Comparative Study of Benzyl and Phenyl Compounds Chemical Structure Effects on the Inhibition of Methane Production by Digested Pig Manure Methanogens

Aims: The present paper aims to study the effect of aromatic structure on the inhibition of biogas production and more specifically the comparative study of benzyl and phenyl compounds chemical structure effects on the inhibition of methane production by digested pig manure methanogens. The objective of this study was also to examine the structure-toxicity relationships of aromatic compounds to acetoclastic methanogens.

Study Design: Anaerobic digestion of pig manure, anaerobic toxicity essay, comparison of benzyl and phenyl compounds structure effects on the inhibition of methane gas biosynthesis.

Place and Duration of Study: Department of Chemistry, University of Kinshasa (DR Congo), between September 2011 and May 2012.

Methodology: The toxicity to acetoclastic methanogenic bacteria was performed with the standard method of serum bottles; digested pig manure was utilized as inoculums and acetate as substrate. The methane gas volume produced was measured by serum bottles liquid displacement systems (Mariotte flask system).

Results: The obtained results indicate exist between the chemical structure of benzyl and phenyl compounds and their inhibitory effects on biosynthesis of methane by methanogenic bacteria. According to these results, α-chlorotoluene and bromobenzene with 0.61 and 2.90 mg/l IC50 values, respectively, are the most toxic compounds, while phenol and benzyl alcohol with IC50 values ​​of 1248.90 and 2391.37 are less toxic.

Comparing the phenyl and benzyl compounds with the same substituents (Cl, Br, H, OH), their behavior was diverse. The benzyl chloride is more toxic than phenyl chloride and benzyl bromide is less toxic than phenyl bromide. Otherwise, benzyl alcohol is less toxic than phenol. A significant correlation between the toxicity of phenyl compounds and their hydrophobicity (R2 = 0.911). But any correlation was not found for benzyl compounds.

Conclusion: The obtained results indicate that relationships exist between the chemical structure of benzyl and phenyl compounds and their inhibitory effects on the production of methane by methanogenic archaea. The behavior of phenyl and benzyl are different in inhibition of methane production by the methanogens because their chemical structures are not identic.