Layered double hydroxide and zirconium phosphate as ion exchangers for the removal of ‘black crusts’ from the surface of ancient monuments

Abstract

The chloride form of MgAl layered double hydroxide (hereafter MgAlCl) as an anion exchanger and the semisodic form of α-zirconium phosphate (hereafter ZrPNaH) as a cation exchanger are proposed as new cleaning agents for the removal of gypsum from ancient monuments. The ability of these exchangers to capture the calcium and sulphate ions of the gypsum powder was first investigated separately and then as a coupled system. MgAlCl/gypsum, ZrPNaH/gypsum and MgAlCl/ZrPNaH/gypsum mixtures were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and dispersive X-ray spectroscopy (EDX). ZrPNaH in the form of a wet paste exhibited a rapid uptake of calcium from gypsum powder via Na⁺ and H⁺/Ca²⁺ cation exchange. Gypsum powder was also successfully dissolved by a wet paste of MgAlCl by exploiting the Cl⁻/SO₄²⁻ anion exchange reaction. However, the dehydration of the paste favoured the reprecipitation of a secondary gypsum that was characterized by lower crystallinity and smaller particle size than the pristine gypsum. The combination of wet MgAlCl and ZrPNaH showed a synergic effect on the dissolution of gypsum and partially prevented the reprecipitation of gypsum in the dry paste. Finally, a preliminary test of the removal of gypsum crust grown on a sandstone sample was performed.