A green analytical assay for the quantitation of bioavailable phosphorus in soils: in situ extraction and smartphone-based measurement

Abstract

Bioavailable phosphate (BAP) is an essential macronutrient for plant biomass formation and agricultural productivity. However, excess phosphate can contribute to the eutrophication of water bodies, exacerbating problems related to water availability. The traditional method for determination of phosphorus in soil requires its solubilization with strong mineral acids and heating for ∼10 h. This paper describes a procedure for extracting BAP that can be performed in 45 min, at ambient temperature, using a portable ultrasonic bath. Use of a conventional benchtop ultrasonic bath was also evaluated. The results were compared with the conventional Mehlich-1 extraction method employing 5.0 g of soil and 50.0 mL of Mehlich-1 solution (0.05 mol L⁻¹ HCl + 0.0125 mol L⁻¹ H₂SO₄), with heating for 12 h. A 2³ factorial design was used to optimize the extraction using the portable device. Certified soil samples were employed in the experiments. Phosphorus extraction using the portable ultrasonic bath was performed for 45 min, using 0.50 g of soil and 5.0 mL of solution (0.05 mol L⁻¹ HCl + 0.0125 mol L⁻¹ H₂SO₄). The reaction to form molybdenum blue used 0.150 mL of the solution obtained from the extraction process. To enable in situ determination of BAP, a chamber was constructed to digitize images of the molybdenum blue dye. When compared to the conventional Mehlich-1 method, the proposed extraction technique presented efficiencies ranging from 105% to 64%. Higher extraction percentages were obtained for clay soils with higher calcium and magnesium ion concentrations, while lower efficiencies, ranging from 64% to 80%, were found for soils richer in iron and aluminum. The new technique is greener than the conventional method, since it can be performed in situ, with reductions of energy consumption, reagent usage, and waste generation.