Els Lesage

Contaminant Removal Processes in Subsurface-Flow Constructed Wetlands: A Review

The main contaminant removal processes occurring in subsurface-flow constructed wetlands treating wastewater are reviewed. Redox conditions prevailing in the wetlands are analyzed and linked to contaminant removal mechanisms. The removal of organic matter and its accumulation in the granular medium of the wetlands are evaluated with regard to particulate and dissolved components and clogging processes. The main biological processes linked to organic matter transformation—aerobic respiration, denitrification, acid fermentation, sulfate reduction, and methanogenesis—are reviewed separately. The processes of removal of surfactants, pesticides and herbicides, emergent contaminants, nutrients, heavy metals and faecal organisms are analyzed. Advances in wetland modeling are presented as a powerful tool for understanding multiple interactions occurring in subsurface-flow constructed wetlands during the removal of contaminants.

Enhanced phytoextraction: in search of EDTA alternatives.

ABSTRACT Enhanced phytoextraction proposes the use of soil amendments to increase the heavy-metal content of above-ground harvestable plant tissues. This study compares the effect of synthetic aminopolycarboxylic acids [ethylenediamine tetraacetatic acid (EDTA), nitriloacetic acid (NTA), and diethylenetriamine pentaacetic acid (DTPA)] with a number of biodegradable, low-molecular weight, organic acids (citric acid, ascorbic acid, oxalic acid, salicylic acid, and NH4 acetate) as potential soil amendments for enhancing phytoextraction of heavy metals (Cu, Zn, Cd, Pb, and Ni) by Zea mays. The treatments in this study were applied at a dose of 2 mmol/kg−1 1 d before sowing. To compare possible effects between presow and postgermination treatments, a second smaller experiment was conducted in which EDTA, citric acid, and NH4 acetate were added 10 d after germination as opposed to 1 d before sowing. The soil used in this screening was a moderately contaminated topsoil derived from a dredged sediment disposal site. This site has been in an oxidized state for more than 8 years before being used in this research. The high carbonate, high organic matter, and high clay content characteristic to this type of sediment are thought to suppress heavy-metal phytoavailability. Both EDTA and DTPA resulted in increased levels of heavy metals in the above-ground biomass. However, the observed increases in uptake were not as large as reported in the literature. Neither the NTA nor organic acid treatments had any significant effect on uptake when applied prior to sowing. This was attributed to the rapid mineralization of these substances and the relatively low doses applied. The generally low extraction observed in this experiment restricts the use of phytoextraction as an effective remediation alternative under the current conditions, with regard to amendments used, applied dose (2 mmol/kg−1 soil), application time (presow), plant species (Zea mays), and sediment (calcareous clayey soil) under study.

Factors affecting metal concentrations in the upper sediment layer of intertidal reedbeds along the river Scheldt

Factors that play a role in determining metal accumulation in sediments of 26 intertidal marshes which are mainly vegetated by reed plants (Phragmites australis) were assessed along the Scheldt estuary (Belgium and The Netherlands). In the upper 20 cm sediment layer, several physico-chemical properties (clay, silt and sand content, organic matter, carbonate and chloride content, pH and conductivity) and aqua regia extractable metals (Cd, Cr, Cu, Ni, Pb, Zn) were determined. The sediments were significantly contaminated with trace metals. The Cd concentrations often exceeded the Flemish soil remediation thresholds for nature areas, whereas Cr, Cu and Zn levels indicated moderate contamination. Pb concentrations occasionally were high, whereas Ni concentrations leaned towards background values. Organic matter was the single most important predictor variable for total metal contents in regression models, except for Cr. Additional significant predictor variables were clay or chloride content, depending on the metal. Observed metal concentrations at sites within a range of a few km from specific point-sources of metals (e.g. shipyards, industrial areas with metallurgic activities, affluents, major motorways) were somewhat higher than predicted by the models, whereas they were lower than predicted at sites which are regularly subjected to flooding by water of high salinity. The ratio between observed and predicted concentrations seems to be a valuable tool for the identification of areas which are specifically impacted by point sources.

Enhanced Phytoextraction: II. Effect of EDTA and Citric Acid on Heavy Metal Uptake by Helianthus annuus from a Calcareous Soil

High biomass producing plant species, such as Helianthus annuus, have potential for removing large amounts of trace metals by harvesting the aboveground biomass if sufficient metal concentrations in their biomass can be achieved. However, the low bioavailability of heavy metals in soils and the limited translocation of heavy metals to the shoots by most high biomass producing plant species limit the efficiency of the phytoextraction process. Amendment of a contaminated soil with ethylene diamine tetraacetic acid (EDTA) or citric acid increases soluble heavy metal concentrations, potentially rendering them more available for plant uptake. This article discusses the effects of EDTA and citric acid on the uptake of heavy metals and translocation to aboveground harvestable plant parts in Helianthus annuus. EDTA was included in the research for comparison purposes in our quest for less persistent alternatives, suitable for enhanced phytoextraction. Plants were grown in a calcareous soil moderately contaminated with Cu, Pb, Zn, and Cd and treated with increasing concentrations of EDTA (0.1, 1, 3, 5, 7, and 10 mmol kg−1 soil) or citric acid (0.01, 0.05, 0.25, 0.442, and 0.5 mol kg−1 soil). Heavy metal concentrations in harvested shoots increased with EDTA concentration but the actual amount of phytoextracted heavy metals decreased at high EDTA concentrations, due to severe growth depression. Helianthus annuus suffered heavy metal stress due to the significantly increased bioavailable metal fraction in the soil. The rapid mineralization of citric acid and the high buffering capacity of the soil made citric acid inefficient in increasing the phytoextracted amounts of heavy metals. Treatments that did not exceed the buffering capacity of the soil (<0.442 mol kg−1 soil) did not result in any significant increase in shoot heavy metal concentrations. Treatments with high concentrations resulted in a dissolution of the carbonates and compaction of the soil. These physicochemical changes caused growth depression of Helianthus annuus. EDTA and citric acid added before sowing of Helianthus annuus did not appear to be efficient amendments when phytoextraction of heavy metals from calcareous soils is considered.

Metal Accumulation in Intertidal Marshes: Role of Sulphide Precipitation

We assessed short-term temporal and spatial variation of metal contents in the upper 100 cm sediment profile of intertidal marshes vegetated by common reed (Phragmites australis) along the Scheldt estuary in Belgium. The upper 0–100 cm sediment profile was sampled in three reedbeds at 56, 94, and 131 km from the river mouth. Sampling was repeated five times, at approximately two month intervals. Sediment properties such as texture and chloride, carbonate and organic matter content differed among locations. Metal accumulation, which is primarily due to association of metals with organic matter and clay in the surface sediment layer, seemed to be supplemented by an accumulation of sulphide precipitates deeper in the sediments. The depth at which sulphide precipitation significantly contributed to metal accumulation depended on the sampling location, and varied from less than 5 cm in clayey, organic sediments to more than 1 m in sandy sediments. Temporal variation of Cu, Cd, Pb, and Zn concentrations could only be linked to newly formed sulphides or sulphide oxidation at the sites with the lowest sulphide content. At sampling sites containing high sulphide amounts, variations should be primarily attributed to metal exchange and the presence of mobile metal complexes. Litter decomposition at the end of the growing season could hereby play a significant role.

Enhanced phytoextraction: I. Effect of EDTA and citric acid on heavy metal mobility in a calcareous soil

Phytoextraction, the use of plants to extract heavy metals from contaminated soils, could be an interesting alternative to conventional remediation technologies. However, calcareous soils with relatively high total metal contents are difficult to phytoremediate due to low soluble metal concentrations. Soil amendments such as ethylene diaminetetraacetate (EDTA) have been suggested to increase heavy metal bioavailability and uptake in aboveground plant parts. Strong persistence of EDTA and risks of leaching of potentially toxic metals and essential nutrients have led to research on easily biodegradable soil amendments such as citric acid. In our research, EDTA is regarded as a scientific benchmark with which degradable alternatives are compared for enhanced phytoextraction purposes. The effects of increasing doses of EDTA (0.1, 1, 10 mmol kg−1 dry soil) and citric acid (0.01, 0.05, 0.25, 0.442, 0.5 mol kg−1 dry soil) on bioavailable fractions of Cu, Zn, Cd, and Pb were assessed in one part of our study and results are presented in this article. The evolution of labile soil fractions of heavy metals over time was evaluated using water paste saturation extraction (∼ soluble fraction), extraction with 1 M NH4OAc at pH 7 (∼ exchangeable fraction), and extraction with 0.5 M NH4OAc + 0.5 M HOAc + 0.02 M EDTA at pH 4.65 (∼ potentially bioavailable fraction). Both citric acid and EDTA produced a rapid initial increase in labile heavy metal fractions. Metal mobilization remained constant in time for soils treated with EDTA, but a strong exponential decrease of labile metal fractions was noted for soils treated with citric acid. The half life of heavy metal mobilization by citric acid varied between 1.5 and 5.7 d. In the following article, the effect of heavy metal mobilization on uptake by Helianthus annuus will be presented.

Factors Affecting Metal Accumulation, Mobility and Availability in Intertidal Wetlands of the Scheldt Estuary (Belgium)

We studied factors affecting the accumulation, mobility and availability of metals in intertidal wetlands of the Scheldt estuary (Belgium), both in greenhouse experiments and under field conditions. The surface layer of the intertidal Scheldt sediments was found to be significantly contaminated with trace metals. The metal concentrations can be predicted from clay or organic matter concentrations. They were somewhat higher than predicted at sites within a range of a few kilometres from specific point-sources of metals, whereas they were lower than predicted at sites which are regularly subjected to flooding by high salinity water. In the deeper sediment layers, sulphides seem to play an important role in the metal accumulation. Salts significantly increased the metal mobility in the oxidised sediment layers in the brackish part of the estuary, which was especially observed for cadmium (U). The decomposition of stems and leaves of reed plants and willows in the upper sediment layer was found to both increase and decrease the metal mobility. The concentrations in the reed litter itself increased significantly during decomposition under field conditions. The hydrological regime to which metal-polluted sediments were subjected, affected the metal concentrations in the pore water to an important extent. Flooding increased the mobility of iron (Fe), manganese (Mn), nickel (Ni) and chromium (Cr) and decreased the mobility of Cd, copper (Cu) and zinc (Zn) in a calcareous substrate. Fluctuating hydrological conditions resulted in fluctuating metal concentrations in the pore water. Reduction and oxidation of Fe and Mn, decomposition of carbonates and the formation and re-oxidation of sulphides were hereby the main controlling processes.

Removal of Heavy Metals from Industrial Effluents by the Submerged Aquatic Plant Myriophyllum spicatum L.

The potential use of Myriophyllum spicatum L. for the removal of Co, Ni, Cu and Zn from industrial effluents was studied. The removal kinetics, tolerance and accumulation capacity of the submerged aquatic plant were assessed. Removal of Cu and Zn was similar and occurred rapidly with time whereas removal of Co and Ni was slower. Plant growth was not adversely affected during the 12 weeks of exposure to the wastewater. Cobalt, Ni, Cu and Zn concentrations of respectively 1,675, 1,529, 766 and 2,883 mg kg(-1) DM were observed in the biomass. M. spicatum is suggested as an efficient plant species for the treatment of metal-contaminated industrial wastewater.