# Influence of Anaerobic Degradation of Organic Matter on the Rheological Properties of Cohesive Mud from Different European Ports

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## Abstract

**:**

## 1. Introduction

_{4}) and carbon dioxide (CO

_{2}) gases [7], which are either trapped in the mud layers or released via the water column. The density and settling–consolidation behavior of mud are known to be significantly influenced by the presence of entrapped gas bubbles. Moreover, owing to the significant influence of clay-organic flocs on the rheological and cohesive properties of mud [6,8,9,10,11,12], anaerobic degradation of organic matter is observed to significantly reduce the rheological properties of mud from Port of Hamburg, Germany [13,14]. However, as the sediment properties including type and content of organic matter, salinity, clay type and content, and particle size distribution, along with the sediment management techniques vary for different ports, different rheological responses to organic matter and its decay can be expected. A systematic analysis of the influence of organic matter degradation on the rheological properties of mud from different ports is still missing.

## 2. Experimental Methods

#### 2.1. Sample Collection

#### 2.2. Wet Bulk Density, Particle Size Distribution and TOC Content

#### 2.3. Anaerobic Degradation of Organic Matter

_{2}, and samples were incubated at 36 °C in the absence of light for 250 days. Anaerobic carbon release was estimated from the increase in headspace pressure and headspace composition, as analyzed by gas chromatography (GC-WLD; Da Vinci Laboratory Solutions). The share of CO

_{2}-C dissolved in the aqueous phase was calculated using the CO

_{2}concentration, the pressure in the bottle headspace, and the temperature-corrected solubility of CO

_{2}in water as given by Henry’s constant (given by Sander [17]). All the samples were incubated and analyzed in triplicate.

#### 2.4. Rheological Characterization

^{−1}, until the shear rate reached 300 s

^{−1}[4,18]. The corresponding rotation of the geometry was measured, which eventually provided the shear rate and apparent viscosity. The amplitude sweep tests were carried out at a frequency of 1 Hz by applying an oscillatory stress/amplitude. The storage (${G}^{\prime}$) and loss (${G}^{\u2033}$) moduli [3] were obtained as a function of oscillatory amplitude. The frequency sweep test was performed within the linear viscoelastic (LVE) regime, from 0.1 to 100 Hz. The outcome of frequency sweep tests was obtained in the form of storage and loss moduli as a function of frequency, which was then converted into complex modulus (${G}^{*}$) and phase angle ($\delta $). The time-dependent test was conducted by performing the shear rate ramp up and ramp down experiment as follows: (i) shear rate ramp-up from 0 to 100 s

^{−1}over 50 s, (ii) constant shear rate of 100 s

^{−1}over 50 s, and (iii) shear rate ramp-down from 100 to 0 s

^{−1}over 50 s. In addition to the time-dependent test, a structural recovery test was performed by using a three-step protocol given in Shakeel et al. [19]. In short, the first step provides the moduli of the mud sample (${G}_{0}^{\prime}$) before pre-shearing, by performing a small amplitude oscillatory time sweep experiment. The second step involves the application of a high shear rate (300 s

^{−1}for 500 s) to completely disturb the sample. The last step allows the sample to recover its structure by again performing a small amplitude oscillatory time sweep experiment for 500 s and recording the moduli (${G}^{\prime}$) as a function of time.

## 3. Results and Discussion

#### 3.1. Anaerobic Degradation Tests

#### 3.2. Wet Bulk Density, Particle Size Distribution and TOC Content

_{50}values (see Table S2). Conversely, the mud samples collected from PoA displayed two different behaviors (Figure S2b), i.e., one was similar to the other ports while another one showed bimodal particle size distribution along with the higher D

_{50}values (see samples A2 and A4 in Table S2), which was linked with the presence of a significant amount of sand sized particles. For these two samples (A2 and A4), the higher sand content was also found in the particle size distribution obtained by the sieving technique [20], which verified the SLS results (see Table S3). Some other sediment properties such as TOC content, electrical conductivity and pH of mud samples from different ports are presented in Table S1.

#### 3.3. Stress Ramp-Up Tests

^{−1}) and $b$ (–) represent the intercept and slope of the line, respectively. The values of these fitting parameters are presented in Table S4. The values of the slope indicated that the yield stresses (static and fluidic) were significantly reduced for mud samples from PoA and PoH due to the degradation of organic matter. In the case of mud samples from PoB and PoR, the reduction in yield stresses after organic matter degradation was less significant, as evident from the smaller values of negative slope (Table S4). The mud samples collected from PoE exhibited smaller densities as compared to the samples collected from other ports (see Table S1). Therefore, the change in yield stresses (degraded–fresh) as a function of yield stresses of fresh mud samples for PoE is compared with the fluid mud samples collected from PoH with similar densities (Figure 2b,d, static and fluidic yield stresses). It is observed that the mud samples from PoE displayed an increase in both static and fluidic yield stress values after organic matter degradation (i.e., positive slope), while in the case of PoH, a pronounced negative slope was found with significant reduction in both static and fluidic yield stresses (Table S4).

_{4}and CO

_{2}, the remaining OM undergoes a transformation toward more stable organo-mineral associations with concurrent changes of the microbial community composition, compared to the fresh sample. As a result, different floc and biofilm properties change, which could result in a different strength. It is assumed that this effect also occurs in the samples from the other ports but might be masked due to the high OM decay and the significant mass loss of carbon, which dominate the rheological response by disrupting mineral bridging. Hence, due to low mass loss and OM decay rates in PoE (see Figure 1), other processes changing the physical architecture of flocs and biofilms might become more relevant.

#### 3.4. Amplitude Sweep Tests

#### 3.5. Frequency Sweep Tests

#### 3.6. Time Dependent and Structural Recovery Tests

^{−1}, in order to investigate the time-dependent behavior of fresh and degraded mud samples from different ports. The results of the time-dependent experiments revealed the existence of a typical clockwise loop at higher shear rates for both fresh and degraded mud samples (Figure S7). However, at lower shear rates, a counterclockwise loop was observed, which may be associated with a shear thickening phenomenon or a structural reorganization due to the shearing action [3,22]. A similar combination of clockwise and counterclockwise loops has already been reported in the literature for mud samples [29].

## 4. Conclusions

_{2}and CH

_{4}) and influences the rheological properties of mud by breaking/weakening the organic bridges between mineral particles. Hence, in this study, the effect of organic matter degradation on the rheological properties of mud samples, collected from different ports, was examined. The mud samples were collected from five different European ports (Port of Antwerp (PoA), Port of Bremerhaven (PoB), Port of Emden (PoE), Port of Hamburg (PoH) and Port of Rotterdam (PoR)), in order to have samples with varying sediment properties. 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.

## Supplementary Materials

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Cumulative carbon release after 250 days of degradation normalized to dry mass for mud samples (

**a**) from different ports, and (

**b**) from PoE and fluid mud samples from PoH. 1.5IQR, factor 1.5 of the interquartile range (25~75%).

**Figure 2.**Change in static yield stress (degraded–fresh) as a function of static yield stress of fresh mud samples (

**a**) from different ports and (

**b**) from PoE and fluid mud samples from PoH; (

**b**) change in fluidic yield stress (degraded–fresh) as a function of fluidic yield stress of fresh mud samples (

**c**) from different ports and (

**d**) from PoE and fluid mud samples from PoH. The dashed line represents the value where the degraded and fresh mud samples have the same yield stresses. The solid line represents the empirical fitting using Equation (1).

**Figure 3.**Percent change in (

**a**) static and (

**b**) fluidic yield stresses $\left(\frac{degraded\u2013fresh}{fresh}\times 100\right)$ of mud samples from different ports. 1.5IQR, factor 1.5 of the interquartile range (25~75%). Negative values represent the percent decrease in yield stresses after degradation, while positive values represent the percent increase in yield stresses after degradation.

**Figure 4.**Change in crossover amplitude (degraded–fresh) as a function of crossover amplitude of fresh mud samples (

**a**) from different ports and (

**b**) from PoE and fluid mud samples from PoH. The dashed line represents the value where the degraded and fresh mud samples have the same crossover amplitude. The solid line represents the empirical fitting using Equation (1).

**Figure 5.**Percent change in crossover amplitude $\left(\frac{degraded\u2013fresh}{fresh}\times 100\right)$ of mud samples from different ports. 1.5IQR, factor 1.5 of the interquartile range (25~75%). Negative values represent the percent decrease in crossover amplitude, while positive values represent the percent increase in crossover amplitude after degradation.

**Figure 6.**Change in complex modulus (degraded–fresh) at 1 Hz as a function of complex modulus at 1 Hz of fresh mud samples (

**a**) from different ports and (

**b**) from PoE and fluid mud samples from PoH. The solid line represents the empirical fitting using Equation (1). The dashed line represents the value where the degraded and fresh mud samples have the same complex modulus at 1 Hz.

**Figure 7.**Percent change in complex modulus at 1 Hz $\left(\frac{degraded\u2013fresh}{fresh}\times 100\right)$ of mud samples from different ports. 1.5IQR, factor 1.5 of the interquartile range (25~75%). Negative values represent the percent decrease in the complex modulus, while positive values represent the percent increase in the complex modulus after degradation.

**Figure 8.**Change in hysteresis area (degraded–fresh) as a function of hysteresis area of fresh mud samples (

**a**) from different ports and (

**b**) from PoE and fluid mud samples from PoH. The dashed line represents the value where the degraded and fresh mud samples have the same hysteresis area. The solid line represents the empirical fitting using Equation (1).

**Figure 9.**Percent change in the hysteresis area $\left(\frac{degraded\u2013fresh}{fresh}\times 100\right)$ of mud samples from different ports. 1.5IQR, factor 1.5 of the interquartile range (25~75%). Negative values represent the percent decrease in the hysteresis area, while positive values represent the percent increase in the hysteresis area after degradation.

**Figure 10.**(

**a**) Change in normalized equilibrium storage modulus, ${G}_{\infty}^{\prime}/{G}_{0}^{\prime}$ (degraded–fresh) as a function of normalized equilibrium storage modulus, ${G}_{\infty}^{\prime}/{G}_{0}^{\prime}$ of fresh mud samples from different ports, and (

**b**) change in characteristic time, ${t}_{r}$ (degraded–fresh) as a function of characteristic time, ${t}_{r}$ of fresh mud samples from different ports. The dashed line represents the value where the degraded and fresh mud samples have same normalized equilibrium storage modulus or characteristic time.

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**MDPI and ACS Style**

Shakeel, A.; Zander, F.; Gebert, J.; Chassagne, C.; Kirichek, A.
Influence of Anaerobic Degradation of Organic Matter on the Rheological Properties of Cohesive Mud from Different European Ports. *J. Mar. Sci. Eng.* **2022**, *10*, 446.
https://doi.org/10.3390/jmse10030446

**AMA Style**

Shakeel A, Zander F, Gebert J, Chassagne C, Kirichek A.
Influence of Anaerobic Degradation of Organic Matter on the Rheological Properties of Cohesive Mud from Different European Ports. *Journal of Marine Science and Engineering*. 2022; 10(3):446.
https://doi.org/10.3390/jmse10030446

**Chicago/Turabian Style**

Shakeel, Ahmad, Florian Zander, Julia Gebert, Claire Chassagne, and Alex Kirichek.
2022. "Influence of Anaerobic Degradation of Organic Matter on the Rheological Properties of Cohesive Mud from Different European Ports" *Journal of Marine Science and Engineering* 10, no. 3: 446.
https://doi.org/10.3390/jmse10030446