Colonization and Biodegradation of Photo-Oxidized Low-Density Polyethylene (LDPE) by New Strains of 'Aspergillus' sp. and 'Lysinibacillus' sp.

The primary objective of this study was the isolation of low-density polyethylene (LDPE)-degrading microorganisms. Soil samples were obtained from an aged municipal landfill in Tehran, Iran, and enrichment culture procedures were performed using LDPE films and powder. Screening steps were conducted using linear paraffin, liquid ethylene oligomer, and LDPE powder as the sole source of carbon. Two landfill-source isolates, identified as 'Lysinibacillus xylanilyticus' XDB9 (T) strain S7-10F and 'Aspergillus niger' strain F1-16S, were selected as super strains. Photo-oxidation (25 days under ultraviolet [UV] irradiation) was used as a pretreatment of the LDPE samples without pro-oxidant additives. The PE biodegradation process was performed for 56 days in a liquid mineral medium using UV-irradiated pure LDPE films without pro-oxidant additives in the presence of the bacterial isolate, the fungal isolate, and the mixture of the two isolates. The process was monitored by measuring the fungal biomass, the bacterial growth, and the pH of the medium. During the process, the fungal biomass and the bacterial growth increased, and the pH of the medium decreased, which suggests the utilization of the preoxidized PE by the selected isolates as the sole source of carbon. Carbonyl and double bond indices exhibited the highest amount of decrement and increment, respectively, in the presence of the fungal isolate, and the lowest indices were obtained from the treatment of a mixture of both fungal and bacterial isolates. Fourier transform infrared (FT-IR), x-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses showed that the selected isolates modified and colonized preoxidized pure LDPE films without pro-oxidant additives

XRD spectra of non-UV and UV-irradiated pure LDPE films before and after incubation in soil with different treatments.

<p>(A) XRD spectra of non-UV-irradiated pure LDPE films without pro-oxidant additives before and after 126 days of incubation in soil with different treatments: (a) blank (no UV irradiation, no incubation); (b) non-UV-irradiated LDPE after incubation in soil in the absence of the selected microorganisms (SP treatment); (c) non-UV-irradiated LDPE after incubation in soil in the presence of the selected microorganisms (SMP treatment). (B) XRD spectra of UV-irradiated pure LDPE films without pro-oxidant additives before and after 126 days of incubation in soil with different treatments: (a) blank (after 25 days' UV irradiation, no incubation); (b) UV-irradiated LDPE after incubation in soil in the absence of the selected microorganisms (SUP treatment); (c) UV-irradiated LDPE after incubation in soil in the presence of the selected microorganisms (SMUP treatment).</p

FT-IR spectra of non-UV-irradiated pure LDPE films before and after incubation in soil in various treatments.

<p>(A) FT-IR spectra of non-UV-irradiated pure LDPE films without pro-oxidant additives before and after 126 days of incubation in soil in the presence and absence of the selected microorganisms from 500–4000 cm-1. (B) The changes in the bands between 500 and 2,000 cm-1 of the FT-IR spectra of non-UV-irradiated pure LDPE films without pro-oxidant additives before and after 126 days of incubation in soil with different treatments: (a) blank (no UV irradiation, no incubation); (b) non-UV-irradiated LDPE after incubation in soil in the absence of the selected microorganisms (SP treatment); (c) non-UV-irradiated LDPE after incubation in soil in the presence of the selected microorganisms (SMP treatment).</p

FT-IR spectra of UV-irradiated pure LDPE films before and after incubation in soil in various treatments.

<p>(A) FT-IR spectra of UV-irradiated pure LDPE films without pro-oxidant additives before and after 126 days of incubation in soil in the presence and absence of the selected microorganisms from 500–4000 cm-1. (B) The changes in the bands between 500 and 2,000 cm-1 of the FT-IR spectra of UV-irradiated pure LDPE films without pro-oxidant additives before and after 126 days of incubation in soil with different treatments: (a) blank (after 25 days' UV irradiation, no incubation); (b) UV-irradiated LDPE after incubation in soil in the absence of the selected microorganisms (SUP treatment); (c) UV-irradiated LDPE after incubation in soil in the presence of the selected microorganisms (SMUP treatment).</p

Mineralisation profile of UV- and non-UV-irradiated pure LDPE films incubated in the soil with various treatments for 126 days.

<p>Each data point represents the average of three replicates ± SD. (SP: Soil + non-UV-irradiated PE; SUP: Soil + UV-irradiated PE; SMP: Soil+ Selected Microorganisms + non-UV-irradiated PE; SMUP: Soil+ Selected Microorganisms + UV-irradiated PE).</p

SEM micrograph of pure LDPE films before and after 126 days of incubation in soil with different treatments.

<p>(a) Blank (no UV irradiation, no incubation). (b) UV-irradiated LDPE film without incubation. (c) non-UV-irradiated LDPE film incubated in soil in the absence of the selected microorganisms (SP). (d) UV-irradiated LDPE film incubated in soil in the absence of the selected microorganisms (SUP). (e) non-UV-irradiated LDPE film incubated in soil in the presence of the selected microorganisms (SMP): (e1) penetration of hyphae into the LDPE matrix; (e2) formation of bacterial biofilm on the surface of LDPE; (e3 and e4) formation of pits and cavities on the surface of LDPE. (f) UV-irradiated LDPE film incubated in soil in the presence of the selected microorganisms (SMUP): (f1 and f2) penetration of hyphae into the LDPE matrix; (f3) formation of bacterial biofilm on the surface of LDPE; and (f4) formation of pits and cavities on the surface of LDPE.</p

Changes in the percentage elongation of UV-irradiated PE films before and after 63 and 126 days of biodegradation in soil in various treatments.

<p><i>ε_r</i> elongation at break (%), <i>*</i> fragile specimens, (Δ %) difference between percentage elongation of films before and after biodegradation process (shown as a percentage). Values accompanied by a similar letter are not significantly different according to Duncan's multiple range test (P = 0.05). Each value represents the average of four replicates ± SD. (SUP: Soil + UV-irradiated PE; SMUP: Soil+ Selected Microorganisms + UV-irradiated PE).</p