Evaluation of Carbonaceous Material Produced from Fireproofed Cotton and Its Adsorption of Methylene Blue

We report herein on the fabrication of carbonaceous material produced from cotton treated with Tricresyl phosphate (TCP) for fireproofing and its evaluation for adsorbing Methylene blue (MB). We prepared two carbonaceous materials: cotton calcined at 900◦C (CT900) and cotton calcined at 900 ◦C with TCP (F-CT900). These materials were evaluated in terms of their scanning electron microscope images, specific surface area, pore volume, mean pore diameter, and solution pH. We found that the specific surface area of F-CT900 (1492 m/g) is greater than that of CT900 (910 m/g), and that the mean pore diameter of F-CT900 (2.41 Å) is smaller than that of CT900 (9.09 Å). The adsorption of MB onto CT900 and F-CT900 reached equilibrium within 5 h. We fitted the experimental data with the pseudo-second-order model and obtained correlation coefficients between 0.993 and 0.999. We found that more MB adsorbed onto F-CT900 (about 650 mg/g) than onto CT900 (about 350 mg/g). We also fitted these experimental data with both the Freundlich and Langmuir equations. Thus, carbonaceous material for MB removal could be produced from fireproofed cotton, and it is useful for the purification of dye solution systems. [DOI: 10.1380/ejssnt.2012.374]


I. INTRODUCTION
Dyes are synthetic organic compounds that are increasingly being produced and used as colorants in many industries worldwide, including textile, plastic, and paper [1,2].Most dyes are toxic and carcinogenic compounds.They are also recalcitrant and are thus stable in the recovering environment, thereby posing a serious threat to human and environmental health [3].Accordingly, to protect humans and help ecosystems recover from contamination, these dyes must be eliminated from wastewater before it is released into the environment [4].Therefore, dye removal techniques are being extensively investigated.To treat dye effluents, several techniques have been developed, such as microbial degradation, chemical oxidation, membrane separation, bioaccumulation, electrochemical treatment, adsorption, and reverse osmosis [5].Among these techniques, adsorption is generally preferred due to easy handling, high efficiency, low energy input, and availability of different adsorbents [6,7].Wastewater treatment has been studied using clay/basic [8], chitosan [9], cotton [10], montmorillonite [11], sepiolite/methyl green, and natural zeolite/basic [12].However, the application of these materials is practically limited because the adsorbent is not available in sufficient amounts.In other words, despite having a significant capacity for dye adsorption, most of these materials are not produced centrally in any country; therefore, they are not available in sufficient bulk quantities to be commercialized for fullscale applications [13].
On the other hand, two million tons of waste textiles (including cotton) are exhausted in Japan every year, with most of this waste being incinerated or sent to landfills.Sustainable development requires a reduction in the use of landfills and incinerators, and a concomitant increase in the reuse of textile wastes as well as other wastes.If waste cotton were to be used as an adsorbent to remove dyes from aqueous solutions, then both the aforementioned problems would be solved.First, the waste textiles would be recycled instead of being incinerated or sent to landfills, and second, improved wastewater purification would be achieved.Moreover, the ability of calcined cotton to adsorb dyes is better than that of virgin cotton (waste cotton), which indicates that the specific surface area of calcined cotton is larger than that of virgin cotton.In fact, studies have shown that because of its high surface area, calcined cotton is effective in adsorbing organic cation contaminants [14].Because calcination gives a very low percent yield of calcined cotton, we selected fireproofing as the calcination treatment.There are many flame retardants, for example, antimony trioxide, aluminum hydroxide, magnesium hydroxide, antimony pentoxide, decabromodiphenyl ether, triphenyl phosphate, tricresyl phosphate and so on.Yield percentage of carbonaceous material (cotton) treated with magnesium hydroxide or antimony trioxide were low.Moreover, magnesium hydroxide and antimony trioxide were harmfuller for human than tricresyl phosphate.Thus, the aim of the current study is to produce an adsorbent from fireproofed waste cotton and to evaluate its capacity to adsorb Methylene blue (MB).We selected MB because it is representative of cationic dyes and is typically used in textile dyeing due to its high-affinity to solid surfaces.We thus used it in the present study as a model to test the adsorption behaviors of treated waste cotton.nace to 900 • C for 2 h (CT900).Fireproofed CT was prepared by adding 5.0 g of CT to 50 mL TCP solution, which was kept for 30 min at room temperature, following which the TCP-treated CT was dried at room temperature.Finally, the TCP-treated CT was heated in a muffle furnace to 900 • C for 2 h (F-CT900).The CT samples were investigated by scanning electron microscopy (SEM) using a JSM-5200 (JEOL, Japan).Specific surface area, pore volume, and mean pore diameter were measured using a NOVA4200e (Yuasa Ionic, Japan) specific surface analyzer.The amount of pH solution added to the CT samples was determined by activated-carbon testing (JIS K1474).

B. Adsorption rate and adsorption isotherm of MB onto CT900 and F-CT900
After adding 0.5 g of the CT sample to 500 mL of 1000 mg/L MB solution, the suspension was shaken at 25 • C from 1 min to 48 h at 100 rpm, after which a 1 mL MB solution was collected.The MB solution was then filtered with a 0.45 µm membrane filter and examined using a UV-1200 spectrophotometer (Shimadzu).The amount of MB adsorbed was calculated by comparing the MB concentration before and after adsorption.The adsorption wavelength for MB was 655 nm.The amount of MB adsorbed was calculated by the following equation: where X is the amount of MB adsorbed (mg/g), C 0 is the initial concentration of MB (mg/L), C e is the equilibrium concentration of MB (mg/L), V is the solvent volume (L), and W is the weight of the CT sample (mg).The adsorption isotherm of MB onto the CT sample was obtained by first adding 0.05 g of CT sample to 50 mL of MB solution at different initial concentrations, and then shaking the suspension at 25 • C for 48 h at 100 rpm.The concentration of the MB solution filtered through a 0.45 µm membrane filter was then measured using the UV-1200 spectrophotometer, and the amount of MB adsorbed onto the CT sample was calculated using Eq.(1).
From these experiments, we confirmed that MB is not adsorbed onto untreated CT.Moreover, the concentration of the MB solution remained constant for 48 h at 25 • C.

A. Properties of the CT samples
The SEM images of CT900 and F-CT900 are shown in Fig. 2. The images show that the CT surface was unaf- fected by calcination and fireproofing.The properties of and F-CT900 are given in Table I, which shows that the specific surface area of F-CT900 (1492 m 2 /g) is greater than that of CT900 (910 m 2 /g), indicating a successful fireproofing treatment.TCP converted to phosphate and polymetaphosphate upon pyrolysis.The CT surface was covered with polymetaphosphate, which cut off the oxygen supply.The fireproofing treatment caused the F-CT900 micropore volume (0.0081 mL/g) and mesopore volume (0.0803 mL/g) to decrease, which indicates that micropores and mesopores converged upon calcination and fireproofing.Thus, we prepared a carbonaceous material from CT with high specific surface area.In general, the pore volume was segregated into micropore (5 < d ≤ 20 Å), mesopore (20 < d ≤ 500 Å), and macropore (d > 500 Å) regimes [15].The evolution of pore volume appears to significantly affect MB adsorption, and carbonaceous material from CT treated with fireproofing demonstrated a similar trend.Further, the mean pore diameter of CT900 (9.09 Å) was larger than that of F-CT900 (2.41 Å).The pH of the CT900 and F-CT900 solutions was 8.42 and 2.46, respectively.The adsorption rate of MB onto CT900 and F-CT900 is shown in Fig. 3.As shown in this figure, the MB adsorption rate for CT900 showed a trend similar to that for F-CT900.The adsorption of MB onto CT900 (approximately 200 mg/g) and F-CT900 (approximately 550 mg/g) reached equilibrium within 5 h.Kinetic models are used to analyze the rate of the adsorption process and potential rate-controlling step.Therefore, in the present work, we analyze the kinetic data with pseudofirst-order [16] and pseudo-second-order [17] kinetic models.The pseudo-first-order equation is expressed as ln (q e − q t ) = ln q e − k 1 t, (2) where q t (mg/g) is the amount of MB adsorbed at time t (min), q e (mg/g) is the equilibrium adsorption, and k 1 (min −1 ) is the rate constant of the pseudo-first-order model.
The pseudo-second-order kinetic equation is given by where k 2 (mg/g/min) is the rate constant of the pseudosecond-order model.The calculated constants are listed in Table II.The correlation coefficients for the pseudofirst-order model and pseudo-second-order model ranged from 0.797 to 0.831 and from 0.993 to 0.999, respectively.The constants k 1 and k 2 for CT900 (F-CT900) were 0.28 min −1 and 0.01 g/mg/min (1.18 min −1 and 0.03 g/mg/min), respectively.Thus, the experimental data fit better with the pseudo-second-order model.

C. Adsorption isotherm
The adsorption isotherms of MB onto CT900 and F-CT900 are shown in Fig. 4. The results show that the amount of MB adsorbed onto F-CT900 (approximately 650 mg/g) was greater than that onto CT900 (approximately 350 mg/g).The most common models used to interpret the solution adsorption data are Langmuir [18] and Freundlich [19] isotherms.Sorption equilibrium provides fundamental physicochemical data for evaluating the applicability of the sorption process as a unit operation.In this study, we used both these models to describe the relationship between the amount of MB adsorbed and its equilibrium concentration.A Langmuir isotherm assumes monolayer adsorption onto a homogeneous surface containing a finite number of adsorption sites, with adsorption occurring via uniform adsorption strategies and with no transmigration of the adsorbate in the plane of the surface.The Langmuir parameters were determined according to the following equation: where q (mg/g) is the amount of MB adsorbed, a (L/mg) is a constant related to the affinity to the binding sites and of the adsorption, W s (mg/g) is the maximum monolayer adsorption capacity, and C e (mg/L) is the equilibrium concentration.
The linear form of the Freundlich isotherm model is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface.The Freundlich parameters were determined using the following equation: where K and 1/n represent the Freundlich capacity factor and the Freundlich intensity parameter, respectively.The Langmuir and Freundlich constants for the adsorption of MB onto CT900 and F-CT900 are listed in Table III.We found that the Langmuir constant a for F-CT900 (22.9 L/mg) is greater than that for CT900 (0.3 L/mg), which indicates that the adsorption of MB onto F-CT900 is greater than onto CT900.The correlation coefficients for CT900 and F-CT900 are 0.938 and 0.925, respectively.
The values for the Freundlich constant 1/n for CT900 and F-CT900 are 0.18 and 0.15, respectively.When 1/n is in the range of 0.1 to 0.  On the other hand, if 1/n > 2, adsorption is considered difficult [20].These results indicate that MB was easily adsorbed onto CT900 and F-CT900.The correlation coefficients for CT900 and F-CT900 are 0.973 and 0.794, respectively.Moreover, the specific surface area of CT900 (910 m 2 /g) is smaller than that of F-CT900 (1492 m 2 /g), which suggests that the amount of MB adsorbed depends on the specific surface area.Thus, we successfully prepared carbonaceous material from fireproofed CT.Furthermore, the material F-CT900 proved useful for removing MB from aqueous solution systems.Table IV shows a comparison of the adsorption capacity of MB onto various adsorbents; it can be seen from the table that F-CT900 is one of the most effective adsorbents for removing MB [21][22][23][24][25].

IV. CONCLUSIONS
Carbonaceous material was prepared from fireproofed cotton (F-CT900) as well as from cotton not treated with fireproofing (CT900).The specific surface area of F-CT900 was greater than that of CT900.SEM imaging of the CT materials indicated that the materials do not change upon fireproofing.The adsorption rate of MB onto the CTs reached equilibrium within 5 h, and the amount of MB adsorbed onto F-CT900 was greater than that adsorbed onto CT900.These results indicate that the amount of MB adsorbed depends on the specific surface area, which was corroborated by fitting the data to the pseudo-second-order model, the Freundlich equation, and the Langmuir equation.Thus, these adsorption experiments indicate that F-CT900 is a very efficient adsorbent for removing MB from aqueous solutions.
FIG. 1: Structures of MB and TCP.

TABLE I :
Properties of CT-900 and F-CT900.

TABLE II :
Adsorption kinetic parameters of MB onto CT900 and F-CT900.

TABLE III :
Freundlich and Langmuir constants for MB adsorption onto CT900 and F-CT900.