Bioleaching of Heavy Metal Polluted Sediment: Influence of Temperature and Oxygen (Part 1)

A remediation process for heavy metal polluted sediment has previously been developed in which the heavy metals are removed from the sediment by solid‐bed bioleaching using elemental sulfur (S 0 ): the added S 0 is oxidized by the indigenous microbes to sulfuric acid that dissolves the heavy metals which are finally extracted by percolating water. In this process, the temperature is a factor crucially affecting the rate of S 0 oxidation and metal solubilization. Here, the effect of temperature on the kinetics of S 0 oxidation has been studied: oxidized Weiße Elster River sediment (dredged near Leipzig, Germany) was mixed with 2 % S 0 , suspended in water and then leached at various temperatures. The higher the temperature was, the faster the S 0 oxidized, and the more rapid the pH decreased. But temperatures above 35 °C slowed down S 0 oxidation, and temperatures above 45 °C let the process – after a short period of acidification to pH 4.5 – stagnate. The latter may be explained by the presence of both neutrophilic to less acidophilic thermotolerant bacteria and acidophilic thermosensitive bacteria. Within 42 days, nearly complete S 0 oxidation and maximum heavy metal solubilization only occurred at 30 to 45 °C. The measured pH( t ) courses were used to model the rate of S 0 oxidation depending on the temperature using an extended Arrhenius equation. Since molecular oxygen is another factor highly influencing the activity of S 0 ‐oxidizing bacteria, the effect of dissolved O 2 (controlled by the O 2 content in the gas supplied) on S 0 oxidation was studied in suspension: the indigenous S 0 ‐oxidizing bacteria reacted quite tolerant to low O 2 concentrations; the rate of S 0 oxidation – measured as the specific O 2 consumption – was not affected until the O 2 content of the suspension was below 0.05 mg/L, i.e., the S 0 ‐oxidizing bacteria showed a high affinity to O 2 with a half‐saturation constant of about 0.01 mg/L. Stoichiometric coefficients describing the relationship between the mass of S 0 , O 2 and CO 2 consumed are scarcely available. The growth of S 0 ‐oxidizing, obligate aerobic, autotrophic bacteria was, therefore, stoichiometrically balanced (by using a yield coefficient of Y X/S  = 0.146 g cells/g S 0 , calculated with data from the literature): 24.14 S 0  + 29.21 O 2  + 27.14 H 2 O + 5 CO 2  + NO 3 – → C 5 H 7 O 2 N + 24.14 SO 4 2–  + 47.28 H + , which resulted in Y   O   2 /S  = 1.21 g O 2 /g S 0 and Y   CO   2 /S  = 0.28 g CO 2 /g S 0 .