Table 1.

XPS peak parameters for Fe 2p, As 3d, S 2p, O 1s, and Al 2p spectra.

Spectral peak	Binding energy (eV)	FWHM	Chemical states
Fe 2p3/2a	707.1 ± 0.1	1.00	Fe(II)–AsS
Fe 2p3/2b	709.7 ± 0.1	1.30	Fe(III)–AsS
Fe 2p3/2b	711.2 ± 0.1	1.50	Fe(III)–O
As 3d5/2c	40.9 ± 0.1	1.00	As(–I)–S
As 3d5/2c	41.5 ± 0.1	1.00	As(0)
As 3d5/2c	43.6 ± 0.1	0.95	As(I)–O
As 3d5/2c	44.4 ± 0.2	0.95	As(III)–O
As 3d5/2c	45.4 ± 0.3	0.95	As(V)–O
S 2p3/2d	161.4 ± 0.3	1.2	Monosulfide (S2–)
S 2p3/2d	162.0 ± 0.1	1.0	Disulfide (S22–)
S 2p3/2d	163.0 ± 0.2	1.6	Polysulfide (Sn2–)
S 2p3/2d	166.3 ± 0.1	1.2	Thiosulfate (SO32–)
S 2p3/2d	167.2 ± 0.2	1.2	Sulfate (SO42–)
O 1s	531.0 ± 0.2	1.6	Lattice oxygen (O2–)
O 1s	532.0 ± 0.3	1.6	Hydroxyl oxygen (OH–)
O 1s	533.2 ± 0.5	1.6	Attached water (H2O)
Al 2p3/2	73.9 ± 0.1	1.0	Al–O in boehmite
Al 2p3/2	74.3 ± 0.1	1.0	Al–OH in bayerite & gibbsite
Al 2p3/2	74.8 ± 0.1	1.0	Al–OH in bayerite

^aThis peak has two multiplets located at lower and higher binding energies with 0.95 eV peak separation.

^bThis peak has three multiplets located at higher binding energies with 0.95 eV peak separation.

^cThis peak has a doublet located at a higher binding energy with 0.7 eV peak separation. The intensity ratio was constrained to two thirds with the same FWHM.

^dThis peak has a doublet located at a higher binding energy with 1.18 eV peak separation. The intensity ratio was constrained to one half with the same FWHM.