Removal of Cu and Pb from contaminated agricultural soil using mixed chelators of fulvic acid potassium and citric acid

High-efficiency combination washing agents with eco-friendliness simultaneously removing Cd, Cu and Ni from soil of e-waste recycling site: A lab-scale experiment

Soil washing technology plays an important role in the removal of heavy metals, and the efficacy of this process depends on the washing agent used. Due to the difficulty in treating soils contaminated by multiple heavy metals, there is still a need for further exploration of efficient washing agents with low environmental impact. Although single washing agents, such as chelators, can also effectively remove heavy metals from soil, combining efficient washing agents and determining their optimal washing conditions can effectively improve their removal efficiency for multiple heavy metals in soil simultaneously. Based on the previous research, the present study was carried out to combine different types of washing agents to remediate contaminated soils at a commonly e-waste recycling site. The objectives were to investigate their efficient washing conditions and assess the impact of the washing process on the speciation distribution and pollution level associated with heavy metals in soil. The results showed that the combination of HEDP (1-hydroxyethylidene-1,1-diphosphonic acid) and FeCl3 at a ratio of 6:4 exhibited the most effective removal of Cd, Cu and Ni from the contaminated soil at an e-waste recycling site. Under optimal washing conditions, with a soil-to-liquid ratio of 1:20 and a washing time of 48 h, the removal rates of Cd, Cu and Ni were 96.72%, 69.91% and 76.08%, respectively. It needed to be emphasized that the combination washing agents were able to remove most of the acid-soluble, reducible and oxidizable fractions of heavy metals, and even the removal rates of the stable residual fraction (e.g., of Cd) was at a relatively high level. In addition, the washing process significantly reduced the pollution level associated with heavy metals in soil. This study aid in the development of combined efficient washing agents and explores optimal washing strategies for the remediation of Cd, Cu, and Ni-contaminated soil at e-waste recycling sites. The findings may play a role in enhancing the remediation capabilities for soils contaminated with multiple heavy metals, due to its characteristics of and high-efficiency and environmental friendliness.

Effect of humic substances on the fraction of heavy metal and microbial response

Contamination of soils by Molybdenum (Mo) has raised increasing concern worldwide. Both fulvic acid (FA) and humic acid (HA) possess numerous positive properties, such as large specific surface areas and microporous structure that facilitates the immobilization of the heavy metal in soils. Despite these characteristics, there have been few studies on the microbiology effects of FA and HA. Therefore, this study aimed to assess the Mo immobilization effects of FA and HA, as well as the associated changes in microbial community in Mo-contaminated soils (with application rates of 0%, 0.5% and 1.0%). The result of the incubation demonstrated a decrease in soil pH (from 8.23 ~ 8.94 to 8.05 ~ 8.77). Importantly, both FA and HA reduced the exchangeable fraction and reducible fraction of Mo in the soil, thereby transforming Mo into a more stable form. Furthermore, the application of FA and HA led to an increase in the relative abundance of Actinobacteriota and Firmicutes, resulting in alterations to the microbial community structure. However, it is worth noting that due to the differing structures and properties of FA and HA, these outcomes were not entirely consistent. In summary, the aging of FA and HA in soil enhanced their capacity to immobilization Mo as a soil amendment. This suggests that they have the potential to serve as effective amendments for the remediation of Mo-contaminated soils.

Remediation of the soil contaminated by heavy metals with nano-hydroxy iron phosphate coated with fulvic acid

Heavy metals pose a serious threat and damage to ecological health when released into the environment. n-HFP is usually used to remediate soils contaminated with heavy metals, but its ability to solidify heavy metals is limited. FA has good ability to trap heavy metals due to its abundant oxygen-containing functional groups. However, the solubility of FA in water limits its application in the field of heavy metal removal. In this paper, n-HFP@FA was prepared by co-precipitation method. Through FT-IR and BET analysis, the oxygen-containing functional groups and specific surface area of n-HFP@FA increased due to the addition of FA. The adsorption behaviour of n-HFP@FA on Pb, Cd, and Cu followed the pseudo-second-order and Langmuir isotherm models. In addition, the maximum adsorption capacities of n-HFP@FA for Pb, Cd, and Cu were 371.1, 190.5, and 129.75 mg/g, respectively. As shown by FT-IR and XPS analysis, the main mechanisms of Pb, Cd and Cu removal by n-HFP@FA are: complexation, electrostatic and precipitation. The n-HFP@FA showed high removal rates of Pb, Cd, and Cu in soil leachates of different pH. In the soil remediation experiments, the BCR method and Pearson correlation analysis showed that the acid-soluble, reducible and oxidizable fractions of Pb, Cd, and Cu in the soil were effectively converted into a more stable residual fraction. This study opens up a prospect for the application of n-HFP@FA composites in the remediation of contaminated soil.

Multi-factor correlation analysis of the effect of root-promoting practices on tobacco rhizosphere microecology in growth stages

Some agronomic practices not only promote the development of crop roots and increase overall plant performance but also affect colonisation by rhizosphere microorganisms. However, the composition and temporal dynamics of the tobacco rhizosphere microbiota under different root-promoting practices are poorly understood. Here, we characterised the tobacco rhizosphere microbiota at the knee-high, vigorous growing, and maturity stages under the application of potassium fulvic acid (PFA), γ-Polyglutamic acid (PGA), soymilk root irrigation (SRI), and conventional fertilization (CK) and its correlation with root characteristics and soil nutrients. The results showed that three root-promoting practices notably improved the dry and fresh root weights. Total nitrogen and phosphorus, available phosphorus and potassium, and organic matter contents in the rhizosphere markedly increased at the vigorous growing stage. The rhizosphere microbiota was changed through root-promoting practices. However, with tobacco growth, the change of rhizosphere microbiota showed a pattern of slow first and then fast and the microbiota of different treatments gradually approached. SRI reduced plant-pathogenic fungi but increased chemoheterotrophic and phototrophic bacteria, and arbuscular mycorrhizal fungi. PFA and PGA markedly increased arbuscular mycorrhizal and ectomycorrhizal fungi at the knee-high stage, which benefitted tobacco nutrient absorption. The correlation between rhizosphere microorganisms and environmental factors varied at different growth stages. Notably, the rhizosphere microbiota was more sensitive to environmental factors at the vigorous growing stage, and the interactions were more complex than in other stages. Furthermore, a variance partitioning analysis showed that the influence of root-soil interaction on the rhizosphere microbiota increased with tobacco growth. Overall, all three root-promoting practices could improve root characteristics, rhizosphere nutrient, and rhizosphere microbiota to varying degrees and increase the tobacco biomass, among which PGA had the most obvious effect and most suitable for tobacco cultivation. Our findings revealed the role of root-promoting practices in shaping the rhizosphere microbiota during plant growth and elucidated the assembly patterns and environmental drivers of crop rhizosphere microbiota driven by the application of root-promoting practices in agricultural production.

Effects of Different Sulfur Compounds on the Distribution Characteristics of Subcellular Lead Content in Arabis alpina L. var. parviflora Franch under Lead Stress

Sulfur plays a vital role in the phytoremediation of lead-contaminated soil. The effects of different sulfur forms (S Na₂S, and Na₂SO₄) on lead (Pb) absorption in hyperaccumulator Arabis alpina L. var. parviflora Franch were studied in a soil pot experiment. The subcellular sulfur and lead enrichment characteristics in A. alpina were studied by adding sulfur in different forms and concentrations (0, 75, and 150 mg·kg^-1) to Pb-contaminated soil. The results show that the root and shoot biomass increased by 1.94 times under Na₂S and Na₂SO₄ treatment, and the root-shoot ratio of A. alpina increased 1.62 times under the three forms of sulfur treatments, compared with the control. Sulfur content in cell walls and soluble fractions of the root and shoot of A. alpina significantly increased 3.35~5.75 times and decreased 5.85 and 9.28 times in the organelles under 150 mg·kg^-1 Na₂SO₄ treatment. Meanwhile, Pb content in the root and shoot cell walls of A. alpina significantly increased by 3.54 and 2.75 times, respectively. Pb content in the shoot soluble fraction increased by 3.46 times, while it significantly reduced by 3.78 times in the shoot organelle. Pb content in the root organelle and soluble fraction decreased by 2.72 and 2.46 times. Different forms and concentrations of sulfur had no regularity in the effect of Pb and sulfur content in the subcellular components of A. alpina, but the bioconcentration and translocation factors of A. alpina increased compared with the control. Under different concentrations of Na₂SO₄, there was a significant positive correlation between the contents of sulfur and Pb in the subcellular components of the root of A. alpina (p < 0.05). These results indicate that sulfur application can enhance the Pb resistance of A. alpina by strengthening the cell wall fixation and vacuolar compartmentalization.

Co-high-efficiency washing agents for simultaneous removal of Cd, Pb and As from smelting soil with risk assessment

Soil washing is considered a highly efficient technology due to its higher removal rate of multiple heavy metals from contaminated soil. However, previous studies on Cd, Pb and As washing agents for soils with complex contaminations did not consider the differences in As and Cd/Pb properties, resulting in the lack of effective washing compounds and washing conditions for soils with complex contaminations. Moreover, most traditional washing agents can cause secondary pollution. In this study, HEDTA and lactic acid (LA) treatments resulted in a higher Cd and Pb removal, while 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) was more effective in As removal. Most importantly, a new washing strategy was proposed with a new combined high-efficiency washing agents consisting of HEDP + LA + FeCl₃ with a ratio of 6:3:1. Considering washing efficiency and consumption under optimal washing conditions, i.e. the soil/liquid (S/L) ratio of 1:20 and washing time of 48 h, the rates of Cd, Pb and As removal were 79.93%, 69.84% and 61.55%, respectively. In addition, washing process could influence the speciation of heavy metals, especially oxidizable and residual Cd and Pb fractions, as well as reducible As fraction. The washing process using the new washing agent can significantly reduce the pollution level and health risk of Cd, Pb and As contamination. The results of this study can provide an efficient washing agent for the remediation of heavy metal-contaminated soils at smelting sites, which will help protect human health.

Effects of potassium fulvic acid and potassium humate on microbial biodiversity in bulk soil and rhizosphere soil of Panax ginseng

Potassium fulvic acid (BSFA) and potassium humate (KHM), as organic fertilizers, can improve soil structure, increase soil nutrient levels and prevent plant diseases. However, knowledge is limited regarding how BSFA and KHM influence soil microbial communities and the interrelationships between community members associated with Panax ginseng. Soil pH and nutrient content increased significantly as a result of the addition of BSFA and KHM. The pH, NH₄⁺-N, NO₃^--N, AP and AK increased by 1.72 %-5.55 %, 70.09 %-108.39 %, 35.38 %-216.20 %, 1.21 %-14.19 % and 3.40 %-5.94 %, respectively, in the BSFA and KHM treatments. The soil nutrient increase may be related to Micrococcaceae and arbuscular mycorrhizal fungi (AMF). The structure of the microbial community also changed radically from that of the control group, and Chloroflexi (2.69 %-3.15 %), Actinobacteria (4.33 %-7.53 %) and Acidobacteria (9.44 %-11.62 %) were the dominant microorganisms at the phylum level in bacteria. In contrast, the dominant fungi at the phylum level were Ascomycota (77.39 %-78.08 %), Glomeromycota (0.36 %-2.68), Olpidiomycota (0.02 %-3.78 %) and Basidiomycota (0.80 %-1.17 %). Fusarium oxysporum and Ascomycota were biomarkers for BSFA and KHM, which may be related to pathogenic bacteria. Network analysis revealed that the association among members of the soil microbial community was more positive than negative following application of KHM, and more positive (62.5 %) than negative (37.5 %) correlations were observed between bacteria, whereas the fungal community exhibited more positive (97.3 %) than negative (2.7 %) correlations. PICRUST predicted the microbial function of adding KHM and BSFA to the soil, and these pathways mainly belong to the degradation and metabolism of organic matter, saprophytic organisms and plant pathogens. In summary, our study demonstrated that the addition of BSFA and KHM increased the nutrients in the ginseng soil and reshaped the microbial function in soils, providing a theoretical foundation for soil improvement and biological control of ginseng diseases. However, due to the limitations of greenhouse cultivation, additional long-term experiments on farmland with different climate changes are recommended.

A spectral characteristic analysis method for distinguishing heavy metal pollution in crops: VMD-PCA-SVM

Exploring the characteristics and types of heavy metal pollution in crops has important implications for food security and human health. In this study, a method for distinguishing heavy metal-polluted elements in corn leaves was proposed. Based on the spectral data obtained from corn leaves polluted by Cu and Pb, the spectra were divided into four characteristic regions. Variational mode decomposition (VMD) was used to decompose the first-order differential spectrum, and the characteristic analysis was transformed from the spectral domain to the frequency domain. Each modal component was processed separately using principal components analysis (PCA) according to the different characteristic regions to obtain the main information on the pollution characteristics, and then a two-dimensional space was constructed to identify the differential characteristics of corn under Cu and Pb stress visually. Finally, the support vector machine (SVM) classifier was used to get the classification line model to distinguish Cu and Pb pollution. This method was named VMD-PCA-SVM. The results show that the method can highlight the spectral response characteristics of heavy metal pollution, which is expected to guide the rapid and non-destructive identification of heavy metal pollution in crops and the formulation of remediation strategies.