The probable use of Jordanian natural zeolitic tuff in wastewater treatment as natural adsorbent for the removal of Cr (VI) ions from aqueous solution in continuous fixed bed columns was tested experimentally and theoretically. The tested zeolitic tuff was obtained from Al Hala volcano (HZ) located in southern part of Jordan and subjected to crushing and sieving only without any further treatment. Experimentally the HZ grains were packed in a fixed bed column. The used grain sizes are HZ1 (1.0–0.60 mm) and HZ2 (0.60–0.30 mm). The adsorption capacity was evaluated using breakthrough curves and by applying the Thomas and Yoon and Nelson models. The Thomas model analysis of the measured breakthrough curves revealed that the adsorbent HZ2 has a higher adsorption capacity to Cr (VI) ions (56.3 mg/g) than HZ1 (35.5 mg/g). The time elapsed to reach 50% breakthrough was determined by the Yoon and Nelson model. The time to reach 50% breakthrough is 318.78 min and 368.18 min for HZ1 and HZ2, respectively. The research results indicate that the small size fraction (HZ2) is more suitable and effective as adsorbent material than the size fraction (HZ1) due to its high surface area.

INTRODUCTION

Contamination of water with heavy metals at certain concentrations due to industrial activities is considered a major concern subjected to environmental laws and regulations due to their toxicity and carcinogenicity, which may cause human health problems (Yan et al. 2008).

One of the most important heavy metals is chromium. It is categorized as a significant pollutant to the environment due to its stability, persistency and low degradability. Naturally, chromium occurs in various oxidation states, but Cr (III) and Cr (VI) are of remarkable concern biologically, so in the human body Cr (III) is an essential trace element which is required for lipid, glucose and amino-acid metabolism as well as a general dietary supplement. However, oxidation of Cr (III) to Cr (VI) is a concern biologically, which is recognized to be highly toxic, mutagenic and carcinogenic for humans. Chromium's hexavalent form Cr (VI) is very toxic and carcinogenic (Rahmaty & Khara 2011). Demand for Cr (VI) is increasing due to its extensive use in many industries such as metal surface finishing, paints, textiles, pigments, leather tanning, wood preservation, reaction catalysis, batteries, magnetic tapes, film and photography, galvanometric, electrical procedures, metal cleaning and corrosion inhibition (Kirk-Othmer Encyclopedia 2007; Ünal  et al. 2010).

A number of treatment technologies such as coagulation, precipitation, ion-exchange, electrochemical methods, extraction, biosorption, and adsorption have been considered for the treatment of contaminated wastewater (Babel & Kurniawan 2003; Kwon et al. 2010; Wang & Peng 2010). Adsorption recently has become one of the attractive treatment techniques for wastewater (Panayotova 2001; Argun 2008; Yadanaparthi et al. 2009).

One of the most important materials used as adsorption and ion exchangeable material is natural zeolites. For heavy metals removal, natural zeolites are widely used as adsorbent material (Bailey et al. 1998; Babel & Kurniawan 2003).

Basically, zeolites as natural low-cost adsorbents have been applied for treatment of contaminated wastewater due to their properties, such as adsorption and ion exchange capacity (Klinkenberg 1948; Benefield et al. 1982; Weng & Huang 1994; Erdem et al. 2004; Zhang 2006).

Zeolites are a group of hydrated aluminum-silicates of the alkali or alkaline earth metals (sodium, potassium, magnesium, calcium) characterized by low mining cost, availability, bulk density, and high resistance to alteration (Mercer & Ames 1978). Zeolites have a three-dimensional crystalline framework of tetrahedral silica or alumina anions strongly bonded at all corners, and they contain channels filled with water and exchangeable cations.

For wastewater treatment, the most important properties of natural zeolite are high cation exchange capacity and good ion selectivity. Colella (1996) and Pansini (1996) found that zeolite removes heavy metals from natural and industrial wastewaters.

Purnomo & Prasetya (2007) studied the adsorption breakthrough curves of Cr (VI) on bagasse fly ash. They measured the breakthrough curve at room temperature using a fixed-bed apparatus. They tried to fit the experimental data to a fixed-bed model for the breakthrough curve. They concluded that if the value of effective diffusivity (De) and overall mass-transfer coefficient (k) can be obtained, it will allow an easier prediction of the behavior of breakthrough curves in specific adsorption operating conditions and column dimensions without doing any adsorption equilibrium experiments.

In Jordan, zeolitic tuff has been characterized and widely used in wastewater treatment. Several investigations were carried out on the adsorption behavior of Jordanian natural zeolites, including: Dwairi (1991), Dwairi (1992), Al Dwairi (2007, 2009), Marashdeh & Al-Haj-Ali (2009), Al Dwairi & Gougazeh (2010), Hussein (2010), Al Dwairi & Al-Rawajfeh (2012a, 2012b), Taamneh & Al Dwairi (2013), Al-Haj-Ali & Marashdeh (2014), Al Dwairi et al. (2014), Al Dwairi et al. (2015), and Khoury et al. (2015).

The most two recent important studies carried out on zeolitic adsorption are Al-Haj-Ali & Marashdeh (2014) and Al Dwairi et al. (2015).

Al-Haj-Ali & Marashdeh (2014) evaluated the capability of northeast Jordanian natural zeolite tuff to remove Cr (III) ions from aqueous solutions by physico-chemical method under specified conditions. The equilibrium adsorption capacity reached 19.6 mg/g and the data fitted well both Langmuir and Redlich-Peterson isotherms. Break points up to 2.5 h were obtained at 5% breakpoint using fixed beds of zeolite at the lower metal concentration and solution velocity but higher pH and bed depth. The mechanism of removal appears to be ion exchange of Cr(OH)2+ at pH ≤ 6.0 and adsorption on zeolite surface of fine Cr(OH)3 precipitate at pH > 6.0.

Al Dwairi et al. studied the kinetic modeling for heavy metal adsorption using natural zeolite obtained from Jabal al Ataitah in southern Jordan. Important kinetic parameters have been investigated to design a column experiment packed with Jordanian natural zeolite beds for the removal of lead and lithium heavy metals. The experimental data were represented using breakthrough curves for the adsorption of metals and modeled using the Thomas model and Yoon and Nelson kinetic models. Comparison studies were carried out for the experimental breakthrough curves and the calculated theoretical curves.

This work investigates the utilization of a low-cost natural zeolite adsorbent, which is available in enormous reserves in Al Hala volcano, southern Jordan, without any treatment except for size reduction and sieving. The study examines its potential as an adsorbent for removal of Cr (VI) using fixed bed columns.

EXPERIMENTAL

Materials

All the experiments were conducted using artificial wastewater stock solution at a concentration of 400 mg/L, which was prepared by dissolving a precisely weighed amount of potassium dichromate (K2Cr2O7) in deionized water. The Cr (VI) stock solution was diluted to specific concentrations and stored at 25 °C in a dark place. The experiments were conducted at an initial pH value of 7.2.

The used adsorbent is a natural zeolite obtained from Al Hala volcano (HZ) located in southern Jordan (Figure 1). This location has been chosen from the southern Jordanian zeolitic tuffs to be used as pollutant control in removing Cr (VI) from wastewater stock solution. This location is new and hasn't been studied before for its industrial or environmental application. In addition, the information available on using this zeolitic tuff in Cr (VI) removal from wastewater is very low in comparison with other heavy metals.
Figure 1

Location map of the southern Jordan basaltic tuff showing the Al Hala volcano (after Al Dwairi 2007).

Figure 1

Location map of the southern Jordan basaltic tuff showing the Al Hala volcano (after Al Dwairi 2007).

HZ was subjected to crushing and sieving only, without any chemical treatment. HZ samples were crushed using a jaw crusher with an aperture of 2 cm and then sieved into the size cuts (1.0–0.60 mm) for HZ1 adsorbent and (0.60–0.30 mm) for HZ2 adsorbent (Table 1). The grain size range between 0.3 and 1 mm was selected because it contains the highest zeolite grade, which ranges between 50 and 60% (Al Dwairi 2007; Al Dwairi et al. 2014). This type of Jordanian zeolite (HZ) is characterized by the high cation exchange capacity range from 189 meq/g to 136 meq/g (Al Dwairi 2014).

Table 1

The characteristics of the used HZ samples as adsorbent materials

Adsorbent Grain size (mm) Mineral content 
HZ1 1.0–0.60 Phillipsite-Chabazite 
HZ2 0.60–0.30 Phillipsite-Chabazite 
Adsorbent Grain size (mm) Mineral content 
HZ1 1.0–0.60 Phillipsite-Chabazite 
HZ2 0.60–0.30 Phillipsite-Chabazite 

The HZ samples contain about 75% zeolites and were characterized in terms of their mineralogical content by applying X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) imaging. A typical XRD pattern and SEM for the HZ zeolitic tuff are displayed in Figures 2 and 3 (Al Dwairi 2014). The characterization analysis indicated that the main zeolitic tuff minerals in the adsorbent are phillipsite and chabazite.
Figure 2

The measured qualitative XRD mineralogical composition of phillipsitic tuff from Al Hala volcano (HZ) used as raw material for the adsorption experiments (Al Dwairi 2014).

Figure 2

The measured qualitative XRD mineralogical composition of phillipsitic tuff from Al Hala volcano (HZ) used as raw material for the adsorption experiments (Al Dwairi 2014).

Figure 3

Scanning electron micrograph showing zeolite crystals from the study area.

Figure 3

Scanning electron micrograph showing zeolite crystals from the study area.

Methods

The HZ1 and HZ2 samples were washed with deionized water several times with constant stirring, to remove the soluble inorganic salts and any adhering materials. The samples were then left to settle, separated from liquid by filtration and then dried at 80 °C for 24 h. An adsorption column apparatus was constructed to perform fixed bed column studies for the adsorption of Cr (VI) ions into HZ1 or HZ2 zeolite samples. The column was selected from glass material with the dimensions of 1 cm2 cross sectional area (i.e. 1.1 cm internal diameter) and length of 40 cm. The Cr (VI) containing solution was fixed at a level higher than the column to allow flow by gravity. A valve was fixed at the exit from the tank to adjust the flow rate to the column. The granulated zeolite (HZ1 or HZ2) was packed in two columns. Bed height of 30 cm was fixed in all experiments. The mass of the adsorbent was constant in all experiments at 30 g, which was used in all tests. The solution was introduced to the column at a constant flow rate of 6.7 mL/min by gravity and adjusted using a valve and flowmeter fixed at the entrance to the column (Figure 4).
Figure 4

Fixed bed column apparatus used for the study of adsorption of Cr (VI) ions into HZ1 or HZ2 zeolite adsorbents.

Figure 4

Fixed bed column apparatus used for the study of adsorption of Cr (VI) ions into HZ1 or HZ2 zeolite adsorbents.

The adsorption study was performed at room temperature, 22 ± 0.5 °C. The effluent samples were collected at specified intervals (one bed volume) and analyzed for the residual ion concentration using a Perkin Elmer UV-vis spectrophotometer (Lambda 25). Column studies were terminated when the column reached exhaustion. The experiments were conducted under constant conditions, and the only variables to be studied are the adsorbent material (HZ1 or HZ2). All continuous adsorption experiments were performed at the same operating conditions (Table 2) but with changing the adsorbent type (HZ1 or HZ2), which differ only in the size. Breakthrough curves were obtained to evaluate the adsorbent performance in a fixed bed column.

Table 2

The experimental conditions used in the fixed bed adsorption experiments for HZ1 and HZ2 adsorbents

Bed height 20 cm 
Mass of adsorbent 20 g 
Flow rate 6.7 mL/min 
Initial Cr(VI) concentration 200 mg/L 
Initial pH for Cr(VI) solution 7.2 
Bed height 20 cm 
Mass of adsorbent 20 g 
Flow rate 6.7 mL/min 
Initial Cr(VI) concentration 200 mg/L 
Initial pH for Cr(VI) solution 7.2 

RESULTS AND DISCUSSION

A series of fixed bed column experiments were directed to examine the performance of the column and the adsorbent characteristics. The adsorption of Cr (VI) ions from its aqueous solution was investigated for the two natural zeolite adsorbents (HZ1 and HZ2), which differ in the size of grains. Experimental breakthrough curves were measured for both adsorbents by keeping all the other experimental conditions constant. The fixed bed experiments were conducted at initial pH value 7.2, initial Cr (VI) ion concentration 200 mg/L, flow rate 6.7 mL/min, bed height 20 cm and adsorbent mass 20 g.

Typical breakthrough curves for the fixed bed adsorption of Cr (VI) ions on HZ1 and HZ2 adsorbents are presented in Figure 5, which displays the variation of the concentration of Cr (VI) ions discharged from the column (Ce) with respect to the feed concentration (C0) with time (t).
Figure 5

Typical experimental breakthrough curves for the adsorption of Cr (VI) ions on the zeolites HZ1 and HZ2 using fixed bed column.

Figure 5

Typical experimental breakthrough curves for the adsorption of Cr (VI) ions on the zeolites HZ1 and HZ2 using fixed bed column.

The experimental breakthrough curves reveal that both adsorbents HZ1 and HZ2 are feasible to be implemented in fixed bed columns for the removal of Cr (VI) from its aqueous wastewater streams. However, a more detailed analysis of the breakthrough curves indicates that the adsorbent HZ2 (the smaller grain size) is more efficient than the adsorbent HZ1 (the larger grain size). Obviously, it can be observed that the breakthrough happened in the case of HZ2 after HZ1, indicating that HZ2 has longer service time and better performance in the column; however, both adsorbents have nearly the same slope of their breakthrough curves.

Analysis using Thomas model

The kinetic parameters, adsorbent capacity and column performance can be evaluated by using the Thomas model through the analysis of the experimental breakthrough curves presented in Figure 5. Such parameters are of great importance for the design and operation of the adsorption fixed bed columns. The Thomas model is based on the bed-depth-service-time model, which can be described mathematically by the following linearized form equation (Baek et al. 2007; Sivakumar & Palanisamy 2009): 
formula
1
where C0 is the adsorbate feed concentration (mg/L), Ce is the adsorbate effluent concentration (mg/L), M is the total mass of the adsorbent (g), Q is volumetric flow rate (mL/min), KT is kinetic Thomas rate constant (mL/min/mg), qo is the maximum adsorption capacity (mg/g) and V is the throughput volume (mL). The mathematical expression described by the mathematical Equation (1) is a modified form of the Thomas model. This modified model assumes a rectangular (irreversible) isotherm when solving the differential equation of mass transfer and is precisely comparable to the Bohart-Adams model (Chu 2010).
According to Equation (1), the slope and intercept of the linear plot of ln [(C0/Ce) − 1] versus the throughput volume V at a given flow rate will be used to evaluate the kinetic parameters, KT and q0 (see Figure 6). The results are tabulated in Table 3.
Table 3

The estimated Thomas model parameters for the fixed bed adsorption of zinc on HZ1 and HZ2 adsorbents

Adsorbent R2 (Eq. 1) KT (mL/min/mg) q0 (mg/g) 
HZ1 0.99 0.2278 35.5 
HZ2 0.98 0.2546 56.3 
Adsorbent R2 (Eq. 1) KT (mL/min/mg) q0 (mg/g) 
HZ1 0.99 0.2278 35.5 
HZ2 0.98 0.2546 56.3 
Figure 6

Plot of the measured breakthrough curves according to the Thomas model.

Figure 6

Plot of the measured breakthrough curves according to the Thomas model.

The analysis of the experimental breakthrough curves of Figure 6 according to the Thomas model indicated that the adsorption capacity of the adsorbent HZ2 (56.3 mg/g) to Cr (VI) ions is much higher than HZ1 (35.5 mg/g). However, both adsorbents display their feasibility to be implemented in fixed bed columns for the removal of Cr (VI) ions from aqueous solutions. On the other hand, HZ2, with smaller size grains, is more feasible due to its advantage of higher adsorption capacity and thus longer service time of operation. In the calculated adsorption capacities of the tested zeolites HZ1 and HZ2, the high adsorption capacity of the tested zeolites are attributed to their porous structure. Zeolites have a cage-like structure with open channels. On the other hand, zeolites have large potential to exchange their alkaline cations for zinc ions.

It is worthwhile to mention that the longer time needed for the adsorbate breakthrough, and thus the enhanced column performance in the case of using HZ2 adsorbent, is attributed to its smaller particle size. Perceptibly, the smaller grain size of adsorbent grains increases bed competences and thus increases breakthrough time. Smaller size adsorbents might have shorter diffusion paths, allowing better mass transfer properties for the adsorbate to reach the adsorbent active sites. Furthermore, reduced grain size of particles provides increased surface area and thus provides active sites that are more available for the adsorption process. On the other hand, grain size has a significant impact on the fluid flow characteristics within the packed bed of the column. The larger size particles lead to smaller hydraulic resistance. This considerably affects the residence time of the adsorbate in the bed, which can be shorter in the case of larger grains. Thus, there is more possibility for adsorption equilibrium to be achieved in the case of the smaller size adsorbent.

A comparison between the calculated breakthrough curves from the Thomas model using the kinetic parameters in Table 3 and the experimental measurements is shown in Figure 7. Therefore, the experimental data for the adsorption of Cr (VI) ions on HZ1 and HZ2 zeolites can be represented to a high degree by the Thomas model.
Figure 7

Comparison between the calculated breakthrough curves from the Thomas model and the experimental measurements for the adsorption of Cr (VI) ions on HZ1 1nd HZ2 adsorbents.

Figure 7

Comparison between the calculated breakthrough curves from the Thomas model and the experimental measurements for the adsorption of Cr (VI) ions on HZ1 1nd HZ2 adsorbents.

The perfect fitting of the experimental data to the Thomas model indicates that the adsorption of Cr (VI) ions to HZ1 and HZ2 zeolites follows a second order reversible mechanism (Thomas 1994) and the adsorption process is controlled by both the ion exchange process and electrostatic adsorption.

Analysis using Yoon and Nelson model

The Yoon and Nelson kinetic model is used to analyze the experimental breakthrough data presented in Figure 5. This model is feasible to study the bed service time. The following linearized equation of Yoon and Nelson model is implemented in this study (Tsai et al. 1999): 
formula
2
where C0 is the adsorbate feed concentration (mg/L), Ce is the adsorbate effluent concentration (mg/L), KYN is the Yoon and Nelson rate constant (min–1), τ is the time required for 50% adsorbate breakthrough (min) and t is the time (min).
According to Equation (2), fitting the experimental values of ln(Ce/C0–Ce) versus time will result in a straight line (Figure 8), which can be used to evaluate the parameters KYN and τ. The calculated parameters are listed in Table 4.
Table 4

The estimated Yoon and Nelson model parameters for the fixed bed adsorption of zinc on HZ1 and HZ2 adsorbents

Adsorbent R2 KYN τ (min) 
HZ1 0.97 0.0136 318.7867647 
HZ2 0.96 0.013 368.1769231 
Adsorbent R2 KYN τ (min) 
HZ1 0.97 0.0136 318.7867647 
HZ2 0.96 0.013 368.1769231 
Figure 8

Analysis of the measured breakthrough curves using the Yoon and Nelson model.

Figure 8

Analysis of the measured breakthrough curves using the Yoon and Nelson model.

Obviously, the experimental breakthrough curves fit the Yoon and Nelson model to a high degree for the adsorbents HZ1 and HZ2, with correlation coefficients of R2 = 0.97 and R2 = 0.96, respectively. By inspecting the results of Table 3, it can be found that the time elapsed to reach 50% breakthrough is 318.78 min and 368.18 min for HZ1 and HZ2, respectively. This indicates that HZ2 provides better column performance with a longer service time. This is in good agreement with the experimental data and the result obtained from the Thomas model.

A comparison between the calculated breakthrough curves from the Yoon and Nelson model using the kinetic parameters in Table 4 and the experimental measurements is presented in Figure 9. Therefore, the adsorption of Cr (VI) ions on HZ1 and HZ2 zeolites can be represented to a high degree by the Yoon and Nelson model.
Figure 9

Comparison between the calculated breakthrough curves from the Yoon and Nelson model and the experimental measurements for the adsorption of Cr (VI) ions on HZ1 1nd HZ2 adsorbents.

Figure 9

Comparison between the calculated breakthrough curves from the Yoon and Nelson model and the experimental measurements for the adsorption of Cr (VI) ions on HZ1 1nd HZ2 adsorbents.

CONCLUSIONS

This work evaluates a Jordanian natural zeolite to be implemented directly after crushing and sieving as adsorbent for treating wastewater contaminated with Cr (VI) ions. The obtained adsorption parameters indicated that a continuous adsorption column process could be implemented successfully to treat the enormous flow rates of wastewater streams due to the long breakthrough time and the adequate adsorption capacity of the tested natural zeolite. The size of the grains was found to be a crucial parameter to enhance the service time and the maximum adsorption capacity. This study is limited to Cr (VI), and future studies are needed to include the effects of different contaminants, changes in pH, and concentration variance on adsorption of chromium.

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