Abstract
Sewage sludge biochars were obtained at different pyrolysis temperatures from 300°C to 900°C and their macro- and microscale properties were analyzed. The biochar's plant-available nutrients and humus-like substances in the water-extractable phase and fixed nutrients in the solid fraction were evaluated for their potential agronomic implications. FT-IR, Raman, XRD, XPS, and SEM techniques were used to investigate the chemical structure, functional groups, and microcrystal structure on the surface of the biochar. The results revealed minor chemical changes and dramatic mass loss in the biochar obtained at 300–500°C, whereas significant chemical changes in the biochar were obtained at 600–900°C. The concentrations of plant-available nutrients as well as fulvic- and humic-acid-like materials decreased in the biochar samples obtained at higher temperatures. These results implied that the biochar samples pyrolyzed at 300–500°C could be a direct nutrient source and used to neutralize alkaline soil. The surface area and porosity of the biochar samples increased with temperature, which increased their adsorption capacity. Rearrangement occurred at higher temperature 600–900°C, resulting in the biochar becoming increasingly polyaromatic and its graphite-like carbon becoming organized.
Highlights
Multiscale visualization of the structural and characteristic changes of sewage sludge biochar oriented towards potential agronomic and environmental implication
Some of the documented changes include those in the yield, volatile contents, pH, electrical conductivity (EC), hardness, bulk density, and element composition that are related to macro-nutrient benefits for agro-application[1,8,9]
The decrease in the biochar yield with the increasing pyrolysis temperature could possibly be related to the cracking and volatilization process, which was reported[8] for sludge biochar pyrolyzed at different temperatures of 450uC, 650uC, and 850uC
Summary
Multiscale visualization of the structural and characteristic changes of sewage sludge biochar oriented towards potential agronomic and environmental implication. The other part of studies have focused on the behavior of heavy metals during low-temperature pyrolysis of sewage sludge at 500uC4, 300–700uC10, 300–500uC11, and 400–450uC12, which are related to the toxicity for agro-application. These two parts of researches were rarely put together and balanced the pros and cons of sludge biochar simultaneously. Song[12] demonstrated that garlic planted in soil amended with biochar obtained at 450uC contained the lowest level of heavy metals, when compared with that planted in soil amended with other biochars obtained at 500–550uC This uncertainty of biochar application should be attributed to the insufficient description about their studied biochar, i.e. only macroscale characteristics were recorded. It is necessary to investigate the macro- and microscale properties of the final sewage sludge biochar, and differentiate them systematically for easier categorization and for better evaluating the potential agronomic application of sludge biochar based on the above investigation
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