Microstructural Characterization and Adsorption Properties of Alkali-Activated Materials Based on Metakaolin

The microstructural characterization and adsorption properties of metakaolin (MK) and alkali-activated metakaolin, known as geopolymer materials (GP) were investigated. The structure and properties of the metakaolin and obtained geopolymer were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Furthermore, based on the analysis of adsorption efficiency, microstructure and mineral structure, the difference between geopolymer and metakaolin on the performance of immobilizing heavy metals have been discussed. The kinetics of adsorption can be represented by pseudo-second order equation. The results of lead ions adsorption experiments were best fitted by Freundlich adsorption isotherm for both investigated adsorbents. The highest removal efficiencies of alkali-activated material based on metakaolin was found 97.5% at pH 4 and metakaolin removal efficiencies was found 92% at pH 5.5.


Introduction
A large number of different methods are used for removal of heavy metals from wastewater, for example coagulation [1], precipitation, ion exchange [2], extraction, membrane filtration [3,4].As the tendency for methods in wastewater treatment is to be more efficient, inexpensive and environmentally responsible, adsorption on natural materials [5][6][7] or modified waste materials becomes relevant and the subject of many researches [8].The influence of the structure of natural domestic materials (kaolin and diatomite) on the adsorption properties of lead were examined in the studies Nenadovic et al. [9,10].There is a worldwide tendency to use cheap materials as a potential sorbents [11,12].Among them, metakaolin and metakaolin-based geopolymers increasingly find applications.Metakaolin is a relatively complex material derived from a naturally occurring mineral and is usually produced by calcination of kaolin clays within a definite temperature range depending on the purity and crystallinity of the precursor clay [13][14][15][16].Serbia has high-quality kaolin clay deposits at Aranđelovac and Kolubara basin, and thus good potential to produce metakaolin from them [10].Geopolymers (alkali-activated metakaolin) are considered to be a group of modern aluminosilicate materials of composition and properties allowing their application in numerous new technologies [17].In recent years, geopolymer materials have attracted much more attention due to their low density, low cost, low curing/hardening temperatures, environmentally friendly nature and excellent thermal stability at high temperatures [18][19][20] The aim of this study is investigation of microstructure and adsorption properties of metakaolin and alkali-activated materials based on metakaolin -geopolymer as the considerably more economical materials for removal of the ionic lead pollution from industrial wastewater.A series of adsorption tests to investigate the influences of different parameters on the effectiveness of metakaolin and alkali-activated metakaolin -geopolymer on lead adsorption as a very toxic element were performed.It is known that lead ions in the heavy metal pollution are one of the important environmental problems in the aqueous system originated from industry.Additionally, the microstructural characterization and some chemical properties of the metakaolin and obtained geopolymer based on metakaolin were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy.

Experimental procedure
Metakaolin (MK) was prepared by calcining kaolinite at 750 °C for 3 h.The used kaolinite is high quality clay obtained from Rudovci, Lazarevac, Serbia.Physicochemical properties of kaolin were done in previous research Nenadovic et al [10].Metakaolin and alkaline solution were mixed 10 min (solid/liquid = 0.85).The alkaline solution was mixed by sodium silicate (technical grade) and 16M NaOH (Sigma Aldrich analytical grade).The samples were cast into 25 mm diameter molds.The alkali-activated material-geopolymer (GP) was formed after sitting at room temperature for one day and at 50 °C for 2 days in a sample drying oven.Upon removal from the molds, the samples were crushed and sieved through sieve with hole of diameters of 355 μm.The contents of SiO 2 and Al 2 O 3 were determined by conventional chemical analysis.The contents of SiO 2 were 55.32% and 47.98 % and, Al 2 O 3 are 26.13 % and 17.40 % in metakaolin and geopolymer respectively.Samples of MK and GP were characterized by X-ray diffractometry (XRD) by using Ultima IV Rigaku diffractometer, equipped by Cu Kα 1,2 radiation, with generator voltage 40.0 kV and generator current 40.0 mA.The range of 10°-80° 2θ was used for all powders in a continuous scan mode with a scanning step size of 0.02° at scan rate of 10 °/min.The morphology and microstructure of the materials were studied by using SEM -JEOL JCM-5800 LV.The functional groups of MK and the GP were studied using FTIR spectroscopy a Perkin Elmer FT-IR spectrometer, Spectrum Two.The specific surface area, S BET , pore size distribution (PSD) for the samples were calculated from the adsorption and desorption isotherms of N 2 and, were measured on samples, at -196°C, using the gravimetric McBain method.The particle size distribution was estimated by applying BJH method [21] to the desorption branch of isotherms while mesopore surface and micropore volume were estimated using the high resolution α s plot method [22][23][24].Micropore surface, S mic , was calculated by subtracting S meso from S BET .
In this study a series of adsorption tests at room temperature to investigate the influences of different parameters on the effectiveness of MK and GP on lead adsorption were performed.The adsorbent was primarily heated in an oven at 100 o C for 16 hours and cooled in a desiccator before using it in the adsorption process.Solution of Pb(II) ion was prepared by dissolving lead nitrate in deionized water.The initial pH of the solutions was adjusted with HCl or NaOH to a desired value.After adsorption, the solution was filtered and the filtrate was analyzed for lead concentration by atomic absorption spectrophotometry (AAS Pye Unicam) at 217.0 nm.Different experimental parameters, such as pH, dosage of adsorbents, initial concentrations of the sample solution and contact time were tested to study their effects on the removal of Pb(II) by the MK and GP.The removal efficiency, R, on metakaolin and geopolymer was calculated with equation ( 1 where c 0 and c e are the initial and equilibrium concentrations of lead ions (mg/L).The adsorption capacity (amount adsorbed per unit mass of adsorbent) q e (mg/g) was calculated with equation ( 2): where V represents volume of the solution and m is the mass of the adsorbent.

Results and Discussions
XRD patterns of metakaolin and alkali-activated material based on metakaolingeopolymer are presented in Fig. 1.

Fig. 1. XRD patterns of a) MK (metakaolin) and b) GP (geopolymer).
X-ray patterns prove that the part of SiO 2 originated from crystallic quartz (SiO 2, ).A broad reflection centered at 2θ = 26.64°attribuTab.to the amorphous metakaolinite along with residual quartz (Fig 1).After dehydration process the kaolin loses its crystal structure and become mainly amorphous material.Similar behavior was commonly noted for geopolymers, with the proportion of the amorphous phase slightly higher compared to metakaolin.FTIR spectroscopy was used to characterize the metakaolin and alkali activated samples as illustrated in Fig. 2.
The analysis of FTIR spectrum of MK (Fig 2a ) shows the strong bands of Si-Si stretching vibrations are present at 485 cm -1 .Bands at 793.9 cm -1 and 562.Geopolymer particles are different in shapes and size and show tendency to agglomerate as well as MK particles do.Some particles of GP tend to have polyhedral shape unlike MK circular like shape particles (this shape has minimum surface relative to the volume of the particles).
Nitrogen adsorption-desorption isotherms of MK and GP are presented in Fig. 4. The main feature of this type of isotherm is the hysteresis loop, which is associated with capillary condensation taking place in mesopores (pore size between 2 and 50 nm) [25].In both samples, the shape of hysteresis loop is of type H2 which indicates a poorly defined shape of pores [24].Pore size distribution (PSD) for GP and MK samples is shown in Fig. 5. GP sample has micro-and mesoporosity while MK has only mesopores.Mesopores are typical pores between geopolymer phases, while micropores exist within the gel network [24].The formation of capillary pores is characteristic of cement systems, while their formation in GP is less distinct, because the gel takes up most of the space [26].Specific surface area calculated by BET equation and for both samples is very similar, about 40 m 2 /g for GP, and for MK 36 m 2 /g (Tab.I).α s plots, obtained on the basis of the standard nitrogen adsorption isotherm are shown in Fig. 6.
The straight line in the high α s region gives a mesoporous surface area including the contribution of external surface, S meso , determined by its slope, and micropore volume, V mic , is given by the interception.Calculated porosity parameters (S meso , S mic , V mic ) are given in Tab.I.

Adsorption properties of metakaolin and alkali-activated metakaolingeopolymer
Different operating parameters, such as pH, contact time, mass of adsorbent and initial concentration of solution were tested to study their effects on the removal of Pb(II) by the MK and GP and to determine adsorption properties of these materials.

Effects of pH and dosage of adsorbents
According to Fig. 7, optimum pH for adsorption is between 4-5 on GP and 5-6 on MK, which is consistent with the previous research works [27].
All other adsorption experiments were performed at pH of maximum removal efficiency, 4.5 for GP and 5.5 for MK.At higher pH, with decrease in concentration of H + ions, the competition between Pb 2+ and H + ions for available positions on the negatively charged surface is smaller.Lead ions have precedence and efficiency is high.For pH values above 6.0, lead hydroxide begins to form resulting in the decrease of Pb(II) adsorption.At higher pH values the surface of the adsorbent is negatively charged as a result of deprotonation, and electrostatic repulsion decreases with increasing pH.At the Fig 8 influence of the adsorbent mass on the removal efficiency of MK and GP samples is showed.
At a low dosage of MK and GP, a large amount of adsorbate is available to be captured by the few available sites.At higher dosage of MK and GP i.e. of their solid particles, the number of binding sites available for the same volume of liquid increases, and thus, the total amount of lead ions that are removed increases.This increase is more pronounced for MK, although for the mass larger than 0.2 g, high R (90%) is achieved.In case of GP, high efficiency is achieved with a much smaller mass of adsorbent.Surface area of GP is higher, possesses more available adsorption sites, and, as it was mentioned above, GP has micro and meso pores.

Effects of contact time and initial lead concentration
Change of the efficiency to time is shown in Fig. 9.
It can be seen that the process occurs in two steps.In the first 10-30 minutes, adsorption is taking place rather quickly, achieving a steady state after approximately 45 minutes for MK and 20 minutes for GP.This initial step of fast adsorption corresponds to ions exchange in micropores and mesopores on the surfaces of GP and mesopores on the surface MK.After the initial step adsorption stay slow due to slower diffusion of ions into the layers of MK and into the channels of GP network.With the increase of concentration, the removal efficiency of MK is decreasing.Higher concentration of metal in the solution leads to the saturation of the adsorbent and the reduction of the free active contact surface causing a lower percentage of adsorption.The results for GP have different tendency.With the increase of concentration, no significant change in removal efficiency is observed.Due to changes in the structure of GP and the existence of network of Si-O groups that are available for binding, the number of adsorption positions increased.In the tested range of initial metal concentrations, the saturation of the adsorbent was not achieved and, therefore, the high R values were obtained in the experiments.Raising the initial metal ion concentration led to an increase in Pb(II) uptake by the used adsorbents [28].

Adsorption kinetics of investigated lead ions
The kinetics of lead (II) ion adsorption on both adsorbents was analyzed by linearized pseudo-first and pseudo-second kinetic models [27].After integration and the application of boundary conditions t = 0 to t = t and q t = 0 to q t = q t , the linear form of the pseudo-first order equation becomes: where q e and q t are the adsorption capacities at equilibrium and at time t, respectively (mg/g) while K 1 is the rate constant for the pseudo-first order adsorption (L/min).
The integrated form of the pseudo-second order mechanism after applying the boundary conditions, t=0 to t=t and q t = 0 to q t = q t is given by: where K 2 is the rate constant of pseudo-second order adsorption (g/mgmin).The plot of t/q t against t is represented in Fig. 11.The plots gave a good fit to the experimental data for both investigated adsorbents.The calculated parameters, constants and corresponding linear regression correlation coefficients for tested kinetics models are presented in Tab.II.High correlation coefficient value for MK shows that pseudo-first kinetic model can be applied to predict the mechanism of adsorption onto this adsorbent.The results show that adsorption of Pb(II) ions on MK and GP are best described by the pseudo-second order kinetics.The higher R² values suggest that the adsorption of a metal by adsorbent may involve a chemical adsorption and its rate is limited by chemisorption.In some previous researches aimed at describing the adsorption of metals on various adsorbents [27,29] pseudo-second order model has proven to be the most successful.

Adsorption isotherms
Among several isotherm models, experimental equilibrium data were fitted to the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich model [27].Equations describing each isotherm are presented in Tab.III, where, q e is the adsorption capacities at equilibrium (mg/g); c e is the equilibrium concentration in solution (mg/L); q m is the maximum adsorption capacity (mg/g); K L is the Langmuir constant (L/mg); F and n are empirical constants; ε is the potential of Polanyi, and is equal to RT ln(1+1/c e ); R is universal gas constant (J/Kmol); T is absolute temperature (K); K' is a constant related to the energy of adsorption (mol 2 /KJ 2 ); b T is Temkin isotherm constant; K T is Temkin isotherm equilibrium binding constant (L/g).
Adsorption isotherm Equation Linear form Langmuir model  Linear form of these equations was used for graphical plots of the experimental data.From the slope and the interception of the plots the models parameters were determined.Results are presented in Tab. 4. Adsorption of Pb(II) ions on MK is best fitted to Freundlich isotherm model.This indicates adsorption in monolayer coverage but with heterogeneity of surface.Adsorption process on GP best described with three models since R 2 is greater then 0.96 for Langmuir, Freundlich and Temkin model.Best fiting is also with Freundlich isotherm model as it can be seen from Tab. IV.The results of the analysis indicate that the major factor for the improved adsorption of GP is the difference in the surface properties of the examined adsorbents.

Conclusion
The structure of metakaolin obtained using natural domestic clay raw appears disordered accordance to X-ray analysis.Similar behavior was commonly noted for geopolymers, with the proportion of the amorphous phase slightly higher compared to metakaolin.Phase evolution for metakaolin and geopolymer monitored by FTIR and, there are substantial changes in intensity, width and position of bands in the spectra of metakaolin and geopolymer.After activation with alkaline activating solution, the geopolymeric gel (spherical aggregates) formed, suggesting that the structure experiences growth.Geopolymer particles are different in shapes and size and show tendency to agglomerate as well as MK particles do.On the basis BET analysis was concluded that GP sample has micro-and mesoporosity while MK has only mesopores.Experimental data has shown that the both of these materials can successfully be used for the adsorption of lead ions from the solution.High values of the removal efficiency (>95%) was achieved due to the changes in the active surface and the internal structure of alkali-activated metakaolin as the number of potential positions for the binding of metal ions has increased.Adsorption processes can be described by Freundlich isotherm and kinetics model of pseudo second order.In both cases, a very good fitting of experimental and model data is obtained.Due to the high efficiency of adsorption, the easy availability of raw materials, low cost and energy consumption during synthesis, metakaolin and alkali-activated metakaolin -geopolymer could be widely used for the removal of lead ions from wastewater.These results are very good base for our further investigation of adsorption properties of used materials as adsorbent of the other heavy metals and many different pollutants of water.
2 are a translational vibration band for Si-O-Al.The band at 804 cm −1 can be related to the change from octahedral coordination of Al 3+ in kaolinite to tetrahedral coordination in metakaolinite.The shift of the peak of the GP was 1024.6 cm -1 compared to 1064 cm -1 for that of the MK indicated the chemical change and geopolymerization of NaOH-activated MK at elevated temperature.The bands observed at 1024.6 and 745 cm -1 represent the stretching bands of Si-O-Si groups and the deformation bands of Si-O respectively.

Fig. 3 .
SEM micrographs of a) MK powder and b) GP sample after grinding.Some of MK particles (Fig 3a) are platy in shape, but they have tendency to create higher agglomerate of circle shape.The process of calcination makes the particles less bulky and less hexagonally shaped, unlike the crystallized kaolinite crystals.Fig 3b shows the microstructure of a GP (after grinding) which are used for further process of adsorption.
Fig 5  show that MK has only mesopores while geopolymer has micropores and mesopores.This is probably one of the reasons that the efficiency of lead ions onto surface of MK is smaller than GP in both of steps of adsorption.The effects of initial lead ions concentration are shown inFig 10.

Fig. 9 .
Fig. 9. Influence of contact time on removal efficiency of MK and GP (0.1 g MK/GP, 50 mg/dm 3 Pb, pH 4.5 for GP and 5.5 for MK).

Fig. 10 .
Fig. 10.Influence of initial concentration on removal efficiency of MK and GP (0.1 g MK/GP, 60 minute, pH 4.5 for GP and 5.5 for MK).
Porous properties of GP and MK samples.
Kinetics model parameters.