The influence of production temperature on physicochemical properties of biochars

Abstract

In the past, polycyclic aromatic hydrocarbons (PAHs) remediation strategies in soils relying on the use of biochar studied sorption or biodegradation of PAHs separately. However, those studies did not acknowledge that sorption and biodegradation of PAHs in soils can occur simultaneously. As a result, biochar productions were conducted using different temperatures and pyrolysis mechanisms, which resulted into different physicochemical properties. In particular, previous studies were not able to sufficiently resolve the scientific mechanisms behind the use of biochar for sorption or stimulation of biodegradation of PAHs by microbes. For example, they relied on production temperatures of 700-800oC, at which organic nitrogen needed for microbial growth and metabolism volatilizes. Similarly, they relied on fast pyrolysis, which yields biochar that have no soil carbon storage value and low aromaticity, necessary for the stronger binding of PAHs. Consequently, biochar production, characterization and application for PAH remediation have been conducted separately and continuously using different production temperatures and pyrolysis mechanisms, without identifying a unique production temperature or pyrolysis mechanism at which these two processes occur simultaneously based on the physicochemical properties of the resulting biochar. The objective of this work was to gain knowledge of the characteristics of biochars at high (650oC) and low (450oC and 350oC) production temperatures, out of which a unique production temperature for the production of biochar for its impact on large-scale petrogenic PAH remediation was identified. The purpose was to contribute to the use of the resulting biochar for PAH remediation in soils. This was achieved by focusing on two topics of concern. Firstly, the characteristics of biochar of slow pyrolysis relevant to petrogenic PAHs remediation in soils were studied. Secondly, the toxicants present in biochar that may hinder microbial activity and lead to soil quality deterioration were quantified and certified. Towards these ends, a novel hypothesis on how biochar production temperature can impact on PAH fate processes in soils simultaneously was formulated. Biochar properties were evaluated by physicochemical, structural and stability characterizations. Characterization of the sample biochar produced at 650oC displayed a greater surface area of 245 m2/g, had a greater organic carbon content of 83%, with greater aromaticity and the most stable with 12% of labile carbon. Differentiation between the carbon storage values and its novel mechanism was achieved, which was in a descending order of sBC+100=587 g kg-1, sBC+100=532 g kg-1 and sBC+100=407 g kg-1, for 650 > 450 > 350 to qualify for Corg (organic carbon) storage classes 4, 4 and 3, respectively. This mechanism is the online IBI classification-CPMAS 13C NMR Spectra-van Krevelen diagram model. Toxicant assessment/enrichment behaviour of heavy metals in biochar was investigated. The results indicated that the lowest metal concentration of 15709 µg/Kg was at the CS650, indicating minimum enrichment. The distribution and stability of heavy metals in biochar was determined. The results indicated a minimum distribution of 620 mg/kg at 650oC, implying greater stability. In conclusion, the study found that the 650oC biochar resulted in better characteristics for the rapid sorption of PAH in soils due to its greater proportion of the nanopores and aromaticity, both of which are responsible for stronger binding. Simultaneously, the greater aromaticity will result in greater biodegradation since the microbes will concentrate on the PAHs alone due to the absence of appreciable labile C substrate, thereby implying that biodegradation proceed at a faster rate.