Enrichment and Migration of Heavy Metals in Mangrove Soil-Plant System from Sea Promenade in Zhanjiang

To explore the distribution, accumulation, and migration characteristics of mangrove soils and mangrove plants, the mass fraction of heavy metals (Cu, Zn, Pb, Cd, Cr, Ni, and As), the source thereof, and factors influencing their presence in the surface soil and different parts of mangrove plants (roots, branches, and leaves) were determined via inductively coupled plasma mass spectrometry (ICP-MS) in the Sea Promenade mangrove wetland in Xiashan District of Zhanjiang. The potential ecological risk index proposed by Hakanson was employed to evaluate the soil risk level. The influencing factors and sources of heavy metals were investigated using correlation and cluster analyses. The enrichment and migration ability of the heavy metals in soil plants were analyzed using the bioconcentration and translocation factors. The results showed that: 1) The order of concentrations (in mg·kg-1) of seven heavy metals in surface soil of the mangrove wetland was Zn (57.48) > Cr (29.31) > Pb (19.23) > Cu (16.62) > Ni (8.18) > As (6.00) > Cd (0.20). Moreover, the average mass fraction of the seven elements did not exceed the risk screening values for soil contamination regarding soil environmental quality, according to the risk control standard for soil contamination of agricultural land (GB15618-2018) (pH ≤ 5.5). The average values of Cu, Zn, Cd, and As were 2.08, 2.74, 6.75, and 1.11 times higher than the soil background values in Guangdong Province. Compared with the results of the investigation on heavy metals in 2015, the present results indicated increases in the concentrations of Cr, Cd, and Ni by 47%, 32%, and 6%, respectively; conversely, the concentrations of Cu, As, Zn, and Pb decreased by 15%, 14%, 13%, and 3%, respectively. 2) The highest potential ecological risk coefficient (Eir) values of Cd were in the range of 70-370, with an average value of 202, indicating a strong degree of potential ecological risk. The Eir values of the other elements were less than 40, indicating a slight degree of potential ecological risk. The mean value of the total potential ecological risk index (RI) of soil heavy metals investigated in the study area was 236, with a medium potential ecological risk degree. 3) The correlation analysis of heavy metals and soil physical and chemical properties showed that, except for Cd and As, there was a significant positive correlation between the heavy metal and organic matter contents, while a correlation between heavy metals and pH was negative but not statistically significant. Heavy metals were positively correlated with clay and silt but negatively correlated with sand. Organic matter, pH, and particle size are important factors that affect heavy metal accumulation in soil. 4) Concentrations of heavy metals in different mangrove plants were different. The order of heavy metal concentrations in Aegiceras corniculatum and Aegiceras corniculatum was Zn > Cr > Cu > Ni > Pb > As > Cd. The order of heavy metal concentrations in Sonneratia apetala was Zn > Cu > Cr > Ni > Pb > As > Cd. 5) The bioconcentration and translocation factors of heavy metals were less than 1 in Aegiceras corniculatum, Sonneratia apetala, and Bruguiear gymnorrhiza, indicating that the enrichment and translocation capacity of heavy metals was not high, except for the root-leaf translocation factor of Zn in Sonneratia apetala, which was greater than 1. Most of the toxic heavy metals were mainly accumulated in the roots, which reduced the risk of their delivery by the food chain