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a b s t r ac tIn this study, the performance and effectiveness of magnetic CaFe 2 O 4 nanoparticles prepared by simple chemical route were evaluated for the adsorption of toxic azo dye Congo Red (CR) from aqua matrix. The prepared CaFe 2 O 4 nanoparticles were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, vibrating sample magnetometer, point of zero charge, and Brunauer-Emmett-Teller surface area measurements. Batch mode adsorption experiments were performed to study the effect of various experimental parameters namely solution pH (4.0-10.0), contact time (2-120 min), adsorbent dose (0.25-1.5 g/L), and initial CR dye concentration (20-150 mg/L) on the adsorption process. Maximum CR dye removal of 99.01% was achieved at solution pH 4.0 and maximum adsorption capacity of 241.16 mg/g was reported at optimum experimental condition. The adsorption equilibrium data strictly follows Langmuir isotherm model and adsorption kinetics was well described by pseudo-second-order model. A three layered artificial neural network (ANN) was applied for the accurate prediction of percentage of CR dye removal by the CaFe 2 O 4 nanoparticles. The Levenberg-Marquardt backpropagation algorithm with "tansig" and "purelin" transfer function in hidden and output layer was used for model development. Optimal ANN architecture (4-9-1) shows high R 2 value (R 2 : 0.995) and very low mean squared error value (0.00042866), confirming the accurate prediction ability of CR dye removal efficiency in this adsorption process.

A facile co-precipitation method was established for synthesis of mesoporous iron-manganese magnetic bimetal oxide (MIMO) and its adsorption property was studied for removal of toxic metal ion hexavalent chromium from aqueous solution. XRD pattern of MIMO confirms the existence of Fe2O3 and Mn3O4 particle, out of which Mn3O4 is ferrimagnetic in nature. Synthesized MIMO has shown high saturation magnetization (23.08 emu/g), high BET surface area (178.27 m2/g) and high pore volume (0.484 cm3/g), which makes it a potential adsorbent. Adsorption process followed second order kinetic and Langmuir isotherm model. Involvement of intra-particle diffusion is also confirmed from kinetic data, which can be attributed to the mesoporous nature of the MIMO. Cr(VI) adsorption shows high pH dependency and maximum adsorption capacity of 116.25 mg/g is reported at pH 2.0. Electrostatic attraction between anionic chromium species and protonated MIMO surface is the predominant mechanism in this adsorption process.

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