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The mechanisms of potassium permanganate on algae removal
ARTICLE IN PRESS
Water Research 39 (2005) 4420–4428 www.elsevier.com/locate/watres
The mechanisms of potassium permanganate on algae removal Jen-Jeng Chena, Hsuan-Hsien Yehb, a Department of Environmental Engineering and Science, Tajen University, Pingtung 907, Taiwan Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan
b
Received 11 February 2005; received in revised form 1 August 2005; accepted 12 August 2005
Abstract The effect of potassium permanganate as preoxidant for algae-laden source water and the mechanism that it causes algae cells aggregation was investigated. Synthetic algae suspensions, prepared from lab-cultured Chodatella sp., were used for batch preoxidation and settling tests. In order to study the effect of water hardness on the function of permanganate, some algae suspensions were spiked with CaCl2 solution. Experiments with preformed MnO2 to look into its effect on algae cell aggregation were also conducted. The results show that preoxidation with potassium permanganate would promote the aggregation of algae cells, and this phenomenon was even more significant with the existence of hardness causing ion, calcium. In addition to incorporating its reducing product, MnO2, into algae floc, and increased its specific gravity, and therefore its settling velocity, permanganate may also induce the release of extracellular organic matters (EOM) from algae cell. Based on SEM observation, EOM probably enhanced the incorporation of MnO2 into algae floc. The role played by calcium ion in promoting the function of permanganate can be explained by charge neutralization and also bridging between negatively charged surfaces. r 2005 Elsevier Ltd. All rights reserved. Keywords: Algae; Permanganate; Flocculation; EOM
1. Introduction A continuing worldwide problem for drinking water treatment industry is the presence of algae in source water. Algae in drinking water supply can cause significant disturbances including taste and odor, production of disinfection by-product (DBP), obstruction to coagulation, clogging of filter, and assimilable organic carbon (AOC) for growth of biofilm (Plummer and Edzwald, 2001; Schmidt et al., 1998; Oliver and Shindler, 1980). Algae removal by conventional treatment is more difficult than inorganic particle, due to their low specific density, motility, morphological Corresponding author. Tel.: +886 6 2757575x65823; fax: +886 6 2752790. E-mail address: [email protected] (H.-H. Yeh).
characteristics and negative surface charge (Pieterse and Cloot, 1997; Bernhardt and Clasen, 1991). Pretreatment with preoxidants are commonly used in drinking water treatment to enhance algae removal (Petrusˇ evski et al., 1996; Steynberg et al., 1996; Mouchet and Bonne´lye, 1998). Numerous studies have showed that preoxidants such as ozone, chlorine dioxide, chlorine, or permanganate can improve algae removal by coagulation and filtration processes (Plummer and Edzwald, 2002; Steynberg et al., 1996). These preoxidants not only served as algaecide to inactivate the algal cell, but also as flocculant aids to improve the removal of taste and odor, color, inorganics and natural organic matters in the purification of plants (Ma et al., 1997; Petrusˇ evski et al., 1996; Saunier et al., 1983). However, other studies also showed that preoxidants may have negative effect on coagulation.
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.08.032
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Preoxidants may cause physiological stress or cell membrane damage resulting in releasing of taste and odor causing compounds or intracellular organic matter (IOM) into the bulk water (Plummer and Edzwald, 2001; Lam et al., 1995). Such intracellular organic matters may be precursors of the disinfection byproducts (DBPs). Sukenik et al. (1987) found that ozone and chlorine at different dosages had distinct effect on algal cell surface architecture, resulting in releasing of cellular organic compounds to the medium. Similar results were also observed by Plummer and Edzwald (2002). The better strategy for applying preoxidant for algae control in drinking water treatment would be removing algae cell intact without cell rupture (Drikas et al., 2001). The effect of preoxidant on algae during drinking water treatment may be varied with its type and dosage. Bernhardt (1989) reported that low ozone dosage may stress algae, and cause release of extracellular organic matter (EOM), which may serve as flocculant aid to improve coagulation; however, there is adverse effect on coagulation at higher ozone dosage. EOM, depending on the concentration and molecular weight, may enhance or hinder flocculation. Higher EOM concentrations and insufficient molecular weight inhibited coagulation, because it cannot bridge the particles and the increase in negative charge hinders the approach of particles. To avoid the formation of chlorinated DBPs and bromate from chlorination and ozonation, permanganate may be used as an alternative preoxidant when treating eutrophic source water. Permanganate has been used as algaecide and disinfectant (Kemp et al., 1966; Cleasby et al., 1964). Recently, attention has been drawn on the role of permanganate in enhancing coagulation and filtration processes. Ma et al. (1997) showed that permanganate preoxidation obviously enhanced the coagulation of several kinds of surface waters. Petrusˇ evski et al. (1996) reported that permanganate pretreatment followed by coagulation with dual coagulants (ferric salt and cationic polymer) distinctly improved particle and algae removal commonly achieved in direct filtration. They indicated that the manganese dioxide produced in situ during permanganate preoxidation played an important role and suggested that manganese dioxide may adsorb natural organic materials in improving the organic particulates removal and also possibly the inorganic fine particles. However, Colthurst and Singer (1982) reported that manganese dioxide did not appreciably adsorb humic substances, except in the presence of Ca2+, probably due to the presence of negative charge on both manganese dioxide and humic substances. Knappe et al. (2004) studied the effect of permanganate dose and contact time on the removal efficiency of algae by conducting experiments with algae-spiked
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natural surface water. They reported that algae removals improved with increasing KMnO4 dose and contact time, and when KMnO4 dose reached 3 mg/L with a contact time of 3 h, algae removals in excess of 80% were observed for Microcystis aeruginosa, Anabaena flos-aquae, and Synura petersenii. Knappe’s group also studied the effect of permanganate preoxidation on alum coagulation of Microcystis aeruginosa, which was spiked into a natural water sample at a concentration of about 10,000 cells/mL. KMnO4 preoxidation was found to effectively enhance algae removal by coagulation. Under same alum dose (40 mg/L), pretreatment with 0.5 and 3 mg KMnO4/L resulted in 75% and 96% algae removal, respectively; compared to 42% removal without pretreatment. Although numerous studies have shown that permanganate preoxidation benefits coagulation and filtration processes, fewer studies have looked directly into the mechanisms of permanganate on algae coagulation. The objectives of this study are (1) to investigate how permanganate reacts with algae cell, and promotes its aggregation, and (2) to study how the background water quality parameters, such as pH value and hardness causing ion (Ca2+), affect the function of permanganate.
2. Materials and methods 2.1. Algal culturing One of the dominant green algal species, Chodatella sp., isolated from local source water, was cultured in a media according to Norris et al. (1955). Axenic cultures of Chodatella sp. were grown in batch mode in 1 L modified serum bottle containing 600 mL sterilized algal media. The bottles were placed in the incubator maintained at 28 1C and continuously provided with 10 000 LUX of illumination. CO2 was supplied to the cultures every day. Cultures were harvested in the log growth phase after 7 days and individual cell size was 3–5 mm. 2.2. Preparation of the algae suspension Stock culture of Chodatella sp. cells was collected by centrifugation at 4000 rpm (3000g) for 5 min. The precipitated algal cells were then washed and resuspended in 0.015 M sodium perchlorate (NaClO4) solution, simulating the ionic strength of source water. Algae cell concentration in suspension solution was determined by the particle counter (Model 8000A, HIAC, Pacific Scientific Instruments, USA) and by measuring absorbance at 684 nm (OD684) with an ultraviolet/visible spectrophotometer (Model U-2001, Hitachi, Japan). Aliquot suspensions of constant algae cell concentration
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Culture of algae, 7 day Resuspended
Centrfigution,
in 0.015 M
3000 rpm, 5 min
NaClO4 4×107 cells/mL algae suspension
KMnO4
Adjustment of pH
Preoxidation 1.2µm filter
With/without calcium SEM, Settling tests
Algal cell
chemical analyses Mn analyses
Extraction of EOM
Fig. 1. Schematic diagram of the experimental procedure.
(4 107 cells/mL) were prepared for experiments. Fig. 1 shows schematic diagram of the experimental procedure. 2.3. Preparation of the colloidal manganese dioxide Colloidal manganese dioxide was prepared through the reduction of potassium permanganate by sodium thiosulfate in 0.015 M sodium perchlorate solution (Perez-Benito et al., 1996). The concentrations of potassium permanganate and sodium thiosulfate solution were 5 103 and 1.88 103 M, respectively. 2.4. Preoxidation and settling tests The stock solution of potassium permanganate was prepared by dissolving crystal potassium permanganate (Merck, Germany) in Milli-Q water (10 g/L) and filtered by 0.2 mm membrane filter (MFS, Japan). Then it was standardized by titration with sodium oxalate and diluted to the concentration for experiments. For preoxidation experiment, predetermined volume of potassium permanganate solution was added to algae suspension water. After 60 min of contact time under slow mixing condition with magnetic stirrer, samples
were collected for zeta potential and other water quality analysis. The zeta potential of algal cell and the floc that formed after permanganate preoxidation was measured by a zeta meter (Zetasizer 2000HSA, Malvern Instruments, UK), using the principle of Laser Doppler Electrophoresis. The water quality parameters measured included nonpurgeable dissolved organic carbon (NPDOC) and potassium. For NPDOC measurement, the sample were first filtered through 0.45 mm filter (MFS, Japan), dropped pH value to lower than 2 by adding phosphoric acid (85%), purged by high purity N2 gas, then injected into a total organic carbon analyzer (Model TOC5000, Shimadzu, Kyoto, Japan), which employed the combustion-infrared detection method. For potassium measurement, samples were first filtered through 0.2 mm membrane filter (MFS, Japan). The filtrate then was acidified with HCl (6 N) to pHo2, and the potassium analyzed by an atomic absorption spectrometer (Model Z-5300, Hitachi, Japan). For algae settling test, the preoxidized algae suspensions were poured into 1 L cone-shaped graduated plastic vessels (Imhoff cones) for quiet settling. At different time intervals, samples were collected at the
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depth of 5 cm below the surface, and residual algae concentration analyzed by OD684 measurement. The blank test without potassium permanganate dosing was also carried out simultaneously. All experiments were performed at room temperature (23 1C) and a majority of pH value at 7.570.5. For selected experiments, pH values of the algae suspensions were adjusted to 4 or 10 by using 0.5 N NaOH or 0.5 N HCl solutions. All pH values were measured by pH meter with glass electrode (Model SP-2200 Suntex, Taiwan). Further, for some experiments, the algae suspensions were spiked with various amount of CaCl2 solution (104 mg/L), to simulate the hardness in natural source water.
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Table 1 Effect of potassium permanganate preoxidation on NPDOC and potassium concentration of aliquot suspension and zeta potential of algae floc KMnO4 dose (mg/L)
NPDOC (mg/L)
K+ increaseda (%)
Zeta potential (mV)
0 1 2 3
0.152 0.147 0.248 0.149
0.0 11.7 18.8 21.9
32.870.8 32.670.9 33.170.6 33.371.8
a The increase in K+ concentration after sterilization by autoclave was 94.2%.
2.5. Extraction of EOM
3. Results and discussion
Table 1. Its effect on surface charge of the cells, as represented by zeta potential, was also included. The invariance of the aqueous NPDOC of algae suspension (4 107 cells/mL) after permanganate preoxidation at dosage up to 3 mg/L indicates that the integrity of algal cells was preserved, as no cellular organic compounds were released. Table 1 also lists the potassium ion concentration under various permanganate dosages and after algae suspension was sterilized by autoclave. As K+ is absorbed into the vacuole of cell and mainly stores as enzyme activator (Stewart, 1974), the release of K+ can be manifested for cell membrane damage (Peterson et al., 1995). The much small percentage increase in potassium ion associated with dosing permanganate, compared with the almost 100% increase after sterilization, also indicates that permanganate probably caused little damage to the cells. In addition, Table 1 also shows the effect of permanganate dosage on surface charge of cell in algal suspensions. Without permanganate preoxidation, the cells were negatively charge with a zeta potential of 32.8 mV. Permanganate preoxidation did not seem to have much effect on surface charge of the cells. The zeta potential of cells was about 33 mV on average at all three permanganate doses tested. Plummer and Edzwald (2002) found that ozone caused a reduction in electrophoretic mobility for Scenedesmus cells (green algae) and suggested that it was due in part to changes in the exterior portions of cell wall. The invariance of zeta potential of cells at permanganate doses of p3 mg/L demonstrates that permanganate preoxidation caused little change on cell surface, and probably also no cell damage. And this is in consistent with the data of NPDOC and K+, which have discussed previously.
3.1. Effect on algae cell damage and surface charge
3.2. Effect of permanganate on algae settling
The effect of permanganate dosage on cell damage, as indicated by release of NPDOC and K+, is presented in
The effect of permanganate preoxidation on cell settling is shown in Fig. 2. As the cell removal rate
Methods for extracting EOM from algae suspensions were adopted with modification from previous study (Paralkar and Edzwald, 1996). The preoxided algae suspensions were first filtered through 1.2 mm filter (MFS, Japan). The retentate was collected and resuspended in deionized water, then 1 N NH4OH solution was added slowly under mild stirring for 30 min. After this chemical stripping, the resuspended algal cells were filtered through a 1.2 mm filter again. The filtrate was collected and its manganese content analyzed by first acidified to pHo2 by HNO3 (6 N), then measured by inductively coupled plasma spectroscopy (ICP spectroscopy: Jobin Yvon, JY 38 PLUS Division d’Instruments S.A.). For EOM quantification, the collected filtrate was first concentrated in a rotary evaporator at 30 1C under a vacuum created by motor aspirator. Then the concentrated EOM was quantified by NPDOC measurement. 2.6. Scanning electron microscopy Algae samples for scanning electron microscopy (SEM) were first filtered through a 0.45 mm nylon membrane filter (MFS, Japan). The membrane filters were placed in phosphate buffer solution (pH ¼ 7) and fixed with 2.5% glutaldehyde at 4 1C over night. After that, the membrane filters were washed by phosphate buffer solution, dehydrated with successively different concentrations of ethanol and dried by critical point drying. Dried samples were mounted on copper stubs and sputter coated with gold–palladium. The specimens were observed and photographed using a scanning electron microscope (JEOL JXA-840, Japan) at 25 kV.
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increased with increasing permanganate dosage, permanganate preoxidation apparently enhanced cell settling. The enhance of cell settling by permanganate can be contributed to the oxidation function of permanganate, and also its reducing product, MnO2 colloids. Once MnO2 colloids were adsorbed onto cell surface, the specific weight of the cells and the floc that formed was increased, and therefore increased their settling velocity. Fig. 3 shows the settling rate of cells under three pH values, namely 4, 7, and 10, and with KMnO4 preoxidation at dosage of 1.5 mg/L. It can be noted that at pH 4 the cell settling rate was much higher than those at pH 7 and 10. This can be partly explained by the property of MnO2 colloids. The isoelectric point of MnO2 was reported to be between 2.8 and 4.5 (Posselt and Anderson, 1968). Therefore, at pH 4 MnO2 may carry some positive charges, and facilitate their adsorption onto the negatively charged cell surface. While at
100
Residue algae (%)
KMnO4 dose (mg/L) 0 1 3
95
pH 7 and 10, both MnO2 and cell surfaces are negatively charged, and adsorption would be hindered by electric repulsive force. However, enhancing cell settling by permanganate should not be contributed solely to MnO2. The comparison between adding KMnO4 and the preformed MnO2 to algae suspension, and look into their effect on cell settling is shown in Fig. 4. It can be seen that both KMnO4 and MnO2 could enhance cell settling. However, the effect of permanganate was more significant than that of MnO2. For the function of KMnO4 on algae cell aggregation, it is speculated that, in addition to the role played by its reducing products, MnO2, which has been elaborated previously, KMnO4 also could put stress on algae, and induce the release of EOM (Leppard, 1997; Paralkar and Edzwald, 1996). The higher cell settling velocity resulting from KMnO4 application, compared to that of MnO2, can be contributed to the function of EOM. First, EOM released after KMnO4 addition were extracted and quantified by NPDOC. The result was shown in Table 2. It can be noticed that the amount of EOM extractable increased with increasing KMnO4 dosage. However, the abrupt increase in NPDOC value when the KMnO4 dosage reached 3.2 mg/L is supposed
100 90
85 0
1
2
3 Time (h)
4
5
6
Fig. 2. Effect of potassium permanganate doses on cell settling.
Residue algae (%)
80 60 Blank 1.5 mg/L preformed MnO2 1.5 mg/L KMnO4
40 20 0
100 80 Residue algae (%)
0
1.5 mg/L KMnO4 pH 4 pH 7 pH 10
1
2 3 Time (h)
4
5
Fig. 4. Comparison of potassium permanganate and preformed manganese dioxide on cell settling.
60 Table 2 Effect of potassium permanganate doses on extractable EOM and its dissolved manganese content
40 20
KMnO4 dose (mg/L)
EOM extracted (mg NPDOC/mg Cell)
Dissolved Mn in EOM (mg/mg Cell)
0 0.6 1.3 3.2
2.0 9.7 12.9 116.4
0.30 0.34 0.41 0.58
0 0
5
10 15 Time (h)
20
25
Fig. 3. Effect of potassium permanganate preoxidation on cell settling under various pH values.
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to be due to the rupture of the cells, and the release of intra-cellular organic matters. The composition of EOM may include protein, polysaccharides, organic acid, carbohydrates, glycolic acid, amino acids and lipids (Stewart, 1974; Wingender et al., 1999). One of the functions of EOM to enhance algae cell aggregation may be due to its complexation or adsorption with MnO2. To prove this point, the Mn concentrations in the extracted EOM under various KMnO4 dosages were measured by ICP spectroscopy. Table 2 also shows that Mn content of EOM increased with increasing permanganate dosage. Therefore, this also proves the interaction between EOM and MnO2. Further, the authors tried to explore the difference in the interaction between KMnO4 and preformed MnO2 with algae using SEM micrographs. Fig. 5A shows the image of MnO2, which were formed by reacting KMnO4 with Na2S2O3 stoichiometrically. Fig. 5B and C were
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SEM micrographs of algae cells taken after algae suspensions have been dosed with preformed MnO2 and KMnO4, respectively. Comparing Fig. 5B and C, the difference in the cell surface between MnO2 and KMnO4 treated samples can be noticed. The surface of the cells with KMnO4 preoxidation seems to be coated with a thick layer of materials. Even the bristles of the cell were coated with similar materials. Furthermore, when the KMnO4 preoxidized and coagulated algae suspension was dosed with trypsin, the well-formed algae floc disintegrated. The SEM micrograph of the disintegrated algae floc (Fig. 5D) also shows less coverage by coating materials, as compared to Fig. 5C. It is speculated that the materials coated on the KMnO4 preoxidized cells are probably the aggregation of EOM and MnO2. As trypsin is an enzyme that acts to degrade protein, it may breakdown protein constituents of the coating material, and, therefore, reduces the thickness of the coating (Keil, 1970).
Fig. 5. SEM micrographs of algal cell surface morphology with and without preoxidation. (A) preformed MnO2, (B) preformed MnO2 adsorbed onto algal cell surface, (C) KMnO4 preoxidized algae cell, (D) after KMnO4 preoxidation, EOM elimination due to trypsin dosing.
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3.3. Effect of Ca on algae aggregation by KMnO4 To study the effect of water hardness on algae aggregation by KMnO4 preoxidation, various amounts of CaCl2 solution were added into the synthetic algae suspensions, to have the hardness value varied from 0 to 250 mg/L as CaCO3. Then KMnO4 preoxidation experiments repeated. Fig. 6 shows the results of one set of these experiments with KMnO4 dosage at 1 mg/L. It clearly shows that the settling velocity of algae cells increased with increasing Ca2+ concentration. Next, experiments were conducted to look into the effect of Ca2+ on algae cell aggregation with preformed MnO2 and without preoxidents at all. The experimental conditions of Fig. 7 were the same as those of Fig. 4, except CaCl2 solution was added into the former to let the hardness of its algae suspension reached 250 mg/L as
100 1 mg/L KMnO4 Calcium (mg/L as CaCO3)
Residue algae (%)
80
0 50 150 250
60 40 20 0 0
1
2
3
4
5
Time (h) Fig. 6. Effect of calcium concentration on cell settling with potassium permanganate preoxidation.
CaCO3. Comparing Fig. 7 with Fig. 4, it can be noticed that the existence of Ca2+ ions improved cell settling for all three cases. Enhancing of algae settling by the existence of Ca2+ ion can, firstly, be attributed to the decrease of surface charge of algae cell. This can be demonstrated by the less negative zeta potential value of the algae floc when calcium existed, compared to system without calcium (Fig. 8). Further, with the case of MnO2 dosing and under neutral pH condition, the surface of both MnO2 and algae cell are negatively charged. Under this condition, in addition to neutralize the surface charges, and the zeta potentials of the surfaces involved (the zeta potentials of aqueous MnO2 with and without Ca2+ spiking with hardness of 250 mg/L as CaCO3 were measures to be 8.1 and 29.8 mV, respectively), Ca2+ may served as bridges to hold the two negatively charged surfaces together (Amirtharajah and O’Melia, 1990). In case of KMnO4 addition, parts of Ca2+ ions may be tightened up with EOM, therefore its improvement on algae settling was not as significant as the case of direct MnO2 addition. The role of Ca2+ ion as bridges between two negatively charged surfaces can be further demonstrated by Fig. 9. The experimental conditions of Fig. 9 were the same as that of Fig. 3, except the former has a hardness of 250 mg/L as CaCO3, also by spiking with CaCl2 solution. First, it can be seen that the existence of hardness improved algae settling at all pH values tested. However, with the existence of Ca2+ the pH 10 system had the highest settling rate, contrary to pH 4 for those without hardness, as shown in Fig. 3. The explanation is, under pH 10, surfaces of both algae cell and MnO2, which formed as the reducing products of KMnO4, are more negatively charged, and Ca2+ could serve as bridge and enhance the adsorption of MnO2 onto cell surface. Therefore, the specific gravity of algae cell floc is increased, and also the settling velocity.
100
-21 Without KMnO4 With KMnO4 (1 mg/L)
-24 Zeta potential (mV)
Residue algae (%)
80 Calcium concentration: 250 mg/L as CaCO3 Blank 1.5 mg/L preformed MnO2 1.5 mg/L KMnO4
60 40 20
-27 -30 -33
0 0
1
2
3
4
5
Time (h) Fig. 7. Comparison of potassium permanganate and preformed manganese dioxide on cell settling (with calcium).
-36 0
50 100 150 200 250 Calcium (mg/L as CaCO3)
300
Fig. 8. Effect of calcium concentration on zeta potential of algae floc with and without permanganate preoxidation.
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100
1.5 mg/L KMnO4 Calcium concentration: 250 mg/L as CaCO3 pH 4 pH 7 pH 10
Residue algae (%)
80
4427
authors also acknowledge the kind help of Mr Songlin Zhou in some water samples analysis.
References
60 40 20 0 0
5
10 15 Time (h)
20
25
Fig. 9. Effect of pH and calcium on cell settling with potassium permanganate preoxidation.
4. Conclusions The effect of potassium permanganate as preoxidant for algae-laden source water and the mechanism that it causes algae cells aggregation was investigated. Synthetic algae suspensions, prepared from lab-cultured Chodatella sp. with background ionic strength similar to local source water, were used for batch preoxidation and settling tests. In order to study the effect of water hardness on the function of permanganate, some algae suspensions were spiked with various amount of CaCl2 solution to render the resulting hardness varied from 0 to 250 mg/L as CaCO3. Experiments with preformed MnO2 to look into its effect on algae cell aggregation were also conducted. The results show that preoxidation with potassium permanganate would promote the aggregation of algae cells, and this phenomenon was even more significant with the existence of hardness causing ion, calcium. When the permanganate dosage did not exceed certain value, the integrity of the cell was probably preserved. In addition to incorporating its reducing product, MnO2, into algae floc, and increased its specific gravity, and therefore its settling velocity, permanganate may also induce the release of extracellular organic matters (EOM) through its oxidation function. Based on SEM observation, EOM probably enhanced the incorporation of MnO2 into algae floc. The role played by calcium ion in promoting the function of permanganate can be explained by both charge neutralization and also bridging between negatively charged surfaces.
Acknowledgments The authors express their appreciation to the National Science Council of Taiwan for providing the funding for this research work (NSC 92-2211-E-006-027). The
Amirtharajah, A., O’Melia, C.R., 1990. Coagulation process: destabilization, mixing, and flocculation. In: Pontius, F.W. (Eds.), Water Quality and Treatment: A Handbook of Community Water Supplies, fourth ed. McGraw-Hill, New York, pp. 268–365 (Chapter 6) Bernhardt, H., 1989. Studies on the treatment of eutrophic water. Water Supply 17, ss.12-1–ss.12-166. Bernhardt, H., Clasen, J., 1991. Flocculation of microorganisms. J. Water Suppl.: Res. Technol.-AQUA 40 (2), 76–87. Cleasby, J.C., Baumann, E.R., Black, C.D., 1964. Effectiveness of potassium permanganate for disinfection. J. Am. Water Works Assoc. 56 (4), 466–474. Colthurst, J.M., Singer, P.C., 1982. Removing trihalomethane precursor by permanganate oxidation and manganese dioxide adsorption. J. Am. Water Works Assoc. 74 (2), 78–83. Drikas, M., Chow, C.W.K., House, J., Burch, M.D., 2001. Using coagulation, flocculation, and settling to remove toxic cyancbacteria. J. Am. Water Works Assoc. 93 (2), 100–111. Keil, B., 1970. Trypsin. In: Boyer, P.D., (Eds.), The Enzymes, third ed. vol. 3. Academic Press, New York, pp. 249–275. Kemp, H.T., Fuller, R.G., Davidson, R.S., 1966. Potassium permanganate as an algicide. J. Am. Water Works Assoc. 58 (2), 255–263. Knappe, R., Detlef, R.U., Belk, C., Briley, D.S., Grandy, S.R., Rastogi, N., Rike, A.H., 2004. Algae detection and removal strategies for drinking water treatment plants. AWWA Research Foundation, Denver, USA. Lam, A.K.Y., Prepas, E.E., Spink, D., Hrudey, S.E., 1995. Chemical control of hepatotoxic phytoplankton blooms: implications for human health. Water Res. 29 (8), 1845–1854. Leppard, G.G., 1997. Colloidal organic fibrils of acid polysaccharides in surface waters: electron-optical characteristics, activities and chemical estimates of abundance. Colloids Surf. A 120, 1–15. Ma, J., Graham, N., Li, G., 1997. Effect of permanganate preoxidation in enhancing the coagulation of surface waterlaboratory case studies. J. Water Suppl.: Res. Technol.AQUA 46 (1), 1–10. Mouchet, P., Bonne´lye, V., 1998. Solving algae problems: French expertise and world-wide application. J. Water Suppl.: Res. Technol.-AQUA 47 (3), 125–141. Norris, L., Norris, R.E., Calvin, M., 1955. J. Exptl. Bot. 6, 64. Oliver, B.G., Shindler, D.B., 1980. Trialomethanes from the chlorination of aquatic algae. Environ. Sci. Technol. 14 (12), 1502–1505. Paralkar, A., Edzwald, J.K., 1996. Effect of ozone on EOM and coagulation. J. Am. Water Works Assoc. 88 (4), 143–154. Perez-Benito, J.F., Arias, C., Amat, E., 1996. A kinetic study of the reduction of colloidal manganese dioxide by oxalic acid. J. Colloid Interface Sci. 177, 288–297. Peterson, H.G., Hrudey, S.E., Cantin, I.A., Perley, T.R., Kenefick, S.L., 1995. Physiological toxicity, cell membrane
ARTICLE IN PRESS 4428
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damage and the release of dissolved organic carbon and geosmin by aphanizomenon flos-aquae after exposure to water treatment chemicals. Water Res. 29 (6), 1515–1523. Petrusˇ evski, B., Van Breemen, A.N., Alaerts, G., 1996. Effect of permanganate pre-treatment and coagulation with dual coagulants on algae removal in direct filtration. J. Water Suppl.: Res. Technol.-AQUA 45 (5), 316–326. Pieterse, A.J.H., Cloot, A., 1997. Algal cells and coagulation, flocculation and sedimentation process. Water Sci. Technol. 36 (4), 111–118. Plummer, J.D., Edzwald, J.K., 2001. Effect of ozone on algae as precursors for trihalomethane and haloacetic acid. Environ. Sci. Technol. 35 (18), 3661–3668. Plummer, J.D., Edzwald, J.K., 2002. Effects of chlorine and ozone on algal cell properties and removal of algae by coagulation. J. Water Suppl.: Res. Technol.-AQUA 51 (6), 307–318. Posselt, H.S., Anderson, F.J., 1968. Cation sorption on colloidal hydrous manganese dioxide. Environ. Sci. Technol. 2 (12), 1087–1093.
Saunier, B.M., Selleck, R.E., Trussell, R., 1983. Preozonation as a coagulant aid in drinking water treatment. J. Am. Water Works Assoc. 75 (5), 239–246. Schmidt, W., Hambsch, B., Petzoldt, H., 1998. Classification of algogenic organic matter concerning its contribution to the bacterial regrowth potential and by-products formation. Water Sci. Technol. 37 (2), 91–96. Stewart, W.D.P., 1974. Extracellular products. In: Hellebust, J.A. (Ed.), Algal Physiology and Biochemistry. University of California Press, Los Angeles (Chapter 30). Steynberg, M.C., Gugleilm, M.M., Geldenhuys, J.C., Pieterse, A.J.H., 1996. Chlorine and chlorine dioxide: pre-oxidants used as algocide in potable water plants. J. Water Suppl.: Res. Technol.-AQUA 45 (4), 162–170. Sukenik, A., Teltch, B., Wachs, A.W., Shelef, G., Nir, I., Levanon, D., 1987. Effect of oxidants on microalgal flocculation. Water Res. 21 (5), 533–539. Wingender, J., Neu, T.R., Flemming, H.-C., 1999. Microbial Extracellular Polymeric Substances, Characterization, Structure and Function. Springer, Berlin, Heidelberg, Germany.
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The mechanisms of potassium permanganate on algae removal Jen-Jeng Chena, Hsuan-Hsien Yehb, a Department of Environmental Engineering and Science, Tajen University, Pingtung 907, Taiwan Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan
b
Received 11 February 2005; received in revised form 1 August 2005; accepted 12 August 2005
Abstract The effect of potassium permanganate as preoxidant for algae-laden source water and the mechanism that it causes algae cells aggregation was investigated. Synthetic algae suspensions, prepared from lab-cultured Chodatella sp., were used for batch preoxidation and settling tests. In order to study the effect of water hardness on the function of permanganate, some algae suspensions were spiked with CaCl2 solution. Experiments with preformed MnO2 to look into its effect on algae cell aggregation were also conducted. The results show that preoxidation with potassium permanganate would promote the aggregation of algae cells, and this phenomenon was even more significant with the existence of hardness causing ion, calcium. In addition to incorporating its reducing product, MnO2, into algae floc, and increased its specific gravity, and therefore its settling velocity, permanganate may also induce the release of extracellular organic matters (EOM) from algae cell. Based on SEM observation, EOM probably enhanced the incorporation of MnO2 into algae floc. The role played by calcium ion in promoting the function of permanganate can be explained by charge neutralization and also bridging between negatively charged surfaces. r 2005 Elsevier Ltd. All rights reserved. Keywords: Algae; Permanganate; Flocculation; EOM
1. Introduction A continuing worldwide problem for drinking water treatment industry is the presence of algae in source water. Algae in drinking water supply can cause significant disturbances including taste and odor, production of disinfection by-product (DBP), obstruction to coagulation, clogging of filter, and assimilable organic carbon (AOC) for growth of biofilm (Plummer and Edzwald, 2001; Schmidt et al., 1998; Oliver and Shindler, 1980). Algae removal by conventional treatment is more difficult than inorganic particle, due to their low specific density, motility, morphological Corresponding author. Tel.: +886 6 2757575x65823; fax: +886 6 2752790. E-mail address: [email protected] (H.-H. Yeh).
characteristics and negative surface charge (Pieterse and Cloot, 1997; Bernhardt and Clasen, 1991). Pretreatment with preoxidants are commonly used in drinking water treatment to enhance algae removal (Petrusˇ evski et al., 1996; Steynberg et al., 1996; Mouchet and Bonne´lye, 1998). Numerous studies have showed that preoxidants such as ozone, chlorine dioxide, chlorine, or permanganate can improve algae removal by coagulation and filtration processes (Plummer and Edzwald, 2002; Steynberg et al., 1996). These preoxidants not only served as algaecide to inactivate the algal cell, but also as flocculant aids to improve the removal of taste and odor, color, inorganics and natural organic matters in the purification of plants (Ma et al., 1997; Petrusˇ evski et al., 1996; Saunier et al., 1983). However, other studies also showed that preoxidants may have negative effect on coagulation.
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.08.032
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Preoxidants may cause physiological stress or cell membrane damage resulting in releasing of taste and odor causing compounds or intracellular organic matter (IOM) into the bulk water (Plummer and Edzwald, 2001; Lam et al., 1995). Such intracellular organic matters may be precursors of the disinfection byproducts (DBPs). Sukenik et al. (1987) found that ozone and chlorine at different dosages had distinct effect on algal cell surface architecture, resulting in releasing of cellular organic compounds to the medium. Similar results were also observed by Plummer and Edzwald (2002). The better strategy for applying preoxidant for algae control in drinking water treatment would be removing algae cell intact without cell rupture (Drikas et al., 2001). The effect of preoxidant on algae during drinking water treatment may be varied with its type and dosage. Bernhardt (1989) reported that low ozone dosage may stress algae, and cause release of extracellular organic matter (EOM), which may serve as flocculant aid to improve coagulation; however, there is adverse effect on coagulation at higher ozone dosage. EOM, depending on the concentration and molecular weight, may enhance or hinder flocculation. Higher EOM concentrations and insufficient molecular weight inhibited coagulation, because it cannot bridge the particles and the increase in negative charge hinders the approach of particles. To avoid the formation of chlorinated DBPs and bromate from chlorination and ozonation, permanganate may be used as an alternative preoxidant when treating eutrophic source water. Permanganate has been used as algaecide and disinfectant (Kemp et al., 1966; Cleasby et al., 1964). Recently, attention has been drawn on the role of permanganate in enhancing coagulation and filtration processes. Ma et al. (1997) showed that permanganate preoxidation obviously enhanced the coagulation of several kinds of surface waters. Petrusˇ evski et al. (1996) reported that permanganate pretreatment followed by coagulation with dual coagulants (ferric salt and cationic polymer) distinctly improved particle and algae removal commonly achieved in direct filtration. They indicated that the manganese dioxide produced in situ during permanganate preoxidation played an important role and suggested that manganese dioxide may adsorb natural organic materials in improving the organic particulates removal and also possibly the inorganic fine particles. However, Colthurst and Singer (1982) reported that manganese dioxide did not appreciably adsorb humic substances, except in the presence of Ca2+, probably due to the presence of negative charge on both manganese dioxide and humic substances. Knappe et al. (2004) studied the effect of permanganate dose and contact time on the removal efficiency of algae by conducting experiments with algae-spiked
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natural surface water. They reported that algae removals improved with increasing KMnO4 dose and contact time, and when KMnO4 dose reached 3 mg/L with a contact time of 3 h, algae removals in excess of 80% were observed for Microcystis aeruginosa, Anabaena flos-aquae, and Synura petersenii. Knappe’s group also studied the effect of permanganate preoxidation on alum coagulation of Microcystis aeruginosa, which was spiked into a natural water sample at a concentration of about 10,000 cells/mL. KMnO4 preoxidation was found to effectively enhance algae removal by coagulation. Under same alum dose (40 mg/L), pretreatment with 0.5 and 3 mg KMnO4/L resulted in 75% and 96% algae removal, respectively; compared to 42% removal without pretreatment. Although numerous studies have shown that permanganate preoxidation benefits coagulation and filtration processes, fewer studies have looked directly into the mechanisms of permanganate on algae coagulation. The objectives of this study are (1) to investigate how permanganate reacts with algae cell, and promotes its aggregation, and (2) to study how the background water quality parameters, such as pH value and hardness causing ion (Ca2+), affect the function of permanganate.
2. Materials and methods 2.1. Algal culturing One of the dominant green algal species, Chodatella sp., isolated from local source water, was cultured in a media according to Norris et al. (1955). Axenic cultures of Chodatella sp. were grown in batch mode in 1 L modified serum bottle containing 600 mL sterilized algal media. The bottles were placed in the incubator maintained at 28 1C and continuously provided with 10 000 LUX of illumination. CO2 was supplied to the cultures every day. Cultures were harvested in the log growth phase after 7 days and individual cell size was 3–5 mm. 2.2. Preparation of the algae suspension Stock culture of Chodatella sp. cells was collected by centrifugation at 4000 rpm (3000g) for 5 min. The precipitated algal cells were then washed and resuspended in 0.015 M sodium perchlorate (NaClO4) solution, simulating the ionic strength of source water. Algae cell concentration in suspension solution was determined by the particle counter (Model 8000A, HIAC, Pacific Scientific Instruments, USA) and by measuring absorbance at 684 nm (OD684) with an ultraviolet/visible spectrophotometer (Model U-2001, Hitachi, Japan). Aliquot suspensions of constant algae cell concentration
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Culture of algae, 7 day Resuspended
Centrfigution,
in 0.015 M
3000 rpm, 5 min
NaClO4 4×107 cells/mL algae suspension
KMnO4
Adjustment of pH
Preoxidation 1.2µm filter
With/without calcium SEM, Settling tests
Algal cell
chemical analyses Mn analyses
Extraction of EOM
Fig. 1. Schematic diagram of the experimental procedure.
(4 107 cells/mL) were prepared for experiments. Fig. 1 shows schematic diagram of the experimental procedure. 2.3. Preparation of the colloidal manganese dioxide Colloidal manganese dioxide was prepared through the reduction of potassium permanganate by sodium thiosulfate in 0.015 M sodium perchlorate solution (Perez-Benito et al., 1996). The concentrations of potassium permanganate and sodium thiosulfate solution were 5 103 and 1.88 103 M, respectively. 2.4. Preoxidation and settling tests The stock solution of potassium permanganate was prepared by dissolving crystal potassium permanganate (Merck, Germany) in Milli-Q water (10 g/L) and filtered by 0.2 mm membrane filter (MFS, Japan). Then it was standardized by titration with sodium oxalate and diluted to the concentration for experiments. For preoxidation experiment, predetermined volume of potassium permanganate solution was added to algae suspension water. After 60 min of contact time under slow mixing condition with magnetic stirrer, samples
were collected for zeta potential and other water quality analysis. The zeta potential of algal cell and the floc that formed after permanganate preoxidation was measured by a zeta meter (Zetasizer 2000HSA, Malvern Instruments, UK), using the principle of Laser Doppler Electrophoresis. The water quality parameters measured included nonpurgeable dissolved organic carbon (NPDOC) and potassium. For NPDOC measurement, the sample were first filtered through 0.45 mm filter (MFS, Japan), dropped pH value to lower than 2 by adding phosphoric acid (85%), purged by high purity N2 gas, then injected into a total organic carbon analyzer (Model TOC5000, Shimadzu, Kyoto, Japan), which employed the combustion-infrared detection method. For potassium measurement, samples were first filtered through 0.2 mm membrane filter (MFS, Japan). The filtrate then was acidified with HCl (6 N) to pHo2, and the potassium analyzed by an atomic absorption spectrometer (Model Z-5300, Hitachi, Japan). For algae settling test, the preoxidized algae suspensions were poured into 1 L cone-shaped graduated plastic vessels (Imhoff cones) for quiet settling. At different time intervals, samples were collected at the
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depth of 5 cm below the surface, and residual algae concentration analyzed by OD684 measurement. The blank test without potassium permanganate dosing was also carried out simultaneously. All experiments were performed at room temperature (23 1C) and a majority of pH value at 7.570.5. For selected experiments, pH values of the algae suspensions were adjusted to 4 or 10 by using 0.5 N NaOH or 0.5 N HCl solutions. All pH values were measured by pH meter with glass electrode (Model SP-2200 Suntex, Taiwan). Further, for some experiments, the algae suspensions were spiked with various amount of CaCl2 solution (104 mg/L), to simulate the hardness in natural source water.
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Table 1 Effect of potassium permanganate preoxidation on NPDOC and potassium concentration of aliquot suspension and zeta potential of algae floc KMnO4 dose (mg/L)
NPDOC (mg/L)
K+ increaseda (%)
Zeta potential (mV)
0 1 2 3
0.152 0.147 0.248 0.149
0.0 11.7 18.8 21.9
32.870.8 32.670.9 33.170.6 33.371.8
a The increase in K+ concentration after sterilization by autoclave was 94.2%.
2.5. Extraction of EOM
3. Results and discussion
Table 1. Its effect on surface charge of the cells, as represented by zeta potential, was also included. The invariance of the aqueous NPDOC of algae suspension (4 107 cells/mL) after permanganate preoxidation at dosage up to 3 mg/L indicates that the integrity of algal cells was preserved, as no cellular organic compounds were released. Table 1 also lists the potassium ion concentration under various permanganate dosages and after algae suspension was sterilized by autoclave. As K+ is absorbed into the vacuole of cell and mainly stores as enzyme activator (Stewart, 1974), the release of K+ can be manifested for cell membrane damage (Peterson et al., 1995). The much small percentage increase in potassium ion associated with dosing permanganate, compared with the almost 100% increase after sterilization, also indicates that permanganate probably caused little damage to the cells. In addition, Table 1 also shows the effect of permanganate dosage on surface charge of cell in algal suspensions. Without permanganate preoxidation, the cells were negatively charge with a zeta potential of 32.8 mV. Permanganate preoxidation did not seem to have much effect on surface charge of the cells. The zeta potential of cells was about 33 mV on average at all three permanganate doses tested. Plummer and Edzwald (2002) found that ozone caused a reduction in electrophoretic mobility for Scenedesmus cells (green algae) and suggested that it was due in part to changes in the exterior portions of cell wall. The invariance of zeta potential of cells at permanganate doses of p3 mg/L demonstrates that permanganate preoxidation caused little change on cell surface, and probably also no cell damage. And this is in consistent with the data of NPDOC and K+, which have discussed previously.
3.1. Effect on algae cell damage and surface charge
3.2. Effect of permanganate on algae settling
The effect of permanganate dosage on cell damage, as indicated by release of NPDOC and K+, is presented in
The effect of permanganate preoxidation on cell settling is shown in Fig. 2. As the cell removal rate
Methods for extracting EOM from algae suspensions were adopted with modification from previous study (Paralkar and Edzwald, 1996). The preoxided algae suspensions were first filtered through 1.2 mm filter (MFS, Japan). The retentate was collected and resuspended in deionized water, then 1 N NH4OH solution was added slowly under mild stirring for 30 min. After this chemical stripping, the resuspended algal cells were filtered through a 1.2 mm filter again. The filtrate was collected and its manganese content analyzed by first acidified to pHo2 by HNO3 (6 N), then measured by inductively coupled plasma spectroscopy (ICP spectroscopy: Jobin Yvon, JY 38 PLUS Division d’Instruments S.A.). For EOM quantification, the collected filtrate was first concentrated in a rotary evaporator at 30 1C under a vacuum created by motor aspirator. Then the concentrated EOM was quantified by NPDOC measurement. 2.6. Scanning electron microscopy Algae samples for scanning electron microscopy (SEM) were first filtered through a 0.45 mm nylon membrane filter (MFS, Japan). The membrane filters were placed in phosphate buffer solution (pH ¼ 7) and fixed with 2.5% glutaldehyde at 4 1C over night. After that, the membrane filters were washed by phosphate buffer solution, dehydrated with successively different concentrations of ethanol and dried by critical point drying. Dried samples were mounted on copper stubs and sputter coated with gold–palladium. The specimens were observed and photographed using a scanning electron microscope (JEOL JXA-840, Japan) at 25 kV.
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increased with increasing permanganate dosage, permanganate preoxidation apparently enhanced cell settling. The enhance of cell settling by permanganate can be contributed to the oxidation function of permanganate, and also its reducing product, MnO2 colloids. Once MnO2 colloids were adsorbed onto cell surface, the specific weight of the cells and the floc that formed was increased, and therefore increased their settling velocity. Fig. 3 shows the settling rate of cells under three pH values, namely 4, 7, and 10, and with KMnO4 preoxidation at dosage of 1.5 mg/L. It can be noted that at pH 4 the cell settling rate was much higher than those at pH 7 and 10. This can be partly explained by the property of MnO2 colloids. The isoelectric point of MnO2 was reported to be between 2.8 and 4.5 (Posselt and Anderson, 1968). Therefore, at pH 4 MnO2 may carry some positive charges, and facilitate their adsorption onto the negatively charged cell surface. While at
100
Residue algae (%)
KMnO4 dose (mg/L) 0 1 3
95
pH 7 and 10, both MnO2 and cell surfaces are negatively charged, and adsorption would be hindered by electric repulsive force. However, enhancing cell settling by permanganate should not be contributed solely to MnO2. The comparison between adding KMnO4 and the preformed MnO2 to algae suspension, and look into their effect on cell settling is shown in Fig. 4. It can be seen that both KMnO4 and MnO2 could enhance cell settling. However, the effect of permanganate was more significant than that of MnO2. For the function of KMnO4 on algae cell aggregation, it is speculated that, in addition to the role played by its reducing products, MnO2, which has been elaborated previously, KMnO4 also could put stress on algae, and induce the release of EOM (Leppard, 1997; Paralkar and Edzwald, 1996). The higher cell settling velocity resulting from KMnO4 application, compared to that of MnO2, can be contributed to the function of EOM. First, EOM released after KMnO4 addition were extracted and quantified by NPDOC. The result was shown in Table 2. It can be noticed that the amount of EOM extractable increased with increasing KMnO4 dosage. However, the abrupt increase in NPDOC value when the KMnO4 dosage reached 3.2 mg/L is supposed
100 90
85 0
1
2
3 Time (h)
4
5
6
Fig. 2. Effect of potassium permanganate doses on cell settling.
Residue algae (%)
80 60 Blank 1.5 mg/L preformed MnO2 1.5 mg/L KMnO4
40 20 0
100 80 Residue algae (%)
0
1.5 mg/L KMnO4 pH 4 pH 7 pH 10
1
2 3 Time (h)
4
5
Fig. 4. Comparison of potassium permanganate and preformed manganese dioxide on cell settling.
60 Table 2 Effect of potassium permanganate doses on extractable EOM and its dissolved manganese content
40 20
KMnO4 dose (mg/L)
EOM extracted (mg NPDOC/mg Cell)
Dissolved Mn in EOM (mg/mg Cell)
0 0.6 1.3 3.2
2.0 9.7 12.9 116.4
0.30 0.34 0.41 0.58
0 0
5
10 15 Time (h)
20
25
Fig. 3. Effect of potassium permanganate preoxidation on cell settling under various pH values.
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to be due to the rupture of the cells, and the release of intra-cellular organic matters. The composition of EOM may include protein, polysaccharides, organic acid, carbohydrates, glycolic acid, amino acids and lipids (Stewart, 1974; Wingender et al., 1999). One of the functions of EOM to enhance algae cell aggregation may be due to its complexation or adsorption with MnO2. To prove this point, the Mn concentrations in the extracted EOM under various KMnO4 dosages were measured by ICP spectroscopy. Table 2 also shows that Mn content of EOM increased with increasing permanganate dosage. Therefore, this also proves the interaction between EOM and MnO2. Further, the authors tried to explore the difference in the interaction between KMnO4 and preformed MnO2 with algae using SEM micrographs. Fig. 5A shows the image of MnO2, which were formed by reacting KMnO4 with Na2S2O3 stoichiometrically. Fig. 5B and C were
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SEM micrographs of algae cells taken after algae suspensions have been dosed with preformed MnO2 and KMnO4, respectively. Comparing Fig. 5B and C, the difference in the cell surface between MnO2 and KMnO4 treated samples can be noticed. The surface of the cells with KMnO4 preoxidation seems to be coated with a thick layer of materials. Even the bristles of the cell were coated with similar materials. Furthermore, when the KMnO4 preoxidized and coagulated algae suspension was dosed with trypsin, the well-formed algae floc disintegrated. The SEM micrograph of the disintegrated algae floc (Fig. 5D) also shows less coverage by coating materials, as compared to Fig. 5C. It is speculated that the materials coated on the KMnO4 preoxidized cells are probably the aggregation of EOM and MnO2. As trypsin is an enzyme that acts to degrade protein, it may breakdown protein constituents of the coating material, and, therefore, reduces the thickness of the coating (Keil, 1970).
Fig. 5. SEM micrographs of algal cell surface morphology with and without preoxidation. (A) preformed MnO2, (B) preformed MnO2 adsorbed onto algal cell surface, (C) KMnO4 preoxidized algae cell, (D) after KMnO4 preoxidation, EOM elimination due to trypsin dosing.
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3.3. Effect of Ca on algae aggregation by KMnO4 To study the effect of water hardness on algae aggregation by KMnO4 preoxidation, various amounts of CaCl2 solution were added into the synthetic algae suspensions, to have the hardness value varied from 0 to 250 mg/L as CaCO3. Then KMnO4 preoxidation experiments repeated. Fig. 6 shows the results of one set of these experiments with KMnO4 dosage at 1 mg/L. It clearly shows that the settling velocity of algae cells increased with increasing Ca2+ concentration. Next, experiments were conducted to look into the effect of Ca2+ on algae cell aggregation with preformed MnO2 and without preoxidents at all. The experimental conditions of Fig. 7 were the same as those of Fig. 4, except CaCl2 solution was added into the former to let the hardness of its algae suspension reached 250 mg/L as
100 1 mg/L KMnO4 Calcium (mg/L as CaCO3)
Residue algae (%)
80
0 50 150 250
60 40 20 0 0
1
2
3
4
5
Time (h) Fig. 6. Effect of calcium concentration on cell settling with potassium permanganate preoxidation.
CaCO3. Comparing Fig. 7 with Fig. 4, it can be noticed that the existence of Ca2+ ions improved cell settling for all three cases. Enhancing of algae settling by the existence of Ca2+ ion can, firstly, be attributed to the decrease of surface charge of algae cell. This can be demonstrated by the less negative zeta potential value of the algae floc when calcium existed, compared to system without calcium (Fig. 8). Further, with the case of MnO2 dosing and under neutral pH condition, the surface of both MnO2 and algae cell are negatively charged. Under this condition, in addition to neutralize the surface charges, and the zeta potentials of the surfaces involved (the zeta potentials of aqueous MnO2 with and without Ca2+ spiking with hardness of 250 mg/L as CaCO3 were measures to be 8.1 and 29.8 mV, respectively), Ca2+ may served as bridges to hold the two negatively charged surfaces together (Amirtharajah and O’Melia, 1990). In case of KMnO4 addition, parts of Ca2+ ions may be tightened up with EOM, therefore its improvement on algae settling was not as significant as the case of direct MnO2 addition. The role of Ca2+ ion as bridges between two negatively charged surfaces can be further demonstrated by Fig. 9. The experimental conditions of Fig. 9 were the same as that of Fig. 3, except the former has a hardness of 250 mg/L as CaCO3, also by spiking with CaCl2 solution. First, it can be seen that the existence of hardness improved algae settling at all pH values tested. However, with the existence of Ca2+ the pH 10 system had the highest settling rate, contrary to pH 4 for those without hardness, as shown in Fig. 3. The explanation is, under pH 10, surfaces of both algae cell and MnO2, which formed as the reducing products of KMnO4, are more negatively charged, and Ca2+ could serve as bridge and enhance the adsorption of MnO2 onto cell surface. Therefore, the specific gravity of algae cell floc is increased, and also the settling velocity.
100
-21 Without KMnO4 With KMnO4 (1 mg/L)
-24 Zeta potential (mV)
Residue algae (%)
80 Calcium concentration: 250 mg/L as CaCO3 Blank 1.5 mg/L preformed MnO2 1.5 mg/L KMnO4
60 40 20
-27 -30 -33
0 0
1
2
3
4
5
Time (h) Fig. 7. Comparison of potassium permanganate and preformed manganese dioxide on cell settling (with calcium).
-36 0
50 100 150 200 250 Calcium (mg/L as CaCO3)
300
Fig. 8. Effect of calcium concentration on zeta potential of algae floc with and without permanganate preoxidation.
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100
1.5 mg/L KMnO4 Calcium concentration: 250 mg/L as CaCO3 pH 4 pH 7 pH 10
Residue algae (%)
80
4427
authors also acknowledge the kind help of Mr Songlin Zhou in some water samples analysis.
References
60 40 20 0 0
5
10 15 Time (h)
20
25
Fig. 9. Effect of pH and calcium on cell settling with potassium permanganate preoxidation.
4. Conclusions The effect of potassium permanganate as preoxidant for algae-laden source water and the mechanism that it causes algae cells aggregation was investigated. Synthetic algae suspensions, prepared from lab-cultured Chodatella sp. with background ionic strength similar to local source water, were used for batch preoxidation and settling tests. In order to study the effect of water hardness on the function of permanganate, some algae suspensions were spiked with various amount of CaCl2 solution to render the resulting hardness varied from 0 to 250 mg/L as CaCO3. Experiments with preformed MnO2 to look into its effect on algae cell aggregation were also conducted. The results show that preoxidation with potassium permanganate would promote the aggregation of algae cells, and this phenomenon was even more significant with the existence of hardness causing ion, calcium. When the permanganate dosage did not exceed certain value, the integrity of the cell was probably preserved. In addition to incorporating its reducing product, MnO2, into algae floc, and increased its specific gravity, and therefore its settling velocity, permanganate may also induce the release of extracellular organic matters (EOM) through its oxidation function. Based on SEM observation, EOM probably enhanced the incorporation of MnO2 into algae floc. The role played by calcium ion in promoting the function of permanganate can be explained by both charge neutralization and also bridging between negatively charged surfaces.
Acknowledgments The authors express their appreciation to the National Science Council of Taiwan for providing the funding for this research work (NSC 92-2211-E-006-027). The
Amirtharajah, A., O’Melia, C.R., 1990. Coagulation process: destabilization, mixing, and flocculation. In: Pontius, F.W. (Eds.), Water Quality and Treatment: A Handbook of Community Water Supplies, fourth ed. McGraw-Hill, New York, pp. 268–365 (Chapter 6) Bernhardt, H., 1989. Studies on the treatment of eutrophic water. Water Supply 17, ss.12-1–ss.12-166. Bernhardt, H., Clasen, J., 1991. Flocculation of microorganisms. J. Water Suppl.: Res. Technol.-AQUA 40 (2), 76–87. Cleasby, J.C., Baumann, E.R., Black, C.D., 1964. Effectiveness of potassium permanganate for disinfection. J. Am. Water Works Assoc. 56 (4), 466–474. Colthurst, J.M., Singer, P.C., 1982. Removing trihalomethane precursor by permanganate oxidation and manganese dioxide adsorption. J. Am. Water Works Assoc. 74 (2), 78–83. Drikas, M., Chow, C.W.K., House, J., Burch, M.D., 2001. Using coagulation, flocculation, and settling to remove toxic cyancbacteria. J. Am. Water Works Assoc. 93 (2), 100–111. Keil, B., 1970. Trypsin. In: Boyer, P.D., (Eds.), The Enzymes, third ed. vol. 3. Academic Press, New York, pp. 249–275. Kemp, H.T., Fuller, R.G., Davidson, R.S., 1966. Potassium permanganate as an algicide. J. Am. Water Works Assoc. 58 (2), 255–263. Knappe, R., Detlef, R.U., Belk, C., Briley, D.S., Grandy, S.R., Rastogi, N., Rike, A.H., 2004. Algae detection and removal strategies for drinking water treatment plants. AWWA Research Foundation, Denver, USA. Lam, A.K.Y., Prepas, E.E., Spink, D., Hrudey, S.E., 1995. Chemical control of hepatotoxic phytoplankton blooms: implications for human health. Water Res. 29 (8), 1845–1854. Leppard, G.G., 1997. Colloidal organic fibrils of acid polysaccharides in surface waters: electron-optical characteristics, activities and chemical estimates of abundance. Colloids Surf. A 120, 1–15. Ma, J., Graham, N., Li, G., 1997. Effect of permanganate preoxidation in enhancing the coagulation of surface waterlaboratory case studies. J. Water Suppl.: Res. Technol.AQUA 46 (1), 1–10. Mouchet, P., Bonne´lye, V., 1998. Solving algae problems: French expertise and world-wide application. J. Water Suppl.: Res. Technol.-AQUA 47 (3), 125–141. Norris, L., Norris, R.E., Calvin, M., 1955. J. Exptl. Bot. 6, 64. Oliver, B.G., Shindler, D.B., 1980. Trialomethanes from the chlorination of aquatic algae. Environ. Sci. Technol. 14 (12), 1502–1505. Paralkar, A., Edzwald, J.K., 1996. Effect of ozone on EOM and coagulation. J. Am. Water Works Assoc. 88 (4), 143–154. Perez-Benito, J.F., Arias, C., Amat, E., 1996. A kinetic study of the reduction of colloidal manganese dioxide by oxalic acid. J. Colloid Interface Sci. 177, 288–297. Peterson, H.G., Hrudey, S.E., Cantin, I.A., Perley, T.R., Kenefick, S.L., 1995. Physiological toxicity, cell membrane
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J.-J. Chen, H.-H. Yeh / Water Research 39 (2005) 4420–4428
damage and the release of dissolved organic carbon and geosmin by aphanizomenon flos-aquae after exposure to water treatment chemicals. Water Res. 29 (6), 1515–1523. Petrusˇ evski, B., Van Breemen, A.N., Alaerts, G., 1996. Effect of permanganate pre-treatment and coagulation with dual coagulants on algae removal in direct filtration. J. Water Suppl.: Res. Technol.-AQUA 45 (5), 316–326. Pieterse, A.J.H., Cloot, A., 1997. Algal cells and coagulation, flocculation and sedimentation process. Water Sci. Technol. 36 (4), 111–118. Plummer, J.D., Edzwald, J.K., 2001. Effect of ozone on algae as precursors for trihalomethane and haloacetic acid. Environ. Sci. Technol. 35 (18), 3661–3668. Plummer, J.D., Edzwald, J.K., 2002. Effects of chlorine and ozone on algal cell properties and removal of algae by coagulation. J. Water Suppl.: Res. Technol.-AQUA 51 (6), 307–318. Posselt, H.S., Anderson, F.J., 1968. Cation sorption on colloidal hydrous manganese dioxide. Environ. Sci. Technol. 2 (12), 1087–1093.
Saunier, B.M., Selleck, R.E., Trussell, R., 1983. Preozonation as a coagulant aid in drinking water treatment. J. Am. Water Works Assoc. 75 (5), 239–246. Schmidt, W., Hambsch, B., Petzoldt, H., 1998. Classification of algogenic organic matter concerning its contribution to the bacterial regrowth potential and by-products formation. Water Sci. Technol. 37 (2), 91–96. Stewart, W.D.P., 1974. Extracellular products. In: Hellebust, J.A. (Ed.), Algal Physiology and Biochemistry. University of California Press, Los Angeles (Chapter 30). Steynberg, M.C., Gugleilm, M.M., Geldenhuys, J.C., Pieterse, A.J.H., 1996. Chlorine and chlorine dioxide: pre-oxidants used as algocide in potable water plants. J. Water Suppl.: Res. Technol.-AQUA 45 (4), 162–170. Sukenik, A., Teltch, B., Wachs, A.W., Shelef, G., Nir, I., Levanon, D., 1987. Effect of oxidants on microalgal flocculation. Water Res. 21 (5), 533–539. Wingender, J., Neu, T.R., Flemming, H.-C., 1999. Microbial Extracellular Polymeric Substances, Characterization, Structure and Function. Springer, Berlin, Heidelberg, Germany.