Evaluating the remediation potential of MgFe2O4-montmorillonite and its co-application with biochar on heavy metal-contaminated soils

Improving soil properties and microbial communities in copper tailings using montmorillonite-based Chlorella gel beads

Copper tailings are the waste left over from copper ore dressing, and their massive pose severe challenges to the ecosystem. In this study, sodium alginate-Chlorella-montmorillonite (SCM) gel beads were prepared by combining sodium alginate, Chlorella, and montmorillonite. The laboratory pot and in-situ field experiment results indicated that the input of SCM gel beads facilitated the formation of larger particle-size soil aggregates and enhanced soil water retention and cation exchange capacities. The pot experiment demonstrated that the application of MMT and Chlorella significantly increased organic carbon content in the tailing soil. The field experiment showed that the application of SCM gel beads at the optimal dosage of 700 g/m2 increased the plant height and fresh weight by 1.77 and 1.22 times, respectively, as well as the chlorophyll content. Furthermore, in SCM group, the proportion of large soil aggregates(particle size >0.25 mm) was increased by 6.66 %, and the R0.25, mean weight diameter (MWD), and geometric mean diameter (GWD) values were also significantly increased, and the increase in large soil aggregates and aggregate stability indicated the improvement of soil structure. Additionally, the dominant microorganisms at the phylum level were Proteobacteria, Acidobacteriota, and Bacteroidota, while they were Sphingomonas, Vicinamibacteraceae, and Candidatus at genus level. These dominant microorganisms were indigenous species to copper tailing. The alpha diversity determination results indicated that SCM gel bead input increased the microbial community richness, but had little effect on their diversity. Our results demonstrated that as soil amendment, SCM gel beads stimulated the growth of tailing native microorganisms, increasing their richness. Overall, the combination of Chlorella, and montmorillonite, as an amendment, improved soil properties of copper tailing and soil microbial community structure. Our findings provide valuable references for developing effective and sustainable soil remediation strategies in tailing areas.

Nanomaterials effectively alleviate cadmium hazards in soil-plant systems: A meta-analysis

Soil cadmium (Cd) contamination is a non-negligible global environmental issue as it may threaten food security and human health through soil-plant interactions. Nanomaterials have a great potential to decrease Cd bioavailability and bioaccumulation, even though the effects have been inconsistent among various studies. Here we compiled data from 137 experiments on the remediation of Cd-contaminated soils by nanomaterials. The effects of experimental design, nanomaterial properties, soil characteristics, and plant attributes on Cd bioavailability and bioaccumulation under nanomaterials application were evaluated. Results showed that incorporating nanomaterials could reduce the bioavailability of Cd in soil by 45.3 % and the bioaccumulation of Cd in plants by 37.6 %. Composite and carbon-based nanomaterials showed the most notable Cd immobilization effects at low and medium application rates, respectively. Cd-contaminated soils with low sand content, neutral pH (6.5 < pH ≤ 7.5), high organic matter (>30 g kg-1), and high cation exchange capacity (>20 cmol kg-1) are more conducive to the efficacy of nanomaterials. Among the four plant tissues, the decrease in Cd accumulation in leaf (39.4 %) and grain (37.7 %) was significantly higher than that in root (31.8 %) and stem (23.3 %) after the application of nanomaterials. Compared to other plant families, nanomaterials significantly suppressed Cd uptake by 54.7 % of Brassicaceae plants. Additionally, the Cd bioavailability showed a significantly positive correlation with Cd accumulation in the root (R2 = 0.46, p < 0.001), leaf (R2 = 0.42, p < 0.001), and grain (R2 = 0.13, p < 0.01). Overall, our results highlighted that nanomaterials are an effective solution to mitigate the hazards of Cd pollution in soil-plant systems.

Mechanisms of heavy metal-induced rhizosphere changes and crop metabolic evolution: The role of carbon materials

To investigate the effects of modified carbon-based materials on soil environmental remediation and crop physiological regulation, this research relied on rice pots with lead (Pb) and cadmium (Cd) composite contamination. Dolomite, montmorillonite, attapulgite and sepiolite modified biochar with different doses have been developed to explore the mechanisms on heavy metal passivation, nutrient improvement, microbial activation, and crop growth. The results indicated that the modified materials effectively reduced heavy metal bioavailability and accumulation in plant tissues through adsorption complexation. Specifically, under montmorillonite and sepiolite modified treatments, the Grains-Pb content significantly decreased by 29.23-30.31% and 27.49-30.58%, compared to the control group (CK). Meantime, carbon-based materials increased available nutrient levels, providing a biological substrate for soil microorganisms metabolism. The content of ammonium nitrogen (NH4+-N) and available phosphorus (AP) in different proportions of montmorillonite modified biochar increased by 10.99-13.98% and 55.76-77.86%, respectively, compared to CK. Furthermore, sepiolite modified biochar enhanced bacterial community diversity, significantly improving the tolerance and resistance of bacterial communities such as Proteobacteria and Acidobacteria to heavy metals. Meanwhile, carbon-based materials enhanced community stability and network complexity, improving microbial stress resistance to adverse environments. In summary, montmorillonite and sepiolite modified biochar regulated microbial community interaction mechanisms by mitigating the physiological toxicity of heavy metals. This process enhanced soil available nutrients and ecological function stability, which had significant implications for improving crop growth and quality.

Efficient and simultaneous immobilization of fluoride and lead in water and tea garden soil by bayberry tannin foam loaded zirconium

Nowadays, human activities intensified the combined pollution of fluoride and lead in acidic tea garden soil. The key to eliminating this combined pollution is to immobilize pollutants simultaneously, thus preventing their migration from tea garden soil to tea trees. In this paper, the natural product bayberry tannin was employed as raw material to fabricate functional materials (TF-Zr) for simultaneous adsorption of fluorine (F) and lead (Pb) in water and soil by the reactivity of tannin with Pb2+ and the affinity of Zr with F. SEM-Mapping, EDS, FT-IR, XPS were utilized to probe the immobilization mechanisms. The results showed that TF-Zr could simultaneously and efficiently adsorb F- and Pb2+ from water with the adsorption capacity of 5.02 mg/g (Pb) and 4.55 mg/g (F). The adsorption processes were both in accordance with the proposed secondary kinetic adsorption model. Besides, the presence of F- promoted the adsorption of Pb2+ by TF-Zr. The materials were applied into tea garden soil to explore its effect on the variation of F and Pb forms in the soil. It was found that the proportion of water-soluble fluorine, exchangeable fluorine and exchangeable lead in the tea garden soil decreased significantly, while the proportion of residual fluorine and lead increased evidently, illustrating TF-Zr possessed eximious fixation effect on the highly reactive fluorine and lead in the soil and facilitated their conversion to the more stable residue state. Therefore, TF-Zr can be used for the efficient and simultaneous immobilization of fluorine and lead in water and tea garden soil.

The role of soil structure on the cracking and cadmium leaching behavior of biochar-amended fine-grained soils

Biochar has shown promising potential for soil remediation, yet its impact on heavy metals (HMs) immobilization often overlooks soil structure, which could influence soil cracking behavior and HMs transport. To address this gap, this study investigates the role of soil structure (dry density and aggregate size) on the cracking and cadmium (Cd) leaching behavior of biochar-amended fine-grained soils. A series of semi-dynamic leaching tests were conducted on samples with and without wetting-drying (W-D) cycles. Based on the proposed improved method for quantifying the effective diffusion coefficient (De) of Cd in unsaturated soils and microstructural analyses, we found that: (1) Higher dry density and larger aggregate generally resulted in smaller De by decreasing soil pore volume. (2) Biochar could connect isolated pores within large aggregates through its internal pores, yielding greater increases in De (294.8%-469.0%) compared to small aggregates (29.1%-77.4%) with 3% biochar. However, further increases in biochar dosage led to decreased De, primarily due to the dense pore structure. (3) Biochar effectively inhibited soil cracking, achieving the highest reduction of 36.8% in surface crack ratio. (4) After W-D cycles, samples exhibited higher De with increasing dry density, with aggravated cracking being the primary cause, suggesting preferential flow within the cracks, particularly those penetrating the soil. This study highlights the importance of careful consideration of soil structure and cracking potential before in situ field application of biochar as a remediation agent for HMs-contaminated fine-grained soils.

Cost estimation of synthesis and utilization of nano-adsorbents on the laboratory and industrial scales: A detailed review

The recent regulations pertaining to the circular economy have unlocked new prospects for researchers. In contrast to the unsustainable models associated with the linear economy, integration of concepts of circular economy braces reducing, reusing, and recycling of waste materials into high-end products. In this regard, adsorption is a promising and cost-effective water treatment technology for handling conventional and emerging pollutants. Numerous studies are published annually to investigate the technical performance of nano-adsorbents and nanocomposites in terms of adsorption capacity and kinetics. Yet, economic performance evaluation is rarely discussed in the literature. Even if an adsorbent shows high removal efficiency towards a specific pollutant, its high preparation and/or utilization costs might hinder its real-life use. This tutorial review aims at illustrating cost estimation methods for the synthesis and utilization of conventional and nano-adsorbents. The current treatise discusses the synthesis of adsorbents on a laboratory scale where the raw material, transportation, chemical, energy, and any other costs are discussed. Moreover, equations for estimating the costs at the large-scale adsorption units for wastewater treatment are illustrated. This review focuses on introducing these topics to non-specialized readers in a detailed but simplified manner.

Comparative efficacy of Parthenium hysterophorus (L.) derived biochar and iron doped zinc oxide nanoparticle on heavy metals (HMs) mobility and its uptake by Triticum aestivum (L.) in chromite mining contaminated soils

In this study we investigated the efficacy of a novel material parthenium weed (Parthenium hysterophorus L.) biochar (PBC), iron doped zinc oxide nanoparticles (nFe-ZnO), and biochar modified with nFe-ZnO (Fe-ZnO@BC) to adsorb heavy metals (HMs) and reduce their uptake by wheat (Triticum aestivum L.) in a highly chromite mining contaminated soil. The co-application of the applied soil conditioners exhibited a positive effect on the immobilization and restricted the HMs uptake below their threshold levels in shoot content of wheat. The maximum adsorption capacity was because of large surface area, cation exchange capacity, surface precipitation, and complexation of the soil conditioners. The scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) showed porous smooth structure of parthenium weed derived biochar that helped in HMs adsorption, increase the efficiency of soil fertilizers and nutrients retention which help in the enhancement soil condition. Under different application rates the highest translocation factor (TFHMs) was obtained at 2 g nFe-ZnO rate followed the descending order: Mn > Cr > Cu > Ni > Pb. The overall TFHMs was found <1.0 indicating that low content of HMs accumulation in roots from soil slight transferred to shoot, thus satisfying the remediation requirements.

Effect of goethite-loaded montmorillonite on immobilization of metal(loid)s and the micro-ecological soil response in non-ferrous metal smelting areas

In this work, the immobilization stabilization and mechanism of heavy metal(loid)s by goethite loaded montmorillonite (GMt) were investigated, and the soil microbial response was explored. The simulated acid rain leaching experiment showed that GMt had a higher acid tolerance and the more stable heavy metal(loid)s fixation ability. The soil incubation demonstrated that GMt significantly decreased the available Cd, Zn, Pb and As concentration. Interestingly, higher immobilization of heavy metals was observed by GMt in highly acid leached and acidic soils. The richness and diversity of bacterial communities improved after the addition of GMt. GMt induced the enrichment of the excellent functional bacteria of the phylum Proteobacteria as well as the genus Massilia and Sphingomonas. The main immobilization mechanisms of heavy metal(loid)s by GMt include electrostatic interaction, complexation, precipitation and oxidation. The addition of the GMt also optimizes the soil bacterial community structure, which further facilitates the immobilization of heavy metal(loid)s. Our results confirm that the novel GMt has a promising application in the immobilization and stabilization of heavy metal(loid)s contaminated soils in non-ferrous metal smelting areas.

Potential of granular complexes of lime and montmorillonite for stabilizing soil cadmium and the underlying mechanisms

Cadmium (Cd) contaminated soils were widely remediated by alkaline materials in powder, while the effects of granular materials are still unknown. This study was conducted to prepare granular materials based on hydrated lime and montmorillonite with ratios of 1:1, 1:2, and 1:3 (LM1, LM2, and LM3); their effects and mechanisms on stabilizing Cd in hydroponic, pot, and field conditions were further explored. The results showed that powdery materials caused intense pH elevations within 30-60 min and dissolved-Cd reductions within 8-100 min. However, granular materials significantly delayed these effects; the highest solution pH and lowest dissolved-Cd occurred after 250 min. The LM1 granules induced a much higher reduction of dissolved-Cd (99.8%) than that in the LM2 (53.6%) and LM3 granules (14.3%) due to the generation of more cadmium carbonate precipitates. Additionally, the soil pH gradually decreased after an intense elevation induced by powdery materials, but the LM1 granules maintained the soil pH at approximately 7.0, resulting in a lower level of CaCl₂-extractable Cd (0.03 mg kg^-1) than the LM1 powder (0.22 mg kg^-1) after 30 d of cultivation. Similar to lime powder, a small spatial variation (Std. of 3.45) of DGT (diffusive gradient in thin films) extractable Cd in soil profile was observed in the LM1 granules, revealing a homogeneous stabilization effect induced by the LM1 granules. Accordingly, the LM1 granules induced a higher reduction in brown rice Cd (50.9%) than that in the LM1 powders (35.1%). Thus, the granular material of hydrated lime and montmorillonite (1:1) h the potential to replace lime powder in the remediation of Cd-contaminated field.