Cubic and tetragonal ferrite crystal structures for copper ion immobilization in an iron-rich ceramic matrix

Abstract

This study proposes a strategy by reusing the incineration ash of municipal wastewater sludge as a ceramic material to immobilize copper. After sintering the mixture of CuO and sludge ash, hematite (α-Fe₂O₃, one major component) incorporated copper into cubic CuFe₂O₄. To observe copper incorporation mechanisms, mixtures of CuO + α-Fe₂O₃ were sintered from 650 to 1050 °C, and different copper incorporation behavior was detected. A low-temperature CuFe₂O₄ phase with a tetragonal structure was detected at 750 °C, and the cubic CuFe₂O₄ developed at 1000 °C. The incorporation efficiencies were first quantitatively determined by Rietveld refinement analysis of the X-ray diffraction data. The maximum copper incorporation into tetragonal and cubic CuFe₂O₄ reached around 80% and 73%, respectively. The leachability analysis pointed to the superiority of both copper ferrites in stabilizing copper, suggesting a promising technique for incorporating copper into the iron-rich ceramic matrix. Both tetragonal and cubic CuFe₂O₄ were observed with incongruent leaching behavior, but the lower copper concentrations and higher [Cu]/[Fe] ratio in tetragonal CuFe₂O₄ leachates indicates its higher capacity for copper stabilization. With a high transformation ratio into CuFe₂O₄ phases and dramatic reduction in metal leachability, the beneficial use of sludge ash to immobilize hazardous metal contaminated soil may be potentially successful.