INTRODUCTION
Soil and water pollution are major environmental issues in the world. Metals are not biodegradable, they generally have little mobility and the ability to persist in natural ecosystems for a long time, even if they are in small amounts in the environment (Chiţimus et al., 2016; Nithiyanantham et al., 2018; Strungaru et al., 2015).
Mining and smelting are important economic activities in Mexico (INEGI, 2010). The disposal of mining by-products, including metals and metalloids, produce considerable adverse environmental effects (Machado-Estrada et al., 2013). Each year, approximately 100 million tons of mining waste is generated in Mexico (SEMARNAT, 2010). Mining industries that extract Ag, Pb and Zn pour their residues into water bodies, which are employed for crop farming (Armienta et al., 2020); therefore, these contaminants might be incorporated into the food chain and pose serious risks for human health (Hazrat et al., 2013). In addition, the presence of these potentially toxic elements (PTE) could reduce land productivity (Prieto-Garcia et al., 2005). Mine tailings in Mexico also represent an important ecological problem due to the dispersion of pollutants (Cortés-Jiménez et al., 2013). The PTE in Mexico are Hg, As, Pb, Cd, Zn and Cr. The states from central Mexico have been disturbed by the pollution of soils and water with PTE (Covarrubias and Peña, 2017; González-Dávila et al., 2012; Hernández-Silva et al., 2012). In addition, intoxication by Hg in humans has been reported (Martínez-Trinidad et al., 2013).
In situ phytoremediation is based on the extraction of organic and inorganic pollutants from the environment, where plants grow in natural conditions and are exposed to different PTE (Figueroa et al., 2008). This strategy is non-disruptive, environmentally-friendly and cost-effective in the long-term. This process considers the level of contamination in the polluted site and the output of contaminants (van der Ent et al., 2013). The physicochemical parameters that influence the efficacy of in situ phytoremediation include pH, dissolved oxygen, sediment type, pollutant concentration, temperature, salinity, organic matter, weather, redox conditions, cation exchange capacity, hydrological cycle and mobilization of these contaminants in soil/water (Anawar, 2015; O’Connor et al., 2019).
A plant considered to be used for phytoremediation must have the following features: high accumulation capacity of contaminants, high biomass production, quick adaptation to prevailing environmental and climatic conditions, capacity for nitrogen fixation, fast growth, deep root system and high pollution translocation from roots to shoots (Hazrat et al., 2013; Maiti and Jaiswal, 2008).
In situ phytoremediation involves the study of several polluting elements, evaluates chemical interactions among ions in the water/soil, and assesses the ability of plants throughout their cycle to survive in contaminated environments (González-Chávez et al., 2017); therefore, these studies can be useful for planning remediation of sites that have been contaminated with PTE. This review summarizes the information available from in situ phytoremediation studies carried out in Mexico.
METHODS
Literature search was performed to analyze studies carried out in Mexico with plants used for in situ extraction of PTE and trace metals from soil and water. The information was taken from Scopus, Web of Science, Scielo, and Pubmed. Scientific reports were searched through the following keywords: plant “or” hyperaccumulator, phytoremediation “and” in situ, phytoextraction, and Mexico. Scientific documents written in Spanish were also included in this review. Publications involved in this paper dated from the last three decades.
USE OF NATIVE PLANTS FOR in situ PHYTOREMEDIATION
Native plants represent a good strategy for in situ phytoremediation studies (Santos-Jallath et al., 2012). Usually, native plants present better rates of survival, growth, adaptation and reproduction under environmental stress conditions, compared to introduced plants (Fernández et al., 2010; Machado-Estrada et al., 2013; Yoon et al., 2006). In spite of their high ability to accumulate heavy metals, many plants are unable to adapt to different climates or different environmental conditions including drought and salinity. In addition, the introduction of new plant species might affect the dynamics of some ecosystems because some plants are considered invasive (Yoon et al., 2006).
The identification and use of native plants with high tolerance and capability to accumulate or stabilize PTE can decrease the spreading of contaminants and help to regenerate vegetation toward remediation of those sites (Carrillo and González-Chávez, 2006; Cortés-Jiménez et al., 2013; Salas-Luevano, 2009; Sánchez-López et al., 2015). Furthermore, native plants could be used as biomonitors, since they provide evidence on the existence of contaminants and as bioremediators of areas that are contaminated with PTE (Jeddi and Chaieb 2018; Khalid et al., 2019; Ngayila et al., 2007; Tzvetkova and Petkova, 2015). Native plants can be considered as models for studying mechanisms of tolerance and accumulation (Carranza-Álvarez et al., 2008).
In situ ACCUMULATION OF TRACE METALS BY PLANTS IN MEXICO
There are many sites in Mexico contaminated with PTE, most of them exceed the levels of trace metals in water/soil considered toxic (Mireles et al., 2004; Puga et al., 2006). Forty-six plant species were found with potential use on in situ phytoremediation studies in Mexico (Table 1). Most of the reports correspond to areas of North Central Mexico polluted with trace metals due to mining activities. This region is characterized by arid and semi-arid climates where small trees grow. The information indicates that members of the Asteraceae family and Flaveria genus are the most cited plant species for in situ phytoremediation in Mexico. This might indicate that they have developed mechanisms to tolerate, accumulate, or to avoid heavy metals.
State | Site of study | Potential plant species | PTE (maximum accumulation reported in mg kg-1) | Reference | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
As | Cd | Cr | Cu | Fe | Hg | Mn | Se | Pb | Zn | ||||
Chihuahua | San Francisco del Oro | Baccharis glutinosa Pers. (Asteraceae) | 88.23 | - | - | - | - | - | - | - | - | 361.46 | Puga et al. (2006) |
Cynodon dactylon (L.) Pers (Poaceae) | - | - | - | - | - | - | - | - | - | 302.18 | |||
Parral, Santa Barbara, Naica | Eleocharis sp. (Cyperaceae) | 301 ± 0.72 | - | - | - | - | - | - | - | - | - | Flores-Tavizón et al. (2003) | |
Ciudad de México | Xochimilco | Eichhornia crassipes (Mart.) Solms (Pontederiaceae) | - | 0.7 ± 9 | 58.1 ± 7 | 27.3 ± 12 | 1660.4 ± 18 | - | 587.3 ±10 | - | 7.7 ± 6 | 135.5 ±17 | Carrión et al. (2012) |
Estado de México | Lerma River | Hydrocotyle ranunculoides L. f. (Araliaceae) | - | - | 15.76 | 30.37 | 20268 | - | 4324 | - | 7.7 | 172 | Zarazúa et al. (2013) |
Guanajuato | San German, León | Scirpus americanus Pers. (Cyperaceae) | 58 | 65 | 971 | - | - | - | - | 90 | - | - | Mauricio et al. (2010) |
Silver and gold mining | Ricinus communis L. (Euphorbiaceae) | - | 0.123 ± 0.008 | - | 2.6 ±0.07 | - | - | - | - | 2.74 ± 0.06 | - | Figueroa et al. (2008) | |
Guerrero | La Concha, Taxco | Gnaphalium chartaceum Greenm (Asteraceae) | - | - | - | 121 | - | - | 744 | - | 2901 | 4906 | Cortés-Jiménez et al. (2013) |
Wigandia urens (Ruiz & Pav.) Kunth (Boraginaceae) | - | - | - | - | - | - | - | - | - | 1027 | |||
Senecio salignus DC (Asteraceae) | - | - | - | - | - | - | - | - | - | 2477 | |||
Zimapán | Zea mays L. (Poaceae) | 4.26 ± 0.96 | 1.50 ± 0.88 | - | - | 1322 ± 284 | - | - | - | 91.7 ± 16.5 | 335 ± 24 | Armienta et al. (2020) | |
Rorippa nasturtium-aquaticum (L.) Hayek (Brassicaceae) | - | - | - | 350 | - | - | - | - | - | - | Carmona-Chit et al. (2016) | ||
Hidalgo | Zimapán | Parietaria pensylvanica Muhl. ex Willd. (Urticaceae) | - | - | - | - | - | - | - | - | - | 7630 | |
Commelina difusa Burm. f. (Commelinaceae) | - | - | - | - | - | - | - | - | - | 5086 | |||
San Francisco and Santa Ana, Zimapán | Viguiera dentata (Cav.) Spreng. (Asteraceae) | - | 21 ±3 | - | - | - | - | 189 ± 57 | - | - | 2231 ± 29 | Sánchez-López et al. (2015) | |
Gnaphalium sp. (Asteraceae) | - | - | - | - | - | - | 338 ± 36 | - | - | - | |||
Cuphea lanceolata W.T. Aiton (Lythraceae) | - | - | - | 352 ± 25 | - | - | - | - | - | - | |||
Nonoalco | Ambrosia psilostachya DC. (Asteraceae) | - | - | - | - | - | - | 89.8 ± 27.07 | - | - | - | Rivera-Becerril et al. (2013) | |
Zimapán | Ricinus communis L (Euphorbiaceae) | - | 8 | - | 48 | - | - | 180 | - | 170 | 590 | Ruiz et al. (2013) | |
Molango | Platanus mexicana Moric. (Platanaceae) | - | - | - | - | - | - | 410.86 ± 5.11 | - | - | - | Juárez-Santillán et al. (2010) | |
Asclepias curassavica L. (Apocynaceae) | - | - | - | - | - | - | 1507.69 ± 9.78 | - | - | - | |||
Solanum diversifolium Dunal (Solanaceae) | - | - | - | - | - | - | 562.57 ± 49.92 | - | - | - | |||
Pluchea symphytifolia (Mill.) Gillis (Asteraceae) | - | - | - | - | - | - | 1062.58 ± 7.0 | - | - | - | |||
Cnidoscolus multilobus (Pax) I.M. Johnst. (Euphorbiaceae) | - | - | - | - | - | - | 1055.80 ± 22.27 | - | - | - | |||
Dos Carlos, Pachuca | Solanum corymbosum Jacq. (Solanaceae) | - | - | - | 6 | - | - | - | - | - | - | Hernández-Acosta et al. (2009) | |
Brickellia veronicaefolia (Kunth) A. Gray (Asteraceae) | - | - | - | - | - | - | - | - | 5 | 20 | |||
Atriplex suberecta I. Verd. (Amaranthaceae) | - | 1 | - | - | - | - | - | - | - | - | |||
Cynodon dactylon (L.) Pers. (Poaceae) | - | - | - | - | - | - | 69 | - | - | - | |||
Zimapán | Prosopis laevigata (Humb. & Bonpl. ex Willd.) M.C. Johnst. (Fabaceae) | 1400 | - | - | - | - | - | - | - | - | - | Armienta et al. (2008) | |
Acacia farnesiana (L.) Willd. (Fabaceae) | 225 | - | - | - | - | - | - | - | - | - | |||
Querétaro | San Joaquín | Zea mays L. (Poaaceae) | - | - | - | - | - | 0.04 - 8.2 | - | - | - | - | Martínez-Trinidad et al. (2013) |
San Joaquín | Zea mays L. (Poaaceae) | - | - | - | - | - | 0.04 -8.7 | - | - | - | - | Hernández-Silva et al. (2012) | |
La Negra Mine | Nicotiana glauca Graham (Solanaceae) | 91.94 | 106.07 | - | 95.17 | - | - | - | - | - | 1984.48 | Santos-Jallath et al. (2012) | |
Flaveria pubescens Rydb. (Asteraceae) | - | - | - | 102.46 | - | - | - | - | 222.89 | 755.82 | |||
Tecoma stans (L.) Juss. ex Kunth (Bignoniaceae) | - | - | - | - | - | - | - | - | - | 942.8 | |||
San Luis Potosí | Villa de la Paz | Euphorbia prostrata Aiton (Euphorbiaceae) | 165 | - | - | 88 | - | - | - | - | 66 | 300 | Machado-Estrada et al. (2013) |
Parthenium incanum Kunth (Asteraceae) | 130 | - | - | 76 | - | - | - | - | 34 | 350 | |||
Zinnia acerosa (DC.) A. Gray (Asteraceae) | 93 | - | - | 70 | - | - | - | - | 49 | 340 | |||
Real de Catorce | Nicotiana tabacum L. (Solanaceae) | - | - | - | 10.33 | - | - | 5.15 | - | - | 6.93 | Levresse et al. (2012) | |
Villa de la Paz | Ambrosia artemisiifolia L. (Asteraceae), | - | - | - | - | - | - | - | - | - | 405.7 | Franco-Hernández et al. (2010) | |
Simsia amplexicaulis (Cav.) Pers. (Asteraceae), | - | - | - | 75.8 | - | - | - | - | - | - | |||
Flaveria angustifolia (Cav.) Pers. (Asteraceae), | 198.5 | - | - | - | - | - | - | - | - | - | |||
Flaveria trinervia (Spreng.) C. Mohr (Asteraceae) | 179.4 | - | - | - | - | - | - | - | - | - | |||
Artificial lagoon | Typha latifolia L. (Typhaceae) | - | 4.6 ± 0.08 | - | - | - | - | - | - | - | - | Carranza-Álvarez et al. (2008) | |
Scirpus americanus Pers. (Cyperaceae) | - | - | 344.6 ± 15.4 | - | 1229.9 ± 20 | - | 3330.2 ± 125 | - | 26.5 ± 3.5 | - | |||
Yucatán | Yum Balam Reserve | Thalassia testudinum Banks & Sol. ex K.D. Koenig (Hydrocharitaceae) | - | 0.2-5 | 0.5-1.1 | - | 141.4 - 504.3 | - | - | - | - | - | Avelar et al. (2013) |
Zacatecas | Asphodelus fistulosus L. (Asphodelaceae) | - | - | - | - | - | - | - | - | 917 ± 1 | 1946 ± 2 | Flores et al. (2018) | |
Dalea bicolor Humb. & Bonpl. ex Willd. (Fabaceae) | - | - | - | - | - | - | - | - | 970 ± 2 | 1296 ± 1 | |||
Fresnillo | Acacia schaffneri (S. Watson) F.J. Herm. (Fabaceae) | 3838 | 17 | - | - | - | - | - | - | 534 | - | Salas-Luévano et al. (2017) | |
Amaranthus hybridus L. (Amaranthaceae) | 2218 | 17 | - | - | - | - | - | - | 842 | - | |||
Arundo donax L. (Poaceae) | 1078 | 7.9 | - | - | - | - | - | - | 272 | - | |||
Asphodelus fistulosus L. (Asphodelaceae) | 4387 | 13 | - | - | - | - | - | - | 512 | - | |||
Buddleja cordata Kunth (Scrophulariaceae) | 3454 | 24 | - | - | - | - | - | - | 1282 | - | |||
Plantago lanceolata L. (Plantaginaceae) | 4150 | 17 | - | - | - | - | - | - | 556 | - | |||
Zacatecas | Guadalupe | Zea mays L. (Poaaceae) | 98.15 | - | - | 213.63 | - | - | 629.71 | - | 293.24 | 849.74 | González-Dávila et al. (2012) |
Francisco I. Madero | Amaranthus hybridus L. (Amaranthaceae) | - | - | - | - | - | - | - | - | 2208 ± 136 | - | Salas-Luevano et al. (2009) | |
Buddleja ser. Scordioides E.M. Norman (Scrophulariaceae) | - | - | - | - | - | - | - | - | 1378 ± 153 | - | |||
Cerdia congestiflora Hemsl. (Caryophyllaceae) | - | - | - | - | - | - | - | - | 1175 ± 126 | - | |||
El Bote, San Martín, Fresnillo and Noria de los Ángeles | Polygonum aviculare L. (Polygonaceae) | - | - | - | - | - | - | - | - | - | 9236 | Carrillo and González-Chávez (2006) | |
Jatropha dioica Sessé ex Cerv. (Euphorbiaceae) | - | - | - | - | - | - | - | - | - | 6249 |
Fuente: PTE: potentially toxic elements. Concentrations of heavy metals obtained in hyperaccumulators plants are shown in bold type.
The order of accumulation of heavy metals among the studied plants is as follows: Fe > Zn > Mn > Pb > As > Cr > Cu > Cd > Se > Hg. Studies with native vegetation growing on PTE polluted areas in México are scarce compared to other regions of the world (Carrillo and González-Chávez, 2006). Only 25 studies have been carried out in Southern Mexico (Avelar et al. 2013), where most areas are covered by rainforest.
Some reports have considered physicochemical characteristics of soil/water and the accumulation of PTE in plants (Carranza-Álvarez et al., 2008; Levresse et al., 2012; Mireles et al., 2004; Santos-Jallath et al., 2012). These characteristics are helpful to evaluate bioavailability of trace elements and their subsequent accumulation, sequestration or immobilization in plant tissues. Accumulation of PTE in plants depends on the chemical species of the PTE in soil/water, pH, age of the plant, mechanisms of mobilization at the sediment-water interface, and especially, the metabolism efficiency of plant ecotypes (Carranza-Álvarez et al. 2008; Levresse et al. 2012).
The highest accumulation of trace metals in roots is a common pattern in many phytoremediation studies in Mexico (Carrión et al., 2012; Martínez-Trinidad et al., 2013, Mauricio et al., 2010; Mireles et al., 2004). The distribution of PTE in roots or aerial parts is affected by variation in the organization of root tissues, the size of the metal influences its movement within plants, and the mobility rate in the transport of PTE from the root to aerial parts (Skorbiłowicz et al., 2016; Vaculík et al., 2012; Yabanli et al., 2014). In some cases, roots can act as a barrier for metal translocation to provide protection against heavvy metal toxicity (Liu et al., 2009). Furthermore, the transportation of PTE from roots to shoots could take months; nevertheless, Puga et al. (2006) found out that most of the trace metals they evaluated (As and Zn) lied in aerial parts of the plants. It is known that athmospheric deposition might be another factor associated to the high accumulation of toxic elements in aerial parts (De Temmerman et al., 2015).
Variations in accumulation patterns of PTE were observed in different plant species gathered from different sites of collection (Carmona-Chit et al., 2016; Franco-Hernández et al., 2010; Levresse et al. 2012; Salas-Luevano et al., 2009); this might be attributed to the varied accumulation/tolerance mechanisms developed by each plant species against PTE toxicity, as well as variations among plant populations (Wan et al., 2013).
HYPERACCUMULATOR PLANTS UNDER FIELD CONDITIONS IN MEXICO
According to van der Ent et al. (2013), plants capable of hyper-accumulating trace metals must meet these criteria: bioaccumulation factor (BF) higher than 1000 μg metal g-1 dry weight and translocation factor (TF) higher than 1. In México, a few metal-tolerant, accumulator, and hyperaccumulator plants have been reported (Table 1). Some of these plant species are cited in this review; for instance, Hydrocotyle ranunculoides, Parietaria pensylvanica and Commelina diffusa could be considered as hyperaccumulators of Zn (Carmona-Chit et al., 2016; Zarazúa-Ortega et al., 2013), whereas Polygonium aviculare accumulated 9230 mg Zn kg-1, which is close to the treshold (10,000 mg kg-1) considered for hyperaccumulator plants (Carrillo and González-Chávez, 2006). Rorippa nasturtium-aquaticum (synonym Nasturtium officinale W.T. Aiton) and Simsia amplexicaulis could be considered as strong accumulators of Cu (Franco-Hernández et al., 2010). Nicotina glauca, Flaveria angustifolia and Flaveria trinervia can be considered accumulators of As, rather than hyperaccumulators (Franco-Hernández et al., 2010; Santos-Jallath et al., 2012). Flaveria trinervia easily adapts to grow on different environments, which is a highly desirable criterion in phytoremediation studies. Buddleja scordioides is a good candidate to be used in phytoremediation of Pb-contaminated soils (Salas-Luevano et al., 2009); however, special attention is needed with Buddleja scordioides and Simsia amplexicaulis since they are also used in traditional medicine in Mexico (Cortés et al., 2006; Sotero-García et al., 2016). Thus, contaminants such as Pb and Cu could be ingested by humans and cause health problems. Puga et al., (2006) studied Baccharis glutinosa for the accumulation of As and Zn. Cynodon dactylon is a potential accumulator of Zn and Mn (Hernández-Acosta et al., 2009; Puga et al., 2006). Juárez-Santillán et al., (2010) studied plants in a mining area of Mn in Hidalgo, Mexico and identified Cnidoscolus multilobus, Platanus mexicana, Solanum diversifolium, Asclepius curassavica and Pluchea sympitifolia as accumulator species.
It is interesting to note that plants cited in this review such as Parietaria pensylvanica, Buddleja scordioides, Flaveria angustifolia and Flaveria trinervia are being reported for the first time for their ability to accumulate metals. This indicates that more studies should be carried out with flora from Mexico to evaluate their use in phytoremediation. From accumulator plant species, Hydrocotyle ranunculoides and Rorippa nasturtium-aquaticum are dispersed worldwide while Parietaria pensylvanica and Commelina diffusa are native to America.
PERSPECTIVES AND FUTURE NEEDS
Many phytoremediation reports are conducted using controlled conditions; however, the ability of plants for accumulating PTE under field conditions remains to be studied in Mexico. This review provides information about the potential plant species to accumulate As, Cd, Cr, Cu, Fe, Hg, Mn, Se, Pb and Zn (Table 1).
As far as we know, the mechanism of metal accumulation by plants cited in this review remains to be studied; thus, it is essential to generate scientific evidence to understand how these plant species accumulate or hyperaccumulate trace elements at the molecular level. Several plants that grow for years under heavy metal-induced stress have used physiological strategies for their adaption and growth under these conditions. Some of these mechanisms include the enhance of xylem loading capacity for PTE, as well as the excretion of phytochelatins, metallothioneins and low molecular weight organic acids (Koźmińska et al., 2018). The efficacy of in situ phytoremediation will rely on understanding these molecular mechanisms.
Some microorganisms such as Streptomyces tendae, Funneliformis mosseae, Bacillus thuringiensis, Microbacterium saperdae, Pseudomonas monteilii, Enterobacter cancerogenes, Serratia marcescens and Rhodotorula mucilaginosa (Babu et al., 2013; Dimkpa et al., 2009; Hassan et al., 2013; Ji et al., 2012; Whiting et al., 2001) can enhance phytoremediation through several pathways: i) accelerating plant growth, ii) increasing bioavailability of PTE, iii) facilitating metal translocation from the soil to the roots, and iv) inducing the translocation from the roots to shoots (Ma et al., 2001; Rajkumar et al., 2012). Mycorrhizal status of PTE-accumulating plants growing in contaminated sites should also be studied. The interactions among Mexican native plants and microorganisms is an interesting topic to study in the framework of in situ phytoremediation. It is important to use herbarium techniques for taxonomic identification of plant species to be incorporated in phytoremediation studies under controlled and field conditions.
Before carrying out a phytoremediation study, the speciation of metals in soil and water should also be considered to evaluate their bioavailability, mobilization processes of these elements in soil-plant or water-plant systems, and their possible interactions with soil particles (Guo et al., 2019; Pan et al., 2019).
CONCLUSIONS
Native plants should be considered as a good strategy for remediation and reclamation of soil and water contaminated with PTE. This review clearly demonstrates that more studies are needed with flora from Mexico to evaluate their use in phytoremediation. There are several crop species (i.e. Amaranthus hybridus and Zea mays) that present tolerance to heavy metal contamination. Some of these plants could be metal excluders. In addition, the molecular mechanisms by which plants hyperaccumulate toxic elements remain to be explored.