Effects of Pyrolysis Conditions on Biosolids Biochar and Heavy Metal Immobilisation in Soil
Creators: name: Aktar, Shefali
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401199
subject: Environmental engineering not elsewhere classified

description: Biosolids, or the treated sewage sludge is the major by-products obtained from wastewater treatment plants and contain different types of organic and inorganic nutrients. Recently, the beneficiary use of biosolids has significantly increased in Australia. However, the presence of heavy metals, pesticides, contaminants and user regulations in the states limit the direct application of biosolids to agricultural land. The research presented in this thesis focuses on the variation in properties of biosolids during pyrolysis processes producing the carbon-rich solid materials, the biochar from biosolids. This research explores the effects of different pyrolysis conditions particularly the pyrolysis temperature and carrier gases on the physicochemical and structural properties of biochar. The effects of pyrolysis temperature (400, 500, and 600 °C) and two different carrier gases (nitrogen, N2 and carbon dioxide, CO2) were determined on biochar, derived from biosolids. The physico-chemical properties were determined by the analytical methods of pH, EC and CEC, FTIR, TGA and CHNS/O analyser, while SEM, BET and XRD were performed to identify the surface images, surface area and surface minerals. The results indicate that high pyrolysis temperature has a less functional group, high surface area, pH and ash content. Biochar produced under CO2 showed higher surface area, and more aromatic and hetero-aromatic structures, while biochar produced under N2 was more alkaline and had higher salinity content. Moreover, biochar produced from biosolids is expected to have a higher metal content after pyrolysis. We measured the total concentration, plant bioavailability and chemical fractionation of heavy metals in resulted biochar using USEPA 3050B, BCR and DTPA methods. Total metal concentration is increased with temperature and the bioavailable metals did not show any significant difference under different pyrolysis conditions. However, biochar produced at higher pyrolysis temperatures under N2 atmosphere can reduce the higher ecological risk of HMs in biochar. Furthermore, P fractionation and the chemical nature of P were investigated in resulted biochar using Hedley chemical sequential extraction and 31P liquid NMR analysis. This method was used to identify plants’ P reactivity or availability and the chemical nature of P compounds. The study determines increased P fixation with higher pyrolysis temperature and decreased short-term P-bioavailability, while the carrier gas shows minimal effects. Biochar produced in CO2 had slightly higher water-soluble P, NaHCO3-Pi, NaOH-Pi and Residual- P than prepared in N2, while HCl-P showed the opposite trend. Additionally, the predominant molecular configuration of P was present in the inorganic form identified by 31P liquid NMR spectra, while organic P transformed into inorganic P with increasing pyrolysis temperature. Finally, the effects of P-rich biochar in the remediation of Pb-contaminated soil were determined. The result identifies the effectiveness of biochar, where we also used traditional phosphate amendments (MCP &amp; NTS) for the immobilisation of Pb-contaminated soil in situ. The results showed that the leachable Pb (determined by TLCP) had a significant after biochar addition, while no significant result was found in bioaccessible Pb similar to traditional phosphate amendments. Overall, the results indicate that the observed changes in properties of biochar can be associated with the characteristic of lower pH, lower Electrical Conductivity (EC) and leachable-Pb in the biochar produced under different environments, although other mechanisms including the formation of stable Pb compounds might play an important role.