Elsevier

Chemical Engineering Journal

Volume 178, 15 December 2011, Pages 60-68
Chemical Engineering Journal

Preparation of macroporous bead adsorbents based on poly(vinyl alcohol)/chitosan and their adsorption properties for heavy metals from aqueous solution

https://doi.org/10.1016/j.cej.2011.10.012Get rights and content

Abstract

A novel macroporous bead adsorbents based on poly(vinyl alcohol)/chitosan (PVA/CS beads) were prepared, characterized and were used for the adsorption of heavy metal ions from aqueous solution. The resulting PVA/CS beads were perfectly spherical in shape and exhibited good mechanical strength and chemical stability. PVA/CS beads with macroreticular structure had been proved with easy separation and excellent adsorption for heavy metal ions. PVA/CS beads adsorbed heavy metal ions in the following order: Cu2+ > Pb2+ > Cd2+ > Zn2+. The equilibrium data of Cu2+, Pb2+ and Cd2+ conformed to the Freundlich isotherm, while those of Zn2+ were best described by Langmuir isotherm. The presence of NaNO3 (0–0.137 mol/L) had little effect on Cu2+ adsorption, but the adsorption of Pb2+, Zn2+ and Cd2+ decreased significantly in the same conditions. The competitive adsorption showed that PVA/CS beads had higher adsorption selectivity for Cu2+ with the coexistence of Pb2+, Zn2+ or Cd2+. The adsorption kinetics of Cu2+, Pb2+, Zn2+ and Cd2+ were found to follow the pseudo-second-order kinetic model. Various thermodynamic parameters were calculated and the results showed that the adsorption of all metal ions onto PVA/CS beads was feasible and endothermic in nature. The results from the sequential adsorption–desorption cycles showed that PVA/CS bead adsorbents held good desorption and reusability, which would be a potential application in the fixed-bed continuous-flow column for the removal of heavy metals.

Highlights

► Novel PVA/chitosan (CS) composite beads were successfully prepared by IPN and cross-linking. ► PVA and CS exhibited a synergistic effect on heavy metals removal from wastewaters. ► PVA/CS beads with macroporous structure showed good mass transfer performance. ► The novel beads were easily separated and reused, which reduced the operation cost. ► The resulting bead was a low-cost, efficient, eco-friendly and reusable adsorbent.

Introduction

Water pollution by heavy metals is a worldwide environmental problem and results in serious threats to ecosystem and human health. Heavy metals can be distinguished from other toxic pollutants, since they are not biodegradable and can be accumulated in living tissues, causing various diseases and disorders even at very low concentrations. These toxic heavy metals, especially Pb2+ and Cd2+, can be accumulated in bone, brain, kidney and muscles and may cause brain, bone, liver and kidney damage, and dysfunction of the central nervous system in human beings due to their cumulative effects [1], [2], [3]. Although Cu2+ and Zn2+ are known to be the essential trace elements to humans, but higher Cu2+ and Zn2+ intake can also cause adverse effects [4], [5]. The maximum acceptable concentration of Pb2+, Cd2+ and Cu2+ recommended by the World Health Organization (WHO) for drinking water is less than 0.01, 0.003 and 2 mg/L [6], respectively, while that of Zn2+ for drinking-water at levels above 3 mg/L may not be acceptable to consumers. Therefore, removal of these toxic heavy metal ions before discharging them into the receiving systems is essential from the standpoint of environment protection and public health.

Adsorption is one of the most effective and economical methods for the removal of heavy metals from aqueous solutions. In recent years, many types of adsorbents including activated carbon [3], chitosan [4], fly ash [5], zeolite [7], perlite [8], kaolinite clay [9], activated alumina [10], grape stalk waste [11], rice husk [12], tea waste [13], various resins [14], [15] and microorganisms [16], [17] have been used to remove various heavy metal ions from aqueous solutions. However, most of these materials are either prepared in certain conditions, e.g., high temperature, controlled pressure and additional chemicals which increase operation costs or of low efficiency, poor mechanical strength and difficult separation from reaction system. Therefore, there is a need to develop low cost, easily available, effective and reusable adsorbents for the removal of heavy metal ions from the aqueous environment.

Poly(vinyl alcohol) (PVA) is a water-soluble material containing large amounts of hydroxyl groups. PVA has been widely applied because it has many advantages such as low cost, non-toxicity, biocompatibility, high durability and chemical stability [18], [19]. In our previous study, a new, economical and eco-friendly way to prepare macroporous PVA beads has been developed in our lab to immobilize microorganism for wastewater treatment [20]. The macroporous PVA-based beads also showed a good removal for heavy metals from aqueous solution [21]. However, it will be very significant if nitrogen with much better coordination property is introduced into PVA materials. In this work, a novel spherical composite of PVA and chitosan (CS) with macroreticular structure, i.e. PVA/CS bead adsorbents, was prepared by the interpenetrating polymer network (IPN) and crosslinking process. It is expected that the hydroxyl and amino groups present in the PVA/CS bead adsorbents would exhibit a synergistic effect on removal of heavy metals from wastewaters. The common metallic pollutants, Cu2+, Pb2+, Cd2+ and Zn2+, were selected as adsorbates. Effects of pH, initial metal concentration, contact time, temperature, ionic strength and competing ions were studied in details for process design and optimization. The Langmuir and Freundlich isotherms were used to evaluate the equilibrium data. The adsorption kinetics were also studied based on the pseudo-first and pseudo-second-order kinetic models. Thermodynamic parameters were also computed. Finally, desorption efficiency and reusability of the adsorbents were investigated based on five consecutive adsorption–desorption cycles.

Section snippets

Materials

Poly(vinyl alcohol) (PVA) with a degree of polymerization of 1750 and an alcoholysis degree higher than 99% was purchased from Lanzhou Vinylon Factory (Gansu, China). Chitosan (CS) with a degree of deacetylation of 90% (80 mesh) was purchased from Yuhuan Ocean Biology Company (Zhejiang, China). All other chemicals were of analytical reagent grade. Distilled water was used to prepare all the solutions.

Preparation of macroreticular PVA/CS bead adsorbents

The macroreticular PVA/CS bead adsorbents were prepared as follows: PVA, sodium alginate (1.3 g)

Characterization of PVA/CS bead adsorbents

In the absence of sodium alginate, the formed beads had a strong tendency to agglomerate and were very difficult to break. In order to form spherical beads, a mixed solution of PVA and sodium alginate and a mixed solution of boric acid and CaCl2 were used in this study. The introduction of sodium alginate could improve the surface properties of the beads by eliminating the agglomeration problem. Water regain factor for I, II, III, IV, V, VI and VII was 85.08, 77.64, 75.42, 77.72, 81.55, 75.29,

Conclusion

Poly(vinyl alcohol)/chitosan (PVA/CS) composite beads were synthesized, characterized and were used for the adsorption of Cu2+, Pb2+, Zn2+ and Cd2+ from aqueous solution. PVA/CS bead adsorbents not only possessed good chemical stability, but also exhibited excellent adsorption ability for Cu2+, Pb2+, Cd2+ and Zn2+. Some factors affecting the adsorption such as pH, initial metal concentration, contact time, temperature and ionic strength were studied. The equilibrium data of Cu2+, Pb2+ and Cd2+

Acknowledgements

The authors gratefully acknowledge financial supports from the National Major Specific Program of Science and Technology on Controlling and Administering of Water's Pollution (2009ZX07212-001-04), the Sustentation Program of Science and Technology of China (2006BRD01B03).

References (43)

Cited by (0)

View full text