Ecological Indicators (Jun 2022)
Risk assessment of groundwater pollution during GLDA-assisted phytoremediation of Cd- and Pb-contaminated soil
Abstract
Using biodegradable chelate agents during phytoremediation of heavy metal contaminated soil is a growing consensus. However, the potential risk of groundwater pollution and the corresponding quantification during biodegradable chelate-assisted phytoremediation is currently ignored. This study provides a risk-assessment framework containing the practical boundaries and the transport model of chelate-metal complexes and metals at the subsoil, to quantify and predict the leaching risk of biodegradable N,N-bis(carboxymethyl)glutamic acid (GLDA)-induced phytoremediation during heavy rainfall. The case of ethylenediaminetetraacetic acid (EDTA)-assisted phytoremediation was also considered. Here, adsorption experiments and percolation experiments were conducted based on the subsoil to obtain adsorption parameters and hydrodynamic parameters. Optimal parameters derived from inverse fitting and the boundary condition originated from pot leaching experiments were used for the framework establishment and Hydrus-1D simulation. Results showed that fewer GLDA-metal complexes were adsorbed on the soil particles compared with metals, inferring their great mobility in the subsoil. However, the chemical and physical non-equilibrium process indicated by the asymmetric breakthrough curves of the percolation experiments would also affect the transport behavior of contaminants in the sub-vadose zone. In addition, adsorption parameters and optimal hydrodynamic parameters aptly described breakthrough curves of the chelate-metal complexes, and could better predict their transport behavior at the subsoil. Opposite to the previous findings, prediction results showed that heavy rainfall during GLDA-assisted phytoextraction resulted in Cd and Pb pollution in shallow groundwater, where the pollution degree was related to properties of the soil horizon, depth of the water table, and types of contaminants. The degradation feature of GLDA-metal complexes would fail to keep groundwater from heavy metal contamination at the sub-vadose zone with high permeability. Thus, we recommend our risk-assessment framework to optimize the operating condition before utilizing the biodegradable chelate-assisted phytoremediation.
