Application of Granulated Nickel–Aluminum–Zirconium Complex Hydroxide in the Flow Method for Recovery of Chromium(VI) Ions

Abstract

A colloidal silicate granulated nickel–aluminum–zirconium (CSG-NAZ) was prepared, and the chromium(VI) (Cr(VI)) ions recovery capacity was evaluated using a sodium sulfate solution in a column experiment. The amount adsorbed and breakthrough time were enhanced by decreasing the flow rate (flow rate is in the order of 3.0 > 2.0 > 0.5 mL). The breakthrough curves and model parameters were estimated using the Thomas and Yoon–Nelson models. The obtained data confirmed to fit both the Yoon–Nelson model (0.858–0.906) and the Thomas model (0.813–0.906). Additionally, Cr(VI) ions that adsorbed onto CSG-NAZ could be desorbed using a sodium sulfate solution in a column experiment. The total recovery percentage of Cr(VI) ions was 80.9% after six repetitions of adsorption/desorption. Finally, the obtained results revealed that CSG-NAZ was a candidate adsorbent for the recovery of Cr(VI) ions owing to its applicability toward a continuous system.

Introduction

Chromium(VI) (Cr(VI)) is one of the major contaminant contributors to water pollution because of its carcinogenic and mutagenic characteristics.^1,2) In addition, chromium is one of the key metal application in electronics especially in Japan.³⁾ Therefore, the Cr(VI) ions recovery from the water environment is necessary.

Numerous physicochemical treatments have been developed for the recovery and/or removal of Cr(VI) ions, including membrane filtration/osmosis, precipitation, oxidation/reduction, ion exchange, and adsorption techniques. Among them, the adsorption techniques are widely utilized for heavy metals removal from a liquid phase. Our previous report was demonstrated that a nickel–aluminum–zirconium (NAZ) complex hydroxide exhibited a great adsorption capability of Cr(VI) ions.⁴⁾ In addition, NAZ was combined with a binder (colloidal silicate) to prepare the granulated agent to remove Cr(VI) ions from the liquid phase.⁴⁾ However, this treatment was conducted via batch operations based on an adsorption treatment, which are limited to treatment in small volumes and becomes an inconvenient approach when larger volumes are considered.^5–7)

Conversely, using fixed-bed columns is a more practical approach in water treatments.^8,9) Few advantages when using this approach include simplicity of operation, ease of handling, and the possibility of in situ regeneration. Additionally, the obtained information from laboratory-scale fixed-bed columns is useful for application in the fields, and fixed-beds can easily be controlled as a function of treated effluent demand profile.^10,11) The ability of fixed-beds is determined by the time or shape of the breakthrough appearance in a breakthrough curve, which is useful in determining the operation and the dynamic response of the adsorption column.¹²⁾

Herein, granulated NAZ with a binder was used as an adsorbent to recover Cr(VI) ions using a column. This study aims to evaluate the packed-bed column dynamic behavior of granulated NAZ for adsorption/desorption of Cr(VI) ions by both models, the Yoon–Nelson model and the Thomas model.^13–16)

Experimental

Materials

An experiment solution was prepared using the standard solution of K₂Cr₂O₇ in 0.1 mol/L HNO₃ (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan). The molar ratio of Ni²⁺/Al³⁺/Zr⁴⁺ = 0.9 : 1.0 : 0.09 was used to prepare NAZ complex hydroxide. The colloidal silicate was used as a binder to prepare granulated NAZ (CSG-NAZ) and its characteristics were already reported in our previous study.¹⁾ The current experiment, one granulated adsorbent was prepared namely CSG-NAZ (particle diameter is 500–1700 µm).

Cr(VI) Ions Recovery Using a Column Packed with CSG-NAZ

Figure 1 depicts the flow method using a column packed with CSG-NAZ. Adsorbents were added to the column (height × inner diameter = 10 × 0.8 cm, chromatography column, Bio-Rad, U.S.A.). The weights of CSG-NAZ were 1.34–1.44 g which the bed length of 4.5 cm. Five milligrams per liter of Cr(VI) ion solution (pH 7) was prepared and pumped upstream at a fixed flow rate, space velocity (S.V., the ratio of feed solution flow rate and adsorbent wet volume, 1/h), and liner velocity (L.V., the ratio of feed solution flow rate and the cross-sectional area of column, m/h). An experimental conditions data is shown in Table 1. The inductively coupled plasma optical emission spectrometer (iCAP-7600 Duo, Thermo Fisher Scientific Inc., Japan) was used to analyze an influent and effluent concentrations of Cr(VI) ions.

Fig. 1. Scheme of the Flow Method Using a Column Packed with Adsorbents

Table 1. Experimental Conditions Using a Column Packed with Adsorbent

Adsorbents	Conditions	Flow rate (mL/min)	Weight (g)	S.V. (1/h)	L.V. (m/h)
CSG-NAZ	1	3.0	1.44	79.6	3.6
	2	2.0	1.34	53.1	2.4
	3	0.5	1.43	13.3	0.60

Next, the adsorption and desorption experiments were repeated in six cycles utilizing CSG-NAZ (each cycle of adsorption and desorption takes 2 and 1 d, respectively; total experimental period is 18 d). The adsorption condition was 1 as indicated in Table 1. For the desorption condition, sodium sulfate solution (FUJIFILM Wako Pure Chemical Corporation) in a concentration of 10 mmol/L and flow rate of 3 mL/min, was used as the desorption solution.

Results and Discussion

Breakthrough Curves for the Adsorption of Cr(VI) Ions at Different Conditions

The ratios between effluent concentration (C_t) and influent concentration (C₀), C_t/C₀ versus effluent volume was plotted to determine a breakthrough curve. The results show the information regarding the adsorbent affinity, adsorbent surface properties, and adsorption pathways. Figure 2 depicts the breakthrough curves for the Cr(VI) ions adsorption at different conditions. The results demonstrated that at a lower flow rate (lower values of S.V. and L.V.), breakthrough curves were clearly delayed (condition 1< condition 2< condition 3). These results indicate that the adsorption column is not easily exhausted. Flow rate significantly affected to the adsorption capability due to the different of contact time between CSG-NAZ and the Cr(VI) ions solution in column.

Fig. 2. Breakthrough Curves for the Adsorption of Cr(VI) Ions at Different Conditions

Initial concentration: 5 mg/L, initial pH:7, flow rate: 0.5–3 mL/min, △: condition 1, 〇: condition 2, ▢: condition 3.

In this study, the Thomas and Yoon–Nelson models were adopted for evaluating the breakthrough curves.¹⁷⁾

The Thomas model is utilized to describe the performance theory of the adsorption process in a fixed-bed column. The following expression describes the linearized form of this model¹⁸⁾:

  

(1)

where C₀ is the initial concentration (mg/L), C_t is the equilibrium concentration at time t (mg/L), K_TH is the Thomas model constant (mL/h/mg), q_TH is the adsorption capacity (mg/g), Q is the flow rate (m³/h), and m is the amount of adsorbent.

The Yoon–Nelson model investigates the breakthrough behaviors of adsorbate gases on activated charcoal.¹⁹⁾ This model is a semi-empirical model with fewer parameters, and it mitigates the errors introduced by the Thomas model. However, detailed information regarding the characteristics of adsorbents is not necessary.^20,21)

  

(2)

where K_YN is the rate constant (1/h) and τ is the time required for 50% adsorbate breakthrough (h).

The calculated parameters predicted from the Thomas and Yoon–Nelson models are presented in Table 2. Inspection of the regressed lines indicates that all conditions were acceptable matches with linear regression coefficients of CSG-NAZ ranging from 0.813 to 0.906. As flow rate decreased (condition 1> condition 2> condition 3), the value of K_TH decreased and the value of q_TH increased. Therefore, lower flow rates enhance the adsorption of Cr(VI) ions on the column packed with adsorbents. Additionally, our previous study reported that a specific surface area and number of hydroxyl groups of granulated NAZ samples were necessary for the adsorption capacity of Cr(VI) ions.¹⁾ The specific surface area and the number of hydroxyl groups of CSG-NAZ were 69.5 m²/g and 1.08 mmol/g, respectively. The Thomas model was suitable for the adsorption process, which indicates that the external and internal diffusions were not the limiting steps under our experimental conditions.^8,22,23)

Table 2. Parameters Predicted from Thomas and Yoon–Nelson Models at Different Conditions

Adsorbents	conditions	Thomas model			Yoon–Nelson model
		KTH (mL/h/mg)	qTH (mg/g)	r	KYN (1/h)	τ (h)	r
CSG-NAZ	1	13.0	13.1	0.906	6.5 × 10−2	20.6	0.906
	2	9.1	19.5	0.888	5.1 × 10−2	40.8	0.888
	3	1.5	45.5	0.813	7.5 × 10−3	436.0	0.858

Additionally, the K_YN value increased and the τ value decreased with increasing flow rate as described in Table 2. The correlation coefficients in the Yoon–Nelson model of CSG-NAZ ranged from 0.858 to 0.906. Our results can conclude here that under our experiment conditions the Thomas model and the Yoon–Nelson model are satisfactory to predict the adsorption performance for the adsorption of Cr(VI) ions in a fixed-bed column.

Recovery of Cr(VI) Ions Based on Adsorption/Desorption Using a Column Packed with CSG-NAZ

The changes in the concentration of Cr(VI) ions based on adsorption/desorption and its recovery percentage using CSG-NAZ are shown in Fig. 3 and Table 3. The adsorption capacity of Cr(VI) ions using CSG-NAZ was not reduced with repetition of adsorption/desorption treatment (6 cycles). In addition, the Cr(VI) ions adsorbed onto CSG-NAZ were easily desorbed (recovered) using the sodium sulfate solution. The total amount adsorbed and total amount desorbed were 166.1 and 134.3 mg/g, respectively. with the total recovery percentage of 80.9%

Fig. 3. Changes in the Concentration of Cr(VI) Ions Using CSG-NAZ

Adsorption condition; initial concentration: 5 mg/L, initial pH:7, flow rate: 3 mL/min, Desorption condition; initial concentration: 10 mmol/L, flow rate: 3 mL/min.

Table 3. Recovery Percentage of Cr(VI) Ions Using CSG-NAZ

Cycles	1	2	3	4	5	6	Total
Adsorption (mg/g)	21.3	26.7	22.2	20.8	24.1	21.4	166.1
Desorption (mg/g)	22.0	15.8	21.2	20.4	17.1	20.9	134.3
Recovery percentage (%)	103.0	59.3	95.4	98.1	71.2	97.7	80.9

Conclusion

This study revealed that a decrease in flow rate enhanced the longevity of CSG-NAZ performance in a column experiment by increasing the breakthrough time and amount adsorbed. From the two models, the obtained data confirmed to fit both the Yoon–Nelson model (0.858–0.906) and the Thomas model (0.813–0.906). In addition, the total recovery percentage of Cr(VI) ions using CSG-NAZ was 80.9% after six cycles of adsorption/desorption. Hence, CSG-NAZ was found to be a promising adsorbent for the recovery of Cr(VI) ions based on adsorption/desorption due to its applicability for a continuous system.

Acknowledgments

This research was funded in part by Kurita Water and Environmental Foundation (21K004) and the Hattori Hokokai Foundation in Japan.

Conflict of Interest

The authors declare no conflict of interest.

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