Abstract
Tetraoxy-anion of iron in +6 oxidation state (FeVIO42−, FeVI), commonly called ferrate, has shown tremendous potential as a green oxidative agent for decontaminating water and air. Encapsulation of solid potassium salt of ferrate (K2FeO4) circumvents the inherent drawbacks of the instability of ferrate under humid conditions. In the encapsulated strategy, controlled release without exposing the solid ferrate to the humid environment avoids self-decomposition of the oxidant by water in the air, and the ferrate is mostly used to decontaminate water efficiently. This study demonstrated the formulation of oxidative microcapsules with natural materials present in chitosan, whose release rate of the core material can be controlled by the type of intermediate hydrocarbon layer and the pH-dependent swelling of chitosan shell. The pH played a pivotal role in swelling chitosan shell and releasing the core oxidant. In a strong acidic solution, chitosan tended to swell quickly and release FeVI at a faster rate than under neutral conditions. Additionally, among the several long-chain hydrocarbon compounds, oleic acid exhibited the strongest “locking” effect when applied as the intermediate layer, giving rise to the slow release of FeVI. Coconut oil and mineral oil, in comparison, allowed FeVI to penetrate the layer within shorter lengths of time and showed comparable degrees of degradation of target contaminant, methylene orange, under ambient temperature and near-neutral conditions. These findings have practical ramifications for remediating environmental and industrial processes.
Highlights
The natural abundance of iron renders the iron-based technologies a highly desirable approach for decontaminating the environment because they do not introduce synthetic material or harmful by-products foreign to natural environment[1,2,3,4,5]
For all challenged methyl orange concentrations, K2FeO4/mineral oil (MO) system consistently produced higher removal rates than K2FeO4/coconut oil (CO), with the gap widening as the initial concentration of methyl orange increased. These results suggest two important properties: (i) The extent of methyl orange removal reflects the FeVI storage capacity displayed in Fig. 1, which shows an enhanced retainment with MO as compared to CO. (ii) Noting the rates of the methyl orange removal only marginally less than those obtained with direct FeVI oxidation, both MO and CO were capable of retaining a large fraction of the oxidative power of FeVI, exhibiting the suitability to act as the buffering medium of choice when synthesizing FeVI capsules
We have successfully demonstrated the potential to use a natural material in chitosan as a wall material to form a shell layer to encapsulate FeVI, separated by another hydrocarbon layer protecting chitosan to be directly attacked by its ferrate content
Summary
The natural abundance of iron renders the iron-based technologies a highly desirable approach for decontaminating the environment because they do not introduce synthetic material or harmful by-products foreign to natural environment[1,2,3,4,5]. Chitosan is chemically stable (e.g., high thermal stability up to 280 °C) and owns a distinct advantage over other encapsulating agents: the possibility to establish covalent or ionic bonds with the crosslinking agents, building a network structure to retain the active substance. These chemical bonds carry advantages regarding controlled release[36,37]. Crosslinked microparticles form non-permanent and reversible networks, allowing chitosan microparticles to exhibit a higher swelling sensitivity to pH changes as compared to the covalently crosslinked counterpart This property extends its potential application since dissolution can occur in extremely acidic or basic pH conditions[38]. The specific objectives of the current paper are: (i) to evaluate the effectiveness of various types of oleochemical (oleic acid and coconut oil) and petrochemical (mineral oil) agents as the buffering medium; (ii) to examine the pH effect on the release behavior of FeVI from microencapsulation; and (iii) to demonstrate the functionality of the encapsulated FeVI to remove contaminants by investigating degradation of methyl orange as the model contaminant
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