Biochar samples were prepared by hydrothermal carbonization (HTC)from municipal sewage sludge (MSS)and textile dyeing sludge (TDS)at different temperatures ranging from 150℃ to 300℃.This study focused on carbon fixation in biochar.The results showed that carbon fixation was significantly affected by sludge type and hydrothermal temperature.The HTC process of MSS was based on decarboxylation

while that of TDS was based on dehydration.Atomic ratios of biochar from MSS varied more greatly with the rise of hydrothermal temperature than those from TDS.Carbon recoveries in biochar from MSS were mainly influenced by biochar yield and decreased from 68.0% to 49.0% as the temperature increased

while those dropped from 100.0% to 76.8% for TDS due to the dominating effect of carbon content.Furthermore

the stable carbon content and its yield

recalcitrant C/total OC (RIc)of MSS biochars increased from 4.19% to 6.62%

from 123.6% to 161.2% and from 48.02% to 64.34% respectively with a rise of temperature

respectively.But the data of those indexes decreased from 3.80% to 1.86%

from 103.0% to 46.5% and from 54.26% to 47.04% respectively for TDS.Raman spectroscopy indicated that polycyclic aromatic hydrocarbon

fused aromatic ring and graphite-like microcrystalline structures led to high stability of MSS biochar.It was obvious that carbon fixation in biochar from municipal sewage sludge was better than that from textile dyeing sludge.