Remediation of diesel-oil contaminated soils using an innovative nanobubble and electrolyzed catalytic system

Abstract

Soil contamination resulting from petroleum hydrocarbons poses a significant environmental challenge. In this research, an innovative electrolyzed catalytic system was devised to produce nanobubble-enriched catalytic water, which was employed for remediating soils contaminated with diesel oil. This novel system harnessed high voltage (220 V) via direct current, utilizing titanium electrodes coated with iridium dioxide. The inclusion of iron-copper hybrid oxide catalysts between the electrodes was instrumental in enhancing the efficiency of radical generation. The study employed the electron paramagnetic resonance (EPR) technique and the Rhodamine B (RhB) method to analyze the species and concentration of the generated radicals. EPR is a versatile tool for investigating materials with unpaired electrons and is capable of quantifying reaction kinetics for transition metals. RhB can be used as a probe for hydroxyl radical concentration determination. The addition of an electrolyte (NaCl or K2SO4) could enhance the electrolytic reaction and improve radical production rate. By incorporating 20 mM of K2SO4 and utilizing 75 g of catalyst per 500 mL batch of nanobubble-contained electrolyzed catalytic water (NECW) production, with a current density of 20 mA/cm2, a highly oxidized nanobubble-infused electrolyzed catalytic water was successfully generated. This water exhibited an oxidation-reduction potential of 887 mV and contained a radical concentration of 7.6 × 10−13 M, encompassing hydroxyl, superoxide, and sulfate radicals. Furthermore, it displayed a negative zeta potential of −39.8 mV, along with nanobubbles averaging 28 nm in diameter and a nanobubble density of 3.7 × 109 particles per milliliter. The presence of nanobubbles in electrolyzed catalytic water, owing to their negative zeta potential and nano-scale attributes, imparts a repulsive force that hinders bubble aggregation and prolongs their existence. When these nanobubbles burst, the resulting radicals contribute to an augmented radical concentration, which, in turn, facilitates the oxidation of petroleum hydrocarbons. Approximately 72.4% of the total petroleum hydrocarbon in soils could be removed (dropped from 3230 mg/kg to 892 mg/kg) after five batches of NECW washing (10 min of each washing time) via the desorption and extended chemical oxidation mechanisms. The total petroleum hydrocarbons in the water phase could be removed entirely after pumping the polluted water into the electrolyzed catalytic system for immediate treatment, indicating that the electrolyzed catalytic system could oxidize the desorbed total petroleum hydrocarbon effectively. The findings demonstrate that the nanobubble-infused catalytic water generated by the innovative electrolyzed system effectively and efficiently remediates petroleum-hydrocarbon-contaminated soils and water, all while minimizing the risk of secondary pollution.