Cationic polyacrylamide (PAM) is a copolymer with ionizable groups on the linear molecular chain. Cationic PAM can be used as a thickening, flocculant, drag-reducing, etc. Because of its ionization in water and the adsorption and bridging of particles in the water body, there is more and more research on cationic polyacrylamide for the water treatment industry.
We analyze the application and research status of cationic polyacrylamide (CPAM) in domestic and foreign industrial wastewater, oilfield, coking, paper, printing and dyeing, fracturing, oil, and grease, etc., in recent years, and make an outlook on the development direction of cationic polyacrylamide in industrial wastewater treatment, to provide reference and reference for related research.
Keywords: cationic polyacrylamide, C-PAM, domestic wastewater, industrial wastewater.
Cationic PAM is a chain polymer compound containing various active groups (mostly quaternary ammonium groups). Cationic polyacrylamide can adsorb many substances (such as colloidal particles, organic matter, etc.) through hydrogen bonding. It is easy to flocculate with negatively charged substances. Therefore, C-PAM has the functions of color removal, turbidity removal, adhesion, and adsorption.
Cationic PAM can treat industrial wastewater and domestic sewage containing more large organic colloids, such as smelting, dyeing, oil field, paper making, urban living, seafood processing, etc. It also performs well in paper making, municipal wastewater, and other industrial sludge dewatering and disposal.
There is a great demand for C-PAM in China. This paper analyzes the research status of domestic and foreign researchers in recent years on domestic wastewater, oilfield wastewater, paper wastewater, printing, and dyeing wastewater, coking wastewater, fracturing wastewater, oil and grease wastewater, etc., where cationic polyacrylamide has been applied more frequently.
1. Current status of research and application.
1.1 Domestic sewage.
The pollutants contained in domestic wastewater are mainly pathogenic microorganisms and (e.g., cellulose, glycolaldehyde, oil, grease, etc.) organic substances. Because cationic PAM has a high density of positive charge in its molecular chain, the dosage is small and non-polluting. Therefore, the application of C-PAM in sludge dewatering interests researchers.
The researchers explored the effect of cationic polyacrylamide on the dewatering performance of sludge from a wastewater treatment plant. The fastest sludge settling rate (Vs=2.83 mL/min) and the best performance were achieved when cationic PAM was 30 mg/L. Cationic polyacrylamide and liquid poly aluminum chloride (LPAC) were compared. The dewatering rate after 30 mg/L CPAM was 95.74%, the sludge cake water content was 84.18% for the same filtration time, and the dewatering rate after 30 mg/L LPAC was 95.33%, and the sludge cake water content was 84.66%. This shows that cationic PAM has better sludge dewatering properties. This is because the positive charge carried by cationic PAM neutralizes the negative charge on the sludge surface. As a result, the repulsive force between sludge particles generated by electrostatic force is reduced. The compressibility is improved. In turn, the sedimentation capacity can be improved.
Another researcher used C-PAM as a flocculant. The effect of C-PAM on the stirring time and dosage was observed when the temperature (20~27℃), raw water turbidity (943~985℃) and pH value were different. The optimal dosage of cationic polyacrylamide in domestic wastewater sludge dewatering treatment was 0.3~0.6 mg/L. The optimal stirring time was 4 min (i.e., 2/3 of the total reaction time).
In the actual dewatering process of North Central sludge, cationic PAM was found to increase the solids content of the cake, reduce the water content of the cake (from 84% to 72%), and reduce the area of the sludge pile.
Some studies found that hydrophobic HACPAM flocculants were prepared by UV-initiated hydro-micellar polymerization to investigate the optimal conditions for sludge flocculation and dewatering. As the molecular weight of HACPAM increased (from 177,392 to 1,047,661), the particle size of sludge flocs increased (from 200 μm to 2,440 μm), and the dewatering performance was enhanced. When the sludge dewatering performance is optimal, 7.578 4 kg/t is the optimum dry solids (DS) dosage. The maximum sludge floc 2 280 μm, the highest free water removal content of 90.9%, and the inflection point of CST change of 59.3 s were observed.
The results showed that the cationic microclusters were enhanced by the repair effect and charge neutralization to produce compact flocs with high mechanical strength. Such compact flocs, as a backbone, will make the filter cake less compressible and more permeable in mechanical dewatering.
At present, it seems that, under normal circumstances, the water content of the cake can be reduced to 55%~80% when cationic PAM treats the slurry of domestic sewage. However, if the supporting equipment is very advanced and perfect, the water content can eventually reach even 10%. Therefore, in-depth studies are needed.
1.2 Oilfield wastewater.
The composition of oilfield wastewater contains emulsified crude oil (0.1~10 μm), solid particles, H2S, salts (e.g., KCL, NaCl, Na2CO3, etc.), and natural impurities such as heavy metals (e.g., copper, lead, zinc, etc.), and additives (e.g., scale inhibition, oxygen removal, disinfection and lubricants, acids flowing into the formation, etc.). These components determine the physical and chemical properties of oilfield-produced water.
In recent years, some scholars have researched oilfield wastewater treatment with cationic polyacrylamide. The effect of cationic PAM dosage on the oil removal effect was investigated. The oil removal rate was obtained by measuring the oil content of the flocculated effluent: the oil removal rate increased rapidly with the increase of C-PAM addition (10~30 mg/L); the oil removal rate (92%~94.9%) increased little within the dosage of 30~50 mg/L; the dosage of 30 mg/L was used to obtain the maximum oil removal rate (94.9%).
The researcher investigated the effect of different cationic degrees (8.97%~93.0%) of cationic PAM on flocculation and oil removal of the recovered oil from the Shengli oilfield: the cationic degrees were investigated in three stages (8.97%~24%, 24%~53% and 53%~93%, respectively, for the Ⅰ, Ⅱ, and Ⅲ stages). Cationic polyacrylamide has the characteristics of strong adsorption and bridging ability, high molecular weight (6~12 million), and many functional groups, etc. Cationic PAM is used in oilfield wastewater treatment with high efficiency, good coagulation effect, a small amount, little influence by acid and alkali, less floating slag, no erosion effect on equipment, and has a broad application prospect.
1.3 Paper wastewater.
Cationic PAM contains a variety of active groups, and paper wastewater contains many anionic micro-particles, suspended solids, and harmful substances. Through polymer chain adsorption bridging and cationic electric neutralization, the microfine particles in paper wastewater will attach with the molecular chain active groups, making them into large flocculent structures, then quickly flocculated and settled by sedimentation net catching action.
Cationic PAM is commonly used in the treatment of paper wastewater. Two types of polymeric flocculants were prepared by cationic polyacrylamide (CPAM) with non-ionic polyacrylamide (NPAM) and anionic polyacrylamide (APAM), respectively. The application of CPAM in paper sludge dewatering was studied. In paper sludge conditioning dewatering, CPAM adsorption bridging produced a higher effect than charge neutralization. When applied with NPAM and APAM, the consumption of CPAM can be reduced. The improvement of dewatering performance is better with APAM than NPAM in combination. However, if over-dosed, it is not beneficial for sludge dewatering. And the adverse effect of NPAM application is less than APAM.
Other researchers have studied the effect of high molecular weight CPAM with different charge densities on the viscoelasticity of coniferous sulfate pulp suspensions. The flocculant increased the flocculation of fibers, probably due to an increase in the number of active sites in the mesh during flocculation, which increased the binding strength of the fiber contact points. This was manifested as an increase in shear modulus and critical strain.
In conclusion, when applied in paper wastewater treatment, cationic PAM significantly affects TSS (total suspended solids), turbidity, COD (chemical oxygen demand), and color treatment. The removal rate can reach more than 90%. However, the performance in removing suspended and dissolved impurities, color or dye molecules, and inorganic or organic matter is yet to be developed.
1.4 Printing and dyeing wastewater.
Cationic PAM is a linear polymer with reactive groups, which can react with many substances to generate hydrogen bonds. It can especially combine with negatively charged colloids to discolor, remove turbidity and adhere.
Researchers investigated COD’s removal rate and color removal ability from printing and dyeing wastewater. The results showed that when the coagulant cationic PAM was used in combination with the flocculant PFS (Poly Ferric Sulphate), the removal rate of COD and color of printing and dyeing wastewater was improved, and the COD removal rate and color removal effect were significantly better than those of cationic PAM and PFS separately.
1.5 Coking wastewater.
Coking wastewater contains high concentrations of hazardous substances that are difficult to decompose. Researchers used modified fly ash compounded with cationic polyacrylamide to study the mixing time, ratio, acidity, and additional amount to remove COD. The results showed that the best COD Cr removal rate was 60.5% for modified fly ash and cationic polyacrylamide with a mixing time of 60 min, a ratio of 10 g/L, and a dosage of 25 g/L.
Some researchers used water glass and aluminum chloride as raw materials to produce poly aluminum silicate coagulants with different aluminum-to-silicon ratios. The decontamination ability of the compound flocculants in removing organic matter from coking wastewater phenol was examined. The flocculation ability of poly aluminum silicate compound cationic PAM, poly aluminum silicate, cationic PAM, and PAC were compared for turbidity and phenol in laboratory wastewater.
The results showed that the combination of PASiCl (aluminum to silicon ratio of 1.0) and a small amount of CPAM increased the coagulation removal of phenol by PASiCl; when the pH value of the water sample was 9, and PASiCl was compounded with 5 mg/L cationic PAM, the removal of phenol and turbidity reached more than 10.3% and 99%, respectively, due to their synergistic effect. This points to a new direction for removing organic matter and decontaminating coking wastewater.
Other researchers polymerized organic monomer AM with chitosan to obtain chitosan derivative (CAM), which was then combined with CPAM to produce CAM-CPAM, which achieved the best dewatering effect with a settling rate of 0.55 cm/s and turbidity below 8 NTU at a mass concentration of 30 mg/L for thickened sludge from wastewater treatment plants.
Cationic PAM can effectively remove suspended particles and colloidal impurities from water by linking and adsorbing other pollutants to colloidal particles, causing colloidal particles to coalesce and form agglomerates. Cationic polyacrylamide is a widely used organic polymer coagulant that can be used as a simple and efficient pretreatment technology to achieve effective pre-reduction of biotoxic substances, thus increasing the biochemical treatment level with reduced water dilution.
1.6 Fracturing wastewater.
Fracturing wastewater treatment in China mainly uses combustion, re-injection, and residual acid pond storage. However, none can overcome the pollution problem from the source due to poor treatment effects and secondary pollution.
Cationic PAM has a high dissolution rate, excellent flocculation, and low viscosity. Especially, the COD removal rate is high after coagulation and air flotation. This has attracted the general attention of scholars at home and abroad.
Researchers have treated oilfield fracturing wastewater with a new cationic polyacrylamide water treatment agent. The results showed that: the water-in-water CPAM with polymerized aluminum chloride was used in the treatment of fracturing wastewater, and the flocculation reaction time was 1 min; the water after flocculation was neutral, and the turbidity was reduced from 362 NTU. As a result, the turbidity was reduced from 362 NTU to 6 NTU, and COD was reduced from 4,973 mg/L before treatment to 530 mg/L. Subsequently, the COD was reduced to 61.3 mg/L by membrane treatment + activated carbon adsorption.
Other data shows that treating oilfield fracturing wastewater with CPAM with polymerized aluminum chloride has a low chemical cost, fast flocculation speed, and low operation cost and can achieve the standard discharge after treatment. Especially, the COD removal rate can reach 88.2%.
1.7 Oil and grease wastewater.
In addition to high concentrations of oil and grease (HDL), organic matter such as phospholipids, soaps, salts, bases, acids, and suspended solids (e.g., sediment, clay, microorganisms, etc.) are present in the wastewater. Both BOD (biological oxygen demand) and COD are high.
Inorganic flocculants (aluminum salts, iron salts, calcium chloride, etc.) are commonly used to treat oil-containing wastewater. These agents have a high cost, large dosage, unsatisfactory treatment effect, many floc residues, and difficult follow-up treatment. With the desire for a good environment and economic development, the development of harmless, efficient, and multifunctional flocculants can not be delayed.
The China University of Geosciences has studied the graft copolymerization of acrylamide and cationic starch for preparing CPAM and conducted flocculation experiments on the wastewater a Shenzhen oil company provided. The optimal preparation conditions were: the mass ratio of cationic starch to acrylamide was 1:3, the pH was 2~3, the mass of kMnO4 elicitor was 0.15% acrylamide, the reaction time was three hours, and the reaction temperature was 50℃. The optimal dosage of the new flocculant was 8 mg/L, and the COD removal rate was more than 45%.
1.8 Other industrial wastewater.
In addition to its application in the above industrial wastewater treatment, cationic PAM has been used in coal chemical wastewater, kaolin wastewater, phenol wastewater, and pharmaceutical wastewater by the polymerization and compounding with other agents to improve the effectiveness of treating wastewater.
2. Conclusion.
Cationic polyacrylamide (CPAM) can effectively treat industrial wastewater mainly because of its good water solubility, high-cost performance, and excellent compounding effect. It has been improved by water dispersion polymerization, microwave-module copolymerization, ultrasonic-initiated template copolymerization, and free radical copolymerization, and the following directions are suggested for future research.
1) CPAM significantly affects the removal of TSS, turbidity, COD, and color in industrial wastewater treatment. Still, it cannot treat suspended matter, dissolved impurities, and dye macromolecules. Therefore, it is recommended to conduct special studies on treating these indicators.
2) The mechanism of flocculation optimization is based on the selection or control of cationic PAM molecular weight range and polymer charge density. Therefore, it is suggested that molecular weight and charge density distribution should be considered in future flocculant preparation studies to prepare better selective flocculants.
3) To conduct an in-depth study on the dissolution properties of cationic PAM from the microscopic perspectives of molecular diffusion, hydrogen bonding, and entanglement.
4) To research CPAM and polysilicicic acid, metal salts, and compound flocculation. Especially, we should strengthen the in-depth study of charge neutralization, bridging, and co-precipitation mechanism. In short, from the viewpoint of performance, cost, characteristics, and compounding, simple, economically feasible processes to produce environmentally friendly and high removal efficiency of cationic polyacrylamide flocculants will have a broad application prospect.