Iron valence state evolution and hydrochar properties under hydrothermal carbonization of dyeing sludge

Iron (Fe) migration mechanisms and hydrochar properties in dyeing sludge hydrothermal carbonization (HTC) are important topics in wastewater treatment. HTC treatment of sludge produces wastewater containing Fe so it is necessary to study the migration behavior of Fe during HTC treatment. This study investigated the basic properties and Fe migration behavior of hydrochar during HTC treatment supplemented with nitric acid (HNO₃). The results showed that the carbonization degree and yield of hydrochar treated with the HNO₃ solution (HHC) were much lower than those of hydrochar treated with ultrapure water (WHC). The variation of total Fe (TF) concentration indicated that the decomposition of organic material and dissolution of minerals in the aqueous release of Fe during the liquid phase, led to much lower TF concentrations compared to the original dyeing sludge. Fe release was further enhanced with the addition of HNO₃ and increase of temperature, rendering a much lower TF concentration of the HHC compared to the WHC. The variations of Fe³⁺ and Fe²⁺ concentrations indicated that the HTC-treated hydrochar contained more Fe²⁺, caused by Fe³⁺ reduction with hydroxyl methyl-furfural and glucose in the liquid and subsequent Fe²⁺/Fe³⁺ transferral to the solid hydrochar phase. X-ray diffraction (XRD) showed that the main Fe content in WHC was FeO(OH), while HHC contained mainly Fe(SO₄)(OH)•2H₂O and Fe₃O₄. XPS and XRF showed that Fe could more easily enter the internal pores of the hydrochar instead of being deposited on the surface. This study provided more insights on Fe migration behavior during HTC treatment.