Biosorption potential of Gliricidia sepium leaf powder to sequester hexavalent chromium from synthetic aqueous solution

Journal of Environmental Chemical Engineering 7 (2019) 103112

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Biosorption potential of Gliricidia sepium leaf powder to sequester hexavalent chromium from synthetic aqueous solution ⁎

Suganya Ea, Saranya Na, Chandi Patrab, Lity Alen Varghesea, , Selvaraju Nb, a b

T

⁎

Department of Chemical Engineering, National Institute of Technology-Calicut, Kozhikode, Kerala, 673601 India Department of Biosciences and Bioengineering, Indian Institute of Technology-Guwahati, Guwahati, Assam, 781039, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Hexavalent chromium Gliricidia sepium leaves Kinetics Isotherms Thermodynamics Desorption-Regeneration

Biosorption potential of Gliricidia sepium leaf powder (GSL) was investigated for elimination of hexavalent chromium (Cr(VI)) from synthetic waste-water. Behavioral effects on sorption of Cr(VI) over the biosorbent surface due to variation in pH, biosorbent-sorbate contact time, biosorbent dosage, initial chromium concentration, batch incubation temperature and agitation speed were analyzed using batch experiments. Characterization of the biosorbent includes point of zero charge evaluation, C-H-N-S analysis, Mercury intrusion porosimetry study, Scanning Electron Microscopy (SEM), Energy-dispersive X-ray (EDX) elemental analysis and Fourier Transform Infrared (FT-IR) analysis. The optimum parameters for maximal elimination (99.90%) of Cr (VI) with residual concentration of 0.48 mg/L were found at pH 2.0, contact time of 120 min, 0.3 g of biosorbent dosage, agitation speed of 100 rpm and 50 mg/L of Cr(VI) as the initial concentration. Freundlich isotherm model gave the best fit with the equilibrium data with adsorption capacity of 35.71 mg/g at pH 2 and 50 mg/L of Cr(VI)initial concentration. Pseudo-second order model best described the adsorption kinetics. Thermodynamic parameters validated the biosorption process as feasible, exothermic and stable. Desorption-regeneration experiments were performed with 0.5 N NaOH showed reasonable performances at equilibrium time. The data depicted the reusability of GSL as a promising, eco-friendly and economic biosorbent for Cr(VI) sequestration.

1. Introduction Generated waste-water with heavy metals due to industrialization is one of the most alarming problems to the eco-system. Improper treatment and effluent discharge from industries in to the water bodies creates deleterious effects to the aquatic flora and fauna, following which it ends up in the food chain and affecting humans too. In aqueous solution chromium occurs in variable oxidation states among which the trivalent and hexavalent forms are the most stable forms [1]. Hexavalent chromium is broadly used in industrial sectors like electroplating, leather, metal finishing, mining, metal cleansing, pigments, wood preservation and chemical manufacturing [2]. As per United States Environmental Protection Agency (USEPA) the maximal acceptable discharge limit for Cr(VI) onto domestic surface water bodies are 0.1 mg/L and 0.05 mg/L for potable water. Above the permissible limit, Cr(VI) tends to be mutagenic and carcinogenic to all life forms and more specifically to humans and animals. Chromium can be held responsible for all forms of mutagenic damages like genotoxicity, mutagenicity and cell transformations. Humans and animals are more exposed to Cr(VI) from drinking water than by any other means [3,4].

⁎

Traditional methods like coagulation, precipitation ion exchange, evaporation, extraction and electrochemical methods are found to have certain drawbacks like incomplete metal recovery at higher concentrations, sludge generation and difficulty in regeneration [5]. Hence a technology that overcomes the above said flaws needs to be implemented for the possible elimination of major Cr(VI) content from the water bodies. Biosorption is a typical biological-surface phenomenon in which the functional group-rich biological waste biomass is utilized for efficient removal of desired sorbate molecules at specific conditions of temperature, pH, initial Cr(VI) concentrations, contact time etc. Wastewater treatment using natural biomass is found to be promising owing to their eco-friendly nature, relative abundance and being economically cheap [6]. Several plants based waste materials have been used earlier by several researchers for Cr(VI) biosorption. Artimisia absinthium [7], Leersia hexandra Swartz [8], Polyalthia longifolia [9], Nicotiana tabacum [10], Tradescantia pallid [11], Polyalthia longifolia [9] are some examples of leaf biomass utilized as potent biosorbents for sequestration of Cr(VI). Leaves are rich in lignocellulose and various phytochemicals; which are a composite of several cationic functional groups and hence can adsorb anionic Cr forms [12]. Gliricidia sepium is a leguminous tree

Corresponding authors. E-mail addresses: [email protected] (L.A. Varghese), [email protected] (S. N).

https://doi.org/10.1016/j.jece.2019.103112 Received 16 January 2019; Received in revised form 18 April 2019; Accepted 26 April 2019 Available online 31 May 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.

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2.3. Point of zero charge (pHpzc) determination

belonging to the family Fabaceae. Its composite leaves can grow up to 30 cm long. These composite leaves are accompanied by small leaflets which are about 2–7 cm long and 1–3 cm wide. G. sepium is natively found in the tropical dry forests of Central American states and Mexico. It is also cultivated in various tropical areas, including the Southeast Asian regions and some parts of India, northern parts of South America and central Africa. The oil found in the leaves of G. sepium consists of aliphatics and terpenoids [13]. GCeMS analysis of the leaf extracts verified the presence of several phytochemicals like pentadecanal, nonanal, methyl linolenate and (Z)-phytol [14]. Since the G. sepium leaves contain immense lignocellulosic content and phytochemicals, they have been selected as biosorbent to check its ability to sequester Cr (VI). Current study deals with the investigation of biosorption ability of Gliricidia sepium leaf powder for the removal of Cr(VI). Influences of variables such as pH, dosage of biosorbent, initial concentration of Cr (VI), incubation temperature, system agitation speed and sorbate-sorbent contact time were analyzed via batch experiments. Biosorption mechanisms of Cr(VI) onto Gliricidia sepium leaves powder were evaluated using isotherm model studies, kinetic constants, and thermodynamic studies. To Evaluate reusability of the biosorbents, desorptionregeneration experiments were performed.

Point of zero charge (pHpzc) relates to the pH for which the number of anionic charges over the surface of adsorbent is equivalent to the number of cationic charges. At this pH, there’s net zero charge on the surface of adsorbent before adsorption. pHpzc was estimated by allowing 0.1 g of biosorbent to contact with different pH solutions of 0.1 mol/L of KNO3 ranging from 1.0 to 9.0 overnight. After 12 h, the pH was calculated using a digital pH meter. pHpzc was evaluated using a plot between initial vs. the variation between the final and initial pH. 2.4. Characterization of biosorbent

2. Materials and methods

Elemental content of the biosorbent was determined using the C-HN-S elemental analyzer (Vario EL III, Elementar, Germany). Porosity and pore volume data were analyzed using Mercury intrusion porosimeter (Quantachrome ASAP 2200, Micromeritics, USA). The predominant functional groups over the surface of the biosorbent involved in the adsorption process were identified using FT-IR (Jasco 1033, FT/ IR-4700 Type A India). Morphology changes of the biosorbent surface after and before biosorption of Cr(VI) was analyzed using Scanning Electron Microscopy (SEM) (JSM – 6390 L V, JEOL, USA). Energy-dispersive X-ray spectroscopy (EDX) (JED–2300, JEOL, USA) was used for analyzing extend of chromium adsorption over the surface of GSL.

2.1. Preparation of biosorbent

2.5. Batch adsorption experiments

Gliricidia sepium leaves were collected locally from the campus of National Institute of Technology-Calicut, Kozhikode, India. The collected leaves were thoroughly washed and cleaned with detergent mixed double distilled water to clean off the soil particles and debris. The washed leaves were dried at normal room temperature for around 72 h, following which it was oven dried at 70 °C for 12–15 h. The dried leaves were pulverized to dry powdered form and then sieved to size of less than 90 μm. Fig. 1(a–c) represents the Gliricidia sepium leaves; before drying, after drying and in powdered form respectively. The obtained powder is then stored for further experiments and named GSL biosorbent.

Batch experiments for biosorption of Cr(VI) using GSL (90 μm) were conducted at variable pH levels (2, 3, 4, 5, 6, 7 and 8), variable dosage of biosorbent (0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 g), variable agitation speeds (60, 80, 100, 120, 140 and 160 rpm), variable initial concentration of Cr(VI) (50, 100, 150, 200 and 250 mg/L) and variable temperatures (303, 313, 323 and 333 K). Several experiments in batch mode were conducted in stoppered conical flasks with desired chromium concentration subjected with desired dosage of biosorbent and were stirred on a shaker incubator (116736GB, GeNei). Chromium solution samples were collected at periodic intervals and were filtered using filter paper. The collected filtrate was then studied for remnant Cr(VI) by reacting it with desired volume of 1,5-diphenylcarbazide in acidic medium thus producing a violet colored mixture. Concentration of Cr(VI) present in the mixture was spectrophotometrically estimated via UV–vis spectrophotometer (2201, Systronics) at 540 nm. Concentration of Cr(VI) ions adsorbed over the biosorbent surface; qt (mg/g) was evaluated as per the following equation:

2.2. Preparation of chromium solution Stock solution for Cr(VI) of 1000 ppm was prepared by adding 2.828 gm of potassium dichromate salt (K2Cr2O7) in to 1000 mL distilled water. Prepared stock solution was appropriately diluted to get the working concentrations viz. 50, 100, 150, 200 and 250 mg/L.

qt =

(C0 − Ct )V m

Fig. 1. leaf (a) before drying (b) after drying and (c) in powdered form. 2

(1)

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Hence the predominant functional groups of GSL participating in Cr(VI) biosorption were hydroxyl (−OH) groups, C–H stretches and eC]Ce stretches of alkanes, NeH groups of primary amines, C–O stretch of alcohols, esters, ethers and carboxylic acids as shown in Fig. 3. The possible mechanisms involved for metal adsorption on cellulosic biosorbents can be chelation, ion exchange and complexation with the functional groups [19]. Physical adsorption can be possibly due to weak Van der Waal’s force of attraction, whereas chemisorption can be due to relatively strong chemical bonding between Cr(VI) species and biosorbent’s surface functional groups as suggested in FT-IR studies [20].

The adsorption efficiency in percentage was evaluated as per the following equation:

C − Ce ⎞ × 100 %Removal = ⎛ o ⎝ Co ⎠ ⎜

⎟

(2)

Here, Co and Ce (mg/L) represents the initial and equilibrium concentrations of Cr(VI) respectively. V represents volume of solvent (L) and m represents the biosorbent’s dosage(g) respectively. Desorption percentages with 0.5 N NaOH were evaluated using the following equation:

%Desorption =

Cdes × 100 Cads

3.2. Effect of variable pH

(3)

Here, Cdes represents the residual concentration of Cr(VI) released with NaOH solution after equilibrium time t and Cads represents the Cr(VI) concentration adsorbed over GSL surface within the same time t.

Point of zero charge was determined as 2.0 from the experiments (Fig. 4). Below this pH the biosorbent is highly protonated and capable of attracting negatively charged ions. Beyond this pH, the biosorbent would acquire negative charge and attracted positively charged ions. Presence of organic functional groups over biosorbent’s surface causes cationic or anionic charges as per desired pH of solution. For pH values, more than pHpzc, the sites tend to dissociate and get negatively charged, while for values less than pHpzc, the sites tend to be in associated form with protons to acquire positive charge [21]. Fig. 5 represents removal biosorption efficiency (%) by GSL leaf at optimum pH. pH of sorbate solution plays a significant role in Cr(VI) biosorption by the biosorbent since pH of sorbate solution controls the biosorbent surface’s charge density and charge of the present metallic species. Batch experiments were done at variable pH viz. 2.0, 3.0, 4.0, 5.0, 6.0, 7.0 and 8.0 for 120 min, 100 rpm and 50 mg/L of Cr(VI) solution. pH 2.0 gave the maximum removal (˜ 90%) and the percentage removal decreased with the rise in pH and it almost fell to 30% at pH 8.0. The reason can be basically due to low pH at which biosorbent surface would be highly protonated. Chromium in aqueous solution undergoes speciation to form anions which preferentially tend to bind with protonated biosorbent surface. Hence lower pH is preferable for the adsorption of Chromate ions [22].

3. Results & discussions 3.1. Characterization of biosorbent Physico-chemical parameters of the biosorbent (GSL) were characterized by analyzing the significant parameters. Elemental content was measured using C-H-N-S analyzer and Mercury intrusion porosimetry. The obtained data is as represented in Table 1. C-H-N-S analysis showed that the biosorbent is cellulosic in nature [15]. Scanning Electron Micrograph shows the morphological changes of GSL surface before and after biosorption. Fig. 2(a) shows that surface of the GSL is coarse and porous. Internal pore size and distribution are crucial for efficient biosorption [16]. SEM image, Fig. 2(b) shows lesser complex morphology which thus determines Cr(VI) molecules adsorbed on GSL surface. EDX spectra of GSL prior and after biosorption [as shown in Fig. 2(a) and (b)] show specific peaks for Cr ions, thus representing that biosorption must have occurred on GSL surface. Fourier Transform Infrared Spectroscopy (FT-IR) analysis determined the probable functional groups involved in Cr(VI) biosorption by the GSL surface. Shift of a broad peak at 3370.90 cm−1 to 3351.60 cm−1 suggest the participation of hydroxyl (−OH) and amino groups (–NH2) [17]. Shift of narrow sharp peak from 2932 cm−1 to 2928.30 cm−1 suggests the contribution of C–H stretches of alkanes. Slight shift of peak at 1569.70 cm−1 suggests the –C]Ce stretches of alkenes. Appearance of sharp narrow peak at 1580.37 cm-1 determine the formation of NeH of primary amines, clearly represents the involvement of amino acids and other nitrogenous compounds of the leaf constituent in biosorption. Shift at 1484.91 cm-1 suggests the contribution of CeC stretch of aromatic compounds of GSL. Presence of small sharp peak at 1359.59 cm-1 represents the contribution of C–H groups of alkanes. Slight shift at 1213.97 cm-1 and 1149.36 cm−1 represent the involvement of C–O stretches of alcohols, esters, carboxylic acids and ethers. Slight drift of peak at 880.34 cm-1 clearly represents chromium adsorption over the surface of the GSL biosorbent [18].

3.3. Effect of variable biosorbent dosage Effects of variable GSL dose was determined using variable doses ranging from 0.1 to 0.6 g with other parameters kept fixed at optimum levels. Removal efficiency of 90.13% for Cr(VI) was attained at 0.3 g dosage of GSL as depicted in Fig. 6. The biosorption efficiency (%) of GSL declined with further rise in biosorbent dosage and this might be due to limited availability of GSL surface area caused due to biosorbent aggregation, overlapping or overcrowding [23]. Also, the decline in biosorption efficiency (%) can be because of non-availability of sufficient Cr(VI) molecules, since sufficient amount is already adsorbed on the surface of available biosorbent. 3.4. Effect of variable agitation speed Variable agitation speed influences the biosorption potential of ions over surface of the biosorbent. The investigation over the agitation speed performed from 60, 80, 100, 120, 140 and 160 rpm and as depicted in Fig. 7. Biosorption efficiency (%) of the biosorbent shows higher capacity values at all different agitation speeds, but showed maximum at 100 rpm. Decrease in removal percentage at greater agitation speeds is probably due to de-sorption of Cr(VI) species from biosorbent’s surface.

Table 1 Physical and chemical characteristics of GSL. Parameter Elemental analysis (%) C% H% N% S% Porosimetry data Surface area (m2/g) Average pore diameter (μm) Bulk density (g/cm3) Particle size (mm)

Value

45.32 4.474 3.885 2.820

3.5. Effect of variable initial Cr(VI) concentration 0.5016 4.17 0.83 90 μm

Effects of variable initial Cr(VI) concentrations upon the sorption potential of GSL biosorbent at variable time intervals keeping other parameters fixed viz. pH 2.0, 100 rpm, 303 K has been shown in the 3

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Fig. 2. SEM-EDAX Gliricidia sepium images of (a) native GSL and (b) after Cr(VI) species biosorption.

Fig. 4. Determination of Point of zero charge of GSL biosorbent.

Fig. 3. FTIR spectra of GSL (a) before Cr(VI) adsorption and (b) after Cr(VI) adsorption.

4

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Fig. 8. Effect of initial chromium concentration at different contact time and optimum conditions.

Fig. 8. The biosorption capacity gradually increased and then achieved equilibrium. After the optimum contact time of 120 min., no considerable increase in the adsorption capacity was found. When a concentration gradient developed between the sorbent molecules and Cr (VI) ions, exchange of ions takes place rapidly and hence the GSL showed an initial rise in adsorption capacity followed by a decline phase and then equilibrium is achieved. Whereas, with the rise in initial Cr(VI) concentration from 50 to 250 mg/L, the removal efficiency (%) reduced owing to the quick depletion of the adsorption sites [24].

Fig. 5. Effect of pH on biosorption efficiency of biosorbent.

3.6. Isotherm analysis Adsorption isotherms analyze the isothermal interactions between the sorbate and sorbent molecules as a function of its pressure (if gas) or concentration (if liquid) at some constant temperature. Biosorption processes are detailed using various two parameter isotherm models as discussed following. Fig. 9 shows the non-linear fitting of the isotherms with the experimental data. Fig. 6. Effect of GSL dosage on biosorption efficiency of biosorbent.

Fig. 7. Effect of agitation speed (rpm) on biosorption of Cr(VI) species by biosorbent. Fig. 9. Isotherm plots of Gliricidia sepium leaf biomass for Cr(VI) removal at optimum conditions. 5

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(

Table 2 Isotherm constants for Cr(VI) biosorption onto the GSL biosorbent. Isotherm Analysis

Parameters

GSL biomass

Langmuir

Q0 (mg/g) KL (L/mg) RL (mg/L) Goodness of fit R2 Number of points Degrees of freedom Residual sum of squares Reduced Chi-Sqr KF (mg/g)(L/mg)1/n nF Goodness of fit R2 Number of points Degrees of freedom Residual sum of squares Reduced Chi-Sqr Qm (mg/g) K(mol2/J2) E (kJ/mol) Goodness of fit R2 Number of points Degrees of freedom Residual sum of squares Reduced Chi-Sqr

33.57 ± 4.177 0.570 ± 0.366 0.33

Freundlich

Dubinin-Radushkevich

0.846 5 3 66.449 22.149 16.046 ± 1.204 4.905 ± 0.600

1 1 + kL C0

0.974 5 3 11.163 3.721 32.23 ± 3.831 1.249 ± 0.65405 0.142

qe = Qm exp−Kε

0.823 5 3 76.675 25.558

2

2

(7) 2

Here, K (mol /KJ ) denotes the adsorption coefficient related to mean free energy, Qm (mg/g) represents the Dubinin-Radushkevich (D-R) maximal adsorptive capacity, ε symbolize the Polanyi potential and it is represented as follows:

1⎞ ε = RT ln ⎛1 + Ce ⎠ ⎝ ⎜

⎟

(8) −1

−1

Here, R (8.314 J mol K ) represents gas constant, T (K) represents temperature. The mean adsorption energy E (kJ/mol) was measured using ‘k’ values as represented in the equation:

E=

1 2k

(9)

Mean adsorption energy (E) provides a meaningful data about the physical/chemical aspects of the biosorption process [28]. If the value of E < 8 kJ/mol, then physisorption influence the biosorption process. If the value of E > 16 kJ/mol, then chemisorption influence the biosorption process. The ion-exchange mechanism lies in the adsorption energy range of 8–16 kJ/mol [29]. Experimental data verify that the calculated value of mean adsorption energy (E) as 0.142 kJ/mol and thus represents physisorption and involvement of weak physical forces between the sorbate-sorbent molecules. The regression value obtained was 0.823 and hence the model represented a least fit for the experimental data.

(4)

(5)

3.7. Kinetic studies

Here, C0 (mg/L) represents the initial Cr(VI) concentration, RL denotes the adsorption nature i.e. favorable if 0 < RL < 1, non-favorable if RL > 1and irreversible if RL = 0. Biosorption capacity was calculated as 33.57 ± 4.177 mg/g. RL = 0.33 denotes favorable process. Coefficient of determination (R2) value obtained is low i.e. 0.846 thus representing a least fit to the Langmuir isotherm. Table 2 clearly represents the calculated Langmuir parameters. Hence the biosorption of Cr(VI) onto the GSL biosorbent surface was predominantly no longer monolayer adsorption.

Kinetics of interaction between Cr(VI) molecules and the surface of the GSL biosorbent have been explored by using pseudo-first order kinetics, pseudo-second order kinetics and intra-particle diffusion models. 3.7.1. Pseudo-first order kinetics Pseudo-first order kinetics can be represented by the following equation [30]:

log(q e−q) = logq e −

3.6.2. Freundlich isotherm model This model defines adsorption as a reversible process and is applicable to multilayer adsorption over the heterogeneous surface. This isotherm directs the surface of the adsorbent as heterogeneous and systemic distribution of the active sites and energies thus related [26]. Freundlich model can be depicted as:

k1 t 2.303

(10)

Here, qe (mg/g) represents the amount of Cr(VI) molecules adsorbed after achieving equilibrium and q (mg/g) represents the amount of Cr (VI) molecules adsorbed at any time t. k1 (min−1) represents pseudofirst order rate constant. Table 3 shows the regression (R2) values for pseudo-first order kinetics and they were found to be lesser than that of other kinetic models further analyzed. Moreover, the evaluated qe values were not in favor with the actual values and hence pseudo-first

1

qe = KF Cen

)

3.6.3. Dubinin-Radushkevich (D-R) isotherm model D-R isotherm model is based on the hypothesis that the sorbent molecules are devoid of homogenous surface and the sorption of sorbate with the adsorbent occurs by pore-filling and not layer-on-layer coverage. This model is based on a theory known as Polanyi potential adsorption theory; according to which the adsorption process is driven by chemical or physical forces [27]. The D–R isotherm can be represented as:

Here, Ce (mg/L) represents Cr(VI) concentration under equilibrium, qe (mg/g) represents the quantity of Cr(VI) adsorbed per unit weight of the biosorbent under equilibrium, Q0 (mg/g) represents Langmuir monolayer coverage capacity and KL (L/mg) represents constant for Langmuir isotherm [25]. The Langmuir isotherm can also be represented in the form of equilibrium parameter RL; referred to as separation factor (dimensionless constant) and can be represented as:

RL =

1

denotes the adsorption intensity. Detailed analysis of the model with the experimental results showed the coefficient of determination value to be 0.947 and thus representing a better fit than Langmuir model. KF value was determined as 16.046 ± 1.204 and low value of 1/n signify biosorption of Cr(VI) over GSL surface is favorable. Hence multilayer adsorption seems dominant for Cr(VI) adsorption onto the biosorbent surface. Non-linear curve fitting as shown in Fig. 9 verifies the same, with Freundlich isotherm model aligning nearest to the experimental data.

3.6.1. Langmuir isotherm model This model assumes the sorption between solid sorbent molecules and solid sorbate molecules concludes to a monolayer formation of the sorbate (heavy metal) over a homogenous and uniform sorbent (biosorbent) surface. According to this model all the available sorption sites are similar and morphologically homogeneous. Langmuir isotherm equation can be expressed as:

Q0 KL Ce qe = 1 + KL Ce

1

Here, KF mg1 − nL ng−1 represents Freundlich isotherm constant and n

(6) 6

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Table 3 Biosorption data evaluated by different kinetic models for the GSL (pH 2.0). Cr(VI) Conc.(mg L−1)

Pseudo first order K1 (min

50 100 150 200 250

0.0023 0.0046 0.0046 0.0046 0.0046

−1

)

qe(cal) (mg g

Pseudo second order −1

1.659 2.07 2.208 1.967 1.936

)

R

2

K2 (g mg

0.795 0.924 0.771 0.731 0.723

−1

min)))min

Intra particle diffusion −1

)

0.0071 0.0019 0.0014 0.0013 0.0011

qe(cal) (mg g

−1

)

9.259 18.86 27.03 33.33 40

R

2

0.996 0.973 0.988 0.986 0.998

kid (mg g−1 min1/2)

R2

0.443 1.001 1.566 1.904 2.153

0.926 0.969 0.942 0.902 0.927

order doesn’t fit well for the experimental results. 3.7.2. Pseudo-second order kinetics Pseudo-second order kinetics can be represented as per the following equation [31]:

t 1 t = + qt qe K2 qe2 −1

(11) −1

Here, K2 (g mg min ) represents pseudo-second order rate constant. Calculated values for qe and K2 for Cr(VI) biosorption by the biosorbent are shown in Table 3. Owing to the coefficient of determination (R2) values, it’s evident that the biosorption phenomenon gave the best fit and thereby followed pseudo-second order kinetics. 3.7.3. Intra-particle diffusion This model represents the mechanistic insight of the adsorptive process and the rate-limiting steps affecting the adsorption kinetics [32]. It can be represented as:

qt = Kid t 1/2 + C

Fig. 10. Effect of temperature on biosorption of Cr(VI) species using GSL. Table 4 Thermodynamic parameters for Cr(VI) removal on GSL biosorbent.

(12) −1

Here, Kid represents the intra-particle diffusion constant (mgg min1/ 2 ). Intra-particle diffusion phenomenon occurs only when qt vs. t1/2 plot is linear. If plot intersects the origin, then the rate determining process is because of intra-particle diffusion. The plot with multi linear curve represented that biosorption phenomenon wasn’t only influenced by diffusion but also by other mechanisms.

Thermodynamic parameters are important to demonstrate the qualitative study of Cr(VI) biosorption. Biosorption equilibrium constant (Kc) in accordance with change in Gibb’s free energy (ΔG) can be represented thermodynamically [33] in form of following equation: (13)

Enthalpy change (ΔH) and entropy change (ΔS) can be represented as:

ln K C = −

ΔH ΔS + RT R

(14)

Here, T represents the absolute temperature in Kelvin and R (8.314 Jmol−1 K−1) represents the universal gas constant. Biosorption equilibrium constant (Kc) can be represented as:

KC =

qe Ce

ΔG° (kJmol−1)

ΔH° (kJmol−1)

ΔS° (kJmol−1 K-1)

303 313 323 333

−13.150 −1.673 2.016 3.292

−0.0078

−2.533

physisorption [34]. Thermodynamic parameters were calculated by plotting the ln kc vs. 1/T gives coefficient of determination value as 0.844. Values of Gibb’s free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) thus calculated were summarized in Table 4. Negative ΔG values represent spontaneous and feasible adsorption of sorbate onto sorbent. The ΔG value at optimum temperature of 303 K is negative; the value increased gradually as the temperature increased and became positive at 323 and 333 K. This showed that the biosorption is not energetically favorable at higher temperatures owing to the increased randomness between the sorbent-sorbate interface which hinder sorbent-sorbate contact. Negative ΔH value clearly represents that adsorption of Cr(VI) onto GSL is exothermic. Moreover, negative ΔS value at optimum conditions implies that the degree of randomness at the Cr(VI) –GSL interface is decreased due to associative biosorption between them [35].

3.8. Thermodynamic studies

ΔG = −RT ln K C

Temperature(K)

3.9. Desorption-regeneration studies

(15)

Desorption-regeneration studies of the used biosorbent after adsorption of Cr(VI) was done using 0.5 N NaOH and then re-using it as a biosorbent. Table 5 shows the desorption-regeneration performance of the GSL biosorbent with NaOH treatment. Desorption of the biosorbent after the first run was 90.10% and gradually reduced to 78.12% and further to 65.12% in consecutive runs respectively. Moreover, the regeneration runs with 50 mg/L of Cr(VI) solution showed 94%, 83% and 72% efficiency in three runs after corresponding desorption runs. This

Temperature plays an influential role on metal ion biosorption. The biosorption efficiency (%) of Cr(VI) was evaluated as function of temperature from 303 to 333 K and as illustrated in Fig. 10. The biosorption efficiency (%) increased when the temperature increased up till 313 K after which it decreased on further increase of temperature up to 333 K. This decline with the rise in temperature elucidates biosorption as an exothermic phenomenon and suggests weak biosorption interaction exhibited between GSL surface and Cr(VI) thus supporting 7

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Table 5 Desorption-Regeneration experiments. Time (mins)

30 60 90 120 150 180 210

Desorption (%)

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Regeneration (%)

1

2

3

1

2

3

5.5 18.34 48.76 63.20 80.16 90.10 90.12

3.50 15.00 35.70 50.90 72.90 78.12 78.0

2.50 7.00 16.50 32.00 63.23 65.12 65.00

32.33 43.76 60.45 74.13 85.65 94.00 94.00

30.0 40.12 55.00 70.60 75.80 83.00 83.00

22.12 32.50 40.50 55.50 63.12 72.00 72.00

Table 6 Previously reported biosorbents with their adsorption capacities for Cr(VI) removal. Biosorbent

Biosorption capacity (mg g−1)

References

Erythrina variegata orientalis leaf powder Ficus carica Eichhornia crassipes Aegle marmelos correa Neem leaves Pineapple leaves Swietenia mahagoni fruit shell Gliricidia sepium Leaf Powder

1.923

[36]

19.68 5.6 17.27 15.95 18.77 2.30 35.71

[37] [38] [39] [40] [41] [42] (Present study)

shows that the GSL biosorbent has a good regeneration capacity that can be utilized for Cr(VI) removal in an effective way from aqueous solutions. 4. Conclusions Lignocellulosic biomass is naturally available in bulk amounts obtained from agricultural by-products and waste biomass from plants. Gliricidia sepium leaf (GSL) is one such biomass that has been exploited as a biosorbent for the effective elimination of Cr(VI) species from synthetic aqueous solutions. Table 6 compares the biosorption potential of GSL biosorbent with the other exploited biosorbents for the removal of Cr(VI) species. Biosorption of Cr(VI) species onto the GSL surface was influenced by factors viz. variable GSL dosage, variable initial concentration of Cr(VI), different agitation speeds and variable temperatures of incubation, in addition to solution pH. Evaluation over the isotherm models revealed that Freundlich isotherm gave the best fit for the biosorption process of Cr(VI) species by GSL surface than the other isotherm models. The pseudo-second order kinetics accurately interpreted the kinetic data. Thermodynamic parameters revealed biosorption as spontaneous, exothermic and stable. Desorption-Regeneration studies showed good performances up to 3 cycles. Hence GSL can be utilized as eco-friendly, cheap and cost-effective biosorbent for the sequestration of Cr(VI) species form synthetic aqueous solutions. Acknowledgement This work was supported by the Kerala State Council for Science, Technology and Environment, India under Grant No. ETP/02/2014/ KSCSTE. References [1] A.K. Shankera, C. Cervantes, H.L. Tavera, S. Avudainayagam, Chromium toxicity in plants, Environ. Int. 31 (2005) 739–753. [2] M. Dinesh, U. Charles, J. Pittman, Activated carbons and low cost adsorbents for remediation of tri and hexavalent chromium for water, J. Hazard. Mater. 137 (2006) 762–811. [3] M. Costa, Potential hazards of hexavalent chromate in our drinking water, Toxicol.

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