Electrokinetic treatment for clayed and sandy soils

Electrokinetic treatment has proven to be an attractive alternative to clean polluted soils. This friendly environmental procedure can be approached by a physicochemical model dealing with the oriented displacement of pollutants by means of the fluid and/or charge flow through a porous media induced by an electric field. Concerning the porous media, soil texture is the main issue to consider, since it is related with physicochemical properties that may define the electrokinetic phenomena. In this paper, two different soil textures were tested under the same experimental conditions. Physicochemical characterization of clayed and sandy unpolluted soils, later mixed with PbCO₃, were done. Zeta potential measurements were performed to approach the soil particles interphase with three aqueous solutions. Clayed and sandy soil, mixed with cerusite, were treated by electrokinetics during 24 h. Results for sandy soil contaminated with Pb, using ammonium acetate and acetic acid as anolyte and catholyte respectively, reported Pb mobility due to a fluid flow from anode to cathode; in accordance with zeta potential indicating more negative values for ammonium acetate solution, compared with EDTA and acetic acid. For the clayed soil, lead mobility was detected, even if no electroosmotic flow appeared. Finally, the same experimental conditions were applied to a real soil containing mining wastes, in this case lead mobility was due to electroosmotic and electromigration flow.     

Electrokinetic Properties of Acid-Activated Montmorillonite Dispersions

In this study, the influence of pH, electrolyte concentration, and type of ionic species on the electrokinetic properties (zeta potential and electrokinetic charge density) of the acid-activated montmorillonite mineral have been investigated using the microelectrophoresis method. The electrokinetic properties of acid-activated montmorillonite dispersions have been determined in aqueous solutions of mono-, di-, and trivalent salts and divalent heavy metal salts. Zeta potential experiments have been performed to determine the point of zero charge (pzc) and potential determining ions (pdi). The zeta potential values of the acid-activated montmorillonite particles were negative and did not vary significantly within the pH range studied. Acid-activated montmorillonite dispersions do not have point of zero charge (pzc). The valence of the electrolytes has a great influence on the electrokinetic behavior of the suspension. A gradual decrease in the zeta potential (from ?25 mV to ?5 mV) occurs with the monovalent electrolytes when concentration increased. Divalent and heavy metal electrolytes have less negative z-potentials due to the higher valence of ions. A sign reversal of z-potential has been observed at AlCl₃, FeCl₃, and CrCl₃ electrolytes (potential determining ions) and zeta potential values have had a positive sign at high electrolyte concentrations.

The electrokinetic charge density of acid-activated montmorillonite has shown similar trends for variation in mono- and divalent electrolyte solutions. Up to concentrations of ca. 10^?3 M, it has remained practically constant at approximately 0.5 × 10^?3 C m^?2 For higher concentrations of monovalent electrolytes more negative values (?16 × 10^?3 C m^?2) were observed. It has less negative values in divalent electrolyte concentrations according to monovalent electrolytes (?5 × 10^?3 C m^?2). For low concentrations of trivalent electrolytes, the electrokinetic charge density of montmorillonite particles is constant, but at certain concentrations it rapidly increased and changed its sign to positive.     

Influence of anolyte and catholyte composition on TPHs removal from low permeability soil by electrokinetic reclamation

Removal of TPHs from polluted soil by electrokientic reclamation was done by using different electrolytes (anolyte and catholyte). The initial concentration of TPHs in soil was 23,000 ppm and removal efficiencies reached almost 90% for a combination of 0.04 M NaOH and 0.1 M Na₂SO₄ in the anode and cathode chambers, respectively. Electroosmotic flow and TPHs desorption were measured under galvanostatic conditions (1.95 mA cm⁻² and electric field <10 V cm⁻¹). The study is supported on the electrokinetic transport model for low permeability soils. Electrolytes (anolyte and catholyte) were maintained at constant ionic composition to keep constant boundary conditions, thus launch a pseudostationary state for fluid and charge transport throughout the soil. It was also observed that electrolyte concentration favored TPHs desorption as well as their transport throughout the soil by electroosmotic flow from anode to cathode. Both, electrolytes concentration and wetting solution helped to maintain a constant pH profile during electroreclamation, thus a sustained fluid flow from anode to cathode.     

Assessment of electrophoresis and electroosmosis in construction materials: effect of enhancing electrolytes and heavy metals contamination

Electrokinetic effects are those that take place by application of an electric field to porous materials, with the zeta potential as the key parameter. Specifically, in the case of contaminated construction materials, the generation of an electroosmotic flux, with the corresponding dragging due to water transport, is a crucial mechanism to succeed in the treatment of decontamination. Therefore, it is of great interest trying to optimize the treatment by the addition of specific electrolytes enhancing the electrokinetic phenomena. Most of the data of zeta potential found in literature for construction materials are based in micro-electrophoresis measurements, which are quite far of the real conditions of application of the remediation treatments. In this paper, electrophoretic and electroosmotic experiments, with monolithic and powdered material respectively, have been carried out for mortar, brick and granite clean and contaminated with Cs, Sr, Co, Cd, Cu and Pb. The electrolytes tested have been distilled water (DW), Na₂–EDTA, oxalic acid, acetic acid and citric acid. The zeta potential values have been determined through the two different techniques and the results compared and critically analysed.     

Effect of clay mineralogy on the feasibility of electrokinetic soil decontamination technology

To evaluate the effect of clay mineralogy on the feasibility of electrokinetic soil remediation technology, we contaminated six soils with Cu(II), Zn(II) and Pb(II) and performed electroremediation for 570 h. Cation exchange resin saturated with H⁺ was placed between soil and cathode to prevent soil alkalinization and trap the migrated heavy metal cations. After the treatment, the heavy metal cations were sequentially extracted with water, 1 M MgCl₂ and hot 6 M HCl. In soils dominated by crystalline clay minerals, Cu(II) and Zn(II) significantly migrated from anode end and accumulated at the cathode end forming sparingly soluble hydroxides. Removal rates of Cu(II) and Zn(II) were highest in a soil dominated with kaolinite and crystalline hematite. In humic–allophanic and allophanic soils, the high pH-buffering capacity of allophane kept the soil pH above 5, even at the anode end, and Cu(II) and Zn(II) did not migrate significantly. In all soils, the migration of Pb(II) was infinitesimal due to the formation of insoluble PbSO₄ and very strong surface complexation at the mineral surfaces. These results show that the reactivity of component clay minerals to H⁺ and heavy metal cations has a crucial effect on the efficiency of the electrokinetic remediation technology and it is not effective for remediation of allophanic soils. The results also indicate that allophanic soils may be useful as a barrier material in landfill sites.     

A Laboratory-Scale Study of Applied Voltage on the Electrokinetic Separation of Lead from Soils

ABSTRACT

The application of electrokinetic (EK) soil-flushing technology to the separation of lead from a nonsynthetic, fine-grained, low permeability soil was examined. In these laboratory-scale experiments the effects of applied voltage (30 and 60 V DC) on cumulative electroosmotic (EO) flow, charge-input, and lead removal were investigated. To develop a more generalized cause-effect relationship, these parameters were studied using three anode/cathode reservoir conditioning schemes: NaNO₃/NaNO₃, NaNO₃/HAc (acetic acid), and HCl/HAc. Charge-input and cumulative EO flow generally increased when the applied voltage was raised. When reservoir pH controls were used, results were more consistent with theoretically predicted outcomes. Increasing the applied voltage increased the electrolysis of water, which increased the fluid conductivity and charge-input. Although cumulative EO flow increased in proportion to the voltage, the advantage of operating at a higher applied voltage diminished as the amount of lead remaining in the soil decreased. The highest lead removal rates for both the 30 and 60 V tests were achieved using the 0.1 M HCl/1.0 M HAc reservoir conditioning scheme. The addition of HCl to the anode reservoir solution enhanced the impact of the acid front, especially during the initial pore volumes of flow which occurred before the oxidation of water could produce significant amounts of H⁺ at the anode. Additionally, HAc in the cathode reservoir prevented the formation of a base front and the subsequent Pb readsorption/precipitation onto soil. The greater cumulative EO flow and charge-input in the experiments conducted with the HCl/HAc reservoir conditioning scheme resulted in faster Pb removal via advection and electrolytic migration. In contrast, the lowest remediation and removal values were obtained with the NaNO₃/NaNO₃ reservoir treatment scheme, which had a low cumulative EO flow, relative to the other tests, and lacked reservoir fluid pH control. To demonstrate the impact of soil pH on Pb removal, soil-bound Pb concentrations as a function of soil pH were also examined. The “critical pH” range necessary to ensure effective Pb removal was between 4 and 4.5.     

Development of electrokinetic-flushing technology for the remediation of contaminated soil around nuclear facilities

The effects of a flushing by a pump on an electrokinetic-flushing remediation of contaminated soil were estimated. The soils were sampled from the sites around nuclear facilities which were built on a high hydro-conductivity of sandstone. An electrokinetic-flushing equipment with a pump was manufactured to estimate the effect of a flushing on an electrokinetic-flushing remediation. In order to select an optimal reagent suitable to the characteristics of a soil near nuclear facilities, 4 experiments were executed with 4 candidate reagents selected from 12 reagents and the results of the experiments are as follows. The removal efficiencies of cobalt and cesium from the contaminated soil with the acetic acid were the highest, which were 92.1% and 83.1%, respectively. The effluent solution volume generated from an electrokinetic remediation was very smaller and it was 5% below that from a soil washing. Next, the results from a comparison of an electrokinetic-flushing remediation and an electrokinetic remediation revealed that the removal efficiencies of Co²⁺ and Cs⁺ by an electrokinetic-flushing remediation for 5 days were increased by 25% and 35% when compared to those by the electrokinetic remediation, but the effect of a flushing by the electrokinetic-flushing equipment started to decrease after 5 days. The removal efficiencies of Co²⁺ and Cs⁺ by an electrokinetic-flushing remediation for 15 days were increased by 6.8% and 7.7% when compared to that by an electrokinetic remediation. Namely, the higher the hydro-conductivity of a soil was, the larger the effect of a flushing was on an electrokinetic-flushing remediation.     

Effect of compost and soil properties on the availability of compost phosphate for white clover (Trifolium repens L.)

Wide variation in results exists in the literature on the effectiveness of composts to sustain the phosphorus (P) nutrition of crops. The aim of this work was to assess the importance of some soil and composts properties on the utilization of compost-P by white clover (Trifolium repens L.). This study was carried out with samples collected from four composts made from solid kitchen and garden wastes, and with two soil samples taken from the A horizon of a P-rich sandy acidic Dystrochrept and of a P-limited clayey calcareous Eutrochrept. Changes in the amount of inorganic P (Pi) isotopically exchangeable within 1 min (E_1min) were measured during 32 weeks in incubated soil-composts or soil-KH₂PO₄mixtures where P sources had been added at the rate of 50 mg P kg^–1 soil. Uptake of compost-P or KH₂PO₄-P by white clover was measured on the same amended soils during 16 weeks. In both soils, the application of composts resulted after 32 weeks of incubation in E_1min values ranging between those observed in the control without P and those observed in the KH₂PO₄treatment, i.e., in values ranging between 4.2 and 5.9 mg P kg^–1 in the sandy acidic soil and between from 1.6 to 4.3 mg P kg^–1 in the clayey calcareous soil. The total coefficient of utilization of compost-P (CU-P) by white clover reached values in both soils for the four composts ranging between 6.5% and 11.6% of the added P while in the presence of KH₂PO₄ the CU-P reached values ranging between 14.5% in the clayey calcareous soil and 18.5% in the sandy acidic soil. Results obtained in the sandy acidic soil suggest, that white clover initially used a fraction of the rapidly exchangeable compost P, while at a latter stage plant roots enhanced the mineralisation of compost organic P and took up a fraction of the mineralized P. These relations were not observed in the clayey calcareous soil probably because of its high sorbing capacity for P. In the sandy acidic soil, composts application increased the uptake of soil P by the plant from 31.4 mg P kg^–1 soil in the control without P to values ranging between 37.9 to 42.7 mg P kg^–1 soil in the presence of composts. This indirect effect was related to a general improvement of plant growth conditions in this soil induced by compost addition (from 9.9 g DM kg^–1 soil in the control without P to values ranging between 14.0 to 16.1 g DM kg^–1 soil in the presence of composts) and/or to the release of Al- or Fe bound soil P to the solution due to soil pH increase following compost application. Finally the total coefficient of utilization of P (CU-P) derived from KH₂PO₄ and composts was related to the total amount of N exported by white clover in the P-limited clayey calcareous soil but not in the P-rich sandy acidic soil. This suggests that in a soil where N₂ biological fixation is limited by low P availability, the CU-P of a compost by white clover is not only related to the forms of P present in the compost but also to its effect on N nutrition. However, it is not clear whether this improved N nutrition was due to compost mineralisation, or to an indirect compost effect on the N₂ biological fixation.     

Simultaneous recovery of Pb and PbO2 from battery plant effluents. Part II

Results are reported of experiments and modelling of cathodic Pb and anodic PbO₂ electrodeposition, aimed at developing a process using both reactions simultaneously for treating aqueous effluents from lead–acid battery recycling plants. Pb(II) solubilities and equilibrium potentials were calculated as functions of pH and sulfate activities. Using kinetic parameters from the literature or determined experimentally, models were developed for current density–potential and current efficiency–potential relationships, together with the current density dependence of specific electrical energy consumptions for Pb(II) recovery. Experimental current density–potential and charge efficiency–potential relationships were in broad agreement with model predictions, with near unity current efficiencies for mass transport controlled PbO₂ deposition from electrolytes containing 1 mol Pb(II) m^–3 at pH 12. However, charge efficiencies for cathodic deposition of lead were typically 0.2 for 1 mol Pb(II) m^–3 + 1 mol O₂ (aq) m^–3; removal of dissolved oxygen was predicted and determined to increase current efficiencies to near unity. Pb(II) concentrations were depleted to <60 ppb in a batch recycle reactor system with graphite felt anodes and graphite or titanium felt cathodes. Simultaneous cathodic Pb and anodic PbO₂ electrodeposition resulted in more rapid Pb(II) depletion than for either reaction separately.     

Electrodeposition of aluminium-copper alloys from alkyl benzene electrolytes

An experimental programme in which aluminium-copper alloys were electroplated from alkyl benzene/ AlBr₃ electrolytes is described. The investigation showed that Al-Cu alloys (0–3.5 wt% Cu) can be plated onto steel and that the resulting coatings are bright, adherent and less porous than equivalent coatings of pure aluminium. The composition of the cathode deposits can be readily controlled by using Al-Cu anodes of specified copper content. About half the copper dissolved from the anodes is transferred to the cathode, the remainder being precipitated from the electrolyte as CuBr. Coating composition varies with current density and this parameter can also be used to control the process. In a parallel investigation, potential differences between aluminium and a number of other metals (Zn, Pb, Cd, Sn, Cu, Ag) were measured. Potential differences in the alkyl benzene electrolytes were found to be 10–30 times less than in aqueous electrolytes which indicates that the alkyl benzenes should be useful for plating other alloys in addition to those of aluminium and copper.     

Transformation kinetics and potential availability of specifically-sorbed phosphate in soils

The improvement of P management in agriculture and environment requires a good understanding of residual effect of applied P in soils. The specific adsorption of P on variable charge minerals has been considered as the major mechanism that leads to a very low utilization of P fertilizer by crops within a growing season in Chinese red soils. Soil incubation and isotope tracing analysis were carried out to examine the transformation kinetics and potential availability of added specifically sorbed ³²P in two pH contrasting light textured soils. The ³²P recovered by 0.5 M NaHCO₃ extraction and microbial biomass-P measurement from the added specifically sorbed ³²P in the soils was well described by a first-order reaction and a Langmuir-type kinetic model, with correlation coefficients (R) being, on average, 0.938 and 0.959, respectively. The half-life (t_1/2, from the first-order model) of the four tested mineral-P complexes ranged from 29 to 47 d in the acid sandy soil and 33 to 105 d in the neutral silty soil. Goethite-P was the most stable among the four tested mineral-P complexes. The potential availability of the mineral complex P (q _m , in percent of total ³²P added) obtained from the Langmuir equation ranged from 43.7 to 90.9% for the four mineral-P complexes, and decreased in the order: Al oxide-P (90.9%) > montmorillonite-P (86.2%) > kaolinite-P (77.5%) > goethite-P (60.2%) in the acid sandy soil, whereas the order was Al oxide-P (89.3%) > kaolinite-P (86.2%) > montmorillonite-P (82.6%) > goethite-P (43.7%) in the neutral silty soil. Based on the release rate and potential availability, kaolinite-P and Al oxide-P could be important sources for residual effect of applied P in variable-charge soils. The goethite-P has the lowest release rate and potential availability among the mineral-P complexes, implying that iron oxides may be the most important variable-charge mineral responsible for P fixation in the Chinese red soils.     

Rheological properties of slips based on polymineral clays with electrolyte additives

The effect of various electrolytes (soda ash, water glass, sodium tripolyphosphate, coal-alkali reactant) as well as their combinations on the rheological properties and electrokinetic potential of ceramic slips for souvenir bottle production based on polymineral clays from Belarus is investigated. It is established that the maximum liquefying of ceramic suspensions is reached under the joint effect of a combination of electrolytes, due to the simultaneous impact of cation exchange and physical adsorption of anions in the system.     

Nitrous oxide emission from soils amended with crop residues

Crop residues incorporated in soil are a potentially important source of nitrous oxide (N₂O), though poorly quantified. Here, we report on the N₂O emission from 10 crop residues added to a sandy and a clay soil, both with and without additional nitrate (NO₃). In the sandy soil, total N₂O emission from wheat, maize, and barley residues was not significantly different from the control. The total N₂O emission from white cabbage, Brussels sprouts, mustard, sugar beet residues and broccoli ranged from 0.13 to 14.6 % of the amount of N added as residue and were higher with additional NO₃ than without additional NO₃. In the clay soil, similar effects of crop residues were found, but the magnitude of the N₂O emission was much smaller than that in the sandy soil: less than 1 % of the residue N evolved as N₂O. The C-to-N ratio of the residue accounted for only 22–34% and the mineralizable N content of the residue for 18–74% of the variance in N₂O emission. We suggest that the current IPCC methodology for estimating N₂O emission from crop residues may be considerably improved by defining crop specific emission factors instead of one emission factor for all crop residues. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electrokinetic remediation of lead‐, zinc‐ and cadmium‐contaminated soil

Electrokinetic remediation of lead‐, zinc‐ and cadmium‐contaminated sand and clayey soils has been investigated under laboratory‐scale conditions. Soil extracts of heavy metals (by 1 M HCl solution) were analysed by optical emission spectrometry. The efficiency of electrochemical remediation was partially dependent on the pH of the soil media. With pH increase, the migration of heavy metal ions toward the cathode was limited. When acetic acid was added to the sandy soil, almost complete remediation was achieved. A clay layer inserted in the cathode area did enhance the remediation rate. The most effective clean‐up was achieved for zinc and cadmium, with less effective clean‐up being achieved for lead. The effectiveness of the electrokinetic remediation of heavy metal‐contaminated clayey soil was low. The appropriate acidity was not achieved using acetic acid because of the high buffering capacity of clay, and metal ion migration was impeded by its sorption onto some clay components. The conclusion was made that clays could be used as immobilizing media for heavy metal ions by electrokinetic remediation of various soils. © 2001 Society of Chemical Industry     

Short-term carbon dioxide emissions and denitrification losses from soils amended with low-P manure from genetically modified pigs

Genetically modified pigs have been developed that are able to use phosphorus (P) in the feed more efficiently and thereby reduce the amount of P excreted in the manure. Improved digestibility would also change the chemical composition of the manure. The objective of this study was to determine how improved P digestibility affects the manure composition and the associated carbon dioxide (CO₂) emissions and nitrogen (N) loss through denitrification when the manure is applied to soil. Aerobic and anaerobic incubation studies were conducted to evaluate CO₂ emissions and denitrification losses from two soils (Brookston clay loam and Harrow sandy loam) amended with pig slurry (115 mg N kg⁻¹ soil) from transgenic pigs as well as from conventional pigs. In addition, both the transgenic and conventional pigs were fed either a low-P diet or a conventional P diet, and the effects of diet type on CO₂ emissions and denitrification losses were examined. Carbon dioxide emissions were 17% lower (P < 0.05) in treatments amended with transgenic pig manure compared with conventional pig manure in the clay loam soil. However, denitrification losses were increased by 37% (P < 0.05) in the clay loam soil amended with manure from transgenic pigs compared to manure from conventional pigs. Neither CO₂ emissions nor N loss through denitrification were affected by pig genotype when the manure was added to the sandy loam soil. The diet type (conventional P diet or low-P diet) did not affect either CO₂ emissions or N losses through denitrification in either the Brookston clay loam or Harrow sandy loam soils.     

Pulse-enhanced electrokinetic remediation of fluorine-contaminated soil

The presence of abundant fluorides in soil can pose potential threats to environment and human health. We investigated the effects of pulsed electric field on electrokinetic remediation of fluorine-contaminated soil by varying the pulse interval. Although the remediation time was increased, experimental results indicated pulse-enhanced electrokinetic remediation could improve the removal efficiency of fluorine better than the conventional electrokinetic remediation in the same intensity of electric field. The fluorine removal efficiency of the former (30 min, pulse interval) was 80.93% and that of normal electrokinetic remediation was 72.40%, because pulsed electric field could reduce the concentration polarization and increase the electrical current, electroosmotic flow and the electrical voltage part in soil cell. Therefore, in the later phase of electrokinetic remediation, when electrical current and the fluorine removal efficiency decreased obviously, at this time the pulse-enhanced method would be a considerable choice to eliminate fluorine in contaminated soils.     

Removal of Pb(II) from aqueous solutions by adsorption onto clayey soil of Indian origin: Equilibrium, kinetic and thermodynamic profile

The feasibility of applying natural, untreated clayey soil as low-cost alternative adsorbent for Pb(II) removal from aqueous solutions was investigated with a batch experimental set-up. Experiments were carried out as a function of initial solution pH (1?C8), contact time (10?C360 min), initial Pb(II) concentration (20?C100 mg L^?1), adsorbent dose (0.5?C5 g) and temperature (303?C333 K). Adsorption equilibrium data were well described by the Langmuir isotherm with maximum adsorption capacity of 121.86 mg g^?1 at 303 K. Adsorption of Pb(II) followed pseudo-second-order kinetics. Gibbs free energy (??G⁰) was spontaneous for all interactions, and the adsorption process exhibited exothermic enthalpy values. The adsorbent was easily regenerated by using 0.1M HNO₃ solution and was reused for five sorptiondesorption cycles without any considerable loss in adsorption capacity. It could be concluded that clayey soil may be used as an inexpensive and effective adsorbent without any treatment or any other modification for the removal of Pb(II) ions from aqueous solutions.     

Electrokinetic decontamination of heavy metals in construction materials: contribution of the different parameters to the global efficiency

To discriminate between the relative weights of each factor involved in the global process of an electrokinetic treatment for remediation, mortar and brick contaminated with Cs, Sr, Co, Cd and Cu were submitted to electrokinetic treatments with different enhancing electrolytes (distilled water, Na₂-EDTA, oxalic acid and citric acid), and the efficiency was analysed in function of the parameters involved in the process. The data obtained in this research, a matrix including 40 cases, have been correlated for the decontamination percentage in function of the key variables of the treatment, and the best correlation found, able to explain a variance of the 78.5% of the data, was a non linear exponential grow regression. An order in the weight of the contributions has been established that, from bigger to smaller contribution, is: constants of chelation equilibriums > precipitation as hydroxide constants > zeta potential > conductivity of the catholyte > pH catholyte > diffusion coefficient > pH of the material.     

Optimizing the preparation of steel slag-metakaolin based geopolymers by response surface methodology and in-depth investigation of Pb immobilization mechanism

To overcome the problems of steel slag (S-Slag) stockpiling and lead (Pb) contamination, in this study, a S-slag-metakaolin based geopolymer (SMBG) was prepared for the immobilization of Pb. The preparation conditions of SMBG were optimized by response surface methodology and immobilization mechanism of Pb was revealed. The optimum conditions for Pb immobilization calculated based on linear regression equations are as follows: S-Slag content 50.3 wt%, liquid–solid ratio 0.31, Baume degrees 51.2°Bé, and modulus 1.39. Under these optimum conditions, the 7-day compressive strength and Pb immobilization efficiency of SMBG were found to be 61.8 MPa and 99.99%, respectively. Subsequently, SMBG with different S-Slag contents were selected for analysis. The characterization analysis results showed that geopolymerization reaction of SMBG containing 50 wt% S-Slag was more complete, and produced more N–A–S–H gels and ettringite. Pb got immobilized in N–A–S–H gels and ettringite by physical encapsulation and ion exchange mechanism. Moreover, Pb reacts directly with silicates to form PbSiO₄ and Pb(OH)₂ precipitates. Further analysis shows that 15–25% of Pb in SMBG was present as precipitate or bound to hydration products, 25–30% of Pb was embedded in the aluminosilicate crystal structure, and the remaining Pb was present as covalent bonds. This study provides an effective solution for the harmless treatment of S-Slag and offers its potential applications in construction field.     

Enhanced electrokinetic remediation of cadmium-contaminated natural clay using organophosphonates in comparison with EDTA

Soil contamination by metals is a worldwide environmental problem. Electrokinetic extraction is a promising technology for in-situ remediation of contaminated soils of low hydraulic permeability. However, the extraction of metals is usually hindered by the high buffer capacity of natural soils. Organophosphonates are strong metal chelates as ethylenediaminetetraacetic acid(EDTA) which has been widely studied in the enhancement of electrokinetic remediation. In this study, batch desorption experiments and bench-scale electrokinetic extraction experiments were carried out to study the effect of two organophosphonates, i.e.,(nitrilotrimethylene)triphosphonate(NTMP) (ethylenedinitrilo)-tetramethylenephosphonate(EDTMP), on the extraction of cadmium from a natural clay in comparison with EDTA. Results of the batch desorption experiments showed that more than 75% of the sorbed cadmium could be dissolved into solution using 0.1 mol·L~(-1) organophosphonates or EDTA in the wide p H range of 1–11. Results of the electrokinetic extraction experiments showed that the cadmium spiked in the specimen migrated towards the anode with the enhancement of NTMP,EDTMP, and EDTA under a constant voltage gradient of approximately 1.0 V·cm-1. Although cadmium mobilization enhanced by EDTA was more efficient than that by the organophosphonates, accumulation of cadmium was observed in the vicinity of the anode. The average removal efficiencies of cadmium from the soil after approximately 5 days of electrokinetic extraction enhanced by 0.1 mol·L-1 NTMP(22.8%) and EDTMP(22.4%) were higher than that by 0.1 mol·L~(-1) EDTA(15.1%).