Two-compartment membrane electrochemical remediation of heavy metals from an aged electroplating-contaminated soil: A comparative study of anodic and cathodic processes

In this study, two-compartment membrane electrochemical remediation (MER) based on the anode process and the cathode process strategies were compared for treating a multi metal -contaminated soil. Remediation effect, as well as energy consumption and risk evaluation of the two strategies under different current density conditions of electroplating-contaminated soil suspension were performed, the following conclusions were drawn. MERs based on both the anode and cathode processes exhibited a synergetic effect because the DC electric field and extractants dissolved more metals from the soil phase into the liquid phase of the suspension compared to a usual soil washing treatment. The maximum Cr, Cu, and Ni removal efficiencies of MERs based on the anode process were 79.5%, 86.2%, and 85.0%, respectively, compared to 27.5%, 72.5%, and 65.9% based on the cathode process. Risk assessment results showed lower soil environmental risk after MER based on the cathode process than after MER based on the anode process. In this study, MER based on the cathode process as an evolving soil remediation strategy was found to present high simultaneous remediation ability for soil heavy metals and leaching materials, showing its advantages of environmental friendliness and economic effectiveness.

Combined biochar and metal-immobilizing bacteria reduces edible tissue metal uptake in vegetables by increasing amorphous Fe oxides and abundance of Fe- and Mn-oxidising Leptothrix species

In this study, a highly effective combined biochar and metal-immobilizing bacteria (Bacillus megaterium H3 and Serratia liquefaciens CL-1) (BHC) was characterized for its effects on solution Pb and Cd immobilization and edible tissue biomass and Pb and Cd accumulation in Chinese cabbages and radishes and the mechanisms involved in metal-polluted soils. In the metal-containing solution treated with BHC, the Pb and Cd concentrations decreased, while the pH and cell numbers of strains H3 and CL-1 increased over time. BHC significantly increased the edible tissue dry weight by 17-34% and reduced the edible tissue Pb (0.32-0.46 mg kg^-1) and Cd (0.16 mg kg^-1) contents of the vegetables by 24-45%. In the vegetable rhizosphere soils, BHC significantly decreased the acid-soluble Pb (1.81-2.21 mg kg^-1) and Cd (0.40-0.48 mg kg^-1) contents by 26-47% and increased the reducible Pb (18.2-18.8 mg kg^-1) and Cd (0.38-0.39 mg kg^-1) contents by 10-111%; while BHC also significantly increased the pH, urease activity by 115-169%, amorphous Fe oxides content by 12-19%, and relative abundance of gene copy numbers of Fe- and Mn-oxidising Leptothrix species by 28-73% compared with the controls. These results suggested that BHC decreased edible tissue metal uptake of the vegetables by increasing pH, urease activity, amorphous Fe oxides, and Leptothrix species abundance in polluted soil. These results may provide an effective and eco-friendly way for metal remediation and reducing metal uptake in vegetables by using combined biochar and metal-immobilizing bacteria in polluted soils.

Application of signaling molecules in reducing metal accumulation in alfalfa and alleviating metal-induced phytotoxicity in Pb/Cd-contaminated soil

The utilization of forages grown on metal-contaminated soil can increase the risk of heavy metals entering the food chain and affecting human health because of elevated toxic metal concentrations. Meanwhile, hydrogen sulfide (H₂S) and nitric oxide (NO) as signaling molecules are known to promote plant growth in metal-contaminated soils. However, the regulatory mechanisms of such molecules in plant physiology and soil biochemistry have not been well-documented. Hence, we investigate the role of the exogenous application of H₂S and NO on alfalfa growth in lead/cadmium (Pb/Cd)-contaminated soil. Our results indicate that the signaling molecules increase the alfalfa chlorophyll and biomass content and improve alfalfa growth. Further, H₂S and NO reduce the translocation and bioconcentration factors of Pb and Cd, potentially reducing the risk of heavy metals entering the food chain. These signaling molecules reduce metal-induced oxidative damage to alfalfa by mitigating reactive oxygen species accumulation and increasing antioxidant enzyme activities. Their exogenous application increases soil enzymatic activities, particularly of catalase and polyphenol oxidase, without significantly changing the composition and structure of rhizosphere bacterial communities. Interestingly, H₂S addition enriches the abundance of plant-growth-promoting rhizobacteria in soil, including Nocardioides, Rhizobium, and Glycomyces. H₂S is more effective than NO in improving alfalfa growth and reducing heavy-metal contamination of the food chain. These results provide new insights into the exogenous application of signaling molecules in alleviating metal-induced phytotoxicity, including an efficient strategy for the safe use of forages.

TiO2/SiO2 decorated carbon nanostructured materials as a multifunctional platform for emerging pollutants removal

Aquatic ecosystem contaminated with hazardous pollutants has become a high priority global concern leading to serious economic and environmental damage. Among various treatment approaches, carbon nanostructured materials have received particular interest as a novel platform for emerging pollutants removal owing to their unique chemical and electrical properties, biocompatibility, high scalability, and infinite functionalization possibility with an array of inorganic nanomaterials and bio-molecules. Within this framework, carbon nanotubes (CNTs) are widely used due to their hollow and layered structure and availability of large specific surface area for the incoming contaminants. Carbon nanotubes can be used either as single-walled, multi-walled, or functionalized nanoconstructs. TiO₂/SiO₂-functionalized CNTs are among the most promising heterogeneous photocatalytic candidates for the degradation of a range of organic compounds, heavy metals reduction, and selective oxidative reactions. Herein, we reviewed recent development in the application of TiO₂ and SiO₂ functionalized nanostructured carbon materials as potential environmental candidates. After a brief overview of synthesis and properties of CNTs, we explicitly discussed the potential applications of TiO₂/SiO₂ functionalized CNTs for the remediation of a variety of environmentally-related pollutants of high concern, including synthetic dyes or dye-based hazardous waste effluents, as polycyclic aromatic hydrocarbons (PAHs), pharmaceutically active compounds, pesticides, toxic heavy elements, remediation of metal-contaminated soil, and miscellaneous organic contaminants. The work is wrapped up by giving information on current challenges and recommended guidelines about future research in the field bearing in mind the conclusions of the current review.

Water managements limit heavy metal accumulation in rice: Dual effects of iron-plaque formation and microbial communities

Understanding the mechanisms on how water management can minimize the concentrations of heavy metals in rice grains is important. Two water managements were concerned in our studies, including continuously flooding and alternate wetting and drying (AWD). Compared to AWD, a continuously flooded culture reduces the concentration of cadmium and other metals in the rice grains by reducing the root-to-shoot translocation and the availability of metals in rhizosphere. In a flooded environment, the rice rhizosphere was characterized by an increased soil pH, reduced fluorescein diacetate (FDA) activity, and lower metal bioavailability. In addition, flooding significantly decreased the iron plaque on the root surface and reduced the affinity for metals in rhizosphere. Water managements significantly changed soil microbial diversity, especially the proportion of anaerobic bacteria, including the iron-reducing bacteria Latescibacteria, Desulfuromonadales, and Geobacteraceae. Interestingly, these bacteria exhibited a significant correlation with cadmium that was adsorbed on the root. This study revealed that continuously flooded culture is a valuable strategy for minimizing heavy metal accumulation in rice grains. By increasing the abundance of unique bacterial community, iron plaque formation and the affinity of metals in rhizosphere were reduced, and the uptake and accumulation of heavy metals in rice plants was finally mitigated.

Ten-year regional monitoring of soil-rice grain contamination by heavy metals with implications for target remediation and food safety

Farmland soil heavy metal contamination could pose potential risks to ecosystems, food safety and human health ultimately. Regional researches on the long-term monitoring of heavy metals in a soil-rice grain system, changed with environmental policy adjustment, have been hindered by limited detailed data. In this study, we collected 169 paired paddy rice grain and corresponding soil samples from a former intensive electronic-waste dismantling region to survey the current status of heavy metal contamination, and to reveal the temporal trends over the past decade based on the previous data obtained in 2006 and 2011. Moderate contaminations of Cd, Cu, Zn and Ni were observed in soil currently. Furthermore, 20.7% of rice grain samples exceeded the Cd threshold value. Cd, Cu, Zn and Pb shared the similar spatial distribution pattern with higher concentrations in northwest, which were contrary to Cr, Ni and As. Risk assessment indicated that much attention is required for the carcinogenic risk of Cr, Cd and As and non-carcinogen risk of Cr. Combining the spatial distribution of heavy metals in soil and rice grains, and the potential ecological risks, with the human health risks, the middle-west rice paddies were identified and proposed as priority areas. Percentage of soil Pb, Cd and Zn decreased in most area and slightly increased in northwest and east. Cu decreased in southwest and increased in central part, while Ni slightly increased in the whole region between 2006 and 2016. With the scrutiny of strict environmental policy, Cd still remained relatively constant levels in soil and rice grains during the last decade, which confirmed that the heavy metals were persisted over the long duration. Target sustainable and ongoing green remediation methods should be adopted urgently in specific area to guarantee food safety and human health for local residents.

Metal-immobilizing Serratia liquefaciens CL-1 and Bacillus thuringiensis X30 increase biomass and reduce heavy metal accumulation of radish under field conditions

In this study, metal-tolerant bacteria Serratia liquefaciens CL-1 and Bacillus thuringiensis X30 were compared for their Cd and Pb immobilization in solution and impacts on biomass and Cd and Pb uptake in a radish in metal-contaminated soils under field conditions. Strains CL-1 and X30 significantly reduced water-soluble Cd and Pb concentrations (45-67%) and increased the pH in solution compared to the controls. These strains significantly increased the biomass (25-99%) and decreased edible tissue Cd and Pb uptake in the radish (37-81%) and DTPA-extractable Cd and Pb contents (18-44%) of the rhizosphere soil compared to the un-inoculated controls. Strain CL-1 had higher potential to reduce edible tissue Cd and Pb uptake in the radish and DTPA-extractable Cd content than strain X30. Also, these strains significantly increased Cd translocation factor and strain CL-1 also significantly increased Pb translocation factor of the radish. Furthermore, strain CL-1 significantly increased the ratio of small soil aggregates (< 0.25 mm and 0.25-0.50 mm) of the rhizosphere soil. The results showed that these strains reduced the edible tissue Cd and Pb uptake through decreasing Cd and Pb availability in the soil and increasing Cd or Pb translocation from the roots to the leaves of the radish. The results also suggested the bacteria-related differences in reduced heavy metal uptake in the radish and the mechanisms involved under field conditions.