Characterization of the Complex Metal-clay Obtained in the Process of Lead Adsorption

This study aims to characterize the complex metal-clay formed by adsorption of lead. In this work, the bentonite clay named Fluidgel was calcined at 750 °C and was used as adsorbent for lead removal. The characterization of this clay and complex metal-clay was carried out by X-ray diffraction (XRD), Fourier transformer infrared spectroscopy (FTIR), surface area (BET method), scanning electron microscopy (SEM) and chemical composition by energy dispersive X-ray (EDX). The removal of lead from aqueous solution was carried out in a fixed bed. Dynamic experiments were performed to evaluate the effect of flowrate on adsorption efficiency. The experiments were accomplished at room temperature, the clay adsorbent particle diameter was of 0.855 mm and the flow rate varied from 0.6 to 2 mL/min. The feed concentration of lead was about 0.24 mmol/L. Comparing the chemical compositions of the clays obtained by EDX before and after removal of the lead, it was concluded that the ion exchange process might be important to lead removal. From the semi-quantitative analysis of chemical composition in clays with and without adsorbed lead it was observed a reduction of the amount of Ca2+, K+ cations and the disappearance of Na+ cations, which was caused by cation exchange process.


Introduction
The environmental impacts mainly originated by the constant discharge of all types of industrial pollutants by using water at some stage of the process reduce the self purifying ability of water.The pollutants present in water interferes with the metabolic cycles, due to the toxic effects they have on microorganisms.Particularly metals pollutants cause an increase in concentration of metals along the food chain (biological amplification), being one of the most problematic pollutants.
Metals have become a current and important topic both in environmental concerns and public health.Although some metals are essential for numerous metabolic processes, all metals are potentially toxic, carcinogenic and mutagenic if used excessively.
Traditional processes have been employed, along with new technologies, with the aim of contributing to the removal of metals present in the effluent wastewater 1 .Among various existing processes there are: ion exchange 2 , the chemical precipitation 3 and ultrafiltration 4 .These methods are not always effective, and usuallyhave high cost and generate waste, thus, requiring a further stage of treatment.
Adsorption is one of the most effective and economical techniques used to remove metals present in wastewater 5 , due to its simplicity when compared with other methods, such as ion exchange and precipitation.This method offers to the project flexibility with the possibility of conducting the metal removing operation in columns.This allows the treatment of large volumes of effluents continuously and under varied low concentration of metals present in feed, in addition to the high efficiency of contaminant removal that can be achieved.
The clays have properties such as high ion exchange capacity 19 , they are easily available and offer a low cost alternative for metal removal 20 , and are reusable 21 .In this context, this study aimed to evaluate the potential of an alternative adsorbent such as sodium bentonite clay, commercially called as Fluidgel, for the removal of Pb 2+ present in aqueous effluents in dynamic system.The study focuses the adsorption of lead under dynamic conditions in a fixed bed column containing chemically and thermally treated (TACal) fluidigel.Evaluation of the behavior of breakthrough curves, kinetic data fitting to the Adams-Bohart model, investigation of removal cycles as well as the elution of lead from Fluidgel are presented in the paper.

Adsorbent material
The Fluidigel clay was used as adsorbent after made by a heat treatment by calcination at 750 °C in muffle furnace and treated with acetic acid 4% in fixed bed.The heat treatment was realized in order to improve physical stability, and treatment with acetic acid was used towards to eliminate sodium carbonate in excess.This clay adsorbent was named Fluidgel TACal.The heat treatment provided a good stability and strength to the particles to be used in fixed bed column.The acid treatment was carried out to eliminate the excess of carbonate and hydroxyl present in the calcined Fluidgel.The calcined clay with an average particle diameter of 0.855 mm was packed inside the column and disposed in contact with deionized water for 2 hours.This washing process was performed before the treatment with acetic acid.

Fixed bed column operations
Sorption experiments under dynamic conditions were performed in a glass column (packed with Fluidigel TACal), jacketed with 1.5 cm internal diameter (ID) and 15 cm height.It was conducted several tests concerning monolithic lead removal having feed concentration of 0.24 mmol/L in this fixed bed column, being the curve of rupture obtained for flowrates of 0.6, 1, and 2 mL/min..In order to evaluate the kinetic behavior of adsorption process of lead in fixed bed columns, the experimental data obtained were fitted to the Adams-Bohart model (quasichemical) 22 .
The process efficiency was analyzed by calculating the mass transfer zone (MTZ), which, according to Geankoplis 23 , is the fraction of the bed in which occurs the transfer of ionic species from the feed to the solid phase.
Useful quantities (q u ) and total (q t ) adsorbed in the bed until the break and saturation points, respectively, were calculated using a mass balance in the column.It was considered the breaking point, the moment in which the concentration of the solution leaving the bed was equivalent to 5% of the concentration at the entrance.The useful (q u ) and total (q t ) quantities removed, in mmol/g, can be calculated by Equations 1 and 2: where C 0 is the lead initial concentration in the liquid phase in mmol/L, V is the volumetric flow of operation mL/min., t b is the time which rupture occurs in the bed and m is the clay adsorbent mass in grams.
Geankoplis 23 presented a method to calculate the MTZ.The parameter δ=q u /q t refers to the fraction of the usable length of the bed to the breaking point.The usable length (H u ) and MTZ are calculated by Equations 3 and 4: where H t is the column total length.
The Adams-Bohart model (quasichemical) 22 was used in the adjustment of kinetic behavior of data of lead adsorption process in fixed bed columns, according to Equation 5. being, , z the bed height, v is the velocity flow, ε L the bed porosity and t is the time.

Desorption tests
Three experimental were performed to obtain the lead rupture curves using a solution of CaCl 2 0.1 mol/L as eluent.
In these experiments the eluent was pumped throughout the Fluidigel TACal packed column at the same flowrate (i.e.1mL/min.).The CaCl 2 was chosen from preliminary batch elution tests, in which were tested NaCl, disodium EDTA, HCl, Thiourea and CaCl 2 .The CaCl 2 was the eluent that presented higher lead elution.
The amount of eluted lead, q el , was calculated by integrating the elution curve.The area under this curve multiplied by the feeding flowrate, V (in mL/min.),per mass of calcined clay, m (in grams), gives the amount of eluted metal according to the Equation 6 [24] .The elution percentage was determined by considering the total amount of lead previously adsorbed as 100% of eluted lead. .
where C el is the metal concentration in the liquid phase in mmol/L.

Characterization
The physical-chemical characterization of Fluidgel clay involved the following techniques: chemical analysis by energy dispersive X-ray spectroscopy (EDX) to identify and quantify the total chemical composition; The surface area was verified by physisorption of N 2 (BET method); The real density measured by helium picnometry; Morphological analysis by scanning electron microscopy (SEM), Identification of particular clay peaks and maintenance of its properties by X-ray diffraction (XRD), and Fourier transformer infrared spectroscopy (FTIR).The above mentioned analyses were performed for samples of commercial Fluidgel clay, calcined Fluidigel TACal before and after lead adsorption.

Mass transfer effect
The different behaviors of the breakthrough curves obtained at different flowrates (Figure 1) indicate the strong influence of the flow on diffusional resistances.The adsorption process for 0.6 mL/min.flowrate presented a strong resistance to saturation of the bed, confirmed by the more elongate breakthrough curve.
In Table 1 the useful (q u ) and total (q t ) quantities of metal retained in the bed were obtained considering the outlet bed concentrations equal to 5 and 95% of the entrance concentration, respectively.
It can be observed (Table 1) that for the flowrate of 1 mL/min.a relative low value of MTZ length (3.4 cm) as well as a low resistance to mass transfer, observed by the jump in the curve after the breakthrough point (Figure 1) for lead removal by the clay, in addition to the greater percentage removal (47.03%).For this reason the flowrate selected to be used in other experiments of lead removal with Fluidgel TACal was 1 ml/min.
The Adams-Bohart model parameters: constant velocity of adsorption linear (K), and the maximum quantity of adsorbate adsorbed by bed volume (q 0 ) for each flowrate are listed in Table 2. Results concerning the fitting of the Adams-Bohart model are presented in Figure 2.
The model did not fit experimental data for the lowest flowrate (0.6 mL/min.)In spite of this, the results in Table 2 shows that with the increase of the flowrate, the constant kinetics increases, which shows that with higher linear adsorption rate, a smaller amount of lead will be retained by this clay.

Beds regeneration
The breakthrough curves obtained with experiments envolving adsorption/desorption cycles, being the adsorvent regenerated using as eluent CaCl 2 0.1 mol/L are shown in Figure 3a.
According to the results, there are different behaviors of the breakthrough curves for the three cycles, in which Table 1.MTZ, q u , q t and %R L for lead removal on Fluidgel TACal.

System
Flow (mL/min.)MTZ (cm) q u (mmol/g) q t (mmol/g) %Rem it can be observed that the greater mass transfer resistance has been obtained in the last cycle with a low removal.The breakthrough time (t b ) remained around 800 min.for the first two cycles.Table 3 summarizes the total and the useful quantities removed, the MTZ and the percentage removal for Pb 2+ /Fluidigel TACal clay.
The lead removal obtained up to bed saturation in the first cycle, was 0.0378 mmol/g, decreasing in the second and third cycles (0.0248 and 0.0206 mmol/g).The curves of the Figure 3a showed different behaviors, indicating the influence of chemical modification in diffusional resistances.
The breakthrough curve for the second cycle of the bed showed a behavior that approaches it to a step function, suggesting a low diffusional resistance compared to the first cycle, obtaining a more favorable removal process.This result indicates that, due to the passage of the eluent, all the exchange sites were located in an area readily accessible to lead ions.Whereas, in the first cycle the breakthrough curves showed weaker inclination rupture, a higher diffusional resistances characteristic.This result indicates that all exchange sites were located on a more intrinsic surface.
In the third cycle, there is a decrease in useful quantities and total removed, and percentage removal, related to the strong stability between the complex of the metal and the clay after two cycles of adsorption.The desorption process through eluting occurs, predominantly, in the first two hours of contact, in which almost all lead is eluated (Figure 3b).Table 4 presents the variables responses of the regeneration of the bed by CaCl 2 for Fluidgel TACal.
The solvent recovery efficiency (SRS) can be evaluated by the ratio of the volume of water spent in the preparation of eluents (V E ) corresponding to the elution time and the volume of purified water (V b ) to the breakthrough point, considering that the eluent has concentrated all lead in the clay after saturation of the bed.Based on breakthrough time and elution time, the volumes V b and V E and SRS were calculated and are presented in Table 5.According to Table 5, most SRS was presented for the first elution, because the second was used more volume of eluent for desorption.After the third adsorption the bed became unstable, with great loss of load, preventing the third elution.
For each of the breakthrough curve the Adams-Bohart model curve (quasichemical) was fitted to the experimental data, as shown in Figure 4.The values of coefficients adjusted are shown in Table 6.It can be observed that the parameter k varies according each clays elution test, meaning that after each elution a new kinetics is established.

Adsorbent characterization
From the semiquantitative analysis of chemical composition showed in Table 7, it can be observed that with the lead adsorption, there was a reduction of the amount of

System
Cycle # MTZ (cm) q u (mmol/g) q t (mmol/g) %Rem Pb/Fluidgel TACal cations Ca 2+ and K + and almost disappearance of cations Na + .This emphasizes the occurrence of cation exchange in the process of lead removal by the Fluidgel TACal.
The XRD analyses for clay samples of Fluidgel TACal, before and after lead adsorption, are shown in Figure 5.In each diffractogram, for the considered band, there are two characteristic peaks for montmorillonite and the others are related to quartz.It can be observed that the Fluidgel clay samples analyzed are not characterized by a highly crystalline structure, as it is found, for example, in zeolites, since the peaks detected in these samples are well defined.The difficulty in identifying peaks is explained by nondiffraction of X-ray, which had a deviation throughout the process, what could characterize the formation of an inter-crystalline joint of clays.
Through the diffractograms, there is a significant deviation of the particular montmorillonite peak (Fluidgel TACal) (2θ = 8.69°) for an angle 2θ higher (9.4°) obtained for the other sample, which indicates shorter plane interlayer distances (d 001 ), besides a reduction in the peaks intensity in the region of Fluidgel TACal clay with adsorbed lead, in relation to the Fluidgel TACal clay without this metal.
From BET analysis, there was a negligible small increase in the surface area of the adsorbent after the removal process from 11 to 13 m 2 /g, which could be possibly due to lead adsorption.The real density reduction after lead removal, from 2.5 to 2.3 g/cm 3 is caused by material losses during the process.
The morphologic analysis of Fluidgel TACal clay particles by scanning electron microscopy provided the micrograph shown in Figure 6, whose dots indicate the mapping of the adsorbed lead on Fluidgel TACal.It can be noticed a homogeneous distribution of the adsorbed lead throughout the whole surface of the clay Fluidigel TACal particle.
The FTIR spectra for plain sample of Fluidgel TACal and sample with adsorbed lead are shown in Figure 7.According to these spectra, the band figures that correspond to the DH-stretching remain the same in both samples, in spite of a small deviation in band positions.The band     in 783-797 cm −1 originated by the existence of quartz in Fluidgel clay did not practically change position after lead adsorbed.According to the FTIR spectra for clay sample contain lead, no bending band for metal-oxygen bond was observed.Consequently, it may be concluded that lead removal by Fluidge TACal clay was mainly caused by ion exchange between the structures that contain clay tetrahedral and octahedral layers in the clay.

Conclusions
The study of the mass transfer parameters, as well as the breakthrough curves, indicate that the most suitable flow ratio was 1 mL/min., with a low value of MTZ length (3.4 cm), and a low resistance to mass transfer for lead removal by Fluidgel TACal.According to the results of the adsorption/desorption, different behaviors of the breakthrough curves for the three cycles, suggest that regeneration of the bed causes changes in the clay reducing its adsorption capacity.The breakthrough time (t b ) remained around 800 min.for the first two cycles.The coefficients parameters of adjustment of the Adams-Bohart model in regenerated beds indicate that the parameter k varies with each elution test and this can be explained because after elution a new kinetics is established.Based on the physical and chemical analysis of Fluidgel TACal clay, it can be considered poly-cationic material, but not characterized by highly crystalline structure.The adsorption spectra in the infrared region showed adsorption bands corresponding to the DH-stretching band, as well as the presence of quartz in clay.Metal-oxygen stretching vibrations in the octahedral and tetrahedral regions were not found.These conclusions combined with the results of the semiquantitative analysis of chemical composition give clear evidence of ion exchange in the lead removal process occurring in the interlayer clay structures.

Figure 4 .
Figure 4. Fitting of the Adams -Bohart model of beds regenerated by CaCl 2 .

Table 2 .
Fitting parameters of the Adams-Bohart model to study flow.

Table 4 .
Parameters regeneration of the columns with CaCl 2 0.1 mol/L.

Table 5 .
Balance recovered water removal -elution of lead.

Table 7 .
Chemical composition by EDX.

Table 6 .
Fitting parameters of Adams-Bohart model for breakthrough curves in beds regenerated by CaCl 2 .