Evaluation of Electrokinetic Removal of Heavy Metals from Tailing Soils

Electrokinetic remediation was studied for the removal of toxic heavy metals from tailing soils. The study emphasized the dependency of removal efficiencies upon their speciations, as demonstrated by the different extraction methods used, which included sequential extraction, total digestion, and 0.1 N HCl extraction. The tailing soils examined showed different physicochemical characteristics, such as initial pH, particle size distribution, and major mineral constituents, and they contained high concentrations of target metal contaminants in various forms. The electrokinetic removal efficiencies of heavy metals were significantly influenced by their partitioning prior to treatment, and the pHs of the tailing soils. The mobile and weakly bound fractions of heavy metals, such as the exchangeable fraction, were easily removed by electrokinetic treatment (more than 90% removal efficiency), but immobile and strongly bound fractions, such as the organically bound species and residual fractions, were not significantly removed (less than 20% removal efficiencies).     

Iodide-Enhanced Electrokinetic Remediation of Mercury-Contaminated Soils

This study investigates using an iodide-enhanced solution at the cathode during electrokinetic treatment to optimize the removal of mercury from soils. The experimental program consisted of testing two types of clayey soils, kaolin, and glacial till, that were initially spiked with 500 mg/kg of Hg(II). Experiments were conducted on each soil type at two voltage gradients (1.0 or 1.5 VDC/cm) to evaluate the effect of the voltage gradient when employing a 0.1 M KI solution. Additional experiments were performed on each soil type to assess the effect of using a higher iodide concentration (0.5 M KI) when using a 1.5 VDC/cm voltage gradient. The tests conducted on the kaolin soil showed that when the 0.1 M KI concentration was employed with the 1.0 VDC/cm voltage gradient, approximately 97% of the mercury was removed, leaving a residual concentration of 16 mg/kg in the soil after treatment. The tests conducted on glacial till indicated that it was beneficial to use the higher (0.5 M KI) iodide concentration and the higher (1.5 VDC/cm) voltage gradient to enhance mercury removal, because, under these conditions, a maximum of 77% of the mercury was removed from the glacial till, leaving a residual concentration of 116 mg/kg in soil after electrokinetic treatment. Compared to kaolin, the lower mercury removal from the glacial till soil is attributed to the more complicated soil composition, such as the presence of carbonates and organic matter, which caused Hg(II) to adsorb to the soil and/or exist as an immobile chemical species.     

Cosolvent-Enhanced Electrokinetic Remediation of Soils Contaminated with Phenanthrene

This research was carried out to evaluate feasibility of using an electrokinetic technique to remove hydrophobic organic pollutants from soils, with the assistance of a cosolvent (n-butylamine, tetrahydrofuran, or acetone) added to the conducting fluid. The experiments were carried out on glacial till clay with phenanthrene as the test compound. Desorption equilibrium was investigated by batch tests. The electrokinetic experiments were conducted using a 19.1 cm long × 6.2 cm inside diameter column under controlled voltage. Water or 20% (volume) cosolvent solution was constantly supplied at the anode. The concentration of phenanthrene in the effluent collected at the cathode was monitored. Each experiment lasted for 100 to 145 days. Results showed that the presence of n-butylamine significantly enhanced the desorption and electrokinetic transport of phenanthrene; about 43% of the phenanthrene was removed after 127 days or 9 pore volumes. The effect of acetone was not as significant as butylamine. The effluent flow in the tetrahydrofuran experiments was minimal, and phenanthrene was not detected in the effluent. The use of water as the conducting solution did not cause observable phenanthrene migration.     

Cyclodextrin-Enhanced Electrokinetic Remediation of Soils Contaminated with 2,4-Dinitrotoluene

The removal of 2,4-dinitrotoluene (2,4-DNT), a munitions waste constituent and an industrial intermediate, from contaminated soils was evaluated using enhanced electrokinetic (EK) remediation. Two model soils were spiked with 480?mg of 2,4-DNT/kg of dry soil for the EK experiments. The spiked soils were kaolin, a low-buffering clayey soil, and glacial till, a high-buffering silty soil. The glacial till was obtained from a field site and contained 2.8% organic matter. Deionized (DI) water and cyclodextrin solutions were used as the EK purging solutions. Cyclodextrin was selected as a nonhazardous solubility enhancer for enhancing the desorption and removal of 2,4-DNT from soils in EK remediation. Two aqueous solutions of hydroxypropyl β-cyclodextrin (HPCD) at concentrations of 1 and 2% were selected for kaolin and glacial till, respectively, based on results for batch extraction of 2,4-DNT from the same soils. During the EK experiments, greater current and electro-osmotic flow were observed for HPCD solutions than for DI water. After the completion of the EK experiments, the soils in the EK cell were extruded and the residual 2,4-DNT in the soils was determined. Less 2,4-DNT remained in the kaolin soil (up to 94% transformed) than in the glacial till soil (20% transformed) due to strong retention of 2,4-DNT by the soil organic matter in glacial till. For kaolin, less 2,4-DNT remained in the soil using HPCD solutions than using DI water. For glacial till, comparable levels of 2,4-DNT remained in the soil for both EK solutions. Since no 2,4-DNT was detected in the effluents from the EK cells, the decrease in 2,4-DNT concentration in the kaolin and glacial till soils was attributed to electrochemical transformation of 2,4-DNT to other species.     

Sequential Electrokinetic Remediation of Mixed Contaminants in Low Permeability Soils

The coexistence of heavy metals and polycyclic aromatic hydrocarbons (PAHs) at many of the contaminated sites poses a severe threat to public health and the environment. Very few technologies, such as soil washing/flushing and stabilization/solidification, are available to remediate such sites; however, these technologies are ineffective and expensive to treat contaminants in low permeability clayey soils. Previous studies have shown that electrokinetic remediation has potential to remove heavy metals and organic compounds when they exist individually in clayey soils. In the present study, the feasibility of using surfactants and organic acids sequentially and vice versa during electrokinetic remediation was evaluated for the removal of both heavy metals and PAHs from clayey soils. Kaolin was selected as a model clayey soil and it was spiked with phenanthrene and nickel at concentrations of 500 mg/kg dry each to simulate typical field mixed contamination. Bench-scale electrokinetic experiments were performed with the sequential anode conditioning with: (1) 1 M citric acid followed by 5% Igepal CA-720; (2) 1 M citric acid followed by 5% Tween 80; and (3) 5% Igepal CA-720 followed by 1 M citric acid. A periodic voltage gradient of 2 V/cm (with 5 days on and 2 days off cycles) was applied in all the tests. A removal of about 96% of phenanthrene was observed in the test with 5% Igepal CA-720 followed by 1 M citric acid sequence. Most of the nickel (>90%) migrated from anode to cathode in this test; however, it precipitated in the section very close to the cathode due to the high pH conditions. Conversely, the removal efficiency of nickel was about 96 and 88% in the tests with 1 M citric acid followed by 5% Igepal CA-720 sequence and 1?M citric acid followed by 5% Tween 80 sequence, respectively. However, the migration and removal efficiency of phenanthrene in both of these tests were very low. Overall, it can be concluded that the sequential use of 5% Igepal CA-720 followed by 1 M citric acid may be an effective remedial strategy to remove coexisting heavy metals and PAHs from clayey soils.     

Sequentially Enhanced Electrokinetic Remediation of Heavy Metals in Low Buffering Clayey Soils

This paper presents the results of an experimental investigation undertaken to evaluate different purging solutions to enhance the removal of multiple heavy metals, particularly chromium, nickel, and cadmium, from a low buffering clay, specifically kaolin, during electrokinetic remediation. Experiments were conducted on kaolin spiked with Cr(VI), Ni(II), and Cd(II) in concentrations of 1,000, 500, and 250 mg/kg, respectively, which simulate typical electroplating waste contamination. A total of five different tests were performed to investigate the effect of different electrode purging solutions on the electrokinetic remedial efficiency. A constant DC voltage gradient of 1 V/cm was applied for all the tests. The removal of heavy metals from the soil using tap water as the purging solution was very low. When 1 M acetic acid was used as the purging solution in the cathode, the removal of chromium, nickel, and cadmium was increased to 20, 19, and 13%, respectively. Using 0.1 M ethylene diamine tetraacetic acid as the purging solution in the cathode, 83% of the initial Cr was removed; however, the nickel and cadmium removal was very low. A sequentially enhanced electrokinetic remediation approach involving the use of water as a purging solution at both the anode and cathode initially, followed by the use of acetic acid as the cathode purging solution and a NaOH alkaline solution as the anode purging solution was tested. This sequential approach resulted in a maximum removal of chromium, nickel, and cadmium of 68–71, 71–73, and 87–94%, respectively. This study shows that the sequential use of appropriate electrode purging solutions, rather than a single electrode purging solution, is necessary to remediate multiple heavy metals in soils using electrokinetics.     

Enhanced Electrokinetic Remediation of Heavy Metals in Glacial Till Soils Using Different Electrolyte Solutions

Previous electrokinetic remediation studies involving the geochemical characterization of heavy metals in high acid buffering soils, such as glacial till soil, revealed significant hexavalent chromium migration towards the anode. The migration of cationic contaminants, such as nickel and cadmium, towards the cathode was insignificant due to their precipitation under the high pH conditions that result when the soil has a high acid buffering capacity. Therefore the present laboratory study was undertaken to investigate the performance of different electrolyte (or purging) solutions, which were introduced to either dissolve the metal precipitates and/or form soluble metal complexes. Tests were conducted on a glacial till soil that was spiked with Cr(VI), Ni(II), and Cd(II) in concentrations of 1,000, 500, and 250 mg/kg, respectively, under the application of a 1.0 VDC/cm voltage gradient. The electrolyte solutions tested were 0.1M EDTA (ethylenediaminetetraacetic acid), 1.0M acetic acid, 1.0M citric acid, 0.1M NaCl/0.1M EDTA, and 0.05M sulfuric acid/0.5M sulfuric acid. The results showed that 46–82% of the Cr(VI) was removed from the soil, depending on the purging solution used. The highest removal of Ni(II) and Cd(II) was 48 and 26%, respectively, and this removal was achieved using 1.0M acetic acid. Although cationic contaminant removal was low, the use of 0.1M NaCl as an anode purging solution and 0.1M EDTA as a cathode purging solution resulted in significant contaminant migration towards the soil regions adjacent to the electrodes. Compared to low buffering capacity soils, such as kaolin, the removal of heavy metals from the glacial till soil was low, and this was attributed to the more complex composition of glacial till. Overall, this study showed that the selection of the purging solutions for the enhanced removal of heavy metals from soils should be primarily based upon the contaminant characteristics and the soil composition.     

Experimental Studies on Heavy Metal Extraction from Contaminated Soil Using Ammonium Citrate as Alkaline Chelate during the Electrokinetic Process

Researchers have performed experimental studies using ammonium citrate (AC) during the electrokinetic (EK) remediation process for the extraction of cadmium (Cd) and copper (Cu) from the contaminated soil. They evaluated the efficiency of ammonium citrate by considering it as a washing solution and a purging solution at the anode electrode compartment. The efficiency of electrokinetic extraction was observed to be significantly influenced by the pH and buffering nature of the soil medium. The experimental studies indicate that the removal of cadmium and copper was 48.9% and 30.0%, respectively, when ammonium citrate was used both washing and purging solution. The solubility of both cadmium (Cd++) and copper (Cu++) in EK-treated soils has also been estimated by sequential extraction studies with deionized (DI) water. The analytical techniques, X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscope (SEM) provide the evidence of migration of cations during treatment of contaminated soil by process of electroosmosis (EO). The SEM images of both cadmium- and copper-contaminated soils show that these soils have a fluffier and more porous structure. This might be caused by the change in surface charges of the clay particles as a result of introduction of heavy metals. The mineralogical compositions of soil are not altered significantly by electrokinetic process.     

Complicating Factors of Using Ethylenediamine Tetraacetic Acid to Enhance Electrokinetic Remediation of Multiple Heavy Metals in Clayey Soils

Batch and electrokinetic experiments were conducted to investigate the removal of three different heavy metals, chromium(VI), nickel(II), and cadmium(II), from a clayey soil by using ethylenediamine tetraacetic acid (EDTA) as a complexing agent. The batch experiments revealed that high removal of these heavy metals (62–100%) was possible by using either a 0.1?M or 0.2?M EDTA concentration over a wide range of pH conditions (2–10). However, the results of the electrokinetic experiments using EDTA at the cathode showed low heavy metal removal efficiency. Using EDTA at the cathode along with the pH control at the anode with NaOH increased the pH throughout the soil and achieved high (95%) Cr(VI) removal, but the removal of Ni(II) and Cd(II) was limited due to the precipitation of these metals near the cathode. Apparently, the low mobility of EDTA and its migration direction, which opposed electroosmotic flow, prevented EDTA complexation from occurring. Overall, this study found that many complicating factors affect EDTA-enhanced electrokinetic remediation, and further research is necessary to optimize this process to achieve high contaminant removal efficiency.     

Electrokinetic Remediation of Cadmium-Contaminated Clay

Electrokinetic extraction has been demonstrated to be very effective in removing heavy metals from Georgia kaolinite. The relatively high removal efficiency depends on the extremely acidic soil environment generated by the electrokinetic process. However, the efficiency observed in Georgia kaolinite cannot be achieved in soils of high acid/base buffer capacity without enhancement. In this study, the effect of ethylenediaminetetraacetic acid (EDTA) to enhance electrokinetic extraction of cadmium from Milwhite kaolinite was examined. The influence of electro-osmotic flow direction on the migration of cadmium, EDTA, and their complexes were also investigated. It was observed that injection of EDTA from the cathode reservoir by a reverse electro-osmotic flow could mobilize the cadmium in the specimen effectively. A less significant mobilization of cadmium was observed when the electro-osmotic flow was directed toward the cathode. However, accumulation of cadmium near the anode was observed regardless of the electro-osmotic flow direction.     

电动力富集电解锰渣中锰的研究

采用电动力技术对电解锰渣中可溶性锰的迁移、转化和富集规律进行研究。结果表明,直接电动力和CO_2辅助电动力均可在阴极区富集锰,反应48h锰富集量分为达到3.8%和4.3%。两种方式作用下,阴极区锰的富集量随反应时间的增加而增加,阳极区锰含量随反应时间的增加逐渐减少。从阴极到阳极,锰含量逐渐减少。直接电动力作用下的反应机制为,锰在阴极区富集并与OH-反应形成Mn(OH)_2、CaMn(OH)SiO_4等难溶物。CO_2辅助电动力作用下的反应机制为,CO_2与阴极区的OH-反应形成CO_3~(2-),CO_3~(2-)进一步与迁移到阴极区的Mn~(2+)反应形成MnCO_3。     

Liquefaction Resistance of Soils from Shear-Wave Velocity

A simplified procedure using shear-wave velocity measurements for evaluating the liquefaction resistance of soils is presented. The procedure was developed in cooperation with industry, researchers, and practitioners and evolved from workshops in 1996 and 1998. It follows the general format of the Seed-Idriss simplified procedure based on standard penetration test blow count and was developed using case history data from 26 earthquakes and >70 measurement sites in soils ranging from fine sand to sandy gravel with cobbles to profiles including silty clay layers. Liquefaction resistance curves were established by applying a modified relationship between the shear-wave velocity and cyclic stress ratio for the constant average cyclic shear strain suggested by R. Dobry. These curves correctly predicted moderate to high liquefaction potential for >95% of the liquefaction case histories and are shown to be consistent with the standard penetration test based curves in sandy soils. A case study is provided to illustrate application of the procedure. Additional data are needed, particularly from denser soil deposits shaken by stronger ground motions, to further validate the simplified procedure.     

Electrokinetic chromatography without electroosmotic flow

Otx1 and Otx2 genes are mouse cognates of a Drosophila head gap gene orthodenticle. The homozygous mutants have previously indicated that Otx2 is essential to development of structures anterior to rhombomere 3, probably reflecting its expression around the early primitive streak stage. Otx2 mutation also exhibits craniofacial defects by haplo-insufficiency. Affected structures correspond to the most anterior and most posterior parts of the Otx2 expression where Otx1 is not, or is only weakly, expressed at the time of brain regionalization. No apparent defects are found in early brain development by the Otx1 mutation, suggesting that the Otx1 and Otx2 functions overlap in the regions where both are expressed. To demonstrate this, the Otx1/Otx2 double heterozygous phenotype was examined in this study. Analyses with molecular markers at 9.5 days post coitus suggested the failure in development of mesencephalon and caudal diencephalon with the expansion of anterior metencephalon. Genes expressed in isthmus exhibited a characteristic lateral stripe normally, although rostrally shifted, except that Fgf8 expression was expanded dorsally. The defects were apparent at the 6-somite stage, but not at the 3-somite stage. Broad Fgf8 expression at the 3-somite stage took place normally, but it did not concentrate into a spot corresponding to future isthmus. The double heterozygous phenotype implicates a previously unsuspected mechanism for development of the mes/metencephalic territory; at the 3- to 6-somite stage Otx1 cooperates with Otx2 to establish the mes/diencephalic domain, allowing for the correct development of isthmus/ rhombomere 1.     

超声波辅助提取花生衣中的白藜芦醇

[目的]以乙醇为溶剂,采用超声波辅助提取的方法提取花生衣中白藜芦醇.[方法]以白藜芦醇的提取率为考察指标,研究花生衣中白藜芦醇提取的优化工艺.[结果]正交试验表明,花生衣中白藜芦醇的适宜提取条件:常温条件下料液比为15:1、乙醇浓度为60%、超声处理时间为60 min、超声功率为150 W,白藜芦醇的平均提取率是0.299%.料液比和乙醇浓度是影响白藜芦醇提取率的显著因素.[结论]该工艺简单快捷,可用于花生衣中白藜芦醇的提取.     

Electrokinetic Remediation Modeling Incorporating Geochemical Effects

Electrokinetic remediation technology is one of the developing technologies that offers great promise for the cleanup of soils contaminated with heavy metals. However, the performance of an electrokinetic remediation system depends on the interaction of a complex set of interrelated system variables and parameters. Many of these interactions were addressed in this study by incorporating geochemical reactions into electrokinetic remediation modeling. A one-dimensional transport model was developed to predict the transport and speciation of heavy metals (chromium, nickel, and cadmium) in soil during electrokinetic remediation as a function of time and space. The model incorporates: (1) pH-dependent adsorption of contaminants to the soil surface; (2) sensitivity of soil surface potential and electroosmotic flow to the pore water properties; and (3) synergistic effects of multiple chemical species on electrokinetic remediation. The model considers that: (1) Electrical potential in the soil is constant with time; (2) surface complexation reactions are applicable in the highly concentrated clay suspensions; (3) the effect of temperature is negligible; and (4) dissolution of soil constituents is negligible. The predicted pH profiles, electroosmotic flow, and transport of chromium, nickel, and cadmium in kaolin soil during electrokinetic remediation were found to reasonably agree with the bench-scale electrokinetic experimental results. The predicted contaminant speciation and distribution (aqueous, adsorbed, and precipitated) allow for an understanding of the transport processes and chemical reactions that control electrokinetic remediation.     

微波辅助法提取山楂黄酮类化合物的工艺条件研究

[目的]探讨微波辅助法提取山楂中黄酮类化合物的工艺.[方法]采用单因素试验和正交试验,研究微波处理时间、微波功率、乙醇浓度和液固比4个因素对提取山楂黄酮类化合物的影响.[结果]最佳提取工艺条件为:液固比60:1、乙醇浓度80%、微波处理时间60 s、微波功率420 W.在此工艺条件下,提取率达到3.47%.[结论]该工艺条件适合于提取山楂中黄酮类化合物.     

Estimating Compaction of Cohesive Soils from Machine Drive Power

To evaluate roller-integrated machine drive power (MDP) technology for predicting the compaction parameters of cohesive soils considering the influences of soil type, moisture content, and lift thickness on machine power response, a field study was conducted with 15-m test strips using three cohesive soils and several nominal moisture contents. Test strips were compacted using a prototype CP-533 static padfoot roller with integrated MDP technology and tested using various in situ compaction measurement devices. To characterize the roller machine-soil interaction, soil testing focused on measuring compaction parameters for the compaction layer. Variation in both MDP and in situ measurements was observed and attributed to inherent variability of the compaction layer and measurement errors. Considering the controlled operations to create relatively uniform conditions of the test strips, measurement variability observed in this study establishes a baseline for acceptable variation in production operations using MDP technology in cohesive soils. Predictions of in situ compaction measurements from MDP were found to be highly correlated when moisture content and MDP-moisture interaction terms were incorporated into regression models.     

TOA-TBP从铁合金萃钯余液中协同萃取分离铂

以等离子炉富集产出的铂钯铑铁合金溶解造液萃钯后的余液为原料,选择TOA-TBP混合萃取剂萃取分离铂。研究单一TBP、TOA以及混合体系对铂的萃取行为。结果表明,对于贱金属较高的溶液体系,100%TBP对铂的萃取率仅有80.9%,单一TOA在高浓度下铂的萃取率接近100%,但铑的共萃率也随之上升,最高可达到82.86%。而95%TBP-5%TOA的混合体系在0.5 mol/L HCI,相比为1,旋转速度100 r/min条件下,铂的萃取率达到99.9%以上,铑的共萃率仅为0.2%。选择稀盐酸洗涤负载有机相,10 mol/L盐酸反萃,铂的反萃率达到97.2%。TOA-TBP混合体系可以实现铂铑高效分离,且该体系对铂的萃取具有协同效应。     

甘草中甘草酸的多级逆流提取技术研究

研究利用多级逆流技术提取甘草酸的新方法.采用正交试验考察提取温度、提取时间、级数和液固比对提取效率的影响,确定多级逆流提取甘草酸的最佳工艺条件;在优选出的最佳工艺条件下,考察了提取溶剂对提取率的影响,并与室温冷浸法、超声波法、索氏提取法和微波提取法做了比较.结果表明:多级逆流提取甘草酸的最佳条件为提取温度70 ℃,单级提取时间60 min,液比6,提取级数为5.在此最佳条件下,多级逆流提取的提取率高于室温冷浸44.3 h、超声波提取40 min、索氏提取4 h、微波萃取54 min的提取率.多级逆流提取具有快速、高效、节能、节约溶剂的特点,用于中草药有效成分的提取,值得推广应用.