Preparation of Freeze-Dried Porous Chitosan Microspheres for the Removal of Hexavalent Chromium

Wei Song,Jian Xu,Jin Tong,Qingzhu Zhang,Lepeng Gao,Lili Ren

doi:10.3390/app11094217

Abstract

Novel porous chitosan microspheres were successfully produced by a freezing–lyophilization drying method in this study and were then used as adsorbents to remove a toxic iron metal, hexavalent chromium (Cr(VI)). The effects of the concentration of the chitosan solution, syringe diameter, and freezing time on the morphologies of porous chitosan microspheres were characterized. The metal ion adsorption for Cr(VI) was also studied. Results showed that freezing chitosan hydrogel beads at a temperature of −20 °C and subsequently lyophilizing the frozen structure allowed to easily obtain the porous chitosan microspheres with rough surfaces and large pores, which were more suitable for adsorption materials to remove metal ions. A chitosan solution concentration of 3% (w/v) and a syringe diameter of 500 μm allowed the porous microspheres to have a good sphericity, thinner pore walls, and small pore sizes. The adsorption capacity of porous chitosan microspheres for Cr(VI) increased with the increase in freezing time. The pH of the initial adsorption solution ranged from 3.0 to 5.0 and was beneficial to the maximum adsorption efficiency for Cr(VI). The porous chitosan microspheres prepared with 3% (w/v) chitosan solution at −20 °C for a freezing time of 72 h had a higher adsorption capacity of 945.2 mg/g for Cr(VI) than the those at 24-h and 48-h freezing times. Kinetic study showed that the adsorption process could be described by a pseudo-second order (PSO) kinetic model. The equilibrium adsorption rate constant and the adsorption amount at equilibrium for the porous chitosan microspheres increased with an increase in the freezing time, and those for the porous microspheres prepared with 3% chitosan solution at −20 °C for a 72-h freezing time were 1.83 × 10−5 g mg−1 min−1 and 1070.5 mg g−1, respectively. The porous chitosan microspheres have good potential to facilitate the separation and recycling of expensive and toxic Cr(VI) from wastewater.

Highlights

The treatment of industrial effluents has become a major concern of humanity, causing extensive losses to society, ecology, and the economy
The increase in freezing time resulted in an increase om Cr(VI) adsorption capacity
The chitosan microspheres prepared with 3% (w/v) chitosan solution at –20 ◦ C for 72 h had a higher adsorption capacity of

Summary

Introduction

The treatment of industrial effluents has become a major concern of humanity, causing extensive losses to society, ecology, and the economy. Large amounts of chromium enter the environment through various industrial effluents, such as chrome ore processing, metal surface treatment, leather tanning, printing, and dyeing, existing in the form of trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)), and are absorbed and accumulated by humans, which can enter drinking water-supply systems via corroding water pipes or containers, or enter groundwater via landfills. Adsorbents are the key in the treatment of heavy metal ion wastewater by adsorption. Due to the advantages of having available raw materials, being renewable and non-toxic, having a low cost, their biodegradability, as well as having diverse active groups and easy modification, natural polymer materials and their derivatives have been prepared using green and environmentally friendly adsorption materials for the heavy metal ion treatment of wastewater [11,12]

Objectives

Methods

Results

Conclusion