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Structure-modulated CaFe-LDHs with superior simultaneous removal of deleterious anions and corrosion protection of steel rebar

The three anionic species; chloride (Cl−), sulfate (SO42−), and carbonate (CO32−), are typical chemical factors that environmentally accelerate failure of concrete structures with steel rebar through long-term exposure. Efficient removal of these deleterious anions at the early stage of penetration is crucial to enhance the lifespan and durability of concrete structures. Here, we synthesize CaFe-layered double hydroxide (CaFe-LDHs) by a simple one-step co-precipitation technique and structural modulation by calcination process. It is applied for the removal of Cl−, SO42−, and CO32− anions as well as corrosion inhibition on steel rebar in aqueous solutions. The synthesized CaFe-LDHs with phase transfer show notable improvement of removal capacity (Qmax) toward Cl− and SO42− over 3.4 times and over 5.69 times, respectably, then those of previous literatures. Furthermore, the steel rebar exposed to an aqueous solution containing the three anionic sources shows a fast corrosion rate (1876.56 × 10−3 mm per year), which can be remarkably inhibited showing 98.83% of corrosion inhibition efficiency when it is surrounded by those CaFe-LDHs. The novel adsorption mechanisms of these CaFe-LDHs-induced crystals and corresponding corrosion protection properties are elucidated drawing on synergy of memory effects and chemical reactions.


Introduction
Building in concrete structures with steel rebar was a remarkable innovation in the construction industry because it adds strength to concrete, providing durability and allowing more specialized designs. 1 However, it is always exposed to corrosion, leading to deterioration in concrete structures, which determines their durability and life span, particularly in natural environments. 2,3 In general, concrete structures present good durability to corrosion because the structure of concrete presents high alkalinity in the pores of the surrounding concrete, forming a protective, passive layer on the surface of the steel rebar. [4][5][6] Nevertheless, corrosion of steel rebar can be readily initiated when passivity is compromised by damage 7 or when the bond between concrete and steel rebar is destroyed by anionic species. Three major anions from different sources are particularly harmful: chloride (Cl À ), sulfate (SO 4 2À ), and carbonate (CO 3 2À ) anions. 8 Corrosion is generally initiated by electrochemical ow of electrons and ions. 9,10 Penetration of Cl À anions to reinforced concrete is signicantly detrimental, especially for structures near marine environments exposed to high concentrations of salts. 11,12 In this case, the Cl À anion can easily penetrate concrete structures through capillary adsorption, hydrostatic pressure, or diffusion, accelerating steel rebar corrosion (eqn (1)). 13,14 It can penetrate into concrete in diverse forms drawing on the chemical sources of SO 4 2À anions from the environment, which also promotes corrosion of steel rebar in concrete. 15 SO 4 2À anion attack occurs by diverse forms drawing on chemical sources, including seawater, acid rain, or combustion gas from industry, leading to destruction of the original properties of concreate (eqn (2)). [16][17][18] Carbonation occurs by CO 3 2À anions, which originate from carbon dioxide, diffusing into the concrete matrix from the air, reacting with calcium hydroxide to form calcium carbonate. This lowers pH and causes general internal volume expansion in the concentrate. When the concrete is neutralized and water and air inltrate, the corrosion of steel and the volume expands, causing cracks in the concrete cover, resulting in loss of the strength and durability of the structure (eqn (3)). 19 This deteriorates the life-span of the concrete structures by providing easy penetration paths for other corrosion species.
Fe 2+ + 2Cl À / FeCl 2 + 2e À 2Fe + 2H 2 SO 4 + O 2 / 2FeSO 4 + 2H 2 O (2) forced concrete structures not only adversely affects safety, but also causes enormous losses to society and the national economy over a long period of time. 20,21 Therefore, it is necessary to selectively and promptly remove deleterious anions as soon as they penetrate the concrete structure to maintain durability and life span.
Layered double hydroxides (LDHs) are hydrotalcite-like compounds with high specic surface area, memory effects, eco-friendly features, and low-cost, widening utilization in the elds of hazardous materials adsorption, catalysts, biomedicine, agriculture, and wastewater treatment. 22 anions on the corrosion protection ability of LDHs has not been considered. Moreover, few studies have investigated the anion adsorption mechanisms of CaFe-based LDHs and their potential for corrosion resistant property. In this study, we report a synthesis of CaFe-LDHs and CaFebased metal oxides by a modied one-step co-precipitation method follow by structural modulation. The removal performance of harmful anionic species including Cl À , SO 4 2À , and CO 3 2À are studied in an aqueous solution containing the nanopowders, initiated by the CaFe-LDHs. The corrosion protection efficiency of steel rebar for the potential improvement of the durability of concrete structures is envisaged relying on structural variation of the CaFe-LDHs. The mechanisms for simultaneous capturing of anion species and anti-corrosion are investigated with respect to phase transfer of CaFe-LDHs drawing on the variation of crystallinity coupled with stoichiometry.

Synthesis of CaFe-LDHs
CaFe-LDHs, which were previously prepared by a typical coprecipitation method, 30,35,36 were prepared via a modied method here. Specically, CaFe-LDH was prepared by one-step synthesis of co-precipitation of two precursor materials, an aqueous solution containing 3 M NaOH (900 mL) was rapidly injected into the second solution (900 mL) containing a mixture of 66.7 mmol of Ca(NO 3 ) 2 $4H 2 O and 33.3 mmol of Fe(NO 3 ) 3 -$9H 2 O at room temperature for 1 min (900 mL min À1 ) and the solution was then aged for 18 h. The concentration was balanced corresponding to xed molar ratios of Ca 2+ : Fe 3+ (2 : 1). During this process, the pH was maintained at 13, and the reaction was always performed at room temperature (25 C). Then, the synthesized CaFe-LDHs were washed with excess quantities of ethanol to remove unwanted nitrate salt.

Structural transformation of CaFe-LDHs
Structural transformation of CaFe-LDHs was carried out by a calcination process. The calcined CaFe-LDHs were prepared under two heat treatment conditions of the original LDHs in a furnace at 400 C (C-400 LDHs) and 700 C (C-700 LDHs) for 3 h in an air atmosphere. Both the un-calcined (pristine LDHs) and calcined LDHs were ground prior to analysis or use in adsorption experiments.

Adsorption test of ionic species in aqueous phase by CaFe-LDHs
(1) Removal of Cl À by CaFe-LDHs. The Cl À solution was prepared by dissolving NaCl in deionized water. The initial concentration of Cl À was 200 ppm. The Cl À removal was then carried out by adding 1.89 g of pristine, C-400 and C-700 LDHs in 200 mL of the aqueous ionic solution at pH 7. Removal of anions in the aqueous solution was conducted by anion exchange reactions during solution mixing assisted by magnetic stirring at 250 rpm, and propagation of the reaction was monitored by analyzing the LDH solution aer sampling at adsorption times of 5, 10, 20, 30, and 60 min. The collected solutions were ltered with a 0.20 micro ltration membrane to separate the LDHs from solution. The solid powder was then dried at 100 C for 3 h in a vacuum oven for further analysis.

Competitive adsorption test of ionic species in aqueous phase by CaFe-LDHs
The anionic solution with Cl À , SO 4 2À , and CO 3 2À was prepared by dissolving NaCl, Na 2 SO 4 , and Na 2 CO 3 in deionized water. The initial concentrations of Cl À , SO 4 2À , and CO 3 2À anions were all 250 ppm. Adsorption was carried out by adding 1 g of pristine, C-400 LDHs, or C-700 LDHs in 200 mL of the aqueous ionic solution at pH 7. Removal of anions in the aqueous solution was monitored as described in Section 2.4.  37 was used in the corrosion experiments. The steel rebar (1.5 mm diameter) was cleaned with ethanol. The test specimen was further coated with a micro-stop coating with a 1 Â 1 cm 2 exposed area to carry out corrosion measurements. Electrolyte solutions were prepared with 250 ppm of NaCl, Na 2 SO 4 , and Na 2 CO 3 solutions, respectively. Corrosion effects were analyzed with/without LDHs (1 g) of various structural transformations. Corrosion was monitored as a function of time by adding 1 g of the structure modulated CaFe-LDHs powders into 200 mL corrosion solutions. The saturated calomel electrode (SCE), platinum electrode (PE) and CaFe-LDHs were connected to reference, counter electrode and working electrodes, respectively. For corrosion testing, the anti-corrosive properties and electrical conductivity of the rebar were determined by electrochemical measurements, including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). The EIS measurements were carried out by applying a sinusoidal potential perturbation of 10 mV over a frequency range of 100 kHz to 10 mHz. The obtained EIS spectra were analyzed with an equivalent circuit using Zman soware (WonATech Co. Ltd., Republic of Korea). All electrochemical measurements were performed under quiescent conditions. No attempt was made to aerate or deaerated the test solutions.

Characterization
The pH was measured using an FE20 pH meter (Mettler Toledo, USA) equipped with a glass electrode (LE438, Mettler Toledo, USA). Concentrations of exchanged liquid were determined by ion chromatography (METROSEP A SUPP 5-250) at a ow rate of 0.7 mL min À1 with an eluent mixture of 3.2 mM Na 2 CO 3 /1.0 mM NaHCO 3 . The solid sample was characterized by Fourier transform infrared spectroscopy (FT-IR) (Thermo, Scientic 380 FT-IR) in the range of 4000 to 500 cm À1 with resolution of 4 cm À1 . X-ray diffraction (XRD) patterns of solid were obtained on a D/max2200 diffractometer (Rigaku Co., Japan) with Cu Ka radiation (l ¼ 0.15406 nm) operating at 40 kV and 100 mA in the range of 2q ¼ 10-80 at a scan speed of 4 min À1 . The morphology of the solid was characterized by scanning electron microscopy (SEM, S-4800, Hitachi Ltd., Japan). The morphologies and sizes of the as-obtained samples were observed by transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) at 200 kV to obtain high-resolution images.

Characterization of CaFe-LDHs by structural transformation
XRD patterns of the as-synthesized Pristine LDHs and their modulated structures are displayed in Fig. 1 , indexed to a rhombohedral structure with a main diffraction peak at 2q ¼ 29.36 , was newly generated. This can be caused by a small amount of CO 2 dissolution from air during the calcination process. [40][41][42] As the calcination temperature increased, the CaCO 3 phase disappeared, and crystallization of the CaO phase increased. When the temperature exceeded 550 C, decomposition ensued. The disappearance of CaCO 3 was explained by the following reaction [eqn (4)]: The FT-IR spectra of CaFe-LDHs were analyzed to identify the structural transformation by comparing chemical bonding of functional groups (ESI 1 †). The characteristic peak at 1640 cm À1 corresponds to the H-O-H bending vibration of the interlayer water in the crystalline structure, and the characteristic sharp absorption band at around 1350 cm À1 is the asymmetric stretching mode of NO 3 À in the interlayer (ESI 1(b) †). The intense broad bands observed at 3585 cm À1 are associated with stretching vibrations of structural OH À groups in Ca(OH) 2 and Fe(OH) 2 (ESI 1(c) †). The peaks at 749 and 580 cm À1 are Because the crystallized water molecules were removed, the band at 1640 cm À1 became weaker aer calcination and completely decomposed in the case of C-400 LDHs. The gradual decrease of NO 3 À stretching vibration intensity at 1350 cm À1 also implies decomposition of the NO 3 À anion, which was completely decomposed in C-700 LDHs. The noticeable decrease in OH À groups in C-400 LDHs implies that the OH À groups of cationic layers, such as Ca(OH) 2 and Fe(OH) 2 , decomposed during thermal treatment at 400 C. Subsequently, the peak intensity increase of Ca-Fe-O at 749 and 580 cm À1 indicates that the pristine LDHs transformed into oxidized metal oxide structures. The commercial CaCO 3 peaks were observed at 1394 cm À1 (y 3 asymmetric CO 3 2À ) along with a sharp band at 871 cm À1 (y 2 asymmetric CO 3

2À
). 45 Those bands decreased in peak intensity with increasing calcination temperature from 400 C to 700 C, and a structural transformation occurred at the C-400 LDHs based on XRD data. Fig. 2 shows the Scanning electron microscopy (SEM) images of Pristine LDHs, C-400 LDHs and C-700 LDHs. As shown, the morphology of C-400 LDHs changed from a smooth surface into one with more roughness and pores because the water of crystallization and the interlayer anions were removed by transformation from pristine LDHs to a metal oxide phase. This morphology change distinctly appears and consists primarily of individual spherical particles in C-700 LDHs, which is related to transformation of CaCO 3 to CaO. In addition, the lateral particle size of CaFe-LDHs conrmed about 1 mm of pristine LDHs, about 500 nm of C-400 LDHs and about 300 nm of C-700 LDHs. The particle size is an important factor regarding anion exchange. 46 Fig. 3 displays the transmission electron microscopy (TEM) images and associated Selected area diffraction (SAED) patterns of CaFe-LDHs. Pristine LDHs revealed the typical crystallographic layered structure of CaFe-LDHs, which had SAED patterns revealing the (300) and (103) planes with lattice constants of 1.70Å and 2.50Å. However, C-400 LDHs indicated the existence of the (006), (110), and (116) planes corresponding to a CaCO 3 rhombohedral structure aer thermal treatment at 400 C. The unique thermal treatment-induced by-product, CaO, was conrmed in C-700 LDHs, and it was identied in XRD as shown in Fig. 1. Thus, the SEAD patterns validate the gradual structural and crystallographic transitions as a function of temperature from 400 to 700 C. Removal

Performance and mechanism of Cl
where C 0 and C e are the initial and equilibrium anion concentration (mg L À1 ), respectively. As shown in Fig. 4(a), the removal (%) of Cl À anions by pristine LDHs, C-400 LDHs, and C-700 LDHs was 13%, 16.98%, and 57.90%, respectively, for 5 min-adsorption time at the initial stage of reaction. The removal of Cl À anions gradually increased at 60 min and reached 13.41% for pristine LDHs, 29.25% for C-400 LDHs, and 79.62% for C-700 LDHs. The removal efficiency by C-700 LDHs was 5.94 times the corresponding value of pristine LDHs, indicating outstanding removal ability of Cl À anions. The effect of SO 4 2À anion removal by employing the CaFe-LDHs is shown in Fig. 4(b). The results show that removal of SO 4 2À anions rapidly increased from 0% to 65.80% for pristine LDHs, 32.96% for C-400 LDHs, and 91% for C-700 LDHs for just 5 min of initial adsorption. The values then reached 92.25% (pristine LDHs), 98.07% (C-400 LDHs), and 99.86% (C-700 LDHs) when adsorption was complete aer 60 min. As shown in Fig. 4(c), the removal of CO 3 2À anions was faster than other anion species for all the  tested LDHs. C-700 LDH exhibited almost 100% of carbonate removal aer 10 minutes of anion adsorption. Consequently, C-700 LDHs showed the highest removal efficiency toward Cl À , SO 4 2À , and CO 3 2À anions, with values of 79.62%, 99.86%, and 99.30%, respectively (Fig. 4(d)).
The FT-IR spectra of CaFe-LDHs including (a) pristine LDHs, (b) C-400 LDHs, (c) C-700 LDHs and their post-adsorption samples aer uptake-tests of Cl À , SO  the CaFe-LDHs decreased with anion adsorption due to the reconstruction phenomenon, which is the so called memory effect. 49,50 As shown in ESI 3(a), † the basal spacing decreased, and the 2q on the (006) plane increased from 10.36 (NO 3 À ) to 11.32 (Cl À ) because the anion ionic radius of Cl À (1.75 A) was smaller than that of the NO 3 À (1.79 A). The Cl À anion was partially substituted with NO 3 À present in the interlayer, which led to two types of interstratied pristine LDHs. Also, the 2q on the (006) plane of CO 3 2À interlayered LDH (11.64 ) was higher than that of pristine LDHs. 51 In a continuation of the pre-adsorption measurements shown in Fig. 1, post-adsorption C-400 LDHs also included secondary crystalline phases of CaCO 3 (2q ¼ 29.36 ) in all CaFe-LDHs samples even aer particle reconstruction (ESI 3(b) †). This is because CaCO 3 is a basic salt with very low solubility in water. Interestingly, a characteristic peak of  In particular, the CaFe-LDHs prepared aer thermal treatment at C-700 LDHs showed a pure crystalline structure of CaFe-LDHs with phase transformation of CaO into both a Ca(OH) 2 phase and CaCl 2 , CaSO 4 , and CaCO 3 phases depending on the anion species. Thus, the C-700 LDHs may undergo reconstruction via the following chemical reaction in water with specic anions [eqn (9)-(11)]: anions To evaluate the removal performance of CaFe-LDHs in a mixture of three anions (Cl À , SO 4 2À , and CO 3 2À ), competitive adsorption was studied by adding the pristine, C-400, and C-700 LDHs to an aqueous anionic solution. As shown in Fig. 5, 100% removal of  To investigate the differences in anion absorption performance for CaFe-LDHs, the adsorption kinetics of Cl À , SO 4 2À , and CO 3 2À anions on CaFe-LDHs were determined by tting the experimental data to the linearized form of pseudo-rst-order (eqn (12)) and pseudo-second-order (eqn (13)) models: Here, Q e and Q t are the amounts of anions adsorbed (mg g À1 ) per unit of LDHs at equilibrium and at specied time t (min), respectively. k 1 and k 2 are the pseudo-rst-order and pseudo second-order rate constants, respectively. Correlation coefficients (R 2 ) were compared in determining the kinetic model for quantitative evaluation. The pseudo-rst-and pseudo-secondorder plots for sorption of anions by CaFe-LDHs as a function of adsorption time are tted in Fig. 6, and the specic model parameters obtained toward Cl À , SO 4 2À , and CO 3 2À anion concentrations are arranged in Table 1.   The data were very well tted when using a pseudo-secondorder plot, as shown in Table 1. The high correlation coefficient values (R 2 ) obtained for all the LDHs adsorbent samples conrmed that the pseudo-second-order model describing the Cl À , SO 4 2À , and CO 3 2À anions kinetics was very well t compared to the pseudo-rst-order. Based on the kinetic parameters, the pseudo-second order model data suggest that the rate limiting step during Cl À , SO 4 2À , and CO 3 2À anion adsorption onto CaFe-LDHs was primarily chemisorption. Therefore, chemisorption, which typically involves electrostatic attraction and chemical bonding between adsorbate and adsorbent, was the dominant factor for adsorbate anions Cl À , SO 4 2À , and CO 3 2À onto CaFe-LDHs. 52

Equilibrium sorption measurements and modelling
The aim of the adsorption isotherms is to correlate the adsorbate concentration in the solution and the adsorbed extent of metal ions at the adsorbate-to-adsorbent interface. Langmuir and Freundlich isotherm models were important for design of sorption systems (Fig. 7). The adsorption of Cl À , SO 4 2À , and CO 3 2À anions onto CaFe-LDHs (here, C-700 LDHs) was investigated, and the applicability of the isotherms described above was examined. The Langmuir model assumes that interactions occur on a homogenous surface by monolayer adsorption between ions. The Langmuir isotherm represents the equilibrium distribution of sorbate between solid and liquid phases and is oen expressed as (eqn (14)): Here, C e (mg L À1 ) is the concentration of sorbate at equilibrium, Q e (mg g À1 ) is the amount sorbed per mass of sorbent at equilibrium, K L (L mg À1 ) is the equilibrium constant related to the sorption energy between sorbate and sorbent, and Q max (mg) is the limiting amount of sorbate that can be taken up per mass of sorbent. The Langmuir parameters were Q max ¼ 881.83 mg g À1 for Cl À , Q max ¼ 706.21 mg g À1 for SO 4

2À
, and Q max ¼ 879.73 mg g À1 for CO 3
The Freundlich equation can be used to estimate the sorption intensity of sorbent toward sorbate as [eqn (15)]: Here, Q e (mg g À1 ) is the amount of sorbed anion, C e is the equilibrium concentration, and K F and n are constants incorporating all parameters affecting the sorption process, such as sorption capacity and intensity. The isotherm constants in eqn (14) and (15), as well as the correlation coefficients (R 2 ) are given in Table 2. Based on the R 2 values for LDHs samples and the maximum capacities, the equilibrium data were better t using the Langmuir isotherm (with a high correlation coefficient) rather than the Freundlich isotherm. The high coefficient values for the Langmuir isotherm indicate monolayer sorption of Cl À , SO 4 2À , and CO 3 2À anions onto CaFe-LDHs.

Corrosion protection of steel rebars with and without CaFe-LDHs
To study the effect of CaFe-LDHs on the corrosion protection behavior of a steel rebar, potentiodynamic polarization (PDP) curves for samples with and without CaFe-LDHs formed by structural transformation in NaCl, Na 2 SO 4 , and Na 2 CO 3 solution are shown in Fig. 8. The values of electrochemical kinetic parameters such as corrosion potential (E corr ), corrosion current (I corr ), anodic and cathodic Tafel slopes (b a and b c ), and corrosion rate (CR) Here, EW is the equivalent weight of Fe (27 g), K is the corrosion rate constant (3272 mm per year per A per cm), r is the density of Fe (g cm À3 ), and A is the surface area of the electrode. The I corr value of the case with CaFe-LDHs was noticeably lower that of the case without CaFe-LDHs. This means that the electrochemical reaction between the electrolyte and sample was reduced, decreasing the participation of electrons in the reaction. The CR value without CaFe-LDHs was 1876.56 Â 10 À3 mm per year for the steel rebar. This value remarkably decreased only because of the presence of CaFe-LDHs. Compared to the steel rebar without CaFe-LDHs, the CR values gradually decreased to 60.68 Â 10 À3 mm per year for pristine LDHs, and the structural modulations further enhanced the anti-corrosion effect to 2.67 Â 10 À3 mm per year for C-400 LDHs and to 2.67 Â 10 À3 mm per year for C-700 LDHs. In other words, the CR of C-700 LDHs (2.67 Â 10 À3 mm per year) implies that, when the steel rebar is exposed to a mixture of anionic species, the speed of steel corrosion is accelerated to 702.6 times faster than when it is protected by the C-700 LDHs (which present the best anion adsorption properties). As can be seen from the Nyquist plot of rebar without and with CaFe-LDHs in Fig. 9(a). The diameter of the semicircle signicantly increased for protective CaFe-LDHs. It is means that the resistance is increased and corrosion is reduced. The corrosion protection with CaFe-LDHs led to an obvious increase in the diameter of the capacitive loop, 53 as conrmed by Bode plots given in Fig. 9(c). The equivalentcircuit model constants in Fig. 9(d) were applied as these account for corrosion protection. In this equivalent circuit model, the steel rebar is considered to have a porous structure and to show capacitive behavior. [54][55][56][57] To compare the inhibition efficiencies (IE) with and without CaFe-LDHs, impedance parameters are shown in Table 4. R s refers to electrolyte resistance, R f and C f are electrolyte resistance inside pores and lm capacitance, respectively. R ct is the charge transfer resistance of electrochemical processes taking place inside the pores, and C dl is the double-layer capacitance. Here, the C dl value observed without LDHs was 524.80 (mF cm À2 ), and this value generally decreased with use of CaFe-LDHs. Specically, it decreased to 157.63 (mF cm À2 ) for pristine LDHs, to 104.32 (mF cm À2 ) for C-400 LDHs, and to 217.95 (mF cm À2 ) for C-700 LDHs. This trend can be explained according to the Helmholtz model, where C dl is expressed as [eqn (17)]: Here, S is the exposed surface area of the steel specimen, d represents the thickness of the double layer, 3 r denotes the dielectric constant of the electrolyte inside the capacitor, and 3 0 is the vacuum dielectric constant. The steel rebar without CaFe-LDHs was expected to have a higher exposed surface area induced by destructive corrosion, leading to the increase in C dl values compared to that with CaFe-LDHs. Because addition of CaFe-LDHs decreased the concentration of aggressive Cl À , SO 4 2À , and CO 3 2À anions, corrosion of the steel specimen was signicantly reduced by decreasing the exposed surface area. In addition, it is well known that the IE of a steel rebar is inversely proportional to the R ct value. In a solution without CaFe-LDHs, the R ct value was 0.38 Â 10 3 U cm 2 . This value was increased to 5.41 Â 10 3 U cm 2 by pristine LDHs, to 14  Here, R ct, with CaFe-LDHs and R ct, without CaFe-LDHs are the charge transfer resistances for the steel specimens in the presence and absence of the CaFe-LDHs, respectively. According to Table 4, the solution containing the steel specimen and CaFe-LDHs showed inhibition effects. Specically, the highest IE% (99.83%) was obtained when C-700 LDHs was applied.

Synergy of simultaneous adsorption of anions and corrosion prevention in steel rebar by CaFe-LDHs with structural modulation
Rapid removal of harmful anions not long aer their introduction into the aqueous solution or into the matrix of steelreinforced concrete is crucial for coating materials or anticorrosion additives. This property can be related to (1) adsorption rate of anions (i.e., removal percentage of anions (%) per given reaction time [see Section 4.2]) and (2) removal capacity (i.e., maximum amount of ionic species removed (mg) per unit mass of LDHs (g) [see Section 4.5]). The anion removal performance is more distinct when we compare the removal capacity of C-700 LDHs with previously released reports ( Table 5). The Q max for Cl À was 881.83 mg g À1 , which is over 3.4 times that of previous studies. Furthermore, the value for SO 4 2À (706.21 mg g À1 ) was over 5.69 times higher than previous reports. 11,16,[58][59][60][61][62] The value for CO 3 2À was 879.73 mg g À1 , which was not previously reported. In addition, no comparable result has been observed in terms of removal capacity of CO 3 2 .
This synergy was induced by (1) anion capture by reconstruction (2) capture by chemical reaction with metal oxide (see Fig. 10). 1 mole of pristine LDHs with crystallinity of Ca 2 -Fe(OH) 6 (NO 3 )$2H 2 O can exchange 1 mole of monovalent anion including Cl À with NO 3 À , while C-700 LDHs can capture 4 moles of Cl À , 1.5 moles of SO 4 2À , and 1.5 moles of CO 3 2À [also see eqn (6)- (11)]. Here, 1 mole of Cl À ions was captured by intercalation between cationic layers during the refolding process, and another 3 moles of Cl À ions were captured by chemical reaction with CaO, which was generated during thermal treatment at 700 C. This metal oxide moiety, initiated by pristine-LDHs, also captures Cl À , SO 4 2À or CO 3 2À ions by chemical reaction other than capturing by the refolding process, which cannot be realized even aer calcination of typical Mg-Al-LDHs or Ca-Al-LDHs. 63 This enhanced adsorption of ionic species in aqueous solution reduced corrosion in the steel specimen in water because of prompt removal of corrosion sources Cl À and SO 4 2À , as well as another deleterious anion, CO 3 2À , upon introduction to reaction media or building matrix.
Hence, the enhanced removal performance of Cl À , SO 4 2À , and CO 3 2À and anti-corrosion effects by CaFe-LDH-induced crystalline structures utilized in this work suggest that CaFe-LDHs are promising materials for superior corrosion protection in concrete structures.

Conclusions
In this study, we fabricated CaFe-LDHs by structural transformation, demonstrated their simultaneous removal of destructive anions (Cl À , SO 4 2À , and CO 3 2À ) in aqueous solutions and superior anti-corrosion efficiency. In the investigation of adsorption kinetics, pseudo-second-order kinetics and Langmuir isotherm models well t the data, which conrms that the target anions were effectively removed with the CaFe-LDHs predominantly by chemisorption through monolayer sorption. Compared with previous reports, the removal capacity (Q max ) of C-700 LDHs was remarkably enhanced. The Q max of Cl À (881.83 mg g À1 ) was over 3.4 times that of previous work, while that of SO 4 2À (706.21 mg g À1 ) was over 5.69 times that of previous literature. This corrosion inhibition efficiency by C-700 LDHs reached 98.83% compared to that of bare rebar without LDHs. This synergy was induced by (1) anion capture by reconstruction (2) capture by chemical reaction with metal oxide of C-700 LDHs. Our data show that the durability and sustainability of buildings can be enhanced by employing the CaFe-LDHs as a coating material, ller in concrete, or additive in the construction industry.

Conflicts of interest
There are no conicts to declare.