Co-pyrolysis of sewage sludge and sawdust/rice straw for the production of biochar

•Co-pyrolysis of sewage sludge and sawdust/rice straw for biochars was conducted.•The yield of biochars decreased, while the content of organic matters increased.•Thermal stability, surface area, and pore volume of biochars were decreased.•The total contents of heavy metals in biochar products were reduced as expected.•The mobility of heavy metals in biochars was not alleviated.

Sewage sludge (SS) has been proved to be a good feedstock material for the production of biochars. However, the problem is that the total/leachable contents of some heavy metal elements in biochar products exceed the corresponding norms. In this work, efforts were made to solve above the mentioned topic through the addition of other biomasses (rice straw (RS) and sawdust (SD)) for the co-pyrolysis with SS. The addition of RS/SD reduced the yield of biochars, while the contents of organic matter in biochars were significantly improved. In addition, the introduction of biomasses, and especially the addition of SD, caused a decrease in the thermal stability, surface area, and pore volume of biochars. The total contents of heavy metals in biochar products, especially their Cu, Zn and Ni contents, were reduced as expected. However, reduction of the mobility of heavy metals in biochars according to the toxicity characteristic leaching procedure was not observed.

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