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Simulated organic acid weathering of granodiorite and basalt

University of British Columbia

Basalt and granodiorite (medium to fine sand particle size) were leached by three low molecular weight aliphatic organic acids, namely citric, oxalic and acetic acid of . 1M concentration. To evaluate the effectiveness of the organic acids in dissolving the rock samples, two control solutions, .005M HCl and distilled water were added to the number of dissolution treatments. Dissolution of ions from granodiorite, by the leaching treatment shown in parenthesis, decreased in the following order: Fe(OX) > Al(OX) ~ Si(OX) > Ca(CIT) ~ Mg(OX) ~ K(OX) > Na(OX) Dissolution of ions from basalt decreased in the following order: Fe(CIT) > Si(CIT) > Mg(CIT) > Ca(CIT) > Al(OX, ACETIC) > Na(OX) > K(HCl) Oxalic acid effectively outcompeted citric acid in the weathering of granodiorite in spite of having lower stability constants for certain elements. This was attributed to differences in pH of the solutions (affecting both concentration of H⁺ ions and anionic species), ionic competition in solution for ligand sites and geometry and oxidation state of ions in the parent mineral. There was no conclusive evidence to indicate that chelation of K⁺ or Na+ took place in any of the experiments. However, mass balance calculations revealed that oxalic acid extracted approximately 40-50% of the K from granodiorite; citric acid extracted approximately 12%. These levels were significantly higher than those extracted by non-sequestering agents. XRD analysis of granodioritic-oxalate precipitate suggested the possible formation of a K-oxalate salt. The chelating acids, citric and oxalic, greatly outcompeted acetic acid and HCl of similar pH for multivalent cations in both basalt and granodiorite. Concentrations of Fe, Al, and Si, in solution were many fold higher than calculated concentrations of those ions in equilibrium with the amorphous oxide in water. Oxalic acid and citric dissolution curves, determined from 11 weeks of leaching, showed initially increasing rates followed by declining rates which approached steady state towards the eleventh week of the experiment. Declining rates followed by steady-state rates were attributed to the presence of hyperfines the build-up of secondary precipitates, the increase of ions in solutions, and to an eroding leached surface layer. Non-chelating acids namely acetic acid, HCl and H₂O revealed dissolution curves that were approximately constant (steady-state) throughout the 11 week weathering period. XRD, XRF, SEM, and EDX analyses of weathered basalt and granodiorite as well as AA spectrophotometric solution analyses provided evidence which indicated incongruent dissolution of granodiorite and basalt by all 5 leaching treatments. SEM and XRF analyses indicated that citric acid was less effective than oxalic acid in forming precipitates from granodiorite. EDX revealed that the amorphous precipitate which did form in citric acid consisted primarily of Fe. EDX analyses of inorganic coatings indicated predominantly Si and Fe in 1:1 ratio. Although citric acid was able to extract greater amounts of Fe from basalt than granodiorite, extractable Fe, Al and Si analysis and SEM detected no organo-amorphous precipitates. Also EDX of inorganic surfaces showed no accumulation of Fe or Al. It was concluded that the Fe extracted from the basalt remained in complexed or soluble form due to the higher pH (3-5) of the basaltic solution. An amorphous precipitate formed from the leaching of granodiorite with oxalic acid. EDX analysis gave evidence that the precipitate consisted primarily of Si and Fe in a 1:1 ratio. An amorphous precipitate formed also from the leaching of basalt with oxalic acid. EDX analysis gave evidence that the precipitate consisted primarily of Mg and Fe in a 1:1 ratio. Data from molar oxide ratios and mass balance calculations as well as XRD, XRF, extractable Fe, Al and Si analyses were relied upon to determine the possible organic and inorganic components of these precipitates.

Text

2010-08-14

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http://hdl.handle.net/2429/27386

Soil Science

University of British Columbia

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