New insights into the sustainable use of soluble straw humic substances for the remediation of multiple heavy metals in contaminated soil

Optimizing extraction conditions to enhance the humification and soil remediation potential of compost-derived dissolved organic matter

Compost tea (CT), primarily composed of dissolved organic matter derived from compost, is widely used in various environmental and agricultural applications. Nevertheless, limited information is available regarding how extraction parameters influence the quality of CT and its efficacy in soil remediation. In this study, a multi-factor orthogonal design L16 (43) was employed to investigate the effects of compost-to-water ratio (CWR), extraction time (ET), and aeration pattern (AP) on nutrient extraction and humification of CT, aiming to optimize the extraction conditions. Results showed that N, P2O5, and K2O extraction efficiencies in all treatments ranged from 10 to 25 %, 10-20 %, and 50-85 %, respectively. The comprehensive humification score was in the range of 1.27-1.60. Among the three parameters, ET showed the most significant influence on CT quality. The optimal treatment for nutrient extraction was T15 (CWR 1:60, ET 48 h, and stirring), while T17 (CWR 1:30, ET 48 h, and aeration) exhibited superior performance on humification. Furthermore, the total Cd removal efficiency of T17 was 83.64 % after multiple washing cycles, which was attributed to an increased number of hydroxyl, carboxyl, and carbonyl functional groups that provided additional binding sites for Cd.

Efficient removal of cadmium (II) and arsenic (III) from water by nano-zero-valent iron modified biochar-zeolite composite

For the removal of Cd(II) and As(III) from water, this study synthesized a nano-zero-valent iron-loaded biochar-zeolite composite material (nZVI-BCZo) using a liquid-phase reduction method, with biochar, zeolite, and FeSO₄·7H₂O as precursors. The successful incorporation of nZVI onto the BCZo was verified through Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Fourier Transform Infrared Spectroscopy (FTIR) analyses, which revealed significant modifications in the surface oxygen-containing functional groups. Batch adsorption experiments were conducted to evaluate the adsorption characteristics and performance of nZVI-BCZo for Cd(II) and As(III). Under optimal conditions (pH 6.0, temperature of 310 K, and an adsorption time of 360 min), the maximum adsorption capacities for Cd(II) and As(III) were found to be 28.09 mg/g and 186.99 mg/g, respectively. The influence of pH on removal efficiency was more pronounced than that of temperature, with nZVI-BCZo exhibiting a higher affinity for As(III) compared to Cd(II). Kinetic analysis showed that the adsorption process is primarily controlled by chemical adsorption and follows a monolayer adsorption mechanism. Regeneration tests demonstrated that nZVI-BCZo retained good adsorption capacity after three cycles, with adsorption efficiencies of 67.78 % for Cd(II) and 53.04 % for As(III), indicating its potential for repeated use in water treatment applications. The economic evaluation revealed that nZVI-BCZo has a lower processing cost. Therefore, this study established nZVI-BCZo as an efficient, reusable, and cost-effective adsorbent for the treatment of heavy metal-laden water.

Changes in the spectroscopic response of soil organic matters by PBAT microplastics regulated the Cd adsorption behaviors in different soils

Contamination of microplastics (MPs) and heavy metals occurs frequently in terrestrial ecosystems, but their interactions remain unclear. A 60-day incubation experiment was conducted to study the behaviors of cadmium (Cd) in polybutylene adipate terephthalate (PBAT) MPs-contaminated soils, with different doses (1, 10%) and sizes (150-300 and 75-150 μm). Soil chemical properties, including the three-dimensional fluorescence of dissolved organic matter (DOM) and microbial diversity in both farmland and woodland soils were analyzed. Results showed that soil properties, especially the components and fluorescence characteristics of DOM varied with soil types and PBAT properties. Higher soil chemical properties and microbial diversity were found in woodland soils. The soluble microbial by-product substances and humic acid-like substance were dominated in soil DOM, while the proportions of fulvic/humic-acid like substances and soil humification decreased with the addition of 10% PBAT. Soil microbial diversity increased with doses of PBAT, but not sensitive to the sizes of PBAT. The adsorption capacity of Cd decreased with the addition of PBAT, especially in the 10% and 75-150 μm PBAT treatments. Both Langmuir and Freundlich models fitted well with the adsorption isotherms of Cd. Multiple correlation analyses showed that low molecular weight fractions, humus index of DOM and soil microbial diversity such as Shannon, Simpson, and Pielou all positively correlated with the adsorption behaviors of Cd in PBAT-contaminated soils. Biodegradable MPs can change soil quality and promote the release of soil Cd, which deserves further research attention.

Environmental colloid behaviors of humic acid - Cadmium nanoparticles in aquatic environments

Humic acid (HA), a principal constituent of natural organic matter (NOM), manifests ubiquitously across diverse ecosystems and can significantly influence the environmental behaviors of Cd(II) in aquatic systems. Previous studies on NOM-Cd(II) interactions have primarily focused on the immobilization of Cd(II) solids, but little is known about the colloidal stability of organically complexed Cd(II) particles in the environment. In this study, we investigated the formation of HA-Cd(II) colloids and quantified their aggregation, stability, and transport behaviors in a saturated porous media representative of typical subsurface conditions. Results from batch experiments indicated that the relative quantity of HA-Cd(II) colloids increased with increasing C/Cd molar ratio and that the carboxyl functional groups of HA dominated the stability of HA-Cd(II) colloids. The results of correlation analysis between particle size, critical aggregation concentration (CCC), and zeta potential indicated that both Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO interactions contributed to the enhanced colloidal stability of HA-Cd(II) colloids. Column results further confirmed that the stable HA-Cd(II) colloid can transport fast in a saturated media composed of clean sand. Together, this study provides new knowledge of the colloidal behaviors of NOM-Cd(II) nanoparticles, which is important for better understanding the ultimate cycling of Cd(II) in aquatic systems.

High-Spin States of Manganese(III) Enable Robust Cold-Adapted Activity of MnO2 Nanozymes

Developing novel cold-adapted nanozymes and elucidating their mechanisms of action remains a great challenge. Inspired by natural oxidases that utilize high-spin and high-valent metal-oxygen intermediates to achieve high efficiency at low temperatures, in this study, a series of MnOx nanomaterials with varied valence and spin states are synthesized. The activity assay revealed that the oxygen vacancy-engineered ε-MnO2 nanozyme displayed excellent cold-adapted oxidase-like properties, and no observable activity loss is observed in the temperature range of -20 to 45 °C. The superior performance is attributed to the high-spin Mn(III)-O species coupled with its induced Jahn-Teller effect, which facilitates the dissociation and activation of oxygen at low temperatures. As a proof of concept, an excellent cold-adapted δ-MnO2 nanozyme can be obtained using Mn3O4 as the precursor by regulating the spin state of Mn(III). Moreover, a novel and effective degradation strategy for corn stalk at low temperature is built based on the robust cold-adapted oxidase-like activity of ε-MnO2. These results not only provide new insights for the rational design of cold-adapted nanozymes but also broaden the application of nanozymes in low-temperature industrial processes.

Crop rotation increases Tibetan barley yield and soil quality on the Tibetan Plateau

Tibetan barley (Hordeum vulgare) accounts for over 70% of the total food production in the Tibetan Plateau. However, continuous cropping of Tibetan barley causes soil degradation, reduces soil quality and causes yield decline. Here we explore the benefits of crop rotation with wheat and rape to improve crop yield and soil quality. We conducted 39 field experiments on the Tibetan Plateau, comparing short-term (≤5 years), 5-10 years and long-term (≥10 years) continuous cropping with rotation of Tibetan barley with wheat or rape. Results showed that Tibetan barley-wheat and Tibetan barley-rape rotations increased yields by 17% and 12%, respectively, while improving the soil quality index by 11% and 21%, compared with long-term continuous cropping. Both Tibetan barley rotations with wheat and rape improved soil quality and consequently yield, mainly by increasing soil microbial biomass nitrogen and microbial biomass carbon and decreasing pH. By contrast, long-term continuous cropping led to decreased soil organic matter, lower microbial biomass nitrogen and increased pH, contributing to yield decline. The benefits of rotations on crop yield and soil quality increased over time. Implementing crop rotation with wheat or rape thus offers a sustainable agricultural strategy for improving food security on the Tibetan Plateau.

Insights into the critical roles of water-soluble organic matter and humic acid within kitchen compost in influencing cadmium bioavailability

Compost has demonstrated potential as a cadmium (Cd) remediation agent, while it still remains unclear about the core components in driving the bioactive transformation of Cd. To address this issue, this study isolated three components-kitchen compost powder (KC), humic acid (HA), and water-soluble organic matter (DOM)-from kitchen compost to regulate soil properties, bacterial community structures and functions, and Cd migration risks. The results revealed that the addition of 20% KC and HA reduced the bioavailability factor of Cd by 47.20% and 16.74%, respectively, with HA contributing 35.47% of the total reduction achieved with KC. Conversely, the application of DOM increased the Cd risk through a reduction in soil pH and an increase in the abundance of Cd-activating bacteria, which adversely affected the stability of Cd complexes. However, the porous structure and organic matter in KC and HA provided adsorption sites for Cd passivation and promoted the growth of Cd-fixing bacteria. This study effectively identifies both the positive and negative effects of key compost components on Cd migration and provides scientific guidance for applying kitchen compost in soil management.

Potential of artificial soil preparation for vegetation restoration using red mud and phosphogypsum

Red mud and phosphogypsum have long been a focus and challenge in global industrial waste management, and their low-cost and large-scale utilization technology has always been an urgent need. This study is based on the strong acid-base neutralization reaction between red mud and phosphogypsum, which contain an elemental composition similar to that of natural soil, red mud itself has characteristic of clay minerals, and other auxiliary materials (i.e. rice husk powder, bentonite, fly ash, polyacrylamide flocculant and microbial suspension) were added, so as to explore the potential of synergistically prepared artificial soil for vegetation restoration. The results showed that the artificial soils exhibited physicochemical characteristics (e.g., pH, moisture content, cation exchange capacity) similar to those of natural soil, along with abundant organic matter, nitrogen, phosphorus, and potassium contents, meeting the growth requirements of plants. The artificial soils were able to support favorable growth of suitable plants (e.g., sunflower, wheat, rye grass), accumulating high levels of diverse enzymatic activities, comparable to those in natural soils (e.g., catalase, urease, phosphatase), or even surpassing natural soils (e.g., sucrase), and rich microorganism communities, such as Cyanobacteria, Proteobacteria, Actinobacteria in the bacteria domain, and Ascomycota in the fungi domain, were initially developed. It's suggested that preparing 1 ton of artificial soil entails synergistic consumption of 613.7 kg of red mud and 244.6 kg of phosphogypsum, accounting for mass proportions of 61.4 % and 24.5 %, respectively. In future, more evaluations on the leaching loss of nutrients and alkalinity and the environmental risks of heavy metals should be conducted to more references for the artificial soil application. In summary, the preparation of artificial soil is a very simple, efficient, scalable and low-cost collaborative resource utilization scheme of red mud and phosphogypsum, which has great potential for vegetation restoration in some places such as tailings field and soil-deficient depression.

Rapid humification of cotton stalk catalyzed by coal fly ash and its excellent cadmium passivation performance

Due to industrialization, soil heavy metal pollution is a growing concern, with humic substances (HS) playing a pivotal role in soil passivation. To address the long duration of the compost humification problem, coal fly ash (CFA) in situ catalyzes the rapid pyrolysis of the cotton stalk (CS) to produce HS to address Cd passivation. Results indicate that the highest yield of humic acid (HA) (8.42%) and fulvic acid (FA) (1.36%) is obtained when the CS to CFA mass ratio is 1:0.5, at 275 ℃ for 120 min. Further study reveals that CFA catalysis CS humification, through the creation of alkaline pyrolysis conditions, Fe2O3 can stimulate the protein and the decomposition of hemicellulose in CS, and then, through the Maillard and Sugar-amine condensation reaction synthesis HA and FA. Applying HS-CS&CFA in Cd-contaminated soil demonstrates a 26.69% reduction in exchangeable Cd within 30 days by chemical complexation. Excellent maize growth effects and environmental benefits of HS products are the prerequisites for subsequent engineering applications. Similar industrial solid wastes, such as steel slag and red mud, rich in Fe2O3, can be explored to identify their catalytic humification effect. It could provide a novel and effective way for industrial solid wastes to be recycled for biomass humification and widely applied in remediating Cd-contaminated agricultural soil.

Short-term high-temperature pretreated compost increases its application value by altering key bacteria phenotypes

Short-term high-temperature pretreatment can effectively shorten the maturity period of organic waste composting and improve the fertilizer efficiency and humification degree of products. To investigate the effect and mechanism of the end products on the saline-alkali soil improvement and plant growth, the short-term high-temperature pretreatment composting (SHC) and traditional composting (STC) were separately blended with saline-alkali soil in a ratio of 0-40 % to establish a soil-fertilizer blended matrix for cultivating Lolium perenne L. The pot experiments combined with principal component analysis showed Lolium perenne L. planted in 20 % SHC-blended saline-alkali soil had the best growth effect, and its biomass, chlorophyll content, and plant height were 109-113 % higher than STC. The soil physicochemical property analysis showed that SHC and STC increased the soil nutrient content, humification degree, and enzyme activity at any blending ratio. The microbial analysis showed that 20 % SHC in the saline-alkali soil stimulated the growth of functional microorganisms and the addition of SHC promoted the sulfur cycle, nitrogen fixation, and carbon metabolism in the soil-plant system. The correlation analysis showed that pH; nutrient contents; and urease, catalase, sucrase, and phosphatase activities in the saline-alkali soil were significantly correlated with plant growth indexes (p < 0.05). Georgenia and norank_f__Fodinicurvataceae had a stronger correlation with four types of enzyme activities (p < 0.01). SHC improved the saline-alkali soil and promoted plant growth by adjusting soil pH, increasing soil nutrients, and influencing soil enzyme activity and dominant flora. This study provides a theoretical basis for applying SHC products in soil improvement.

Investigation on humic substance and tetracycline interaction mechanism: biophysical and theoretical studies and assessing their effect on biological activity

Tetracycline (TC) is a widely used antibiotic, and evaluating its interaction with humic substances (HS) that act as a complexing agent in the environment is essential to understanding the availability of this contaminant in the environment. This study evaluated the interaction between HS and TC using different spectroscopic techniques, theoretical studies, and biological assays simulating environmental conditions. TC interacts with HS, preferably by electrostatic forces, with a binding constant of 9.2 × 103 M-1 (30 °C). This process induces conformational changes in the superstructure, preferably in the HS, like protein fraction. Besides, studies using the 8-anilino-1-naphthalene sulfonate (ANS) probe indicated that the antibiotic alters the hydrophobicity degree on HS's surface. Synchronized fluorescence shows that the TC interaction occurs preferentially with the protein-like fraction of soil organic matter (KSV = 26.28 ± 1.03 M-1). The TC epitope was evaluated by 1H NMR and varied according to the pH (4.8 and 9.0) of the medium, as well as the main forces responsible for the stabilization of the HS-TC complex. The molecular docking studies showed that the formation of the HS-TC complex is carried out spontaneously (ΔG = -7.1 kcal mol-1) and is stabilized by hydrogen bonds and electrostatic interactions, as observed in the experimental spectroscopic results. Finally, biological assays indicated that HS influenced the antimicrobial activity of TC. Thus, this study contributed to understanding the dynamics and distribution of TC in the environment and HS's potential in the remediation of antibiotics of this class in natural systems, as these can have adverse effects on ecosystems and human health.