Introduction

Air and water pollution is one of the fundamental problems that pose a great threat to the ecosystem and human health. In recent years, industrial development has undergone tremendous growth in various parts of the world, and consequently, increased water and air pollution can occur due to release of effluents containing chemicals into water bodies and release of toxic gases released into the atmosphere [1]. Organic dyes are one of the chemicals that are widely used in many industries such as textiles, pharmaceuticals, plastics, cosmetics, paper, and leather [2]. The release of these dyes into freshwater environments causes many problems for biota, such as reducing the effectiveness of photosynthesis, in addition to human health problems [3]. Methyl orange (MO) and reactive orange 16 (RO16) are among the azo dyes that are typical of anionic model organic pollutants [4, 5]. These dyes are non-degradable which makes them a great danger to aquatic environments, including their health hazards [6].

On the other hand, sulfur dioxide (SO2) is considered one of the most harmful gases, which is produced from several industries such as the combustion of fossil fuels (e.g., coal, gas, and oil) in the power plants, and the smelting of mineral ores [7]. This gas causes several environmental phenomena such as photochemical smog, acid rain, and haze [8]. These phenomena can negatively affect climatic systems (e.g., the atmosphere, rivers, snow cover, and sea ice), plant communities, human health, and animal growth [8, 9]. In light of the foregoing, the treatment of wastewater containing organic pollutants and the removal of toxic gases before release into the environment is an urgent matter to preserve the ecosystem and human health. Several methods have been reported in the treatment of dyeing wastewater including adsorption [10], nanofiltration [11], photocatalytic degradation [12], and electrocoagulation [13], while in air pollution with SO2 gas, the common methods applied for capturing SO2 employ liquid alkaline solutions (e.g., Ca(OH)2, and Mg(OH)2), and adsorbent materials (e.g., activated carbon) [7]. In general, adsorption technology is a preferred method for pollutant removal from water and air. Adsorption is characterized by unique and favourable features (e.g., simplicity of design, low investment cost, lack of secondary pollution, removal efficiency, and regeneration ability) that position it at the forefront of methods applied to air and water pollution control [9, 14]. Activated carbon (AC) is one of the most popular adsorbents in the field of adsorption of air and water pollutants due to its amazing properties such as porosity, high adsorption capacity, high surface area, and thermal stability [15, 16]. Despite these impressive properties of AC, it is economically expensive to produce. For this reason, research efforts have shifted to low-cost, renewable, and environmentally friendly adsorbents.

Chitosan (CHI) is one of the most environmentally friendly, renewable, and efficient adsorbents employed in adsorption technology [17]. In acidic media, CHI is a natural cationic polysaccharide that consists of glucosamine units [18]. CHI has several unique advantages over other common adsorbents, such as its ease of modification, biodegradability, super absorbency, hydrophilicity, and nontoxicity [19]. The chemical structure of CHI is characterized by abundant amino and hydroxyl groups, which are likely to be active adsorption sites for the removal of acid gases such as SO2 and acid dyes [20, 21]. Despite the foregoing, some of the properties of CHI require further development such as the effective surface area, chemical stability, and mechanical strength. Several strategies have been employed in developing the CHI properties such as composite formation with inorganic materials [22], grafting with organic groups [23], and crosslinking [24]. The development of CHI by composite formation with inorganic materials has attracted much attention due to its extraordinary properties like improved surface area, absorption capacity, chemical stability, and mechanical strength [22].

Carbon-doped TiO2 (known as Kronos) is one of the most promising materials that can be employed in modifying the properties of CHI due to its distinctive characteristics such as its surface area, chemical stability, mechanical strength, photo activity, and adsorption efficiency of inorganic/organic contaminants [25, 26]. Many studies have reported on the development of CHI by TiO2 and its utilization in various applications such as wastewater treatment [27], biosensor [28], and antimicrobial [29], but with carbon-doped TiO2 (C–TiO2), no studies were reported thus far. Furthermore, the CHI -based composites for capturing hazardous gases were recently explored, for example, CHI grafted Leca biocomposite for the capture of carbon dioxide (CO2) [30], adsorption of formaldehyde gas by CHI crosslinked with MOF-199@ aminated graphene oxide aerogel [31], CHI extracted from shrimp shells loaded with Cd ions for the capture of hydrogen sulfide (H2S) [32], CHI aerogel for the removal of ammonia (NH3) gas [33], and ferrocene- CHI for sorption of carbon monoxide (CO) gas [34].

Herein, this study aims to develop a CHI biopolymer by loading different wt% of C-doped TiO2 onto CHI to attain an efficient adsorbent, referred to as CHI/C-doped TiO2 (CHI/C–TiO2). The synthesized composites were fully characterized by BET, FTIR, XRD, TEM, SEM–EDX, pHpzc, and pH-potentiometric titrations. The fabricated materials were applied for the removal of organic pollutants (MO and RO16) and SO2 capture. Statistical modeling represented by Box–Behnken design (BBD) was utilized for optimization of the impacts of the C–TiO2 particles loading, dose, pH, and temperature on the adsorption of MO and RO16 dyes. In addition, several kinetic and isothermal models have been utilized to investigate the adsorption properties and mechanism of the CHI/C–TiO2 composite.

Materials and Methods

Materials

Chitosan (CHI, deacetylation ≥ 75%), and Carbon-doped TiO2 (C–TiO2, Kronos) were supplied by Sigma–Aldrich. The organic dyes including methyl orange (MO; C14H14N3NaO3S; MW: 327.32; λmax = 464 nm), and reactive orange 16 (RO16; C20H17N3Na2O11S3; MW: 617.54 g/mol; λmax = 493 nm) were obtained from ACROS, Organics. The chemicals including acetic acid (CH3COOH), sodium chloride (NaCl), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were obtained from R&M Chemicals.

Synthesis of CHI/C–TiO2

The synthesis of CHI/C–TiO2 was based on the approach described in previous work [35]. Initially, 1 g of CHI flakes was poured into 60 ml of CH3COOH acid solution (5% v/v) and stirred at 500 rpm on a magnetic stirrer for 24 h at 25° C to dissolve the CHI flakes. Then, the CHI solution was added drop-wise into 1000 ml NaOH (2 M) solution to obtain CHI beads. The resulting CHI hydrogel beads were washed with distilled water until the pH became neutral. For comparison, CHI/C–TiO2 composites were fabricated by loading different wt. % of C–TiO2 with CHI before adding to the solution of CH3COOH, i.e. 25:75 wt% (C–TiO2:CHI), which was termed as CHI/C–TiO2-25 and 50:50. wt% (C–TiO2:CHI), which was termed as CHI/C–TiO2-50. Finally, the fabricated composites were crushed and then sieved to particle size (250 μm) for the dye removal tests, along with SO2 capture.

Characterization

The textile properties (e.g., specific surface area, and average pore width) of the CHI/C–TiO2-50 were obtained through Brunauer–Emmett–Teller (BET) method (Micromeritics ASAP 2060). The morphological characteristics of CHI/C–TiO2-50 before and after adsorption of MO and RO16 dyes were achieved by recording scanning electron microscope (SEM, Zeiss Supra 40 VP) images. The energy dispersive X-Ray analysis (EDX) was performed to the chemical qualitative analysis of the CHI/C–TiO2-50 before and after adsorption of MO and RO16 dyes. X-ray powder diffraction spectra (XRD, X’Pert PRO, PANalytical) were used to evaluate the phase determination of the CHI/C–TiO2-50 composite. Fourier Transform Infrared (FTIR) spectra (Perkin-Elmer, Spectrum RX I) was used to determine the surface chemical characteristics of the CHI/C–TiO2-50 composite before and after MO and RO16 dyes adsorption. The free amino (-NH2) groups in the CHI/C–TiO2-50 was computed by pH-potentiometric titrations [36]. Zero-point of charge (pHpzc) analysis was employed to determine the surface charge of the CHI/C–TiO2-50 [37].

Experimental modeling

BBD was used in this research work as an efficient and reliable statistical method to provide MO removal (%) and RO16 removal (%) as responses based on several independent factors that affect the adsorption properties: C–TiO2 loading, pH, adsorbent dosage, and temperature. The statistical analysis and numerical demonstration of the experimental results related to MO and RO16 removal were acquired through the software program of Design-Expert (Stat-Ease, version 13). Table 1 shows the parameters evaluated inside the BBD model, along with their levels and symbols. A polynomial equation (Eq. 1) was applied to correlate the experimental results and estimate the MO removal (%) and RO16 removal (%).

Table 1 Coded and actual variables and their levels in BBD
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$$Y={\beta }_{0}+\sum{\beta }_{i}{X}_{i}+\sum{\beta }_{ii}{X}_{i}^{2}+\sum\sum{\beta }_{ij}{X}_{i}{X}_{j}$$
(1)

where Y, and \({(X}_{i}\) and \({X}_{j})\) symbol the forecasted response (MO removal (%) or RO16 removal (%)); and the evaluated factors. \({\beta }_{0}\) and \({\beta }_{i}\), denote the regression coefficients of intercept, and the linear; whereas, \({\beta }_{ii}\) and \({\beta }_{ij}\) signify quadratic and interaction effects. The BBD model produced 29 runs to examine the influence of the following factors: A: C–TiO2 loading (0–50%), B: adsorbent dose (0.04–0.15 g), C: pH (4–10), and D: temperature (30–50 °C) on MO and RO16 dyes adsorption process. The actual values of responses (MO and RO16 removal) along with their experimental conditions are recorded in Table 2. In 250 ml flasks that contain 100 ml of the dye solution, a constant quantity of the adsorbent was added. Then, these sealed flasks were transferred into a shaker water bath (WNB7-45, Memmert) at a speed of up to 90 strokes/min. The next step after the adsorption process was to remove the adsorbents from solutions with a syringe filter (0.45 μm). Then, the residual concentrations of MO and RO16 in solutions was estimated using spectrophotometry with a UV–Vis spectrometer (HACH DR 2800) at λmax 464 nm and 493 nm, respectively. The removal efficiency (R, %) of MO and RO16 dyes were calculated using Eq. 2 shown below.

Table 2 The 4-variables BBD matrix and experimental data for MO removal and RO16 removal
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$$R \%=\frac{\left({C}_{o}-{C}_{e}\right)}{{C}_{o}} \times 100$$
(2)

where \({C}_{o}\) (mg/L) and \({C}_{e}\) (mg/L) represent the dye concentrations (MO or RO16) before and after the adsorption process, respectively.

Adsorption study of MO and RO16 dyes on CHI/C–TiO 2 -50

Determining the adsorption ability of CHI/C–TiO2-50 toward MO and RO16 dyes was accomplished by use of batch experiments. As determined in Table 2, the maximum removal of MO (81.6%) and RO16 (90.4%) dyes was accomplished in run eighteen under the following conditions: C–TiO2 loading = 50%, dosage = 0.095 g (adjusted to 0.1 g in adsorption tests), pH 4, and temperature = 40 °C. In this way, batch tests were carried out to study the uptake the MO and RO16 dyes at these previously mentioned ideal conditions over a range of initial concentrations (20–200 mg/l). The calculation of the adsorption capacity (qe, mg/g), of MO and RO16 dyes on the surface of the CHI/C–TiO2-50 at equilibrium employed Eq. 3.

$${q}_{e}=\frac{\left({C}_{o}-{C}_{e}\right)V}{W}$$
(3)

where W (g) and V (L) are the amount of CHI/C–TiO2-50 and the volume of the dye solution (MO and RO16), respectively.

SO2 adsorption tests

The SO2 adsorption tests were carried out by Ultramat 23 Gas Analyzer integrated with computer dynamic simulation to adjust the gas flow rate of SO2 for the tests. 1 g of the synthesized adsorbent was placed in the reaction vessel and the temperature was maintained at 87 °C. Before SO2 capture, N2 (99.99%) was passed through the prepared adsorbent to exclude any undesired compounds. After that, a stream of SO2 (1000 ppm) gas was fed to the column containing adsorbent meanwhile keeping on the feed flow at 500 ml/min. Equation 4 was utilized to calculate the breakthrough adsorption capacity (S, mmol/g) of SO2 gas.

$$S=\frac{{10}^{-6}q{\int }_{0}^{{t}_{m}}\left({C}_{0}-{C}_{i}\right)dt}{22.4G}$$
(4)

where q, tm, and G signify the gas flow rate (ml/min), is the breakthrough time (min), and the amount of the adsorbent (g). Co and Ci represent the initial concentration (ppm), and the outlet concentration (ppm) of SO2 gas.

Results and discussion

Characterization of the CHI/C–TiO2-50

Table 3 lists the physicochemical properties of the CHI/C–TiO2, specifically the amine content (%) and surface area characteristics. pH-potentiometric titration is a vital method used to estimate the proportion of primary amino groups (-NH2) existing in the backbone of the CHI/C–TiO2-50, which symbolizes adsorptive sites for anionic dyes such as MO and RO16 dyes. The –NH2 content in CHI/C–TiO2-50 determined by potentiometric titration reached 38.41%. This finding suggests that the CHI/C–TiO2-50 still has plenty of –NH2 to bind with MO and RO16 dyes. The results of the BET analysis showed that the surface area of the CHI/C–TiO2-50 composite is significantly higher than the cross-linked CHI composites containing 50% of TiO2 nanoparticles. For example, cross-linked CHI-glyoxal/TiO2 (17.74 m2/g) [38] and cross-linked CHI-tripolyphosphate/TiO2 (2.75 m2/g) [35] indicate that C–TiO2 possesses a high surface area that plays a significant role in improving the surface properties of CHI/C–TiO2-50 through supporting onto the CHI biopolymer matrix. As per IUPAC, CHI/C–TiO2-50 is a mesoporous material due to its average pore diameter of 7.31 nm.

Table 3 The physicochemical properties of the CHI/C–TiO2-50
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The X-ray diffraction (XRD) profile of the CHI/C–TiO2-50 composite is illustrated in Fig. 1. The broad peak located at 20.3° in Fig. 1 relates to the semi-crystalline nature of CHI biopolymer [39]. In Fig. 1, several peaks at 2θ of 25.28°, 37.8°, 48.05°, 53.89°, 55.1°, 62.6°, 68.7°, 70.3° and 75.01° correspond to the [101], [004], [200], [105], [211], [204], [116], [220], and [215] crystalline phases of C–TiO2, respectively with the Miller indices (JCPDS file No. 00-021-1272) [40].

Fig. 1
figure 1

XRD pattern of CHI/C–TiO2-50

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TEM analysis was performed to accurately determine the particle size of C–TiO2. The TEM images of C–TiO2 particles at several wavelengths (500 nm, 200 nm, 100 nm, and 50 nm) are illustrated in Fig. 2a–d, respectively. According to the TEM results in Fig. 2, the size of C–TiO2 particles is 8.60 nm (Fig. 2d). These results concur with the results of the BET analysis, which support that the surface area of the CHI/C–TiO2-50 was 89.34 m2/g. The presence of nano-sized particles (C–TiO2) in the formula designed (CHI/C–TiO2-50) for adsorption of anionic dyes and SO2 gas impart a high adsorption efficiency to remove organic and inorganic pollutants.

Fig. 2
figure 2

The TEM images of C–TiO2 particles at variable magnification namely a 500 nm, b 200 nm, c 100 nm, and d 50 nm

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The surface functional groups of the CHI/C–TiO2-50, and CHI/C–TiO2-50 after MO and RO16 dyes adsorption were determined by FTIR analysis. The FTIR spectra of CHI/C–TiO2-50, and CHI/C–TiO2-50 after the adsorption of MO and RO16 dyes are shown in Fig. 3a–c. The characterized bands of CHI/C–TiO2-50 composite (Fig. 3a) at 3698 cm−1, 3500–3650 cm−1, 2270 cm−1, 1660 cm−1, 1450 cm−1, 1380 cm−1, 1075 cm−1, 770 cm−1, and 540 cm−1 are attributed to the hydroxyl groups of TiO2, –OH and –NH2 stretching, stretching of C≡C, N–H bending vibration, deformations of C–O–H, C–N stretching, C–O–C stretching, stretching vibration of Ti–O–Ti and Ti–O bond, respectively [41, 42]. The FTIR spectra of the CHI/C–TiO2-50 composite after MO (Fig. 3b) and RO16 (Fig. 3c) adsorption showed a remarkable change in the bands of –NH2 and –OH groups, which is notably shifted, and broadened, confirming that the –OH and NH2 groups participate in the MO and RO16 adsorption process. Besides, the peak at 1500 cm−1 can be attributed to the aromatic rings (C=C) of the MO and RO16 dyes further confirm the loading or adsorption of MO and RO16 dyes onto the CHI/C–TiO2-50 surface.

Fig. 3
figure 3

FTIR spectra of a CHI/C–TiO2-50, and CHI/C–TiO2-50 after adsorption of b MO and c RO16 dyes

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Exploration of the surface morphology of the CHI/C–TiO2-50 before and after adsorption of MO and RO16 dyes as well as estimates of its chemical composition was achieved by SEM–EDX. Figure 4a–c provides SEM–EDX images of (a) CHI/C–TiO2-50, and CHI/C–TiO2-50 following adsorption of (b) MO and (c) RO16 dyes. As seen in Fig. 4a, the surface features for the 50% C–TiO2 particles loaded onto CHI reveal that the CHI/C–TiO2-50 had a granular surface containing tiny pores, which illustrates the embedding of C–TiO2 particles onto the polymer matrix of CHI. The EDX spectrum of the CHI/C–TiO2-50 reveals the presence of Ti, O, C, and N. The appearance of a Ti signature in the EDX spectrum is further evidence that the C–TiO2 particles were effectively incorporated onto the CHI matrix. The CHI/C–TiO2-50 surface was altered into a smooth, packed, and uniform surface after adsorption of MO (Fig. 4b) and RO16 (Fig. 4c), consistent with the uniform dye surface coverage. This result was confirmed by EDX, which showed that the content of C (%) was increased because of loading the dye (MO or RO16) molecules.

Fig. 4
figure 4

SEM–EDX images of a CHI/C–TiO2-50, and CHI/C–TiO2-50 after adsorption of b MO and c RO16 dyes

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Model validation

A statistical analysis of variance (ANOVA), including the statistical evaluation of the significant impacts of the factors and their interactions related to the MO and RO16 dyes removal was carried out. The ANOVA findings for the removal of the MO and RO16 dyes are listed in Table 4. The MO and RO16 removal models have F-values of 23.63 and 18.12, respectively, implying that the models are critical [43]. The correlation coefficients (R2) of 0.96 (MO removal) and 0.95 (RO16 removal) indicate that the models are well fitted and that there is a favourable agreement between theoretical and actual removal values [44]. Statistically, the BBD model terms have statistical significance when the p-value ≤ 0.05. Accordingly, A, B, C, AB, AC, BC, A2, and D2 are critical terms in the removal process of the MO and RO16 dyes. Along these lines, the linkage between MO and RO16 removal with the examined variables was empirically fitted by the quadratic polynomial formula, as expressed in Eqs. 5 and 6, respectively.

Table 4 Analysis of variance (ANOVA) for MO removal and RO16 removal
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$$\mathrm{MO} \; \mathrm{removal } \; (\mathrm{\%}) \,{=+18.08+25.23\mathrm{A}+12.20\mathrm{B}-15.13\mathrm{C}+11.20\mathrm{AB}-11.65\mathrm{AC}-9.20\mathrm{BC}+11.40{\mathrm{A}}^{2}+6.51{\mathrm{D}}^{2}}$$
(5)
$$\mathrm{RO}16 \; \mathrm{ removal } \; (\mathrm{\%})\, {=+23.94+26.77\mathrm{A}+12.75\mathrm{B}-17.70\mathrm{C}+11.82\mathrm{AB}-10.88\mathrm{AC}-12.63\mathrm{BC}+11.44{\mathrm{A}}^{2}+8.39{\mathrm{D}}^{2}}$$
(6)

Analysis of the diagrams inclusive of real versus predicted, where the normal probability of the residuals might be utilized to successfully verify the experimental findings of dye removal for MO and RO16. The real vs. expected graphs of MO and RO16 removal values are shown in Fig. 5a, b, where the real points were largely near the predicted points, demonstrating that the BBD model can optimize the adsorption process for these dyes. The normal probability charts of the MO and RO16 removal dyes are depicted in Fig. 5c, d. As seen from Fig. 5c, d, the typical distribution of dots around the straight-line denote the independence of the residuals [41].

Fig. 5
figure 5

Plots the relationship between the actual and predicted values of a MO and b RO16 removal; the normal probability of residuals for c MG and d RO16 removal

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Adsorption optimization of MO and RO16 dyes

3D response surfaces plots are as one of the main graphical representations that provide an understanding of the nature of the fundamental interactions among the two investigated variables and their impacts on the responses denoted by the MO removal and RO16 removal. The cumulative impacts of C–TiO2 loading and dosage (termed as AB interaction) on the MO and RO16 removal were meaningful. Figure 6a, b displays the 3D surface plot of the cumulative impacts of AB interaction on the MO and RO16 removal at the accompanying fixed circumstances: pH (7) and temperature (40 °C). The findings shown in Fig. 6a, b found that increasing the adsorbent dosage and C–TiO2 loading enhances the removal of MO and RO16 dyes. Such findings are related to the excessive surface area coming from C–TiO2 particles and the adequate quantity of reactive sites available on the CHI/C–TiO2 surface, which are attracted with MO and RO16 molecules to obtain the adsorption process.

Fig. 6
figure 6

3D surface plots of the cumulative impacts of AB interaction toward a MO and b RO16 removal

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The cumulative impacts of C–TiO2 loading vs. pH (termed as AC interaction) and dose vs. pH (termed as BC interaction) on the MO and RO16 removal were meaningful. Figure 7a–d displays the 3D surface plot of the cumulative impacts of AC and BC interactions on the MO and RO16 removal at the accompanying fixed conditions: dose (0.095 g) and temperature (40 °C) for AC interaction; and C–TiO2 loading (25%) and temperature (40 °C) for BC interaction. As demonstrated in Fig. 7a–d, the MO and RO16 (%) improved simultaneously with a pH drop from 10 to 4. As shown in Fig. 7e, pHpzc of the CHI/C–TiO2-50 equals 7.0. This observation indicates that the CHI/C–TiO2 surface acquires negative charges at pH values larger than pHpzc. As a result of the rise in H+ concentration in the solution, the CHI/C–TiO2 surface becomes positively charged at an acidic medium. Therefore, the electrostatic attractions between the sulfonate (–SO3−) groups belonging to the MO and RO16 dyes and cationic groups of CHI/C–TiO2-50, as shown in Eq. 7.

Fig. 7
figure 7

3D surface plots of AC (a MO and b RO16) and BC (c MO and d RO16) interactions on the MO and RO16 removal; along with, e pHpzc of CHI/C–TiO2-50

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$${\mathrm{CHI}/\mathrm{C}-{\mathrm{TiO}}_{2}^{+} + {\mathrm{Dye}-\mathrm{SO}}_{3}^{-} \leftrightarrow{\text{ CHI}}/{\text{C}}-{\text{TiO}}_{2}^{+} \dots {}{}^{-}{\mathrm{O}}_{3}\mathrm{S}-\mathrm{Dye}}$$
(7)

where the \({\mathrm{Dye}-\mathrm{SO}}_{3}^{-}\) indicates either the MO or the RO16 dye species.

Adsorption study

An investigation on the effect of initial concentration on the adsorption behaviour of MO and RO16 dyes by the CHI/C–TiO2-50 composite was performed at several dye adsorbate concentrations (20–200 mg/l); whereas, the mass of the adsorbent (0.095 g), volume of the dye solution (100 ml), pH (4), and temperature (40 °C) were held constant. The adsorption property of CHI/C–TiO2 towards MO and RO16 dyes as a function of contact time are shown in Fig. 8a, b. In Fig. 8a, b, the adsorption capacity of CHI/C–TiO2-50 was found to increase from 17.9 to 148.4 mg/g for MO and from 20.4 to 193.7 mg/g for RO16, across a range of dye concentration (20 to 200 mg/l). The higher concentration range for MO and RO16 provides an important driving force that contributes to the transport of MO and RO16 molecules to the available active sites on the CHI/C–TiO2-50 surface [45].

Fig. 8
figure 8

Effect of the contact time on a MO and b RO16 adsorption at different initial concentrations (dosage 0.1 g, pH of solution 4, temperature 40 °C, agitation speed = 90 strokes and volume of solution = 100 ml)

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SO2 capture

The removal of SO2 over CHI biopolymer in the presence and absence of variable C–TiO2 loadings was investigated. Figure 9 displays the breakthrough curves of SO2 capture over CHI biopolymer, CHI/C–TiO2-25, and CHI/C–TiO2-50. According to the breaking curves exhibited in Fig. 9, the saturation time of CHI biopolymer, CHI/C–TiO2-25, and CHI/C–TiO2-50 was 11 min, 30 min, and 35 min, respectively. These results concur with the values of SO2 adsorption capacity listed in parentheses as follows: CHI/C–TiO2-25 (0.6140 mmol/g), CHI/C–TiO2-50 (0.7622 mmol/g), and CHI biopolymer (0.1342 mmol/g). Therefore, the SO2 adsorption data revealed that SO2 capture by the CHI/C–TiO2-25 and CHI/C–TiO2-50 biomaterials was significantly improved compared to the CHI biopolymer. The improvement in adsorption performance of CHI biopolymer (without C–TiO2 particles) upon loading with C–TiO2 loading (especially 50%) for CHI/C–TiO2-50 (89.34 m2/g) can be attributed to the greater surface area effects of the C–TiO2 particles. In addition to the above, and from a chemical point of view, CHI/C–TiO2-50 possesses active adsorption sites such as Ti–O, C–O, –OH, and –NH2, which can participate in the binding of SO2 molecules [8, 20]. Fundamentally, the SO2 capture occurred at a high temperature of 360 K, which signifies that the surface sulfate-like species (SO42−) was formed from the SO2 capture process is activated by elevated temperature [46].

Fig. 9
figure 9

Breakthrough curves of SO2 capture over CHI, CHI/C–TiO2-25, and CHI/C–TiO2-50

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Kinetic study

In order to understand the adsorption behavior and determine the control mechanism responsible for the adsorption of MO and RO16 dyes on the surface of CHI/C–TiO2-50, the experimental data were evaluated by common kinetics model equations, via the pseudo-first-order (PFO) [47] and pseudo-second-order (PSO) models [48]. The terms of the kinetics including PFO and PSO were extracted by the non-linear formulas listed in Eqs. 8 and 9, respectively.

$$ q_{t} = q_{e} (1 - \exp^{{ - k_{1} t}} ) $$
(8)
$$ q_{t} = \frac{{q_{e}^{2} k_{2} t}}{{1 + q_{e} k_{2} t}} $$
(9)

where qe (mg/g) and qt (mg/g) represent the adsorption capacities at equilibrium and time (t), respectively. k1 (1/min) and k2 (g/mg min) are the rate constants of the PFO and PSO kinetic models, respectively. The calculated kinetic parameters are recorded in Table 5, where it can be concluded that the adsorption process for both dyes are described by the PFO model over that of the PSO model. This trend is supported by the relatively high R2 values of PFO compared to the R2 values of PSO, in addition to the experimental qe values, which are closer to the calculated qe values of the PFO model, in comparison with the calculated qe values of PSO. The results reported herein support that the adsorption of the MO and RO16 dyes on CHI/C–TiO2-50 is physisorption that involves electrostatic attractions [49]. The latter occurs between the cationic groups present on the surface of CHI/C–TiO2-50 and the -SO3− groups of the MO and RO16 dyes, respectively.

Table 5 PFO and PSO kinetic parameters for the adsorption MO and RO16 on CHI/C–TiO2-50
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Isotherm study

An account of the mode of interaction between CHI/C–TiO2-50 and the MO and RO16 dye species can be obtained by analysis of the equilibrium isotherms. The adsorption capacity of CHI/C–TiO2-50 for the uptake MO and RO16 dyes from the liquid phase was determined through the fitting of the experimental results to equilibrium isotherms like Langmuir, Freundlich, and Temkin [50,51,52]. The terms of the isotherms including Langmuir, Freundlich, and Temkin were extracted by the non-linear formulae listed in Eqs. 10–12, respectively.

$$ q_{e} = \frac{{q_{\max } K{}_{a}C_{e} }}{{1 + K_{a} C_{e} }} $$
(10)
$$ q_{e} = K_{f} C_{e}^{1/n} $$
(11)
$${q}_{e}=\frac{RT}{{b}_{T}}\mathrm{ln}({K}_{T}{C}_{e})$$
(12)

Ka (L/mg), KT (L/mg), and Kf (mg/g) (L/mg)1/n, are the adsorption constants of Langmuir, Temken, and Freundlich isotherms. qmax (mg/g) is the adsorption capacity of the CHI/C–TiO2-50. n is the exponent term for the Freundlich model. bT (J/mol) is a term related to the heat adsorption. T (K) and R (8.314 J/mol K) denote the temperature and universal gas constant, respectively. The curves acquired from adsorption isotherms for MO and RO16 dyes are exhibited in Fig. 10a, b while the calculated terms of the isotherms are given in Table 6. Depending on the R2 values, it can be concluded that the adsorption of MO and RO16 dyes by CHI/C–TiO2-50 fit better with the Langmuir isotherm model, based on the highest R2 value compared to the other models. These findings signify the homogeneous character of the adsorbent surface for CHI/C–TiO2-50 with the MO and RO16 dyes [53]. The qmax values for MO and RO16 dyes on CHI/C–TiO2-50 were estimated as 196.6 mg/g and 270.5 mg/g at 40 °C, respectively. In order to get a better idea of the CHI/C–TiO2-50's adsorption capacity, it was compared with the adsorption capabilities of other materials studied for the removal of such anionic dyes (MO and RO16), according to the values listed in Table 7. These results show that the CHI/C–TiO2-50 poses a good ability to remove organic pollutants, specifically anionic dyes from polluted water.

Fig. 10
figure 10

Adsorption isotherms of a MO and b RO16 by CHI/C–TiO2-50 (dosage 0.1 g, pH of solution 4, temperature 40 °C, agitation speed = 90 strokes and volume of solution = 100 ml)

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Table 6 Parameters of the Langmuir, Freundlich and Temkin isotherm models for the adsorption of MO and RO16 on CHI/C–TiO2-50 at 40 °C
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Table 7 Comparison of the adsorption capacity of CHI/C–TiO2-50 toward the adsorption MO and RO16 dyes with various adsorbents
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Adsorption mechanism of MO and RO16 dyes

The surface of CHI/C–TiO2 is characterized by an enormous amount of active groups such as hydroxyl (–OH), amino (–NH2), and (Ti–OH), as per the FTIR spectrum, which is consistent with other studies [35, 38]. Because of the existence of such functional groups, the mechanism of MO and RO16 dye adsorption on the surface of CHI/C–TiO2 was deduced, as seen in Fig. 11. One of the key pathways contributing to the adsorption of MO and RO16 dyes on the surface of CHI/C–TiO2 is electrostatic forces that achieve by the attraction between the negative charge of the MO and RO16 dyes and the negatively charged groups of the CHI/C–TiO2 surface as shown in Fig. 11. The interaction between the hydrogen on the CHI/C–TiO2 surface and the nitrogen and oxygen atoms in the MO and RO16 dye structures is another probable interaction contributing to the adsorption of MO and RO16 dyes, which is called H-bonding. The n-π stacking is also implicated in the adsorption of the MO and RO16 dyes via the interface that occurs between the CHI/C–TiO2 surface (oxygen and nitrogen groups) and the aromatic rings of the MO and RO16 dyes [66].

Fig. 11
figure 11

Illustration of the probable interaction between CHI/C–TiO2 surface and MO and RO16 dyes including electrostatic forces, n-π stacking, and H-bonding

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Conclusion

A composite adsorbent with improved adsorption properties over pristine chitosan was reported herein. The composite is comprised of chitosan/C-doped TiO2 (CHI/C–TiO2) was found to be suitable for the removal of organic pollutants (methyl orange, MO; and reactive orange 16, RO16) and capture of an inorganic gas (SO2). The best experimental conditions for the design of the composite adsorbent herein and the adsorption conditions for the highest removal of MO (81.6%) and RO16 (90.4%) dyes are as follows: C–TiO2 loading = 50%, adsorbent dosage = 0.095 g, pH 4, and temperature = 40 °C. The results from the BBD model reveal that the highest MO and RO16 removal were obtained upon consideration of the following interactions: AB (C–TiO2 loading × dose), AC (C–TiO2 loading × pH), and BC (dose × pH). The results for the adsorption isotherms and the kinetic profiles showed that the adsorption process of MO and RO16 dyes onto the surface of CHI/C–TiO2-50 is primarily monolayer adsorption and physisorption in nature. The maximum adsorption capacities of CHI/C–TiO2-50 (containing 50% C–TiO2) were found to be 196.6 mg/g and 270.5 mg/g of MO and RO16 dyes, respectively. The developed biocomposite material (CHI/C–TiO2-50) was found to be an effective adsorbent for environmental applications such as the treatment of wastewater, along with gas capture, as shown for SO2.