# Adsorption of Lead and Nickel on to Expanded Graphite Decorated with Manganese Oxide Nanoparticles
## 1 Introduction  
With the growth of human activities, heavy metal contamination of soil and groundwater has become a great concern. Heavy metals are highly toxic and carcinogenic due to their persistence, bioaccumulation, and risk of reappearance according to changes of aquatic chemistry. Among various heavy metals, lead and nickel are considered the most dangerous contaminants when they are released into natural waters from various industrial activities including mining, leather processing, and metal plating. Lead can easily accumulate in human organs, causing anemia and severe damage to the nervous system, reproductive system, liver, brain, and kidneys [1]. Nickel is another toxic metal ion commonly present in steel and battery wastewater, causing vomiting, chest pain, and shortness of breath [2]. Various methods for heavy metal removal have been developed including reverse osmosis, ion exchange, electrodialysis, electrolysis, and adsorption. Among these technologies, adsorption has been acknowledged as a fast, inexpensive, and widely applicable technique [3; 4; 5; 6]. In this regard, many studies have been focused on the effectiveness of various types of adsorbents, including biomass, clays, activated carbon, and metal particulates [7; 8; 9; 10]. Among the listed adsorbents, adsorption onto naturalmaterials and biomass have been the most commonly tested techniques due to their cost-effectiveness when compared to those of other adsorbents such as metal particulates and activated carbon, while specific adsorption capacities have generally been found to be lower than that of metal particulates [7]. Moreover, effluent concentration for heavy metals during water treatment is very low in most cases, and the adsorption capacity is not the only criterion; the affinity between adsorbent and heavy metals should also be considered [1]. Nano-sized metal oxide particles have fascinated many researchers due to their high adsorption capacities as well as their extremely fast adsorption of and high affinity for heavy metals [11; 12]. Among them, manganese oxide nanoparticles (MONPs), with notably high surface area and uniform size, are of great importance due to their novel applications in heavy metal removal [13; 14]. However, due to the strong tendency of MONPs to form aggregates, synthesis of particles of well-controlled shape and size has been critical for maintaining their high affinity for heavy metals and their surface area. To minimize the aggregation of MONPs, various substrates with large surface area and uniform surface energy have been considered [15]. Specifically, graphene oxide (GO), a two-dimensional (2D) atomically thick carbon crystal with a honeycomb lattice, huge surface area, mechanical and thermal stability, and durability, has attracted great scientific and technological interest because of its potential applications in various areas including as a substrate for novel nano-particle deposition [16; 17]. However, it is difficult to obtain homogeneous metal deposition on the hydrophobic surface because graphene sheets are not wetted by water, which leads to the infeasibility of traditional metal loading methods such as wet impregnation. Although GO is suitable and versatile, able to decorate various organic and inorganic chemicals, the subsequent reduction of GO to grow metal nanoparticles has been regarded as a complex, time-consuming, and highly toxic process [18; 19]. Many efforts have been made to immobilize MONPs on various supporting materials such as sand, zeolite, and crushed brick [20; 21; 22; 23], but their adsorption performance for heavy metals removal are still facing multiple challenges as discussed above. Recently, several groups have investigated the use of expanded graphite (EG) because EG could achieve the requirements for nanoparticle support, including high porosity, large surface area, and low price [24; 25].