Effect of organic matter degradation in cohesive sediment: a detailed rheological analysis

Abstract

Purpose The presence of organic matter in cohesive sediment results in the formation of clay-organic flocs, which eventually impart complex rheological behavior including shear-thinning, viscoelasticity, thixotropy and two-step yielding to mud. In this study, the influence of microbial degradation of sediment organic matter on the rheological properties of mud samples, having similar densities, was examined. Materials and methods Mud samples were collected from three different locations in the Port of Hamburg, Germany, displaying varying organic matter content. The rheological analysis of fresh and degraded mud samples was performed with the help of several tests including stress ramp-up tests, amplitude sweep tests, frequency sweep tests, time-dependent tests, and structural recovery tests. Results and discussion The results showed a significant decrease in rheological properties including yield stresses, complex modulus, etc. for degraded mud samples as compared to the fresh mud samples. The slopes of the line, correlating the change (degraded − fresh) in the above-mentioned rheological properties as a function of the same rheological property of the fresh mud, varied within the range of −0.28 to −0.49. The structural recovery tests displayed a better recovery (i.e., stronger system) in mud after the pre-shearing step for the degraded mud samples as compared to the fresh mud samples. The effect of degradation time on the rheological properties of mud samples showed two critical time periods (3 days and 150 days) after which a significant change in rheological properties of mud samples was observed. Conclusions This study provided a useful understanding about the influence of organic matter degradation on the rheological properties of mud, which can be used to optimize sediment management strategies in ports and waterways.

Effects of organic matter degradation in cohesive sediment: linking sediment rheology to spatio-temporal patterns of organic matter degradability

“…The later was supported by Jommi et al (2019) who found that the presence of gas must be accounted for when studying the reduction in shear stresses by organic matter decay. It was found that the SOM decay caused a significant decrease in the rheological properties including yield stress, crossover amplitude, complex modulus, and thixotropic hysteresis area (Shakeel et al 2022). For the given data set, the changes in density during the period of incubation observed in this study were only minor, which means that the detected changes in the rheological behavior are not caused by changes in the material's density.…”

“…In literature, higher yield stress values of fine-grained sediments were typically associated to either higher density or higher organic matter content (Van Kessel and Blom 1998;Soltanpour and Samsami 2011;Xu and Huhe 2016;Shakeel et al 2019Shakeel et al , 2020aShakeel et al , 2022 with organic matter assumed to result in a stronger structure, i.e., higher yield stresses, of fine-grained sediment (Shakeel et al 2019). Shear stresses of long-term incubated (250 days) samples correlated best with long-term SOM degradability, seen in Fig.…”

“…Static yield stress was obtained from the first decline in viscosity as a function of shear stress, which is linked to the breakage of the inter-connected network of flocs. On the other hand, the second decline in viscosity (i.e., fluidic yield stress) was associated to the breakage of reorganized flocs into smaller flocs or individual particles upon further shearing (Shakeel et al 2022). The rheological analysis of both fresh and degraded mud samples was performed using a HAAKE MARS I rheometer (Thermo Scientific, Germany) with concentric cylinder (Couette) geometry (gap width = 1 mm).…”

“…It is therefore concluded that the initial SOM decay is most relevant for the yield stress change, breaking up the organic bridging between particles (for a conceptual model see Shakeel et al 2020b). In a parallel study investigating the structural recovery of sheared samples, it could be shown that after breakdown of SOM the samples exhibit a better recovery after shearing (Shakeel et al 2022). This is due to the fact if the SOM bridges are broken, the elastic properties of the samples are greatly reduced and, in line with what is observed for sediment samples without organic matter, these samples can fully recover their pre-shearing state.…”

Sediment challenges and opportunities due to climate change and sustainable development

“…The reduced static and fluidic yield stress of sediments can enhance port maintenance dredging and navigability of ports and waterways. Shakeel et al ( 2022 ) studied the impact of microbial degradation of organic matter on the rheological properties of mud samples by exposing fresh and degraded mud samples collected from three different locations in the Port of Hamburg to various stress tests (stress ramp-up tests, amplitude sweep tests, frequency sweep tests, time dependence tests, and structural recovery tests). They confirmed that degradation of organic matter indeed weakens the strength of mud and found that the strongest degradation occurs after 3 and 150 days.…”