The Preparation, Modifications And Adsorption Properties Of Amine-Rich Porous Schiff-Base Polymer

The problem of water pollution caused by organic pollutants has become increasingly serious.Chlorophenols in wastewater are difficult to be degraded and can be enriched through the food chain,and cause the teratogenic,carcinogenic and mutagenic effects to organism.Azo dyes in sewage water are refractory,light-resistant,acid-fast,alkali-tolerant,and anti-microbial,enriched in biological body lead to cancer,and affect photosynthesis of aquatic plants.Therefore,the removal of the chlorophenols and azo dyes in the water has become one of the most urgent tasks at this stage.Adsorption is one of the most common ways to deal with them.Traditional adsorbent materials are greatly limited in their large-scale applications because of their high cost,poor recovery,and poor efficiency.Porous organic polymers have high specific surface area,abundant voids,good stability and easy regeneration.Thus,in recent years,it has become one of the most important and concerned adsorbent materials for research workers.In this work,we first prepared a rich-amine porous organic polymer（RAPOP）,because of its serious agglomeration,the specific surface area was limited.Therefore,two methods were applied to improve the specific surface area,and comprehensively investigated their structure and adsorptive performance towards azo dyes（methyl orange（MO））and chlorophenol（2,4-dichlorophenol（2,4-DCP））.The details are as follows:Firstly,the RAPOP was synthesized via the Schiff-base reaction with melamine（MA）and terephthalaldehyde（TA）as the monomers.The characterization results confirmed that the nanoparticles were constructed first and then assembled to form a porous structure with a high specific surface area of 368.05 m²·g^-1 and a large pore volume of 0.651 cm³·g^-1arising from highly cross-linked aminal networks.Its functional group had been successfully obtained and had excellent thermal stability,and it mainly consists of C and N elements.The adsorption results show that solution pH 3 was an optimal condition for MO while the adsorption of 2,4-DCP was effective at pH 7,and the RAPOP dosages were decided accurately at 5 mg for MO and 10 mg for 2,4-DCP.The adsorption kinetics indicated that the adsorption equilibrium times were about 40 seconds for 2,4-DCP and 240minutes for MO,and both followed a pseudo-second-order model.The isotherm data shown that it agreed well with the Langmuir isotherm with maximum adsorption amounts of454.545 mg·g^-1 and 188.697 mg·g^-1 for MO and 2,4-DCP at 298 K,respectively.The adsorptions of MO and 2,4-DCP on RAPOP were both spontaneous and endothermic.Secondly,the graphitic carbon nitride（g-C₃N₄）/RAPOP were synthesized via one-pot polymerization by using varying molar ratios of MA/TA/g-C₃N₄ with 4/4/1,4/4/2 and 4/4/4.The g-C₃N₄ was used as a supporting scaffold,its stratified structure provided a skeleton,the polymerization between MA and TA was mostly occurred on the g-C₃N₄ surface,which formed a porous spatial structure.Especially for MA/TA/g-C₃N₄（4/4/2）,the surface areas and pore volumes reached 540.36 m²·g^-1 and 1.502 cm³·g^-1,respectively.They had good thermal stability,and mainly composed of C and N elements.The zero potential of MA/TA/g-C₃N₄（4/4/2）was at pH=5.8,g-C₃N₄ was crystalline state and RAPOP was amorphous state.From the adsorption results,for MO and 2,4-DCP,the optimal pH values were 3 and 7,The best dosages were 5 mg and 10 mg,and the equilibrium times were 40seconds and 240 minutes,respectively.The presence of Na⁺and Cl^-ions had no adverse effects on adsorption process,while humic acid（HA）led to decrease slightly on 2,4-DCP adsorption.And they both agreed well with the pseudo-second-order model and Langmuir isotherm.At 298 K,the maximum adsorption capacity could reached 625.00 mg·g^-1 and270.27 mg·g^-1,respectively.Thermodynamic test indicated the adsorptions reacted spontaneously and endothermically.Thirdly,the polyimide（PI）blocked RAPOP were synthesized via polymerization by using varying molar ratios of MA/TA/pyromellitic anhydride（PMDA）with 4/3/1,4/2/2and 4/1/3.The characterization results displayed that PI-b-RAPOP were successfully synthesized and had a good thermal stability,and mainly consisted of C,N and O elements.PI and RAPOP particles were connected with each other in various ways to form a porous three-dimensional network structure.Compared with RAPOP,the agglomeration was greatly reduced,the specific surface area and total pore volume were observably increased,especially for MA/TA/PMDA（4/2/2）,they reached 487.27 m²·g^-1 and 1.169 cm³·g^-1,respectively.The adsorption results showed that the optimal pH values were 3 and 7,the best dosage were 5 mg and 10 mg,and the equilibrium time was 40 seconds and 240minutes for MO and 2,4-DCP,respectively.The presence of Na⁺and Cl^-ions had no adverse effects on the adsorptions,and they could be described well by the pseudo-second-order model and Langmuir isotherm.At 298 K,the maximum adsorption capacity could reached 636.94 mg·g^-1 and 282.49 mg·g^-1,respectively.The adsorption was spontaneous and endothermal.In summary,RAPOP itself had relative porous structures and adsorption properties.After its modification,the specific surface area was increased evidently,and the adsorption capacity was also greatly improved.Moreover,they all could maintain the high performance and stability after five adsorption desorption cycles.The adsorption of MO mainly depended on the strong electrostatic attraction between the protonated adsorbents and the MO anions,while theπ-πinteraction,hydrogen bonding interaction and pore trapping were mainly responsible for adsorption of 2,4-DCP.