Mechanisms Involved In Lmwoas-Caused Inhibition On Ferrous Iron Oxidation By Acidithiobacillus Ferrooxidans

The sludge bioleaching using Acidihiobacillus ferrooxidans is an effective way for removing heavy metals from wastewater sludge. However, obvious inhibition has often been observed during the sludge bioleaching process. This inhibition could be attributed to low molecular weight organic acids (acetic and propionic acid) which are produced during anaerobic degradation of organic sludge material. The present paper is aimed to exploring the mechanisms involved in low molecular weight organic acid-caused inhibition on iron oxidation by the bioleaching bacterium Acidihiobacillus ferrooxidans.Results from the present study showed that the presence of≥1 mmol/L acetic acid or≥0.5 mmol/L propionic acid led to obvious inhibition on bacterial iron oxidation by Acidihiobacillus ferrooxidans ANYL-1. Iron oxidation could be completely inhibited by≥5 mmol/L acetic or propionic acid. Leakage of cellular materials especially DNA and total sugar was observed when bacterial culture was treated with different concentraions of acetic and propionic acids. Cellular material leakage increased with the concentration of either acid and maximum leakage could be reached within 1 h after the addition of organic acids. pH also influence the organic acid-caused cellular material leakage. Leakage occurred mainly in acidic environment at pH 2-3. Leakage decreased as pH increased from 2 to 3 and stoped at pH. However iron oxidation and viable counts of this bioleaching bacterium remained unchanged when the acid was removed. This indicated the cellular materials leakage was not responsible for the inhbition on iron oxidation caused by acetic and propionic acids.Inhibitory concentraions of both acetic and propionic acids were significantly influenced by the cell density of Acidithiobacillus ferroxidans ANYL-1. When the inoculum density was increased by 10 times 5 mmol/L of both acids can no longer inhibit the iron oxidation by ANYL-1 and the inhibitory levels of acetic and propionic acids were increased from 1-4 mmol/L and 0.5-4 mmol/L to 10-50 mmol/L, respectively. Iron oxidation rate of the organic acid-treated bacteria was also compared at elevated inoculum density in the range of pH 2-4. The results showed that iron oxidation rate of the bacterium remained unchanged if the acid was removed through centrifugation after 24 h exposure to 10-20 mmol/L of acetic/propionic acid. But significant reduction in iron oxidation rate was observed for the bacterium treated with≥30 mmol/L of acetic or propionic acid. Reduction in in iron oxidation rate was more obvious as the acidity decreased from pH 4 to pH 2, indicating stronger toxicity of both acids at low pHs.The effects of acid treatment on bacterial cellular shape and structure were also investigated in the present study. The results showed that cellular morphology was not significantly affected by 24 h treatment of the bacterium with 30 mmol/L of either acetic or propionic acid. The images of cellular envelopes including cell wall and cell membrance were quite clear under transmission electronic microscope. This further confirmed that cellular material leakage did not lead to cell disruption and therefore was responsible for the inhibition on iron oxidation by A. ferrooxidans ANYL-1.Results from SDS-PAGE electrophorensis showed that no difference existed between the whole cell protein of the acid-treated bacterial cells and the control cells. Whether the acids influence the expression of enzymes or proteins other than ferrous iron oxidase is still unclear. Therefore further stuies are required to investigate the mechanisms using 2D electrophorensis and proteomics methods.