Valorization of the use of waste agricultural materials for the anodic oxidation of Amaranth Red (E123) 1 using SS/PbO 2 anodes elaborated by pulsed mode current

9 The present paper aims to valorize the use of cheap agricultural waste materials for polluted water 10 decontamination. An evaluation of the efficiency of coupling anodic oxidation (AO) using SS/PbO 2 electrodes 11 with biosorption by Luffa cylindrica (L.C) for the removal of Amaranth Red (E123) from aqueous solution was 12 investigated. The effects of pH, contact time, and initial concentration were studied. The regeneration of L.C was 13 estimated based on biosorption /desorption tests. The performance of the coupling process was evaluated based 14 on the color, chemical organic carbon (COD), Total organic carbon (TOC) removals, the energy consumed, and 15 the time required for the degradation of Amaranth. A comparison between the efficiency of the AO and the 16 coupling process for the increase of the lifetimes of the anodes used was done. 54.1, 97.8, and 99.9% of 50 mg.L - 17 1 of Amaranth were removed respectively after 85, 65, and 50 min by biosorption, AO, and coupling AO with 18 biosorption. An increase in the percentages of COD, TOC, germination indexes (GI), and Amaranth removals 19 were observed when adopting the coupling process. Furthermore, a decrease in the release of Pb 2+ ions was 20 observed confirming the good stability of the elaborated anodes during the coupling process. Atomic absorption 21 analysis showed that the Pb 2+ ions reached about 0.020 mg.L -1 , after the total removal of Amaranth dye (60 min) 22 and 0.051 after (80 min) respectively, for coupling AO with biosorption and the AO process. These values are 23 inferior to those allowed by the Standards. Phytotoxicity tests confirmed the possibility of the reuse of the treated solutions.


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The discharge of wastewater laden with polluting substances into the receiving environment without or with 28 unsuitable treatment is a cause for growing concern given the undesirable effects; it can have on the environment 29 and the health of living beings (Iloms et al. 2020). The protection of water resources against this growing 2 pollution is the subject of several research studies for the implementation of more efficient, inexpensive, and 31 environmentally friendly technologies (Othmani et al. 2020b). The decontamination of water resources can have 32 various origins; they can be inorganic or organic (Garcia-Segura et al. 2015). Wastewater is characterized by 33 strong variations in pH, high concentration of organic matter , and high chemical oxygen demand (Choi et al.

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However, the short lifetime of the deposited layer of PbO2 into the substrate these last can be one of the main 48 drawbacks of their use (Mohd and Pletcher 2006). In this context, several methods have been devoted to this 49 utility. However, most of them are expensive and required the use of several products and types of equipment.

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Among the proposed solutions; the doping methods have been successfully applied for enhancing the stability of 51 the elaborated electrodes (Dao et al. 2020). Coupling two efficient processes to enhance the effectiveness of each 52 process alone has been successfully applied. Among the performing coupling processes studied, we can cite 53 coupling ultrasonic cavitation with electrochemical oxidation, which leads to enhance the effectiveness of the 54 studied parameters like the reaction rate, the energy consumed, and the removal efficiency. ).

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Others have chosen to modify the PbO2 electrodes with an environmentally friendly conductive carbon black.

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The results showed an increase of about 24.66% in the removal efficiency of metronidazole compared to the raw 57 PbO2 electrodes. Furthermore, an improvement in the current efficiency and the generation of hydroxyl radicals 58 was achieved (Wang et al. 2020). Based on the literature most of the solutions proposed for the enhancement of 59 the stability of electrodes based on PbO2 are oriented for the doping methods. However, more cheap and clean 60 3 alternatives can take place for this utility. Among these alternatives the one proposed by (Othmani et al. 2019) 61 who have offered a new alternative based on the use of alternating current (AC) instead of direct current (DC) 62 for the anodic oxidation of methylene blue (MB) using SS/PbO2 and Pb/PbO2 electrodes. Results showed an 63 enhancement of the stability of the used electrodes. Furthermore, atomic absorption analysis confirmed the 64 decrease of the release of (Pb 2+ ) ions to much lower values compared to DC and those allowed by the Standards.

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A year later, the same team has proposed a cheap alternative based on coupling biosorption using L.C with AO 66 using SS/PbO2 anodes. Results showed that 98.7 and 80.02% of MB were removed, respectively, after 60 and 67 120 min for AC and DC when using the coupling process. Otherwise, 62.84 and 46.87 % of the same dye were 68 removed by the AO process, respectively, after 120 and 180 min for AC and DC (Othmani et al. 2020a). These 69 findings encourage the use of cheap agricultural waste materials for the removal of more hazardous pollutants.

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According to the literature, the production of azo dyes is estimated at around 350,000 tons per year (Pagga and 71 Brown 1986; Radhakrishnan 2014). In practice, this category of dye is currently the most common. It represents 72 more than 50% of worldwide production (Khadhraoui et al. 2009). During the dyeing procedures, 10-15% of the 73 dyes used in the initial quantities can be lost and discharged without prior treatment in the effluents (Singh and 74 Arora 2011). These latter are carcinogenic and refractory (Bauer et al. 2001; Barros et al. 2014). Therefore,

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Amaranth Red (E123) was chosen as a model in this study. Many points were studied in the present paper. First,

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we tried to encourage the use of cheap and abundant agricultural waste materials for hazardous pollutant removal 77 to replace the use of expensive adsorbent. Second, enhance their capacity uptake and reaction speed by coupling 78 biosorption with anodic oxidation. Third, to enhance the lifetime of the elaborated electrodes used and to 79 minimize the generation of Pb 2+ ions by a safe and economic method based on the coupling process. The 80 characterization of the biosorbent used and the SS/PbO2 anodes were done using X-ray diffraction (XRD) and

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The used L.C has a Tunisian origin. It is composed of 54% of cellulose, 11% of lignin, 5% of pectin, 7% of fats 92 and waxes, and 23% hemicelluloses (Othmani et al. 2020b). The preparation of this biosorbent consisted of 93 cutting the fibers finely, washing those last several times to remove all impurities, and drying them at 70°C until 94 the material was completely dried constantly. The dried sample was ground and sieved to obtain a uniform

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The stock solution of Amaranth Red (E123) (purchased from Sigma-Aldric, purity≥ 99.5%) was prepared by 114 dissolving 1 g of the dye in 1 L of distilled water. Test amaranth red solutions were prepared by diluting the 115 stock dye solution.

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The desired concentrations were obtained by successive dilution. The adjustment of pH was done using dilute  125 Qe is the adsorbed quantity at equilibrium time, C0 and Ct (mg.L -1 ), correspond respectively to the initial and

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The desorption rates were calculated according to Eq.(4) If TD >50%, the retention of Amaranth by the L.C is weak which means it is physisorption,

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If TD <50% the retention of Amaranth by the L.C is important which means it is chemisorption.

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The evaluation of all experiments was performed based on various parameters, like chemical Oxygen Demand

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Therefore, this porous structure of L.C leads to the retention of Amaranth Red (E123).

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Concerning the coupling process, a combination between the possible pathway of the retention of Amaranth dye 235 onto the available sites of L.C and the electrochemical degradation of this last has taken place in harmony.

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Therefore, the removal of Amaranth was faster than each process alone. The presence of L.C has given some

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The degradation of Amaranth Red (E123) was promoted by the hydroxyl radicals formed at the SS/PbO2 surface 263 from the water oxidation and other weak oxidizing species formed in the sulphate medium. The efficiency of the 264 process depends mainly on the ability of the SS/PbO2 anodes to produce • OH reagents (Skoumal et al. 2008).

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The electrochemical oxidation of Amaranth Red (E123) can be divided into two main stages (Baddouh et al.       The regeneration ability of the used L.C through three consecutive biosorption/desorption cycles was performed 356 for a biosorbent mass of 3 g, an initial concentration of Amaranth of 10 mg.L -1 , V=0.5 L, and T=25°C.Tests

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were carried out at different pH media; 2, 7, and 10. Based on Fig.8, the best desorption was obtained at pH=2 358 for the biosorption and biosorption coupling AO. As seen, at pH 7 and 10 all the desorption rates for the three 359 cycles were higher than 50% indicating that the retention forces of Amaranth by the L.C for both processes and

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Despite this gradual decrease, these findings confirmed the good regeneration ability of the used L.C even after 372 three continuous cycles.  divided into two main steps: the first one as described in Fig.8 is specific to the formation of hydroxyl radicals 384 which is evolved at SS/PbO2 anodes upon water discharge reaction, after that the hydroxyl radicals will be 385 adsorbed on the surface of the used anodes. The second step consists of the oxidation of the Amaranth dye by 386 these last. As for the coupling process, and based on results obtained during this study, the mechanism can be 387 divided into two stages. The first one is specific to the fixation of the Amaranth onto the available sites onto the 388 surface of L.C until the saturation phase. As the dominant functional groups onto the L.C are positively charged, 389 the main pathway of the fixation of Amaranth is the electrostatic interaction (especially that the Amaranth is 390 negatively charged at pH =2). The second one is specific to the AO process were the same mechanism that is 391 described before has taken place for the degradation of the remaining molecules of Amaranth dye.