Test Method To Simulate The Turtle Digestion.

Degradation Of Different Plastic Films.

Introduction

It is known that the marine debris ingestion by sea turtles is a serious enviromental trouble.  Plastic items littered into the sea can be ingested by sea turtles and cause severe health problems.  Plastic debris can be responsible for the death of turtles by obstructing the gut.  Plastics can be also the cause of sub-lethal  effects, including absorption of toxins from the debris, mechanical abrasion or blockage of absorptive surfaces in the digestive tract and displacement of nutritious food with debris so that nutrient gain is decreased ("nutrient dilution").

The use of biodegradable plastics in place of traditional ones is considered as a possible means to avoid such problems.

The plastic items after ingestion by sea turtles will be subjected to a digestive process which will lead to a degradation consisting in a loss of mechanical properties & weight.  The degraded plastic items will therefore become weaker, suitable to transit through the gut lumen, and more easily displaced.

How is it possible to evaluate these properties & make a comparison between the effect of biodegradable plastics & the effect of the traditional ones?  An in vivo study would be the more informative, giving the possibility to directly evaluate the effect of the ingestion under real conditions & testing real animals.  This would imply the use of discrete numbers of turtles, to allow a statisical analysis;  it would also imply some casualties within the turtle population.  Since turtles were not available to us, and also trying to set up an in vitro experimental scheme to reach significant conclusions without sacrificing living animals, we decided to simulate the turtles digestion in the laboratory & study the degradation of plastic specimens under in vitro conditions.

Materials & Methods

The digestive process of turtles is divided into two phases:

• Stomach digestion, under acidic conditions (pH = 1-2)
• Intestinal digestion, under neutral conditions (pH = 6.4 - 7.5).  Anaerobic fermentation processes take place in the intestine.  The end products formed by these fermentations are considered to be absorbed & to represent very important energy sources for turtles.

The movement of digesta through the gut of reptiles is generally slow.  Transit times are reported to be in the range from 2.8 days to 21.2 days.  The transit times are affected by the diet and by the external temperature.

On the basis of this information, the following experimental test conditions have been applied.

Materials

• LDPE film from a package (thickness = 22UM) (PE)
• Film from a EPI ®  compostable bag (thickness = 30UM) RIC514 (EPI)
• Mater Bi ® NF01U film (batch 220041330) (thickness = 30UM) (NF01U)

Test Method

(1)  SPECIMENS.  Film were cut to obtain tapes of standard dimensions (ASTM 882), suitable for the ensuing mechanical characterisation.  The films were conditioned under standard conditions (23^oC 55% r.h. for 1 day) & weighed.  The specimens were closed within pockets made with a net, in order to facilitate their recovery after digestion.

(2)  SIMULATION OF STOMACH DIGESTION.  The nets have been submerged into acidic deionised water (5 litres), brought to pH = 2 with HCI.  Digestion time = 1 or 2 days.

(3)  WASHING.  A part of the specimens has been washed with deionised water after stomach digestion & submitted to phase (4).  Another part has been washed, dried, conditioned & characterised.

(4) SIMULATION OF GUT DIGESTION.  The specimens have been submerged into an anaerobic sludge sample (5 litres) withdrawn from the anaerobic digestor of the city of Novara.  The sludge has been maintained in a closed vessel and once a day the vessel manually shaken to allow homogeneity of the sludge.  The closure allows a release of overpressure, if present.  Under these conditions the specimens are exposed to an anaerobic, fermenting environment.  Digestion time = 2 or 7 days.

(5)  WASHING.  After phase (4) the films have been washed, dried, conditioned and characterised.

Experimental Set Up

In a first test, the specimens were first treated for 24 hours under acidic conditions and then for 48 hours under anaerobic conditions (1 day + 2 days).

In a second test, the specimens were first treated for 2 days under acidic conditons and then for 7 days under anaerobic conditions (2 days + 7 days).

At least 5 specimens were tested for each determination.

Characterisation

Mechanical Characterisations.  Performed with an Instron Mod. 4502

Weight Loss (WL).  Measured with an analytical balance as: 

                              WL = (Mi -  Mf) 

                                           Mi

Where:  Mi = initial mass of specimens            Mf = final mass of specimens

Results

The results of the characterisation performed on the specimens are summarised in the following tables:

Conclusion

Both PE and EPI were not substantially affected by the applied treatments.  On the other hand, the Mater-Bi NF01U showed a loss of the mechanical properties after the 24 hour acidic digestion (simulation of gastric digestion).  The average decrease of the main mechanical properties after the 2 day + 7 day treatment was of about 30%.  The weight loss reached a 8.95% value after the 1 day + 2 day treatment and a 14.8% after the 2 day + 7 day treatment.

The results indicate that the Mater-Bi NF01U is susceptible of degradation under conditions simulating the digestive tract of a turtle, while the PE and EPI films are not.  Therefore, it can be reasonably supposed that the ingestion of a film whose mechanical properties decrease within the intestine can be less harmful for a turtle that the ingestion of a recalcitrant film.

References

All the information on turtle digestion used in the present study has been obtained from two reviews:

• Bjorndal K.A. (1996) Foraging Ecology and Nutrition of Sea Turtles In: (Lutz P.L. and Musick J.A. Editors) The Biology of Sea Turtles p.200-231 CRC Press
• Bjorndal K.A. (1997) Fermentation in Reptiles and Amphibians In: (Mackie R.I. and White B.A., Editors) Gastrointestinal microbiology, Vol. I Gastrointestinal ecosystems and fermentations. Chapman and Hall, New York, p.199-231

Study performed by:  I. Guanella & C. Russo (Mechanical Characterisation Lab.);  M. Tosin (Biodegradation Lab.) in the Novamount's laboratories located in Novara, via Fauser 8, Italy, from 16/09/02 till 10/10/02.  Report and study supervision:  Dr. F. Degli-Innocenti.  Report finalised on the 16th October, 2002.