# Wastewater Treatment Using Alkali-Activated-Based Sorbents Produced from Blast Furnace Slag
###### Abstract
Therefore, both academia and industry are putting in much effort to transform wastes or byproducts that could be used as sorbents in wastewater treatment. Such an approach will positively impact environmental preservation and the responsible use of resources, including water, fulfilling the \"Sustainability Agenda\" goals (namely, goals number 3, 6, 9, 12, 13, 14, and 15) [15]. Among others, by-products and residual wastes of steel manufacturing have gained attention as possible sorbent starting materials, due to their composition and their large availability, which poses strong environmental issues [16]. Therefore, actions allowing the reduction of the environmental impact, such as for instance red mud and blast furnace slag valorization as potential sources of raw materials, are of course welcome [17]. In this respect, the use of waste-based geopolymers or alkali-activated materials [18] as sorbents can be a very good opportunity to contribute to a cleaner production and to mitigate the greenhouse effects of the steel-making industry. The term \"geopolymer\" identifies a class of ceramic-like and closely related materials, which are obtained via metakaolin activation with alkali silicate solutions. Recently, the term has been used also to refer to the most varied starting precursors, which can be a source of aluminosilicates. Alkali activation allows for the achievement of Al-O-Si bonded mineral polymers. For their production, the use of various industrial waste streams as precursors, such as for instance fly ash, granulated blast furnace slag waste materials, metallurgical slags, glass wastes, has been proposed [19; 20]. Alkali-activated materials synthesis can be performed with a quite low energy input, keeping the alkaline activation at low values, and performing the reaction at room temperature, mitigating the release of greenhouse gases. Moreover, they could be reused in other applications after exhaustion when used as sorbents for water depollution, e.g., as an aggregate in the production of new building materials, which is not possible for materials in use presently. Fly ashes have been found to be a potential and versatile sorbent material for the removal of toxic metals, like lead or copper, from aqueous solutions [21], but they show a quite low adsorption capacity if used just as raw fly ashes [22]. However, to become part of an alkali-activated geopolymer, they need to be reacted with an alkali activator, thus requiring the need for further chemical treatments [19]. Fly ashes-based by-products from coal power plants are also interesting as their high silica content makes them possible precursors of nano-porous materials with high surface area and large pore volume, which can be exploited not only in wastewater treatment but also in carbon dioxide capture. Considering the alumina-silicate nature of fly ash, it can be potentially regarded as a source of different sorbent materials, depending on its origin and chemical activation, for several industrial processes. It has also been successfully employed as a precursor of fillers for the development of mixed matrix membranes (MMMs) to separate carbon dioxide from methane [23]. Moreover, it could also be considered a suitable substitute for lime to enrich soils [24]. Recently, conversion of fly ashes into geopolymer systems has arisen as a very interesting opportunity for wastewater treatment because of their effective absorption and immobilization of heavy metals. The structure of geopolymers is very similar to that of zeolites, as it consists of a negatively charged aluminosilicate framework with charge-balancing exchangeable cations, but, unlike zeolites, geopolymers are amorphous or only partially crystalline. Blast furnace slag (i.e., the by-product generated during pig-iron production) is another low-cost sorbent showing the ability to adsorb heavy metals. Blast furnaces generate a large volume of granular blast furnace slag that is also being used as a constituent of concrete or in the production of slag cement.