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doi:10.3850/38WC092019-0255

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EFFECTS OF IONIC SURFACTANTS ON THE SETTLING VELOCITY OF FINE COHESIVE SEDIMENTS IN YANGTZE RIVER

Liu Yue-xiao(1), Huang Zhuo*(1,2), Wang Wei-ke(1), Cao Hui-qun(1,2), Xie Ling-xian(1), Lin Li(1,2)

(1) Basin Water Environmental Research Department, Changjiang River Scientific Research Institute, Wuhan 430010, China;

(2) Key Lab of Basin Water Resource and Eco-environmental Science in Hubei Province, Changjiang River Scientific Research Institute,

Wuhan 430010, China

ABSTRACT Representing as the anionic and cationic surfactant respectively, Linear Alkylbenzene Sulfonate (LAS) and Cetyltrimethyl Ammonium Bromide (CTAB) is extensively employed in the formulation of laundry and cleaning products. Due to its properties, it could aggregated to the interface of solid-liquid and have a tendency to absorb on sediments with a result of altering its physicochemical characteristics. Undoubtedly, it have some effects on the behavior of sediment dynamics once discharged into water column. However, there are no research studied on the influence of ionic surfactants on the settling velocity of natural sediment particles in available literature. This study examined the effect of ionic surfactants on the settling velocity of sediments via the McLaughlin method. Several parameters including settling velocity, zeta potential and surface tension were measured to determine their influences. The results showed that anionic surfactants have little effect on the sedimentation rate of fine sediment. Additionally, Cationic surfactants accelerate the flocculation of fine sediment by changing the surface potential of natural particles. Keywords: cohesive sediment; LAS; CTAB; settling velocity 1 INTRUDUCTION

Surfactants refer to a class of substances with certain properties, structures and interfacial adsorption properties, which can significantly reduce the surface tension of solvents or the interfacial tension of liquid-liquid, liquid-solid. Generally, they are divided into ionic surfactant and nonionic surfactant according to chemical structure. Ionic surfactants can be divided into anionic surfactants (such as LAS) and cationic surfactants (such as CTAB). Surfactants are amphiphilic. They can form micelles at a specific surfactant concentration. Another feature is that they tend to adsorb on the interface, mainly directional adsorption, which can change the surface characteristics of particles (Koopal 2012). Surfactants are widely used in chemical industry due to their unique properties, such as detergents and soaps in daily life, emulsifiers and foaming agents in industry, chemical fertilizer anticoagulants in agriculture, and soil remediation (Haigh 1996, Myers 2005, Mulligan et al. 2001, Mao et al. 2015, Munehideet al. 2016, Tadros 1994).With the wide application of surfactants, the discharged domestic wastewater and industrial wastewater contain a large amount of surfactants, which seriously affect the water environment (WANG 2007).Surfactants exist in the environment for a long time and are difficult to degrade(Scott and Jones 2000, Ying 2006). They are easy to accumulate on the surface of water body, generate foam or emulsifying phenomenon, and block oxygen exchange in water body. This will not only lead to deterioration of water quality, but also produce certain toxicity to organisms (KIIKUCHI et al. 1979, Pan et al. 2001, WANG et al. 2007).

The fine sediment in estuary is generally under 63 µm in diameter and widely exists in natural water areas such as rivers, lakes and reservoirs (Desmond et al. 2000, Forsberg et al. 2018, Portela et al. 2013). Due to the physical and chemical effects on the surface of fine sediment particles, flocculation will occur in water containing electrolytes, which will affect the transport, sedimentation and resuspension of sediment (CHEN et al. 2002).Fine sediment is different from large sediment. Settling velocity of fine sediment is affected by a variety of factors such as salinity and suspended sediment concentration, which is not fully applicable to Stokes' law (Lai et al. 2018, Yangming et al. 2016, L.I. et al. 2013, Wim 1999).In recent years, with the increasing of pollutants in natural water bodies, the influence of electrolytes on sediment settlement of fine particles has not only become a hot spot for scholars at home and abroad, but also promoted the development of disciplines. In recent years, the research on the influencing factors of sediment settlement mainly focuses on the material composition, such as mineral composition, particle size composition and sediment concentration (Goldberg 1991, Manning et al. 2010,Cole et al. 2014); medium conditions, such as salinity, temperature and pH (Tan 2014, Portela et al. 2013, Grabowski 2011, Zbik 2008); dynamic conditions, such as turbulence intensity , flow velocity, (Ha et al. 2010,JIANG 2002) and other aspects

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(CHEN 2000, CHEN 2001, J.C. Winterwerp 2002). However, studies on organic pollutants such as ionic surfactants have not been reported. In general, sediments are not only reservoirs, but also potential sources of pollutants and nutrients (McCave 1984). Therefore, it is very important to fully understand the relationship between ionic surfactant and sediment settling behavior in still water.

In this paper, we selected the anionic surfactant sodium dodecyl benzene sulfonate (LAS) and cationic surfactant dodecyl trimethyl ammonium bromide (CTAB) these two kinds of typical material. Natural graded fine sediment (particle size≤63 µm) in Wuhan section of Yangtze River was selected as the research object. The influence mechanism of ionic surfactant on fine sediment settlement was discussed by measuring settlement speed and comparing the changes of Zeta potential, specific surface area, surface tension and scanning electron microscopy before and after settlement. The research results have important theoretical and engineering significance for the in-depth understanding of the movement law of sediment deposition, and have important guiding significance for solving a series of problems such as river regulation and maintenance, sedimentation of reservoirs and ports and waterways, and protection and treatment of water ecological environment.

2 MATERIALS AND METHODS 2.1 Materials and instruments

Materials and instruments used in the hydrostatic sedimentation test: Zeta Potentiometer (Malvern Instrument Co., UK); Mastersizer 3000 laser particle size analyzer (Malvern Instrument Co., Ltd.); Kino fully automatic surface tensiometer (USA KINO Industry Co., Ltd); JW-BK122F specific surface and aperture meter (Beijing Jingwei Gaobo Technology Co. Ltd.); BSD-YX2400 air bath thermostatic oscillator (Shanghai Bo Xun Medical Bio-instrument limited); SY-2 constant temperature sand bath (Changzhou Yineng Experimental Instrument Factory); HS-70W-1JH one-thousandth stopwatch (Japan CASIO company); one-tenth of a balance (Sartorius, Germany) Company); KQ-300VDE type dual frequency CNC ultrasonic cleaning (Kunshan Ultrasonic Instrument Co., Ltd.); 1000 mL sand core filter (Tianjin Jinteng Experimental Equipment Co., Ltd.), 50 mL pycnometer (Sichuan Yak Glass Instrument Co., Ltd.), 1000 mL measuring cylinder, 15 mL pipette, 0.45 μm microporous membrane (Shanghai Xinya Purification Device Factory), particle size meter, sand cup, three-layer stirring rod; thermometer; 1000 mg/L LAS solution, 1000 mg/L CTAB solution. 2.2 Sand sample collection and processing

The sand samples used in the test were collected from a section of Wuhan section in the middle reaches of the Yangtze River. The sampling depth was 0~30cm from the bottom of the bed. 2.2.1 Sand cleaning

The natural sand sample cleaning process was divided into three stages. In the first stage, a certain amount of crude pure water mixed with sand samples was put into a 250mL corkscrew conical flask, the pH value was adjusted to 2 with HCl, and the sample was soaked and shaken for 6h in a constant temperature oscillator. After confirming the full reaction, the sample was washed with crude pure water until pH was 4. In the second stage, the sediment suspension was mixed with 30% H2O2 and 0.02 mol/L HNO3 solution until pH was

2; the sediment suspension was placed in water-bath water at 85℃ constant temperature and heated 2 h,

stirred with a glass rod. In this stage, a large number of bubbles were produced, and at the same time, heat and gas with pungent smell were released. The process was repeated and sand sample was washed with crude pure water until there was no more foam floating on the surface of the solution. In the third stage, the filtered sediment was added to the crude pure water and placed in a 1L beaker. The mixture was stirred and allowed to stand, and the supernatant was removed. Repeat this step until the pH was 7, and make sure that the sediment particles were basically cleaned. Sand samples were dried in oven for 8 h at 105℃, and then transferred to a dryer and left for 12 hours. 2.2.2 Sand properties The BET multipoint mass specific surface area of the sediment sample was determined to be 5.704 m2/g; the total pore volume of BJH (Barret, Joyner and Halenda method) adsorption-desorption was 0.035 cm3/g; the average pore diameter of BJH adsorption was 18.146 nm, and the average pore diameters of desorption were 15.868 nm and 23.412 nm. At a temperature of 17 °C, the bulk density of the cohesive sediment passing the 250 mesh measured by the pycnometer method is 2.739 g/cm3, and the pH value of the sediment solution is between 7.25 and 8.27 when measured under the condition of an undisturbed water-sediment ratio of 2.5:1.

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Figure 1. Grain-size distribution of the sediment sample.

2.3 Methods 2.3.1 Effect of surfactant on sedimentation of fine sediment

According to the "Procedures for Analysis of River Sediment Particles", the static sedimentation test of fine sediment was completed in a 1000 mL measuring cylinder (having an inner diameter of 6.6 cm and a height of 40 cm) by the McLaughlin method. The test conditions were set as follows.

Table 1. Test conditions for sediment deposition under different surfactant concentrations

Serial number

initial surfactant concentration

(mg/L)

initial sediment concentration

(g/L)

solution temperature

(°C)

sampling time (min)

1 0

3 18 ± 0.5 0.25 min, 0.5 min, 1 min, 5 min, 10

min, 15 min, 20 min, 35 min, 50 min, 65 min, 105 min, and 145 min.

2 5

3 10

4 15

5 20

Firstly, the surfactant solution with corresponding concentration was poured into the measuring cylinder, 3

g (D< 0.062 mm) sand sample was weighed and shaken in ultrasonic wave for 30min to fully disperse, then the suspended sand solution was incorporated into the measuring cylinder, and pure water was added to the 1000 mL scale line. A three-layer spiral stirring rod was used to stir vigorously for 20 s until there was no visible sand at the bottom of the measuring cylinder, and then we stirred up and down for 1min (about 30 times of reciprocation) to make the sediment in the measuring cylinder evenly distributed. Stopped stirring and sampled at regular intervals. Used a pipette with a volume of 10mL and a piston switch to vertically insert the sample from the center of the measuring cylinder to a depth of 20 cm below the liquid level. Used an ear-sucking ball to assist the pipette to take the sample and transferred it to a sand cup. The operation time was controlled within 15 s. After the sand sample in the sand receiving cup was filtered, the real-time sediment concentration was measured by drying and weighing method. 2.3.2 The measurement of zeta potential and surface tension

The fine sediment suspension with different concentrations of surfactant added was placed in a measuring cylinder and mixed evenly. After sediment suspension being placed for 24 hours, 10 mL of the solution was taken from 10 cm below the liquid level and passed through a 0.45 μm water system microporous filter membrane, which was then transferred into a clean sample cell. The Zeta Potentiometer was preheated for 30 minutes, and the average value was measured three times to record the zeta potential of the solution under the influence of different concentrations of surfactant.

The platinum ring method was used to measure the surface tension of the mixture of sediment and surfactant aqueous solution. 2.3.3 Scanning electron microscope observation

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Sand samples with balanced adsorption under different concentrations of surfactant were placed in a glass evaporating dish and transferred to a vacuum freeze dryer for lyophilization for 48h. A portion of the sand sample was scraped off with a spoon, vacuum sprayed with gold on the sample preparation table, and then placed under a scanning electron microscope to obtain an SEM image with a grayscale of 256 and gray value range scale of 0 to 255.

3 Results and discussion 3.1 Effect of ionic surfactant concentration on sedimentation of fine sediment

Under the condition of different CTAB concentrations, the variation law of sediment concentration at 20cm below the liquid level with time is shown in Figure 2. In the first 30 s, the variation law of sediment content under the influence of different CTAB concentrations is irregular. The sediment particles are strongly influenced by Brownian motion and gravity, and the sediment is in the separation flocculation section. After 1min, with the increase of CTAB concentration, the sediment content decreases sharply, and the change rate of sediment content after 35 min is less than the stage of 1 min to 35 min, which indicates that with the increase of CTAB concentration, the sediment settlement speed shows an overall accelerating trend.

Under different concentrations of LAS, the variation of sediment concentration with time at 20cm below the liquid level is shown in Figure 3. In the first 10 min, the variation of sediment concentration with time is relatively severe, and there is no obvious clear and turbid water boundary in the suspended sediment solution at this stage. After 50 min, the particle size of the sediment above 20cm below the liquid level is small, and stratification can be seen faintly in the measuring cylinder. The upper water body is light yellow and the lower water body is brown. In the first 1 min, the change trend of surfactant content to sediment concentration is relatively small. It can be judged that this stage is in the separation settling stage, and sediment enters the flocculation settling stage after 1min. From 5 min to 10 min, the change trend of sediment concentration with time is larger, while LAS concentration has no apparent change trend of sediment concentration.

Figure 2. Evolution of averaged suspended sediment concentration (g L-1) in the five experiments with different

concentrations of CTAB

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Figure 3. Evolution of averaged suspended sediment concentration (g L-1) in the five experiments with different

concentrations of LAS Figure 2 and Figure 3 show that under the conditions of the same surfactant concentration and the same

initial sediment concentration, the cationic surfactant CTAB has a more obvious influence on the sediment settling velocity and has a greater influence degree.

3.2 Settling velocity of sediment in the presence of surfactants

The speed at which the sediment sinks at a constant speed in the static water is the settlement speed, and the average settlement time is the time when the sediment content reaches 50 % of the initial sediment content within 0.5 t. That is, when the vertical coordinate of the sand stage distribution curve is taken to 50 %, the corresponding transverse coordinate represents the sedimentation path, and the corresponding sedimentation speed is the average sedimentation speed of the sediment under this condition. The relationship between sediment content and corresponding sediment content curve under different surfactant concentrations is determined, and the corresponding median particle size is calculated. Substituting the particle size into the McLaughlin formula, the corresponding velocity is calculated.

dtt

tmc 5.0

05.0

%50

1）（ [1]

The scatterplot of surfactant concentration and corresponding settling rate in table 2 is made to obtain Figure 4, and the correlation curve between the surfactant concentration and the representative settling velocity is fitted by a function. The fitting formula is as follows:

CTAB .97233.000395.000395.03532.0 267159.9/91312.7/ Reey xx ， [2]

LAS .95389.000132.035872.0 2 Rxy ， [3]

The correlation coefficient of CTAB is 0.97233. The results show that exponential function fitting is more

suitable for describing the variation of sediment settling velocity and CTAB concentration of fine particles, indicating that the settling velocity is exponentially increasing with the increase of CTAB concentration, and the fitting result. The correlation coefficient of LAS is 0.9589. The linear function is more suitable for describing the change law between the sediment settling velocity of fine particles and the LAS concentration. The results show that with the increase of LAS concentration, there is a linear decreasing trend between settling velocity and concentration.

Table 2. Equivalent median diameter and the corresponding settling velocity under conditions of different

concentrations

CTAB Concentration (mg/L)

Median Diameter (mm) The Corresponding Settling Velocity (cm/s)

0 0.0604 0.3611

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5 0.0606 0.3682

10 0.0610 0.3819

15 0.0624 0.3910

20 0.0660 0.4300

LAS Concentration (mg/L)

Median Diameter (mm) The Corresponding Settling Velocity (cm/s)

0 0.0604 0.3611

5 0.0586 0.3484

10 0.0591 0.3456

15 20

0.0593 0.0595

0.3402 0.3321

Figure 4. Comparison of the median setting velocity with mass-weighted mean settling velocities (cm s-1)

3.3 Zeta potential test results analysis

Zeta potential reflects a measure of the intensity of mutual repulsion or attraction between particles. The smaller molecules or dispersed particles are, the higher the absolute value (positive or negative) of Zeta potential is, the more stable the system is, that is, dissolution or dispersion can resist aggregation. On the contrary, the lower the Zeta potential (positive or negative), the more prone to coagulation or agglomeration, that is, the attraction force exceeds the repulsive force, and the dispersion is destroyed to coagulation or agglomeration. As shown in Figure 5, with the increase of CTAB concentration, the absolute value of Zeta potential decreases, the solution particles tend to agglomerate, and the settling speed increases. With the increase of LAS concentration, the absolute value of Zeta potential increases and the settling velocity decreases.

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Figure 5. The correlation between different LAS and CTAB concentrations and the zeta potential of the water-sediment system under the same initial sediment concentration.

3.4 Scanning electron microscope test results analysis

Figure 6 is an SEM image of sediment particles magnified 500 times under different surfactant environments. As shown in Figure 6(a), the sediment particles after lyophilization in pure water are not uniform in size and gaps. Figure 6(b) also shows SEM images of the sediment after lyophilization LAS water and sediment suspension. The sediment particles are finer than the undisturbed sediment, which indicates that some sediment particles are smaller in size and the sediment distribution is more dispersed than the undisturbed sediment. When 100 mg/L CTAB solution is added, part of the sediment deposited to form larger flocs, which significantly changes the physical and chemical properties of the floc surface, such as sediment porosity and particle size, and slightly increases the settling velocity of flocs. The settling rate is greater than that of single particle. Therefore, under the same gravity, the flocculation degree is improved

(a) (b) (c)

Figure 6. SEM images (500 times magnification) of sediment with and without surfactants (a. Sediment in the initial state; b. sediment after the adsorption of the surfactant LAS; c. sediment after

the adsorption of the surfactant CTAB)

4 CONCLUSIONS In this paper, we use a series of laboratory instruments and sedimentation columns to study the influence

of two typical ionic surfactants on the settling velocity of natural viscous sediments in the Yangtze River. The following conclusions can be drawn:

(1) With the increase of CTAB concentration, the settling velocity of fine particles decreases exponentially. With the height of the anionic surfactant LAS, the sediment settling rate decreases slightly and presents a linear trend.

(2) As the concentration of CTAB increases, the absolute value of Zeta potential increases, the system tends to be unstable, and sediment particles tend to aggregate with each other. With the increase of LAS concentration, the absolute value of Zeta potential decreases slightly, and the sediment particles have a tendency of dispersion. The change rule is more consistent with the sedimentation rule reflecting the change of

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velocity. Thus, changes in the sedimentation behavior are driven by changes in the settling velocity. (3) SEM results show that CTAB aggregates sediment particles into larger particles, and the size of

sediment flocs increases obviously.

ACKNOWLEDGMENT This work is supported by a grant from the National Natural Science Foundation of China (51379016).

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