Sodium-Modified Vermiculite for Calcium Ion Removal from Aqueous

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Sodium-Modified Vermiculite for Calcium Ion Removal from Aqueous Solution Roberto R. C. Lima,* Paulo Douglas S. de Lima,* Vitor Rodrigues Greati,* Paulla B. F. de Sousa, and Gabriel V. S. Medeiros

Downloaded by BUFFALO STATE at 16:09:18:127 on May 24, 2019 from https://pubs.acs.org/doi/10.1021/acs.iecr.9b00045.

Laboratório de Pesquisa em Recursos Naturais, Instituto Federal de Educaçaõ , Ciência e Tecnologia do Rio Grande do Norte, Rua Brusque, Potengi, Natal, 59112-490, Brazil

ABSTRACT: The treatment and recuperation of hydric resources portray recurrent challenges and demand alternative processes and technologies. The removal of hardness and metal ions from water are necessary in certain sources of human consumption and for industrial purposes. In this work, sodium-modified vermiculite was investigated toward the removal of calcium ions in aqueous medium. The effects of adsorbent dosage, contact time, initial concentration of calcium ions, initial pH, and temperature were systematically studied for both raw and modified vermiculite. pH 10 was shown to be the most appropriate to conduct the experiments. Raw, sodium-modified, and postadsorption vermiculite were characterized by microstructure analysis (scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray fluorescence, and X-ray diffraction). The maximum adsorption capacity was found in sodium-modified vermiculite, resulting in about 80 mg g−1. Kinetic studies were carried out to relate the experimental data to pseudo-first-order, pseudo-second-order, and Elovich models. Isotherm models (Freundlich, Langmuir, and Redlich−Peterson) were employed to describe the softening process, and thermodynamic parameters ΔG°, ΔH°, and ΔS° were determined. Results revealed a spontaneous endothermic adsorption process with pseudo-second-order kinetics and corroborated the sodium-modified vermiculite adequacy for water softening.

■

INTRODUCTION

water treatment efforts. Additionally, the severe eventual droughts in those places aggravate such problems,4 so available water is of poor quality and in low quantity. Such inconveniences along with the considerable demand surrounded by sensitive social aspects justify the pursuit toward water softening. A common approach for hardness control consists in precipitation using lime soda. Despite its highly available sources, easiness of operation, and prominent hardness removal efficiency, this method generates high amounts of sludge with high water content, which may cause environmental damages and increase the costs due to post sludge-dewatering processes.5,6 Another process for water softening is electrochemical precipitation, which works by generating a high pH environment

The hardness property of water refers to the presence of divalent cations, mainly calcium, magnesium and manganese. Hardness levels between 120 and 180 mg L−1 are considered high and above 180 mg L−1 are very high. Such concentrations are not desirable for general domestic consumption.1 Reasons regarding this observation comprise a set of adverse effects caused by the excess of the cited ions, such as incrustations in boiler and household facilities, bad influence on detergent’s cleaning capacity and the formation of precipitates, which may cause, among others, stains in dishes and clothes, decrease of fluid rate, and bursting of water pipelines. Noticeably, these effects can also compromise industrial scenarios. This whole set of negative facets establishes high water hardness levels as a worldwide concern.2,3 In particular, some semiarid regions in northeastern Brazil are served exclusively with hard water extracted from wells. Also, water supply systems in such localities have to deal with excessive hardness levels, increasing maintenance costs, and © XXXX American Chemical Society

Received: January 3, 2019 Revised: April 6, 2019 Accepted: May 20, 2019

A

DOI: 10.1021/acs.iecr.9b00045 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

procedures are not only recommended, but generally necessary.10 They fundamentally consist in putting the clay sample in contact with a saline solution, in order to saturate the clay sites with cations of interest, such as sodium or lithium. Vermiculite has been applied for heavy metals removal,23,24 but research on calcium ion adsorptionand hence in water softeningis scarce in the current literature. In view of this, the present work contributes with an investigation of the sodiummodified vermiculite capacity for calcium adsorption by means of equilibrium, kinetics, and thermodynamics studies under different operational conditions. The modified clay was fully characterized, and batch experiments were conducted for measuring its adsorptive performance. Results asserted the feasibility of sodium-modified vermiculite for water softening with respect to calcium ions.

at a cathode surface, inducing the precipitation of the hardness components. The main benefits include the independence of additional chemicals and environmental friendliness.7 Nevertheless, the maintenance of the cathodes, due to the adherence of most of the deposits in them,8 demands considerable costs and increases the complexity of the method, making it unsuitable for simpler, nonindustrial contexts, such as household facilities. Besides those two techniques, adsorption has been extensively studied as a framework for the application of various materials as adsorbents in water softening processes. Main reasons for its adoption are the relatively low costs, flexibility, and simplicity in process design, operations, and maintenance.9,10 These advantages depend mainly on the adopted adsorbent, and the main requisites to guide this choice are high efficiency, low cost, and eco-friendliness.11 Among the low-cost adsorbents, natural materials and agricultural wastes have been extensively investigated.9 Various examples can be found in the literature: wasted polystyrene converted into adsorbent by heterogeneous sulfonation;12 sugarcane bagasse and coffee husk modified with alkali activated carbon;13 natural and alkaline modified pumice stones;14 and pine cone treated with citric acid and saponification.15 Recently, clay minerals have also been considered low-cost adsorbents for hardness ion adsorption.3 In this context, vermiculite represents a promising candidate for such an application, presenting the common advantageous features of adsorbents based on clay minerals,16 such as high availability,17 low cost, low hazardousness, chemical versatility, eco-friendly nature, and high potential for repeatable reuse by regeneration.18,19 Regarding its formation and structure, vermiculite is an expandable 2:1 phylosilicate formed as a product of modifications on biotite or phlogopite, by means of intemperism or hydrothermic reactions. Vermiculite presents a high permanent negative charge due to substitutions of both Al3+ and Si4+ for Mg2+/Fe2+ in the octahedral sheet and Al3+ in the tetrahedral sheet, respectively. Also, the layers in vermiculite’s structure are continuous in the width and length directions, but the weak bonds between them allow the intercalation of water and other molecules in the existing interlayer space.19 The literature presents different chemical formulas for vermiculite, being (Na0.21, K0.39, Mg0.19, Ca0.13, 6H2O)(Mg5, Fe0.22+, Fe0.83+){Si5.5, Al2.5, O20}(OH)4 a common one. The first segment represents the ion permutable layer, the second describes the cations in the octahedral sheet, and the last are the ones of the tetrahedral sheet.20 Vermiculite’s high adsorption capacity results from its large surface area and numerous reactive sites, enabling it as an ionic permutator of metallic ions in aqueous solutions. In nature, the clay’s permanent negative charge is counterbalanced by the Ca2+ and Mg2+ cations in the interlayer regions and basal sites, where cation exchange and complexation occur during metal adsorption. In addition, in vermiculite’s crystal edges, hydroxyl groups formed by Al and Si hydrolysis can interact with metallic cations resulting in complexes.21 The first one, cation exchange, is the main adsorption mechanism in this case, by which cations in aqueous solution adhere to the clay’s surface and those like Ca2+, K+, Na+, and Mg2+ present in the clay are released to the solution. The contribution of each clay site for the global adsorption capacity highly depends on the solution’s pH.17 In order to best explore vermiculite’s capacities for specific applications, chemical and physical treatments are commonly employed.22 When envisioning adsorption, modification

■

EXPERIMENTAL SECTION Materials. Concentrated vermiculite (CVT), the precursor material used in this work, was provided by the company Brasil Minérios (Goiás, Brazil). The purity of CVT is between 85 and 95%, and most of its particle sizes (60−90%) were around 0.5 μm (35 mesh). In addition, its chemical formula is (Mg,Fe3+,Al)3(Si,Al)4O10(OH)2·4H2O. The preparation of the synthetic calcium standard solution employed CaCO3 (Isofar 90%) and HCl (Atriom 37%). The solution used to adjust the pH of the calcium solutions used NH4OH (Vetec 28−30%). The cation exchange capacity (CEC) determination employed BaCl2·2H2O (Dinâmica 99%), H2SO4 (Vetec 95−99%), and NaOH (Impex 99%). Vermiculite modification used NaCl (Vetec 99%). Adsorption experiments employed EDTA AR (CRQ). Preparation of Calcium Standard Solutions (CSS). To simulate water hardness, synthetic calcium standard solutions (CSS) Ca2+ were prepared by dissolving dry CaCO3 in 1 mol L−1 HCl solution with subsequent boiling and adding bidistilled water. The pH adjustment was done by adding concentrated NH4OH solution or 1 mol L−1 HCl solution. Vermiculite Modification. The sodium-modified vermiculite (CVT-Na) preparation was based on Zang et al.25 Initially, 20 g of CVT was dispersed in 200 mL of 1 mol L−1 NaCl solution and stirred for 60 min in a magnetic stirrer at room temperature. In order to guarantee complete clay saturation with Na+ ions, this procedure was carried out in triplicate. After that, the vermiculite was washed three times with bidistilled water, by sedimentation and decantation. The vermiculite was then transferred to a melting pot and dried in a furnace at 373 K for 24 h. Finally, the resultant modified vermiculite was carefully pulverized to pass through a 16 mesh sieve. Determination of Surface Charge Properties. Variations in the surface charge properties of CVT and CVT-Na were determined by zeta potential (ZP) measurements (Zeta-Meter Inc., USA), as a function of the pH. For this purpose, based on Nunes et al.,26 0.5 g of both clays were dispersed in 50 g of 0.001 mol L−1 NaNO3 (Synth) solution under ultrasonic treatment (20 min) at room temperature. The sample was kept at rest for 24 h and ZP measurements were performed in a range of pH 2− 10. The initial pH was adjusted by 0.01 mol L−1 HNO3 (Neon) and NaOH solutions. For each pH value, 10 measurements were computed and average data was reported. CEC Determination. The CEC measurements were based on the barium chloride (BaCl2) method25,27 with a slight modification. Initially, 0.5 g of both CVT and CVT-Na were centrifuged with 10 mL of 0.1 mol L−1 BaCl2 solution for 5 min B


Article


In addition, the effect of the initial concentration was also verified by 500, 800, 1000, 1500, and 2000 mg L−1 CSS in contact with CVT and CVT-Na on optimal operating parameters (adsorbent mass, time, and pH) previously determined. In the isotherm experiments, the initial concentration of the calcium solution was the same utilized at the optimum condition mentioned above at temperatures of 298, 308, and 318 K. After each test, the solution was filtered in 2.0 μm quantitative filter paper (Nalgon, Germany) to separate adsorbent and adsorbate. The concentration of filtrate was determined by standard EDTA titration method.28 The EDTA volume was measured with 0.05 mL of precision. The adsorptive removal efficiency R (%), as well as the calcium uptake at equilibrium, qe (mg g−1), and that at time t, qt (mg g−1), were calculated by the following equations:

at 2000 rpm. The supernatant was discarded thereafter. This procedure was conducted in quintuplicate in order to completely exchange the cations of clay for Ba2+ ions. After that, 10 mL of 0.1 mol L−1 H2SO4 solution, previously titrated by standard 0.1 mol L−1 NaOH solution, was centrifuged with the clay under the same conditions. Finally, the supernatant was titrated and the CEC (mmol/100 g) values were determined by the following equation: CEC =

2C(V1 − V2) 5 ·10 m

(1) −1

where m (g) is the mass of clay, C (mol L ) is the concentration of NaOH solution used in titration, and V1 (L) and V2 (L) are the volumes of NaOH consumed in H2SO4 titration and the supernatant, respectively. The experiment was conducted in triplicate, and standard deviation (SD) was considered as a statistical parameter. Sodium-Modified Vermiculite Characterization. The X-ray diffraction (XRD) data of samples were analyzed by a diffractometer (D2 PHASER Bruker), with the 2θ angle ranging from 5 to 80° at a scanning rate of 5 deg min−1, using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 40 mA to analyze the interlayer space. The samples were introduced in a rectangular aluminum holder to fill the cylindrical unevenness with 2 cm diameter and 1.5 mm depth, leveled with glass plate. The interplanar distances, mainly those related to inflection peaks d(002), were calculated according to Bragg’s condition: mλ = 2d sin θ

R=

C0 − Ct ·100% C0

(3)

qe =

C0 − Ce V m

(4)

qt =

C0 − Ct V m

(5)

−1

−1

where C0 (mg L ) and Ct (mg L ) are the initial concentration and the concentration at time t, respectively. m (g) is the mass of CVT or CVT-Na, and V (L) is the volume of the solution. Statistical Analysis. EDTA titration was carried out in triplicate for each adsorption experiment. The average of each triple was considered. The criteria for deciding the best-fit model in the isotherm studies were the coefficient of determination (R2, eq 6) and the chi-squared test (χ2, eq 7):

(2)

where d is the interplanar distance, θ is the angle between the incident beam and the reflecting surface, λ is the wavelength of the X-ray beam, and m is the diffraction order. The clay’s chemical composition was determined using X-ray fluorescence (XRF) by dispersive energy in a spectrometer (Shimadzu EDX-720). The infrared spectra were recorded using the attenuated total reflectance (ATR-FTIR) technique with ZnSe crystal, in the range of 400−4000 cm−1, with 8 cm−1 resolution and 20 scans (IRTracer-105 FT-IR, spectral range 350−7800 cm −1 , DLATGS detector, Shimadzu). Scanning electron microscopic (SEM) analysis was realized in a tabletop microscope (model TM-3000, Hitachi) with 15 kV acceleration, 38 800 nA emission current, 1750 mA filament, and 8400 μm working distance. Adsorption Experiments. Adsorption experiments were performed using a batch equilibrium technique for both CVT and CVT-Na clays. Initially, the effect of adsorbent dosage was investigated in the softening of the simulated hard water by adding 0.01, 0.5, 1.0, and 1.5 g of clay to a series of 250 mL Erlenmeyer flasks containing 50 mL of 500 mg L−1 CSS. The flasks were covered with plastic films and stirred at 200 rpm for 150 min at a controlled temperature of 298 K. The influence of contact time was verified aiming to determine the equilibrium time and to further kinetic study. The tests were conducted on optimum condition of adsorbent mass in flasks containing 50 mL of 500 mg L−1 CSS. The solutions were stirred in the same previous conditions. Then, the samples collected at 1, 5, 10, 15, 30, 60, 90, 120, 150, and 180 min were analyzed. Thereafter, the effect of pH in the adsorption process was analyzed in pH values of 4, 6, 8, 10 and 11, and conducted on optimal conditions of adsorbent mass and contact time previously determined, in flasks containing 50 mL of 500 mg L−1 CSS.

R2 = 1 −

2

χ =

∑

∑ (qe,exp − qe,cal)2 ∑ (qe,exp − qe,mean)2

(6)

(qe,exp − qe,cal)2 qe,cal

(7)

where qe,cal (mg/g) is the uptake calculated by the model under consideration, qe,exp (mg/g) is the experimental uptake at equilibrium, and qe,mean (mg/g) is the mean of the qe,exp values. In general, greater values of R2 indicate fits of higher quality. However, in the nonlinear case, this is not enough to reach the right conclusions,29 and the chi-squared is necessary: the closer χ2 is to zero, the better the fit. Reconditioning Experiments. The setup for the reconditioning experiments was preparing CVT-Na according to Vermiculite Modification and executing the adsorption experiments as in Adsorption Experiments, keeping the clay residue. Then, the procedure consisted of running the following steps in two cycles: (a) perform the modification procedure again with the obtained residue and (b) execute an adsorption experiment with the result of (a) and keep the residue for an eventual repetition. Experiments considered the optimal adsorption conditions found in the adsorption studies, namely, 1500 mg L−1 CSS at pH 10 and stirring time of 90 min.

■

RESULTS AND DISCUSSION Characterization. XRD Analysis. XRD analysis revealed the characteristic displacement reflection of vermiculite, referring to

C


Article


Table 1. XRF Analysis for Calcium and Sodium Ions in CVT, CVT-Na, and CVT-Na-Ca Samples CVT CVT-Na CVT-Na-Ca

Ca ions (%)

SD

Na ions (%)

SD

97.9 80.9 99.1

0.706 0.364 0.338

2.11 19.6 0.859

5.436 2.065 1.58

confirmed the presence of hydrated Na+ ions with the apperance of the reflections 2θ 7.2 and 29.4° (Figure 1). The diffractograms obtained also allowed the inference that the vermiculite lamellar structure was significantly preserved, although the treatment and adsorption assays were realized with magnetic stirring. Fourier Transform Infrared (FTIR) Analysis. The FTIR analysis curves (Figure 2) signalized aspects of typical vermiculite structure, such as the range around 3650 cm−1 of OH stretching vibrations present in the mineral clay’s tetrahedral and octahedral sheets,32 and the peaks at 995 and 920 cm−1, both related to the asymmetric stretching vibrations Si−O and Si−OH.31 The broad band around 3500 cm−1 and the peak at 1650 cm−1 correspond to the water molecule OH stretching vibration and the angular deformation of the OH bonds of the water adsorbed by hydrogen bonds, respectively. The peak at 2970 cm−1 evinces the presence of condensation water33 that is dispersed as hydration water in CVT-Na and CVT-Na-Ca. SEM Analysis. The resulting images from the scanning electron microscope corroborated that the clay’s typical lamellar structure was preserved after the sodium-modification process and Ca2+ ion adsorption (Figure 3). Futhermore, the images also emphasize the morphology of typical overlapping phyllosilicate sheets and compacted structures arranged in blocks of irregular shapes. X-ray Fluorescence. The relative values obtained from XRF analysis (Table 1) proved the insertion efficiency of Na+ ions in the vermiculite modification process, by comparing CVT and CVT-Na sodium ion amounts. Also, they corroborate the Ca2+ ion exchange of the CSS by adsorption assays. CEC Determination. The CEC calculated from eq 1 provided values about 101.7 ± 0.9 mmol/100 g for CVT and 118.8 ± 5.1 mmol/100 g for CVT-Na. These results showed a major cation exchange capacity after sodium modification. Similar results were found in Zang et al.25 and Abate et al.34 Adsorption Experiments. Effects of Adsorbent Dosage. Adsorbent dosage is an important parameter in the softening water process because it determines the adsorption capacity for a given set of operational conditions. Figure 4, which shows the

Figure 1. XRD patterns of vermiculite (CVT) and sodium-modified vermiculite (CVT-Na).

Figure 2. FTIR spectra of CVT, CVT-Na, and CVT-Na-Ca.

the 002 basal reflection of 1.42 nm19,30,31 to 1.45 nm, validating the hypothesis that the treatment with NaCl would cause an average increase of the interlayer space by inserting hydrated alkaline ions, with structural ordering reduction. A wider comparison of the diffractograms showed the crystallinity pattern of the samples’ ionization before and after and also

Figure 3. SEM images of (a) CVT, (b) CVT-Na, and (c) CVT-Na-Ca. D


Article


Figure 4. Effects of (a) CVT and (b) CVT-Na dosage in Ca2+ ion removal. Experimental initial conditions: C0 = 500 mg L−1, pH 10, T = 298 K, and t = 150 min.

Figure 5. Effect of contact time in Ca2+ ion removal onto CVT and CVT-Na. Experimental initial conditions: C0 = 500 mg L−1, pH 10, and T = 298 K.

Figure 6. Initial pH effect on Ca2+ removal for CVT and CVT-Na. Experimental initial conditions: C0 = 500 mg L−1, pH 10, T = 298 K, t = 150 min for CVT and t = 90 min for CVT-Na.

effect of adsorbent dosage for CVT and CVT-Na, demonstrates that the increment of adsorbent dose implied a higher number of adsorption sites available for calcium uptake which increases the removal efficiency and at the same time descreases the adsorption capacity. Calcium removals of 36.5% for CVT and 97.1% for CVT-Na were obtained using a 20 g L−1 dose of adsorbent. A further increase of the adsorbent dose had no significant effect on the removal efficiency. Thus, for economical purposes, the adopted adsorbent dosage in the remaining work was 20 g L−1. Effects of Contact Time and Kinetic Modeling. Figure 5 illustrates the effect of contact time on calcium removal. The removed amount swiftly increased over the initial 30 min of contact time interval and abruptly turned constant until reaching

equilibrium at approximately 90 min. Instantaneous adsorption phenomena demonstrated that CVT-Na had a strong affinity toward calcium cations. In view of this, 90 min was used as the equilibrium time in subsequent experiments. In the kinetic study, pseudo-first-order,35 pseudo-secondorder,36 and Elovich37 models were applied in experimental data. The respective nonlinear forms of those models are mathematically expressed as qt = qe(1 − e−k1t )

qt =

(8)

qe 2k 2t 1 + k 2qet

(9)

Table 2. Kinetic Parameters of Calcium Adsorption onto CVT and CVT-Na at 298 K pseudo first order CVT CVT-Na

pseudo second order

qe,cal

k1

R

χ

8.450 24.27

0.2139 0.9775

0.7804 0.8043

0.1646 0.3415

2

2

2

qe,cal

k2

R

8.935 24.98

0.03598 0.05852

0.8993 0.9639

E

Elovich χ

α

β

R2

χ2

0.07544 0.06297

18.57 31821

0.8368 0.5715

0.9457 0.8816

0.04068 0.2066

2


Article


Effects of Initial pH. Adsorption studies under different initial pHs (Figure 6) and zeta potential analysis (Figure 7) indicated that, for CVT, the greater the pH under which adsorption takes place, the greater the adsorption capacity, given the increase in the surface negative charge. For CVT-Na, surface charge was negative in the whole considered pH range and considerably lower than the one of CVT, confirming that the sodium treatment significantly improved the cation adsorption capacity. In particular, regarding CVT in acid, neutral, or soft alkaline medium, the hydration enthalpy of the Ca2+ ions favored the release of this species to the aqueous solution, hindering the adsorption onto the clay, a phenomenon that only starts around pH 10.16 On the other hand, CVT-Na, even when in acid medium, presented a high and nearly constant cation exchange capability until near pH 10. The smaller influence of pH in CVTNa adsorption behavior might be due to the ionic radius and hydration enthalpies21 near the ones of H3O+ and Na+ present in solution. Additionally, in strongly alkaline medium (pH >10), formation of Ca(OH)+ complexes starts to influence the availability of exchangeable Ca2+ cations.38 This fact justifies the choice for pH 10 in the adsorption studies, since pH >11, despite the impression of better adsorption, is not an appropriate condition for evaluating the adsorption capacity of Ca2+ under the considered experimental procedures. Effects of Initial Concentration and Temperature. The effects of initial calcium concentration on CVT and CVT-Na adsorption are presented in Figure 8. Notably, the adsorption capacity was improved by the increase of the initial Ca2+ ion concentration and started to turn constant for CVT-Na and decreased for CVT around 1500 mg L−1. This result, in agreement with the obtained curve, can be interpreted as the saturation of the clay adsorption sites for CVT-Na and transient behavior for CVT. Moreover, the calcium uptake for each initial concentration rises with the increase of temperature for CVT and CVT-Na, although for the CVT this effect is more attenuated. This increase of calcium uptake for CVT-Na is unclear at 500 mg L−1, which can be attributed to higher adsorption capacity in this concentration and the adsorbed calcium measurement precision.

Figure 7. Zeta potential curves for vermiculite (CVT) and sodiummodified vermiculite (CVT-Na).

qt =

1 ln(1 + αβt ) β

(10)

where k1 (min−1) and k2 (g mg−1 min−1) are the rate constants of the pseudo-first-order and pseudo-second-order equations, respectively. The α (g mg−1 min−1) constant is the initial adsorption rate, and β (mg g−1) is the desorption constant in the experiments. Table 2 summarizes the corresponding parameters of calcium adsorption onto CVT and CVT-Na at 298 K. For CVT-Na, the pseudo-second-order model exhibited higher determination coefficients (R2 = 0.9639) and lower chi-squared (χ2 = 0.06297) values in comparison with the pseudo-first-order (R2 = 0.8043 and χ2 = 0.3415) and Elovich models (R2 = 0.8816 and χ2 = 0.2066), while for CVT, the Elovich model exhibited higher determination coefficients (R2 = 0.9457) and low chi-squared (χ2 = 0.040) values when compared to the other two models, meaning the calcium adsorption in CVT has a slower adsorption rate.

Figure 8. Effect of initial Ca2+ concentration in removal capacity onto (a) CVT and (b) CVT-Na. Experimental initial conditions: pH 10, t = 150 min for CVT, and t = 90 min for CVT-Na. F


Article


used in this work in order to describe the softening process. Nonlinear equation for these models are presented respectively: qe =

0 Q max KLCe

1 + KLCe

(11)

qe =

KFCen

(12)

qe =

KRPCe 1 + aRPCeg

(13)

Q0max

where qe and Ce are obtained from eq 4; is the adsorbent maximum saturated monolayer adsorption capacity; KL (L mg−1) is the Langmuir constant related to the affinity between an adsorbent and adsorbate; KF ((mg/g)/(L/mg)n) is the Freundlich constant, which characterizes the adsorption strength; n is a Freundlich intensity parameter; KRP (L g−1), aRP (mg L−1)−g, and g are the Redlich−Peterson constants. The calcium adsorption isotherms onto CVT and CVT-Na at different temperatures are exhibited in Figure 9, and the corresponding parameters are listed in Table 3. For CVT, the higher determination coefficients (R2 > 0.8013) and lower chisquared values (χ2 < 0.60) showed that the experimental equilibrium data is adequately described by the Langmuir model. The Q0max derived from the Langmuir model shows lower capacity of calcium adsorption for CVT when compared with CVT-Na. In the case of CVT-Na, the statistical parameters induced the choice of the Freundlich model as the best fit for experimental equilibrium data, showing that the adsorption process was favorable (n < 1) and occurred in a multilayered heterogeneous surface, in agreement with ref 25. Moreover, the lower values of KRP and aRP, and g = 1 for CVT, and the higher values of KRP and aRP, and the proximity of g to unity for CVTNa, show that experimental data can also be approximated by the Redlich−Peterson model in some cases. Adsorption Thermodynamics. Thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were calculated in order to determine the adsorption mechanisms. The Gibbs free surface energy change was computed by the equation ΔG° = −RT ln Kc

(14) −1

−1

where R is the universal gas constant (8.314 J mol K ), T is temperature, and Kc is the equilibrium constant. The equilibrium constant Kc was derived from the Freundlich constants (KF and n) as suggested in refs 39 and 40. ρKF jij 106 zyz Kc = j z 1000 jjk ρ zz{

n

(15) −1

where ρ is the pure water density (∼1.0 g mL ). The relationship among ΔG°, ΔH°, and ΔS° is described as ΔG° = ΔH ° − T ΔS°

(16)

The combination of expressions 14 and 16 provides the van’t Hoff equation: ln Kc = −

Figure 9. Ca2+ ion removal isotherms onto CVT and CVT-Na at (a) 298, (b) 308, and (c) 318 K. Experimental initial conditions: C0 = 500− 2000 mg L−1, pH 10, t = 150 min for CVT, and t = 90 min for CVT-Na.

ΔH ° ΔS° + RT R

(17)

The ΔH° and ΔS° values were determined from the slope and intercept of the plot of ln Kc against T−1, as illustrated in Figure 10. Table 4 summarizes the calculation of the aforementioned thermodynamic parameters. The Kc values increased with the temperature, confirming the hypothesis that the adsorption process was more favorable at a

Adsorption Isotherms. The adsorption isotherm indicates the adsorbate distribution at the equilibrium state. The Langmuir, Freundlich, and Redlich−Peterson models were G


Article

Industrial & Engineering Chemistry Research Table 3. Isotherm Parameters for Ca2+ Ion Removal by CVT and CVT-Na Freundlich Model T (K)

adsorbent

KF

n

R2

χ2

298

CVT CVT-Na CVT CVT-Na CVT CVT-Na

0.3406 14.57 0.3689 18.46 0.5096 21.70

0.4647 0.1756 0.4644 0.1569 0.4237 0.1395

0.7488 0.9880 0.8032 0.9286 0.7687 0.8890

0.7524 0.5037 0.6615 4.1667 0.7381 6.9277

T (K)

adsorbent

Q0max

298


15.98 0.0012 45.43 0.04812 16.99 0.00124 49.16 0.08319 16.59 0.00146 53.19 0.07551 Redlich−Peterson Model

308 318

Langmuir Model

308 318

KL

R2

χ2

0.8013 0.7919 0.8736 0.8085 0.8519 0.9707

0.5950 8.7135 0.4250 11.184 0.4727 1.8312

T (K)

adsorbent

KRP

aRP

g

R2

χ2

298


0.01913 26.44 × 1045 0.01741 11.26 × 1045 0.02414 44.93 × 1045

0.0012 2.133 × 1045 0.0012 6.087 × 1044 0.0015 2.077 × 1045

1 0.8246 1 0.8435 1 0.8600

0.8013 0.9880 0.8736 0.9286 0.8519 0.8890

1.3388 1.1333 0.9562 9.3759 1.0636 15.589

308 318

Figure 11. Reconditioning experiment results for CVT-Na under optimal conditions.

Figure 10. Plot of ln Kc versus T−1 for calcium adsorption onto CVT and CVT-Na.

can be associated with the derivation of thermodynamics paramaters from the Freundlich model. Reconditioning. Reconditioning results (Figure 11) revealed that the sodium-modified clay, after being used for calcium ion removal from aqueous solution, can, in fact, be reused for the same purpose by applying the same sodium modification process, which reconditions the clay by means of a simple ion exchange mechanism. Moreover, repeating the sodium modification causes an intensification of the adsorption capacity, which may be explained by a granulometry reduction leading to an increase in the contact surface.

Table 4. Thermodynamic Parameters of Calcium Adsorption onto CVT and CVT-Na from Freundlich Constant ΔG° (kJ mol−1) adsorbent

T = 298 K

T = 308 K

T = 318 K

ΔH° (kJ mol−1)

ΔS° (J mol−1)

CVT CVT-Na

−13.71 −34.92

−17.10 −37.29

−18.89 −39.65

37.85 35.54

178.42 236.45

■

higher temperature and, therefore, endothermic. The more negative values of ΔG° for CVT-Na suggested that the adsorption phenomena occurred more spontaneously for this clay. Moreover, the positive values of ΔH° and ΔS° indicated the endothermic and random nature of the process in both cases. In particular, the lower coefficient determination value for CVT

CONCLUSIONS The methodology used in the present study allowed traceable results regarding the adsorption properties of sodium-modified vermiculite clay. The obtained material revealed a great capacity H


Article


(8) Jin, H.; Yu, Y.; Zhang, L.; Yan, R.; Chen, X. Polarity reversal electrochemical process for water softening. Sep. Purif. Technol. 2019, 210, 943−949. (9) Alaei Shahmirzadi, M. A.; Hosseini, S. S.; Luo, J.; Ortiz, I. Significance, evolution and recent advances in adsorption technology, materials and processes for desalination, water softening and salt removal. J. Environ. Manage. 2018, 215, 324−344. (10) Yu, M.; Gao, M.; Shen, T.; Wang, J. Organo-vermiculites modified by low-dosage Gemini surfactants with different spacers for adsorption toward p-nitrophenol. Colloids Surf., A 2018, 553, 601−611. (11) Chen, L.; Wu, P.; Chen, M.; Lai, X.; Ahmed, Z.; Zhu, N.; Dang, Z.; Bi, Y.; Liu, T. Preparation and characterization of the eco-friendly chitosan/vermiculite biocomposite with excellent removal capacity for cadmium and lead. Appl. Clay Sci. 2018, 159, 74−82. (12) Bekri-Abbes, I.; Bayoudh, S.; Baklouti, M. The removal of hardness of water using sulfonated waste plastic. Desalination 2008, 222, 81−86. (13) Ayaliew Werkneh, A. Removal of Water Hardness Causing Constituents Using Alkali Modified Sugarcane Bagasse and Coffee Husk at Jigjiga City, Ethiopia: A Comparative Study. International Journal of Environmental Monitoring and Analysis 2015, 3, 7. (14) Sepehr, M. N.; Zarrabi, M.; Kazemian, H.; Amrane, A.; Yaghmaian, K.; Ghaffari, H. R. Removal of hardness agents, calcium and magnesium, by natural and alkaline modified pumice stones in single and binary systems. Appl. Surf. Sci. 2013, 274, 295−305. (15) Altundoğan, H. S.; Topdemir, A.; Cakmak, M.; Bahar, N. Hardness removal from waters by using citric acid modified pine cone. J. Taiwan Inst. Chem. Eng. 2016, 58, 219−225. (16) Cherian, C.; Kollannur, N. J.; Bandipally, S.; Arnepalli, D. N. Calcium adsorption on clays: Effects of mineralogy, pore fluid chemistry and temperature. Appl. Clay Sci. 2018, 160, 282−289. (17) Padilla-Ortega, E.; Leyva-Ramos, R.; Mendoza-Barron, J. Role of electrostatic interactions in the adsorption of cadmium(II) from aqueous solution onto vermiculite. Appl. Clay Sci. 2014, 88−89, 10−17. (18) Uddin, M. K. A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade. Chem. Eng. J. 2017, 308, 438−462. (19) Wang, J.; Gao, M.; Ding, F.; Shen, T. Organo-vermiculites modified by heating and gemini pyridinium surfactants: Preparation, characterization and sulfamethoxazole adsorption. Colloids Surf., A 2018, 546, 143−152. (20) Fuks, L.; Herdzik-Koniecko, I. Vermiculite as a potential component of the engineered barriers in low- and medium-level radioactive waste repositories. Appl. Clay Sci. 2018, 161, 139−150. (21) Han, B.; He, B.; Geng, R.; Zhao, X.; Li, P.; Liang, J.; Fan, Q. Ni(II) sorption mechanism at the vermiculite-water interface: Effects of interlayer. J. Mol. Liq. 2019, 274, 362−369. (22) Barabaszová, K. C.; Valásǩ ová, M. Characterization of vermiculite particles after different milling techniques. Powder Technol. 2013, 239, 277−283. (23) de Freitas, E. D.; de Almeida, H. J.; Vieira, M. G. A. Binary adsorption of zinc and copper on expanded vermiculite using a fixed bed column. Appl. Clay Sci. 2017, 146, 503−509. (24) Kebabi, B.; Terchi, S.; Bougherara, H.; Reinert, L.; Duclaux, L. Removal of manganese (II) by edge site adsorption on raw and milled vermiculites. Appl. Clay Sci. 2017, 139, 92−98. (25) Zang, W.; Gao, M.; Shen, T.; Ding, F.; Wang, J. Facile modification of homoionic-vermiculites by a gemini surfactant: Comparative adsorption exemplified by methyl orange. Colloids Surf., A 2017, 533, 99−108. (26) Nunes, J. S.; de Vasconcelos, C. L.; Dantas, T. N. C.; Pereira, M. R.; Fonseca, J. L. C. Preparation of Acrylic Latexes with Dispersed Magnetite Nanoparticles. J. Dispersion Sci. Technol. 2008, 29, 769−774. (27) Ciesielski, H.; Sterckeman, T.; Santerne, M.; Willery, J. P. A comparison between three methods for the determination of cation exchange capacity and exchangeable cations in soils. Agronomie 1997, 17, 9−16.

of calcium ion adsorption, which was expected in view of the obtained CEC values. It was also found that the Langmuir isotherm best described the equilibrium adsorption for CVT, while Freundlich isotherm was the best model for CVT-Na. Thermodynamic parameters evidenced an endothermic and spontaneous process. Moreover, reconditioning experiments attested that the sodium-modified vermiculite can be recovered after being used for calcium ion removal, a fact that corroborates its ecofriendly nature and feasibility for small-scale usage in socially vulnerable contexts (as by families in Brazilian drought regions) and large-scale industrial applications. Further studies are needed to investigate the observed increase in the adsorption capacity after successive executions of the reconditioning procedure. In summary, sodium-modified vermiculite was demonstrated to be an efficient, low-cost, and recyclable adsorbent candidate for hard water softening. Further works on this topic may explore more of the vermiculite adsorption capacities by investigating the effectiveness of alternative modification procedures for the adsorption of calcium and other hardness ions, such as sodium and magnesium.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: paulo.douglas.lima@fisica.ufrn.br. *E-mail: [email protected]. ORCID

Roberto R. C. Lima: 0000-0003-4409-0071 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by the Brazilian Research Council (CNPq). The authors are thankful for the support from the Graduate Program in Materials Science and Engineering (PPGCEM-UFRN) and the Federal Institute of Rio Grande do Norte Campus Zona-Norte. The authors also thank Dr. Sibele Pergher for her useful suggestions.

■

REFERENCES

(1) WHO. Hardness in Drinking-Water; World Health Organization: 2011. (2) Kadir, N. N. A.; Shahadat, M.; Ismail, S. Formulation study for softening of hard water using surfactant modified bentonite adsorbent coating. Appl. Clay Sci. 2017, 137, 168−175. (3) Hailu, Y.; Tilahun, E.; Brhane, A.; Resky, H.; Sahu, O. Ion exchanges process for calcium, magnesium and total hardness from ground water with natural zeolite. Groundwater for Sustainable Development 2019, 8, 457−467. (4) Agostinho, L. C. L.; Nascimento, L.; Cavalcanti, B. F. Water Hardness Removal for Industrial Use: Application of the Electrolysis Process. J. Cancer Sci. Ther. 2012, 1, 460. (5) Mahasti, N. N.; Shih, Y.-J.; Vu, X.-T.; Huang, Y. H. Removal of calcium hardness from solution by fluidized-bed homogeneous crystallization (FBHC) process. J. Taiwan Inst. Chem. Eng. 2017, 78, 378−385. (6) Sis, H.; Uysal, T. Removal of heavy metal ions from aqueous medium using Kuluncak (Malatya) vermiculites and effect of precipitation on removal. Appl. Clay Sci. 2014, 95, 1−8. (7) Hasson, D.; Sidorenko, G.; Semiat, R. Calcium carbonate hardness removal by a novel electrochemical seeds system. Desalination 2010, 263, 285−289. I


Article

Industrial & Engineering Chemistry Research (28) AWWA. Standard Methods for the Examination of Water and Wastewater, 5th ed.; American Public Health Association: Washington, DC, 2005. (29) Ho, Y.-S. Selection of optimum sorption isotherm. Carbon 2004, 42, 2115−2116. (30) dos Anjos, V. E.; Rohwedder, J. R.; Cadore, S.; Abate, G.; Grassi, M. T. Montmorillonite and vermiculite as solid phases for the preconcentration of trace elements in natural waters: Adsorption and desorption studies of As, Ba, Cu, Cd, Co, Cr, Mn, Ni, Pb, Sr, V, and Zn. Appl. Clay Sci. 2014, 99, 289−296. (31) Ritz, M.; Zdrálková, J.; Valásǩ ová, M. Vibrational spectroscopy of acid treated vermiculites. Vib. Spectrosc. 2014, 70, 63−69. (32) Somasundaran, P. Encyclopedia of Surface and Colloid Science, 2nd ed.; Taylor & Francis: New York, 2006. (33) Stawiński, W.; Wȩgrzyn, A.; Mordarski, G.; Skiba, M.; Freitas, O.; Figueiredo, S. Sustainable adsorbents formed from by-product of acid activation of vermiculite and leached-vermiculite-LDH hybrids for removal of industrial dyes and metal cations. Appl. Clay Sci. 2018, 161, 6−14. (34) Abate, G.; dos Santos, L. B.; Colombo, S. M.; Masini, J. C. Removal of fulvic acid from aqueous media by adsorption onto modified vermiculite. Appl. Clay Sci. 2006, 32, 261−270. (35) Lagergren, S. About the Theory of So-Called Adsorption of Soluble Substances. K. Sven. Vetenskapsakad. Handl. 1898, 24, 1−39. (36) Blanchard, G.; Maunaye, M.; Martin, G. Removal of heavy metals from waters by means of natural zeolites. Water Res. 1984, 18, 1501− 1507. (37) Chien, S. H.; Clayton, W. R. Application of Elovich Equation to the Kinetics of Phosphate Release and Sorption in Soils. Soil Sci. Soc. Am. J. 1980, 44, 265−268. (38) Atesok, G.; Somasundaran, P.; Morgan, L. Adsorption properties of Ca2+ on Na-kaolinite and its effect on flocculation using polyacrylamides. Colloids Surf. 1988, 32, 127−138. (39) Ghosal, P. S.; Gupta, A. K. An insight into thermodynamics of adsorptive removal of fluoride by calcined Ca−Al−(NO 3) layered double hydroxide. RSC Adv. 2015, 5, 105889−105900. (40) Tran, H. N.; You, S.-J.; Chao, H.-P. Thermodynamic parameters of cadmium adsorption onto orange peel calculated from various methods: A comparison study. J. Environ. Chem. Eng. 2016, 4, 2671− 2682.

J


Sodium-Modified Vermiculite for Calcium Ion Removal from Aqueous

Recommend Documents