Investigation of the adsorption behavior of Pb(II) onto natural-aged microplastics as affected by salt ions

Adsorption behavior of UV aged microplastics on the heavy metals Pb(II) and Cu(II) in aqueous solutions

As the "vector" of heavy metals in the aquatic environment, microplastics (MPs) have a great influence on the migration and transformation of heavy metals. In this study, the adsorption of polypropylene (PP), polyethylene (PE) and polystyrene (PS) on two models of heavy metals after UV aging and environmental variables (ionic coexistence, pH, salinity, and fulvic acid) were comprehensively explored on adsorption. The results show that new oxidation functional groups are formed and their hydrophilicity is enhanced after MPs aging. As a result, the adsorption experiments showed that the adsorption of contaminants by UV aged MPs exceeds that of pristine MPs. The adsorption amounts of Pb(II) and Cu(II) by PP, PE and PS increased by 1.45, 1.46, 1.25 and 1.63, 1.39, 1.22 times, respectively. Adsorption kinetic data were more consistent with the pseudo-second-order kinetic model, proving chemisorption to be the mechanism governing the interaction between metal ions and MPs. The Freundlich model could accurately predict the heavy metal adsorption isotherms on MPs, showing that non-homogeneous multilayer adsorption dominates the process. In Pb(II)-Cu(II) binary composite system, metal ion adsorption capacity on MPs is less than that of the single system adsorption capacity, which proves that there is a specific inhibitory effect between coexisting ions. Additionally, external factors like pH, salinity, and fulvic acid content have a big impact on adsorption behavior. According to mechanism analysis, the adsorption process mainly relies on electrostatic interaction, surface complexation, and van der Waals force.

Assessment of microplastics pollution in aquatic species (fish, crab, and snail), water, and sediment from the Buriganga River, Bangladesh: An ecological risk appraisals

Current work focus on microplastic (MPs) occurrence in the water, sediment, and aquatic species (fish, crab, and snail) of the Buriganga River, Bangladesh, with an ecological risk assessment perspective. It also includes the distribution of MPs in different river ecosystem segments and the presence of heavy metal (loid)s (HMs) in water, sediments, and MPs surface. The MPs were inspected by stereomicroscope to identify the shapes, color, and size, and Fourier transform infrared (FTIR) spectroscopy was used to characterize polymer types. The samples concentration of four HMs viz., As, Cd, Cr, and Pb were determined by atomic absorption spectrometry (AAS). The possible MPs content in water, sediment, fish, crab, and snail were varied from 0.250 to 0.117 MPs/mL, 3.5-8.17 MPs/g, 0.65-3.82 MPs/g, 3.75-4.28 MPs/g, and 0.84-1.12 MPs/g, respectively. Fibers and fragments were the most dominant shape, less than 0.5 mm was dominant in size, and blue was the dominant color. In the evaluation of the chemical composition of MPs in water, sediment, fish, snail, and crab samples, Polyethylene terephthalate (PETE), Ethylene-vinyl acetate (EVA), High-density polyethylene (HDPE), Acrylonitrile butadiene styrene (ABS), Cellulose acetate (CA), and Nylon were identified. Regarding HMs load, the river demonstrated a highly polluted environment following the abundance pattern Cr > Pb > As>Cd. SEM-EDAX of MPs was conducted to investigate the surface MP's surface and elemental composition. It reveals that the MPs surface has characteristic flakes, cracks, and adhering particles along with Si, K, Au, C, and O on the surface studied MPs. There is no significant relationship found among the ecosystem segments. However, Ompok bimaculatus species show a negative relationship of MPs distribution with water and sediment. Moreover, according to the ecological risk of MPs pollution in the Buriganga River, it was in category-I, indicating considerable pollution load due to the presence of MPs.

New insights into the adsorption behavior of thiacloprid at the microfibers/water interface: Role of humic acid

Dissolved organic matter regulates the interaction between microplastics (MPs) and organic pollutants. Here, this paper investigated the effect and mechanism of humic acid (HA) on the adsorption behavior of thiacloprid at two microfibers (MFs)/water interface, and compared the differences in the performance of MFs and pure MPs. The results showed that 10 mg L^-1 HA decreased the adsorption capacity and the partition coefficient K_D of thiacloprid on MFs and pure MPs. Spectral analysis showed that HA could form hydrogen bonds and van der Waals forces with both MPs and thiacloprid, ultimately affecting the adsorption behavior of thiacloprid at MPs/water interface via competitive adsorption and bridging effect. Furthermore, two-dimensional correlation spectroscopy demonstrated that thiacloprid was preferentially adsorbed onto MPs compared with HA. Finally, density functional theory calculation demonstrated that phenolic-OH, -COOH, and alcoholic-OH played critical roles in competing adsorption and bridging effect. This study offers a theoretical foundation for a better comprehension of the adsorption behavior of organic pollutants at the MPs/water interface.

Polyamide microplastics as better environmental vectors of Cr(VI) in comparison to polyethylene and polypropylene microplastics

The adsorption, desorption, and their influence factors of Cr(VI) by microplastics (MPs) in the solution was investigated in this study. The results demonstrated that UV aging promoted adsorption, while the increase of salinity and natural organic matter (NOM) inhibited adsorption. The particle size affected the total Cr(VI) active adsorption sites on MPs, while the pH changed the electrostatic force. The Cr(VI) adsorption by MPs conformed to the pseudo-first and pseudo-second order kinetic models, and was monolayer and inhomogeneous. SEM-EDS, XPS and ATR-FTIR analyses demonstrated that the physical adsorption dominated the adsorption process. Especially, the highest adsorption capacity of Cr(VI) by polyamide (PA) MPs was attributed to their hydrophilic amide groups. However, Cr(VI) was only released from PA MPs, of which desorption rate followed the sequence of seawater (79.5 %) > fresh water (66.6 %) > deionized water (34.8 %). Thus, PA MPs might be non-negligible environmental vectors for the transport of Cr(VI).

Aging of polylactic acid microplastics during hydrothermal treatment of sewage sludge and its effects on heavy metals adsorption

Microplastics' (MPs) aging process and environmental behavior have attracted extensive attention due to the potential long-term ecological impact. MPs enriched in sludge may accelerate aging during sludge treatment and the affecting environmental behavior, i.e., adsorption performance for pollutants. However, the related studies have not been well researched, especially for the biodegradable MPs. This study revealed the influences of hydrothermal treatment on the characteristics of polylactic acid microplastics (PLA-MPs) and the consequences on heavy metals adsorption. The changes in PLA-MPs' physiochemical properties were characterized and compared. PLA-MPs' surface became irregular, and the oxygen-containing functional groups increased through FTIR and XPS analysis. Meanwhile, the molecular weight and crystallinity of PLA-MPs decreased significantly with the rising in hydrothermal temperature. Accordingly, the adsorption capacity of PLA-MPs for Pb²⁺ increased from 93.97 μg g^-1 for the raw PLA-MPs to 1058.03 μg g^-1 for the aged PLA-MPs. Multiple adsorption kinetics and isotherms were discussed for the Pb²⁺ adsorption onto PLA-MPs with different aging of the PLA-MPs. The adsorption mechanisms of Pb²⁺ relate to electrostatic interaction and complexation. The main difference is that the adsorption for raw PLA-MPs is dominated by physical and chemical adsorption, whereas the adsorption for the aged PLA-MPs prefers chemical adsorption. In addition, we carefully evaluated the influences of pH, dissolved organic matter, and ionic strength on the PLA-MPs adsorption. The present study highlighted the significance of hydrothermal treatment on the MPs aging and the adsorption performance.

Heavy metal-mediated adsorption of antibiotic tetracycline and ciprofloxacin on two microplastics: Insights into the role of complexation

Microplastics (MPs) have recently become an emerging environmental concern. Nevertheless, limited information is known about the adsorption of MPs for organic contaminants under combined heavy metals pollution, with an emphasis on the role of complexation. Thus, this study aims to comprehensively compare and investigate the adsorption performance of antibiotic tetracycline (TC) and ciprofloxacin (CIP) on two polar MPs (polyamide (PA) and polyvinyl chloride (PVC)) affected by Cu(II) and Cd(II) with contrasting complexation abilities. Batch adsorption experiments were used in combination with speciation calculation, zeta potential determination, FTIR spectroscopy characterization and investigation of the affinity of MPs for heavy metals. Results showed that the sorption kinetics and isotherms of TC and CIP on PA and PVC could be well fitted to pseudo-second-order and Langmuir models, respectively, both in the absence and presence of Cu and Cd, suggesting that multiple interactions and monolayer adsorption played an important role in the adsorption process. The presence of Cu substantially improved TC and CIP adsorption and obviously changed the pH dependence of their adsorption onto both MPs, which may result from the Cu-induced strong complexation with TC and CIP. The presence of Cd slightly enhanced TC adsorption on both MPs while reduced CIP adsorption especially on PVC, which may be ascribed to the Cd-induced cationic bridging effects in TC adsorption and the competitive adsorption of Cd in CIP adsorption. Therefore, the heavy metal-mediated complexation effects may play a dominant role in antibiotic adsorption by MPs only in the presence of heavy metals with strong complexation ability while the adsorption performance in the presence of heavy metals with negligible complexation capacity may be influenced by effects other than complexation. This study helps further understand the heavy metal-mediated adsorption behavior of organic contaminants on polar MPs and the role of complexation reactions therein.

The weathering process of polyethylene microplastics in the paddy soil system: Does the coexistence of pyrochar or hydrochar matter?

This study is based on a particular test site to simulate the weathering process of microplastics (MPs) in paddy soil. A substantial amount of plastic waste, especially MPs, inevitably accumulates in agricultural soil due to the high consumption and short average use of plastics. Recently, MP pollution has become a global environmental concern. However, insight into the soil weathering process of MPs in paddy soil, particularly in the presence of biochar, is lacking. In this study, the physicochemical properties of polyethylene (PE) MPs were determined through a 24-week weathering system conducted in paddy soil, paddy soil with pyrochar, or hydrochar. Moreover, the sorption of original and weathered PE MPs toward three typical pollutants (cadmium/Cd, bisphenol A/BPA, and dimethyl phthalate/DMP) was investigated. The surface of PE MPs was fractured, 1.1-fold rougher, yellow-colored (11.7 units), and 1.8-fold more oxidized after paddy soil weathering. In addition, the crystallinity, negative charge, and stronger hydrophilicity of weathered PE MPs increased compared to original PE MPs. Weathering in a pyrochar or hydrochar system caused fissures, extensive destruction of amorphous areas, and accelerated chemical or bio-oxidation processes for PE MPs, resulting in a more noticeable change in roughness (1.4-2.2-fold), yellow color (12.7-13.7), crystallinity (1.2-1.5-fold), and oxygen content (2.5-3.6-fold). Weathered PE MPs facilitated the sorption with Cd and BPA, attributed to larger specific surface area, abundant polar functional groups, and increased negatively charged sites. However, sorption of DMP to PE MPs was highly influenced by their hydrophobicity, resulting in decreased hydrophobic partition sorption on weathered PE MPs. Overall, paddy soil weathering affected the properties of PE MPs and enhanced sorption of Cd and BPA but reduced sorption of DMP. The coexistence of biochar exacerbated the paddy soil weathering effect. The insight gained from this study assists in better understanding the weathering process of PE MPs in agricultural soils.

Low-temperature aerobic carbonization and activation of cellulosic materials for Pb2+ removal in water source

Targeting the removal of Pb²⁺ in wastewater, cellulosic materials were carbonized in an aerobic environment and activated via ion exchange. The maximum adsorption capacity reached 243.5 mg/g on an MCC-derived adsorbent activated with sodium acetate. The modified porous properties improved the adsorption capacity. The capacity could be completely recovered five times through elution with EDTA. Because of the negative effects of Ni, Mg, and Ca elements, the adsorption capacities of activated carbonized natural materials were lower than that of pure cellulose. N₂ adsorption measurement showed that the adsorbent had a large specific surface area as well as abundant micropores and 4-nm-sized mesopores. FTIR and surface potential results proved that carboxyl group was generated in the aerobic carbonization, and was deprotonated during ion exchange. This adsorbent consisted of C-C bonds as the building blocks and hydrophilic groups on the surface. XPS results demonstrated that the Pb 4f binding energies were reduced by 0.7-0.8 eV due to the interaction between Pb²⁺ and the activated adsorbent, indicating that the carboxylate groups bonded with Pb²⁺ through coordination interactions. Pseudo-second-order and Elovich kinetic models were well fitted with the adsorption processes on the pristine and activated carbonized adsorbents, indicative of chemisorption on heterogeneous surfaces. The Freundlich expression agreed well with the data measured, and the pristine and activated adsorbents had weak and strong affinities for Pb²⁺, respectively. The Pb²⁺ adsorption process was exothermic and spontaneous, and heat release determined the spontaneity. The adsorption capacity is attributed to the carboxylate groups and pores generated in the aerobic oxidation and ion exchange procedures.

The potential of polyethylene microplastics to transport copper in aquatic systems: Adsorption and desorption studies

Heavy metals and microplastics are two types of general contaminants that can cause significant damage to water systems and organisms. However, the interaction of heavy metal ions with microplastic in aquatic systems received less attention compared with that of organic contaminants. This study aims to explore the interaction of copper (II) ions with microplastic (polyethylene) in aquatic systems. The adsorption experiments were performed by varying several operational parameters including equilibrium pH, initial Cu (II) concentrations, and contact times. The microplastic was characterized using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared. The results confirmed the porous nature of the microplastic surface and the existence of various binding sites. The maximum Cu (II) uptake by microplastic was recorded as 1.23 mg/g at pH 5, according to the Langmuir adsorption isotherm. The experimental isotherm data exhibited a good fit to the Toth model, followed by the Langmuir and Freundlich equations, according to the correlation coefficient and %error values. The pseudo-first kinetics equation showed a better fit to copper (II) kinetics data compared with the pseudo-second kinetics equation. Elution of copper (II) ions from copper (II)-loaded microplastic was attempted using several elutants, and the results indicated that 0.01 M HNO₃ performed well with elution efficiency over 99.5%. Thus, the elution experiments furnished proof that Cu-loaded microplastic may leach Cu (II) ions under rich acidic conditions, thereby aiding the transport of Cu (II) ions into the digestive tracts of aquatic organisms. PRACTITIONER POINTS: Polyethylene microplastics showed potential to sorb copper ions. The mechanism was electrostatic interaction between microplastics and metal ions. Maximum copper adsorption by microplastic was recorded as 1.23 mg/g. Once desorbed, Cu(II) transferred into the digestive tracts of aquatic organisms.

Quantitative assessment of interactions of hydrophilic organic contaminants with microplastics in natural water environment

The interaction between microplastics (MPs) and hydrophilic organic contaminants (HOCs) in natural water environment has recently raised great public attentions due to the potential toxicity to humans. However, the quantitative assessment is less studied. In this study, the interaction between ciprofloxacin (CIP) and ofloxacin (OFL) (two important HOCs) and virgin and aged polystyrene (PS) was investigated. The aged PS showed higher adsorption rate and capacity than the virgin PS, due to its larger surface area and more O-containing groups. The pH-dependent adsorption of CIP was higher than OFL on both virgin and aged PS; the maximum adsorption for both HOCs occurred at pH 5. The sequential orders of functional groups for the adsorption were discovered according to the study by the 2D correlation Fourier transform infrared spectroscopy. Several mechanisms existed for the interaction: (1) at 3.0 < pH < 5.0, the electrostatic attraction (EA) was inhibited while H-bond (HB) was dominant, accounting for > 60% of the total uptake; (2) at 5.0 < pH < 8.0, the contribution of EA increased to around 50-60% while HB decreased to 30-40%; (3) at 8.0 < pH < 10.0, EA, HB and π-π conjugation caused 30-40%, 25-40% and 20-45% of the total uptake, respectively; (4) at 10.0 < pH < 12.0, π-π conjugation accounted for 90-100%. Notably, higher adsorption of CIP was mainly attributed to the presence of secondary amino groups and its higher pK_a value, correspondingly leading to the additional ordinary HB and negative charge-assisted HB, and EA interactions with PS. This study further provides clear evidences on the risk of MPs and HOCs on humans and aqueous living organisms.

Aging relieves the promotion effects of polyamide microplastics on parental transfer and developmental toxicity of TDCIPP to zebrafish offspring

Understanding the role of microplastics (MPs) in the biological fate and toxicity of organic pollutants in food webs is vital for its risk assessment. However, contradictory results and the neglect of MP aging as a factor have led to a research gap, which needs to be filled. Our study discovered that polyamide (PA, a ubiquitous MP in water) MPs clearly facilitated bioaccumulation of tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) in the F0 zebrafish gonads and parental transfer of TDCIPP to the F1 offspring. Rapid TDCIPP desorption in the gut and intestine barrier dysfunction triggered by MPs were the causes for the phenomenon. In contrast to the pristine forms, aged PA with higher hydrophilcity exhibited stronger binding and polar interactions with TDCIPP, and the intestine damage was neglectable, resulting in increased intestinal immobilization and prevented parental transfer of TDCIPP. Additionally, the aggravated body weight loss and decreased length of TDCIPP offspring were relieved after PA aging. The recovery of subintestinal venous plexus angiogenesis, yolk lipid utilization, and ATP synthesis were responsible for the mitigated transgenerational toxicity. Our results highlight the significance of aging on the role of MPs with respect to coexisting pollutants and have great implications for understanding MP-associated risks.

Photochlorination-induced degradation of microplastics and interaction with Cr(VI) and amlodipine

Wastewater treatment plants (WWTPs) are the important source of microplastics (MPs) in the environment, and disinfection processes bear high potential to degrade MPs. This study investigated the physicochemical degradation, dissolved organic products and interaction with co-existed pollutants (heavy metal and pharmaceutical) on polyethylene (PE), polypropylene (PP) and polystyrene (PS) MPs during simulated disinfection processes. Compared to photo or chlorination, photochlorination significantly resulted in the physicochemical degradation, including morphology alteration, fragmentation, and chemical oxidation on PP and PS MPs, but showed relatively low effect on PE, indicating the different resistance among polymers to disinfected treatment. Photochlorination also caused the formation of chain-scission organic compounds and even chlorinated products from MPs (e.g. C₁₁H₁₉O₄Cl for PP and monochlorophenol, dichlorophenol, chloroacetophenone and chlorobenzoic acid for PS), which may form disinfection byproducts to induce healthy risk. The adsorption potentials of MPs for Cr(VI) or amlodipine were enhanced by photochlorination since the cracking and formed oxygen functional groups enhanced the pore filling and surface precipitation of Cr(VI), and the electrostatic attraction and hydrogen bonding with amlodipine. The findings indicated the physicochemical degradation of MPs and the combined pollution with co-existed pollutants, highlighting the health risks of MP-derived organic products during the disinfection treatments (even in normal dosage) in WWTPs.

Microplastics aging in wastewater treatment plants: Focusing on physicochemical characteristics changes and corresponding environmental risks

Microplastics (MPs) have been frequently detected in effluent wastewater and sludge in wastewater treatment plants (WWTPs), the discharge and agricultural application of which represent a primary source of environmental MPs contamination. As important as quantitative removal is, changes of physicochemical characteristics of MPs (e.g., shapes, sizes, density, crystallinity) in WWTPs are crucial to their environmental behaviors and risks and have not been put enough attention yet. This review is therefore to provide a current overview on the changes of physicochemical characteristics of MPs in WWTPs and their corresponding environmental risks. The changes of physicochemical characteristics as well as the underlying mechanisms of MPs in different successional wastewater and sludge treatment stages that mainly driven by mechanical (e.g., mixing, pumping, filtering), chemical (e.g., flocculation, advanced oxidation, ultraviolet radiation, thermal hydrolysis, incineration and lime stabilization), biological (e.g., activated sludge process, anaerobic digestion, composition) and their combination effects were first recapitulated. Then, the inevitable correlations between physicochemical characteristics of MPs and their environmental behaviors (e.g., migration, adsorption) and risks (e.g., animals, plants, microbes), are comprehensively discussed with particular emphasis on the leaching of additives and physicochemical characteristics that affect the co-exist pollutants behavior of MPs in WWTPs on environmental risks. Finally, knowing the summarized above, some relating unanswered questions and concerns that need to be unveiled in the future are prospected. The physicochemical properties of MPs change after passing through WWTP, leading to subsequent changes in co-contaminant adsorption, migration, and toxicity. This could threaten our ecosystems and human health and must be worth investigating.