Wat. Res. Vol. 34, No. 5, pp. 1487±1494, 2000 # 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
REMOVAL OF HUMIC SUBSTANCES BY LAYERED DOUBLE HYDROXIDE CONTAINING IRON YOSHIMI SEIDA* and YOSHIO NAKANO Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8502, Japan (First received 1 December 1998; accepted in revised form 1 July 1999) AbstractÐHydrotalcite and hydrotalcite-like compounds containing iron were synthesized. The eect of their physicochemical structure on the adsorption properties for humic substances were elucidated following the characterization of the compounds. The compounds and their heat-treated ones showed a high adsorption capacity for the humic substances dissolved in water. The following adsorption and/or removal mechanisms were suggested through the series of the studies. The removal of the humic substances occurs by both the intercalation of humic substances into the positively charged innerlayer (ion-exchange) and the adsorption onto hydroxyl groups of the compounds. The buering pH function of the compounds enlarges the removal by producing hydroxides through their slight dissolution. The produced hydroxides coagulate the humic substances onto the compounds, resulting in a sludge with low water content. The additive salt increases the amount of adsorption. The compounds were found to be eective insoluble adsorbents for the removal of humic substances. # 2000 Elsevier Science Ltd. All rights reserved Key wordsÐhydrotalcite, pyroaurite, humic acid, adsorption, separation, water treatment
It has been understood that humic substances constitute a major fraction of organic matter in natural water and euents such as from lakes and rivers. Their presence has been a problem in the water industry and in environmental puri®cation such as soil remediation. The contamination of humic substances has been known to induce a deterioration of adsorbents and a prevention of adsorption onto the adsorbents. As seen in the removal process of heavy metal contaminants in soil remediation (Clapp et al., 1991; Jelinek et al., 1998) and the removal process of organic pollutants in drinking water treatment (Ellis and Korth, 1993), the humic substances often reduce the removal of the target substances through their adsorption onto adsorbents and/or a formation of a complex with the target substances (Rav-Acha and Rebhn, 1992). The removal of the humic substances by conventional adsorbents is, however, dicult due to their water soluble formation and their wide range of distribution in molecular weight and size. The research of insoluble adsorbents for those hardly removable targets would be of great interest for the concerning area. Many potential insoluble adsor*Author to whom all correspondence should be addressed. Tel.: +81-45-924-5419; fax: +81-45-924-5441; e-mail: [email protected]
bents such as zeolite, clay, metal oxide and resins have been studied so far to elucidate the adsorption phenomena of humic substances onto them due to their engineering and scienti®c interests (Schulthess and Huang, 1991; Johns et al., 1993; Karan®l et al., 1994; Zhou et al., 1994). Recently, Amin and Jayson (1996) studied the potential of hydrotalcite clays and pillard cationic clays as insoluble adsorbents to remove humic substances from upland surface water (Amin and Jayson, 1996). They examined the hydrotalcites with nitrate and carbonate intercalants prepared by several synthesis methods. The hydrotalcite clay was shown to be eective for this purpose. Hydrotalcite and hydrotalcite-like compounds have a layered double hydroxide (abbreviated as LDH) structure and have been known as inorganic anion-exchangers with the formula 3+ nÿ 2+ of M2+ =Mg2+, x My (OH)2(x+y )Ay/nmH2O (M 2+ 3+ 3+ 3+ nÿ Zn ; M =Al , Fe ; A is an intercalated anion. In the case of hydrotalcite; M2+=Mg2+, M3+=Al3+, Anÿ=CO2ÿ 3 ). The LDH compounds also have a cation adsorption property (Miyata, 1983) as well as the anion exchange property due to their surface hydroxyl groups. As for the synthetic conditions of the LDHs, there has been a lot of studies (Feitknecht et al., 1942; Allmann et al., 1970; Reiche, 1986; Drits et al., 1987; Sato et al., 1988). Some reports studying the
Yoshimi Seida and Yoshio Nakano
Fig. 1. X-ray powder diraction patterns of the synthesized compounds (a) Mg2+/Fe3+ type and (b) Ca2+/Fe3+ type, .: CaCO3.
physical property of the LDHs have indicated that the anion-exchange and cation adsorption properties of the LDHs depend on the physicochemical structure of the compounds (species and compositions of metal ions and intercalated anions) (Cavani et al., 1991; Shin et al., 1996). Physicochemical properties of those compounds as well as adsorption properties for organic pollutants from a viewpoint of their physicochemical structures have not been well clari®ed, in spite of their engineering importance. The eect of physicochemical structure on the adsorption property for humic substances should also be of the interests. In water treatment plants of drinking water, aluminum-based coagulants have been widely used to reduce organic contaminants from water source. Research of aluminum free adsorbents and/or coagulants would attract any attention in future because it has been hypothesized that aluminum exposure is a risk factor for the development or acceleration of onset of Alzheimer disease in humans (Guidelines for drink-
ing water quality, 1998). The goal of this study is to evaluate the potential of layered double hydroxide formed by toxic-free metal ions as an insoluble adsorbent for the removal of organic matter dissolved in water. In the present study, the hydrotalcite and hydrotalcite-like compounds containing iron (Fe3+-compounds) were synthesized. The characterization of the synthesized compounds in the relation to their physicochemical structures (species and compositions of metal ions) was ®rst carried out. The adsorption of humic substances on the compounds and their heat-treated ones (abbreviated as HT) were then performed following the characterization. The eect of the physicochemical structure of the compounds on the adsorption property of the humic substances were investigated. The adsorption mechanism of the humic substances onto the compounds and their HTs were also examined from the viewpoint of the physicochemical structure of the compounds and the size of the humic substances.
Removal of humic substances
Fig. 2. X-ray powder diruction patterns of the original compound, the HT one and the HT one treated with humic substances (Mg2+/Fe3+ type).
The potential of Fe3+-compounds for the removal of the humic substances was evaluated. MATERIALS AND METHODS
Sample preparation The hydrotalcite and Fe3+-compounds were synthesized at room temperature by slow addition of 10 wt% NaOH to a mixed metal (M2+; Mg2+ or Ca2+ and M3+; Al3+ or Fe3+) chloride solution with molar M2+/M3+ ratios of 1/1, 1/0.5, 1/0.33 and 1/0.25. The alkali solution was added until a ®nal pH of 11 was achieved in the synthesis of hydrotalcite. In the case of the Fe3+-compounds, the alkali solution was added until the ®nal pH of above 13 was achieved to form LDH phase with a minimum of other coproducts such as hydroxides and carbonates. The reaction was carried out under atmosphere. A large amount of carbon dioxides dissolve into the alkaline solution rapidly from the atmosphere under this condition and produce carbonate ions in the solution. As a result, most of the anions intercalated in the innerlayer of the produced LDHs are carbonate ions under the synthetic conditions (Seida et al., 1998a). The reaction mixtures were thoroughly stirred during the reaction at 353 K. The obtained compounds were further hydrothermally treated to improve their crystallinity for 1 day at 353 K. The precipitates were washed by centrifugation and dialysis to remove dissolved ions. The synthesized compounds were sieved to collect the particles of 71±140 mm in diameter after drying at 353 K. Half of them were heat-treated at 773 K (hydrotalcite) and 723 K (Fe3+-compounds) for 5 h in the air to obtain the heat-treated compounds (HT). The compounds synthesized by M2+ and M3+ ions are abbreviated as M2+/M3+ type compounds. Characterization of synthesized compounds The structures, thermal behaviors and compositions of synthesized compounds (hydrotalcite and Fe3+-compounds) were analyzed by XRD (X-ray diraction detector; Rigaku, RAD-R), TG (thermal gravimetry; Seiko Instr. Inc.; SSC5200) and ICP plasma emission analyzer (Seiko Instr. Inc., SPA4000), respectively. The cationic
compositions of those synthesized compounds were analyzed after being dissolved by 1 N HCl to evaluate the molar M2+/M3+ ratio in those synthesized compounds. Adsorption of humic acid Humic substances (HS) purchased from Aldrich Co. were used for the adsorption experiment. 0.05 g of the synthesized compounds were put into 25 ml of 1±200 mg/l solutions of the humic substances. The solutions were kept at room temperature for over 24 h. The concentration of humic substances in the solution was measured by ultraviolet spectrophotometer (Hitachi UV-2000, wavelength; 280 nm, measured without any puri®cation). The amount of the humic substances adsorbed onto the compounds was calculated by the mass balance. The removal properties of hydroxides (Al(OH)3, Fe(OH)3, Mg(OH)2, Ca(OH)2) and oxides (FeO, MgO, CaO) for humic substances were also examined to understand the eect of LDH formation on the removal. The LDH can be accepted as an inorganic polytype compound polymerized from hydroxides. The hydroxides and oxides employed are those of the basic units constituting the LDHs and HTs synthesized in this study. All the hydroxides and oxides used were of reagent grade.
RESULTS AND DISCUSSION
XRD diractogram The synthesized compounds were identi®ed as LDH from X-ray diractograms except for Ca2+/ Fe3+=1 type compounds. In the case of Ca2+/ Fe3+=1 type compounds, the LDH structure did not form. The produced compound was mainly CaCO3. Figure 1(a) and (b) shows XRD diractograms for the series of Fe3+-compounds. The crystallinity of the compounds was improved by increasing the M2+/M3+ ratio in the compounds. The diractograms of the Ca2+/Fe3+ type compounds contain several peaks meaning the copro-
Yoshimi Seida and Yoshio Nakano
Fig. 3. M2+/M3+ ratios of the reaction mixtures in the preparation stage and those of synthesized compounds.
duction of some compounds involving CaCO3. The coproduct compounds, however, were not removed by a simple wash using a large amount of distilled water. Figure 2 shows the diractograms of the original compound, the HT compound and the HT one treated with the humic substances. The layered structure of the original compound decomposed by the heat-treatment was again reconstructed in the solution of the humic substances. The crystallinity of the reconstructed LDH was, however, lowered. The d-spacing of the reconstructed LDH after the treatment with the humic substances showed a slight increase compared to those of original compounds. This implies the intercalation of humic substances into the innerlayer of the LDHs (table in Fig. 2). The humic substances are acidic molecules and have anionic sites in them. The intercalation of humic substances into the positively charged innerlayer of the LDHs is quite reasonable. The humic substances used in this study have a wide range of sizes, from several sub-nm up to 80 nm, which we con®rmed by dynamic light scattering (Otsuka Electric Company, DLS700). In the case of reconstruction of LDH structure in pure water, the HT compounds reproduce the LDHs with almost the same crystallinity as the original ones (Fig. 2). The diractograms of the reconstructed compounds with the broad and low intensity diraction peaks also indicate the intercalation of humic substances. Cationic composition of synthesized LDH Figure 3 shows the relationships between the cationic molar composition (M2+/M3+ ratio) in the preparation stage and in the synthesized compounds. The M2+/M3+ ratios of the compounds are almost the same as those of the reaction mixtures in the preparation stage. In the case of the Ca2+/Fe3+ type compounds, there exists the upper limit of the M2+/M3+ ratio (=1) independent with the cationic compositions of the reaction mixtures in the preparation stage. Large metal cation is di-
Fig. 4. TG diagnosis of the synthesized compounds. (a) Mg2+/Fe3+ type and (b) Ca2+/Fe3+ type.
cult to be accommodated in the holes of the close packed con®guration of the OH group in the brucite-like host layer (Cavani et al., 1991). It has been considered that the large dierence in molecular size between Ca2+ and Fe3+ ions makes it dicult to form a layered structure (brucite-like layered structure), resulting in the formation of the compounds with a low M2+/M3+ ratio. Although the cationic compositions (Ca2+/Fe3+ ratio) are close to each other for the series of the compounds, they showed a adsorption property for humic substances dierent from each other, depending on their cationic compositions in the preparation stage as shown in Section Eect of type and composition of LDH. Thermal analysis Figure 4(a) and (b) shows TG diagrams for the series of Fe3+-compounds treated with humic substances. The observed decomposition temperatures were around 673, 653 and 533 K for Mg2+/Al3+, Mg2+/Fe3+ and Ca2+/Fe3+ type compounds, respectively. The decomposition temperature of the compounds treated with humic substances were almost the same with the original ones. The de-
Removal of humic substances Table 1. Fractional removal of humic substances by metal hydroxides and oxides from 100-ppm solution Compounds Mg(OH)2 Ca(OH)2 Fe(OH)2 MgO CaO FeO
Weight (g) 0.05 0.10 0.15 0.01 0.05 0.10 0.05 0.10 0.15 0.05 0.10 0.15 0.05 0.10 0.15 0.05 0.10 0.15
0.44 0.71 0.92 0.83 0.91 0.95 0.05 0.28 0.47 0.92 0.96 0.98 0.93 0.96 0.97 0 0 0
10.4 10.4 10.5 11.6 12.2 12.4 9.0 8.6 8.3 10.7 10.7 10.6 12.2 12.4 12.4 8.4 8.8 8.7
composition temperature shifted towards higher temperatures with increasing the M2+/M3+ ratio in the cases of Mg2+/Al3+ and Mg2+/Fe3+ type compounds. Adsorption property for humic substances Removal property of hydroxides and oxides. Table 1 shows the fractional removal of the humic substances (100 mg/l solution) for the series of hydroxides and oxides. Although the hydroxides coagulated the humic substances, the formed sludge hardly precipitated and contained a large amount of water showing colloidal suspensions. Actually, the concentration of humic substances in the solutions were measured after ®ltrating them by 0.45mm membrane ®lter. The oxides also coagulated the humic substances by forming hydroxide except for FeO. The formed sludge contained the large amount of colloidal suspensions as seen in the case of hydroxides.
Eect of type and composition of LDH. The adsorption of humic substances (HS) onto the LDHs and HT compounds occurred by two phases (Fig. 5). In the ®rst phase, the adsorption occurred rapidly within a few hours. Sedimentation of the compounds was also within a few hours. In the HT compounds, the reconstruction of the LDH structure occurs simultaneously accompanied with the adsorption of humic substances in the ®rst phase. The pH of the solutions increased gradually up to 8±9 during the ®rst phase. In the second phase, the small amount of hydroxides produced by a slight dissolution of the compounds promoted the coagulation of humic substances onto the compounds. The hydroxide-induced coagulates adsorbed onto the compounds resulting in a sludge with very low water content. The slow ion-exchange between carbonate ions in the compounds and humic substances would also occur in the second phase. The second phase occurred taking a long time (few days or more longer). Figure 6(a) and (b) shows the amount of adsorption for the series of compounds and their HT ones. The Mg2+/Al3+ and Mg2+/ Fe3+ type compounds show isotherms with similar patterns and comparable adsorption capacity each other. The amount of adsorption increased with increasing the M2+/M3+ ratio. Although the M3+ ions in the brucite-like layer produce the anion exchange site, the amount of adsorption increased with decreasing the content of M3+ ion in the compounds. The results of adsorption in Fig. 6 implies the adsorption of humic substances at the hydroxyl groups of M2+ as well as at the innerlayer anion exchange site. The surface hydroxyl groups of the compounds would work as coagulation sites. The humic substances with large molecular size should be adsorbed onto the surface of the LDHs and not intercalate in the innerlayer of the LDHs due to the larger size of them. The HT compounds show a lar-
Fig. 5. Schematic diagram of adsorption mechanism.
Yoshimi Seida and Yoshio Nakano
Fig. 6. Adsorption isotherms. (a) Mg2+/Al3+ type, (b) Mg2+/Fe3+ type and (c) Ca2+/Fe3+ type.
ger adsorption capacity especially in the lower concentration region as shown in Fig. 6(a) and (b) by the dotted lines. In our recent study of intercalation of anionic surfactants into the LDH innerlayer (Seida et al., 1998b), the HT-compounds are able to intercalate a large amount of and a larger size of guest molecules during their reconstruction of the layered structure compared to the LDHs. In contrast to Mg2+/Al3+ and Mg2+/Fe3+ type compounds, the Ca2+/Fe3+ type ones showed quite a dierent adsorption property as shown in Fig. 6(c). The Ca2+/Fe3+ type compounds showed a very large adsorption capacity for the humic substances, except for the compound with M2+/M3+ ratio=1. The amount of adsorption decreased with increasing M2+/M3+ ratio. The humic substances adsorbed were almost expelled by the heat-treatment above 773 K, which was con®rmed from both TG/DTA diagrams of the compounds and surface color of the heat-treated ones as shown in Fig. 7(a) and (b). The exothermic peak of the combustion of humic substances (HS) at around 1073 K disappeared in the DTA curves of LDHs treated with the humic substances. The dark brown color of Mg/Al type LDH treated with the humic substances turned to white of its original color after the heat-treatment above 773 K, which means the combustion of the humic substances. The HT compounds again showed an adsorption capacity almost equivalent to the initial HT ones in the second cycle of the adsorption as shown in Table 2. The humic substances were not expelled completely by the heat-treatment below 673 K. The heat-treatment below 673 K resulted in lower adsorption capacity in the second cycle (Table 2). The binding force of humic substances with the compounds is very strong and the desorption of them hardly occurs. Small fraction of saturately adsorbed humic substances were desorbed from the compounds by treatments with NaOH and Na2CO3 at 363 K for 2 h as shown in Table 3.
Table 2. Adsorption of humic substances in the 2nd cycle Compound
Mg/Al=2, HT Mg/Fe=2, HT Mg/Al=2, HT Mg/Fe=2, HT
773 K, 773 K, 623 K, 623 K,
6h 6h 6h 6h
q before HT (qb) (mg/g)
q after HT (qa) (mg/g)
89.4 71.5 99.4 73.9
99.4 73.9 62.4 30.4
1.11 1.03 0.63 0.41
Table 3. Desorption of humic substancesa Compound
Mg/Al=2 Mg/Fe=2 a
Percentage of desorbed humic substances (%) 1M Na2CO3
0.1 g LDHs treated with HS in 10 ml of each solution.
Removal of humic substances
Fig. 7. (a) and (b) TG/DTA diagrams of humic substances (HS), LDH and LDH treated with the humic substance.
Eect of salt and pH. Table 4 shows the eects of NaCl and pH on the fractional removal of the humic substances (100 mg/l solutions). The fracTable 4. Eects of pH and NaCl on the fractional removal of humic substances NaCl (M)
Day 1 0 0.1 0.3 0.5 Day 3 0 0.1 0.3 0.5
0.26 0.48 0.48 0.46
0.13 0.13 0.16 0.19
0.27 0.15 0.17 0.17
0.85 0.91 0.92 0.91
0.72 0.76 0.80 0.77
0.72 0.71 0.72 0.69
Day 1 8.7 3.0 2.5 2.0 Day 3 8.7 3.0 2.5 2.0
0.26 0.40 0.95 0.95
8.12 5.72 5.06 3.69
0.13 0.23 0.33 0.92
8.84 7.71 7.28 6.58
0.85 0.85 0.77 1.00
8.21 7.47 7.21 6.63
0.72 0.78 0.80 0.99
8.91 8.55 8.41 8.14
tional removal of the humic substances increased with increasing additive NaCl. The additive salt and/or increase of ionic strength in the solution is considered to enhance the adsorption of the humic substances onto the compounds eectively, which is often observed as a salting out eect in a coagulation system using inorganic coagulants. To study the eect of pH on the adsorption, the initial pH of the solution of the humic substances was adjusted because the LDHs buer the pH through their slight dissolution. As the binding force of humic substances with LDHs is very strong and the rate of adsorption is rapid, the pH adjustment after putting the LDHs into the solution may results in lesser estimation of the pH eect on the adsorption. The solution of humic substances adjusted the pH at 2±3 were used in the experiment. The ®nal pHs after the adsorption were shown in Table 2. The amount of adsorbed humic substances decreased with increasing the ®nal solution pH. This would be due to the coagulation eect of hydroxides produced during the buering.
Hydrotalcite and Fe3+-compounds were found to be eective insoluble adsorbent for the removal of humic substances from water due to their high adsorption capacity and low water content of their
Yoshimi Seida and Yoshio Nakano
resulting sludge. Heat treated compounds were also found to be eective due to their higher adsorption capacity per unit weight. The adsorption capacity of the compounds makes use of the anion exchange capacity of the innerlayer space of the compounds. Contribution of the surface hydroxyl groups of the compounds to the adsorption of the larger size of humic substances was also suggested. The adsorption capacity increases with an increasing M2+/M3+ ratio in the cases of Mg2+/ Al3+ and Mg2+/Fe3+ type compounds. To the contrary, the capacity decreases with an increasing ratio in the Ca2+/Fe3+ type compounds. The removal of humic substances by the compounds is enhanced with increasing additive salt and with decreasing pH. The pH buering eect of the compounds was considered to enhance the removal of humic substances. The Fe3+-compounds are eective for their adsorption capacity, comparable to that of aluminum-based adsorbents (hydrotalcite). REFERENCES
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