a 0.5 M HCl is a moderately corrosive first step and therefore it is not possible to differentiate the release of divalent ions from loosely sorbed / exchangeable phase or as a result of dissolution of amorphous or soluble phases . Furthermore , the first step was originally used for analysis of Fe from within a different sediment environment . The implications regarding these points are included in the results / discussion section as the authors consider the results provide valuable supporting data .