For green energy transition, the industry seeks alternative resources for nickel and cobalt, the main ingredients for energy storage devices and other applications. Polymetallic nodules lying at the abyssal plain are rich in these mineral resources, which leads to an increased interest in deep-sea mining (DSM) of polymetallic nodules. During the DSM operation, the seabed will be disturbed, resulting in a suspended sediment plume. Such a plume can have a significant environmental impact. As a result, defining the main processes and quantifying sediment plume dispersion is vital for predicting the possible ecological implications. Flocculation could play a key role in minimizing and better prediction of dispersion of turbidity flows generated by deep-sea mining equipment. In this work, we study the effect of flocculation on the propagation of deep-sea sediment plumes by means of conducting a series of lock exchange experiments using artificial deep-sea sediment. Experiments were conducted in fresh and saline water with different clay and synthetic organic matter concentrations. Experiments are conducted in freshwater for comparison with saline water. The head velocity is measured via video analysis. At the end of the lock exchange experiments, subsamples at various run-out lengths are collected for particle size and settling velocity measurements. When experiments are conducted with synthetic organic matter in saline water, the results show that the head velocity reduces significantly compared to freshwater conditions due to the formation of dense flocs. @en

Flocculation between inorganic sediment, salt ions and microscopic organic
matter present in the marine environment might play an important role in the
dynamics of turbidity currents. The ability to predict, understand, and potentially
leverage the effect of flocculation on turbidity currents will help to minimize the
impact of human interventions such as dredging, trenching, and deep-sea
mining. To better characterize the effect of flocculation on the benthic turbidity
currents generated by these activities, a series of laboratory experiments were
performed. Turbidity currents were created by means of lock exchange
experiments. The present work focuses on the flocculation of clays that are
representative for abyssal regions where deep-sea mining is performed, but
most of the conclusions of this work are generic and can be applied to other
types of benthic flows, occuring in harbours and channels. The effect of salt and
organic material as flocculant agent was investigated. Various concentrations of
clay and organic flocculant were tested. Video analysis was used to determine
the head velocity of the plume. Samples at different run-out lengths were
collected at the end of the lock exchange experiments for particle size and
settling velocity measurements. The velocities of the turbidity currents in fresh
and saline water (when no organic matter was present) were found to be similar,
which was expected considering the timescales of salt-induced flocculation
(about 30 min or more compared to the duration of lock exchange
experiment <60 s). It was however demonstrated that, in presence of
organic matter, flocculation occurred during the short time (30–60 s) of the
experiment, leading to a reduced current propagation and a significant change
in floc sizes (from 20 to 1,000 μm) and settling velocities (from 1 to 60mms−1).
Salt ions contributed to flocculation in the sense that flocculation with organic
matter was improved in the presence of salt.@en

Turbidity currents are generated as a result of various processes such as dredging and deep-sea mining. In this work, we generate a turbidity current in a lock exchange setup [1] by using 100 g/l illite, as shown in figure 1. Two different flocculant dosages (0.25 mg/g & 0.75 mg/g of clay) were used with this illite. The material was mixed in the mixing section of the lock exchange before the lock gate was opened. Experiments were done both in fresh and salt water. The samples were collected after the end of the experiment, and their rheological properties were measured using a HAAKE MARS I rheometer (Thermo Scientific, Germany). Rheological studies were carried out using Couette geometry with a gap of 1mm. The sample was gently stirred before rheological measurements. Higher yield stress values were observed in freshwater experiments compared to saltwater experiments, which can be attributed to a larger floc size in freshwater. In addition, the structural recovery of the flocs was also found to be higher in freshwater than in salt water.@en

Renewable energy installations and energy storage solutions require a significant amount of critical raw materials such as nickel, cobalt and rare earth elements. The supply chains of these raw materials face many challenges, e.g., these materials are often found at lower grades on land. These complications motivate the search for new resources. Therefore, the deep sea is looked into as a potential source for such minerals. However, sea bed mining is expected to affect the mined area. One of the concerns is the so-called mining-generated turbidity current, which can cause a negative impact on the deep-sea environment. For that reason, in order to characterize the generated turbidity current, we investigate the generated current experimentally, where cohesive and noncohesive sediment types are tested using a lock-exchange set-up. Three non-cohesive sediment types are tested in order to investigate the effect of the particle size and initial concentration on the propagation velocity of the current. Moreover, one cohesive sediment, i.e illite, is used to compare the propagation velocity in both saline and fresh water. Finally, we used flocculating agents as a proxy to biological matter, to test its influence on the flocculation process. The results show that using or generating larger particle sizes effectively results in a reduced propagation velocity of the current. In addition, the propagation velocity increases in case of higher initial concentrations. In case of cohesive sediment, natural flocculation (i.e flocculation without using flocculants ) occurs faster in saline water than the fresh water. Moreover, using organic flocculants would increase the process of the flocs formation, which results in a lower front velocity and an effectively reduced plume dispersion. @en