The potential of using iron-oxide-rich soils for minimizing the detrimental effects of H₂S in freshwater aquaculture systems

The potential of using soil rich in iron for controlling hydrogen sulfide concentrations in aquaculture systems was investigated. The sulfide removal capacity of two local soils (Terre Rosse and Bazalt) was compared with four commercially available hematite ores. Terre Rosse soil, with iron content of 8% by mass, showed high reactivity towards sulfide. ⁵⁷Fe Mössbauer analysis of the soil revealed that the iron content is distributed as hematite (43%), goethite (33%) and iron silicates (24%). Dissolution potential experiments conducted at a high sulfide concentration (1200 mg S l^-1 at the end of the reaction) showed that 1 kg of Terre Rosse soil can remove up to 25 g of H₂S-S at pH values typical in aquaculture practice (near neutral), and that the reactive iron phase (i.e. the part that reacts with sulfide) is ∼40% of the total iron mass in the soil. The dissolution potential experiments were considered in conjunction with the analysis of the temperature evolution (300-5 K) of the Mössbauer data that also revealed ∼40% of the iron content to exist as nano-structured iron-oxides phases. The anticipated high surface-to-volume ratio and enhanced reactivity of such nanophase iron oxides, compared to bulk "micronic" materials is supposed to be the likely cause of the high reactivity of this soil. In an experiment performed with gradual addition of low sulfide concentrations (up to about 10 mg S l^-1) to simulate biological sulfide formation in anaerobic sediments, sulfide breakthrough occurred after the removal of 9.5 g S (kg soil)^-1. Under anaerobic conditions, sulfide removal was found to proceed via a redox reaction (end product: elemental sulfur) followed by FeS precipitation. Reaction kinetics was found to be pseudo-first order with respect to the total sulfide concentration. The iron oxides in the soil react rapidly in the presence of sulfide - the reactive iron fraction in the soil dissolves almost completely after about 1 day in the presence of a high sulfide concentration (>0.9 g S l^-1). At low concentrations, 2 g of Terre Rosse reduced the sulfide concentration from 200 to about 5 mg S l^-1 in around 120 h (solution volume=63 ml). In practical terms, it is hypothesized that an addition of 3 to 4 kg-soil m^-2-sediment prior to seasonal de-stratification of fish farming reservoirs will significantly reduce the toxic effects and rapid oxygen consumption rates associated with sulfide dispersal following abrupt mixing of the water column.