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

Soil Research ◽  
2013 ◽  
Vol 51 (3) ◽  
pp. 203 ◽  
Author(s):  
Chamindra L. Vithana ◽  
Leigh A. Sullivan ◽  
Richard T. Bush ◽  
Edward D. Burton
Keyword(s):  
Correction Factor ◽  
Soil Samples ◽  
Acid Sulfate Soils ◽  
Peroxide Oxidation ◽  
Acid Base ◽  
Acid Sulfate ◽  
Soils And Sediments ◽  
Actual Acidity ◽  
Sulfate Soils

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.

The measurement of actual acidity in acid sulfate soils and the determination of sulfidic acidity in suspension after peroxide oxidation

Soil Research ◽  
2002 ◽  
Vol 40 (7) ◽  
pp. 1133 ◽  
Author(s):  
Angus E. McElnea ◽  
Col R. Ahern ◽  
Neal W. Menzies
Keyword(s):  
Regression Line ◽  
Correction Factors ◽  
Acid Sulfate Soils ◽  
Peroxide Oxidation ◽  
Acid Sulfate ◽  
Inorganic Sulfur ◽  
Actual Acidity ◽  
Sulfate Soils ◽  
The Relationship

Improvements to the routine methods for the determination of actual acidity in suspension for acid sulfate soils (ASS) are introduced. The titratable sulfidic acidity (TSA) results using an improved peroxide-based method were compared with the theoretical acidity predicted by the chromium reducible sulfur method for 9 acid sulfate soils. The regression between these 2 measures of sulfidic acidity was highly significant, the slope of the regression line not significantly different from unity (P = 0.05) and the intercept not significantly different from zero. This contrasts with results of other workers using earlier peroxide oxidation methods, where TSA substantially underestimated the theoretical acidity predicted by reduced inorganic sulfur analysis. Comparison was made between the 2 principal measurements from the improved peroxide method (TSA and SPOS), with SPOS converted to theoretical sulfidic acidity to allow comparison. The relationship between these 2 measurements was highly significant. The effects of titration in suspension, as well as raising titration end points to pH 6.5, were investigated, principally with respect to the titratable actual acidity (TAA) result. TAA results obtained by KCl extraction were compared with those obtained using BaCl2, MgCl2, and water extraction. TAA in 1 M KCl suspensions titrated to pH 6.5 agreed well with titratable actual acidity measured using the 25-h extraction approach of the Lin et al. (2000a) BaCl2 method. Both BaCl2 and KCl solutions were ineffective at fully recovering acidity from synthetic jarosite without repeated extraction and titration. The application of correction factors for the estimation of total actual acidity in ASS is not supported by the results of this investigation. Acid sulfate soils that contain substantial quantities of jarosite or other acid-producing but relatively insoluble sulfate minerals continue to prove problematic to chemically analyse; however, an approach for estimating this component is discussed.

An improved analytical procedure for determination of total actual acidity (TAA) in acid sulfate soils

2000 ◽  
Vol 262 (1-2) ◽  
pp. 57-61 ◽  
Author(s):  
C. Lin ◽  
K. O’Brien ◽  
G. Lancaster ◽  
L.A. Sullivan ◽  
D. McConchie
Keyword(s):  
Analytical Procedure ◽  
Acid Sulfate Soils ◽  
Acid Sulfate ◽  
Actual Acidity ◽  
Sulfate Soils

Oxidation rate of pyrite in acid sulfate soils: in situ measurements and modelling

Soil Research ◽  
2004 ◽  
Vol 42 (6) ◽  
pp. 499 ◽  
Author(s):  
F. J. Cook ◽  
S. K. Dobos ◽  
G. D. Carlin ◽  
G. E. Millar
Keyword(s):  
Oxidation Rate ◽  
Volume Ratio ◽  
Soil Surface ◽  
Biological Processes ◽  
Acid Sulfate Soils ◽  
Acid Sulfate ◽  
The Difference ◽  
Actual Acidity ◽  
Sulfate Soils

The generation of acidity from oxidation of pyrite in acid sulfate soils requires the transport of oxygen into the soil profile. The sink for this oxygen will not only be the chemical reaction with pyrite but the biological processes associated with both microbial and plant respiration. The biological sinks in burning the oxygen (O2) will release CO2. The respiratory quotient which is the molar volume ratio of O2 : CO2 varies between 1.3 and 0.7 depending on the source of the organic matter being oxidised, but is generally 1.0. The oxidation of pyrite by oxygen will, by comparison with the biological processes, produce minor amounts of CO2 (if any) by reaction with intrinsic carbonate minerals. Gas samplers were installed into the soil at various depths and samples collected from these at approximately fortnightly intervals. The samples were analysed by gas chromatography and the CO2 and O2 profiles obtained. The flux of these gases was calculated and the difference between these attributed to the oxidation of pyrite. The flux difference varied over the period of sampling and on average gave an in situ oxidation rate of 11.5 tonnes H2SO4/ha.year. This is considerably more that the rate of export of acidity from this site and would explain the considerable actual acidity storage in these soils. A model is developed for steady state transport of oxygen into soils with an exponentially decreasing biological sink with depth and an exponentially increasing chemical (pyrite) sink with depth. The model is developed in non-dimensional variables, which allows the relative strengths and rates of increase or decrease in sink terms to be explored. This model does not explicitly treat the flow of oxygen in macropores. Other models that do explicitly calculate macropore flow are compared and found to give similar results. These results suggest that the use of biological or other sinks near the soil surface could be a useful method for reducing the oxidation rate of pyrite in acid sulfate soils.

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