Preparation of Activated Carbon From Sunflower Straw by H3PO4 Activation and Its Application for Acid Fuchsin Adsorption


 In this work, sunflower straw (SS) was used as the raw material, H3PO4 was used as the activator, and the sunflower straw activated carbon (SSAC) was prepared by the one-step activation method under the impregnation ratio of 1:1, 1:2, 1:3, 1:5 (SS/H3PO4, g/g). The adsorption of acid fuchsin (AF) simulated dye wastewater by SSAC prepared under different immersion ratios has been studied. As the impregnation ratio increases, the pore structures of SSAC changed greatly. SSAC3 had the largest specific surface area (1794.01 m2/g), and SSAC4 had the smallest microporosity (0.0527 cm3/g) and the largest pore volume (2.549 cm3/g). The adsorption kinetics of four types of SSAC to AF were more in line with the quasi-second-order adsorption kinetic model. The Langmuir isotherm model was suitable for describing SSAC3 and SSAC4, and the Freundlich isotherm model was suitable for describing SSAC1 and SSAC2. Thermodynamics showed that the adsorption process was spontaneous and endothermic. At 303 K, SSAC4 showed a removal rate of 97.73% for 200 mg/L AF, and the maximum adsorption capacity of 2763.36 mg/g, which was the highest among the four types of SSAC. This study shows that the sunflower straw activated carbon prepared by the H3PO4 one-step activation method is a green and efficient carbon material and has great application potential in the treatment of dye-containing wastewater.


Introduction 31
The application of dyes is very common in the industry fields of textiles, printed matter, rubber, 32 plastics, cosmetics and so on. The annual production of dyestuff in the world exceeds 7× 10 5 t 33 (Zhang et al. 2011), resulting in a large amount of dyestuff wastewater containing toxic chemicals, 34 carcinogenic substances, which is difficult to biodegrade (Greer et al. 2015; Radjenovic et al. 35 2015). Unfortunately, these colored dyes will seriously reduce the light transmittance of water, 36 thus blocking the photosynthesis of aquatic plants (Yang et al. 2015a). Triphenyl methane dye is 37 one of the three most used dyes (azo dye, anthraquinone dye, and triphenyl methane dye) ( specific surface area by physical or chemical methods. Compared with physical approach, 57 chemical method has lower synthesized temperature and time (Ahmed 2016), and the prepared AC 58 has larger specific surface area and controllable microporosity in a small range (Ahmed 2017). 59 The activators used for chemical activation usually include KOH, ZnCl2, H3PO4. To ensure 60 porosity, air or nitrogen can be introduced during the pyrolysis process. (Vladov et al. 2019 Vashishtha 2020). 67 As a large agricultural country, China produces a large amount of agricultural waste every 68 year, and sunflower straw is one of the main sources. Taking September 2016 as an example,  69 Chinese production is about 175.8 million tons of sunflower seeds, and 527.4 million tons of 70 sunflower stalks could be produced according to the ratio of the output of sunflower stalks to 71 sunflower in China ). Most of these stalks are burned and deposited as farmland 72 fertilizer, but the resulting PM2.5, SOx and NOx will pollute the atmosphere . 73 Therefore, using sunflower straw waste to prepare biomass activated carbon can not only reduce 74 environmental pollution, but also further develop the economic benefits and utilization potential of 75 sunflower straw to achieve the purpose of waste treatment. In this study, sunflower straw was used 76 as raw material, H3PO4 was used as activator, and SSAC was prepared by one-step activation 77 method at the impregnation ratio of 1:1 to 1:5. To characterize SSAC, investigate the surface 78 physical and chemical properties of SSAC. The adsorption kinetics, isothermal adsorption 79 equilibrium and thermodynamics methods were used to study the properties of SSAC prepared at 80 different impregnation ratios and its effect on AF adsorption and adsorption properties. 81 analytical pure, and all solutions were prepared with ultra-pure water. 88

89
Sunflower straw was cleaned with tap water, then rinsed with pure water for 3-4 times, and then 90 put into the oven to dry completely at 115 ℃. Break the completely dried sunflower straw into 91 small pieces and grind it in the grinder. Put the grinded straw powder through a 90-mesh sieve and 92 put it into a plastic sealed bag. Weighed 4 portions of straw powder, each 15 g, into 4 crucibles, 93 added different amounts of 85% concentrated phosphoric acid, and adjusted the impregnation ratio 94 to 1:1, 1:2, 1:3 and 1:5. Stirred fully, sealed it with plastic wrap and stood for 12 h, then put it into 95 an electric blast drying oven and heated it for 150 min at 180 ℃. After being cooled to room 96 temperature, placed in a tube furnace with 10 ℃‧min -1 in the heating rate in N2 protection under 97 the 600 ℃, 150 min activation, the 4 samples after cooling were washed with ultrapure water and 98 supplemented with 0.1 mol/L NaOH solution until the pH was neutral. They were dried in an oven 99 at 85 ℃ for 2 h, then crushed with a mortar and passed through a 60-mesh sieve. The four types of 100 SSAC obtained were marked as SSAC1, SSAC2, SSAC3 and SSAC4, and they were put into 101 plastic bags for use. 102

103
The elemental composition of materials was detected by an elemental analyzer (Costech ECS 104 4010/4024, Italy). The morphology of the material was characterized by scanning electron 105 microscope (SEM, HITACHI SU8010, Japan). Specific surface and pore size characteristics were 106 determined at 77 K using an automatic specific surface area and pore size analyzer (Micromeritics 107 ASAP2460, USA) using N2 as adsorbent. The functional groups of the materials were determined 108 by Fourier infrared spectroscopy (FT-IR, Nicolet6700, USA). 109

110
In this study, batch measurement method was used to investigate the adsorption effect of 111 self-made AC on AF, and different adsorption thermodynamic equation and adsorption kinetic 112 equation were selected for fitting. AF removal rate (%), equilibrium adsorption capacity (qe/ 113 mg·g -1 ) and adsorption capacity (qt/ mg·g -1 ) at time t (min) were calculated by the following 114 equation: 115 where C0 (mg/L), Ct (mg/L) and Ce (mg/L) are the initial, at time t (min) and equilibrium 119 concentrations of AF solutions, V is the volume of AF solution (L), and m is the SSAC amount (g). 120 1000 mg‧L -1 acid fuchsin reserve solution was prepared for later use. Because AF will fade 121 with the increase of pH, in order to reduce the influence of its own fading, the tests are all carried 122 out under the condition of the initial pH of AF (pH=2.89). with UV spectrophotometer. 134

136
Elemental analysis was performed on four SSAC samples, and the specific data were shown in 137 Table 1. The data showed that the main elements of SSAC were C (60.33%-67.34%) and O 138 (30.73%-38.15%), and the content of H (1.35%-1.54%) and N (0%-0.39%) was very low. When 139 the impregnation ratio increased from 1:1 to 1:3, the C content in the three types of SSAC 140 increased with the increase in the impregnation ratio, while the O content decreased. When the 141 impregnation ratio was 1:3, the highest C content (67.34%) and the lowest O content (30.73%) 142 were reached, indicated that the use of H3PO4 as an activator can remove oxygen-containing 143 functional groups on the surface of sunflower straw and the hard-to-remove cellulose remaining in 144 the straw through its hydrolysis (Vladov et al. 2019). But when the impregnation ratio increased to 145 1:5, the C content of SSAC4 decreased (62.55%), which might be caused by excessive H3PO4 146 attacking carbon materials. 147 The the four kinds of SSAC changed slightly, but with a small range (≤0.02), indicated that the change 151 of aromaticity was not obvious. As the impregnation ratio increased from 1:1 to 1:3, the surface 152 hydrophilicity and polarity of SSAC decrease. When the impregnation ratio increased to 1:5, the 153 hydrophilicity and polarity of SSAC4 were greater than that of SSAC2 and less than that of 154 SSAC1. Therefore, the increase of the impregnation ratio can affect the abundance of functional 155 groups (decrease at 1:1 to 1:3, increase at 1:5), and the most abundant functional groups exist on 156 the surface of SSAC1. 157 Scanning electron microscopy and characterization of pore structure 159 Fig. 1 showed four types of SSAC scanning electron microscopes. SEM showed that when the 160 immersion ratio is 1:1, dense circular holes with a diameter of 1-2 μm appear on the surface of 161 SSAC1. When the impregnation ratio increased to 1:2, the number of pores on the surface of 162 SSAC2 was greatly reduced, but the diameter of the pores that appeared increased to about 5 μm. 163 As the impregnation ratio increased to 1:3 and 1:5, the pores changed from round to larger and 164 narrow cracks (>5 μm). At the same time, the increase in the impregnation ratio also makes the 165 surface of SSAC rougher. 166 Fig. 2a showed the N2 adsorption-desorption isotherms of four SSAC. It could be seen from 167 the figure that the four types of SSAC were basically in line with the characteristics of the type IV 168 isotherm in the six physical adsorption isotherms divided by IUPAC (Sing et al. 1985), and the 169 adsorption isotherm tended to the Y axis under lower pressure, showed it was the H4 hysteresis 170 loop, which was the typical feature of the material containing both micropores and mesopores 171 ). This could also be seen from the pore size distribution diagram in Fig. 2b. 172 The pore structure property data of SSAC was shown in Table 2. From the data in the table, it 173 could be seen that the four types of SSAC prepared by using H3PO4 as the activator all had a large 174 specific surface area (SBET>1300 m 2 ‧g -1 ). As the impregnation ratio increased, the SBET of SSAC 175 also increases, and reached the maximum (1794.01 m 2 ‧g -1 ) at 1:3. Among them, SSAC2 was about 176 20% larger than SSAC1, and SSAC3 was compared with SSAC2. The increase was small, about 177 5%. However, when the impregnation ratio is increased to 1:5, the SBET of SSAC4 is significantly 178 reduced, slightly smaller than that of SSAC2, reaching 1643.21 m 2 /g. This is due to the expansion 179 of micropores due to the high impregnation ratio, which is caused by super-activation before the 180 pore wall is destroyed (Hwang et al. 2008). As the impregnation ratio increased, the specific 181 surface area (SMI) of SSAC decreased and the specific surface area (SEXT) increased. The total pore 182 volume increased with the increase of the impregnation ratio, while the micropore volume 183 decreased, on the contrary the mesopore volume increased. The formation of the pores of the 184 material is due to the oxygen-containing functional groups in the sunflower straw reacting with the 185 phosphoric acid vapor generated by the activator H3PO4 at high temperature and the C atoms in   as removal rate R%). The results were shown in Table 3. As the impregnation ratio increases, the 220 removal rate of SSAC to AF also increased, and the removal rate of SSAC4 reached the maximum 221 (97.73%). According to the results of characterization analysis, SSAC1 had the most abundant 222 surface functional groups, SSAC3 had the largest specific surface area, and SSAC4 had the 223 smallest microporosity, largest and average pore size. Since the adsorption capacity was related to 224 the pore volume and specific surface area, the removal effect was the best. The surface functional 225 groups and specific surface area of SSAC4 were not the largest, so it could be considered that for 226 SSAC, the pore size might be the main factor in determining the effect of removing AF. In order to 227 further investigate the adsorption performance of SSAC to AF, four types of SSAC adsorption 228 kinetic models and thermodynamic models of AF were studied below. 229

Effect of time and adsorption kinetics 231
The pseudo-first-order kinetics (Eq.4) (Lagergren 1898), the pseudo-second-order kinetics (Eq.5) 232 (Ho 2006) and the intra-particle diffusion model (Eq.6) ( Tan where, qe (mg/g) and qt (mg/g) are the adsorption capacity of SSAC on AF at adsorption 238 equilibrium and t (min) time respectively; k1 (min -1 ), k2 (g/(mg‧g)) and ki (mg/(g‧min 1/2) ) are the 239 adsorption rate constants, Ci is the constant related to the thickness of the boundary layer. 240 The adsorption kinetic curves of the four types of SSAC on AF are shown in Fig. 4a. SSAC1 241 and SSAC2 were both fast adsorption at 0-40 min, SSAC1 turned to slow adsorption at 40-140 242 min, then the adsorption of SSAC1 to AF basically reached equilibrium; while the slow adsorption 243 of SSAC2 occurred at 40-120 min, and the adsorption was basically reached after 120 minutes 244 balance. The fast adsorption stage of SSAC3 and SSAC4 was 0-15 min, and the slow adsorption 245 stage was 15-80min. After 80min, the adsorption of SSAC3 and SSAC4 to AF basically reached 246 equilibrium. Compared with the four types of SSAC, SSAC3 and SSAC4 took the shortest time to 247 reach the slow adsorption and the final adsorption equilibrium. When the adsorption reached 248 equilibrium, the saturated adsorption capacity of the four types of SSAC to AF was 249 SSAC4>SSAC3> SSAC2>SSAC1. 250 The experimental data were fitted using the pseudo-first-order kinetic model, the 251 pseudo-second-order kinetic model and the intra-particle diffusion model. The results are shown in 252 Table 4 and Table 5. According to the kinetic parameters shown in Table 3, it can be seen that 253 under the same concentration of AF, the correlation coefficient R 2 of the pseudo-second-order 254 kinetic models of the four kinds of SSAC models was all greater than 0.98, while the correlation 255 coefficient R 2 of the pseudo-first-order kinetic model was poor. Moreover, the calculated results of 256 the pseudo-second-order kinetic model were closer to the experimental values, so it could be 257 considered that the pseudo-second-order kinetic model was more suitable to explain the adsorption 258 kinetic model of SSAC on AF, so the adsorption process of SSAC on AF included chemical 259 adsorption (Liu et al. 2017). 260 Intra-particle diffusion model was used to further investigate the adsorption mechanism of 261 SSAC on AF. It can be seen from Fig. 4b that the process of the four kinds of SSAC on AF is 262 presented in three stages, indicating that the adsorption process is composed of multiple steps. It 263 can be seen from the data in Table 5 that the correlation coefficient R 2 of the three processes in the 264 intra particle diffusion model was all above 0.86, and all the Ci values were not 0, so it could be 265 concluded that intra particle diffusion was involved in the adsorption process, but it was not the 266 only control step (Wu et al. 2014). ki values of the three stages of the four ACs decreased 267 successively (ki1 >ki2 >ki3), indicating that in the first stage, AF molecules controlled by molecular 268 diffusion and membrane diffusion were transferred from the solution to the surface of SSAC; in 269 the second stage, AF molecules were transferred from the surface of SSAC to the pore; this 270 process was intra-particle diffusion; in the third stage, adsorption reached equilibrium (Zeng et al. 271 2019). It can be concluded that the diffusion rate of AF was limited by both membrane diffusion 272 and intra-particle diffusion. 273   Fig. 4 Fitting of pseudo-first-order and pseudo-second-order kinetic models of AF adsorption by 278 SSAC (a); intra-particle diffusion model (b) 279

287
where Ce (mg/L) is the concentration of AF in adsorption equilibrium, qe (mg/g) is the adsorption 288 capacity of SSAC to AF in adsorption equilibrium, 1/n is the heterogeneous factor, BT (L/min) is 289 the equilibrium binding constant, kL (L/mg), kF ((mg/g)‧(L/mg) 1/n ). 290 The adsorption isotherms of four types of SSAC to AF are shown in Fig. 5, and the fitting 291 data are shown in Table 6. It can be seen from the adsorption isotherm that as the SSAC 292 immersion ratio increases, the equilibrium adsorption capacity of the four SSAC for AF also 293 increases, and the equilibrium concentration of AF in the solution decreases, and the immersion 294 ratio increases from 1:1 When it reached 1:3, the equilibrium adsorption capacity increases more 295 obviously, while the impregnation ratio was expanded from 1:3 to 1:5, and the equilibrium 296 adsorption capacity increases slightly. This may be because the increase in the AF concentration 297 provides more for the adsorption process. Driving force to control the resistance of AF transfer 298 from liquid to solid (Qiao et al. 2016;Yang et al. 2020).

299
From the data in Table 6, for SSAC3 and SSAC4, the Langmuir adsorption isotherm model 300 fits better, indicating that the adsorption of these two types of SSAC were more inclined to 301 monolayer adsorption and chemical adsorption, and 0<kL<1, indicating this was a favorable 302 adsorption process (Foo and Hameed 2010). 303 For SSAC1 and SSAC2, the correlation coefficient R 2 obtained from the Freundlich 304 adsorption isotherm model were the highest, indicating that the two types of SSAC were 305 non-uniform surfaces, adsorbed as multi-molecular layers, and had adsorption behavior of 306 physical adsorption. The heterogeneity factor 1/n<1 indicated that Easy to adsorb and the 307 adsorption process was favorable (Adamson and Gast 1997). 308 Since the Temkin adsorption isotherm model fits the four types of SSAC well (R 2 >0.96), it 309 indicated that there was chemical adsorption. In summary, the three adsorption models of 310 Langmuir, Freundlich and Temkin had high correlation coefficients for the four types of SSAC 311 (R 2 >0.96). It can be considered that the adsorption behavior of AF on the SSAC surface includes 312 both physical and chemical adsorption. 313 According to the fitting data, at 303 K, the maximum adsorption capacity of the four types of 314 SSAC for AF were greater than 600 mg/g, of which SSAC4 had the highest maximum adsorption 315 capacity for AF, which was 2763.36 mg/g. Table 7 compared the adsorption of different materials 316 to AF. The data showed that compared with other materials, the four types of SSAC all exhibit 317 better adsorption performance to AF. Therefore, SSAC has great potential for the adsorption of 318 AF. 319 where Kd is a distribution coefficient, R is the gas constant in an ideal state (8.314 J/(mol‧K)), T is 333 absolute temperature (K), ΔG is Gibbs free energy change (kJ/mol), ΔH (kJ/mol) is a reactive 334 enthalpy, ΔS (kJ/(K‧mol)) is the entropy change of the reaction. It can be seen from

345
In this experiment, H3PO4 was used as the activator, and the sunflower straw activated carbon was 346 successfully prepared using the one-step activation method under the preparation conditions of 347 14 600 ℃. It can be seen from the characterization results that the impregnation ratio has a great 348 influence on the performance of SSAC. When the impregnation ratio increased within a certain 349 range, the specific surface area of SSAC would increase, and it reached the maximum when the 350 impregnation ratio was 1:3 (1794.01 m 2 ‧g -1 ), but when the impregnation ratio increased to 1:5, the 351 specific surface area would decrease (1643.21 m 2 ‧g -1 ) due to the collapse of micropores caused by 352 hyper-activation. At the same time, the microporosity rate was also reduced from 50.33% at 1:1 to 353 2.07% at 1:5. It showed that the SSAC prepared by using H3PO4 to activate sunflower straw was 354 mainly mesoporous. SSAC prepared with different impregnation ratios all showed higher 355 adsorption capacity for AF. The quasi-second-order kinetic model was suitable for describing the 356 adsorption kinetics of the four types of SSAC. The Langmuir adsorption isotherm model was 357 suitable for describing the adsorption process of SSAC1, SSAC3 and SSAC4, while SSAC2 was 358 more in line with the Freundlich model. Generally, the three adsorption isotherm models of 359 Langmuir, Freundlich and Temkin have good correlation coefficients. The adsorption of AF on the 360 four types of SSAC was a process of spontaneous heat absorption, so the adsorption of SSAC to 361 AF had two effects: physical adsorption and chemical adsorption. Among the four types of SSAC, 362 SSAC4 had not the largest specific surface area, but had the lowest microporosity (2.07%), the 363 largest total pore volume (2.549 cm 3 ‧g -1 ) and average pore size (6.21 nm). SSAC4 had the best 364 removal effect on AF dye wastewater. Under the condition of 303 K, the removal rate of 200 mg/L 365 AF wastewater was 97.73%, and its maximum adsorption capacity for AF was 2763.36 mg/g, 366 which was the highest among the four types of SSAC. Therefore, SSAC4 prepared with an 367 immersion ratio of 1:5 at 600 °C was a potential SSAC material for removing AF in water. SSAC 368 had simple preparation method and low cost, realizes the secondary utilization of agricultural 369 waste, and achieves the purpose of treating waste with waste. 370 Funding This work was supported in part by the Major Science and Technology Project in Inner 371 Mongolia Autonomous Region (2019ZD002). 372 Availability of data and materials The datasets used and/or analyzed during the current study are 373 available from the corresponding author on reasonable request. 374