Formation and properties of inorganic silica-contaminant-compounds in soil

• Applicant: Professor Dr. Thilo Rennert, since 10/2019
• Subject Area: Soil Sciences
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 319417306

Several studies demonstrate the positive effect of Si-fertilizer on the resistance of some plant species to soil contamination. For example, the use of Si-fertilizer led to a significant reduction of the effect of Pb, Cd and Cu toxicity in some crops. One of the considerable reasons is the immobilization of contaminants through complexation in soil after Si-addition. The contaminants are thus bonded to silica in a plant unavailable form. However, this process has previously not been systematically investigated and clarified.The objective of the proposed project is to clarify the processes of immobilization of the metal cations of Pb, Cd and Cu in soil by means of silica. It should be investigated which bonding mechanisms occur between silica and soil contaminants, and which role play dissolved organic and inorganic substances and surfaces naturally occurring in soil.It is assumed that Pb, Cd and Cu cations form thermodynamically stable compounds with silica due to a polymerization reaction, after Si is added to the soil. The metal cation content of such silica-contaminant-compounds is characterized by an incorporated, an occluded and an adsorbed fraction. The incorporated and occluded metal cations are considered as long-term immobilized fractions and thus as hardly available or non-available for plants. It is assumed that the degree of metal incorporation into the polymer network of the silica in soil depends on the metal cation, Si-metal ratio and the type and amount of dissolved ions occurring in the soil, determining the amount, composition, as well as the thermodynamic stability of the silica-contaminant-compounds. The solid soil components provide surfaces for the aggregation of silica-contaminant-compounds, strongly diminishing their relocatability in the soil.The interactions between the dissolved silica and the metal cations in aqueous solution should be investigated in a series of precipitation experiments with different initial concentrations with and without the addition of two dissolved ions, which occur in soils of temperate-humid climate. The time dependence of the growth of formed particles will be determined by the measurement of the charge and particle size distribution. The formation and mobility of silica-contaminant-compounds in soil should be tested by soil column experiments using metal-contaminated arable soil. The adsorbed and occluded metal cations and the total metal content of the silica-contaminant-compounds will be determined in the aqueous solution after the precipitation experiments by extractions and total digestions. The metal incorporation in the silica polymer network will be identified by 29Si-NMR spectroscopy.The findings of this research project will provide fundamental knowledge on the use of silica (through Si-application) for the reduction of plant toxic inorganic metal ions in agricultural soils.

• DFG Programme: Research Grants
• Ehemalige Antragstellerin: Dr. Anna Georgiadis, until 10/2019