3.1 Goethite physical-chemical characterization
Goethite was synthesized in the laboratory to be used as simulating the iron (hydr)oxide present naturally in groundwater and being characterized by several techniques to confirm its structure. XRD diffractogram of synthesized goethite (Figure 1a) exhibited peaks at 21.22°, 33.16°, 34.71°, 36.67°, 41.27°, 53.29°, and 59.09° ascribed to the presence of α-FeOOH (JCPDS-29-0713) .
Figure 1b shows SEM micrographs at 2000x and EDS spectrum of synthesized goethite revealing irregular flake-shape morphology and surficial presence of iron, oxygen, and carbon.
Observing the XPS survey spectrum (Figure 2a), it was evidenced the presence of signals attributed to Fe 2p, C 1s (due to adventitious carbon), O 1s, and S 2p (coming from residual FeSO4) while the high-resolution spectrum of Fe 2p (Figure 2b) exhibited two signals at 711.6 and 725.8 eV often assigned to Fe3+2p3/2 and Fe3+ 2p1/2in goethite [27–29].
Mössbauer measurements (Figure 3a) showed an asymmetric sextet with broadened lines, a hyperfine magnetic field (Bhf) around 33 T, and a quadrupole splitting (QS) of 0.45 mm s-1matching very well with goethite structure . Broadened lines and asymmetry could be related to poor crystallinity as it was confirmed by XRD diffractograms and the presence of Fe(II) respectively, the latter coming from synthesis precursor FeSO4. DRS spectrum revealed a broad UV-vis light absorption comprised between 300-700 nm typical of iron (hydr)oxides such as goethite (Figure 3b). The Goethite valence band consists of O 2p and occupied Fe 3d orbitals while its conduction band is composed of unoccupied Fe 3d orbitals. Both bands are separated through a band gap energy of 2.7 eV [31].
N2 adsorption-desorption isotherms showed a typical type IV isotherm with hysteresis cycle H-3 corresponding to mesoporous materials according to the IUPAC convention, an average pore width around 16 nm (Figure 4) and BET-specific surface area of 98.31 m2 g-1. Similar textural features have also been reported for goethite .