**2. Raw materials and preparation of GP/AAM materials for water treatment applications**

This section summarizes different types of aluminosilicate precursors, occuring naturally or derived from industrial processes. Materials, which are currently abundant and/or urgent to dispose of, fall within the ambit of the section, but cover only water treatment applications not geopolymer production for construction industry, e.g. substitutes for Portland cement or as tailings' covering.

### **2.1 Conventional and new sources of aluminosilicate precursors for GP/AAM preparation**

**Ashes.** *Fly ashes (FA)* are prominent materials used as alkaline-activation binders. FAs are abundant yet complex materials, the composition of which is affected by co-incinerated material. Produced mostly by coal-fired electric and steam generating plants, coal FAs represent the greater part of generated FAs with the estimated flow of approximately 750 Mt. in 2015. The utilization rates of FAs

differ greatly worldwide. Thus, rates in the developed countries equal 96%, 85%, 50% and 65% from produced streams for Japan, Germany, UK and USA, respectively [7]. For the developing countries with growing economics such India and China, the utilization rates are 38% and 45%, yet rate 66% for Asia in general are reported. Russian Federation and Africa implement FA as secondary resource less than 20% [7].

The composition of FA varies widely as it is derived initially from various primary sources: municipal waste/sludge co-incineration, different coal types, or subspecialized byproducts from industrial treatment plant (paper, forestry industry or agriculture). The combustion and cooling processes have profound impact on the characteristics of FA (particles size, shape, surface area, uniformity, etc.) as well as its composition and impurities' inclusion.

Mainly, ASTM C 618 specification is applied to indicate the class of FA used for geopolymer preparation; however, a local/field or an unspecified labelling is also common. Coal FA (class F [8, 9] and C [10]) has been extensively considered as an aluminosilicate source for GP production, while the exploitation of biomass and coincinerated FAs is less common [11, 12]. On the other hand, the utilization of these FAs particularly in the GP production for water treatment sector might be also beneficial. It would reduce the FA accumulation in landfills, and improve adsorbents' LCA in comparison with metakaolin-based GPs.

Although FAs were studied as adsorptive materials previously [7, 13, 14], concerns on potential toxicity of impurities and convenience of use have encouraged to seek more suitable forms of FA-based materials for water treatment sector.

*Municipal waste incineration bottom ash* (IBA) has been traditionally considered as solid waste [15, 16]**.** IBA is mainly composed of Si, Al, Ca, and Na oxides, and could be classified as a hazardous or non-hazardous waste depending on the amount of toxic metal(oid)s. IBA's main applications are in the engineering field as secondary materials in form of weathered bottom ash (after outdoor ageing for 2–3 months for pH stabilization). However, new applications of IBA have emerged in recent years as use as an aluminosilicate source for GPs/AAMs [17] including adsorbents [15, 18–20]. IBA by itself also was investigated as an adsorptive material for metal removal [21, 22]. In most of the studies, mixtures of IBA with various aluminosilicate precursors (BFS, FA, metakaolin) are used in order to obtain desired mechanical characteristics [23]. For the water treatment applications, IBA as raw material for GPs could have hidden benefits as the aluminum presented in it reacts with the alkaline activator and forms hydrogen gas, which leads to an increase in porosity [24]. Moreover, since a compressive strength of the resulting materials could be lower than for construction applications, the high porosity of materials and *in situ* stabilization of concomitant hazardous impurities via encapsulation could be attractive options [9, 19].

Pre-treatment of FAs and IBAs with various chemicals were suggested in order to reduce their toxicity and to meet the environment requirements of pristine materials or/and GPs/AAMs based on them [14, 25–27].

**Steel industry waste.** *Blast furnace slag (BFS), dust, and sludge.* BFS is another copious industrial nonmetallic by-product that is used widely as GP precursor in civil engineering [28]. Similar to ashes, steel industry wastes are mainly composed of Si and Al oxides, while Mg and Ca oxides could consist up to 35–60% of the material by weight [29]. Iron and sulfur are the major impurities in BFS, derived from the iron-smelting process. Ground granulated BFS has a high specific area due to the small particle size distribution, which makes it an excellent candidate as adsorptive material [13, 30]. However, in order to avoid the leaching of heavy metals from BFS during deployment of these materials in water remediation techniques, alkaline activation were suggested for entrapment/binding impurities

within the GPs' matrix [31–34]. The presence of significant amounts of silica, aluminosilicates, and calcium-alumina-silicates in a pristine material makes the geopolymerization process rapid and effective resulting in rigid and enduring compositions. Thus, BFS was used to enhance the stability of metakaolin- and FA-based GPs [35].

*Basic oxygen and electric arc furnace slag.* These materials are sub-categorized, depending on the process of their formation. Both basic oxygen furnace slag (BOFS) and electric arc furnace slag (EAFS) are formed during the steelmaking process [36, 37]. These types of slags are similar in composition to BFS, except for their iron, manganese, chromium and sulfur contents, which are higher in BOFS and EAFS. EAFS was modified by alkali activation [38] and its adsorption properties towards copper were compared to raw EAFS*.* Significant amount of posnjakite were detected in the crystalline phase after adsorption of copper that could explain the drastically high removal efficiency of this AAM.

The accumulation of BOFS has become a significant issue due to its generation in large quantity, high disposal costs, and unsuitableness in cement industry due to high iron oxide content. Sarkar et al. adopted BOFS as a raw material for obtaining of GPs and investigated Ni2+ [39], Zn2+ [40], and F [41] removal. BOFS was used by Sithole et al. as a precursor for AAM preparation [42, 43]. In order to achieve highly porous structures for percolation column tests, a foaming agent (hydrogen peroxide) was added.

**Red mud, silica fume and ore materials.** *Bauxite* is a sedimentary rock, rich in aluminum oxide minerals and accompanied by kaolinite, quartz, and iron oxides. The amount of impurities varies depending on the place of origin. Bauxite itself has recently been tested in water treatment applications for purification of fluor- and arsenic-contaminated waters [44, 45]. However, even keener interest is observed in the valorization of bauxite residues, rich in iron and aluminum, as GPs/AAMs [46] and their water purification applications [47, 48].

*Alumina* manufactured through Bayer process from bauxite mainly goes to aluminum metal production. The rest (up to 10% of the whole production flow) is used as fillers in construction, in glass production, abrasive materials, and catalysts. Depending on the field of application, aluminum oxide could be called alumina, aloxide, aloxite, or alundum, and their respective waste materials would conform to these terms. Aloxite, for instance, is used as catalyst and/or catalyst support in organic chemistry due to its physicochemical stability and unique surface properties. As waste material, it could be accumulated in immense amounts from gas purification, decolorization, and catalytic processes as well as refining and desulfurization of petroleum oils and waxes. One of the ways of its valorization is in the design of eco-friendly GPs [49] and adsorbents for wastewater treatment [50, 51]. Thus, the addition of aloxite to analcime before geopolymerization showed an increase in the specific surface area and pore volume [50].

*Silica fume* is an amorphous form of silicon dioxide, collected as a by-product of the silicon and ferrosilicon alloy production. Historically, the main field of application is as pozzolanic material for high performance concrete of high strength and low porosity, though its applications for designing of foamed GPs and immobilization of cesium are reported [52, 53].

*Mine tailings* are mining and mineral wastes. The proper disposal of tailings has gotten under strong scrutiny for environmental preservation during the last decades. In many cases, the tailings are fine particles, containing silica together with iron oxides, alumina, and other minor minerals. This constitution makes tailings an excellent source of material for GPs. Iron ore tailings were mixed with FA to produce GP for copper removal [54], while gold mine tailings with Al2O3 additive were the source for GP production used for lead removal [51]. Prophyllite waste

materials obtained from mine were converted to prophyllite GPs by Panda et al. [55], and tested as adsorbent for Co, Cd, Ni, and Pb removal from model solutions. Magnesite tailings from talcum mines show a reactivity dependent on the calcination temperature: light burnt (700 – 1000°C), hard burnt (1000 – 1500°C), and dead burnt (1500 – 2000°C). For use in water treatment and as a GP precursor, a light burnt magnesite in the form of periclase MgO is suitable.

**Natural materials: zeolites, clays, sedimentary rocks.** Kaolin is a rock rich in kaolinite, a clay mineral, and the source of production for the most widely used GP precursor called metakaolin. Metakaolin, a disordered, activated, and dehydroxylated form of kaolinite, is obtained through calcination of kaolinite over 600°C. The temperature of calcination has direct impact on reactivity of metakaolin, and as a consequence, on the crystallinity of the produced GPs [56]. However, raw kaolin/kaolinite were also used in GP production [57], yet for water treatment applications just recently [58]. Moreover, valorization of spent metakaolin could be beneficial and decline the cost of metakaolin-based GP [59]. Other clay materials have also been utilized recently for GP design [60]. Thus, bentonite clay calcined at 700–800°C were used by Maleki et al. [61] for obtaining a magnetic GP for heavy metal removal. Laterite clay-based GP were proposed by Ghani et al. [62] as a promising adsorptive material for Ni and Co removal. Laterite was activated at 900°C prior the geopolymerization. Volcanic tuff is another naturally available material with high porosity and with a high potential for ionexchange. It was used in [63, 64] as an abundant yet low-cost material for GP production, and subsequent Zn removal from water.

*Zeolites and zeolitic materials* are well-known microporous materials. Found in nature or obtained through synthetic procedure, they are considered to be selective adsorbents [65–67], catalysts [68], carriers in biotreatment [69] due to their unique structure. Although, naturally occurred zeolites are readily available, they generally show lower surface area than synthetic ones.

Recently, much attention has also been paid on how zeolite could be synthesized from low-cost materials [70]. GP-zeolite composites and zeolite-like GPs are two different categories of adsorptive materials, which have recently attracted increased interest [71]. GP-zeolite composites are hybrid materials, unite the advantages of both constituents. The GP here serves as a durable support, while the zeolite provides a high surface area, porosity, and adsorption capacity. For instance, metakaolinite–zeolitic tuff GPs have been proposed in [72]. The report clearly showed the beneficial influence of the zeolitic tuff addition into a starting mixture on the microstructure and the adsorption potential of GPs. Andrejkovičová et al. [4] prepared metakaolin-based GPs blended with by 25, 50 and 75% of Nižný Hrabovec zeolite. It was shown that the zeolite particles are responsible for the higher amount of crystalline phases, producing a more compact and firm microstructure of blended GPs. The amount of blender has significant influence on the order of adsorbed metals and on the adsorption capacities of the formulations. Hayashi et al. [63] incorporated clinoptilolite into GPs though sol–gel protocol in order to further use of the resulting coatings for heavy metal ion adsorption.

It should be noted that zeolitic phase could be incorporated into GPs'structures not only externally. Zeolite-like crystalline phases could be derived from synthesis routes through fusion method or even at moderate temperatures leading to zeolitelike GP structures. Javadian et al. [64] converted FA into a mesoporous aluminosilicate adsorbent through a fusion method at 600°C. Deng et al. showed that a hydrothermal synthesis of zeolite-like materials from IBA with higher crystallinity than through a fusion method is possible [73]. Similarly, Visa [74] converted FA into zeolite through a hydrothermal process. Rios et al. synthesized zeolite-like GPs from metakaoline at 100°C through the hydrothermal procedure [75]. Studies reported

indicate that such materials have higher surface area and porosity than GPs/AAMs obtained through simple alkaline activation. Although the ultimate set of preferable conditions to form a GP instead of a zeolite are still under discussion, ratios Si:Al > 1.5 have been empirically established as providing more amorphous structures [60].

*α-Analcime* is a reject from spodumene refining at a Finnish lithium hydroxide plant, currently in piloting stage, and estimated to start the production in 2024, but is also found as a natural zeolite [3]. The small cavity size of analcime facilitates ionexchange only for small mono- and divalent cations such as ammonium and Cu2+, and also K<sup>+</sup> , Ag+ , Tl+ , Rb+ (at elevated temperatures), and with low adsorption capacity. Raw analcime is inert to alkaline activation and analcime requires either chemical activation by 3–5MH2SO4 or thermal activation above 700°C [3].

Not infrequently, industrial side streams cannot be used alone for geopolymerisation due to disharmonious Si/Al molar ratios. Therefore, by-products are commonly used as mixtures of aluminosilicate sources [76]. **Table 1** summarizes the studies on different compositions of GPs/AAM that have been proposed for water and wastewater treatment applications. An afford was made to collect and match the precursors, synthetic protocol specificity, and distinctive characteristics resulting materials.

#### **2.2 Forms and manufacturing techniques of GPs/AAMs for water treatment**

Originally, a basic composition applied for manufacturing GP/AAM adsorbents consisted of an alumosilicate precursor, an alkali, and an additional source of silicate in a form of water glass. Initially, both sodium and potassium forms of alkaline activators were used to induce geopolymerization. In the vast majority of the research reviewed, sodium alkaline and water glass are used in the activation process. It was shown by Bakharev that dissolution rates of the minerals was higher when a sodium form is used [148]. Luukkonen et al. [149] found that adsorption characteristics of metakaolin-based GP prepared with NaOH is better than with KOH in case of ammonium removal. An in-depth discussion of G chemistry and vivid explanations could be found in the latest reviews [57, 150, 151].

Forms and manufacturing techniques of GPs/AAMs for water treatment application are emerging and evolving constantly. In the first instance, *powdered forms* were mostly used for gaining of adsorption characteristics of materials. At first, GPs/AAMs were manufactured in bulk forms to be crashed to powder or rubbles after curing procedures. However, these materials have relatively low porosity, and addition of foaming agents were appealing for increasing surface area, pore volume, and porosity. In **Table 1** forms of GPs/AAMs reported and specific additives listed along aside with their synthetic procedures and properties gained. For bulk samples, species are usually sealed with plastic films to prevent moisture evaporation since the presence of water increases porosity. Curing and aging are usually carried out at temperatures ranging from 20 to 105°C. Commonly, an industrial application of powdered forms requires instance pressure filtration and an additional separation step after adsorbent exhaustion. Both these processes increase a cost of treatment, its complexity, limiting a regeneration ability causing sludge accumulation.

*Granular forms of GPs/AAMs* are more preferable for large-scale applications. FA-based GPs supported on inert substrate were proposed with the aim of overcome these limitations [107]. A simple technique similar to the conventional GP preparation was applied to design floatable light granules for phosphorous removal. Moreover, spherical granules could be produced by *in situ* geopolymerization during granulation by a high-sheer granulator, where a liquid alkaline activator acts as a binding liquid [59, 152]. A granule size distribution is a function of a liquid to solid ratio, granulation time, and a rotation speed. While the amount of liquid required



#### *Advances in Geopolymer-Zeolite Composites - Synthesis and Characterization*







*Advances in Geopolymer-Zeolite Composites - Synthesis and Characterization*



**Table 1.**

*\*\*Calculated*

 *using the XRF of product.*

*GP/AAMs compositions for water and wastewater treatment reported in literature.*

depends on the wetting behavior and particle size of the precursors, a good starting point is L/S of 1/3. The production method is easy to upscale that makes feasible large-scale water treatment applications with granular GPs. A more complicated procedure enables the development of granular adsorbents through a suspension and solidification method, resulting in microspheres (<100 μm) [33, 34] or highly porous GPs [93]. Composites of GPs with biopolymers, for instance alginate that possesses the ability to solidification in presence of calcium ions, were also obtained in granulated form [83, 93].

*Porous GP/AAM adsorbents* contain relatively high volume of voids or pores. The pore sizes usually range from nanometers up to millimeters with the total pore volume ranging from 30 to 90% [153]. Direct foaming, either chemically or mechanically, is seen as the most widely applied foaming approach. The common additives that have been used in the direct foaming methods are hydrogen peroxide [11, 33, 43, 54, 78, 93], Al [131, 154] or Si [155] powders along aside with stabilization agents such surfactants or oils.

*Pervious GP/AAM* is another promising form of an adsorptive material for water purification. Thus, AAM-based membranes with potential to remove alkaline earth metals [156], and nickel [157] have been reported*.* Development of porous/pervious GPs/AAMs led to variety of fabricated adsorptive forms such as monoliths, membranes, granules, and self-supported filters. That is opening the versatility of approaches, conventional in water and wastewater treatment practice, yet challenging if the powdered forms used. Separation, regeneration, and surface modification are no longer restricted by the form of production. Porous/pervious materials can be used directly in packed bed adsorbers, and be easily regenerated or retrieved after adsorbent saturation with target substances or contaminants.
