Experimental investigations of dynamic sorption of diesel from contaminated water

Surface wastewater pollution due to accidental runoff or release of oil or its products is a longstanding and common environmental problem. The aim of the study was to investigate the impact of concentrations of oil products (diesel) and suspended solids, the sorbent type, the water flow rate and the interfering factors (chlorides) on the dynamic sorption of diesel and to test regeneration of polypropylene after its use for sorption. The sorbents used for study included common wheat straw (Triticum aestivum), polypropylene and sorbents modified with hydrogen peroxide solution. Standard methods were used for the determination of the investigated parameters and an in-house procedure employing a gas chromatograph was used for the determination of diesel concentration. The following factors that impact the sorption of diesel were investigated during the study: diesel concentration, concentration of suspended solids; type of sorbent (common wheat straw (Triticum aestivum), wheat straw modified with hydrogen peroxide, and polypropylene), water flow rate; and influence of the interfering factors (chlorides). Filtration speed in the range of investigated speeds does not affect the efficiency of diesel removal. Removal efficiency does not depend on the concentration of diesel before the sorbent reaches its maximum sorption capacity. Filling containing 50% of polypropylene and 50% of wheat straw was used for the study. It was found that polypropylene and wheat straw do not remove chlorides and suspended solids from solution. The study found that the solution of hydrogen peroxide boosts the hydrophobic properties of common wheat straw, but does not affect the sorption of diesel. The recommended number of regenerations of polypropylene should be limited to two. Experimental investigations of dynamic sorption of diesel from contaminated water 31 (Yalcinkaya et al. 2020, Lurchenko et al. 2019, Voronov et al. 2018, Kwasny et al. 2018). Sorption with a solid sorbent is one of the possible treatment methods. Sorbents can be used for the removal of oil products and organic pollutants from polluted water (Gushchin et al. 2018, Akpomie and Conradie 2021, Quím 2020, Rudkovsky et al. 2016, Paulauskiene 2018, Króla and Rożek 2020, Baiseitov et al. 2016). Clay minerals and modified clay minerals are also widely used for removing oil products and organic pollutants from water and soil (Li et al. 2016, Moshe and Rytwo 2018, Bandura et al. 2017). Scientists have tested polypropylene and other types of plastic for the removal of hydrocarbons and other organic pollutants as well (Baig and Saleh 2019, Karyab et al. 2016, Thilagavathi and Das 2018, Mohammadi et al. 2020). The sorption efficiency of oil depends on the porosity of the sorbent: sorbents with higher porosity have higher sorption capacity. There are three stages of sorption: the first stage is the initial one, where sorption is most intensive during the first minute. This represents the most intensive process of oil products removal in the whole sorption process. The second stage is a transitional phase in which sorption is slowed down. The third stage is a constant phase in which sorption no longer occurs even with increasing contact time. The aim of the study was to investigate the influence of oil products (diesel) and suspended solids concentrations, sorbent type, water flow rate and interfering factors (chlorides) on the dynamic sorption of oil products (diesel). Materials and methods By using a mixture of tap water, cellulose and sodium chloride, contaminated water was artificially prepared to simulate the surface effluent and placed in a 100 l tank (Fig. 1). The flow rate was determined and the effluent level in the tank was kept constant by adding tap water, in order for the pressure not to affect the flow rate. Water was supplied by a pump from the tank to a pipe where it was mixed with oil products. Dieseline was lifted from a smaller diesel tank by help of a rotary pump and mixed with the water stream. The mixture flowed through a pipe in which there were steps with 15 cm distance between them to cause stream turbulence and create better conditions for mixing water and diesel. Next, the mixture flowed out of the tube through a grid that distributes the water stream over the entire diameter of the column for more accurate estimation of sorption capacity. The test water, filtered through the layer of sorbent in the column, flowed through the valve at the bottom of the column. In order to evaluate sorption, sampling of the water mixed with oil products was performed before it entered the column and after it exited the column at the bottom. Two kinds of sorbent were used in the experiment, namely, common wheat straw as a natural sorbent, chosen due to its ability to remove water-immiscible organic matter; and polypropylene as a synthetic sorbent, chosen due to its ability to efficiently remove high concentrations of organic matter and solvents. Sorption of oil products was tested in three cases. In all the cases, the column height and filling height were 100 cm and 90 cm, respectively (Figure 2): 1. The column was filled with polypropylene. 2. The column was filled with common wheat straw. 3. The column was filled with a combination of polypropylene and common wheat straw (1:1 by height). The experimental studies used two types of artificially contaminated aqueous solutions simulating road surface Fig. 1. Scheme of laboratory test bench: 1 – tank with water simulating surface wastewater; 2 – water pump; 3 – rotary pump for oil products; 4 – place of water and oil products supply; 5 – oil and water mixing pipe; 6 – water outlet; 7 – column with sorbent; 8 – treated water outlet