**3.3 Sorbents**

Another method of oil and oil products recovery is represented by sorbents. Sorbents are used to clean smaller oil spills, for final removing of oil substances or in places where skimmers cannot be used. Sorbents are solid, in water-insoluble material, which can absorb gaseous or liquid substances due to their physical and chemical composition. They work through the adsorption or absorption mechanism. Adsorbents become coated with oil substances on the surface, in pores and capillaries, binding can be both physical and chemical in nature. In order to be effective in oil spills disposal, sorption materials must be partly oleophilic and at the same time water-repellent. Absorbents pick up and retain liquids distributed throughout its molecular structure, typically is the volume increasing [2, 14, 32].

Sorbent material can be divided in three basic groups: natural organic material, natural inorganic, and synthetic.

Natural organic sorbents are mostly carbon-based products, such as straw, sawdust, coconut fibers, peat moss, etc., which can be in granular or fibrous form. These materials can adsorb more three times their weight of oil. Some of organic sorbents tend to break down to smaller particles, and it could be complicated to collect it. Other organic materials could sink to the bottom due to water absorption [14, 32].

Natural inorganic sorbents are used as granular materials such as perlite, clay, sand, ash. They can adsorb more than four times their weight in oil. As with organic sorbents, these materials also can have collection problems due to the breakdown into smaller particles.

Synthetic sorbents are composed of man-made materials, especially plastics such as polyurethane, polypropylene, polyethylene modified to adsorb liquids like a sponge. Other synthetic sorbents include cross-linked polymers and rubber materials, which absorb liquids into its solid structure, causing the sorbent material to swell [14]. Synthetic sorbents can absorb up 40–50 times their own weight in oil [14, 33].

### *3.3.1 Oil sorbent sorption capability tests*

The most significant property of all sorbents is their ability to bind several times the amount of contaminant relative to their own weight, the sorption capability. The sorption capacity of the sorbent mainly depends on the size of the surface area to which the dangerous substance, oil product can adhere as well as surface type (surface structure). The size of the surface of the sorbents and the increase in their sorption capacity are influenced by various cracks such as pores and capillaries. The average surface area of the sorbent is expressed in units of area (m2 ) per unit mass (g, kg). Sorbents have a surface area most often about 1,000,000 m2 kg<sup>−</sup><sup>1</sup> [33, 34]. The sorption capacity expresses how many times the sorbent binds more liquid to its weight or how many grams of contaminant bind 1 g of sorbent.

The American Society of Testing and Materials unified the methods of testing the sorption capacity of sorbents—ASTM F 726: Standard Test Method for Sorbent Performance of Adsorbents [35]. This test method describes laboratory tests that describe the "Short-term Oil-Absorption Test" and other non-emulsion floating, nonemulsion liquids from the water level. The comparison of sorption capability different sorbent material is given in **Table 2**.

### *3.3.2 Water contamination after cleanup process*

No cleaning process can be 100%. Various studies document that after mechanical cleaning, 10–15% of oil substances remain in the water [14, 32]. We made an experiment, where the samples of surface water (1 liter) were contaminated with diesel fuel and engine oil in amount of 1 g, 5 g, and 10 g. The layer of petroleum substance was built on the bottom of water. The oil film was stronger after application of engine oil. On the oil substances was applicated sorbent (expanded perlite) in the way to cover entire bottom of water level, which treated on oil substance. Contaminated sorbent was removed after 3 (diesel fuel) or 10 min (engine oil), and the amount of NES was determined in the water samples. The results are in **Table 3**.

Comparing the determined amounts of NES in "cleaned" water samples with legislative limits, we can say that all samples are still contaminated. The highest amount of NES was determined in the sample contaminated with diesel fuel (contamination with 10 g.L<sup>−</sup><sup>1</sup> ). Amount of NES was in all samples higher than 0.1 mg.L<sup>−</sup><sup>1</sup> , what according to research of Hybská et al. could lead to toxicological treat of aquatic organisms in the first step and cumulation of oil substances in other environment components [25].
