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Evidence of lead ions on palygorskite surface after adsorptive process: kinetic and isotherms studies

ABSTRACT

Our previous work reported that palygorskite has potential for application as metal cations adsorbent due to its chemical and mineralogical properties. In this work, kinetic study and adsorption isotherms were performed in order to evaluate Pb (II) ions adsorption rate, maximum capacity and type of adsorption by using palygorskite as adsorbent. Adsorption tests were performed in batch, using pH of 5, 2 g of palygorskite, 40 mL of synthetic effluent solution and stirring for 1 hour. Kinetic experiments were performed using 34 mg L-1 of a lead synthetic effluent at same mass and pH conditions. Furthermore, after adsorption studies, the sample was characterized by X-ray fluorecence (XRF), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) coupled with Energy-dispersive X-ray spectroscopy (EDS) analysis, in order to verify and understand the interaction of lead ions in palygorskite. The results demonstrated that palygorskite presented an equilibrium time of 10 min with 99.14% of removal, following pseudo-second order kinetic. The maximum adsorption capacity was 21.65 mg g-1 and Gibbs’ adsorption-free energy was -21.39 KJ mol-1, with Langmuir model being the most suitable for adjustment of the data. Thus, its presence was confirmed by characterization techniques, indicating that the metal is distributed homogeneously on palygorskite surface, proving its efficiency as lead ions adsorbent.

Keywords
Brazilian palygorskite; kinetic; isotherm; characterization

1. INTRODUCTION

Pollution by heavy metals originating from anthropogenic activities, such as those in industries, commerce and agriculture, poses a threat to ecosystems and human health, and has gradually drawn wide concern across the globe because of their increased discharge, acute toxicity, persistence, bioaccumulation and biomagnification through food chains [11 XU, P., ZENG, G.M., HUANG, D.L., et al. “Adsorption of Pb (II) by iron oxide nanoparticles immobilized phanerochaete chrysosporium: equilibrium, kinetic, thermodynamic and mechanisms analysis”, Chemical Engineering Journal. v. 203, pp. 423-431, Sep. 2012.

2 BARZEGAR, R., MOGHADDAM, A.A., ADAMOWSKI, J., et al. “Assessing the potential origins and human health risks of trace elements in groundwater: A case study in the Khoy plain, Iran”, Environmental Geochemistry and Health. v. 41, pp. 981-1002, Sep. 2018.
-33 MGBENU, C.N., EGBUERI, J.C., “The hydrogeochemical signatures, quality indices and health risk assessment of water resources in Umunya district, southeast Nigeria”, Applied Water Science. v. 9, n. 22, Jan. 2019.].

Industrialization and population growth are identified as key factors responsible for the increase in pollution by heavy metals, expanding the need to treat effluents containing such polluting species [44 SIMÕES, K.M.A., NOVO, B.L., FELIX, A.A.S., et al. “Ore Dressing and Technological Characterization of Palygorskite from Piauí/Brazil for Application as Adsorbent of Heavy Metals”, In: ed. Springer International Publishing, Characterization of Minerals, Metals and Materials, chapter 7, California, USA, 2017., 55 PYRGAKI, K., MESSINI, P., ZOTIADIS, V., “Adsorption of Pb and Cu from Aqueous Solutions by Raw and Heat-Treated Attapulgite Clay”. Geosciences. v. 8, n. 5, pp. 157, Apr. 2018.]. These toxic ions are frequently present in wastewater due to industrial activities such as dyeing textile, hydrometallurgical, tanning, smelting of ores, metal plating, battery industries, fertilizers and herbicides production, motor, electrochemical, house paint, gasoline additives and plumbing pipes. [66 JAISHANKAR, M., TSETEN, T., ANBALAGAN, N., et al. “Toxicity, mechanism and health effects of some heavy metals”, Interdisciplinary Toxicology. v. 7, n. 2, pp. 60-72, Nov. 2018.] Variety of clays and clay minerals play an important role in the environment and used as an effective adsorbent material for the removal of toxic metal ions from water solution [55 PYRGAKI, K., MESSINI, P., ZOTIADIS, V., “Adsorption of Pb and Cu from Aqueous Solutions by Raw and Heat-Treated Attapulgite Clay”. Geosciences. v. 8, n. 5, pp. 157, Apr. 2018., 77 UDDIN, M.K., “A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade”. Chemical Engineering Journal. v. 308, pp. 438–462, Jan. 2017.].

Lead is a naturally occurring element, found in abundance in the earth crust, almost always as lead sulphide (galena) [88 AZEVEDO, F.A., CHASIN, A.A.M., “Chumbo”, In: Atheneu, Metais: gerenciamento da toxicidade, 1 ed., chapter 12, Rio de Janeiro, Brazil, 2003.]. Due to its vast industrial application in production of batteries, solder, metal alloys, cable shields, pigments, rust inhibitors, ammunition, glassware and plastic stabilizers, it is one of the most commonly found heavy metals in water bodies [99 AXTELL, N.R., CLAUSSEN, K., STERNBERG S.P.K., “Lead and nickel removal using Microspora and Lemna minor”, Bioresource Technology. v. 89, n. 1, pp. 41–48, Aug. 2003., 1010 MASINDI, V., MUEDI, K.L., “Environmental Contamination by Heavy Metals”, In: Hosam El-Din M. Saleh and Refaat F. Aglan, Heavy Metals, Jun. 2018.].

In Brazil, the region of Alto Vale do Ribeira, in the extreme southeast of São Paulo State and northeast of Paraná State, as well as the region of Boquira, in the State of Bahia, are the main Brazilian cases of contamination by this metal. Boquira region covers the most worrying case of lead contamination in Brazil, where lead-zinc mining worked for approximately 30 years, until it was suddenly abandoned. The waste generated, containing toxic metals such as lead, zinc, silver, barium, copper, chromium and nickel, was improperly deposited close to the city [1111 DALTRO, R.R., DOS ANJOS, J.A.S.A., GOMES, M.C.R., “Avaliação de metais pesados nos recursos hídricos do município de Boquira, no semiárido baiano – Brasil”, Geosciências. v. 39, n. 1, pp. 139-152, May. 2020.], damaging not only the commercial activities, but also the living conditions of the population. For this reason, the remediation of soil and water, aiming to minimize environmental risks and human exposure to these contaminated wastes are targets of study until nowadays [1212 FANG, C., ACHAL, V., “The Potential of Microbial Fuel Cells for Remediation of Heavy Metals from Soil and Water-Review of Application”, Microorganisms. v. 7, n. 12, pp. 697, Dec. 2019., 1313 MARQUES, J.P., RODRIGUES, V.G.S., RAIMONDI, I.M., et al. “Increase in Pb and Cd adsorption by the application of peat in a tropical soil”, Water, Air & Soil Pollution. v. 231, pp. 136, Mar. 2020.]. In addition, there is a concern to raise awareness and encourage companies in the mining sector, in order to invest in safety measures, as well as comply with the legislative requirements that compete in the companies operating in this sector [1414 MIRANDA, L.S., ANJOS, J.A.S.A., “Occupational impacts and adaptation to standards in accordance with Brazilian legislation: The case of Santo Amaro, Brazil”, Safety Science. v. 104, pp. 10-15, Apr. 2018.].

Due to this, a few technologies for the removal of heavy metals from aqueous solutions have been studied nowadays. These technologies include chemical precipitation, ion exchange, membrane filtration, carbon adsorption and coprecipitation/adsorption [1515 MARTINS, R.J.E., VILAR, V.J.P., BOAVENTURA, R.A.R., “Kinetic modelling of cadmium and lead removal by aquatic mosses”, Brazilian Journal of Chemical Engineering. v. 31, n. 1, pp. 229-242, Mar. 2014.

16 KHULBE, K.C., MATSUURA, T., “Removal of heavy metals and pollutants by membrane adsorption techniques”, Applied Water Science. v. 8, pp. 19, Jan. 2018.
-1717 DUAN, C., MA, T., WANG, J., et al. “Removal of heavy metals from aqueous solution using carbon-based adsorbents: A review”, Journal of Water Process Engineering. v. 37, Oct. 2020.]. However, these techniques have inherent limitations in practice (such as complicated treatment process, high cost and energy requirement) or pose danger of secondary pollution [1818 SÖNMEZAY, A., SALIM ÖNCEL, M., BEKTAŞ, N., “Adsorption of lead and cadmium ions from aqueous solutions using manganoxide minerals”, Transactions of Nonferrous Metals Society of China. v. 22, n. 12, pp. 3131-3139, Dec. 2012., 1919 BHAGAT, S.K., PARAMASIVAN, M., AL-MUKHTAR, M., et al. “Prediction of lead (Pb) adsorption on attapulgite clay using the feasibility of data intelligence models”, Environmental Science and Pollution Research. v. 28, pp. 31670-31688, Feb. 2021.].

Adsorption is an effective method of removing a wide range of contaminants from effluents, especially when adsorbate and adsorbent combination is the best possible [2020 TEODORO, L., PARABOCZ, C.R.B., ROCHA, R.D.C., “Caracterização da argila vermiculita expandida: avaliação dos padrões físico-químicos e mineralógicos para aplicação como adsorvente”, Matéria, v. 25, n. 4, pp. 1-8, 2020.]. However, the main disadvantage of adsorption methods is the high price of the adsorbents, which increases the cost of wastewater treatment. The most generally used solid adsorbent is activated carbon [2121 SALEEM, J., SHAHID, U., HIJAB, M., et al. “Production and applications of activated carbons as adsorbents from olive stones”, Biomass Conversion and Biorefinery. v. 9, pp. 775-802, Aug. 2019.]. However, activated charcoal is expensive and for effluents containing metallic ions it requires chelating agents to enhance its performance, thus increasing treatment cost [2222 DENIZ, F., KEPEKCI, R.A., “Dye biosorption onto pistachio by-product: A green environmental engineering approach”, Journal of Molecular Liquids. v. 219, pp. 194-200, Jul. 2016., 2323 SILVA, T.L., RONIX, A., PEZOTI, O., et al. “Mesoporous activated carbon from industrial laundry sewage sludge: Adsorption studies of reactive dye Remazol Brilliant Blue R”, Chemical Engineering Journal. v. 303, pp. 467–476, Nov. 2016.]. Therefore, the need of alternative low cost adsorbents has prompted the search for new and cheaper sorption processes for aqueous efluent treatment, since these materials could significantly reduce the wastewater-treatment cost [2424 YAVUZ, O., ALTUNKAYNAK, Y., GUZEL, F., “Removal of copper, nickel, cobalt and manganese from aqueous solution by kaolinite”, Water Research. v. 37, n. 2, pp. 948-952, Feb. 2003.

25 ERDEM, E., KARAPINAR, N., DONAT, R., “The removal of heavy metal cations by natural zeolites”, Journal of Colloid and Interface Science. v. 280, n. 2, pp. 309-314, Dec. 2004.

26 MESHKO, V., MARKOVSKA, L., MARINKOVSKI, M., “Experimental study and modelling of zinc adsorption by granular activated carbon and natural zeolite”, International Journal of Environment and Pollution. v. 27, n. 4, pp. 285-299, Jan. 2006.
-2727 LEAL, T.W., LOURENÇO, L.A., SCHEIBE, A.S., et al. “Textile wastewater treatment using low-cost adsorbent aiming the water reuse in dyeing process”, Journal of Environmental Chemical Engineering. v. 6, n. 2, pp. 2705-2712, Apr. 2018.].

Thus, the use of clay minerals as adsorbent materials for various contaminants present in aquatic environment emerges as an economically viable alternative [2020 TEODORO, L., PARABOCZ, C.R.B., ROCHA, R.D.C., “Caracterização da argila vermiculita expandida: avaliação dos padrões físico-químicos e mineralógicos para aplicação como adsorvente”, Matéria, v. 25, n. 4, pp. 1-8, 2020., 2828 SILVA, V.C., ARAÚJO, M.E.B., RODRIGUES, A.M., et al. “Adsorption Behavior of Acid-Treated Brazilian Palygorskite for Cationic and Anionic Dyes Removal from the Water”, Sustainability 2021, 13, 3954.]. Palygorskite, for example, has been investigated in recent decades as an adsorbent for the removal of organic pollutants and metallic ions from solutions [2929 AGUIAR, M.R.M.P., NOVAES, A.C., GUARINO, A.W.S., “Remoção de metais pesados de efluentes industriais por aluminossilicatos”, Química Nova. v. 25, n. 6B, pp. 1145-1154, Dec. 2002.

30 POTGIETER, J.H., POTGIETER-VERMAAK, S.S., KALIBANTONGA, P.D., “Heavy metals removal from solution by palygorskite clay”, Minerals Engineering. v. 19, n. 5, pp. 463-470, Apr. 2006.

31 CHEN, H., WANG, A., “Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay”, Journal of Colloid and Interface Science. v. 307, n. 2, pp. 309-316, Mar. 2007.

32 FAN, Q., LI, Z., ZHAO, H., et al. “Adsorption of Pb (II) on palygorskite from aqueous solution: Effects of pH, ionic, strength and temperature”, Applied Clay Science. v. 45, n. 3, pp. 111-116, Jul. 2009.

33 HE, M., ZHU, Y., YANG, Y., et al. “Adsorption of cobalt (II) ions from aqueous solutions by palygorskite”, Applied Clay Science. v. 54, n. 3-4, pp. 292-296, Dec. 2011.

34 OLIVEIRA, A.B.M., COELHO, L.O., GOMES, S.S., et al. “Brazilian Palygorskite as Adsorbent of Metal Ions from Aqueous Solution – Kinetic and Equilibrium Studies”, Water Air Soil Pollution. v. 224, n. 1687, Aug. 2013.
-3535 WANG, J., SUN, T., SALEEM, A., et al. “Enhanced adsorptive removal of Cr (VI) in aqueous solution by polyethyleneimine modified palygorskite”, Chinese Journal of Chemical Engineering. v. 28, pp. 2650-2657, Oct. 2020.]. Palygorskite has been used for this purpose due to an excellent adsorptive property, which are inherent in its porous structure associated with its high surface area, generally between 125 and 210 m2 g-1 [3636 LUZ, A.B., ALMEIDA, S.L.M., “Atapulgita e Sepiolita”, In: CETEM/MCT, Rochas e Minerais Industriais, Usos e Especificações, 2 ed., chapter 10, Rio de Janeiro, Brasil, 2008.], giving it an advantage in the adsorption of heavy metals. Besides that, the increasing interest in the use of this material as adsorbent is related to its high worldwide availability, low cost and possibility of reuse.

The chemical composition of palygorskite unit cell is (Mg,Al)5Si8O20(OH)2(OH2)4.4H2O. Mineralogically, palygorskite belongs to the group of 2:1 phyllosilicates and has a three-dimensional structure consisting of a double layer composed of silicon oxide tetrahedra bound by octahedral magnesium ions [3737 SANTOS, P. S., In: Edgard Blucher, Ciência e Tecnologia de Argilas, 2 ed, São Paulo, Brazil, 1989., 3838 BERTOLINO, L.C., SILVA, F.A.N.G., BRANDÃO, V.S., et al. “Use of Brazilian palygorskite as adsorbent of lead and cadmium ions from aqueous solutions”, In: XVI International Clay Conference, pp. 82, Granada, Jul. 2017.]. These tapered layers are joined at the ends by Si-O-Si bonds, resulting in a porous structure with channels [3939 HADDEN, W., SCHWINT, I., “Attapulgite: its properties and applications”, Industrial Engineering. v. 59, n. 9, pp. 58-69, Sep. 1967., 4040 GALAN, E., “Properties and applications of palygorskite-sepiolite clays”, Clay Minerals. v. 31, n. 4, pp. 443-453, Dec. 1996.] which have exchangeable cations and water [4141 OLIVEIRA, C., “Caracterização tecnológica de atapulgitas do Piauí”, In: XX Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa, p. 49-56, Florianópolis, Jun. 2004.].

In palygorskite structure, the most common substitutions are Si4+ in the tetrahedra layer, by trivalent cations (Al3+ or Fe3+) and Al3+ cations in the octahedra layer, by divalent cations (Mg2+ or Fe2+), leading to a deficiency of positive charges and to a negative potential on the clayey surface [4242 GUERRA, D.L., LEMOS, V.P., ANGÉLICA, R.S., et al. “Influência de argilas pilarizadas na decomposição catalítica do óleo de andiroba”, Eclética Química. V. 32, n. 4, pp. 19-26, Nov. 2007., 4343 EL-MOFTY, S.E., ASHOUR, F.H., EL-SHALL, H., “Adsorption mechanism of toxic metal ions by clay (attapulgite)”, In: Twelfth International Water Technology Conference, p. 403–414, Alexandria, Mar. 2008.].

Palygorskite is a typical fibrous clay, whose fibers have active sites for adsorption, with the silanol group (SiO-H) being the predominant site in palygorskite. These sites can interact through hydrogen bonds to adsorb cationic species or accumulated molecules at the interfaces [2323 SILVA, T.L., RONIX, A., PEZOTI, O., et al. “Mesoporous activated carbon from industrial laundry sewage sludge: Adsorption studies of reactive dye Remazol Brilliant Blue R”, Chemical Engineering Journal. v. 303, pp. 467–476, Nov. 2016.].

In Brazil, the main palygorskite deposits are in the municipality of Guadalupe, in Piauí’s state [4444 BALTAR, C.A.M., BALTAR, L.M., BEZERRA, F.J., et al. “Influence of morphology and surface charge on the suit ability of palygorskite as drilling fluid”, Applied Clay Science. v. 42, n. 3-4, pp. 597-600, Jan. 2009., 4545 BERTOLINO, L.C., ALMEIDA, S.L.M., LUZ, A.B., et al. “Estudo de ativação ácida da atapulgita do Piauí para clarificação de óleos”, In: XXIV Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa, p. 530-36, Salvador, Oct. 2011.] and in the region of Alcântara, in the state of Maranhão [4646 AMORIM, K.B., ANGELICA, R.S., “Mineralogia e geoquímica da ocorrência de palygorskita de Alcântara, Bacia de São Luis-Grajaú, Maranhão”, Cerâmica. v. 57, pp. 483-490, Dec. 2011. , 4747 RODRIGUES, G.M.A., NEVES, R.F., ANGÉLICA, R.S., “Beneficiamento de uma argila tipo paligorskita da bacia de S. Luis-Grajaú, região de Alcântara, MA, e sua utilização como adsorvente de fósforo”, Cerâmica, v. 60, pp. 117-126, Mar. 2014.]. Among Brazilian palygorskite occurrences, the reserves in the Guadalupe region have the greatest potential for economic use. However, samples from this region are not used for more noble commercial purposes, being a national raw material, free from more technological economic interests. Thus, in this work, the efficiency of the processed Guadalupe’s palygorskite sample as an adsorbent for the removal of Pb2+ from aqueous solutions was investigated by batch tests.

A kinectic study was conducted in order to determine the necessary time to reach the equilibrium of the system, aiming to assess relevant information about its adsorption rate. Besides that, the relationship between the effluent concentration and the amount of adsorbed ions were described by Langmuir and Freundlich isotherms models.

After adsorption tests, X-ray fluorecence (XRF), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) coupled with Energy-dispersive X-ray spectroscopy (EDS) of the system were conducted in order to verify the presence of lead ions in palygorskite.

2. MATERIALS AND METHODS

2.1 Preparation and characterization of palygorskite (adsorbent)

The preparation and characterization of palygorskite was performed as previously described [44 SIMÕES, K.M.A., NOVO, B.L., FELIX, A.A.S., et al. “Ore Dressing and Technological Characterization of Palygorskite from Piauí/Brazil for Application as Adsorbent of Heavy Metals”, In: ed. Springer International Publishing, Characterization of Minerals, Metals and Materials, chapter 7, California, USA, 2017.]. In this work, it was added the textural properties study, which is an important characteristic of the adsorbent material.

The textural properties were determined by nitrogen (N2) physisorption at -196ºC. Analyses were performed with a Micrometrics using approximately 400 mg of sample. Pretreatment of the samples consisted of drying at 100°C under vacuum at 1x10-6 mmHg for 24 hours for the elimination of physically adsorbed water. Then the actual analysis was carried out, in which the adsorption and desorption isotherms were obtained by varying the partial pressure of N2. With the isotherms, the surface area was calculated by the B.E.T method and the pore size distribution was obtained from the desorption isotherm of N2 by the B.J.H. (Barret-Joyner-Halenda) method.

2.2 Adsorption experiments

Kinetic experiments were previously performed using 34 mg L-1 of a lead synthetic effluent prepared with lead II nitrate salt (Pb(NO3)2) from Sigma-Aldrich and 2 g of palygorskite at pH 5. For different tested times (from 10 to 50400 min), the samples were agitated on an IKA model KS 4000i orbital shaker table with incubation, rotating at 250 rpm at room temperature (approximately 21°C), then centrifuged for 10 minutes in Cientec CT-6000 digital microprocessed bench centrifuge. The supernatant was filtered with qualitative filter paper and submitted to flame atomic absorption spectrometry (FAAS) for Pb2+ ions quantification. The analyses were performed with a Varian 50B model with acetylene flame and nitric acid solutions (HNO3) were used to construct the analytical curve.

To batch adsorption tests, palygorskite mass was weighed and transferred to a Falcon tube with 40 mL of synthetic effluent. The pH was adjusted with 0.1 mol L-1 NaOH and HCl solutions using the Metrohm model 827 pH meter. The samples were then subjected to ultrasonic scattering to disperse the particles in homogeneous condition and then agitated on the same shaker and at equal conditions described for kinectics experiments. A schematic representation of adsorption process steps can be seen in Fig. 1.

Figure 1
Representative scheme of adsorption experiments steps.

The lead adsorption isotherm was studied in order to understand the relationship between the effluent’s concentration and the amount of Pb2+ ions adsorbed. The experiments were carried out using the lead solution concentration from 34.1 to 3360 mg L-1. The contact time was defined according to the kinetic study, where 2 g of the sample was used at pH 5. All readings were performed in triplicate and accompanied by an analytical blank.

The metal ion adsorption capacity of the adsorbent in batch test was calculated by the Equation (1)

q e = ( C a C e ) V m (1)

Where:

  • qe is the equilibrium capacity of lead on the adsorbent (mg g−1);

  • Co the initial concentration of lead solution (mg L−1);

  • Ce the equilibrium concentration of lead solution (mg L−1);

  • m the mass of adsorbent used (g);

  • V the volume of lead solution (L).

2.3 Characterization after adsorption

In order to characterize the metal ions, present on palygorskite after adsorption process, X-ray fluorecence (XRF), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) coupled with Energy-dispersive X-ray spectroscopy (EDS) analyses were conducted.

X-ray fluorecence (XRF) was performed on an X-ray fluorescence spectrometer, (WDS-1), model AXIOS MAX (Panalytical) at X- Ray Fluorescence Laboratory in CETEM. The analysis was conducted with 0.200 g which was subjected to muffle furnace with a temperature of 1,000ºC for 16 hours. After cooling in desiccator, the samples were weighed to check the loss of ignition (LOI). For chemical analysis, approximately 5.0 g were placed in an oven at 100ºC for 24 hours and cooled in a desiccator. Then 3.0 g of the sample was added to 0.3 g of boric acid and pressed in an automatic press VANEOX (20 mm mold) for 3 series of 10 seconds with a pressure of 20 tons.

Fourier Transform Infrared Spectroscopy (FTIR) was performed in the Laboratory of Instruments and Research of the Chemistry Institute at UFRJ using Nicolet 6700 FT-IR spectrophotometer, with records from 4,000 to 400 cm-1 and 4 cm-1 of resolution in KBr tablets.

Scanning electron microscopy (SEM) analysis was performed on Tabletop microscope equipment, brand HITACHI TM303 Plus. 1.0 g of the sample was pressed with carbon in FLUXANA press (Vaneox® Technology) and analyzed in backscattered electron modules and energy dispersive spectrometry (EDS) for chemical elements qualitative determination. The mapping of lead element distribution was carried out in 200 seconds with 5000x of approximation.

3. RESULTS AND DISCUSSION

3.1 Determination of Textural Properties

Table 1 presents the textural properties (surface area, pore volume and average pore size). According to the results, palygorskite sample has a surface area of 71.29 m2 g-1. In the kinetic and isothermal study of lead adsorption, the surface area obtained using a Chinese palygorskite of 200 mesh (75 μm) was 48.66 m2 g-1 [3131 CHEN, H., WANG, A., “Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay”, Journal of Colloid and Interface Science. v. 307, n. 2, pp. 309-316, Mar. 2007.]. Thus, it is possible to conclude that previous ore dressing corroborates with the increase of the surface area. The pore volume is 0.24 cm3 g-1, which is in accordance with the values obtained for this clay mineral in literature [4848 ZHANG, J., WANG, Q., CHEN, H., et al. “XRF and nitrogen adsorption studies of acid-activated palygorskite”, Clay Minerals, v. 45, n. 2, pp. 145-156, Jun. 2010., 4949 WANG, Y., QIN, Z., GUO, P., et al. “Preparation of attapulgite carriers with different pore structures and their effects on thermophysical properties of composite phase change materials”, AIP Advances. v. 9, Oct. 2019.]. The average pore size is 12.23 nm, which characterizes this clay mineral as a mesoporous material, which covers pores with an internal width between 2 and 50 nm [5050 SING, K.S.W., EVERETT, D.H., HAUL, R.A.W., et al. “Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity”, Pure and Applied Chemistry. v. 57, n. 4, pp. 603-619, 1985].

Table 1
Textural properties of Guadalupe’s palygorskite.

According to the International Union of Pure and Applied Chemistry (IUPAC) [5050 SING, K.S.W., EVERETT, D.H., HAUL, R.A.W., et al. “Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity”, Pure and Applied Chemistry. v. 57, n. 4, pp. 603-619, 1985], isotherms can be classified into six different characteristic types according to the nature of the solid. The hysteresis presence, observed in two types of isotherm, occurs because the saturation pressures are not the same for condensation and evaporation inside the pores, that is, the adsorption and desorption isotherms do not coincide. The isotherm profile obtained (Fig. 2) is classified as type IV, characteristic of mesoporous materials.

Figure 2
Isotherm profile obtained by B.E.T method.

It can be observed in the adsorption isotherm, that the initial process occurs in monolayer at low pressures, followed by adsorption in multilayers. At this point, the amount of gas adsorbed increases rapidly with little pressure variation. Considering that the predominant class of pores in a solid material is the one where the largest amount of N2 was released to fill the pores volume with a certain diameter, and that for palygorskite sample, the average pore size largest distribution was between the range of 2 to 50 nm, this sample is classified as being a mesoporous material, corroborating with the obtained isotherm. Hysteresis provides important information about pores shape. The type of hysteresis in the studied sample according to IUPAC was classified as hysteresis type H3 and indicates the presence of pores in cracks, which format is associated with non-rigid particles aggregates in the form of plates originating this type of pore.

3.2. Kinetics studies

Fig. 3 shows the amount of lead ions adsorbed by palygorskite sample according to contact time. Equilibrium was reached in 10 min, with 1.2492 mg g-1 of Pb2+ adsorbed, representing a removal of 99.14%. The same values were found for 60 min of contact and in order to guarantee the equilibrium conditions of the system, the tests were performed with a longer equilibrium time than that observed, since 60 min could be considered a viable time for the development of the process on an industrial scale. Besides that, it is a way of ensuring a complete adsorption of lead ions to the active sites present in palygorskite structure.

Figure 3
Equilibrium time of palygorskite sample.

According to CHEN and WANG [3131 CHEN, H., WANG, A., “Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay”, Journal of Colloid and Interface Science. v. 307, n. 2, pp. 309-316, Mar. 2007.], in their lead ions adsorption kinetic study by a 200 mesh (75 µm) palygorskite determined that the equilibrium time was reached in 8 hours of contact time and remained constant. FAN et al. [3232 FAN, Q., LI, Z., ZHAO, H., et al. “Adsorption of Pb (II) on palygorskite from aqueous solution: Effects of pH, ionic, strength and temperature”, Applied Clay Science. v. 45, n. 3, pp. 111-116, Jul. 2009.], on the other hand, using the same clay mineral granulometry, verified that the equilibrium was reached at 3 hours of contact. Although both palygorskite samples are from the same location (Gansu, China), the shorter equilibrium time obtained by FAN et al. [3232 FAN, Q., LI, Z., ZHAO, H., et al. “Adsorption of Pb (II) on palygorskite from aqueous solution: Effects of pH, ionic, strength and temperature”, Applied Clay Science. v. 45, n. 3, pp. 111-116, Jul. 2009.], may be associated with the heat treatment carried out, since with the increase in temperature the loss of water molecules occurs, providing more adsorption sites.

POTGIETER et al. [3030 POTGIETER, J.H., POTGIETER-VERMAAK, S.S., KALIBANTONGA, P.D., “Heavy metals removal from solution by palygorskite clay”, Minerals Engineering. v. 19, n. 5, pp. 463-470, Apr. 2006.], investigated the removal of Pb (II) from an aqueous solution of palygorskite and found that the equilibration time for this ion was only 30 min for Pb (II) concentrations of 20 to 100 mg L-1. According to the authors the balance was quickly reached, indicating that the adsorptive sites were more exposed. The rapid adsorption (10 min) obtained in this study may be associated with the previous ore dressing carried out on palygorskite sample [44 SIMÕES, K.M.A., NOVO, B.L., FELIX, A.A.S., et al. “Ore Dressing and Technological Characterization of Palygorskite from Piauí/Brazil for Application as Adsorbent of Heavy Metals”, In: ed. Springer International Publishing, Characterization of Minerals, Metals and Materials, chapter 7, California, USA, 2017.], promoting a purification and reduction of particle size (20 µm) that allowed the active sites to become more available for the studied ion.

Aiming to investigate palygorskite adsorption process characteristics on lead removal and the controlling step of the potential rate (chemical reaction, diffusion control and mass transfer), the kinetic data were analyzed by using pseudo-first order and pseudo-second order linear models, as presented in Equations 2 e 3, respectively [5151 NASCIMENTO, R.F., LIMA, A.C.A., VIDAL, C.B., et al. Adsorção: aspectos teóricos e aplicações ambientais, Fortaleza, Imprensa Universitária, 2014.].

L og   ( q e q t ) = log   q e k 1 2 , 303 t (2)
t q t = 1 k 2 q e 2 + t q e (3)

Where:

  • qe is the amount adsorbed per gram of adsorbent at equilibrium (mg g-1);

  • qt is the amount adsorbed per gram of adsorbent at time t (mg g-1).

  • t is the contact time;

  • k1 is the constant rate of pseudo-first order adsorption (min-1);

  • k2 is the constant rate of pseudo-second order adsorption (g mg-1 min-1);

The values of k1 and qe were calculated from slope and intercept of the plot of log (qe-qt) vs. t, while the values of k2 and qe were evaluated from the intercept and slope of a plot t/qt vs. t. Both kinetic constants k1 and k2 are interpreted as a time scale factor, which aim to indicate the speed with which the system reaches equilibrium. The applicability of the model with the best fit of the data is conditioned to a linear relationship close to 1.

Table 2 shows the kinetic parameters obtained for both kinetic models and the linearized graphs of pseudo-first order and pseudo-second order models are presented in Figure 4, respectively.

Table 2
Kinetic parameters of pseudo-first order and pseudo-second order models.
Figure 4
Linearized pseudo-first order model (A) and linearized pseudo-second order model (B).

According to analysis of the results, it is observed that the rate correlation coefficients of pseudo-second order kinetic model (R2 = 1.0000) is higher than that obtained for pseudo-first order model (R2 = 0.8178). In addition, pseudo-second order parameters qex and qc demonstrated a good agreement with each other, indicating that this model is the one that best describes the adsorption kinetics of lead ions by palygorskite.

As stated by WANG and GUO [5252 WANG, J., GUO, X., “Adsorption kinetic models: Physical meanings, applications, and solving methods”, Journal of Hazardous Materials. v. 390, May. 2020.], a system that better fits a pseudo-second order model indicates that the adsorbent is abundant with active sites. This fact corroborates with the short equilibrium time obtained in this work that was due to an efficient ore dressing step that increased the adsorptive sites availability. This result indicated that the sorption process was complex and involved more than one mechanism [5353 JINTAKOSOL, T., NITAYAPHAT, W., “Adsorption of Silver (I) From Aqueous Solution Using Chitosan/Montmorillonite Composite Beads”, Materials Research. v. 19, n. 5, pp.1141-1121, Jul. 2016.].

3.3. Adsorption studies

In order to study the relationship between the amount of ions adsorbed by work sample and lead solution concentration (adsorption isotherm), the contact time of 60 min was fixed, and solution concentration was varied until reaching balance. Among mathematical models of isotherms used in liquid systems that involve the treatment of wastewater and industrial effluents, the main models to describe the adsorption equilibrium are the Langmuir and Freundlich isotherms [5454 SILVA, J.C., “Desenvolvimento de processo integrado utilizando processos de separação por membrana e adsorção em carvão ativado para o tratamento de água associada à produção de petróleo”, M.Sc. Dissertation., COPPE/UFRJ, Rio de Janeiro, RJ, Brazil, 2010.].

Langmuir isotherm is a theoretical equilibrium isotherm which relates the amount of solute adsorbed on a surface to the concentration of the solute in the solution. This model is based on the hypothesis that the interaction forces between the adsorbed molecules are negligible and that each site can be occupied by only one molecule. All molecules are adsorbed on a fixed number of sites. For high concentrations of solute, the equation predicts a limited capacity by the formation of the monolayer.

Freundlich isotherm is an empirical adsorption isotherm for non-ideal adsorption on heterogeneous surfaces, as well as for multilayer adsorption.

The adsorption capacity of a saturated monolayer (Langmuir) can be represented by Equation 4 [3131 CHEN, H., WANG, A., “Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay”, Journal of Colloid and Interface Science. v. 307, n. 2, pp. 309-316, Mar. 2007.]. The Equation 5 represents the linearized form for the Freundlich model [3131 CHEN, H., WANG, A., “Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay”, Journal of Colloid and Interface Science. v. 307, n. 2, pp. 309-316, Mar. 2007.].

C e q e = 1 b q m + C e q m (4)
log   q e = log   K f + 1 n log   C e (5)

Where:

  • Ce is the concentration of the solution in equilibrium (mg L-1);

  • qe is the amount of solute adsorbed at equilibrium (mg g-1 of adsorbent);

  • qm represents the maximum amount of solute adsorbed to form a complete monolayer (mg g-1);

  • b is the adsorption constant of Langmuir, which is related to free energy of adsorption;

  • Kf and n are Freundlich’s constants;

  • Kf indicates the adsorption capacity of the adsorbent and n indicates the effect of concentration on the adsorption capacity and represents the adsorption intensity.

Adsorption isotherms were obtained from the experimental data using Equations 4 and 5. The isotherms constants and the correlation coefficients were calculated from the linearization of the Langmuir and Freundlich equations by plotting Ce/qe vs. Ce and log qe vs. log Ce. These results are shown in Table 3 and Figure 5.

Table 3
Langmuir and Freundlich constants and correlation coefficients associated with Pb (II) adsorption isotherms by palygorskite.
Figure 5
Linearized adsorption isotherm for lead ions using Langmuir model (A) and Freundlich model (B).

For the adsorption of lead ions, the most suitable model was the Langmuir isotherm model, considering the higher value of R2 (0.9943) in relation to the Freundlich isotherm (0.9838). This can occur due to the homogeneous distribution of active sites on the palygorskite surface. The maximum adsorption value (qm) of 21.65 mg g-1 and the value of b equal to 0.0185 indicate that the studied sample can be used to adsorb lead ions in effluents.

The adsorption data of lead ions by palygorskite also fit in the Freundlich model, but not as well as in the Langmuir model. The adsorption profile of palygorskite is well described by the Langmuir equation [3030 POTGIETER, J.H., POTGIETER-VERMAAK, S.S., KALIBANTONGA, P.D., “Heavy metals removal from solution by palygorskite clay”, Minerals Engineering. v. 19, n. 5, pp. 463-470, Apr. 2006.

31 CHEN, H., WANG, A., “Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay”, Journal of Colloid and Interface Science. v. 307, n. 2, pp. 309-316, Mar. 2007.
-3232 FAN, Q., LI, Z., ZHAO, H., et al. “Adsorption of Pb (II) on palygorskite from aqueous solution: Effects of pH, ionic, strength and temperature”, Applied Clay Science. v. 45, n. 3, pp. 111-116, Jul. 2009., 3434 OLIVEIRA, A.B.M., COELHO, L.O., GOMES, S.S., et al. “Brazilian Palygorskite as Adsorbent of Metal Ions from Aqueous Solution – Kinetic and Equilibrium Studies”, Water Air Soil Pollution. v. 224, n. 1687, Aug. 2013., 5555 ÁLVAREZ-AYUSO, E., GARCÍA-SÁNCHEZ, A., “Palygorskite as a feasible amendment to stabilize heavy metal polluted soils”, Environmental Pollution. v. 125, n. 3, pp.337-344, Oct. 2003.].

The coefficient correlation value was very close to the unit (R2 = 0.9943) indicating that the obtained adsorption curve really obeys the Langmuir model. Therefore, the apparent equilibrium constant can be estimated to be 5630.40 L mol-1 and Gibbs’ adsorption-free energy, calculated according to Equation 6, using T= 298K, was -21.39 KJ mol-1. This calculated ΔG value indicated that in this case a chemical and spontaneous adsorption process occurs through interactions between the adsorbed cation and the basic centers of the adsorbent [4242 GUERRA, D.L., LEMOS, V.P., ANGÉLICA, R.S., et al. “Influência de argilas pilarizadas na decomposição catalítica do óleo de andiroba”, Eclética Química. V. 32, n. 4, pp. 19-26, Nov. 2007., 5656 CHEN, H., ZHAO, J., “Adsorption study for removal of Congo red anionic dye using organo-attapulgite”, Adsorption. v. 15, pp. 381-389, Feb. 2009.], at 25°C.

ΔG = RT   ln   K (6)

Where R is the gas constant (8.31451 J K-1 mol-1) and T the temperature in Kelvin.

3.4. Characterization after adsorption

Table 4 presents the chemical composition of palygorskite sample after adsorption. In our previous work [44 SIMÕES, K.M.A., NOVO, B.L., FELIX, A.A.S., et al. “Ore Dressing and Technological Characterization of Palygorskite from Piauí/Brazil for Application as Adsorbent of Heavy Metals”, In: ed. Springer International Publishing, Characterization of Minerals, Metals and Materials, chapter 7, California, USA, 2017.], the sample before adsorption presented 50.50, 15.50, 4.50, 7.10 and 18.6% w/w of SiO2, Al2O3, MgO, Fe2O3 and Loss on Ignition (LOI), respectively. It can be observed that before adsorption, the content of these oxides increased due to a decrease in (LOI). The presence of lead oxides (PbO), not present before, confirms its adsorption onto palygorskite.

Table 4
XRF of palygorskite sample after lead adsorption.

According to Figure 6, it is possible to observe the results obtained for spectroscopy in the Infrared Region with Fourier transform (FTIR) for the systems before adsorption (BA) and after adsorption (AA). According to the BA spectrum, there is a 3699 cm-1 band that can be attributed to the vibrational stretching of Mg-OH palygorskite group, followed by the same vibrational species of Al-O-H group at 3621 cm-1 characteristics of palygorskite. The bands at 3550 and 3428 cm-1 can be attributed to coordination and zeolitic waters [5757 SUÁREZ, M., GÁRCIA-ROMERO, E., “FTIR spectroscopic study of palygorskite: Influence of the composition of the octahedral sheet”, Applied Clay Science. 31, n. 1-2, pp. 154-163, Jan. 2006.], while the band at 1651 cm-1 was attributed to water molecules angular deformation [5858 SRASRA, N.F., SRASRA, E., “Acid treatment of south Tunisian palygorskite: Removal of Cd (II) from aqueous and phosphoric acid solutions”, Desalination. v. 250, n. 1, pp. 26-34, Jan. 2010.]. Characteristic bands of Si-O-Si vibrations can be seen in 1033, 913 cm-1 and the bands in 517 and 469 cm-1 referes to the vibration of the Si-O-Al and Si-OH respectively [4242 GUERRA, D.L., LEMOS, V.P., ANGÉLICA, R.S., et al. “Influência de argilas pilarizadas na decomposição catalítica do óleo de andiroba”, Eclética Química. V. 32, n. 4, pp. 19-26, Nov. 2007.], The bands at 694 and 428 cm-1 correspond to the Si-O-Mg vibrations [5959 LAZAREVIÉ, S., JANKOVIÉ-CASTVAN, I., DJOKIÉ, V., et al. “Iron-modified sepiolite for Ni2+ sorption from aqueous solution: An Equilibrium, kinetic, and thermodynamic study”, Journal of Chemical Engineering Data. v. 55, n. 12, pp. 5681-5689, Nov. 2010.].

By spectra analysis, displacements of the bands are observed in the AA spectrum, which may indicate the presence of lead ions on the clay mineral surface. These displacements are probably due to ion exchange between Pb2+ and H+ presented in –OH groups of palygorskite, since the bands containing deformations and stretches related to this functional group were precisely those that obtained the greatest displacements, in addition, to a possible electrostatic interaction between Pb2+ with the silanol (Si-OH) and aluminol (Al-OH) groups present on the palygorskite surface. Besides that, such changes in wavelength and intensity of transmittance are addressed in literature as being indicative of the presence of contaminants in the adsorbent [6060 MOYO, M., NYAMHERE, G., SEBATA, E., et al. “Kinetic and equilibrium modelling of lead sorption from aqueous solution by activated carbon from goat dung”, Desalination and Water Treatment. v. 57, n. 2, pp.765-775, Oct. 2014.

61 CHATTERJEE, S., DE, S., “Application of novel, low-cost, laterite-based adsorbent for removal of lead from water: Equilibrium, kinetic and thermodynamic studies”, Journal of Environmental Science and Health. Part. A, v. 51, n. 3, pp. 1-11, Dec. 2015.

62 TANG, C., SHU, Y., ZHANG, R., et al. “Comparison of the removal and adsorption mechanisms of cadmium and lead from aqueous solution by activated carbons prepared from Typha angustifolia and Salix matsudana”, RSC Advances. v. 7, n. 26 , pp 16092-16103, Mar. 2017.
-6363 BOMBUWALA DEWAGE, N., FOWLER, R.E., PITTMAN, C.U., et al. “Lead (Pb2+) sorptive removal using chitosan-modified biochar: batch and fixed-bed studies”, RSC Advances. v. 8, n. 46, pp. 25368-25377, Jul. 2018. ].

Figure 6
FTIR of palygorskite sample after lead adsorption.

Mapping and EDS obtained by SEM for palygorskite sample after lead adsorption is shown in Fig. 7 and 8, respectively. The blue image represents the mapping of Mg, characteristic palygorskite element, and the red image represents the lead present on the sample surface. Thus, it was observed that the metal is distributed homogeneously on palygorskite surface. Besides that, EDS result confirmed the presence of lead, corroborating with the results of adsorption, XRF and FTIR presented, evidencing that lead had been adsorbed by the clay mineral.

Figure 7
Mapping of magnesium (blue) and lead (red) distribution for palygorskite sample.
Figure 8
EDS of palygorskite sample after lead adsorption.

It is worth mentioning that modification of clay minerals, such as thermal activation [6464 TORRES-LUNA, J.A., CARRIAZO, J.G., “Porous aluminosilicic solids obtained by thermal-acid modification of a commercial kaolinite-type natural clay”, Solid State Sciences. v. 88, pp. 29-35, Feb. 2019.], acid treatment [6565 ESPAÑA, V.A.A., SARKAR, B., BISWAS, B., et al. “Environmental applications of thermally modified and acid activated clay minerals: Current status of the art”, Environmental Technology & Innovation. v. 13, pp. 383-397, Feb. 2019.] and pillarization [6666 BARAKAN, S., AGHAZADEH, V., “The advantages of clay mineral modification methods for enhancing adsorption efficiency in wastewater treatment: a review”, Environmental Science and Pollution Research. v. 28, pp. 2572–2599, Oct. 2020.], can be achieved by using any of the available modification methods, but there are challenges associated with each method that needs to be considered before making a decision. Although clay mineral for heavy metal remediation is being used because it is cost-effective and environmentally friendly, however, modification may result in additional costs and release of new chemical agents into the environment [6767 ROES, A.L., MARSILI, E., NIEUWLAAR, E., et al. “Environmental and Cost Assessment of a Polypropylene Nanocomposite”, Journal of Polymers and the Environment. v. 15, pp. 212-226, Aug. 2007., 6868 LI, J., WANG, X., ZHAO, G., et al. “Metal-organic framework-based materials: superior adsorbents for the capture of toxic and radioactive metal ions”, Chemical Society Reviews. v. 47, pp. 2322-2356, Mar. 2018.]. Thus, a previous physical treatment, such as an ore dressing to reduce granulometry and consequently existing impurities, already consists of an efficient process to increase the removal capacity of these contaminants, especially lead ions, as presented in this work.

4. CONCLUSIONS

The determination of the textural properties indicates that Brazilian palygorskite has a surface area of 71.29 m2 g-1, a pore volume of 0.24 cm3 g-1 and an average pore size of 12.23 nm, characteristic of mesoporous materials.

The kinetic study revealed that Pb (II) adsorption onto palygorskite clay reached rapid equilibrium (10 min) and was well represented by pseudo-second order kinetic model. Batch adsorption assays were efficient (99.14%) for lead ions with 60 min shaking on an orbital shaker table, and pH = 5.

Langmuir isotherm model was most suitable to fit the data obtained in the adsorption process, since the R² value was higher than that obtained for the Freundlich model. Thus, the results obtained suggest that the adsorption process occurs in a monolayer on the active sites of each adsorbent. The studied sample presented adsorption capacity of 21.65 mg g-1 and Gibbs’ adsorption-free energy was -21.39 KJ mol-1.

Therefore, the data obtained for both kinetic study and adsorption isotherm indicate a chemisorption process, and according to the free energy of adsorption, the reaction occurs spontaneously, evidencing palygorskite adsorptive potential.

XRF, FTIR and mapping coupled with EDS results after adsorption confirmed the presence of the metal on clayey surface, ratifying palygorskite efficiency in lead ions removal.

5. ACKNOWLEDGEMENTS

The authors thank the collaborators in this research, since without the help and contribution it would not be developed. Thanks go to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for their financial support.

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Publication Dates

  • Publication in this collection
    13 Jan 2023
  • Date of issue
    2022

History

  • Received
    17 Aug 2021
  • Accepted
    01 Apr 2022
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