Fracionamento químico do fósforo no sedimento dos reservatórios de gargalheiras e carnaúba, localizados na Região Tropical Semiárida

Abstract

Knowing the chemical composition of the sediment in order to determine the mobile fractions of phosphorus (P) is an essential measure in the recovery of eutrophic lakes. The P accumulated in the sediment can be released to the water column in a process called internal fertilization, which is also responsible for intensifying the eutrophication process in aquatic bodies. In the semiarid region, in periods of scarcity of rainfall, internal fertilization acts as one of the most relevant causes of the eutrophication process in water bodies, making it essential to understand the composition and mobility of the P fractions in the sediments of these systems. In view of the above, the objective of this study was to quantify the fractions of P in the sediment and to evaluate the potential contribution of these internal phosphorus loads in maintaining the eutrophication of two reservoirs in the semiarid region, namely Gargalheiras and Carnaúba. Sediment samples were collected in January 2016, with the aid of a Kajak sampler/corer, these samples were frozen and lyophilized. Chemical fractionation analyses were performed to determine the P fractions present in the sediments, following the methodology proposed by Paludan and Jensen (1995), which consists of sequential extraction in 5 stages, with some modifications carried out by Cavalcante et al (2018). The fractions are classified as P weakly adsorbed to the surface of minerals and dissolved in interstitial water (P-Water), P associated with Fe and Mn oxides (P-BD), P bound to Al oxides (P-NaOH), P linked to organic matter (P-Humic), P bound to calcium (P-HCl) and residual P (P-Residential). As for the chemical composition of the fractions, aluminum oxides (Al2O3), calcium (CaO), iron (Fe2O3), magnesium (MgO), manganese (MnO) and silica (SiO2) were found through X-ray fluorescence spectrometry. (FRX). The total amount of phosphorus in the sediments was 1064.93 mg / kg, in Carnaúba, and 648.89 mg / kg in Gargalheiras. The P-BD fraction predominated in both reservoirs, with a value of 436 mg / kg (± 18.51) for the Carnaúba reservoir and 203.50 mg / kg (± 0.66) for Gargalheiras, followed by the inorganic part of the P-NaOH fraction, which was the second largest fraction found in the reservoirs. The smallest fraction found was P-Water with 20.65 mg / kg (± 0.90) in Carnaúba and 15.87 mg / kg (± 0.022) in Gargalheiras. As for oxides, the highest percentages are linked to Iron, Silica and Aluminum, respectively. In Carnaúba,a large/high amount of mobile P was found (53% of the total P load), these fractions can contribute to the eutrophication process, as they tend to be released from the sediment to the water column. Gargalheiras reservoir showed a higher percentage of non-mobile fractions, which are equivalent to 57% of the total P, these fractions are hardly released, tending to remain immobilized in the sediment. However, if we consider the high load of mobile fractions in its sediment, we can suggest that the Gargalheiras sediment also has a high potential for internal fertilization. Therefore, the sediments of Gargalheiras and Carnaúba have a high potential to act as internal sources of P, which can contribute to maintaining the eutrophication of these environments.