Alginate-Based Sustainable Green Composites of Polymer and Reusable Birm for Mitigation of Malachite Green Dye: Characterization and Application for Water Decontamination

Abstract

Environmental sustainability appraisal of adsorption for exclusion of the malachite green toxic dye was the center of attention in this work. The influenced goals were to analyze the consequences of novel composites fabricated by sodium alginate with guar gum and birm (SA@GG@B composites) by ion gelation. This work not only explains the feasibility of the sorbent and its application for the removal of dye stuff but also proclaimed various effects of different parameters affecting the removal efficiency. Adsorption processes were carried out in the batch process. The composite was characterized by SEM, which revealed that the irregular surface of composites has pores present for high adsorption, FTIR (for functional groups detection) reveals the presence of –OH group which provides attachment sites for dye, and BET (surface analysis) with a surface area of 5.01 m²/g shows that it has a wide surface area for greater adsorption process. Adsorption was performed on synthetic composites by varying different parameters like contact time, the concentration of sorbent and sorbate, and pH. Maximum adsorption was achieved (92.7%) at 100 ppm initial concentration, 120 min interaction time, and pH 9. Adsorption isotherms (Freundlich, Langmuir, Dubnin, and Elvoich isotherm) were applied in this work and evaluated the adsorption phenomenon and nature of adsorption. Freundlich adsorption capacity K_F (9.45) reveals effective adsorption of dye by the proposed adsorbent. The kinetics models show that it was better with the pseudo-second-order reaction. Effective removal of malachite green by synthesized composites reveals their importance for the industrial water purification from hazardous dyes.

1. Introduction

Green synthesis is versatile for ecofriendly approaches as it relates to natural product utilization, which is effective and economical. The economy of the world relies on the progress of industries. In Pakistan, the textile industry is a major contributor to the country’s exports [1]. It is also a fact that these industries release a huge amount of polluted water that carry different dyes and pigments [2]. Textile dye leftovers cause drastic water contamination, which is either discharged directly into the environment or omitted by industry wastewater-treatment systems, particularly in areas near textile mills [3]. The textile industry uses 125 to 150 L of water for creating approximately 1 kg of fabric product. The textile sector annually consumes 1000 metric tons of dyes with 100 tons of dyes being seeped into freshwater [4]. Malachite green is an azo dye, and it is one of the most toxic dyes used in the textile industry. Azo dyes are chemical molecules having a functional group in the form of RN=NR′, and along with this R and R′ often aryl group attached [5]. These are a significant class of azo compounds, which have the C-N=N-C connection [6]. Azo dyes are commonly employed in the treatment of textiles, leather goods, and some foods. Azo colorants, which are not soluble in water and other solvents, are chemically linked to azo dyes [7]. Malachite green is an illuminating bright green and crystalline solid dye [8]. It is synthesized by condensing a single part of acetic anhydride with two parts of dimethylaniline. This process is carried out in strong concentrated sulfuric acid or zinc chloride [9]. Malachite green is a toxic dye that causes several health issues like eye infection and several skin-related problems [10]. For the exclusion of these harmful toxins, various wastewater treatment methods have been employed [11,12], and several methods have been introduced by scientists [13,14]. In terms of efficiency and ease of construction, the use of biological materials to remove toxins from the water habitat is thought to be superior to other strategies [15]. Pollutants accumulate on the adsorption material’s surface, which is a surface phenomenon [16]. The type of binding depends on the sorbent and the sorbate, although physiosorption or chemosorption are the most common types [17]. Alginate shows high adsorption for the removal of toxins, so it is widely used for making different hydrogels and water-decontamination purposes [18]. Bacterial alginates can be used to make micro- or nanostructures that can be used in medicinal applications [19].

Natural gums, which belong to the polysaccharide family, are often used to increase the efficiency of the remediation of pollutants and colorants. Guar gum has particular importance in this regard, as it may increase the viscosity of solutions, even at low concentrations [20]. Natural gums show hydrophobic behavior and are mostly obtained from plants or bacteria. Because the gum molecules are biological, they have a wide range of linear and branching features with variations in molecular weight and other characteristics [21]. Natural gums hydrolyze to produce a combination of xylose, arabinose, galactose, mannose, and uronic acids and act as a good binder as well as a good adsorbent [22].

The term birm is an abbreviation for the “Burgess iron-removal method.” For the synthesis of birm, manganese salt is impregnated with aluminum silicate to prepare granular filter media of a dark grey color. This media was applied for the adsorption of metal ions from aqueous media [23,24]. Studies performed on birm show its effective use for water purification [25].

To forecast how the sorbent will behave under various experimental circumstances, it is required to establish the correlation of different adsorption conditions [26]. The equilibrium isotherms were used to create this equilibrium correlation. The type of interaction between the sorbent and dye, whether monolayer or multilayer, is shown by this isotherm [27,28].

This study intended to examine the influence of pH, contact time, starting concentration, and temperature on the ability of (SA@GG@B) composites to adsorb MG. Kinetic studies were carried out to study the reaction nature, and application of the linear form of the equilibrium isotherm should establish the appropriate isotherm for the target [14,29]. Synthetic composites were found to be an efficient and green source for the removal of MG from wastewater supplies.

2. Materials and Methods

To assure the precision and correctness of the research findings, all the chemicals used for the synthetic purpose were of analytical grade. By using distilled water and methanol, all equipment was properly cleaned before and after use. Birm clack in granule form was converted to powder form by using a pestle and mortar and was passed through fine sieves to obtain very fine particles.

2.1. Preparation of SA@GG@B Novel Composites

For the synthesis of sodium alginate composites, birm and guar gum were taken in a ratio 10: 10: 1. The solution of each component was made in a separate beaker. In a solution of sodium alginate, a magnetic bar was inserted and it was heated for 10 h over a hot plate. Similarly, birm and guar gum solution was also heated on the hot plate at 90 °C, and after the required time all components were combined and again stirred with the help of a magnetic bead on a hot plate for an hour (Figure S1).

Ion-gelation involves the synthesis of micro- and nanoparticles based on electrostatic interaction [30]. For solidification purposes, the mixture synthesized was taken in a microsyringe and dropped in a 3% calcium chloride solution, the cross-linking process of which produced composites. Solid composites were obtained, which were separated after 12 h from the calcium chloride mixture by filtration. After proper drying of composites, they were washed with distilled water to remove any remains of calcium chloride.

2.2. Adsorbent Characterization

Characterization of the sorbent is required to ascertain the different physical and chemical factors that influence adsorption [31]. Therefore, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to carry out physical and chemical characterization. Similarly, Brunauer, Emmett, and Teller (BET) surface analysis was performed, which provides a specific surface area of m²/g for synthetic composites.

A scanning electron microscope with the JEOL JSM5910 model number was used to analyze surfaces. SEM gives information about the surface morphology of the adsorbent [32,33].

A Fourier transform infrared spectrophotometer was employed to assess the functional groups that were present in the sorbent structure by using an FTIR instrument (model Shimadzu AIM-8800). These functional groups are responsible for the attachment of dye on the sorbent surface, and recognizing these groups can help to identify the type of binding relationship that occurs between the sorbate and the composite surface [34]. The study was conducted by utilizing the diffuse reflectance infrared (DRIFT) technique by mixing the samples with KBr.

In most cases, the gas adsorption data are evaluated by using the BET theory, which yields a specific surface area result that is reported in unit area per sample mass (m²/g). BET is performed by physically adsorbing a gas (often nitrogen, krypton, or argon) onto the specimen surface at a cryogenic temperature, the material’s surface area is calculated (usually liquid nitrogen or liquid argon temperature).

2.3. Batch Process

For this purpose, the parameter-optimization technique was applied, and the following factors were varied to optimize the conditions:

➢

Initial Concentration of the Adsorbate (20–100 ppm),

➢

Amount of Adsorbent (0.3 gm),

➢

PH (3–12), and

➢

Contact Time (30–150 min).

The optimization of parameters is necessary to obtain best-suited conditions for the removal of dye by applying synthetic composites.

2.4. Kinetics and Isotherm Study

Kinetic studies include details on ideal circumstances, sorption mechanisms, and potential rate-limiting phases for batch adsorption systems [35]. The adsorption data was subjected to pseudo-first-order and pseudo-second-order linear kinetics for this purpose. The MG solution was prepared with an initial concentration of 100 mg/L, and 100 mL of this sample was utilized to investigate the impact of contact time (30–150 min) on adsorption. Sorbent (0.3 g) was added to this MG solution, and it was shaken vigorously at 135 rpm. After a specific interval of time, each adequate example was removed and, after filtration, the amount of dye was determined in the remaining filtrate through a UV-visible spectrophotometer.

To investigate the adsorption process and the equilibrium relationship between sorbent and sorbate, an appropriate adsorption isotherm must be designed [36]. For many sorption systems, isotherms forecast the appropriate parameters and behavior of the sorbent [37]. To study isothermal behavior for the removal of MG through synthesized composites, the initial concentration of the dye was changed (20–100 ppm) by keeping all other parameters constant.

3. Results & Discussion

3.1. Characterization of Adsorbent

In order to study the surface of the composites, SEM is considered a well-suited technique [38]. The surface morphology of birm and guar gum-based sodium alginate composites obtained through SEM indicates irregular, puffy, and rough surfaces available on the composite which enhance the speed of dyes adsorption [39]. Multilayer and intense adsorption can occur on the surface of the sorbent due to its rough, protruding surface, which is possible as opposed to a smooth surface. These cavities come in various sizes (Figure 1). Magnesium oxide composites (Birm) covered with trapped plastic in significant proportions appear in SEM. The SEM results are consistent with the data that was presented.

The attachment of different dye molecules on the surface of the composites can be evaluated by FTIR analysis. Spectra given in Figure 2 shows different functional groups that are present on the surface of the composites before and after adsorption [40].

FTIR was obtained in the range of 4000–550 cm⁻¹. Here, a broad band is identified in the range of 3200 cm⁻¹ to 3950 cm⁻¹ which appears due to the presence of the –OH group. Because the width of the –OH group band in this region indicates the presence of hydrogen bonds in these compounds, this is caused by the stretching vibration of –OH hydroxyl functional groups, including hydrogen bonds. A decrease in the intensity of this band is observed after the attachment of dye to the composites. As –OH is one of the important groups responsible for the attachment of dye molecules on the sorbent [38]. A C–H band appears in the region between 2910 cm⁻¹ to 2922 cm⁻¹. C–O show presence in 1303 to 1350 on both adsorbents. Peaks at 1791 and 1784 are due to the presence of the C=O group. C=C group can be detected at 1616–1622 cm⁻¹. Peak intensity varies after adsorption of the dye which shows that functional groups present on the surface of the composites are occupied by the sorbent molecule.

A clean composite made up of sodium alginate, guar gum, and birm underwent BET, as shown in Figures S2 and S3. The type II isotherm with an H4 hysteresis loop in the N₂ adsorption isotherm is seen due to the presence of metal salts in birm, which is a major part of composites. Then, 5.192 m²/g of surface area and 0.452 nm of pore size, as measured by the SF technique, were found (4.45). The pore volume was 0.058 cc/g, whereas the pore size, as calculated by the HK technique, was 0.432 nm. The outcomes and the provided data agree fairly well (

Table 1. Summary of BET.

BET Summary
Slope (1/g)	Intercept (1/g)	Correlation Coefficient	, r C Constant	Surface Area
643.057 1/g	2.730 × 101 1/g	0.997356	24.558	5.195 m2/g

). Studies on pore size and surface area support the adsorption uptake potential of synthetic composites. A large surface area and suits for attachment of the dye on the adsorbent surface. Literature on the adsorption of dyes also supports these findings [38].

Zero-point charge (ZPC) is used to determine the charge on the surface of the adsorbent. To determine ZPC adsorption was recorded in the pH range 3–11 by taking 20 mL of dye solution with 0.1 M concentration and 0.3 g of composites in each flask. A shaking speed of 135 rpm was maintained for 2 h and contents were filtered. The pH of the filtrate was again determined, and the difference in pH before and after adsorption was recorded. A plot of initial pH vs. difference in initial and final pH was recorded (Figure S4) and the slope intersection shows the ZPC (6.5). The value of ZPC shows that composites have a negative charge on the surface [41]. Positively charged dye attached on this negative surface of the composites and efficient uptake of dye took place at equilibrium.

3.2. Adsorption Studies

To find the ideal conditions for eliminating MG from aqueous solutions, adsorption tests using the batch adsorption method were conducted by adjusting some of factors, including pH, contact time, and initial sorbate concentration.

3.2.1. Effect of Initial Concentration

The initial concentration of dye is an important parameter that describes the adsorption capacity of the synthetic composites. For this purpose, 0.3 gm of adsorbent was added in 20 ppm, 40 ppm, 60 ppm, 80 ppm, and 100 ppm solution in a conical flask, and the orbital shaker was maintained with 135 rpm at a constant temperature. After the required time interval, all solutions were filtered and the remaining amount of dye was examined by a UV-visible spectrophotometer and compared with standard solutions of dye. Results given in Figure 3 indicate that maximum adsorption of dye was achieved with 100 ppm dye solution (92%). Adsorption continuously increases with an increase in the amount of dye showing that maximum sites are available on the surface of the adsorbent for the attachment of dye molecules [42].

3.2.2. Effect of pH

We kept all parameters the same and performed the experiment for pH by changing pH in the range of 3 to 12. The whole process was repeated to determine the maximum absorbance. The effectiveness of the adsorption is significantly influenced by the pH of the solution. For the current study, the solution’s pH range is set from 3 to 12. The adsorption action has been significantly influenced by varying pH values. As malachite green is a basic dye, it shows a high removal effect in acidic media as given in Figure 3, which shows an approximately 92% removal of malachite green basic dye under this basic condition.

3.2.3. Effect of Contact Time

To determine the optimum time required to obtain the best adsorption, the whole process was repeated by varying time intervals from 30–150 min by keeping all other parameters constant (Figure 3). The results obtained show that by increasing contact time adsorption also increases to 120 ppm, but a further increase in contact time at 150 min of adsorption was found to decrease. This may be due to the reason that all active sites are occupied at that time, and a further increase in time has no significant effect in terms of adsorption [43].

3.3. Kinetic and Isothermal Analysis

Adsorption rate, efficiency, and influencing factors, as well as an analysis of adsorption mechanisms, can all be determined through the study of adsorption kinetics. To fit experimental data for adsorption, pseudo-first-order kinetic equations and pseudo-second-order kinetic equations are typically used (Equations (1) and (2)). We have

$${\ln (\mathrm{Q}}_{\mathrm{e}}{-\mathrm{Q}}_{\mathrm{t}}{)=\ln (\mathrm{Q}}_{\mathrm{e}}{)-\mathrm{k}}_1\mathrm{t}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} \mathrm{Pseudo} \mathrm{First} \mathrm{Order}$$

(1)

$$\frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{\mathrm{Q}}_{\mathrm{e}}{}^2}+(\frac{1}{{\mathrm{Q}}_{\mathrm{e}}}) \mathrm{t}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} \mathrm{Pseudo} \mathrm{Sec}\mathrm{ond} \mathrm{Order},$$

(2)

where Q_e and Q_t are MG adsorption capacity at any time, respectively (mg/g) and t (mins) is the adsorption time.

• k₁ is the rate constant of pseudo first-order reaction.
• k₂ is the rate constant of pseudo second-order reaction.

From the curves log(Q_e − Q_t) and t or (t/Q_t) and t, the appropriate parameters of the pseudo-first-order dynamics model can be calculated. When the log(Q_e − Q_t) is plotted against the time interval for the pseudo-first-order kinetic model, the value of k is determined from the slope of the line and Q_e from the intercept (Figure 4).

Second-order kinetics are applied at small initial concentrations to determine the initial adsorption rate. Various forms of linear pseudo-second-order reactions are discussed below. Figure 4 shows the adsorption rate of the adsorbent and four different types of pseudo-second-order kinetic models which were applied to the results. Type 1 gives a high coefficient of determination with R² value of 0.958. Results reveal that the adsorption of MG follows Type I of pseudo-second-order kinetics.

The equilibrium isotherm, which represents the quantity of solute adsorbed per unit weight of the sorbent, can be used to design the adsorption system. The equilibrium concentration of the sorbent at constant temperature is used by the isotherm. The adsorption isotherm is a specific design that is optimized to generate the correct correlation to the experimental data to remove the effluent from the system. In this context, many isotherms based on adsorption systems have been proposed in the literature, including Langmuir, Freundlich, Dubnin, and Elovich.

The Langmuir adsorption isotherm is based on the monolayer adsorption theory, and it makes use of the constant energy assumption. The initial concentration was adjusted from 10 to 100 ppm with 0.3 g of sorbent and an hour of agitation time to compute the Langmuir pattern. Equation (3) is used to calculate the distribution of ions between the liquid and solid surfaces. C_e/C_ad vs. C_e was plotted to produce the Langmuir isotherm (Figure 5). The R² values for the plots were deemed adequate and showed that the model was appropriate for adsorption studies. We have

$$\frac{{\mathrm{C}}_e}{{\mathrm{C}}_{ad}}=\frac{1}{{\mathrm{bQ}}_{°}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{Q}}_{°}}$$

(3)

The Langmuir constant, b, and Q_o stand for dye ion absorption per unit mass of adsorbent (mg/g). The starting concentration determines whether the model is appropriate for a given system and is used to derive the dimensionless constant RL by using the Langmuir constant. The system is deemed appropriate for adsorption applications if the RL value is in the range of 0 and 1. This model is applicable for this work because the experimental data and forecasts obtained for it are closely related to the low sum of squares of residuals (0.618) value.

The concept of multilayer adsorption on the surface of the sorbent is provided by the Freundlich adsorption isotherm, which was created for the heterogeneous system (Figure 5). By using its linear form, parameters for the Freundlich isotherm are derived by plotting log C_ad versus log C_e. The intercept and slope yield the K_F and n values, respectively. An indication of a system’s suitability for adsorption is its Freundlich adsorption capacity (K_F). Adsorption is deemed promising if the K_F value is in the range of 1 to 20, and the findings of the current study show that K_F was 9.45 for the linear approach of the Freundlich adsorption isotherm (

Table 2. Values of constant for adsorption isotherms for the removal of MG.

Adsorption Isotherm	Values of Constant
Langmuir Isotherm	Qօ	b	R2
	47.16	0.186	0.613
Freundlich Isotherm	KF	n	R2
	9.45	4.71	0.96
Dubnin Isotherm	Kad	Qs	R2
	2.522	111.41	0.814
Elovich Isotherm	Ke	Qm	R2
	1.167	79.36	0.969

). Similarly, if the value of n is greater than 1, then it shows the model’s suitability for adsorption purposes. The R² value obtained was 0.961. This outcome demonstrates how well-fit this model is.

An empirical model for vapor adsorption on solid surfaces was established, and it is called the Dubinin–Radushkevich isotherm. The adsorption of heterogeneous systems, including solids and liquids, can be studied by its use. Due to the homogenous surface and assumption that there is no absorption potential in the derivative, this model is thought to be more general than Langmuir’s. The information is used to connect ln C_ad to 2, where 2 is the Polanyi potential determined by the equilibrium concentration, natural gas constant, and temperature as given in the equation. We have

$${\ln \left(\mathrm{Cad}\right)=\ln (\mathrm{q}}_{\mathrm{s}}{)-\mathrm{k}}_{\mathrm{ad}}{\mathsf{\epsilon }}^2.$$

(4)

The graph’s intercept is qs, and its slope yields the value of k_ad. The model exhibits good nonlinear applicability to the adsorption system and has a high R² value. Numerous intriguing uses of the Dubinin–Radushkevich isotherm for figuring out the chemical or physical characteristics of sorption have been discovered. We have

E = 1/√2 kad.

(5)

The value of E calculated was 0.4454 and suggests the physical nature of sorption.

For Elovich adsorption, isotherm C_ad was plotted against ln C_ad/C_e (Figure 5). The intersection and slope of the graph yield K_E and Qm, respectively. Q_m is the adsorption constant, whereas K_E denotes the initial adsorption rate. The linear version of the Elovich model’s initial sorption rate yields: (1166) and R² value of 0.969 suggests linearity. Results reveal high initial uptake of the adsorbent and are in good agreement with reported data [44].

3.4. Mode of Adsorption on Composites

Malachite green carries a positive charge on the surface due to the positively charged nitrogen present in its structure. This positive charge makes it feasible to attach on the negative surface of the composites through electrostatic attraction so maximum adsorption is achieved as shown in Figure 6. A similar mode of interaction of malachite green is also reported in previous studies [45]. ZPC also reveals that composites carry a negative charge at the equilibrium step which supports dye attachment on the surface.

3.5. Reusability of Adsorbent

Synthetic composites can be regenerated and reused more than one time with good adsorption, as shown in Figure 7. To regenerate, used composites were thoroughly washed with methanol at 40 °C for 12 h. After desorption, it was applied for removal of MG and adsorption was found at 90.14% and the second reuse give adsorption of 88.34%, respectively. Results reveal that synthetic composites can be used many times with a quite significant rate of dye uptake.

4. Conclusions

Adsorption is an attractive and promising phenomenon for the removal of harmful and toxic dyes from water. In this work, we study the malachite green exclusion process by employing newly synthesized composites of sodium alginate and birm. SEM and FTIR were employed for characterization and show active site availability and functional group presence for attachment of dye molecule on the surface of composites. The adsorption phenomenon was found promising with 92.7% adsorption of dye. Four adsorption isotherms, including Freundlich, Langmuir, Dubinin–Radushkevich, and Elovich were employed to analyze the experimental data and results suggest the physical nature of adsorption. The Freundlich adsorption capacity K_F (6.17) indicates efficient adsorption of the dye on the synthetic sorbent. Adsorption study can be done along with kinetics, which shows how a pseudo-second-order reaction can be fitted in this work (R² = 0.958). Sodium alginate with guar gum and birm is found an excellent choice for the elimination of toxic dyes from aqueous media. Removal of dyes by the synthetic composite of sodium alginate, guar gum, and birm is yet not reported to our knowledge, so this study shares a significant contribution toward dye remediation from wastewater by employing these novel composites.