Fig. 7 The attenuated total reflectance–Fourier transform infrared (ATR–FTIR) difference spectra of am-Fe(OH)₃ equilibrated at pH 5.0 with (a) 0.1 M Na₂SeO₄ + 1.0 M NaCl, (b) 0.05 M Na₂SeO₄ + 1.0 M NaCl, and (c) 1.0 M NaCl alone. The BET N₂ surface area of am-Fe(OH)₃ was 250 m² g^-1.The subtracted reference spectra were the supernatant solutions. The initial solid concentration was 200 g L^-1 before centrifugation and the solid concentration in the ATR reservoir was 1000 g L^-1. The interfacial selenate spectra were presented as (a–c) and (b–c), respectively