Transformation Behavior Of Phosphorus During The Thermal Treatment Of Solid Wastes And Its Utilization

Thermal processing (pyrolysis or incineration) of solid wastes can substantially reduce the waste volume and simultaneously properly reuse the energy and resource contained in wastes. Phosphorus, one of the most important and nonrenewable nutrient elements contained in solid wastes, has received more interests. The distribution and form of P in biochar or ash (the solid product derived from the thermal treatment of solid wastes) are essential for the disposal (reutilization) of biochar or ash. Herein, the thermochemical behaviors of P in different solid wastes (e.g., sewage sludge and plastics) in thermal treatment were investigated. Moreover, the release of P from biochar of rice husk to the environment was also explored and the environmental factors affecting the release of P were evaluated.1. Investigations into the transformation and migration of P in the sewage sludge under various thermal treatment conditions show that the temperature significantly influenced the species and content of P in the sewage sludge char or ash (SSC/A), while the atmosphere of thermal treatment had a slight effect on the fate of P. The sequential extraction,31P-NMR, and XRD analysis indicate that P was mainly migrated to the medium-term plant available P pool (poolNaOH) when treating the sewage sludge at low temperatures (400-600℃), while it was migrated to long-term plant available P pool (poolhcl) when treated at high temperatures (700-800℃).2. To investigate the behaviors. of tris(2-butoxyethyl) phosphate (TBEP) in co-pyrolysis, the mixture of TBEP and lignin was used to simulate the feedstock of the co-pyrolysis of wood biomass and waste plastics. In fast pyrolysis, the proportion of P distributed in char was significantly influenced by temperature. With an increase in temperature (400-500℃), the proportion of P distributed in char decreased from76.6%to51%. In slow pyrolysis, temperature had little effect on the proportion of P in char (all of them exceeded70%). The dose of CaCl2and MgCl2could significantly increase the P distributed in char through the formation of Ca, Mg-P compounds. The XPS analysis shows that the P species in char were orthophosphate, pyrophosphate (or P4O10), and the P associated with the aromatic rings in char (Ar-P), among which Ar-P was the most stable species and hardly to be extracted. Compared with the P in the char produced in fast pyrolysis, the P in the char derived from the slow pyrolysis could hardly be extracted, which was caused by the higher Ar-P content in the char of slow pyrolysis. The TG/FTIR/MS analysis suggests that the thermochemical reactions between the intermediates of both lignin and TBEP were responsible for the diversity of the P species in chars.3. The effects of environmental (soil) conditions on the release of different P species from rice husk biochar in a biochar-water system were explored. About2.2mg g-1P in the form of inorganic orthophosphate and pyrophosphate was released from a raw biochar (contained4.7mg P g-1) at initial pH of9.0in the initial8h. The release of orthophosphate was significantly enhanced by the coexisting anions of Cl-, NO3-or SO42-, attributed to the effect of ion exchange competition and the elevated ionic strength, while the release of pyrophosphate (P2O74-) was not influenced by the introduction of anions, which might be attributed to the formation of stable complexes. The introduction of Hoagland nutrient solution led to the decrease in release of P due to the formation of precipitates between dissolved P and excessive Ca2+, Mg2+, and NH4+.