RESUMO
Phytoextraction has been proposed in many papers as a low-cost method for remediating contaminated soil. However, if national regulation is based on total metal(loid) concentrations in soil, phytoextraction is generally infeasible because of the long time required for remediation. Assessing phytoextraction requires determination of the dynamic rate of metal removal from soil. Phytoextraction may be feasible if the main goal is to reduce the soluble fraction of the metal(loid) with the goal of reducing bioavailability. However, it has been reported that there is a large mass balance mismatch between the reduction of the soluble metal fraction in contaminated soil and metal uptake by plants. Several studies report that the decrease of soluble fraction of metals in soil is higher than can be accounted for by plant uptake. In other words, studies generally overestimate the feasibility of bioavailable contaminant stripping. Therefore, a more rigorous approach is advisable to ensure that papers on bioavailable contaminant stripping include relevant information on mass balances. Furthermore, to implement the concept of bioavailable contaminant stripping, regulations must distinguish between the bioavailable fraction and the total metal concentration in soil. Environ Toxicol Chem 2023;42:558-565. © 2022 SETAC.
Assuntos
Metais Pesados , Poluentes do Solo , Estudos de Viabilidade , Biodegradação Ambiental , Poluentes do Solo/análise , Metais/análise , Solo , PlantasRESUMO
Several studies have attempted to predict the so-called "phytoavailable" fraction by correlating plant responses with different soil metal pools. Most of the data derived from these studies tend to be inconsistent, making interpretations difficult. Thus, the main objective of this study was to determine which soil Cu pool (free Cu2+, salt-exchangeable Cu or total Cu) controls Cu phytotoxicity in soils near a Cu smelter in central Chile. We studied the following traits of the local plant community grown spontaneously on the study site: species richness, shoot biomass, and plant cover. The site was dominated by four early plant colonizers: Eschscholzia californica Cham., Hirschfeldia incana (L.) Lagr.-Fossat, Lolium perenne L., and Vulpia bromoides (L.) Gray. We determined exchangeable soil Cu and activity of free Cu2+ in 0.1â¯M KNO3 extracts using soil/solution ratio of 1/2.5. The effect of total soil Cu on plant responses was not significant (pâ¯>â¯0.05). In our field-collected soil series, exchangeable Cu was a better indicator of soil phytotoxicity than either total soil Cu or free Cu2+ in the soil solution. We determined upper critical threshold values for Cu exposure using the three plant traits cited above. The mean values of EC10, EC25, and EC50 (effective concentration at 10%, 25%, and 50%, respectively) of exchangeable soil Cu (in µg L-1) were 255, 391, and 533, respectively. The mean EC10, EC25 and EC50 values of pCu2+ were 7.5, 6.8, and 5.9, respectively. We highlight the importance of further studies on Cu phytotoxicity using actual field-contaminated soils.