Desenvolvimento e validação de métodos analíticos para determinação de micronutrientes e contaminantes em fertilizantes orgânicos

Abstract

In this work, four analytical methods were developed and validated for the determination of contaminants and micro-nutrient in organic fertilizers employing Fast Sequential Flame Atomic Absorption Spectrometry - FS FAAS (Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb e Zn) and Hydride Generation Atomic Absorption Spectrometry HG AAS (As e Se). The sample preparation step was the main focus, seeking higher analytical frequency, lower reagent consumption and less waste, moreover with adequate accuracy and precision. Different energy sources were employed to make available the analytes: conventional heating in open system (block digester), microwave radiation in closed system (microwave oven) and ultrasonic energy in reactor cup-horn. Univariate and multivariate strategies were used in the methods optimization. After optimization, the decomposition procedure assisted by microwave consisted of adding 4.50 mL of concentrated HNO3 in 200.0 mg of the sample, followed by addition of 2.00 mL of H2O2 and 1.5 mL of HCl. Then the flasks were closed and subjected to heating program (10 min until 200 °C and 15 min at 200 °C). The recoveries and precision of the method evaluated by analysis (n = 7) of the certified reference material of sewage sludge (CRM029-50 FLUKA) ranged 84-98% and 2-5%, respectively, for the determined elements (Cd , Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb and Zn). The extraction procedure in block digester for determining the total content of analytes involved the addition of 2.50 mL of aqua regia in 250.0 mg sample, 20 min contact, heating at 130 °C for 50 min, addition of 2.00 mL of H2O2 and further heating at 130 °C for 25 min. The recoveries of the analytes present CRM029-50 sample were greater than 93% and relative standard deviation (RSD) were less than 4% for the analyzes conducted with seven replicates. The sequential determination of As and Se by HG-AAS was performed from the extracts of the digestion block procedure. The optimized conditions for hydrides generation were: extract samples in HCl 7.0 mol L-1, solution reductive: NaBH4 4% (v m-1) + KI 10% v m-1 in 0.5% NaOH (v m-1) and acidic solution of HCl 6.0 mol L-1. The recoveries for the CRM029-50 were 97 and 111% for As and Se, respectively. Precision was 4 e 5% for As and Se, respectively. The optimized procedure of extraction assisted by ultrasound consisted of adding 500 L of acid mixture (HNO3-HCl-HCl, 5:3:3, v:v:v) in 120.0 mg of sample, 20 min contact, adding 500 L deionized water and sonication for 10 min with 50% amplitude. The recoveries for the CRM029-50 were 100, 109, 96, 92, 101, 104 and 102% for Cd, Cr, Cu, Mn, Ni, Pb and Zn, respectively. The developed methods presented suitable performance to meet the Brazilian legislation. Moreover, these methods has advantages as lower reagent consumption and higher analytical frequency when they compared to recognized methods by the Brazilian agencies.