Investigation of radium-radioisotopes removal in produced water associated with petroleum extraction using powdered activated carbon: Isotherm, thermodynamic and kinetic studies

Abstract

The huge volume waste of the produced water (PW) associated with petroleum extraction poses significant hazards to the surrounded environment due to its complex composition and the presence of various hazardous pollutants, including organic, inorganic, biological contaminants, and natural occurring radioactive materials (NORM). This study was conducted to investigate the factors affecting the removal of the long-lived radium isotopes, i.e., ²²⁶Ra (1600 y) and ²²⁸Ra (5.8 y), from PW associated with oil and natural gas production in certain Egyptian oilfield. The total radium concentration (²²⁶Ra + Ra²²⁸) in PW was found to be 164 Bq/L, i.e., 3.145 ng/L. Isotherm, thermodynamic, kinetic and adsorption mechanism were carried out for the removal of Ra-isotopes in PW using powdered activated carbon (PAC) as adsorbent material. The removal efficiency of Ra-isotopes was found to be affected by pH, adsorbent dosage, temperature, and contact time. The maximum removal efficiency exceeded 97 ± 3 % under optimal conditions (pH∼5, 23 ± 2 °C and 3 h contact time). Moreover, it was revealed that the adsorption mechanism of Ra isotopes removal in PW is heterogeneous process controlled by both Freundlich (R² = 0.998) and Dubinin-Radushkevich (R² = 0.970) isotherms. The process followed the pseudo-second-model (R² = 0.998), and was endothermic up to temperatures below 40 °C. At the optimal conditions, the maximum adsorption capacity (q_e) of Ra from PW onto PAC reached 0.77 ng/g (40 Bq/g) after 3 h at pH 5 $\pm$ 0.02 and 23$\pm$ 2 °C. At these conditions, removal of Ra-isotopes from PW using PAC could be mainly due to surface complexation mechanism. As a result, the activity concentration (Bq/L) and natural radiation dose rate ($\mu$ Sv/h) of total radium (²²⁶Ra + ²²⁸Ra) in PW were reduced below the exemption limits (<10 Bq/L, <0.1 $\mu$ Sv/h) recommended internationally. Furthermore, the data is efficient and promising for designing a prototype for NORM removal from PW generated in petroleum industry prior to its discharge into the surrounded environment.