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Sample pre-treatment and the determination of some chemical properties of acid sulfate soil materials.

Introduction

Acid sulfate soils (ASS) are a prominent feature of many low-lying coastal areas of Australia and around the world, with estimates of 20 million hectares of ASS worldwide (Dent 2000), and around 4 million hectares in Australia. The oxidation of ASS produces a plethora of environmental problems, including the degradation of aquatic ecosystems, fish kills (Sammut and Lines-Kelly 2000), scalding (Rosicky et al. 2002), and the release of toxic metals (Sundstrom et al. 2002). Effective management of ASS to avoid such problems relies on an accurate assessment of existing and potential acidity. Determination of the reduced inorganic sulfur content (also referred to as the oxidisable sulfur content) of the soil, using the chromium-reducible sulfur ([S.sub.CR]) method, can predict the maximum potential acidity risk using stoichiometric relationships (Sullivan et al. 2001).

The reduced inorganic sulfur content has also been used to trigger the need for the preparation of ASS Management Plans in Australia (Stone et al. 1998). As the action criterion for this property is as low as 0.03% [S.sub.CR] for sands, the method should be capable of delivering a suitably high degree of accuracy and precision. An overestimation or underestimation of [S.sub.CR] compounds may result in a misidentification of soil materials or the adoption of inappropriate treatments such as excessive or insufficient liming application rates (Sullivan et al. 1999; Ward et al. 2002).

The grinding of dried soil samples prior to analyses for [S.sub.CR] is a widely used sample preparation technique. The main reason for grinding samples is that the resulting finer particles have a greater relative surface area giving them greater contact with the reagents. Grinding has also been used to increase the homogeneity of the soil, providing a more representative sample (Ahern et al. 1996; McElnea et al. 2002).

Despite the regular adoption of grinding prior to analyses, very little research has been conducted to examine systematically the effect of drying and grinding on ASS properties. The purpose of this research was to provide a comparison between sample pre-treatment techniques such as freezing, drying, and duration of grinding on the determination of [S.sub.CR] and water-soluble sulfate in 4 ASS materials.

Methods and materials

Four soil materials with widely varying properties were collected from 3 different sites in northern New South Wales, including the Tuckean Swamp, Bungawalbin Swamp, and 2 soil materials from McLeods Creek (Table 1).

Samples of each ASS material were collected using a Russian D-section corer, then placed in thick plastic bags to prevent oxidation. Upon receipt at the laboratory, a portion of each soil material was placed immediately in a freezer, while the remainder was oven-dried at 85[degrees]C. A subsample of the fresh soil material and the 85[degrees]C dried material was subsequently oven-dried to 105[degrees]C in a fan-forced oven to determine the moisture content ([theta]g).

The oven-dry soils were gently hand-ground and sieved to <1 mm size. A 60-g subsample of each soil material was then placed in a ring mill grinder for 30 s. From the ground soil, 4 replicate 0.2 g samples were taken to be analysed fur [S.sub.CR] by the [S.sub.CR] method of Sullivan et al. (2000). Another 4 replicate 0.2-g samples were taken for water-soluble sulfate analysis. A 2.0-g subsample was also dried to 105[degrees]C to determine the [theta]g value of the ground material. The remaining soil was returned to the grinder for another 30 s before further subsamples (as for the 30-s grinding time) were extracted. The process was repeated after 5 and 10 min. Four replicate samples of the hand-ground soil and the frozen soil were also analysed for [S.sub.CR] and water-soluble sulfate. The [theta]g values were used to convert all results to an oven-dried soil mass.

In addition, 3 ASS materials, one from the McLeods Creek A soil layer and 2 from the Tuckean Swamp (0.60-0.75 and 1.00-1.85 m depth layers) that had been incubated at field moisture content for 333 days during a previous study (NJ Ward, unpublished data), were also analysed. These soil samples had been incubated until their [S.sub.CR] content (measured on freeze-dried, hand-ground samples) was negligible. Duplicate samples of each of these soil materials were analysed for Sea, while the remainder of the material was placed in the ring mill grinder for 30 s and subsequently analysed for [S.sub.CR].

Water-soluble sulfate was determined turbidimetrically using flow injection analysis colourimetry (Lachat QuickChem 8000). The oven-dried soil was suspended in a 1 : 40 soil/water extract, shaken for 1 h, and centrifuged for 5 min at 3500 r.p.m. The liquid was extracted and filtered though a 0.45-[micro]m filter.

Results

Effect of pre-treatment on the determination of [S.sub.CR]

Figure 1 shows that oven drying of the soil samples followed by hand grinding (time zero) resulted in much lower [S.sub.CR] values in all 4 soil materials compared with the [S.sub.CR] values for samples that were analysed frozen. For the Tuckean and Bungawalbin Swamp soil materials, the [S.sub.CR] values after oven drying and hand grinding were lower than those for the frozen samples by 20% and 21%, respectively. For both McLeods Creek samples, the oven-dried samples had [S.sub.CR] values lower by ~5%.

[FIGURE 1 OMITTED]

Figure 1 also shows that ring mill grinding of oven-dried samples resulted in increased [S.sub.CR] values in all 4 soil materials compared with the dried and hand-ground samples. These increases ranged from 8% for the McLeods Creek B soil material to 17% for the Bungawalbin Swamp soil material. The duration of ring mill grinding for optimum [S.sub.CR] values in the dried soil materials was different for each soil sample and varied from 30 s to 5 min.

The highest [S.sub.CR] values for Tuckean and Bungawalbin Swamp were achieved by analysing frozen samples. For both McLeods Creek samples, however, there was little difference in [S.sub.CR] results between analysis of frozen samples and analysis of oven-dried samples that had been ring mill ground for at least 30 s. For the Tuckean and Bungawalbin Swamp samples, the [S.sub.CR] values of the frozen samples were 15% and 8% higher, respectively, than those of the optimally ring mill ground, oven-dried samples.

Effect of pre-treatment on the determination of water-soluble sulfate

Figure 2 shows that oven drying of the soil samples followed by hand grinding resulted in higher values of water-soluble sulfate in all 4 ASS materials compared with the frozen samples. For the Tuckean and Bungawalbin Swamp soils, the relative increase in water-soluble sulfate values after oven drying was 170% and 500%, respectively, whereas for the McLeods Creek A and B soil materials the increase in water-soluble sulfate values after oven drying was 70% and 19%, respectively. Comparison between Figs 1 and 2 shows that for each ASS material, the increase in water-soluble sulfate sulfur after oven drying was not as great as the decrease in the [S.sub.CR] fraction. Assuming that the increase in water-soluble sulfate was due to sulfide oxidation during the oven drying procedure, then the proportion of the total [S.sub.CR] oxidised during oven drying was 3-5% in the Tuckean Swamp, Bungawalbin Swamp, and McLeods Creek A soils and ~0.6% in the McLeods Creek B soil material. Figure 2 also shows that after oven drying, ring mill grinding resulted in relatively minor changes in the water-soluble sulfate values.

[FIGURE 2 OMITTED]

Effect of ring mill grinding on the [S.sub.CR] values' of long-term incubated ASS materials

Table 2 shows that ring mill grinding the 3 long-term incubated ASS materials after freeze drying did not result in higher [S.sub.CR] values compared with those that were not ring mill ground.

Discussion

The results show that [S.sub.CR] analysis of frozen samples gave higher values than oven drying and hand grinding for all 4 soils analysed. For the Tuckean and Bungawalbin Swamp soils, the [S.sub.CR] values of the frozen samples were also higher than those for oven-dried and ring mill ground samples. For both McLeods Creek samples, the [S.sub.CR] values of the frozen samples were similar to those of the oven-dried and ring mill ground samples. The results indicate that the differences between frozen and oven-dried samples were most likely due to several factors rather than a single factor.

One factor responsible for the generally lower [S.sub.CR] results in the oven-dried samples is that the [S.sub.CR] fraction in the soil materials oxidised during oven drying. Even in efficient fan-forced ovens, these results show that oxidation of 3-5% of the [S.sub.CR] occurred in 3 of the 4 ASS materials.

Another likely factor for the enhanced [S.sub.CR] results from analysis of frozen samples is that ice crystal growth during freezing may have contributed to the disruption of coatings such as organic matter, which Bush and Sullivan (1999) demonstrated are often found around pyrite grains in Australian AS S. This process may result in a greater exposure of sulfide grains to reagents in the [S.sub.CR] analytical procedure.

The results also show that for ASS materials that have been oven-dried, aggressive grinding (e.g. use of a ring mill) resulted in higher [S.sub.CR] values (by up to 17%) than hand grinding. As with freezing samples, the increased [S.sub.CR] values associated with this pre-treatment are most likely due to the abrasion of coatings found around write grains (Bush and Sullivan 1999). These results indicate that in order to optimise [S.sub.CR] values in oven-dried ASS materials, samples should be ring mill ground for a least 1 min in mineral ASS and for at least 5 min in peaty ASS.

It is possible that coatings around sulfide grains, as well as occluding sulfides from reagents during analytical procedures, may also provide protection against oxidation in the environment and that sulfide grains so coated are not of environmental concern. This may lead to an assumption that the maximum value from [S.sub.CR] analysis on ASS materials is not desirable and that analysing either frozen or ring mill ground ASS samples may result in an overestimation of the 'reactive' [S.sub.CR] content of that material. Analysis of the long-term incubated ASS materials, however, shows that ring mill grinding did not result in an enhanced [S.sub.CR] value (Table 2). This indicates that coated sulfide grains in ASS materials, although perhaps not reactive over the time scales of laboratory analytical techniques (i.e. minutes to hours) are reactive over longer time scales (i.e. months) and can be expected to be oxidisable in the natural environment.

The results also show that oven drying substantially increases the amount of water-soluble sulfate in ASS samples. Such artifacts of sample pre-treatments may be important in the identification and interpretation of ASS materials as well as in assessing sulfate availability to plants. Failing to pre-treat ASS materials appropriately could lead to an underestimation of the potential acidity risk of these materials in the environment.

Conclusions

The results of this study show that higher [S.sub.CR] values in ASS materials were gained from the analysis of frozen samples rather than from oven-dried and hand-ground samples. If the soil material is oven-dried, then in order to maximise the [S.sub.CR] values, it should also be ring mill ground optimally. From this study, the optimum ring mill grinding time for oven-dried, mineral-dominated ASS materials is 1 min, whereas for the peat ASS material it is 5 min. However, the data also indicate that once a sample is oven-dried, even optimal ring mill grinding may result in a considerable underestimation of [S.sub.CR]. A further finding of this study is that greatly enhanced water-soluble sulfate contents can be an artifact of oven drying ASS materials.
Table 1. General description of sampling sites

 Sampling
Site Location depth (m) Description

Tuckean Swamp 28[degrees]56'00"S, 0.70-1.80 Unoxidised actual
 153[degrees]23'30"E ASS (pH <4.0)
Bungawalbin Swamp 29[degrees]06'27"S, 1.20-2.60 Peaty clay
 153[degrees]13'30"E
McLeods Creek A 28[degrees]18'7"S, 1.10-1.25 Partially
 153[degrees]30'50"E oxidised ASS
McLeods Creek B 28[degrees]18'7"S, 1.90-2.40 Unoxidised
 153[degrees]30'50"E potential ASS

Table 2. Effect of ring mill grinding on long-term incubated ASS
materials

Sample Mean [S.sub.CR] (%S)

 Hand ground Ring mill ground

McLeods Creek A 0.01 0.01
Tuckean Swamp (0.60-0.75 m) 0.02 0.02
Tuckean Swamp (1.00-1.85 m) 0.01 0.01


References

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Sundstrom R, Anstrom M, Osterholm P (2002) Comparison of the metal content in acid sulfate soil runoff and industrial effluents in Finland. Environmental Science and Technology 36, 4269-4272. doi: 10.1021/ES020022G

Ward NJ, Sullivan LA, Bush RT (2002) Sulfide oxidation and acidification of acid sulfate soil materials treated with CaC[O.sub.3] and seawater-neutralised bauxite refinery residue. Australian Journal of Soil Research 40, 1057-1067. doi: doi:10.107 I/SR01119

Crystal A. Maher (A, B), Leigh A. Sullivan (A), and Nicholas J. Ward (A)

(A) Centre for Acid Sulfate Soil Research, Southern Cross University, Lismore, NSW 2480, Australia.

(B) Corresponding author; email: cmaher@scu.edu.au

Manuscript received 16 May 2003, accepted 18 August 2004
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Author:Maher, Crystal A.; Sullivan, Leigh A.; Ward, Nicholas J.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Sep 1, 2004
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