Selective removal of thiosulfate from thiocyanate-containing water by a three-dimensional structured adsorbent: a calcined NiAl-layered double hydroxide film

Abstract

NiAl-layered double hydroxide (NiAl-LDH) platelets were uniformly grown on a porous Ni foam substrate by a facile in situ hydrothermal method. By subsequent calcination, the three-dimensional (3D) structured adsorbent, a calcined NiAl-layered double hydroxide film/Ni foam (NiAl-LDO/NF) was obtained. Batch experiments were carried out to investigate the selective adsorption performance of the obtained material for thiosulfate and thiocyanate anions in water, compared with the powder NiAl-LDO adsorbent. The results show that the NiAl-LDO/NF has highly selective adsorption for S₂O₃²⁻ in the mixture solution with a maximum adsorptive capacity of about 209.4 mg g⁻¹ at room temperature, while the removal capacity for SCN⁻ was only about 15.9 mg g⁻¹. At the same time, the resultant 3D structured adsorbent exhibited higher preferential adsorption and easier separation performance from the solution than the corresponding NiAl-LDO powder. It was found that the adsorbability and selectivity of this material was well maintained after more than 10 regeneration cycles. Therefore, the obtained 3D hierarchical NiAl-LDO/NF can be considered as a potential structured adsorbent in environmental applications for the selective removal of S₂O₃²⁻ from SCN⁻-containing aqueous solution.

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Introduction

As reported, sulfur-contaminated wastewater endangers the lives of creatures and is likely to be responsible for environmental deterioration in some ways. Sulfur-containing pollutants have a severe hazardous influence on aquatic organisms and the ecosystem when sulfur-effluents are released without proper treatment. This is because the presence of sulfuric anions (sulfate, thiosulfate, thiocyanate, etc.) inhibit the function of enzymes in organisms and lower the pH value of the corresponding aqueous solutions.^1,2 Meanwhile, the cycling and effective utilization of sulfur resources are globally significant and have received considerable attention.³ Sulfur-containing anions are always present in wastewater from diverse industrial processes including coking, photo finishing, textile manufacturing, electroplating, dyeing, and mining.^2,4–8 The effective separation of thiocyanate and thiosulfate has potential industrial applications and commercial value, because both thiocyanate and thiosulfate play important roles in the pharmaceutical and agro- industries, respectively.^9–11 It is difficult to separate the two anions by traditional methods, such as the resource wasting, high-cost and low efficiency methods of electrodialysis, ion exchange, wet-oxidation and biodegradation.^1,6,12–16 Adsorption is believed to be a feasible and efficient approach for pollutant removal from contaminated solutions.^17–22 However, the post-processing and regeneration of powder adsorbents are remaining challenges to achieve greater economical accessibility in practical applications.

The layered double hydroxides (LDHs), which can be represented by the general formula [M_1−x²⁺M_x³⁺(OH)₂]^x+(Aⁿ⁻)_x/n·yH₂O, where M²⁺ represents a divalent metal cation, M³⁺ a trivalent metal cation, Aⁿ⁻ an anion, x ranges from 0.15 to 0.33 for pure LDH formation, and y is typically of the order 1–2.^23–29 In the past few decades, LDHs with characteristic lamellar structures, specific anion exchange properties and large surface areas (about 20–120 m² g⁻¹), have attracted great attention in various fields such as catalysis,²³ electrical materials,²⁴ light-emitting materials,²⁵ and adsorption.^26,27 The products obtained from calcination of the LDH precursors at about 400–500 °C,^28,29 have been widely used as adsorbents for the removal of anionic contaminants, such as SCN⁻,^16,19 PO₄³⁻,³⁰ boron,³¹ Cr(VI),³² acid dyes,³³ etc. In previous work, our group has successfully separated S₂O₃²⁻ and SCN⁻ anions (the ratio of [S₂O₃²⁻] and [SCN⁻] was changed from 1 : 1 to 1 : 7)¹¹ by using calcined MgAl-LDH powder as an adsorbent. However, the restraining factors for comprehensive application of LDO powder adsorbents are obvious: the aggregation of the adsorbent particles, laborious separation post-processing, and difficulties in the recycling process. Therefore, some researchers have focused on the fabrication and application of monolithic structured adsorbents with controllable morphology and mechanical stability.³⁴

Porous foam metals as a style of materials which integrate function and construction performance, have attracted much attention in both industrial separation processes and electrochemical applications.^35–37 Their cross-linked network endows the structural materials with high geometric surface areas and low mass transportation resistance.^38,39 LDH films with hierarchical architectures have been successfully prepared on foam metal substrates by in situ crystallization techniques and sol–gel nano-copying processes, and showed fantastic performances in removing organic and inorganic pollutants during water treatment.^40,41 The network substrates consisted of interconnected channels and macro-pores which are beneficial for the dispersion and fixation of LDH flakes, which further improved the mechanical stability as well as avoiding aggregation.

In this work, the as-prepared NiAl-LDO/NF was used as a structured adsorbent for the selective adsorption of S₂O₃²⁻ from SCN⁻-containing wastewater. At the same time, we investigated the adsorption–regeneration of the structured adsorbent during the selective adsorption process. Moreover, the structured NiAl-LDO/NF adsorbent with a high selective adsorption capacity, good reusability and fine solid–liquid separability was proved to be a promising material in water treatment for the selective separation of different anionic pollutants.

Experimental section

Materials

Nickel foams (purity: 99.5%) were purchased from Kunshan Desi Electronic Technology Co., Ltd. The other analytical grade chemicals, including Ni(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, urea, NaSCN and Na₂S₂O₃ were obtained from Beijing Chemical Reagent Company and used without further purification.

Synthesis of structured and powder adsorbents

Nickel foams (30 mm × 40 mm × 1 mm, weight 0.517 g) with 3D networks were used as the substrates for structured adsorbents. They were degreased in ethanol for 10 min, ultrasonically cleaned in 3 mol L⁻¹ hydrochloride acid for 10 min, washed by deionized water 3 times, and then dried at 60 °C in N₂ atmosphere in an oven. After pretreatment, a piece of dried nickel foam was vertically immersed into an 80 mL aqueous solution of Ni(NO₃)₂·6H₂O (0.009 mol), Al(NO₃)₃·9H₂O (0.003 mol) and urea (0.054 mol) that dissolved in the deionized water. This mixture was sealed in a 100 mL Teflon-lined autoclave and maintained at 120 °C for 12 h. The prepared NiAl-LDH film on nickel foam (denoted as NiAl-LDH/NF) was washed three times with deionized water and dried at 60 °C for 10 h. After being calcined at 400 °C for 3 h with a heating rate of 5 °C min⁻¹, the final structured adsorbent (denoted as NiAl-LDO/NF) was obtained by cooling the sample. The powder NiAl-LDH sample was prepared by a coprecipitation method with the same conditions, without nickel foam being added into the reaction vessel.

Batch selective adsorption tests

To investigate the selective adsorption behavior of NiAl-LDO/NF, batch adsorption tests were carried out in 200 mL glass vessels on magnetic stirring apparatus. The initial concentration of S₂O₃²⁻, equal to that of SCN⁻ ranged from 0.05 to 0.5 mmol L⁻¹ in 100 mL of the SCN⁻-containing solution. One piece of NiAl-LDO/NF (with a size of 30 mm × 40 mm × 1 mm) was suspended in the solution and stirred with an agitation speed of 100 rpm at 25 ± 2 °C for 240 min. Four hours was found to be enough to reach adsorption equilibrium. During the adsorption process, ca. 2 mL of the solution was taken out by syringe and filtered through a 0.22 μm organic membrane filter at regular intervals. The concentrations of S₂O₃²⁻ and SCN⁻ in the filtrate were measured by an ion chromatography instrument (Dionex ICS-90A, USA) with an AS 25 column (Dionex, USA) and a conductivity detector. The mobile phase was a sodium hydroxide solution (36 mmol L⁻¹) at a flow rate of 0.9 mL min⁻¹. For comparison with the powder samples and the pure Ni foam substrate, controlled experiments were performed under the same conditions except the adsorbent used. The pure substrate, with the same size as the as-prepared NiAl-LDO/NF, was calcined at 400 °C before being immersed into the solution.

The adsorption capacity (Q_e) and removal percentage (R%) of S₂O₃²⁻ and SCN⁻ can be calculated by the following equations:

where Q_e (mg g⁻¹) is the equilibrium adsorption capacity, C₀ and C_e (mg L⁻¹) denote the initial and equilibrium concentration of the corresponding anions. V (L) and m (g) represent the volume of solution and the mass of adsorbent, respectively. R (%) is the anion removal percentage.

Desorption and regeneration tests

After the adsorption process reached equilibrium, the structured adsorbent was taken out of the sulfur-containing solution and washed with deionized water to remove any un-adsorbed thiosulfate and thiocyanate anions. The desorption of S₂O₃²⁻ from the structured adsorbent was carried out by dipping the samples in a 100 mL 0.02 mol L⁻¹ Na₂CO₃ solution with continuous stirring for 6 h in air at room temperature. The anion exchanged NiAl-LDH/NF was rinsed thoroughly with deionized water 3 times, and calcined at 400 °C for 3 h. Then, the regenerated structure adsorbent was immersed into a mixed S₂O₃²⁻ and SCN⁻ aqueous solution with the same concentrations as the first adsorption process. This procedure of the adsorption–regeneration cycle was repeated 10 times. The concentration of residual S₂O₃²⁻ and SCN⁻ after each adsorption cycle was detected by the same ion chromatography method.

Characterization

The powder X-ray diffraction (XRD) patterns of the samples were recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ = 0.15418 nm, 40 kV, 30 mA) in the 2θ range of 3–70° with a scanning rate of 10° min⁻¹. Fourier transform infrared (FTIR) spectra were collected using the KBr pellet technique (sample : KBr = 1 : 100, mg mg⁻¹) on a Bruker Vector 22 Fourier transform spectrometer in the range of 4000–400 cm⁻¹ with a resolution of 2 cm⁻¹. Scanning electron microscopy (SEM) images were obtained using Hitachi S4700 apparatus with an applied voltage of 20 kV. To avoid a charging effect, the surfaces of the samples were covered with a thin platinum layer by spray-platinum treatment. The elemental analysis for the metal ions of the adsorbents was performed on an Oxford Link-Isis 300 Energy Dispersive X-ray spectrometer (EDX). Inductively coupled plasma atomic emission spectroscopy (ICPS-7500) was also used to indirectly measure the loaded capacity of LDH or LDO on the substrate. Solutions were prepared by dissolving a small amount of powder samples that were scraped from the NiAl-LDH/NF or NiAl-LDO/NF in dilute HNO₃ solution. The low-temperature N₂ adsorption–desorption experiments were carried out using a Quantachrome Autosorb-1 system. Samples were outgassed at 373 K for 8 h. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) method based on the N₂ adsorption isotherms.

Results and discussion

Structures and morphologies of the as-prepared NiAl-LDO/NF and its precursor

The XRD patterns of the NiAl-LDH/NF and NiAl-LDO/NF samples are shown in Fig. 1. Two sharp diffraction peaks of Ni at 44.9° and 52.4° (JCPDS card no. 04-0850), superimposed on the broad feature due to the substrate (Fig. 1a), are observed in the case of the NiAl-LDH/NF sample (Fig. 1b). Comparison of the XRD patterns of powder scraped from the NiAl-LDH/NF sample (Fig. 1c), JCPDS card no. 15-0087, and the literature^42,43 shows that the low angle peaks correspond to the basal reflection of CO₃²⁻-containing NiAl-LDH (d₀₀₃ = 0.78 nm). The other peaks can be indexed as the (006), (012), (015), (018), (110) and (113) reflections of an LDH phase. For NiAl-LDO/NF (Fig. 1d), there are no peak of Ni was superimposed on the XRD pattern of NiAl-LDO powder scraped from NiAl-LDO/NF sample (Fig. 1d). After being calcined at 400 °C, the two LDO samples exhibited the characteristic peaks of a poorly crystalline NiO structure (Fig. 1d and e) in accordance with reports in the literature.⁴⁴ Compared with that of NiAl-LDH/NF (Fig. 1b), NiAl-LDO/NF (Fig. 1e) displays the disappearance of characteristic (00l) reflections for the LDH phase and the presence of typical reflections of (111), (200), (220) planes for the NiO phase (JCPDS no. 65-2901)⁴⁵ which indicates the LDH phase had almost disappeared after 3 h of calcination at 400 °C in air. The FTIR spectra of NiAl-LDH powder and NiAl-LDO powder scraped from the nickel foam substrate are shown in Fig. 2. In Fig. 2a, the broad band between 3700–3000 cm⁻¹ is assigned to the stretching vibrations of O–H groups in the brucite-like layers of the LDH phase. The adsorption band at ca. 1357 cm⁻¹ in the spectrum of the NiAl-LDH powder is attributed to the characteristic stretching vibration of CO₃²⁻ groups, which is consistent with the XRD analysis. The NiAl-LDO sample obtained after calcination (Fig. 2b) shows the disappearance of the carbonate absorption band at 1357 cm⁻¹, which indicates that the interlayer carbonate was delaminated by the calcination process.

Fig. 1 XRD patterns of (a) the Ni foam substrate, (b) NiAl-LDH/NF, (c) NiAl-LDH powder prepared by coprecipitation, (d) NiAl-LDO powder scraped from NiAl-LDO/NF and (e) NiAl-LDO/NF.

Fig. 2 FTIR spectra of (a) NiAl-LDH powder and (b) NiAl-LDO powder scraped from the nickel foam substrate.

The growth of NiAl-LDH platelets on the 3D nickel foam substrate was based on the precipitation from homogeneous solutions using the hydrolysis of urea at 120 °C, which leads to an increase in the concentration of CO₃²⁻ and pH (the latter associated with the formation of ammonia), resulting in crystallized NiAl-LDH platelets. Nickel foam has a 3D network structure (inset of Fig. 3a), which greatly increases the surface area of the substrate. Fig. 3a and b display the morphology of the NiAl-LDH film on the nickel foam substrate. It can be seen that the NiAl-LDH platelets have grown on the nickel foam and that, in general, the (00l) planes of the NiAl-LDH platelets were almost perpendicular to the surface of the Ni foam with each platelet having a hexagonal shape (Fig. 3b). However, the NiAl-LDH particles are chemically immobilized throughout the Ni foam substrate. Through the EDX spectrum of the NiAl-LDH/NF surface (inset of Fig. 3b), it can be calculated that the Ni/Al molar ratio was about 2.9, which approximates to the original addition molar ratio of Ni/Al (about 3.0). NiAl-LDO/NF was obtained after the process of thermal decomposition of NiAl-LDH/NF at 400 °C to remove the interlayer CO₃²⁻. As shown in Fig. 3c and d, the NiAl-LDO film layer retained the staggered sheet morphology of NiAl-LDH and an open-pore structure. This is mainly a response to the dehydration, dehydroxylation and CO₃²⁻ delamination carried out in the process of thermal decomposition. It was found that the NiAl-LDO platelets were not detached from the Ni foam after calcination. Fig. 3c is the cross section image of the NiAl-LDO/NF. It was also found that the NiAl-LDO film was attached to the substrate tightly, there is only one platelet layer of NiAl-LDO on the surface and the thickness of the NiAl-LDO layer was about 300 nm. We found that the whole weight of the obtained NiAl-LDH/NF (ca. 0.467 g, 30 mm × 40 mm × 1 mm) was slightly lighter than that of the pure nickel foam (ca. 0.517 g), which could be attributed to a little Ni dissolved in the alkaline ammonium solution. Therefore, the loading of NiAl-LDH on the Ni foam cannot be directly calculated by weighing the difference of obtained NiAl-LDH/NF and the initial substrate. The weight percentage of NiAl-LDH grown on the Ni foam was calculated to be approximately 3.34 wt% by the results of ICP measurement for the amounts of the Ni and Al elements (the Ni element resourced from the substrate has been considered). The weight loss was about 0.33 g per gram of NiAl-LDH when NiAl-LDH was calcined to NiAl-LDO. Therefore, the weight percentage of the active component NiAl-LDO on the structured adsorbent was about 2.44 wt%. According to the low-temperature N₂ adsorption–desorption experiments, the specific areas of pure Ni foam, NiAl-LDO/NF and NiAl-LDO powder are 21.8, 27.1 and 92.9 m² g⁻¹, respectively. By deducting the Ni foam, the specific area of NiAl-LDO on NiAl-LDO/NF was ca. 121.1 m² g⁻¹. It was found that the specific area of NiAl-LDO on NiAl-LDO/NF was obviously higher than that of NiAl-LDO powder, which is beneficial to improve the adsorption performance of NiAl-LDO.

Fig. 3 SEM images of the as-prepared NiAl-LDH/NF (a) at low magnification and (b) high magnification, (c) side view and (d) top view of NiAl-LDO/NF at high magnification (insets: (a) pristine Ni foam substrate and (b) EDX of NiAl-LDH/NF surface).

Selective adsorption behavior of NiAl-LDO/NF

The selective adsorption ability of the structured adsorbent NiAl-LDO/NF from the mixed aqueous solution was studied with adsorption time and the initial concentrations of the anions. The adsorption curves are shown in Fig. 4 and 5, both the results show that NiAl-LDO/NF is able to preferentially remove S₂O₃²⁻ from the SCN⁻-containing solution. From Fig. 4, we can see that when one piece of NiAl-LDO/NF was set in the solution, the removal percentage of S₂O₃²⁻ by NiAl-LDO/NF was always much higher than that of SCN⁻. When the initial concentrations of SCN⁻ and S₂O₃²⁻ were 0.2 mmol L⁻¹ respectively, the removal percentage ratio was [S₂O₃²⁻] : [SCN⁻] = 65.4% : 2.1% after adsorption for 240 min. The high preferential adsorption of NiAl-LDO/NF for S₂O₃²⁻ over SCN⁻ can be attributed to the higher charge density of S₂O₃²⁻ anions, leading to stronger affinity between these ions and the positive layers of the NiAl-LDH host.⁴⁶ After 180 min of selective adsorption by the structured adsorbent, the residual SCN⁻ in the mixture had a concentration 2.8 times that of S₂O₃²⁻. Additionally, the residual concentration ratio of [SCN⁻] : [S₂O₃²⁻] can be substantially increased by adding more NiAl-LDO/NF pieces, which would be beneficial in obtaining high quality thiocyanate by subsequent crystallization.

Fig. 4 Effect of contact time on the removal percentage of S₂O₃²⁻ and SCN⁻ by NiAl-LDO/NF ([SCN⁻] = [S₂O₃²⁻] = 0.2 mmol L⁻¹).

Fig. 5 Effect of initial concentration on the adsorption of (a) S₂O₃²⁻ and (b) SCN⁻ by NiAl-LDO/NF ([SCN⁻] = [S₂O₃²⁻], adsorption time was 240 min).

The initial concentrations of anions in the SCN⁻-containing solution also influenced the selective adsorption performance, as is shown in Fig. 5 for a piece of NiAl-LDO/NF and varying (but equimolar) concentrations of S₂O₃²⁻ and SCN⁻. At a low concentration, [S₂O₃²⁻] = [SCN⁻] = 0.05 mmol L⁻¹, the removal of S₂O₃²⁻ reached about 79.9% despite the fact that the adsorption capacity (Q_e) was only about 47.3 mg g⁻¹ (Fig. 5a), the removal of SCN⁻ was only about 5.1% and the Q_e was ca. 1.2 mg g⁻¹ (Fig. 5b), which resulted in a [SCN⁻] : [S₂O₃²⁻] ratio in the remaining solution of about 4.7 : 1. When the concentrations of SCN⁻ and S₂O₃²⁻ in the mixed solution were increased, both the removal percentage of SCN⁻ and S₂O₃²⁻ decreased. When the concentrations of the anions were increased to 0.6 mmol L⁻¹, the removal percentage of S₂O₃²⁻ fell to only about 15.8% with a very high Q_e (ca. 209.4 mg g⁻¹), but the removal percentage of SCN⁻ fell to only about 1.6% and the Q_e was about 15.9 mg g⁻¹. However, it was proved that the resulting structured adsorbent NiAl-LDO/NF shows a good selective adsorption of S₂O₃²⁻ in SCN⁻-containing solutions.

The kinetics of S₂O₃²⁻ removal at various initial concentrations, as an important characteristic to define the adsorption efficiency, was studied to understand the adsorption behavior of the as-prepared structured adsorbent NiAl-LDO/NF (Fig. 6). The kinetics of SCN⁻ removal cannot be given because of the influence of the selective S₂O₃²⁻ adsorption. Among the kinetic models tested, a pseudo-second-order kinetic model (eqn (3)) gave a satisfactory description of the S₂O₃²⁻ removal by NiAl-LDO/NF with high correlation coefficients (R² > 0.99).

where Q_e and Q_t are the capacity of adsorbate (mg g⁻¹) on the NiAl-LDO/NF at equilibrium and time t (min), respectively; k₂ (g mg⁻¹ min⁻¹) is the pseudo-second-order kinetic rate constant. According to the slope and intercept of the lines presented in Fig. 6, the kinetic parameters were calculated and listed in Table 1. The experimental amount (Q_e,exp) of S₂O₃²⁻ adsorbed by NiAl-LDO/NF matches well with the calculated one (Q_e,cal) on the basis of the pseudo-second-order kinetic model. A decrease in the rate constant k₂ values can be seen in the Table 1, with the increase of the initial concentration. The pseudo-second order equation is based on the uptake capacity of the solid phase and is in agreement with a chemisorption mechanism being the rate determining step. Another advantage of the pseudo-second order model is that it predicted the behavior over the whole range of the adsorption process.²

Fig. 6 Pseudo-second-order kinetic plots for adsorption of S₂O₃²⁻ by NiAl-LDO/NF.

Table 1 Kinetic parameters for the adsorption of S₂O₃²⁻ by NiAl-LDO/NF based on the pseudo-second-order model

C0 (mmol L^−1)	Qe,exp (mg g^−1)	k2 (g mg^−1 min^−1)	Qe,cal (mg g^−1)	R^2
0.05	47.3	0.003772	48.5	0.9994
0.10	78.1	0.003005	78.6	0.9997
0.20	143.4	0.001026	144.7	0.9991
0.25	165.4	0.000590	170.4	0.9958
0.60	209.4	0.000880	207.9	0.9984

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The equilibria of thiosulfate separated between the liquid and solid phase were analyzed by two general-purpose adsorption isotherm models: the Langmuir (eqn (4)) and Freundlich models (eqn (5)). The parameters are listed in Table 2. As is shown, the equilibrium isotherm typically follows the Langmuir model, judging from the correlation coefficients. The Langmuir isotherm is applicable to a homogeneous surface where all potential adsorption sites have equal affinity for the guest.⁴⁸

Table 2 Comparison of the Langmuir and Freundlich model parameters

Adsorbent	Langmuir			Freundlich
	Qmax (mg g^−1)	KL (L mg^−1)	R^2	n	KF (L g^−1)	R^2
NiAl-LDO/NF	229.4	0.1895	0.9993	2.47	49.934	0.8865

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For comparison purposes, the selective adsorption performances of NiAl-LDO/NF, NiAl-LDO powder and pure Ni foam in the SCN⁻ and S₂O₃²⁻ mixed solution are shown in Fig. 7. The dosage of NiAl-LDO powder was 0.011 g, accounting for about 2.44 wt% of a piece of NiAl-LDO/NF (about 0.451 g) used in the controlled experiment. The size of pure Ni foam was the same as NiAl-LDO/NF (30 mm × 40 mm × 1 mm). From Fig. 7, it is revealed that there was a little removal of S₂O₃²⁻ and SCN⁻ by the pristine Ni foam. With an adsorption duration of 240 min for the structured NiAl-LDO/NF adsorbent, the removal percentage of S₂O₃²⁻ reached ca. 80.1%, whereas the removal of SCN⁻ was 5.1%. However, for the NiAl-LDO powder adsorbent, this was about 38.9% for S₂O₃²⁻ and 9.5% for SCN⁻. Obviously, the removal of S₂O₃²⁻ by structured NiAl-LDO/NF is higher than that of NiAl-LDO powder and the removal of SCN⁻ by NiAl-LDO/NF is lower than that of NiAl-LDO powder, suggesting that the selectivity for S₂O₃²⁻ with NiAl-LDO powder was not as good as with structured NiAl-LDO/NF, though both of them could realize a successful separation of these mixed pollutants. The results unambiguously indicated that the network Ni foam substrate consisted of interconnected channels and macro-pores with a high geometric surface area which improved the dispersion as well as the avoidance of aggregation of NiAl-LDO flakes. At the same time, there was a lower mass transportation resistance for the 3D structured NiAl-LDO/NF during the adsorption process than that found for the NiAl-LDO powder. We also investigated the influence of the initial pH of the mixed solution on the selective adsorption. As is shown in Fig. 8, in the pH range from 5 to 11, there is no obvious change in the selective adsorption of S₂O₃²⁻ or SCN⁻ on the structured NiAl-LDO/NF.

Fig. 7 Adsorption curves of S₂O₃²⁻ and SCN⁻ vs. time using NiAl-LDO/NF, NiAl-LDO powder and pure Ni foam as absorbents in the mixed solution of S₂O₃²⁻ and SCN⁻ ([SCN⁻] = [S₂O₃²⁻] = 0.05 mmol L⁻¹).

Fig. 8 The effect of initial pH on the selective removal of S₂O₃²⁻ and SCN⁻ by NiAl-LDO/NF.

The great advantage of using such a structured adsorbent was the avoidance of adsorbent–water separation which is generally encountered with liquid-phase adsorption and the high adsorbate–active phase contact surface due to the structure and size of adsorbent. In Fig. 9 the adsorption processes using NiAl-LDO/NF and NiAl-LDO powder as adsorbents can be visually observed. These results demonstrated that the structured NiAl-LDO/NF could be easily separated from the solution (Fig. 9a and b) compared with the powder adsorbent (Fig. 9c and d). In addition, during the adsorption process with violent stirring, no NiAl-LDO particles were found exfoliated from NiAl-LDO/NF to the solution, suggesting that NiAl-LDO/NF is a mechanically stable structured adsorbent with a favorable selective adsorbability for S₂O₃²⁻ in an SCN⁻-containing solution.

Fig. 9 Photographs of a S₂O₃²⁻ and SCN⁻ mixed solution in the presence of NiAl-LDO/NF (a and b) and NiAl-LDO powder (c and d) adsorbents during the adsorption process and at the end of the experiments.

Regeneration and recycling of NiAl-LDO/NF

The development of an efficient and low-cost regeneration method for adsorbents is economically important.⁴⁷ By anion-exchange, S₂O₃²⁻ desorption from the used NiAl-LDO/NF was carried out in a Na₂CO₃ solution. It is easy for CO₃²⁻ to replace the adsorbed S₂O₃²⁻ owing to the higher affinity of CO₃²⁻ with the layers of LDH materials than that of S₂O₃²⁻.⁴⁸ By subsequently calcining, the structured NiAl-LDO/NF was reconverted to a selective adsorption material for reuse. Consecutive adsorption–regeneration cycles for the SCN⁻ and S₂O₃²⁻ mixed solution with the NiAl-LDO/NF adsorbent were repeated 10 times under the same experimental conditions. Fig. 10 shows that at the first regeneration cycle of the structured adsorbent, the S₂O₃²⁻ removal percentage was more than 80% and the adsorption of SCN⁻ was hardly observed. The removal percentage of S₂O₃²⁻ by NiAl-LDO/NF diminished progressively with each cycle of regeneration and that of the SCN⁻ removal remained lower than 6.0%. The outstanding adsorption–regeneration performance of the NiAl-LDO/NF is related to the 3D hierarchical structure which facilitates a high dispersion of NiAl-LDO platelets. Moreover, on comparison of the material after the first adsorption–regeneration (Fig. 11a) and after 10 consecutive cycles of adsorption–regeneration (Fig. 11b), some tiny changes in its morphology were observed: the surface of the latter increased in roughness and some cracks appeared because of the calcination. These changes may be caused by thermal expansion and contraction of the Ni foam during anion-exchange and calcination processes, which resulted in a slight loss of NiAl-LDO particles from the Ni foam substrate and reduced the removal percentage of S₂O₃²⁻. However, by the last cycle, the structured adsorbent still had an S₂O₃²⁻ removal percentage of more than 51% and maintained a very high selectivity.

Fig. 10 Recycling experiment with 10 consecutive cycles of adsorption–regeneration by NiAl-LDO/NF ([SCN⁻] = [S₂O₃²⁻] = 0.05 mmol L⁻¹).

Fig. 11 SEM images of NiAl-LDO/NF after the first adsorption–regeneration (a) and 10 consecutive cycles of adsorption–regeneration (b).

Conclusions

A 3D hierarchical NiAl-LDH/NF has been prepared by an in situ hydrothermal growth method using urea as a precipitant. By subsequent calcination, the as-prepared NiAl-LDO/NF was a good structured adsorbent for the preferential removal of S₂O₃²⁻ from a mixed aqueous solution of S₂O₃²⁻ and SCN⁻. S₂O₃²⁻ was adsorbed by the structured material, and SCN⁻ was left in the solution. The adsorption capacity of S₂O₃²⁻ can reach about 209.4 mg g⁻¹ while it reaches only about 15.9 mg g⁻¹ for SCN⁻. Furthermore, the structured NiAl-LDO/NF can be easily separated from the mixed solution with high mechanical stability during the selective adsorption process. An adsorption–regeneration cycle study demonstrated that the structured adsorbent can be regenerated and reused at least 10 times. It can be concluded that the as-prepared 3D hierarchical NiAl-LDO/NF is an efficient and potential structured adsorbent with highly selective adsorbability and reusability in the recycling of industrial S₂O₃²⁻ and SCN⁻ mixed effluent.

Acknowledgements

This work was supported by the 973 Program (no. 2014CB932104), National Natural Science Foundation of China, Program for New Century Excellent Talents in Universities, Fundamental Research Funds for the Central Universities (ZZ1501 and YS1406) and Program for Changjiang Scholars, Innovative Research Team in University (no. IRT1205) and Beijing Engineering Center for Hierarchical Catalysts of P. R. China.

Notes and references

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