Phosphorus sequestration in Fe-rich sediments from two Brazilian tropical dam reservoirs

Abstract

This study investigates the important role of Fe-oxide/hydroxide-rich bottom sediments, from two Brazilian tropical reservoirs, on P sequestration from the overlying waters, which decreases the risk of eutrophication. Data on P fractionation indicates that P adsorption capacity of sediments is predominantly associated with Fe, followed by orthosphosphate adsorbed onto Al hydr(oxides) and onto different forms of apatite (Ca–P). The soluble and loosely bound P-forms are low. To have a better understanding of the role of Fe oxides on the adsorption and sequestration of P in the reservoirs, data obtained by sequential extraction of P have been related to concentrations of total Fe and of various Fe-fractions extracted through an optimized fractionation scheme. These analyses indicated high levels of Fe bound to oxides/ hydroxides. These mineral oxides, and the preponderance of pH-dependent charged clay minerals (kaolinite) in the clay fraction, induce the sequestration of the P entering the lakes, mainly in particulate form, by strong retention of PO3 4 through surface adsorption and precipitation phenomena. Phosphorusretention provided by the replacement of OH groups on the basal plane of the minerals, for P anions, decreases the negative impact of P inputs in these water systems. There is significant correlation between the concentrations of soluble reactive P in the water (SRP) and major chemical components of sediments (SiO2, Fe2O3, Al2O3, K2O, P2O5), which are related to the dominant bedrock geology. The interaction of SRP with bed-sediments and the chemical conditions of the environment (decrease of redox potential with depth and, in the dry period, a low O2 abundance), can explain fluctuations of SRP and TP (total P) in the water column. Due to the dynamic equilibrium in the sediment–water interface, in the dry season, reducing conditions in hypoliminia enhance the reduction of Fe(III) to the more soluble Fe(II) with subsequent release of the strongly retained P. This mechanism increases the soluble P fraction in sediments and the soluble reactive P in the water column, in this period. The straight linkage between geochemical properties of sediments and P concentration in the water column reported in this article suggests that study of the mineralogical and geochemical composition of bottom sediments could help to improve model predictions of P concentrations in surface waters.