Functionalized Mesoporous Silicon Nanomaterials in Inorganic Soil Pollution Research: Opportunities for Soil Protection and Advanced Chemical Imaging

"Innovative actions towards a pollution free-planet" is a goal of the United Nations Environment Assembly (UNEA). Aided by both the Food and Agricultural Organisation (FAO) and its Global Soil Partnership under the 3rd UNEA resolution, a consensus from > 170 countries have agreed a need for accelerated action and collaboration to combat soil pollution. This initiative has been tasked to find new and improved solutions to prevent and reduce soil pollution, and it is in this context that this review provides an updated perspective on an emerging technology platform that has already provided demonstrable utility for measurement, mapping, and monitoring of toxic trace elements (TTEs) in soils, in addition to the entrapment, removal, and remediation of pollutant sources. In this article, the development and characteristics of functionalized mesoporous silica nanomaterials (FMSN) will be discussed and compared with other common metal scavenging materials. The chemistries of the common functionalizations will be reviewed, in addition to providing an outlook on some of the future directions/applications of FMSN. The use of FMSN in soil will be considered with some specific case studies focusing on Hg and As. Finally, the advantages and developments of FMSN in the widely used diffusive gradients-in-thin films (DGT) technique will be discussed, in particular, its advantages as a DGT substrate for integration with oxygen planar optodes in multilayer systems that provide 2D mapping of metal pollutant fluxes at submillimeter resolution, which can be used to measure detailed sediment-water fluxes as well as soil-root interactions, to predict plant uptake and bioavailability.

Metal partitioning and speciation in a mining-impacted estuary by traditional and passive sampling methods

This study deals with the metal partitioning and bioavailability of metal/loids in the estuary Ria of Huelva (SW Spain) which is strongly affected by historical mining and industrial activities. To address this issue, traditional (i.e., grab samples) and passive sampling (i.e., diffusive gradient in thin films, DGTs) was carried out in the outer part of the estuary during different tidal cycles in order to determine the dissolved and particulate metal/loid concentrations. The dissolved concentrations exceeded, by several orders of magnitude, those reported in other estuaries worldwide that are affected by anthropogenic activities. A spatial pattern was observed in the metal distribution; a decrease seaward was recorded for some of the elements associated with mining (e.g., Cu, Zn, and Cd), the opposite tendency is observed for others associated with harbor emissions (e.g., Sn, Ni, or Pb). A different metal/loid partitioning pattern was also observed; Fe, and to a lesser extent Pb and Sn, were chiefly found in the particulate matter, while the rest of the elements were mainly found in the dissolved form. The bioavailability of the metal/loids was studied by speciation using both geochemical modeling and DGTs; while concentrations in DGTs supported metal/loid speciation for Zn, Cd, Mn, Co, As, and Sb according to their affinity to form strong or weak complexes, some discrepancies were observed for other elements such as Cu, V, Fe, and Pb, which are prone to forming strong complexes. The main reason behind the unexpectedly high Fe and Pb DGTs concentrations may be associated with their presence in the colloidal particles passing through the DGT. There was a strong positive correlation between dissolved and DGT concentrations for Cd and Mn, and to a lesser extent for Fe and Cu, highlighting the direct relationship between the concentrations in water and availability to living organisms in the estuary.

In situ simulation of thin-layer dredging effects on sediment metal release across the sediment-water interface

Dredging is widely applied to remediate contaminated sediments in aquatic ecosystems. However, the efficiency of thin-layer dredging for metal pollution control remains uncertain and even controversial. This study conducted an in-situ simulation experiment in Lake Taihu to investigate dredging effects on sediment metal release based on metal fractions, diffusion flux and kinetics parameters of metal resupply, using diffusive gradient in thin films (DGT), multi-microelectrode, and European Community Bureau of Reference (BCR) sequential extraction scheme. Results indicated that the exchange fluxes of metals did not necessarily correspond to total sediment metal concentrations or the contents of different sequentially-extracted metal fractions; there were appreciable decreases in Ni, Cd, Cu and Zn in terms of total sediment metal concentrations and metal fractions, whereas the bioavailability and release fluxes of labile Ni, Cu and Zn (but not Cd) were all notably promoted (by 136, 128 and 149%, respectively) in dredged area compared to those in un-dredged sediments. Further analysis on the kinetics of metal resupply by DGT technique and DGT-induced fluxes in sediments model (DIFS) showed higher concentrations of labile metals, with a larger resupply ability from sediments after dredging. Therefore, thin-layer dredging had the possibility to increase metal release from sediments to the water column. This was attributed to the remobilization of metal sulfides in anoxic deep sediments, as oxidation increased after dredging due to the introduction of oxygenated water, causing subsequent dissolution of sulfide-bound metals. In conclusion, dredging may not mitigate metal contamination, although it can reduce the total pollution load. Our findings indicated dual effects of dredging and provided new insights into the remobilization mechanism of metal release induced by dredging.