Characterization of Pb accumulation and factors influencing Pb mobility in rhizosphere soils of wetland plants

Objectives Characterization of Pb accumulation and factors influencing Pb mobility in rhizosphere soils of wetland plants could provide theoretical guidance on the remediation of heavy metal polluted water in constructed wetland.

Methods A rhizobox experiment was conducted to compare iron (Fe) oxidation and variation of pH, redox potential (Eh) and fractions of lead (Pb) in rhizosphere and non-rhizosphere soils of five emergent-rooted wetland plants, Echinodorus macrophyllus, Eleocharis geniculata, Hydrocotyle vulgaris, Jussiaea linifolia and Veronica serpyllifolia.

Results The results showed that all the wetland plants decreased pH and concentrations of Fe(II) and Fe(Ⅲ) but increased the Eh in the rhizosphere soils. The pH in rhizosphere soils was significantly lower (P<0.05) than that in non-rhizosphere soils, with differences in a range of 0.1–0.4 pH units. Concentrations of extractable Fe(II) and Fe(Ⅲ) in the rhizosphere soils were significantly lower than those in the bulk soils, with differences in a range of 0.6–2.7 mmol/kg. In rhizosphere and non-rhizosphere soils, Pb was mainly in RES (36.39%–47.54%) and OX (30.16%–41.64%), followed by OM (8.85%–15.08%) and WSA (6.89%–12.46%), and EX was again below the detection limit. Lead was transformed from unstable fractions (WSA) to more stable fractions (OX) in the rhizosphere soils, so decreased their potential metal mobility factors (MFs). Among the five plants, E. macrophyllus with higher iron oxidation ability on root surface and in rhizosphere possessed the greatest ability to reduce the MFs of Pb in the rhizosphere soils.

Conclusions Pb in the five plants is mainly distributed in roots. The amounts of Fe plaque on root surfaces were significantly higher than the amounts of Mn plaque on root surfaces. Wetland plants, with higher iron oxidation ability in rhizospheres and root surfaces, may thus be effective in decreasing potential long-term heavy metal bioavailability. Our results will provide strong theoretical basis for the restoration of heavy metal polluted water by constructing wetland.