Designing Magnesium Phosphate Cement for Stabilization/Solidification of Zn-Rich Electroplating Sludge

Electroplating sludge is a hazardous waste due to its high potential to leach toxic elements into the natural environment. To alleviate this issue, we tailored magnesium phosphate cement (MPC) as a low-carbon material for stabilization/solidification (S/S) of Zn-rich electroplating sludge. The interaction between MPC and ZnO was investigated to clarify the precipitate chemistry, microstructure transition, and chemical environment of Zn species in the MPC-treated Zn sludge system. Comprehensive characterization (by X-ray diffraction (XRD), P-31 nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure spectroscopy (EXAFS)) and thermodynamic modeling results revealed that the incorporated ZnO preferentially reacted with phosphate to form Zn-3(PO4)(2)center dot 2H(2)O/Zn-3(PO4)(2)center dot 4H(2)O, changing the orthophosphate environment in the MPC system. Stronger chemical bonding between Zn and phosphate in comparison to the bonding between Mg and phosphate also resulted in the formation of amorphous Zn-3(PO4)(2)center dot 2H(2)O/Zn-3(PO4)(2)center dot 4H(2)O. Zn-3(PO4)(2)center dot 4H(2)O precipitate appears to predominate at high {K+}{H+}{HPO42-} values, and the formation of Zn-3(PO4)(2)center dot 2H(2)O/Zn-3(PO4)(2)center dot 4H(2)O competed for the Mg sites in the MPC system, leading to the inhibition of formation of Mg- phosphate precipitates. Overall, this work uncovers the precipitate chemistry and microstructure transition of Zn species in the MPC system, providing new insights into the sustainable S/S of Zn-contaminated wastes by adopting MPC.

Automated summary

The interaction between magnesium phosphate cement (MPC) and zinc oxide (ZnO) in the stabilization/solidification (S/S) process of Zn-rich electroplating sludge was investigated, revealing changes in precipitate chemistry and microstructure transition. The formation of amorphous Zn-3(PO4)(2)center dot 2H(2)O/Zn-3(PO4)(2)center dot 4H(2)O inhibited the formation of Mg-phosphate precipitates.