Iron (Fe) is an important trace element as it is utilised by all life on Earth. Fe(III) reduction and microaerophilic Fe(II) oxidation is well understood, however, much less is known about anaerobic Fe(II) oxidation. Under anoxic conditions bacteria are capable of coupling nitrate reduction to Fe(II) oxidation. In this thesis the extent to which Fe(II) oxidation is either chemically or biologically catalysed is quantitatively determined using process-based numerical models at the laboratory and field-scale. Furthermore, the controlling processes determining Fe(III) mineral transformation pathways and rates were also investigated to understand how Fe(III) minerals may evolve after their initial formation.