Chemical contaminant reactions and assessment of soil cleanup levels for protection of groundwater

About 70 percent of hazardous waste sites listed in the National Priority List (NPL) have some groundwater contamination that may require remediation. Such remediation is inadequate if the unsaturated soils above will continue to act as a source of groundwater contamination. Consequently, for most of these sites, it becomes necessary to determine what the cleanup levels for contaminants in soils should be so that subsequent contribution of contaminants from these soils to groundwater would not exceed groundwater protection levels. Representation of the dynamics of interactions between contaminants and soils is very complex, requiring among others, a thorough understanding of the chemical processes that influence the behavior of the contaminant once it enters the subsurface. Because of such complexities, environmental professionals frequently utilize methods with very simple assumptions that tend to err on the conservative side. While the public may feel protected, the needless spending of dollars could be avoided if attempts are made to incorporate, where possible, such complexities in the modeling efforts so that the system is represented as accurately as possible.     

Co-contamination of water with chlorinated hydrocarbons and heavy metals: challenges and current bioremediation strategies

Chlorinated hydrocarbons can cause serious environmental and human health problems as a result of their bioaccumulation, persistence and toxicity. Improper disposal practices or accidental spills of these compounds have made them common contaminants of soil and groundwater. Bioremediation is a promising technology for remediation of sites contaminated with chlorinated hydrocarbons. However, sites co-contaminated with heavy metal pollutants can be a problem since heavy metals can adversely affect potentially important biodegradation processes of the microorganisms. These effects include extended acclimation periods, reduced biodegradation rates, and failure of target compound biodegradation. Remediation of sites co-contaminated with chlorinated organic compounds and toxic metals is challenging, as the two components often must be treated differently. Recent approaches to increasing biodegradation of organic compounds in the presence of heavy metals include the use of dual bioaugmentation; involving the utilization of heavy metal-resistant bacteria in conjunction with an organic-degrading bacterium. The use of zero-valent irons as a novel reductant, cyclodextrin as a complexing agent, renewable agricultural biosorbents as adsorbents, biosurfactants that act as chelators of the co-contaminants and phytoremediation approaches that utilize plants for the remediation of organic and inorganic compounds have also been reported. This review provides an overview of the problems associated with co-contamination of sites with chlorinated organics and heavy metals, the current strategies being employed to remediate such sites and the challenges involved.     

Fe(Ⅱ)/铁氧化物表面结合铁系统还原有机污染物的研究进展

在土壤和沉积物的自然厌氧环境中,铁氧化物可被铁还原菌等微生物异化还原产生Fe(Ⅱ),形成的Fe(Ⅱ)/铁氧化物表面结合铁系统具有还原活性,可使有机污染物还原转化。综述了含卤和含硝基有机污染物的非生物还原转化过程和表面结合铁系统与有机污染物之间的界面反应机理,进而揭示了污染物在环境中的赋存状态和迁移转化规律;重点分析了影响该还原过程的因素,如铁氧化物类型、pH值、Fe(Ⅱ)与铁氧化物接触时间,以及过渡金属、腐殖酸等竞争因子对反应过程的影响。强化自然界中天然的Fe(Ⅱ)/铁氧化物表面结合铁系统在有机污染治理中的作用,在受污染环境修复领域具有广阔的应用前景。 [HT5H]关 键 词:[HT5K]     

Ultrasonically enhanced electrokinetic remediation for removal of Pb and phenanthrene in contaminated soils

Electrokinetic and ultrasonic remediation technologies were studied for the removal of heavy metal and polycyclic aromatic hydrocarbon (PAH) in contaminated soils. The study emphasized the coupled effects of electrokinetic and ultrasonic techniques on migration as well as clean-up of contaminants in soils. The laboratory soil flushing tests combined electrokinetic and ultrasonic technique were conducted using specially designed and fabricated devices to determine the effect of both techniques. The electrokinetic technique was applied to remove mainly the heavy metal and the ultrasonic technique was applied to remove mainly organic substance in contaminated soil. A series of laboratory experiments involving electrokinetic and electrokinetic and ultrasonic flushing tests were carried out. Natural clay was used as a test specimen and Pb and phenanthrene were used as contaminants. An increase in out flow, permeability and contaminant removal rate was observed in electrokinetic and ultrasonic tests. Some practical implications of these results are discussed in terms of technical feasibility of in situ implementation of electrokinetic ultrasonic remediation technique.     

Investigation of elemental enrichment and ecological risk assessment of surface soils in two industrial port cities,southwest Iran

In recent years, there has been a rapid growth in the two industrial port cities of Iran, the port of Mahshahr and the port of Imam Khomeini. In spite of their importance for the economy, the two cities require monitoring and assessment from an environmental and human health perspective. In this study, environmental quality and heavy metals pollution of soils influenced by human activities were investigated. A total of 30 soil samples were collected and subjected to detail physicochemical characterization. The results showed high levels of heavy metals in the urban soils of port of Mahshahr (POM) and elevated respective levels in surface soils of the port of Imam Khomeini (PIK) with Ni being significantly higher than those in POM. The integrated pollution index (IPI) values of metals ranged from 1.22 to 1.87 in POM and 1.52 to 3.31in PIK, 50% of soil samples in PIK were classified as highly polluted, and all of the soil samples in PM were classified as moderately polluted. These results are in accordance with Enrichment Factor (EF) values, which showed the role of anthropogenic activities in the soil heavy metal enrichment. Moreover, the highest potential ecological risk index (RI) was found in the sites of petrochemical industries in PIK indicating serious metal contamination. Statistical analysis showed that heavy metals were mainly controlled by human activities. The results showed more dense industrial activity in PIK is responsible for higher pollution. This study establishes a benchmark against which future monitoring and remediation programs can be based on. Because of proximity to the Persian Gulf, continued the release of contaminants into the region, could have adverse biological health effects.     

Influence Factors on the In-situ Remediation of Halogenated Organic Compounds by Nanoscale Zerovalent Iron

The pollution of soil and groundwater by halogenated organic compounds (HOCs) is more and more severe. HOCs are of strong toxicity and difficult to be biodegraded. Due to its unique advantages, nanoscale zerovalent iron (NZVI) has become a hot research topic in the field of in situ remediation around the world. In this paper, basic reaction theories and kinetics of HOCs degradation by NZVI are briefly summarized. The influence factors on the in situ remediation of HOCs by NZVI are comprehensively discussed. The influence factors include the intrinsic properties of NZVI due to its different preparation and modification methods, and environment factors, such as pH, dissolved oxygen, ionic species, metals, nonreactive hydrophobic and natural organic compounds, concentrations and components of HOCs, microorganisms and subsurface heterogeneity. The effects of all these factors on NZVI stability, deliverability, targeting ability, and reactivity during in situ remediation are emphasized. Finally, the practical application of this technology are summarized and prospected.     

Mathematical model for hydraulically aided electrokinetic remediation of aquifer and removal of nonanionic copper

One of the most cost-effective in situ technologies for soil and groundwater (i.e., aquifer) remediation is electrokinetic remediation. In electrokinetic remediation, electromigration due to electric field is combined with hydromigration due to hydraulic flow by purge water to remove pollutants from aquifers through the pore water. This study aims at investigating theoretically the role of electromigration (as active movement) of pollutants and the role of hydromigration (as passive movement) of pollutants in electrokinetic remediation, and making it clear that the control variables for electrokinetic remediation are the applied voltage and the hydraulic flow rate. These aims are pursued by construction of a mathematical model based on physico-chemical considerations and by model simulations of the electrokinetic remediation applied to the virtual aquifer polluted by heavy metals of copper sulfate. According to numerical simulations with the model: (1) heavy metal (nonanionic copper) is removed from the upstream anode region and accumulated in the downstream cathode region; (2) to carry away the heavy metal outside the aquifer (global removal), hydromigration by purge water flow is essential; and (3) electromigration contributes mainly to the redistribution of heavy metals within the aquifer (local removal and local accumulation).     

Investigation of potential accumulation and spatial distribution of heavy metals in topsoil surrounding the cement plant of Meftah (southeastern Algiers region,Algeria)

Cement’s dust particles contamination on soil has received a good amount of attention due to toxicity persistence in the environment. It must be noted that cement dust falls are enriched in toxic heavy metals, which can spread throughout a large area by wind and rain then accumulate on plants, animals, and soils, thus ultimately affecting human health. For this purpose, a study was achieved to evaluate the potential accumulation and spatial distribution of heavy metals in topsoils surrounding the Meftah cement factory, situated at the southeastern of Algiers region. Sixty six soil samples were collected following six directions (east, northeast, north, northwest, west, and southwest) from cement factory on a radius of 3.5 km approximately. The physicochemical parameters, such as the pH, the conductivity, and the organic matter of these samples were determined. The concentration of the Cd, Cu, Pb, and Zn elements, in soil samples, were determined with flame atomic absorption spectrophotometry (FAAS) method and the data generated were analyzed statistically. Thus, the enrichment factor (EF), pollution index (PI), contamination factor (CF), pollution load index (PLI), geoaccumulation index (Igeo), and principal component analysis (PCA) was performed to the dataset in order to calculate and estimate the enrichment of metal concentrations in soils. Moreover, mapping of the spatial distribution of heavy metals was carried out using geographic information system (GIS), to determine more polluted areas and accomplish the objectives of the study. The results of our analysis indicate that soils were polluted most significantly by metals such as lead, zinc, copper, and cadmium. It is thus important to keep systematic and continuous monitoring of the bag filter to reduce the cement dust emission. As possible, remediation activities be carried out on the soil of heavy metals and their derivatives to manage and suppress such pollution.     

中国典型有机污染场地土层岩性和污染物特征分析

目前我国存在大量待修复场地,其分布具有一定的规律性和区域性,场地地层系统结构复杂、渗透性空间异质性显著,污染物种类复杂。总结场地典型土层结构和典型污染物有助于有针对性地开展修复技术的研发。为此,本文收集并整理了全国136处有机污染场地相关资料,对其地域性、地层及污染物特征总结如下:目前我国已经开展调查与修复的有机污染场地主要集中在京津冀和沪宁杭地区;有机污染场地土层基本都含有黏土等低渗透介质,而且都具有非均质性,其中67%场地土层有强非均质性;沪宁杭地区场地土层渗透性总体低于京津冀和辽中南地区,此外我国京津冀和辽中南地区场地调查深度(20.3 m)总体大于沪宁杭地区(12.8 m);我国有机污染场地地下水中最常见的有机污染物种类为氯代溶剂,依次为氯代烷烃类(场地数量占比84%)、氯代苯类(场地数量占比46%)和氯代烯烃类(场地数量占比33%);最常见的3种氯代溶剂依次为二氯乙烷、一氯苯和三氯甲烷。     

Remediation technologies for metal-contaminated soils and groundwater: an evaluation

Metals including lead, chromium, arsenic, zinc, cadmium, copper and mercury can cause significant damage to the environment and human health as a result of their mobilities and solubilities. The selection of the most appropriate soil and sediment remediation method depends on the site characteristics, concentration, types of pollutants to be removed, and the end use of the contaminated medium. The approaches include isolation, immobilization, toxicity reduction, physical separation and extraction. Many of these technologies have been used full-scale. This paper will review both the full-scale and developing technologies that are available. Contaminants can be isolated and contained to minimize further movement, to reduce the permeability of the waste to less than 1×10⁻⁷ m/s (according to U.S. guidelines) and to increase the strength or bearing capacity of the waste. Physical barriers made of steel, cement, bentonite and grout walls can be used for isolation and minimization of metal mobility. Another method is solidification /stabilization, which contains the contaminants in an area by mixing or injecting agents. Solidification encapsulates contaminants in a solid matrix while stabilization involves formation of chemical bonds to reduce contaminant mobility. Another approach is size selection processes for removal of the larger, cleaner particles from the smaller more polluted ones. To accomplish this, several processes are used. They include: hydrocyclones, fluidized bed separation and flotation. Addition of special chemicals and aeration in the latter case causes these contaminated particles to float. Electrokinetic processes involve passing a low intensity electric current between a cathode and an anode imbedded in the contaminated soil. Ions and small charged particles, in addition to water, are transported between the electrodes. This technology have been demonstrated in the U.S. full-scale, in a limited manner but in Europe, it is used for copper, zinc, lead, arsenic, cadmium, chromium and nickel. The duration of time that the electrode remains in the soil, and spacing is site-specific. Techniques for the extraction of metals by biological means have been not extensively applied up to this point. The main methods include bioleaching and phytoremediation. Bioleaching involves Thiobacillus sp. bacteria which can reduce sulphur compounds under aerobic and acidic conditions (pH 4) at temperatures between 15 and 55°C. Plants such as Thlaspi, Urtica, Chenopodium, Polygonum sachalase and Alyssim have the capability to accumulate cadmium, copper, lead, nickel and zinc and can therefore be considered as an indirect method of treating contaminated soils. This method is limited to shallow depths of contamination. Soil washing and in situ flushing involve the addition of water with or without additives including organic and inorganic acids, sodium hydroxide which can dissolve organic soil matter, water soluble solvents such as methanol, nontoxic cations, complexing agents such as ethylenediaminetetraacetic acid (EDTA), acids in combination with complexation agents or oxidizing/reducing agents. Our research has indicated that biosurfactants, biologically produced surfactants, may also be promising agents for enhancing removal of metals from contaminated soils and sediments.

In summary, the main techniques that have been used for metal removal are solidification/stabilization, electrokinetics, and in situ extraction. Site characteristics are of paramount importance in choosing the most appropriate remediation method. Phytoremediation and bioleaching can also be used but are not as well developed.     

Environmental response to ore deposits and mining of the Diaojiang River basin, Guangxi （South China）

Mining activities in the upper reaches of the Diaojiang River basin have caused severe soil environment changes, especially in soils along the Diaojiang River. The flooding has resulted in the deposition of mine wastes on the arable land, and caused severe heavy metal contamination of soils along the Diaojiang River. There are obvious mine wastes in soils of the upper reaches, so it is relatively easy to identify the polluted area, but in the lower reaches, mine wastes are not so obvious, although the concentrations of pollutants in soils are still high. The plough sole of rice land can obstruct the transport of heavy metals downwards. The concentrations of Cd, Pb and Zn in rice and corn exceed the national food standards, and may have serious detriment to public health. The plants on the mine sites contain high levels of heavy metals which may lead to more serious detriment to the local ecosystem.     

An Integrated Geostatistical Approach for Contaminated Site and Soil Characterisation

For several years, abandoned or stopped industrial sites increasingly arouse the interest of politics and investors. Having a high social and economic estate value, these sites generally reveal contaminated soils that must be remediated first before receiving any new use. Due to financial, environmental or human health stakes, heuristic methods appear inappropriate because they do not provide reliable estimations of contaminated soil volumes and ignore spatial uncertainties. Problems at hand may be very complex, involving multiple correlated contaminants for which spatially varying pollutant grades are to be estimated and confronted to various regulatory thresholds, depending on redevelopment target areas. In such conditions, geostatistics provides effective methods to quantify local and global uncertainties about soil contamination and contaminated soil volumes. By quantifying uncertainties, geostatistical models are useful as support for decision-making about redevelopment scenarios or remediation techniques. Specific approaches are required, however, to overcome particular modelling issues as related to the skewness of pollutant grade distributions or change of support. Making use of our practical experience, such an integrated geostatistical approach is proposed for modelling contaminated sites. It is illustrated by application to a recent actual case study.     

Fundamental aspects of prolonged electrokinetic flows in kaolinites

Electrokinetic flows include the migration of fluid, chemicals, fine particles, bacteria, and electrons through a soil–fluid–chemical system driven by an externally applied direct-current (d.c.) electric field. The promising potential of electrokinetic remediation of contaminated fine-grained soils has stimulated recent interest in the study of electrokinetic flow processes, in particular the impact of a prolonged application of a d.c. electric field on fine-grained soils. The results of a bench-scale laboratory experimental investigation on some of the fundamental aspects of prolonged electrokinetic flows in kaolinites, including gas generation rates at power electrodes, electrochemical behaviour of the soil–fluid–chemical system, time variation of electric current intensity flowing through the soil, pH gradient development in the soil, effects of reservoir chemistry on electro-osmotic fluid volume flow rate and flow direction through the soil, time variation of electro-osmotic volume flow rate, and energy requirements per unit volume of fluid transported, are presented in this paper. Effects of soil type and pore fluid chemistry on these aspects were also studied.     

土壤修复过程中重金属形态的研究综述

重金属污染土壤的修复是现阶段污染土壤治理中的难点之一,在土壤修复过程中对重金属的形态研究已在多个领域中开展,并且在重金属形态及其与生物有效性和毒性等研究领域取得了一定的成果。本文综述了现阶段在污染土壤修复过程中对重金属形态研究的主要领域,分析研究重金属形态的必要性,总结出土壤修复过程中重金属形态方面应当从重金属在土壤与植物中的存在形态入手,研究重金属元素在不同界面间的迁移转化规律,通过阻断重金属元素在污染源、土壤、生物之间的传递链条,以阻止重金属对生物体造成危害,从而为土壤重金属污染的治理修复提供理论基础。     

江西贵溪冶炼厂重金属环境污染特征及生态风险评价

金属矿物开采、冶炼等活动引起土壤金属污染是世界普遍存在的问题,由此引起的人类健康问题和环境生态的恶化,越来越受到世界各国环境学家关注。本文调查了冶炼厂周围农田重金属元素的纵向分布,并利用连续浸提法分析主要污染元素的化学形态,结果表明,土壤Cu,Zn,Cd和Pb的非残渣态含量远远高于未污染土壤,污染元素的生物可利用性高;同时采用多指标生态评价体系,对铜矿冶炼厂周围农田土壤重金属污染进行综合的潜在生态风险评价。根据风险指标体系和空间分异规律,将冶炼厂周围农田分成3种生态风险功能区。     

The Effect of pH on Electro-osmotic Flow in Argillaceous Rocks

The use of electro-osmosis in geotechnical engineering has been considered since 1930. Its application has been tested in several sectors like the fight against rising damp in porous materials, the consolidation of soils as well as the remediation of the soils contaminated by oil and heavy metals, etc. The paper presents an experimental study of the electrokinetic phenomena resulting from the application of an electric field to argillaceous sandstones. The electroosmotic tests required the development of a completely new experimental device composed of a confining cell, of a system of measurement and control of the pH of the electrolyte near the electrodes, of a system of control and measurement of the voltage and current at the edges of the sample, and of a system of measurement of inlet (anodic) and outlet (cathodic) flows. Various boundary conditions are considered with and without control of pH. In addition, a micro structural modelling of the electrokinetic phenomena makes it possible to quantify the electro osmotic conductivity with a good agreement with experiments.     

Mapping potential risks of long-term wastewater irrigation in alluvial soils,Egypt

In Egypt, wastewater has been used for irrigation in areas with fresh water scarcity; however, continuous applications may cause potential risks. Thus, the current study aims to map the spatial distribution of soil contamination and human risks of long-term wastewater irrigation due to the exposure of heavy metals. Soils from nine sites in Al-Qalyubia Governorate, Egypt, were sampled and analyzed. Wastewater irrigation resulted in a buildup of heavy metals in soils compared to Nile fresh water-irrigated soil. The pollution index (PI) showed the decreasing order of Cd?>?Zn?>?Ni?>?Cu?>?Co?>?Pb?>?Cr. The soils were out of the safe domain, as the integrated pollution index of Nemero’s (IPIN) exceeded the safe limit of 0.7. The enrichment factor (EF) exceeded 1.0, proving anthropogenic effects. The geo-accumulation index (I_geo) indicated high threats associated with Cd. The calculated hazard index (HI) indicated that humans exposed to such contaminated soils would have a potential health risk, particularly children. It is recommended to perform a treatment to the wastewater used in irrigation to avoid such threats.     

The effect of organic materials on the mobility and toxicity of metals in contaminated soils

Organic materials such as compost are often proposed as suitable materials for the remediation of contaminated brownfield sites intended for soft end-use. In addition to vitalising the soil, they are also believed to immobilise metals thereby breaking contaminant-receptor pathways and reducing the ecotoxicity of the contaminants. However, some research has demonstrated contradictory effects between composts on metal immobilisation. In the present study, four different composts and a liming product containing organic matter (LimeX70) were tested to examine both their metal retention and toxicity reduction capabilities on three different metal contaminated soils. Leaching tests, a plant growth test with Greek cress (Lepidium sativum), an earthworm (Eisenia fetida) survival and condition test and a bacterial toxicity test using Vibrio fischeri were carried out. The leaching test results showed that spent mushroom compost caused an increase in metal concentration in the leachates, while LimeX70 caused a decrease. The variation in behaviour between different amendments for each soil was high, so a generic conclusion could not be drawn. Toxicity tests showed significant reduction of metal bioavailability and toxicity for Greek cress, earthworms and bacteria. The results also suggest that more research should be undertaken to understand the mechanisms involved in metal complexation using different types of organic matter, in order to optimise the use of organic materials like compost for soil remediation.     

Metal content and environmental risk assessment around high-altitude mine sites

Gold mining activities in Apolobamba area, northwest of La Paz, Bolivia have created serious environmental concern and great risk to human health. The current methods used to extract gold are too primitive resulting in metal contamination of soil and water. The objectives of this study were to: (1) determine the degree of metal pollution, and (2) assess the risk to human health and environment in the Apolobamba area. Soil, water, sediment samples, and mine spills were collected and analyzed. Metals including Pb, Cu, Zn, Cd, and Hg concentrations were higher in surface soils than in subsurface soils indicating active atmospheric deposition of metals. Sediment samples had elevated levels of metals probably from mine spills discharged into the Sunchulli River. Surface soils in the Sunchulli community show the highest levels of Pb and Hg in all soil samples and may pose a risk to the health of the human population and environment.     

Arsenic hyperaccumulation by brake ferns and their potential in phytoremediation

Municipal solid waste incinerator （MSWI） fly ash is world-widely defined as hazardous waste because of its high concentration of heavy metals and high toxic equivalents of dioxin-like compounds. Therefore, if not properly disposed, it would pose a risk of being released into the environment including soil and groundwater. Heavy metals in the MSWI fly ash are considered as the most dangerous component owing to their leaching characteristics and migration capability. To get a better knowledge of their releasing behavior, it is important to assess the potential environmental mobility of heavy metals in MSWI fly ash and to investigate the factors affecting their leaching characteristics. Fly ashes from four MSW incinerators were characterized and leaching of heavy metals according to both Chinese HVEP and USA TCLP were performed. A geochemical modeling computer program, PHREEQC （version 2） was used to calculate the equilibrium concentration, the speciation and possible precipitation of heavy metals in the leaching systems. In the modeling, the heavy metals, Cd, Cr, Cu, Ni, Pb, Zn, were input as master aqueous species in solutions, while the major and accessory components, Ca, Na, K, Cl, S, etc., were defined as finite solids. The agreement between the experimental results and the modeling ones varies for different metals as well as for different fly ashes. In general, the fitness of the modeling for different heavy metals is： Pb〉Zn〉Cd〉Cu. The modeling is excellent for fly ashes with low calcium but high chloride content, or with high concentration of heavy metals, which favor the implicit postulation of component input in the modeling, i.e., high availability of heavy metals.