Unravelling the chemistry behind the toxicity of oil refining effluents: from characterisation to treatment

Abstract

Adequate wastewater management is a crucial element to achieve water sustainability in the petroleum refining sector, as their operations produce vast quantities of wastewater with potentially harmful contaminants. Treatment technologies are therefore pivotal for stopping these chemicals from entering the environment and protecting receiving environments. However, refining effluents are still linked to serious pollution problems, partly because little progress has been made in determining the causative agents of the observed biological effects, resulting in non-targeted treatment. Here it is shown that naphthenic acids, which have been reported as toxic and recalcitrant, are important components of refining wastewater resulting from the processing of heavy crude oil and that they have a significant contribution to the toxic effects exerted by these effluents. Furthermore, it was found that their chemical stability makes them highly resistant to remediation using Pseudomonas putida and H2O2/Fe-TAML (TetraAmido Macrocyclic Ligands) systems under laboratory conditions, and only sequential aliquots of Fe-TAML catalysts and H2O2 showed to partially degrade naphthenic acids (50 mg/L) within 72 hours. Results suggest that a combinatorial approach of Fe-TAML/H2O2 followed by biodegradation might improve current treatment options, but further optimisation is required for the biological treatment. These results can serve as a starting point for better environmental regulations relevant to oil refining wastewater resulting from heavy crude oil, as naphthenic acids are not currently considered in the effluent guidelines for the refining sector. Furthermore, the degradation of naphthenic acids under mild conditions using Fe-TAML/H2O2 systems indicates that these catalysts hold promise for the remediation of refining wastewater in real-life scenarios.