PYRITE OXIDATION, CARBONATE WEATHERING, AND GYPSUM FORMATION IN A DRAINED POTENTIAL ACID SULFATE SOIL

To quantify the rate of pyrite oxidation, weathering of carbonates and the formation of gypsum in a potential acid sulfate soil exposed to drainage, a column experiment was executed for 175 d. During the experiment, 24 g of FeS2, of the originally present 1327 g, oxidized within 70 d. Weathering of carbonates prevented acidification; an estimated 21 g of CaCO3 and 17 g of CaMg(CO3)2 dissolved simultaneously within the first 70 d. Rising Ca2+ and SO2-4 concentrations initiated gypsum precipitation from around Day 70 until the end of the experiment. A numerical model was used to simulate the experimental results. Estimated and simulated quantities of pyrite oxidation and carbonate weathering, and measured and simulated soil solution concentrations appeared to agree well. Model simulations indicate the formation of 63 g CaSO4 between Days 70 and 175 in the upper 60 cm of the soil profile.