Removal of polycyclic aromatic hydrocarbons from offshore produced water by advanced oxidation technologies

As the largest waste stream from offshore oil and gas industry, offshore produced water contains dissolved toxic organic pollutants that are hard to be removed by conventional wastewater treatment technologies. Among those pollutants, polycyclic aromatic hydrocarbons (PAHs) are of growing concern due to their high toxicity and persistence in the marine and coastal environments. Removal of PAHs from produced water before disposal is thus essential for offshore oil and gas production. However, the offshore operation and facilities (e.g., platforms and ships) usually have many special technical and economic constraints that limit the applications of many treatment technologies. Since advanced oxidation processes (AOPs) are featured with high cost-efficiency, small footprints, and eco-friendliness which well match with the requirements of offshore operation and present a promising treatment option for offshore wastewater (e.g., produced water). However, limited research efforts have been reported in investigating AOPs’ mechanisms, performance and applicability in treating offshore produced water. In order to help fill the knowledge and technical gaps, this research aimed at development of advanced oxidation technologies for removal of PAHs from offshore produced water treatment and examination of the oxidation processes and kinetics, and effluent toxicity and biodegradability. To ensure efficient, reliable, and acurate analysis results, a refined analytical method, Vortex and Shaker Assisted Liquid-liquid Microextraction (VSA-LLME), was first developed, tested and adopted in the analysis of 16 priority PAHs recommended by U.S. Environmental Protection Agency. Under the optimized condition, the enrichment factors ranged from 68 to 78. The recoveries of the method were 74 to 85%, and the limits of detection were as low as 2 to 5 ng/L. The linearity results (R² values) for 16 PAHs were all above 0.99 with the relative standard deviations (RSD%) of 6 to 11%. This method also creatively utilized the organic constitutes in produced water as dispersive solvents to reduce the solvent consumption. Its straightforward procedure and excellent performance showed a strong potential for application in research and regulatory and industrial practice. The photolysis of 16 PAHs in offshore produced water was then thoroughly investigated in this research. The results indicated much more complex kinetics in the removal of PAHs from produced water than those in stilled water, mianly due to the complex chemical constitutions of the substrate. The experiment disclosed the unique mechanisms including direct photolysis, dynamic light screening, and radical induced organic synthesis. A novel kinetic model involving dynamic light screening was developed and approved to support the mechanism analysis, and a semi-empirical model was also established to simulate the photolysis process. The proposed mechanisms and kinetics not only helped answered some scientific questions but also showed strong practical significance for further AOP development and applications. The performance of ozonation in removing polycyclic aromatic hydrocarbons (PAHs) from offshore produced water (OPW) was studied. The experimental results showed that ozone dose had positive effect due to enhancement in ozone decomposition, and radical yield. On the other hand, the removal was suppressed at increased bubble size and pH, which may be attributed to the reduction of interfacial area as well as stronger radical scavenging effect, respectively. Microtox tests showed that the acute toxicity of OPW was reduced after ozonation, which was highly correlated with the removal of PAHs. Such reduction was inhibited at high ozone doses, possibly due to the formation of disinfection by-products via reactions with halogens. As compared to control, ozonated OPW had higher oxygen uptake and less organic residual after biodegradation, indicating more bioavailable organics were formed after ozonation. Results from this study can be used as good references for designing new or upgrading existing OPW treatment systems using ozonation. Based on the experimental results, the three major mechanisms affecting the PAHs removal through AOP treatment were proposed in the first time. Novel kinetic models based on the dynamic oxidant competitiveness was developed and validated. The model was able to simulate the oxidation processes, quantify the effects of different operational parameters. The testing result also indicated that insufficient treatment could lead to carcinogenetic by-products. On the other hand, proper advanced oxidation technologies could significantly increase biodegradability, showing strong potential of combining with conventional biological treatment in practice.