# Removal of Chromium from Synthetic Wastewater Using Modified Maghemite Nanoparticles
## 1 Introduction  
The three most common forms of chromium that exist in our environment are metallic (Cr\({}^{0}\)), trivalent (Cr\({}^{4}\)), and hexavalent (Cr\({}^{+6}\)). Metallic chromium does not occur naturally in elemental form. Trivalent chromium ion, Cr (III), is found in trace amounts in humans and is an essential nutrient for insulin, sugar, and lipid metabolism. Hexavalent chromium species (Cr\({}^{+6}\)) are highly toxic agents that act as carcinogens, mutagens, and teratogens in different biological systems [1]. Hexavalent chromium is included in the World Health Organization's (WHO) list of 10 chemicals of major public health concern and the maximum level of total chromium in drinking water has been set as 0.05 mg/L [2]. The United States Environmental Protection Agency (USEPA) prescribes that the levels of chromium in water should be reduced to 0.1 mg/L using appropriate technologies [3]. Hexavalent chromium (Cr+6) usually exists in wastewater in the form of oxyanions, such as chromate (CrO4\({}^{2-}\)) and dichromate (Cr2O7\({}^{2-}\)), and does not precipitate easily using ordinary precipitation techniques. Cr(VI), a hazardous material, is also found in solution as the monomeric ions H2 CrO4\({}^{0}\), HCrO4\({}^{-}\) (bichromate), and CrO4\({}^{2-}\) (chromate), or as the dimeric ion Cr2O7\({}^{2-}\) (dichromate) [4; 5]. According to the literature, chemical redox followed by precipitation can be applied for Cr+6 removal from contaminated water streams [6]. In this work, adsorption technology was considered as the most promising technique for chromium removal from industrial wastewater. [PERSON] et al. [7] reported the removal of chromium from industrial wastewater using maghemite nanoparticles as the adsorbent. This technique has been used in many industrial case studies, whereby various adsorbents were found to be effective for heavy metal removal [1; 8; 9]. In principle, adsorption not only removes heavy metals but also recoups and recycles them back into the industrial process [10]. The adsorbed material, which is either toxic or valuable, is recouped in a concentrated form for disposal or reuse. The solid adsorbent can often be regenerated for reuse [11]. Several natural and synthetic materials have been used as Cr(VI) adsorbents, such as activated carbons, biological materials, zeolites, chitosan, and industrial wastes [12; 13; 14]. However, these adsorbents also have several disadvantages and limitations, including high cost, low adsorption capacity, and difficulties associated with the separation and removal following the treatment steps. Therefore, in this study, maghemite (\(\gamma\)-Fe2O3) was used as the adsorbent because of its high surface area, magnetic properties, and chemical stability [15]. Maghemite, a common magnetic material, is a promising environmentally friendly adsorbent for heavy metal removal because it is inexpensive, readily available, and it can be easily separated and recovered [16; 17; 18]. Various surface modification techniques have been used to suitably modify surface properties and study the resultant changes in adsorption efficiency. Nanoparticles (NPs) have been of special interest to researchers and scholars due to their magnetic properties, availability, and susceptibility to modification. For example, bentonite-supported zero-valent iron nanoparticles and imine functionalized magnetic nanoparticles have been exploited for the removal of Cr (VI) from landfill leachate and Zn\({}^{2+}\) and As\({}^{3+}\) from wastewater, respectively [19; 20]. Based on a preliminary literature review, it was noticed that sodium dodecyl sulfate (SDS), a common synthetic anionic surfactant, has never been used before with NPs for heavy metal ion removal, despite its proven potential for substantially and suitably altering the surface properties of nanoparticles. In this study, the adsorption potential of NPs modified with SDS for chromium heavy metal ions was investigated.