Phosphorus and nitrogen co-doped carbon derived from Cigarette Filter for adsorption of methylene blue dye from aqueous solution

Global access to sanitary water is of utmost importance to human health. Presently, 14 textile dye water pollution and cigarette pollution are both plaguing the environment. 15 Herein, waste cigarette filters are converted into useful carbon-based adsorbent 16 materials via a facile, microwave-assisted carbonization procedure. The cigarette filters 17 are co-doped with phosphorus and nitrogen using ammonium polyphosphate to 18 enhance their surface characteristics and adsorbent capability. The adsorbents are 19 characterized physically to examine their surface area, elemental composition, and 20 surface charge properties. Batch adsorption experiments were performed to determine 21 the maximum adsorption capacity of the adsorbents. Additionally, the effects of various 22 adsorption parameters— temperature, adsorbent dosage, pH, and time—on adsorption 23 process were examined. The doped adsorbent showed a maximum adsorption capacity 24 of 303.3 mg g -1 respectively, which is three times that of the methylene blue adsorption 25 capacity of commercially available activated carbon (~100 mg g -1 ). Thus, the 26 phosphorus and nitrogen co-doped carbonized waste cigarette filter adsorbent shows a profound potential as a sustainable solution to combat textile dye water pollution and cigarette filter pollution simultaneously, due to its low cost, simple preparation, and versatility in application. This work aims to convert used CF into useful carbon adsorbents via inexpensive and facile microwave-assisted approach. The phosphorus and nitrogen heteroatom – doped carbon contains functional groups which may have a synergistic effect on the adsorption capacity. Both undoped and doped carbons from CF are characterized in 68 detail. Methylene blue (MB) dye is used to investigate the adsorption characteristics of the newly developed phosphorus and nitrogen co-doped carbon from cigarette filter (DCCF) and undoped carbonized cigarette filter (CCF). To the best of our knowledge, no has the use of this one-step microwave-assisted method for producing and its use as an adsorbent. second order adsorption process which is indicative of a chemisorptive process. Adsorption onto the adsorbents is spontane ous (ΔG<0) and endothermic in nature. This work highlights an 367 inexpensive and green method to convert a common litter source into a useful material for water 368 remediation.


Introduction 32
The textile industry contributes to a considerable amount of water pollution 33 worldwide. According to the National Resources Defense Council, textile mills 34 generate approximately 20% of industrial water pollution, during which an approximate 35 20,000 different chemicals contaminate water [1]. It is estimated that 500,000 textile 36 dyes are produced yearly; these synthetic dyes are immensely stable to light, 37 temperature, and chemical treatment. Moreover these synthetic dyes are resistant to 38 biodegradation under aerobic conditions, and exhibit a high level of solubility in 39 aqueous solutions with visibility to the naked eye at concentrations as low as 1 ppm [2]. 40 Due to these characteristics, current large-scale effluent treatments are ineffective for 41 dye removal and high in cost, resulting in an estimated 20% of dyes being released into 42 the environment. 43 Several prominent chemical and physical methods are used currently for textile dye 44 removal from effluents, such as coagulation-flocculation, aerobic degradation, and 45 adsorption [3]. Adsorption is a common choice to remove dye from solution as it is a 46 7 Physical characterization of CF and DCCF were performed using several 98 techniques. A JSM-7000F scanning electron microscope (SEM) was utilized to 99 determine the morphology and percent elemental composition of the bulk materials. 100 For SEM imaging, a small amount of sample was placed on a double-sided carbon tape 101 on aluminum mount substrate before analysis. ASAP 2020 Micrometrics surface area 102 and porosity analyzer with Brunauer-Emmett-Teller (BET) method was used to 103 analyze surface area and pore size of materials via nitrogen adsorption/desorption 104 studies at a bath temperature of 77 K. Thermo K-Alpha X-ray photoelectron 105 spectrometer (XPS) system was used to determine surface elemental composition of 106 dried carbonized samples. Fourier transform infrared (FTIR) spectroscopy was 107 performed using a Thermo Scientific Nicolet 6700 Spectrometer to confirm that MB 108 was adsorbed onto adsorbent surface and not degraded during adsorption. To determine 109 point of zero charge (PZC), a simple salt addition method was used. Briefly, a 110 suspension of adsorbent (0.01 g) was prepared in 0.1 M NaNO3 aqueous solution in 111 different reaction vessels (5 g L -1 CCF or DCCF). 112

Adsorbent dosage 114
To optimize the adsorbent dosage for adsorption, various masses (5-20 mg) were 115 contacted with 50 mL of 10 ppm MB solution. After reaching equilibrium (24 h), the 116 treated MB solutions were centrifuged at 3800 RPM for 10 min to separate the dye 117 solution from the adsorbent. Absorption spectrophotometry (Lambda 850 UV-vis 118 spectrophotometer with 1 cm path length quartz cuvette) was utilized to determine the 119 concentration of MB remaining in solution at equilibrium. From this, the percent 120 removal and adsorption capacity were determined. 121

Initial concentration 122
To analyze the effects of MB concentration on the adsorption onto CCF and DCCF 123 from CFs, batch adsorption tests were performed. Using the MB stock solution, 50 mL 124 MB solutions were prepared with varying concentration (5 ppm to 100 ppm). 10 mg 125 samples of the carbon from the CF and a magnetic stir bar were placed into each of the 126 MB solutions. The solutions were stirred at room temperature at constant speed until 127 they reached equilibrium. The equilibrium data was then fitted into Langmuir and 128 Freundlich isotherm models to determine how the MB interacts with CCF and DCCF. 129

Adsorption kinetics 130
To understand how the uptake of MB by CCF and DCCF is affected by time, 131 adsorption kinetic tests were performed. Samples containing 50 mL of various initial 132 concentration of MB solution (5-100 ppm) were prepared and contacted with 10 mg of 133 adsorbent powder and stirred using a magnetic stir bar after which a timer was 134 immediately started. After specified time intervals, an aliquot of the MB solution was 135 analyzed by absorbance spectroscopy. This data was fit into pseudo-first and second 136 order equation to determine the primary mechanism of adsorption for the two materials.     However, DCCF is significantly more mesoporous (87.6% mesopores by volume). This 172 could be due to larger reducing gas formation when combined with APP. The overall 173 results of BET analysis are tabulated in Table 1. negative charge). In order to determine PZC, a salt addition method was used as 197 described previously. A plot of ΔpH versus pHinitial was formed where ΔpH=0 was 198 deemed to be the PZC (Figure 4). The PZC of CCF and DCCF were found to be 7.25 199 and 3.23, respectively. This implies that MB adsorption will be more favorable onto Where C is the concentration of MB in solution initially and at equilibrium. The amount 221 of dye adsorbed at equilibrium, Qe (mg g -1 ) was calculated by using the following 222 Where V is the volume of solution and W is the weight of adsorbent.   The effect of initial concentration of MB adsorption onto the adsorbents was 235 investigated by contacting a fixed mass of 10 mg adsorbent to varying concentration of 236 MB from 5-100 ppm. For both adsorbents, it was observed that the adsorption capacity 237 increased as the initial concentration of MB increased (Figure 7). This is due to the 238 mass transfer driving force between liquid-solid interface involved in heterogeneous  Where KF and n are constants related to adsorption capacity and intensity, respectively. 275 Neither adsorbent was well represented by the model with R 2 values less than 0.88 in 276 both cases. However, both adsorbents possess an n value greater than 1 which is 277 indicative of a favorable process of adsorption. The n value of DCCF is greater than 278 that of CCF (Table 3) which can be correlated to a stronger interaction between 279 adsorbate and adsorption sites on the surface of the adsorbent material. KF values are 280 related to adsorption capacity and, like Langmuir modelling of the data, DCCF exhibits 281 a higher KF value. Langmuir and Freundlich fitting plots are displayed in Figure S2. 282 To investigate the spontaneity of the adsorption onto the two different adsorbents, 283 Gibbs free energy was calculated from Equation 5. 284 where R is the gas constant (8.314 J mol -1 K -1 ) and T is the temperature (298 K), and 286  Table 3. 290 [ Table 3] 291

Kinetics 292
In order to gain insight on the kinetic mechanism of adsorbents, kinetic adsorption 293 was fitted into pseudo-first and pseudo-second order kinetic models. Experiments were 294 performed by varying initial concentration from 5 to 100 ppm of MB at natural pH and 295 recording the dye concentration at various time intervals after contact with adsorbent 296 (Figure 8a and 8b). The pseudo-first order model was used in the linear form (Equation 297 6) in order to evaluate the first order rate constant, k1. 298 Where Qt is the amount of dye adsorbed at time, t. R 2 values were used to determine 300 the better fitting model. The pseudo-second order equation was also used in the linear 301 form (Equation 7) to determine second order rate constant, k2. 302 Plotting the linear form of Equation 6 and 7 generates a straight line in which k can 304 be directly calculated (Figure 8c, 8d, S2a, and S2b). CCF average R 2 value is 0.88 305 (Table S1) while the average value of R 2 for second order fitting is 0.99. This indicates 306 a better fitting of CCF adsorption to pseudo-second order kinetics. DCCF average R 2 307 value for pseudo-first order plot is 0.90 while for pseudo-second, the value average is 308 0.99. This indicates that the adsorption mechanism is better correlated to a pseudo-309 second order process for both samples. This process is characterized by strong 310 chemisorptive adsorption between MB and CCF and DCCF adsorbents. In addition, the 311 standard deviation was calculated from Equation 8: 312 For CCF adsorption, ∆ was found to be between 1.03-8.48 for second order 314 compared to 6.98-19.90 for first order (Table S1) Table 4. From this 324 data, it can be concluded that CCF and DCCF show exceptional application as 325 adsorbent materials for removal of the cationic MB dye as their adsorption capacities 326 are higher than many other waste-derived carbons. Additionally, many of the 327 previously used waste precursors require chemical activation, whereas this work 328 highlights a simplistic method to produce doped carbons with desirable adsorption 329 characteristics. 330 [ Table 4] 331

Effect of temperature: 332
In order to evaluate the temperature effect on adsorption onto the cigarette-based 333 samples, experiments were performed at four different temperatures ranging from 25 334 to 55 o C. It is observed that for both adsorbent materials, the adsorption capacity, Qe is 335 increasing as the temperature increases (Figure 9). This is due to an increased diffusion  The effect of solution pH on MB adsorption was determined by contacting 10 mg 344 of adsorbent to 50 mL of 30 ppm MB solution (Figure 10). CCF adsorption is greatly 345 dependent on pH, ranging from 77.6 mg/g to 99.7 mg/g in a pH range from 3.9 to 10.2. 346 This is due to the neutral PZC of CCF. Below pH 7, the adsorption is unfavored because 347 of the repulsive forces between MB and the positive adsorbent surface. In contrast, 348 DCCF adsorption capacity remains relatively unchanged over the pH range 3.

Conclusions 355
Useful adsorbent materials were successfully prepared from waste CCF using a simple and

Availability of data and materials 371
The datasets generated during and/or analyzed during the current study are available 372 from the corresponding author on reasonable request.