Field evaluation of intensive compost application on Cd fractionation and phytoavailability in a mining-contaminated soil

Unravelling the molecular and biochemical responses in cotton plants to biochar and biofertilizer amendments for Pb toxicity mitigation

Over the past few years, there has been a rising interest in employing biochar (BC) and biofertilizers (BF) as a means of restoring soils that have been polluted by heavy metals. The primary objective of this study was to examine how the application of BC and BF affects the ability of cotton plants to withstand Pb toxicity at varying concentrations (0, 500, and 1000 mg/kg soil). The findings revealed that exposure to Pb stress, particularly at the 1000 mg/kg level, led to a decline in the growth and biomass of cotton plants. Pb toxicity triggered oxidative damage, impaired the photosynthetic apparatus, and diminished the levels of photosynthetic pigments. By increasing the expression of Rubisco-S, Rubisco-L, P5CR, and PRP5 genes and regulating proline metabolism, BC and BF increased the levels of proline and photosynthetic pigments and protected the photosynthetic apparatus. The application of BC and BF resulted in an upregulation of genes such as CuZnSOD, FeSOD, and APX1, as well as an increase in the activity of the glyoxalase system and antioxidant enzymes. These changes enhanced the antioxidant capacity of the plants and provided protection to membrane lipids from oxidative stress caused by Pb. The inclusion of BC and BF offered protection to photosynthesis and other essential intracellular processes in leaves by minimizing the transfer of Pb to leaves and promoting the accumulation of thiol compounds. This protective effect helped mitigate the negative impact of the toxic metal Pb on leaf function. By improving plant tolerance, reducing metal transfer, strengthening the antioxidant defense system, and enhancing the level of protective substances, these amendments show promise as valuable tools in tackling heavy metal pollution.

Evaluation of three common alkaline agents for immobilization of multi-metals in a field-contaminated acidic soil

We investigated three common alkaline agents (NaOH, CaO, and Mg(OH)₂) for immobilization of four heavy metals (Pb, Zn, Cu, and Cd) in a field-contaminated soil and elucidated the underpinning principles. NaOH caused the highest pH spike in the soil, while CaO and Mg(OH)₂ served as a longer-lasting source of OH^-. Amending the soil with CaO or Mg(OH)₂ at ≥0.1 mol as OH^- (kg·soil)^-1 for 24 h was able to immobilize all four metals, while NaOH failed. NaOH leached up to 3 times more organic carbon than CaO and Mg(OH)₂, resulting in elevated leachability of the metals. Column elution tests showed that amendments by CaO and Mg(OH)₂ lowered the leachable Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺ by 52-54%, 71-75%, 69-73%, and 68%, respectively, after 1440 pore volumes of elution. Sequential extraction revealed that the soil amendments converted the exchangeable fraction of the metals to the much less available forms. XRD and FTIR analyses indicated that formation of metal oxide precipitates and complexation with soil organic matter were responsible for the metals immobilization. Taken together the chemical cost, technical effectiveness, and environmental impact, CaO is the most suitable alkaline agent for remediation of soil contaminated with heavy metals.

Geochemical Anomaly Characteristics of Cd in Soils around Abandoned Lime Mines: Evidence from Multiple Technical Methods

Lime mines are a potential source of pollution, and the surrounding soil environment is generally at threat, especially in abandoned lime mines. This paper focuses on the study area in eastern Anhui, attempting to analyze whether Cd enrichment is related to abandoned mines. On the basis of geological investigation, this study systematically used XRD, XRF, GTS and universal Kriging interpolation to determine the distribution law of Cd in the study area, and evaluated the potential ecological risk of Cd. The results showed that the main mineral types of soil samples of red clastic rock soil parent material (RdcPm) and soil samples of carbonate soil parent material (CPm) were not completely the same. Correlation analysis showed that CaO, MgO and Cd were positively correlated with the CPm. Human activities led to the accumulation of Cd in the study area. High Cd was mainly concentrated in the northwest of the study area, which was correlated with abandoned mines and soil parent materials. The study area was dominated by slight potential risks, although some areas had medium potential risks and high potential risks. All potential high risks were in the CPm field. This study provides a scientific basis for the comprehensive utilization and development planning of soil in the study area.

Energetic Value of Elymus elongatus L. and Zea mays L. Grown on Soil Polluted with Ni2+, Co2+, Cd2+, and Sensitivity of Rhizospheric Bacteria to Heavy Metals

Plants, and microorganisms associated with them, offer an effective tool for removing pollutants, such as heavy metals, from the soil environment. The aim of this study was to determine changes caused by Ni2+, Co2+, and Cd2+ in the genetic diversity of soil-populating bacteria and the effect these heavy metals on the heating value of elongated coach grass (Elymus elongatus L.) and maize (Zea mays L.). Microorganisms support plants in removing heavy metals from soil. These plants can then be used for energetic purposes. The study aim was accomplished by determining counts of microorganisms and their resistance (RS) to Ni2+, Co2+, Cd2+, their colony development index (CD), ecophysiological diversity index (EP), and diversity established with the next generation sequencing (NGS) method. Further analyses aimed to establish test plants resistance to pollution with heavy metals and their heating value. Organotrophic bacteria turned out to be the most resistant to Co2+, whereas actinobacteria—to Cd2+ effects. At all taxonomic levels, the genetic diversity of bacteria was most adversely influenced by Cd2+ in the soil sown with Zea mays L. Bacteria belonging to Arthrobacter, Rhodoplanes, Kaistobacter, Devosia, Phycicoccus, and Thermomonas genera showed high tolerance to soil pollution with Ni2+, Co2+, and Cd2+, hence they should be perceived as potential sources of microorganisms useful for bioaugmentation of soils polluted with these heavy metals. Ni2+, Co2+, and Cd2+ had no effect on the heating value of Elymus elongatus L. and Zea mays L. The heating value of 1 kg of air-dry biomass of the tested plants was relatively high and ranged from 14.6 to 15.1 MJ. Elymus elongatus L. proved more useful in phytoremediation than Zea mays L.

The Response of the Soil Microbiome to Contamination with Cadmium, Cobalt and Nickel in Soil Sown with Brassica napus

Soil fertility is determined by biological diversity at all levels of life, from genes to entire biocenoses. The aim of this study was to evaluate bacterial diversity in soil contaminated with Cd2+, Co2+ and Ni2+ and sown with Brassica napus. This is an important consideration because soil-dwelling microorganisms support phytoremediation and minimize the adverse effects of heavy metals on the environment. Microbial counts, the influence (IFHM) of Cd2+, Co2+ and Ni2+ on microorganisms, the colony development (CD) index, the ecophysiological diversity (EP) index and genetic diversity of bacteria were determined under controlled conditions. Soil contamination with Cd2+, Co2+ and Ni2+ significantly influenced microbial diversity and increased the values of CD and EP indices. The tested heavy metals decreased the genetic diversity of bacteria, in particular in the phyla Actinobacteria and Proteobacteria. Bacteria of the genera Arthrobacter, Devosia, Kaistobacter, Paenibacillus, Phycicoccus, Rhodoplanes and Thermomonas were identified in both contaminated and non-contaminated soil. These bacteria are highly resistant to soil contamination with Cd2+, Co2+ and Ni2+.

Effects of biochar and biofertilizer on cadmium-contaminated cotton growth and the antioxidative defense system

Consistent use of large amounts of fertilizers, pesticides, and mulch can cause the accumulation of harmful substances in cotton plants. Among these harmful substances, cadmium (Cd), an undegradable element, stands out as being particularly highly toxic to plants. The objective of this study was to evaluate the ability of biochar (3%) and biofertilizer (1.5%) to decrease Cd uptake, increase cotton dry weight, and modulate the activities of photosynthetic and peroxidase (POD), superoxide dismutase (SOD), catalase enzyme (CAT) in cotton (Gossypium hirsutum L.) grown in Cd-contaminated soil (0, 1, 2, or 4 mg Cd kg-1 soil) in pots. These studies showed that, as expected, exogenous Cd adversely affects cotton chlorophyll and photosynthesis. However, biochar and biofertilizer increased cotton dry weight by an average of 16.82% and 32.62%, respectively. Meanwhile, biochar and biofertilizer decreased the accumulation of Cd in cotton organs, and there was a significant reduction in the amount of Cd in bolls (P < 0.05). Biochar and biofertilizer have a positive impact on cotton chlorophyll content, net photosynthesis, stomatal conductance, transpiration rate, and intercellular CO2 concentration. Thus, the addition of biochar and biofertilizer promote cotton growth. However, biochar and biofertilizer increased the SOD activity of leaves (47.70% and 77.21%), CAT activity of leaves (35.40% and 72.82%), SOD activity of roots (33.62% and 39.37%), and CAT activity of roots (36.91% and 60.29%), respectively, and the addition of biochar and biofertilizer decreased the content of MDA and electrolyte leakage rate. Redundancy analyses showed that biochar and biofertilizer also improved SOD and POD activities by reducing the heavy metal-induced oxidative stress in cotton and reducing Cd uptake in cotton organs. Therefore, biochar and biofertilizer have a positive effect on the growth of cotton.

Cadmium Uptake by Wheat (Triticum aestivum L.): An Overview

Cadmium is a toxic heavy metal that may be detected in soils and plants. Wheat, as a food consumed by 60% of the world’s population, may uptake a high quantity of Cd through its roots and translocate Cd to the shoots and grains thus posing risks to human health. Therefore, we tried to explore the journey of Cd in wheat via a review of several papers. Cadmium may reach the root cells by some transporters (such as zinc-regulated transporter/iron-regulated transporter-like protein, low-affinity calcium transporters, and natural resistance-associated macrophages), and some cation channels or Cd chelates via yellow stripe 1-like proteins. In addition, some of the effective factors regarding Cd uptake into wheat, such as pH, organic matter, cation exchange capacity (CEC), Fe and Mn oxide content, and soil texture (clay content), were investigated in this paper. Increasing Fe and Mn oxide content and clay minerals may decrease the Cd uptake by plants, whereas reducing pH and CEC may increase it. In addition, the feasibility of methods to diminish Cd accumulation in wheat was studied. Amongst agronomic approaches for decreasing the uptake of Cd by wheat, using organic amendments is most effective. Using biochar might reduce the Cd accumulation in wheat grains by up to 97.8%.

Retraction Note: Field evaluation of intensive compost application on Cd fractionation and phytoavailability in a mining-contaminated soil

The Editor-in-Chief has retracted this article (Li et al. 2016). An investigation by the Journal has not been able to confirm the identity and affiliation of the author David Raleve. This author was the corresponding author on submission but changed the corresponding authorship at the proof stage. The current corresponding author was not aware of the publication of this article. As the appropriate authorship for this article cannot be determined, the Editor-in-Chief no longer has confidence in this article. Ibrahim Mohamed disagrees with this retraction. Ming Li and Wenli Chen did not respond to any correspondence about this retraction. Qiaoyun Huang did not respond to any correspondence about this retraction notice.

Soil amendments for immobilization of potentially toxic elements in contaminated soils: A critical review

Soil contamination by potentially toxic elements (PTEs) has led to adverse environmental impacts. In this review, we discussed remediation of PTEs contaminated soils through immobilization techniques using different soil amendments with respect to type of element, soil, and amendment, immobilization efficiency, underlying mechanisms, and field applicability. Soil amendments such as manure, compost, biochar, clay minerals, phosphate compounds, coal fly ash, and liming materials are widely used as immobilizing agents for PTEs. Among these soil amendments, biochar has attracted increased interest over the past few years because of its promising surface properties. Integrated application of appropriate amendments is also recommended to maximize their use efficiency. These amendments can reduce PTE bioavailability in soils through diverse mechanisms such as precipitation, complexation, redox reactions, ion exchange, and electrostatic interaction. However, soil properties such as soil pH, and clay, sesquioxides and organic matter content, and processes, such as sorption/desorption and redox processes, are the key factors governing the amendments' efficacy for PTEs immobilization in soils. Selecting proper immobilizing agents can yield cost-effective remediation techniques and fulfill green and sustainable remediation principles. Furthermore, long-term stability of immobilized PTE compounds and the environmental impacts and cost effectiveness of the amendments should be considered before application.

Influence of amendments on metal environmental and toxicological availability in highly contaminated brownfield and agricultural soils

The immobilizing effects of wood biochar (BW2%) and iron grit (Z1%) applied alone or in combination (BW2% + Z1%) to agricultural (M750) and brownfield (MAZ) soils highly contaminated by metals were assessed in a greenhouse experiment. The results showed that Z1% and BW2% + Z1% were the most efficient amendments to reduce Cd, Cu, Pb, and Zn mobility, environmental availability, and phytoavailability in the M750 soil. The oxidation of Z1% allowed part of the Cu and Zn pools present in exchangeable or carbonate-bound forms (labile fraction) to complex in less mobile forms. In this soil, the metal chemical extractions (0.01 M CaCl₂ and 0.05 M EDTA) and the DGT (diffusive gradient in thin films) devices to assess metal in soil solution and soil pore water (SPW) also highlighted the immobilizing characteristic of Z1%. In most cases, the addition of BW2% to Z1% (BW2% + Z1%) did not improve this effect, except for the dissolved Pb and Zn concentrations in the M750 soil solution. It was also observed that Cd, Pb, and Zn passed throughout DGT mimicking the biological cell membrane were reduced by all amendments of the M750 soil corroborating metal concentrations measured in rye grass shoots. In the MAZ soil, metals were less available as shown by their low extractability rate, low capacity of metal resupply from the solid phase to pore water, and low phytoavailability. The poor metal availability could be explained by the high levels of carbonate and organic matter contents in this soil. Nevertheless, a decrease of the Cu environmental availability and the Cu concentrations in rye grass shoots grown on the MAZ soil was also observed in the soil amended with Z1% alone or in combination with BW2%. From a health point of view, the most effective amendment to reduce human exposure through ingestion of soil particles for the M750 and MAZ soils was BW2% for Cd and BW2% + Z1% for Pb. However, the presence of rye grass minimized the amendments' beneficial effects.

Inhibition of Cd accumulation in winter wheat (Triticum aestivum L.) grown in alkaline soil using mercapto-modified attapulgite

Cd contamination in agricultural soils threatens the safety of agricultural products and poses human health risk via food chain. However, the remediation of Cd polluted alkaline soils has not drawn the public concern, and the corresponding efficient amendments that can reduce Cd accumulation in crop grains are relatively few. In current study, mercapto-modified attapulgite (MA in abbreviation) was selected as the amendment to conduct winter wheat (Triticum aestivum L.) cultivation pot experiment to investigate the effect of MA on Cd accumulation in winter wheat and Cd bioavailability in alkaline soil. MA had no adverse impact on the normal growth of winter wheat but could inhibit Cd accumulation in wheat grain of both cultivars grown in alkaline soil with a maximum reduction of 75%, while pH-regulating amendment sepiolite had no reduction effect. In the term of soil chemistry, MA could decrease the zeta potential of soil particles and enhance the sorption amount of Cd on soil particles, resulted in the increase of Fe-Mn-oxides bounded Cd fraction in alkaline soil. The enhanced sorption effect combined with complexation effect of MA itself, made the exchangeable and bioavailable Cd concentrations in the soil decrease. In the term of plant uptake, MA could inhibit the uptake of Cd via roots from the soil, and hinder Cd transfer from roots to grains. MA had environmental friendliness and capability in the aspect of soil pH, effective cation exchange capacity and available micronutrients in the soil. The high performance of MA in inhabitation of Cd in winter wheat revealed that it was an efficient immobilization agent with great application potential for Cd-polluted alkaline soil.

Bio-organic stabilizing agent shows promising prospect for the stabilization of cadmium in contaminated farmland soil

In situ immobilization of cadmium (Cd) has been considered as a cost-effective and non-disruptive remediation technique for Cd-contaminated soils. In this study, several immobilization approaches were compared in a Cd-contaminated agricultural farmland. The soil was treated with different combinations of the immobilizing agents such as biochar (C), rice straw (RS), lime (L), and engineered bacteria P. putida X4/pIME (B). The plant yield and Cd uptake of lettuce as well as soil Cd fractionations were measured. The Cd content in lettuce leaves and roots decreased by 46.8~67.2% and 36.8~60.2%, respectively. Among the five treatments, combined rice straw, lime, and engineered bacteria treatment showed the lowest Cd concentration in lettuce leaves (0.14 mg/kg) and the highest plant yield (21.5 t/ha). The alleviating effects are assigned to the significant transformation of water soluble and exchangeable Cd to humic substance bound, strong organic bound and residual Cd in the soils. This study suggests that this bio-organic stabilizing agent is more cost-effective than some other immobilization agents reported previously, and shows a great application prospect in improving agriculture production of heavy metal-polluted agricultural soils.

An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade

Soil contamination by heavy metals and metalloids has been a major concern to human health and environmental quality. While many remediation technologies have been tested at the bench scale, there have been only limited reports at the field scale. This paper aimed to provide a comprehensive overview on the field applications of various soil remediation technologies performed over the last decade or so. Under the general categories of physical, chemical, and biological approaches, ten remediation techniques were critically reviewed. The technical feasibility and economic effectiveness were evaluated, and the pros and cons were appraised. In addition, attention was placed to the environmental impacts of the remediation practices and long-term stability of the contaminants, which should be taken into account in the establishment of remediation goals and environmental criteria. Moreover, key knowledge gaps and practical challenges are identified.

Influence of Zn, Cd, and Cu fractions on enzymatic activity of arable soils

The aim of this study was to investigate the heavy metal effect on enzymatic activity in acidic soil samples during spring, summer, and autumn. The four metal fractions, acid-soluble and exchangeable (F1), reducible (F2), oxidizable (F3), and residual (F4) using BCR method in soil samples, were evaluated. The highest percentage share of zinc and copper was determined in F4 (45.8, 54.9%, respectively) and cadmium in F3 (45.6%). The enzymatic activity in soil was differentiated in seasons. During spring, the significant relationship was noted between F1/zinc/dehydrogenase, during summer F2/cadmium/phosphatase as well as F4/cadmium/dehydrogenase and autumn F3/zinc/dehydrogenase. Fraction F1/zinc/copper influenced phosphatase activity, whereas F3/Zn increased dehydrogenase and F2/Cd protease activity. The results indicate that the heavy metals affected dehydrogenase activity the most.

Effects of Pulp and Na-Bentonite Amendments on the Mobility of Trace Elements, Soil Enzymes Activity and Microbial Parameters under Ex Situ Aided Phytostabilization

The objective of this study was to explore the potential use of pulp (by-product) from coffee processing and Na-bentonite (commercial product) for minimizing the environmental risk of Zn, Pb and Cd in soil collected from a former mine and zinc-lead smelter. The effects of soil amendments on the physicochemical properties of soil, the structural and functional diversity of the soil microbiome as well as soil enzymes were investigated. Moreover, biomass of Festuca arundinacea Schreb. (cultivar Asterix) and the uptake of trace elements in plant tissues were studied. The outdoor pot set contained the following soils: control soil (initial), untreated soil (without additives) with grass cultivation and soils treated (with additives) with and without plant development. All of the selected parameters were measured at the beginning of the experiment (t₀), after 2 months of chemical stabilization (t₂) and at the end of the aided phytostabilization process (t₁₄). The obtained results indicated that both amendments efficiently immobilized the bioavailable fractions of Zn (87–91%) and Cd (70–83%) at t₁₄; however, they were characterized by a lower ability to bind Pb (33–50%). Pulp and Na-bentonite drastically increased the activity of dehydrogenase (70- and 12-fold, respectively) at t₁₄, while the activities of urease, acid and alkaline phosphatases differed significantly depending on the type of material that was added into the soil. Generally, the activities of these enzymes increased; however, the increase was greater for pulp (3.5-6-fold) than for the Na-bentonite treatment (1.3–2.2-fold) as compared to the control. Soil additives significantly influenced the composition and dynamics of the soil microbial biomass over the experiment. At the end, the contribution of microbial groups could be ordered as follows: gram negative bacteria, fungi, gram positive bacteria, actinomycetes regardless of the type of soil enrichment. Conversely, the shift in the functional diversity of the microorganisms in the treated soils mainly resulted from plant cultivation. Meanwhile, the highest biomass of plants at t₁₄ was collected from the soil with Na-bentonite (6.7 g dw^-1), while it was much lower in a case of pulp treatment (1.43–1.57 g dw^-1). Moreover, the measurements of the heavy metal concentrations in the plant roots and shoots clearly indicated that the plants mainly accumulated metals in the roots but that the accumulation of individual metals depended on the soil additives. The efficiency of the accumulation of Pb, Cd and Zn by the roots was determined to be 124, 100 and 26% higher in the soil that was enriched with Na-bentonite in comparison with the soil that was amended with pulp, respectively. The values of the soil indices (soil fertility, soil quality and soil alteration) confirmed the better improvement of soil functioning after its enrichment with the pulp than in the presence of Na-bentonite.