Composite cationic flocculants based on dual gradients of molecular weight and charge density, and their preparation and application methods.

Abstract

This invention discloses a composite cationic flocculant based on a dual gradient of molecular weight and charge density, along with its preparation and application methods. The flocculant is composed of a medium-molecular-weight, high-charge-density poly(diallylammonium chloride)-acrylamide copolymer (PDA) and a high-molecular-weight, medium-low-charge-density cationic polyacrylamide (CPAM), with an effective mass ratio of 10:1 to 20:1. The mass charge density of PDA is 2 to 3 times that of CPAM. A gradient shear mixing preparation process and a time-sequential synergistic flocculation application method based on differences in diffusion kinetics are employed. Through a relay effect of rapid charge neutralization (0-30 seconds) and slow bridging (30-180 seconds), the flocculant can form a skeleton-filled floc structure, achieving adaptive interception of dissolved organic matter and complexation-encapsulation and fixation of heavy metals. This invention can significantly shorten plate and frame filter press time and reduce the moisture content of the filter cake and the concentration of pollutants in the effluent.

Description

Technical Field

[0001] This invention relates to the field of water treatment and sediment disposal technology, and in particular to a composite cationic flocculant based on a dual gradient of molecular weight and charge density, and its preparation and application methods. Background Technology

[0002] River and lake dredged sediment is characterized by high water content, high fine particle content, and complex colloidal composition, making it a typical multiphase system that is difficult to dewater. Achieving efficient solid-liquid separation and deep dewatering of the sediment is a crucial pre-processing step for subsequent volume reduction treatment. Flocculants, as core chemical additives for adjusting the rheology and dewatering performance of slurry, directly determine the treatment efficiency of mechanical dewatering processes such as plate and frame filter presses, the final water content of the sludge cake, and the environmental safety of the effluent by interacting with colloidal particles, based on their floc microstructure, density, and filtration resistance.

[0003] Currently, sediment conditioning mainly employs a combined process of inorganic coagulants (such as polyaluminum chloride, PAC) and organic polymeric flocculants (such as polyacrylamide, PAM). Among these, cationic polyacrylamide (CPAM) is widely used due to its long-chain adsorption and bridging capabilities; polydimethyldiallyl ammonium chloride (PDA), as a high charge density polymer, is often used for charge neutralization and destabilization. Existing technologies often involve adding both in stages or preparing compound agents through simple physical mixing, attempting to combine the bridging function of CPAM with the charge neutralization function of PDA, utilizing the long chains of CPAM to capture large particles and using PDA to assist in the treatment of fine colloids.

[0004] However, when treating sediment with high organic matter content, existing compound systems suffer from several core problems, including severe competitive adsorption between components, loose floc microstructure, and poor resistance to organic interference. Simple physical compounding ignores the gradient matching of different polymers in terms of molecular weight and charge density. This leads to long-chain molecules often occupying adsorption sites before short-chain molecules in the initial reaction phase, causing competitive adsorption or chain segment entanglement, preventing the formation of a dense framework-filling structure. This results in high floc porosity and weak shear resistance. The large amount of dissolved organic matter (DOM) in the sediment ineffectively consumes cationic charges. Without targeted gradient design, high-charge components are easily preferentially shielded by DOM, leading to low reagent utilization. Furthermore, existing processes often employ constant-speed stirring, ignoring the differences in diffusion kinetics between components with different molecular weights. This easily causes uneven diffusion of small-molecule components or the strong shearing of large molecular chains after floc formation, limiting further improvements in dewatering efficiency. Summary of the Invention

[0005] Objective of the invention: To provide a composite cationic flocculant based on a dual gradient of molecular weight and charge density, and its preparation and application methods, in order to solve one of the problems existing in the prior art.

[0006] Technical solution: A composite cationic flocculant based on a dual gradient of molecular weight and charge density, the flocculant comprising polydimethyl diallyl ammonium chloride (PDA) and cationic polyacrylamide (CPAM):

[0007] PDA is a medium molecular weight, high charge density component with a weight-average molecular weight (Mw) of 100,000 to 1,000,000 Da and a mass charge density of 3.5 to 6.0 mmol / g.

[0008] CPAM is a high molecular weight, medium to low charge density component with a weight-average molecular weight (Mw) of 6 million to 12 million Da and a mass charge density of 1.0 to 3.0 mmol / g.

[0009] Based on the dry weight of the active ingredients, the mass ratio of PDA to CPAM is 10:1 to 20:1.

[0010] Beneficial effects: This invention can significantly shorten the plate and frame filter press time and reduce the moisture content of the filter cake and the concentration of pollutants in the effluent. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the components of the composite cationic flocculant based on a dual gradient of molecular weight and charge density in the embodiments of this application.

[0012] Figure 2 This is a schematic diagram of the components of the Lewis base coordination sites in the embodiments of this application.

[0013] Figure 3 This is a flowchart illustrating the steps of preparing the composite cationic flocculant in the embodiments of this application.

[0014] Figure 4 This is a diagram showing the three timing stages of gradient shearing and mixing in an embodiment of this application. Detailed Implementation

[0015] To solve these problems, combined with Figures 1 to 4 The present invention will be specifically described through the following embodiments.

[0016] This invention provides a composite cationic flocculant (hereinafter referred to as PDAC) based on a dual gradient of molecular weight and charge density, which is suitable for rapid dewatering and synergistic treatment of pollutants in river and lake dredging sediment.

[0017] In this embodiment, the chemical raw materials used are defined as follows:

[0018] PDA: refers to poly(diallyldimethylammonium chloride-co-acrylamide) copolymer, in which PDA is a medium molecular weight, high charge density component, with a weight-average molecular weight (Mw) of 100,000 to 1,000,000 Da and a mass charge density of 3.5 to 6.0 mmol / g. Unless otherwise specified, the monomer ratio can be adjusted as needed.

[0019] CPAM: refers to cationic polyacrylamide, preferably a copolymer of acrylamide and cationic monomers (such as methacryloyloxyethyltrimethylammonium chloride, DMC). CPAM is a high molecular weight, medium-low charge density component with a weight-average molecular weight (Mw) of 6 million to 12 million Da and a mass charge density of 1.0 to 3.0 mmol / g.

[0020] Effective content: Unless otherwise stated, the dosage and ratio are calculated based on the mass of the polymer dry basis.

[0021] To ensure the reproducibility of the technical solution, the key physicochemical parameters involved in this invention were all measured according to the following standard methods:

[0022] Weight-average molecular weight (Mw): determined by gel permeation chromatography (GPC). The mobile phase was a buffer solution of 0.2 mol / L NaNO3 and 0.01 mol / L NaH2PO4 (pH 7.0), the flow rate was 0.5 mL / min, the detector was a differential refractive index detector, and polyethylene oxide (PEO) was used as the standard.

[0023] Charge density: determined by colloidal titration. Titration was performed at pH 7.0 using a standard polyvinyl sulfate (PVSK) solution with toluidine blue as the indicator. The number of moles of positive charge per unit mass of polymer (mmol / g) was calculated.

[0024] fractal dimension (d) f The static light scattering (SLS) method was used for determination. A laser particle size analyzer (such as the Malvern Mastersizer) was used to collect the relationship between the scattered light intensity I and the scattering vector q, which was then determined using the formula ln(I) = -dq. f The fractal dimension is obtained by fitting the slope of ×ln(q)+C.

[0025] Zeta potential: The surface potential of colloidal particles in the supernatant was measured using electrophoretic light scattering with a Malvern Zetasizer Nano ZS instrument at 25°C.

[0026] To accurately predict the on-site effects of the project, this embodiment employs the following two simulation test methods:

[0027] Method A: Plate and frame simulation mud infeed test.

[0028] The conditioned sludge was pumped into a small plate and frame filter press simulation device, with the feed pressure set at 0.6~0.8MPa and the feed time uniformly set at 20min. The total sludge feed volume (g) and the oven-dry sludge feed volume (g) were recorded to evaluate the dewatering rate of the flocculant and the sludge-water separation performance.

[0029] Method B: Model tablet compression test.

[0030] A measured amount of conditioned sludge was placed in a filter press mold and pressed directly at a pressure of 1.0~1.5MPa for 50 minutes, regardless of the feeding time. The sludge cake was then removed and its thickness (cm) and moisture content (%) were measured to evaluate the floc strength and final dewatering depth formed by the flocculant.

[0031] Example 1: This example demonstrates the process of preparing a standard composite flocculant using a stepwise dilution-gradient shear mixing process.

[0032] Synthesis of PDA colloids.

[0033] PDA is a medium molecular weight, high charge density component with a weight-average molecular weight (Mw) of 100,000 to 1,000,000 Da and a mass charge density of 3.5 to 6.0 mmol / g.

[0034] In a 500 mL four-necked flask equipped with a mechanical stirrer, a condenser and a nitrogen delivery tube, dimethyl diallyl ammonium chloride (DMDAAC) monomer and acrylamide (AM) monomer were weighed in a molar ratio of 1:9.

[0035] Dissolve in deionized water, and then purge with high-purity nitrogen gas for 30 minutes while stirring.

[0036] Add 0.1% of potassium persulfate (K2S2O8) initiator solution by total monomer mass, and control the dropping rate to complete the dropping within 30 minutes.

[0037] Stop nitrogen flow and heat the water bath to 50°C for 2 hours; then heat to 60°C and continue the ripening reaction for 3 hours.

[0038] After cooling, the product yielded a viscous PDA colloid. The weight-average molecular weight (Mw) of this PDA was determined to be 500,000 Da, with a cationicity of 10% (molar ratio), and a calculated mass charge density of approximately 3.8 mmol / g (based on the molecular weights of DMDAAC (161.5) and AM (71).

[0039] Pre-dilution of CPAM.

[0040] CPAM is a high molecular weight, medium to low charge density component with a weight-average molecular weight (Mw) of 6 million to 12 million Da and a mass charge density of 1.0 to 3.0 mmol / g.

[0041] The molar percentage of cationicity of PDA is 40% to 80%, and its molecular chain contains 20% to 60% molar percentage of acrylamide units; the side chain of the acrylamide unit contains an amide group (-CONH2), which can serve as a Lewis base coordination site.

[0042] The molar percentage of cation content of CPAM is 10%~30%; its molecular chain extension length is 10~20 times that of PDA molecular chain; CPAM exhibits a linear long chain conformation in aqueous solution, forming a three-dimensional physical trapping framework, which embeds the coordination complex formed by PDA and heavy metal ions inside the floc, forming a complex-embedding dual fixation structure.

[0043] The heavy metal ions are selected from one or more of copper ions, lead ions, zinc ions or cadmium ions; the stability of the coordination complex inside the flocs ensures that the leaching concentration of the above heavy metal ions in the treated effluent meets the environmental discharge standards.

[0044] Commercially available cationic polyacrylamide (CPAM) powder was selected, with a weight-average molecular weight (Mw) of approximately 10 million Da, a cationicity of 20%, and a mass charge density of approximately 2.1 mmol / g.

[0045] Add CPAM dry powder to deionized water, stir to dissolve and dilute to a mass concentration of 0.2%, and let stand for 1 hour to allow the molecular chains to fully extend in the water.

[0046] Gradient shear mixing.

[0047] Based on the dry weight of the active ingredients, the mass ratio of PDA to CPAM is 10:1 to 20:1; and the mass charge density of PDA is 2 to 3 times that of CPAM.

[0048] A three-stage variable-speed stirring process was used to compound the dilute CPAM solution and PDA colloid at an effective content mass ratio of 15:1.

[0049] First stage: Add 30% to 40% of the total amount of dilute CPAM solution to PDA colloid, control the stirring speed to 100 to 140 r / min, stir for 3 to 8 minutes to achieve initial dispersion of CPAM in PDA matrix;

[0050] Second stage: Add the remaining dilute CPAM solution, reduce the stirring speed to 70~90 r / min, stir for 8~12 minutes to promote the uniform distribution of the two polymer segments;

[0051] Third stage: Stop feeding, further reduce the stirring speed to 30~50 r / min, stir for 10~20 minutes to eliminate bubbles generated during mixing and stabilize the system structure.

[0052] In this embodiment, the specific value ranges of each technical parameter are as follows:

[0053] First stage (rapid dispersion): Add 1 / 3 of the total volume of the above dilute CPAM solution to the PDA colloid, and stir at a speed of 120 rpm for 5 minutes. This stage utilizes high shear force to rapidly disperse CPAM into the PDA matrix, preventing localized gelation.

[0054] Second stage (uniform distribution): Add the remaining 2 / 3 of the dilute CPAM solution, reduce the stirring speed to 80 rpm, and stir for 10 minutes. This stage promotes the uniform intercalation of the two polymer segments.

[0055] The third stage (eliminating bubbles and stabilizing): stop feeding, further reduce the stirring speed to 40 r / min, stir for 15 min to eliminate mixed bubbles and stabilize the system.

[0056] It is stable after maturation.

[0057] Adjust the pH of the system to 7.0 using a 0.1 mol / L NaOH or HCl solution.

[0058] The final composite cationic flocculant PDAC-1 was obtained by allowing it to stand and mature at 25℃ for 3 hours.

[0059] Testing revealed that the PDAC-1 system was homogeneous and transparent with no phase separation, and the mass charge density ratio of PDA to CPAM was approximately 1.8 (3.8 / 2.1), meeting the preset gradient requirements.

[0060] Example 2: Preparation of the high charge density gradient variant PDAC-2. Specifically, a variant with a higher charge density gradient was prepared by adjusting the monomer ratio during PDA synthesis.

[0061] PDA Synthesis: PDA colloid was prepared by copolymerization of dimethyl diallyl ammonium chloride (DMDAAC) and acrylamide (AM) as monomers via an initiator.

[0062] CPAM pre-dilution: Dissolve and dilute cationic polyacrylamide (CPAM) to a dilute solution with a mass concentration of 0.1% to 0.3%, allowing the CPAM molecular chains to fully extend in the solvent;

[0063] Gradient shear mixing: The dilute CPAM solution is added to the PDA colloid in stages, and the stirring speed is controlled to decrease in a stepwise manner to prevent the high molecular weight CPAM segments from entanglement and wrapping in the early stage of mixing;

[0064] Maturation and stabilization: After mixing, adjust the pH of the system to 6.5~7.5 and let it stand to mature, thus obtaining a stable composite flocculant.

[0065] PDA synthesis adjustment: The molar ratio of DMDAAC to AM is adjusted to 5:5.

[0066] The obtained PDA (denoted as PDA-High) has a cationicity of 50%, and its calculated mass charge density is approximately 4.3 mmol / g.

[0067] Compound formulation: CPAM (charge density 2.1 mmol / g) was selected.

[0068] Mix according to the effective content mass ratio of 20:1 (for low DOM scene formula).

[0069] The mixing process is the same as in the above embodiments. In the prepared composite flocculant PDAC-2, the mass charge density ratio of PDA to CPAM is 2.05 (4.3 / 2.1), which meets the preferred range of 2 to 3 times gradient characteristics.

[0070] Comparative Example 1: Preparation by gradient-free shearing process.

[0071] To verify the importance of the preparation process, this comparative example uses the traditional one-step mixing method.

[0072] Process difference: Add 0.2% CPAM solution to PDA colloid in one go, and stir at a constant speed of 60 r / min for 30 min.

[0073] Results: A small number of tiny gel clumps were observed in the prepared product (denoted as C-PDAC), and slight stratification occurred after standing for 24 hours, indicating that the lack of gradient shearing process led to uneven dispersion of high molecular weight CPAM and chain segment entanglement.

[0074] Example 3: This example aims to verify whether the prepared composite flocculant PDAC-1 possesses a dual-gradient structure, a skeleton-filled microstructure, and heavy metal complexation-embedding functions.

[0075] Verification of dual gradient structure parameters.

[0076] The PDAC-1 sample was used for component separation and determination. The molecular weight of heavy metals in each component was determined by gel permeation chromatography (GPC). The results showed that there were two obvious molecular weight distribution peaks in the system: the main peak was located near 500,000 Da (corresponding to PDA), and the secondary peak was located near 10,000,000 Da (corresponding to CPAM). The weight-average molecular weights of the two peaks differed by about 20 times, which confirmed the existence of a molecular weight gradient.

[0077] The charge density was determined using charge density titration. The mass charge density of the PDA component was measured to be 3.8 mmol / g, and that of the CPAM component was measured to be 2.1 mmol / g. Calculations show that the mass charge density of PDA is approximately 1.81 times that of CPAM. If PDAC-2 (PDA charge density 4.3 mmol / g) is used, this ratio reaches 2.05 times. The above data confirm that the composite flocculant of the present invention does indeed construct a charge density gradient in its microstructure, with the high charge density PDA acting as a charge neutralization agent, while the low charge density CPAM acts as a bridging framework.

[0078] Fractal dimension characterization of floc microstructure.

[0079] To validate the skeleton-filling model, PDAC-1 and the ordinary CPAM from the comparative example were used to treat kaolin suspensions. After flocculation reached stability, the fractal dimension (d) of the generated flocs was determined using static light scattering (SLS). f ).

[0080] Test results show that the fractal dimension d of the flocs generated by CPAM treatment alone is... f The fractal dimension d is approximately 1.75, indicating that the floc structure is relatively loose and porous, similar to a spool of yarn; while the flocs generated using the PDAC-1 of this invention have a fractal dimension d. f It reached 2.42.

[0081] Thanks to the complexation-embedding dual fixation structure and the filling effect of PDA on the skeleton gaps, the final flocs formed by the composite cationic flocculant when treating bottom sediment have high density, with a fractal dimension of 2.2~2.5, which is higher than the fractal dimension of flocs formed by using CPAM alone.

[0082] According to fractal theory, d f A higher value indicates a denser structure. This increase (from 1.75 to 2.42) directly confirms the mechanistic hypothesis of this invention: high molecular weight CPAM segments form a loose primary framework, while the complex formed by medium molecular weight PDA and fine particles effectively fills the voids in the framework, creating a dense structure similar to reinforced concrete. This dense structure is the physical basis for the significant reduction in cake moisture content (<30%) in subsequent embodiments.

[0083] Verification of the complexation-embedding mechanism of heavy metals.

[0084] Formulating a mixture containing copper (Cu) 2+ Lead (Pb) 2+ Zinc (Zn) 2+ Simulated heavy metal wastewater at concentrations of 10 mg / L was treated with PDAC-1. Functional group shifts before and after flocculation were analyzed using Fourier Transform Infrared Spectroscopy (FTIR).

[0085] The results showed that the characteristic absorption peak of the amide group (-CONH2) on the PDA molecular chain underwent a significant red shift (approximately 15-20 cm⁻¹) after interaction with heavy metals. -1 This indicates that the amide group, acting as a Lewis base, underwent a coordination complexation reaction with heavy metal ions.

[0086] Simultaneously, speciation analysis of the formed flocs using the BCR continuous extraction method revealed that heavy metals mainly existed in organically bound and residual states, with a significant reduction in the proportion of the originally unstable exchangeable state. This demonstrates that heavy metals were not only chemically complexed by PDA but also physically embedded within the flocs by the entrapment effect of CPAM, forming a dual fixation that effectively reduced the bioavailability and mobility of heavy metals.

[0087] Example 4 describes how to accurately calculate the optimal PDA to CPAM dosage ratio based on the dissolved organic matter (DOM) content in the sediment supernatant using the adaptive formula proposed in this invention.

[0088] The basis of orthogonal experiments for formula optimization.

[0089] To determine the parameters in the formula, the flocculation effect was investigated in sediments with different PDA to CPAM ratios (8:1, 12:1, 16:1, 20:1) at different DOM concentrations (characterized by COD, 40, 80, and 120 mg / L).

[0090] Experimental data shows:

[0091] When COD is 40 mg / L (low interference), a 20:1 ratio results in the lowest residual DOM in the supernatant;

[0092] When COD is 80 mg / L (medium interference), the 12:1 ratio is the most effective.

[0093] When COD is 100 mg / L (medium interference), the 11:1 ratio is the most effective.

[0094] When COD is 120 mg / L (high interference), the 8:1 ratio is optimal.

[0095] This pattern indicates that as the DOM content increases, the relative proportion of CPAM must be increased (i.e., the PDA / CPAM ratio must be reduced) to utilize the macromolecular network of CPAM to intercept more organic interfering substances.

[0096] Application and calculation of adaptive adjustment formula.

[0097] Based on the above principles, this embodiment uses a modified linear adaptive adjustment formula for formula design.

[0098] In this embodiment, the effective mass ratio of PDA to CPAM is denoted as R, the reference mass ratio is denoted as R0, and the dissolved organic matter content of the sediment supernatant is denoted as C. DOM The baseline organic matter content is denoted as C0, and the adjustment sensitivity coefficient is denoted as k.

[0099] ;

[0100] Where, k is the adjustment coefficient, ranging from 0.3 to 0.5; R0 is the baseline ratio, with a value of 15; C0 is the baseline DOM content, with a value of 50 mg / L; when C DOM When the concentration is greater than 50 mg / L, the mass ratio R is reduced to increase the relative proportion of CPAM.

[0101] The parameters were set as follows: R0=15, i.e., the baseline ratio is 15:1; C0=50mg / L, the baseline DOM concentration; k=0.4, the adjustment coefficient, which was obtained by fitting the orthogonal experimental data.

[0102] The effective ratio of PDA to CPAM is adjusted according to the dissolved organic matter content of the sediment to be treated. When the COD of the sediment supernatant is 25~50 mg / L, the ratio is 15:1~20:1; when the COD is 50~100 mg / L, the ratio is 10:1~15:1.

[0103] Application Scenario A: Treatment of sediment with high organic matter content.

[0104] Suppose the COD content (C) of the supernatant of dredged sediment in a certain river channel is measured. DOM The concentration was 100 mg / L.

[0105] The calculations are as follows: relative deviation rate = (100-50) / 50 = 1.0; correction factor = 1 - 0.4 × 1.0 = 0.6; optimal ratio R = 15 × 0.6 = 9.

[0106] Specifically, for this high-organic-matter sediment, the mass ratio of PDA to CPAM should be adjusted to 9:1. This increases the amount of CPAM used, which aligns with the technical logic that high interference requires a strong framework for interception. This is consistent with the trend of the ratio of approximately 11:1 when COD=100mg / L, as shown in the experimental data above (the difference comes from a slight adjustment of the k value; in this embodiment, k=0.4 is preferred to obtain stronger anti-interference ability).

[0107] Application scenario B: Treatment of sediment with low organic matter content.

[0108] Suppose the COD content (C) of the supernatant of the sediment in a certain lake is measured. DOM The concentration is 25 mg / L. The calculation process is as follows:

[0109] Relative deviation rate = (25-50) / 50 = -0.5; Correction factor = 1 - 0.4 × (-0.5) = 1 + 0.2 = 1.2; Optimal ratio R = 15 × 1.2 = 18.

[0110] For this type of sediment with low organic matter content, the mass ratio of PDA to CPAM should be adjusted to 18:1. This reduces the amount of CPAM used, avoiding waste, while utilizing a high proportion of PDA for deep charge neutralization.

[0111] As can be seen from the above embodiments, the adaptive formula of the present invention can be used to quickly calculate the optimal formula based on the on-site water quality indicators, overcoming the problem of poor adaptability of traditional fixed ratios (such as a constant 15:1) in the treatment of complex and variable river and lake sediments.

[0112] According to one aspect of this application, other formulas can also be used for fitting, such as: .

[0113] It should be noted that different fitting formulas and different parameters will result in slightly different optimal parameter fitting for different intervals. You can use a table lookup method, or select a formula and adjust the parameters according to the actual situation.

[0114] Example 5 describes a specific process for treating river and lake sediments using the composite flocculant PDAC-1. This method, based on the differences in diffusion kinetics of polymers with different molecular weights, strictly controls the timing and intensity of stirring to achieve optimal flocculation results.

[0115] The process flow was designed using riverbed dredging sediment with a solids content of 30% as the treatment target. The initial slurry's Zeta potential was measured to be -32mV, indicating a stable negatively charged colloidal state. PDAC-1 was prepared as a 1.0% solution and added at 1.2% of the oven-dry sediment mass, i.e., the composite cationic flocculant was added to the water-containing sediment slurry. The flocculated sediment slurry was then transported to a plate and frame filter press for dewatering, yielding a filter cake with a moisture content of 20%–30%. Time-controlled flocculation involved applying hydraulic shear to the slurry system after addition and controlling the stirring process within three kinetic time windows:

[0116] First window (rapid charge neutralization period): Within 0-30 seconds after addition, high shear stirring is applied to allow PDA molecules with a larger diffusion coefficient to preferentially penetrate the water layer and rapidly adsorb onto the surface of colloidal particles to complete charge neutralization and destabilization; the high shear stirring speed in the first window is 250-300 r / min.

[0117] Specifically, the time is set from 0 to 30 seconds after addition. During this stage, the agitator speed is controlled at 280 r / min (high shear). The kinetic mechanism of this stage is to utilize high-intensity hydraulic shear to accelerate the Brownian motion and convective diffusion of the medium molecular weight (approximately 500,000 Mw) PDA component, allowing it to rapidly penetrate the water layer and reach the surface of the negatively charged colloidal particles. At the end of the first window (rapid charge neutralization period), through the rapid adsorption and charge neutralization effect of PDA, the Zeta potential on the surface of the sediment colloidal particles increases from the initial -25mV to -35mV to the destabilization range of -5mV to +5mV.

[0118] Process monitoring data showed that at the end of the 30th second, the Zeta potential of the supernatant rapidly increased from the initial -32mV to -3mV. This indicates that within just 30 seconds, the PDA had completed the charge neutralization and destabilization of the vast majority of colloidal particles, causing the particles to lose their electrostatic repulsion and preparing them for subsequent flocculation. If the stirring is too slow during this stage (e.g., <100r / min), uneven PDA diffusion will lead to localized over-adsorption and incomplete overall destabilization.

[0119] Second window (slow bridging growth period): Within 30~180 seconds, the stirring speed is reduced to medium shear to match the slower diffusion rate of CPAM molecules and the long chain extension time. The long chains of CPAM are used to capture fine flocs for adsorption and bridging to form coarse flocs. The medium shear stirring speed in the second window is 80~120 r / min.

[0120] Specifically, the time was set to 30–180 seconds. During this stage, the stirring speed was reduced to 100 r / min (medium shear). The kinetic mechanism at this stage is that the diffusion rate of the high molecular weight (Mw approximately 10 million) CPAM component is slow, and the long chain extension requires time. Moderate stirring intensity ensures contact and collision between the long CPAM chains and the fine flocs while preventing excessive shear force from breaking the newly formed polymer bridges. During this period, it was observed that the fine flocs gradually aggregated into coarse flocs with a particle size exceeding 200 μm.

[0121] The third window (densification period): After 180 seconds, the stirring speed is reduced to low shear to promote the rearrangement and compaction of the floc structure, forming high-density flocs and completing the flocculation treatment; the low shear stirring speed in the third window is 30~50 r / min.

[0122] Specifically, the time is set to 180 to 600 seconds. During this stage, the stirring speed is further reduced to 40 r / min (low shear). The kinetic mechanism of this stage is to utilize the gentle vortex generated by low-speed stirring to promote the drainage of water and structural rearrangement within the flocs. Because the PDAC of this invention has a skeleton-filled structure, the flocs become denser and the settling speed is accelerated under low shear.

[0123] Multi-stage variable speed stirring prevents PDA diffusion from being hindered due to insufficient shear in the first window period, or CPAM long chain breakage due to excessive shear in the second window period.

[0124] In a comparative experiment, if a constant stirring speed of 150 r / min was used throughout the process, the results showed that the Zeta potential only rose to -15 mV within 0-30 seconds (insufficient charge neutralization), and after 180 seconds, the average particle size of the flocs was only 60% of that in this embodiment, and the turbidity of the supernatant was high. This demonstrates that segmented time-series control based on kinetic differences is the key to maximizing the performance of the composite agent of this invention.

[0125] Example 6: Based on actual river dredging project field data, the actual performance of the PDAC-1 agent of this invention and the traditional PAC+PAM agent in the plate and frame filter press dewatering process was compared.

[0126] Test conditions and filter press equipment: XMZ500 / 1500-U type plate and frame filter press. Raw mud properties: density 1.15 g / cm³. 3The moisture content is approximately 70% (i.e., solids content 30%). Comparative scheme (PAC+PAM): 6 kg / tDS of polyaluminum chloride (PAC) is added as a filter aid, and 1 kg / tDS of anionic polyacrylamide (APAM) is added as a flocculant. Scheme of this invention (PDAC-1): Only 12 kg / tDS (i.e., 1.2%) of PDAC-1 is added, without adding other inorganic salts. Wherein, kg / tDS refers to kilograms per ton of dry sludge.

[0127] Efficiency and effectiveness data comparison: Based on multiple on-site operation records, the data comparison is as follows:

[0128] Feeding and pressing time (efficiency indicators): PAC+PAM scheme: Feeding time requires 4000 seconds, pressing time requires 400 seconds, totaling 4400 seconds to complete one batch. PDAC-1 scheme (optimized): Feeding time requires only 1500 seconds, pressing time requires only 100 seconds, totaling 1600 seconds. Conclusion: The scheme of this invention reduces the core operation time of a single batch by 63.6%. This is because the flocs formed by PDAC-1 have a high fractal dimension (dense and porous), ensuring unobstructed filtration channels and reducing filtration resistance (specific resistance).

[0129] Mud cake properties (quality indicators): PAC+PAM scheme: mud cake thickness 3.2 cm, moisture content 38.2%, mud cake texture is relatively soft. PDAC-1 scheme: mud cake thickness 3.8 cm, moisture content 26.7%, mud cake is well-formed and firm, with no sticking. Conclusion: The scheme of this invention not only dehydrates more thoroughly (moisture content <30%), but also produces a larger amount of mud per batch (increased thickness), facilitating subsequent transportation and disposal.

[0130] Overall construction efficiency: Calculates the amount of sludge processed per unit time (m³). 3 / min), the PAC+PAM scheme is 1.60m 3 / min, while the PDAC-1 scheme achieves 5.25m. 3 / min. Conclusion: The overall construction efficiency of this invention is 3.29 times that of the traditional method, reducing the time cost and equipment occupancy cost of the project.

[0131] Example 7: The key environmental indicators in the effluent (filtrate) discharged after pressure filtration were analyzed to verify the synergistic removal capability of the present invention for pollutants.

[0132] Nitrogen and phosphorus removal efficiency: Total nitrogen (TN): The original sludge supernatant was 11.76 mg / L. After PAC+PAM treatment, the effluent was 12.8 mg / L (even slightly higher). After PDAC-1 treatment, the total nitrogen in the effluent decreased to 4.69 mg / L, achieving a removal rate of 60%. Total phosphorus (TP): The original sludge supernatant was 0.57 mg / L. After PAC+PAM treatment, the effluent was 0.22 mg / L. After PDAC-1 treatment, the total phosphorus in the effluent decreased to 0.15 mg / L, achieving a removal rate of 73%. Analysis suggests that the high charge density PDA component in PDAC-1 can undergo strong charge neutralization and adsorption precipitation with negatively charged phosphate ions and nitrogen-containing organic colloids, achieving simultaneous and deep removal of nitrogen and phosphorus.

[0133] The results for the heavy metal indicators of concern in the sediment are as follows: Arsenic (As): 3.51 mg / L in raw sediment, 3.11 mg / L in effluent after PDAC-1 treatment. Cadmium (Cd): 0.91 mg / L in raw sediment, 0.53 mg / L in effluent after PDAC-1 treatment, removal rate approximately 41%. Hexavalent Chromium (Cr6+): 33.34 mg / L in raw sediment, 28.81 mg / L in effluent after PDAC-1 treatment. For the highly toxic cadmium (Cd), this invention exhibits a significant immobilization effect. This confirms the complexation-encapsulation mechanism: the amide groups of the PDA side chain form stable complexes with heavy metal ions and are firmly locked within the dense flocs, significantly reducing the risk of them being discharged into water bodies with the effluent. In contrast, the traditional PAC+PAM scheme mainly relies on physical adsorption, and its ability to immobilize dissolved heavy metals is relatively weak.

[0134] The residual amounts of acrylamide monomer and polymer in the effluent were measured. The residual amount of the PAC+PAM scheme was 0.20 mg / kg, while the residual amount of the PDAC-1 scheme was only 0.13 mg / kg. This indicates that the adaptive formulation and sequential flocculation process of the present invention result in a more complete reagent reaction, higher utilization rate, and lower risk of secondary pollution.

[0135] Example 8: Using laboratory simulation methods, without involving a large-scale feeding system, the solid-liquid separation limit of the flocculant on sludge was investigated, and the optimal dosage range was determined.

[0136] PDAC-1 flocculant was used. Sediment with a solids content of 30% was selected, and 200g of sediment sample (70g oven-dried) was taken from each group. The filtration time was set to 60min (simulating extreme pressing conditions), and the pressure was 1.5MPa. Three dosage gradients were set: 1kg / tDS (0.1%), 2kg / tDS (0.2%), and 3kg / tDS (0.3%). It should be noted that the units used in this embodiment differ from those used in field applications and are only exploratory parameters for a small-scale laboratory test.

[0137] At a dosage of 1 kg / tDS: the cake weight was 94.57 g, thickness was 13.04 mm, and moisture content was 24.56%. The cake formed well and had high hardness. At a dosage of 2 kg / tDS: the cake weight was 93.99 g, thickness was 13.08 mm, and moisture content was 25.56%. The effect was comparable to the 1 kg / tDS group, with no significant improvement. At a dosage of 3 kg / tDS: an over-cationic stickiness was observed on the cake surface. The excessive cationic charge caused colloid restabilization and entanglement between polymer chains, which hindered moisture drainage and made demolding difficult.

[0138] This embodiment demonstrates that: (a) the PDAC flocculant of the present invention achieves excellent dehydration effect (moisture content <25%) at a low dosage, which is superior to the PAC+PAM control group (moisture content approximately 27-29%). (b) there is an upper limit to the optimal dosage. That is, in practical applications, excessive dosage should be avoided to prevent over-cation.

[0139] Example 9 describes the process adaptability of the composite flocculant of the present invention (taking PDAC-3 and PDAC-4 as examples, i.e., PDA:CPAM=20:1 formulation) under different combinations of feed and pressing time.

[0140] The extreme time test of PDAC-3 (20:1 ratio) under the extreme condition of reducing the dosage to 0.8% (octane-dry mud mass ratio) and compressing the feeding + pressing time to 20 min + 6 min showed that the mud cake thickness still reached 3.0 cm and the moisture content was 36.3%. Although the mud cake was slightly loose in the center (not completely dry), it still met the requirements for unloading and transportation. In contrast, the PAC+PAM solution, under a long cycle of 40 min + 30 min, had a mud cake thickness of only 2.5 cm. This proves that the high charge density PDA component of this invention (with a very high PDA content in the 20:1 formulation) has an extremely fast charge neutralization rate, making it suitable for short-cycle, fast-paced construction scenarios.

[0141] The robustness test of PDAC-4 (20:1 ratio) was conducted under the standard condition that the dosage was restored to 1.2%, and three time gradients were tested: (1) 35+15min; (2) 25+10min; (3) 20+6min.

[0142] The results showed that the moisture contents of the three groups of sludge cakes were 24.2%, 25.3%, and 27.3%, respectively. This indicates that although the moisture content increased slightly with the shortening of the process time, it was always kept below 30%. This demonstrates that the skeleton-filled floc structure formed by the composite flocculant of the present invention has excellent permeability and compressibility, and can maintain stable dewatering output under different filtration times, providing significant flexibility for adjusting parameters on-site according to the project schedule.

[0143] According to one aspect of this application, a cationic flocculant for river dredging is provided, which utilizes the synergistic flocculation effect of molecular weight gradient in a multiphase system of complex sludge to improve sludge flocculation efficiency, and provides a ratio adjustment formula to adapt to different DOM content changes in bottom sediment to achieve the best flocculation effect.

[0144] The effective content ratio of PDA to CPAM components is 10:1 to 20:1.

[0145] The pH value of the flocculant system is 6.5~7.5. Within this pH range, the quaternary ammonium groups of PDA and the modified cationic groups of CPAM are both in a fully ionized state, and the PDA colloid remains stably dispersed in the solution network formed by CPAM without phase separation or gelation.

[0146] A cationic flocculant for river dredging is prepared as follows: (1) Weigh DMDAAC and AM according to a certain ratio and add them to a four-necked flask with a stirrer. Stir and pass nitrogen to deoxygenate for 30 min; (2) Add a certain amount of potassium persulfate initiator solution and adjust the dropping speed so that the initiator solution is dropped completely within 30 min; (3) After the initiator is dropped completely, stop passing nitrogen, heat the water bath to a certain reaction temperature, react for 2 h, heat to the ripening temperature of 60℃, and continue to react for 3 h to obtain the product PDA colloid; (4) Dilute CPAM with deionized water to a dilute solution with a mass concentration of 0.1%~0.3% to fully extend the CPAM molecular chain; (5) Three-stage gradient mixing process: First stage: Take 1 / 3 Add the CPAM dilute solution to the PDA colloid and stir rapidly at 120r / min for 5min to achieve initial dispersion; Second stage: add the remaining 2 / 3 of the CPAM dilute solution and stir at 80r / min for 10min to promote uniform distribution; Third stage: reduce the speed to 40r / min and stir slowly for 15min to eliminate bubbles and stabilize the system; (6) After mixing, adjust the pH of the system to 6.5~7.5 with dilute hydrochloric acid or sodium hydroxide solution to ensure that both polyelectrolytes are in the optimal ionization state; (7) let it stand for 2~4h at 25±2℃ to allow the PDA and CPAM molecular chains to reach a balanced conformation and obtain a stable composite flocculant PDAC, which is a composite cationic flocculant.

[0147] Furthermore, a cationic flocculant used for river dredging exhibits a synergistic flocculation kinetic principle after addition, whereby PDA and CPAM undergo a fast-slow relay flocculation process due to their molecular weight differences.

[0148] First time window (0~30 seconds) – PDA rapid neutralization period:

[0149] Medium molecular weight PDA, with its high diffusion coefficient (approximately 3-4 times that of CPAM), rapidly diffuses to the surface of colloidal particles, completing the main charge neutralization process within 10-30 seconds. This causes the colloidal zeta potential to rise from -25 to -35 mV to -5 to +5 mV, achieving destabilization. During this stage, a large number of fine flocs (particle size 10-50 μm) are formed.

[0150] Second time window (30~180 seconds) – CPAM slow bridging growth period:

[0151] High molecular weight CPAM diffuses more slowly, gradually reaching the surface of fine flocs within 30–180 seconds. It then connects the fine flocs into larger flocs (particle size 200–1000 μm) through long-chain adsorption bridging. The delayed arrival of CPAM precisely avoids competing with PDA for initial adsorption sites.

[0152] Third time window (>180 seconds) – floc compaction period:

[0153] Under slow stirring, the flocs rearrange, the long chains of CPAM shrink, and the loose flocs are compacted to form high-density, high-strength final flocs.

[0154] Based on the time-series synergistic mechanism, the flocculation stirring speed was adjusted as shown in Table 1, revealing the kinetic division of labor mechanism of the compound flocculant and providing a theoretical basis for process parameter optimization.

[0155] Table 1. Flocculation and stirring time and speed during the dosing process.

[0156]

[0157] The main application of cationic flocculants is in river and lake dredging projects, where they are used for dewatering and flocculating bottom sediments and conditioning sludge, which helps in the deep treatment of sludge.

[0158] The effective content ratio of PDA and CPAM components in the composite flocculant can be adaptively adjusted to match the changes in DOM content in different sediments. By establishing a quantitative relationship between the ratio of PDA and CPAM and the DOM content in the sediment, the formulation can be adaptively adjusted to the sediment characteristics, achieving the optimal flocculation effect under different DOM content conditions.

[0159] Formula for adjusting proportions (Formula 1): ;

[0160] Where: R PDA / CPAM R0: The actual ratio of effective PDA to CPAM content used; R0: The baseline ratio, taken as 15:1 (midpoint of the range); C DOM: DOM content of sediment supernatant (characterized by COD, mg / L); C0: baseline DOM content, taken as 50 mg / L; k: adjustment coefficient, taken as 0.3~0.5;

[0161] Application rule: When C DOM When the concentration is 50 mg / L, R PDA / CPAM =15:1; when C DOM When R = 100 mg / L, PDA / CPAM ≈11:1 (increasing the CPAM ratio); when C DOM When the concentration is 25 mg / L, R PDA / CPAM ≈20:1 (reducing the CPAM ratio).

[0162] Table 2 Orthogonal Experiment and Flocculation Effect of Mix Proportion Optimization Design

[0163]

[0164] The flocculation effect of different PDA:CPAM ratios (8:1, 12:1, 16:1, 20:1) under different DOM contents (COD: 40, 80, 120 mg / L) was investigated, and the experimental results are shown in Table 2. According to Formula 1, the k adjustment parameter for each group was calculated. When the k value was between 0.3 and 0.5, the residual DOM content was at its lowest, indicating the best flocculation effect, thus proving the effectiveness of the adaptive formulation. Furthermore, in the three batches of experiments, the optimal DOM removal rate reached 70-83%, demonstrating that the composite flocculant has a good interception effect on DOM.

[0165] In this embodiment, the cationic flocculant PDAC developed in this invention, when used on-site, significantly shortened the time for treating river silt compared to the original solution (PAC+PAM), and resulted in a thicker sludge cake with lower moisture content, which is beneficial for subsequent sludge disposal. Overall, the efficiency of treating river silt is more than three times that of the original solution (PAC+PAM).

[0166] Example 10: Comparison between the PAC+APAM scheme and the new flocculant PDAC scheme.

[0167] In the initial test, considering that CPAM would have a better effect on sludge dewatering, five PAC+CPAM schemes with different ionic degrees and five PDAC schemes with different ionic degrees were set up to investigate the effect of different ionic degrees on the dewatering effect. The results showed that the higher the ionic degree of CPAM, the higher its total sludge feed rate and the oven-dry sludge feed rate, and the better the sludge feed effect.

[0168] Three schemes were selected for plate and frame sludge infeed tests: PAC+APAM, PAC+CPAM-90 with higher ionization degree, and PDAC (50 ionization degree). The test results are shown in Table 3.

[0169] The effect of PAC+APAM is slightly better than that of PAC+CPAM-90, mainly because there are many suspended particles in the sludge. The addition of APAM can break their stable state, accelerate particle settling, and quickly separate sludge into water, resulting in the highest total sludge intake. Although CPAM has better dewatering performance, its settling performance for suspended particles is weaker than that of APAM, so the sludge intake effect is slightly worse than that of APAM. In PDAC(50), the addition of CPAM can effectively resist the consumption of negatively charged colloids. While maintaining a good flocculation effect, it can effectively condition the sludge, resulting in better dewatering performance and a higher total sludge intake than CPAM.

[0170] Table 3. Plate and frame simulated mud infeed test data

[0171]

[0172] Model pressing test: The actual on-site pressing time is 20 min feeding time + 50 min pressing time. The laboratory simulation test involves no pipeline transportation of sludge, and the feeding time is not considered. Only the plate and frame pressing time is considered, and the pressing time of the plate and frame is controlled at 50 min.

[0173] Table 4. Model tablet compression test data

[0174]

[0175] The data from the tableting simulation test in Table 4 show that the tableting effects of the PAC+PAM and PDAC combinations are not significantly different. The moisture content of PDAC is lower than that of the PAC+PAM combination. However, the dosage of PDAC3 (dosage) is too high, which leads to the sludge cake being too sticky and not conducive to on-site filtration. Therefore, PDAC3 (dosage) will not be considered in subsequent tests.

[0176] The results of the pressing clearly show that the raw sludge is relatively soft and easily deformed, with nearly 30% of the sludge escaping. Therefore, the raw sludge cannot be directly filtered and requires the addition of flocculants to condition the sludge before filtration.

[0177] Determination of filtration effluent parameters: The results of ammonia nitrogen, total nitrogen, total phosphorus, COD, and flocculant residue in the filtration effluent are shown in Table 5. The removal effects of PAC+APAM, PAC+CPAM-90, and PDAC on ammonia nitrogen were generally moderate. PDAC showed the highest removal efficiency for total nitrogen and total phosphorus, at 36.9% and 64.9%, respectively. PAC+CPAM-90 was the next most effective, at 30.9% and 52.6%, respectively. This is mainly because PDA and CPAM both contain positively charged active groups, which are effective against NO3- in sludge. - PO4 3- Negative ions have excellent adsorption properties and can effectively remove nitrogen and phosphorus. The three methods have little impact on COD. PAC+APAM and PAC+CPAM-90 produce more flocculated residue than PDAC, mainly due to the synergistic effect of PDA and CPAM, creating a system dominated by stable cationic charges. This helps overcome the performance instability that may occur when using PAM products alone due to hydrolysis, resulting in more efficient flocculation and dewatering. Combined with sludge floc, it results in less residue in the effluent.

[0178] Table 5. Results of Filter Press Effluent Indicators

[0179]

[0180] Table 6 shows the cake measurement results for both PAC+PAM and PDAC processes. PDAC showed better results in reducing cake moisture content, superior to the PAC+PAM combination, while PDAC3 (dosage) resulted in cake formation difficulties. Therefore, PDAC is more suitable for on-site optimization.

[0181] Table 6 Results of mud cake testing

[0182]

[0183] Based on the results of multiple plate and frame filter press tests, PAC+APAM has a slightly better sludge feeding effect than PAC+CPAM-90, with the highest total sludge feeding volume. PDAC, with the addition of CPAM, can effectively resist the consumption of negatively charged colloids, effectively conditioning the sludge while maintaining a good flocculation effect, thus exhibiting better dewatering performance, and its total sludge feeding volume is higher than that of CPAM.

[0184] Based on the comparative test results of the filter press effluent, among the three schemes PAC+APAM, PAC+CPAM-90 and PDAC, PDAC has the highest removal efficiency for total nitrogen and total phosphorus and the least flocculant residue, while PAC+CPAM-90 is the second most effective. None of the three schemes have a good removal effect on ammonia nitrogen, but the final deep treatment results can all meet the standards. Therefore, the PDAC process is more in line with the needs of the site.

[0185] Based on the results of the comparative test of filter cakes, among the PAC+APAM and PAC+CPAM-90 schemes, the PAC+CPAM-90 process produces filter cakes with higher moisture content but thicker cakes, resulting in higher treatment efficiency. The PDAC process produces filter cakes with better moisture content than the PAC+PAM combination, and the cake thickness is basically the same as that of the PAC+CPAM-90 process. Therefore, the PDAC process is suitable for on-site pressure filtration.

[0186] According to one aspect of this application, the pharmaceutical preparation process is as follows:

[0187] Weigh DMDAAC and AM at a molar ratio of 1:9 and add them to a four-necked flask equipped with a stirrer. Stir and purge with nitrogen for 30 minutes to remove oxygen.

[0188] Add 0.1% of the total amount of potassium persulfate initiator solution, and adjust the dropping rate so that the initiator solution is added completely within 30 minutes;

[0189] After the initiator was completely added, nitrogen flow was stopped, and the water bath was heated to a certain reaction temperature. The reaction was carried out for 2 hours, and then the temperature was raised to the ripening temperature of 60°C. The reaction was continued for 3 hours to obtain the product PDA colloid.

[0190] A stepwise dilution-gradient mixing process was adopted: the dissolved CPAM solution was slowly added to the PDA colloid in three stages at a decreasing stirring speed (120→80→40r / min) at a ratio of 1:10 to the effective content of PDA. The pH was adjusted to 6.5~7.5 and the mixture was allowed to mature for 2~4 hours to obtain a stable composite flocculant PDAC-1.

[0191] Experimental steps:

[0192] Add water to the reagent and stir for 10 minutes to fully dissolve the reagent, then dilute to a 10% solution (the residual PAC+PAM before the first group of tests does not react with the reagent and is ignored).

[0193] Add the ingredients according to the experimental plan ratio, and stir for 10 minutes after addition to ensure a complete reaction.

[0194] After filtration, the moisture content and thickness of the filter cake were analyzed, and the subsequent experimental parameters were determined based on the results.

[0195] Table 7 Test Results

[0196]

[0197] The on-site unloading conditions and cake condition are described below: With the same dosage (1.2% of the oven-dry mud mass), the feeding time + filtration time were 2400 + 200 seconds, 1800 + 100 seconds, and 1500 + 100 seconds, respectively. The cakes after filtration were well-formed and compacted in all three groups, with thicknesses of 3.8 cm, 4.3 cm, and 3.8 cm, respectively. The 3.3 cm thickness meets the on-site unloading requirements, and the feeding and filtration times, cake thickness, and moisture content are significantly better than the PAC+PAM scheme.

[0198] Assuming the PAC+PAM scheme results in non-stick mud cakes that do not affect transportation, and all schemes have a uniform unloading time of 600 seconds, with approximately 180 seconds for backflushing, venting, and unloading, the feed rate per unit time within one cycle is compared in Table 8. With a new agent PDAC-1 dosage of 1.2%, adjusting the feeding and pressing time from 2400 seconds + 200 seconds to 1500 seconds + 100 seconds resulted in well-formed and firm mud cakes in all three test groups, with stable cake thickness and moisture content. At a feeding and pressing time of 1500 seconds + 100 seconds, the mud cake thickness was 3.8 cm, the moisture content was 26.7%, and the construction efficiency was 5.25 m. 3 / min, which is 3.29 times that of the original scheme.

[0199] Table 8 Comparison of construction efficiency between the new agent and the PAC+PAM scheme

[0200]

[0201] Compared with the original sludge, the total nitrogen in the effluent after sludge conditioning and filter pressing with the new agent PDAC-1 is reduced by 52%~60%, the total phosphorus is reduced by 72%~82%, the COD remains largely unchanged, and the SS, i.e., suspended solids, is significantly reduced.

[0202] Compared with the PAC+PAM scheme, total nitrogen is reduced by 56%~63%, COD is reduced by 33%~57%, SS is significantly reduced, flocculant residue is slightly reduced, and the three heavy metal data are not much different.

[0203] Overall, after the use of the new agent PDAC-1, the total nitrogen and SS in the effluent decreased significantly, while the total phosphorus, COD, and heavy metals decreased slightly.

[0204] Table 9 Tailwater Indicators

[0205]

[0206] Example 11

[0207] Weigh DMDAAC and AM at a molar ratio of 3:7 and add them to a four-necked flask equipped with a stirrer. Stir and purge with nitrogen for 30 minutes to remove oxygen.

[0208] Add 0.2% of the total amount of potassium persulfate initiator solution, and adjust the dropping rate so that the initiator solution is added completely within 30 minutes;

[0209] After the initiator was completely added, nitrogen flow was stopped, and the water bath was heated to a certain reaction temperature. The reaction was carried out for 2 hours, and then the temperature was raised to the ripening temperature of 60°C. The reaction was continued for 3 hours to obtain the product PDA colloid.

[0210] A stepwise dilution-gradient mixing process was adopted: the dissolved CPAM solution was slowly added to the PDA colloid in three stages at a decreasing stirring speed (120→80→40r / min) at a ratio of 1:15 to the effective content of PDA. The pH was adjusted to 6.5~7.5 and the mixture was allowed to mature for 2~4 hours to obtain a stable composite flocculant PDAC-2.

[0211] Experimental steps:

[0212] Add water to the reagent and stir for 10 minutes to fully dissolve the reagent, then dilute to a 10% solution (the residual PAC+PAM before the first group of tests does not react with the reagent and is ignored).

[0213] Add the ingredients according to the experimental plan ratio, and stir for 10 minutes after addition to ensure a complete reaction.

[0214] After filtration, the moisture content and thickness of the filter cake were analyzed, and the subsequent experimental parameters were determined based on the results.

[0215] Table 10 Test Results

[0216]

[0217] The on-site sludge unloading situation and sludge cake condition are as follows: Under the same addition amount (1.2% of the oven-dry sludge mass), the feeding time + filtration time were 4000+1200 seconds, 3500+1000 seconds, and 3000+800 seconds, respectively. Looking at the sludge cakes after filtration, the first two groups of test cakes were well-formed. The batch with a 4000+1200 second filtration time had a firm sludge cake without a loose core, with a thickness of 3.5 cm; the batch with a 3500+1000 second filtration time had a slightly loose core, with a thickness of 3.3 cm, also meeting the on-site sludge unloading requirements; the batch with a 3000+800 second filtration time had some sludge formation, but the sludge cake was very runny, requiring an increased filtration time.

[0218] Assuming the PAC+PAM scheme does not cause the mud cake to stick to the plate and does not affect the mud unloading, the mud unloading time for all schemes is uniformly 600 seconds, and backflushing, venting, and unloading take about 180 seconds. The comparison of the feed rate per unit time within one cycle is shown in Table 11.

[0219] When the sludge density is 1.18 g / cm³ 3With a dosage of 1.2%, the combined feeding and pressing time significantly affects the formation and thickness of the mud cake. When the combined feeding and pressing time decreased from 4000+1200 seconds to 3500+1000 seconds, the mud cake thickness decreased from 3.5cm to 3.3cm, and a slight loose core appeared. When the combined feeding and pressing time decreased from 3500+1000 seconds to 3000+800 seconds, the mud cake became very thin, with a thickness dropping to approximately 3.0cm, and its condition no longer met the requirements for on-site mud unloading. For reference, at 3800+1000 seconds, the batch mud cake thickness was 3.4cm, indicating good condition, falling between the 4000+1200 and 3500+1000 seconds range.

[0220] With a chemical dosage of 1.2%, the sludge cake formed and became compacted. Therefore, the feeding time was reduced from 4000 seconds to 3000 seconds, and the pressing time from 1200 seconds to 800 seconds. The construction efficiency increased from 0.85 to 0.99, which is 1.16 times that of the original plan. The decrease in batch construction efficiency at reference time 3800+1000 seconds indicates that the feeding and pressing times were sufficient and could be reduced.

[0221] Table 11 Comparison of construction efficiency of PAC+PAM scheme

[0222]

[0223] Compared with the original sludge, the total nitrogen in the effluent after sludge conditioning and filter pressing with the new agent PDAC-2 was reduced by 47%~52%, the total phosphorus by 63%~72%, the COD remained largely unchanged, and the SS was significantly reduced.

[0224] Compared with the PAC+PAM scheme, total nitrogen is reduced by 51%~56%, COD is reduced by 30%~50%, SS is significantly reduced, and the data on flocculant residue and heavy metals are not much different;

[0225] Overall, after the use of the new agent PDAC-2, the total nitrogen and SS in the effluent decreased significantly, while the total phosphorus, COD, and heavy metals decreased slightly.

[0226] Table 12 Tailwater Indicators

[0227]

[0228] Example 12

[0229] Weigh DMDAAC and AM at a molar ratio of 5:5 and add them to a four-necked flask equipped with a stirrer. Stir and purge with nitrogen for 30 min to remove oxygen.

[0230] Add 0.2% of the total amount of potassium persulfate initiator solution, and adjust the dropping rate so that the initiator solution is added completely within 30 minutes;

[0231] After the initiator was completely added, nitrogen flow was stopped, and the water bath was heated to a certain reaction temperature. The reaction was carried out for 2 hours, and then the temperature was raised to the ripening temperature of 60°C. The reaction was continued for 3 hours to obtain the product PDA colloid.

[0232] A stepwise dilution-gradient mixing process was adopted: the dissolved CPAM solution was slowly added to the PDA colloid in three stages at a decreasing stirring speed (120→80→40 r / min) at a ratio of 1:20 to the effective content of PDA. The pH was adjusted to 6.5~7.5 and the mixture was allowed to mature for 2~4 hours to obtain a stable composite flocculant PDAC-3.

[0233] Experimental steps:

[0234] Add water to the reagent and stir for 10 minutes to fully dissolve the reagent, then dilute to a 10% solution (the residual PAC+PAM before the first group of tests does not react with the reagent and is ignored).

[0235] Add the ingredients according to the experimental plan ratio, and stir for 10 minutes after addition to ensure a complete reaction.

[0236] After filtration, the moisture content and thickness of the filter cake were analyzed, and the subsequent experimental parameters were determined based on the results.

[0237] Table 13 Flocculation Test Results

[0238]

[0239] On-site sludge unloading and cake condition: When the dosage was reduced from 1.2% (1.2% of the oven-dry sludge mass) to 1.0%, the feeding time + filtration time remained at 25 + 10 min. The sludge cakes after filtration were all well-formed and compacted. When the dosage was reduced to 0.8%, and the feeding time + filtration time was reduced to 20 + 6 min, the batch of cakes showed slight loosening in the center, with a thickness of 3.0 cm, which also met the on-site sludge unloading requirements. The PAC+PAM solution produced compact cakes, but the feeding time + filtration time was too long, and the cake thickness was only 2.5 cm, resulting in a worse performance compared to PDAC-3.

[0240] The PAC+PAM scheme prevents the mud cake from sticking to the plate and does not affect mud unloading. The unloading time for all schemes is uniformly 5 minutes, while backflushing, venting, and unloading take about 3 minutes. The comparison of feed rate per unit time within one cycle is shown in Table 14.

[0241] When the sludge density is 1.2 g / cm³ 3With a PDAC-3 dosage of 1.2%, the feed volume was relatively large, and the feed + pressing time was shortened compared to the PAC + PAM scheme. The sludge cakes were all formed and hard, indicating that the feed and pressing times were sufficient. Subsequently, when the PDAC-3 dosage was reduced to 1.0% and 0.8%, and the feed and pressing times were shortened to 20 min and 6 min respectively, the sludge cakes were still formed and the effect was good.

[0242] With a PDAC-3 dosage of 1.2%, the mud cake is well-formed and firm, and the construction efficiency can reach up to 5.77, which is 2.73 times that of the original scheme. Therefore, in order to reduce costs, the dosage was reduced to 0.8%, and the feeding and pressing time was adjusted to 20+6 min. The mud cake had a slightly sandwiched core and a moisture content of 36.3%, which still met the requirements for on-site mud unloading. The construction efficiency was still 5.18, which is 2.45 times that of the original scheme.

[0243] Table 14 Comparison of construction efficiency of PAC+PAM schemes

[0244]

[0245] Compared with the original sludge, the total nitrogen in the effluent after sludge conditioning and filter pressing with the new agent PDAC-3 is reduced by 31%~40%, the total phosphorus is reduced by 51%~63%, the COD remains largely unchanged, and the SS is significantly reduced.

[0246] Compared with the PAC+PAM scheme, total nitrogen is reduced by 37%~45%, COD is reduced by 20%~40%, SS is significantly reduced, flocculant residue is slightly reduced, and the three heavy metal data are not much different.

[0247] Overall, after the use of the new agent PDAC-3, the total nitrogen and SS in the effluent decreased significantly, while the total phosphorus, COD, and heavy metals decreased slightly.

[0248] Table 15 Tailwater Indicators

[0249]

[0250] Example 13

[0251] Weigh DMDAAC and AM at a molar ratio of 1:9 and add them to a four-necked flask equipped with a stirrer. Stir and purge with nitrogen for 30 minutes to remove oxygen.

[0252] Add 0.2% of the total amount of potassium persulfate initiator solution, and adjust the dropping rate so that the initiator solution is added completely within 30 minutes;

[0253] After the initiator was completely added, nitrogen flow was stopped, and the water bath was heated to a certain reaction temperature. The reaction was carried out for 2 hours, and then the temperature was raised to the ripening temperature of 60°C. The reaction was continued for 3 hours to obtain the product PDA colloid.

[0254] A stepwise dilution-gradient mixing process was adopted: the dissolved CPAM solution was slowly added to the PDA colloid in three stages at a decreasing stirring speed (120→80→40r / min) at a ratio of 1:20 to the effective content of PDA. The pH was adjusted to 6.5~7.5 and the mixture was allowed to mature for 2~4 hours to obtain a stable composite flocculant PDAC-4.

[0255] Experimental steps:

[0256] Add water to the reagent and stir for 10 minutes to fully dissolve the reagent, then dilute to a 10% solution (the residual PAC+PAM before the first group of tests does not react with the reagent and is ignored).

[0257] Add the ingredients according to the experimental plan ratio, and stir for 10 minutes after addition to ensure a complete reaction.

[0258] After filtration, the moisture content and thickness of the filter cake were analyzed, and the subsequent experimental parameters were determined based on the results.

[0259] Table 16 Flocculation Test Results

[0260]

[0261] The on-site sludge unloading and cake condition are as follows: With the dosage maintained at 1.2% (1.2% of the oven-dry sludge mass), the feed time + filtration time was reduced from 35 + 15 min to 20 + 6 min. The filtered cakes were well-formed and compact, especially the batch with a feed time + filtration time of 20 + 6 min, which achieved a cake thickness of 2.9 cm, demonstrating excellent flocculation. The PAC+PAM solution produced compact cakes, but the feed time + filtration time was too long, and the cake thickness was only 2.5 cm, resulting in a poorer performance compared to PDAC-3.

[0262] (1) 1.2% PDAC-4 (feed + press 35 + 15 min); (2) 1.2% PDAC-4 (feed + press 25 + 10 min); (3) 1.2% PDAC-4 (feed + press 20 + 6 min); (4) PAC+PAM scheme (feed + press 40 + 30 min)

[0263] The PAC+PAM scheme prevents the mud cake from sticking to the plate and does not affect mud unloading. The unloading time for all schemes is uniformly 5 minutes, while backflushing, venting, and unloading take about 3 minutes. The comparison of feed rate per unit time within one cycle is shown in Table 17.

[0264] When the sludge density is 1.2 g / cm³ 3With a reagent dosage of 1.2%, the feed rate reached a maximum of 259L when the feeding time was 35 minutes. As the time decreased, the feed rate and cake thickness decreased slightly, but the cakes were all well-formed and firm, indicating that the feeding and pressing times were sufficient; the moisture content increased slightly, but both met the on-site process requirements.

[0265] With a reagent dosage of 1.2%, the mud cake is formed and hard, and the construction efficiency can reach up to 5.77, which is 2.73 times that of the original plan. Therefore, in order to reduce costs, the dosage was reduced to 0.8%, and the feeding and pressing time was adjusted to 20+6min. The mud cake was slightly sandwiched and the moisture content was 36.3%, which still met the requirements for unloading mud on site. The construction efficiency was still 5.18, which is 2.45 times that of the original plan.

[0266] Table 17 Comparison of construction efficiency of PAC+PAM schemes

[0267]

[0268] Compared with the original sludge, the total nitrogen in the effluent after sludge conditioning and filter pressing with the new agent PDAC-4 is reduced by 50%~55%, the total phosphorus is reduced by 65%~70%, the COD remains largely unchanged, and the SS is significantly reduced.

[0269] Compared with the PAC+PAM scheme, total nitrogen is reduced by 54%~59%, COD is reduced by 37%~50%, SS is significantly reduced, flocculant residue is slightly reduced, and the three heavy metal data are not much different.

[0270] Overall, after the use of the new agent PDAC-4, the total nitrogen and SS in the effluent decreased significantly, while the total phosphorus, COD, and heavy metals decreased slightly.

[0271] Table 18 Tailwater Indicators

[0272]

[0273] In summary, the addition of PDA to sludge neutralizes and adsorbs negatively charged colloidal particles, while some colloidal particles redisperse and stabilize due to charge repulsion, leading to a decrease in treatment efficiency. Therefore, it is considered to polymerize PDA using DMDAAC and AM as monomers, and then add CPAM, which is also a cationic component. By adopting the principle of molecular weight gradient synergistic flocculation, the two polyelectrolytes, PDA and CPAM, form a graded capture mechanism due to their molecular weight difference, reducing the flocculation consumption of negatively charged colloids and dissolved organic matter, and ensuring the flocculation effect of flocculant in complex sludge systems.

[0274] In the PDA-CPAM compound system, the two polyelectrolytes form a hierarchical capture mechanism due to their molecular weight difference:

[0275] The first stage—the macromolecular interception effect of CPAM: High molecular weight CPAM (greater than 6 million) forms an extended molecular network in the sediment suspension, preferentially capturing coarse particles (>10 μm) and some DOM molecules through physical entanglement and van der Waals forces, forming a primary floc skeleton. Due to the low charge density of CPA4.3M, its binding with DOM is mainly through physical encapsulation, and it does not consume a large amount of the system's cationic charge.

[0276] The second stage—the fine charge neutralization effect of PDA: PDA with medium molecular weight and high charge density can penetrate the primary flocculent network formed by CPAM to efficiently neutralize fine colloidal particles (<10μm). Since CPAM has pre-isolated some DOM interfering substances, the cationic groups of PDA can act more fully on the target colloid.

[0277] The third stage – secondary bridging enhancement: The fine particles that have been neutralized and destabilized by PDA undergo secondary bridging and aggregation with the assistance of CPAM long chains, forming a dense, high-strength final-state floc.

[0278] Based on the Taihu Lake dredging project, this study compared the existing PAC+PAM flocculant scheme with sludge moisture content, sludge cake and effluent indicators, and developed a new composite cationic flocculant to achieve the dewatering of bottom sludge and the treatment of nitrogen and phosphorus indicators in effluent, thereby improving the overall benefits of the project.

[0279] According to one aspect of this application, when the solid content of the bottom sediment is less than 15%, the amount of flocculant added should be increased to 1.5% to 2.0% of the dry sediment mass; when the solid content of the bottom sediment is higher than 40%, it should be diluted to 30% to 35% before flocculation treatment.

[0280] When the ambient temperature is below 10℃, the PDA synthesis reaction time should be extended to 4-5 hours and the ripening time should be extended to 4-6 hours.

[0281] If excessive stickiness is observed, the addition should be stopped immediately and the amount of PDA should be reduced appropriately.

[0282] To address the issues of severe competitive adsorption between components and loose floc microstructure, this solution employs a material construction approach with dual gradients in molecular weight and charge density. Through the compounding of medium molecular weight / high charge density PDA with high molecular weight / medium-low charge density CPAM, examples have demonstrated that the two can form a unique framework-filled dense structure (significantly improved fractal dimension), effectively overcoming the defects of traditional compounded agents caused by disordered adsorption, such as loose structure and weak shear resistance.

[0283] To address the issue of ineffective charge consumption by dissolved organic matter (DOM), this solution introduces an adaptive DOM formulation adjustment method. The example demonstrates how a quantitative formula dynamically adjusts the ratio of the skeleton to the filler components based on the organic matter content of the sediment, achieving targeted interception and shielding of organic interferences and solving the problem of low agent efficiency under high organic matter conditions.

[0284] To address the kinetic mismatch issue caused by constant-speed stirring, this solution implements a kinetic-time synergistic flocculation process. By strictly defining the stirring shear rate during the rapid charge neutralization period and the slow bridging period, the rapid diffusion of small-molecule PDA and the slow extension characteristics of large-molecule CPAM are precisely matched, eliminating localized reaction inhomogeneity and long-chain breakage.

[0285] Ultimately, the plate and frame filter press time was shortened, efficiency was increased by more than 3 times in some scenarios, the moisture content of the filter cake was reduced to below 30%, and deep solidification of heavy metals was achieved.

[0286] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all fall within the protection scope of the present invention.

Claims

1. A method for treating river and lake dredging sediment using a composite cationic flocculant based on a dual gradient of molecular weight and charge density, characterized in that... The flocculant contains polydiallyl ammonium chloride (PDA) and cationic polyacrylamide (CPAM): PDA is a medium molecular weight, high charge density component with a weight-average molecular weight (Mw) of 100,000 to 1,000,000 Da and a mass charge density of 3.5 to 6.0 mmol / g. CPAM is a high molecular weight, medium to low charge density component with a weight-average molecular weight (Mw) of 6 million to 12 million Da and a mass charge density of 1.0 to 3.0 mmol / g. Based on the dry weight of the active ingredients, the mass ratio of PDA to CPAM is 10:1 to 20:1; Poly(diallyldimethylammonium chloride) PDA is a copolymer of diallyldimethylammonium chloride (DMDAAC) and acrylamide (AM). The molar percentage of cationicity of PDA is 40%~80%, and its molecular chain contains 20%~60% molar percentage of acrylamide units; The side chain of the acrylamide unit contains an amide group (-CONH2), which can serve as a Lewis base coordination site. The effective content mass ratio R of PDA to CPAM and the dissolved organic matter content C of the supernatant of the sediment to be treated. DOM Based on COD, the following adaptive matching relationship is satisfied: ； Where, k is the adjustment coefficient, ranging from 0.3 to 0.5; R0 is the baseline ratio, with a value of 15; C0 is the baseline DOM content, with a value of 50 mg / L; when C DOM When the concentration is greater than 50 mg / L, the mass ratio R is reduced to increase the relative proportion of CPAM.

2. The method for treating river and lake dredging sediment with a composite cationic flocculant according to claim 1, characterized in that, The molar percentage of cation content of CPAM is 10%~30%; its molecular chain extension length is 10~20 times that of PDA molecular chain; CPAM exhibits a linear long chain conformation in aqueous solution, forming a three-dimensional physical trapping framework, which embeds the coordination complex formed by PDA and heavy metal ions inside the floc, forming a complex-embedding dual fixation structure.

3. The method for treating river and lake dredging sediment with a composite cationic flocculant according to claim 1, characterized in that, The pH value of the flocculant system is 6.5~7.

5. Within this pH range, the quaternary ammonium groups of PDA and the modified cationic groups of CPAM are both in a fully ionized state, and the PDA colloid remains stably dispersed in the solution network formed by CPAM without phase separation or gelation.

4. The method for treating river and lake dredging sediment with a composite cationic flocculant according to claim 1, characterized in that, The stepwise dilution-gradient shear mixing process includes the following steps: PDA Synthesis: PDA colloid was prepared by copolymerization of dimethyl diallyl ammonium chloride (DMDAAC) and acrylamide (AM) as monomers via an initiator. CPAM pre-dilution: Dissolve and dilute cationic polyacrylamide (CPAM) to a dilute solution with a mass concentration of 0.1% to 0.3%, allowing the CPAM molecular chains to fully extend in the solvent; Gradient shear mixing: The dilute CPAM solution is added to the PDA colloid in stages, and the stirring speed is controlled to decrease in a stepwise manner to prevent the high molecular weight CPAM segments from entanglement and wrapping in the early stage of mixing; Maturation and stabilization: After mixing, adjust the pH of the system to 6.5~7.5 and let it stand to mature, thus obtaining a stable composite flocculant.

5. The method for treating river and lake dredging sediment with the composite cationic flocculant according to claim 4, characterized in that, Gradient shearing mixing specifically includes the following three time stages: First stage: Add 30% to 40% of the total amount of dilute CPAM solution to PDA colloid, control the stirring speed to 100 to 140 r / min, stir for 3 to 8 minutes to achieve initial dispersion of CPAM in PDA matrix; Second stage: Add the remaining dilute CPAM solution, reduce the stirring speed to 70~90 r / min, stir for 8~12 minutes to promote the uniform distribution of the two polymer segments; Third stage: Stop feeding, further reduce the stirring speed to 30~50 r / min, stir for 10~20 minutes to eliminate bubbles generated during mixing and stabilize the system structure.

6. The method for treating river and lake dredging sediment with a composite cationic flocculant according to claim 1, characterized in that, The method utilizes the difference in molecular diffusion kinetics between PDA and CPAM to achieve time-sequential synergistic flocculation, and includes the following steps: Dosing: Add the composite cationic flocculant to the water-containing sediment slurry; Time-controlled flocculation: Hydraulic shear is applied to the added mud system, and the stirring process is controlled within three kinetic time windows: First window: Within 0-30 seconds after addition, apply high shear stirring to allow PDA molecules with a larger diffusion coefficient to preferentially penetrate the water layer and quickly adsorb onto the surface of colloidal particles to complete charge neutralization and destabilization. Second window: Within 30~180 seconds, reduce to medium shear stirring to match the slower diffusion rate of CPAM molecules and the long chain extension time, and use the long chains of CPAM to capture fine flocs for adsorption and bridging to form coarse flocs. Third window: After 180 seconds, reduce to low shear stirring to promote floc rearrangement and compaction, forming high-density flocs and completing the flocculation treatment.

7. The method according to claim 6, characterized in that, The stirring speed is controlled as follows: The high-shear stirring speed in the first window is 250~300 r / min; The medium shear stirring speed in the second window is 80~120 r / min; The low-shear stirring speed in the third window is 30~50 r / min; Multi-stage variable speed stirring prevents PDA diffusion from being hindered due to insufficient shear in the first window period, or CPAM long chain breakage due to excessive shear in the second window period.

8. The method according to claim 6, characterized in that, The method further includes: The flocculated bottom mud slurry is transported to a plate and frame filter press for dewatering to obtain mud cake; the moisture content of the mud cake is 20%~30%.

Images