Enhanced photocatalytic oxidation of polycyclic aromatic hydrocarbons in offshore produced water

Liu, Bo (2018) Enhanced photocatalytic oxidation of polycyclic aromatic hydrocarbons in offshore produced water. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The growing amount and environmental impact of offshore oily wastewater especially offshore produced water (OPW) have drawn significant attention in recent years. The petroleum hydrocarbons in wastewater can have severe negative effects in a long term on coastal and marine ecosystems if without sufficient treatment before discharge. Polycyclic aromatic hydrocarbons (PAHs) as a representative of dissolved chemical compounds or environmental pollutants in oily wastewater have been a major issue of marine environments due to their carcinogenic or mutagenic, toxic, persistent and bio-accumulative properties. To reduce the negative impact of produced water to the marine ecosystem, it is required to remove all toxicants especially PAHs before discharge. Various challenges have been identified in implementing conventional technologies (e.g., physical separation, chemical oxidation and biological remediation) for treating the dissolved organic pollutants (e.g., PAHs). Therefore, the research and development of more effective technologies to address these concerns are much desired. Photocatalysis generates powerful oxidative radicals which can rapidly mineralize organics especially aromatic compounds, offering a great potential use in removing PAHs from oily wastewater. However, the photocatalytic degradation of organics can be dramatically inhibited by the complex matrix of OPW. Limited in-depth studies were reported on the behaviors and interactions of different components in produced water during photocatalysis. The mechanisms of the interferences are of utmost importance to the development of highly efficiency treatment technologies. The generation of intermediates caused by the complex matrix and inhibited treatment process could further lead to the increase in the toxicity of treated effluent to the marine ecosystem, and consequently reduce its potential in natural attenuation. In addressing these challenges and fulfill the knowledge gaps, this research is focused on the evaluation of the key factors and the mechanisms of OPW matrix in photocatalysis, and the development of enhanced photocatalytic oxidation processes to aid the OPW treatment, thus can achieve both high efficiency in removal of PAHs, and low toxicity and high biodegradability of the effluent. The matrix effect was first investigated in a suspensive photocatalytic oxidation system, in which the synthesized TiO2 nanoparticles were used. It is indicated that the degradation of PAHs was inhibited by the impurities in OPW matrix in many ways: the alkaline-earth cations caused the flocculation of the particle; the insoluble particulate matters competed with PAHs in the adsorption on TiO2; the competition and the fouling effect of other dissolved organic matters were deteriorating the process. To enhance the treatment process, immobilized TiO2 was used instead and it was compared with the TiO2 nano-particles. Improvements were found in both naphthalene adsorption and degradation in the immobilized photocatalytic oxidation system, indicating immobilized TiO2 was more efficient and durable than TiO2 nanoparticles in oily wastewater treatment. The competition of hydrocarbons especially phenols played a key role in the degradation of PAHs. The fouling on the catalyst surface was verified by the scaling of alkaline-earth metals and the deposition of organic matters. Further improvement was aimed at developing a novel UV-light-emitted diode (UV-LED)/TiO2 nanotube array (TNA)/ozonation process for treating OPW. The involvement of ozone was to reduce the competition of other organics and enhance the degradation efficiency. The TNA with hollow 1-D tubular nano-structures was applied because of the combined advantage of nano-particle and immobilization, as well as high quantum yield. UV-LED has the advantage of high energy efficiency and long-life time. In the integrated system, the removal of PAHs can be achieved within 30-min treatment with the half-lives reduced to less than 10 mins. Factorial analysis demonstrates that the best dose of TNA is 0.2 g/L. Light intensity affects the generation of iodine radicals, which is a strong scavenger of ozone thus reduces the efficiency of PAHs removal. Ozone dose is a dominated factor that promotes the degradation. Further results indicate that the degradation of phenols and PAHs with higher solubility favors to undergo to ozone-inducted oxidation, while PAHs with lower solubility are more likely oxidized on the catalyst surface. The toxicity and biodegradability of OPW treated by photocatalytic oxidation were investigated during and after the treatment. Studies on the intermediates formed during the photocatalytic ozonation treatment in the presence of halogen ions reveal the mechanism and various reaction pathways of aromatic compounds. Iodization and bromination were the dominant interfering reactions in sequential stages. Two multivariable regression models were developed to quantify the contributions of key toxicants (e.g., total PAHs, total phenols, dibromo-pentane and bromoform) to the acute toxicity of OPW during the treatment processes. It was observed that by removing the total PAHs and total phenols, the acute toxicity was increased from 3% to 57%, and the biodegradability (BOD₂₈/COD ratio) was doubled more than 80% by the integrated UV-LED/TNA/ozonation process. Further, the biodegradation rate of bromoform was much faster than those of phenols, indicating that the proposed technology features high efficiency and has low impact on marine environment. In this research, I have investigated the matrix effect of OPW on photocatalysis and the impacts to the suspended and immobilized TiO₂. A novel integrated UV-LED/TNA/ozonation process was developed to treat OPW. The efficiency of the process, the effects of operational parameters, the intermediates and degradation pathways, and their contribution to the acute toxicity and biodegradability of treated effluent were investigated. The scientific contributions of the research are: 1) revealing and summarizing the key mechanisms of OPW matrix and their key effects on photocatalysis, 2) understanding the interactions of OPW composition with catalyst surface, 3) fulfilling the knowledge gaps on the removal of PAHs from OPW by the UV-A (365 nm) photocatalytic ozonation process, including the interactive mechanisms of the adsorption and photocatalytic oxidation, the behaviors of halogenic ions, and the effects of the operational factors, 4) proposing the altered photodegradation pathways of aromatic organic matter in the presence of halogen ions, and 5) proposing toxicity contribution models targeted on the most toxic compounds in OPW with/without photocatalytic ozonation. The findings of this thesis work also help 1) develop a better strategy to countermeasure the difficulties in the application of photocatalytic oxidation for treating OPW, 2) develop an advanced alternative option for the OPW management, and 3) monitor the composition and toxicity changes during the process and hence the production of by-products in the OPW treatment practice.

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