A novel indole-based conjugated microporous polymer for highly effective removal of heavy metals from aqueous solution via double cation–π interactions

A novel indole-based conjugated microporous polymer (PTIA) with three coplanar indole units, designed and synthesized by an oxidative coupling reaction, was utilized as a platform for removing heavy metals. Owing to the conjugation of the three coplanar indoles, the highly electron-rich large π planes can simultaneously attract six heavy metal atoms via double cation–π interactions, endowing this microporous material with remarkable heavy metal adsorption capacity and efficiency.


Main materials and measurements
were supplied by Aladdin. The rest of the materials and reagents were obtained from different commercial sources and used without further purification.
FT-IR spectrum was recorded on a Nicolet 6700 FTIR spectrometer. Solidstate cross-polarization magic-angle-spinning (CP/MAS) NMR spectra were recorded on a Bruker Avance III 400 NMR spectrometer. 1 H NMR and 13 C NMR spectra were recorded using Bruker AMX600 MHz NMR spectrometers in DMSO-d 6 . The elemental analysis characterization technique was performed using a Vario EL III apparatus. UV-Visible spectroscopies were analyzed on the UV-3150 instrument. Scanning electron microscopy (SEM) was recorded on an S-4800 (Hitachi Ltd) field emission scanning electron microscope. Morphological observation was performed with a Tecnai G2 F20 S-TWIN (FEI Company) transmission electron microscope. Digital photos were taken using a Cannon 600D camera. The nitrogen adsorption and desorption isotherms were measured at 77 K using a Autosorb IQ instrument. The sample was treated at 120 °C for 24 h before the measurement. The surface area was calculated by Brunauer-Emmett-Teller S3 (BET) equation (0.01 < P/P0 < 0.1). The pore-size-distribution (PSD) curve was obtained from the adsorption branch using non-local density functional theory (NL-DFT) method. The total pore volume (V total ) and micropore volume (V micro ) were estimated from the adsorbed amount at a relative pressure P/P 0 of 0.995 and 0.1, respectively. The metal ion concentrations in solutions before and after adsorption were measured using ICP-AES (Jarrel-ASH, ICAP-9000) and ICP-MS

Heavy metal uptake experiments
The heavy metal uptakes from aqueous solutions with various concentrations were Regeneration study: After the equilibrium study, metal-loaded PTIA was collected and washed with deionized water and dried to the constant weight. Then the adsorbents (0.1 g) were shaken with 50 ml of HCl (1 mol L -1 ) in 250 mL Erlenmeyer flasks at 150 rpm on an orbital shaker at 25°C for 2 h. After washing with deionized water, the treated PTIA was ready for the next use.

Synthesis and characterization of TAT
TAT molecule was prepared with a slight modification to what was published in previous literature. [1] A mixture of 2-indolinone (2.0 g, 15 mmol) and 10 ml of POCl 3 was heated at 100°C for 8 h. Then, the reaction mixture was poured into ice and neutralized carefully with NaOH until pH 7-8. After neutralization, the precipitate was filtered to give the crude product as a brown solid. The crude solution in methanol (MeOH) was absorbed on silica-gel, dried, loaded and eluated through a thick silica-gel pad with a dichloromethane (DCM) as a mobile phase.

Simulation method
The Density functional theory (DFT) calculation was utilized to investigate the adsorption mechanism. All the calculations reported here were fulfilled with Materials Studio DMol3 program (Accerlrys. USA). [17][18][19] For the exchange correlation term of the energy functional, the generalized gradient corrected functional GGA and PW91 functional as implemented, were applied for all the geometry optimizations. [18] The