APPLICATION OF SPECTROMETRIC METHODS TO INVESTIGATE Cr, Cu, Fe, Mn, Pb and Zn DISTRIBUTION IN ORGANIC FERTILIZER

Organic fertilizers can be prepared from various raw materials, such as plants and animal scraps, manure and sewage sludge. To be applied as organic fertilizers, these materials must undergo a composting process, which may alter elements chemical composition, affecting some properties such as mobility, availability and phytotoxicity. Total elements concentration cannot supply enough data, being fractionating studies necessary to evaluate lability and bioacessibility of nutrients and contaminants in compost and organic fertilizers. Thus, in this work the particle size fractionating to investigate the lability of Cr, Cu, Fe, Mn, Pb and Zn associated to the different particle size fractions is presented. Organic fertilizer samples were fractionated using mechanical sieves (<500, 500-400, 400-300, 300-200, 200-100, 100-50 e >50 µm); each fraction was digested in a closed vessel microwave oven, using 3mL HNO3 + 1mL HBF4 + 2mL H2O. The concentrations of Cr, Cu, Fe, Mn, Pb and Zn were determined by ICP-OES. Carbon and nitrogen contents were also evaluated on each fraction, since the ratio C/N is and indicative of the humification degree. The elements content increased for lower particle size, as well as the C/N ratio decreased. There was a noticeable split on results obtained for particle sizes greater than 300 µm and lower than 300 µm (i.e. Pb from 3.5 mg kg-1 in >300 µm to 10.5 mg kg-1 in <300 µm). XRD analysis showed that fraction >300 µm was mostly formed by amorphous materials while fraction <300 µm presented quartz along with amorphous structures. These results indicated that the studied contaminants and nutrients on organic fertilizer come from a mineral fraction, which also had the most humified organic fraction. To investigate the lability of elements, extractions using different pHs (2.5; 4.0; 6.2 and 9.2) was executed and analyzed by ICP-MS. For particle size <300 µm, it was observed that Cr was almost non extracted. Cu and Pb were slightly extracted only on pH 9.2 (2.6% and 3.4% respectively), indicating that on this particle size fraction there is some of these compound that might be bound to humic substances (humic and fulvic acids are soluble on basic media). Mn and Zn had higher extraction on lower pH 2.5 (78% and 39%), respectively, indicating that there are species weakly bound to matrix.