Enhancing plastic biodegradation process: strategies and opportunities

Figures & data

Figure 1. Strategies that enhance the overall efficiency of plastic biodegradation process.

Figure 2. Effects of UV pretreatment on the surface morphologies of LDPE films. (a) Control (no UV pretreatment, no incubation with microorganisms). (b) After UV pretreatment for 25 days. (c) Formation of pits and cracks on UV-pretreated LDPE films after incubation with soil microorganisms for 126 days [14].

Figure 3. Detailed mechanisms on the photo-oxidation of various plastics [18–22].

Table 1. Effects of UV, thermal, and chemical pretreatments on the biodegradation of plastics.

Substrate	Process conditions	Observations of pretreatment effects on plastic biodegradation^a		References
		After pretreatment	After biodegradation process
UV irradiation
LDPE	UV wavelength at 312 nm 70 °C 60 h	Increased formation of carbonyl groups	Reduction of carbonyl groups Increased dry weight loss by 39%	[16]
	55 W UV lamps 25 days	Higher carbonyl and double bond indices Increased polymer crystallinity	Higher reduction in tensile strength Higher soil microbial biomass carbon Higher CO2 production	[14]
	UV wavelength at 310 nm 70 °C 29 days	Increased crystallinity Increased formation of carbonyl, hydroxyl, and double bond groups Reduced mean crystallite size	Higher CO2 production	[23]
	UV wavelength at 254 nm 1 h	No formation of carbonyl groups	No change in growth rate and cell densities of bacteria	[28]
	Natural sunlight 120 h	Increased surface roughness and the formation of pits and cavities Increased formation of functional groups	Increased weight loss Increased survival rate of waxworms	[13]
HDPE	UV wavelength of 312 nm 70 °C 60 h	Increased crystallinity	Reduced crystallinity Increased keto carbonyl index Enhanced surface modification and corrosion Increased formation of degradation products such as carboxylic acids and alkanes Increased polymer weight loss	[24]
PP	UV wavelength at 225 nm 6 days	Increased hydrophilicity	Further increase in hydrophilicity Increased formation of carbonyl groups Enhanced biofilm formation Pits and cracks were observed	[29]
	UV wavelength at 300 nm 10 days		Increased formation of degradation products Cracks and grooves were observed Increased carbonyl index Increased gravimetric and thermogravimetric weight loss	[12]
	UV wavelength at 225 nm 6 days		Increased hydrophilicity Increased weight loss	[30]
	UV chamber 10 days	No change in surface morphology Increased formation of ester groups	Increased formation of double bond groups Increased microbial colonization	[25]
PET	UV wavelength at 250 nm 14 days	Increased formation of carboxyl groups Reduced average molecular mass Increased crystallinity	Reduced weight loss	[26]
Thermal treatment
LDPE	105 °C 120 h	Reduced melting point and onset melting temperature Reduced crystallinity Increased formation of carbonyl and hydroxyl groups Reduced mean crystallite size	Higher CO2 production compared to non-pretreated	[23]
	70 °C 10 days	Increased hydrophilicity Increased formation of carbonyl and hydroxyl groups	Further increase in hydrophilicity Increased hyphal growth	[31]
	80 °C 10 days	Increased carbonyl index	Reduction in carbonyl index Reduced tensile strength and crystallinity Increased weight loss Increased biofilm growth Increased surface roughness	[32]
	Autoclave	Increase in carbonyl absorption band in FTIR spectrum	Decrease in C–H stretch bond in FTIR spectrum	[33]
HDPE	50 °C 72 h	Increased polymer crystallinity	Reduced crystallinity Enhanced surface modification and corrosion Increased formation of degradation products such as carboxylic acids and alkanes Increased keto carbonyl bond index Increased weight loss	[24]
	80 °C 10 days	Increased carbonyl index	Reduced carbonyl index Increased weight loss Increased surface roughness Increased biofilm growth	[32]
	70 °C 10 days	Increased formation of carbonyl groups	Reduced carbonyl groups Increased weight loss	[34]
PP	100 °C 8 days	Increased hydrophilicity	Further increase in hydrophilicity Increased formation of carbonyl groups Enhanced biofilm formation	[29]
	100 °C 10 days		Increased formation of degradation products Cracks and grooves were observed Increased carbonyl index No change in gravimetric weight loss Increased thermogravimetric weight loss	[12]
	100 °C 8 days		Increased hydrophilicity Increased weight loss	[30]
	100 °C 15 min	Rough and uneven surface was observed	Increased formation of ester, hydroxyl, and double bond groups	[25]
	80 °C 10 days	Increased crystallinity Increased formation of ester groups Increased tensile strength	Reduced ester groups Increased surface roughness Increased biofilm growth Increased weight loss by 10 folds	[15]
Chemical treatment
HDPE	0.25/0.5 mol L^–1 of KMnO4/HCl 45 °C 8 h	Increased crystallinity	Reduced crystallinity Reduced keto carbonyl bond index Enhanced surface modification and corrosion Increased formation of degradation products such as carboxylic acids and alkanes Increased weight loss	[24]
	10% citric acid 45 °C 8 h		Reduced keto carbonyl bond index Increased growth of fungal biofilm Enhanced surface modification and corrosion Increased formation of degradation products such as carboxylic acids and alkanes Increased weight loss	[24]
PP	Aqua regia 3 days		Increased hydrophilicity Increased weight loss	[30]
	Fenton’s reagent salt 7 days		Increased hydrophilicity Increased weight loss	[30]
	HNO3 100 °C 15 min	Formation of cracks and granules	Increased formation of ester, hydroxyl, and double bond groups Increased microbial colonization	[25]
PS	Inorganic acid and oxidizing agent Room temperature 2 h	Increased formation of carbonyl and hydroxyl groups	Complete microbial colonization	[35]

^a The pretreated plastic samples were compared to the non-pretreated plastic samples or to the pristine plastic without subject to biodegradation process.

Table 2. Comparison of plastic pretreatment approaches.

Pretreatment technique	Equipment/substances used	Strengths	Drawbacks
UV irradiation	UV lamp UV chamber Weathering tester Photochemical reactor	More effective compared to thermal treatment	Prolonged treatment duration Effective only when the plastic surface is directly exposed to the irradiation source Limited scalability; plastic sheet or film requires treatment on both sides [43]
Thermal treatment	Hot air oven Autoclave	Practical in large-scale plastic biodegradation Safe process due to the use of moderate temperatures and ease of operation	Prolonged treatment duration
Chemical treatment	Concentrated chemicals or strong oxidizing agents	Practical in large-scale plastic biodegradation Feasibility of chemicals reusability may promote an economic outcome in large-scale plastic biodegradation	Requires proper disposal of concentrated chemicals upon pretreatment process
Gamma irradiation	Gamma ray equipment	High penetrating power of gamma rays eliminates the need for direct exposure of plastic surface to the radiation source Shorter treatment duration	Compromised safety due to harmful effects of gamma radiation
Plasma treatment	Diffuse coplanar surface barrier discharge (DCSBD) type equipment	Shorter treatment duration Green technology [39]	Effective only when the plastic surface is directly exposed to the irradiation source

Table 3. Effects of additives on plastic biodegradation.

Substrate	Additives	Pretreatment technique	Observations^a	References
Pro-oxidants
LLDPE	Pro-oxidant (undisclosed compound)	UV irradiation (93 days) and thermal treatment at 45, 55, and 65 °C (200 days)	Increased carbonyl index upon UV exposure and further increase was observed upon subsequent thermal treatment Increased fungal colonization after abiotic treatments Increased CO2 production	[51]
LDPE	UV photosensitizer (undisclosed compound)	UV irradiation (60 h)	Higher weight loss upon biological treatment	[16]
	0.1% w/w cobalt stearate	UV-B irradiation (600 h)	No change in mechanical properties but reduction in mechanical strength was observed upon UV irradiation Increase in the formation of carbonyl groups Increased the formation of degradation products Increased crystallinity upon UV exposure Increased weight loss upon biological treatment Increased hydrophilicity of LDPE upon UV exposure and further increase upon biological treatment Improved bacterial adhesion and biofilm formation	[48]
PP	Metal ion	Thermal treatment at 100 °C (10 days) or UV irradiation (10 days)	Increased weight loss by 8% Higher surface colonization Higher degradation products such as hydrocarbons, esters, and aldehydes	[12]
Natural polymers
LDPE	12–15% starch		Higher weight loss Increased hydrophilicity Reduced crystallinity Reduced tensile strength	[32]
PP	Starch	UV-C irradiation (10 days) or thermal treatment at 100 °C (10 days)	Higher carbonyl index upon pretreatment Higher weight loss	[12]
PVC	40% cellulose		Enhanced colonization of Trichocladium sp. and Chaetomium sp. Higher weight loss	[52]

^a The pretreated plastic samples were compared to the non-pretreated plastic samples or to the pristine plastic without subject to biodegradation process.

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