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Interactions between organic matter and iron oxyhydroxysulfates / iron sulfides during remediation of acid sulfate soils

Subject Area Soil Sciences
Term from 2015 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 274961325
 
Final Report Year 2024

Final Report Abstract

When acid sulfate soils dry, oxidation of pyrite can cause acidification and formation of iron (Fe) oxyhydroxy sulfate phases such as jarosite. Remediation via re-establishment of reducing conditions requires submergence and available organic carbon (OC) to stimulate activity of reducing bacteria. We identified acid sulfate soil layers low in biodegradable carbohydrates and proteins, but high in lignin and lipids that cannot be remediated by rewetting alone. Consequently, not only the amount of OC, but rather its biodegradability is decisive for the remediation success. Follow-up experiments showed that sufficient addition of fresh plant residues can activate reducing bacteria, likely due to the release of readily available soluble organic matter. However, the effectiveness of soluble organic matter from different plant residues has not been tested so far. Furthermore, it was unclear whether an additional pH adjustment to values >4.5 was necessary to activate microbial reducers (especially sulfate reducers). In contrast to previous studies, our incubation studies showed that sufficient addition of dissolved OC (DOC) induced reduction processes that consumed protons, leading to increased pH values without any pH pre-adjustments. Thus, remediation of sulfuric soils can be achieved by combining submergence and addition of plant-derived DOC, with wheat straw and litter of Phragmites australis being most suitable. Due to its potential to migrate deep into soil, application of DOC from crop residues, such as wheat straw, is a promising measure to remediate also sulfuric subsoils. We expected that the reduction processes and the increase in pH would cause jarosite to dissolve and Fe sulfides to form. Our results confirmed the dissolution of jarosite, but we did not detect the formation of Fe sulfides in any incubation experiment. Instead, we observed formation of Fe oxyhydroxides (e.g., nano-sized goethite), as well as Fe2+/Fe3+ either sorbed to minerals or associated with OC. The observed formation of Fe oxyhydroxides is advantageous with regard to the remediation of sulfuric soils containing jarosite as it reduces the risk of leaching Fe2+ to neighbouring environments and minimizes renewed acidification in the case of future aeration. An increase in mineral–organic associations was detected in all incubation experiments. Presumably, the observed formation of Fe oxyhydroxides led to a proportion of the DOC being bound to their large, reactive surfaces. The sorption of DOC on reactive mineral surfaces was particularly evident in an experimental remediation of a clayey sulfuric soil. Here, rapid sorption removed so much DOC from the soil solution that the reduction processes were greatly delayed and only started after increased extra additions of DOC. Sorption contributes to an increase in the soil OC concentration, but the OC availability for microbial processes is greatly reduced. In an accompanying study, it was shown that in addition to sorption, the encapsulation of organic matter also greatly reduces its availability: When wetlands containing Fe sulfides dry, jarosite precipitates preferentially along root channels and almost completely fill the pore space around the roots. Due to this encrustation, the organic matter of the root becomes isolated from the soil matrix and thus inaccessible to microbes. Consequently, the amount of DOC added to remediate sulfuric soils needs to be high enough to overcome sorption processes and to unlock native root organic matter encrusted by jarosite. Overall, the project provides novel insight into processes governing the biogeochemistry of sulfuric soils. These have immediate implications for developing improved strategies to remediate acid sulfate soils: application of DOC from crop residues is environmentally friendly, can also be used for subsoils and, if dosed appropriately, leads to the conversion of jarosite to Fe oxyhydroxides, which minimizes the risk of renewed acidification in the case of future aeration.

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