Leigh A Sullivan

A modified chromium-reducible sulfur method for reduced inorganic sulfur: optimum reaction time for acid sulfate soil.

Reaction times for 16 acid sulfate soil materials analysed using a modified chromium-reducible sulfur method varied between 10 and 15 min, regardless of whether the samples had been dried and ground prior to analysis or were analysed without pretreatment. The reaction time for a ground (<63 mm) pyritic rock sample was 20 min. An optimum reaction time of 20 min is recommended for analysing acid sulfate soil using the modified method; this reaction time is much less than the 1 h reaction time used in previous methods.

Quantitative elemental microanalysis of rough-surfaced soil specimens in the scanning electron microscope using a peak-to-background method

Scanning electron microscopy is very useful for morphological examination of pedofeatures. Whenever quantitative elemental microanalysis of pedofeatures has been required during such morphological examinations, either thin sections or polished resin-impregnated blocks across similar pedofeatures have had to be prepared to satisfy the smooth flat surface requirement of currently used electron microanalytical methods. This prerequisite has been accompanied by both conceptual and technical problems. A direct method for obtaining quantitative elemental microanalyses of rough-surfaced soil specimens in the scanning electron microscope is examined here. This method is based on the use of peak-to-background ratios in energy dispersive X-ray spectra. Analysis of a pyrite standard at a range of geometrys of analysis and of the iron sulphide minerals in two Holocene sediments indicates that the peak-to-background method can be used to obtain reliable quantitative elemental compositions of rough-surfaced soil specimens in the scanning electron microscope. Under ideal operating conditions (i.e., flat, polished specimens), the peak-to-background method will not be as accurate as the currently used peak integral methods ; however, the results here indicate that the peak-to-background method has, for the analysis of soil, the important advantage of being able to directly provide reliable quantitative elemental microanalyses of pedofeatures on rough-surfaced soil specimens in the scanning electron microscope.

Seawater-induced mobilization of trace metals from mackinawite-rich estuarine sediments

Benthic sediments in coastal acid sulfate soil (CASS) drains can contain high concentrations (~1-5%) of acid volatile sulfide (AVS) as nano-particulate mackinawite. These sediments can sequester substantial quantities of trace metals. Because of their low elevation and the connectivity of drains to estuarine channels, these benthic sediments are vulnerable to rapid increases in ionic strength from seawater incursion by floodgate opening, floodgate failure, storm surge and seasonal migration of the estuarine salt wedge. This study examines the effect of increasing seawater concentration on trace metal mobilization from mackinawite-rich drain sediments (210-550 μmol g⁻¹ AVS) collected along an estuarine salinity gradient. Linear combination fitting of S K-edge XANES indicated mackinawite comprised 88-96% of sediment-bound S. Anoxic sediment suspensions were conducted with seawater concentrations ranging from 0% to 100%. We found that mobilization of some metals increased markedly with increasing ionic strength (Cu, Fe, Mn, Ni) whereas Al mobilization decreased. The largest proportion of metals mobilized from the labile metal pool, operationally defined as Σexchangeable + acid-extractable + organically-bound metals, occurred in sediments from relatively fresh upstream sites (up to 39% mobilized) compared to sediments sourced from brackish downstream sites (0-11% mobilized). The extent of relative trace metal desorption generally followed the sequence Mn > Ni ≈ Cu > Zn > Fe > Al. Trace metal mobilization from these mackinawite-rich sediments was attributed primarily to desorption of weakly-bound metals via competitive exchange with marine-derived cations and enhanced complexation with Cl⁻ and dissolved organic ligands. These results have important implications for trace metal mobilization from these sediments at near-neutral pH under current predicted sea-level rise and climate change scenarios.

Acidity fractions in acid sulfate soils and sediments: contributions of schwertmannite and jarosite

In Australia, the assessment of acidity hazard in acid sulfate soils requires the estimation of operationally defined acidity fractions such as actual acidity, potential sulfidic acidity, and retained acidity. Acid–base accounting approaches in Australia use these acidity fractions to estimate the net acidity of acid sulfate soils materials. Retained acidity is the acidity stored in the secondary Fe/Al hydroxy sulfate minerals, such as jarosite, natrojarosite, schwertmannite, and basaluminite. Retained acidity is usually measured as either net acid-soluble sulfur (SNAS) or residual acid soluble sulfur (SRAS). In the present study, contributions of schwertmannite and jarosite to the retained acidity, actual acidity, and potential sulfidic acidity fractions were systematically evaluated using SNAS and SRAS techniques. The data show that schwertmannite contributed considerably to the actual acidity fraction and that it does not contribute solely to the retained acidity fraction as has been previously conceptualised. As a consequence, SNAS values greatly underestimated the schwertmannite content. For soil samples in which jarosite is the only mineral present, a better estimate of the added jarosite content can be obtained by using a correction factor of 2 to SNAS values to account for the observed 50–60% recovery. Further work on a broader range of jarosite samples is needed to determine whether this correction factor has broad applicability. The SRAS was unable to reliably quantify either the schwertmannite or the jarosite content and, therefore, is not suitable for quantification of the retained acidity fraction. Potential sulfidic acidity in acid sulfate soils is conceptually derived from reduced inorganic sulfur minerals and has been estimated by the peroxide oxidation approach, which is used to derive the SRAS values. However, both schwertmannite and jarosite contributed to the peroxide-oxidisable sulfur fraction, implying a major potential interference by those two minerals to the determination of potential sulfidic acidity in acid sulfate soils through the peroxide oxidation approach.