Quantification of heavy metals in agricultural soils: the influence of sieving in standard analytical methods

Conservation of agriculture soils is a topic of major concern, namely through the increase of soil organic matter. SoilCare project (https://www.soilcare-project.eu/) aims to enhance the quality of agricultural soils in Europe, through the implementation and testing of Soil Improving Cropping Systems in 16 study sites. In Portugal, the application of urban sewage sludge amendments in agriculture soils has been investigated. However, this application is a sensitive topic, due to the risk of long term accumulation of heavy metals and consequent contamination of the soil. The recent Portuguese legislation (Decret-Law 103/2015) is more restrictive than the precedent one (Decret-Law 276/2009) in terms of maximum concentrations of heavy metals in agricultural soils. The analytical quantification of heavy metals, however, raises some methodological questions associated with soil sample pre-treatment, due to some imprecisions in standard analytical methods. For example, the ISO 11466 regarding the extraction in Aqua Regia provides two pre-treatment options: (i) sieve the soil sample with a 2 mm mesh (but if mass for analyses is <2g, mill and sieve the sample <250µm is required), or (ii) mill and sieve the soil sample through a 150µm mesh. On the other hand, the EN 13650 requests soil samples to be sieved at 500µm. Since heavy metals in the soil are usually associated with finer particles, the mesh size used during the pre-treatment of soil samples may affect their quantification.

This study aims to assess the impact of soil particle size on total heavy metal concentrations in the soil. Soil samples were collected at 0-30cm depth in an agricultural field with sandy loam texture, fertilized with urban sludge amendment for 3 years. These samples were then divided in four subsamples and sieved with 2mm, 500µm, 250µm and 106µm meshes (soil aggregates were broken softly but soil wasn’t milled). Finer and coarser fractions were weighted and analyzed separately. Heavy metals were extracted with Aqua Regia method, using a mass for analyze of 3g, and quantified by atomic absorption spectrophotometer with graphite furnace (Cd) and flame (Cu, Ni, Pb, Zn and Cr).

Except for Cu, heavy metals concentrations increase linearly with the decline of the coarser fraction. This means that analyzing heavy metals content only in the finest fractions of the soil leads to an over estimation of their concentrations in the total soil. Results also show that coarser fractions of soil comprise lower, but not negligible, concentrations of heavy metals. Calculating heavy metal concentrations in the soil based on the weighted average of both fine and coarse fractions and associated concentrations, provide similar results to those driven by the analyses of heavy metals in the <2mm fraction. This indicates that milling and analyzing finer fractions of the soil did not influence the quantification of heavy metals in total soil. Clearer indications on analytical procedures should be provided in analytical standards, in order to properly assess heavy metal concentrations and compare the results with soil quality standards legislated.