average pK values of various iron oxides (!) from literature ([APQ67] (goethite and haematite), [S79] (goethite), [BL71] (haematite), [AM68] (magnetite); SCM in literature sometimes not clear; PZC=1/2(pK1+pK2); C fixed at 1.06 F/m^2 which was optimal for g-Al2O3 [WH80]; SSA taken from [S79]