Kinetics and mechanism of radium-isotopes dissolution in TENORM scale waste associated with petroleum production...

The present study is conducted to explore the dissolution as inferred from the kinetic mechanism for radium-isotopes (²²⁸Ra, ²²⁶Ra, and ²²⁴Ra) in the TENORM scale waste deposited in oilfield pipes and equipment, Gulf of Suez, Egypt. The main efficiency factors for Ra²⁺-compound dissolution by lactic acid (LA) solution, e.g., reactive organic carbon (i.e., electron-donor source), have been investigated, and optimum chemical conditions have been determined. The obtained data were also employed to predict the leaching kinetics and mechanism of the Ra²⁺-isotopes removal by three shrinking core models (SCM, liquid film process-chemical controlled process-diffusion controlled process) and Arrhenius model. The maximum leaching percentage of Ra²⁺-isotopes reached to 55-60% at the optimal leaching conditions (0.3 M LA, 5 h, 25 °C, ϕ < 1 mm, S/L ratio 10/50 g mL^-1). The Ra-isotopes removal proceeds kinetically by diffusion-controlled process. Activation energy (E_a) of the leaching process was 10.51 kJ mol^-1. This value conforms that the leaching process for removal of Ra²⁺-isotopes in the TENORM scale waste by LA solution is controlled by a diffusion process. Values of thermodynamic parameters (∆G^o, ∆H^o, ∆S^o) were determined and indicate that dissolution of Ra²⁺-isotopes in the studied waste is non-spontaneous and temperature dependent. Moreover, the leaching mechanism may be attributed to the dissolution of soluble exchangeable and acidic species of Ra²⁺-species and/or these due conversions of insoluble Ra-sulfate to more soluble Ra-sulfide and/or Ra-hydrogen sulfide by LA solutions.