Experience with evaluating biodegradability of lubricating base oils

ARTICLE IN PRESS Tribology International 41 (2008) 1212– 1218

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Experience with evaluating biodegradability of lubricating base oils Elz˙bieta Beran  ´ ska 7/9, 50-344 Wroc!aw, Poland Faculty of Chemistry, Wroc!aw University of Technology, Division of Fuels Chemistry and Technology, ul. Gdan

a r t i c l e in f o

a b s t r a c t

Article history: Received 19 November 2007 Received in revised form 25 February 2008 Accepted 12 March 2008 Available online 1 May 2008

Biodegradability analysis of lubricants by standardised tests provides valuable information for both legislation purposes and assessment of how chemical structure influences biodegradability. The choice of an appropriate test for evaluating the ultimate biodegradability of oils raises serious problems as the majority of lubricating base oils display a poor water solubility. The paper summarises the experience gained and the results achieved from the evaluation of primary and ultimate biodegradability of lubricating base oils differing in chemical structure, such as rapeseed oil, synthetic polyolester oils, poly(a-olefin) oils (PAO 4 and PAO 6) and the conventional mineral oil. Primary biodegradability was evaluated using the CEC L-33-A-93 test. To evaluate the ultimate biodegradability of oils in an aerobic aquatic environment, use was made of the testing methods OECD 301 B and OECD 310 for ready biodegradability. Oils that fail to fulfil the ‘‘ready biodegradability’’ criterion, e.g. pentaerythrite tetra(sec-capronate) oil (polyolester with steric hindrance around the ester bonds), PAO 4, PAO 6, and mineral oil, were evaluated for inherent biodegradability using the OECD 302 D (draft) test and the OECD 302 B method where the test vessels were prepared via a modified procedure. The oils belonging to this group differred in inherent biodegradability. Thus, PE tetra(sec-capronate) oil reached an extent of biodegradation amounting to 65%, that of PAO 4 and mineral oil being equal to 48% and 38%, respectively. Experiments have shown that ISO 14593 offers a convenient method for evaluating the ready biodegradability of base oils according to OECD 310 and makes it possible to evaluate (with the same apparatus and reagents) the inherent biodegradability of oils when the conditions and criteria recommended by the OECD 302D (draft) CONCAWE test are satisfied. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Lubricant Base oils Biodegradability Standardised test methods

1. Introduction Technological advance, on the one hand, and any actions as regards environmental protection, on the other hand, have raised public awareness of the importance of ecological information, which makes it possible to assess the risks (and related effects) involved with the penetration of chemical products into the environment. Ecological information, particularly that pertaining to the assessment of the biodegradability, ecotoxicity and bioaccumulation of chemical substances or preparations in the aquatic ecosystems, should be made available from paragraph 12 of the safety data sheet for chemical products. The manufacturer or supplier of the product has the responsibility to provide such information in compliance with the relevant standard [1,2]. The results of biodegradability assessments are particularly important when the chemical products enter the natural environment during service, or due to inadequate use or utilisation. The same holds true for lubricants [3,4]. The type, quality and performance of lubricants depend on their main

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components—the base oils, which account for 75–80% in engine oils and up to 99% in industrial lubricants. In sum, base oils are the principal contributory factors in the biodegradability of lubricating materials [5,6]. What stimulates improvements in the quality and changes in the type of the base oils being produced is not only technological advance but also the ever increasing tendency to reduce the environmental impact of the oils and to ensure an optimal use of the available resources [7,8]. It is essential to note, however, that these days approximately 75% of world’s annual base oil production (estimated at 37–38 m tons) comprises conventional, crude oil-derived mineral oils (Group I base oils according to API classification) regarded as low-biodegradability products hazardous to the environment [9]. About 15% of global base oil production per annum includes Group II, Group II+ and Group III oils. They all are derived from crude oil but, owing to the use of advanced technologies, their composition and hydrocarbon structure have been modified so they are classified as nonconventional mineral oils. Some of these, specifically lowviscosity oils [5], display a high biodegradability potential. The remaining 10% of world’s annual base oil production covers poly(a-olefin) oils (PAO) defined as Group IV base oils, as well as the other synthetic base oils referred to as Group V base oils.

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According to the assessments reported by Murphy et al. [10], approximately 80% of the synthetic base oil market has been dominated by three groups of compounds: polyalphaolefins (45%), organic esters (25%) and polyalkyleneglycols (10%). It is expected, however, that in near future there will be a rise in the manufacture of synthetic ester base oils both in the form of modified vegetable oils and in the form of synthetic ester oils produced on the basis of plant and animal raw materials [7,11,12]. The increasing demand for ‘biolubricants’ on the oil market is attributable to the ‘ecolable’ programme being implemented for lubricating oils, as well as to the continuing actions undertaken by the West-European countries with the aim to direct public attention to the need of reducing the environmental impact of lubricants. It is worthnoting that every year 13–15% of the oils produced are made use of in open lubricating systems and that they will unavoidably penetrate into the environment in the form of ‘total loss lubricants’. That is why the application of biodegradable base oils to the manufacture of such lubricants is taking on a sense of importance to the control of environmental pollution. In 2004, lubricants manufactured from synthetic ester oils (approx. 114,000 tons) and vegetable oils (58,000 tons) accounted for 3.6% of the oil market in Western Europe [13]. Even if in the nearest future the production of ‘biolubricants’ increased twofold, this would not cover the demand for ‘total loss lubricants,’ whose annual use in the European Union has been assessed at approximately 600,000 tons [14,15]. Taking into account the deficiency of readily biodegradable oils, it is necessary to make use of lubricating materials produced from base oils that differ in their inherent biodegradability. Thus, the general characteristics for each of the base oils produced should specify their biodegradability (evaluated by standard tests) in addition to their service properties. Ecological information is to facilitate, on the one hand, the choice of optimal lubricating materials for the applications desired, and, on the other hand, the design of new lubricants, less hazardous to the environment as compared to mineral oils. The results presented further on (which have been obtained in our research onto the biodegradability of oils classified as poorly water-soluble organic compounds), as well as the author’s own observations, may be of support to the laboratory staff in coping with the assessment of the ecological and service properties of lubricants.

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 Polyolester oil PRIOLUBE 3999—obtained by synthesis of 

 

trimethylolpropane and modified fatty acids; a product of UNIQEMA. Rapeseed oil—low-erucic rapeseed oil with oleic acid, linolic acid, linolenic acid and other C16 and C18 acid structures accounting for 63%, 19%, 7% and 11%, respectively; a product of Z. T. Kruszwica S.A. Polyolester oil PE tetracapronate—obtained by esterification of pentaerythrite and n-hexanoic acid [16]. Polyolester oil PE tetra(sec-capronate)—obtained by esterification of pentaerythrite and 2-methyl-pentanoic acid [16].

Polyolester oils obtained via esterification of PE with carboxylic acids (n-hexanoic or 2-methyl-pentanoic acid) of different chemical structures are characterised by the same molecular weight and elemental composition. There are, however, differences in the physicochemical properties of the oils, which are to be attributed to the structure of the acids. Thus, the kinematic viscosity at 40 1C amounts to 19.4 mm2/s for PE tetracapronate and 22.3 mm2/s for PE tetra(sec-capronate). While planning relevant tests, it was expected that they would show to what degree the steric hindrance at the ester bonds might influence the extent of biodegradation in particular test. 2.2. Methods used for the evaluation of oil biodegradability 2.2.1. CEC L-33-A-93: primary biodegradability test Primary biodegradability was tested according to the CEC L-33A-93 [17] procedure, using triplicate flasks containing the test oil, triplicate flasks containing DITA as the reference substance, duplicate poisoned flasks and duplicate neutral flasks, all of them being prepared for each duration of the test (0, 7, 14 and 21 days). The inoculum made use of in the tests was a coarse-filtered effluent from the mechanical stage of the Wroclaw Wastewater Treatment Plant Jano´wek, with bacterial levels of 107 CFU/ml. IR spectroscopic grade carbon tetrachloride (Merck) was used both as the solvent for the test oils and reference material (DITA) and as the extraction solvent. The maximum absorption of CH3– and CH2– bonds was measured at 2930 cm1 using the BRUKER FTIR Vector 22 spectrophotometer.

 Mineral oil of viscosity grade ISO VG 32—oil of SAE 10/95-type

2.2.2. Ready biodegradability tests 2.2.2.1. OECD 301 B: CO2 evolution (modified Sturm test). The oils were tested according to the OECD 301 B [18] method (which is similar to the ISO 9439 procedure [19]), using Erlenmeyer flasks of a 1.5 dm3 volume, which permit magnetic stirring and aeration. Duplicate flasks were used for each of the test oils, duplicate flasks for the blank test and one flask for the test with aniline as the reference substance. The test solutions (of a concentration varying between 30 and 40 mg C/l) were obtained by direct dosing of the test oil into the mineral salts medium with a micropipette. The solutions were treated with 10 ml portions of the inoculum prepared via the route applied in CEC L-33-A-93. The CO2 evolved was absorbed in a 0.0125 M Ba(OH)2 solution (three bottles in series) and CO2 evolution was assessed by titrating the Ba(OH)2 (with 0.05 M HCl) that remained in the absorption bottles.

obtained by a conventional method at the petroleum refinery ˜ ska S.A. Rafineria Gdan Poly(a-olefin) oils PAO 4 and PAO 6—obtained in the form of the following products: Nexbase 2004 (viscosity, 4 mm2/s at 100 1C) and Nexbase 2006 (viscosity, 6 mm2/s at 100 1C) from Fortum Oil N.V. Polyolester oil NYCOBASE 3118—obtained in the form of trimethylolpropane-trioleate, marketed under the brand name TMP trioleate; a product of NYCO.

2.2.2.2. OECD 310: CO2 in sealed vessels (headspace test). Biodegradability tests were performed according to the OECD 310 [20] and ISO 14593 [21] procedures, where sealed serum bottles (600 ml in volume) containing the test solutions (of a 400 ml volume) were used as test vessels for incubation. In this way, a headspace of an air-to-liquid volume ratio of 1:2 has been obtained. The test solutions (of a concentration ranging between 20 and 25 mg C/l) were prepared by adding watch glasses (f 18 mm)

2. Materials and methods 2.1. Oils tested The oils to be tested were chosen from a group of commercial base oils and from a group of polyolester oils synthesised in laboratories with the aim to demonstrate how the chemical structure affects their biodegradability. According to API classification, the test oils chosen represent Group I, Group IV and Group V base oils:





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with weighed quantities of the test oils into the vessels that contained the mineral salts medium. Prior to the addition of the inoculum, the solutions were made subject to dispersion with ultrasounds (25 kHz, 20 1C). The inoculum was a 1 ml dose of activated sludge (108 CFU/ml) collected from the aeration tank at the Wroclaw Wastewater Treatment Plant Jano´wek. The test bottles (two items for each duration, tx, of the test: 7, 14, 21 and 28 days) were incubated in the dark at 2272 1C, using orbital bio-shakers. The quantity of the total inorganic carbon (TIC) produced after time tx of biodegradation was calculated in terms of the CO2 concentration in the headspace gas that was taken for chromatographic analysis after pH adjustment (with orthophosphoric acid) in the medium to values lower than 3. The percentage of biodegradation was calculated from the quantity of the TIC evolved in the oil-containing test flasks (after having subtracted the quantity of the IC evaluated in the blank test) in relation to the theoretical amount of inorganic carbon (ThIC) based on the amount of the test compound added initially. Aniline was used as the reference substance for the control of the biodegradation process.

the test), 7, 14, 21 and 28 days. For each period, there were duplicate flasks which contained the oil tested, one flask with ethylene glycol as the reference substance and one blank flask. Each flask contained 150 ml of the dispersion tested (with an oil concentration of 200 mg/l), which was obtained by adding watch glasses with weighed quantities of the test oils into the flasks that contained the mineral medium. Prior to inoculation with 10 ml of activated sludge (the same as in the OECD 310 test), the solution was made subject to ultrasonic dispersion (25 kHz, 20 1C) in compliance with ISO 10634 [26]. During the test, the flasks were incubated in the dark at 2071 1C in a mechanical bio-shaker. After a defined incubation time, the aqueous medium in the flask (prepared for the test duration tx) underwent ultrasonic dispersion and two samples of 50 ml volume were collected for COD determination according to the ISO 6060 method [27]. The extent of biodegradation was calculated in terms of the mean COD values obtained from the analysis of two simultaneously incubated test flasks.

3. Results and discussion 2.2.3. Inherent biodegradability tests 2.2.3.1. OECD 302 D (draft) CONCAWE test. The inherent biodegradability of base oils was evaluated according to the test procedure described in the draft document OECD 302D [22], making use of the same apparatus (e.g. serum bottles, orbital shaker, gas chromatograph for analysis of CO2 in the headspace gas) and the same reagents as those used in the OECD 310 test. For the sake of compliance with the procedure of the CONCAWE test [22,23], the following modifications had to be made in the mode of inherent biodegradability evaluation as compared to the OECD 310 and ISO 14593 tests:

 the inoculum was prepared in the form of a mixed population

  

of microorganisms (from soil and activated sludge collected at the Wroclaw Wastewater Treatment Plant Jano´wek), which was pre-exposed to the test oils according to OECD 302D (draft); the mineral salt medium contained higher concentrations of ammonium chloride (2.0 g/l stock solution) in order to provide a sufficient quantity of nitrogen in the medium; the number of adequately prepared test bottles had to be increased in order to ensure an incubation period of 56 days, n-hexadecane and low erucic rapeseed oil were used as reference substances for test control.

Using the ISO 14593 procedure modified in the way shown above, inherent biodegradability evaluation was performed for the base oils that failed to meet the ready biodegradability criterion in the OECD 310 test. 2.2.3.2. OECD 302 B Zahn–Wellens/EMPA test. The method described in OECD 302 B [24] complies with that of ISO 9888 [25] and enables the inherent biodegradability of water-soluble organic compounds to be evaluated by analysing dissolved organic carbon (DOC) reduction or measuring chemical oxygen demand (COD) variations in the test solutions. Because this test was designed for soluble substances, the test for poorly water-soluble oils required the application of our own, appropriately modified method for the preparation of the test vessels. The primary objective of the modification was to reduce the errors resulting from the insufficient size of the samples being collected, which failed to represent the mean concentrations of the test oils in the aqueous dispersion. The tests were conducted in 250 ml Erlenmeyer flasks, which had been prepared for the periods (tx) of 3.5 h (at the beginning of

Although the biodegradability of organic substances can be evaluated using a wide variety of testing procedures, the greatest significance is put to the standard tests specified in the OECD Guidelines for the testing of chemicals and in the ISO standardised methods [28]. The biodegradation process can be analysed under aerobic or anaerobic conditions, in fresh water or seawater, or in soil. In the majority of instances, biodegradability is evaluated by analysing the aerobic biodegradability of a substance in an aqueous medium. The principles that underlie the biodegradability evaluations in standard screening tests are depicted in Fig. 1. The choice of an appropriate test is an essential prerequisite for the determination of the biodegradability of lubricants [15]. The CEC L-33-A-93 test [17], for example, enables the evaluation of primary biodegradability while the OECD test permits the ultimate biodegradability to be assessed [29]. The OECD tests for ready biodegradability (OECD 301 series and OECD 310) play an important role in the environmental classification of chemicals. A substance is considered as readily biodegradable if it has reached a sufficient level of degradation in one of these tests, 70% in the case of DOC removal or 60% for CO2 or BOD. The pass-levels have to be reached within the 28-day test period by the end of a 10-day window, which begins when biodegradation reaches 10%. When the substance fails to meet the ready biodegradable criterion, it can be made subject to inherent biodegradability tests, which are used to assess whether a substance has any potential for biodegradation. Biodegradation rates above 20% (measured as ThCO2, ThOD, DOC or COD) may be regarded as evidence of inherent, primary biodegradability, whereas biodegradation rates above 60% ThCO2, ThOD or 70% DOC or COD may be regarded as evidence of inherent, ultimate biodegradability. It has been widely acknowledged that the testing of lubricants for biodegradability raises problems and trouble. The majority of lubricating base oils are poorly water-soluble. The water-solubility of synthetic ester oils, for instance, is lower than 1 mg/l [7], and the concentrations recommended for the substance examined in the test solution are higher than the limit water-solubility values for oils. What is more, lubricants generally constitute a mixture of compounds differing in their chemical structure. That is why a careful choice of the test for evaluating their biodegradability, as well as the application of an additional procedure for the preparation of the test solutions that contain the oils to be examined, has become an issue of particular significance [15,26]. It is essential to underline, however, that the choice of the biodegradability test has a number of limitations. Thus, OECD

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Primary biodegradation: microorganisms-induced alteration in the chemical structure of the substance resulting in the loss of a specific property. CEC L-33-A-93 test for lubricants new biomass

microorganisms (N, P) organic substance

H2O, O2

mineralisation products

intermediate products

microorganisms

A → B → C → D

CO2 + H2O inorganic salts

H2O, O2

TEST METHODS FOR EVALUATION OF ULTIMATE BIODEGRADABILITY in aerobic aquatic environment

Ready biodegradability tests Reduction in dissolved organic carbon (DOC)

OECD 301A: DOC Die-Away-Test OECD 301E: Modified OECD Screening Test

Biochemical oxygen demand related to theoretical oxygen demand for total mineralisation (BOD/ThOD)

*OECD 301D: Closed Bottle Test *OECD 301F: Manometric Respirometry Test OECD 301C: Modified MITI Test (I)

Measured quantity of CO2 evolved related to theoretical quantity of CO2 evolved for total mineralisation (CO2/ThCO2) *OECD 301B: CO2 Evolution Test *OECD 310: CO2 in Sealed Vessels (Headspace Test)

Inherent biodegradability tests OECD 302A : Modified SCAS Test OECD 302B: Zahn-Wellens/EMPA Test

OECD 302C: Modified MITI Test (II)

*OECD 302D (draft): CONCAWE Test

Fig. 1. Principles of evaluating the biodegradability of organic substances in aerobic aquatic media by means of screening tests [29]. *Tests recommended for evaluating the biodegradability of poorly water-soluble substances (including lubricating oils).

301A and E, and OECD 302 A and B (Fig. 1) are based on the analysis of reduction in DOC in the test solution; OECD 301 C and 302 C are applicable in Japan (MITI I and II). Therefore, the tests that are of choice include four screening tests for ready biodegradability evaluation (OECD 301 D and F, based on the analysis of BOD/ThOD variations, as well as OECD 301 B and OECD 310, based on the analysis of CO2/ThCO2 variations) and only one screening test for the determination of inherent biodegradability, namely the OECD 302 D draft test [22], also known as the CONCAWE Test [23]. To evaluate the ultimate biodegradability of the set of various oils, we made use of the OECD 301 B and OECD 310 tests alongside with the CEC L-33-A-93 test for assessing primary biodegradability. It has been found that, compared to OECD 310, the OECD 301B procedure is a labour-consuming one (both at the stage of

preparation and the stage of test control), the laboratory setup demands a large space, and the results suggest that the CO2 values are underrated (Fig. 2). The plots in Fig. 2 illustrate the course of primary biodegradation (CEC L-33-A-93) and that of ultimate aerobic biodegradation (OECD 301B and OECD 310). Their analysis makes it clear that of the two polyolester oils (having the same molecular mass and elemental composition but differing in chemical structure) only the one characterised by the pentaerythrite tetracapronate structure (Fig. 2, oil a) belongs to the class of readily biodegradable. When use was made of the OECD 301B and OECD 310 tests, the extent of biodegradation obtained for this oil exceeded 60% (as determined from the CO2 evolved during 28 days in either of the two tests). What is more, the biodegradation plot obtained in the OECD 310 test indicates that the oil has reached the sufficient

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level of biodegradation within the time of the so-called 10-day window. The fact that the oil failed to meet this criterion in the OECD 301B test suggests an underestimation of the CO2 evolved. This is because CO2 is trapped and determined outside of the system. Seemingly, the large volume of the flasks in the OECD 301 B test, an insufficient stirring of the solution and the sophistication of the testing system have contributed to the persistence of some CO2 portions in the test vessels and solutions [30]. It is worthy to note that this does not occur when use is made of the OECD 310 test, which seems to be better suited for determining the ultimate aerobic biodegradation of poorly water-soluble substances. Polyolester oil of a pentaerythrite tetra(sec-capronate) structure cannot be classified as readily biodegradable since none of the tests performed provided the desired extent of biodegradation (exceeding 60%) during 28 days (Fig. 2, oil b). This oil exhibits steric hindrance around the ester bonds, which is attributable to the use of the branched 2-methyl-pentanoic (i.e. sec-caproic) acid for its synthesis. The comparison of the biodegradation curves presented in Fig. 2 clearly shows that the steric hindrance around the ester bonds exerts an inhibiting effect on the initial stage of

100 OECD 301B test OECD 310 test CEC L-33-A-93 test

90 80

oil (a) Biodegradation [%]

70 60 50 40 30 20 oil (b) 10 0 0

7

14 Time [days]

21

28

Fig. 2. Extent of biodegradation related to time, obtained with different tests for two polyolester oils: PE tetracapronate oil (a) and PE tetra(sec-capronate) oil (b).

ester oil biodegradation, which involves enzymatic hydrolysis of the ester bonds and then is followed by the stage of b-oxidation of carboxylic acids leading to their ultimate biodegradation. Literature data have shown that the extent of primary biodegradation of synthetic ester fluids is higher than the extent of ultimate biodegradation and that the differences can result from the chemical structure of esters [6,9]. In the case of PE tetra(sec-capronate) (Fig. 2, oil b), ultimate biodegradability was unexpectedly found to be higher than primary biodegradability. Primary biodegradability was expressed in % as the difference in residual oil content between the poisoned flask and respective test flasks (according to the CEC test). An abiotic degradation (which reached 20%) was observed in the poisoned flasks. Thus, chemical hydrolysis can support the process of biological degradation of PE tetra(sec-capronate) oil and can be responsible for the higher extent of ultimate than of primary biodegradation. And so we see that, contrary to the common belief that ester oils are readily biodegradable, it has to be agreed that their biodegradability may noticeably vary from one ester oil to another according to their chemical structure. The oils that failed to meet the ready biodegradability criterion were subjected to the CONCAWE test [22] in order to assess their inherent biodegradability. For the purpose of comparison, some of the oils were tested using OECD 302 B [24] alongside a procedure modified at our laboratory. The fact that aqueous dispersions of lubricating oil were used instead of real water solutions has been taken into account. The method was standardised using model aqueous systems of water-soluble substances (ethylene glycol, pentaerythritol and caproic acid) and aqueous systems applied as dispersions of poorly water-soluble substances (pentaerythrite tetracapronate, di-isotridecyl adipate, TMP trioleate, PAO 4 oil and mineral base oil). Thus, the solutions were homogenised by ultrasonic dispersion in order to minimise the effect of the hydrophobic oil properties manifesting in the adsorption of oil droplets on the vessel walls and in the formation of agglomerates on the surfaces of the aqueous dispersions. In order to verify the dosing concentrations of base oils (200 mg/l) in the aqueous systems (of 150 ml volume), the study was conducted with the most popular analytical techniques for COD measurement, namely the miniaturised instrumental Hach Dichromate COD Method 8000, approved by US EPA [31], and the ISO 6060 method [27] using conventional COD determination by titration. The results obtained are summarised in Table 1. The comparison of the mean COD values (obtained for the substance tested) with the ThOD value (calculated in terms of the molecular formula and reaction stoichiometry) shows the efficiency

Table 1 Comparison of ThOD with COD determined for model aqueous systems of water-soluble and poorly water-soluble substances by two analytical methods Test substance and molecular formula

Ethylene glycol C2H6O2 Pentaerythritol (PE) C5H12O4 Caproic acid C6H12O2 Pentaerythrite tetracapronate C29H52O8 DITA, di-isotridecyl adipate C32H62O4 TMP trioleate C60H110O6 PAO 4 C29,2H59,6 Mineral oil C28H52 a

ThODa (g O2/g)

1.290 1.412 2.207 2.303 2.855 2.920 3.442 3.381

CODc determined according to Hach’s method

CODd determined according to ISO 6060

Mean value (7s. d.)b (g O2/g)

COD/ThOD (%)

Mean value (7s. d.)b (g O2/g)

COD/ThOD (%)

1.277 1.403 2.170 1.313 1.613 1.606 1.893 1.961

99 99 98 57 56 55 55 58

1.280 1.398 2.162 2.285 2.626 2.745 3.166 3.077

99 99 98 99 92 94 92 91

(70.014) (70.012) (70.092) (70.362) (70.311) (70.436) (70.268) (70.339)

ThOD calculated from molecular formula. 7s. d.: standard deviation (n ¼ 6). c COD measured by analysing small-sized samples of test solution (1.33% v/v). d COD measured by analysing large-sized samples of test solution (33% v/v). b

(70.012) (70.016) (70.083) (70.193) (70.279) (70.080) (70.206) (70.170)

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oils differing in potential inherent biodegradability

100 oil( b) OECD 302B test oil (c) OECD 302B test oil (d) OECD 302B test

90

oil (b) OECD 302D draft test oil (c) OECD 302D draft test oil (d) OECD 302D draft test

28 days with OECD 302B test

70

Biodegradability [%]

Biodegradation [%]

80 56 days with OECD 302D draft test

60 50 40 30 20 10

readily biodegradable oils

100 90 80 70 60 50 40 30 20 10 0

CEC L-33-A-93 test OECD 310 test OECD 302D (draft) test

1

0 0

7

14

21

28 35 Time [days]

42

49

56

Fig. 3. Extent of biodegradation related to time, obtained with two inherent biodegradability tests (OECD 302B and OECD 302D draft) for three base oils: PE tetra(sec-capronate) oil (b), conventional mineral oil (c) and PAO 4 oil (d).

(expressed as COD/ThOD (%)) with which the content of the test substance in the aqueous system was evaluated. The data in Table 1 make it clear that in the case of watersoluble substances the two analytical techniques produce comparable COD values and low standard deviations. The comparison of the COD/ThOD ratios shows that the efficiency of assessing the content of the test substances in water solutions with both the methods is very high (98–99%). The application of Hach’s method to oil samples displaying hydrophobic properties has produced COD/ThOD ratios below 60%. The COD values determined (too low as compared to those of ThOD), as well as the high standard deviations, disclose considerable errors in the assessment of dispersed model solutions. What should be blamed for those errors is the fact that the samples collected for the analysis of COD in model aqueous systems containing poorly water-soluble substances were very small in size (1.33% v/v), so they fail to represent the real concentrations of the substance tested, the most likely cause being the heterogeneity of dispersion. More representative were the samples of lubricating oil dispersion collected for the evaluation of COD by the ISO 6060 method, where the size of the model solution sample for analysis approached 33%v/v. The agreement between the COD values established via this route and the ThOD values exceeded 90%. These results have prompted us to evaluate the inherent biodegradability of lubricating oils by making use of the OECD 302 B Zahn-Wellens/EMPA Test, where the test vessels were prepared via a modified procedure based on our own concept. Fig. 3 shows the biodegradation curves obtained for PE tetra(sec-capronate) oil, PAO 4 oil and conventional mineral oil. Subjected to the ready biodegradability test, the oils have not achieved the required extent of biodegradation (460% ThCO2). In this context, they had to be assessed (in compliance with the OECD guidelines) for inherent biodegradability using the OECD 302 test. As can be inferred from the test results (Fig. 3), the PE tetra(sec-capronate) oil may be regarded as having an inherent ability to undergo ultimate biodegradation under aerobic conditions (the criterion of a biodegradability higher than 60% has been fulfilled) although it cannot be classified as readily biodegradable. The other oils are to be classified as having an inherent ability to undergo primary biodegradation (the criterion of biodegradability exceeding 20% has been satisfied). As for the PAO 4 oil, no plateau phase was achieved after 56 days, which suggests that the extent of ultimate biodegradation will be substantially higher if the duration of the test is extended. A similar rise in the extent of primary biodegradation determined for PAO 4 and PAO 8 during

2

3

4

5

6

7

8

Fig. 4. Biodegradability of lubricating base oils differing in chemical structure: rapeseed oil (1), NYCOBASE 3118 oil (2), PRIOLUBE 3999 oil (3), PE tetracapronate oil (4), PE tetra(sec-capronate) oil (5), PAO 4 oil (6), PAO 6 oil (7) and conventional mineral oil ISO VG 32 (8). The tests in the left-hand panel were repeated three times; the tests in the right-hand panel were repeated three times (CEC and OECD 302D) or twice (OECD 310).

the time-extended CEC L-33-T-82 test has been reported by Carpenter [32]. Comparing the results of the OECD 302D (draft) test with those of the OECD 302B test we can assume that the OECD 302D (draft) test has produced more reliable results since it has been designed for poorly water-soluble substances, such as lubricating base oils. It should, however, be stressed that although the OECD 302D (draft) test [22] for inherent biodegradability and the OECD 310 test [20] for ready biodegradability involve the same procedure of the ISO 14593 test [21], they obviously differ in the conditions of incubation and in the principle that underlies the interpretation of the results obtained. Finally, the whole set of the oils tested was examined. The results obtained from assessing the primary and ultimate biodegradability of these oils are depicted in Fig. 4. As shown by the bars, rapeseed oil, polyolester oils NYCOBASE 3118 and PRIOLUBE 3999, and PE tetracapronate can be classified as readily biodegradable substances whereas the other oils differ in their inherent biodegradability and can be ordered as follows: PE tetra(sec-capronate)4PAO 44PAO 6 and mineral oil ISO VG 32. The results visualised in Fig. 4 make it clear that the CEC test fails to reveal the relationship between chemical structure and biodegradability in the case of readily biodegradable ester oils. The OECD 310 (ISO 14593) test discloses the effect of chemical structure on the biodegradability of the oil, as can be inferred from the lowest ultimate biodegradability value for PRIOLUBE 3999, which is attributable to the branching in the structure of the acid residue in the polyolester molecule. As for the oils that are not classified as readily biodegradable (e.g. oils 5–8 in Fig. 4), the determination of their inherent biodegradability provides the data needed for the assessment of the potential environmental impact, as well as for the optimal choice of the base oils for the manufacture of lubricants that are to be used in devices operated in protected areas.

4. Conclusions Evaluation of the biodegradability of lubricating oils by standard screening tests provides information that is useful to legislators but primarily to the staff of research laboratories helping them to acquire a better knowledge of how the chemical structure influences the biodegradability of lubricants. The choice of an appropriate standard test for the evaluation of ultimate biodegradability is troublesome since the majority of lubricating base oils do not dissolve in water. The ISO 14593 test is

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a useful tool of practical significance to the staff of research laboratories where lubricants are extensively tested for their ecological and service properties. The procedure of the ISO 14593 test, combined with the consideration of the conditions and criteria made use of in the OECD 310 testing method, permits the ready biodegradability of the oils to be evaluated. Using the same apparatus and reagents, and taking into account the conditions and criteria of the OECD 302D (draft) testing method, it is possible to assess the inherent biodegradability of the oils. As more than 95% of the base oils applied to the manufacture of lubricants fail to meet the ready biodegradability criterion, the determination of their inherent biodegradability may be of great practical importance to the designers of new lubricating materials.

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