The main objective of this study was to perform laboratory experiments on calcium nitrate addition to sediments of a tropical eutrophic urban reservoir (Ibirité reservoir, SE Brazil) to immobilize ...the reactive soluble phosphorus (RSP) and to evaluate possible geochemical changes and toxic effects caused by this treatment. Reductions of 75 and 89% in the concentration of RSP were observed in the water column and interstitial water, respectively, after 145 days of nitrate addition. The nitrate application increased the rate of autotrophic denitrification, causing a consumption of 98% of the added nitrate and oxidation of 99% of the acid volatile sulfide. As a consequence, there were increases in the sulfate and iron (II) concentrations in the sediment interstitial water and water column, as well as changes in the copper speciation in the sediments. Toxicity tests initially indicated that the high concentrations of nitrate and nitrite in the sediment interstitial water (up to 2300 mg L−1 and 260 mg L−1, respectively) were the major cause of mortality of Ceriodaphnia silvestrii and Chironomus xanthus. However, at the end of the experiment, the sediment toxicity was completely removed and a reduction in the 48 h-EC50 of the water was also observed. Based on these results we can say that calcium nitrate treatment proved to be a valuable tool in remediation of eutrophic aquatic ecosystems leading to conditions that can support a great diversity of organisms after a restoration period.
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► Nitrate addition cause iron oxidation, which reduces the RSP flow from sediments to water column. ► AVS oxidation, alkalinity and nitrate consumption indicate autotrophic denitrification process. ► Nitrate addition as sediment restoration technology may cause toxicity to aquatic organisms.
This study aimed to analyze laboratory and field data to assess the ecotoxicological risks of calcium nitrate exposure to freshwater tropical biota. Short-term laboratorial tests resulted in ...estimated EC50 values of 76.72 (67.32–86.12)mg N-NO3-L−1 for C. silvestrii and 296.46 (277.16–315.76)mg N-NO3-L−1 for C. xanthus. Long-term laboratorial tests generated IC25 values of 5.05 (4.35-5.75) and 28.73 (26.30–31.15)mg N-NO3-L−1 for C. silvestrii and C. xanthus, respectively. The results from in situ mesocosm experiments performed in the Ibirité reservoir (a tropical eutrophic urban water body located in SE Brazil) indicated that C. silvestrii and C. xanthus were not under severe deleterious acute impact due to the treatment because the higher nitrate concentrations determined were 5.2mg N-NO3-L−1 (t=24h; sediment-water interface) and 17.5mg N-NO3-L−1 (t=600h; interstitial water). However, an abrupt decrease in the densities of Cyanophyceae members and other benthic taxa was observed. In summary, the present work contributes greatly to the toxicity data linked to two taxonomically distinct organisms that have never been screened for calcium nitrate sensitivity. Furthermore, considering the problem of the management and restoration of eutrophic environments, our study reports a comprehensive field assessment that allows the elucidation of the possible toxic impacts caused by the addition of calcium nitrate (a remediation technique) on aquatic and benthic organisms as well as the implications on the aquatic ecosystem as a whole, which may greatly allow expanding the current knowledgebase on the topic.
•Ceriodaphnia silvestrii: 48h-EC50=76.72 (67.32–86.12)mg N-NO3−L−1.•Ceriodaphnia silvestrii: 7d-IC25=5.05 (4.35–5.75)mg N-NO3−L−1.•Chironomus xanthus: 96h-EC50=296.46 (277.16–315.76)mg N-NO3−L−1.•Chironomus xanthus: 20d-IC25=28.73 (26.30–31.15)mg N-NO3−L−1.•No acute effect was observed to C. silvestrii and C. xanthus in treated mesocosms.
The performance of three configurations of microalgae-based systems used to treat a nutrient-rich anaerobic effluent was evaluated: a horizontal tubular photobioreactor covered with 40% sunshade mesh ...(FT); a vertical flat panel photobioreactor covered with 40% sunshade mesh (FPc); and a vertical flat panel photobioreactor directly exposed to solar radiation (FPd). The photobioreactors were inoculated with
Chlorella sorokiniana
and carbon dioxide (
CO
2
) supplementation was not provided. Physical, chemical and biological characteristics were monitored during a seven-week period. Pairwise correlation of operational parameters was performed to define the easiest way of monitoring treatment systems. Results indicated that the absence of
CO
2
injection was not a limiting factor for
Chlorella sorokiniana
growth in any of the tested configurations; however, biomass increase and nutrient removal were negatively influenced by N-nitrite and N-nitrate production. Moreover, direct exposure to solar radiation probably inhibited nitrifying activity in FPd (N-nitrate concentration in this photobioreactor was 80.5% lower than in FPc and 81.8% lower than in FT). Optical density (682 nm) presented a high correlation to microalgae cell counting (
R
2
= 0.9063) and total chemical oxygen demand (
R
2
= 0.9221), therefore it was indicated as the method of choice to estimate biomass growth.
