Tae-Hyun
Kim
,
In Taek
Hong
and
Jae-Min
Oh
*
Department of Chemistry & Medical Chemistry, College of Science & Technology, Yonsei University, Wonju, Gangwon-do 26493, Republic of Korea. E-mail: jaemin.oh@yonsei.ac.kr
First published on 28th November 2017
We evaluated the effects of particle size and surface charge on the algal flocculation activity of layered double hydroxides (LDHs). LDHs with different lateral sizes of 50, 250, and 2000 nm were prepared by adjusting reaction methods and parameters during co-precipitation of a metal solution under alkaline conditions. LDH particles of 2000 nm readily flocculated ∼100% of Microcystis aeruginosa within 60 min, whereas smaller LDHs did not show significant algal flocculation. We controlled the surface charge of 2000 nm LDHs to be highly positive, almost neutral, or negative using organic moiety coatings of L-serine, succinate, and citrate, respectively. Negatively coated LDHs did not flocculate algae, suggesting the importance of surface charge in the algae–LDH interaction. Taking into account the size and surface charge parameters for algal flocculation, we prepared LDHs with a lateral size of ∼500 nm and a positive surface charge from industrial waste slag, a by-product of alloy manufacturing, from an electric arc furnace. These LDHs showed ∼40% flocculating activity within 60 min, whereas the slag ground into powder showed little flocculating activity after 240 min.
Environmental significanceAlgal bloom is a serious environmental problem in fresh water. Many techniques have been developed to remove algae, and sedimentation upon adsorption is practically applied. Generally utilized adsorbents include clays but their adsorption efficiency is not optimized yet. Based on the hypothesis that the algal–adsorbent interaction is governed by surface interaction, we fine-tuned the particle size and surface charge of synthetic clays to investigate algal flocculation. We found that the particle size (∼2000 nm) and surface charge (positive) are key factors that control algal–clay flocculation. Considering applicability, we suggested a method to prepare synthetic clays with large size and positive charge from industrial waste slag. The obtained clay powder showed a much higher algal flocculation than the conventional clays. |
To reduce HABs, various algal removal techniques have been developed, such as flocculation,9–12 ultrasonic aggregation,13,14 froth flotation,15,16 filtration,17,18 and algicide treatment.19,20 Among these, flocculation is the most practical because of its ease of handling and cost-effectiveness.3 In fact, yellow loess, a natural clay, is currently used to treat HABs in South Korea and Japan,21–23 and many efforts have been made to improve its functionality in algal flocculation.12,24 Clay materials can prohibit the motility of blue-green algae through particle–cell interactions that result in flocculation and effective removal.25 Thus, the first step towards optimizing the functionality of algal flocculating clays is sufficient comprehension of the surface interaction between blue-green algae and its flocculants and how that interaction varies with the physicochemical properties of clay. We here describe the relationships between the physicochemical properties of clay and algal flocculation activity. Studies on the interaction between microorganisms and inorganic nanoparticles are not simply an issue in algal flocculation. In the recent decade, many scientists tried to understand the interaction between small organisms and inorganic nanoparticles in order to develop antibacterial agents,26,27 drug delivery systems28 and vaccine adjuvants.29,30 Therefore, the current study on algal–clay interaction would be scientifically important to develop various bio-functionalized materials.
A layered double hydroxide (LDH), also known as anionic clay, is composed of positively charged nanolayers (M(II)1−xM(III)x(OH)2x+; M(II), M(III): divalent and trivalent metal cations, 0.2< x <0.4) and charge-compensating interlayer anions.31,32 General clays, such as kaolinites and smectites, have fixed physicochemical properties because they are usually obtained from nature and purified. On the other hand, the physicochemical properties of LDHs, such as the particle size, surface charge, and composition, can be easily controlled by adjusting the synthetic conditions. Thus, LDHs can be good model materials for studying the relationship between a clay's properties and algal flocculation. In this study, we prepared LDHs with a well-defined lateral size and surface charges using reagent grade chemicals and evaluated the relationships between these physicochemical properties and algal flocculation activity with MA cells. Then we tried to synthesize an LDH algal flocculant from slag, a by-product of steel or alloy manufacturing. Taking into account that industry produces approximately 50 million tons33 of slag annually, and that the annual consumption of algal flocculant is up to hundreds of thousands of tons,34 converting slag into an algal flocculant would result in a high-value industry. Because it has hydraulic properties and contains various metal species (Fe, Mn, Ca, Mg, Al, and Si), slag has been used as a road base material, a concrete admixture, for soil improvement, or as calcium silicate fertilizer, as-produced or ground to a powder.35–37 However, these applications are quite simple and add little value considering the useful metal components. Several improved applications that use slag for heavy metal adsorption,38 phosphate removal,39 and marine forest restoration40 are in progress. However, the undefined chemical structure and inhomogeneous composition of slag limit their versatility.33 Recently, it was reported that blast furnace slag can be a source for LDH synthesis by extracting metal components via acid treatment.41 Inspired by that work, we analyzed the physicochemical properties of electric arc furnace slag and used it to synthesize LDHs. We then studied the MA flocculation activity of the LDHs in terms of the lateral size and surface charge, the effects of which we had established using LDHs with well-controlled parameters.
To control the surface charge of LDH-3 to be negative, moderate, or highly positive (preservation of the pristine surface charge), we applied coatings of trisodium citrate, disodium succinate, and L-serine, respectively. First, 0.5 g of pristine LDH powder was dispersed into 200 mL of a solution containing one of the three organic coating agents (0.025 mol L−1) and reacted for 12 h at room temperature. The products were collected by centrifugation, washed several times with deionized water, and then lyophilized.
Amount of Chlorophyll-a = 11.85E664 − 1.54E647 − 0.08E630 | (1) |
The flocculation activity was calculated using the following equation:
Flocculation activity (%) = 1 − (1 − A/B) × 100 | (2) |
(A: the amount of chlorophyll-a in the supernatant at a specific time; B: the amount of chlorophyll-a in the supernatant at 0 min).
The size-dependent MA flocculating activity of LDH is displayed in Fig. 2(A). For this experiment, the concentrations of LDHs and cells were fixed to 500 ppm and 1.18 × 107 cells per mL, respectively. The LDH particles of tens or hundreds of nanometers (LDH-1: 50 nm, ○; LDH-2: 270 nm, △) did not show significant flocculating activity compared with the control group (□), even at a long incubation time of up to 240 min. However, LDH-3 (◇), with a lateral size of 2200 nm, flocculated ∼90% of MA within 10 min and almost 100% of MA at 60 min.
Therefore, we evaluated the effect of LDH concentration on algal flocculation using LDH-3 (Fig. 2(B)). The flocculating activity was highly dependent on LDH concentration; 50, 100, 200, and 500 ppm of LDH-3 exhibited 6.5, 15.9, 34.8, 79.0, and 99.7% flocculation, respectively, at 120 min. To evaluate the effect of cell concentration on flocculation, we varied the initial cell numbers while fixing the LDH-3 concentration at 500 ppm. As shown in Table 1, the flocculation ratio decreased as the initial cell number increased, showing 89.0, 50.6, and 25.0% for initial MA cell numbers of 5.90 × 109, 1.19 × 1010, and 2.56 × 1010, respectively. It is worth noting that the number of flocculated MA per 1 mg of LDH was not seriously affected by the initial cell number; the calculated values were 2.17 × 107, 2.42 × 107, and 2.56 × 107 cells per mg LDH-3 for initial MA cell numbers of 5.90 × 109, 1.19 × 1010, and 2.56 × 1010, respectively. Rough calculations indicate that there were 1.1 × 109 particles in 1 mg of LDH-3; thus, approximately 46 LDH-3 particles (calculated from 1.1 × 109 particles per 2.4 × 107 cells) were required to flocculate one cell regardless of the LDH:MA ratio.
Initial cell number in 500 mL | 5.90 × 109 | 1.19 × 1010 | 2.56 × 1010 |
Flocculated cell | 5.25 × 109 | 6.05 × 109 | 6.40 × 109 |
Flocculation ratio (%) | 89.0 | 50.6 | 25.0 |
Flocculated cells per 1 mg of LDH | 2.1 × 107 | 2.42 × 107 | 2.56 × 107 |
The SEM images of the MA/LDH flocculates showed that the LDH particles covered the MA cells. The MA cells were found to be spherical and 1700 nm in diameter (Fig. 3A). After treatment with LDHs, the MA cells (white dotted circles in Fig. 3B–D) assembled with the LDH particles (white arrows in Fig. 3B–D). We attribute the high algal flocculation activity of LDH-3 to two factors. First, LDHs inherently possess a positive surface charge that facilitates effective adsorption onto the negatively charged algal surface.43 Second, because the zeta potentials of the three LDH samples were similar to one another (Fig. S1†), their different flocculating activities are surely attributable to size. LDH particles tend to agglomerate;44 small LDH particles agglomerated with one another rather than interacting with MA, whereas agglomerates of LDH-3 resulted in a “house-of-cards” structure, whose inter-particle cavity fitted an MA cell (Fig. 3). It could therefore be concluded that LDH-3, which has a lateral dimension comparable to that of MA, could incorporate MA cells through surface–surface interactions more effectively than LDH-1 and LDH-2 (Fig. 3).
Because we found that the LDH particle size was a determining factor for MA flocculation and that the surface charge could also be important, we systematically evaluated the surface charge effect. We controlled the surface charge of LDH-3 to be highly positive, moderately positive, or negative by coating it with L-serine, succinate, and citrate, respectively. All coating agents have a carboxylate moiety that attach to the positive LDH surface. L-Serine, a zwitterionic amino acid with an amine and a carboxylic acid, was thought to preserve the original surface charge of LDH. Succinate and citrate, with two and three carboxylates, respectively, shifted the surface charge negatively. As expected, the zeta potential of citrate-, succinate-, and serine-coated LDHs was well controlled at −35.3 mV, 15.7 mV, and 26.6 mV, respectively. Note that the zeta potential of pristine LDH was 32.6 mV (Fig. 4). There was neither significant alteration of particle size nor serious aggregation of LDH-3 particles upon surface coating, as shown in the SEM images (Fig. 4).
Fig. 4 Zeta potential distributions at pH 7.0 (left) and scanning electron microscopy images (right) of (A) LDH-3, (B) LDH-3-cit, (C) LDH-3-suc, and (D) LDH-3-ser. |
The algal flocculating activity of surface charge-controlled LDH-3 is displayed in Fig. 5. The results indicate that the flocculation depends on the surface charge. Negatively charged LDH particles did not show effective flocculation, whereas the positive LDH-3-suc and LDH-3-ser exhibited almost the same algal flocculating activity as the uncoated LDH-3.
The important role of electrostatic interaction in algal flocculation has been demonstrated in various studies. Metal cations such as trivalent aluminium are widely used to flocculate algae.45,46 Adding positive polymers such as chitosan to negatively charged clays or minerals dramatically enhances the algal removal efficiency.12,24,47 MA cells are known to have abundant anionic moieties (such as carboxylate and phosphoryl) on their surface. Thus, the electrostatic interaction between positive LDH and negative MA forms MA–LDH flocculates. To double check the role of surface charge in algal flocculation, we compared the MA flocculation activity of LDH-3 with that of several representative clay minerals (kaolinite, yellow loess, and sepiolite). The respective zeta potentials of kaolinite, yellow loess, and sepiolite are −34.7 mV, −38.2 mV, and −19.9 mV. In spite of their large hydrodynamic radii (kaolinite: ∼4000 nm, yellow loess: ∼3000 nm, and sepiolite: 4100 nm as dispersed), the algal flocculation activities of these clays were not significant even after 240 min, whereas LDH-3 showed ∼100% efficacy at that time (Fig. 6). Therefore, we conclude that both a large size and a positive surface charge are required for effective algal flocculation.
Fig. 6 Flocculating activity of LDH-3 (◇), kaolinite (△), yellow loess (▽), sepiolite (○), and negative control (□) on MA algae (1.5 × 107 cells per mL). |
Based on our finding that the particle size and surface charge were critical factors for algal flocculation using clay particles, we tried to find a way to recycle industrial waste slag for the high-value added application of algal removal. Slags are by-products generated during iron or steel manufacturing. Because slag is mass produced and causes environmental problems, several recycling methods have been developed, including its use as a contaminant scavenger48,49 and fertilizer and algal flocculation.12,50 However, due to slag's amorphous phase, inhomogeneous distribution of elements, and possible leakage of heavy metal species, its direct application has several drawbacks. Therefore, several groups have tried to synthesize well-defined structures from slag using a series of processes.41,51
After we found that large LDH particles with a positive surface charge were effective in algal removal, we tried to prepare such particles from slag, namely SL, for use as algal flocculants. We determined that the slag in this study consisted mainly of SiO2, MnO2, CaO, MgO, and Al2O3 (Table 2). All metal species except Si can be used in LDH; thus, we extracted metal species from the slag under acidic conditions. The extract was determined to contain Mn, Ca, Mg, and Al. To optimize the pH conditions to precipitate as many metal species as possible into the LDH framework, we investigated the base–pH titration curve for the extract solution (Fig. S2†). There were three plateaus at around pH 4, 8, and 11, which we attributed to the buffering effect during the hydrolysis of Al3+, Mg2+, and Ca2+, respectively. The extract was titrated until pH ∼11.5, which allowed all metal species in the extract (Mn2+/Mn3+, Ca2+, Mg2+ and Al3+) to be effectively incorporated into the LDH framework.
Materials element | Pristine slag | Leaching solution | SL |
---|---|---|---|
Si | 2.97 | N.D. | N.D. |
Mn | 0.78 | 0.49 | 1.14 |
Ca | 0.67 | 0.78 | 1.58 |
Mg | 0.64 | 0.31 | 0.75 |
Al | 1 | 1 | 1 |
As shown in Fig. 7(A), the XRD pattern of pristine slag showed amorphous peaks from SiO2. On the other hand, SL exhibited sharp (hkl) peaks at 11.24°, 22.65°, 23.51°, and 31.06° that correspond to (002), (004), (202), and (110), respectively, of hydrocalumite (JCPDS No. 31-0245), which is a type of LDH with hepta-coordinated calcium hydroxide.52 The chemical formula of SL, evaluated by ICP-OES and EDS, was CaII1.58MgII0.75AlIII1.00MnIV1.14(OH)8.94Cl3.28·mH2O.
The hydrodynamic radius (data not shown) and zeta potential (Fig. 7B) of the pristine slag and SL were ∼1211 nm at −23.7 mV and ∼470 nm at 33.5 mV, respectively. Although the size was not sufficiently large for effective algal flocculation, the highly positive surface charge of SL is advantageous in algal flocculation. In addition, we expected the hard metal Mn4+ in the SL framework to have a strong affinity to hard bases, such as the carboxyl and phosphoryl on the MA surface. The SEM results show that the prepared SL had a significantly different morphology than the pristine slag (Fig. 7C). The SL showed a 2-dimensional layered morphology with a lateral size of ∼500 nm (Fig. 7), which matched well with the hydrodynamic radius. As shown in Fig. 7D, SL (△) showed moderate flocculation activity of up to ∼45% with 500 ppm treatment, whereas the pristine slag (○) showed no significant flocculation. Our LDH and SL flocculating activity results reveal that a highly positive surface charge and a hydrodynamic radius similar to that of algal cells could enhance flocculation for algal removal.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7en00809k |
This journal is © The Royal Society of Chemistry 2018 |