Investigation of organic matter entrapped in synthetic carbonates
Résumé
Organic matter (OM) entrapped in calcite is regularly used for environmental studies; however, insertion mechanisms and types of interaction remain poorly understood. The present study used a new methodology to investigate interactions between OM and the calcite matrix during crystallization processes with humic acid (Aldrich and natural) entrapment. As a first step, we conducted a detailed study of the effect of HS adsorption on calcite during crystallization processes, and the effect of this adsorption on the HS. We then investigated the ways in which the structure of the HS evolves as it is absorbed into the calcite matrix. We monitored the physico-chemical properties of the HS using fluorescence and x-ray absorption spectroscopy (XAS). Morphological and structural analyses at various scales combined with spectroscopic measurements confirmed that OM is both adsorbed onto the calcite surface and incorporated into the calcite lattice during crystallization. From a quantitative point of view, we showed that incorporation of HS is proportional to the HS concentration of the precipitating solution and, through the combined use of x-ray and FTIR spectroscopy, we demonstrated the partitioning of HS during crystallization. Structural investigations of the calcite matrix did not reveal any effect of OM insertion on calcite structure. Based on synchrotron analysis, we also developed a new method in which sulfur is used as a tracer element for entrapped humic acid and to localize the OM electrostatically adsorbed onto the calcite surface. Changes in the sulfur environment, determined using XANES, indicated partitioning during calcite crystallization due to the effect of the matrix on OM insertion. Desorption experiments showed the stability of the OM atomic structure and revealed the layered nature of that structure. These results allowed us to draw up a general model of OM insertion in calcite, and then a comparison between the behavior of Aldrich HS and natural HS could be given.