pH-controlled sulfuric acid washing enhancing heavy metal stabilization in lightweight aggregate production from municipal solid waste incineration fly ash

Municipal solid waste incineration fly ash (FA) is a pressing issue in urban development. In FA treatment, washing is a common dechlorination pre-treatment. During washing, considerable heavy metal would leach due to the abundant alkaline substances, posing challenges to the heavy metal stabilization. Also, with high alka-linity, FA has potential to co-dispose of waste sulfuric acid (hazardous waste). Addressing these, our study presented a novel pH-controlled sulfuric acid washing for FA dechlorination during the sintering preparation of lightweight aggregate (LWA). It was aimed at enhancing heavy metals stabilization and enabling the co-recycling of FA and waste sulfuric acid. By controlling a pH of 10.50 during sulfuric acid washing, the leaching rate of Pb, Cd, Zn, Cu, Ba, and Cr was below 2%, which was a marked improvement over conventional water washing. Sulfuric acid washed FA (SFA) can be used to produce standard-compliant LWA by sintering with bentonite and SiC. The SFA-based LWA stabilized heavy metals more effectively than LWA using FA and water-washed FA. At a 30 wt% addition rate, the stabilized rate of Pb and Cd increased from 11.7% (using FA) to 72.4% (using SFA), and from 6.3% (using FA) to 49.0% (using SFA), respectively. During washing and sintering, XPS analysis further investigated the transformation of Pb, Cu, and Zn into stable states (Pb-glass, Cu2O/CuO, and ZnAl2O4), enhancing their stabilization. Our study introduces a novel method for co-recycling FA and waste sulfuric acid, offering enhanced environmental sustainability and deeper insights into the mechanism of heavy metal stabilization.

Automated summary

This study presents a novel pH-controlled sulfuric acid washing method for municipal solid waste incineration fly ash (FA) to enhance heavy metal stabilization and enable the co-recycling of FA and waste sulfuric acid. The leaching rate of heavy metals was significantly reduced by controlling the pH during sulfuric acid washing, and the use of sulfuric acid washed FA (SFA) in lightweight aggregate production improved heavy metal stabilization. XPS analysis further investigated the transformation of heavy metals into stable states during washing and sintering. This study offers insights into the mechanism of heavy metal stabilization and provides an environmentally sustainable solution for FA treatment.