Cross-linked chitosan/marble powder composites for the adsorption of Dimozol Blue

Several industries such as textile, food, cosmetic, pharmaceutical products, rubber, leather, and so on use dyes for their products. Industrial wastewater contaminated with dyes affects all living organisms in aquatic life because it has toxic and carcinogenic properties. Many techniques, such as filtration, oxidation, ozonation, adsorption, photocatalysis, and electrochemical treatment, have been employed to remove dyes from wastewater (Su et al. 2016). Among these techniques, adsorption stands out due to its low cost, easy operation, flexibility, and simplicity of design. Activated carbons are generally regarded as the most effective adsorbents used in adsorption processes due to their adsorptive properties and high surface areas (Yuliani et al. 2017). However, activated carbon has a high cost and difficulty in regeneration (Nesic et al. 2012). In the literature, relatively cheap alternative materials derived from industrial waste (Silva et al. 2016), agricultural waste (Singh et al. 2017), plant biomass (Bayramoglu et al. 2013), minerals (Dotto et al. 2016; Marrakchi et al. 2016; Arica et al. 2017), and resin (Bayramoglu et al. 2009; Yavuz et al. 2011) have been intensively investigated as alternatives for activated carbon.

Marble powder is a waste material produced in the millions of tons each year around the world. The wastes can cause three main problems: environmental pollution, environmental health issues, and economic loss (Alyamac et al. 2017). The reuse of waste marble powder as an adsorbent will provide an opportunity to obtain environmental and economic gains. However, unmodified marble powder shows less adsorption capacity for the removal of dye. A linear copolymer of glucosamine and N-acetyl glucosamine, chitosan is easily obtained by thermo-chemical deacetylation of crustacean chitin. Chitosan has excellent properties for adsorption of dyes due to the presence of amino (NH₂) and hydroxyl (OH) groups in the polymer matrix (Nesic et al. 2012). Moreover, natural abundance, biocompatibility, anti-bacterial properties, and biodegradability are other important characteristic features of chitosan, making it unique among other adsorbent materials (Khan et al. 2016). However, high cost, low chemical stability and dissolution in an acidic medium are drawbacks associated with the use of pure chitosan as an adsorbent. Immobilizing chitosan on a low-cost material is an effective modification method to provide enhanced mechanical, thermal or adsorption properties compared with any of its components used alone. Several studies have investigated the removal of dyes and metal ions from aqueous solution using chitosan-based composites such as chitosan/bentonite composite (Zhang et al. 2016), chitosan/clinoptilolite composite (Dinu & Dragan 2010) and chitosan/clay composite (Auta & Hameed 2014). However, it is observed that there is no report on the utilization of chitosan/marble powder composite as an adsorbent specifically for the removal of the dye molecules.

The objective of this study was to synthesize the chitosan/marble powder composites and analyze their adsorptive effectiveness in the removal of Dimozol Blue from aqueous solutions as potential low-cost, high performance adsorbents. The factors influencing the adsorption capacity of the composites, such as the contact time, pH value of dye solution, adsorbent dosage, initial concentration of dye solutions and temperature, were investigated. In order to examine the mechanism of the adsorption process, the kinetics, adsorption isotherms and thermodynamics were evaluated.

The 75–85% deacetylated chitosan with a viscosity of 200–800 cP measured by Brookfield viscometer in 1% of chitosan solution in 1% acetic acid was purchased from Aldrich Chemical Corporation, Germany. The average molecular weight of chitosan is 190,000–310,000 Da. Aqueous acetic acid (Merck) solution was used as a solvent for the chitosan. Glutaraldehyde solution (Fluka, 50%) was used as a cross-linker. The marble powder was collected from the local marble cutting/processing industry in Bilecik, Turkey. The obtained marble powder was dried in an oven at 80 °C for 24 h and then passed through a sieve to obtain 90 μm size before use. Diamozol Blue BRF %150 (C.I. Reactive Blue 221) (purity of 99%) was obtained from a dye factory in Bursa, Turkey. The solubility of the dye is high in aqueous solution (100 g/L), even in the presence of alkali it has very good solubility. All chemicals were of analytical grade, and no further purification was required.

To obtain a homogenous mixture, 2 g of chitosan was dissolved in 75 mL of 5% v/v acetic acid with constant stirring. The marble powder was added and the mixture was left overnight with continuous stirring on a magnetic stirrer. Then, the dispersion was taken in a syringe and allowed to fall slowly and dropwise into 500 mL of 1 M sodium hydroxide solution with gentle stirring, resulting in the formation of the chitosan/marble powder composites, which were kept in the same solution overnight with continuous stirring. To remove any residual sodium hydroxide, the wet chitosan/marble powder composites were washed many times with distilled water to attain a neutral pH. Subsequently, the composites were shaken in 2.5 w % glutaraldehyde ethyl alcohol solution for 15 h at 60 °C. In chitosan crosslinking with glutaraldehyde, the high process temperature (60 ± 1 °C) resulted in the formation of glutaraldehyde condensates that were also capable of covalent crosslinking and, additionally, could affect the enlargement of the surface of the composites (Jóźwiak et al. 2017). After the cross-linking reaction, the composites were washed using distilled water to remove free glutaraldehyde and then freeze-dried for 24–48 h. In this study, a series of chitosan/marble powder composites with different weight ratio percentages were synthesized, viz., 100:0 wt.% (henceforth denoted as C100M0), 70:30 wt.% (C70M30), 50:50 wt.% (C50M50), and 30:70 wt.% (C30M70).

The surface morphology of the composites was observed with a scanning electron microscope (SEM, Bruker, Germany). To analyze the C, O, and Ca element content of the composites, elementary characterization was carried out using energy-dispersive spectroscopy (EDS, Bruker, Germany). Surface area of the chitosan/marble powder composites was measured by a Micromeritics (ASAP 2020) BET (Brunauer, Emmett and Teller) instrument using the nitrogen intrusion technique. The microstructure of the sorbent was characterized using physical adsorption/desorption of nitrogen at 77 K. Spectra of Fourier transform infrared spectroscopy (FTIR) for the bare chitosan and marble powder and chitosan/marble powder composites were obtained by a FTIR spectrophotometer (Agilent Technologies, Cary 630 FTIR). The spectra were acquired in the wavenumber range from 4,000 to 400 cm⁻¹.

SEM images of the cross-linked chitosan/marble powder composites envisioned at magnifications of 74 × (100 μm) and 520 × (20 μm) are shown in Figures 1 and 2. From the SEM micrograph in Figure 1, it is observed that the all composites exhibit a porous texture.

As shown in Figure 2, the composites with lesser amounts of marble powder have a smooth surface.

In the adsorption process, the surface area of the adsorbents is the most important factor. In the BET analysis, the effects of marble powder and chitosan contents on the surface areas of the adsorbents were detected. As shown in Table 1, the surface area of the adsorbent increases from 17.9 m²/g to 34.3 m²/g as the amounts of marble powder in the adsorbent matrix are increased from 0 to 30 wt. %. An additional increase in the marble powders indicates a decline in the surface of C50M50 and C30M70 (i.e. 23.1 m² g⁻¹ and 0.2 m² g⁻¹, respectively). The sharp decrease in surface area of the composites is considered to be due to an increase in agglomeration of the marble powders on the top layer and inside the channel of the adsorbent matrix (Gebru & Das 2017).

The EDS characterization was used to analyze the elemental composition of the composites. As can be seen in Figure 3, C and O elements are detected in all composites, whereas Ca element is only detected in C70M30, C50M50, and C30M70 composites due to the marble powder content of the composites.

In order to study the functional groups' content in the chitosan/marble powder composites, the composites were characterized using FTIR. The FTIR spectrum of the composites were compared with FTIR spectrum of chitosan and marble powder. The comparison of these spectra was shown in Figure 4. Figure 4(b) shows the FTIR spectrum of chitosan. It confirmed the presence of saturated hydrocarbons in the structure of chitosan. Stretching vibrations of NH₂ occurred at wave number 3,489 cm⁻¹. Stretching vibrations of −OH appeared at wave number 3,213 cm⁻¹. FTIR spectra of marble powder display characteristic absorption bands in the region 1,850–650 cm⁻¹ corresponding to C–O bond vibrations. FTIR spectrum of the chitosan/marble powder composites (Figure 4(a)) shows the combination of chitosan and marble powder. The wavenumbers of functional groups in the structure of chitosan and marble powder also appeared in the composites (Marrakchi et al. 2016).

Contact time is an important variable in adsorption processes. In the literature, most studies about chitosan-based adsorbents show that the adsorption of dyes is fast in the initial stages of the treatment time due to the availability of a large number of vacant surface sites. Thereafter, the adsorption process becomes slower near the equilibrium because of difficulty in occupying the remaining vacant surface sites due to the repulsive forces between dye molecules adsorbed on the adsorbent surface and those in the solution phase. Thus, the contact time for equilibrium to be attained is in the range of 3–5 days for most dye molecules (Crini & Badot 2008).

The effects of contact time on the adsorption uptake of different chitosan/marble powder composites (C100M0, C70M30, C50M50, and C30M70) for Dimozol Blue were examined at various time intervals, and the results obtained are presented in Figure 5. It was observed that the adsorption capacity increased with increasing contact time and reached equilibrium after 72 h. The adsorption amount of Dimozol Blue dye onto the adsorbents at various time intervals followed the order of C70M30 > C50M50 > C100M0 > C30M70. At equilibrium, the adsorption capacity of Dimozol Blue onto C100M0, C70M30, and C50M50 was about 27 mg/g and higher than that onto C30M70. C50M50 composites were more economical than C100M0 and C70M30 due to the higher marble powder content and hence were selected as an adsorbent for all the other studies.

The parameters of kinetic models and correlation coefficients (R²) for Dimozol Blue dye adsorption on different chitosan/marble powder composites are depicted in Table 2. It is obvious that the corresponding linear regression correlation coefficient (R²) value for the pseudo-second-order model is relatively higher than that for the pseudo-first-order. Moreover, the calculated q_e values obtained from the pseudo-second-order kinetic model agree with the experimental data (q_e,exp) better than the ones obtained from pseudo-first-order. It is demonstrated that the adsorption of the Dimozol Blue dye onto the chitosan/marble powder composites can be described by the pseudo-second-order kinetic much better than the pseudo-first-order model.

The pH of the solution affects the surface charge of the adsorbents as well as the degree of ionization of dye molecules. The effect of the pH of Dimozol Blue solution on the adsorption capacity of C50M50 was investigated in the pH range 3–11 (Figure 6). The initial pH ranged from 11 to 5, the adsorption capacity of Dimozol Blue onto the C50M50 composites increased with pH until the pH reached 5, and then the adsorption capacities slightly decreased with a further decrease in the pH value. The highest amounts of adsorbed Dimozol Blue were observed at pH 5. At pH values lower than 5, the adsorbents may dissolve, thereby it may cause a decrease in the number of available adsorption sites on the chitosan/marble powder composites, leading to a lower adsorption capacity.

The effect of adsorbent dosage was investigated by the batch equilibrium technique using 50 mL dye solution by varying the adsorbent mass (0.010, 0.025, 0.050, and 0.100 g), and the results of this study are graphed in Figure 7. The adsorbed amount decreased from 155.6 mg/g to 28.0 mg/g when the adsorbent dosage was increased from 0.01 g to 0.1 g. The decrease of adsorption was due to the concentration gradient between adsorbent and adsorptive.

Four different concentrations (40, 60, 80 and 100 mg/L) were selected to investigate the effect of initial dye concentration on the adsorption of Dimozol Blue dye onto the C50M50 composites. Figure 8 shows the adsorption isotherm for Dimozol Blue adsorption on the C50M50 composites. It reveals that the amount of dye adsorbed increased from 111.3 mg/g to 234.5 mg/g when the initial dye concentration was increased from 40 mg/L to 100 mg/L.

The adsorption isotherm models were applied to understand the interaction between adsorbate and adsorbent, whether it was a single layer (Langmuir) or a multilayer (Freundlich) adsorption. The linear forms of Freundlich and Langmuir isotherm models are shown in Equations (4) and (5), respectively (Crini & Badot 2008).

Adsorption parameters obtained from the Freundlich and Langmuir isotherm models are given in Table 3. The Langmuir isotherm model assumes monolayer coverage of adsorbates onto adsorbents and uniform energies of adsorption on the surface of adsorbent. The correlation coefficient result for the Langmuir isotherm (0.9487) is not a satisfactorily good correlation between the model prediction and the experimental data. The results showed that the Freundlich model, with the higher value of correlation coefficient (R² = 0.9943) fitted better than the Langmuir isotherm model. The Freundlich isotherm model assumes the adsorption process occurs as multilayer coverage on the heterogeneous surface of the adsorbent due to the diversity of adsorption sites (Tadjarodi et al. 2016). Moreover, the value of n is greater than 1, indicating that the adsorption process is favorable.

The effect of temperature on the adsorption isotherm was investigated under isothermal conditions in the temperature range of 298–318 K. The adsorption capacity decreased with the increase in temperature.

The negative values of ΔG° confirm the feasibility of the process and the spontaneous nature of the adsorption of Dimozol Blue molecules onto the C50M50 composites. ΔG is more negative with decreasing temperature from 318 K to 298 K, which suggests that lower temperature makes the adsorption easier. The negative (−37.4 kJ/mol) for Dimozol Blue adsorption is an exothermic process in accordance with the decreasing adsorption capacity with increasing temperature. Generally, ΔH for physical adsorption ranges from −4 to −40 kJ/mol, compared to that of chemical adsorption ranging from −40 to −800 kJ/mol (Crini & Badot 2008). Therefore based on the ΔH° value, the adsorption of Dimozol Blue onto the C50M50 composite surface is physical adsorption. In addition, the negative value of ΔS° (−89.2 J/mol K) suggests that the randomness decreased at the solid/solution interface as a results of Dimozol Blue adsorption onto the C50M50 composite surface.

For comparison, the different adsorbents for adsorption of Reactive Blue 221 dye are listed in Table 5. The adsorption capacity of the cross-linked chitosan/marble powder composites in this work is considerably higher than that of others reported in the literature.

In this study, the chitosan/marble powder composites with different weight ratio percentage (C100M0, C70M30, C50M50 and C30M70) were synthesized and used as adsorbents for Dimozol Blue dye. The experimental results indicated that chitosan/marble powder composites, especially C100M0, C70M30 and C50M50, were effective adsorbents for the removal of Dimozol Blue dye from aqueous solution. The adsorption experimental data agreed perfectly with the pseudo-second-order kinetic equation, where the regression coefficients were greater than 98% (R² > 0.98) for all composites. The results showed that the adsorption was also influenced by factors including contact time, pH value, adsorbent dosage, initial concentration of dye, and temperature. Under the optimal conditions (contact time: 72 hours; pH:5; initial dye concentration: 100 mg/L; adsorbent dosage: 0.01 g; 25 °C), the experimental maximum adsorption capacity achieved was 234.5 mg/g. The attractive features of marble powder and chitosan make the composites effective, economic, and environmentally friendly adsorbents.