1 Introduction

Water pollution is getting worse and has been one of the severest environmental challenges (Zhou et al., 2019). Particularly, dyes from the textile, printing, and dyeing industries are one of the primary sources of water pollution (Tang et al., 2020b; Popoola et al., 2021). With the development of industrialization, the use of dyestuffs is increasing, resulting in a corresponding increase in the discharge of dyestuff wastewater. Dye wastewater poses a significant threat to aquatic organisms due to its complex composition and high toxicity (Wang et al., 2005; Kismir et al., 2011; Cao et al., 2018). Congo red (C₃₂H₂₂N₆Na₂O₆S₂) is a significant pollutant in printing and dyeing effluents and is a commonly used azo-anionic dye (Cui et al., 2021). Aquatic organisms can metabolize Congo red into benzidine, a carcinogen that can harm humans (Velkova et al., 2018). Due to its complex aromatic structure, it is typically difficult to be degraded (Soni et al., 2020). The coloration of Congo red hurts the photosynthesis of aquatic plants, affecting the ecosystem (Chen et al., 2016). The urgent issue of Congo red wastewater contamination needs to be addressed.

Printing and dyeing wastewater can be treated using various methods, including biological processes (Wu et al., 2023), chemical methods (Cui et al., 2021), adsorption methods (Extross et al., 2023), and photocatalytic degradation methods (Taha et al., 2019). Physical adsorption is widely considered as the most promising method for treating wastewater due to its simple, low-cost, highly efficient, environmentally friendly property (Dinh et al., 2019). Adsorption is a process that transfers hazardous substances from wastewater to the solid phase, which can reduce their potential damage to the aquatic environment (dos Santos et al., 2014). Li and Zhai (2023) shows that modified peanut shells effectively adsorbed Cr³⁺ from wastewater. El Jery et al. (2024) synthesized an activated carbon adsorbent from rice straw precursor to remove methylene blue dye from wastewater. Adebayo et al. (2022) developed a composite material using coconut shells, raw clay, Fe(Ⅱ), and Fe(Ⅱ) compounds to adsorb Congo red dye from wastewater. Paluch et al. (2023) utilized biochar derived from fennel seeds to eliminate methyl red from an aqueous solution. Using biowaste as an adsorbent has emerged as a new direction in recent years.

In recent years, there has been a significant increase in global aquaculture, with shellfish accounting for a considerable portion (Sicuro et al., 2021). The increasing amount of shellfish farming also generates significant shell waste. It is often discarded as an industrial by-product and food waste (Qin et al., 2016). Shell waste has been a significant environmental issue in coastal regions; however, they are a valuable renewable mineral resource. Shells comprise 95% CaCO₃ and 5% organic matter (Srichanachaichok and Pissuwan, 2023). After CO₂ emissions and substance decomposition, calcined shells form a complex porous structure that serves as the foundation for their adsorption properties (Dai et al., 2018; Rodríguez-Arellano et al., 2021). Furthermore, shells settle faster, making them more suitable for separating and recovering adsorbents (El Jery et al., 2024).

Mytilus edulis is widely distributed worldwide. In recent years, the scale of M. edulis farming has significantly increased due to technological advancements. M. edulis shells are produced in large quantities as waste products. Finding practical methods to use them is an important issue. In recent years, M. edulis shells have been widely used in many fields, such as catalyst carriers for photocatalytic reactions (Cao et al., 2023), soil conditioners (Hannan et al., 2021), and adsorbents (Trinh et al., 2023). Wang et al. (2021) reported that M. edulis shell powders could adsorb Pb(Ⅱ) from aqueous solution. M. edulis shell is a good adsorbent due to its low cost, high efficiency, and minimal side effects. However, the adsorption result of dye, such as Congo red, onto M. edulis shells is still unknown.

Utilizing waste resources such as shells for decolorizing dye wastewater can not only help address the wastewater treatment issue, but also promote the efficient use of shell waste resources. Thus, the current study aims to employ discarded M. edulis shells for the removal of Congo red. The characteristics of M. edulis shell powders adsorbing Congo red dye were also investigated, including the adsorption conditions, kinetics, and isothermal adsorption models. This research provides a theoretical basis for developing adsorbents using M. edulis shell waste.

2 Materials and Methods 2.1 Materials

M. edulis were collected from Tuandao Seafood Market in Qingdao, Shandong, China. Congo red was from Tianjin Damao Chemical Reagents Factory, and other reagents are analytically pure.

2.2 Preparation and Thermal Modification of M. edulis Shell Adsorbent

The shells of M. edulis were washed to eliminate any insoluble impurities, such as sediment. The shells were dried in an oven at 70℃ for 12 h and then calcined in a muffle furnace at temperatures of 550℃, 700℃, and 900℃ for 3 h respectively. After completing the calcination process, the samples were cooled down to room temperature and weighed to determine their yields. The shells were crushed separately before and after calcination using a high-speed multifunctional grinder. The shell powder was sieved through a 200 mesh sieve, and the powder that passed through was collected.

2.3 Structural Characterization of Shell Powder

The characteristics of M. edulis shell adsorbent was studied with a SU8020 scanning electron microscope. The powdered shells before and after calcination were coated with gold for SEM observation.

Fourier transform infrared spectroscopy (FTIR) of shell powder was conducted using KBr pressing method. The functional groups of the modified adsorbent were determined through this process. The shell powder was analyzed using a Thermo Scientific Nicolet iS10 Fourier infrared spectrometer with a 4000 – 400 cm^–1 scanning range.

2.4 Adsorption Experiments

A series of standard solutions of Congo red at different concentrations of 5, 20, 35, 50, and 70 mg L^–1 were prepared, and their absorbance was measured at 498 nm. The plot of Congo red concentration versus absorbance was illustrated to create the standard curve.

Batch adsorption experiments were conducted using various shell powders. The study investigated the impact of temperature, contact time, and Congo red concentration on adsorption process. The batch experiment steps are outlined as below: 0.1 g of shell powders and 50 mL of Congo red solution at 250 mg L^–1 were added into a 100 mL conical flask. The adsorption experiments were conducted at 25℃ with a rotation speed of 150 r min^–1 for 12 h in a constant temperature shaker. Subsequently, the adsorbed solution was centrifuged at 10000 r min^–1 for 3 min, and then the absorbance of the supernatant was measured. The concentration of Congo red in the adsorbed solution was determined using the standard curve of Congo red. The samples underwent three sets of repetitive parallel adsorption experiments. The average value was calculated, and the adsorption capacity was determined using the following equation (Tang et al., 2020a):

$ {Q_{\text{e}}} = \frac{{({C_0} {C_t})V}}{m}, $

(1)

where C₀ is the initial concentration of Congo red solution (mg L^–1), C_t is the concentration of Congo red in the solution after adsorption at time t (mg L^–1), t is the adsorption reaction time (min), V is the volume of the solution (L), and m is the mass of shell powder (g).

The kinetics of adsorption was conducted at 25℃ with a concentration of 250 mg L^–1. The concentration of Congo red in the supernatant was measured at 5, 10, 20, 30, 60, 90, 120, 150, and 180 min, respectively. The study of adsorption kinetics can aid in comprehending the mechanisms of adsorption. Adsorption kinetics is concerned with the dispersion characteristics of adsorbent particles. The data is often fitted using the pseudo-first-order and pseudo-second-order kinetic models (Srilakshmi and Saraf, 2016).

The pseudo-first-order kinetic model assumes that the adsorption rate is proportional to the number of unoccupied sites on the adsorbent (Acemioğlu, 2005):

$ \log ({Q_{\text{e}}}-{Q_t}) = \log {Q_{\text{e}}}-\frac{{{k_1}}}{{2.303}} \times t, $

(2)

where Q_e and Q_t are the unit adsorption capacity (mg g^–1) at equilibrium and time t, respectively; k₁ is the pseudo-first-order adsorption rate constant (min^–1), and t is the adsorption reaction time (min).

The pseudo-second-order kinetic model assumes that the adsorption rate is proportional to the square of the number of unoccupied adsorption sites (Ho and Mckay, 1999):

$ \frac{t}{{{Q_t}}} = \frac{1}{{{k_2}Q_{\text{e}}^2}} + \frac{1}{{{Q_{\text{e}}}}}t, $

(3)

where k₂ is the rate constant for quasi-secondary adsorption (g mg^–1 min^–1). The Q_e and k₂ values are determined by fitting a linear relationship to a graph with t and t/Q_t as horizontal and vertical coordinates, respectively.

2.5 Adsorption Isotherms

The impacts of varying shell powder concentrations on the adsorption of Congo red were investigated at 25℃, 35℃, and 40℃, respectively. The shell powder was initially presented in 200, 500, 1000, 1500, 2000, and 2500 mg L^–1 concentrations. The adsorption isotherm shows the correlation between the adsorbent's equilibrium amount and the solution's remaining concentration. Adsorption equilibrium is achieved when the adsorbate concentration in a bulk solution is in dynamic equilibrium with the adsorbate concentration at the adsorbent interface (Ho and Mckay, 2000). Two adsorption isotherm models, Langmuir and Freundlich equations, were further used to simulate the change of adsorption capacity of shell powder at different initial concentrations of Congo red. The Freundlich model assumes the presence of multiple layers of adsorption sites, and the activity of the adsorption sites is not homogeneous (Li et al., 2015). The equation is:

$ \log {Q_{\text{e}}} = \log {k_{\text{f}}} + \frac{1}{n}\log {C_{\text{e}}}, $

(4)

where C_e (mg L^–1) is the concentration of Congo red at adsorption equilibrium, Q_e (mg g^–1) is the adsorbed amount at the corresponding equilibrium concentration, while n and k_f (L g^–1) are the heterogeneity factor and Freundlich constant, respectively. The k_f and n values are determined by fitting a linear relationship to a graph with logC_e and logQ_e as horizontal and vertical coordinates, respectively.

The Langmuir model assumes homogeneous adsorption site adsorption and describes homogeneous adsorption in monomolecular layers (Langmuir, 1916). The equation is:

$ \frac{{{C_{\text{e}}}}}{{{Q_{\text{e}}}}} = \frac{{{C_{\text{e}}}}}{{{Q_{\max }}}} + \frac{1}{{{K_{\text{L}}}{Q_{\max }}}}, $

(5)

where Q_max (mg g^–1) is the maximum adsorption capacity, and K_L (L g^–1) is the equilibrium constant. The Q_max and K_L values are determined by fitting a linear relationship to a graph with C_e and C_e/Q_e as horizontal and vertical coordinates, respectively.

2.6 Statistical Analyses

The statistical evaluation was performed using the SPSS 27.0 software package. The data are presented as the mean ± the standard deviation of three repetitions. No significant differences were found between groups sharing the same letters. Different letters indicated significant differences in multiple comparisons between groups.

3 Results and Discussion 3.1 Characterization of Calcined M. edulis Shell Powder

Fig.1 displays the impact of various calcination temperatures on M. edulis shell mass. The shell powder yields were 96% and 92% after calcination at 550℃ and 700℃, respectively. Both of these temperatures just had a negligible effect on changes in shell mass. However, the calcination treatment at 900℃ had a more significant impact on the residue of the shell, resulting in a yield of 60% after calcination.

The structural changes in shell powder were characterized using FTIR. Fig.2 shows that the absorption peak at 2512 cm^–1 is caused by the vibration of C-H containing organic matter in the uncalcined feedstock. The 1795 cm^–1 and 1402 cm^–1 peaks correspond to the antisymmetric stretching vibrations of CO₃^2–. The peaks at 712 cm^–1 and 873 cm^–1 correspond to the in-plane bending vibration and out-of-plane bending vibration of CO₃^2–, respectively (Wu et al., 2022; Zhang et al., 2024). In comparison, the peak observed at 2512 cm^–1 disappeared after calcination at 900℃, indicating complete degradation of the organic matter at this temperature. The primary constituent of shell powder is CaCO₃. The disappearance of the CO₃^2– peak after calcination is primarily due to its conversion to CaO. The 1795 cm^–1 and 712 cm^–1 peaks persisted after calcination at 550℃ and 700℃ but vanished after calcination at 900℃. After calcination at 900℃, the heights of the peak at 1402 cm^–1 and the peak at 873 cm^–1 were significantly reduced. These results suggest that shell powder calcined at 550℃ and 700℃ was primarily composed of a mixture of CaCO₃ and CaO. At 900℃, CaCO₃ decomposed almost entirely into CaO. Furthermore, the shell powder calcined at 900℃ exhibits a significant peak at 3640 cm^–1, which was absent in the remaining three shell powders. This peak corresponds to the free-OH stretching vibration. This is because CaO reacts readily with H₂O in air to form Ca(OH)₂. Therefore, the spectrum of calcined shell powder exhibits a characteristic -OH peak. This group shows good hygroscopicity (Parvin et al., 2019). In comparison with shells treated with 1 mol L^–1 KOH at 80℃ (Malik et al., 2020), it was found that the calcined shells in this case had a more complete removal of organic matter. As a result, their pores are more numerous, and their adsorption capacity is relatively higher.

The adsorption capacity of an adsorbent depends not only on the chemical reactivity of the surface functional groups but also on its porosity. The morphological structure of shell powder was measured using SEM as presented in Fig.3. From Figs.3(A) and (B), the raw material of the M. edulis shell powder before calcination is rod-shaped. It has a tighter structure and a smoother surface with almost no pores. Figs.3(C) and (D) show there is no significant change in the M. edulis shell powder after being calcined at 550℃ compared to the raw material. It has a rougher surface after calcination. Figs.3(E) and (F) show that after calcination at 700℃, M. edulis shell powder contains a higher number of micropores, with fuzzy angles and a loose texture. Figs.3(G) and (H) show the morphological structure of shell powder with calcination treatment at 900℃. Its surface is rough and has a loose organization with a significantly increased specific surface area and many pore structures. The FTIR results indicate that CaCO₃ from the shell decomposed and released CO₂ due to the high temperature of 900℃. And the high temperature causes the organic matter to decompose, creating a complex and porous structure on the surface. These results are also in accordance with the significantly decreased yield of shells calcined at 900℃ as shown in Fig.1.

3.2 Adsorption Characteristics of Congo Red on Calcined Shell Powder 3.2.1 Impact of calcination temperature on shell powder's capacity to adsorb Congo red

The shells underwent modification through calcination at 550℃, 700℃, and 900℃. Subsequently, experiments were conducted to determine the adsorption of Congo red dye to the modified shells. Fig.4 shows the supernatant obtained after centrifugation. There was no significant dye adsorption observed for shell powder calcined at 550℃ and 700℃ in comparison to uncalcined shell powder. The adsorption capacity of the raw shell material and shells with calcination at 550℃ and 700℃ are 10.77, 21.54, and 11.84 mg g^–1, respectively. In contrast, the adsorption of Congo red by shell powder calcined at 900℃ was highly increased and the Congo red solution almost become colorless. Fig.5 shows more than 90% of Congo red has been removed from aqueous solution by calcinated shell powder at 900℃ according to the absorbance difference before and after adsorption treatment. In combination with the SEM and FTIR results, shell powder calcined at 900℃ showed a large pore size and adsorption surface area. Its distinctive O-H bond also may facilitate hydrogen bonding with Congo red molecules, enhancing its adsorption (Yen and Li, 2015). Thus shell powder calcined at 900℃ was selected for further study.

3.2.2 Adsorption kinetics

Fig.6 illustrates the fluctuation of the adsorption capacity of shell powder calcined at 900℃ for Congo red at various time intervals. In the first 10 min, shell powder removed about 77.6% of the Congo red. Then, the rate of adsorption slows down as time increases. This might be because the surface active sites of the adsorbent decrease with decreasing concentration of Congo red. The equilibrium for adsorption was achieved after 150 min, after which the adsorption equilibrium was established and the adsorption capacity remained constant at 112.1 mg g^–1 of Congo red. After the equilibrium, 96.2% of Congo red in the solution was adsorbed onto calcined shell powder. It is observed that the adsorption process consists of two stages. The first stage was very fast within the first 10 min, which corresponded to the surface adsorption. In the second stage (10 – 150 min), the adsorption capacity still increased but it has been slowly. Intra-particle diffusion is supposed to occur in this stage (Qadeer, 2012). The potential practical application of M. edulis shell powder is demonstrated by its fast adsorption rate.

Adsorption kinetics studies are informative to elucidate the nature of adsorption process. The results of fitting the adsorption kinetic model can be observed in Figs.7(a) and (b), as well as Table 1. The pseudo-first-order kinetic model showed a low correlation coefficient (R² = 0.8111), indicating a poor model fit. The correlation coefficient of the pseudo-second-order kinetic model was much higher (R² = 0.9976), indicating an agreement between the pseudosecond-order kinetic model and the adsorption process. The calculated adsorption capacity Q_e (112.4 mg g^–1) also closely matches the experimental value (112.1 mg g^–1). The results suggest that chemisorption plays a role in the adsorption process through electron sharing or exchange between adsorbent and adsorbate (El Jery et al., 2024).

3.2.3 Temperature effect and adsorption isotherms

Adsorption behavior of shell powder may be influenced by temperature depending on the property of the adsorbate and adsorbent. Effect of temperature on the adsorption and the adsorption isotherms of shell powder on Congo red were obtained at 25℃, 35℃, and 40℃ in Fig.8. It is demonstrated that the adsorption capacity of Congo red on shell powder at equilibrium decreases as the temperature increases. The results suggest that the adsorption is an exothermic process. Additionally, it appears that adsorption at 25℃ is more effective, which is also favorable for practical application.

The adsorption isotherm model fit results are presented in Figs.8(b) and (c), as well as in Table 2. The Freundlich model showed good correlation coefficients ranging from 0.9598 to 0.9980 at the three temperatures, which obviously outperformed the Langmuir model. The results suggest that the adsorption process is multiphase, and each active site binds multiple molecules of the adsorbent simultaneously. Both physical and chemical adsorption are present, with chemical adsorption being the primary form. The parameter n represents the reactivity and heterogeneity of the active sites of the adsorbent. If n = 1, adsorption follows a linear pattern. If n > 1, the adsorption process is primarily chemical (Conde-Cid et al., 2019). The value of n ranges from 1 to 10 for favorable adsorption (Bhaumik et al., 2012). At the three temperatures, the adsorption is multiphase favorable adsorption with inhomogeneous adsorption sites, where 1 < n < 10. The FTIR data indicate that the primary functional group present in the adsorbent is -OH. The adsorbate is removed from the aqueous solution by the adsorbent through various interactions, such as hydrogen bonding and electron interactions. At the same time, the porous structure enhances the surface diffusion rate. The reaction takes place within 10 min, and results in a high adsorption efficiency.

In particular, at 25℃ and an initial concentration of 2500 mg L^–1 Congo red, M. edulis shell powder exhibited a very high adsorption capacity of 1015 mg g^–1, as shown in Fig. 8(a). The adsorption capacity may also be increased if a higher initial concentration of Congo red is used. This data showed huge potential of M. edulis shell powder in the application of dye removal. Malik et al. (2020) used 0.5 g of modified rice husk to adsorb 1000 mg L^–1 of Congo red, achieving an adsorption ratio of 92.3%. In this case, M. edulis shell powder adsorbed 87.8% of 1000 mg L^–1 Congo red (Fig.8(a)), but only 0.1 g absorbent was used in our experiments. The M. edulis shell powder seems to have a comparable or even higher adsorption capacity compared to rice husk. It is also much higher than the adsorption capacity of potato peel powder, while only 72% of the 30 mg L^–1 Congo red was removed using 0.6 g of potato peel powder (Akram and Javed, 2023). Additionally, Zhou et al. (2018) treated shrimp shells with NaOH to obtain a modified adsorbent for Congo red, but its maximum adsorption capacity is just 288.2 mg g^–1. In the meanwhile, producing modified shrimp shells requires a large amount of water to be repeatedly rinsed until neutral. This process is also cumbersome and has negative impacts on environment. Some new materials were also synthesized to adsorb Congo red. For example, Sakshi et al. (2023) prepared a Chitosan/Moringa oleifera gum hydrogel but its Q_max for Congo red is only 50.25 mg g^–1, which is significantly lower than that of M. edulis shell powder in this study. In another experiment, the Ackee apple seed-bentonite composite was tested to adsorb Congo red, with a relatively high Q_max of 1439.9 mg g^–1 (Adebayo et al., 2020). However, preparing adsorbents requires significant raw materials and is of high-cost. In contrast, the process preparing the M. edulis shell adsorbent is simple and inexpensive. This shows that M. edulis shell is a promising adsorbent for the removal of Congo red dye.

4 Conclusions

1) The adsorption of Congo red was the highest in shell powder that was calcined at 900℃. At equilibrium, 96.2% of the Congo red in the solution is adsorbed. 2) The adsorption process follows the pseudo-second-order model, indicating the presence of chemisorption. 3) The adsorption of Congo red on M. edulis shell powder is exothermic, and lower temperatures promote the adsorption process. At 25℃, M. edulis shell powders showed at least adsorption capacity of 1015 mg g⁻¹. The adsorption isotherm is consistent with the Freundlich model, indicating multiphase-favorable adsorption. Mussel shell powder calcinated at 900℃ can be employed in adsorbing Congo red dye.

Acknowledgements

This study was funded by the National Key Research and Development Program of China (No. 2023YFD2401105), and the Fujian Science and Technology Planning Project-STS Program (No. 2021T3013).

Author Contributions

Xin Wang, methodology, data analysis, raw material preparation, writing-original draft, data curation writing-review and editing. Xiangyun Ge and Siqi Zhu, software and surveys. Weixiang Liu, raw material preparation. Ronge Xing, supervision and funding acquisition. Pengcheng Li, administration and supervision. Kecheng Li, conceptualization, investigation, data analysis, writing-original draft, writing-review and editing, funding acquisition, supervision, project administration.

Data Availability

The data and references presented in this study are available from the corresponding author upon reasonable request.

Declarations

Ethics Approval and Consent to Participate

This article does not contain any studies with human participants or animals performed by any of the authors.

Consent for Publication

Informed consent for publication was obtained from all participants.

Conflict of Interests

The authors declare that they have no conflict of interests.