Effect of zinc ion on synthesis of chromium-containing spinel from wastewater and insight into the synthesis mechanism

Stable spinel can be synthesized in chromium-containing wastewater by ferrite process, which can not only realize the purification of chromium-containing wastewater, but also make chromium resource in the wastewater recycle. However, zinc ion often exists in the chromium-containing wastewater, and easily enters the spinel structure when the wastewater is treated by the ferrite process. Therefore, the effect of zinc ion on the synthesis of chromium-containing spinel from the wastewater was investigated, and the synthesis mechanisms was revealed. The quality of the filtrate, stability and magnetic properties of products synthesized at different initial concentrations of zinc ion were examined, and the results verified that moderate zinc ion enhanced the stability of chromium and magnetic properties of synthetic products, but excessive zinc ion reduced the quality of the filtrate. The synthetic path of products was illustrated by examining the stability and crystal evolution of synthetic product, monitoring the potential and pH, testing the dissolved oxygen, and analyzing the contents of metal ions in the filtrate. The synthesis mechanism was further confirmed by SEM-EDS and mössbauer spectra, and the results demonstrated that the gained solid products were mainly composed of goethite and spinel. It was deduced that the molecular formula of synthetic spinel wasFe3.243+(Fex2+⋅Fe2-x3+⋅Cry3+)O1.5y-0.5x+7.86 in the absence of zinc ion. However, the molecular formula of synthetic spinel was (Zn0.52+⋅Fe2.743+)(Fex2+⋅Fe2-x3+⋅Cry3+)O1.5y-0.5x+7.61 in the presence of zinc ion. The study laid a theoretical foundation for the industrial application of ferrite process in the chromium-containing wastewater.