How to clean soil?

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In the last couple of centuries humans have done a strange thing: we’ve dug the biggest pits, the deepest holes, and the longest tunnels the world has ever seen, all to find the most insidious and subtle poisons known to our mammalian bodies, remove them from deep inside rocks where they had lain sequestered for eons, and concentrate them in the places where most of us live. We’re starting to think this maybe wasn’t a good idea.

Take lead, which last- century humans put into containers, car parts, pipes, paints and many other products — and even in petroleum, spreading lead-tainted exhaust across a world. Lead causes brain damage and erratic behaviour if absorbed into the human body, and its rise and fall correlates with the US crime rate in the 20th century – the more lead was around children, the more crime appeared a generation later. It’s been banned from paints and auto fuel, of course, but it lingers on old buildings and in soil.

Or take mercury: burning coal releases it into air and water, and thence into animals like fish – a 2009 study by the US Geological Survey tested 300 streams across the USA and found that every fish tested contained mercury, a quarter at unsafe levels.

You could go on with a list of such heavy metals – cadmium, zinc, copper – right down the periodic table. Most of all, we have pulled out coal and oil and used it not just to fuel up the car and turn on the lights, but to generate hundreds of thousands of petrochemicals with unpronounceable names as long as sentences and often-unpleasant effects.

I say “we,” of course, but this isn’t a guilt trip; most of this was before your time, and you didn’t vote for it anyway. You and I use small amounts of heavy metals and fossil fuels in our own lives – driving, flying, heating, buying plastic products, just looking at this on a computer – but it’s very difficult to avoid doing so and still living in the modern world.

The consequence of so many people doing so many of these things, though, is that any urban area — and many rural ones – will have splotches on the map with large quantities of toxic materials in the ground. If you live where a gasoline station used to be, or a factory, a garbage dump, or any number of other things, you might have things in your soil you don’t want in your stroganoff.

If you think you just won’t live in places, or just move away from them, congratulations: you’re thinking the same thing as everyone else. That presents a problem, as everyone who can live somewhere else will do so, and everyone who can’t live somewhere else will live on contaminated sites. Realistically, this means the poor, the elderly and other vulnerable people have to live with everyone else’s toxic waste – which is often the case already.

Other methods, like removing tonnes of contaminated soil, involve years of work and vast sums of money we don’t have anymore. If you could remove all the affected soil, moreover, where would you put it, aside from somewhere else that would then be contaminated?

What we need is a device that can suck toxins out of the soil and either turn them into something harmless, or concentrate them in something lightweight and removable. No one has much money lying around to invent such a device, though, much less to manufacture millions of them and send them to sites around the world for free. Thus, these hypothetical devices would be even better if they already appeared around the world.

It would be best, in fact, if these machines cost nothing to create, and once created could make more of themselves, at an exponential rate. While we’re at it, it would also be nice if the devices also prevented soil erosion, fed bees and other pollinators, and provided shade, beauty, a home for wildlife, and possibly firewood.

Thankfully, we have these machines now. Certain plants, it turns out, have a particular gift for sucking up specific chemicals, either as a quirk of their biology or as a way to make themselves poisonous and avoid being eaten. When these plants are sown on contaminated ground, they absorb the contaminants into their tissues, gradually reducing the amount in the soil until it is safe for humans.

Called phyto-remediation, this process has become one of the newest and most promising fields of biology. Similar methods use mushrooms in what is called myco-remediation, or use bacteria and have unfortunate names like bio-sparging, bio-slurping and bio-venting, but we’ll restrict ourselves here to plants.

The basic method is straightforward: find out what toxins lurk in your patch of ground, and come up with a regimen of plants appropriate for the climate that hyper-accumulate those particular toxins.

“Toxins,” of course, covers a lot of ground, and the vagueness of the word allows it to be used in all kinds of unproductive ways – for example, every fake New Age cure that claims to rid your body of unspecified “toxins.” So to get more specific, let’s separate toxins into two of the most common categories: metals and petrochemicals.

Petrochemicals generally have familiar atoms like carbon, hydrogen and oxygen, the same things that make up chocolate sundaes, flower gardens, testosterone, newspaper, and most of the world around us. Those same elements in different combinations, however, make common but un-tasty compounds like gasoline, or lethal poisons like Agent Orange — it’s all in how many atoms are put together in what arrangement.

If a plant can absorb, let’s say, the cancer-causing benzo-pyrene – C20H12, found in coal tar – with some oxygen (O) and then separate it into C12H22O11 and H2O, the petroleum-based poison would become sugar water. I’m not saying this is the actual chemical process, by the way – just an example of how chemical combinations can make something deadly or delicious.

When the toxins are metals, of course, they cannot be broken down into other elements any more than lead could be changed to gold. Some plants can absorb the metal and metabolise it into some kind of molecule, however, making it less easy to be absorbed by the human body and thus safer to be around. Sometimes the metals can even help us; some biologists have even proposed using certain edible plants to accumulate zinc from contaminated soils and feeding the plants to people with a zinc deficiency.

After the plants are harvested with the metals concentrated in their tissues, they can be burned, and the metal stays in the ash – a small amount of space and weight to dispose of, compared to the tonnes of contaminated earth. The ash might even be able to be mined for the metals, for complete recycling.

One example comes from Brazil, where abandoned gold mines are leaking mercury and other heavy metals into the soil and water. Mercury is one of the most toxic of heavy metals, and once in the soil it is soaked up by grass, which is eaten by cows, which are eaten by … you get the idea. Farmers are now growing maize and canola plants in the area, though, which soak up heavy metals quite nicely – gold as well as mercury. One scientists overseeing the project estimated farmers could get a kilogram of gold per hectare from doing this, which would help pay for the clean-up.

Mustard greens were used to remove 45% of the excess lead from a yard in Boston to ensure the safety of children who play there. Pumpkin vines were used to clean up an old Magic Marker factory site in Trenton, New Jersey, while Alpine pennycress helped clean up abandoned mines in Britain. Hydroponically grown sunflowers were used to absorb radioactive metals near the Chernobyl nuclear site in the Ukraine as well as a uranium plant in Ohio.

Blue Sheep fescue helps clean up lead, as do water ferns and members of the cabbage family. Smooth water hyssop takes up copper and mercury, while water hyacinths suck up mercury, lead, cadmium, zinc, cesium, strontium-90, uranium and various pesticides. Sunflowers slurp a wide range of compounds – not just the uranium and strontium-90 from radioactive sites, but also cesium, methyl bromide and many more. Bladder campion accumulates zinc and copper, while Indian mustard greens concentrate selenium, sulphur, lead, chromium, cadmium, nickel, zinc, and copper.

Perhaps the most magnificent hyperaccumulator, though, is the simple willow tree, Salix viminalis; it slurps up copper, zinc, cadmium, selenium, silver, chromium, uranium, petrochemicals and many others. Also, once its bio-mass has concentrated the heavy metals, it can be harvested and used for many practical things.

Of course, phytoremediation operates under certain limitations; the plants have to be able to grow in that climate, and should not be an invasive species that will take over the landscape, as kudzu did in the American South. The plants can only remove toxins as deep as their roots, so the technique might not solve groundwater contamination.

Most importantly, plants move at a different speed than we do, and even after the plants are harvested they are not likely to have eliminated the toxin. Reducing a toxin to safe levels takes time, and phytoremediation doesn’t remove a problem overnight.

Perhaps the most appealing aspect of this new field, though, is its scale, that the work to clean up toxic-waste sites could be done with no massive government project or corporate funding, with no bulldozers or construction equipment, without advanced and delicate technology beyond that to measure the toxin levels. The principles could be taught to every schoolchild or practiced by every land-owner, so that if anyone detects a certain toxin on their property, they will know what to plant to gradually remove it. The seeds and plants could be sold by any gardening or farm-supply store, so that some of our society’s most grandiose mistakes can be fixed by ordinary people, using natural means, using home-made experiments, hard work and patience, to restore our land to what it once was.

Thanks to Dr. David Leung of the University of Canterbury, New Zealand for his assistance in checking this article.

Survey of US streams: “Mercury Found in Every Fish Tested, Scientists Say,” New York Times, August 19, 2009.

Effects of lead on crime: “America’s Real Criminal Element: Lead,” Mother Jones magazine, January 2013

Effects of lead on crime: “How Lead Exposure Relates to Temporal Changes in IQ, Violent Crime, and Unwed Pregnancy,” Rick Nevin, Environmental Research, Volume 83, Issue 1, May 2000, Pages 1-22.

Effects of lead on crime: “Hazards of heavy metal contamination,” British Medical Bulletin, Volume 68, Issue 1, p. 167-182

Phytoremedation: Recent Advances Toward Improved Phytoremediation of Heavy Metal Pollution, Bentham Books, 2013.

Gold mines and mercury: Phytoremediation of Mercury-Contaminated Mine Wastes, Fabio Netto Moreno, Massey University 2004.

Playground in Boston: “New Jersey company cultivates pollution-eating plants Mustard greens, alfalfa help to clean up ravages of industry,” Baltimore Sun, March 30. 1997.

Playground in Boston: Blaylock, M.J., S. Dushenkov, D. Page, G. Montes, D. Vasudev, and Y. Kapulnik. Phytoremediation of a Pb-contaminated brownfield site in New Jersey. (1996), pp. 497-498. In Emerging Technologies in Hazardous Waste Management VIII, 1996 Extended Abstracts for the Special Symposium, Birmingham, Alabama, Industrial & Engineering Chemistry Division, American Chemical Society, September 9-11, 1996.

Blue Sheep Fescue: Phytoremediation: A Green Technology to Remove Environmental Pollutants, p. 71 – 86, American Journal of Climate Change 2013.

“Metal armour protects plants from disease,” Planet Earth Online, 10 September 2010.

“Improving Plants for Zinc Acquisition,” Prachy Dixit and Susan Eapen, Bioremediation Technology: Recent Advances, M. H. Fulekar, Springer, 2010.

Bio-remediation and Bio-fortification: Two Sides of One Coin, by X. Yin and L. Yuan, Springer 2012.

Soil Contamination Inspection

by Nick Gromicko, CMI®
How does soil become contaminated? Soil at a residential property can become contaminated when man-made chemicals come into contact with clean soil. Other sources of soil contamination may be wastes that leach from operational or closed landfills, runoff from livestock manure, direct dumping of hazardous industrial waste, waste piles from mining operations, septic systems and leach fields that breach their boundaries, and storage cisterns that burst underground. Inspectors, as well as homeowners, should use extreme caution if they suspect that such conditions exist at a property. InterNACHI inspectors should recommend further evaluation by a qualified professional. Contaminated particles pollute soil either by becoming attached to the soil itself or by residing in the spaces between the soil particles. Sometimes, the contamination may be from a “point source,” such as when pollutants are dumped directly on soil or buried underneath. In other cases, soil becomes contaminated as liquids or gasses from point sources migrate elsewhere, contaminating residential properties downstream. A less visible example of this is when a factory emits hazardous fumes out of its smokestacks, which then travel on the air and eventually fall to the ground farther away.

The three most widespread pollutants in urban and rural residential soils are lead, arsenic and cadmium. These elements were in widespread use in paints and construction practices in the past and persist in soils today because, as heavy metals, they do not readily break down. The following is a brief description of them.

  • The primary source of lead contamination in soil is from paint that contains lead. Paint residue falls to the ground and contaminates the soil as precipitation wears away a home’s exterior. The area with the highest contamination and of greatest concern is the “drip zone,” which extends 6 feet out from the perimeter of a home. Paint residue that fell into the soil decades ago may still persist today, even though leaded paint was banned in the late 1970s. Paint chips may also have become dislodged more recently if, for example, the home’s exterior was power-washed or sand-blasted. Lead may also be deposited in soil as demolished or abandoned structures eventually fall to the ground. Another common source of lead contamination is from leaded gasoline of years past. Soil that is close to a roadway with heavy traffic has the highest risk for this type of contamination.
  • Arsenic is another contaminant that is commonly found in residential soil. Arsenic was a widely used preservative for wood used in exterior structures, such as children’s playgrounds, walkways and gazebos constructed from the mid- to late 1900s until 2004. Arsenic is likely to have leached into the soil surrounding these wooden structures, especially in areas that have heavy precipitation. Areas with old, abandoned wooden structures on them are of concern. Arsenic was also a common ingredient in pesticides, herbicides and fertilizers. It is likely to persist in the soil of historic orchards and agricultural areas where these chemicals have not been sprayed in decades.
  • Cadmium is a common contaminant that has entered the environment and, consequently, the soil at properties as a byproduct of the burning of fossil fuels and municipal wastes, and from the smelting of zinc, lead, and copper.

Why is contaminated soil hazardous? Contaminated soil is dangerous for humans and animals. People are at high risk for poisoning when they come into direct contact with soil. This may occur when a person conducts any activity in the soil, such as digging, gardening and landscaping, and when soil is tracked into the home. Soil contaminants may be inhaled when soil is kicked up in the air, such as while mowing grass. Children are at particularly high risk because of their propensity for mouthing objects and putting their hands into their mouths without washing.

Pets and wild animals come into contact with contaminated soil when burrowing, eating and drinking from the ground. Changes in soil chemistry affect creatures at the lower end of the food chain, such as arthropods and tiny micro-organisms. Consequently, this puts entire ecosystems at risk, since it may cause a ripple effect through the food chain.

Another serious hazard is that drinking water supplies may become contaminated from contaminated soil. This is of primary concern when residents rely on underground water wells and aquifers for their fresh water supply.

There is a variety of health risks associated with contaminated soil, depending on the level of exposure, the type of pollutant, and the vulnerability of the affected population. Chronic exposure to heavy metals and other contaminants may put exposed individuals at a higher risk for cancers, neural disorders, reproductive disorders, and birth defects. Other less serious side effects may occur with light exposure, such as headaches, dizziness, fatigue and rashes. Contaminated soil also has a negative effect on plant growth and crop yields. Contaminated fruits and vegetables may be hazardous to consume, especially if they are not properly handled and washed prior to eating. 
 How can you tell if soil at a property is contaminated? There may or may not be visible indications of soil contamination. One obvious sign of lead contamination may be paint chips around the exterior of the house, if the paint is known to be leaded. Apparent discolorations in soil and strong odors are other indications that the soil may be hazardous. Another possible clue of contamination may be if vegetation fails to thrive in a certain area of soil, although this depends on the vegetation and whether the soil’s natural content is hospitable to such growth. One of the first things to consider when trying to determine if soil is contaminated is the history of the land. If the land or adjacent areas have ever been used for agriculture, industry, mining, waste disposal or gas dispensing, then there may be a lingering problem. If there have ever been wooden structures on the property prior to the late 1970s, then the land has a higher risk for lead and arsenic contamination. The only sure way to tell if soil is contaminated is to sample the soil and have a certified laboratory test it. A certified local soils engineer or professional should be employed to conduct soil sampling. Exhaustive testing may be exorbitantly expensive, however, but tests for the most common contaminants are usually affordable. Sampling for lead, arsenic and cadmium is usually less than $100. What are the options for remediating contaminated soil? Complete soil mapping and remediation can be a prohibitively expensive venture. Many contaminated industrial and commercial sites qualify for grants based on their “brownfield” status. However, this does not necessarily apply to residential sites. Local authorities should be consulted to see if there is financial assistance available. Funding may be provided for remediation procedures, depending on the level of risk to the surrounding community.Soil remediation methods include the following:

  • Excavation is the most comprehensive and expensive method of remediating soil. After contaminated soil has been identified, it may be removed from the property and transferred to a landfill for disposal. New topsoil is tested, trucked in, and distributed throughout the property to replace the old soil.
  • “Soil blending,” whereby contaminated soil is mixed with fresh soil, results in a mix that has a lower concentration of contaminants and meets local guidelines for acceptable pollutant levels.
  • The soil can be excavated, treated, and then replaced, after it has been deemed safe. Various methods for treating soil may be employed in situ or after excavation. These include applying aeration, heat and/or water, or treating it with chemicals that change the hazardous substances into ones that will biodegrade over time.
  • Bioremediation is a process whereby specific plants or fungi are utilized that naturally break down hazardous materials.
  • Problem areas may be covered or paved over so as to avoid exposure when excavating or treating soil is not an option. This is not a long-lasting solution, although it does help to inhibit the contaminants from spreading further.
  • “Capping” is a procedure whereby problem areas are covered by more than a foot of new topsoil. This may be an adequate temporary solution but, eventually, plant roots may penetrate into the contaminated soil.

What precautions should be taken with soil that is or is suspected to be contaminated? It is important to take special precautions with contaminated soil, especially when remediation techniques are not feasible.

  • Hazardous areas should be partitioned off, and direct exposure should be minimized.
  • Children’s play areas should be situated away from areas that may be hazardous, such as around the drip zone of a house, or near roadways where gasoline deposits may have accumulated.
  • Shrubbery may be planted around the drip zone of a house to discourage traffic in that area. Consider covering bare soil with mulch if grass will not grow on patches of contaminated soil.
  • Edible gardens should always be located away from hazardous areas. Building raised garden beds with clean topsoil minimizes the possibility of plants growing in unfit soil. Gardeners should always be sure to wash their hands thoroughly after working in the soil, as well as washing vegetables that will be consumed. Special attention should be paid to scraping root vegetables that have come into direct contact with contaminated soil. 

  • When mowing grass or working in areas with hazardous soil, wear personal protective equipment, including clothing that adequately covers exposed skin, a dust mask or respirator to prevent inhaling contaminated dirt and airborne particles, and protective eyewear.

In summary, soil may become contaminated in a myriad of ways, and special caution must be taken to minimize the associated risks. InterNACHI inspectors who suspect that soil or groundwater at a property is contaminated may want to recommend that their client have such conditions investigated and evaluated by a qualified soils engineer or other professional.

A new approach to cleaning heavy metals out of soil

When poisonous heavy metals like lead and cadmium escape from factories or mines, they can pollute the nearby soil.

With no easy ways to remove these contaminants, fields must be cordoned off to prevent these toxins from entering the food chain where they threaten human and animal health.

According to the Environmental Protection Agency, heavy metals have been found at thousands of locations nationwide. While some have been cleaned up through a combination of federal, state and private efforts, the need remains for new technologies to address heavy metal contamination

Now a research team led by Stanford materials scientist Yi Cui has invented a way to wash heavy metals from contaminated soils using a chemical process that’s a bit like brewing coffee.

As they describe in Nature Communications, the researchers started by rinsing contaminated soil with a mixture of water and a chemical that attracts heavy metals. When that mixture percolates through the soil, the chemical pulls heavy metals loose. The team members then collected this toxic brew and ran it through an electrochemical filter that captured the heavy metals out of the water. In this way they cleansed the soil of heavy metals and recycled the water and chemical mixture to percolate through more contaminated ground.

“This is a new approach to soil cleanup,” said Cui, who is a professor of materials science and engineering and photon science. “Our next step is a pilot test to make sure that what works in the lab is practical in the field, and to figure out how much this process will cost.”

So far, his team has cleansed soils contaminated with lead and cadmium, two prevalent and dangerous toxins, as well as with copper, which is only dangerous in high concentrations. Cui believes this process of chemical cleansing and electrochemical filtering will work with other dangerous heavy metals like mercury and chromium, but further lab experiments are needed to demonstrate that.

No more sacrificial plants

Cui said the project began two years ago when he and graduate student Jinwei Xu brainstormed about how to solve the basic problem: heavy metals bind to the soil and become virtually inextricable. Today, Cui said, cleanup may involve digging up contaminated soils and sequestering them somewhere. Agricultural researchers have also developed phytoremediation techniques — growing sacrificial plants in contaminated soil to absorb heavy metals, then harvesting these crops and taking them to an extraction and disposal facility. But phytoremediation can take many years of repeated harvests.

Seeking a quick, cost-effective way to extract heavy metals from contaminated fields, the researchers tried washing toxic soil samples with plain water. They soon realized that plain water couldn’t break the chemical bond between the heavy metals and the soil. They needed some additive to pry the contaminants loose. They found the answer in a common chemical known by its initials: EDTA.

In retrospect, EDTA was the obvious choice because this same chemical is used to treat patients poisoned with lead or mercury. Negatively charged EDTA bonds so strongly to positively charged heavy metal particles that it pulls the lead or mercury from the patient’s tissues. The researchers reasoned that, when dissolved in water, EDTA’s negative hooks would rip heavy metals loose from soils. Experiments bore this out. When EDTA-treated water percolated through contaminated soil, it carried the heavy metals away.

But the team’s job was only half done. The soil was clean, but the treated water was still toxic. They needed a way to separate the EDTA from the heavy metals in the rinse water and capture those toxins once and for all.

The scientists knew that EDTA remained strongly negative even after it captured a positively charged metal particle. So, the researchers built a sieve with the electrical and chemical properties to pull the negatively charged EDTA and positively charged heavy metals apart. The result was isolated heavy metals and a mixture of water and EDTA ready to purify more soil.

In addition to lead and cadmium, the researchers tested the process on copper. Next Cui would like to run the experiment on other heavy metals like mercury, which are so toxic they require special handling to protect the researchers. But he thinks the chemistry is so sound that he is confident of success in the lab. The bigger question is whether the process can be scaled up to treat tons of contaminated soil. The researchers have sought to patent the process through the Stanford Office of Technology Licensing and would like to find an opportunity to run a pilot project in a contaminated field.

“We really have no good remediation technology for heavy metals,” Cui said. “If this proves practical on a large scale it will be a significant advance.”

Yi Cui is also a senior fellow of the Precourt Institute and a member of Stanford Bio-X and the Wu Tsai Neurosciences Institute. Other Stanford authors include Tong Wu, Jing Tang, and Kai Liu, postdoctoral scholars in materials science and engineering. Former Stanford researchers Chong Liu, Po-Chun Hsu and Jie Zhao, also contributed to this research.

Contaminated Soil Treatment – How To Clean Contaminated Soils

The key to growing a healthy garden is clean, healthy soil. Contaminants in soil can quickly lead to an array of problems, so determining possible causes of soil contamination beforehand and learning how to clean contaminated soils is very important.

What is Soil Contamination?

Before you begin to plan and construct your garden, it’s always wise to have a soil sample analyzed. The quality of soil can be affected by many things. It is important to determine what nearby land was used for in the past and assess the impact of any nearby industry.

Oftentimes, the causes of soil contamination result from dangerous chemicals that find their way into the soil and disrupt the soil structure. Contaminants in soil that is taken up by plants or comes in contact with garden fruits and vegetables can cause health problems. Soil test results will indicate the quality of the soil and the causes of soil contamination, if any.

Possible Contaminants in Soil

Urban dwellers should be particularly concerned with a number of possible soil contaminants including lead, which has been used in paint and as an additive to gasoline; cadmium, which results from burning coal and garbage; arsenic, which is used in wood preservatives; weed killers; pesticides and fertilizers.

If you live close to an industrial or commercial site, it’s wise to have your soil checked for metals and cyanides, benzene, toluene and other chemicals associated with gas station leaks. Rural residents should also check for past and present industries and pesticides.

How to Clean Contaminated Soils

While cleaning contaminated soil is not “literally” possible, some things can be done to reduce the toxic impact. Adjusting the soil pH to as close to neutral as possible will help reduce the negative impact of contaminants.

Contaminated soil treatment also includes adding plenty of rich organic matter to the soil and a healthy top-dress of peat moss, compost or aged manure. This practice will help protect plants from damage.

Always be sure to wash any fruits or vegetables before you eat them. If contaminants are a problem, you can also plant in raised beds made with untreated lumber. This will allow you to add your own healthy soil.

Taking appropriate measures for cleaning contaminated soil beforehand can lead to a healthy garden for you and your family.

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