2022 Plant Talk 12: Phytoremediation and Mycoremediation
8.27.22
marc@botanyeveryday.com
Plants for Phytoremediation
What’s Blooming
Some herbaceous wildflowers blooming currently around southern Appalachia:
Angelica (Angelica spp.), Autumn Joy (Hylotelephium sp. syn Sedum sp.), Bee Balm (Monarda spp.), Beeblossoms (Oenothera spp. syn Gaura spp.), Bluebeard (Caryopteris sp.), Canna Lily (Canna sp.), Cardinal Flower (Lobelia cardinalis), Catnip (Nepeta cataria), Chaste Tree (Vitex spp.), Clover (Trifolium spp.), Comfrey (Symphytum officinale), Corn (Zea mays), Crown Vetch (Securigera varia syn Coronilla v.), Culver’s Root (Veronicastrum virginicum), .), Cuspidate Knotweed (Polygonum cuspidatum syn. Reynoutria japonica), Dodder (Cuscuta spp.), Devil’s Walkingstick (Aralia spinosa), Fennel (Foeniculum vulgare), Foxtail Millet (Setaria italica), Everlasting Pea (Lathyrus latifolius), Evening Primrose (Oenothera spp.), Groundnut (Apios spp.), (Hibiscus spp.), Honeysuckles (Lonicera spp.), Hydrangea spp., Hyssops (Agastache spp.), Indian Tobacco (Lobelia inflata), Mallow (Malva neglecta), Meadowbeauty (Rhexia spp.), Jewelweeds (Impatiens spp.), Jumpseed (Polygonum virginianum, Tovara virginiana), Lambsquarters/Goosefoots (Chenopodium spp.), Lady’s Tresses (Spiranthes spp.), Monkeyflower (Mimulus ringens), Morninglory (Convolvulus spp.), Motherwort (Leonurus spp.), Mountain Mint (Pycnanthemum spp.), Nasturtium (Tropaeolum spp.), Passionflower (Passiflora spp.), Phlox spp. Rose Gentian (Sabatia spp.), Sages (Salvia spp.), Shiso (Perilla frutescens), Skullcaps (Scutellaria spp.), Shoo Fly Plant (Nicandra physalodes), Spider Plant (Cleome sp.), Tobacco (Nicotiana spp.), Turk’s Cap Lily (Lilium superbum), Turtlehead (Chelone spp.), Virgin’s Bower (Clematis spp.), Wild Carrot (Daucus carota), Windflower (Anemone spp.).
The Asteraceae are by far the most prolific native bloomers from this time until killing frost. Some current plants in flower in southern Appalachia are included below.
Asters (Doellingeria, Eurybia, Oclemena, Symphyotrichum), Bearsfoot (Smallanthus uvedalius) syn Polymnia sp.), Beggar’s Ticks/Spanish Needles (Bidens spp.) Boneset (Eupatorium perfoliatum), Chicory (Cichorium intybus), Compass Plant (Silphium spp.), Cosmos sp., Dahlia spp., Burnweeds/Pileworts (Erechtites spp.), Mountain Dwarf Dandelion (Krigia montana), Echinacea spp., Galliant Soldier (Galinsoga spp.), Goldenrod (Solidago spp.), Ironweed (Vernonia noveboracensis), Joe Pye Weed (Eutrochium spp.), Marigolds (Tagetes spp.), Mexican Sunflower (Tithonia sp.), Ragweeds (Ambrosia spp.), Rattlesnake Root/White Lettuce (Prenanthes spp. syn Nabalus), Snakeroot (Ageratina spp.), Sochane (Rudbeckia laciniata), Sunflowers (Helianthus spp.), Thistles (Cirsium spp.), Tickseed (Coreopsis spp.), Wild Lettuce (Lactuca spp.), Wild Quinine (Parthenium integrifolium), Wingstems (Verbesina spp. syn Actinomeris), Yarrow (Achillea millifolium), and Zinnia spp.
Food Ready For Harvest
Some things in abundance around me follow below.
Woody Fruits
Apples (Malus spp.), Blueberries (Vaccinium spp.), Figs (Ficus carica), Pears (Pyrus spp.), Rose Hips (Rosa spp.) and Spicebush (Lindera benzoin).
Vegetables
Some of the main out of the garden include members of the Cucurbitaceae, Fabaceae, Poaceae and Solanaceae.
What can you think of?
Phytoremediation
Well, on to the topic at hand… Our world is increasingly subject to all sorts of pollution. Humans have introduced tens of thousands novel compounds into the environment just since World War II and very few of these have been adequately tested for safety (McDonough & Braungart, 2002). It makes sense that even fewer plants have been tested in regards to dealing with toxins. All i could dig up in a semester of graduate school and as time has allowed since is below. Much increase in literature has happened in the last decade or so in particular. It is very humbling to do research into such a technical realm. i hope to dive in further as time goes on and greatly encourage anybody willing and able to join me. Would love to know more about the threat from foraging or community gardening in polluted urban environments in particular!
Plants for Phytoremediation
Phytoremediation entails the use of plants to mitigate the effects of some type of environmental toxin or damage (Chandra et al., 2017; Cuypers & Vangronsveld, 2017; Desai et al., 2019; Golubev, 2011; Hakeem et al., 2018; Karuna, 2019; Kennen & Kirkwood, 2015; Leeson & Alleman, 1999; Leung, 2018; Matichenkov, 2017; McCutcheon & Schnoor, 2003; Parker, 2009; Raskin & Ensley, 2000; Shmaefsky, 2020).
Fairly recently the academic publisher Springer has released a whole series on the issue of Phytoremediation (Ansari et al., 2014, 2016a, 2016b, 2017).
Phytoremediation may be used to remove contaminants from soil, toxins from air, water or simply re-vegetate and stabilize a disturbed area. In the process, phytoremediation may offer a suite of benefits familiar to those who work with plants including carbon sequestration, increased water quality, aesthetic value, food for wildlife, craft materials etc. Below i treat a number of different areas regarding how plants and some fungi have been employed for remediation primarily of soil and water with a particular focus on the coal areas in Appalachia. A study of plants that improve indoor air quality follows.
The Case of Coal in Appalachia
The main cause for phytoremediation currently in Appalachia are the effects of coal mining. It remains to be seen what the effects of fracking may call for! Coal mining in Appalachia is a very controversial and complex issue. The effects of coal mining on society and the economy of Appalachia have been studied thoroughly for decades (Baker, 2008; Butler, 2009; Eller, 1982; Erikson, 1976; Fisher, 1993; Hamby, 2020; Shifflett, 1991; Zipper & Skousen, 2020). The effects on the environment are also present in the books mentioned above. However, a separate literature deals with the mitigation of environmental damage to areas once they have been mined.
Techniques in mining have evolved radically since the start of Appalachian coal extraction in the 1800’s. Initially mining consisted of digging tunnels and a lot of manual labor. Over time methods employing machines were developed for strip mining. Much of the Appalachian phytoremediation literature addresses the strip mining method. West Virginia was the first state to develop a strip mine reclamation law in 1939 (Mellinger et al., 1966). Currently workers for coal companies practice mountain top removal (MTR). With this method the “overburden” covering the coal is exploded and then dumped into surrounding valleys often covering streams. In full disclosure i don’t agree with mountain top removal and wish it would stop. No amount of planting will bring buried streams back. Plants can however help prevent erosion, offer food for wildlife, and possibly mitigate toxic chemicals. Over 500 MTR sites have already been developed to date and need to me remediated in whatever ways possible (Voices, 2012). Google Earth is a resource that has a specific layer for exploration of this large scale tragedy.
Control of water sources, proper fertility, negation of acidic conditions sometimes caused by mine spoil and proper grading are all part of effective revegetation. Fly ash or fluidized coal residue from coal power plants and municipal sewage sludge have been researched as a means of raising pH and fertility (Joost et al., 1987; E. M. Taylor & Schuman, 1988; Topper & Sabey, 1986). Raising pH also tends to immobilize lead at a higher rate (Berti, Cunningham, & Cooper, 1998).
A relatively short list of plants has been developed for the task of phytoremediation from coal mining. Typically revegetation uses a combination of woody species and grasses. Woody species include Autumn Olive (Elaeagnus angustifolia), Black Locust (Robinia pseudoacacia), Scotch Pine (Pinus sylvestris), Red Pine (Pinus resinosa), Short Leaf Pine (Pinus echinata), Virginia Pine (Pinus virginiana) and White Pine (Pinus strobus). Pines tend to acidify soils which may increase mobility of heavy metals like cadmium and zinc if they are present (Bergkvist et al., 1989). Plants that have the ability to fix nitrogen and/or have low nutrient needs are at a distinct advantage. Plant tolerance to heavy metals which are often at mine sites is also helpful. Fast growth is ideal and part of the reason for selection of early succession species which typically exhibit such a pattern. Eventually, some areas may take on more diverse floras (Carter & Ungar, 2002). A study on the ability of Robinia pseudoacacia, Arizona Cypress (Cupressus arizonica) in error referred to as Hesperocyparis arizonica by PLANTS USDA and Fraxinus rotundifolia to take up Pb and Cd in particular has been published (Khodakarami et al., 2014).
Proper pH is even more important for herbaceous plants. Plants that have been employed and are useful in parts of Appalachia include Alfalfa (Medicago sativa), Bird’s foot trefoil (Lotus corniculatus), Tall Fescue (Festuca arundinacea syn Schdonorus arundinaceus), Sericea Lespedeza (Lespedeza cuneata), Crown Vetch (Securigera varia) syn (Coronilla varia), Flat Pea (Lathyrus sylvestris), Goutweed (Aegopodium podagraria), Mugwort (Artemisia vulgaris), Sweet Clover (Melilotus albus syn Melilotus officinalis), Red Clover (Trifolium pratense), Red Top (Agrostis gigantea), Switchgrass (Panicum virgatum), Big Blue Stem (Andropogon gerardii), Little Blue Stem (Schizachyrium scoparium), Bermudagrass (Cynodon dactylon), Deer tongue (Dichanthelium clandestinum), and Bahiagrass (Paspalum notatum) (Karlen et al., 2003; Saminathan et al., 2015). Many of these plants are introduced to the U.S. and some may become invasive. Proper care is required. Some of these plants are also used for hay and forage. One study has shown that hay may be more viable than pasture. However, hay also costs more to establish than trees (Baker, 2008). Whether that hay might take up heavy metals is one question to ask. A bibliography that has been developed for the production of forages on reclaimed surface mine lands may offer more insight (G. C. Anderson & Schubert, 1981).
The role of phytoremediation for coal areas has a special significance for Appalachia. Native plants are preferred to exotics due to the potential for escape by introduced species. Prevalent phytoremediation families may be analyzed to provide a ready pool of possible detoxifying plants. John Todd (Todd, 2008) won a $100,000 grant from the Buckminster Fuller Institute for his proposal to remediate the damage from coal mining. His plan may lead to a more robust treatment of coal mine remediation than has historically been practiced. https://www.toddecological.com/ . In Australia the EDEN project which already made an incredible garden from an old clay mine in England is working to do something similar in an area once used to mine coal.
Phytoremediation for Heavy Metals and Other Toxins in Soil
Soil phytoremediation has only been developed in the last 40 years (Lasat). A prolific literature of phytoremediation in the soil has been developed in that time (Berti et al., 1998; Brooks, 1998a, 1998b; Grobelak et al., 2017; Macci et al., 2013; Marchiol et al., 2004; McIntyre, 2003; Raskin & Ensley, 2000; A. Singh & Ward, 2004; Varma & Sherameti, 2011). Phytoremediation may be referred to as variously “phytorestoration”, “phytostabilization”, and “agronomic stabilization. The goal of soil focused phytoremediation often has to do with the presence of heavy metals such as nickel (Ni) lead (Pb), mercury (Hg), copper (Cu), zinc (Zn), chromium (Cr), aluminum (Al) cadmium (Cd) and the metalloid arsenic (As) (McIntyre, 2003). The sources of these toxins include metal working, coal combustion, sewage sludge, pesticides, and fertilizers. Prolonged cooking in stainless steel cookware can also be a source of heavy metal exposure to Cr and Ni (Kamerud et al., 2013).
Practitioners of phytoremediation have also turned to dealing with organic solvents such as Trichloroethylene, Polychlorinated Biphenyls, and various products of war (Best, 1997a, 1997b; Fiorenza et al., 1999; Megeed, 2013; Stroo & Ward, 2010; Wickramanayake, 2000). The compounds collectively known as PFAS which are in everything from Gore Tex to Teflon and many of surface resistant materials are a new frontier on known toxins that need remediation in this case often in water supplies in many parts of the world (De Silva et al., 2021; Gagliano et al., 2020).
The selection of ground based phytoremediation techniques depends on site; size, location, history, soil characteristics, type and physical state of contaminants, degree of pollution, desired final land use, environmental, legal, and social issues as wells as technical and financial means available. (Vangronsveld & Cunningham, 1998). Other methods for remediation include excavation, soil washing, thermal treatment, electro-reclamation, chemical, and other biological techniques. However, all of these tend to be more expensive than phytoremediation.
Heavy metals may take several forms. Methyl mercury is the most dangerous form of Hg and is typically created in an anaerobic environment such as the bottom of water bodies. It may then enter the food chain and bio-accumulate through fish. Lead is one of the most worrisome metals and it can be consumed through inhalation or ingestion. Lead can cause a vast array of neurological disorders especially in children. However, Zinc and Cadmium tend to be more bioavailable in soil than lead (Lasat, n.d.).
Soil additives such as synthetic chelates are sometimes used in concert with plants to enhance uptake of Pb in particular (Henry, 2000). Some additives are used to simply complex and stabilize pollutants in place. Plants that exhibit tolerance for metal contaminated soils may then be used for soil stabilization instead of phytoextraction (Schat & Verkleij, 1998). Additives include agents that change pH such as liming agents or acidifiers. Phosphates, aluminosilicates and steel shots are also used to affect soil chemistry (Vangronsveld & Cunningham, 1998).
A hyperacculmulator is a plant species capable of accumulating 100 times more metal than a typical plant (Hassanien, 2009). Typically hyperaccumulators will take up greater than 1% of their mass in metals (Vangronsveld & Cunningham, 1998). Most hyperaccumulators that have been identified take up Ni while some have been shown to accumulate Pb, Cd, Co, Cu, Se, and Zn. Hyperaccumulators for Hg are uncommon (Henry, 2000; Millán & Gamarra, 2006). Alyssum is a common plant that has been shown to be a hyperaccumulator of Ni (Schat & Verkleij, 1998). Here is an interesting article from the BBC about such plants being used in Indonesia.
Plant family trends may elucidate patterns in hyperaccumulation. Research shows that the Mustard (Brassicaceae) and Carnation (Caryophyllaceae) families contain many obligate metallophytes but are usually nonmycorrhizal (Colpaert, 1998). A whole book has been published on the Brassicaceae related to phytoremediation in particular (Anjum et al., 2012). Poinsettia (Euphorbiaceae) and Snapdragon (Scrophulariaceae) families have also been shown to accumulate metals (Henry, 2000). Metallophytes in the subtropical and tropical parts of the world often belong to Bean (Fabaceae), Mint (Lamiaceae), Sunflower (Asteraceae) and Grass (Poaceae) families (Colpaert, 1998). The grass family has some specific literature around phytoremediation (Lapviboonsuk, 2011; Pandey & Singh, 2020).
Most plants that hyperaccumulate metals have been identified for other areas than the U.S.A. (Brooks, 1998b) Relevant genera to the U.S.A. and Appalachia include Penny cress (Thlaspi spp.), Alyssum (Alyssum spp.), and Astragalus (Astragalus spp). Ultramafic soils of which Appalachia has some, represent one place on which such plants are typically found. Therefore, potential may exist to discover some indigenous Appalachian hyperaccumulators. Such soils are rare in Appalachia and therefore the flora inhabiting these spaces as well and the plants represented from such places would need to be bred and propagated in order to be applied for use.
Plant breeding has been pursued to increase the efficacy of phytoremediation. One study found that the total flora of plants adapted to metalliferous soils in western and central Europe is no more than fifty higher plant species and many of these are not fit for the work of phytoremediation (Schat & Verkleij, 1998). Traditional breeding of plants has shown promise in providing faster growth and greater tolerance to a larger range of metals (Schat & Verkleij, 1998). Transgenic experiments in phytoremediation plants has been carried out as well (Maestri & Marmiroli, 2011). Research has also been conducted on using transgenic plants for removal of mercury specifically (Henry, 2000; Lasat, n.d.; Sasaki et al., 2006). Some plants that are metal tolerant may not provide effective ground cover or have unknown cultural needs (Vangronsveld & Cunningham, 1998).
Some plants are naturally tolerant to metalliferous soils. Within these species intra-specific tolerance can vary greatly. Plants also vary in their transport of metals from root to shoot which may be an issue with grazing animals. Paper birch (Betula papyrifera) has been shown to accumulate twice as much copper as many other trees while only translocating 20% of the Cu into its foliage versus 60% for the other trees studied (Lepp & Dickinson, 1998). Plants that deposit metals in their foliage can act as a vector of toxic exposure for wildlife and even ultimately people. Soil additives can help decrease the level of translocation from the soil into foliage (Vangronsveld & Cunningham, 1998). Certain mycorrhizal relationships may also serve to sequester metals in the root zone rather than the foliage (Colpaert, 1998; Kanwal, 2016).
Phytoremediation studies have also been conducted on antibiotics from animal waste and from the textile dyeing process as well (Govindwar & Kagalkar, 2010; Gujarathi, 2009).
Mycoremediation
Paul Stamets (2005) has explored the role of fungal relations in mycoremediation and relation to phytoremediation. Mycologist Trad Cotter (2014) released a book in this regard as well. A number of other publications from other authors have come out addressing the field of mycoremediation too (McCoy, 2016; Prasad, 2018a, 2018b; Schindler, 2014; H. Singh, 2006). Mycorrhizal relationships play an important role in the tolerance of plants for metals and the potential for remediation. It may be possible to identify certain fungal organisms that may make tree establishment easier on metal contaminated soils (Lepp & Dickinson, 1998). Arbuscular mycorrhizal (AM) fungi in particular seem to colonize a number of different metalophyte plants across a range of plant families (Colpaert, 1998; Thangaswamy et al., 2017). Crown vetch (Securigera varia) plants that were colonizing anthracite wastelands in Pennsylvania were heavily colonized with AM fungi (Colpaert, 1998). One study has looked at the ability of mycoremediation of spent drilling mud in particular (Umolo et al., 2017).
Actinorhizal plants such as Alder (Alnus spp.), Sea Buckthorn (Hippophae spp.), Bog Myrtle (Myrica spp. syn Morella spp.), and She Oak (Casuarina spp.) may also have a role to play in phytoremediation due to their tolerance of marginal environments.
Some plants show tolerances to several different heavy metal types while others are more specific. Plants that can tolerate and remediate metallic soils may be different than plants used to clean up organic compounds (Vangronsveld & Cunningham, 1998). According to the literature Canada has set up a database called PHYTOREM that covers the phytoremediating ability of vascular plants, fungi, bryophytes, lichen, algae and bacteria (McIntyre, 2003). Within the database 775 species in 76 families are stated to be included though i can find nothing about it online.
Phytoremediation of Water and Wetlands
Most plants show accumulation, tolerance or hyperaccumulation of only one metal. Water plants show the largest ability to accumulate multiple metals. These include Hydrilla (Hydrilla verticillata), Duckweed (Lemna minor), Water Lettuce (Pistia stratiotes) Water Fern (Salvinia molesta) Giant Duckweed/Common Duckmeat (Spirodela polyrhiza), Water Hyacinth (Eichhornia crassipes) Water Hyssop/Herb of Grace (Bacopa monnieri) Water Fern/Pacific Mosquito Fern (Azolla filiculoides) and Tape Grass (Vallisneria americana) (Bennicelli et al., 2004; McIntyre, 2003). These plants represent a plethora of families. Most of these are exotic to North America with the exception of Duckweed and Tape Grass. The science of using Duckweed for Chromium remediation has benefitted from an individual study (Nasir et al., 2016). Many have escaped cultivation and are now very challenging problems all over the world. Several edible plants have been shown to accumulate metals. Water Hyacinth is edible however care should be taken not to consume it when metal contamination is a concern (Couplan, 1998). Phytoremediation of water and wetlands has a literature all its own (Dhir, 2013; Dhulap, 2012; Diego & Leeson, 2002; Ghazi et al., 2019; Mishra, 2017; Rai & Singh, 2015; Stephen, 2011; Terry, 1999). A particular study looked at the use of plants in remediating the effluent from tanneries (Alemu et al., 2012).
Some common edible terrestrial plants also have been shown to accumulate multiple metals including Indian mustard (Brassica juncea), Sunflower (Helianthus annuus), Bent Grass (Agrostis castellana) and Alpine Penny Cress (Thlaspi caerulescens) (Kent Kobayashi et al., 2007; B. C. Wolverton, 1997; B. C. Wolverton & Wolverton, 1993). An individual study has been conducted on Sunflowers in particular (Francis & Ndukwu, 2017).
Toxins in Urban Environments
See below some links on the subject of lead in urban soils
http://www.livescience.com/27531-produce-from-urban-gardens-could-contain-lead.html
https://www.soils.org/discover-soils/story/lead-contamination-garden-soils
http://civileats.com/2015/11/11/is-urban-foraging-cities-safe-to-eat-boston/
http://www.urbanleadpoisoning.com/
http://civileats.com/2014/12/05/foragers-delight-can-wild-foods-make-city-dwellers-healthier/
Research into accumulation of metals in urban environment food plants has a specific literature (Buiso, 2014; Cruz et al., 2014; Finster et al., 2004; Kalat, 2014; McBride et al., 2013, 2014; Wharton, 2012). It has been shown through one study that common plants such as Cilantro, Collard Greens, Epazote, Lemon Balm and Mint may accumulate significant amounts of lead (Finster et al., 2004). In general it appears that lead tends to accumulate in the roots and possibly leaves of plants but rarely the fruits (Raloff, 2003). A study on lead and arsenic contamination in orchard soil and effect on it by organic amendments has been conducted as well (Fleming et al., 2013). The following linked article focuses on Hunting Down Hidden Dangers and Benefits of Urban Fruit. More recent research out of Berkeley has shown the potential for foraging some urban growing vegetables without risk of metal pollution present being taken up (Stark et al., 2019).
Several limitations to the application of phytoremediation have been identified. Phytoremediation is limited to areas with low concentrations of toxins. This technique only tends to remove toxins relatively close to the surface. The resulting plant material from phytoremediation may have to be classified as hazardous waste. The site soil type and climatic conditions can also limit the efficacy of phytoremediation.
Benefits from phytoremediation abound. Phytoremediation is economical. It reduces waste going to the landfill by up to 95%. Soil disturbance is reduced lowering chance of cross contamination. It is easy to implement, environmentally friendly, and aesthetically pleasing (Henry, 2000). Overall, the techniques of phytoremediation may offer an additional tool in the suite of techniques used to mitigate toxic elements in the environment.
Overview of Air Quality issues and the Role of Phytoremediation
Many toxins that occur in the home environment need to be removed to facilitate good human health. Various plant species have been shown to remove indoor air pollutants (IAPs) (Irga et al., 2018; Kent Kobayashi et al., 2007; Pettit et al., 2018; Torpy et al., 2017; B. C. Wolverton, 1997; B. C. Wolverton & Wolverton, 1993). Some of The IAP substances that researchers have removed with plants include Formaldehyde, Xylene, Toulene, and Benzene.
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Sources of some major common indoor air pollutants |
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Xylene |
Gasoline, marker pens, photocopiers, printers, adhesives, joint compounds, floor coverings, solvents, dyes and kerosene smoke |
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Toulene |
Synthetic carpet, wood floor finishes, cigarette smoke, printers, photocopiers, solvents, adhesives, wallpaper, joint compound, vinyl flooring, caulking compound, paint and kerosene smoke |
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Formaldehyde |
Cigarette smoke, glues, resins, carpets, curtains, facial tissues, floor coverings, gas stoves, grocery bags, paints, paper towels, particle board, permanent press clothing, plywood, stains, varnishes and foam insulation |
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Benzene |
Solvents, tobacco smoke, paints, finishes and caulk |
Sources: (E. L. Anderson & Albert, 1999; Godish, 2001; Murphy, 2006; Warde, 1997; B. C. Wolverton, 1997).
Several of the plants that remove indoor toxins are relatively easy to propagate (Bryant, 2006). Propagation is the process by which a plant is reproduced. However, some of the plants that remove toxins from the air are themselves potentially also toxic. The toxicity of plants is mostly of concern in the case of accidental ingestion by young children or pets (Alber & Alber, 1993). Therefore, house plant owners must be as aware of plant toxicity as they are of air toxicity. When possible poisoning by children is a factor less toxic plants could be chosen.
The study described here focused on learning the techniques to propagate plants that help alleviate the issues of indoor air pollution. The presence of plants in the indoor environment also helps alleviate a host of other societal ills. For instance, plants in the work environment can increase happiness and worker productivity (Kent Kobayashi et al., 2007). Working with plants can also help provide a connection with nature that many people in our society desperately lack (Louv, 2005; Nabhan & Trimble, 1994).
Future sustainability depends on the combination of the old and the new. Much of the natural world has been discounted in the relentless pursuit of sophisticated modern technological solutions. Sometimes these solutions cause problems that are just as bad as what they are trying to remedy (Carson, 2002). The natural world often can offer solutions to problems without deleterious side effects (Bradley & Barbara, 1997).
The model of Appropriate Technology (AT) serves to underpin the initiative espoused in this class (Hazeltine, 2003; Kaplinsky, 2011; Régnier et al., 2022). AT seeks simple solutions that use inexpensive locally attainable ideally indigenous materials (Schumacher, 1973). A resource explaining AT in an Appalachian context has also been published (Fritsch & Gallimore, 2007). Plant propagation is an ancient practice used by the Romans and even societies before them. Many air detoxifying plants are readily available to the everyday building dweller. In fact several species of detoxifying plants already commonly occur within the built environment.
The propagation of some of these plants is known due to its extreme ease and utility. Yet, how many people still go to some big box store to purchase plants from afar that could be propagated for free from local stock? It stands to reason that the planting of these particular useful plants would increase were the knowledge of the air detoxifying power known more widely. Propagation lowers the barrier of accessibility for people whom plants can sometimes be too costly to buy. FEMA trailer residents and others in manufactured housing are examples of populations that might benefit from such knowledge. Manufactured housing in general tends to have the highest residential levels of formaldehyde (Godish, 2001). This type of housing is also prevalent for low income people in Appalachia as well. The provision of plants that detoxify the air for such situations may help avoid excessive exposure. The prevalence of high exposure levels to toxic chemicals by disadvantaged groups represents a social justice issue often known by the name of environmental racism (Bullard, 2000; Cole & Foster, 2001; Murphy, 2006; D. Taylor, 2014; Washington, 2020; Zimring, 2017). Propagation of plants can also serve as a form of income for these people and others.
Humans have developed many methods for propagating plants. The right technique/s for each type of plant is necessary to ensure success. The physical product of some of my research was the propagation of six different species of air cleaning houseplants. The plants for this project were propagated using offsets, cuttings and divisions. Other species and other propagating methods were also explored to give the reader a fuller sense of the technological potential. Many of the most effective remediating plants are more difficult to propagate and/or harder to acquire.
The plants included for my study were Air Plant (Chlorophytum comosum), English Ivy (Hedera helix), Golden Pothos (Epipremnum aureum) Mother in law’s tongue (Sansevieria trifasciata) Aloe (Aloe vera syn A. barbadensis) and Rubber Plant (Ficus elastica syn F. robusta). These plant types represent air cleaning species that were the easiest to obtain cheaply while also seeming easy to propagate according to the literature.
Literature Review for Houseplant Phytoremediation of Poor Air Quality
The main purpose of the current review is to unify the disparate literatures of house plants, air quality phytoremediation, plant propagation and plant toxicity within the framework of plant family trends. Writings on plant propagation are numerous and have a long history. Phytoremediation on the other hand as has been stated already is a rather new science. Work on the detoxifying properties of house plants in particular rests largely on the shoulders of a researcher named Wolverton (B. Wolverton et al., 1984; B. C. Wolverton, n.d.; B. C. Wolverton et al., 1989; B. C. Wolverton & Wolverton, 1996). Experiments by Wolverton and others were carried out on behalf of the National Aviation and Space Administration (NASA) as a means of preserving air quality in space. Wolverton ultimately ranked 50 plants on their ability to clean the air, ease of cultivation, susceptibility to insect infestation, and transpiration rate (B. C. Wolverton, 1997). Wolverton also demonstrated that soil microorganisms play a significant role in the detoxification of air along with the plants in his studies (B. C. Wolverton & Wolverton, 1993). More research on phytoremediation for air quality using biotechnology has been conducted as well (Omasa et al., 2002).
English Ivy (Hedera helix) has been shown to remove formaldehyde the most followed by Spider Plant (Chlorophytum comosum), Snake Plant (Sansevieria trifasciata) and Aloe (Aloe vera) regarding the plants in this study. However Spider plant removes the most Xylene followed by Snake Plant and English Ivy (B. C. Wolverton & Wolverton, 1993). In another study Golden Pothos (Epipremnum aureum) was also shown to remove comparable amounts of formaldehyde to English Ivy (B. C. Wolverton et al., 1989). In the same study English Ivy was the best at removing benzene and Golden Pothos was third. Many of the studies by Wolverton use varying techniques and plant materials so a standardized comparison is hard to do. Nonetheless, the overall summation is that these plants mentioned above are some of the most effective at detoxifying while also factoring in their ease of propagation.
Many other plants were shown by Wolverton to be better at detoxifying than the ones in this study. However, most of these plants are either more expensive to acquire, harder to propagate, or harder to take care of. Monkey grass (Liriope spicata) is another potential good detoxifying plant but not included in this study. It is second overall to the plants studied in formaldehyde and xylene removal as well as a strong remover of ammonia (B. C. Wolverton & Wolverton, 1993). Florist’s mum (Chrysanthemum x morifolium) is another potentially readily available good detoxifier of formaldehyde, xylene and ammonia. Typically ferns, members of the Ficus genus, and members of the Aroid (Araceae) Palm (Arecaceae) and Agave (Agavaceae) families have been shown to be the best at detoxifying in general for formaldehyde, xylene, ammonia, and benzene (B. C. Wolverton et al., 1989; B. C. Wolverton & Wolverton, 1993; B. C. Wolverton, 1997). A few of the most effective detoxifying plants may significantly increase the air quality of the average enclosed office (B. C. Wolverton & Wolverton, 1993).
Health Effects of Indoor Air Pollution
This section also offers a brief overview of the health effects from various chemicals. Our society is becoming increasingly saturated with synthetic substances. The more these substances can be removed from the environment the healthier society will be (May, 2006; Murphy, 2006).
Modern age construction uses air tight building envelopes that encourage energy efficiency. These impermeable barriers can sometimes lead to the trapping of deleterious substances inside the buildings. “Sick building syndrome” is evidence of the unwanted effect that comes from such tight insulation (Yanagisawa et al., 2017). Maladies such as asthma, allergies, and chemical sensitivities can be traced to this modern building phenomenon but are hard to prove (May, 2006; Murphy, 2006). Different people may react differently to the same level of chemical exposure. Animal testing normally involves acute dosages and cannot be directly correlated to humans.
The methodology of Indoor Air Quality (IAQ) risk assessment is very diverse and many studies do not correlate well in general. There is an overall lack of proven connection between Volatile Organic Compounds (VOCs) and sick building syndrome. However, this may be due to the lack of adequate testing protocols amongst other variables.
The Environmental Protection Agency (EPA) initially spent much more time regulating outside air quality than inside (E. L. Anderson & Albert, 1999). However, the indoor air environment may be up to ten times more polluted than the outdoors (B. C. Wolverton, 1997). Some people spend as much as 90% of their time indoors (E. L. Anderson & Albert, 1999). With statistics like these it is not hard to see why an increase in illness related to IAQ might be occurring.
Modern society is filled with novel toxins that did not exist until the industrial revolution (McDonough & Braungart, 2002). The catch phrase “better living through modern chemistry” has been shown to have significant down side effects. The book Silent Spring served as a warning over 50 years ago in regard to potential deleterious effects from various chemicals (Carson, 2002). Many potential toxins exist in the indoor environment. Mold, radon, asbestos, tobacco smoke, particulates from combustion, carbon monoxide and various microscopic insects all represent potentially toxic elements in the indoor environment (Spengler et al., 2001). Airborne toxins such as formaldehyde, benzene, toluene and xylene are the current focus. Nonetheless, plants have been shown to remove some microbial contaminants as well (B. C. Wolverton & Wolverton, 1996).
Aldehydes such as formaldehyde are irritants to the mucous membranes of the eyes and upper respiratory tract (Godish, 2001). Formaldehyde is one of the most toxic of this group of chemicals. It may cause neurological symptoms and evidence points to the fact that formaldehyde may aggravate asthmatic problems as well. Significant deleterious effects have been shown to occur at concentrations as low as 1 part per million (ppm) after just 90 minutes (Godish, 2001). Formaldehyde has also been found to be carcinogenic (E. L. Anderson & Albert, 1999; Moeller, 2005). The level of .20 - .40 parts per million (ppm) allowable by the U.S. Department of Housing and Urban Development for mobile homes is well above the threshold of .05 – .02 ppm under which the above mentioned symptoms may start to occur (Warde, 1997).
Formaldehyde, toluene and xylene are all types of Volatile Organic Compounds (VOCs). Hundreds of VOCs have been identified in indoor air of which about 50 are common (Kostiainen, 1995; Warde, 1997). Formaldehyde has been studied more than the other VOCs. Toulene is considered as a possible cause of anemia and a sensitizer that lowers dosages of other VOCs that are required to produce symptoms (Warde, 1997). Xylene is an eye and respiratory irritant and central nervous system depressant. Xylene may also cause liver, kidney, and heart damage (Warde, 1997).
Other Mitigation Techniques for Indoor Air Toxins
Detoxifying plants represent just one method of relieving the problem of Indoor Air Pollution (IAP). A mixture of techniques is most prudent to avoid the effects of VOCs. Formaldehyde emissions naturally decrease significantly over time (Godish, 2001). Proper air circulation can help remove these gases. Air circulation in combination with carbon filtration and/or plants also increases the efficacy of plants in detoxification (Leviton, 2001; B. C. Wolverton, 1997). Heat and moisture increase off gassing and may be used to speed up the release of gases prior to habitation. Using products that are low in VOCs will also decrease the total load. People who employ methods of decreasing toxins typically follow a certain pattern. First, research is done to see if a substance might be eliminated or substituted. Next, the process involving the substance is possibly altered or equipment is changed. Isolating the substance follows and finally personal protective equipment is employed. The field of Industrial Ecology (IE) deals with such design processes (Ayres & Ayres, 2002; Blokdyk, 2022; Clift & Druckman, 2016; Dupont et al., 2016; Graedel & Allenby, 2011; Kleindorfer, 2002; Manahan, 1999; Spencer, 2018). Ultimately, more stringent regulations are necessary to stimulate such research especially if mitigation techniques may cost manufacturing businesses more money. However, it has been shown that significant savings by increased productivity may be had for businesses that are willing to mitigate the effects of poor indoor air quality on their workforce (Fisk, 2001).
Testing for VOCs varies for each compound. Many techniques have been developed to determine the level of toxins in the air (American Society for Testing and Materials, 1999). For formaldehyde diffusion tube testers and DNPH devices are the most common. The DNPH devices are considered to be the most accurate (Warde, 1997). In either case the testers must be sent off for analysis. However, only a few prominent VOC’s even have developed tests.
Methods of propagation
Many techniques are available to the potential propagator. These actions can be simple and mundane such as cutting a piece of plant and putting it in soil or water until it roots. However, complex laboratory techniques that need special equipment and absolute sterility are necessary for some plants. The techniques for the plants in this study were all simple and are variations of taking cuttings.
Most propagation books describe a multitude of ways to propagate plants. Some of these books do a better job than others. Visual imagery and step by step instructions can greatly assist the understanding of how certain plants are propagated (Toogood, 1999). As might be understood, older references may not provide as many visual aids or refer to modern innovations (Kains, 1942). Very few of the propagation books refer to the uses of the plants whether for improved environmental quality or aesthetic beauty. Toxicity is also rarely mentioned except in a specialized literature dealing with that subject alone.
Houseplant Toxicity and Plant Family Trends
The toxicity of certain houseplants is well known. Plants that both clean the air and contain inherent toxicity tend to come from the families Araceae, Araliaceae, and Euphorbiaceae (Frohne & Pfander, 2005). Knowledge of which plants are toxic and which ones aren’t would help alleviate the phenomenon of unintentional poisonings that occur every year across the world. What follows is a chart of the plants from the current study with their related toxicity.
Toxic elements of selected air detoxifying plants
|
Scientific Name |
Common Name |
Plant Family |
Toxicity |
|
English Ivy |
Araliaceae |
Saponins |
|
|
Spider plant/Air plant |
Liliaceae |
None known |
|
|
Mother in law's tongue |
Agavaceae |
Saponins when consumed in very high doses |
|
|
Aloe |
Asphodelaceae |
Barbaloin is a purgative |
|
|
Golden Pothos |
Araceae |
Rahphides of calcium oxalate |
|
|
Rubber plant |
Moraceae |
None known |
Source: (Alber & Alber, 1993; Frohne & Pfander, 2005; Turner & Aderkas, 2009)
Analysis of plants at the family level may also allow the discovery of other potential air purifying plants. Out of the 50 plants discussed by Wolverton (1997) only about 20 families are represented.
The Araceae family alone accounts for 24% of the total while the Palm family (Arecaceae) and the Agave family (Agavaceae) account for another 10% each. For many families only one individual species has been identified. Typically many members of a family will exhibit similar characteristics such as uses for food, medicine, or poisonous properties (Elpel, 2013). This phenomenon might be extended to detoxifying abilities. All the plants included from the Araceae family are potentially toxic (Alber & Alber, 1993; Frohne & Pfander, 2005). Exploration in the benefits of other species within underrepresented relatively non-toxic families such as the Sunflower (Asteraceae), Begonia (Begoniaceae), Prayer plant (Marantaceae) and Fig (Moraceae) groups might be a good direction for further research. New non-toxic families of houseplants that could be explored include the Mint (Lamiaceae) and Geranium (Geraniaceae) groups. Different species of the same genus often act similarly as is demonstrated by Wolverton’s inclusion of four Philodendron spp., four Dracaena spp. and three Ficus spp. in his treatment of the fifty plants (B. C. Wolverton, 1997). However, Removal rates were shown to vary by species.
Data Analysis of an Experiment on Propagation of Air Detoxifying Plants
An analysis of the data leads to the idea that English Ivy (Hedera helix) is by far the best prospect. It is an exotic invasive that can be readily found for free in the environment. It is easy to propagate, and it is one of the most effective detoxifiers. However, English Ivy is also toxic and should not be planted outdoors due to its invasiveness. Snake Plant (Sansevieria trifasciata) is easy to propagate by division but hard to propagate by cuttings. Spider Plant (Chlorophytum comosum), Aloe (Aloe vera) and Pothos (Epipremnum aureum) are easy to propagate but not as good at toxin removal.
Many house plants grow slowly or are hard to propagate and it might be difficult to reproduce a plant that one might find in the store economically. English Ivy (Hedera helix), Spider Plant (Chlorophytum comosum), and Pothos (Epipremnum aureum) are notable exceptions.
Propagation of air detoxifying plants offers a beneficial economics angle. People that need jobs might make a living by propagating and selling plants to institutions like businesses, schools, hospitals, restaurants and churches. This concept goes far beyond just plants that may be used for air purification. Disadvantaged people can have the cost of plants subsidized or they may be encouraged to do work trade, or come to a workshop and create their own. English ivy (Hedera helix), Spider Plant (Chlorophytum comosum) and Pothos (Epipremnum aureum) can be propagated easily and often found already growing in many people’s houses. Monkey grass (Liriope sp.) and florist Mums (Chrysanthemum sp.) may also be easy to propagate. Members of the Palm family (Arecaceae) may be worth purchasing and eventually dividing due to their superior abilities to detoxify the air.
Conclusion
Most work on soil phytoremediation has been done outside the country. Development of knowledge for appropriate plants indigenous to Appalachia and other areas for phytoremediation of metal contaminants in particular offers an excellent opportunity for further study. Plants that are used for phytorestoration of denuded coal areas mainly come from the Grass (Poaceae) and Bean (Fabaceae) families. These plants may be grown for wildlife, pasture or hay. Some plants that have been employed for such purposes are exotic invasives whose use might be questioned and discontinued.
A combination of propagating easily reproduced plants that are very effective at cleaning the air coupled with purchasing plants that offer the most detoxification for the lowest price is probably the most effective strategy.
Study at the plant family level clearly shows trends in the ability of certain groups to help detoxify the environment. Similarly, many plants that detoxify the environment can in fact be toxic themselves. Focusing at the plant family level may be helpful in introducing people to such concepts and provide directions for further study in a simplified manner.
|
Potential New Families for further Air Detoxification Use |
|
|
||||
|
|
|
|
|
|||
|
Family |
Scientific name |
Common Name |
Toxicity |
|||
|
Lamiaceae |
Solenostemon syn Plectranthus |
Coleus |
Low |
|||
|
Caryophyllaceae |
Dianthus, Carnation |
Low |
||||
|
Acanthaceae |
Fittonia vershaffeltii |
Mosaic plant |
Not known |
|||
|
Potential New Genera for further Air Detoxification Study |
|
|
|
|||
|
|
|
|
|
|
||
|
Family |
Scientific name |
Common Name |
Toxicity |
|
||
|
Araceae |
Elephant's ear |
High |
|
|||
|
Agavaceae |
Cordyline fruticosa syn C. terminalis |
Ti plant |
Low |
|
||
|
Asteraceae |
Cosmos |
Low |
|
|||
|
Araliaceae |
Dizygotheca elegantissima |
False aralia |
Not known |
|
||
|
Araliaceae |
Fatsia |
Not known |
|
|||
|
Asteraceae |
Velvet plant |
Low |
|
|||
Article for next time Mother Jones: Foodies about to Ruin Kale
For the next class we will cover Exotic Invasive Plants and it will be posted around September 8th
Below are items to think about/comment on. Please write me directly at marc@botanyeveryday.com or leave information in the commentary under this class.
I WOULD REALLY LOVE TO HEAR WHAT YOU HAVE TO SAY!!!
- Look up some of the families mentioned in this post in Botany in a Day and share some information about them with the group. Or provide info from your personal experience
- Read one article on Phytoremediation and do your part to add to our collective knowledge on this important topic
- Post any clear photos of question plants to Facebook or send in an email.
Praises to all that have donated to the cause!!! i encourage everyone reading this to donate as they are able financially, commentarialy, or energetically... Your contributions greatly help me continue this crucial work of ethnobotanical research and education. Please let me know your thoughts in general and any way i can help this class serve you best.
Thanks, marc
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