Most of us use sorbents every day, though we probably do so without thinking about it. Sorbents are, quite simply, materials that are good at absorbing liquid.
In the home, you'll encounter sorbents when you use paper towels, sponges, and even cat litter. But in industry, they're made of stronger stuff and can be used for dealing with oil and chemical drips, leaks, and spills.
While they have many uses, oil clean up is the most common industrial application for sorbents, and that’s what we will focus on here.
What Are Sorbents?
Sorbents are insoluble materials used to absorb or adsorb liquids. What’s the difference? While absorbents pick up and retain liquid, causing the material to swell by 50 percent or more, liquids coat adsorbent materials on the surface.
Sorbents are often used to clean up small spills, remove final traces of oil, and clean hard-to-reach areas that can be accessed by skimmers. They're broken down into three categories: natural organic, natural inorganic, and synthetic.
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Characteristics
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Examples
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Natural organic sorbents
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- Can absorb 3 to 15 times their weight in oil
- Tend to absorb water and sink (this can be addressed by using flotation devices)
- Are often loose particles that are difficult to collect after use (can be wrapped in mesh to prevent spreading)
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- Peat moss
- Straw
- Hay
- Sawdust
- Ground corncobs
- Feathers
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Natural inorganic sorbents
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- Absorb 4 to 20 times their weight in oil
- Inexpensive and readily available in large quantities
- Not used on the surface of water
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- Clay
- Perlite
- Vermiculite
- Glass wool
- Sand
- Volcanic ash
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Synthetic sorbents
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- Can absorb up to 70 times their weight in oil
- Man-made materials similar to plastics, which absorb liquid onto their surfaces
- Cross-linked polymers and rubber materials absorb liquid into their solid structure and swell
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How Sorbents Work
To successfully combat oil spills, it’s important that sorbents be oleophilic and hydrophobic; that is, they must attract oil and repel water.
Adsorbents
Adsorbents are perhaps the more common of the two types, and most sorbents tend to fall into this category.
Oil must wet the material and then spread over its surface in preference to water. To be effective, the sorbent must have surface tension less than water (60-65 mN/m) but greater than oil (20 mN/m). For example, polypropylene, with a value of 29 mN/m, makes an ideal oil absorbent.
The speed with which sorbents absorb oil varies depending on the viscosity of the liquid and the size of the sorbent pores.
- Low viscosity oils like light crude can usually penetrate within seconds and are best used with smaller pores
- High viscosity ones like heavy fuel oil can take hours and require coarser cell structures
- Using high viscosity oils with fine pores will result in clogs; coarse cells are unlikely to sufficiently hold low viscosity oils
These materials also need a high surface area to volume ratio, as their capacity is directly related to the available surface area – including both internal and external surfaces.
Use caution when adsorbing volatile liquids. Spreading the liquid over the entire surface area of the material can increase the release of vapor, which could pose a risk to combustion and negatively affect human health.
Absorbents
Absorbents allow liquid to diffuse into their matrix, causing the material to swell and hold the liquid such that it cannot leak or be squeezed out under pressure.
Since these sorbents reduce the surface area of the liquid, they are excellent for use with volatile products.
Absorption time can be lengthy for oils, so absorbents are most often used with low viscosity liquids and chemical spills.
Sorbents come in four general forms: bulk loose material, enclosed in mesh (pillows and booms), continuous (sheets and rolls), and loose fibers.
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Material
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Advantages
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Disadvantages
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Bulk loose sorbents
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Organic, inorganic, or synthetic
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- Naturally abundant or widely available as industrial process by-products
- Low cost
- Good for small oil spills on land
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- Difficult to control and retrieve (easily spread by wind) and, therefore, have limited marine use
- Oil and sorbent mixture difficult to pump
- Limited disposal options
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Enclosed sorbents
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Organic, inorganic, or synthetic enclosed in mesh or nets
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- Easier to deploy and retrieve than loose sorbents
- Greater surface area than continuous booms
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- Limited structural strength
- Quickly become saturated and sink
- Limited oil retention
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Continuous sorbents
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Synthetic (usually polypropylene)
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- Easy to deploy and retrieve
- Long-term storage
- High oil recovery ratio if used to full capacity
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- Limited efficiency with higher viscosity oils
- Do not decompose
- Limited disposal options
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Loose fibers
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Synthetic (usually polypropylene)
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- Most effective for high viscosity oils
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- Limited effectiveness with light and medium oils
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Things to Consider When Selecting Sorbent Products
While form is certainly one consideration, there are a number of other things you’ll need to think about when selecting the right sorbent for your application.
Saturation
Once sorbents have reached their saturation points, they are no longer able to recover any further oil and must be removed as soon as possible. Use booms with a relatively small diameter to reduce the chance of discarding booms that aren’t yet fully saturated. Sorbent sheets saturate quickly and should only be used for small-scale incidents with limited recovery.
Oil Retention
Good retention ensures that captured oil won’t be re-released into the environment it was just taken out of. Organic materials are particularly prone to oil retention problems, while sorbent materials with fine pores generally retain oil quite well.
Strength and Durability
If sorbents are being left for an extended period, consider opting for a highly durable material. It’s not uncommon for sorbent booms to degrade and fall apart after just a few hours due to waves or rock abrasion, which can lead to secondary contamination.
Ease of Application
Consider whether the sorbents will be applied manually or mechanically, such as with a fan. Organic sorbents that are loose (like clay and vermiculite) are difficult to apply in windy conditions and can be hazardous to workers if inhaled.
Cost
The biggest factor affecting cost is the type of material used. While organic and inorganic sorbents are generally less expensive than synthetic ones, they are also less effective, meaning you’ll have to purchase more.
Don’t forget to also factor in the cost of disposal.
Conclusion
It’s important to remember that sorbents are a last resort. Companies should focus on reducing the risk of oil or chemical spills in the first place.
But of course, no matter how many precautions you take, accidents happen. By understanding the properties, benefits, and drawbacks of the various types sorbents, you can be ready for the clean up operation when a spill occurs.
Download our 3 Steps to Choosing Your Absorbent guide for a handy, user-friendly reference.