Thermophilic Composting as a Means to Evaluate the Biodegradability of Polymers Used in Cosmetic Formulations

Abstract

In the last decade, a growing demand for sustainable cosmetic ingredients has yielded numerous biodegradation protocols. While OECD (Organization for Economic Co-operation and Development) aquatic assays are suitable for water-borne chemicals, it is crucial for the personal care industry to consider the persistence of plastics in soil, compost, and municipal sludge. Adopting this cradle-to-grave holistic approach would strengthen product appeal while increasing the accuracy and ethical integrity of green product labeling. The aim of our study was to employ quantitative CO₂ detection and thermophilic composting protocols specified in ASTM D5338, along with pass level criteria outlined in ASTM D6400, to assess the mineralization of plastics commonly formulated into personal care products. Our results indicate that many cellulose ethers, cationic guars, starches, proteins, and labile polyesters demonstrate satisfactory disintegration, biodegradation, and seed germination rates to secure an ASTM D6400 compostability claim. By contrast, macromolecules designed with carbon–carbon backbones resisted acceptable mineralization in composting experiments, advocating that unadulterated municipal compost lacks the microbial diversity to enzymatically digest many synthetically derived resins. Additionally, polymers that demonstrated acceptable biodegradability in internal and published OECD aquatic studies, including chitosan and polyvinyl alcohol, exhibited limited respiration in local municipal compost; hence, untested correlations between aquatic, soil, and compost testing outcomes should never be assumed.

1. Introduction

The successful commercialization of plastics began in the 1950s, and global production has gradually increased to more than 330 million metric tons (MMT) per annum [1]. Consequently, plastics pollution has paralleled global production. According to a 2018 EPA report, 32 MMT of discarded plastic was generated in the United States [2]. Of this total, about 75% was disposed of in regulated dumpsites and landfills, which underscores the pressing need for sustainable waste reduction and disposal methods. In nearly every region of the world, successful cosmetics and personal care products are packaged in single-use rigid plastic containers, including vessels composed of polyethylene terephthalate (PET), polyethylene, polyvinyl chloride (PVC), polystyrene (PS), and polylactic acid (PLA). In addition, each year, approximately 0.7 MMT of plastics is blended into personal care formulations, where dilute resins function as rheology modifiers, emulsifiers, film formers, opacifiers, and cleansing aids [3]. Although a few macromolecules in finished chassis are discernible to the naked eye, most polymers in liquid formulations (PLFs) are too soluble or minuscule to be visibly perceived. When rinsed from the body, these overlooked plastics, including polyacrylates, crosslinked thickeners, and polyethylene, flow down drains and enter cesspools, septic tanks, and public sewer systems. In septic systems, a fraction of the soluble or dispersed plastics may circumvent residential containment tanks and percolate through leach fields and into the water table, or in towns with public sewer systems, micronized debris may elude sewage remediation and enter neighboring rivers and oceans [4,5]. Consequently, when recalcitrant plastics reach local springs and waterways, PLFs and other microparticles may be inadvertently consumed by organisms such as zooplankton, coral, arthropods, shellfish, cephalopods, and marine mammals while accumulating in our food chain [6,7]. In response to heightened environmental concerns, the North American Oceanic and Atmospheric Administration (NOAA) and European Chemicals Agency (ECHA) have led efforts to rigorously investigate the ecological impact of carbon-based synthetic microplastics, which are loosely defined as water-insoluble plastic particulates less than 5 mm in length [8].

Polymers used in personal care formulations are synthetically or naturally derived. Synthetic polymers, such as polyacrylates, polyethylene glycol (PEG), and polycaprolactone (PCL), are routinely polymerized using non-renewable petrochemicals. By contrast, higher molecular weight (MW) natural polymers (bioplastics) are typically produced by purifying and chemically modifying extracts sourced from renewable resources, including terrestrial florae (e.g., starch, cellulose, zein, guar, PLA, and pectin), seaweed (e.g., alginates and carrageenan), shellfish (e.g., chitin and chitosan), and faunae (e.g., collagen, keratin, and gelatin) [9]. Synthetic and natural polymers may be further divided into degradable and non-degradable plastics. Degradable plastics deteriorate in the environment through mechanical, hydrolytic, thermal, photooxidative, and/or enzymatic mechanisms, whereas non-degradable resins are designated as persistent, with half-lives measured in years. Many synthetic personal care polymers are designed with thermodynamically stable carbon–carbon (-C—C-) backbones and resist ambient photooxidative and hydrolytic degradation processes. Notable exceptions include specific grades of polyvinyl alcohol (PVA) and a select group of polyesters and polyurethanes that have hydrolytically labile backbones [10]. In the absence of sunlight, however, aquatic or landfill temperatures must exceed 200 °C to thermo-oxidatively degrade these plastics in a timely manner—which is highly unlikely [11,12,13]. By contrast, most bioplastics resist abiotic hydrolysis but are susceptible to abiotic fragmentation via photooxidation and enzymatic degradation by adapted aquatic and soil microbes.

Biodegradation is the breakdown of organic substances by microorganisms, and it occurs in well-characterized stages, including biodeterioration, biofragmentation, bio-assimilation, and mineralization [14]. Biodeterioration is a quasi-abiotic step, where tandem weathering processes, including extreme temperatures, mechanical work, abiotic hydrolysis, and photooxidation, cause surface imperfections in the plastic, resulting in an increase in bioavailable surface area. Once the polymer matrix is compromised, biofragmentation ensues as macroorganisms (e.g., nematodes, earthworms, and snails) and microorganisms (e.g., bacteria, fungi, algae, and protozoa) colonize the polymer surface and employ microbial gut symbionts or secreted enzymes to cleave accessible macromolecular chains into shorter oligomers and monomers [15]. These smaller molecules are then digested by macroorganisms or are assimilated and/or mineralized by cells. The assimilation and mineralization steps terminate when the microbiota chemically transform all bioavailable polymer fragments into usable biomass, biogas, inorganic salts, or waste. Enzymatic carbon mineralization processes occur aerobically or anaerobically, whereupon aerobic mineralization routes produce CO₂ and water, and slower anaerobic digestion yields methane, CO₂, water, and small amounts of H₂S and ammonia. Each stage of biodegradation is influenced by the properties of the ecosystem, including the biodiversity of its indigenous microbes. Hence, biodegradability assays and relevant claims should be tailored to the elements of the targeted local environment (e.g., fresh water, raw sewage, seawater, marine sediment, compost, and landfill) [16]. Although native microorganisms cannot produce enzymes to biodegrade many synthetic polymers, they have suitably developed enzymes to biofragment and mineralize natural polymeric materials. For example, cellulose is a linear semi-crystalline biopolymer composed of D-anhydroglucose units that are covalently joined with β-1,4 glycosidic linkages, and, due to numerous intra- and intermolecular hydrogen bonds, cellulose forms insoluble and impenetrable fibrils that are difficult to enzymatically hydrolyze [17]. Nevertheless, over millions of years of co-evolution with woody plants, microbes have adapted collections of lytic enzymes to productively degrade lignocellulose [17,18]. Similarly, enzymes have been naturally adapted by microorganisms to mineralize other agropolymers, including guar gum [19], chitosan [20], xanthan gum [21], and starch [22].

Consumer demand for environmentally friendly goods has sparked interest in biodegradation testing and innovative strategies for developing sustainable raw materials for the personal care industry. Current aquatic biodegradability testing protocols are set forth by international standards organizations such as ASTM International, International Organization for Standardization (ISO), and OECD and are routinely used to establish ecolabeling claims [23,24,25,26,27,28]. Arguments for using aquatic OECD testing standards to solely establish biodegradability claims include the understanding that ca. 99% of treated municipal wastewater is discharged to local waterways as rejuvenated water; however, one could dispute that the fate of the accumulated 1% (w/w) municipal sludge (biosolids) should be considered as well [29]. Approximately three-quarters of US households use municipal wastewater treatment facilities to treat sewage [30]. In these treatment facilities, wastewater influent receives primary, secondary, activated sludge, disinfection, anaerobic digestion, and filtration treatments [31]. After releasing the rejuvenated water, which may contain numerous soluble and dispersed polymer fragments, the residual biosolids are condensed and subsequently (a) deposited as fertilizer in home gardens and US farms (55%); (b) sent to landfills (27%); (c) incinerated (16%); or (d) used in land reclamation or other projects (2%) [31,32,33]. In land use, sludge treatments are typically added to the soil surface as a top dressing or are blended with water, air, wood chips, straw, and decaying plant matter to produce enriched compost—in which composting is defined as the conversion of mixed organic waste into materials that are beneficial to plant life [34]. The good news is that composting effectually recycles biowaste and amends agricultural soil by improving its aeration, water-holding capacity, pH, and soil structure, as well as its bioavailable carbon, nitrogen, and micronutrient composition. Furthermore, composting is an important solution for communities looking to economically manage large volumes of organic refuse, including kitchen, agricultural, industrial, and municipal waste [34]. The potentially concerning news is that each batch of biosolids added to farmland likely introduces enduring oligomers and microplastics [35,36,37,38,39,40,41,42].

To model the municipal composting of plastics in a laboratory setting, respirometry studies are accomplished in an aerobic milieu using municipal compost and standard biodegradability testing protocols such as ASTM D5338, ISO 14855-1, and ISO 14855-2 [43,44,45,46,47,48]. For each of these standard methods, ASTM D6400 and EN 13432 define the required disintegration, mineralization, and terrestrial safety pass level criteria to label a plastic as compostable in municipal or industrial facilities [46,47,49]. In this study, compost testing protocols outlined in ASTM D5338 and pass level criteria defined in ASTM D6400 are used to evaluate the biodegradability of several biopolymers and synthetic plastics in an inoculum of local municipal compost.

2. Materials and Methods

2.1. Source of Municipal Compost

Municipal compost was provided by NaturCycle LLC (Plainville, NY, USA) and sampled from the Morris County Municipal Utilities Authority (MCMUA) facility in Parsippany, NJ, USA. The compost was analyzed externally by MCMUA and met the seal of approval of the US Composting Council, using Test Methods for the Examination of Composting and Compost (TMECC) standard testing assays to assess trace heavy metals, percent seedling emergence, compost maturity, pH, electrical conductivity, select pathogens, particle size, moisture, organic matter, and ash content. As per specifications in ASTM D5338, the compost was sifted at the collection site to a particle size of less than 10 mm using a USA Standard 10 mm brass test sieve (Dual Manufacturing Co., Franklin Park, IL, USA), and residual inert materials, including small sticks and stones, plastic particles, and glass, were manually removed.

2.2. List of Tested Polymers with Measured Moisture Levels, Ash Content, and Maximum Theoretical CO₂ Production

A description of the commercial polymers and their measured moisture levels, ash content, and maximum theoretical CO₂ production are specified in

Table 1. Overview of chemical/trade names and common/INCI names for the various grades of examined polysaccharides, proteins, and synthetic resins. All tabulated results were produced in this study and include the initial moisture, ash content, and maximum theoretical CO₂ from complete polymeric carbon mineralization.

Chemical/Trade Name	INCI/Common Name	Moisture (%)	Ash (%)	CO2 (g) *
Aqualon 7L2F a	Cellulose Gum ^	7.5	36.4	139
Aqualon 7H4F a	Cellulose Gum ^	6.9	36.0	144
Aquasorb A380 a	Cellulose Gum ^	5.1	35.3	142
Aquasorb A500 a	Cellulose Gum ^	6.0	35.5	142
Avicel PH101 b	Microcrystalline Cellulose	5.4	0.2	154
Avicel PH102 b	Microcrystalline Cellulose	6.0	0.1	165
Bovine gelatin c	Gelatin	11.0	1.0	164
κ-Carrageenan c	Carrageenan	6.9	33.9	84
Cassia gum a	Cassia Gum	10.8	2.0	145
Chitin c	Chitin	5.4	1.1	164
Chitosanc	Chitosan, 50–190 KDa, 76% deacetylation	7.1	0.3	153
Citrus peel pectin c	Pectin	7.2	15.1	146
Crosslinked pAA d	Carbomer	8.6	0.1	174
Guar gum c	Guar Gum	11.3	1.1	171
Keratin (bleached hair) e	Bleached European dark brown hair	12.7	6.5	153
Keratin (virgin hair) e	Virgin European dark brown hair	11.7	1.7	163
Locust bean gum c	Ceratonia Siliqua Gum	12.5	1.0	147
Maize amylopectin c	Amylopectin	11.9	0.6	147
N-Hance 3196 a	Guar Hydroxypropyltrimonium Chloride	9.5	1.0	151
Poly(OAA/Acrylates/BAEM) f	OAA/Acrylates/BAEM Copolymer	3.6	2.4	226
Polycaprolactam c	Nylon 6	0.5	0.1	212
Polycaprolactone (80 kDa) c	Polycaprolactone	1.2	0.1	234
Polyethylene c	Polyethylene	0.0	0.3	313
Polylactic acid (60 kDa) c	Polylactic Acid	0.6	0.1	186
Polystyrene c	Polystyrene	0.1	0.1	335
Potato starch c	Potato Starch	8.1	0.6	151
PVA, ≥85% hyd. c	Polyvinyl Alcohol, 30 kDa, ≥85% hydrolyzed	3.4	0.8	197
PVA, ≥98% hyd. c	Polyvinyl Alcohol, <50 kDa, ≥98% hydrolyzed	3.4	0.2	194
SigmaCell Type 20 cellulose c	Cellulose (cotton linters)	5.0	0.1	156
Sodium alginic acid c	Alginic Acid, Sodium Salt	13.2	23.2	114
Soluble starch (potato) c	Soluble Starch	17.0	0.9	138
Soy protein acid hydrolysate c	Hydrolyzed Soy Protein	11.7	18.1	146
Styleze ES-1 a	Guar Hydroxypropyltrimonium Chloride	9.3	2.8	165
Styleze ES-Dura a	Guar Hydroxypropyltrimonium Chloride	7.7	3.0	153
Xanthan gum c	Xanthan Gum	15.0	0.6	131
Zein protein c	Zein	6.8	1.8	220
α-Cellulose c	Microcrystalline Cellulose	7.5	0.4	153

^a Ashland Inc. (Wilmington, DE, USA); ^b IFF Inc. (New York, NY, USA); ^c Merck KGaA (Darmstadt, Germany); ^d Lubrizol (Wickliffe, OH, USA); ^e European dark brown hair (1–2 mm fiber snippets) from International Hair Importers and Products, Inc. (Glendale, NY, USA); ^f Nouryon (Amsterdam, The Netherlands); pAA = polyacrylic acid; OAA = tert-octylacrylamide; BAEM = tert-butylaminoethyl methacrylate; Acrylates = methyl methacrylate, acrylic acid, hydroxypropyl methacrylate; ^ sodium carboxymethyl cellulose; * based on the carbon content from 100 g of as received polymer.

. In addition,

Table 2. Cross-reference of the polymer ID used in the text against its commercial trade name.

Polymer ID	Commercial Trade Name
CMC-1	Aqualon 7L2F
CMC-2	Aqualon 7H4F
CMC-3	Aquasorb A380
CMC-4	Aquasorb A500
MCC-1	Avicel PH101
MCC-2	Avicel PH102
CGG-1	N-Hance 3196
CGG-2	Styleze ES-1
CGG-3	Styleze ES-Dura

contains a cross-reference of the polymer identification used in the text against its marketed trade name. Unless otherwise mentioned, the biodegradation of all materials was examined in the as-received physical form of the tested sample (e.g., powders, films, and small segments of hair fibers).

2.3. Polymer Ash Content

The ash content (±0.2% (w/w)) in Table 1 was determined using a TA Instruments (New Castle, DE, USA) Q5000 thermogravimetric analyzer (TGA) with the following protocol: switch purge gas to dry air (25 mL/min); isothermal at 40 °C for 5 min; ramp 10 °C/min to 600 °C; isothermal for 30 min (n = 3). TA Instruments Universal Analysis v4.5A software was used to quantify the residue.

2.4. Compost and Polymer Dry Solids Measurements

Prior to commencing respirometry studies, ASTM D5338 indicates that the dry solids of compost–polymer admixtures should be adjusted to 50–55% (w/w) and loss on drying (LOD) protocols defined in Standard Method 2540D be applied for adjusting the moisture content [43,49]. In Standard Method 2540D, the sample is dehydrated to constant weight in a drying oven set to 103–105 °C for no less than 60 min, and the weight difference is used to calculate the initial dry solids. To expedite moisture analyses, we instead developed a 15 min higher temperature (140 °C) evaporation method using a Sartorius (Göttingen, Germany) MA-30 moisture analyzer. The results are presented in Table 1. To validate the protocol,

Table 3. Compost solids determination using various standard LOD techniques. Note that the average dry solids of the collected compost lots was 31.5 ± 1.3% (w/w) (n = 4).

Drying Method	Sample Size (g)	Temperature (°C)	Time (h)	Dry Solids (%)
Sartorius MA 30	4–6	140	0.25	47.9 ± 1.1 (n = 10)
TGA Q5000 isotherm	0.02	175	1.0	46.2 ± 1.3 (n = 3)
Forced-air oven (2540D)	25–35	105	24	48.3 ± 1.1 (n = 5)
Vacuum oven (2540D)	25–35	105	24	48.1 ± 0.3 (n = 5)
DVS isotherm *	0.02	25	24	48.8 ± 1.5 (n = 4)

* TA Instruments Q5000 SA (25 °C; 0% RH using dry N₂; 24 h isotherm).

contrasts neat compost dry solids results using Standard Method 2540D and findings from other commonly employed LOD procedures.

2.5. Effect of Drying and Heat on the Compost Microbial Biomass

The microbial biomass was determined for three distinct compost samples: (1) as-received compost (32% (w/w) solids); (2) compost tray-dried at 23 °C in a laboratory hood to 50% (w/w) solids (as per ASTM D5338); and (3) neat compost aerobically composted for 90 days at 58 °C (53% (w/w) solids). The microbial content of each compost inoculum was then determined by dispersing 1.0 g of compost in 9 mL of sterile saline solution. An aliquot of liquid from the compost suspension was serially diluted in sterile saline. Each dilution was subsequently pour-plated onto Letheen agar for bacteria counts (B) and potato dextrose agar for mold (fungi) counts (F). After solidification, the Petri dishes were incubated for 48 h at 35 °C for bacteria and 5 days at 28 °C for fungi.

The following results are reported in mass-normalized colony forming units (cfu/g), wherein F/B represents the unitless biomass ratio. For the pre-dried compost, B = 2.1 × 10⁸ cfu and F = 5.9 × 10⁴ cfu (F/B = 2.8 × 10⁻⁴). After tray drying to 51% (w/w) solids, B and F decreased to 2.6 × 10⁷ cfu and 2.3 × 10⁴ cfu, respectively (F/B = 8.8 × 10⁻⁴). Finally, after thermally treating neat compost in a respirometry vessel for 90 days, both F and B populations further decreased to 9.4 × 10⁶ cfu and 5.9 × 10¹ cfu, respectively (F/B = 6.3 × 10⁻⁶).

2.6. CHN Analysis of Tested Polymers

Using carbon–hydrogen–nitrogen (CHN) analysis, average measurements (n ≥ 2) were used to assess the maximum theoretical CO₂ produced in the aerobic biofragmentation of the as-received polymers. The % carbon results were mathematically transformed to the maximum theoretical CO₂ produced (i.e., assuming 100% mineralization) and are presented in Table 1. For each replicate, ca. 1.5 mg of sample was weighed into a tin sample capsule and analyzed directly with a Thermo Fisher Scientific Flash 2000 CHNS/O combustion analyzer (Waltham, MA, USA) using the following instrumental parameters: helium carrier = 140 mL/min; oxygen = 250 mL/min; helium reference = 100 mL/min; sample run time = 720 s with 12 s sampling delay; oxygen injection end = 5 s; oven temperature = 65 °C; and furnace temperature = 950 °C. The measurement was calibrated linearly using a 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene standard. In addition, CHN analyses estimated the C/N ratio of the compost as ca. 19:1 (w/w), which is acceptable for composting carbon-rich polymers [43].

2.7. ASTM D5338 and ASTM D6400: Biodegradability Controls, Calculations, and Compostability Labeling Claims

The composting apparatus described in ASTM D5338 incorporates a minimum of 12 vessels, in which the test material, blank (compost only), negative control, and positive control are run in triplicate at 58 ± 2 °C in diffuse light. Note that respirometry experiments using ASTM D-5338 are conducted at 58 °C to emulate the expected thermophilic temperatures observed within municipal composting windrows. The blank vessels are used for zeroing CO₂ contributions provided by the indigenous carbonaceous matter in the compost. Since it takes 10–600 years for polyethylene to naturally biodegrade [41], ASTM D5338 specifies polyethylene as the negative control, wherein the physical form of the negative control should match that of the test sample—powders, films, and cylindrical slugs were used in our studies. The universal positive control is defined as ≤20 μm high-purity microcrystalline cellulose (MCC) powder. ASTM D5338 requires that the positive control biodegrades ≥70% in ≤45 days or the composting assay is judged as invalid.

The % biodegradation of the test polymer is defined by Equation (1), as follows:

$$\% biodegradation=\frac{{\mathrm{C}\mathrm{O}}_2 polymer \left(\mathrm{g}\right)-{\mathrm{C}\mathrm{O}}_2 blank \left(\mathrm{g}\right)}{{\mathrm{C}\mathrm{O}}_2 theoretical \left(\mathrm{g}\right)}\times 100,$$

(1)

in which CO₂ polymer is the detected CO₂ mass (44 g/mol) produced by the mineralized test polymer in the compost; and CO₂ blank is the detected mass of CO₂ produced by the indigenous compost. In Equation (2),

$${\mathrm{C}\mathrm{O}}_2 theoretical (\mathrm{g})=polymeric carbon \left(\mathrm{g}\right)\times \frac{44}{12}$$

(2)

CO₂ theoretical is the maximum theoretical CO₂ evolved by the biodegraded test polymer, which assumes that all polymeric carbon (12 g/mol) is catabolized to gaseous CO₂ [43]. For our calculations, the polymeric carbon was determined empirically using the CHN analysis method detailed in Section 2.6. The CO₂ theoretical values for each tested polymer are itemized in Table 1. According to ASTM D5338, the % biodegradation may be reported in absolute form (i.e., Equation (1)) or defined relative to the performance of the positive control.

ASTM D6400 defines the following criteria to meet the requirements for labeling materials as biodegradable and/or compostable in aerobic municipal and industrial composting facilities: (1) in tandem with environmental conditions outlined in ASTM D5338, materials must naturally degrade ≥ 90% in ≤6 months; (2) after 84 days, no more than 10% of its dry weight is retained after passing the composted plastic through a 2 mm sieve; and (3) composted plastics must successfully demonstrate acceptable terrestrial safety [43,44,45,46,47].

2.8. Composting of Polymers: Thermophilic Respirometry

A Columbus Instruments 24-vessel Micro-Oxymax respirometer (Columbus, OH, USA), equipped with a computer and software to archive and display the biodegradation results, was used for the composting studies. The instrumentation includes a nondispersive mid-IR (NDIR) 0–10% CO₂ sensor, expansion interfaces to direct sampled gas through a common CO₂ sensor, fresh air pump, water vapor condensers, and an environmental enclosure for monitoring and controlling respirometry studies. The 4.26 μm NDIR sensor was calibrated using a certified 9.02 ± 0.02% CO₂ (with N₂ gas diluent) primary standard (Praxair, Morrisville, PA, USA). Fresh ambient air was continuously introduced to the vessels using a linear diaphragm pump with a calibrated flow rate of 250 mL/min. Other timed instrumental settings include sensor purge = 240 s; purge measurement = 60 s; sample circulation = 240 s; and sample measurement = 60 s.

Polymer samples were added to the compost as powders, cylindrical slugs, or films, in which the dimensions of the resin particles were always less than 2 cm × 2 cm, as per ASTM D5338. After adjusting the dry solids of the compost to ca. 50% solids, exactly 25, 30, or 100 g of polymer was mixed with 600 g of compost (dry basis). The compost mixture was subsequently added to a 5 L Pyrex glass respirometry vessel. Twenty-four sample vessels were then placed in a heated environmental enclosure (58 ± 0.5 °C) and incubated for ≥45 days (positive control) or ≤180 days. Every seventh day, the vessels were removed from the chamber and manually shaken to inhibit air and water vapor channeling. In our investigation, the Micro-Oxymax was operated in open circuit mode at ambient atmospheric pressure, wherein the system purges measured CO₂ from the system after successively logging the differential CO₂ concentration between the atmosphere and each composting vessel. The production of CO₂ was automatically logged every 4 h.

2.9. Differential Scanning Calorimetry (DSC)

Using a TA Instruments Q2000, DSC experiments were performed to determine the glass transition (T_g) and melting temperatures (T_m) of PCL, PLA, and Nylon 6 by evaluating the second heats, wherein the polymers were heated in perforated aluminum DSC hermetic pans from −100 °C to 200 °C or 300 °C at 10 °C/min under a 50 mL dry nitrogen purge. TA Instruments Universal Analysis v4.5A software was used to evaluate T_g and T_m.

2.10. Preparation of Hair Fibers for Protein Biodegradation

For studies of the biodegradation of human hair (i.e., keratin protein), a comparison of the compostability of virgin and bleached hair was performed. European dark brown hair was purchased from International Hair Importers, Inc. (Glendale, NY, USA). Hair was bleached using Clairol Professional Basic White (The Wella Corporation, Calabasas, CA, USA) bleaching powder containing potassium persulfate, ammonium persulfate, sodium metasilicate, sodium stearate, silica, hydroxypropyl methylcellulose, aluminum distearate, sodium lauryl sulfate, and disodium EDTA. The bleaching powder (120 g) was mixed with 147 mL of Salon Care Professional 20 volume developer (Arcadia Beauty Labs LLC, Reno, NV, USA), according to the manufacturer’s instructions. The developer contained water, 6% (w/v) H₂O₂, and phosphoric acid. Prior to placing the hair in the respirometer vessels for the biodegradation studies, the fibers were cut into shorter 3–5 mm snippets to optimize the homogeneity of the resultant compost matrix.

2.11. Seed Types Used in Germination Studies

To appraise the terrestrial safety of successfully biodegraded plastics, Champion radish (JD Hardware LLC, Wellsville, UT, USA), Purple Top White Globe turnip (Burpee, Warminster, PA, USA), and Heirloom Robust barley (The Plant Good Seed Company LLC, Ojai, CA, USA) seeds were chosen for the germination studies.

2.12. Compost Viability: Seed Germination Studies

ASTM D6400 and OECD Guideline 208 specify that materials may be designated as compostable if the compost with degraded plastic passes terrestrial safety testing protocols [47,50]. One directive involves monitoring seed germination, where trends in compost viability factors, including the visual assessment of germination, fresh shoot weight or height, and plant health, are contrasted with seedling studies implemented with untreated compost [47]. ASTM D6400 indicates that the germination rate of the composted mixture shall be no less than 90% of the germination rate observed in blank compost. ASTM D6400 adds modifications to OECD Guideline 208, which are found in Annex E of EN 13432. One such modification states that 25–50% of the potting soil mixture should include compost containing the disintegrated polymers [46].

In the assays, 300 g of compost containing degraded polymers from the respirometric studies was thoroughly blended with 600 g of fresh compost. Radish, turnip, and barley seeds were then placed in contact with the mixture, and compost viability factors were documented after ca. 50% of the seedlings emerged in the vessel containing only untreated compost (control). Each viability test was performed in triplicate, where six seeds were added to three 4-inch diameter plastic gardening pots (n = 18) that had been filled with blended compost. In each pot, the six seeds were evenly dispersed and covered with approximately ¼ inch of compost. After watering generously, a transparent humidity dome was placed on each pot to minimize moisture evaporation. After germination, the humidity domes were removed from the gardening pots to boost light exposure for the developing florae (Figure 1). A broad spectrum (400–840 nm) 30 W full-spectrum LED grow light with a 12 h on/off schedule setting (Jinhongtu, Zhongshan City, China) was used to simulate sunlight. Before germination, the light source was positioned 4 inches above the surface of the seeded compost mixture and the intensity of the visible light spectrum was measured with a BTMeter (Zhuhai City, China) BT-881E digital lux meter (9010–10,430 lux). After germination, the LED source was raised to accommodate the growing plants, whereupon the measured light intensity at 10 inches from the compost surface was diminished to 2960–3163 lux. The ambient conditions in the laboratory were 23 ± 2 °C and 33–46% RH. Compost viability factors were documented for ≥15 days.

3. Results and Discussion

Throughout aerobic biofragmentation, assimilation, and carbon mineralization, microbes use enzymes and processes such as the electron transport cycle to systematically convert polymeric intramolecular bonding energy into fuel, which is subsequently used for cellular activities including reproduction, cell growth, ion transport, and chemical synthesis. The net catabolic activity of cellular respiration includes the incorporation of nutrients and energy into cells (i.e., assimilation) and the sequential oxidation (i.e., mineralization) of oligomeric carbon into CO₂ gas or carbonate salts. In the present work, we used respirometry and guidelines in ASTM D5338 and ASTM D6400 to examine the carbon mineralization rates of several common polymers using municipal compost and quantitative CO₂ gas detection.

3.1. Biodegradation of Common Personal Care Polymers

The 90-day composting results are summarized below, detailing the relative biodegradability, as well as the absolute rate and extent of mineralization for the composted polysaccharides, proteins, and synthetic polymers. The variation in the biodegradation trials (n ≥ 3) is indicated by the sample standard deviation (sd) at 90 days of thermophilic composting. Generally, the precision between trials decreases at shorter times and increases as mineralization rates plateau.

3.2. Biodegradation of Natural Polymers

Table 4. Thermophilic biodegradation of polysaccharides using municipal compost.

Polymer ID	Relative Biodegradability (%)	Absolute Biodegradability (%)
		5	10	20	30	60	90 days
Sodium alginic acid	100	50	96	122	175	177	179 ± 4.7
Soluble starch (potato)	100	57	89	109	119	134	144 ± 3.0
Potato starch	100	87	96	99	101	128	140 ± 4.9
Xanthan gum	100	39	67	98	102	121	122 ± 5.7
Citrus peel pectin	100	51	63	74	78	100	120 ± 6.2
Guar gum	100	35	60	89	100	101	102 ± 6.2
MCC-2	100	16	33	57	83	94	99 ± 4.7
MCC-1	100	19	32	58	81	90	97 ± 5.1
Cassia gum	100	77	89	92	95	95	97 ± 4.7
Locust bean gum	100	71	80	84	86	89	94 ± 5.2
Amylopectin (maize)	100	69	80	84	86	92	93 ± 2.7
α-Cellulose	100	6	13	28	48	82	88 ± 6.7
CMC-4 (powder)	100	5	10	38	65	85	87 ± 6.4
Chitin	100	17	39	55	63	75	85 ± 2.4
CMC-1	100	5	12	33	55	72	84 ± 7.7
SigmaCell Type 20 cellulose	100	58	73	79	81	81	81 ± 4.2
CGG-2	98	12	43	58	66	74	79 ± 5.7
CGG-1	95	19	33	51	63	70	77 ± 4.4
CMC-4 (film shards)	93	14	21	46	57	64	75 ± 8.7
κ-Carrageenan	90	3	13	36	49	67	75 ± 6.6
CMC-3	90	39	59	68	73	73	73 ± 6.9
CGG-3	90	37	42	49	56	63	72 ± 4.7
Chitosan (pH = 6.0)	22	2	4	8	10	12	18 ± 3.7
Chitosan (as received)	0	0	0	0	0	0	0

presents biodegradation data for several classes of natural and modified carbohydrates, including starches, marine carbohydrates, natural galactomannans, cationic guar gum (CGG), MCCs, and sodium carboxymethyl cellulose (CMC). The examined starches comprised natural potato, soluble potato, and maize amylopectin, whereas the composted marine carbohydrates included chitin, chitosan, carrageenan, and sodium alginate. In addition, the biodegradability trends of natural galactomannans, including guar gum, locust bean gum, and cassia gum, are compared to the those of CGG (i.e., CGG-1, CGG-2, and CGG-3). Lastly, Table 4 contrasts the composting results of several MCC powders against the biodegradation rates of various CMC grades (i.e., CMC-1, CMC-3, and CMC-4).

At first glance, Table 4 suggests that several polysaccharides biodegraded more than 100% on an absolute basis; however, the phenomenon is rationalized by considering the priming effect, which expedites the decomposition of indigenous organic matter upon adding monosaccharides or simple polysugars, including polymeric glucose, to neat compost. Although the mechanism is not completely understood, it is known that priming with simple carbohydrates introduces surplus chemical energy that stimulates microbial activity [51,52,53]. Our results suggest that complex carbohydrates such as starch, sodium alginate, xanthan gum, citrus peel pectin, and hydrolyzed soy protein also induce positive priming (see Table 4 and

Table 5. Thermophilic biodegradation of proteins using municipal compost.

Polymer ID	Relative Biodegradability (%)	Absolute Biodegradability (%)
		5	10	20	30	60	90 days
Soy acid hydrolysate	100	78	99	118	135	147	151 ± 8.4
Zein	100	78	85	95	98	98	98 ± 7.4
Bovine gelatin	100	82	90	91	91	91	91 ± 6.7
Keratin (bleached hair)	100	45	76	87	87	87	87 ± 7.8
Keratin (virgin hair)	11	1	5	8	9	9	9 ± 2.5

).

To elucidate the priming contributions, slugs of sodium alginate, xanthan gum, potato starch, soluble starch, citrus peel pectin, and hydrolyzed soy protein were manually compressed into 8–10 mm thick cylinders using a rubber mallet and 9 mm diameter stainless-steel flat-faced tableting punches. The slugs were then composted, whereby slug disintegration was used as a visual cue for monitoring the disintegration rates of each bioplastic. In contrast to the attrition rate of SigmaCell Type 20 slugs, in which erosion in compost was slow and absolute biodegradation plateaued at 82–86%, thermophilic respirometry of sodium alginate, natural potato starch, xanthan gum, soluble potato starch, pectin, and hydrolyzed soy protein slugs induced rapid slug disintegration and ≫100% absolute biodegradation. Indirectly, the results suggest that the disintegrated bioplastics wholly mineralized; however, it is also possible that priming preferentially mineralized compost organic matter. To discriminate between the CO₂ released from catabolized bioplastics and the CO₂ liberated from biodegraded compost carbon, sodium alginate, xanthan gum, natural potato starch, citrus peel pectin, and hydrolyzed soy protein were composted in carbon-free vermiculite using guidelines in ISO 14855-2 [45]. As an example, Figure 2 presents the biodegradation rate data for xanthan gum, which is a high MW biopolymer of glucose, mannose, and glucuronic acid. The overlay demonstrates that xanthan gum powder and xanthan gum slugs degraded > 120% when mineralized in municipal compost, whereas the biodegradation of xanthan gum powder composted in activated vermiculite plateaued at ca. 100%. The results indicate that xanthan gum introduced positive priming, in which xanthan gum likely mineralized, while surplus CO₂ biogas was generated by augmented mineralization of indigenous carbon in the neat compost. Like xanthan gum, other tested bioplastics exhibited positive priming in composting studies but demonstrated 45-day biodegradation plateaus at ca. 100% mineralization in activated vermiculite, including sodium alginate = 100.4%; potato starch = 100.7%; citrus peel pectin = 94.6%; and hydrolyzed soy protein = 93.8%.

Not surprisingly, Table 4 indicates that starches exhibit positive priming in municipal compost. Natural potato and corn starches are polymers of glucose and contain ca. 25% amylose and 75% amylopectin. Amylose is the linear fraction and is polymerized with α-1,4 glycosidic linkages. Similarly, amylopectin contains glucosyl units joined with α-1,4 glycosidic bonds but has a much higher degree of polymerization (DP) than amylose, including intermittent branching with α-1,6 glycosidic side chains. In our composting studies, soluble potato and natural potato starches demonstrated similar rates and extents of hydrolysis, suggesting that enzymatic digestion may be linked to amylopectin content [54]. However, although the initial fragmentation rates of maize amylopectin and natural potato starch were similar, the mineralization rate of maize amylopectin decreased markedly after 7–10 days, indicating that higher DP chains in amylopectin likely influenced the rate of hydrolysis at >80% biodegradability. Moreover, the immediate hydrolysis of soluble potato and natural potato starches (50–90% in ≤5 days) suggests the presence of thermally stable exocellular amylases in the compost at the start of testing, where the rapid enzymatic production of glucose monomers and oligomers in the compost likely activated positive priming.

Table 4 also details the mineralization rates of several marine carbohydrates, including sodium alginate, chitin, carrageenan, and chitosan. Sodium alginate is a linear polysaccharide derived from brown seaweed and is a biopolymer of the sodium salts of mannuronic and guluronic acids. Like starch, sodium alginate demonstrated significant priming effects, including rapid initial mineralization, as well as 179% absolute biodegradability after 90 days of thermophilic composting. The rapid initial biodegradation of sodium alginate suggests that active exocellular hydrolytic enzymes may be present in the compost, demonstrating that previous secretions from the compost inocula “recognized” the added carbohydrate. Moreover, the microorganisms appear to possess the enzymatic machinery to mineralize mannuronic and guluronic acids while simultaneously increasing the biofragmentation of indigenous carbon in the compost. Although compositionally different, like sodium alginate, carrageenan is another high MW extract derived from (red) seaweed. As shown in Figure 3, the κ-isomer of carrageenan may be described as a sulfated (one ester sulfate per repeating unit) anionic gum composed of alternating units of β-D-galactose and 3,6-anhydro-α-D-galactose, which are linked by α-(1,3) and β-(1,4) glycosidic bonds. Although κ-carrageenan was ultimately consumed by composting microbes, Table 4 shows that the initial biodegradability rate was slow and there was a lack of priming compared to sodium alginate, indicating that the indigenous compost microbiome initially lacked the ability to rapidly mineralize κ-carrageenan.

Table 4 also summarizes the biodegradation results for shrimp chitin and two composting experiments using chitosan. Chitin is the second most abundant polysaccharide found in nature—cellulose is the first. It plays an important role in the structural integrity of the cell walls of fungi and is also a key element of the exoskeleton of crustaceans and insects. The chitin used in our study is a water-insoluble carbohydrate isolated from the exoskeleton of marine shrimp and is a polymer of N-acetylglucosamine (Figure 4a). Like chitin, our chitosan was sourced from shrimp shells and is compositionally the deacylated form of chitin (Figure 4b). In our experiments, chitin steadily biodegraded > 95%, revealing no priming effects. Curiously, in opposition to published results, low MW linear chitosan with 76% deacetylation exhibited limited mineralization in municipal compost [55]. Instead, chitosan remained persistent until the powder was slurried in an aqueous HCl-adjusted pH 6 solution, dried, and then introduced to the compost. Ultimately, the acid-treated chitosan slowly biodegraded ca. 40% in 180 days, clearly substantiating that deacetylating chitin decelerates mineralization kinetics.

Galactomannans are polysaccharides with β-(1,4)-mannopyranose (M) backbones and α-(1,6)-galactopyranose (Ga) linked side groups, whereupon the M/Ga ratio of the seed extract varies depending on the plant source from which it is derived. Table 4 documents the biodegradability results for guar, locust bean, and cassia gums, which have M/Ga ratios of 2:1, 4:1, and 5:1, respectively. The data suggests that higher mannose content leads to greater initial biodegradation (absolute), in the order of cassia gum (89%) > locust bean gum (80%) > guar gum (60%) after 10 days of composting. However, after 90 days of thermophilic composting, each polysaccharide similarly mineralized ca. 100% (relative). Interestingly, the initial biodegradability of galactomannans in compost contrasts with trends in water solubility, wherein the solubility of galactomannans increases with increasing galactose content. CGG is a derivatized form of guar gum that includes a (2-hydroxypropyl)trimethylammonium chloride group on the pendant D-galactose unit (Figure 5). Note that the three grades of CGG enumerated in Table 4 have similar cationic charge densities; however, the marketed 1% (w/w) Brookfield viscosity values are ranked as follows: CGG-1 > CGG-3 ≫ CGG-2. Additionally, Table 4 documents the mineralization results for CGG-1, CGG-2, and CGG-3, in which the CGGs were comparably degraded by exocellular gumases and/or galactomannanases ca. 60–70% (absolute) in less than 30 days, while mineralizing >85% (relative) in ≤90 days. Comparing the mineralization rates of natural guar gum to the examined CGG grades, quaternization of guar gum appears to slow but not obstruct the extent of biodegradation.

Additionally, Table 4 contrasts the biodegradability of MCC against commercial grades of CMC. MCC-1 (DP = 309), MCC-2 (DP = 309), and SigmaCell Type 20 (DP = 321) MCC grades have much lower DP values than α-cellulose (DP = 4389); furthermore, the average particle sizes for the powders vary between D_V50 = 20 and 120 μm, with MCC-2 and α-cellulose having the largest particles [56,57]. Consequently, the trends in Table 4 demonstrate that higher MCC powder mineralization rates are more associated with lower MW cellulose grades rather than a specific particle size distribution. CMC is derived from cellulose, a polysaccharide that is the most abundant biopolymer in nature and is found in the woody parts and cell walls of plants. CMC is used extensively as a rheology modifier, film former, and water retention agent in cosmetics and oral care preparations. The MW trends are as follows: CMC-4 (10⁶ Da) > CMC-3 ≫ CMC-1 (10⁵ Da). As indicated by the structural formula in Figure 6, there are several possible chemical structures that may evolve during the carboxymethyl substitution of cellulose. For example, carboxymethylation can take place at any of the three hydroxyl substituents on each anhydrous cellulose ring, wherein the degree of substitution (DS) is the average number of hydroxyl groups on the ring that have been substituted with carboxymethyl moieties. The examined CMC lots possessed statistically similar DS. In distinction, there were significant particle size differences between the CMC samples: CMC-3 powder (ca. D_V50 = 500 μm) and CMC-4 film shards (ca. 1 cm × 1 cm × 2 mm) have >10-fold larger particle sizes than CMC-1 and CMC-4 powders. Comparison of the mineralization rates between the CMC grades demonstrates that vessels containing larger CMC particles showed higher initial biodegradation rates: in 10 days, CMC-3 (59%) and CMC-4 film shards (21%) mineralized polymeric carbon more rapidly than CMC-1 (12%) and CMC-4 powders (10%). One likely reason for the initial trend is that larger CMC particles less intimately complex compost particles, wherein rapid CMC dissolution can raise the bulk compost viscosity and deleteriously affect air and moisture diffusion in the composting vessels.

Consequently, higher initial mineralization rates for the CMC grades are more associated with particle morphology than DP, MW, or chemistry; however, we concluded that while MCC mineralized more rapidly, the CMC bioplastics—specifically CMC-1 powder, CMC-3 powder, CMC-4 powder, and CMC-4 film shards—still achieved a high degree of mineralization, exceeding 90% within 90 days. More than likely, the slower erosion of larger particles in CMC-3 powder and CMC-4 film shards limited their initial biofragmentation rates, but the polymers still reached substantial mineralization. This suggests that while municipal compost microorganisms are more adapted to breaking down unmodified cellulose, CMC materials can still be effectively biodegraded, although at slower mineralization rates.

The remaining carbohydrate in Table 4 is citrus peel pectin. Citrus peel pectin is a structural heteropolysaccharide and is used as a gelling agent, especially in the food industry. Pectins consist of linear backbones of α-(1,4)-D-galacturonic acid residues partially esterified with methanol. In addition, neutral sugar side chains and periodic L-rhamnose residues introduce irregularities in the backbone [58]. Citrus peel pectin demonstrated positive priming by mineralizing 120% in 90 days, suggesting that pectin increases the microbial degradation of compost carbon while lytic enzymes such as exocellular esterases, depolymerases, and pectinases successfully mineralize pectin chains [59].

3.3. Biodegradation of Proteins

Table 5 summarizes the biodegradability of several proteins in municipal compost. Proteins with predominantly linear polyamide backbones, including zein, bovine gelatin, and hydrolyzed soy protein, degraded ≥90% in less than 20 days. In addition, hydrolyzed soy protein demonstrated positive priming, in which ca. 15% excess of the maximum theoretical CO₂ was measured at 15 days. Interestingly, bleached keratin fibers fully mineralized, on a relative basis, whereas virgin hair only decomposed ca. 11% after 90 days. The results suggest that our municipal compost microbiome lacked specialized enzymes such as disulfide reductase for catalyzing cystine breakage in virgin human hair fibers; however, the microorganisms evidently secreted exocellular proteases and keratinases to effectively mineralize protein chains devoid of disulfide crosslinks that had been chemically oxidized to cysteic acid.

3.4. Biodegradation of Synthetic Polymers

Conventionally, synthetic polymers have been used in personal care products due to their purity, performance benefits, tunability, and acceptable toxicological profiles.

Table 6. Thermophilic biodegradability of synthetic polymers using municipal compost.

Polymer ID	Relative Biodegradability (%)	Absolute Biodegradability (%)
		5	10	20	30	60	90 days
PCL (80 kDa), 30 g	100	5	13	39	70	99	102 ± 5.4
PLA (60 kDa)	100	10	11	31	35	61	90 ± 3.4
PCL (80 kDa), 100 g	100	6	13	27	30	32	32 ± 6.4
Poly(OAA/Acrylates/BAEM)	15	10	12	12	12	12	12 ± 3.4
Polycaprolactam	12	8	10	10	10	10	10 ± 1.2
PS	6	5	5	5	5	5	5 ± 0.4
PVA, ≥31 kDa, ≥98% hyd.	4	3	3	3	3	3	3 ± 0.4
Crosslinked pAA	3	2	2	2	2	2	2 ± 1.5
PVA, 30 kDa, ≥85% hyd.	1	1	1	1	1	1	1 ± 0.4
Polyethylene	0	0	0	0	0	0	0

details the composting results for the synthetic polymers. As expected, polyethylene and PS powders showed negligible biodegradability in municipal compost. Interestingly, the two grades of 30 kDa PVA (ca. 85 and 98% hydrolyzed) demonstrated insignificant biodegradability (ca. 1–3%) as well. The low biodegradability of PVA in compost differed from the outcomes reported in published enhanced OECD 301B aquatic studies, which reported > 60% degradation in less than 60 days [60]. In OECD biodegradation studies, PVA is studied in dilute aqueous solution (100–400 ppm), and the PVA chains are less likely to aggregate into crystal structures; however, in laboratory composting, semicrystalline PVA is ca. 3–8% (w/w) of the composting mixture. Consequently, in our compost testing, we observed that PVA particles remained intact, suggesting that the available moisture and exocellular lytic enzymes in the compost had little effect on disrupting strong interchain hydrogen bonding and the inherent crystallinity of PVA. Hence, it is probable that the successful biodegradation of PVA in OECD aquatic studies is linked to particle morphology, solubility, and acclimated aquatic sludge microbiota and implies that our local municipal compost lacked the compulsory microbiome and adapted exocellular enzymes to disrupt intra- and interchain hydrogen bonding and hydrolyze 1,3-diol repeating units in PVA.

Like polyethylene, PS, and PVA synthetics, crosslinked pAA, and poly(OAA/acrylates/BAEM) contain persistent -C—C- backbones and are common polyacrylates that are routinely formulated into commercial hair care styling gels and sprays. Although the crosslinked anionic thickener showed negligible biodegradability, poly(OAA/acrylates/BAEM) demonstrated 12% absolute mineralization in municipal compost; however, increasing degradation ceased after ca. ten days. The remaining synthetics, including PCL, PLA, and polycaprolactam, contained hydrolyzable ester and amide chain backbones. PCL (30 g) showed the swiftest biodegradability in municipal compost, fully degrading in <60 days. PCL is a thermoplastic polyester derived from petroleum and, due to its low glass transition (T_g = −60 °C) and melting temperatures (T_m = 60 °C), the polymer softens at the ASTM D5338 thermophilic testing temperature. Although PCL (30 g) mineralized, it should be noted that PCL charged at 100 g solids did not fully degrade and instead plateaued at 32% mineralization. One plausible explanation for poorer mineralization at higher PCL polymer solids may be souring of the compost, in which hydrolysis of numerous ester bonds lowered the pH of the compost matrix from pH 7.4 to pH 5.7. Compared to PCL (30 g), PLA (30 g) also fully degraded, but the rate of enzymatic hydrolysis was much slower, requiring > 90 days to fully mineralize. The slower mineralization rate of PLA may be partially linked to differences in thermal properties, wherein PLA is a bio-based thermoplastic with higher T_g (60 °C) and T_m (170 °C) values.

Polycaprolactam is a tough, hydrolytically stable polyamide (Nylon 6) with T_g = 50–55 °C and T_m = 260 °C. In contrast to polyesters, polycaprolactam degraded only 10%, indicating that unfragmented polycaprolactam remained solid and semicrystalline during thermophilic composting. This result advocates that exocellular enzymes such as lipases, esterases, and proteases may selectively degrade proteins but do not necessarily catalyze chain scission in all polyamides.

3.5. Seed Germination Studies

We conducted phytotoxicity assessments of the biodegradable bioplastics and synthetic polymers by determining the seed germination and initial growth behavior of radish, turnip, and barley seeds in a mixture containing composted plastics.

Table 7. Compost terrestrial safety of biodegradable polymers: seed germination success summary (15-day growth). Note that the absolute germination success (%) is reported (see Section 3.5 for pass/fail criteria).

Polymer ID	% Germinated (Radish)	% Germinated (Turnip)	% Germinated (Barley)	Pass/Fail
Amylopectin	94	89	94	Pass
CMC-1	83	83	89	Pass
CMC-3	89	89	100	Pass
CMC-4	83	83	83	Pass
MCC-1	94	89	89	Pass
MCC-2	89	89	89	Pass
κ-Carrageenan	89	89	94	Pass
Cassia gum	89	89	94	Pass
Chitin	94	94	83	Pass
Compost	89	83	89	Pass
Gelatin	94	89	89	Pass
Guar gum	100	89	94	Pass
Keratin (bleached hair)	94	94	100	Pass
Locust bean gum	94	94	89	Pass
CGG-1	100	89	94	Pass
PCL (30 g)	94	89	89	Pass
Pectin	89	89	94	Pass
PLA (30 g)	94	94	94	Pass
Potato starch	94	100	100	Pass
SigmaCell Type 20	94	94	89	Pass
Sodium alginate	89	83	94	Pass
Soluble starch	83	89	83	Pass
Soy protein acid hydrolysate	94	94	89	Pass
CGG-2	94	100	89	Pass
CGG-3	94	89	100	Pass
Xanthan gum	94	83	83	Pass
Zein	94	94	83	Pass
α-Cellulose	100	94	94	Pass

provides the seed germination success results for all biodegradable polymers, revealing that each plastic mineralized and produced compostable mixtures with suitable seed germination properties. Specifically, ASTM D6400 and OECD Guideline 208 require that ≥78% of radish seeds, ≥77% of turnip seeds, and ≥82% of barley seeds germinate in a timely manner to pass terrestrial safety standards. In addition, for polymers demonstrating successful biodegradability and germination, Figure 7 illustrates the requisite average growth rates for radish, turnip, and barley seedlings in our neat compost (n = 18), wherein each curve is bounded by ± 10% pass criteria limits defined by ASTM D6400 and OECD Guideline 208. In our studies, all seeds that germinated also demonstrated successful 15-day shoot growth rates [47,50].

It should be noted that although PCL (30 g) was deemed compostable, the polyester showed unacceptable biodegradation and germination performance when the mass of PCL in the compost was increased to the ASTM D5338 recommended 100 g. Akin to ineffective biodegradation, seed germination failure was likely related to increased hydronium concentration and souring of the bulk compost matrix, whereupon the composted PCL matrix was evaluated as weakly acidic (pH = 5.7).

3.6. Comparing Aquatic and Composting Degradation Rates

Unlike OECD aquatic biodegradability testing, ASTM D6400 does not incorporate acceptability criteria for labeling a system as inherently, readily, or ultimately biodegradable by simply assessing short-term mineralization rates. Instead, a system passes or fails ASTM D5338, and ASTM D6400 subsequently labels the plastic as biodegradable if the polymeric carbon is mineralized ≥90% in ≤6 months. Nevertheless, for research purposes, we informally contrasted internal 28-day OECD aquatic degradation rate data (unpublished) against 28-day composting results.

Figure 8 compares select polymer mineralization outcomes from aquatic testing using OECD 301D [23], OECD 306 [24], and OECD 302B [26] against respective composting results from ASTM D5338 composting studies. Table 4 and Figure 8 indicate that CGG-1, CGG-2, cellulose, and CMC-2 demonstrated steady biodegradation in municipal compost. Additionally, keeping in mind that OECD testing is typically restricted to 28 days at 20–28 °C, it is fair to conclude that CGG-1, CGG-2, cellulose, and CMC-2 demonstrated meaningful biodegradability in aqueous media treated with activated sludge using OECD 301D or OECD 302B methodology. Furthermore, the results revealed that compost inocula mineralized water-insoluble cellulose more rapidly than the sludge microbiota; however, the data in Figure 8 also indicate that higher MW chains (DP = 4389) inhibited the biofragmentation rate of cellulose in both municipal compost and aqueous media inoculated with activated sludge. In distinction, the aqueous mineralization of CGG in compost was significantly slower than the fragmentation rates measured in water that had been inoculated with activated sludge and the OECD 302B biodegradation testing protocol. Finally, composting and aquatic biodegradability rates were contrasted for PCL and PVA, in which aquatic OECD 306 and OECD 301D biodegradation methods were used, respectively, to assess aquatic biodegradability. Interestingly, PCL displayed inherent biodegradability in seawater while also degrading quickly in compost (>60% mineralization in 28 days using 30 g PCL). Conversely, PVA yielded mixed results, in which PVA exhibited persistence in municipal compost while demonstrating inherent biodegradability in an aquatic environment using activated sludge and OECD 301D.

OECD aquatic biodegradation protocols are performed mesophilically (10–42 °C), whereas ASTM D5338 testing is accomplished thermophilically (>42 °C); hence, we expected notable differences in biodegradation that could be linked to the 58 °C testing temperature, physical phase of the tested polymers, active microbiome diversity, and distribution of lytic enzymes. Interestingly, the results generally indicate that materials which degrade in aqueous media also degrade in municipal compost. Nonetheless, comparisons between composting and OECD biodegradation results for PVA, PCL, and chitosan (discussed in Section 3.2) suggest that positive correlations between OECD aquatic and ASTM D5338 outcomes should be measured rather than assumed. Furthermore, standard OECD aquatic protocols use ≤ 400 ppm of polymer, whereas our composting studies included ca. 3–9% (w/w) resin; consequently, polymer degradation rates in water and compost should not be expected to correlate linearly, with CMC-2 being a notable exception (R² = 0.99). That is, in most cases, we observed that the initial mineralization rates of dilute and biodegradable polymer chains in aqueous media superseded the biodegradation rates of solid polymer aggregates dispersed in a solid matrix of damp municipal compost particles.

Although aquatic testing biodegradation protocols are primarily used to assess the enzymatic degradation of polymeric waste from rinsed-off personal care formulations, we believe that it is worthwhile to consider the fate of municipal sludge and its potential use in downstream home garden and agricultural applications. For example, in research performed in Spain, the number of microplastics isolated from sludge-treated crop fields increased by ca. 700 microplastic particles per kilogram of soil with each additional batch of sludge, and the distribution of microplastic contaminants in the soil was ubiquitous, where non-degrading plastic particles were isolated from 97% of analyzed soil samples [35,36]. Results from a second study suggest that microplastics may indeed influence the vigor of beneficial soil macro- and microorganisms—in the analysis, polyethylene particles were added to sludge and were found to negatively affect the health and production of vermicomposting earthworms, as well as the intrinsic biodiversity of bacteria in the biosolids [37]. Other researchers have examined the sluggish mineralization kinetics of modern synthetic plastics; using carbon isotope assays of personal care polymers, Gaytán and Wilske reviewed the fate of recalcitrant polyacrylates and demonstrated that mineralization rates in cultured aqueous media (0.200%/day) were much different than decomposition rates observed in soil (0.001%/day) [38,39,40,41].

4. Conclusions

In our studies, the majority of examined bioplastics demonstrated satisfactory biodegradation, seed germination, and shoot growth rates, indicating they are compostable according to ASTM D5338 methodology and ASTM D6400 criteria. However, macromolecules with -C—C- backbones resisted carbon mineralization in composting experiments, unlike most carbohydrates, proteins, and labile polyesters. This suggests that our municipal compost lacks the adapted thermophilic microbiome and enzymatic activity needed to hydrolyze carbon-carbon chain backbones effectively.

Regarding the applicability of ASTM D5338 for modeling the genuine deposition and mineralization of personal care polymers from sludge into compost-amended soil—dissolution, dilution, and diffusion of resin chains in municipal waste streams likely reduce the influence of bulk particle properties on polymer biodegradation rates. In addition, ASTM D5338 was designed to examine the biodegradation of plastics in municipal compost at 58 °C, whereas the authentic range of bioactivity in soil is ca. 10–30 °C. Therefore, to research the biodegradability of very dilute polymer concentrations in sludge, compost, or soil effectively, ASTM standard biodegradation protocols should be adapted to minimize polymer chain aggregation while targeting the expected biodegradation environment and optimizing the instrumental CO₂ detection limit.