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Study of phenols and petrochemicals adsorption from model solutions using various sorbents

The article studies petrochemicals and phenols adsorption in static and dynamic conditions from model petrochemical solutions. It was established that the adsorption of resorcinol occurs in micropores by volumetric filling, this process is best described by the Dubinin-Astakhov equation. The adsorption of petrochemicals occurs on the sorbent surface, forming a monolayer, this process is best described by the Langmuir equation. Based on the kinetic curves of petrochemicals and resorcinol adsorption, DEC (dynamic exchange capacity) and FDEC (full dynamic exchange capacity) have been calculated for activated carbons and fibrous sorption material. Using the Shilov equation, kinetic constants of the adsorption dynamics for resorcinol and petrochemicals on various sorbents are calculated. The study proved the promising use of sorbents for wastewater treatment of various industries.


Introduction
Phenols and petrochemicals are one of the most dangerous pollutants contained in wastewater of various industries. According to the publications [1]. For fishery reservoirs, the following maximum permissible concentrations are established: for petrochemicals -0.05 mg/dm 3 , for phenols -0.1 mg/dm 3 .
An analysis of methods used for treating wastewater from petrochemical products and phenols shows that none of them allows reaching concentrations in the treated water that correspond to the maximum permissible concentrations for fishery reservoirs [2][3][4][5][6][7]. Most of the methods used either require energy or chemical reagents, or the secondary pollution is formed. One of the most effective methods of wastewater treatment from phenols and petroleum products, in our opinion, is the adsorption method using affordable and cheap sorbents.

Materials and methods
As the model wastewater system containing petrochemicals, we took finely dispersed system wastewater-gasoline. The system was composed in a proportion of 1:15 (gasoline-distilled water) so that the initial concentration of petrochemicals did not exceed 250 mg/dm 3 . Gasoline AI-95 was being mixed with distilled water during the day with periodic stirring. The aqueous fraction was separated from gasoline residues using a separator funnel.
An aqueous solution of resorcinol (m-dihydroxybenzene) was used as a model phenol solution. The concentration of petroleum products and resorcinol was determined by fluorimetric method.
As adsorbents, we used activated carbons of two fractions (0.30-0.84 and 0.40-1.68 mm) produced by Ecofresh Carbon and NWC and fibrous sorption material produced by JSC "VNIISV". In dynamic conditions, the adsorption was performed from model solutions of petrochemicals with a concentration of 200 mg/dm 3 and of resorcinol with concentration of 10 mmol/dm 3 at constant linear speed of 4 cm/min with sorbent layer height of 6 and 12 cm).
The adsorption equilibrium constants in resorcinol-sorbent solution systems have been calculated using the Dubinin-Astakhov equation and in petroleum-sorbent solution systems -using the Langmuir equation.

Results
The study of petrochemicals and phenols adsorption in static conditions was performed on model solutions of oil products with a concentration of 20-100 mg/dm 3 and of resorcinol with a concentration of 0.01-20.0 mmol/dm 3 . The petrochemicals adsorption was studied at the ratio solution : sorbent = 100:1. The resorcinol adsorption was studied at the ratio solution : sorbent = 1000:1. The time to establish the adsorption equilibrium in the model solution-sorbent system was 48 hours (figures 1, 2).  There are several theories of adsorption [8], each of them is described by its own equation. The appropriateness of applying adsorption theories is determined by the linearized forms of the corresponding adsorption equations for experimental data. The experimental data approximation showed that the adsorption of resorcinol occurs in micropores by volumetric filling and the Dubinin-Astakhov equation is most suitable for describing this process (figures 3-5). The adsorption of petrochemicals occurs on the sorbent surface, forming a monolayer, and the Langmuir equation is most acceptable for describing this process (figure 6).       The adsorption of petrochemicals and phenols under dynamic conditions was studied on model solutions of petrochemicals with a concentration of 200 mg/dm 3 , and of resorcinol -with a concentration of 10 mmol/dm 3 at a constant linear speed of 4 cm/min and sorbent layer height of 6 and 12 cm (figures 7-11).      Kinetic curves of the petrochemicals and resorcinol adsorption allow us to calculate the dynamic exchange capacity (capacity before the breakthrough) and the full dynamic exchange capacity for activated carbon and fibrous sorption material (table 2). The kinetic curves shape corresponds to the parallel transfer mode, which allows the kinetic constants to be calculated using the Shilov equation to describe the process. Kinetic constants were calculated from the adsorption dynamics output curves by the least squares method in the computer algebra system MathCAD (table 3).

Conclusion
The study on the phenols and petrochemicals adsorption on activated carbon and fibrous sorption material under static and dynamic conditions showed the promise of using sorbents for wastewater treatment in various industries from phenols and petroleum products with the aim of releasing treated wastewater into water bodies, including fishery reservoirs.