Article
Mild oxidative degradation of spent auricularia auricular substrate and molecular composition of carboxylic acids in the resulting soluble portion

https://doi.org/10.1016/j.cjche.2019.07.014Get rights and content

Highlights

  • SAAS can be degraded to produce CAs, which consists of aliphatic acids and aromatic acids.

  • Aliphatic acids represent the predominant species, and NADAs are the most abundant.

  • AHPO/AAH ratio and temperature have impact on the composition and yield of CAs.

  • radical dotOH, CH3COOradical dot, and HOOradical dot play crucial roles in the SAAS oxidative degradation.

Abstract

Spent auricularia auricular substrate (SAAS) was oxidatively degraded with aqueous hydrogen peroxide (AHPO)/acetic anhydride (AAH) to produce carboxylic acids (CAs) under mild conditions. The results show that up to 53.6% of the organic matter in SAAS was converted to the soluble species (SSs). In total 122 CAs were detected in the SSs by the analysis with a gas chromatograph/mass spectrometer, which can be classified into 29 group components, mainly being aliphatic acids and along with small amount of aromatic acids. Among the aliphatic acids, normal alkanedioic acids are the most abundant. The detected aromatic acids include benzoic acids, phthalic acids, trimellitic acids, pyromellitic acids, and their derivatives. The synergistic oxidation and the released radical dotOH, CH3COOradical dot, and HOOradical dot induced by AHPO/AAH play crucial roles in oxidatively degrading SAAS.

Introduction

Spent auricularia auricular substrate (SAAS) is a media residue after artificially growing auricularia auricular with wheat bran, oak wood sawdust, and other raw materials [1]. In most cases, SAAS has not been effectively developed and utilized, either directly placed in the open air or simply combusted, leading to serious environmental problems and huge waste [2], [3].

Like other lignocellulosic biomass, SAAS contains abundant oxygen-functional moieties that can be used to produce value-added oxygenated chemicals such as carboxylic acids (CAs) through mild oxidative degradation. The prevailing oxidants mainly consist of O2 [4], [5], [6], RuO4 (RuCl3 as precursor) [7], [8], NaOCl [9], [10], [11], [12], H2O2 [13], [14], and aqueous hydrogen peroxide/acetic anhydride (AHPO/AAH) [15], [16], [17], [18]. Among them, AHPO/AAH is relatively cheap, ecofriendly, and without other elements involved, and has relatively strong oxidizability. However, few reports are issued on the oxidative degradation of SAAS with AHPO/AAH. According to previous reports [19], [20], [21], [22], the oxidative degradation of AHPO/AAH mainly acts on aromatic carbon. Due to the action of peroxidase and laccase during the growth of auricularia auricular, the cellulose and hemicellulose in SAAS have been partially degraded. Therefore, SAAS contains 78% oak wood sawdust, which is rich in lignin. The p-hydroxyphenylpropane, guaiacol, and eugenol (H, G & S) are generally considered to be the major unit components of macromolecules from lignin [23], which can be converted into valuable CAs. Therefore, it is feasible for SAAS to be oxidatively degraded by AHPO/AAH.

In this study, we investigated oxidative degradation of SAAS using AHPO/AAH as the oxidant under mild conditions and analyzed the molecular compositions of the CAs obtained. We tried to explore the mechanisms for the oxidative degradation of SAAS and to optimize the reaction conditions for obtaining CAs in high yield and simple composition.

Section snippets

SAAS and reagents

SAAS consisting of 78% oak wood sawdust, 20% wheat bran, 1% sucrose, and 1% lime was collected from an edible mushroom planting base in Dunhua, Jilin Province, China. It was pulverized to pass through an 80-mesh sieve (particle size ≤ 180 μm) followed by desiccation in a vacuum at 80 °C for 24 h before use. Table 1 shows the proximate, ultimate, and group composition analyses of SAAS. AHPO (30%), AAH, acetone, CH2N2/diethyl ether solution, sodium thiosulfate, and potassium permanganate used in

Effect of the AHPO/AAH ratio

As Fig. 2 exhibits, ASP yield increased to peak, whereas the residue yield decreased to minimum at the AHPO/AAH ratio of 1:2. Nevertheless, when the AHPO/AAH ratio decreased to 2:1, ASP yield also decreased from 47.5% to 26.7%, while the GP yield increased rapidly to 24.2%, indicating that HOradical dot is derived from HPO decomposition of which acts on the oxidatively degraded products to continue to degrade into smaller molecules until it is a gas, so ASP yield is lowered and the GP yield is increased.

Conclusions

Up to 53.6% of the organic matter in SAAS was converted to the SPs, most of which was CAs. NADAs have much higher yield than the other group components, among NADAs, malonic acid, succinic acid, and nonanedioic acid are the most predominant. The composition and content of CAs obtained were significantly affected by the operating AHPO/AAH ratio and temperature. A series of acetoxy compounds indicate that CH3COOradical dot is also involved in the oxidative degradation of SAAS using AHPO/AAH. The oxidative

Nomenclature

    AAs

    alkenoic acids

    AAAs

    acetoxy alkanoic acids

    AADAs

    alkylalkanedioic acids

    AAH

    acetic anhydride

    ABAs

    acetoxy benzoic acids

    ADAs

    alkenedioic acids

    AHPO

    aqueous hydrogen peroxide

    AISPs

    acetone-insoluble portions

    AMBA

    5-acetyl-2-methoxy benzoic acid

    ASPs

    acetone-soluble portions

    ATAs

    alkanetricarboxylic acids

    ATAs′

    alkenetricarboxylic acids

    BAs

    benzoic acids

    BDCAs

    benzene dicarboxylic acids

    BMCAs

    benzene monocarboxylic acids

    BR

    benzene ring

    BTAs

    benzenetrioic acids

    BTAs′

    benzenetetroic acids

    BPCAs

    benzenepolycarboxylic acids

    CAs

Acknowledgments

This work was supported by the Seed Fund from Jilin Agricultural Science and Technology University ([2016] No.Z02) and Undergraduate Scientific and Technical innovation of Jilin Province (Grant [2016] No. 030).

References (29)

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