Immobilization Rates Research Articles

Study on the effectiveness of sulfate-reducing bacteria to remove Pb(II) and Zn(II) in tailings and acid mine drainage.

In view of water and soil getting polluted by Pb(II), Zn(II), and other heavy metals in tailings and acid mine drainage (AMD), we explored the removal effect of sulfate-reducing bacteria (SRB) on Pb(II), Zn(II), and other pollutants in solution and tailings based on the microbial treatment technology. We used the scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray fluorescence (XRF), to reveal the mechanism of SRB treatment of tailings. The results showed that SRB had a strong removal capacity for Zn(II) at 0-40 mg/L; however, Zn(II) at 60-100 mg/L inhibited the growth of SRB. Similarly, SRB exhibited a very strong ability to remove Pb(II) from the solution. At a Pb(II) concentration of 10-50 mg/L, its removal percentage by SRB was 100%. SRB treatment could effectively immobilize the pollutants leached from the tailings. With an increase in the amount of tailings added to each layer, the ability of SRB to treat the pollutants diminished. When 1 cm of tailingssand was added to each layer, SRB had the best effect on tailing sand treatment. After treatment, the immobilization rates of , Fe(III), Mn(II), Pb(II), Zn(II), Cu(II), and total Cr in the leachate of #1 tailing sand were 95.44%, 100%, 90.88%, 100%, 96.20%, 86.23%, and 93.34%, respectively. After the tailings were treated by SRB, although the tailings solidified into a cohesive mass from loose granular particles, their mechanical strength was <0.2 MPa. Desulfovibrio and Desulfohalotomaculum played the predominant roles in treating tailings by mixing SRB. The S2- and carbonate produced by mixing SRB during the treatment of tailings could metabolize sulfate by combining with the heavy metal ions released by the tailings to form FeS, MnS, ZnS, CuS, PbS, Cr2S3, CaCO3, MnCO3, and other precipitated particles. These particles were attached to the surface of the tailings, reducing the environmental pollution of the tailings in the water and soil around the mining area.

Synthesis and characterization of electrospun nanofibers for the immobilization of a cocktail of native laccases from Pycnoporus sanguineus CS:43 and their evaluation in the biotransformation of 2,4,6-trinitrotoluene

Enzymatic biotransformation stands as an eco-friendly strategy for eliminating toxic contaminants like 2,4,6-trinitrotoluene (TNT). Moreover, the application of immobilized biocatalysis is relevant for industrial applications due to the improved stability and reusability of the immobilized enzymes. In the present study, an enzymatic cocktail containing laccases from a native fungus from the northeastern of Mexico (Pycnoporus Ssanguineus CS43) was immobilized in nanofibers. The study explores three biopolymer mixtures—polyalcohol vinyl (PVA), PVA/alginate (PVA/SA), and PVA/chitosan (PVA/CS)—for nanofiber preparation and employs three distinct immobilization strategies: entrapment, adsorption, and covalent bonding. Among them entrapped laccases in PVA/SA nanofibers showed the highest immobilization rate (96.92%) and retained 77.85% activity after five cycles using 2,2-azino- bis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) as substrate. Preliminary kinetic parameters and optimal pH/temperature were determined, comparing the cocktail of immobilized laccases to free ones. The application of this system resulted in a substantial 93.43% TNT removal at pH 7 and 25 °C within four days, preserving 24.13% activity after five cycles. Significantly, this study marks the pioneering use of immobilized laccase for TNT biotransformation, offering a promising eco-friendly strategy for tackling persistent pollutants such as TNT.

Micro-assessment of heavy metal immobilization within alkali-activated copper tailings-slag geopolymer

To address the challenge of mitigating heavy metal pollution stemming from the utilization of industrial solid wastes like copper tailings, this study focused on the development of copper tailings-granulated blast furnace slag (CT-GBFS) geopolymer through alkali activation. The primary objective was to effectively immobilize heavy metals, specifically copper (Cu) and zinc (Zn), within the geopolymer. This investigation encompassed an exploration of how Cu and Zn impacted the mechanical properties of CT-GBFS geopolymer, along with an analysis of the leaching behavior of these heavy metals in various environmental conditions. The study also delved into the underlying mechanisms responsible for immobilizing these heavy metals. The experimental findings revealed that the inclusion of up to 2% heavy metals could enhance the mechanical properties of geopolymer. However, as the heavy metal content increased, a detrimental effect on the mechanical properties became evident. Furthermore, CT-GBFS geopolymer exhibited excellent heavy metal immobilization capabilities across different environmental conditions. Specifically, the immobilization rate for copper exceeded 98%, while zinc exhibited an immobilization rate exceeding 93%. The geopolymer has a physical encapsulation fixation mechanism for both Cu and Zn. Additionally, copper was observed to form a covalent bond with non-bridging oxygen in the geopolymer, manifesting as a Si–O–Cu structure, thereby effectively immobilizing the metal. Conversely, zinc was immobilized within the geopolymer through ionic bonding. In conclusion, the technology proposed in this study represents a significant step toward achieving the safe and resourceful utilization of copper tailings by effectively immobilizing heavy metals, thus mitigating their environmental impact.

Pore structure characteristics and leaching mechanism of Cu and Cr in sustainable porous ceramisite produced from contaminated soil and biomass waste

Contaminated soil (CS) and biomass waste are potential resources. Their secure, scale, resource, and high-value utilization will bring significant socio-economic benefits. In present work, the basic properties, crystalline phases, microstructure and leaching behavior of porous ceramisite (PC) via CS and biomass waste were investigated systematically. The link of pore structure characteristics with leaching behavior of Cu and Cr was revealed. The environmental and economic benefits were also assessed. The results showed that the addition of 5∼15% pore-forming agent (carbon powder or biomass waste) made the matrix looser and the leaching amounts of heavy metals increased. However, the heavy metals leaching amounts was still lower than the critical limits of nonhazardous materials with immobilization rates higher than 97.5%. The leaching mechanism of Cu in the PC was controlled by initial wash-off, diffusion and depletion. The time period of the depletion dominated leaching is prolonged with increasing porosity. The leaching mechanism of Cr in the PC was mainly initial wash-off and depletion. The wash-off dominated the total heavy metal leaching amount, accounted for more than 80% of the total leaching amount. The leaching of Cu and Cr showed a positive correlation with (effective) pore volume of pores large than 1 μm, while demonstrating a negative correlation with the fractal dimension. The economic benefits of PC throughout the production process were analyzed, which showed that the production of PC using CS and biomass waste could eliminate raw material costs and could be subsidized by the government. This study provides an idea for the 100% utilization of CS and biomass waste as well as theoretical support for the disposal of solid waste.

Cs+ Promoting the Diffusion of K+ and Inhibiting the Generation of Newberyite in Struvite-K Cements: Experiments and Molecular Dynamics Simulation Calculations.

Struvite-K cements, also called magnesium potassium phosphate cements (MKPCs), are applicable for particular applications, especially the immobilization of radioactive Cs+ in the nuclear industry. This work focuses on how Cs+ affects the hydration mechanism of struvite-K cements because newberyite and brucite in the hydration products are deemed to be risky products that result in cracking. Experiments and molecular dynamics simulations showed that Cs+ promoted the diffusion of K+ to the surface of MgO, which greatly facilitates the formation of more K-struvite crystals, inhibiting the formation of newberyite and brucite. A total of 0.02 M Cs+ resulted in a 40.44%, 13.93%, 60.81%, and 32.18% reduction in the amount of newberyite and brucite, and the Cs immobilization rates were 99.07%, 99.84%, 99.87%, and 99.83% when the ratios of Mg/P were 1, 3, 5, and 7, respectively. This provides new evidence of stability for struvite-K cements on radioactive Cs+ immobilization. Surprisingly, another new crystal, [CsPO3·H2O]4, was found to be a dominating Cs-containing phase in Cs-immobilizing struvite-K cements, in addition to Cs-struvite.

Machine learning-based prediction of heavy metal immobilization rate in the solidification/stabilization of municipal solid waste incineration fly ash (MSWIFA) by geopolymers

Geopolymer is an environmentally friendly solidification/stabilization (S/S) binder, exhibiting significant potential for immobilizing heavy metals in municipal solid waste incineration fly ash (MSWIFA). However, due to the diversity in geopolymer raw materials and heavy metal properties, predicting the heavy metal immobilization rate proves to be challenging. In order to enhance the application of geopolymers in immobilizing heavy metals in MSWIFA, a universal method is required to predict the heavy metal immobilization rate. Therefore, this study employs machine learning to predict the heavy metal immobilization rate in S/S of MSWIFA by geopolymers. A gradient boosting regression (GB) model with superior performance (R2 = 0.9214) was obtained, and a graphical user interface (GUI) software was developed to facilitate the convenient accessibility of researchers. The feature categories influencing heavy metal immobilization rate are ranked in order of importance as heavy metal properties > geopolymer raw material properties > curing conditions > alkali activator properties. This study facilitates the rapid prediction, improvement, and optimization of heavy metal immobilization in S/S of MSWIFA by geopolymers, and also provides a theoretical basis for the resource utilization of industrial solid waste, contributing to the environmental protection.

Exploiting bifunctional 3D-Printed geopolymers for efficient cesium removal and immobilization: An approach for hazardous waste management

3D-printed geopolymer lattices (3DGPLs) have recently been considered as promising candidates for hazardous waste management due to their low cost, porous nature, and excellent environmental stability. However, the ion adsorption capacity influenced by usage conditions such as multi-ion systems and radiation remains largely unexplored. Furthermore, effective strategies to immobilize adsorbed ions for long-term disposal are still lacking. This work tackles these questions by studying the Cs+ adsorption and immobilization properties of 3DGPLs in the presence of competing ions and gamma-ray radiation. Our results reveal the inhibitory intensity of Cs+ adsorption capacity by five common metal cations in seawater, ranked in the following order: Na+ > K+ > Ca2+ > Mg2+ > Sr2+. After exposure to different doses of gamma-ray radiation, the Cs+ adsorption capacity of 3DGPLs shows no significant change, demonstrating their outstanding radiation resistance. By encapsulating the Cs-containing 3DGPL with a homologous GP coating, the Cs+ immobilization rate reaches 98.545% after leaching in NaCl solution at 60 °C for 5 days. The already-high Cs+ immobilization rate can be further increased to 99.228% by sintering treatment due to the densification effect. This study not only provides microscopic insights into the adsorption of hazardous ions by 3DGPLs under usage conditions, but also offers a feasible approach to prevent the leakage of loaded ions through GP coating technology.

Accelerated soil nitrogen cycling in response to a whole ecosystem acid rain mitigation experiment

Acid deposition has declined substantially over the last thirty years in the developed world. In forested watersheds previously impacted by acid deposition, evidence suggests that soils are slowly beginning to recover their alkalinity and base cation fertility. Research in these recovering ecosystems suggests that these changes in soil chemistry may result in decreased ecosystem retention and greater hydrologic export of nitrogen (N), although the drivers behind this enhanced export remain poorly understood. A whole-watershed acid rain mitigation experiment at Hubbard Brook Experimental Forest (New Hampshire, USA) offers a unique opportunity to examine how forest N cycling might change as soil recovery progresses over the coming decades. In this experiment, researchers added 1168 kg ha−1 of wollastonite (CaSiO3) to an 11.8 ha watershed in 1999, leading to sustained increases in soil pH and increasing Ca fertility. By 2008, the experimental watershed began to export significant quantities of nitrogen, becoming a net nitrogen source for the first time since measurements began in 1963. We sought to understand whether shifts in soil N cycling could explain this watershed phenomenon. Across repeated soil sampling campaigns in 2015 and 2016, we found that soil inorganic nitrogen pools in the treated watershed were between 1.8 and 2.6 times higher (p < 0.001) than in a nearby reference watershed. Gross N mineralization rates and immobilization rates were ∼40% higher in the litter layer (Oie horizon) of the treated watershed(p < 0.001). We conclude that an accelerated N cycle in this litter layer, with faster turnover between organic and inorganic N pools, likely resulted in faster replenishment of inorganic N pools that were susceptible to hydrologic loss, resulting in higher N export. Nitrogen cycling rates were uncorrelated with soil acid-base properties, suggesting that direct geochemical controls are not the primary driver of the altered N cycle.

Acute toxicity of nanoscale zeolitic imidazolate framework 8 (ZIF-8) to saltwater planktonic species Artemia salina and Nannochloropsis oculata.

Zeolitic imidazolate framework-8 nanoparticles (ZIF-8 NPs) are metal-organic frameworks (MOFs) that have gained significant attention in various fields due to their unique properties. They have potential applications in drug delivery, gas storage, and catalysis. However, their increasing use raises concerns about their potential environmental impact. Our study evaluates the effects of ≈90nm ZIF-8 NPs in two planktonic species, the green microalga Nannochloropsis oculata and the brine shrimp Artemia salina. After synthesis and characterization (SEM, EDS, BET, and DLS) of nanoporous ZIF-8 NPs, a growth inhibition test on microalgae (72h) and acute immobilization test on instar I and II of Artemia nauplii (48h) were conducted following, OECD 201 and ISO/TS 20787, respectively. The toxicity of ZIF-8 NPs to both species was time- and concentration-dependent. The 72-h median inhibitory concentration (IC50) of ZIF-8 NPs for N. oculata based on average specific growth rate and yield were calculated as 79.71 ± 8.55mg L-1 and 51.73 ± 5.16mg L-1, respectively. Also, the 48-h median effective concentration (EC50) of ZIF-8 NPs on immobilization rate of instar I and II were calculated as 175.09 ± 4.14mg L-1 and 4.69 ± 0.34mg L-1, respectively. Moreover, the swimming type of non-immobilized animals was affected by ZIF-8 NPs. These findings provide a good insight into the toxicity of nanoparticulate ZIF-8 to saltwater planktons and also confirm that instar II Artemia is more sensitive than instar I. This study demonstrated that ZIF-8 NPs, despite all their advantages, could have toxic effects on aquatic organisms. More studies are required to assess their potential environmental impact and develop strategies to mitigate their toxicity.

Immobilization of fluorinion in phosphorus tailings-blast furnace slag based geopolymers activated by waste alkalis

Calcium carbide reduce-mirabilite was used as an alkaline activator to prepare geopolymer materials based on phosphorus tailings (PT) and blast furnace slag (BFS) for immobilization of anion fluorine (F). The influence of free F on mechanical properties, microstructure, and microtopography of geopolymers was discussed. The immobilization effect and mechanism of geopolymers on free F were studied. The results show that free F will cause more pores in the geopolymer, increase the proportion of unreacted particles and N-A-S-H gel, and reduce the proportion of C-(A)-S-H gel, resulting in a slight decline in mechanical properties. The leaching test shows the F leaching concentration gradually decreases while immobilization rate gradually increases with increasing BFS proportion. The immobilization mechanism of free F is reflected in that Ca2+ in geopolymer reacts with free F to form insoluble precipitates CaF2 and Ca5(PO4)3F, and the further immobilization of free F by the physical encapsulation of gel in geopolymer.

Simultaneous Immobilization of Heavy Metals in MKPC-Based Mortar-Experimental Assessment.

Heavy metal contamination, associated with the increase in industrial production and the development of the population in general, poses a significant risk in terms of the contamination of soil, water, and, consequently, industrial plants and human health. The presence of ecotoxic heavy metals (HMs) thus significantly limits the sustainable development of society and contributes to the deterioration of the quality of the environment as a whole. For this reason, the stabilization and immobilization of heavy metals is a very topical issue. This paper deals with the possibility of the simultaneous immobilization of heavy metals (Ba2+, Pb2+, and Zn2+) in mortar based on magnesium potassium phosphate cement (MKPC). The structural, mechanical, and hygric parameters of mortars artificially contaminated with heavy metals in the form of salt solutions were investigated together with the formed hydration products. In the leachates of the prepared samples, the content of HMs was measured and the immobilization ratio of each HM was determined. The immobilization rate of all the investigated HMs was >98.7%, which gave information about the effectiveness of the MKPC-based matrix for HM stabilization. Furthermore, the content of HMs in the leachates was below the prescribed limits for non-hazardous waste that can be safely treated without any environmental risks. Although the presence of heavy metals led to a reduction in the strength of the prepared mortar (46.5% and 57.3% in compressive and flexural strength, respectively), its mechanical resistance remained high enough for many construction applications. Moreover, the low values of the parameters characterizing the water transport (water absorption coefficient Aw = 4.26 × 10-3 kg·m-2·s-1/2 and sorptivity S = 4.0 × 10-6 m·s-1/2) clearly demonstrate the limited possibility of the leaching of heavy metals from the MKPC matrix structure.

Elevated atmospheric CO2 promotes the contribution of autotrophic nitrification to N2O emissions in a typical summer maize field

Elevated atmospheric carbon dioxide (CO2) concentration generally stimulates nitrous oxide (N2O) emissions from upland soils, leading to a promoted feedback on climate change. However, the underlying mechanisms on how elevated CO2 (eCO2) stimulates N2O emissions from nitrogen (N)-fertilized upland soils remain poorly known. Here, we investigated the effects of eCO2 on soil N2O production pathways and associated gross N transformation rates, as well as on nitrifying and denitrifying microbes. Based on a 13 years of free-air CO2 enrichment (FACE) platform, we found that N2O emissions were significantly increased by 46.7% under eCO2 in a typical summer-maize field. The 15N tracing experiment showed that autotrophic nitrification rate was significantly increased by 11.6% under eCO2, coincided with the significantly higher abundance and shift composition of ammonia-oxidizing bacteria (AOB) under eCO2. Also, eCO2 enhanced gross ammonium (NH4+) immobilization rate but reduced gross nitrate (NO3−) immobilization rate. Autotrophic nitrification accounted for approximately 78% of N2O emissions, followed by heterotrophic nitrification (about 19%) and denitrification (about 3%). It was found that eCO2 increased carbon (C) deposition in soil, which could affect the availability of soil labile organic C and N and alter the composition and activity of nitrifiers. These changes promoted the N2O production derived from autotrophic nitrification, resulting in higher N2O emissions under eCO2. We could deduce that nitrifier denitrification contributed to the increased N2O emissions greatly under eCO2, derived by strongly stimulated ammonia oxidation and microbial respiration (i.e. higher soil CO2 emissions) caused suboxic conditions (i.e. lower soil oxygen (O2) concentration), along with higher abundance ratios of (nirK+nirS)/nosZ. Our results suggest that controlling autotrophic nitrification appropriately is crucial for mitigating N2O emissions from upland soils under rising atmospheric CO2 concentration.

Coexistence of microplastics and Cd alters soil N transformation by affecting enzyme activity and ammonia oxidizer abundance

Interactions between heavy metal and microplastics represent a serious threat to ecosystems and human health, but the effect of their coexistence on the soil N transformation processes is unclear. The mechanism in which metal-polluted soil reacts to additional microplastics stress and their toxicology interactions on soil N transformation were determined by investigating the dynamics of soil microbial N transformation in response to Cd stress and different doses of polythene (PE) microplastics by conducting a 14 days aerobic 15N microcosmic incubation experiment. The gross nitrification rates (n_gross) were decreased by 7.47% and 12.5% in the 1% and 2% (w/w) PE groups, respectively, through the direct effect on enzyme activity (β-glucosidase, N-acetylglucosaminidase, and leucine-aminopeptidase) and the abundance and community composition of ammonia oxidizer. It also exerted indirect effect by reducing nitrification substrate concentrations. PE microplastics (>1% [w/w]) significantly increased the gross N immobilization rate, and this change could have been driven by C/N stoichiometry. Cd stress alone led to a rapid short-term mineralization–immobilization turnover (1.67 times of the control). However, such effect was offset when Cd coexisted with PE microplastics, possibly because Cd was directly adsorbed by PE microplastics, and/or microplastics satisfied the C demand by microorganisms under Cd stress. Our findings demonstrated that the coexistence of microplastics and Cd significantly altered soil N nitrification and immobilization, which would change the N bioavailability in soil and alter the effect N cycling on the ecological environment.

Immobilization of Lipase on Agarose Beads for Enzymatic Hydrolysis/Transesterification of Castor Oil

Lipase was immobilized on agarose beads and used for enzyme-catalyzed hydrolysis/transesterification of castor oil at room temperature without surfactants and salts. The immobilization yield was evaluated by reacting p-nitrophenyl butyrate and immobilized lipase. The reaction was performed in a 25 mL phosphate buffer medium (pH 7.0) and incubated for 2 hours. The absorbance of the formed p-nitrophenol was measured at 346 nm. The highest immobilization rate was obtained using 5.0 mg lipase with 2.5 mL glyoxal agarose beads. The reaction was performed in 25.0 mL sodium bicarbonate buffer pH 10 for 12 h. The hydrolysis/transesterification of castor oil was performed in phosphate buffer (pH 7) – ethanol medium. The main products were ricinoleic acid and ethyl ricinoleate. The highest hydrolysis/transesterification yield was 87% using 1.0 g castor oil and 2.5 mL of lipase immobilized agarose beads in the phosphate buffer (pH 7): ethanol (1: 9, V: V) at 72 hours. The reaction products were analyzed using FTIR and RP-HPLC.

Utilizing thiourea-modified biochars to mitigate toxic metal pollution and promote mustard (Brassica campestris) plant growth in contaminated soils

This study investigated the potential of using biochars made from sugarcane bagasse, banana peel, and watermelon rind that were modified with 1 % thiourea through pyrolysis at 500 °C to make sugarcane bagasse + thiourea modified biochar (STMB), banana peel + thiourea modified biochar (BTMB), and watermelon rind + thiourea modified biochar (WTMB) to immobilize toxic metals such as Cd, Ni, Cu, Pb, Cr, and Zn in polluted soils from Gadap town and Malir cantonment. The modified biochars were used at different dosages (1.25 %, 2.5 %, and 5 %) and the results showed that they were effective in immobilizing the toxic metals (TMs), with immobilization rates ranging from 43.75 % to 93.98 % with application of with BTMB 2.5 % and BTMB 5 %. The addition of these biochars also led to improved growth of mustard plants, with the highest biomass observed with STMB at a 1.25 % dosage. The study also found that soil pH, SOC, CaCO3, and TOC played key roles in reducing the concentration of toxic metals in the polluted soils. The results suggest that a dosage of 1.25 % can be recommended for better plant growth while 2.5 % and 5 % dosage can be recommended for immobilization of TMs in industrial wastewater-polluted soils.

Microbial gene abundance mirrors soil nitrogen mineralization intensity across an age gradient in Chinese-fir plantations

Stand development affects soil properties, nitrogen (N) dynamics, and soil microbial community composition, but the question remains whether differences in N mineralization rates are mirrored by the abundance of relevant functional genes. In this study, we used the 15N pool-dilution method to estimate N mineralization (i.e., ammonification and nitrification) rates across a Chinese fir (Cunninghamia lanceolata) chronosequence, with stands aged 7, 16, 29, 36, and >80 years. Gene copy numbers of bacteria (16S rRNA), fungi (ITS), ammonia-oxidizing archaea (AOA) and bacteria (AOB) (amoA), denitrifiers (nirS, nirK), N2 fixers (nifH) and organic N decomposers (chiA) were quantified by qPCR. Gross ammonification and nitrification rates increased linearly with stand age in the topsoil (0–5 cm depth) and were strongly positively correlated with the abundance of the bacterial 16S rRNA gene and AOA amoA, respectively. Higher net nitrification but lower NO3− immobilization rates in older stands (32 and > 80 years) drove higher N availability for vegetation than in young stands (7 years). Older stands also had higher rates of NH4+ consumption than younger stands due to the increased fungal ITS abundance and higher microbial biomass N (MBN), and AOA amoA was more abundant and active than AOB amoA due to the more acid conditions characteristic of mature forests. Redundancy analysis showed that functional gene abundance was strongly affected by soil properties such as pH, NH4+-N content, and MBN. We also found that microbial N storage potential was lower, and the NO3−-N leaching and gaseous N loss potential were higher in older stands than in younger stands. Collectively, stand developmental stage gave rise to the observed spatial gradient of gross ammonification and nitrification rates by altering the abundance of microbial functional genes, which affected plantation productivity via its modulation of the supply of bioavailable N.

Effective reducing the mobility and health risk of mercury in soil under thiol-modified biochar amendment

Soil mercury contamination poses health risks. However, effective immobilization techniques are lacking with challenges including low efficiency, limited long-term stability, susceptibility to multi-medium interference, and difficulty in controlling health risks. This study confirmed the feasibility of thiol-modified biochar, and elucidated the underlying mechanisms. Within 32 days of treatment, the leachable mercury decreased from 184.7 μg/L to below the hazardous waste threshold (100 μg/L, HJ/T299–2007, China). After 198 days of treatment, the soil achieved a safe ecological state with a mercury immobilization rate of 79.8–98.2% and a 50% reduction in available methylmercury. Thiol-modified biochar facilitated the conversion of active mercury species (exchangeable, carbonate, and oxide) into stable forms (organic and residual) through complexation and precipitation (e.g., HgS). Soil quality improvements were observed, including enhanced cation exchange capacity, available nitrogen, and total organic carbon. Thiol-modified biochar exhibits long-term effectiveness. After one and two years of treatment, the leachable mercury remained within acceptable health risk limit (hazard quotient < 1) for adults and children, respectively. After three years, the leachable mercury met the Level III groundwater quality standard (< 1 μg/L, GB 14848–2017, China). This study demonstrates an effective strategy for long-term diminishing mercury mobility and health risks in soil.

The regulation of gross nitrogen transformation rates in greenhouse soil cultivated with cucumber plants under elevated atmospheric [CO2] and increased soil temperature

Climate change scenarios forecast both increase in atmospheric CO2 concentration ([CO2]) and temperature. Elevated [CO2] (e[CO2]) and soil temperature (eST) significantly increased plant growth and nitrogen (N) uptake. However, studies on the combined effect of these factors on soil N transformation and microbial community are scarce in agricultural intensification. In the present work, we cultivated cucumber plants under e[CO2] or/and eST, followed by a paired labeling 15N tracer experiment (15NH4NO3 and NH415NO3) to quantify soil gross N transformation rates. Results showed that e[CO2] decreased soil gross N nitrification rate, indicated by a decline in ammonia-oxidizing bacteria (amoA-AOB) and nitrite oxidoreductase alpha subunit (nxrA) gene copy numbers, and changed microbial taxonomic composition under ambient soil temperature (aST). Furthermore, eST increased chitinase gene (chiA) and amoA-AOB gene copy numbers and alpha diversity and changed the microbial taxonomic composition to improve soil gross N mineralization and nitrification rates. The combination of e[CO2] and eST promoted soil gross N mineralization, nitrification, and immobilization rates compared with ambient [CO2] and aST. The study showed that the combination of e[CO2] and eST increased the primary gross N transformation rates and accelerated soil N circulation, which could ensure plant N demand. Therefore, combining e[CO2] and eST provided a promising way to help cultivators promote vegetable growth in winter, mitigate soil N loss, and achieve sustainable development in greenhouse vegetable cultivation.

Deciphering the role of particulate organic matter in soil nitrogen transformation in rice–rapeseed and rice–wheat rotation systems

Crop rotation affects the decomposition of soil organic matter (SOM) and thereby alter the composition of SOM fractions. It remains unclear how different SOM fractions impact soil nitrogen (N) transformation in various rotation systems. The aim of this study was to ascertain the role of particulate organic matter (POM)-a labile SOM fraction-in soil N transformation under various crop rotations. A paired plot experiment was conducted under two common cropping patterns, i.e., rice–rapeseed rotation (RR) vs. rice–wheat rotation (RW). Soil chemical composition and organic matter fraction before rice transplanting were compared between RR and RW systems after four years of crop rotations (2017–2021). With the same N inputs, the rice yield and N uptake under RR were 16.4 % and 13.2 % higher than those under RW, respectively. Compared with RW, RR resulted in higher carbon (C) and N contents in soil POM, despite minimal differences in total SOM. A larger potentially mineralizable N pool and a higher N mineralization rate occurred under RR than under RW, based on the results of soil net mineralization experiment. When POM was incubated alone, its contribution to potentially mineralizable N was 65.1 % and 61.3 % in RR and RW soils, respectively. Infrared spectroscopy revealed that in contrast with RW, RR promoted the accumulation of organic matter with high bioavailability (e.g., amides, carbohydrates, polysaccharides) in soil POM. This might be responsible for the higher gross mineralization and nitrification rates but lower gross immobilization rate under RR than under RW. Consequently, RR not only increased the contents of POMC and POMN but also improved the quality of POM fraction in soils. Findings of the present study demonstrate that POM plays a distinct role in soil N mineralization in various rotation systems. The discrepancy in POM content and composition resulting from various crop rotations leads to differences in soil N mineralization, which in turn affects the N supply and rice yield.

Optimizing raw material composition and alkali content in low silicon-aluminum tailings-based geopolymers

This study delves into the effective utilization of low silicon–aluminum tailings, exploring their potential for large-scale applications in one-part (‘just add water’) geopolymers. Optimizing the raw material composition and alkali content is crucial. Higher slag content enhanced gel production and polymerization, resulting in a more homogeneous structure dominated by C-(A)-S-H gels with stronger bonding. While increased metakaolin content forms more zeolite phases and exhibited an incompatible structure filled with N-A-S-H gels with weaker bonding. Alkali concentrations also play a significant role in promoting tailings dissolution and gel polymerization. Excessive levels, however, hinder the process and restrict strength development. Ultimately, a reasonable mechanical strength of 52.8 MPa was achieved. The geopolymer paste effectively immobilizes Pb and Zn, demonstrating impressive immobilization rates of 97% and 91%, respectively. The chemical speciation analysis reveals the conversion from unstable to stable state. These findings highlight the potential applicability of low silicon–aluminum tailings in geopolymers.