## 1 Introduction  
Dyes are widely used in many industrial sectors such as textile and paper companies [1]. These industries dump large amounts of wastewater into streams without any prior treatment. Their discharges present a source of pollution that can be very harmful to human and environmental health. The protection of the environment and the improvement of water quality by means of the treatment of these industrial effluents have become an issue of concern in recent years. Methylene blue (MB) is a synthetic dye considered a suspected carcinogenic agent. It can cause breathing difficulties, nausea and vomiting [2; 3], as well as allergic dermatitis and skin irritation [4], so the efficient removal of this contaminant from water is of high scientific relevance. Several technologies to remove these dyes from wastewater have been reported, such as adsorption and membrane separation [5; 6; 7]. Among these techniques, activated carbon adsorption is an effective method for treating contaminated wastewater, widely supported in the literature. Conventional activated carbon materials are commonly used as adsorbents; however, the difficulties for the separation after treatment in some applications limit their use and, in some cases, make the process unfeasible for industrial applications [7; 8]. As it has been widely reported in the literature, conventional activated carbons, synthesized from a single precursor, are economical materials used in the adsorbent processes, and even finding specific applications, such as supercapacitors [9] and catalysts [10], due to their relatively low cost. Recently, nanomaterials have been intensively used as adsorbents to remove organic substances from wastewater, showing great potential in the environmental remediation field due to their moderate-large specific surface area and opened-porous structure [11; 12; 13]. Among these nanomaterials, carbon nanotubes (CNTs) [14; 15] and magnetic iron oxides [16; 17] are considered as excellent adsorbents for dyes removal. CNTs can be classified attending to two categories, depending on their size and structure: single-walled carbon nanotubes (SWCNTs, 0.7 \(<\) d \(<\) 2 nm) and multi-walled carbon nanotubes (MWCNTs, 1.4 \(<\) d \(<\) 150 nm) [13]. MWCNTs have showed excellent properties for the adsorption of dyes, pesticides, heavy metal ions and other organic pollutants from water. They possess small radial size, excellent mechanical properties and exceptional electrical and thermal conductivity [18]. Chemical modifications such as chemical oxidation of MWCNTs lead to incorporate new functional groups on their surface, enhancing their affinity and selectivity towards the target pollutants [14]. In addition, these functionalities can increase the MWCNTs solubility, which help to increase the interactions between MWCNTs surface and the contaminant [18]. However, the use of MWCNTs as nano-adsorbents is limited due to their separation and regeneration difficulties. The most used iron oxides in different technological applications are hematite (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)), magnetite (Fe\({}_{3}\)O\({}_{4}\)) and maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)) [17]. These oxide materials demonstrated a huge potential for their application in wastewater treatment due to their, magnetic properties, relevant selectivity, reactivity and biocompatibility abilities [19]. They have also been used in different research areas including catalysis, environmental remediation, magnetic storage media, biomedicine, ferrofluid, magnetic resonance imaging and electrochemical applications [20]. However, the disadvantages of these materials, in particular magnetite and maghemite nanoparticles (MNPs), is that they tend to aggregate between them due to magnetic dipole-dipole attraction forces. In addition, MNPs are prone to oxidation [21], corrosion by acidic or basic medium [22] and release of metal ions [23].