Assembly of Polyacrylamide-Sodium Alginate-Based Organic-Inorganic Hydrogel with Mechanical and Adsorption Properties

Abstract

Hydrogels have been widely used in water purification. However, there is not much discussion and comparison about the effects of different nanofillers on the reinforcement and adsorption performances of hydrogels, which can be subjected to rapid water flow and possess strong adsorption ability. In this work, polyacrylamide (PAAM)-sodium alginate (SA) interpenetrating polymer network-structured hydrogels were prepared by in situ polymerization. PAAM formed the first flexible network and SA constructed the second rigid network. Three kinds of inorganic nanoparticles including carbon nanotubes (CNTs), nanoclays (NCs), and nanosilicas (NSs) were incorporated into a PAAM-SA matrix via hydrogen bond. The obtained hydrogels exhibited a macroporous structure with low density (≈1.4 g/cm³) and high water content (≈83%). Compared with neat PAAM-SA, the hydrogels with inorganic nanoparticles possessed excellent mechanical strengths and elasticities, and the compression strength of PAAM-SA-NS reached up to 1.3 MPa at ε = 60% by adding only 0.036 g NS in a 30 g polymer matrix. However, CNT was the best filler to improve the adsorption capacity owing to its multi-walled hollow nanostructure, and the adsorption capacity of PAAM-SA-CNT was 1.28 times higher than that of PAAM-SA. The prepared hydrogels can be potential candidates for use as absorbents to treat wastewater.

Keywords: adsorption properties; hydrogel; inorganic nanofillers; interpenetrating polymer network; mechanical behavior.

Figure 1

The morphology of hydrogels: (a) schematic illustration of the hydrogel fabrication; (b) SEM photographs of freeze-dried interpenetrating polymer network (IPN)-structured hydrogels; amplified view of (c) carbon nanotubes (CNTs), (e) nanoclays (NCs), and (g) nanosilicas (NSs) distributed in polymer matrix; photographs of (d) PAAM-SA-CNT, (f) PAAM-SA-NC and (h) PAAM-SA-NS; (i) appearance of CNT, NC, NS suspensions after 2 h of settling; (j) water content and density of hydrogels.

Figure 2

The morphology of inorganic nanoparticles: TEM pictures of (a) CNT, (b) NS, and (c) NC, respectively; the dimension histogram of (d) CNT, (e) NS, and (f) NC, respectively; and (g) the statistical parameters in Gaussian distribution for inorganic nanoparticles.

Figure 3

(a) FTIR spectra of PAAM-SA, PAAM-SA-CNT, PAAM-SA-NC, PAAM-SA-NS; (b) XPS spectrum of nanoclay; XPS spectra of curve-fitted (c) O1s and (d) Al2p peaks.

Figure 4

Schematic illustration of (a) the 3D network of the PAAM-SA-based hydrogels; (b) the formation of the PAAM and MBA network, the SA and Ca²⁺ network, and the interaction between the PAAM and SA networks; (c) the formation of hydrogen bonding in PAAM-SA-NC; and (d) the intermolecular bonding between the PAAM-SA matrix and the inorganic nanofillers.

Figure 5

Compression stress–strain behavior of hydrogels: (a) compression plots; (b) energy absorption curves; (c) hysteresis of hydrogels for five cycles; and (d) stresses at 50% strain under five successive loadings.

Figure 6

Dynamic viscoelasticity performance of the hydrogels at 25 °C: (a) strain dependence of storage modulus of hybrid hydrogels and 0.1% inorganic nanoparticle suspension at frequency of 1.0 Hz; (b) frequency dependence of storage and loss modulus of hydrogels; (c) frequency dependence of storage and loss modulus of 0.1% inorganic nanoparticle suspension; and (d) schematic illustration of the mechanism of fracture and rebuilding of hydrogen bonds.

Figure 7

The adsorption capacity of hydrogels: (a) Langmuir adsorption isotherms and experimental results of Cu²⁺ on hydrogels; (b) Freundlich adsorption isotherms and experimental results of Cu²⁺ on hydrogels; (c) linearized Langmuir isotherms determined from 1/q_e versus 1/c_e; and (d) mechanism of Cu²⁺ adsorption on hydrogels.

Figure 8

Four adsorption–desorption cycles of Cu²⁺ on the prepared hydrogels.