Speciation evolution and transformation mechanism of P during microwave hydrothermal process of sewage sludge

Microwave hydrothermal sulfuric acid leaching of spent cathode carbon from aluminum electrolysis for high efficiency removal of insoluble calcium fluoride

Toxic substances, like fluoride salts present in spent cathode carbon (SCC), have been a great risk to the environment and public health. Our approach involves alkali leaching to eliminate soluble fluoride, followed by microwave hydrothermal acid leaching to efficiently remove insoluble CaF2 from SCC. The optimized conditions, including a temperature of 353 K, a solid-liquid ratio of 1:20, and a 60-minute reaction time, resulted in an impressive 95.6 % removal of fluoride from SCC. Various characterization techniques were employed to analyze the composition, micro-morphology, and elemental content of the materials before and after the leaching process. Furthermore, critical process parameters on the leaching separation of insoluble CaF2 during microwave hydrothermal acid leaching were systematically investigated. The study removal mechanism revealed the transformation of insoluble CaF2 in the process of microwave oxidation insertion-hydrothermal acid leaching for SCC. The kinetic characteristics of the two-stage leaching process of CaF2 at different temperatures were analyzed according to the shrinkage kernel model. The results indicate that the two-stage leaching process of CaF2 is affected by mixing control and by diffusion control, severally. The expansion of the graphite flake layer of SCC through oxidative intercalation was identified as a critical process for the thorough removal of CaF2. Microwave hydrothermal acid leaching demonstrated a 17 % improvement over traditional hydrothermal acid leaching within the same reaction time, showcasing a noteworthy enhancement in fluoride removal. Consequently, the microwave oxidizing intercalation-hydrothermal acid leaching treatment of SCC, as explored in this study, offers an effective approach for achieving deep defluoridation of SCC.

Fundamental properties and phosphorus transformation mechanism of soybean straw during microwave hydrothermal conversion process

Microwave hydrothermal (MHT) conversion is emerging as a promising technology for the disposal and reutilization of biowastes. This study investigated the fundamental properties and phosphorus transformation mechanism of soybean straw during the MHT conversion process. The oxygen-containing functional groups in soybean straw were stripped, and a trend of dehydration was observed as the temperature increased during the MHT process. Cellulose was identified as the major component of the MHT solid products at high temperature. Glucose and glucuronic acid in the MHT liquid products were gradually converted to formic acid and acetic acid with increasing temperature and holding time. The characteristics of the MHT products directly affected the changes in P speciation and transformation. Most of the P was distributed in liquid products and the impact of holding time was not significant on P distribution at low MHT temperature. With the increase in temperature and holding time, P gradually transferred into the solid products. The proportion of organic phosphorus and soluble inorganic phosphorus in soybean straw was high, and it decreased noticeably after the MHT process. The increase in MHT temperature promoted the conversion of OP and AP into IP and NAIP respectively. P K-edge X-ray absorption near edge structure analysis reveals that Ca5(PO4)3(OH) was the major component of soybean straw and more Ca5(PO4)3(OH) was formed at lower MHT temperature. This study provides fundamental knowledge on the property changes of soybean straw and the transformation of phosphorus during MHT conversion process, which is essential for its disposal and further utilization.