Remediação de sedimentos eutróficos da Represa Ibirité (MG) pela adição de nitrato e Phoslock : experimentos em laboratório

Abstract

Ibirité Reservoir (Ibirité/Sarzedo, MG) receives untreated effluents and raw sewage from the city of Ibirité and surroundings, which are the main cause of the eutrophication of this ecosystem. Past studies have shown that sediment is a significant internal source of nutrients (ammonium and orthophosphate ions) to the water column. Within this context, the main objective of this study was to perform laboratory experiments (microcosms) using calcium nitrate and Phoslock as sediment remediation technologies to immobilize orthophosphate ions. Total incubation time was 145 days for the experiments with nitrate and 41 days using Phoslock . The results of the experiments involving the addition of calcium nitrate showed a reduction in the concentration of orthophosphate in the water column (75%) and interstitial water (89%). After 145 days the sediment was significantly oxidized and there was almost complete abatement of acid volatile sulfide in the sediments, leading to a significant increase in sulfate concentrations. The system s nitrogen mass balance revealed the occurrence of an intense process of denitrification. The ecotoxicological assays initially showed that the high concentrations of nitrate (up to 2,300 mg L-1 and 270 mg L-1 in the sediment s interstitial waters, repectively) were responsible for the mortality of test organisms. However, at the end of the experiment, sediment toxicity disappeared completely and there was a significant reduction in the water s EC50 values at 48h. The results of the assays involving the addition of Phoslock showed an 82% reduction in the concentration of orthophosphate in the water column immediately after application of the product and a 64% reduction in interstitial water after 41 days of treatment. The application of the Phoslock suspension increased water column turbidity (150 NTU), which, however, declined by 75% just 6 hours later. The concentration of dissolved oxygen in the water column increased significantly 360 hours into the experiment. The application of Phoslock caused a significant increase in the concentration of dissolved lanthanum in the water column. At the time of application (t = 0), the concentration of La in the water column was 1.64 mg L-1; however, at the end of the assay, the concentration of this element had dropped to less than 0.05 mg L-1. The ecotoxicological tests showed that at t = 0 the application of Phoslock caused mortality of the test organisms in the water column samples, which may have been due to an increase in turbidity and La concentration during this period. Consequently, at the end of these assays, the toxicity was completely eliminated. Considering the general characteristics of the assays performed here, it is postulated that both the technologies studied showed good application potential, i.e., the results obtained indicated that both technologies were efficient in removing P from the water. However, the chances of success of these treatments in the long run will depend (among other factors) on the abatement of the external sources and on the long-term maintenance of such abatement. Since both the technologies investigated appeared to perform similarly in this respect, the use of Phoslock to remediate the eutrophic sediments of the Ibirité reservoir may be more favorable than that of nitrate, if one considers the cost estimates presented in this work from the standpoint of the approximations or forecasts employed here.