Phytoremediation—from Environment Cleaning to Energy Generation—Current Status and Future Perspectives

: Phytoremediation is a technology based on the use of green plants to remove, relocate, deactivate, or destroy harmful environmental pollutants such as heavy metals, radionuclides, hydrocarbons, and pharmaceuticals. Under the general term of phytoremediation , several processes with distinctively different mechanisms of action are hidden. In this paper, the most popular modes of phytoremediation are described and discussed. A broad but concise review of available literature research with respect to the dominant process mechanism is provided. Moreover, methods of plant biomass utilization after harvesting, with particular regard to possibilities of “bio ‐ ore” processing for metal recovery, or using energy crops as a valuable source for bio ‐ energy production (bio ‐ gas, bio ‐ ethanol, bio ‐ oil) are analyzed. Additionally, obstacles hindering the commercialization of phytoremediation are presented and discussed together with an indication of future research trends.


Introduction
The rapid development of the global industry, especially in the field of energy supply, agriculture, mining, metal, and chemical production or transport, contributes to the increase of air, soil, and water pollution [1][2][3][4].On the other hand, the rising environmental awareness of people affects pro-ecological activities, aiming to improve the quality and purity of the environment.It is crucial to maximize the progress in pollution-free production methods and minimize the level of contaminants by applying effective treatment ways.Contaminated soils and residues can be treated by various methods, including: isolation, incineration, stabilization, vitrification, thermal treatment, solvent extraction, chemical oxidation and many more.Above mentioned methods are usually expensive and in most cases result in the generation of secondary waste.Phytoremediation techniques seem to be a sustainable alternative to less environmentally friendly traditional methods of soil purification [5][6][7].Phytoremediation is the use of plants and their associated microorganisms with the aim of removal, degradation or isolation of toxic substances from the environment.The word "phytoremediation" derives from the Greek "phyton" and the Latin "remedium", which mean "plant" and "to correct", respectively [8].
Phytoremediation involves using plants as a purifying agent in situ for soil and water systems remediation.The cleanup process may utilize various mechanisms, including phytoextraction, phytovolatilization, phytostabilization, phytodegradation, rhizodegradation, or rhizofiltration [9,10].Phytoremediation has been widely studied for the removal of heavy metals, radionuclides, hydrocarbons, pesticides, or recently also pharmaceuticals [11][12][13][14][15].The critical point, ensuring the profitability and effectiveness of this method, may be achieved by appropriate selection of plant to the type and concentration of contaminants as well as the characteristics of the polluted site.Previous studies indicate that plants used for phytoremediation processes should have high adaptability and significant tolerance to contaminant exposure.Moreover, the high biomass yield, smooth spreading and harvesting along with strong ability to uptake and accumulate pollutants are required [16][17][18].
Two main parameters determine the plantʹs ability to remove contaminants from the substrate and to transport them from underground to above-ground parts of plants.The first parameter is the bioconcentration factor (BCF), defined as follows [19]: where: Croots is concentration of a contaminant in roots, Cshoots is concentration of a contaminant in shoots and Cmedium is concentration of contaminant in growing medium (i.e., soil or aqueous solution).
The second parameter, called translocation factor (TF), may be calculated using Formula (3) [19]: where: Croots is concentration of a contaminant in roots, Cshoots is concentration of a contaminant in shoots.This paper is a comprehensive review of the recent state-of-the-art in the field of phytoremediation.In this paper, we present a detailed look at the phytoremediation mechanism-plant-contaminant proper selection.This review is organized as follows: the second section focuses on different mechanisms of phytoremediation with an emphasis on the examples of pollutants that may undergo remediation processes.The third section deals with examples of plants and contaminants assigned to the specific phytoremediation mechanism, summarized in the form of a table.In the next part, we describe major factors and the latest promising solution, which influence the efficiency of phytoremediation.Finally, we discuss the challenges and benefits of phytoremediation, and we point out future directions in this field, including the promising processing pathway of post-harvested plants towards bioenergy production.Although, only several reviews in the subject of phytoremediation have been published.Previous works have mainly focused on a single mechanism of phytoremediation, plant, or contaminant.There are also brief reviews describing all of the phytoremediation mechanisms without detailed analysis of plants or pollutants [9,[20][21][22].While this paper includes an elaborate description of phytoremediation topics taking into account the newest trends to overcome the phytoremediation limitations.

Mechanisms of Phytoremediation
The main mechanism of phytoremediation with examples of potentially treated contaminants which may undergo corresponding processes are shown in Figure 1.

Phytoextraction
Phytoextraction, also called phytoaccumulation, is a technique applied to remove contaminants from the environment and mainly focuses on the removal of heavy metals and radionuclides.Mechanism of phytoextraction follows several steps, starting from contaminants uptake from soil or water, which are further transported from roots to above-ground plant tissues and finally accumulated therein.Then, plants with collected contaminants are harvested [23,24].Plants with both TF and BCF >1 have the potential to be used in phytoextraction process [19].A series of recent studies have investigated the effect of phytoremediation on single-contaminated soil.Yang and Shen [25] characterized the potential of Typha latifolia on the remediation of cadmium in wetland soils.Researchers observed that the plant exhibits excellent tolerance for contaminants, however higher concentration of metal in roots than in shoots indicates weak ability to transport accumulated cadmium.Holubík et al. [26] investigated thallium removal by Sinapis alba during a hydroponic or semi-hydroponic experiment.In this case, shoots had the highest metal concentration, which indicates that use of Sinapis alba in phytoremediation is sufficient to harvest above-ground biomass.Saleem et al. [27] performed research on phytoremediation of copper-contaminated soil by Linum usitatissimum.Copper concentration in the soil above 400 mg/kg inhibited plant growth and biomass accumulation.Meanwhile, 140 days after sowing, mainly shoots accumulated copper, and 39-43% of initial Cu concentration was removed from the soil.Zhang and Liu [28] investigated the phytoremediation ability of Gypsophila paniculata to accumulate cesium.This plant showed good tolerance for the presence of metal in soil.The highest accumulation was observed in leaves and shoots with translocation factor above 1.After 75 days level of cesium extraction from soil was around 10-12%.
The effects of phytoremediation on multi-contaminated soil have been discussed in many scientific papers.Marathe and Ravichandran [29] investigated Helianthus annuus potential to remove heavy metals from soil contaminated by landfill leachate.There were no signs of damage to the plants, and the removal efficiency was about 68% for Pb, 100% for As, and 97% for Hg.Marchiol et al. [30] characterized the ability of Brassica napus and Raphanus sativus for phytoextraction of Cd, Cr, Cu, Ni, Pb, and Zn from contaminated soil.The Raphanus sativus was more tolerant and presented higher metal accumulation in shoots than Brassica napus.However, both of these species showed low phytoremediation effect.Gupta and Sintha [31] reported the ability of Chenopodium album to accumulate metals from soil amended with tannery sludge.Accumulation of the metals in the plants was as the following: Fe > Mn > Zn > Cr > Cu > Pb > Ni > Cd.The highest concentration of Cr, Pb Fe, and Cd was observed in leaves, while for Mn and Zn in roots.Keeling et al. [32] compared the phytoextraction efficiency of Berkheya coddii for single-contaminated soil with Ni or Co, and soil contaminated with both Ni and Co. Plant accumulation of both metals from single-contaminated soil indicate that the bioconcentration factor increases as total metal concentrations increase.Plants readily accumulated cobalt, irrespectively on the presence of nickel.However, the co-existence of an equal mass concentration of cobalt limited the removal of nickel.
Hyperaccumulator plants are of particular importance when phytoextraction is considered.These plants can accumulate high concentrations of heavy metals.The term hypperacumulator was first introduced for a species that might accumulate in aerial parts > 1000 mg/kg Ni [33].Examples of these plants are Alyssum pintodasilvae [34] and Alyssum murale [35] for nickel, Pteris vittata [36] for arsenic, Solanum nigrum [37] for cadmium, Arabidopsis helleri [38] for zinc.Meanwhile, Thlaspi caerulescens is a plant that exhibits hyperaccumulation for both metals-zinc and cadmium [39].

Phytovolatilization
Phytovolatization concerns mainly the volatile organic compounds [40][41][42] or volatile inorganic contaminants such as Hg, As, or Se [43][44][45][46].This mechanism of phytoremediation may involve two possible ways of volatilization.These are direct phytovolatilization by steam or leaves and indirect phytovolatilization from the root zone.Direct phytovolatilization includes uptake of contaminants from soil, transforming them into volatile compounds, which are further excreted to the atmosphere by leaves transpiration or radial diffusion through stem tissues [47].The mechanism of indirect phytovolatilization involves volatile organic contaminants flux from the subsurface as an effect of plant roots activities e.g., increasing soil permeability, lowering the water table, chemical transport via hydraulic redistribution, or water table fluctuations [47].Plants applied for phytovolatilization of organic compounds commonly belong to deciduous trees such as Poplarand Salix, or coniferous trees like Pinus [40,[48][49][50][51]. Sakakibara et al. [45] investigated the phytovolatilization effect of Pteris vitatta on soil contaminated with arsenic.Plants converted about 90% of total arsenic uptake into arsenite and arsenate.Brassica juncea, Chara canesces, and Salicornia bigelovii were applied for phytovolatlization of selenium compounds [52,53].The researchers proved that plants transformed toxic selenium compounds into the relatively non-toxic chemical form of dimethyl selenide.Heaton et al. [54] implemented genetically engineered plants with a modified bacterial mercuric reductase gene-Arabidopsis thaliana and Nicotiana tabacum for phytoremediation of soil contaminated with mercury and methylmercury.They found that toxic Hg(II) is converted into much less toxic Hg(0) and volatilized from the plants.Moreover, transgenic plants were able to volatilize 3 to 4 times more Hg(0) than control plants without the mercuric reductase gene.Phytovolatilization technique is also widely applied for the treatment of soil irrigated by groundwater with trichloroethylene, tetrachloroethylene, or perchloroethylene [55][56][57][58][59].It was observed that above 90% of trichlotoethylene (TCE) or perchloroethylene (PCE) might be removed from soil.However, a large proportion of contaminants is volatilized right out of the soil.The researchers pointed out that dehalogenation is the primary mechanism responsible for the removal of chlorinated compounds, which was confirmed by the presence of free chloride in the soil after the phytoremediation process.Furthermore, among organic compounds, phytovolatilization may be applied for the removal of methyl tert-butyl ether (MTBE) by Pinus or Salix [40,50].The authors observed that signs of Salix phytotoxicity related to the reduction of normalized relative transpiration occurred for a high dose of MTBE (400 mg/L).The principal mechanism responsible for MTBE removal was plant transpiration, while plant tissues accumulated the only little amount of organic compound.However, they found that MTBE was not metabolized during transport in the plant and released in the unchanged chemical form.

Phytodegradation
Phytodegradation, also referred to as phytotransformation, is applied for petroleum hydrocarbons, pesticides, insecticides, surfactants, or pharmaceuticals degradation in soil and water.Phytodegradation includes uptake of contaminant from the substrate and its breakdown to lowmolecular-weight intermediates via metabolic processes within the plants [60,61].Enzymes i.e., dehalogenases, oxygenases, and reductases, secreted by plant tissues, are responsible for catalyzing degradation pathways of contaminants [62][63][64].Susarla et al. [65] compared the effect of phytodegradation on the sand and aqueous experiment upon contamination with perchlorate by Myriophyllum aquaticum.The uptake of perchlorates was 5 times faster for the aqueous environment than for soil, and this probably was due to the influence of chloride ions desorbed from the sand.Degradation pathways of perchlorate were suggested as a stepwise manner to form chloride.Similarly, Myriophyllum aquaticum was tested for phytodegradation of trinitrotoluene in aqueous treatment.In this case, the plant removed about 70% of trinitrotoluene (TNT) from the solution [66].Yoon et al. [67] investigated the mechanism of phytotransformation of 2,4-dinitrotoluene (DNT) in liquid medium by Arabidopsis thaliana.The authors observed that plants degrade about 95% of initial 2,4-DNT concentration after 15 days and transformed and incorporated a small part of organic compounds in plant tissue, possibly as a such of lignin and cellulose.Moreover, monoaminonitrotoluene and other unknown metabolites were detected as an intermediate product of 2,4-DNT transformation.
Most recent trends in the studies of phytodegradation processes concern the treatment of aqueous solution contaminated with pharmaceuticals.Regular literature studies involve phytodegradation of antibiotics and nonsteroidal anti-inflammatory drugs [68][69][70].One of the examples is the phytodegradation of Ibuprofen by Phragmites australis [71].The researchers concluded that this plant was able to uptake, translocate, and degrade Ibuprofen (IBP).No symptoms of phytotoxicity were observed, and the plant removed the total amount of initial contaminant concentration within 21 days.Moreover, in the plant tissue, especially in steam and leaves, four types of intermediate products were detected.These were: hydroxy-IBP, 1,2-dihydroxy-IBP, carboxy-IBP, and glucopyranosyloxy-hydroxy-IBP.A similar degradation pathway of Ibuprofen by Typha Latifolia observed Li et al. [72].The authors detected Ibuprofen carboxylic acid, 2-hydroxy-IBP, and 1-hydroxy-IBP as intermediates.Singh et al. [73] investigated the effect of antibiotic-ofloxacin phytodegradation by Spirodela polyrhiza.In this case, after 7 days, about 93-98% of ofloxacin was removed.Other studies concern phytoremediation of an antibiotic -tetracycline.Datta et al. [74] reported that Chryspogon zizanioides exhibit high potential for degradation of tetracycline (TC) from aqueous media.After 40 days, TC was completely removed, moreover, some unknown metabolites of TC were detected in the plant root and shoot tissues.Comparable research focused on the degradation of tetracycline by Lemna gibba L. performed Topal et al. [75].In this case, the maximum removal efficiency for TC in the planted reactor was determined as 79.6% at day 10.Li et al. [76] and Ryšlavá et al. [77] evaluated the phytodegradation of carbamazepine by Zea mays, Helianthus annuus, Daucus carota L., and Apium graveolens L. Carbamazepine (CBZ) was not readily uptaken by plants, probably due to its hydrophobic properties.However, some intermediates such as amino-CBZ-10, 11-epoxide were detected, thus the epoxidation process was confirmed as one of the possible degradation pathways.
Another group of contaminants, which may be effectively removed from water by phytodegradation, are plant protection products, especially pesticides and insecticides.Xia and Ma [78] investigated the potential of Eichhornia crassipe to remove a phosphorus pesticide-ethion.The Authors concluded that the plant removed about 69% of ethion by accumulation.Moreover, after one week of incubation, the concentration of phosphorus pesticide in plant tissue significantly decreased, which indicates the degradation of this compound by the metabolism of the plant.Rani et al. [79] observed phytotransformation of toxic insecticides-phorate by Brassica juncea.During this experiment, above 68% of phorate was removed within 5 days.Only phorate sulfoxide was detected as an intermediate product, which confirms that sulfoxidation is an essential transformation pathway for organophosphate insecticides in the plant.

Phytostabilization
Phytostabilization is widely used for the treatment of soil, wetlands, or mining waste contaminated with metals such as Zn, Pb, Cd, Mn, Cu, Cr, Fe, As, and Ni [80][81][82][83][84].This method refers to limitation of contaminants bioavailability and its immobilization to avoid bulk erosion, reduce air-borne transport or leaching and thus to prevent the distribution of toxic contaminants to other areas [85].Phytostabilization involves rhizosphere-induced adsorption and precipitation processes, sorption, or complexation [86][87][88][89].In this process, plants called excluders play a fundamental role, which actively limits metal uptake.These plants can absorb and accumulate metals in their roots and (BCF > 1), on the other hand, are characterized by low root-shoot translocation factor (TF < 1) [90,91].One of the solutions to enhance the efficiency of phytostabilization is the so-called aided phytostabilization [92,93].It is related to the application of inorganic or organic amendments to the substrate.The most common examples are manure compost, biochar, biosolids, activated carbon, diatomite, chalcedonite, dolomite, sand, limestone, bone mill, bottom ash, furnace slag, and red mud [93][94][95][96].Pérez-Esteban et al. [94] observed that the addition of manure had an impact on the reduction of metal concentration in shoots of B.juncea, decreasing the value of bioconcentration factor and copper and zinc bioavailability in soil and high accumulation of both metals in roots.This effect, as explained, was related to high pH and the presence of organic matter, which acts as fertilizer.A similar effect was reported by Meeinkuirt et al. [97], who examined the influence of manure on the Cd phytostabilization potential of Eucalyptus camaldulensis.The application of amendments improved plant growth and biomass production.Furthermore, plants grown on amended soils had lower Cd accumulation than those grown on the Cd soil alone.The same effects were described by Phusantisampan et al. [98], who demonstrated the potential of Vetiveri zizanioides for phytostabilization of cadmium in soil, enhanced by mixing soil with manure.Meanwhile, Lee at al. [92] compared the effect of the bone mill, bottom ash, furnace slag, and red mud application in aided phytostabilization of Pb/Zn mine tailings by Miscanthus sinensi and Pteridium aquilinum.M.sinensis accumulated heavy metals mostly in the roots and had a lower value of translocation factor compared to P. aquilinum.Furthermore, Fe-rich amendments such as furnace slag or red mud significantly reduced the amount of soluble and extractable heavy metals.
Another possible direction to improve phytostabilization processes includes roots inoculation with fungus resulting in symbiotic interaction between fungi and plants.Application of mycorrhizal fungus was investigated by Chen et al. [99] and Gu et al. [100] in the phytostabilization of zinc, lead, copper, cadmium, or uranium.In the case of uranium, mycorrhizal decreased concentration of U in shoots and increased U concentration in roots.A comparable effect was observed for Pb, Zn, Cu, and Cd with a higher concentration in roots than in shoots.The Authors concluded that mycorrhizal fungi inoculation reduces metal bioconcentration and translocation factors.Ouaryi et al. [101] also observed that the inoculation of mycorrhizal fungi onto Eucalyptus camaldulensis might improve the plant growth and its tolerance to high copper concentration in soil.

Rhizodegradation
Rhizodegradation also known as a phytostimulation refers to the decomposition of pollutants such as PAHs (polyaromatic hydrocarbons), hydrocarbons or perchlorates, due to the activity of microorganisms in the rhizosphere.This type of phytoremediation may be referred to plantmicroorganism cooperation, strongly depending on the interactions between these group of species.Plant exudates being a carbon source, provide beneficial conditions for growth and development of soil microflora, while microorganism such as bacteria and fungus are able to degrade hazardous contaminants to nontoxic products via enzymatic and metabolic processes [102][103][104].Rhizodegradation may be an effective way for cleanup of soil contaminated by petroleum, diesel, or oily sludge [105][106][107].Maqbool et al. [106] compared the impact of bioaugmentation on rhizodegradation of petroleum hydrocarbons by Sesbania cannabina.The plant degraded about 75% of hydrocarbons in the rhizosphere within 120 days due to natural plant-microorganism interaction, whereas bioaugmentation did not improve organic contaminants removal.Comparable research performed Ramos et al. [108], who noticed that after 60 days, reduction of hydrocarbons concentration by Sebastiania commersoniana was 60% and after 424 days above 94%.Moreover, vegetated and contaminated soil presented higher microbial density and diversity.Hydrocarbons found in petroleum may be used by microorganism as a carbon source.Lu et al. [109] and Jia et al. [110] investigated rhizodegradation of phenanthrene (Ph) and pyrene (Py), respectively, by Kandelia candel and Avicennia marina.Researchers stated that the dissipation of phenanthrene and pyrene were significantly higher in the rhizosphere compared to nonrhizosphere zones of sediments.Kandelia candel was able to remove 47.7% of Ph and 37.6% of Py after 60 days, while Avicennia marina 71-86% Ph and 63-79% Py after 120 days.In both cases, plant root promoted dissipation significantly exceeded uptake and accumulation of hydrocarbons in plant tissue.Some authors have also suggested that perchlorates may be readily removed by rhizodegradation.Yifru et al. [111] examined the potential of Salix nigra for biostimulation of perchlorates rhizodegradation.As an electron source, the authors proposed natural and artificial carbon products.Addition of dissolved organic carbon reduced time required for degradation of total perchlorate from 70 days to 9 days.Mwegoha et al. [112] evaluated the effect of biostimulation for perchlorate rhizodegradation by Salix babylonica using chicken manure.They observed that the addition of dissolved organic carbon reduces perchlorate uptake and phytoaccumulation in plant tissue.

Rhizofiltration
Rhizofiltration is a phytoremediation technique which involves using a plant able to adsorb contaminants occurring in the rhizosphere on the surface of roots or absorb into roots tissue, concentrate and precipitate them [113,114].Plants with long, fibrous root system covered with root hairs and having high surface area are particularly desirable to provide effective remediation [115,116].This method is especially used for the treatment of groundwater or wastewater polluted with heavy metals or radionuclides, such as Ra, U, and Cs [115,[117][118][119]. Uranium rhizofiltration by Helianthus annuus and Phaseolus vulgaris was investigated by Lee and Yang [113].Researchers observed that Helianthus annuus removed about 80% of initial concentration within 24 hours, while Phaseolus vulgaris from 60 to 80%.Moreover, it was concluded that the highest removal efficiency occurred at pH 3-5.Roots analysis revealed that the mechanism of uranium rhizofiltration might be strongly dependent on the adsorption, precipitation, and exchangeable sorption on the root surface.Eapen et al. [116] examinated potential of Brassica juncea and Chenopodium amaranticolor for rhizofiltration of uranium.Plant root systems genetically transformed by Agrobacterium rhizogenes have evolved hairy root cultures used as a bioadsorbant.B. juncea uptake was 20-23% of uranium from the solution, while C. amaranticolor showed only 13% uptake.Tomé et al. [115] applied rhizofiltration for removing uranium and radium by Helianthus annuus.In this case, after 2 days plant removed about 50% of uranium and 70% of radium, which accumulated in the roots with deficient translocation factor.In contrast, a specific part of radionuclides was bounded as copious white precipitate.Analogous analysis was performed by Yang et al. [118] for the removal of uranium by Phaseolus vulgaris.Obtained result suggested that optimal conditions for uranium removal occurred at moderately acidic pH conditions (pH 3-5) when uranyl cation is the predominant uranyl species, which is readily translocated to plant roots.At pH 5, the Phaseolus vulgaris decreased the uranium concentration by 90.2% within 12 h and by 98.9% within 72 h.Root analysis confirmed that rhizofiltration mechanism at pH 7 is based on the adsorption and precipitation on the root surface in the form of insoluble uranium compounds.Vesel´y et al. [120] proposed treatment of highly polluted solution contaminated by cadmium and lead by rhizofiltration using Pistia Stratiotes.The plant exhibited high tolerance to heavy metal stress and excellent capability for metal accumulation.The experiment showed that Pistia Stratiotes was able to remove up to 95% of metals after 7 days, and the concentration of Cd and Pb were about 10-fold higher in roots than in leaves.
Information provided in Table 1 summarizes the literature research on phytoremediation.Collected information is grouped according to the dominant mechanism of phytoremediation.Valuable information on the details of experimental work together with main conclusions are presented in concise manner.10 wt% biochar-amended soil had a positive effect on promoting plant growth and seed yield while 15 wt% biochar had an adverse effect on plant growth.
• Cadmium and Zn bioavailability in soil decreased with an increasing biochar addition.

•
Biosolids application increased the dry biomass production of L. perenne.

•
Metals were mostly accumulated in the roots and only a small part was translocated to the shoots.

•
With the addition of biosoilds accumulation of copper in the roots was even 166-times higher, than in the shoots of the plants.[

•
The accumulation of Pb in plantswas the highest during the first 4 days 10 times higher in roots than in leaves.

•
Maximal Cd accumulation in roots was 3923 mg/kg (at 14th day) while for Pb was 42,862 mg/kg (at 4th day). [120] DAS-day after sowing, DOC-dissolved organic carbon, DTPA-diethylenetriaminepentaacetic acid, DW-dry weight, EDDS-ethylene diamine disuccinic acid, EDTA-ethylenediaminetetraacetic, HEDTA-hydroxyethylethylenediaminetriacetic acid, MGWL-monosodium glutamate waste liquid, NTA-nitriloacetic acid, PAH-polyaromatic hydrocarbons, TPH-total petroleum hydrocarbo Taking into consideration percentage of research papers assigned to individual continents it can be conluded that greatest interest in the topic of phytoremediation occurs in Asian countries, followed by Europe and America.However, notable lower share of Africa may be associated with the limitation of the ability to perform phytoremediation process for example due to the tropical weather conditions.

Parameters Affecting Phytoremediation Process
The cleanup process of contaminated soils, which involves the application of plants, depends on variety of factors.The most important of these include soil parameters, properties, concentration and phytoavailability of pollutants, and also plant species [157,158].

Soil pH
pH is one of the factors affecting the efficiency of soil phytoremediation.Soil pH affects the adsorption and desorption of contaminants in soil.It is the parameter which controls metal solubility.Generally, metals present higher mobility under acidic and reducing conditions than under alkaline and oxidizing conditions [159].The capacity of soil to adsorb cationic metals increases with pH increasing [160].At high pH values, the metal ions are virtually non available for plants [161].Moreover, it was observed that variable pH conditions are crucial for plant growth.Willscher et al. [162] investigated the growth and ability of Helianthus tuberosus for the phytoextraction of heavy metals (Cd, Cu, Fe, Mn, Ni, Zn, Pb) under different pH conditions (4.0, 4.5, 5.0, 5.5, and 6.0).The best growth of roots achieved control plants at pH 5, and for leaves and stems it was obtained at pH 5.0 and 5.5 for controls, whereas at slight (highest metal concentration equal 62.5 mg/kg) and medium (highest metal concentration equal 125 mg/kg) heavy metal concentrations, best growth of roots were obtained at pH 5.5 and 6.For slightly and medium contaminated soil, the highest accumulation of all metals in shoots occurred at pH 4.0 and was considerably higher than at pH 5.0 or 6.0.Bagga and Peterson [163] compared the accumulation of arsenic by Asparagus Fern from contaminated soil (300 pm) growing on the soil at pH 4.5, 5, 6, and 7.The highest concentration of arsenic in plant tissue occurred at pH 5 (480 ± 17.37 mg/kg), followed by pH 4 > pH 6 > pH 7. A similar effect evaluated Brown et al. [164] for zinc and cadmium uptake by plants concerning soil pH (5.06-7.04).Thlaspi caerulescens exhibits metal stress only in low pH treatments.For Zn and Cd contaminated yard soil, the highest concentration of Zn and Cd in shoots was observed at pH 5.07 and steadily declined with the increase in pH.Research carried out by Chen et al. [165] concerning the addition of citric acid to lowering soil pH and decreased the adsorption of Pb and Cd.The effect of decreased in soil adsorption was more evident for cadmium than for lead.Moreover, addition of citric acid stimulated metals transportation from root to shoot of radish.Hattori et al. [166] compared the effect of soil pH (3.5 and 5.0) on cadmium uptake by Helianthus annuus, Hibiscus cannabinus, and Sorghum vulgare.It was observed that when the soil pH decreased, the amount of Cd dissolved in soil water increased.In case of Helianthus annuus and Hibiscus cannabinus at low pH (3.5) Cd accumulation increased above twofold compared to the control soil (pH 5), while for Sorghum vulgare decreased due to roots sensitivity for low pH.In a study, Saleh [167] controlled the uptake of radionuclides 60 Co and 137 Cs by Eichhornia crassipes at variable pH conditions (2.9, 4.9, 8.9 and 10.9).Researcher concluded that Eichhornia crassipes might tolerate pH values from 4 to 10, while the highest uptake rate for both radionuclides was observed at pH 4.9.Meanwhile, at low pH value (2.9), protons might compete for the plant adsorption sites with the radioisotope cations, respectively Cs + and Co 2+ .Singh et al. [168] investigated potential of Lemna minor for removal of Pb at pH 5.0-9.0.The lowest toxic effect of Pb was found at pH 5, but in contrast the highest percent of removal occurred at pH 9.However, the maximum bioconcentration factor (0.9) was observed at pH 6 for lead content 10 mg/L.

Inorganic Fertilizers
Fertilization may promote plant growth and biomass production.Moreover, the addition of fertilizers may enhance the phytoremediation process.The first group of amendments is inorganic fertilizers which supply three main nutrients: nitrogen (N), phosphorus (P), and potassium (K).Fertilization of N is essential for promoting leaves growth and forms protein and chlorophyll.Phosphorus plays important role in roots and flower formation, while potassium is responsible for steam and root development [169,170].However, for optimal plant growth, important is not only the amount of added fertilizer but also the ratio between N, P, and K, which depends on nutrient deficiency in soil and plant requirements.Moreover, inappropriate ratios of N, P, and K fertilizer may have negative effect on the absorption and utilization of nutrients.[171].Wu et al. [172] investigated the effect of nutrient addition on the phytoremediation efficiency of Cu contaminated soil by Brassica juncea.The addition of fertilizer N (urea) and P (superphosphate) significantly increased plant shoot yield.Nitrogen and phosphorus increased the amount of chlorophyll in the leaves.Moreover, N and P applied at 100 and 200 mg/kg, respectively resulted in the highest Cu uptake.Schwarz et al. [173] characterized phytoextraction of cadmium with Thlaspi caerulescens, enhanced by N-fertilization.Soils were amended with increasing rates of N-nitrate (NaNO3) or N-ammonium ((NH4)2SO4) in the amount of 0, 20, 80, 200 mg N/kg.T. caerulescens responded positively to the increasing nitrogen fertilization on both soils and the nitrogen supply significantly improved metal extraction.However, the ammonium fertilization led to a lower biomass production than nitrate.Similar studies by Liao et al. [170] investigated the effect of a different form of nitrogen fertilizers for arsenic accumulation by Pteris vittata.Research evaluated the potential of several nitrogen suppliers i.e., NH4HCO3, (NH4)2SO4, Ca(NO3)2, KNO3, and urea.As accumulation was greater due to higher biomass, when N fertilizer was added, especially with the addition of NH4 + -N source.Furthermore, the total arsenium uptake and/or accumulation within the plants grown under different forms of N fertilizer, decreased as NH4HCO3 > (NH4)2SO4 > urea > Ca(NO3)2 > KNO3 > control.Jacobs et al. [174] evaluated the efficiency of KNO3 and NH4NO3 fertilizers addition on phytoremediation of Cd and Zn contaminated soil by Noccaea caerulescens.A slight favorable effect of nitrogen fertilization on biomass production occurred only in soils with low initial nitrogen content (under 25 μg/g NO3 − ).Above this concentration, fertilization caused decrease in shoot Cd and Zn concentration.Moreover, there was no difference with biomass increase with application of the two N fertilizers (KNO3 or NH4NO3).Research performed by Di Luca et al. [175] focused on the improvement of chromium phytoremediation by Pistia stratiotes applying a different concentrations of P and N nutrients.P and N concentrations were 5 mg/L or 10 mg/L.It was observed that nutrients addition significantly increased Cr removal and enhanced Cr translocation to leaves.The decrease in the relative growth rate due to Cr exposure was reduced by nutrient addition at 5 mg/L of P or N, suggesting an improving effect of nutrient enrichment on the Cr tolerance of P. stratiotes, whereas the addition of 10 mg/L of P or N increased Cr toxicity.Fertilization may be also effective way to improve phytoremediation process of soil contaminated with crude oil.Merkl et al. [176] studied the influence of fertilizer level on plant growth and oil dissipation.Fertilizer was applied twice in a concentration of 200, 300, and 400 mg/kg soil, each of N, P, and K (commercial fertilizer .The medium fertilizer concentration (300 mg/kg) resulted in the highest root growth and maximal oil dissipation (18.4%) after 22 weeks.While, the highest fertilizer level produced best shoot growth and highest oil dissipation after 14 weeks, but it reduced root biomass production.Application of controlled-release fertilizers (CRF) is another approach presented in the study Cartmill et al. [177] to enhance phytoremediation of petroleum-contaminated soil (3000, 6000 and 15,000 mg/kg).It was found that plant adaptation to contaminants, growth, photosynthesis, and chlorophyll content of Lolium multiflorum were improved by the addition of CRF (4, 6, or 8 kg/m 3 ).Moreover, soil contaminated with 6000 or 15,000 mg/kg had enhanced petroleum hydrocarbons degradation with fertilization.In contrast, the application of fertilizers not always give the desired effect.Jayaweera et al. [178] investigated removal of Fe (9.27 mg/L) from synthetic wastewaters by Eichhornia crassipes.Plants were growing under variable nutrient conditions with 28 mg/L of total nitrogen (TN) and 7.7 mg/L of total phosphorous (TP).Another experiments were performed with 2-fold, 1/2-fold, 1/4-fold and 1/8-fold of these nutrient concentrations.Plants grown without nutrition showed the highest phytoremediation efficiency of 47% after the 6th week of growth, with the highest accumulation of 6707 Fe mg/kg DW.These studies proved that Eichhornia crassipes grown under nutrient-poor conditions is very good Fe accumulator.Similarly, Ji et al. [179] observed that the addition of fertilizers (NH4NO3 and Ca(H2PO4)2) for phytoremediation of Cd-contaminated soil by Solanum nigrum had no significant effect on plant biomass.

Organic Amendments
Organic matter content is one of the most important soil component, and it has the ability to retain heavy metals in soil, due to metal-organic matter interactions and thus limits metal phytoavailability [180].It has been proved that increase of organic matter in the soil might reduce the metal ions [181].The poor organic matter content in contaminated soil may limit the plant growth, slow colonization, and microbial activity [182,183], which inhibit natural succession and remediation effects [184].Organic amendments may include i.e., chicken, cow, horse and pig manure, compost, sludge, biochar, humic acids.Pillai et al. [185] investigated the effect of organic manure addition (1 g powdered cow dung/kg soil) on the phytoremediation potential of Vetiveria zizanioides, growing on soil contaminated with chromium.High biomass production was observed in soil with organic manure.Moreover, the addition of cow dung prevents chromium toxicity, which was visible as a yellowing of the plant leaves growing on soil without organic additives.Chromium uptake by Vetiveria zizanioides was improved with cow manure.Wai Mun et al. [184] applied chicken manure to improve cleanup of sand tailings contaminated with Pb by Hibiscus cannabinus.It was stated that the application of organic fertilizer promoted biomass production as well as higher accumulation capacity of Pb in plant tissues.Application of pig manure compost for phytoremediation of PAHcontaminated soil was studied in the research performed by Cheng et al. [186] and Wang et al. [187].It has been found that addition of organic compost increased shoot biomass yield.Wang et al. proved that the dissipation of PAHs: phenanthrene, pyrene, and anthracene were greatly improved with using soil-manure composition, while Cheng et al. observed enhancement in dissipation only for pyrene.Several studies performed by Wei et al. [188][189][190] compared the effect of addition of urea (0.5, 1, and 2 g/kg) and chicken manure (50, 100, and 200 g/kg) on cadmium accumulation different plants.First research applied Solanum nigrum for phytoremediation of soil contaminated with cadmium (10, 25, and 50 mg/kg).It was concluded that application of fertilizers led to increase of the Cd phytoextraction efficiency of S. nigrum by enhancing its shoot biomass production.Moreover, urea did not affect Cd concentration in plant tissue, while chicken manure decreased the Cd phytoavailability and tissues cadmium concentration.It suggests that the application of urea may be suitable as fertilizer for the phytoextraction process and chicken manure for metal phytostabilization in soil.These results were compared with another study concerning the potential of Taraxacum mongolicum for phytoremediation of Cd-contaminated soil (2.5, 5, 10 and 25 mg/kg).The same effect was observed, with increasing plant biomass with fertilization.It was also confirmed that chicken manure significantly decreased Cd concentration in plant tissue by decreasing extractable Cd in soil.Similar research performed by Nwaichi et al. [191] focused on cadmium accumulation by Mucunapruriens and Sphenostylis stenocarpa, growing under urea and chicken manure fertilization (0.8 g/pot).In contrast, the researcher observed that the addition of chicken manure significantly improved the translocation factor by >1, while urea did not affect metal transport from roots to shoots.Moreover, the chicken manure treatment remarkably increased the shoot Cd concentration, while its application decreased Cd solubilization in comparison to urea addition.
Recent research showed that using biochar may strongly influence the cleanup efficiency of contaminated soil by plants.Houben et al. [192] concluded that the addition of biochar (1, 5, and 10 wt%) to Cd, Pb, Zn-contaminated soil might increase metals bioavailability and biomass yield of Brassica napus.The reduction of metal content in soil reached 71%, 87% and 92% for Cd, Zn, and Pb respectively in the presence of 10 wt% biochar.However, addition of organic amendment caused reduction in metal concentrations in shoots, but the biomass production was remarkably improved as a result of the soil fertility improvement.Meanwhile, Han et al. [193] performed a greenhouse pot experiment for the phytoremediation of soil contaminated with total petroleum hydrocarbons (TPHs) by ryegrass.In contrast, negative impact on the growth of ryegrass and the degradation of TPHs by ryegrass was observed.Moreover, Saum et al. [194] used biochar or its mixture with compost to enhance phytoremediation of oil-contaminated soil by petroleum hydrocarbons.It was observed that addition of biochar to contaminated soil suppressed the population of oil-degrading bacteria.Lower level of petroleum degradation in the biochar treatment probably mightbe related to the smaller populations of PAH-metabolizing microbes.Several studies showed that the addition of humic acid to contaminated soil might also be a possible solution to enhance phytoremediation efficiency.Humic acids, which contain acidic groups such as carboxyl and phenolic OH functional groups, play an essential role in the transport, phytoavailability, and solubility of heavy metals [195].Angin et al. [196] studied the effect of humic acid (100,200, and 400 kg/ha) in enhancing boron and lead accumulation by Vetiveria zizanioides.Humic acid application increased Pb phytoavailability in soil and improved Pb removal.The highest boron uptake was for plant growing under the addition of 400 kg/h.However, incorporation of HA to Pb or B contaminated soil did not influence the translocation from roots to shoots.HA addition might increase permeability of root cell membranes and thus allowing for more effortless transfer of metals [197].Also, it was confirmed for Cu accumulation in Chrysopogon zizanioides tissues by Vargas et al. [198].Researcher concluded that addition of humic acids (10 and 20 g/kg) promoted root growth and increased Cu concentrations in plant roots due to formation of soluble metal-organic complexes, while translocation factor was reduced.In contrast, Evangelou et al. [195] observed that using of humic acids (2 g/kg soil) enhanced Nicotiana tabacum uptake of cadmium from contaminated soil.Moreover, upon organic amendment cadmium concentration in shoots increased.It was suggested that this effect was related to pH reduction and higher cadmium availability.A study of Wong et al. [199] showed adverse effects of humic acid addition on phytoremediation of pyrene-contaminated sediments by Kandelia candel.The pyrene removal was tremendously higher for sediments without organic amendment (decreased from 89% to 29%).Moreover, total plant biomass was reduced by 50% for humic acid addition.Only for roots pyrene accumulation was slightly higher growing on soil with HA.

Contaminant Concentration
Uptake of contaminants from soil depends on its concentration.Some of the contaminants, especially at higher concentration may compete with micro-and macronutrients such as P, Ca, Mg or Fe and thus present toxicity effect on plant growth or vital processes [200,201].Generally higher pollutant concentration makes it more difficult for a plant to accumulate or degrade them.Gomes et al. [202] investigated the effect of variable Cd concentration (0, 15, 25, 45, 90 μmol/m 3 ) on plant growth and phytoremediation capacity of Eucaliptus camaldulenses.Shoots and roots biomass production was negatively affected by an increase in Cd concentration.Moreover, the highest cadmium concentration caused visible symptoms of phytotoxicity, such as yellowing of leaves or blackening and thickening of roots.These symptoms might be related to deficiencies of several nutrients essential for the formation, expansion, and operation of chloroplasts.Decreases in total K, Ca, and Mg contents might be related to competition for bivalent ion binding sites by Cd.For Cd concentration in soil equal 45 μmol/m 3 , metal concentration in shoots and roots was the lowest.Studies performed by Dheeba and Sampathkumar [203] concerning influence of variable Cr concentration (10,20,30,40,50 mg/kg) on growth and accumulation of metal by five species Helianthus annuus, Zea mays, Sorghum bicolour, Vigna radiate and Arachis hypogaea.It was shown that plants had different tolerance to chromium pollution.Vigna radiate and Arachis hypogaea shoot length were reduced by more than 50% when compared to control with the increase in contaminants concentration, and also fresh weight was reduced by about 50% at 50 mg/kg Cr concentration.Meanwhile, for all plants, pigment levels significantly decreased at 10-50 mg/kg of chromium in comparison to control.The greatest Cr concentration in roots was in the order of Zea mays > Sorghum bicolour > Helianthus annus > Arachis hypogaea > Vigna radiate.Impact of cadmium concentration (0.1 and 30 mg/kg) on growth and removal efficiency of Typha latifolia was presented by Yang and Shen [25].The highest plant shoot and root lengths were for 1 mg/kg Cd treatment with 89.4 and 18.3 cm, while growth at 30 mg Cd/kg treatment reduced biomass production, but plants did not show any toxicity symptoms.The Cd concentration in plant roots and shoots of Cd were 51.6 and 26.0 mg/kg (for 1 mg/kg) and 279 and 131 mg/kg (for 30 mg/kg), respectively.Phytoextraction potential of Linum usitatissimum growing on soil differentially spiked with copper (200,400, and 600 mg/kg soil) was evaluated by Saleem et al. [27].Results suggested that plant was able to grow up to 400 mg Cu/kg level without any inhibition in growth.Further increase of Cu concentration caused a reduction in plant growth, total chlorophyll and carotenoids content, and biomass production.However, even at high concentration plant was able to accumulate a significant amount of Cu in roots and shoots.

Mobility, Bioavailability and Chelating Agents
The problem of low mobility and bioavailability of metals is an essential factor affecting phytoextraction efficiency.A new approach demonstrates the possibility of performing chelate assisted phytoextraction.Chelators may enhance the solubility of metals in soil and thus improve its phytoavailability as well as boost metal translocation from roots to above-ground plant parts [127,137,204].There is a wide choice of chelating agents for enhancing phytoextraction described in the literature.The classification consists of natural and synthetic substances [205].The group of natural chelators includes mainly low-molecular-weight organic acids such as citric acid, vanillic acid, gallic acids, oxalic acid, or tartaric acid, whereas the synthetic group contains among other things EDTA, EDDS, DTPA, HEDTA, and NTA [127,[205][206][207][208][209][210][211].Meers et al. [127] compared the effectiveness of synthetic aminopolycarboxylic acids with low molecular weight, biodegradable organic acids on the phytoextraction ability of Zea mays planted on the soil contaminated by Cu, Zn, Cd, Pb, and Ni.Only the addition of EDTA and DTPA increased metal accumulation in above-ground plant tissue and higher the value of the translocation factor.Moreover, the application of chelating agents 10 days after germination more efficient than before sowing of plant for higher accumulation of metal in shoots.The addition of NTA or acids did not have the expected results, mainly due to rapid mineralization and too low dosage.A similar effect observed Kos and Lestan [212] with an application of citric acid on vineyard soil contaminated with copper.While, Quartacci et al. [213] observed that the overall accumulation of Cd, Zn, and Cu in Brassica juncea has been improved upon NTA treatment.Several studies also compared the performance of EDTA and EDDS.The authors concluded that EDDS might be more active on multi-contaminated soil and assist in metal translocation from roots to shoots.Furthermore, Luo et al. [214] suggested the combined application of EDTA and EDDS for phytoextraction of Cu, Pb, Zn, and Cd by Zea mays.The most efficient ratio was 2:1 of EDTA: EDDS, which led to significantly increased the concentration of heavy metals in shoots and total metal uptake.
Although synthetic chelators can enhance phytoextraction efficiency by increasing metal solubility and bioavailability, their application may adversely affect plant growth, and biomass as well as promote necrosis and chlorosis symptoms.These might be the consequence of excessive metal concentration in soil and its toxicity to plants [132].Moreover, the application of poor biodegradable synthetic chelators such as EDTA or DTPA that may be persistent in the environment, will be a highly risky action for the due to the possibility of secondary pollution of groundwater [214][215][216].

Plant Growth, Biomass Production and Accumulation Capacity
Plants used for phytoremediation of the pollutant from contaminated soil should exhibit a fast growth rate and high biomass production.Moreover, an extended root system for exploring large soil areas is favorable.Excellent tolerance and resistance to stress induced by the high contaminant concentration in the soil are necessary.Phytoextraction also requires the capability to accumulate a high concentration of contaminants (hyperaccumulator), simultaneously with high translocation factor from roots to above-ground plant tissues.Keller et al. [217] compared various high biomass plants (Brassica juncea, Nicotiana tabacum, Zea mays,and Salix viminalis) with hyperaccumulator plant (Thlaspi caerulescens) growing on soil contaminated with Zn, Cd, and Cu.T. caerulescens,characterized by small biomass was the most efficient plant for Cd and Zn removal with very high concentrations in the shoots.Among plants with high biomass production, Salix viminalis was able to accumulate a high concentration of Cd and Zn, while Nicotiana tabacum effectively removed Cd and Cu.Moreover, the difference between root system distribution was observed.T. caerulescens formed a shallow root system, which was able to remove contaminants from shallow soil zones (0.2 m), whereas Zea mays and Salix viminalis colonized the soil at depth and thus were more suitable for deep contamination (0.7 m).These results indicate the importance of proper choosing plant species to type and concentration of contaminants.Similar research performed Zhuang et al. [135] investigated the potential of high biomass plants (Vertiveria zizanioides, Dianthus chinensis, Rumex K-1 (Rumex upatientia × R. timschmicus), Rumex crispus, and Rumex acetosa) in comparison to metal hyperaccumulators (Viola baoshanensis and Sedum alfredii).Among the high biomass plants, R. crispus extracted Zn and Cd of 26.8 and 0.16 kg/ha, respectively, which was comparable to Zn accumulation by Sedum alfredii and Cd by Viola baoshanensis.However, Vertiveria zizanioides, which presented the highest biomass, accumulated only a small amount of each metal, with a much higher concentration in root than in shoot.Tolra`et al. [218] compared phytoremediation properties of hyperaccumulator Thlaspi praecox and non-hyperaccumulator species of Thlaspi arvense.Thlaspi arvense exposed to Cd exhibited toxicity symptoms in leaves in the form of chlorosis, while any symptoms were observed in T. praecox.Moreover T. praecox presented considerably higher root elongation rates than T. arvense under control conditions.T. arvense accumulated higher root Cd concentrations than T. praecox, while shoot Cd accumulation was significantly higher in T. praecox, which was above 2500 μg Cd/g DW.Similarly, Shen et al. [219] compared uptake and transport of Zn in the hyperaccumulator Thlaspi caerulescence and non-hyperaccumulator Thalspi ochroleucum.T. caerulescence was able to tolerate 500 mmol/m 3 Zn in solution without affecting growth and up to 1000 mmol/m 3 with a 25% decrease in dry weight, while in case of T.ochroleucum severe toxicity was observed for Zn concentration 500 mmol/m 3 .Moreover, T. caerulescence accumulated a higher concentration of Zn in shoots, while T.ochroleucum in roots.Presented results indicated that T. caerulescence exhibit a strongly expressed constitutive sequestration mechanism, which detoxifies a large amount of Zn in plant tissue.
Meanwhile, plants applied for phytostabilization treatment should avoid excessive uptake and transport of contaminants thus present low accumulation in steams (excluders) with a low value of the translocation factor [80].Moreover, in this process, the crucial requirements are related to morphology, density, and penetration depth of root [97,220,221].Plants with high root biomass, or fibrous rooting system are excellent candidates for metal stabilization in soil [222,223].
Favorable candidates for phytoremediation might be engineered plants.Transgenic plants with unique genes promotes fast growth rate, development of deep rooting system, abilities to detoxify hazardous pollutants, or tolerance to various, very often harsh climatic conditions.However, this technique despite the many benefits might bring potential environmental risk due to the possibility for invasion into natural plant communities [224].The transgenic Beta vulgaris L. with gene that synthesizes glutathione have been reported by Liu et al. [225] as an efficient agent for removal of Cd, Zn, and Cu from aqueous solution.The modified plants presented higher tolerance to heavy metals and stronger accumulation than wild-type plant.Similar, He et al. [226] overexpressed bacterial γglutamylcysteine synthetase in the cells of Populus tremula x P.alba.Transgenic plants were characterized by higher Cd uptake, accumulation in aerial parts, and tolerance to the presence of metal in nutrient solution.Meanwhile, Sharma and Yeh [227] proved that genetic engineered Arabidopsis and Nicotiana tabacum showed 4-7 times higher accumulation of Fe than wild-type plants.

Microbial Activity
Microbial activity in the rhizosphere has been considered as important parameter that has strong functions in plant growth and metal uptake.Microbes are involved in many significant processes associated with nutrient acquisition, cell elongation, metal detoxification and alleviation of stress in plants [230].A group of microbes in the soil, participating in phytoremediation, includes such as plant growth promoting bacteria (PGPB), phosphate-solubilizing bacteria (PSB), mycorrhizal-helping bacteria (MHB), and arbuscular mycorrhizal fungi (AMF) [231].Jeong et al. [232] tested the ability of phosphate-solubilizing bacteria for enhancing Cd bioavailability and phytoextraction potential of Brassica juncea and Abutilon theophrasti.Phosphate-solubilizing bacteria solubilize insoluble phosphates of soil into soluble plant available forms by secreting various organic acids, and therefore are able to stimulate plant nutrition and growth [233].However, performed analysis revealed that inoculation with Bacillus megaterium increased Cd accumulation by two folds compared to uninoculated plants, while did not remarkably affect plants biomass.Meanwhile, it has been suggested that the incorporation of plant growth promoting bacteria seems to be useful approach to improve plant growth and shoot and root biomass production [234,235].These bacteria could make the plants more tolerant to harmful contaminants, lower stress ethylene levels and decreased concentrations of proline and malondialdehyde [236].Marques et al. [237] found that inoculation of Helianthus annuus with PGPB reduced biomass losses growing on Zn and Cd contaminated soil.However, bacterial community decreased Zn and Cd accumulation in plant tissue.This strategy may be a reliable approach in phytostabilization.Meanwhile, Rajkumar and Freitas [238] observed that inoculation Brassica juncea with PGPB facilitate above-ground biomass production and at higher Ni concentration (300 mg/kg) in soil increased metal uptake by shoot and root compared to the uninoculated plant.Moreover, it was concluded that bacteria strains protect the plants against the toxic effects of nickel, probably due to the production of phytohormone-indole acetic acid (IAA), siderophore, and solubilization of phosphate.Several researches have been demonstrated the effect of arbuscular mycorrhizal fungi (AMF) on phytoremediation potential of plants.In the study Bhaduri and Fulekar [239] investigated the potential of Ipomea aquatica supported by AMF for Cd contaminated soil phytoremediation.Results showed that AMF enhanced accumulation of cadmium in plant tissues.Furthermore, inoculated plants exhibited improved Cd tolerance and resistance under stress conditions and thus lower reduction of biomass growing on contaminated soil than non-AMF plants.Similar research were performed by Gunathilakae et al. [240] with AMF inoculated Eichhornia crassipes.AMF colonization enhanced plant growth, biomass production, relative growth rate and Cd concentration in roots and shoots.

Phytoremediation-Benefits and Limitations
Phytoremediation involves a group of cost-effective and eco-sustainable green processes based on several mechanisms, which finally led to pollutants removal from aqueous or soil ecosystems and is a promising alternative for traditional remediation technologies.The estimated cost of phytoremediation amounts to $5-$40/ton of contaminated soil [241].Calculation prepared by Wan et al. [242] proved that total cost of phytoremediation of soil contaminated with arsenic, cadmium, and lead was $37.7/m 3 ,which is significantly lower than costs for other remediation techniques such as solidification ($87-$190/m 3 ) [243], extraction ($240-$290/m 3 ) [243], or vitrification ($75-$425/ton) [241].The significant advantage of this method is a vast variety of plants demonstrating the potential for accumulation, degradation, or stabilization of a satisfactory amount of contaminants.This group of plants includes diverse species i.e., grasses, legumes, aquatic and marsh plants, deciduous, and coniferous trees.Moreover, the broad spectrum of pollutants such as heavy metals, radionuclides, polyaromatic hydrocarbons, surfactants, pesticides, or pharmaceuticals may be subject to phytoremediation.The advantage of in-situ performed remediation is a limitation of contaminants spread with air or water, and prevention of secondary pollution.Despite many advantages phytoremediation technique has not still become Worldwide used technology.However, information provided for example by the U.S. Environmental Protection Agency (EPA) indicate that phytoremediation has been successfully used at many sitesacross the country.Moreover EPA pointed out that this techniques is used because requires less equipment and work than other remediation methods as well as helps control soil erosion and improves air quality.Also in the U.S exist dedicated companies offering commercial phytoremediation services targeting particular contaminants.One of the main disadvantage of phytoremediation is related to the duration of treatment processes.It might be a slowand time-consuming process, which last from months to several years [244].It should be mentioned that another obstacle is related to seasonality of the phytoremediation, which loses efficiency during the winter season.Moreover, phytoremediation may be limited by agronomic challenge-quality of the soil.Poor soil structure and low nutrition level might be the factors which have significant impact on remediation efficiency.Thereby proper preparation of field including irrigation and fertilization is required and in turn, phytoremediation costs might increase [245].The majority of previous research in the field of phytoremediation has only focused on a greenhouse experiment, maintained at special conditions (temperature, humidity, photoperiod), so recreating of this condition in the field experiment may be problematic.Furthermore, the phytoremediation process may be affected by several factors, for example, soil texture, soil pH, fertilization, coexistent pollutants, and climatic conditions, thus fields have to be appropriately adapted to provide high removal effectiveness.Moreover, harvested plants with accumulated contaminants may be recognized as a hazardous waste.A challenging area in the field of phytoremediation is plant disposal and thus suitable utilization methods are required.Some researchers proposed composting and compaction as a post-harvested plant management [246].Moreover, application of crops as a "bio-ore"-high grade and useful material for metal recovery was investigated.Thermal, thermochemical and chemical methods were used for extraction of nickel from biomass with obtaining respectively, ferronickel, Ni 2+ salts, and Ni 0 [247].Vaughan et al. [248] proposed combustion and leaching of nickel from tropical hyperaccumulator plant.This process led to producing unique impurity nickel hydroxide.A step forward in plant utilization is also novel adsorption-pyrolysis technology for recovering copper and cadmium from contaminated biomass after the phytoremediation process [249].The possibility of using a contaminated biomass as an adsorbent with functional groups, able to react with metals and retain them within biomass was proved.

Energy Generation from Harvested Plants
Recently, utilization of plant biomass as a non-fossil material for renewable, clean energy production progressively increases and currently is the 4th largest energy source in the world [250,251].Biomass may be easily converted into bio-solid (chips, pellets, briquettes), bio-liquid (methanol, ethanol, diesel), and bio-gaseous (hydrogen, biogas, syngas) fuels using thermochemical or biological methods.The conversion methods include such as combustion, pyrolysis, gasification, fermentation, and anaerobic decomposition [252].Biomass might be called "CO2-neutral" or "zero CO2-emission" energy source, since equal or even higher amount of carbon dioxide is used during plant photosynthesis processes than released when it is burned [253].In this case, there is no net increase in the atmospheric CO2 correlated with plant biomass use as fuel, in contrast to fossil fuels.Energy crops for bioenergy production should be characterized by high yield, fast growth, low fertilizer input, low energy input to its production, and low costs [254].Similar requirements are imposed for plant applied for phytoremediation process.Thus, recent studies suggested that also contaminated plant biomass from phytoremediation might be a promising source for bioenergy production [255,256].Some authors hypothesized that energy crops with high biomass such as Populus, Salix, Pinus, Helianthus annuus might be not only promising candidates as phytoremediation species, but also their biomass can be economically valorized for renewable energy production (bioethanol, bio-diesel, bio-gas, or bio-energy) [257].Table 2 shows examples of plant efficient in phytoremediation processes as well as in bio-energy production.
Witters et al. [258] predicted that Silage maize might be used for phytoremediation of soil contaminated with Cd with simultaneous application of post-harvest biomass as a source for renewable energy production.Silage maize biomass production was 20 Mg DW per hectare per year with Cd accumulation 0.022 kg per hectare per year.Performed life cycle analysis (LCA) revealed that Silage maize biomass might be converted by anaerobic digestion to biogas, with a production of 12,459 MJ energy per hectare per year, pointed out the positive effect of metals on energy production.Hunce et al. [259] compared the potential of Helianthus annuus and Silybum marianum growing on contaminated and non-contaminated soil, for biogas production.It was concluded that the presence of trace elements in plant biomass did not limit the potential of energy recovery.The biogas production potential of S. Marianum biomass (194-223 mL/g) was higher than that from H. annuus (134-154 mL/g).Meanwhile, Meers et al. [260] performed the field experiment using Zea mays for removal of Cd, Zn and Pb from contaminated soil.Application of energy Zea mays will valorized potential of phytoremediation techniques due to its biomass conversion to biogas via anaerobic digestion as a sustainable waste management.It was estimated that Zea mays biomass from field experiment might be converted in 33,000-46,000 kWh of renewable energy per hectare per year, which as a substituent of fossil energy, that will help reduce up to 21,000 kg per hectare per year CO2.In the study, Balsamo et al. [261] investigated effectiveness of grasses for remediation of lead contaminated soil, and biofuel production from their contaminated biomass.The Authors stated that the presence of lead in the grass material feedstock did not adversely affect the outcomes of the conversion processes.Furthermore, it was concluded that grasses might be a promising candidate for bioethanol or bio-crude oil production.

Summary
A large number of papers published in recent years indicate that phytoremediation is gaining interest both for scientists as well as for practical purposes.Searching for new plant species, contaminants that can be removed via phytoremediation techniques, or novel methods to enhance biomass yield and efficiency of the cleanup process are still in the developing stage.However, despite successes of phytoremediation confirmed by laboratory-scale greenhouse experiments, there is a gap in the field research, where the phytoremediation process is depending on real conditions and may be affected by numerous factors.Thus, there is a need to investigate phytoremediation at the field scale.Furthermore, an essential aspect of phytoremediation, which supposed to be envisaged, is the economic and ecological valorisation of contaminated biomass of plants after harvesting.There is still a need for further experiments to develop a productive and profitable method for plant biomass processing, when "bio-ore" generation with metal recovery is considered.An approach of combining phytoremediation aiming at biomass generation and its utilization as energy source should be more intensively investigated.This two-track approach for interconnection of phytoremediation processes with renewable bioenergy production from contaminated crops might bring tangible benefits, especially related to simultaneous clean up-process of large areas and thus significant amount of alternative energy production from waste, also taking into account reduction of CO2 production in comparison to using fossil fuel.This will allow to call phytoremediation "zero waste" sustainable environmental technology for soil remediation.Moreover, an interesting approach of research may be related to investigations on mutual symbiotic interactions between various plant or microbial activity in terms of enhancing plant growth, and thus phytoremediation efficiency.Key aspect is also development in the field of plant engineering, which provide plant unique features.Furthermore, a focus may be on investigations of factors affecting plant growth and plant selection for obtaining valuable products possible to be extracted (not only e.g., metals, but even biologically active compounds).

Figure 1 .
Figure 1.Mechanisms of phytoremediation with examples of removed pollutants.

Table 1 .
Summary of research on phytoremediation with respect to the main process mechanism.B.napus accumulated about 20 mg Cu, 2 mg Pb and 120 mg Zn/kg DW in shoots and 130 mg Cu, 30 mg Pb and 180 mg Zn/kg DW in roots of the control plants.EDDS was the most productive to rise the concentrations of Cu, Pb, Zn and Cd in the shoots and TF value.•Thesimultaneous application of EDTA and EDDS (ratio of 2:1) let to obtain the greatest Pb concentration of 647 mg kg DW in the shoots.
•The addition of 9 mmol EDTA/kg soil, led to a rise of Pb concentration in Poplar aerial parts(251 ± 35mg/kg). 89]

1
Figure 2 presents percentage distribution of research papers included inTable 1 for individual continents.
Figure 2. Percentage of research papers from Table1for the individual continents.

Table 2 .
Examples of plants applied in phytoremediation and bioenergy production.