Biochar from co-pyrolysis of urban organic wastes—investigation of carbon sink potential using ATR-FTIR and TGA

Urban organic wastes (UOW) strain the infrastructures for solid waste treatment (SWT) in emerging economies. This study investigated biochar gained from three major UOW sources in India—banana peduncles (BP), a fibrous waste, from fruit markets; sewage sludge (SS) from wastewater treatment plants; and anaerobic digestate (AD) from food and market waste processing facilities—in terms of its potential to sequester and become long-term carbon sink in soils. Herein, the chemical properties (using ATR-FTIR) and thermal oxidative stability (using TGA) of biochars derived from these UOW and their three blends were examined. Biochar from SS and AD and the blends were found to possess more ash content, Cl, and alkali and alkaline earth metals (AAEM) than that from BP. The conventional recalcitrance index (R50) could not quantify and compare the stability of these mineral- and ash-rich biochars. Hence, a modified thermal oxidative recalcitrance index (TORi) is proposed. All the biochar from blends prepared at highest treatment temperature of 650 °C shows similar aromaticity. However, biochar from blend of 50% SS, 30%BP, and 20% AD exhibits the highest recalcitrance (TORi = 0.193) to become a long-term carbon sink in soil. More than aromaticity, the influence of Si, Fe, and AAEM on the biochar matrix affects its recalcitrance. Variations in the structural properties and recalcitrance of biochars from blends are attributable to the synergy among their constituents SS, AD, and BP. The determined TORi confirms the potential of biochar from the blends of UOW as a long-term carbon sink.

Phenomenological Kinetics of Isothermal Wet Air Oxidation: Operating Severity Factor and Recalcitrance Index

In the present investigation, the kinetics governing wet air oxidation (WAO) of organic pollutants is modeled by two distinct parallel kinetic terms corresponding to a readily oxidizable and a recalcitrant structural parts of the pollutant molecule involving the concept of thermodynamic severity factor. The developed model was successfully applied to several classes of organic water pollutants including phenolic compounds as well as mono- and di-carboxylic acids. In the offshoot of the model, a recalcitrance index was proposed classifying the pollutant molecules according to their resistance to oxidation. For the carboxylic acids, it was found that the recalcitrance towards WAO was inversely correlated to the molecular size and weight. Acetic acid proved to be the more recalcitrant component. As for the phenolic compounds, the oxidation efficiency was shown to be affected by the nature of the substitutents present on the phenolic ring. The electron donating substitutents such as alkyl groups (CH3 or –C2H5) confer to the molecule an enhanced reactivity towards WAO. On the contrary, electron-accepting substitutents, such as methoxy groups, lower the oxidation and consequently enhance the recalcitrance.