Award Date

5-1-2013

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geoscience

First Committee Member

Elisabeth M. Hausrath

Second Committee Member

Eugene Smith

Third Committee Member

Scott Nowicki

Fourth Committee Member

Gary Cerefice

Number of Pages

35

Abstract

Phosphate is an essential element for life on Earth, and therefore if life exists or ever existed on Mars it may have required phosphate. Amorphous Al- and Fe-phosphates rapidly precipitate from acidic solutions and amorphous Al-phosphates likely control phosphate concentrations in some natural waters on Earth. Amorphous phases may be even more important on Mars than on Earth, and amorphous phosphates are therefore likely important in the phosphate cycle on Mars. Despite this importance, however, few dissolution rates exist for amorphous Al- and Fe- phosphates. In this study, dissolution rates of amorphous Al- and Fe-phosphates were measured in flow-through reactors from steady state concentrations of Al, Fe and P. A pH -dependent rate law was calculated from the dissolution rates log R = log k - npH, where Ri s the dissolution rate, k is intrinsic rate constant and n is the rate dependence on pH. For amorphous Al-phosphate, log k= -6.539 ± 1.529, and n= 2.391 ± 0.493. For amorphous Fe-phosphate, log k= -13.031 ± 0.558, and n= 1.376 ± 0.221. Amorphous Al-phosphate dissolves stoichiometrically under all conditions, and amorphous Fe-phosphate dissolves non-stoichiometrically, approaching stoichiometric dissolution as pH decreases, due potentially to Fe-oxides precipitating and armoring grain surfaces. Perhaps due to these effects, amorphous Al-phosphate dissolution rates are approximately three orders of magnitude faster than amorphous Fe-phosphate dissolution rates. Amorphous Al-phosphate dissolution rates measured in this study are also faster than published variscite dissolution rates. Dissolution rates of amorphous Al- and Fe-phosphates in this study therefore imply rapid phosphate release into acidic environments, suggesting phosphate mobility under Mars-relevant conditions.

Keywords

Aluminum phosphide; Amorphous substances; Astrobiology; Exobiology; Iron, Phosphoric salts of; Mars (Planet); Phosphate; Soils; Weathering

Disciplines

Astrophysics and Astronomy | Geochemistry | Geology

Language

English


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