Potential Influence of Ocean Acidification on Deep-Sea Fe–Mn Nodules: Results from Leaching Experiments

With the continuous rise in CO₂ emissions, the pH of seawater may decrease extensively in the coming centuries. Deep-sea environments are more vulnerable to decreasing pH since sediments in deep oceans below the carbonate compensation depth (CCD) are often completely devoid of carbonate particles. In order to assess the potential risk of heavy metal release from deep-sea deposits, the mobility of elements from ferromanganese (Fe–Mn) nodules and pelagic clays was examined by means of leaching experiments using phosphate buffer solutions ranging in pH from 7.1 to 8.6 (NBS scale). With decreasing pH, the results showed an enhanced leaching of elements such as Li, B, Mg, Si, Sc, Sr, Ba, Tl, and U, but a reduced leaching of V, Cu, Mo, Cd, and W. Elements in leachates originate mainly from exchangeable fractions, and tend to be affected by sorption–desorption processes. Concentrations of most elements did not exceed widely used international water quality criteria, indicating that changes in pH caused by future ocean acidification may not increase the risk of heavy metal release during deep-sea nodule mining operations.