Microscopic single particle characterization of zeolites synthesized in a soil polluted by copper or cadmium and treated with coal fly ash

In the perspective of the development of new soil remediation technologies, zeolites can be directly synthesized in soil from fused coal fly ash to reduce heavy metal mobility and availability. Such a process promotes the formation of metal hydroxide/oxide precipitates which can be also occluded inside the structure of the forming minerals. In this study, different types of zeolites (zeolite X, P, and A) were synthesized by treating soil samples, artificially contaminated by high concentrations of Cu or Cd ions, with fused coal fly ash at 30 and 60 °C. The formed zeolites were characterized for their amount, structure, chemical composition and size. To accomplish this survey, besides quantitative X-ray diffraction analyses (XRD), an automated single particle analysis (ASPA) method using electron probe X-ray microanalysis (EPXMA) was employed for the first time for direct zeolite characterization in soil samples. The obtained results provide new information for assessing the role of heavy metals in zeolite crystallization in coal fly ash-treated soils. Heavy metal contamination, while not significantly hindering the zeolite formation process, can influence crystal size and preferentially drive zeolite synthesis toward the formation of sodalite unit-based zeolite X and zeolite A, even at 60 °C. The presence and nature of metal precipitate occlusions inside the forming zeolite minerals might have also favored the preferential synthesis of certain zeolitic structures.