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Novel porous fly-ash containing geopolymer monoliths for lead adsorption from wastewaters
Accepted Manuscript Title: Novel porous fly-ash containing geopolymer monoliths for lead adsorption from wastewaters Author: Rui M. Novais L.H. Buruberri M.P. Seabra J.A. Labrincha PII: DOI: Reference:
S0304-3894(16)30691-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.07.059 HAZMAT 17915
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
30-3-2016 15-7-2016 23-7-2016
Please cite this article as: Rui M.Novais, L.H.Buruberri, M.P.Seabra, J.A.Labrincha, Novel porous fly-ash containing geopolymer monoliths for lead adsorption from wastewaters, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.07.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Novel porous fly-ash containing geopolymer monoliths for lead adsorption from wastewaters
Rui M. Novais a,*, L. H. Buruberri a, M. P. Seabra a, J. A. Labrincha a a
Department of Materials and Ceramic Engineering / CICECO- Aveiro Institute of
Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
*Corresponding author: Tel.: +351234370262; fax: +351234370204 E-mail address: [email protected] (Rui M. Novais)
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Graphical abstract
2
Highlights
Porous fly ash containing-geopolymer monoliths for lead adsorption were developed. Geopolymers’ porosity and pH of the ion solution controls the adsorption capacity. Lead adsorption by the geopolymer monoliths up to 6.34 mg/g was observed. These novel adsorbents can be used in packed beds that are easily collected. The reuse of biomass fly ash wastes as raw material ensures waste valorization.
Abstract In this study novel porous biomass fly ash-containing geopolymer monoliths were produced using a simple and flexible procedure. Geopolymers exhibiting distinct total porosities (ranging from 41.0 to 78.4%) and low apparent density (between 1.21 and 0.44 g/cm3) were fabricated. Afterwards, the possibility of using these innovative materials as lead adsorbents under distinct conditions was evaluated. Results demonstrate that the geopolymers’ porosity and the pH of the ion solution strongly affect the lead adsorption capacity. Lead adsorption by the geopolymer monoliths ranged between 0.95 and 6.34 mglead/ggeopolymer. More porous geopolymers presented better lead removal efficiency, while higher pH in the solution reduced their removal ability, since metal precipitation is enhanced. These novel geopolymeric monoliths can be used in packed beds that are easily collected when exhausted, which is a major advantage in comparison with the use of powdered adsorbents. Furthermore, their production encompasses the reuse of biomass fly-ash, mitigating the environmental impact associated with this waste disposal, while decreasing the adsorbents production costs.
Keywords: geopolymers; biomass fly ash; monoliths; lead adsorption; porosity. 3
1. Introduction The alarming increase of water demand over recent decades is a major societal problem. By 2025, around 1800 million people will be living in regions with absolute water scarcity, while two-thirds of the world population could be under stress conditions [1]. As a result, specific legislation has been adopted by the EU through the Water Framework Directive [2]. In this context, wastewater treatment and subsequent reuse can provide an important increase in water supply to cope with water scarcity. The presence of heavy metals in industrial wastewaters is a serious and long-lasting environmental problem [3]. Lead is one of the most common heavy metals in industrial wastewaters, and it is known to be toxic for humans and plants [4]. Therefore, its removal from wastewaters is of the foremost importance. Several methods have been carried out for heavy metals removal, such as chemical precipitation, coagulation, ion exchange and adsorption [5]. Amongst these, adsorption has attracted increasing attention due to its simplicity, effectiveness and low cost [6]. Common adsorbents such as activated carbon, alumina and silica present high efficiency levels; however, their high production cost hinders wider application. Therefore, the development of alternative and low cost adsorbents is eagerly pursued. The synthesis of geopolymers involves mixing aluminosilicate materials with strong alkaline [7] or acidic activators at relatively low temperatures. The geopolymers’ chemical structure is composed of a negatively charged aluminosilicate framework where charge-balancing cations can be exchanged with cations in the solution [8], which suggests the possibility of using them as adsorbents. Nevertheless, the use of geopolymers as adsorbents is fairly recent (Figure 1), and the existing literature is scarce [9-11]. Metakaolin-based geopolymers [6] and coal fly-ash geopolymers [9] exhibited a
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lead uptake capacity around 100 mg/g and 80 mg/g, respectively. The latter demonstrates the feasibility of using geopolymers as heavy metals adsorbents. The vast majority of studies focused the use of geopolymer powders, which cannot be directly used in packed beds and are not easily retrieved [12, 13]. On the contrary, geopolymer monoliths can be directly used in packed beds as membranes, yet surprisingly this strategy has been neglected. Waste materials have been used as precursors in the production of geopolymers (e.g. fly ash and blast furnace slag). Coal fly-ash (FA) has been extensively considered as an aluminosilicate source, while the incorporation of biomass FA is less common. Nevertheless, the growing interest in the production of energy through the use of biomass [14] has raised the volume of biomass FA generated. Current management strategies, including addition to forest soils and incorporation in cement production, are unable to consume the huge production volumes of these wastes, and as a result large quantities are disposed in landfills [15]. The feasibility of using biomass FA in the production of porous geopolymers was recently demonstrated by the authors [16]. The incorporation of biomass fly ash in the geopolymers production, for structural and nonstructural applications (where the wastewater treatment is an increasing contributor), can reduce the FA environmental impact, while simultaneously reduce the geopolymers’ production cost in comparison with MK-based geopolymers. In this work, porous biomass FA-containing geopolymers showing distinct pore structures were produced and then evaluated as lead adsorbents. To the best of our knowledge, this is one of the first investigations concerning the use of geopolymeric monoliths as adsorbents. Adsorption of Pb2+ by the porous geopolymers is studied as a function of porosity, contact time and pH.
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These fly ash-containing porous geopolymers monoliths can be easily handled and recycled, presenting interesting potential for the continuous treatment of industrial wastewaters. Furthermore, using biomass FA reduces both the production cost and carbon footprint of these novel adsorbent materials.
2. Experimental
2.1. Materials Metakaolin (MK) was purchased under the name of Argical™ M1200S from Univar®, while biomass FA was collected at the electrostatic precipitator of a Portuguese industrial co-generation plant with bubbling fluidized combustor, that consumes a mixture of eucalyptus bark with minor amounts of other residual biomass from forestry operations as fuel. For the alkaline activation, a mixture of sodium silicate solution (Chem-Lab, Belgium) and NaOH (reagent grade, 97%, Sigma Aldrich) was used in a 2:1 proportion (in weight). The 12 M NaOH solution was prepared by dissolution of 20-40 mesh sodium hydroxide beads in distilled water. A 3% wt./wt. hydrogen peroxide (H2O2) solution was used as blowing agent. The 50 ppm Pb2+ ion stock solution was prepared by dissolving Pb(NO3)2 (analytic grade reagent) in distilled water.
2.2. Geopolymers preparation Geopolymers were prepared using a mixture of 2/3 MK and 1/3 biomass FA (in weight) as a source of aluminosilicate. Four formulations were prepared using distinct amounts of hydrogen peroxide to evaluate its influence on the geopolymers’ pore structure. In
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these compositions, sodium silicate was substituted by 0.30, 0.60, 0.90 and 1.2 wt.% of H2O2. A fifth formulation was also prepared without using hydrogen peroxide. The mixing was carried out following a procedure described elsewhere [16], than can be summarized by the schematic representation shown in Fig. 2. After mixing the slurry was transferred to plastic moulds and sealed with a plastic film. The samples were cured in controlled conditions (40 ºC and 65% relative humidity) using a climatic chamber for 24 hours. Afterwards, the specimens were demoulded and kept sealed in the same curing conditions until the 7th curing day. Then the samples were left in sealed bags at ambient temperature (closed conditions) until the 28th curing day. After geopolymerization free alkalis remain in the structure [17], thus when immersing the geopolymers in aqueous solution an increase on pH is expected. Indeed, previous investigation by the authors [18] has shown a significant leaching of OH- ions from these porous geopolymers. The pH increase will affect the lead solubility [19], and thus its uptake by the geopolymer monolith. To evaluate the influence of the geopolymer free alkalis content on the lead removal efficiency, the samples were divided into two batches: i) non-washed geopolymers (hereafter coded as n-wFA) and ii) washed geopolymers (coded as wFA). Prior to adsorption tests the wFA where washed in 50 ºC distilled water (60 mL) until the pH of the wash water was kept at 7.0 ± 0.5 for at least 24 h.
2.3. Lead adsorption test Lead adsorption tests were performed with geopolymer monoliths, testing influencial variables on the lead removal efficiency such as the contact time, pH, and porosity of the bodies. Cylindrical discs, 22 mm diameter and 3 mm thickness, were immersed in 50 ppm Pb2+ ion solution (60 mL) of a specific pH (5 and 7 for wFA and 7 for n-wFA)
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and shaken for between 3 and 24 hours at room temperature. The Pb2+ concentration was determined using an atomic absorption spectrometer (Avanta PV, GBC). The quantity of lead uptake by the geopolymer was determined according to the following equation: (1) where
is the quantity of lead uptake by the geopolymer (mglead/ggeopolymer),
initial concentration of lead (ppm), (ppm),
is the
is the remaining equilibrium lead concentration
is the solution volume (L) and
is the mass of geopolymer (g).
2.4. Lead desorption tests Desorption tests were also performed on the samples obtained from the adsorption test. Water treatment (60 mL distilled water; 24 h contact time) and acid treatment (60 mL 0.1 M HCl; 3 h contact time) were applied to study the leaching characteristics of the lead-containing specimens at room temperature. The leaching efficiency (quantity of lead recovered from the geopolymer) was determined according to the following equation:
where
%
100
(2)
is the initial mass of lead adsorbed in the geopolymer (mglead/ggeopolymer) and
is the mass of extracted lead from the geopolymer with desorption (mglead/ggeopolymer).
2.5. Materials characterization Scanning electron microscopy (SEM - Hitachi S4100 equipped with energy dispersion spectroscopy, EDS – Rontec) was used, at 15 kV, to investigate the microstructure of the geopolymers before and after alkalis leaching, while EDS maps were obtained using SEM-Hitachi SU-70. Optical analysis (Leica EZ4HD microscope) was used for the 8
morphological analysis of the porous geopolymers. Samples were cut from 28 daycured geopolymers using a Struers Secotom-10 table-top cutting machine. The mineralogical compositions of MK, FA and geopolymer specimens (cured for 28 days) were assessed by X-ray powder diffraction (XRD). The XRD was conducted on a Rigaku Geigerflex D/max-Series instrument (Cu Kα radiation, 10–80°, 0.02° 2θ stepscan and 10 s/step), and phase identification by PANalytical X’Pert HighScore Plus software. The chemical composition of FA and MK was obtained by using X-ray fluorescence (Philips X´Pert PRO MPD spectrometer). The loss on ignition (LOI) at 1000 C was also determined. The Archimedes method (using water as immersion fluid) was employed to evaluate the water absorption of the samples, while the bulk density was measured by the geometric method. The true density of the geopolymer prepared without H2O2, 2.05 g/cm3, was determined by the helium pycnometer technique (Multipycnometer, Quantachrome). The total porosity of the geopolymers prepared with distinct addition of hydrogen peroxide was then calculated as suggested by Landi et al. [20]. The chemical analysis (for Pb) of the water solutions obtained from the adsorption/desorption tests was performed using atomic absorption spectroscopy (Avanta PV, GBC).
3. Results and discussion
3.1. Geopolymers characterization
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The XRD patterns of the MK, FA and the geopolymer adsorbents prepared with 1.2 wt.% H2O2 are presented in Fig. 3, while the chemical composition of the raw materials is shown in Table 1. The geopolymer XRD shows a broad hump, between 18º and 38º (2), which provides evidence of the geopolymerization reaction. This hump was observed for all compositions (not shown here for the sake of brevity), indicating that the H2O2 content does not modify the geopolymer mineralogical composition [18]. A typical image of the porous geopolymer produced using hydrogen peroxide is shown in Fig. 4a, while Fig. 4b presents images of samples cuts produced with distinct H2O2 content used in the lead adsorption tests. As depicted, the H2O2 content dictated the number and volume of the produced pores, which is expected to strongly affect the geopolymers lead adsorption capacity. Indeed water absorption and total porosity values increase and apparent density decreases when the blowing agent content rises (see Table 2). Figs. 5 and 6 present optical and SEM micrographs of the n-wFA and wFA geopolymers. As illustrated, the washing procedure applied to remove free alkalis remaining in the structure induces significant changes on the geopolymer microstructure for all compositions. Table 2 shows a relative increase between 33% and 86% on the water absorption, suggesting an increase on the porosity level upon washing. Indeed, Fig. 7 shows that total porosity increases for all compositions, while the apparent density decreases. This increase in porosity is expected to enhance the lead adsorption, due to the higher number of adsorption sites for Pb2+ ions.
3.2. Lead adsorption tests For the lead adsorption tests cylindrical discs with identical geometry and dimensions were immersed in a Pb2+ ion solution. The EDS map and spectrum of FA-containing
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geopolymer after the lead adsorption test is shown in Fig. 8. Fig. 8a clearly shows the presence of Pb2+ in the geopolymer after the adsorption (absent before the test), which is supported by the EDS spectrum. The amount of lead removed for a contact time of 24 h is presented in Fig. 9. Previous investigations have shown that at low pH values there is an excess of H+ ions in the solution, that compete with the Pb2+ ions for the adsorption sites [9]. As the pH increases, the competition between the H+ and Pb2+ ions for active sites decreases, and Pb2+ ions are the prevalent species [9]. pH increments beyond 5.0 decrease the lead adsorption efficiency [19] due to the formation of lead hydroxides that might precipitate. Indeed, results (see Fig. 9) for the wFA show an increase on the lead uptake between 8 and 38% when the pH of the ion solution drops from 7.0 to 5.0. The lead removal efficiency was also found to be dependent on the geopolymer porosity: an increase up to 68% being evident when the total porosity rises from 52.0% to 78.4%. The latter was attributed to the higher number of active sites available for lead adsorption in the higher porosity geopolymers. This result suggests that the lead adsorption can be further enhanced if the geopolymers porosity increases. As shown in Figs. 5 and 6, the hydrogen peroxide content controls the geopolymer’s porosity level, so increasing its incorporation content would induce higher porosity levels. Moreover if the H2O2 concentration threshold is exceeded coalescence between the pores would open the geopolymers structure, and therefore enhance the adsorption amount. In Table 3 we show a comparison between the published lead adsorption capacity of various powdered adsorbents, and the highest lead adsorption FA-containing geopolymers reported here (prepared with 1.2 wt.% H2O2). The maximum lead uptake here (6.34 mglead/ggeopolymer) is significantly smaller than that reported for marine algae [21], sepiolite [28], metakaolin-based [6] or fly ash-based geopolymers [9] (in powder),
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which is explained by the much lower specific surface area of the monoliths. Nevertheless, these novel porous geopolymer monoliths show similar adsorption in comparison to that of corn chaff [26], and higher than that of bagasse fly ash [22], coconut [25], rice husk [24], and hen and duck egg shell [27]. These results clearly demonstrate the potential of these large shape specimens as lead adsorbents, still further increase on their removal ability is feasible (e.g. raising the adsorbent dose, initial concentration of contaminant and temperature) [29]. Moreover these innovative adsorbents can be used in packed beds, making it easier to collect and recycle them when exhausted. This is a major practical advantage in comparison with the use of powdered adsorbents. Furthermore the multifunctionality exhibited by these porous geopolymeric monoliths suggests the possibility of being applied first as pH buffering material [18] causing metals precipitation and then, when the solution pH decreases below a certain level, the material will act as adsorbent which is an environmental friendly approach. As for the n-wFA, a significant change in the pH of the ion solution, from 7 to around 9, was observed when the geopolymeric bodies were immersed for 24 h. The pH increase is attributed to the OH- leaching from the geopolymers. Recently, Novais et al. [18] demonstrated that the leaching rate of OH- from FA-containing geopolymers is very high in the first day, which would explain the observed pH fluctuation. The observed increase in pH led to the precipitation of lead hydroxides, which hinders the adsorption process. In fact, all n-wFA bodies showed worst performance in comparison with wFA (see Fig. 9), which indicates that the free alkalis remaining after geopolymerization must be extracted prior to adsorption to maximize the lead adsorption capacity. Despite this, the possibility of using these n-wFA bodies to promote separation by chemical precipitation presents interesting potential, and will be addressed in future work. In fact,
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the observed pH fluctuation is expected to induce significant metal precipitation. Therefore, we can expect two competing, sequential action for these n-wFA bodies, where adsorption is the prevailing mechanism at low pH (<7), while at higher pH chemical precipitation will be the dominant one. Nevertheless it should be highlighted that the poorer results obtained for n-wFA can also be partially attributed to their lower porosity level (see Table 2). Previous investigations have shown that the heavy metals adsorption is time dependent [9, 30]. In the initial period, of time a large number of active sites are available [30] and the heavy metal adsorption rate is high, while thereafter the adsorption rate decreases until the maximum adsorption is reached. The removal of Pb2+ ions occurs rapidly, for FA-geopolymer powders an equilibrium contact time of 120 min was reported by AlZboon et al. [9]. Considering the low specific surface area of the porous geopolymer monoliths, higher contact times were used here (3 and 24 h). The influence of contact time on the geopolymer’s adsorption capacity is illustrated in Fig. 10. For the wFA geopolymers, two distinct behaviours are perceived depending on the pH of the ion solution: for pH=7 an increase on the lead uptake up to around 56% is observed; while minor fluctuations are apparent for pH=5. These results show that the lead adsorption rate is significantly higher for the lower pH value, for which the time to reach the maximum lead adsorption is ≤ 3 h. To provide a better insight on the influence of contact time, the lead adsorption differential between the samples immersed in the ion solution for 24 and 3 h were determined and presented in Fig. 10f. This way, the influence of pH and geopolymers porosity is quite obvious. Indeed for pH=7 the less porous geopolymers take longer to reach equilibrium, which is demonstrated by the increase of around 56, 31 and 27% for the geopolymers prepared respectively with 0.00, 0.30 and 0.60 wt.% H2O2. The latter
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was attributed to the higher number of pores obtained when the H2O2 content rises, which enhances the number of available sites for lead adsorption. On the contrary, the smaller number of pores shown by less porous bodies hinders the adsorption rate, prolonging the equilibrium contact time to 24 h. A distinct behaviour was observed for the n-wFA with a decrease on the lead uptake (up to 66%) with prolonged contact time. In the first 3 h of the adsorption test the pH of the ion solution shifts from 7 to 10 due to the alkalis leaching from the geopolymer. Interestingly the pH of the ions solution drops to around 9 when the contact time rises to 24 h (in all compositions). One possible explanation for this pH variation is the formation of lead hydroxides between the released HO- ions and the previously adsorbed Pb2+ ions, hence decreasing the adsorption.
3.3. Lead desorption tests To evaluate the lead extraction efficiency of the lead-containing geopolymer monoliths, the samples were submitted to water and acid-treatment. For this evaluation bodies that showed the highest adsorption levels (see Fig. 9) were selected. Fig. 11 illustrates the effect of water and acid treatment on the leaching efficiency. Results show that the 24 h water treatment cannot remove lead from the geopolymer monoliths; an insignificant recovery rate (up to 0.09%) was obtained, suggesting a highly effective lead fixation. The extraction is strongly improved if acidic (0.1 M HCl) rather than neutral conditions are employed. Lead recovery up to 10% was obtained, which can be attributed to an ion exchange between H+ and Pb2+ ions or to dissolution of precipitated lead. Still this highly acidic environment induces some damage on the geopolymer structure which hinders their reutilization as adsorbents. Interestingly the lead leaching was affected by the geopolymer porosity, decreasing from around 10% (less porous bodies) to 1.5%
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(more porous samples) when the porosity rises. This behaviour might be explained by differences on the pore size distribution. During adsorption, the Pb2+ ions diffuse into the geopolymer locking into the pores, which decreases the possibility of leaching [9]. As the porosity rises the diffusion of Pb2+ into the geopolymer structure increases, hindering its detachment. Furthermore the adsorption of lead ions into the surface was also found dependent on the geopolymers porosity. The EDS maps shown in Fig. 12 clearly demonstrate a higher surface adsorption of lead for the less porous geopolymer in comparison with the more porous one. As depicted, the lead is homogeneously distributed in the surface of the less porous geopolymer (see Fig. 12b), while in the more porous geopolymer, lead is absent from the largest pores at the surface. Considering the lead uptake results (see Fig. 9), it is obvious that Pb2+ ions have diffused into the geopolymers and are therefore less available for leaching, which explains the lower leaching levels of the more porous geopolymers. In fact EDS maps performed in transversal section of the monoliths show differences among the samples depending on their porosity level. Lead was detected throughout the sample in the higher porosity bodies (see Fig. 13), while the adsorption was mainly superficial in the less porous geopolymers. The relatively low leaching levels can also be attributed to chemical interaction between the Pb2+ ions and the geopolymer matrix as reported in previous works [31, 32]. The regeneration of the geopolymer monoliths was not considered here, since the highly acidic environment induced structural damage on the geopolymers hindering their reutilization. Nevertheless their regeneration ability in less acidic conditions will be addressed in future work.
4. Conclusions
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In the present work, porous biomass FA-containing geopolymer monoliths were used as lead adsorbents from synthetic wastewaters. Results demonstrate that the geopolymer porosity crucially affects the monoliths adsorption efficiency, an increase up to 68% being evident when the porosity rises from 52.0% to 78.4%. Furthermore the geopolymers porosity also affects the adsorption rate, higher porosity induces a faster adsorption. An increase on pH (from 5 to 7) decreased the monoliths removal ability (up to 38%), which was associated with the formation of lead hydroxides at pH higher than 5. Results also show the need to extract, through washing, the free alkalis remaining after geopolymerization prior to lead adsorption. Washing the porous geopolymers with water enables the HO- ions removal from the geopolymer, which highly increases the monoliths porosity and therefore their adsorption capacity. The latter suggests the possibility of being used sequentially: first as pH buffering material causing metals precipitation and secondly as heavy metal adsorbents. Lead desorption from the monoliths in water is ineffective, with insignificant recovery rates (0.09%), suggesting a strong lead fixation. Acidic desorption conditions enhance the lead recovery (up to 10%) while inducing some damage on the geopolymers structure, which hinders their reuse. The recovery rate may be enhanced by prolonging the duration time and temperature, while the use of less severe acidic conditions may allow the geopolymer monoliths regeneration. The maximum lead uptake here reported (6.34 mglead/ggeopolymer) demonstrates the potential of these innovative materials as adsorbents, still further increase on their removal capacity is feasible (e.g. raising the geopolymers porosity). These novel geopolymeric monoliths can be directly used in packed beds and easily collected when
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exhausted, which is a major advantage in comparison with the conventional geopolymer powdered adsorbents. In addition these novel adsorbents allow the reuse of biomass fly ash decreasing their production cost, while mitigating this waste environmental impact.
Acknowledgements: This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The authors acknowledge the assistance of Dr. R.C.Pullar with editing English language in this paper.
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Pokethitiyook,
P.
Panyawathanakit, Removal of lead from battery manufacturing wastewater by egg shell. Songklanakarin Journal of Science and Technology 29 (2007) 857-868. 28. N. Bekta, B.A. Ağim, S. Kara, Kinetic and equilibrium studies in removing lead ions from aqueous solutions by natural sepiolite, Journal of Hazardous Materials 112 (2004) 115-122. 29. K.K. Al-Zboon, B.M. Al-smadi, S. Al-Khawaldh, 2016. Natural volcanic tuff-based geopolymer for Zn removal: adsorption isotherm, kinetic, and thermodynamic study, Water, Air, & Soil Pollution, 227-248. 30. W. Tang, H. Huang, Y. Gao, X. Liu, X. Yang, H. Ni, J. Zhang, Preparation of novel porous adsorption material from coal slag and its adsorption properties of phenol from aqueous solution, Materials and Design 88 (2015) 1191-1200. 31. M.T. Jin, Z.F. Jin, C.J: Huang, Immobilization of heavy metal Pb2+ with geopolymer, Huan Jing Ke Xue 32 (2011) 1447-1453 (in Chinese). 32. J. Zhang, J.L. Provis, D. Feng, J.S.J. van Deventer, Geopolymers for immobilization of Cr6+, Cd2+, and Pb2+, Journal of Hazardous Materials 157 (2008) 587-598.
21
Fig. 1. Works published for “adsorbents” and “geopolymer adsorbents” (data obtained from Scopus on February 2016).
22
Fig. 2. Schematic representation of the procedure used for the geopolymers preparation.
23
Fig. 3. XRD patterns of fly ash (FA), metakaolin (MK) and the synthesized geopolymer.
24
Fig. 4. a) Geopolymer cylindrical specimen (22 mm diameter and 48 mm length) and b) sample cuts (used for lead adsorption) produced with distinct pore forming agent content.
25
Fig. 5. Optical microscopy characterization of FA-containing geopolymers before (a-e) and after (f-j) alkalis leaching (washing in 50 ºC water until constant pH) produced with distinct hydrogen peroxide content: 0.00 wt.% (a, f), 0.30 wt.% (b, g), 0.60 wt.% (c, h), 0.90 wt.% (d, i) and 1.20 wt.% (e, j).
26
Fig. 6. SEM characterization of FA-containing geopolymers before (a-e) and after (f-j) alkalis leaching (washing in 50 ºC water until constant pH) produced with distinct hydrogen peroxide content: 0.00 wt.% (a, f), 0.30 wt.% (b, g), 0.60 wt.% (c, h), 0.90 wt.% (d, i) and 1.20 wt.% (e, j).
27
Fig.7. Apparent density and total porosity of FA-containing geopolymers before and after alkalis leaching.
28
Fig. 8. EDS map (a) and spectrum (b) of the surface of FA-containing geopolymer prepared with 1.2 wt.% H2O2 after lead adsorption.
29
Fig. 9. Effect of pH on the lead adsorption by porous geopolymers prepared with distinct hydrogen peroxide content (contact time 24 h).
30
Fig. 10. Effect of contact time and pH on the porous geopolymers lead adsorption efficiency.
31
Fig. 11. Effect of water and acid treatment on the lead extraction efficiency from the porous geopolymers.
32
Fig. 12. SEM micrograph and EDS map of the surface of FA-containing geopolymer prepared with (a, b) 0.0 wt.% and (c, d) 1.2 wt.% H2O2 after lead adsorption. 33
Fig. 13. EDS map (a) and spectrum (b) of a transversal section of a geopolymer monolith prepared with 1.2 wt.% H2O2 after lead adsorption.
34
Table 1 Chemical composition of metakaolin (MK) and fly ash (FA).
Oxides (wt.%)
MK
FA
SiO2
54.40
25.34
TiO2
1.55
0.60
Al2O3
39.40
6.05
Fe2O3
1.75
4.15
MgO
0.14
3.61
CaO
0.10
36.72
MnO
0.01
0.42
Na2O
-
0.95
K2O
1.03
5.84
SO3
-
3.84
P2O5
0.06
4.70
LOI
2.66
5.12
Ratio of SiO2/ Al2O3
1.38
4.19
35
Table 2 Water absorption, apparent density and total porosity of geopolymers before (n-wFA) and after alkalis leaching (wFA).
Sample reference
n-wFA
wFA
Water
Apparent
absorption
density
(%)
(g/cm3)
0.00
28.6
1.21 ± 0.03
41.0
0.30
34.1
0.79 ± 0.02
61.6
0.60
38.7
0.71 ± 0.06
62.8
0.90 1.20 0.00 0.30 0.60 0.90 1.20
43.0
0.66 ± 0.03
67.9
50.4 38.1 56.5 65.5 77.0 93.6
0.56 ± 0.01 0.98 ± 0.11 0.62 ± 0.08 0.56 ± 0.06 0.48 ± 0.04 0.44 ± 0.02
72.5 52.0 69.7 72.7 76.4 78.4
H2O2 (wt.%)
Total porosity (%)
36
Table 3 Lead adsorption capacity of various adsorbents.
*
Material
q (mg/g)
Reference
Algae marine, nonliving biomass
126.5
21
Bagasse fly ash
3.8
22
Biomass FA-geopolymer monolith *
6.3
-
Cashew nut shell
17.8
19
Cereal chaff
12.5
23
Chitosan
8.3
24
Coconut
4.4
25
Corn chaff
6.7
26
Fly ash-based geopolymer (powder)
81.0
9
Metakaolin-based geopolymer (powder)
100.0
6
Natural hen egg shell
1.5
27
Natural duck egg shell
1.6
27
Rice husk
5.7
24
Sepiolite
185.2
28
The bold identifies the results obtained in this work.
37
S0304-3894(16)30691-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.07.059 HAZMAT 17915
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
30-3-2016 15-7-2016 23-7-2016
Please cite this article as: Rui M.Novais, L.H.Buruberri, M.P.Seabra, J.A.Labrincha, Novel porous fly-ash containing geopolymer monoliths for lead adsorption from wastewaters, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.07.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Novel porous fly-ash containing geopolymer monoliths for lead adsorption from wastewaters
Rui M. Novais a,*, L. H. Buruberri a, M. P. Seabra a, J. A. Labrincha a a
Department of Materials and Ceramic Engineering / CICECO- Aveiro Institute of
Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
*Corresponding author: Tel.: +351234370262; fax: +351234370204 E-mail address: [email protected] (Rui M. Novais)
1
Graphical abstract
2
Highlights
Porous fly ash containing-geopolymer monoliths for lead adsorption were developed. Geopolymers’ porosity and pH of the ion solution controls the adsorption capacity. Lead adsorption by the geopolymer monoliths up to 6.34 mg/g was observed. These novel adsorbents can be used in packed beds that are easily collected. The reuse of biomass fly ash wastes as raw material ensures waste valorization.
Abstract In this study novel porous biomass fly ash-containing geopolymer monoliths were produced using a simple and flexible procedure. Geopolymers exhibiting distinct total porosities (ranging from 41.0 to 78.4%) and low apparent density (between 1.21 and 0.44 g/cm3) were fabricated. Afterwards, the possibility of using these innovative materials as lead adsorbents under distinct conditions was evaluated. Results demonstrate that the geopolymers’ porosity and the pH of the ion solution strongly affect the lead adsorption capacity. Lead adsorption by the geopolymer monoliths ranged between 0.95 and 6.34 mglead/ggeopolymer. More porous geopolymers presented better lead removal efficiency, while higher pH in the solution reduced their removal ability, since metal precipitation is enhanced. These novel geopolymeric monoliths can be used in packed beds that are easily collected when exhausted, which is a major advantage in comparison with the use of powdered adsorbents. Furthermore, their production encompasses the reuse of biomass fly-ash, mitigating the environmental impact associated with this waste disposal, while decreasing the adsorbents production costs.
Keywords: geopolymers; biomass fly ash; monoliths; lead adsorption; porosity. 3
1. Introduction The alarming increase of water demand over recent decades is a major societal problem. By 2025, around 1800 million people will be living in regions with absolute water scarcity, while two-thirds of the world population could be under stress conditions [1]. As a result, specific legislation has been adopted by the EU through the Water Framework Directive [2]. In this context, wastewater treatment and subsequent reuse can provide an important increase in water supply to cope with water scarcity. The presence of heavy metals in industrial wastewaters is a serious and long-lasting environmental problem [3]. Lead is one of the most common heavy metals in industrial wastewaters, and it is known to be toxic for humans and plants [4]. Therefore, its removal from wastewaters is of the foremost importance. Several methods have been carried out for heavy metals removal, such as chemical precipitation, coagulation, ion exchange and adsorption [5]. Amongst these, adsorption has attracted increasing attention due to its simplicity, effectiveness and low cost [6]. Common adsorbents such as activated carbon, alumina and silica present high efficiency levels; however, their high production cost hinders wider application. Therefore, the development of alternative and low cost adsorbents is eagerly pursued. The synthesis of geopolymers involves mixing aluminosilicate materials with strong alkaline [7] or acidic activators at relatively low temperatures. The geopolymers’ chemical structure is composed of a negatively charged aluminosilicate framework where charge-balancing cations can be exchanged with cations in the solution [8], which suggests the possibility of using them as adsorbents. Nevertheless, the use of geopolymers as adsorbents is fairly recent (Figure 1), and the existing literature is scarce [9-11]. Metakaolin-based geopolymers [6] and coal fly-ash geopolymers [9] exhibited a
4
lead uptake capacity around 100 mg/g and 80 mg/g, respectively. The latter demonstrates the feasibility of using geopolymers as heavy metals adsorbents. The vast majority of studies focused the use of geopolymer powders, which cannot be directly used in packed beds and are not easily retrieved [12, 13]. On the contrary, geopolymer monoliths can be directly used in packed beds as membranes, yet surprisingly this strategy has been neglected. Waste materials have been used as precursors in the production of geopolymers (e.g. fly ash and blast furnace slag). Coal fly-ash (FA) has been extensively considered as an aluminosilicate source, while the incorporation of biomass FA is less common. Nevertheless, the growing interest in the production of energy through the use of biomass [14] has raised the volume of biomass FA generated. Current management strategies, including addition to forest soils and incorporation in cement production, are unable to consume the huge production volumes of these wastes, and as a result large quantities are disposed in landfills [15]. The feasibility of using biomass FA in the production of porous geopolymers was recently demonstrated by the authors [16]. The incorporation of biomass fly ash in the geopolymers production, for structural and nonstructural applications (where the wastewater treatment is an increasing contributor), can reduce the FA environmental impact, while simultaneously reduce the geopolymers’ production cost in comparison with MK-based geopolymers. In this work, porous biomass FA-containing geopolymers showing distinct pore structures were produced and then evaluated as lead adsorbents. To the best of our knowledge, this is one of the first investigations concerning the use of geopolymeric monoliths as adsorbents. Adsorption of Pb2+ by the porous geopolymers is studied as a function of porosity, contact time and pH.
5
These fly ash-containing porous geopolymers monoliths can be easily handled and recycled, presenting interesting potential for the continuous treatment of industrial wastewaters. Furthermore, using biomass FA reduces both the production cost and carbon footprint of these novel adsorbent materials.
2. Experimental
2.1. Materials Metakaolin (MK) was purchased under the name of Argical™ M1200S from Univar®, while biomass FA was collected at the electrostatic precipitator of a Portuguese industrial co-generation plant with bubbling fluidized combustor, that consumes a mixture of eucalyptus bark with minor amounts of other residual biomass from forestry operations as fuel. For the alkaline activation, a mixture of sodium silicate solution (Chem-Lab, Belgium) and NaOH (reagent grade, 97%, Sigma Aldrich) was used in a 2:1 proportion (in weight). The 12 M NaOH solution was prepared by dissolution of 20-40 mesh sodium hydroxide beads in distilled water. A 3% wt./wt. hydrogen peroxide (H2O2) solution was used as blowing agent. The 50 ppm Pb2+ ion stock solution was prepared by dissolving Pb(NO3)2 (analytic grade reagent) in distilled water.
2.2. Geopolymers preparation Geopolymers were prepared using a mixture of 2/3 MK and 1/3 biomass FA (in weight) as a source of aluminosilicate. Four formulations were prepared using distinct amounts of hydrogen peroxide to evaluate its influence on the geopolymers’ pore structure. In
6
these compositions, sodium silicate was substituted by 0.30, 0.60, 0.90 and 1.2 wt.% of H2O2. A fifth formulation was also prepared without using hydrogen peroxide. The mixing was carried out following a procedure described elsewhere [16], than can be summarized by the schematic representation shown in Fig. 2. After mixing the slurry was transferred to plastic moulds and sealed with a plastic film. The samples were cured in controlled conditions (40 ºC and 65% relative humidity) using a climatic chamber for 24 hours. Afterwards, the specimens were demoulded and kept sealed in the same curing conditions until the 7th curing day. Then the samples were left in sealed bags at ambient temperature (closed conditions) until the 28th curing day. After geopolymerization free alkalis remain in the structure [17], thus when immersing the geopolymers in aqueous solution an increase on pH is expected. Indeed, previous investigation by the authors [18] has shown a significant leaching of OH- ions from these porous geopolymers. The pH increase will affect the lead solubility [19], and thus its uptake by the geopolymer monolith. To evaluate the influence of the geopolymer free alkalis content on the lead removal efficiency, the samples were divided into two batches: i) non-washed geopolymers (hereafter coded as n-wFA) and ii) washed geopolymers (coded as wFA). Prior to adsorption tests the wFA where washed in 50 ºC distilled water (60 mL) until the pH of the wash water was kept at 7.0 ± 0.5 for at least 24 h.
2.3. Lead adsorption test Lead adsorption tests were performed with geopolymer monoliths, testing influencial variables on the lead removal efficiency such as the contact time, pH, and porosity of the bodies. Cylindrical discs, 22 mm diameter and 3 mm thickness, were immersed in 50 ppm Pb2+ ion solution (60 mL) of a specific pH (5 and 7 for wFA and 7 for n-wFA)
7
and shaken for between 3 and 24 hours at room temperature. The Pb2+ concentration was determined using an atomic absorption spectrometer (Avanta PV, GBC). The quantity of lead uptake by the geopolymer was determined according to the following equation: (1) where
is the quantity of lead uptake by the geopolymer (mglead/ggeopolymer),
initial concentration of lead (ppm), (ppm),
is the
is the remaining equilibrium lead concentration
is the solution volume (L) and
is the mass of geopolymer (g).
2.4. Lead desorption tests Desorption tests were also performed on the samples obtained from the adsorption test. Water treatment (60 mL distilled water; 24 h contact time) and acid treatment (60 mL 0.1 M HCl; 3 h contact time) were applied to study the leaching characteristics of the lead-containing specimens at room temperature. The leaching efficiency (quantity of lead recovered from the geopolymer) was determined according to the following equation:
where
%
100
(2)
is the initial mass of lead adsorbed in the geopolymer (mglead/ggeopolymer) and
is the mass of extracted lead from the geopolymer with desorption (mglead/ggeopolymer).
2.5. Materials characterization Scanning electron microscopy (SEM - Hitachi S4100 equipped with energy dispersion spectroscopy, EDS – Rontec) was used, at 15 kV, to investigate the microstructure of the geopolymers before and after alkalis leaching, while EDS maps were obtained using SEM-Hitachi SU-70. Optical analysis (Leica EZ4HD microscope) was used for the 8
morphological analysis of the porous geopolymers. Samples were cut from 28 daycured geopolymers using a Struers Secotom-10 table-top cutting machine. The mineralogical compositions of MK, FA and geopolymer specimens (cured for 28 days) were assessed by X-ray powder diffraction (XRD). The XRD was conducted on a Rigaku Geigerflex D/max-Series instrument (Cu Kα radiation, 10–80°, 0.02° 2θ stepscan and 10 s/step), and phase identification by PANalytical X’Pert HighScore Plus software. The chemical composition of FA and MK was obtained by using X-ray fluorescence (Philips X´Pert PRO MPD spectrometer). The loss on ignition (LOI) at 1000 C was also determined. The Archimedes method (using water as immersion fluid) was employed to evaluate the water absorption of the samples, while the bulk density was measured by the geometric method. The true density of the geopolymer prepared without H2O2, 2.05 g/cm3, was determined by the helium pycnometer technique (Multipycnometer, Quantachrome). The total porosity of the geopolymers prepared with distinct addition of hydrogen peroxide was then calculated as suggested by Landi et al. [20]. The chemical analysis (for Pb) of the water solutions obtained from the adsorption/desorption tests was performed using atomic absorption spectroscopy (Avanta PV, GBC).
3. Results and discussion
3.1. Geopolymers characterization
9
The XRD patterns of the MK, FA and the geopolymer adsorbents prepared with 1.2 wt.% H2O2 are presented in Fig. 3, while the chemical composition of the raw materials is shown in Table 1. The geopolymer XRD shows a broad hump, between 18º and 38º (2), which provides evidence of the geopolymerization reaction. This hump was observed for all compositions (not shown here for the sake of brevity), indicating that the H2O2 content does not modify the geopolymer mineralogical composition [18]. A typical image of the porous geopolymer produced using hydrogen peroxide is shown in Fig. 4a, while Fig. 4b presents images of samples cuts produced with distinct H2O2 content used in the lead adsorption tests. As depicted, the H2O2 content dictated the number and volume of the produced pores, which is expected to strongly affect the geopolymers lead adsorption capacity. Indeed water absorption and total porosity values increase and apparent density decreases when the blowing agent content rises (see Table 2). Figs. 5 and 6 present optical and SEM micrographs of the n-wFA and wFA geopolymers. As illustrated, the washing procedure applied to remove free alkalis remaining in the structure induces significant changes on the geopolymer microstructure for all compositions. Table 2 shows a relative increase between 33% and 86% on the water absorption, suggesting an increase on the porosity level upon washing. Indeed, Fig. 7 shows that total porosity increases for all compositions, while the apparent density decreases. This increase in porosity is expected to enhance the lead adsorption, due to the higher number of adsorption sites for Pb2+ ions.
3.2. Lead adsorption tests For the lead adsorption tests cylindrical discs with identical geometry and dimensions were immersed in a Pb2+ ion solution. The EDS map and spectrum of FA-containing
10
geopolymer after the lead adsorption test is shown in Fig. 8. Fig. 8a clearly shows the presence of Pb2+ in the geopolymer after the adsorption (absent before the test), which is supported by the EDS spectrum. The amount of lead removed for a contact time of 24 h is presented in Fig. 9. Previous investigations have shown that at low pH values there is an excess of H+ ions in the solution, that compete with the Pb2+ ions for the adsorption sites [9]. As the pH increases, the competition between the H+ and Pb2+ ions for active sites decreases, and Pb2+ ions are the prevalent species [9]. pH increments beyond 5.0 decrease the lead adsorption efficiency [19] due to the formation of lead hydroxides that might precipitate. Indeed, results (see Fig. 9) for the wFA show an increase on the lead uptake between 8 and 38% when the pH of the ion solution drops from 7.0 to 5.0. The lead removal efficiency was also found to be dependent on the geopolymer porosity: an increase up to 68% being evident when the total porosity rises from 52.0% to 78.4%. The latter was attributed to the higher number of active sites available for lead adsorption in the higher porosity geopolymers. This result suggests that the lead adsorption can be further enhanced if the geopolymers porosity increases. As shown in Figs. 5 and 6, the hydrogen peroxide content controls the geopolymer’s porosity level, so increasing its incorporation content would induce higher porosity levels. Moreover if the H2O2 concentration threshold is exceeded coalescence between the pores would open the geopolymers structure, and therefore enhance the adsorption amount. In Table 3 we show a comparison between the published lead adsorption capacity of various powdered adsorbents, and the highest lead adsorption FA-containing geopolymers reported here (prepared with 1.2 wt.% H2O2). The maximum lead uptake here (6.34 mglead/ggeopolymer) is significantly smaller than that reported for marine algae [21], sepiolite [28], metakaolin-based [6] or fly ash-based geopolymers [9] (in powder),
11
which is explained by the much lower specific surface area of the monoliths. Nevertheless, these novel porous geopolymer monoliths show similar adsorption in comparison to that of corn chaff [26], and higher than that of bagasse fly ash [22], coconut [25], rice husk [24], and hen and duck egg shell [27]. These results clearly demonstrate the potential of these large shape specimens as lead adsorbents, still further increase on their removal ability is feasible (e.g. raising the adsorbent dose, initial concentration of contaminant and temperature) [29]. Moreover these innovative adsorbents can be used in packed beds, making it easier to collect and recycle them when exhausted. This is a major practical advantage in comparison with the use of powdered adsorbents. Furthermore the multifunctionality exhibited by these porous geopolymeric monoliths suggests the possibility of being applied first as pH buffering material [18] causing metals precipitation and then, when the solution pH decreases below a certain level, the material will act as adsorbent which is an environmental friendly approach. As for the n-wFA, a significant change in the pH of the ion solution, from 7 to around 9, was observed when the geopolymeric bodies were immersed for 24 h. The pH increase is attributed to the OH- leaching from the geopolymers. Recently, Novais et al. [18] demonstrated that the leaching rate of OH- from FA-containing geopolymers is very high in the first day, which would explain the observed pH fluctuation. The observed increase in pH led to the precipitation of lead hydroxides, which hinders the adsorption process. In fact, all n-wFA bodies showed worst performance in comparison with wFA (see Fig. 9), which indicates that the free alkalis remaining after geopolymerization must be extracted prior to adsorption to maximize the lead adsorption capacity. Despite this, the possibility of using these n-wFA bodies to promote separation by chemical precipitation presents interesting potential, and will be addressed in future work. In fact,
12
the observed pH fluctuation is expected to induce significant metal precipitation. Therefore, we can expect two competing, sequential action for these n-wFA bodies, where adsorption is the prevailing mechanism at low pH (<7), while at higher pH chemical precipitation will be the dominant one. Nevertheless it should be highlighted that the poorer results obtained for n-wFA can also be partially attributed to their lower porosity level (see Table 2). Previous investigations have shown that the heavy metals adsorption is time dependent [9, 30]. In the initial period, of time a large number of active sites are available [30] and the heavy metal adsorption rate is high, while thereafter the adsorption rate decreases until the maximum adsorption is reached. The removal of Pb2+ ions occurs rapidly, for FA-geopolymer powders an equilibrium contact time of 120 min was reported by AlZboon et al. [9]. Considering the low specific surface area of the porous geopolymer monoliths, higher contact times were used here (3 and 24 h). The influence of contact time on the geopolymer’s adsorption capacity is illustrated in Fig. 10. For the wFA geopolymers, two distinct behaviours are perceived depending on the pH of the ion solution: for pH=7 an increase on the lead uptake up to around 56% is observed; while minor fluctuations are apparent for pH=5. These results show that the lead adsorption rate is significantly higher for the lower pH value, for which the time to reach the maximum lead adsorption is ≤ 3 h. To provide a better insight on the influence of contact time, the lead adsorption differential between the samples immersed in the ion solution for 24 and 3 h were determined and presented in Fig. 10f. This way, the influence of pH and geopolymers porosity is quite obvious. Indeed for pH=7 the less porous geopolymers take longer to reach equilibrium, which is demonstrated by the increase of around 56, 31 and 27% for the geopolymers prepared respectively with 0.00, 0.30 and 0.60 wt.% H2O2. The latter
13
was attributed to the higher number of pores obtained when the H2O2 content rises, which enhances the number of available sites for lead adsorption. On the contrary, the smaller number of pores shown by less porous bodies hinders the adsorption rate, prolonging the equilibrium contact time to 24 h. A distinct behaviour was observed for the n-wFA with a decrease on the lead uptake (up to 66%) with prolonged contact time. In the first 3 h of the adsorption test the pH of the ion solution shifts from 7 to 10 due to the alkalis leaching from the geopolymer. Interestingly the pH of the ions solution drops to around 9 when the contact time rises to 24 h (in all compositions). One possible explanation for this pH variation is the formation of lead hydroxides between the released HO- ions and the previously adsorbed Pb2+ ions, hence decreasing the adsorption.
3.3. Lead desorption tests To evaluate the lead extraction efficiency of the lead-containing geopolymer monoliths, the samples were submitted to water and acid-treatment. For this evaluation bodies that showed the highest adsorption levels (see Fig. 9) were selected. Fig. 11 illustrates the effect of water and acid treatment on the leaching efficiency. Results show that the 24 h water treatment cannot remove lead from the geopolymer monoliths; an insignificant recovery rate (up to 0.09%) was obtained, suggesting a highly effective lead fixation. The extraction is strongly improved if acidic (0.1 M HCl) rather than neutral conditions are employed. Lead recovery up to 10% was obtained, which can be attributed to an ion exchange between H+ and Pb2+ ions or to dissolution of precipitated lead. Still this highly acidic environment induces some damage on the geopolymer structure which hinders their reutilization as adsorbents. Interestingly the lead leaching was affected by the geopolymer porosity, decreasing from around 10% (less porous bodies) to 1.5%
14
(more porous samples) when the porosity rises. This behaviour might be explained by differences on the pore size distribution. During adsorption, the Pb2+ ions diffuse into the geopolymer locking into the pores, which decreases the possibility of leaching [9]. As the porosity rises the diffusion of Pb2+ into the geopolymer structure increases, hindering its detachment. Furthermore the adsorption of lead ions into the surface was also found dependent on the geopolymers porosity. The EDS maps shown in Fig. 12 clearly demonstrate a higher surface adsorption of lead for the less porous geopolymer in comparison with the more porous one. As depicted, the lead is homogeneously distributed in the surface of the less porous geopolymer (see Fig. 12b), while in the more porous geopolymer, lead is absent from the largest pores at the surface. Considering the lead uptake results (see Fig. 9), it is obvious that Pb2+ ions have diffused into the geopolymers and are therefore less available for leaching, which explains the lower leaching levels of the more porous geopolymers. In fact EDS maps performed in transversal section of the monoliths show differences among the samples depending on their porosity level. Lead was detected throughout the sample in the higher porosity bodies (see Fig. 13), while the adsorption was mainly superficial in the less porous geopolymers. The relatively low leaching levels can also be attributed to chemical interaction between the Pb2+ ions and the geopolymer matrix as reported in previous works [31, 32]. The regeneration of the geopolymer monoliths was not considered here, since the highly acidic environment induced structural damage on the geopolymers hindering their reutilization. Nevertheless their regeneration ability in less acidic conditions will be addressed in future work.
4. Conclusions
15
In the present work, porous biomass FA-containing geopolymer monoliths were used as lead adsorbents from synthetic wastewaters. Results demonstrate that the geopolymer porosity crucially affects the monoliths adsorption efficiency, an increase up to 68% being evident when the porosity rises from 52.0% to 78.4%. Furthermore the geopolymers porosity also affects the adsorption rate, higher porosity induces a faster adsorption. An increase on pH (from 5 to 7) decreased the monoliths removal ability (up to 38%), which was associated with the formation of lead hydroxides at pH higher than 5. Results also show the need to extract, through washing, the free alkalis remaining after geopolymerization prior to lead adsorption. Washing the porous geopolymers with water enables the HO- ions removal from the geopolymer, which highly increases the monoliths porosity and therefore their adsorption capacity. The latter suggests the possibility of being used sequentially: first as pH buffering material causing metals precipitation and secondly as heavy metal adsorbents. Lead desorption from the monoliths in water is ineffective, with insignificant recovery rates (0.09%), suggesting a strong lead fixation. Acidic desorption conditions enhance the lead recovery (up to 10%) while inducing some damage on the geopolymers structure, which hinders their reuse. The recovery rate may be enhanced by prolonging the duration time and temperature, while the use of less severe acidic conditions may allow the geopolymer monoliths regeneration. The maximum lead uptake here reported (6.34 mglead/ggeopolymer) demonstrates the potential of these innovative materials as adsorbents, still further increase on their removal capacity is feasible (e.g. raising the geopolymers porosity). These novel geopolymeric monoliths can be directly used in packed beds and easily collected when
16
exhausted, which is a major advantage in comparison with the conventional geopolymer powdered adsorbents. In addition these novel adsorbents allow the reuse of biomass fly ash decreasing their production cost, while mitigating this waste environmental impact.
Acknowledgements: This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The authors acknowledge the assistance of Dr. R.C.Pullar with editing English language in this paper.
17
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17. R.R. Lloyd, J.L. Provis, J.S.J. van Deventer, Pore solution composition and alkali diffusion in inorganic polymer cement, Cement and Concrete Research 40 (2010) 1386– 1392. 18. R.M. Novais, L.H. Buruberri, M.P. Seabra, D. Bajare, J.A. Labrincha, Novel porous fly ash-containing geopolymers for pH buffering applications, Journal of Cleaner Production 124 (2016), 395-404. 19. P.S. Kumar, Adsorption of lead (II) ions from simulated wastewater using natural waste: a kinetic thermodynamic and equilibrium study, Environmental Progress & Sustainable Energy 33 (2014) 55-64. 20. Landi, E., Medri, V., Papa, E., Dedecek, J., Klein, P., Benito, P., Vaccari, A., 2013. Alkali-bonded ceramics with hierarchical tailored porosity. Applied Clay Science 73, 56–64. 21. R. Jalali, H. Ghafourian, Y. Asef, S.J. Davarpanah, S. Sepehr, Removal and recovery of lead using non-living biomass of marine algae, Journal of Hazardous Materials 92 (2002) 253-262. 22. V.K. Gupta, I. Ali, Removal of lead and chromium from wastewater using bagasse fly ash – a sugar industry waste, Journal of Colloid and Interface Science 271 (2004) 312328. 23. R. Han, J. Zhang, W. Zou, J. Shi, H. Liu, equilibrium biosorption isotherm for lead ion on chaff, Journal of Hazardous Materials 125 (2005) 266-271. 24. M.M.D. Zulkali, A.L. Ahmad, N.H. Norulakmal, N.S. Sharifah, Comparative studies of Oryza Sativa L. husk and chitosan as lead adsorbent, Journal of Chemical Technology and Biotechnology 81 (2006) 1324-1327.
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25. S. Gueu, B. Yao, K. Adouby, G. Ado, Kinetics and thermodynamics study of lead adsorption on to activated carbons from coconut and seed hull of the palm tree, International Journal of Environmental Science & Technology 4 (2007) 11-17. 26. R. Han, J. Zhang, W. Zou, H. Xiao, J., Shi, H. Lin, Biosorption of copper (II) and lead (II) from aqueous solution by chaff in a fixed-bed column, Journal of Hazardous Materials 133 (2006) 262-268. 27. C.
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Fig. 1. Works published for “adsorbents” and “geopolymer adsorbents” (data obtained from Scopus on February 2016).
22
Fig. 2. Schematic representation of the procedure used for the geopolymers preparation.
23
Fig. 3. XRD patterns of fly ash (FA), metakaolin (MK) and the synthesized geopolymer.
24
Fig. 4. a) Geopolymer cylindrical specimen (22 mm diameter and 48 mm length) and b) sample cuts (used for lead adsorption) produced with distinct pore forming agent content.
25
Fig. 5. Optical microscopy characterization of FA-containing geopolymers before (a-e) and after (f-j) alkalis leaching (washing in 50 ºC water until constant pH) produced with distinct hydrogen peroxide content: 0.00 wt.% (a, f), 0.30 wt.% (b, g), 0.60 wt.% (c, h), 0.90 wt.% (d, i) and 1.20 wt.% (e, j).
26
Fig. 6. SEM characterization of FA-containing geopolymers before (a-e) and after (f-j) alkalis leaching (washing in 50 ºC water until constant pH) produced with distinct hydrogen peroxide content: 0.00 wt.% (a, f), 0.30 wt.% (b, g), 0.60 wt.% (c, h), 0.90 wt.% (d, i) and 1.20 wt.% (e, j).
27
Fig.7. Apparent density and total porosity of FA-containing geopolymers before and after alkalis leaching.
28
Fig. 8. EDS map (a) and spectrum (b) of the surface of FA-containing geopolymer prepared with 1.2 wt.% H2O2 after lead adsorption.
29
Fig. 9. Effect of pH on the lead adsorption by porous geopolymers prepared with distinct hydrogen peroxide content (contact time 24 h).
30
Fig. 10. Effect of contact time and pH on the porous geopolymers lead adsorption efficiency.
31
Fig. 11. Effect of water and acid treatment on the lead extraction efficiency from the porous geopolymers.
32
Fig. 12. SEM micrograph and EDS map of the surface of FA-containing geopolymer prepared with (a, b) 0.0 wt.% and (c, d) 1.2 wt.% H2O2 after lead adsorption. 33
Fig. 13. EDS map (a) and spectrum (b) of a transversal section of a geopolymer monolith prepared with 1.2 wt.% H2O2 after lead adsorption.
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Table 1 Chemical composition of metakaolin (MK) and fly ash (FA).
Oxides (wt.%)
MK
FA
SiO2
54.40
25.34
TiO2
1.55
0.60
Al2O3
39.40
6.05
Fe2O3
1.75
4.15
MgO
0.14
3.61
CaO
0.10
36.72
MnO
0.01
0.42
Na2O
-
0.95
K2O
1.03
5.84
SO3
-
3.84
P2O5
0.06
4.70
LOI
2.66
5.12
Ratio of SiO2/ Al2O3
1.38
4.19
35
Table 2 Water absorption, apparent density and total porosity of geopolymers before (n-wFA) and after alkalis leaching (wFA).
Sample reference
n-wFA
wFA
Water
Apparent
absorption
density
(%)
(g/cm3)
0.00
28.6
1.21 ± 0.03
41.0
0.30
34.1
0.79 ± 0.02
61.6
0.60
38.7
0.71 ± 0.06
62.8
0.90 1.20 0.00 0.30 0.60 0.90 1.20
43.0
0.66 ± 0.03
67.9
50.4 38.1 56.5 65.5 77.0 93.6
0.56 ± 0.01 0.98 ± 0.11 0.62 ± 0.08 0.56 ± 0.06 0.48 ± 0.04 0.44 ± 0.02
72.5 52.0 69.7 72.7 76.4 78.4
H2O2 (wt.%)
Total porosity (%)
36
Table 3 Lead adsorption capacity of various adsorbents.
*
Material
q (mg/g)
Reference
Algae marine, nonliving biomass
126.5
21
Bagasse fly ash
3.8
22
Biomass FA-geopolymer monolith *
6.3
-
Cashew nut shell
17.8
19
Cereal chaff
12.5
23
Chitosan
8.3
24
Coconut
4.4
25
Corn chaff
6.7
26
Fly ash-based geopolymer (powder)
81.0
9
Metakaolin-based geopolymer (powder)
100.0
6
Natural hen egg shell
1.5
27
Natural duck egg shell
1.6
27
Rice husk
5.7
24
Sepiolite
185.2
28
The bold identifies the results obtained in this work.
37