砷污染土壤微生物修复机制及其研究进展

土壤砷污染是一个全球性问题。越来越多的研究表明,自然界中的微生物在砷的迁移转化过程中发挥了重要作用。微生物修复是目前研究的热点,也是治理砷污染土壤的主要手段之一。综述了土壤中砷污染现状及其赋存形式,重点分析讨论了砷污染土壤的微生物修复机制以及提高微生物修复效率的方法。土壤砷污染修复是一项复杂的系统工程,单一的修复技术很难实现显著的效果。只有建立在以微生物修复为主的基础上,辅之以物理化学、植物及农业生态等措施,才能大大提高微生物修复效率。总结了近年来国内外微生物修复砷污染土壤技术的研究进展,以期为深入研究微生物代谢砷的机制及其在砷污染治理中的应用提供参考。     

重金属污染土壤原位钝化修复材料的最新研究进展

土壤环境关系到人们的身体健康,重金属污染土壤的修复一直以来是全球环境领域关注的焦点和研究的热点。重金属原位钝化修复技术是一种有前景的解决方案。其中,钝化材料是关键。着重介绍了不同类型重金属污染土壤原位钝化修复材料的修复原理、修复效果及应用情况,简要分析了修复材料的成本和效益,为重金属污染土壤原位钝化修复研究提供相关信息。     

表面活性剂冲洗修复多氯联苯污染土壤多相流研究

多氯联苯(PCBs)是一种具有持久性、抗生物降解性、脂肪溶性和明显的生物毒性等特性的持久性有机污染物,PCBs在土壤中难于准确定位、难被分解和强烈吸附,去除土壤中PCBs比较困难.表面活性剂冲洗法可以通过提高PCBs溶解度和降低水-PCBs界面张力来实现PCBs从土壤中去除;表面活性剂冲洗PCBs污染土壤涉及气相、水相、NAPLs相和固相等物质,是多相共存并相互发生作用的过程,发生相对渗透率、饱和度和毛细压力的变化;另外,为研究表面活性剂冲洗土壤中PCBs的去除机理,并降低PCBs对研究人员的危害,采用微观孔隙结构网络模型是一种较新颖的和效果显著的研究方法.开展表面活性剂冲洗PCBs污染土壤多相流研究,可以为PCBs污染场地修复提供理论基础和实验支持,并促进我国POPs履约工作的顺利进行.     

土壤重金属钝化材料生物炭的研究进展

原位钝化法作为一种快速有效的土壤重金属污染治理方法得到了广泛的应用。生物炭是由生物质在缺氧环境下热解而成的一种含碳材料,具有精细的孔隙结构、较大的比表面积和丰富的表面官能团,能够有效地钝化土壤中的重金属,降低其生物有效性,是一种应用前景广阔的钝化材料。综述了影响生物炭对土壤重金属钝化效果的主要因素、钝化机制以及生物炭的改性方法。寻找钝化持效性好的生物炭材料,深化研究生物炭与不同形态重金属的作用机制,有利于更好地将生物炭钝化材料应用于重金属污染土壤的修复。     

土壤有机污染的原位修复技术

介绍了土壤气相抽提、生物通风和空气喷射等3种土壤原位修复技术的概念，简述了它们修复受污染土壤的适用范围和修复机理，并对影响3种修复方法的各种因素进行了详细论述。展望了土壤气相抽提、生物通风和空气喷射技术在我国的应用前景。     

土壤有机污染的原位修复技术

介绍了土壤气相抽提、生物通风和空气喷射等3种土壤原位修复技术的概念,简述了它们修复受污染土壤的适用范围和修复机理,并对影响3种修复方法的各种因素进行了详细论述.展望了土壤气相抽提、生物通风和空气喷射技术在我国的应用前景.     

天然海泡石-二乙基二硫代氨基甲酸钠复合体对土壤中镉的钝化机制

通过材料的表征、吸附实验及钝化实验,探讨了天然海泡石-二乙基二硫代氨基甲酸钠复合体(SEP-DDTC)对土壤中镉的钝化效果和机制。结果显示,SEP-DDTC与镉的反应机制除了静电吸附、离子交换吸附和羟基配位吸附外,还存在CSS—基团螯合配位吸附作用。海泡石改性后对镉的饱和吸附容量明显提高;在2种土壤中,添加0.10%(质量分数)SEP-DDTC能够不同程度地降低土壤中有效态镉含量,有效态镉含量分别比对照降低59.4%、18.8%。表明SEP-DDTC是一种潜在的土壤重金属镉修复材料。     

原位热处理技术修复重质非水相液体污染场地研究进展

重质非水相液体(DNAPLs)是土壤及地下水中广泛存在的有机污染物,原位热处理技术是目前修复受DNAPLs污染土壤及地下水的最具潜力的技术之一。综述了国内外常用原位热处理技术的基本原理及其影响因素,介绍了相关现场应用实例,并展望了该技术未来的应用前景和发展趋势,以期为中国污染土壤及地下水的原位修复提供有益借鉴。     

芬顿和氯化铁协同修复三价砷污染土的稳定化效果

由于铁盐稳定化修复砷污染土通常不考虑砷价态对毒性和迁移性的影响,导致了修复效果欠佳。针对三价砷污染土提出了预氧化-稳定化协同修复方法,先使用芬顿将三价砷氧化为低迁移性的五价砷,再利用氯化铁将其稳定化。通过合成沉降淋滤试验、生物可利用性、pH测试、连续萃取试验和光谱分析研究了协同修复的稳定化效果和机理。结果表明,与未经处理的污染土相比,1%用量的芬顿和1倍铁砷比的氯化铁进行协同修复的稳定化处理可使浸出浓度降低99.6%,生物可利用性砷的质量分数降低99.4%,修复效率得到提高;芬顿氧化和氯化铁稳定化都使土壤pH降低,但1倍铁砷比下协同修复后的pH为5.82,高于6倍铁砷比下只用氯化铁修复的pH (3.78),由此可一定程度上避免过量使用铁盐造成的土壤酸化;连续萃取试验表明协同修复后更多不稳定态的砷转变为稳定形态;光谱分析发现协同修复可以将土壤中三价砷全部氧化为五价,并通过铁氧化物/氢氧化物吸附和砷酸铁沉淀的形式固定砷。本研究结果可为三价砷污染土修复提供理论和技术支持。     

污染土壤电动修复增强方法研究进展

污染土壤电动修复是一项新兴绿色原位修复技术。其原理是在土壤上施加直流电场 ,利用电迁移和电渗去除污染物 ,土壤pH、Zeta电位以及土壤化学性质等因素影响电动修复效果。为了提高修复效率和扩大电动修复应用范围 ,现在已经发展了针对不同类型土壤和污染物的增强修复技术。本文归纳总结了 1995年以来土壤电动修复中常用增强处理效果的 8种方法 ,即酸碱中和法、阳离子选择膜法、电渗析法、络合剂法、表面活性剂法、氧化 还原法、EK 生物联用和LasagnaTM法 ,且对每种方法的典型实验装置、增强原理、方法特点和适用范围等进行了分析和讨论 ,为以后的实验设计提供了有益的参考     

A methodological approach for the identification of arsenic bearing phases in polluted soils

A methodological approach is used to characterize arsenic pollution in three soils and to determine arsenic speciation and association with solid phases in three polluted soils. HPLC-ICP-MS was used for arsenic speciation analysis, SEM-EDS and XRD for physical characterization of arsenic pollution, and sequential chemical extractions to identify arsenic distribution. Arsenic was concentrated in the finest size fractions also enriched in iron and aluminium. Total arsenic concentrations in soils are close to 1%. Arsenic was mainly present as arsenate, representing more than 90% of total arsenic. No crystallised arsenic minerals were detected by XRD analysis. SEM-EDS observations indicated arsenic/iron associations. Modified Tessier's procedure showed that arsenic was mainly extracted from amorphous iron oxide phase. The results of this methodological approach lead to predict the formation of iron arsenates in the case of one of the studied soils while arsenic sorption on iron amorphous (hydr)oxides seemed to be the determinant in the two other soils.     

Biological oxidation of arsenite: batch reactor experiments in presence of kutnahorite and chabazite

Arsenic represents a threat to all living organisms due to its toxicity which depends on its speciation. This element is carcinogenic, teratogenic and is certainly one of the most important contaminants affecting millions of people around the world. Abiotic and biotic processes control its speciation and distribution in the environment. We have previously shown that a new bacterial strain named ULPAs1 performed oxidation of As(III) (1.33 mM) to As(V) in batch cultures. In order to develop new methods to remove arsenic from contaminated effluents or waste, by bacterial oxidation of As(III) to As(V) followed by its sorption, the conservation of oxidative properties of ULPAs1 was investigated when cultivated in batch reactors in the presence of two solid phases, chabazite and kutnahorite, already used as microorganisms immobilizing materials in biological remediation processes. In parallel, the retention efficiency of these solid phases toward arsenic ions and particularly arsenate was studied. Pure quartz sand was used as a reference material. Kutnahorite efficiently sorbed As(V), chabazite alone performed As(III) oxidation and pure quartz sand did not sorb arsenic at all. The arsenite oxidative properties of ULPAs1 were conserved when cultivated in the presence of quartz or chabazite.     

Arsenic in the soils of Zimapán, Mexico

Arsenic concentrations of 73 soil samples collected in the semi-arid Zimapán Valley range from 4 to 14 700 mg As kg(-1). Soil arsenic concentrations decrease with distance from mines and tailings and slag heaps and exceed 400 mg kg(-1) only within 500 m of these arsenic sources. Soil arsenic concentrations correlate positively with Cu, Pb, and Zn concentrations, suggesting a strong association with ore minerals known to exist in the region. Some As was associated with Fe and Mn oxyhydroxides, this association is less for contaminated than for uncontaminated samples. Very little As was found in the mobile water-soluble or exchangeable fractions. The soils are not arsenic contaminated at depths greater than 100 cm below the surface. Although much of the arsenic in the soils is associated with relatively immobile solid phases, this represents a long-term source of arsenic to the environment.     

Enhanced solubilization of arsenic and 2,3,4,6 tetrachlorophenol from soils by a cyclodextrin derivative

The application of extracting aqueous solutions with cyclodextrins in several soil remediation technologies has been increasingly studied but little is known about their removal capacities toward the inorganic species. Herein, the effectiveness of cyclodextrins (CDs) in extracting arsenic, copper, and iron from a mining soil is presented. In a preliminary test of four types of CD aqueous solutions, only the addition of carboxylmethyl-beta-cyclodextrin CMCD (a cyclodextrin derivative) led to a significant enhancement in arsenic removal. An increase in the concentration of copper and iron in the leachates was also observed with CMCD. Kinetic study of arsenic release was carried out at two temperatures (20 and 35 degrees C). The arsenic concentration in the leachates increases with increasing cyclodextrin concentration. At an 80 mM CMCD concentration, arsenic, copper, and iron released in filtrates were about 20-, 1,000-, and 4,000-fold greater, respectively, than that obtained using deionized water. In the soil system, the CMCD capacity removal was found to be higher for cations than for arsenic. Because the tetrachlorophenol can co-occur with arsenic and copper in several contaminated sites, its solubilization by CMCD was also investigated. Extraction experiments were performed to extract 2,3,4,6 tetrachlorophenol (TeCP) in spiked soil with CMCD. The results of batch experiments have shown that CMCD could significantly increase the TeCP extraction from soil. CD sorption on soils as quantified by a fluorescence technique was low, indicating no significant loss of CD during the leaching experiments. The use of CMCD as a flushing agent to enhance the removal of both inorganic and organic pollutants from mixed-contaminated soils appears as a promising remediation method.     

Kinetic constraints on theIn-situ remediation of soils contaminated with organic chemicals

Cleanup of contaminated soils to comply with soil quality limits currently receives much interest.In-situ remediation of contaminated soils relies on the ability of the techniques employed to enhance the rate of release of contaminants from the soil-sorbed and nonaqueous phase liquid (NAPL) phases into the aqueous or gaseous phases from which they can be more readily removed and treated. Contaminant concentrations in these “environmentally mobile” forms usually decline over time so that the economic efficiency and the overall success of remediation technologies are subject to the “law of diminishing returns”. In this paper we consider the “state of the art” in our understanding of NAPL dissolution and transport, desorption of soilsorbed contaminants and fluid flow in porous media. The extent to which these processes may constrain the success of bioremediation, pump-and-treat remediation and soil venting in relation to established soil quality limits is addressed. Finally, we suggest directions for future research and comment on legislative considerations.     

Soil remediation time to achieve clean-up goals II: Influence of natural organic matter and water contents

This work reports a relatively rapid procedure for the forecasting of the remediation time (RT) of sandy soils contaminated with cyclohexane using vapour extraction. The RT estimated through the mathematical fitting of experimental results was compared with that of real soils. The main objectives were: (i) to predict the RT of soils with natural organic matter (NOM) and water contents different from those used in experiments; and (ii) to analyse the time and efficiency of remediation, and the distribution of contaminants into the soil matrix after the remediation process, according to the soil contents of: (ii1) NOM; and (ii2) water. For sandy soils with negligible clay contents, artificially contaminated with cyclohexane before vapour extraction, it was concluded that: (i) if the NOM and water contents belonged to the range of the prepared soils, the RT of real soils could be predicted with relative differences not higher than 12%; (ii1) the increase of NOM content from 0% to 7.5% increased the RT (1.8-13 h) and decreased the remediation efficiency (RE) (99-90%) and (ii2) the increase of soil water content from 0% to 6% increased the RT (1.8-4.9 h) and decreased the RE (99-97%). NOM increases the monolayer capacity leading to a higher sorption into the solid phase. Increasing of soil water content reduces the mass transfer coefficient between phases. Concluding, NOM and water contents influence negatively the remediation process, turning it less efficient and more time consuming, and consequently more expensive.     

Sequential soil washing techniques using hydrochloric acid and sodium hydroxide for remediating arsenic-contaminated soils in abandoned iron-ore mines

Sequential washing techniques using single or dual agents [sodium hydroxide (NaOH) and hydrochloric acid (HCl) solutions] were applied to arsenic-contaminated soils in an abandoned iron-ore mine area. We investigated the best remediation strategies to maximize arsenic removal efficiency for both soils and arsenic-containing washing solution through conducting a series of batch experiments. Based on the results of a sequential extraction procedure, most arsenic prevails in Fe-As precipitates or coprecipitates, and iron exists mostly in the crystalline forms of iron oxide. Soil washing by use of a single agent was not effective in remediating arsenic-contaminated soils because arsenic extractions determined by the Korean standard test (KST) methods for washed soils were not lower than 6mg kg(-1) in all experimental conditions. The results of X-ray diffraction (XRD) indicated that iron-ore fines produced mobile colloids through coagulation and flocculation in water contacting the soils, containing dissolved arsenic and fine particles of ferric arsenate-coprecipitated silicate. The first washing step using 0.2M HCl was mostly effective in increasing the cationic hydrolysis of amorphous ferrihydrite, inducing high removal of arsenic. Thus, the removal step of arsenic-containing flocs can lower arsenic extractions (KST methods) of washed soils. Among several washing trials, alternative sequential washing using 0.2M HCl followed by 1M HCl (second step) and 1M NaOH solution (third step) showed reliable and lower values of arsenic extractions (KST methods) of washed soils. This washing method can satisfy the arsenic regulation of washed soil for reuse or safe disposal application. The kinetic data of washing tests revealed that dissolved arsenic was easily readsorbed into remaining soils at a low pH. This result might have occurred due to dominant species of positively charged crystalline iron oxides characterized through the sequential extraction procedure. However, alkaline extraction using NaOH was effective in removing arsenic readsorbed onto the surface of crystalline minerals. This is because of the ligand displacement reaction of hydroxyl ions with arsenic species and high pH conditions that can prevent readsorption of arsenic.     

Soil remediation time to achieve clean-up goals I: Influence of soil water content

The current models are not simple enough to allow a quick estimation of the remediation time. This work reports the development of an easy and relatively rapid procedure for the forecasting of the remediation time using vapour extraction. Sandy soils contaminated with cyclohexane and prepared with different water contents were studied. The remediation times estimated through the mathematical fitting of experimental results were compared with those of real soils. The main objectives were: (i) to predict, through a simple mathematical fitting, the remediation time of soils with water contents different from those used in the experiments; (ii) to analyse the influence of soil water content on the: (ii(1)) remediation time; (ii(2)) remediation efficiency; and (ii(3)) distribution of contaminants in the different phases present into the soil matrix after the remediation process. For sandy soils with negligible contents of clay and natural organic matter, artificially contaminated with cyclohexane before vapour extraction, it was concluded that (i) if the soil water content belonged to the range considered in the experiments with the prepared soils, then the remediation time of real soils of similar characteristics could be successfully predicted, with relative differences not higher than 10%, through a simple mathematical fitting of experimental results; (ii) increasing soil water content from 0% to 6% had the following consequences: (ii(1)) increased remediation time (1.8-4.9h, respectively); (ii(2)) decreased remediation efficiency (99-97%, respectively); and (ii(3)) decreased the amount of contaminant adsorbed onto the soil and in the non-aqueous liquid phase, thus increasing the amount of contaminant in the aqueous and gaseous phases.     

Effects of amended compost on mobility and uptake of arsenic by rye grass in contaminated soil

Arsenic poses a major environmental and human health problem because of its carcinogenic nature and effect on the ecosystem. Therefore, a cost effective and socially acceptable technique is needed for its remediation. The effect of different combinations of compost amended with zeolite and/or iron oxide (up to 20% w/w) was tested on a contaminated soil with high arsenic levels (34470 mg kg(-1)). The bioavailability of arsenic was determined in terms of uptake by rye grass (Lolium perenne L.) under greenhouse experimental conditions. The results indicated that the arsenic concentrations in the rye grass was reduced to 2 mg kg(-1) dry weight by using 15% compost with 5% iron oxide and 15% compost with 5% zeolite. Less than 0.01% of the total arsenic content in the soil was being taken up by the plants. Both treatments were effective in establishing significantly higher plant growth on the contaminated soil compared to other treatments. The results from sequential extraction tests indicated that in all the compost-amended soils, there was a reduction in the soluble fraction (10-37%). Arsenic in soil was examined using Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy. The results indicated that arsenic was distributed mostly within the matrix of iron and oxygen in treated samples. Amongst various treatment mixtures tested, high percent of compost (15%) with zeolite (5%) and/or iron oxide (5%) is effective in reducing arsenic uptake by plants and establish re-vegetation on the contaminated soil.     

Soil vapor extraction in sandy soils: influence of airflow rate

Airflow rate is one of the most important parameters for the soil vapor extraction of contaminated sites, due to its direct influence on the mass transfer occurring during the remediation process. This work reports the study of airflow rate influence on soil vapor extractions, performed in sandy soils contaminated with benzene, toluene, ethylbenzene, xylene, trichloroethylene and perchloroethylene. The objectives were: (i) to analyze the influence of airflow rate on the process; (ii) to develop a methodology to predict the remediation time and the remediation efficiency; and (iii) to select the most efficient airflow rate. For dry sandy soils with negligible contents of clay and natural organic matter, containing the contaminants previously cited, it was concluded that: (i) if equilibrium between the pollutants and the different phases present in the soil matrix was reached and if slow diffusion effects did not occur, higher airflow rates exhibited the fastest remediations, (ii) it was possible to predict the remediation time and the efficiency of remediation with errors below 14%; and (iii) the most efficient remediation were reached with airflow rates below 1.2 cm(3)s(-1) standard temperature and pressure conditions.