As the field of phytoremediation has evolved over the last 10 years, there has been a growing interest in the use of vegetative covers as alternative landfill caps. Consequently, a number of vegetative caps (grass, shrubs, and trees) have been installed at landfills across the United States, with tree caps being the focus of this paper. The two primary objectives of vegetative caps are to minimize water percolation into landfill waste via a “sponge and pump” mechanism and to prevent surface soil erosion. The soil pores hold precipitation like a sponge until plant roots can access the water; plants take up this water for growth and release it into the atmosphere by transpiration. The erosion control objective is achieved by canopy interception of rainfall, minimizing the “splash effect,” and the growing of a root matrix that interlaces through soil particles. Secondary benefits of vegetative caps include aesthetic appeal for the neighboring community, sequestration of greenhouse gases, reduced construction costs, reduced operation costs, and creation of wildlife habitat. When properly designed and managed, vegetative caps can minimize landfill owner liability while creating a future asset for the surrounding community. The Ecolotree Cap (ECap) (Ecolotree, Inc., North Liberty, Iowa) is a patented phytoremediation system that consists of carefully designed cover soils planted with perennial trees and understory grasses. This system has been installed at 13 landfills in the United States (in nine states) and at one landfill in Europe (Slovenia). The sites include pre-Subtitle D landfills, Subtitle D demonstrations, a construction debris landfill, and a landfill on the Superfund National Priorities List. The ECap cover is currently being evaluated for Subtitle D landfill closure at two sites by the U.S. Environmental Protection Agency Alternative Cover Assessment Program.

High concentrations of Al, Fe, Ca, Mg, and Zn in the tanning agent are detrimental to the properties of tanned leather. To facilitate removal of chromium from the tannery sludge and recycle to the tanning agent, these five metals must first be removed as much as possible from the acid extract of tannery sludge. This work proposes a three-step process for the separation of Cr(III) from these interfering element metals present in the acid extract (leachate) of tannery sludge. In step 1, the leachate was appropriately diluted, and the Al was efficiently removed by precipitation in the form of hydroxide at 21°C and adjusting the pH between 4.6 and 4.8. The optimum dilution ratio for the tannery sludge leachate at a solids concentration 80 (g/L) was 6.3. Under the above operating conditions, 50–80% of Al, 30–40% of Fe, and 22–40% of Cr were removed, and the discrimination ratio between Al and Cr was between 2.0 and 2.2, depending on the pH. In step 2, the Al and Fe that remained in the filtrate after step 1 were further reduced at 21°C by reacting with an organic agent cupferron (Y). The precipitation reactions of cupferron with all of metals were completed in 10 min. The optimum pH for the removal of Al, Fe, and Zn was between 3 and 4.83. The optimum molar ratio of cupferron to the sum of Al, Fe, and Zn was between 2.78 and 4. Under these operating conditions, about 90% of Fe, 80% of Al, 70% of Zn, and 7% of Cr were precipitated. During step 2, most of Fe, Al, and Zn originally present in the tannery leachate were separated from Cr(III), Ca, and Mg. In step 3, the Ca and Mg left in the filtrate after step 2 were separated from Cr(III) by adjusting the pH of the filtrate to 7.0 with the solution of sodium hydroxide at 21°C. The removal efficiency of Cr(III), Ca, and Mg were 99.6, 13.7, and 4.9%. After step 3, the molar percentages of Al, Ca, Cr, Fe, Mg, and Zn in the precipitate formed at pH 7.0 and 21°C in a typical case were 2.94, 0.055, 95.649, 0.302, 1.013, and 0.041, respectively. The overall removal efficiencies of the targeted metals in a typical case were as follows: Al, 77%; Fe, 84.4%; Ca, 99.2%; Mg, 98.5%; and Zn, 61%. About 42% of Cr(III) in total (step 1 + step 2) was removed as precipitates with other metals. The hydroxide precipitates of Cr(III) (from step 1) can be recovered efficiently by the conventional oxidation method. The spent cupferrons as cupferrates can be recovered and recycled. The separation process was simple since it does not involve the oxidation of Cr(III) and the use of ionic exchange resin.

Three fast-growing, freshwater microalgal species, Scenedesmus subspicatus, Selenastrum capricornutum, and Nannochloropsis sp., were used for bioaccumulating barium from water. Batch studies indicated that all three species exposed to 5 mg/L barium concentrations can bioaccumulate up to 88 to 99% of barium within 10 days, depending on the species. The average algal growth rates of treated cultures were not significantly different (P > 0.05) from the controls. Furthermore, the growth curves of Scenedesmus seem to indicate a preference for moderate levels of barium (up to 5 mg/L). The metal-laden microalgae can be harvested with an efficiency of nearly 99% by coagulation with 200 mg/L ferric chloride. The discharge water had less than 0.64 mg/L of iron for 200 mg/L ferric chloride dosage; thereby endorsing the applicability of the overall microalgal phytoremediation process. Unlike land-plant-based phytoremediation, which often has a duration of treatment spanning a few decades, the entire microalgal phytoremediation process is anticipated to last from 8 to 14 days under favorable microalgal growth conditions. Heavily contaminated waters and sediments may require additional recycling and reculturing of microalgae until the desired residual contaminant levels are achieved. With appropriate modifications, this technique may be extended to sites with other toxic metals, such as arsenic, lead, cadmium, nickel, chromium, and zinc, and possibly to sites with mixed radioactive and toxic metal wastes.

Groundwater circulation well (GCW) techniques are an innovative remedial technology that may effectively remediate contaminated groundwater in areas unfavorable to more traditional technologies. Traditional pump-and-treat systems may be problematic where concentrated hot spots of contamination are present, aquifer drawdown due to remedial activities is unacceptable, surface discharge of treated groundwater is unwanted, or property acquisition is challenging. The vertical component of groundwater flow induced by GCW systems presents a new challenge to practitioners evaluating their potential performance. Modifications were made to two familiar pump-and-treat analysis tools—capture zone type curves and MODFLOW numerical groundwater flow modeling—to predict site-specific effectiveness of GCW technologies. These techniques were used to predict the GCW treatment area dimensions, which included the capture zone, recharge zone, and circulation cell diameter. Additionally, GCW design parameters including GCW spacing, screen length, screen placement, and circulation flow rate were determined. The state of the practice is to use these predictive tools to design an instrumentation scheme that can effectively evaluate the performance of GCW systems during full-scale pilot testing.

Gaseous emissions of volatile organic compounds (VOCs) and semi-VOCs (SVOCs) from slurry and solid phase bioremediation processes were evaluated. The specific processes considered were those used for the bioremediation of concentrated industrial wastes and of contaminated soils and sediments. The purpose of the evaluation was to document the extent to which such processes emit VOCs and SVOCs. Such information is needed to answer questions such as (1) what are the VOC and SVOC emissions that occur during the operation of these processes?; (2) should emission controls be considered for such processes?; and (3) to what extent does volatilization contribute to the loss of organic compounds in what is considered a bioremediation process? These questions are relevant to understanding the performance of aerobic processes treating concentrated hydrocarbons. The results of this evaluation indicated that, although volatile losses of VOCs and SVOCs can occur from slurry and solid phase bioremediation units, in such units, biodegradation is the primary method of SVOC loss. In well-acclimated bioremediation units, biodegradation of xylene/somers also can be a primary chemical loss mechanism. In situations where mass balances of SVOCs could be determined, volatile losses of SVOCs such as polycyclic aromatic hydrocarbons and polychlorinated biphenyls were <0.1% and frequently <0.01% of the mass of such chemicals in the bioremediation units. Greater volatile losses occurred in the initial phase of units operated as batch bioreactors. This is likely due to higher concentration driving forces from the liquid phase to the air phase coupled with biomass that is not fully acclimated to degrading the added VOCs and SVOCs. To minimize volatilization losses from bioreactors, the bioreactors should be designed and operated as well-acclimated continuously fed units rather than as intermittent batch units. There are many fundamental and operational factors that can affect gaseous emissions from bioremediation units. The more important factors are the partitioning of organic chemicals to solids in the unit, rate of microbial activity, volatility of the chemicals, and manner in which the unit is operated.

Concern over the potentially negative impacts of climate change has resulted in a search for techniques to reduce anthropogenic emissions of methane, a primary greenhouse gas. Microbial oxidation of methane in soils may serve as an inexpensive technique for reducing CH₄ emissions from sources such as landfills and heavy oil wells. To gain a better quantitative understanding of the biological and physical processes limiting CH₄ oxidation in soils and biofilters, a numerical reactive-transport model was developed. The model inputs include CH₄ source strength, soil bulk density, moisture content, and biological kinetic parameters. The outputs consist of gas concentration profiles, CH₄ oxidation rates, and surface flux rates. A series of soil column and batch incubation experiments were performed on a variety of soil types to calibrate and verify the model. The model was also verified by reproducing experimental results found in the literature.