Oil spills enhanced dispersion and transport of microplastics in sea water and sand at coastal beachheads.

The coastal zone is being under the threat by accumulation of microplastics (MPs), with much of MPs ending up on the beachhead. Oil spills, which frequently happen in coastal zones due to oil pipe leakage or oil drilling, may affect the behavior of MPs in the beachheads. Herein, sea water and sea sand were collected from three different coastal beachheads including Bohai Sea (BS), East Sea (ES), and South Sea (SS), China, to investigate how the oil spills affect the dispersion and transport of MPs in sea water and sand. The oil spills greatly enhanced the dispersion of MPs in all three sea waters by forming MPs-oil-dispersant agglomerates, which increased the electrostatic repulsion and steric hindrance between MPs particles. Accordingly, the aggregation rates of MPs were reduced from 1.7-8.86 nm min-1 to 0.39-1.29 nm min-1. The lowest salinity and highest dissolved organic carbon content in SS sea water favored the highest dispersion of MPs, compared to BS and ES sea water. The improved dispersion of MPs with oil spills enhanced their transport in sea sand with an increase of effluent rates from 0-18.8 % to 5.78-42.2 % for BS and from 30.5-45.2 % to 35.0-60.0 % for SS one. However, the transport of MPs in ES sea sand was lower than 3.62 %, even with oil spills, which was attributed to the strong adsorption of MPs by the rich Fe/Al oxides in ES sea sand through electric attraction. Modeling also showed that oil spills increased the migration rate of 10 mg g-1 MPs accumulated in the surface 0-1 cm sea sand from 6.50-13.8 cm year-1 to 8.17-16.7 cm year-1 after 1500 mm rainfall for 3 years, and the strongest transport of MPs was observed in SS sea sand, with the highest cumulative flux and the longest maximum migration depth as 0.089-0.120 mg/cm2 and 50 cm, respectively. These results indicated that the dispersion and transport of MPs can be enhanced by oil spills, but regulated by sea water salinity for MPs dispersion and sea sand Fe/Al oxides for MPs transport, which advanced our understanding of the transport and transformation of MPs in coastal zones.

Colloid formation and facilitated chromium transport in the coastal area soil induced by freshwater and seawater alternating fluctuations.

Seawater seasonal fluctuation results in its close interaction with freshwater in the coastal area, which may affect behavior of contaminants there. This study was conducted to explore the transport and transformation of soil colloid and associated Cr during freshwater and seawater alternating fluctuations by laboratory experiment and numerical simulation. Such a fluctuation brought downward migration of Cr from upper contaminated soil and induced reduction of Cr(VI) into Cr(III). An obvious increase of retained Cr(III) was observed at the lower layers of soil due to the reducing environment. More importantly, the colloids with average sizes between 800-1500 nm was formed during the fluctuation and mainly composed of microcline and Fe/Mn oxides minerals, which determined the Cr transport. Compared with the previous freshwater fluctuation, seawater fluctuations generated more and larger-sized colloids due to its high ionic strength. These colloids carried over 94% Cr in the effluent and Cr(III) accounted for over 95% of total Cr. A colloid-facilitated Cr transport modeling showed that the soil retained Cr decreased by about 14% after eight rounds of fluctuation on an actual soil-contaminated site scale. Our study provides insight for the understanding of geochemical process of Cr in the coastal area under freshwater and seawater fluctuation conditions.

Dispersion and transport of microplastics in three water-saturated coastal soils.

The coastal area is one of the key zones for transport and fate of microplastics (MPs). This study investigated the transport behaviors of different sized MPs in three water-saturated coastal soils, with the aim to explore effects of properties of three different coastal soils on the dispersion and migration of three-sized MPs (0.3, 0.5, and 1 µm). All three-sized MPs had the strongest dispersion in Soil 3 solution, followed by that in Soil 1 solution and then that in Soil 2 solution. The strongest dispersion of MPs in Soil 3 solution was attributed to the lowest ionic strength. Such a high dispersion favored MPs movement in soil solution but readily be sorbed and fixed by rich Fe and Al oxides in Soil 3 solid through strong electrostatic attraction, leading to the lowest transport rate (20.5-41.2%). The high ionic strength in the Soil 1 solution decreased the dispersion of MPs, but the presence of high content of humic acid enhanced the electrostatic repulsion and steric hindrance between MPs and soil particles, resulting in the highest transport ability of MPs in Soil 1 (39.4-72.5%). The large amount of dissolved Ca2+ and Mg2+ in Soil 2 solution favored MPs bridged with fulvic acid, resulting in the highest aggregation of MPs and relatively lower transport ability (34.1-49.6%). Large-sized MPs had higher electrostatic repulsion between the particles, thus increasing the dispersion and transport capacity of MPs in soil. Modeling showed the experiment-consistent results that Soil 3 had the lowest MPs transport after 600 mm of heavy rainfall, with the maximum migration distance of 7.50-10.5 cm, which was smaller than that in Soil 2 (8.10-12.0 cm) and that in Soil 1 (9.00-18.3 cm). These results indicated that MPs transport in coastal soil is significant and soil solution and solid composition plays an important role in the dispersion and transport of MPs, respectively. These findings afforded a great basis for the assessment of the fate and risk of MPs in coastal areas.

Biochar nanoparticles with different pyrolysis temperatures mediate cadmium transport in water-saturated soils: Effects of ionic strength and humic acid.

Biochar is advocated as an environment-friendly and cost-effective material for removing both heavy metals and organic contaminants in soil remediation. However, our understandings on the cotransport potential of contaminants with the nanoscale biochar downward along soil profiles (e.g., potential environmental risks towards groundwater) remain largely unknown. This study investigated the effects of wheat straw-derived biochar nanoparticles pyrolyzed at 350 °C and 500 °C (BNP350 and BNP500) on the transport of cadmium (Cd(II)) in water-saturated soil packed columns. Different ionic strengths (ISs) without/with humic acid (HA) were tested to mimic the scenarios during soil remediation. BNPs could act as a vehicle mediating Cd(II) transport in soils. At a low IS (1.0 mM KCl), compared to the limited transport of individual Cd(II), BNP500 enhanced (69 times) Cd(II) transport (Cd(II) mass recovery (M) = 7.59%) in soils, which was greater than that by BNP350 (54 times, M = 5.92%), likely due to the higher adsorption of Cd(II) onto BNP500. HA further increased the Cd(II) transport by BNPs (M = 8.40% for BNP350 and M = 11.95% for BNP500), which was mainly due to the increased mobility of BNPs carrying more absorbed Cd(II). In contrast, at a high IS (10 mM KCl), BNP500 dramatically inhibited the transport of Cd(II) (M = 12.9%), decreasing by about 61.6%, compared to the BNPs absence (M = 33.6%). This is because a large amount of BNP500-Cd(II) was retained in soils at a high IS. This inhibition effect of Cd(II) transport by BNPs was reinforced with the presence of HA. Our findings suggest that the pyrolysis temperature of biochar should be carefully considered when applying biochar for in-situ remediation of soils contaminated by heavy metals such as Cd(II) under various organic matter and IS conditions.

Further reuse of phosphorus-laden biochar for lead sorption from aqueous solution: Isotherm, kinetics, and mechanism.

Biochar and engineered biochar have been used for phosphorous recovery from wastewater, but the resulted phosphorous-laden (P-laden) biochar needs further disposal. In this study, the feasibility of reusing P-laden biochar for Pb immobilization as well as the underlying mechanism was explored. Three types of engineered biochar, i.e., Ca modified biochar, Mg modified biochar, and Fe modified biochar, were selected to sorb P and then the exhausted biochar was further used for Pb sorption. Results showed that Mg and Ca modified biochar exhibited considerable Pb sorption capacity after P sorption with the maximum value of 3.36-4.03 mmol/g and 5.49-6.58 mmol/g, respectively, while P-laden Fe modified biochar failed to sorb Pb due to its acidic pH. The removal of Pb by P-laden Mg modified biochar involved more precipitation including PbHPO4, Pb5(PO4)3(OH), and Pb3(CO3)2(OH)2 because of its higher P sorption capacity and more -OH group on the surface. Cation exchange with CaCO3 to form PbCO3 was the main mechanism for Pb removal by P-laden Ca modified biochar despite the formation of Pb5(PO4)3(OH) precipitate. Our results demonstrate that waste P-laden biochar can be further used for the effective removal of Pb, which provides a potential approach for waste adsorbent disposal.

Chemical and photo-initiated aging enhances transport risk of microplastics in saturated soils: Key factors, mechanisms, and modeling.

Microplastics (MPs) inevitably undergo aging transformation and transport process in environmental compartments. In this study, the polystyrene MPs were aged via three different oxidation methods including persulfate oxidation (PS), UV irradiation (UV), and UV irradiated persulfate oxidation (UVPS). All three treatments induced the great transformation of MPs, with the significant increase in surface roughness and in oxygen-containing functional groups, i.e., COOH or COOC. The UVPS aging showed synergetic effect due to the strengthened photo-initiated chemical oxidation, compared to UV and PS alone. All aged MPs exhibited the enhanced transport (34.9%-89.2%) in sandy and clay loam soils than pristine MPs (30.5%), and the synergetic effect was also observed in the transport behaviors of the UVPS MPs. Higher transport of MPs and aged MPs occurred in sandy soil than that in clay loam soil since the latter one contained high Fe minerals that tend to retain MPs, which was confirmed by the model quartz sand column experiment. Modeling on the migration of MPs retained in soil under a rainstorm scenario showed that the aged MPs had the stronger remobility and greater proportion of cumulative flux than pristine ones in the soil profile. These findings provided new insights on the fate and transport of MPs in natural soil and their potential risk to groundwater contamination.

Microplastics in the soil-groundwater environment: Aging, migration, and co-transport of contaminants - A critical review.

Microplastic contamination in soil has received increasing attention since excessive plastic debris has been emitted directly into the terrestrial environment. Once released into the terrestrial environment, microplastics can be aged via photo- and thermally-initiated oxidative degradation, hetero-aggregation, and bioturbation. Aging affects the physiochemical properties of microplastics with the increase of surface roughness and oxygen-containing groups, which could enhance the sorption and mobility of microplastics in the soil and groundwater environment. However, the interactions among aging, sorption, and transport of microplastics in the terrestrial system have not been unveiled. This review clarifies the key processes of microplastics transport pathways in soil and groundwater ecosystems influenced by aging and sorption under various scenarios. Co-transport of microplastics and sorbed contaminants are also addressed to help understand the risks associated with heavy metals, organic contaminants, and engineered nanoparticles in the soil environment. Overall, this review elaborates the most pressing research limitations on the present literature and highlights the future perspectives to investigate the possible broad transport pathways of microplastics in soil.

Soil colloids affect the aggregation and stability of biochar colloids.

The stability of biochar colloids plays an important role in the transport and fate of contaminants and nutrients in soil. This study aimed to investigate the effects of main soil components, kaolin (Kao), goethite (Goe), and humic acid (HA) colloids on the aggregation kinetics of biochar colloids derived from dairy manure (DM), sewage sludge (SS), and wheat straw (WS). The WS biochar colloid had the highest critical coagulation concentration (CCC) (624 mM) than that of SS (200 mM) and DM (75 mM) due to its richest hydroxyl and carboxyl groups, showing the highest stability. Kao markedly improved the stability of DM and SS biochar colloids with 171% and 52.5% increase of CCC, respectively, by increasing the electrostatic repulsion of the system. However, the WS biochar colloid became more aggregated in the presence of Kao since the hydroxyl and carboxyl functional groups in WS biochar colloid could complex with Kao, generating electrostatic shielding. Goe could rapidly combine with biochar colloids via electrostatic attraction, resulting in the aggregation of SS and WS, while the aggregation rate of DM/Goe mixed colloids was inhibited. The HA increased the electrostatic repulsion of all biochar colloids through adsorbed on the surface of biochar colloids, resulting in the increased steric hindrance and stability of biochar colloids, with the CCC increased from 75 to 624 mM to 827-1012 mM. Our findings reveal that soil kaolin, goethite, and humic acid colloids have remarkable effects on the stability and aggregation of biochar colloid, which will advance understanding of the potential environmental fate and behaviors of biochar colloids.

Biochar-activated persulfate for organic contaminants removal: Efficiency, mechanisms and influencing factors.

Turning biomass into biochar as a multifunctional carbon-based material for water remediation has attracted much research attention. Sawdust and rice husk were selected as feedstock for biochar (BC) production, aiming to explore their performance as a catalyst to activate persulfate (PS) for degrading acid orange 7 (AO7). There was an excellent synergistic effect in the combined BC/PS system. Sawdust biochar (MX) showed a faster and more efficient performance for the AO7 degradation due to its abundant oxygen functional groups, compared to rice husk biochar (DK). In the BC/PS system, AO7 was well decolorized and mineralized. Based on the two-dimensional correlation analysis method, the azo conjugation structure and naphthalene ring of AO7 molecule changed first then benzene ring changed during the reaction. Moreover, AO7 decolorization efficiency increased with the increase of PS concentration and biochar dosage, and the deacrease of pH. Biochar deactivated after used twice. When the biochar reached its adsorption equilibrium of AO7, the AO7 could not be degraded in the BC/PS system. SO4- and OH participated in the reaction together and OH played the main role in activating PS to AO7 decolorization based on the radical scavengers experiment. All of results indicate using biochar to activate PS for degradation of AO7 contaminated water is a promising method.

Effect of pyrolysis temperature on the composition of DOM in manure-derived biochar.

Dissolved organic matter (DOM) plays an important role in the migration and transformation of nutrients and pollutants. Recently, DOM derived from biochar has the potential to determine the application of biochar and has attracted much researcher's attention. However, the effects of pyrolysis temperature on the composition evolution of DOM in manure-derived biochar are still unclear. In this study, DOM solutions extracted from a series of biochars derived from three kinds of manure (chicken, swine and dairy) at six pyrolysis temperature (200-700 °C) were analyzed using UV-Visible, Fourier transform infrared and fluorescence spectroscopy, aiming to investigate the effects of pyrolysis temperature on the composition evolution of DOM. The results showed that, with the increased of pyrolysis temperature, the dissolved organic matter (DOC) content sharply declined to reach stable. High DOC content was obtained at low pyrolysis temperature. Moreover, the DOM mainly contained humic acid-like and protein-like substances. With the pyrolysis temperature increased, the protein-like substances firstly decreased and then increased, while there was an opposite trend for the humic acid-like substances. Moreover, functional groups evolution of DOM depended on the pyrolysis temperature and manure type, evidenced by the Fourier transform infrared spectroscopy with two-dimensional correlation analysis. This study highlights the importance of optical analysis and may provide valuable information regarding the characteristics evolution of biochar-derived DOM.

Effect of calcium dihydrogen phosphate addition on carbon retention and stability of biochars derived from cellulose, hemicellulose, and lignin.

Pyrolysis of biomass with phosphate compound is a promising method to improve biochar characteristics. However, how phosphate compound affects the three components of biomass during the biochar formation is still unclear. In this study, a typical phosphate compound, calcium dihydrogen phosphate (Ca(H2PO4)2), was premixed with cellulose, hemicellulose, and lignin reagent, at the ratio of 20% (w/w) for biochar production through pyrolysis, aiming to investigate the effects of Ca(H2PO4)2 addition on biochar formation. Results show that, with Ca(H2PO4)2 additions, carbon retention of biochars from cellulose (MCBC) and hemicellulose (MHBC) increased by 63.4% and 48.3%, respectively, but that of lignin (MLBC) decreased by 6.7% due to the reactions between lignin and Ca(H2PO4)2. Moreover, the stable carbon proportion in the biochar decreased by 10.2% for MCBC, almost unchanged for MHBC, and increased by 6.15% for MLBC based on the potassium dichromate oxidation. During the pyrolysis process, Ca(H2PO4)2 addition fixed more volatile and/or labile carbon in biochar, resulting in greater carbon retention. Declined carbon stability of biochar might be caused by the inhibited formation of aromatic-C, evidenced by the Fourier transform infrared spectroscopy analysis. This study highlights the importance and potential mechanisms of calcium dihydrogen phosphate influencing the carbon retention and stability of biochar derived from three biomass components.