Development of Nonlinear Group Contribution Method for Prediction of

Chapter 9

Development of Nonlinear Group Contribution Method for Prediction of Biodegradation Kinetics from Respirometrically Derived Kinetic Data 1

2

Downloaded by UNIV OF ALBERTA on June 7, 2013 | http://pubs.acs.org Publication Date: February 22, 1993 | doi: 10.1021/bk-1993-0518.ch009

Henry H. Tabak and Rakesh Govind 1

Risk Reduction Engineering Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH 45268 Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221 1

The fate of organic chemicals in the environment depends on their susceptibility to biodegradation. Recent studies have attempted to correlate the kinetics ofbiodegradationwith the compound's molecular structure. This has led to the development of structure-biodegradation relationships (SBRs) using the group contribution approach. In this paper, a non-linear group contribution method has been developed using neural networks, which is trained using literature data on the first order biodegradation kinetic rate constant for a number of priority pollutants. The trained neural network is then used to predict the biodegradation kinetic constant for a new list of compounds. It has been shown that the nonlinear group contribution method using neural networks is able to provide a superior fit to the training set data and produce a lower prediction error than the previous linear method. The number and amount o f s y n t h e t i c o r g a n i c chemicals produced commercially i s large and i n c r e a s i n g every year. The presence o f many o f these chemicals i n the ecosystem i s a s e r i o u s p u b l i c h e a l t h problem. biodegradation i s an important mechanism f o r removing these chemicals from n a t u r a l ecosystems (1) . biodegradation can e l i m i n a t e hazardous chemicals by b i o t r a n s f o r m i n g them i n t o innocuous forms, o r completely degrading them by m i n e r a l i z a t i o n t o carbon d i o x i d e and water. K i n e t i c data f o r c a l c u l a t i n g biodegradation r a t e s i n n a t u r a l ecosystems a r e important f o r s e v e r a l reasons. Kinetic data i s needed t o estimate the role of biodegradation in t h e presenece o f other competing 0097-6156/93/0518-0159$09.00/0 © 1993 American Chemical Society In Emerging Technologies in Hazardous Waste Management III; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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mechanisms, such as v o l a t i l i z a t i o n and a d s o r p t i o n , on the d i s t r i b u t i o n and c o n c e n t r a t i o n of organic compounds i n the environment. Information r e g a r d i n g the extent and the r a t e of biodegradation of o r g a n i c chemicals i s v e r y important i n e v a l u a t i n g r e l a t i v e p e r s i s t e n c e of the chemical i n the environment, and f o r r e g u l a t i n g t h e i r manufacture and use. Due t o the l a r g e number of chemicals o b t a i n i n g t h i s information i s labor intensive, time consuming and expensive. Thus, there i s a need t o develop c o r r e l a t i o n s and p r e d i c t i v e techniques t o assess b i o d e g r a d a b i l i t y (2). S t r u c t u r e - a c t i v i t y r e l a t i o n s h i p s (SARs) are used t o p r e d i c t i n t r i n s i c p r o p e r t i e s of many chemicals and t o estimate the k i n e t i c constants f o r important t r a n s f o r m a t i o n processes. SARs approach can be e f f e c t i v e l y used t o shorten the l i s t of thousands of chemicals t o a few hundred key chemicals, f o r d e t a i l e d l a b o r a t o r y and f i e l d t e s t i n g . In a recent review of SARs (3) i t was concluded t h a t a p p l i c a t i o n of SARs has great p o t e n t i a l i n p r e d i c t i n g the f a t e o f organic chemicals and these techniques are being accepted t o a g r e a t e r extent by r e g u l a t o r y agencies i n d e c i s i o n making and p o l i c y implementation. In t h i s paper, a q u a n t i t a t i v e s t r u c t u r e - b i o d e g r a d a t i o n r e l a t i o n s h i p (SBR) has been developed u s i n g the group c o n t r i b u t i o n approach. T h i s method i s widely used i n chemical engineering thermodynamics t o estimate pure compound p r o p e r t i e s such as liquid densities, heat c a p a c i t i e s and c r i t i c a l constants. The group c o n t r i b u t i o n method i s s i m i l a r t o the FreeWilson model (4,5) widely used i n pharmacology and m e d i c i n a l chemistry. Using t h i s method, a very l a r g e number o f chemicals can be c o n s t i t u t e d from perhaps a few hundred f u n c t i o n a l groups. Using t h i s method, the compound's property i s p r e d i c t e d from i t s molecular s t r u c t u r e , which i s s t r u c t u r a l l y decomposed i n t o groups or fragments, each group or fragment having a unique c o n t r i b u t i o n towards the s p e c i f i c value of the p r o p e r t y . Techniques

f o r Measuring

biodegradation

Kinetics

Techniques f o r e v a l u a t i n g biodegradation k i n e t i c s have been reviewed i n the l i t e r a t u r e (6,7) and hence would not be repeated here. In the f o l l o w i n g s e c t i o n , the e l e c t r o l y t i c r e s p i r o m e t r i c method w i l l be presented i n some d e t a i l , s i n c e t h i s method was used i n o b t a i n i n g experimental data on biodegradation k i n e t i c s u t i l i z e d i n the development of the n o n - l i n e a r model. E l e c t r o l y t i c Respirometry S t u d i e s . T h i s study was conducted u s i n g an automated continuous oxygen uptake and BOD measuring V o i t h Sapromat B-12 (12 u n i t system). The instrument c o n s i s t s of a temperature c o n t r o l l e d waterbath, c o n t a i n i n g measuring u n i t s , an o n - l i n e microcomputer f o r data sampling, and a cooling unit for continuous r e c i r c u l a t i o n of waterbath volume. Each measuring u n i t

In Emerging Technologies in Hazardous Waste Management III; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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c o n s i s t s o f a r e a c t i o n v e s s e l , c o n t a i n i n g the m i c r o b i a l inoculum and t e s t compound, an oxygen generator, comprised of an e l e c t r o l y t i c c e l l c o n t a i n i n g copper s u l f a t e and s u l f u r i c a c i d s o l u t i o n , and a pressure i n d i c a t o r which t r i g g e r s oxygen generation. The carbon d i o x i d e produced i s absorbed by soda lime, contained i n t h e r e a c t i o n f l a s k stopper. Atmospheric pressure f l u c t u a t i o n s do not a f f e c t the r e s u l t s s i n c e the measuring u n i t forms an a i r sealed system. The uptake o f oxygen by the microorganisms i n the sample during biodegradation i s compensated by the e l e c t r o l y t i c generation o f oxygen i n the oxygen generator, connected t o the r e a c t i o n v e s s e l . The amount o f oxygen s u p p l i e d by the e l e c t r o l y t i c c e l l i s p r o p o r t i o n a l t o i t s amperage requirements, which i s continuously monitored by the microcomputer and the d i g i t a l recorder. M a t e r i a l s and Methods. The n u t r i e n t s o l u t i o n used i n our s t u d i e s was an OECD s y n t h e t i c medium (8) c o n s i s t i n g of measured amounts per l i t e r of d e i o n i z e d d i s t i l l e d water of (1) mineral s a l t s s o l u t i o n ; (2) t r a c e s a l t s s o l u t i o n ; and (3) a s o l u t i o n (150 mg/1) o f yeast e x t r a c t as a s u b s t i t u t e for vitamin solution. The m i c r o b i a l inoculum was an a c t i v a t e d sludge from The L i t t l e Miami wastewater treatment p l a n t i n C i n c i n n a t i , Ohio, r e c e i v i n g municipal wastewater. The a c t i v a t e d sludge sample was aerated f o r 24 hours before use t o b r i n g i t t o an endogenous phase. The sludge biomass was added t o the medium a t a concentration of 3 0 mg/1 t o t a l s o l i d s . T o t a l volume o f the s y n t h e t i c medium was 250 ml i n t h e 500 ml capacity reaction vessels. The t e s t and c o n t r o l compound c o n c e n t r a t i o n i n the media were 100 mg/1. A n i l i n e was used as the biodegradable reference compound, a t a concentration o f 100 mg/1. In a t y p i c a l experimental run, d u p l i c a t e f l a s k s were used f o r the t e s t compound, and reference compound, a n i l i n e , a s i n g l e f l a s k f o r t o x i c i t y c o n t r o l ( t e s t compound p l u s a n i l i n e a t .100 mg/1 each) and an inoculum c o n t r o l . The r e a c t i o n v e s s e l s were incubated i n the dark a t 25° C i n the temperature controlled bath and s t i r r e d continuously throughout the run. The microbiota o f the a c t i v a t e d sludge were not pre-acclimated t o the s u b s t r a t e . The i n c u b a t i o n p e r i o d of the experimental run was between 28-50 days. A more comprehensive d e s c r i p t i o n o f t h e procedural steps involved i n the r e s p i r o m e t r i c t e s t s has been presented elsewhere ( 9 ) . Evaluation

of

biodegradation

Kinetics

F i r s t Order Rate Constants. biodegradation r a t e s were q u a n t i f i e d by measuring the r a t i o of the net b i o l o g i c a l oxygen demand (BOD) values i n mg/1 (oxygen uptake v a l u e s o f test compound minus endogenous oxygen uptake v a l u e s [inoculum c o n t r o l ] ) t o the t h e o r e t i c a l oxygen demand (ThOD) of substrate i . e . , the r a t i o BOD/ThOD. The v a l u e s o f


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t h e o r e t i c a l oxygen demand (ThOD) were c a l c u l a t e d by u s i n g the s t o i c h i o m e t r i c balanced o x i d a t i o n equation. The BOD/ThOD curves c a l c u l a t e d from e l e c t r o l y t i c r e s p i r o m e t r i c data were c h a r a c t e r i z e d by four i n d i c e s (10) (1) t h e l a g time (t,) which g i v e s the adaptation time; (2) the r a t e constant (k) ; (3) the biodegradation time ( t ) before the endogenous r e s p i r a t i o n p e r i o d ; and (4) r a t i o o f BOD/ThOD a t time t . The values o f k can be c a l c u l a t e d from t h e slope o f the s t r a i g h t l i n e obtained by p l o t t i n g log (BOD) vs time (t) f o r values of t such t h a t t < t < t . The a p p r o p r i a t e equations f o r c a l c u l a t i n g t h e value o f k are given as f o l l o w s : d

d


1

d(B0D)/dt = k'(BOD) l o g (BOD) = (k'/2.3)t = k t + constant where t ^ t p-methyl phenol > pchlorophenol > p - n i t r o p h e n o l . P i t t e r (24) has reported the f o l l o w i n g order : phenol > methyl phenols > dimethyl phenols > chlorophenols > n i t r o p h e n o l s > d i c h l o r o p h e n o l s > 2-chloro-4-nitrophenol > trichlorophenol. Pitter has reported the degradation as percentage of t h e o r e t i c a l oxygen demand (ThOD) achieved. The values of the Monod r a t e constant f o r compounds (17-24) i n Table IX agrees with t h i s trend. The p r e d i c t e d Monod r a t e constants decreases with increase i n molecular weight of phthalates (compounds 25-30 i n Table IX). Urushigawa and Yonezawa (39), Wolfe e t a l . (22) and Sugatt et a l . (40) have reported the degradation r a t e constants f o r p h t h a l a t e s . Sugatt et a l . (40) found t h a t d i b u t y l phthalate was o u t l i e r and d i d not f o l l o w the trend but other two s t u d i e s and experimental r e s u l t s of t h i s study confirm the r e s u l t s p r e d i c t e d by the group c o n t r i b u t i o n method. The molecular weight of both d i o c t y l p h t h a l a t e and b i s ( 2 - e t h y l hexyl) p h t h a l a t e i s 391 but b i s ( 2 - e t h y l hexyl) phthalate i s more branched than d i o c t y l p h t h a l a t e . Hence, b i s ( 2 - e t h y l hexyl) phthalate should be more refractory than dioctyl phthalate. The group c o n t r i b u t i o n approach p r e d i c t s negative Monod r a t e constant f o r both these phthalates, i n d i c a t i n g t h a t none of them would degrade. The p r e d i c t e d r a t e constant of b i s ( 2 - e t h y l hexyl) p h t h a l a t e i s g r e a t e r than d i o c t y l p h t h a l a t e , which


m

η


9. TABAKANDGOVIND


187

suggests t h a t b i s ( 2 - e t h y l hexyl) p h t h a l a t e should be more d i f f i c u l t t o degrade than d i o c t y l p h t h a l a t e . The agrees with the f i n d i n g s of above three s t u d i e s .


Conclusions The r e s u l t s of t h i s research study show t h a t the group c o n t r i b u t i o n approach can p r e d i c t f i r s t order k i n e t i c s and Monod r a t e constants. The extent of degradation does f o l l o w the trends reported i n l i t e r a t u r e f o r d i f f e r e n t groups o f compounds. More data are r e q u i r e d t o p r e d i c t the unique c o n t r i b u t i o n due t o p o s i t i o n a l e f f e c t s o f the groups and t o include more f u n c t i o n a l groups i n SAR. The developed f a t e p r e d i c t i v e group c o n t r i b u t i o n model w i l l c l o s e l y p r e d i c t the r e s u l t s on biodegradation k i n e t i c s found experimentally f o r many organic p o l l u t a n t s with varied molecular s t r u c t u r e configurations and v a r i e d chemical groups composing the molecule. I t was demonstrated t h a t both group c o n t r i b u t i o n models can be used f o r estimating the biodegradation k i n e t i c constants f o r chemically r e l a t e d compounds and t h a t the n o n l i n e a r model gives b e t t e r r e s u l t s than the l i n e a r approach. I t was shown t h a t the n o n - l i n e a r model i s able t o provide a s u p e r i o r f i t t o the t r a i n i n g s e t data (goodness of f i t between the experimental and computed values) and t h a t i t produces lower per cent e r r o r between the p r e d i c t e d and experimental value than the l i n e a r group c o n t r i b u t i o n approach. Further research on biodegradation of organic compounds with d i f f e r e n t f u n c t i o n a l groups i s needed t o extend the a p p l i c a b i l i t y of the p r e d i c t i v e biodegradation group c o n t r i b u t i o n models. Further work i s needed t o extend the n o n l i n e a r a n a l y s i s t o a d d i t i o n a l compounds. T h i s would r e q u i r e electrolytic respirometric measurements for other compounds, s i n c e c u r r e n t l y there i s a l a c k of s u f f i c i e n t biodegradation k i n e t i c s data t o extend our a n a l y s i s t o a d d i t i o n a l compounds. E v e n t u a l l y i t would be p o s s i b l e t o obtain estimates of biodegradation k i n e t i c constants f o r a v a r i e t y of compounds using the nonlinear method presented i n t h i s paper. Furthermore, t h i s type of a n a l y s i s would aid i n the "designing" of molecules with favorable biodegradation kinetics.

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Development of Nonlinear Group Contribution Method for Prediction of

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