Magnetic carboxyl-coated silica iron oxide nanoparticles (Fe 3 O 4 @SiO 2 -COOH NPs) were successfully synthesized, characterized, and then applied as a nanoadsorbent for removal of malachite green (MG) from aqueous solutions. According to the experimental results, about 97.5% of MG could be removed from aqueous solutions using an adsorbent amount of 0.5 g/L at pH = 9 in 120 min. The kinetics and equilibrium adsorptions is well-described by the pseudo-secondorder kinetics and Langmuir model with the maximum absorption capacity of 263.16 mg/g, respectively. Thermodynamic studies showed that the adsorption of the hazardous MG dye was spontaneous and endothermic with a random process.

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