# Arsenic (V) Removal by an Adsorbent Material Derived from Acid Mine Drainage Sludge
## 3 Results and Discussion  
### Adsorbent Characterization  
Powder X-ray diffractometry (PXRD) was used to probe the crystal structures of the adsorbents of powdered GFH and MIRESORBTM. The XRD patterns of both adsorbent materials are shown in Figure 1. According to the manufacturer, GFH consists of akgeneite (\(\beta\)-FeO(OH)) and iron hydroxide (Fe(OH)\({}_{3}\)). The PXRD pattern of GFH clearly shows a diffraction pattern that corresponds to akageneite [24], while patterns attributed to iron hydroxide could not be resolved due to overlapping peak positions with akageneite (Figure 1). By comparison, the PXRD pattern of MIRESORBTM shows only weak diffraction peaks originating from iron hydroxide [25]. The diffraction patterns of both GFH and MIRESORBTM, however, exhibit low peak intensities that reflect low crystallinity. The PXRD analyses establish that the GFH and MIRESORBTM predominantly consist of akageneite and iron hydroxide, respectively. Scanning electron microscope images of the grains and grain surfaces of MIRESORBTM and GFH are displayed in Figure S2. The surfaces of both samples have large grain sizes and are rough because of fine particles attached to the grain surface. Interestingly, the GFH adsorbent had smaller fine particles than MIRESORBTM, which may influence the surface area. The elemental compositions of the adsorbents were estimated by XRF analysis, as summarized in Table 1. Iron is the most abundant element in GFH and MIRESORBTM and accounts for the crystalline phases observed in the PXRD patterns. The GFH has a higher iron content of 93.0% versus the 78.1% iron content in MIRESORBTM. Magnesium and aluminum are minor elements in GFH adsorbents. MIRESORBTM contains a broader range of elements, which can vary because the composition of the precursor sludge depends on the source of mine drainage [19]. The composition of MIRESORBTM includes Fe (78.1%), Al (9.20%), Ca (5.73%), and Mg (4.0%), which are the major constituents of AMDS. These analyses indicate that MIRESORBTM materials have high amounts of iron oxide and low Ca, Si, Al, and Mn contents. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Adsorbent** & **Mg** & **Al** & **Si** & **Ca** & **Fe** & **Ni** & **Zn** \\ \hline MIRESORBTM & 4.00 & 9.20 & 1.92 & 5.73 & 78.1 & 0.12 & 0.12 \\ GFH & 4.40 & 2.19 & 0.15 & 0.15 & 93.0 & ND & ND \\ \hline \end{tabular} Note, elemental composition less than 0.05% was regarded as ND.  
\end{table} 
Table 1: Elemental compositions estimated by X-ray fluorescence spectrometry. Figure 1: Powder X-ray diffractogram of granular ferric hydroxide (GFH) (**black line**) and MIRESORBTM (**red line**). The specific surface area (SSA) and pore area are important factors for adsorption and were estimated for the MIRESORBTM and GFH by analyzing N2 adsorption/desorption isotherms by using the BET method (Figure 2). The SSAs of powdered MIRESORBTM and GFH were established as 233 and 306 m\({}^{2}\)/g, respectively (Table 2). The GFH exhibits approximately 24% higher SSA than MIRESORBTM, which is due to the smaller fine particles on the surface, as evidenced by SEM images (Figure S2). As shown in Figure 2a, the N2-adsorption isotherms of GFH and MIRESORBTM have different types of hysteresis loops, namely, type II and type IV for MIRESORBTM and GFH, respectively. This indicates that MIRESORBTM adsorbents likely have a macropore structure and promote unrestricted mono-multilayer adsorption as compared with the type IV hysteresis of GFH, provides evidence of capillary condensation in mesopores [26]. The pore volume and pore size of the adsorbents were estimated by the Barrett, Joyner, Halenda (BJH) method and are summarized in Table 2. The pore volume was calculated to be 0.33 cm\({}^{3}\)/g for GFH materials and 0.57 cm\({}^{3}\)/g for MIRESORBTM materials. Moreover, the pore volume distribution graph shows that MIRESORBTM materials have a larger fraction of pores with diameters over 10 nm (Figure 2b). The pore sizes in MIRESORBTM materials are also approximately 66% larger than that for GFH materials.