Comparison of different extracting agents for the recovery of Pb and Zn through electrokinetic remediation of mine tailings

Electrokinetic Remediation of Cu- and Zn-Contaminated Soft Clay with Electrolytes Situated above Soil Surfaces

Electrokinetic remediation (EKR) has shown great potential for the remediation of in situ contaminated soils. For heavy metal-contaminated soft clay with high moisture content and low permeability, an electrokinetic remediation method with electrolytes placed above the ground surface is used to avoid issues such as electrolyte leakage and secondary contamination that may arise from directly injecting electrolytes into the soil. In this context, using this novel experimental device, a set of citric acid (CA)-enhanced EKR tests were conducted to investigate the optimal design parameters for Cu- and Zn-contaminated soft clay. The average removal rates of heavy metals Cu and Zn in these tests were in the range of 27.9-85.5% and 63.9-83.5%, respectively. The results indicate that the Zn removal was efficient. This was determined by the migration intensity of the electro-osmotic flow, particularly the volume reduction of the anolyte. The main factors affecting the Cu removal efficiency in sequence were the effective electric potential of the contaminated soft clay and the electrolyte concentration. Designing experimental parameters based on these parameters will help remove Cu and Zn. Moreover, the shear strength of the contaminated soil was improved; however, the degree of improvement was limited. Low-concentration CA can effectively control the contact resistance between the anode and soil, the contact resistance between the cathode and soil, and the soil resistance by increasing the amount of electrolyte and the contact area between the electrolyte and soil.

Highly alkaline electrokinetic extraction: Characteristics of chromium mobilization, conversion and transport in high alkalinity soil

In site chromium (Cr) contaminated soil characterized by high alkalinity and carbonate content, protons are not effectively targeted for Cr(III) mobilization but rather accelerate the reduction of easily transportable Cr(VI) within the acidification electrokinetic (EK) system. As an alternative, the highly alkaline extraction conditions (HAECs) maintained by anolyte regulation are explored owing to the ability to desorb strong binding Cr(VI) and form anionic Cr(III)-hydroxides (Cr(OH)4-, Cr(OH)52-). The results demonstrate that HAECs were more efficient in mobilizing ions in severe alkalinity and electrical conductivity soil compared to organic acid acidifying extraction conditions (OAECs). Simultaneously, a limited amount of soluble Cr(III) was produced; however, its transportation was hindered and more noticeable in the case of Cr(VI), displaying a distinct retention phase within the intermediate soil chamber. The antagonistic interplay between electromigration and electroosmotic flow was considered the main responsible factor. The conversion intensity of Cr(VI) to Cr(III) was inhibited at HAECs. The promising mobilization and low conversion intensity contributed to total Cr removal. At HAECs, enhanced electromigration and electroosmotic flow combined with a favorable oxidation environment may facilitate in situ delivery of oxidants, offering practical implications for the EK detoxification of high alkalinity site soil contaminated with Cr. The practicability of HAECs is likely to be enhanced when the cost-benefit balance of providing a simultaneous energy supply during site treatment is resolved.

Restoration of degraded estuarine and marine ecosystems: A systematic review of rehabilitation methods in Europe

This article provides a comprehensive study of ecosystem rehabilitation methods widely used in the 21st century, focusing on Europe. The review covers the evolution and trends in scientific article publication, identification of European countries demonstrating high publication outputs, collaboration patterns, leading journals, and thematic areas. Additionally, it examines primary stressors in European aquatic ecosystems, and different methods and treatments commonly employed for remediation purposes. The analysis of selected articles revealed a significant increase in studies over time, driven by public awareness and financial incentives from national, European and global organizations. Italy, Portugal and Spain were the leading countries in degraded ecosystem rehabilitation studies, mainly focusing on remediating contaminated areas where metals were identified as the primary stressor (chemical pollution). Chemical remediation method emerged as the most used, closely followed by biological remediation method, which have gained prominence in recent years due to their ecological, economic, and social combined benefits. Furthermore, recent studies demonstrate a growing trend towards the combined use of more than one treatment/method to rehabilitate ecosystems, particularly with biological treatments. This combined approach has the potential for synergistic effects in achieving more effective rehabilitation and their sustainability in the long term, thus, a focus for future research.

Different responses of Sinorhizobium sp. upon Pb and Zn exposure: Mineralization versus complexation

Lead (Pb) and zinc (Zn) have been discharged into environment and may negatively impact ecological security. Rhizobia has gained attention due to their involvement in the restoration of metal polluted soils. However, little is known about the responses of rhizobia under Pb and Zn stress, especially the roles played by extracellular polymeric substances (EPS) in the resistance of these two metals. Here, Sinorhizobium sp. C10 was isolated from soil around a mining area and was exposed to a series of Pb/Zn treatments. The cell morphology and surface mineral crystals, EPS content and fluorescent substances were determined. In addition, the extracellular polysaccharides and proteins were characterized by attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS). The results showed that Zn stress induced the synthesis of EPS by C10 cells. Functional groups of polysaccharides (CO) and proteins (C-O/C-N) were involved in complexation with Zn. In contrast, C10 resisted Pb stress by forming lead phosphate (Pb3(PO4)2) on the cell surface. Galactose (Gal) and tyrosine played key roles in resistance to the Zn toxicity, whereas glucosamine (N-Glc) was converted to glucose in large amounts during extracellular Pb precipitation. Together, this study demonstrated that C10 possessed different strategies to detoxify the two metals, and could provide basis for bioremediation of Pb and Zn polluted sites.

Removal of heavy metals from mine tailings by in-situ bioleaching coupled to electrokinetics

This work utilizes a combined biological-electrochemical technique for the in-situ removal of metals from polluted mine tailings. As the main novelty point it is proposed to use electrokinetics (EK) for the in-situ activation of a bioleaching mechanism into the tailings, in order to promote biological dissolution of metal sulphides (Step 1), and for the subsequent removal of leached metals by EK transport out of the tailings (Step 2). Mine tailings were collected from an abandoned Pb/Zn mine located in central-southern Spain. EK-bioleaching experiments were performed under batch mode using a lab scale EK cell. A mixed microbial culture of autochthonous acidophilic bacteria grown from the tailings was used. Direct current with polarity reversal vs alternate current was evaluated in Step 1. In turn, different biological strategies were used: biostimulation, bioaugmentation and the abiotic reference test (EK alone). It was observed that bioleaching activation was very low during Step 1, because it was difficult to maintain acidic pH in the whole soil, but then it worked correctly during Step 2. It was confirmed that microorganisms successfully contributed to the in-situ solubilization of the metal sulphides as final metal removal rates were improved compared to the conventional abiotic EK (best increases of around 40% for Cu, 162% for Pb, 18% for Zn, 13% for Mn, 40% for Ni and 15% for Cr). Alternate current seemed to be the best option. The tailings concentrations of Fe, Al, Cu, Mn, Ni and Pb after treatment comply with regulations, but Pb, Cd and Zn concentrations exceed the maximum values. From the data obtained in this work it has been observed that EK-bioleaching could be feasible, but some upgrades and future work must be done in order to optimize experimental conditions, especially the control of soil pH in acidic values.

Study on mechanical properties and damage characteristics of cemented waste rock-tailing backfill

Tailing and waste rock-cemented filling is an effective way to solve the problem solid waste in mines. In this paper, the effects of waste rock content and cement-sand ratio on the properties of tailing-waste rock-cemented filling materials and cemented backfill were analyzed based on the single-factor multi-level experimental design method. The results show that with the increase of waste rock content, the fluidity of the filling slurry increases first and then decreases, the bleeding rate increased gradually, and the compressive strength of the backfill increases first and then decreases. When the waste rock content is 60% and the cement-sand ratio is 1:4, the cemented backfill has higher compressive strength. With the increase of waste rock content, the interface failure area between waste rock particles and cementitious matrix under loading gradually increases, the crack extension is more complex, and the acoustic emission (AE) ringing count is higher. Microstructural analysis showed that the main hydration products in the cemented backfill were calcium silicate hydrated (C-S-H) gels, ettringite (AFt), and calcium hydroxide (Ca(OH)2). Because there is more content of hydration products, the microstructure of the cemented backfill was denser and the compressive strength was higher. Based on the results of uniaxial compression tests, the damage constitutive model of cemented backfill with different waste rock contents and cement-sand ratios was established, which could provide guidance for the design and safety production of phosphate rock filling engineering.

Shoot-root signal circuit: Phytoremediation of heavy metal contaminated soil

High concentrations of heavy metals in the environment will cause serious harm to ecosystems and human health. It is urgent to develop effective methods to control soil heavy metal pollution. Phytoremediation has advantages and potential for soil heavy metal pollution control. However, the current hyperaccumulators have the disadvantages of poor environmental adaptability, single enrichment species and small biomass. Based on the concept of modularity, synthetic biology makes it possible to design a wide range of organisms. In this paper, a comprehensive strategy of "microbial biosensor detection - phytoremediation - heavy metal recovery" for soil heavy metal pollution control was proposed, and the required steps were modified by using synthetic biology methods. This paper summarizes the new experimental methods that promote the discovery of synthetic biological elements and the construction of circuits, and combs the methods of producing transgenic plants to facilitate the transformation of constructed synthetic biological vectors. Finally, the problems that should be paid more attention to in the remediation of soil heavy metal pollution based on synthetic biology were discussed.

Incidence of Electric Field and Sulfuric Acid Concentration in Electrokinetic Remediation of Cobalt, Copper, and Nickel in Fresh Copper Mine Tailings

In the present study, the assessment of heavy metal contaminant migration from fresh mine tailings was conducted using the electrokinetic remediation technique (EKR). In this sense, a pilot EKR cell was designed to evaluate the recovery potential of copper, nickel, and cobalt species. In particular, the focus was on the impacts of electric field intensity and pH in initial mixture and testing their interaction in copper, nickel, and cobalt migration. Experiments were made using a 22 factorial experimental design with a central point, using DC electric fields from 1.0 to 2.0 V cm−1 and H2SO4 pretreatment solutions from 1.0 to 2.0 mol L−1, along with an ANOVA test with error reduction. The metal removal rates were approximately 7% for cobalt, neglectable for copper, and 6% for nickel. In the best cases, the highest concentrations by migration at the cathodic zone were 11%, 31%, and 30%, respectively. According to ANOVA tests, factor interaction was proven for each metal in the half cell near the cathode and the closest zone from the cathode specifically. Both factors affected metal concentrations, which indicates that when the goal aims for species accumulation in a narrower section, each factor has a significant effect, and their interaction makes a proven enhancement. Thus, using 2.0 V cm−1 and 2.0 mol L−1 showed a high improvement in metal concentration in the cathodic zone.

Assisted Phytostabilization of Mine-Tailings with Prosopis laevigata (Fabaceae) and Biochar

Phytoremediation is a cost-effective technique to remediate heavy metal (HM) polluted sites. However, the toxic effects of HM can limit plant establishment and development, reducing phytoremediation effectiveness. Therefore, the addition of organic amendments to mine wastes, such as biochar, improves the establishment of plants and reduces the bioavailability of toxic HM and its subsequent absorption by plants. Prosopis laevigata can establish naturally in mine tailings and accumulate different HM; however, these individuals show morphological and genetic damage. In this study, the effect of biochar on HM bioaccumulation in roots and aerial tissues, HM translocation, morphological characters and plant growth were evaluated, after three and six months of exposure. Plants grown on mine tailings with biochar presented significantly higher values for most of the evaluated characters, in respect to plants that grew on mine tailing substrate. Biochar addition reduced the bioaccumulation and translocation of Cu, Pb, and Cd, while it favored the translocation of essential metals such as Fe and Mn. The addition of biochar from agro-industrial residues to mine tailings improves the establishment of plants with potential to phytoextract and phytostabilize metals from polluted soils. Using biochar and heavy metal accumulating plants constitutes an assisted phytostabilization strategy with great potential for HM polluted sites such as Cd and Pb.

Combining Soil Vapor Extraction and Electrokinetics for the Removal of Hexachlorocyclohexanes from Soil

This paper focuses on the evaluation of the mobility of four hexachlorocyclohexane (HCH) isomers by soil vapor extraction (SVE) coupled with direct electrokinetic (EK) treatment without adding flushing fluids. SVE was found to be very efficient and remove nearly 70 % of the four HCH in the 15-days of the tests. The application of electrokinetics produced the transport of HCH to the cathode by different electrochemical processes, which were satisfactorily modelled with a 1-D transport equation. The increase in the electric field led to an increase in the transport of pollutants, although 15 days was found to be a very short time for an efficient transportation of the pollutants to the nearness of the cathode. Loss of water content in the vicinity of the cathode warns about the necessity of using electrokinetic flushing technologies instead of simple direct electrokinetics. Thus, results point out that direct electrokinetic treatment without adding flushing fluids produced low current intensities and ohmic heating that contributes negatively to the performance of the SVE process. No relevant differences were found among the removal of the four isomers, neither in SVE nor in EK processes.

Migration and Removal of Labile Cadmium Contaminants in Paddy Soils by Electrokinetic Remediation without Changing Soil pH

Electrokinetic remediation (EKR) is a viable, advanced cleaning strategy that can permanently reduce the toxicity of soil contaminants. However, EKR is prone to causing changes in soil pH. The negative impacts must be minimized if field-scale application is to be realized. In this study, EKR with polarity reversal was used to avoid soil pH polarization and to clean up cadmium (Cd)-contaminated paddy soils. Results showed that Cd desorbed from oxidizable and residual fractions to labile and easily available parts. Soil moisture content above 0.35 g g⁻¹ was conductive to achieving the desirable Cd-migration rate. The exchangeable Cd phase eventually migrated from both ends of that soil compartment towards the intermediate. Moreover, the addition of citric acid at the concentration of 0.1 mol L⁻¹ was an effective enhancement strategy. The methodology enriched Cd contaminants to specific sites. The technology can be used for electrokinetic-assisted phytoremediation during the rice growing period. Hyperaccumulator is planted in the intermediate area to remove the Cd contaminants. On the other hand, Cd removal is achieved in the region close to the electrodes. The present study provides a theoretical basis for in situ remediation. It has a wider significance for field-scale application.

An overview of in-situ remediation for nitrate in groundwater

Faced with the increasing nitrate pollution in groundwater, in-situ remediation has been widely studied and applied on field-scale as an efficient, economical and less disturbing remediation technology. In this review, we discussed various in-situ remediation for nitrate in groundwater and elaborate on biostimulation, phytoremediation, electrokinetic remediation, permeable reactive barrier and combined remediation. This review described principles of each in-situ remediation, application, the latest progress, problems and challenges on field-scale. Factors affecting the efficiency of in-situ remediation for nitrate in groundwater are also summarized. Finally, this review presented the prospect of in-situ remediation for nitrate pollution in groundwater. The objective of this review is to examine the state of knowledge on in-situ remediation for nitrate in groundwater and critically evaluate factors which affect the up-scaling of laboratory and bench-scale research to field-scale application. This helps to better understand the control mechanisms of various in-situ remediation for nitrate pollution in groundwater and the design options available for application to the field-scale.

Cleanup of arsenic, cadmium, and lead in the soil from a smelting site using N,N-bis(carboxymethyl)-L-glutamic acid combined with ascorbic acid: A lab-scale experiment

Chemical washing has been carried out to remediate soil contaminated with heavy metals. In this study, the appropriate washing conditions for N,N-bis(carboxymethyl)-L-glutamic acid (GLDA) combined with ascorbic acid were determined to remove As, Cd, and Pb in the soil from the smelting site. The mechanism of heavy metal removal by the washing agent was also clarified. The results showed that heavy metals in the soil from the smelting site can be effectively removed. The removal percentages of As, Cd, and Pb in the soil from the smelting site were found to be 34.49%, 63.26%, and 62.93%, respectively, under optimal conditions (GLDA and ascorbic acid concentration ratio of 5:20, pH of 3, washing for 60 min, and the liquid-to-solid ratio of 10). GLDA combined with ascorbic acid efficiently removes As, Cd, and Pb from the soil through synergistic proton obstruction, chelation, and reduction. GLDA can chelate with iron and aluminum oxides while directly chelate with Cd and Pb. Ascorbic acid can reduce both Fe(III) to Fe(II) and As(III) to As⁰. The dissolution of As was promoted by indirectly preempting the binding sites of iron and aluminum in the soil while those of Cd and Pb were improved by directly interrupting the binding sites. This study suggested that GLDA combined with ascorbic acid is an effective cleanup technology to remove As, Cd, and Pb simultaneously from contaminated smelting site soils.