Optimization of the spent activated carbon regeneration with the radical based 1 advanced oxidation processes: Preference of the most suitable process by PROMETHEE 2 approach

11 In this study, regeneration of spent granular activated carbon (GAC) with reactive dye by hydroxyl and sulfate 12 radical based advanced oxidation processes (Microwave (MW) +Persulfate (PS)), (Fe(II)+ PS), and (O 3 + H 2 O 2 ) 13 were evaluated. The adsorption of the dye to the GAC surface was characterized by chemisorption and Langmuir 14 isotherm. Regeneration processes have been optimized by the Response Surface Methodology to determine the 15 operating conditions that will provide the highest adsorptive capacity. The optimum conditions of (MW + PS), 16 (Fe (II) + PS), and (O 3 + H 2 O 2 ) processes were (process PS anion of 45.52 g/L, pH of 11.4, MW power of 126 17 W, duration of 14.56 min), ( Fe (II) of 3.58 g/L, PS anion of 73.5 g/L, duration of 59.8 min, pH of 10.9) and 18 (H 2 O 2 of 2.8 mole/L, ozone dose of 98%, duration of 32.8 min, pH of 5.3), respectively. For (MW + PS), (Fe (II) 19 + PS), and (O 3 + H 2 O 2 ) processes, the adsorptive capacity under optimum conditions were found as 4.36, 8.89 20 and 8.12 mg dye / g GAC respectively. For (Fe (II) + PS) and (O 3 + H 2 O 2 ) processes these values are 21 approximately equal to the adsorptive capacity of raw GAC (8.01 mg dye / g GAC). The predicted values of the 22 adsorption capacities by the obtained models were in good agreement with the actual experimental results. 23 PROMETHEE approach was used in the preference of the appropriate regeneration process. The adsorptive 24 capacity of regenerated GAC, operating cost of the regeneration process, change in the adsorptive capacity 25 during the regeneration cycle and carbon mass loss criteria were taken into account. The order of preference of 26 regeneration processes was determined as (Fe (II) + PS)> (MW + PS)> (O 3 + H 2 O 2 ) considering all criteria.


Introduction
Reactive dyes are highly preferred due to their high photolytic stability, high resistance to microbial shrinkage and superior fastness to the applied fabric (Charnkeitkong and Phoophuangpairoj 2020). The 45 wastewater generated in industries using reactive dyes is intensely colored due to the low fixation degree 46 of these dyes to fabrics, their low adsorption capability and high water solubilities. and low adsorption capabilities (Vakili et al., 2020). Approximately 70% of reactive dyes have one or more chromophoric     Table  1. fed into the GAC-water mixture with a porous diffuser at a flow rate of 6 L / min. The maximum (100%)

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Ozone content of the ozone generator is 3 g/h and the amount of ozone pumped into the reactor has been 189 studied at certain levels as a percentage. The maximum (100%) ozone content of the ozone generator is

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GAC adsorption capacity was calculated according to the following eqn (8).
where q is adsorption capacity (mg/g), C 0 and C t (mg / L) are the SR-S3B concentrations at the initial and t,

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respectively. V is the volume of the solution (L) and M is the mass of the adsorbent (g).

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The carbon mass loss of adsorbents was determined by the difference between the mass of raw GAC and the 203 mass of regenerated GAC after evaporating free water at 105 o C.

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Response Surface Modelling (RSM) is a statistical and mathematical technique useful for improving,

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developing and optimizing processes in which the response in a process is affected by various variables.

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The goal is to optimize the response. RSM describes the effect of independent variables on processes,

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In order to investigate the mechanism of adsorption were used kinetic models. To study the mechanism 304 of adsorption of SR-S3B onto GAC and to fit the kinetic data pseudo-first and pseudo-second order 305 kinetic models were used. Kinetics and isotherms parameters were calculated using models listed in 306 with an increase in the initial SR-S3B concentration.
Between the Langmuir and Freundlich isotherms, the Langmuir isotherm fits the experimental data

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In the (MW + PS) regeneration process (Fig. 2 (a)), an average adsorptive capacity of 4.6 mg / g GAC is

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predicted at MW 126 W and duration of 10 to15 min. Similarly, an average adsorptive capacity of 4.7 mg dye / g GAC is predicted at MW 700 W and duration of 5 to 9 min. In the process, the lowest value of the adsorptive capacity (1.9 mg dye / g GAC) was predicted at the highest conditions of MW power 364 and duration. In the (Fe (II) + PS) regeneration process ( Fig. 2 (b)), an average adsorptive capacity of 365 6.5 mg / g GAC is predicted at Fe (II) of 4 to 6 g/L and duration of 40 to 60 min. The lowest adsorptive 366 capacity value (3.4 mg dye/g GAC) was predicted at the highest Fe (II) concentration and the lowest 367 duration.