Interaction of the Gardanne Bayer effluent with seawater (Mediterranean Sea): Influence on the trace metals behaviour

Abstract

In the global effort to evaluating the environmental consequences of deep-sea tailing placement, our study focused on understanding the formation of hydrotalcite concretions at the outfall of the Gardanne effluent into the Mediterranean. Through a combination of in situ sampling and laboratory experiments, we explored the interaction between the clarified Bayer effluent and seawater. Our results indicated that, when mixed with seawater, 1 L of effluent can form 20–25 g of (wet) particles. Hydrotalcite formation proved effective in removing metals and metalloids such as Al, Co, Pb, As, Ni, Cu and Ti from the effluent. In contrast, Cr and V were either partially removed or remained in the dissolved phase. Anomalies in Al and As concentrations within the plume at sea further supported their active removal. While hydrotalcite's capacity for contaminant removal is significant, its non-stability in seawater suggested that these particles will dissolve or evolve in the water column beyond the plume, raising concerns about their long-term environmental impact. The fate of the columns of concretion formed at the outfall remains also a central concern, especially given that since 2019, the plant has reduced both the alkalinity and Al concentrations in its discharges to ensure compliance with environmental regulation.