Bioleaching of Heavy Metal Polluted Sediment: Kinetics of Leaching and Microbial Sulfur Oxidation

摘要

Remediation of heavy metal polluted sediment through bioleaching using elemental sulfur (S{sup}0 ) as the leaching agent can be regarded as a two-step process: firstly, the microbial oxidation of the added S{sup}0 to sulfuric acid and, secondly, the reaction of the produced acid with the sediment. Here, both subprocesses were studied in detail independently: oxidized river sediment was either suspended in sulfuric acid of various strengths, or mixed with various amounts of finely ground S{sup}0 powder (diameter of the S{sup}0 particles between 1 and 17.5 μm with a Rosin-Rammler-Sperling-Bennet (RRSB) distribution and an average diameter of 35 μm) and suspended in water. The leaching process was observed by repeated analysis of the suspension concerning pH, soluble sulfate and metals, and remaining S{sup}0. In the case of abiotic leaching with H{sub}2SO{sub}4, the reaction between the acid and the sediment resulted in a gradual increase in pH and a solubilization of sediment-borne heavy metals which required some time; 80% of the finally solubilized heavy metals was dissolved after 1 h, 90% after 10 h, and 100% after 100 h. In the case of bioleaching, the rate of S{sup}0 oxidation was maximal at the beginning, gradually diminished with time, and was proportional to the initial amount of S{sup}0. Due to its very low solubility in water, S{sup}0 is oxidized in a surface reaction catalyzed by attached bacteria. The oxidation let the particles shrink, their surface became smaller and, thus, the S{sup}0 oxidation rate gradually decreased. The shrinking rate was time-invariant and, at 30 ℃, amounted to 0.5 μm/day (or 100 μg/cm{sup}2/day). Within 21 days, 90% of the applied S{sup}0 was oxidized. Three models with a different degree of complexity have been developed that describe this S{sup}0 oxidation, assuming S{sup}0 particles of uniform size (I), using a measured particle size distribution (II), or applying an adapted RRSB distribution (III). Model I deviated slightly from the measured data but was easy to handle, Model II fitted the measured data best but its simulation was complicated, and Model III was intermediate. The amount of soluble sulfate was smaller than the amount of H{sub}2SO{sub}4 added or microbially generated as the H{sub}2SO{sub}4 reacted with the sediment to form in part poorly soluble sulfates. A model has been developed that describes the pH and the soluble sulfate and metals at equilibrium, depending on the amount of H{sub}2SO{sub}4 applied or microbially generated, and that is based on the condition of electrical neutrality, a global metal/proton exchange reaction, and a sulfate-fixation reaction. In suspension, bioleaching with S{sup}0 required considerably more time than abiotic leaching with H{sub}2SO{sub}4, but the final pH and metal solubilization were identical when equimolar amounts of leaching agents were applied.