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Removal of Metals in Constructed Wetlands, Review

Removal of Metals in Constructed Wetlands:Review

T.Y.Yeh1

Abstract:Constructed wetlands are growing in popularity as a natural and economical alternative for purifying contaminated water worldwide.This paper summarizes the results of implementation of constructed wetlands in the treatment of metal containing water and evaluates their performance with respect to various metal removal mechanisms.Metal contaminated waters including acid mine drainage, storm runoff,sewage,and industrial wastewater are also evaluated regarding their removal ef?ciency within various types of constructed wetland systems.The role of vegetation within wetland systems is also examined to determine the possibility of enhancing the metal removal ef?ciency for future applications.

DOI:10.1061/?ASCE?1090-025X?2008?12:2?96?

CE Database subject headings:Wetlands;Heavy metals;Mining;Drainage;Storms;Runoff;Acids.

Introduction

Constructed wetlands possess the merits of low cost and low maintenance and are capable of removing various pollutants in-cluding heavy metals,nutrients,organic matters,etc.from various contaminated waters?Kadlec1995;Kadlec and Knight1996; Greenway2005;Vymazal2007?.Constructed wetlands are engi-neered systems that have been designed to employ natural pro-cesses including vegetation,soil,and microbial activity to enhance water quality.In general,constructed wetlands can be classi?ed based on the type of macrophyte growth including emergent,submerged,free?oating,and rooted with?oating leaves.Further classi?cation is based on the water?ow scheme, namely surface?ow,subsurface vertical,or horizontal?ow systems.

Constructed wetlands for water quality enhancement involve the use of engineered systems that are designed and constructed to utilize natural processes.Constructed wetlands have gained much attention due to their pollution removal,recreational assets, and landscape aesthetics values in tropical regions.The warm tropical climates are conducive for higher biological activity and productivity,hence there is better pollutant removal performance of wetland systems.The processes of biogeochemical cycling within constructed wetlands provide storage and conversions for numerous pollutants including organic substances,nutrients,and heavy metals.

The potential of employing constructed wetlands to treat in-dustrial wastewater,particularly metal-containing wastewater,has received increased attention?Lim et al.2003;Sheoran and Sheo-ran2006?.Wetland soils characterized by their reduced condition and high organic matter content can accumulate heavy metals. Recently,constructed wetlands have been used for different kinds of metal containing waters including land?ll leachate,industrial wastewater,sewage,storm runoff,and mine drainage?Kivaisi 2001;Song et al.2006?.This paper summarizes the results of implementation of constructed wetlands in the treatment of metal containing waters and evaluates their performance with respect to various metal removal mechanisms.

Metal Removal Mechanisms within Wetlands

Metal removal within constructed wetlands has been attributed to various mechanisms including sedimentation,?ltration,chemical precipitation and adsorption,microbial interactions,and uptake by vegetation.Speci?cally,the major processes are responsible for metal removal in constructed wetlands including binding to sediments and soils,precipitation as insoluble salts,and uptake by plants and bacteria?Kadlec and Knight1996?.

The metal removal within wetlands occurs mainly in compart-ments including soil and its overlying water,and vegetation.Met-als introduced into wetland systems are either in dissolved or particulate forms.The free metal ion is the most biologically available https://www.sodocs.net/doc/3a5283424.html,plexation of metals with organic ligands is considered to be the important abiotic factor mitigating metal toxicity.Sediments might serve as sinks and reservoirs for metals in wetland systems.Chemical reactions including sorption,pre-cipitation,and complexation contribute to the?ux of metals to and from the sediments.Mixing,uptake,and aeration may dis-perse contaminants through sediments and release metals to the overlying water column.

The property of wetland soils usually characterized as the re-duced condition and high organic contents that enhance heavy metal removal to transform into less remobilization acting as natural sink systems.The major metal retention processes in wet-land soils include cation exchange,complex with organics,and precipitation as oxides,carbonates,and sul?des?Tam and Wong 1996?.The last process is speci?cally important for surface?ow wetland systems receiving high sulfate contents,because the soil was subjected to anaerobic environments where sulfate reducing bacteria produce sul?des that may precipitate metals as metal sul?des.Metal sul?de precipitations are more inert and may be

1Professor,Dept.of Civil and Environmental Engineering,National Univ.of Kaohsiung,Kaohsiung811,Taiwan,Republic of China.E-mail: tyyeh@https://www.sodocs.net/doc/3a5283424.html,.tw

Note.Discussion open until September1,2008.Separate discussions must be submitted for individual papers.To extend the closing date by one month,a written request must be?led with the ASCE Managing Editor.The manuscript for this paper was submitted for review and pos-sible publication on October1,2007;approved on October4,2007.This paper is part of the Practice Periodical of Hazardous,Toxic,and Radio-active Waste Management,V ol.12,No.2,April1,2008.?ASCE,ISSN 1090-025X/2008/2-96–101/$25.00.

permanently bound to the soil leading to unavailability to the aqueous biota.Heavy metals associated with different fractions of the soil have different impacts on the aquatic https://www.sodocs.net/doc/3a5283424.html,anic rich sediments have low redox values under which a release of metals may occur because of organic matter decomposition or through microbiologically mediated iron and manganese oxide reduction?Tarutis and Unz1995?.

The in?uence of plant activities modi?es the distribution of trace metals between the solid and aqueous phases.Wetland plants transfer oxygen from the surface to the roots.A fraction of the oxygen diffuses into the sediments where it can reoxidize reduced electron acceptors?i.e.,sul?de,ferrous?formed during the degradation of organic matter in the anaerobic sediments.Oxi-dation of sediment sul?de may release associated metals to the water column.On the other hand,plants release organic carbon into sediments and the organic carbon may further degrade lead-ing the sediment to a more reduced condition.Therefore,wetland plants may result in the extensive sulfur cycling and metal trans-port between oxidizing and reducing conditions.Research has reported that metal retained was markedly increased in sediments due to the in?uence of vegetation.Concentrations of metals were signi?cantly higher in the vegetated sediments than in the non-vegetated sediments?Choi et al.2006?.Stein et al.?2007?re-searched the use of constructed wetlands for sulfate reduction and subsequent metal precipitation in cold climates.The results indi-cated that plants can provide both oxygen and organic carbon and their net effect was to reduce sulfate reduction especially in winter.However,metal sul?de might be reoxidized and release associated metal into the aquatic environments.The role of mac-rophytes is prominent for metal removal within wetland systems.

The metal removal performance of the wetland treatment sys-tem depends on the in?ow water quality and the metal removal mechanisms.A mass balance was performed on trace metals at Sacramento Constructed Wetlands.Annual vegetation harvest does not account for more than5%of annual trace metals mass removal.Metal mass removal ranged from27to81%within the wetland.An average of7.6kg/ha/year,or54%of in?uent metals loadings,is sequestered within the internal wetland compartments ?Dombeck et al.1998?.Unlike organic pollutants,metals cannot be removed from wastewater directly by biological processes. The processes including uptake by plants,physical interactions with the substrate,formation of complexes,and subsequent pre-cipitation determine the ef?ciency of constructed wetlands in the removal of metals.Ghermandi et al.?2007?summarized the re-moval of heavy metals in free water surface wetland systems and concluded moderate removal ef?ciency for copper,mercury, silver,and zinc.

Muray-Gulde et al.?2005?evaluated bioavailable copper re-moval mechanisms in a constructed wetland.Approximately7% of the copper removed from the water column was associated with the roots and shoots of vegetation.Sul?des and other min-erals in the sediment were responsible for36and25%,respec-tively,of the copper being sequestered from the water column.In addition,copper measured in the sediments and plants was not evenly distributed along the treatment scheme of the wetland.

Major processes responsible for metal removal in constructed wetlands are retaining onto sediments and uptake by plants.The extent of different removal mechanisms might vary depending on speciation of in?ow metals,physicochemical water parameters ?e.g.,redox,pH?,and types of wetland systems.Heavy Metal Reduction in Storm Water

Constructed wetlands are ef?cient in removing low levels of met-

als from large volumes of polluted water.In contrast to uniform or

predictable?ows and loadings of the other water,storm runoff is

a highly variable and intermittent feedstock treated by constructed

wetlands.There has been increasing interest in the use of con-

structed wetlands for remediate storm runoff,which often

contains high levels of metals mainly including Cu,Cd,Zn,and

Pb?Pontier et al.2004?.

Studies demonstrated that metal removal ef?ciency might be

varied in different types of wetland systems and water metal load-

ings.Walker and Hurl?2002?studied the heavy metal?Zn,Cu,

Pb,Cr,and As?removal within wetlands receiving storm water.It

was found that Zn,Pb,and Cu concentrations decreased57,71,

and48%,respectively,while Cr remained relatively constant and

that of As increased by150%.The discrepancy of these results

might be attributed to factors of chemical behavior and the role of

organic matter in the wetland.This study pointed out that heavy

metals in storm water tend to be associated with particulate

matter.Therefore,sedimentation is the primary process for the

removal of heavy metals from storm water.In particular metal

removal by sedimentation was strongly dependent on the associa-

tion with organic matter.Other processes including?ltration by

plants,adsorption,biological assimilation,decomposition,chemi-

cal transformation,and volatilization might also be signi?cant.

Lee and Scholz?2006?assessed the heavy metal removal per-

formance in experimental constructed wetlands treating urban

runoff.Reduction rates of Cu ranged from91.8to97.4of planted

wetland.In contrast,Ni reduction rates of the unplanted?lter ?89.4%?were higher than those of the planted?lter?81.5%?.Both Cu and Ni exhibited signi?cant removal rates.In addition,nickel

was likely to leach under high conductivity in combination with

low pH condition while copper concentration in the ef?uent was

not correlated with both conductivity and pH environmental

variables.

Mungur et al.?1997?studied the heavy metal removal perfor-

mance by a laboratory-scale wetland system treating storm runoff.

The removal ef?ciencies and rates for metals monitored ranged

from81.7to91.8%and36.6–372.7mg/m2/day for Cu, 75.8–95.3%and30.8–387mg/m2/day for Pb,and82.8–90.4% and33.6–362.1mg/m2/day for Zn,respectively.Results for the storm simulation showed that the metal loadings leaving the sys-tem remained very low with the wetland system retaining over 99%of the metals.Wetland receiving stormwater can be an ef?-cient sink for heavy metals.

Though numerous research has demonstrated promising metal

removal results by wetland systems receiving storm water,the

removal ef?ciency still varied.The variability in metal removal

performance might be attributed to factors including shortcircuit-

ing,short detention and contact times,pollutant remobilization,

and seasonal vegetation effects?Revitt et al.2004?.It is necessary

to address these factors to design an effective wetland system for

the treatment of metal contaminated storm water.

Heavy Metal Reduction for Industrial Wastewater and Sewage

Historically,the discharge of heavy metals into the aquatic envi-ronment by industrial wastewater was one of the primary threats to the ecosystem.Conventionally,heavy metals can be removed by a range of physicochemical treatment technologies such as

precipitation,ion exchange,electrochemical,and membrane pro-cesses.However,the aforementioned technologies are expensive and energy intensive.Constructed wetlands have been proposed to offer a low cost and low maintenance treatment alternative for industrial ef?uent,especially in developing countries?Kivaisi 2001?.

Hadad et al.?2006?assessed the feasibility of treating indus-trial wastewater within pilot-scale wetland in Argentina.Average metal removal ef?ciencies were83,82,69,and55%for Fe,Cr, Ni,and Zn,respectively.In addition,metal concentration in mac-rophyte tissues increased signi?cantly where metal concentration in the roots was2–3times higher than in leaves.However,only a small fraction of metal retained?7,2,and4%of the Cr,Ni,and Zn,respectively?in the wetland was stored in the macrophyte tissue.Retained metals were stored mainly in the sediment com-partment.Lesage et al.?2007a,b?studied the accumulation of metals in a horizontal subsurface?ow constructed wetland treat-ing domestic wastewater of350population equivalent after3 years of operation.Removal ef?ciencies of Al,Zn,and Cu were higher than84%.Removal ef?ciencies are strongly dependent on in?uent concentrations and hydraulic loading rates.The sediment in the inlet area was signi?cantly contaminated with Zn,Cu,and Cd.The sediment metal contamination problem was situated within the inlet area.Metal concentrations in the sediment de-creased towards background values further along the treatment path.Less than2%of the metal mass removal from the wastewa-ter after passage through the reed bed is accumulated in the above ground reed biomass.The sediment acts as the primary sins for metals.Sulfate reduction occurs due to oxygen consumed rapidly for the aerobic degradation process.Precipitation as metal sul-phide complexes was thought to be an important removal process for metals in the inlet area where the organic loading is high.

Aslam et al.?2007?tested compost-based and gravel-based vertical?ow constructed wetland treatment systems to treat wastewater from an oil re?nery in Pakistan.The results indicated that the purifying ef?ciency was low at the beginning but it im-proved gradually with the growth of plants and bio?lm.The sig-ni?cant removal of heavy metals including Fe2+,Cu2+,and Zn2+ was observed.Total amounts in whole Phragmites plants?roots, rhizomes,and culms?,in the compost-based and gravel-based constructed wetlands,respectively,were35.5and27.6mg/m2for copper,82.4and58.9mg/m2for iron,and18.5and13.6mg/m2 for zinc.Plant tissues took up signi?cant amounts of metals ?35–56%?in this study.

Sawaittayothin et al.?2007?investigated the feasibility of ap-plying constructed wetlands to treat a sanitary land?ll leachate in Thailand.Cadmium removal was effective resulting in the ef?u-ent concentration of less than1?g/L.The primary metal re-moval mechanisms were attributed to precipitation and adsorption while0.3%of the Cd input was uptake by the plant biomass.Due to the high Cd content in the leachate,the Cd content of the cattail biomass in the experimental unit was found to be4.52mg/kg.

The research?ndings?Lim et al.2003?reported that heavy metals such as Cu,Cd,Zn,Pb,Ni,and Co could be readily removed by constructed wetland systems though the metal re-moval ef?ciency seems to be in?uenced by the types of media used and the types of wastewater to be treated.Four laboratory-scale gravel-?lled subsurface-?ow constructed wetland units planted with cattails were fed with primary treated domestic wastewater at a constant?ow rate of25mL/min.The carbon oxygen demand?COD?removal ef?ciency was independent of increasing metal loading while nitrogen removal ef?ciency dete-riorated progressively with increasing metal loading.The relative effect of the heavy metals was found to increase in the following order:Zn?Pb?Cd.The metal seems to exhibit some inhibitory effect on nitrogen uptake by cattail plants.However,the uptake of Zn,Pb,Cd,and Cu by cattail plants was insigni?cant compared to other removal pathways based on the mass balance analysis.This study provides useful information for some metal containing industrial wastewater combined with sewage for treatment by constructed wetlands.

The distribution of heavy metals was investigated in a con-structed wetland system designed for processing domestic sew-age.The concentrations of metals in the ef?uents were well below the permitted limits for treated sewage discharged to surface water.The heavy metal content of suspended matter decreased along the course of treatment.The main mechanism for removal of dissolved metals was sorption on particulate matter suspended. The calculation of the total load of metal retained in the wetland system can be employed as the basis to estimate the life expect-ancy of the system?Obarska-Pempkowiak and Klimkowska 1999?.

The potential of constructed wetland treating industrial waste-water and sewage is enormous based on these studies.In order to enhance metal removal ef?ciency,proper control of in?ow metal loading,selection of vegetation species,and design of wetland systems need to be further investigated.

Heavy Metal Reduction for Acid Mine Drainage

Acid mine drainage is often characterized by low pH water with elevated concentration of heavy metals.Inactive mining sites have limited funds for construction and maintenance of elaborate treatment systems.The use of constructed wetlands as alternative options for the treatment of acid mine drainage has developed https://www.sodocs.net/doc/3a5283424.html,pared to conventional physicochemical treatment processes,constructed wetlands offer an alternative treatment technology with minimum inputs,low investment costs,low operating costs,and no external energy input.The special micro-bially induced sulphate and iron reduction occur in wetland sedi-ments that facilitate the removal of metals from acid mine water through increasing pH.The elevation of pH results in precipita-tion of the metals either as hydroxides or as sulphides.Mays and Edwards?2001?studied metal accumulations in sediments and plants of constructed and natural wetlands receiving acid mine drainage.The results indicated that Mn,Zn,Cu,Ni,B,and Cr were accumulated in the plants,although accumulation of metals by plants accounts for a small percentage of the removal of the annual metal load.The role of vegetation in metal retention may include serving as sites for metal precipitation and/or sedimenta-tion.Plant uptake and decay,and strong retention of metals by inorganic and organic soil components,are the primary mecha-nisms of accumulation of metal ion near the soil surface.

Recent work on wetlands receiving metal contaminated mine water has demonstrated that organic matter in the form of fallen leaf and stem debris from P.sustralis plays an important role in the immobilization of metals within the wetland system.The metal uptake was higher in the roots than the stems.The metal concentrations in the debris samples were markedly higher than the roots of the reeds.The values for Fe,Al,and As in debris were several orders of magnitude larger than the in?uent mine water ?Barley et al.2005?.The sediment retained most of the in?ow metal within wetland systems receiving mine water.

Water from weathered sulphidic mine tailings with a low pH often contains high concentration of metals.Stoltz and Greger

?2006?studied wetland plants on weathered acidic mine tailings and their effect on pH in tailings.The amendments,sewage sludge,and an ashes-sewage sludge mixture were employed as plant nutrition.Their in?uence on the metal and metal concentra-tions of plant shoots was analyzed.The increased pH levels caused by the addition of ashes induced better plant growth since the increase in pH reduces the solubility of metals and makes the environment less harmful to the plants.The highest tissue con-centrations of the metals within plant species were around8,1, 23,38,and800mg/kg for As,Cd,Cu,Pb,and Zn,respectively. The experimental results indicated that the addition of a pH rais-ing amendment resulted in better plant growth and lower metal concentrations in the shoots.

Researches reported that constructed wetlands successfully treated metal loaded acid mine drainage.The removal mecha-nisms mainly involve sedimentation,adsorption,and precipitation into insoluble compounds.Though vegetation may contribute to metal reduction,the sediment retained most of in?ow metal within wetland systems.

Signi?cance of Macrophytes within Constructed Wetlands

Vegetation is one of the principal components to remove pollut-ants within wetland systems.Macrophytes may assimilate pollut-ants directly into their tissues,provide an environment for microorganisms to grow,and usually act as catalysts for puri?ca-tion reactions especially in their rhizosphere.The role of plants in metal removal is also mediated through plant uptake,reduction of water?ow,and plant-induced chemical changes in the rhizo-sphere that increase the storage capacity of the sediment.

Wetland plant species differ greatly in their abilities to accu-mulate and translocate metals.Metal removal can be signi?cantly enhanced by the selection of appropriate plant species.In general, macrophytes produce a good yield in green biomass and their roots can reach a considerable depth.They have been reported as feasible plants for metal removal.The following processes are assumed to in?uence metal accumulation rates in plants:mobili-zation and uptake from the soil,compartmentalization and sequestration within the root,ef?ciency of xylem loading and transport,distribution between metal sinks in the aerial parts, sequestration,and storage in leaf cells.

Species of vegetation with regard to their tolerance and adap-tation to metal containing water have been studied.Calheiros et al.?2007?investigated the application of different plant species in constructed wetlands receiving tannery https://www.sodocs.net/doc/3a5283424.html,tifolia and P.australis were the species better adapted to tannery waste-water in terms of survival and propagation.The chromium con-centration at the out?ow was below the detection limit with the average in?ow concentration of0.01mg/L.These low levels are due to the fact that the leather manufacture plant does not use chromium in high amounts.Differences in chromium removal might have been achieved due to different vegetation having dif-ferent capacities to remove heavy metals.Regardless of their ori-gin,wetland plants are able to grow in high metal concentrations and this has led to the theory that wetland plants have an innate tolerance to heavy metals.Wetland plants including Typha latifo-lia,Phragmites australis,Glyceria?uitans,and Eriophorum an-gustifolium were found to be tolerant to high concentrations of metals?Matthews et al.2005?.

Research regarding the metal removal mechanism via macro-phytes has been conducted.Sune et al.?2007?determined chro-mium and cadmium bioaccumulation processes of two free-?oating macrophytes used in wetlands for water treatment: Salvinia herzogii and Pistia stratiotes.The results indicated that the most important processes of Cd uptake are biological ones in S.herzogii and adsorption,chelation,and ionic exchange are in P.stratiotes.In addition,the main processes of Cr uptake in both macrophytes are adsorption,chelation,and ion exchange.Lesage et al.?2007a,b?studied the submerged plant Myriophyllum spicatum for the treatment of metal-contaminated industrial wastewater.The investigation focused on the sorption/desorption characteristics of the surface of M.spicatum for Co,Cu,Ni,and Zn.Batch experimental results showed that the metal removal process by plants involves a combination of rapid sorption on the surface and slow accumulation and translocation in the biomass. For Co,Ni,and Zn,the sorption process was well described by the Langmuir model,whereas sorption of Cu was better described by the Freundlich model.The biomass had the highest af?nity for Cu and Zn.A Langmuir sorption maximum of Co,Ni,and Zn were2.3,3.0,and6.8mg/g,respectively.At the highest initial concentration of100mg/L,a maximum of29mg/g of Cu was sorbed onto the surface of the biomass.

Fritioff and Greger?2006?examined the uptake of Zn,Cu,Cd, and Pb by Potamogeton natans in the translocation from organs of uptake to other plant parts.The results showed that Zn,Cu,Cd, and Pb were taken up by the leaves,stems,and roots,with the highest accumulation found in the roots.Between24and59%of the metal content was bound to the cell walls of the plant.No translocation of the metals to other parts of the plant was found except for Cd.Dispersion of metals from sediment to water through P.natans is unlikely.

Vymazal et al.?2007?studied trace metals in macrophytes growing in constructed wetlands receiving municipal sewage. Metal concentrations were much lower as compared to those found in plants growing wetlands receiving acid mine drainage waters,waters from smelters,or highway runoff.Zinc concentra-tions in leaves,stems,roots,and rhizomes are20.3,11.6,61.3, and22.7mg/kg,respectively.The average concentrations of cop-per analyzed for leaves,stems,roots,and rhizomes are19.1,10.1, 28,and33mg/kg,respectively.Metal concentrations decrease in the order of roots?rhizomes?leaves?stems.Below ground/ above ground plant tissue concentration ratios varied from2.2to 32for Cu and Cr,respectively,with the average value of9.9. Metal retained in various parts of vegetation was reported.

The studies conducted to examine the extents of metal removal ef?ciency are summarized as follows.Contradictory results of metal removal extents have been reported.Bragato et al.?2006?studied the accumulation of heavy metals and nutrients in macro-phytes in newly constructed Italian wetlands.In order to maxi-mize heavy metal mitigation within the wetlands,harvest should be conducted to remove maximum plant tissues.The distance from the inlet of the wetlands did not affect either nutrients or heavy metals shoot content.However,the heavy metal contents in the macrophytes were highest in late autumn after senescence. Mantovi et al.?2003?researched two horizontal subsurface?ow reed beds treating dairy parlor ef?uent and domestic sewage in an Italian rural settlement.Copper and zinc content were evaluated in P.australis tissues and showed that plant tissues took up only 1–2%of metals entering the treatment systems.However,the accumulation ratios for copper and zinc were high:108and171 for copper and56and63for zinc,in culms and root systems, respectively.

A constructed wetland comprising a vertical?ow chamber with Cyprus alternifolius followed by a reverse-vertical?ow

chamber with Villarsia exaltata was assessed for decontamination of arti?cial wastewater polluted by heavy metals.The in?ow chamber was responsible for the decontamination process of more metal species with?nal concentration far below World Health Organization drinking water standards.About one third of the Cu and Mn were absorbed,predominantly by lateral roots of C.al-ternifolius.Contents of Cd,Cu,Mn,and Zn in soil were highest in top layer that can be removed mechanically.A vertical?ow constructed wetland with C.alternifolius is an effective tool in phytoremediation for treatment of water polluted with heavy met-als?Cheng et al.2002?.

Scholz?2003?investigated the treatment ef?ciencies of vertical-?ow?lters containing macrophytes and granular media of different adsorption capacities.The use of macrophytes and adsorption media did not enhance heavy metal reduction after13 months of monitoring;however,no breakthrough of metals was recorded.Expensive or time-consuming variables?e.g.,biochemi-cal oxygen demand?can be predicted with less expensive ones ?e.g.,dissolved oxygen and temperature?to reduce monitoring cost.

Lee and Scholz?2007?assessed the role of the macrophyte in experimental wetlands treating storm runoff from urban areas. The amount of metals removed by plant harvesting was negligible ??1%on average?when compared to those retained in the?lter. In other words,most of the heavy metal loads were accumulated in the sediment rather than taken up by vegetation.Though the wetland systems remove signi?cant in?uent mental,higher ef?u-ent Ni concentrations were recorded in the planted?lters.More-over,higher Cu out?ow concentrations were also recorded for planted?lters in comparison to unplanted?lters after applying shock loadings of copper.Findings indicate that metal retention in the wetland?lters is more susceptible to the change of environ-mental conditions such as pH and redox potential.The presence of vegetation,namely P.australis,has an impact on metal reten-tion,speci?cally by lowering the pH in aged wetland?lters.

Contradictory results have been reported regarding the extent of metal removal ef?ciency.Research has shown that macro-phytes contribute insigni?cantly to heavy metal removal?Mays and Edwards2001;Scholz2003;Mantovi et al.2003;Lee and Scholz2007?.Contrary to the aforementioned research a signi?-cant role of macrophytes in heavy metal removal was also reported?Cheng et al.2002;Southichak et al.2006?.The discrep-ancy might be attributed to factors including loadings of in?ow metals,types of wetland systems,and species of vegetation. Conclusions

Constructed wetlands possess the potential for mitigating metal contaminated waters.The metal removal mechanisms within wet-land systems mainly include sedimentation,?ltration,chemical precipitation and adsorption,microbial interactions,and uptake by vegetation.Though metal removal ef?ciency might be varied among different types of wetland systems,retained metals within constructed wetlands were reported to be stored mainly in the sediment compartment.Contradictory results have been reported regarding the role of macrophytes in heavy metal removal.Nev-ertheless,vegetation is one of the principal components to remove metals within wetland systems.Further research is needed to en-hance the removal ef?ciency within constructed wetland receiv-ing metal polluted waters.References

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