"The Global Oil Supply, Biodiversity and the Changing Climate." Talk at Braziers Park by Professor Chris Rhodes, Sunday May 15th.

An Eco-Parable for Our Time: "Hippy the Happy Hippopotamus!"

Awarded as a Winner in the "50 Great Writers You Should be Reading" contest, Chris Rhodes was interviewed on The Authors Show (by Don McCauley), about his children’s book, “Hippy the Happy Hippopotamus”, 10.00 Eastern Standard Time (3.00 PM, UK Time), January 23rd, 2016.

Described as an Eco-Parable, an edited version of the recording was broadcast on The Authors Show from February 9th (4.00 A.M. UK time) to February 10th (4.00 A.M. UK time).


INTERVIEW QUESTIONS:

DM: Can you tell us about the book?

CR: “Hippy the Happy Hippopotamus” is a children’s picture book, about a young hippo growing up with his family and friends in Africa. Hippy has the mind of a five year old boy, and so the events that happen in his various adventures are as they might appear to a five year old. How he sees his changing world.


DM: Who did you write your book for?

CR: Well, I wrote the original story for my step-niece, Chloe, when she was about 4. She’s 21, now, and so her interests have shifted somewhat, but she still likes the story of the little hippo! However, its purpose seems to have grown, rather, and “Hippy the Happy Hippopotamus” is now the first book in a series of books, aimed at a general audience of children between 5 and 8 years of age.


DM: Is there a central message in the book?

CR: Yes, the message is the importance of cooperation and working together, in order to solve problems and adapt to a changing environment. In the later books, topics such as the damming of rivers, flooding, climate change, soil erosion, conflict between humans and animals over habitat are covered directly. So the whole project addresses different aspects of "sustainability".


DM: If you had to choose, what would you say is the single most important idea you're sharing in your book that is really going to add value to the reader's life?

CR: In a nutshell, to get over to children, the idea that cooperation with others is the best way to solve problems, rather than conflict.


DM: If you could compare this book with any book out there we might already be familiar with, which book would it be and why?

CR: I am not aware of an exact match between this book and any other, but I might draw a comparison with “The Lorax”, by Dr Seuss. The Lorax is a mythical creature and protector of the trees, who is very unhappy when people cut the trees down, to process them in a factory. This has been interpreted more generally as being about a call for environmental protection, and so “Hippy” has similar themes, especially in the subsequent books, about the vulnerability of the environment to how we treat it, and that we (humans) are a part of the environment rather than being something separate from it. This is reinforced by the theme of cooperation between animals (hippos) and also between animals and humans, that runs throughout the series of stories.


DM: Since your background is mainly in science and environmental journalism, why did you decide to write a children's picture book?

CR: That’s a good question. When I first wrote “Hippy the Happy Hippopotamus”, I was working in a university as a professor of chemistry, but I was really deciding to move on with my life and do something else.

I had previously made a trip to Prague (in the Czech Republic), and visited the zoo there, where there is a family of hippos. I found these really quite fascinating creatures, and it struck me that it might be nice to write a children’s story about them. Then, a few years later, I found the words just came to me, and I scribbled the story down in about 20 minutes, but clearly it had been working in my subconscious for several years beforehand. I think it was all driven by various changes that were going on in my life at the time, which brought the story up from the depths. A bit like poppy-seeds, which only grow in disturbed ground, when they are exposed to light!

[Hence the poppies of Flanders Fields, where the ground had been disturbed by the shelling in World War I].


DM: The book has been described as a modern day "eco-parable". What is meant by this?

CR: This was a description given to the book by a leading British permaculturist, Steve Jones, who is promoting it on his website. I believe he was actually thinking of “The Lorax” by Dr Seuss, which is an eco-parable (story with an environmental message) about protecting the trees, and the environment more generally. Steve saw “Hippy” as a similar eco-parable with the message that empathy and cooperation are the best ways to solve problems in a changing world.


DM: Of all the animals you could have chosen as a main character for the book, why did you decide upon a hippopotamus?

CR: I think it was spending time with the hippo family in Prague zoo, and I then started reading about the hippopotamus, and discovered that they are “vulnerable”, because their population is decreasing, mainly as a result of poaching and that their habitat is being destroyed by humans for farming. So, a story about a young hippo seemed to be a good metaphor for environmental change, and a way of illustrating environmental issues, e.g. climate change, conflict between humans and animals over habitat, damming of rivers, pollution issues and so on. But in a way that children could understand.


DM: Do you think that the book would be of particular value in schools?

CR: Very much so! We have done quite a number of readings in schools... well, I read the stories and Jeanette shows the kids the pictures. She also teaches them to draw – hippos and other things. They love the illustrations, and they connect easily with the storyline. In a classroom setting, the book also helps to teach children to read English (particularly in multi-cultural schools, as they tend to be in East London, where many of the kids don't have English as their first language), but the book also sets a natural mood for discussions about environmental topics.


DM: How does the book relate more broadly to your work on "sustainability"?

CR: This began about 10 years ago, when I started my blog, and began writing various articles about “sustainability” in the broadest sense. I then got involved with permaculture and the Transition Towns movement, and a whole host of other things, and it struck me that the future that our children will find is likely to be quite different from how things are now, in terms of how we use resources, and in very many other ways.

And for example, because the oil supply is going to fail at some point over the next few decades, we will need to do more on the local scale, not the global, and so we will need strong communities where people have to cooperate with each other. So, the book (and the whole series of books) is aimed to bring these ideas to young children, since really it is their future that’s at stake. They are ones that matter - more than me, at this stage in my life!


DM: The book is linked to your charity, "For Our Children's Earth". What does this do?

CR: The charity works to promote ideas about sustainability, the need to preserve the environment (both locally and globally), and to reconnect children with nature. We do a lot of outreach work in schools, talking about environmental topics, and also reading the hippo-stories to the kids. We run workshops in schools and communities, I give public lectures, and the longer-term aim is to build a “centre” where actual hands-on activities can be done. Permaculture is a strong part of this. Our funding comes from sales of the books, and money we raise from talks etc. So, the link between the book and the charity is both conceptual/ideological and financial.


DM: Tell us your most rewarding experience since publishing your work?

CR: Without doubt, it’s the reaction of children when they see the pictures in the book, and have the stories read to them. It’s lovely to see the joy in a child’s face, that the book gives them. In fact, everyone seems to love the pictures, which are entirely due to the talent of Jeanette Cole, the book’s illustrator. But when I say this to her, she says that she finds it very easy to draw the pictures from the way I write the stories. So we are a good team!

I was also extremely moved by the overall flow of goodwill toward the book which we have received from very many and entirely different kinds of people: children, parents, teachers, and people with “green” interests. There has been a wonderful response via Facebook to Hippy becoming a winner on the Authors Show. So he has a good following, and people are rooting for him, which I find very heart-warming, and encouraging


DM: If someone wrote a book about your life, what would the title be?

CR: I remember being asked this same question some time ago, and the title that popped instantly into my head was, “On the Edge of Adversity.” Now, of course, this can also mean being on the edge of “opportunity”. So, while my life has certainly had its ups and downs, I feel that I have been very fortunate in that people have come along, at various critical points, who have nudged me in the right direction, or been able to open a door for me, which has led on to something good.

And I’m very grateful for that, as it could have all gone very badly wrong at various points, especially when I was in my teens, but more recently too, deciding to make a break from the academic world, and yet I have found a new role, and been able to explore a lot of different dimensions that I never would have done otherwise. Indeed, it was pure chance that I met Jeanette, the book’s illustrator, and yet amazingly, she seems able to “see” the same images that I saw in my mind when I wrote the stories in the first place. But she can draw them. So, it’s as though we were floating around looking for each other, until fate managed to connect us.


DM: How would you describe your writing style?

CR: That depends on what I’m writing. I think there is humour in how I write, albeit somewhat on the dark side at times. For example, my novel “University Shambles”, which is very much a black comedy, albeit a surreal one. I’ve written very many scientific papers. I’ve also published a collection of poetry, quite a lot of journalism (mostly on-line), and now the series of children’s stories. I also had a shot at writing a radio play, recently. So, my words are in quite a mixed bag, really!


DM: Who influenced your writing the most?

CR: As a child, I loved the books by Enid Blyton (“The Mountain of Adventure”, “The Island of Adventure” and so on), and “The Famous Five”.

My first urge toward creative writing began when I was 18, inspired by an abandoned book of poems by Walter Scott, which lead me on to writing my own poetry, and I was further influenced by W.H.Auden and Dylan Thomas. In terms of novels I was particularly influenced by D.H.Lawrence and Thomas Hardy, and I was very much drawn by the social commentaries of George Orwell (“Down and Out in Paris and London”, for example).

In terms of the campus novel genre, I was probably influenced by Malcolm Bradbury (“The History Man”), and there was a B.B.C. television drama (“A Very Peculiar Practice”) written by Andrew Davies, which probably put into my mind the idea of writing my own novel (“University Shambles”) based on my own experience of the commercialisation (...destruction maybe), of the British University system.

So, any influences are probably highly various.


DM: Are your characters pure fiction, or did you draw from people you know?

When I wrote my novel, “University Shambles”, at the beginning some of the characters were based on the traits of some people I knew, but I found that as the writing-momentum gathered, all of the characters developed a life of their own and finally became entirely fictional, rather than parodies of old familiar faces. Having said that, it is striking that various people, having read the book, seemed to recognise the characters as people that they know!

Now, in the case of “Hippy the Happy Hippopotamus”, I can say that the “characters” are based on a family of hippos that I was able to visit in Prague Zoo, over several years! And, I suppose, the anthropomorphic elements (their human characteristics) are based on the ideal elements of a happy, loving family, all trying to do their best in a changing and unpredictable world. But there is an innocence to it all, because the things that happen are as seen through the eyes of young “Hippy” who has the mind of a 5-year old boy.


DM: Are you more of a character artist or a plot-driven writer?

CR: I would say more of a “character artist”. However, the plot and the characters weave together into a broad tapestry, and the characters and events intermingle and borrow structure from one another. But the initial idea – the first thought in my mind - is that of one person (or a young hippo, in this case!), and I try to feel how they would feel, in their immediate surroundings and situation, then roll out the strands and develop the plot from there.


DM: Other than selling your book, what do you hope to accomplish with it?

CR: To get the message over to kids that empathy and cooperation are the best ways to solve problems. This is particularly important because the next generation is likely to be challenged in all manner of ways as the world changes profoundly, and their ability to work together to solve problems is likely to become paramount. We also aim to get the book onto the school National Curriculum (U.K.) and its equivalent in other European countries and in the U.S. A proportion of the sales revenue from the books goes to support the charity, “For Our Children’s Earth”.


DM: Who should buy this book?

CR: The book is aimed at children aged between 5 and 8 years old, and their parents and teachers. The primary geographic target area is Europe and the United States. The benefits of buying this book are its beautiful illustrations, and simple text which children easily connect with. It is an ideal book for parents to read to their children, helped by the pictures. In a classroom situation, the book is very helpful for teaching children to read English, and it is a natural prompt for discussions on environmental topics. Also, because of the book’s theme of sustainability and social harmony, it is of interest to community and environmental groups.


DM: Where can readers find you and your book?

CR: My webpage is http://www.rba.co.uk/cjr/ or simply google “professor chris rhodes” and you will find me. My email is on the webpage, but is cjrhodes@fresh-lands.com and please get in contact with me directly about buying the books, and indeed anything else.

We have e-versions of “Hippy the Happy Hippopotamus” available and also the next two books in the series, “Visitors for Hippy” and “Hippy Eco-Hippo”. We are seeking a printer for all three books at a reasonable price.

COP21 and "4 per thousand" - Storing Carbon in the Soil.

It would have been a remarkable oversight, had not our use of the land and its soils featured among the discussions about climate change mitigation in Paris at COP21. However, at the conference was hosted a side-event and official launch of the "4 per thousand" initiative, which aims to increase soil carbon over a 25 year period, with the effect of halting the annual increase in CO2 in the atmosphere. It is important to be aware of what "4/1000" means: it is not an increase in the overall soil carbon by an annual 4 grams per 1000 grams of soil as has been claimed, but an increase in the existing carbon in the topsoil by 0.4%/year. This has been described from an Australian perspective:

"Let us start with the analogy of a football field (Soccer, not rugby!). Imagine it is a fifth larger than normal – making it one hectare in size. The top layer of soil on the field, 30 cm deep, is known as the topsoil.

"Carbon is the main ingredient of organic matter, so organic matter is often referred to as ‘soil organic carbon’. In Australian soils, this organic matter makes up on average, between 1 and 3 percent of the topsoil. For the purpose of the exercise, we will assume that the topsoil on the football field contains 1.5 percent carbon. This equates to 58 tonnes of carbon in the topsoil across the whole football field.What the French Government is calling for is to increase that 58 tonnes by 0.4 percent per annum – in our imaginary football field that would equate to an increase of 0.2 tonnes (or 200 kg) of carbon in the topsoil each year."

Thus, the annual carbon increase is 0.4% of 1.5%, or 0.006%, giving a total soil carbon content of 1.506% after year 1, and 1.65% after 25 years, with around 6 tonnes of carbon having been captured per hectare. Done on the global scale, the impact could be enormous. The “4/1000 Initiative: Soils for Food Security and Climate” aims to integrate agriculture as part of the climate change solution, rather than being the major problem it is often deemed to be, which along with forestry and other land-use, contributes 24% of global greenhouse gas emissions. The total amount of carbon stored in soils is reckoned at 2,400 billion tonnes, making it the largest terrestrial carbon pool. The total carbon emissions by humans amounts to an annual 8.9 billion tonnes, and so the ratio 8.9/2,400 = 0.4%, which is where the "4/1000" figure comes from.

However, it is the annual rate of carbon sequestration per hectare which is the critical determinant of how successful the strategy is likely to be. As has been noted:

"The land area of the world has 149 million km2, and it would be estimated that on average there are 161 tonnes of C per hectare. So 0.4% of this equates to an average sequestration rate to offset emissions at 0.6 tonnes of C per hectare per year. We know that soil varies widely in terms of C storage, for example peat soils in the tropics hold about 4000 tonnes of C per hectare, while sandy soils in arid regions may only hold 80 tonnes of C. The type of above ground vegetation and how quickly the soil biota uses the carbon also can affect this rate. Taking this into account, we would need to add about 4 times the amount of organic matter to meet this sequestration rate."

Previous studies have concluded that a global mean storage rate of 0.5 tonnes of carbon/hectare/year is possible, and research from the Rodale Institute concluded that if their regenerative practices were carried out across the world's agricultural lands, it would be possible to capture all human carbon emissions. Thus, while achieving a global "4/1000" poses an appreciable challenge, even approaching this target would be of considerable benefit, not only in terms of helping to balance the global carbon books, but in improving and restoring the quality of the world' soils. The world's cultivated soils are estimated have lost between 50 and 70% of their original carbon content, a trend that can be reversed by using defined agricultural methods. The result is more productive, carbon-rich soils, and so the strategy is able to “reconcile food security and climate change.”


The essential methods for 4/1000:

• Avoid leaving the soil bare in order to limit carbon losses
• Restore degraded crops, grasslands and forests
• Plant trees and legumes which fix atmospheric nitrogen in the soil
• Feed the soil with manure and composts
• Conserve and collect water at the feet of plants to favour plant growth

If good practices and introduced and sustained, it is expected that the carbon capture will continue for 20 to 30 years.

Applied to the surface horizon of the world's soils, which contain 860 billion tonnes of carbon, the 4‰ target would result in 3.4 billion tonnes carbon being stored annually, which amounts to around 40% of anthropogenic CO₂ emissions. The majority of soils, not only agricultural soils, could be so addressed, including forests. The above practices could be undertaken by almost half the world's population, those living in rural areas, working 570 million mostly small farms.

The likely costs?

For crops, $20 to $40 (US) per tonne of CO₂. For grasslands and forests, $50 or $80 (US) per tonne of CO₂.

Phytoremediation - Using Plants to Cleanse the Earth.

Phytoremediation^1,2 may be defined as the treatment of environmental problems by using plants in situ to avoid the need to excavate the contaminated material for disposal elsewhere. It can be applied to the amelioration of contaminated soils, water, or air, using plants that can contain, degrade, or eliminate metals, pesticides, solvents, explosives, crude oil and its derivatives (refined fuels), and related contaminating materials. Phytoremediation has been used successfully for the restoration of abandoned metal-mine workings, and cleaning up sites where polychlorinated biphenyls have been dumped during manufacture, and for the mitigation of on-going coal mine discharges. Phytoremediation uses the natural ability of particular plants (“hyperaccumulators”, described below) to bioaccumulate, degrade, or otherwise reduce the environmental impact of contaminants in soils, water, or air. Those contaminants that have been successfully mitigated in phytoremediation projects worldwide are metals, pesticides, solvents, explosives, and crude oil and its derivatives, and the technology has become increasingly popular and has been employed at sites with soils contaminated with lead, uranium, and arsenic. A major disadvantage of phytoremediation is that it takes a relatively long time to achieve, because the process rests upon the ability of a plant to thrive in an environment that is not normally ideal for plants.

Advantages and limitations of phytoremediation.

• Advantages:
• The cost of phytoremediation are lower than those of traditional processes, both in situ and ex situ.
• The plants can be easily monitored.
• There is the possibility of the recovery and re-use of valuable metals (by companies specializing in “phyto-mining”).
• It is potentially the least harmful method because it uses naturally occurring organisms and preserves the environment in a more natural state.
• Trees may be used in phytoremediation, since they grow on land of marginal quality, have long life-spans and a high flood tolerance. Willows and poplars are most commonly used, and can grow 6-8 feet (ca 2 metres) per year. For deep contamination, hybrid poplars with roots extending 30 feet deep have been used, which penetrate microscopically sized pores in the soil matrix and each tree can cycle 100 L of water per day, functioning almost as a solar powered and self-contained pump and treatment system.
• Phytoscreening is possible, in which plants may be used as biosensors for particular types of contaminants, thus giving a signal of underlying contaminant plumes, e.g. trichloroethene has been detected in the trunks of trees.
• Genetic engineering may confer improvements to phytoremediation, e.g. genes encoding a nitroreductase from a bacterium, when inserted into tobacco, increased the resistance of the plant to the toxic effects of TNT and the uptake of the material. Plants may be genetically modified to grow in soils even when the pollution levels in the soil are lethal for non-treated plants, and to absorb a greater concentration of the contaminant.

• Limitations:
• Phytoremediation is limited to the surface area and depth occupied by the plant roots.
• Slow growth and low biomass require a long-term commitment.
• Using plants, it is not possible to prevent entirely the leaching of contaminants into the groundwater (without the complete removal of the contaminated ground, which in itself does not resolve the problem of contamination).
• The survival of the plants is affected by the toxicity of the contaminated land and the general condition of the soil.
• Bio-accumulation of contaminants, especially metals, into plants which then pass into the food chain, from primary level consumers upwards, or that the safe disposal of the affected plant material is required, i.e. if the plants might be eaten by animals.
• The procedure is slow.

Hyperaccumulators and biotic interactions. 

If a plant is able to concentrate a particular contaminant, to a given minimum concentration (> 1000 mg/kg of dry weight for nickel, copper, cobalt, chromium or lead; or > 10,000 mg/kg for zinc or manganese), it is categorized as a hyperaccumulator. This capacity for accumulation is a result of genetic adaptation over many generations in hostile environments. Metal hyperaccumulation can affect various different factors, such as protection, interferences between different species of plants, mutualism (e.g. mycorrhizae, pollen and seed dispersal), commensalism, and biofilm.

Different possible phytoremediation methods.

Various processes that are mediated by plants or algae might be used to address environmental problems:

• Phytoextraction — uptake and concentration of substances from the environment into the plant biomass.
• Phytostabilization — reducing the mobility of substances in the environment, for example, by limiting the leaching of substances from the soil.
• Phytotransformation — chemical modification of environmental substances as a direct result of plant metabolism, often resulting in their inactivation, degradation (phytodegradation), or immobilization (phytostabilization).
• Phytostimulation — enhancement of soil microbial activity for the degradation of contaminants, typically by organisms that associate with roots. This process is also known as rhizosphere degradation. Phytostimulation can also involve aquatic plants supporting active populations of microbial degraders, as in the stimulation of atrazine degradation by hornwort.
• Phytovolatilization — removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile and/or less polluting substances.
• Rhizofiltration — filtering water through a mass of roots to remove toxic substances or excess nutrients. The pollutants remain absorbed in or adsorbed to the roots. 

Phytoextraction.
In phytoextraction (or phytoaccumulation) plants or algae are used to extract contaminants from soils, sediments or water into harvestable plant biomass (those organisms that take larger-than-normal amounts of contaminants from the soil are called hyperaccumulators). Phytoextraction has been used more often for extracting heavy metals than for organic contaminants. The plants absorb contaminants through the root system which they then contain in the root biomass and/or move them into the stems and/or leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest, a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. The process can be repeated to affect further decontamination. There are two forms of phytoextraction:

• Natural hyper-accumulation, where plants take up the contaminants in soil unassisted.
• Induced (assisted) hyper-accumulation, in which a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily. In many cases natural hyperaccumulators are metallophyte plants that can tolerate and incorporate high levels of toxic metals.

Examples of phytoextraction:

• Arsenic, using the Sunflower (Helianthus annuus), or the Chinese Brake fern (Pteris vittata), a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.
• Cadmium, using willow (Salix viminalis): willow has a significant potential as a phytoextractor of cadmium (Cd), zinc (Zn), and copper (Cu), as willow has some specific characteristics like high transport capacity of heavy metals from root to shoot and huge amount of biomass production; can be used also for production of bioenergy in the biomass energy power plant.
• Cadmium and zinc, using Alpine pennycress (Thlaspi caerulescens), a hyperaccumulator of these metals at levels that would be toxic to many plants, although its growth appears to be inhibited by copper.

• Lead, using Indian Mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia), Hemp Dogbane (Apocynum cannabinum), or Poplar trees, which sequester lead in their biomass.
• Salt-tolerant (moderately halophytic) barley and/or sugar beets are commonly used for the extraction of sodium chloride (common salt) to reclaim fields that were previously flooded by sea water.
• ¹³⁷Cs and ⁹⁰Sr contaminating a pond were removed using sunflowers, following the 1986 Chernobyl accident.
• Mercury, selenium and organic pollutants including polychlorinated biphenyls (PCBs) have been removed from soils by transgenic plants containing genes for bacterial enzymes.
• Recovery of phosphate from wastewaters by algae.

Phytostabilization.
In phytostabilization the intention is to stabilize, or contain the pollutant over the long-term. There may be a number of contributing factors to this, e.g. the reduction of wind (soil) erosion by the body of the plant, but the roots of the plant can resist water (soil) erosion, immobilize the pollutants by adsorption or accumulation, and provide a zone around the roots where the pollutant can be deposited in an immobilized form. In contrast with phytoextraction, phytostabilization aims mainly to sequester pollutants in soil around the roots but not in the plant tissues. Hence the pollutants are increasingly less bioavailable, such that exposure to livestock, wildlife, and humans is reduced. Mine tailings may be stabilized by growing a vegetative cap.

Phytotransformation. 

 Some plants, e.g. cannas, are able to detoxify organic pollutants - pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances  - by metabolising them. The metabolic functions of microorganisms living in association with plant roots may also metabolize these substances, as present in soil or water. Due to the complex and recalcitrant nature of many of these compounds, they cannot be broken down entirely (mineralised) to basic molecules (H₂O, CO₂, etc.) by plants and hence the term phytotransformation represents molecular alterations rather than the complete decomposition of the compound. Phytotransformation may be viewed¹ as a "Green Liver" because plants behave analogously to the human liver in processing these xenobiotic compounds, introducing polar groups such as –OH to them. This is known as Phase I metabolism, similar to the way that the human liver increases the polarity of drugs and foreign compounds. In plants, it is enzymes such as nitroreductases which carry out these transformations, whereas in the human liver it is enzymes such as the Cytochrome P450s that perform the task. Phase II metabolism in the second step in phytotransformation, in which the polarity of the xenobiotic molecule is increased by combination with plant biomolecules such as glucose and amino-acids. This is called “conjugation”, and is once more similar to processes such as glucoronidation (addition of glucose) and glutathione addition reactions, catalysed by appropriate enzymes. The effect of the two metabolic steps may serve to detoxify the xenobiotic and aid its mobilization via aqueous channels. In Phase III metabolism, the xenobiotic becomes sequestered, by incorporation in a complex “lignin-type” structure, where it is kept apart from the normal functioning of the plant. The phytotransformation of trinitrotoluene (TNT) has been well studied, and a detailed mechanism proposed for it.

Phytostimulation and rhizoremediation. 

This term identifies the process where compounds released from plant roots enhance microbial activity in the rhizosphere, which is the narrow region of soil around the roots of plants, and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. In rhizoremediation, microorganisms degrade soil contaminants in the rhizosphere. It is usual that those soil pollutants which are remediated by this method are highly hydrophobic xenobiotics, and are hence unable to enter the plant. Rather than the plant being a main protagonist in this process, it creates a haven in which microorganisms in the rhizosphere are able to perform the degradation.The plant acts as a solar-powered pump, which draws in both water and the xenobiotic agent, simultaneously producing substrates (e.g. root exudates and root turnover) that assist the growth of the microbes which act as pollutant degrading agents. Microbial activity is stimulated in the rhizosphere through a number of different routes: (i) exudates, e.g. sugars, carbohydrates, amino acids, acetates, and enzymes, nourish indigenous microbe populations; (ii) root systems bring oxygen into the rhizosphere, meaning that aerobic transformations are supported; (3) the available organic carbon is enhanced through the growth of fine-root biomass; (4) mycorrhizae fungi, which are an essential component of the rhizosphere, provide unique enzymatic pathways lending the capacity to degrade pollutant molecules that would not be degraded by bacteria alone; and (5) the presence of plants (and their roots) creates a domain for microbial populations, which are activated in the rhizosphere. There have been five enzyme systems identified in soils: (i) dehalogenase (which acts in dechlorination reactions of chlorinated hydrocarbons); (ii) nitroreductase (essential for the initial step of nitroaromatic degradation); (iii) peroxidase (a critical catalyst for oxidation reactions); (iv) laccase (able to begin the decomposition of otherwise robust aromatic ring structures); (v) nitrilase (another key factor in oxidation processes). The method is limited in that when there are high concentrations of pollutants present, the plants may be overwhelmed and die. The successful use of phytostimulation has been demonstrated in the remediation of chlorinated solvents from groundwater, petroleum hydrocarbons from soil and groundwater and PAHs from soil. 

Phytovolatilization.

Probably, this is the most controversial of the phytoremediation technologies, since it involves the release of contaminants either directly, or in a metabolically modified form, into the atmosphere. Phytovolatilization³ has been used principally for the removal of Hg²⁺ ions which are transformed into less toxic elemental mercury⁴. Tritium (³H), a radioactive isotope of hydrogen with a half-life of about 12 years, decaying to helium, has also been removed by phytovolatilization⁵. A good deal more research is necessary before this strategy becomes mainstream, since there are various negative features to be addressed. For example, mercury that is released into the atmosphere from plants is likely to be recycled by precipitation and thus returned the ecosystem, and the method is restricted both to sites where the concentration of contaminants is toward the low side, and where the contamination is no deeper than the roots of the plants being used. 

Rhizofiltration. 
Rhizofiltration⁶ involves filtering contaminated water through a mass of roots for the extraction of contaminants, or excess nutrients, e.g. phosphorus. The contaminated water can either be collected from a waste site and taken to where plants are being hydroponically cultivated, or the plants may be planted in the area directly. In both cases, the roots draw up the water and its associated contaminants. This process is very similar to phytoextraction in that the contaminants become sequestered in the form of harvestable plant biomass. Then new plants are grown and harvested until a satisfactory degree of decontamination is achieved. It is the concentration and precipitation of heavy metals that is sought principally. While noting these similarities, the fundamental difference between the two approaches is that rhizofiltration is used in aquatic environments, while phytoextraction is applied to the decontamination of soils. There are limitations to rhizofiltration. As usual in phytoremediation methods, any contaminant that is below the rooting depth will not be extracted, and if the level of contamination is too high the plants will not grow. Depending on the type of plant and contaminant, the process may need to be continued over a protracted period, before regulatory levels are achieved. It is generally true that many different kinds of contaminants will be present – in some cases a mixture of organics and heavy metals – and thus the use of rhizofiltration alone is unlikely to succeed. Importantly, the plants chosen should be non-fodder crop to minimize poisoning animals, which might eat them in contaminated form.  That noted, the effective removal of heavy metal cations, e.g. Cu²⁺, Cd²⁺, Cr⁶⁺, Ni²⁺, Pb²⁺, and Zn²⁺ from aqueous solutions has been demonstrated⁷, and the removal of low-level radionuclides, from liquid streams⁸. In that latter application, a “feeder layer” of soil is suspended above the stream through which plants grow, from which the plant roots extend downward into the water. In this way, fertilizer can be used to help the plants to grow, while avoiding adding to the contamination of the stream, while the latter is cleansed of heavy metal cations⁹. Rhizofiltration is cost-effective when large volumes of water must be treated containing low concentrations of contaminants. Inclusive of the costs of the capital outlay and final waste disposal, the cost of removing radionuclides from water using sunflowers was reckoned (at 1996 prices) at $2─6 per thousand gallons of water treated¹⁰.
 

References. 
(1) Burken, J.G. (2004), "2. Uptake and Metabolism of Organic Compounds: Green-Liver Model", in McCutcheon, S.C.; Schnoor, J.L. (Eds.), Phytoremediation: Transformation and Control of Contaminants, A Wiley-Interscience Series of Texts and Monographs, Hoboken, NJ: John Wiley, p. 59, doi:10.1002/047127304X.ch2, ISBN 0-471-39435-1.
(2) http://en.wikipedia.org/wiki/Phytoremediation.
(3) http://www.unep.or.jp/Ietc/Publications/Freshwater/FMS2/2.asp.
(4) http://tede.ibict.br/tde_busca/arquivo.php?codArquivo=431.
(5) Dushenkov, S. (2003) “Trends in phytoremediation or radionuclides.” Plant and Soil, 249, 167. (6) http://en.wikipedia.org/wiki/Rhizofiltration.
(7) EPA, (1998) “A Citizen's Guide to Phytoremediation,.” U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response,” EPA 542-F-98-011, August. 
(8) Dushenkov, V., Motto, H., Raskin, I. and Nanda Kumar, P.B.A. (1995) "Rhizofiltration: the Use of Plants to Remove Heavy Metals From Aqueous Streams." Environmental Science Technology 30, 1239. 
(9) Raskin, I., Smith, R.D. and Salt, D.E. (1997) "Phytoremediation of Metals: Using Plants to Remove Pollutants from the Environment." Current Opinion in Biotechnology. 8, 221. 
(10) Cooney, C. M. (1996) "Sunflowers Remove Radionuclides From Water in Ongoing Phytoremediation Field Tests." Environmental Science and Technology 30, 194.