Heavy metals in sediments from constructed wetlands treating municipal wastewater

Abstract

Constructed wetlands are commonly used for treatment of municipal sewage. The treatment is usually aimed at removal of organics, suspended solids, nutrients and microbial pollution. The information on removal and fate of heavy metals is very limited. The purpose of this study was to evaluate the amount of sediments and heavy metal concentration in the sediments in filtration beds of seven constructed wetlands with horizontal subsurface flow treating municipal sewage with various length of operation. The results revealed that concentrations of Cd, Ni, Pb, Cu, Cr and Zn in the sediment are mostly comparable with concentrations occurred in natural unpolluted or slightly polluted wetlands. The concentrations are much lower than those found in wetlands impacted with mine drainage waters or wastewater from industrial operations. Concentrations of studied heavy metals exceeded only occasionally limits set by the Czech legislation. However, when heavy metal concentrations are evaluated within the filtration material the concentrations are well below the limits set for soils in the Czech Republic. The results also revealed that concentrations of heavy metals in the sediment do not reflect the time of operation probably due to buildup of sediments from suspended solids contained in wastewaters. However, the sediment mass increases during the course of operation and consequently the metal mass increases as well.

Metal content and distribution in surface sediments in an industrial region

“…This stream receives all discharges from one of the main urban centers and around its site there are electroplating, metallurgical and chemical industries. Cu was extracted mainly in reducible (1-24%) and oxidized (6-29%) fractions of the sequential extraction procedure, according to results from other studies that evaluated urban and industrial influences (Davidson et al 1994, Kubová et al 2008, Davidson et al 2006, Rodrigues and Formoso 2006, Hang et al 2009, Li et al 2009, Vymazal et al 2010, Davutluoglu et al 2011. In addition, a high percentage in the exchangeable phase (21%) was also observed in the Luiz Rau stream, in accordance to studies by Irabien and Velasco (1999), Hang et al (2009), and Liu et al (2009.…”

“…The Pb association to these phases has been reported by several authors e.g. Davidson et al (1994Davidson et al ( , 2006, Kubová et al (2008), Boughriet et al (2007), Li et al (2009), Vymazal et al (2010, Davutluoglu et al (2011), andDundar et al (2012). Pb oxidizable fraction presence may be due to the complex-metal formation with humic substances and/or metal sulfides insoluble in the sediments (Davutluoglu et al 2011).…”

Assessment of heavy metals in sediments of the Sinos river basin, southern Brazil

“…From Figure 1 on, we see that there is an increase in the potential mobility as we reach the mouth of the Sinos River, especially at the Luiz Rau stream, where it reaches 60 %. Cu was extracted mainly in the reducible and oxidizable fractions of the sequential extraction procedure, as reported by Davidson et al (1994) and Vymazal et al (2010).…”

“…Zn, frequently associated with Fe and Mn oxides (hydroxides and oxyhydroxides), was extracted mainly in the reducible phase (Vymazal et al, 2010). Moreover, this element also occurred in the exchangeable phase.…”

“…As the metals highlighted above, there is an increase in the mobile fractions (Figure 1) from point SI 066 on. Cr was extracted mainly in the oxidizable fraction, indicating its binding to organic matter (Rodrigues & Formoso, 2006;Vymazal et al, 2010). As for Portão stream, 1039 mg kg -1 , equivalent to 81 % of the total concentration of this metal, were extracted in the third step of the sequential extraction.…”