Freeze-thaw Aging Research Articles

FeAl-LDH-modified biochar (FeAl-LDH@BC): A high-efficiency passivator for hexavalent chromium (Cr(VI)) reduction and immobilization in contaminated soil

A composite FeAl-LDH@BC obtained by successfully loading FeAl-LDH onto biochar (BC) is used to the Cr(VI)-contaminated soil remediation. The efforts of loading, initial pH, concentration and dosage are investigated. The results indicate that the immobilization process of Cr(VI) in soil could be well described by the pseudo-second-order kinetic model and maximum adsorption capacity of FeAl-LDH@BC is 42.78 mg/g at a temperature of 298 K. Additionally, The physicochemical properties of FeAl-LDH@BC are characterized by SEM, EDS, XRD, FTIR and XPS. The characterization results suggest that the immobilization mechanism involved adsorption and reduction. Cr(VI) is immobilized by adsorption to the layers of FeAl-LDH, the reduction of Cr(VI) to Cr(III) is then accomplished using Fe(II) as a reducing agent. In order to demonstrated the stability of composites for environmental restoration, the TCLP tests, dry-wet, freeze-thaw aging cycles, soil phytotoxicity tests, and microbial community are analyzed. The study shows that we provided a material for soil immobilization with high performance for the remediation of Cr(VI)-contaminated soil.

Freeze-thaw aging increases the toxicity of microplastics to earthworms and enriches pollutant-degrading microbial genera

Freeze-thaw (FT) aging can change the physicochemical characteristics of microplastics (MPs). The toxic impacts of FT-aged-MPs to soil invertebrates are poorly understood. Here the toxic mechanisms of FT-aged-MPs were investigated in earthworms after 28 d exposure. Results showed that FT 50 µm PE-MPs significantly increased reactive oxygen species (ROS) by 5.78–9.04 % compared to pristine 50 µm PE-MPs (41.80–45.05 ng/mgprot), whereas FT 500 µm PE-MPs reduced ROS by 7.52–7.87 % compared to pristine 500 µm PE-MPs (51.44–54.46 ng/mgprot). FT-PP-MPs significantly increased ROS and malondialdehyde (MDA) content in earthworms by 14.82–44.06 % and 46.75–110.21 %, respectively, compared to pristine PP-MPs (40.56–44.66 ng/mgprot, 0.41–2.53 nmol/mgprot). FT-aged PE- and PP-MPs caused more severe tissue damage to earthworms. FT-aged PE-MPs increased the alpha diversity of the gut flora of earthworms compared to pristine MPs. Earthworm guts exposed to FT-aged-MPs were enriched with differential microbial genera of contaminant degradation capacity. FT-PE-MPs affected membrane translocation by up-regulating lipids and lipid-like molecules, whereas FT-PP-MPs changed xenobiotic biodegradation and metabolism by down-regulating organoheterocyclic compounds compared to the pristine PE- and PP-MPs. This study concludes that FT-aged MPs cause greater toxicity to earthworms compared to pristine MPs.

Glycidyl polyhedral oligomeric silsesquioxane-enhanced flexible aminosiloxanes to protect sandstone monuments

To protect weathered ancient stone monuments from surface cracking and surface damage by powders, strong materials containing flexible chains are required. Herein, a facile strategy was proposed to produce hybrid octa-glycidyl polyhedral oligomeric silsesquioxane (GPOSS)-enhanced flexible siloxanes by the ring-opening reaction of GPOSS using three aminosiloxanes (bis(3-aminopropyl) terminated poly (dimethyl siloxane) (NH2-PDMS), 3-aminopropyltriethoxysilane (APTS), and N-(6-aminohexyl) aminopropyl trimethoxysilane (AHAPTMS). This produced three different protective materials (POSS-PDMS, POSS-APTS, and POSS-AHAPTMS) that greatly improved the weather resistance of sandstone monuments, especially their resistance to water, salt, temperature, and humidity. All three protective materials presented an adhesion of up to 2.4 MPa. The resistance of freeze-thaw aging cycles was much higher for the protected sandstone (104–133 cycles) compared with the unprotected sandstone (20 cycles). The three protective materials improved the salt weather resistance of protected sandstone (9–60 cycles) compared with unprotected sandstone (3 cycles), with the long-chain aminosiloxane AHAPTMS providing the best protection. The hydrophobicity of three protective materials on the sandstone surface was improved. Furthermore, the water absorption, water vapor permeability, pore size distribution, mechanical strength, light transmittance, and glass transition temperature (Tg), were all improved. The three GPOSS-enhanced flexible siloxanes are novel and eco-friendly materials for protecting sandstone monuments.

Freeze-thaw aged polyethylene and polypropylene microplastics alter enzyme activity and microbial community composition in soil

In cold regions, microplastics (MPs) in the soil undergo freeze-thaw (FT) aging process. Little is known about how FT aged MPs influence soil physico-chemical properties and microbial communities. Here, two environmentally relevant concentrations (50 and 500 mg/kg) of 50 and 500 µm polyethylene (PE) and polypropylene (PP) MPs treated soils were subjected to 45-day FT cycles (FTCs). Results showed that MPs experienced surface morphology, hydrophobicity and crystallinity alterations after FTCs. After 45-day FTCs, the soil urease (SUE) activity in control (MPs-free group that underwent FTCs) was 33.49 U/g. SUE activity in 50 µm PE group was reduced by 19.66 %, while increased by 21.16 % and 37.73 % in 500 µm PE and PP groups compared to control. The highest Shannon index was found in 50 µm PP-MPs group at 50 mg/kg, 2.26 % higher than control (7.09). Compared to control (average weighted degree=8.024), all aged MPs increased the complexity of network (0.19–1.43 %). Bacterial biomarkers of aged PP-MPs were associated with pollutant degradation. Aged PP-MPs affected genetic information, cellular processes, and disrupted the biosynthesis of metabolites. This study provides new insights into the potential hazards of MPs after FTCs on soil ecosystem in cold regions.

Surface alteration on biochar in long-term application: Insights into pyrolysis, freeze-thaw aging, and dissipation

Biochar, a carbonaceous material produced by pyrolysis, is subject to physical and chemical changes over time in its long-term application. Pollutants (e.g., TBBPA) through the environmental fate and transport can be immobilized on the biochar surface, causing an unexpected environmental impact. The objective of this study is to investigate the alteration of surface in freeze-thaw cycles to explore the long-term behaviors of biochar interactions with pollutants. The aging process of biochar is elucidated through characterization using X-ray photoelectron spectroscopy (XPS) and surface charge density analysis. These findings indicate that the aging process promotes the redistribution and re-equilibration of surface functional groups, resulting in the emergence of carboxyl groups and the disruption of aromatic structures. Using 2D-COS approaches, it was observed that the freeze-thaw aging of biochar resulted in a sequence of aromatic alkane (CH) > hydroxyl groups (-OH) > polysaccharide groups (CH) > ether and ester groups (CO-C) > carboxyl and lipid groups (C = O). The study also revealed a dissipative process with a specific sequence, depending on the level of pyrolysis of the biochar. This research contributes to an enhanced comprehension of the long-term use of biochar, particularly in the context of freeze-thaw aging processes and contaminant dissipation. This understanding can inform applications, regulations, and policies related to biochar in the context of long-term utilization.

Competitive adsorption behaviors and mechanisms of Cd, Ni, and Cu by biochar when coexisting with microplastics under single, binary, and ternary systems

In this study, the effects of coexistence with microplastics and co-ageing with the soil on adsorption behaviors and mechanisms of biochar for heavy metals were investigated. Adsorption experiments of Cd, Ni, and Cu by microplastics, biochar, and their combination were conducted in single, binary, and ternary systems. The results indicated that the heavy metal adsorption by microplastics was ranked as Ni > Cd > Cu, which increased with decreasing particle size, and the adsorption capacity of microplastics was enhanced after dry-wet and freeze-thaw ageing. Biochar preferentially adsorbed Cd in the single system, while the maximum adsorption of Cu was observed in the binary and ternary systems due to the minimizing impact of competition on the Cu adsorption by biochar. The heavy metal adsorption by the combination of microplastics and biochar was less than that by single biochar, and the smaller the particle size of microplastics, the greater the negative effects on heavy metal adsorption. Coexistence with microplastics reduced Cd adsorption of biochar by 0.72 %–50.35 %, Ni adsorption by 1.17 %–30.43 %, and Cu adsorption by 5.78 %–47.88 %, respectively. Moreover, coexistence with microplastics exacerbated the adverse impacts of competition on biochar adsorption for heavy metals. The contribution percentages of biochar mineral mechanisms for heavy metal adsorption were ranked as Cu > Cd > Ni. When coexisting with microplastics or after ageing, the mineral mechanisms of heavy metal adsorption by biochar significantly decreased. This study investigated the competitive adsorption behaviors and mechanisms of heavy metals by biochar when coexisting with microplastics, which highlighted that the application of biochar for the remediation of heavy metal pollution should be concerned with the impacts of microplastics.

Aging rice straw reduces the bioavailability of mercury and methylmercury in paddy soil

Straw amendment is a prevalent agricultural practice worldwide, which can reduce air pollution and improve soil fertility. However, the impact of aging straw amendment on the bioavailability of mercury (Hg) and methylmercury (MeHg) in paddy soil remains unclear. To investigate this, incubation experiments were conducted using the diffusive gradient in thin-film technique. Results showed that amendments of dry-wet aging (DRS), photochemical aging (LRS), and freeze-thaw aging rice straw (FRS) reduced the bioavailable MeHg in paddy soil by 2.2–27.6%, 13.5–69.8%, and 23.5–86.1%, respectively, compared to fresh rice straw (RS) amendment. This result could be due to changes in soil properties such as soil pH and overlying water Fe and Mn as well as microbial abundance (including Clostridiaceae, Firmicutes, and Actinobacteriota). Simultaneously, The LRS and FRS amendments reduced bioavailable Hg in paddy soil by 20.0–40.8% and 17.1–48.6%, respectively, while DRS increased the bioavailable Hg by 15.8–120.0%. This could be attributed to changes in soil oxidation-reduction potential and overlying water SO42− content. Additionally, the results of sand culture experiments showed that the concentrations of Hg uptake by rice seedlings were 97.1–118.2%, 28.1–35.6%, and 198.0–217.1% higher in dissolved organic matter (DOM) derived from DRS, LRS, and FRS than RS, indicating that aging straw leached DOM may promote the Hg bioavailable when straw amendment. This result could be due to lower molecular weight and higher CO functional group content. These results provide new insight into how aging straw amendment affects the bioavailability of Hg and MeHg in paddy soil under different climates.

New Insights into the Enhancement Effect of Exogenous Calcium on Biochar Stability during Its Aging in Farmland Soil

Converting biowaste into biochar and incorporating it into soil is recognized as an effective method for carbon (C) sequestration. However, biochar inevitably undergoes an aging process in soil, which influences its stability, and ultimately threatens its carbon sequestration ability. This study selected CaCl2 as an exogenous additive of sewage sludge and bone dreg for producing Ca-rich biochars, and both surface C and bulk C stability in them were investigated under three aging processes (dry–wet aging, freeze–thaw aging, and natural aging in farmland soil). The results showed that the above three aging processes resulted in oxidized surface C in Ca-rich biochar decreasing by 10~23%, 28~41%, and 0~74%, respectively, compared with that in pristine biochar, while oxidized bulk C decreased by 6~10%, 0~1%, and 0~35%, respectively. This was attributed to the “protective effect” of Ca-containing crystals on the biochar surface, including CaO, Ca5(PO4)3Cl, Ca5(PO4)3(OH), Ca8H2(PO4)6·H2O, and Ca10(PO4)6(OH)2, which intervened in the oxidation of C–C/C=C in biochar but also reduced the generation of a fragmented C structure. By comprehensively assessing surface C and bulk C stability under three aging processes, final C sequestration in Ca-rich biochar increased to 27~80%, compared to that in pristine biochar (23~74%). Therefore, Ca-rich biochar is more dominant than pristine biochar, considering C sequestration potential during long-term aging in soil.

Effects of physical aging processes on the bioavailability of heavy metals in contaminated site soil amended with chicken manure and wheat straw biochars

The physicochemical properties of biochars undergo slow changes in soils due to the natural aging processes, which influences their interaction with heavy metals. The effects of aging on immobilization of co-existing heavy metals in contaminated soils amended with fecal and plant biochars possessing contrasting properties remain unclear. This study investigated the effects of wet–dry and freeze–thaw aging on the bioavailability (extractable by 0.01 M CaCl2) and chemical fractionation of Cd and Pb in a contaminated site soil amended with 2.5% (w/w) chicken manure (CM) biochar and wheat straw (WS) biochar. Compared to that in the unamended soil, the contents of bioavailable Cd and Pb in CM biochar-amended soil decreased by 18.0% and 30.8%, respectively, after 60 wet–dry cycles, and by 16.9% and 52.5%, respectively, after 60 freeze–thaw cycles. CM biochar, which contained significant levels of phosphates and carbonates, effectively reduced the bioavailability of Cd and Pb and transformed them from the labile chemical fractions to the more stable ones in the soil during the accelerated aging processes, mainly through precipitation and complexation. In contrast, WS biochar failed to immobilize Cd in the co-contaminated soil in both aging regimes, and was only effective at immobilizing Pb under freeze–thaw aging. The changes in the immobilization of co-existing Cd and Pb in the contaminated soil resulted from aging-induced increase in oxygenated functional groups on biochar surface, destruction of the biochar's porous structure, and release of dissolved organic carbon from the aged biochar and soil. These findings could help guide the selection of suitable biochars for simultaneous immobilization of multiple heavy metals in co-contaminated soil under changing environmental conditions (e.g., rainfall, and freezing and thawing of soils).

Remediation of di(2-ethylhexyl) phthalate (DEHP) contaminated black soil by freeze-thaw aging biochar

Di(2-ethylhexyl) phthalate (DEHP), a complex structure with high toxicity, is a common organic pollutant. This study investigated the effects of fresh biochar (FBC), and freeze-thaw cycled aged biochar (FTC-BC) on DEHP-contaminated soils using a pot experiment. The specific surface area of FBC increased from 145.20 to 303.50 m2/g, and oxygen-containing functional groups increased from 1.26 to 1.48 mol/g after freeze-thaw cycles, greatly enhancing the adsorption of DEHP by biochar in the soil. The comprehensive radar chart evaluation showed that FBC and FTC-BC reduced DEHP growth stress and improved the soil properties. Compared with FBC, FTC-BC performed better in protecting the normal growth of pakchoi and improving soil properties. In addition, the application of biochar increased the diversity and abundance of bacteria in the DEHP-contaminated soil and changed the composition of the soil bacterial community. The partial least squares path model (PLS-PM) showed that adding biochar as a soil remediation agent significantly positively impacted soil nutrients and indirectly reduced the DEHP levels in soil and plants by increasing soil microbial diversity. Compared with FBC, FTC-BC creates a more satisfactory living environment for microorganisms and has a better effect on the degradation of DEHP in the soil. This study provides a theoretical basis for future biochar remediation of DEHP-contaminated soils in cold high-latitude regions.

Artificial aging induced changes in biochar,s properties and Cd2+ adsorption behaviors.

Fresh biochar has been widely applied to the remediation of heavy metals in soil by its property of adsorption, but the changes in its physicochemical properties and in situ adsorption performance over time cannot be ignored. In this study, the sorption of Cd2+ by corn straw biochars (CB) and municipal sludge biochars (SB) produced at 350 °C and 650 °C before and after H2O2 oxidation, and dry-wet and freeze-thaw aging were investigated using batch sorption experiments. The changes of physicochemical properties of biochar before and after aging were analyzed by various characterization methods. Based on these results, the impact of aging on the Cd2+ adsorption behavior could be clarified, which showed that CB650 was able to display the highest adsorption capacity in fresh biochars. Aging treatments reduced the ash content and pH value of CB, and significantly diminished the adsorption performance of Cd2+. These changes indicated that precipitation was a critical factor in the adsorption of Cd2+ on CB. The adsorption capacity of SB was enhanced after H2O2 oxidation, but weakened after dry-wet and freeze-thaw aging. This was closely related to the increase or decrease in the content of oxygen-containing functional groups, which in turn enhanced or inhibited its ability to compound with heavy metals. These results are of great significance for evaluating its long-term application prospects in the natural environment.

Immobilization of Cr(Ⅵ) in polluted soil using activated carbon fiber supported FeAl-LDH

Layered double hydroxides (LDHs) have great potential as new amendments for the remediation of Cr(Ⅵ) polluted soil. However, the large-scale application of LDHs is limited by the difficulties of separating from the environment and the metal leaching during application. Herein, a carbon-based composite, FeAl-LDH@ACF, was synthesized by assembled FeAl-LDH onto activated carbon fiber (ACF). The resulting FeAl-LDH@ACF had an inherent affinity of FeAl-LDH toward Cr(VI) and the outstanding performance of the ACF, which could easily separate from the soil. Immobilization experiments showed that FeAl-LDH@ACF had good performance for the immobilization of Cr(VI) in polluted soil. Products before and after immobilization were characterized by SEM, EDS, XRD, FTIR and XPS. A Cr(VI) removal mechanism by FeAl-LDH@ACF involving immobilization and reduction processes had been proposed. Cr(VI) was immobilized to the FeAl-LDH@ACF by anion exchange then the Cr(VI) was reduced to the less toxic Cr(Ⅲ) by Fe2+ on the main laminate of the material. The influencing factors affecting the immobilization performance of FeAl-LDH@ACF for Cr(VI) were explored, for example, the contact time, the dosage, the initial concentration, and the initial pH. In addition, the TCLP extraction tests, dry-wet cycle, freeze-thaw aging cycle, and phytotoxicity tests were considered to investigate the stability and suitability of materials used in the environment remediation.

Stability improvement technology of SBS/crumb rubber composite modified asphalt from Xinjiang China

Previous studies have shown that single modified asphalt is difficult to meet the requirements of road quality at complex service environments. The service environment of roads in Xinjiang, China is harsh, and the stability of modified asphalt in Xinjiang is poor, which cannot meet the requirements of road quality. For the purpose of solving the technical problems such as poor stability after modification of local asphalt in Xinjiang China, improving the comprehensive performance of modified asphalt and recycling waste tire crumb rubber, SBS/crumb rubber composite modified blended Xinjiang asphalt was prepared. Based on the basic performance of asphalt, microstructure test, aging test and rheological test, the stability improvement methods of composite modified asphalt were studied systematically, and the mechanism of asphalt stability improvement was confirmed. The SBS/CR-S asphalt and SBS/CR-N asphalt composite modified asphalts which were suitable for hot and cold regions in Xinjiang were determined. The environmental durability of SBS/CR-S asphalt, SBS/CR-N asphalt, SBS asphalt and rubber asphalt used commonly in Xinjiang under different conditions of light-thermal and freeze-thaw were compared and evaluated. The results showed that the stability improvement methods such as asphalt blending and additives compounding supplemented the lightweight components of asphalt, which promoted the reaction and cross-linking between modifier and asphalt, and improved the stability and comprehensive performance of asphalt. After stability improvement, the 5 °C ductility of SBS/CR-N asphalt and SBS/CR-S asphalt was 32.6 cm and 31.5 cm respectively, and the segregation softening point difference was 1.5 °C and 1.8 °C. After light-thermal and freeze-thaw aging, SBS/CR-S asphalt and SBS/CR-N asphalt still exhibited outstanding high and low temperature performance. SBS/CR-S asphalt and SBS/CR-N asphalt had excellent storage stability, high and low temperature performance and environmental durability.

Mobility of exogenous lead in acidic soil treated with wheat straw biochar after aging process of freeze-thaw cycles

ABSTRACT Although biochar has been investigated extensively to stabilize soil heavy metals, there were limited investigation on the stabilization of soil heavy metals by using biochar under the natural ageing processes of alternate-cooling/heating. In this study, wheat-straw-biochar-treated soil samples contaminated with Pb(II) ions/PbO were subjected to 12 h-freeze/12 h-thaw cycles simulating the natural alternate-cooling/heating process. After the freeze-thaw process, the mobile fractions of leadreduced by 30.0% (500 mg·kg−1 Pb(II) spiked soil) and 19.5% (PbO spiked soil) due to biochar’s addition evaluating via 0.01 M HCl batch extraction. Compared with the control, the mobile Pb in treated groups decreased in column leaching . Exchangeable lead decreased by 6.4% – 80.9% due to biochar addition. Freeze-thaw ageing had no significant effects on lower lead leaching and lower exchangeable lead resulted from biochar’s addition. Therefore, wheat-straw-biochar treatment is helpful for stabilization of both dissolved Pb and particulate Pb in soil under the alternate-cooling/heating ageing process.

Influence of Freeze-Thaw Aging on the Impact Performance of Damped Carbon Fiber Reinforced Plastics for Automotive Applications

The increasing use of composite materials in the automotive field requires more attention with regards to the appearance of noise, vibration and harshness (NVH) study in cars construction. However, in car door panels production, impact characteristics need to be evaluated in sandwich laminates. Furthermore, it is important to consider the effect of prolonged environmental aging on crashworthiness properties. The innovative content of the work is the hygrothermal effects evaluation on impact performance for two damped CFRP sandwich laminates. In this paper, two damping materials, Kraibon HHZ9578/99 and SUT9609/24, were used as core between two skins of CFRP for sandwich composite production. Freeze-Thaw aging treatment according to IEC 60068, specific for Automotive, was performed to investigate environmental effects on components. Up to 750 h, it was demonstrated that water absorption is regulated by Fick’s Law. The low-velocity impact behavior of the damped sandwiches has been studied according to ASTM D7136 throughout drop dart test equipment. Both main peak forces and energy absorption characteristics are negatively affected by aging condition. The introduction of damping core inside the composite structure of vehicle components can satisfy NVH constrictions. By contrast, at least same operating conditions must be assured in relation to not-damped components.

An Eco-Friendly Polymer Composite Fertilizer for Soil Fixation, Slope Stability, and Erosion Control.

In the Loess Plateau region, the poor structure and properties of loess slopes will cause many types of geological disasters such as landslides, mudflow, land collapse, soil erosion, and ground cracking. In this paper, an eco-friendly polymer composite fertilizer (PCF) based on corn straw wastes (CS) and geopolymer synthesized from loess was studied. The characterization by FT-IR of the PCF confirmed that graft copolymer is formed, while morphological analysis by scanning electron microscopy and energy dispersive spectroscopy showed that geopolymer and urea were embedded in the polymer porous network. The effects of PCF contents on the compressive strength of loess were investigated. The PCF was characterized in terms of surface curing test, temperature and freeze-thaw aging property, water and wind erosion resistance, and remediation soil acidity and alkalinity property, which indicates that PCF can improve loess slope fixation and stability by physical and chemical effects. Moreover, the loess slope planting experiment showed that PCF can significantly increase the germination rate of vegetation from 31% to 68% and promote the survival rate of slope vegetation from 45.2% to 67.7% to enhance biological protection for loess slopes. The PCF meets the demands of building and roadbed slope protection and water-soil conservation in arid and semi-arid regions, which opens a new application field for multifunctional polymer composite fertilizers with low cost and environmental remediation.

Natural field freeze-thaw process leads to different performances of soil amendments towards Cd immobilization and enrichment

Cadmium (Cd) soil pollution is a global issue affecting crop production and food safety. Remediation methods involving in-situ Cd immobilization have been developed, but their effectiveness can diminish under seasonal freeze-thaw aging processes. In this study, we assessed the field performance of four soil treatments at a seasonally frozen rice paddy. Amendments were applied at 2 wt%, including: (i) sepiolite (a 2:1 clay mineral), (ii) superphosphate, (iii) biochar (produced by rice husk at 500 °C for 2 h), and (iv) joint application of biochar & superphosphate (1:1 mixture by weight). Immobilization performance was determined as DTPA extractable Cd and plant uptake in various organs. Overall, the four treatments significantly reduced Cd bioavailability during the plant growth period, with average DTPA-extractable concentrations decreasing by 43%, 34%, 39% and 45% for the four treatments, respectively, relative to untreated soil (control). Rice grain yields from the superphosphate and the joint application treatments increased by 8.0% and 11.8%, respectively, and Cd accumulation within those grains reduced by 14.3% and 48.9%, respectively. During the winter non-growth period, freeze-thaw aging facilitated Cd mobilization, with DTPA-extractable Cd increasing by 16.9% in the control soil, relative to the initial period. However, this reduced to 10.9%, 14.4%, 7.6% and 5.0%, for the sepiolite, superphosphate, biochar and joint application treatments, respectively. Overall, the joint application of biochar and superphosphate provided the best performance in terms of both long-term Cd immobilization and rice production enhancement, offering a green remediation option for risk management at Cd contaminated rice paddies in seasonally frozen regions.

Enhanced immobilization of cadmium and lead adsorbed on crop straw biochars by simulated aging processes

Aging is an important natural process affecting the physiochemical properties of biochar, while mechanistic understanding of its effect on the adsorbed heavy metals is still lacking. After adsorption of Cd2+ and Pb2+, biochars produced from wheat straw (WS) and maize straw (MS) at 300 and 500 °C (denoted as WS300, WS500, MS300, and MS500, respectively) were subjected to 60 cycles of wet–dry or freeze–thaw aging. The results showed that simulated aging treatment transformed the Cd2+ and Pb2+ adsorbed on the low-temperature biochars from the readily and potentially bioavailable fractions into the non-bioavailable one, while the fractionation of Cd2+ and Pb2+ adsorbed on WS500 and Pb2+ on MS500 barely changed. Spectroscopic characterization revealed that simulated aging enhanced the complexation of Cd2+ and precipitation of Pb2+ on the biochars. These findings suggest that heavy metals could be effectively immobilized on low-temperature biochars amended to contaminated soils in the long term.

Contrasting effects of dry-wet and freeze-thaw aging on the immobilization of As in As-contaminated soils amended by zero-valent iron-embedded biochar

Zero-valent iron-embedded biochar (ZVI/BC) is considered as an effective material for arsenic (As) immobilization in soil, but the stability of As after remediation against aging remains unknown. Herein, the effects of dry-wet and freeze-thaw aging on the immobilization of As in two As-contaminated soils amended by ZVI/BC were evaluated. ZVI/BC showed high immobilization capacity for As-contaminated soils with an over 82% decrease of bioavailable As, mainly due to the As-Fe co-precipitation accompanied with ZVI oxidation. The aging of dry-wet and freeze-thaw had an opposite effect on the bioavailability of As. After 35 rounds of dry-wet aging, bioavailable As concentration increased from 1.25‐9.50 to 1.83‐21.75 mg/kg, because of the oxidation dissolution of ZVI and the formation of mobile reduced As(III). By contrast, the crystallization of amorphous iron with the structural incorporation of sorbed As and the oxidation of As(III) into stable As(V) occurred during the 35 rounds of freeze-thaw aging, leading to the decrease of bioavailable As concentration from 9.50‐1.25 to 5.42‐0.45 mg/kg. Our results revealed that the stability of soil As after remediation by ZVI/BC varied with the different aging process, which needs more consideration for the long-term soil As immobilization in the different whether areas.

Aging reduces the bioavailability of copper and cadmium in soil immobilized by biochars with various concentrations of endogenous metals

Biochar is widely used for environmental remediation. However, the effects of aging on the bioavailability of trace metals in biochar-amended soil remain largely unknown, especially for the biochars with various concentrations of endogenous metals. In this study, three biochars marked as BB, MB, and HB were produced from the straws of Pennisetum sinese grown in background soil, moderately-, and highly-polluted soils by trace metals, respectively. We distinguished the effects of dry-wet (DW) and freeze-thaw (FT) aging on the bioavailability of copper (Cu) and cadmium (Cd) from soil particles, the biochar interior, and the surface of biochar. The adsorption capacities of Cu2+ and Cd2+ followed the order of BB > MB > HB. DW and FT aging both increased the adsorption capacity of Cu2+, but decreased that of Cd2+ in the three biochars, resulting in a reduction in Cu bioavailability and increase in Cd bioavailability in the biochars after the saturated adsorption of Cu2+ and Cd2+. The incorporation of the three biochars decreased Cu bioavailability compared to the control after incubation for 30d, while the addition of MB increased Cd bioavailability. DW and FT aging decreased Cu bioavailability in biochar-amended soil by decreasing the bioavailability of Cu adsorbed on the biochar surface and immobilized by soil particles. Meanwhile, aging decreased Cd bioavailability by decreasing the bioavailability of Cd immobilized by soil particles. Overall, environmental risk would be increased by the application of biochars with high endogenous Cd. The major implications are that biochar dosage and environmental risk should be carefully assessed before large-scale, continuous application, especially for biochars containing high contents of endogenous trace metals.