Original Research Paper Cobalt ferrite nanoparticles prepared by microwave hydrothermal synthesis and adsorption efficiency for organic dyes: Isotherms, thermodynamics and kinetic studies S. Mishra a , S.S. Sahoo a , A.K. Debnath c , K.P. Muthe c , N. Das b,1 , P. Parhi a,1,⇑ a Department of Chemistry, Ravenshaw University, Cuttack, Odisha, India b Department of Chemistry, Utkal University, Bhubaneswar, Odisha, India c Technical Physics Division, Bhaba Atomic Research Centre, Mumbai 400 085, India article info Article history: Received 10 May 2020 Received in revised form 4 September 2020 Accepted 12 October 2020 Available online 12 November 2020 Keywords: Nanoferrites Magnetic nano-adsorbent Organic dyes Adsorption isotherms, regeneration abstract Magnetic nanoferrites (MFe 2 O 4 , M = Co, Ni) were successfully synthesised through microwave- hydrothermal route, characterised and used for adsorption of Eriochrome Black T (EBT) and Bromophenol Blue (BRB) dyes from their aqueous solution. The powder XRD patterns confirmed the for- mation of cubic spinel structure for both the ferrites. Under identical conditions, the adsorption efficiency of CoFe 2 O 4 was found relatively higher than the corresponding NiFe 2 O 4. Further characterisations revealed that CoFe 2 O 4 sample was nearly spherical in size (8–9 nm) with narrow size distribution. The sample showed superparamagnetic behaviour with saturation magnetization (M s ) value (66.4 emu/g). BET surface area calculated for the synthesized cobalt ferrite as 70.9 m 2 /g. Batch adsorption experiments as a function of initial dye concentration, pH, contact time and adsorbent dose showed the adsorption of dyes depends on pH. Equilibrium adsorption data were well explained by both Langmuir and Freundlich isotherm models. The maximum monolayer adsorption capacities (Q o ) were found to be 82.6 and 25.6 mg/g for EBT and BRB dyes, respectively. Kinetics of the adsorption was best described by pseudo-second-order model. Various thermodynamic parameters such as DG, DH and DS derived from adsorption data over the temperature range 20–50 °C, accounted for a favourable, spontaneous, endothermic physisorption process. The materials showed potential for repeated use without significant decrease in adsorption capacity after proper regeneration. Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction In the waste water bodies the major pollutants are organic dyes which are generally self- resistant to degradation due to their com- plex molecular structures [1]. Most of these dyes are toxic, carcino- genic and, cause harm to both human and aquatic lives. Thus, removal of these dyes from their effluents in an efficient and eco- nomical way is highly desirable before their discharge into the water bodies [2,3]. Among different physical, chemical and biolog- ical methods such as coagulation, chemical oxidation, membrane separation, electrochemical processes, photo-degradation, nanofil- tration and microbial degradation, proposed or used in practice for treatment of wastewater containing dyes, the adsorption based techniques are regarded as one of the most successful and compet- itive methods due to their operational simplicity, flexibility, cost- effectiveness and ability to treat the dyes in concentrated forms [2–7]. There have been continuous efforts in search of alternative adsorbents rather than conventional adsorbents (carbon and silica based materials, synthetic resin etc.) as these adsorbents suffer from drawbacks like high cost, difficulty in regeneration and non-selectivity [8–12]. As a result a large variety of alternative adsorbents including nanostructured materials have been investi- gated for removal of dyes [13–15]. In recent past, nanometer-sized materials have attracted substantial interest in the scientific com- munity due to their usefulness, and as fast and efficient adsorbent for different water contaminants including dyes [16–22]. Unfortu- nately the separation/recovery of nanostructured adsorbents in heterogeneous systems is still a difficult thing to achieve and thus remains a steep challenge. Use of magnetic nanoadsorbents is a https://doi.org/10.1016/j.apt.2020.10.001 0921-8831/Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. ⇑ Corresponding author. E-mail address: pparhi@ravenshawuniversity.ac.in (P. Parhi). 1 Equal Author Advanced Powder Technology 31 (2020) 4552–4562 Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt