Oil and gas pipelines installed in the deep sea have a finite lifetime, at the end of this period a strategy is required for the decommissioning of this infrastructure [1]. From an industrial perspective, this approach must be time and cost effective but equally, must adhere to regulations which require that the marine environment is protected to the greatest degree possible [2]. Current regulations on mercury (Hg) cannot be specific as any threat of contamination is unknown with limited available data on Hg accumulation in pipelines. It is essential that the concentrations of Hg and its species present on deep sea infrastructure, and the threat to the environment that they pose is understood. We have previously stated that Hg adsorption on pipeline surfaces may represent a percent level increase oceanic Hg concentration [3]. Now, through the application of analytical techniques such as LA-ICP-TOFMS, SP ICP-TOFMS, atomic fluorescence spectrometry and XANES, we were able to show that the speciation on contaminated steel surfaces can be more complex than previously estimated.

It is expected that the species which should be found in oil and gas systems will be insoluble in the sea (i.e., HgS, Hg⁰). However, the data presented here may now suggest that a significant amount of the Hg which interacts on steel pipelines will be water-soluble species. Analysis of pipeline material exposed to both Hg⁰ and H₂S found that over 50 % of the total Hg which accumulated on carbon steel segments was soluble in seawater. The remaining Hg was then found to be either present as a Cu-Hg amalgam or Hg/S nanoparticles. In the case of the amalgam, this species had never been identified in oil and gas before and the long-term stability in the deep sea is unknown. The Hg/S nanoparticles, which accounted for up to 1 % of the total Hg, pose an interesting environmental question, as HgS particles have shown increased methylation of HgS on the nanoscale [4]. As such, it is feasible that even compounds such as HgS which are largely insoluble may prove to be problematic in the context of oil and gas decommissioning. In summary, we were able to identify several Hg species which can be generated and accumulate throughout the lifetime of an active pipeline. These compounds could result in significant contamination of the deep sea following the use of improper decommissioning strategies. It is essential that we evaluate the full extent of the chemical speciation of Hg within pipelines and to, therefore, evaluate the bioaccumulation potential of these compounds should they be released from a pipeline left in-place following its decommissioning.

References
[1] B. Sommer, A. M. Fowler, P. I. Macreadie, D. A. Palandro, A. C. Aziz, D. J. Booth, Science of the Total Environment 2019, 658, 973–981.
[2] M. L. Fam, D. Konovessis, L. S. Ong, H. K. Tan, Ocean Engineering 2018, 160, 244–263.
[3] L. Paton, P. Crafts, D. Clases, T. Lindsay, A. Zimmer, H. Siboni, R. G. de Vega, J. Feldmann, J Hazard Mater 2023, 458, 131975.
[4] T. Zhang, B. Kim, C. Levard, B. Reinsch, G. Lowry, M. Deshusses, H. Hsu-Kim, Environmental Science and Technology 2012, 46, 6950-6958.