Removal of toxic metals from wastewater in constructed wetlands as a green technology; catalyst role of substrates and chelators

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv

Review

Removal of toxic metals from wastewater in constructed wetlands as a green technology; catalyst role of substrates and chelators Ammara Batoola,∗, Tawfik A. Salehb,∗∗ a b

National University of Sciences and Technology, Islamabad, Pakistan King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia

A R T I C LE I N FO

A B S T R A C T

Keywords: Substrates Chelators Phytoremediation Wastewater Catalyst

In recent years knowledge in regard to phytoremediation for removal of metals from wastewater has been extensively developed. Despite advance treatment methods; different plants were widely used for wastewater treatment that may affect the efficiency of plants by stressing their natural ability. Therefore, this paper reviews the catalytic role of constructed wetlands, spiking of chelators and substrates to enhance phytoremediation for removal of metals. Catalytic combination of substrates, chelators with plants helped to remove different metals from wastewater simultaneously without compromising the plant's health. Moreover, this paper summarizes the interaction mechanism of plants with the chelators and substrates within constructed wetlands. In addition, this paper also discusses the potential research needs for this field.

1. Introduction Wastewater/leachate treatment is one of the difficult problems as it is loaded with multiple metal ions and compounds from different industries; the municipal sector, hospital wastewater, etc are contaminated with metals. Trace metal in environment cannot be degraded but transform into different compounds either more reactive or inert (Batool and Zeshan, 2017). Different plant species play an important role in uptake metals and metabolize them. Trace metals like Zn, Pb, Fe, and Cu play a significant role in the physiology of plants in acceptable concentrations (Wu, 2019; Angassa et al., 2018). Whereas high concentration of metal in environment may induce toxic effects on plants which may affect regulation of vital ions ultimately disrupting metabolic activity. Hyperaccumulator species have capacity to accumulate metal in high concentrations in their tissues. Different species like Arundo donax, Imperata cylindrical, Cassia tora, Canna indica have been used for removal of different nutrients (Zhu and Haynes, 2010). The most widely used hyperaccumulators are Phragmites australis and Typha latifolia due to dense root systems, high biomass, and rapid detoxification (Kumari and Tripathi, 2015). Plants used in constructed wetlands are provided with an environment feasible to uptake the targeted metals by using different substrates or spiked with chelators to increase bioavailability of metals. Use of efficient substrates also played a vital role for enhancing phytoremediation; Palm tree (Herrera-Cárdenas

∗

et al., 2016), rice husk, palm tree mulch, limestone cocopeat mixture, crushed brick and steel slag (Batool and Zeshan, 2017) have recently used as substrates in constructed wetlands. Substrates provide support for plant roots, microbial organisms and also, storage for many pollutants. It is assumed that the removal capacity of the substrate mainly depends on its sorption capacity (conditioned by its organic composition specific surface, mineral) and on the chemical and physical conditions in the CWs (Sheoran and Sheoran, 2006). Gravel and sand (Calheiros et al., 2008), slag (Ge et al., 2015; He et al., 2017), crushed brick (Batool and Zeshan, 2017), limestone were used for removal of nutrients and metals in lab scale CWs. In this review, the advance treatment options in comparison to natural treatment will be first described, in order to give an overall contextual before getting into the comprehensive discussion later. The types of constructed wetlands and removal of metals by various plants will then be summarized. Subsequently, the interactions of chelators and substrates with plants and their role as catalyst for removal of metals in the constructed wetland will be discussed. Finally, future research perspectives associated with the use of chelators and substrates in CWs will be pointed out. 2. Labscale new treatment methods There are several options for treatment of wastewater/leachate like biological treatment (aerobic & anaerobic), physical/chemical

Corresponding author. Corresponding author. E-mail addresses: [email protected] (A. Batool), tawfi[email protected] (T.A. Saleh).

∗∗

https://doi.org/10.1016/j.ecoenv.2019.109924 Received 7 September 2019; Received in revised form 3 November 2019; Accepted 4 November 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.

Please cite this article as: Ammara Batool and Tawfik A. Saleh, Ecotoxicology and Environmental Safety, https://doi.org/10.1016/j.ecoenv.2019.109924

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

Table 1 Different advance leachate treatment methods. S.#

Authors

Method

Organic & Inorganic

Removal (%)

1 2 3 4 5 6 7 8

Xie et al. (2014) Moradi and Ghanbari (2014) Chemlal et al. (2014) Hilles et al. (2016) Abu Amr et al. (2013) Kabuk et al. (2014) Zhang et al. (2014) He et al. (2015)

Anaerobic membrane bioreactor Coagulation, Fenton Advanced oxidation biological process Sodium persulfate/H2O2 based oxidation process Ozone per sulphate advanced oxidation Electrocoagulation Sequencing batch biofilm Micro-aerobic bioreactor

COD BOD, COD BOD, COD COD, NH3–N COD, NH4 TOC, TSS, N, NH3–N BOD, COD, TOC COD, TN

65 60, 75, 63, 80, 82, 78, 58,

treatment and natural treatment systems (Table 1). Physical treatment includes settling of leachate is mandatory so that chemical compounds and solid particles may be collected at the bottom before it goes for a second treatment. Chemical precipitation, coagulation, flocculation, ammonium removal, activated carbon adsorption, and advance oxidation process may be applied. Removal of organic compounds that are non biodegradable in nature and heavy metals is done by adding coagulant shown by (Moradi and Ghanbari, 2014). Whereas precipitation of organic matter with heavy metals after addition of precipitating reagents, water filtration may remove the remnants from leachate. Adhesion of gas bubbles with solid particles in suspension makes them float at surface is known as floatation process. The use of granular activated carbon or powdered activated carbon to adsorb particles to its surface with its attraction forces is also an effective way to remove suspended matter from leachate. Removal of ammonia is commonly done by ammonia stripping at pH 7 or below. At pH 11 ammonia gas formed and released from wastewater. Another process is the exchange of ions without inducing any change in structure. This ion exchange process is operated in batch and continuous mode in which resins are stirred in wastewater and removed after it settled down. Electrochemical methods include electrocoagulation, electro-oxidation, and electro floatation. These methods are effective in recovering nutrients and metals from solutions (Kabuk et al., 2014).

72 90 50 68 85, 70, 75 84, 80 74

Fig. 1. Different types of constructed wetlands based on the direction of flow.

combined in order to achieve higher treatment effect, especially for nitrogen. Hybrid systems comprise most frequently VF and HF systems arranged in a staged manner but, in general, all types of constructed wetlands could be combined in order to achieve more complex treatment efficiency. The flow pattern of leachate/wastewater classifies the type of wetland. Two main categories are surface flow and subsurface flow constructed wetland. Removal of toxicity (metals and inorganics) and chemical processing are common functions of vegetation. Sustainable growth of plants ensures the efficient working of wetland (Kadlec and Wallace, 2008) and growth habits of plants are categorized by surface of water.1) Emergent woody plants 2) Emergent soft tissue plant 3) Floating mats 4) Floating plants 5) Submerged aquatic plants. Dominating macrophytes are emergent soft tissue plants as they have rhizome system and extensive root. Common reed (P. australis), cattail (T. latifolia), bulrush (Scirpus).

3. Constructed wetlands as natural cost-effective treatment method Comparatively, natural treatment systems can remove organic and inorganic pollutants from landfill leachate at low cost. Natural systems are based on the biological systems for purification of wastewater without any external source of energy except solar energy. The process of purification is very slow because of higher retention time as compared to conventional treatment systems. Significance of natural treatment systems are: 1) Natural systems remain reliable in extreme weather and high loading rates of organic content. 2) Aesthetically good and provides habitat to wildlife, 3)Easily maintained without any specific skills 4) Cost-effective. Constructed wetlands are engineered systems that have been constructed and designed to utilize the natural processes involving soils, wetland vegetation, and the associated microbial communities to assist in treating wastewaters. Constructed wetlands are designed to take benefit of many of the same processes that occur in natural wetlands but in a controlled environment. The first experiments aimed at the possibility of wastewater treatment by wetland plants were undertaken by Käthe Seidel in Germany in the early 1950s at the Max Planck Institute (Seidel, 1955). Seidel then carried out numerous experiments aimed at the use of wetland plants for treatment of various types of wastewater, including phenol wastewaters (Seidel, 1976). Most of experiments were carried out in constructed wetlands with either horizontal or vertical subsurface flow, but the first fully constructed wetland was built with free water surface in the Netherlands in 1967 (De Jong, 1976). Various types of constructed wetlands (Fig. 1.) may be

3.1. Free water surface system Modified natural lagoons of 0.3–0.4 m depth are free water surface system constructed wetlands (FWSCW). Wastewater/leachate flows through stems and leaves of plants. Microbes also play an important role in rhizomes of plants. Coverage of plants in free water surface system may not be uniform and homogenous (Fig. 2 a). Burgoon et al. (1999) employed a series of free surface wetlands followed by a vertical flow wetland and denitrifying free surface wetlands to provide treatment of potato processing wastewater (Table 2). The system was subjected to 0.5 kg/m3d COD loading. Overall NH4–N, COD, organic N, and TN removals ranged between 72, 92 and 94%, and 84%, 66 and 63%, and 42 and 60%, respectively. The performance of FWS CW system was also studied for domestic wastewater treatment with theoretical hydraulic retention times of 7, 5 and 10 days. Important parameters, such as NH4–N, BOD5, COD, TSS, PO4–P, DO, pH and fecal coliforms in both raw and treated wastewaters were monitored during a macrophytes life cycle. Based on the studies, it is concluded that minimum 5 days hydraulic retention time (HRT) is necessary for the treatment of wastewater in FWSCW using C. Lilies or T. latifolia (Shrikhande et al., 2014). Compared to other intensive (high-rate) anaerobic and aerobic 2

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

Fig. 2. Types of constructed wetland (Garcia et al., 2006; Vymazal, 2001, 2010).

et al., 2018) evaluated the performance of pilot-scale vertical subsurface flow constructed wetlands planted with three plant species, i.e. C. alternifolius, T. latifolia, and C. dactylon, in removing heavy metals from secondary treated refinery wastewater. Whereas removal of ibuprofen, acet-aminophen, diclofenac, tonalide, oxybenzone, triclosan, ethinylestradiol, bisphenol A was also studied in subsurface vertical flow constructed wetland (Avila et al., 2014). Sand media provided a larger available surface area for microbial growth, as well as higher oxygen availability which worked efficiently for removal of pollutants. Ge et al. (2015) Investigated removal of nutrients by combining free surface and horizontal subsurface wetland in presence of macrophytes. The effect of

treatment options (e.g. activated sludge), constructed wetlands are natural systems, which work efficiently and extensively. However, treatment may require land and time, but it is cost-effective and require lower operation with no electricity input. 3.2. Subsurface flow system Water flow in contact with roots and substrates and wetland depth is 0.3–0.9 m. This system provides an environment for the proliferation of biofilm and for the removal of pollutants. Common reed and bulrush are commonly used in subsurface flow systems. A study by (Mustapha 3

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

Table 2 Type of wastewater treated in different types of constructed wetlands. Studies

Trace metals

Type of CW

Retention time (days)

Substrates

Types of wastewater

Plant Species

Removal %

1.

Ashraf et al. (2018)

VF

45

Coconut Shavings

Tannery effluent

B. mutica

2.

Mustapha et al. (2018)

VSFCW

14

Gravel & sand

Refinery wastewater

T. latifolia

3.

Sima et al. (2017)

HSSF

25

Gravels

Municipal wastewater

P. australis

4.

He et al. (2015)

HSSF

5

Zeolite

Leachate

T. latifolia, P. australis

95 82 86 84 83 77 95 50 86 71 90

5.

Fountoulakis et al. (2017)

Cr COD Cu Cr Zn Pb Cd Pb Cd Cr Cr As Cd Hg Pb B

HF

4

Coarse gravels

Domestic wastewater

6.

Gill et al. (2017)

FSCW

Soil

Highway runoff

7.

Papaevangelou et al. (2017)

Cd Cu Pb Zn Cr

T.Parviflora L.Monopetal J. Acutus P. australis T. latifolia

8.

Bakhshoodeh et al. (2016)

Cu, Pb, Zn, Cr

9.

Rai et al. (2015)

10.

Kumari and Tripathi, 2015

Pb Cu Zn As Cr Cu Pb Zn

HSF VF HSSF

6, 8, 20

Fine gravels

Municipal wastewater

P. australis

5

Gravels

Compost leachate

T. latifolia P. australis

HSSF

14

Gravels

Urban sewage

T. latifolia P. australis

HSSF

15

Gravel & sand

Secondary treated wastewater

T. latifolia P. australis

41

95 88 86 95 90 80 86 84 83 82 81 86 84 83 82 81 55 50 71

interspecific competition was notable for P. australis, whereby it showed the highest growth performance in both FWS and SSF wetlands (Table 2).

removed in vertical flow system. Efficient removal of nutrients, metals in vertical flow constructed was investigated by several authors (Saeed and Sun, 2011).

3.3. Horizontal flow system

3.5. Hybrid systems

Through granular bed water flows horizontally with a depth of 0.3–0.9 m with 0.05–0.1 cubic meter flow rate (García, 2011). A horizontal flow system has a good distribution network of pipes and varies with feasibility (Fig. 2 b). Coarse gravels are filled inlet and outlet for filtration purposes. P. australis is commonly planted in HSF (Table 2). Papaevangelou et al. (2017) reported removal of chromium in horizontal and vertical flow constructed wetland planted with macrophytes. Results showed that pilot-scale vertical flow units exhibited lower performance in comparison to horizontal flow units, with lower removal capacity and higher effluent concentrations. Removal of chromium can be performed in two units of horizontal flow wetland with high chromium removal in horizontal flow constructed wetland.

Hybrid systems are a combination of horizontal and vertical SSFs for the removal of nitrogen and nitrates (Fig. 2 d). Similarly different other combinations are also possible using different types of wetland vertical SSFs with FWS etc. Recently multistage hybrid system was used by (Vymazal and Březinová, 2016) for removal of nitrogen and COD. The system achieved 83% and 79% removal of COD and nitrogen. He et al. (2017) investigated two horizontal subsurface flow constructed wetlands (HSSF CWs, down-flow (F1) and up-flow for removal of COD, ammonia, and metals. Results showed significant removal of ammonia as compared to other parameters. 4. Catalytic role of constructed wetland for phytoremediation process

3.4. Vertical flow system Removal mechanisms of toxic compounds, metals, organic and inorganic pollutants rely on physical, biological, chemical processes in different types of CW. Death, decay and growth of plants play a role in biogeochemical cycle. Overall, constructed wetlands provide a suitable environment for plants to play a healthy and positive role for removal of metals from wastewater without impairment to their health. During the growth of plants, maximum pollutants are removed by macrophytes and also providing suitable environment to microbes for proliferation. Macrophytes assimilate metals in their tissues of roots and

Water flows vertically downward through a bed of substrates and not flooded permanently (Fig. 2 c). As compared to horizontal flow treatment capacity of vertical flow system is high with same organic loading rate (Table 2). At the same time underground piping systems can be clogged as pipes are 0.05–0.01 buried under soil. Yalcuk and Ugurlu (2009) investigated the removal of COD, ammonia, and metals in presence of two vertical flow units in combination with one horizontal flow unit. Results showed that ammonia was efficiently 4

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

There is a diverse array of organic matter characterizing volatile solids and total carbon gives the measurement of total organic matter, COD is organic matter which is chemically oxidized and biologically degradable is BOD. In free water surface wetland, dissolved organic matter is degraded by photolysis, sorption, oxidation, and biodegradation. Macrophytes also help in supplying small amount of oxygen. Oxygen is depleted with increase of depth in CW with dense population of macrophytes. Whereas aerobic conditions have been observed in CW with submerged plants. The surface has aerobic conditions in horizontal SSF and biological processes are anaerobic in nature. In vertical constructed wetlands organic matter is degraded aerobically. Filtration in horizontal SSF retains particulate matter by inlet. Abiotic fragmentation converts particulate matter to smaller particles and extra cellular enzymes hydrolyze them. Fermentative bacteria or aerobic heterotrophs excrete these enzymes. Aerobic heterotrophs oxidize organic compounds of low molecular weight generated by hydrolysis.

Fig. 3. Interrelated factors in constructed wetland.

leaves. Constructed wetland is used to treat industrial, municipal, acid mine drainage and agricultural wastewater. The mechanism for removal of pollutants through the constructed wetland is described below.

4.3. Inorganic matter Nitrogen, phosphorous and metals are inorganic in nature. The cycle of nitrogen is complex in wetlands and its control is challenging. It is found in organic nitrogen and ammonia form constituting peptides, proteins, urea, and nucleic acid. Depending upon pH and temperature NH4–N is found in ionized form NH4 which is predominant. Eutrophication is caused by excess nitrogen by discharging wastewater and leachate to ground and surface water. In the constructed wetland, organic ammonia is removed in same way as of TSS by physical processes (filtration, adsorption). Through microbial mechanism and depletion of oxygen, nitrification is converted to denitrification. Carbon and nitrogen cycles are coupled through process of denitrification. Autotrophs bacteria in presence of oxygen convert organic ammonia to nitrate and nitrite. While denitrification is carried out by heterotrophic bacteria.

4.1. Suspended solids Solids retained on filter paper of glass fiber with a pore size of 1.2 μm are termed as total suspended solid including settable solids and dissolved solids. TSS is removed naturally in CW by interception and sedimentation. Chemical precipitation of pollutants combined with internally generated SS including plant's detritus and microbes (Fig. 3). Denser and larger particles are removed in CW by settling theory. Filtration is not a significant process in FWS wetland but particles adhere to plant surfaces. Usually, plant surfaces are coated with periphyton biofilm. In horizontal subsurface flow wetland removal of TSS takes place near inlet. As wastewater/leachate flows through gravel bed suspended solids are exponentially decreased, removal was achieved up to 90% with 20 mg L−1 hydraulic loading rate. At the same time high HLRs may clog the filter media by reducing the removal efficiency.

4.4. Metals Removal of metals in a constructed wetland is a complicated process including plant uptake, abiotic and biotic reaction (flocculation, sedimentation, precipitation, exchanges of cations and anions, reduction and oxidation). Total removal of metals is not possible but their physical and chemical properties are changed. Work on removal of metals

4.2. Organic matter The main constituent of raw wastewater is organic matter. Removal of organic matter involves biological, chemical and physical processes. Table 3 Metal removal studies in different types of constructed wetlands. Trace Metals

Type of CW

Retention time (days)

Number of chambers

Removal (%)

References

Cu, Zn, Pb Zn, Pb, Cd Fe, Zn, Cu, Hg Cr, Zn, Ni

SF HSSF SF,HSSF SF

120 – 504 168

2 1 3 11

Crites et al. (1997) Lim et al. (2003) Kamal et al. (2004) Hadad et al. (2006)

Cu, Pb, Zn Cr, Cu, Pb Cr, Ni, Fe, Zn Zn, Cr Cu, Pb, Cr, Ni

SF BR SF BR SF

48 360 168–288 48–360 40

1 6 3 8 11

Cu, Pb, Zn, Cr

HSSF

90

2

Cr, As, Ni, Cd, Pb

HSSF

–

2

78, 81, 73 80, 87, 66 99.8, 76.7, 33.9,41.6 82, 55, 69 85, 71, 88 75, 83, 69 66, 72, 61, 70 76, 61 48, 50, 89, 74 39, 29, 35, 52 85, 38, 83, 90, 97

5

Nelson et al. (2006) Zhang et al. (2007) Maine et al. (2009) Mishra and Tripathi (2009) Khan et al. (2009)

Bakhshoodeh et al. (2016)

He et al. (2017)

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

wetland. In a mixed-culture, P. australis demonstrates superiority in terms of competitive interactions for space between plants (Batool, 2019). Other plant species like C. indica, V. zizanioides are also planted in wetlands for removal of metals. A variety of different emergent, macrophytes have experimented in constructed wetlands for the removal of metals. T. latifolia and P. australis are most important among all hyperaccumulators. Vymazal and Březinová (2016) found properties of P. arundinacea similar to P. australis. Organics, inorganics, and metals are efficiently removed in presence of these plants (Scholz and Hedmark, 2010) by absorption and storage in roots. Moreover, fewer metals are translocated to shoots of plant. Long-distance translocation of metals between roots and shoots is described by Lu et al. (2013). Meager translocation of metals by roots of plants may be due to sequestration in vacuoles of roots thus natural action of defense to remediate potential toxic effects of metals (Shanker et al., 2005). Acetate, oxalate, malonate, oxalate, and citrate are anions excreted as root exudates performing as chelators for metallic ions (Ryan et al., 2001). Substrates help plants in mechanism of rhizodeposition of metals and perform role of catalyst involving organic acids. Detoxification mechanism towards metals stress is multigenic adaptability of a plant (Rai et al.. 2015). Biomineralization leading towards precipitation of metals is the resistance mechanism of plants toward excessive exposure. Another mechanism is complex formation with enzyme glutathione (GSH) and transportation to vacuoles of roots. Thirdly plant produces organic ligands enriched innon protein (NP– SH) thiols such as metallothioneins and phytochelatins and cysteine (Verbruggen et al., 2009). Phytochelatins help in chelating complexes and metals, later transporting them to vacuoles. Metallothioneins protect against oxidative stress and help to bind metals and maintain homeostasis (Palmer and Guerinot, 2009). Monferrán et al. (2012) found that short exposure of Cu to P. pusillus responded by producing antioxidant enzymes named peroxidase, glutathione reductase, and glutathione peroxidase. Constitutive feature of wetland species is metal tolerance, for example, T. latifolia and P. australis have constitutive tolerance towards Zn (Vymazal and Březinová, 2016). Bioconcentration and translocation factors (TF) are the best way to measure the efficiency of plants for trace element accumulation and

in CW and mesocosms by different researchers is shown in Table 3. Mesocosms are simulation of CW at labscale (Batool and Zeshan, 2017). Vegetation plays an important role in removal of metals either by process of phytostabalization, phytoaccumulation, phytovolatization. Mustapha et al. (2018) reported efficient removal of chromium and iron by T. latifolia. The metals were taken up into the (stem, leaves, and roots) parts of the plants, with the roots being the most significant. Recently various studies revealed importance of plants for removal of metals (Khan et al., 2009). At the same time, bioavailability of metals also influences their removal by plants in constructed wetland (Vymazal and Březinová, 2016). Detail of metal removal by plants and substrates is given in the following section. 4.5. Role of plants in constructed wetlands for removal of contaminants Vegetation plays a fundamental role in wetland treatment system by transferring oxygen through their roots to the bottom of treatment wetlands, and by providing a medium beneath the water surface for the attachment of microorganisms that perform the biological treatment. The plants used frequently in constructed wetlands include water hyacinth, cattails, reeds, duckweed, and rushes. Reeds grow along the shoreline and in water up to 1.5 m but are poor competitors in shallow waters; they are selected for SFS systems because the depth of rhizome penetration allows for the use of deeper basins (Mustapha et al., 2018). Water Hyacinth (E. crassipes) is an aquatic plant that grows very vigorously and uses highly the nutrients in the environment. The growth rate of water hyacinth is affected by water quality, nutrient content, harvesting interval, and solar constructed wetland radiation. Aquatic plants have ability to uptake trace metals; this phenomenon has brought wetlands to new scale of treatment. Usually native plants are chosen for removal of pollutants. Plants should be readily available in case of severe plant damage or harvesting. Perennial plants should be selected to guarantee continuity of treatment and operation with a sustainable growth of two seasons. P. australis and T. latifolia have good growth with properties of hyperaccumulation for different heavy metals (Table 4). Zhang et al. (2007) reported interspecies competition was notable for P. australis, whereby it showed the highest growth performance in both SSF and FWS Table 4 Metal accumulation by different plants in constructed wetlands. Species Name

Common name

Metals

Type of CW

Influent mg L−1

Shoots mg kg−1

Roots mg kg−1

Location

Reference

P. australis

Reed

HSSF

Calheiros et al. (2008) Maddison et al. (2009)

Reed

Italy

Galletti et al. (2010)

T. latifolia

Cattail

Gahna

Anning et al. (2013)

T. latifolia

Cattail

14.4 1.62 4.20 55 470 17 66.2 30 38 5.8 38.30 9.00 4.00

Portugal Estonia

P. australis

4.38 0.14 1.26 4.1 8.4 11.2 111.1 65 30 4 2.60 <1 < 1.5

Peverly et al. (1995)

Reed Cattail

28.5 99.9 150

USA

P. australis T. latifolia

France

Salem et al. (2014)

V. zizanioides V. zizanioides

Vetiver grass Vetiver grass

–

–

Pakistan Zimbabwe

Batool and Baig (2015) Mudhiriza et al. (2015)

V. zizanioides

Vetiver grass

Cu Pb Cr Cu Zn Cu Zn Cu Zn Pb Cu Cr As Cu Zn Mg Ni Cr Ni Pb Zn Mn Ni Cu Zn Pb

3.52 9.54 3.93 7.51

16.50 21.50 20.50 17.52

Iran

Bakhshoodeh et al. (2016)

India

Bhattacharya et al. (2006)

Scripus littoralis

HSSF FWSF HSSF SSF

6800, 173000 0.31 0.42 0.19

Integrated CW Mesocosm Labscale

HF

63.7 31.3 51.4 0.38 2.60 1.76 37.92 494.9 56.37 144.9 207.9 93.08

6

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

translocation from contaminated environments. The translocation and bioconcentration of C. esculenta, G. sagittatum and H. psittacorum in constructed wetlands, showed the important difference in levels of metals found in plant tissues may imply low mobility of metals from the root to shoots, a condition that was equally validated against the low TF values that in general terms remained < 1. In spite of the good bioaccumulation of the metals, it revealed a decreasing tendency in the order of Pb > Cd > Cr (VI) > Hg; and in general terms accumulation decreased in leaves. Rai et al. (2015) reported bioconcentration and translocation of three macrophytes in summer and winter season which showed that translocation factor was low (< 1) in all the studied macrophytes, T. latifolia, P. australis and C. esculenta in winter season except for Mn (1.01) in T. latifolia and As (1.25) in C. esculenta. Low TF indicates immobilization of trace elements from root to shoot or vacuole as in agreement with the findings of various authors (Maine et al., 2009).

Fig. 4. Different factors supported by substrates in constructed wetlands.

activity. Substrates with poor infiltration capacity reduce efficiency of system. Chemical characteristics of substrates will determine its adsorption capacity. Substrates with Al and Fe ions may help in reducing phosphate from wastewater. Substrates are cost-effective way to enhance phytoremediation without impairing any damage to plant's health like in case of chelators. Therefore substrates can be considered as efficient and harmless catalyst to enhance and improve phytoremediation process including Phytostabilization. The partitioning coefficient (Kd) of trace metal is defined as the ratio of trace metal concentration in the substrate to interstitial water/ leachate (Chapman and Wang, 2001) and is affected by environmental conditions (redox, pH, salinity, and temperature), biogeochemical processes and hydrodynamics. Commonly used substrates in constructed wetlands are gravel and sand. Washed gravels increase the filtration of wetlands and minimize clogging. For reed bed sandstone (Fluvio-glacial) are ideal. Gravel size for reed bed range between (3–12 mm). Gravels increase nitrification process and higher denitrification occurs in reed systems with soil as substrate. Plant growth can be supported by gravels, sand, and soil. Crushed stone may also provide support to plant's growth and also provide surface for microbial growth and ion exchange. Research on removal of pollutants from leachate by different substrates is shown in Table 5. Sorption of heavy metals involves reaction between cations and anions on the surface of adsorbent. Different waste materials like alum sludge, oyster shell, organic wood mulch, gravel wood mulch, zeolite, quartz, sand, gravels, slag, and crushed brick are used as adsorbent in different studies shown in Table 5. These waste materials were used as in situ immobilized sorbents for heavy metal and inorganics. Surface precipitation and specific adsorption by minerals present on the surface of these waste material determine the efficiency of these sorbents. Similarly, on organic molecules like lignin, humic substance, chitin, reaction of functional groups (carboxyl, hydroxyl, amine) with heavy metals explain the capacity of sorption of heavy metals. With surface ligands, anions and cations of heavy metals are exchanged to form partial covalent bonds with ions on the charged surface areas of the adsorbent. Studies in Table 5 explained that surface structure also played an important role as crystalline and amorphous surfaces have Mn, Fe, Al oxides with alumino-silicates. Blast furnace slag consists of calcium alumino-silicates with Al and Fe oxides whereas steel slag analysis showed presence of Fe oxides and calcium – iron oxides. Crushed brick (red mud) contains oxides of iron along with oxides of aluminum. Fly ash contains ash particles with amorphous ferro-alumino silicates (Zhu and Haynes, 2010). Outer and inner-sphere complexes are formed in adsorption reactions. Complexes in the out sphere are formed with one molecule of aqueous solution interposed between bound ions and functional groups involving electrostatic bonding between them. In inner-sphere complexes no solvent molecule is involved instead direct covalent bond is formed with functional group of surface. Proximity of distribution of ions of charged surface can be termed as “electrical double layer”. The

5. Spiking of chelators as a catalyst to enhance bioavailability In order to enhance bioconcentration chemical amendment like the addition of chelators is being used to improve the phytoextraction process. However, chelator assisted phytostabilization has widely been used to decontaminated soil. High accumulation of metals in shoots of plants with 0.5 mMol application of EDTA is indicated with less growth of plant as compared to control. Therefore, chelator may improve bioavailability of metals by compromising sustainable plant growth. Hyperaccumulator species are naturally capable to accumulate metals at high concentrations without using chelators (Kumari and Tripathi, 2015). Plants are categorized as hyperaccumulators based on their accumulation capacity of metals. Accumulation capacity of Cu and Pb is 1000 mg kg-1 and 10,000 mg kg-1 Zn (Anning et al., 2013). Whereas different environmental factors may affect their tolerance towards metals like salinity, temperature, and geography. Application of macrophytes like P. australis, T. latifolia, S. littoralis in constructed wetland is a promising solution for the removal of pollutants from different types of wastewater/effluent. Macrophytes with higher biomass production accumulate significant amount of metals in roots, shoots, and flowers. Marchand et al. (2010) reported that removal rates of metals are higher than 70% in constructed wetlands planted with common macrophytes like P. australis, T. latifolia. P. australis is a well-known hyperaccumulator of Cu, Cd, Ni, Cr and Pb (Kumari and Tripathi, 2015; Vymazal and Březinová, 2016). T. Latifolia has thick mass of roots which are an efficient accumulator of metals especially Cu, Zn, Pb, and Cd. Vetiver is a grass species strong affiliation for metals in soil and waste. V. zizanioides are known for hyperaccumulation of Cu and Pb. Rhodes grass has tolerance of Pb. Exposure to higher level of Pb may Pb to bending and swelling of roots.E. globulus is hyperaccumulator of Pb and Cu. It grows in all seasons. Pulp of E.globulus is also used for metal adsorption from biodiesel. Bark of E. globulus is used for adsorption of chromium. C. indica is used for removal of nutrients in constructed wetlands (Calheiros et al., 2007). Their easy growth and wide availability also make them an appropriate choice. The selection of appropriate plant species along with efficient substrates may significantly affect the performance of constructed wetlands. Besides techniques like biostimulation, bioaugmentation, genetically modified microbes or plants; progressive approach toward enhancing the accumulation property of macrophytes can be employed by introducing novel substrates that may improve mechanism of phytostabilization of metals in roots. 6. Role of substrates as a catalyst to enhance phytoremediation Substrates play the role of filtration, adsorption, sedimentation, flocculation, precipitation and ion exchange. Hydraulic permeability and adsorption capacity are the main characteristics of substrates (Fig. 4). They also provide foundation for plant's growth and microbial 7

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

Table 5 Studies of different substrates in constructed wetlands. Substrate

Type of CW

Metals

Sand & dolomite

VF

–

P

Oyster Shells Organic wood mulch, gravel wood mulch Alum Sludge

Integrated VF, HF

– –

N, P, TSS TN, TP, NH4–N

Integrated

Zeolite

Mesocosm

Alum sludge

Pilot scale CW

Sand, gravel Organic manure Gravels Sand & gravel Sand, gravel

VFCW VF IVF Mesocosm HSSF

Slag Slag, crushed brick

HSSF Mesocosm

– – – As, Zn Cr, Co, Pb, As – Cu, Zn

Gravels

mesocosm

B, Se

Cu, Fe, Zn, Mn –

Removal (%)

60, 40, 75, 58

65, 80 81, 76, 86, 82 80, 77 26-45, 50–69

Inorganics

Organic

Removal (%)

Reference

–

45

85, 98,94 97, 60, 99

BOD BOD

92 71

Prochaska and Zouboulis (2006) Park and Polprasert (2008) Saeed and Sun (2011)

TSS, TP, NH4–N NH4–N

97, 88, 87

BOD

96

Wu et al. (2011)

TN, TP, NH4–N

11-78, 75–94, 49-93

TP, TN TP, TKN TN, TP NH4–N, PO4–P TP – –

Removal (%)

–

97, 98 15, 52 91, 94 23

COD, BOD5 TOC, COD COD COD BOD BOD, COD COD

Zhao et al., 2011) 57-84, 36–84 96 62.8

–

–

(Zhao et al., 2011) (Y. J. Zhao et al., 2011) Korboulewsky et al. (2012) Chang et al. (2012) Arroyo et al. (2013) Rai et al. (2015) Ge et al. (2015) (Batool and Zeshan, 2017) Zhu and Bañuelos (2017)

Fig. 5. Mechanism of metal uptake by plants in presence of efficient substrates acting as catalyst.

Analysis of sand by X-ray diffraction revealed presence of hallousite (Al2Si2O5 (OH)4·2H2O), silicon oxide SiO2, iron oxide (FeO), and silicon phosphate oxide (SiP2O7), and anorthite (CaAl2Si2O8). Where XRD analysis of dolomite found calcite (CaCO3) and (CaMg(CO3)2). Authors found that oxide of iron performed efficiently in removal of phosphorous as compared to oxide of aluminum (Zhao et al., 2011). used alum sludge as substrates in constructed wetland for removal of phosphorous with high concentration of organic matter. A variety of waste materials have been used for removal of metals, organics and inorganics from different effluent. Substrates supports plants, bacteria, biofilms in constructed wetland along with mechanism of adsorption, absorption, sedimentation and filtration, Fig. 5. Presence of plants in efficient substrates provides them with organic matter thus reducing sulphate production with immobilization of metals (Marchand et al., 2010). Therefore appropriate selection of substrates may improve the removal efficiency of constructed wetlands.

process of adsorption can be visualized in three planes i) charged adsorbent surface ii) plane of adsorbent iii) thirdly in near surface water layer balancing of different ions in outside plane. A combination of adsorption of ions in inner sphere and outer sphere ions makes a charge on adsorbent surface. Charged surface maintains its electro neutrality by countering indifferent ions with equal and opposite in magnitude and charge to surface charge. Whereas pH of aqueous solutions also plays important role in these adsorption reaction in both inner and outer spheres. In particular steel slag has a highly reactive charged surface and therefore known for efficient removal of Ni, Cu, and Zn and also act as catalyst in combination to plants (Batool, 2018). A column experiment conducted at laboratory scale to reveal metal retention capacity of slag. Total 300L was fed to columns and > 60% of copper and zinc and 20% Ni was achieved respectively (Prochaska and Zouboulis, 2006). experimented sand and dolomite as substrates in constructed wetland. 8

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh

Batool, A., Baig, M.A., 2015. ‘Growth behavior comparison of three species exposed to municipal solid waste leachate in microcosm constructed wetland’. pp. 10–13. Batool, A., Zeshan, 2017. ‘Effect of chelators and substrates on phytoremediation of synthetic leachate for removal of trace elements’, soil and sediment contamination: an international journal. Taylor & Francis 26, 220–233. https://doi.org/10.1080/ 15320383.2017.1274958. Bhattacharya, T., Banerjee, D.K., Gopal, B., 2006. ‘Heavy metal uptake by scirpus littoralis schrad. from fly ash dosed and metal spiked soils’. Environ. Monit. Assess. 121, 363–380. https://doi.org/10.1007/s10661-005-9133-1. Burgoon, P.S., Kadlec, R., Henderson, M., 1999. Treatment of potato processing wastewater with engineered natural systems. Water Sci. Technol. 40 (3), 211–215. Calheiros, C.S.C., Rangel, A.O.S.S., Castro, P.M.L., 2007. ‘Constructed wetland systems vegetated with different plants applied to the treatment of tannery wastewater’. Water Res. 41 (8), 1790–1798. https://doi.org/10.1016/j.watres.2007.01.012. Calheiros, C.S.C., Rangel, A.O.S.S., Castro, P.M.L., 2008. ‘The effects of tannery wastewater on the development of different plant species and chromium accumulation in P. australis australis’. Arch. Environ. Contam. Toxicol. 55 (3), 404–414. https://doi.org/ 10.1007/s00244-007-9087-0. Chang, J., Wu, S., Dai, Y., Liang, W., Wu, Z., 2012. ‘Treatment performance of integrated vertical-flow constructed wetland plots for domestic wastewater’. Ecol. Eng. 44, 152–159. https://doi.org/10.1016/j.ecoleng.2012.03.019. Chapman, P.M., Wang, F., 2001. ‘Annual review assessing sediment contamination in estuaries’. Environ. Toxicol. Chem. 20 (1), 3–22. https://doi.org/10.1002/etc. 5620200102. Chemlal, R., Azzouz, L., Kernani, R., Abdi, N., Lounici, H., Grib, H., Mameri, N., Drouiche, N., 2014. ‘Combination of advanced oxidation and biological processes for the landfill leachate treatment’. Ecol. Eng. 73, 281–289. https://doi.org/10.1016/j. ecoleng.2014.09.043. Crites, R.W., Dombeck, G.D., Watson, R.C., Williams, C.R., 1997. ‘Removal of metals and ammonia in constructed wetlands’, water environment research. Water Environment Federation 69 (2), 132–135. https://doi.org/10.2307/25044854. De Jong, J., 1976. The Purification of Wastewater with the Aid of Rush or Reed Ponds. In: Tourbier, J., Pierson, R.W. (Eds.), Biological Control of Water Pollution. University of Pennsylvania press, pp. 133–139. Fountoulakis, M.S., Sabathianakis, G., Kritsotakis, I., Kabourakis, E.M., Manios, 2017. Halophytes as vertical flow constructed wetland vegetation for domestic wastewater treatment. Sci. Total Environ. 583 (1), 432–439. Galletti, A., Verlicchi, P., Ranieri, E., 2010. ‘Removal and accumulation of Cu, Ni and Zn in horizontal subsurface flow constructed wetlands: contribution of vegetation and filling medium’. Sci. Total Environ. 408 (21), 5097–5105. https://doi.org/10.1016/j. scitotenv.2010.07.045. García, J., 2011. Dvances in pollutant removal processes and fate in natural and constructed wetlands. Ecol. Eng. 37 (5), 663–665. Garcia, T., Angarita, S., Rodríguez, M., 2006. Classical subsurface flow wetland optimization to heavy metal removal. In: 10th Int. Conf. Wetl. Syst. Water Pollut. 23rd 29th Sept, Control.Lisbon, Portugal). Ge, Y., Wang, X., Zheng, Y., Dzakpasu, M., Zhao, Y., Xiong, J., 2015. ‘Functions of slags and gravels as substrates in large-scale demonstration constructed wetland systems for polluted river water treatment’, Environmental Science and Pollution Research. Springer Berlin Heidelberg 22 (17), 12982–12991. https://doi.org/10.1007/s11356015-4573-9. Gill, W.L., Casey, P.R.B., Higgins, N.M.P., Johnston, P.M., 2017. Longterm heavy metal removal by constructed wetland by treating rainfall runoff from a motorway. Sci. Total Environ. 601–601 (1), 32–44. Hadad, H.R., Maine, M.A., Bonetto, C.A., 2006. ‘Macrophyte growth in a pilot-scale constructed wetland for industrial wastewater treatment’. Chemosphere 63 (10), 1744–1753. https://doi.org/10.1016/j.chemosphere.2005.09.014. He, R., Wei, X.-M., Tian, B.-H., Su, Y., Lu, Y.-L., 2015. ‘Characterization of a joint recirculation of concentrated leachate and leachate to landfills with a microaerobic bioreactor for leachate treatment’. Waste Manag. 46, 380–388. https://doi.org/10. 1016/j.wasman.2015.08.006. He, H., Duan, Z., Wang, Z., Yue, B., 2017. ‘The removal efficiency of constructed wetlands filled with the zeolite-slag hybrid substrate for the rural landfill leachate treatment’, Environmental Science and Pollution Research, pp. 1–9. https://doi.org/10.1007/ s11356-017-9402-x. Herrera-Cárdenas, J., Navarro, A.E., Torres, E., Herrera-C Ardenas, J., 2016. ‘Effects of porous media, macrophyte type and hydraulic retention time on the removal of organic load and micropollutants in constructed wetlands’, Journal of Environmental Science and Health, Part A Toxic/Hazardous Substances and Environmental EngineeringOnline). J.ISSN Homepage 1093–4529. https://doi.org/10.1080/ 10934529.2015.1120512. Hilles, A.H., Abu Amr, S.S., Hussein, R.A., El-Sebaie, O.D., Arafa, A.I., 2016. ‘Performance of combined sodium persulfate/H2O2 based advanced oxidation process in stabilized landfill leachate treatment’. J. Environ. Manag. 166, 493–498. https://doi.org/10. 1016/j.jenvman.2015.10.051. Kabuk, H.A., İlhan, F., Avsar, Y., Kurt, U., Apaydin, O., Gonullu, M.T., 2014. ‘Investigation of leachate treatment with electrocoagulation and optimization by response surface methodology’, CLEAN. Soil, Air, Water 42 (5), 571–577. https://doi. org/10.1002/clen.201300086. Kadlec, R.H., Wallace, S., 2008. Treatment Wetlands 2nd Edition. CRC Press, Boca Raton. Kamal, M., Ghaly, A.E., Mahmoud, N., Côté, R., 2004. ‘Phytoaccumulation of heavy metals by aquatic plants’. Environ. Int. 29 (8), 1029–1039. https://doi.org/10.1016/ S0160-4120(03)00091-6. Khan, S., Ahmad, I., Shah, M.T., Rehman, S., Khaliq, A., 2009. ‘Use of constructed wetland for the removal of heavy metals from industrial wastewater’. J. Environ. Manag. 90 (11), 3451–3457. https://doi.org/10.1016/j.jenvman.2009.05.026.

6.1. Future perspectives The present approach to enhance phytoremediation by the catalytic role of substrates, chelators and constructed wetland still have a potential need for future research. 1. Bioavailability of metals by the use of chelators develops healthy competition for metal removal by plants and substrates. This complex mechanism needs detailed research to understand underlying complex chemical exchanges of ions along with a breakdown of complicated metal compounds. 2. Life cycle assessment and fate of new substrates also need to be determined to explore the recycling perspectives of cost-effective substrates. 3. A dose of chelators in the presence of new substrates needs to be assessed; to reduce treatment cost as well as poor health effects on plants. 4. The role of microbial community must be considered while studying the interactive ecological environment of plants with substrates in a constructed wetland. 7. Conclusions Overall, this paper reviews phytoremediation of metals from wastewater, summarizes factors enhancing the phytoremediation efficiency and elucidates the treatment. Chelators enhance bioavailability of metals while substrates trap metals and reduce metal stress on plants. A suitable growing environment for plants is provided by different types of constructed wetlands. pH, the temperature in constructed wetland also plays an important role in effect the mechanisms of metals removal by chelators, substrates, and plants. Although more research is needed to understand the detailed mechanism of metal removal by use of chelators and substrates in constructed wetlands to further elucidate the treatment process. Conflict of interest There is no conflict of interest. References Abu Amr, S.S., Aziz, H.A., Adlan, M.N., 2013. ‘Optimization of stabilized leachate treatment using ozone/persulfate in the advanced oxidation process’. Waste Manag. 33 (6), 1434–1441. https://doi.org/10.1016/j.wasman.2013.01.039. Angassa, K., Leta, S., Worku, Mulat, Kloos, H., Meers, E., 2018. Organic matter and nutrient removal performance of horizontal subsurface flow constructed wetlands planted with phragmite karka and V. zizanioides zizanioide for treating municipal wastewater. Env Processes 5, 115–130. Anning, A.K., Korsah, P.E., Addo-Fordjour, P., 2013. ‘Phytoremediation of wastewater with Limnocharis flava, Thalia geniculata and T. latifolia latifolia in constructed wetlands.’. Int. J. Phytoremediation 15 (5), 452–464. https://doi.org/10.1080/ 15226514.2012.716098. Arroyo, P., Ansola, G., Miera, L.E.S., de, 2013. ‘Effects of substrate, vegetation and flow on arsenic and zinc removal efficiency and microbial diversity in constructed wetlands’. Ecol. Eng. 51, 95–103. https://doi.org/10.1016/j.ecoleng.2012.12.013. Ashraf, S., Afzal, M., Rehman, K., Naveed, M., Zahir, A.Z., 2018. Plant-endophyte synergism in constructed wetlands enhances the remediation of tannery effluent. Water Sci. Technol. 5, 1262–1270. Avila, C., Nivala, J., Olsson, L., Kassa, K., Headley, T., Mueller, R.A., Bayona, J.M., Garci'a, J., 2014. ‘Emerging organic contaminants in vertical subsurface flow constructed wetlands: influence of media size, loading frequency and use of active aeration’. Sci. Total Environ. 211–217. https://doi.org/10.1016/j.scitotenv.2014.06. 128. Elsevier B.V., 494–495. Bakhshoodeh, R., Alavi, N., Soltani Mohammadi, A., Ghanavati, H., 2016. ‘Removing heavy metals from Isfahan composting leachate by horizontal subsurface flow constructed wetland’. Environ. Sci. Pollut. Control Ser. 23 (12), 12384–12391. https:// doi.org/10.1007/s11356-016-6373-2. Batool, Ammara, 2018. Phytoaccumulation of heavy metals from solid waste leachate using constructed wetland. PhD Thesis. National University of Sciences and Technology, Islamabad (Pakistan HEC Repository). Batool, A., 2019. Metal accumulation from leachate by polyculture in crushed brick and steel slag using pilot-scale constructed wetland in the climate of Pakistan. Environ. Sci. Pollut. Res. 1 (1), 10–65.

9

Ecotoxicology and Environmental Safety xxx (xxxx) xxxx

A. Batool and T.A. Saleh Korboulewsky, N., Wang, R., Baldy, V., 2012. ‘Purification processes involved in sludge treatment by a vertical flow wetland system: focus on the role of the substrate and plants on N and P removal’. Bioresour. Technol. 105, 9–14. https://doi.org/10.1016/ j.biortech.2011.11.037. Kumari, M., Tripathi, B.D., 2015. Efficiency of Phragmites australis and Typha latifolia for heavy metal removal from wastewater. Ecotoxicol. Environ. Saf. 112, 80–86. https:// doi.org/10.1016/j.ecoenv.2014.10.034. Lim, P.E., Mak, K.Y., Mohamed, N., Noor, A.M., 2003. ‘Removal and speciation of heavy metals along the treatment path of wastewater in subsurface-flow constructed wetlands.’. Water Sci. Technol.: a J. Int. Assoc. Water Pollut. Res. 48 (5), 307–313. http://www.ncbi.nlm.nih.gov/pubmed/14621178, Accessed date: 11 July 2017. Lu, L., Tian, S., Yang, X., Peng, H., Li, T., 2013. ‘Improved cadmium uptake and accumulation in the hyperaccumulator Sedum alfredii: the impact of citric acid and tartaric acid.’. J. Zhejiang Univ. - Sci. B 14 (2), 106–114. https://doi.org/10.1631/jzus. B1200211. Zhejiang University Press. Maddison, M., Soosaar, K., Mauring, T., Mander, Ü., 2009. ‘The biomass and nutrient and heavy metal content of cattails and reeds in wastewater treatment wetlands for the production of construction material in Estonia’. Desalination 246 (1–3), 120–128. https://doi.org/10.1016/j.desal.2008.02.040. Maine, M.A., Hadad, N.S.H., Sanchez, G., Bonetto, C., 2009. ‘Influence of vegetation on the removal of heavy metals and nutrients in a constructed wetland’, Journal of Environmental Management Journal of Environmental Management 90, 355-363. Marchand, L., Mench, M., Jacob, D.L., Otte, M.L., 2010. ‘Metal and metalloid removal in constructed wetlands, with emphasis on the importance of plants and standardized measurements: a review’. Environ. Pollut. 158 (12), 3447–3461. https://doi.org/10. 1016/j.envpol.2010.08.018. Elsevier Ltd. Mishra, V.K., Tripathi, B.D., 2009. ‘Accumulation of chromium and zinc from aqueous solutions using water hyacinth (Eichhornia crassipes)’. J. Hazard Mater. 164 (2–3), 1059–1063. https://doi.org/10.1016/j.jhazmat.2008.09.020. Monferrán, M.V., Pignata, M.L., Wunderlin, D.A., 2012. Enhanced phytoextraction of chromium by the aquatic macrophyte Potamogeton pusillus in presence of copper. Environ. Pollut. 161, 15–22. https://doi.org/10.1016/j.envpol.2011.09.032. Moradi, M., Ghanbari, F., 2014. ‘Application of response surface method for coagulation process in leachate treatment as pretreatment for Fenton process: biodegradability improvement’. J. Water Process Eng. 4, 67–73. https://doi.org/10.1016/j.jwpe.2014. 09.002. Mudhiriza, T., Mapanda, F., Mvumi, B., Wuta, M., 2015. ‘Removal of nutrient and heavy metal loads from sewage effluent using vetiver grass, Chrysopogon zizanioides (L.) Roberty’, Water SA. Water Research Commission (WRC) 41 (4), 457. https://doi.org/ 10.4314/wsa.v41i4.04. Mustapha, H.I., Van Bruggen, J.J.A., Lens, P.N.L., 2018. Fate of heavy metals in vertical subsurface flow constructed wetlands treating secondary treated petroleum refinery wastewater in Kaduna, Nigeria. Int J. Phyt 20 (1), 44–53. Nelson, E.A., Specht, W.L., Knox, A.S., 2006. ‘Metal removal from water discharges by a constructed treatment wetland’, engineering in life sciences. WILEY-VCH Verlag 6 (1), 26–30. https://doi.org/10.1002/elsc.200620112. Palmer, C.M., Guerinot, M. Lou, 2009. ‘Facing the challenges of Cu, Fe and Zn homeostasis in plants’. Nat. Chem. Biol. 5 (5), 333–340. https://doi.org/10.1038/ nchembio.166. Papaevangelou, V.A., Gikas, G.D., Vryzas, Z., Tsihrintzis, V.A., 2017. Treatment of agricultural equipment rinsing water containing a fungicide in pilot-scale horizontal subsurface flow constructed wetlands. Ecol. Eng. 101, 193–200. Park, W.H., Polprasert, C., 2008. ‘Roles of oyster shells in an integrated constructed wetland system designed for P removal’. Ecol. Eng. 34 (1), 50–56. https://doi.org/10. 1016/j.ecoleng.2008.05.014. Peverly, J.H., Surface, J.M., Wang, T., 1995. ‘Growth and trace metal absorption by P. australis australis in wetlands constructed for landfill leachate treatment’. Ecol. Eng. 5 (1), 21–35. https://doi.org/10.1016/0925-8574(95)00018-E. Prochaska, C., Zouboulis, A.I., 2006. ‘Removal of phosphates by pilot vertical-flow constructed wetlands using a mixture of sand and dolomite as substrate’. Ecol. Eng. 26 (3), 293–303. https://doi.org/10.1016/j.ecoleng.2005.10.009. Rai, U.N., Upadhyay, A.K., Singh, N.K., Dwivedi, S., Tripathi, R.D., 2015. ‘Seasonal applicability of horizontal sub-surface flow constructed wetland for trace elements and nutrient removal from urban wastes to conserve Ganga River water quality at Haridwar, India’. Ecol. Eng. 81, 115–122. https://doi.org/10.1016/j.ecoleng.2015. 04.039. Ryan, P., Delhaize, E., Jones, D., 2001. ‘Function and m echanism of o rganic a nion e xudation from p lant r oots’, Annual Review of Plant Physiology and Plant Molecular Biology. Annual Reviews 4139 El Camino Way, P.O. Box 10139, Palo Alto, CA 943030139. USA 52 (1), 527–560. https://doi.org/10.1146/annurev.arplant.52.1.527. Saeed, T., Sun, G., 2011. ‘A comparative study on the removal of nutrients and organic

matter in wetland reactors employing organic media’. Chem. Eng. J. 171 (2), 439–447. https://doi.org/10.1016/j.cej.2011.03.101. Salem, Ben, Z., Laffray, X., Ashoour, A., Ayadi, H., Aleya, L., 2014. ‘Metal accumulation and distribution in the organs of Reeds and Cattails in a constructed treatment wetland (Etueffont, France)’. Ecol. Eng. 64, 1–17. https://doi.org/10.1016/j.ecoleng. 2013.12.027. Scholz, M., Hedmark, Å., 2010. ‘Constructed wetlands treating runoff contaminated with nutrients’. Water Air Soil Pollut. (205), 323–332. https://doi.org/10.1007/s11270009-0076-y. Seidel, K., 1955. Die flechtbinse scirpus lacustris. In: Okologie, " Morphologie und Entwicklung, ihre Stellung bei den Volkern und ihre wirtschaftliche Bedeutung. Schweizerbart˅ısche Verlagsbuchnadlung. Germany, Stuttgart, pp. 37–52 ISBN 9783-510-40730-9. Seidel, K., 1976. Macrophytes and Water Purification. In: Tourbier, J., Pierson, R.W. (Eds.), Biological Control of Water Pollution. Pennsylvania University Press, Philadelphia, pp. 109–122. Shanker, Cervantes, Lozatavera, Avudainayagam, 2005. ‘Chromium toxicity in plants’. Environ. Int. 31 (5), 739–753. https://doi.org/10.1016/j.envint.2005.02.003. Sheoran, A.S., Sheoran, V., 2006. Heavy metal removal mechanism of acid mine drainage in wetlands: a critical review. Miner. Eng. 19, 105–116. Shrikhande, A.N., Nema, P., Mhaisalkar, V.A., 2014. Performance of free water surface constructed wetland using typhalatifolia and Canna lilies for the treatment of domestic wastewater. J. Environ. Sci. Eng. 56 (1), 93–104. Sima, J., Svoboda, L., Šeda, M., Krejsa, J., Jahodová, J., 2017. ‘Removal of selected risk elements from wastewater in a horizontal subsurface flow constructed wetland’, Water and Environment Journal, (Ranieri 2004). https://doi.org/10.1111/wej. 12269. Verbruggen, N., Hermans, C., Schat, H., 2009. ‘Mechanisms to cope with arsenic or cadmium excess in plants’. Curr. Opin. Plant Biol. 12 (3), 364–372. https://doi.org/ 10.1016/j.pbi.2009.05.001. Vymazal, J., 2001. Types of constructed wetlands for wastewater treatment: their potential for nutrient removal. In: Vymazal, J. (Ed.), Transformations of Nutrients in Natural and Constructed Wetlands. Backhuys Publishers, Leiden, The Netherlands, pp. 1–93. Vymazal, J., 2010. Constructed wetlands for wastewater treatment. Water 2, 530–549. https://doi.org/10.3390/w2030530. Vymazal, J., Březinová, T., 2016. ‘Accumulation of heavy metals in aboveground biomass of P. australis australis in horizontal flow constructed wetlands for wastewater treatment: a review’,. Chem. Eng. J. 290, 232–242. https://doi.org/10.1016/j.cej. 2015.12.108. Wu, S., Vymazal, J., Brix, H., 2019. Critical review: biogeochemical networking of iron in constructed wetlands for wastewater treatment. Environ. Technol. 53, 7930–7944. Wu, S., Austin, D., Liu, L., Dong, R., 2011. ‘Performance of integrated household constructed wetland for domestic wastewater treatment in rural areas’. Ecol. Eng. 37 (6), 948–954. https://doi.org/10.1016/j.ecoleng.2011.02.002. Xie, Z., Wang, Z., Wang, Q., Zhu, C., Wu, Z., 2014. ‘An anaerobic dynamic membrane bioreactor (AnDMBR) for landfill leachate treatment: performance and microbial community identification’. Bioresour. Technol. 161, 29–39. https://doi.org/10.1016/ j.biortech.2014.03.014. Yalcuk, A., Ugurlu, A., 2009. ‘Comparison of horizontal and vertical constructed wetland systems for landfill leachate treatment’. Bioresour. Technol. 100 (9), 2521–2526. https://doi.org/10.1016/j.biortech.2008.11.029. Elsevier Ltd. Zhang, X., Liu, P., Yang, Y., Chen, W., 2007. ‘Phytoremediation of urban wastewater by model wetlands with ornamental hydrophytes.’. J. Environ. Sci. 19 (8), 902–909. http://www.ncbi.nlm.nih.gov/pubmed/17966845, Accessed date: 11 July 2017. Zhang, D.-B., Wu, X.-G., Wang, Y.-S., Zhang, H., 2014. ‘Landfill leachate treatment using the sequencing batch biofilm reactor method integrated with the electro-Fenton process’, Chemical Papers. Versita 68 (6), 782–787. https://doi.org/10.2478/s11696013-0504-8. Zhao, Y.Q., Babatunde, A.O., Hu, Y.S., Kumar, J.L.G., Zhao, X.H., 2011. ‘Pilot field-scale demonstration of a novel alum sludge-based constructed wetland system for enhanced wastewater treatment’. Process Biochem. 46 (1), 278–283. https://doi.org/ 10.1016/j.procbio.2010.08.023. Zhu, H., Bañuelos, G., 2017. ‘Evaluation of two hybrid poplar clones as constructed wetland plant species for treating saline water high in boron and selenium, or waters only high in boron’. J. Hazard Mater. 333, 319–328. https://doi.org/10.1016/j. jhazmat.2017.03.041. Zhu, Y.-F., Haynes, R.J., 2010. ‘Sorption of heavy metals by inorganic and organic components of solid wastes: significance to use of wastes as low-cost adsorbents and immobilizing agents’. Crit. Rev. Environ. Sci. Technol. 40 (11), 909–977. https://doi. org/10.1080/10643380802586857.

10