Formation of porous wollastonite-based ceramics after sintering with yeast as the pore-forming agent

In this paper, synthesis of porous wollastonite-based ceramics was reported. Ceramic precursor, methylhydrocyclosiloxane, together with micro-sized CaCO3, was used as starting material. After 20 min of ultrasound treatment, and calcination at 250 oC for 30 min, yeast as a pore-forming agent was added to the as-obtained powders. Sintering regime was set up based on the results obtained by differential thermal analysis. Prepared mixture was pressed into pallets and sintered at 900 oC for 1 h. After the sintering regime, porous wollastonite-based ceramics was obtained. The phase composition of the sintered samples as well as microstructures was analyzed by X-ray diffraction method and SEM. In a batch test, the influence of pH, contact time and initial ion concentration on adsorption efficiency of As+5, Cr+6, and phosphate ions on synthesized wollastonite-based ceramics were studied. Time-dependent adsorption was best described by pseudo-second-order kinetic model and Weber-Morris model that predicted intra-particle diffusion as a rate-controlling step of overall process. High adsorption capacities 39.97, 21.87, and 15.29 mgg–1 were obtained for As+5, Cr+6, and phosphate ions, respectively.

Wollastonite is widely used in ceramic fabrication, as a high frequency insulator, filler material in resins and plastics, in civil construction, metallurgy, as paint and frictional (Dr.Nina Obradović) products regarding to its properties, such as low dielectric constant, low dielectric loss, thermal stability, low thermal expansion and low thermal conductivity [2][3][4].Furthermore, wollastonite owns possibility of applying as the medical material for artificial bones and dental roots due to biocompatibility, bioactivity and degradability [5,6].
On the other hand, porous materials find nowadays many applications as final products and in several technological processes regarding their special properties and features that usually cannot be achieved by their conventional dense shape.
Macro-porous materials have a wide application in many human life segments including porous ceramics for water purification.Recently, an increasing need for porous ceramics, especially for environments where high temperatures, extensive wear and corrosive media are involved, have appeared.The many advantages of using porous ceramics are observed such as high melting point, tailored electronic properties, high corrosion, and wear resistance [7].One of the ways to obtain porous ceramic is addition of either inorganic or organic pore-forming agents.Inorganic pore-forming agents are ammonium carbonate, ammonium bicarbonate and ammonium chloride salts or other high temperature decomposable inorganic carbon such as graphite, coal ash, etc.; organic pore-forming agent include natural fibers, polymers, such as sawdust, shell and corn flour, starch, polystyrene (PS), polymethyl methacrylate (PMMA), and yeast [8][9][10].
Wollastonite preparation is very important in scientific terms because of all mentioned potential applications.It has been synthesized using a few techniques such as precipitation, sol-gel, solid-state reaction, etc.All of them require high temperatures of sintering 950-1400 o C [11][12][13][14].Another, very useful method for obtaining wollastonite ceramics is synthesis from preceramic precursors.Siloxanes, as a source of silicon, represent often investigated type of preceramic polymers.Polymers are subjected to transformation into ceramics during heat treatment [15].This method offers some advantages such as possibility of combining shaping and synthesis of ceramics, in a very easy way and with lower applied temperatures, up to 900 o C in wollastonite preparation.
In this study, porous wollastonite-based ceramics synthesized by addition of yeast, as an organic pore-forming agent already used for the synthesis of ceramic materials, were investigated [16][17][18].The aim of this paper was to demonstrate influence of yeast on phase composition and microstructure.The objective of the present work was to use wollastonitebased ceramics for the removal of As +5 , Cr +6 , and phosphate ions from aqueous solutions.The studies have been carried out at various concentrations, retention times and pH.

Adsorbents preparation 2.2.1. Yeast isolation
Yeast was isolated from soil sample (experimental field Radmilovac, Serbia) using serial dilution method.YPD (yeast extract-peptone-dextrose) medium which consists of 1 % yeast extract, 2 % peptone, and 2 % dextrose, was used for the isolation at the temperature of 28 ºC/48 h [19].100 ml of YPD medium was inoculated by the yeast isolate.Incubation was performed in orbital shaker (Biosan-20KS, Latvia) for two days at the temperature of 28 ºC and at 150 rpm.The final yeast concentration in medium was 1⋅10 8 CFU ml -1 , which corresponds to the absorbance of 0.5 at 780 nm (T70 UV / VIS Spectromer, PG Instruments Ltd., UK).

Synthesis of wollastonite-based adsorbent
Wollastonite-based adsorbents were synthesized in two-step process.In the first step 7.7905 g siloxane (methylhydrocyclosiloxane) was dissolved in 100 ml isopropyl alcohol at magnetic stirrer for 10 min.12.9220 g of micro-sized CaCO 3 was added and mixed for another 10 min, followed by ultrasound treatment for 20 min and drying overnight at 80 o C. The obtained paste was calcined in furnace at 250 o C during 30 min, with 5 o Cmin -1 heating rate.The second step was mixing the as-prepared powder with yeast (20 wt.%), an organic pore-forming agent.

Adsorption and kinetic experiments
Batch adsorption experiments of As +5 , Cr +6 , and phosphate ions, under magnetic mixing, was applied to evaluate effect of diffusion processes on the performance of wollastonite-based ceramics.The adsorbent material, m/V = 200 mgl -1 suspension, was placed in vials containing 10 ml of standard solutions of As +5 , Cr +6 , and phosphate ions at different initial concentrations, C i , from 0.1, 0.5, 1.0, 2.5, and 5.0 mgl -1 , being in the range of the maximum allowable concentrations of these ions in drinking water.Maximum contaminant level of total chromium, arsenate and phosphate ions in drinking water are 0.1, 10.0, and 0.5 mgl -1 , respectively for public water systems [20][21][22].Effect of pH was studied in the range from 3 to 10. Adsorption and kinetic experiments were performed at 25 °C.The adsorption kinetic were studied by varying the As +5 , Cr +6 , and phosphate ions solution contact time in the range 5-120 min at C i = 1 mgl -1 .Mean value from three determination was used for processing of experimental data.The percentage of adsorbed As +5 , Cr +6 , and phosphate ions was calculated by using Eq. ( 1): where q is adsorption capacity in mgg -1 , C i and C f are initial and final As +5 , Cr +6 , and phosphate ions in mgl -1 , respectively, V is volume of solution in l, and m is mass of adsorbent in g.

Characterization methods
The thermal behavior and characteristic temperatures were determined by simultaneous TG-DTA (Setsys, SETARAM Instrumentation, Caluire, France) in the temperature range between 25 and 1100 o C, under the air flow of 20 mlmin -1 , in an Al 2 O 3 pan.Based on those results, sintering regime was set up (see section 2.4.).The X-ray powder diffraction pattern was obtained using a Philips PW-1050 diffractometer with λCu-K α radiation and a step/time scan mode of 0.05 o s -1 .The measurement was taken at room temperature from 10-90 o in air atmosphere.The morphology of the sintered powder was characterized by scanning electron microscopy (JEOL JSM-6390 LV).The pallets were crushed and covered with gold in order to perform these measurements.
Fourier transforms infrared spectroscopy (FTIR) spectra of the wollastonite based adsorbents were recorded in absorbance mode using a Nicolet™ iS™ 10 FT-IR Spectrometer (ThermoFisherSCIENTIFIC) spectrometer with Smart iTR™ Attenuated Total Reflectance (ATR) Sampling accessories, within a range of 400-4000 cm -1 , at a resolution of 4 cm -1 and in 20 scan mode.
Determination of textural properties of the wollastonite-based adsorbents was performed by collecting the isotherm of low-temperature nitrogen adsorption-desorption; Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were applied to process data.Micromeritics ASAP 2020 V4.O3 surface area and porosity analyzer was used.

Sintering process
The binder-free powders were compacted using the uniaxial double-action pressing process in an 8 mm diameter tool (Hydraulic press RING, P-14, VEB THURINGER).The applied pressure was 392 MPa, and an amount of 0.5 g of powders was used for each pressed sample.The compacts were placed in an alumna boat and heated in a tube furnace (Lenton Thermal Design Typ 1600).The compacts were sintered isothermally at 900 o C in air atmosphere for 60 min (RT-300 o C with 5 o Cmin -1 heating rate, 300-600 o C with 1 o Cmin -1 heating rate, 600-900 o C with 3 o Cmin -1 heating rate, 900 o C 1 h).

Characterization of wollastonite-based adsorbent
DTA-TG curves of prepared mixture containing yeast are shown in Fig. 1.Mixture possesses two exo-thermal and one endo-thermal effects, followed by mass loss.First exothermal peak at around 300 o C belongs to decomposition of organic compound [23].Mass loss that follows this decomposition process is 5 %.Sharp exo-thermal effect in the range 500-600 o C corresponds with decomposition of siloxane [24,25].Endo-thermal effect at around 800 o C corresponds to CaCO 3 decomposition [26].Mass losses that follow CaCO 3 decomposition is 30 %.Based on these results, sintering regime was set up.No peaks were detected in DTA-TG curves up to 200 o C, so first part of sintering regime was non-isothermal heating from RT-300 o C, with 5 o Cmin -1 heating rate.All organic decompositions occur in the range from 300-600 o C, where second part of sintering regime was isothermal heating from 300-600 o C, with 1 o Cmin -1 heating rate.Third part of sintering regime was isothermal heating from 600-900 o C with 3 o Cmin -1 heating rate, due to slowly CaCO 3 decomposition.In order to obtain wollastonite, all samples were sintered isothermally on 900 o C for 1 h, and then cooled down to RT. XRD pattern of sintered samples are presented in Fig. 2. All obtained intensities were identified by JCPDS cards (042-0547 for wollastonite CaSiO 3 , and 077-0409 for larnite Ca 2 SiO 4 ).Two-phase system was detected: wollastonite and larnite.Mixture contains 51.1 % CaSiO 3 , and 48.9 % Ca 2 SiO 4 .Sharp and intensive peaks indicate at recrystallization process.
Scanning electron micrographs of sintered samples are presented in Fig. 3. Uniform porosity is present in sintered sample.Pores are spherical, few microns in size, and interconnected with a network of channels, approx. 1 μm in size.Besides formation of network channels, yeast makes surface erosion of wollastonite structure.Those small pores are connected with contact necks, 500-600 nm thickness.Wollastonite structure is very complex, made of agglomerates that contain pores and particles.Tough agglomerates are 400 nm sized, larger ones are present as well, but hollow and few microns sized.The agglomerates interior is consisted of nano-sized particles and great porosity.Width of agglomerates is around 200 nm.
ATR-FTIR analysis of CaCO 3 -Sil-yeast sorbent confirmed wollastonite-contained ceramics structure and shows the characteristic bands at 1022 cm −1 and 1072 cm −1 originate from stretching bridging Si-O(Si), and bands at 898 cm −1 , 873 cm −1 , and 845 cm −1 originate from stretching non-bridging Si-O.Low intensity band at 714 cm −1 originates from stretching bridging Si-O(Si) which are characteristic for the presence of 3-membered ring in wollastonite-contained ceramics.The most intense band at ≈ 1460 cm −1 is assigned to the carbonate ion vibrational modes in bulk calcite [27].
The textural properties of the wollastonite-based adsorbent were determined using low-temperature nitrogen adsorption/desorption; the obtained textural parameters are preented in Tab.I, while the isotherm for CaCO 3 -Sioxane-yeast sample is shown in Fig. 4. Accordingly to IUPAC classification [28], the part of isotherm between p/p 0 = 0.3 and 1 can be described as type V with a H3 type hysteresis, what can be comprehended as the indication of hierarchical structure containing both meso-and macropores, similar result has been already published in the case of wollastonite [29].Besides, low-pressure hysteresis (p/p 0 < 0.3) indicates the existence of some microporosity.However, due to the low specific surface area, it can be supposed that porosity appears on the surface of material.)

Adsorption study 3.2.1. Effect of pH on adsorption efficiency
The pH influences state of equilibrium of ionic sorbate species and protonation/deprotonation of the functional groups present at adsorbent surface.It is known that presence of hydrogen/hydroxide ion could modify the redox potential of both sorbates and sorbents, and provoke dissolution of the sorbent.The results of the determination of influence of pH on adsorption efficiency of As +5 , Cr +6 , and phosphate ions indicated that adsorption of As +5 , Cr +6 , and phosphate ions on CaCO 3 -Sil-yeast sorbent was independent in the pH range 4-8 with > 90 % removal.Decrease of As +5 removal was observed at pH>8.According to pH-dependent ionization of triprotic weak arsenic acid (H 3 AsO 4 ), i.e. pKa(1) of 2.3, pKa(2) of 7.0, and pKa(3) of 11.5, it was proved that in the range of highest adsorption As +5 ionic species shows the most effective removal at pH in the vicinity of pKa.Positively charged adsorbent surface attracts the negatively charged sorbate ions by electrostatic interactions contributing to more efficient arsenic removal.Electrostatic repulsion between negatively charged arsenate species, H 2 AsO 4 − and HAsO 4 -2 ions, and adsorbent surface at higher pH contribute to decrease of adsorption efficiency.Similar behavior stand for Cr +6 , and phosphate in the studied pH range.Selection of optimal pH value 6 for As +5 and phosphate removal, and pH 5 for Cr +6 was made, and these pH was used throughout of adsorption experiments.

Adsorption study
The state of adsorption equilibrium can be described by fitting experimental data with various adsorption isotherms [30], and statistical criteria used to evaluate the quality of model fitting of adsorption data.Since the linear regression lines, obtained using Langmuir 1 adsorption isotherms, have highly significant correlation coefficients (R 2 > 0.992 for all As +5 , Cr +6 , and phosphate ions adsorption) it indicate a good fit to the Langmuir 1 equation.The values of Langmuir 1 adsorption isotherm parameters at 25 °C for As +5 , Cr +6 , and phosphate ions adsorption using CaCO 3 -Sil-yeast sorbent, are presented in Tab.II.The Langmuir 1 isotherm model is given by Eq. ( 2) and linearized form, by Eq. ( 3): where: C e is the equilibrium concentration of As +5 , Cr +6 , and phosphate ions remaining in the solution (moll -1 ); q e is the amount of ions sorbed per weight unit of solid at equilibrium (molg -1 ); b is Langmuir constant related to sorption affinity, and q max is maximum sorption capacity (molg -1 ).

Effect of time on adsorption capacity
The removal of As +5 , Cr +6 , and phosphate ions by adsorption on CaCO 3 -Sil-yeast increased with time up to 90 min; thereafter it became constant.The kinetic parameters of As +5 , Cr +6 , and phosphate ions adsorption, presented in Tab.III, showed that CaCO 3 -Sil-yeast sorbent possess high affinity with respect to As +5 , Cr +6 , and phosphate ions, and satisfactory rate at which system attain equilibrium.The analysis of kinetic data, using pseudo-second-order kinetic method, resulted in significantly higher second-order rate constant and similar adsorption capacity values for As +5 adsorption compare with Cr +6 , and phosphate ions adsorption using CaCO 3 -Sil-yeast sorbent.Kavitha and co-workers found that results of kinetic parameters of the adsorption of Cr +6 on wollastonite based adsorbent (quartz/ feldspar/wollastonite -QFW) is best described by the pseudo-second order equation suggesting that the rate-limiting step may be the adsorption mechanism but not mass transport [31].Results of intra-particle Weber-Morris diffusion model showed two step adsorption process with fast kinetic in first step for As +5 , Cr +6 , and phosphate ions adsorption.
First kinetic step demonstrates external mass transfer from bulk solution, related not only to instantaneous adsorbate bonding at the most readily available adsorbing sites at outer surface, but also could be due to the contribution of adsorption at surface of the pores with largest diameter close to the particle surface [32].While in the course of final stage, i.e. second step, slow transport of As +5 , Cr +6 , and phosphate ions inside micro pores dominate and attainment of adsorption/desorption equilibrium denote overall saturation of available adsorptive sites.
Various solid materials including activated carbons, silica gels, activated clays, activated alumina and other oxides, zeolites, bauxite, different biosorbents have been used for As +5 , Cr +6 , and phosphate ions removal from waters.The application of wollastonite based adsorbents was reported in literature as well.As +5 , Cr +6 , and phosphate ions adsorption capacities (value of q e derived from Langmuir equation) of various wollastonite based adsorbents are summarized in Tab.IV.Importantly, the adsorption capacities of wollastonite-based materials used in this work are similar or in the same range of values as those already reported for other materials: the insight into literature data reveals that capacities for Cr 6+ [35], As 5+ [36][37][38], or phosphate ions [39,40] are in the range of units or tens of mgg −1 , what is comparable with the results reported in this work.

Conclusion
Porous wollastonite-based ceramics were synthesized using yeast as a porogen agent in order to obtain appropriate porosity of obtained adsorbents.DTA gave us characteristic temperatures, we chose appropriate sintering regime up to 900 o C. XRD measurements confirmed wollastonite phase, while SEM confirmed macro and micro porosity.Synthesized CaCO 3 -Siloxane-yeast sorbent was used for As +5 , Cr +6 , and phopshate removal, and results showed that pH is important parameter which control effectiveness of pollutant removal.The quality of the isotherm modeling of adsorption data was judged by the correlation coefficients and error functions, and the best adsorption model was found to be Langmuir isotherm.Time dependent study showed that the best fitting kinetic model is parabolic or Weber-Morris model giving the highest values of correlation coefficients than the other investigated models.The kinetic data of the sorption was well fitted with the pseudo-secondorder kinetic and Weber-Morris models, suggesting that the rate-limiting step was diffusion rather than chemical sorption.
Comparison of adsorbents.