Electrokinetic remediation of soils contaminated with heavy metals

This paper describes the results obtained from an electrokinetic treatment of a real soil polluted by lead acetate. Powdered soil samples were pressed with a consolidometer till 100 kPa to obtain cylindrical specimens with the same characteristics as a subsoil. Tests were carried out in a Perspex electrochemical cell where soil specimens were introduced with a hollow punch. A low intensity direct current was applied in order to remove contaminants, due to electrophoresis and electroosmosis phenomena. The water flow, conductivity, apparent electroosmotic coefficient, as well as other characteristic parameters, were measured throughout the test. The water content and degree of pore saturation were estimated at the beginning and end of the test. The soil slab was divided into four slices and the Pb concentration profile determined. The main factor governing the extraction of contaminant was found to be the pH in the acidic range. Indeed, under these conditions high removal efficiencies could be reached. These results could lead to the design of a new electrochemical treatment cell equipped with a cationic membrane to expand the region of favourable pH within the soil.     

Rationale of lead immobilization by ball milling in synthetic soils and remediation of heavy metals contaminated tailings

In this work, the use of mechanical milling for the remediation of heavy metals in synthetic soils and tailings sampled from the mining area of “Barraxiutta”, SW of Sardinia, Italy is investigated. Specifically, Pb(II) contaminated synthetic soils of sandy, bentonitic and kaolinitic type are taken into account following the results obtained in previous works. Suitable sequential extraction procedures have been performed on both untreated and treated synthetic soils. It is found that mechanical loads which occur during collisions among milling media and soils are able to modify the distribution of Pb(II) onto the different solid fractions of contaminated synthetic soils. Specifically, for sandy soils the milling treatment induces a significant increase in Pb(II) content in the Fe–Mn oxides fraction. On the other hand, for bentonitic and kaolinitic soils, Pb(II) content in both carbonate and Fe–Mn oxide fractions is augmented after treatment. Such phenomena may contribute to Pb(II) immobilization efficiency since the heavy metal trans-speciation occurs in favor of fractions characterized by stronger bonds and lower solubility.As for the heavy metals contaminated tailings, their immobilization is obtained using both ball and attrition milling devices under specific ball to powder ratio values. The degree of metals immobilization is evaluated by analyzing the corresponding leachable fraction obtained through the Synthetic Precipitation Leaching Procedure (SPLP) proposed by EPA. X-ray diffraction and granulometric analyses revealed no significant alterations of the intrinsic character of the tailings after milling except for a relatively small increase in particles size.The increase in immobilization efficiency when tailings are mechanically treated may be due to specific phenomena induced during milling such as entrapment of heavy metals into aggregates, solid diffusion of metals into the crystalline reticulum of soil particles, the formation of new fresh surfaces onto which heavy metals may be irreversibly adsorbed as well as metal trans-speciation onto tailings fractions characterized by stronger bonds and lower solubility.     

土壤重金属污染联合修复技术研究进展

从土壤重金属污染治理的现状入手,介绍了以生物、物理、化学修复技术为基础的各联合修复体系,主要包括以植物修复为核心的微生物-植物、化学-植物、动物-植物、植物-植物以及以电动修复和化学淋洗为核心的各种联合修复体系。结果表明,相对于单一重金属污染修复技术,联合修复体系可显著提高重金属污染土壤的修复效率,尤其是以植物修复为核心的联合修复技术,以其低能耗、环境友好等优势,具有良好的发展潜力和应用前景。分析了目前土壤重金属修复过程中存在的问题,并对今后的研究方向进行展望。     

Screening the possibility for removing cadmium and other heavy metals from wastewater sludge and bio-ashes by an electrodialytic method

Both wastewater sludge and fly ash from combustion of biomass (bio-ash) have traditionally been applied to agricultural land in Denmark. However, Cd concentrations often exceed limiting values. The present study is a preliminary investigation of the possibility for reducing the Cd concentration in wastewater sludge and bio-ashes (straw and wood) using an electrodialytic method. The waste products were treated as stirred suspensions. During the remediation the suspension was acidified from water splitting at the anion exchange membrane and the acidification mobilized Cd that was removed to the electrode compartments. Even though the matrices were very different the remediation was successful in all cases. After treatment the Cd concentration in the ashes allowed for spreading at agricultural land and the limiting concentration of 0.8 mg Cd/kg for the wastewater sludge was almost reached (0.84 and 0.88 mg Cd/kg). The main differences of the waste products influencing the remediation process were: the sludges had a high content of organic particles that were mobilized by electrophoresis and fouled the anion exchange membrane; the straw-ash contained a lot of chloride, which formed anionic complexes with Cd, and the wood ash had a high initial pH (13.3). The mass of wastewater sludge and bio-ashes decreased during treatment but the concentration of other heavy metals (Pb, Ni, Cu and Zn) was not increased to exceed limiting values in remediated matrix.     

Melanin nano-pigments for heavy metal remediation from water

Melanins are water insoluble polyphenol compounds. The metal ion chelating property of natural melanin is exploited for removal of heavy metals from contaminated water. We optimized biosynthesis of melanin from marine bacterium using different growth media, media components, and operating conditions. Optimized medium yielded 513 mg/L melanin at 36 h of incubation, which was 3.15 times higher than the yield before optimization. Particle size analysis of the biosynthesized melanin indicated a size of 32 ± 0.98 nm. Preliminary investigation indicated that melanin nanoparticles could adsorb different heavy metals such as chromium, selenium, and lead from very low initial concentrations.     

Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal: an overview

Rice husk as a low-value agricultural by-product can be made into sorbent materials which are used in heavy metal and dye removal. It has been investigated as a replacement for currently expensive methods of heavy metal removal from solutions. Currently, the study of rice husk as a low-cost sorbent for removing heavy metals has regained attention. The heavy metals being studied are: As(V) [1], Au [2,3], Cr(IV) [4], Cu and Pb [5,6], Fe, Mn, Zn, Cu [7] and Cd(II) [8-10]. Rice husk is also being used to treat textile dyes such as like malachite green [11,12] and acid yellow 36 [13]. The treatment and preparation of rice husk activated carbon are of importance and became a subject of study [14-16]. In this review an extensive list of previous and current literature on rice husk activated carbons in removing heavy metals and dyes, their preparation or treatment and isotherms studies were complied to provide a summary of available information on rice husk and its potential as a low-cost sorbent.     

Effect of inorganic materials on the solidification of heavy metal sludge

Portland cement, cement-fly ash and lime-fly ash binders were used to solidify a synthetic heavy metal sludge containing nitrates of Cr, Ni, Cd and Hg. The sludge to binder (cement, cement-fly ash and lime-fly ash) ratio was kept at 3.33, 1.43 and 1.25, respectively. In addition inorganic substances like Cu, Zn, Pb, Sodium hydroxide and sodium sulfate were added. The molded samples were cured at room temperature for 28 days. The solidified samples with and without interference were examined for the change in their bulk density and compressive strength at definite time intervals during curing. All the metals and sodium salts added increased the average bulk density of the final product with increase in concentration (2% to 8%) with all the binder systems. The samples containing copper and lead decreased the compressive strength at all the concentrations added with CFA and LFA binders. Zn had the largest effect on all the three binder systems, lowering the strength of all samples at all the days and concentrations except the 2% Zn with CEM binder. However, Pb had only minor effect on the compressive strength with CEM binder and values remained almost constant at all the times and concentrations studied. In contrast, the effect of sodium sulfate was less marked while sodium hydroxide increased the rate of set and 28-day compressive strength of samples containing cement as binder. These observations confirm the need for specific studies of the waste and binder prior to the selection of a solidification process for the treatment of hazardous wastes. The results provide a better understanding of materials that may interfere with the immobilization of waste constituents and provide information on the possible mechanism of the interfering effects.     

Modelling of heavy metal adsorption into activated carbon from apricot stones in fluidized bed

The purpose of the present study is to investigate the behaviour of activated carbon from apricot stones (ACAS) in a closed circuit fluidized bed (CCFB) for removal of Pb, Cu, Cd and Zn ions from aqueous solution. The expansion characteristics of the bed are studied. A homogeneous solid phase diffusion model was used to describe the mass transfer inside the adsorbent particles. A piston-dispersion model was applied for the liquid phase in the bed and compared to the calculations, assuming perfect mixing conditions in the whole CCFB adsorber. The close correspondence between the experimentally determined adsorption in batch and fluidized bed operation modes support the observation that the CCFB can be described by a stirred batch adsorption model. The concentration profile at the bed exit is satisfactorily predicted by the model, thus confirming the equilibrium and kinetic parameters, determined from the laboratory batch adsorption runs.     

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.     

Modelling heavy metal and phosphorus balances for farming systems

Accounting for agricultural activities such as P fertilization in regional models of heavy metal accumulation provides suitable sustainable management strategies to reduce nutrient surpluses and metal inputs in agricultural soils. Using the balance model PROTERRA-S, we assessed the phosphorus (P), cadmium (Cd) and zinc (Zn) flux balances in agricultural soils of a rural region in Switzerland for different farm types and crop types. The P requirements of crops on arable farms were mainly supplied by commercial fertilizers and sewage sludge, while on animal husbandry farms P fertilizer demands were met by animal manure alone. Metal accumulation in soil was very different between the balance units. Estimated net Cd fluxes ranged between 1.0 and 2.3 g ha^–1 yr^–1 for arable farm types, 0.6 and 2.0 g ha^–1 yr^–1 for dairy and mixed farm types, and 9.1 and 17.8 g ha^–1 yr^–1 for animal husbandry farm types. Largest net Zn fluxes of 17.9–39.8 kg ha^–1 yr^–1 were estimated for animal husbandry farms, whereas for arable farm types net Zn fluxes of 101–260 g ha^–1 yr^–1 and for dairy and mixed farm types of 349–3360 g ha^–1 yr^–1 were found. The results indicate that P management is a primary factor determining the variation of these net Cd and net Zn fluxes. The latter were highly sensitive to the Zn/P concentration ratio in animal manure, atmospheric deposition and crop concentrations. Variation of net Cd fluxes resulted mainly from uncertainty in crop concentrations, atmospheric deposition, leaching parameters and uncertainty in Cd/P concentration ratio of commercial fertilizers. In addition, element balances were sensitive to empirical assumptions on fertilization strategy of farmers, such as the partitioning of manure between balance units.     

Study of the potential valorisation of heavy metal contaminated biomass via phytoremediation by fast pyrolysis: Part I. Influence of temperature, biomass species and solid heat carrier on the behaviour of heavy metals

Presently, little or no information of implementing fast pyrolysis for looking into the potential valorisation of heavy metal contaminated biomass is available. Fast pyrolysis of heavy metal contaminated biomass (birch and sunflower), containing high amounts of Cd, Cu, Pb and Zn, resulting from phytoremediation, is investigated. The effect of the pyrolysis temperature (623, 673, 773 and 873 K) and the type of solid heat carrier (sand and fumed silica) on the distribution of the heavy metals in birch and sunflower pyrolysis fractions are studied. The goal of the set-up is “concentrating” heavy metals in the ash/char fraction after thermal treatment, preventing them to be released in the condensable and/or volatile fractions. The knowledge of the behaviour of heavy metals affects directly future applications and valorisation of the pyrolysis products and thus contaminated biomass. They are indispensable for making and selecting the proper thermal conditions for their maximum recovery. In view of the future valorisation of these biomasses, the amounts of the pyrolysis fractions and the calorific values of the obtained liquid pyrolysis products, as a function of the pyrolysis temperature, are determined.     

Removal of heavy metals from water by lignite-based sorbents

Calcium-loaded lignite was used to remove heavy metals from wastewater by ion exchange. The following quantities were determined experimentally: calcium-loading capacity for different types of lignite, selectivity coefficients of Cu, Zn, Pb, Cd, Ni, Co and Fe(II) ions, kinetic sorption dependences and the effect of flow rate on the shape of break-through curves in column experiments. Sorption kinetics was relatively slow and consequently, the sorption dynamics was influenced by this phenomenon. A numerical code taking into account the sorption kinetics was used for modelling, which proved its applicability for scaled-up columns.The process is very efficient especially in the case of low concentrations of pollutants in water, where common methods are either economically unfavourable or technically complicated. This is very often the case of various metals in the environment, and thus the method could be convenient for their removal.     

Behaviour of heavy metals in the partial oxidation of heavy fuel oil

The behaviour of heavy metals in the partial oxidation of heavy fuel oils under a pressure of up to 100 bar (10 MPa) has been investigated. The tests were carried out in a 5 MW HP POX (High Pressure Partial Oxidation) test plant, that is operated by the IEC (Department of Energy Process Engineering and Chemical Engineering, TU Bergakademie Freiberg) in cooperation with Lurgi GmbH. In several test campaigns preheated oil with a viscosity of up to 300 cSt (= 300 mm²/s) at the burner inlet has been gasified. The heavy metals nickel Ni, iron Fe and vanadium V occur in heavy residual oils in considerable concentration and may seriously impact the gasification itself and the synthesis gas conditioning and usage. While iron is largely recovered in the gasification residue, the recovery rates of nickel and vanadium depend on the process conditions. Volatile nickel compounds were detected in the raw synthesis gas. It was found that an incomplete carbon conversion enables the capture of nickel Ni and vanadium V in the solid residue phase and can thus mitigate the problem of volatile metal compounds in the raw synthesis gas.     

重金属污染的生物修复研究进展

目前,对重金属污染土壤进行生物修复已成为一个世界性的环境问题,引起了人们的高度关注。本文对重金属污染土壤修复技术的进展情况作了简要介绍和述评,重点介绍了植物修复技术和微生物修复技术,为土壤重金属污染的生物修复提供参考。     

Removal of heavy metal ions from aqueous solution by multi-walled carbon nanotubes modified with 8-hydroxyquinoline: Kinetic study

Carbon nanotubes were modified with 8-hydroxyquinoline and used for the removal of Cu(II), Pb(II), Cd(II), and Zn(II) from aqueous solutions. Different instrumentation parameters and methods of development for determining metal ions from aqueous solutions using differential pulse anodic stripping voltammetry were studied. The adsorption of heavy metals from aqueous solution by the pristine and modified MWCNTs was studied kinetically using different kinetic models, and the results showed that the adsorption process best fitted the pseudo-second-order model and the Elovich model. The mechanism of adsorption was explored using the intra-particle diffusion model and the liquid-film model.     

Study of the leaching behaviour of paving concretes: quantification of heavy metal content in leachates issued from tank test using demineralized water

The leaching behaviour of concretes made from porphyry aggregate, river sand and Portland cement or blast furnace slag cement has been studied by means of a tank test using demineralized water in accordance with NEN 7345. The results show that the amount of heavy metals leached is small, much lower than the parametric values specified by the European Directive defining the quality of drinking water, and becomes negligible after prolonged immersion.At the end of the tank test, the fraction of heavy metals leached by the concrete represents less than 1% of the total heavy metal content of the cement. To refer to the bulk content as a criterion of environmental quality is therefore unjustified and unduly restrictive.The results suggest that the controlled use of alternative fuels and raw materials to replace natural materials does not in any way alter the leaching behaviour. Similarly, the replacement of a defined fraction of the clinker with blast furnace slag does not compromise the environmental compatibility of the concrete in terms of the heavy metals it releases.A second finding of these experiments is that the risk of contamination due to release of heavy metals from concrete on-site appears small. The total heavy metal content of concrete is of the same order of magnitude as (or even smaller than) that of a soil considered as being unpolluted in Belgium. Moreover, the fraction of heavy metals released by the concrete as a proportion of this total heavy metal content is nonsignificant.     

重金属离子脱除技术进展

综述了脱除水中重金属离子的各类主要方法 ,侧重介绍了近年来开发的若干新技术。     

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.     

改性木质素吸附重金属离子的研究与应用进展

木质素有很强的吸附能力,通过对木质素进行改性,可以提高其对重金属的吸附能力。综述了改性木质素在吸附处理重金属废水中的研究与应用,并展望了木质素基吸附剂在处理重金属废水中的发展趋势。