Bioremediation of nitriles using endophytes isolated from Phragmites Australis and Zantedeschia Aethiopica plants found in the Blesbokspruit Wetland, South Africa

Abstract

Organonitriles are organic compounds which contain the cyano group (C = N) in their structure and are commonly used in Lhe synlhesis of pharmaceulicals, solvenls, rubbers, and peslicides, amongsl olhers. Acetonitrile (ACN), benzonitrile and acrylonitrile are a group of organonitriles which are widely utilised in these industries. ACN is of particular importance since it is widely utilised in laboratories as a mobile phase during HPLC analyses and as a solvent in the extraction of fatty acids from animal and vegetable oils. In addition, the increase in the global demand for pharmaceuticals and the increasing production of pesticides has resulted in the average annual increase of ACN utilisation by 5% -6% annually. This increase in demand has culminated in subsurface water and groundwater contamination by ACN due to the improper disposal practices, which necessitates remediation since ACN is known to be carcinogenic and mutagenic. In most instances, when ACN is discharged into the environment, it can be converted into hydrogen cyanide, the most toxic form of cyanide species thus posing significant risk to public health. Bioremediation is a process of degrading chemical pollutants making use of the natural environment. Microbes isolated from soil play an important role in the biotransformation of most pollutants that includes great diversity of compounds, ranging from metals, pharmaceutical substances, and hydrocarbons. The strategy is to isolate and characterise microorganisms from the contaminated environment as they have proven to have a diverse metabolic versatility. Microorganisms possess a mechanism that breaks down and detoxifies a wide spectrum of organic nitriles and inorganic cyanide. Biodegradation technology of nitriles significantly produces multiple chemicals such as carboxylic acid, amides, and ammonia. Therefore, the optimum bioremediation requires a considerable selection of microorganisms with high tolerance and high degradation capacity for nitriles. This degradation follows different biochemical routes and thus produces non-toxic products such as ammonia, and carbon dioxide. Therefore, a bioremediation approach needs to be undertaken such that the ACN-laden wastewater can be decontaminated. This study focused on using bacterial endophytes to bioremediate nitriles at optimum conditions subsequent to their application in a single stage airlift bioreactor. Bacterial endophytes were isolated from the plants: Phagmites australis and Zantedeschia aethiopica in the Blekbokspruit wetland, Gauteng, South Africa. It was detected that the wetland was contaminated with various nitrile compounds. Water samples were collected from the site and wre analysed for organic toxicants using the Liquid Chromatography Quadrupole-Time-of-flight Mass Spectrometry (LC-QTOF-MS) instrument. MetFrag was used to analyse the fragments and the results were confirmed on PubChem and ChemSpider databases and various nitrile compounds were detected on the water sample, including ACN. The biodegradation of ACN was undertaken using the bacterial endophytes and these endophytes were dominated by Bacillus sp., Pseudomonas sp., and Enterobacter sp. These organisms were mixed to form a microbial consortium for the biodegradation of ACN. Response Surface Methodology was used to achieve optimum physicochemical parameters which were then used in a single stage airlift bioreactor, in continuous system. The physicochemical parameters such as temperature, pH, and ACN concentration were optimised using response surface methodology (RSM) and the optimised conditions were 34.99 °C, 7.03 and 127.23 mg/L respectively, where 99% biodegradation efficiency could be achieved. Using the optimised data, the biodegradation of ACN in a single-stage airlift biofilm reactor was evaluated in continuous mode where the ACN concentration was increased from 150 mg/L to 500 mg/Lover a period of 122 days, and the observed biodegradation efficiency exceeded 99% throughout the operation of the bioreactor system. The total nitrogen that was produced from the biodegradation of ACN was completely utilised by the microbial consortium. Therefore, the bacterial endophytes consortium isolated form Phragmites australis and Zantedeschia aethiopica sampled in the contaminated environment possess the machinery to biodegrade ACN.