Influence of hydrological and geochemical processes on the transport of chelated metals and chromate in fractured shale bedrock

Field-scale processes governing the transport of chelated radionuclides in groundwater remain conceptually unclear for highly structured, heterogeneous environments. The objectives of this research were to provide an improved understanding and predictive capability of the hydrological and geochemical mechanisms that control the transport behavior of chelated radionuclides and metals in anoxic subsurface environments that are complicated by fracture flow and matrix diffusion. Our approach involved a long-term, steady-state natural gradient field experiment where nonreactive Br- and reactive 57Co(II)EDTA2- 109CdEDTA2-, and 51Cr(VI) were injected into a fracture zone of a contaminated fractured shale bedrock. The spatial and temporal distribution of the tracer and solutes was monitored for 500 days using an array of groundwater sampling wells instrumented within the fast-flowing fracture regime and a slower flowing matrix regime. The tracers were preferentially transported along strike-parallel fractures coupled with the slow diffusion of significant tracer mass into the bedrock matrix. The chelated radionuclides and metals were significantly retarded by the solid phase with the mechanisms of retardation largely due to redox reactions and sorption coupled with mineral-induced chelate-radionuclide dissociation. The formation of significant Fe(III)EDTA byproduct that accompanied the dissociation of the radionuclide-chelate complexes was believed to be the result of surface interactions with biotite which was the only Fe(III)-bearing mineral phase present in these Fe-reducing environments. These results counter current conceptual models that suggest chelated contaminants move conservatively through Fe-reducing environments since they are devoid of Fe-oxyhydroxides that are known to aggressively compete for chelates in oxic regimes. Modeling results further demonstrated that chelate-radionuclide dissociation reactions were most prevalent along fractures where accelerated weathering processes are expected to expose more primary minerals than the surrounding rock matrix. The findings of this study suggest that physical retardation mechanisms (i.e. diffusion) are dominant within the matrix regime, whereas geochemical retardation mechanisms are dominant within the fracture regime.     

Multispecies transport of metal–EDTA complexes and chromate through undisturbed columns of weathered fractured saprolite

Laboratory-scale tracer experiments were conducted to investigate the geochemical and hydrological processes that govern the fate and transport of organically chelated radionuclides and toxic metals in undisturbed saturated columns of weathered, fractured shale saprolite. Three long-term, reactive contaminant injections were pulsed onto three separate soil columns, with the following influent mixtures: (1) ¹⁰⁹CdEDTA²⁻, (2) ¹⁰⁹CdEDTA²⁻ and ^57,58Co(II)EDTA²⁻, and (3) ¹⁰⁹CdEDTA²⁻, ⁵⁷Co(III)EDTA⁻, and H⁵¹CrO₄⁻. Both single and multiple species experiments were conducted to determine the importance of interaction between the contaminants and competition for surface sites. Flow interruption was used to identify physical and chemical non-equilibrium (PNE and CNE) which were caused by multiple pore-region flow and rate-limited chemical reactions, respectively. Reactive contaminant transport through the fractured, weathered shale was affected by sorption, redox, and dissociation reactions, which were mediated by soil organic matter and surficial oxides of Fe, Mn, and Al. The transport of CdEDTA²⁻ was significantly influenced by ligand-promoted dissolution of subsurface Fe and Al sources, resulting in the liberation of Cd²⁺, Al(III)EDTA⁻ and Fe(III)EDTA⁻. Flow interruption confirmed that the surface-mediated dissociation reaction was time-dependent, with the stability of the CdEDTA²⁻ complex dependent on its residence time within the soil. The migration of Co(II)EDTA²⁻ was dominated by oxidization to the highly stable Co(III)EDTA⁻ species, and elevated effluent Mn²⁺ suggested that surficial Mn(IV) oxides likely catalyzed the redox reaction, though Fe-oxides may have also contributed to the reaction. Dissociation (12%) of the Co(II)EDTA²⁻ complex was first observed during flow interruption, indicating that rate-limited dissociation of the complex by Fe-oxides may be significant under equilibrium conditions. The transport of HCrO₄⁻ was significantly altered by the reduction of mobile Cr(VI) to irreversibly bound Cr(III). The reduction reaction was catalyzed by surface-bound natural organic matter and flow interruption confirmed that the reaction was time-dependent. There was little evidence of competitive effects between the various contaminants in the multispecies experiments, since each was influenced by a different geochemical process during transport through the soil. The results of this study further support research findings that suggest anionic toxic metals and radionuclide–organic complexes can be significantly influenced by soil geochemical processes that can both enhance and impede the subsurface migration of these contaminants.     

pH-Dependent fate and transport of NTA-complexed cobalt through undisturbed cores of fractured shale saprolite

The codisposal of toxic metals and radionuclides with organic chelating agents has been implicated in the facilitated transport of the inorganic contaminants away from primary waste disposal areas. We investigated the transport of Co(II)NTA through undisturbed cores of fractured shale saprolite. Experiments were conducted across the pH range 4 to 8 by collecting cores from different locations within the weathering profile. Aqueous complexation, adsorption, dissociation and oxidation reactions influenced Co(II)NTA transport. The suite of reaction products identified in column effluent varied with experimental pH. At low pH and in the presence of abundant exchangeable aluminum, Co transport occurred predominantly as the Co2+ ion. At higher pH, Co was transported primarily as Co(II)NTA and the Co(III) species Co(III)(HNTA)2 and Co(III)(IDA)2. The formation of the geochemical oxidation products (Co(III) species) has far reaching implications as these compounds are kinetically and thermodynamically stable, are transported more rapidly than Co(II)NTA, and are resistant to biodegradation. These results demonstrate that natural minerals, in the physical structure encountered naturally, can be more important in the formation of mobile, stable contaminant forms than they can be for the retardation and dissociation of the contaminants.     

Acceleration of the Fe(III)EDTA(-) reduction rate in BioDeNO(x) reactors by dosing electron mediating compounds

BioDeNO(x), a novel technique to remove NO(x) from industrial flue gases, is based on absorption of gaseous nitric oxide into an aqueous Fe(II)EDTA(2-) solution, followed by the biological reduction of Fe(II)EDTA(2-) complexed NO to N(2). Besides NO reduction, high rate biological Fe(III)EDTA(-) reduction is a crucial factor for a succesful application of the BioDeNO(x) technology, as it determines the Fe(II)EDTA(2-) concentration in the scrubber liquor and thus the efficiency of NO removal from the gas phase. This paper investigates the mechanism and kinetics of biological Fe(III)EDTA(-) reduction by unadapted anaerobic methanogenic sludge and BioDeNO(x) reactor mixed liquor. The influence of different electron donors, electron mediating compounds and CaSO(3) on the Fe(III)EDTA(-) reduction rate was determined in batch experiments (21mM Fe(III)EDTA(-), 55 degrees C, pH 7.2+/-0.2). The Fe(III)EDTA(-) reduction rate depended on the type of electron donor, the highest rate (13.9mMh(-1)) was observed with glucose, followed by ethanol, acetate and hydrogen. Fe(III)EDTA(-) reduction occurred at a relatively slow (4.1mMh(-1)) rate with methanol as the electron donor. Small amounts (0.5mM) of sulfide, cysteine or elemental sulfur accelerated the Fe(III)EDTA(-) reduction. The amount of iron reduced significantly exceeded the amount that can be formed by the chemical reaction of sulfide with Fe(III)EDTA(-), suggesting that the Fe(III)EDTA(-) reduction was accelerated via an auto-catalytic process with an unidentified electron mediating compound, presumably polysulfides, formed out of the sulfur additives. Using ethanol as electron donor, the specific Fe(III)EDTA(-) reduction rate was linearly related to the amount of sulfide supplied. CaSO(3) (0.5-100mM) inhibited Fe(III)EDTA(-) reduction, probably because SO(3)(2-) scavenged the electron mediating compound.     

Experimental Zn(II) retention in a sandy loam soil by very small columns

The use of column experiments, usually performed to better approximate field conditions, may provide information that is not available from batch experiments. In such experiments heavy metals are often adsorbed until saturation followed by desorption experiments. When the affinity of the metal to soil is high, the retention factor (R) could be greater than thousands and the duration of experiments can become impractically long. In order to use reasonable laboratory time, the flow rate should be increased or the column size decreased. The increase in flow rate produces undesirable kinetic and dispersion effects, so we used very small soil columns (pore volume = 0.31–0.70 ml) and relatively high flow rates (0.03–0.12 ml min⁻¹) in studies of Zn(II) adsorption and retention in soils. Conservative tracer flow column experiments under saturation conditions were carried out to determine flow parameters for different flow rates. Column pore volume (V_p), Peclet numbers (Pe) and longitudinal dispersion coefficients (D_L) were determined from breakthrough curves. The effect of type of electrolyte and ionic strength on the Zn(II) retention onto soil was determined. The influence of flow rate and bed height on the retention coefficient and on the mass transfer zone was also studied. The effect of different influent Zn(II) concentrations on the R values obtained was analyzed. Freundlich parameters from column experiments were compared with batch ones. The leaching efficiency of different electrolytes, salts of weak organic acids and EDTA was also studied.     

Transport of simazine in unsaturated sandy soil and predictions of its leaching under hypothetical field conditions

The potential contamination of groundwater by herbicides is often controlled by processes in the vadose zone, through which herbicides travel before entering groundwater. In the vadose zone, both physical and chemical processes affect the fate and transport of herbicides, therefore it is important to represent these processes by mathematical models to predict contaminant movement. To simulate the movement of simazine, a herbicide commonly used in Chilean vineyards, batch and miscible displacement column experiments were performed on a disturbed sandy soil to quantify the primary parameters and processes of simazine transport. Chloride (Cl(-)) was used as a non-reactive tracer, and simazine as the reactive tracer. The Hydrus-1D model was used to estimate the parameters by inversion from the breakthrough curves of the columns and to evaluate the potential groundwater contamination in a sandy soil from the Casablanca Valley, Chile. The two-site, chemical non-equilibrium model was observed to best represent the experimental results of the miscible displacement experiments in laboratory soil columns. Predictions of transport under hypothetical field conditions using the same soil from the column experiments were made for 40 years by applying herbicide during the first 20 years, and then halting the application and considering different rates of groundwater recharge. For recharge rates smaller than 84 mm year(-1), the predicted concentration of simazine at a depth of 1 m is below the U.S. EPA's maximum contaminant levels (4 microg L(-1)). After eight years of application at a groundwater recharge rate of 180 mm year(-1) (approximately 50% of the annual rainfall), simazine was found to reach the groundwater (located at 1 m depth) at a higher concentration (more than 40 microg L(-1)) than the existing guidelines in the USA and Europe.     

A comparative study of the effects of chloride, sulfate and nitrate ions on the rates of decomposition of H2O2 and organic compounds by Fe(II)/H2O2 and Fe(III)/H2O2

The effects of chloride, nitrate, perchlorate and sulfate ions on the rates of the decomposition of hydrogen peroxide and the oxidation of organic compounds by the Fenton's process have been investigated. Experiments were conducted in a batch reactor, in the dark at pH < or = 3.0 and at 25 degrees C. Data obtained from Fe(II)/H2O2 experiments with [Fe(II)]0/[H2O2]0 > or = 2 mol mol(-1), showed that the rates of reaction between Fe(II) and H2O2 followed the order SO4(2-) > ClO4(-) = NO3- = Cl-. For the Fe(III)/H2O2 process, identical rates were obtained in the presence of nitrate and perchlorate, whereas the presence of sulfate or chloride markedly decreased the rates of decomposition of H2O2 by Fe(III) and the rates of oxidation of atrazine ([atrazine]0 = 0.83 microM), 4-nitrophenol ([4-NP]0 = 1 mM) and acetic acid ([acetic acid]0 = 2 mM). These inhibitory effects have been attributed to a decrease of the rate of generation of hydroxyl radicals resulting from the formation of Fe(III) complexes and the formation of less reactive (SO4(*-)) or much less reactive (Cl2(*-)) inorganic radicals.     

Interfacial transfer velocities of ozone dry deposition over agricultural soils: experimental and theoretical analysis

A mathematical dry deposition model was developed and an experiment performed to verify that the interfacial transfer velocity (V(S)) of ozone dry deposition includes the interfacial reactive velocity (V(Sr)) and interfacial kinetic velocity (V(Sk)), as determined by measuring the ozone depletion over agricultural field soils in a dry deposition chamber. Experimental results indicate that the chemical reaction (O3 + NO --> NO2 + O2) produces the reactive velocity. Observed interfacial transfer velocities depend on nitrogen oxide emission (NO) and soil temperature (T(S)). Additionally, observed kinetic velocities of conditioned field soils increased linearly with soil temperature. Moreover, observed reactive velocities of field soils increased exponentially with soil temperature, and depend on the emission rate of nitrogen oxide. Results in this study demonstrate that interfacial transfer velocities are variable velocities for long-term transportation, that influenced factors are chemical kinetics, thermodynamics and biochemical mechanisms.     

A parametric transfer function methodology for analyzing reactive transport in nonuniform flow

We analyze reactive transport during in-situ bioremediation in a nonuniform flow field, involving multiple extraction and injection wells, by the method of transfer functions. Gamma distributions are used as parametric models of the transfer functions. Apparent parameters of classical transport models may be estimated from those of the gamma distributions by matching temporal moments. We demonstrate the method by application to measured data taken at a field experiment on bioremediation conducted in a multiple-well system in Oak Ridge, TN. Breakthrough curves (BTCs) of a conservative tracer (bromide) and a reactive compound (ethanol) are measured at multi-level sampling (MLS) wells and in extraction wells. The BTCs of both compounds are jointly analyzed to estimate the first-order degradation rate of ethanol. To quantify the tracer loss, we compare the approaches of using a scaling factor and a first-order decay term. Results show that by including a scaling factor both gamma distributions and inverse-Gaussian distributions (transfer functions according to the advection-dispersion equation) are suitable to approximate the transfer functions and estimate the reactive rate coefficients for both MLS and extraction wells. However, using a first-order decay term for tracer loss fails to describe the BTCs at the extraction well, which is affected by the nonuniform distribution of travel paths.     

Experimental determination of sorption in fractured flow systems

Fracture "skins" are alteration zones on fracture surfaces created by a variety of biological, chemical, and physical processes. Skins increase surface area, where sorption occurs, compared to the unaltered rock matrix. This study examines the sorption of organic solutes on altered fracture surfaces in an experimental fracture-flow apparatus. Fracture skins containing abundant metal oxides, clays, and organic material from the Breathitt Formation (Kentucky, USA) were collected in a manner such that skin surface integrity was maintained. The samples were reassembled in the lab in a flow-through apparatus that simulated approximately 2.7 m of a linear fracture "conduit." A dual-tracer injection scheme was utilized with the sorbing or reactive tracer compared to a non-reactive tracer (chloride) injected simultaneously. Sorption was assessed from the ratio of the first temporal moments of the breakthrough curves and from the loss of reactive tracer mass and evaluated as a function of flow velocity and solute type. The breakthrough curves suggest dual-flow regimes in the fracture with both sorbing and non-sorbing flow fields. Significant sorption occurs for the reactive components, and sorption increased with decreasing flow rate and decreasing compound solubility. Based on moment analysis, however, there was little retardation of the center of solute mass. These data suggest that non-equilibrium sorption processes dominate and that slow desorption and boundary layer diffusion cause extensive tailing in the breakthrough curves.     

Mechanisms of electron acceptor utilization: implications for simulating anaerobic biodegradation

Simulation of biodegradation reactions within a reactive transport framework requires information on mechanisms of terminal electron acceptor processes (TEAPs). In initial modeling efforts, TEAPs were approximated as occurring sequentially, with the highest energy-yielding electron acceptors (e.g. oxygen) consumed before those that yield less energy (e.g., sulfate). Within this framework in a steady state plume, sequential electron acceptor utilization would theoretically produce methane at an organic-rich source and Fe(II) further downgradient, resulting in a limited zone of Fe(II) and methane overlap. However, contaminant plumes often display much more extensive zones of overlapping Fe(II) and methane. The extensive overlap could be caused by several abiotic and biotic processes including vertical mixing of byproducts in long-screened monitoring wells, adsorption of Fe(II) onto aquifer solids, or microscale heterogeneity in Fe(III) concentrations. Alternatively, the overlap could be due to simultaneous utilization of terminal electron acceptors. Because biodegradation rates are controlled by TEAPs, evaluating the mechanisms of electron acceptor utilization is critical for improving prediction of contaminant mass losses due to biodegradation. Using BioRedox-MT3DMS, a three-dimensional, multi-species reactive transport code, we simulated the current configurations of a BTEX plume and TEAP zones at a petroleum-contaminated field site in Wisconsin. Simulation results suggest that BTEX mass loss due to biodegradation is greatest under oxygen-reducing conditions, with smaller but similar contributions to mass loss from biodegradation under Fe(III)-reducing, sulfate-reducing, and methanogenic conditions. Results of sensitivity calculations document that BTEX losses due to biodegradation are most sensitive to the age of the plume, while the shape of the BTEX plume is most sensitive to effective porosity and rate constants for biodegradation under Fe(III)-reducing and methanogenic conditions. Using this transport model, we had limited success in simulating overlap of redox products using reasonable ranges of parameters within a strictly sequential electron acceptor utilization framework. Simulation results indicate that overlap of redox products cannot be accurately simulated using the constructed model, suggesting either that Fe(III) reduction and methanogenesis are occurring simultaneously in the source area, or that heterogeneities in Fe(III) concentration and/or mineral type cause the observed overlap. Additional field, experimental, and modeling studies will be needed to address these questions.     

The effect of EDTA on Helianthus annuus uptake, selectivity, and translocation of heavy metals when grown in Ohio, New Mexico and Colombia soils

The use of two EDTA concentrations for enhancing the bioavailability of cadmium, chromium, and nickel in three natural soils (Ohio, New Mexico and Colombia) was investigated. The resulting uptake, translocation and selectivity with Helianthus annuus after mobilization were also examined. In general, plants grown in the sandy-loam Ohio soil had a higher uptake that resulted in a selectivity and total metal content of Cd>Cr>Ni and 0.73 mg and Cr>Cd>Ni and 0.32 mg for 0.1 and 0.3 g kg-1 EDTA, respectively. With the silty-loam New Mexico soil, although the total metal uptake was not statistically different the EDTA level did alter the selectivity; Cd>Cr>Ni (0.1 g kg-1 EDTA) and Cd>Cr>Ni (0.3 g kg-1 EDTA). Conversely, with the Colombian (sandy clay loam) soil increasing the EDTA level resulted in a higher total metal uptake (0.62 mg) than the 0.1 g kg-1 (0.59 mg) treatment. For all three soils, the translocation of Cd was limited. Evaluating the mobile metal fraction with and without EDTA determined that the chelator was capable of overcoming mass transfer limitations associated with the expandable clay fraction in the soils. Root wash results and root biomass concentrations indicated that Cd sorption was occurring. Therefore limited Cd translocation was attributed to insufficient phytochelatin levels.     

Simultaneous oxidation of EDTA and reduction of metal ions in mixed Cu(II)/Fe(III)-EDTA system by TiO2 photocatalysis

Both the photooxidation of EDTA and the photoreduction of metal ions in metal-EDTA systems were investigated. EDTA oxidation by TiO(2) photocatalysis occurred sequentially as Cu(II)-EDTA>Cu(II)/Fe(III)-EDTA>Fe(III)-EDTA. For Cu(II)-EDTA, EDTA was completely decomposed after only 60min of irradiation. The rate of EDTA decomposition was directly correlated with the initial Cu(II) concentration in the case of a mixed Cu(II)/Fe(III)-EDTA system. The metal ions in a single metal-EDTA complex were removed following significant decomposition of EDTA. For a mixed Cu(II)/Fe(III)-EDTA system, however, no copper was removed whereas almost all of the iron was removed. The iron and copper species deposited on the TiO(2) surface were identified via EPR and XPS as mixed FeO/Fe(3)O(4) and Cu(0)/Cu(2)O, respectively.     

Process evaluation for optimization of EDTA use and recovery for heavy metal removal from a contaminated soil

This study aimed to establish an optimized, closed loop application of ethylenediaminetetraacetic acid (EDTA) in heavy metal removals from a contaminated soil through integrating EDTA recovery/regeneration and metal precipitation processes in the treatment train. Three divalent heavy metals were investigated, namely, Pb, Cd, and Ni. The extractability of the metals by EDTA followed the decreasing order of CdPb>Ni. The first part of this study was to search for the optimal use of the fresh EDTA in removing these heavy metals from the contaminated soil. The second part of this study was devoted to the recovery/regeneration of the spent EDTA which followed the sequential processes involving (1) complex destabilization by adding ferric ion (Fe(III)) to liberate Pb, Cd, and Ni, (2) precipitation of the liberated Pb, Cd, and Ni in phosphate (PO4(3-)) forms, and (3) precipitation of the excess Fe(III) which eventually produced free EDTA for reuse. The process variables were dosages of Fe(III) and PO4(3-), pH and reaction times. Laborious trial experiments would be needed in searching for the optimum conditions for the above processes. To expedite this exercise, a geochemical equilibrium model, MINTEQA2, was used to find the thermodynamically favorable conditions for recoveries of both EDTA and heavy metals. This was then followed by experimental examination of the process kinetics to observe for the optimal reaction time for each thermodynamically favorable process. This study revealed that 2 h of reaction time each for the complex destabilization reaction and the metal phosphate precipitation reaction was sufficient to achieve equilibrium. With the optimized process condition identified in this study, a total of 95%, 89% and 90% of the extracted Pb, Cd and Ni, respectively, could be precipitated from the spent EDTA solution, with 84% EDTA recovery. The reused EDTA maintained more than 90% of its preceding extraction power in each cycle of reuse.     

Single-well reactive tracer test and stable isotope analysis for determination of microbial activity in a fast hydrocarbon-contaminated aquifer

Single-well reactive tracer tests, such as the push-pull test are useful tools for characterising in-situ bioattenuation processes in contaminated aquifers. However, the analytical models that are used to interpret push-pull data may be over-simplified, and potentially overlook important processes responsible for the frequent discrepancy between predicted and observed results obtained from push-pull tests. In this study, the limitations underlying the push-pull test methodology were investigated and were supported with results from a push-pull test conducted in a sulphate-reducing aquifer contaminated by crude oil. Poor (<7%) mass recovery was achieved when the push-pull test was performed in a fast-flowing aquifer, preventing a quantifiable reaction rate to be determined. Breakthrough curve data were unexplainable using simplified analytical models, but exhibited trends analogous with tests conducted by others, when >20% mass recoveries were achieved. Push-pull test data collected from sulphate-reducing aquifers indicate that the assumption of a well-mixed batch reactor system is incorrect and that reaction rates obtained from push-pull tests in such systems may be affected by the extraction regime implemented. Evidence of microbial respiration of the reactive tracer was provided by stable sulphur isotope analysis, from which an isotope fractionation factor of +9.9 +/- 8.1 per thousand was estimated. The stable isotope data support the argument that reaction rates calculated using push-pull tests are not uniformly distributed in space and time and are likely to be influenced by heterogeneities in the flow field.     

Application of microbial hot spots enhances pesticide degradation in soils

Through transfer of an active, isoproturon degrading microbial community, pesticide mineralization could be successfully enhanced in various soils under laboratory and outdoor conditions. The microbes, extracted from a soil having high native ability to mineralize this chemical, were established on expanded clay particles and distributed to various soils in the form of microbial "hot spots". Both, diffusion controlled isoproturon mass flow towards these "hot spots" (6microg d(-1)) as well as microbial ability to mineralize the herbicide (approximately 5microg d(-1)) were identified as the main processes enabling a multiple augmentation of the native isoproturon mineralization even in soils with heavy metal contamination. Soil pH-value appears to exert an important effect on the sustainability of this process.     

An evaluation of the green chelant EDDS to enhance the stability of hydrogen peroxide in the presence of aquifer solids

Hydrogen peroxide is a widely used in situ chemical oxidation reagent which relies on catalysts to generate the suite of reactive species that are required to aggressively remediate contaminated soils and groundwater. In the subsurface environment these catalysts are usually transition metals that are added to the injected solution, or are naturally occurring. Chelating agents are widely used to maintain an adequate dissolved transition metal concentration in near-neutral pH conditions; however, they can also be used to improve the persistence of H(2)O(2) in situations when the aquifer solids have sufficient transition metal content. Ethylenediamine tetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) have been considered to be the most effective chelants and therefore are the most widely used. While previous research efforts have focused on the chelating agent efficiency, the long-term fate of these chelants in the natural subsurface environment is a concern since both EDTA and NTA are non-readily biodegradable. The focus of this investigation was to evaluate the potential of using the environmentally friendly or green chelating agent ethylenediaminedisuccinate (EDDS) as an alternative to EDTA or NTA to suppress the catalytic activity of naturally-occurring transition metals. A series of batch reactor and column experiments were performed using five different aquifer materials and the results demonstrate that EDDS has a comparative chelating efficiency to that of EDTA. The addition of EDDS was able to reduce the H(2)O(2) decomposition rates in the presence of the aquifer materials used in this investigation by 24-97% in well-mixed batch systems, and by 20% and 38% in the column trials where H(2)O(2) was detected in the effluent.     

Pre-concentration and separation of heavy metal ions by chemically modified waste paper gel

Iminodiacetic acid was immobilized on waste paper by chemical modification in order to develop a new type of adsorption gel for heavy metal ions. Adsorption behavior of the gel was investigated for a number of metal ions, specifically Cu(II), Pb(II), Fe(III), Ni(II), Cd(II), and Co(II) at acidic pH. From batch adsorption tests, the order of selectivity was found to be as follows: Cu(II)  Fe(III) > Pb(II) > Ni(II)  Co(II) > Cd(II). Column tests were carried out for pairs of metal ions to understand the separation and pre-concentration behavior of the gel. It was found that mutual separation of Ni(II) from Co(II) and that of Pb(II) from Cd(II) can be achieved at pH 3. Similarly, selective separation of Cu(II) from Cu(II)–Fe(III) and Cu(II)–Pb(II) mixtures at pH 1.5 and 2, respectively, was observed by using this new adsorption gel. In all cases, almost complete recovery of the adsorbed metal was confirmed by elution tests with HCl.     

Efficient abatement of an iodinated X-ray contrast media iohexol by Co(II) or Cu(II) activated sulfite autoxidation process

Efficient abatement of an iodinated X-ray contrast media iohexol by an emerging sulfite autoxidation advanced oxidation process is demonstrated, which is based on transition metal ion–catalyzed autoxidation of sulfite to form active oxidizing species. The efficacy of the combination of sulfite and transition metal ions (Ag(I), Mn(II), Co(II), Fe(II), Cu(II), Fe(III), or Ce(III)) was tested for iohexol abatement. Co(II) and Cu(II) are proven to show more pronounced catalytic activity than other metals at pH 8.0. According to the quenching studies, sulfate radical (SO₄^??) is identified to be the primary species for oxidation of iohexol. Increasing dosages of metal ion or sulfite and higher pH values are favorable for iohexol abatement. Inhibition of iohexol abatement is observed in the absence of dissolved oxygen, which is vital for the production of SO₅^?? and subsequent formation of SO₄^??. Overall, activation of sulfite to produce reactive radicals with extremely low Co(II) or Cu(II) concentrations (in the range of μg L^?1) in circumneutral conditions is confirmed, which offers a potential SO₄^??-based advanced oxidation process in treatment of aquatic organic contaminants.