Environ Monit Assess (2015) 187:763 DOI 10.1007/s10661-015-4984-6

Natural biosorbents (garlic stem and horse chesnut shell) for removal of chromium(VI) from aqueous solutions Şerife Parlayıcı & Erol Pehlivan

Received: 13 July 2015 / Accepted: 10 November 2015 # Springer International Publishing Switzerland 2015

Abstract The biosorption of Cr(VI) by the garlic stem (GS)-Allium sativum L. and horse chesnut shell (HCS)-Aesculus hippocastanum plant residues in a batch type reactor was studied in detail for the purpose of wastewater treatment. The influence of initial Cr(VI) concentration, time, and pH was investigated to optimize Cr(VI) removal from aqueous solutions and equilibrium isotherms and kinetic data. This influence was evaluated. The adsorption capacity of the GS and the HCS for Cr(VI) was determined with the Langmuir and Freundlich isotherm models, and the data was fitted to the Langmuir. The adsorption capacity of the GS and the HCS was found to be 103.09 and 142.85 mg/g of adsorbent from a solution containing 3000 ppm of Cr(VI), respectively. The GS’s capacity was considerably lower than that of the HCS in its natural form. Gibbs free energy was spontaneous for all interactions, and the adsorption process exhibited exothermic enthalpy values. The HCS was shown to be a promising biosorbent for Cr(VI) removal from aqueous solutions. Keywords Hexavalent chromium . Equilibrium . Garlic stem . Horse chestnut shell

Ş. Parlayıcı : E. Pehlivan (*) Department of Chemical Engineering, Selcuk University, Campus, 42079 Konya, Turkey e-mail: [email protected]

Introduction Rapid industrialization has led to a tremendous increase in the use of chromium over the last decades and inevitably resulted in an increased flux of toxic substances in the aquatic medium. The chromium enters tissues through the food pipe or other pathways and accumulates in the human tissue. If the chromium is ingested beyond the permitted concentration, it can cause serious health disorders. Therefore, it is necessary to treat chromium-polluted wastewater prior to discharging it to the environment. The primary means of human exposure to chromium are inhalation, ingestion, and skin contact. It can be inhaled when hexavalent chromium (Cr(VI)) particles are present as dust, mist, and fumes in the air. Particles of Cr(VI) dust can contaminate hands, clothing, food, beverages, and facial hair. Cr(VI) is a substance that has carcinogenic properties and can get into very common place sources of ingestion such as water sources if it is not controlled properly. Inhaling chromium compound can cause asthma sympstoms such as wheezing and shortness of breath. Exposure of Cr(VI) can occur for people living near uncontrolled hazardous waste sites containing chromium or industries that use chromium. Cr(VI) is classified as a carcinogen, in particular lung cancer caused by Cr(VI) compounds formed in the environmet. The World Health Organization and the EPA have classified Cr(VI) as a human carcinogen. The maximum permissible limit of Cr(VI) for the discharge into inland surface water is 0.1 mg/L and in to potable water is 0.05 mg/L (EPA 1990).

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Chromium can exist both in trivalent and hexavalent forms, making its removal from a stream very difficult. In most of methods, first Cr(VI) was reduced to chromium(III) (Cr(III)) and then precipitated out from aqueous phase. Cr(VI) is typically present as an anion, and its direct precipitation is not usually practical. Chromium compounds are widely used in many different industries, and Cr(VI) is a toxic form of element among them. Some of the major industrial sources include chromic pigments and dyes, paints, inks, and plastics. Cr(VI) is a toxic form of element chromium, and Cr(VI) compounds are mainly used in many different industries. A number of cleanup technologies cover precipitation, adsorption, ion exchange, chemical oxidation-reduction, reverse osmosis, electrodeposition, ultrafiltration, solvent extraction, etc. are being used to remove Cr(VI) from effluent (Guell et al. 2008; Gode and Pehlivan 2005; Saordohan et al. 2010; Yanhui et al. 2011; Edebali and Pehlivan 2014). All these methods have their inherent advantages and limitations. Biosorption of Cr(VI) from effluents is a rather new separation technique which was proven to be very promising in the removal of contaminants. These materials derived from low-cost plants can be used for the effective removal of Cr(VI) ions. Many types of agricultural wastes and plant residue have been investigated for their chromium adsorption properties. These are coconut husk, teak tree bark, rice straw, rice bran, rice husk, hyacinth roots, neem bark, saw dust of teakwood origin, neem leaves, coconut shells, (Singha et al. 2011), and chestnut shells (Ya et al. 2010). Furthermore, modified spruce bark (Liang et al. 2014), mangosteen peel (Huang et al. 2014), sesame stems (Selen et al. 2014), Portulaca oleracea (Mishra et al. 2014), barks of Acacia albida trees and leaves of Euclea schimperi (Gebrehawaria et al. 2014), chitosan, carbon nanotubes (Jung et al. 2013), and chestnut shell (Vazquez et al. 2009) can be used as chromium adsorbents. A mass yield was produced from agricultural wastes during the harvest. Garlic is an annual herb native to Turkey, but its widespread use as a condiment and medicinal properties is cultivated in all places of the world. Since ancient times, garlic, among the oldest cultivated plants, has been used both as common foods and for medicinal applications. GS is usually consisting of cellulose (30–50 %), hemicellulose (10–30 %), and lignin (15–30 %) (Parka et al. 2012; Virginia 2006). In

Environ Monit Assess (2015) 187:763

Turkey, a huge amount of HCS and GS is planted. The seeds of HCS have been used as an emergency provision since ancient times. Recently, the seeds have been utilized traditionally in Japan as a confectionery ingredient in rice cakes and rice (Ogawa et al. 2008). HCS is generated as a residue and has no significant industrial and commercial uses, but it becomes an issue and contributes to prevent environmental water pollution by using them in wastewater treatment units as adsorbent. The aim of this study was to compare the use of the GS and of the HCS in the removal of Cr(VI) from aqueous solution. The research involved the investigation of some experimental conditions such as pH of solution, contact time, adsorbent loading, and temperature as it relates to adsorption of the Cr(VI) by the adsorbents. Freundlich and Langmuir isotherms were used to fit the equilibrium data using the operational conditions of experiments. Kinetic study using pseudofirst-order and pseudo-second-order kinetic models were also used to investigate the mechanism of adsorption.

Experimental Preparation of GS and HCS All the chemicals and reagents used in the experiments were of analytical grade, and double-distilled water was used wherever required. A stock solution of Cr(VI) ions with a concentration of 5000 ppm was prepared by dissolving K2Cr2O7 salt (Merck) in double-distilled water. NaOH and HCl solutions were purchased from Merck. Both GS and HCS were collected from agricultural fields in Kastamunu and Konya, respectively. They were cut into small pieces and blended and washed with distilled water and 1 M H2SO4 to remove dirt and color which were present in the biosorbents. The samples were ground and screened to 200 mm size then dried for 24 h at 100 °C to reduce the water content. Each portion of biosorbent was stored in a desicator prior to analysis. Characterization of the biosorbents C, H, and N contents of the bisorbents were analyzed by the METU Central Laboratory R&D Training and Measurement Center in Ankara, Turkey ( LECO, CHNS-932) (Table 1).

Environ Monit Assess (2015) 187:763 Table 1 C–H–N analysis of HCS and GS

Page 3 of 10 763 HCS

GS

C content (%)

47.46

22.14

H content (%)

5.77

3.52

N content (%)

–

1.02

A scanning electron microscope (SEM) that produces images of a sample by scanning it with a focused beam of electrons was used for the biosorbents before and after adsorption of Cr(VI) (Fig. 1). The SEM of biosorbents was taken to study the surface morphology. The surface morphology and texture of the metal-loaded biosorbents were completely different compared to the natural ones. A certain change on the beads’ surface

after the adsorption of Cr(VI) was observed. The SEM of the pure biosorbents showed a particular flakes shape (Fig. 1). The surface morphology of GS and HCS appears to change significantly following dispersion of Cr(VI). As seen in the Fig. 1, the micrograph shows that the porous nature of the outer surface of the biosorbents adsorbed Cr(VI) ions from the solution. Fouier transform infrared spectra (FT-IR) (Bruker EQUINOX 55, Germany) was used to investigate the changes in vibration frequency in the functional groups of the biosorbents [7]. The functional group is one of the key factors to understand the mechanism of the Cr(VI) binding process on the GS and on the HCS. Each pretreated GS and HCS before and after Cr(VI) adsorption were mixed separately with KBr to prepare

Fig. 1 The SEM micrograph of pure and Cr(VI)-loaded biosorbents. Powders a HCS, b HCS-Cr(VI), c GS, and d GS-Cr(VI)

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translucent sample pellets. The spectra were recorded using a FT-IR (Fig. 2). These spectra indicated a number of adsorption peaks showing the complex nature of the biosorbents. The spectral analysis before and after metal binding also indicated that, in the case of Cr(VI), there was an intensity change and shift to a lower frequency of the CO bands in carboxylic acids or their esters. The spectra was measured within the range of 750– 4000 cm−1 and given in Fig. 2. Some changes in the characteristic peaks ranging from 3600 to 900 cm−1 and some visible shifts of peak locations of the biosorbents were observed before and after adsorption. There was a clear shift from wavenumber of 3375 cm−1 HCS to 3250 cm−1 (Cr(VI)-loaded HCS), 3375 cm−1 GC to 3300 cm−1 (Cr(VI)-loaded GC) which indicated that surface -OH group was one of the functional groups responsible for the adsorption of Cr(VI). Aliphatic C–H stretching may be responsible for Cr(VI) adsorption onto GC as wave number shift from 2900 to 2890 cm−1. Unsaturated groups such as alkenes were found to have major shifts of wave numbers from 1620 to 1610 cm−1 for the adsorption of Cr(VI) by the HCS and 1620 to 1630 cm−1 for the adsorption of Cr(VI) by the GC. FT-IR spectrum of rice straw also showed intense bands around 1300 cm−1 which shifted to 1400 cm−1 for Cr(VI)-loaded GC. This showed that the carboxylate anion is responsible for the adsorption

Fig. 2 FTIR spectra of a HCS, b GS, c HCS-Cr(VI), and d GS-Cr(VI)

Environ Monit Assess (2015) 187:763

on GC (Singha et al. 2011; Pehlivan et al. 2013a). The peaks around 1735.6 cm−1 show the carbonyl (C=O) stretching vibration of the carboxyl groups, while the peaks at 1400 cm−1 were initiated by the stretching of the carboxylate group (–COO–). The band around 1730 cm−1 is due to a C=O group of a carboxylic acid or its ester (Pavan et al. 2006). The peaks ranging from 1300 to 1000 cm−1 are ascribed generally to the C–O stretching vibration in carboxylic acids and alcohols.

Results and discussions Effect of contact time, initial pH, Cr(VI) concentration, and amount of biosorbent Figure 3 shows the variation in the percentage of removal of Cr(VI) with contact time at 25 °C, initial pH (≈2.0), and an initial concentration of approximately 3000 mg L−1. It was observed that the adsorption percentage increased with the increased contact time (about 6 h for GS and 2 h for HCS) until equilibrium was obtained. The highest value obtained was nearly 72 % for the GS, and 95 % HCS) for the Cr(VI) ions. Figure 4 shows the effect of initial solution pH on the adsorption % of Cr(VI) ions by the two biosorbents. It can be seen that the adsorption efficiencies toward

Environ Monit Assess (2015) 187:763 Fig. 3 Adsorption of Cr(VI) versus with the time for GS and HCS

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b 14

14

12

12

10

10

pHpzc

pHpzc

a

8 6

4

2

2 0 0

20

40

NaOH (ml)

Cr(VI) ions of biosorbents were high (>72 for GS and 95 % for HCS) at low pH (1.5–2.0). Lower pH was beneficial for Cr(VI) ion removal because the active sites on the biosorbents were protonated and inclined to adsorb Cr(VI) ions, which exist as anion (mainly HCrO4−) in solution. It suggests that high concentrations of H+ ions facilitate adsorption. Whereas, high concentrations of OH− ions suppress the adsorption reaction, thus accounting for the decrease in the percentage of adsorption of Cr(VI) ions at high pH. Point zero charge (pzc) is a related concept in chemistry and the idea connected to the phenomenon of adsorption, and it shows the sitiuation when the electrical charge density on a surface of the adsorbent is zero. The pzc of raw GS and HCS were plotted by changing the electrolyte’s pH, and the pzc values were assigned to the biosorbents (Fig. 5). The pzc of raw biosorbents indicates the electrical neutrality of the biosorbent surface at a particular value of pH. Determination of pHpzc was done to investigate the surface behavior of them. The pzc of GS and HCS was calculated by titrimetric method and that was 10.8 for HCS and 11.5 for GS (Fig. 3). An decrease in pH results in more positive 160 140 120 100

60

80

(GS) Sorption %

40

60

(HCS) sorption % 40

(GS) qe

20

20

(HCS) qe 0 0

500

1000

0 1500

time(min)

Fig. 4 Adsorption versus with the initial pH for GS and HCS

qe (mg g -1 )

Sorption %

80

6

4

0

100

8

60

0

20

40

60

NaOH (ml)

charges on the adsorbent surfaces that is probably to attract the Cr(VI) ions to bind to their surfaces resulting in increased adsorption. At high pH values, Cr(VI) adsorption will be inhibited due to the overall surface negatively charges caused a repulsion between the surface and adsorbate. For the biosorption of Cr(VI) from the solution phase to the biosorbent surface, another explanation can be provided on considering the pHpzc of HCS and GS. CrO42− ions exist when pH > 6.5 and HCrO4− and Cr2O72− are prominent in the pH range from 0 to 6.5. At a pH below 10 (pHpzc) for both biosorbents, the surfaces of them are positively charged due to protonation. This protonation effect is more pronounced at low pH values of about 2.0 due to the presence of high concentrations of H+ ions in the solution phase, and results in more favorable for Cr(VI) biosorption at a lower pH value owing to the electrostatic extraction between both positively charged biosorbent surface and the Cr(VI) ions. Cr(VI) is adsorbed through electrostatic attraction and via the binding of HCrO4− to acidic functional groups on the surface of the biosorbents. In addition, the number of protons available on the surface of the adsorbate increases when the pH of the solution becomes less than the pHpzc. This condition results in the increase of the removal efficiency (%). Similar results for the relation between Cr(VI) adsorption from aqueous solutions and pH values have been reported in other research studies (Singha et al. 2011). Such high adsorption of Cr(VI) on the biosorbents should be attributed to the high pHpzc of them, since the adsorption of the anionic Cr(VI) species are only favored at low pH values. Generally speaking, the prominent form of Cr(VI) at pH 1.0 is the acid chromate ion species (HCrO4−) and the increasing pH moves the concentration of HCrO4− to different structures (e.g.,

Environ Monit Assess (2015) 187:763

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Fig. 5 pHpzc of a HCS and b GS

(GS) Cr(VI) Sorption % (GS) Cr(III) Sorption % (HCS) Cr(VI) Sorption % (HCS) Cr(III) Sorption % (GS) Cr(VI) qe (GS) Cr(III) qe (HCS) Cr(VI) qe (HCS) Cr(III) qe

100

Sorption %

80

60

160 140 120 100 80

40

60

qe (mg g-1)

763

40 20 20 0

0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

pH

CrO42−). Subsequently, Cr(VI) is easily hydrolyzed in water and the prominent Cr(VI) species for concentrations less than 500 mg/L are the HCrO4− and CrO42− anions form (Miretzky and Fernandez 2010). In order to understand the adsorption mechanism, the time dependency of solution pH as well as the concentration of Cr(VI) ions, Cr(III) ions were measured under different initial pH amounts. The results depicted in Fig. 4 showed that all adsorbents exhibited similar adsorption trends. After pH 2, when a certain amount of time passed, instead of Cr(VI) ions, the Cr(III) ions, which were not initially present, appeared in the solution. This revealed that the reduction of Cr(VI) ions to Cr(III) ions occurred during the adsorption process according to the following reactions (Pehlivan et al. 2013b; Miretzky and Fernandez 2010):

surface of the adsorbent was positively charged. It can be calculated that removal efficiencies of Cr(VI) ions by GC, HCS were 72 and 95 %, respectively. Equilibrium data, generally known as adsorption isotherms, are basic requirements to provide insight into the biosorption mechanism, the surface properties and the affinity of the biosorbent and can be modeled using different simple adsorption models such as Langmuir and Freundlich isotherms (Liang et al. 2014; Pehlivan et al. 2009). The Langmuir adsorption isotherm which assumes that adsorption occurs at specific homogeneous sites within the biosorbent was applied in its nonlinear form (Eq. 2) and in the most commonly used linear form, the type 1 (Eq. 3): The traditional usage of this model is as follows: Langmuir equation

HCrO4
þ 7Hþ þ 3e
→ Cr3þ þ 4H2 O

qe ¼

ð1Þ

The solution pH changed a little because there were enough H+ ions for reduction after pH 2.0. The reduced Cr(III) ions were present in the solution as cations, which could not be easily adsorbed at such low pH when the

a

ð2Þ

where As (mmol/g) and Kb (L/mol) are the coefficients, qe is the Cr(VI) ion amount adsorbed per unit mass of adsorbent and Ce is the equilibrium Cr(VI) concentration in solution phase.

b

200

120 100

150

80

qe(mg/g)

qe(mg/g)

Fig. 6 Adsorption isotherms for Cr(VI) on a HCS and b GS

As K b C e 1 þ K bCe

100 50

60 40 20 0

0 0

200

400

Ce(mg/L)

600

0

500 1000 1500 2000

Ce (mg/L)

Environ Monit Assess (2015) 187:763 Table 2 Isotherm parameters for Cr(VI) adsorption onto HCS and GS

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Bio-sorbent

Langmuir As (mg g−1)

Kb (L mg−1)

R2

k (mg g−1 L1/n)

n

R2

HCS

142.85

0.056

0.99

166.40

2.23

0.96

GS

103.09

0.003

0.99

99.49

1.86

0.98

By rearranging Eq. 2, Eq. 3 was obtained. Ce Ce 1 ¼ þ qe As As K b

ð3Þ

The empirical Freundlich isotherm based on biosorption on heterogeneous surfaces was applied in nonlinear (Eq. 4) and logarithmic forms (Eq. 5): Freundlich equation x ð4Þ ¼ kC e 1=n m

log

Freundlich

x

1 ¼ log k þ log C e m n

ð5Þ

where k and 1/n are the Freundlich constants indicating the relative adsorption capacity and the intensity of adsorption, respectively. X/m is the amount of Cr(VI) ions adsorbed per unit amount of biosorbent, and Ce is Cr(VI) concentration at equilibrium in aqueous phase. The equilibrium adsorption isotherms of GS and HCS at a certain initial pH were evaluated by plotting the adsorbed Cr(VI) ion (qe) against the equilibrium concentration of chromium ions (C e ) in solution (Fig. 6). It was shown in Table 2 that the correlation coefficient obtained from the Freundlich adsorption isotherm was 0.96 and 0.98 for GS and HCS, respectively. It was lower than that in Langmuir sorption isotherm. It can be concluded that the Langmuir adsorption isotherm was more appropriate for explaining equilibrium than Freundlich adsorption isotherm. The isotherm profiles of different biosorbents showed the same trend: with the

increase of Cr(VI) concentration, the adsorption capacity increased until adsorption saturation was reached. The adsorption capacity of Cr(VI) ions by GC and HCS increased with the decrease of initial solution pH. This is because the reduction reaction of Cr(VI) ions consumed a large amount of H+, and higher concentration of Cr(VI) ions needed more H+. At the initial solution pH around 2, the highest adsorption capacities of Cr(VI) ions on GC and HCS were nearly 103.09 and 142.85 mg g−1 biosorbent (Table 2). These capacities were much higher than the selected high adsorption capacities for Cr(VI) ion removal by other biosorbents reported in the literatures (Wang et al. 2009; Aoyama et al. 2005; Chand et al. 2009). The RL value (Eq. 6) shows the type of isotherm to be either unfavourable (RL >1), linear (RL =1), or irreversible (RL =0). RL ¼ 1=1 þ K L C o

ð6Þ

The calculated RL values were determined to be 0.06 and 0.12 for the HSN and for the GS, respectively. RL values drops into the range 0–1, and this confirms the favourable adsorption of Cr(VI) on the adsorbents. Also, the desorption of Cr(VI) from them are rather easy. Biosorption thermodynamics Values of thermodynamic parameters for the adsorption of Cr(VI) ions by the GS and the HCS are displayed in Table 3. Equilibrium constant versus temperature for GS and HCS was given in the Fig. 7.

Table 3 Thermodynamic parameters for the adsorption of Cr(VI) onto GS and HCS Bio-sorbent

HCS GS

ΔH°

ΔS°

ΔG° (J mol−1)

(J mol−1)

(J K−1 mol−1)

T=293.15 K

T=303.15 K

T=313.15 K

T=323.15 K

−24450.9

108.4

−7328.1

−8412.2

−9496.2

−10580.3

−3708.0

17.2

−1326.3

−1498.0

−1669.7

−1841.4

763

Environ Monit Assess (2015) 187:763

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Fig. 7 Equilibrium constant versus temperature for GS and HCS

a

b 0.3 0.25

1.6

log K

log K

0.2 0.15 0.1

1.2 0.8 0.4

0.05 0 0.003

0 0.003

0.0032 0.0034 0.0036

1/T

The Gibbs free energy change of the adsorption process is related to the equilibrium constant by the classic Van’t Hoff equation (Eq. 7):

0.0032

0.0034

0.0036

1/T (K-1)

(K-1)

value of H° implies that the adsorption phenomenon is exothermic. Biosorption kinetics



ΔG ¼ −R T ln K

ð7Þ

The pseudo-first-order reaction equation was widely used for the adsorption of liquid/solid system on the basis of solid capacity (Veeram et al. 2003). The first and pseudo-second-order kinetic models were applied to the experimental data, and a good agreement between the experimental and calculated data was found when using the pseudo-first-order and pseudo-second-order models (Figs. 8 and 9), as indicated by the correlation coefficients higher than 0.99 obtained using this models (Gandhi et al. 2011). Pseudo-first-order kinetic model and pseudo-secondorder kinetic model for Cr(VI) adsorption onto GS and HCS is in Table 4.

where G° is the standard free energy change (J mol−1), T is the absolute temperature (K), and R is the gas constant (J mol−1 K−1), and K (L g−1) is an equilibrium constant obtained by multiplying the Langmuir constants As and Kb. According to thermodynamics, the Gibbs free energy change is related to the entropy change and heat of adsorption at constant temperature by the following equation (Eq. 8): 



ð8Þ

where H° is the enthalpy change (J mol−1) and S° is the entropy change (J mol−1 K−1). H° and S° can be calculated from the G° versus T plot. The calculated thermodynamic parameters for the adsorption of Cr(VI) by HCS are given in Table 3. The negative value of G° indicates the feasibility of the process and indicates the spontaneous nature of the adsorption. The negative

Interfering ion effect on the adsorption was investigated with KNO3 ion and low ionic concentration was

a

b 6

t/qt (min/mg/g)

Fig. 8 Kinetics of a GS and b HCS

Effect of ionic strength and desorption and reusing of GS and HCS

5

500ppm

4

1000ppm

3

2000ppm

2

4000ppm

1 0 0

50

100

t(min)

log(qe-qt)



ΔG ¼ H −T ΔS

3 2 1 0 -1 0 -2 -3 -4 -5 -6

50

100

150 500ppm 1000ppm 2000ppm 4000ppm

150

t(min)

Environ Monit Assess (2015) 187:763

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a

Fig. 9 Kinetic model for Cr(VI) adsorption onto a GS and b HCS

b

14

2 500ppm

10

1000ppm

8

2000ppm

6

4000ppm

4 2

500ppm

1.5

log(qe-qt)

t/qt(min/mg/g)

12

1000ppm

1

2000ppm

0.5

4000ppm

0

-0.5

0

100

200

300

0 0

100

200

300

-1

t(min)

t(min)

removal for each concentration ranges can be achieved within 6 h for GS and 2 h for HCS. Maximum removal percentage was 72 and 95 % GS and HCS, respectively, in the adsorption procedure. Many mechanism played roles for the adsorption, and mainly adsorption, electrostatic interaction, and complexation are paramount instruments for the adsorption process. Equilibrium data agreed well with Langmuir isotherm model. Further investigations using FTIR and SEM reveal that these biosorbents have a unique structure and morphology, which favors higher Cr(VI) ion adsorption. The kinetic data were found to follow the pseudo-firstorder model and the pseudo-second-order model for GS and HCS, respectively. Gibbs free energy was spontaneous for all interactions, and the adsorption process exhibited exothermic enthalpy values. A limited number of adsorption-desorption cycles indicated that the GS and HCS biosorbents could be regenerated and reused to remove Cr(VI) from waste streams.

taken in the experimental steps (0.01 M KNO3). The electrostatic attraction displayed a negligible effect at low concentration of KNO3 for the uptake of Cr(VI) ion for GS and HCS. Cr(VI) uptake was almost independent on the concentration of salt, indicating that Cr(VI) removal was not influenced by electrostatic interactions. Desorption studies were carried for the biosorbents to further use it in the adsorption process. Desorption efficiency of Cr(VI) ions from the biosorbents was studied with 0.1 M HCl and 0.1 M NaOH.

Conclusions This study focused on the GS and the HCS as biosorbents due to their nontoxicity, capability of holding Cr(VI) ions, and biodegradability. The ability of GS and HCS biosorbents to remove Cr(VI) from aqueous solution was investigated in equilibrium, kinetics, and thermodynamics. Maximum

Table 4 Kinetic parameters for the removal of Cr(VI) by GS and HCS Biosorbent

HCS

GS

C0 (mg L−1)

Pseudo-first-order

Pseudo-second-order

qe (mg g−1)

k1 ×10−3 (min−1)

R2

qe (mg g−1)

k2 ×10−3 (g mg−1 min−1)

h0 (mg g−1 min−1)

R2

500

11.06

111.70

0.98

25.06

35.94

22.57

1.00

1000

18.56

43.99

2000

43.37

20.27

0.97

50.76

5.27

13.57

0.99

0.98

96.15

1.37

12.69

4000

83.16

0.99

6.22

0.95

149.25

1.02

22.78

0.99

500 1000

11.41

16.35

0.98

21.23

16.96

7.65

0.99

22.03

14.05

0.96

38.91

1.97

2.98

0.99

2000

15.89

11.05

0.91

68.49

0.17

0.78

0.99

4000

28.03

13.13

0.89

95.24

0.06

0.51

0.99

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Acknowledgments The authors thank to Turkish Academia of Sciences (TÜBA) and Selçuk University (BAP) (number 14101008) for the financial support.

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