Biochar from vacuum pyrolysis of agricultural residues : characterisation and its applications.

[摘要] ENGLISH ABSTRACT: According to recent studies, biochar has the potential to improve soil fertility, mitigate climate change, reduce off-site pollution and assist in managing wastes. The application of biochar to soil is not a new concept; Amazonian dark earths are carbon-rich soils with high soil fertility that were created before 1541. Vacuum pyrolysis is a thermo-chemical conversion technique in which biomass is transformed into bio-oil, biochar and non-condensable gas. The objective of this work was to investigate the chemical and physical properties of biochar produced from vacuum pyrolysis of black wattle, vineyard annual prunings and sugar cane bagasse for their potential as soil amendment and adsorbent.The vacuum pyrolysis of black wattle, vineyard prunings and sugar cane bagasse (pyrolysis temperature: 460°C, pressure: 8kPaabs, heating rate: 17°C/min) resulted in biochar yields of 23.5%, 31.0% and 19.7% on a weight basis, respectively. The nature of the biomass had a substantial effect on yields of the products. High ash content combined with high lignin composition led to higher biochar yields for vineyard prunings.The highest surface acidity was observed for sugar cane bagasse (2.3 mmol/g), whereas the lowest surface acidity was observed for vineyard biochar (1.67 mmol/g). Consequently, the pH of the biochars was in the order: vineyard (10.43)> black wattle (9.74)> sugar cane bagasse (6.56). The cation exchange capacities (CEC) of biochars were 122 cmol/kg, 101 cmol/kg and 65 cmol/kg for sugar cane bagasse, black wattle and vineyard, respectively. The electrical conductivities (EC) were highly correlated with feedstock nature. The Ca and K rich vineyard biochar resulted in the highest EC (0.83 dS/m), whilst EC values of black wattle and sugar cane bagasse were 0.67 dS/m and 0.17 dS/m, respectively. Biochars contained substantial amounts of plant-available nutrients, while being low in toxic inorganic content (Pb, As, Cd). The BET surface areas of sugar cane bagasse, black wattle and vineyard were 259 mª/g, 241 mª/g and 91 mª/g, respectively.The adsorption capacity was found to increase with increased contact time and initial solution concentration. The experimental equilibrium time were found to be 3505 min, 1350 min and 150 min for adsorption of 20 mg/L methylene blue solution for vineyard, black wattle and sugar cane bagasse, respectively. Equilibrium data were well fitted to Langmuir and Freundlich isotherms. The maximum adsorption capacities were found to be 15.15 mg/g, 14.49 mg/g and 19.23 mg/g for vineyard, black wattle and sugar cane bagasse when modelled with Langmuir isotherms. The adsorption kinetics was found to follow the pseudo-second order kinetic model.In summary, biochar from sugar cane bagasse is a promising adsorbent for the removal of basic dyes due to its high surface area and microporous structure. This biochar can be applied to slightly acidic soils for nutrient retention and the exchange of nutrients. On the other hand, possessing high amounts of nutrients, biochars from black wattle and vineyard are potential soil amendentment agents. Biochar from black wattle is more beneficial compared to biochar from vineyard due to its higher surface area, microporosity and cation exchange capacity.

[发布机构] Stellenbosch University