Abstract
The lack of a standard method to regulate heavy metal determination in Brazilian fertilizers and the subsequent use of several digestion methods have produced variations in the results, hampering interpretation. Thus, the aim of this study was to compare the effectiveness of three digestion methods for determination of metals such as Cd, Ni, Pb, and Cr in fertilizers. Samples of 45 fertilizers marketed in northeastern Brazil were used. A fertilizer sample with heavy metal contents certified by the US National Institute of Standards and Technology (NIST) was used as control. The following fertilizers were tested: rock phosphate; organo-mineral fertilizer with rock phosphate; single superphosphate; triple superphosphate; mixed N-P-K fertilizer; and fertilizer with micronutrients. The substances were digested according to the method recommended by the Ministry for Agriculture, Livestock and Supply of Brazil (MAPA) and by the two methods 3051A and 3052 of the United States Environmental Protection Agency (USEPA). By the USEPA method 3052, higher portions of the less soluble metals such as Ni and Pb were recovered, indicating that the conventional digestion methods for fertilizers underestimate the total amount of these elements. The results of the USEPA method 3051A were very similar to those of the method currently used in Brazil (Brasil, 2006). The latter is preferable, in view of the lower cost requirement for acids, a shorter digestion period and greater reproducibility.
Highlights
The increasing application of fertilizers to meet the crops’ nutritional requirements has caused the annual addition of hundreds of tons of heavy metals to the soil, due to the impurities present in these substances (Lottermoser, 2009)
The highest rates of average metal recovery from National Institute of Standards and Technology (NIST) SRM 695 were obtained by United States Environmental Protection Agency (USEPA) 3052 (Table 2), due to HF addition, which solubilizes the metals contained in environmentally inaccessible fractions, recovering a higher metal content
It is noteworthy that the metal content of the certified material is determined by non-destructive methods, such as neutron activation, or by methods involving the complete dissolution of the sample (USEPA, 1996), whereas the MAPA method and USEPA 3051A
Summary
The increasing application of fertilizers to meet the crops’ nutritional requirements has caused the annual addition of hundreds of tons of heavy metals to the soil, due to the impurities present in these substances (Lottermoser, 2009). Commercial fertilizers contain an extremely varied range of products from different raw materials, resulting in various types of matrices (Kane & Hall Jr., 2006) that are accessed differently by chemical extractions. In this context, the use of effective methods for monitoring these metals in fertilizers is very important for the assessment of potential risks for soils. Due to the lack of a standard method for the extraction of heavy metals from fertilizers, various methods of acid digestion have been used instead These methods can use concentrated hydrochloric acid (HCl) in an open system (Brasil, 2006) or hydrochloric and nitric acids in closed (microwave) systems (Campos et al, 2005)
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