Geochemical and microbial responses to limestone and peat treatment of incubated hypermonosulfidic sediments

Fine-grained hypermonosulfidic sediments are widespread on the coastal plains of the northern Baltic Sea that when drained, cause the formation and dispersion of acid and toxic-metal species. In this study, a 30-month laboratory oxidation experiment with such a sediment was performed in incubation cells. To minimize or prevent acidification, limestone was applied in two grain sizes: agricultural limestone with particles that were all <3.15 mm and half of them <0.80 mm, and fine-grained limestone with a median grain size of 2.5 μm. The amount of limestone applied corresponded to the theoretical acidity contained in the sulfides, as well as four times that amount. Another treatment included addition of peat to the low limestone dose to test its effects on immobilizing sufhur and metals. The pH of the drainage water and solid phase decreased to pH <4.0 in the control, and to pH <5.0 in the coarse-grained low-limestone treatment, but remained near-neutral in the other treatments. Hence, the fine-grained limestone effectively hindered acidity formation in contrast with the coarse-grained limestone when applied in amounts corresponding to the potential acidity held in the sulfides. The limestone treatments further overall decreased the rate of pyrite oxidation, slowed down the movement of the oxidation front, strongly minimized the formation of dissolved and solid-phase labile Al, and caused formation of gypsum as well as more labile secondary Fe(III) phases than corresponding Fe phases formed in the control. The limestone and peat treatments also caused shifts in the 16S rRNA gene-based microbial communities, where the control developed acidophilic iron and sulfur oxidizing communities that promoted acidity and metal release. Instead, the limestone-treated unacidified incubations developed acid tolerance to neutrophilic communities of iron and sulfur oxidizers that promoted sulfate formation without acidity release. The results showed that limestone treatments have several biogeochemical effects, and that using a fine-grained limestone as amendment was favourable in terms of minimizing acidity formation and metal release.