Mechanism and Kinetics of Dehydration of Epsomite Crystals Formed in the Presence of Organic Additives

Abstract

The thermal dehydration of epsomite (MgSO4*7H2O) crystals grown in the presence and absence of organic additives (phosphonates, carboxylic acids, and polyacrylic acid derivatives) was studied by means of thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray thermodiffraction (XRTD), and environmental scanning electron microscopy (ESEM). In situ XRTD analyses (in air, 30% relative humidity) show an -->epsomite hexahydrite (MgSO4*6H2O) transition at 25-38 degrees C, followed by formation of amorphous phase(s) at T > 43-48 degrees C, and MgSO4 crystallization at approximately 300 degrees C. Kinetic parameters (E(alpha) and A) were determined for the main dehydration step (25-160 degrees C), which corresponds to a MgSO4*7H2O-->MgSO4*H2O transition, by applying two isoconversional methods to nonisothermal TG data obtained at different heating rates (beta= 1, 3, and 5 K*min-1). In situ, hot-stage ESEM observations of the thermal dehydration of epsomite crystals are consistent with the nonisothermal kinetic study and, along with XRTD results, allow us to propose a dehydration mechanism which includes an early nucleation and growth event, followed by the advancement of the reaction interface (3D phase boundary reaction). Both E(alpha) and A values increase in the presence of the most effective crystallization inhibitors tested. H-bonding between additives and epsomite crystal surfaces is consistent with Fourier transform infrared spectroscopy (FTIR) and may account for this effect. The increase of E(alpha) values can be related to the excess energy required to break additive-water bonds in the reactant. These results are likely to further our understanding of the interaction mechanisms between salt hydrates and organic additives which act as growth inhibitors/modifiers.

Molecular dynamics study on thermal dehydration process of epsomite (MgSO4·7H2O)

“…The RDF profiles in the final configuration differ from the RDF profiles of crystalline MgSO 4 ( Figure 6(a),(d)), indicating that the newly formed anhydrous MgSO 4 possesses a disordered structure and therefore has a different RDF. Both van Essen et al [2] and Ruiz-Agudo et al [30] mentioned a recrystallisation process of the dehydrated amorphous precursor. However, this possibly slow reorganisation of the lattice structure was not observed in the simulation.…”

Interaction between Epsomite Crystals and Organic Additives

“…Therefore, it should be considered that the salt crystal surface becomes increasingly negative and may thus electrostatically repel the ionized inhibitor molecules as pH rises. It has been claimed that the latter effect accounts for the reduction in nucleation inhibition at high pH in the case of BaSO 4 39 and Na 2 SO 4 · 10H 2 O. 29 Another possibility is that the crystal surface loses hydration water protons, therefore limiting its capacity to establish hydrogen bonds with the inhibitor molecule.…”

“…The sulfate anion presents four normal modes in the infrared region: a nondegenerate symmetric stretch υ 1 , a doubly degenerate symmetric bending υ 2 , and two (triply degenerate) asymmetric stretching and bending, υ 3 and υ 4 , respectively. 61 Changes in solvation, metal complexation and protonation of SO 4 2-can modify S-O bond length and, as a result, may change the symmetry of the anion. This leads to a shift in the vibrational bands to different wavenumbers and causes the degenerate vibrations to become nondegenerate.…”

“…32,33 Some phosphonates inhibit the crystallization of mirabilite (NaSO 4 · 10H 2 O), thus representing a potential treatment for materials affected by this damaging salt. 29 However, little is known about 4 On the other hand, although much research has been dedicated to the study of the interaction between sparingly soluble alkali earth salts and organic additives, very little is known about the interaction of organic additives with divalent cations in highly soluble salts such as magnesium sulfates. To our knowledge only two papers studied the effects of organics, a surfactant (sodium dodecyl sulfate) 50 and urea, 5 on the crystallization of epsomite.…”

Can drying and re-wetting of magnesium sulfate salts lead to damage of stone?

“…At least 8 identified metastable crystalline phases with 1, 1.25, 2, 3, 4, 5, 11 and 12 water molecules can form by hydration or dehydration of the stables salts (e.g. see Chipera and Vaniman 2007;Paulik et al 1981) as well as one amorphous phase Ruiz-Agudo et al 2007b).…”