Heavy metals, which are toxicity for most creatures, have caused serious environmental problems due to their non-biodegradability and acceleration in food chain for decades. [1–3] Cadmium, which commonly presents in the environment, has been considered as one of general heavy metals exhausted in industrial wastewater, especially from nickel cadmium battery production process. Continuous exposure to Cd-rich sources may lead to functional disorder in human’s kidneys, liver, lungs, cardiovascular, immune and reproductive systems. [4–6]
Sewage sludge is an inevitable by-product of sewage treatment plants, which contains pathogens, heavy metals and organic pollutants. With the development of industrialization and modernization, the annual output of sewage sludge in various countries is also increasing. Some schemes try to reuse sewage sludge incineration residues directly for building materials, but the concentration of heavy metals containing in the untreated sewage sludge incineration residues usually exceeds the relevant standards in leaching process. Before utilization, sewage sludge incineration residues require stabilization process to comply with the law related to final applications. [7–9] Chemical precipitation, coagulation–flocculation, flotation, ion-exchange and membrane processes are technologies that have been widely used to remove cadmium from industry waste and reduce the biological hazards of cadmium. However, the removal processes also produce sludge which aggregates large amount of cadmium and causes serious secondary pollution. Landfilling is a traditional method to treat sewage sludge. However, landfilling is not anymore suitable for treatment of Cd-laden waste, because it takes up too much space, and the reduction of available land for landfill has already become a critical issue. Besides, high concentrations of organic matter and heavy metals in leachates will contaminate the surrounding water and soil once released in long term environment erosion. Therefore, it is of great importance to develop an effective strategy to treat the cadmium bearing waste. [11–14]
In recent years, sewage sludge management methods based on thermal treatment are becoming more and more popular. The volume of sewage sludge will be reduced by 85%, thus saves a lot of space. At the same time, viruses, bacteria and organic matter will be removed after thermal treatment. [16, 17] As a matter of fact, ceramic sintering has become one of the most efficient way to stabilize poisonous heavy metals. Previous studies have reported the utilization of sewage sludge incineration residues as precursor for zinc stabilization at high temperature. Spinel phase with formula of ZnAlxFe2−xO4 was greatly enhanced during thermal treatment process at elevated temperatures, and the leachability of zinc substantially declined because of the formation of the spinel structure. In system for co-stabilization of Pb/Cu/Zn, heavy metals achieved coimmobilization through ceramic sintering together with sewage sludge incineration residues, which significantly reduced the leachability of Pb, Cu and Zn in acid environment. The formation of crystal compounds such as ZnxCu1−xFeyAl2−yO4, PbAl2Si2O8, Pb3(PO4)3, and CuFe2O4 contributes to the stabilization of Pb/Cu/Zn metals at high temperatures (1000℃-1600℃) by adding silicon-containing compound to sewage sludge containing heavy metals. Further leaching experiment proves long term stabilization of heavy metals in ceramic products, therefore their threats to the environment are eliminated. [19, 20]. However, the clay minerals used in ceramic systems has attracted attention due to the excessive exploitation of resources and the consequent environment pollution, so it is essential to find alternatives to replace nonrenewable clay minerals.
Owing to the high content of aluminum and silicon, the residues from incineration of sewage sludge may be reused as ceramic raw materials in stabilizing cadmium-laden waste. [22–25] The following achievements can be made. Firstly, cadmium was incorporated in ceramic system with matrix rich in aluminum and silicon to achieve stabilization. Secondly, it will reduce the storage of sewage sludge, and realize resource utilization. Thirdly, sewage sludge incineration residues have the potential to replace raw materials in the ceramic industry and eliminate the stress in excessive exploitation of clay minerals in ceramic systems. The other benefit of using sewage sludge as ceramic raw materials is that there’s always abundant of sewage sludge in the world. For example, in 2004, Japan produced about 4.14 × 108m3 of sewage sludge, and the volume increased by 170% from 1990 to 2004.  The sewage sludge occupies a lot of space and has long term toxicity, therefore requires instant properly handling. The recycling of sewage sludge incineration residues as ceramic precursor will relieve the environmental burden and provide a ‘waste-to-resource’ strategy to realize the sustainable management of sewage sludge.
In this study, sewage sludge incineration residues were used as ceramic raw materials, and Cd(NO)3 and CdO were added to the sewage sludge incineration residues to simulate Cd-laden industrial sludge, which has yet to be stabilized. Sintering temperature range is 500℃-1000℃, and the duration is 2 hours. The phase constitution of each product will be characterized to explain the phase transformation and cadmium binding mechanism during sintering. The leachability of cadmium will be tested by leaching experiments to further evaluate the stabilizing effect of cadmium in sintered ceramic products.