The Cd concentrations in the leachates of CdSiO 3 , Cd 2 SiO 4 , Cd 3 SiO 5 , CdAl 4 O 7 ,
and CdFe 2 O 4 phases are shown in Fig. 7.8. The CPLT results show that the
concentrations of Cd in the leachates of these product phases, particularly for
CdSiO 3 and CdFe 2 O 4 , were remarkably lower than that in the leachate of CdO
after 120 min of leaching by nitric acid at constant pH of 4.0. This indicates that
superior stabilization and detoxification could be achieved by the formation of
silicates, aluminate, and spinel crystal structures. The leachability of Cd 2 SiO 4 and
Cd 3 SiO 5 phases is more significant than that of other Cd-hosting product phases.
The leaching behavior of Cd 2 SiO 4 and Cd 3 SiO 5 phases tends toward congruent
dissolution, whereas the others (CdSiO 3 , CdAl 4 O 7 , and CdFe 2 O 4 ) display incongruent dissolution. The crystal structures of Cd 2 SiO 4 and Cd 3 SiO 5 together with the
high content of Cd (over 70 wt.%) in these two phases suggest that Cd atoms may be
more exposed to the leaching fluid, which may lead to a high dissolution of Cd from
Cd 2 SiO 4 and Cd 3 SiO 5 phases. For Cd detoxification, converting Cd into CdSiO 3 and
CdFe 2 O 4 can reliably prevent Cd leaching under acidic conditions, compared with
the CdO phase and other Cd-hosting product phases.
4 Conclusion
This chapter summarizes the emergence of heavy metal contamination, particularly
Cd contamination, the current and new technologies for the control of Cd pollution,
the potential ceramic matrices for Cd detoxification, and the quantitative approaches
to assessments of Cd incorporation and detoxification. The possibility and feasibility
of beneficially using various low-cost and easily attainable ceramic matrices to
convert Cd-bearing waste into stable crystal structures via a ceramic sintering
process is considered. Ceramic sintering (a type of thermal treatment process) has
been demonstrated to be a reliable strategy for transforming metal-contaminated
substances into preferable crystalline phases. Four types of common ceramic matrix
(SiO 2 , γ-Al 2 O 3 , α-Fe 2 O 3 , and Fe 3 O 4 ) were thermally reacted with Cd, and their
incorporation capabilities were quantified with a combination of the XRD technique
and the Rietveld refinement method. The results show that CdAl 4 O 7 was the only
Cd-hosting product of sintering with γ-Al 2 O 3 . Silicates CdSiO 3 , Cd 2 SiO 4 , and
Cd 3 SiO 5 were the predominant Cd-hosting product phases in the CdO + SiO 2
system. The formation of silicates was greatly affected by the sintering temperature
and the Cd/Si molar ratios (1/1, 2/1 or 3/1). CdFe 2 O 4 spinel was the only Cd-hosting
product phase in sintered CdO + α-Fe 2 O 3 and CdO + Fe 3 O 4 systems. The CPLT
results revealed that CdFe 2 O 4 spinel is the most stable phase, suggesting that the
formation of CdFe 2 O 4 spinel is the most appropriate for Cd detoxification.
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and CdFe 2 O 4 phases are shown in Fig. 7.8. The CPLT results show that the
concentrations of Cd in the leachates of these product phases, particularly for
CdSiO 3 and CdFe 2 O 4 , were remarkably lower than that in the leachate of CdO
after 120 min of leaching by nitric acid at constant pH of 4.0. This indicates that
superior stabilization and detoxification could be achieved by the formation of
silicates, aluminate, and spinel crystal structures. The leachability of Cd 2 SiO 4 and
Cd 3 SiO 5 phases is more significant than that of other Cd-hosting product phases.
The leaching behavior of Cd 2 SiO 4 and Cd 3 SiO 5 phases tends toward congruent
dissolution, whereas the others (CdSiO 3 , CdAl 4 O 7 , and CdFe 2 O 4 ) display incongruent dissolution. The crystal structures of Cd 2 SiO 4 and Cd 3 SiO 5 together with the
high content of Cd (over 70 wt.%) in these two phases suggest that Cd atoms may be
more exposed to the leaching fluid, which may lead to a high dissolution of Cd from
Cd 2 SiO 4 and Cd 3 SiO 5 phases. For Cd detoxification, converting Cd into CdSiO 3 and
CdFe 2 O 4 can reliably prevent Cd leaching under acidic conditions, compared with
the CdO phase and other Cd-hosting product phases.
4 Conclusion
This chapter summarizes the emergence of heavy metal contamination, particularly
Cd contamination, the current and new technologies for the control of Cd pollution,
the potential ceramic matrices for Cd detoxification, and the quantitative approaches
to assessments of Cd incorporation and detoxification. The possibility and feasibility
of beneficially using various low-cost and easily attainable ceramic matrices to
convert Cd-bearing waste into stable crystal structures via a ceramic sintering
process is considered. Ceramic sintering (a type of thermal treatment process) has
been demonstrated to be a reliable strategy for transforming metal-contaminated
substances into preferable crystalline phases. Four types of common ceramic matrix
(SiO 2 , γ-Al 2 O 3 , α-Fe 2 O 3 , and Fe 3 O 4 ) were thermally reacted with Cd, and their
incorporation capabilities were quantified with a combination of the XRD technique
and the Rietveld refinement method. The results show that CdAl 4 O 7 was the only
Cd-hosting product of sintering with γ-Al 2 O 3 . Silicates CdSiO 3 , Cd 2 SiO 4 , and
Cd 3 SiO 5 were the predominant Cd-hosting product phases in the CdO + SiO 2
system. The formation of silicates was greatly affected by the sintering temperature
and the Cd/Si molar ratios (1/1, 2/1 or 3/1). CdFe 2 O 4 spinel was the only Cd-hosting
product phase in sintered CdO + α-Fe 2 O 3 and CdO + Fe 3 O 4 systems. The CPLT
results revealed that CdFe 2 O 4 spinel is the most stable phase, suggesting that the
formation of CdFe 2 O 4 spinel is the most appropriate for Cd detoxification.
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M. Su et al.