Karst water is rich in calcium ions (Ca 2+ ) and exhibits poor metal availability and low biodegradation efficiency. This study sought to analyze the effects and mechanisms of Ca 2+ on lead (Pb) removal and absorption by Eichhornia crassipes (a floating plant common in karst areas). Moreover, the morphology and functional groups of E. crassipes in water were characterized via SEM, and FTIR. The results demonstrated that the removal rate of Pb in karst water (85.31%) was higher than that in non-karst water (77.04%); however, the Pb bioconcentration amount (BCA) in E. crassipes roots in karst water (1763 mg/kg) was lower than that in non-karst water (2143 mg/kg). With increased Ca 2+ concentrations (60, 80, and 100 mg/L) in karst water, the Pb removal rate increased (85.31%, 88.87%, and 92.44%), the Pb BCA decreased (1763, 1317, and 1095 mg/kg), and the Ca BCA increased (6801, 6955, and 9368 mg/kg), which was attributed to PbCO 3 and PbSO 4 precipitation and competitive Ca and Pb absorption. High Ca 2+ concentrations increased the strength of cation exchange, alleviated the fracture degree of fibrous roots, reduced the atrophy of vascular bundles, protected the cell wall, promoted C-O combined with Pb, enhanced the strength of O-H, SO 4 2− , C=O, and reduced the oxidization of alkynyl acetylene bonds.industrial wastewater discharged by the lead smelting industry. Pb remains and accumulates in the food chain after entering the environment, which causes serious toxicity to organisms. Moreover, Pb affects plant photosynthesis and transpiration capacity [5,6]; therefore, Pb remediation has become the focus of many studies worldwide. Eichhornia crassipes is an aquatic floating plant that is widely distributed all over the world and occurs in large quantities in karst areas in China. E. crassipes has broad, thick, glossy, ovate leaves and stems, which are important for photosynthesis, food production, gas exchange, and water transpiration [7]. This plant is known to adapt to a wide variety of environmental conditions and even grows well in sewage water. Importantly, E. crassipes possesses a high heavy metal tolerance and uptake efficiency and has therefore become popular for water phytoremediation [8]. E. crassipes can reproduce quickly and absorb various pollutants [9]. Suryandari et al.[10] also demonstrated Pb removal efficiencies of up to 99.71% in E. crassipes harvested after nine days.Calcium (Ca) is an essential element for plants; it is an important regulator of plant growth and supports the structure and stability of cell walls, membranes, and membrane-binding proteins. Ca is also a second messenger that regulates the response of plants to environmental changes [11]. Moreover, certain Ca 2+ concentrations could promote growth and development, metabolism, cell structure, and adaptability to heavy metal stress [12][13][14]. Ca 2+ could also inhibit the transport of heavy metals from underground to aboveground plant structures [15]. Increased Ca 2+ concentrations could reduce heavy metal solubility [16] and allevia...
