When exposed to microplastics, the microbial activities in wastewater treatment systems will be threatened. A research team led by Prof. Bing-Jie Ni and Dr Wei Wei from the University of Technology Sydney (UTS) found that plastic particles can disrupt functional microbial aggregates. It can even lead to cell death during wastewater treatment, thus inhibiting treatment efficiency.
Prof. Ni is a worldwide pioneer in modelling and wastewater management. Dr Wei is a young lecturer at the School of Civil and Environmental Engineering. She is a renowned pioneer in revolutionising the science and practice of wastewater/sludge management. This team has developed an innovative way to mitigate the inhibitory effects of microplastics using biochar, carbon-rich materials produced from biomass.
“Existing research has only focused on the impacts of microplastics in biological wastewater treatment systems. Working out how to relieve the microbial toxicity is still in its infancy,” said Prof. Ni. “We believe that our study will promote a revolutionary solution to the inhibited organic removal efficiency during wastewater treatment and can stimulate more innovations in this field.”
The researchers have successfully prepared biochar materials from diverse biomasses and tested their efficient mitigating performance on the biotoxicity of different types and sizes of microplastics. The research results are published in Water Research, Chemical Engineering Journal, and Journal of Hazardous Materials.
How Biochar Mitigates Toxicity of Microplastics?
The researchers took the granular sludge system as the representative biological wastewater treatment process. They used low-value and available biomass substrates as the precursors of biochar to explore the deep mitigation mechanism through physicochemical characterisation and microbial community analysis.
“We measured the elemental composition, properties of surface functional groups, and porosity characteristics of biochar prepared under different conditions,” said Dr Wei.
The team concluded that biochar’s unique physical and chemical properties possess a stronger affinity for microplastic particles than microorganisms in biological wastewater treatment systems.
“Microplastics that would otherwise contact microbial aggregates and pose toxicity to cellular activity are captured by biochar due to adsorption, thus ensuring the pollutant removal capacity of the biological system,” said Chen Wang, co-author and PhD student from UTS. “We also simulated the adsorption process of biochar and microorganisms on microplastics, and the accurate simulation of adsorption kinetic models confirmed the adsorption effects during biochar mitigation.”
Assessing microbial responses of biotoxicity of microplastics with biochar
To assess microbial responses with biochar involvement, the researchers also tested the compositions of microbial aggregates and the changes in secretion in microbial self-protection mechanisms. Chen said they judged the distribution of living and dead cells in the mixed microbial system through the cell staining method, followed by microbial sequencing to understand the effect of biochar on the relative abundances of functional microorganisms.
“Biochar is effective in maintaining the cell viability in wastewater treatment systems. We saw an increase of live cell percentages in biochar mitigation systems compared to microplastic exposure systems,” she said. “When exposed to microplastics, the abundances of microorganisms associated with pollutant degradation in the microbial community will be negatively affected, but fortunately, biochar can ensure the proportion of corresponding microorganisms.”
So, when being affected by biologically harmful environments such as microplastics, what defence activities will the microbial system do to maintain the activity of the original wastewater treatment system?
“Microplastics, the tiny particles, can cause excessive oxidative stress to functional microorganisms during biological wastewater treatment,” said Chen. “They can even possibly penetrate cell membranes. To this end, the microorganisms release extracellular polymers, acting like an armour to protect the cell.” The team confirmed that adding biochar effectively enhanced the secretion of these polymers in microbial systems after exposure to microplastics.
Where will the follow-up research focus?
“Based on ensuring the mitigation effect on the negative shock of microplastics, improving the environmental sustainability of biochar utilisation is crucial for the practical engineering application of this technology,” Dr Wei said.
“In our study, biochar can capture most of the microplastics. It can elevate the pollutant removal rate back to the level of the original level without microplastic exposure,” she said. “However, human society increasingly depends on plastic products. Thus, more and more tiny-sized particles will be received by wastewater. It puts forward higher requirements for the adsorption performance of biochar.”
Therefore, more in-depth exploration is necessary for biochar materials’ future modification and dosage modification.
They also believe that the research on biochar recovery method is essential. It is related to whether this technology can be economically applied in engineering. “According to the adsorption-desorption principle, it is not difficult to find that solvent rinsing and heat treatment can recover carbon-based adsorption materials. These basic theories provide broad prospects for the engineering practice of biochar application in wastewater treatment to alleviate the biotoxicity of microplastics,” said Dr Wei.
“We look forward to taking our innovations to the next level with industrial partners for efficient application in wastewater treatment plants.”
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