Degradation of polycyclic aromatic hydrocarbons from coal tar aqueous solutions using photo-biocatalysts

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are amongst the most predominant pollutants that are currently released into the environment. Despite the myriad of environmental regulations enforced by governments in countries around the world, various industries have continued to release PAHs in various water sources. The petroleum industry has been identified as the industry that released high level of PAHs into the environment. However, a considerable amount of PAH is also released from coal tar produced by the coking industry. The interaction of coal tar with various aqueous solutions in the environment is often a factor determining the release of PAHs into receiving water. In this study, environmental solutions such as acid mine drainage, alkaline mine drainage and sewage wastewater were considered to simulate the release of PAHs from coal tar into the environment. A mass of 5g of coal tar obtained from the coking process was found to contain 2916.47 mg/L of PAHs. The concentration of PAHs in coal tar was determined by using gas chromatography spectrometry. Naphthalene (788 mg/L), phenanthrene (632 mg/L), fluoranthene (395 mg/L), acenaphthylene (356 mg/L), fluorene (327 mg/L), pyrene (266 mg/L), and anthracene (245 mg/L) were the most concentrated PAHs in coal tar. Amongst these PAHs, it has been reported that naphthalene (NAP) is the most abundant PAH in the environment. The coal tar was immersed in a designed column, containing 300 mL of either acidic mine drainage, alkaline drainage or sewage wastewater. The leaching process conducted over a period of 8 weeks using acidic mine drainage, sewage wastewater and alkaline mine drainage resulted in the leaching of 7.1 mg/L, 1.2 mg/L and 0.32 mg/L of PAHs respectively at room temperature. The PAHs that were highly concentrated were considered to be dissolved at a higher rate. It was also reported that the higher molecular weight of a PAH is life-threatening and more carcinogenic as compared to the lower molecular weight . The dissolution of the PAHs in the different water samples was influenced by the dissolved organic matter and pH amongst others to influence the leaching process. The acidic mine drainage released the most PAHs, followed by sewage wastewater and lastly alkaline mine drainage. After 8 weeks of leaching, 19.3 mg/L, 1.62 mg/L and 1.81 mg/L PAHs in acidic mine drainage, alkaline mine drainage, and sewage wastewater respectively was released. The release of PAHs from the coal tar was observed to increase over time. The PAHs with lower weight between 2 to 4 rings were observed to be highly dissolved in the samples. The presence of PAHs in the environment demands the implementation of an urgent technique that will remove them completely. Various studies have reported that PAH can only be destabilized under the influence of temperature and photoactive material. Metal oxides were selected as the most effective materials due to their optical, antifouling, and antibacterial properties and thermal resistance to remediate PAH pollutants. In the study, zinc oxide was selected as the photocatalyst for degradation of PAHs. The band gap of ZnO is roughly 3.2eV and in this study, it was recommended to implement agrowastes to improve the photodegradation efficiency of the ZnO/Ag photocatalyst. Various biological agrowastes such as moringa oleifera seed (MO), groundnut shells (GS) and apatite (A) derived from cow bones were used

to enhance the desired property of the ZnO. In this study, the band gap of ZnO was found to be 3.35 eV; it was found that due to the relatively high content of metals, including rare earth metals with high photocatalytic activity such as strontium (Sr) that are present in A, GS and MO at a concentration of 387273 μg/kg, 31469 μg/kg and 22730 μg/kg respectively, that they are highly proficient in obtaining a band gap of 1.59 eV and 2.96 eV for ZnO/Ag and MO/GS/A/ZnO/Ag for novel bioheterophotocatalysts. Many photocatalysts were synthesized from the agrowastes, ZnO/A, ZnO/GS and ZnO/MO with a band gap of 2.15 eV, 2.51 eV and 2.74 eV respectively. It was proven from the result that the Sr content in A, followed by GS and lastly MO strongly contributed to a lesser band gap and a higher photoactivity. MO/GS/A/ZnO and ZnO/Ag materials were tested for the photodegradation of PAHs and it was proven that MO/GS/A/ZnO/Ag was the most effective material to degrade 69.59%, 61.07 % and 61.68% of PAHs as compared to 52.62%, 37.46% and 44.30% using ZnO/Ag in acidic mine drainage, alkaline mine drainage and sewage wastewater in 60 min. Furthermore, another experiment was conducted and this time, a self-made photoreactor was introduced that achieved a photodegradation efficiency of 64%, 55%, and 58%. Without its application, a photodegradation efficiency of 53%, 33%, and 39% was obtained in 60 min under solar irradiation using acidic mine drainage, alkaline mine drainage and sewage wastewater respectively. It was then concluded that the self-made photoreactor was effective even compared with ZnO photocatalyst alone. The photodegradation was conducted with and without a flat-bed reactor or photocatalyst under solar irradiation. The self-made photoreactor was introduced with the MO/GS/A/ZnO/Ag synthesized photocatalyst. It was found that almost 100%, 97% and 98% degradation were obtained, as compared to 89%, 94% and 92% degradation using ZnO/Ag at 60 min under solar irradiation in acidic mine drainage, alkaline mine drainage and sewage. This indicates that the introduction of MO, GS and A was effective in degrading PAHs in various water sources when using a self-made photoreactor in the presence of solar irradiation. Most significantly, the introduction of the flatbed and even the solar irradiation alone could degrade the PAHs in various water sources.