Introduction

This decade is marked with the gradual advancement of technology and ever-increasing industrial activities leading to water pollution caused by discharge of industrial effluents1. Contamination of water due to synthetic pigments is a major issue, as they are mutagenic and carcinogenic in nature2,3. Approximately 70% of dyes used around the globe are azo dyes containing single or multiple chromophore groups4. According to Ecological and Toxicological Association of Dyestuff Manufacturing Industry (ETAD) the highest rates of toxicity (LD 50) are discovered among azo dyes5. These dyes are stable to optical, physiochemical, thermal and microbial effects due to their aromatic structures6,7.This persistence of dye- laden effluent imparts distinguishable coloration to water bodies, restraining the passage of sunlight and interfering in the process of photosynthesis, thereby retarding the growth of aquatic flora8.

Thus, various methods are employed that can sequester dyes from polluted water, including flocculation/coagulation9, biodegradation10, photodegradation11, ion-exchange12, membrane filtration13and adsorption14. Among these methods, adsorption is most favorable due to its economic feasibility, insensitivity to pollutants, simplicity of design, no sludge production, and performance15. The process involves the transfer of mass and accumulation of materials at the interface of solid-liquid phase. The dye (liquid phase) from aqueous solution is attracted and retained onto the surface of biodegradable adsorbent (solid phase) via intermolecular or intramolecular interactions16. Therefore, the choice of suitable adsorbent for sequestration of dye is of paramount importance for effective adsorption process without producing secondary pollutant.

Numerous singular source adsorbents have been studied due to their specific properties. However, their wide range applications are limited due to chemical specificity, versatility, cost efficiency, and efficacy17. Hashem et al. used Syzygium cumini bark for removal of chromium18, Tirkey et al. used Syzygium cumini leaf ash with calcium carbonate for removal of fluoride19. However, there is limited literature available utilizing Syzygium cumini leaf powder as a part of bio-composite for dye removal application. Extensive research has been carried out to prepare cost-effective adsorbents which demonstrate exceptional affinity for binding and capturing hazardous coloring matter from contaminated wastewater20. This pursuit to provide low cost, sustainable dye removal has led to the utilization of discarded agricultural and industrial wastes21. Designing protocols for discoloration of phthalocyanine dyes such as biodegradation, Fenton, TiO2/UV, and photo-Fenton processes have been discussed in literature, but degradation points to low efficiency of these techniques22,23. Therefore, adsorption provides the simple and cost-effective alternative.

In this experimental study reactive copper phthalocyanine dye, Remazol Turquoise Blue G-133 was studied. Reactive phthalocyanine dyes are derivatives of reactive dye and are crucial due to their tinctorial strength, excellent hue, wet fastness and chemical stability24. Sodium alginate was used as a substrate for composite material due to its availability, tunability, biodegradability, biocompatibility and non-toxicity25. It is an effective anionic biopolymer26containing carboxyl (-COOH) and hydroxyl (-OH) groups distributed throughout polymeric chain27. Its ability to form water insoluble gel by ionotropic gelation allows it to encapsulate macromolecular agents by forming beads with potentially large number of active sites27,28. Therefore, friendly remediation of nanocomposite material was fabricated in the form of beads for separating Remazol Turquoise Blue G-133 from wastewater. The treated biomass was obtained from Syzygium cumini leaves powder, in which cellulose is chosen as a model component with its simplest unit comprising of β-1,4-glycosidic linkage29. It was incorporated into sodium alginate with granite. The resulting biopolymer was characterized using SEM, FTIR, and XRD, furthermore adsorption efficiency was determined by optimizing the adsorbent dosage, contact time, and pH. The kinetic, isotherm, and thermodynamic studies were carried out and practical efficiency was tested through desorption studies.

Materials and methods

Preparation of granite samples

The granite sample was divided into calcined and non-calcined material. For calcination, the samples of granite were placed in muffle furnace (range 30–3000 °C) at 750 °C for 4 h. The calcined and non-calcined granite samples were further converted into (i) water paste then calcined in muffle furnace for 4 h at 750 °C, and (ii) treated with excess water then filtered and air dried, half of dried sample was stored while the other half was again calcined in muffle furnace for 4 h at 750 °C. A total of 8 samples were obtained due to this physical modification. The overall process is presented in Scheme 1. The adsorbent was prepared using Syzygium cumini leaf powder, with and without granite samples by the above-described method.

Preparation of GR-SC@SAB and adsorption studies

Granite powder obtained from marble factory was sieved and pretreated in oven at 80 °C to remove moisture. 100mL water was heated to dissolve Sodium alginate (2.0 g) with constant stirring. After the solution cooled down, CPC granite (1.0 g/100 mL) and Syzygium cumini leaves powder (1.0 g/100 mL) were added and stirred until a homogenous mixture of solution was obtained. To obtain uniform beads of immobilized of GR-SC@SAB, the above mixture was dropped into 0.1 M CaCl2 solution using burette. The beads were washed with distilled water and stored in 0.05 M CaCl2 solution at 10 °C30. Later, sample was oven dried in conventional over to be used in further studies.

Dye solutions were prepared at range of concentration from 5 to 50 ppm at pH 6. 10 mL of dye solution was filled in each test tube with 0.02 g of GR-SC@SAB at the speed of 120 rpm for 3 h31. For thermodynamic studies 30–50 °C temperature was employed at 10 ppm concentration using above mentioned conditions. The characterization studies were conducted using Hitachi S-4500 FE-SEM, Agilent Cary 630 FTIR, 721D UV/Visible spectrophotometer, and Shimadzu XRD-6000 for XRD analysis. Conventional oven was used and shaking incubator model IST-3075 was used during adsorption experiments.

Scheme 1
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Flow sheet of Granite sample preparation.

Results and discussion

Percentage removal of RTB using different granite samples

Granite samples in aqueous solution of dye were shaken at 120 rpm shaking speed for 180 min. For dye, the % removal for these eight granite samples were measured and the highest % removal was calculted for CPC granite, as indicated in Fig. 1. Therefore, CPC granite was used for preparation of GR-SC@SAB adsorbent.

Fig. 1
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Graphical representation of RTB dye % removal before and after addition of eight granite samples.

Effect of adsorbent dosage, contact time and pH

Adsorbent dosage, contact time of adsorbent and adsorbate and pH of solution are crucial factors for determining the optimum dye uptake percentage32. It was noted that by increasing the amounts of Syzygium cumini leaf powder and granite the removal efficiency improved, as indicated in Fig. 2a. At 1.0 g the adsorbent achieved optimum removal efficiency for both leaf powder and granite. Therefor, 1.0 g of Syzygium cumini leaf powder and 1.0 g of granite was used in preparation of GR-SC@SAB along with 2.0 g of sodium alginate. It was noted that by increasing the contact time of adsorbent and dye the removal efficiency was improved. Figure 2b indicates that the adsorption efficiency observed steady increased till 180 min until equilibrium was achieved. pH also effects the removal efficiency of dyes, therefore, a pH range of 3–9 was studied, as indicated in Fig. 2c. The small values of removal efficiencies at lower pH have been attributed to the presence of solvated protons which occupy the active adsorption sites and hinder adsorption. There was a sharp rise in dye removal from pH 3 to 6 attributed to decrease in the concentration of hydrogen ions and activation of surface functional groups which helps in binding the GR-SC@SAB with the adsorbate which increases adsorption efficiency33.

Fig. 2
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(a) Optimizing adsorbent dosage by varying the amount of Syzygium cumini leaves powder and CPC Granite on elimination of dye, (b) influence of contact time by varying contact time of adsorbent with RTB dye, and (c) influence of pH on dye removal.

Physical characterization of biodegradable adsorbent

SEM images of the surface of unmodified GR-SC@SAB adsorbent reveals irregular heterogenous surface with deep ridges and cavities. SEM image in Fig. 3a and b shows rough, crease and coarse surface. The presence of cervices and cracks on the outer surface facilitates the diffusion and entrapment of RTB molecules within surface cavities, thereby enhancing adsorption capacity. These observations align with the morphological research studies reported in literature34,35,36,37. The IR spectrum in Fig. 3c shows presence of several functional groups such as −OH, −CH3 and −C = O. The peak at 3250 cm− 1 indicates −OH bond stretching due to Si−OH and Al−OH in granite38. The indication of peak at 2950 cm− 1 is due to stretching vibrations of carbonyl functional groups. The peaks at 1750 cm− 1 can be assigned to the asymmetric stretching vibrations of carboxylate group. The peak around 1070 cm− 1 correspond to −C−O−C− bonds in the anhydro-glucose unit of cellulose. The UV-Visible spectrum of RTB dye reveals λmax at 620 nm, as shown in Fig. 3d.

The XRD spectrum of GR-SC@SAB of biodegradable adsorbent in Fig. 3e reveals two sharp peaks at 8.56° and 20.39°. The alginate derived peak (8.56°) confirms the successful formation of a stable, cross-linked bead matrix that immobilizes the other components. The peak at 20.39° is classical signature of (002) crystallographic phase of cellulose. It provides the evidence for the crystalline reinforcing scaffold contributed by biomass, which creates the rough, porous morphology observed in SEM. The broad hump after 40.8° verifies the incorporation of amorphous granite particles within the matrix. This elucidates semi-crystalline nature of adsorbent advantageous for adsorption as the amorphous network provides a high density of functional groups and porous pathways while the crystalline domain offers structural stability to the biodegradable adsorbent. This combination enhances the capacity of GR-SC@SAB to bind RTB dye molecules. Literature studies have also reported similar results39,40. The chemical structure of RTB dye is given in Fig. 3f. The presence of light blue color in the aqueous solution of RTB is attributed to the presence sulfonate groups (−SO3−) anions in the structure of reactive dye38, as indicated in inset of Fig. 3e.

Fig. 3
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(a,b) SEM, (c) FTIR of GR-SC@SAB, (d) Spectrophotometric analysis of RTB dye before treatment, (e) XRD of GR-SC@SAB, and (f) Chemical structure of RTB inset shows RTB solution after adsorption by GR-SC@SAB.

Kinetic studies

To gain an insight into the mechanisms and underlying factors responsible for dye adsorption, kinetic models were fitted to the experimental data, as indicated in Table S1. The kinetic studies revealed that overall adsorption rate of RTB on the surface of GR-SC@SAB is exclusively dependent upon the adsorption rate of dye on the surface of modified adsorbent, and external mass transport whereas intraparticle diffusion is negligible. The plots of non-linearized form of Pseudo First Order (PFO), Pseudo Second Order (PSO), Elovich Kinetic Model (EKM) and Intraparticle Diffusion Model (IPDM) are compared in Fig. 4a. The non-linear regression coefficient and reduced χ2 comparison of these models in Table 1 indicates the following pattern of model fitting: PSO > PFO> EKM> IPDM. The equilibrium constant K2 for pseudo-second order is also greater than pseudo-first order. This experimental data is supported by sorption kinetic studies conducted by other researchers41. The results predict that the interaction between RTB and GR-SC@SAB was chemisorption42. PFO, on the other hand is indictive of physisorption. Unlike the first two kinetic models Elovich model assumes that rate of adsorption changes overtime. It takes adsorption as complex phenomenon, predicting decrease in rate of adsorption with passage of time, as sites get occupied. It is useful in cases where adsorption is not merely physical as in present case. The comparative literature study to analyze rate constants of various adsorbents have been summarized in Table S3.

Table 1 Values of kinetic parameters involving RTB adsorption onto GR-SC@SAB.
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Isotherm modeling

The isotherm curves, to predict the relationship between the effect of initial concentration of RTB and GR-SC@SAB are depicted in the Fig. 4 (b-d) based on the isotherm model fittings given in Table S2. It can be inferred that the ability of synthesized adsorbent to remove dye, is proportional to the initial concentration of RTB dye until equilibrium is achieved. In this regard, mass transfer can be considered as primary driving factor due to increase in concentration of dye, which enables adsorbent to better adhere to dye43.

Langmuir isotherm among two parameter, Khan isotherm among three parameters, and Marczewski-Jaroneic among four parameters on basis of values of correlation coefficient R2 larger than other models and reduced chi-square χ2 better fits the adsorption data. The Qm values of all the isotherms is listed in the Table 2. The highest value of correlation coefficient (R2 = 0.9975) and low value of reduced chi-square (χ2 = 0.0148) for Langmuir model, signifies monolayer adsorption of RTB on the homogenous surface of GR-SC@SAB. Similar results were reported by Islam et al.44. According to Freundlich isotherm, the model exponent (1/nF=0.4192) was found to be less than 1, indicating that sorbent has good adsorption capacity. In case of Temkin isotherm where binding energies are uniformly distributed, the values of constant (BT=2.4067 J/mol) indicates that as heat of adsorption decreases with increasing surface coverage, it does so at moderate level. The process of adsorption can lead to the linear decrease in adsorption energy overtime. Jovanovic isotherm assumes negligible interactions between adsorbent/adsorbate. The model implies that finite number of adsorption sites are available and once they are occupied by adsorbate, no further adsorption can occur. The comparative analysis of Qm in Langmuir and Jovanovic isotherm, reproduces that Qm in case of Langmuir isotherm is greater. The corresponding R2 as well as reduced χ2 further verifies the assumption. Overall, the order of model fitting can be predicted as: Langmuir> Freundlich> Temkin> Jovanovic.

Table 2 Values two- parameter isotherm for DB HS-RL adsorption onto GR-SC@NAB.
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As the adsorption data shows an indigenous shift from Langmuir to Freundlich isotherm therefore, to understand the complex adsorption system three parameter isotherms were studied in this research. These isotherms reduce either to Langmuir or Freundlich isotherm based on the value of model exponent. Equilibrium results were examined for Hill, Khan, Koble-Corrigan, Jossens, Toth and Redlich-Peterson isotherm were used. The Hill model exponent (nH=1.1306) being closer to 1.0, assumes that binding interaction between RTB and GR-SC@SAB occurred in the form of positive cooperativity. Khan adsorption model is particularly useful to describe equilibrium relationship between concentration of adsorbate in liquid phase and adsorbed amount on the solid phase. The model is generalized form of Langmuir isotherm, assuming adsorption site as monolayer where each adsorption cavity can hold only one molecule of adsorbate. A value of nK greater than 1, signifies surface with higher heterogeneity, where some sites have much stronger adsorption energy than the other. In case of Koble-Corrigan isotherm, the value of model exponent (nKC=1.1307) was found to be greater than 1.0, signifying Langmuir characteristics. The Hill isotherm, is based on the binding ability of adsorbates onto adsorbents on basis of homogenous adsorption. Jossens adsorption isotherm considers the distribution of interaction energy between adsorbate-adsorbent on the site of adsorption. The Jossens and Redlich-Peterson have high correlation between parameters. The Redlich-Peterson isotherm, is empirical in nature indicating that it is derived from experimental data rather than theoretical understanding of adsorption process. The βRP values were found to be close to unity (βRP = 1.2557) indicating that adsorption process analyzed in this experiment was primarily due to Langmuir isotherm45. Similarly, the values of Toth model exponent are greater than 1, signifying homogenous characteristics. The isotherm fitting order is as follows: Khan> Redlich-Peterson, Jossens> Koble-Corrigan > Toth > Hill.

Fig. 4
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(a) Kinetic Models (b)-(d) Isotherm Models.

The presence of multiple parameters i.e., two parameters and two model exponents are adequate to explain adsorption behavior on basis of four parameter isotherms. Herein, two adsorption models were studied, both competitive of each other. In summary, Fritz-Schlunder IV showed slightly less fit compared to Marczewski-Jaroneic isotherm. The model is particularly useful for porous materials or materials with high surface irregularities. According to Table 2, the magnitude of parameters AFS/BFS are indicative of adsorptive strength, whereas the exponents αFS and βFS provide information about type of adsorption. If the model exponents are close or greater than unity, the adsorption process conforms to Langmuir isotherm, as indicated in present study. The four parameter Marczewski-Jaroneic isotherm, provides highest value for \(\:{Q}_{{m}_{MJ}}\)(29.0889 L/mg). In contrast, the values of nMJ and mMJ indicates that the given adsorption isotherm can be transformed to Langmuir isotherm.

To gain comprehensive understanding of adsorbate and adsorbent interactions El-Awady (Fig. 5a), Fowler Guggenheim (Fig. 5b), and Frumkin (Fig. 5c) isotherms were studied. The linearized expression of El-Awady isotherm is presented in Table S2. The isotherm parameter KEA is indictive of strength between adsorbent and adsorbate. The parameter 1/y quantitatively analyzes the possibility of multilayer formation. If 1/y > 1 conforms single adsorbate can occupy multiple adsorption sites, whereas 1/y < 1 indicates positive cooperativity and strong adsorbent-adsorbate interactions. As the value of 1/y is less than 1 strong attractive forces exist between molecules.

Frumkin equation otherwise an extension of Langmuir isotherm, indicates that already adsorbed molecules also influence further adsorption either by attracting or repelling the molecules. The interactive forces between adsorbed molecules are characterized by attractive, repulsive and zero interaction based on αF > 0, αF < 0 and αF = 0 where isotherm reduces to Langmuir equation. In present case study, as value of linear regression coefficient indicates that isotherm cannot fit experimental data. Fowler-Guggenheim isotherm parameters are highlighted in the Table 2. The isotherm equation describes whether lateral interactions exist between molecules. The slope αFG determines the strength of interactions in the form of attractive (αFG is negative), repulsive (αFG is positive) and no interaction (αFG is zero). Regardless of good parameter values the regression coefficient indicates poor fit of experimental data. The comparative literature study to analyze adsorption capacities of various adsorbents have been summarized in Table S4 (Table 3).

Table 3 Values for three- four- parameter models and isotherm on basis of surface coverage for RTB adsorption onto GR-SC@SAB.
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Fig. 5
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Linear fitting of (a) El-Awady, (b) Frumkin, (c) Fowler-Guggenheim isotherm, and (d) \(\:ln{K}_{c}\) and\(\:\:\frac{1}{T}\) thermodynamic parameters.

Thermodynamic studies

The adsorption process was studied for a temperature range from 30 to 50 °C. The thermodynamic parameters were calculated from the following equation35:

$$\:\varDelta\:{G}^{o}=\:\varDelta\:{H}^{o}-\:T\varDelta\:{S}^{o}$$
$$\:\varDelta\:{G}^{o}=-RTln{K}_{c}$$
$$\:{K}_{c}=\frac{{q}_{e}}{{C}_{e}}$$

The \(\:\varDelta\:{S}^{o\:}\)and \(\:\varDelta\:{H}^{o}\) values can be calculated from intercept and slope values by plotting a graph versus \(\:ln{K}_{c}\) and\(\:\:\frac{1}{T}\), as indicated in Fig. 5d. The linearized form of van’t-Hoff equation can be written as:

$$\:ln{K}_{c}=\:\frac{\varDelta\:{H}^{o}}{RT}+\frac{\varDelta\:{S}^{o}}{R}$$

The negative values of enthalpy change (ΔH°) given in Table S5, conforms the exothermic nature of adsorption process involving strong physical adsorption (physisorption) or weak chemical interactions (chemisorption)46. The negative value of entropy change (ΔS°) indicates a decrease in randomness at the GR-SC@SAB-RTB interface during adsorption of dye molecules. The negative values of ΔG° at all temperatures indicates the spontaneity and feasibility of the adsorption with the increase in temperature47.

Proposed adsorption mechanism

The process of adsorption depends upon difference in concentration between dye solution and adsorbate. Figure 6 illustrates plausible mechanism for adsorption which involves various types of interactions involving hydrogen bonding, π-bonding, weak electrostatic attractions depending upon type of groups interacting with adsorbate. In present study, various isotherms and kinetic models were used to develop an adsorption mechanism. Each of these models provides a different perspective on the adsorption process and is useful for characterizing various types of adsorption behaviors. The adsorbent molecules of RTB dye adsorb to the surface of GR-SC@SAB adsorbent in the form of monolayer (Langmuir) with varying heat of adsorption (Temkin) accounting for various affinities (Redlich-Peterson, Toth), wherein chemisorption (pseudo-second order) is key factor in adsorption of molecules. Hill isotherm predicted that significant interaction exists between adsorbate molecules on the surface. The affinities between adsorption sites are variable (Toth) which may be attributed to surface irregularities. The rate of adsorption also decreased with time (Elovich).

Fig. 6
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Adsorption mechanism illustrating RTB removal from bulk solution.

Desorption studies

The main goal of desorption studies was to evaluate the regeneration of GR-SC@SAB adsorbent48. The reusability of adsorbent is crucial for practical applicability of adsorbent to enhance the overall efficiency of adsorption process49. Desorption experiments were conducted with 0.1 M HCl and 0.1 M NaOH. Figure 7 summarizes the % desorption efficiencies up to five cycles. The results accomplished that desorption of RTB dye is more favorable in alkaline media using 0.1 M NaOH.

Fig. 7
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% Desorption of GR-SC@SAB studied at 5 cycles 0.1 M HCl and 0.1 M NaOH.

Conclusion

In summary, we successfully designed and applied GR-SC@SAB as biodegradable adsorbent for removing RTB. This hybrid adsorbent was composed of left over agricultural and industrial wastes i.e., cellulosic Syzygium cumini leaves and granite followed by various pretreatments and additionally sodium alginate. The comprehensive characterization of GR-SC@SAB followed by SEM, FTIR, and XRD techniques confirmed the integration of components, yielding a semi-crystalline material with heterogenous porous surface with active functional groups essential for dye uptake. The adsorbent dosage, contact time and pH were optimized and adsorption process was found to be concentration dependent. Kinetic and isotherm modeling revealed adsorption process follows pseudo-second order and dye molecules form monolayer around GR-SC@SAB adsorbent following Langmuir isotherm. Thermodynamic studies revealed the exothermic and spontaneous nature of the process. A key finding in the practical applications is the desorption study of GR-SC@SAB in alkaline media regeneration efficiency of adsorbent up to five cycles. Therefore, GR-SC@SAB, derived from abundant renewable resources exhibits high potential as a sustainable, effective and reusable biosorbent for the remediation of dye contaminated water, contributing to circular economy approach in wastewater treatment.