Effects of iron on physicochemical and nutrient properties of biochar derived from sewage sludge

Abstract

Iron plays a vital role in determining the properties of sewage sludge-based biochar. In this study, sewage sludges with Fe content of 0.3, 3.5 and 9.2 wt% were obtained and pyrolyzed at 300 and 500 °C. Properties of the sewage sludge biochar (SSB) were characterized to explore the effects of Fe-level and pyrolysis temperature. Thermogravimetric analysis showed retarded weight loss at medium temperature range for high Fe-level sludge. Hematite was formed at high Fe-level but the crystallization of other minerals was disrupted. The porous structure was affected by Fe-level depending on temperature. The contents of C, H and N decreased in Fe-dosed SSB, while the H/C and C/N ratios varied with Fe-level. With the rise of Fe-level, the yield and ash content increased but the cation exchange capacity decreased. Due to catalytic effect and surface covering, iron-oxides promoted the dissolved and unstable organic carbon. The availability of P was decreased heavily even at medium Fe-level, caused by the formation of Fe−P species and the entrapment effect of iron-oxides. The available Ca and Mg were lowered in Fe-dosed SSB but N and K were less affected. When high Fe-level sludge was pyrolyzed at 500 °C, a recalcitrant iron-oxides layer appeared and lowered nutrient availabilities. The findings are useful for evaluating the role of Fe in determining SSB properties. The level of Fe should be controlled when SSB is produced aiming at land use, carbon sequestration and other purposes.