INTRODUCTION:

Dyes represent one of the problematic groups, disposed into wastewaters from various industrial branches mainly from the dye manufacturing, textile finishing, food coloring, paper, and carpet industry^1,2. Due to these disposed dye effluents, the natural streams become toxic and they inversely affect aquatic life³. Also. Dyes bearing effluents inhibit the sunlight penetration into the water and reduce the photosynthetic activity⁴.

Several chemical or biological techniques are used for the treatment of groundwater, surface water, and wastewater such as coagulation, flocculation, filtration, ion exchange, membrane separation, photocatalysis, and photo-oxidation^5-7BC, whose major drawbacks are high cost and secondary waste generation, sludge⁸. Hence, the safe secondary waste disposal of sludge becomes another rising issue for industries. In recent times, the passive removal of dyes by biological materials has been greatly developed as a complementary technique for wastewater treatment. Earlier studies show that adsorption has proved to be an effective technology in the industrial and environmental remediation of toxic dyes^9,10. This technique has several advantages over other techniques, such as it is easy to implement, it has a potent regeneration capacity and the operation is sludge free and has high removal efficiency of dyes¹¹. Among diverse types of adsorbents (i.e. bacteria, fungi, industrial waste, etc.), algal biomass has proven to be a highly effective and economical technology for its ability to remediate toxic dyes¹².

MO is an acidic dye and is widely used in a variety of industries. It is a highly toxic dye and could cause harmful effects and it is, therefore, essential to properly treat industrial wastewater containing this dye. Literature review shows that there are reports showing the removal of MO from aqueous solution using bacteria Streptomyces fradiae¹³, silkworm exuviae¹⁴, aminated pumpkin seeds¹⁵, and activated carbon derived from Phragmites australis, a reed¹⁶. Our lab has also reported previously, the adsorptive elimination of MO dye using Vaucheria sp., a yellow-green alga¹⁷.

Chara sp. was chosen in the present study, since it is easily available and so relatively cheap adsorbent for the adsorption process. In our earlier studies, we have reported that this alga had considerable potential for the removal of Methylene blue dye from an aqueous solution¹⁸. Chara sp., a green alga, is found in freshwater, particularly in limestone areas where they grow submerged, attached to the muddy bottom. They are multicellular and superficially resemble land plants because of stem and leaf-like structures.

Hence, the main objective of the present study was to evaluate the potential of green alga Chara sp. for the removal of MO from synthetic wastewater. Characterization of adsorbent was evaluated using FTIR, SEM, and BET methods. Various experimental variables were optimized to study the adsorption capacity of Chara sp. on MO dye. To understand the adsorption process, different adsorption isotherms and kinetic models were also studied. The dye-loaded adsorbents have toxic effects on humans and the environment, therefore, the used adsorbents must be released into the environment only after the recovery of the dye completely. Considering the need for dye desorption and recovery, this paper summarizes the efficiency of various regenerating agents also.

MATERIAL AND METHODS:

Chemicals and Equipment:

Technical grade MO of 98% purity was used without further purification. All the chemicals used were of AR grade either from SD Fine Chem Ltd., India, or from Merck, Germany. Doubly distilled water was used throughout the experiments. The spectrometric determination of dye was done on a UV-visible spectrophotometer-119 (Systronics India Ltd.). The pH measurement was made using a pH meter (PERFIT, India). Infrared spectra were recorded using KBr pellets on a Thermo Nicolet FTIR (Germany) within 4000-400cm^-1. ZEISS EVO 40 EP (Cambridge, UK) with analytical software – Quantax 200 instrument was used to examine the morphological characteristics of algal biomass before and after adsorption of MO dye. Carbon, Hydrogen, Nitrogen, and Sulphur analysis of the adsorbent was done on an Elementar analyze system Vario MICRO CHNS V3.1.1 (GmbH, Germany). The Brunauer-Emmett-Teller (BET) surface area of the adsorbents was measured by a Micrometrics ASAP 2010 Surface area analyzer (England, UK).

Preparation of Adsorbent:

The sample of green algae Chara sp. was collected from running water in Dehradun. In order to remove the filth and undesired materials, the algae were washed thoroughly with double distilled water and then kept on a filter paper to reduce water content. The algal biomass was then Sun-dried for two days followed by drying in an oven at 343 K for 24 hrs. Later the algal biomass was ground on an agate stone pestle mortar and then sieved to obtain a particle size of 100µm mesh size. The powdered adsorbent was then maintained in vacuum desiccators prior to use.

Preparation of adsorbate (Synthetic wastewater):

MO dye used as an adsorbate in this study is an acidic dye having molecular formula.: C₁₄H₁₄N₃NaO₃S, molecular weight.: 327.33g mol-1 and λ_max 463nm. The stock of the dye solution (1g/L) was prepared according to the standard procedure by dissolving the required amount of MO in double-distilled water. Synthetic wastewater containing the desired concentration of dyes was prepared by diluting the above-prepared stock solution with a suitable volume of distilled water. The initial pH was adjusted with 0.1M HCl and 0.1M NaOH solution using a digital pH meter calibrated with standard buffer solutions.

Adsorbent Characterization:

The surface area of the adsorbents was determined by the BET method¹⁹ and the surface morphology after gold coating was studied using Scanning Electron Microscope (SEM) images. To examine the percentage composition of C, H, N, and S elements, elemental analysis was carried out and the functional groups present at the surface of the adsorbent were identified by Infrared analysis.

Batch adsorption studies:

Adsorption experiments were conducted in batch mode to investigate the effects of various process parameters such as contact time (0-140 min), pH (1-12), adsorbent dose (1-10mg/L), and temperature (298, 308, and 318K) on the adsorption of MO. The experiments were performed in triplets and the average values were expressed. Standard deviations were never found to be within ±1.0%. The error bars for the figures were so small as to be smaller than the symbols used to plot the graphs and, hence, not shown.

The adsorption capacity was obtained by the mass balance equation given as:

Where q_e is the adsorption capacity of alga (mg/g), C_oand C_eare the initial and the equilibrium concentration of dye (mg/L), V is the volume of the reaction mixture (L) and M is the mass of adsorbent used (g).

For predicting and comparing the adsorption performance of adsorbent, the equilibrium data were fitted using different isotherm models namely Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R)^20-23. Equilibrium experiments under optimized conditions were performed at three different temperatures (298, 308, 318 K). Two different concentrations (100 and 200 ppm) at 318 K were used to carry out the kinetic studies of adsorption by algal biomass; the extent of adsorption was analyzed at regular time intervals.

Adsorption-Desorption and Regeneration studies:

In order to reduce the secondary pollution and check adsorbent reusability repeated adsorption and desorption experiments were carried out using HCl (acid), NaOH (base), and EDTA (Ethylenediaminetetraacetic acid) (chelating agent). The dried mass was suspended in 50ml of the desorbing agents at 318K for 100 min, for consecutive cycles using the same adsorbent. A single cycle comprises of adsorption followed by desorption. After adsorption, the algal biomass loaded with MO dye was again filtered and reintroduced into the desorption solvent. The amount of dye desorbed was determined using UV-Vis Spectrophotometer. The desorption ratio is given as the amount of dye desorbed to the amount of dye adsorbed multiplied by 100.

RESULTS AND DISCUSSION:

Physiochemical characterization of adsorbent:

To explain the behavior and mechanism of the adsorption process, physiochemical parameters provide essential insights into the adsorption of MO dye. According to the results obtained by the BET method, the specific surface area of Chara sp. was 0.855m²/g. The percentage of major elements C, N, H, and S in the powdered form of algae Chara sp. are 13.55, 0.32, 0.477, and 0.426% respectively. The morphology of algal biomass was characterized using SEM. The massy thick surface of the green alga was observed, and the surface had a number of scattered amorphous particles. The SEM images (Figure 1a and b) of before and after MO dye adsorption showed that rough and irregular algal surfaces are more occupied due to dye molecules being trapped or adsorbed on the surface.

Figure 1. SEM micrographs of (a) Chara sp. before adsorption (b) Chara sp. after adsorption of MO dye

FTIR analysis before and after adsorption was done to depict the various peaks arising due to different functional groups present on the surface of the adsorbent. The major peaks were observed at 3416.05 cm^-12926.42 cm^-1, 2515 cm^-1, 1610.00 cm^-1, 1410 cm^-1, 1096.81 cm^-1 and 873.85 cm^-1. The spectra revealed adsorbent heterogeneity, evidenced by different characteristic peaks with the possible presence of amino, carboxylic, hydroxyl and carbonyl groups. A comparison of FTIR spectra before and after adsorption of MO dye by algal biomass displays significant changes in the peaks (Table 1). These shifting of peaks suggested the dye-binding process taking place on the surface of the algal biomass. It also gives an idea of the mechanism of adsorption, which is dependent on the functional groups, especially the carboxyl and amino groups. Similar results were also reported for the removal of Lanaset Red G by macroalga Chara contraria²⁴.

Table 1: IR adsorption band and corresponding possible functional groups present on algal biomass Chara sp.

Before adsorption of MO (cm^-1)	After adsorption of MO (cm^-1)	Bonds indicative of functional groups
3416.05	3421.61	Carboxylic/OH stretch and N-H stretch
2515.66	2515.31	Phenolic/ carboxylic
1096.81	1144.06	≡C-N<
873.85	873.76	C-N-S scissoring

Adsorption studies:

Various batch studies were performed to investigate the effects of different parameters that affect the adsorption of MO dye on algal biomass such as contact time, pH, adsorbent dose, and temperature. The results are discussed below.

Impact of contact time:

The adsorption efficiency of Chara sp. for MO dye was evaluated as a function of time at two different concentrations (100 and 200mg/L) depicted in Figure 2. The perusal of Figure 2 showed the rapid adsorption rate in the beginning, and it decreased gradually until equilibrium was attained after 90 min. The change in the adsorption rate can be regarded as the availability of a large number of free surface sites during the initial stage and later there was a reduction of sites available for adsorption and an increase in the repulsive forces among adsorbed dye molecules and those present in the solution. Similar results of contact time were also reported for adsorption of Acid Orange 7 dye by soil and Orange –G and Methyl Violet dye onto bagasse fly-ash^25,26.

Figure 2 Effect of contact time on adsorption of MO dye on Chara sp.

Impact of solution pH:

Dye adsorption is a pH-dependent process. The pH of the solution influences the properties of algal biomass affecting the adsorption mechanism and dissociation of the dye molecules. In the present study, the effect of pH on the adsorption of MO onto algal biomass was studied at pH ranging from 1 to 12. Figure 3 shows that at acidic pH (2), the adsorption of MO dye was maximum. This could be understood by the fact that at pH 2, a considerable high electrostatic attraction exists between the positively charged surfaces of the adsorbent, due to the ionization of the functional group of adsorbent and negatively charged anionic dye molecules. A similar trend was observed in the adsorption of dyes like Acid Blue, 290, and Acid Blue 324 by green algae Spirogyra sp²⁷.

Figure 3 Effect of pH on adsorption of MO dye on Chara sp.

Impact of adsorbent dose:

Keeping the fixed value of pH, contact, time, and temperature the adsorption experiment was carried out at various adsorbent doses (1-10g/L) (Figure 4). The study in Figure 4 shows the effect of adsorption dose on the adsorption of MO dye. It was observed that with the increase in adsorption dose, the adsorption capacity also increased until it reached a maximum value, after that on the further increase, the adsorption capacity decreased. This is due to the reduction of total adsorbent surface area as there was an increase in particle interaction and aggregation. A comparable tendency was also observed on the removal of Methyl Orange by waste materials²⁸.

Figure 4 Effect of adsorbent dose on adsorption of MO dye on Chara sp.

Impact of temperature:

Figure 5 shows the adsorption of MO dye on Chara sp. at three different temperatures (298, 308, and 318K). The maximum dye uptake was acquired at 318K, explaining the endothermic nature of adsorption. The perusal of Figure 5 indicates the maximum adsorption increased on raising the temperature from 298 to 318K. This phenomenon can be understood by the fact that the increase in temperature decreases the boundary layer thickness of the adsorbent which leads to the increase in the availability of active sites and porosity, and also increases the kinetic energy of dye molecules. Similar behavior was also reported for the biosorption of Reactive Red 120 on Chara contraria²⁹.

Figure 5: Effect of temperature on adsorption of MO dye on Chara sp.

Adsorption Isotherms:

The adsorption isotherms are significant in the design of the adsorption process³⁰. The values of the isotherm models constants at three different temperatures (298, 308, and 318K) and their correlation coefficient are presented in Table 2.

The Langmuir isotherm explains the uniform adsorption with no cross-migration of adsorbate on the surface having a finite number of adsorption sites that are monolayer adsorption²⁰. To ensure equilibrium conditions the linear form of the Langmuir isotherm model, equation (3) was applied to the experimental data as:

where q_e is the amount adsorbed (mg/g), C_eis the equilibrium dye concentration of adsorbate i.e dye (mg/L), Q₀is the Langmuir constant related to maximum monolayer adsorption capacity (mg/g) and b is the constant related to free energy. The values of these constants were calculated by plotting a graph of 1/q_eversus 1/C_e(Table 2). Dimensionless separation factor, R_L, can be calculated from Langmuir constant, b, using below equation:

The values of R_L were computed at different temperatures, and the nature of the adsorption isotherm depends on the criterion that if the R_L value is greater than 1 then it is considered to be Linear, if the value of R_L falls between 0 and 1 it is a favorable and in case the value is equal to 0 it is considered to be irreversible. In this case, the R_L value is found to be less than unity and confirms the favorable behavior of the adsorption process.

The Linearized form of Freundlich isotherm is given as:

Where K_Fis a Freundlich constant (mg^1-1/nL^1/ng^-1) and n is a dimensionless factor related to adsorption intensity and surface heterogeneity. The plot between ln q_e versus ln C_e was drawn and the intercept and slope were used to calculate the value of K_Fand n. The magnitude of K_Fincreased with increasing temperature value from 8.13 mg g^-1 to 16.11 mg g^-1 and showed easy adsorption of dye molecules on the algal biomass (Table 2). The high values of n (2.09-2.78) indicated high affinity between the solute molecules and the adsorbent and the correlation coefficient values indicate heterogeneous surface conditions.

In order to find certain indirect adsorbate-adsorbent interactions on the dye adsorption and that the fall in the heat of adsorption is linear rather than logarithmic, Temkin isotherm was modeled. Temkin model is given by:

Where R is the gas constant, and T is the absolute temperature in Kelvin. From the slope and intercept of q_e versus ln C_e curve, the constants b_T(g kg mg^-1 mol^-1) and A_T(L mg^-1) associated with the heat of adsorption were calculated. The low value of regression coefficient (R² ) and A_Tvalues, indicate that Temkin isotherm poorly describes the adsorption of MO dye on Chara sp. (Table 2).

The Dubinin and Radushkevich (D-R) isotherm were studied to estimate the porosity and apparent free energy. The linear form of the D-R isotherm equation is given as:

where q_mis maximum adsorption capacity(mg/g), β is the constant related to adsorption energy (mol²kJ^-2), ε is a Polanyi potential, and R is the gas constant (8.314Jmol^-1K^-1), T is the temperature(K). The mean adsorption energy can be calculated from the formula

Celekli et al reported earlier that, the adsorption process is chemically controlled, if the value of E falls between 8kJ/mol and 16kJ/mol, and physically controlled if the value of E is < 8 kJ/mol²⁶. In this study, the calculated value of E is lower than 8 kJ/mol, which confirms that the adsorption of MO on to test alga is physical adsorption.

Table 2 shows the value of the constants and the correlation coefficients for all four isotherms. It can be observed from the table that good monolayer adsorption capacity of 119.04 mg/g at 318K is obtained from Langmuir Isotherm. Also, the correlation coefficient values were high at different temperatures for Langmuir Isotherm followed by the Freundlich model and then Temkin and D-R model.

Table 2: Isotherm parameters for the adsorption of MO dye onto algal biomass (Chara sp.)

Isotherm Parameters	Methyl Orange
Isotherm Parameters	298 K	308 K	318 K
Langmuir Isotherm
b (L mg^-1)	0.0313	0.0352	0.0357
q_e (mg g^-1)	109	114.94	119.04
R²	0.990	0.991	0.996
Dimensionless separation factor
R_L	0.008	0.725	0.008
Freundlich isotherm
N	2.096	2.447	2.780
K_F (mg g^-1)	8.133	11.553	16.119
R²	0.939	0.939	0.963
Temkin isotherm
A_T	0.88	1.028	0.998
b_T	68.661	73.554	82.615
R²	0.904	0.956	0.957
D-R isotherm
q_m ( mg g^-1)	123	135	176
E (kJ mol^-1)	0.091	0.100	0.112
R²	0.957	0.951	0.988

Thermodynamics study:

Thermodynamic variables such as standard free energy changes (ΔG^o, kJ/mol), enthalpy (ΔH^o, kJ/mol), and entropy (ΔS^o, kJ/mol/K), were studied to understand the feasibility of adsorption of MO dye on algal biomass at three different temperatures (298, 308, 318 K) using the following equations.

Where b₁ and b₂ are Langmuir constants at different temperatures and other terms have their usual meanings. The negative value ΔG^o reflects the feasibility of adsorption and vice-versa, whereas positive and negative ΔH^o values show the endothermic or exothermic nature of adsorption respectively. Similarly, positive or negative ΔS^o gives an idea about the increase or decrease in randomness. The values of these parameters are compiled in Table 3.

In the present study, the decrease in ∆G^o values with increasing temperature explains an increase in the feasibility and spontaneity of the adsorption at a higher temperature. The positive ∆H^ovalue clearly explains the endothermic nature of adsorption. The increased randomness at the solid-solution interface during the fixation of these dyes on the active sites of the adsorbents is indicated by the positive value of entropy (∆S^o). Similar results were shown by other algae for the removal of other dyes²⁸ where the entropy increased with temperature.

Table 3: Thermodynamic parameters for the adsorption of MO dye onto Chara sp:

Dye		298 K	308 K	318 K
Methyl Orange	ΔG^o	-22.880	-23.952	-30.713
	ΔS^o	0.094	0.096	0.1129
	ΔH^o*	5.210

*ΔH^o Measured between 298 and 318K

Kinetic studies:

The kinetic studies are necessary to optimize the different operating conditions for the adsorption³¹ and the simplest kinetic models that are used to test the experimental data were pseudo-first-order and pseudo-second-order,^32,33.

The linear form of the pseudo-first-order equation can be expressed as follows.

where q_t(mg/g) is the amount of dye adsorbed by biomass at equilibrium time t, k₁ is the pseudo-first-order rate constant (min^-1). The graph between log (q_e–q_t) versus log t was plotted in order to calculate the constants. The q_e(cal) which is maximum equilibrium adsorption and pseudo-first-order rate constant K₁ for each dye was calculated from intercept and slope respectively. The values of q_e(cal), K_1,and R² are reported in Table 4. The R² values were found to be low (0.926). The values of q_e(cal) did not agree well with q_e(exp) values.

The linear form of pseudo-second-order model can be written as follows.

Where q_e and q are the amount of the dye adsorbed by the algal biomass (mg/g) at equilibrium and time t, respectively, and k₂(g mg^-1 min^-1) is the rate constant of second-order adsorption. q_eand k₂can be computed from the slope and intercept obtained from the plots of t/q versus t. The values of maximum equilibrium adsorption (q_e) and pseudo-second-order rate constant (K₂) for each dye were calculated from the intercept and slope of the plot respectively.

In comparison between the results of various kinetic models as summarized in Table 4, it has been observed that the values of correlation coefficient R² for the pseudo-second-order adsorption model are relatively high, and the adsorption capacities calculated by this model are also close to those determined by the experiments. Therefore, it has been concluded that the pseudo-second-order adsorption model is more suitable for describing the adsorption kinetics of dye on Chara sp. Similar results have earlier been reported by Nemr et al. for the removal of Chrysophenine dye (DY-12) by Ulva Lactuca, green algae³⁴.

Regeneration studies:

Regeneration of adsorbent for repeated use is of crucial importance to make it cost-effective and user-friendly and the adsorbent should regenerate to reuse for further dye adsorption. In order to show the reusability of the adsorbent, the adsorption-desorption cycle of the dyes was repeated five times using the same adsorbent. Experiments conducted explain that HCl is a better reagent for desorption in comparison to various other reagents used. The adsorption capacity of the tested alga did not noticeably change (only a maximum 20-25% change was observed) during repeated adsorption-desorption operation. Stirk and Staden found that acids are more effective for desorbing as compared to other eluents³⁵.

Desorption efficiency decreases with an increase in the number of cycles due to the decrease in the adsorption capacity as shown in Figure 6. Hu and Shipley concluded that due to the stronger chelating property of EDTA that made it was difficult to reverse adsorption after continuous cycles both adsorption and desorption decreased with repeated regeneration cycles³⁶. Thus, the reuse of biomass is an important feature for its possible utilization in continuous systems in industrial processes.

Figure 6. Adsorption/Desorption cycles of MO dye on Chara sp. using HCl

CONCLUSION:

The test alga Chara sp for Methyl Orange dye adsorption, showed a good maximum adsorption capacity of 119.04mg/g at optimized conditions of pH 2, contact time 90 min, dose 3g/L, and 318K temperature. The Langmuir isotherm was found to be the best fit with the equilibrium experimental data followed by Freundlich, Temkin, and D-R isotherm. Thermodynamic parameters showed that the adsorption of MO dye onto algal biomass was feasible, spontaneous, and endothermic under studied conditions. Higher regression coefficient value for second-order kinetics suggested that the adsorption data followed pseudo-second-order kinetic in the case of MO dye and intraparticle diffusion was not the only rate-limiting step. The interaction between the dye molecules and the functional groups on the cell wall surface of the biomass was confirmed by FTIR analysis, which indicated the participation of carboxylic, amino, amide, and hydroxyl functional groups in the dye adsorption. The adsorbent was reused for five consecutive adsorption and desorption cycles with a negligible decrease in their adsorption capacities. This study demonstrated that the biomass of Chara sp. has the potential to be used as efficient biomass for the treatment of MO dye-containing wastewater.

ACKNOWLEDGEMENT:

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

CONFLICT OF INTEREST:

The authors declare no conflicts of interest.

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Received on 13.04.2022 Modified on 25.06.2022

Asian J. Research Chem. 2022; 15(5):319-326.

DOI: 10.52711/0974-4150.2022.00057

Dye	Initial dye Conc. (mg L^-1)	q_e(exp) (mg g^-1)	First-order model			Second-order model
Dye	Initial dye Conc. (mg L^-1)	q_e(exp) (mg g^-1)	K₁(x10^-3min^-1)	q_e(cal) (mg g^{- 1})	R²	K₂ x 10^-3(g mg^-1min^-1)	q_e(cal) (mg g^-1)	R²
MO	100	94.94	0.690	79.588	0.896	0.122	112.35	0.998
MO	200	112.5	1.1515	93.35	0.926	1.127	117.64	0.995