Co-impacts of cation type and humic acid on migration of polystyrene microplastics in saturated porous media.

The aging process of microplastics (MPs) could significantly change their physical and chemical characteristics and impact their migration behavior in soil. However, the complex effects of different cations and humic acids (HA) on the migration of aged MPs through saturated media are not clear. In this research, the migration and retention of pristine/aged PSMPs (polystyrene microplastics) under combined effects of cations (Na+, Ca2+) (ionic strength = 10 mM) and HA (0, 5, 15 mg/L) were investigated and analyzed in conjunction with the two-site kinetic retention model and DLVO theory. The findings showed that the aging process accelerated PSMPs migration under all tested conditions. Aged PSMPs were less susceptible to Ca2+ than pristine PSMPs. Under Ca2+ conditions, pristine/aged PSMPs showed higher retention than under Na+ conditions in the absence of HA. Furthermore, under Na+ conditions, the migration of aged PSMPs significantly increased at higher concentrations of HA. However, under Ca2+ conditions, the migration of aged PSMPs decreased significantly at higher concentrations of HA. In higher HA conditions, HA, Ca2+, and PSMPs interact to cause larger aggregations, resulting in the sedimentation of aged PSMPs. The DLVO calculations and two-site kinetic retention models' results showed the detention of PSMPs was irreversible under higher HA conditions (15 mg/L) with Ca2+, and aged PSMPs were more susceptible to clogging. These findings may help to understand the potential risk of migration behavior of PSMPs in the soil-groundwater environment.

Effects of solution chemistry and humic acid on transport and deposition of aged microplastics in unsaturated porous media.

Microplastics (MPs) are susceptible to aging in the environment, and aged MPs are highly migratory in soil due to their smaller particle size and more negative surface charge, but the effects of soil environmental factors on the fate and transport of aged MPs are still unclear. In this study, the transport behavior of pristine/aged MPs in unsaturated sandy porous media was examined under different ionic strength (IS), cationic type (Na+, Ca2+) and humic acid (HA) conditions. The results indicated that the surface charge, surface oxygen-containing functional groups and surface morphology of MPs changed significantly after aging, and that the mobility of aged MPs was significantly enhanced than the pristine MPs under all test conditions. The retention amounts of pristine/aged MPs in unsaturated porous media increased with IS, and IS had a less inhibitory effect on the transport of aged MPs than pristine MPs. The mobility of pristine/aged MPs in Ca2+ solutions was significantly weaker than that in Na+ solutions due to enhanced straining and electrostatic adsorption. HA promoted the mobility of pristine/aged MPs in unsaturated porous media under all IS Na+ (1, 10, and 25 mM) solutions and lower IS (1 mM) Ca2+ solutions, and the ability of HA to promote the transport of aged MPs was significantly stronger than that of pristine MPs due to the higher adsorption of HA on the surface of aged MPs. However, at higher IS (10 mM) Ca2+ solution conditions, the bridging effect of Ca2+ led to the formation of HA-MPs complexes, which altered the hydrophobicity of the pristine/aged MPs surface and the pristine/aged MPs were mainly retained on the air-water interface (AWI). CFT theory and two-site kinetic retention models indicated that the retention of pristine/aged MPs in unsaturated media was dominated by monolayer adsorption, straining and clogging effects. The current research findings may provide insights into the fate and transport of aged MPs in soil and their potential risk of groundwater contamination.

Development of chitosan-magnetic sawdust hydrochar for Pb and Zn immobilization process on various soil conditions.

A series of 60-day soil immobilized incubations were performed to explore the impacts of various factors (incubation time, chitosan modified magnetic sawdust hydrochar (CMSH) dosages, initial pH values, moisture contents, and humic acid (HA)) on CMSH immobilization of Pb and Zn. DTPA and BCR extraction techniques were undertaken to study the distribution of form transformations of Pb and Zn. CMSH showed significant immobilization ability for both DTPA-Pb and DTPA-Zn, and the highest removal rates were shown to be 57.40% and 90.00% for Pb and Zn respectively. After 60 days of incubation, the residual Pb was enhanced by 34-61% and residual Zn increased by 25-41%, which indicated that CMSH was effective in immobilizing Pb and Zn. Meanwhile, the immobilization efficiency improved with increasing incubation time, CMSH dosage, HA dosage, and initial solution pH. In particular, 5% HA application increased the soil TOC and accelerated the metal stabilization processes, with the residual forms of Pb and Zn eventually reaching a maximum of 73% and 71%, respectively. In addition, the alkaline initial solution promoted the ion exchange, surface complexation reaction, and cationic-π interaction, resulting in a better immobilization of Pb and Zn by CMSH. Finally, according to the orthogonal analysis of BCR results, HA dosage was the major factor affecting Pb and Zn immobilization by CMSH compared to soil pH and moisture content in this study.

Transport characteristics of polystyrene microplastics in saturated porous media with biochar/Fe3O4-biochar under various chemical conditions.

Magnetically modified biochar, with a rougher surface and more positive surface charge, may interact with microplastics (MPs) after being applied to soil, potentially altering the fate and transport of MPs in porous media. In this study, the transport and retention behavior of polystyrene microplastics (PSMPs) in a sandy porous media mixed with biochar/Fe3O4 modified biochar (Fe3O4-biochar) was investigated under various chemical conditions (humic acid (HA), ionic strength (IS) and cationic types (Na+/Ca2+)). The results showed that the addition of biochar and Fe3O4-biochar can hinder the transport of PSMPs in porous media without HA, and that Fe3O4-biochar was more effective in inhibiting the transport of PSMPs through electrostatic adsorption and complexation, with an optimum retention efficiency of 92.36 %. HA significantly attenuated the retention of PSMPs in both porous media through electrostatic repulsion, steric resistance and competitive adsorption under 1 mM Na+ solutions, and the mobility of PSMPs in Fe3O4-biochar/sand was enhanced more significantly than in biochar/sand with the increase of HA concentration. IS significantly inhibited the transport of PSMPs in both porous media in the absence of HA, but there was an antagonistic effect of HA and IS on the transport of PSMPs in the presence of HA, with the facilitative effect of HA being stronger than the inhibitory effect of IS. Ca2+ was consistently more effective in inhibiting the transport of PSMPs than Na+ under all test conditions, and HA promoted the transport of PSMPs in all Na+ solutions, while it inhibited the transport of PSMPs in high IS (10 mM) with Ca2+ solutions. In addition, HA, Fe3O4-biochar and PSMPs tend to form larger aggregates under the complex interactions of Ca2+, leading to increased retention of PSMPs in porous media. The two-site kinetic retention models suggested that the retention of PSMPs in porous media with biochar was predominantly reversible attachment effect, while retention in porous media with Fe3O4-biochar was predominantly an irreversible straining effect.

Organics composition and microbial analysis reveal the different roles of biochar and hydrochar in affecting methane oxidation from paddy soil.

Biochar and hydrochar, as valuable and eco-friendly soil remediation materials from greenwaste, have potential to enhance methane oxidation in paddy soil. But the mechanism of biomass carbon on the improvement of methane-oxidizing bacteria communities in paddy soil has not been adequately elucidated. In the present study, the effect of different-temperature rice straw-based biomass carbon (RB400, RB600, RH250 and RH300) on methane oxidation were investigated by analyzing the soil dissolved organic matter (DOM), physicochemical properties and changes in microbial community structure. The results of the 17-day incubation experiment showed that the methane oxidation rate increased under all types of biomass carbon in the first 6 days. The enhancement of methane oxidation rate was more pronounced for biochar compared to hydrochar, with RB600 being the most effective treatment. The result of excitation-emission matrix (EEM) fluorescence spectroscopy showed that less DOM were released from the soil in the biochar treatments compared to the hydrochar treatments and protein-like were detected only in the hydrochar group. Microbial analysis further showed that hydrochar inhibited the growth of Bacillus, Methylobacter, and Methylocystis, while RB600 significantly increased the relative abundance of methanotrophs (responsible for methane oxidation), such as Methylocystis and Methylobacter, which was consistent with their different effects on the methane oxidation rate. Moreover, from the analysis of principal component analysis (PCA) and canonical correspondence analysis (CCA), Methylobacter and Methylocystis were negatively respond to H/C of biomass carbon. The present study provides a deeper understanding of the effect of biomass carbon obtained by different processes on methane oxidation when applied to soil from the perspective of organic matter and microbial communities.

Immobilization of Cd and Pb in soil facilitated by magnetic biochar: metal speciation and microbial community evolution.

The preparation of magnetic biochar from sewage sludge and rice straw for heavy metal contaminated soil remediation has greater application prospects, but its remediation mechanism was rarely considered by combining soil physicochemical properties with microbial community. In this study, the effects of magnetic sewage sludge biochar (SSB) and rice straw biochar (RSB) on Cd and Pb immobilization in paddy soil were compared and analyzed by 60-day soil incubation experiments. The results illustrated that DTPA-Cd and DTPA-Pb were reduced by 51.53% (43.07%) and 53.57% (50.47%), while the percentage of residual fraction of the BCR procedure was enhanced by 31.27% (30.78%) of Cd and 27.25% (23.22%) of Pb in the SSB (RSB) treatment, respectively. Fe was detected on both SSB and RSB surfaces, but SSB had rougher and a larger specific surface area compared to RSB. The addition of SSB and RSB in paddy soil increased soil pH and TOC content, and affected the diversity and species of soil microbial community. Compared with the CK group, the relative abundance of Proteobacteria, Bacteroidota, and Lysobacter decreased, and the relative abundance of Actinobacteriota, Pontibacter, and Alkaliphilus increased with SSB and RSB treatments, all of which reflected the bioavailability of Cd and Pb reduction.

Transport and retention of microplastics in saturated porous media with peanut shell biochar (PSB) and MgO-PSB amendment: Co-effects of cations and humic acid.

Biochar particles are extensively used in soil remediation and interact with microplastics (MPs), especially metal oxide-modified biochar may have stronger interactions with MPs. The mechanism of interactions between humic acid (HA) and different valence cations is different and the co-effect on the transport of MPs is not clear. In this study, the co-effects of HA and cations (Na+, Ca2+) on the transport and retention of MPs in saturated porous media with peanut shell biochar (PSB) and MgO-modified PSB (MgO-PSB) were systematically investigated. Breakthrough curves (BTCs) of MPs were fitted by the two-site kinetic retention model for analysis. In the absence of HA, the addition of PSB and MgO-PSB significantly hindered the transport of MPs in saturated porous media, and the retention of MPs increased from 34.2% to 59.1% and 75.5%, respectively. In Na+ solutions, the HA concentration played a dominant role in controlling MPs transport, compared to the minor role of Na+. The transport capacity of MPs always increased gradually with the increase of HA concentration. Whereas, in Ca2+ solutions, Ca2+ concentrations had a stronger effect than HA. The transport ability of MPs was instead greater than that in Na+ solutions as the HA concentration increased at low ionic strength (1 mM). However, the transport capacity of MPs was significantly reduced with increasing HA concentrations at higher ionic strength (10, 100 mM). The two-site kinetic retention model indicated that chemical attachment and physical straining are the main mechanisms of MPs retention in the saturated porous media.