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Controlling harmful algae blooms using aluminum-modified clay
Marine Pollution Bulletin 103 (2016) 211–219
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Controlling harmful algae blooms using aluminum-modified clay Yang Liu a,b,c, Xihua Cao a,c,⁎, Zhiming Yu a,c,⁎, Xiuxian Song a,c, Lixia Qiu a,b,c a b c
Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, People's Republic of China University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China Laboratory of Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266071, People's Republic of China
a r t i c l e
i n f o
Article history: Received 18 August 2015 Received in revised form 16 December 2015 Accepted 17 December 2015 Available online 4 January 2016 Keywords: HABs control Aluminum-modified clay Dispersion medium Suspension pH Surface charge Removal mechanism
a b s t r a c t The performances of aluminum chloride modified clay (AC-MC), aluminum sulfate modified clay (AS-MC) and polyaluminum chloride modified clay (PAC-MC) in the removal of Aureococcus anophagefferens were compared, and the potential mechanisms were analyzed according to the dispersion medium, suspension pH and clay surface charges. The results showed that AC-MC and AS-MC had better efficiencies in removing A. anophagefferens than PACMC. The removal mechanisms of the three modified clays varied. At optimal coagulation conditions, the hydrolysates of AC and AS were mainly monomers, and they transformed into Al(OH)3(am) upon their addition to algae culture, with the primary mechanism being sweep flocculation. The PAC mainly hydrolyzed to the polyaluminum compounds, which remained stable when added to the algae culture, and the flocculation mainly occurred through polyaluminum compounds. The suspension pH significantly influenced the aluminum hydrolysate and affected the flocculation between the modified clay and algae cells. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction The frequency, intensity, and geographic extent of harmful algae blooms (HABs) have increased worldwide in recent years and caused profound and deleterious effects on aquatic ecosystems, aquaculture, tourism and public health (e.g., Anderson et al., 2012; Liu et al., 2013; Al Shehhi et al., 2014), which has increased the focus on developing effective, environmentally friendly and relatively inexpensive methods of controlling HABs. The control of HABs and the mitigation of their effects using clay is a promising method worldwide (e.g., Shirota, 1989; Yu et al., 1994a; Anderson, 1997; Sengco and Anderson, 2004; Kim, 2006), and it has been fully implemented in the field, with good results reported in Japan, South Korea and China (Shirota, 1989; Sengco and Anderson, 2004; Lee et al., 2008; Song et al., 2010). However, unmodified clay was inefficient in the removal of algae cells, and such processes require a large amount of clay (Sengco et al., 2001; Yu et al., 2004), which complicates field applications; in addition, the high loads of clay may cause ecological concerns (Lee et al., 2008; Orizar et al., 2013). Thus, research has been conducted to increase the removal efficiency and reduce the amount of clay required for removal. Maruyama et al. (1987) found that inorganic acid-treated clay improved the removal efficiency, and Yu et al. (1994c) used polyaluminum chloride (PAC) to modify the clay surface and subsequently proposed the clay surface modification theory (Yu et al., 1994a, 1994b, 1994c, 1995). A number of materials and methods used to modify clay surfaces have improved ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Cao), [email protected] (Z. Yu).
http://dx.doi.org/10.1016/j.marpolbul.2015.12.017 0025-326X/© 2016 Elsevier Ltd. All rights reserved.
the HAB removal efficiency (e.g., Yu et al., 1994c, 1999; Sengco et al., 2001; Sun et al., 2004a; Sun et al., 2004b; Pierce et al., 2004; Lee et al., 2008; Liu et al., 2010), and researches related to HAB control have focused on screening for more cost-effective clay modification materials and methods and exploring the corresponding removal mechanisms (e.g., Sun et al., 2004a). Aluminum chloride (AC), aluminum sulfate (AS) and PAC (e.g., Yu et al., 1994c; Sengco et al., 2001; Pierce et al., 2004; Sengco et al., 2005; Hagström and Granéli, 2005; Pan et al., 2011) can be used to increase HAB removal efficiency. The differences in the molecular structure and properties of these compounds could lead to significant differences in the modified clay suspension, which would influence the removal efficiency. Previous studies have shown that the removal efficiency of PAC-MC prepared with seawater (SW) was substantially lower than that with deionized water (DW) (Sengco et al., 2001; Yu et al., 2004), and the sulfate in the suspension was found to be an important factor. However, the difference between the removal efficiencies of AC-MC or AS-MC prepared by DW and SW is still unclear. In addition, few studies have compared the removal efficiencies of the three aluminummodified clays. Comparisons of the removal efficiency of each modified clay suspension prepared using different dispersion mediums may contribute to a better understanding of the flocculation mechanisms and optimum application conditions in the field. Brown tide is a type of HAB caused by the picophytoplankton Aureococcus anophagefferens, and such phenomena have occurred in the United States (1985) (Sieburth et al., 1988), South Africa (1997) (Probyn et al., 2001) and China (2009) (Zhang et al., 2012). Since the first report of brown tide, this algae has continuously bloomed in
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Hebei Province coastal waters and caused significant ecological and economic losses. In 2011, a southward trend in brown tide was observed, and HABs were observed in Yantai coastal waters, Shandong Province. Previous studies indicated that clay showed little removal efficiency because of its small size and high density (Sengco et al., 2001; Yu et al., 2004). The wide distribution of A. anophagefferens from the surface to the bottom also increased the difficulty in controlling brown tide. Therefore, studies seeking more effective control methods have received great attention. Based on the results of the cited studies, we compared the ability of three types of aluminum-modified clay to remove A. anophagefferens and drew contour diagrams of the modified clay surface charge and removal efficiency (alum dosage versus suspension pH). The potential mechanisms underlying the efficiency of each modified clay were analyzed from the suspension pH, clay particle surface charge and alum hydrolysis products. A uniform algae flocculation mechanism model was developed for the three modified clays, and the relationships among the alum dosage; modified clay surface charge in the clay suspension, algae culture and clay-algae flocs; and the removal efficiency (%) were established. The differences in the ability of each modified clay to remove freshwater and seawater HABs and the practical implications of increasing the removal efficiency were also discussed.
ZS, Malvern Instruments, Malvern, UK) of the clay particles and the pH values of the clay suspensions were also determined. 2.3. Effects of dispersion medium and suspension pH on the removal efficiency of the three types of aluminum-modified clay The effects of the pH of the three modified clay suspensions (prepared with DW or SW) on the efficiency at which A. anophagefferens was removed was determined. Modified clay suspensions containing the same alum dosage were prepared with DW and SW. The clay concentration in the suspension was 25 g/L, and the total alum was 2.85 × 10− 2 mol/L. The pH of the clay suspension was adjusted to 3, 5, 7, 9, or 11 by adding aqueous HCl or NaOH. The pH obtained at this stage was within ±0.05 of the target pH. The clay particles continued exchanging ions with the dispersion medium; therefore, the pH values were measured and recorded before the removal experiments. The pH adjustment was repeated if the final pH was far from the target pH, and 0.10 g/L clay and 0.20 g/L clay were added to the algae culture to test its removal efficiency. The removal experiments were run in triplicate. The zeta potentials of the clay particles at various pH conditions were also measured (Zetasizer Nano ZS, Malvern Instruments). 2.4. Removal efficiencies of the three modified clay at different alum dosages
2. Materials and methods 2.1. Algae culture and clay preparation An A. anophagefferens (CCMP 1984) culture was bought from the US National Center for Marine Algae and Microbiota (East Boothbay, ME, USA). Each A. anophagefferens cell was approximately 2 μm to 3 μm in diameter and unable to move freely. The algae was cultured in L1 culture medium (Guillard and Hargraves, 1993) at 20 ± 1 °C under a light intensity of approximately 60–65 μmol photons m−2 S−1 and a 14:10 light:dark cycle. The growth of the cultured algae was monitored by measuring the fluorescence using a TD-700 fluorometer (Turner Designs, Sunnyvale, CA, USA), and the measurement method was calibrated by counting cells in a blood cell counting chamber. The algae removal experiments were performed using cultures in the mid-to-late exponential growth phase, with cell densities of 8.0 × 109 cells/L to 11.0 × 109 cells/L. The clay used in this experiment was collected from Jiangsu Province, China, and it showed a relatively high removal efficiency for A. anophagefferens in a previous study (Zhang et al., 2013). The reagents used in this experiment were all of analytical grade. The AC and AS were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China), and the PAC was obtained from Guangfu Fine Chemical (Tianjin, China), and the Al2O3 contents were 20.42%, 15.22% and 28.59%, respectively, as measured by the zinc chloride standard solution titration method. The modified clay was prepared as described by Yu et al. (1994c). The DW was obtained from a Milli-Q water purification system (Millipore, Billerica, MA, USA). The SW was collected offshore at Qingdao, China and then passed through 0.45 μm membrane filter (Whatman, Buckinghamshire, UK) to remove the particulate matter.
Removal efficiency diagrams at different pH values and alum dosages of the three aluminum-modified clays were drawn. The clay concentration in the suspensions was 25 g/L, and the tested concentrations of Al were 7.14 × 10−3 mol/L, 1.43 × 10−2 mol/L, 2.85 × 10−2 mol/L, 4.28 × 10−2 mol/L and 5.71 × 10−2 mol/L. The target pH values in this experiment were 3, 5, 7, 9 and 11, and the pH adjustment method was as described in Section 1.3. pH adjustments were repeated if the final pH was far from the target pH. The removal experiments were conducted at a clay concentration of 0.25 g/L and run in triplicate. The zeta potentials of the clay particles at different pH conditions were also measured (Zetasizer Nano ZS, Malvern Instruments). 2.5. Measurements of the removal efficiency, clay surface potential and pH Each removal experiment was conducted in a 50 mL test tube at 20 ± 1 °C. A 50 mL aliquot of the algae culture in the mid-to-late exponential growth phase was placed in the tube, and the required amount of the modified clay suspension was added, with the mixture briefly shaken by hand until the clay was thoroughly dispersed. The tubes were placed in racks and allowed to stand for 3 h under the culture conditions described above. A 45 mL aliquot of the upper layer of the algae culture was then transferred to another tube to measure the fluorescence of the culture (TD-700 fluorometer, Turner Designs). Each experiment was performed in triplicate. The removal efficiencies were later presented as the mean ± standard deviation. The differences between the mean removal efficiencies were tested for statistical significance using the unpaired Student's t-test. The statistical analysis was performed using SPSS 13.0 software (IBM, Armonk, NY, USA). The cell removal efficiency was calculated using the following equation:
2.2. Comparison of the removal efficiencies of the three types of aluminummodified clay
Removal efficiency ð%Þ
The removal efficiency of the three aluminum-modified clays at the same alum dosage was determined. The modified clay suspension was prepared using DW and SW, with the clay concentration in suspension at 25 g/L and the alum concentration at 2.85 × 10− 2 mol/L. A predetermined clay dosage was added to the algae culture, and the removal efficiency was determined at 0.10 g/L clay, 0.25 g/L clay, 0.50 g/L clay, 1.0 g/L clay, 1.5 g/L clay and 2.0 g/L clay. The removal experiments were run in triplicate. The zeta charges (Zetasizer Nano
100%;
¼ ½1−ðfinal fluorescence of treatment final fluorescence of controlÞ
where the final fluorescence of the control was used to account for cell sinkage. The pH values were measured using a LEICI PHS-3C digital pH meter (INESA Scientific Instrument Co., Ltd., Shanghai, China). The unit was calibrated at the beginning of each testing day using a commercially prepared buffer.
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The zeta charge of the clay was determined using a Zetasizer Nano ZS zeta charge analyzer (Malvern Instruments). The instrument automatically provided three measured values when each sample was analyzed. Each sample was measured twice, and the mean and standard deviation were determined.
3. Results
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Table 1 pH and zeta charges of the three modified clay suspensions prepared with DW and SW. Total alum concentration: 2.85 × 10−2 mol/L. pH
CLAY AC-MC AS-MC PAC-MC
Zeta charges (mV)
DW
SW
DW
SW
3.99 3.20 ± 0.01 3.04 4.14 ± 0.02
5.42 ± 0.02 3.50 ± 0.01 3.39 ± 0.02 4.27 ± 0.01
−4.81 ± 1.24 16.03 ± 0.51 −5.31 ± 0.46 56.29 ± 3.80
−13.37 ± 0.64 −3.83 ± 0.80 −9.66 ± 0.90 6.09 ± 0.41
3.1. Removal efficiency of A. anophagefferens by the three aluminummodified clays The efficiency at which algae cells were removed greatly improved after surface modifications. The algae cell removal efficiencies of the three modified clays prepared with DW were similar (Fig. 1a), whereas the algae cell removal efficiencies of AC-MC and AS-MC were similar and significantly higher than that of PAC-MC when the suspension was prepared with SW (Fig. 1b). The pH values of the three modified clay suspensions with the same aluminum dosage prepared with DW and SW were all acidic. The pH of the modified clay suspension prepared with SW was slightly higher than that prepared with DW (Table 1). Large differences were observed among the surface charges of the three modified clay suspensions. PAC-MC particles showed the highest surface charge at 56.29 ± 3.80 mV, whereas the surface charge of AS-MC was −5.31 ± 0.46 mV when prepared with DW. The surface charge of the modified clay decreased significantly when the suspension was prepared with SW. The surface charge of PAC-MC was only 6.09 ± 0.41 mV, and the surface charges of AC-MC and AS-MC were both negative (Table 1).
3.2. Effects of the dispersion medium on the removal efficiencies of the three aluminum-modified clays Different pH values were observed for each aluminum-modified clay suspension prepared with DW and SW (Table 1). The effects of the dispersion medium (DW or SW) on the removal efficiency of each aluminum-modified clay were compared by adjusting each suspension to the same pH. At most of the pH values tested (3 to 11), the removal efficiency of AC-MC or AS-MC prepared with DW were lower than that prepared with SW (Fig. 2a, b). When the pH was below 6.5, the removal efficiency of PAC-MC prepared with DW was higher than that prepared with SW, whereas the clay suspension prepared with SW showed a higher removal efficiency above pH 6.5 (Fig. 2c).
The dispersion medium had a significant impact on the particle surface charge at different pH values (Fig. 2). When the suspension pH of AC-MC was above 7.8 and the pH of AS-MC was above 3.9, the surface charge of the modified clay suspension prepared with SW was higher than that prepared with DW (Fig. 2a, b). The surface charge of the PAC-MC suspension prepared with DW was higher than that prepared with SW at all of the tested pH values (3 to 11) (Fig. 2c).
3.3. Effects of the suspension pH on the removal efficiencies of the three aluminum-modified clays 3.3.1. Removal efficiency The removal efficiency of AC-MC and AS-MC decreased as the pH increased when prepared with DW. The removal efficiency of PAC-MC decreased in the pH range from 3 to 7 and increased from 20% to 40% (0.25 g/L clay) in the suspension pH above 7 (Fig. 3a, c). At the pH range from 3 to 5, the removal efficiency of AC-MC and AS-MC decreased from 95% to 30%, whereas the removal efficiency of PAC-MC was still high at approximately 90%. At the pH range from 5 to 7, the removal efficiency of AC-MC and AS-MC decreased from 30% to 5% and from 35% to 22%, respectively, and the removal efficiency of PAC-MC decreased from 90% to 20%. At the pH range from 7 to 11, the removal efficiency of PAC-MC was higher than that of AC-MC and AS-MC (Fig. 3c). The removal efficiency of the three aluminum-modified clays decreased and then increased as the suspension pH increased (Fig. 3b, d). The removal efficiency of AC-MC and AS-MC showed similar tendencies and differentiated greatly from that of PAC-MC, especially at pH N 5 (Fig. 3b, d). The removal efficiency of AC-MC and AS-MC decreased from 95% to approximately 15% in the pH range from 3 to 7 and gradually increased to approximately 70% at pH 7 to 11. However, the removal efficiency of PAC-MC decreased from 95% to approximately 40% in the
Fig. 1. A. anophagefferens removal efficiencies by the three aluminum-modified clays with the same aluminum dosage prepared with (a) DW and (b) SW. Total alum concentration: 2.85 × 10−2 mol/L. (■)AC-MC (●) AS-MC (▲) PAC-MC (▼) CLAY (unmodified clay).
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pH range from 3 to 5 and increased to approximately 75% (0.25 g/L) as the pH increased (Fig. 3d).
3.3.2. Clay particle surface charge The surface charge of the AC-MC, AS-MC and PAC-MC particles first increased, reaching a maximum value at pH 5, and then decreased when prepared with DW at the suspension pH of 3 to 11. The surface charges of unmodified clay were all negative in the pH range studied (3 to 11) and decreased gradually as the pH increased. The isoelectric point of AC-MC in DW was at approximately pH 7.8 (Fig. 3e). The surfaces charges of the three modified clay particles showed an increasingdecreasing-increasing trend when prepared with SW. The charge increased between pH 3 and pH 5, decreased between pH 5 and pH 7 and then increased again between pH 7 and pH 11, reaching a maximum at pH 5 and a minimum at pH 7. The surface charges showed a tendency of PAC-MC N AC-MC N AS-MC at the same pH and aluminum dosage (Fig. 3f).
3.4. Coagulation diagrams for harmful algae removal by the three aluminummodified clays We studied the removal efficiency of the three aluminum-modified clays prepared with DW and SW at different pH values and alum dosages to determine the optimal conditions. From the results of these experiments, a removal efficiency diagram and a clay particle surface charge diagram was drawn for each aluminum-modified clay prepared with DW and SW. From the diagrams, the domains of the removal efficiencies and corresponding zeta charge values of each modified clay suspension were easily identified at different suspension pH values and alum conditions. The removal efficiency and the surface charge of the clay particles both decreased as the pH of the suspension increased when prepared with DW. The increased alum dosage in the clay suspension led to a high removal efficiency at higher pH values (Fig. 4a). The increased removal efficiency and surface charge values of AC-MC and AS-MC were not as obvious as those of PAC-MC as the alum dosage increased in the suspension at the same pH value (Fig. 4a, b). The maximum clay particle surface charge area (+50 mV) was found at a suspension pH of 3.5 to 5 and a −lg[AlT] (negative logarithm of the alum dosage) of 1.4 to 1.8 in suspension for PAC-MC (Fig. 4c-3). However, at this condition, the corresponding removal efficiency was not high at approximately 20% to 60% (Fig. 4a-3). The addition of modified clay decreased the pH of the algae culture, and the final algae culture pH was related to the pH of the clay suspension and alum dosage (Fig. 4c). The contour diagrams of the removal efficiency of the three modified clay suspensions prepared with SW at different pH values and alum dosage conditions showed a minimum removal efficiency area (less than 20%) at pH values from 5 to 7, and the removal efficiency increased as the suspension pH decreased or increased (Fig. 5a). The removal efficiencies of AC-MC (Fig. 5a-1) and AS-MC (Fig. 5a-2) were similar and showed significant differences from that of PAC-MC (Fig. 5a-3). The surface charge pattern for SW-prepared aluminum-modified clay was more complicated when compared with that of the DW-prepared suspension (Fig. 5b). At each pH value, the surface charge gradually increased for AC-MC and PAC-MC with increasing alum dosage; however, for AS-MC, a relative high area (−3 mV) occurred at the middle of the contour distribution pattern (Fig. 5b). The minimum removal efficiency area and the minimum surface charge area were not consistent for AC-MC and AS-MC but were consistent for PAC-MC (Fig. 5a, b). 4. Discussion 4.1. Comparison of the removal efficiencies of the three aluminum-modified clay
Fig. 2. Effects of the dispersion medium (DW and SW) on the removal efficiency (0.25 g/L clay) and zeta charge of (a) AC-MC, (b) AS-MC and (c) PAC-MC. (■) Removal efficiency of modified clay prepared with DW; (●) removal efficiency of modified clay prepared with SW; (□) zeta charge of modified clay prepared with DW; and (○) zeta charge of modified clay prepared with SW.
Previous studies have shown that the removal efficiency of PAC-MC prepared with SW is substantially lower than that with DW (Sengco et al., 2001; Yu et al., 2004), and the results in this paper verified this finding (Fig. 1). The results also showed that the effects of the dispersion medium on the removal efficiency of aluminum-modified clay were alum-type dependent (Fig. 2). The removal efficiency of PAC-MC greatly decreased when prepared with SW, whereas the removal efficiency of AS-MC and AC-MC were not significant affected by SW (Fig. 1). The removal efficiencies of the AC-MC and AS-MC suspensions prepared with SW were significantly higher than that of the PAC-MC suspension prepared with SW (Fig. 1). These results suggested that more effective control of marine HABs would occur using AC-MC or AS-MC. PAC-MC prepared with SW showed a limited removal efficiency (Fig. 1), and Sengco et al. (2001) believed that this low efficiency was the result of the high ion intensity of SW. Yu et al. (2004) found that SW decreased the surface charge of PAC-MC, and PAC-MC prepared with DW and SW showed differences in the suspension density and surface soakage. In this study, we noticed that the suspension pH values of the three modified clay suspensions prepared with DW and SW were different,
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Fig. 3. Removal efficiencies (tested at 0.10 g/L and 0.25 g/L clay) and zeta charges of the three aluminum-modified clays at different suspension pH values prepared with DW and SW: (a) DW 0.10 g/L; (b) DW 0.25 g/L; (c) SW 0.10 g/L; (d) SW 0.25 g/L; (e) DW zeta charge; and (f) SW zeta charge. Total alum concentration: 2.85 × 10−2 mol/L.
and the pH of the modified clay suspension prepared with SW was higher than that with DW (Table 1). All of the pH values of the suspensions were between the range of 3 and 5, and the SW-prepared PAC-MC showed the highest pH values (Table 1). In this pH range (3 to 5), the removal efficiency decreased as the pH values increased (Fig. 3). We analyzed the effects of the dispersion medium and suspension pH on the removal efficiency of the three modified clays. We found that the effects of the dispersion medium on the removal efficiency were related to the type of modified clay. The removal efficiency of the AC-MC and AS-MC suspensions prepared with DW were lower than those of the suspensions prepared with SW, although the removal efficiencies were generally equivalent at a pH below 7 (Fig. 3). However, the PAC-MC suspension prepared with DW showed a significantly higher removal efficiency than that prepared with SW when the suspension pH was below 6.5 (Fig. 3). The difference in the removal efficiency of suspensions prepared with DW and SW was most likely caused by sulfate in the SW.
When the three modified clays were prepared with the same dispersion medium (SW), the three modified clays showed similar removal efficiencies at a pH range of 3 to 5 (Fig. 3b, d). The DW-prepared PACMC showed a higher removal efficiency compared with AC-MC and AS-MC. Whether prepared with DW or SW, the removal efficiencies of AC-MC and AS-MC were similar and differentiated from PAC-MC, which may suggest that AC-MC and AS-MC shared the same removal mechanism and had a different mechanism from PAC-MC. 4.2. Possible mechanism of aluminum-modified clay The surface charge of the modified clay particles was influenced by the suspension pH. Similar removal efficiencies of AC-MC and AS-MC were obtained at the same pH values; however, the surface charge showed great differences (Table 1), and the surface charge of AS-MC was negative at all of the pH values. Differences in the surface charge did not lead to differences in the removal efficiency, which indicated that there were
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Fig. 4. The contour diagrams of the removal efficiency (a) (n = 3) and the surface charge (b) of the three modified clays prepared with DW and pH values of algae culture after the addition of clay (c) at different pH values and alum amounts. a-1, a-2 and a-3 represent the removal efficiency of AC-MC, AS-MC and PAC-MC, respectively, which was tested using 0.25 g/L clay. b-1, b-2 and b-3 show the surface charge of AC-MC, AS-MC and PAC-MC, respectively. c-1, c-2 and c-3 show the pH values of the algae culture after the addition of AC-MC, AS-MC and PAC-MC, respectively. In this diagram, the horizontal axis is the pH of the clay suspension and the vertical axis is the −lg[AlT] in suspension. The curve labels indicate the removal efficiency (%) (a), zeta charge values (mV) (b), and pH values of the algae culture after the addition of clay (c).
likely additional flocculation mechanisms affecting the AC-MC and AS-MC particles in suspension than charge neutralization. The removal efficiency was positively correlated with the surface charge of PAC-MC (Yu et al., 1999). The surface charge of PAC-MC (prepared with DW or SW) at pH 3 was lower than that at pH 5 (Fig. 3e, f); however, the removal efficiency was higher at pH 3 (Fig. 3a, b, c, d). The surface charge of the modified clay was influenced by the alum hydrolysis products attached to the clay particles. Although the relationship of the removal efficiency and surface charge was complex for the different types of modified clay, the removal efficiency and surface charge of the three modified clays at different pH conditions were consistent with the hydrolysis products of different types of alum. The alum hydrolysis products at different pH conditions could be used to explain the flocculation process and removal efficiency. The hydrolysis products of alum were influenced by the suspension pH (Wang et al., 2004). The hydrolysis products of AC and AS were mainly monomers with a low charge density at pH 3. When the monomers that occurred in the AC-MC and AS-MC suspensions were added to the algae culture at pH value above 8.5, large amounts of amorphous aluminum hydroxide (Al(OH)3(am)) rapidly formed (Amirtharajah and Mills, 1982). The removal of algae cells mainly occurred by sweep flocculation of Al(OH)3(am). The hydrolysis products of PAC were monomers (16%) and polynuclear aluminum compounds (84%) in this experiment. Polynuclear aluminum compounds are composed of alum and OH− and partly dissolved at pH 3 (Wang et al., 2004). The surface charge of PACMC was lower at pH 3 than at pH 5 (Fig. 3e, f). The alum monomers
hydrolyzed by PAC were also transformed to Al(OH)3(am) when added to the algae culture. Therefore, the removal of algae cells by PAC-MC at pH 3 was through Al(OH)3(am) and polynuclear aluminum compounds. Alum monomers were sensitive to pH, and the increased suspension pH decreased the concentration of monomers and the effects of sweep flocculation. However, the surface charge of the modified clay increased with increasing pH values in the pH range of 3 to 5, and a higher surface charge usually indicated a higher charge neutralization ability. However, the removal efficiency declined at pH 5 compared with that at pH 3, which most likely indicated that sweep flocculation caused by alum monomers played a more important role in the removal of algae cells by PAC-MC at pH 3 to 5, especially at pH 3. In the weak acidic pH ranges (5 to 7), the alum monomers in AC-MC and AS-MC decreased to a minimum (Wang et al., 2004) and the sweep flocculation effects disappeared. The hydrolysis product was mainly a mid to high polymer with a hexahydroxy ring structure (not Al13), and its charge density was weak (Wang et al., 2004). The hydrolysis products of AC and AS showed a tendency to transform into aluminum hydroxide precipitates with increasing pH values, and the removal ability decreased. Alum monomers in the PAC-MC suspension disappeared and the surface charge decreased as the pH increased; thus, the removal efficiency decreased (Fig. 3a, b, c, d). In the neutral to alkaline pH range (7 to 11), AC and AS showed a strong tendency to hydrolyze to aluminum hydroxide precipitates with weak charge densities. The hydrolysis products were mainly Al(OH)− 4 when the suspension pH was above 9. The hydrolysis product of PAC was relatively stable, although it could also combine with OH− in
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Fig. 5. Contour diagrams of the removal efficiency (a) (n = 3) and the surface charge (b) of the three modified clays prepared with SW at different pH values and alum dosage conditions. a-1, a-2 and a-3 represent the removal efficiency of AC-MC, AS-MC and PAC-MC, respectively, which were tested using 0.25 g/L clay. b-1, b-2 and b-3 show the surface charge of AC-MC, AS-MC and PAC-MC, respectively. In this diagram, the horizontal axis is the pH of the clay suspension and the vertical axis is the −lg[AlT] in suspension. The curve labels indicate the removal efficiency (%) (a) and zeta charge values (mV) (b).
suspension to occur alkaline dissolution (Wang et al., 2004). Therefore, when the three modified clays were prepared with DW, the removal ability decreased in the suspension pH range of 7 to 11 (Fig. 3a, c). However, when the modified clays were prepared with SW, the surface charge of the three types of aluminum-modified clay increased as the suspension pH increased, which was likely the result of the formation of precipitates, such as CaCO3, MgCO3 and Mg(OH)2 by Ca2+ and Mg2+ in SW at high pH values (Ayoub et al., 1986; Semerjian and Ayoub, 2003; Vandamme et al., 2012). The formed Mg(OH)2 precipitation adsorbed on the clay particles and increased the surface charge of the clay particles after absorbing positively charged ions in suspension. Therefore, the increased removal efficiency of the modified clay prepared with SW at a suspension pH above 9 was most likely the result of enhanced charge neutralization. Therefore, the removal ability of the modified clay was influenced by differences in the alum hydrolysis products at different suspension pH values. In the process of controlling HABs, the flocculation of harmful algae cells can be divided into two steps. The first step is preparing a clay suspension. In this step, the aluminum was hydrolyzed, and certain hydrolysis products were absorbed on the clay particles and changed the surface charges. The surface charge changed from negative to positive when enough positively charged polynuclear alum compounds were absorbed. The unabsorbed hydrolysis products remained uncombined in the suspension. In the second step, the modified clay particles in suspension were added to the algae culture, and flocculation occurred between the modified clay particles and the algae cells. In the flocculation process, conditions that could cause the highest removal efficiencies were identified as the optimal removal conditions of the modified clay. At the optimal flocculation conditions, the hydrolysis products of
AC and AS were mostly monomers (at low pH values), and they transformed into Al(OH)3(am) upon addition to the SW algae culture. The removal of algae cells only occurred in the “optimal sweep flocculation” area of alum sulfate (Amirtharajah and Mills, 1982). Therefore, AC-MC and AS-MC mainly functioned through Al(OH)3(am) formed upon addition to the algae culture to remove algae cells. The PAC hydrolyzed to monomers (Ala) and polynuclear compounds (Alb and Alc). The behaviors of Ala were similar to that of AC and AS, and Ala underwent great changes with increasing pH values. Alb and Alc attached to clay particles in the clay suspension and were relatively stable in the pH range of 3 to 11 (Wang et al., 2004). PAC-MC removed algae cells through Al(OH)3(am) and polynuclear aluminum compounds. Therefore, significant differences occurred in the removal mechanisms of PAC-MC and AC/AS-MC. The suspension pH influenced the aluminum hydrolysates; thus, it also influenced the removal efficiency. The continuity of the two steps in the removal process made it possible to establish relationships between the alum dosage, modified clay surface charge in suspension and algae cell removal efficiency. Dentel (1988, 1991) suggested that the mechanism of AS flocculation occurred through the effects of charge neutralization caused by the attachment of Al(OH)3(am) to the clay particles, and they proposed a flocculation model, with the main principle stating that after aluminum hydroxide attached to the clay surfaces, the total charge density could be regarded as the mixture of charges from the two components (Dentel, 1988, 1991). Wang and Tang (2006) modified this flocculation model according to the characteristics of PAC and advanced a flocculation model for PAC. The fundamental principles underlying these models were also suitable for the control
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of HABs by modified clay. Therefore, the two models could also be used to analyze the removal behaviors of the three modified clays at different suspension pH conditions. In the first step, ζ1;2 C1 ; 2 ¼ ζ1 C1 þ ζ2 C2
ð1Þ
where ζ1,2 is the surface charge of the modified clay at the different suspension pH conditions; C1,2 is the total amount of modified clay particles at the different suspension pH conditions; ζ1 is the surface charge of the clay particles at the different suspension pH conditions; C1 is the amount of clay particles at the different suspension pH conditions; ζ2 is the surface charge of the alum compounds absorbed by the clay particles or insoluble alum compounds at the different suspension pH conditions; and C2 is the amount of alum compounds absorbed by the clay particles or insoluble alum compounds at the different suspension pH conditions. For PAC-MC, C2 can be regarded as Alb, Alc and the amount of alum in Ala that exceeds the solubility and becomes aluminum hydroxide: C2 ¼ CT −CN;M
ð2Þ
where CT is the total amount of alum in the clay suspension; and CN,M is the dissolved alum at different pH conditions; the dissolved alum or Al(OH)− is mainly alum monomers, such as Al(H2O)3+ 6 4 , and their values could be obtained by the alum solubility diagram (Duan and Gregory, 2003). And for PAC-MC: CT ¼ Ala þ Alb þ Alc :
ð3Þ
In the second step, When AC-MC and AS-MC were added to the algae culture, the alum unabsorbed to the clay suspension transformed to Al(OH)3(am). The polynuclear alum compounds (transformed by monomers) absorbed on the clay particles in the suspension transformed to aluminum hydroxide when added to the algae culture. This amount of alum was still absorbed on the clay particles after transformation, and the clay particles and absorbed alum could be considered together. For PACMC, the unabsorbed alum transformed to aluminum hydroxide upon addition to the algae culture, and the polynuclear compounds (Alb and Alc) that were adsorbed onto the clay particles in the clay suspension were still attached to the clay particles and could be considered together. Subsequently, ζ1;2;3 C3 þ C01;2 þ C0N;M ¼ ζ4 C0N;M þ ζ3 C3 þ ζ01;2 C01;2
ð4Þ
where ζ1,2,3 is the surface charge of the modified clay-algae flocs after the addition of the modified clay; ζ 4 is the surface charge of the aluminum hydroxide at the pH of the algae culture after the addition of modified clay; C′ N,M is the amount of dissolved alum that was added to the algae culture and occurred in suspension as monomers and was transformed to aluminum hydroxide upon addition of algae culture; ζ 3 is the surface charge of the algae cells at the pH of the algae culture after the addition of the modified clay; C3 is the amount of algae cells in the flocs after the addition of modified clay, and its value is related to removal efficiency (%) of algae cells; ζ′1,2 is the surface charge of the modified clay particles at the pH of the algae culture after the addition of the modified clay; and C′1,2 is the amount of modified clay that was added to the algae culture. The model above was suitable for the control of algae cells by modified clay suspensions prepared with DW and SW. The model showed relationships among the alum dosage; modified clay surface charge in suspension, algae culture, and clay-algae flocs; and the removal efficiency (%). The model also showed that the removal efficiency of the
modified clay was related to the surface charge of the modified clay (ζ’1,2) at the pH of the algae culture after the addition of clay. For PACMC, the surface charge of the modified clay upon addition to the algae culture (ζ’1,2) is equivalent to the surface charge of the modified clay in suspension (ζ1,2) minus the charge loss caused by the compression of the double electrical layer in the SW minus the charge loss caused by the attachment of anion adsorption (such as SO2− 4 ). Therefore, the removal efficiency of the modified clay was correlated with the surface charge of the modified clay in suspension, which is consistent with the results found by Yu et al. (1999). At optimal flocculation, the alum monomers of AC-MC and AS-MC in suspension transformed to Al(OH)3(am), and their removal efficiencies were related to the surface charges of aluminum hydroxide (ζ’1,2). Therefore, when Ala occurred in the suspensions of AC-MC and AS-MC, even if the surface charge of the modified clay in the suspensions was negative, a high removal efficiency was still achieved (Fig. 3). In addition, the model provided an approach to calculating the number of algae cells that could be removed by 1 g modified clay. 4.3. Practical implications of this study These mechanisms also indicated that there were differences in the removal efficiencies of the three aluminum-modified clays in relation to freshwater and seawater blooms. When applied in the field, the practice is to prepare a modified clay suspension using the algae bloom water. When using AC-MC and AS-MC, whether the algae cells could be removed efficiently is dependent on whether Al(OH)3(am) is able to form in the algae culture. The pH and buffering capacity of freshwater is low; therefore, when treating freshwater algae blooms, it is advisable to avoid low pH conditions at the water surface after adding the clay suspension to form aluminum hydroxide. The PAC-MC suspension prepared with freshwater could remove algae cells effectively due to the high charge neutralization ability of polynuclear alum compounds. However, when using PAC-MC to remove seawater algae blooms, significantly decreased removal efficiencies were observed. The mechanism of AC-MC and AS-MC differed from that of PAC-MC, and PAC-MC prepared with SW presented limited removal effects. Therefore, it is better to control SW blooms by AC-MC and AS-MC. These results also present a number of practical implications for increasing the removal efficiency of HABs. Although the removal mechanism was not the same for the three modified clays, they each had a high removal efficiency at low suspension pH values. At high seawater quantities and strong buffering capacities and blending abilities, the pH of the surface seawater presented minimal changes, and the suspension pH could be reduced to increase the removal efficiency, especially for AC-MC and AS-MC. However, when controlling freshwater blooms using PAC-MC, the polynuclear alum compounds played the main role, and the optimal suspension pH was most likely 5. In recent years, the further enhancement of flocculation functions, such as bridging effects and charge neutralization effects, or the introduction of other functions (e.g., oxidation effects and killing effects) based on clay surface modifications have been the focus of HABs control researches. The effects of the introduced components in the composite application on alum hydrolysis and the corresponding removal mechanisms may be the subject of future research. 5. Conclusions In this study, we compared the removal efficiency of three aluminum-modified clays in controlling A. anophagefferens blooms and discussed their removal mechanisms according to the dispersion medium and suspension pH. We found that the removal efficiency of the PAC-MC suspension prepared with SW was significantly lower than that of AC-MC and AS-MC. Compared with PAC-MC, the dispersion medium showed little effect on AC-MC and AS-MC at a suspension pH below 7. Significant differences were observed in the removal
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mechanisms of PAC-MC compared with that of AC-MC and AS-MC. At optimal coagulation conditions, the hydrolyzed products of AC and AS were mainly monomers, and they transformed into Al(OH)3(am) upon addition to the algae culture, and the primary mechanism was sweep flocculation. PAC hydrolyzed to monomers (Ala) and polynuclear compounds (Alb and Alc). The behaviors of Ala were similar to that of AC and AS. The Alb and Alc attached to the clay particles remained stable upon their addition to the algae culture. The removal of algae cells by PAC-MC mainly occurred through Al(OH)3(am) and polynuclear aluminum compounds. The alum hydrolysis products were significantly influenced by the suspension pH, and the flocculation process between the modified clay particles and algae cells was subsequently influenced. Special attention should be paid to the suspension pH during field applications. The results also indicated that AC-MC and AS-MC were better suited to controlling seawater algae blooms. Acknowledgments We would like to thank Dr. Guangyuan Lu for his kind advice during the writing of this paper. This work was supported by the National Natural Science Foundation of China (NSFC)-Shandong Joint Fund for Marine Science Research Centers (Grant No. U1406403), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA11020302) and the Chinese State Oceanic Administration public welfare project (Grant No. 201305003-3) and the National Natural Science Foundation of China (NSFC) (Grant No. 41576119). The authors declare that they have no conflicts of interest. References Al Shehhi, M.R., Gherboudj, I., Ghedira, H., 2014. An overview of historical harmful algae blooms outbreaks in the Arabian Seas. Mar. Pollut. Bull. 86 (1–2), 314–324. http:// dx.doi.org/10.1016/j.marpolbul.2014.06.048. Amirtharajah, A., Mills, K.M., 1982. Rapid-mix design for mechanisms of alum coagulation. J. Am. Water Works Assoc. 74 (4), 210–216. http://dx.doi.org/10.2307/41271001. Anderson, D.M., 1997. Turning back the harmful red tide. Nature 388 (6642), 513–514. Anderson, D.M., Cembella, A.D., Hallegraeff, G.M., 2012. Progress in understanding harmful algal blooms: paradigm shifts and new technologies for research, monitoring, and management. Ann. Rev. Mar. Sci. 4 (4), 143–176. http://dx.doi.org/10.1146/annurevmarine-120308-081121. Ayoub, G.M., Lee, S.-I., Koopman, B., 1986. Seawater induced algal flocculation. Water Res. 20 (10), 1265–1271. http://dx.doi.org/10.1016/0043-1354(86)90157-0. Dentel, S.K., 1988. Application of the precipitation-charge neutralization model of coagulation. Environ. Sci. Technol. 22 (7), 825–832. http://dx.doi.org/10.1021/es00172a013. Dentel, S.K., 1991. Coagulant control in water treatment. Crit. Rev. Environ. Control. 21 (1), 41–135. http://dx.doi.org/10.1080/10643389109388409. Duan, J., Gregory, J., 2003. Coagulation by hydrolysing metal salts. Adv. Colloid Interf. Sci. 100–102, 475–502. http://dx.doi.org/10.1016/S0001-8686(02)00067-2. Guillard, R.R.L., Hargraves, P.E., 1993. Stichochrysis immobilis is a diatom, not a chrysophyte. Phycologia 32 (3), 234–236. http://dx.doi.org/10.2216/i0031-8884-32-3-234.1. Hagström, J.A., Granéli, E., 2005. Removal of Prymnesium parvum (Haptophyceae) cells under different nutrient conditions by clay. Harmful Algae 4 (2), 249–260. http:// dx.doi.org/10.1016/j.hal.2004.03.004. Kim, H.G., 2006. Mitigation and controls of HABs. In: Granéli, E., Turner, J. (Eds.), Ecology of Harmful AlgaeEcological Studies vol. 189. Springer, Berlin Heidelberg, pp. 327–338. Lee, Y.-J., Choi, J.-K., Kim, E.-K., Youn, S.-H., Yang, E.-J., 2008. Field experiments on mitigation of harmful algal blooms using a sophorolipid—yellow clay mixture and effects on marine plankton. Harmful Algae 7 (2), 154–162. http://dx.doi.org/10.1016/j.hal.2007. 06.004. Liu, G., Fan, C., Zhong, J., Zhang, L., Ding, S., Yan, S., Han, S., 2010. Using hexadecyl trimethyl ammonium bromide (CTAB) modified clays to clean the Microcystis aeruginosa blooms in Lake Taihu, China. Harmful Algae 9 (4), 413–418. http://dx. doi.org/10.1016/j.hal.2010.02.004. Liu, L., Zhou, J., Zheng, B., Cai, W., Lin, K., Tang, J., 2013. Temporal and spatial distribution of red tide outbreaks in the Yangtze River Estuary and adjacent waters, China. Mar. Pollut. Bull. 72 (1), 213–221. http://dx.doi.org/10.1016/j.marpolbul.2013.04.002. Maruyama, T., Yamada, R., Usui, K., Suzuki, H., Yoshida, T., 1987. Removal of marine red tide planktons with acid treated clay. Nippon Suisan Gakkaishi 53 (10), 1811–1819.
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Controlling harmful algae blooms using aluminum-modified clay Yang Liu a,b,c, Xihua Cao a,c,⁎, Zhiming Yu a,c,⁎, Xiuxian Song a,c, Lixia Qiu a,b,c a b c
Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, People's Republic of China University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China Laboratory of Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266071, People's Republic of China
a r t i c l e
i n f o
Article history: Received 18 August 2015 Received in revised form 16 December 2015 Accepted 17 December 2015 Available online 4 January 2016 Keywords: HABs control Aluminum-modified clay Dispersion medium Suspension pH Surface charge Removal mechanism
a b s t r a c t The performances of aluminum chloride modified clay (AC-MC), aluminum sulfate modified clay (AS-MC) and polyaluminum chloride modified clay (PAC-MC) in the removal of Aureococcus anophagefferens were compared, and the potential mechanisms were analyzed according to the dispersion medium, suspension pH and clay surface charges. The results showed that AC-MC and AS-MC had better efficiencies in removing A. anophagefferens than PACMC. The removal mechanisms of the three modified clays varied. At optimal coagulation conditions, the hydrolysates of AC and AS were mainly monomers, and they transformed into Al(OH)3(am) upon their addition to algae culture, with the primary mechanism being sweep flocculation. The PAC mainly hydrolyzed to the polyaluminum compounds, which remained stable when added to the algae culture, and the flocculation mainly occurred through polyaluminum compounds. The suspension pH significantly influenced the aluminum hydrolysate and affected the flocculation between the modified clay and algae cells. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction The frequency, intensity, and geographic extent of harmful algae blooms (HABs) have increased worldwide in recent years and caused profound and deleterious effects on aquatic ecosystems, aquaculture, tourism and public health (e.g., Anderson et al., 2012; Liu et al., 2013; Al Shehhi et al., 2014), which has increased the focus on developing effective, environmentally friendly and relatively inexpensive methods of controlling HABs. The control of HABs and the mitigation of their effects using clay is a promising method worldwide (e.g., Shirota, 1989; Yu et al., 1994a; Anderson, 1997; Sengco and Anderson, 2004; Kim, 2006), and it has been fully implemented in the field, with good results reported in Japan, South Korea and China (Shirota, 1989; Sengco and Anderson, 2004; Lee et al., 2008; Song et al., 2010). However, unmodified clay was inefficient in the removal of algae cells, and such processes require a large amount of clay (Sengco et al., 2001; Yu et al., 2004), which complicates field applications; in addition, the high loads of clay may cause ecological concerns (Lee et al., 2008; Orizar et al., 2013). Thus, research has been conducted to increase the removal efficiency and reduce the amount of clay required for removal. Maruyama et al. (1987) found that inorganic acid-treated clay improved the removal efficiency, and Yu et al. (1994c) used polyaluminum chloride (PAC) to modify the clay surface and subsequently proposed the clay surface modification theory (Yu et al., 1994a, 1994b, 1994c, 1995). A number of materials and methods used to modify clay surfaces have improved ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Cao), [email protected] (Z. Yu).
http://dx.doi.org/10.1016/j.marpolbul.2015.12.017 0025-326X/© 2016 Elsevier Ltd. All rights reserved.
the HAB removal efficiency (e.g., Yu et al., 1994c, 1999; Sengco et al., 2001; Sun et al., 2004a; Sun et al., 2004b; Pierce et al., 2004; Lee et al., 2008; Liu et al., 2010), and researches related to HAB control have focused on screening for more cost-effective clay modification materials and methods and exploring the corresponding removal mechanisms (e.g., Sun et al., 2004a). Aluminum chloride (AC), aluminum sulfate (AS) and PAC (e.g., Yu et al., 1994c; Sengco et al., 2001; Pierce et al., 2004; Sengco et al., 2005; Hagström and Granéli, 2005; Pan et al., 2011) can be used to increase HAB removal efficiency. The differences in the molecular structure and properties of these compounds could lead to significant differences in the modified clay suspension, which would influence the removal efficiency. Previous studies have shown that the removal efficiency of PAC-MC prepared with seawater (SW) was substantially lower than that with deionized water (DW) (Sengco et al., 2001; Yu et al., 2004), and the sulfate in the suspension was found to be an important factor. However, the difference between the removal efficiencies of AC-MC or AS-MC prepared by DW and SW is still unclear. In addition, few studies have compared the removal efficiencies of the three aluminummodified clays. Comparisons of the removal efficiency of each modified clay suspension prepared using different dispersion mediums may contribute to a better understanding of the flocculation mechanisms and optimum application conditions in the field. Brown tide is a type of HAB caused by the picophytoplankton Aureococcus anophagefferens, and such phenomena have occurred in the United States (1985) (Sieburth et al., 1988), South Africa (1997) (Probyn et al., 2001) and China (2009) (Zhang et al., 2012). Since the first report of brown tide, this algae has continuously bloomed in
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Hebei Province coastal waters and caused significant ecological and economic losses. In 2011, a southward trend in brown tide was observed, and HABs were observed in Yantai coastal waters, Shandong Province. Previous studies indicated that clay showed little removal efficiency because of its small size and high density (Sengco et al., 2001; Yu et al., 2004). The wide distribution of A. anophagefferens from the surface to the bottom also increased the difficulty in controlling brown tide. Therefore, studies seeking more effective control methods have received great attention. Based on the results of the cited studies, we compared the ability of three types of aluminum-modified clay to remove A. anophagefferens and drew contour diagrams of the modified clay surface charge and removal efficiency (alum dosage versus suspension pH). The potential mechanisms underlying the efficiency of each modified clay were analyzed from the suspension pH, clay particle surface charge and alum hydrolysis products. A uniform algae flocculation mechanism model was developed for the three modified clays, and the relationships among the alum dosage; modified clay surface charge in the clay suspension, algae culture and clay-algae flocs; and the removal efficiency (%) were established. The differences in the ability of each modified clay to remove freshwater and seawater HABs and the practical implications of increasing the removal efficiency were also discussed.
ZS, Malvern Instruments, Malvern, UK) of the clay particles and the pH values of the clay suspensions were also determined. 2.3. Effects of dispersion medium and suspension pH on the removal efficiency of the three types of aluminum-modified clay The effects of the pH of the three modified clay suspensions (prepared with DW or SW) on the efficiency at which A. anophagefferens was removed was determined. Modified clay suspensions containing the same alum dosage were prepared with DW and SW. The clay concentration in the suspension was 25 g/L, and the total alum was 2.85 × 10− 2 mol/L. The pH of the clay suspension was adjusted to 3, 5, 7, 9, or 11 by adding aqueous HCl or NaOH. The pH obtained at this stage was within ±0.05 of the target pH. The clay particles continued exchanging ions with the dispersion medium; therefore, the pH values were measured and recorded before the removal experiments. The pH adjustment was repeated if the final pH was far from the target pH, and 0.10 g/L clay and 0.20 g/L clay were added to the algae culture to test its removal efficiency. The removal experiments were run in triplicate. The zeta potentials of the clay particles at various pH conditions were also measured (Zetasizer Nano ZS, Malvern Instruments). 2.4. Removal efficiencies of the three modified clay at different alum dosages
2. Materials and methods 2.1. Algae culture and clay preparation An A. anophagefferens (CCMP 1984) culture was bought from the US National Center for Marine Algae and Microbiota (East Boothbay, ME, USA). Each A. anophagefferens cell was approximately 2 μm to 3 μm in diameter and unable to move freely. The algae was cultured in L1 culture medium (Guillard and Hargraves, 1993) at 20 ± 1 °C under a light intensity of approximately 60–65 μmol photons m−2 S−1 and a 14:10 light:dark cycle. The growth of the cultured algae was monitored by measuring the fluorescence using a TD-700 fluorometer (Turner Designs, Sunnyvale, CA, USA), and the measurement method was calibrated by counting cells in a blood cell counting chamber. The algae removal experiments were performed using cultures in the mid-to-late exponential growth phase, with cell densities of 8.0 × 109 cells/L to 11.0 × 109 cells/L. The clay used in this experiment was collected from Jiangsu Province, China, and it showed a relatively high removal efficiency for A. anophagefferens in a previous study (Zhang et al., 2013). The reagents used in this experiment were all of analytical grade. The AC and AS were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China), and the PAC was obtained from Guangfu Fine Chemical (Tianjin, China), and the Al2O3 contents were 20.42%, 15.22% and 28.59%, respectively, as measured by the zinc chloride standard solution titration method. The modified clay was prepared as described by Yu et al. (1994c). The DW was obtained from a Milli-Q water purification system (Millipore, Billerica, MA, USA). The SW was collected offshore at Qingdao, China and then passed through 0.45 μm membrane filter (Whatman, Buckinghamshire, UK) to remove the particulate matter.
Removal efficiency diagrams at different pH values and alum dosages of the three aluminum-modified clays were drawn. The clay concentration in the suspensions was 25 g/L, and the tested concentrations of Al were 7.14 × 10−3 mol/L, 1.43 × 10−2 mol/L, 2.85 × 10−2 mol/L, 4.28 × 10−2 mol/L and 5.71 × 10−2 mol/L. The target pH values in this experiment were 3, 5, 7, 9 and 11, and the pH adjustment method was as described in Section 1.3. pH adjustments were repeated if the final pH was far from the target pH. The removal experiments were conducted at a clay concentration of 0.25 g/L and run in triplicate. The zeta potentials of the clay particles at different pH conditions were also measured (Zetasizer Nano ZS, Malvern Instruments). 2.5. Measurements of the removal efficiency, clay surface potential and pH Each removal experiment was conducted in a 50 mL test tube at 20 ± 1 °C. A 50 mL aliquot of the algae culture in the mid-to-late exponential growth phase was placed in the tube, and the required amount of the modified clay suspension was added, with the mixture briefly shaken by hand until the clay was thoroughly dispersed. The tubes were placed in racks and allowed to stand for 3 h under the culture conditions described above. A 45 mL aliquot of the upper layer of the algae culture was then transferred to another tube to measure the fluorescence of the culture (TD-700 fluorometer, Turner Designs). Each experiment was performed in triplicate. The removal efficiencies were later presented as the mean ± standard deviation. The differences between the mean removal efficiencies were tested for statistical significance using the unpaired Student's t-test. The statistical analysis was performed using SPSS 13.0 software (IBM, Armonk, NY, USA). The cell removal efficiency was calculated using the following equation:
2.2. Comparison of the removal efficiencies of the three types of aluminummodified clay
Removal efficiency ð%Þ
The removal efficiency of the three aluminum-modified clays at the same alum dosage was determined. The modified clay suspension was prepared using DW and SW, with the clay concentration in suspension at 25 g/L and the alum concentration at 2.85 × 10− 2 mol/L. A predetermined clay dosage was added to the algae culture, and the removal efficiency was determined at 0.10 g/L clay, 0.25 g/L clay, 0.50 g/L clay, 1.0 g/L clay, 1.5 g/L clay and 2.0 g/L clay. The removal experiments were run in triplicate. The zeta charges (Zetasizer Nano
100%;
¼ ½1−ðfinal fluorescence of treatment final fluorescence of controlÞ
where the final fluorescence of the control was used to account for cell sinkage. The pH values were measured using a LEICI PHS-3C digital pH meter (INESA Scientific Instrument Co., Ltd., Shanghai, China). The unit was calibrated at the beginning of each testing day using a commercially prepared buffer.
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The zeta charge of the clay was determined using a Zetasizer Nano ZS zeta charge analyzer (Malvern Instruments). The instrument automatically provided three measured values when each sample was analyzed. Each sample was measured twice, and the mean and standard deviation were determined.
3. Results
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Table 1 pH and zeta charges of the three modified clay suspensions prepared with DW and SW. Total alum concentration: 2.85 × 10−2 mol/L. pH
CLAY AC-MC AS-MC PAC-MC
Zeta charges (mV)
DW
SW
DW
SW
3.99 3.20 ± 0.01 3.04 4.14 ± 0.02
5.42 ± 0.02 3.50 ± 0.01 3.39 ± 0.02 4.27 ± 0.01
−4.81 ± 1.24 16.03 ± 0.51 −5.31 ± 0.46 56.29 ± 3.80
−13.37 ± 0.64 −3.83 ± 0.80 −9.66 ± 0.90 6.09 ± 0.41
3.1. Removal efficiency of A. anophagefferens by the three aluminummodified clays The efficiency at which algae cells were removed greatly improved after surface modifications. The algae cell removal efficiencies of the three modified clays prepared with DW were similar (Fig. 1a), whereas the algae cell removal efficiencies of AC-MC and AS-MC were similar and significantly higher than that of PAC-MC when the suspension was prepared with SW (Fig. 1b). The pH values of the three modified clay suspensions with the same aluminum dosage prepared with DW and SW were all acidic. The pH of the modified clay suspension prepared with SW was slightly higher than that prepared with DW (Table 1). Large differences were observed among the surface charges of the three modified clay suspensions. PAC-MC particles showed the highest surface charge at 56.29 ± 3.80 mV, whereas the surface charge of AS-MC was −5.31 ± 0.46 mV when prepared with DW. The surface charge of the modified clay decreased significantly when the suspension was prepared with SW. The surface charge of PAC-MC was only 6.09 ± 0.41 mV, and the surface charges of AC-MC and AS-MC were both negative (Table 1).
3.2. Effects of the dispersion medium on the removal efficiencies of the three aluminum-modified clays Different pH values were observed for each aluminum-modified clay suspension prepared with DW and SW (Table 1). The effects of the dispersion medium (DW or SW) on the removal efficiency of each aluminum-modified clay were compared by adjusting each suspension to the same pH. At most of the pH values tested (3 to 11), the removal efficiency of AC-MC or AS-MC prepared with DW were lower than that prepared with SW (Fig. 2a, b). When the pH was below 6.5, the removal efficiency of PAC-MC prepared with DW was higher than that prepared with SW, whereas the clay suspension prepared with SW showed a higher removal efficiency above pH 6.5 (Fig. 2c).
The dispersion medium had a significant impact on the particle surface charge at different pH values (Fig. 2). When the suspension pH of AC-MC was above 7.8 and the pH of AS-MC was above 3.9, the surface charge of the modified clay suspension prepared with SW was higher than that prepared with DW (Fig. 2a, b). The surface charge of the PAC-MC suspension prepared with DW was higher than that prepared with SW at all of the tested pH values (3 to 11) (Fig. 2c).
3.3. Effects of the suspension pH on the removal efficiencies of the three aluminum-modified clays 3.3.1. Removal efficiency The removal efficiency of AC-MC and AS-MC decreased as the pH increased when prepared with DW. The removal efficiency of PAC-MC decreased in the pH range from 3 to 7 and increased from 20% to 40% (0.25 g/L clay) in the suspension pH above 7 (Fig. 3a, c). At the pH range from 3 to 5, the removal efficiency of AC-MC and AS-MC decreased from 95% to 30%, whereas the removal efficiency of PAC-MC was still high at approximately 90%. At the pH range from 5 to 7, the removal efficiency of AC-MC and AS-MC decreased from 30% to 5% and from 35% to 22%, respectively, and the removal efficiency of PAC-MC decreased from 90% to 20%. At the pH range from 7 to 11, the removal efficiency of PAC-MC was higher than that of AC-MC and AS-MC (Fig. 3c). The removal efficiency of the three aluminum-modified clays decreased and then increased as the suspension pH increased (Fig. 3b, d). The removal efficiency of AC-MC and AS-MC showed similar tendencies and differentiated greatly from that of PAC-MC, especially at pH N 5 (Fig. 3b, d). The removal efficiency of AC-MC and AS-MC decreased from 95% to approximately 15% in the pH range from 3 to 7 and gradually increased to approximately 70% at pH 7 to 11. However, the removal efficiency of PAC-MC decreased from 95% to approximately 40% in the
Fig. 1. A. anophagefferens removal efficiencies by the three aluminum-modified clays with the same aluminum dosage prepared with (a) DW and (b) SW. Total alum concentration: 2.85 × 10−2 mol/L. (■)AC-MC (●) AS-MC (▲) PAC-MC (▼) CLAY (unmodified clay).
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pH range from 3 to 5 and increased to approximately 75% (0.25 g/L) as the pH increased (Fig. 3d).
3.3.2. Clay particle surface charge The surface charge of the AC-MC, AS-MC and PAC-MC particles first increased, reaching a maximum value at pH 5, and then decreased when prepared with DW at the suspension pH of 3 to 11. The surface charges of unmodified clay were all negative in the pH range studied (3 to 11) and decreased gradually as the pH increased. The isoelectric point of AC-MC in DW was at approximately pH 7.8 (Fig. 3e). The surfaces charges of the three modified clay particles showed an increasingdecreasing-increasing trend when prepared with SW. The charge increased between pH 3 and pH 5, decreased between pH 5 and pH 7 and then increased again between pH 7 and pH 11, reaching a maximum at pH 5 and a minimum at pH 7. The surface charges showed a tendency of PAC-MC N AC-MC N AS-MC at the same pH and aluminum dosage (Fig. 3f).
3.4. Coagulation diagrams for harmful algae removal by the three aluminummodified clays We studied the removal efficiency of the three aluminum-modified clays prepared with DW and SW at different pH values and alum dosages to determine the optimal conditions. From the results of these experiments, a removal efficiency diagram and a clay particle surface charge diagram was drawn for each aluminum-modified clay prepared with DW and SW. From the diagrams, the domains of the removal efficiencies and corresponding zeta charge values of each modified clay suspension were easily identified at different suspension pH values and alum conditions. The removal efficiency and the surface charge of the clay particles both decreased as the pH of the suspension increased when prepared with DW. The increased alum dosage in the clay suspension led to a high removal efficiency at higher pH values (Fig. 4a). The increased removal efficiency and surface charge values of AC-MC and AS-MC were not as obvious as those of PAC-MC as the alum dosage increased in the suspension at the same pH value (Fig. 4a, b). The maximum clay particle surface charge area (+50 mV) was found at a suspension pH of 3.5 to 5 and a −lg[AlT] (negative logarithm of the alum dosage) of 1.4 to 1.8 in suspension for PAC-MC (Fig. 4c-3). However, at this condition, the corresponding removal efficiency was not high at approximately 20% to 60% (Fig. 4a-3). The addition of modified clay decreased the pH of the algae culture, and the final algae culture pH was related to the pH of the clay suspension and alum dosage (Fig. 4c). The contour diagrams of the removal efficiency of the three modified clay suspensions prepared with SW at different pH values and alum dosage conditions showed a minimum removal efficiency area (less than 20%) at pH values from 5 to 7, and the removal efficiency increased as the suspension pH decreased or increased (Fig. 5a). The removal efficiencies of AC-MC (Fig. 5a-1) and AS-MC (Fig. 5a-2) were similar and showed significant differences from that of PAC-MC (Fig. 5a-3). The surface charge pattern for SW-prepared aluminum-modified clay was more complicated when compared with that of the DW-prepared suspension (Fig. 5b). At each pH value, the surface charge gradually increased for AC-MC and PAC-MC with increasing alum dosage; however, for AS-MC, a relative high area (−3 mV) occurred at the middle of the contour distribution pattern (Fig. 5b). The minimum removal efficiency area and the minimum surface charge area were not consistent for AC-MC and AS-MC but were consistent for PAC-MC (Fig. 5a, b). 4. Discussion 4.1. Comparison of the removal efficiencies of the three aluminum-modified clay
Fig. 2. Effects of the dispersion medium (DW and SW) on the removal efficiency (0.25 g/L clay) and zeta charge of (a) AC-MC, (b) AS-MC and (c) PAC-MC. (■) Removal efficiency of modified clay prepared with DW; (●) removal efficiency of modified clay prepared with SW; (□) zeta charge of modified clay prepared with DW; and (○) zeta charge of modified clay prepared with SW.
Previous studies have shown that the removal efficiency of PAC-MC prepared with SW is substantially lower than that with DW (Sengco et al., 2001; Yu et al., 2004), and the results in this paper verified this finding (Fig. 1). The results also showed that the effects of the dispersion medium on the removal efficiency of aluminum-modified clay were alum-type dependent (Fig. 2). The removal efficiency of PAC-MC greatly decreased when prepared with SW, whereas the removal efficiency of AS-MC and AC-MC were not significant affected by SW (Fig. 1). The removal efficiencies of the AC-MC and AS-MC suspensions prepared with SW were significantly higher than that of the PAC-MC suspension prepared with SW (Fig. 1). These results suggested that more effective control of marine HABs would occur using AC-MC or AS-MC. PAC-MC prepared with SW showed a limited removal efficiency (Fig. 1), and Sengco et al. (2001) believed that this low efficiency was the result of the high ion intensity of SW. Yu et al. (2004) found that SW decreased the surface charge of PAC-MC, and PAC-MC prepared with DW and SW showed differences in the suspension density and surface soakage. In this study, we noticed that the suspension pH values of the three modified clay suspensions prepared with DW and SW were different,
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Fig. 3. Removal efficiencies (tested at 0.10 g/L and 0.25 g/L clay) and zeta charges of the three aluminum-modified clays at different suspension pH values prepared with DW and SW: (a) DW 0.10 g/L; (b) DW 0.25 g/L; (c) SW 0.10 g/L; (d) SW 0.25 g/L; (e) DW zeta charge; and (f) SW zeta charge. Total alum concentration: 2.85 × 10−2 mol/L.
and the pH of the modified clay suspension prepared with SW was higher than that with DW (Table 1). All of the pH values of the suspensions were between the range of 3 and 5, and the SW-prepared PAC-MC showed the highest pH values (Table 1). In this pH range (3 to 5), the removal efficiency decreased as the pH values increased (Fig. 3). We analyzed the effects of the dispersion medium and suspension pH on the removal efficiency of the three modified clays. We found that the effects of the dispersion medium on the removal efficiency were related to the type of modified clay. The removal efficiency of the AC-MC and AS-MC suspensions prepared with DW were lower than those of the suspensions prepared with SW, although the removal efficiencies were generally equivalent at a pH below 7 (Fig. 3). However, the PAC-MC suspension prepared with DW showed a significantly higher removal efficiency than that prepared with SW when the suspension pH was below 6.5 (Fig. 3). The difference in the removal efficiency of suspensions prepared with DW and SW was most likely caused by sulfate in the SW.
When the three modified clays were prepared with the same dispersion medium (SW), the three modified clays showed similar removal efficiencies at a pH range of 3 to 5 (Fig. 3b, d). The DW-prepared PACMC showed a higher removal efficiency compared with AC-MC and AS-MC. Whether prepared with DW or SW, the removal efficiencies of AC-MC and AS-MC were similar and differentiated from PAC-MC, which may suggest that AC-MC and AS-MC shared the same removal mechanism and had a different mechanism from PAC-MC. 4.2. Possible mechanism of aluminum-modified clay The surface charge of the modified clay particles was influenced by the suspension pH. Similar removal efficiencies of AC-MC and AS-MC were obtained at the same pH values; however, the surface charge showed great differences (Table 1), and the surface charge of AS-MC was negative at all of the pH values. Differences in the surface charge did not lead to differences in the removal efficiency, which indicated that there were
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Fig. 4. The contour diagrams of the removal efficiency (a) (n = 3) and the surface charge (b) of the three modified clays prepared with DW and pH values of algae culture after the addition of clay (c) at different pH values and alum amounts. a-1, a-2 and a-3 represent the removal efficiency of AC-MC, AS-MC and PAC-MC, respectively, which was tested using 0.25 g/L clay. b-1, b-2 and b-3 show the surface charge of AC-MC, AS-MC and PAC-MC, respectively. c-1, c-2 and c-3 show the pH values of the algae culture after the addition of AC-MC, AS-MC and PAC-MC, respectively. In this diagram, the horizontal axis is the pH of the clay suspension and the vertical axis is the −lg[AlT] in suspension. The curve labels indicate the removal efficiency (%) (a), zeta charge values (mV) (b), and pH values of the algae culture after the addition of clay (c).
likely additional flocculation mechanisms affecting the AC-MC and AS-MC particles in suspension than charge neutralization. The removal efficiency was positively correlated with the surface charge of PAC-MC (Yu et al., 1999). The surface charge of PAC-MC (prepared with DW or SW) at pH 3 was lower than that at pH 5 (Fig. 3e, f); however, the removal efficiency was higher at pH 3 (Fig. 3a, b, c, d). The surface charge of the modified clay was influenced by the alum hydrolysis products attached to the clay particles. Although the relationship of the removal efficiency and surface charge was complex for the different types of modified clay, the removal efficiency and surface charge of the three modified clays at different pH conditions were consistent with the hydrolysis products of different types of alum. The alum hydrolysis products at different pH conditions could be used to explain the flocculation process and removal efficiency. The hydrolysis products of alum were influenced by the suspension pH (Wang et al., 2004). The hydrolysis products of AC and AS were mainly monomers with a low charge density at pH 3. When the monomers that occurred in the AC-MC and AS-MC suspensions were added to the algae culture at pH value above 8.5, large amounts of amorphous aluminum hydroxide (Al(OH)3(am)) rapidly formed (Amirtharajah and Mills, 1982). The removal of algae cells mainly occurred by sweep flocculation of Al(OH)3(am). The hydrolysis products of PAC were monomers (16%) and polynuclear aluminum compounds (84%) in this experiment. Polynuclear aluminum compounds are composed of alum and OH− and partly dissolved at pH 3 (Wang et al., 2004). The surface charge of PACMC was lower at pH 3 than at pH 5 (Fig. 3e, f). The alum monomers
hydrolyzed by PAC were also transformed to Al(OH)3(am) when added to the algae culture. Therefore, the removal of algae cells by PAC-MC at pH 3 was through Al(OH)3(am) and polynuclear aluminum compounds. Alum monomers were sensitive to pH, and the increased suspension pH decreased the concentration of monomers and the effects of sweep flocculation. However, the surface charge of the modified clay increased with increasing pH values in the pH range of 3 to 5, and a higher surface charge usually indicated a higher charge neutralization ability. However, the removal efficiency declined at pH 5 compared with that at pH 3, which most likely indicated that sweep flocculation caused by alum monomers played a more important role in the removal of algae cells by PAC-MC at pH 3 to 5, especially at pH 3. In the weak acidic pH ranges (5 to 7), the alum monomers in AC-MC and AS-MC decreased to a minimum (Wang et al., 2004) and the sweep flocculation effects disappeared. The hydrolysis product was mainly a mid to high polymer with a hexahydroxy ring structure (not Al13), and its charge density was weak (Wang et al., 2004). The hydrolysis products of AC and AS showed a tendency to transform into aluminum hydroxide precipitates with increasing pH values, and the removal ability decreased. Alum monomers in the PAC-MC suspension disappeared and the surface charge decreased as the pH increased; thus, the removal efficiency decreased (Fig. 3a, b, c, d). In the neutral to alkaline pH range (7 to 11), AC and AS showed a strong tendency to hydrolyze to aluminum hydroxide precipitates with weak charge densities. The hydrolysis products were mainly Al(OH)− 4 when the suspension pH was above 9. The hydrolysis product of PAC was relatively stable, although it could also combine with OH− in
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Fig. 5. Contour diagrams of the removal efficiency (a) (n = 3) and the surface charge (b) of the three modified clays prepared with SW at different pH values and alum dosage conditions. a-1, a-2 and a-3 represent the removal efficiency of AC-MC, AS-MC and PAC-MC, respectively, which were tested using 0.25 g/L clay. b-1, b-2 and b-3 show the surface charge of AC-MC, AS-MC and PAC-MC, respectively. In this diagram, the horizontal axis is the pH of the clay suspension and the vertical axis is the −lg[AlT] in suspension. The curve labels indicate the removal efficiency (%) (a) and zeta charge values (mV) (b).
suspension to occur alkaline dissolution (Wang et al., 2004). Therefore, when the three modified clays were prepared with DW, the removal ability decreased in the suspension pH range of 7 to 11 (Fig. 3a, c). However, when the modified clays were prepared with SW, the surface charge of the three types of aluminum-modified clay increased as the suspension pH increased, which was likely the result of the formation of precipitates, such as CaCO3, MgCO3 and Mg(OH)2 by Ca2+ and Mg2+ in SW at high pH values (Ayoub et al., 1986; Semerjian and Ayoub, 2003; Vandamme et al., 2012). The formed Mg(OH)2 precipitation adsorbed on the clay particles and increased the surface charge of the clay particles after absorbing positively charged ions in suspension. Therefore, the increased removal efficiency of the modified clay prepared with SW at a suspension pH above 9 was most likely the result of enhanced charge neutralization. Therefore, the removal ability of the modified clay was influenced by differences in the alum hydrolysis products at different suspension pH values. In the process of controlling HABs, the flocculation of harmful algae cells can be divided into two steps. The first step is preparing a clay suspension. In this step, the aluminum was hydrolyzed, and certain hydrolysis products were absorbed on the clay particles and changed the surface charges. The surface charge changed from negative to positive when enough positively charged polynuclear alum compounds were absorbed. The unabsorbed hydrolysis products remained uncombined in the suspension. In the second step, the modified clay particles in suspension were added to the algae culture, and flocculation occurred between the modified clay particles and the algae cells. In the flocculation process, conditions that could cause the highest removal efficiencies were identified as the optimal removal conditions of the modified clay. At the optimal flocculation conditions, the hydrolysis products of
AC and AS were mostly monomers (at low pH values), and they transformed into Al(OH)3(am) upon addition to the SW algae culture. The removal of algae cells only occurred in the “optimal sweep flocculation” area of alum sulfate (Amirtharajah and Mills, 1982). Therefore, AC-MC and AS-MC mainly functioned through Al(OH)3(am) formed upon addition to the algae culture to remove algae cells. The PAC hydrolyzed to monomers (Ala) and polynuclear compounds (Alb and Alc). The behaviors of Ala were similar to that of AC and AS, and Ala underwent great changes with increasing pH values. Alb and Alc attached to clay particles in the clay suspension and were relatively stable in the pH range of 3 to 11 (Wang et al., 2004). PAC-MC removed algae cells through Al(OH)3(am) and polynuclear aluminum compounds. Therefore, significant differences occurred in the removal mechanisms of PAC-MC and AC/AS-MC. The suspension pH influenced the aluminum hydrolysates; thus, it also influenced the removal efficiency. The continuity of the two steps in the removal process made it possible to establish relationships between the alum dosage, modified clay surface charge in suspension and algae cell removal efficiency. Dentel (1988, 1991) suggested that the mechanism of AS flocculation occurred through the effects of charge neutralization caused by the attachment of Al(OH)3(am) to the clay particles, and they proposed a flocculation model, with the main principle stating that after aluminum hydroxide attached to the clay surfaces, the total charge density could be regarded as the mixture of charges from the two components (Dentel, 1988, 1991). Wang and Tang (2006) modified this flocculation model according to the characteristics of PAC and advanced a flocculation model for PAC. The fundamental principles underlying these models were also suitable for the control
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of HABs by modified clay. Therefore, the two models could also be used to analyze the removal behaviors of the three modified clays at different suspension pH conditions. In the first step, ζ1;2 C1 ; 2 ¼ ζ1 C1 þ ζ2 C2
ð1Þ
where ζ1,2 is the surface charge of the modified clay at the different suspension pH conditions; C1,2 is the total amount of modified clay particles at the different suspension pH conditions; ζ1 is the surface charge of the clay particles at the different suspension pH conditions; C1 is the amount of clay particles at the different suspension pH conditions; ζ2 is the surface charge of the alum compounds absorbed by the clay particles or insoluble alum compounds at the different suspension pH conditions; and C2 is the amount of alum compounds absorbed by the clay particles or insoluble alum compounds at the different suspension pH conditions. For PAC-MC, C2 can be regarded as Alb, Alc and the amount of alum in Ala that exceeds the solubility and becomes aluminum hydroxide: C2 ¼ CT −CN;M
ð2Þ
where CT is the total amount of alum in the clay suspension; and CN,M is the dissolved alum at different pH conditions; the dissolved alum or Al(OH)− is mainly alum monomers, such as Al(H2O)3+ 6 4 , and their values could be obtained by the alum solubility diagram (Duan and Gregory, 2003). And for PAC-MC: CT ¼ Ala þ Alb þ Alc :
ð3Þ
In the second step, When AC-MC and AS-MC were added to the algae culture, the alum unabsorbed to the clay suspension transformed to Al(OH)3(am). The polynuclear alum compounds (transformed by monomers) absorbed on the clay particles in the suspension transformed to aluminum hydroxide when added to the algae culture. This amount of alum was still absorbed on the clay particles after transformation, and the clay particles and absorbed alum could be considered together. For PACMC, the unabsorbed alum transformed to aluminum hydroxide upon addition to the algae culture, and the polynuclear compounds (Alb and Alc) that were adsorbed onto the clay particles in the clay suspension were still attached to the clay particles and could be considered together. Subsequently, ζ1;2;3 C3 þ C01;2 þ C0N;M ¼ ζ4 C0N;M þ ζ3 C3 þ ζ01;2 C01;2
ð4Þ
where ζ1,2,3 is the surface charge of the modified clay-algae flocs after the addition of the modified clay; ζ 4 is the surface charge of the aluminum hydroxide at the pH of the algae culture after the addition of modified clay; C′ N,M is the amount of dissolved alum that was added to the algae culture and occurred in suspension as monomers and was transformed to aluminum hydroxide upon addition of algae culture; ζ 3 is the surface charge of the algae cells at the pH of the algae culture after the addition of the modified clay; C3 is the amount of algae cells in the flocs after the addition of modified clay, and its value is related to removal efficiency (%) of algae cells; ζ′1,2 is the surface charge of the modified clay particles at the pH of the algae culture after the addition of the modified clay; and C′1,2 is the amount of modified clay that was added to the algae culture. The model above was suitable for the control of algae cells by modified clay suspensions prepared with DW and SW. The model showed relationships among the alum dosage; modified clay surface charge in suspension, algae culture, and clay-algae flocs; and the removal efficiency (%). The model also showed that the removal efficiency of the
modified clay was related to the surface charge of the modified clay (ζ’1,2) at the pH of the algae culture after the addition of clay. For PACMC, the surface charge of the modified clay upon addition to the algae culture (ζ’1,2) is equivalent to the surface charge of the modified clay in suspension (ζ1,2) minus the charge loss caused by the compression of the double electrical layer in the SW minus the charge loss caused by the attachment of anion adsorption (such as SO2− 4 ). Therefore, the removal efficiency of the modified clay was correlated with the surface charge of the modified clay in suspension, which is consistent with the results found by Yu et al. (1999). At optimal flocculation, the alum monomers of AC-MC and AS-MC in suspension transformed to Al(OH)3(am), and their removal efficiencies were related to the surface charges of aluminum hydroxide (ζ’1,2). Therefore, when Ala occurred in the suspensions of AC-MC and AS-MC, even if the surface charge of the modified clay in the suspensions was negative, a high removal efficiency was still achieved (Fig. 3). In addition, the model provided an approach to calculating the number of algae cells that could be removed by 1 g modified clay. 4.3. Practical implications of this study These mechanisms also indicated that there were differences in the removal efficiencies of the three aluminum-modified clays in relation to freshwater and seawater blooms. When applied in the field, the practice is to prepare a modified clay suspension using the algae bloom water. When using AC-MC and AS-MC, whether the algae cells could be removed efficiently is dependent on whether Al(OH)3(am) is able to form in the algae culture. The pH and buffering capacity of freshwater is low; therefore, when treating freshwater algae blooms, it is advisable to avoid low pH conditions at the water surface after adding the clay suspension to form aluminum hydroxide. The PAC-MC suspension prepared with freshwater could remove algae cells effectively due to the high charge neutralization ability of polynuclear alum compounds. However, when using PAC-MC to remove seawater algae blooms, significantly decreased removal efficiencies were observed. The mechanism of AC-MC and AS-MC differed from that of PAC-MC, and PAC-MC prepared with SW presented limited removal effects. Therefore, it is better to control SW blooms by AC-MC and AS-MC. These results also present a number of practical implications for increasing the removal efficiency of HABs. Although the removal mechanism was not the same for the three modified clays, they each had a high removal efficiency at low suspension pH values. At high seawater quantities and strong buffering capacities and blending abilities, the pH of the surface seawater presented minimal changes, and the suspension pH could be reduced to increase the removal efficiency, especially for AC-MC and AS-MC. However, when controlling freshwater blooms using PAC-MC, the polynuclear alum compounds played the main role, and the optimal suspension pH was most likely 5. In recent years, the further enhancement of flocculation functions, such as bridging effects and charge neutralization effects, or the introduction of other functions (e.g., oxidation effects and killing effects) based on clay surface modifications have been the focus of HABs control researches. The effects of the introduced components in the composite application on alum hydrolysis and the corresponding removal mechanisms may be the subject of future research. 5. Conclusions In this study, we compared the removal efficiency of three aluminum-modified clays in controlling A. anophagefferens blooms and discussed their removal mechanisms according to the dispersion medium and suspension pH. We found that the removal efficiency of the PAC-MC suspension prepared with SW was significantly lower than that of AC-MC and AS-MC. Compared with PAC-MC, the dispersion medium showed little effect on AC-MC and AS-MC at a suspension pH below 7. Significant differences were observed in the removal
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mechanisms of PAC-MC compared with that of AC-MC and AS-MC. At optimal coagulation conditions, the hydrolyzed products of AC and AS were mainly monomers, and they transformed into Al(OH)3(am) upon addition to the algae culture, and the primary mechanism was sweep flocculation. PAC hydrolyzed to monomers (Ala) and polynuclear compounds (Alb and Alc). The behaviors of Ala were similar to that of AC and AS. The Alb and Alc attached to the clay particles remained stable upon their addition to the algae culture. The removal of algae cells by PAC-MC mainly occurred through Al(OH)3(am) and polynuclear aluminum compounds. The alum hydrolysis products were significantly influenced by the suspension pH, and the flocculation process between the modified clay particles and algae cells was subsequently influenced. Special attention should be paid to the suspension pH during field applications. The results also indicated that AC-MC and AS-MC were better suited to controlling seawater algae blooms. Acknowledgments We would like to thank Dr. Guangyuan Lu for his kind advice during the writing of this paper. This work was supported by the National Natural Science Foundation of China (NSFC)-Shandong Joint Fund for Marine Science Research Centers (Grant No. U1406403), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA11020302) and the Chinese State Oceanic Administration public welfare project (Grant No. 201305003-3) and the National Natural Science Foundation of China (NSFC) (Grant No. 41576119). The authors declare that they have no conflicts of interest. References Al Shehhi, M.R., Gherboudj, I., Ghedira, H., 2014. An overview of historical harmful algae blooms outbreaks in the Arabian Seas. Mar. Pollut. Bull. 86 (1–2), 314–324. http:// dx.doi.org/10.1016/j.marpolbul.2014.06.048. Amirtharajah, A., Mills, K.M., 1982. Rapid-mix design for mechanisms of alum coagulation. J. Am. Water Works Assoc. 74 (4), 210–216. http://dx.doi.org/10.2307/41271001. Anderson, D.M., 1997. Turning back the harmful red tide. Nature 388 (6642), 513–514. Anderson, D.M., Cembella, A.D., Hallegraeff, G.M., 2012. Progress in understanding harmful algal blooms: paradigm shifts and new technologies for research, monitoring, and management. Ann. Rev. Mar. Sci. 4 (4), 143–176. http://dx.doi.org/10.1146/annurevmarine-120308-081121. Ayoub, G.M., Lee, S.-I., Koopman, B., 1986. Seawater induced algal flocculation. Water Res. 20 (10), 1265–1271. http://dx.doi.org/10.1016/0043-1354(86)90157-0. Dentel, S.K., 1988. Application of the precipitation-charge neutralization model of coagulation. Environ. Sci. Technol. 22 (7), 825–832. http://dx.doi.org/10.1021/es00172a013. Dentel, S.K., 1991. Coagulant control in water treatment. Crit. Rev. Environ. Control. 21 (1), 41–135. http://dx.doi.org/10.1080/10643389109388409. Duan, J., Gregory, J., 2003. Coagulation by hydrolysing metal salts. Adv. Colloid Interf. Sci. 100–102, 475–502. http://dx.doi.org/10.1016/S0001-8686(02)00067-2. Guillard, R.R.L., Hargraves, P.E., 1993. Stichochrysis immobilis is a diatom, not a chrysophyte. Phycologia 32 (3), 234–236. http://dx.doi.org/10.2216/i0031-8884-32-3-234.1. Hagström, J.A., Granéli, E., 2005. Removal of Prymnesium parvum (Haptophyceae) cells under different nutrient conditions by clay. Harmful Algae 4 (2), 249–260. http:// dx.doi.org/10.1016/j.hal.2004.03.004. Kim, H.G., 2006. Mitigation and controls of HABs. In: Granéli, E., Turner, J. (Eds.), Ecology of Harmful AlgaeEcological Studies vol. 189. Springer, Berlin Heidelberg, pp. 327–338. Lee, Y.-J., Choi, J.-K., Kim, E.-K., Youn, S.-H., Yang, E.-J., 2008. Field experiments on mitigation of harmful algal blooms using a sophorolipid—yellow clay mixture and effects on marine plankton. Harmful Algae 7 (2), 154–162. http://dx.doi.org/10.1016/j.hal.2007. 06.004. Liu, G., Fan, C., Zhong, J., Zhang, L., Ding, S., Yan, S., Han, S., 2010. Using hexadecyl trimethyl ammonium bromide (CTAB) modified clays to clean the Microcystis aeruginosa blooms in Lake Taihu, China. Harmful Algae 9 (4), 413–418. http://dx. doi.org/10.1016/j.hal.2010.02.004. Liu, L., Zhou, J., Zheng, B., Cai, W., Lin, K., Tang, J., 2013. Temporal and spatial distribution of red tide outbreaks in the Yangtze River Estuary and adjacent waters, China. Mar. Pollut. Bull. 72 (1), 213–221. http://dx.doi.org/10.1016/j.marpolbul.2013.04.002. Maruyama, T., Yamada, R., Usui, K., Suzuki, H., Yoshida, T., 1987. Removal of marine red tide planktons with acid treated clay. Nippon Suisan Gakkaishi 53 (10), 1811–1819.
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