Improvements to Peroxide Oxidation Procedures for Analysis of Self-Neutralizing Acid Sulfate Soils.
Angus E. McElnea, Col R. Ahern, and Niki P. Finch. Queensland Dept of Natural Resources and Mines, Block C, 80 Meiers Rd, Indooroopilly, Brisbane, Australia
Analytical techniques based on hydrogen peroxide oxidation of sulfides have frequently been used to assess acid generation in both Acid Rock Drainage and Acid Sulfate Soil environments. However, problems can arise with this approach when appreciable quantities of carbonates are present (either naturally, or as result of ameliorant application). Zhang and Evangelou (1998) noted than peroxide oxidation of pyrite becomes less efficient at alkaline pH, which they suggested was a result of coatings and low concentrations of iron in solution. Some Acid Sulfate Soils (ASS) that contain substantial amounts of sedimentary pyrite may never generate any net acidity (and are termed self-neutralizing soils). At the East Trinity property in Cairns Australia, the site of a government-funded ASS remediation project, some soil horizons naturally contain an excess of acid neutralizing capacity due to the presence of fine shell material. This paper discusses experiments performed on soils from East Trinity aimed at improving analytical methodology by maximizing oxidation of pyrite by hydrogen peroxide in self-neutralizing ASS. This research led ultimately to modifications to the methodology of McElnea et al. (2002) that were incorporated into the Acid Sulfate Soils Laboratory Methods Guidelines (Ahern et al. 2004) and draft Australian Standards (Draft AS4873). A number of self-neutralizing soils were selected from profiles taken at East Trinity. Soils had been frozen after sampling, then dried in a fan-forced oven at 85°C for 48 h before being ground to <1 mm in a hammer and then made into a powder by grinding them in a ceramic ring and puck mill. Four approaches aimed at ensuring more complete pyrite oxidation by peroxide were investigated. The first approach (tested in duplicate on 8 soils) extended the digest procedure of McElnea et al. (2002) by incorporating two further additions of 30% H2O2 and associated two x 1 h heating steps. This approach proved unsuccessful, though less pyrite remained in the soil residue than following digestion using the conventional McElnea et al. (2002) procedure. The next approach tested employed the extended digestion procedure on 5 of the soils, but using a smaller sample mass (e.g. one half or one quarter of the original 2 g sample mass). While this procedure resulted in less pyrite remaining in the soil residue following digestion, oxidation was still not quantitative. In the third approach, 4 of the soils were digested using the approach of McElnea et al. (2002) (with 3 of the soils also digested using half the usual mass), but were then titrated with 0.5M HCl (using an auto-titrator) to pH 5.5 to dissolve excess carbonate and digested for a further 1 h following addition of 25 mL of 30% H2O2. This process proved to be little better than the extended peroxide digest procedure in oxidizing the pyrite present in the soils. Carbonate analysis of the soils combined with acid-base accounting calculations indicated that the titration to pH 5.5 with 0.5M HCl did not dissolve all the carbonate present in the soils. In the final approach, the pH endpoint for the 0.5M HCl titration used in the previous procedure was lowered to 4.0. The effect of this was to ensure essentially complete oxidation of pyrite by the peroxide. Subsequent experiments in which ASS not containing carbonates were spiked with AR grade CaCO3 indicated that the procedure recovered at least 96.5% of the added CaCO3. This procedure is capable of identifying self-neutralizing soils and can quantify their excess acid-neutralizing capacity. The procedure above is part of a decision-tree based approach for analysis of ASS (which has been given the acronym SPOCAS, standing for Suspension Peroxide Oxidation Combined Acidity and Sulfur Method) and is detailed in Ahern et al. 2004. The SPOCAS method allows a full acid-base account to be conducted on ASS and determination of a soil's net acidity.