These sorbents are usually chemically resistant, they absorb different liquids, are heavy and non-inflammable.
These sorbents absorb various liquids, petroleum products and water solutions. If adequately processed, they do not absorb water for some time. This increases the price and diminishes their absorptive capacity. That is why these sorbents are characterized by diverse absorptive capacity (from 70% to 400%). In most cases these sorbents should not be used for absorption of aggressive liquids, such as concentrated acids, bases and oxidizing substances. These are flammable sorbents, requiring adequate storage conditions.
They are characterized by very large absorptive capacity. They are very light, which extends the time of absorption of spilled liquid – sorbent floats on surface and in order to facilitate the process of sorption, one ought to weigh down the sorbent. The bulk density around 0.10 kg/l causes these sorbents to be blown by wind. Study results limit the use of sorbents of this type to almost windless weather, which is why their use is limited to confined spaces. Oil-soaked natural organic sorbents (especially peat ones) are slippery and sticky and hard to collect from the bed. Very often they release the absorbed liquid if weighed down.
Properly prepared organic sorbents are usually hydrophobic and can be used for the collection of oil spills on water. If used in powder form, they are very hard to collect (fast current, sedimentation on coastal plants). Limited time of remaining on water surface (sometimes the sorbent sinks after 24 hours) causes secondary pollution of water and the bottom of water reservoirs.
After absorption of liquid, large amounts of waste material remain to be neutralized.
They usually have the following form:
Synthetic sorbents have very high absorptive capacities. They are produced as universal or hydrophobic sorbents. In most cases, they release the absorbed liquid under pressure.
Unisafe is a universal sorbent with very high absorptive capacity. It can be used for collecting all liquids. It includes an indicator which changes colour if exposed to acids or bases. In order to be used for collecting oils or petroleum products, it should be activated by water prior to use. This largely affects the time of collection of spilled liquid and the amount of waste generated as a result. Because of that, this sorbent is not suited for large spills during rescue actions in accidents and breakdowns. In Western Europe, this type of sorbent is used mainly in laboratories.
Hard, grinded polyurethane foams are water-repellent sorbents which collect only oils, petroleum products and other liquids immiscible with water. They do not absorb water. Their packing density (depending on sorbent preparation) varies from 0.102 kg/l to 0.45 kg/l. If it is light, the sorbent behaves similarly to organic sorbents, i.e. remains on spilled liquid surface for a long time and in order to facilitate the absorption process, one should additionally weigh it down.
Due to its packing density (up to approx. 0.36 kg/l) it may be blown by wind. Tests limit its use to almost windless weather. The absorptive capacity of such sorbents (depending on their specific gravity) varies from 100 to 300%. The only exception is Nonaqua sorbent. These sorbents are flammable and can release toxic gases when on fire (to be extinguished wearing a breathing apparatus). Because of its chemical composition, polyurethane requires special means of neutralization, performed by specialized companies.
Polypropylene sorbents - cotton wool and multilayer cloth of various thicknesses. Polypropylene sorbents are characterized by very large absorptive capacity, at the level of 1400%. They may be universal or hydrophobic (absorbing only oils and not absorbing water). Polypropylene sorbents are used mostly for the limitation and collection of oil and petroleum product spills on water. Such sorbents are selective and unsinkable even at the highest saturation . They are flammable, but combustion products are not toxic (carbon dioxide - CO2 and water vapour).
Spillage on water surface
Spillage on the ground
Classification of sorbents as heavy and light
Both these groups are characterized by very diverse usable properties and by their absorptive capacity.
Light sorbents are generally of organic origin (synthetic and natural ones). Heavy sorbents are inorganic ones, of mineral origin. Both these groups differ in density. Heavy sorbents of inorganic origin have higher densities. Thanks to that, they have more durable structure, they do not release the absorbed liquid, in other words they are hard. Light sorbents are quite different. They do not retain the absorbed liquid in their structure and release the liquid very easily under pressure. In other words, they are soft.
Light and heavy sorbents differ also in chemical composition. This has a direct influence on their reactivity. Inorganic heavy sorbents are chemically inactive; in turn, light sorbents react easily with many chemical compounds – e.g. with strong acids,bases and oxidizing compounds. A model sorbent should have features typical for different types of sorbents, but unfortunately it does not exist as yet.
used:
While usable aspects can be easily compared between the two groups of sorbents (light and heavy ones), the comparison of absorptive capacity should be performed within one group only. Comparison of absorptive capacities (within one group, naturally) is only reliable when we use one specific method and procedure.
In Poland there is no generally applicable and standardized method for testing absorptive capacity of sorbents. Because of that we often end up comparing “chalk and cheese”, relying on unverified or even outright invented data.
In Europe, sorption tests are usually performed using the Westinghouse method. This method is also used by Poland’s leading fire protection laboratory in Józefów.
The absorptive capacity gives us an indication of what amount of liquid we can collect with 1 kg of sorbent.
For the user, the cost of spill removal is very important.
Direct costs are connected with the volume of the sorbent purchased and (what we often tend to forget) the amount of waste material generated in the form of used sorbent, saturated with the collected liquid.
Indirect costs are connected with sorbent transportation costs, storage of sorbent, area of storage, and transportation and neutralization of generated waste material.
The calculation of direct costs can be easily demonstrated with the example of removal of 100 kg of Diesel oil by mineral sorbents with different absorptive capacities, including the cost of generated waste material neutralization.
As the above calculation shows, the volume of resultant waste material is very closely dependent on the absorption capacity of the sorbent. The higher capacity, the less waste material. This relationship directly influences the economy of purchase.
Direct cost of removing 100 l of spilled oil can be calculated using the following formula:
And what is the result? … What happened to indirect costs?
We have described above, in very general terms, the concept of waste material, i.e. used sorbent saturated with collected liquid. According to the Waste Materials Act, all sorbents saturated with dangerous substances are dangerous waste materials and should be handled in the same way as the absorbed medium.
There is a lot of contradictory information and questionable interpretations provided by manufacturers and distributors with regard to handling this type of waste material. Some of them recommend storing it in municipal landfills, others suggest composting. In the case of dangerous waste formation, the above ways of handling waste are against the law in force (Waste Materials Act dated April 27, 2001 as amended, Environmental Protection Law dated April 27, 2001 as amended). The way of neutralizing waste should be selected according to the type of sorbent and the absorbed substance.
Consequently, the use of sorbents is inseparably linked to the formation (in most cases) of dangerous waste, hence it has to comply with the Waste Materials Act. Chapter 1, Article 1.1. of the Act defines the principles of handling waste materials in a manner ensuring the protection of human life and health and protection of the environment, according to the principle of sustained development, and in particular the prevention of waste material formation or limitation of waste volume and their negative impact on the environment, as well as recovery or neutralization of waste.
The Waste Material Act in Chapter 2 imposes among others: