Pretreatment of spiramycin fermentation residue by thermally activated peroxydisulfate for improving biodegradability: Insights into matrix disintegration and antibiotics degradation
Introduction
The spiramycin fermentation residue (SFR) is the precipitation of Streptomyces ambofaciens [1] fermentation broth after extraction the antibiotic of spiramycin (SPM). The SFR is solid fermentative biowaste mainly contains unused culture medium, mycelium, secondary metabolites and the incompletely extracted SPM. The SFR was classified as hazardous waste in China because of the potential risk of causing microbial antibiotic resistance once the residual high level of SPM was exposed to the environment. The innoxious disposal of the increasing production of SFR due to the increasing demand for SPM with the growing global medical demand has become one of the main challenges for the bio-pharmaceutical industry [2].
Anaerobic digestion [3], [4], [5] and aerobic composting [6] are considered as promising resource utilization technologies for antibiotic fermentation residues (AFRs) in consideration of abundant high quality microbial nutrients including polysaccharides, proteins, various amino acids and trace elements. However, the direct biological treatment of untreated antibiotic bacterial residue could not achieve satisfactory efficiency due to its poor biodegradability resulted by the biodegradation resistance of agglomerated mycelium and extracellular polymeric substances (EPS) [4] and inhibitory effect of residual antibiotics on microorganisms[5], [7] and even promote the development of bacterial resistance. In order to improve the biodegradability of SFR, it is necessary to use appropriate pretreatment technology to disintegrate its matrix structure and degrade the high level of residual SPM.
There are many different types of pretreatment strategies have been investigated to improve AFRs biodegradability such as advanced oxidation processes (AOPs) [8], [9], thermal treatment [10], [11], [12], [13] and irradiation treatment (e.g., microwave, gamma and ionization) [14], [15], [16]. Among these approaches, thermal treatment required a higher temperature (usually >120 °C) and even combined with alkaline/acid conditions to achieve the disruption of microbial aggregates. The irradiation treatments equipped with more advanced equipment have been proved to be sufficient for degradation of antibiotics in AFRs, but their practical applications were limited by complex procedures and high operating costs.
Advanced oxidation processes (AOPs) are effective for the degradation of refractory organics and antibiotics [17], [18], [14], [9], [19] due to the highly reactive hydroxyl radical (OH) (E0(OH, H+/H2O) = 2.81 VNHE) [20]. In recent years, sulfate radical anion (SO4−) based oxidation technologies utilize the highly reactive of SO4− (E0(SO4−/SO42−) = 2.5–3.0 VNHE) [21], [22] generated by the activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS) through manifold pathways [23], [24], [25], [26], [27] have been regarded as promising alternatives to degrade organic pollutants and improve the waste activated sludge dewatering performance by disintegrating the hydrophobic organic substances (e.g., protein and humic acid) in EPS [28], [29]. In addition, the SO4− could exhibit stronger oxidizing at a wide pH range (3 ~ 9). Several studies have shown that Fe(II)-activated, biochar-activated and microwave-activated persulfate oxidation could destroy the flocs structure and EPS of sludge to improve its biodegradability by micropollutant degradation and sludge solubilization[28], [29], [30]. Furthermore, as compared with other conventional oxidants including H2O2, O3, K2FeO4 and PMS, PDS has the cheapest price, the longest lifetime in water, easier to store and transport and the lowest activation energy [31]. However, practical experience is still insufficient with thermal activated persulfate (TAP) oxidation on AFRs treatment even though the satisfactory degradation rate could be achieved by thermal activated persulfate for most refractory organic contaminants especially at high levels [32]. Furthermore, the thermal activation can be achieved at mild temperature to avoid high heating costs (Eq. (1)) and without additional catalyst [33]. Therefore, TAP treatment is proposed as a pretreatment for SFR to improve biodegradability by simultaneous mycelium disintegration and SPM degradation.S2O82− + heat → 2SO4− (30 °C < T < 99 °C)
In addition, it is necessary to systematically investigate the characteristics of SFR disintegration and SPM degradation mechanism.
In this study, the characteristic behaviors of SFR disintegration by TAP oxidation treatment under different PDS dosages in terms of the structure and composition, particle size distribution, filterability, centrifuged weight reduction (CWR) and SFR solubilization (dissolved organic carbon (DOC), total dissolved nitrogen (TDN), ammonium ion (NH4+), polysaccharide and protein and fluorescent substances) were firstly studied in detail. Then the experimental parameters (PDS dosage, activation temperature and initial pH) on SPM degradation were investigated. The possible degradation pathways of SPM were hypothesized based on the determination of degradation products. Furthermore, the inactivation of SPM related antibiotic resistance genes (ARGs) in SFR during TAP oxidation treatment was tested. This paper comprehensively evaluated the feasibility of TAP as a pretreatment method for SFR, which has important significance for the efficient and safe utilization of subsequent bioprocess.
Section snippets
Materials and chemicals
The SFR was collected from a biopharmaceutical factory located in Henan Province, China. The collected SFR sample was stored at −25 °C and transferred to the fresh room of refrigerator (4 °C) for thawing before to the experiment. The characteristics of SFR were listed in Table S1. The residual SFR concentration was 2.11 ± 0.05 g/kg. The standard of SPM (HPLC ≥ 90%) and potassium peroxydisulfate (K2S2O8, ACS, ≥99.0% (RT)) were obtained from Aladdin Chemistry Co. China. All the other chemicals
Morphology and structural characterization
The SEM images of raw and treated SFR were shown in Fig. 1. The color of SFR gradually lightened due to the discoloration of radicals on colored organic compounds with the increase of PDS dosages (Fig. S2). As shown in Fig. 1a, the raw SFR exhibited a stone-like appearance with mycelium and EPS clinging to its surface. The morphology of SFR did not change under PT-50 treatment (Fig. 1b). However, the surface of SFR became smooth after thermal treatment (Fig. 1c). In the TAP processes, SFR
Conclusions
In this study, the performance of TAP treatment for SFR disintegration, SPM degradation and ARGs inactivation were investigated in detail. The matrix structure of SFR was effectively destroyed by TAP treatment with release of water-soluble organic matters. However, the mineralization of dissolved organic matters by oxidative radicals was increased in TAP process at high PDS dosage. The high residual antibiotic of SPM in SFR were efficiently removed by TAP treatment and its degradation rate was
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work is financially supported by the National Natural Science Foundation of China (Grant No. 51978497).
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