Desorption of Methylene blue dye from brown macroalga: Effects of operating parameters, isotherm study and kinetic modeling

Journal of Cleaner Production 152 (2017) 443e453

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Desorption of Methylene blue dye from brown macroalga: Effects of operating parameters, isotherm study and kinetic modeling Ehsan Daneshvar a, b, Arya Vazirzadeh a, *, Ali Niazi c, Masoud Kousha d, Mu. Naushad e, Amit Bhatnagar b, ** a

Department of Natural Resources and Environmental Engineering, School of Agriculture, Shiraz University, Shiraz, 71441-65186, Iran Department of Environmental and Biological Sciences, University of Eastern Finland, P.O. Box 1627, FI-70211, Kuopio, Finland Institute of Biotechnology, School of Agriculture, Shiraz University, Shiraz, 71441-65186, Iran d Department of Fisheries, Faculty of Animal Science and Fisheries, Sari Agricultural Sciences and Natural Resources University, Km 9 Darya Boulevard, P.O. Box, 578, Sari, Iran e Department of Chemistry, College of Science, Bld#5, King Saud University, Riyadh, Saudi Arabia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 October 2016 Received in revised form 17 March 2017 Accepted 18 March 2017

In this study, desorption of Methylene blue (MB) dye from brown macroalga, Nizamuddinia zanardinii was investigated. Batch experiments were conducted to determine the effects of various operating parameters namely, the effect of seventeen different eluents, eluents concentrations, dye saturated alga weight, different HCl:1-butanol ratios, different particle sizes, different alga mass washing modes (after sorption experiment), initial dye concentration in the sorption experiment and contact time on the desorption efficiency of MB dye from brown macroalga. Among the seventeen eluents tested, hydrochloric acid (HCl) was found to be the most effective eluent for dye desorption. The mixture of 1 M HCl (25%):1 M 1-butanol (75%) was found to increase the dye desorption efficiency up to 63.67 ± 1.67%. Dye desorption percentage increased by increasing the alga particle sizes from <75 mm to >250 mm. The highest and lowest desorption efficiencies were attained by wet and dry masses after water washing by shaker, respectively. Both isotherm and kinetic data were obtained and fitted very well with the Sips isotherm model and pseudo-second-order kinetic model. After five sorption/desorption cycles, the dye sorption efficiency decreased from 96.99 ± 0.90% to 48.16 ± 1.98%, and the dye desorption efficiency decreased from 68.70 ± 2.03% to 46.83 ± 1.49%. Overall, this study has demonstrated that brown macroalga is a promising sorbent for the removal of MB dye from aqueous solutions. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Brown macroalga Methylene blue dye Desorption Isotherm study Kinetic modeling

1. Introduction Wastewaters due to their toxic nature represent serious threat to the environment and human health. Wastewater containing dyes is one of the major sources of water pollution and must be treated before discharge to the natural water bodies. Among various techniques employed for dye removal (such as, chemical and electrochemical coagulation, flocculation, ion exchange, membrane separation, advanced oxidation processes, adsorption, photocatalysis and photooxidation), the sorption process has shown a great potential for the removal of dyes from effluents (Alqadami

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A. Vazirzadeh), amit.bhatnagar@uef. fi (A. Bhatnagar). http://dx.doi.org/10.1016/j.jclepro.2017.03.119 0959-6526/© 2017 Elsevier Ltd. All rights reserved.

et al., 2016; Kumar et al., 2014). However, dealing with the dyesloaded sorbents is still a challenging issue. To tackle the problem attributing to solid mass loaded with pollutants, the use of appropriate regenerating agents is very important. The regeneration of sorbents is critical to keep the treatment cost low, minimizing the disposal problem and it also opens the probability for recovering the valuable materials. Desorption mechanism may involve ion exchange or complexation processes where sorbed dyes are desorbed from the sorbent to yield a small volume of concentrated dye solution (Aldor et al., 1995; Davis et al., 2003). It will also be useful for further reuse of the adsorbents and discarding them safely. Dye desorption can be achieved with ion exchange using relatively inexpensive mineral and organic acids and bases such as acetic acid (Yu et al., 2013) and diluted NaOH (Akar and Celik, 2011; Dahri et al., 2014), and by applying photocatalytic agents i.e. acid TiO2 hydrosol (Yu et al., 2012). An efficient eluent should desorb the

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dye entirely without destruction of the biomass structure and functional groups. Various eluents have been used for desorption of dyes from the dried biomass of bacteria (Vijayaraghavan et al., 2008b), yeast (Song et al., 2015; Yu et al., 2009, 2013), fungi (Akar et al., 2013), microalgae (Gupta et al., 2014), plant (Mahmoodi et al., 2011), polyethyleneimine (PEI)-treated peanut husk (Sadaf et al., 2015), walnut shell (Vijayaraghavan et al., 2008a) and sugarcane bagasse (Yu et al., 2012). Nizamuddinia zanardinii, a species of the family of Sargassaceae belongs to the class of Phaeophyceae, is a tropical brown macroalga which is distributed to the north-western Indian Ocean and in southwest Asia including Iran, Oman, Qatar, Pakistan and Yemen (Fariman et al., 2016; Kemp, 1998). Here, this macroalga was used in the dye sorption and desorption studies. The first objective of this study was to compare the dye desorption efficiency of seventeen different desorbing agents in order to determine the most effective one. Since there are currently not much published studies on the effect of important factors i.e. dye saturated algal mass, eluent volume, effect of different particle sizes, effect of different types of washing modes (before their usage in desorption experiment) on MB desorption, therefore, the second objective of this study was to investigate desorption efficiency in terms of these operational parameters for the selected eluents. The third objective was to verify the efficient regeneration of sorbent with the best desorbing agent by conducting isotherm and kinetic studies in order to determination of maximum desorption capacity and suitable kinetic and isotherm models. Reusability of the brown macroalga for the dye sorption for five consecutive sorption-desorption cycles was also studied. 2. Materials and methods 2.1. Algal biomass preparation The brown macroalga, N. zanardinii was used for MB dye sorption and desorption experiments. Macroalga biomass was collected from the Oman Sea coasts of Chabahr, Iran. Collected algal biomass was washed with tap water and then with distilled water several times to remove extraneous debris and salts. The washed algal biomass was dried in an oven at 50
C for 24 h. Finally, the obtained dried algal biomass was chopped and sieved. Particle size of 106e250 mm was used for both sorption and desorption studies. 2.2. Characterization of sorbent The characterization of brown macroalga was performed using Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). For FT-IR analysis, samples of brown macroalga were dried and coated with KBr to form pellets and analyzed using PerkinElmer FT-IR (model Spectrum RXI) within the range of 400e4000 1/cm. The macroalga morphology after gold coating was studied using scanning electron microscope (SEM, Tescan, Czech Republic).

spectrophotometer at optimum wavelength of 664 nm. All the experiments were performed in duplicate and all values are expressed as the mean ± standard deviation (SD). The MB dye sorption capacity (qe, mg/g) and sorption efficiency were calculated by determining the absorbance of the amount of adsorbed dyestuff before and after sorption process. qe,

sorption

¼ V (Ci  Ce)/M

(1)

R (%)¼(Ci  Ce)/Ci  100

(2)

where, qe, sorption is the amount of dye sorbed per gram of sorbent at equilibrium (mg/g), R (%) is MB removal efficiency (%), Ci and Ce are the initial and equilibrium MB dye concentrations in the solution (mg/L), respectively, V is the MB solution volume (L) and M is the alga mass (g). The collected samples were washed with distilled water and oven dried at 50
C for 24 h and later used for desorption studies. 2.4. Desorption experiments Seventeen different chemical reagents including hydrochloric acid (HCl), phosphoric acid (H3PO4), nitric acid (HNO3), boric acid (H3BO3), sodium hydroxide (NaOH), sodium chloride (NaCl), potassium iodide (KI), sodium thiosulfate (Na2S2O3), sodium sulphate (Na2SO4), methanol (CH3OH), ethanol (C2H5OH), 1-butanol (C4H6OH), glycerin (C3H8O3), xylene (C8H10), isooctane (C8H18), chloroform (CHCl3) and acetone ((CH3)2CO) were tested as eluents. The concentration of 0.5 M of each eluent was used in the experiments. Desorption experiments were conducted using 100 mL capped Erlenmeyer flasks containing 25 mL of eluents solution. 5 mg of MB dye loaded macroalga powder was suspended in 25 mL eluents solution (200 mg/L). The mixture was stirred at 130 rpm at 27
C for 120 min. Algal mass was separated from the mixture by centrifuging at 4000 rpm for 4 min. The MB dye desorption capacity (qe, desorption, mg/g) was calculated as follows: qe,

desorption

¼ V (Cf)/M

(3)

where, qe, desorption is the amount of dye desorbed from per gram of dye saturated sorbent at equilibrium (mg/g), Cf is the MB dye concentration in the desorbing solution (mg/L), V is the eluent solution volume (L) and M is the dye saturated alga weight (g). Desorption efficiency (%) of MB dye was calculated using Eq. (4): D %¼ (qe,

desorption/

qe,

sorption)

 100

(4)

where, D % is MB dye desorption efficiency (%) and qe, desorption and qe, sorption are MB dye desorption and sorption capacity (mg/g), respectively. All the experiments were performed in duplicate and all values are expressed as the mean ± standard deviation (SD).

2.3. Sorption experiments

2.5. Effect of different operating variables on MB dye desorption efficiency

The sorption experiments were conducted in 500 mL Erlenmeyer flasks on a thermostatic shaker in batch mode. Each flask was filled with 250 mL of 200 mg/L MB dye solution and 1.5 g brown alga biomass (6 g/L) was added in dye solution. The mixture was agitated at 150 rpm at 27
C for 120 min. In the sorption experiments, the solution was used without pH adjustment (pH around 6.5). After completing the experiment, MB solution was filtered through centrifuging at 4000 rpm for 4 min and residual dye concentration was measured using an UV/vis

2.5.1. Effect of eluents concentrations To investigate the effect of different concentrations of eluents on desorption efficiency, three different eluents e.g. HCl, NaCl and (CH3)2CO were selected. Different concentrations of each eluent viz., 0.125, 0.25, 0.5 and 1 M were prepared. Desorption experiment was conducted as explained in section 2.4. Among the different concentrations of mentioned eluents, 1 M HCl showed the highest dye desorption efficiency and further dye desorption experiments were conducted by this eluent.

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2.5.2. Effect of dye loaded alga weights Different weights of dye saturated alga mass varying from 100 to 500 mg/L were used to determine the effect of alga weight on dye desorption efficiency. Except the alga mass, the other variables were kept constant and desorption study was conducted as explained in section 2.4. 2.5.3. Effect of eluent volumes Desorption experiments were performed using different volumes of eluent varying from 10 to 40 mL. Except the volume of eluent, the other variables were kept constant and desorption study was conducted as explained in section 2.4. 2.5.4. Effect of different HCl:1-butanol ratios To investigate the effect of different ratios of HCl:1-butanol (1 M) on dye desorption efficiency, various mixtures of solutions (HCl:1-butanol) were prepared as follows: 100:0 HCl:1-butanol, 75:25 HCl:1-butanol, 50:50 HCl:1-butanol, 25:75 HCl:1-butanol, 0:100 HCl:1-butanol. Except the eluent, the other variables were kept constant and desorption study was conducted as explained in section 2.4. 2.5.5. Effect of different particle sizes of alga Dried algal mass was crushed and sieved to obtain three different particle sizes, specifically <75, 106e250 and > 250 mm. Sorption experiments were conducted with 200 mg/L of N. zanardinii biomass in 100 mL Erlenmeyer flasks containing 25 mL of solutions with 110 mg/L initial dye concentrations. The mixture was agitated at 130 rpm at 27
C for 120 min. Desorption experiments were conducted using three above mentioned particle sizes of algal mass. Except the alga particle size, the other variables were kept constant and desorption study was conducted as explained in section 2.4. 2.5.6. Effect of washing of dye loaded alga after sorption process Sorption experiments were carried out with 1.25 g/L of N. zanardinii in 500 mL Erlenmeyer flasks containing 200 mL of solutions with 80 mg/L initial dye concentration. The mixture was agitated at 150 rpm at 27
C for 120 min. Before desorption experiment, dye saturated alga were washed by three washing modes including no-washing, water washing using wash bottle to remove MB dye on the alga surface and water washing by keeping the mixture in shaker for 30 min. Dye loaded alga samples were collected from different types of washing modes. Desorption experiments were conducted with wet and dry dye-saturated alga mass. To use the similar weight of wet and dry masses, the weight of oven dried wet masses were calculated. Desorption study was conducted as explained in section 2.4.

445

Peterson (Redlich and Peterson, 1959) and Sips (1948) models, were studied (Eqs. (5)e(8)). qe ¼ Q0 b Ce/1 þ b Ce

(5)

qe ¼ KF C1/n e

(6)

qe ¼ KR Ce/1 þ aR Cge

(7)

. b b qe ¼ Ks Ce S 1 þ aS Ce S

(8)

where, qe is the solid-phase dye concentration (mg/g), Ce is the equilibrium dye concentration in the dye (for sorption isotherm) and eluent solution (for desorption isotherm) (mg/L), b is Langmuir isotherm constant (dm3/mg) related to the energy of sorption/ desorption and Q0 is the maximum sorption/desorption capacity (mg/g), KF is the Freundlich isotherm constant (mg/g)(dm3/g)n, n is a dimensionless constant related to sorption/desorption intensity, KR is the RedlichePeterson isotherm constant (L/g), aR is the RedlichePeterson isotherm constant (1/mg), g is the RedlichePeterson isotherm exponent, Ks is the Sips isotherm model constant (L/g), bS is Sips isotherm model exponent and aS is the Sips isotherm model constant (L/mg). The MATLAB software (Version 7.11.0 (R2010b)) was used to run non-linear analysis of isotherm models. 2.7. Effect of contact time Kinetic desorption study was carried out in 500 mL Erlenmeyer flasks containing 200 mL HCl and mixture of HCl:1-butanol (25:75) (1 M) as eluents. Experiments were conducted in a rotary shaker at 130 rpm for various time intervals between 5 and 420 min. Samples were taken at different time intervals, filtered and analyzed for desorbed dye content. The kinetic data were analyzed using pseudo-first-order (Chang et al., 2016; Njikam and Schiewer, 2012) and pseudo-second-order (Chang et al., 2016; Njikam and Schiewer, 2012) kinetic models. 2.8. Reusability Batch experiment was employed for five successive cycles of MB dye sorption and desorption process. Desorption experiment was performed in 100 mL Erlenmeyer flasks containing 5 mg of alga mass at 27
C for 120 min using 25 mL of HCl:1-butanol (25:75) (1 M) solution as an eluent. The sorption and desorption efficiency is calculated by using Eqs. (2) and (4). 3. Results and discussion

2.6. Effect of different initial dye concentrations

3.1. Dye sorption and sorbents characteristics

Batch experiments were performed to determine the effects of various initial dye concentrations. Equilibrium experiments were carried out by using 1 g/L of alga mass in 100 mL Erlenmeyer flasks containing 25 mL of dye solution with different initial concentrations (20e320 mg/L). Mixtures were shaken at 150 rpm for 120 min. After sorption process, alga mass was centrifuged and separated from the dye solution. Dye sorption capacity was calculated and dye saturated mass was saved to determine the effect of different initial dye concentration on desorption efficiency. Except the initial dye concentration for sorption experiment, the other variables were kept constant and desorption study was conducted as explained in section 2.4. The most widely used isotherm equations, including the nonlinear form of Langmuir (1916), Freundlich (1906), Redlich-

For sorption experiment, 1.5 g of brown alga was added to 250 mL MB dye (200 mg/L) solution. Dye sorption capacity and efficiency were observed as 35.06 ± 0.71 mg/g and 83.78 ± 1.93%, respectively (data not shown). MB dye sorption capacity and efficiency were also studied as a function of initial dye concentration (20e320 mg/L), while the initial pH, biomass loading, temperature and contact time were kept constant. Dye sorption capacity increased with increasing initial dye concentration (data not shown) which might be due to the fact that the higher initial dye concentration provides the driving force to overcome the mass transfer resistance between aqueous and solid phases (Daneshvar et al., 2012). On the other hand, the decrease in dye removal efficiency by increasing the initial dye concentration could be related to the saturation of algal biomass after equilibrium. The dye loaded

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alga were used for desorption and reusability experiments. Alga cell wall contains different types of chemical functional groups such as carboxyl, amine, imidazole, phosphate, sulphate, sulfhydryl and hydroxyl that play a key role in the sorption process. Previously, this brown alga has been used in heavy metals and acidic dyes sorption studies (Esmaeli et al., 2013; Kousha et al., 2012; Koutahzadeh et al., 2013). Esmaeli et al. (2013) have reported that hydroxyl and amine groups on the surface of N. zanardinii are the most important functional groups for biosorption of Acid black 1 (AB1) dye. Also, according to the FT-IR results, Kousha et al. (2012) suggested that eOH and eNH groups have the major role for AB1 dye sorption onto N. zanardinii biomass. In this study, the sharp and broad peak (Fig. 1 (a)) was found at 3394.05 1/cm that shows the presence of both surface hydroxyl (OeH) stretching of carboxylic groups and also stretching of amido groups (NeH). The other peaks were found at 1637.12 and 1034.70 1/cm which could be related to eC]C and aromatic ethers, aryl-O stretching (Coates, 2000). SEM image demonstrated that N. zanardinii has a lot of granules on its rough surface (Fig. 1 (b)). These nano granules create a huge surface area and according to the surface to volume ratio, there is large surface area for MB sorption due to this granular surface of alga. In addition, a number of pores are formed by the aggregation of these tiny particles that can increase the sorption capacity. The high dye removal capacity and efficiency of brown alga, N. zanardinii might be related to the presence of different functional groups and also distinctive morphology on its cell wall (Daneshvar

et al., 2017). 3.2. Effect of different eluents The adsorbents can be used in the multiple sorption/desorption cycles by applying desorption process, and it notably reduces the cost of treatment procedure, adsorbents supply and the disposal problems of exhausted adsorbents (Gupta and Rastogi, 2008). In the present study, seventeen different eluents have been used to investigate MB desorption from brown macroalga (Table 1). As can be seen from Table 1, NaOH and HCl solutions indicate the minimum (0%) and maximum (40.72 ± 0.72%) desorption percentages of MB, respectively. The reason of minimum amount of MB dye desorption by NaOH solution might be due to the formation of aviolet-colored complex between eluent and desorbed MB dye in the experimental solution and difficulty of measuring dye concentration in optimum wavelength of 664 nm. Desorption efficiency by distilled water as control eluent was found to be 2.36 ± 0.27%. The number of positively charged sites on the biomass increases under acidic conditions. It can increase the desorption efficiency of MB dye due to the electrostatic repulsion between positively charged sites on the algal mass and cationic dye molecules (Vijayaraghavan et al., 2008a). Another reason might be due to the abundance of Hþ ions in the acidic solution and its exchange with the MB ions on N. zanardinii (Abdallah and Taha, 2012). Thus, the desorbing agents that can generate more cations in the solution especially hydrogen ions (Hþ) are more effective eluents

Fig. 1. The characteristics of brown macroalga N. zanardinii, (a) FT-IR spectra and (b) SEM image.

Table 1 Characterization of used eluents in MB dye desorption experiments. Name

Chemical formula

Chemical group

Desorption efficiency (%)±SDa

Distilled water Sodium hydroxide Isooctane Sodium sulphate Chloroform Glycerine Xylene Sodium thiosulfate Boric acid Potassium iodide Sodium chloride Ethanol Methanol Acetone 1-Butanol Nitric acid Phosphoric acid Hydrogen chloride

H2O NaOH C8H18 Na2SO4 CHCl3 C3H8O3 C8H10 Na2S2O3 H3BO3 KI NaCl C2H5OH CH3OH (CH3)2CO C4H6OH HNO3 H3PO4 HCl

e Base Alkane Salt Organic compound Aromatic alcohol Aromatic Hydrocarbon Salt Inorganic acid Salt Salt Alcohol Alcohol Ketone Alcohol Inorganic acid Inorganic acid Inorganic acid

2.36 ± 0.12 0.00 12.14 ± 0.64 14.03 ± 1.41 16.62 ± 0.91 17.50 ± 0.84 18.86 ± 1.06 19.27 ± 0.54 19.51 ± 1.67 20.71 ± 1.65 21.28 ± 1.69 21.92 ± 1.23 24.05 ± 1.09 27.11 ± 1.41 31.71 ± 1.78 35.07 ± 0.23 35.66 ± 1.58 40.72 ± 0.92

a

Standard Deviation.

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for cationic dye desorption. Moreover, acid dissociation constant (Ka), also known as acidity constant or acid-ionization constant, is a quantitative measure of the acidity strength of a solution, which can be written as follows (Roberts and Caserio, 1977):

HA þ H2 O4A þ H3 Oþ

(9)

where, HA is a generic acid that dissociates into A, known as the conjugate base of the acid and a hydrogen ion (Hþ) which combines with a water molecule to make an hydronium ion (H3Oþ). The dissociation constant is usually written as follows (Roberts and Caserio, 1977):

 Ka ¼

  A H3 Oþ ½HA½H2 O

447

increased desorption efficiency from 0.55 ± 0.09 to 26.49 ± 2.58% for (CH3)2CO, 11.85 ± 0.26 to 22.12 ± 0.34% for NaCl and 30.75 ± 1.89 to 41.20 ± 0.86% for HCl. Increasing HCl concentration results in increasing Hþ ions concentration, which lead to the increase of dye desorption efficiency. Chen et al. (2011) indicated that desorption percentage of butyl Rhodamine B dye sharply increased from 1.97 to 99.70% along with an increase in ethanol concentrations from 0 to 80%. The desorption efficiency of Cd, Zn, and Cu onto immobilized activated sludge increased by increasing eluent concentrations concentration, and the optimal dose for maximum metals desorption percentage was found at 1 M nitric acid (6.3 g/100 mL) _ (Kuczajowska-Zadrozna and Filipkowska, 2016). 3.4. Effect of dye saturated alga weight

(10)

Therefore, higher the H3Oþ ions concentration in the aqueous solution, higher will be the Ka value. The Ka values of chemical groups generally follow: inorganic acids > carboxylic acids > alcohols > ketones > amines > alkanes > bases (Table 1) (Roberts and Caserio, 1977). Different studies have demonstrated different desorbing efficiencies due to diverse biomass structures, specific modification and experimental conditions. The findings are in agreement with previous works where the maximum desorption percentages of MB using nonviable fungus biomass Aspergillus fumigatus was achieved 80% at pH 3.0 (Abdallah and Taha, 2012). Moreover, the maximum MB desorption by diluted HCl using different biomass were obtained from 53.3 to 61.4% in the first cycle, and from 14.3 to 21.0% in the seventh cycle, which indicated that MB desorption efficiency using the studied biosorbents decreased after few sorption-desorption cycles (Rosas-Castor et al., 2014). 3.3. Effect of eluents concentrations on dye desorption efficiency Effect of different concentrations of three eluents viz., NaCl, HCl and (CH3)2CO ranging from 0.125 to 1.0 M was examined on the dye desorption efficiency from N. zanardinii mass (Fig. 2). It was found that desorption efficiency was increased by increasing the eluent concentrations. The maximum desorption percentage was obtained using 1.000 M HCl, whereas 0.125 M (CH3)2CO demonstrates the minimum desorption efficiency. Increasing eluents concentrations

Desorp on efficiency (%)

Hydrochloric acid

The effect of alga biomass loading on desorption efficiency of MB dye was also investigated by varying the biomass loading ranging from 100 to 500 mg/L, while temperature and contact time were kept constant at 27
C, and 120 min, respectively. Fig. 3 (a) shows that the MB dye desorption percentage significantly reduced by increasing the dose of biomass loading and it ranged between 40.24 and 28.05 ± 0.22% with an increase of biomass loading from 100 to 500 mg/L. The decrease in dye desorption efficiency with increasing biomass amount was suggested to be the result of increasing particle interaction and aggregation, leading to a reduction of total biomass surface and an increase in diffusional path length under constant elution concentration and volume (Daneshvar et al., 2015). By increasing the alga loading, there is a lower eluent volume for higher dye loaded alga mass and the effect of eluent on the dye desorption is decreased. 3.5. Effect of eluent volumes on dye desorption efficiency The effect of different volumes (10e40 mL) of 1 M HCl on desorption efficiency of MB dye from N. zanardinii biomass was studied, while eluent concentration, alga loading, temperature and contact time were kept constant at 1 M, 200 mg/L, 27
C and 120 min, respectively. As can be seen from Fig. 3 (b), MB dye desorption efficiency increased from 30.22 ± 0.30 to 36.23 ± 0.76% with an increase in the eluent volume between 10 and 25 mL, then it reached a plateau up to 40 mL (37.49 ± 1.34%). The reasons of this observation can be attributed to the fact that the increasing eluent

Sodium chloride

Acetone

50 40 30 20 10 0 0.125

0.25 0.5 Eluent concentra on (M)

1

Fig. 2. Effect of different concentrations of three eluents viz., HCl, NaCl and (CH3)2CO on MB dye desorption efficiency (%) from N. zanardinii. [Sorption exp.: Initial solution pH: Notadjusted (approx. 6.5), alga mass: 6 g/L, initial dye conc.: 200 mg/L, agitation speed: 150 rpm, particle size: 106e250 mm, T: 27
C, time: 120 min; Desorption exp.: Eluents conc.: 0.125e1 M, Eluents volume: 25 mL, Initial solution pH: Not-adjusted, alga mass: 200 mg/L, agitation speed: 130 rpm, particle size: 106e250 mm, T: 27
C, time: 120 min].

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volume provided a more effective interaction between the aqueous and solid phases. By increasing the eluent volume, the ratio of eluent volume in relation to the alga mass is increased. It means that there is a higher eluent volume for a known dye loaded alga biomass and the impact of eluent on the dye desorption efficiency is increased. The plateau pattern at higher volumes (32.5 and 40 mL) shows that increasing the eluent volume more than the optimum value had no significant effect on the dye desorption efficiency. 3.6. Effect of acid to alcohol ratios on dye desorption efficiency The effect of different acid to alcohol ratios of 100:0, 75:25, 50:50, 25:75 and 0:100% of HCl:1-butanol was investigated, when eluent concentration, volumes, temperature, and time were kept constant at 1 M, 25 mL, 27
C, and 120 min, respectively. The dye desorption efficiency was dramatically increased by increasing the alcohol percentage (Fig. 3 (c)). The maximum dye desorption

a)

efficiency of 63.67 ± 1.67% was obtained at acid: alcohol ratio of 25:75%, then the dye desorption efficiency sharply decreased to 34.08% by increasing the alcohol ratio to 100%. The lowest MB dye desorption percentages by 100% 1-butanol indicate that the predominant mechanism of sorption is a strong electrostatic attraction between the dye and biomass functional groups. On the other hand, mixture of 1-butanol and HCl significantly improved the dye desorption which indicates the MB dye is bound to the alga mass not only by electrostatic attraction but also by hydrophobic interaction and H-bonding (Smaranda et al., 2009). In another study 1 M HCl/50% methanol and 1 M NaOH/50% methanol solutions increased the desorption percentage of Acid orange 7 dye about 44 and 27%, respectively from Amberlite IRA-958 resin (Greluk and Hubicki, 2011). 3.7. Effect of different particle sizes on dye desorption efficiency Operating variables such as different particle sizes of sorbents

(d) (e)

(b) (f)

(c) Fig. 3. Effect of the operating parameters on MB dye desorption efficiency (%) from N. zanardinii. (a)Effect of different dye saturated alga loading, (b) Effect of different eluent volume, (c) Effect of the different ratios of HCl:1-butanol, (d) Effect of different particle sizes, (e) Effect of the different washing modes of dye saturated alga mass after sorption experiment, (f) Effect of different initial dye concentrations in sorption experiment. [Sorption exp.: Initial solution pH: Not-adjusted (approx. 6.5), alga mass: 6 g/L, initial dye conc.: 200 mg/L [(f): 20e320 mg/L], agitation speed: 150 rpm, particle size: 106e250 mm [(d): <75, 106e250 and > 250 mm)], T: 27
C, time: 120 min; Desorption exp.: Eluent conc.: 1 M HCl [(c): 1 M HCl: 1 M 1-butanol], eluent volume: 25 mL [(b): 10e40 mL], initial solution pH: Not-adjusted, alga mass: 200 mg/L [(a): 100e500 mg/L], agitation speed: 130 rpm, particle size: 106e250 mm [(d): <75, 106e250 and > 250 mm)], T: 27
C, time: 120 min].

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can also affect the dye removal efficiency. In order to study the effect of particle sizes, batch experiments were carried out using three sizes of brown macroalga (<75, 106e250 and > 250 mm) and an initial MB dye concentration of 110 mg/L at room temperature (data not shown). The dye removal percentage onto <75, 106e250 and > 250 mm particle sizes was observed as 73.89, 86.23 and 85.16 ± 1.14%, respectively. There were no significant differences in the dye removal efficiency of adsorbent with medium (106e250 mm) and bigger (250 mm) particle sizes. However, an increase in the particle size from <75 to 106e250 and > 250 mm leads to a significant decrease in the sorption percent to 73.89 ± 0.85%. Sorption is a surface phenomenon and sorption efficiency is generally related with surface area of sorbent. It is generally favored by reduction in particle size due to larger surface area of smaller particles for the same amount of sorbent. But some studies have reported the reverse results that sorption percentage increased by increasing the particle sizes (Deokar et al., 2016a, 2016b), which is consistent with our results of this study. Deokar et al. (2016a) investigated the relation between particle size and surface area. According to their findings, larger particles have deeper pores and smaller opening (cylindrical shape), while smaller ones have shallow pore and wider opening (bowl shape). When the size of particles decreases, the pore walls contact with each other and create a bigger pore. So larger particles have smaller pores and smaller ones have larger and lesser internal surface area. Also, according to Deokar et al. (2016b), the bigger particles have higher BrunauereEmmetteTeller (BET) surface area than smaller particles and can adsorb more compared to smaller particles. Fig. 3 (d) illustrates that desorption efficiency increased by the increasing the particle sizes of adsorbent. Desorption percent for <75, 106e250 and > 250 mm particles were found to be 38.33 ± 2.00, 61.26 ± 2.88 and 63.04 ± 2.89%, respectively. Our findings can be explained by the following reasons: i) lower the sorption efficiency, lower is the desorption efficiency from smaller particles, ii) the breaking of large particles can change the availability of reactive groups, present on internal layers that have more affinity toward MB dye molecules and stronger bonds, iii) according to the surface/volume ratio, there is more pressure on the surface of smaller particle sizes that increases the diffusion resistance to mass transport. Finally, higher sorption and desorption efficiency onto larger particle sizes suggest that studied macroalga can be used as an efficient sorbent for dye removal from aqueous solution. Using the bigger particles in wastewater treatment seems to be more advantageous due to reduction cost of grinding and easier recovery of them as compared to smaller particles. 3.8. Effect of different types of washing modes on dye desorption efficiency After sorption experiments, there are different ways of using dye saturated mass for desorption experiments. Here, we used the dye loaded mass for desorption experiments in different modes: nowashing, water washing with wash bottle (to remove extraneous dye on the surface) and water washing by using a shaker (30 min). After collecting the algal mass by these different washing modes, they were applied in wet and dried forms. Four main points can be drawn from Fig. 3 (e): i) the desorption efficiency increased in the following order for wet mass: no-washing < water washing-wash bottle < water washing-shaker, ii) the desorption efficiency increased in the following order for dry mass: no-washing > water washing-wash bottle > water washing-shaker, iii) in the nowashing mode, higher desorption efficiency was recorded onto the dry mass, while in the water washing-wash bottle and shaker modes, higher desorption efficiency was observed onto the wet mass, and iv) the highest and lowest desorption percentage is

449

related to the wet and dry masses after water washing-shaker mode, respectively. The reason of higher dye desorption efficiency of wet mass in comparison to dry mass can be related to turgidness and plasmolysis process. After sorption process, a lot of dye molecules pass into the alga cells (turgidness). When alga cells exposure to distilled water (water washing-shaker), water flows into the alga cells with higher dye concentration and alga tissue will be expanded due to water sorption. Subsequently, in desorption experiments, eluent solution like HCl will be able to diffuse and react by the alga particles easily and desorb dye molecules. The reverse situation will be happened for dry mass. In dye sorption process, dye and water molecules pass through alga particles (plasmolysis). During oven drying period, due to plasmolysis and water evaporation, alga tissue will be contracted. Consequently, dye molecules will be trapped in the inside layers and eluent solution diffuses and reacts by the plasmolysed particles difficultly and dye desorption efficiency will be decreased. As can be seen from Fig. 3 (e) in the no-washing mode, dry mass showed higher desorption efficiency in comparison with wet mass. Reduction in desorption percentage might be due to hydrophobic interactions including pep interactions between the aromatic rings of the dye and the alga that can decrease MB dye desorption efficiency onto brown alga cell walls (Greluk and Hubicki, 2011). 3.9. Effect of different initial dye concentrations and desorption isotherms Fig. 3 (f) depicts the effect of different initial MB dye concentrations in the sorption process on dye desorption efficiency. It can be seen that the difference between desorption percentage of three initial dye concentrations viz., 20, 40 and 80 mg/L was not significant (43.36 ± 1.43, 43.44 ± 0.58 and 40.60 ± 1.42%, respectively). Dye desorption efficiency for higher initial dye concentrations of 160 and 320 mg/L was also not significant (79.59 ± 1.33 and 78.13 ± 2.74%, respectively). However, the dye desorption efficiency between three initial dye concentrations (20, 40 and 80 mg/L) and two higher initial dye concentrations (160 and 320 mg/L) was significant. Increasing initial dye concentration in sorption experiment leads to increasing dye sorption capacity (mg/g) and consequently dye desorption efficiency. Increasing initial dye concentration has doubled the dye desorption efficiency (from 40.60 to 78.13 ± 2.74%). The equilibrium sorption and desorption isotherms are of fundamental importance in the design of sorption and desorption systems. In this study, four isotherm models were selected to describe the sorption/desorption of MB onto brown macroalga at 27
C. These include two-parameters (Langmuir and Freundlich) and three-parameters (Redlich-Peterson and Sips) isotherm models. These models were fitted with the experimental data by non-linear modeling. The models’ parameters were measured and optimized by non-linear regression modeling by MATLAB (R2012a). The Langmuir model assumes monolayer sorption onto a completely homogeneous surface without any lateral interaction between adsorbed dye molecules on neighboring sites. According to this model, there are finite identical number of binding sites with the same sorption energy on the sorbent surface. The Freundlich isotherm model is another empirical equation that is widely employed to describe solideliquid adsorption. Unlike the Langmuir, this model endorses a multilayer sorption onto heterogeneous sorption surfaces with non-equivalent energy active centers (binding sites) and interactions between adsorbed molecules. RedlichePeterson isotherm model is a hybrid model which incorporates the features of the Langmuir and Freundlich isotherms into a single equation. To provide a wide concentration range, this model has a linear dependence on concentration in the numerator

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and an exponential function in the denominator. Due to its versatility, it can be applied either to homogenous or heterogeneous systems. Isotherm data were also fitted with Sips, another three parameters (KS, bS and aS) model. This model also known as LangmuirFreundlich isotherm and the name derives from the limiting behavior of the equation. At low and high sorbate concentrations, the Sips isotherm model approaches the form of the Freundlich and Langmuir models, respectively. Hereby, Sips isotherm model is expected to describe heterogeneous surfaces much better than the Langmuir and Freundlich isotherm models. The corresponding Langmuir, Freundlich, Redlich-Peterson and Sips parameters for MB dye sorption and desorption are listed in Table 2. The values of determination coefficients (R2), non-linear error functions, the residual root mean square error (RMSE) and sum of squares error (SSE) obtained from these models were used as the fitting criteria for comparing these isotherms. The value of R2 from all models fitting was found to be 0.999. Thus, all studied models have shown good fitness with the experimental data. Higher values of R2 and lower values of RMSE indicate that the Sips and Freundlich models are more suitable for describing the sorption equilibrium of MB dye on the brown alga. The good fitting of Sips isotherm model to the experimental data suggested that dye sorption takes place both on homogenous and heterogeneous surface of alga biomass. A desorption isotherm is a mathematical expression that relates the amounts of dye remained in the solid phase (qe, desorption) after desorption at the interface to equilibrium dye concentrations in the aqueous phase (Ce, desorption) at a constant temperature. As known, the higher R2 value (close to 1.000) confirms that the equation fits the experimental data better. On the other hand, the smaller RMSE and SSE values indicate that the data calculated from isotherm models are similar to the experimental data. The calculated values of all four models parameters are presented in Fig. 4 (a). From Table 2 and Fig. 4 (a), it was noted that three-parameter desorption isotherm models such as Sips and Redlich-Peterson exhibited better performance than two-parameter isotherm models e.g., Langmuir and Freundlich. On the basis of obtained R2 of 0.947 and RMSE of 6.46, desorption experimental data was better described by the Table 2 Langmuir, Freundlich, Redlich-Peterson and Sips isotherm models for MB dye sorption and desorption onto/from N. zanardinii. Isotherm models

Parameters

Sorption

Desorption

Langmuir

Q0 (mg/g) b (dm3/mg) R2 a RMSE b SSE KF (mg/g) (dm3/g)n n R2 RMSE SSE KR (L/g) aR (1/mg)

95.45 0.0003 0.999 3.093 28,70 1.134 1.05 0.999 2.76 22.85 3.909 2.478 0.061 0.999 3.382 22.88 1.134 0.953 0.0003 0.999 2.76 22.85

66.79 0.2215 0.809 10 300 20.72 3.34 0.626 14 588 8.86 0.20 1.547 0.914 8,24 135.9 1.576 2.73 0.028 0.947 6.46 83.42

Freundlich

RedlichePeterson

bR

Sips

R2 RMSE SSE KS (L/g)

bS aS (L/mg) R2 RMSE SSE

a b

RMSE: Root Mean Square Error. SSE: Sum of Squares Error (SSE).

Fig. 4. Modeling for MB dye desorption onto N. zanardinii, (a) Isotherm models and (b) Kinetic models. [Sorption exp.: Initial dye conc.: (Isotherm studies: 20e320 mg/L, kinetic studies: 200 mg/L), initial solution pH: Not-adjusted (approx. 6.5), agitation speed: 150 rpm, particle size: 106e250 mm, T: 27
C, time: 120 min; Desorption exp.: Eluents conc.: (Isotherm studies: 1 M HCl, kinetic studies: 1 M HCl and HCl:1-butanol (25:75) (1 M) solution), Eluent(s) volume: (Isotherm studies: 25 mL, Kinetic studies: 200 mL), initial solution pH: Not-adjusted, alga mass: 200 mg/L, agitation speed: 130 rpm, particle size: 106e250 mm, T: 27
C, time: (Isotherm studies: 120 min, kinetic studies: 440 min)].

Sips isotherm model followed by Redlich-Peterson model. The Langmuir model fitted the experimental data better than Freundlich model, indicating desorption of MB onto brown alga mass tended to monolayer desorption. Other studies have also shown the successful prediction of sorption and desorption isotherm data using Sips isotherm as compared to Langmuir and Freundlich isotherms. The reason for this behavior might be due to that Sips isotherm is a hybrid form of Langmuir and Freundlich isotherm models and is able to predict a wide sorbate concentration ranges (Ahmed and Dhedan, 2012). According to the three fitness parameters (R2, RMSE and SSE), the three-parameter Sips isotherm model is the most suitable model for understanding the sorption and desorption mechanisms of MB dye onto brown macroalga. Therefore, it was established that experimental data fitted well in the following order: Sips ¼ Freundlich > RedlichePeterson > Langmuir for sorption isotherm, and Sips > RedlichePeterson > Langmuir > Freundlich for desorption isotherm.

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3.10. Effect of contact time and kinetic models Fig. 4 (b) shows the effect of contact time at initial eluents concentration of 0.5 M. For both eluents (HCl and mixture of HCl and 1-butanol), the dye desorption was rapid in the first 30 min (about 60%), then continued with a slower rate during 30e180 min and finally remained constant up to 300 min. In the beginning of experiment, the dye concentration in the solid phase is high. Also, the purity of eluents in the initial phase likely results in the increased driving force of the concentration gradient. By passing of time, reverse situation takes place when the dye concentration decreases in solid phase and increases in liquid phase. Consequently, dye desorption rate decreases until reaching equilibrium. The maximum MB dye desorption by HCl and mixture of HCl and 1butanol were 17.31 and 30.10 mg/g, respectively. The desorption kinetics is an important parameter for design and regeneration of adsorbent and is required for selecting the optimum operating conditions for pilot-scale process. The desorption kinetics were recorded experimentally and fitted to both pseudo-first order and pseudo-second-order kinetic equations. For first and second order kinetics, the kinetic equations can be written as (Chang et al., 2016):

q ¼ q0 ek1D t

(11)

1/q ¼ 1/q0þk2Dt

(12)

Table 3 Pseudo-first-order and pseudo-second-order kinetic models for MB dye desorption from N. zanardinii. Models

Parameters

Eluents HCl

HCl:1-butanol (25:75)

Pseudo-first-order

qe, experimental (mg/g) KP1D (1/min) q0D (mg/g) RMSE R2 KP2D (1/min) q0D (mg/g) RMSE R2

17.4 0.032 16.09 1.22 0.923 0.003 17.86 0.47 0.988

30.1 0.032 27.8 2.25 0.908 0.001 30.81 1.01 0.982

Pseudo-second-order

where, k1D (1/min) and k2D (g/mg. min) are the reaction rate constant for first and second order kinetics, respectively and t is the reaction time. The Lagergren’s pseudo-first order and pseudo-second order kinetic equations can be adapted for desorption by assuming that the rate of desorption is proportional to the number of dye molecules-filled sites and the square of the number of dye molecules-filled sites, respectively (68). The pseudo-first order and pseudo-second order kinetic equations are given as (Chang et al., 2016; Njikam and Schiewer, 2012):

  qD ¼ q0D 1  ekP1D t

(13)

qD ¼ q20D.kP2D.t/1þkP2D.q0Dt

(14)

where, qD (mg/g) and q0D (mg/g) are the amount of MB dye molecules that have been desorbed and the amount of desorbed MB dye molecules at desorption equilibrium, respectively. kP1D (1/min) and kP2D (g/mg. min) are the reaction rate constants for the pseudofirst order and pseudo-second order desorption reaction, respectively. The conformity between the experimental data and the model-predicted values were determined from determination coefficients (R2) and the non-linear error values of RMSE (Table 3). Higher model R2-values (i.e. 0.988 for HCl eluent and 0.982 for mixture of HCl and 1-butanol) and lower model RMSE-values (i.e. 0.47 for HCl eluent and 1.01 for mixture of HCl and 1-butanol) demonstrate that the pseudo-second order model is significant. The pseudo-second order equation showed more accurate prediction of equilibrium desorption capacities (qe, calculation close to qe, experimental values) than the pseudo-first order equation (Fig. 4 (b)). Thus, based on the kinetic results, desorption was found to follow pseudo-second order reaction. Consequently, based on these results, desorption process was found to be the rate-limiting step (Njikam and Schiewer, 2012).

3.11. Reusability The main aim of desorption studies includes sludge management and adsorbent reusability after sorption process. The reusability of adsorbents in the multiple sorption/desorption cycles is a remarkable benefit for their practical and economical applications.

Sorp on Sorption/Desorption efficiency (%)

451

Desorp on

100 90 80 70 60 50 40 30 20 10 0 1

2

3

4

5

Cycle Fig. 5. Sorption/desorption cycles of MB dye onto/from N. zanardinii. [Sorption exp.: Initial solution pH: Not-adjusted (approx. 6.5), alga mass: 200 mg/L, initial dye conc.: 110 mg/L, agitation speed: 150 rpm, particle size: 106e250 mm, T: 27
C, time: 120 min; Desorption exp.: Eluent conc.: HCl:1-butanol (25:75) (1 M) solution, Eluent volume: 25 mL, initial solution pH: Not-adjusted, alga mass: 200 mg/L, agitation speed: 130 rpm, particle size: 106e250 mm, T: 27
C, time: 120 min].

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In the current study, the mixture of HCl:1-butanol (25:75) (1 M) solution was used as eluent to investigate the possibility of reusing the brown macroalga. Five cycles of successive sorption/desorption experiments were conducted in the batch mode. The percentage of dye sorption and desorption rates are presented in Fig. 5. From the first to the second cycle, dye sorption efficiency decreased remarkably from 96.99 ± 0.90% to 61.54 ± 2.04%. Then, dye sorption percentage decreased to 48.16 ± 1.98% in the last cycle. This might be due to several reasons such as: i) the protonation of certain sorption sites or functional groups present on the alga surface by acidic eluent, ii) the saturation of the available binding sites on the sorbent by MB dye molecules, iii) decomposition of alga particles during consecutive sorption/desorption cycles (Kyzas et al., 2012). The rate of the dye desorption also decreased gradually from 68.70 ± 2.03% (first cycle) to 46.83 ± 1.49% (fifth cycle). The lower desorption efficiency as compare to sorption efficiency, in sorption/ desorption cycles, indicates that some heterogeneity exists on the alga biomass surface and a strong interaction is possible between dye molecules and some binding sites with higher energy (Kyzas et al., 2012). 4. Conclusions The desorption study of saturated adsorbents is important in wastewater treatment studies. The present study reports the results of MB dye desorption efficiency using seventeen different acids, base, salts, alcohols, and organic solvents onto brown macroalga. The effect of different operating variables on desorption efficiency were also investigated. Among different used eluents, the maximum percentage of desorption efficiency was found to be 40.08% by HCl. The result indicates that Hþ ion exchange takes place in the acidic medium with adsorbed MB dye molecules on the surface of macroalga mass. The mixture of acid and alcohol increased dye desorption efficiency due to the promotion of the ionic interactions. It shows that some other interactions such as, hydrophobic interactions and hydrogen bonding might also be involved in the sorption process. Dye desorption percentage increased by increasing eluents concentration, eluent volume, initial dye concentration in sorption experiment and particle sizes. Water washing-shaking mode of dye saturated alga increased dye desorption efficiency remarkably. The desorption equilibrium data were fitted well to Sips isotherm model that demonstrates the heterogeneous dye desorption process. The reusability of alga mass during five consecutive sorption/desorption cycles was successfully achieved. The findings of this study suggest that studied macroalga can favorably use as environmentally friendly and efficient sorbent for the removal of basic dyes from aqueous solution. Acknowledgment Thanks to Dr. Ashkan Ezhdehakoshpour, from Off-shore Fisheries Research Center, IFRO, Chabahar, Iran, for his assistance in collection and characterization of studied macroalga species. This work was financially supported by Research Council of Shiraz University (Postgraduate Studies), Shiraz, Iran. References Abdallah, R., Taha, S., 2012. Biosorption of methylene blue from aqueous solution by nonviable Aspergillus fumigatus. Chem. Eng. J. 195, 69e76. Ahmed, M.J., Dhedan, S.K., 2012. Equilibrium isotherms and kinetics modeling of methylene blue adsorption on agricultural wastes-based activated carbons. Fluid Phase Equilib. 317, 9e14. Akar, T., Arslan, S., Akar, S.T., 2013. Utilization of Thamnidium elegans fungal culture in environmental cleanup: a reactive dye biosorption study. Ecol. Eng. 58, 363e370. Akar, T., Celik, S., 2011. Efficient biosorption of a reactive dye from contaminated

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