Double-function ZnO/starch biodegradable hydrogel composite for methylene blue adsorption and photocatalytic degradation

An eco-friendly material for the removal of dyes from wastewater was developed. Biodegradable polymers (BP), cassava starch and poly(vinyl alcohol), were used to replace polyacrylamide. The hydrogel containing 50 wt% of BP (BP50) could absorb 34 times its dry weight of water. The hydrogel could adsorb Zn2+ and ZnO photocatalyst particles could be formed via a simple precipitation method. The incorporation of ZnO did not affect the adsorption efficiency of the ZnO/BP50 hydrogel composite towards methylene blue (MB). At initial concentrations (Co) below 4500 mg/g, the hydrogel composite removed ∼99 % of MB from solution in 3 h. The highest adsorption capacity of 1170 mg/g was obtained when Co was 6000 mg/g and at a dose of 0.10 g/20 mL. The hydrogel composite degraded 95 %-98 % of adsorbed MB at rates of 0.19 h-1 and 1.77 h-1 under UV irradiation and sunlight, respectively, with exposure times of 16 h for UV but only 2 h for sunlight. The material remained effective for at least 10 cycles of photodegradation under sunlight and removed 86 % of MB in solution on the 10th cycle. The composite also showed antibacterial activities and biodegradability in soil. These results indicated this material would not generate after-process toxic waste.

Fabrication porous adsorbents templated from aqueous foams using astragalus membranaceus and attapulgite as stabilizer for efficient removal of cationic dyes

The water-based foam stabilized by the natural surfactant applied in the fabrication of porous materials has attracted extensive attention, as the advantages of cleanness, convenience and low cost. Particularly, the development of a green preparation method has became the main research focus and frontier. In this work, a green liquid foam with high stability was prepared by synergistic stabilization of natural plant astragalus membranaceus (AMS) and attapulgite (APT), and then a novel porous material with sufficient hierarchical pore structure was templated from the foam via a simple free radical polymerization of acrylamide (AM). The characterization results revealed that the amphiphilic molecules from AMS adsorbed onto the water-air interface and formed a protective shell to prevent the bubble breakup, and APT gathered in the plateau border and formed a three-dimensional network structure, which greatly slowed down the drainage rate. The porous material polyacrylamide/astragalus membranaceus/attapulgite (PAM/AMS/APT) showed the excellent adsorption performance for cationic dyes of Methyl Violet (MV) and Methylene Blue (MB) in water, and the maximum adsorption capacity could reach to 709.13 and 703.30 mg/g, respectively. Furthermore, the polymer material enabled to regenerate and cycle via a convenient calcination process, and the adsorption capacity was still higher than 200 mg/g after five cycles. In short, this research provided a new idea for the green preparation of porous materials and the treatment of water pollution.

Synthesis and Characterization of Orange Peel Modified Hydrogels as Efficient Adsorbents for Methylene Blue (MB)

In recent years, due to the developments in the textile industry, water contaminated with synthetic dyes such as methylene blue (MB) has become an environmental threat based on the possible impacts in terms of chemical and biochemical demand, which leads to disturbance in aquatic plants photosynthesis, besides their possible toxicity and carcinogenicity for humans. In this work, an adsorbent hydrogel is prepared via free radical polymerization comprising acrylic acid (PAA) as a monomer and orange peel (OP) as a natural modifier rich in OH and COOH present in its cellulose and pectin content. The resulting hydrogels were optimized in terms of the content of OP and the number of cross-linkers and characterized morphologically using Scanning electron microscopy. Furthermore, BET analysis was used to follow the variation in the porosity and in terms of the surface area of the modified hydrogel. The adsorption behavior was found to follow pseudo-second-order as a kinetic model, and Langmuir, Freundlich, and Temkin isotherm models. The combination of OP and PAA has sharply enhanced the adsorption percent of the hydrogel to reach 84% at the first 10 min of incubation with an adsorption capacity of more than 1.93 gm/gm. Due to its low value of pHPZc, the desorption of MB was efficiently performed at pH 2 using HCl, and the desorbed OP-PAA were found to be reusable up to ten times without a decrease in their efficiency. Accordingly, OP-PAA hydrogel represents a promising efficient, cost-effective, and environmentally friendly adsorbent for MB as a model cationic dye that can be applied for the treatment of contaminated waters.

Removal of methylene blue from wastewater using ternary nanocomposite aerogel systems: Carboxymethyl cellulose grafted by polyacrylic acid and decorated with graphene oxide

In this study, environmentally-friendly nanocomposite hydrogels were fabricated. These hydrogels consisted of semi-interpenetrating networks of carboxymethyl cellulose (CMC) molecules grafted to polyacrylic acid (PAA), as an eco-friendly and non-toxic polymer with numerous carboxyl and hydroxyl functional groups, which were reinforced with different levels of graphene oxide particles (0.5, 1.5 or 3% wt). Field-emission electron scanning microscopy (FESEM) images indicated that the pore size of the nanocomposites decreased with increasing graphic oxide concentration. The presence of the graphic oxide increased the storage modulus and thermal stability of the nanocomposite hydrogels. The hydrogels had an adsorption capacity of 138 mg/g of a model cationic dye pollutant (methylene blue) after 250 min. Moreover, a reusability test showed that the adsorption capacity remained at around 90% after 9 cycles. Density functional theory (DFT) simulations suggested that the adsorption of methylene blue was mainly a result of π-π bonds, hydrogen bonds, and electrostatic interactions with graphene oxide. Our results indicated that the nanocomposite hydrogels fabricated in this study may be eco-friendly, stable, efficient, and reusable adsorbents for ionic pollutants in wastewater treatment.

Synthesis of a novel arginine-modified starch resin and its adsorption of dye wastewater

In this work, corn starch (St) was firstly grafted with polyacrylamide (PAM) to obtain StAM, which was subsequently immobilized with arginine to obtain a guanidine-containing starch-based resin, StAM–Arg. The synthesized products were characterized via Fourier transform infrared spectroscopy (FT-IR), ¹³C-NMR nuclear magnetic resonance (¹³C-NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). StAM–Arg exhibited a significantly enhanced adsorption capacity for acid fuchsin (AF), acid orange G (AOG), and acid blue 80 (AB80) compared with zeolite, diatomite, St and StAM, and it also exhibited broad-spectrum adsorption for different dyes. Weak acidic conditions were favorable for the resin to adsorb acid dyes. The decolorization rate (DR) by StAM–Arg for mixed wastewater reached 82.49%, which was higher than that of activated carbon (DR = 58.09%). StAM–Arg showed high resistance to microbial degradation, resulting in significantly improved structural stability for the resin. Its antibacterial rate (AR) for E. coli was up to 99.73%. After 7 days in simulated natural water, the weight loss ratio (WR) of StAM–Arg was 14.5%, which was much lower than that of St (WR = 66.53%). The introduced guanidine groups were considered to be the major reason for the observed improvements. Furthermore, the cationic guanidine could trap the acid dyes via ion-exchange reactions, while effectively inhibiting or eliminating the growth of bacteria on the adsorbent surface. The above advantages, including good dyestuff adsorption properties, high structural stability and prolonged service life, make StAM–Arg overcome the inherent drawbacks of the existing natural polymer adsorbents and have good application prospect in the treatment of textile wastewater.

In the side reaction, the two aldehyde groups in the glutaraldehyde molecules should undergo an aldol condensation reaction with the hydroxyl group in the starch molecule, which has been corrected.

Preparation of a poly(acrylic acid) based hydrogel with fast adsorption rate and high adsorption capacity for the removal of cationic dyes

A biocompatible Dex-MA/PAA hydrogel was prepared through copolymerization of glycidyl methacrylate substituted dextran (Dex-MA) with acrylic acid (AA), which was applied as the adsorbent to remove cationic dyes from aqueous solutions. Dex-MA/PAA hydrogel presented a fast adsorption rate and the removal efficiency of Methylene Blue (MB) and Crystal Violet (CV) reached 93.9% and 86.4%, respectively within one minute at an initial concentration of 50 mg L-1. The adsorption equilibrium data fitted the Sips isotherm model well with high adsorption capacities of 1994 mg g-1 for MB and 2390 mg g-1 for CV. Besides, dye adsorption occurred efficiently over the pH range 3-10 and the temperature range 20-60 °C. Moreover, the removal efficiencies for MB and CV were still >95% even after five adsorption/desorption cycles which indicates the robust nature of the Dex-MA/PAA hydrogel and its potential as an eco-friendly adsorbent for water treatment.

Evaluation of the structural morphology of starch-graft-poly(acrylic acid) on its scale-inhibition efficiency

The development of phosphorus-free and biodegradable scale inhibitors has been paid considerable attention. Two series of starch-graft-poly(acrylic acid) (St-g-PAA) samples with different grafting ratios and grafted-chain distributions, that is, the number and length of grafted PAA chains on the starch backbone, were designed and prepared in this study. Fourier transform infrared and ¹H nuclear magnetic resonance spectra were used to further characterize the molecular structures of the St-g-PAAs. In addition to dose, the effects of the structural morphologies of St-g-PAA, namely, grafting ratio and grafted-chain distribution, on the scale-inhibition performance against calcium carbonate were investigated systematically. Structural morphology significantly influenced the scale-inhibition performance of St-g-PAA. St-g-PAA with relatively low grafting ratio (≤97%) displayed better scale-inhibition effect than samples with similar grafted-chain distributions. Meanwhile, under the similar grafting ratios, samples with higher number of branched chains with shorter grafted chains displayed better antiscaling performance. Thus, higher scale-inhibition rate and lower corresponding optimal dose were obtained. Different scale-inhibition mechanisms were involved in the effects of the structural morphology. These mechanisms were investigated in detail from the molecular levels using scanning electron microscopy and X-ray diffraction.

Scale-inhibition and flocculation dual-functionality of poly(acrylic acid) grafted starch

Natural-polymer based water treatment agents have recently received much more attention due to their environmental friendliness, widespread availability, and prominent structural features. Starch-graft-poly(acrylic acid) (St-g-PAA) is a simple natural-polymer based material that can be obtained easily by a one-step graft copolymerization. When used as a water treatment agent, St-g-PAA exhibits both effective scale-inhibition performance and high turbidity removal efficiency. The scale-inhibition efficiency of St-g-PAA against calcium carbonate (CaCO₃) is approximately 94% at the optimal dose in a static test of approximately 40 mg/L. Dispersion, crystal lattice distortion, and chelating effects all play important roles in the scale inhibition. When St-g-PAA is used as a coagulant aid for polyaluminum chloride (PAC) in the flocculation of a real hairwork wastewater, the highest reduction of the optimal PAC dose is more than 30% while the turbidity reduction is about 97% at the same time, both floc size and compactness increase, and the final settling efficiency also improves evidently. The efficient bridging flocculation effects account for the effective turbidity removal. The prominent scale-inhibition and flocculation dual-functionality of St-g-PAA is intrinsically ascribed to its distinct anionic linear branched-chain structure.