The Important Role of Layered Double Hydroxides in Soil Chemical Processes and Remediation: What We Have Learned Over the Past 20 Years

Soils are a nonrenewable resource and particularly vulnerable to long-term environmental contamination. Sorption reactions on soil mineral surfaces affect dissolved metal mobility and influence the fate of metal contaminants in soils and groundwater. Since the early 1980s, the Environmental Soil Chemistry group at the University of Delaware has studied sorption reactions of metal contaminants and plant nutrients on clay minerals and soil components using a multitude of spectroscopic, microscopic, thermochemical, kinetics, and wet chemical techniques to identify adsorption species and surface precipitates. Of particular significance has been the formation of layered double hydroxide (LDH) surface precipitates. Many research areas focus on LDHs; however, this review highlights environmentally related LDHs due to their impact on limiting metal contaminant mobility in soils and sediments. This chapter, in three sections, consists of (1) a literature review of LDH research done by this group and others, (2) the structure and thermodynamic properties of LDHs, and (3) the kinetics and proposed mechanisms of environmental LDH formation in geochemical systems such as in soils, sediments, and mineral surfaces. This review mainly discusses Ni, Zn, and Fe(II)–Al LDHs. Particular emphasis is placed on comparing proposed mechanisms of formation and distinguishing between isomorphous substitution and heteroepitaxial growth. Ultimately, evidence from quick-scanning X-ray absorption spectroscopy and Al precipitation kinetics yields the conclusion that heteroepitaxial growth (surface precipitation) on clay mineral surfaces is the manner in which environmental LDHs form.