A revised method for determining existing acidity in re-flooded acid sulfate soils
Introduction
Titratable actual acidity (TAA) is a fundamental and widely used standard technique for estimating the pool of exchangeable H+ in acid sulfate soils (ASS) (Andriesse, 1993, Lin et al., 2000, McElnea et al., 2002a, Vithana et al., 2013). TAA is part of a suite of standardised assessment procedures required by various State Government agencies in Australia during developments that involve disturbance of ASS (Ahern et al., 1998, McElnea and Ahern, 2004). It is intended to provide an estimate of existing soluble and exchangeable acidity and thus provide an indication of the magnitude of the actual (ie. currently manifest) acidity hazard posed by the soils and, if appropriate, to provide part of the basis for calculating neutralisation requirements.
The current widely adopted TAA technique is based on a titrating a 1M KCl soil suspension (1:40) with a weak base (0.1 M NaOH) to a known pH end-point (6.5) (McElnea et al., 2002a). The origins of the technique as it is currently applied to ASS can be traced to a field method initially developed by Konsten et al. (1988), that was subsequently refined by Dent and Bowman, 1996, Lin et al., 2000 and finally by McElnea et al. (2002a). The current method, that has been widely adopted by various Government agencies and industry (ie. McElnea and Ahern, 2004), is based on the approach outlined by McElnea et al. (2002a). The technique employs rapid drying of soil at 85 °C in a fan-forced oven and relies on the assumption that this will not generate additional exchangeable H+ via oxidation of reduced inorganic sulfur (RIS) or iron species. Many years of application of rapid oven-drying of ASS materials demonstrate that this assumption appears to be generally valid for most oxic, sulfuric horizons and also for many typical sulfidic materials (e.g. Burton et al., 2008, McElnea et al., 2002b, Sullivan et al., 2000, Ward et al., 2002).
When ASS are subject to long term re-flooding as part of wetland remediation, former sulfuric horizons are subject to redox conditions that promote iron and sulfate reduction (Burton et al., 2011a, Johnston et al., 2014, Johnston et al., 2009a, Johnston et al., 2009b). This can lead to the development of high concentrations of porewater and solid-phase Fe(II) and a variety of nano-particulate RIS species including pyrite, mackinawite, greigite and elemental sulfur (Burton et al., 2011a, Burton et al., 2011b, Johnston et al., 2014, Johnston et al., 2011, Keene et al., 2011). Re-flooding of ASS is often accompanied by large increases in the field pH of soils (Johnston et al., 2014, Johnston et al., 2009b). When such re-flooded ASS sediments are subject to rapid oven-drying, there is potential for oxidation of Fe(II) and reactive RIS species. Oxidation may lead to the generation of H+ and thereby introduce artefacts that contribute to an overestimate of existing exchangeable H+. However, the potential for such artefacts in this context has not been systematically investigated. This is important to resolve, especially given the standard TAA approach is used as one of the tools to assess the efficacy of re-flooding as a wetland remediation approach (Johnston et al., 2009b).
In this study, we compare TAA via the standard approach using oven-dried soil (1M KCl; 1:40 soil–water suspension; 4 h extract; McElnea et al., 2002a, McElnea and Ahern, 2004) with an identical approach using wet-sediment (correctly weighed to obtain the same dry mass equivalent) and employing strict O2-free extraction procedures. We apply both approaches to a series of soils collected from the former sulfuric horizons of re-flooded freshwater ASS wetlands. We aim to compare the approaches in terms of TAA and also compare differences in key RIS species and Fe fractions that may help explain any differences in TAA results. The underlying intention is to identify the most suitable approach for assessing TAA in re-flooded ASS.
Section snippets
Soil sample collection
Soils were collected from two freshwater re-flooded ASS wetlands in which iron and sulfate reduction have led to the contemporary (within the last 10 years) formation of substantial Fe(II) and RIS species in former sulfuric horizons (Johnston et al., 2014). Soil collection procedures are detailed in Johnston et al. (2014). Soil samples were selected to span a range of concentrations in organic carbon, Fe(II) and RIS species. Samples were sealed in air-tight polyethylene bags, completely filled
TAA
TAAD values exceeded those of TAAW in 85% of cases (Fig. 1a). The magnitude of additional exchangeable H+ measured from the oven-dried method [TAAD/TAAW] was up to 12× that of the wet, O2-free approach (Fig. 1b; geometric mean = 2.8×). The distribution of these relative differences in TAA approached log-normal (Fig. 1b) and log transformed data were positively skewed (Skewness = 0.478).
Sulfur species
The total S content of sediments ranged from <10 to ∼500 μmol g−1. There was a positive linear correlation between
Discussion
The standard oven-drying approach to prepare soils for measuring TAA is clearly poorly suited to some re-flooded ASS materials. In the majority of cases examined here, oven-drying caused positive artifacts that substantially over-estimate soluble and exchangeable H+ compared with wet sediments at field-moisture conditions. This is important to recognize given that re-flooding of ASS is increasingly being adopted as a remediation technique (Johnston et al., 2014, Johnston et al., 2009b, Powell
Conclusions
This study demonstrates that oven-drying of soil, as currently employed in the widely adopted TAA method of McElnea et al., 2002a, McElnea et al., 2002b, is not necessarily appropriate for assessing existing soluble and exchangeable acidity in re-flooded ASS and may result in substantial overestimates. This is an important limitation in its application, yet it can easily be overcome by using wet-sediments and O2-free extraction procedures to prevent liberation of acidity by oven-drying.
Acknowledgements
Research expenses and salary support for Scott Johnston was provided by the Australian Research Council Future Fellowship (Grant No. FT110100130). Salary support for Ed Burton was provided by the Australian Research Council (Grant No. DP110100519). This project was funded by the Australian Research Council (Grant no. LP120100238), Great Lakes Council, Port Macquarie Hastings Council and Division of Research, Southern Cross University.
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