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A new adsorbent for boron removal from aqueous solutions a

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Joanna Kluczka , Teofil Korolewicz , Maria Zołotajkin , Wojciech Simka & Malwina Raczek

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Faculty of Chemistry, Department of Chemistry, Inorganic Technology and Fuels , Silesian University of Technology , Gliwice , Poland Accepted author version posted online: 19 Nov 2012.Published online: 04 Jan 2013.

To cite this article: Joanna Kluczka , Teofil Korolewicz , Maria Zołotajkin , Wojciech Simka & Malwina Raczek (2013) A new adsorbent for boron removal from aqueous solutions, Environmental Technology, 34:11, 1369-1376, DOI: 10.1080/09593330.2012.750380 To link to this article: http://dx.doi.org/10.1080/09593330.2012.750380

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Environmental Technology, 2013 Vol. 34, No. 11, 1369–1376, http://dx.doi.org/10.1080/09593330.2012.750380

A new adsorbent for boron removal from aqueous solutions Joanna Kluczka∗ , Teofil Korolewicz, Maria Zołotajkin, Wojciech Simka and Malwina Raczek Faculty of Chemistry, Department of Chemistry, Inorganic Technology and Fuels, Silesian University of Technology, Gliwice, Poland

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(Received 4 March 2012; final version received 5 November 2012 ) A new adsorbent based on natural clinoptilolite and amorphous zirconium dioxide (ZrO2 ) was prepared for the uptake of boron from fresh water. The sorption behaviour of this adsorbent for boron was investigated using a batch system and found to obey Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models. The ZrO2 loading level, pH, temperature, contact time, initial boron concentration and adsorbent dose, on the removal of boron were studied. It was found that the removal of boron increased while the adsorbent dose increased and the temperature decreased at an optimum pH (pH = 8) and a contact time of 30 min. At optimum conditions, the maximum boron percentage removal was 75%. According to the D–R model, the maximum capacity was estimated to be >3 mg B/g of the adsorbent. The adsorption energy value (calculated as 9.13 kJ/mol) indicated that the adsorption of boron on clinoptilolite modified with ZrO2 was physical in nature. The parameters of the adsorption models and the pH investigations pointed to the possibility of a chemisorption process. The thermodynamic parameters (standard entropy S ◦ , enthalpy H ◦ , and free energy G ◦ changes) of boron adsorption were also calculated. The negative value of S ◦ indicated a decreased randomness at the solid–solution interface during the boron adsorption. Negative values of H ◦ showed the exothermic nature of the process. The negative values of G ◦ implied that the adsorption of boron on clinoptilolite modified with amorphous ZrO2 at 25◦ C was spontaneous. It was considered that boron dissolved in water had been adsorbed both physically and chemically on clinoptilolite modified with 30% ZrO2 . Keywords: boron adsorption; clinoptilolite; amorphous zirconium dioxide

Introduction Boron is an essential element for plant growth and human health. In contrast, it was found that there is little difference between boron deficiency and its toxicity levels [1]. A very low boron content is required in irrigation water for certain metabolic activities, but if its concentration is only slightly higher, plant growth will exhibit effects of boron poisoning, which include yellowish spots on the leaves and fruits, accelerated decay and ultimately plant death [2]. Referring to Nable et al. [3], safe concentrations of boron in irrigation water are 0.3 mg/L for sensitive plants, 1–2 mg/L for semi-tolerant plants, and 2–4 mg/L for tolerant plants. Furthermore, high boron levels in drinking water can be toxic to humans (boron has been shown to cause male reproductive obstructions in laboratory animals) [4]. Boron occurs naturally throughout the environment and it is commonly found in water. Its concentration in natural waters is diversified and ranges from 0.007 to 5 mg/L in freshwater to approx. 4 mg/L in seawater [5]. During the production of boron compounds and its applications, such compounds can be introduced into the environment in the form of waste. Boron compounds penetrate into surface and underground waters as well as sea and ocean waters, which might result in its excessive concentration in natural water reservoirs. From 2009 the maximum boron level in drinking ∗ Corresponding

author. Email: [email protected]

© 2013 Taylor & Francis

water for humans is given as 2.4 mg/L (Drinking-Water Quality Committee). The United State Environmental Protection Agency is considering adopting 0.6 mg/L as the standard for boron in drinking water. The European Union regulation suggests a guideline of 1.0 mg B/L in drinking water. In Poland, the boron level has to be limited to 1 mg/L in the case of water discharged to the environment. The above limits cause serious shortcomings when treating boron-containing water. For boron removal, the main processes that have been studied are: (1) coagulation and coprecipitation [6–8], (2) ion-exchange using boron-selective ion-exchange resins [9–11], (3) solvent extraction [12,13], (4) membrane technologies [14–16] and (5) adsorption on activated carbon [17–20]. Among them, the use of boronselective resins is the most efficient method, but in contrast it is not economical because of high regeneration costs and expensive resins [21,22]. The adsorption methods using natural sorbents, biosorbents or wastes as a sorbent seem to have a future. In recent literature new adsorbents such as fly ash [23,24], clay [25,26] and some inorganic materials and wastes [27–30] were used to remove boron from aqueous solutions. Natural zeolites are potential adsorbents because of their high capacity and low cost. These adsorbent materials have, in their internal structure, channels and cavities interconnected of molecular dimensions where counter

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cations exist, allowing the ion-exchange. Moreover, zeolites can be modified by the introduction of new functional groups in order to improve its activity and selectivity for the removal of several substances [31,32]. Clinoptilolite being the most common natural zeolite belongs to the heulandite family or a structural variation of the zeolite mineral group and has the following total chemical formula (Na,K,Ca)4 Al6 Si30 O72 ·24H2 O. The effect of particle size and application rate of clinoptilolite on boron adsorption in sewage sludge amended soil was examined by Öztürk et al. [33]. They concluded that clinoptilolite has a high potential for preventing boron leaching. Kavak investigated boron removal from aqueous solution using clinoptilolite by batch adsorption methods [34]. The best adsorption results of 10 mg/L of boron were found at 25◦ C, pH 7 and 1 g of clinoptilolite per 25 mL solution. At these conditions the boron adsorption percentage was approximately 61%. Previously, several works relating to boron-containing wastewater treatment using zirconium hydroxide prepared using the sol–gel method have been cited in the literature [35,36]. Ryabinin et al. [37] studied the sorption of boron on granulated zirconium hydroxide prepared using a freezing technique. They showed that the maximum adsorbed amount of B2 O3 from seawater was 8 mg/g ZrO2 . The high capacity values were also observed for the boron ions adsorbed by zirconium hydroxide impregnated on the anion exchanger [38,39]. Although compounds of Zr(iv) are efficient precipitants of borate, their cost would most probably preclude their use in an industrial scale [40]. In the present study, the removal of boron from water by adsorption using a new adsorbent was investigated for the purpose of treating freshwater polluted with boron compounds. For adsorbent preparation, clinoptilolite and ZrO2 were used. The preliminary experiments showed that amorphous ZrO2 had the ability to adsorb boron in contrast to the crystalline form of ZrO2 , which showed no affinity for oxoborate ions [41]. Therefore in the present study, amorphous ZrO2 prepared in our laboratory was used for the first time as modifier of clinoptilolite to adsorb boron. The influence of the composition and dosage of the new adsorbent, boron concentration, pH, time, and temperature on boron adsorption in the batch system have been examined.

Experimental Materials A basic standard solution of boron in the form of borax 1 g B/L; oxalic acid, analytical grade, zirconyl chloride, analytical grade; sodium hydroxide, analytical grade solution concentration of 0.1 mol/L, hydrochloric acid, analytical grade solution concentration of 1 mol/L. All reagents were analytically pure and supplied by POCh situated in Gliwice (Poland). Brenntag Polska provided the natural zeolite (95% clinoptilolite). The characteristics and the results of analysis of the adsorbent are given in the Table 1.

Table 1. Technical (Brenntag Polska).

specification

of

Parameter Crystallinity (%) Brightness (MgO 550 nm) (%) Content of water (1 h at 800◦ C) (%) Ion-exchange capacity (mg CaO/g dry) Content of Na2 O (%) Content of Al2 O3 (%) Content of SiO2 (%) Iron total content (ppm) Free iron total content (ppm) pH (1% suspension) Density (g/dm3 ) 15 μm Mean particle size in 50% (μm) Sieve residue 45 μm (%) BET surface area (m2 /g) Grain size distribution (%):

clinoptilolite Specification ≥95 ≥92 18–22 ≥160 Type 22 Type 36 Type 42 ≤120 ≤10 10–12 200–500 ≤3 ≤8 ≤2.5 ≤6.0 ≤0.5 34.88

Figure 1. Percentage adsorption of boron as a function of adsorbent amount and composition, initial boron concentration: 10.0 mg/L, solution volume: 50 mL, temperature: 25◦ C, adsorption time: 24 h, pH = 9.

Clinoptilolite modified with zirconium dioxide was used as an adsorbent in the present study. ZrO2 was produced in our own laboratory. In the first step, zirconyl oxalate was precipitated following ammonium oxalate and zirconyl chloride mixing in the molar ratio of 1.1 to 1. In the second step, the mixture was dried at room temperature and heated at 420◦ C for 1 h in muffle furnace [42]. In this way an amorphous ZrO2 was prepared, see the upper layer of adsorbent in Figure 1. After powdering clinoptilolite with amorphous ZrO2 in an agate mortar for 0.5 h, the preparation of the adsorbent was complete. Brunauer–Emmett–Teller

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(BET) analysis of the adsorbent was performed. In the experiments several weight ratios of clinoptilolite to amorphous ZrO2 were used: 9 : 1; 7 : 3; 5 : 5, 3 : 7 (clinoptilolite with 10% ZrO2 , 30% ZrO2 , 50% ZrO2 and 70% ZrO2 ). The adsorbents were dried for 2 h before the adsorption experiments.

on the adsorbent used in the present study. The Langmuir model assumes that the adsorption occurs in surface sites where the energy is equal in each size. The Langmuir equation is given by Equation (2):

Apparatus and laboratory equipment A spectrophotometer CARY 50 SCAN (VARIAN), a scanning electron microscope Hitachi S-3400 N (Hitachi High-Technologies, Thermo Noran), ASAP 2020 automatic adsorption/chemisorption analyzer (Micromeritics), HTL micropipettes (Dom Handlowy Nauki, Cracow); a WU-4 universal shaker (PREMED, Warsaw); a WPE 120 electronic balance (Radwag, Radom); an analytical balance WPA 60/C (Radwag, Radom); a test-tube centrifuge MPW350 (MPW Med. Instruments), a drier Promed KBC G100/250 (MPW Med. Instruments); a MILL-547 shaker with heating bath (AJL ELEKTRONIC), pH-meter Basic 20+ (CRISON); laboratory glassware and small equipment: conical flasks with ground glass joint, measuring flasks, beakers, chemical funnels were used in this work.

where qm and B are the Langmuir parameters, qm is the adsorption capacity [mg/g], expressed as the maximum amount of boron that can be adsorbed by the adsorbent as a monolayer, and B is an equilibrium constant that corresponds to the adsorption energy [L/mg]. The Freundlich model allows for several kinds of adsorption sites in the solid, each having a different energy of adsorption. The Freundlich model is usually applied to adsorption processes on heterogeneous surfaces. The Freundlich isotherm is represented by Equation (3):

Methods A series of batch-mode adsorption studies were conducted to evaluate the effects of the constitution and dose of adsorbent, initial boron concentration, pH, temperature and time on boron adsorption. The experiments were carried out with 0.1–2 g of adsorbent and 50 mL of boron solution of concentration 5–50 mg B/L at pH 2–10 at 25◦ C and 45◦ C and 80◦ C for 5–1440 min in 250 mL conical flask with a ground glass joint. The boron solution and adsorbent were shaken at 120 rpm mixing rate in a mechanical shaker. At the end of the experiment, the suspension was centrifuged and filtered through medium paper filters. The filtrate was analyzed for boron concentration by ultravioletvisible (UV-vis) spectrometry using Azometyne H [43]. Each adsorption experiment was repeated three times in order to have average values. Adsorption isotherms The boron adsorption (q) per unit mass of adsorbent [mg/g] was calculated from the experimental data in each sample according to Equation (1): q=

c0 − c V0 m

(1)

where c0 is the initial concentration of boron in the solution [mg/L], c is the final concentration of boron in the solution [mg/L], V0 is the volume of the solution [L], and m is the mass of the adsorbent [g]. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherms were checked for the adsorption of boron

q = qm

B·c 1+B·c

q = Kc1/n

(2)

(3)

where the parameters K [mg/g] and n correspond to the relative adsorption capacity and the adsorption intensity of the adsorbent, respectively. Both Langmuir and Freundlich data fitting was done by linearization of Equations (2) and (3) as shown by Equations (4) and (5), respectively: 1 1 1 = + q q m · B · c qm

(4)

log q = log K + 1/n log c

(5)

Plotting 1/q versus 1/c gives a curve with inclination 1/qm and an intersection 1/B · qm . Plotting log q versus log c results in a straight line with inclination 1/n and an intersection log K. The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL , which is defined by Equation (6): RL =

1 1 + Bc0

(6)

According to Hall et al. [44], the parameter RL indicates the shape of the isotherm accordingly: RL > 1, unfavourable; RL = 1, linear; 0 < RL < 1, favourable; and RL = 0, irreversible. Similarly, the fitness of using the Freundlich equation to describe the adsorption can be assessed by the constant, n. If 1 < n < 10, the Freundlich equation is adequate for use [45]. In order to explain the adsorption type, the equilibrium data was applied to the D–R isotherm. The D–R isotherm is given by Equation (7): q = xm exp −(kε 2 )

(7)

where ε is the Polanyi potential, which is equal to RT ln(1 + 1/c), xm is the adsorption capacity [mol/g], k is a constant

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related to the adsorption energy [mol2 /kJ2 ], T is the temperature [K], and R is the gas constant [kJ/molK]. This expression can be linearized as shown in Equation (8): ln q = ln xm − kε 2

(8)

The xm and k values were obtained by plotting ln q versus ε2 at various temperatures. The adsorption energy (the energy required to transfer 1 mol of adsorbate species to the surface of the adsorbent from infinity in the bulk of the solution) was obtained from Equation (9):

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E = −(2k)−0.5

(9)

If E is less than 20 kJ/mol, the adsorption is physical in nature due to weak van der Waals forces. The energy for chemisorption lies in the range 40–800 kJ/mol [46]. Thermodynamics of adsorption In order to understand the effect of temperature on the adsorption process thermodynamic values (standard entropy S ◦ , enthalpy H ◦ , and free energy G ◦ changes) were calculated. The molar free energy change of the adsorption process is related to the equilibrium constant (Kc ) and calculated from Equation (10): G ◦ = −RT ln Kc

(10)

where R is the gas constant (8.314 J/molK), T is the temperature [K]. The Kc values were estimated (Equation (11)) as: Kc = (c0 − c)/c (11) Each Kc value was the average of all experimental values (c0 and c) obtained at a constant temperature at which the adsorption experiments were carried out. H ◦ and S ◦ of adsorption can be estimated using Equation (12): ln Kc = −H ◦ /RT + S ◦ /R

(12)

Plotting ln Kc versus 1/T produces a straight line with a slope of −H ◦ /R and an intersection of S ◦ /R. Results and discussion Efficiency of clinoptilolite in boron adsorption The samples of zeolite modified with ZrO2 (natural clinoptilolite loaded with 10, 30, 50 and 70% of amorphous ZrO2 ) were tested for their boron adsorption efficiency. As can be seen from Figure 1, the percentage of boron adsorption dramatically increased until ZrO2 content increased to 30% (clinoptilolite with 30% ZrO2 ). However, increasing the ZrO2 content further had little effect in the enhancement of the boron adsorption. It was also determined that unmodified clinoptilolite had little boron adsorption and ZrO2 without clinoptilolite had

Figure 2. Scanning electron microscope image of clinoptilolite modified with amorphous ZrO2 .

less boron adsorption as compared to the new adsorbent based on clinoptilolite and ZrO2 . Hence for economical purposes, clinoptilolite with 30% ZrO2 was used in the subsequent investigations. The BET data (determined on the basis of BET instrument Micromeritics ASAP 2020 V3.01 H) showed the increasing of the BET surface area of the clinoptilolite after its modification with 30% of ZrO2 . The BET surface area, pore volume and average pore size were found to have values of 81.46 m2 /g, 0.135 cm3 /g and 2.248 nm, respectively. From the scanning electron microscope image (Figure 2) it can be seen that the clinoptilolite is an amorphous ZrO2 carrier.

Effect of pH on adsorption of boron The pH value of the solution was an important controlling parameter in the adsorption process. The form of boron in solution depends strongly on the pH and takes the form of B(OH)3 at low pH or B(OH)− 4 at high pH. The adsorption of boron as a function of pH for clinoptilolite modified with 10% ZrO2 30% ZrO2 and unmodified clinoptilolite at 25◦ C, with an adsorbent concentration of 20 g/L and an initial boron concentration of 10 mg/L (0.995 mmol/l) and adsorption time of 24 h is presented in Figure 3. The results show that boron removal by all adsorbents depended on the pH of the solution. However, a more significant influence of pH is evident for clinoptilolite modified with high concentrations of ZrO2 than for clinoptilolite modified with lower concentrations of ZrO2 or without ZrO2 . This can be explained by the possibility ZrO2 reacting with H3 BO3 , the species which is the dominant form of boron in aqueous solution at pH values of less than 9, and the formation of sparingly soluble and stable compounds, e.g. Na[ZrO(OH)x (B4 O7 )n ] [36]. It has been found that the maximum adsorption of boron takes place in a pH range of 6–9; and the most favourable pH = 8. Decreased adsorption values were observed at lower pH values. The amount of boron removed was higher in the case of modified clinoptilolite.

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Figure 3. Percentage adsorption of boron as a function of pH, temperature: 25◦ C, adsorbent concentration: 20 g/L, initial boron concentration: 10 mg/L, adsorption time: 24 h.

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Figure 4. Adsorption kinetics of boron on unmodified clinoptilolite and clinoptilolite modified with ZrO2 , temperature: 25◦ C, adsorbent concentration: 20 g/L, initial boron concentration: 10 mg/L, pH = 8.

When the pH = 8, boron removal by clinoptilolite modified with 30% ZrO2 increased to about 60%. Therefore, all adsorption experiments were conducted at pH = 8. Boron removal by unmodified clinoptilolite reached to maximum values of about 13% at the same pH values. Effect of temperature on adsorption of boron S ◦ and H ◦ of the adsorption of boron on clinoptilolite modified with ZrO2 were calculated as, S ◦ = −0.10 kJ/mol K and H ◦ = −31.31 kJ/mol, respectively. The negative value of S ◦ indicated a decreased randomness at the solid–solution interface during the adsorption of boron on the adsorbent. The negative value of H ◦ showed the exothermic nature of the process. As can be seen from Table 2, boron adsorption and Kc (equilibrium constant) values decreased with an increase in temperature. In contrast, the G ◦ values increased when the temperature increased. The negative values of G ◦ implied that the adsorption of boron on modified clinoptilolite was spontaneous. The negative S ◦ values correspond to a decrease in the degree of freedom of the adsorbed species. In addition to this, since the values of G ◦ increased with an increase in temperature, the spontaneous nature of adsorption can be said to be inversely proportional to the temperature. Effect of adsorption time The effect of time on the boron adsorption was studied using 1 g of unmodified clinoptilolite and clinoptilolite modified

Figure 5. Isotherms of boron adsorption by unmodified clinoptilolite and clinoptilolite modified with ZrO2 , temperature: 25◦ C, adsorbent concentration: 20 g/L, initial boron concentration: 5–50 mg/L, adsorption time: 30 min, pH = 8.

with ZrO2 and an initial boron concentration of 10 mg/L at pH = 8, 25◦ C and a shaking time of 5 min to 8 h (Figure 4). The maximum boron adsorption was reached after 30–60 min for clinoptilolite with 10% ZrO2 while the adsorption–desorption equilibrium was attained after about 2 h. There was almost no change in the adsorption during 2–8 h. 30 min was accepted as the optimum contact time.

Table 2. Thermodynamic parameters of boron adsorption on clinoptilolite modified with ZrO2 at different temperatures. Temperature (◦ C) 25 50 80

Boron adsorption (%)

Kc

G ◦ (kJ/mol)

H ◦ (kJ/mol)

S ◦ (kJ/mol K)

64.15 38.23 19.97

1.79 0.58 0.25

−1.44 1.46 4.07

−31.31

−0.10

7 × 10−9 7 × 10−9 6 × 10−9 5 × 10−9 0.447 2.069 3.044 1.705 0.9334 0.9947 0.9645 0.9709 1.885 1.656 1.812 2.020 0.019 0.061 0.139 0.117 0.9398 0.9906 0.8577 0.9324 0.569 0.602 0.351 0.217 0.0759 0.0662 0.1852 0.3607 0.1606 0.6731 0.8081 0.4643 0.9448 0.9956 0.9665 0.9633 20 20 20 40

qm (mg/g) B (L/mg) RL C0 = 10 mg/L Concentration of adsorbent (g/L) Adsorbent

Clinoptilolite Clinoptilolite with 10% ZrO2 Clinoptilolite with 30% ZrO2 Clinoptilolite with 30% ZrO2

xm (mol/g) K (mol2 /kJ2 ) K (mg/g)

n

R2 Freundlich parameters Langmuir parameters

R2 R2

Langmuir, Freundlich and D–R isotherm parameters for unmodified clinoptilolite and clinoptilolite modified with ZrO2 at 25◦ C.

Adsorption isotherms The study of the adsorption isotherm is helpful in determining the maximum adsorption capacity of adsorbate for a given adsorbent and in explaining the mechanism of adsorption. Correlations between the equilibrium adsorption of boron (the amount of boron adsorbed per unit mass of the adsorbent), qe [mg/g], and the residual boron concentration in the solution, ce [mg/L], are graphically depicted in Figure 5. The uptake of boron was studied in the concentration range 5–50 mg/L while the concentration of clinoptilolite was held constant at 20 g/L. The adsorption isotherms were regular, positive and concave to the concentration axis for unmodified and modified with ZrO2 clinoptilolite. As can be seen in Figure 5, the isotherms for clinoptilolite (unmodified) and modified with 10% ZrO2 can be classified as the L-type according to the classification of Giles et al. [47]. In the L-type isotherm, the amount of the adsorbed component increases steadily with concentration until a plateau is reached where the surface of the adsorbent is practically saturated. No further adsorption occurs at this stage. L-type behaviour can generally be explained with the Langmuir isotherm. For the clinoptilolite modified with 30% ZrO2 , the adsorption was rapid and increased even at high concentrations. In the present study the applicability of the Langmuir, Freundlich and D–R isotherms were studied. The data obtained from the adsorption experiments was fitted into the linearly transformed Langmuir, Freundlich and D–R isotherms. The parameters obtained from the linear fits and correlation coefficients obtained at 25◦ C are presented in Table 3. It has been found that the experimental data satisfy the Langmuir, Freundlich and D–R models. The Langmuir isotherm provided excellent correlation of the experimental equilibrium data, yielding correlation coefficient values of R2 = 0.99 and 0.96 for clinoptilolite with 10% ZrO2 and clinoptilolite with 30% ZrO2 , respectively. The calculated Langmuir parameters for the maximum capacity (qm ) and B were 0.67 mg/g and 0.06 L/mg, 0.81 mg/g and 0.19 L/mg for clinoptilolite with 10% ZrO2 and clinoptilolite with 30% ZrO2 , respectively. The equilibrium constant B corresponds to the adsorption energy and was higher for clinoptilolite with 30% ZrO2 than the other adsorbents. The calculated equilibrium parameters RL (for c0 = 10 mg/L) in the range 0–1 indicate favourable adsorption for boron on the clinoptilolite with 10% ZrO2 and clinoptilolite with 30% ZrO2 . The Freundlich model is less suitable than the Langmuir model for the representation of the adsorption data, as reflected by the lower correlation coefficients (R2 ) obtained from the linear fits of the data in all cases. The values of the Freundlich constants, K and n, were 0.06 mg/g and 1.6, 0.14 mg/g and 1.8, respectively. The values of n > 1 reflects the occurrence of physical nature of adsorption. According to Treybal [45] n values between 1 and 10 represent beneficial adsorption. The value of K corresponds

D–R parameters

J. Kluczka et al.

Table 3.

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Figure 6. Effect of adsorbent dosage for boron adsorption, temperature: 25◦ C, adsorbent concentration: 10–40 g/L, initial boron concentration: 10 mg/L, adsorption time: 30 min, pH = 8.

to the maximum capacity of the clinoptilolite sites and indicates that efficiency of adsorption increases with the increase of ZrO2 in the clinoptilolite. The equilibrium adsorption data was fitted to the linear model of the D–R equation. In all modified clinoptilolite materials, the correlation coefficients (R2 ) for the linear regression fits were found to be >0.96. The values of the parameter xm (maximum amount of boron that can be adsorbed by the clinoptilolite) were estimated to be 2 and 3 mg/g of the clinoptilolite with 10% ZrO2 and clinoptilolite with 30% ZrO2 , respectively. The adsorption energy values were calculated as 8.45 and 9.13 kJ/mol, which indicate that the adsorption of boron on clinoptilolite modified with ZrO2 is physical in nature. However, the isotherm models and pH point to the possibility of a chemisorption process (due to the reaction of ZrO2 with H3 BO3 present in solution) and the formation of sparingly soluble compounds, e.g. Na[ZrO(OH)x (B4 O7 )n ]. Thus, boron dissolved in water has been adsorbed both physically and chemically on clinoptilolite modified with 30% of ZrO2 . Effect of adsorbent dosage To investigate the effect of adsorbent dosage on boron removal, the concentrations of clinoptilolite modified with ZrO2 were changed to between 10–40 g/L while the solution concentration of boron and pH were held constant at 10 mg/L and 8, respectively. The effect of adsorbent dosage on boron removal is shown in Figure 6. The adsorption of boron increased with an increase in the adsorbent dosage. The boron removal efficiencies varied from 33 to 61% using clinoptilolite with 10% ZrO2 and from 46 to 75% using clinoptilolite with 30% ZrO2 . An increase in adsorption with the adsorbent dosage can be attributed to greater surface area and availability of more adsorption sites. Conclusion A clinoptilolite-type natural zeolite was modified using amorphous ZrO2 and used as a novel adsorbent for boron

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removal. The results showed that temperature, pH, and mass of adsorbent affected the boron removal by adsorption. The boron removal increased with an increase in pH (until pH = 9) and adsorbent dosage, but decreased with an increase in temperature. The optimum conditions were found as pH = 8, 25◦ C, contact time = 30 min, and adsorbent dosage = 2 g/50 mL solution. At these conditions, the boron adsorption percentage was approximately 75% using clinoptilolite modified with 30% ZrO2 in contrast to 13% of boron adsorption for unmodified clinoptilolite. The boron adsorption into clinoptilolite modified with ZrO2 can be explained using the linear Langmuir, Freundlich and D–R isotherm models. These models fit well for the experimental results and suggest that the boron adsorption on clinoptilolite modified with ZrO2 is favourable. Moreover, the results indicate that the mechanism of boron on clinoptilolite modified with 30% of ZrO2 is both physical and chemical in nature. The thermodynamics studies indicate a spontaneous and exothermic process of adsorption. The experimental results showed that this kind of new adsorbent had a higher adsorption amount of boron than the precursor. The pH of the suspension, content of ZrO2 in clinoptilolite and dose of the adsorbent are three critical factors in determining the removal efficiency. The efficient adsorption takes place in the pH 6–9 range; the maximum adsorption of boron on clinoptilolite modified with 30% of ZrO2 takes place at pH = 8. This indicates that boron has been removed by chemisorption following the physisorption either as B(OH)3 or as B(OH)− 4 . As a result, sparingly soluble and stable compounds, e.g. Na[ZrO(OH)x (B4 O7 )n ] have been formed in the clinoptilolite. Furthermore, the adsorption of boron is not only due to the distribution of boron species but also to the type and number of active sites that may vary with changing pH, composition of solution matrix, and surface properties of the solid. The experiments should also be performed by using original wastewater. References [1] J.L. Parks and J.L. Edwards, Boron in the environment, Environ. Sci. Technol. 35 (2005), pp. 81–114. [2] A. Kabata-Pendias and H. Pendias, Biogeochemistry of Trace Elements, PWN, Warsaw, 1999 (in Polish). [3] R.O. Nable, G.S. Banuelos, and J.G. Paull, Plant and Soil, Kluwer Academic Publishers, The Netherlands, 1997. [4] E. Weinthal, Y. Parag, A. Vengosh, A. Muti, and W. Kloppmann, The EU drinking water directive: The boron standard and scientific uncertainty, Eur. Environ. 15 (2005), pp. 1–12. [5] B.J. Alloway and D.C. Ayres, Chemical Principles of Environmental Pollution, Stanley Thornes, London, UK, 1998. [6] P. Remy, H. Muhr, E. Plasari, and I. Ouerdiane, Removal of boron from wastewater by precipitation of a sparingly soluble salt, Environ. Progress 24 (2005), pp. 105–110. [7] M. Turek, D. Dydo, J. Trojanowska, and A. Campen, Adsorption/co-precipitation reverse osmosis system for boron removal, Desalination 205 (2007), pp. 192–199. [8] A.E. Yilmaz, R. Boncukcuo˘glu, and M.M. Kocakerim, A quantitative comparison between electrocoagulation and

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