Heavy Metals in Water & Soil: Methods for Treatment
The ultimate faceoff: Human engineering versus plant power
We’re back with Part 2 on heavy metals! If you haven’t read Part 1 yet, be sure to do so to ensure you’ve got all the background info down.
As discussed in Part 1, you can be exposed to heavy metals through bioaccumulation in food. Though bioaccumulation often starts with microorganisms in aquatic environments, contaminants can also enter agricultural crops through contaminated soil or irrigation water in crop fields. Similarly, humans can be directly exposed to heavy metals through drinking contaminated water.
There are many methods available for reducing heavy metals at their source–before they enter our food and bodies. Chemically engineered methods such as reverse osmosis, ion exchange, and chemical precipitation have been in use for a while, but there are also some emerging technologies that utilize the natural power of plants to absorb heavy metals directly from the environment. Some of these technologies can only be used at treatment plants, while some of them are more consumer friendly and can be used at home. We review them both, below.
Chemically Engineered Methods
Contamination Pathway: Water
Reverse OsmosisUsed For: At Home Water Treatment for City & Well Water; Water Treatment Plants
Reverse osmosis (RO) can be used to treat many heavy metals such as chromium, copper, lead, and arsenic. RO technology uses added pressure to push water through a semipermeable membrane, which blocks contaminants larger than 0.0001 micrometers from passing through.
Removal rates depend on several factors including the post- and pre-treatment steps, but RO removal efficiencies are high for metals, with upwards of 99.4% removal for metals like cadmium and copper. Though reverse osmosis is a very effective method, it is expensive (systems cost anywhere from $150-$1000) and creates a high volume of wastewater. Not only will this likely increase your water bill (by about $50/month), but the wastewater stream contains a high concentration of contaminants can harm the environment if not properly disposed of.
Ion ExchangeUsed For: At Home Water Treatment for City & Well Water; Water Treatment Plants
Ion exchange is another common heavy metal treatment method, which can reduce nickel, mercury, lead, cadmium, chromium, and copper in water. When water passes through an ion exchange resin, heavy metal ions are attracted to the resin surface and easily swap places with charged surface particles called ions. The swapped ions are harmless ions such as hydrogen, making the discharge water safe to drink and use for irrigation.
As with all technologies, ion exchange has some drawbacks, including: it requires diligent cleaning (through backwash and brine regeneration) of the resin to work properly, it cannot handle highly concentrated metal solutions, it is not selective to heavy metals, and it is sensitive to your water’s pH. Ion exchange resins being “non-selective” means that they can also filter out beneficial ions in water such as minerals, like calcium and iron, that your body needs.
Chemical PrecipitationUsed For: Water Treatment Plants
Chemical precipitation is one of the most widely used methods for heavy metal removal. Contaminated water is mixed with a chemical solution, often containing lime, that reacts with the heavy metal ions. This causes the heavy metals to precipitate into solids. The precipitate is then large enough to filter out, and the heavy metal removal is complete!
Chemical precipitation is well established and low maintenance, but also creates concentrated wastewater sludge and can require complicated chemical dosing.
Ecologically Engineered Methods
Contamination Pathway: Soil & Water
PhytoremediationUsed For: Water Treatment Plants; Soil Remediation Projects
Phytoremediation enompasses any method that uses plants to uptake contaminants from soil or water. It is more affordable than chemically engineered methods and more environmentally friendly, because it does not create concentrated wastewater sludge that can harm ecosystems.
Engineered wetlands are an example of phytoremediation in action: water passes through and sits in wetlands while contaminants collect on the floor of the wetland and get taken up by plants.
However, phytoremediation is time consuming, limited by the age of the plant, root depth, level of contamination, weather conditions, etc, and there still exists a concern for proper contaminated plant disposal.
A few phytoremediation methods include:
Phytoextraction & Phytovolatilization
Phytoextraction refers to plants’ ability to uptake/absorb heavy metals through their roots and transport them to the above ground parts of the plant, called shoots. The shoots can then be burned to both generate energy and extract heavy metals from the ash to be recycled. Or, in the case of phytovolatilization, the heavy metals are released into the atmosphere.
Certain plant species can immobilize heavy metals present in soil and groundwater by “adsorption and accumulation in plant tissues, adsorption onto roots, or precipitation within the root zone” (Tangahu). Once the plant has latched onto the heavy metals, they can no longer move throughout the soil or be swept away by erosion.
Bio-sorption refers to the adsorption of contaminants onto modified agriculture such as hazelnut shells, pecan shells, maize cob or husk, rice husk, or jackfruit. The foods are heated into activated carbon, then used to adsorb heavy metals.
What about Contaminated Soil at Home?
Phytoremediation requires expertise to ensure that the correct plant species is being used and that contaminants are being removed effectively. At home, you’ll need a simpler approach–but you’ll want to know what’s in your soil first. Soil testing is an essential first step to understanding what is in your soil.
If you find you have contaminated soil with heavy metals, there are a few simple methods you can use to remediate the problem. You can create raised beds for your crops that are above the contaminated soil, or you can add a thick layer of organic material such as mulch or compost on top of your soil to create a physical barrier from the contamination.
Much water and soil treatment happens behind the scenes at treatment plants and soil remediation projects–but some of these methods are applicable at home. At Simple Water, we mostly focus on helping people protect themselves from heavy metals by providing advanced at-home tests for water and soil, so people can get a sense of what you’re dealing with and what type of treatment may be appropriate for their situation.
As always, if you have any questions, feel free to email us at email@example.com!