Sustainable Conversion of Agriculture and Food Waste into Activated Carbons Devoted to Fluoride Removal from Drinking Water in Senegal

The objective of this study was to investigate the production of activated carbons (AC) from cashew shells, and millet stalks and their efficiency in fluoride retention. These agricultural residues are collected from Senegal. It is known that some regions of Sénégal, commonly called the groundnut basin, are affected by a public health problem caused by an excess of fluoride in drinking water used by these populations. The activated carbons were produced by a combined pyrolysis and activation with water steam; no other chemical compounds were added. Then, activated carbonaceous materials obtained from cashew shells and millet stalks were called CS-H2O and MS-H2O respectively. CS-H2O and MS-H2O show very good adsorbent features, and present carbon content ranges between 71 % and 86 %. The BET surface areas are 942 m2 g and 1234 m2.g for CS-H2O and MS-H2O respectively. A third activated carbon produced from food wastes and coagulation-flocculation sludge (FW/CFS-H2O) was produced in the same conditions. Carbon and calcium content of FW/CFS-H2O are 32.6 and 39.3 % respectively. The kinetics sorption were performed with all these activated carbons, then the pseudo-first equation was used to describe the kinetics sorption. Fluoride adsorption isotherms were performed with synthetic and natural water with the best activated carbon from kinetics sorption, Langmuir and Freundlich models were used to describe the experimental data. The results showed that carbonaceous materials obtained from CS-H2O and MS-H2O were weakly efficient for fluoride removal. With FW/CFS-H2O, the adsorption capacity is 28.48 mg.g with r2 = 0.99 with synthetic water.


Introduction
In recent years, many studies have been done in order to remove the high concentration of many contaminants from drinking water such as fluoride, lead, arsenic, copper and nitrate (Sud, Mahajan, & Kaur, 2008).It is known that fluoride is an essential element in drinking water.This trace element is actively involved in the health of teeth, especially in the prevention of dental cavities and plays an important role in bone strength.However, when the fluoride concentration in drinking water is higher than 1.5 mg.L -1 , it may cause harmful effects on human health namely dental fluorosis and skeletal fluorosis at concentrations above 4 mg.L -1 (Srivastav, Singh, Srivastava, & Sharma, 2013;Diallo, Diop, Diémé, & Diawara, 2015).In the world, it is estimated that more than 200 million people are exposed to drinking water with a fluoride concentration that exceeds the WHO guideline (1.5 mg.L -1 ) (Bhatnagar, Kumar, & Sillanpää, 2011).Many countries around the world such as India, Bangladesh, Nepal, Sénégal, USA, and Mexico are concerned by fluoride exposure because of the high numbers of their populations that present the effects caused by the high concentration of fluoride in their drinking water (Fawell et al., 2006;Bibi, Farooqi, Hussain, & Haider, 2015).Thus, several processes for fluoride removal have been developed.Some are based on membrane technologies such as nanofiltration and reverse osmosis ( Diop, Diémé, & Diawara, 2015), while others use adsorption technologies such as adsorption into clay or activated carbon (Yadav, Abbassi, Gupta, & Dadashzadeh, 2013).Membrane technology is well known for its effectiveness in fluoride removal, but its high operating cost may remain a problem for developing countries.Many papers also report the use of the adsorption process to remove fluoride in drinking water.Activated carbons are widely used as adsorbents for pollutant removal due to their interesting physical and chemical properties.Moreover, activated carbons could be low-cost materials if produced from agricultural waste collected freely in the fields.Previous papers studied the efficiency of activated carbon in fluoride uptake.Consequently, for enhancing the uptake of fluoride by activated carbon these authors have modified the adsorbent by impregnating it with the calcium solution (Hernández-Montoya, Ramírez-Montoya, Bonilla-Petriciolet, & Montes-Morán, 2012).
The purpose of this study is to produce and characterize low-cost activated carbon and to study their efficiency in fluoride sorption.The activated carbons were produced from millet stalk, cashew shell and a mixture of food waste and coagulation-flocculation sludge (FW/CFS).Then these three carbonaceous materials were used to study fluoride adsorption efficiency.The Langmuir and Freundlich models were used to describe the isotherms experimental data and the pseudo first order equation was also used to describe the kinetic data.

Production of Carbonaceous Materials
Millet stalks and cashew shells named respectively MS and CS, were used as precursors.Millet stalks were cut into small pieces to facilitate their introduction to the reactor, while cashew shells were left in their original state.The carbonization (or pyrolysis) was conducted under an inert atmosphere (0.5 L/min of N 2 ) up to 850 °C with a temperature ramp of 10 °C/min in a batch quartz rotary furnace (HTR 11/150,Carbolite).At 850 °C, the step of activation was started with an injection of steam (0.7 mL of water.min - ) as activating gas for 80 minutes.The cooling of the furnace was still realized under inert atmosphere.For the FW/CFS, first, pyrolysis char from 50 wt % FW and 50 wt % CFS was produced in semi-continuous screw reactor by slow pyrolysis (heating rate of 22 °C.min - ) at 700 °C during 30 min.The details of the experimental procedure were described in a previous paper (Mura, Debono, Villot, & Paviet, 2013).The char was then activated with steam to produce FW/CFS-H 2 O.The same experimental procedure as that described above was used for the activation process.AC were washed with deionized water, and dried at 105 °C before being characterized.This method was adapted from the previous work realized by the research team (Torres-Perez, Gerente, & Andres, 2012).

Characterization of Activated Carbon
Elemental analysis of CHNSO was performed using the apparatus Flash EA 1112, Thermofinnigan.The total ash content and pH PZC (point of zero charge) determination of each activated carbon were carried out following a methodology previously described (Torres-Perez, Gerente, & Andres, 2012).For the pH PZC , 100 mL of 0.01 mol.L -1 NaCl solution was placed in a closed polyethylene bottle.The pH was adjusted between 2 and 12 by adding HCl or NaOH 0.1 mol.L -1 solution.Then, 0.05 g of each sample was added in the closed polyethylene bottle that was set stirring for 5 days at the room temperature before measuring the final pH.Then the final pH was plotted against the initial pH, and the point where this curve crosses the line pH final = pH initial represents the pH PZC .
The porous properties of activated carbon were deduced from nitrogen adsorption isotherms at 77 K (ASAP 2020 Micromeritics).The scanning electron microscopy (SEM) was carried out using the apparatus JOEL JSM 5800LV, allowing the observation of the porous structure of carbonaceous materials.To determine the presence of the other elements such as iron and calcium quantitative analysis was performed by using EDX-800HS apparatus.

Fluoride Adsorption
All the measurements of fluoride concentration were performed by using a UV-1800 spectrophotometer.Firstly, the suitable wavelength for these measurements was determined.Thus, a solution of 2 mg.L -1 was prepared from an initial fluoride solution of 0.2 g.L -1 prepared by dissolution of NaF in deionized water (Milli-Q Millipore 18.0 MΩcm -1 , resistivity).Then a square cell sample was filled with 2 mg.L -1 of fluoride solution up to the mark before it was placed in the sample holder.From there, the peak wavelength was determined from the spectrum curve.Thus the maximum wavelength used in this study was 618.3 nm.Analytical measurements were obtained with a quantification limit of 0.25 mg L -1 and a detection limit of 0.12 mg L -1 .The quantification limit is the lowest level that can be reliably measured.
All three activated carbons were used in the fluoride adsorption in deionized water, before the better of these adsorbents were used for the isotherm adsorption both in deionized and natural water.
For the sorption kinetic experiments, batch contact time experiments were conducted at 21°C by stirring 0.8 g of sorbent with 1000 mL of fluoride solution (5 mg L -1 ) at 250 rpm.The pH was measured before adding the sorbent in the polyethylene reactor and measured at the end of kinetic.Then the equilibrium time between the solid and the solution was determined by plotting the fluoride concentration versus time.The pseudo first order sorption model proposed by (Ho & Mckay, 1998) was used to describe the kinetic curves as indicate by the following equation: Where e q and t q are the sorption capacities at equilibrium and at time t respectively (mg.g -1 ) and 1 k is the rate constant of pseudo first order sorption (min -1 ).Then the integration with the conditions follow, t = 0 to t = t and t q = 0 to t q = t q , the linear form obtained is expressed as follow: The parameters e q and 1 k were calculated by plotting   t e q q  log versus t .
Bath adsorption isotherms were conducted at 21 °C with 250 mL of synthetic solution from 3 to 25 mg.L -1 of fluoride and 0.175 g of adsorbent.Then the reactors have been stirred for 120 min at 250 rpm.The pH was measured before adding the sorbent and at the end of the experiment, the values ranged between 5 and 9. Langmuir and Freundlich models were used to describe the experimental data of the isotherms (Freundlich, H.M., 1906); (Langmuir, I., 1918).The Langmuir equation is describes below: Where b is the equilibrium constant of the reaction (L.mg -1 ), m q and e C are the maximum adsorption capacity (mg.g -1 ) and the amount of fluoride at equilibrium (mg.L -1 ), respectively.
The Freundlich equation is given below: Where f K (mg.g -1 )/(mg.L -1 ) 1/n and n the Freundlich isotherm constant related to the adsorption capacity.
In sorption processes, it is know that the presence of other ions can contribute to a competitive effect between the ions, leading to a modification of the adsorption capacities.Thus the isotherm adsorption was performed with natural water, the composition of which is given in Table 1.The potential effect of these ions into the adsorption capacity will be discussed below.

Characterization of Activated Carbon
The chemical characterization (elemental analysis and pH PZC ) and the physical characterization (BET surface area analysis) are given in the Table 2.

Table 1 .
Mineral content of natural water

Table 2 .
Properties of activated carbons from the mixture of food waste and coagulation-flocculation sludge (FW/CFS-H 2 O) is low (32.6 %).This carbon content may negatively impact on the BET surface area.As an identical production method was used, the differences in the properties of the activated carbons are only assigned to the precursor nature.The second major element of the activated carbon is oxygen.Its content is 12.2 %, 21.0 % and 6.6 % for the MS-H 2 O, CS-H 2 O and FW/CFS-H 2 O respectively.These values of oxygen content are close to those obtained by Torres-Perez, Gerente, & Andres, (2012) that have characterized two commercial granular activated carbons.e q