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A New Approach for Removing Anionic Organic Dyes from Wastewater Based on Electrostatically Driven Assembly Sira Sansuk, Somkiat Srijaranai, and Supalax Srijaranai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00919 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 29, 2016

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Environmental Science & Technology

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A New Approach for Removing Anionic Organic

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Dyes from Wastewater Based on Electrostatically

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Driven Assembly

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Sira Sansuk, Somkiat Srijaranai, and Supalax Srijaranai*

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Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for

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Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002,

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Thailand

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ACS Paragon Plus Environment


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TABLE OF CONTENT:

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ABSTRACT:

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A conceptually new approach for an efficient removal of anionic organic dyes from wastewater

33

using layered double hydroxides (LDHs) through their formation is presented. Acid yellow 25

34

(AY25) was used as anionic organic dye model molecules. As a result of the electrostatic

35

induction, the removal mechanism involved a concurrent incorporation of AY25 molecules into

36

the interlayer of LDHs during their structural arrangement, where Mg2+ and Al3+ ions were

37

utilized to construct the base of LDHs in an alkaline solution. It was found that the molar

38

stoichiometry of all precursors was a key factor affecting the removal efficiency. Within 5 min

39

removal time, this method still maintained high removal efficiency of over 97% and provided a

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removal capacity of ~186 mg g-1, comparable to that of other LDH-based methods. Also, almost

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complete dye recovery was simply achieved by anionic exchange with common anions (Cl-,

42

NO3-, and CO32-). Additionally, the present technique is straightforward, cost effective, and

43

environmentally friendly since it avoids the synthesis step of sorbents, thus significantly saving

44

time, chemicals, and energy. Hence, this strategy not only exhibits the alternative exploitation of

45

LDHs, but also provides new insights into the removal of contaminants from wastewater.

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■ INTRODUCTION

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Water pollution caused by synthetic dyes is an important environmental issue, and receives

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major worldwide attention.1,2 These water soluble organic dyes can be classified into three main

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groups as anionic (direct, acid and reactive dyes), cationic (basic dyes), and nonionic dyes

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(disperse dyes), and they have been widely used in various industries such as textile, printing,

57

cosmetics, leather, food, pharmaceuticals, etc.3-8 The presence of released dyes into

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environmental water has been a major cause for concern as they are highly visible, non-

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biodegradable, toxic, and potentially able to be transformed into carcinogenic, teratogenic, and

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even mutagenic agents, creating a serious threat to human health and marine organisms.9,10

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Therefore, the development of alternative purification technology for the effective separation and

62

removal of water contaminants is urgently required.

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The removal of organic dyes from industrial effluents is very difficult as a result of their high

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solubility in water. Most of the common water treatment technologies such as biological

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treatment,11 membrane filtration,12 coagulation,13 ion exchange,14 photocatalytic degradation,15

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etc. are either energy intensive or restrictive due to high operational costs, complexity, and the

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possible production of more toxic byproducts. Among the aforementioned methods, adsorption is

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considered the most reliable and effective means for dye removal from discharged wastewater

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owing to its simplicity, ease of operation, economic feasibility, recyclability of adsorbents, as

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well as the availability of a wide range of adsorbents such as activated carbon,16 zeolites,17 metal

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organic frameworks (MOFs),18 polymers,19 etc. Even though a variety of sorbent materials are

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available, it is still challenging to search for effective, readily available, reusable, and

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economically feasible materials with high adsorption capacities and removal rates. Most current

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research efforts have been devoted to increasing the efficiency of dye removal using


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nanomaterials,20,21 nanocomposites,22 as well as supramolecular gels.23 These materials have

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demonstrated their capability as effective sorbents for the removal of organic contaminants due

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to their specific morphologies and structures. However, these engineered sorbents require a high-

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cost fabrication route, and suffer from low water dispersibility. Additionally, attention has been

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paid most recently to the synthesis and design of sorbents from sustainable sources such as

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cellulose from mulberry branches,24 waste hydrochar,25 and chitosan.26 The use of these raw

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materials for the adsorbent preparation not only resolves the environmental issues, but also offers

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a new way to prepare promising low-cost adsorbents for the efficient removal of organic dyes

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from industrial wastewater. Besides a high adsorption capability, the environmental toxicity of

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materials is a key factor that needs to be further considered in order to utilize them effectively

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and confidently.27

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Layered double hydroxides (LHDs), a large class of natural and synthetic ionic lamellar

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compounds, have been considered as eco-friendly and effective sorbents, since they have unique

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hydrophobic and anion exchange properties.28,29 They are made up of positively charged brucite-

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like layers with an interlayer region containing charge compensating anions and water

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molecules. The composition of LDHs can be represented by the general formula [M2+1-

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xM

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respectively. x is the molar ratio between M2+ and M3+, while An- is the interlayer anion with

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charge n. The use of intercalated LDHs as adsorbents for the removal of contaminants from an

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aqueous solution has been extensively studied.30-35 The removal is generally based on different

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mechanisms including electrostatic attraction,36 ion exchange,37 van der Waals force,38 and

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hydrogen bonding interactions.39 For the removal of anionic organic dyes, hydrothermally and

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sonochemically synthesized LDHs have been employed.40,41 However, the removal kinetics,

3+

x+ nx(OH)2] [A ]x/n.mH2O,

where M2+ and M3+ are divalent and trivalent metal cations,



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mainly based on the adsorption or anion exchange of dye molecules on the sorbents, is relatively

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slow. Therefore, the modification of LDH materials is required in order to improve their removal

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capacity. Recently, LDHs functionalized with surface passivated carbon dots (LDH-CD

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composites) have been used as an environmentally friendly composite for the removal of anionic

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organic dyes.42 An addition of these nanoparticles with abundant oxygen-containing functional

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groups onto the positively charged surface of LDHs results in an enhanced interaction with dye

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molecules, thus improving the removal efficiency and removal rate. However, this synthetic

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route of these nanocomposites is complicated, time consuming, and involves a large

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consumption of chemicals, as well as energy. For these reasons, the development of a new,

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simple, and environmentally benign approach based on the efficient exploitation of LDHs is still

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challenging.

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In this study, a new approach for the application of LDHs as a simple and green treatment for

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the removal of anionic organic dyes from an aqueous solution has been demonstrated. This

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removal simultaneously occurs with the formation of LDHs, thus the traditional synthetic step of

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LDHs is not required. As a result of an electrostatic induction, an assembly removal was simply

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established through the simultaneous incorporation of acid yellow 25 (AY25), used as an anionic

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dye model molecule, into the interlayer galleries of inorganic LDHs during their structural

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arrangement. The parameters affecting the removal efficiency were studied. A removal of AY25

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in the presence of other organic dyes and other anions was investigated to test the selectivity and

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capability of the proposed method. Also, the recovery of anionic dyes from LDHs was evaluated

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through the anion exchange process with the other common anions.

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■ EXPERIMENTAL SECTION

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Materials. All the chemical reagents were obtained from commercial suppliers and used as

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received without further purification. Acid yellow 25 (AY25), brilliant crocein moo (BCM),

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methylene blue (MB), and acid blue 29 (AB29) were purchased from Merck (China).

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Magnesium chloride hexahydrate (MgCl2.6H2O), and aluminium chloride hexahydrate

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(AlCl3.6H2O) used as sources of metal cations were obtained from Sigma-Aldrich (China). NaCl,

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Na2SO4, Na2CO3, NaNO3, Na3PO4, NaOH, and sodium dodecyl sulfate (SDS) were purchased

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from Univar (Australia). Deionized water produced from RiOsTM Type I Simplicity 185

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(Millipore water, USA) with a specific resistivity of ~18.2 MΩ cm was used throughout. A 1000

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mg L-1 stock solution of AY25 was prepared by dissolving 0.1000 g of AY25 in 1 L of water.

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Different working solutions of AY25 were prepared daily by stepwise dilution of the stock

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solution with water. To prepare a mixed solution of Mg2+ and Al3+ ions with mole ratio of 3:1,

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0.6099 g MgCl2.6H2O (3 mmol) and 0.2414 g AlCl3.6H2O (1 mmol) were dissolved in 10 mL of

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water.

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Removal of Anionic Organic Dye. The removal experiment was carried out as a batch test by

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rapidly introducing a mixed solution of Mg2+ and Al3+ ions into a 10 mL dye solution containing

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100 mg L-1 of AY25, previously conditioned with 1 M NaOH. After the contact time elapsed, the

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colloidal solution was centrifuged at 4000 rpm for 3 min to completely isolate the precipitate

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from the solution. Then, the residual AY25 concentration was analyzed by measuring the

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absorbance at 393 nm. The removal efficiency, expressed as a dye removal (%), was calculated

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using the following expression: removal (%) = [(Ao - At)/Ao] x 100, where Ao and At are the

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absorbance of AY25 before and after the removal at any time, respectively. All parameters that

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affect the removal efficiency including molar stoichiometry of all precursors (Mg2+, Al3+, and



143

OH-) and removal time were studied. Moreover, the effect of the initial dye concentrations on the

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removal ability was investigated in a range from 10 to 300 mg L-1 AY25 using an optimal

145

amount of Mg2+:Al3+:OH- for 5 min removal time. In addition, the selectivity of this removal

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strategy was also carried out in simulated dye wastewater containing AY25, BCM (anionic), and

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MB (cationic). Also, the influence of anion ions on the removal was studied at 100 mg L-1 AY25

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and 0.010 mol L-1 sodium salts. The tested salts included NaNO3, NaCl, Na2SO4 Na2S, Na2CO3,

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Na3PO4, and SDS. All measurements were carried out three times at room temperature.

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Recovery of Anionic Organic Dye. The experiments were performed, based on

151

exchangeability towards an anion exchange reaction, by introducing different anions into the

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system containing the AY25-LDH precipitate. The releasing kinetics of AY25 from the gallery

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of LDH precipitate was investigated with an excess 200 times concentration of common anions

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(Cl-, NO3-, and CO32-) and the absorbance of the released AY25 was subsequently measured as a

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function of time. The releasing rate was defined as a ratio percentage between the absorbance of

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AY25 released at any time (At) and the original absorbance of AY25 contained in LDH

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precipitate (Ao). Moreover, wet LDH precipitate obtained after the anion exchange was further

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reused as a sorbent for the removal of an azo dye, AB29, to test its recyclability.

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Measurement and Characterization. Absorption spectra of AY25 were obtained on an

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Agilent 8453 UV-vis spectrophotometer with a 1 mL quartz microcell. Fourier transformed

161

infrared (FT-IR) spectra of the precipitates were recorded using a Bruker Tensor 27

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spectrometer. Scanning electron microscopy (SEM) was carried out using a LEO-1450VP

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microscope.

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■ RESULTS AND DISCUSSION

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Scheme 1. A schematic illustration for the removal of anionic organic dyes by electrostatic

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assembly and their recovery by the anionic exchange.

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Proof of Dye Removal Concept. The procedure for assembly removal of anionic organic

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dyes through the formation of LDHs is illustrated in Scheme 1. Generally, various metal ions

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including Mg2+, Ni2+,Co2+, Fe2+, Mn2+,Cu2+, Al3+, Co3+, and Cr3+ can be used to prepare LDHs.51

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When considering their toxicity and environmental impact, Mg2+ and Al3+ ions were selected as

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representative divalent and trivalent metal ions, respectively, as these cations as well as

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hydroxide ions were essential for forming the LDH sheets. First, to prove the concept study, the

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removal of anionic organic dyes, AY25, from an aqueous solution by electrostatic assembly was

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performed. The UV-vis spectra of AY25 before and after 5 min of an introduction of metal

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cation precursors with a fixed mole ratio (Mg2+:Al3+) of 3:1, typically used to obtain LDHs,43 are

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presented in Figure 1. It was obvious that the characteristic absorption peaks of AY25

180

disappeared promptly after the removal, as can be seen in Figure 1b. This observation indicated

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the concurrent removal of AY25 molecules during the formation of LDHs. Given the presence of

182

a very high concentration of Cl- ions (9 mmol) contained in metal chloride salts used as



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precursors, an almost complete removal of AY25 was still obtained within only 5 min, possibly

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due to the reactive nature of AY25 and a very strong negative charge of its sulfonate functional

185

group. The corresponding photo images (inset in Figure 1) showed that the dye solution was

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almost colorless after 5 min of the introduction of mixed metal ions. Moreover, it was found that

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a removal of AY25 was unattainable, when Mg2+ or Al3+ was used individually. These results

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indicated that the removal of anionic dye was successfully achieved by this technique, as it could

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be simultaneously incorporated into the interlayer of LDHs during their formation as a result of

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electrostatic attraction.

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192 193

Figure 1. UV-vis absorption spectra with corresponding photo image (Inset) of AY25 solution at

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an initial concentration of 100 mg L-1 before (a) and after 5 min removal time (b).

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Removal Kinetics. A set of experiments was performed in order to study the kinetics of dye

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removal using 60:20 µmol of Mg2+:Al3+ ions and an initial AY25 concentration of 100 mg L-1.

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The supernatant was withdrawn at desired time intervals ranging from 15 s to 20 min and the

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absorption measurement at 393 nm was carried out. The removal rate was assessed as a ratio of


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the absorbance at any time t (At) of removal and the initial absorbance (Ao) before the removal (i.

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e. introduction of metal cations). Figure 2 illustrates the removal rate and efficiency of 100 mg L-

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1

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stage (during the first min), whereas it became much slower in the later stage. After 5 min, the

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removal rate was constant, indicating that the equilibrium was reached. As can be seen in Figure

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2b, the efficiency rose sharply within the first min, and subsequently increased slowly up to 96%

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until 5 min; afterwards, the efficiency no longer changed even when the contact time was

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extended to 60 min. The initial fast removal rate or high removal efficiency possibly

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corresponded to the electrostatic attraction of present ions in the solution. This fast dye removal

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capacity has significant practical importance for the efficient treatment of water. However, a

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complete removal was unachievable. The feasible reason was the competition of Cl- ions,

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contained in a mixture of metal precursor solution, and water molecules that can be incorporated

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into the structure of LDHs, affecting the removal efficiency.43 In addition, the influence of CO32-

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ions should be taken into account since LDHs are typically synthesized by coprecipitation at

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constant pH under a nitrogen atmosphere to exclude CO2.28

AY25 as a function of time. As shown in Figure 2a, the removal was very fast in the initial

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216 217

Figure 2. The removal kinetics (a) and corresponding efficacies (b) of AY25.



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Selectivity and Interference. The effluents of dye-bearing wastewater often contain a mixture

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of various kinds of dyes. Hence, it is highly necessary to investigate the removal ability of this

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technique for the mixed dye solution. The removal experiments were carried out in simulated

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dye wastewater containing AY25, BCM, and MB. The UV-vis spectra of the control and treated

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dye are presented in Figure 3A revealed the ability of this method for the removal of

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multicomponent organic dyes. All absorption peaks could be assigned as follows; 393 nm for

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AY25 (yellow solution), 343 and 510 nm for BCM (red solution), 670 nm for MB (blue solution)

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and a combined peak at 627 nm for MB and BCM. Both anionic dyes, AY25 and BCM, were

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completely eliminated from the aqueous solution within 5 min, whereas cationic dye, MB, was

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still in the solution. A slight reduction of the absorbance of MB was possibly due to its

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degradation in alkaline conditions, and adsorption onto the surface of sorbent as a consequence

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of hydrophobic interaction (hydrogen bonding interaction) formed between the free hydroxyl

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groups in the LDH sheets and the nitrogen or oxygen-containing groups of MB. The

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corresponding color images (Inset in Figure 3A) of the dye solution clearly revealed the selective

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removal of anionic dyes. This demonstrated that the present method has a potential to selectively

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remove negatively charged dye molecules from wastewater.

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The influence of other competitive anions was also studied. As shown in Figure 3B, the effect

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of monovalent anions (i.e. Cl-, NO3-) on AY25 removal was negligible. In contrast, the divalent

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(SO42-, S2-, CO32-) and trivalent (PO43-) anions had a profound influence, indicating the

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occurrence of the competition between AY25 molecules and these inorganic anions.43 In

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addition, the lowest removal efficiency of AY25 was obtained in the presence of anionic

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surfactant SDS, possibly due to a very strong competition as well as high surface tension or high

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viscosity of the solution that obstructed the movement of ions. These findings implied the


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existence of electrostatic induction during the formation of LDHs. Nevertheless, even with the

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competition of other interference anions during the capture of AY25 molecules, it was found that

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removal efficiency was still over 80%. This result could be possibly clarified by a subsequent

244

adsorption of AY25 molecules onto the positively charged surface of the precipitate, as other

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anions were potentially incorporated into the structure of LDHs due to their higher charge

246

density than that of AY25.

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248 249

Figure 3. The removal selectivity (A) with corresponding photo image (Inset) of simulated dye

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solution containing AY25, BCM, and MB at 50 mg L-1 each and the effect (B) of other



251

competitive anions (10 mmol L-1) on the removal efficiency of AY25 at an initial concentration

252

of 100 mg L-1.

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253 254

Characteristics of Precipitates. The functionality of the hybrid precipitates was investigated

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to assure the incorporation of anionic dye molecules into the structure of LDHs. Figure 4A

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shows FT-IR spectra of LDHs obtained from a blank solution (without the presence of AY25),

257

and AY25-LDHs obtained from a AY25 solution. Similar characteristic bands were obtained.40

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The weak bands at 3368 and 3429 cm-1 represented the stretching vibration of hydrogen bonding

259

and interlayer water molecules of Cl--LDHs and AY25-LDHs, respectively. A slight shift was

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possibly due to the influence of incorporated AY25 molecules in LDHs. The weak bands at 1637

261

cm-1 were attributed to the bending vibration of interlayer water molecules. In LDHs, the peak at

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1365 cm-1 was attributed to the stretching vibration of chloride ions, while in AY25-LDHs the

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same peak position, but stronger intensity, was ascribed to the stretching vibration of anionic

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AY25 molecules. Additionally, a difference in the color of LDH precipitates was distinguishable.

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The yellowish precipitate was obtained after the removal of AY25 from the solution, while white

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precipitate was obtained from the blank solution (without AY25). This observation was also

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confirmed by the disappearance of the yellow solution of AY25 after the removal by the

268

proposed approach, as shown in Figure 1. These results signified the successful incorporation of

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AY25 into the structure of LDHs during their structural arrangement, confirming the removal of

270

anionic organic dyes.

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SEM images were also used to compare the morphological feature and surface characteristics

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of the LDH precipitates obtained in the presence and absence of AY25 dye molecules. The Cl--

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LDHs exhibited a smooth and thick surface (Figure S1), while AY25-LDHs, illustrated in Figure


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4B, showed a rough surface with aggregation of particles. The change in surface morphology of

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the precipitates obtained after the removal possibly resulted from the incorporation of AY25

276

molecules influenced stacking and corrosion of the hydroxide layers, resulting in low

277

crystallinity.

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280 281

Figure 4. FT-IR spectra (A) of Cl--LDHs (a) and AY25-LDHs (b) and SEM image (B) of AY25-

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LDHs.

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Effect of Compositional Precursors on Removal Efficiency. This method eliminates the

285

synthesis step of sorbents as AY25 molecules are simultaneously captured during LDH

286

formation. Hence, the amount of all precursors needs to be considered as a major factor that

287

affects the removal efficiency. In this work, the effect of hydroxyl ions was studied, instead of

288

the solution pH, because of their participation in the base formation of LDHs. Figure 5A presents

289

the effect of OH- ions on the AY25 removal efficiencies. The removal increased when the

290

291 292

Figure 5. Effect of hydroxide ions (A) and metal ions precursors (B) on the removal efficiency

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of AY25 at an initial concentration of 100 mg L-1.


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amount of OH- ions increased. However, increasing the amount of OH- ions beyond 200 µmol

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resulted in a reduction in the incorporation of AY25. This decrease could be explained by the

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strong competition of OH- ions during LDH formation.43 Also, an influence of Mg2+ and Al3+

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ions at a fixed mole ratio of 3:1 was investigated in the range of 30:10-180:60 µmol and a similar

298

trend to hydroxide ions was observed, as shown in Figure 5B. The removal efficiency increased

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when the amount of metal cations increased up to 60:20 µmol. Then, the removal flattened out,

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indicating an excess of metal cations. For this reason, an amount of 60, 20, and 200 µmol of

301

Mg2+, Al3+, and OH- ions, respectively, was chosen as optimal compositional precursors for

302

further experiments.

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Effect of Initial Dye Concentrations on Removal Efficiency. The influence of the initial

304

AY25 concentration on the removal efficiency was carried out using a molar ratio of 60:20:200

305

µmol, i.e. 3:1:10, of Mg2+:Al3+:OH- ions. Figure 6 shows that the removal of AY25 was almost

306

complete at concentrations lower than 100 mg L-1, while a decrease in the removal efficiency

307

was observed at a higher concentration. This observation pointed out that the removal of anionic

308

dye molecules was dependent on the concentrations of Mg2+:Al3+:OH- ions consumed in order to

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form the base structure of the hybrid precipitate. Therefore, a removal at high dye concentrations

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required a high amount of all precursors.

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Figure 6. Effect of initial concentrations of AY25 on the removal efficiency.

317

In general, the adsorption capacity of adsorbents can be obtained when the adsorption process

318

reaches equilibrium.44 However, in this study, the removal mechanism was different as it relied

319

on electrostatic incorporation of dye molecules into the structure of LDHs, while they were being

320

formed, and molar stoichiometry of all precursors for LDH formation limited dye removal

321

quantity. Still, the removal capacity can be estimated using the optimal concentration of

322

Mg2+:Al3+:OH- at a molar ratio of 3:1:10 for the removal of 100 mg L-1 AY25 in 10 mL solution.

323

The removal capacity of 186 mg g-1 was achieved for the proposed approach. However, this

324

value is dependent on the charge and mass of dye molecules, known as charge density, as it

325

dictates the ease of dye removal.45 The comparison of the removal capability of this technique

326

with the other adsorbent-based methods for anionic dye removal is summarized in Table 1. It

327

was obvious that the present approach did not require the exploitation of sorbent materials,

328

showing a great promise for the removal of anionic organic dyes within the shortest time,

329

compared to the other techniques, where their removal kinetics is chiefly based on adsorption.

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Moreover, the removal capacity of this approach was also found to be higher, when especially


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compared to LDH counterparts.41,42 Hence, the present approach could be a potentially

332

alternative route to remove other contaminants from wastewater.

333 334

Table 1. The comparison of the proposed method with other sorbent-based techniques for

335

the removal of anionic organic dyes. Sorbent

Anionic

Removal

Removal

Ref.

material

organic dye

time (min)

capacity (mg/g)

Magnetite

Eriochrome

15

67.1

46

(Iron oxide NPs)

black T

Bentonite

Congo red

60

158.7

47

Activated carbon

Acid yellow 17

240

215.05

14

CeO2−δ nanopowder

Reactive orange 16

40

91

45

Methyl orange

60

113

Mordant blue 9

180

101

NiO nanosphere

Congo red

90

440

48

Nanospinel ZnCr2O4

Reactive blue 5

60

41.3

49

Cr-doped ZnO NPs

Methyl orange

120

310.56

50

NiAl-LDHs-C250SC

Remazol brilliant

6

150

41

violet LDH-CD composites

Methyl blue

20

180

42

LDHs

Acid yellow 25

5

186

This work

(concurrent formation and dye removal) 336



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Dye Recovery and Recyclability. The structural flexibility of AY25-LDHs precipitate was

338

evaluated via the anionic exchange. The exchangeability of AY25-LDH precipitate, expressed in

339

terms of dye recovery (%), can be estimated by comparing the amount of released AY25 to the

340

original AY25 extracted in LDHs. Figure 7A shows the representative absorption spectra of

341

AY25 after introducing various anions for a 1 h exchange reaction time. It was found that the

342

recoveries of AY25 after exchange with Cl-, NO3-, and CO32- were 23, 48, and 74%, respectively.

343

These values mainly relied on both charge density and the affinity of anions into the gallery of

344

LDHs.51 The kinetics of AY25 release was further investigated using the absorbance of released

345

AY25 as a function of time over 10 hr. The releasing rate, defined as dye recovery (%) as a

346

function of time is shown in Figure 7B. It was obvious that a releasing rate of AY25 dye

347

molecules initially increased up to 5 h and was then stable, indicating a complete recovery of

348

AY25. As shown in Figure 7B (Inset), a colourless solution turned yellow after 5 h. Also, when

349

compared to Cl- and NO3- ions, CO32- ions had the strongest affinity to LDHs, and a releasing

350

rate of AY25 by these anions was ordered as CO32- > Cl- > NO3-, corresponding to their charge

351

density and affinity to LDHs.51

352

In addition, the reusability of the precipitates, obtained after the recovery of AY25, was tested

353

and NO3--LDHs were used as a representative sorbent for the removal of AY25 from the

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solution. It was found that a complete removal within a 5 min contact time was unachievable,

355

compared to the proposed method. The possible reason was due to the low charge density of

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AY25 as well as the positively-charged surface of the recovered LDHs since the removal at this

357

stage occurred through adsorption and/or the ion exchange process, which required a longer

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contact time.14,45,48 Furthermore, the recyclability of the recovered NO3--LDHs was further

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examined for the removal of AB29 (a dianionic azo dye with two sulfonate groups). UV-vis


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spectra and the corresponding colour image of the control and treated dye solutions are presented

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in Figure S2. It was obvious that the absorbance maxima of the AB29 diminished following the

362

treatment process and the corresponding colour image (Inset in Figure S2) clearly illustrated an

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efficient removal of AB29 with over 90% removal efficiency. This result showed good recycling

364

ability of the sorbent materials for the removal of organic dyes from wastewater.

365

366

367 368

Figure 7. UV-vis spectra (A) of AY25 released after 1 h anionic exchange with different anions

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and releasing rate (B) with corresponding photo image (Inset) of AY25 solution before and after

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5 h anionic exchange.



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In summary, this work presents an alternative approach for the efficient removal of anionic

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organic dyes from wastewater. Based on the electrostatic induction during the LDH formation, a

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removal of anionic organic dye molecules from the solution could be accomplished

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simultaneously. The removal efficiency of this method depends upon the amounts at a certain

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mole ratio of all precursors (metal cations and hydroxides ions) because of their participation in

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an assembly of layered hybrid precipitates. Compared to the other sorbent-based techniques, the

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proposed method is superior in terms of simplicity, rapidity, cost-effectiveness, and

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environmental responsibility, as the removal can be accomplished without the need for sorbent

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preparation. Hence, this technique could be further developed for the removal of anionic

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pollutants from aqueous solutions in environmental management.

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■ ASSOCIATED CONTENT

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Supporting Information

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SEM image of Cl--LDH precipitate and UV-vis absorption spectra with corresponding photo

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image of acid blue 29 solution at an initial concentration of 100 mg L-1 before and after 30 min

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removal time by using NO3--LDHs as sorbent. This information is available free of charge via

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the Internet at http://pubs.acs.org.

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■ AUTHOR INFORMATION

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Corresponding Author

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* Tel.: +66 43 202222 41 ext. 12243, Fax; +66 43 202373, E-mail: [email protected]

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Notes

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The authors declare no competing financial interest.

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■ ACKNOWLEDGMENTS

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We gratefully acknowledge The Development and Promotion of Science and Technology

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Talents Projects (DPST), the Royal Thai Government, and the Center of Excellence for

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Innovation in Chemistry (PERCH-CIC), the Office of the Higher Education Commission,

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Ministry of Education.

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