Adsorption of Congo red by fibrous xonotlite prepared from waste silicon residue

The rapid development of industry has brought not only high economic benefits, but also environmental challenges. Waste aqueous effluent containing heavy metal ions (Chen & Wang 2006), phosphorus and fluorine (Kang et al. 2011) and organic dye (Ismail et al. 2019) causes serious environmental problems. The organic dye generated from the textiles and paper enterprises is considered extremely serious for their non-degradability and accumulation. Low concentration discharged may cause great pollution. Removing the organic dyes from industrial wastewater has become an urgent task (Feng et al. 2015). Many solutions have been developed, such as membrane separation (Yoon et al. 2014), ion co-precipitation (Liu et al. 2014), photocatalysis (Shen et al. 2020a, 2020b), adsorption (Xuan et al. 2014) and so on. Among them, adsorption is one of the promising and efficient methods owing to its simplicity and high efficiency (Wang et al. 2017).

The adsorbent plays an important role during the adsorption process. Up to now, micro-nano materials are promising adsorbents due to their unique physical and chemical properties and geometric structure (Shen et al. 2020a, 2020b). Generally, morphological structures of materials can significantly influence their performance, therefore, controllable synthesis of materials with special morphology has been given more attention. Many adsorbents with special shapes were reported, including rod-shaped (Yun et al. 2020), flower (Shen et al. 2020a, 2020b), hollow spherical (Qiao et al. 2012), fiber-shaped (Chen et al. 2018) etc. Meanwhile, some related synthesis methods, such as the sol-gel (Chen et al. 2019), hydrothermal (Wang et al. 2015), vapor deposition (Tofighy & Mohammadi 2011) and so on, also have been studied. The hydrothermal process is widely used in the preparation of micro-nano materials with the superiorities of simplicity, high yield, controllability and easy amplification. (Bao et al. 2016).

In recent years, silicate materials with special morphologies have attracted much attention in the field of wastewater treatment, and have been adopted to treat Pb²⁺ (Liu et al. 2021), PO₄³⁻ (Guan et al. 2013) and Methylene blue (Maeda & Ishida 2011) etc. Notably, calcium silicate materials are non-toxic and environmentally friendly. Xonotlite, as a member of the calcium silicate family, has excellent adsorption properties. However, few reports are available on the synthesis of fibrous xonotlite from metallurgical residue. On the other hand, the metallurgical solid waste containing silicon causes great pressure to the environment due to huge quantity as well as the potential release of heavy metal ions and non-neutral wastewater. Therefore, the preparation of fibrous xonotlite from metallurgical residue is regarded as a feasible route towards environmental sustainability and resource utilization efficiency.

In this work, Na₂SiO₃ solution obtained from alkaline leaching waste residue was purified and used as raw material. Fibrous xonotlite was synthesized using the purified Na₂SiO₃ solution and self-made active Ca(OH)₂ in the hydrothermal process. The adsorption performance of fibrous xonotlite for Congo red was discussed. The adsorption mechanism was determined, and the isotherm study was carried out using linear Langmuir and Freundlich models.

The main mineral phases in the silicon residue from nickel hydrometallurgy are quartz, lizardite, magnetite, amorphous SiO₂, etc. Na₂SiO₃ solution was obtained through the reaction of residue and alkali. The obtained Na₂SiO₃ solution was purified. SiO₃²⁻ solution concentration was adjusted to 0.05 mol·L⁻¹. Active Ca(OH)₂ emulsion with a concentration of 150 g·L⁻¹ was prepared. The distilled water was homemade. Congo red purchased from Sinopharm Group was of analytical grade.

120 mL of 0.05 mol·L⁻¹ Na₂SiO₃ solution was added into a beaker fixed in a water bath with the pre-heated temperature of 40 °C. 3 mL Ca(OH)₂ emulsion was slowly added according to Si/Ca molar ratio of 1:1 and held for 30 min under magnetic stirring. The slurry was transferred into a Teflon lining autoclave and aged for 1 h before 8 mL of 10 wt.% C₁₂H₂₅SO₄Na (SDS) solution was added. The autoclave was placed in an oven preheated to 220 °C for 9 h. When the reaction was completed, the autoclave was naturally cooled down to room temperature. After filtration, the filter cake was washed four times using distilled water and three times using absolute ethanol. The fibrous xonotlite was obtained after being dried at 100 °C for 8 h. The filtrate was recycled to leach the residue for obtaining raw solution.

In the kinetics adsorption, 50 mL Congo red solution with concentrations ranging from 100 to 200 mg·L⁻¹ was employed. According to preset time intervals, the solutions were centrifuged and the concentrations of Congo red were measured before calculating the adsorption capacity.

In the thermodynamics adsorption, at the selected temperatures of 0, 20 and 40 °C, the experiments were implemented with the Congo red concentrations ranging from 100 to 300 mg·L⁻¹.

The morphology of the specimen was observed by a Hitachi 8010 scanning electron microscope. The structure of the specimen was identified by a Japan Rigakn UltimaIV X-ray diffraction, employing Cu Kα radiation with a voltage of 40 kV, at a scanning rate of 8° min⁻¹ with 2θ ranging from 5° to 90°. The FT-IR spectrum of the specimen was recorded on a Nicolet 380 infrared spectroscope. The concentrations of Congo red were measured using a TAS-990 atomic absorption spectrophotometer.

Figure 1 shows the X-ray diffraction (XRD) pattern and scanning electron microscopy (SEM) image of fibrous xonotlite. XRD shows that all diffraction data are in good agreement with JCPDS file No. 230125, indicating that the obtained specimen is monoclinic xonotlite (Ca₆Si₆O₁₇(OH)₂). Fibrous xonotlite with high crystallinity exhibits good dispersibility and high aspect ratio.

The adsorption capacity of fibrous xonotlite for Congo red versus time using 30 mg xonotlite is presented in Figure 2(a). High adsorption capacities have been achieved in 1 min and adsorption capacities are 133.09, 213.60 and 278.15 mg·g⁻¹. The adsorption almost reached the equilibrium in 10 min with the equilibrium adsorption capacities of 156.33, 237.72 and 316.77 mg·g⁻¹ at the varied Congo red concentrations of 100–200 mg·L⁻¹, respectively. Rapid and efficient adsorption makes fibrous xonotlite a potential adsorbent. The adsorption capacities of fibrous xonotlite for Congo red at varied initial concentrations ranging from 100 to 300 mg·L⁻¹ and different temperatures within 10 min were studied, and the results were plotted in Figure 2(b). The adsorption capacities increase with increase of the temperature, implying that the adsorption is an endothermic process. Increasing temperature is favorable for adsorption. Furthermore, the adsorption capacities increase with the increase of Congo red concentration. This is because the increase of the Congo red molecules in solution improves the occupying opportunities of active sites of fibrous xonotlite. The equilibrium adsorption capacities at Congo red concentration of 300 mg·L⁻¹ are 404.17, 445.86 and 454.16 mg·g⁻¹ at temperatures of 0, 20 and 40 °C, respectively.

Figure 4(a) shows the fitting plots of the intra-particle diffusion model for the adsorption of Congo red on fibrous xonotlite. It can be clearly seen that the whole adsorption process can be divided into three stages. The external diffusion with the fastest adsorption rate occurred in the first stage of 1 min. In the second stage, the adsorption rate decreased obviously. The Congo red molecules might be transferred to the inner surface through intra-particle diffusion. In the last stage, the final adsorption equilibrium was reached. In the adsorption processes of macromolecular dyes, the rate of internal diffusion generally determines the adsorption rate of the whole process (Walker & Weatherley 2001). However, the fitting lines do not cross the ‘zero’, which reveals that internal diffusion is the main rate control step, but not the only one (Mohan et al. 2002). Other researchers also believed that if intra-particle diffusion was the only step, the internal diffusion rate k_p would be linear with the square root of the initial solution concentration (Allen et al. 1992). To verify this conclusion, the experimental data are processed to obtain Figure 4(b). Clearly, there is no linear relationship between k_p and the square root of the initial solution concentration. Therefore, the adsorption rate is controlled by the high-rate external diffusion and internal diffusion.

To further investigate the effect of the temperature on Congo red adsorption, the experimental data were fitted by the Langmuir and Freundlich models. The results are shown in Figure 5, and the corresponding parameters are listed in Table 2.

It can be seen from Figure 5 that both of the isotherm models fitted well. By comparison, the R² values of the Langmuir model are higher than those of the Freundlich model. In addition, the experimental maximum adsorption capacities (q_max) are basically consistent with those obtained from the Langmuir model, which indicates that the Langmuir model is more suitable to describe the adsorption. The adsorption of Congo red by fibrous xonotlite is monolayer, in agreement with the kinetic analysis. The maximum adsorption capacity reaches 574.71 mg·g⁻¹ at 20 °C.

The FT-IR spectra of fibrous xonotlite before and after adsorption are displayed in Figure 7. The peak at 970 cm⁻¹ is due to the antisymmetric tensile vibration of Si-OH. The broad bands at 1,204 and 1,074 cm⁻¹ are attributed to the Si–O–Si asymmetric stretching vibration. The characteristic peak of the Si–O–Si bond is observed at 664 cm⁻¹. The new adsorption band at 1,236 cm⁻¹ is attributed to S = O stretching vibration and the bands appearing at 742 and 694 cm⁻¹ can be assigned to the characteristic aromatic skeleton after Congo red adsorption (Hou et al. 2012). These results reveal that the Congo red molecules in solution have been effectively bound by fibrous xonotlite.

The synthesized xonotlite has a fibrous structure with high aspect ratio, which not only provides a large specific surface area and a large number of active sites but also exhibits excellent hydroxyl releasing ability. According to the pH_pzc of fibrous xonotlite, the surface of xonotlite is positively charged under pH less than 7, which can protonate the surface hydroxyl groups (Zhang et al. 2018). As a typical anionic organic, Congo red has a strong attraction to protonated hydroxyl groups (-OH₂⁺) due to electrostatic attraction, which promotes the adsorption. Therefore, electrostatic attraction is considered to be the main mechanism of Congo red adsorption on fibrous xonotlite, as schematically presented in Figure 8.

• (1)

  Fibrous xonotlite with a high aspect ratio was successfully prepared at a Na₂SiO₃ concentration of 0.05 mol·L⁻¹, Si/Ca molar ratio of 1:1, temperature of 220 °C and time of 9 h.

• (2)

  Fibrous xonotlite shows excellent adsorption capacity for Congo red. The adsorption equilibrium can be reached in 10 min. The calculated maximum adsorption capacity is 574.71 mg·g⁻¹ at 20 °C.

• (3)

  The Langmuir model can describe the adsorption well. The adsorption kinetics follows the pseudo-second-order model. The adsorption rate is controlled by high-rate external diffusion and internal diffusion.

This work was supported by the National Natural Science Foundation of China (Nos. 51774070 and 52004165).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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