Environmental Remediation Technologies for Metal-Contaminated Soils

Heavy metal (HM) pollution of soils has been observed on local, regional, and global scales, and is likely to increase worldwide with growing industrial and agricultural activities. The HM pollution of soil is a significant environmental issue, because HM is responsible for causing adverse effect on human health through food chain contamination. The HM may originate and reach soils through pedogenic as well as anthropogenic processes. Once entered into the soil environment, the HM undergoes a number of chemical changes over time. The HM dynamics in soil is complex, and the bioavailability, mobility, and toxicity of metals in the soil fractions are influenced by different factors, including the properties of both the soil and the metal. This book chapter reviews the effect and significance of soil properties on the metal contamination of soils, which will help us to improve our understanding of the mechanisms involved in the transfer and mobilization of HM in soils.

Heavy metals, such as arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg) and zinc (Zn), are major environmental problem due to their toxic nature, nonbiodegradability and accumulative behaviors. Once in the estuarine/coastal and marine environments, sources predominantly form industrial, agricultural and hydrocarbon-related activities, scrap metal recycling, commercial ports and sewage, these contaminants accumulate in sediments and soils. Thus, heavy metals concentrations in coastal areas around shipyards, ports and industrial sites with refineries, smelters and milling facilities are often far exceed their background values or standard limits that can be toxic. The toxicity of heavy metals may negatively affects marine biodiversity as higher concentration is detected in fish and other organisms. Due to their persistence, through bioaccumulation and biomagnification along the aquatic food chain, heavy metals contamination ultimately affects human health. Here, the sources and impacts of heavy metals pollution in living systems are discussed.

The Chernobyl nuclear power plant accident in 1986, the Mayak reprocessing plant accident in 1957, and the Fukushima nuclear power plant accident in 2011 have released various radionuclides. Spatial distribution of radionuclides in surface soil constitutes fundamental information related to radiation exposure and to post-accident sources for secondary dispersion of radionuclides. The spatial and vertical distributions of ¹³⁷Cs, ⁹⁰Sr, and Pu from those three nuclear accidents were reviewed to elucidate the fate of the radionuclides in the soil environment. This chapter specifically presents information about the dynamics of radiocesium (¹³⁴Cs and ¹³⁷Cs) in the soil of contaminated areas because of its long half-life and its major contribution to the overall external radiation dose. Along with an examination of global fallout, for soils of various types, the existence forms and geochemical behavior of radiocesium in surface soil after the Fukushima accident are reviewed, as are similar results from the Chernobyl and Mayak nuclear accidents.

Test methods for the evaluation of heavy metals contained in contaminated soil were summarized. The soil environmental standards, criteria for soils in each country, and setup backgrounds were studied. The methods for the extraction of heavy metals were classified into three categories: (1) digestion/decomposition, (2) extraction, and (3) leaching test. Regarding these three analytical methods, the scientific meaning of each method and concrete test procedures were described. Regarding the extraction method, the SCE (sequential chemical extraction) procedure for chemical forms of metals and extraction for bioavailability were discussed. In the section with regard to the leaching test, the relationship between operational factors and the leaching concentration was discussed and the availability test was also introduced. Furthermore, the movement of unifying the test methods in the ISO (International organization for standardization) was introduced. The environmental standards of soils and the criteria of 13 countries were compared. In addition, the setup backgrounds of the environmental standards and the environmental criteria for soils in Canada, Australia, the Netherlands, and Germany were investigated and discussed.

This chapter analyzed existing international and regional legal developments to deal with soil contamination and remediation measures. It further examined characteristics of different national legal approaches in the field of soil contamination and remediation process. It concluded that there are little international initiatives for the adoption of an international soil regime to deal with soil protection including issues of contamination and possible remediation measures. Absence of international soil protection regime and hence lack of concerted global and national efforts on remediation action on contaminated sites could pose not only serious health risks but also long-term sustainable development challenge. However, a credible, comprehensive model law on soil contamination could make a considerable progress for remediation of contaminated sites globally and hence could reduce health and environmental risks for future generations.

Decontamination of hazardous discards by immobilization of toxic components is a longstanding approach for managing waste, while it gained much attention in recent years due to the increasing number of statutes and regulations favouring the technology. The solidification/stabilization (S/S) technique is the commonly adopted immobilization option to treat the contaminated soils, which employ additives to convert the hazardous waste to non-hazardous mass in accordance with the legitimate landfill provisions. The discussion is further extended to the stabilization of toxic elements in contaminated soils using chemical amendments. The current paper presents a summarized overview on the application of S/S technique in managing metal-contaminated soil, including information about the frequently used additives for the purpose, and the steps involved in the implementation of S/S remediation.

Estimation and immobilization of fluoride and heavy-metals in contaminated soil are important approaches for the remediation of fluoride and heavy-metal polluted soil. In this review, firstly, we described recent achievements about the on-site estimation of various pollutants by simple chemical reaction without special skills for operators. The in-situ immobilization of fluoride was also described. For immobilization of fluoride, dicalcium phosphate dihydrate (DCPD) was selected as functional material because it reacts with fluoride ion effectively and forms stable fluorapatite (FAp). Both laboratory and field tests showed that the DCPD is useful to immobilize fluoride in polluted soil.

Over centuries, industrial, mining and military activities, agriculture, farming, and waste practices have contaminated soils and wetlands in many countries with high concentrations of toxic metals. In addition to their negative effects on ecosystems and other natural resources, toxic metals pose a great danger to human health. Unlike organic compounds, metals cannot be degraded, and clean-up usually requires their removal. Most of the conventional remedial methods have lost economic favor and public acceptance because they are expensive and cause degradation of soil fertility that subsequently results in adverse impacts on the ecosystem. Conventional methods of environmental remediation do not solve the problem; rather they merely transfer it to future generation. Obviously, there is an urgent need for alternative, cheap, and efficient methods to clean-up sites contaminated with toxic metals.

Phytoremediation, a plant-based green technology, is cost effective, environmental friendly, aesthetically pleasing approach for the remediation of toxic metals. Due to its elegance and the extent of contaminated areas, phytoremediation approaches have already received significant scientific and commercial attention. Two approaches have been proposed for the phytoremediation of toxic metals from soils and wetlands: natural and induced phytoremediation. Natural phytoremediation refers to the use of hyper-accumulating plants and associated soil microbes, while the induced phytoremediation refers to the use chemicals, especially synthetic chelating ligands, for the increase of metal bioavailability and uptake in plants. Recently, genetically modified plants (GMPs) have been proposed to use in phytoremediation technology; however, this approach is being hindered by ideology-driven restrictive legislation over the use of GMPs. We will discuss the concepts and practical applications of phytoremediation technologies for the restoration of contaminated soils and wetlands.

The immobilization or removal of toxic components using aqueous extractants, with or without additives, is one of the commonly practiced techniques for the treatment of metal-contaminated soils. However, rather than the use of water alone, the solution with chemical-additives is preferred due to the less time requirement and better separation effectiveness. There is a long-favored list of additives that have been used for the chemical-induced washing remediation of soils, which include acids, bases, chelants, surfactants, and so forth. The objective of this chapter is to provide a brief overview of the chemical-assisted soil washing approaches.

Soil is an important part of environment which is under threat due to various types of contaminations happening since last few decades. Recovery and regeneration of soil have become a global problem. Recently nanotechnology has emerged as an efficient, cost effective, environment friendly and promising technology for soil remediation. This technology has significant potentiality to remove contaminants from environment by various approaches like adsorption, redox reaction, conversion, stabilization, etc. Various types of nanomaterials and devices are used to remove contaminants from soil. Therefore, soil could be remediated effectively by utilizing nanotechnology based concepts, processes and products which cannot be achieved from conventional methods. One of the most hazardous soil contaminants is heavy metal. Like other contaminants, heavy metals could be removed from soil using nanotechnology based approaches. The applications of nanotechnology to remove heavy metals from the contaminated soil are discussed in this chapter.

In Japan, environmental standards for contaminants in groundwater and in leachate from soil are set with the assumption that they are used for drinking water over a human lifetime. Where there is neither a well nor groundwater used for drinking, the standard is thus too severe. Therefore, remediation based on these standards incurs excessive effort and cost. In contrast, the environmental assessment procedure used in the USA and the Netherlands considers the site conditions (land use, existing wells, etc.); however, a risk assessment is required for each site. This chapter shows a framework for validating contamination by considering the merits of the environmental standards used and a method for risk assessment. The framework involves setting risk-based concentrations (RBCs), which are attainable remediation goals for contaminants in soil and groundwater. The framework was then applied to a model contaminated site for risk management. RBCs of Cr(VI) in a contaminated site were set according to the site conditions. The RBCs of Cr(VI) with/without drinking water in residential area are calculated. Second, an experiment for contaminated soil was introduced by using column equipment. The equipment was designed by applying water permeability test. And then, variation of concentration of the contaminant was simulated using an advection-diffusion model. Simulation by the mathematical model is also useful for monitored natural attenuation or in situ treatment, because the simulation can estimate clean-up time at the contaminated site. Even though the estimated clean-up time is not exact time, the cost of in situ treatment is not expensive. And then land owners can choose the in situ treatment.