Cadmium Isotope Fractionation during Adsorption onto Edge Sites and Vacancies in Phyllomanganate.

Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd2+ adsorption on edge sites (Δ114/110Cdedge-solution = -1.54 ± 0.11‰) is smaller than that on vacancies (Δ114/110Cdvacancy-solution = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

Enhanced reduction of Cd uptake by wheat plants using iron and manganese oxides combined with citrate in Cd-contaminated weakly alkaline arable soils.

Iron (Fe) and manganese (Mn) oxides can be used to remediate Cd-polluted soils due to their excellent performance in heavy metal adsorption. However, their remediation capability is rather limited, and a higher content of available Mn and Fe in soils can reduce Cd accumulation in wheat plants due to the competitive absorption effect. In this study, goethite and cryptomelane were first respectively used to immobilize Cd in Cd-polluted weakly alkaline soils, and sodium citrate was then added to increase the content of available Mn and Fe content for further reduction of wheat Cd absorption. In the first season, the content of soil-available Cd and Cd in wheat plants significantly decreased when cryptomelane, goethite and their mixture were used as the remediation agents. Cryptomelane showed a better remediation effect, which could be attributed to its higher adsorption performance. The grain Cd content could be decreased from 0.35 mg kg-1 to 0.25 mg kg-1 when the content of cryptomelane was controlled at 0.5%. In the second season, when sodium citrate at 20 mmol kg-1 was further added to the soils with 0.5% cryptomelane treatment in the first season, the content of soil available Cd was increased by 14.8%, and the available Mn content was increased by 19.5%, leading to a lower Cd content in wheat grains (0.16 mg kg-1) probably due to the competitive absorption. This work provides a new strategy for the remediation of slightly Cd-polluted arable soils with safe and high-quality production of wheat.

High-efficiency electrochemical removal of Cd(II) from wastewater using birnessite-biochar composites: Performance and mechanism.

Birnessite has been widely used for electrochemical removal of heavy metals due to its high pseudocapacitance. Incorporation of carbon-based materials into birnessite can enhance its conductivity and stability, and synergistically improve the electrochemical adsorption capacity due to the double-layer capacitor reaction derived from carbon-based materials. In this study, biochar was successfully incorporated with birnessite at various ratios to synthesize composites (BC-Mn) for effective electrochemical removal of cadmium (Cd(II)) from water. The effects of cell voltage, initial pH, and recycling performance of BC-Mn were evaluated. As a result, the electrosorption capacity of BC-Mn for Cd(II) exhibited gradual increases with increasing birnessite content and reached equilibrium at a Mn content of 20% (BC-Mn20). The Cd(II) adsorption capacity of BC-Mn20 rose at higher cell voltage, and reached the maximum at 1.2 V. At pH 3.0-6.0, the electrosorption capacity initially rose until pH 5.0 and then approached equilibrium with a further increase in pH value. The Cd(II) electrochemical adsorption capacity of BC-Mn20 in the solution could reach 104.5 mg g-1 at pH 5.0 for 8 h at 1.2 V. Moreover, BC-Mn20 exhibited excellent reusability with a stability of 95.4% (99.7 mg g-1) after five cycles of reuse. Due to its superior heavy metal adsorption capacity and reusability, BC-Mn20 may have a promising prospect in the remediation of heavy metal polluted water.

Preparation, adsorption performance and mechanism of MgO-loaded biochar in wastewater treatment: A review.

Biochar is a low cost, porous and solid material with an extremely high carbon content, various types of functional groups, a large specific surface area and many other desirable characteristics. Thus, it is often used as an adsorbent or a loading matrix. Nano-magnesium oxide is a crystalline material with small particles and strong ion exchangeability. However, due to the high surface chemical energy, it easily forms agglomerates of particles. Therefore, to combine the advantages of biochar and magnesium, metal magnesium nanoparticles can be loaded onto the surface of biochar with different modification techniques, resulting in biochars with low cost and high adsorption performance to be used as an adsorption matrix (collectively referred to as Mg@BC). This review presents the effects of different Mg@BC preparation methods and synthesis conditions and summarizes the removal capabilities and adsorption mechanisms of Mg@BC for different types of pollutants in water. In addition, the review proposes the prospects for the development of Mg@BC to solve various problems in the future.

Removal of As(V) from wastewaters using magnetic iron oxides formed by zero-valent iron electrocoagulation.

Electrocoagulation of zero-valent iron has been widely applied to the removal of dissolved arsenic, but the solid-liquid separation of arsenic-containing precipitates remains technically challenging. In this work, zero-valent iron was electrochemically oxidized to magnetic iron oxides for the removal of As(Ⅴ) from simulated and actual mining wastewaters. The results indicated that lepidocrocite was formed when zero-valent iron was oxidized by dissolved oxygen, but ferrihydrite and green rust were first formed and then transformed to magnetic iron oxides (mainly magnetite and maghemite) in the electrochemical oxidation from 0 to 0.9 V (vs. SCE), which facilitates the adsorption of As(V) and subsequent solid-liquid separation under a magnetic field. In simulated As(V)-containing solution with initial pH 7.0, zero-valent iron was electrochemically oxidized to magnetite and maghemite at 0.6 V (vs. SCE) for 2 h. The As(V) concentration first decreased from 5127.5 to 26.8 µg L-1 with a removal ratio of 99.5%. In actual mining wastewaters, zero-valent iron was electrochemically oxidized to maghemite at 0.6 V (vs. SCE) for 24 h, and the As(V) concentration decreased from 5486.4 to 3.6 µg L-1 with a removal ratio of 99.9%. The removal ratio of As(V) increased slightly with increasing potential, and increased first and then decreased with increasing initial pH. Compared with that of SO42- and NO3-, the presence of Cl- significantly enhanced the removal of As(V). This work provides a highly efficient, facile and low-cost technique for the treatment of arsenic-containing wastewaters.

Photooxidation of Fe(II) to schwertmannite promotes As(III) oxidation and immobilization on pyrite under acidic conditions.

Pollution of arsenic (As) in acid mine drainage (AMD) is a universal environmental problem. The weathering of pyrite (FeS2) and other sulfide minerals leads to the generation of AMD and accelerates the leaching of As from sulfide minerals. Pyrite can undergo adsorption and redox reactions with As, affecting the existing form and biotoxicity of As. However, the interaction process between As and pyrite in AMD under sunlight radiation remains unclear. Here, we found that the oxidation and immobilization of arsenite (As(III)) on pyrite can be obviously promoted by the reactive oxygen species (ROS) in sunlit AMD, particularly by OH. The reactions between hole-electron pairs and water/oxygen adsorbed on excited pyrite resulted in the production of H2O2, OH and O2-, and OH was also generated through the photo-Fenton reaction of Fe2+/FeOH2+. Weakly crystalline schwertmannite formed from the oxidation of Fe2+ ions by OH contributed much to the adsorption and immobilization of As. In the mixed system of pyrite (0.75 g L-1), Fe2+ (56.08 mg L-1) and As(III) (1.0 mg L-1) at initial pH 3.0, the decrease ratio of dissolved total As concentration was 1.6% under dark conditions, while it significantly increased to 69.0% under sunlight radiation. The existence of oxygen or increase in initial pH from 2.0 to 4.0 accelerated As(III) oxidation and immobilization due to the oxidation of more Fe2+ and production of more ROS. The present work shows that sunlight significantly affects the transformation and migration of As in AMD, and provides new insights into the environmental behaviors of As.

Electrosorption of cadmium and arsenic from wastewaters using nitrogen-doped biochar: Mechanism and application.

Biochar has been widely applied as an adsorbent, whose electrochemical capacity and heavy metal adsorption performance can be improved by nitrogen doping. In this work, nitrogen-doped biochar (NBC) was synthesized by calcinating sodium humate with sodium bicarbonate (NaHCO3) and urea as the activation agent and nitrogen source, respectively. The NBC was then used to electrochemically adsorb Cd(II) and As(III,V) from simulated and actual wastewaters, respectively. The results indicated that NaHCO3 activation and nitrogen doping could increase the surface area and nitrogen content of the biochar, contributing to the enhancement of adsorption performance for Cd(II) and As(III,V). The electrosorption capacities for Cd(II) and total arsenic (As(T)) increased first and then reached equilibrium with increasing nitrogen content, increased first and then decreased with increasing calcination temperature, and consistently increased with increasing voltage. The Cd(II) electrosorption capacity (79.0 mg g-1) and As(T) removal ratio (94.0%) at 1.2 V in actual As-contaminated wastewater (1.16 mg L-1) were about 4 and 2.6 folds of their inorganic adsorption capacities, respectively. After five cycles of reuse, the Cd(II) and As(T) removal ratio could be maintained at 65.8% and 51.7% of the initial electrosorption capacity. This work expands the application of NBC for heavy metal removal.

Solar Irradiation Induced Transformation of Ferrihydrite in the Presence of Aqueous Fe2.

Ferrihydrite commonly occurs in soils and sediments, especially in acid mine drainage (AMD). Solar irradiation may affect Fe(II)-catalyzed transformation of metastable ferrihydrite to more stable iron oxides on AMD surface. We investigated the Fe(II)-catalyzed transformation process and mechanism of ferrihydrite under light irradiation. In nitrogen atmosphere, Fe2+aq could be oxidized to goethite and lepidocrocite by hydroxyl radical (OH•), superoxide radical (O2•-) and hole (hvb+) generated from ferrihydrite under ultraviolet (UV) irradiation (300-400 nm) at pH 6.0, and O2•- and hvb+ were mainly responsible for Fe2+aq oxidation. In addition, the ligand-to-metal charge-transfer (LMCT) process between Fe(II) and ferrihydrite could be promoted by UV irradiation. Goethite proportion increased with increasing Fe2+aq concentration. Both visible (vis) and solar irradiation could also lead to the oxidation of Fe2+aq to goethite and lepidocrocite, and the proportion of lepidocrocite increased with increasing light intensity. Fe2+aq was photochemically oxidized to schwertmannite at pH 3.0 and 4.5, and the oxidation rate was higher than that under dark conditions in air. The photochemical oxidation rate of Fe2+aq decreased in the presence of humic acid. This study facilitates a better understanding of the formation and transformation of iron oxides in natural environments and ancient Earth.

Photochemical Formation and Transformation of Birnessite: Effects of Cations on Micromorphology and Crystal Structure.

As important components with excellent oxidation and adsorption activity in soils and sediments, manganese oxides affect the transportation and fate of nutrients and pollutants in natural environments. In this work, birnessite was formed by photocatalytic oxidation of Mn2+aq in the presence of nitrate under solar irradiation. The effects of concentrations and species of interlayer cations (Na+, Mg2+, and K+) on birnessite crystal structure and micromorphology were investigated. The roles of adsorbed Mn2+ and pH in the transformation of the photosynthetic birnessite were further studied. The results indicated that Mn2+aq was oxidized to birnessite by superoxide radicals (O2•-) generated from the photolysis of NO3- under UV irradiation. The particle size and thickness of birnessite decreased with increasing cation concentration. The birnessite showed a plate-like morphology in the presence of K+, while exhibited a rumpled sheet-like morphology when Na+ or Mg2+ was used. The different micromorphologies of birnessites could be ascribed to the position of cations in the interlayer. The adsorbed Mn2+ and high pH facilitated the reduction of birnessite to low-valence manganese oxides including hausmannite, feitknechtite, and manganite. This study suggests that interlayer cations and Mn2+ play essential roles in the photochemical formation and transformation of birnessite in aqueous environments.

Effects of human management on black carbon sorption/desorption during a water transfer project: Recognizing impacts and identifying mitigation possibilities.

Water resources management is an important public concern. In this study, we examined the extent of sorption/desorption of trace pollutants to soil black carbon (BC) in the water level fluctuation zone (WLFZ) of the middle route of the South to North Water Transfer Project in China. In addition, we investigated the main management measures affecting these processes during the project. The results showed that the pseudo second-order model adequately describes the sorption/desorption of phenanthrene on the soil BC in the WLFZ. Water level fluctuation may indirectly influenced BC sorption/desorption by altering water chemistry. Water level residence time had negative effects on BC sorption in short-term experiments (days to months), but the impact gradually diminished with increased residence time. The results suggested that long-term field monitoring of water chemistry is urgent. During the initial period of water transfer, delaying the water supplies as drinking water source or directly irrigating crops could mitigate the adverse impacts. Future research should focus on the water-soluble products of BC degradation. The findings of this study should be useful in improving sustainable management of water resources for water transfer projects.

Influence factors for the oxidation of pyrite by oxygen and birnessite in aqueous systems.

The oxidation of exposed pyrite causes acid mine drainage, soil acidification, and the release of toxic metal ions. As the important abiotic oxidants in supergene environments, oxygen and manganese oxides participate in the oxidation of pyrite. In this work, the oxidation processes of natural pyrite by oxygen and birnessite were studied in simulated systems, and the influence of pH, Fe(II) and Cr(III) on the intermediates and redox rate was investigated. SO4(2-) and elemental S were formed as the major and minor products, respectively, during the oxidation processes. Ferric (hydr) oxides including Fe(OH)3 and goethite were formed with low degree of crystallinity. Low pH and long-term reaction facilitated the formation of goethite and ferric hydroxide, respectively. The rate of pyrite oxidation by birnessite was enhanced in the presence of air (oxygen), and Fe(II) ions played a key role in the redox process. The addition of Fe(II) ions to the reaction system significantly enhanced the oxidation rate of pyrite; however, the presence of Cr(III) ions remarkably decreased the pyrite oxidation rate in aqueous systems. The introduction of Fe(II) ions to form a Fe(III)/Fe(II) redox couple facilitated the electron transfer and accelerated the oxidation rate of pyrite. The present work suggests that isolation from air and decreasing the concentration of Fe(II) ions in aqueous solutions might be effective strategies to reduce the oxidation rate of pyrite in mining soils.

Interaction mechanisms and kinetics of ferrous ion and hexagonal birnessite in aqueous systems.

BACKGROUND: In soils and sediments, manganese oxides and oxygen usually participate in the oxidation of ferrous ions. There is limited information concerning the interaction process and mechanisms of ferrous ions and manganese oxides. The influence of air (oxygen) on reaction process and kinetics has been seldom studied. Because redox reactions usually occur in open systems, the participation of air needs to be further investigated. RESULTS: To simulate this process, hexagonal birnessite was prepared and used to oxidize ferrous ions in anoxic and aerobic aqueous systems. The influence of pH, concentration, temperature, and presence of air (oxygen) on the redox rate was studied. The redox reaction of birnessite and ferrous ions was accompanied by the release of Mn2+ and K+ ions, a significant decrease in Fe2+ concentration, and the formation of mixed lepidocrocite and goethite during the initial stage. Lepidocrocite did not completely transform into goethite under anoxic condition with pH about 5.5 within 30 days. Fe2+ exhibited much higher catalytic activity than Mn2+ during the transformation from amorphous Fe(III)-hydroxide to lepidocrocite and goethite under anoxic conditions. The release rates of Mn2+ were compared to estimate the redox rates of birnessite and Fe2+ under different conditions. CONCLUSIONS: Redox rate was found to be controlled by chemical reaction, and increased with increasing Fe2+ concentration, pH, and temperature. The formation of ferric (hydr)oxides precipitate inhibited the further reduction of birnessite. The presence of air accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite, which was not completely reduced and dissolved after 18 days. As for the oxidation of aqueous ferrous ions by oxygen in air, low and high pHs facilitated the formation of goethite and lepidocrocite, respectively. The experimental results illustrated the single and combined effects of manganese oxide and air on the transformation of Fe2+ to ferric oxides. Graphical abstract:Lepidocrocite and goethite were formed during the interaction of ferrous ion and birnessite at pH 4-7. Redox rate was controlled by the adsorption of Fe2+ on the surface of birnessite. The presence of air (oxygen) accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite.

Formation of todorokite from "c-disordered" H(+)-birnessites: the roles of average manganese oxidation state and interlayer cations.

BACKGROUND: Todorokite, a 3 × 3 tectomanganate, is one of three main manganese oxide minerals in marine nodules and can be used as an active MnO6 octahedral molecular sieve. The formation of todorokite is closely associated with the poorly crystalline phyllomanganates in nature. However, the effect of the preparative parameters on the transformation of "c-disordered" H(+)-birnessites, analogue to natural phyllomanganates, into todorokite has not yet been explored. RESULTS: Synthesis of "c-disordered" H(+)-birnessites with different average manganese oxidation states (AOS) was performed by controlling the MnO4 (-)/Mn(2+) ratio in low-concentrated NaOH or KOH media. Further transformation to todorokite, using "c-disordered" H(+)-birnessites pre-exchanged with Na(+) or K(+) or not before exchange with Mg(2+), was conducted under reflux conditions to investigate the effects of Mn AOS and interlayer cations. The results show that all of these "c-disordered" H(+)-birnessites exhibit hexagonal layer symmetry and can be transformed into todorokite to different extents. "c-disordered" H(+)-birnessite without pre-exchange treatment contains lower levels of Na/K and is preferably transformed into ramsdellite with a smaller 1 × 2 tunnel structure rather than todorokite. Na(+) pre-exchange, i.e. to form Na-H-birnessite, greatly enhances transformation into todorokite, whereas K(+) pre-exchange, i.e. to form K-H-birnessite, inhibits the transformation. This is because the interlayer K(+) of birnessite cannot be completely exchanged with Mg(2+), which restrains the formation of tunnel "walls" with 1 nm in length. When the Mn AOS values of Na-H-birnessite increase from 3.58 to 3.74, the rate and extent of the transformation sharply decrease, indicating that a key process is Mn(III) species migration from layer into interlayer to form the tunnel structure during todorokite formation. CONCLUSIONS: Structural Mn(III), together with the content and type of interlayer metal ions, plays a crucial role in the transformation of "c-disordered" H(+)-birnessites with hexagonal symmetry into todorokite. This provides further explanation for the common occurrence of todorokite in the hydrothermal ocean environment, where is usually enriched in large metal ions such as Mg, Ca, Ni, Co and etc. These results have significant implications for exploring the origin and formation process of todorokite in various geochemical settings and promoting the practical application of todorokite in many fields.Graphical abstractXRD patterns of Mg(2+)-exchanged and reflux treatment products for the synthetic "c-disordered" H(+)-birnessites.

Mechanisms for synergistically enhancing cadmium remediation performance of biochar: Silicon activation and functional group effects.

This work proposes an advanced biochar material (ß-CD@SiBC) for controllable transformation of specific silicon (Si) forms through endogenous Si activation and functional group introduction for efficient cadmium (Cd) immobilization and removal. The maximum adsorption capacity of ß-CD@SiBC for Cd(II) reached 137.6 mg g-1 with a remarkable removal efficiency of 99 % for 200 mg L-1Cd(II). Moreover, the developed ß-CD@SiBC flow column exhibited excellent performance at the environmental Cd concentration, with the final concentration meeting the environmental standard for surface water quality (0.05 mg L-1). The remediation mechanism of ß-CD@SiBC could be mainly attributed to mineral precipitation and ion exchange, which accounted for 42 % and 29 % of the remediation effect, respectively, while functional group introduction enhanced its binding stability with Cd. Overall, this work proposes the role and principle of transformation of Si forms within biochar, providing new strategies for better utilizing endogenous components in biomass.

Mechanism for Fe(III) to decrease cadmium uptake of wheat plant: Rhizosphere passivation, competitive absorption and physiological regulation.

The presence of dissolved Fe(III) and Fe(III)-containing minerals has been found to alleviate cadmium (Cd) accumulation in wheat plants grown in Cd-contaminated soils, but the specific mechanism remains elusive. In this work, hydroponic experiments were conducted to dissect the mechanism for dissolved Fe(III) (0-2000 µmol L-1) to decrease Cd uptake of wheat plants and study the influence of Fe(III) concentration and Cd(II) pollution level (0-20 µmol L-1) on the Cd uptake process. The results indicated that dissolved Fe(III) significantly decreased Cd uptake through rhizosphere passivation, competitive absorption, and physiological regulation. The formation of poorly crystalline Fe(III) oxides facilitated the adsorption and immobilization of Cd(II) on the rhizoplane (over 80.4 %). In wheat rhizosphere, the content of CaCl2-extractable Cd decreased by 52.7 % when Fe(III) concentration was controlled at 2000 µmol L-1, and the presence of Fe(III) may reduce the formation of Cd(II)-organic acid complexes (including malic acid and succinic acid secreted by wheat roots), which could be attributed to competitive reactions. Down-regulation of Cd uptake genes (TaNramp5-a and TaNramp5-b) and transport genes (TaHMA3-a, TaHMA3-b and TaHMA2), along with up-regulation of the Cd efflux gene TaPDR8-4A7A, contributed much to the reduction of Cd accumulation in wheat plants in the presence of Fe(III). The inhibitory effect of Fe(III) on Cd uptake and transport in wheat plants declined with increasing Cd(II) concentration, particularly at 20 µmol L-1. This work provides important implications for remediating Cd-contaminated farmland soil and ensuring the safe production of wheat by using dissolved Fe(III) and Fe(III)-containing minerals.

Amino acid promoted oxidation of atrazine by Fe3O4/persulfate.

In the present study, we demonstrated that the presence of cysteine could remarkably enhance the degradation of atrazine by Fe3O4/persulfate system. The results of electron paramagnetic resonance (EPR) spectra confirmed the combination of cysteine and Fe3O4 exhibited much higher activity on activation of persulfate to generate more SO4•- and •OH than Fe3O4 alone. At pH of 3.0, SO4•- and •OH contributed to about 58.2 % and 41.8 % of atrazine removal respectively, while •OH gradually dominated the oxidation of atrazine from neutral condition to alkaline condition. The co-existing Cl- and HCO3- could quench SO4•-, resulting in the inhibition of atrazine degradation. The presence of low natural organic matters (NOM) concentration (0-2 mg L-1) could enhance the atrazine removal, and high concentration (>5 mg L-1) of NOM restrained the atrazine degradation. During the Cysteine/Fe3O4/Persulfate process, cysteine served as a complexing reagent and reductant. Through acidolysis and complexation, Fe3O4 could release dissolved and surface bound Fe2+, both of which contributed to the activation of persulfate together. Meanwhile, cysteine was not rapidly consumed due to a regeneration process, which was beneficial for maintaining Fe2+/Fe3+ cycle and constantly accelerating the activation of persulfate for atrazine degradation. The reused Fe3O4 and cysteine in the Cysteine/Fe3O4/Persulfate process exhibited high stability for the atrazine degradation after three cycles. The degradation pathway of atrazine included alkylic-oxidation, dealkylation, dechlorination-hydroxylation processes. The present study indicates the novel Cysteine/Fe3O4/Persulfate process might be a high potential for treatment of organic polluted water.

Simulation of vertical migration behaviors of heavy metals in polluted soils from arid regions in northern China under extreme weather.

Heavy metal migration behaviors and mechanisms in soils are important for pollution control and remediation. However, there are few related studies in arid areas under extreme weather patterns. In this study, we developed a one-dimensional continuous point source unsaturated solute transport model, and utilized Hydrus-1D to simulate the transport of Cu, As and Zn, in the pack gas zones of soils within the impact areas of two typical mining areas in Inner Mongolia. The results show that the soil has a significant interception capacity, with a short heavy metal vertical migration distance of ≤100 cm. Soil texture and heavy metal sorption affinity are two key factors that influence heavy metal transport. In soils with high contents of sands but low contents of clays, heavy metals have large mobility and thus migrate deeper and are more evenly distributed in the soil profile. The migration of different heavy metals in the same soil also varies considerably, with large migration depth for metals having low binding affinities onto soils. Scenario analysis for extreme drought and rainfall shows that, rainfall amount and intensity are positively correlated with heavy metal transport depth and negatively correlated with the peak concentration. Increasing rainfall/intensity results in a more uniform distribution of heavy metals, and lower profile concentrations owing to enhanced horizontal dispersion of surface runoff. When the total amount and intensity of rainfall remain constant, continuous or intermittent rainfall only affects the transport process but has almost no effect on the final pollutant concentration redistribution in the soil. These results provide theoretical data for estimating the degree of heavy metal pollution, and help design control and remediation strategies for polluted soils.

Sustainable and reagent-free cathodic precipitation for high-efficiency removal of heavy metals from soil leachate.

Heavy metal pollution of soils has become a serious environmental problem. Soil washing with degradable reagents is an effective remediation technique of heavy metal pollution, and the generated leachate must be appropriately treated before discharge. However, the existing methods usually have the problems of large consumption of regents, high cost, and secondary pollution. This study proposed a reagent-free electrochemical precipitation method to remove mixed heavy metal ions extracted from soils by citrate using inert electrodes (IrO2-Ta2O5/Ti anode and graphite cathode). The results showed that the low potential of cathode led to the electrodeposition of Cd; the local alkaline environment provided by electro-mediated water reduction caused the hydrolytic precipitation of Zn and Pb; and the precipitation of Fe washed out from Fe-rich soil resulted in the coprecipitation of As on cathode surface. These combined cathodic precipitation processes decreased the concentrations of toxic heavy metals by over 99.4% after 12 h of electrolysis at 26 mA cm-2. The electrodes exhibited high stability after multiple successive cycles of reuse. The concentrations of As, Zn, Pb and Cd in the leachate decreased to below the limits of industrial wastewater discharge in each cycle, and those in soils could be reduced by 53.8%, 58.8%, 25.5%, and 70.2% at the initial concentrations of 1549, 1016, 310 and 50 mg kg-1, respectively. The heavy metal removal rate increased with increasing current density in the range of 0-52 mA cm-2. This work provides an efficient and sustainable method for the remediation of site soils polluted by mixed heavy metals.

Efficient removal of As(III) from groundwaters through self-alkalization in an asymmetric flow-electrode electrochemical separation system.

It remains a great challenge to efficiently remove As(III) from groundwater using traditional technologies due to its stable electroneutral form. This study constructed an asymmetric flow-electrode electrochemical separation (AFES) system, which overcomes the drawback of H+ release from anodic carbon oxidation and achieves continuous self-alkalization function and highly efficient removal of As(III) from groundwater. At the applied voltage of 1.2 V and initial pH 7.5, the system could rapidly decrease the total As (T-As) concentration from 150.0 to 8.9 µg L-1 within 90 min, with an energy consumption of 0.04 kWh m-3. The self-alkalization was triggered by the generation of H2O2 from dissolved oxygen reduction and the adsorption of H+ on the cathode in the feed chamber, which significantly promoted the dissociation and oxidation of As(III), resulting in the removal of T-As predominantly in the form of As(V). The removal performance of T-As was slightly affected by the initial pH and coexisting ions in the feed chamber. The AFES system also exhibited considerable stability after 20 cycles of continuous experiments and superior performance in treating As-containing real groundwater. Moreover, the pH of the alkalized solution can be restored to the initial level by standing or aeration operation. This work offers a novel and efficient pathway for the detoxication of As(III)-contaminated groundwaters.

Rethinking terrestrial dissolved organic matter in dam reservoirs before mixing: Linking photodegradation and biodegradation and the phenanthrene binding behavior.

With the increased construction of dam reservoirs and the demand for water security, terrestrial dissolved organic matter (DOM) has received attention because of its role in regulating water quality, ecological functions, and the fate and transport of pollutants in dam reservoirs. This study investigated the transformations of soil DOM and vegetation DOM of dam reservoirs following photodegradation and biodegradation before conservative mixing, as well as the resultant effects on phenanthrene binding. Based on the results, terrestrial DOM could undergo transformation via photodegradation and biodegradation before conservative mixing in dam reservoirs. Although both processes resulted in substantial decreases in DOM concentrations, the changes in chromophoric DOM and fluorescent DOM depended on the original DOM sources. Furthermore, the photodegradation of terrestrial DOM resulted in more pronounced photobleaching than photomineralization. In addition, photodegradation of terrestrial DOM resulted in the generation of DOM-derived by-products with low molecular weight and low aromaticity, whereas the biodegradation of terrestrial DOM resulted in DOM-derived by-products with low molecular weight and high aromaticity. Subsequently, the photodegradation and biodegradation of terrestrial DOM substantially enhanced the binding affinity of phenanthrene. Soil DOM is prior to vegetation DOM when predicting the ecological risk of HOCs. These results indicate that the terrestrial DOM in dam reservoirs should be reconsidered before conservative mixing. Further studies on the coupling effects of both biogeochemical processes, as well as on the relative contributions of soil DOM and vegetation DOM after transformation to the aquatic DOM in dam reservoirs, are required. This study provides information on the environmental effects of dam construction from the perspective of biogeochemical processes.