A especiação do ferro e sua relação com a mobilização geoquímica do fósforo no reservatório Castanhão

Abstract

In recent years, the Castanhão reservoir has lost volume due to a prolonged drought, which increased the severe eutrophication of the reservoir, which potentially changes the oxygenation from both the aquatic environment and the sediment. This work, carried out in the Coastal Biogeochemistry Laboratory of the Institute of Marine Sciences of Federal University of Ceará, aimed to determine the speciation of dissolved iron in surface and pore waters to evaluate their importance in the geochemical mobility of the phosphorus in natural water and sediments of the reservoir. In August of 2015, the average concentrations of Fe+3 in surface water were 5µg/L ± 1,68, Fe+2 from 10,1 µg/L ± 3,9, total phosphorus (TF) was 115,58 µg/L ± 6,8, and reactive dissolved phosphorus (RDF) from 11,51 µg/L ± 4,71. In February of 2016, average concentrations of Fe+3 in surface water weren’t detected, Fe+2 from 38,94 µg/L ± 8,18, TF from 96,33 µg/L ± 29,24 and RDF from 15,3 µg/L ± 3,44. In deep waters, were recorded concentrations from 0 to14,2 µg/L of Fe+3, 72 to 545,2 µg/L of Fe+2, 141 to 147,4 µg/L of TF. In April of 2016, at a single point, were recorded on surface water, concentrations of 13,8 µg/L of Fe+3, 9,3 µg/L of Fe+2, 150 µg/L of total phosphorus TF, 16,2 µg/L of RDF and on pore waters were recorded concentrations of 1296,5 µg/L of Fe+2 , weren’t detected Fe+3 and 218 µg/L of reactive dissolved phosphorus. The concentrations of sediments varied from 10 to 36,62 mg/g, total iron from 0,16 to 0,07 mgP/g of total phosphorus in August 2015 and varied from 13,2 to 37,8 mg/g of total iron and 0,19 to 0,23 in February 2016.The results showed that the hydro chemical of Castanhão Reservoir favors the occurrence of the species Fe+2, which influences the mobilization of the pellet matrix of phosphorus, aggravating the reservoir eutrophication chart which showed hypereutrophic conditions. Statistical analysis showed that the most frequent Fe+2 , in February 2016, favored the occurrence of more bioavailable phosphorus form, so that the FRD concentrations were responsible for increased of primary productivity without being registered depletions in FRD concentrations as was checked, in August 2015.The processes responsible for the reduction of iron in the water column need a better evaluation. However, on one of the occasions evaluated, the speciation of Fe+3was found to be pH dependent, and not in levels of dissolved oxygen.