Uptake and Speciation of Arsenic in Hydroponically Exposed Horsetail Plants (Equisetum Sp.)

Abstract

Our study examined the arsenic uptake abilities of three horsetail species ( Equisetum fluviatile , Equisetum hyemale and Equisetum scirpoides ) and the speciation of arsenic in their tissues when exposed to 1 and 10 mg/L of arsenate (As(V)) under hydroponic, high bioavailability conditions. We assessed which Equisetum sp. is best suited for planting in constructed wetlands treating arsenic contaminated water. We have shown that arsenic concentrations increased with increasing arsenic dose and that roots accumulated more arsenic than shoots At 1 mg/L As(V), E. fluviatile , E. hyemale and E. scirpoides plants accumulated up to 8.8, 16 and 14 mg/kg dw of arsenic corresponding to plant BCFs of 8.8, 16, and 14 respectively. At 10 mg/L As(V), E. fluviatile , E. hyemale and E. scirpoides plants accumulated up to 60, 170 and 120 mg/kg dw of arsenic corresponding to plant BCFs of 6, 17 and 12 respectively. Arsenic translocation to shoot tissues decreased with increasing As(V) dose, except in E. fluviatile . From X-ray absorption near-edge structure (XANES) analysis of E. fluviatile and E. hyemale plant tissues, E. fluviatile root tissues exposed to the highest arsenic dose produced the highest proportion of reduced arsenic species, namely As(III) and As(III)-S, matched to As(III) triglutathione [As(SG) 3 ]) but the same was not true for E. hyemale where most of the arsenic in the roots remained as As(V). E. fluviatile may be dealing with arsenic toxicity by forming As(SG) 3 and storing it primarily in root vacuoles. Shoot tissues at the highest arsenic dose also had a higher proportion of As(III) in both Equisetum sp. We conclude that the Equisetum sp. tested are arsenic tolerant plants, similar to other arsenic tolerant fern species, but are not arsenic hyperaccumulators. We also believe that E. hyemale is the best candidate for use in constructed wetlands treating arsenic contaminated water.

Keywords: horsetails, hydroponic uptake, bioavailability, arsenic speciation, Constructed wetlands