The influence of carbonization temperature on the modification of TiO₂ in the removal of methyl orange from aqueous solution by adsorption

Abstract

This work investigated the adsorption ability of unmodified and carbon-modified TiO₂ nanoparticles for the removal of methyl orange (MO) from aqueous solution. Carbon–TiO₂ was obtained by carbonization of ethanol vapors at three different temperatures (200, 300, and 400°C), and their adsorption was compared with unmodified TiO₂ nanoparticles. The Freundlich adsorption model was found to fit for TiO₂ and C–TiO₂-200, while carbon modification of TiO₂ at a high temperature fitted the Langmuir–Freundlich model (C–TiO₂-300 and C–TiO₂-400). Generally, the carbonization of C–TiO₂ increased the adsorption capacity of TiO₂ nanoparticles, however the BET surface of modified and pristine TiO₂ was almost the same. The zeta potential of modified TiO₂ is higher than unmodified TiO₂, which leads to efficient adsorption of MO onto modified TiO₂.

Keywords:

• Methyl orange
• Dye
• TiO₂
• Carbon modification
• Zeta potential
• Adsorption

1. Introduction

Environmental protection is the practice of protecting the natural environment, which is also of benefit to humans. Azo dyes are among the thousands of industrial dyes that break down to toxic products harmful to ecosystems and human health [1]. Thus, the efficient removal of these dye pollutants from wastewaters is very important.

Many types of biological and chemical process including ozonation [2], flocculation [3], coagulation [3,4], and adsorption [5–7] have been used to remove dye pollutants. Adsorption is simple, low cost, and in many cases the most effective of these.

Although activated carbon is one of the most efficient adsorbents for different types of pollutant, it is expensive, usually time-consuming, and complicated to prepare. Thus, many attempts have been made to find economical yet effective adsorbents. Nano-adsorbents are highly efficient and have been extensively used for dye removal. The very high surface area to volume ratio of many nanomaterials makes them extremely advantageous as adsorbents. For example, Raveendra et al. [8] synthesized nano ZnTiO₃ ceramic for the removal of azo dyes from aqueous solutions. Iron oxide nanospheres were synthesized by Khosravi et al. and used to remove anionic dyes from water [9]. Wu et al. used a hydrothermal procedure to prepare magnetic Fe₃O₄@C nanoparticles for dye adsorption [10].

Nanostructured titanium dioxide (TiO₂), which is a semiconductor material, is nontoxic, inexpensive, active in reactions, and chemically stable [11–14]. Therefore, it has many applications, including its use as an adsorbent [7,15,16]. TiO₂ has also been applied to remove anionic dye pollutants from wastewaters. The main issue in applying TiO₂ as an adsorbent is the problem of separation from an aqueous medium. Separation techniques, such as membrane filtration [17] are expensive. Yet commercial TiO₂ does not show a very high adsorption capacity for dye removal compared to activated carbons [6]. Doping the other materials to TiO₂ is an effective method to improve its adsorbent and photocatalytic character, also enhancing the separation efficiency.

The modification of TiO₂ with carbon creates a synergetic effect established by the adsorption capacity of activated carbon and the photoactivity of TiO₂ [11,18,19]. Carbon-modified TiO₂ has been used to adsorb azo dyes, such as reactive red 198 and direct green 99, [1] or reactive black 5 diazo dye [20]. These studies show that the combination of activated carbons and TiO₂ nanoparticles can produce a proper adsorbent which has both high adsorption capacity and excellent photocatalytic activity.

In the present study, carbon-modified TiO₂ (C–TiO₂) was prepared by ethanol carbonization and used for the removal of methyl orange (MO) dye pollutant from aqueous solution. The characterization of the prepared adsorbents was carried out by SEM images, N₂ adsorption/desorption isotherm, and XRD patterns. UV–vis spectroscopy was used to follow the adsorption process. The adsorption equilibrium, adsorption kinetics, and the effect of pH on dye removal efficiency were investigated. The effect of carbonization temperature on the zeta potential (ZP) and removal efficiency of MO by the prepared adsorbents was also studied. The ability to recover an alternative adsorbent is important from both an economic and a practical perspective. Thus, desorption studies were also performed.

2. Experimental

2.1. Materials

MO sodium salt dye was supplied by J.T. Baker and used without further purification. TiO₂ (nanopowder avg size = 25 nm with a chemical purity of ≥99.5) was purchased from Sigma–Aldrich and used as a precursor for the modification of TiO₂ (C–TiO₂).

2.2. Instruments

MO concentration was analyzed by UV–vis spectroscopy using a Jasco V-670 spectrophotometer (Jasco, Japan). The X-ray diffraction (XRD) patterns of modified TiO₂ were obtained with a PANalytical X́Pert PRO alpha 1 diffractometer. Cu Kα1 radiation (1.5406 Å; 45 kV, 30 mA) was used, generated by primary beam Johansson monochromator. The Hitachi S-4800 (Hitachi, Japan) Ultra-High Resolution Scanning Electron Microscope (SEM) was used to analyze the morphology and size of the adsorbents.

N₂ adsorption–desorption isotherms were obtained with an ASAP2010 (Micromeritics). The samples were outgassed at 100°C for 12 h, before nitrogen adsorption. The pore volume distributions as a function of pore size was calculated based on the Dubinin–Radushkevich equation using adsorption branches of the measured isotherm.

FTIR type Vertex 70 by Bruker Optics (Germany) was used to identify the surface functional groups of the prepared adsorbents. The FTIR spectra were recorded at 4 cm⁻¹ resolution from 400 to 4,000 cm⁻¹ and 100 scans per sample. The surface charge and point of zero charge of the modified and unmodified TiO₂ were determined by isoelectric point titration as a function of pH using a Zetasizer Nano ZS (ZEN3500, Malvern). The determination of carbon was obtained using an Organic Elemental Analyzer Flash 2000 (Thermo Scientific, Germany).

2.3. Preparation of carbon-modified TiO₂

The carbon-modified TiO₂ was prepared using the method described by Janus et al. [21]. The installation setup used for adsorption preparation is schematically shown in Fig. 1.

Fig. 1. Schematic presentation of installation setup for the synthesis of C–TiO₂ [21].

Notes: (1) Gas cylinder with argon, (2) controller, (3) Dreschel bottle with ethanol, (4) pipe furnace, (5) combustion boat with powder sample, and (6) scrubber.

Briefly, 1 g of TiO₂ in a porcelain boat was modified with carbon by heating up to the specified temperature with a pipe furnace under ethanol vapor for 40 min and maintaining that temperature for 1 h. The ethanol vapor was prepared by bubbling pure argon through ethanol at room temperature.

Commercial TiO₂ consists of a mixture of anatase/rutile phases [22]. Previously, we have shown that up to 600°C the phase ratio of anatase to rutile remained nearly constant and phase transition was not observed [6]. Thus, to investigate only the influence of carbonization temperature on the removal efficiency of C–TiO₂ adsorbents, these were synthesized at temperatures of 200, 300, and 400°C. The C–TiO₂ samples were identified as C–TiO₂-200, C–TiO₂-300, and C–TiO₂-400, based on their carbonization temperatures.

2.4. Batch adsorption experiments

2.4.1. Adsorption kinetics and equilibrium

In order to evaluate the adsorption characterization of the synthesized adsorbents and to determine their adsorption capacity, adsorption kinetic and equilibrium studies were performed.

For the kinetic adsorption study 10 mL of a solution of 10 mg/L MO with 0.02 g of each C–TiO₂ adsorbents was placed in a shaker (200 rpm and 25°C) and the residual MO concentration at the proper time intervals, C_t, was determined at 473 nm. The amount of adsorbed MO per unit mass of the adsorbents at time t, q_t, was calculated by Eq. (1(1) ):(1)

where C₀ (mg/L) is the initial concentration of the dye, V (L) is the solution volume, and m (g) is the mass of the adsorbent.

For the equilibrium experiments, a series of solutions with different initial concentrations (4–18 mg/L) were prepared, then 10 mL of each solution with 0.02 g of the desired adsorbent was placed in a shaker (200 rpm and 25°C) for 24 h. The equilibrium concentration of MO and C_e was measured at 473 nm, and the amount of adsorbed dye per unit mass of the adsorbent at equilibrium, q_e, was calculated using Eq. (2(2) ):(2)

2.5. Effect of pH

To investigate the adsorption efficiency of C–TiO₂ adsorbents at different pH, a series of MO solutions (6 mg/L) with different initial pH (2–10) were prepared, and 10 mL of each solution with 0.02 g of the desired adsorbent was shaken overnight. The removal percentage of MO was calculated by Eq. (3(3) ):(3)

where A₀ and A are the initial and equilibrium absorbance of MO at 473 nm, respectively.

2.6. Zeta potential

ZP is the electrical potential in the double layer at the interface between a particle, which moves in an electrical field and the surrounding liquid. Measuring ZP is also a method of characterizing the particle surface in the processes running on this surface. In this work, triton was used as a dispersing agent to pre-disperse the adsorbents in deionized water. To determine the ZP, 0.053 g of adsorbent and 0.25 mL of triton were added to 25 mL of deionized water and stirred for 10 min. This was then filled up to 50 mL with deionized water and subsequently mixed for 10 min. Next, the solution with a dilution ratio of 1:50 was stirred for 20 min.

2.7. Desorption experiments

Five adsorption/desorption cycles were performed to evaluate C–TiO₂ adsorbent efficiency after recovery.

Firstly, 0.2 g of the desired adsorbent was loaded onto 50 mL of the MO solution with an initial concentration of 200 (mg/L), overnight. Then the adsorbent was separated and washed several times to remove unadsorbed MO traces. The adsorbent was dried in an oven at 80°C overnight. The dried adsorbent was then washed with NaOH solution (1 M) several times until the elute pH became neutral. The adsorbent was washed with distilled water again and dried in an oven (80°C, overnight). In order to evaluate the removal efficiency of the recovered adsorbent, the above procedure was repeated another four times. Since an amount of adsorbent is lost during elution, the ratio between the mass of the adsorbent and the volume of the MO solution was kept at 4 (). After each cycle, the removal percentage of MO by recovered C–TiO₂ adsorbents was calculated by Eq. (4(4) ):(4)

where A₀ and A are the initial and equilibrium absorbance of MO at 473 nm in each cycle.

3. Results and discussion

3.1. Characterization of C–TiO₂ adsorbents

The surface structures of the C–TiO₂ adsorbents were studied by SEM images. It can be seen from Fig. 2 that, all the C–TiO₂ samples consist of nanometer-size particles, like the TiO₂ precursor (Fig. 2(a)). The particle sizes of the TiO₂, C–TiO₂-200, and C–TiO₂-300 samples are about 30 and 50 nm (most particles are sized > 50 nm), respectively, and C–TiO₂-400 adsorbent (Fig. 2(d)) consists of particles with various sizes (20–60 nm). Thus, increasing the carbonization temperature to 300°C increased the particle size (Fig. 2(b–d)).

Fig. 2. SEM of (a) unmodified TiO₂, (b) C–TiO₂-200, (c) C–TiO₂-300, and (d) C–TiO₂-400. The scale bar represents 500 nm.

Fig. 3 shows the nitrogen adsorption/desorption isotherm of the obtained adsorbents. The type IV isotherm and textural properties of C–TiO₂ samples listed in Table 1 reveal that the obtained adsorbents have a mesoporous structure. Table 1 also shows that the total surface area was slightly increased by increasing the carbonization temperature. These data correspond to the SEM images.

Fig. 3. Nitrogen adsorption/desorption isotherm of (a) TiO₂, (b) C–TiO₂-200, (c) C–TiO₂-300, and (d) C–TiO₂-400.

Table 1 Textual properties of TiO₂ and C–TiO₂ adsorbents

Adsorbent	SBET (m^2/g)	Vtotal 0.95 (cm^3/g)	Vmikro DR (cm^3/g)	Vmezo (cm^3/g)
TiO2	44	0.0163	0.016	0.003
C–TiO2-200	51	0.710	0.020	0.690
C–TiO2-300	52	0.49	0.020	0.47
C–TiO2-400	53	0.790	0.020	0.770

The effect of carbonization temperature on the crystalline phase of C–TiO₂ samples was studied by XRD technique. Fig. 4 shows the XRD patterns of the prepared adsorbents and TiO₂ precursor. It can be concluded that when TiO₂ nanoparticles were heated up to 400°C, no phase transition occurred, which is in agreement with our previous work [6].

Fig. 4. XRD patterns of the prepared adsorbents and TiO₂ precursor.

The FTIR spectra of the modified and unmodified TiO₂ are presented in Fig. 5. It can be observed that with increasing temperature, a new peak appeared in the range of 2,850–3,000 cm⁻¹ assigned to the methyl group. This indicates not only the presence of ethanol in carbonization, but also that temperature can be related to the increased amount of carbon on modified TiO₂ nanoparticles. This finding is in agreement with the obtained elemental analysis results: The highest carbon content was found in C–TiO₂-400 (6.3%), compared to 0.12 and 4.53% for C–TiO₂-200 and C–TiO₂-300, respectively. This marked increase in carbon content also corresponds to the introduction of a methyl group on the TiO₂ surface.

Fig. 5. FTIR spectra of modified and unmodified TiO_2, (a) C–TiO₂-200, (b) C–TiO₂-300, (c) C–TiO₂-400, and (d) TiO₂.

3.2. Kinetic and equilibrium studies

A high adsorption rate is very desirable in water purification applications.

Fig. 6 shows the time dependency of the adsorption of MO by C–TiO₂ adsorbents. In each case, the dye removal was achieved within less than 60 min. The adsorption rate increased further with rising carbonization temperature. This result corresponds to the results of the nitrogen adsorption/desorption isotherm and SEM images. Indeed, the increase in adsorbent surface area with rising carbonization temperature leads to the increase in adsorption rate.

Fig. 6. Time dependency of the adsorption of MO by C–TiO₂ adsorbents. Solid and dashed lines represent the predicted q_t values by PSO model.

The kinetic data were analyzed with pseudo-first-order (PFO) and pseudo-second-order rate equations [6]. The integrated form of the PFO model can be expressed as:(5)

where q_t and q_e are the amounts of the dye adsorbed at time t and at equilibrium e, respectively, and k₁ is the PFO rate coefficient.

The following equation shows the pseudo-second-order (PSO) model [6]:(6)

where k₂ is the pseudo-second-order rate coefficient.

According to the obtained correlation coefficients, the experimental kinetic data were best fitted to the pseudo-second-order equation for the adsorption of MO onto the C–TiO₂ adsorbents. The fitting results are listed in Table 2. The solid lines in Fig. 6 show the predicted values of q_t based on the PSO model as a function of time.

Table 2 Obtained constants of PFO and PSO rate equations for the adsorption of MO onto C–TiO₂ adsorbents

Adsorbent	PFO				PSO
	qe (mg/gads)	k1 (1/min)	r^2		qe (mg/gads)	k2 (gads/mg min)	r^2
C–TiO2-200	201.3	0.0050	0.9642		200.00	0.00038	0.9996
C–TiO2-300	403.9	0.0044	0.8962		400.00	0.00006	0.9955
C–TiO2-400	520.1	0.1506	0.8644		588.235	0.00030	0.9956

Fig. 7 shows the equilibrium data of MO adsorption onto the prepared adsorbents. These data show that while carbonization at 300 and 400°C increases the adsorption capacity of C–TiO₂-300 and C–TiO₂-400 in comparison to unmodified TiO₂, carbonization at 200°C has no significant effect on C–TiO₂-200 adsorption capacity. Thus, it can be concluded that, although, carbon modification of TiO₂ at 200°C increases its surface area (Table 1), which could also improve the adsorption efficiency, a large surface area does not always indicate efficient adsorption. The surface charge of adsorbents also plays a significant role in the adsorption process, and seems more important for high adsorption capacity, as discussed below.

Fig. 7. Adsorption isotherm of MO onto TiO₂ and C–TiO₂ adsorbents at 25°C.

Langmuir, Freundlich, Langmuir–Freundlich, and Temkin isotherms were used to analyze the experimental equilibrium data, and to obtain the maximum adsorption capacity of synthesized C-TiO₂ adsorbents for MO dye removal.

The Langmuir isotherm can be expressed as follows [23]:(7)

where q_m (mg/g) and K_L (L/mg) are the maximum adsorption capacity and Langmuir constant, respectively.

The following equation shows the Freundlich isotherm which described the multisite adsorption [24]:(8)

where K_F (mg/g) (L/mg)^1/n is the Freundlich constant, and the parameter n describes the system heterogeneity.

The Langmuir–Freundlich (Sips) isotherm, which is a combination of the Langmuir and Freundlich isotherms, can be expressed as follows [25]:(9)

where K_s (l/mg)^1/n is the adsorption constant.

The Temkin isotherm, [26] which is a two-parameter adsorption isotherm, can be expressed as below:(10)

where α and β are the isotherm constants.

The results of fitting to the adsorption isotherm models are listed in Table 3. According to the obtained correlation coefficients, the experimental equilibrium data for the adsorption of MO onto TiO₂ and C–TiO₂-200 were fitted to the Freundlich isotherm and those for adsorption onto C–TiO₂-300 and C–TiO₂-400 were best fitted to the Langmuir–Freundlich isotherm. The predicted values of q_e based on the Langmuir–Freundlich equation are presented in Fig. 7 as solid lines.

Table 3 Obtained isotherm constants for adsorption of MO onto TiO₂ and C–TiO₂ adsorbents

Isotherm models		TiO2	C–TiO2-200	C–TiO2-300	C–TiO2-400
Langmuir	KL (L/mg)	3.000	4.367	137.475	182.936
	qm (mg/g)	0.0288	0.018	0.0007	0.0003
	r^2	0.8875	0.9584	0.7260	0.9668
Freundlich	KF (L mg^(1−1/n)/g)	0.1822	1.059	141.982	321.436
	n	0.442	0.452	0.479	0.393
	r^2	0.9304	0.968	0.9307	0.9905
Langmuir–Freundlich	KL–F (L/mg)	0.0624	0.057	0.0015	0.0018
	qm (mg/g)	47.619	171.428	500.000	909.090
	n	0.90	0.90	0.25	0.25
	r^2	0.8714	0.9488	0.9982	0.9990
Tempkin	b	180.66	53.373	189.000	354.43
	r^2	0.6968	0.433	0.9271	0.8516

3.3. Effect of pH and ZP

The pH of the solution is an effective function in dye removal efficiency, especially in the case of semiconductor adsorbents. Thus, the removal percentages of MO by C–TiO₂ adsorbents were investigated at different pH.

Fig. 8(a–c) show the removal percentage of MO by carbon-modified TiO₂ adsorbents at different pH. The maximum removal efficiency was observed at pH 6 and 4 for C–TiO₂-200 and C–TiO₂-300, respectively. In the case of C–TiO₂-400, the maximum removal percentage was achieved in an acidic medium up to pH 4, and then decreased slightly. For C–TiO₂-200, C–TiO₂-300, and C–TiO₂-400, the point of zero charge (pH_PZC) was found to be at pH 6.53, 5.19, and 4.48, respectively, while the pH_PZC for unmodified TiO₂ was around 6.13 (data not shown). A change in pH_PZC after modification was caused by the presence of methyl groups on the surface of the modified TiO₂.

Fig. 8. Removal percentage of MO by (a) C–TiO₂-200 (b) C–TiO₂-300 and (c) C–TiO₂-400 at different pH.

The adsorbent surface has a negative charge at solution pH higher than pH_PZC, while the surface charge is positive at pH lower than pH_PZC [27].

Fig. 9 shows the chemical structure of MO in acidic and basic media. In the case of C–TiO₂-200 and C–TiO₂-300, high removal efficiency was reached in an acidic medium, while basic pH decreased the removal percentages. Thus, in acidic media (lower than pH_pzc of 6.53 and 5.19 for C–TiO₂-200 and C–TiO₂-300, respectively), the strong attractive interaction between the – group of MO and positive surface of the mentioned adsorbents causes maximum removal efficiency. At basic pH, the repulsion interaction between the negative end of the MO molecules and negative surface of the C–TiO₂-200 and C–TiO₂-300 decreased the removal percentages.

Fig. 9. MO structure at (a) acidic and (b) basic mediums.

As shown in Fig. 8(c), the removal percentage of MO by C–TiO₂-400 almost remains unchanged at acidic and basic pH. This suggests that MO molecules are not adsorbed through the ionic – group [5]. It seems that MO adsorption is due to the interaction between the central nitrogen or benzene rings and the surface of C–TiO₂-400.

ZP was measured to illustrate the mechanism of the pH effect on MO removal efficiency by C–TiO₂ adsorbents. It was very difficult to make this type of modified TiO₂ nanoparticle disperse homogeneously into the aqueous solution, especially as dispersion takes a long time. Triton was therefore used to pre-disperse the adsorbents in deionized water. As can be seen from Fig. 10, the TiO₂ surface has a highly negative ZP in suspension, leading to repulsion between particles. In contrast, the modified TiO₂ had a significantly lower ZP (the ZP of TiO₂, C–TiO₂-200, C–TiO₂-300, and C-TiO₂-400 are −32.2, −23.3, −15.2, and −10.8 mV, respectively). Thus, the decline in negative ZP not only leads to flocculation [1], but also causes better adsorption of MO onto modified TiO₂ than unmodified TiO₂. Furthermore, carbonization at 200°C led to a slight decrease in the ZP of C–TiO₂-200 as compared to unmodified TiO₂, while adsorbents prepared at 300 and 400°C demonstrated a large change in ZP. Thus, the adsorption capacity of unmodified TiO₂ and C–TiO₂-200 is almost the same (Fig. 7). The much lower ZP of C–TiO₂-300 and C–TiO₂-400 is due to their higher adsorption capacity. This may suggest that the electrostatic interaction between the dye-(C–TiO₂) complex and negatively charged surface of heat-treated TiO₂ becomes dominant. In addition, the interaction mechanism of dye molecules and the sample calcined at different temperatures are different; the interaction is stronger at high temperatures.

Fig. 10. ZPs of TiO₂ and C–TiO₂ adsorbents.

3.4. Desorption and regeneration studies

The results of the adsorption/desorption experiments are shown in Fig. 11. These results approve the electrostatic interaction between MO and C–TiO₂ adsorbents, since this interaction is limited by NaOH solution.

Fig. 11. Regeneration studies of C–TiO₂ adsorbent.

Although the removal efficiency decreased by 30, 24, and 19% after five cycles for C–TiO₂-200, C–TiO₂-300, and C–TiO₂-400, respectively, it remained above 50% for all C–TiO₂ adsorbents. Thus, all prepared adsorbents, especially C–TiO₂-400 (with its 19% efficiency decrease), could be reused at least five times, which is both highly economical and practical.

3.5. Adsorption mechanism

The Weber–Morris intraparticle diffusion model was applied to identify the adsorption mechanism of MO adsorption onto the modified TiO₂. This model, which includes terms to account for both intraparticle diffusion and boundary layer diffusion, is expressed as follows:(11)

where K_dif (mg/g min^−0.5) is the rate constant of intraparticle diffusion and C_i (mg/g) stands for thickness of the boundary layer.

The plot of q_t vs. t^0.5 for MO adsorption on C–TiO₂-200, C–TiO₂-300, and C–TiO₂-400 are illustrated in Fig. 12. It can be seen from Fig. 12(a) that q_t was plotted against t^0.5 in a straight line with an intercept (C_i). This indicates that the intraparticle diffusion was important, but not the only rate-determining step in the adsorption process. Moreover, the obtained intercept value (C_i = 1.579 mg/g) is positive, indicating rapid adsorption in a short period of time. Furthermore, the rate constant (K_dif = 0.145 mg/g min^0.5) indicates a rapid diffusion process. Fig. 12(b) and (c) shows the intraparticle diffusion plots for MO adsorption onto C–TiO₂-300 and C–TiO₂-400, which reveal the same general features as the multi-linear q_t vs. t^0.5 plot. The whole adsorption process for C–TiO₂-400 and C–TiO₂-300 consist of two and three steps, respectively. In C–TiO₂-300, the initial sharply sloped portion of 2–6 min may occur due to the small particle size noted above, indicating external surface adsorption. The second step is the gradual adsorption stage from 6 to 114 min, during which intraparticle diffusion may be the rate-limiting step. The third and final equilibrium stage beyond 22 h corresponds to the slowing down of intraparticle diffusion due to low solute concentrations in the solution. When adsorption has reached saturation on the exterior surface, the MO ions may have entered the pore surface, which is hydrated in liquid phase. In C–TiO₂-400 (Fig. 12(c)) an initial curved portion representing increased sorption is followed by a linear portion as equilibrium is attained. The primary linear portion at 2–30 min may be attributed to the external surface, and the second curved portion (beyond 22 h) may be ascribed to the pore diffusion or intraparticle diffusion. Two distinct linear regions of differing slopes are visible on the graph, indicating that intraparticle diffusion is involved in the adsorption process, but is not the only rate-limiting mechanism. Other mechanisms may also simultaneously control the rate of MO adsorption onto C–TiO₂-400; further investigation is required in order to describe these.

Fig. 12. The intraparticle diffusion plot for adsorption of MO onto (a) C–TiO₂-200, (b) C–TiO₂-300 and (c) C–TiO₂-400.

4. Conclusions

Carbon-modified TiO₂ adsorbents were prepared at different carbonization temperatures (200–400°C), and used for the adsorptive removal of MO dye pollutant from aqueous solution. Our results show that MO dye pollutant was rapidly removed by all C–TiO₂ adsorbents; the adsorption process took less than 1 h. It was revealed that the MO adsorption capacity of TiO₂ increases by carbon modification. Changing the carbonization temperature had no significant effect on the surface area of TiO₂, but did enhance maximum adsorption capacity, which might be due to the increase in adsorbent ZP. The Freundlich adsorption model was found to fit well for TiO₂ and C–TiO₂-200, while carbon-modified TiO₂ fitted the Langmuir–Freundlich model at higher temperatures. All the prepared adsorbents could be regenerated at least five times and used as economical adsorbents in purification applications.