# Arsenic Removal from Water by Adsorption onto Iron Oxide/Nano-Porous Carbon Magnetic Composite
## 1 Introduction  
Arsenic (As) contamination of groundwater is of major concern in the various parts of the world, where the water supply is primarily dependent on groundwater. A higher concentration of arsenic in groundwater has been reported in several countries, including Argentina, Chile, Mexico, China, Argentina, USA, Hungary, Australia, Switzerland, Cambodia, Thailand, Vietnam, Mongolia, Taiwan, Pakistan, as well as in the Indian State of West Bengal, Bangladesh, and Nepal [1]. In Southeast Asia, millions of people are exposed to groundwater with As concentrations higher than the WHO guideline value of 10 \(\upmu\)g/L [2]. The highest concentration of As is being found in India (0.003-3700 \(\upmu\)g/L), followed by China (220-2000 ug/L) and Bangladesh (10-1000 ug/L). The largest population at risk is in Bangladesh, followed by West Bengal in India [3]. In Nepal, nearly 47% (11 million) of the total population inhabits in southern lowlands of Nepal (the so-called Terai), and over 90% of the population draws groundwater from tube wells for drinking, household use, and irrigation. The possibility of arsenic contamination in tube well water of somedistricts of Terai region was first identified in 1999. Later studies also documented that the region of As contamination extends into 24 districts in Nepal, including 20 districts of Terai region and four districts of the hilly region [4]. The most severe As contamination is prevailing in Nawalparasi, Bara, Parsa, Rautahat, Rupandehi, and Kapalivastu districts [5]. It is estimated that 0.5 million people in Terai are at risk of consuming water with As level greater than 50 \(\mu\)g/L, while 3.5 million have been exposed to As levels between 10 to 50 \(\mu\)g/L [4]. Arsenic contamination of groundwater of Nepal is mainly of natural (geogenic) origin, and its elevated concentrations are considered to be caused by reductive dissolution of ferro-oxyhydroxide [6]. Arsenic can exist in four oxidation states: \(-\)3, 0, +3, and +5. In natural water, As exists primarily as oxyanions representing two oxidation states: arsenic (+3) and arsenate (+5). Usually, inorganic As is more toxic than organic As. Inorganic As is mostly found in water as trivalent arsenite or pentavalent arsenate. Arsenite (As (III)) is more mobile and toxic than the arsenate (As (V)) due to its tendency to react like a soft metal with thiols. Solubility of Arsenic in ground water depends on pH, redox conditions, temperature, and solution composition. [7; 8]. The reduced form (As(III)) is normally found in groundwater under anaerobic conditions, while arsenate, the oxidized form (As (V)), is found in surface water under aerobic conditions. At near neutral pH, the predominant species are H\({}_{2}\)AsO\({}_{4}\)- and HASO\({}_{4}\)\({}^{2-}\) for arsenate and uncharged H\({}_{3}\)AsO\({}_{3}\) for arsenite. In the Terai region of Nepal, groundwater appears mostly to be neutral to alkaline with a pH range of 6.1-8.1. Moreover, the variations of redox potential (Eh) between \(-\)0.2 to \(-\)0.11 V also suggested fairly reduced conditions in the aquifers [9]. Hence, arsenite, the reduced trivalent form (As (III)), is mostly present in the groundwater of the Terai region. Long-term exposure to arsenic from drinking-water can cause diverse types of cancers, including skin, lungs, urinary bladder, kidney liver, and prostate cancers. The non-cancerous effects that are associated with arsenic exposure include hyperkeratosis, pigmentation changes, cardiovascular diseases, hypertension, and respiratory, neurological, liver, and kidney disorders, as well as diabetes mellitus [10]. To prevent health problems associated with As, an effective technique for the removal of As from groundwater before using it for drinking purpose is very essential. Several methods have been applied for the removal of As from water which include oxidation [11], alum/iron coagulation [12; 13], adsorption/ion-exchange (activated alumina, iron-coated sand, and ion-exchange resin) [14; 15], membrane filtration [16], reverse osmosis [16].