Hydrogels and Biodegradable Polymers for Bioapplications

Chapter 10

Biodegradability of Polycarboxylates: Structure—Activity Studies M. B. Freeman, Y. H. Paik, G. Swift, R. Wilczynski, S. K. Wolk, and Κ. M. Yocom Downloaded by NORTH CAROLINA STATE UNIV on October 9, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0627.ch010

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Rohm and Haas Company, 727 Norristown Road, Spring House, PA 19477

It is generally recognized that in order to improve the environmental acceptability of household laundry detergents, biodegradble replacements for the polycarboxylate components of these detergent formulations are needed. Although the poly(acrylic acid) homopolymers and copolymers currently in use are not considered harmful to the environment, they are not readily biodegradable; they enter environmental compartments beyond the sewage treatment plant. Poly(aspartic acid), on the other hand, has recently emerged as a benchmark for the biodegradability of synthetic polycarboxylates which exhibit acceptable detergent performance. Semi-continuous activated sludge (SCAS) removability tests and modified Sturm CO production tests show that poly(α,β-D,L-aspartates) prepared via a phosphoric acid-catalyzed thermal polymerization process are rapidly and completely biodegraded by municipal treatment plant microorganisms. Polyaspartates prepared via uncatalyzed thermal polymerizations or maleic anhydride/ammonia-based processes are only partially biodegradable in these tests. Structural analysis indicates that a linear polyamide backbone is key to the total biodegradability of the acid-catalyzed polyaspartates. For the poly(acrylic acids) (C-C backbone), modified Sturm tests show that molecular weights must be in the oligomer range for total biodegradability. 2

Polycarboxylates - in particular, poly(acrylic acid) and copolymers of acrylic acid and maleic acid (Figure 1) - are widely used in low-phosphate and phosphate-free household laundry detergents. These polycarboxylates are generally found in zeoliteor soda ash-built detergent formulations at about 2-5 wt. % as builder assists (or cobuilders). As such they improve the cleaning performance of these powdered detergents by dispersing soils (thus improving soil removal and helping prevent soil redeposition) and by inhibiting the crystal growth of inorganic salts (thus preventing the incrustation of salts, e.g., calcium carbonate, on fabrics). Polycarboxylates are also known to function as process aids in the production of powdered laundry 1

Corresponding author

0097-6156/96/0627-0118$15.00/0 © 1996 American Chemical Society

In Hydrogels and Biodegradable Polymers for Bioapplications; Ottenbrite, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Downloaded by NORTH CAROLINA STATE UNIV on October 9, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0627.ch010

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detergents. To a lesser extent, polycarboxylates find utility as dispersants in such products as automatic dishwashing detergents and industrial and institutional cleaners. The switch from phosphate-built laundry detergents to the no-P formulations has been driven primarily by environmental concerns, namely by the need to minimize the eutrophication of lakes and streams. Currently the use of phosphates in home laundry detergents is banned in many areas. The environmental fate and effect of all the components of laundry detergents (and other cleaning products as well) are coming under closer scrutiny. Polycarboxylates are no exception. Recently reported studies on the environmental fate and ecotoxicity of the polycarboxylates currently used in household laundry detergents conclude that these materials have no adverse impact on the environment and are of extremely low toxicity (7). The necessity for these studies stems from the fact that the polycarboxylates commonly used in the U.S. (i.e., acrylic acid homopolymers, Mw -4500) and in Europe (i.e., acrylic acid/maleic acid copolymers, Mw -70,000), are not totally biodegradable. The favorable environmental assessment of these materials results primarily from the findings that they are, in large part, removed from wastewater by precipitation and/or adsorption on sewage sludge. (Some polymer does enter environmental compartments beyond the sewage treatment plant - in the aqueous effluent and/or with the sludge - thus the need for the toxicitiy testing.) Although not critical from an environmental fate/toxicity standpoint, it is generally recognized that the environmental acceptability of household laundry detergents could be improved if totally biodegradable replacements for the polycarboxylate components of these detergent formulations were available. Ideally these polymers would be totally degraded to gaseous products, minerals, and biomass at a rapid rate, i.e., before they leave the treatment plant. Since it is unlikely that an effective detergent polymer will biodegrade within the hydraulic retention time of a treatment plant (4-8 hrs.) (2), it is advantageous to have a polymer which is highly adsorptive on sewage sludge so that biodégradation can occur within the sludge retention time (-5-15 days) (2) - a more realistic goal. In the search for such a biodegradable water-soluble detergent polymer, poly(aspartic acid) has recently emerged as a benchmark (3). Poly(a,p-D,Laspartates) (Figure 1) can be prepared which exhibit acceptable detergent performance, are totally removed in the presence of sewage sludge, and are rapidly and completely biodegraded by municipal treatment plant microorganisms. We have assessed the biodegradability of polyaspartates prepared via several different synthetic routes by carrying out semi-continuous activated sludge (SCAS) removability tests and modified Sturm (CO2 production) tests. For comparison, we have also carried out some modified Sturm tests on low molecular weight poly(acrylic acids). In addition to the primary objective of synthesizing polyaspartates which are rapidly and completely biodegraded, an understanding of the structure-biodégradation relationships for the polyaspartates and the poly(acrylic acids) was also pursued. Experimental Materials. The sodium salts of poly(aspartic acid) used in the biodégradation studies were obtained by hydrolyzing polysuccinimide (poly(anhydroaspartic acid)). The polysuccinimides were prepared using several different synthetic routes (Figure 2). The synthetic methods used to prepare the polysuccinimides, and to hydrolyze them, are described below. Synthesis of polysuccinimide without an acid catalyst. L-aspartic acid was heated at either 240°C or 270°C at ambient pressure in air. Reaction times were on the order of 5-9 hours, depending on the scale of the reaction and the type of



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COOH

COOH Poly(Acrylic Acid)

COOH

COOH

Poly(Acrylic Acid-co-Maleic Acid)

COO" Na v

COO"Na

+

+

Poly(Aspartic Acid), Sodium Salt Figure 1. Detergent polycarboxylates.




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equipment used. Small lab-scale reactions were done in a beaker and the material was stirred at regular intervals. Larger scale reactions were done in a rotary tray dryer wherein the material was mixed continously throughout the reaction. Conversion to the desired product (a tan powder) was confirmed by NMR. Synthesis of polysuccinimide with phosphoric acid as the catalyst. L-aspartic acid was mixed with 85% phosphoric acid and heated at 240°C at ambient pressure in air; reaction times -2-7 hours. The amount of phosphoric acid used depended on the molecular weight desired. The higher molecular weight samples examined in this study were prepared by using up to 20 wt.% H3PO4 (85% aq. solution) in the reaction. These lab-scale reactions were generally carried out in beakers; the clumps which formed early in the reaction were ground with a mortar and pestle; thereafter the powder was stirred at regular intervals. A light tan powder was obtained. Complete conversion to polysuccinimide was confirmed by NMR. Synthesis of polysuccinimide from maleic anhydride and ammonia. Maleamic acid was prepared in toluene from maleic anhydride and ammonia gas, similarly to known methods (4). The isolated and purified maleamic acid was converted to polysuccinimide by heating it to 190-200°C for one hour. These reactions were carried out either in a plow mixer or a double planetary mixer. Complete conversion of maleamic acid was confirmed by NMR. Polysuccinimide was also prepared on a laboratory scale by passing molten maleic anhydride and anhydrous ammonia through a stainless steel tube at ~235°C. Hydrolysis of polysuccinimide to poly(aspartic acid), sodium salt. Typically, polysuccinimide powder in water was heated to 90°C and an aqueous solution of sodium hydroxide (50 wt. %) was added dropwise while maintaining the pH of the solution at about 11. After all the polysuccinimide was dissolved and the NaOH addition was complete, the solution was held at 90°C for an additional 0.5 nr. Some hydrolyses were done at lower temperatures and/or slightly higher pH. For biodégradation testing, all solutions were lyophilized to give solid sodium polyaspartate. In some cases, removal of the catalyst salts from samples prepared with phosphoric acid was desired. This was accomplished by thoroughly washing the insoluble polysuccinimide with water to remove the soluble phosphate species. This was done prior to hydrolysis. Complete removal of the phosphates was confirmed by elemental analysis. The poly(acrylic acid) homopolymers used in this study were laboratory versions of commercially available polymers. They were prepared by the free radical polymerization of acrylic acid in aqueous solution. The acrylic acid oligomer samples were obtained from a mixture of poly(acrylic acid) homopolymer solutions (Mw's -500 and 1000) by chromatographic isolation of oligomer-containing fractions (TosoHaas TSK Gel G2500PW column). The glacial acrylic acid was obtained internally (Rohm and Haas product); 2-methyl glutaric acid was obtained from Aldrich Chemical Company. Analytical Methods. All polyaspartate samples tested for biodegradability were characterized by nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), and elemental analysis. Selected samples were also subjected to additional analyses (e.g., infrared spectoscopy, mass spectrometry) for structural characterization. Poly(acrylic acids) were characterized primarily by GPC and NMR. Gel Permeation Chromatography. Aqueous GPC was used to measure the weight average molecular weights (Mw) and number average molecular weights (Mn) of both the sodium polyaspartates and the sodium salts of the poly(acrylic acid) homopolymers. A Progel TSK GMPWXL gel column, 30 cm χ 7.8 mm (Supelco, Inc., Bellefonte, PA) was used, with a 0.1 M NaS04 mobile phase and a flow rate of


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1.0 mL/min. The Mw and Mn values reported are relative to a Mw 4500 poly(acrylic acid) in-house standard, and were calculated using proprietary software. The composition of the pAA oligomer solutions was determined using six GPC columns with higher resolution for low molecular weight species, i.e., AllTech Macrosphere GPC 60 columns, 25 cm χ 4.6 mm each, with a pH 7 phosphate buffer mobile phase and a flow rate of 1.0 mL/min. Detection was by refractive index in all cases. Nuclear Magnetic Resonance Spectroscopy. Both proton ( H) and carbon-13 ( C) NMR spectroscopy were used to extensively characterize the poly(aspartic acid) samples. The levels of residual aspartic acid present during synthesis, the levels of residual succinimide following hydrolysis, the ratio of α to β linkages in the polyaspartates, and the extent of branching were all determined by NMR. All samples were analyzed on a Bruker AMX500 MHz spectrometer. Polysuccinimide samples were dissolved in DMSO-cfc, and sodium polyaspartate samples in D2O. The details of these analyses are described in Reference 5. Elemental Analysis. Lypholized samples of sodium polyaspartate were analyzed for C, Η, N, Na (and P, where applicable) prior to biodégradation testing. Analyses were done by Galbraith Laboratories, Inc. Good agreement of observed values with theory was obtained. Good agreement was also always observed between the carbon values obtained by elemental analysis on the lyophilized samples and those obtained by TOC analysis on the biodégradation stock solutions prepared from these samples. !


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Biodégradation Test Methods. All biodégradation tests were run at Roy F. Weston, Inc. (West Chester, PA) using standard test procedures and analytical protocols. The sludge for the tests was obtained from local residential/light industrial sewage treatment plants. Semi-Continuous Activated Sludge (SCAS) Removability Test. The SCAS test procedure as carried out on the sodium polyaspartate samples was similar to the OECD Guidelines Test 302A (92% and final CO2 evolution values of >78% of theory. CO2 evolution and carbon removal profiles (at 20 mg/L) for an uncatalyzed polyaspartate sample and a maleic/NH3 polyaspartate sample are shown in Figure 6. Although the rates of biodégradation for these uncatalyzed and maleic/NH3 samples as measured by CO2 evolution (0.20 day and 0.47 day , respectively) are similar to the rates of biodégradation seen for the catalyzed samples (Table 1, Figures 4 and 5), the extent of biodégradation is significantly less. In Figure 6, the C 0 evolution of the uncatalyzed sample reached only -63% of theory with 71% carbon removal; the maleic/NH3 sample values are 49% CO2 and 55% carbon removal. The bar chart in Figure 7 summarizes the final CO2 evolution and corresponding carbon removal data obtained on numerous samples of sodium polyaspartate prepared by the three different synthetic processes. The bars marked with an asterisk indicate the samples discussed above (Figures 4-6). One replicate test on a catalyzed sample is depicted (PC-2); the remaining bars represent unique samples. With the exception of samples PC-3 and PC-4, the inoculum used in each modified Sturm test was obtained from the SCAS unit used for acclimating and testing the removability of the same polymer. The inoculum for PC-3 and PC-4 was obtained from the SCAS unit used to acclimate sample PC-2. These summary charts (Figure 7) show that the H3P04-catalyzed polyaspartate samples are essentially totally biodegradable - the mean carbon removal for these samples is 98%. Mineralization of the polymeric carbon to C 0 accounts for most of the carbon removal observed - the C 0 evolution mean for these samples is 89%. In contrast, the uncatalyzed and maleic/NH3 polyaspartate samples are only partially biodegradable in this test. The mean carbon removal and CO2 evolution values for the uncatalyzed samples are 73% and 73%, respectively; for the maleic/NH3 samples, 60% carbon removal and 57% CO2. This data is summarized in Table 1 along with the mean CO2 evolution rate data for these samples. Within each group of samples, the variability of the CO2 evolution data was found to be higher than the variability of the carbon removal data. It is postulated that this may merely be a reflection of greater sample-to-sample variability in the balance of carbon converted to CO2 versus the carbon converted to biomass, while the total amount of carbon converted (i.e., removed) is relatively invariant across the samples within each group. Most of the samples tested at 20 mg/L in the modified Sturm test were also tested at 40 mg/L. At 40 mg/L the relative biodégradation behavior of the different 1


1

1

1

2

2

2



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30 35 40 45

Figure 5a. Kinetics of CO2 evolution during biodégradation of phosphoric acid-catalyzed sodium polyaspartates with and without the catalyst salts removed: Mw 8000 - with catalyst salts ( · , Sample PC-5); Mw 8000 - catalyst salts removed (•, Sample PC-6); Mw 16,000 - with catalyst salts (•, Sample PC-7); Mw 16,000 - catalyst salts removed (A, Sample PC-8). Test concentration = 20 mg active/L; SCAS inoculum.

0

5

10

15 20 25 Day

30 35 40 45

Figure 5b. Kinetics of soluble organic carbon removal during biodégradation of phosphoric acid-catalyzed sodium polyaspartates with and without the catalyst salts removed: Mw 8000 - with catalyst salts ( · , Sample PC-5); Mw 8000 - catalyst salts removed (•, Sample PC-6); Mw 16,000 - with catalyst salts (•, Sample PC-7); Mw 16,000 - catalyst salts removed (A, Sample PC-8). Test concentration = 20 mg active/L; SCAS inoculum.


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100

0

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15

20 25 Day

30

35

40

45

Figure 6a. Kinetics of CO2 evolution during biodégradation of uncatalyzed sodium polyaspartate, Mw 4000, (•, Sample UC-1) and maleic anhydride/ ammonia-based sodium polyaspartate, Mw 2000 (A, Sample MA-3) at 20 mg active/L using SCAS inoculum. 100

1 1 c* 8

80

0

CO

60 40 20 0 0

5

10

15

20 25 Day

30

35

40

45

Figure 6b. Kinetics of soluble organic carbon removal during biodégradation of uncatalyzed sodium polyaspartate, Mw 4000, (•, Sample UC-1) and maleic anhydride/ammonia-based sodium polyaspartate, Mw 2000 (A, Sample MA-3) at 20 mg active/L using SCAS inoculum.


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100


Ό

Ί>

80

ω es Ο

60

» Ο

40 20

« Μ Ν

τ»· m so r»

^ P-Catalyzed Ε Uncatalyzed Κ Maleic/NH3 Figure 7a. Summary of final CO2 evolution values from modified Sturm tests on sodium polyaspartates at 20 mg active/L using SCAS inoculum.

100100100100100

* * * • »

Τ

P-Catalyzed • Uncatalyzed S Maleic/NH3 Figure 7b. Summary of final soluble organic carbon removal values from modified Sturm tests on sodium polyaspartates at 20 mg active/L using SCAS inoculum.


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polyaspartates was the same as it was at 20 mg/L, i.e., catalyzed>uncatalyzed> maleic/ammonia, however, the extent of biodégradation was slightly less at 40 mg/1 than at 20 mg/L (Table 1). From a practical standpoint, this observed concentration dependence is of little concern since environmentally realistic concentrations of detergent polycarboxylates in U.S. sewage treatment influent is

Hydrogels and Biodegradable Polymers for Bioapplications

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