Adsorption of Congo Red on Nano MgO Particles Prepared by Molten Salt Method

Nano-substances show many surface properties due to their high activity and high surface area. This study concentrates on the possibility of using nano-MgO (NMO) for removing Congo red (CR) dye from wastewater. The effects of equilibrium time, pH, dye concentration and temperature have been investigated. Isotherm studies revealed the favorability of the adsorption process and the energy of adsorption (10.38<E<11.60 kJ/mol) suggest a mechanism controlled by chemical processes. The overall process was spontaneous and endothermic in nature with a maximum adsorption capacity of 1100 mg/g at 40C as estimated from Langmuir isotherm. The adsorption kinetics was found to follow pseudo second-order rate equations.


Introduction
Nano magnesium oxide has a wide range of applications in different fields of science and technology; this is because the physical properties of nano-materials are completely different from the bulk material that makes the nano-material more applicable [1].Surface properties like porosity, morphology and size of the particles define the specific surface area and adsorption capacity of the nano-material.On the other hand, the surface properties are controlled by the preparation method and preparation conditions of the nanomaterial [2].Due to the large surface area of nanoparticles, they have been used as adsorbent widely.In the gaseous phase, nano-metal oxides were used efficiently as adsorbents and destruction of hazardous chemicals [1].Nanomaterials have also shown an excellent efficiency in removing heavy metal ions from aqueous solutions [3].One of the polluting industrial wastes is dye effluents from textile, rubber, plastics [4].The concentrations of dye stuff in waste effluents are in the range   carcinogenic [5] and most of these synthetic dyes are non-biodegradable [6].Benzidine, a human carcinogen, is a product of Congo red (CR) metabolism.Due to the significant impact of CR as an industrial dye waste on aquatic system and human health, it has to be treated before discharge [7].Among all physical, chemical and biological treating strategies, adsorption still remains the most simple, low cost and efficient treating method for dye effluents [8].Recently, researchers found that nano-metal oxides are selective sorbents against heavy metal ions with high sorption capacity [9].Furthermore, nano crystalline MgO [5,10] and MgO nano-composites [11] have been used in removing dyes from waste water effectively.In the present study, a simple method (molten salt) was used to synthesis nano-MgO, and the nano-product was examined for its efficiency as an adsorbent to remove Congo red from aqueous solutions.Optimization of some key factors, like contact time, initial pH, initial concentration of dye and temperature were performed as well.Kinetics and isotherm studies are necessary for proposing the possible mechanism of the adsorption process which is important in transferring the current study to a large scale.

Materials and Methods
Materials used in this research were all of analytical reagent grade and used without further purification.Congo red (CR) was obtained from Fluka-Guaranties.The stock solution of CR 2000 mg/L (C32H22N6Na2O6S2) was prepared with distilled water, and the desired experimental concentrations of CR solutions were prepared by consecutive dilutions of the stock solution.In a previous work of the authors, molten salt method was used for the synthesis of NMO particles and the product was well characterized using XRD and SEM [12].

Adsorption Studies
Batch method was used in adsorption measurements to obtain kinetics and equilibrium data.The effect of temperature on adsorption was done by operating the adsorption process at various temperatures; 298, 303, 313, and 318.

Equilibrium Time and Initial Dye Concentration Study
The equilibrium time is a function of initial dye concentration [13].

Effect of Initial pH
The initial pH of the dye solution can significantly affect the adsorption capacity of the dye because it affects the charge distribution of the surface of the adsorbent (NMO) as well as the adsorbate (the dye molecules).Fig. 2 shows that in acidic medium the adsorption capacity is maximum till pH=6, after which the adsorption capacity decreased slightly with the increase of initial pH of CR solution.This result is agreed with literature for the adsorption of CR on various materials like red mud [14], zeolite and kaolin [15] and anilinepropylsilicaxerogel [16].

Adsorption Isotherm
The benefit behind the sorption isotherms is to explore the relation between the adsorbate concentration in the bulk (at equilibrium) and the amount adsorbed at the surface.Five commonly used isotherm models (Langmuir, the Freundlich, the Temkin, the Dubinin-Kaganer-Radushkevich and BET) were applied to the experimental data to explain the dye-NMO interaction.The linear forms of the isotherms are presented below: For monolayer adsorption, Langmuir isotherm is the most important model: Where Ce is the equilibrium concentration of CR (mg/L), qm the monolayer maximum adsorption capacity of the adsorbent (mg/g) and KL is the Langmuir adsorption constant (L/mg) [17].The problem of Langmuir isotherm is assuming a homogeneous surface with identical adsorptions sites and their binding energies and neglecting any interactions between adsorbed ions, atoms or molecules [18].From the intercept and slope of the plots in Fig. 3, the Langmuir constants qm and KL were calculated at different temperatures and are presented in Table 1.A dimensionless constant separation factor of Langmuir isotherm (RL) is used to determine the favorability of adsorption for Langmuir type process which is defined as: For multilayer adsorption and adsorption on heterogeneous surfaces, Freundlich isotherm was formulated [19].The linear form of the isotherm is given below: Where Kf (mg g -1 (mg L -1 ) -1/n ) and n are representing adsorption capacity and nonlinearity coefficients respectively.A more common isotherm than Langmuir is D-R isotherm was proposed by Dubinin which does not assume a homogenous surface of sorbent.It is applied to determine the adsorption mechanism (physical or chemical).The linear form of D-R isotherm can be expressed as follows [21].
ln   = ln   −    2 (7) qD is the maximum adsorption capacity (mg/g), BD is the adsorption energy constant (mol 2 /J 2 ), is the Polani potential defined as: Prognostication of the adsorption mechanism (physical or chemical sorption) can be done by calculating the value of the mean sorption energy, E (J/mol) from the relation below: If the values of E were less than 8 kJ/mol, the mechanism maybe a physical adsorption, while E values between 8-16 kJ/mol assumes the adsorption to be controlled by ion exchange and E greater than 16kJ/mol presume a particle diffusion mechanism (chemical process).values of qD (mol/g), BD (mol 2 /J 2 )and E (J/mol) were calculated as shown in Fig. 7according to Eq. (7 and 9).The above isotherm parameters are listed in Table 1.Freundlich constants (n) at all temperatures were greater than one, pointing to the favorability of the adsorption.Langmuir constants (qm and KL) were found to increase slightly with temperature and the results were close to that reported for adsorption of toxic dyes on nano-MgO.Flower like nano-MgO [22] showed 1500 mg/g adsorption capacity for Cd(II) 3 .The adsorption energies (E) calculated from Dubinin isotherm were found to be (10.38<E<11.60kJ/mol) which presumes a mechanism controlled by chemical process [23,24].

Thermodynamic Study
Table 1 show that the maximum adsorption capacity calculated from Langmuir isotherm increases with increasing temperature, this can be used to determine the thermodynamic parameters.The thermodynamic equilibrium constants (Kc) at different temperatures for the adsorption of CR on g1 NMO adsorbent were calculated from the intercept of the plots of ln(qe/Ce) versus qe by extrapolating to qe = 0 (Khan plot) [25] as shown in Fig. 8.The free energy of the adsorption process at the studied temperature range were negative and changed with the rise in temperature, this imply that the adsorption was spontaneous and thermodynamically favorable.The positive ΔHº value indicates the endothermic of the adsorption process in nature.The positive value of ΔSº show increased randomness at the solid/solution interface and an affinity of the nano-MgO toward Congo red dye.Mi-Hwa found analogous results for the adsorption of Malachite green onto degreased coffee bean [11].

Adsorption Kinetics
In order to assess the rate of the adsorption for CR, experiments were carried out for different initial concentrations (250, 500, 1000 and 2000 ppm).Both pseudo first order and pseudo second order kinetics were applied to the adsorption data.Pseudo first order (Lagergren) rate equation [26] given as: Where qe and qt (mg.g -1 ) are the amounts of dye adsorbed at equilibrium and at time t respectively, k1is the pseudo-first order rate constant (min -1 ).The pseudo second order equation (Ho and McKay) [27] expressed by: Where k2 (g/mg.min) is the pseudo second order rate constant.
The plots of the equations were examined for best fit by comparing their correlation coefficients.The effect of initial concentration of the CR dye was further explored by plotting both (k2 and qm) versus initial concentration of CR.Fig. 12 shows that qm increases linearly with increasing initial concentration of the dye, while k2 showed no regular pattern.Results show that the equilibrium is reached at higher rates at elevated temperatures, while the maximum adsorption capacity remained almost unchanged which is consistent with the chemisorption mechanism of the process.Pseudo second order kinetic plots at different temperatures are presented in Fig. 14 from which the pseudo second order rate constant (k2) and maximum adsorption capacity (qcalc.)were calculated and the results are presented in Table 3.The calculated activation parameters are given in Table 4 below.The magnitude of Ea may give an indication of whether a physical or chemical adsorption process is involved.In physical adsorption (physisorption), the interaction is easily reversible, equilibrium is rapidly attained and its energy requirements are small and Ea is usually in the range of 5-40 kJ/mol, this because of weak intermolecular forces are involved.However, with chemical adsorption (chemisorption) much stronger bonding forces are involved and Ea range is from 40-800 kJ/mol.In the present study, the high value of Ea (175.77kJ/mol) is consistent with the chemisorption mechanism for the adsorption process 11 .

Conclusions
The results of the present work show that nano-MgO can be considered as an effective adsorbent for the treatment of CR from waste water.In batch experiments, the influence of initial dye concentration and temperature were shown to be effective, while initial pH effect was found to be insignificant on adsorption capacity.In the acidic medium, the efficiency of adsorption was maximum (≈50%) and decreased slightly in neutral and basic medium to (≈47.3%).Results of the thermodynamic studies indicated a spontaneous and endothermic adsorption.A pseudo second-order rate model might have followed by the adsorption process as supported by correlation coefficients of the linear plots, and also the qcalc.Were very close to the qexp for the pseudo second-order rate kinetics.The overall adsorption experiments above suggest that NMO can be considered as an effective adsorbent to remove CR from industrial waste water.As the adsorption process is the first step in catalytic photodegradation, the prepared NMO might be useful as a photo-catalyst in degradation of CR after adsorption.
photosynthesis, most of these dyes (especially synthetic dyes) are toxic, mutagenic and Adsorption experiments were performed at temperature 30 °C and initial CR solution pH 4.3, exclude those of which the effects of pH and temperature were investigated.Adsorption experiments were carried out in a series of polyethylene bottle containing 0.1 g of MgO adsorbent and 50 ml of CR solution of desired concentration in a water bath shaker for 150 minutes shaking.At the end of the adsorption period, the suspensions were separated by centrifugation at a rate of 3500 rpm for 12 minutes.The amount of CR adsorbed on NMO was Vol: 13 No:4 , October 2017 DOI : http://dx.doi.org/10.24237/djps.1401.345CP-ISSN: 2222-8373 E-ISSN: 2518-9255 determined by using spectrophotometer (TU-1800S UV-VIS spectrophotometer) at λ= 497 nm.The adsorbed amount of CR on NMO was computed from Eq. (1) L) is the volume of solution, m (g) is the mass of adsorbent, Co and Ce (mg/L) are initial and equilibrium concentrations of the CR dye in the solution.Equilibrium time and kinetic studies was performed in batch adsorption experiments using a serial of 50ml of dye solutions contacted with 0.1g of NMO.At premeasured contact times (5, 10, 15, 30, 60, 90,120,180, 240,300,360, 420 and 1440 minute), a sample bottle was withdrawn from the water bath shaker for analysis.pH effect was examined in the range of pH 2.5-12 using 50ml of 2000 mg/L dye solutions (a sample bottle for each pH).The pH of the solutions were controlled with approximately 0.1MHCl and 0.1M NaOH solution as per required.Effect of initial dye concentrations on kinetic measurements were examined using 250, 500, 100 and 2000 ppm CR concentration.Dye solutions at a concentration range 50 to 2000 mg/L were mixed with 0.1 g NMO and shacked for 150 minutes at constant temperature in isotherm studies.

Figure 1 Figure 1 :
figure demonstrates that equilibration was reached after 120 minutes; therefore, a period of 150 minutes of equilibration was selected for the next studies to ensure complete equilibration.

Figure 2 :
Figure 2: Effect of initial pH of dye solution (500 ppm initial concentration) on the adsorption of CR on NMO

Figure 3 :Figure 4 :
Figure 3: Langmuir adsorption isotherm of CR sorption on NMO at different temperatures

Fig. 5 showsFigure 5 :Figure 6 :
Figure 5: Freundlich adsorption isotherm of CR adsorption on NMO at different temperatures From the slope and intercept of the plots of  2 versus lnqe of the linear expression of D-R isotherms, the

Figure 7 :
Figure 7: D-R adsorption isotherm for the adsorption of CR on NMO at different temperatures.

Figure 8 : 10 )
Figure 8: Khan plots for the adsorption of CR on nano-MgO at different temperatures

Figure 9 :
Figure 9: Van't Hoff plot of adsorption of CR on nano-MgO Fig. (10 and 11) are typical examples of pseudo first order and pseudo second order plots of the adsorption of CR on NMO at 30 o C. The correlation coefficients of the linear curves of both kinetics shows that the process more likely follows a second order kinetics.

Figure 10 :Figure 11 :Vol
Figure 10: Pseudo first order plots at different initial CR concentrations

Figure 12 : 6 .
Figure 12: Effect of initial concentration of CR (ppm) on k2 and qm at 30 o C

Figure 13 :
Figure 13: Effect of temperature on the adsorption kinetics of CR (500mg/L) on NMO

Figure 14 :Table 3 :
Figure 14: Effect of temperature on the pseudo second order kinetics of the adsorption of CR on nanoMgO

Figure 15 :Figure 16 :
Figure 15: Arrhenius plot for the adsorption of CR on NMO

Table 1 :
The calculated adsorption parameters of the four used isotherms D-

Table 2 :
Thermodynamic parameters for adsorption of CR on NMO

Table 4 :
Activation parameters of the adsorption of CR on NMO with an initial concentration of 500 ppm.