Industrial waste, commonly thrown into aquatic environments, contains both inorganic and organic contaminants, representing a global concern for many countries (Akhil et al. 2021). Among the various inorganic contaminants, antimony (Sb) is considered pollutant non-biodegradable and that can cause many environmental damages. Furthermore, carcinogenic contaminants cause health problems, especially for human beings (Viczek et al. 2020). The main industrial activities that can contribute to the discharge Sb in aquatic environments come from the industries of fire retardants, pigments, mining, and ceramics (Aquino et al. 2016; Chu et al. 2019; Meng et al. 2020). Some countries, such as China, Grace, and India, have suffered with problems of contamination in groundwater by Sb, which can cause damage to human health (Antoniadis et al. 2019; Xu et al. 2019). In this sense, the development of methodologies that are efficient for the Sb remediation from wastewater is essential.
Among the various approaches in the literature for the Sb removal (Xiang-Xue et al. 2019; Chen et al. 2020), adsorption technology is most commonly method used, and several adsorbents have been developed (Zhao et al. 2010). Despite the various absorbent materials already available in the literature, it is still necessary to find an absorbent material that has the following characteristics: (i) low-cost, eco-friendly, and sustainable; (ii) good physical and chemical stability; (iii) excellent textural and structural features; and (iv) high selectivity, so on (Costa and Paranhos 2020; Costa et al. 2020b). In this context, the synthesis mesoporous materials has attracted a great deal of interest for the adsorption process and has already been used with success for removal of the organic compounds (Santos et al. 2019; Costa et al. 2020d, c, 2021b, a) and inorganic constituents (de Sá et al. 2020; Costa et al. 2020a).
The mesoporous materials were synthesized for the first time in the early 1990s, and have since been used in the most diverse technological applications. The best known and/or studied mesoporous materials are those of the M41S family, represented by MCM-41 (hexagonal phase), MCM-50 (lamellar phase), and MCM-48 (cubic phase) (Costa et al. 2015, 2017a, b; Santos et al. 2019), which is the focus of this approach.
The mesoporous structures have attractive features, such as good thermal and mechanical stability, and high surface area, which allows the diffusion and/or adsorption process of the organic and inorganic compounds through their uniform pores and high mesoporous arrangement, as well as ease in the synthesis and functionalization process of these ordered structures (Costa et al. 2014, 2015, 2017a, b). The silica-based mesoporous arrays are synthesized via the hydrothermal method from the use of a surfactant (directing agent), a catalyst (acid or basic), and mainly from a silica source, which is responsible for forming the framework of the amorphous material (Costa et al. 2015, 2017b).
In the literature there are several works showing the synthesis of these mesoporous materials from the use of the commercial silica sources, mainly tetraethylorthosilicate (TEOS) (Costa et al. 2014, 2017b; Ambursa et al. 2017), silica gel (Santos et al. 2019), and sodium silicate (Costa et al. 2017a; Santos et al. 2019). However, there are some works that show the preparation of the mesoporous structures from the use of the alternative, sustainable, and eco-friendly materials as a silica source, such as fly ash (Castillo et al. 2018), rape straw (Li et al. 2019), straw ash (Ma et al. 2016), bamboo leaf ash (Arumugam et al. 2018), rice husk (Sohrabnezhad and Daraie Mooshangaie 2019), sedge ash (Ghorbani et al. 2013), and sugarcane bagasse (Norsuraya et al. 2016), so on. In the present approach, we use the rice husk ash (RHA) as an alternative, inexpensive, eco-friendly, low-cost, abundant, and accessible source of amorphous silica for the synthesis of the mesoporous material with a cubic phase (MCM-48 (RHA)), which was later used as an adsorbent material in the Sb removal in aqueous media.
The most of the approaches found in the literature, which are dedicated to the adsorption studies, are carried out from the univariate optimization of the adsorption tests, which aim at understanding the adsorption mechanism between the adsorbent material and the adsorbate, especially from the correlation of experimental adsorption data with kinetic and isothermal theoretical models (Costa et al. 2014; Costa and Paranhos 2019). However, these approaches are laborious and requires expertise analyst.
Recently, a demand has emerged for the optimization step of the procedures are fast and with reduced number of experiments (Ferreira et al. 2018). In this sense, the multivariate optimization techniques have been shown powerful to evaluation the variables that affect the analytical response in order to obtain the best conditions of optimization to ensure the procedure reliability. Among the multivariate optimization tools factorial design is more employed and allows a preliminary evaluation of the variables for development of linear models (Costa et al. 2019; Gamela et al. 2020). These tools have numerous advantages, such as: (i) possibility of evaluating synergistic and antagonistic interactions between variables; (ii) possibility of forecasting the system under study in a condition that has not been tested in practice; and (iii) reduces the generation of chemical waste which contributes to the principles of green chemistry (Ferreira et al. 2017; Costa et al. 2018). Factorial designs have been used in several areas, but its use in absorption procedures has not been explored sufficiently.
In this context, the factorial design was employed to optimize a procedure for Sb remediation in wastewater. In addition, the adsorbent material used was obtained from a cleaner, low-cost, and eco-friendly approach from the use of alternative amorphous silica extracted from RHA.