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Organic matter increases jarosite dissolution in acid sulfate soils under inundation conditions.

Introduction

Jarosite (K[Fe.sub.3][[OH].sub.6][[S[O.sub.4]].sub.2]) is a common product of pyrite oxidation (van Breemen 1973; Dent 1986) and may occur in substantial amounts in well-drained acid sulfate soils (Lin et al. 2001; Ward et al. 2004). Each mole of jarosite carries 3 moles of retained acidity, and therefore the formation of jarosite leads to partial retention of the acid generated from pyrite oxidation (Sullivan et al. 2002). Acid buffering through jarosite formation is important in terms of minimising the environmental hazards caused by acid drainage from acid sulfate soils. However, jarosite is only stable under sufficiently oxidised conditions (van Breemen 1973). A drop in redox potential (Eh) of soils to a critical level could unstabilise jarosite and result in its dissolution and subsequent release of acid. In such a case, jarosite acts as a secondary source of soluble acid to the receiving environments (Lin et al. 1998).

Acid sulfate soils are distributed in coastal floodplains subject to frequent flooding (Johnston et al. 2004; Lin et al. 2004). Water inundation of acid sulfate lands could cause reduction in Eh value of the soils, which may have impacts on the stability of jarosite contained in acid sulfate soils. In many acid sulfate soil areas, rotation of paddy rice with dryland crops is a general practice for better use of agricultural land (Lin and Melville 1994; Li et al. 2002; Le Quang and van Mensvoort 2004). During the period when dryland crops are grown, pyrite tends to oxidise to produce sulfuric acid and jarosite. However, in the following period of inundation for paddy rice cultivation, soil Eh decreases due to waterlogged conditions. In addition, reflooding of acid sulfate soils has been considered as a potential technique for remediation of actual acid sulfate soils (Dent 1986, 1992; Smith and Yerbury 1996). However, the possible release of acid through jarosite dissolution under water inundation conditions has not been well addressed.

This work examines the effects of organic matter and thickness of 'floodwater' on the stability of jarosite in a coastal acid sulfate soil. The objective is to understand the roles of these 2 factors on the jarosite-related acid release in acid sulfate soils and to provide information that can be used to guide the management of coastal acid sulfate soils.

Materials and methods

A column experiment was conducted. The soil used for the experiment was collected from the bund of a fishpond excavated in a site where the soil was identified as acid sulfate soil (Wang and Luo 2002). The soil was previously used for paddy rice cultivation and turned into a fishpond about 3 years prior to sample collection in May 2004. The soil contained large amount of jarosite (confirmed by XRD), and the inorganic reduced sulfur originally contained in the soil has been completely oxidised (confirmed by the LECO method; Lin et al. 1996). The soil sample was air-dried and crushed to pass a 2-mm sieve. Some major soil characteristics are given in Table 1. ++++++++++++++++ The experiment was conducted using columns 0.5 m high, with an internal diameter of 0.15 m. In each column, 2.5 kg of the soil was placed on a thin layer of fine sand. Dried grass clippings chopped to about 3 cm long were used as an organic additive. Five treatments (each in triplicate) were set by considering 2 factors: (1) amount of organic material added, and (2) thickness of the overlying water layer (see Table 2). Treatment CK without addition of any organic material acted as a control. For Treatments 1 and 3 (T1 and T3, respectively), 50g of the dried grass clippings was placed on the top of the soils. For Treatments 2 and 4 (T2 and T4, respectively), 125 g of the dried grass clippings was placed on the top of the soils. Deionised water was added to each column with CK, T1, and T2 having a 0.15 m thick water layer overlying the soil, and T3 and T4 having a 0.25-m-thick water layer overlying the soil. The inundation experiment was separated into 2 stages. The first stage of experiment was undertaken from 13 August 2004 to 24 October 2004. In the first 6 days, in situ measurements of pH and dissolved oxygen (DO) in the surface water were made every day; from day 7 to day 21, the measurements were made every 3 days; from day 22 to day 73, the measurements were made every week. At the same time as pH and DO in the surface water were measured, 60 mL of leachate was collected from the bottom of each column for analyses of pH, Fe, and S[O.sub.4.sup.2-] (henceforth called S[O.sub.4]). At the end of the first inundation experiment, the water in each column was allowed to completely drain and a small amount of soil sample was collected from T4 for XRD analysis. The XRD pattern was compared with that of the original soil to see whether there was any change in quantity of jarosite contained in the soil after the treatment.

The second stage of inundation experiment was from 29 October 2004 to 28 March 2005. The amounts of the added water were exactly the same as in the first stage of the experiment. For the surface water, pH was measured, and for the leachate, pH, Fe, and S[O.sub.4] were determined. The sampling interval was monthly.

The pH was measured using a calibrated pH meter (pH S-25 meter); DO was measured using a portable water quality tester (TPS 90-FLMV). Sulfate concentration was determined turbidimetrically (Rhoades 1982), and Fe concentration was determined by atomic adsorption spectrometer.

The data are expressed as the mean of 3 replicates and significant treatment differences were tested at P = 0.05 by using Duncan's Multiple Range Test.

Results

First stage of inundation experiment

pH in surface water

The pH in surface water of the control (CK) fluctuated between 3 and 4 during the entire first stage of the inundation experiment. In contrast, all other treatments with the added organic material show a trend for pH to increase with increasing time of inundation (Fig. 1). Statistical analysis showed that no significant difference (P > 0.05) existed among the treatments with organic material added (data not shown).

[FIGURE 1 OMITTED]

DO in the surface water

The DO in surface water of the control dropped from 7.32 mg/L on day 1 to 4 mg/L on day 4 and then became relatively stable. However, DO in the surface water rapidly decreased from 5 mg/L on day 1 to < 1.5 mg/L for T1 and <0.5 mg/L for the remaining treatments on day 2, and then gradually decreased to about 0.3 mg/L on the last day of measurement (Fig. 2).

[FIGURE 2 OMITTED]

Leachate pH

In contrast to the surface water, leachate pH in the control increased with increasing time of inundation, whereas there was a trend for leachate pH in other treatments to decrease with increasing time of inundation following an initial increase in the first a few days. Within the 4 treatments with added organic matter, the leachate pH tended to be in the following decreasing order: T3 > T1 > T4 > T2 (Fig. 3).

[FIGURE 3 OMITTED]

Leachate S[O.sub.4]

Within the first 10 days of inundation, leachate S[O.sub.4] sharply decreased to a very low level. For the control, the leachate S[O.sub.4] was at a low level throughout the remainder of the experiment, although fluctuation did occur. However, for all treatments with organic matter added, leachate S[O.sub.4] increased after the initial drop; S[O.sub.4] concentration rapidly increased with increasing time of inundation until day 45 and then became stable (Fig. 4).

[FIGURE 4 OMITTED]

In general, leachate S[O.sub.4] of the treatments with 125 g added organic matter was higher than that of the treatments with 50 g added organic matter. At the same level of added organic matter, there was no significant difference in leachate S[O.sub.4] between the treatment with a 0.15-m thick-water layer and the treatment with a 0.25-m-thick water layer (P > 0.05; data not shown).

Leachate Fe

Leachate Fe for the control was almost undetectable during the entire experiment. Leachate Fe for treatments T1 and T3, with 50 g added organic matter, maintained a low level before a slight increase observed after day 45 following inundation. For treatments T2 and T4, with 125 g added organic matter, leachate Fe increased much earlier and the concentration of Fe was much higher than in T1 and T3 (Fig. 5).

[FIGURE 5 OMITTED]

XRD analysis

The XRD analysis (Fig. 6) shows that the peak (3.t3) corresponding to jarosite in the original soil was not identifiable for the soil after the most intensive treatment (T4, 125 g added organic matter and 0.25 m water depth).

[FIGURE 6 OMITTED]

Second stage of experiment

pH in surface water

The pH in the surface water for all the treatments decreased initially (in the first month) and then increased with increasing time of inundation. The pH of the control fluctuated between 3 and 4, whereas all organic matter-added treatments had a pH >5 (Fig. 7). In most cases, there was no significant difference in pH among the organic matter-added treatments (P > 0.05; data not shown).

[FIGURE 7 OMITTED]

Leachate pH

Unlike the strong contrast in the first stage of the experiment, leachate pH v. time trend in the control was similar to that in organic matter-added treatments. Initially the leachate pH in all treatments (including the control) was low but increased within the first month of the experiment (Fig. 8).

[FIGURE 8 OMITTED]

Leachate S[O.sub.4]

Leachate S[O.sub.4] in all the treatments fluctuated but there is no clear trend during the second stage of the experiment (Fig. 9). Except for the control, 804 concentration in all treatments was much lower in the second stage than in the first stage of the experiment (refer to Fig. 4). In general, there was no significant difference in S[O.sub.4] between the control and any individual organic matter-added treatment (P > 0.05; data not shown).

[FIGURE 9 OMITTED]

Leachate Fe

Variation in leachate Fe with time was minor for all the treatments. However, marked differences m the concentration of leachate Fe existed among different treatments and generally it showed the following decreasing order: T2 > T4 > T3 > T1 > CK. Treatment CK contained very little Fe (Fig. 10). However, the Fe concentration was much lower in the second stage than the first stage of the experiment (refer to Fig. 5).

[FIGURE 10 OMITTED]

Discussion

The results (Fig. 1) obtained from this experiment suggest that the activity of [h.sup.+] diffused into the overlying water from the soil could remain high (pH remained low) if no organic matter was added on the soil surface, indicating that reduction reactions that lead to consumption of [h.sup.+] were weak in such a situation. However, when organic matter was added onto the soil, a decrease in [h.sup.+] activity (increase in pH) with time occurred following an initially short period of low pH. This was accompanied by a drop in DO (Fig. 2) that was possibly caused by decomposition of organic matter. Under reduced conditions as indicated by depletion of dissolved oxygen, transformations of Fe(III) into soluble Fe(II) and S[O.sub.4] into [h.sub.2]S may take place, and these chemical reactions consume [h.sup.+], resulting in an increase in pH (Lin et al. 2003).

[FIGURES 1-2 OMITTED]

Unlike the surface water, there was a clear trend for leachate pH in the control to increase with increasing time of inundation (Fig. 3). This suggests that [h.sup.+]-consuming reactions did take place in the soil pore water, and the longer the flooding, the more [h.sup.+] was consumed. In contrast to the control, the pH in all organic matter-added treatments decreased with increasing time of inundation following an initial increase in the first a few days (Fig. 3). The reason for the initial increase in pH is the same as for the control, reflecting [h.sup.+] consumption in the soil pore water. The subsequent decrease in leachate pH indicates the release of acid from the soil. The increases in S[O.sub.4] after about 10 days following inundation (Fig. 4) suggest that the releasing acid is of jarosite origin. The drop in S[O.sub.4] during the first 10 days corresponded well with the increase in pH, suggesting that sulfate reduction was involved in the [h.sup.+] consumption that took place in the soil pore water during this period. The stronger release of S[O.sub.4] in T2 and T4 (addition of 125g organic matter) than in T1 and T3 (addition of 50g organic matter) suggests that an increase in the amount of added organic matter significantly enhances the dissolution of jarosite, while the insignificant difference between T1 and T2 or T3 and T4 indicates that thickness of overlying water layers at the same level of added organic matter is not important in controlling the dissolution of jarosite.

[FIGURES 3-4 OMITTED]

The increase in leachate Fe with increasing time of inundation (Fig. 5) further confirms the dissolution of jaroste. However, unlike leachate S[O.sub.4], leachate Fe did not show an initial drop during the first 10 days. This is probably because the soil contained a very limited amount of soluble Fe. It is interesting to note that the S[O.sub.4] ratio of T2 to T1 or T4 to T3 was <2, whereas the Fe ratio of T2 to T1 or T4 to T3 was > 5. This imbalance may be due to the following reasons: (a) in T2 and T4, which contained larger amounts of added organic matter, the stronger reducing conditions also enhance the dissolution of iron compounds (e.g. iron oxides) rather than jarosite; (b) in T1 and T3, which had higher pH, the solubility of Fe was relatively low, which results in the precipitation of some soluble Fe following oxidation of ferrous form to ferric form in the leachates.

[FIGURE 5 OMITTED]

The XRD results (Fig. 6) suggest that under the experimental conditions in T4, almost all the jarosite contained in the soil was decomposed. Because the S[O.sub.4] released from the soil was much less in T1 and T3 than in T4, it is reasonable to assume that part of the jarosite in T1 and T3 did not dissolve in the first stage of inundation experiment. After being completely drained, the acid generated by jarosite was removed from the soil in the column.

[FIGURE 6 OMITTED]

Following inundation in the second stage, the pH in the surface water was >5 for all the treatments, except that the pH of the control remained at ~4 for most of time (Fig. 7). The leachate pH was initially low (3.5-4.1), possibly because the first leachate contained more residual acid accumulated in the bottom of soil colunm. The increase in pH with increasing time of inundation (Fig. 8) indicates that [h.sup.+] consumption dominated acidification-deacidification reactions. This suggests that with the removal of jarosite-related acid materials from the soil after drainage, the remaining soil of all organic matter-added treatments had very weak, or even no, capacity to release soluble acid upon further inundation. Clearly, although T1 and T3 still contained some jarosite, it did not appear that the jarosite further dissolved to a significant degree in the second stage of the inundation experiment. Possibly this is because the organic matter remaining in the soil was not sufficient to drive the dissolution of the remaining jarosite. The S[O.sub.4] concentration in the leachate was much less in the second stage than the first stage of the experiment (Fig. 9). This further supports the above point that there was no further significant dissolution of jarosite. Because the Fe in the leachate could be derived from a variety of iron compounds other than jarosite, it does not give reliable indication of jarosite dissolution, although the relatively higher concentration of Fe in T2 and T4, compared with T1 and T3, in both stages of the experiment (Fig. 10) did indicate that the increased amount of added organic matter allowed enhanced release of Fe from the soil to the solution.

[FIGURES 7-10 OMITTED]

Conclusion and recommendations

The presence of organic matter overlying the soil appears to be a triggering factor that controls the dissolution of jarosite contained in the acid sulfate soil under the inundation conditions described in this experiment. The increase in amount of added organic matter enhances jarosite decomposition, whereas the increase in the thickness of the overlying water layer has limited effects on jarosite dissolution. The results here have implications for the management of coastal acid sulfate soils. In coastal floodplains subject to extended flood inundation, dissolution of jarosite may significantly contribute to acid drainage from acid sulfate soils if the soil contains substantial amounts of jarosite and there is sufficient supply of organic debris from the plants killed during flooding. Therefore, acid release from jarosite dissolution should be taken into account when developing acid sulfate soil remediation strategies. In theory, removal of biomass from the land surface prior to major flood events may assist in minimising jarosite-derived acid risk. However, this strategy may not be practical in the broadacre acid sulfate lands. Alternatively, it is possible to eliminate the jarosite-related acidity by a 2-step treatments: (1) destabilisation of jarosite by inundating acid sulfate soils in the presence of substantial organic matter for an extended period, and (2) intensive treatment of draining water that contains soluble acids generated from jarosite dissolution.

Acknowledgments

This work was financially supported by the Natural Science Foundation of China (Project No. 40471067). The authors would also like to thank Associate Professor Leigh Sullivan at the Southern Cross University and Associate Professor Zhang Jiaen at the South China Agricultural University for their assistances in the filed.

References

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Manuscript received 12 July 2005, accepted 31 October 2005

Chengxing Chu (A), Chuxia Lin (A,B), Yonggui Wu (A), Wenzhou Lu (A), and Jie Long (A)

(A) College of Resources and Environment, South China Agricultural University, Guangzhou 510642, China.

(B) Corresponding author. Email: cxlin@scau.edu.cn
Table 1. Major characteristics of the soil used for the experiment

Parameter Reference

Soil texture Loam
pH 3.62 1 : 5 (soil : water)
EC (dS/m) 4.45 1 : 5 (soil : water)
Water-extractable acidity 34.01 Lin et al. (2002)
 (mmol/kg)
1 M KCl extractable-acidity 117.47 Lin et al. (2002)
 (mmol/kg)
Total actual acidity (mmol/kg) 181.9 Lin et al. (2000)
Water-extractable S[O.sub.4] (mg/kg) 19.6 Rhoades (1982)
Organic matter content (%) 3.20 Nelson and Sommers
 (1982)

Table 2. Experimental design of the treatments

Treatment CK T1 T2 T3 T4

Organic material added (g) 0 50 125 50 125
Thickness of overlying 0.15 0.15 0.15 0.25 0.25
 water layer (m)
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Author:Chu, Chengxing; Lin, Chuxia; Wu, Yonggui; Lu, Wenzhou; Long, Jie
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:May 1, 2006
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