# Low-Cost Goethite Nanorods for As (III) and Se (VI) Removal from Water
## 3 Results and Discussion
### Structural and Morphological Study
However, a sharp loss of water molecules was observed in the 150-375 \({}^{\circ}\)C temperature range, attributed to lattice and interlayer water losses [31]. Consequently, a little amount of weight loss was also observed in a wider temperature range of 370-675 \({}^{\circ}\)C. This slight loss is attributed to the structural deformation, indicating a complete phase transformation of \(\alpha\)-_FeOOH_ to \(\alpha\)-_Fe\({}_{2}\)O\({}_{3}\)_ at temperature 375 \({}^{\circ}\)C. Also, oxidation of the organic and surface residues is represented by exothermic bands at 155 to 370 \({}^{\circ}\)C [32]. DSC curve (black curve of Figure 2d) supports the findings of TGA analysis. It shows a similar two critical points. The first transition at 160 \({}^{\circ}\)C is due to the endothermic nature of water molecules removal, indicating that this powder contains a large proportion of H\({}_{2}\)O molecules, plausibly due to low temperature drying. Likewise, the second transition observed at \(\sim\)375 \({}^{\circ}\)C represents a complete phase transformation to \(\alpha\)-_Fe\({}_{2}\)O\({}_{3}\)_[31; 32]. This decomposition temperature is larger than many previous studies, since it is dependent on the particle size and morphology. In other words, nanosize particles of \(\alpha\)-_FeOOH_ are thermally more stable than larger particles [10]. Zeta potential experiment (Nanoparticle analyzer, Horiba Scientific, Minami-ku, Kyoto Japan) was conducted using a light scattering method by a zeta analyzer. Figure 2e shows a typical representation of surface charge density that is commonly used to evaluate the dispersion of nanoparticles at varied pH. In fact, this is a commonly used procedure to present the surface charge observations in an attempt to study the interactions between the particles and their aqueous surroundings, and therefore their stability as well. The point of zero charges (PZC) was measured to be at pH 5.2. However, the results implied that zeta potential, in general, has relatively low magnitude values with a range of 52 mV at lower pH and \(-\)32 mV at higher pH values. Indeed, the lower the value of PZC, the weaker is the repulsive force (higher attractive forces) between particles, and therefore aggregation of the particles is plausibly the feasible phenomenon [19]. Accordingly, an evident decrease in the specific surface area in contact with pollutant ions leading to a decrease in As and Se removal. \(\alpha\)-_FeOOH_ NRs have an affinity to aggregation at different conditions, which in turn affects its interactions with the surface contaminants [19]. At certain pH, the hydroxyl ions are deprotonated, resulting in a negative surface charge (exothermic at room temperature on \(\alpha\)-_FeOOH_ NRs. These exothermic surfaces of \(\alpha\)-_FeOOH_ were experimentally verified for As(III) and Se(VI) oxyanions and the results showed weaker interaction even with the increase in solution temperatures [33; 34]. At these conditions, the interaction forces get weaker and the adsorption sites are occupied by hydroxyl ions. Therefore,adsorption is significantly decreased at these pH values. Finally, Figure 2f depicts the energy dispersive spectroscopy (EDS) measurements. The results show that iron and oxygen molecules are the only constituents of the powder indicating that no other contaminants in the powder and thus the purity of \(\alpha\)-FeOOH polymorph. Figure 3 shows the morphological and structural characterization of \(\alpha\)-FeOOH. Particularly, Figure 3a displays plane view field emission scanning electron microscope (FE-SEM, Zeiss ultra 55) micrograph of bulk \(\alpha\)-FeOOH NRs, clearly depicting the uniform nanorods. Of note, the micrograph shows very small spherical-shaped particles attached to the nanorods with typical dimensions of ~10 nm. These observations correlate well with crystallite size calculated from the XRD diffractogram.