Gloria Sanchez Galvan and Eugenia J Olguin contents

10.1 Introduction 389

10.2 Rhizofiltration 390

10.3 Constructed Wetlands and Lagoons 393

10.3.1 Lagoons with Free-Floating Plants 394

10.3.2 Surface Flow Constructed Wetlands 396

10.3.3 Subsurface Flow Constructed Wetlands 397

10.4 Bioadsorbents-Based Systems 397

10.5 Conclusions 402

References 402

10.1 I NTRODUCTION

Surface water and groundwater may become contaminated with hazardous compounds as a consequence of natural and human activities. Pollutants of concern are both inorganic (heavy metals, radionuclides, nitrogen, phosphorous, etc.) and organic compounds (fuels, solvents, explosives, pesticides, herbicides, chemical and petrochemical compounds, etc.).1 Organic pollutants are mostly man-made and xenobiotic to organisms. They are released into the environment via spills, military activities, agriculture, industry, wood treatment, and so on. Inorganic pollutants occur as natural elements in the earth's crust or atmosphere, and human activities such as mining, industry, traffic, agriculture, and military activities promote their release into the environment.2 Heavy metals and nutrients such as nitrogen and phosphorous are the inorganic pollutants of major concern worldwide.3-5

The release of heavy metals into the environment presents a serious threat. Over recent decades, the annual worldwide release of heavy metals reached 22,000 T for cadmium, 939,000 T for copper, 783,000 T for lead, and 1,350,000 T for zinc.3 Because of their high solubility in the aquatic environments, heavy metals can be absorbed by living organisms and enter the food chain.6 Exposure to high levels of these metals has been linked to cytotoxic, mutagenic, and carcinogenic effects on

FIGURE 10.1 Different types of systems used in phytofiltration.

human health and wildlife.7 As a consequence, their elimination from contaminated waters has become a major topic of research in recent years.8

Different technologies have been developed in recent years to treat the wastewaters contaminated with heavy metals. Chemical precipitation, coagulation-flocculation, flotation, ion exchange, and membrane filtration can be employed to remove heavy metals from contaminated wastewater.6 However, they have inherent limitations in application mainly due to the lack of economical feasibility for the treatment of large volumes of water with a low metal concentration. Furthermore, the major disadvantage of conventional technologies is the production of sludge.9

Due to the above-mentioned constraints of conventional technologies, the biological treatment of metals, especially phytoremediation, is becoming a more attractive alternative. It is defined as the use of plants and their associated microbes to remove, reduce, degrade, or immobilize environmental pollutants from soil and water, thus restoring contaminated sites to a relatively clean, nontoxic environment. A variety of polluted waters can be phytoremediated, including sewage and municipal wastewater, agricultural runoff/drainage water, industrial wastewater, coal pile runoff, landfill leachate, mine drainage, and groundwater plumes. Phytoremediation includes various strategies and all of them are promising, cost-effective, and environmentally friendly technologies.10

Phytofiltration, a specific strategy of phytoremediation, is the use of plants to remove contaminants from water and aqueous waste streams. Three different systems (Figure 10.1) can be considered within this strategy: (a) rhizofiltration (the use of hydroponically cultivated plant roots),31112 (b) constructed wetlands (CWs) and lagoons, and (c) bioadsorbents-based systems.1

It is worth noting that there are preparation stages of the plant biomass before they can be used for pollutants removal, in the case of rhizofiltration and bioadsorbents-based systems (Figure 10.2), which may increase the investment and operational costs. On the contrary, the lagoons and CWs are designed to process the influents in one single stage.

This chapter is aimed at presenting an overview of the state of the art in phytofiltration of heavy metals using any of the three different treatment systems. It has been considered useful to discuss the three alternatives in one single document, since usually, information for each of the systems is reviewed separately, missing the advantages of a holistic discussion.

10.2 RHIZOFILTRATION

The system or process termed rhizofiltration is the use of hydroponically cultivated plant roots of several terrestrial plants to absorb, concentrate, or precipitate toxic metals from polluted effluents

(a) Rhizofiltration

(a) Rhizofiltration

Hydroponic cultivation of terrestrial plants

Hydroponic cultivation of terrestrial plants

(bj) Lagoons

Use of enhanced root system for the removal of pollutants (•).

Use of enhanced root system for the removal of pollutants (•).

Water

(b2) Constructed wetlands

Wastewater with pollutants

(b2) Constructed wetlands

Wastewater with pollutants

Treated effluent

(c) Biosorbent-based systems

(c) Biosorbent-based systems

Water

Production of biomass

Water

Production of biomass

Drying process

Polluted^-? influent

Treated effluent

Use of nonliving plant material for the removal of pollutants.

FIGURE 10.2 Preparation and operative stages in rhizofiltration, lagoons, CWs, and biosorbent-based systems.

and was initially promoted by Dushenkov and his group.13 An extensive review on this topic and the use of various plants for the removal of heavy metals has been already published.14 More recently, other reviews have been published, mainly related to modeling systems15 and radionucleotides removal.12 Thus, in this chapter, an effort has been made in order to refer only to the more recent work related to the removal of heavy metals through rhizofiltration, in the strict sense of the definition described above, and also in relation to the use of terrestrial plant systems for metal removal from aqueous solutions and wastewaters.

During rhizofiltration, the plant roots sorb, concentrate, and/or precipitate the contaminants present in the irrigated wastewater through the soil plant root system into the harvestable parts of the roots and above-ground shoots.15 Subsequent volatilization of contaminants can also occur. As they become saturated with the contaminants, roots or whole plants are harvested for disposal. Terrestrial plants are preferred over aquatic plants, since they produce long, more substantial, often fibrous root systems with large surface area for pollutant sorption.3

Roots of many hydroponically grown terrestrial plants, for example, Indian mustard (Brassica juncea (L.) Czern.), sunflower (Helianthus annuus L.), and various grasses, have proved to remove effectively toxic metals from aqueous solutions. Sunflower (Helianthus annuus L.) is one of the most promising environmental crops that is being used in diverse situations for environmental clean-up.16 It has been shown to be very efficient in the uptake of Cd(II) and Pb(II).17 The latter was concentrated in both leaf and stem at the region of vascular bundles with greater amounts in the leaf portion. Lead granules were also found in the root tissue from the epidermis layer to the central axis.18 The influence of a chelating agent such as (S,S)-N,N0-ethylenediamine disuccinic acid (EDDS) on the accumulation of Cu(II), Zn(II), and Pb(II) by sunflowers from nutrient solution has also been assessed. The uptake of Pb(II) in shoots was enhanced, whereas that of essential metals, such as Cu(II) and Zn(II), was decreased. These results show that synthetic chelating agents do not necessarily increase uptake of heavy metals, when soluble concentrations are equal in the presence and absence of chelates.19

Plant groups such as Brassica have members with the ability to extract selenium from soil. Brassica juncea, in particular, has been the focus of much research due to its relatively large biomass and its fast growth cycle.20 In hydroponics, it has shown an exceptionally high accumulation of Pb(II) (138 g/kg) being restricted largely to root tissue. Examination using scanning transmission electron microscopy-energy dispersive spectroscopy revealed substantial and predominantly intracellular uptake at the root tip.21 This plant is also able to remove Hg(II) from contaminated solutions. The main removal mechanism was volatilization that occurred from the roots, while only little Hg(II) was translocated to the shoots (0.7-2% of the total metal accumulated).22

Sedum alfredii has proved to be a Zn/Cd-hyperaccumulator23 and Pb-accumulating plant.24 Studies developed in hydroponics have suggested that the naturally occurring rhizospheric bacteria may be useful in S. alfredii tolerance to heavy metal toxicity, and also accelerate the metal removal from contaminated water.25 The effect of Pb(II) on hyperaccumulating and nonhyperaccumulating ecotypes of this plant has also been evaluated. Although growth, leaf physiology, and ultrastructure of both the ecotypes were affected by Pb treatment, deleterious effects were more pronounced in the nonhyperaccumulating ecotype.26

It is known that lead phytoextraction can be economically feasible only when the developed systems employ high biomass plants that can accumulate >1% of the metal in their shoots. Sahi and coworkers27 demonstrated that Sesbania drummondii, a leguminous shrub occurring in the wild, is able to accumulate >4% of Pb(II) in shoots when it is grown hydroponically in a Pb-contaminated nutrient solution. Pb(II) granules were found in the plasma membrane and cell wall, and also in the vacuoles. S. drummondii is also able to accumulate Hg(II) from water in roots (998 mg Hg/kg) and shoots (41,403 mg Hg/kg). It has been suggested that this plant uses effective antioxidative defense mechanisms such as the modulating nonenzymatic antioxidants (glutathione and nonprotein thiols) and enzymatic antioxidants: superoxide dismutase, ascorbate peroxidase, and glutathione reductase.28

The potential efficiency of Fagopyrum esculentum, common buckwheat, in removing chromium from wastewaters has also been assessed. Plants grown in Cr(III) showed a higher tissue concentration and a higher chromium removal efficiency than those grown in Cr(VI). Buckwheat was able to retain their capacity for Cr removal even though it showed strong toxicity symptoms.29

Buddleja asiatica (wild species) and B. paniculata (cultivated species) are plants of fast growth, containing an extensive root system. Evaluations under hydroponic conditions were carried out to compare their phytoremediation potential of Cd(II), Zn(II), and Pb(II). B. asiatica accumulated more Pb(II) and Cd(II) than B. paniculata in both shoots and roots. An extremely high Pb content (12,133-21,667 mg/kg) was observed in roots of both species.30 On the other hand, Sesuvium portu-lacastrum and Mesembryanthemum crystallinum are halophytes from Aizoaceae family that show tolerance to Cd(II) in aqueous solutions. It was found that Cd(II) accumulation was significantly higher in the roots than in the shoots. However, the metal content in the shoots reached values characteristic of Cd hyperaccumulator plants (350-700 ^g/g).31

Pteris vittata, a terrestrial fern, has been described as an As hyperaccumulator.32 Additionally, it has been also grown hydroponically to assess its effectiveness in As removal from contaminated groundwater. Several short-term studies have shown that it is able to reduce effectively the arsenic concentration in 3 d (from 46 to <10 mg/L). After this time, reused plants can continue to take up arsenic from the groundwater but at a slower rate.33 At field scale, Natarajan and coworkers34 evaluated some factors such as plant density, nitrogen (N) and phosphorous (P) addition, and reuse of plants to improve the effectiveness of P. vittata roots system. Results suggested that a higher plant density and lower P levels may enhance the As depletion (from 130 to 1.7 ^g/L in five weeks). At the same scale, a continuous flow phytofiltration system with the genus Pteris consistently produced water having an arsenic concentration less than the detection limit of 2 mg/L, at flow rates as high as 1900 L/d for a total treated water volume of approximately 60,000 L throughout the 84 d demonstration period.35

Vetiveria zizanioides is a fast growing, perennial, tussock grass belonging to the family Poaceae. It has the ability to extract metals from the soil and water. Its high tolerance for metals and metalloids is often attributed to its capability to accumulate metals in above-ground tissues that do not affect the roots and shoot growth, and to the mycorrhizal association within its roots that makes it sturdy enough to withstand high toxic metal concentration in soils.36 Boonyapookana and coworkers18 reported that after four weeks of growth, a 17-fold increase in shoot Pb(II) content was observed in plants grown in a solution containing 2.5 mmol/L of Pb(II) and in the presence of EDTA. A Bioconcentration Factor (BCF) of 88 was obtained.

10.3 CONSTRUCTED WETLANDS AND LAGOONS

An extensive number of manuals and books related to the different aspects of performance and design of CWs are available in the literature.37-44 On the contrary, reviews concerning metal removal using lagoons with floating macrophytes are scarce. In this review, brief information about CWs is presented and a major emphasis is given to the use of lagoons with aquatic floating plants for metal removal.

CWs are engineered systems that have been designed to treat wastewaters taking advantage of many of the processes that occur in natural wetlands, although its design allows a more controlled environment. Natural processes involve wetland vegetation, soils, and their associated microbial assemblages.45 CWs have been used for the purification of domestic, industrial, and agricultural wastewater, and stormwaters. They are furthermore applied to strip nutrients from polluted surface waters before these are discharged into vulnerable nature reserves.41

Many are the advantages of CWs for treating wastewater and runoff. They are a cost-effective and technically feasible technology. The expenses of operation and maintenance (energy and supplies) are low, requiring only a periodic, rather than continuous, on-site labor. CWs are tolerant to fluctuations in flow and facilitate water reuse and recycling. Additionally, they provide habitat for many wetland organisms and benefits to wildlife habitat.37

In developing countries, additional advantages of using CWs can be obtained. They may provide economic benefits and could encourage small communities to maintain natural wastewater treatment systems. The production of plant biomass can provide economic returns to communities through production of biogas, animal feed, compost, and fiber for paper according to the type of pollutant.46

The treatment mechanisms in CWs are numerous and often interrelated. Metals are removed at different stages from the water column by a series of physicochemical and biological processes (Table 10.1).

TABLE 10.1

Mechanisms Involved in the Improvement of the Water Quality in a CW

TABLE 10.1

Mechanisms Involved in the Improvement of the Water Quality in a CW

Mechanism

Type of Process

Pollutant

Settling of suspended particulate matter

Physical

Organic matter

Filtration and chemical precipitation through contact of the water with the

Physicochemical

Metals

substrate and litter

Adsorption and ion exchange on the surfaces of plants, substrate, sediment,

Physicochemical

Metals

and litter

Direct uptake by plants and microorganisms. Microbial removal of metals

Biological

Metals

Breakdown and transformation of pollutants by microorganisms and plants

Biological

Organics

Uptake and transformation of nutrients by microorganisms and plants

Biological

N, P

Predation and natural die-off of pathogens

Biological

Pathogens

Source: Environmental Protection Agency (EPA), A Handbook of Constructed Wetlands. Available at http://www.epa.gov/

OWOW/wetlands/pdf/hand.pdf, 2000b and Greenway, M., The role of macrophytes in nutrient removal using constructed wetlands, in Environmental Bioremediation Technologies, Singh, S.N. and Tripathi, R.D., Eds, Springer, Berlin, Heidelberg, 2007, pp. 331-351.

OWOW/wetlands/pdf/hand.pdf, 2000b and Greenway, M., The role of macrophytes in nutrient removal using constructed wetlands, in Environmental Bioremediation Technologies, Singh, S.N. and Tripathi, R.D., Eds, Springer, Berlin, Heidelberg, 2007, pp. 331-351.

Type of flow

Type of plants

Type of support

Surface flow constructed wetlands

Various types of plants

A) Free-floating

B) Floating-leaved

C) Submerged

D) Emergent

Soil

Sub-surface flow constructed wetlands

A) Downflow

B) Upflow

C) Tidal

Emergent plants

Pea gravel crushed rock, etc.

FIGURE 10.3 Types of CWs according to the type of flow, plant, and supporting media.

The basic classification of CWs is based on the type of flow regime and macrophytic growth. In general terms, two types can be described (Figure 10.3). The selection of the most appropriate option shall be according to various operational factors and to the plants available in the region of establishment of the system.37384044

10.3.1 Lagoons with Free-Floating Plants

Lagoons with free-floating macrophytes consist of one or more shallow ponds in which plants float on the surface. Several free-floating plants (FFP) have been tested to purify water by removing nutrients and metals (Table 10.2). They range from large plants with rosettes of aerial and/or floating leaves and well-developed submerged roots to minute surface-floating plants with few or no roots.44 The metal removal in this kind of systems is mainly due to plant uptake.69,70 Adsorption to the roots or surface plant, translocation, and intracellular accumulation have been described as the main removal mechanisms.71,72 The majority of works have been carried out in single-metal microcosms in batch-operated systems. The genus Azolla has been widely assessed for removing metals at low initial metal concentrations. Bennicelli and coworkers73 reported that A. californiana was able to remove Hg(II) (75-93%) and Cr(III) (74-91%) from synthetic water solutions. The concentration of metals ranged from 71 to 964 mg/kg dry weight in the plant tissues. Similar results for Hg (90%, 94%, and 80%) and Cd(II) (>80%) removal were found using A. pinnata. The metal content in the biomass was directly related to that of the solution, being 667 and 740 mg/kg, for Hg(II) and Cd(II), respectively at an initial concentration of 3.0 mg/L. In all cases, the metal presence inhibited the plant growth at 20-30%.74 Likewise, A. filiculoides is an excellent accumulator of Pb(II) (1.8% of dry weight). More in-depth studies carried out to characterize the mechanisms of such Pb(II) accumulation and storage showed that Pb(II) uptake in Azolla leaves takes place in the cell wall and vacuoles. In mature Azolla leaves, lead accumulated in larger aggregates than in young leaves. The tonoplasts may be involved in lead accumulation through secondary ion transporters in the vacuoles via H+ -ATPase activity.75

The phytofiltration of Pb(II) and Cd(II) has been also studied using species of Salvinia. S. minima Baker is a small free-floating aquatic fern native to Mexico, Central America and South America. It has been proved to be an excellent aquatic phytoremediator and hyperaccumulator of Cd(II) and Pb(II).72,76 The relevance of using a compartmentalization analysis (CA) complementary to the use of BCFs and metal removal kinetics by plants has been demonstrated using S. minima

Table 10.2

Types of Plants Utilized in CWs and Lagoons

Type of Plants Reference

Free-floating Eichhornia crassipes 48

Pistia stratiotes 49

Lemnaceae 50

Azolla filiculoides 51

Ipomoea aquatica 52

Bacopa monnieri 53

Salvinia minima 54

Floating-leaved Nymphaea spontanea 55

Nymphaea aurora 56

Hydrocotyle umbellata 57

Submerged Nuphar variegatum 58

Elodea canadensis 59

Potamogeton natans 59

Hydrilla sp. 60

Vallisneria spiralis 61

Emergent Phragmites australis 62

Bolboschoenus maritimus 62

Zizania latifolia 63

Typha latifolia 59

Alisma plantago-aquatica 59

Sagittaria sagittifolia L. 59

Juncus effusus 64

Typha domingensis 65

Phragmites australis 66

Phalaris arundinacea 66

Spartina alterniflora 67

Carex rostrata 68

Eriophorum angustifolium 68

exposed to Pb(II) as a model system.72 The CA is used to define the fate of the metal within four compartments in the microcosm (surface of the plant, intracellular space, water column, and sediments) by using a series of EDTA washings. Recently, based on the use of this methodological tool, a bioadsorption factor (BAF) and an intracellular accumulation factor (IAF) were proposed in order to gain a full insight into the hyperaccumulating lead capacity of S. minima. It was clear that such an ability was mainly due to a strong adsorption capacity (BAF in the range of 780-1980) compared to a weaker one for intracellular accumulation (IAF in the range of 57-1007). Surprisingly, the ability of S. minima to accumulate the metal into the cells was not inhibited at concentrations as high as 28.40 ± 0.22 mg Pb(II)/L.77

Phetsombat and coworkers78 found significant Cd(II) and Pb(II) increases in the accumulation of these two metals by S. cucullata, when exposure time (2-8 d) and concentration were increased (from 0.5 to 4.0 and from 5 to 40 mg/L of Cd(II) and Pb(II), respectively). The roots of S. cucullata had higher metal contents than leaves suggesting that the metals were bound to the root cells and were partially transported to the leaves. At the same time, there were significant decreases in the relative growth, biomass productivity, and total chlorophyll content when the exposure time and concentration were increased. Other studies have demonstrated the potential of S. minima to remediate Cu(II) in concentrations 100 times above that currently found in freshwater environments.79

Reports on the evaluation of FFP for metal removal at mesocosms level are scarce. The following discussion provides some of the more relevant reports related to this topic. Eichhornia crassipes, Pistia stratiotes, Lemna minor, A. pinnata, and S. polyrhiza were tested for their heavy metal removal capacity from the secondary treated municipal wastewater (150 L). Such a wastewater contained metals in various concentrations (mg/L), such as Cr (1.2), Cd (0.09), Cu (0.11), Zn (0.92), Fe (1.8), and Ni (0.07). The aquatic plants showed metal tolerance and surprisingly the secondary treated municipal wastewater promoted their growth. E. crassipes was the most efficient accumulator removing up to 70% of Fe(II) and 59% of Ni(II). Metals were mostly accumulated in roots than in leaves, according to the translocation factors, which in general were <1. The highest translocation factor was obtained in L. minor for Fe (0.94) and the lowest for Zn in A. pinnata (0.48). The biomass produced may be used for biogas production, papermaking, and so on, while treated wastewater may be of possible use for irrigation. Maximum removal at 20 d hydraulic retention period and decreasing trend after that indicate that aquatic plants should be harvested every 20 d for wastewater treatment. This technology is highly recommendable for tropical wastewaters where sewage is mixed with industrial effluents.80

Aquatic floating plants have been also tested for the removal of heavy metals from the coal mining effluent in mesocosms. The high removal efficiency (>60% for Fe, Cr, Cu, Cd, and Zn) found when a combination of E. crassipes and L. minor was used may be due to preferential higher absorption capacities of each plant.81 Experimental sets containing only E. crassipes removed the highest concentration of heavy metals. The translocation factor indicated lower transportation of heavy metals from roots to leaves. A lower accumulation of metals in leaves than in root can be associated with protection of photosynthesis from toxic levels of trace elements.82 No symptom of metal toxicity was found; therefore this method can be applied to the large-scale treatment of wastewaters in which metal concentrations are low. The mining effluents treated by this method can be used for various purposes in industry and agriculture or can be safely discharged into surrounding water bodies.81

Jayaweera and coworkers83 described the different mechanisms involved in the phytoremedia-tion of Fe-rich industrial wastewaters by water hyacinth grown under different nutrient conditions in batch-type lagoons. Fe removal was largely due to an uptake process and chemical precipitation of Fe2O3 and Fe(OH)3 followed by flocculation and sedimentation. Chemical precipitation was more significant especially during the first three weeks of the study. Plants grown without any nutrient addition, other than Fe as a heavy metal, showed the highest removal efficiency of 47% with the highest accumulation of 6707 Fe mg/kg dry weight. Active effluxing of Fe back to the wastewater at intermittent periods was a key mechanism to avoid Fe phytotoxicity in the plant cultivated in all nutrient conditions. It was concluded that water hyacinth grown under nutrient-poor conditions is ideal to remove Fe from wastewaters with a hydraulic retention time of approximately six weeks.

A discussion of the use of floating aquatic plants for metal removal at large scale in surface flow constructed wetlands (SFCWs) is provided below.

10.3.2 Surface Flow Constructed Wetlands

This type of wetlands consists of a shallow sealed basin or sequence of basins, containing 20-30 cm of rooting soil, with a water depth <0.4 m. Dense emergent vegetation covers usually more than 50% of the surface. However, floating-leaved attached macrophytes, that is, plants with roots in the sediment and floating leaves, submerged macrophytes, and floating macrophytes are also found.44,47 In tropical regions, treatment wetlands are often dominated by floating aquatic plants rather than emergent macrophytes that are more common in temperate regions.84 The most commonly emergent species used for SFCWs are Phragmites australis (Common reed), Typha spp. (Cattail), Scirpus spp. (Bulrush), Sagittaria latifolia (Arrowhead), and so on.

The advantages of SFCWs are that their capital and operating costs are low and their construction, operation, and maintenance are not complicated. Their main disadvantage is that they generally require a larger land area than other systems.38

Not many reports on the simultaneous use of different plants in SFCWs are available. Maine and coworkers85 reported the treatment of wastewater from a tool manufacturing plant at large scale (100 m3/d). Three floating (P. stratiotes, E. crassipes, and S. rotundifolia) and eight emergent plants (Cyperus alternifolius, P. elephantipes, Thalia geniculata, Polygonum punctatum, Pontederia cor data, Pontederia rotundifolia, Typha domingensis, and Aechmea distichantia) were transplanted to the CW. Cr, Ni, and Fe concentrations were reduced by 86%, 67%, and 95%, respectively. However, soluble reactive phosphate removal was not efficient. The FeS precipitation probably caused the high retention of Fe (95%). Phosphate and ammonium were not retained within the wetland, while 70% and 60% of the nitrate and nitrite were removed. The assessment of the removal efficiency during the different macrophyte dominance stages was also carried out. During E. crassipes dominance, metals were retained in the macrophyte biomass. On the other hand, when E. crassipes together with T. domingensis were dominant, sedimentation was the main removal mechanism. Finally, during T. domingensis dominance stage, pollutants were retained in both sediment and macrophyte biomass. Removal efficiency did not show significant differences among the three vegetation stages even though removal mechanisms were different. Therefore, the choice of the most suitable species depends on the tolerance of the macrophytes to the conditions of the wastewater.65

10.3.3 Subsurface Flow Constructed Wetlands

Subsurface flow wetlands are gravel and/or soil/sand-filled trenches, channels, or basins with no standing water, which support emergent vegetation. They are also known as vegetated submerged bed systems or reed-bed or root-zone wastewater treatment systems.47 There are two types of subsurface flow constructed wetlands (SSFCWs), the horizontal flow CW (HFCW) and the vertical flow CW (VFCW). In the HFCW, the wastewater flows slowly through the bed in a relatively horizontal path and comes into contact with a network of aerobic, anoxic, and anaerobic zones. The aerobic zones occur around roots and rhizomes that leak oxygen into the substrate. On the other hand, VFCWs are fed intermittently to flood the surface and wastewater, then gradually percolate down through the bed and are collected by a drainage network at the base. The bed drains freely and it allows air to refill the bed.44

The SSFCWs have several advantages over the SFCWs. They have greater cold tolerance, promote a minimization of pest and odor problems, and, possibly, have greater assimilation potential per unit of land area, which results in a smaller requirement of land for the same volume of waste-water. In tropical regions, one of their great advantages is that they do not promote mosquitos proliferation. On the other hand, SSFCWs are more expensive to construct and may be more difficult to regulate than SFCWs. Furthermore, maintenance and repair costs are generally higher. Clogging and unintended surface flows problems have been also reported for this kind of system.38

Metal removal in SSFCWs has been recently focused on metal elimination from synthetic water and different wastewaters,66 86 on the evaluation of the effects of season, temperature, plant species, and chemical oxygen demand (COD) loading on metals removal,87 and on the accumulation of metals in wetland plant species and sediments.88 89 Recent reviews on heavy metal phytoremediation wetlands are also available.48

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