Treatment of high-ash industrial sludge for producing improved char with low heavy metal toxicity

[Display omitted] •Co-pyrolysis of high-ash industrial sludge and rice straw was conducted.•Aromaticity and pore structure of char are improved after adding rice straw.•Moderate rice straw proportions promote the retention of heavy metals in char.•Correlations between heavy metals speciation and char properties were proposed.•The char has superior application potential with low heavy metal toxicity. Industrial sludge is a type of solid waste that is known for its high ash content and heavy metal concentrations. In this study, industrial sludge was used for co-pyrolysis with rice straw at different temperatures (400, 500, and 600 °C) and mixture proportions (1:0, 9:1, 4:1, and 3:2, w/w). The relationship between heavy metal speciation and char properties was characterized in detail to investigate the effects of co-pyrolysis. Results show that the addition of rice straw can decrease the ash content and increase thermal stability and pore structure of char due to its high organic content. Simultaneously, co-pyrolysis decreases heavy metal concentrations and enhances the transformation of Cr, Zn, and Cd to a more stable fraction, with higher alkalinity, aromaticity, and specific surface area of char. Moreover, the leaching toxicity of Zn with high content is decreased to below threshold values, and the potential ecological risk of char is reduced to the lowest level under the condition of 600 °C with a mixing proportion of 4:1. This work demonstrates that co-pyrolysis provides an effective industrial sludge treatment for improving char characteristics and immobilizing heavy metals, indicating its superior application potential.