In Situ hydrochar regulates Cu fate and speciation: Insights into transformation mechanism

Cu is one of the dominant heavy metals toxic to human health and environmental ecosystems. Understanding its fate and chemical speciation is of great importance for hydrothermal liquefaction (HTL) of Cu-rich hazardous streams. Herein, we investigated its evolution during the HTL of wastewater algae through ICP-MS, XRD, XANES, and EXAFS. Cu-cysteine complexes (51.5%) and Cu2S (40.4%) were the main components of Cu in algae, whereas the predominant form was CuS (70.9%) in 220 ?C-hydrochar. Model compound experiments indicated that Cucysteine could be converted into CuS, while Cu2S was stable during HTL. However, Cu2S was partially converted into CuS in the hydrochar. Subsequently, the positive Gibbs free energy (36.8 KJ/mol) indicates that the oxidation from Cu+ to Cu2+ can?t occur spontaneously. Furthermore, cyclic voltammograms demonstrated that hydrochar facilitated the oxidation of Cu2S due to its higher capability of electron acceptance. All these results prove that hydrochar serves as a catalyst for the conversion of Cu2S to CuS during HTL. This study firstly elucidated that Cu2S was oxidized into CuS in the presence of hydrochar, and Cu-cysteine was converted into CuS under HTL. This study provides a critical insight into the transformation mechanism of Cu during the HTL of hazardous streams.

Automated summary

The study demonstrated that Cu- cysteine complexes and Cu2S were the main components of Cu in wastewater algae, while CuS was the predominant form in 220°C-hydrochar. Model compound experiments showed that Cu- cysteine could be converted into CuS, while Cu2S was stable during hydrothermal liquefaction. The positive Gibbs free energy indicated that the oxidation from Cu+ to Cu2+ cannot occur spontaneously. Cyclic voltammograms showed that hydrochar facilitated the oxidation of Cu2S due to its higher capability of electron acceptance.