Heat Carbonization and ZnCl2 Functionalization of Date Stone as an Adsorbent: Optimization of Material Fabrication Parameters and Adsorption Studies

42 The scientific community gave a lot attention to prepare adsorbents from different natural 43 agriculture-based materials to be used alternative to commercial activated carbon. However, 44 less studies on the optimization of fabrication parameters to obtain activated carbon with highly 45 surface area and adsorption capacity. Herein, we report the synthesis conditions optimization 46 of adsorbent based on date stone and modified with ZnCl 2 . To obtain a highly adsorption ability 47 of the materials, three systematic parameters were evaluated such as the activation temperature, 48 activation time and the functionalization ratio by ZnCl 2 . The optimization study showed that 49 the best factors to fabricate an adsorbent from date stone are 700°C, 120 min and 2.0 (g/g), 50 wherein, the specific surface area was found to be 1036 m 2 /g. While, the iodine and phenol 51 numbers were 928.5 mg/g and 2.1 mmol/g, respectively. To further understand the effect of 52 synthesis parameters, the raw and the as-synthesized activated carbon were characterized via 53 Fourier transmission infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential 54 thermal analysis (DTA) and differential scanning calorimetry (DSC). Batch sorption tests to 55 remove MB from water showed a maximum adsorption capacity of 384.6 mg/g using the 56 prepared activated carbon at pH 6 and room temperature (25 ± 2 °C). It was found also that the 57 kinetic adsorption data obeyed the pseudo-second order and, both external diffusion and intra- 58 particle diffusion control the adsorption. Based on the obtained results, the optimization of 59 synthesis conditions through experimental and mobilization studies may help the transfer of 60 technology in terms of agriculture-based materials valorisation towards the environmental 61 remediation.


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The recent huge socio-agro-industrial revolution development has led to an eco-system and 68 environmental disequilibrium as a result of the huge irregular liquid, solid and gas pollution 69 discharged in our planet. It became an inevitable to create novel ideas and find solutions to the 70 existing problems. The scientific and health communities are recommending to apply green and 71 clean technologies for the production processing, that respecting our environment from one side  Negroiu et al. 2021). The valorisation of agro-wastes has two main advantages, the first is the 82 reduction of solid pollution and waste management in the environment, and the second is the 83 economic value of obtained adsorbents due to the local availability of several raw agro-ratio should be investigated. In the present study, the optimization of activation parameters 117 (thermal and chemical) was investigated experimentally and also using planning experiments 118 method via design expert. By the use of a such software and based on the experimental data 119 (Sulaiman et al. 2018), a better optimization can be obtained which further facilities the large-120 scale synthesis processing and the transfer of the use agro-wastes technology to real-world 121 application. Three parameters were controlled to fabricate the activated carbon from date stone 122 waste such as temperature, time and the ratio of ZnCl2. The as-prepared activated carbon-based 123 adsorbent was characterized using several techniques to evaluate the morphology and structural 124 proprieties. The sorption ability was evaluated to adsorb methylene blue in batch system. The raw material (date stones) was obtained from a local date manufactory. Firstly, date stones 128 were washed several times with water and distilled water, and dried at 105 °C for 24 h. After 129 that, the raw material was mechanically crushed and particles with size of about 300 μm were 130 recovered by sieving. The activation of date stones powder was carried out by controlling three 131 parameters: values of calcination (400 and 700°C), time of calcination (30 and 120 min) and 132 ratio of ZnCl2 (0.25 and 2 w/w). The planification of preparation experiments were studies and 133 optimized using 2 3 full factorial design (FFD) software in order to identify the role of applied 134 parameters as a function of the adsorption capacity of MB as shown in Table 1. 135 Date stones powder was impregned in ZnCl2 solution with different ratio (w/w) for 24 h at 136 110°C. After that, the sample is transferred to a furnace and heated at different temperature and 137 time under a constant N2 (99.99%) flow rate of 120 cm 3 / min. The obtained activated and then 138 hot distilled water until the pH of the washed solution reached a value around 7. Finally, the 139 sample were dried at 110°C for 24 h, ground and sieved to get a particle size of around 125 μm.  The effects of main variables governing the removal efficiency of MB such as solution pH,  The effects of synthesis factors and their interactions were invistigated by using full factorial 186 design (FFD). As it can be seen from

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In addition, the ratio of ZnCl2 showed a great effect towards the activation of DSAC. Certainly,

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the heating activation is a main factor, together with ZnCl2 activation as shown in Figure 1.a.

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The polynomial model equation, adopted in this study, is given as follows: The positive sign in front of the terms indicates a synergetic effect, whereas a negative sign     and pore diamater of as-prepared DSAC, respectively.

Effect of operating parameters on the adsorption capacity 256
The capacity of adsorption of DSAC towards MB was studied from the range of 5 to 9 at a 257 concentration of 83.5 ppm and a mass of DSAC of 0.2 g/L (Figure 4.a). The results showed 258 that the capacity of adsorption was in the range of 321-313 mg/g at the pH range of 5-9. At this 259 pH range, the charge of DSAC might be relatively negative within this pH range which allows

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In order to study the isotherm adsorption of MB, a set of experiments was carried out by adding coefficients. It can be seen that the correlation coefficient R 2 of Langmuir equation (R 2 = 303 0.9991) is higher than those of Freundlich (R 2 = 0.956) and Temkin (R 2 = 0.8713) equations.

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Hence, it seems that the adsorption isotherm data are well described by the Langmuir model 305 which can be attributed to the homogeneous distribution of the active sites on the DSAC 306 surface. The monolayer adsorption capacity, according to the Langmuir isotherm, is found to 307 be 384.6 mg/g at 25 o C. Furthermore, the value of 1/n gives by Freundlich assessed as 0.0778 308 proves the efficiency of the MB adsorption.  Table 3. Langmuir, Freundlich and Temkin isotherm constants and correlation coefficients for 312 adsorption of MB onto the DSAC.

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To investigate the adsorption mechanism of the MB on the DSAC, kinetic models such as the 315 pseudo -first order, the second order and the intra-particle diffusion were applied to study the Where e q and t q (mg/g) are the MB adsorbed amount at equilibrium and at time t (min), 320 respectively. 1 k (l /min) is the pseudo-first-order rate constant.

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The pseudo-second-order kinetic equation is described using Equation 11. Where ext k is the external diffusion coefficient and t C is the concentration at time t.

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The intra-particle diffusion model is expressed by Equation 13.
Where p K is the intra-particle diffusion rate constant (mg/g. min 0.5 ) and C is the constant 331 related to the thickness of the boundary layer (the intercept). In fact, the greater the value of 332 this constant, the higher is the effect of the boundary layer. The slope and the intercept values of each linear plot are listed in Table 4. result suggests that the adsorption data fit well with the pseudo-second-order kinetic model.

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The values of kp and kext obtained respectively from external and intra-particle diffusion models 344 are listed in Table 4 with their correlation coefficient values. It was concluded that the 345 adsorption of MB on the surface of DSAC was achieved through two steps, the first step (during 346 the first 90 min) fits with the external diffusion, while the second step, after 90 min, was in 347 agreement with the intra-particle diffusion, which involves the insertion of MB molecules.