Treatment of oil/water emulsions using seawater-assisted microwave irradiation

Waste oil emulsions are generated in several manufacturing processes. Such emulsions not only affect the efficacy of wastewater treatment but also influence the water quality of the effluent. Thus, many processes have been developed for demulsifying such materials and salt-assisted microwave irradiation has been shown to be most effective in this respect. In the present study, we propose that artificial seawater can be an economical source of the cations required in this process, termed seawater-assisted microwave demulsification. Our experiments include tests of emulsion characteristics and the effects of microwave operating conditions on demulsification rate and separation efficiency of three oil-in-water (O/W) emulsions. We conducted batch demulsification tests on 50-mL aliquots with an initial oil content of 10 g/L, and found that the separation efficiencies of a cutting-oil emulsion, an olive-oil emulsion, and a cutting-oil/olive-oil mix reached 93.1%, 92.6%, and 93.2%, respectively, using our optimum operating conditions, which were 40 s of microwave irradiation at 700 W, a 60 min settling time, and addition of 12%, 32%, and 20% (all v/v) of artificial seawater, respectively. Using this set of operating conditions, a decrease in solution pH was found to significantly increase the demulsification efficiency after addition of inorganic acid, whereas an increase in the concentration of surfactant, sodium dodecyl sulfate (SDS), resulted in a decrease in efficiency. In addition, our test data were explored using a stepwise regression method, yielding a multivariable equation. This empirical equation was able to describe separation efficiency rather well, after exclusion of tests showing separation efficiency below 40%.

Research highlights▶ In this study, a demulsification process termed ‘seawater-assisted microwave irradiation’ was proposed. This process utilizes seawater as an economical source of various salts in salt-assisted microwave irradiation process. ▶ During executing seawater-assisted microwave irradiation process in treating O/W emulsions, “alkaline flocculation” was found to be effective in improving the separation efficiency with high concentrations of magnesium ion (from artificial seawater) at high pH. ▶ Most of previous reports studied the microwave demulsification on W/O (water/oil) mineral oil emulsions. As such, addition of inorganic salts has been shown mostly to improve the separation efficiency of W/O emulsions with microwave irradiation, while little is known about the effects of salts on the O/W (oil/water) emulsions. This study could provide essential information related as three O/W emulsions (mineral oil, vegetable oil, and mineral-oil/vegetable-oil mix) were investigated. ▶ Results from the present study demonstrated that the seawater-assisted microwave irradiation process may apply in the demulsification of O/W emulsions of mineral oil and vegetable oil, and seemingly possible to demonstrate the feasibility of applying the batch-scale results on in situ wastewater treatment.