Study on the transfer, transformation and toxic leaching of heavy metals in sludge shale composite ceramics from the perspective of environmental protection

Using sludge to produce expanded clay aggregate is an effective way to achieve resource recovery and commercialization of sludge. However, current sludge-based clay aggregates face challenges such as low strength and inadequate pore structure, which hinder their industrial application. To address these issues, this study proposes a new approach for preparing porous lightweight aggregates for environmental remediation by using sludge and shale as aggregates, biomass and Fe2O3 as foaming agents, and K2CO3 as a fluxing agent, with high- temperature sintering. The migration behavior of heavy metals such as Cr, Cu, Ni, Pb, and Zn in the sludge during the sintering process was analyzed, and the forms and impact mechanisms of these heavy metals were discussed. The toxicity leaching of the aggregates was evaluated. The study found the following: When the sludge-to-shale ratio is 3:7, with 10 % K2CO3, biomass, and Fe2O3, the comprehensive performance of the aggregates is optimal. At this composition, the compressive strength and porosity of the aggregates reach 76.22 MPa and 67.24 %, respectively, representing improvements of 90.56 % and 158.62 % compared to commercial aggregates. The content of weak-acid extractable heavy metals in the samples significantly decreases, while the content of residual states increases, especially at 1250 degrees C: the residual state contents of Cr, Cu, Ni, Pb, and Zn increase by 5.29, 33.2, 10.7, 112.3, and 106.4 times, respectively, while the weak-acid extractable contents decrease by 1.36, 0.69, 6.92, 0.22, and 28.4 times, respectively. This indicates that heavy metals present a higher environmental risk in their untreated state. After high temperature sintering, the extractable state of weak acid of these heavy metals is obviously reduced, and the residual state is greatly increased, which reduces the pollution to the environment. After sintering, the leaching concentration of Cr, Cu, Ni, Pb, and Zn decreases significantly with increasing pH, which greatly reduces the environmental hazard. XRD analysis reveals that the melting and sintering of the samples at high temperatures form potassium feldspar and other framework silicate crystals and glassy amorphous substances. This study provides theoretical basis and data support for sludge recycling, preparation of light porous aggregate based on sludge, stability and control of heavy metals and toxic leaching. The sludge composite ceramide prepared in this paper can greatly reduce the pollution of sludge to the environment.