Effect of industrial sludge pre-washing on heavy metals leaching and properties of the char produced via pyrolysis process

Effectivness of pre-washing of industrial sludge (IS) which contained high concentrations of heavy metals on the immobilisation of heavy metals in the pyrolysis derived char was investigated. IS was washed using five different pH solutions; namely, 2, 4, 7, 10 and 12. Two sets of pyrolysis were conducted at temperatures 500 and 800 °C. Other than pH 2 washing, the removal amount of metals, especially Cu, Mn and Zn, was insignificant by the washing process. In addition, the leaching of the metals increased from the washed samples at all the conditions. Thermogravimetric analysis (TGA) results confirm these observations as the decomposition patterns of the samples were fairly similar to each other. However, the yield from pyrolysis experiment shows that the washing could improve the yield of the char due to high ash content. The increase in temperature increased the ash content but decreased the yields of the char for all samples. Van Krevelen diagram indicates that the dehydration and decarboxylation were the essential reactions that occurred during the pyrolysis process for all the samples. All the experimented heavy metals were supposed to be enriched in the produced char at the pryolysis temperature range due to their high thermal stability. Pyrolysis of samples at 500 °C clearly converts the metals into more stable forms as the leaching of metals was low, compared to that of the raw samples. However, at 800 °C, the temperature seems to have negative impacts on the metals’ leaching as the leaching of Cu, Mn and Zn significantly increased, compared to their leaching from the raw samples. Surprisingly, the washed samples have exhibited higher leaching compared to non-washed samples at all the experimental conditions. These observations were also confirmed from the sequential extraction data. From the study, it appears that the washing of IS rather results in negative impacts during the pyrolysis process.Bachelor of Engineering (Environmental Engineering