Permeable Reactive Barriers for Containment Control in Beneficial Use Applications in Highways

Reactive barriers are used to remediate contaminated sites where the potential for leaching of heavy metals exists. Permeable reactive barriers (PRB) could be used in highway applications below either embankments or base courses constructed with recycled materials where a possibility of contaminant leaching exists. The objective of this project is to develop technologies that use apatite minerals as PRB applied in highway construction that use recycled materials in alternative construction applications. Sorption of heavy metals and radionuclides onto apatite has been well demonstrated. Interest in apatite is primarily due to the high stability of the metal-phosphate minerals that result from the interaction between the contaminants and the apatite. However, the exact molecular scale mechanism of removal is not completely understood. This research sought to investigate the sorption mechanism using a combination of methods, including batch tests and surface analytical techniques, to study how Zn2+ is immobilized by apatite. Three different synthetic apatites were used in this investigation: hydroxyapatite (Ca5(PO4)3OH), fluoroapatite (Ca5(PO4)3F) and carbonate apatite (Ca5(PO4,CO3)3(OH)). BET isotherms indicate that at low Zn2+ concentrations sorption proceeds as surface complexation and at increased concentrations it is followed by the formation of a co-precipitate and in turn a pure precipitate. The zeta potential analysis is in agreement with the proposed complexation – co-precipitation/precipitation mechanism. In this study two distinct rates of change were observed in the surface charge (mV) of the apatite particle as a function of Zn2+ loading (mmol Zn/g Apatite). The spectroscopic analysis indicates that co-precipitation begins to occur even at concentrations well below the solubility limit of Zn – minerals. The XAS spectra of HAP and CAP at 0.0016 mM had very strong similarities to the mineral structure of scholzite, a naturally occurring Zn-phosphate mineral and to synthetic zinc phosphate, respectively. At 0.92 - 0.99 mmol Zn/g apatite XPS identified scholzite and hopeite as likely species present at the particle surface. It is proposed that dissolution-precipitation reactions account for the formation of a Zn-phosphate solid solution, this indicates that co-precipitation begins to occur well before the maximum sorption capacity, qmax, predicted by the Langmuir isotherm. At the elevated Zn2+ loadings of all three apatites, 8.22 mmol/g HAP, 4.48 mmol/g CAP and 2.74 mmol/g FAP, XAS evidenced similarities to hydrozincite, (Zn5(CO3)2(OH)6) and XPS identified the likely presence of zinc carbonate hydroxide, [ZnCO3]2·[Zn(OH)2]. The formation of a zinc carbonate hydroxide implies that dissolved CO3 plays an important role in the complexation of Zn2+, under the given experimental conditions. Results from this research suggest that surface complexation, surface precipitation, co-precipitation and precipitation all play an important role in the sorption mechanism of Zn2+ onto apatite. This identification of a Zn-phosphate co-precipitate and hydrozincite emphasize that although the formation of Zn-phosphates does occur, competing reactions also lead to the formation of other Zn complexes such as Zn carbonates and/or hydroxides.

Language

  • English

Media Info

  • Media Type: Digital/other
  • Edition: Final Report
  • Features: Appendices; Bibliography; Figures; Tables;
  • Pagination: 91p

Subject/Index Terms

Filing Info

  • Accession Number: 01531745
  • Record Type: Publication
  • Files: TRIS, ATRI, USDOT
  • Created Date: Jul 18 2014 1:10PM