Adsorption Performance on As(III) from Aqueous Solution Using the Complex Nickel–Aluminum Hydroxides

Abstract

In this study, complex nickel–aluminum hydroxides were prepared at different molar ratios (NA12, NA11, NA21, NA31, and NA41), and their adsorption capability on arsenic ions (As(III)) from aqueous media was assessed. The physicochemical properties such as morphology, X-ray diffraction pattern, specific surface area, numbers of hydroxyl groups, and surface pH were investigated. In addition, the effect of contact time, temperature, and pH on the adsorption capability on As(III) was also evaluated. NA41 exerted the highest adsorption capability on As(III) comparable to other prepared adsorbents. However, the specific surface area and numbers of hydroxyl groups did not significantly affect the adsorption capability on As(III). The equilibrium adsorption of As(III) using NA41 was achieved within 24 h, and the obtained results corresponded to a pseudo-second-order model with correlation coefficient value of 0.980. Additionally, the adsorption isotherms were well described by both the Langmuir and Freundlich equations. The optimal pH condition for removal of As(III) using NA41 was found to be approximately 6–8. Finally, the adsorption mechanism of As(III) was assessed by analyzing the binding energy and elemental distribution, which indicated that the electrostatic interaction and ion exchange influenced the adsorption of As(III) under experimental conditions. These results demonstrated the potential candidate of NA41 as an effective adsorbent on As(III) removal from aqueous media.

Introduction

Seventeen goals of the sustainable development were adopted by the United Nations in 2015 as part of the 2030 Agenda for Sustainable Development, which set out a 15-year plan to achieve these goals.¹⁾ Among these, Goal 6 (clean water and sanitation) and Goal 14 (life below water) are aimed at a global improvement in water quality. This is important for mitigating water pollution, including that from heavy metals such as arsenic (As), which is the twentieth most abundant element in the earth’s crust.^2,3) Arsenic occurs through a combination of natural processes (such as volcanic activities and weathering reactions) as well as through a range of anthropogenic activities (such as agricultural and industrial activities).^4,5) Arsenite (As(III)) and arsenate (As(V)) are common in natural water bodies,⁶⁾ and As(III) is more toxic, soluble, and mobile than As(V).^7,8) The toxicity of arsenic causes skin, lung, liver, kidney, and bladder cancer.⁹⁾ The relationships between arsenic exposure and asthma or nervous system functions have been evaluated in previous studies.^10,11) Therefore, arsenic is classified under group A (human carcinogen) and category 1 (carcinogenic to humans) by the U.S. Environmental Protection Agency¹²⁾ and the International Association for Research on Cancer,¹³⁾ respectively. Additionally, the WHO has adopted 10 µg/L as the maximum permissible level of arsenic in drinking water.¹⁴⁾ Thus, arsenic pollution is of critical concern globally and need to develop the novel technologies to reduce arsenic level from aqueous media.

Numerous treatment techniques have been studied and developed for removing arsenic from aqueous media, such as adsorption, ion-exchange, precipitation, coagulation (or flocculation), reverse osmosis, membrane technology, as well as biological processes.²⁾ Among them, adsorption methods using organic and inorganic adsorbents have gained attention due to simple operation, cost effectiveness, and adsorbent regeneration.¹⁵⁾ Our previous studies found that complex metal hydroxides (inorganic materials based on aluminum, iron, magnesium, and zirconium) were useful in removing heavy metals from aqueous media.^16–21) Additionally, the nickel–aluminum complex hydroxide showed superior adsorption performance on phosphate ions from an aqueous solution system.^22,23) However, there are lack of the reports regarding arsenic adsorption from aqueous solution systems. Arsenic adopts chemical forms similar to those of phosphorus (P). Previous studies have been reported that most adsorbents were effective against As(V), but failed in a form of As(III).^24–27) In drinking water treatment, As(III) transformed to As(V) by prechlorination, and subsequently coagulation process using aluminum-based coagulant. This treatment needs to two steps for removal of As(III) from aqueous media. Finally, secondly pollutants are generated after treatment. Therefore, it is important to removal of As(III) directly from aqueous media. There is urgent demand for an effective adsorbents with high adsorption capability on As(III) from aqueous media. The aim of this study was to evaluate the effect of various factors including contact time, temperature, pH, and adsorption mechanism on the adsorption capability on As(III) from aqueous media using a novel adsorbents prepared by different molar ratios of nickel and aluminum.

Experimental

Materials

A standard solution of As(III) (As₂O₃ and NaOH in water at pH 5.0 with HCl) was purchased from FUJIFILM Wako Pure Chemical Corporation, Japan. The different molar ratios of nickel–aluminum complex hydroxides were prepared and obtained from Kansai Catalyst Co., Ltd., Japan. The molar ratios of Ni²⁺ to Al³⁺ were 4.0, 3.0, 2.0, 1.0, and 0.5, which were denoted as NA41, NA31, NA21, NA11, and NA12, respectively. Additionally, sulfate ions (SO₄²⁻) are included in the interlayer of NA adsorbents in this study. These sulfate ions in the interlayer of NA adsorbents could be easily exchanged with target anions in the aqueous media (Our previous study confirmed that NA adsorbents had the ability to exchange target anions with sulfate ions).²²⁾ The physicochemical properties of these materials have been reported in a previous publication.²²⁾ Briefly, the morphologies and crystallinities of adsorbents were measured using a scanning electron microscope, SU1510 (Hitachi High-Technologies Co., Japan) and a powder X-ray diffractometer (XRD), MiniFlex II (Rigaku, Japan), respectively. The specific surface area was analyzed by a specific surface analyzer, NOVA4200e (Quantachrome Instruments Japan G.K., Japan). The numbers of surface hydroxyl groups were measured by a previously reported method (fluoride ion adsorption method).²⁸⁾ Finally, the surface pH of the prepared adsorbent was measured using the followed method: 0.01 g of the prepared adsorbent was mixed with 50 mL of distilled water (pH 7.0), and the solution was shaken at 25 °C for 2 h at 100 rpm. Subsequently, the suspension was filtered through a 0.45 µm membrane filter (ADVANTEC Co., Ltd., Japan). The pH in solution was measured using a digital pH meter, F-73 (Horiba, Ltd., Japan). Additionally, Ni(OH)₂, Al(OH)₃, HCl, and NaOH (special reagent grade) were also obtained from FUJIFILM Wako Pure Chemical Corporation.

Adsorption Capability of As(III)

A 0.05 g of each adsorbent, namely NA41, NA31, NA21, NA11, NA12, Ni(OH)₂, and Al(OH)₃ was mixed with 50 mL solution of As(III) in the concentrations of 10 or 50 mg/L (sample solution was prepared using the purified water, and the pH of the sample solution was approximately 4.0). The reaction solution was shaken at 25 °C at 100 rpm for 24 h, and the resulting solution was filtered through a 0.45 µm membrane filter (ADVANTEC Co., Ltd., Japan). The concentration of As(III) was measured using inductively coupled plasma optimal emission spectrometry, iCAP 7600 Duo (Thermo Fisher Scientific Inc., Japan). The quantity of As(III) adsorbed was calculated via the difference between its levels before and after adsorption.

Effect of Contact Time, Temperature, and pH on the Adsorption of As(III)

First, in order to evaluate the effect of contact time, 0.05 g of NA41 was mixed with a 50 mL solution of 50 mg/L As(III). The reaction solutions were shaken at 25 °C for 0.5, 1, 3, 5, 20, 24, 30, 40, and 48 h at 100 rpm. Second, in order to evaluate the effect of temperature, 0.05 g of NA41 was mixed with a 50 mL solution of 10, 20, 30, 40, 50, and 100 mg/L As(III). The reaction solutions were shaken at 10, 25, and 45 °C at 100 rpm for 24 h. Third, in order to evaluate the effect of pH, 0.05 g of NA41 was mixed with a 50 mL solution of 50 mg/L As(III). Hydrochloric acid or sodium hydroxide were used to adjusted initial solution pH to 2, 4, 6, 8, and 10. The reaction solutions were shaken at 25 °C at 100 rpm for 24 h. The quantity of As(III) adsorbed was calculated using the same method as mentioned previously. The results were represented as mean ± standard deviation (S.D.) < 0.5.

Finally, in order to evaluate the adsorption mechanism, the elemental distribution and the binding energy before and after adsorption, and ion exchanges with sulfate ions in the interlayer of NA41 were assessed in this study. An electron probe microanalyzer, JXA-8530F (JEOL Ltd., Japan), with an accelerating voltage of 15.0 kV and a beam diameter of 5 µm was used for elemental analysis. X-ray photoelectron spectroscopy with an AXIS-NOVA system (Shimadzu Co., Ltd., Japan) using a conventional Al-Kα source, and electrical current and voltage of 10 mA and 15 kV, respectively was used to monitor the binding energy of arsenic and sulfur on the adsorbent surface.

Ion chromatograph, DIONEX ICS-900 (Thermo Fisher Scientific Inc., Japan) was used to measure the concentration of sulfate ions. The regenerant and mobile phase were 12.5 mmol/L sulfuric acid solution and 2.7 mmol/L sodium carbonate solution +0.3 mmol/L sodium hydrogen carbonate solution, respectively. The flow rate was set at 1.0 mL/min and the injection volume was set at 10 µL under ambient conditions. Anion mixed standard solution IV was purchased from Kanto Chemical Co., Inc., Japan. In our experiment, the elution time of sulfate ion was 8.4 min.

Results and Discussion

Physicochemical Properties of the NA Series

We have reported the physicochemical properties of NA12, NA11, NA21, NA31, and NA41 in previous publications.^20,22) An irregular crystal system (NA41 and NA31) was observed with increasing concentrations of aluminum, indicating that the constituent metals such as nickel and aluminum affected the crystal system under our experimental conditions. The XRD patterns of NA11 and NA12 showed an amorphous structure, whereas NA41, NA31, and NA21 had a highly crystalline structure. The specific surface area of NA12, NA11, NA21, NA31, and NA41 was 26.4, 22.8, 15.6, 11.7, and 14.6 m²/g, respectively. In addition, the amount of hydroxyl groups was 1.62, 1.92, 1.05, 0.82, and 0.80 mmol/g, respectively. Surface pH did not vary between each adsorbent (surface pH: 6.50–7.98).

Amount of As(III) Adsorbed by NA Series

The amount of As(III) adsorbed is shown in Fig. 1. The quantity of As(III) adsorbed at the initial concentration of 50 mg/L As(III) was higher comparable to 10 mg/L As(III). In addition, the quantity adsorbed using NA41 or NA31 was greater than that using other adsorbents including Ni(OH)₂ and Al(OH)₃. These results indicate that complex metal hydroxides were useful in removing As(III) from aqueous media. The correlation coefficients between the quantity of As(III) adsorbed and physicochemical properties such as specific surface area and number of hydroxyl groups were −0.566 and −0.640, respectively. Thus, these factors did not strongly affect to the adsorption capacity on As(III) (especially for initial concentration was 50 mg/L As(III)). From the preliminary results, NA41 was chosen to evaluate the adsorption capability on As(III) in the following experiments.

Fig. 1. Amount of As(III) Adsorbed onto Adsorbents, ■/●: 10 mg/L, □/〇: 50 mg/L, Initial Concentration: 10 and 50 mg/L, Sample Volume: 50 mL, Adsorbent: 0.05 g, Temperature: 25 °C, Contact Time: 24 h, 100 rpm

Effect of Contact Time on the Removal of As(III) Using NA41

Figure 2 exhibits the effect of contact time on the removal of As(III) using NA41. The equilibrium adsorption of As(III) was achieved within 24 h. Trends similarly were reported by previous studies using an iron-zirconium binary oxide adsorbent⁸⁾ and Fe-exchanged natural zeolite.²⁹⁾ If the adsorption mechanism was solely controlled by electrostatic processes, it usually occurred at a rapid rate (on the order of seconds).³⁰⁾ In this study, the adsorption capacity on As(III) using NA41 was on the order of hours, indicating that the adsorption of As(III) using NA41 was not solely controlled by electrostatic processes under our experimental condition.

Fig. 2. Effect of Contact Time on the Adsorption of As(III) onto NA41, Initial Concentration: 50 mg/L, Sample Volume: 50 mL, Adsorbent: 0.05 g, Temperature: 25 °C, Contact Time: 0.5, 1, 3, 5, 20, 24, 30, 40, and 48 h, 100 rpm

The adsorption kinetics was investigated using the pseudo-first-order Eq. (1) and pseudo-second-order Eq. (2) models.^31,32)

  

(1)

  

(2)

where q_e and q_t represent the amount of As(III) adsorbed at equilibrium and at time t (mg/g), respectively, and k₁ and k₂ are the pseudo-first-order rate constant (1/h) and the pseudo-second-order rate constant (g/mg/h), respectively.

The parameters of kinetic models and the correlation coefficients are shown in Table 1. The adsorption kinetics of NA41 was strongly described by the pseudo-second-order model (correlation coefficient value of 0.980). The pseudo-second-order model bases on the assumption that the rate-limiting step may be chemical sorption or chemisorption involving valency forces through sharing or exchange of electrons between sorbent and sorbate. Therefore, the adsorption of As(III) using NA41 was related to chemical sorption and/or chemisorption under our experimental condition.³²⁾ In addition, a previous research reported that the low value of k₂ indicated that the As(III) adsorption occurred rapidly³³⁾ resulting in NA41 (the k₂ value of 6.0 × 10⁻⁴) was more efficient in removing As(III) from aqueous media comparable to other tested adsorbents (the k₂ values of from 2.6 to 1.3 × 10⁻³).^8,34,35) Finally, NA41 will be granulated with binder and it fills in the column (flow method) for the application of NA41 in the field application. Therefore, further studies are necessary for elucidate the application of their adsorbents in detail.

Table 1. Fitting Results of Kinetic Data Using Pseudo-First-Order Model and Pseudo-Second-Order Model

Adsorbent	qe,exp (mg/g)	Pseudo-first order model			Pseudo-second order model
		k1 (1/h)	qe,cal (mg/g)	r	k2 (g/mg/h)	qe,cal (mg/g)	r
NA41	19.9	0.09	16.2	0.860	6.0 × 10−4	24.9	0.980

The Quantity of As(III) Adsorbed Using NA41 at Different Temperatures

The different temperatures adsorption isotherm of As(III) using NA41 are shown in Fig. 3. The results demonstrated that the quantity of As(III) adsorbed increased with increasing temperatures (10 °C <25 °C <45 °C), indicating that adsorption of As(III) using NA41 was endothermic. To assess the relationship between adsorbate and adsorbent in the liquid medium at equilibrium, both the Langmuir (3) and Freundlich (4) equations were applied.^36,37)

  

(3)

  

(4)

Fig. 3. Adsorption Isotherms of As(III) onto NA41 ◆: 10 °C, ●: 25 °C, ▲: 45 °C, Initial Concentration: 10, 20, 30, 40, 50, and 100 mg/L, Sample Volume: 50 mL, Adsorbent: 0.05 g, Temperature: 10, 25, 45 °C, Contact Time: 24 h, 100 rpm

where q_e is the quantity of As(III) adsorbed at equilibrium (mg/g) and C_e is the equilibrium concentration of As(III) (mg/L). W_s and a are fitting parameters representing the maximum adsorption capacity of As(III) and the adsorption constant, respectively. k and n are the Freundlich constants related to the adsorption of the adsorbent and the intensity of adsorption, respectively.

Table 2 summarizes the Langmuir and Freundlich constants for the adsorption capability on As(III). The adsorption data could be well represented reasonably by both models with the correlation coefficients value of the Langmuir and Freundlich models were 0.918–0.997 and 0.932–0.989, respectively. The value of W_s increased with increasing temperature, which corroborated the adsorption isotherm data shown in Fig. 3. Additionally, As(III) was easily adsorbed onto the NA41 surface when 1/n ranged between 0.1–0.5, but not when 1/n > 2.³⁸⁾ The data obtained (the value of 1/n was 0.46–0.62) showed that the As(III) adsorption onto the NA41 surface occurred readily. Thus, the adsorption of As(III) on the NA41 could be attributed to monolayer adsorption.

Table 2. Langmuir and Freundlich Constants for the Adsorption of As(III)

Adsorbent	Temperature (°C)	Langmuir isotherm model			Freundlich isotherm model
		Ws (mg/g)	a (L/mg)	r	1/n	log k	r
NA41	10	11.4	0.02	0.918	0.62	−0.30	0.932
	25	30.8	0.08	0.997	0.46	0.64	0.938
	45	37.2	0.18	0.986	0.47	0.85	0.989

Effects of pH on the Removal of As(III)

The effect of pH on the removal of As(III) is shown in Fig. 4. The quantity of As(III) adsorbed gradually increased with raising initial solution pH and became maximum at an equilibrium pH of approximately 7–8 under experimental conditions. In addition, the quantity adsorbed As(III) at an equilibrium pH of approximately 7–8 was higher than that at an equilibrium pH below 6. A trend similarly was observed in a previous research.²⁹⁾ In our study, the solution pH neutralized after adsorption of As(III), indicating that NA41 may be a potential candidate as a neutralizing agent to treat wastewater that includes As(III). A previous study reported that the adsorption capability on weak acids by metal oxides usually reached a maximum at pH values closed to the pk_a1 of the acid.³⁹⁾ The pk_a1 value of H₃AsO₃ is 9.2, which suggests that the predominant monoanionic and neutral (H₂AsO₃⁻ and H₃AsO₃) species could have been responsible for the adsorption of As(III) from aqueous media. Additionally, the value of pH_pzc in NA41 was approximately 6.7. Thus, the electrostatic interaction between negative charge of As(III) and positive charge of NA41 occurred at an initial pH below 10. Moreover, the ion exchange, one of the adsorption mechanisms of As(III), with sulfate ions in the interlayer of NA41 was occurred our experimental conditions (see the next section).

Fig. 4. Effect of pH on the Adsorption of As(III) onto NA41, □: Amount Adsorbed, ●: Equilibrium pH in Solution, Initial Concentration: 50 mg/L, pH in Solution: 2, 4, 6, 8, 10, Sample Volume: 50 mL, Adsorbent: 0.05 g, Temperature: 25 °C, Contact Time: 24 h, 100 rpm

Adsorption Mechanism of As(III) Using NA41

To assess the adsorption mechanism on As(III) using NA41, the elemental distribution and binding energy were analyzed (Fig. 5). According to the elemental distribution of As, the arsenic intensity increased after adsorption treatment. Additionally, the As binding energy peaks (3d (42 eV), 3p (141 eV), and LMMa (260 eV)) were undetectable before adsorption, but were evident after adsorption. The surface characteristics of NA41 are important during its interactions with As(III). In contrast, the binding energy peak of sulfur (2p, 164 eV) decreased after adsorption treatment comparable to that before the adsorption treatment. The adsorption mechanism of anions using the complex metal hydroxides showed electrostatic attraction, ligand exchange, and ion exchange.^21,40) This suggests that ion exchange was related to the adsorption mechanism in this study. The relationship between the amount of As(III) adsorbed and the amount of sulfate ions released from NA41 was also evaluated (Fig. 6); a correlation coefficient value of 0.866 was found. Thus, ion exchange involving sulfate ions from NA41 was one of the adsorption mechanisms of As(III) from aqueous media.

Fig. 5. Surface Analysis of NA41 before and after Adsorption of As(III), Initial Concentration: 100 mg/L, Sample Volume: 50 mL, Adsorbent: 0.05 g, Temperature: 25 °C, Contact Time: 24 h, 100 rpm

(Color figure can be accessed in the online version.)

Fig. 6. Correlation between Amount of As(III) Adsorbed and Amount of SO₄²⁻

Released, initial concentration: 10, 20, 30, 40, 50, and 100 mg/L, sample volume: 50 mL, adsorbent: 0.05 g, temperature: 25 °C, contact time: 24 h, 100 rpm.

Conclusion

In this study, various parameters including contact time, temperature, and pH on the adsorption capability and adsorption mechanism of As(III) using complex nickel–aluminum hydroxides were evaluated. Among the complex nickel–aluminum hydroxides, NA41 showed a highest As(III) adsorption capability comparable to other tested adsorbents. Kinetic studies revealed that the pseudo-second-order model accurately described the adsorption of As(III) with a correlation coefficient value of 0.980. Isotherm studies demonstrated that the adsorption capacity depended on the adsorption temperatures (10 °C <25 °C <45 °C). These isotherm data were strongly described by both Langmuir with a correlation coefficient value of 0.918–0.997 and Freundlich models with a correlation coefficient value of 0.932–0.989. The pH studies showed that the optimal pH condition for removal of As(III) using NA41 from aqueous media was approximately 7–8. Finally, the adsorption mechanism of As(III) was related to the electrostatic attraction and ion exchange under our experimental conditions. Our results suggested that NA41 could be potential candidate for removal of As(III) from aqueous media.

Acknowledgment

This research was funded by The Research Foundation for Pharmaceutical Science.

Conflict of Interest

The authors declare no conflict of interest.

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