Land, surface waters, and ground water worldwide, are increasingly affected by contaminations from industrial, research experiments, military, and agricultural activities either due to ignorance, lack of vision, carelessness, or high cost of waste disposal and treatment. The rapid build-up of toxic pollutants (metals, radionuclide, and organic contaminants in soil, surface water, and ground water) not only affects natural resources, but also causes major strains on ecosystems. Interest in phytoremediation as a method to solve environmental contamination has been growing rapidly in recent years. This green technology that involved “tolerant plants” has been utilized to clean up soil and ground water from heavy metals and other toxic organic compounds. Phytoremediation involves growing plants in a contaminated matrix to remove environmental contaminants by facilitating sequestration and/or degradation (detoxification) of the pollutants. Plants are unique organisms equipped with remarkable metabolic and absorption capabilities, as well as transport systems that can take up nutrients or contaminants selectively from the growth matrix, soil or water. As extensive as these benefits are, the costs of using plants along with other concerns like climatic restrictions that may limit growing of plants and slow speed in comparison with conventional methods (i.e., physical and chemical treatment) for bioremediation must be considered carefully. While the benefits of using phytoremediation to restore balance to a stressed environment seem to far outweigh the cost, the largest barrier to the advancement of phytoremediation could be the public opposition. The long-term implication of green plant technology in removing or sequestering environmental contaminations must be addressed thoroughly. As with all new technology, it is important to proceed with caution.
The success of green technology in phytoremediation, in general, is dependent upon several factors. First, plants must produce sufficient biomass while accumulating high concentrations of metal. In some cases, an increased biomass will lower the total concentration of the metal in the plant tissue, but allows for a larger amount of metal to be accumulated overall. Second, the metal-accumulating plants need to be responsive to agricultural practices that allow repeated planting and harvesting of the metalrich tissues. Thus, it is preferable to have the metal accumulated in the shoots as opposed to the roots, for metal in the shoot can be cut from the plant and removed. This is manageable on a small scale, but impractical on a large scale. If the metals are concentrated in the roots, the entire plant needs to be removed. Yet, the necessity of full plant removal not only increases the costs of phytoremediation, due to the need for additional labor and plantings, but also increases the time it takes for the new plants to establish themselves in the environment and begin accumulation of metals.
The availability of metals in the soil for plant uptake is another limitation for successful phytoremediation. For example, lead (Pb2+), an important environmental pollutant, is highly immobile in soils. Lead is known to be “molecularly sticky” since it readily forms a precipitate within the soil matrix. It has low aqueous solubility, and, in many cases, is not readily bioavailable. In most soils capable of supporting plant growth, the soluble Pb2+ levels are relatively low and will not promote substantial uptake by the plant even if it has the genetic capacity to accumulate the metal. In addition, many plants retain Pb2+ in their roots via absorption and precipitation with only minimal transport to the aboveground harvestable
plant portions. Therefore, it is important to find ways to enhance the bioavailability of Pb2+ or to find specific plants that can better translocate the Pb2+ into harvestable portions [
Although there are some challenges associated with the phytoremediation, it remains a very promising strategy and feasible alternative. However, in many situations, soil contamination may have unique factors that require special evaluation. Some plants may only accumulate these essential elements and prevent all others from entering. For plants termed as “hyperaccumulators” can extract and store extremely high concentrations (in excess of 100 times greater than non-accumulator species) of metallic elements [
The principal application of phytoremediation is for lightly contaminated soils and waters where the material to be treated is at a shallow or medium depth and the area to be treated is large. This will make agronomic techniques economical and applicable for both planting and harvesting. In addition, the site owner must be prepared to accept a longer remediation period. Plants that are able to decontaminate soils does one or more of the following: 1) plant uptake of contaminant from soil particles or soil liquid into their roots; 2) bind the contaminant into their root tissue, physically or chemically; and 3) transport the contaminant from their roots into growing shoots and prevent or inhibit the contaminant from leaching out of the soil.
Moreover, the plants should not only accumulate, degrade or volatilize the contaminants, but should also grow quickly in a range of different conditions and lend themselves to easy harvesting. If the plants are left to die in situ, the contaminants will return to the soil. So, for complete removal of contaminants from an area, the plants must be cut and disposed of elsewhere in a nonpolluting way. Some examples of plants used in phyoremediation practices are the following: water hyacinths (Eichornia crassipes); poplar trees (Populus spp.); forage kochia (Kochia spp); alfalfa (Medicago sativa); Kentucky bluegrass (Poa pratensis); Scirpus spp, coontail (Ceratophyllum demersum L.); American pondweed (Potamogeton nodosus); and the emergent common arrowhead (Sagittaria latifolia) amongst others [
Four heavy metal concentrations in soils (Cu, Cr, As, and Pb) were examined to see if removal through the process of phytoremediation was possible. Tomato and mustard plants were able to extract different concentrations of each heavy metal from the soils. The length of time that the soils were exposed to the contaminants affected the levels of heavy metals accumulation. Today, many institutions and companies are funding scientific efforts to test different plants' effectiveness in removing wide ranges of contaminants. Scientists favor Brassica juncea and Brassica olearacea, two members of the mustard family, for phytoremediation because these plants appeared to remove large quantities of Cr, Pb, Cu, and Ni from the soil [
Vetiver (Vetiveria zizanioides L.) belongs to the same grass family as maize, sorghum, sugarcane, and lemon grass. It has several unique characteristic as reported by the National Research Council [
Vetiver grass is highly suitable for phytoremedial application due to its extraordinary features. These include a massive and deep root system, tolerance to extreme climatic variations such as prolonged drought, flood, submergence, fire, frost, and heat waves. It is also tolerant to a wide range of soil acidity, alkalinity, salinity, sodicity, elevated levels of Al, Mn, and heavy metals such as As, Cr, Ni, Pb, Zn, Hg, Se, and Cu in soils [
Various uses of vetiver grass are known worldwide. In South Africa, it was used effectively to stabilize waste and slime dams from Pt and Au mines [
Cogon grass, generally occurs on light textured acid soils with clay subsoil, and can tolerate a wide range of soil pH ranging from strongly acidic to slightly alkaline [
The roots can penetrate to a soil depth of about 58 cm in alluvial soil. More than 80 percent of shoots can originate from rhizomes less than 15 cm below the soil surface. The average number of shoots of cogon grass was about 4.5 million per hectare, producing 18,500 kg·ha−1 of leaves and rhizomes (11,500 kg of leaves and 7000 kg of rhizomes) [
Carabao grass is a vigorous, creeping perennial grass with long stolons and rooting at nodes. Its culms can ascend to about 40 to 100 cm tall, branching, solid, and slightly compressed where new shoots can develop at every rooted node. Under a coconut plantation, a yield of about 19,000 kg·ha−1 of green materials was obtained. It grows from near sea-level up to 1700 m altitude in open to moderately shaded places. It is adapted to humid climates and found growing gregariously under plantation crops and also along stream banks, roadsides, and in disturbed areas. This grass can adapt easily to a wide range of soils [
Phytoremediation is described as a natural process carried out by plants and trees in the cleaning up and stabilization of contaminated soils and ground water. It is actually a generic term for several ways in which plants can be used for these purposes. It is characterized by the use of vegetative species for in situ treatment of land areas polluted by a variety of hazardous substances [
Garbisu [
Several types of phytoremediation are being used today. One is phytoextraction, which relies on a plant’s natural ability to take up certain substances (such as heavy metals) from the environment and sequester them in their cells until the plant can be harvested. Another is phytodegredation in which plants convert organic pollutants into a non-toxic form. Next is phytostabilization, which makes plants release certain chemicals that bind with the contaminant to make it less bioavailable and less mobile in the surrounding environment. Last is phytovolitization, a process through which plants extract pollutants from the soil and then convert them into a gas that can be safely released into the atmosphere [
Phytoremediation is a naturally occurring process recognized and documented by humans more than 300 years ago [
Phytoremediation of heavy metals from the environment serves as an excellent example of plant-facilitated bioremediation process and its role in removing environmental stress. Traditionally, when an area becomes contaminated with heavy metals, the area must be excavated and the soil should be removed and put to a landfill site [
Many human diseases result from the buildup of toxic metals in soil, making remediation crucial in protecting human health. Lead is one of the most difficult contaminants to be removed from the soil and one of the most dangerous. According to Lasat [
In 2005, Cortez [
Xia and Ma [
Letachowicz et al. [
The Philippines is blessed to have relatively high mangrove diversity having 35 species [
Mangroves are higher plants, which are found mostly in the intertidal areas of tropical and subtropical shorelines and show remarkable tolerance to high amounts of salt and oxygen poor soil. The mechanisms of mangrove to keep the salt away from the cytoplasm of the cell were through the excretion of salt in their salt glands found in the leaves and roots and through storage of salts in the mature leaves, bark and wood [
Few studies were conducted about phytoremediation potential of mangroves and other wetland plant species. However, those researchers paved the way to explore more species of mangroves particularly the native species present in the area, for their feasibility to accumulate heavy metals. Zheng et al. [
MacFarlane and Burchett [
Cheng [
Sari et al. [
Shete et al. [
Nazli and Hashim [
Qui et al. [
The research of Nirmal et al. [
Phytoremediation is a cleanup technology for metal contaminated soils, specifically Pb. In order for this type of remediation strategy to be successful, it is necessary to utilize metal accumulating plants to extract environmentally toxic metals from the soil, such as Pb, Ni, Cr, Cd and Zn. Certain plants have been identified not only to accumulate metals in the plant roots, but also to translocate the accumulated metals from the root to the leaf and to the shoot. While many plants performed this function, some plants, known as “hyperaccumulators”, can accumulate extremely high concentrations of metals in their shoots (0.1% to 3% of their dry weight) [
Bioremediation process would be extremely slow because the rate of bioemediation is directly proportional to growth rate while the total amount of bioremediation is correlated with a plant’s total biomass. No plant has been discovered yet capable of meeting all the ideal criteria of an effective phytoremediator. These criteria are fast growing, deep and extensive roots, high biomass, easy to harvest and hyperaccumulators of a wide range of toxic metals. A Pb absorption study by Huang and Cunningham [
Two main amendment techniques have been used to increase the bioavailability of Pb in soils and the mobility of Pb within plant tissue by lowering soil pH and adding synthetic chelates. Soil pH is a significant parameter in the uptake of metal contaminants because soil pH value is one of the principal soil factors controlling metal availability [
The physiological and biological mechanisms involved in Pb uptake of plants involving root to shoot transport of Pb may require some time to develop and become functional. Since plant species can differ significantly in Pb uptake and translocation, the success of using plants to extract Pb from contaminated soils requires the following: 1) the identification of Pb accumulating plants that can survive in the presence of contaminants; 2) the measurement of the concentration of pollutant in the soil, and 3) knowledge of chemistry (availability or speciation) of the metal in the soil matrix. The combination of soil amendment and foliar fertilizer application to plants capable of absorbing and translocation of Pb may be an effective means of remediating an area with varying levels of Pb concentrations.
Other model of phytoremediators includes various varieties of transgenic trees. Trees are ideal in the remediation of heavy metals because they can withstand higher concentrations of pollutants due to their large biomass. As such, they can accumulate large amounts of the contaminants in their systems because of their size capable of reaching huge area and great depths due to their extensive root systems. Furthermore, they can stabilize an area, prevent erosion, and minimize spread of contaminant because of their perennial presence. They can also be easily harvested and removed from the area with minimal risk, effectively taking with them a large quantity of the pollutants that were once present in the soil [
Earlier discussion has illustrated many advantages and disadvantages of transgenic phytoremediation. The primary advantages of using plants in bioremediation are as follows: it is more cost-effective; more environmentally friendly; and more aesthetically pleasing than conventional methods. The conventional methods are usually expensive and environmentally disruptive [
More benefits are derived through phytoextraction. It enables scientists to reclaim and recycle usable materials, including a wide variety of precious metals from the soil [
Moreover, concerns have been raised regarding the potential for contaminants to move up the food chain more quickly. This problem may occur if toxic materials are sequestered in consumable sources such as plants [
The global problem concerning contamination of the environment as a consequence of human activities is increasing. Most of the environmental contaminants are chemical by-products such as Pb. Lead released into the environment makes its way into the air, soil and water. Lead contributes to a variety of health effects such as decline in mental, cognitive, and physical health of the individual. An alternative way of reducing Pb concentration from the soil is through phytoremediation. Phytoremediation is an alternative method that uses plants to clean up contaminated area. Hence, Paz-Alberto et al. [
Results of the study (
§Means in respective columns (1 and 2) with the same letter(s) are not significantly different at 5% level of significance. †Means in respective rows (1 and 2) with the same letter(s) are not significantly different at 5% level of significance.
grass is more tolerant to Pb-contaminated soil compared with carabao grass. The important implication of the findings of this study is that vetiver grass can be used for phytoextraction on sites contaminated with high levels of heavy metals, particularly Pb [
A field survey was conducted by Bautista [
As discussed previously, there are several different methods through which phytoremediation can occur. However, in order to maximize the success of a phytoremediation strategy, it is critical to have significant metal bioavailability at a contaminated site as well as a large quantity of plant biomass with high rates of growth. Metal contaminants that are not soluble, may limit the success of phytoremediation. In most Pb contaminated soils usually less than 0.1% of the total Pb present is bioavailable for plant uptake. The plants grown in a contaminated soil accumulated less Pb in both the roots and shoots than the plants grown hydroponically in a solution with a similar Pb concentration. The difference in uptake was because the Pb in the solution was much more bioavailable to the plants.
It should be noted that while hydroponic tests do not reflect accurately the accumulation potential in terrestrial applications, these tests could be valuable in the screening for Pb accumulating plant species and tolerance levels. The second limitation in Pb phytoextraction is the poor translocation of the metal from the roots to the harvestable shoots. In the plants that do translocate Pb, translocation is less than 30% [
Research has been conducted in the field to improve both the uptake and translocation of Pb through induced hyperaccumulation, which involves soil pH adjustments or the application of synthetic chelates. In general, the more biomass that the plant has, the more metal can be accumulated since the metal uptake is a function of the overall biomass [
The focus of this study were on the accumulation of heavy metals in plants most commonly found in mine tailings of Victoria, Manlayan, Benguet, Philippines and identification of the different plant species within the area of the study. These plant species were assumed to be potential phytoremediation species [
The heavy metals extracted from the plants in the mine tailing were Cu, Cd, Pb and Zn. The fourteen plant species that were identified within the study were: Eleusine indica L.; Amaranthus spinosus L.; Alternathera sessilis L.; Portuluca oleracea L.; Fimbristylis meliacea L., Vahl, Mikania cordata ((Burm. F.) B. l. Robins; Polygonun barbatum L.; Achyranthes aspera L., Blumea sp., Cyperus alternifolus L.; Crassocephalum crepidioides (Benth.) S. Moore; Cyperus compactus Retz.; Desmodium sp. and Muntingia calabura L. These plants absorbed certain metals at low and high levels. Among the plants speciesA. spinosus was found to have almost all the metals extracted in large amounts particularly Pb. The other plant species with high concentration of Pb were A. sessilis, Desmodium sp., P. oleracea, and A. aspera. E. indica has the highest concentration of Zn together with M. cordata, C. compactus, F. maliacea and A. spinosus. In contrast, Cd was found in trace amount in soil, but high in the following species: C. crepidioides, P. oleracea, A. sessilis, and C. alternifolius. Nickel was found high only in A. sessilis and Blumea sp. but trace amount in Desmodium sp. and F. meliacea. Also, high Cu concentrations were found in A. spinosus and P. oleracea.
In this study, the phytoremediation potential was dependent on population within species. The potential of the surveyed species mentioned for phytoremediation was remarkable and promising because of the presence of heavy metals suspected to have accumulated in the soil. Root system of these plants showed higher root to shoot ratios compared to other plants found in the area indicating high translocation of metals to the shoot. These species also plays an important role in the phytostabilization of metals to reduce leaching and run off. Also, these may be transformed to less toxic forms. These typical plants have dense root systems which can be effective for phytostabilization and elimination of contaminants such as Pb, Cd, Zn, As, Cu, and Ni in mine tailing sites.
A similar study conducted in Poland was worth including in this section. Wislocka et al. [
Research and development has its own benefits and inconveniences. One of the inconveniences is the generation of enormous quantity of diverse toxic and hazardous wastes and its eventual contamination to soil and groundwater resources. Ethidium Bromide (EtBr) is one of the commonly used substances in molecular biology experiments. It is highly mutagenic and moderately toxic substance in DNA-staining during electrophoresis. Interest in phytoremediation as method to solve chemical contamination has been growing rapidly in recent years. The technology has been utilized to clean up soil and groundwater from heavy metals and other toxic organic compounds in many countries like the United States, Russia and most of European countries. Phytoremediation requires somewhat limited resources and is very useful in treating a wide variety of environmental contaminants. It is in this context that Uera et al. [
This study used tomato (Solanum lycopersicum), mustard (Brassica alba), vetiver grass (Viteveria zizanioides), cogon grass (Imperata cylindrical), carabao grass (Paspalum conjugatum) and talahib (Saccharum spontaneum) to remove EtBr from laboratory wastes. The six tropical plants were planted in individual plastic bags containing 10% EtBr-stained agarose gel. The plants were allowed to establish and grow in the soil for 30 days. Ethidium Bromide content of the test plant s and the soil were analyzed before and after soil treatment. Ethidium Bromide contents of the plants and soils were analyzed using an UV VIS spectrophotometer.
Results showed a highly significant (p ≤ 0.001) difference in the ability of the tropical plants to absorb the EtBr from the soils. Mustard registered the highest absorption of EtBr (1.4 ± 0.12 µg·kg−1) followed by tomato and vetiver grass with average uptake of 1.0 ± 0.23 and 0.7 ± 0.17 µg·kg−1 EtBr, respectively. Cogon grass, talahib, and carabao grass had the least amount of EtBr absorbed (0.2 ± 0.6 µg·kg−1). Ethidium bromide content of the soil planted with mustard was reduced by 10.7%. This was followed by tomato with an average reduction of 8.1%. Only 5.6% reduction was obtained from soils planted to vetiver grass. Soils planted to cogon grass, talahib and carabao grass had the least reduction of 1.52% from its initial EtBr content (
Environmentally hazardous and health risk substances in animals and humans in the environment have increased as a result of continuing anthropogenic activities. Exam ples of these activities are food processing, laboratory, food production, industrial and other relative activities that use various forms of acrylamide. All acrylamide in
§Means in column followed by a common letter(s) are not significantly different from each other at p ≤ 0.05.
the environment are man-made. It is the building block for the polymer, polyacrylamide, which is considered to be a non-toxic additive. However, if the polymerization process is not perfect and complete, the polyacrylamide may still contain acrylamide which is toxic and may pose risks and hazards to the environment. Another form of acrylamide may pose danger as well in the environment is the acrylamide monomer, also a very toxic organic substance that could affect the central nervous system of humans and is likely to be carcinogenic.
Phytoremediation could be a tool to somehow absorb this neurotoxic agent and lessen the contamination in the soil. This technology could lessen the soil and water contamination by acrylamide thereby limiting the exposure of animals and humans. This technique may also help solve the problem of disposing of contaminated acrylamide waste materials. Thus, Paz-Alberto et al. [
Among the plants tested, the highest concentration of acrylamide was absorbed by the whole plant of mustard (6512.8 mg·kg−1) compared with pechay (3482.7 mg·kg−1), fern (2015.4 mg·kg−1), hogweeds (1805.3 mg·kg−1), vetiver grass (1385.4 mg·kg−1) and snake plants (887.5 mg·kg−1). Results of the study regarding the acrylamide absorption of the whole plants of mustard and pechay conformed to previous findings of other studies (
All the test plants planted in soil without acrylamide had survival rate of 100%. The 100 percent survival rate of vetiver grass and snake plant was due to the tolerance of these plants to acrylamide (
Results of the study proved that all the test plants are potential phytoremediators of acrylamide. However, mustard and pechay were the most effective as they absorbed the highest acrylamide concentrations in their roots, shoots and the whole plants. On the other hand, vetiver grass and snake plant had the highest uptake of acrylamide even though these plants did not absorb the highest acrylamide concentration. Therefore, these two plants can be considered as the best phytoremediator of acrylamide because they are perennial plants with heavier biomass with long, dense and extended root system. As such, these plants are capable of absorbing acrylamide in the soil for a long period of time.
As preventive measures and for application purposes, vetiver grass and snake plants could be planted along and around the wastewater treatment ponds of laboratories
using polyacrylamide gel. These plants can prevent further migration of pollutants to the environment aside from making the ponds more resistant to soil erosion. Further studies are suggested to evaluate acrylamide contaminations from laboratory washing, primary treatment pond, and seepage ponds that have earth dikes. Vetiver grass and snake plants are recommended for further phytoremediation studies for longer period of time to test the reduction of acrylamide in soil. Moreover, the outcome of acrylamide accumulation in the plants is also recommended for further study in conjunction with labeled-carbon tracer to determine its effects on the plants.
Phytoremediation using “green plants” has potential benefits in restoring a balance in stressed environment. It is an emerging low cost technology, non-intrusive, and aesthetically pleasing using the remarkable ability of green plants to metabolize various elements and compounds from the environment in their tissues. Phytoremediation technology is applicable to a broad range of contaminants, including metals and radionuclides, as well as organic compounds like chlorinated solvents, polycyclic aromatic hydrocarbons, pesticides, explosives, and surfactants. However, phytoremediation technology is still in its youthful development stages and full scale application is still inadequate. As with all new technology, it is important to proceed with caution.
The largest barrier to the advancement of phytoremediation, however, may be public opposition to genetic modification in general. Because all natural hyperaccumulator species are small in size, genetic modification can be used to introduce this technology to other species or to increase the biomass of the natural hyperaccumulators in order to create effective phytoremediators. This public opposition was the same fears that surround the issue of genetic modification of crops, and includes concerns regarding decreased biodiversity, the entry of potentially harmful genes into products consumed by humans, and the slippery slope created by introducing and transferring novel, foreign DNA between non-related species. Nonetheless, the benefits of using phytoremediation to restore balance to a stressed environment seem to far outweigh the costs.