Polyacrylamide-Phytic Acid-Polydopamine Conducting Porous Hydrogel for Efficient Removal of Water-Soluble Dyes

Removal of toxic dyes from wastewater has become a hot topic in both academic and industrial fields since there is growing concern about the threat of sewage to human health. Herein, we demonstrate that the three-dimensional porous polyacrylamide-phytic acid-polydopamine (termed as PAAM/PA/PDA) hydrogel can be served as reusable adsorbent with high efficiency for either anionic or cationic dyes. Using methyl blue (MB), methylene blue (YMB), methyl violet (MV) and neutral red (NR) as model dyes, we investigate the effect of pH, temperature, dye concentration, and PAAM/PA/PDA hydrogel mass on the adsorption. The experimental maximum adsorption capacities are more than 350.67 mg g−1 for four selected dyes. Adsorption kinetic and thermodynamic analysis suggests that the dyes are adsorbed on the PAAM/PA/PDA hydrogel through the strong π-π stacking and anion-cation interaction, and the adsorption process satisfies a pseudo-second-order model. Furthermore, the free-standing PAAM/PA/PDA hydrogel can be easily removed from water after adsorption process, and regenerated by adjusting solution pH values.

Optimization and evaluation of dye adsorption capacity. The effects of temperature, pH value, adsorbent dosage and initial concentration of dyes on the adsorption capacity have been investigated. The surface charge of the PAAM/PA/PDA hydrogel is strongly dependent on the solution pH value and the zero point charge (pzc) of PAAM/PA/PDA hydrogel is calculated as pH 5.3 (as shown in Fig. S2). As shown in Fig. 1, the negatively charged PAAM/PA/PDA hydrogel exhibits high equilibrium absorption capacities for cationic dyes (NR, MV, and YMB) while positively charged PAAM/PA/PDA hydrogel has high equilibrium absorption capacity for anionic dye (MB). The maximum adsorption capacities were 317.05 mg g −1 for NR at pH 5.5, 161.18 mg g −1 for MB at pH 4.5, 314.55 mg g −1 for MV at pH 10.0 and 242.46 mg g −1 for YMB at pH 10.0, respectively. The experimental result indicates that electrostatic interaction plays important role in the dye adsorption. The phenomenon is consistent with the literature report 34 .
As shown in Fig. S3, the equilibrium absorption capacities are decreased from 1008.65 to 298.64 mg g −1 for NR, 393.25 to 168.82 mg g −1 for MB, 810.67 to 251.30 mg g −1 for MV and 553.37 to 224.93 mg g −1 for YMB with increasing PAAM/PA/PDA hydrogel dosage from 2.5 to 10.0 mg, respectively, while the concentrations of the dyes are kept as constants. In contrast, the adsorption efficiencies are increased from 65.5% to 92.5% for NR, 51.8% to 93.2% for MB, 51.3% to 89.3% for MV and 54.9% to 91.5% for YMB with increasing hydrogel dosage from 2.5 to 10.0 mg, respectively. This phenomenon can be explained by adsorption equilibrium between dye and PAAM/ PA/PDA hydrogel [35][36][37] . Large dosage of PAAM/PA/PDA hydrogel can provide high amount of adsorption sites, resulting in decreased significantly the concentration of free dye in the solution. The utilization rate of adsorption sites is decreased by the low concentration of free dye, which leads to low equilibrium absorption capacity. Figure S4 demonstrates the effect of initial concentrations of dyes on the equilibrium absorption capacity while dosage of PAAM/PA/PDA hydrogel is kept as constant. As can be seen, with the increasing concentrations of dyes from 5 to 50 mg mL −1 , the maximum adsorption capacities of PAAM/PA/PDA hydrogel are increased to 566.61, 381.35, 529.96 and 341.29 mg g −1 for NR, MB, MV and YMB while the adsorption efficiencies are decreased to 91.4%, 92.1%, 87.9% and 90.2% for NR, MB, MV and YMB, respectively. This phenomenon can be explained as the high driving force for mass transfer and the saturation of the active adsorption sites, respectively 36,37 .

Discussion
It is well known that the rate of dye adsorption depends on the contact time of the solid and liquid as well as on the diffusion processes 50 . During adsorption process, dye molecules migrate to the surface of the PAAM/PA/PDA hydrogel firstly, and then diffuse in the boundary layer and eventually diffuse from the hydrogel surface into the internal sites via pore diffusion. The pseudo-first-order, pseudo-second-order and intra-particle diffusion models were investigated for analyzing the adsorption kinetics (see Supporting Information 2.2 for details). The fits of the experimental results (as shown in Tables S2,S3) show that the pseudo-second-order model possesses a higher R 2 value, compared with the pseudo-first-order model (as shown in Fig. S6). This phenomenon indicates that the adsorption kinetics can be described as a pseudo-second-order model (as shown in Fig. 2) which depends on the amount of dyes adsorbed on the surface of hydrogel and the amount adsorbed at equilibrium 36 . Figure S7 presents the intra-particle diffusion kinetics model of Weber-Morris 51 , and the fits of the experimental results are given in Table S4. There are two linear slopes in each curve, suggesting that at least two steps are involved during adsorption process. The first linear slope represents boundary layer diffusion attributed to the transfer of dye to the surface of the PAAM/PA/PDA hydrogel. The second linear slope indicates a steady adsorption step, corresponding to intra-particle diffusion of dye molecules throughout the porous surface of the hydrogel 51,52 . In addition, there is the thickness of the boundary layer since the curves possess intercepts 53 . The presence of multilinearity and the boundary layer thickness suggests that surface adsorption may also play significant role in the dye uptake process in combination with intra-particle diffusion model.
The thermodynamic parameters of dyes on PAAM/PA/PDA hydrogel were analyzed by Arrhenius equation and Eyring equation (see Supporting Information 2.3 for details). As shown in Fig. 3a energies are higher than activation energy of physical adsorption process (c.a., 5-40 kJ mol −1 ), suggesting that the dye are interacted with PAAM/PA/PDA hydrogel through chemical adsorption (e.g., π-π stacking) 20 Fig. S8), indicating the strong π-π stacking interaction between hydrogel and dyes 55 . As shown in Fig. S9, after adsorbed with dyes, the C = C stretching peak of PAAM/PA/PDA hydrogel is changed from 1674 to 1641 cm −1 for NR, 1674 to 1647 cm −1 for MB, 1674 to 1642 cm −1 for MV, and 1674 to 1646 cm −1 for YMB, respectively, also indicating that π-π stacking interaction is involved in adsorption process.  Semi-empirical PM6-D3H4 method was employed to further figure out the adsorption mechanism between the PAAM/PA/PDA hydrogel and dye molecules by using Mopac2012 program since weak interactions in molecular system includes hydrogen bonding, dipole-dipole interactions, π-π interactions, steric repulsion and London dispersion 56,57 . The gradient isosurfaces were colored according to the corresponding values of sign(λ 2 )ρ 58 , which is found to be a good indicator of interaction strength. The gradient isosurfaces provide a rich visualization of noncovalent interactions as broad regions of real space, rather than simple pairwise contacts between atoms. The reduced density gradient (s = 0.5 au) of the dyes-PAAM/PA/PDA hydrogel generated by program Multiwfn 59 . The surfaces are colored on a blue-green-red scale according to values of sign(λ 2 )ρ, ranging from −0.04 to 0.02 au. The low-gradient isosurfaces for dye molecules adsorbed to PAAM/PA/PDA hydrogel shows the complex webs of noncovalent interactions between the hydrogel and active sites (as shown in Fig. 4). The anion-cation interactions exist in all the dyes-PAAM/PA/PDA hydrogel labeled by red circle, but only part of them can be seen from the Fig. 4. Strong π-π stacking interactions between the dye molecules (NR and MV) and PAAM/PA/PDA hydrogel are clearly manifested by isosurfaces filling the interlayer spaces (as shown in Fig. 4a,c, labeled by black circle), which are stronger than the π-π stacking interactions between MB and hydrogel (as shown in Fig. 4b). As shown in Fig. 4d, steric clashes have been offset by stronger π-π stacking effect and anion-cation interactions forming the YMB-PAAM/PA/PDA hydrogel structure.
For testing it regeneration ability, the adsorption and desorption of dye with the PAAM/PA/PDA hydrogel was performed (as shown in Fig. 5 and Fig. S10) and summarized in Fig. 6. The dye desorption can be easily achieved by adjusting solution pH values (i.e., pH 2.0 for cationic dye (NR, MV and YMB) and pH 11.0 for anionic dye (MB)). After 7 times of adsorption-desorption, there are negligible changes of the equilibrium absorption capacities (as shown in Fig. 5). This result indicates that the dye removal and recovery were stable. The adsorption/desorption of dyes on the PAAM/PA/PDA hydrogel can be further proved by DPV curves using the PAAM/PA/PDA hydrogel-based electrochemical sensor since the dyes have electrochemical activity. As shown in Fig. 7 and Fig. S11, the peak currents are increased with increasing concentration of dyes. The different dye molecules give different positions of current peaks (−0.60 V for NR, −0.45 V for MB, −0.65 V for MV, and −0.25 V for YMB), respectively, which are agreement with the literature reported values [61][62][63][64] . After treated NR, MV, YMB at pH 2.0 and MB at pH 11.0, no current peaks appear on the DPV curves, indicating that the dyes are desorbed from the hydrogel. Furthermore, the dyes can be sensitively detected by the electrochemical sensor with a low detection limit (5 mg L −1 , S/N = 3) and wide linear range (from 5 mg L −1 to 750 mg L −1 ) (as shown in Fig. 7), indicating that the electrochemical sensor could be used to monitor dye molecules with reasonable assay performance.  In summary, we demonstrate that the PAAM/PA/PDA hydrogel can be used as high efficiency and pH sensitive broad spectrum adsorbent for organic dyes. The high efficiency of the hydrogel is attributed to the abundant functional groups and the three-dimensional porous structure. The PAAM/PA/PDA hydrogel exhibits high adsorption capabilities for a family of dyes with aromatic structure. The adsorption capacities toward four selected dyes are higher than most of reported adsorbents. In particular, the good regeneration highlights the utility of PAAM/PA/PDA hydrogel for environmental remediation.  Table S1. All other reagents (analytical grade) used without further purification were purchased from Beijing Chemical Reagents Co. (Beijing, China). All dye aqueous solutions were prepared by dissolving a certain amounts of dyes in deionized water. Milli-Q water (18.2 MΩ cm) was used in all experiments. The pH value of water was adjusted by 1.0 mol L −1 HCl and/or 1.0 mol L −1 NaOH.

Materials and reagents.
Characterization and instruments. The Fourier Transform Infrared (FTIR) spectra were recorded with a Bruker Vertex 70 FTIR spectrometer (Bruker, Germany). The pH value was determined by Orion 4-Star Plus pH/ISE Benchtop Multiparameter Meter (Shanghai, China). The zeta potential of PAAM/PA/PDA hydrogel was determined in the pH range from 1 to 10 using Zetasizer Nano ZS (Malvern, UK). The absorbance of dyes at maximum absorption wavelength were recorded using the XS2 microplate reader (BioTek, USA), and the concentrations of the dyes were defined corresponding to calibration curves, respectively. The differential pulse voltammetry (DPV) measurements were performed on a CHI 660D electrochemical workstation (Shanghai, China) with a desired potential range. A conventional three-electrode system was used with an Ag/AgCl (saturated KCl) electrode as the reference electrode, a platinum sheet as the counter electrode and glassy carbon electrode (GCE, 3 mm in diameter) modified with the PAAM/PA/PDA hydrogel film as the working electrode. Dye adsorption and desorption studies. The PAAM/PA/PDA conductive hybrid hydrogel was synthesized according to our previously reported strategy (see Supporting Information 1.1 for details) 21 . Dye adsorption experiments on the PAAM/PA/PDA hydrogel were performed by mixing 8 mL dye solutions (50 mg L −1 ) with 5 mg PAAM/PA/PDA hydrogel in a water bath at desired temperatures (285, 298, 308 and 318 K), respectively. In order to monitor the changed amount of dyes in solution, 20 μL supernatants were continuously taken out from the mixture at appropriate time intervals (0, 2,5,8,10,15,20,30,45,60,90,120,180,240, 300, 400, 600 and 720 min), respectively. After diluted with 180 μL water, the maximum absorbance of supernatants were measured for calculating the adsorbed amounts of dyes. After the adsorption process, the hydrogel was moved out from water solution, transferred into 50 mL desorption solution, incubated for 12 hours and washed with 30 mL water (3 times, 30 minutes every time). After dried at 75 °C, the recovered PAAM/PA/PDA hydrogel was ready for next cycle adsorption. The adsorption-desorption procedure was repeated 7 times.
For studying the effect of pH on adsorption capacity, 8