Micro (nano) plastics in wastewater: A critical review on toxicity risk assessment, behaviour, environmental impact and challenges.

With millions of tonnes of plastic pollution generated every year, small-sized plastic particles, including micro- and nanoplastics, end up in freshwater systems. Due to the very small size and very large specific surface area of nanoplastics, they are known to be persistent and toxic in our environment. These particles are also known to react with other water-borne contaminants and cause acute toxicity in organisms. Nanoplastics are prone to biomagnification and can be transported to humans through various pathways. This study aims to contribute towards understanding the behaviour of nanoplastics in our environment, specifically through identification of various sources, detection techniques, toxicity estimation, health risk in humans, environmental fate, recovery and reuse, and future challenges and limitations. Detailed review on the toxic effects of nanoplastics on various organisms and their degradation rates in soil and water matrices are provided. The suitability of small- and large-scale separation techniques for the removal of nanoplastics in wastewater treatment plants is also discussed. Current challenges and future perspectives in understanding the fate and transport of nanoplastics in the environment are also discussed. Research gaps, including the development of quantification techniques, estimation of degradation mechanisms, transport in marine ecosystems, and development of sensors to examine nanoplastics in the environment, are explored. Finally, we can limit the release of nanoplastics to the environment through reduction, reuse and recycling (3 Rs) of bulk plastic products.

Biochar applied to soil under wastewater irrigation remained environmentally viable for the second season of potato cultivation.

The environmental effectiveness of plantain peel biochar in the second season of its application to soil was studied using outdoor lysimeters (0.45 m diameter x 1.0 m height) packed with sandy soil, cultivated with potatoes (Solanum tuberosum) and irrigated with wastewater. Biochar (1% w/w) was amended in the soil one-time in the first season. For two seasons, the biochar improved the soil properties, immobilized the heavy metals in the soil, and reduced their uptake by the crop. The CEC of the biochar-amended soil (WW + B) for example, as compared to the unamended treatment (WW-B), was significantly higher (p<0.05; >65%) for both seasons due to higher pH which controls the availability of cations in soils, influencing their CECs. The soil sampled in the second season showed accumulation of all the heavy metals in the topsoil, while only Zn, Pb and Fe moved to the 0.1 m depth. The Fourier transform infra-red spectra of the soil and soil-biochar mix were similar and suggested that oxygen-containing functional groups were partly responsible for binding the heavy metals. The heavy metals translocated to all the potato parts (flesh, peel, root, stem and leaves). The concentrations of the heavy metals in potato parts under freshwater were lower than those under wastewater irrigated condition. After the second season of being in the soil, biochar significantly reduced (p < 0.05) the concentrations of Cd, Cu, Cr, Pb and Zn in the edible flesh suggesting that biochar immobilized wastewater-laden heavy metals in soil and reduced their uptake in potatoes for at least two seasons.

Effect of biochar on heavy metal accumulation in potatoes from wastewater irrigation.

In many developing countries water scarcity has led to the use of wastewater, often untreated, to irrigate a range of crops, including tuber crops such as potatoes (Solanum tuberosum L.). Untreated wastewater contains a wide range of contaminants, including heavy metals, which can find their way into the edible part of the crop, thereby posing a risk to human health. An experiment was undertaken to elucidate the fate and transport of six water-borne heavy metals (Cd, Cr, Cu, Fe, Pb and Zn), applied through irrigation water to a potato (cv. Russet Burbank) crop grown on sandy soil, having either received no biochar amendment or having top 0.10 m of soil amended with 1% (w/w) plantain peel biochar. A non-amended control, irrigated with tap water, along with the two contaminated water treatments were replicated three times in a completely randomized design carried out on nine outdoor PVC lysimeters of 1.0 m height and 0.45 m diameter. The potatoes were planted, irrigated at 10-day intervals, and leachate then collected. Soil samples collected two days after each irrigation showed that all heavy metals accumulated in the surface soil; Fe, Pb and Zn were detected at 0.1 m depth, while only Fe was detected at 0.3 m depth. Heavy metals were not detected in the leachate. Tested individually, all portions of the potato plant (tuber flesh, peel, leaf, stem and root) bore heavy metals. Biochar-amended soil significantly reduced only Cd and Zn concentrations in tuber flesh (69% and 33%, respectively) and peel compared to the non-amended wastewater control (p < 0.05). Heavy metal concentrations were significantly lower in the tuber flesh than in the peel, suggesting that when consuming potatoes grown under wastewater irrigation, the peel poses a higher health risk than the flesh.