Porous geopolymer spheres as novel pH buffering materials

Accepted Manuscript Porous geopolymer spheres as novel pH buffering materials Rui M. Novais, M.P. Seabra, J.A. Labrincha PII:

S0959-6526(16)32056-X

DOI:

10.1016/j.jclepro.2016.12.008

Reference:

JCLP 8583

To appear in:

Journal of Cleaner Production

Received Date: 5 August 2016 Revised Date:

15 November 2016

Accepted Date: 2 December 2016

Please cite this article as: Novais RM, Seabra MP, Labrincha JA, Porous geopolymer spheres as novel pH buffering materials, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2016.12.008. 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.

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Porous geopolymer spheres as novel pH buffering materials Rui M. Novais 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,

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Portugal

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E-mail address: [email protected] (Rui M. Novais)

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*Corresponding author: Tel.: +351234370262; fax: +351234370204

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ACCEPTED MANUSCRIPT Abstract The production of biogas from anaerobic digestion is emerging as an alternative to fossil fuels due to its lower carbon footprint. However problems such as low methane yield and process instability arise when using low pH waste streams, hindering the

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technology widespread. In this study, porous metakaolin-based and biomass fly ashbased geopolymer spheres were produced using a simple and low cost technology. Afterwards, the pH buffer capacity of the produced spheres was evaluated. Results

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demonstrate that the geopolymers alkalis leaching can be controlled by the content of fly ash in the geopolymers, higher content promoting greater leaching. The fly ash-

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based spheres showed superior pH buffer capacity than their metakaolin-based counterparts, with a narrow pH fluctuation (1.6-1.9) over the 30 days measurements. The maximum leaching of hydroxyl ions from the geopolymer spheres (0.0317 mol/dm3.g) demonstrates the high pH buffer capacity of this innovative material,

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suggesting their use as pH regulator in anaerobic digestion.

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Keywords: geopolymer spheres; biomass fly ash; buffer capacity; alkalis leaching.

1. Introduction

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Despite the increasing awareness regarding the greenhouse gases emissions the global primary energy supply mainly relies on fossil fuels (82%) (IEA, 2015). A paradigm shift to cleaner and more energy-efficient technologies is, therefore, inevitable. Biogas technology has become a hot topic in recent years since it can significantly mitigate the greenhouse gases emissions in comparison with traditional fossil fuels (Cherubini et al., 2009). Moreover different waste materials can be used as feedstock (e.g. municipal and industrial organic waste, agricultural by-products and animal waste) (Hijazi et al., 2016)

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ACCEPTED MANUSCRIPT which further contribute to reduce its carbon footprint, while preventing the wastes landfilling. Biogas is produced by anaerobic digestion of organic material, being methane, carbon dioxide and organic acids the most abundant products. Anaerobic digestion is a complex process governed by key parameters such as temperature, pH,

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volatile fatty acids, nutrients, among other (Zhang et al., 2014a). A major limitation of anaerobic digestion is the rapid acidification of the substrates (Xu et al., 2014), caused by the high organic loads, which inhibits the methanogenesis and therefore the systems

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efficiency (Saddoud et al., 2007). To overcome the pH drop the common methodology is buffering these wastes with sodium hydroxide (Dai et al., 2016). Nevertheless an

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innovative and environmental friendlier alternative is here suggested, using waste-based materials exhibiting high pH buffering capacity that can provide prolonged pH adjustment.

Geopolymers are amorphous to semi-crystalline structures (Davidovits, 1994)

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synthesized by the chemical reaction between aluminosilicate precursors and alkaline solutions. They have been extensively investigated in the past years as a promising and environmental friendlier alternative to Portland cement (Davidovits, 2015), and for that

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reason the focus was directed to their mechanical performance. However, these materials exhibit other interesting properties, suggesting their use in innovative

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applications such as wastewater treatment (Al-Zboon et al., 2011), thermal insulation (Novais et al., 2016a), pH buffering (Novais et al., 2016b) and toxic waste immobilization (Zhang et al., 2008). The geopolymer framework contains free alkalis available for leaching (Lloyd et al., 2010), which suggest the possibility of using them as pH buffering materials. Nevertheless this topic has been rather neglected with few studies performed to date. Bumanis and Bajare (2014) observed that the alkalis leaching as mainly affected by the alkali activator solution-solid ratio, while Bumanis et al.

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ACCEPTED MANUSCRIPT (2015) studied the effect of particle size and mixture composition on the pH buffer ability of alkaline materials. Geopolymers with high pH buffering capacity have been recently reported by the authors, but using monoliths (Novais et al., 2016b). Nevertheless additional

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investigations would shed light on the influencing parameters affecting the alkalis leaching by the geopolymers. In particular the influence of the raw materials chemical composition on the geopolymers leaching ability remains unclear.

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Previous investigations have shown that the geopolymers leaching rate is affected by the nature and concentration of the activator (Zhang et al., 2014b), geopolymers’

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porosity (Novais et al., 2016b) and liquid-to-solid ratio (Bumanis and Bajare, 2014). Moreover the Na+ leaching rate in deionised water is time dependent, following a pseudo-second-order kinetic model (Aly et al., 2012). Therefore gradual and prolonged leaching may be obtained through the optimization of such parameters.

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Several waste streams (e.g. fly ash (FA), metallurgical slag, mining wastes) have been used as raw materials in the geopolymerization technology which decreases the geopolymers production cost in comparison with metakaolin (MK)-based formulations.

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Longhi et al. (2016) studied the possibility of using kaolin mining waste as raw material in geopolymers’ production, reporting compressive strengths up to 70 MPa after one

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day of curing. Nazari and Sanjayan (2015) used aluminium and grey cast iron slags respectively as alumina and silica sources, while an extensive review on the preparation of FA-based geopolymers was recently reported by Zhuang et al. (2016). The production of biomass FA has steeply increased in the last decade (Tarelho et al., 2015) which is attributed to the renewed interest in the production of electrical energy through the use of biomass. These ashes are still classified as a solid waste (Commission Decision, 2000) and a common practice is their landfill disposal, which is obviously

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ACCEPTED MANUSCRIPT perceived as unsustainable. In this context, the reuse of biomass FA, as proposed in this study, mitigates the wastes environmental impact while decreasing the geopolymers production cost. In this work, porous MK-based and biomass FA-based geopolymer spheres were

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synthesized using a simple, low cost and green technology. These innovative spheres can be easily handled, and collected when exhausted. Most importantly they increase process simplicity and efficiency by preventing the need of continuous pH

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adjustment/control which is a major advantage in comparison with the use of commercial alkaline materials. Moreover these spheres present much higher specific

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surface area than the previously reported monoliths (Novais et al., 2016b) which should enhance the alkalis leaching and therefore their pH buffer capacity. The influence of the MK/FA ratio on the geopolymers leaching behavior was evaluated to reduce the knowledge gap concerning the role of the raw materials chemical

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composition on the leaching ability. The gradual and prolonged leaching exhibiting by the FA-based geopolymer spheres indicates that they can be an effective solution to reduce the process instability and increase the efficiency of the anaerobic digestion

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when using low pH waste streams. Moreover the incorporation of biomass FA as raw materials in geopolymers may prevent huge amounts of this waste from being

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landfilled.

2. Experimental Conditions

2.1. Materials MK was purchase under the name of Argical™ M1200S from Univar®, while biomass FA was supplied by a Portuguese industrial co-generation plant. The FA are generated

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ACCEPTED MANUSCRIPT from the biomass burning (especially Eucalyptus forest waste) in a fluidized bed combustor. Prior to mixing the FA particles were sieved and then only those below 75 µm were used. Industrial grade sodium silicate solution (solid mass fraction = 33.9 wt. %;

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SiO2/Na2O=3.09) was purchased from Chem-Lab (Belgium). The NaOH (ACS reagent, 97%), sodium dodecyl sulfate (ACS reagent, 99%) and polyethylene glycol (PEG-600)

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were supplied by Sigma Aldrich.

2.2. Geopolymers preparation

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The porous geopolymeric spheres were produced through suspension solidification, following a previous described procedure (Tang et al., 2015). Distinct compositions were prepared in which MK was partly (33, 50, 66, and 75 wt.%) substituted by FA. The alkaline activator was prepared by adding 13.22 g of NaOH to 100.00 g of sodium

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silicate solution. The alkaline solution (24.38 g) and distilled water (4.15 g) were added to 15 g of the aluminosilicate precursor (MK or a mixture of MK and FA depending on the formulation) and then the mixture of the blend was carried out in a planetary mixer.

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Afterwards the foaming agent (0.59 g), sodium dodecyl sulfate, was added to the blend, and mixed to obtain foamed slurry. Finally the slurry was injected into polyethylene

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glycol medium (under a bath temperature of 85 ± 5 ºC) to produce geopolymeric spheres. The spheres were then collected and cured in controlled conditions (40 °C and 65% relative humidity) using a climatic chamber for 24 hours. Afterwards, the spheres were cured at room temperature for 4 days.

2.3. Materials characterization

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ACCEPTED MANUSCRIPT Scanning electron microscopy (SEM - Hitachi S4100 equipped with energy dispersion spectroscopy, EDS – Rontec) was used, at 25 kV, to investigate the microstructure of the porous geopolymer spheres. The mineralogical compositions of MK, FA and geopolymer spheres were assessed by

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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 step-scan and 10 s/step), and phase identification by PANalytical X’Pert HighScore Plus software.

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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

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also determined.

To evaluate the geopolymers pH buffer capacity 5 g of geopolymer spheres (cured for 5 days) were immersed in distilled water (60 ml) during 30 days. On a daily basis the specimens were moved to a new distilled water bath. Leaching rate of OH- ions was

formulation).

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measured every day for the prepared compositions (two specimens for each

Leaching of OH- ions (mol/dm3.g of geopolymer) was determined by acid-base titration

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method by using an 0.035 M HCl solution as titrant and phenolphthalein as acid-base

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indicator. The pH of the medium was calculated using the concentration of OH-, determined by acid base titration (Novais et al., 2016b).

3. Results and discussion 3.1. Raw materials characterization The chemical composition, presented in Table 1, shows that the most abundant components in biomass FA are CaO, SiO2, Al2O3 and K2O, followed by SO3 and P2O5. The ashes present high alkali (6.22 wt.%) and alkali earth metals content (43.98 wt.%),

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ACCEPTED MANUSCRIPT which limit their addition to forest soils (Barbosa et al., 2013). Moreover the chloride content (2.06 wt.%) also hinders its incorporation in cement production. Still these ashes are an inert solid waste (trace elements provided as supplementary material). The latter demonstrates the need to find alternative managements strategies such as incorporation

detrimental to durability and performance.

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in geopolymers production, as proposed here, where these components are not

The XRD patterns of FA and MK are presented in Fig. 1. Significant differences are

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perceived from the patterns in terms of crystallinity. The biomass FA contains crystalline phases such as quartz, muscovite, calcite, lime and microcline, while MK contains

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essentially amorphous silica and alumina. The irregular shape of the FA particles is shown by the SEM micrograph presented in Fig. 2, while the compositional heterogeneity of the ashes is demonstrated by the two EDS spectrum. Nevertheless, the

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characteristic elements are in agreement with their chemical composition (see Table 1).

3.2. Geopolymers characterization

Fig. 3 presents a typical photograph of the produced FA-containing geopolymer spheres,

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which had between 2 and 4 mm in diameter.

The XRD patterns for all produced FA-containing geopolymer spheres are presented in

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Fig. 4. The position of the diffraction peaks in the FA-containing geopolymers coincides with those of the MK and the FA, while the peaks intensity is increased as the FA content rises. All the patterns show a broad hump between 20 and 40º (2θ), while the centre of the hump is shifted towards higher 2θ as the FA content rises (from ca. 26º (2θ) to 30º (2θ)). The microstructure of distinct FA-containing geopolymeric spheres is shown in Fig. 5, while the EDS spectrum of two compositions prepared with 0 wt.% and 75 wt.% FA is

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ACCEPTED MANUSCRIPT presented in Fig. 6. As expected, the geopolymer composition reflects the nature of the raw materials. The MK-based geopolymer matrix is mainly composed of Si, Al and Na, while the FA-based also contains Ca. The intensity of the Na peak increased in comparison with that evaluated in the raw materials (see Fig. 2), which indicates that the

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Na is incorporated into the geopolymer structure (Luukkonen et al., 2016a) or that is present in the framework cavities to maintain the electrical charge balance (Cui et al., 2008). Interestingly the Na content is significantly higher on the FA-based geopolymers

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in comparison with the MK-based one, which may affect the geopolymers’ leaching behaviour. In fact, major differences are perceived from the cumulative OH- leaching

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shown in Fig. 7. The leaching percentage, calculated according to equation 1, was also included in Fig. 7 to provide a better insight on the OH- leaching from the porous geopolymer spheres. 

∑[  ] [  ]

× 100

(1)

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ℎ (%)=

For the compositions containing FA substantial OH- leaching was observed in the first couple of days, while afterwards the amount/rate of leached OH- ions decreased.

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Interestingly the leaching amount was dependent of the geopolymers FA content, higher contents promoting greater leaching. There are two explanations for these results: (i) the

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higher alkalis content in the FA (see Table 1); and (ii) the lower geopolymerization extension for the FA-based compositions, due to the lower reactivity of FA in comparison with that of MK, which increased the amount of alkalis available for leaching. For example, the total OH- leaching (see Table 2) of the spheres containing 66 wt.% FA was around 54% higher than that observed for those containing 33 wt.% FA. As for the composition prepared without FA, the leaching on the first day (0.009 mol/dm3.g) was similar to that of the composition prepared with 50 wt.% FA. However

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ACCEPTED MANUSCRIPT in the next days the OH- leaching rate strongly reduced. Indeed, the leaching percentage at the 1st day (see Table 2 and Fig. 7) for the MK-based spheres corresponded to circa 45% of the total leaching, while at the 10th day it reached around 95%. For comparison, the spheres containing 50 wt.% FA leached 30.8 % and 77.8%, respectively at the 1st and

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10th day.

Fig. 8 allows a comparison between the values here reported and other geopolymer systems. The maximum OH- leaching (0.0337 mol/dm3.g) observed for the 75 wt.% FA-

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containing spheres is substantially higher than those reported in previous investigations: around 87% (Novais et al., 2016b) and 25% (Bajare and Bumanis, 2014), respectively

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for porous 33 wt.% FA-containing geopolymers and for glass-modified alkali activated material (AAM), which is explained by the much higher specific surface area of the spheres in comparison with the monoliths. The latter demonstrates the potential of these innovative materials for pH buffering applications.

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Furthermore geopolymers may immobilize heavy metals (Novais et al., 2016c), ammonium (Luukkonen et al., 2016b) and other common inhibitors present in anaerobic digesters, thus promoting the process efficiency.

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The huge differences on the spheres leaching behaviour will affect the pH buffering capacity of the geopolymer spheres as clearly illustrated in Fig. 9, where the pH values

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of the water solution after geopolymers immersion over time are shown. As observed the pH decreased along the 30 days measurements for all compositions. Nevertheless, the shape of the pH curve, the initial (1st day) and final (30th day) pH values (see Fig. 10) differ among the investigated compositions. In the compositions containing FA the initial and final pH values slightly increased when the FA content rises, which was attributed to higher alkalis leaching. In these

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ACCEPTED MANUSCRIPT compositions the pH decrease over time could be described by a logarithmic equation (see the insert graph in Fig. 9): y = -0.496 ln(x) + 11.832

(2)

The pH decay/gradient (see Fig. 10) between the initial and final pH, for these

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compositions ranged from 1.6 (lower FA content) to 1.9 (higher FA content).

In the composition prepared without FA (MK-based spheres) the shape of the pH curve is rather distinct from those of FA-containing formulations, being well described by a

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polynomial equation: y = 0.0016 x2 - 0.1501 x + 11.708

(3)

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Moreover the initial pH was slightly lower than that observed for the FA-containing spheres. Most important, the final pH was markedly lower (see Table 2), which indicates reduced amount of free alkalis in this composition. In fact, the total leaching of OH- ions increased by around 78% when 66 wt.% of MK is substituted by FA in the geopolymers

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preparation. This striking increase on the total leaching crucially affects the geopolymers ability to control the pH over time. The higher pH decay (3.7) registered with MK-based spheres indicates a lower pH buffer capacity in comparison with the FA-based spheres,

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which is disadvantageous for this application. On the contrary, the narrow pH fluctuation (1.6-1.9) obtained with the FA-based spheres demonstrates their potential as pH

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regulators. This pH fluctuation is smaller than those reported by Bumanis et al. (2015) (∼3-4) and Bumanis and Bajare (2014) (∼4-5), being similar to that previously reported by the authors (Novais et al., 2016b) (∼0.9-1.5). Nevertheless, as abovementioned the cumulative leaching levels reported in this investigation are also higher. Moreover in this investigation MK was replaced by 75 wt.% biomass FA, which may prevent huge amounts of this unexplored waste from being landfilled.

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ACCEPTED MANUSCRIPT Table 2 presents the geopolymers weight loss after the leaching test. The weight loss was quite similar among the distinct compositions, ranging between 36 and 38%. Despite the significant weight losses all geopolymeric spheres (with MK or FA) preserved their integrity, as demonstrated by the SEM micrographs shown in Fig. 11. The comparison

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between the SEM micrographs taken before (see Fig. 4) and after leaching (Fig. 9) shows that porosity increases, which was associated with the free alkalis leaching from the spheres.

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Fig. 12 presents the EDS spectrum of 50 wt.% FA-containing spheres before and after alkalis test. After 30 days water immersion the intensity of the Na peak was significantly

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reduced which provides evidence of Na leaching, thus explaining the observed porosity increase.

4. Conclusions

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In this work, porous MK-based and biomass FA-based geopolymer spheres were produced using a suspension solidification procedure, and then their pH buffer capacity was evaluated. The pH buffering behaviour has found dependent on the raw materials

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chemical composition. The leaching of OH- ions from the MK-based spheres in the 1st day was high, but significantly decreased in the next few days which is disadvantageous

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for pH buffering applications. On the contrary, FA-based spheres exhibited gradual and prolonged leaching which reduced the pH fluctuation over time from 3.7 (MK-based spheres) to 1.6. The latter was attributed to the significantly higher OH- leaching (up to ∼78%) of the FA-based spheres in comparison with their MK-based counterparts. The narrow pH fluctuation exhibited by the FA spheres, among the lowest ever reported for geopolymers, is quite smaller than those reported by Bumanis et al. (2015) (∼3-4) and

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ACCEPTED MANUSCRIPT Bumanis and Bajare (2014) (∼4-5), which demonstrates their potential as pH buffering materials. Results also show that the content of FA in the geopolymers affects the total OH- ions leaching, higher FA content promoting greater alkalis leaching (up to 54%). The

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maximum alkalis leaching here reported (0.0317 mol/dm3.g) is substantially higher than those reported by Novais et al. (2016) for FA-containing geopolymer monoliths (87%) and by Bajare and Bumanis (2014) for glass-modified AAM (25%). The latter suggest

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their use as pH regulators in anaerobic digestion. These innovative spheres can be easily handled and collected, and due to their gradual and prolonged leaching prevent the need

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of continuous pH adjustment (e.g. sodium hydroxide) increasing process simplicity and efficiency. Nevertheless the comparison between the developed geopolymer spheres and commercial alkaline material (e.g. sodium hydroxide) in bioreactors would be important to quantify the increase on systems efficiency and cost-savings. This topic will be

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addressed in future work.

Furthermore the FA incorporation as raw material in geopolymers (up to 75 wt.%) may

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prevent the landfill disposal of this unexplored waste.

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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.

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Fig. 1. XRD patterns of fly ash (FA) and metakaolin (MK).

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Fig. 2. SEM micrograph and EDS spectrum at two different plots of fly ash. 19

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Fig. 3. Porous FA-based geopolymer spheres prepared using 50 wt.% FA.

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Fig. 4. XRD patterns of geopolymers produced with distinct FA content.

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Fig. 5. SEM micrographs of geopolymer spheres produced with distinct FA incorporation content. The first column corresponds to the exterior surface of the spheres, while the second to the pore microstructure. 22

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Fig. 6. EDS spectrum of FA-based geopolymer spheres prepared with 0 wt.% and 75

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wt.% FA.

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Fig. 7. Cumulative leaching amount of OH- ions from the porous geopolymer spheres produced with distinct FA incorporation content. The insert graph illustrates the

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leaching percentage.

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Fig. 8. Comparison between the cumulative leaching amount of OH- ions from the MK-

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based and the 75 wt.% FA-containing spheres with data from other investigations.

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Fig. 9. Temporal variation of pH of the water solution contacting with distinct FAcontaining geopolymer spheres. The insert graph illustrates the logarithmic and the

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FA, respectively.

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polynomial fitting of pH decay with time for the samples with 33 wt.% FA and without

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Fig. 10. pH of water solution after 1 and 30 days immersion of geopolymer spheres

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prepared with distinct FA incorporation content.

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Fig. 11. SEM micrographs of FA-based geopolymer spheres produced with 0 wt.% (a

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and b) and 50 wt.% (c and d) FA after the leaching test.

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Fig. 12. EDS spectrum of FA-based geopolymer spheres (prepared with 50 wt.% FA)

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before and after alkalis leaching.

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ACCEPTED MANUSCRIPT Table 1 Chemical composition of metakaolin (MK) and fly ash (FA).

FA

SiO2

54.40

18.91

TiO2

1.55

0.45

Al2O3

39.40

5.54

Fe2O3

1.75

2.56

MgO

0.14

3.58

CaO

0.10

MnO

0.01 -

0.85

1.03

5.37

-

4.89

0.06

4.35

-

2.06

LOI

2.66

10.31

Ratio of SiO2/ Al2O3

1.38

3.41

K 2O SO3 P2O5

0.48

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Cl

40.40

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Na2O

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MK

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Components (wt.%)

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Table 2.

st

1 day

30 day after leaching test

11.93

8.24

33

11.78

10.15

50

11.94

10.20

66

12.09

10.24

75

12.08

10.15

1st day (%) 45.2

-

Leaching of OH (mol/dm3.ggeopolymer)

Weight loss after leaching (%)

0.0189

38.0

27.8

0.0219

39.4

30.8

0.0282

37.0

36.7

0.0337

36.0

37.9

0.0317

36.0

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0

Leaching of OH- at

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th

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FA content (wt.%)

pH at

pH at

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pH values, leaching of OH- and weight loss of porous geopolymer spheres after 30 days of immersion in distilled water.

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ACCEPTED MANUSCRIPT Highlights

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• • • •

Porous biomass fly ash-based geopolymer spheres for pH regulation were developed. The raw materials chemical composition dictates the pH buffering capacity. Fly ash-based spheres exhibiting gradual and prolonged leaching were produced. The narrow pH decay suggests their use as pH regulators in anaerobic digestion. Biomass fly ash as raw material in geopolymers ensures waste valorization.

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•