Regulation Mechanisms Of Surface Hydrophobicity/Hydrophilicity And Flotation Separation Of Microplastics

Microplastics refer to plastic fragments and particles with a diameter smaller than 5 mm that have infiltrated into the natural environment,posing potential hazards to organisms and ecosystems.The extensive consumption of plastic products has directly resulted in the accumulation of plastic waste in the environment,leading to increasingly severe microplastic pollution and persistent ecological risks.Therefore,how to treat microplastic pollution in the environment has become an inevitable scientific challenge.Unlike conventional waste plastic,microplastics in the environment have undergone complete changes in their surface properties due to the influences of environmental factors such as metal ions,organic compounds,sediment particles,and natural ageing.Achieving effective flotation separation of microplastics should focus on two aspects:firstly,understanding the"hydrophilicity induction"mechanisms of oxidation,adsorption,and coating on surfaces under the influence of environmental factors;secondly,exploring the mechanisms and application of"hydrophobicity regulation"under the action of surfactants to regulate hydrophilic microplastic surfaces.This work focuses on typical microplastics,including polyvinyl chloride（PVC）,polypropylene（PP）,polystyrene（PS）,polyethylene terephthalate（PET）,and acrylonitrile-butadiene-styrene copolymer（ABS）,as research objects.Based on analysis methods such as microplastic surface wettability,elemental chemical states,and interfacial interaction free energy calculations,the interactions and"hydrophilicity induction"behaviors between microplastics and common environmental factors,such as metal ions,organic pollutants,sediment particles,and the combinations of various environmental factors（natural aging）,are investigated.The order of hydrophilic induction strength of environmental factors is as follows:metal ions≈organic matter<sediment<natural aging.Due to the low adsorption capacity（<1 mg/g）of metal ions and organic matter on the surface of microplastics,and the fact that they do not alter the surface morphology and functional groups of microplastics,the wettability of microplastic surfaces is slightly affected by the effect of metal ions and organic matters.In contrast to metal ions and organic pollutants,the"hydrophilicity induction"effect of sediment particles is direct and significant,leading to a significant increase in hydrophilicity of microplastics（the contact angle decreases from 102-107°to 15-24°）.Calculation results of the free energy of interaction between particles indicate that the characteristic metal ions of the sediment weaken the energy barrier between them,resulting in a reduction of total interaction free energy by more than 3.0×10^-12 m J,thereby promoting spontaneous deposition of sediment particles onto the surface of microplastics.The"hydrophilicity induction"of natural aging is a more complex process that involves the effects of multiple environmental factors.After undergoing natural aging for 6 months,the surface hydrophilicity of microplastics is significantly enhanced（the contact angle of microplastics decreases from79-86°to 29-36°）,and a hydrophilic shell is formed on the surface,which contains oxygen-containing functional groups and is covered by sediment particles.Therefore,unlike the conventional understanding of hydrophobic surfaces of plastics,microplastics in the natural environment are influenced by various inducing factors,and their surfaces have been transformed into highly hydrophilic surfaces.In order to determine the relative strength and effective range of hydrophilic induction factors,this study further systematically investigated the effects of the above"hydrophilicity induction"factors on the flotation behavior of microplastics.Considering the strength of interactions between microplastics and various environmental factors,the flotation behavior of most microplastics is not affected by monovalent and divalent metal ions such as K⁺,Na⁺,Ca²⁺,and Mg²⁺,but trivalent Al³⁺significantly inhibits the flotation behavior of microplastics.Organic matter has minimal influence on the flotation behavior of microplastics,and even under the effects of high concentrations of humic acid and catechol（50 mg/L）,the average flotation ratio of microplastics still exceeds 70%.The hydrophilicity induction of sediment has a significant impact on the flotation behavior of microplastics,and it is strongly correlated with factors such as ion strength and microplastic particle size.Under high ion strength conditions,the flotation ratio of microplastics in lake sediment can decrease to as low as48.6%.Except for small-sized microplastics which still maintain high flotation ratio,the flotation ratio of microplastics with larger particle size（3-4 mm）decreases to 0.The impact of natural aging on the flotation behavior of microplastics is influenced by multiple hydrophilicity induction factors in the environment.After 6 months of natural aging,the hydrophilic shell formed on the surface of microplastics causes the flotation ratio of polypropylene microplastics with density less than water to decrease to 10.7%,and the flotation ratio of PVC,PS,and PET to decrease to 0.In order to achieve efficient flotation separation of hydrophilic microplastics,the transition of microplastic surface from hydrophilic to hydrophobic,which is favorable for flotation,is crucial.In this study,a targeted approach is developed for hydrophobicity regulation through the use of anionic and cationic collectors,as well as metal ion activation.Sodium oleate（Na OL）and dodecyltrimethylammonium chloride（DTAC）are selected as anionic and cationic surfactants,respectively,to regulate the hydrophobicity of aged microplastic surfaces.Surface characterization,adsorption experiments,and density functional theory calculations are used to determine the hydrophobicity regulation mechanism of surfactants on microplastic surfaces.The results show that both types of collectors significantly enhance the hydrophobicity of microplastic surfaces.Due to the longer hydrophobic chain of Na OL,the adsorption capacity of Na OL is higher than that of DTAC.As a result,the functional groups on the microplastic surface undergo significant changes after treatment with Na OL.The result of density functional theory calculations shows that both types of surfactant molecules stack and wrap around the surface of microplastics through van der Waals dispersion forces.Flotation experiments showed that the use of 0.10 mg/L Fe³⁺to activate the hydrophilic microplastic surface and the addition of 3.00 mg/L Na OL to regulate hydrophobicity could achieve efficient separation of naturally aged microplastics in the optimized process（>90%）.Taking water samples from Yubai River,Xiangjiang River,and landfill leachate containing microplastics as research objects,the flotation separation for microplastics developed in the paper is used,achieving removal ratio of 85.6%,75.1%,and 82.8%for Yubai River water,Xiangjiang River water,and landfill leachate,respectively.In summary,this paper has revealed the mechanism of hydrophilicity induction derived from various environmental factors on microplastic surfaces,proposed the"hydrophobicity regulation"method based on collectors and metal ion activation,and successfully develops the mechanism of hydrophilicity and hydrophobicity regulation on microplastics surface and technology of flotation separation.This technology achieves efficient flotation separation of microplastics in natural water.The research results are expected to alleviate the dilemma of widespread mesh-based surveys in the field of microplastics without effective solutions,and provide prototypes of theoretically advanced and technologically sound microplastics disposal techniques,creating conditions for reducing the existing stock,controlling the increment,and centralized disposal of microplastics.