Characterization of metal stabilization effect by X-ray diffraction technique and nano-indentation

Abstract

The technological development and application of waste-to-resource strategy is

significantly critical and crucial in both environmental and manufacturing industries, via

which we do not only provide practical treatments to toxic waste materials but also

translate them into usable products. It has been considered as a preferred method which

should be applied in future wastewater treatment strategies.

In this study, we investigated the process of incorporating cadmium oxide and nickel

oxide into ceramic-based materials with the phases of CdAl4O7, and CdFe2O4. Such

products are of less harmful effect to the natural environment and can also be of

beneficial use with their good mechanical properties identified by nano-indentation. We

proved the possibility and provided an opportunity to convert the waste from

wastewater treatment process to a new material resource.

XRD is preferred for characterizing solid mixtures to determine the relative

abundances of crystalline phases during the reaction process. As a result, we can obtain

the relative abundance information on the growth of the crystalline products, such as

CdAl4O7, and CdFe2O4 according to the change of fabrication temperatures. In that case,

the starting reaction temperature and the optimized temperature (at which the completed

reaction could be achieved) could be revealed. In Cd-Al system, the starting temperature

for CdAl4O7 formation is 900 °C, and the optimized formation temperature is around

1020 °C. On the other hand, for Cd-Fe system, such temperatures are of 700 °C and

850 °C correspondingly.

In our research, it is shown that the ceramic-process is an effective strategy to

stabilize the waste hazardous metals (cadmium and nickel) by materials such as

aluminate, ferrite, and kaolin commonly used in ceramic industry. Through this method,

the difficult-to-treat wasted metals would become reusable and applied in building and

infrastructure projects. Products containing CdAl4O7, CdFe2O4 and NiAl2O4 have

shown higher resistance to acidic leaching, comparing to CdO and NiO used as the

starting materials to simulate the waste metal forms discharged from the industrial

stream. Furthermore, similar measurements by alkaline attack on the sintered products

(NiAl2O4 containing samples) were also studied.

Besides the investigation to leaching behavior, the mechanical properties are also

measured by nano-indentation in our work. The incorporation of metal waste into the

fabrication of ceramic products is valuable due to the preferred stabilization

mechanisms of crystal structures and the large volume of ceramic products needed by

the construction industry. Furthermore, because the product safety and functionality

should not be compromised, a fundamental understanding of the surface properties of

metal containing phases should be further established, rather than relying solely on data

from regulatory tests on bulk samples. Therefore, the results of this study demonstrate

the superior mechanical properties of nickel spinel containing products, comparing to

the cristobalite silica matrix, under severe acid attack.