The impact of humic acid on chromium phytoextraction by aquatic macrophyte Lemna minor

• Cr(VI) decreases growth rate and content of photosynthetic pigments in Lemna minor.
• Cr(VI) is reduced into Cr(III) in presence of humic acid.
• Cr(III) is less toxic and less bioavailable.
• Presence of humic acid affects the efficiency of Cr phytoextraction.

Studies assessing chromium phytoextration from natural waters rarely consider potential implications of chromium speciation in the presence of ubiquitous humic substances. Therefore, the present study investigated the influence of environmentally relevant concentration of humic acid (TOC = 10 mg L−1) on chromium speciation (Cr = 0.15 mg L−1) and consequently on phytoextraction by aquatic macrophyte duckweed Lemna minor. In absence of humic acid, only hexavalent chromium was present in water samples and easily taken up by L. minor. Chromium uptake resulted in a significant reduction of growth rate by 22% and decrease of chlorophyll a and chlorophyll b contents by 48% and 43%, respectively. On the other hand, presence of humic acid significantly reduced chromium bioavailability (57% Cr uptake decrease) and consequently it did not cause any measurable effect to duckweed. Such effect was related to abiotic reduction of hexavalent chromium species to trivalent. Hence, findings of our study suggest that presence of humic acid and chromium speciation cannot be neglected during phytoextraction studies.

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