Study On Adsorption Characteristics Of Nitrate And Phosphate From Aqueous Solution On Modified Activated Carbon

Excessive presence of nitrogen and phosphorus species in water bodies has become a serious problem in the world. Nitrate and phosphate can contribute significantly to eutrophication and be hazardous to human health. Thus, it is meaningful to find an effective treatment method to remove nitrate and phosphate from wastewater before their discharge into water bodies. Activated carbon has been recognized as one of the oldest and widely used adsorbent for water treatment, but the unmodified activated carbon has little or no affinity for nitrate and phosphate removal. In this study, four kinds of modified activated carbon adsorbents have been prepared to investigate the removal of nitrate and phosphate. Both the adsorption characteristics and mechanisms have been detailed in this study. These modified activated carbon included cetylpyridinium chloride(CPC) modified activated carbon(CPMAC), cetyltrimethylammonium chloride(CTAC) modified activated carbon(CTMAC), lanthanum and CTAC modified activated carbon(La CTMAC), zirconium and CTAC modified activated carbon(Zr CTMAC).The adsorption of nitrate from aqueous solution on CPMAC was investigated in batch mode. The CPMAC exhibited much higher nitrate adsorption capacity than the unmodified activated carbon. The nitrate adsorption capacity for CPMAC increased with increasing its CPC loading. The adsorption kinetics of nitrate on CPMAC followed a pseudo-snecond-order kinetic model. The equilibrium adsorption data of nitrate on CPMAC could be described by the Langmuir isotherm model. Based on the Langmuir isotherm model, the maximum nitrate adsorption capacity for CPMAC with CPC loading amount of 444 mmol per 1 kg activated carbon was determined to be 16.1 mg·g-1. The nitrate adsorption capacity for CPMAC decreased with increasing solution p H. The presence of competing anions such as chloride, sulfate and bicarbonate reduced the nitrate adsorption capacity. The nitrate adsorption capacity for CPMAC slightly decreased with increasing reaction temperature. Almost 95% of nitrate molecules adsorbed on CPMAC could be desorbed in 1 mol·L-1 Na Cl solutions. The main mechanisms for the adsorption of nitrate on CPMAC are anionic exchange and electrostatic attraction. The results of this work indicate that CPMAC is a promising adsorbent for removing nitrate from aqueous solution.Batch experiments were conducted to investigate the removal of nitrate from aqueous solution on CTMAC. The CTMAC was effective for removing nitrate from aqueous solution. The equilibrium adsorption data of nitrate on CTMAC could be described by the Langmuir isotherm model. Based on the Langmuir isotherm model, the maximum nitrate adsorption capacity for activated carbon loading per kg amount of 395 mmol CTAC was determined to be 11.8 mg·g-1. The adsorption kinetics of nitrate on CTMAC followed the pseudo-second-order kinetic model. The nitrate adsorption capacity for CTMAC decreased with increasing solution p H. The presence of competing anions such as chloride, sulfate and bicarbonate reduced nitrate adsorption capacity. Almost 90% of nitrate molecules adsorbed on CTMAC could be desorbed in 1 mol·L-1 Na Cl solutions in batch experiments. The main mechanisms for the adsorption of nitrate on CTMAC were anionic exchange and electrostatic attraction.The adsorption of nitrate and phosphate from aqueous solution on La CTMAC was investigated in batch mode. Results showed that the La CTMAC exhibited high adsorption capacity for nitrate and phosphate in aqueous solution. The kinetic studies showed that the adsorption of nitrate and phosphate fitted well with a pseudo-second-order kinetic model. The equilibrium adsorption date could be described by the Langmuir, Freundlichand Dubinin-Radushkevich(D-R) isotherm models. According to the Langmuir isotherm, the predicted maximum nitrate and phosphate adsorption capacities for La CTMAC were 11.2mg·g-1 and 4.15 mg·g-1, respectively. The nitrate adsorption capacity for La CTMAC decreased with increasing p H from 3 to 12. The phosphate adsorption capacity for La CTMAC increased with increasing p H from 3 to8, but decreased with increasing p H from 8 to 12. The presence of coexisting anion such as Cl-, HCO3- and SO42- had a negative effect on nitrate and phosphate adsorption onto La CTMAC. The presence of coexisting nitrate in solution inhibited the adsorption of phosphate on La CTMAC, and the presence of coexisting phosphate in solution inhibited the adsorption of nitrate on La CTMAC. The desorption experiments showed that 97% of nitrate and 71% of phosphate adsorbed by La CTMAC could be desorbed in 1 mol·L-1 Na Cl solution and 1 mol·L-1 Na OH solution, respectively. The mechanisms for the adsorption of nitrate on La CTMAC were the anion exchange and electrostatic attraction.The mechanisms for the adsorption of phosphate on La CTMAC were the anion exchange, electrostatic attraction, ligand exchange and Lewis acid-base interaction.Zr CTMAC was prepared to remove nitrate and phosphate from aqueous solution. Results showed that the Zr CTMAC was effective for nitrate and phosphate removal from aqueous solution. The pseudo-second-order kinetic model fitted both the nitrate and phosphate kinetic experimental date quite well. The equilibrium isotherm data of nitrate adsorption onto the Zr CTMAC were well fitted to the Langmuir, Dubinin-Radushkevich(D-R) and Freundlich isotherm models. The equilibrium isotherm data of phosphate adsorption onto the Zr CTMAC could be described by the Langmuir and Freundlich isotherm models. According to the Langmuir isotherm model, the maximum nitrate and phosphate adsorption capacities for the Zr CTMAC were 7.58mg·g-1and 10.9mg·g-1, respectively. High p H was unfavorable for nitrate and phosphate adsorption onto the Zr CTMAC. The presence of Cl-、HCO3- and SO42-in solution reduced the nitrate and phosphate adsorption capacities for the Zr CTMAC. The nitrate adsorption capacity for the Zr CTMAC was reduced by the presence of coexisting phosphate insolution, and the phosphate adsorption capacity for the Zr CTMAC was also reduced by the presence of coexisting nitrate in solution. About 90% of nitrate adsorbed on the Zr CTMAC could be desorbed in 1 mol·L-1 Na Cl solution, and about 78% of phosphate adsorbed on the Zr CTMAC could be desorbed in 1 mol·L-1 Na OH solution. The adsorption mechanism of nitrate on the Zr CTMAC included the anion exchange interactions and electrostatic attraction, and the adsorption mechanism of phosphate on the Zr CTMAC included the ligand exchange interaction, electrostatic attraction and anion exchange interaction.These results revealed that the surfactant-modified activated carbon adsorbents such as CPMAC and CTMAC were effective for nitrate removal from aqueous, the metal/surfactant-modified activated carbon adsorbents such as La CTMAC and Zr CTMAC were both effective for nitrate and phosphate removal from aqueous solution. It is expected that these noval adsorbents will have a good prosprect in water treatment application.