The use of sequential extraction to examine iron and trace metal mobilisation and the case-hardening of building sandstone: a preliminary investigation

Abstract

Mobilisation of iron in building stone due to acid rainfall and chemical cleaning may cause its uneven precipitation, discoloration of the stone and the formation of an indurated iron-cemented surface crust. Breaching of the crust can result in accelerated internal excavation, by, for example, salt weathering. Therefore, to understand the decay of iron-rich sandstone, the conditions necessary for iron mobilisation and precipitation and how amorphous iron oxides bind trace elements within the stone must be known. This preliminary study moves away from traditional total-element analysis procedures that provide limited information on internal elemental distributions and mobility. A selective extraction protocol, developed to study element speciation in soils and palaeosols, was modified to extract metals from operationally defined phases within a weathered, iron-rich sandstone from a ca. 150-year-old building in Belfast, N. Ireland. These phases include amorphous Fe and Mn oxides that are known to be effective scavengers of trace metal ions and constitute the principal components of surface stains and cement. Knowledge of the relationships between Fe and Mn and associated trace metal suites may eventually allow the latter to be used as tracers for Fe/Mn movement in stone.