Solar pyrolysis tars treatment using oxidation processes – Efficiency and toxicity analysis

Abstract

This paper presents oxidation and photocatalytic oxidation as the methods for elimination of toxic activity of biomass pyrolysis liquids subjected to the environment – water. With the increasing interest in biomass pyrolysis-derived fuels and chemicals, there is still a need for determining the environmental impact of unwanted leakage of the pyrolysis liquids to the environment via inappropriate handling or containers damage and proposing adequate solutions to solve it. Tars from solar pyrolysis of three biomass types (waste straw – WS, wood chips – WC, and dried sewage sludge – DSS) were treated using selected oxidation processes (ozonation – O3, photolysis – UV, and photocatalysis – UV-TiO2). The decomposition of organic substances present in the tars was investigated by changing the absorbance (Abs), total organic carbon (TOC), and phenol index (PI). Tar solutions before and after treatment were also submitted to an ecotoxicological assessment. It was determined that the tars organic substances content depended on the type of biomass subjected to solar pyrolysis. The highest concentration of organics was denoted in the liquid products from the solar pyrolysis of the sewage sludge. The decomposition of organic substances took place in the process of photocatalysis and ozonation, with higher effectiveness of the latter. The organic compounds present in the solar pyrolysis tar depend on the biomass feedstock. After 60 min of photocatalysis (TiO2 dose 1500 mg/L), phenolic compounds from tars coming from the solar pyrolysis of waste straw and wood chips were completely removed. In turn, the reduction in Abs and TOC was 37 and 19% for WS and 41 and 28% for WC, respectively. Abs in tars from the solar pyrolysis of sewage sludge in the photocatalysis process did not change, and the reduction of TOC and PI was 8 and 86%, respectively. However, the toxicity of post-process solutions requires attention. Tar solutions from the solar pyrolysis of wood chips after the photocatalysis process were non-toxic against bacteria, but tar solutions coming from the solar pyrolysis of waste straw and sewage sludge were low-toxic and toxic. Additionally, tars from the solar pyrolysis of sewage sludge after the photocatalysis process were highly toxic to crustaceans.