Bone char from an invasive aquatic species "devilfish" as a sustainable adsorbent for 1 the removal of fluoride in water for human consumption

26 In this study, bone chars were obtained from an alien acuatic species “devilfish” bones by 27 pyrolysis of 500-800 °C. Bone chars were evaluated as a sustainable adsorbent of fluoride, it 28 was found pyrolysed bone char at 500 °C adsorbed the most amount of fluoride. The effect of 29 pH indicated that the adsorption capacity increased as the pH decreased. Thermodynamic 30 parameters of fluoride adsorption on devilfish bone chars were estimated as  H°= 7.213 kJ mol - 31 1 ,  G°= 23.61 kJ mol -1 and  S° = 103.4 J mol -1 K -1 indicating that adsorption is endothermic, 32 spontaneous and with great affinity of fluoride on bone char from devilfish. The fluoride 33 desorption study showed that fluoride is desorbed from the material of 0.24 to 20.06 %, so the 34 adsorption is considered to be partly reversible. The regeneration of the bone char at 400, 500 35 and 600 °C was studied and it was noted that its adsorption capacity decreases slightly so it 36 could be considered appropriate for the use in water treatment technologies. Adsorption of 37 fluorides from drinking well water of a rural community with dental fluorosis problems and 38 high levels of fluoride in water, revealed that by increasing the amount of the bone char of 0.05 39 to 0.8 g, the disposal of fluoride increases from 69.1 to 98.7 %. Lastly, it was established that 40 the bone char synthesized from devilfish is a low-cost, viable a sustainable material to remove 41 fluorides from water and represents an environmental management strategy of this alien


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The presence of invasive aquatic specie affects the imbalance of biodiversity and it is a problem 49 detected in ecosystems worldwide, also it has even generated economic collateral consequences The synthesis of the bone chars by means of a pyrolysis process was conducted in a tube furnace 120 Carbolite, model CTF-1200°C, at temperatures of 400, 500, 600, 700 and 800 °C, with a flow 121 of N2 of 100 mL min -1 , 10 °C min -1 and a pyrolysis time of one hour at the specified synthesis 122 temperature.

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The pyrolysed bones at 400, 500, 600, 700 and 800 °C were labelled as C400, C500, C600, 124 C700 and C800, respectively. 125 The yield of the production of the bone chars, %R, was determined with the next equation: The amount of fluoride remaining in te adsorbent was estimated by applying a mass balance: where q0 is the amount of fluoride on the adsorbent at the outset of the desorption, mg g -1 .

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Additionally, thermal regeneration of bone char synthesized at 500 °C saturated with fluoride 164 was evaluated at temperatures of 400, 500 and 600 °C using the same synthesis conditions as 165 described in section 2.2. The materials were identified as CR400, CR500 and CR600, 166 respectively. These materials were contacted with fluoride solutions to determine their 167 adsorption capacity. The adsorption of fluoride contained in the water of a well in La Laborcilla rural community in 171 the municipality of Villa de Arriaga, San Luis Potosi in Mexico was studied (Fig. 2). The 172 fluoride concentration of the drinking well water from this community had a concentration of of bone char that varied from 0.05 to 0.8 g, and a volume of 100 mL of water. The determination 175 of the amount of fluoride adsorbed was described in section 2.4. Also, the % removal of total 176 hardness, chlorides and sulphates from water was evaluated as they can compete in fluoride 177 removal on bone char. The % removal, %Re, was defined according to the following equation where C0 is the concentration of the anion in water and Cf is the concentration to the end of the 180 adsorption experiment.    Fig. 3.

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The N2 adsorption isotherm of sample Bone (Fig. 3a) showed that it is type IIb, which 201 corresponds to non-rigid aggregates, cements, pigments. Furthermore, it presents a hysteresis  be considered that the conditions used in this study to carry out bone pyrolysis were adequate.

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The constants of these isotherms were evaluated using a method of least squares based on the 316 Rosenbrock-Newton optimisation algorithm and using a criterion of minimum percentage 317 standard deviation, which is defined as where qexp (mg g -1 ) is the experimental amount of adsorbed fluoride and qcal (mg g -1 ) is the 320 amount of adsorbed fluoride predicted.

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The isotherm constant values and their %D for the fluoride adsorption equilibrium on C500 are 322 shown in Table 3. As it can be seen, the experimental data were better interpreted by three-323 parameter isotherm model, as the %D were less than 8.70, 11.37 and 19.84 % for the Prausnitz-

Adsorption capacity and its effect by the solution pH 328
The fluoride adsorption on C500 was studied at 25 °C and at different solution pH (5.0, 7.0 and 329 9.0) to evaluate its effect on the adsorption capacity, the results are shown in Fig. 9 where (∆Hads)q represents the isosteric heat of adsorption (J mol -1 ), R is the universal constant 361 of the ideal gases (J mol -1 K -1 ), C1 and C2 (mg L -1 ) are the fluoride concentrations at T1 and T2, 362 respectively, at the same value of q and T1 and T2 (K) are the temperatures at conditions 1 and At a mass of fluoride adsorbed on C500 of q= 7.0 mg g -1 the equilibrium fluoride concentrations 365 were C1= 2.624 mg L -1 and C2= 1.163 mg L -1 , at temperatures T1=288.15 K and T2= 308.15 K, 366 respectively. The isosteric heat calculated was 30.038 kJ mol -1 . This indicates that fluoride 367 adsorption on C500 is an endothermic and physical process (<83 kJ mol -1 ).  Table 4: The thermodynamic parameters evaluated for the fluoride adsorption process on bone char are 378 shown in Table 4  This reveals that fluoride is desorbed to a greater extent from C500 at more basic solution pH.

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That is, the desorption at pH 5.0 shows us that the fluoride that is desorbed goes from 0.24 to 403 3.06 %, while for desorption at pH 9.0 it goes from 1.03 to 20.06 %, this indicates that the 404 fluoride desorption process in C500 at both pHs is partially reversible, therefore the adsorption 405 is not completely physical, there is also the adsorption by chemisorption or by ion exchange.

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The adsorption capacities of the regenerated materials as well as the C500 before regeneration 407 are shown in Figure 12. The results show that the fluoride adsorption capacities are maintained 408 after thermal regeneration as the adsorption capacities vary slightly compared to C500 and show 409 a similar variation to the study of the effect of the pyrolysis temperature of bone char.  NVP NVP and the values suggested by the WHO are fluorides whose concentration is 4.5 mg/L, 3 times 440 higher than the permitted concentration (1.5 mg L -1 ).
441 Table 6 shows that the % of fluoride removal increases from 69.1 to 98.7 % as the dose of C500 442 increases from 0.05 to 0.8 g, until almost all the fluoride is removed from the water.

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The increase in adsorption with the amount of the adsorbent may be due to a higher specific 444 surface area and concentration of adsorption sites.