Adding value to aluminosilicate solid wastes to produce adsorbents, catalysts and filtration membranes for water and wastewater treatment

Abstract

Geopolymers are a class of inorganic polymers that have attracted attention in recent years, especially in the construction sector, due to their promising mechanical properties, as well as simple and low-cost fabrication. These materials also stand out for being more environmentally friendly, not only because of their lower CO₂ emissions during production, but also because industrial by-products can be incorporated in their synthesis. Recent studies have investigated porous geopolymers, allowing expansion of their potential use to several other applications. Meanwhile, application of GPs to efficient water and wastewater treatments, such as nanofiltration and advanced oxidation processes, remains a challenge, especially due to high operational costs. Thus, this paper provides a comprehensive review of the current state of knowledge of geopolymers produced from aluminosilicate wastes, showing the main promising advances in their applications in three technological fields: (1) adsorption, (2) membrane filtration and (3) catalysis (as both catalyst or catalyst support).

Graphic abstract

Introduction

Geopolymers are inorganic polymers produced by the polycondensation of aluminosilicate materials, which is promoted by an alkali activator [1]. The reaction consists of three main steps: dissolution, gelation and polycondensation [2]. The reaction begins by the dissolution of the aluminosilicates, which occurs due to hydrolysis (water consumption) of the alkali solution, which forms two distinct monomeric tetrahedral structures: aluminates (AlO₄) and silicates (SiO₄). It is important to point out that geopolymerization is assumed to occur as a consequent dissolution of solid particles at the surface, leading to the release of aluminate and silicate to the solution [3]. These tetrahedral units then begin to link by sharing oxygen atoms, forming polymeric bonds of Si–O–Al–O, resulting in a complex mixture of silicates, aluminates and aluminosilicates. The solution is quickly supersaturated with formed oligomers, leading to a gel formation. Finally, the oligomers condense and eliminate water, developing large network structures, producing an amorphous or semi-crystalline geopolymer, which presents an equilibrium of both negative (Al) and positive (Na) charges [3,4,5,6]. A schematic mechanism is presented in Fig. 1.

Figure 1

Reprinted with permission from [7]. Copyright 2020 Elsevier

Simplified mechanism of geopolymerization.

The great interest in geopolymers research is due to the lower cost of the sources used for their synthesis (such as industrial wastes) and more environmentally friendly production techniques, combined with the possibility to customize their properties to the application of the final material [8, 9]. Among the characteristics of geopolymers, when compared to Portland cement, stand out: their greater mechanical properties (such as compressive, tensile and flexural strengths, elastic performance and fracture failure), fire-proof and better thermal resistance, chemical inertness, improved resistance to carbonation and frosting, greater durability and less shrinkage during drying [7, 10,11,12].

There are several recent review articles about the geopolymeric materials discussing general properties, chemistry, possible raw materials and applications [2, 7, 13,14,15,16,17,18,19,20,21]. Moreover, innovative applications in water and wastewater treatment have been proposed with promisor results in separation processes (adsorption/ion exchange, filtration media and membranes), oxidation processes (peroxidation, ozonation, photocatalytic degradation) or combined processes. Therefore, in this review article, we present a critical panorama about geopolymer uses in separation and reaction processes for water and wastewater treatment.

Most of GP-based materials are based on alkali-activated metakaolin (MK), different types of fly ash (FA), blast furnace slags (BFS) or industrial solid wastes. As the composition and microstructure of these aluminosilicates vary in a wide range, the adsorptive capacity, the textural characteristics or mechanical strength vary widely as well [7, 22]. Recent important reviews describe the effect of the source material and curing conditions on the geopolymer characterization [7, 9, 23,24,25]. However, they did not emphasize the potential applications and drawbacks in membrane separation processes, catalytic membrane applied in advanced oxidation processes or adsorptive processes for water and wastewater treatment.

Metakaolin (MK) is produced by the calcination of kaolinite (KT) at 600–700 °C, which leads to a dehydroxylation of kaolinite (Eq. 1) [26]. MK has been largely applied in GP synthesis, since it has a higher purity than other aluminosilicate materials, such as fly ash [27]. Furthermore, MK is an appropriate source for producing zeolite-like materials, which are promising for application in filtration membranes, due to combined high mechanical strength and elevated adsorptivity [28]. In addition, Rasaki et al. [2] reinforce that MK-based geopolymers may have small particle sizes, high surface area, nanostructure surfaces and effective electron transference, enabling them suitable for use as catalysts or for co-catalyst support.

$$\begin{array}{*{20}l} {{\text{Al}}_{ 2} {\text{O}}_{ 3} \cdot 2 {\text{SiO}}_{ 2} \cdot 2 {\text{H}}_{ 2} {\text{O}}} \hfill & \to \hfill & {{\text{Al}}_{ 2} {\text{O}}_{ 3} \cdot 2 {\text{SiO}}_{ 2} + {\text{ 2H}}_{ 2} {\text{O}} \uparrow } \hfill \\ {\left( {\text{Kaolinite}} \right)} \hfill & {} \hfill & {\left( {\text{Metakaolin}} \right)} \hfill \\ \end{array} \,$$

(1)

Moreover, fly ashes (FA), bottom ashes (BA), palm oil fuel ash (POFA) and blast furnace slags (BFS) have also attracted attention alone or incorporated into GP, due to their low cost (considered a hazardous industrial waste) and appropriate chemical composition. Moreover, Gollakota et al. [23] point out the fineness of FA provides a higher glass phase, therefore generating a greater geopolymerisaton rate, thus producing materials with greater mechanical strength properties. Several high-strength values have been reported for FA-based GP, up to 65 MPa [9, 25, 29]; this is very relevant for applications as concretes/cement, adsorbents and filtration membranes, which can be exposed to elevated pressures. In addition, this kind of waste has already been applied to several different wastewater treatments, to serve as coagulants, adsorbents, membrane filters, Fenton catalysts and photocatalysts [30]. Although, it is worth mentioning that the chemical composition of each type of ash can vary significantly, therefore needing attention on synthesis, in order to produce homogeneous materials [29].

However, the incorporation of some non-conventional sources has also been recently focused on, such as: pozzolan [31], glass [10], natural zeolites [11], biomass fly ash [32], silicomanganese fume [33], magnetite [34], red mud [35] and others. It is important to emphasize that the incorporation of industrial wastes not only can benefit the GP’s overall properties, but also has financial and environmental advantages. The choice of these sources will be influenced by availability, cost and application [9, 36].

Si et al. [10] were able to incorporate significant amounts of waste glass powder (up to 20%) in metakaolin-based geopolymers. The authors found that the increase in this waste led to denser microstructure formation, as well as a smaller porous range distribution, with a well-defined pore diameter peak of 30 nm. In addition, a sample with 10% of waste glass powder remarkably decreased the water loss rate under drying conditions, i.e., it reduced drying shrinkage of the synthesized materials.

Moreover, Rossato et al. [34] synthesized a novel magnetic geopolymer (MGP) by adding 5% of magnetite (Fe₃O₄) to its composition (Fig. 2), this facilitated separation of the solids from the solution with a magnet. The authors also reported impressive adsorption capacity (400 mg g⁻¹) of acid green 16 dye (300 mg L⁻¹), although catalytic activity in this process has not been evaluated. Magnetite has interesting catalytic activity on many AOP [37,38,39]; however, a GP based on this material has not yet been studied, thus their behavior in these reactions remains unknown.

Figure 2

Reprinted with permission from Rossato et al. [34]. Copyright 2020 Elsevier

Magnetization curves of MGP and Fe₃O₄ (a) and MGP particles when attracted by a magnet (b).

Similarly, red mud, which is rich in Al and Fe, has been widely investigated as a catalyst to AOP, for example, as a substitute for Fe²⁺ ions in Fenton-like reactions. Although GP preparations with red mud have not focused on these types of reactions [40]. Hu et al. [35] investigated the role of Fe species during geopolymerization and observed that the binding energies of Al–O and Si–O increased; due to Fe³⁺ replacement by Al³⁺ in the geopolymer matrices. Thus, the fabricated materials showed a potential catalytic application and may also reduce leaching, since the Fe atoms are arranged in the GP’s network.

Furthermore, the activity of the geopolymer is highly associated with the aluminosilicate sources, as is their chemical composition, soluble Si/Al content, particle size, presence of inert particles and glass phase (which is inert in water, requiring the addition of an activator for geopolymerization) [41].

Several alkaline activators can be applied to geopolymerization processes; as consequence of the cations different sizes and charges density, distinct properties are obtained in the final material. Commonly hydroxides (NaOH or KOH) and silicates (Na₂SiO₃ or K₂SiO₃) or a mixture of them are used, because of their lower cost and high efficiency [7]. Normally, K⁺-based activators provide faster solidification than Na⁺, because of their larger ionic radius, although they are also more susceptible to cracking and provide lower porosity [42].

Moreover, Xin et al. [43] showed that concrete GP activated by NaOH and Na₂CO₃ generates more microdefects than that with composed of the combination of NaOH and Na₂SiO₃, which implies that this mixture produces more suitable materials. Other studies have indicated that the combination of Na₂SiO₃:NaOH produces materials with higher compressive strength than each activator alone [41, 44]. The adequate concentration of the activators depends on the aluminosilicate sources, but Zhang et al. [7] reviewed 173 studies and found that the most common proportion is 2–2.5 of Na₂SiO₃:NaOH.

Furthermore, studies have revealed that the concentration of the alkaline activator also influences the final properties of the GP matrix, having an optimum value, like the other precursors. If this value is too low, the dissolution of the aluminosilicates is diminished (few OH⁻ ions in the medium), while a value that is too high decreases the mechanical properties [45, 46].

As mentioned, GP’s microstructure and mechanical properties depend not only on the aluminosilicate sources but also on the composition and concentrations of the alkaline activator, such as, for example, Si/Al, M₂O/H₂O, M₂O/SiO₂ and M₂O/Al₂O₃ molar ratios (where M is the corresponding alkaline cation Na⁺ or K⁺) [36]. Normally, these values follow the parameters Davidovits et al. [47] established as ideal for obtaining high mechanical properties (Table 1).

Table 1 Proposed oxides ratio for GP formulation.

Another influential parameter are the curing conditions, the chosen temperature and its duration will also modify the GP’s mechanical properties. Normally, curing is divided into two periods: heated and room-temperature stages. The heated stage generally occurs at temperatures between 40 and 75 °C for 24 h, in order to initiate the reaction, since higher temperatures are needed to overcome the energetic [48, 49]. Contradictory results have been reported for GP curing temperatures, Aliabdo et al. [50] showed that as temperature increases, compressible strength, tensile strength and elastic modulus initially increase, but decrease after reaching a maximum. Yet, Zhang et al. [48] found similar behavior for compressible strength, while tensile strength had an initial constant with temperature elevation and then decreased. In terms of curing time, it cannot be too short, because nucleation and crystallization processes are reduced, producing a material with low geopolimerization. However, prolonged periods (normally, more than 48 h) cause the granular semi-crystalline structure to break, leading to dehydration and, consequently, gel constriction [23]. Thus, the heat curing time and temperature need to be optimized for each composition, for example for metakaolin-based GP studies found that maximum strength is obtained at 60 °C for 24 h [51, 52].

Yet, the room-temperature step normally takes 7–28 days and water curing is generally applied if the geopolymer usage would be in wet or submersed areas [7]. Furthermore, GP cured under water have higher absorptivity and porosity, although as a consequence the compressive strength is reduced [53].

It is important to comment on the efflorescence phenomena found during GP curing. Efflorescence occurs due to a high concentration of the alkaline activator applied during the synthesis. The unreacted excess diffuses to the material surface where it reacts with atmospheric CO₂ leading to the formation and accumulation of carbonates on the GP surface [54]. This phenomenon not only changes the geopolymers’ appearance, but also their mechanical properties [55]. Thus, strategies are used to minimize efflorescence, such as altering: chemical formulation, particle size, type of activator, additives mixing and hydrothermal cure [56, 57]. An interesting study was conducted by Xue et al. [58], who modified the FA-based GP surface by coating it with octyltriexysilane, transforming its surface from hydrophilic to hydrophobic, which reduced the leaching of ions, suppressing the efflorescence.

As a consequence of the impressive characteristics of GP, they have attracted attention in several industries including: construction [12, 29], aeronautics, aerospace, shipbuilding and automotive [59,60,61,62], nuclear and oil/gas cementing [63, 64], archaeological research [65], acoustic and thermal insulation [66, 67], ceramics [68, 69], pharmaceuticals [70], gaseous and aqueous effluents treatment (by adsorption, membrane filtration and/or chemical catalysis) [34, 71,72,73] and many others.

Davidovits [65] suggested an ideal proportion of Si/Al depending on its application (Table 2), which could be expanded considering the most recent advances in the application of geopolymers, as shown.

Table 2 Proposed oxides ratio for GP formulation.

Due to particular properties, geopolymers are being considered for use in membrane filtration or catalytic reactions. For these applications, some textural and physico-chemical properties must be adjusted, since GPs normally have low porosity and low surface area.

A simple method for increasing porosity is to add a small percentage of a porogenic agent (such as H₂O₂), which generates gaseous products (O₂ and H₂, for example), leading to a more porous material. Other strategies have been investigated, such as direct gas bubbling, the insertion of a sacrificial filler or by introducing an additive to the GP synthesis. However, these methodologies are not often applied since they are complex, expensive and raise environmental concerns [83].

Geopolymeric adsorbents (GPA)

Many studies have been published emphasizing the application of GPs as adsorbents [21, 69, 83,84,85], for this reason, this topic will point out the adsorption mechanisms (also very relevant for membrane filtration), but only some highlights about recent achievements in this field will be briefly discussed.

Adsorption processes promote the removal of the contaminants by using the interactions of the contaminants and a solid surface, in this review geopolymers. The molecules bonding may occur by two distinct forms: physisorption or chemisorption, depending on van der Waals or chemical interactions, respectively [2].

In general, the mechanism of heavy metals adsorption on GPA occurs by physisorption, due to electrostatic interactions or ionic exchange properties of the GP surface properties, which facilitate the adsorbent regeneration by simple or steam washing, chemical or thermal treatment, but also generates negative effect to multiple ions adsorption, due to sites competition [2, 84]. The equilibrium of adsorption is frequently described according to the Langmuir model (for toxic metals removal, such as Zn²⁺, Cu²⁺, Mn²⁺, Pb²⁺) [84] and the adsorptive capacity is comparable to that on natural zeolites [86].

Yet, for other substances, as CO₂ [13, 87] or organic molecules [76, 88, 89] chemisorption has an important role. In fact, Al-Zeer and MacKenzie [76] showed that the chemisorption of pyridine on the Lewis and Brønsted acidic sites on their produced fly ash-based GP contributed on the enhancement to their acylation reactions when compared to other metal-zeolite and metal-mesoporous silicate.

Initially, it is important to mention that most investigations have been conducted with dyes or heavy metals as contaminants, some of which even have superior adsorbent capacity than commercial solids [21, 24, 90]. Extensive and comprehensive reviews have reported several advantages of geopolymeric materials as adsorbents, such as, low cost, high adsorptive and/or ion-exchange capacity and chemical stability [2, 16, 18, 19, 21, 69]. However, as emphasized by Luukkonen et al. [24] other pollutants of interest should also be further evaluated, such as pharmaceuticals, oils and fats, phenolic compounds, micro-pollutants, among others. For example, a remarkable adsorption capacity was found by Siyal et al. [89] in their studies using a geopolymer synthetized with fly ash residues to remove an anionic surfactant (sodium dodecyl benzene sulfonate, SDBS) from water. A maximum removal of 714.3 mg g⁻¹ was measured. These results show promising application for the separation of similar surfactants from wastewaters.

Recently, Song et al. [77] concluded that high adsorption played an important role in oil separation during geopolymeric membrane filtration, since the droplets were affected by van der Waals forces and a hydrophobic effect. Additional studies should be performed using real water (ground water, surface water, etc.) and wastewaters where competitive and simultaneous cations/anions, organic pollutants and natural organic matter could affect adsorption efficiency [6].

Although most applications of GP in adsorption processes are focused on liquid phases, recent studies have shown their potential for applications in gaseous phases. An important and emergent application is the removal of CO₂ to avoid the greenhouse effect [87, 91,92,93]. Minelli et al. demonstrated that their synthesized metakaolin-based geopolymers not only had an adsorption capacity comparable to other materials, but could selectively remove CO₂, since the adsorption capacity of N₂ or CH₄ are low.

Overall, the literature indicates that GPs have a prosperous future as adsorbents. However, impediments remain to their large-scale application, such as their performance when exposed to several different contaminants (industrial wastewater effluents, for example), long-term durability, reusability, the length of regeneration cycles and other operating conditions, etc.

Geopolymeric membranes (GPM)

Membrane filtration is a technology developed to separate molecules according to their characteristics (chemical, size, hydrophilicity, etc.) (Fig. 3) and is normally driven by a pressure difference, allowing some molecules to pass through the membrane (permeate) while retaining others (retentate) [94,95,96].

Figure 3

Reprinted with permission from Khanzada et al. [97]. Copyright 2020 Elsevier

Membrane filtration removal mechanisms: a size exclusion, b hydrophobicity, c electrostatic interaction and d adsorption.

Inorganic membranes have attracted attention due to their advantageous properties: chemical and thermal stability, fouling resistance, mechanical strength and long service life, making them widely applicable, especially those made of ceramic materials [98, 99]. Catalytic membranes have also been developed to enhance removal of contaminants [100], minimize fouling (incrustation accumulation, which reduces transmembrane flow) [101] and provide a self-cleaning material [102].

However, inorganic membranes have a high manufacturing cost and are difficult to model. Thus, geopolymer membrane (GPM) materials offer an opportunity to solve these adversities [73]. Recently, Shao et al. [103] analyzed the fabrication costs of nanofiltration membranes with fly ash-based GPM, which they had synthesized. The authors concluded that the production cost of conventional membranes is over $1000 m⁻², while for the geopolymeric materials, this cost is reduced to $31.8 m⁻². They also emphasized that the operational pressure used in the experiments (0.1 MPa) was the lowest level applied among efficient nanofiltration procedures.

Moreover, Bai and Colombo [69] and Zhu et al. [104] highlight that GPs, especially porous ones, are promising low-cost materials for use in technological applications, such as adsorbents, catalyst supports or filtration membranes for liquids or gases.

There has been an increasing amount of studies involving GPMs and their new applications (Table 3). One application that should be highlighted is separation by pervaporation; a process that has been used for water desalination and ethanol purification. GPMs are suitable for this procedure due to their high thermal resistance and long durability.

Table 3 Geopolymers and their application in water and wastewater treatment

GPMs have also been investigated for several environmental applications such as, air depollution, oil/water separation, as well as the removal of organic pollutants and heavy metals (Table 3). Shao et al. [103], for example, studied fly ash-based GPM in the filtration of several dyes and pharmaceuticals, obtaining impressive separations (greater than 95%).

Geopolymer-based membrane micro-, ultra- and nanofiltration

This section reviews the most important applications of GPMs to aqueous solution filtration. The results are summarized in Table 4. As can be seen, the majority of GPM studies synthesized flat ultraporous membranes. The work of Li el al. [131] is an exception because they produced a membrane composed mainly of nanopores (< 2 nm), not only did the produced material reject 99% of Cd²⁺ ions, but its operating pressure was also relatively low compared to the other studies examined. In terms of the operating pressure of the GPM, He et al. [130] reported the lowest condition. They applied 10 kPa to a fly ash-based geopolymer membrane and were able to remove 91% of Cr⁴⁺ cations. However, due to the small average pore sizes (12 nm) and the low applied pressure, this study also presented one of the lowest permeate fluxes. The sufficient and suitable compressive strengths (≥ 10 MPa) of GPM enable them to be used in filtration processes [28, 103, 115, 123, 130].

Table 4 GPM applied to micro-, ultra- and nanofiltration

Furthermore, high separation efficiency can be achieved using GPM. Xu et al. [127] studied the retention of suspended solids in the green liquor stream (Fig. 4) (from the paper industry) by microfiltration (all pores < 0.28 μm) and found a rejection of ~ 100%, reducing their concentrations from 186 to 0.79 mg L⁻¹, which is far below the requirements for green liquor recycling (< 20 mg L⁻¹).

Figure 4

Reprinted with permission from Xu et al. [127]. Copyright 2019 Elsevier

Support (left) and inorganic composite membrane (right) (a); and green liquor before and after filtration (b).

The applicability of FA-based membranes to remove turbidity, color and suspended solids from produced water [124] and household wastewater [125] was recently reported by Naveed et al. (2019). Besides, significant rejection rate from oils present in the produced water was acquired (78%), reducing its initial very high value (597 mg L⁻¹) [125].

Another relevant issue is the application of GPMs for the removal of dyes or heavy metals from water and wastewater (Table 4). Shao et al. [103] and Song et al. [77] reported especially interesting studies, since they examined not only the types of molecules mentioned, but also the removal of other refractory organic compounds (tetracycline, p-nitrophenol, tetracycline and polystyrene) and water/oil separation (hexadecane). All the results showed high removals (≥ 97%) of the contaminants, except for p-nitrophenol (61%), because of its low molecular size [103].

Moreover, distinct mechanisms were observed for the contaminants removal. Several researches describe the main pathway for the pollutants rejections as a combination of adsorption kinetics with sieve segregation [71, 73, 127, 131], i.e., molecules (larger than the membranes pores) are barred due to size exclusion, simultaneously with molecules bonding into the surface, due to their interactions affinity. Besides, electrostatic interactions were also considered important on membranes filtration [77, 130], Song et al. [77] proposed that organic cationic molecules (crystal violet) and heavy metal ions were attracted by the negatively surface of the GP, being easily removed. At the same time, the authors concluded that, for congo red and crystal violet dyes removal, chelation or hydrogen bonding (hydrophilicity) also had important participation, once the existence of amino, hydroxyl and carboxyl groups on their produced GP facilitated the molecules bonding.

Thus, the reviewed studies showed promising results in terms of pollutants separation efficiency permeate flow/operational pressure relationship, ultra- and nanoporous distribution, versatile removal mechanisms, aside from having low cost and easy production. These results are thus of great interest for further research and possible commercial applications.

Geopolymer-based catalysts

Catalytic geopolymers (CGP) are geopolymers that are functionalized and/or modified to enhance specific reaction kinetics. Mechanical strength, thermal stability, acid-basic properties, permeability and durability are some of the properties for which they are used in catalysis, as catalysts themselves or as catalyst support [83, 135].

Furthermore, the microstructure and morphology of CGPs are also interesting, given that some authors have synthesized GPs with structures similar to zeolites (in terms of surface area, pore size and volume, number of active sites, affinity for reactants and stability) (Fig. 5), which are widely studied for their applications in catalysis [21, 74, 136].

Figure 5

Reprinted with permission from Chen et al. [74]. Copyright 2019 American Chemical Society

Nano-geopolymeric zeolites, where the aluminosilicate source is: faujasite (a), cancrinite (b), sodalite (c) and Linde-Type A (d).

As explained above, during polycondensation, tetrahedral silicates and aluminates connect randomly and, some cations (Na⁺, K⁺ and Ca²⁺) simultaneously interact with them, promoting an electronic balance. Thus, in some studies, these cations are substituted by other transition metal and/or rare metal cations to produce CGPs with enhanced activity [85].

The most common methods for incorporating these metals in the GP are ion exchange, surface coating/impregnation, incorporation into the composition during synthesis and thermal activation. These treatments alter the materials’ porosity, surface area, available active sites, band gap, electronic surface charge and many other properties, which improve their catalytic activity [2, 85].

Few studies have been done with CGP, a search in SCOPUS with the words catalytic AND geopolymer yielded only 54 results in the past 10 years. Although there was a sharp increase, in 2019 when 11 studies were published (~ 20%) [137]. Some of the publications on CGP are listed in Table 5.

Table 5 Summary of geopolymer-based catalysts and their application

In a very recent work, Chen et al. [71] demonstrated that the incorporation of Cr₂O₃ not only enhanced the membrane flux (compared to an undoped process) with high rejection (~ 100%), but also presented low fouling during the filtration process. Moreover, the incorporation of the catalyst led to a high photocatalytic performance for the degradation of green dye (30 mg L⁻¹), and almost complete oxidation was obtained in 100 min.

Considering the studies reported in the literature (Table 5), geopolymer-based catalytic membranes are promising, since they combine adjustable porosity, diverse morphology, easy modulation, excellent mechanical and thermal properties and are simple to prepare, environmentally friendly and have lower manufacturing costs.

Similarly to the application of CGP as membranes, the main advantage of using these materials for catalyst support is their mechanical and thermal characteristics, as well as high surface area, porosity and, in some cases, photo-activity, which increase interest in these materials [69, 72]. Asim et al. [85] emphasize that the surface modification of geopolymers is beneficial to their catalytic applications.

Several of the studies cited (Table 5) used a catalyst supported in GP for organic syntheses reactions. Alzeer and MacKenzie [166] reported that the CGP produced (with NH₄⁺ ion exchange) had superior catalytic activity than other commonly used aluminosilicate supports such as zeolite M and mesoporous molecular sieves.

The production of H₂ has also been studied through the application of CGP materials (Table 5), mainly by the incorporation of metal oxides (CeO₂, CuO, In₂O₃–NiO, CaWO₄) or graphene as photocatalysts. These materials are able to act as electron acceptors and improve the catalytic performance of geopolymers, consequently raising the reaction rate.

Zhang et al. [176] studied both H₂ production and dye removal using ZnO/graphene CGP. The H₂ yield was up to 30% higher due to the incorporation of ZnO, because of the synergistic effects of this semiconductor with graphene and the geopolymer composition itself. Zhang et al. [176] also noted a considerable separation of photo-generating e⁻/h⁺ pairs, promoting efficient reduction reactions. Moreover, they observed that an incorporation of 15 wt% of ZnO to the geopolymer enhanced the degradation kinetics constant by approximately 17 times, possessing a high activity under visible light.

Moreover, the majority of the studies utilized CPG in photocatalysis (Table 5), promoted high oxidation of dyes, volatile molecules and nitrogen oxides [34, 71, 72, 143, 157, 158]. Metal oxides, metal alloys and sulfides were used to coat GP and enhanced their absorbance under visible light [78, 148, 157]. Ancora et al. [139] and Chen et al. [75] reported impressive photocatalytic performance not only on the surface, but also in the bulk of the material, as a consequence of its high porosity.

As seen above, the chemical composition, porosity and water adsorption make geopolymers suitable for other AOP reactions as catalysts or for catalyst support, a still little explored field. Studies that applied CGPs for these purposes will be discussed below.

Geopolymer-based catalysts applied to advanced oxidation processes

Initially, it is important to point out that some authors did not introduce catalysts to their geopolymeric materials (Table 6), showing photocatalytic activity due to the composition of raw materials used in their synthesis [88, 143, 151]. These three studies found a high degradation of dye compounds (≥ 96%) when exposed to UV light, which is attributed in particular to the presence of Fe₂O₃ in their composition, a widely studied semiconductor, since it is rapidly excited by electron transfer and has a relatively low band gap [183].

Table 6 Photocatalytic and Fenton-like degradation results by catalytic geopolymers application

In addition, only one CGP has been applied to a Fenton-like reaction (Table 5), all of the other studies focused on photocatalysis. Huang et al. [178] produced a biochar/geopolymer composite membrane (denominated as BC/GM), coated with alkaline lignin, which was simultaneously carbonated and self-activated. It was applied to a highly oxidizing reaction in the presence of H₂O₂ (a Fenton-like process). Pharmaceutical tetracycline (50 mg L⁻¹) was almost completely degraded with 150 mg L⁻¹ of the GP (10 mL L⁻¹ H₂O₂, pH 5.0, 60 °C, 5 h). Moreover, electron paramagnetic resonance (EPR) analysis revealed the formation of a hydroxyl radical (·OH) by solid materials. The authors concluded that graphitized carbon, ketone, quinone moieties and defect structures contributed to the generation of radicals. The proposed reaction mechanism is presented in Fig. 6.

Figure 6

Reprinted with permission from Huang et al. [178]. Copyright 2020 Elsevier

Schematic mechanism of ·OH radicals formation and tetracycline degradation.

As expected, the catalyst most commonly added to photocatalytic GP is TiO₂ (Table 6). The reported results showed a lower degradation or a higher reaction time than other studies that utilized distinct catalysts, probably a consequence of TiO₂’s high band gap (~ 3.20 eV).

To overcome this difficulty, Falah et al. (2015) [149] used the Cu₂O/TiO₂ heterojunction in the synthesized CGP, producing a material with a reduced band gap of 2.17 eV, which led to high removal of methylene blue (1000 mg L⁻¹, 98%). The incorporation of other metal oxides was also studied by Kang et al. [156], Zhang et al. [170] and Chen et al. [71] (CeO₂, CdO/graphene and Cr,₂O₃ respectively) and showed an almost complete degradation of three different contaminants, malachite green, direct fast bordeaux dye and basic green, respectively.

Moreover, graphene is commonly reported for CGP (Table 5), in general as part of the composite, producing catalysts that act with Z-scheme mechanisms, which enhances photocatalytic activity by promoting electron transfers between the higher conduction band and the lower valence band of each semiconductor, resulting in elevated generation of e⁻/h⁺ pairs and consequent higher degradation potentials [184].

The work of Zhang et al. [171] also deserves emphasis. These authors incorporated carbon black (CB) into their GP samples, the presence of this compound produced a solid with one of the lowest reported band gaps for CGP (2.53 eV) and also increased the absorption in the visible region (Table 6 and Fig. 7). This is highly associated with the increased electroconductivity of the photocatalysts, improving the e⁻/h⁺ pair segregation by facilitating transmission to the network of CB.

Figure 7

Reprinted with permission from Zhang et al. [171]. Copyright 2018 Multidisciplinary Digital Publishing Institute

UV–Vis diffuse reflectance spectra of the samples with different amounts of CB.

Unfortunately, studies that only examined TiO₂ semiconductor loading did not calculate the band gap values to provide a better comparison. Actually, few investigations reported this parameter value, 7 of the 26 (Table 6), which is probably because GPs are composed of several materials, making it difficult to estimate it. In addition, the reported values considered the GP to be a pure semiconductor, to facilitate obtaining its value [88].

Furthermore, although few studies were conducted under visible light, their results were promising. Zhang et al. [176], Chen et al. [71] and Maiti et al. [179] observed organic contaminant degradation of 93%, ~ 100% and 95%, respectively. The band gap of CGP used by Zhang et al. [176] is considered high, yet the material demonstrated great photocatalytic efficiency.

It is also important to comment on the mechanical properties of the CGP. Various studies demonstrated enhanced values of compressible strength and flexural modulus [78, 144, 159, 171, 176, 179], many of them even higher than those reported for membrane filtration applications. Zhang et al. [144, 176], for example, obtained, in this order, 86 MPa and 59 MPa of compressive strength, while the flexural strengthens were 2 MPa and 5.2 MPa, respectively. It is known that some compounds such as TiO₂ and graphene are able to increase these properties [155, 182], which make them very interesting for application as catalytic membranes.

The literature review indicated that few studies have applied CGP to photocatalysis or peroxidation and none in another AOP. Moreover, these studies focused mainly on the degradation of dyes, while other pollutants have been little explored. Thus, it is not only important to study CGP performance in other advanced oxidation processes, but also the behavior of other contaminants when submitted to these materials, as emerging contaminants, which have been widely reported to degrade with AOP.

Final remarks

In general, from the results presented in this review of the literature, it can be concluded that geopolymeric materials are promising for a number of their characteristics (compressive and flexural strength, thermal resistance, porosity, water absorption, electronic transference, surface area, catalytic activity, etc.). Thus, they appear as an effective solution for minimizing costs for both membrane filtration and advanced oxidation processes, making them more economically viable. This is not only because of the lower production costs for GPMs, but also due to the fact they possibly need none or lower catalyst loads, as well as shorter time and lower temperature conditions in manufacturing than others adsorbents, inorganic membranes precursors or catalyst supports, such as zeolites.

Therefore, a combination of these processes using catalytic geopolymeric membranes may be used in the future for many applications, including water and wastewater treatments. However, more research is still needed in this field, since few studies have been published that consider large-scale applications. In particular, studies involving other important pollutants, different AOP reactions (aside from photocatalysis), metal leaching, reuse, maintenance costs (replacement of materials, cleaning, fouling, etc.), chemical resistance and synthesis homogeneity (as a consequence of the variability of the industrial waste samples) would be important to clarify some essential issues that would allow further expanding development of GPMs.