A preliminary investigation of the removal of rust stains from globigerina limestone surfaces using coupled chelating-reducing systems

Abstract

This study focuses on the removal of orange-brown iron oxy-hydroxide stains from the surface of artificially stained Globigerina Limestone test slabs. Two cleaning treatments were evaluated: (i) a 1:1 by mass sodium dithionite: tri-ammonium citrate solution and (ii) a thioglycolate solution. The two cleaning solutions were buffered at pH 8.5 using ammonium carbonate - ammonium bicarbonate buffer. The concentration of the cleaning solution and the contact time between solution and rust stain were varied in an attempt to determine the optimal application conditions. Stain removal was evaluated by visual observation and via a colorimetric method. As expected, the higher the concentration of dithionite—citrate solution, the more efficient was the extraction of iron stains although long treatment times with this reagent did not improve cleaning. Anomalous results were obtained when the stained stone was treated with thioglycolate. In general, the best results were obtained using medium concentrations (1-2%) of cleaning agents and short treatment periods (10-30 minutes). Application of cleaning solutions to unstained Globigerina Limestone control slabs caused considerable orange-brown staining of the stone surface when the stronger concentrations of cleaning agent or longer reaction times were employed. This was interpreted as being caused by extraction at depth following re-precipitation at the surface of the iron minerals present naturally in the stone. Such mobilization of iron species appears to explain the anomalous results obtained when stronger concentrations of cleaning agent and longer retention times were used on the stained test slabs.