Sorption and desorption studies on silver ions from aqueous solution by coconut fiber

Accepted Manuscript Sorption and desorption studies on silver ions from aqueous solution by coconut fiber Paweł Staroń, Jarosław Chwastowski, Marcin Banach PII:

S0959-6526(17)30339-6

DOI:

10.1016/j.jclepro.2017.02.116

Reference:

JCLP 9042

To appear in:

Journal of Cleaner Production

Please cite this article as: Paweł Staroń, Jarosław Chwastowski, Marcin Banach, Sorption and desorption studies on silver ions from aqueous solution by low-cost biosorbent, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.02.116 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Possible application of biosorbent for Ag ions sorption.

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The effects of several physiochemical parameters were investigated.

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The sorption kinetics were used to explain the sorption mechanism.

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The thermodynamic studies of sorption process were performed.

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The coconut fiber has a great potential for removal of Ag+ ions.

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ACCEPTED MANUSCRIPT Wordcount: 7113 Sorption and desorption studies on silver ions from aqueous solution by coconut fiber Paweł Staroń*, Jarosław Chwastowski, Marcin Banach

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Department of Engineering and Chemical Technology, Cracow University of Technology, 24 Warszawska St., 31-155 Cracow, Poland *

Corresponding author:

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[email protected] Tel: +48 12 628 20 92

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Fax: +48 12 628 20 36

Abstract

This paper presents a method for removing silver ions from aqueous solutions using sorption on biological material (coconut fiber). Sorption was carried out in a bath-type bed.

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The effect of several parameters, such as initial concentration, pH, temperature, etc. on the degree of removal of Ag+ from the solution was examined. The results obtained at the equilibrium state allowed us to fit an isotherm sorption model and to calculate the maximum

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sorption capacity. The models that best fit the results for the sorption of silver ions on coconut fiber at different temperatures are the Langmuir and Freundlich isotherm models, but at various pH-values the highest fit ratio is characterized by the Langmuir isotherm model. The

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results of the sorption kinetics revealed that the pseudo second-order model is best suited for the sorption process of Ag+ on coconut fiber, and on that basis it can be concluded that the limiting step of the sorption process is chemisorption. The thermodynamic calculations indicate that adsorption of silver on coconut fiber occurs in two different ways (sorption and biosorption), depending on the concentration of silver. Desorption of silver from coconut fiber showed that the best eluents for this process are organic acids (citric and acetic acids). The use of NaOH as eluent allows silver particles of nanometric size to be obtained on the surface of the sorbent.

Keywords: coconut fiber, silver, sorption, desorption, nanoAg 1

ACCEPTED MANUSCRIPT 1. Introduction Silver is one of the sub-components of the earth's crust, in which its average content is equal to 0.1 mg/kg (Purcell and Peters, 1998). Silver has been used in various industries for many years. It has been used since ancient times for jewelry, tableware, drinking vessels, coins, for the production of batteries and bearings, as well as in dental and medical

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applications (Bondarenko et al. 2013). Silver photography was widely used by the 1850s. Each use of silver caused waste generation (silver waste), resulting in increased contamination of waste water. Based on data from silverinstitute.org, in 2014 the demand for silver was 33,000 t per annum. About 2,500 t of silver enters the aquatic environment, of

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which at least 150 t goes into the sludge from wastewater treatment plants, and 80 t are released into surface waters (Kadukova, 2016). In the literature, there are discussions of the dangers associated with silver ions in the environment. Ag+ ions are biologically active,

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interacting with proteins, amino acids, anions, free receptors, and the cell membranes of eukaryotes. They are assimilated in the human body in the form of a protein complex, which is then removed by the liver and kidneys. In addition, the silver ion is toxic to various microbes and marine invertebrates, impairing the enzymes Na/K adenosine triphosphatase and carbonic anhydrase (Chansoo and Yufeng, 2012). Silver is bioaccumulated by several species of marine phytoplankton, macroalgae, and invertebrates. The degree of accumulation

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is related to the chemical speciation. The monovalent ion, Ag+, is recognized as one of the most harmful chemical species in water systems. It has been shown that the toxicity of silver to freshwater phytoplankton is directly related to the intracellular accumulation of this ion

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(Syed, 2016).

There are many methods for removing silver ions from the aqueous phase. These include chemical precipitation, coagulation, reverse osmosis, solvent extraction, ion

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exchange, and adsorption. Adsorption is one of the most widely used methods for the removal of silver ions from contaminated water. There are many different sorption materials, including organic polymers, inorganic materials, biomass, and synthetic polymers; the latter are among the most widely studied due to the possibility of altering their synthetic routes in order to obtain different functional groups (Bhattarai et al. 2016). Biosorption is an independent metabolic process, which is fast and reversible (Machalová et al. 2015). Biosorption mechanisms are based on physico-chemical interactions between the metal ion and the functional groups present on the cell surface. Generally, the term biosorption may be used to describe any system where the solid surface of a biological

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ACCEPTED MANUSCRIPT material interacts with the sorbate, resulting in lower concentrations in solution (Fomina and Gadd, 2014). The aim of this study was to examine the ability of coconut fiber (CF) to act as a biosorbent to remove silver ions from model solutions. The main components of coconut fiber are cellulose, hemi-cellulose, and lignin. The amount of lignin in coconut fiber is very high.

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Cellulose and hemi-cellulose are polysaccharide compounds, and lignin can be assigned to the group of macromolecular polyphenol compounds. The strength and ductility of the fiber depend mainly on the type of bonding between the fiber surface and the fiber structure (Arsyad et al. 2015). This study involved the determination of the sorption capacity of

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coconut fiber in relation to silver ions, the fitting of a suitable sorption model, and the determination of the sorption kinetics and thermodynamic parameters of the process. The calculated parameters of the sorption process gave basic information about the mechanisms of

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the sorption process. Moreover, desorption of silver ions from coconut fiber with water and acidic solutions (organic acids) was carried out. This research, involving the use of sorbents of biological origin for the removal of silver ions, can lead to the development of new technologies for removing metal ions from the environment or for reducing their negative

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impact on it.

2. Materials and methods Materials

In this study, coconut fiber was purchased at a commercial store. Silver ion solution

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was prepared by dissolving AgNO3 in deionized water. All reagents used in the study were of analytical grade (Sigma-Aldrich).

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Sorption process

The sorption process of silver ions was conducted in a dynamic “bath”-type bed. The

variable process parameters were: Ag+ ion concentration, the pH of the solutions, the time, and the process temperature. 0.5 g of coconut fiber was added to a polypropylene dish with a capacity of 60 cm3. In the next step, 20 cm3 of a solution containing Ag+ ions at various concentrations (1,000, 2,000, 4,000, 6,000, and 8,000 mg/dm3) and having a pH value of either 2, 3, 4, 4.5, or 5 (HNO3 was used for pH adjustments) was added. The sorption process was carried out at constant temperatures of 20, 30, 40, 50, and 60°C (the constant temperature was maintained with the use of water bath) over 0.5, 1, 2, 4, 6, 8, 10, and 15 minutes. After the process was 3

ACCEPTED MANUSCRIPT completed, samples were filtered under reduced pressure, and the resulting filtrate was analyzed for its silver content by ICP-OES. The sorption process was repeated three times, and the results were averaged. According to Equations 1-4, the sorption capacity at a given time (qt) and at the state of equilibrium (qe), the degree of removal of silver (Re), and the distribution coefficient (Kd, cm3/g) for the coconut fiber were determined (Rahmani-Sani et (

)

∙

(1)

=

(

)

∙

(2)

=

(

)

∙ 100

(3)

=

(

)

∙

(4)

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where:

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=

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al. 2015).

qt – mass of adsorbed silver over time “t” (mg/g)

qe – mass of adsorbed silver at equilibrium (mg/g)

C0 – initial concentration of silver in the aqueous solution (mg/dm3) Ct – concentration of silver after time “t” (mg/dm3)

Ce – concentration of silver at equilibrium (mg/dm3)

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V – volume of solution (dm3)

w – mass of coconut fiber used in sorption process Re – percentage removal of silver at equilibrium (%)

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Kd – distribution coefficient (cm3/g)

Sorption equilibrium

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Equilibrium parameters for silver ions were modeled according to the following

sorption models: Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich.

Langmuir isotherm

This quantitatively describes the formation of a monolayer on the outer layer of the

adsorbent. The Langmuir model represents the equilibrium distribution of metal ions between the liquid and solid phases. The Langmuir isotherm is correct for a single-layer adsorption on a surface including a finite number of identical locations. Based on these assumptions, the Langmuir isotherm is represented by Equation 5 (Dada et al. 2012). =

+

∙

(5) 4

ACCEPTED MANUSCRIPT where, qe – sorption capacity at a state of equilibrium (mg/g) qmax – maximum sorption capacity (mg/g) b – Langmuir constant (L/mg)

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Freundlich isotherm This is used to describe the adsorption properties of heterogeneous surfaces. It is assumed that the sorption process is imperfect, reversible, and multi-layered. The model is represented by Equation 6 (Mousa et al. 2016). +

where: qe – sorption capacity of silver at equilibrium (mg/g)

(6)

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Kf – Freundlich constant (mg1-(1/n)(dm3)1/ng-1)

!

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=

n – heterogeneity factor

Temkin isotherm

This isotherm model assumes that the heat of adsorption of all the particles in the layer

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decreases linearly rather than logarithmically with the coating layer, which is caused by an interaction between the sorbate and the sorbent. The sorption process is characterized by a uniform distribution of energy reaching a particular maximum. The Temkin isotherm model is represented by Equation 7 (Can et al. 2016).

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="#

"=



%$(7) (8)

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where:

+ " #!

Kt – the equilibrium binding constant corresponding to the maximum binding energy (dm3/g) B – constant related to the heat of sorption (J/mol) R – gas constant (8.314 Jmol/K) T – temperature (K) bt – Temkin isotherm constant

Dubinin-Radushkevich isotherm 5

ACCEPTED MANUSCRIPT The Dubinin-Radushkevich (D-R) isotherm model (Eq. 9) is commonly used to express the mechanism of adsorption of a sorbent on non-homogeneous porous structures (Setiabudi et al. 2016). = #

− (" ' ( )

(9)

' = ) #(1 + )

(10)

where: qd – theoretical maximum isotherm saturation capacity (mg/g)

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#

Bd – Dubinin–Radushkevich isotherm constant related to the sorption energy (mol2/J2)

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' – Polanyi potential

The mean sorption energy magnitude can give an idea about the type of sorption process,

*= where: E – the mean sorption energy (J/mol)

Sorption kinetics

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whether it is physical or chemical.

(11)

+(,-

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The contact time is one of the most important parameters affecting the effectiveness of the adsorption process. In order to investigate the effect of contact time and kinetic behavior on the sorption process, the coconut fiber sorption capacity was measured at different times

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for different initial concentrations. The sorption process was carried out from 20 to 60°C, each successive run differing by 10°C. The data modeling the rate of sorption was used for

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the following models.

Pseudo-first order rate model In this model, it is assumed that the rate of discharge of silver over time is directly

proportional to the difference between the mass of adsorbed silver in the equilibrium state and the mass of adsorbed silver on the sorbent at a given time (qe-qt). The linear equation for this model is represented by Equation 12 (Yoosefian et al. 2017). log(

−

)=

1

2 − (.454 6

(12)

where: qe – amount of silver sorbed at equilibrium

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ACCEPTED MANUSCRIPT qt – amount of silver sorbed at time t k1 – the pseudo-first order rate constant (1/min) t – time (min)

Pseudo-second order rate model

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The pseudo-second order model assumes that the process of sorption process is of a pseudo-chemical nature. In this model, the driving force is the difference between the equilibrium capacity of the sorbent and the mass of adsorbate adsorbed in a given time (qe-qt). The overall rate of sorption is proportional to the square of the driving force. The pseudo-

2016). 7

7

+

(13)

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=1

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second order kinetics sorption model is expressed by the following Equation (Tadjarodi et al.

where:

k2 – the pseudo-second order rate constant (g/mgmin) Elovich model

The Elovich equation is one of the most common heterogeneous kinetic equations

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describing the adsorption of gases on solid surfaces (de Vries et al. 2015). The model assumes a multilayer adsorption and that the number of active sites increases exponentially with sorption. The Elovich model is described by Equation 14 (Inyang et al. 2016). (14)

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= 8 ln(:;) + 8 ln (6)

where:

: – the initial sorption rate (mg/gmin)

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; – the desorption constant (g/mg)

The values α and β are related to the degree of surface coverage and the activation energy for adsorption.

Intra-particle diffusion model (Weber-Morris model) The intramolecular diffusion model assumes that the effect of diffusion of the sorbate is a phase determining the sorption process. Intermolecular diffusion is constant, and its direction is radial. This model was used to calculate the intermolecular diffusion constant. The model Equation is presented below (Muthu Kumara Pandian et al. 2016): = => 6 5.? + @

(15) 7

ACCEPTED MANUSCRIPT where: kid – the intra-particle diffusion rate constant (mg/g min0.5) I – the values are proportional to the boundary layer

Sorption thermodynamics

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The determination of the effect of temperature on the sorption of silver ions was carried out on the basis of the equilibrium and kinetic experiments at 20, 30, 40, 50, and 60°C. The distribution constant, Kd, calculated for these temperatures was used to calculate thermodynamic parameters by the following Equations (Shahverdi et al. 2016): ∆B = − ) # ∆C

= − $% +

where:

$

(17)

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∆B – the change in the Gibbs free energy (J/mol)

∆D

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#

(16)

∆E – the change in standard enthalpy (J/mol)

∆F – the change in standard entropy (J/molK)

Desorption

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The desorption process was carried out as follows: 20 g of coconut fiber was added to a glass container of 1,000 cm3 volume. In the next step, 800 cm3 of silver ion solution at a concentration of 8,000 mg/dm3 (pH=4) was added. The process was carried out at a temperature of 20°C over 15 min. After the sorption process, the sorbent was filtered under

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reduced pressure and dried at a temperature of 30°C over 24 h. The acquired filtrate was analyzed for the presence of silver (the mass of adsorbed Ag on the coconut fiber was

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established according to Equation 2). After being dried and homogenized, the coconut fiber was treated with eluents in order to desorb the Ag ions present. The following eluents were used:

a) H2O

b) Citric acid 0.1 mol/dm3 c) Citric acid 1 mol/dm3

d) Acetic acid 0.1 mol/dm3 e) Acetic acid 1 mol/dm3 f) Sodium hydroxide 0.1 mol/dm3 g) Sodium hydroxide 1 mol/dm3 8

ACCEPTED MANUSCRIPT To 0.5 g of dry sorbent after sorption, 20 cm3 of eluent was added and the desorption process was carried out with a “bath”-type bed for 20 min. After completion of the desorption, the sorbent was separated from the eluent by filtration under reduced pressure. The filtrate was analyzed for silver content. The desorption process was repeated three times,

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and the results were averaged. Based on Equation 18, the degree of desorption of silver from coconut fiber was designated Rdes. HI

= HI JKL ∙ 100% JKM

where: Agsol – silver content in solution after desorption (mg)

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Agsor – silver content in sorbent after desorption (mg)

(18)

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Methods

The examination of the surface area was carried out using a Micrometrics ASAP 2010 apparatus, with the option of measuring micropores and a deaerator station. Prior to measurement, the samples were dried at 110°C in a helium atmosphere for 8 h and then at

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100°C under a vacuum of 0.001 torr for 8 h.

The surface analysis was performed with a LEO 1430 VP (Electron Microscopy Ltd) scanning electron microscope equipped with an EDX X-ray microanalyzer. In order to define the characteristic chemical bonds present in the sorbent, materials

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before and after the sorption process were subjected to Fourier transform infrared spectroscopy using a Nicolet 380 spectrometer.

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The analysis of the silver content was performed with a Perkin Elmer OPTIMA 7300 DV ICP-OES emission spectrometer with inductively coupled plasma. The phase composition of the coconut fiber, before and after sorption, was determined

by X-ray diffractometry using an X'Pert Philips diffractometer equipped with a PW 17521700 graphite monochromator.

3. Results and discussion 9

ACCEPTED MANUSCRIPT 3.1 Sorbent characterization Figure 1a shows the SEM micrographs of the coconut fiber before the process of adsorption. It can be seen that the coconut is characterized by a heterogeneous and porous

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structure. The BET surface area of the adsorbent was 3.1477 m2/g.

Fig. 1. SEM microphotography of coconut fiber: a – before the sorption process, b – after the sorption process

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On the micrographs shown, it can be seen that the surface of the adsorbent after the sorption process is covered by silver (Fig. 1b). In order to confirm the presence of silver on

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the surface of the sorbent, EDX analysis was performed (Fig. 2b).

Fig. 2. EDX analysis of coconut fiber: a – before the sorption process, b – after sorption process

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ACCEPTED MANUSCRIPT Based on the EDX analysis it was stated that all of the coconut fiber surface was covered with silver, in contrast to coconut fiber before the sorption process, where no silver was present (Fig. 2a). Figure 3 shows the FTIR spectra of the coconut before and after the sorption process. The spectrum of the coconut exhibits the characteristic peaks of the cellulose structure. The

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large, broad peak at 3,427 cm-1 corresponds to the vibration of hydroxyl groups (OH). The peak at 1,032 cm-1 is due to the presence of amino groups (NH), while the peak at 1,622 cm-1 is consistent with the presence of carbonyl groups (COO), a peak at 2,920 cm-1 corresponds to

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the presence of CH2 (asymmetric stretch) (Kramer et al. 2014).

Fig. 3. FTIR spectra of coconut fiber before and after the adsorption process 3.2 Influence of the contact time and the initial concentration Contact time is an important parameter for complete sorption, which is dependent,

among other factors, on the type of sorbent and the sorption mechanism. As the duration of contact of the sorbent with a solution containing silver ions increases, the sorption process tends towards completion. Figure 4 shows the effect of sorption time on the process, and the effect of the concentration of the initial solution on the sorption capacity of the coconut fiber. In the initial stage, the rate of removal of silver ions from the total initial concentration, which ranged from 1,000-8,000 mg/dm3, was very high. With the passage of time, the sorption rate 11

ACCEPTED MANUSCRIPT decreased, reaching a state of equilibrium after 6 min. Based on the data presented, it can be concluded that the sorption capacity of coconut fiber in relation to the silver ions increases with the increase in the concentration of the initial solution from 1,000 to 8,000 mg/dm3. The initial high speed and a higher sorption capacity at higher starting concentrations were due to the greater number of moles of silver ions relative to the number of available active sites in

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the sorbent.

Fig. 4. Sorption of Ag+ ions on coconut fiber over time at different initial concentrations

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(T=20°C, pH=4)

The amount of sorbent is one of the important parameters in the sorption process,

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because with its increasing mass the number of active sites also increases. It was apparent that increasing the amount of silver ions in solution when the number of active sites was constant (with a constant amount of sorbent), the degree of removal was lowered as a result of the filling of the available active sites (Fig. 5). It can be seen that at a concentration of 2,000 mg/dm3 the highest removal levels occurred with the sorption processes carried out at temperatures of 40 and 50°C. Due to the degradation of proteins existing in the cell membranes of microorganisms that are present in the coconut fiber, which can influence the sorption capacity of the material and the degradation of compounds present in the plant material, the removal of silver ions at a temperature above 50°C was greatly lowered (Kołoczek et al. 2015). 12

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temperature (t=15 min, m=0.5 g, pH=4)

4.3. Equilibrium studies

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Fig. 5. Percent removal of silver ions from the solution depending on the concentration and

Examination of the equilibrium state allows for the modeling of the sorption process and elucidates the sorption mechanism. The Sorbate can be sorbed from the aqueous solution

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on the surface of solid sorbent by several types of mechanism. The sorption mechanism can depend on many factors, including the type of sorbent, the sorbate and their mutual affinity, the surface properties of the sorbent, the nature of the active sites, as well as the properties of the aqueous solutions (e.g. pH). In order to determine the best isotherm model, sorption

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experiments of silver ions on coconut fiber were carried out under optimal conditions. To evaluate different isotherms and their ability to correlate experimental data at different

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temperatures and pH values, four models of sorption were used: Langmuir, Freundlich, Temkin, and Dubinin and Radushkevich (D-R) (Fig. 6, 7).

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Fig. 6. Line graphs of different models of sorption isotherms for silver ions on coconut fiber

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at different temperatures

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Fig. 7. Line graphs of different models of sorption isotherms for silver ions on coconut fiber

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at different pH values

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The values of the isotherm model parameters were obtained on the basis of simple equations, which are listed in Tables 1 and 2. The results indicated that the best models for describing the sorption of silver ions on coconut fiber are the Langmuir and Freundlich isotherm models. The Langmuir isotherm was characterized by a higher correlation coefficient, R2, in the sorption carried out at temperatures of 20, 50, and 60°C, while at temperatures of 30 and 40°C the process was better characterized by the Freundlich isotherm. In the case of sorption processes of silver ions on the coconut fiber carried out at the different pH values, the highest correlation coefficient was characterized by the Langmuir isotherm. Additionally, the complete monolayer sorption was calculated with respect to the sorption process carried out at different temperatures and pH values. It can be seen that the sorption

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ACCEPTED MANUSCRIPT capacity of the sorbent increased with temperature until 40°C, while above that value a reduced sorption capacity occurred. The lowering of the sorption capacity at higher temperatures can be explained by changes in the thermodynamic conditions of the process. During the sorption process at different pH values, the highest sorption capacity was observed at pH=4.5 and 5 and the lowest at pH=2 (more than 2 times lower). The highest values of

which is about 4.5.

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sorption capacity were observed when the pH was close to the natural pH of the coconut,

Table 1. Parameters and the equations of the isotherms for the sorption of silver ions on

Equation and parameters

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qmax (mg/g) 71.117 72.406 82.787 79.918 65.089 1-(1/n) KF (mg (dm3)1/ng-1) 10.356 10.812 8.409 8.479 9.080 KT (dm3)/g) 0.1043 0.1070 0.0495 0.0531 0.0677 E (kJ/mol) 0.009425411 0.010962153 0.009525016 0.008157882 0.007910202

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R2 0.9933 0.9885 0.9884 0.9993 0.8727 R2 0.9683 0.9927 0.9971 0.9622 0.7600 R2 0.9813 0.9679 0.9825 0.9914 0.6530 R2 0.8906 0.7349 0.7792 0.8917 0.5460

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Equation y=0.0141x+5.5202 y=0.0138x+6.7444 y=0.0121x+6.6775 y=0.0125x+5.2348 y=0.0154x+31.6644 Equation y=0.2196x+1.0152 y=0.2121x+1.0339 y=0.2580x+0.9248 y=0.2586x+0.9284 y=0.2101x+0.9581 Equation y=10.5806x-23.9182 y=10.3626x-23.1589 y=13.4460x-40.4164 y=13.4037x-39.3238 y=9.4520x-25.4275 Equation y=-0.0056x+4.1122 y=-0.0042x+4.0645 y=-0.0055x+4.1709 y=-0.0075x+4.2114 y=-0.0080x+3.9067

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Isotherm model Langmuir 20 30 T 40 (°C) 50 60 Freundlich 20 30 T 40 (°C) 50 60 Temkin 20 30 T 40 (°C) 50 60 D-R 20 30 T 40 (°C) 50 60

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coconut fiber at varying temperatures (pH = 4, t = 10 min)

KL (dm3/mg) 0.002547233 0.002047799 0.001808956 0.002390309 0.000485203 1/n 0.2196 0.2121 0.2580 0.2586 0.2101 B 10.5806 10.3626 13.4460 13.4037 9.4520 qd (mg/g) 61.082 58.235 64.774 67.454 49.732

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ACCEPTED MANUSCRIPT Table 2. Parameters and the equations of the isotherms for the sorption of silver ions on coconut fiber at varying pH values (T=20°C, t=10 min) Equation and parameters KL (dm3/mg) 0.054444369 0.002035001 0.002547233 0.002562075 0.003032065 1/n 0.1711 0.2690 0.2196 0.2530 0.2632 B 4.3700 13.0057 10.5806 12.8055 14.1380 qd (mg/g) 34.525 62.496 61.082 65.216 69.540

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qmax (mg/g) 32.332 76.350 71.117 77.392 81.045 KF (mg1-(1/n)(dm3)1/ng-1) 8.039 7.179 10.356 8.608 8.475 KT (dm3)/g) 0.5203 0.0424 0.1043 0.0585 0.0507 E (kJ/mol) 0.004700998 0.008280314 0.009425411 0.009384942 0.009487152

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R2 0.9834 0.9987 0.9933 0.9987 0.9906 R2 0.4873 0.9823 0.9683 0.9793 0.9396 R2 0.4292 0.9962 0.9813 0.9908 0.9237 R2 0.8681 0.8317 0.8906 0.8378 0.7743

Equation y=0.0309x+0.5681 y=0.0131x+6.4361 y=0.0141x+5.5202 y=0.0129x+5.0433 y=0.0123x+4.0694 Equation y=0.1711x+0.9052 y=0.2690x+0.8561 y=0.2196x+1.0152 y=0.2530x+0.9349 y=0.2632x+0.9282 Equation y=4.3700x-2.8555 y=13.0057x-41.1028 y=10.5806x-23.9182 y=12.8055x-36.3427 y=14.1380x-42.1572 Equation y=-0.0226x+3.5417 y=-0.0073x+4.1351 y=-0.0056x+4.1122 y=-0.0057x+4.1777 y=-0.0056x+4.2419

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Isotherm model Langmuir 2 3 pH 4 4.5 5 Freundlich 2 3 pH 4 4.5 5 Temkin 2 3 pH 4 4.5 5 D-R 2 3 pH 4 4.5 5

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Based on the Langmuir equation, the constant RL (separation coefficient) can be calculated, on the basis of which it can be concluded that the process of sorption is favorable

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or unfavorable. Equation 19 shows the model on the basis of which the separation factor was calculated (Zheng et al. 2009). O

=

PQR

(19)

For RL parameters:

RL=0 – sorption is irreversible, 01 – unfavorable conditions for sorption.

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ACCEPTED MANUSCRIPT The collected data indicate that the separation coefficient was always positive and lower than 1. This indicates a favorable sorption of silver ions on coconut fiber at different temperatures and pH values.

4.4. Kinetic modeling

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Kinetic studies are of great importance, as they enable information to be obtained on the physical chemistry of the processes of sorption and sorption systems’ design. Generally, the sorption of silver ions on the coconut surface takes several stages. Initially during the sorption, the transfer of silver ions onto the surface of sorbent occurs through a process of

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diffusion. This is sequentially followed by the diffusion of silver ions on the outer surface of the sorbent (film diffusion) and migration into the pores (pore diffusion). On the inner surface of the pores, mutual interaction of silver ions with the free active sites occurs. The rate of

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sorption is limited by one of these steps, or by a combination of them. In order to obtain information on the sorption kinetics, the results of the effect of contact time and the initial concentrations were used. These data allowed for results from several models to be obtained and discussed. The correlation coefficient (R2) was used to compare models and their correlation at various concentrations and times. Visual analysis of the charts obtained led to

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the conclusion that the model with the best fit was the pseudo-second order model (Fig. 8). These visual observations were confirmed by the correlation coefficients obtained for each of the models used (Table 3). As can be seen from the data in Table 3, the model with the pseudo-second order kinetics was very successful in explaining the sorption data, and its fit

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was the highest for all of the studied concentrations. Based on the results obtained for the sorption kinetics, it can be concluded that the rate-limiting step for the sorption of silver ions on the coconut fiber was chemisorption, which is based on the interaction of valence forces

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through the sharing or exchange of electrons between silver and coconut fiber (Ho and McKay, 1999).

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(T=20°C)

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Fig. 8. Line graphs of different models of sorption kinetics for silver ions on coconut fiber

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ACCEPTED MANUSCRIPT Table 3. Kinetic parameters of different sorption models of silver ions on coconut fiber (T=20°C) Ag concentration Co (mg/dm3) 2,000 4,000 6,000

52.2330 0.0581 0.9999

63.2603 0.0534 0.9994

3,380 0.1676 0.8993

6,474 0.1456 0.7811

32.8564 5.7974 0.7310

4.5. Thermodynamic studies

41.7400 6.4803 0.5988

8,000

38.6695 0.4305 0.8718

24.5599 0.1970 0.7293

64.4851 0.0219 0.9983

70.3440 0.0230 0.9962

558 0.1050 0.9566

1,169 0.1068 0.9148

29.9660 9.6438 0.8455

36.7913 9.3702 0.7892

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15.2869 0.3322 0.7454

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16.0233 0.3878 0.9130

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Kinetic model 1,000 Pseudo first-order rate model qe 5.2600 k1 0.6997 2 R 0.8049 Pseudo second-order rate model qe 33.1960 k2 0.2347 R2 0.9998 Elovich model α 100,649 β 0.3836 2 R 0.7492 Intra-particle diffusion model I 25.6287 Kid 2.4077 R2 0.5506

Due to the fact that the influence of the working temperature is very important in the

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study of the sorption of metal ions, a thermodynamic study was conducted to determine the effect of process temperature on the sorption of silver ions on the surface of coconut fiber. As is well known, there are two major effects of temperature changes in sorption processes. A

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reduction in the solution viscosity and an increase in the mobility of the metal ions in solution are observed resulting in an increase of the temperature and the rate of diffusion of metal ions to the outer surface of the sorbent and its internal pores. Changes in the temperature will

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change the equilibrium constant and sorption capacity of coconut fiber. On the basis of equilibrium and kinetic studies, the influence of the process temperature at various silver ion concentrations can be measured. The designated thermodynamic parameters were used to determine the spontaneity of

the sorption process. Using the value of fixed distribution, Kd, the following thermodynamic parameters were calculated: enthalpy change (∆H), entropy change (∆S), and Gibbs free energy (∆G) for the sorption of silver ions on coconut fiber; the dependence of lnKd on 1/T is shown in the graphs (Fig. 9). The calculated thermodynamic parameters for the adsorption of silver on coconut fiber are shown in Table 4. The negative value of the Gibbs free energy showed the high affinity of silver ions to coconut and the spontaneous nature of the sorption 20

ACCEPTED MANUSCRIPT process. Decreasing values of ∆G along with increasing temperature suggested that the adsorption was more favorable at higher temperatures (Rahman and Haseen, 2014). The presented graphs show that the adsorption of silver on coconut fiber occurs in two different ways (sorption and biosorption), depending on the concentration of silver. For the lowest concentration (1,000 mg/dm3), sorption was an exothermic process. The initial rise in the

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removal ratio (between 20 and 30°C) was associated with the increased activity of microorganisms present in the organic material, whereas at a temperature of 40°C, a drop in the degree of removal due to their degradation was observed. For concentrations in the range 2,000-6,000 mg/dm3 at 50°C, an endothermic sorption process was observed, due to the

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increase in the ion transport and the associated increased availability in the pores. Moreover, the sorption percentages were greater at higher temperatures due to the greater equilibrium constants. Above 50°C, changes in the sorption type and in the nature of the thermal process,

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which became exothermic (negative values of ∆H), were observed. These results were due to the temperature degradation of biological matter (proteins, cellular structures, inhibition of enzymatic activity). Moreover, the negative ∆S value corresponded to a decrease in the degree of freedom of the adsorbed species (Othman et al. 2011). Positive ∆S values for the adsorption process indicated an irregular increase in the randomness at the coconut fiber–

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solution interface during the adsorption (Sukpreabprom et al. 2014).

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Fig. 9. Plots of lnKd vs 1/T for determination of thermodynamic parameters (C0=1,000-8,000 mg/dm3)

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ACCEPTED MANUSCRIPT Table 4. Thermodynamic parameters for silver ion sorption on coconut fiber

4,000

4.6. Desorption

293 303 313 323 333 293 303 313 323 333 293 303 313 323 333 293 303 313 323 333 293 303 313 323 333

8,796

14

-13,116

-58

6,236

-4

-47,406 7,090

-170 -8

-4,744 -4,606 -5,034 -5,614 -6,194 -7,385 -7,424 -7,463 -7,503 -9,176 -9,404 -9,483 -9,562 -9,641 -11,707 -10,294 -10,440 -10,586 -10,733 -12,806 -10,879 -10,952 -11,024 -11,650 -12,316

-58,211

-210

6,003

-15

-52,854

-197

8,754

-7

-9,886

-67

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8,000

∆G (kJ/mol)

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6,000

∆S (J/molK)

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2,000

∆H (kJ/mol)

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1,000

T (K)

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Ag Co (mg/dm3)

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4.6.1 Desorption with the use of organic acids and water Table 5 shows the results of analyses for the eluting silver ions from coconut fiber

using various types of eluents.

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ACCEPTED MANUSCRIPT Table 5. Analysis of the eluting of silver ions from coconut fiber Eluent – Concentration (mol/dm3)

Content of silver

Content of

before elution

silver after

(mg/g)

elution (mg/g)

Degree of elution (%)

66.986

3.16

Citric acid - 0.1

48.644

29.68

Citric acid - 1

47.205

Acetic acid - 0.1

69.172

56.592 49.919

Sodium hydroxide - 0.1

69.056

Sodium hydroxide - 1

68.825

31.76

18.19

27.83 0.12

0.35

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Acetic acid - 1

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H2O

From the obtained data, the dependence of the degree of elution of silver from coconut fiber on the type and concentration of the eluent could be observed. This was due to chemical reactions between groups of eluent and silver ions. Citric acid was characterized by a high degree of washing. It belongs, along with acetic acid, to the group of organic acids that are capable of forming various types of complexes; in addition, as strong chelating agents, they

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can lead to greater ion desorption from the sorbent to the eluent (Wang et al. 2013). It was noted that, depending on the type of sorbent, a different mechanism of action of the organic acids on the sorption / desorption occurred (Shan et al. 2002). According to Najafi and Jalali (2015), citric acid had a greater ability to complex cadmium compared with acetic acid. Also,

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Wang et al. (2009) observed, in their study on the effects of low molecular-weight organic acids on Cu(II) sorption onto hydroxyapatite nanoparticles, that the presence of citric acid

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reduced the sorption of copper. An increase in the concentration of the organic acid increased the number of organic acid residues in solution, and the organic ligand-chelated metal could increase the negative charge in the solution, which led to the metal being repelled from the sorbent or to the formation of metal complexes in the solution. Moreover, increasing the concentration of the organic acid could cause competition with the metal for the free sites on the surface of the sorbent (Hu et al. 2007). The effect of selected organic acids on cadmium sorption by variable- and permanently-charged soils was investigated by Yuan et al. (2007). They noted that the pH effect, along with the complexation of metals by organic acids, should not be ignored. The desorption of copper and cadmium from soils was enhanced by organic acids. In our results, it was found that increasing the concentration of acetic acid (lowering of 24

ACCEPTED MANUSCRIPT pH) caused an increase in the removal of silver from a coconut by almost 10%. Due to the favorable adsorption conditions of silver on coconut fiber (measured separation coefficients), and the chemical nature of the sorption (high binding energy between the sorbent and sorbate), water was not a good eluent for the silver elution process. As an alternative to organic acid eluents, sodium hydroxide was used. From the data

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presented in Table 5, it can be concluded that the use of this type of eluent is not preferred, due to the minimal degree of silver elution from the coconut surface. However, the application of NaOH allows nanometric-sized silver to be obtained on the surface of the organic material. One of the methods of preparing nano-silver is a chemical reduction carried out by organic or inorganic reducing agents. These reducing agents reduce Ag+ and lead to the

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formation of metallic silver (Ag0), which is followed by agglomeration into oligomeric clusters. These clusters result in the formation of colloidal particles of silver metal. It is

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important to use protective measures in order to stabilize the dispersion of nanoparticles, avoiding their agglomeration (Iravani et al. 2014). A photomicrograph of the coconut after the

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desorption processes with 0.1 and 1 mol/dm3 NaOH is presented in Fig. 10.

Fig. 10. SEM photomicrograph of coconut fiber after desorption with NaOH: a – 0.1 mol/dm3,

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b – 1 mol/dm3

In the presented micrographs, it can be seen that the surface of coconut is covered by

silver particles. The resulting silver nanoparticles had an average particle size of 20 to 100 nm. EDX analysis of the images confirmed the presence of silver on the surface of the sorbent (Fig. 11).

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Fig. 11. EDX analysis of coconut fiber after desorption process with NaOH: a – 0.1 mol/dm3, b – 1 mol/dm3

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XRD analyses of the coconut desorption process showed differences in the qualitative composition of the sorbent compared to its initial state. The analysis showed the presence of

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peaks characteristic for silver on the surface of coconut after desorption with 1 mol/dm3

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NaOH (Fig. 12).

Fig. 12. XRD pattern of the sorbent: a – the raw material, b – after desorption with 1 mol/dm3 NaOH

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ACCEPTED MANUSCRIPT 4. Conclusion This work showed that coconut fibers have a high sorption capacity for silver and can be effectively used for the rapid removal of silver ions from aqueous solutions. The sorption of silver depended on several physical and chemical parameters, such as the initial concentration, contact time, temperature, and pH. The sorption process occurred quickly, and

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the sorption equilibrium was reached in a time of 10 min for all tested concentrations and temperatures. Equilibrium studies showed that at temperatures of 20, 50, and 60°C the Langmuir isotherm model was a better fit, and at temperatures of 30 and 40°C the Freundlich isotherm was more accurate. In contrast, sorption carried out at different pH values was

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characterized by the Langmuir isotherm. Sorption kinetic studies showed that the sorption of silver on coconut fiber is best described by a pseudo-second order model. On this basis, it can be assumed that the sorption is of a chemical nature. Thermodynamic studies allowed us to

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calculate the thermodynamic parameters, such as ∆H, ∆G, and ∆S. It was found that sorption occurred as an endo or exothermic process, depending on the silver concentration. At concentrations of 1,000 mg/dm3 the process was exothermic, and at higher concentrations it changed to endothermic. The difference is related to, among others, the harmful effects of silver to microorganisms present in coconut fiber.

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Desorption studies showed that the highest degree of elution of silver was characterized by organic acids (acetic and citric). In the case of citric acid, changes in its concentration did not affect the level of elution (29.68-31.76%), as was the case with acetic acid (18.19-27.83%). The lowest level of elution was obtained with the use of water (~3%).

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Using an alternative eluent (sodium hydroxide) led to silver nanoparticles being obtained on the surface of the organic material. The average size of nanoparticles formed was in the range of 20-100 nm, and moreover their distribution was on the entire surface of the

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sorbent; this can be considered to be a nanoparticle biosynthesis.

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