Municipal sewage sludge (MSS) and hazelnut shell were used for co-pyrolysis by chemical activation with ZnCl2. The surface characteristics and potential ecological risk evaluation of heavy metals in the bio-char produced by co-pyrolysis were analyzed by surface analyzer and BCR sequential extraction. When raw materials with ZnCl2 (3 mol/L) were co-pyrolyzed at 500 degrees C for 90 min, specific surface area of the bio-char is 598.73 m(2)/g, and iodine absorption number is 607.85 mg/g. For microcosmic surface of the bio-char, the ratio of micropore area is stabilized from 0.74 to 0.80 of the total specific surface area, and hazelnut shell is effective to generate microporous construction. For the migration and transformation behavior of heavy metals, pyrolysis promoted mobile fraction (F1 and F2) to stable fraction (F3 and F4) with increasing pyrolysis temperature. The potential ecological risk of heavy metals transforms from considerable risk to low risk after pyrolysis at 500 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.