The Application of Zeolites for Fixation of Cr(VI) Ions in Sediments

Abstract

The aim of the study was to investigate the fixation of Cr(VI) ions from contaminated sediments using synthetic zeolite 4A and natural zeolite clinoptilolite. Parameters such as pH, contact time, adsorption mass and temperature were investigated. If the ions of the heavy metals were mobile, they would become toxic to the environment. After sediment digestion, the initial and final concentrations of Cr(VI) were measured in sediment samples with or without zeolite. Inductively coupled plasma with optical emission spectroscopy (ICP-OES) and X-ray diffraction (XRD) were used to characterize the material. The adsorption kinetics were investigated using a pseudo-first order model, a pseudo-second order model, and an intra-particle diffusion model. The results showed that the zeolites enhanced the fixation of Cr(VI). Chemisorption was the main mechanism when using acid-modified zeolite.

1. Introduction

Since heavy metals accumulate in sediments in various forms, it is likely that a portion of these metals will be mobilized during use and become toxic to the environment. Among the exchangeable fractions of heavy metals released from sediments, less than 1% is safe for the environment, while more than 50% of the total amount poses a high risk and can enter the food chain [1]. Industrialization, urbanization, and intensification of agriculture reveal a constantly increasing level of pollution in our environment. Pollution comes from several different sources, including mobile sources (vehicles), stationary sources (agriculture, industrial facilities and factories), natural sources (wildlife, volcanic activity), and many more [2]. Various methods have been adopted in recent years to remove harmful substances from water, air and soil, including adsorption and ion exchange [3]. Organic matter only accounts for 1–10% of all soil components, but nevertheless play critical roles in the physical, chemical, and biological processes within sediments [4]. Recent study assessed the metal pollution in China, and further the associated coupling mechanisms with organic matter in sediments [5]. There was no observable correlation between dissolved organic carbon (DOC) and Cr(VI), but a positive correlation was found for other metals, such as Cd(II), Pb(II), and Zn(II). Metals entering the environment undergo various physical and chemical reactions with sediments, such as precipitation-dissolution, ion exchange, adsorption, redox reactions, complexation, and biological processes, which alter their speciation and chemical properties [6]. Zeolites contain molecules or clusters of atoms in the cages of zeolite structures [7]. In addition to their commercial use as molecular sieves, they are also widely used for their cation exchange properties. Cations (such as Na(I), K(I), and Ca(II)) are only weakly bound to the tetrahedral framework and can be easily removed or exchanged by washing with a strong solution of another ion. In addition, zeolites exhibit cation exchange properties with high ion exchange capacity, selectivity, and compatibility with the natural environment [8]. The cations are only weakly bound to the tetrahedral framework and can be easily removed or exchanged with other ion during from a solution. Only a few zeolites are available in sufficient quantity and purity to be used as a commercial product, thus increasingly, synthesized zeolites are used. Large structural cavities and the entry channels leading into them contain water molecules that form hydration spheres around exchangeable cations [9]. Very specific zeolite structures (with unique channel diameters and/or cation exchange properties) are produced for a variety of industrial applications [10]. Recently, zeolites have received considerable attention from the scientific community due to their unique physicochemical properties, easy availability, and low cost. They are increasingly used in sewage sludge, sediments, and water treatment [1]. Clinoptilolite was modified with NaOH and HCl for improved fixation of Pb(II), Cu(II), Zn(II), and Cd(II). The highest efficiency 80% was achieved for all studied metals fixation [9]. Numerous studies on the application of natural zeolites as adsorbents in water and wastewater treatment [11]. Other studies focused on similar metals and the mobility of heavy metals decreased after fixation with zeolites [12].

Although some studies exist on heavy metals’ fixation, very few studies on Cr(VI) ions fixation in sediment were reported. The aim of the research was to study the fixation of Cr(VI) ions from contaminated sediments using synthetic zeolite 4A and natural zeolite clinoptilolite. It was reported that acidic modification improved the adsorption without structural changes in zeolite [13]. Therefore, acid modification was applied for both types of zeolites. Sampling of sediments with high Cr(VI) concentrations near factory were performed. Fixation was improvised in the laboratory at different conditions, such as temperature, sediment/zeolite ratio, and pH values. Additionally, the influence of organic carbon in sediment on fixation of Cr(VI) was studied.

2. Materials and Methods

The natural clinoptilolite was found in Krapina, Croatia, with bulk density 1025 ± 100 g/L. Among zeolites, clinoptilolite is the most abundant natural zeolite [11]. Chemically it is hydrated alkali aluminosilicate. The synthetic powdered zeolite (ZP-4A) was applied for the study kindly provided from Silkem company, Kidričevo, Slovenia. pH value was 11–12 and bulk density was 350 ± 100 g/L [11]. There are eight sodium ions located near the center of the six rings.

Zeolites adsorption could be negligible because of the negative charge and a modification by a cationic metallic reagent is needed. Zeolite was modified by HCl [14], achieved with an HCl solution (0.5 M) using a 1:3 acid zeolite ratio for 1 h at 70 °C. The zeolite was subsequently washed with distilled water for 1 h at 70 °C, dried at 105 °C. The influence of pH, temperature, initial concentration, adsorption time, and adsorbent dose were studied. pH was measured using WTW Multimeter 3410. Cr(VI) was determined by the ISO 22036 standard method [15]. The initial and final concentrations of Cr(VI) in sediment was determined by inductively coupled plasma with optical emission spectroscopy (ICP-OES), Varian 710-ES ICP (Varian, Palo Alto, CA, USA). Firstly, powdered samples (2.5–3 g) were mixed with 21 mL HCl (37%, Fluka, Buchs, Switzerland) and 7 mL HNO₃ (65%, Fluka) and heated for 2 h under reflux. The sample was then filtered through a cellulose filter into a 100 mL flask and diluted to the mark with deionized water. Then, ICP analyses were performed.

Total organic carbon (TOC) was performed by using Nanocolor TOC 30 test and photometer PF-12^PLUS. (LLG, Meckenheim, Germany).

All chemicals used were p.a. quality.

Sediments were sampled in city Zreče, Slovenia (exact location is not permitted to be reported). Firstly, the water content in sediments was determined by the gravimetric method by weighing wet and dry sediment at constant mass (at 105 °C).

The fixation was performed in bottles: firstly 100 g of sediment was weighed, 1 g of modified zeolite or clinoptilolite was added and diluted into 250 mL of water. The second set of measurements was performed at the same conditions; the only difference was the increased mass of 10 g of zeolite, which was added into the sediment. In the third set, the difference to the first set was in adjusting the pH value to 4 when 1 g of zeolite was mixed with the sediment. In control samples, only 100 g of sediment was weighed into 250 mL of water. After each sampling of dried sediment and analyses of the initial Cr(VI) concentration, fresh deionized water was once again added to the original bottles to sustain the leaching process.

TOC analysis was performed using Nanocolor TOC 30 tube tests (MN, Düren, Germany).

Adsorption kinetics were evaluated by classical pseudo-first order (PFO) and pseudo-second order (PSO) kinetic equations. The kinetic Equations (1)–(3) of these models are given below [16]. The parameters of the PSO kinetic model (q_e and k₂) were determined from the slope and the intercept of the linear plots t/q_t versus t, whilst for the pseudo-first order model, the linear plots of log(q_e–q_t) against t were applied [16].

q_t = q_e(1 − e^−k1t)

(1)

where

• q_t is the sorption capacity at time t (mg/g);
• q_e is the sorption capacity at equilibrium (mg/g);
• k₁ is the PFO reaction rate constant (d⁻¹).

$$q_{\mathrm{t}}={\displaystyle \frac{{ k}^2{ q_e}^{2}t}{{ 1+k}^2{q}_{e}t}}$$

(2)

where k₂ is the pseudo-second order reaction rate constant g/(mg.d)

The intra-particle diffusion (IPD) equation was chosen to study the intra-particle diffusion and can be expressed as follows [16]:

q_t = k × t^0.5 + c_id

(3)

where k (mg(g d^0.5)) and C_id (mg/g) are regarded as a constant that characterizes the mass transfer over the boundary layer. The parameters of intra-particle diffusion model were quantified from the plots of q_t versus t^0.5.

3. Results

3.1. Characterization of Zeolites

Table 1. Chemical composition of clinoptilolite and zeolite ZP-4A.

Compound	Clinoptilolite wt [%]	ZP-4A wt [%]
SiO2	64.9	34
Al2O3	13.4	30
Fe2O3	2.0	0
MgO	1.1	0
CaO	2.0	0
Na2O	2.4	19
K2O	1.3	0

shows main chemical composition of clinoptilolite and ZP-4A.

Clinoptilolite was found to be mainly SiO₂. The Si/Al ratio > 4 is typical for a clinoptilolite [14]. Clinoptilolite in this study has a Si/Al ratio of 4.84. For ZP-4A, SiO₂ and Al₂O₃ were the main oxides. Additionally, CaO was found in clinoptilolite at 2.0% and Na₂O in clinoptilolite and ZP-4A at 2.4 and 19%, respectively. The Si/Al ratio in the synthetic zeolite after activation with HCl showed negligible variations as in other studies [14]. The rest was water.

The XRD of zeolite ZP-4A is shown in Figure 1. It is in accordance with literature (PDF No. 00-047-1870) [17]. It is recognized by the data of sodium-alumo-phosphate silicate hydrate with formulae Na₂O∙Al₂O₃∙1.71SiO₂∙0.24P₂O₅∙4H₂O. The red dots denote the hydrate peaks.

The natural zeolite was analyzed by X-ray powder diffraction to determine the phase composition. Figure 2 shows the diffractogram of a natural zeolite. The red and blue lines show the peaks of two clinoptilolites from the database (PDF No. 00-039-1383 and PDF No. 00-047-1870). The black line shows the phases of the present quartz, SiO₂ (PDF No. 00-047-1870) [17].

3.2. Characterization of Sediment

The sediment contained 23.0 ± 3.0 w% of water. The initial concentration in sediment was determined by ICP-OES after aqua regia digestion. The measured initial concentration of Cr(VI) was 23.6 ± 0.2 mg/kg.

The results of zeta potential and particle size measurements that we found are shown in

Table 2. Zeta potential and particle size measurement.

Sample	d [nm]	w [%]	ζ [mV]
Control	878.5	100.0	−15.1
Sediment:4A = 100:1	140.5 722.1	86.4 13.6	−37.8
Sediment:4A = 10:1	267.3 876.9	58.2 41.8	−47.5
Sediment: clinop = 100:1	185.5 841.1	91.4 8.6	−42.9
Sediment: clinop = 10:1	168.3 963.5	90.9 9.1	−49.6

.

We found that the particle size is uniform in control, while the absolute value of the zeta potential is the lowest, which indicates less stable suspensions. In the other samples, two separate phases are clearly visible. It could be assumed that two phases have not been homogenized with each other. Larger particles are the same size as the particles in the sludge without zeolite, while smaller ones have the size of clinoptilolite or powdered zeolite, thus having a larger surface area, which affects both the larger absolute value of the zeta potential and the stability of the suspension [18]. Sample with the ratio sediment:clinoptilolite = 10:1, has the highest absolute value of the zeta potential at −49.6 mV, so it is also the most stable among all the samples. The increase in the zeta potential is a consequence of the addition of zeolite and the adsorption of Cr(VI) ions on the surface of the zeolites. Based on the more negative zeta potential a higher mass of zeolite, e.g., at ratio 1:10, better metals fixation could be expected.

3.3. Leaching Experiments

The results of leaching experiments are presented in Figure 3a–c.

Figure 3a–c show decreasing concentrations of Cr(VI) in control samples at all conditions. The initial concentration decreased from 23 mg/kg to15 mg/kg after three months. In the control sample, only Cr(VI) ions are present, and fixation partly took place, which is seen from Table 2, namely a negative zeta potential, meaning a negative charge due to anions in the sediment. Due to small size of sediment, the fixation is quite high in accordance with another study [19]. Figure 3a–c represents the results of 1 g of zeolite addition per 100 g of sediment. The difference is that by using synthetic zeolite the fixation is slightly less efficient. The most efficient fixation was found at samples at pH adjusted to 4 and with natural zeolite. It was calculated that the concentration of Cr(VI) ions decreased only by 8.5% with adjusted pH to 4 and natural zeolite, while at 4 °C and pH = 4 the result improved by an additional 2.2%. The concentration decreases are the highest after 2 months. The reason could be the infiltration of water into the sample, creating interconnected pathways for increased leaching [20]. After three months, the concentration remained the same. These results confirmed the long-term effectiveness of zeolite in stabilizing heavy metals [20]. Figure 3b also shows the overlapping of the measured concentrations when adding natural or synthetic zeolite, meaning the same fixation using natural or synthetic zeolite.

The influence of the pH value was investigated at pH = 4 and pH = 7 (Figure 3a,b). At lower pH values the concentration of H⁺ ions is higher. The modified zeolite surface is mainly positively charged and there is an electrostatic interaction between Cr(VI) ions and positive H-ions [21]. Therefore, fixation was better at pH = 4. Since the negative zeta potential was determined when either clinoptilolite or ZP-4A was added to the sediment, as seen from Table 2, it can be concluded that most of the sites on zeolite were occupied by Cr(VI).

In the next phase of the experiment, the mass of zeolites was increased from 1 g to 10 g per 100 g of sediment. However, the results showed the same efficiency of Cr(VI) fixation. The measurements show a negligible difference in Cr(VI) fixation. When the sediment/zeolite ratio was increased to 10:1, the zeta potential increased slightly (Table 2). The concentration of Cr(VI) in the solution remains similar to a ratio of 100:1. With a large amount of zeolite as an adsorbent, mass transfer limitations may occur [22]. This could be the reason why fixation is not improved. It is more likely that the decrease in the amount of Cr(VI) adsorbed per unit mass of zeolite is due to the aggregation of the particles, which reduces the surface area [23]. This is consistent with our results in Table 2. As the sediment to zeolite ratio increased from 100:1 to 10:1, the particle size increased from 720 to 877 µm for ZP-4A and from 841 to 964 µm for clinoptilolite, respectively.

3.4. Results of Kinetics Study

The results are presented in Figure 4a–c. PFO assumes that the adsorption rate depends solely on the number of unoccupied sites [24]. This kinetic model for zeolite ZP-4A and clinoptilolite did not fit the kinetic data. The PFO kinetic model better described the fixation of Cr(VI) in sediment itself, with R² = 0.989. PFO indicated that the process is controlled by physisorption. An even slightly better agreement was obtained with an intra-particle model. The plot has a zero intercept, indicating that the diffusion of Cr(VI) species into the pores of sediment is the dominant factor controlling the mechanism of the process. The addition of zeolites changed the kinetics. The PSO kinetic model fits to the kinetic data best and describes the fixation of Cr(VI) more accurately. R² was determined to be 0.978.

The PSO adsorption kinetics are used to accommodate some adsorption processes that require longer time to fill the adsorption sites [25]. PSO indicated that the process was in a monolayer and controlled by chemisorption. The kinetic parameters of the intra-particle diffusion model were calculated using Equation (3). The kinetic plots mostly show two linear segments [26]. If the first segment of the plot is linear it suggested the contribution of boundary layer effect. If the second segment is linear it revealed the involvement of intra-particle diffusion in the uptake of Cr(VI). Since there were not two segments, the diffusion-based intra-particle model did not fit to kinetic data of sediment/zeolite mixtures. Several mechanisms are simultaneously active in sediment that are governing the adsorption processes. The observation is in accordance with Chang, in which the kinetics of sediment–Cr (VI) interactions was very complex [27]. On the other hand, in the zeolite mixture, it seemed that PSO is the primary mechanism governing Cr(VI) adsorption [22].

After organic matter enters sediment, it forms heavy metal–organic complexes due to abundant carboxyl, hydroxyl, amine, and other active groups on their surfaces [24]. Organic matter can adsorb onto zeolites, potentially blocking adsorption sites that would otherwise be available for Cr(VI) [28]. This competition can reduce the adsorption capacity of soils for Cr(VI). The organic matter in sediment could also react with Cr(VI) [29]. The hydrogen bonding and hydrophobic and electrostatic forces are involved in the binding process of Cr(VI) and humic compounds [30]. Since humic compounds represent a major part of TOC, the TOC analyses were performed. The results showed very low TOC content as seen from Figure 5. The TOC was measured at 12.4 mg/L in the control sediment sample and decreased to 10.0 mg/L with a sediment/zeolite ratio of 10:1 with both zeolites. The differences in measured TOC are small. It was assumed that they were a result of different ratios of sediment/zeolite. Although the TOC content is very low for Cr(VI), it could be assumed that the organo-complexes in sediment itself were formed due to relatively high fixation determined at 70%. This claim is in agreement with another study, where it was indicated that a weak interaction between Cr(VI) and humic acid exists [30]. Furthermore, not only one kind of interaction force was involved in the binding interaction of Cr(VI) with humic acid, confirming our results of the kinetic analysis of the control sediment sample [30].

3.5. Future Research

Many authors emphasize the importance of the detailed carbonate, sesquioxide, silicate minerals, and TOC analyses in sediments and their interactions with heavy metal ions [31]. Therefore, future research directions may be directed toward detailed protocol analyses in sediments to better understand the metal-binding behavior in sediment. Protocol analysis is a multi-step extraction of the metals that are present in exchangeable and acid soluble (i.e., bound to carbonates), reducible (bound to Fe/Mn oxides), oxidizable (bound to organic matter), and residual forms in the sediment or soil samples.

4. Conclusions

The sampled sediments contained a high concentration of Cr(VI) ions. The contents of Cr(VI) ions in the sampled sediments were, in initial sample, 22.6 ± 0.3 mg/kg. The fixation of Cr(VI) ions was tested at different pHs and temperatures. The fixation proceeded better at acidic pH = 4 with the use of a 100:1 ratio of added zeolite mixture. The fixation efficiency of Cr(VI) ions was 88.0%. The sample with a 10:1 ratio of added zeolite, at acidic pH, showed only a slight improvement of the Cr(VI) fixation at 91.3%. In the sediment without the added zeolite mixture, the fixation of Cr(VI) ions reached 70.0%. The results indicated that sediment fixation tendency was probably due to organo-interfaces. Governing adsorption mechanism in sediments with zeolite was pseudo-second order reaction. The research showed that the use of zeolites for the fixation of Cr(VI) ions is a promising choice for the remediation of contaminated sediments.