Recent advances in microplastic removal from drinking water by coagulation: Removal mechanisms and influencing factors.

Microplastics (MPs) are emerging contaminants that are widely detected in drinking water and pose a potential risk to humans. Therefore, the MP removal from drinking water is a critical challenge. Recent studies have shown that MPs can be removed by coagulation. However, the coagulation removal of MPs from drinking water remains inadequately understood. Herein, the efficiency, mechanisms, and influencing factors of coagulation for removing MPs from drinking water are critically reviewed. First, the efficiency of MP removal by coagulation in drinking water treatment plants (DWTPs) and laboratories was comprehensively summarized, which indicated that coagulation plays an important role in MP removal from drinking water. The difference in removal effectiveness between the DWTPs and laboratory was mainly due to variations in treatment conditions and limitations of the detection techniques. Several dominant coagulation mechanisms for removing MPs and their research methods are thoroughly discussed. Charge neutralization is more relevant for small-sized MPs, whereas large-sized MPs are more dependent on adsorption bridging and sweeping. Furthermore, the factors influencing the efficiency of MP removal were jointly analyzed using meta-analysis and a random forest model. The meta-analysis was used to quantify the individual effects of each factor on coagulation removal efficiency by performing subgroup analysis. The random forest model quantified the relative importance of the influencing factors on removal efficiency, the results of which were ordered as follows: MPs shape > Coagulant type > Coagulant dosage > MPs concentration > MPs size > MPs type > pH. Finally, knowledge gaps and potential future directions are proposed. This review assists in the understanding of the coagulation removal of MPs, and provides novel insight into the challenges posed by MPs in drinking water.

Revealing the environmental hazard posed by biodegradable microplastics in aquatic ecosystems: An investigation of polylactic acid's effects on Microcystis aeruginosa.

The influence of petroleum-based microplastics (MPs) on phytoplankton has been extensively studied, while research on the impact of biodegradable MPs, derived from alternative plastics to contest the environmental crisis, remains limited. This study performed a 63 days co-incubation experiment to assess the effect of polylactic acid MPs (PLA-MPs) on the growth, physiology, and carbon utilization of M. aeruginosa and the change in PLA-MPs surface properties. The results showed that despite PLA-MPs induced oxidative stress and caused membrane damage in M. aeruginosa, the presence of PLA-MPs (10, 50, and 200 mg/L) triggered significant increases (p < 0.05) in the density of M. aeruginosa after 63 days. Specifically, the algal densities upon 50 and 200 mg/L PLA-MPs exposure were increased by 20.91% and 36.31% relative to the control, respectively. Meanhwhile, the reduced C/O ratio on PLA-MPs surface and change in PLA-MPs morphological characterization, which is responsible for substantially increase in the aquatic dissolved inorganic carbon concentration during the co-incubation, implying the degradation of PLA-MPs; thus, provided sufficient carbon resources that M. aeruginosa could assimilate. This was in line with the declined intracellular carbonic anhydrase content in M. aeruginosa. This study is the first attempt to uncover the interaction between PLA-MPs and M. aeruginosa, and the finding that their interaction promotes the degrading of PLA-MPs meanwhile favoring M. aeruginosa growth will help elucidate the potential risk of biodegradable MPs in aquatic environment.

The competitive advantage of Microcystis aeruginosa over Scenedesmus obliquus weakened by exposure to polylactic acid microplastics.

Aquatic ecosystems are heavily affected by microplastics (MPs), and its impacts on aquatic life have received extensive attention. However, it is still unclear how biodegradable MPs influence the growth and competition of phytoplankton. In this study, the response of growth dynamics, alternation in algal cell morphology and toxin-producing capability, and changes in the extracellular process of Microcystis aeruginosa (M. aeruginosa) and Scenedesmus obliquus (S. obliquus) were systematically studied in monoculture and co-culture conditions with and without the presence of polylactic acid MPs (PLA-MPs). The results indicated that although the loss of cell integrity was observed, PLA-MPs addition (50 mg/L) caused a 1.40- and 1.36-fold increase in cell densities of M. aeruginosa and S. obliquus in monoculture systems, respectively. This suggests the PLA-MPs more favored the growth of M. aeruginosa. This effect was manifested in co-culture conditions, because the degradation of PLA-MPs provided additional inorganic carbon in the experimental systems and benefited the growth of both M. aeruginosa and S. obliquus. Meanwhile, the synthesis of microcystins from the toxic M. aeruginosa was substantially reduced upon PLA-MPs exposure, which reduced the competitive advantage of M. aeruginosa over S. obliquus. Thus; the cell density of M. aeruginosa and S. obliquus showed 1.15- and 1.88-folds increasing compared to that without PLA-MPs addition. This interaction between PLA-MPs and algae weakened the competitive advantage of M. aeruginosa over S. obliquus, but their competitive outcomes remained unchanged. The results provided new insights into understanding the potential ecological risks of biodegradable plastics in aquatic ecosystems.

Controlling internal nutrients loading at low temperature using oxygen-loading zeolite and submerged macrophytes enhances environmental resilience to subsequent high temperature.

Nutrients releasing from anoxic sediment can be enhanced in summer because the dissolved oxygen (DO) consumption, nitrogen (N) and phosphorus (P) migration are susceptible to temperature. Herein, we proposed a method to hinder the aquatic environmental deterioration in warm seasons through consecutive application of oxygen- and lanthanum-modified zeolite (LOZ) and submerged macrophytes (V. natans) at low temperature scenario (5 °C, with depleted DO in water), and the effect was examined with drastic increasing the ambient temperature to 30 °C. The investigation was conducted in a microcosm scale including sediment cores (with a diameter of 11 cm, height of 10 cm) and overlying water (with depth of 35 cm). During the 60 days experiment, application of LOZ at 5 °C facilitated slower releasing and diffusion of oxygen from LOZ and the growth of V. natans. Thereby, when the temperature was increased to 30 °C and maintained for 35 days, the DO reached 10.01 mg/L, and the release of P and N from the sediment was reduced by 86% and 92%, respectively. This was achieved from the joint efforts of adsorption, biological conversion, chemical inactivation, and assimilation. Also, the LOZ inhibited 80% N2O, 75% CH4, and 70% CO2 emissions primary by promoting V. natans growth and reshaping microbiota. Meanwhile, the colonization of V. natans benefited the sustainable improvement in the water quality. Our results addressed the time that the remediation of anoxic sediment can be applied.

The Inhibition of Microcystin Adsorption by Microplastics in the Presence of Algal Organic Matters.

Microplastics (MPs) could act as vectors of synthetic chemicals; however, their influence on the adsorption of chemicals of natural origin (for example, MC-LR and intracellular organic matter (IOM), which could be concomitantly released by toxic Microcystis in water) is less understood. Here, we explored the adsorption of MC-LR by polyethylene (PE), polystyrene (PS), and polymethyl methacrylate (PMMA). The results showed that the MPs could adsorb both MC-LR and IOM, with the adsorption capability uniformly following the order of PS, PE, and PMMA. However, in the presence of IOM, the adsorption of MC-LR by PE, PS, and PMMA was reduced by 22.3%, 22.7% and 5.4%, respectively. This is because the benzene structure and the specific surface area of PS facilitate the adsorption of MC-LR and IOM, while the formation of Π-Π bonds favor its interaction with IOM. Consequently, the competition for binding sites between MC-LR and IOM hindered MC-LR adsorption. The C=O in PMMA benefits its conjunction with hydroxyl and carboxyl in the IOM through hydrogen bonding; thus, the adsorption of MC-LR is also inhibited. These findings highlight that the adsorption of chemicals of natural origin by MPs is likely overestimated in the presence of metabolites from the same biota.

Regulating autogenic vegetation in the riparian zone reduces carbon emissions: Evidence from a microcosm study.

Riparian zones have been found to be hot spots of greenhouse gas (GHG) production and have attracted increasing attention in recent decades. The occurrence of autogenic vegetation in riparian zones is prevalent, but little information is available concerning the influence of the occurrence and decomposition of this vegetation on carbon mitigation. We conducted a 220-day (110 days for the dry season and 110 days for the flooded season) microcosm experiment to study the mitigation and transformation of carbon regulated by the vegetation. The results revealed that there was a carbon dioxide (CO2) flux in the treatment with vegetation, and that without vegetation harvesting (835.58 mg/m2/h) was close to that with vegetation harvesting (796.22 mg/m2/h) under the simulated dry season conditions, but it was significantly higher than that without vegetation seedlings (411.55 mg/m2/h). After being flooded, the decomposition of the vegetation residues increased the total organic carbon (TOC) content of the sediment, the dissolved organic carbon (DOC) concentration of the water, and the dissolved CO2 and methane (CH4) contents of the sediment. This effect was reversed by harvesting the vegetation biomass. During the flooded season, the CO2 flux reached 222.95 mg/m2/h in the vegetation seeded treatment, but it decreased to -53.71 mg/m2/h when the vegetation biomass was harvested before being submerged. This was due to the decrease in the substrate available for CO2 production, the altered microorganism communities, and the decrease in the abundance of carbon metabolizing related enzymes. As a result, vegetation harvesting reduced the net carbon emissions by 48 % compared to that without vegetation regulation during the 220-day incubation period. The results of this study are significant to implementing measures to reduce GHG emissions from the riparian zone.

Microplastics benefit bacteria colonization and induce microcystin degradation.

Microplastics (MPs) can sorb toxic substances and be colonized by microorganisms. However, the interactions between the adsorbed toxic substances and the MPs biofilm remains inadequately understood. Here, a 37-days microcosm experiment was conducted to investigate the influence of polystyrene microplastics (PS-MPs) on microcystin (MC-LR) behavior in turbulent scenarios. The results revealed that adsorption by PS-MPs was the primary process that led to a quick reduction of aquatic MC-LR concentrations. With the colonization of microorganisms on the PS-MPs, the attached biofilm altered the surface properties of PS-MPs, which enhanced the bio-adsorption of MC-LR. Meanwhile, microcystins degrading bacteria, such as Sphingomonadaceae and Methylophilaceae, inhabited in the biofilm, which facilitated the MC-LR biodegradation; this was also demonstrated by the identified MC-LR degradation products. Thus, the MC-LR concentration in water was constantly decreased, with a maximum removal capability of 35.8% in PS-MPs added groups. In addition, a 25% reduction of MC-LR was recorded in PS-MPs added static water. This suggested that the interaction between PS-MPs, biofilm, and MC-LR may be prevalent in natural waters. Our results indicate MPs as vectors for toxic substances could be a double-edged sword (adsorption and biodegradation), which provides new insights for understanding the ecological risks of microplastics.