipitated iron sludge were also investigated. The biotically precipitated iron was shown to be oxidised and precipitated with a rate about 60 times faster than the traditional abiotic process in spite of the much poorer physicochemical conditions for the process. The faster kinetics indicate a catalytic activity due to the presence of exopolymers from Gallionella ferruginea. A model is proposed for the relationship between the dierences in characteristics of the iron precipitate and the kinetics of the precipitation. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐiron ®ltration, Gallionella ferruginea, morphology, kinetics, physicochemical conditions
INTRODUCTION
Groundwater normally has an iron and manganese content which necessitates precipitation before the water can be released into the freshwater supply systems. This especially applies to the Ribe formation in the south-western parts of Jutland, Denmark, which has a high content of iron (1± 19 mg/l) in the groundwater. The normal chemical way to decrease the iron content is by aeration, which initiates iron precipitation. The waters are almost saturated with oxygen to about 8 mg/l before ®ltering. This strips o excess carbon dioxide, methane and some ammonia. The ®lter systems are usually composed of two sections. In the ®rst ®lter, only iron is precipitated, whereas in the second ®lter, manganese is precipitated together with residual iron. An alternative method for iron precipitation is biological, where bacteria are actively involved in *Author to whom all correspondence should be addressed. Tel.: +45-7912-7666; fax: +45-7545-3643; e-mail: [email protected]
the process (Frischherz et al., 1985; Badjo and Mouchet, 1989; Bourgine et al., 1994). The eciency of the method depends on the chemical and physical properties of the water and the additives, which have to be adjusted for optimal performance. According to Mouchet (1992), a shift from abiotic to biotic precipitation can increase the water plant capacity substantially and reduce operation costs by up to 80%. Redox potential Eh and pH are the main factors that determine whether the precipitation is going to take place biotically or abiotically. Iron precipitating bacteria such as Gallionella, Leptothrix, and Siderocapsa, demand a lower pH and a lower Eh than normally used for abiotic precipitation (Hem, 1961; Hanert, 1992; Mouchet 1992). Biological iron removal is best at an rH2 greater than 14, however the boundary between physico-chemical and biological iron precipitation is not well de®ned (DegremoÂnt, 1991). rH2 ÿlog pH2 Eh =0:0296V 2pH corresponding to a rewriting of the Nernst equation for the reduction potential of H+ (see e.g. Ehrlich, 1990). G. ferruginea is an autotroph able to derive
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energy from the reduction of HCOÿ 3 /CO2 while oxidising another source possibly Fe(II) (Hanert, 1992; Ghiorse, 1984; Madigan et al., 1997). It is an obligate autotroph according LuÈtters-Czekalla (1990). The carbon dioxide used by the bacteria is expected to come from oxidising reactions of organic compounds in the ground waters (Stumm, 1992). Chemical autooxidation is important above pH 5 (Ghiorse and Ehrlich, 1992). However, G. ferruginea is only reported in slightly acidic environments with relatively low concentrations of Fe(II), up to 25 mg/ l (LuÈtters and Hanert, 1989). Gradient-loving bacteria such as Gallionella are reported to depend on sulphate reducing and other anaerobic bacteria to maintain the micro-aerophilic conditions in their environments (Ghiorse, 1984). Some of the Fe(II) in the anoxic zone of the ground water is chelated with the organic compounds from the destruction of humic substances (Stumm and Lee, 1961; Stumm 1992). Through reduction involving organic ligands as an electron bridge, the Fe(II)-organic chelates are expected to increase the rate of solvating for iron(III) hydroxides in the anoxic zone of the groundwater, therefore, contributing in part to the iron content in the groundwater (Wherli et al., 1989). In the boundary between oxic and anoxic waters, the amorphous iron(III)hydroxides in this way have become more soluble through autocatalysis from Fe(II) and ligand formation (Hering and Stumm, 1990). Fe(II) only creates weak complexes with all ligands except the bisulphide ion, while Fe(III) forms strong (innersphere) complexes with most ligands (Langmuir, 1997). Therefore, the resulting Fe(III) product of the Fe(II)/Fe(III) oxidation process is the more stable part of the couple. In this study, we compared iron sludge from three ®lter systems belonging to three dierent Danish freshwater plants. In Denmark, the ®rst of two freshwater plants, Astrup, was designed to precipitate iron biologically. Forum is operated using the traditional method, where iron is precipitated abiotically. The third of these plants, Grindsted, was originally designed to precipitate iron abiotically. However, this study showed that the iron in the ®lters of Grindsted is precipitated biotically. This research aimed to characterise the precipitated iron sludge, mainly in its original wet condition and to investigate the correlation between the conditions for precipitation and the rate of precipitation. The backwash water iron precipitates were analysed by a variety of microscopic techniques e.g. visible light microscopy, X-ray microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) together with energy dispersive Xray analysis (EDX). For comparison we have investigated the conditions for abiotic and biotic precipitation by means of selected analytical chemical and colloidal physical methods.
METHODS AND MATERIALS
Iron sludge samples were collected from three freshwater plants Astrup, Forum, and Grindsted (south-west Jutland, Denmark). Containers were ®lled with backwashing water directly from the ®lters at the beginning of the backwashing period, at dierent times during and at the end of the period. Samples were obtained from more than one backwashing period and from dierent ®lters. On the macroscopic and microscopic level no dierences were observed between precipitates collected from a given ®lter system. Samples were stored in the containers at 38C until analysis which was completed within 24 h. No ageing eect of the samples was observed after several months of storage. All samples from all three freshwater plants were analysed by the following methods. Visible light microscopy A Leica visible light microscope (LM) with phase contrast and dierential interference contrast (DIC) was used. Images were recorded electronically with a video camera or with a camera using photo®lms. X-ray microscopy A full ®eld imaging X-ray microscope (XRM) located at ISA, Aarhus, Denmark (Medenwaldt and Uggerhùj, 1998) was used to obtain the presented X-ray micrographs. The microscope was equipped with circular diraction gratings, so called zone plates, as optical elements, thereby providing a resolution of around 30 nm. The high resolution could be combined with high contrast in the micrographs by choosing an X-ray wavelength of 2.4 nm, where the absorption in organic (carbon and nitrogen containing) material was much higher than in water. The optical setup of the X-ray microscope was equivalent to visible light transmission microscopes. Radiation from a synchrotron source was focused by a condenser lens onto the sample. An objective lens behind the sample formed a transmission image on a CCD detector which was stored digitally for subsequent analysis. During imaging, samples were kept in a sealed chamber between two thin silicon foils at room temperature and atmospheric pressure. Typical imaging times were of the order of 30 s. Transmission electron microscopy Samples were air-dried onto copper grids and viewed with a Philips CM20 transmission electron microscope. The electron diraction option, SAED, was used to determine whether a sample was crystalline or amorphous. Scanning electron microscopy An aliquot of sample was air-dried onto aluminium stubs and imaged with a CamScan MaXim scanning electron microscope (SEM) equipped with an EDX system. This was used to determine the elemental composition of the samples. z-potential measurements and particle size distributions The z-potentials were measured by means of Malvern ZetaMasterS Version PCS: v1.27. The particle size distributions were analysed with a Dantec/Invent's Particle Analyser system based on the phase Doppler principle. Only distributions of particles smaller than 30 mm were measured and only spherical particles were accepted. Kinetic calculations The dierential equation described by Davison and Seed (1983) was used to calculate the speed of precipitation in kinetic calculations ÿr kFe II pO2 OH ÿ 2 ,
1
Iron precipitation in Danish freshwater plants where r is the rate of the oxidation of Fe(II) to Fe(III), [Fe(II)] is the molar concentration of Fe(II), [OHÿ] is the molar concentration of the hydroxide, and pO2 is the partial pressure of O2 above the water which is in equilibrium with its gas. For natural and synthetic waters, they found a rate constant of k 2 1013 Mÿ2 atmÿ1 minÿ1. This value describes the maximum possible rate of iron precipitation if the mechanism is abiotic without added catalysts.
RESULTS
Table 1 shows the analytical data collected from each of the three ®lter systems. Figures 1±4 show representative images of sludge from the three freshwater plants. Light microscopy images gave a general overview of the sludge structure. X-ray microscopy allowed imaging of wet samples at atmospheric pressure with a resolution an order of magnitude better than that achieved by light microscopy, whilst retaining the gentle sample preparation methods. X-ray microscopy revealed details of the fully-hydrated sludge on the submicron level. Electron microscopy provided an even higher resolution, but this came at a cost of drying the samples. However, it was seen by comparing XRM, SEM and TEM that drying gently at room temperature in a desiccator did not alter the structure. In Fig. 1(a), showing sludge from Astrup, all precipitated iron is connected to exopolymers of ironprecipitating bacteria. The characteristic shape with the twisted stalks makes it most likely to be G. ferruginia. The exopolymers are relatively small and some of them have a less de®ned structure, which could be fractions of former intact polymers. In contrast, the sludge from Forum (Fig. 1(b)) generally had a homogeneous appearance consisting mainly of calcite (determined by EDX) resulting
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from added calcium hydroxide. Single bacteria sheaths, usually of the Gallionella type, could be found only rarely in the sludge. In Grindsted (Fig. 1(c)), the exopolymers were generally smaller than in Astrup, but the appearance of the sludge was very similar. Though Gallionella was probably the dominating bacteria, other iron-precipitating bacteria types were observed. The sludge from Astrup (Fig. 2(a)) appears as a network of exopolymers linking the smaller particles of the iron precipitate together. It is possible to see that the stalks are composed of smaller parallel threads that seem to be twisted, which is characteristic for the bacteria G. ferruginea (Ghiorse and Ehrlich, 1992). XRM images of the Forum sludge (Fig. 2(b)) resembled that of the LM images. However, at this scale the sludge does not look totally homogeneous. No distinct structure of this colloidal substance could be seen. Two examples of structures found in the sludge from Grindsted are shown in Fig. 2(c) and (d). In this sludge, the iron was found in lumps connected to exopolymers. The twisted structure in Fig. 2(c) is typical for G. ferruginea, but in contrast to the Astrup exopolymers, the single threads are not observed, which might be due to a higher iron content in the exopolymers. Figure 2(d) shows an exopolymer, which most likely belongs to a dierent bacteria species. The transmission electron micrograph in Fig. 3(a), is an image from the Astrup sludge showing threads from the stalks of G. ferruginea. The iron precipitate is localised within the threads and is also found as big lumps in connection with the sheaths. By electron diraction techniques (SAED), we found that the iron precipitated in the threads is amor-
Table 1.
Number of borings Treated raw water Number of 1st ®lters Filter height and area Filter material and grain size Max. Filtering velocity Backwashing frequency Aeration before 1st ®lter Typical raw water pH and Eh Typical Eh after 1st ®lter Typical backwash water pH z-potential Mean particle size of iron precipitate from ®lter Typical iron content in raw water Iron content of dry precipitate Density of wet precipitate Typical MnOÿ 4 consumption for raw water Typical MnOÿ 4 consumption for potable water Typical CO2 content in raw water Typical CO2 content in pot. Water
Astrup (biological)
Forum (chemical)
Grindsted (biological)
7 250, max 325 m3/h 2 1.5 m, 15 m2 Quarts 2.5±3 mm (top) and 3±5 mm (bottom) 11.6 m/h 3500 m3/®lter 2±3 mg/L O2 by free fall
5 250, max 310 m3/h 4 0.9 m, 14.2 m2 Anthracite (top 30 cm) and quarts 2±5 mm 5.5 m/h 1750 m3/®lter 8 mg/L by aeration with compressed air pH=5.5 Eh=160 mV Eh=155 mV pH=8.2 ÿ23.827.0 mV 2.90 mm 2 mg/L
4±5 300, max 400 m3/h 6 0.9 m, 17 m2 Quarts 2.4±4 mm
pH=7.3 Eh=ÿ60 mV Eh=195 mV pH=7.7±7.8 ÿ24.726.8 mV 2.09 mm 3 mg/L
15%22% 0.01120.001 g/ml 2±3 mg/L 1.5 mg/L 60 mg/L Not measurable
40%25% 0.09820.005 g/ml 2±6 mg/L 1 mg/L 15 mg/L 2 mg/L
pH=7 Eh=ÿ60 mV Eh=100 mV pH=7.2±7.3 ÿ24.526.9 mV 1.43 mm 11±13 mg/L max: 20 mg/L 45%25% 0.06820.005 g/ml 10 mg/L 1.5 mg/L 20 mg/L 4 mg/L
8 m/h 1250 m3/®lter 1±2 mg O2 by free fall
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Fig. 1. Light microscopy images of iron sludge from (a) Astrup, )b) Forum, and (c) Grindsted.
phous, while the iron in the lumps is microcrystalline. The parallel appearance of the threads resembles the impression of the stalks from the X-ray micrograph (Fig. 2(a)). The Forum sludge did not seem to have any de®nite structure on the smaller scale in the TEM images, however, the iron precipitate and the calcite shown in Fig. 3(b) were found to be microcrystalline as could be concluded from the electron diraction pattern. One of the exopolymers in the sludge from Grindsted can be seen in Fig. 3(c). Again, we see the typical stalks of Gallionella. In the twisting area, the precipitation seems to be more condensed. In the enlarged part of Fig. 3(c), shown in Fig. 3(d), we observe an area with a relatively large amount of precipitated iron. In comparison to Fig. 3(a), the threads are smaller and more compact, which probably is due to their higher iron content. This com-
pactness is also re¯ected in the X-ray micrograph of Fig. 2(c) where the single threads are not as distinct as in Fig. 2(a). The twisted structures in the centre of Fig. 4(a), again, are typical for G. ferruginea. EDX measurements showed a relatively large iron content in the precipitates that are connected to the exopolymers. Very little calcium was found neither in the granular precipitate nor in the exopolymers. In contrast, the composition of the chemically precipitated sludge of Forum was rich in calcium; Fig. 4(b) shows the dried calcite precipitate with the collected iron oxide. In the sludge from Grindsted (Fig. 4(c)), similar to the Astrup sludge, exopolymers and iron oxide particles were connected, however, the exopolymers were typically smaller than in the Astrup sludge. Using SEM-EDX all samples were found to contain Ca, Cl, Fe, Na, P, S, and Si. The mean size of the particles, see Table 1, was substantially larger for the abiotic precipitation. To investigate the in¯uence of the calcite on the Forum particle distribution, a second sample of raw water from Forum (before addition of calcium hydroxide) was aerated to saturation in the laboratory before the particle size was measured for comparison. The mean size of the particles in this case (4.97 mm) was even larger. The measured z-potentials were all negative with mean values about ÿ24 mV disregarding precipitation method and dierences in pH. The biological iron precipitate was much more dense than the abiotic iron precipitate by a factor of 7±9 times. A pH of 7 was measured in raw water decreasing to about 6.7 during ®ltration. The value of pO2 was calculated for an oxygen content of 3.4 mg/l, (measured at the entrance of the ®lter). The oxygen content decreases through the ®lter to 2.3 mg/l, corresponding to pO2 0:058 atm. By keeping [OHÿ] and pO2 constant in equation (1), we get a pseudo-®rst order reaction with the solution ln Fe II 0 =Fe II k 0 t, where k 0 kOH ÿ pO2 : From the average treated amount of water (250 m3/h) and the ®lter capacity in Astrup of 45 m3, we get residence time t 45=250 h 10:8 min. Inserting Fe II 0 10 mg/l and Fe II 0:05 mg/l in the above expression yields k 0 0:49 minÿ1. With OH ÿ 10 ÿ7 and pO2 0:084 atm T 8:5 ^ C we end up with kAstrup 58 1013 atmÿ1 Mÿ2 minÿ1. In Grindsted, where the pH is 7 as in Astrup, but where the oxygen content is particularly low, 0.037 atm, the calculated k-value is about the same as in Astrup, kGrindsted 60 1013 atmÿ1 Mÿ2 minÿ1. In contrast, the kinetic of precipitation of iron oxides in Forum with pH 7.6 after aeration and pO2 0:21 atm gives a much lower k-value of kForum 0:9 1013 atmÿ1 Mÿ2 minÿ1. This estimate shows that bio®ltration is expected to be much more ecient (60 times or more) than purely abiotic precipitation.
Iron precipitation in Danish freshwater plants
Fig. 2. X-ray micrographs of iron sludge from (a) Astrup, (b) Forum, and (c) and d) Grindsted.
Fig. 3. TEM images of iron sludge from (a) Astrup, (b) Forum, and (c) Grindsted.
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Fig. 4. SEM images of iron sludge from (a) Astrup, (b) Forum, and (c) Grindsted. DISCUSSION
In Astrup, the conditions for biotic iron precipitation are excellent despite variations in water quality from dierent water boring sites. Raw water at the inlet has a Eh of about ÿ60 mV to ÿ100 mV, a pH of about 7, and a negligible oxygen content. To obtain the best conditions for biotic iron precipitation, raw water from two borings are usually mixed during on-line measurements of Eh and pH. The sludge resulting from biotic precipitation is much denser than after abiotic precipitation (Table 1), having a number of advantages: The voluminous amount of sludge is smaller, so that ®lters do not clog as frequently and the backwashing frequency of the ®lters is reduced. The required amount of backwash water per kg precipitated iron is smaller than in normal sand®lters and the iron sludge in the backwash water is easy to reprecipitate. This gives a unique possibility at
Astrup, that the backwash water can be ®ltered again in a special closed sand ®lter built for this purpose and radiated with UV-light, aerated and sent back to join the raw water between the ®rst and second ®lter. Furthermore, slurry of calcium hydroxide and a computer-regulated amount of carbon dioxide can be added to the raw water before its entrance to the second ®lter for manganese precipitation. Why is the sludge so much denser after biological treatment? In Forum, calcium hydroxide is added to the raw water in order to reduce the amount of aggressive CO2. The resulting calcium carbonate is then precipitated together with the iron oxides as a low-density colloid sludge in the ®rst ®lter. The negative z-potential keeps the iron precipitate in colloidal solution for large portions of the iron precipitate. Conversely, in Astrup, calcium hydroxide is not added to the raw water until after the ®rst ®lter. The biotic precipitation in the ®rst ®lter then results in a very dense structure of the iron sludge constituents. The reason for the interaction of bacteria with iron oxides and the mechanism involved is not well understood. However, a number of factors in¯uence the precipitation process when the bacteria compete with abiotic precipitation. The highest content of HCOÿ 3 /CO2 was found at Forum, where aeration is done before ®ltering, so most of the carbon dioxide is already stripped o when the water reaches the ®lters. In Astrup and Grindsted, on the other hand, the aeration and addition of calcium hydroxide is done between the ®lters, leaving the CO2 for bacterial growth in the ®rst ®lter. At Astrup and Grindsted the pH is 7 and the iron concentration 11 and 3 mg/l, respectively. How can these iron-precipitating bacteria thrive so successfully in an environment where they have to compete with autooxidation? An intracytoplasmic membrane system in G. ferruginea is thought to be a protection mechanism against chemical autooxidation of the limited amount of Fe(II) (LuÈtters and Hanert, 1989). Hallbeck and Pedersen (1990), however, think that stalk formation will only occur under increased levels of oxygen and also has a protective function. The negative z-potentials of the biotic precipitate implied that Fe(II) was easily bound within the stalks. Iron precipitate in exopolymers results in a mineralised cellular matrix containing iron oxides that are not easily re-dissolved (Beveridge and Fyfe, 1985). In Forum, it was the colloid precipitated autooxidised sludge with its calcite matrix that carried a negative z-potential. The Eh is another important factor governing iron precipitation and bacterial growth. G. ferruginea does not form stalks at an Eh less than ÿ40 mV (Hallbeck and Pedersen, 1990) and the preferred Eh-conditions are above +200 mV (Ehrlich, 1990). Hanert (1992) reports the optimal conditions to be
Iron precipitation in Danish freshwater plants
between +200 mV to +320 mV with an oxygen content of 0.1±1 mg/l O2. However, an oxygen content of 0.1 mg/l corresponds to an Eh of 370 mV at pH 7 when oxygen is the sole oxidation component and therefore Eh-dominating. rH2 26:5: In Astrup, the Eh rises from about ÿ60 mV to about 100 mV after the iron-precipitating ®lter and in Grindsted from ÿ60 mV to about 200 mV, which would correspond to very low oxygen contents. However, the measured oxygen content in the ®lters was higher. Therefore oxygen alone cannot be the Eh-dominating factor in the ®lters. At the measured pH of 7 these values correspond to rH2-values between 12 and 21. The sulphate content of the raw water at Astrup was about 40 mg/l and the sulphide content was below the detection limit for the analytical procedure used (0.1 mg/l). A calculation based on these numbers gives an Eh of ÿ35 mV, provided sulphate were the only oxidation component. This shows that the Eh-dominating species in the raw water was sulphate because of its higher amount relative to iron. As a conclusion, in the sand ®lter, the dominating role changes gradually from sulphur to oxygen with an increase in Eh as a result. Some of the sulphate in the water from Astrup was used during the ®ltering, because the initial measured content (40 mg/l) decreased. In one case it fell to about 20 mg/l during the ®rst ®lter without any detectable sulphide release into the water. Some of the sulphate could have been reduced to HSÿ and re-oxidised to non-soluble compounds, So or metallic polysulphides (Langmuir, 1997). Indeed, by EDS we found sulphur in the sludge, but in small amounts. So, it is still an open question as to where the majority of the sulphur ends up, as it is neither found in the exopolymers, nor in the bacteria at such high levels. However, total biomass was not determined. Freshwater treatment plants use permanganate consumption as an indication of the level of organic substances in the water. In this study, the permanganate consumption was higher in the two biologically treated water plants than in the chemically treated plant, which could indicate the presence of more organics in the Astrup and Grindsted cases. Purely physical±chemical oxidation of Fe(II) has been observed to be retarded in synthetic and natural waters when Fe(II) is chelated with organics and the O2 partial pressure is kept at 0.5 atm (Theis and Singer, 1974). Furthermore, Sung and Morgan (1980) investigated the eect of Na2SO4 (0.165 M) on the rate of oxidative precipitation of Fe(II). They found that Na2SO4 reduces the rate by a factor of 100 compared to solutions of a freshwaterlike composition. Our sulphate contents are much lower (about 0.4 mM) so this direct physical±chemical eect is expected to be almost negligible. A tenfold increase in ionic strength reduces the rate constant to about half the value (Sung and Morgan,
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1980). This indicates that the speed of biological precipitation relative to the physicochemical case may be even higher than expected from our kinetics calculation if the raw waters from the borings of the three freshwater plants were identical with respect to organics, complexing cations, and ionic strength. The increased precipitation rate in the sand ®lter seen in the bacteriological case could be congruent with the fact that when iron is bound to organic compounds the redox potential of the Fe(III)/Fe(II) couple normally decreases substantially relatively to the O2/H2O couple (Stumm, 1992). However, chelating organics normally reduce the abiotic precipitation rate (Theis and Singer 1974), indicating another reason for the increased rate of oxidationprecipitation. It is known that complexation of Fe(II) to OH± or the adsorption of Fe(II) to surface hydroxyl groups of the precipitated hydrous oxide, e.g. ferrihydrite, lowers the redox potential of the iron couple relatively to the O2/H2O couple. This gives a relative increase in reaction rate constants of several orders of magnitude (Wherli, 1990). In fact, the rate determining step of the O2 reduction is thought to be the ®rst step, O2 e ÿ 4 O2ÿ , E o ÿ0:16 V. The next step, H HO2 e ÿ 4 H2 O2 , has a slightly higher Eo than the O2/H2O couple (Wherli, 1990). In the biotic case, this eect of complexation between Fe(II) and OHÿ is much lower than in the abiotic case due to the lower pH. Despite the fact that the pH is lower the precipitation rate is higher in the biotic case. An explanation of the increased rate of oxidationprecipitation is likely to be a chelate creation between Fe(II) and the exopolymers of Gallionella. The exopolymers are composed mainly of carbohydrates, which contain rows of hydroxyl groups with electron lone pairs for chelating with Fe(II). The Fe(II) will be kept in a ®xed position by the exopolymers in readiness for oxidation. After oxidation the amorphous iron(III)hydroxides are partly kept in the stalks of the exopolymer and partly released to start crystallisation as micro crystallites. In this way, the exopolymers act as a catalyst in the biotic oxidation of Fe(II) contrary to the inhibiting eect of Fe(II)-organic chelates in aqueous solution during abiotic oxidation. The enhanced precipitation speed could be a side eect of the concentration and incorporation of the precipitates in the exopolymers. It could, however, also be such that the bacteria actively participate in the oxidation process as a part of their metabolism. CONCLUSION
In this work, we have analysed iron precipitates from the three freshwater plants in order to evaluate biotic iron hydroxide precipitation as compared to abiotic precipitation. We propose that a probable explanation for the increased precipitation rate in
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the biological ®lters relative to the physicochemical ®lters could be that precipitation of iron hydroxides takes place in contact with the exopolymers of the bacterium G. ferruginea. The exopolymers act as a catalyst for the oxidation-precipitation process of iron and prevent re-dissolution of the iron hydroxides in spite of the oxic/anoxic conditions found in the biological ®lters. The precipitated iron hydroxides, e.g. ferrihydrite, autocatalyse the oxidation/ precipitation of Fe(II) by reducing the redox potential of the Fe(OH)3 (amorphous)/Fe(II) couple relative to the Fe(III)/Fe(II) couple in aqueous solution. That the iron-precipitating bacterium G. ferruginea is chemolithotropic with respect to the oxidation of Fe(II) is not proven, but it seems plausible. Though many arguments and interpretations have been presented here to elucidate the functioning of bacteria in the ®lter systems, there are still a number of open questions that need further investigation. However, there are clear advantages in using bacteria for iron precipitation in freshwater plants. AcknowledgementsÐThe authors express their thanks to the personnel at Astrup, Forum and Grindsted freshwater plants, especially thanks to Svend Laursen and Holger Brinckmann for their help in the collecting of samples. Thanks to engineer J. C. Blandford, KruÈger A/S, Horsens, Denmark for discussions about Astrup. Finally, thanks to Jacques Chevallier, Institute for Physics and Astronomy, University of Aarhus, Denmark for his help with TEM and SEM. REFERENCES
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