Polyacrylonitrile (PAN) is a polymer made from the monomer acrylonitrile, which has a linear structure with a repeating unit of -CH2-CH(CN)-. Since the carbon-carbon bonds in the polymer backbone are single bonds, the chain is flexible and can move to a certain extent, which facilitates the material's ability to be processed into fibers or membranes. Also important is the presence of the -CN group, which is polar, meaning it attracts water molecules, making PAN somewhat hydrophilic or water-absorbent. In addition, the nitrile group can participate in hydrogen bonding, which gives the material additional properties
Polyacrylonitrile is chemically resistant, which makes PAN films useful in environments where there is a possibility of exposure to such chemicals. Polyacrylonitrile is a polymer composed of repeating units of the acrylonitrile monomer. The non-polar nature of PAN films minimizes interactions with polar inorganic chemicals, which results in increased electrical resistance. In addition, PAN films can be cross-linked, which involves chemically linking adjacent polymer chains, and cross-linking enhances the stability and chemical resistance of the membrane. It reduces the mobility of the polymer chains, making the membrane less susceptible to swelling or dissolution by inorganic chemicals. PAN has good thermal stability, which makes it suitable for applications that may be exposed to high temperatures. It can withstand a wide temperature range, usually operating effectively in the range of about -20°C to 80°C, maintaining its structural integrity and filtration performance without significant degradation. As can be seen from its structure, the nitrile group makes the PAN membrane hydrophilic. This means that the PAN membrane has good water absorption and can be used in filtration applications; similarly, it also has good mechanical strength, membrane durability and the ability to resist strong physical stress.
A study based on the chemical enhanced primary treatment (CEPT) coupled Fe@aminated polyacrylonitrile (Fe-NH2-PAN) adsorption process built a pilot-scale platform for phosphorus recovery on-site at a sewage treatment plant, achieving the recovery of high-purity battery-grade FePO4 from complex biogas slurry. The platform stably removed more than 80% of PO43− in the biogas slurry during long-term operation. The removed PO43– was enriched in chemical sludge and Fe-NH2-PAN adsorbent, and then leached into H2SO4 solution. FePO4 crystals were successfully recovered from the leachate by supplementing Fe3+ and adjusting the solution pH, with a recovery rate of more than 75% (0.33 kg-FePO4/m3-biogas slurry). Importantly, the lithium-ion battery prepared from the recovered FePO4 exhibited excellent and stable electrochemical performance (discharge specific capacity was stable at 100.4–106.7 mAh/g in 100 charge-discharge cycle tests at a current density of 1.0 C). This study provides a new approach for large-scale phosphorus recovery in wastewater treatment plants.
CEPT coagulation-sedimentation coupled Fe-NH2-PAN adsorption process achieves continuous phosphorus removal with a treatment capacity of 10 tons/day. The biogas slurry produced from the anaerobic digestion unit of the wastewater treatment plant on a certain day was selected as the influent for the first run test of the pilot platform. The PO43− concentration of the biogas slurry was as high as 123.75 ± 4.65 mg/L (chemical oxygen demand COD: 6320 ± 80 mg/L, suspended solids SS: 2240 ± 70 mg/L). After CEPT treatment, the coagulation effect of PFS flocculant reduced the SS concentration to 34 ± 12 mg/L and the COD concentration to 3080 ± 40 mg/L, alleviating the interference of the respective impurities on the adsorption of phosphate by Fe-NH2-PAN. The effluent PO43− concentrations after SBR, CEPT and Fe-NH2-PAN adsorption treatment were 116.41 ± 5.01 mg/L, 39.43 ± 2.13 mg/L and 24.63 ± 0.78 mg/L, respectively. The adsorption performance of Fe-NH2-PAN in raw biogas slurry (2.0 ± 1.9%) and SBR effluent (5.1 ± 0.9%) was extremely poor, which may be due to the interference of various suspended and organic matter on the adsorption of phosphorus. Fe-NH2-PAN can effectively adsorb PO43− in CEPT effluent, with a removal efficiency of 44.6% ± 1.6%. Overall, the pilot platform was able to remove more than 80% of PO43− in the biogas slurry, and the removed PO43− was enriched in PFS chemical sludge and Fe-NH2-PAN.
By adjusting the pH value (pH = 2) and supplementing Fe3+, FePO4 crystals were successfully recovered from the high-concentration phosphate (822.35 ± 15.91 mg/L) leachate. After separating the FePO4 crystals from the solution, the residual solution (173.48±11.12 mg/L) was discharged back to the leaching tank, sulfuric acid was added to adjust the pH to about 1, and the next phosphorus leaching was carried out. Subsequently, the secondary leachate (717.52±10.47 mg/L) was discharged into the FePO4 crystallization unit, and the above operations (supplementing Fe3+ and adjusting pH) were repeated to complete the secondary recovery. Using waste liquid as a supplement to the leachate can save a large amount of H2SO4/Fe2(SO4)3 solution, solving the problem of waste liquid treatment.
The FePO4 recovery process has been verified on a pilot scale, but it is still a long way from actual large-scale production and application. It is possible to recover higher purity FePO4 at a lower cost in some wastewaters with fewer impurities and higher PO43− concentrations. In addition, in order to identify areas for improvement and minimize negative environmental impacts, a life cycle assessment is also needed to fully evaluate the environmental impact of the FePO4 recovery process. Compared with the traditional mining of natural phosphate ore to produce FePO4, recovering FePO4 from wastewater has obvious environmental benefits.
