Abstract
ABSTRACT It is common the use of 3051A and aqua regia methods to assess the pseudo-total and total P contents in soil and in its fractions. However, previous study has shown the formation of large amounts of short-range order materials (SRO) in the residues of these extractions. The objective of this work was to evaluate whether phosphate resorb in the SRO formed in the residue of 3051A and aqua regia extractions in clay soil fraction. Residues obtained from the 3051A and aqua regia were recovered from filter papers. To characterize the SRO in these residues, sequential extractions were performed with 0.2 mol L-1 ammonium oxalate and 0.5 mol L-1 NaOH. The maximum pseudo-total P content extracted by aqua regia was 6800 mg kg-1. If SRO formed during this acid extraction was not extracted , 698 mg kg-1 P would have been not accounted (underestimation of 10.3%). This high level of P resorption in the 3051A and aqua regia extraction lead to misinterpretations of P reserve in clay fraction. Phosphate resorption was mainly in SRO extracted by ammonium oxalate for both 3051A and aqua regia residues. The chemical environment of the 3051A and aqua regia extractions facilitated the inner sphere adsorption of H2PO4 - in ferrol and aluminol groups of the SRO- ammonium oxalate. There was larger resorption of H2PO4 - in the 3051A-NaOH in relation to NaOH- aqua regia.
Highlights
EPA 3051A is the most used method to determine the pseudo-total content of elements in soils with environmental interesse (Chen et al, 1991; Dudka; Markert, 1992; Salonen; Korkka-Niemi, 2007; Souza et al, 2015; Melo et al, 2017)
Quantity and chemical composition of short-range order material (SRO) formed during 3051A and aqua regia extractions were detailed by Guedes, Melo and Batista (2020), and some important findings can be highlights: i) the amount and composition of short-range order materials (SRO) was dependent of clay mineralogy
The residue of 3051A and aqua regia extractions were composed by almost 100 % of SRO in the smectitic samples (Adrianópolis/site – samples 1 to 6); ii) chemical composition of SRO of aqua regia and 3051A residues: ammonium oxalate extracted mainly Al2O3 and Fe2O3, and the SRO solubilized by NaOH are composed of SiO2 and residual amount of Al2O3
Summary
EPA 3051A is the most used method to determine the pseudo-total content of elements in soils with environmental interesse (Chen et al, 1991; Dudka; Markert, 1992; Salonen; Korkka-Niemi, 2007; Souza et al, 2015; Melo et al, 2017). EPA 3051A involves digestion in a closed microwave oven system with concentrated. HNO3 and HCl (3:1) or only concentrated HNO3 (United State Environment Agency – USEPA, 1998). Aqua regia is another methodology frequently used to extract pseudo-total contents of heavy metals in soils (Öztam; Düring 2012). This method involves digestion in an open system (Sastre et al, 2002) and uses the same concentrated acids specified by EPA 3051A; MELO, V.
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