Metal Distribution and Stability in Constructed Wetland Sediment

Abstract

The A-01 wetland treatment system (WTS) is a surface flow wetland planted with giant bulrush [Schoenoplectus californicus (C.A. Mey.) Palla] that is designed to remove Cu and other metals from the A-01 National Pollution Discharge Elimination System (NPDES) effluent at the Savannah River Site near Aiken, SC. Copper, Zn, and Pb concentrations in water were usually reduced 60 to 80% by passage through the treatment system. The Cu concentrations in the wetland sediments increased from about 4 to 205 and 796 mg kg(-1), respectively, in the organic and floc sediment layers in cell 4A over a 5-yr period. Metal concentrations were higher in the two top layers of sediment (i.e., the floc and organic layers) than in the deeper inorganic layers. Sequential extraction was used to evaluate remobilization and retention of Cu, Pb, Zn, Mn, and Fe in the wetland sediment. Metal remobilization was determined by the potentially mobile fraction (PMF) and metal retention by the recalcitrant factor (RF). The PMF values were high in the floc layer but comparatively low in the organic and inorganic layers. High RF values for Cu, Zn, and Pb in the organic and inorganic layers indicated that these metals were strongly bound in the sediment. The RF values for Mn were lower than for the other elements especially in the floc layer, indicating low retention or binding capacity. Retention of contaminants was also evaluated by distribution coefficient (Kd) values. Distribution coefficient (Kd) values were lower for Cu and Zn than for Pb, indicating a smaller exchangeable fraction for Pb.

Bioavailability of Metals in Contaminated Sediments

“…Sediments can be sequentially extracted using chemicals of decreasing pH and increasing oxidizing strength to remove these geochemical phases and potentially relate them to the bioavailable metal fraction in the total contaminant pool. For example, the recalcitrant fraction, which consists of metals contained within the crystal structure of minerals, is typically unavailable to biota (Knox et al, 2006).…”

“…The site-specific desorption K d of metals in sediments identifies the metal fraction that is readily available for entry into interstitial water or surface water; i.e., the potentially bioavailable fraction (PBF) (Knox et al, 2006). It can be operationally defined by selective extractions, with the intent of providing a conservative (high) estimate of the amount of a contaminant that may be released from the sediment.…”

Phytoextraction of potentially toxic elements by Indian mustard, rapeseed, and sunflower from a contaminated riparian soil

“…Many riparian and wetland soils worldwide and in Germany are highly contaminated with PTEs (Knox et al 2006;Shaheen and Rinklebe 2014;Rinklebe and Shaheen 2014;Frohne et al 2014). Due to the high concentrations of PTEs in riparian soils, these elements might be released and mobilized (Shaheen et al 2014a, b, c), leading to soil and groundwater contamination what increase the possibility of entering into the grassland and food chain (Overesch et al 2007;Licina et al 2010).…”

Comparative analysis of the composition of bacterial communities from two constructed wetlands for municipal and swine wastewater treatment

“…They are a costeffective alternative to conventional wastewater treatment (Knox et al 2006). They are designed to take advantage of many of the same processes that occur in natural wetlands, but do so within a more controlled environment (Vymazal 2005).…”