Fenton chemistry encompasses reactions of hydrogen peroxide in the presence of iron to generate highly reactive species such as the hydroxyl radical and possibly others. In this review, the complex mechanisms of Fenton and Fenton-like reactions and the important factors influencing these reactions, from both a fundamental and practical perspective, in applications to water and soil treatment, are discussed. The review covers modified versions including the photoassisted Fenton reaction, use of chelated iron, electro-Fenton reactions, and Fenton reactions using heterogeneous catalysts. Sections are devoted to nonclassical pathways, by-products, kinetics and process modeling, experimental design methodology, soil and aquifer treatment, use of Fenton in combination with other advanced oxidation processes or biodegradation, economic comparison with other advanced oxidation processes, and case studies.

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Advanced Oxidation Processes for Organic Contaminant Destruction Based on the Fenton Reaction and Related Chemistry

Scattered literature is harnessed to critically review the possible sources, chemistry, potential biohazards and best available remedial strategies for a number of heavy metals (lead, chromium, arsenic, zinc, cadmium, copper, mercury and nickel) commonly found in contaminated soils. The principles, advantages and disadvantages of immobilization, soil washing and phytoremediation techniques which are frequently listed among the best demonstrated available technologies for cleaning up heavy metal contaminated sites are presented. Remediation of heavy metal contaminated soils is necessary to reduce the associated risks, make the land resource available for agricultural production, enhance food security and scale down land tenure problems arising from changes in the land use pattern.

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Heavy Metals in Contaminated Soils: A Review of Sources, Chemistry, Risks and Best Available Strategies for Remediation

Biodegradation aspects of Polycyclic Aromatic Hydrocarbons (PAHs): A review

Toxic metal pollution of waters and soils is a major environmental problem, and most conventional remediation approaches do not provide acceptable solutions. The use of specially selected and engineered metal-accumulating plants for environmental clean-up is an emerging technology called phytoremediation. Three subsets of this technology are applicable to toxic metal remediation: (1) Phytoextraction--the use of metal-accumulating plants to remove toxic metals from soil; (2) Rhizofiltration--the use of plant roots to remove toxic metals from polluted waters; and (3) Phytostabilization--the use of plants to eliminate the bioavailability of toxic metals in soils. Biological mechanisms of toxic metal uptake, translocation and resistance as well as strategies for improving phytoremediation are also discussed.

Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants

The major potential environmental impacts related to landfill leachate are pollution of groundwater and surface waters. Landfill leachate contains pollutants that can be categorized into four groups (dissolved organic matter, inorganic macrocomponents, heavy metals, and xenobiotic organic compounds). Existing data show high leachate concentrations of all components in the early acid phase due to strong decomposition and leaching. In the long methanogenic phase a more stable leachate, with lower concentrations and a low BOD/COD-ratio, is observed. Generally, very low concentrations of heavy metals are observed. In contrast, the concentration of ammonia does not decrease, and often constitutes a major long-term pollutant in leachate. A broad range of xenobiotic organic compounds is observed in landfill leachate. The long-term behavior of landfills with respect to changes in oxidation-reduction status is discussed based on theory and model simulations. It seems that the somewhere postulated enhanced release of accumulated heavy metals would not take place within the time frames of thousands of years. This is supported by a few laboratory investigations. The existing data and model evaluations indicate that the xenobiotic organic compounds in most cases do not constitute a major long-term problem. This may suggest that ammonia will be of most concern in the long run.

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Present and long-term composition of msw landfill leachate: a review

The chemical interactions of hydrophobic organic contaminants (HOCs) with soils and sediments (geosorbents) may result in strong binding and slow subsequent release rates that significantly affect remediation rates and endpoints The underlying physical and chemical phenomena potentially responsible for this apparent sequestration of HOCs by geosorbents are not well understood This challenges our concepts for assessing exposure and toxicity and for setting environmental quality criteria Currently there are no direct observational data revealing the molecular-scale locations in which nonpolar organic compounds accumulate when associated with natural soils or sediments Hence macroscopic observations are used to make inferences about sorption mechanisms and the chemical factors affecting the sequestration of HOCs by geosorbents Recent observations suggest that HOC interactions with geosorbents comprise different inorganic and organic surfaces and matrices, and distinctions may be drawn along these lines,

Sequestration of hydrophobic organic contaminants by geosorbents

We often think of plants primarily as a source of wood, food, and fiber Secondarily we may also appreciate their presence for aesthetic reasons as well as for "altruistically" providing habitat for other species Increasingly, however, their value as an environmental counterbalance to industrialization processes is being appreciated These processes include the burning of fossil fuels, generation of wastes (sewage, inorganic and organic solids, and effluents), and general water flow and processing Plants have long been recognized for their consumption of CO2 and, more recently, of other gaseous industrial byproducts (Simonich and Hites, 1994) Recently, their role in slowing the rate of global warming has been further appreciated in both the scientific and popular press Their use as a final water treatment and for disposal of sludge resulting from waste water treatment is centuries old (Hartman, 1975) The extensive literature concerning water and sludge treatment and the emerging field of air pollution abatement with plants will not be discussed here Instead, we focus on an emerging concept, "phytoremediation," the use of plants to remediate contamination of soil with organic or inorganic wastes Remediation of soil contamination by conventional engineering techniques often costs between $50 and $500 per ton Certain specialized techniques can exceed costs of $1000 per ton With an acre of soil (to a 3-foot depth) weighing approximately 4500 tons, this translates to a minimum cost of about a quarter million dollars per acre (Cunningham et al, 1995) It is not surprising that the cleanup of contaminated sites has not been proceeding at a rapid pace There is an active effort to develop new, more costeffective technologies to remediate contamination of such soils For the most part these efforts are being led by engineers and microbiologists More recently, however, green plant-based processes have begun receiving greater attention It has long been known that the life cycle of a plant has profound effects on the chemical, physical, and biological processes that occur in its immediate vicinity In the process of shoot and root growth, water and mineral acquisition, senescence, and eventual decay, plants can profoundly alter the surrounding soil The effects of many of these processes are apparent on the restoration of land at physically and chemically altered sites, ranging from road cuts to the site of the Mount St Helen's eruption These same plant-driven processes also occur in areas heavily impacted by industrial, mining, and urban activities One of the greatest forces driving increased emphasis on research in this area is the potential economic benefit of an agronomybased technology Growing a crop on an acre of land can be accomplished at a cost ranging from 2 to 4 orders of magnitude less than the current engineering cost of excavation and reburial There have been perhaps two dozen field tests to date; however, in many ways phytoremediation is still at its initial stages of research and development A comforting thought for plant biologists is that much of the research effort will be expected to center on a deeper understanding of basic plant processes So how do we envision phytoremediation working? The theory appears to be simple Agronomic techniques will be used to ready the contaminated soil for planting and to ameliorate chemical and physical limitations to plant growth Plants will then directly or indirectly absorb, sequester, and/or degrade the contaminant Plants and irrigation, fertilization, and cropping schemes will be managed to maximize this remedial effect By growing plants over a number of years, the aim is to either remove the pollutant from the contaminated matrix or to alter the chemical and physical nature of the contaminant within the soil so that it no longer presents a risk to human health and the environment As people who work in the remediation, herbicide development, and farming industries will attest, many weed species are remarkably tolerant of a wide range of organic and inorganic toxins Plants can thrive in soil contaminated to levels that are often orders of magnitude higher than current regulatory limits These limits are often set relatively independent of plant tolerance limits and are most often derived from human health and aquatic toxicology end points Ironically, many remediation plans begin with the destruction of the existing vegetation

Promises and Prospects of Phytoremediation

Lead phytoextraction, using plants to extract Pb from contaminated soils, is an emerging technology. Calculations of soil Pb mass balance suggest that this technology will be economically feasible only if systems can be developed to employ high biomass plants that can accumulate greater than 1% Pb in their shoots. In this study, we investigated the potential of adding chelates to Pb-contaminated soils to increase Pb accumulation in plants. The addition of chelates to a Pb-contaminated soil (total soil Pb 2500 mg kg-1) increased shoot Pb concentrations of corn (Zea mays L. cv. Fiesta) and pea (Pisum sativum L. cv. Sparkle) from less than 500 mg kg-1 to more than 10000 mg kg-1. The surge of Pb accumulation in these plants was associated with the surge of Pb level in the soil solution due to the addition of chelates to the soil. For the chelates tested, the order of the effectiveness in increasing Pb desorption from the soil was EDTA > HEDTA > DTPA > EGTA >EDDHA. We also found that EDTA significantly increas...

Phytoremediation of Lead-Contaminated Soils: Role of Synthetic Chelates in Lead Phytoextraction

https://www.cell.com/trends/biotechnology/pdf/S0167-7799(00)88987-8.pdf

Scott D. Cunningham

Papers

Sequestration of hydrophobic organic contaminants by geosorbents

Promises and Prospects of Phytoremediation

Phytoremediation of Lead-Contaminated Soils: Role of Synthetic Chelates in Lead Phytoextraction

Phytoremediation of contaminated soils

Phytoremediation of soils contaminated with organic pollutants.