Overlooked synergisms of flotation organic reagents and toxic metals in water: Formation of tridentate cadmium-xanthate complexes

Mining activities release a substantial quantity of toxic metals and organic flotation reagents into the aquatic environment, causing severe pollution and potential health risks. However, the synergisms and mechanisms associated with these mixed pollutants are largely overlooked. The combined application of atomic force microscopy, high-resolution mass spectrometry, nuclear magnetic resonance spectroscopy, isothermal titration calorimetry, and quantum chemical calculations revealed that that cadmium (Cd) is capable of forming tridentate coordination complexes with typical organic beneficiation reagents, namely xanthates (ethyl xanthate, isopropyl xanthate, and butyl xanthates). The resulting complexes exhibited symmetrical trigonal pyramidal structures, with the complexation composed of 4 σ bonds and 1.5 π bonds. The formation of such complexes significantly enhanced the bioavailability of Cd, thereby increasing the accumulation of Cd in hydroponically grown Chinese cabbage seedlings by up to 2.8-fold. Furthermore, it significantly inhibited the xanthates degradation, thereby augmenting their environmental persistence. Therefore, the coexistence of Cd and xanthates, particularly ethyl xanthate, caused significant synergistic toxic effects on Chinese cabbage seedlings. This was characterized by exacerbation of membrane lipid peroxidation damages and suppression of photosynthesis and transpiration, achieved through the inhibition of electron transport, stomatal conductance, and transpiration rate. These findings offer novel insights into the synergisms and mechanism of flotation organic reagents and toxic metals, which are essential to ensuring aquatic environment safety and human health.