Biodegradation Study of Some Commonly Used Plasticizers (Phthalate Esters) and Development of a Vegetable Oil-Based Plasticizer

Abstract

In the present study, an attempt was made to lessen the environmental pollution burden of commonly used plasticizers, phthalate esters (PEs). The largely consumed low molecular weight PEs, dimethyl phthalate (DMP) and diethyl phthalate (DEP), were of primary focus to explore their biodegradation. Two aerobic bacterium strains tolerating high concentrations of PEs were isolated from soil contaminated with municipal wastewater. Based on the morphological, biochemical characteristics, and 16S rRNA sequencing, the isolates were identified as Bacillus sp. KS1 and Micrococcus sp. KS2. To enhance the biodegradation efficiency of isolates, process parameter optimization was performed by applying Plackett-Burman design and response surface methodology (RSM) based central composite design (CCD). Batch biodegradation experiments were performed in shake flask at optimum levels of process parameters. Bacillus sp. KS1 showed tolerance up to 1700 ± 12.5 mg/l of DMP and 1650 ± 17.8 mg/l of DEP while Micrococcus sp. KS2 exhibited tolerance up to 1500 ± 23.5 mg/l of DMP and DEP. The growth kinetic study of the high concentrations of PEs was performed by using growth kinetic models where the Haldane model was found to fit well with the experimental data suggesting the substrate inhibition effect. Analysis of PEs degradation residuals by Gas chromatography-Mass spectrometry (GC-MS) revealed the presence of monomethyl phthalate (MMP), monoethyl phthalate (MEP) and phthalic acid (PA) as the PEs degradation metabolites. To treat PEs containing synthetic wastewater, an internal loop airlift bioreactor (ILABR) was fabricated with an overall tank capacity of 4.7 liters and a working volume of 3.8 liters. For low viscous Newtonian liquids, viscosity showed a substantial positive effect on the gas holdup, liquid velocity, and mass transfer. Batch experiments of DMP and DEP degradation were carried out by mixed culture and combined system comprising suspended biomass and biofilm of pure cultures of isolates in an ILABR set up independently. The mixed culture showed tolerance up to 2250 ± 32.4 mg/l of PEs while the combined system exhibited enhanced efficiency of PEs degradation, as the tolerance capability was increased up to 2500 ± 28.9 mg/l of PEs. Thus, the study established the potential of microbes for treating PEs containing wastewater. The present study was also focused on the development of a vegetable oil based substitute for PEs. Citrullus lanatus (watermelon) seed oil (CLO) was considered for this objective since it is still unexplored in the perspective of epoxidation and its application as a plasticizer. CLO was epoxidized in the presence of acetic acid, sulphuric acid, and hydrogen peroxide. Epoxidized Citrullus lanatus seed oil (ECLO) with the maximum conversion of iodine value of 86.7% and relative conversion to oxirane oxygen of 84.2% was attained. FTIR, 1H NMR, and 13C NMR analysis confirmed the maximum conversion of the double bond to the oxirane group of an epoxide. Further, soft polyvinyl chloride (PVC) films plasticized with ECLO showed better surface morphology and improved mechanical properties. The plasticization effect was observed based on exudation, migration stability, thermal stability, and reduction in glass transition temperature (Tg). The comparison study of ECLO and dioctyl phthalate (DOP) as a plasticizer for soft PVC films established the effectivity of ECLO and confirmed its exploitation as a partial substitute for DOP. The biodegradable polymer is a current insistence to counter the hurdle of plastic materials disposal. In the present study, the biodeterioration of soft polyvinyl chloride (PVC) films plasticized with ECLO was explored in vitro by isolated strains, Bacillus sp. KS1 and Micrococcus sp. KS2 independently. Post 90 days of inoculation with bacterial isolates, morphological, and structural alterations of PVC films were observed. The changes in thermal stability, weight loss, and molecular modifications of PVC films exhibited evident microbial utilization of ECLO. However, the plasticized PVC films without inoculating by isolates displayed no significant deterioration. The obtained results confirmed that the isolates could effectively deteriorate PVC films by utilizing a developed plasticizer (ECLO) as a source of energy from the polymer matrix. Thus, the present research work delivered a potential plasticizer (ECLO) as an alternative for toxic PEs.