Abstract

This study evaluates the application of Cr(VI) adsorption from the prepared synthetic solution by black sesame (Sesamum indicum L.) seed pulp (BSSP) and chitosan (Cts)-coated black sesame seed pulp beads (Cts-BSSP). BSSP and Cts-BSSP were used as an adsorbent without any chemical or physical treatment to remove Cr(VI) from an aqueous medium. The results indicated that the Cr(VI) removal was pH-dependent and reached an optimum at pH 2.0. It has been observed that the percentage of adsorption increased from 62% to 95% when the amount of Cts-BSSP increased from 0.0125 g to 0.0250 g. The required adsorbent amount for the maximum removal was 0.05 g and 0.1 g for Cst-BSSP and BSSP, respectively. The contact time for the adsorption was 120 min and 90 min for BSSP and Cst-BSSP, respectively. Scanning electron microscopy and Fourier transform infrared spectroscopy were used to explore the possible adsorption mechanism for Cr(VI). The equilibrium data for the BSSP and Cts-BSSP were used with the Langmuir and Freundlich adsorption isotherm models to assess the adsorption capacity and relevant mechanism. The adsorption capacity of the Cts-BSSP for Cr(VI) is relatively high compared to BSSP. The monolayer maximum adsorption capacities for Cr(VI) ions were 31.44 and 18.32 mg/g for Cts-BSSP and BSSP, respectively.

NOMENCLATURE AND UNITS

     
  • As

    Langmuir constant related to maximum adsorption capacity (mg or mmol)

  •  
  • Ci

    Initial Cr(VI) concentration in the solution phase (mol/L)

  •  
  • Cf

    Final Cr(VI) concentration in the solution phase (mol/L)

  •  
  • Ce

    Final Cr(VI) concentration in the solution phase at the equilibrium (mol/L)

  •  
  • KF

    Freundlich constant indicative of the relative adsorption capacity of the adsorbent [(mg/g) (L/g)1/n]

  •  
  • KL

    Constant related to the free energy of adsorption (L/mg)

  •  
  • m

    Weight of adsorbent (g)

  •  
  • qe

    Amount of Cr(VI) adsorbed per unit mass of adsorbent (mg/g)

  •  
  • qm

    Maximum adsorption capacity (mg/g)

  •  
  • R2

    Correlation coefficient

  •  
  • t

    Contact time (min)

  •  
  • T

    Temperature (°C)

INTRODUCTION

Heavy metals can accumulate in aqueous systems and they harm the natural ecosystems. The most familiar toxic pollutants are chromium, lead, cadmium, copper, and mercury. Chromium is considered a major hazardous pollutant for industrial wastewater and its contamination in an aquatic medium is a critical issue (Lasheen et al. 2016). When chromium enters soil, groundwater and surface water, serious human health problems can result. It is the sixth most abundant element in the Earth's crust where it exists in the combination of iron and oxygen in the form of chromite or chromium in three stable valence states; zero, three, and six. One of the oxidation states of chromium, Cr(VI), is a hard steel-gray metal that is resistant to the oxidation. Cr(III) is biologically more significant than Cr(VI) for human life. Cr(III) is an essential dietary mineral in low doses. As an essential dietary nutrient, chromium is expected to potentiate insulin dose and is required for normal glucose metabolism. Cr(III) deficiency has been linked with cardiovascular disease. Cr(VI) is carcinogenic and a thousand times more toxic than Cr(III) (Nandi et al. 2017). Chromium is used in industries such as metallurgical, chrome plating leather tanning, welding, paint pigments, wood treatments, drilling muds, copy-machine toners and refractory. In the metallurgical industry, it is an important component of stainless steels and various metal alloys. Cobalt–chrome has a very high specific strength and is commonly used in gas turbines, dental implants and orthopaedic implants. Refractory factories use magnesium and chrome firebrick for metallurgical furnace linings, and granular chromite for various other heat-resistant applications (Rangabhashiyam et al. 2016). People do not know what level of Cr(VI) could cause health problems but health officials know that cancer cells were observed in rats and mice after drinking certain amounts of Cr(VI). The World Health Organization has set the allowable specific limit as 0.05 mg/L for total chromium in drinking water (Owlad et al. 2009). Some scientists say that even small amounts of Cr(VI) can damage children, infant, and foetuses (Economou-Eliopoulos et al. 2012).

Chemical extraction, solvent extraction, filtration, ion exchange, oxidation/reduction, ceramic membrane separation, and adsorption are methods of remediation or removal of Cr(VI) (Sivakami et al. 2013; Adam et al. 2018; Dragan et al. 2018). These methods have some disadvantages, and limitations as well, for water purification or remediation. However, due to the specific nature of industrial effluents, these methods do not meet the needs of developing countries because of the high maintenance cost (Jain & Ali 2000; Ahluwalia & Goyal 2007). Compared with the other removal techniques, adsorption is the most commonly used method because of its low price, high efficiency, and ability to regenerate (Parlayici & Pehlivan 2015; Salih & Ghosh 2018). The traditional methods applied in wastewater plants can lead to high operating costs of the treatment process. Therefore, inexpensive adsorbents are preferred in the adsorption process. Several adsorbents are available for the adsorption process, and these can be either a natural adsorbent or modified adsorbent. The use of natural materials that are available in large quantities has received much attention for the adsorption. Agricultural waste materials, being economic and eco-friendly due to their unique chemical composition, availability in abundance, renewable nature, low cost and good efficiency, seem to be a viable option for the chromium removal. Some natural adsorbents such as natural solids, clays, chitosan, zeolites, dolomite, metal ores, activated carbon, and mesoporous silica (Aguado et al. 2008; Javadian 2014; Kaya 2017; Zheng et al. 2017) were applied in the wastewater treatment systems. Agricultural materials such as sawdust, rice husks, almond shells, horse chestnut shell, garlic stem, rye husk, cactus leaves, oakwood and nut shells have been applied for the removal of chromium from wastewater (Dakiky et al. 2002; Guo et al. 2002; Mohan et al. 2011; Parlayici & Pehlivan 2015; Altun et al. 2016).

Black sesame seeds are a very popular dietary therapy food for the traditional Turkish and Chinese cultures. Africa produces a variety of sesame seeds. Sesame is the oldest oilseed plant that belongs to the family of Pedaliaceae, domesticated well over 3,000 years ago. The other name of sesame is gingelly. It is a drought-tolerant rainfed crop containing a high amount of oil in the seeds. Sesame oil is rich in omega-6 fatty acids and contains 47% oleic acid as well as 25% protein. The oil contains a high number of natural antioxidants. Black sesame seed pulp (BSSP) has flavonoids, alkaloids, tannins, phenol, and phytate in the structure (Omosun & Oyedemi 2010). Tannin is a polyphenolic compound containing sufficient hydroxyls and other suitable groups of carboxyls to form strong complexes with various metals. Phytate contains phosphate groups and they can interact with chromium ions after reduction.

Chitosan (Cts) is a linear polysaccharide consisting of randomly distributed B-(1-4)-linked d-glucosamine and N-acetyl-d-glucosamine (Druet et al. 2015). Cts, made from chitin deacetylation (DA), is a typical alkaline polysaccharide and contributes significantly to the cellulosic matrix in the BSSP structure and exhibits several perfect chemical properties, such as biocompatibility, being non-toxic, and non-harmful (Wang et al. 2017). Cts is cheap and highly available, and does not harm the environment and humans. This feature indicates that Cts is a cationic polysaccharide. Unlike other natural polymers, Cts is positively charged (due to the weakly basic groups), and is a hydrophilic polymer. The amino group in Cts has a pKa value of 6.5, which leads to a protonation in acidic to neutral solution with a charge density dependent on pH and DA percentage. This makes Cts water-soluble and a bioadhesive that binds readily to negatively charged groups. Many naturally occurring materials such as Cts have surfaces that are populated with both an amino (-NH2) group and a hydroxyl (-OH) group or carbonyl (-C=O) group which ionize in the presence of water to an extent dependent on the pH of the medium. Therefore they play an important role in many applications because of their unique character. Also, they allow the adsorption of heavy metal ions by electrostatic attraction or ion exchange. At low pH, amino groups pick up a proton to become -NH3+, giving the surface a positive surface charge, while at high pH, the carboxyl groups ionize to give it a negative charge. Cts has been used for the removal pollutants, such as pesticides, herbicides, amines, dye and toxic metal ions from wastewater (Rashid et al. 2018). The coating of sesame seeds with Cts can improve the adsorption capacity because this composite has different functional groups in the adsorption mechanism (El-Reash 2016). Therefore, there is a strong tendency to use the composite (Cts-covered sesame) for the chromium removal applications.

Since the combination of two or more efficient adsorbents in a unique form improves the adsorption capacity, there is a strong tendency for using composite adsorbents in recent studies in the literature. Coating biomaterials with Cts also enhances the functional groups of the composite. Cts is soluble in acidic aqueous media but when combined with BSSP, it resists the acidic medium and can find applications in a slightly acidic medium. Cts has amino and –OH groups and Cts-coated BSSP (Cts-BSSP) can improve the capacity of the composite adsorbent. Cts has some useful qualities in terms of porosity, surface area, adsorption sites and mechanical properties (Zhang et al. 2016). Despite a lot of research work being published on natural adsorbents for pollutants removal from wastewater, there is yet little literature containing a full study of the removal of Cr(VI) by using Cts and its coated forms with BSSP.

For this purpose, in this study, the aim was to prepare composites by coating BSSP with Cts and use them for the removal of Cr(VI) from aqueous solutions. BSSP has been favoured for this research because it can be easily harvested, and is economical and abundant in nature. This natural material was joined with Cts and formed a strong skeleton with various functional groups. The effects of contact time, pH, and initial Cr(VI) ion concentration were optimized for the elimination actions.

MATERIALS AND METHODS

Materials

Cts flakes (degree of deacetylation = 75–85%) were obtained from Sigma-Aldrich. All other chemicals were purchased from Merck Company. A stock solution of Cr(VI) with a concentration of 1,000 ppm was prepared from potassium dichromate (K2Cr2O7). All the chemicals applied in the experiments were of analytical grade, and double distilled water was used to prepare the required solutions. For pH adjustment of solution phase, 0.1 M NaOH and HCl solutions were added in the prepared Cr(VI) solutions.

For adsorption experiments, an Orion 900S2 pH meter, GFL 3033 thermostatted shaker, and IKAMAG-RO15 magnetic stirrer were used for the control of pH and mixing velocity of the solutions. A UV-visible (UV-Vis) spectrophotometer (Shimadzu UV-1700) was used for the determination of chromium concentration. The Fourier transform infrared (FTIR) spectra for BSSP and Cts-BSSP, before and after Cr(VI) adsorption, were recorded with a Bruker-Platinum ATR-vertex 70 (Germany) at 550–4,000 cm−1 wavenumbers at a resolution of 4 cm−1 using an attenuated total reflectance accessory. The microstructure of the Cts-BSSP and Cr(VI)-loaded Cts-BSSP was examined by scanning electron microscopy (SEM, Nova Nano SEM 200, FEI Company). The samples were coated with thin gold and sputtered at 20 kV. The Brunauer–Emmett–Teller (BET) surface area of Cts-BSSP was determined by nitrogen gas applied at 77.3 K using a surface area analyzer (Quantachrome Corporation, USA). The BET surface area was found to be 12.7 m2/g.

Preparation of raw material

BSSP was obtained after milling the black sesame seeds in a cold press mill. Then, BSSP was washed several times with 0.1 M HCl and pure water. The prepared pulp was dried at room temperature for 24 hours. The material was powdered to obtain a different particle size and reduced to a particle size of 125 μm.

Preparation of Cts-BSSP

Three grams of Cts was mixed with 300 mL of a 1% acetic acid solution in a magnetic stirrer until the slurry was gelled, and after mixing with 3 g of BSSP, the slurry was stirred for about 1 hour using a magnetic stirrer. Two hundred millilitres of 5% NaOH and 300 mL of 6% ethyl alcohol were poured into a beaker and the mixture was dropped into a basic solution with a syringe. The particle formation in the spheres was observed by adjusting the height of the droplets and the particle radius in the solution phase. Then, the agglomerated particles were washed with pure water until the pH was neutral. The slurry was reacted with 3 mL of glutaraldehyde in an oven at 60–70 °C for 30 minutes to form a covalent bond with Cts, and the number of functional groups in the structure of adsorbent was increased (Figure 1) and then the slurry was allowed to dry for 24 hours.

Figure 1

Preparation of Cts-BSSP.

Figure 1

Preparation of Cts-BSSP.

Adsorption studies

The batch method was used to perform an adsorption experiment. In a 100 mL Erlenmeyer flask, 50 mL of Cr(VI) solution was taken at a constant pH and a certain amount of adsorbent was added and allowed to shake for 2 hours at 200 rpm. All experiments were performed with blank tests and the results were checked to remove the errors. The samples were tested twice to ascertain the accuracy, reliability, and reproducibility of the data obtained from the experimental results. After the equilibrium was obtained, the filtrate was analyzed for Cr(VI) concentration using the UV-Vis spectrophotometer (540 nm) with 1,5-diphenyl carbazide as a complexing agent. This agent reacts with Cr(VI) in low pH medium and Cr(III)–diphenylcarbazone complex was obtained (Equation (1)).  
formula
(1)

H4L: 1,5-diphenylcarbazide

H2L: diphenylcarbazone

Percent adsorption of Cr(VI) was calculated as in Equation (2) and the adsorption capacity per unit mass of adsorbent (qe) was calculated using Equation (3).  
formula
(2)
 
formula
(3)
where Ci and Cf are the initial and final Cr(VI) concentrations, respectively, V is volume of sample solution and m is the mass of dry adsorbent.

RESULTS AND DISCUSSION

Characteristics of Cts-BSSP using FTIR

The spectra of biomasses were measured within the range of 550–4,000 cm−1 and are displayed in Figure 2. Some changes in the characteristic peaks ranging from 900 to 3,600 cm−1 and some visible shifts of peak locations of the biomasses were observed before and after adsorption of Cr(VI). As seen in the FTIR spectrum, hydroxyl group (-OH) appeared at 3,324 cm−1. CH2 and CH3 group aliphatic C–H stretching vibrations appeared at 2,971 cm−1. -NH at 1,637 cm−1 and -NH2 line at 1,425 cm−1 were placed due to the bending band (Chui et al. 1996; Kaminski & Modrzejewska 1997). This proves the presence of the functional groups such as –NH2, –OH, and –CO–. They are involved in binding of Cr(VI) to Cts-BSSP. In the structure of Cts, alcohol groups (COH) appeared at 1,369 cm−1 and –CO stretching vibration in –COH appeared at 1,020 cm−1. Some clear shifts in the matrix of the adsorbents were seen after the adsorption of Cr(VI). These were a changing of wavenumber from 3,324 cm−1 (Cts-BSSP) to 3,293 cm−1 (Cr(VI)-loaded Cts-BSSP) and from 3,278 cm−1 (BSSP) to 3,334 cm−1 (Cr(VI)-loaded BSSP). The -OH group in the surface matrix 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 BSSP as wavenumber shifted from 2,923 to 2,918 cm−1. Unsaturated groups such as alkenes were found to have major shifts of wavenumbers from 1,635 to 1,666 cm−1 for the adsorption of Cr(VI) by the Cts-BSSP and 1,635 to 1,734 cm−1 for the adsorption of Cr(VI) by BSSP. The FTIR spectrum of the adsorbent showed intense bands around 1,369 cm−1 which shifted to 1,425 cm−1 for Cr(VI)-loaded Cts-BSSP. This showed that the carboxylate anion was responsible for the adsorption on Cts-BSSP (Singha et al. 2011). The peaks ranging from 1,300 to 1,000 cm−1 are ascribed generally to the C–O stretching vibration in carboxylic acids and alcohols.

Figure 2

The FTIR spectral characteristics of BSSP and Cts-BSSP before and after Cr(VI) adsorption.

Figure 2

The FTIR spectral characteristics of BSSP and Cts-BSSP before and after Cr(VI) adsorption.

Characteristics of Cts-BSSP using SEM

The synthesized Cts-BSSP was characterized using SEM. SEM studies were made in order to explore the surface morphology of the Cts-BSSP used for the present work. The surface morphology of Cts-BSSP using SEM is shown in Figure 3. It was observed that the structure of Cts-BSSP was similar to a honeycomb. On the other hand, the adsorption of Cr(VI) revealed that the pores of Cts-BSSP were filled and there were retained Cr(VI) ions on the surface of the adsorbent.

Figure 3

SEM images of Cts-BSSP before and after Cr(VI) adsorption.

Figure 3

SEM images of Cts-BSSP before and after Cr(VI) adsorption.

Effect of contact time on the adsorption of Cr(VI)

Twenty-five millilitres of 50 ppm Cr(VI) solution was added to 0.1 g of BSSP and 0.05 g of Cts-BSSP, separately, for the adsorption process. The mixture was stirred in the magnetic stirrer for the specified time intervals (5, 15, 30, 60, 90, 120, 180, 240 min) and the Cr(VI) contents of the remaining solution after the filtration were measured with the UV-Vis spectrometer. The percentage of adsorption of Cr(VI) ions by BSSP and Cts-BSSP was calculated. Due to the large surface area of the Cts-BSSP and the existence of functional groups in the matrix, the first part (15 min) of the adsorption is fast because Cr(VI) ions tend to interact with the functional groups of the adsorbent.

It can be seen in Figure 4 that the adsorption by BSSP increased over time and then reached a stationary value and, after this stage, the adsorption remained constant. The adsorption was fast in the first 5–120 minutes and then reached a stable condition in 120–240 minutes. The longer contact time was not effective for the adsorption, and the contact time was taken as 120 minutes. The adsorption of Cr(VI) ions was low with BSSP but when it was reacted with Cts, the adsorption increased. By using Cts-BSSP, the adsorption rapidly increased in the first 5–90 minutes and reached a stable value in 90–240 minutes. The longer contact time was not effective for the adsorption. For that reason, the contact time for this modified adsorbent was taken as 90 minutes in the experiments. Cr(VI) adsorption stage of Cts-BSSP came to the equilibrium in a shorter time than BSSP. The BSSP in the raw form and coated form (Cts-BSSP) showed 70.0% and 93.0% adsorption efficiency, respectively. The Cts-BSSP showed the best Cr(VI) removal efficiency.

Figure 4

Effect of contact time on the adsorption of Cr(VI) by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; pH 2; adsorbent amount, 0.1 g; temperature, 25 ± 1 °C for BSSP; and concentration of Cr(VI), 50 ppm; pH 2; adsorbent amount, 0.05 g; temperature, 25 ± 1 °C for Cts-BSSP).

Figure 4

Effect of contact time on the adsorption of Cr(VI) by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; pH 2; adsorbent amount, 0.1 g; temperature, 25 ± 1 °C for BSSP; and concentration of Cr(VI), 50 ppm; pH 2; adsorbent amount, 0.05 g; temperature, 25 ± 1 °C for Cts-BSSP).

Effect of pH on Cr(VI) ions adsorption

In adsorption analyses conducted in the solution phase, it is known that the pH value of the solution significantly affects the adsorption capacity due to the change of the properties of the adsorbent applied. The pH of the solution plays an important role in the entire adsorption process and affects the adsorption capacity of the adsorbent. The pH of the solution influences both the binding sites and functional groups on the surface of the adsorbent and the charge profiles of the adsorbate species, and thus has an important effect on the interactions between adsorbent and adsorbate species. Cts has weak acid groups such as –COOH and weak basic groups such as –NH2, and their degree of ionization (hence surface charge density) is a function of pH, producing a situation of relevance to Cr(VI). Different pH values of the solution phase were considered for adsorption of Cr(VI). The distinctive mechanical, chemical and electrical properties of interfaces of the solid and solution phase create great and highly varied effects on the adsorbent behaviour. The surface layer of the Cts-BSSP exhibits an electrical charge effect in the response section of Cr(VI) and the adsorbent. Interfaces dividing electrically neutral bulk phases can appear to bear a charge; positive or negative charges spread relative to the adsorbent and Cr(VI) interface, resulting in the formation of an electrical double layer (Berg 2010).

In the adsorption experiments, the adsorption values of Cr(VI) ions were found to be maximum around pH 2.1 for the pH range (1.5, 2, 3, 4, 5, 6). The similar pH-dependent trend was also observed by some other researchers for the Cr(VI) removal by various adsorbents (Anandkumar & Mandal 2010; El-Reash 2016).

Figure 5 shows the extent of elimination of Cr(VI) from the solution phase as a function of pH, and the Cr(VI) removal efficiency was higher at lower pH and the removal of Cr(VI) decreased considerably at higher pH. The acquisition of charge by the adsorbent surface facilitates the interaction with the species of opposite charges.

Figure 5

Effect of pH on the adsorption of Cr(VI) by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; adsorbent amount, 0.1 g; contact time, 120 min; temperature, 25 ± 1 °C for BSSP; and concentration of Cr(VI), 50 ppm; adsorbent amount, 0.05 g; contact time, 90 min; temperature, 25 ± 1 °C for Cts-BSSP).

Figure 5

Effect of pH on the adsorption of Cr(VI) by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; adsorbent amount, 0.1 g; contact time, 120 min; temperature, 25 ± 1 °C for BSSP; and concentration of Cr(VI), 50 ppm; adsorbent amount, 0.05 g; contact time, 90 min; temperature, 25 ± 1 °C for Cts-BSSP).

In acidic solution, the equilibrium is as follows (Equations (4) and (5)):  
formula
(4)
 
formula
(5)
The equilibrium in alkaline pH is given as (Equations (6) and (7)):  
formula
(6)
 
formula
(7)

The better adsorption capability discovered at a low hydrogen ion concentration value is related to a large number of H+ ions present at around low pH values, which successively neutralize the negatively charged hydroxyl group (–OH) ions causing an increasing hindrance to the diffusion of positively charged dichromate ions. After increasing pH values from 2.0 to 6.0, HCrO4 gradually converts to the divalent CrO42−. As pH increases, the surface of adsorbent carries a lot of negatively charged ions and therefore the number of positively charged sites diminishes. This causes an increased repulsion between Cr(VI) and Cts-BSSP. A negatively charged surface site on the adsorbent does not favour the adsorption of Cr(VI), because of the static repulsion. Adjusting the pH of the solution to greater than 6.0 by adding NaOH causes flocculation and deprotonation of the adsorbent (Rinaudo 2006). Additionally, at pH lower than 1.5, the amine group of Cts is easily protonated, which causes electrostatic repulsion to Cr(VI) ion. The amine groups (-NH2) in Cts are mainly responsible for Cr(VI) adsorption and can be protonated to NH3+ in a slightly acidic medium (Salih & Ghosh 2018).

Effect of Cr(VI) concentration on adsorption of Cr(VI) ions

Cr(VI) is a general water contaminant at industrial installations. Some industrial effluents can contain chromium at concentrations ranging from 10 to 100 mg/L (Rengaraj et al. 2003). Therefore, adsorption of Cr(VI) at various concentrations (25, 50, 100, 150, 200, 250, 300, 350 ppm) was carried out to determine the effect of initial adsorbate concentrations on the adsorption (Figure 6). The adsorption experiments were carried out by treating the solutions with BSSP and Cts-BSSP. Freundlich and Langmuir models (Table 1) were applied for the equilibrium study (Saha et al. 2013). The Freundlich isotherm shows physical adsorption. According to Freundlich, the adsorption fields on the surface of an adsorbent are heterogeneous. The R2 values we obtained from Langmuir isotherm plots are 0.98 for BSSP and 0.97 for Cts-BSSP, respectively (Table 2). The R2 values obtained from the Freundlich isotherm are 0.87 for BSSP and 0.94 for Cts-BSSP, respectively. The Langmuir isotherm was found to be suitable for the adsorption. If the value of RL lies between 0 and 1, the adsorption process is favourable; if RL is greater than 1, the process is unfavourable. The RL values in this study (Table 2) lie between 0 and 1, which points to a high attraction of Cts-BSSP for Cr(VI) ions. The Langmuir isotherm describes chemical and single layer adsorption. This model is defined as the simplest theoretical model for single layer adsorption. From this isotherm, the maximum adsorption capacity was found to be 31.44 mg/g for Cts-BSSP and 18.32 mg/g for BSSP. Rangabhashiyam and colleagues concluded that by using Ficus auriculata leaves powder, adsorption capacity for the metal was 13.33 mg/g (Rangabhashiyam et al. 2015). Anandkumar and Mandal obtained an adsorption capacity of 17.27 mg/g by using bael fruit shell activated carbon (Anandkumar & Mandal 2010). The adsorbent (Cts-BSSP) has a more effective capacity for Cr(VI) ions and the arrival time to the equilibrium stage was shorter, compared with the results of Rangabhashiyam et al. (2015) and Anandkumar & Mandal (2010).

Table 1

Cr(VI) adsorption isotherm equations

Isotherm model Equation Plots 
Freundlich  
formula
(8)
 
Log qe versus log Ce 
Langmuir  
formula
(9)
 
(Ce/qe) versus Ce 
Isotherm model Equation Plots 
Freundlich  
formula
(8)
 
Log qe versus log Ce 
Langmuir  
formula
(9)
 
(Ce/qe) versus Ce 
Table 2

Cr(VI) adsorption isotherm equation parameters and correlation coefficients

Adsorbent Langmuir
 
  Freundlich
 
qm KL R2 RL KF n R2 
BSSP 18.32 0.028 0.98 0.418 2.23 2.52 0.87 
Cts-BSSP 31.44 63.6 0.97 0.0003 21.01 10.68 0.94 
Adsorbent Langmuir
 
  Freundlich
 
qm KL R2 RL KF n R2 
BSSP 18.32 0.028 0.98 0.418 2.23 2.52 0.87 
Cts-BSSP 31.44 63.6 0.97 0.0003 21.01 10.68 0.94 
Figure 6

Effect of initial Cr(VI) concentration on Cr(VI) adsorption: (a) BSSP and (b) Cts-BSSP (adsorption conditions: pH 2; adsorbent amount, 0.1 g; contact time, 120 min; temperature, 25 ± 1 °C for BSSP; and pH 2; adsorbent amount, 0.05 g; contact time, 90 min; temperature, 25 ± 1 °C for Cts-BSSP).

Figure 6

Effect of initial Cr(VI) concentration on Cr(VI) adsorption: (a) BSSP and (b) Cts-BSSP (adsorption conditions: pH 2; adsorbent amount, 0.1 g; contact time, 120 min; temperature, 25 ± 1 °C for BSSP; and pH 2; adsorbent amount, 0.05 g; contact time, 90 min; temperature, 25 ± 1 °C for Cts-BSSP).

Many investigations have been carried out on the effective removal of Cr(VI) from aqueous solution using various adsorbents (Table 3). Cts-BSSP has a higher capacity for the elimination of Cr(VI) compared to other adsorbents.

Table 3

Comparison of the adsorption capacities of Cr(VI) ions onto various adsorbents

Adsorbent Qm (mg/g) References 
Hazelnut shell 17.70 Cimino et al. (2000)  
Tamarind seeds 29.70 Gupta & Babu (2009)  
Banana waste 10.00 Sharma et al. (2016)  
Hydrolysed crosslinked chitosan 11.30 Vieira et al. (2014)  
Chitosan 22.09 Aydın & Aksoy (2009)  
BSSP 18.32 This study 
Cts-BSSP 31.44 This study 
Adsorbent Qm (mg/g) References 
Hazelnut shell 17.70 Cimino et al. (2000)  
Tamarind seeds 29.70 Gupta & Babu (2009)  
Banana waste 10.00 Sharma et al. (2016)  
Hydrolysed crosslinked chitosan 11.30 Vieira et al. (2014)  
Chitosan 22.09 Aydın & Aksoy (2009)  
BSSP 18.32 This study 
Cts-BSSP 31.44 This study 

Effect of the amount of adsorbent on the adsorption of Cr(VI) ions

In the experiments, while the initial concentrations of Cr(VI) were kept constant, the masses of the adsorbents applied were increased. Increasing the mass of adsorbent increased the effective surface area for the adsorption. This increase in the surface area resulted in greater adsorption of Cr(VI), and the adsorption capacity for Cr(VI) ions in the range of 0.025–0.1 g of adsorbent were increased. The adsorption capacity for Cr(VI) ions in the adsorbent range of 0.1–0.2 g stayed constant and the adsorption capacities of the two adsorbents became closer to each other as the mass of adsorbent was increased (Figure 7). The adsorption capacity of Cr(VI) ions in the range of 0.025–0.05 g increased for both adsorbents. The maximum adsorption mass was found as 0.05 g and 0.1 g for Cst-BSSP and BSSP, respectively. Thus, shorter contact time and a smaller amount of Cst-BSSP resulted in nearly two times higher adsorption capacity of Cst-BSSP compared to BSSP.

Figure 7

Effect of adsorbent dosage on the adsorption capacity and percentage removal of Cr(VI) ions by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; pH 2; contact time, 120 min; temperature, 25 ± 1 °C for BSSP; and concentration of Cr(VI), 50 ppm; pH 2; contact time, 90 min; temperature, 25 ± 1 °C for Cts-BSSP).

Figure 7

Effect of adsorbent dosage on the adsorption capacity and percentage removal of Cr(VI) ions by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; pH 2; contact time, 120 min; temperature, 25 ± 1 °C for BSSP; and concentration of Cr(VI), 50 ppm; pH 2; contact time, 90 min; temperature, 25 ± 1 °C for Cts-BSSP).

Effect of temperatures on the adsorption of Cr(VI) ions and thermodynamic study

The effect of temperature on the adsorption of Cr(VI) to BSSP and Cts-BSSP between 25 °C and 55 °C at pH 2.0 was studied in the batch reactor. The temperature has a direct influence on the adsorption process (Figure 8). It can be seen that Cr(VI) adsorption decreased as temperature increased from 25 °C to 55 °C. This shows that the adsorption process was exothermic in nature.

Figure 8

Effect of temperature on the removal of Cr(VI) ions by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; pH 2; contact time, 120 min; adsorbent amount, 0.1 g; for BSSP; and concentration of Cr(VI), 50 ppm; pH 2; contact time, 90 min; adsorbent amount, 0.05 g; for Cts-BSSP).

Figure 8

Effect of temperature on the removal of Cr(VI) ions by BSSP and Cts-BSSP (adsorption conditions: concentration of Cr(VI), 50 ppm; pH 2; contact time, 120 min; adsorbent amount, 0.1 g; for BSSP; and concentration of Cr(VI), 50 ppm; pH 2; contact time, 90 min; adsorbent amount, 0.05 g; for Cts-BSSP).

Thermodynamic considerations of an adsorption process are necessary to conclude whether the process is spontaneous or not. The adsorption of Cr(VI) on BSSP and Cts-BSSP was carried out with different temperatures (25 °C, 35 °C, 45 °C and 55 °C) for the thermodynamic analysis. The experimental data obtained were used in the calculation of the thermodynamic parameters: Gibbs free energy change (ΔG°), entropy change (ΔS°) and enthalpy change (ΔH°) (Table 4).

Table 4

Thermodynamic parameters

  ΔSo ΔHo ΔGo (J/mol)
 
R2 
Adsorbent (J/(K·mol)) J/mol T = 298.15 K T = 308.15 K T = 318.15 K T = 328.15 K 
BSSP −56.0 −18,444.5 −1,747.4 −1,187.3 −627.3 −67.3 0.979 
Cts-BSSP −369.4 −126,216 −16,090 −12,396.3 −8,702.7 −5,009.1 0.985 
  ΔSo ΔHo ΔGo (J/mol)
 
R2 
Adsorbent (J/(K·mol)) J/mol T = 298.15 K T = 308.15 K T = 318.15 K T = 328.15 K 
BSSP −56.0 −18,444.5 −1,747.4 −1,187.3 −627.3 −67.3 0.979 
Cts-BSSP −369.4 −126,216 −16,090 −12,396.3 −8,702.7 −5,009.1 0.985 

The value of ΔH° is negative, indicating that the adsorption reaction is exothermic. The negative value of the standard entropy change reflects a decreased value in the randomness at the solid/solution interface during the adsorption process. ΔG° values were found to be negative. The negative ΔGo values with temperature suggest that the process is feasible and with a spontaneous nature of adsorption with a high preference for Cr(VI) on BSSP and Cts-BSSP. Similar observations were reported for Cr(VI) adsorption on various adsorbents (Saha et al. 2013; Rangabhashiyam et al. 2015).

CONCLUSION

This research related to the adsorptive removal of Cr(VI) from an aqueous medium. The functional groups such as amino, carboxyl and hydroxyl groups on the surface of Cts-BSSP played an important role in the binding of Cr(VI) from the medium. Cts-BSSP displayed a higher adsorption capacity than BSSP. Cts-BSSP adsorption capacity of 31.44 mg/g for the removal of Cr(VI) from the aqueous medium was derived from the qm values calculated from the adsorption isotherm data. pH 2.1 was an optimum value for the adsorption of Cr(VI). The time to reach the equilibrium stage was recorded as 90 min. The amount of adsorbent sufficient for Cr(VI) removal from the solutions was found to be 0.05 g. The coordination, electrostatic attraction and complexation of Cr(VI) with the functional groups can be accepted for the interaction. The Langmuir isotherm model best described the adsorption process (R2 is 0.97) and the values of the RL factor indicating a strong binding at the active sites of the Cts-BSSP. It was found that Cr(VI) adsorption followed the Langmuir monolayer adsorption. The adsorption process was exothermic and this affected Cr(VI) adsorption. Therefore, the adsorption was decreased in high temperatures. The thermodynamic calculations of ΔGo gave negative values, and this proved that the Cr(VI) adsorption was spontaneous during the reaction period. Cts-BSSP has effectively removed Cr(VI), and 93% removal efficiency of Cr(VI) in synthetic water samples was achieved. Cts-BSSP is recommended for the treatment plants to remove the toxic Cr(VI) species from effluents.

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