Plant, aquatic plant

Azolla caroliniana Willd.

Carolina mosquito fern

A small floating fern living in symbiosis with a nitrogen-fixing cyanobacterium and growing fast on nitrogen-poor water. In the laboratory it removes mercury and chromium from wastewater; the related Azolla filiculoides has also been studied as dead sorbent biomass.

SB_Johnny, CC BY-SA 3.0

How it works

Mechanisms: rhizofiltrationbiosorption

Contaminants

Contaminant	Evidence	What was shown
Mercury	laboratory	Twelve days in solution with Hg(II), Cr(III) or Cr(VI) at 0.1, 0.5 and 1.0 mg/dm³: metals in solution fell by 74–100%; in tissues from 71 to 964 mg/kg dry matter; growth inhibited by 20–31%, most with mercury (Bennicelli et al. 2004). Bennicelli et al., The ability of Azolla caroliniana to remove heavy metals (Hg(II), Cr(III), Cr(VI)) from municipal waste water, Chemosphere 55:141, 2004
Hexavalent chromium	laboratory	In the same test removal reached 100% for chromate at 0.1 mg/dm³; the highest tissue value came with Cr(III) (Bennicelli et al. 2004). Bennicelli et al., Chemosphere 55:141, 2004
Arsenic	laboratory	It accumulates arsenic, but 20 µM As cuts biomass by 30% (Rofkar et al. 2014). In A. filiculoides, at pH 5 and 30 µM As, 15 µg/g fresh weight with moderate growth; above 40 µM growth turns negative (Biswas and Ganesan 2024). Rofkar et al., Uptake and toxicity of arsenic, copper, and silicon in Azolla caroliniana and Lemna minor, Int. J. Phytoremediation 16:155, 2014 Biswas and Ganesan, Evaluation of arsenic phytoremediation potential in Azolla filiculoides Lam. plants under low pH stress conditions, Plant Physiol. Biochem. 214:108956, 2024

Contaminant

Evidence

What was shown

Mercury

laboratory

Twelve days in solution with Hg(II), Cr(III) or Cr(VI) at 0.1, 0.5 and 1.0 mg/dm³: metals in solution fell by 74–100%; in tissues from 71 to 964 mg/kg dry matter; growth inhibited by 20–31%, most with mercury (Bennicelli et al. 2004).

Bennicelli et al., The ability of Azolla caroliniana to remove heavy metals (Hg(II), Cr(III), Cr(VI)) from municipal waste water, Chemosphere 55:141, 2004

Hexavalent chromium

laboratory

In the same test removal reached 100% for chromate at 0.1 mg/dm³; the highest tissue value came with Cr(III) (Bennicelli et al. 2004).

Bennicelli et al., Chemosphere 55:141, 2004

Arsenic

laboratory

It accumulates arsenic, but 20 µM As cuts biomass by 30% (Rofkar et al. 2014). In A. filiculoides, at pH 5 and 30 µM As, 15 µg/g fresh weight with moderate growth; above 40 µM growth turns negative (Biswas and Ganesan 2024).

Limits

Laboratory data only, at concentrations of 1 mg/L or less; even at those doses growth falls by a fifth or more. The sources consulted include no pilot plants. Biomass must be harvested and disposed of, or it releases the metals. American azollas are alien in Europe and can cover canals and ponds.