Comparative adsorption mechanism of doxycycline and Congo red using synthesized kaolinite supported CoFe 2 O 4 nanoparticles * Sunday J. Olusegun * , Nelcy D.S. Mohallem Universidade Federal de Minas Gerais, Departamento de Química, Laborat orio de Materiais Nanoestruturados, Belo Horizonte, Brazil article info Article history: Received 14 October 2019 Received in revised form 4 January 2020 Accepted 17 January 2020 Available online 20 January 2020 Keywords: Doxycycline Congo red Adsorption mechanism Kaolinite CoFe 2 O 4 abstract Kaolinite supported CoFe 2 O 4 (KCF) was synthesized and employed to adsorb doxycycline (DOX), an antibiotic and Congo red (CR), a dye from aqueous solution. The prepared KCF nanocomposite was treated in a muffle furnace at 300, 500 and 700  C, and thereafter characterized. X-ray diffractogram revealed structural damage of kaolinite and appearance of distinct peaks of CoFe 2 O 4 with an increase in calcination temperature, while transmission electron microscopy (TEM) images showed that CoFe 2 O 4 nanoparticles were supported on the lamellar surface of kaolinites. Comparative adsorption mechanism of the two targeted contaminants showed that adsorption of DOX was influenced by hydrogen bond and n-p interaction, while that of CR was due to hydrophobic interaction and hydrogen bond. However, the adsorption of the two contaminants was best fitted to the isotherm that was proposed by Langmuir, with a monolayer maximum adsorption capacity of 400 mg g 1 at 333 K for DOX, and 547 mg g 1 at 298 K for CR. The removal of DOX from aqueous solution was favored by an increase in temperature (endothermic), while that of CR was exothermic. Thermodynamics studies confirmed that the adsorption of the two contaminants is feasible and spontaneous. The presence of natural organic matter (NOM) did not affect the removal of the two contaminants. Regeneration and reusability study showed that KCF is econom- ically viable. Therefore, introducing inorganic particles like cobalt ferrite into the matrix of kaolinites provides a composite with promising adsorption capacity. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Indiscriminate disposal of contaminants such as pharmaceutical wastes, dyes, heavy metals among others into water bodies has been one of the major environmental challenges that is confronting the entire globe (Nghiem et al., 2005). Widespread of these con- taminants could be traced to a geometric increase in population growth, rapid industrialization, lack of adequate education on the proper discharge of contaminant and the nonchalant attitude of the populace (Fonseca Couto et al., 2018; Li et al., 2019; Salehi et al., 2016). Wastewater from industries such as chemical manufacturing, mining, pharmaceutical, textile industries, etc. contains high levels of toxic materials, organic pollutants and many other complex compounds that have compromise the integrity of both surface and groundwater (Hasan et al., 2019; Keshmirizadeh et al., 2018; Raper et al., 2018; Zhao et al., 2019). Among these contaminants are doxycycline (antibiotic) and Congo red (dye) whose presence in the environment is traced to effluents mainly from pharmaceutical and textile industries respectively. Doxycycline (classified under tetracyclines) is a drug that is widely use for the wellness of humans and animals. In humans, it is effective in curing infections that are related to res- piratory trait, poor renal function, lung, among others (Gao et al., 2018). It functions as additives in animals’ food to hasten their growth and is often preferred over other tetracyclines (Kogawa et al., 2014). It was stated that very little of the antibiotics admin- istered into humans and animals are consumed by the body while the majority is excreted without modification (Alsager et al., 2018). In addition, the unused antibiotics are discarded to municipal wastewater, and, hence, wastewater treatment plants could not remove antibiotics totally. Then, polluted effluents flow to surface water, seep into groundwater and finally into drinking water (Wang et al., 2019). Congo red is an azo dye commonly used in textile in- dustries. It is credited to be the first dye with the capability of dyeing cotton, it is highly soluble in water and frequently used in textiles, plastics and printing industries (Chawla et al., 2017; Madan et al., 2019; Mahmoud et al., 2018; Purkait et al., 2007). Congo red and doxycycline become contaminants when they are discharged * This paper has been recommended for acceptance by Baoshan Xing. * Corresponding author. E-mail address: arewasegun@ufmg.br (S.J. Olusegun). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol https://doi.org/10.1016/j.envpol.2020.114019 0269-7491/© 2020 Elsevier Ltd. All rights reserved. Environmental Pollution 260 (2020) 114019