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Effects of Plants on the Removal of Hexavalent Chromium in Wetland Sediments

Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544

View larger version (20K): [in a new window]   Fig. 1. Pore water Cr(VI) concentrations: (A) after 6 mo of operation; and (B) after 11 mo of operation. The Phragmites australis (without acetate) microcosm was only sampled once after 6 mo of operation. Lines represent nonlinear regression curves of the concentrations against depth. Chromium(VI) removal rates were calculated based on the regression curves to minimize the effects of experimental variations.

View larger version (27K): [in a new window]   Fig. 2. Conceptual graph showing the mechanism by which plant transpiration concentrates conservative dissolved species. For a thin layer of sediment with transpiration rate of Qtranspiration and inflow rate Qinflow with concentration of Cinflow, the outflow rate Qoutflow = Qinflow – Qtranspiration, and its concentration in the outflow Coutflow = (QinflowCinflow)/(Qoutflow) > Cinflow. With the known drainage rate and the vertical concentration profile of a conservative species, the transpiration and inflow rates for a specific layer can be estimated: QN – 1 = CNQN/CN – 1; QN – 1t = QN – 1 – QN; Qi – 1 = CiQi/Ci – 1; Qi – 1t = Qi – 1 – Qi. Here N represents total number of layers, while i is the sequential number of one specific layer; i = 0 for the sediment right beneath the water–sediment interface; QN – 1 is the inflow rate of layer N – 1, QN – 1 is the outflow rate of layer N – 1, and QN – 1t represents the transpiration rate in layer N – 1.

View larger version (18K): [in a new window]   Fig. 3. Pore water SO42– profiles: (A) after 6 mo of operation; and (B) after 11 mo of operation. The Phragmites australis (without acetate) microcosm was only sampled once after 6 mo of operation. Lines represent nonlinear regression curves of the concentrations against depth.

View larger version (18K): [in a new window]   Fig. 4. Relationship between aqueous Cr(VI) concentrations and Cr(VI) removal rates: (A) after 6 mo of operation; and (B) after 11 mo of operation.

View larger version (19K): [in a new window]   Fig. 5. Concentrations of Cr(III) precipitated in the sediment.

View larger version (15K): [in a new window]   Fig. 6. Relationship between Cr(III) precipitation and Cr(VI) removal rate.

View larger version (17K): [in a new window]   Fig. 7. Loss of Cr(VI) in the incubation experiment with 100 mL of degassed Hoagland solution with 5 mM Na acetate and 2.0 mg L–1 Cr(VI): A = solution only; B = autoclaved solution only; C = solution with 4 g of sediment samples (10–12 cm) from the Typha latifolia microcosm; D = same as C but autoclaved; E = solution with 4 g of sediment samples (10–12 cm) from the Phragmites australis microcosm that received 2 mM acetate from the nutrient solution; and F = same as E but autoclaved. Error bars are smaller than symbols.