Ligand-enhanced electrokinetic remediation of metal-contaminated marine sediments with high acid buffering capacity

Geochip 5.0 insights into the association between bioleaching of heavy metals from contaminated sediment and functional genes expressed in consortiums

The heavy metal contamination in river and lake sediments endangers aquatic ecosystems. Herein, the feasibility of applying different exogenous mesophile consortiums in bioleaching multiple heavy metal-contaminated sediments from Xiangjiang River was investigated, and a comprehensive functional gene array (GeoChip 5.0) was used to analyze the functional gene expression to reveal the intrinsic association between metal solubilization efficiency and consortium structure. Among four consortiums, the Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans consortium had the highest solubilization efficiencies of Cu, Pb, Zn, and Cd after 15 days, reaching 50.33, 29.93, 47.49, and 79.65%, while Cu, Pb, and Hg had the highest solubilization efficiencies after 30 days, reaching 63.67, 45.33, and 52.07%. Geochip analysis revealed that 31,346 genes involved in different biogeochemical processes had been detected, and the systems of 15 days had lower proportions of unique genes than those of 30 days. Samples from the same stage had more genes overlapping with each other than those from different stages. Plentiful metal-resistant and organic remediation genes were also detected, which means the metal detoxification and organic pollutant degradation had happened with the bioleaching process. The Mantel test revealed that Pb, Zn, As, Cd, and Hg solubilized from sediment influenced the structure of expressed microbial functional genes during bioleaching. This work employed GeoChip to demonstrate the intrinsic association between functional gene expression of mesophile consortiums and the bioleaching efficiency of heavy metal-contaminated sediment, and it provides a good reference for future microbial consortium design and remediation of river and lake sediments.

Temperature-dependent dynamics of electrokinetic conservative and reactive transport in porous media: A model-based analysis

Electrokinetic techniques employ direct current electric fields to enhance the transport of amendments in low permeability porous media and have been demonstrated effective for in situ remediation of both organic contaminants and heavy metals. The application of electric potential gradients give rise to coupled chemical, hydraulic and electric fluxes, which are at the basis of the main transport mechanisms: electromigration and electroosmosis. Previous research has highlighted the significant impacts of charge interactions and fluid composition, including temperature-dependent properties such as electrolyte conductivity and density, on these transport phenomena. However, current models of electrokinetic applications often assume isothermal conditions and overlook the production of heat resulting from Joule heating. This study provides a detailed model-based investigation, systematically exploring the effects of temperature on electrokinetic conservative and reactive transport in porous media. By incorporating temperature-dependent material properties and progressively investigating the impact of temperature on each transport mechanism, we analyze the effects of temperature variations in both 1D and 2D systems. The study reveals how temperature dynamically influences the physical, chemical and electrostatic processes controlling electrokinetic transport. A temperature increase results in a higher speed of amendments delivery by both electromigration and electroosmosis and increases the kinetics of degradation reactions. The simulations also reveal a feedback mechanism in which higher aqueous conductivity results in increased Joule heating, leading to a faster temperature rise and, subsequently, to higher electrolyte conductivity. Finally, we estimate the electric energy requirements of the system at varying temperatures and show how these changes impact the rate of contaminant removal.

Potential phytoremediation system using macrophyte Limnobium laevigatum to remove in situ Cr from contaminated bottom sediments

The contamination of aquatic environments with heavy metals is an important issue, and in turn, it is crucial to study remediation techniques that can be applied in situ. In this work, the use of a containment system with macrophytes Limnobium laevigatum is explored in the laboratory to evaluate the remotion of Cr in contaminated sediments. The roots of the plants were placed in contact with the bottom sediment through a containment system. The concentration of Cr in macrophyte and sediment samples exposed to different exposure times (1, 4 and 7 days) was determined by laser-induced breakdown spectroscopy technique. The initial concentration of Cr in the sediment was 112 ± 5 mg/kg and decreased by 65% to the control (p < 0.05) after 24 h of exposure. The removal continued throughout the study time until reaching values of 23 ± 1 mg/kg. In macrophytes, the Cr concentration increased from 20 ± 5 mg/kg to 2066 ± 216 mg/kg after seven days of exposure. The correlation coefficient between Cr concentrations in both matrices was -0.96. Finally, the bioaccumulation factor of Cr in L. laevigatum was 95.22 ± 8.51. Therefore, the system studied could be a potential tool to remedy the bottom sediments of streams and lakes contaminated with heavy metals in situ.

Effects of sulfur dosage on continuous bioleaching of heavy metals from contaminated sediment

The bioleaching technology has been considered as a promising green technology for remediation of contaminated sediments in recent years. Bioleaching technology was generally conducted in the batch bioreactor; however, the continuous bioreactor should be developed for the application of bioleaching technology in the future. The purposes of this study were to establish a continuous bioleaching process, and to evaluate the effects of sulfur dosage on the efficiency of metal removal during this continuous bioleaching process. The obtained results show that the pH decrease, sulfate production and metal removal efficiency all increased with increasing sulfur dosage in the continuous bioleaching process due to high substrate concentration for sulfur-oxidizing bacteria. After 30 days of operation time, the maximum solubilization efficiencies for Zn, Ni, Cu and Cr were found to be 78%, 90%, 88% and 68%, respectively, at 5% of sulfur dosage. After the bioleaching process, heavy metals bound in the carbonates, Fe-Mn oxides and organics/sulfides in the sediment were effectively removed and the potential ecological and toxic risks of treated sediment were greatly reduced. The results of bacterial community analyses demonstrated that this continuous bioleaching process were dominated by several acidophilic sulfur-oxidizing bacteria; S. thermosulfidooxidans, At. thiooxidans/At. ferrooxidans, S. thermotolerans and At. albertensis, whereas the percentage of less-acidophilic sulfur-oxidizing bacteria (T. thioparus and T. cuprina) was lower than 15% of total bacteria. In addition, the cell numbers of sulfur-oxidizing bacteria increased as the sulfur dosage was increased in the continuous bioleaching process.

Enhanced electrokinetic remediation for Cd-contaminated clay soil by addition of nitric acid, acetic acid, and EDTA: Effects on soil micro-ecology

Enhanced electrokinetic remediation (EKR) allows the rapid remediation of heavy metal-contaminated clay, but the impacts of this process on soil micro-ecology have rarely been evaluated. In this study, nitric acid, acetic acid, and EDTA were applied for enhancement of EKR and the effects on Cd removal, soil enzyme activity, and soil bacterial communities (SBCs) were determined. Nitric acid and acetic acid allowed 93.2% and 91.8% Cd removal, respectively, and EDTA treatment resulted in 40.4% removal due to the formation of negatively charged EDTA-Cd complexes, resulting in opposing directions of Cd electromigration and electroosmosis flow and slow electromigration rate caused by low voltage drop. Activities of soil beta-glucosidase, acid phosphatase, and urease, were all reduced by enhanced EKR treatment, especially nitric acid treatment, by 46.2%, 58.8% and 57.7%, respectively. The SBCs were analyzed by high-throughput sequencing and revealed significantly increased diversity for acetic acid treatment, no effect for EDTA treatment, and reduced diversity for nitric acid treatment. Compared with nitric acid and EDTA, acetic acid treatment enhanced EKR for higher Cd removal and improved biodiversity.

Enhanced electrokinetic remediation (EKR) for heavy metal-contaminated sediments focusing on treatment of generated effluents from EKR and recovery of EDTA

Electrokinetic remediation (EKR) is one of the most successful remediation techniques to treat the sediments contaminated with heavy metals. EDTA is the most widely used enhancing agent to improve the transport process in EKR. But often the generated effluents from EKR contains a high concentration of heavy metals, which cannot be disposed of without treatment. The major objective of this study includes the estimation of optimal concentration of chelating agent EDTA, followed by treatment of contaminated sediments by EKR technique for heavy metal removal. The effluents generated from EKR were further studied for recovery and reuse of EDTA and for safe discharge of heavy metals. The optimum concentration of EDTA was found as 0.05 M with a solid-to-liquid ratio as 1:10. When fresh EDTA was used as enhancing agent the average removal of heavy metals obtained as 74.8% with EKR, whereas the application of recovered EDTA in treatment process in first, second, and third cycle showed the slight reduction of heavy metals of about 71.1%, 63.5%, and 52.1%, respectively. The heavy metal removal by recovered EDTA was effective in reduction of heavy metals up to three cycles of re-use while reducing the ecological risk in sediments. PRACTITIONER POINTS: Treatment of contaminated sediments with heavy metals achieved by electrokinetic remediation (EKR) technique enhanced with EDTA. The recovery of EDTA and heavy metal reduction from the generated effluents during EKR treatment were performed by the addition of FeCl₃ and Na₂ PO_4, and optimized concentration was evaluated. This study found that the use of recovered EDTA in EKR treatment has effectively reduced the risk associated with heavy metals.

Experimental study on removing heavy metals from the municipal solid waste incineration fly ash with the modified electrokinetic remediation device

The MSWI fly ash which contains a large number of heavy metal substances is a subsidiary product of waste incineration power generation technology. If the MSWI fly ash is disposed improperly, heavy metal pollutants will pose a great threat to environmental safety and human health. Based on the technology of electrokinetic remediation, the feasibility of removing heavy metal pollutants from the MSWI fly ash using a modified electrokinetic remediation device - cylinder device was evaluated in this study. Differing from the traditional cuboid device with the volume ratio of the cathode chamber to the anode chamber being 1:1, the volume ratio of the cathode chamber to the anode chamber of the cylinder device was 16:1. Changes in parameters, such as pH values and conductivity in the cathode and the anode chambers as well as current and voltage in the sample area were analysed under the voltage gradient of 2 V/cm. After the experiment, the average removal efficiencies for Zn, Pb, Cd and Cu in the sample area were 53.2%, 31.4%, 42.3% and 30.7%, respectively. It indicates that the cylinder device is effective in removing heavy metals from the MSWI fly ash. Adopting the cylinder device for the experimental study on the electrokinetic remediation technology could provide a better way of thinking for the future engineering practices and applications.

Ferric-enhanced chemical remediation of dredged marine sediment contaminated by metals and petroleum hydrocarbons

Sediments nearby harbors are dredged regularly, and the sediments require the stringent treatment to meet the regulations on reuse and mitigate the environmental burdens from toxic pollutants. In this study, FeCl₃ was chosen as an extraction agent to treat marine sediment co-contaminated with Cu, Zn, and total petroleum hydrocarbons (TPH). In chemical extraction process, the extraction efficiency of Cu and Zn by FeCl₃ was compared with the conventional one using inorganic acids (H₂SO₄ and HCl). Despite the satisfactory level for extraction of Cu (78.8%) and Zn (73.3%) by HCl (0.5 M) through proton-enhanced dissolution, one critical demerit, particularly acidified sediment, led to the unwanted loss of Al, Fe, and Mg by dissolution. Moreover, the vast amount of HCl required the huge amounts of neutralizing agents for the post-treatment of the sediment sample via the washing process. Despite a low concentration, extraction of Cu (70.1%) and Zn (69.4%) was done by using FeCl₃ (0.05 M) through proton-enhanced dissolution, ferric-organic matter complexation, and oxidative dissolution of sulfide minerals. Ferric iron (Fe³⁺) was reduced to ferrous iron (Fe²⁺) with sulfide (S^2-) oxidation during FeCl₃ extraction. In consecutive chemical oxidations using hydrogen peroxide (H₂O₂) and persulfate (S₂O₈^2-), the resultant ferrous iron was used to activate the oxidants to effectively degrade TPH. S₂O₈^2- using FeCl₃ solution (molar ratio of ferrous to S₂O₈^2- is 19.8-198.3) removed 42.6% of TPH, which was higher than that by H₂O₂ (molar ratio of ferrous to H₂O₂ is 1.2-6.1). All experimental findings suggest that ferric is effectively accommodated to an acid washing step for co-contaminated marine sediments, which leads to enhanced extraction, cost-effectiveness, and less environmental burden.

The influence of Magnafloc10 on the acidic, alkaline, and electrodialytic desorption of metals from mine tailings

Repparfjorden in northern Norway has been partly designated for submarine mine tailings disposal when the adjacent Cu mine re-opens in 2019. In order to increase sedimentation, the flocculant, Magnafloc10 is planned to be added to the mine tailings prior to discharge into the fjord. This study investigated the feasibility of reducing the Cu concentrations (375 mg/kg) in the mine tailings by applying electrodialytic extraction, including potential optimisation by adding Magnafloc10. In the acidic electrodialytic treatment (pH < 2), Magnafloc10 increased the extraction of Cu from the mine tailings particles from 76 to 86%, and the flocs with adsorbed metals were separated from the tailings solids by the electric field (1 mA/cm²). The electric energy consumption increased with the use of Magnafloc10 (from 17 to 30 kWh/g Cu extracted), due to lower conductivity in the liquid phase and clogging of the membrane by the flocs. In the alkaline electrodialytic treatment (pH > 12), Magnafloc10 reduced the extraction of Cu from 17% to 0.7%, due to the flocs remaining in the tailing slurries. The electric energy consumption per extracted Cu was similar in the acidic and alkaline electrodialytic treatments without the addition of Magnafloc10. In the alkaline electrodialytic treatment, the extraction of other metals was low (<2%), however longer treatment time is necessary to achieve similar Cu extraction as in the acidic electrodialysis. Depending on the target and timescale for treatment, acidic and alkaline electrodialysis can be employed to reduce the Cu concentration in the mine tailings thereby reducing the metal toxicity potential.

Operating optimization for the heavy metal removal from the municipal solid waste incineration fly ashes in the three-dimensional electrokinetics

The three-dimensional (3D) electrode has been widely studied and curiously pushed forward for the treatment of various wastewater. However, the reports on the electro-remediation of municipal solid waste incineration fly ashes using 3D electrochemical platform are scarce. In this research, the removal of heavy metals from the municipal solid waste incineration fly ashes was comprehensively investigated using a 3D electrokinetic system. Central composite design method was used based on the 31 trials and desirability functions in the 3D electrochemical process to evaluate the main effect and their interacted effect of parameter-variables on the remedial efficiencies. The maximum removal efficiencies of zinc (Zn), lead (Pb), copper (Cu), and cadmium (Cd) were 79.54%, 67.34%, 81.34%, and 59.11%, respectively. The 3D electrode system was demonstrated to be more effective in removing heavy metals from samples compared with the 2D electrokinetic process. The intensification of hydrolysis reaction and the enhancement of electromotive force were concluded based on the mechanism analysis as two main reasons in improving heavy-metal removals with a 3D electrode in the electrokinetic remediation.

Efficient Removal of Polycyclic Aromatic Hydrocarbons, Dyes, and Heavy Metal Ions by a Homopolymer Vesicle

It is an important challenge to effectively remove environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs), dyes, and heavy metal ions at a low cost. Herein, we present a multifunctional homopolymer vesicle self-assembled from a scalable homopolymer, poly(amic acid) (PAA), at room temperature. The vesicle can efficiently eliminate PAHs, cationic dyes, and heavy metal ions from water based on π-π stacking, hydrophobic effect, and electrostatic interactions with the pollutants. The residual concentrations of PAHs, cationic dyes, and heavy metal ions (such as Ni²⁺) in water are lower than 0.60 and 0.30 parts per billion (ppb) and 0.095 parts per million (ppm), respectively, representing a promising adsorbent for water remediation. Furthermore, precious metal ions such as Ag⁺ can be recovered into silver nanoparticles by in situ reduction on the membrane of PAA vesicles to form a silver nanoparticle/vesicle composite (Ag@vesicle) that can effectively catalyze the reduction of toxic pollutants such as aromatic nitro-compounds and be recycled for more than ten times.

Mild electrokinetic treatment of cadmium-polluted manure for improved applicability in greenhouse soil

Applications of cadmium (Cd) and salinity-containing manures contribute to Cd pollution and salinization in greenhouse soils. In this study, chicken manure polluted with Cd (5.6 mg/kg) was mildly electrokinetically treated (0.25 V/cm) for 48 h with intermittent replacement of catholyte with 20 mM acetic acid solution to remove Cd and salinity for application without need of post-treatment in greenhouse soil. The electrokinetic treatment created pH conditions mainly ranging from 5.0 to 8.0 within the manure for minimizing re-precipitation of desorbed Cd and evaporative loss of ammonium. However, without manure pre-acidification, electrokinetic treatment resulted in negligible removal of total Cd but 61.7% of increase in the small fraction of exchangeable Cd, due to poor desorption but enhanced formation of exchangeable Cd. In contrast, manure pre-acidification with 20 mM acetic acid favored Cd desorption, leading to electrokinetic removal of exchangeable, carbonate-bound, and total Cd by 32.2%, 34.5%, and 14.5%, respectively. Mild electrokinetic treatment of manure with and without pre-acidification resulted in similar removal of salinity (72.3% and 68.0%), similar pH condition (7.2 and 7.4), and basically same evaporative loss of ammonium (14.6% and 14.2%). Overall, the mild electrokinetic treatment considerably lowered the risk of Cd and the salinity from the pre-acidified manure for improved applicability in greenhouse soil, and more studies are needed to enhance the performance of electrokinetic Cd removal from manure.

Multispecies reactive transport modelling of electrokinetic remediation of harbour sediments

We implemented a numerical model to simulate transport of multiple species and geochemical reactions occurring during electrokinetic remediation of metal-contaminated porous media. The main phenomena described by the model were: (1) species transport by diffusion, electromigration and electroosmosis, (2) pH-dependent buffering of H⁺, (3) adsorption of metals onto particle surfaces, (4) aqueous speciation, (5) formation and dissolution of solid precipitates. The model was applied to simulate the electrokinetic extraction of heavy metals (Pb, Zn and Ni) from marine harbour sediments, characterized by a heterogeneous solid matrix, high buffering capacity and aged pollution. A good agreement was found between simulations of pH, electroosmotic flow and experimental results. The predicted residual metal concentrations in the sediment were also close to experimental profiles for all of the investigated metals. Some removal overestimation was observed in the regions close to the anode, possibly due to the significant metal content bound to residual fraction.