Synthesis of geopolymer for the removal of hazardous waste: a review

Water is the most essential material on Earth; any pollution of water can have adverse effects on living beings. Today, the elimination of harmful pollutants from water is one of the most critical concerns; in this respect, the application of a minimum quantity of chemicals or the use of natural substances for pollutant elimination is of serious importance. This review discusses the study work that is carried out on adsorbent synthesis from different natural Substances, such as (kaolinite, metakaolin, fly ash, etc). This adsorbent is known as geopolymer. An attention to the geopolymer was paid to its specific three-dimensional network structure, with defined pore sizes. The review shall include the elemental and sorts of materials used in the creation of adsorbents and the impact of parameters like as (pH, contact time, adsorbent dose, temperature and initial concentration) on the adsorption capacity of hazardous waste. In addition to enhance the adsorption characteristics of natural substances and producing high-quality adsorbents, different synthesis techniques have been produced. One of the most successful methods is to precipitate nanoparticles on the geopolymer surface. This development would improve the adsorbent surface area, mechanical strength and resistance to chemical compounds, by means of that improving its ability for adsorption of contaminants.


Introduction
A great variety of contaminants such as heavy metals, [1], dyes, pharmaceuticals [2] [3], pesticides, surfactants and home healthcare items. have polluted water supplies. These contaminants are environmentally harmful to humans and animals [4]. Various wastewater treatment techniques like flocculation, membrane processes, adsorption, absorption, ion exchange, electrolytic isolation, reduction, and reverse osmosis techniques. However, these systems have a variety of drawbacks, i.e., expensive facilities, higher maintenance and high operating costs. [5]. Adsorption is considered to be the most effective method for removing contaminants in wastewater, because it is simple and costeffective method [6]. The adsorbent matter can be mineral, organic or biological. Activated carbon is the preferred, conventional material at the industrial scale [7]. Recently, most of the materials used as adsorbent, like fly ash [8], metakaolin [9], kaolin and slag [10] Can be used as one of the developing materials as an adsorbent based geopolymer [11]. Davidovits was the first person who coined the term geopolymer in 1978 to characterize a wide materials range, which characterized by inorganic molecules' networks or chains. Geopolymers a promising category of materials is intended to remove toxic substances from industrial and household waste. Over the last decade, alkali-activation (or geopolymer) technology has gained a lot of interest in its possible applications in the treatment of water and waste water. [12]. The characteristics of geopolymers depend mainly on the properties of the precursor materials (chemical composition, glass phase, the quantity of soluble silicon and aluminum, morphology of particles, mineralogical structure, distribution of particle size, and existence of inert particles). The excellent foundation materials should have sufficient spherical glass bead content and have an extremely amorphous structure. [13]. For the reason that many adsorbents are inexpensive and have lower resistance to environmental variations like as pH. In this sense, improvement of the surface by chemically resistant materials of cost effective natural adsorbents like geopolymers is considered to be a key factor in overcoming these challenges [14]. The aim of this review, is to present the production of a low-cost adsorptive media for the removal of hazardous waste and the precursor for the synthesis of this media from waste or raw material with high capacity, uniform properties and ease of preparation such as geopolymer. This review shows effect of modification on the geopolymer and other natural materials on the adsorption capacity by precipitating or coating of nanomaterial on their surface.

Geopolymer structure
The suggested terminology of the geopolymer structure categorizes geopolymers into three elementary forms based on their Si/Al ratios, which are poly (sialate-disiloxo), poly (sialate-siloxo) and poly (sialate), as seen in the Figure 1

Synthesis of geopolymer
There are two synthesize routes used in the synthesis of geopolymer [16].
1-Alkali route: the geopolymer is synthesized in this route using alkali medium containing one or two or more hydroxide solutions (Na+, Ca+7+, K+, Cs+). 2-Acidic route: in which phosphoric acid (H3PO4) is used for the development of polyaluminum phosphate geopolymer as a medium The most suggested mechanism for geopolymer synthesis shall include the following four stages.: [17] 1) Dissolution of Si and Al from solid alumino silicate materials in extremely alkaline aqueous solution as shown in equation (1) Where: M symbolize Na or K. 2) Creation of Si and / or Si-Al oligomers in the aqueous phase. Equations (2), (3) and (4) 3) Polycondensation of the oligomers to form a three-dimensional aluminosilicate framework. as it is presented by chemical Equations (5a) and (5b): Linking of the solid particles into the geopolymeric structure and hardening into a final solid polymeric structure of the overall system. Within the Chemical Equation (6).
Where: (< T) indicate to Si surface or Al sites Geopolymers are inorganic polymeric materials produced by the alkali activation of aluminosilicate materials in metakaolin, fly ash and granulated blast furnace slag (GBFS). Widely used alkali activators contain sodium and potassium hydroxide with sodium and potassium silicate is also applied to geopolymer preparation [11]. The formulation of geopolymers involves different stages: the release of silicate and a luminate, gelation, polymerization and hardening. There is various material used as precursor for synthesis geopolymer as shown below:

Kaolinite
Kaolinite (Kaol) is primarily a layered white clay mineral denoted as shown in figure (2) by the chemical composition Al2Si2O5(OH)4 [18]. Impurities or significant amounts of other minerals such as montmorillonite or iron oxides are commonly found in colored forms of kaolinite clay. Kaolinite formation occurs by hydrothermal alteration or the breaking down of acid igneous rocks containing silicate aluminum, such as feldspars and muscovites. As well as it can be used in granite and gneiss. Kaolinite-rich rock is usually referred to as kaolinite [19]. Kaolin is a phyllosilicate, composed of alternating layers of silica and aluminum in octahedral and tetrahedral coordination. This electrically neutral crystalline layer formation, which is a typical feature of clay minerals, contributes to a fine particle size and a plate-like morphology, allowing the particles to pass through. Easily across each other, Giving rise to physical properties such as softness, soapy feeling and basic cleavage [20].

Metakaolinite
Metakaolin rather than kaolinite as a source material provides specific advantages in terms of high reactivity and purity. [21]. Metakaolin is an anhydrous aluminosilicate clay mineral with the chemical composition Al2Si2O5(OH)4 formed by tetrahedral silica layer and one octahedral alumina layer. This also results in a geopolymer with higher compressive strength, high surface area and large porous surfaces [22]. The properties i.e. purity, crystalline high surface areas, and metal oxides) make geopolymers based on metakaolin a very interesting group. [23] [24].Table (1) illustrated the chemical composition of metakaolin based geopolymer adsorbent.

Fly ash
Fly ash is one of the residues produced during the combustion of coal in the coal power plant [26]. Fly ash performance in geopolymers is highly affected by its physical, chemical and mineralogical properties. The mineralogical and chemical composition as shown in Table 1 depends mainly on coal composition [27]. Fly ash can be classified into two groups depending on its source and constitution. Class F of the fly ash is generated of the burning of bituminous or anthracite coal and follows this chemical composition ( 2 + 2 3 + 2 3 ) ≥ "70%" as shown in Table 2. Class C is usually formed from the combustion of sub-bituminous or lignite coal and has the following chemical composition. ( 2 + 2 3 + 2 3 ) ≥ "50%" [28].
[29]prepare geopolymer successfully from calcined fly ash and using it for adsorption of various pollutant.

Slag
Blast furnace slag (BFS) is a non-metallic residual material used in the manufacture of steel. Blast furnaces are fed with controlled mixtures of iron ore ( 2 3 + 2 ), coke (C), and limestone ( 3 ), and work at temperatures about 2000°C. The resulting material is steel and residual slag [30].
[31] used a blast furnace slag (BFS) with a fly ash (FA) based geopolymer adsorbent to remove 2 + from aqueous solutions which were synthesized using the hydrothermal process at 60 C for 24 h, and then cured at 25 C for a further six days. [10] use the metakaoline with slag as the base of the geopolymer with the chemical composition of the slag. as shown in table (3): -

Rice husk ash
Because of the raised generation of biomass ash as a waste of thermal biomass. alternative reuse mechanisms for ash helps to verify the economic viability of biomass-based energy [32]. Rice husk ash is one of the wastes of biomass used as the precursor for geopolymer synthesis with other raw material such as metakaolin [33] and fly ash. [32] determined the effect of the biomass ash combine on the main engineering characteristics of the geopolymer and the conformity of these characteristics with industrial standards.

Adsorption experiments on the geopolymer
Recent times, geopolymers have been commonly used for wastewater treatment due to porous composition and relatively inexpensive. Geopolymers have also been successfully used for the adsorption of heavy metals such as Cd, Ni, Pb ions, boron, fluoride, phosphate, radionuclide 137 Cs and 90 Sr, 2+ , and 2+ [34] [35] and dyes like methylene blue (MB) [36].Table (4) illustrated the effect of using geopolymer from different precursor for treatment of different types of adsorbate on adsorption capacity.

Modification on the natural material
Recently, several researchers have been concentrating their attention to enhance adsorption properties of natural materials and developing high-quality adsorbents. A variety of synthesis techniques have been developed to address this issue. One of the most successful methods was to coat the adsorbent surface with nanoparticles. This development would improve the adsorbent surface area, mechanical strength and resistance to chemical compounds. Thus, its potential for adsorption contaminants [37] [38]. In recent years , one of the nano size particles synthesis and utilized in adsorption process is iron oxide Fe3O4 because of its scale in nano-range, high surface-to-volume ratios and super para magnetism [39]. However, the use of pure 3 4 nanoparticles as adsorbent media are not desirable for economic purposes. Moreover, nanoparticles tend to agglomerate, resulting in a major loss of reactivity [40]. To overcome this challenge and improve the quality of treatment and the ability of natural material to eliminate pollutants, a developed method of precipitating or coating nano-sized particles onto the surface of the natural material has been adopted. Like the Geopolymer, zeolite [38], bentonite [41]. Table 5 present the effect of modification on the natural material on the performance of adsorption capacity.

Conclusion
This paper identified various forms of geopolymers for the adsorption of hazardous waste, like dyes and heavy metals, and illustrated the geopolymer synthesis mechanism. High performance, mechanical durability, environmentally friendly and cost-effectiveness has drawn a great deal of attention to geopolymers as the adsorption materials for these contaminants. Its application needs to be applied to other water and wastewater contaminants such as medicines, oil and grease (O&G), phenols, micropollutants, and others. Geopolymer efficiency requires additional improvement of adsorption capacity and rate of adsorption; for future work, geopolymers should be modified by precipitating nanomaterial on their surface to increase adsorption capacity. This could render the geopolymers the next innovative material of the future.