Experimental techniques are described for conducting greenhouse tests to examine the pathways for phytoremediation of volatile organic compounds (VOCs), distinguishing between uptake of the VOC along with water transpired by living plants and the passive, physical process of volatilization. The experimental techniques were applied to assess the potential for phytoremediation of methyl-tertiary-butyl-ether (MTBE). Experimental data indicate that MTBE was readily taken up from water by hybrid poplar saplings, yielding 25% reduction in aqueous MTBE concentration and 30% reduction in MTBE mass over a 1-week period. These reductions in plant systems were significantly greater than in controls, indicating great potential for MTBE phytoremediation in the field. Engineering parameters for phytoremediation were determined from the experiments, yielding an MTBE transpiration stream concentration factor of 1 and a root concentration factor of 0.7–1.4. Mass balance studies showed good closure on MTBE mass balance, indicating no significant degradation of MTBE in the young poplar saplings used in this study. These results suggest that phytovolatilization may be the primary pathway for MTBE phytoremediation.
Subsurface spills of high-molecular weight, multicomponent, dense nonaqueous-phase liquids (DNAPLs) are intractable for remediation by conventional techniques. This paper introduces the concept of biostabilization of the DNAPL source region as a means of achieving risk reduction at DNAPL-contaminated sites. Successful biostabilization depends upon the interplay among dissolution, degradability, and toxicity of various DNAPL constituents, difficult to predict a priori for the mixture. Bench-scale screening tests are proposed for identifying those DNAPLs that are amenable to biostabilization. The screening protocols compare four criteria: (1) microbial activity; (2) composition of the DNAPL residue; (3) aqueous phase contaminant concentrations; and (4) aggregate aqueous phase toxicity—across unbiotreated controls and in mixed versus unmixed biometers. The unmixed system represents slow dissolution from DNAPL pools in the quiescent subsurface. The protocols are developed and evaluated with DNAPL coal tar in the first paper of this set (Part I). Unmixed coal tar biometers, characterized by slow mass transfer and low-level microbial activity, exhibited reduced, aqueous-phase contaminant concentrations and aggregate toxicity, as well as stable DNAPL composition, consistently indicating favorable potential for in situ biostabilization.
Aroclors are dense nonaqueous-phase liquids (DNAPLs) composed of polychlorinated biphenyls, which are common subsurface contaminants. Because complete remediation of Aroclor is very difficult, biostabilization may offer an alternative where risk reduction can be achieved without destruction of the DNAPL mass. The potential for aerobic in situ biostabilization of Aroclor 1242 was evaluated using laboratory protocols similar to those described in the companion paper. Total microbial concentrations increased and stabilized in both mixed and unmixed systems, while the respiring cells did not stabilize in either system. After 100 days, the DNAPL in mixed biometers was depleted in dichlorobiphenyls; the DNAPL composition in unmixed biometers did not change significantly. The total aqueous polychlorinated biphenyl concentration was lower in the unmixed than mixed biometers; both were below the predicted equilibrium concentration. After 100 days, the chronic toxicity of the aqueous phase to Cerodaphnia was greater in the biotreated systems than in the unbiotreated systems. The results indicate that aerobic microbiological activity may be insufficient to fully stabilize Aroclor in the subsurface, in contrast to the clear biostabilization potential of coal tar.
Limited information is available on screening and selection of terrestrial plants for uptake and translocation of uranium from soil. This article evaluates the removal of uranium from water and soil by selected plants, comparing plant performance in hydroponic systems with that in two soil systems (a sandy-loam soil and an organic-rich soil). Plants selected for this study were Sunflower (Helianthus giganteus), Spring Vetch (Vicia sativa), Hairy Vetch (Vicia villosa), Juniper (Juniperus monosperma), Indian Mustard (Brassica juncea), and Bush Bean (Phaseolus nanus).
Plant performance was evaluated both in terms of the percent uranium extracted from the three systems, as well as the biological absorption coefficient (BAC) that normalized uranium uptake to plant biomass. Study results indicate that uranium extraction efficiency decreased sharply across hydroponic, sandy and organic soil systems, indicating that soil organic matter sequestered uranium, rendering it largely unavailable for plant uptake. These results indicate that site-specific soils must be used to screen plants for uranium extraction capability; plant behavior in hydroponic systems does not correlate well with that in soil systems. One plant species, Juniper, exhibited consistent uranium extraction efficiencies and BACs in both sandy and organic soils, suggesting unique uranium extraction capabilities.
This paper develops batch-mixed treatment with zero-valent iron as a point-of-use technology, appropriate for arsenic removal from water stored within rural homes in Bangladesh and West Bengal, India, where arsenic poisoning has affected an estimated 20 million people. Batch tests with iron yielded the following results: (1) High arsenic removal (>93%) was achieved from highly arsenated waters (2000 μg/L) over short contact times (0.5–3 h) with iron filings added at doses ranging from 2500 to 625 mg/L; (2) Most rapid arsenic removal was observed in head-space free systems with sulphates present in solution, while phosphate buffers were observed to inhibit arsenic removal by iron; (3) The arsenic removed from water was found to be strongly bound to the elemental iron filings, such that the treated water could be decanted and iron could be reused at least 100 times; (4) Some iron dissolved into water over the contact period, at concentrations ranging from 100 to 300 μg/L, which are within safe drinking water limits. These results indicate that, with appropriate assessment of water chemistry in the affected region, treatment with metallic iron followed by simple decantation can be used as a practical, in-home, point-of-use technique for reducing human exposure to arsenic in drinking water.
A modeling framework is developed that addresses mass transfer, bioavailability, and potential biotreatment rates that may be achieved under stable microbial conditions in slurry systems containing multi-component non-aqueous-phase liquids (NAPLs). The framework is applied to describe biotreatment of polynuclear aromatic hydrocarbons (PAHs) released from coal tar NAPL in solid−slurry and liquid−liquid dispersion systems. A multi-step mass transport−degradation model considers equilibrium partition ing of PAH compound at the NAPL−water interface, followed by three sequential kinetic processes occurring in the aqueous phase: micropore sorption−diffusion, bulk aqueous-phase transport, and first-order biodegradation of bulk-phase substrate. Dynamic changes in NAPL−water equilibria due to depletion of PAH compound from the NAPL are incorporated into the model. Model results indicate that the overall rate of biotransformation of organic compounds from NAPLs is controlled by NAPL−water equilibrium processes represented by a dimensionless solubility factor, as well as the slowest of three aqueous-phase kinetic processes determined by pair-wise analysis of the dimensionless Biot number, the Thiele modulus, and the Damkohler number. Analytical equations and computer simulations demonstrate the utility of the dimensionless parameters in quantifying bioavailability, identifying dominant rate-limiting processes, and developing simpler models for biotransformation in NAPL−slurry systems. Some aspects of the modeling framework are evaluated in a companion paper using data from controlled laboratory experiments.
This paper develops experimental protocols for evaluating the impact of physicochemical mass transfer phenomena on bioavailability and biotreatment rates in slurry systems containing multi-component non-aqueous-phase liquids (NAPLs). The experiments are conducted with two coal tar NAPL samples obtained from field sites. Experimental evalu ations consist of abiotic mass transfer tests and independent biomineralization studies. The mass transfer tests measure equilibrium partitioning, dynamic changes in equilibrium partitioning, and dissolution kinetics for two polynuclear aromatic hydrocarbon (PAH) compounds, naphthalene and phenanthrene, released from coal tar NAPL in solid−slurry and liquid−liquid dispersion systems. Companion bimineralization tests assess initial rates of mineralization of naphthalene from coal tar NAPL. The results are used to evaluate the performance of a dissolution−degradation model, developed in the preceding paper, that addresses mass transfer and initial biodegrada tion rates of PAH compounds in multi-component coal tar (NAPL)−slurry systems. It is shown that independent equilibrium and kinetic dissolution tests aid in quantifying PAH bioavailability and potential initial biotransformation rates.
This research presents the first demonstration of substantial microbial degradation and depletion of naphthalene from coal tar, a multicomponent, aromatic, dense nonaqueous phase liquid (NAPL). The rates and extents of microbial degradation of naphthalene from coal tar and from a two-component NAPL simpler in composition than coal tar were evaluated in gently mixed, NAPL−water batch systems. The rate of degradation of naphthalene, the principal constituent in coal tar, was found to be significantly influenced by the rate of external surface mass transfer from the coal tar. Results show that the rate of mass transfer may control the overall rate of biotransformation in mixed systems where coal tar is present as a globule (≃11 mm diameter). Mass transfer is relatively rapid and does not limit biodegradation in slurry systems when coal tar is distributed among a large number of small microporous silica particles (≃250 μm diameter). These results were obtained for conditions favorable for biodegradation and provide an indication of the maximum potential rates for microbial degradation of naphthalene from coal tar. The microbial degradation process is dependent on relationships between the NAPL composition and the equilibrium aqueous naphthalene concentration, the naphthalene mass transfer rate between the NAPL and the aqueous phases, and the intrinsic rates of microbial degradation of naphthalene. These relationships have been incorporated in a dissolution−degradation framework, and the rate-limiting phenomena for the biodegradation process was evaluated using this framework.
A conceptual stochastic model is developed that describes the natural spatial variability of nonreactive solute concentrations in large groundwater systems. The spatial variation in aqueous concentration is associated with dissolution from source areas of high mineral enrichment in the aquifer matrix. The stochastic model considers randomly varying inputs of a solute species from source deposits that occur as a two‐dimensional spatial Poisson process. A steady state advective‐dispersive transport equation is utilized to predict the downgradient movement of the solute from the source areas. The total groundwater concentration at any location is calculated from the superposition of the individual contributions from each source area in the aquifer. A spatially varying concentration field results, described mathematically by a filtered Poisson process model. The theoretical concentration field is nonstationary, with the mean and variance increasing, and the coefficient of variation decreasing, in the direction of groundwater flow. Gaussian fields for abundant elements and highly skewed probability distributions for trace elements are indicated by the filtered Poisson process model. Evaluation of elemental concentration data from the Sherwood aquifer in England demonstrates how field data may be analyzed in the context of the stochastic model.
This study examines the role of physico-chemical mass transfer processes on the rate of biotransformation of polycyclic aromatic hydrocarbon (PAH) compounds released from non-aqueous phase liquid (NAPL) coal tar present at residual saturation within a microporous medium. A simplified coupled dissolution-degradation model is developed that describes the concurrent mass transfer and biokinetic processes occurring in the system. Model results indicate that a dimensionless Damkohler number can be utilized to distinguish between systems that are mass transfer limited, and those that are limited by biological phenomena. The Damkohler number is estimated from independent laboratory experiments that measure the rates of aqueous phase dissolution and biodegradation of naphthalene from coal tar. Experimental data for Stroudsburg coal tar imbibed within 236 μm diameter silica particles yield Damkohler numbers smaller than unity, indicating, for the particular system under study, that the overall rate of biotransformation of naphthalene is not limited by the mass transfer of naphthalene from coal tar to the bulk aqueous phase. There is a need for investigation of mass transfer for larger particles and/or other PAH compounds, and study of microbial rate-limiting phenomena including toxicity, inhibition and competitive substrate utilization.