Development of a method for sequential Si extraction from soils

Abstract

In this paper, we introduce a new method for sequential extraction of different silicon (Si) fractions from soils. The method has been developed based on several series of extraction experiments on well-characterized isolated soil compounds and selected soil samples. Results and implications of these test series are presented, and reasons for the choice of methods for the single steps of the sequential extraction procedure are given.The sequential extraction method separates seven Si fractions. The first four extraction steps are performed on four replicates, which are then split into two by two replicates subjected to two different treatments in the following step. 1) The mobile Si fraction is obtained by extraction by weak electrolyte solution of CaCl2. 2) Si in adsorbed silicic acid is extracted by acetic acid through anion exchange. 3) Si in soil organic matter (SOM) is released by SOM oxidation with H2O2. 4) Si in pedogenic oxides and hydroxides is obtained by treatment with ammonium oxalate and oxalic acid solution under UV-light. 5) In step 5 amorphous silica of biogenic and minerogenic origin is specified. This separation is done in three sub-steps: 5.1) Two samples are directly subjected to NaOH extraction to obtain the total amorphous silica fraction; 5.2) the two other samples are first subjected to bio-opal separation with sodium polytungstate; 5.3) NaOH extraction is then applied to the bio-opal samples to obtain their Si content. The difference between the amounts of Si in the extracts of sub-steps 5.1 and 5.3 is interpreted as Si from minerogenic amorphous silica forming precipitates on surfaces of mineral grains. 6) In addition, total Si concentration is either determined by fusion of sub-samples with lithium borate, dissolution in nitric acid and ICP-OES analysis or by X-ray fluorescence analysis. The share of crystalline silicates is calculated as the difference between total Si and the sum of the Si fractions obtained in extraction steps 1 to 5.Potential drawbacks of the method include i) varying efficiency of the second extraction step, strongly depending on soil composition, ii) overestimation of Si in soil organic matter due to partial dissolution of clay minerals, pedogenic oxides and amorphous silica by H2O2, iii) underestimation of Si in pedogenic oxides and hydroxides due to incomplete destruction of highly crystalline pedogenic oxides or nodules.