A comparative sorption study of Cr3+ and Cr6+ using mango peels: kinetic, equilibrium and thermodynamic

Atta ul Haq; Muhammad Saeed; Muhammad Usman; Muhammad Yameen; Majid Muneer; Saiqa Tubbsum

doi:10.1515/gps-2019-0001

Article Open Access

A comparative sorption study of Cr³⁺ and Cr⁶⁺ using mango peels: kinetic, equilibrium and thermodynamic

Atta ul Haq , Muhammad Saeed , Muhammad Usman , Muhammad Yameen , Majid Muneer and Saiqa Tubbsum

Published/Copyright: March 25, 2019

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From the journal Green Processing and Synthesis Volume 8 Issue 1

Abstract

The present study investigates a comparative study of the sorption of Cr³⁺ and Cr⁶⁺ from water using an agricultural by-product; mango peels in batch system under the effect of initial metal ion concentrations, solution pH, temperature, sorbent dose and contact time. Characterization of the mango peels was done before and after sorption of Cr³⁺ and Cr⁶⁺ using scanning electron microscopy, surface area pore size analyzer and FTIR spectroscopy. The pH study revealed that that maximum removal of Cr³⁺ and Cr⁶⁺ was obtained at pH 5.0 and 7.0 respectively. Among various kinetic models, pseudo-2^nd order well explained the data owing to the higher values of R² and the nearness between the values of experimental and calculated sorption capacities. The isotherms study revealed that Freundlich is the suitable isotherm for explanation of the equilibrium data due to higher R² values. The monolayer sorption capacity of mango peels was found to be 98.039 mg g^-1 for Cr³⁺ and 66.666 mg g^-1 for Cr⁶⁺. The spontaneity and exothermic nature of the sorption process of Cr³⁺ and Cr⁶⁺ using mango peels was reflected from thermodynamic study.

Keywords: mango peels; kinetics; equilibrium; thermodynamics

1 Introduction

Scientists have focused their attention much more on the contamination of heavy metals. The accumulation of these metals in the bodies of flora and fauna causes serious problems [1]. The effluents of various industries such as electroplating, tanning, metallurgy, pesticide, ceramic, batteries and mining consist of heavy metals in high amount [2,3]. Among these metals, chromium is considered as toxic and usually found in the form of Cr³⁺ and Cr⁶⁺. The literature showed that Cr³⁺ is an important nutrient necessary for the metabolism of sugar and fat, and its toxicity is less than Cr⁶⁺ [4]. However, maximum exposure of the chromium causes serious health problems such as cancer, allergic skin reaction, kidney and lung diseases [2]. Hence, the removal of chromium from the industrial effluents is necessary before discharging into the environment. Various methods have been applied so far, to treat polluted water like ion exchange, precipitation, solvent extraction, electrochemical deposition, membrane separation, adsorption, reduction etc. However, there are a number of considerable drawbacks of these methods such as high price, energy consumption, partial removal and further production of toxic sledge due to which they have not gained much attention. Therefore, it is imperative to investigate an inexpensive and eco-friendly method to remove chromium and other contaminants from water [5, 6, 7, 8].

Recently, biosorption has become one of the alternative technologies that have been widely utilized for the decontamination of wastewater. On comparison to other conventional wastewater treatment technologies, biosorption has many potential marketing advantages such as high efficiency, low cost, no additional nutrient requirement, low biological sledge formation, eco-friendly, recycling of biosorbents, high metal binding ability and high possible metal recovery [9]. It has been reported that those materials which are obtained from various peels of the fruit have a high potential to remove heavy metal ions [10, 11, 12, 13].

Mango is one of the delicious fruit and commonly grows in those regions of the world which occur on tropical and subtropical. The annual production of the mango is about 27 million tons which makes the mango on fifth position among major fruits [14]. The edible part of the mango is the pulp making approximately 33 to 85% of the total fruit and used in many products include juices, jams and jelly powders [15]. About 7-14% of the total fruit weight is the peels of mango and generated in a huge amount during the season. These peels have no proper use and the disposal of these wastes is also a serious problem.

The major components of mango peels are pectin, lipids, cellulose, hemicellulose, polyphenols and carotenoids [16]. Cellulose and pectin contain various functional groups such as carboxylic and hydroxyl. Due to these functional groups, the mango peels sequester different types of pollutants, especially metal ions [17, 18, 19]. Recently, different parts of the mango such as mango seeds, mango peels, unfertilizable fruiting buds of mango plant and phosphate treated sawdust of mango tree have been employed to sequester dyes, phenol, heavy metal ions [20, 21, 22, 23, 24, 25].

The present study was performed to compare the potential removal of Cr³⁺ and Cr⁶⁺ using mango peels from aqueous solution. The study was performed regarding the influence of pH, time of contact, dosage of sorbent, initial metal ions concentration and temperature. Kinetics, equilibriums and thermodynamics of sorption were studied to recognize the nature and mechanism of Cr³⁺ and Cr⁶⁺ sorption.

2 Materials and methods

2.1 Preparation of biosorbent

A bulk quantity of mango peels were collected from juice shops in Faisalabad, Pakistan. These peels were first thoroughly washed with tap water to remove any dirt particle and then washed with excess of distilled water. After washing, the materials were cut with knife to obtain small pieces of the peels and dried in sunlight. These materials were then ground in electrical grinder and made powder of the peels. Sufficient amount of the powder was then transferred in to a beaker containing distilled water and allowed the materials to settle down in the water. Distilled water was then decanted after settled down of the powder and decantation was repeated till the water become transparent. These materials were filtered and dried at a temperature of 120 oC in oven until the weight become constant. The dried materials were then stored for further study in air tight bottle.

2.2 Chemicals and reagents

In this study sulfuric acid, boric acid, phosphoric acid, hydrochloric acid, acetic acid and sodium hydroxide were used. Solution pH was adjusted with Briton Robinson buffer solutions of different pH.

2.3 Standard solutions of Cr³⁺ and Cr⁶⁺

Stock solution of Cr³⁺ was prepared from chromium nitrate and Cr⁶⁺ was prepared from potassium dichromate in distilled water. These stock solutions were then used for preparation of working solutions using dilution formula.

2.4 Instruments

Analytical balance (Sartorius-GC 2012 Germany) was used to weigh various solid materials. Sorbent was heated in oven (Memmert Celsius 2005) and pH of the solutions was determined with the help of pH meter (WTW-Inolab 720 Series). The raw materials were ground with the help of grinder (Frtsch-Pulverisette 2 of Japan), and for shaking of the suspensions, orbital shaker was used. The amount of Cr³⁺ and Cr⁶⁺ was determined using atomic absorption spectrophotometer (Hitechi Polarized Zeeman AAS, Z-8200, Japan).

2.5 Sorption studies

The sorption studies were performed by taking 0.1 g mango peels powder in separate conical flaks and added 10 mL of Cr³⁺ and Cr⁶⁺ of a particular concentration (100 mg L^-1). Britton Robinson buffer was used to adjust the solution pH with optimum value and the volume of the suspensions was kept up to 25 mL. The suspensions were then shaken for two hours in an orbital shaker. After equilibration, the suspensions were filtered and the amount of Cr³⁺ and Cr⁶⁺ in filtrates was determined with atomic absorption spectrophotometer.

Percentage removal and sorption capacity were evaluated with the help of following equations:

(1)Sorption(%)=[ci−cfci]×100

(2)qe=[ci−cfm]V

In the above equations C_i and C _f are the initial and final concentrations of metal ions (mg L^-1), m denotes mass (g) of mango peels powder, V is the volume (mL) of metal ions solution and q_e is the sorption capacity (mg g^-1).

3 Results and discussion

3.1 Characterizations of mango peels

To characterize the mango peels various techniques have been employed like scanning electron microscopy, surface area pore size analyzer and Fourier transform infrared spectroscopy.

3.1.1 Scanning electron microscopic analysis

Important information regarding the surface structure has been obtained from scanning electron microscopy. Figures 1-3 showed the SEM images of the mango peels prior to and after sorption of Cr³⁺ and Cr⁶⁺ at a magnification of 2000x. Before sorption study, a roughness and

Figure 1

SEM of mango peels before removal of Cr³⁺ and Cr⁶⁺.

Figure 2

SEM of mango peels after removal of Cr³⁺.

Figure 3

SEM of mango peels after removal of Cr⁶⁺.

irregularity was found on the surface of mango peels. However, the coarseness of the surface of the mango peels was vanished after sorption of these metal ions which suggests an indication of Cr³⁺ and Cr⁶⁺ sorption.

3.1.2 Surface analysis of mango peels

The removal efficiency is greatly affected by surface area and volume of pores. Therefore, surface analysis data of the mango peels powder were obtained using nitrogen adsorption isotherm. The results of the surface analysis of mango peels powder before and after removal of Cr³⁺ and Cr⁶⁺ are summarized in Table 1. The surface area of mango peels powder was decreased after removal of Cr³⁺ and Cr⁶⁺ which confirms the adsorption of these metal ions.

Table 1

Surface areas, pore radius and pore volume of mango peels powder.

Parameter		After removal
		Cr³⁺	Cr⁶⁺
Surface area (m² g^–1)	13.455	8.751	8.345
Pore volume (mL g^–1)	0.055	0.0453	0.0385
Pore radius (Å)	17.065	18.755	19.235

3.1.3 Fourier Transform Infrared analysis

The functional groups play a very important role in the sorption process. Therefore, to indentify the involvement these functional groups FTIR analysis was carried out prior to and subsequent to sorption of Cr³⁺ and Cr⁶⁺. The spectrum of the mango peels powder before removal of Cr³⁺ and Cr⁶⁺ is shown in Figure 4 which consists of major peaks at 3323, 2359, 2331, 1595, 1507 cm^-1 corresponding to -OH, –CH stretching of –CH₂ and –CH₃, N-H bending and C-N stretching [26, 27, 28]. However, after removal of these metal ions the peaks intensities were somewhat decreased as shown in Figure 5 which indicates the involvement of these functional groups [29].

Figure 4

FTIR spectrum of mango peels before removal of Cr³⁺ and Cr⁶⁺.

Figure 5

FTIR spectrum of mango peels after removal of Cr³⁺ and Cr⁶⁺.

3.2 Effect of pH

Solution pH is considered as one of the chief experimental variables which affect the sorption of heavy metals [30]. To observe the effect of pH, the sorption experiments were performed at pH values varying from 3 to 11 and other parameters were kept constant. Figure 6 presents a slight increase in sorption of Cr³⁺ with each increment of pH up to pH 5.0 but after that a small decrease in sorption was observed. A competition develops between H⁺ and Cr³⁺ on sorption sites of mango peels at low pH owing to

Figure 6

Effect of pH (conditions for experiment: range of pH 3-11, initial Cr³⁺ and Cr⁶⁺ concentration 40 μg mL^-1, dosage of sorbent 0.1 g, time of contact 1 hour, volume for sorption of Cr³⁺ and Cr⁶⁺ 25 mL).

which the sorption of Cr³⁺ was low but this completion was decreased with the progress in pH. Thus, maximum sorption of Cr³⁺ was obtained at pH 5.0. However, at high pH, a decline in sorption of Cr³⁺ was noted which may be owing to the conversion of Cr³⁺ into Cr (OH)^-₄ and Cr (OH)₃ in alkaline medium. Consequently, an electrostatic repulsion develops between the negative charges on both chromium species and the surface of mango peels at higher pH. Conversely, in case of Cr⁶⁺, the sorption onto mango peels was more effective at low pH and become all most remain constant between 5.0 and 7.0, followed by a major decrease in sorption of Cr⁶⁺. The sorption of Cr⁶⁺ is high at low pH may be owing to the presence of Cr⁶⁺ in various anionic forms in acidic medium such as [HCrO₄]^-, [Cr₃O₁₀]^2-, [Cr₂O₇]^2- and [Cr₄O₁₃]^2-. Therefore, a strong attractive force created between the anionic species of Cr⁶⁺ and sorption sites of mango peels due to which the sorption process is high at low pH. However, a considerable decrease in sorption of Cr⁶⁺ was noted at higher pH. It may be perhaps due to the completion of anionic species of Cr⁶⁺ and OH^- for sorption sites of mango peels [31]. Consequently, pH 5.0 was then used for further study as optimum pH for Cr³⁺ while pH 7.0 for Cr⁶⁺ removal.

3.3 Effect of sorbent dose

To scrutinize the effect of mango peels dosage on sorption of Cr³⁺ and Cr⁶⁺, the experiments were conducted by varying the amount of mango peels powder from 0.1 to 0.7 g with a constant concentration (40 mg L^-1) of both metal ions. Figure 7 demonstrates that the percent sorption was increased with each increment of sorbent dose. Such type of trends is commonly reflects that surface area increases with increase in sorbent dose and more sites are available for binding of metal ions. After certain point, however, the sorption process become steady and no further significant increase in sorption was observed due to keeping constant the concentration of metal ions.

Figure 7

Effect of sorbent dose (conditions for experiment: dose range 0.1-0.7, initial Cr³⁺ and Cr⁶⁺ concentration 40 μg mL^-1, time of contact 1 hour, volume for sorption of Cr³⁺ and Cr⁶⁺ 25 mL, pH 5.0 for Cr³⁺ and 7.0 for Cr⁶⁺).

3.4 Effect of contact time

One of the most significant variables in sorption study is contact time because it gives information about practical usage of sorbent as well as the rate of sorption process [32]. Therefore, the influence of contact time on sorption was scrutinized by varying it from 10 to 70 min and the result is depicted in Figure 8. An increase in sorption of Cr³⁺ and Cr⁶⁺ onto mango peels powder was observed with the progress of time of contact and equilibrium was established within 50 min. After 50 min time of contact no prominent change in sorption was observed. A steady increase in percent sorption was noted with agitation of the suspension of metal ions and sorbent. In fact the sorptive sites exposed more to the metal ions with agitation, resultantly; the metal ions sorption was increased. Therefore, the sorption of Cr³⁺ was increased from 85% to 98% while Cr⁶⁺ was increased from 97% to 98% with agitation of the metal ions and sorbent.

Figure 8

Effect of contact time (conditions for experiment: range of contact time 10-70, initial Cr³⁺ and Cr⁶⁺ concentration 40 μg mL^-1, dosage of sorbent 0.1 g, volume for sorption of Cr³⁺ and Cr⁶⁺ 25 mL, pH 5.0 for Cr³⁺ and 7.0 for Cr⁶⁺, shaking speed 100 rpm).

3.5 Sorption kinetics

Important information about designing and modeling of sorption process can be obtained from the prediction of kinetic parameters and these parameters can also be used to select the optimized conditions for batch process [33, 34, 35, 36]. The nature and pathway of the sorption of Cr³⁺ and Cr⁶⁺ onto mango peels powder was elucidated by applying pseudo-1^st order, pseudo-2^nd order, Elovich, intra-particle and liquid film diffusion models.

3.5.1 Pseudo-1^st order

According to pseudo-1^st order, the difference in sorption capacity at equilibrium and at a given time is directly related to the rate of change of sorption capacity with regard to time [37]. The following equation shown pseudo-1^st order:

(3)log(qe−qt)=log qe−K1t2.303

In this equation K₁ denotes the rate constant (min^-1), q_t and q_e (mg g^-1) represent the sorption capacities at time t and at equilibrium respectively.

3.5.2 Pseudo-2^nd order

This model explains that the nature of sorption process is a chemisorption which often occurs due to the exchange of electrons or some time due to sharing of valance force between sorbate and sorbent [38]. The following equation expresses pseudo-2^nd order:

(4)tqt=tqe+1K2qe2

In this equation K₂ denotes the rate constant (g mg^-1 min^-1), q_t and q_e represent the sorption capacities (mg g^-1) at time t and at equilibrium respectively.

3.5.3 Intraparticle diffusion model

Diffusion of sorbate species in to the pore of adsorbent is explained by intraparticle diffusion model and considers this step as the rate determining step. This model describes a direct association between the sorption of sorbate species and t^0.5. To apply this model the following equation is used:

(5)qt=Kint t1\2+C

In the above equation K_int (mg g^-1 min^-1/2) represents the rate constant and C is the intercept and associated to the thickness of the boundary layer. If a straight line is obtained from the plot of q_t versus t^1/2 having zero intercept, then the rate determining step is the intraparticle diffusion [39]. However, it may be suggested that intraparticle diffusion is not involved in our study because the line does not passing through the origin.

3.5.4 Elovich model

Zeldowitsch developed the Elovich kinetic model which explains the heterogeneity of the sorbent surface energetically and no interactions taking place among sorbed species [40]. This model can be expressed as given:

(6)qt=1βln(αβ)+1βln(t)

In this equation α (mg g^-1 min^-1) denotes the initial sorption rate and β (g mg^-1 min^-1) is the desorption coefficient.

3.5.5 Liquid film diffusion model

This model is commonly applied in those situations in which the boundary plays a significant function in transportation of solute from liquid to the solid phases [41]. The following equation expressed this model:

(7)ln(1−F)=−Kfdt

In this equation K_fd is the rate constant for sorption and F expresses the fractional achievement of equilibrium and is equal to q_t/q_e.

The plot of –ln(1-F) against time having zero intercept indicates fitness of kinetic data into liquid film diffusion model [42]. However in case of our study, the intercept is not zero which may be suggested from this result that some other mechanisms controlled the sorption of these metal ions.

Constant parameters of each model were evaluated from slopes and intercepts of their respective linear equations and summarized in Table 2. By comparing all these kinetic models, pseudo-2^nd order is one of the most suitable models to explain the sorption process of Cr³⁺ and Cr⁶⁺ onto mango peels due to highest value of R². Moreover, the closeness between the values of experimental sorption capacity and calculated sorption capacity of the pseudo-2^nd order kinetic model further confirms that pseudo-2^nd order is a suitable model to express the sorption process.

Table 2

Comparison of the kinetic parameters for the sorption of Cr³⁺ and Cr⁶⁺ onto mango peels.

Model	Parameter	Cr³⁺	Cr⁶⁺
Pseudo-1^st-order	q_e, mg g^–1, (exp)	9.032	9.925
	q_e, mg g^–1, (cal)	1.969	5.780
	K¹, min^–1	0.009	0.023
	R²	0.9350	0.9264
Pseudo-2^nd- order	K₂, g mg^–1min^–1	0.027	0.017
	q_e, mg g^–1	9.165	10.504
	R²	0.9973	0.9982
Intraparticle diffusion	K_int, mg g^–1 sec^1/2	0.223	0.371
	C	6.765	6.589
	R²	0.9610	0.9772
Elovich	α, mg g^–1 min^–1	5630.761	113.738
	β, g mg^–1	1.545	0.9195
	R²	0.9323	0.9674
Liquid film diffusion	K_fd, min^–1	0.021	0.053
	Intercept	1.523	0.540
	R²	0.9350	0.9264

3.6 Effect of initial metal ion concentration

Among those variables which significantly affect the sorption process is initial concentration of sorbate because it provides a driving force which over comes on the mass transfer resistance of molecules [43]. Therefore, the effect of initial Cr³⁺ and Cr⁶⁺ concentration on removal (%) was studied by changing the concentration of these metal ions from 20 to 180 μg mL^-1 and the result is depicted in Figure 9. This figure showed that % sorption of Cr³⁺ and Cr⁶⁺ was decreased with each increment in initial metal ion concentration. Possibly, this type of results may take place owing to the occupation of sites on the sorbent as the initial concentration of these metal ions was increased [44].

Figure 9

Effect of initial metal ion concentration (conditions for experiment: initial metal ion concentration range 20-180 μg mL^-1, dosage of sorbent 0.1 g, volume for sorption of Cr³⁺ and Cr⁶⁺ 25 mL, pH 5.0 for Cr³⁺ and 7.0 for Cr⁶⁺, time of contact 1 hour).

3.7 Adsorption isotherm studies

A number of isotherms are available in literature which can be used to analyze the sorption process. The sorption equilibrium isotherm models are characterized by certain parameters whose values describe the affinity of sorbent for various solutes and the surface properties of sorbents. In the present study, various isotherms such as Freundlich, Langmuir, Tempkin and Dubinin-Radushkevich (D-R) were used to interpret the sorption data of Cr³⁺ and Cr⁶⁺.

3.7.1 Freundlich isotherm

This isotherm considered multilayer sorption on a surface of heterogeneous in nature and is given in the following equation:

(8)log qe=log KF+1nlog Ce

In this equation K_F represents Freundlich isotherm constant which indicates sorbent capacity (mg g^-1) while n (g L^-1) is the Freundlich exponent; use to measure the sorption intensity. Similarly, q_e represents the amount of Cr³⁺ and Cr⁶⁺ sorbed (mg g^-1) while C_e denotes the concentration Cr³⁺ and Cr⁶⁺ at equilibrium (μg mL^-1).

The magnitude of n represents the nature of sorption either it occurs chemically or physically. The value of n for chemical sorption is less than one and for physical sorption its value is greater than one [45].

3.7.2 Langmuir isotherm

This isotherm assumes that solutes are sorbed in the form of a monolayer on a surface having fixed number of sorptive sites with homogeneous energy and its linear form is following:

(9)Ceqe=1K1Qm+CeQm

Where K¹ is related with sorption free energy (L mg^-1) while Q_m is theoretical sorption capacity (mg g^-1).

3.7.3 Temkin isotherm

This isotherm describes the interaction of sorbate with sorbent and expressed that the sorption energy decreases with surface coverage and given in the following equation:

(10)qe=BTlnAT+BTlnCe

Where B_T is related with sorption heat and A_T is related with maximum binding energy.

3.7.4 Dubinin-Radushkevich (D-R) isotherm

This isotherm describes sorption mechanism i.e. whether the sorption occur chemically or physically on a heterogeneous surface [46]. This isotherm can be represented in the following equation:

(11)lnqe+lnQm−Kε2

Where Q_m (mg g^-1) denotes theoretical saturation capacity, K is associated to the sorption energy (mol² (kJ²)^-1) and ε represents Polanyi potential and its value can be evaluated as:

(12)ε=RTln(1+1Ce)

Whether the sorption process is physical or chemical can be revealed from the magnitude of mean free energy of sorption (E). The magnitude of E may be calculated as:

(13)E=12K

The process which occurs physically has E value below 8 (KJmol^-1) and the process which occur through chemical has E value between 8 and 16 (KJmol^-1) [47].

The linear form of these equations were used for calculation of the constant parameters of each isotherm and listed in Table 3 which revealed that R² values of Freundlich isotherm are highest among the studied isotherms suggesting that equilibrium data followed Freundlich isotherm. It can be seen from this table that n is greater than one which point out that the sorption of Cr³⁺ and Cr⁶⁺ onto mango peels is physical in nature. The linear form of Langmuir isotherm was used to evaluate the monolayer sorption capacity of the mango peels for Cr³⁺ and Cr⁶⁺ and was found to be 98.039 and 66.666 mg g^-1 respectively. The table also showed that E has greater value than 8 KJmol^-1 for both Cr³⁺ and Cr⁶⁺ suggesting that the sorption of Cr³⁺ and Cr⁶⁺ is physical in nature.

Table 3

Comparison of the isotherm constant parameters for the sorption of Cr³⁺ and Cr⁶⁺.

Isotherm	Parameter	Cr³⁺	Cr⁶⁺
Freundlich	K_F, mg g^-1	3.057	4.303
	n	1.239	1.417
	R²	0.9991	0.9669
Langmuir	K₁, L mg^-1	0.0264	0.053
	Qo, mg g^-1	98.039	66.666
	R²	0.9114	0.9406
Temkin	A_T, L g^-1	0.5869	0.8512
	B_T, mg L^-1	13.007	11.588
	R²	0.9158	0.9292
Dubinin-Radushkevich	Q_m, mg g^-1	25.267	25.508
	K	1 x 10-6	9 x 10-7
	E, kJ mol^-1	0.707	0.745
	R²	0.7611	0.7779

3.8 Effect of temperature

The exothermic and endothermic nature of the sorption was determined by checking the influence of temperature on sorption of Cr³⁺ and Cr⁶⁺ using mango peels. The temperature was varied from 303 to 353 K while the rest of parameters were kept unchanged. Figure 10 showed that % sorption of Cr³⁺ and Cr⁶⁺ onto mango peels powder decreased with the increment of temperature demonstrating the exothermic nature of the sorption process. Infect, a decrease in the thickness of the boundary layer took placed owing to the escaping of metal ions from the surface of sorbent into the bulk of solution. Consequently, decrease in sorption of Cr³⁺ and Cr⁶⁺ was observed with increase in temperature [48].

Figure 10

Effect of temperature (conditions for experiment: range of temperature 303-353 K, initial concentration of Cr³⁺ and Cr⁶⁺ 40 μg mL^-1, dosage of sorbent 0.1 g, volume for sorption of Cr³⁺ and Cr⁶⁺ 25 mL, pH 5.0 for Cr³⁺ and 7.0 for Cr⁶⁺, time of contact 1 hour).

3.9 Thermodynamic studies

The spontaneity and favorability of the sorption process of Cr³⁺ and Cr⁶⁺ using mango peels powder was determined with the help of enthalpy, free energy and entropy [49]. These parameters were evaluated using the following equations:

(14)KD=qeCe

(15)ΔG∘=−RTlnKD

(16)lnKD=ΔH∘RT+ΔS∘R

Where K_D represents distribution coefficient (L g^-1) while ΔS^o, ΔG^o and ΔH^o represent entropy, free energy and enthalpy of the system respectively. Similarly, the absolute temperature is denoted by T (K) and R is the general gas constant.

The values of these parameters are listed in Table 4 which revealed that all the values of ΔG^o are negative suggest that the sorption of Cr³⁺ and Cr⁶⁺ using mango peels powder is spontaneous and feasible. The value of ΔH^o is negative which demonstrates the exothermic nature of the sorption process of Cr³⁺ and Cr⁶⁺ while the negative value of ΔS^o suggests a reduction in the randomness at the solid/solution interface during the sorption process [50, 51, 52, 53, 54].

Table 4

Thermodynamic parameters for the sorption of Cr³⁺ and Cr⁶⁺ onto mango peels.

Temperature (K)	ΔG⁰(kJ mol^-1)		ΔH⁰(kJ mol^-1)		ΔS⁰(kJ mol^-1)

	Cr³⁺	Cr⁶⁺	Cr³⁺	Cr⁶⁺	Cr³⁺	Cr⁶⁺

303	7.783	6.618
313	4.282	3.987
323	4.100	2.781
333	4.022	2.199	-9.083 -	28.259 -	0.0018 -	0.0094
343	3.805	1.253
353	3.660	0.880

4 Conclusions

The present work demonstrates the usability of mango peels powder for the sorption of Cr³⁺ and Cr⁶⁺ from aqueous media in batch system. Solution pH was found to be the effective parameter and maximum sorption of Cr³⁺ and Cr⁶⁺ was achieved at pH 5.0 and 7.0 respectively. Sorption kinetics fitted well in to pseudo-2^nd order among different kinetic models owing to high R² values. Due to high values of the correlation coefficient, sorption data followed Freundlich isotherm as compared to other isotherm models. Maximum monolayer sorption capacities for Cr³⁺ and Cr⁶⁺ were found to be 98.039 and 66.666 mg g^-1 respectively. The sorption process of Cr³⁺ and Cr⁶⁺ was exothermic and spontaneous as indicated by values of thermodynamic parameters. It can be concluded from these results that mango peels powder can be used as an efficient and eco-friendly sorbent for the sorption of Cr³⁺ and Cr⁶⁺ from aqueous media.

Acknowledgements

The authors highly acknowledge the support of Higher Education Commission Pakistan.

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Received: 2018-08-31

Accepted: 2018-09-11

Published Online: 2019-03-25

Published in Print: 2019-01-28

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Articles in the same Issue

https://doi.org/10.1515/gps-2019-0001

Keywords for this article

mango peels; kinetics; equilibrium; thermodynamics

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A comparative sorption study of Cr3+ and Cr6+ using mango peels: kinetic, equilibrium and thermodynamic

Article

Abstract

1 Introduction

2 Materials and methods

2.1 Preparation of biosorbent

2.2 Chemicals and reagents

2.3 Standard solutions of Cr3+ and Cr6+

2.4 Instruments

2.5 Sorption studies

3 Results and discussion

3.1 Characterizations of mango peels

3.1.1 Scanning electron microscopic analysis

3.1.2 Surface analysis of mango peels

3.1.3 Fourier Transform Infrared analysis

3.2 Effect of pH

3.3 Effect of sorbent dose

3.4 Effect of contact time

3.5 Sorption kinetics

3.5.1 Pseudo-1st order

3.5.2 Pseudo-2nd order

3.5.3 Intraparticle diffusion model

3.5.4 Elovich model

3.5.5 Liquid film diffusion model

3.6 Effect of initial metal ion concentration

3.7 Adsorption isotherm studies

3.7.1 Freundlich isotherm

3.7.2 Langmuir isotherm

3.7.3 Temkin isotherm

3.7.4 Dubinin-Radushkevich (D-R) isotherm

3.8 Effect of temperature

3.9 Thermodynamic studies

4 Conclusions

Acknowledgements

References

Articles in the same Issue

Articles in the same Issue

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A comparative sorption study of Cr³⁺ and Cr⁶⁺ using mango peels: kinetic, equilibrium and thermodynamic

2.3 Standard solutions of Cr³⁺ and Cr⁶⁺

3.5.1 Pseudo-1^st order

3.5.2 Pseudo-2^nd order