Insight into the mechanisms of combined toxicity of cadmium and flotation agents in luminescent bacteria: Role of micro/nano particles

Currently, risk assessment and pollution management in mines primarily focus on toxic metals, with the flotation agents being overlooked. However, the combined effects of metals and flotation agents in mines remain largely unknown. Therefore, this study aimed to evaluate the combined effects of Cd and two organic flotation agents (ethyl xanthate (EX) and diethyldithiocarbamate (DDTC)), and the associated mechanisms. The results showed that Cd + EX and Cd + DDTC exhibited synergistic toxicity. The EC50 values for luminescent bacteria were 1.6 mg/L and 1.0 mg/L at toxicity unit ratios of 0.3 and 1, respectively. The synergistic effects were closely related with the formation of Cd(EX)2 and Cd(DDTC)2 micro/nano particles, with nano-particles exhibiting higher toxicity. We observed severe cell membrane damage and cell shrinkage of the luminescent bacteria, which were probably caused by secondary harm to cells through the released CS2 during their decomposition inside cells. In addition, these particles induced toxicity by altering cellular levels of biochemical markers and the transcriptional levels of transport proteins and lipoproteins, leading to cell membrane impairment and DNA damage. This study has demonstrated that particulates formed by Cd and flotation agents contribute to the majority of the toxicity of the binary mixture. This study helps to better understand the complex ecological risk of inorganic metals and organic flotation agents in realistic mining environments.