**1. Introduction**

Alkali-activated materials (AAMs) are inorganic, amorphous compounds that contain [SiO4] <sup>4</sup><sup>−</sup> and [AlO4] <sup>5</sup>−, which can be prepared by using aluminosilicates in addition to hydroxides, carbonates, or silicates of alkali and alkaline earth metals. The calcium content affects the AAM structure; therefore, materials with a low Ca content comprise a three-dimensional, highly interconnected aluminosilicate framework (also known as a geopolymer) [1], while those with a high Ca content comprise a cross-linked and non-cross linked structure that resembles that of tobermorite [2]. The use of AAMs, and especially geopolymers, has been investigated in the building industry as a more environmentfriendly alternative to Portland cement, due to their chemical and physical stability [3–5], as well as in more advanced applications, such as adsorbents for the removal of impurities from wastewater [6–9] and composite materials [4]. Moreover, owing to the fact that the structure of AAMs is similar to that of zeolites, their use as catalytic materials can also be exploited. Compared with the synthesis of zeolites, that of AAMs can be performed at ambient pressure and room temperature, using cost-effective raw materials (kaolin clay or industrial waste, such as fly ash), making AAMs fascinating, environment-friendly alternatives to commercial zeolite. Only a few years ago, Sazama et al. reported the synthesis of AAM-based catalysts [10] by the modification of metakaolin geopolymers for the catalytic reduction of nitrogen oxides by ammonia, as well as the total oxidation of volatile hydrocarbons. Furthermore, metakaolin-based geopolymers and steel slagcontaining AAMs also have been examined for photocatalytic applications [11,12] and biodiesel production [13]. Therefore, AAMs are interesting alternatives as catalysts, as well as for water-phase applications like catalytic wet peroxide oxidation (CWPO).

Bisphenol A (BPA) is an estrogenic compound commonly used in the production of polycarbonate plastic and epoxy resin, which are further utilized in several daily-use

**Citation:** Heponiemi, A.; Pesonen, J.; Hu, T.; Lassi, U. Alkali-Activated Materials as Catalysts for Water Purification. *Catalysts* **2021**, *11*, 664. https://doi.org/10.3390/catal 11060664

Academic Editor: Roberto Fiorenza

Received: 7 May 2021 Accepted: 22 May 2021 Published: 23 May 2021

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plastic products, such as drinking bottles, containers, thermal paper, etc. [14]. BPA can be spread into water bodies during the manufacturing process, and also from daily-use plastic products. Due to its endocrine disrupting character for humans and environment [15], the removal of it from wastewaters is essential. Several techniques, such as activated sludge treatment [16], membrane bioreactors [17], and sorption [18,19] have been used for the removal of BPA from wastewaters. In addition of these, advanced oxidation processes (AOPs) have been effectively used for the oxidation of BPA in wastewaters [20]. Techniques like photolysis [21] and ozonation [22], as well as hybrid processes like UV/H2O<sup>2</sup> [23], UV/O<sup>3</sup> [24], UV/TiO<sup>2</sup> [25], and O2/H2O<sup>2</sup> [26], have been successfully used for the oxidation of BPA. In this study, one of AOPs, CWPO, is studied for the removal of BPA from water. In CWPO, hydrogen peroxide is used as an oxidizing agent to decompose organic compounds from wastewater. Transition metals, typically Fe and Cu, are used as catalysts in the reaction to decompose hydrogen peroxide to active hydroxyl radicals (Equation (1)) [27].

$$\rm M^{n+} + H\_2O\_2 \rightarrow M^{(n+1)+} + HO^- + HO^\bullet \tag{1}$$

The formed hydroxyl radicals can further oxidize organic compounds, according to Equation (2):

$$\text{RH} + \text{HO}^{\bullet}\text{-}2 \rightarrow \text{R}^{\bullet} + \text{H}\_{2}\text{O}\_{2} \tag{2}$$

To obtain the reduced form of the active metals, they must be dispersed on suitable supports [28,29]. Various materials have been used as supports in CWPO. Carbon-based materials, such as activated carbon [30], graphite, carbon black [31], carbon nanotubes [32], and biomass-based carbons [33], have been successfully used for the degradation of organics with CWPO. Moreover, zeolites [34,35] and clay materials [36,37] also have been applied as supports for Fe and Cu; therefore, AAMs with a chemical composition similar to those of zeolites and clay minerals are interesting alternatives as carriers for use in CWPO.

However, for the use of industrial side streams as raw materials in a catalyst, the prepared material must exhibit stability, especially when the prepared material is used for water treatment applications. Typically, catalyst stability for photocatalytic experiments has been evaluated in consecutive tests [38] and by characterization of the used materials after experiments, e.g., by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) [39,40].

In this study, the industrial side stream from the steel industry, i.e., blast furnace slag (BFS), was applied as a raw material to produce cost-effective catalytic materials for water purification. Catalysts were prepared by mixing different ratios of BFS and NaOH, followed by their consolidation at 20 ◦C and 60 ◦C. Moreover, iron was impregnated as the active metal in the AAMs via ion exchange. The as-prepared materials were characterized, e.g., by XRD, and their surface area and catalytic activity were examined for the CWPO of a bisphenol A (BPA) aqueous solution. Particular attention has been focused on the stability of materials; therefore, the possible leaching of the main elements (such as Ca, Si, Al, Mg, and Na) has been investigated before the CWPO of BPA under 2 MPa and 150 ◦C. Furthermore, the concentrations of the main elements of the as-prepared materials was analyzed after oxidation experiments from water samples, as well as those from the used catalysts.
