Fig. 1. X-ray diffraction patterns: (a) 79/05R, reference sample formed by rapid oxidation of a FeCl₂ solution at pH 5.5; (b) 79/05; (c) 79/07, same as 75/05R, but in the presence of dissolved organic matter (DOM); and (d) 74/2, for comparison, four-line ferrihydrite, precipitated by rapid hydrolysis of a Fe(NO₃)₃ solution at pH 7 (Schwertmann et al., 2004).