Coprecipitation of Pb and As in apatite and applications to environmental remediation

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The crystal chemistries of apatite group minerals (a.k.a. apatites) have been widely investigated. They can accommodate numerous substitutions, including cations, polyanions, and monoanions. Thus, they play a significant role in many geochemical and environmental processes such as elemental cycling, availability of nutrients, and contaminant transport and fate. Formation of apatites has been studied as a means for contaminant remediation, especially for heavy metals such as Pb, Zn, Cd, Cu, and Ni, through sequestration by sorption or coprecipitation, and has been shown to be effective due to its high affinity for these metals and its low solubility under earth?s surface conditions. Thus, once co-precipitated in the highly stable apatite structure metals are no longer easily dispersible or bioavailable. However, there is still a great deal to be learned about the sequestration of other metals and the effect of combined metal uptake. One metal of enormous environmental significance and with great potential to be remediated through apatite sequestration is As. However, there are a limited number of studies focused on coprecipitation of As in apatite and the properties of solid solutions involving this metal. Furthermore, Pb and As often occur together in the environment due to natural processes (e.g. weathering of ore deposits) and human activities (e.g. smelting), and even less work on the simultaneous coprecipitation of Pb and As in apatite exists, despite the fact that natural Pb-As apatites occur (mimetite, clinomimetite). This proposal focuses on I) synthesis and elucidation of the chemical and physical properties of apatite group minerals in the Ca-Pb-PO4-AsO4-OH-Cl system for evaluation of their application to simultaneous As and Pb remediation and II) testing the application of Ca-PO4 solutions for sequestration of As, and combined As and Pb, through apatite precipitation in contaminated sediments. PI will investigate the extent of possible solid solutions, the kinetics of crystallization reactions, the possible pathways of formation, the structure and crystal chemistry, and the thermodynamics of synthesis reaction products. He will conduct a site-specific case study at a former smelting facility with known As and Pb contamination of local sediments. Engineered contaminated sediments will also be investigated to help elucidate reactions taking place in the natural samples. The results of this study will greatly increase our knowledge of the fundamental mineralogy of some of the least understood apatite group minerals with wide geologic implications, for example the study of ore deposit petrogenesis, and the general geochemical cycling of As and Pb. Furthermore, the results may lead to the effective application of these minerals to environmental remediation. The broader impacts of this proposal are many. The work has broad environmental implications (heavy-metal sequestration and stabilization) that are of immense importance to society. It may also lead to a more cost effective remediation technology for As and Pb. The proposed research will include the participation and education of undergraduate and graduate students. Outreach and education of the general public, specifically targeting the amateur scientist and mineral enthusiast communities, will be facilitated through talks at museums and amateur societies, and publications in journals for a diverse non-professional audience.