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

Abstract

"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.

Keywords: Diffusive gradient in thin films (DGT); Functional mesoporous silicon nanomaterials (FMSN); Heavy metals; Soil pollution; X-ray fluorescence spectrometry (XRF).

Fig. 1

Copper removal by iMoLboX FMSN and its high absorbing capacity. a Partitioning of Cu from 15 ml 98% anhydrous ethanol solution (pH 4–6) containing 30 mg L⁻¹ CuSO₄ into 0.025 g FMSN, removal efficiency reached 92.97%. The color change of the solution is caused by bromophenol blue (0.1–0.3 ml). The color of FMSN changed from colorless to blue after the adsorption was completed. b Si crystal image before and after palladium recovery from a solution (pH 2–5) of 250 mg L⁻¹ PbCl₂. c and d Si dosed with 30 mg L⁻¹ CuSO₄ solution (pH 4–6). e Si crystal image before and after iron recovery from a solution (pH 3–5) of 150 mg L⁻¹ FeCl₃/FeCl₂. Si crystal particle size is 180–600 microns. The images of FMSN particle were obtained through compound optical microscope (LEICA DM500), provided by GoldenKeys High-tech Materials Co., Ltd.

Fig. 2

The cumulative number of published papers featuring SBA-15 and MCM-41 FMSN (Data source: Scopus)

Fig. 3

Two-dimensional representation of Se^IV and O₂ around a set of rice root. a Photograph of rice root grown in Se-contaminated soil. b Visualization of Se^IV around a set of rice roots. The outlined position of root that is featured in (c) is indicated by gray dash markings. c Se^IV species in the soil solution with distance from the root zone. d Oxygen distribution imaged by an O2 planar optode sensor [103]. Reprinted with permission from Shi X, Fang W, Tang N, Williams P, Hu X, Liu Z, Yin D, Ma L, and Luo J. In situ selective measurement of Se(IV) in waters and soils: diffusive gradients in thin-films with bi-functionalized silica nanoparticles. Environmental Science & Technology. 2018;52(24):14140-14148.). Copyright (2020) American Chemical Society

Fig. 4

Solute fluxes around a set of four-week-old rice roots with SPR-IDA DGT and O₂ planar optodes. a Image of O₂ distribution obtained before the deployment of the sandwich sensor. The horizontal-dashed lines show the soil-water interface (SWI). b 3D plot of O₂ distribution in the rice rhizosphere with the sandwich sensor, O+R indicates the aerobic rhizosphere, O-B indicates the anaerobic bulk area. c 3D plot of As fluxes in rice rhizosphere with the sandwich sensor. FM indicates flux maxima around the root tip apice. D indicates the flux depletion zones. d Fe fluxes in the rhizosphere. The green box shows the corresponding data extraction region/transect used for PCA analysis. e Mn fluxes in the rhizosphere. The yellow circles indicate flux microniches (label as M1). f PCA plot of elements in different regions, aerobic rhizosphere (O+R), non-rhizosphere/anaerobic soil (O-B), and flux maximal around root tip apice (FM1). For all images, the metal fluxes (f_DGT, pg cm⁻² s⁻¹) and oxygen concentration (percent air saturation) increased sequentially with the color scale shown from blue to white. The scales in the figure represent the following ranges from 0 to 100% for O₂, from 0.004 to 0.126 for As, from 0 to 42.144 for Fe, and from 0.71 to 22.39 for Mn [104]. Reprinted with permission from Yin D, Fang W, Guan D, Williams P, Moreno-Jimenez E, Gao Y, Zhao F, Ma L, Zhang H, and Luo J. Localized intensification of arsenic release within the emergent rice rhizosphere. Environmental Science & Technology. 2020;54(6):3138-3147.). Copyright (2020) American Chemical Society

Fig. 5

Photograph of rice root after planting in the blank soil (C) and the soil treated with 100 mg kg⁻¹ sulfur (S) (top). High-resolution 2D profiles of Sb^III and dissolved sulfide in the rhizosphere of rice obtained by MSBA-DGT (middle) and AgI-DGT (bottom) for 24-h deployments, respectively [74]. Reprinted with permission from Fang W, Shi X, Yang D, Hu X, Williams P, Shi B, Liu Z, and Luo J. In situ selective measurement based on diffusive gradients in thin films technique with mercapto-functionalized mesoporous silica for high-resolution imaging of Sb(III) in soil. Analytical Chemistry. 2020;92(5):3581-3588.). Copyright (2020) American Chemical Society