Adsorption of reactive blue 49 onto cross-linked chitosan-based composites containing waste mussel shell and waste active sludge char

Synthetic dyes used in various industries like rubber, leather, textile, plastic, paint, paper, cosmetics, etc., for the coloration of products are becoming common water pollutants. These dyes have complex molecular structures and are often highly soluble in water, resistant to aerobic digestion, and quite stable to sunlight and oxidizing agents (Zhao et al. 2018). Reactive Blue 49 (RB49) is known as one of the most widely used reactive dyes, which is hazardous and may affect living organisms’ life, causing different diseases (Amin Radi et al. 2015; Alrobayi et al. 2017). Several techniques used in dye removal include coagulation (Morshedi et al. 2013), flotation (Liu et al. 2010), biological treatment (Khouni et al. 2020; Liu et al. 2020), reverse osmosis (Nataraj et al. 2009), ion exchange (Raghu & Basha 2007), and adsorption (Crini & Badot 2008; Mohammed et al. 2020). Among these methods, adsorption is the most promising, easy, and economical method for its high quality of treated effluents and simple design. Many kinds of adsorbents have been reported, such as magnetic iron oxide nanocomposites (Abdullah et al. 2019), cellulose-based materials (Putro et al. 2019), clay-based minerals (Ngulube et al. 2017), agricultural wastes (Dai et al. 2018), carbon nanomaterials (Gusain et al. 2020), chitosan and its derivatives (Vakili et al. 2014). Chitosan-based composites exhibit excellent adsorption performance for dyes because of the large surface area (Zhou et al. 2019). Chitosan-based composites have also been proven to be bio-compatible and non-toxic.

Sludge is a byproduct of water discharged from numerous industrial wastewater plants (Hadi et al. 2015). Sludge, as a carbon-rich material, is an alternative for the production of activated carbon-based adsorbents. In the past few years, a few studies have focused on the activation and reuse of sludge as a value-added adsorbent for the removal of contaminants from air and water (Rivera-Utrilla et al. 2013; Björklund & Li 2017; Akin Sahbaz et al. 2019).

Mussel shell is a carbonate-rich waste product of the seafood processing industry. Some factories have developed industrial processes in order to transform shells into rich products. In these processes, shells are washed, crushed, and sieved, and sometimes also calcined (Paradelo et al. 2016). In recent years, calcined mussel shells as low cost, easily obtainable, and biodegradable adsorbent materials to treat dyes and heavy metals in wastewater have attracted great interest (Pena-Rodriguez et al. 2010; El Haddad et al. 2014). However, there is no information currently regarding the use of calcined waste mussel shells (WMS) beside waste active sludge (WAS) as a component of chitosan-based composites, or there is limited information on the removal of RB49 from contaminated solutions using chitosan-based composites, in the literature.

The aim of this study was to prepare the chitosan/waste mussel shell (C/WMS) and chitosan/waste mussel shell/waste active sludge char (C/WMS/WASC) composites as adsorbents for the removal of reactive dyes. The fabricated composites were analyzed using a scanning electron microscope (SEM), Brunauer, Emmett and Teller (BET), and energy-dispersive X-ray spectroscopy (EDX) techniques were subjected for adsorption of RB49 molecules by the batch method. The influence of various adsorption parameters such as contact time, adsorbent dosage, solution pH, initial RB49 concentration, and temperature for the removal of RB49 molecules were determined. The adsorption mechanism, isotherm, kinetic, and thermodynamic parameters were also investigated.

Chitosan (with a deacetylation degree of 75–85%) was purchased from Aldrich. The acetic acid (glacial, %100) was used as a solvent to synthesize the composites purchased from Merck. Glutaraldehyde solution (50%) was used as a crosslinker and supplied by Fluka. WAS was gathered from the membrane bioreactor (MBR) system, containing Trametes Versicolor and used for the decolorization of simulated textile wastewater in the laboratory. WMS was obtained from a seafood processing industry in Bilecik, Turkey. RB49 (purity of 99%) dye used as adsorbate was obtained from a dye factory in Turkey. The pH of the adsorption solutions was adjusted by hydrochloric acid (HCl, Carlo Ebra) and sodium hydroxide (NaOH, Merck) solutions. All chemicals were of analytical grade, without further purification.

Firstly, WMS was calcinated and WAS were carbonized by chemical activation methods (El Haddad et al. 2014; Dandil et al. 2019a). After these processes, C/WMS and C/WMS/WASC composites were prepared using the following procedure (Dandil et al. 2019b): 1 g of chitosan was dissolved into 75 ml of 5% acetic acid solution under constant stirring in order to get homogeneous mixtures. Certain amounts of WMS and WASC were added in chitosan solutions and the mixtures were stirred with a magnetic stirrer (MR Hei-Standard, Heidolph) overnight, resulting in the formation of the dispersion. The quantities of chitosan, WMS and WASC used for the C/WMS and C/WMS/WASC composites were 1:1 and 1:0.5:0.5, respectively. The C/WMS and C/WMS/WASC mixtures were dripped into alkaline (1 M NaOH) solution with gentle stirring in order to form composite beads. The composite beads were kept in the alkaline mixture for 24 h with continuous stirring. Then, the composite beads were removed from the alkaline solution and washed with distilled water many times to attain a neutral pH. Subsequently, the composites were shaken in glutaraldehyde solution (2.5 w %) for 15 h at 60 °C. After the cross-linking reaction, the composites were washed using distilled water to remove free glutaraldehyde and left to dry at room temperature. Finally, they were placed in a glass bottle and stored in a desiccator until further use in adsorption studies.

The surface morphology of the C/WMS and C/WMS/WASC composites was investigated with an SEM (Leo 1430 VP, Zeiss). To analyze the composition of the composites, elementary characterization was carried out using an EDX (Leo 1430 VP, Zeiss) analyzer. Surface areas of the composites were measured by a BET (ASAP 2020, Micromeritics) instrument using the nitrogen intrusion technique. The microstructures of the composites were characterized using physical adsorption/desorption of nitrogen at 77 K.

Batch adsorption tests were performed on a certain amount of C/WMS and C/WMS/WASC composites using 50 mL of the RB49 dye solutions of known concentration. The effect of pH on RB49 adsorption was investigated in the pH ranges between 1 and 5. To optimize the adsorbent dosage for the adsorption capacity and removal of RB49 dye from aqueous solution, the adsorbent dosage was varied from 0.01 to 0.1 g/50 mL for C/WMS and C/WMS/WASC composites. Initial concentrations of RB49 dye solutions in the range of 20 mg/L to 100 mg/L were investigated. Finally, the adsorption experiments were carried out at 25, 35, and 45 °C to determine the effect of temperature on the adsorption process and the thermodynamic parameters of the process. In adsorption processes, the dye solutions were agitated in a thermostat shaker (H11960, Termal) at 150 rpm until adsorption reached equilibrium. Afterward, the suspension of the adsorbent was centrifuged in order to obtain the supernatant. The concentration of dye ions remaining in the supernatant solutions was analyzed spectrophotometrically at 586 nm wavelength using a UV–vis spectrophotometer (Cary 60 UV-Vis, Agilent). All experiments were performed in triplicate.

The morphology and surface variations of the C/WMS and C/WMS/WASC composites were investigated at different magnifications (125 ×, 500 ×, and 3000 ×) using SEM analysis and micrographs as shown in Figure 1. The microscopic observations showed that both C/WMS and C/WMS/WASC composites have spherical shapes and porous structures.

The EDX analyses of C/WMS and C/WMS/WASC composites were employed to depict the elemental composition of the composites. As can be seen in Figure 2, carbon (C), oxygen (O), calcium (Ca), and chlorine (Cl) elements were detected in all composites, whereas silicon (Si), phosphorus (P), and iron (Fe) elements were only detected in the C/WMS/WASC composites due to the waste active sludge char content of the composites. The waste active sludge contains a trace amount of inorganic compound due to the biological treatment of textile wastewater in the MBR system.

The quantitative analysis of the composites is given in Table 1. The mass fraction of Ca, C and O are 22.30%, 25.61%, 51.67% and 9.38%, 35.96%, 54.46% for the C/WMS and C/WMS/WASC composites, respectively.

In the adsorption process, the surface area of the adsorbents is the most important factor. When the surface area is high, there will be more active surface sites for adsorption of the dye molecules. In the BET analysis, the surface areas of the C/WMS and C/WMS/WASC composites were determined as 15.0 m²/g and 7.6 m²/g, respectively. The higher adsorption capacity of C/WMS than that of C/WMS/WASC composites could be linked to the higher BET value resulting in more availability of active adsorption sites (Kanakaraju et al. 2020).

Figure 3 presents the RB49 adsorption by C/WMS and C/WMS/WASC composites as a function of contact time at different solution pH. The RB49 dye adsorption capacities of C/WMS and C/WMS/WASC composites increase by time up to the adsorption equilibrium. While the adsorption of dye is quite rapid initially, the rate of adsorption becomes slower with time and reaches a constant value at equilibrium. The equilibrium time is 1,620 min for both C/WMS and C/WMS/WASC composites. The results indicate that the C/WMS and C/WMS/WASC composites have nearly the same adsorption rates for RB49 dye. The rapid uptake at the beginning of the adsorption processes could be better explained by the high affinity of C/WMS and C/WMS/WASC composites towards RB49 dye molecules. Moreover, this adsorption behavior could be due to the adsorption of RB49 dye onto the external surface C/WMS and C/WMS/WASC composites and the availability of a large excess of adsorption binding sites. The following slow adsorption behavior is attributed to the penetration-diffusion of RB49 molecules into the inner surface of C/WMS and C/WMS/WASC composites (Hsini et al. 2020a).

Two widely used kinetic models, pseudo-first-order and pseudo-second-order kinetic models, were applied to the experimental data in order to investigate the adsorption dynamics of RB49 on the C/WMS and C/WMS/WASC composites, as well as the mechanisms involved in the adsorption system.

The results of the kinetic parameters for RB49 adsorption of C/WMS and C/WMS/WASC composites as calculated from the linear plots of the two kinetic models are presented in Table 2. The pseudo-second-order kinetic model fitted the experimental data of both C/WMS and C/WMS/WASC composites better than the pseudo-first-order model, resulting in high correlation coefficients. Additionally, the theoretical q_e (q_e,cal) values of C/WMS and C/WMS/WASC composites were both very close to experimental q_e (q_e,exp) values, indicating that the adsorption process followed the pseudo-second-order kinetic model and the RB49 dye adsorption of C/WMS and C/WMS/WASC composites was dominated by a chemical process.

To optimize the adsorbent dosage for the adsorption capacities of C/WMS and C/WMS/WASC composites and the percentages of RB49 removal from aqueous solution, the effect of the adsorbent dosage was investigated and shown in Figure 6.

The percentage of dye adsorption increased from 18.2 to 48.0% and from 12.9 to 45.8%, by increasing the adsorbent mass from 0.01/50 mL to 0.1 g/50 mL for C/WMS and C/WMS/WASC composites, respectively. This increase may be due to the number of adsorption sites increased with the number of adsorbents. The adsorption capacities decreased from 54.7 mg/g to 14.4 mg/g and from 38.8 mg/g to 13.7 mg/g by increasing the adsorbent mass from 0.01/50 mL to 0.1 g/50 mL for C/WMS and C/WMS/WASC composites, respectively.

The adsorption capacity of C/WMS and C/WMS/WASC composites for reactive blue dye ions are substantially high compared to other adsorbents, as seen in Table 3.

Table 4 shows the Freundlich and Langmuir isotherm constants. The comparison of the correlation coefficient (R²) values shows that the Freundlich isotherm model yields a better fit to the experimental data of the adsorption behavior of C/WMS and C/WMS/WASC composites than the Langmuir isotherm model. The Freundlich model is based on the assumption that adsorption occurs on multilayer and heterogeneous surfaces of the adsorbent due to the diversity of adsorption sites (Crini & Badot 2008). Moreover, as shown in Table 4, the values of n are greater than 1, confirming that the adsorption processes are favorable. The lower values of both K_F and n for C/WMS composites compared with C/WMS/WASC composites reflect the lower affinity of RB49 for adsorption surfaces (Hua et al. 2018).

Table 5 indicates the determined values of thermodynamic parameters for RB49 dye adsorption. A decrease in randomness is indicated by negative values of entropy change whereas the exothermic nature of the process is indicated by the negative values of enthalpy. Positive ΔG° values indicate that the adsorption process is non-spontaneous in the temperature range of 298–318 K (Asgher & Bhatti 2012). Furthermore, the increasing pattern of ΔG° magnitudes upon rising the temperature further suggests that the RB49 molecules adsorption becomes more feasible at lower temperatures (Hsini et al. 2020b).

The present study demonstrates that the chitosan-based composites containing WMS and WASC can be used as an effective alternative and low-cost adsorbent for the treatment of water/wastewater contaminated with RB49 dye. It has been observed that the two adsorbents have high resistance to acidic conditions and are very effective in removing the organic pollutants in an acidic environment.

Approximately 48.0% and 45.8% RB49 dyes were removed from the aqueous solution using the C/WMS and C/WMS/WASC composites in 1,620 minutes, respectively.

The maximum RB49 adsorption capacity of C/WMS and C/WMS/WASC composites were 54.7 and 38.8 mg/g, respectively. The pseudo-second order kinetic model and the Freundlich isotherm model were successfully fitted to describe the adsorption mechanism for both composites.

Thermodynamic parameters for the adsorption of RB49 onto the C/WMS and C/WMS/WASC composites like ΔG°(298 K), ΔH°, and ΔS° changes have been calculated as (2.198 kJ·mol⁻¹) and (2.313 kJ·mol⁻¹); (−78.54 kJ·mol⁻¹) and (−41.72 kJ·mol⁻¹); (−0.2709 kJ·mol⁻¹·K⁻¹) and (−0.1478 kJ·mol⁻¹·K⁻¹), respectively.

All in all, the C/WMS and C/WMS/WASC composites are quite promising adsorbents and may be used as alternative approaches for RB49 dye removal from contaminated water.

All relevant data are included in the paper or its Supplementary Information.