Microplastics: A potential proxy for tracing extreme flood events in estuarine environments.

The transport of microplastics (MPs) is susceptible to being influenced by catchment hydrology; however, there is a notable lack of research on their retention and responses to flood events in estuarine sedimentary records. Herein, we collected two cores in the Yangtze Estuary to explore their microplastic pollution, influencing factors and linkage to flood events. MP abundance exhibited a decreasing trend from the top to the bottom in both cores. Both plastic production and sediment mean grain size showed a significant positive correlation with MP abundance. The sedimentary record displayed a marked surge in MP abundance during the extreme flood period, suggesting a direct influence of flooding on MP deposition. The resuspension of upstream MPs and erosion of land-based MPs by heavy rain might be responsible for this increase. Furthermore, our study identified significant periodicities in MP abundance, closely aligned with the hydrological patterns of the Yangtze River. This study highlights the role of floods in fluvial MP distribution and proposes MPs as a proxy of extreme floods from the 20th century in estuarine environments.

Effects of microplastics on the toxicity of co-existing pollutants to fish: A meta-analysis.

Aquatic ecosystems are among the main destination for microplastics (MPs) in the environment. MPs that enter aquatic ecosystems can contribute to pollution together with other co-existing pollutants. However, whether such pollution results in higher or lower toxicity to fish than that caused by co-existing pollutants alone remains controversial. This study aimed at closing this research gap based on 1380 biological endpoints under the background of environmental MP concentrations collected from 55 laboratory studies. Overall, MPs in co-existing pollutant solutions significantly increased the toxicity to fish. Specifically, MPs elevated negative effects on the immune system, metabolism, and oxidative damage. Subgroup analysis indicated that changes in toxicity were related to fish life stage and MP size, but not to co-existing pollutant or MP type. Meta-regression analysis indicated that changes in toxicity were not related to the logarithm of the octanol-water partition coefficient (logKow) or exposure time. Finally, the differences between laboratory research and the actual aquatic environment were discussed from four aspects: MPs, co-existing pollutants, environmental factors, and experimental objects. Our study provides a basis for further understanding the potential impact of MPs on aquatic organisms from a combined pollution perspective. Moreover, our results can provide a reference for the conservation and management of aquatic ecosystems.

Distribution of organophosphate ester fractions in sediment of the Eastern China Marginal Seas and the influencing factors.

Organic pollutant fractions should be closely investigated because of their different ecological risks. In this study, we examined the distribution of organophosphate ester (OPE) fractions (labile, stable-adsorbed, and tight-adsorbed fractions) in sediments from Eastern China Marginal Seas (ECMSs) and assessed the influencing factor of the fractions. The mean values of total OPEs in ECMSs are 13.70 ± 6.16 µg L-1 in seawater and 32.04 ± 14.31 µg kg-1 in sediment. The results showed that OPE concentration decreased from the northern to the southern ECMSs, and tris(1-chloro-2-propyl) phosphate and tris(1,3-dichloro-2-propyl) phosphate were the primary OPEs. The mean contents of labile, stable-adsorbed, and tight-adsorbed fractions in the ECMSs were 9.50, 11.29, and 11.71 µg kg-1, respectively. Labile OPEs were predominant in offshore waters; the percentage of stable- and tight-adsorbed fractions increased progressively with offshore distance in ECMSs. The specific surface area and surface functional groups of sediment were not consistent with the fraction concentrations from the correlation coefficient, but the gross domestic product per coastline agreed well with the fraction distribution. Based on this, we conclude that human activity, rather than physicochemical characteristics of sediments, may be the primary influencing factor of the relative distribution of different OPE concentration fractions in ECMSs.

Can microplastics in offshore waters reflect plastic emissions from coastal regions?

Marine microplastic pollution is a major environmental challenge that threatens marine ecosystems and human health. Several models have been used to calculate and predict the theoretical amount of plastic waste discharged into the sea by coastal countries. Unlike earlier theoretical models of source discharge, we used the method of data normalisation to focus on the actual distribution of microplastics and their potential ecological risk in offshore surface waters. Our findings indicate that the average normalised abundance of microplastics in near-shore region of Bohai Sea was greater than the average normalised abundance of microplastics in the seas near the Yangtze River Delta urban agglomeration and the Pearl River Delta urban agglomeration. Moreover, the average amount of plastic waste discharged from terrestrial sources to the ocean per kilometre exhibited the following order: Bohai Rim urban agglomeration (150.90) (tonnes km-1) < the Pearl River Delta urban agglomeration (274.30) (tonnes km-1) < Yangtze River Delta urban agglomeration (577.44) (tonnes km-1). Further, the average microplastics abundance in offshore areas of different countries and the amount of plastic discharged per kilometre of the coastline were significantly negatively correlated, implying that microplastics were not necessarily abundant in coastal areas where large amounts of plastic are discharged into the sea. Hydrodynamic conditions had the greatest influence on the distribution of microplastics in offshore surface waters. The transport of nutrient salts from terrestrial areas to offshore waters was also influenced by hydrodynamics, with enrichment patterns in offshore areas exhibiting similar to those of microplastics. Therefore, when the offshore microplastic accumulation area overlapped with the nutrient salt enrichment zone, the health risk associated with the consumption of edible fish from offshore communities increased. In view of these findings, coastal countries must implement policies to reduce marine plastic waste emissions and develop management strategies based on their local pollution levels.

Distribution pattern and influencing factors for the microplastics in continental shelf, slope, and deep-sea surface sediments from the South China Sea.

Marine microplastic pollution has become a major global concern in recent years and the fate of microplastics in the ocean is a hot issue of research. We investigated microplastic pollution in surface sediments in the northern South China Sea to explore its distribution characteristics and influencing factors across the continental shelf, continental slope, and deep-sea environments. It was found that the microplastic abundance of surface sediments was 130.56 ± 40.48 items/kg. The average abundance of microplastics in all three topographic areas gradually decreased with increasing distance offshore. However, the differences in microplastic diversity indices between the three areas were not significant and were higher than those in other seas of the world, indicating that the waters of the northern South China Sea are rich in microplastics from complex sources, with more pollution input channels. In the continental shelf, fibrous and low density microplastics accounted for the largest amount, with a low degree of microplastic aging, and were mostly transported by suspended-load. These microplastics were mainly influenced by human activities. In the deep sea, microplastics with higher density were the most abundant and the number of fibrous microplastics was fewer, while the average size was larger, mainly influenced by the bottom currents. These microplastics underwent long-term bedload transport. In the continental slope, the main factors affecting the distribution of microplastics were more complex. In addition to pollution by human activities, the slope also receives microplastic materials carried by bottom currents; therefore, the composition of microplastics in the slope combines those characteristics of microplastics in both the continental shelf and deep-sea areas. The findings of this study indicate that the South China Sea is affected by complex pollution sources under the dual effects of human activities and natural conditions; in particular, the pollution situation in the deep-sea area needs extensive attention.

Assessing ecological risk of organophosphate esters released from sediment with both of total content and desorbable content.

This study introduced fractions (labile, stable-adsorbed, tight-adsorbed fractions) of organophosphate esters (OPEs) into ecological risk assessment to evaluate the potential risks of organophosphate esters that released from sediment, and conduct a case study to verify it. The content of desorbable fractions was get from adsorption-desorption experiments. Adsorption process can be divided into fast sorption, gradual sorption, and final equilibrium stage, and labile, stable-adsorbed, tight-adsorbed fractions were formed during adsorption. Approximately 86.21% labile, 73.41% stable-adsorbed, and 43.01% tight-adsorbed TPhP-D can be desorbed from sediments in desorption experiments. According to the results, the value of hazard quotient (HQ) that calculated by desorbable fractions reduced by 13.88% than HQ calculated by the sum of fractions, and result of 29.76% decrease for ∑HQs. The isotherm results demonstrated that the contents of labile and stable-adsorbed fractions increased faster than tight-adsorbed fraction when the concentration of contaminant in water increased from 50 to 400 µg/L, which means ecological risk in areas with high concentration of contaminants are higher than the discreet value. In case study, an additional hotspot was found in areas, where characterized with high proportion of labile fraction, when ecological risk was calculated by desorbable fractions. Indicating that revised ecological risk assessment takes both of total content and desorbable content into consideration.

Changes in surface characteristics and adsorption properties of 2,4,6-trichlorophenol following Fenton-like aging of biochar.

Fenton-like system formed in a natural soil environment deemed to be significant in the aging process of biochar. Aged biochars have distinct physico-chemical and surface properties compared to non-aged biochar. The aged biochar proved to be useful soil amendment due to its improved elements contents and surface properties. The biochar aging process resulted in increased surface area and pore volume, as well as carbon and oxygen-containing functional groups (such as C=O, -COOH, O-C=O etc.) on its surface, which were also associated with the adsorption behavior of 2,4,6-trichlorophenol (2,4,6-TCP). The biochar aging increased the adsorption capacity of 2,4,6-TCP, which was maximum at pH 3.0. The 2,4,6-TCP adsorption capacity of aged-bush biochar (ABB) and aged-peanut shell biochar (APB) was increased by 1.0-11.0% and 7.4-38.8%, respectively compared with bush biochar (BB) and peanut shell biochar (PB) at the same initial concentration of 2,4,6-TCP. All biochars had similar 2,4,6-TCP desorption rates ranging from 33.2 to 73.3% at different sorption temperatures and times. The desorbed components were mainly 2,4,6-TCP and other degraded components, which were low in concentration with small molecule substance. The results indicated that the aged-biochar could be effective for the long-term remediation of naturally organic polluted soils.

Simulated photocatalytic aging of biochar in soil ecosystem: Insight into organic carbon release, surface physicochemical properties and cadmium sorption.

Photochemical/photocatalytic reaction, one of the aging pathway of biochar in soil, not only changed the physicochemical properties of biochar, but also affected the migration and transformation of pollutants. Wheat straw biochar was photocatalytic aged in a Fenton-like system using organic acid as buffer solution under light sources, the organic carbon release and surface chemical changes of biochar were investigated to illustrate the adsorption behaviors. With Fe(III) or α-Fe2O3 added, the total organic carbon (TOC) of aged biochar solution was influenced more by buffer system than light sources, with the highest of 420.59 mg L-1 in citric acid system. The production of the hydroxyl radical (OH·) at citric/Fe(III) system was higher than the oxalic/Fe(III) system under the Hg lamp and showed an increasing trend with time. With light exposure, the porous structure of the biochar altered and surface area increased from 7.613 to 29.74 m2 g-1. Meanwhile, the adsorption of cadmium ion by biochar aged in citric/Fe(III) system also showed an increased adsorption capacity with a maximum of 73.54 mg g-1. So, a well understanding of biochar physicochemical properties changes under natural ecosystem was undoubtedly useful for scientific assessment the long-term feasibility of biochar as soil remediation.

Insights into the effects of long-term biochar loading on water-soluble organic matter in soil: Implications for the vertical co-migration of heavy metals.

Although interest in biochar remediation is growing, the effects of long-term biochar loading on soil environments have not been clearly confirmed. The contents and characteristics of water-soluble organic matter (WSOM) from soils after eight years of biochar remediation were investigated, and the vertical co-migration of heavy metals controlled by interactions between WSOM, soil and contaminants were also analyzed. The results showed that biochar-leaching WSOM featured high aromaticity. Fluorescence excitation-emission matrix (EEM) spectrophotometry was employed, and three primary components, including fulvic-acid-like (FA-like), tryptophan, and humic-acid-like (HA-like) compounds, were identified in the EEM spectra via parallel factor analysis models. With increasing biochar loading, FA-like and HA-like greatly increased, but tryptophan showed a weak response. Furthermore, the WSOM was freeze dried and analyzed with Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, and the results demonstrated that the BC treatment increased oxygen-containing functional groups and enhanced the complexation capability of the WSOM. Finally, the Cd and Pb concentrations in the WSOM were investigated, and Cd was found to decrease in top-soil WSOM with added BC because of increased complexation, but the Pb content increased because exchangeable and carbonate Pb converted into organic Pb. Further, the Cd and Pb concentrations decreased in sub-soil WSOM. These findings suggest that more efforts should be devoted to studying the effects of long-term biochar loading on soil environments.

Effects of laboratory biotic aging on the characteristics of biochar and its water-soluble organic products.

Effects of biotic aging on the characteristics of biochar and its water-soluble organic products were determined through a one-year laboratory incubation study. Biochar had a positive influence on microbial population size. Without microbial addition, biochars showed little change, except for an obvious increase in oxygen content from 3.2% to 6.3% after one year. By contrast, the carbon (C) content of the biologically-aged biochars continually decreased throughout the incubation at two humidity levels, suggesting that microbes consumed biochar C or encouraged organic matter solubilization. Fourier Transform Infrared Spectroscopy (FTIR) analysis indicated that all aged biochar surfaces showed increases in oxygen-containing functional groups and TG-DTG analysis showed that biologically-aged biochars were less stable than the corresponding abiotically-aged one. The release of dissolved organic matters from biologically-aged biochar logarithmically increasing with time, corresponded with of the pattern of microbe production, suggesting microbial involvement in solubilizing biochar. Combined three-dimensional excitation-emission matrix (3DEEM) and parallel factor (PARAFAC) analyses revealed that fulvic and humic acid-like components were the main water-soluble products of biologically-aged biochar, and these became increasingly rich in O-containing functional groups, i.e. humified, over time. These results highlight the importance of microbes in chemically transforming biochar and the dissolved products of biochar during aging.

Characteristics of organo-mineral complexes in contaminated soils with long-term biochar application.

Long-term studies on the environmental effects following biochar additions to soils, while plentiful, are predominantly focused on the soil fertility, whereas few are on the soil organo-mineral complexes. This study examines the changes of organo-mineral complexes in an acidic paddy soil and a saline-alkali soil which were remediated using biochar for approximately 8 years and 3 years, respectively. The results showed that loosely combined humus increased by 30.1% and 25.1% with the application of 40 t ha-1 biochar in the acidic paddy soil and the saline-alkali soil, respectively. Meanwhile, an increase of cement (Fe-oxides) was the contributor to the rise of the complexes content. Complex iron in the saline-alkali soil were 30% higher than in the acidic paddy soil with the application of 40 t ha-1 biochar. Fourier Transform Infrared Spectroscopy showed oxygen-containing functional groups on the surface of the biochar separated from the remediated field. X-ray diffraction analysis indicated that both complexation and sedimentation were involved in heavy metal immobilization. It was found that biochar amendment mitigated the effect of acid rain leaching and reduced vertical migration of the Fe/Al-bound complex, which can prevent soil from podzolization and thus improve its fertility.

Effects of Wet Oxidation Process on Biochar Surface in Acid and Alkaline Soil Environments.

Biochar has been studied for remediation of heavy metal-contaminated soils by many researchers. When in external conditions, biochar in soils ages, which can transform its structural properties and adsorption capacity. This study was conducted with two oxidation processes, HNO3/H2SO4 and NaOH/H2O2, to simulate the effects of biochar in acid and alkaline soil conditions. The results show that the oxygen-containing functional groups increased in aged biochar, which led to improve the ratio of oxygen and carbon (O/C). Nitro functional groups were found in the acid-oxidation treated biochar. Destroyed ditches and scars were observed on the surface of aged biochar and resulted in growth in their specific surface area and porosity. Specific surface area increased by 21.1%, 164.9%, and 63.0% for reed-derived biochar treated with water washing, acid oxidation, and basic oxidation, respectively. Greater peaks in the Fourier Transform Infrared Spectroscopy (FTIR) results were found in C⁻O and O⁻H on the surface of field-aged biochar. Meanwhile, mappings of energy-dispersive spectroscopy showed that biochar aged in soil was abundant in minerals such as silicon, iron, aluminum, and magnesium. In summary, biochar subjected to wet oxidation aging had an increased capacity to immobilize Cd compared to unaged biochar, and the adsorption capacity of oxidized biochar increased by 28.4% and 13.15% compared to unaged biochar due to improvements in porosity and an increase in functional groups.

Effects of chemical oxidation on surface oxygen-containing functional groups and adsorption behavior of biochar.

Biochar is a beneficial soil amendment but the changes in its surface properties during the aging process, especially the oxygen-containing functional groups and the associated adsorption behaviors, are not well documented. In this paper, the aged wheat straw biochar was simulated by chemical oxidation with HNO3-H2SO4 and NaOH-H2O2 systems. Characterization results showed that carbon loss and oxygen incorporation ran throughout the aging process. Surface oxygen-containing functional groups were found to be increased in all treated biochars, especially for carboxyl. Much more developed mesopores were observed in aging biochar, specific surface area was increased by 126% for biochar treated with NaOH-H2O2, and 226% for biochar treated with 40% of HNO3-H2SO4. Thermogravimetric analysis showed that the increasing oxygen-containing functional groups led to 14% and 30% mass loss by treating biochar with alkali and acid, respectively. The improved biochar surface through the increase of oxygen-containing functional groups enhanced the cadmium sorption capacity, and the sorption capacity increased by 21.2% in maximum. Roughed surface from oxidation was another reason for increasing cadmium adsorption. Results indicated that the adsorption performance of biochar on pollutant would be changed during aging process along with the changing surface properties.