The Effects of Ferric Sulfate (Fe2(SO4)3) on the Removal of Cyanobacteria and Cyanotoxins: A Mesocosm Experiment

Abstract

Cyanobacterial blooms are a global concern. Chemical coagulants are used in water treatment to remove contaminants from the water column and could potentially be used in lakes and reservoirs. The aims of this study was to: 1) assess the efficiency of ferric sulfate (Fe₂(SO₄)₃) coagulant in removing harmful cyanobacterial cells from lake water with cyanobacterial blooms on a short time scale, 2) determine whether some species of cyanobacteria can be selectively removed, and 3) determine the differential impact of coagulants on intra- and extra-cellular toxins. Our main results are: (i) more than 96% and 51% of total cyanobacterial cells were removed in mesocosms with applied doses of 35 mgFe/L and 20 mgFe/L, respectively. Significant differences in removing total cyanobacterial cells and several dominant cyanobacteria species were observed between the two applied doses; (ii) twelve microcystins, anatotoxin-a (ANA-a), cylindrospermopsin (CYN), anabaenopeptin A (APA) and anabaenopeptin B (APB) were identified. Ferric sulfate effectively removed the total intracellular microcystins (greater than 97% for both applied doses). Significant removal of extracellular toxins was not observed after coagulation with both doses. Indeed, the occasional increase in extracellular toxin concentration may be related to cells lysis during the coagulation process. No significant differential impact of dosages on intra- and extra-cellular toxin removal was observed which could be relevant to source water applications where optimal dosing is difficult to achieve.

Keywords: coagulation; cyanobacteria; cyanobacterial blooms; cyanotoxins; ferric sulfate; mesocosms; microcystins; water treatment.

Figure 1

Total taxonomic cell counts in control, 20 mgFe/L, and 35 mgFe/L mesocosms (Mean ± standard deviation): A) 10–12 September 2018; B) 24–26 September 2018; C) 13–15 August 2019; a) 26–28 June 2019; b) 24–26 July 2019; c) 05–07 August 2019.

Figure 2

Violin plot shows the distribution of taxonomic cell counts at level genus in control, 20 mgFe/L, and 35 mgFe/L mesocosms. (Others include Chroococcus, Coelosphaerium, Merismopedia). The bottom and top of the box shows the lower and upper quartiles, the band in between them shows the median, whiskers show the minimum and maximum (excluding outliers) and circles show the outliers. Outliers are values more than 1.5 times the length of the interquartile range greater than the upper quartile or smaller than the lower quartile.

Figure 3

Concentration of total intracellular microcystins in control, 20 mgFe/L, and 35 mgFe/L mesocosms (Mean ± standard deviation): A) 10–12 September 2018; B) 24–26 September 2018; C) 13–15 August 2019; a) 26–28 June 2019; b) 24–26 July 2019; c) 05–07 August 2019.

Figure 4

Violin lot shows the distribution of intracellular toxin concentration of individual cyanotoxins in control, 20 mgFe/L, and 35 mgFe/L mesocosms in Missisquoi Bay (MB) and Petit Lac St. Francois (PLSF). The bottom and top of the box show the lower and upper quartiles, the band in between them shows the median, whiskers show the minimum and maximum (excluding outliers) and circles show the outliers. Outliers are values more than 1.5 times the length of the interquartile range greater than the upper quartile or smaller than the lower quartile.

Figure 5

Concentration of total extracellular microcystins in control, 20 mgFe/L, and 35 mgFe/L mesocosms (Mean ± Standard deviation). A) 10–12 September 2018; B) 24–26 September 2018; C) 13–15 August 2019; a) 26–28 June 2019; b) 24–26 July 2019; c) 05–07 August 2019.

Figure 6

Violin plot shows the distribution of extracellular toxin concentration of individual cyanotoxins in control, 20 mgFe/L, and 35 mgFe/L in Missisquoi Bay (MB) and Petit Lac St. François (PLSF). The bottom and top of the box show the lower and upper quartiles, the band in between them shows the median, whiskers show the minimum and maximum (excluding outliers) and circles show the outliers. Outliers are values more than 1.5 times the length of the interquartile range greater than the upper quartile or smaller than the lower quartile.