The interfacial electrochemistry of goethite (α-FeOOH) Part II. Ion adsorption and HSAB: Mercury is “soft” but goethite is “hard”

Abstract

The chemical behaviour of species can be qualitatively predicted from their “hardness” through the HSAB principle which states that complexes of “hard” (Lewis)) acids with “hard” (Lewis) bases (or of “soft” acids with “soft” bases) have an added stability. The behaviour of mercury indicates that it is “soft”—for example the very”hard“ ions F − , H + and OH − are indifferent, at least in the region of the pzc, whereas the “soft” ion I − is specifically adsorbed. In contrast F − , H + and OH − are potential-determining for goethite. In this paper the hypothesis that goethite is “hard” is proved through comparison of the surface charge density/pH functions (and their first derivatives, which are akin to differential capacitance/surface potential functions) of CO 2 -free goethite suspended in NaI and NaCl solutions and CO 2 -contaminated goethite suspended in NaI solution.