生物修复固废

ita si ri Ann inee 3 M Environmental problems posed by municipal solid waste (MSW) are well documented. Scientifically designed landfills and/or open dumpsites are used to dispose MSW in many developed and developing countries. Non-availability of land and need to reuse the dump- environment, as evident from the diversity of refuse compo- may also be used in leachate treatment (Maurice, 1998). Landfill vegetation often shows signs of damage commonly and as favorable to root growth as is necessary to achieve 2. Phytoremediation Exhaustive information on the state of the science and engineering of phytoremediation is available inMcCutcheon and Schnoor (2003). These authors have approached the subject from the perspectives of biochemistry, genetics, * Corresponding author. Tel.: +91 44 22301283; fax: +91 44 22354717. E-mail addresses: nag_nag@hotmail.com, rnagendran@gmail.com (R. Nagendran). Waste Management 26 sition with respect to location and time. Landfills hold wastes containing a wide range of organic molecules of both natural and xenobiotic origin. In many developed countries, municipal solid wastes (MSW) are dumped in scientifically designed sanitary landfills. In many developing countries, they are dumped in an uncontrolled manner without any precaution to deal with gas emissions and leachate generation, which pose a threat to the environment. Natural or planted vegetation on a landfill has an important role in erosion control and removal of contam- inants, besides imparting aesthetic value. Moreover, it desired plant performance (Vogel, 1987). Although reviews on phytoremediation of sites contam- inated with a variety of contaminants are readily available (Siciliano and Germida, 1998a; Lasat, 2002; Schwitzguebel et al., 2002), the applicability of this technology in remedi- ation and rehabilitation of municipal solid waste dumpsites has not been given its due. The present review, an off-shoot of studies on rehabilitation of municipal solid waste dump- sites, attempts to fill this gap by leaning on research find- ings, especially those reported in the last two decades. site space, especially in urban areas, call for rehabilitation of these facilities. A variety of options have been tried to achieve the goals of rehabilitation. In the last couple of decades, phytoremediation, collectively referring to all plant-based technologies using green plants to remediate and rehabilitate municipal solid waste landfills and dumpsites, has emerged as a potential candidate. Research and develop- ment activities relating to different aspects of phytoremediation are keeping the interest of scientists and engineers alive and enriching the literature. Being a subject of multi-disciplinary interest, findings of phytoremediation research has resulted in generation of enormous data and their publication in a variety of journals and books. Collating data from such diverse sources would help understand the dynamics and dimensions of landfill and dumpsite rehabilitation. This review is an attempt in this direction. � 2006 Elsevier Ltd. All rights reserved. 1. Introduction A landfill is an extremely variable and heterogeneous caused by the presence of landfill gas (LFG) in the root zone. The goal for the reconstruction of a suitable medium for landfill revegetation is to provide a capping that is deep Phytoremediation and rehabil landfills and dump R. Nagendran a,*, A. Selvam a, Ku a Centre for Environmental Studies, b Department of Environmental Eng Accepted Abstract 0956-053X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2006.05.003 tion of municipal solid waste tes: A brief review an Joseph a, Chart Chiemchaisri b a University, Chennai 600025, India ring, Kasetsart University, Thailand ay 2006 www.elsevier.com/locate/wasman (2006) 1357–1369 toxicology, and pathway analysis. Their work covers the following aspects of phytoremediation: overview of science and applications; fundamentals of phytotransformation and control of contaminants; science and practice for aromatic, phenolic, and hydrocarbon contaminants; transformation and control of explosives; fate and control of chlorinated solvents and other halogenated compounds; modeling, design, and field pilot testing; and latest advances. Phytoremediation, collectively referring to all plant- based technologies, uses green plants to remediate contam- inated sites (Sadowsky, 1999). This technology draws its inspiration from the myriad of physical, chemical and bio- logical interactions occurring between plants and the envi- ronmental media (Fig. 1). Phytoremediation is evolving into a cost-effective means of managing wastes, especially excess petroleum hydrocarbons, polycyclic aromatic hydrocarbons, explosives, organic matter, and nutrients. Applications are being tested for cleaning up contaminated soil, water, and air (McCutcheon and Schnoor, 2003). Sev- eral features make phytoremediation an attractive alterna- tive to many of the currently practiced in situ and ex situ technologies. These include: low capital and maintenance costs, non-invasiveness, easy start-up, high public accep- tance and the pleasant landscape that emerges as a final product (Boyajian and Carreira, 1997). In the last several decades, phytoremediation strategies have been examined as a means to clean up a number of organic and inorganic pollutants, including heavy metals (Kumar et al., 1995; Salt et al., 1995; Chaney et al., 1997), chlorinated solvents (Wal- ton et al., 1994; Haby and Crowley, 1996), agrochemicals (Anderson et al., 1994; Hoagland et al., 1997; Kruger et al., 1997), polycyclic aromatic hydrocarbons (Aprill and Sims, 1990; Reilly et al., 1996), polychlorinated biphe- nyls (Brazil et al., 1995; Donnelly and Fletcher, 1995), munitions (Schnoor et al., 1995) and radio nuclides (Entry et al., 1997). These soluble organic and inorganic contam- inants, which move into plant roots or rhizosphere by the mass flow process of diffusion, appear to be most amenable to the remediation process (Schnoor et al., 1995; Cunning- ham et al., 1996). In several instances, plants and/or their attendant rhizosphere microbes have been shown to trans- form some chemical compounds to some degree (Walton et al., 1994; Crowley et al., 1996; Siciliano and Germida, 1998b). 1358 R. Nagendran et al. / Waste Management 26 (2006) 1357–1369 Fig. 1. Plant–environment interactions. Source: Licht and Isebrands (2005). Plants are known to sequester, degrade and stimulate the degradation of organic contaminants in soil (Ander- son et al., 1993; Shimp et al., 1993). The sequestration of heavy metals by plants is an effective method of reduc- ing heavy metal contamination in soil (Cunningham et al., 1995). Sequestration of toxicants by plants is an impor- tant area of phytoremediation research. Plants are known to accumulate a variety of toxicants from soil (Paterson et al., 1990) and if the toxic chemical is metabolically stable and mobile, it may be transferred via apoplast or symplast compartments, or both, throughout most of the plant as parent compound and stored at highly bioconcentrated levels (McFarlane et al., 1987). However, the mechanisms by which plants stimulate the disappear- ance of hazardous organics from soil are not fully understood. In view of its demonstrated potential, phytoremediation has been gaining importance in rehabilitation of contami- nated sites including MSW dumpsites. Many types of phy- toremediation processes have been described based on the kind of mechanism. These include: phytoextraction, rhizo- filtration, phytovolatilization, phytodegradation, rhizo- sphere biodegradation, hydraulic pumping, phytosorption and phytocapping. Fig. 2 outlines the common processes involved in phytoremediation. Processes and contaminants dealt with by different phytoremediation processes are pre- sented in Table 1. The selection of plant and the type of phytoremediation depends on the type of contaminants to be treated and the nature of the site. 3. Interactions between plants and microbes Municipal solid waste contains a large microbial popu- lation and may be heavily contaminated with pathogenic microorganisms (Gaby, 1975). Municipal solid waste land- fills often contain animal remains and feces, hospital wastes eb. as ts g etal rac sorb ease bes ts g ate e u rom n o R. Nagendran et al. / Waste Management 26 (2006) 1357–1369 1359 Fig. 2. Processes involved in phytoremediation. Source: http://oldw Table 1 Types and processes involved in phytoremediation Type Contaminant Process Phytoextraction Heavy metals: arsenic, cadmium, chromium, copper, mercury, lead, zinc High biom in shoots Rhizofiltration Plant roo Phytostabilization Heavy-m Phytovolatilization Plants ext foliage Phytodegradation Plants ab Rhizosphere biodegradation Plants rel the micro Hydraulic pumping Plant roo polluted w Phytovolatilization Plants tak released f Phytosorption Adsorptio movement Phytocapping Plants consum northampton.ac.uk/aps/env/landfillleachate/images/phytorem. jpg. s, metal hyperaccumulators extract metals from soil and accumulate them rowing in polluted water precipitate and concentrate metals tolerant plants stabilize the metal in soil and render them harmless t volatile metals like Hg and Se from the soil and volatilize them from the the contaminants and degrade them within the plant system exudates and enzymes which directly degrade the pollutant and/or induce which are involved in degradation row to the water table, take up water and prevents the migration of r p the pollutants along with water, pollutants pass through xylem and are foliage f pollutants by plant roots and leaves and prevention of the pollutant e water from the rainfall and reduce leaching and pollutant movement ana and domestic sewage sludge that pose a potentially signifi- cant health hazard. There can be a significant bacterial population associ- ated with municipal landfill leachates. The acute bacterial content of leachate, particularly the members of coliforms and fecal streptococci vary with the age and the chemical properties of the leachate (Senior, 1990). A limited number of bacterial pathogens have been found in leachates from commercial and experimental landfills and experimental lysimeters (Reinhart and Grosh, 1998). Ware (1980) found increase in bacterial mortality with time of leaching or refuse age due to the bactericidal effects of the leachates of the landfill. Relatively high temperatures achieved in the aerobic stage of refuse biodegradation can inhibit bac- terial growth and survival (Reinhart and Grosh, 1998). Besides sequestering or metabolizing contaminants, plant roots may increase contaminant degradation in situ via their root systems. Plant roots and their exudates increase microbial numbers in the soil surrounding them by one or two orders of magnitude, thus increasing micro- bial activity (Siciliano and Germida, 1998c). Donnelly et al. (1994) suggest that plants specifically increase degradation of certain contaminants in soil by providing the soil microflora with polyphenolic com- pounds. These compounds, in turn, will induce bacterial enzymes that can degrade a variety of pollutants such as trichloroethylene (TCE) or polychlorinated biphenyls (PCBs). These authors have screened a wide range of plants for production of polyphenolics that support PCB-degrad- ing bacteria and identified mulberry (Morus rubia L.) as a possible plant species suited to remediate PCB-contami- nated soil sites (see also Fletcher and Hegde, 1995; Hegde and Fletcher, 1996). However, it is not clear if these plants would increase exudation in the presence of contaminants. In contrast, other workers have suggested that stimulation of bacteria may occur indirectly owing to nutrients released from roots i.e., a non-specific relationship (Schnoor et al., 1995). These nutrients, often low molecular weight organic acids, increase microbial biomass and activity but do not normally induce specific enzymatic processes that degrade xenobiotics. Consequently, plant species with deep fibrous roots that can grow in stressed environments are used in phytoremediation studies. Plants and bacteria are known to form specific associa- tions in which the plants provide the bacteria with a specific carbon source that induces the bacteria to reduce the phy- totoxicity of the contaminated soil (Siciliano and Germida, 1998b). Alternatively, plants and bacteria can form non- specific associations in which normal plant processes stim- ulate the microbial community, which in the course of nor- mal metabolic activity degrades contaminants in soil (Zablotowicz et al., 1994). Similarly, the biochemical mech- anisms have been reported to increase the degradative activity of bacteria associated with plant roots. In return, bacteria can augment the degradative capacity of plants 1360 R. Nagendran et al. / Waste M or reduce the phytotoxicity of the contaminated soil (Aprill and Sims, 1990). The specificity of the plant–bacteria inter- action is dependent upon soil conditions (Baker et al., 1991; Brown et al., 1994; McGrath et al., 1997), which can alter contaminant bioavailability (Marschner, 1995; Baker et al., 2000), composition of root exudates (Ma and Nomoto, 1996) and nutrient levels (Mathys, 1977; Still and Williams, 1980; Kramer et al., 1996). This aspect in respect of rehabilitation of MSW dumpsites assumes great significance owing to variations in municipal solid waste characteristics. In addition, the metabolic requirements for contaminant degradation may also dictate the form of the plant–bacteria interaction i.e., specific or non-spe- cific (Anderson et al., 1993; Shimp et al., 1993). Siciliano and Germida (1998c) have reported that no systematic framework that can predict plant–bacteria interactions in a contaminated soil has emerged, but it appears that the development of plant–bacteria associations that degrade contaminants in soil may be related to the presence of alle- lopathic chemicals in the rhizosphere. Investigations on plants that are resistant to or produce allelopathic chemi- cals may throw much light on this interesting bacterial association. Nicholas et al. (1997) found that the number of bacteria capable of degrading the contaminant increases in contam- inated soil. Higher populations of bacteria in contaminated compared with non-contaminated rhizosphere cannot dem- onstrate selective enhancement of degrading populations. Similarly, increased levels of degrading bacteria in the rhi- zosphere compared with the bulk soil cannot be taken as proof of selective enhancement. Proponents of non-specific interactions argue that specific stimulation of selected bac- terial groups in soil may not be necessary for the plant to enhance contaminant degradation. Specific plant–bacteria interactions still occur in phyto- remediation, but may not be based on the strict genetic alteration seen between legumes and rhizobia. For exam- ple, Siciliano and Germida (1997) found that a combina- tion of pseudomonas enhanced the phytoremediation activity of three different forage grasses while having no effect on other grass species. One of these strains was iso- lated as a plant-growth-promoting rhizobacteria (PGPR) of wheat, whereas the other was isolated from soil contam- inated with 2-chlorobenzoic acid. It is unlikely that genetic alterations in the plant or bacteria are the basis for the enhanced phytoremediation activity seen when these two organisms are combined (see also Guerinot, 2000). Walton et al. (1994) propose that plants produce specific signals in response to specific contaminants. As a result, bacteria detoxify contaminants in soil and the plant pro- vides root exudates that either supply energy source or in some other way increase microbial detoxification activity in the rhizosphere. The key point to this association is that the plant alters its behaviour in contaminated soil to stim- ulate microbial communities that degrade contaminants. Plants that encounter toxicants in soil will not survive unless they can find a way to detoxify the contaminant. gement 26 (2006) 1357–1369 Over the millennia, plants have developed means of using rhizobacteria as a method to detoxify toxins in soil. 4. Vegetation at dumpsites Plants are known to increase nutrient availability by secreting cationic chelators, organic acids, or specific enzymes such as phosphatase into the soil systems. Compe- tition for these nutrients by degrading and non-degrading species will influence the amount of contaminant degraded (Steffensen and Alexander, 1995). Increases in nutrient availability brought about by plant growth may be one mechanism by which plants stimulate biodegradation. Sup- porting this, Cheng and Coleman (1990) found that living roots and fertilizers had equivalent stimulatory effects on straw decomposition. Furthermore, atrazine degradation by an inoculated consortium was similar in treatments receiving fertilizer and those in which corn plants were grown (Alvay and Crowley, 1996). Besides increasing the availability of nutrients, plants may also increase the bioavailability of the contaminant. This feature is of significance in the context of landfill veg- etation. Contaminant bioavailability often limits biodegra- dation, and increasing it can stimulate degradation (Siciliano and Germida, 1998a). Root exudates can accounts for the occurrence of landfill plants (Maurice, 1998). Similarly, in some cases the landfill plant species are related to human activities in the feeder area (Example: Kalix in Sweden – Stenberg, 1997). Maurice et al. (1995) have reported that plants belong- ing to four families viz., Poaceae, Asteraceae, Polygonaceae and Chenopodiaceae dominate, while other species occur only sporadically in Stockholm, Malmo and Helsingborg landfills of Sweden. Their observations further indicate that the species diversity decreases with the age of the land- fill. Dwyer et al. (2000) have quantified the plant species occurring in Albuquerque, USA, with reference to different landfill covers. According to them, the perennial grass and annual weeds were abundant in different landfill covers. At Kodungaiyur and Perungudi dumping grounds in Chennai, India, the dominant plant species recorded were Acalypa indica, Lycorpersicon esculentum, Parthenium hys- teroporus, Cynodon dactylon and Cucurbita maxima (Study of the authors of this review). Reviewing plant species occurring at different landfills facilitates the selection of suitable plant species to deal with a range of contaminants together. It is interesting to note R. Nagendran et al. / Waste Management 26 (2006) 1357–1369 1361 increase contaminant bioavailability by competing with the contaminant for binding sites on the soil matrix. A good starting point for selection of appropriate plant species for the remediation and rehabilitation of dumpsites is to employ endemic species. Although landfills only cover a limited surface, they often offer a large diversity of envi- ronmental niches for species. Several fluxes of waste and cover materials with different origins end up at landfills and create microhabitats on which a certain type of vegeta- tion will have a competitive advantage and develop while other species will be rare. The age of the cover also Fig. 3. Influence of different types that the species diversity is influenced by the nature of ori- gin of wastes, local flora and the conditions prevailing at the landfill. Hence, a single species cannot be identified as a universal indicator and the plant selection should be based on the climatic conditions and the native plants occurring in a particular landfill. 5. Factors influencing landfill vegetation Reclamation of a landfill site must include the objective of containing the material within. This is because the pro- of contaminants on vegetation. ana cesses that take place after the compaction and the cover- ing of the waste in the site produce p