## 1 Introduction  
The 20 th most abundant element on the earth's crust is arsenic [1], which has become the reason of death in many countries [2]. Arsenic poisoning has claimed the lives of millions of people, and millions more are suffering from its toxicity [3]. Now, the most serious problem is arsenic intake with drinking water [4]. The elevated levels of arsenic in drinking water are associated with several anthropogenic and natural activities [5]. Arsenic is commonly present as arsenite (As(III)) in groundwater while presenting as arsenate (As(V)) in surface water [6]. Arsenite is more poisonous and dangerous to humans than arsenate because of its sensitivity to cell-binding sites [7]. The arsenic limit in drinking water according to WHO is 0.01 ppm [8]. Therefore, the decrease of arsenic concentration in water below this limit is a challenge. Various methods have been applied for the removal of arsenic from water such as the adsorption [9], reverse osmosis [10], ion exchange [11], membrane separation [12], electro-coagulation [13], flocculation [14], and precipitation [15]. The adsorption process has been considered as the commonly used method due to several advantages (eco-friendly, efficient, reversibility, economic, and simplicity) [16; 17; 18; 19; 20; 21]. In this context, different materials have been used for arsenic removal from water via adsorption method. These materials include metal oxides [22], agricultural wastes [23], biochar [24], clay-based adsorbents [25], aluminum-based adsorbents [26], commercial activated carbon [27], and many other materials. In recent years, nanomaterials have attracted interest as adsorbents due to their exceptional properties [28; 29; 30; 31; 32; 33]. Although adsorption efficiency is offered by the nanomaterials, it is not easy to recollect the used adsorbents after the pollutant adsorption. Interestingly, several magnetic nanomaterials have been used for the adsorption of contaminants from aqueous media due to their ease of separation by an external magnet, large surface area, and simple functionalization [34; 35; 36; 37; 38; 39]. By using magnetic nanomaterials, filtration and centrifugation conducted after the adsorption process become unnecessary as they are replaced with an easy magnetic separation. Several reports have described the synthesis and the application of low-cost and eco-friendly magnetic adsorbents for water treatment. Among these adsorbents, Fe\({}_{3}\)O\({}_{4}\) nanoparticles are one of the main selected materials due to their high surface area, high saturation magnetization, high affinity, low cost, ease of surface modifiability, biocompatibility, and low toxicity [40]. However, the small size of the magnetic nanoparticles (NPs) is an obstacle to their application in adsorption systems. To overcome this problem, Fe\({}_{3}\)O\({}_{4}\) nanoparticles were combined with biopolymeric matrix as a supporting platform. Mainly, chitosan (CS) provides great adsorption ability to different types of pollutants such as heavy metals [41], proteins [42], and dyes [43] because of the presence of high hydroxyl and amino groups in its structure. Additionally, chitosan has many advantages that encourage its application in water treatment such as no harmful by-products, high affectivity, and accessibility. However, the practical applications of chitosan face certain difficulties including its dissolution in acidic medium below pH 5.5, and cannot be recovered. Many materials have been impeded by integratation with CS support to overcome this disadvantage by cross-linking (e.g., glutaraldehydehe as cross-linker). This integration increases the chitosan adsorption efficiency and decreases its solubility in acidic medium. So, the cross-linked CS can be used as excellent adsorbents for the removal of different types of pollutants including anions [44] and clofibric acid [45] from water, especially if modified with proper amines such as polyethylenimine (PEI).