Sulfidjord – Metod för hållbar hantering av uppschaktad sulfidjord : Redoxprocesser och försurningshastighet

Earthworks in sulphide soil areas almost invariably leads to more expensive project costs. A key factor is that almost all sulphide soil is classed as potentially acidifying which has halted the progress of developing alternative management methods for different types of sulphide soil. In addition, there are also uncertainties concerning the legal and environmental aspects of how excavated sulphide soil should be managed in a sustainable way. Technical development with regard to the assessment of the acidification rate and how to assess the acidification state is needed for a more efficient and sustainable management of sulphide soil.Along the entire northern coast of Sweden there are areas with post-glacial sulphide sediment, which as a result of land uplift now could be found at an altitude of 0-80 m above the sea level. Below the water table the sulphides in the soil are stable, and the pore water pH above 7. When the soil is disrupted or when the water table is lowered oxygen may diffuse in the pores and start oxidizing the sulphides. The oxidation process may result in a lowering of the pH and heavy metals bound in the soil starts to leach out. The purpose with this thesis was to increase the awareness about the acidification rate and redox processes in excavated sulphide soil and how current management could be improved without harming the local environment. This project is based on a literature and laboratory study. The literature part deals with important concepts and properties regarding sulphide soil, e.g. how today’s characterization method and management of sulphide soil is carried out, as well as factors affecting the chemical behavior in aerobic- and anaerobic environments. The laboratory study includes sampling of sulphide soil material from Skurholmsfjärden, Luleå, and development of a control method for evaluating the acidity state and acidification rate of excavated sulphide soil. Below the water table the sampled soils were black colored by the presents of iron monosulphides. The results showed that sulphide soil does not have notably higher concentrations of heavy metals than other comparable sediments. According to the Swedish current assessment system for classifying sulphide soil the sampled soils from Skurholmsfjärden was classified as “strongly acidifying” and showing no neutralization potential. At oxidizing conditions and low pH, S, V, Ni, Mn, Co, Ba, Sr and Cu were mobilized up to 23-98 % of the total soil content. This study showed that the soils initial oxidation state has a great influence on the acidification rate. “Actual acid sulfate soil” that already has been affected by oxidation resulted in a low pH (<6) within a few days at aerated conditions while “potential acid sulfate soil” (PASS) is not acidified instantaneously, where the pH after 10 days at aerobic conditions decreased from an average of 7.4 to 6.1 regardless of the number of wet/ dry cycles. After 81 days of aerobic conditions and 10 wet cycles the pH of anaerobic soil had dropped to a level of ~5.9 units. For the aerated PASS the element concentrations were lowered in the leachate after 10 days in aerated conditions and suggest that metal ions were adsorbed on different Fe-, Mn, and Al oxides when redox increased. The results indicate that PASS does not need to be managed immediately, while already oxidized sulphate soil with pH