Abstract

In the water treatment process of natural water bodies, a large amount of dredged slurry with high water content is generated and required for treatment. The coagulation–flocculation method can improve the efficiency of separation, and a suitable scheme is of great significance. It is unclear whether there is a significant difference in flocculation and separation of dredged slurries from different sources and which constituents dominate this process. Facing these problems, the tests were conducted for dredged sediments from 10 different sources, including rivers, lake, and ocean. Under the same flocculation conditions, the difference in the increment of particle size d10, the specific resistance of filtration, and the suspended solids (SS) of the supernatant after sedimentation are 0–4.6 times, 0–2.4 orders of magnitude, and 0–4 orders of magnitude, respectively. It was found that the main constituents in the dredged slurries, such as clay minerals, fulvic acid and humic acid, impact on flocculation and separation effects by affecting the zeta potential of the particles. However, there is no single constituent in the dredged slurry which dominates the flocculation and separation effect. When these constituents are incorporated, the zeta potential exhibited in the slurry determines the difference in flocculation and separation effects.

INTRODUCTION

A large amount of dredged slurry with high water content is produced in dredging projects every year (Koerner et al. 2016; Kasmi et al. 2017). Rapid dewatering of the dredged slurry is a hot issue. Recently, the coagulation–flocculation method has been successfully used in engineering to accelerate the soil-water separation efficiency of the dredged slurry (Berilgen & Bulut 2016; Bhatia 2017; Wang et al. 2018), which is originally used in water treatment and sludge treatment. For example, Berilgen & Bulut (2016) studied the dewatering effects of four types of polymers on the Golden Horn dredged slurry. It was found that cationic polymers have better performance for the Golden Horn dredged slurry than anionic polymers. Liu et al. (2019) studied the sedimentation of six types of coagulants or flocculants on the dredged slurry from WenRuitang river. Among these additives, FeCl3 or anionic polyacrylamide (PAM) was more effective in the separation of the slurry. Lots of existing research focused on the selection of flocculants to improve the separation of local dredged slurries. However, it is unknown whether the same coagulation–flocculation scheme has a universal effect on dredged slurries from different sources. Therefore, it is of great significance to study the influencing mechanism of coagulation–flocculation performances based on dredged slurry properties.

According to the environment near rivers and lakes, the main constituents of dredged sediment are clay minerals and humus decomposed by plant residues. Therefore, it is necessary to research the influence of clay minerals and humus on the flocculation and separation. Some related research is summarized as follows.

  • Clay minerals: In the research of soil and water conservation, it has been found that the clay minerals significantly impacted on the flocculation effect. Using the same flocculant dosage, the turbidity removal degree of kaolinite-based slurry was better than that of smectite-based slurry (McLaughlin & Bartholomew 2007). Yan & Zhang (2014) carried out adsorption experiments with three different PAMs adsorbed on two pure clays and found that various PAMs adsorbed on Na-montmorillonite consistently were more effective than Na-kaolinite. Mamedov et al. (2010) used a PAM as a stabilizing agent for 16 soil samples with various clay minerals. It showed that the PAM efficacies in improving aggregate stability were followed in the order of kaolinitic < illitic < smectitic soils.

  • Humus: Relevant studies showed that the types and characteristics of humus had a significant effect on flocculation. It was necessary to increase the amount of flocculant to maintain flocculation and separation performances when the humus content was increased (Ali et al. 1985). Fulvic acid, humic acid, humic acid sodium and citric acid improved the stability of the colloid in the raw water, which lead to the difficulty of flocculation, the decrease of floc size, and the larger specific resistance to filtration (Xu et al. 2013). Sillanpää et al. (2018) concluded that particle size, charge, hydrophobicity, divalent cations and destabilizing anions of humus had impacts on flocculation results.

  • Coexistence of clay and humus: It has been found that the results of flocculation were changed when clay minerals and humus presented simultaneously. In the studies about the clay particles removal from water by flocculation, it was found that the presence of organic matter, such as humic acid, diminished the separation effect of clay particles (Gibbs 1983) and increased the consumption of flocculant (Petzold et al. 2004). In studies addressing the removal of humic acid from water by flocculation, it was determined that the presence of kaolin clay increased the removal degree of humic acid (Ma et al. 2017).

Besides, the particle size distribution (PSD) of the slurry might affect the results of flocculation. McLaughlin & Bartholomew (2007) carried out flocculation tests on slurries of 13 types of subsoils and analyzed the Pearson correlation between the turbidity reduction of slurry after flocculation and particle size contents including the sand, silt and clay. They found that increasing sand content had a negative effect on turbidity reduction. However, there is limited research about the influence on the flocculation by the PSD as an individual variable. The impacts of the PSD on flocculation results is not yet certain.

In summary, clay minerals, humus, and PSD could have impacts on flocculation and separation. For dredged slurries, the incorporated influence of these factors on flocculation and separation is unknown. It is also unclear which constituent in the dredged slurry is the sensitive factor affecting flocculation and separation.

According to the above problems, this paper used 10 types of dredged sediments from different sources to test and study the difference in flocculation and separation. In addition, different modified samples mixed by clay samples, humus, and PSDs were used to simulate the dredged slurry, and flocculation and separation tests were carried out to further explore the influences of these constituents. Based on these results, the mechanism that affects the flocculation and separation of dredged slurry was discussed and obtained.

MATERIALS AND METHODS

Dredged sediments

In order to study the difference in the flocculation and separation of dredged slurry, 10 different dredged sediments from various sources were selected for the tests. Eight types of sediments (HH-01 W, ATB-01N, QJD, SZX-01N, ZB-01E, SZX-03S, HH-02E, and SZX-02M) were obtained from eight different urban rivers in Nanjing, Jiangsu Province, China. Sediments from these rivers were captured by a small grab sampler. One of the sediment types (TAIHU) was obtained from a storage yard near Taihu Lake in Wuxi, Jiangsu Province, China, and was dredged from Taihu Lake by cutter suction dredging in the previous year. The final sediment sample (DALIAN) was from marine water and was collected by a large dredged grab (Wu et al. 2018) from the proposed dredging area in Dalian Bay, in Dalian, Liaoning Province, China.

The properties of humus composition and PSDs of these 10 dredged sediments are shown in Table 1. The total organic carbon (TOC) was determined by a TOC tester TOC-CPN SSM-5500 manufactured by Shimazu, Japan. The humus constituents were determined by the potassium dichromate oxidation–volumetric method (Ministry of Agriculture of the PRC 2010). The PSD was tested by a laser particle size analyzer, Malvern Mastersizer 2000. It can be seen from Table 1 that the TOC of dredged sediments from 10 different sources ranges from 0.72 to 4.75%, and the d50 ranges from 9.90 to 55.15 μm. Compared with the properties of the dredged sediments in other regions (Berilgen & Bulut 2016; Wilke & Cantre 2017; Liu et al. 2019), these sediments are representative.

Table 1

Ten types of dredged sediments

Sediment sampleTOC (%)Fulvic acid contenta (%)Humic acid contenta (%)Particle size (μm)
d10d50d90
HH-01 W 1.08 0.22 0.14 6.90 55.15 176.85 
ATB-01N 4.75 0.62 0.62 7.98 49.55 354.41 
QJD 2.77 0.35 0.13 3.04 18.43 145.86 
SZX-01N 1.72 0.31 0.09 4.84 43.30 431.72 
ZB-01E 1.76 0.35 0.13 5.47 36.29 117.56 
SZX-03S 1.81 0.31 0.06 3.04 17.91 110.51 
HH-02E 2.61 0.40 0.25 3.68 23.32 118.50 
SZX-02M 1.82 0.30 0.00 2.29 12.69 76.17 
TAIHU 0.72 0.27 0.01 2.51 9.90 47.36 
DALIAN 0.75 0.25 0.06 2.30 13.22 57.21 
Sediment sampleTOC (%)Fulvic acid contenta (%)Humic acid contenta (%)Particle size (μm)
d10d50d90
HH-01 W 1.08 0.22 0.14 6.90 55.15 176.85 
ATB-01N 4.75 0.62 0.62 7.98 49.55 354.41 
QJD 2.77 0.35 0.13 3.04 18.43 145.86 
SZX-01N 1.72 0.31 0.09 4.84 43.30 431.72 
ZB-01E 1.76 0.35 0.13 5.47 36.29 117.56 
SZX-03S 1.81 0.31 0.06 3.04 17.91 110.51 
HH-02E 2.61 0.40 0.25 3.68 23.32 118.50 
SZX-02M 1.82 0.30 0.00 2.29 12.69 76.17 
TAIHU 0.72 0.27 0.01 2.51 9.90 47.36 
DALIAN 0.75 0.25 0.06 2.30 13.22 57.21 

aThe fulvic acid or humic acid content refers to the mass fraction of the carbon of fulvic or humic acid in the dry mass of the sediment.

Clays

In order to study the effects of clay minerals on flocculation and separation in dredged slurries, three clay samples (Hebei bentonite, Jiangning clay, and Speswhite kaolin (Shu et al. 2018)) were selected for tests. For the preparation of clay particles, three clay samples were dried at 105 °C for 24 h to remove bulk water, then cooled to room temperature. To study the effect of PSD on flocculation and separation, Hebei bentonite (HB) was sieved by 75 μm and 50 μm sieves for 15 min to obtain the bentonite having different PSDs (HB above 75 μm sieve, HB below 75 μm and above 50 μm sieve, HB below 50 μm sieve). The three PSDs of the HB slurries were characterized by d50. The properties and mineral constituents of the clay samples are shown in Table 2 and Figure S1 (supplementary data, available with the online version of this paper), respectively. The mineral constituent was determined by X'TRA X-ray diffractometer manufactured by Thermo. Figure S1 shows that the main clay minerals in Speswhite kaolin (SK) are kaolinite, and there is very little illite and quartz. The content of quartz in the Jiangning clay (JC) is higher, and some illite and albite is observed. Hebei bentonite (HB) mainly contains montmorillonite, cristobalite, calcite and quartz.

Table 2

Three types of clay samples

Clay samplesSpecific gravity GsLiquid limit WL (%)Plastic limit WP (%)Particle size (μm)
d10d50d90
HB (below 50 μm) 2.75 181 53 3.14 12.86 33.24 
HB (50–75 μm) – – – 3.80 20.69 68.75 
HB (above 75 μm) – – – 5.24 53.24 130.68 
JC 2.72 48 24 2.72 13.23 30.62 
SK 2.61 69 38 1.79 6.70 51.40 
Clay samplesSpecific gravity GsLiquid limit WL (%)Plastic limit WP (%)Particle size (μm)
d10d50d90
HB (below 50 μm) 2.75 181 53 3.14 12.86 33.24 
HB (50–75 μm) – – – 3.80 20.69 68.75 
HB (above 75 μm) – – – 5.24 53.24 130.68 
JC 2.72 48 24 2.72 13.23 30.62 
SK 2.61 69 38 1.79 6.70 51.40 

Humus

In order to study the flocculation and separation effects by clay minerals adsorbed with humus, fulvic acid and humic acid were selected as representative humus types. Fulvic acid is produced by Shanghai Yuanye Biological Co., Ltd, the molecular formula is C14H12O8, and the molecular weight is 308. Humic acid is produced by Beijing Cool Chemical Company, the molecular formula is C9H9NO6, and the molecular weight is 227. To characterize the adsorption properties of two humus types on three clay types, the adsorption isotherms were determined by ASTM (2016), as shown in Figure 1. The distribution coefficient Kd indicates the adsorption capacity was obtained based on linear fitting of the adsorption isotherm. In Figure 1, C0 is the initial humus concentration, and Cs is the amount of humus adsorbed by a unit mass of clay sample, which is calculated in Equation (1).
formula
(1)
where V is the volume of the initial slurry, Ce is the equilibrium concentration of humus, and m is the dry mass of the slurry.
Figure 1

The adsorption isotherms of two humus types on three clay types.

Figure 1

The adsorption isotherms of two humus types on three clay types.

Flocculant

Commercial polymer, cationic flocculant PAM (Wshinefloc 312VS manufactured by Shanghai Wshine Chemical Co., Ltd), was used for flocculation. The flocculant was reconstituted every day, at a concentration of 0.1% (w/w).

Flocculation and separation tests of dredged slurries

Ten dredged sediments from different sources were tested to study the difference in flocculation and separation. Sediment was diluted with tap water to provide a concentration of 5% (w/w). Then, 5% slurry was added to a 0.05% PAM dosage for flocculation. The PAM dosage is calculated as the ratio of the dry mass of the flocculant to the dry mass of the slurry. Flocculation was conducted in a 500 mL breaker at an agitation speed of 450 rpm for 2 min. The PSD after flocculation was measured immediately to get d10. The increment of d10 is defined as the difference between d10 after and before the flocculation. The increment of d10 is used as an index for evaluating the flocculation effect. After flocculation was completed, the slurry was precipitated for 1 min, then 50 mL of supernatant was extracted to determine the suspended solids (SS). SS was measured using the standard analytical procedure (APHA 1998). SS is as an index for evaluating the sedimentation separation effect. Simultaneously, another 50 mL of the supernatant was sampled and used to determine the zeta potential. Zeta potential was measured through electrophoretic light scattering (Holmes et al. 2015), by using a Zetasizer Nano ZSP (Malvern Instruments, UK). The remaining slurry was used to determine the specific resistance to filtration (SRF). SRF was used as an index for evaluating the filtration effect. Filtration test was conducted with a Buchner funnel using 0.45 μm filter paper. The effective filtration area of the funnel was 67.17 cm2, and SRF was calculated by measuring the filtrate volume over time, shown in Equation (2).
formula
(2)
where ΔP is the vacuum pressure (setting at 60 kPa), A is the filter area, is the kinetic viscosity, denotes dry solid weight per unit volume cake on the filtrate media, and b is the slope of the curve obtained by plotting the ratio of the time to the filtration volume versus the filtrate volume.

The PSD tests, the SS tests, and the zeta potential tests were all conducted in parallel three times. The specific resistance test was performed once.

Flocculation and separation tests of modified slurries

Modified slurries with different PSDs, clay samples and humus were used to explore the effects of these constituents on the flocculation and separation results.

HB clays with three PSDs were diluted with tap water to achieve a concentration of 5% (w/w). Then, 5% slurry of each distribution was added to PAM at different dosages (0, 0.0125%, 0.025%, 0.0375%, 0.05%, and 0.0625%) for flocculation.

SK, JC, and HB (below 50 μm) clay samples were diluted with tap water to achieve a concentration of 5% (w/w). Then, 5% slurry of each clay sample was added to PAM at different dosages (0, 0.0125%, 0.025%, 0.0375%, 0.05%, 0.0625%) for flocculation.

The clay samples of HB (below 50 μm), JC and SK were mixed with different amounts (0, 0.1%, 0.3%, 0.5%, 0.7%) humus (fulvic acid or humic acid) to study the influence of humus content on flocculation and separation results, and the description of the dosages is shown in Table 1,a. Table 1 demonstrates that the carbon contents of fulvic acid or humic acid in 10 dredged sediments are less than 0.62% of the total dry matter mass. Therefore, for the modified clay samples with humus, the upper limit of the humus content was set as 0.7% of the dry matter mass. Then the modified samples were diluted with tap water to 5% concentration (w/w) and uniformly mixed. The slurry was left to stand at 25 °C for 12 h. Then the slurry was stirred uniformly, and a 0.05% dosage of PAM was added for flocculation.

After flocculation tests, PSD, SS, zeta potential and SRF of all the modified slurries were determined as before.

RESULTS and DISCUSSION

Flocculation and separation results of 10 dredged slurries

The results of d10 increment, SRF, SS and zeta potential from 10 dredged slurries are shown in Figure 2.

Figure 2

Flocculation and separation results of dredged slurries. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 2

Flocculation and separation results of dredged slurries. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 2 indicates that the flocculation and separation effects are different in dredged slurries from 10 different sources. The d10 increment varies from 16.6 to 93.3 μm, the difference in d10 increment is 0–4.6 times. SRF changes from 1.7E + 11 to 5.7E + 13 m/kg, showing a difference of 0–2.4 orders of magnitude. SS ranges from 0 to 15,897.4 mg/L, showing a difference of 0–4 orders of magnitude. The zeta potential after flocculation varies from −26.2 to −9.1 mV. It indicates that if the same flocculation scheme is adopted, the dredged slurries in some areas might not be well flocculated and separated. For example, when using the 0.05% PAM dosage, the flocculation and separation effect of Dalian Bay slurry is very poor, d10 increment is only 16.6 μm, SRF is 5.7E + 13 m/kg, and SS is 15,897.4 mg/L. Therefore, it is necessary to further explore the reasons for the differences in flocculation and separation of different slurries.

Flocculation and separation results of modified slurries

Different PSDs

The results of d10 increment, SRF, SS and zeta potential of HB slurries with three PSDs are shown in Figure 3.

Figure 3

Flocculation and separation results of HB slurries with three PSDs. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 3

Flocculation and separation results of HB slurries with three PSDs. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 3 demonstrates that the slurries with the different particle size d50 ranging from 12.86 to 53.24 μm have limited influence on d10 increment, SS and SRF. For 10 dredged slurries, d50 is between 9.90 and 55.15 μm, which is close to the range of 12.86 to 53.24 μm. It can be inferred that PSD is not the main reason for the differences in the flocculation and separation of dredged slurries.

Different clay samples

The results of d10 increment, SRF, SS and zeta potential of three types of clay samples are shown in Figure 4.

Figure 4

Flocculation and separation results of three types of clay samples. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 4

Flocculation and separation results of three types of clay samples. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 4 indicates that the influences of clay samples with different mineral constituents on flocculation and separation have significant differences. The d10 increment of JC is larger than SK than HB. SRF and SS of JC are smaller than SK than HB. Thus, it can be known that the difference in clay mineral constituents of the dredged slurries is a possible reason that affects the difference in flocculation and separation results.

Different clay samples with humus

The results of the d10 increment, SRF, SS, and zeta potential for different clay samples with fulvic acid and humic acid are shown in Figures 5 and 6.

Figure 5

Flocculation and separation results for different clay samples and fulvic acid. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 5

Flocculation and separation results for different clay samples and fulvic acid. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 6

Flocculation and separation results of different clay samples and humic acid. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 6

Flocculation and separation results of different clay samples and humic acid. (a) d10 increment, (b) SRF, (c) SS, (d) zeta potential.

Figure 5 shows that, for SK and JC, the addition of fulvic acid has limited effect on flocculation and separation. However, when fulvic acid is added to HB slurry, the effect of flocculation and separation is slightly enhanced, which has not been found in the literature.

Figure 6 shows that, for the SK and JC slurries, the effect of humic acid on flocculation and separation is significant compared to the effect of fulvic acid. With an increase in humic acid addition, d10 increment decreases rapidly, SRF and SS increase. For the HB slurry, humic acid has limited effect on flocculation and separation, and there is no obvious change in d10 increment, SRF, and SS.

According to the results shown in Figures 5 and 6, it is known that the interaction between the clay minerals and the humus have influences on flocculation and separation to varying degrees. Thus, it can be known that the difference in clay mineral constituents and humus of the dredged slurries is a possible reason that affects the difference in flocculation and separation results.

Mechanism of the differences of flocculation and separation

Based on the above results, it can be found that clay minerals and humus are possible factors influencing the differences in flocculation and separation in dredged slurries. To determine which constituent in dredged slurries is the sensitive factor affecting flocculation and separation, all the previous tests with a PAM dosage of 0.05% were analyzed. The fulvic acid content, humic acid content, and particle size(d50) of slurries conducted a Pearson correlation analysis with the d10 increment, SRF, and SS after flocculation, respectively. The results are shown in Table 3. Due to the difficulties in quantifying clay minerals, the Pearson correlation analysis of clay minerals was not conducted. According to the results shown in Figures 26, zeta potential has a significant correlation with the flocculation and separation effect. To eliminate the influence of flocculant addition on zeta potentials of slurries, Table 3 shows the zeta potential before flocculation of various types of slurries analyzed by a Pearson correlation with various indicators.

Table 3

Pearson correlation coefficients between slurry properties and indicators

Slurry propertiesr
d10 incrementSSSRF
Initial d50 0.37 −0.37 −0.08 
Fulvic acid content 0.30 −0.35 −0.07 
Humic acid content −0.28 0.24 −0.05 
Zeta potential before flocculation 0.86 −0.86 −0.21 
Slurry propertiesr
d10 incrementSSSRF
Initial d50 0.37 −0.37 −0.08 
Fulvic acid content 0.30 −0.35 −0.07 
Humic acid content −0.28 0.24 −0.05 
Zeta potential before flocculation 0.86 −0.86 −0.21 

From Table 3, the correlations between the initial d50, the fulvic acid, humic acid content and indicators were low (r ≤ 0.37). Although the humus has a significant effect on the results of flocculation and separation in Figures 5 and 6, when the various constituents are combined, there is no constituent dominating the flocculation and separation effect. However, Table 3 shows that the zeta potential before flocculation has a high correlation with d10 increment (r = 0.86) and SS (r = −0.86). To demonstrate this correlation, the relationship between zeta potentials and indicators are shown in Figure 7.

Figure 7

Relationships between zeta potential and indicators. (a) Zeta potential and d10 increment, (b) zeta potential and SS, (c) zeta potential and SRF.

Figure 7

Relationships between zeta potential and indicators. (a) Zeta potential and d10 increment, (b) zeta potential and SS, (c) zeta potential and SRF.

Figure 7 shows that the zeta potential of the slurry can well characterize the effect of flocculation and separation. At the same time, previous results mentioned that the clay mineral constituents and humus would impact the flocculation and separation effects. By incorporating those two results, it could be conjectured that all these constituents of slurries affect the zeta potential to some extent and the new zeta potential of slurry has the influence on effects of flocculation and separation. To verify this conjecture, the test results in Figures 36 are further discussed. The zeta potentials of HB clay samples with different PSDs are similar (−33.6 to −32.75 mV), which indicates that PSD has limited impact on the zeta potential. Meanwhile, flocculation and separation results of different PSDs are also similar in Figure 3. The quartz-based JC slurry has the lowest absolute value of zeta potential, which is −16.20 mV. It is followed by kaolinite-based SK slurry with a zeta potential of −22.33 mV. The HB slurry, which is composed of montmorillonite and cristobalite, has the highest absolute value of zeta potential (−30.90 mV). Referencing Figure 4(a)–4(c), largest d10 increment, optimal sedimentation effect (SS), and optimal filtration effect (SRF) were obtained by the JC slurry, which has the lowest absolute value of zeta potential. In contrast, the highest absolute value of zeta potential in the HB slurry led to the smallest d10 increment, the worst sedimentation effect (SS) and the worst filtration effect (SRF). It means that the difference in mineral constituents causes the difference in zeta potentials of the slurries, which leads to the differences in flocculation and separation effects. Figure S2 (supplementary data, available with the online version of this paper) indicates that the zeta potentials perform differently with different combinations of clay samples and humus. The different adsorption properties could be the possible reason. In order to discuss the relationship between the adsorption properties, the differences of zeta potential, and the changes in flocculation and separation effects, Table 4 summarizes the distribution coefficients of different clay samples and humus (from Figure 1), the differences in zeta potentials of slurries before and after adsorption, and the differences in flocculation and separation effects before and after adsorption. Table 4 shows that, for the combination of the clay and humus with a higher distribution coefficient, the test has a high degree of humus adsorption. The change of zeta potential is also significant, and the corresponding flocculation and separation effects also change drastically.

Table 4

Changes of zeta potentials and changes of flocculation and separation effects

AdsorbatesaAdsorbentsDistribution coefficient Kd (mL/g)Difference in zeta potential before and after adsorptionb (mV)Difference in flocculation and separation effects before and after adsorption
Humic acid JC 21.33 −10.8 Sharp decline 
Humic acid SK 16.80 −13.85 Sharp decline 
Humic acid HB 7.33 −2.25 No change 
Fulvic acid HB 14.95 +3.95 Slightly rising 
Fulvic acid JC 8.31 −0.2 No change 
Fulvic acid SK 6.28 +0.2 No change 
AdsorbatesaAdsorbentsDistribution coefficient Kd (mL/g)Difference in zeta potential before and after adsorptionb (mV)Difference in flocculation and separation effects before and after adsorption
Humic acid JC 21.33 −10.8 Sharp decline 
Humic acid SK 16.80 −13.85 Sharp decline 
Humic acid HB 7.33 −2.25 No change 
Fulvic acid HB 14.95 +3.95 Slightly rising 
Fulvic acid JC 8.31 −0.2 No change 
Fulvic acid SK 6.28 +0.2 No change 

aThe amount of fulvic or humic acid added is 0.07%.

bThe values refer to the zeta potential before flocculation.

The above discussion determines that the clay minerals and humus in slurry have influences on the flocculation and separation effect by affecting the zeta potential of the slurry. When these constituents present simultaneously (the state of the dredged slurry), various constituents are incorporated and exhibit a new zeta potential. The difference in the new zeta potential dominates the difference in flocculation and separation effects, as shown in Figure 2. According to the above discussion, the results shown in Table 3 can be explained. Although humic acid strongly affects the flocculation and separation effect in Figure 6, the correlation between the humic acid content and the flocculation and separation effects of all the slurries is not strong in Table 3. It is because other constituents also change the zeta potential of the slurry. Therefore, a corresponding insight for engineering applications is evident. When the dredged slurry is processed by the coagulation–flocculation method, the determination of the zeta potential is more necessary instead of considering each constituent of the slurry. The zeta potential can then be adjusted to converge to 0 by applying a coagulation–flocculation method to achieve good flocculation and separation effects.

CONCLUSIONS

  • (1)

    There are significant differences in the flocculation and separation of dredged slurries from different sources. Under same flocculation conditions, the difference of increment of particle size d10, specific resistance of filtration, and SS of the supernatant after sedimentation are 0–4.6 times, 0–2.4 orders of magnitude, and 0–4 orders of magnitude, respectively.

  • (2)

    The clay mineral constituents, fulvic acid, and humic acid all impact on the flocculation and separation effects. The differences of the effects can be characterized by the zeta potential, where the lower absolute values of zeta potential correlate with better flocculation and separation effects. When the characteristic particle diameter, d50, of the slurry is in the range of 12.86–53.24 μm, the particle size does not affect flocculation and separation.

  • (3)

    There is no single constituent in the dredged slurry dominating the flocculation and separation effect. When the constituents are incorporated, the new zeta potential exhibited in the slurry determines the difference in flocculation and separation effects.

  • (4)

    The engineering proposal for flocculation and separation of dredged slurries is to improve the flocculation and separation effect by adjusting the zeta potential of the slurry. When using the same type of coagulant or flocculant, the lower absolute value of the zeta potential after addition to the slurry contributes to better flocculation and separation effects.

ACKNOWLEDGEMENTS

This work was supported by the National Program on Key Basic Research Project of China (973 Program) (2015CB057803); the National Science and Technology Major Project of the Ministry of Science and Technology of China (2017ZX07603-003-04).

SUPPLEMENTARY MATERIAL

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2019.428.

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Supplementary data