DESIGN OF A FIXED BED ADSORPTION COLUMN AND MODELLING OF OPERATING PARAMETERS FOR THE REMOVAL OF METHYLENE BLUE IN DYNAMIC MODE

This study investigates the potential of using the mixture of titaniferous sand and attapulgite as adsorbents in a fixed bed adsorption process to remove a synthetic dye such as methylene blue in aqueous media. The different adsorbents were characterised by X-ray fluorescence spectroscopy and infrared spectroscopy. The different physico-chemical


INTRODUCTION
Dyes are organic compounds that are widely used in many industries, including textile, paper, food, cosmetics and paint, resulting in large amounts of water-soluble dye molecules in wastewater (Al Arni et al., 2021; Laskar & Kumar, 2018).In recent decades, as a result of industrial increasing and technological advances, coloured effluents have caused damage to the aquatic ecosystem and posed a significant threat to human health due to their toxicity (Karaoǧlu et al., 2010;Wazir et al., 2020).To date, several techniques have been identified for the removal of dyes, including solvent extraction, photocatalytic degradation, coagulation, membrane filtration, chemical and biological oxidation and adsorption (Léonce Kouadio et al., 2022;Wang et al., 2022).Compared to other treatment systems, adsorption is an inexpensive, accessible and feasible method for the decontamination of wastewater with dyes (Al-Mahbashi et al., 2022; Blanco-Flores et al., 2016).Activated carbon is the most widely used adsorbent due to its high adsorption capacity for organic materials (Bouchemal & Achour, 2007;SEHILI, 2008).However, this adsorbent has a high cost and is difficult to regenerate (Dutta & Basu, 2014;Shah et al., 2021).The search for another effective and less expensive adsorbent is therefore interesting.Fixed bed column adsorption also has many advantages due to its ease of use and high removal efficiency.In addition, it can be easily scaled up to laboratory or industrial scale (Sylvia et al., 2018;Vasques et al., 2009).Continuous mode adsorption is generally characterized by the so-called breakthrough curves, representation of the effluent concentration at the column outlet as a function of the time profile in a fixed bed column (Patel, 2019 (Maleki et al., 2021).On the other hand, the studies of Patel, H showed that fixed bed column adsorption techniques are considered as the best methods due to their practical advantages and a set of columns is used to have a proper industrial perspective and get better results (Patel, 2019).GHINWA MELODIE NAJA et al. showed that columns can be operated in series to better control adsorption or in parallel to increase the capacity of the system.The advantages of this system include the simple form, the ability to perform continuous flow operations, the lack of limitation in increasing its scale, the lack of need for solid-liquid separation and the ability to perform regeneration and washing (Naja et al., 2010).
In this work, firstly, a fixed bed adsorption column was designed and then a mixture of titaniferous sand and attapulgite was used as an effective adsorbent to remove methylene blue in aqueous media.In addition, this study examined the effects of operating conditions such as bed height, initial dye concentration and flow rate.The experimental data obtained from the dye adsorption was modelled using the Thomas and Adam-Bohart models.

Adsorbat
The pollutant was methylene blue, also called tetramethylthionine hydrochloride.It is a cationic dye which belonging to the group of quinone imides belonging to the Thiazine section.It is a sulphur dye in which two benzene rings are joined by a closed ring of one nitrogen atom, one sulphur atom and four carbon atoms.

Preparation of adsorbents
Titaniferous sand and attapulgite were used as adsorbents.The titaniferous sand is a residue of a mining industry located in Senegal.The treatment was carried out by bringing it into contact with a sulphuric acid solution of concentration 4mol/L and a volume of 100 mL.The mixture was stirred for 4 hours to ensure maximum contact with the sulphuric acid.At the end of the operation, the sand is recovered and washed several times with distilled water until a pH close to neutrality is obtained.It is then dried in an oven for 24 hours and placed in flasks for later use.The other material, namely attapulgite, has not undergone any prior treatment and is used in a raw state in the work.It was collected in Mbour, one of the departments of Senegal.

Sizing of the adsorption column
The sizing of the adsorption column was based on results from the removal of methylene blue in batch mode (Kalidou BA et al., 2021).After optimizing the operating parameters that influence the pollutant removal efficiency, the adsorption isotherms were studied in order to calculate the material transfer zone (MTZ) using the most suitable isotherm, the initial concentration and the maximum tolerated concentration (Cb).The various calculations were carried out in Excel.
Considering that the adsorption takes place on a fixed bed, the sizing will therefore consist in finding the length of the material transfer zone as well as that of the column.For this, the following assumptions will be used: The

•
The density of the fluid is assumed to be that of water and is f =1000g/dm3 ;

•
The density of the adsorbent is assimilated to that of titanium sand and is a =2209g/dm3 ; The algorithm used to size the fixed bed column is given below : The required adsorbent mass Ma is calculated using Eq.(1).
Where : Y is a multiplicative factor to move from experimental to real values ; M a exp : the optimal mass found experimentally ; Q : the actual volume flow rate of the column ; Q exp : the experimental volume flow rate.
The fraction L'/V' is determined from Eq. ( 2) : with : L' : mass flow rate of the adsorbent ; V' : Volume flow of the effluent.
q 1 is calculated by replacing C e with C i in the equation for the equilibrium curve and this leads to Eq. (3).
The saturation concentration C S is calculated using Eq.(4).
The number of transfer units NUT is obtained from the Eq. ( 5).C e : the concentration at equilibrium.The transfer unit height HUT is calculated through Eq. ( 6).

NUT = ∫ 1 C − C e dC
HUT = Q Ky.S (6) with : K y : the material transfer coefficient ; S : the section of the column.
The height of the material transfer zone Z a is obtained using Eq. ( 7).
The unsaturated fraction f of the adsorbent was also calculated using Eq. ( 8).
Where  is a change of variable such that : The calculation of ZF is done using Eq.(10).
with :  ′ Flow rate of the inert fluid ; F : the saturated fraction of the column ; Z : the height of the column ;   : the density of the adsorbent.The height Z of the adsorption column is obtained from Eq. ( 11).Z = ZF + f.Za (11) The cycle time is deduced from Eq. ( 12)

Description of the adsorption pilot
The pilot consisted of a glass column (1m height and 3cm in diameter), inside which there was a support to allow attachment of the adsorbent bed.The effluent with a known initial concentration was sent to the top of the column by a peristaltic pump calibrated beforehand.The dye was passed through the adsorbent bed and recovered at the  C 0 : Initial concentration of the pollutant (mg.L -1 ) ; C t : Effluent concentration at time (t) at the column outlet (mg.L -1 ) ; m : Mass of the bed (g) ; K Th : Thomas constant (L.mg -1 .min -1 ) ; q 0 : Maximum adsorption capacity of the solute on the adsorbent (mg.g -1 ); F : Solution feed rate to the column (L.min -1 ) ; t : Breakthrough time relative to the concentration C t at the column outlet (min).

The Adams Bohart model
It is one of the first models to describe the breakthrough curves of an adsorbent bed.The breakthrough time is related to the operating parameters of the adsorber by Eq. ( 14) (Alardhi et al., 2020).

RESULTS AND DISCUSSION Physico-chemical characterization
Before the two materials were used as adsorbents, they were characterized in order to determine their physicochemical properties.The different values are given in Table 1.

Table 1 : Physico-chemical properties of adsorbents
The values obtained show on the one hand that titaniferous sand treated with sulphuric acid is very dense, somewhat acidic and has a relatively low specific surface area, whereas attapulgite is basic, less dense than titaniferous sand and has a higher specific surface area.The combination of these two materials will go a long way to promoting good interaction with the cationic dyes.X-ray fluorescence characterisation X-ray fluorescence spectroscopy was used to characterise the composition of the titaniferous sand and attapulgite (figure.2aand 2b).The analysis of titaniferous sand treated with sulphuric acid shows that titanium, iron and aluminium are the predominant elements which are present at concentrations of 300000 ppm, 63423.27ppm and 18676.65 ppm respectively.This confirms the presence of oxides such as titanium dioxide, iron oxide and alumina, which are essential elements in the adsorption of refractory organic and inorganic compounds.However, for attapulgite, the major elements are silicon, iron, calcium with respective concentrations of 408028.69ppm, 17829.26ppm and 11952.21ppm.The literature has revealed a great importance given to these elements in the context of surface adsorption of organic compounds and heavy metals (Bhattacharyya & Gupta, 2007).

Sizing of the adsorption column
The values obtained from the adopted sizing algorithm are presented in Table 2.These were used for the design of the fixed bed adsorption column.The calculation of the column sizing was done using the Excel spreadsheet.

Effect of initial dye concentration
The breakthrough curves of methylene blue adsorption by the adsorbent mixture studied under the conditions of bed height equal to 3cm, flow rate equal to 18mL/min, show that the increase in concentration from 50mg/L to 100mg/L affects the bed saturation and breakthrough time.This observation can be explained by the fact that the concentration gradient is a driving force for the proper functioning of the mass transfer from the liquid phase to the adsorbent surface.Thus, an increase in the concentration of methylene blue causes a decrease in the breakthrough time of the dye.This can be verified from figure 4a, at increasing dye concentration, the adsorption sites are occupied very quickly and the adsorbent bed is saturated in a shorter period of time.Identical results were reported by Sarici-Ozdemir (Sarici-Ozdemir & Onay, 2018) and Aydın et al. (Aydın et al., 2021).

Effect of adsorbent bed height
The breakthrough curves for the adsorbent mixture with two different bed heights (2 and 3 cm), with an initial concentration of 100 mg.L-1 and a flow rate of 18 mL.min-1are presented in figure 4b.For the same concentration and fixed feed rate, increasing the bed height from 2cm to 3cm leads to an increase in the amount of BM adsorbed and the time required to reach saturation of the column increases.Thus, the variation in height is proportional to the breakthrough time, the higher the height, the better the breakthrough time.By increasing the bed height, the height of the material transfer zone increases, the exchange rate between solute and solid decreases.Similar results were found by M.D. Yahya (Yahya et al., 2020) and Dereje Tadesse Mekonnen (Mekonnen et al., 2021).

Effect of dye feed rate on breakthrough curve
Breakthrough curves are obtained by maintaining the initial concentration at 100 mg.L-1, the bed height at 3cm and varying the feed rate from 18 to 20 mL.min-1.
The results obtained are shown in figure 4c.These variations show that when the feed rate is increased, when the residence time of the solute molecules in the column is decreased, the height of the matter transfer zone decreases, and thus a loss of efficiency of the adsorbent to immobilise the dye molecules.This leads to an increase in the exchange rate between the dye molecules and the adsorbent, allowing the saturation rate of the bed to increase rapidly, resulting in a decrease in breakthrough time and retention of the

Thomas' model
The parameter values obtained after modelling using the Thomas model are collected in Table 3.The width of the adsorption front requires a long time to reach equilibrium in the packing layers.The values of the adsorbed amount under different conditions show that increasing the feed rate decreases the adsorption capacity of the BM and increases the Thomas adsorption constant (KTH).However, varying the concentration of BM from 50 to 100 mg.L-1 results in a decrease in the adsorption capacity from 334.7 to 291.36 mg.g-1 and a small increase in adsorption constant from 3.6 to 3.7 mL/min/mg (KTH ).The values of the correlation coefficients R2 are higher than 0.90 except for the concentration of the dye equal to 50mg/L which has a determination coefficient R2 equal to 0.88 (figure 5a et 5b).Therefore, the Thomas model describes well all the BM breakthrough curves studied.These results show that the Thomas model is representative of the system studied.

Bohart and Adams model
The parameters obtained from the Bohart and Adams model for the dye studied on the composite material are shown in Table 4. From the results obtained it can be seen that as the bed height increases, the dynamic adsorption capacity for the BM on the adsorbent mixture decreases.However, increasing the concentration of BM from 50 to 100 mg.L-1 also leads to a decrease in the dynamic adsorption capacity from 236.75 to 206.10 mg.g-1 and a small increase in adsorption constant from 3.6 to 3.7 mL/min/mg (KAB ).

The
American Journal of Applied sciences (ISSN -2689-0992) VOLUME 05 ISSUE 05 Pages: 05-21 SJIF IMPACT FACTOR (2020: 5. 276) (2021: 5. 634) (2022: 6. 176) (2023: 7. 361) OCLC -1121105553 Publisher: The USA Journals bottom of the column using a tank.The change in methylene blue concentration was monitored as a function of time.The samples were collected in 2h intervals and were put inside glass tubes to avoid any kind of light modification of the solution or any kind of bacterial contamination.The residual concentration of methylene blue was determined by a spectrophotometer at a wavelength of 664nm.The different parameters as a function of time were the feed flowrate to the column, the initial concentration of methylene blue and the height of the adsorbent bed.Finally, different mathematical models such as Thomas, Adams and Board were used to study the behaviour of methylene blue on adsorbent beds.Figure1shows the process used to remove methylene blue in dynamic mode.

Figure 1 .
Figure 1.Methylene blue removal process in dynamic modeModelling of breakthrough curvesIt consists of using numerical models to access the concentration profile of the dye at the outlet of the adsorption column.Various simple mathematical models such as the Bohart-Adams and Thomas models have been developed to predict the dynamic behaviour of the column and to estimate some kinetic coefficients through the experimental data obtained

Figure 2 . 21 SJIF
Figure 2. X-ray spectrum of titaniferous sand (a) and attapulgite (b) Characterizations with IR spectroscopy IR characterisation was carried out in order to determine the different surface functions of attapulgite and titaniferous sand (figure.3aand 3b).We note the existence of a band located at 3550.19 cm-

Figure 4 .
Figure 4. Effects of adsorbate concentration (a), adsorbent bed height(b) and feed rate (c) on the breakthrough curve