New ways to use the red mud waste as raw material for inorganic- organic hybrid hydrogels Teodor Sandu a , Andrei Sarbu a, ⁎, Rodica Zavoianu b , Catalina Paula Spatarelu a , Mihaela Florea b , Corina Bradu c , Eleonora Luminita Mara d , Dumitru Valentin Dragut d , Elvira Alexandrescu a , Anamaria Zaharia a , Anita Laura Radu a a National Research-Development Institute for Chemistry and Petrochemistry (INCDCP-ICECHIM), Splaiul Independentei 202, 6th District, Bucharest 060021, Romania b University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis, 4-12 Regina Elisabeta, 3rd District, Bucharest 030018, Romania c University of Bucharest, Faculty of Biology, 4-12 Regina Elisabeta, 3rd District, Bucharest 030018, Romania d National Research-Development Institute for Non-ferrous and Rare Metals-IMNR, Bulevardul Biruintei 102, Pantelimon, Ilfov County, Romania abstract article info Article history: Received 6 February 2017 Received in revised form 19 October 2017 Accepted 9 November 2017 Available online 15 November 2017 The present paper was aimed at developing innovative hybrid composites by using cheap filler, like red mud (RM). The study importance refers to RM recycling, a recent trend in environmental science. Besides its causticity, large amounts of RM are produced yearly, calling for recovery measurements. Herein, poly (acrylic acid)- based inorganic-organic hybrid hydrogels (PAA hydrogels) incorporating RM were synthesized. Furthermore, the possibility of involving inorganic additives (kaolin and/or sodium silicate) together with RM was investigated. Incorporation of all three inorganic compounds (RM + Kaol + Sil) in hydrogels was also eval- uated, novel hybrids being prepared. The obtained hybrids were characterized by different techniques (DR UV- VIS; DRIFT; TGA/DTG; SEM) and in terms of swelling behavior. DRIFT and DR UV-VIS confirmed the occurrence of characteristic bands of raw materials. Final properties (water uptake, thermal stability, porosity) can be con- trolled by modifying preparation conditions. Hydrogels yielded, by calcinations, iron- rich ceramic foams, able to promote water removal of sulfide ions from wastewaters. © 2017 Elsevier B.V. All rights reserved. Keywords: Red Mud Kaolin Sodium silicate Acrylic hydrogels Hybrid materials Wastes recovery 1. Introduction Red Mud (RM), a solid brick red slimy waste residue resulting from caustic digestion of bauxite ores in the process of alumina (Al 2 O 3 ) pro- duction, occurs as alkaline slurry (pH = 10–13) containing various ox- ides (Al 2 O 3 , Fe 2 O 3 , Na 2 O, SiO 2 , CaO and TiO 2 ). Large amounts of RM are generated worldwide yearly, its disposal becoming a major problem [Wang et al., 2008; Ribeiro et al., 2012; Ramprasath et al., 2014; Biswas and Satapathy, 2009; Jena and Satapathy, 2012; Banjare et al., 2014; Zhang et al., 2014; Lv et al., 2013; Balomenos et al., 2011; Smiljanic et al., 2011]. Currently, Al is produced using a two-stage pro- cess: (I) high metallurgical grade Al 2 O 3 is obtained from bauxite (Bayer, 1888) and then (II) Al 2 O 3 is reduced to Al (Hall-Heroult). De- spite of its high alkalinity (it bears various hydroxides and, also, due to NaOH treatment), RM is classified by EC as a non-hazardous. However, RM causes a significant environment impact (it forms small powder particles by drying, it bears various chemical and mineralogical species, it occurs in large amounts), recovery of useful compounds from RM gaining significant importance. Bayer himself suggested iron recovery in 1892. Considering both the high content in iron oxides as well as the presence of TiO 2 in RM, this solid could be utilized for obtaining ox- idation catalysts. One of the most important properties of a catalyst is the dispersion of active sites on its surface. The higher the dispersion de- gree, the catalytic activity is higher. Since RM exhibits a low surface, it would be expected to show also a low dispersion of active sites. A mo- dality to increase the dispersion degree could be the incorporation of the RM in a polymeric matrix. Using this approach, red mud ceramic foams with hierarchical porous organization of active phase can be ob- tained following the calcinations of the polymeric hydrogel [Ramprasath et al., 2014; Balomenos et al., 2013; Smiljanic et al., 2011]. Hydrogels represent three-dimensional hydrophilic polymer net- works, either chemically or physically crosslinked, able to absorb plenty water, without dissolution [Ray et al., 2009; Zaharia et al., 2015a; Zaharia et al., 2015b]. One important pollution source consists of dissolved sulfides rising from various industry branches such as: oil processing; fossil fuel gasifi- cation; dyes and pigments; kraft paper; tanneries; viscose rayon pro- cessing; sewer networks, as a result of anaerobic sulfate reduction processes. These sulfides are further released in wastewaters and are International Journal of Mineral Processing 169 (2017) 111–118 ⁎ Corresponding author at: National Research-Development Institute for Chemistry and Petrochemistry-ICECHIM, Splaiul Independentei 202, 6th District, 060021, Bucharest, Romania. E-mail address: andr.sarbu@gmail.com (A. Sarbu). https://doi.org/10.1016/j.minpro.2017.11.005 0301-7516/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro