Critical perspectives on the use of oceanic manganese nodule-sodium alginate (OMN@SA) composite microspheres for deep-sea mining applications

Section snippets

Deployment challenges of the microspheres

The deployment of microspheres (with a diameter of 3 mm) presents significant challenges, particularly given the vast scale of polymetallic nodule (PMN) mining operations. Nodule concentrations vary widely across different regions of the world's oceans: from 15 to 75 kg/m² in the Clarion-Clipperton Zone (CCZ), 10 kg/m² in the Peru Basin, 25 kg/m² in Penrhyn Basin and 3.85–6.71 kg/m² in the Central Indian Ocean Basin (CIOB). Additionally, the estimated mining areas are extensive, covering

Resource efficiency and economic feasibility

A large amount of oceanic manganese nodules (OMN) would be needed to produce OMN@SA microspheres. This raises concerns about resource efficiency and sustainability. The vast mining areas would require a continuous supply of OMN, which could take these resources away from other useful applications. Producing and spreading the microspheres across large mining zones would also be very expensive. These high costs could make the approach uneconomical. As a result, contractors may avoid using this

Uncertainty in sediment resuspension and metal release

The processes governing sediment resuspension during deep-sea mining and the associated release of trace and toxic metals from metal-sediment complexes remain poorly understood. Current approaches, such as kinetic speciation (Chakraborty et al., 2014), kinetic fractionation, or sequential extraction protocols (Chakraborty et al., 2021), are operationally defined. This means they are designed for controlled environments and may not accurately reflect real-world conditions in the deep sea. As a

Long time required for metal removal by OMN@SA

A drawback of this micro-composite is the time required to effectively remove dissolved metals, which exceeds 5 hours. This duration is considered too long for practical applications in mining operations, where efficiency and speed are critical. Therefore, optimizing the micro-composite to reduce the time needed for metal removal is essential for enhancing its feasibility and effectiveness in treating effluents in a timely manner.

Combined impact of pressure and salinity

The extreme conditions of high pressure and high salinity in deep-sea environments pose significant challenges for the performance of OMN@SA microspheres.Alginates are polysaccharides composed of β-D-mannuronic (M) and α-L-guluronic (G) acid residues. The cation-binding affinity and physicochemical properties of alginates are determined by the block-wise arrangement of these residues. In biological systems, alginates form negatively charged biogels, generating a Donnan potential that drives ion

Potential ecological impacts on deep-sea benthic organisms

If OMN@SA microspheres are sprayed/dispersed on the surface of the ocean at mining site locations, they could absorb dissolved trace and heavy metals, reducing the bioavailability of essential micronutrients in the photic depth (∼surface to 200 m). This could negatively impact marine ecosystems, harm primary productivity, and disrupt CO₂ sequestration. Deploying these microspheres in the surface ocean could raise serious concerns. The authors did not propose their use in surface waters. The

CRediT authorship contribution statement

Parthasarathi Chakraborty: Conceptualization, Data curation, Formal analysis, Investigation, Supervision, Writing – original draft, Writing – review & editing. Lamjahao Sitlhou: Data curation, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank Editor-in-Chief, Prof. Christian Sonne, for recognizing the importance of this correspondence and the need for its publication in Environmental Pollution. We appreciate his support and the opportunity he has provided to contribute to the journal. The authors thank the Ministry of Earth Sciences, Government of India, for their encouragement support. They also extend sincere gratitude to Dr. N.H. Khadge (retired) from CSIR-National Institute of Oceanography, Goa for his support.