Comparison of the feasibility of different washing solutions for combined soil washing and phytoremediation for the detoxification of cadmium (Cd) and zinc (Zn) in contaminated soil

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.

Chelate facilitated phytoextraction of Pb, Cd, and Zn from a lead-zinc mine contaminated soil by three accumulator plants

This study aims to evaluate the enhancement of phytoextraction of heavy metals (Pb, Cd, and Zn) by species Marrubium cuneatum, Stipa arabica, and Verbascum speciosum, through EDTA amendment. Assisted phytoextraction pot experiments were performed at different EDTA dosages (0, 1, 3, and 5 mmol kg-1 soil). The DTPA-extractable metal content increased in the presence of EDTA, followed by their contents in the tissues of all three studied species. Resulting from oxidative stress, the activity of antioxidant enzymes such as glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT) increased when the chelating agent was added. EDTA in higher doses partially decreased chlorophyll concentration, and 5 mmol kg-1 of that reduced the biomass of the studied species. The bioconcentration factor (BCF) for Cd was notably high in all studied plants and considerably elevated for Zn and Pb with the addition of EDTA in M. cuneatum and S. arabica (BCF > 1), whilst an accumulation factor greater than one (AF > 1) was found for Cd in all species and for Pb in the case of S. arabica. In general, the results demonstrated that EDTA can be an effective amendment for phytoextraction of Cd, Zn, and Pb by M. cuneatum, V. speciosum and S. arabica in contaminated soils.

Impact of soil treatment with Nitrilo Triacetic Acid (NTA) on Cd fractionation and microbial biomass in cultivated and uncultivated calcareous soil

Purpose

The aim of this study was to evaluate the effectiveness of nitrilotriacetic acid (NTA) on cadmium (Cd) fractions and microbial biomass in a calcareous soil spiked with Cd under cultivated (Zea mays L.) and uncultivated regime subject to soil leaching condition. Expanding investigations related to soil-plant interactions on metal-contaminated soils with insights on microbial activity and associated soil toxicity perspective provides novel perspectives on using metal-chelating agents for soil remediation.

Methods

The experimental factors were three levels of Cd contamination (0, 25, and 50 mg kg-1 soil) and three levels of NTA (0, 15, and 30 mmol L-1) in loamy soil under maize-cultured and non-cultured conditions. During the experiment, the adding NTA and leaching processes were performed three times.

Results

The results showed that the amount of leached Cd decreased in cultivated soil compared to uncultivated soil due to partial uptake of soluble Cd by plant roots and changes in Cd fractions in soil, so that Cd leached in Cd50NTA30 was 9.2 and 6.1 mg L-1, respectively, in uncultivated and cultivated soils. Also, Cd leached in Cd25NTA30 was 5.7 and 3.1 mg L-1 respectively, in uncultivated and cultivated soils. The best treatment in terms of chemical and microbial characteristics of the soil with the high percentage of Cd removed from the soil was Cd25NTA30 in cultivated soil. In Cd25NTA30 compared to Cd25NTA0 in cultivated soil, pH (0.25 unit), microbial biomass carbon (MBC, 65.0 mg kg-1), and soil respiration (27.5 mg C-CO2 kg-1 24 h-1) decreased, while metabolic quotient (qCO2, 0.05) and dissolved organic carbon (DOC, 20.0 mg L-1) increased. Moreover, the changes of Cd fractions in Cd25NTA30 in cultivated soil compared to uncultivated soil were as follows; the exchangeable Cd (F1, 0.27 mg kg-1) and Fe/Mn-oxide-bounded Cd (F4, 0.15 mg kg-1) fractions increased, in contrast, carbonate-Cd (F2, 2.67 mg kg-1) and, organically bounded Cd (F3, 0.06 mg kg-1) fractions decreased. NTA had no significant effect on the residual fraction (F5).

Conclusion

The use of NTA, especially in calcareous soils, where most of the Cd is bound to calcium carbonate, was able to successfully convert insoluble fractions of Cd into soluble forms and increase the removal efficiency of Cd in the phytoremediation method. NTA is a non-toxic chelating agent to improve the accumulation of Cd in maize.

Best management practices for minimizing undesired effects of thermal remediation and soil washing on soil properties. A review

The use of remediated soils as end-of-life materials raises some challenges including policy and regulation, permits and specifications, technological limitations, knowledge and information, costs, as well as quality and performance associated with using them. Therefore, a set of procedures must be followed to preserve the quality and fundamental properties of soil during a remediation process. This study presented a comprehensive review regarding the fundamental impacts of thermal desorption (TD) and soil washing (SW) on soil characteristics. The effects of main operating parameters of TD and SW on the physical, chemical, and biological properties of soil were systematically reviewed. In TD, temperature has a more remarkable effect on physic-chemical and biological characteristics of soil than heating time. Therefore, decrease in temperature within a suitable range prevents unreversible changes on soil properties. In SW, more attention should be paid to extraction process of contaminants from soil particles. Using the right dosage and type of chelating agents, surfactants, solvents, and other additives can help to avoid problems with recovery or treatment using conventional methods. In addition, this review introduced a framework for implementing sustainable remediation approaches based on a holistic approach to best management practices (BMPs), which, besides reducing the risks associated with different pollutants, might provide new horizons for decreasing the unfavourable impacts of TD and SW on soil.

Biochar applications for treating potentially toxic elements (PTEs) contaminated soils and water: a review

Environmental pollution with potentially toxic elements (PTEs) has become one of the critical and pressing issues worldwide. Although these pollutants occur naturally in the environment, their concentrations are continuously increasing, probably as a consequence of anthropic activities. They are very toxic even at very low concentrations and hence cause undesirable ecological impacts. Thus, the cleanup of polluted soils and water has become an obligation to ensure the safe handling of the available natural resources. Several remediation technologies can be followed to attain successful remediation, i.e., chemical, physical, and biological procedures; yet many of these techniques are expensive and/or may have negative impacts on the surroundings. Recycling agricultural wastes still represents the most promising economical, safe, and successful approach to achieving a healthy and sustainable environment. Briefly, biochar acts as an efficient biosorbent for many PTEs in soils and waters. Furthermore, biochar can considerably reduce concentrations of herbicides in solutions. This review article explains the main reasons for the increasing levels of potentially toxic elements in the environment and their negative impacts on the ecosystem. Moreover, it briefly describes the advantages and disadvantages of using conventional methods for soil and water remediation then clarifies the reasons for using biochar in the clean-up practice of polluted soils and waters, either solely or in combination with other methods such as phytoremediation and soil washing technologies to attain more efficient remediation protocols for the removal of some PTEs, e.g., Cr and As from soils and water.

Green remediation of Ni, Zn, and Cu in an electroplating contaminated site by wood vinegar with optimization and risk assessment

Wood vinegar (WV) is a renewable organic compound, possessing characteristics such as high oxygenated compound content and low negative impact on soil. Based on its weak acid properties and complexing ability to potentially toxic elements (PTEs), WV was used to leach Ni, Zn, and Cu contaminated soil in electroplating sites. In addition, the response surface methodology (RSM) based on the Box-Behnken design (BBD) was established to clarify the interaction between each single factor, and finally completed the risk assessment of the soil. The amounts of PTEs leached from the soil climbed with the increase of WV concentration, liquid-solid ratio, and leaching time, while they surged with the decrease of pH. Under optimal leaching circumstances (the concentration of WV= 100 %; washing time= 919 min; pH= 1.00), the removal rates of Ni, Zn, and Cu could reach 91.7 %, 57.8 %, and 65.0 %, respectively, and the WV-extracted PTEs were mainly from the Fe-Mn oxides fraction. After leaching, the Nemerow integrated pollution index (NIPI) decreased from an initial value of 7.08 (indicating severe pollution) to 0.450 (indicating no pollution). The potential ecological risk index (RI) dropped from 274 (medium level) to 39.1 (low level). Additionally, the potential carcinogenic risk (CR) values reduced by 93.9 % for both adults and children. The results revealed that the washing process significantly reduced the pollution level, potential ecological risk, and health risk. Coupled with FTIR and SEM-EDS analysis, the mechanism of WV removal of PTEs could be explained from three aspects: acid activation, H⁺ ion exchange, and functional group complexation. In summary, WV is an eco-friendly and high-efficiency leaching material for the remediation of PTEs polluted sites, which will maintain soil function and protect human health.

The application of chemometrics in metals source of identification in Brunei Bay surface sediment

Brunei Bay is a unique ecosystem which offers a vast biodiversity. This study was carried out to define the source of metals in the surface sediment of Brunei Bay to ensure the bay's health. The secondary data were analysed using chemometrics analysis to verify the possible factors that influence metals distribution in Brunei Bay sediment. Samples were collected several times during 2013 to 2014 using Ponar grab at 16 stations within the bay. Samples were then dried, pre-treated, digested and analysed using Inductively Coupled Plasma Mass Spectrometry (ICPMS) in the laboratory. Overall, the mean concentration of metal, sediment pH and clay fraction were significantly changed during different sampling periods, as the changes were presumed affected by seasonal changes. The Pearson correlation has pointed that metals were dominantly derived by natural input; however, the total organic carbon was proven to be derived by anthropogenic sources. Moreover, the principal component analysis has verified that the distribution of metals in the bay's sediment was dominantly influenced by natural processes. However, the utilization and manipulation of marine resources are slightly affecting the bay's ecosystem which may deteriorate the ecosystem health soon.

Heavy metal remediation using chelator-enhanced washing of municipal solid waste compost based on spectroscopic characterization

Due to high metal toxicity, mixed municipal solid waste (MSW) compost is difficult to use. This study detected the presence of heavy metals (Cd, Cu, Pb, Ni, and Zn) in MSW compost through mineralogical analysis using X-ray diffraction (XRD) and performed topographical imaging and elemental mapping using a scanning electron microscope and energy dispersive X-ray analysis (SEM-EDX). Ethylenediaminetetraacetic acid (EDTA), a typical chelator, is tested to remove heavy metals from Indian MSW compost (New Delhi and Mumbai). It deals with two novel aspects, viz., (i) investigating the influence of EDTA-washing conditions, molarity, dosage, MSW compost-sample size, speed, and contact time, on their metal removal efficiencies, and (ii) maximizing the percentage removal of heavy metals by determining the optimal process control process parameters. These parameters were optimized in a batch reactor utilizing Taguchi orthogonal (L₂₅) array. The optimization showed that the removal efficiencies were 96.71%, 47.37%, and 49.94% for Cd, Pb, and Zn in Delhi samples, whereas 45.55%, 79.52%, 59.63%, 82.31%, and 88.40% for Cd, Cu, Pb, Ni, and Zn in Mumbai samples. Results indicate that the removal efficiency of heavy metals was greatly influenced by EDTA-molarity. Fourier-transform infrared spectroscopy (FTIR) confirmed the presence of hydroxyl group, which aids heavy metal chelation. The results reveal the possibility of EDTA to reduce the hazardous properties of MSW compost.

Leaching and characterization studies of heavy metals in contaminated soil using sequenced reagents of oxalic acid, citric acid, and a copolymer of maleic and acrylic acid instead of ethylenediaminetetraacetic acid

In this work, the removal performance of three environmentally friendly reagents, oxalic acid (OA), citric acid (CA), and a copolymer of maleic and acrylic acid (PMAA), on heavy metals in polluted soil was studied at the optimum conditions and compared their sequenced performance. The results showed that the consecutive washing with the individual acids significantly improved the removal percentage of heavy metals in the soil compared to that of EDTA (10.2%, 71.3%, 29.8%, 61.6%, and 52.4% removal for As, Cd, Cu, Pb, and Zn, respectively). The removal of As, Cd, Cu, Pb, and Zn in the sequence of CA-OA was 65.6%, 79%, 59.1%, 64.6%, and 63.5%, respectively. In addition, the organic acids had little influence on the soil physicochemical properties after washing with slight reductions of acidity (pH) and soil organic matter (SOM), which are the major determinants of the usability of washed soils for plant growth. The germination rate of Sorghum bicolor in CA-OA-washed soils reached over 70% on the 7th day. CA-OA-washed soils collectively stand out in using washed soils for plant growth with the following advantages: simultaneous removal of cationic and anionic metals, less harmful impact on soil properties, and successful support for the germination of crops. Based on the findings, we recommend the CA-OA sequence as the best alternative to EDTA with higher metal removal efficiency and germination success.

Effect of multiple washing operations on the removal of potentially toxic metals from an alkaline farmland soil and the strategy for agricultural reuse

Few studies have carried out soil washing experiments using pot experiments to simulate in situ soil washing operations, particularly for alkaline soils. This study explored the effects of multiple washing operations using pot experiments on the removal efficiencies of potentially toxic metals (PTM) from alkaline farmland soil and the reuse strategy of washed soil for safe agricultural production. The results showed that the removal efficiencies of Cd, Pb, Cu, and Zn after seven washings with a mixed chelator (EDTA, GLDA, and citric acid) were 41.1%, 47.1%, 14.7%, and 26.5%, respectively, which was close to the results of the EDTA treatment. For the alkaline soil studied, the second washing with the mixed chelators most effectively removed PTM owing to the activation of them after the first washing operation. The mixed chelator more effectively increased the proportion of stable fraction of PTM and maintained soil nutrients (e.g., nitrogen content) than EDTA, indicating little disturbance of alkaline soil quality after washing with the mixed chelator. After the amendment of the washed soil, there was no visible difference in the biomass weight of crops from the soils washed with different agents, indicating that the inhibitory effect of both washing agents on plant growth was effectively alleviated. The Cd and Pb contents in Z. mays were below the threshold of Hygienical Standard for Feeds of China (GB 13078-2017) (1 and 30 mg·kg^-1). Moreover, after three cropping operations, the available concentrations of PTM in the soil washed with the mixed chelator were lower than those in the soil washed with EDTA, indicating the value and potential of agricultural reuse of alkaline farmland soil washed with the mixed chelator.

Reduction of cadmium bioavailability in paddy soil and its accumulation in brown rice by FeCl3 washing combined with biochar: A field study

Cadmium (Cd) removal from paddy soil to reduce Cd accumulation in brown rice is essential for agroecology, food safety, and human health. In this study, we demonstrate that ferric chloride (FeCl₃) washing combined with biochar treatment efficiently remediates Cd-contaminated paddy soil in field trials. Our results showed that 30.9 % of total Cd and 41.6 % of bioavailable Cd were removed by the addition of 0.03 M FeCl₃ at a liquid/soil ratio of 1.5:1. The subsequent addition of 1 % biochar further reduced bioavailable Cd by 36.5 and 41.5 %, compared with FeCl₃ washing or biochar treatment alone. The principal component regression analysis showed that the Cd content in brown rice was primarily affected by the bioavailable Cd in soil. The combined remediation contributed to the decreased Cd contents in brown rice by 45.5-62.5 %, as well as a 2.7-11.8 % increase in rice yield. The Cd contents in brown rice decreased to 0.12 and 0.04 mg kg^-1 in two cultivars of rice (Zhuliangyou189 and Zhuliangyou929), lower than the national food safety standard limit value of China (0.2 mg kg^-1). Meanwhile, the combined remediation promoted the restoration of soil pH and organic matter as well as the improvement of available nutrients. This finding suggests that the combination of FeCl₃ washing and biochar is an effective remediation strategy to minimize Cd bioavailability in paddy soil, and improves soil quality, thus contributing to food safety.

BioClay nanosheets infused with GA3 ameliorate the combined stress of hexachlorobenzene and temperature extremes in Brassica alboglabra plants

Environmental pollutants and climate change are the major cause of abiotic stresses. Hexachlorobenzene (HCB) is an airborne and aero-disseminated persistent organic pollutants (POP) molecule causing severe health issues in humans, and temperature extremes and HCB in combination severely affect the growth and yield of crop plants around the globe. The higher HCB uptake and accumulation by edible plants ultimately damage human health through the contaminated food chain. Hence, confining the passive absorbance of POPs is a big challenge for researchers to keep the plant products safer for human consumption. BioClay functional layered double hydroxide is an effective tool for the stable delivery of acidic molecules on plant surfaces. The current study utilized gibberellic acid (GA₃) impregnated BioClay (BioClay _GA ) to alleviate abiotic stress in Brassica alboglabra plants. Application of BioClay _GA mitigated the deleterious effects of HCB besides extreme temperature stress in B. alboglabra plants. BioClay _GA significantly restricted HCB uptake and accumulation in applied plants through increasing the avoidance efficacy (AE) up to 377.61%. Moreover, the exogenously applied GA₃ and BioClay _GA successfully improved the antioxidative system, physiochemical parameters and growth of stressed B. alboglabra plants. Consequently, the combined application of BioClay and GA₃ can efficiently alleviate low-temperature stress, heat stress, and HCB toxicity.

A review for recent advances on soil washing remediation technologies

Contaminated soils have caused serious harm to human health and the ecological environment due to the high toxicity of organic and inorganic pollutants, which has attracted extensive attention in recent years. Because of its low cost, simple operation and high efficiency, soil washing technology is widely used to permanently remove various pollutants in contaminated soils and is considered to be the most promising remediation technology. This review summarized the recent developments in the field of soil washing technology and discusses the application of conventional washing agents, advanced emerging washing agents, the recycling of washing effluents and the combination of soil washing and other remediation technologies. Overall, the findings provide a comprehensive understanding of soil washing technology and suggest some potential improvements from a scientific and practical point of view.

Improvement of the Cd and Zn phytoremediation efficiency of rice (Oryza sativa) through the inoculation of a metal-resistant PGPR strain

Cadmium (Cd) and zinc (Zn) in contaminated soil inhibit rice yield and produce toxic effects on human body through rice accumulation. Plant growth promoting rhizobacteria (PGPR) assisted phytoremediation is an effective ecological measure to improve the remediation efficiency of heavy metal contaminated soil. The purpose of this study was to investigate the efficiency of the combination of rice and Cd/Zn-tolerant PGPR strain Bacillus sp. ZC3-2-1 for the remediation of Cd-Zn contaminated soil. Moreover, the effects of inoculations on rhizosphere bacterial communities and ion homeostasis of rice under Cd-Zn exposure will also be explored. The results showed that compared with the treatment without inoculation, ZC3-2-1 decreased the bioavailable Cd and Zn concentrations in soil by 39.3% and 32.0%, respectively, and increase the phytoextraction of Cd²⁺ and Zn²⁺ by rice to 48.2% and 8.0%, respectively. This inoculation process significantly increased the rice biomass, resulting that the contents of Cd²⁺ and Zn²⁺ per biomass unit of rice didn't change significantly. This fact meant that ZC3-2-1 could improve the phytoremediation efficiency of Cd-Zn contaminated soil by promoting the phytoextraction and immobilization of the metal, while might not affect the crop food safety. Besides, through regulation of the Na⁺ and Mg²⁺ concentration in rice, ZC3-2-1 played a positive role in maintaining ion homeostasis which was disrupted by Zn or Cd. Moreover, ZC3-2-1 could modulate the beneficial bacterial communities in rice rhizosphere soil, and then enhanced Cd-Zn immobilization and enzyme activities in soil, leading to the enhancement of rice growth and phytoremediation efficiency. Above all, this study provided novel insights into developing an efficient phytoremediation system and safe production of rice in Cd-Zn contaminated soil with the application of Bacillus sp. ZC3-2-1, as well as advance our understanding of the principles of rhizosphere bacterial community assemble and maintaining ion homeostasis in rice during this phytoremediation process.

Integrated soil washing and bioreactor systems for the treatment of hexachlorocyclohexane contaminated soil: A review on enhanced degradation mechanisms, and factors affecting soil washing and bioreactor performances

Investigations about the remediation of Hexachlorocyclohexane (HCH), a persistent organic pollutant of global concern, have been extensively reported to treat the HCH contaminated soil. The difficulty arising due to desorption and long ageing procedures of this hydrophobic organic compound in the soil, make it necessary to exploit techniques like soil washing or addition of surfactants, for enhancing the mass transfer rate of hydrophobic compounds. However, this technique gives rise to the generation of a large quantity of waste solution containing the pollutant and various other toxic substances. Moreover, it is challenging to deal with the complex soil washing solution, and thus a follow-up treatment of such washing solution is essentially required before its discharge. This follow-up treatment could be the bioreactor system to efficiently treat the pollutant in the wash solution, thereby reducing the amount of contaminated soil that has to be treated. Among many suggested remediation methods and treatment technologies, integrated soil washing and post-treatment with the bioreactor system could be an environmentally viable method for the remediation of HCH contaminated sites. This review focuses on the soil washing procedures applied so far for the HCH contaminated soil and various factors affecting the efficiency of separation of the target pollutant. Furthermore, the environmental and reactor design-related factors are also discussed for degradation of HCH in the reactor system. Finally, advantages and environmental feasibility of this proposed combined technology and the challenges that need to be encountered are envisaged.

Effective immobilization of heavy metals via reactive barrier by rhizosphere bacteria and their biofilms

As the portal of plants, rhizosphere microorganisms play an essential role in controlling the species, transformation, and bioavailability of heavy metals, yet the potential passivation mechanism is still unclear. In this study, two heavy metal resistant and growth-promoting rhizosphere bacteria were screened, and their mechanisms in dealing with external stress and immobilizing heavy metal were explored. The results showed that heavy metals inhibited the ability of Pseudomonas sp. H13 and Brevundomonas sp. H16 to promote plant growth, but stimulated the production of extracellular polysaccharides and inorganic labile sulfide, and enhanced biofilm formation, thereby significantly improved the removal efficiency of Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺. Compared with H16, the biofilm of H13 disintegrated rapidly in the later stage, so more metal ions were adsorbed on the planktonic cells. The C-OH and PO groups related to polysaccharides play a crucial role in heavy metal adsorption, and the immobilization mechanism of the planktonic cell is mainly ion exchange and group complex, but for H16, intracellular enrichment cannot be ignored. Functional group complexes played a dominant role in biofilm, and the immobilized heavy metals were more difficult to release into the environment. This study highlighted the potential application prospects of biofilm bacteria in heavy metal remediation and explained the reactive barrier of rhizosphere bacteria to heavy metals.

Development of phytoremediator screening strategy and exploration of Pennisetum aided chromium phytoremediation mechanisms in soil

Screening of chromium (Cr) phytoremediators (i.e., hyperaccumulator plants and accumulation plants) is essential for the phytoremediation of Cr-contaminated soils but less tackled previously. In this study, we proposed a stepwise strategy for screening Cr phytoremediators and explored tolerance mechanism of the screened species. To achieve effective screening of Cr phytoremediators, seed germination, hydroponic, and pot experiment were performed sequentially, and an improved indicator system was established accordingly. Pennisetum was selected from nine plants, with its high growth rate and Cr remediation efficiency successfully demonstrated in the field. Antioxidant enzymes (i.e., superoxide dismutase (SOD) and catalase (CAT)) and photosynthesis under Cr stress were monitored for tracking the tolerance mechanism. Results showed that the enhanced SOD and CAT contributed to the strong tolerance of Pennisetum to Cr. The SOD and CAT were positively correlated with net photosynthetic rate (Pn), resulting in a phenomenon that Cr had no significant effect on Pn of Pennisetum even at 400 mg kg^-1. The research findings helped obtain powerful Cr phytoremediators, deepen our understanding of the tolerance mechanisms associated with phytoremediation, and eventually facilitate effective Cr removal in soil.

Role of coconut shell biochar and earthworm (Eudrilus euginea) in bioremediation and palak spinach (Spinacia oleracea L.) growth in cadmium-contaminated soil

The contamination of soil with heavy metals is known to affect the yield the soil fertility, which in turn affects the growth of agricultural crops. This study investigates the role of coconut shell biochar (CSB) and earthworms (Eudrilus euginea) in the bioremediation and growth of Palak spinach (Spinacia oleracea L.) in cadmium (Cd) contaminated soil. The soils were amended with different combinations of CSB and earthworms and incubated for 35 days. Later, the soil samples were analyzed for the changes in the soil properties, soil enzyme activity, and heavy metal contents. It is observed that the treatments with both CSB and earthworms resulted in the improvement of soil properties and soil enzyme activity which was directly related to soil fertility. Meanwhile, the maximum removal of 94.38% of total Cd content in the soil was obtained for the soil sample contain both CSB and earthworms. The improved soil properties resulted in a higher germination percentage of Spinacia oleracea L. seeds in the Cd contaminated soil.

Roles of exogenous plant growth regulators on phytoextraction of Cd/Pb/Zn by Sedum alfredii Hance in contaminated soils

Plant growth regulators (PGRs) assisted phytoextraction was investigated as a viable phytoremediation technology to increase the phytoextraction efficiency in contaminated soils. This study aimed to evaluate the cadimum (Cd)/lead (Pb)/zinc (Zn) phytoextraction efficiency by a hyperaccumulator Sedum alfredii Hance (S. alfredii) treated with 9 PGRs, including indole-3-acetic acid (IAA), gibberellin (GA₃), cytokinin (CKs), abscisic acid (ABA), ethylene (ETH), brassinosteroid (BR), salicylic acid (SA), strigolactones (SL) and jasmonic acid (JA), in slightly or heavily contaminated (SC and HC, respectively) soil. Results demonstrated that PGRs were able to improve S. alfredii biomass, the most significant increases were observed in GA₃ and SL for HC soil, while for SC soil, IAA and BR exhibited positive effects. The levels of Cd, Pb and Zn in the shoots of S. alfredii treated with ABA and SL were noticeably greater than in the CK treatment in HC soil, while the uptake of metals were increased by IAA and CKs in SC soil. Combined with the results of biomass and metal contents in shoots, we found that ABA showed the highest Cd removal efficiency and the maximum Pb and Zn removal efficiency was observed with GA₃, which was 62.99%, 269.23%, and 41.18%, respectively higher than the control in HC soil. Meanwhile, compared to control, the maximum removal efficiency of Cd by IAA treatment (52.80%), Pb by JA treatment (165.1%), and Zn by BR treatment (44.97%) in the SC soil. Overall, our results suggested that these PGRs, especially, ABA, SL, IAA, BR and GA₃ had great potential in improving phytoremediation efficiency of S. alfredii grown in contaminated soils.

Bioaugmentation and bioaugmentation-assisted phytoremediation of heavy metal contaminated soil by a synergistic effect of cyanobacteria inoculation, biochar, and purslane (Portulaca oleracea L.)

In recent decades, soil contamination with heavy metals has become an environmental crisis due to their long-term stability and adverse biological effects. Therefore, bioremediation is an eco-friendly technology to remediate contaminated soil, which the efficiency requires further research. This study was designed to comparatively investigate two strategies: bioaugmentation by using a cyanobacterial species (Oscillatoria sp.) and bioaugmentation-assisted phytoremediation by using Oscillatoria sp. and purslane (Portulaca oleracea L.) for the bioremediation of soil contaminated by heavy metals (Cr (III), Cr (VI), Fe, Al, and Zn). Various quantities of biochar (0.5, 2, and 5% (w/w)) were used as an amendment in the experiments to facilitate the remediation process. The results of the bioaugmentation test showed that applying biochar and cyanobacteria into contaminated soil significantly increased the chlorophyll a, nitrogen, and organic carbon contents. In contrast, the extractable fractions of Cr (III), Cr (VI), Zn, Al, and Fe declined compared with those of the control treatment. The highest reduction content (up to 87 %) in the extractable portion was obtained for Cr (VI). The development of longer root and hypocotyl lengths and vigour index from lettuces and radish seeds grown in the remediated soil confirmed the success of remediation treatments. Moreover, the findings of the bioaugmentation-assisted phytoremediation test displayed a reduction in the bioavailable fraction of Cr (III), Cr (VI), Zn, Al, and Fe. Cr (III) presented the highest reduction (up to 90 %) in metal bioavailability. With cyanobacteria inoculation and biochar addition, the shoot and root lengths of purslane grew 4.6 and 3-fold while the heavy metal accumulation decreased significantly. Besides, these treatments enhanced the tolerance index (TI) quantities of purslane whereas diminished its bioaccumulation coefficient (BAC) and bioconcentration factor (BCF) values. For all heavy metals (except Zn), translocation factor (TF) and BAC values were found to be less than 1.0 at all treatments, indicating the successful phytoextraction by the purslane. These results suggest that the purslane can be considered an excellent phytoextracting agent for soils contaminated with heavy metals.

The cadmium decontamination and disposal of the harvested cadmium accumulator Amaranthus hypochondriacus L

The heavy metal accumulated biomass after phytoremediation needs to be decontaminated before disposal. Liquid extraction is commonly used to remove and recycle toxic heavy metals from contaminated biomass. In this study, we examined the cadmium (Cd) removal efficiency using different chemical reagents (hydrochloric acid, nitric acid, sulfuric acid, and ethylenediaminetetraacetic acid disodium) of the post-harvest Amaranthus hypochondriacus L. biomass. The purifications for the extracted liquids and ecological risk assessments for the extracted residues were also investigated. We have found that 77.8% of Cd in stems and 62.1% of Cd in leaves were removed by 0.25 M HCl after 24 h. In addition, K₂CO₃, KOH, and 4 Å molecular sieve could remove ≥89.0% of Cd in the extracted liquids. Finally, after we returned the extracted residues to the earthworm-incubated soil, the extracted biomass negatively affected the growth (weight loss ≥ 11.0%) and survival (mortality ≥ 33.3%) of Eisenia fetida. It should be noted that earthworms decreased soil available Cd concentrations from 0.14-0.05 mg kg^-1 to 0.11-0.04 mg kg^-1 and offset the negative effects of the Cd-contaminated biomass on soil microbes. Overall, given the cost of reagents, the Cd removal efficiency, and the ecological risks of the extracted biomass, using 0.25 M HCl for liquid extraction and K₂CO₃ for purification should be recommended. This work highlights the potential of liquid extraction for immediately and directly removing the Cd from fresh contaminated accumulator biomass and the resource cycling potential of the extracted liquids and biomass after purification.

Calcium Nanoparticles Impregnated With Benzenedicarboxylic Acid: A New Approach to Alleviate Combined Stress of DDT and Cadmium in Brassica alboglabra by Modulating Bioacummulation, Antioxidative Machinery and Osmoregulators

At present, the alleviation of stress caused by climate change and environmental contaminants is a crucial issue. Dichlorodiphenyltrichloroethane (DDT) is a persistent organic pollutant (POP) and an organochlorine, which causes significant health problems in humans. The stress caused by cadmium (Cd) and the toxicity of DDT have direct effects on the growth and yield of crop plants. Ultimately, the greater uptake and accumulation of DDT by edible plants affects human health by contaminating the food chain. The possible solution to this challenging situation is to limit the passive absorption of POPs into the plants. Calcium (Ca) is an essential life component mandatory for plant growth and survival. This study used impregnated Ca (Bd_Ca) of benzenedicarboxylic acid (Bd) to relieve abiotic stress in plants of Brassica alboglabra. Bd_Ca mitigated the deleterious effects of Cd and reduced DDT bioaccumulation. By increasing the removal efficacy (RE) up to 256.14%, Bd_Ca greatly decreased pollutant uptake (Cd 82.37% and DDT 93.64%) and supported photosynthetic machinery (86.22%) and antioxidant enzyme defenses (264.73%), in applied plants. Exogenously applied Bd also successfully improved the antioxidant system and the physiochemical parameters of plants. However, impregnation with Ca further enhanced plant tolerance to stress. This novel study revealed that the combined application of Ca and Bd could effectively relieve individual and combined Cd stress and DDT toxicity in B. alboglabra.

Evaluation of the cadmium phytoextraction potential of tobacco (Nicotiana tabacum) and rhizosphere micro-characteristics under different cadmium levels

In this study, a field-scale and pot experiment were performed to evaluate the remedial efficiency of Cd contaminated soil by tobacco and explore rhizosphere micro-characteristics under different cadmium levels, respectively. The results indicated that tobacco could remove 12.9 % of Cd from soil within a short growing period of 80 d. The pot experiment revealed that tobacco could tolerate soil Cd concentrations up to 5.8 mg kg-1 and bioaccumulate 68.1 and 40.8 mg kg-1 Cd in shoots and roots, respectively. The high Cd bioaccumulation in tobacco might be attributed to strong acidification in the rhizosphere soil and the increase in Cd bioavailability. Rhizobacteria did not appear to be involved in Cd mobilization. In contrast, tobacco tended to enrich sulfate-reducing bacteria (such as Desulfarculaceae) under high Cd treatment (5.8 mg kg-1) but enrich plant growth-promoting bacteria (such as Bacillus, Dyadobacter, Virgibacillus and Lysobacter) to improve growth under low Cd treatment (0.2 mg kg-1), suggesting that tobacco employed different microbes for responding to Cd stress. Our results demonstrate the advantages of using tobacco for bioremediating Cd contaminated soil and clarify the rhizosphere mechanisms underlying Cd mobilization and tolerance.

Effects of Herbaspirillum sp. p5-19 assisted with alien soil improvement on the phytoremediation of copper tailings by Vetiveria zizanioides L

Microbial assisted phytoremediation and reclamation are both potential contaminated soil remediation technologies, but little is known about the combined application of the two technologies on real contaminated soils. This study investigated the potential of Herbaspirillum sp. p5-19 (p5-19) assisted with alien soil improvement on improving stress tolerance and enhancing the accumulation of Mn, Cu, Zn, and Cd by Vetiveria zizanioides L. in copper tailings. Phytoremediation potential was evaluated by plant biomass and the ability of plants to absorb and transfer heavy metals. Results showed that the biomass was increased by 19.64-173.81% in p5-19 inoculation treatments with and without alien soil improvement compared with control. Meanwhile, photosynthetic pigment contents were enhanced in co-inoculation treatment (p5-19 with alien soil improvement). In addition, the malondialdehyde (MDA) content was decreased, and the activities of antioxidant enzymes such as ascorbate peroxidase (APX), superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were increased in p5-19 treatment, thereby alleviating the oxidative stress. Moreover, co-inoculation significantly (p < 0.05) increased the concentrations of Mn, Cu, Zn, and Cd in the roots and shoots of V. zizanioides. In particular, the highest concentrations of Mn, Zn, and Cd in the shoots (roots) were obtained in covering 10 cm combined with p5-19 inoculation treatment, which were 4.44- (2.71-), 4.73- (3.87-), and 5.93- (4.35-) fold as that of the controls, respectively. These results provided basis for the change of phytoremediation ability of V. zizanioides after inoculation. We concluded that p5-19 assisted with alien soil improvement was a potential strategy for enhancing phytoremediation ability in tailings.

Remediation of zinc-contaminated soils by using the two-step washing with citric acid and water-soluble chitosan

Remediation of heavy metal contaminated soil with appropriate washing agents is crucial to the decline in the harmfulness of contaminated soil by heavy metals to the environment and human health. In this study, citric acid (CA) and water-soluble chitosan (WSCS) as natural and degradable washing agents were used to remove Zn in the soil by two-step washing method. Results indicated that the two-step washing with CA and WSCS were found to be suitable for the removal of Zn from the contaminated soils, which significantly decreased the total concentration of Zn in the soil. After the remediation process with two-step soil washing, the contents of Zn in different chemical species decreased, especially for the carbonate-bound fraction. Therefore, the two-step soil washing with CA and WSCS was advisable for the remediation of Zn-contaminated soils. The washing mechanism could include the acid dissolution, ion exchange and complexation reaction between zinc ions and functional groups such as hydroxyl, carboxyl, amine and amide groups. This study provided the theoretical support for the exploitation and application of suitable washing agents used for the remediation of contaminated soils by heavy metals.

The use of previous crops as sustainable and eco-friendly management to fight Fusarium oxysporum in sesame plants

Sesame (Sesamum indicum L.), the “Queen of oil seeds” is being infected with pathogens, i.e., fungi, bacteria, virus and nematodes. Fusarium oxysporum sp. sesami (Zap.), is one of the fiercest pathogens causing severe economic losses on sesame. This work aimed to evaluate the impact of the cultivation of some preceding crops and seed inoculation with antagonistic predominant rhizospheric bacteria and actinomycetes on the incidence and development of Fusarium damping-off and wilt disease. Results showed that the lowest pre and/or post-emergence damping-off and wilt of sesame were recorded after onion and garlic, followed by wheat compared to clover in both the 2019 and 2020 seasons. In vitro, soil extracts from plots where onion and garlic have been cultivated slightly decreased the conidia germination and mycelium radial growth of F. oxysporum. The numbers of sesame rhizospheric F. oxysporum and fungi were lower after the cultivation of onion and garlic than those after wheat and clover. However, the numbers of actinomycetes and bacteria were higher in the onion, garlic, and clover rhizosphere than wheat. Among all isolated bacteria and actinomycetes associated with sesame roots cultivated after preceding plants, the Tricoderma viride and Bacillus subtilis (isolate No.3) profoundly reduce F. oxysporum mycelial growth in vitro. When sesame seeds were inoculated with Tricoderma viride, Bacillus subtilis, Streptomyces rochei and Pseudomonas fluorescens, the disease incidence of damping-off and wilt significantly decreased in the greenhouse and field trials conducted in both tested growing seasons, also had highly significant on plant health and growth parameters. Therefore, the current study suggested that using the preceding onion and garlic plants could be used for eco-friendly reduction of damping-off and wilt disease of sesame.

Fruit extracts from Phyllanthus emblica accentuate cadmium tolerance and accumulation in Platycladus orientalis: A new natural chelate for phytoextraction

A key challenge for phytoextraction is the identification of high efficiency, growth-supporting, and low cost chelating agents. To date, no substance has satisfied all above criteria. This study investigated nine traditional Chinese herbs and found that Phyllanthus emblica fruit (FPE) extract could be utilised as an optimal chelate for the phytoextraction of cadmium (Cd)-contaminated soils. FPE application into soil at a ratio of 0.1% (w/w) significantly increased extractable Cd (by 43%) compared to the control. The success of FPE as a chelating agent was attributed to high quantities of polyphenol compounds (0.76%) and organic acids (9.6%), in particular, gallic acid (7.6%). Furthermore, antioxidative properties (1.4%) and free amino acids in FPE alleviated Cd-induced oxidant toxicity and enhanced plant biomass. FPE promoted 78% higher phytoextraction efficiency in Platycladus orientalis compared to traditional chelating agents (EDTA). Furthermore, 76% of FPE was degraded 90 days after the initial application, and there was no difference in extractable Cd between the treatment and control. FPE has been commercially produced at a lower market price than other biodegradable chelates. As a commercially available and cost-effective chelator, FPE could be utilised to treat Cd-contaminated soils without adverse environmental impacts.

Kinetics of Cu, Pb and Zn removal during soil flushing with washing agents derived from sewage sludge

This paper presents the first tests of Cu (7875 mg/kg), Pb (1414 mg/kg) and Zn (566 mg/kg) removal from contaminated soil with sewage-sludge-derived washing agents (SS_WAs) (dissolved organic matter, DOM; soluble humic-like substances, HLS; soluble humic substances, SHS) and Na₂EDTA (as a standard benchmark) in column experiments. Flow rates of 0.5 ml/min and 1 ml/min were used. Using a 1. order kinetic model, the kinetic constant (k), the maximum concentrations of each metal removed (C_max), and the initial rates of metal removal (r) were established. At both flow rates, stable flow velocity was maintained for approximately eight pore volumes, for flushing times of 8 h (1.0 ml/min) and 16 h (0.5 ml/min). Although the flow rate did not influence k, it influenced C_max: at 1 ml/min, C_max values were higher than at 0.5 ml/min. For Cu and Zn, but not Pb, k was about twofold higher with Na₂EDTA than with SS_WAs. Although Na₂EDTA gave the highest k_Cu, C_max,Cu was highest with DOM (Na₂EDTA, 66%; DOM 73%). For Pb removal, HLS was the most effective SS_WA (77%; Na₂EDTA was 80% effective). k_Zn was about twofold higher with Na₂EDTA than with SS_WAs. C_max,Zn was highest with HLS. The quick mobilization of Cu, Pb and Zn with most of the WAs corresponded to efficient metal removal from the exchangeable (F1) fraction.

Karrikinolide alleviates BDE-28, heat and Cd stressors in Brassica alboglabra by correlating and modulating biochemical attributes, antioxidative machinery and osmoregulators

In this study, we have evaluated the role of karrikin (KAR¹) against the absorption and translocation of a persistent organic pollutant (POP), 2,4,4'-Tribromodiphenyl ether (BDE-28) in plants, in the presence of two other stressors, cadmium (Cd) and high temperature. Furthermore, it correlates the physiological damages of Brassica alboglabra with the three stresssors separately. The results revealed that the post-germination application of KAR¹ successfully augmented the growth (200%) and pertinent physiochemical parameters of B. alboglabra. KAR¹ hindered air absorption of BDE-28 in plant tissues, and reduced its translocation coefficient (TF). Moreover, BDE-28 was the most negatively correlated (-0.9) stressor with chlorophyll contents, while the maximum mitigation by KAR¹ was also achieved agaist BDE-28. The effect of temperature was more severe on soluble sugars (0.51), antioxidative machinery (-0.43), and osmoregulators (0.24). Cd exhibited a stronger inverse interrelation with the enzymatic antioxidant cascade. Application of KAR¹ mitigated the deleterious effects of Cd and temperature stress on plant physiological parameters along with reduced aero-concentration factor, TF, and metal tolerance index. The phytohormone reduced lipid peroxidation by decreasing synthesis of ROS and persuading its breakdown. The stability of cellular membranes was perhaps due to the commotion of KAR¹ as a growth-promoting phytohormone. In the same way, KAR¹ supplementation augmented the membrane stability index, antioxidant defense factors, and removal efficiency of the pollutants. Consequently, the exogenously applied KAR¹ can efficiently alleviate Cd stress, heat stress, and POP toxicity.

Effect of phosphorus supplementation on growth, nutrient uptake, physiological responses, and cadmium absorption by tall fescue (Festuca arundinacea Schreb.) exposed to cadmium

Cadmium is a common heavy metal pollutant. In some plants, its absorption is inhibited by exogenous phosphorus. Here, the effect of P supplementation on the growth of tall fescue exposed to Cd was evaluated in a hydroponic culture experiment. Plants were exposed to five concentrations of P (0, 0.25, 0.5, 0.75, and 1.0 mmol L^-1) and three concentrations of Cd (50, 100, and 150 mg L^-1), and plant growth, Cd content, absorption, physiological characteristics, and nutrient accumulation were investigated. P supplementation significantly reduced the Cd content, Cd translocation factor (TF), Cd removal efficiency, plant P absorption, chlorophyll content, glutathione levels, glutathione reductase levels, and superoxide dismutase (SOD) activity in tall fescue under Cd stress (P < 0.05). Moreover, it increased the vertical growth rate and biomass of tall fescue. At a constant P concentration, the biomass and vertical growth rate significantly decreased with an increasing Cd concentration, and the shoot Cd content, SOD activity, and TF significantly increased (P < 0.05). High P supplementation (0.75 and 1.0 mmol L^-1) ameliorated the damage caused by 150 mg L^-1 Cd stress, and the biomass, vertical shoot and vertical root growth rates were increased by 72.06-82.06%, 250.00-316.67%, 300.00-312.00%, respectively. In the plants subjected to 50 mg L^-1 Cd stress, 0.5 mmol L^-1 P supplementation enhanced biomass, vertical shoot and vertical root growth rates by 29.99%, 20.41%, and 21.43%, respectively, and reduced the Cd content in shoots (45.85%) and roots (9.71%). Except for the total potassium content and catalase activity, different concentrations of Cd negatively affected all parameters tested. Such negative effects were limited by P supplementation. Optimizing the nutrient composition and concentrations could minimize the potential negative impacts of Cd on plant growth.

Role of microbial community and metal-binding proteins in phytoremediation of heavy metals from industrial wastewater

This review illustrated the role of metal-binding proteins (MBPs) and microbial interaction in assisting the phytoremediation of industrial wastewater polluted with heavy metals. MBPs are used to increase the accumulation and tolerance of metals by microorganisms via binding protein synthesis. Microbes have various protection mechanisms to heavy metals stress like compartmentalization, exclusion, complexity rendering, and the synthesis of binding proteins. MBPs include phytochelatins, metallothioneins, Cd-binding peptides (CdBPs), cysteines (gcgcpcgcg) (CP), and histidines (ghhphg)₂ (HP). In comparison with other physico-chemical methods, phytoremediation is an eco-friendly and safe method for the society. The present review concentrated on the efficiency of phytoremediation strategies for the use of MBPs and microbe-assisted approaches.

Bioremediation of Cd-spiked soil using earthworms (Eisenia fetida): Enhancement with biochar and Bacillus megatherium application

In this study, we evaluated the influence of biochar and Bacillus megatherium on Cd removal from artificially contaminated soils using earthworms (Eisenia fetida). Within a 35-days remediation period, over 30% of Cd was removed by earthworms from the contaminated soil (with Cd at ∼ 2.5 mg kg^-1), and both additives facilitated Cd removal. Additionally, over 22% reduction in the extractable Cd contents was also achieved by earthworms. Cd accumulated in earthworms steadily increased through remediation, and the accumulated Cd decreased in the order of earthworm + biochar (T3) > earthworm + Bacillus megatherium (T4) > earthworm alone (T2). The bioaccumulation factors (BCF) were above 1, indicating the enrichment of Cd in earthworms, and there were higher BCF for both T4 (944%) and T3 (845%). The ingestion of metal-bonded biochar particle and the elevated Cd mobility would be the main reason for the enhanced Cd-remediation by earthworms under T3 and T4, respectively. Through remediation, microbiota communities in both, soil and earthworm guts, demonstrated high similarity, while a lower level of bacterial abundance was observed in earthworm guts compared with that in soils. Eventually, soils became more fertile and demonstrated higher enzyme activities after remediation. Therefore, we concluded that earthworm, alone or combined with biochar or Bacillus megatherium could be an alternative method for Cd-contaminated soil remediation.

Petroleum hydrocarbon-contaminated soil bioremediation assisted by isolated bacterial consortium and sophorolipid

Pollution in soil by petroleum hydrocarbon has become a global environmental problem. The bioremediation of petroleum hydrocarbon-contaminated soil was enhanced with the combination of an isolated indigenous bacterial consortium and biosurfactant. The biodegradation efficiency of total petroleum hydrocarbon (TPH) was increased from 12.2% in the contaminated soil to 44.5% and 57.7% in isolated consortium and isolated consortium & 1.5 g sophorolipid (SL)/kg dry soil, respectively. The half-life of TPH degradation process was decreased from 32.5 d in the isolated consortium reactor to 20.4 d in the isolated consortium & 1.5 g SL/kg dry soil. The addition of biosurfactant into contaminated soils improved the TPH desorption from solid matrix to the aqueous solution and the subsequent solubilization, which ultimately improved the bioavailability of TPH in contaminated soils. Biosurfactant also served as carbon sources which contributed to the stimulation of cell growth and microbial activity and accelerated the biodegradation process via co-metabolism. The enzyme activities and quantities of functional genes were demonstrated to be incremented in SL reactors. The biosurfactant improved the TPH bioavailability, stimulated the microbial activities and participated in the co-metabolism. The combination of bioaugmentation and SL benefitted the bioremediation of petroleum hydrocarbon-contaminated soil.

Effects of composited organic mobilizing agents and their application periods on cadmium absorption of Sorghum bicolor L. in a Cd-contaminated soil

Organic mobilizing agents have been advocated for phytoremediation of heavy metals contaminated soils, while the effects of application period of such agents remain unclear. A pot experiment was conducted, with two composited organic agents (oxalic acid or citric acid + dissolved organic fertilizer (OA + DOF and CA + DOF)) and four application periods (seeding, jointing, flag leaf and heading stages) of sorghum (Sorghum bicolor L.), to investigate their impacts on Cd bioavailability in soil. Results indicated that application of the two composited agents increased soil dissolved organic carbon (DOC) and DTPA extractable Cd by 7.31-49.13%, Cd contents in roots and shoots by 21.49-72.10%, bioaccumulation factor (BCF) and translocation factor (TF) of shoots by 4.44-71.99%, while reduced soil pH by 0.25-0.53 units, respectively. Most of these indices increased with the application periods, and largely peaked with their application during the flag leaf to heading stages. Meanwhile, the maximum sorghum biomass (132.84 g pot^-1) and Cd bioaccumulation quantity (BCQ, 0.71 mg pot^-1) in shoots were obtained for the CA + DOF applied at the heading. The DTPA extractable Cd was closely related to soil pH and DOC. Similar close relationships were observed between the Cd contents in shoots and soil DTPA extractable Cd, pH and DOC. The BCQ of Cd was positively related to the shoots biomass rather than their Cd contents. Therefore, the sorghum combined with the CA + DOF may be advocated as an alternative phytoremediation mode in Cd-contaminated soils, and the mobilizing agent should be primarily applied at the heading stage.

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

This study investigated the application of a specific soil washing method to remove Cu and Pb from contaminated agricultural soil. To develop an efficient leaching agent of heavy metal compounds for use in farmland soil, a mixed chelator (MC) was prepared using potassium fulvic acid (PFA, 3.2%) and citric acid (CIT, 0.16 M) in a volume ratio of 4:1 (PFA:CIT = 4:1); the optimal solid-liquid ratio (S/L = 1:20), initial pH value (4.51) and contact time (360 min) were also explored. Under optimal conditions, the removal efficiencies of MC for Cu and Pb were 42.92% and 50.46%, respectively, both of which performed better than PFA (27.86% of Cu and 17.91% of Pb) and CIT (42.04% of Cu and 41.46% of Pb). The effective states, bioavailability and relative mobilities of Cu and Pb in soil were also efficiently reduced by MC, which also increased the stability of these elements, thereby lowering the risk to soil health. More importantly, MC not only had little effect on the soil physicochemical properties (e.g., pH, organic matter (OM), cation exchange capacity (CEC), ammonium nitrogen (AN), available phosphorus (AP) and rapidly available potassium (AK)), but also improved the restored soil. Furthermore, soil structure, surface elements and the enzyme activity did not exhibit significantly loss. Therefore, MC has great potential for remediating agricultural soil.

Iron-modified rice husk hydrochar and its immobilization effect for Pb and Sb in contaminated soil

Cationic and anionic heavy metal contamination sometimes co-exists in soil systems, such as mining areas and shooting ranges, seriously threatens human health and ecological stability. In this study, iron-modified rice husk hydrochar showed commendable ability to immobilize both heavy metal cation (Pb) and anion (Sb) simultaneously in soils. Iron-modified rice husk hydrochar (HC12.5-180) (5%) amendment reduced the bioavailability (EX- and CB-fraction) of Pb and Sb by 25 and 40%, respectively, which were 8 and 5 times higher than that of pristine rice husk hydrochar (HC0-180) (5%) amendment. The cation (Pb) immobilization mainly depends on cation exchange with mineral components (K⁺, Ca²⁺, Na⁺, Mg²⁺), precipitation with nonmetallic anions (Cl- and SO₄^2-), and complexation. Meanwhile, the iron oxides (FeO, Fe₂O₃, Fe₃O₄), formed during hydrothermal process, can be easily combined with anion (Sb) to form geochemically stable minerals. In conclusion, this work offered a practical and cost-effective technology based on the iron modification rice husk hydrochar for the immobilization of both anionic and cationic heavy metal contaminants in soils.

Bioremediation of Cd-contaminated soil by earthworms (Eisenia fetida): Enhancement with EDTA and bean dregs

The remediation of cadmium (Cd) contaminated soil has become a global problem due to its toxicity to living organisms. In this study, earthworm (Eisenia fetida) alone or combined with EDTA or bean dregs were used for Cd removal from soils. The total and available Cd in soils, soil physicochemical and biological (soil enzyme) properties, Cd accumulation in the earthworm and its antioxidant responses towards Cd, were determined during the 35 days of soil incubation experiment. Our results showed that earthworms were capable of removing Cd from soils, and the remediation process was accelerated by both EDTA and bean dregs. By translocation of Cd from soils, the content of Cd in earthworm steadily increased with the exposure time to 8.11, 12.80, and 9.26 mg kg^-1 on day 35 for T2 (earthworm alone), T3 (EDTA enhancement), and T4 (bean dregs enhancement), respectively. Consequently, a great reduction in the Cd contents in soils was achieved in T3 (36.53%) and T4 (30.8%) compared with T2 (28.95%). The concentrations of water/DTPA extractable Cd were also reduced, indicating the low Cd mobility after amendment. Finally, the soil became more fertile and active after wermi-remediation. The soil pH, EC, NO₃^--N, available P, and K contents increased, while soil SOM, DOC, and NH₄⁺-N contents were decreased. There were higher soil enzyme activities (including acid phosphatase activity, β-glucosidase activity, and urease activity) among treatments with earthworms. Additionally, the operational taxonomic units (OTUs) increased by 100-150 units, and the higher chao1 and Shannon indexes indicated the enhanced microbial community after wermi-remediation, especially among treatment with EDTA and bean dregs. Therefore, we concluded that earthworms, alone or combined with EDTA and bean dregs, are feasible for the remediation of Cd-contaminated soil.

Remediation of a metal-contaminated soil by chemical washing and repeated phytoextraction: a field experiment

Agricultural soil contaminated with potentially toxic metals poses great health risk to humans and it requires long-term remediation. Here, we investigate the remediation of metal-polluted agricultural soil by combining chemical washing with repeated phytoextraction. The polluted field was initially washed with 40 mmol L^-1 FeCl₃ (F) or 20 mmol L^-1 FeCl₃ + 40 mmol L^-1 citric acid (F + C). After the application of organic fertilizer (O), lime (L), and sepiolite (S), Sedum plumbizincicola was cultivated for three successive crops from 2017 to 2019. Results showed that the soil washed with FeCl₃ had high removal efficiencies of Cd (35.2%), Pb (24.3%), and Zn (26.6%). Although the shoot biomass and metal concentrations of S. plumbizincicola decreased significantly in the first crop, there were no significant differences in the subsequent two crops. Throughout the remediation process, the higher total removal efficiencies of Cd, Pb, and Zn were conducted in F + OLS treatment which observed in 71.0, 34.0, and 47.7%, respectively. The results, therefore, conclusively indicated that combining chemical washing with repeated phytoextraction showed considerable potential for the remediation of agricultural soils polluted with multiple metals. However, further studies are required to focus on the amelioration of the degraded soil quality and safe agricultural production.

Modification-bioremediation of copper, lead, and cadmium-contaminated soil by combined ryegrass (Lolium multiflorum Lam.) and Pseudomonas aeruginosa treatment

The principal objective of this study was to investigate the strengthened remediation effect and relevant mechanism of P. aeruginosa on ryegrass (Lolium multiflorum Lam.) for soil contaminated by Cu-Pb-Cd compound heavy metals. The results showed that the complex heavy metals' contamination had remarkable inhibiting effect on the growth of plants (P < 0.01), and the biomass of ryegrass's stem and leaves declined by 28.2%, while that of roots decreased by 34.7% after 45 days. The inoculation of P. aeruginosa promoted the growth of ryegrass in polluted soil, in which the biomass recovered to the same level of that in normal plant; the activity of both catalase and urease in the soil also increased strikingly (by 29.3% and 75.7%, respectively); the ratio of residual heavy metals in the soil decreased, while the acid extractable heavy metals increased notably. Therefore, the absorption and accumulation of ryegrass to the heavy metals in soil were improved to some extent; the bioconcentration factor of Cu, Pb, and Cd in ryegrass increased by 35.9%, 55.6%, and 283.5%, respectively. The exterior microorganism allowed the accumulation of Cu, Pb, and Cd in shoots of ryegrass increasing remarkably, while in roots, only the accumulation of Pb increased by 16.3%, and that of both Cu and Cd decreased. Besides, in the P. aeruginosa-inoculated system, the transfer factor of Cu and Cd in plants increased strikingly, while that of Pb decreased.

Heavy metal concentration, potential ecological risk assessment and enzyme activity in soils affected by a lead-zinc tailing spill in Guangxi, China

In 1976, a tailing dam collapse accident at the Xingping Lead-Zinc Mine in Guangxi Province, China, led to the spillage of mining wastewater and sludge into downstream agricultural lands in Side Village. Heavy metal concentrations, soil pollution and soil enzyme activity were analysed to understand the pollution characteristics of the agricultural lands along the Side stream by this accident. The tailing soil (TS), natural forest soil without spill contamination (NFS) and four representative agricultural soils were selected. The four agricultural soil sites located at the entrance of the stream to the agricultural soil (EnS), the upstream region (US), the middle stream (MS) and at the exit of the stream (ExS) from the village, respectively. The results showed that the soil pH values and heavy metal concentrations were in the order of tailing soil TS > ExS > MS > US > EnS > NFS. The concentration of Pb ranked highest among the analysed elements followed by Zn, Cd, Mn and Cu. The concentrations of Pb, Zn, Cd Mn and Cu 10530.41, 1708.58, 8.32, 885.61 and 104.51 mg kg^-1, respectively. Soil pollution assessments by single pollution index, synthesis pollution index and individual/comprehensive potential ecological risk analysis explicated all the soils reached the heavily polluted level and presented extremely high ecological risk grades. Pb, Zn and Cd were the dominant pollutants. The soil enzyme activities of invertase, protease and urease exhibited the opposite distribution pattern as those of the heavy metal concentrations, while the inversely results were observed for the activity of catalase.

Application of alkyl polyglycosides for enhanced bioremediation of petroleum hydrocarbon-contaminated soil using Sphingomonas changbaiensis and Pseudomonas stutzeri

Bioremediation is considered a cost-effective and environmentally sound method for degradation of petroleum hydrocarbons in contaminated soils. This study investigated the effects of biosurfactant alkyl polyglycosides (APG) on enhanced biodegradation of petroleum hydrocarbon-contaminated soils using Sphingomonas changbaiensis and Pseudomonas stutzeri and explored the mechanism responsible for the enhanced petroleum hydrocarbon degradation. To accomplish this, the following treatments were evaluated: (1) bioaugmentation with Sphingomonas changbaiensis; (2) bioaugmentation with Pseudomonas stutzeri; (3) a combination of Sphingomonas changbaiensis and APG; and (4) a combination of Pseudomonas stutzeri and APG. The results showed that the degradation rates of total petroleum hydrocarbons (TPH) in contaminated soil samples bioaugmented with S. changbaiensis and P. stutzeri for 30 days were 39.2 ± 1.9% and 47.2 ± 1.2%, respectively. The addition of biosurfactant APG enhanced the bioremediation processes and improved the biodegradation rates. The biodegradation rate at 1.5 g/kg APG in soil samples bioaugmented with S. changbaiensis was 52.1 ± 2.0%, while the rate at 1.5 g/kg APG in soil samples bioaugmented with P. stutzeri was 59.0 ± 1.8%. The half-life decreased from 39.7 d to 24.5 d and from 29.6 to 20.1 d when the dosage of APG was 1.5 g/kg in contaminated soil samples bioaugmented with S. changbaiensis and P. stutzeri, respectively. Mechanism studies showed that the addition of APG can increase the TPH solubility, promote the sorption of TPH onto microbial cells and subsequent trans-membrane transport by APG-induced structural changes, stimulate microbial activities and participate in the co-metabolism. Therefore, the combination of bioaugmentation and APG is an effective method for remediation of petroleum hydrocarbon-contaminated soil.

The combined effects of Cu and Pb on the sex-specific growth and physiology of the dioecious Populus yunnanensis

In recent years, pollution by heavy metals (HM) has become an increasingly serious problem in forest ecosystems, making their remediation a primary research focus in China. Poplars are ideal candidates for phytoremediation because of their great commercial value, ability to produce large biomass, and high capacity for HM uptake. The individual and combined effects of copper (Cu) and lead (Pb) on Populus yunnanensis growth and physiology were tested for both male and female potted plants in four treatment groups: control, Pb only (1,000 mg kg^-1 PbAc dry soil), Cu only (400 mg kg^-1 CuSO₄·5H₂O dry soil), and combined Pb and Cu. Each treatment group contained 25 male and 25 female individuals. The experimental duration was 3 months. Compared with the control plants, the Cu and Pb treatment groups experienced reduced leaf, stem, root, and total biomass for both sexes, but the impact on growth rate was more severe in females than in males. The cellular ultrastructure of leaves was extensively damaged in both male and female trees but was more severely damaged in females. Male trees demonstrated a stronger Cu absorption ability with a bioconcentration factor 2.30 times that of females. Significant changes in pigment content, membrane lipid peroxidation, and protein oxidation (carbonyl) also indicated that females were more sensitive than males to Cu- and Pb-induced stress. The higher Cu and Pb tolerance in males correlated with better H₂O₂ scavenging ability and proline accumulation. Nevertheless, the combined stress from both Cu and Pb yielded greater negative effect on the growth and physiology of P. yunnanensis for both sexes.

EDTA-enhanced phytoremediation of heavy metals from sludge soil by Italian ryegrass (Lolium perenne L.)

Landscaping of sludge is a kind of recycling disposal, but the potential heavy metal risks limit its application. In this paper, the sludge soil was remediated by ryegrass, and the effect of ethylene diamine tetraacetic acid (EDTA) was studied through pot experiments. Italian ryegrass was planted in the sludge soil treated with six gradients concentrations of 0, 1, 2, 3, 4, 5 mmol kg^-1 of EDTA, and the planting conditions were kept the same. After 45 days of planting, compared with the control group (without EDTA treated), the application of 1-5 mmol kg^-1 EDTA decreased ryegrass biomass by 2-43%, reduced soil pH value by 0.21-0.34 unit, and reduced 4.1-9.7% capacity of exchange cation, but increased 1.4-8.6% soil organic matter. After growing ryegrass, the contents of heavy metals decreased by 10% for Cu, 15% for Zn, 6% for Ni, 14% for Cd and 44% for Pb; and after spraying EDTA decreased again by 33% for Cu, 31% for Zn, 56% for Ni, 24% for Cd, and 68% for Pb. In ryegrass, the uptake heavy metals were enhanced, and bio-concentration factor of Cu, Zn, Ni, Cd, and Pb of EDTA treated groups were 1.9, 1.6, 4.1, 2.7, and 4.8 times of the control group, respectively. However, EDTA only significantly increased transfer factor values of Cu and Zn, and made bio-extraction factor value of Cu greater than 1. The remediation factor values were used to comprehensive assess accumulation capacity of heavy metals by ryegrass under EDTA treating, and they ordered in Zn > Cu > Ni > Cd > Pb, and the best dose was 2 mmol kg^-1 EDTA. Prediction models for bio-concentration factor were established by using stepwise multiple linear regression, explaining 94.9-99.3% of the corresponding elements with soil organic matter, EDTA dosage, and/or pH value (p < 0.005). This paper provided effective heavy metals remediation data for municipal sludge landscape and the prediction models.

Phytoremediation of cadmium (Cd) and uranium (U) contaminated soils by Brassica juncea L. enhanced with exogenous application of plant growth regulators

This investigation was made to examine the role of indole-3-acetic acid (IAA), gibberellin A3 (GA₃), 6-Benzylaminopurine (6-BA), and 24-epibrassinolide (EBL) in improving stress tolerance and phytoremediation of the cadmium (Cd) and uranium (U) by mustard (Brassica juncea L.). The optimum concentrations of IAA, GA₃, 6-BA, and EBL were determined based on plant biomass production, metal uptake, translocation, and removal efficiency. The biomass and total chlorophyll content decreased under Cd and U stress. Nevertheless, the application of IAA, GA₃, and 6-BA significantly (p < 0.05) increased the growth and total chlorophyll content of mustard. The malondialdehyde (MDA) and H₂O₂ content of mustard were enhanced under Cd and U stress, but they were significantly (p < 0.05) decreased in plant growth regulators (PGRs) treatments (except for EBL). PGRs treatments increased activities of antioxidant enzymes such as superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase, thus reducing the oxidative stress. Furthermore, the shoot uptake of Cd and U of IAA and EBL treatments was significantly (p < 0.05) higher than that of other treatments. IAA and EBL also have more significant effects on the translocation and remediation of Cd and U compared to GA₃ and 6-BA. The removal efficiency of Cd and U reached the maximum in the 500 mg L^-1 IAA treatment, which was 330.77% and 118.61% greater than that in the control (CK), respectively. These results suggested that PGRs could improve the stress tolerance and efficiency of phytoremediation using B. juncea in Cd- and U- contaminated soils.

A Soluble Humic Substance for the Simultaneous Removal of Cadmium and Arsenic from Contaminated Soils

With abundant oxygen-containing functional groups, a humic substance (HS) has a high potential to remediate soils contaminated by heavy metals. Here, HS was first extracted from a leonardite and analyzed for its chemical compositions and spectroscopic characteristics. Then it was assessed for its ability as a washing agent to remove Cd and As from three types of soils (red soil, black soil, and fluvo-aquic soil) that were spiked with those contaminants (Cd: 40.5–49.1 mg/kg; As: 451–584 mg/kg). The operational washing conditions, including the pH and concentration of the HS, washing time and cycles, and liquid–soil ratio, were assessed for Cd and As removal efficiency. At pH 7, with an HS concentration (3672 mg C/L) higher than its critical micelle concentration and a liquid–soil ratio of 30, a single washing for 6–12 h removed 41.9 mg Cd/kg and 199.3 mg As/kg from red soil, 33.5 mg Cd/kg and 291.5 mg As/kg from black soil, and 30.4 mg Cd/kg and 325.5 mg As/kg from fluvo-aquic soil. The removal of Cd and As from the contaminated soils involved the complexation of Cd and As with the carboxyl and phenolic groups of HS. Outcomes from this research could be used to develop a tailor-made HS washing agent for the remediation of Cd- and As-contaminated soils with different properties.

A novel method for removing heavy metals from composting system: The combination of functional bacteria and adsorbent materials

The aim of this study was to remove heavy metals from composting system using a novel method combined functional bacteria with adsorbent materials. Four types of adsorbent materials were selected in this study. Results showed that Cr had significant removal efficiency, especially in sponge treatment (19.09%) and cotton treatment (26.36%). In addition, a significant movement of heavy metals from the outside to adsorbent column was also observed. RDA results indicated that bands 1, 2, 10, 18, 19 and 20 had negative correlations with six types of heavy metals, which contributed to the removal of heavy metals. Structural equation models further confirmed functional bacteria can directly affect the removal of Cu, Cd and Cr. In addition, it can also indirectly remove Pb and Cr by changing native bacteria. In summary, this study suggested the combination of functional bacteria and adsorbent materials was effective to remove heavy metals from composting system.