The dissolution rates and mechanism of tetragonal ferrous sulfide (mackinawite) in anoxic aqueous systems

Citation

Pankow, James Frederick (1979) The dissolution rates and mechanism of tetragonal ferrous sulfide (mackinawite) in anoxic aqueous systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/31FS-0Q87. https://resolver.caltech.edu/CaltechETD:etd-03112009-074718

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. An experimental study was carried out on the rates and mechanisms of the non-oxidative dissolution of mackinawite (tetragonal FeS) in anoxic aqueous systems of varying pH (3-7), T (5-35°C), and ionic strength (0.05-0.60 M [M is underscored]). A special glass and teflon dissolution reactor was constructed. The main design criterion was the need to keep the reaction medium absolutely free of oxygen. The flux F[subscript S] (moles/cm[superscript 2]-min) from the surfaces of pellets pressed from [...]lμ FeS particles was found to obey the rate equation F[subscript S] = k[subscript 1]a[subscript H superscript +] + k[subscript 2] k[subscript 1] and k[subscript 2] are rate constants and a[subscript H superscript +] is the activity of the hydrogen ion At 25°C, k[subscript 1] = 0.22 cm/min with a relative standard deviation of 18%. The value of k[subscript 2] was measured to be 1.9 moles/cm[superscript 2]-min with a relative standard deviation of 22%. The actual exposed FeS surface area A[...] was related to the projected area A by the expression A[...] = 1.7A. The factor of 1.7 accounts for the roughness of the surface. The actual rate constants k[...] and k[...] were therefore obtained by dividing k[subscript 1] and k[subscript 2] by 1.7: k[...] = 0.13/min and k[...] = 1.1 x 10[superscript -9] moles/cm[superscript 2]-min. The k[...] term dominates at pH < 4.3 and k[...]dominates at pH > 5.6. pH 4.3-5.6 is a transition region. It is argued that these rate constants are relatable to dissolution mechanisms at the FeS crystal surface. k[...] is envisioned to arise from an attack at the surface by H[superscript +]. k[...] is linked to a mechanism which relies upon normal thermal vibrations and H20 solvation effects to liberate lattice constituents. Both k[…] and k[…] are thought to reflect a rate-limiting departure of surficial S(-II) lattice constituents. The activation entropies [...] for these two processes are calculated accordingly. The activation enthalpies […] were determined from the E[subscript a] data. Concentrations of Cu(II) in the ppm range were found to strongly inhibit the dissolution rate. The k[...] term would dominate at pH's typical of natural waters. This term was used to determine how quickly FeS dissolves when anaerobic sediments are disturbed. When anoxic and oxic sediments are adjacent to one another, the dissolution of FeS in the anoxic portion takes place primarily near the oxic front. If FeS-containing anaerobic sewage sludge is discharged to anoxic receiving waters, the particles of FeS may be expected to dissolve. Expressions were developed by which the lifetimes of such particles may be calculated. Both transport and non-transport controlled situations are analyzed. The effects which the agglomeration of such FeS with other sewage particles would have on the dissolution rate were considered. The implications which this work holds for sediment pyritization rates were also discussed.

Thesis Files