Abstract:As an important strategy for sustainable development, bioremediation technology has been widely used in soil contamination remediation due to the advantages of environmental friendliness, excellent metal removal efficiency and free of secondary pollution. This special issue with a collection of 16 papers covers the research aspects from phytoremediation, microbial repair, combined remediation, molecular mechanisms of heavy metals absorption and accumulation, to beneficial reuse of feedstock resources, presents the recent advances as well as the future prospects involved in bioremediation for soil contamination. We aim to provide useful insights to help future development of bioremediation technology.

Abstract:Arsenic is a toxic metalloid. Arsenic pollution in soils affects food safety and threatens human health. Pteris vittata L. has enormous application value in phytoremediation of arsenic-contaminated soil for its high arsenic hyperaccumulation ability. Understanding the arsenic hyperaccumulation molecular mechanism of P. vittata is the core theoretical basis of phytoremediation technology. This review introduces the omics study on arsenic hyperaccumulation mechanisms, as well as important molecular component that is involved in arsenic hyperaccumulation of P. vittata. Further research directions and trends are also discussed.

Abstract:Common submerged plants (Vallisneria natans (Lour.) Hara., Potamageton crispus L., Myriophyllum spicatum L., Ceratophyllum demersum L. and Hydrilla verticillata (L.f.) Royle) have shown a tremendous potential for arsenic removal from water. The studies on the accumulation of arsenic by submerged plants are important for the control of arsenic pollution in water and protection of human health. The enrichment ability of submerged plants to arsenic is correlated with arsenic species and concentration in the water, as well as co-existing ions such as PO43– and Zn2+. The tolerance and enrichment ability of submerged plants to arsenic can be enhanced by suitable chemical and biological methods, which is useful for removing arsenic from water by submerged plants. However, the diversity of plant genes and the complex of water environment, as well as the subsequent treatment of submerged plants, are still the issues that need to be focused in the future.

Abstract:Phytoremediation is one of the important methods for restoring heavy-metal contaminated soils. Using high-biomass economic plants to restore heavy-metal contaminated soils can have both ecological and economic benefits, with great application prospects. Based on the analysis of current situation and existing problems of phytoremediation, we propose the advantages of high-biomass economic plants in contaminated soil remediation, and summarize the recent advances and mechanisms involved in absorbing heavy metals in high-biomass economic plants. Furthermore, the possible methods for improving the remediation efficiency of high-biomass economic plants are also discussed, to provide insights for improving the economic benefits of phytoremediation and promoting its widespread application in the future.

Abstract:With the development of modern industry, heavy metal pollution of soil becomes more and more serious. Heavy metals can cause serious health problems to humans and animals even at very low concentrations. Thus, it is urgent to remediate the contaminated soils by effective methods. Different remediation strategies have been reported to remove heavy metals from contaminated soil, among which phytoremediation is the most important one. Understanding the mechanisms underlying heavy metal accumulation and detoxification in plants is one of the key points for phytoremediation. In this review, we try to summarize the progresses and trends on phytoremediation and related molecular mechanisms, and discuss the prospects for the future research.

Abstract:Phytomining technology cultivates hyperaccumulator plants on heavy metal contaminated soils, followed by biomass harvesting and incineration to recover valuable metals, offering an opportunity for resource recycling and soil remediation. Large areas of ultramafic soils, naturally rich in nickel (Ni), are present in numerous places around the world. As an environmentally friendly and cost-effective soil remediation technology, phytomining has a broad application prospect in such areas and thus has attracted great attention from global researchers. The key processes of phytomining include: (1) high-selectivity response of hyperaccumulator plants to Ni the underlying mechanisms involved in the rhizosphere; (2) underlying mechanisms of high-efficiency uptake and translocation of Ni in hyperaccumulators; and (3) resource recycling of high-added value Ni products from the Ni-rich bio-ore of hyperaccumulators. In recent 30 years, phytomining practices have successfully carried out in United States, Albania and Malaysia. However, the research and application of this technology in China are still in the fledging stage. This paper reviews the key processes and research progress of phytomining, and points out the bottleneck, to provide theoretical basis and technical guidance for phytomining.

Abstract:Arsenic-resistant microorganisms are abundant in surface and near-surface arsenic-contaminated environment, substantially affecting the fate and transport of arsenic in the environment. Siderophores produced by microorganisms under iron-limiting conditions have high affinity for ferric iron and enhance the uptake of iron to the microorganisms. Siderophores help sequester Fe, which is needed to activate AsIII oxidase as Fe is unavailable at physiological pH. Although the principal role of siderophores is to chelate ferric iron, they can also chelate wide array of toxic metals. Inoculation of plants with siderophore-producing bacteria has been found to either promote or reduce heavy metal uptake. Therefore, the siderophore-producing and arsenic-resistant bacteria might have the potential in the remediation of arsenic contamination environment.

Abstract:Soil is the material basis for human survival. However, in China, soils are wildly polluted by heavy metals, which poses serious health risks to humans. Bioremediation of heavy-metal contaminated soil is widely considered as a sustainable remediation strategy, but low remediation efficiency is still a scientific bottleneck of bioremediation. There are abundant microorganisms, plants and animals living in soils. Among these soil biota, there are complex interactions to form an intricate food web through material circulation and energy transfer. These interactions among soil biota affect the transportation and transformation of pollutants in soil, and consequently influence the bioremediation efficiency. The synergistic remediation by soil biota combines the advantages of diferent organisms to enhance the efficiency of bioremediation. In this paper, the interactions among soil biota and their influence on heavy-metal transportation and transformation, as well as bioremediation efficiency are reviewed. We also propose perspectives for future researches, including targeted regulating the structure of soil food web, improving the bioremediation efficiency of heavy-metal contaminated soil, and building a synergistic remediation technology with multi-organisms based on food web.

Abstract:With the acceleration of industrialization and agriculture, soil heavy metal pollution has become increasingly serious. Copper, though an essential trace element for the growth and development of living organisms, is one of the main heavy metal pollutants. Excess copper in soil is toxic to plants and threatens human health via food chain. Bioremediation has received extensive attention as a novel technology for heavy-metal-contaminated soils. Here, we review the development of bioremediation technologies, including phytoremediation, microbial remediation, plant-microbe combined remediation, and animal remediation, for copper-contaminated soils, in order to provide a theoretical basis for the effective management of heavy-metal-contaminated soils and the sustainable development of agriculture.

Abstract:Coastal and inland saline-alkali soil is important reserve land resources. However, some parts of saline land are now under the threat of heavy metals such as cadmium (Cd), lead (Pb) and the light metal lithium (Li). Phytoremediation with halophytes could be the most economical and effective way to restore the contaminated saline soil. In this study, the growth, physiological and biochemical indexes and ion contents of halophyte Salicornia europaea under different concentrations of Cd (0?50 mmol/L), Pb (0?50 mmol/L) and Li (0?400 mmol/L) were investigated to evaluate the tolerance and accumulation of the metal contaminations. The results showed that plant height, fresh weight and dry weight of S. europaea decreased significantly with the increase of Cd and Pb concentration. Low concentration of Li (< 20 mmol/L) promoted the growth of S. europaea, while the growth of plants was inhibited under higher concentration of Li (> 20 mmol/L). The tolerance order of S. europaea to Cd, Pb and Li was Li > Pb > Cd. Cd, Pb and Li stresses may negatively affected Na and K uptake and transport in S. europaea to affect plant growth. In addition, the antioxidant enzyme system synergistically responsed to resist the oxidative toxicity of different ions. The contents of Cd, Pb, Li in roots and shoots of S. europaea also increased with the increase of treatment concentration. Furthermore, Cd and Pb contents in roots were significantly higher than in shoots, while more Li accumulated in shoots than in roots. The aforementioned results showed that S. europaea had strong tolerance along with a high accumulate ability to Cd, Pb and Li, indicating its application potential in restoring Cd, Pb and Li contaminated saline soil. This study laid a basis for further exploration of the tolerance mechanism of S. europaea to Cd, Pb and Li stresses, and gave a new perspective for the usage of S. europaea to remediate Cd, Pb and Li pollutants in high-salinity alkali soils.

Abstract:Nickel and copper, as high toxic heavy metals (HMs), are the most serious contaminants in Jinchuan mining area, China. In this paper, the influence of combined HMs stress on the growth of widespread plant-S. salsa has been studied. The stress gradient of combined Ni-Cu was set based on the local environment and pre-experiment. Seed germination, growth, physiological characteristics of S. salsa were investigated by the control test, and its heavy metal bioaccumulation capacity was investigated by samples collected from field platform. The growth of S. salsa was promoted at lower concentration (≤40 mg/L) and inhibited at higher concentration (≥80 mg/L) under the single HMs stress and combined HMs stress (Cu20/Ni20). The malondialdehyde (MDA) content was increased with increasing concentration, and the soluble protein and free proline content in stress group were higher than that of in control group. Under single HMs stress, the peroxidase (POD) activity increased with increasing concentration; while under combined HMs stress, the POD activity increased initially and then reduced. Cu320 and Ni320 combined HMs stress inhibited the growth of S. salsa at all concentrations. The average translocation factors (TF) of S. salsa were greater than 1.00, and higher in leaves compared to stems. The results of bio-concentration factors (BCF) of S. salsa show that BCF of leaves were larger than that of roots and stems. At lower concentration, the combined HMs stress promoted the growth of S. salsa in comparison to single HMs stress, however, opposite results were obtained at higher concentration. Overall, S. salsa showed high tolerance to Cu and Ni and stronger capabilities of HMs uptake and translocation, and therefore, it can be used as an alternative plant for the bioremediation of heavy metal pollution in mining area.

Abstract:In this study, pot and field experiments were conducted to study the enrichment of soil cadmium by Kochia scoparia. Further, rotations in pot experiments were carried out with four varieties of Brassica rapa to verify the remediation effect of Kochia scoparia on cadmium contamination in soil. The enrichment capacity of Kochia scoparia was leaf > root > stem with bioconcentration factors (BCFCd) of 15.07, 5.44 and 2.96, respectively. The total cadmium in soil decreased by 6.02% to 13.60% after planting Kochia scoparia, and the activities of soil urease and acid phosphatase also increased. The results of pot cultivation shows that the above-ground cadmium content of Brassica rapa in Kochia scoparia-Brassica rapa rotation system decreased by 17.21% on average compared with the control group without rotation, whereas the biomass increased slightly, and the above-ground translocation factors (TFCd) did not change significantly. These results suggest that the rotation of Brassica rapa with Kochia scoparia could increase the yield of Brassica rapa, and effectively reduce the cadmium content in edible parts of Brassica rapa, toward the purpose of realizing the green agricultural concept of “harnessing while producing”.

Abstract:In order to explore the effect of intercropping on the uptake of heavy metal cadmium (Cd), pot experiments were undertaken using three different planting methods: monoculture, restrictive intercropping and intercropping. The effects of Cd accumulation in different plant parts, and their causes, were examined using a plant species regarded as a relatively high heavy metal accumulator (tomato: Lycopersicon esculentum var. Zhongshu 4) and a species regarded as a relatively low heavy metal accumulator (maize: Zea mays L. var. Jinzhumi). Cd levels for all experiments were 3.70 mg/kg. Results indicate that restricted intercropping and intercropping of tomato and maize increased the accumulation of Cd (from 13.52 mg/kg to 24.94 mg/kg and 27.30 mg/kg in tomato leaf, respectively). Compared with the control group, pH levels in soil surrounding tomato roots in the intercropped samples decreased and the activity of acid phosphatase increased, while the activity of urease decreased. Intercropping can also change the structure of the crop root microorganism population, increase the abundance of microbiological species that promote the uptake of heavy metals, and finally achieve high accumulation of Cd in tomatoes. Our research results provide reference for controlling soil heavy metal pollution and ensuring food safety by using an intercropping model.

Abstract:To explore the effects of some chemical amendments on the plant growth and phytoextraction efficiencies of cadmium (Cd)/zinc (Zn) hyper accumulator Sedum plumbizincicola in acid soils with high aluminum (Al) toxicity, a greenhouse pot experiment was conducted. Different kinds and dosages of amendments including calcium- magnesium-phosphorus fertilizer (CMP), magnesium carbonate (MgCO3), potassium dihydrogen phosphate (KH2PO4) were added. The results showed that CMP and MgCO3 increased soil pH and decreased soil exchangeable Al concentration to some extent, while KH2PO4 reduced soil exchangeable Al concentration but had little effect on increasing soil pH. Proper application (9.39 mg/kg) of CMP could improve the biomass and Cd and Zn phytoextraction efficiencies by S. plumbizincicola but it would inhibit plant growth and phytoextraction performance when exceeding 9.39 mg/kg. MgCO3 addition enhanced plant metal uptake while KH2PO4 presented an opposite effect. It suggests that using CMP and MgCO3 could alleviate Al toxicity to S. plumbizincicola in acid soils and maintain relatively high metal extraction efficiency.

Abstract:Hyperaccumulators can hyper-accumulate and -tolerate heavy metals, thus are not only an ideal model to explore the mechanisms of ion transport and toxicity tolerance, but also play an irreplaceable role in the development and application of phytoremediation. Sedum plumbizincicola is a recently identified cadmium (Cd)/zinc (Zn) hyperaccumulator in the Crassulaceae family in China. Here we report the construction and screening of its yeast-expressing cDNA library. We identified a metallothionein protein encoding gene SpMT2. SpMT2 is localized in yeast cytoplasm and expression of it in yeast specifically enhanced resistance to Cd. Further analysis showed that SpMT2 did not affect Cd absorption in yeast, but greatly inhibited Cd transport into vacuoles, indicating that SpMT2 may reduce Cd toxicity via chelation in cytoplasm. qRT-PCR analyses indicated that SpMT2 was highly expressed both in roots and shoots, and did not respond to Cd treatment. Taking together the results that SpMT2 was also cytoplasm-localized in plants, we proposed that SpMT2 may chelate/detoxify Cd and retain the complex in cytosol, which renders higher mobility of Cd thus promoting long-distance Cd transport in S. plumbizincicola.

Abstract:Sedum plumbizincicola is a native cadmium/zinc (Cd/Zn) hyperaccumulator in China. At present, it has been applied to remediation of Cd contaminated soils. As the large differences in the removal efficiency of Cd and Zn for different soil conditions, the enhancement measure is important for the phytoremediation process. There have been many studies on the effects of earthworms on plant growth. But the effects on the growth of S. plumbizincicola and heavy metal removal efficiency have been rarely reported. There were 2 pot experiments: S. plumbizincicola was planted on 3 types of soils: Perudic Luvisols, Stagnic Anthrosols, and Udic Cambisols inoculated with Eisenia foetida to explore the effect of Eisenia foetida on the growth and Cd/Zn absorption of S. plumbizincicola. Stagnic Anthrosols with higher Cd effectiveness was selected in the second season pot experiment for further research the combined effect of earthworm and rich straw. The results of the first pot experiment showed that the addition of earthworms in acidic Perudic Luvisols increased the shoot biomass of S. plumbizincicola by 106% compared with the control treatment, and the Cd and Zn uptake increased by 72.0% and 36.0%, respectively. The soil available Cd was reduced by the addition of earthworms. The other two soils inoculated with earthworms had no enhancement on phytoremediation, addition of earthworms together with straw could improve the growth of S. plumbizincicola and the Cd/Zn uptake in Stagnic Anthrosols. The above results indicated that adding earthworm can enhance the phytoremediation of Cd/Zn hyperaccumulator, and the addition of straw is an important synergistic technique for earthworm-enhanced phytoremediation of S. plumbizincicola.

Abstract:In order to solve the problem of soil, water pollution and sensitive crop drug damage caused by chlorosulfuron residue, and to provide degradation strain resources for microbial remediation of contaminated soil, a chlorimuron-ethyl-degrading strain T9DB-01 was isolated from chlorosulfuron contaminated soil by the method of enrichment culture and step by step domestication. Strain T9DB-01 was identified as Pseudomonas sp. by morphological characteristics, physiological and biochemical analysis and 16S rDNA gene sequence analysis. The effects of temperature, pH value, substrate concentration, medium volume, and inoculation volume on the degradation of chlorsulfuron-methyl by strain T9DB-01 were investigated by single factor experiment. The degradation conditions of chlorosulfuron by strain T9DB-01 were optimized by orthogonal test and verification. Results show that 30 °C, pH 8.0, inoculum 4%, liquid volume 100 mL/250 mL, substrate concentration of 200 mg/L, cultured for 5 d, the strain degraded 93.7% chlorsulfuron-methyl. The degrading strain has certain application potential for bioremediation of chlorsulfuron-contaminated soil.