Zhong-yue Li, Jin-feng Zhang, , Qing-he Zhang, Xiao-teng Shen , Tong-qing Chen

1. State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300072,China

2. Key Laboratory of Earthquake Engineering Simulation and Seismic Resilience of China Earthquake Administration, Tianjin University, Tianjin 300350, China

3. Key Laboratory of Ministry of Education for Coastal Disaster and Protection, Hohai University, Nanjing 210098, China

4. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing 210029, China

Abstract: The flocculation and the settling of the cohesive sediment play an important role in the sediment transport process, and they are affected by many factors. On the coasts and in the estuaries, the organic matter and the salinity are two significant factors. In this study, experiments are carried out to evaluate the flocculation and the settling of the kaolinite in cases of various organic matter and salinity solutions in quiescent waters. The optical multi-channel suspended concentration monitoring system is used to measure the sediment concentration and the floc size. The results show that the settling velocity decreases with the increase of the organic matter content, and the floc size increases with the increase of the organic matter content. In addition, an empirical formula for the settling velocity under the influence of the organic matter and the salinity based on the experimental results is proposed.

Key words: Kaolinite, humic acid, salinity, settling velocity, floc size distribution

Introduction

The cohesive sediment widely exists in the muddy coast, as a significant issue in the estuaries and the harbors, the contaminant transport, and the marine structural design. The cohesive sediment transport is complicated on the coasts and in the estuaries, mainly due to the flocculation of the cohesive sediment[1].

Many factors influence the flocculation and the settling of the cohesive sediment. Some factors are related to the properties of the sediment particles themselves, such as the sediment particle size, the suspended sediment concentration and the sediment mineral composition[2-4]. Other factors are related to the properties of the solution, such as the salinity, the pH value, the organic matter content, the temperature,and the turbulence intensity[5-8]. In the context of the interaction of various factors, the complicated properties of the flocculation and the settling behavior of the cohesive sediment play a very important role in the cohesive sediment transport[9]. The organic matter is one of the most significant issues on the coasts and in the estuaries. The effect of the organic matter on the floc properties is mainly reflected in the floc size, the effective density and the settling velocity of the flocs.The organic matter would increase the floc size,therefore, the organic matter content plays an important role in the floc size[10]. The organic matter,such as the humic acid, makes it more easily for the sediment particles to bind together to form a loose floc,with a larger floc size[11-12]. Organic matter also changes the effective density of a floc. As seen from the formation process of the floc, when the volume of the floc increases, the porosity increases gently, reducing the effective density. The organic floc density is 50% lower than the floc density without the influence of the organic matter, mainly because of the loose floc structure formed by the organic matter, and the density of the attached organic matter itself (900.0 kg/m3-1 300.0 kg/m3) is also much smaller than the density of the particles themselves (2 300.0 kg/m3-2 700.0 kg/m3)[13-14]. In the organic induced flocculation process, although the floc size is large, the settling velocity may also decrease as the effective density is greatly reduced, because the organic coating attached to the surface of the particles increases the bonding probability of the particles, and the permeability of the flocs is also enhanced[15].

The effect of the salinity on the flocculation and the settling of the cohesive sediment also cannot be ignored. With a relative low salinity, the settling velocity increases with the increase of the salinity[16-17].But when the salinity reaches a critical value, the settling velocity decreases with the increase of the salinity[18].

On the coasts and in the estuaries, the flocculation of the cohesive sediment can be affected by various factors. But most of the researches only considered one single factor[19]. Therefore, to fully consider the influence of both the organic matter and the salinity on the flocculation of the cohesive sediment, , in this paper, the experiments are carried out. A settling column with a temperature control device and a high-pressure gas stirring device is established. The sediment concentration and the floc images are measured by the optical multi-channel suspended concentration monitoring system. During the experiments, the floc size distributions and the settling velocities are obtained in order to clarify the flocculation and the settling of the cohesive sediment under the combined influence of the salinity and the organic matter.

1. Materials and methods

1.1 Experiment facility

A settling column is established to study the flocculation and the settling of the cohesive sediment.Figure 1 shows the arrangement of the settling column,which is a transparent cylindrical acrylic pipe with a height of 1 m and an internal diameter of 0.48 m in order to ensure that the enough residence time of the flocs and to avoid the interference of the side walls.Five sample taps are arranged on the side wall of the settling column with a distance of 0.20 m between two adjacent taps. Inside the settling column, six electric heating rods are arranged along the tube wall, and therefore the water body in the settling column can be heated to a relatively uniform temperature distribution.A temperature sensor is arranged in the center of the settling column, so that the temperature of the water in the settling column can be monitored and adjusted simultaneously through the heating rod. The high-pressure nitrogen gas is exhausted from the bottom of the settling column to the inside forming a bubble to achieve the agitation, so that a uniform initial field in the settling column is generated.

1.2 Experimental material

The kaolinite is most susceptible to the salinity,followed by the illite, and the montmorillonite[20].Furthermore, the kaolinite is widely distributed on the coasts and in the estuaries. In this experiment, the kaolinite is selected as the experimental material. The median diameter of the primary particles is 2.56 μm as shown in the Fig. 2, where D is the grain size. The initial concentration in this experiment is chosen to be 0.5 kg/m3. Four salinities (1 ppt, 3 ppt, 5 ppt and 10 ppt) are selected by preparing different concentrations of the NaCl solution. The water environments are rich in the organic matter, such as the chitin, the humic acid and the extracellular polymeric substances(EPS). The sodium humate is prepared for the organic matter in the settling column. The organic matter concentration is set as in 13 groups (0 mg/L, 5 mg/L,10 mg/L, 15 mg/L, 20 mg/L, 30 mg/L, 40 mg/L,50 mg/L, 60 mg/L, 70 mg/L, 80 mg/L, 90 mg/L and 100 mg/L) .

Fig. 1 (Color online) Experimental device

Fig. 2 (Color online) The particle size distribution of kaolinite in the experiment

1.3 Measuring instrument

The optical multi-channel suspended concentration monitoring system is used to measure the sediment concentration and the floc size[21-22]. This system is made up of four parts: a light source, an optical sensor, a CCD camera and some fibers. During the experiment, the light source and the optical sensor are put into the water. When the CCD camera is turned on, the light passes through the water body and reaches the optical sensor. The image of the floc between the light source and the optical sensor is captured by the CCD camera (Fig. 3). By processing the floc images, the suspended sediment concentration and the floc size can be obtained.

Fig. 3 The image of sediment floc when the kaolinite concentration is 0.5 kg/m3

The relationship between the image grayscale and the sediment concentration is established. The variation of the grayscale value is largely due to the scattering of the light. The grayscale value of the image can be expressed as[21]

where G is the grayscale value of the image, g and Gbare the conversion coefficients, I0is the intensity of the light source, L is the distance of the light passing through the water body, C is the mass concentration and D*is the specific scattering coefficient, which is defined as the ratio of the scattering cross-section to the total particle mass.

In the experiment, the light is a LED with a color temperature of 5 000 K-8 300 K. The light intensity will influence the concentration measurement[22].According to Ref. [21], the range of the concentration is 0 kg/m3-100.0 kg/m3for the particle sizes of 13.00 μm-106.00 μm. Huang et al.[23]concluded that the range of the concentration is 0 kg/m3-50.0 kg/m3for the silicon with the sizes of 38.00 μm-250.00 μm.In this study, we also calibrate the relationship between the greyscale and the concentration from 0 kg/m3to 3.0 kg/m3for the kaolinite (Fig. 4). It can be seen that the grayscale value of the image is exponentially related to the sediment concentration, in which the correlation coefficient of the curve fitting is 0.99.

Fig. 4 (Color online) Fitting curve of the concentration grayscale value and sediment concentration for the detector 1

The experiments are conducted with the initial sediment concentration of 0.5 kg/m3for the kaolinite.Six probes are placed at the water depths of 0 m,0.2 m, 0.4 m, 0.6 m, 0.8 m and 1.0 m, respectively, to measure the sediment concentration, every 60 min according to the settlement duration in the fresh water and every 10 min-30 min for the salinity water. Each group of experiments is repeated 10-20 times.

ImageJ[24]is used to statistically determine the area of the floc, by analyzing the obtained image. And the diameter d is calculated as

where A is the area of the floc.

1.4 Settling velocity calculation

The settling velocity is calculated from the sediment concentration at each depth of the water at various times through the Mclaughlin formula[25-26].

According to the continuity equation of the sediment

where C is the sediment concentration, t is the time, z is the vertical coordinate and ω is the average settling velocity.

Integrate Eq. (3) along the water depth h

By measuring the sediment concentration at each depth of the water at different times, the settling velocity of the sediment particles at different depths can be calculated by Eq. (4). Generally, we take the average settling time t0.5(the time required for the sediment concentration to reach 50% of the initial sediment concentration) to represent the settling speed at the water depth h, and it is expressed as

whereiω is the settling velocity at the time i.According to the experimental data, in the present study, t0.5is in a range of 1.5 h-12 h.

2. Results and discussions

2.1 Settling time

The settling time is defined as the time it takes for the sediment concentration at the water depth 0.8 m to reduce to 50% of the initial sediment concentration. Figures 5, 6 show the effect of the organic matter on the settling time of the kaolinite of different salinities. It can be seen that the settling time increases with the increase of the organic matter and the settling time decreases with the increase of the salinity. With the combined influence of the salinity and the organic matter, the settling time in the fresh water is much larger than that in the salt water.

Fig. 5 The settling time of kaolinite in fresh water

Fig. 6 The settling time of kaolinite in salt water

For the fresh water and the salt water with salinity of 1 ppt, 3 ppt, 5 ppt and 10 ppt, the settling times for the total organic matter content of 20 mg/L are 3.4%, 18.2%, 26.7%, 37.7% and 95.6% longer than those without the organic carbon. The combined influences of the salinity and the organic matter on the flocculation processes are complicated. When the organic matter is higher than 20 mg/L, the increasing trend of the settling time slows down under the high salinity conditions (3 ppt, 5 ppt and 10 ppt). In the fresh water, when the TOC content is 100 mg/L, the settling time reaches its maximum, which is 21.2% longer than the minimum settling time for the zero TOC content. When the salinity is 1 ppt, 3 ppt, 5 ppt and 10 ppt, the settling time increases with the increase of the organic matter content. With the increase of the salinity, the inhibition of the cohesive sediment by the organic matter becomes more intense.For the salinity of 1 ppt, the settling time of the TOC content of 100 mg/L is 82.9% longer than that of the TOC content of 20 mg/L. For a salinity of 10 ppt, it is 104.1% longer.

2.2 Settling velocity

Figures 7, 8 show the effect of the organic matter on the settling velocity of the kaolinite at different salinities. With the increase of the organic matter, the settling velocity of the kaolinite gradually decreases.

Fig. 7 The settling velocity of kaolinite in fresh water

Fig. 8 The settling velocity of kaolinite in salt water

In the fresh water, the maximum settling velocity of the cohesive sediment is observed when the TOC content approaches 0 mg/L, and the maximum settling velocity is 0.0156 mm/s, the minimum settling velocity is observed when the TOC content becomes 100 mg/L, and the minimum settling velocity is 0.0123 mm/s. In the fresh water, with the increase of the organic matter content of the aqueous solution, the negatively charged organic matter adheres to the surface of the sediment particles, which increases the negative charge of the sediment particles. The double electric layer inhabits the flocculation of the cohesive sediment and reduces the floc settling velocity. On the other hand, the organic matter and the sediment particles have a bridging effect, and the organic matter adheres to the surface of the sediment particles to form an organic coating layer, which reduces the effective density of the sediment particles and slows down the settling.

When the salinity is 1 ppt, 3 ppt, 5 ppt and 10 ppt,the maximum settling velocity of the cohesive sediment is also observed at the TOC content of 0 mg/L,and the maximum settling velocity is 0.0423 mm/s,0.0588 mm/s, 0.0723 mm/s and 0.1180 mm/s,respectively. Under the condition of a large salinity of the aqueous solution, the cation in the aqueous solution is more likely to bind to the negatively charged organic matter than to the negative charge on the surface of the sediment particle. As the organic matter content in the aqueous solution increases, more and more cations are combined with the negatively charged organic matter to enhance the repulsive force of the double electric layer between the sediment particles and to slow down the settling process. It can be seen that the increase of the organic matter in the aqueous solution generally inhibits the settling of the kaolinite.

2.3 Floc size

In this paper, the ImageJ is used to analyze the image of the sediment floc and to obtain the floc size distribution (Fig. 9). The representative particle size(i.e., the median sizeD50) is listed in the Table 1. The minimum floc size generally ranges from 13.00 μm-20.00 μm. However, as the salinity and the organic matter content in the water body increase, the floc size increases rapidly.

Fig. 9 (Color online) The floc size distribution at S = 0 ppt

Table 1 The representative particle sizes (i.e., the median size D50) in various cases (μm)

For the salinity of 1 ppt, 3 ppt, 5 ppt and 10 ppt(Figs. 10-13), as the content of the organic matter in the water body increases, the median size of the floc increases. Under the high salinity condition, the growth of the floc size is slower than that under the lower salinity condition.

Fig. 10 (Color online) The floc size distribution at S =1ppt

Fig. 11 (Color online) The floc size distribution at S =3ppt

Fig. 12 (Color online) The floc size distribution at S =5ppt

Fig. 13 (Color online) The floc size distribution at S =10ppt

When the organic matter content of the aqueous solution is 0 mg/L, the floc size increases gradually with the increase of the salinity. At the salinity of 10 ppt (Fig. 13), the floc size is 45.00 μm, which is 2.5 times larger than that in the fresh water environment.

With the increase of the organic matter content in the water body, the organic matter is more easily adhered on the surface of the kaolinite to form an organic coating, which changes the original physicochemical properties of the sediment particles, making it easier to form a larger floc. Furthermore, the organic matter in the solution changes the flocculation process dominated by the salinity flocculation, and the bridge flocculation and the net flocculation become dominant.With these two flocculation mechanisms, larger flocs can easily be formed.

In the process of the floc image acquisition, the single wire diameter of the optical fiber is 13.00 μm and the pixel of the camera is 6.00 μm, so we can obtain the floc image when the floc size is larger than 13.00 μm. When the floc size is smaller than 13.00 μm, the equipment will not give proper results. In future, the optical fiber with the 3.80 μm wire diameter will be used in the equipment, the image resolution will be significantly improved.

The settling velocity of the floc can be calculated based on the Winterwerp’s equation[27]. However, this equation includes the parameters of the floc effective density and the floc fractal dimension, which are hard to be determined from the experimental data. In future,we can achieve the high-resolution images and establish the image correlation to determine the settling velocity directly.

2.4 Empirical formula

An empirical formula is obtained from the experiment data. Taking the organic matter content and the salinity as the independent variables, the empirical formula is obtained from fitting the expression

This formula is an empirical formula for the flocculation under the combined effect of the organic matter and the salinity. And it can be applied under the salinity and the organic matter conditions. The valid scope of the formula is 0 ppt to 10 ppt for the salinity and 0 mg/L to 100 mg/L for the TOC content.

3. Conclusion

Two significant parameters (the salinity and the organic matter) with respect to the settling velocity and the floc size of the sediment are studied in this experimental study. The results show that the settling velocity decreases with the increase of the organic matter content (TOC=5 mg/L, 10 mg/L, 15 mg/L,20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L, 60 mg/L,70 mg/L, 80 mg/L, 90 mg/L and 100 mg/L) for the fresh water or the salinity water (S=1 ppt, 3 ppt,5 ppt and 10 ppt). However, the settling velocity deceases faster in the fresh water as compared to the salinity water with the increase of the organic matter content. The floc sizes are also analyzed, and it is indicated that the floc size increases with the increase of the organic matter content. In the fresh water, the increase of the organic matter content can make the floc size of sediment increasing more rapidly.Furthermore, the empirical formula of the settling velocity under the influences of the organic matter and the salinity is obtained by fitting the experimental results. In view of the fact that the settling of the cohesive sediment can be influenced by many factors,so in future, we will consider more influencing factors such as the temperature and the turbulence.

Acknowledgements

This work was supported by the Open Research Fund of State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University(Grant No. HESS-1917), the Belt and Road Special Foundation of the State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering (Grant No. 2018490911).