Roles of reactive iron mineral coatings in natural attenuation in redox transition zones preserved from a site with historical contamination.

In this study, a protocol was developed to identify reduction-oxidation (redox) transition zones in an effort to exploit natural source zone depletion processes. A sediment core with a total length of 18-m was collected from a site with historical contamination that includes chlorinated benzenes where the redox condition was preserved. In the four redox transition zones investigated, reactive iron coatings are characterized with a suite of analyses under anaerobic conditions. To distinguish surface coating mineralogy, X-ray diffraction, X-ray fluorescence, and field-emission scanning electron microscopy with an energy dispersive X-ray analyzer were applied along with a six-step sequential extraction process. The cycling of Fe and S, as an important contribution and indicator of ongoing natural attenuation processes for constituents of concern (COC), was delineated by using data from multiple and complementary analyses for isolating and identifying iron phases. Along with groundwater chemistry, contaminant concentrations, and microbial genera, attenuation of COCs is expected to be active and sustainable in redox transition zones, where there is an abundance of reactive iron mineral coatings cycling through biogeochemical reactions. Reactions in other redox transition zones may be limited where iron mineral coatings are not dominant.

Impacts of cryogenic sampling processes on iron mineral coatings in contaminated sediment.

This study focused on comparing iron mineral coatings found in contaminated sediments from a cryogenic (Cryo Core) core versus an Anaerobic Core (collected under oxygen-free and ambient conditions). After thawing the Cryo Core in an oxygen-free glovebox, a suite of analyses was applied on sediments from both cores: pH, redox potential, X-ray fluorescence (XRF), X-ray diffraction (XRD), and field-emission scanning electron microscopy (FESEM) with energy dispersive X-ray analyzer (EDX). Among the iron minerals identified, crystalline pyrite was found throughout the Cyro Core sediment samples, which is in contrast to that observed for the Anaerobic Core. Moreover, mackinawite and greigite that were ubiquitous in the Anaerobic Core were not observed in Cryo Core samples. To better understand why the metastable minerals were not present, a freeze/thaw process was simulated on Anaerobic Core samples using a liquid­nitrogen quench with surface coatings characterized by FESEM/EDX. In these quenched samples, mackinawite was no longer observed, and in its place was pyrite. In addition, both greigite and pyrite were found to be unique morphologically after quenching. Dissolution and re-precipitation of iron sulfide coatings during the freeze/thaw process appears to affect the geochemistry of the pore water through two main mechanisms of freeze-concentration and freezing potential.

Application of hardwood biochar as a reactive capping mat to stabilize mercury derived from contaminated floodplain soil and riverbank sediments.

Hardwood biochar (pyrolyzed at 700 °C), a potential candidate for Hg removal, has been proposed for use as reactive capping mats along groundwater discharge zones or riverbanks to control release of Hg from contaminated riverbank sediments. Frequent flooding and drainage in fluvial settings can influence the effectiveness of remediation systems in contaminated riverbank sediments and floodplain soils. This study evaluated the effectiveness of Hg removal using hardwood biochar under hydrogeochemical conditions representative of those present within a reactive capping mat installed in a fluvial setting. Two sets of treatment columns, containing 50% v.v biochar and quartz sand, were subjected to 100 weekly wetting/drying cycles that included dry air, water-saturated air, and drainage using leachate derived from two source columns as input solutions: 1. Passing simulated acid rain water through floodplain soil, 2. Passing river water through riverbank sediment. In both treatment columns, >80% of the Hg was retained on the biochar without promoting Hg methylation and the release of other unintended dissolved constituents (including N, P, DOC). Results from solidphase extraction analyses suggest that Hg accumulated near the air/biochar-sand interface (0-2 cm) in the treatment columns at low loadings but was present at greater depths at higher loadings. Results of micro X-ray fluorescence (µ-XRF) mapping and micro X-ray absorption near edge structure (µ-XANES) for the biochar collected at depths 0-2 cm in treatment columns suggest retention of Hg-bearing particles derived from riverbank sediment and floodplain soil within the pore structure of the biochar. Sulfur K-edge XANES analysis of the unused biochar and the biochar after treatment suggest formation of Hg complexes on the biochar surface. These results indicate that hardwood biochar is potentially an effective media for application in reactive mats for controlling Hg discharging from contaminated riverbank sediments.

Medicaid personal care services for children with intellectual disabilities: what assistance is provided? When is assistance provided?

We report on the nature and timing of services provided to children with an intellectual disability (ID) identified by a new comprehensive assessment and care planning tool used to evaluate children's needs for Medicaid Personal Care Services (PCS) in Texas. The new assessment procedure resulted from a legal settlement with the advocacy community. Participants in the study were 1,109 children ages 4-20 with an intellectual disability diagnosis who were assessed between January and April of 2010. The need for assistance is higher on Saturday and Sunday, when school services are not available. We report differences in service patterns for children who vary in ID severity. Finally, we consider the implications of our results for policies and programs that serve families with children with an ID.

Personal care services provided to children with special health care needs (CSHCN) and their subsequent use of physician services.

BACKGROUND: Medicaid Personal Care Services (PCS) help families meet children's needs for assistance with functional tasks. However, PCS may have other effects on a child's well-being, but research has not yet established the existence of such effects. OBJECTIVES: To investigate the relationship between the number of PCS hours a child receives with subsequent visits to physicians for evaluation and management (E&M) services. METHODS: Assessment data for 2058 CSHCN receiving PCS were collected in 2008 and 2009. Assessment data were matched with Medicaid claims data for the period of 1 year after the assessment. Zero-inflated negative binomial and generalized linear multivariate regression models were used in the analyses. These models included patient demographics, health status, household resources, and use of other medical services. RESULTS: For every 10 additional PCS hours authorized for a child, the odds of having an E&M physician visit in the next year were reduced by 25%. However, the number of PCS hours did not have a significant effect on the number of visits by those children who did have a subsequent E&M visit. A variety of demographic and health status measures also affect physician use. CONCLUSIONS: Medicaid PCS for CSHCN may be associated with reduced physician usage because of benefits realized by continuity of care, the early identification of potential health threats, or family and patient education. PCS services may contribute to a child's well-being by providing continuous relationships with the care team that promote good chronic disease management, education, and support for the family.

Summarizing activity limitations in children with chronic illnesses living in the community: a measurement study of scales using supplemented interRAI items.

BACKGROUND: To test the validity and reliability of scales intended to measure activity limitations faced by children with chronic illnesses living in the community. The scales were based on information provided by caregivers to service program personnel almost exclusively trained as social workers. The items used to measure activity limitations were interRAI items supplemented so that they were more applicable to activity limitations in children with chronic illnesses. In addition, these analyses may shed light on the possibility of gathering functional information that can span the life course as well as spanning different care settings. METHODS: Analyses included testing the internal consistency, predictive, concurrent, discriminant and construct validity of two activity limitation scales. The scales were developed using assessment data gathered in the United States of America (USA) from over 2,700 assessments of children aged 4 to 20 receiving Medicaid Early and Periodic Screening, Diagnostic and Treatment (EPSDT) services, specifically Personal Care Services to assist children in overcoming activity limitations. The Medicaid program in the USA pays for health care services provided to children in low-income households. Data were collected in a single, large state in the southwestern USA in late 2008 and early 2009. A similar sample of children was assessed in 2010, and the analyses were replicated using this sample. RESULTS: The two scales exhibited excellent internal consistency. Evidence on the concurrent, predictive, discriminant, and construct validity of the proposed scales was strong. Quite importantly, scale scores were not correlated with (confounded with) a child's developmental stage or age. The results for these scales and items were consistent across the two independent samples. CONCLUSIONS: Unpaid caregivers, usually parents, can provide assessors lacking either medical or nursing training with reliable and valid information on the activity limitations of children. One can summarize these data in scales that are both internally consistent and valid. Researchers and clinicians can use supplemented interRAI items to provide guidance for professionals and programs serving children, as well as older persons. This research emphasizes the importance of developing medical information systems that allow one to integrate information not only across care settings but also across an individual's life course.

A Greenhouse Gas and Soil Carbon Model for Estimating the Carbon Footprint of Livestock Production in Canada.

To assess tradeoffs between environmental sustainability and changes in food production on agricultural land in Canada the Unified Livestock Industry and Crop Emissions Estimation System (ULICEES) was developed. It incorporates four livestock specific GHG assessments in a single model. To demonstrate the application of ULICEES, 10% of beef cattle protein production was assumed to be displaced with an equivalent amount of pork protein. Without accounting for the loss of soil carbon, this 10% shift reduced GHG emissions by 2.5 TgCO2e y(-1). The payback period was defined as the number of years required for a GHG reduction to equal soil carbon lost from the associated land use shift. A payback period that is shorter than 40 years represents a net long term decrease in GHG emissions. Displacing beef cattle with hogs resulted in a surplus area of forage. When this residual land was left in ungrazed perennial forage, the payback periods were less than 4 years and when it was reseeded to annual crops, they were equal to or less than 40 years. They were generally greater than 40 years when this land was used to raise cattle. Agricultural GHG mitigation policies will inevitably involve a trade-off between production, land use and GHG emission reduction. ULICEES is a model that can objectively assess these trade-offs for Canadian agriculture.

Severity of children's intellectual disabilities and Medicaid personal care services.

OBJECTIVES: This research investigated the relationship between a child's reported intellectual disability (ID) level and caregivers' reports of the child's health status to predict Medicaid Personal Care Services (PCS) hours authorized for that child. We also investigated how activity limitations in the home varied with the level of ID. DESIGN: The sample included 1,108 community-residing children with a reported level of ID in the Texas Medicaid system and who were assessed for the PCS program. All data were collected with the Personal Care Assessment Form (PCAF), an instrument developed by the authors for evaluating children's PCS needs. Case managers completed the PCAF in the child's home with the child and primary caregivers present. Structural equation modeling (SEM) was used to test a model reflecting the role of ID and other characteristics of the child in determining the number of PCS hours authorized. Additional analyses revealed the degree to which variation among the case managers affected the number of hours authorized. RESULTS: ID level and other individual characteristics had a significant effect on reports of a child's activity limitations (R2 = .67), which in turn affected the hours of PCS authorized (R2 = .27). We found no significant direct relationship between ID level and PCS hours: ID level had an indirect relationship on PCS hours through activity limitations. When the variance in hours authorized was decomposed, individual characteristics accounted for 20% of the variance and case managers accounted for 14%. CONCLUSIONS: Assessments of caregiver and child strengths and limitations in the home are critical in the allocation of Medicaid home-based services, above and beyond the information conveyed by demographic and diagnostic data. Implications for home-based assessments of functional limitations and needs for family caregivers and their children with ID are discussed.

Medicaid Personal Care Services and caregivers' reports of children's health: the dynamics of a relationship.

OBJECTIVE: To investigate the relationship between Medicaid Personal Care Services (PCS) and caregivers' reports of activity (activities of daily living [ADL]) limitations for children with chronic health problems. DATA SOURCES/STUDY SETTING: Primary data collected in 2008 and 2009. A state Medicaid program was the setting. The focus was children receiving Medicaid PCS. DATA COLLECTION: Medicaid case managers assessed children to determine their need for PCS, using information provided by the child or informal caregivers. Two thousand seven hundred assessments were provided to researchers directly from case managers. PRINCIPAL FINDINGS: Medical conditions and impairments explained 58 percent of the variance in the child's activity limitations. Activity limitations and problem behaviors explained 28 percent of the variance in PCS hours authorized. Which case manager completed the assessment also played a substantial role in determining hours of care. CONCLUSIONS: Caregivers' reports of the severity of a child's activity limitations effectively summarize the effects of conditions and impairments on the child's ADL performance and have a significant impact on the level of services provided. Assessors often respond differently to children's characteristics and circumstances as they move from assessment to decisions concerning care provision. Our results imply that the provision of appropriate services may be enhanced when both case managers and caregivers play an active role in decisions concerning care provision.

Reliability and the measurement of activity limitations (ADLs) for children with special health care needs (CSHCN) living in the community.

PURPOSE: Meeting the personal care challenges of children with special health care needs (CSHCN) living in the community demands a reliable assessment of their abilities to perform activities of daily living (ADLs). This research investigates factors affecting the inter-rater reliability of functional assessments of CSHCN conducted in the home. METHODS: Dual-standardised assessments were conducted with 236 community-dwelling CSHCN seeking or receiving Medicaid Personal Care Services (PCS) in a single state in USA. RESULTS: Analyses revealed that assessments of CSHCN with the greatest or least amount of activity limitations exhibited the greatest agreement. The greatest disagreement occurred when assessors faced children with moderate to moderately severe activity limitations. Specific ADLs, where the greatest and least agreements occurred, varied by the level of the child's overall activity limitation. CONCLUSIONS: These results imply that the most serious challenges to the reliability of home-based assessment of ADLs among CSHCN occur in cases of children with moderate or moderately severe activity limitations.

Modeling caregivers' perceptions of children's need for formal care: physical function, intellectual disability, and behavior.

BACKGROUND: Like most caregivers, informal caregivers for children (typically parents) with special needs supply the majority of the care provided. Formal care is sometimes needed to supplement informal caregivers' efforts. For those interested in children with special needs, there is a paucity of research on those factors affecting the amount of formal care that caregivers' request. OBJECTIVES/HYPOTHESES: This research investigates factors affecting the amount of Medicaid personal care services (PCS) requested by primary caregivers for children with special needs. The research focuses especially on the roles played by the child's functional status, intellectual abilities, and behaviors in determining the level of assistance requested by caregivers. METHODS: The data used in this research are cross-sectional information on 262 children with special needs who were Medicaid recipients in a single southwestern state. These data were collected in 2007 by master's trained social workers or registered nurses using a standardized assessment instrument. RESULTS: The results indicate that a child's ability to independently perform activities of daily living (ADLs) tasks plays the major role in determining caregivers' perceptions of the need for PCS. The severity of a child's intellectual disability, like some other factors investigated, has an effect on caregivers' perceptions, but it is an indirect effect that operates through the level of the child's ADL impairment. A child's age and behaviors have direct effects on caregivers' perceptions of need, as does the presence of barriers to the caregiver providing care. DISCUSSION: Much of the research on children with special needs has emphasized the importance of the child's medical or behavioral diagnoses. Little attention has been given to modeling caregivers' perceptions of children's needs. This analysis of caregivers' requests for formal PCS brings to the forefront the role of ADL or functional status in this process. Many factors that one would expect to directly affect caregivers' perceptions of need, instead, have indirect effects filtered through their impact on the child's functional status.

Bidentate complexation modeling of heavy metal adsorption and competition on goethite.

Accurate sorption modeling is critical for environmental risk assessment and development of sound remedial technologies. Adsorption to iron oxide phases is one of the important sorption processes regulating the bioavailability and toxicity of metal ions in natural systems. In this study, we used spectroscopically derived bidentate surface species to constrain surface complexation modeling in addressing Ni(ll) and Zn(ll) adsorption and competition on goethite surfaces. The 2-pK(a) triple layer model successfully predicted adsorption in single adsorbate systems. The curvature in adsorption isotherms was accurately depicted using two types of sites: high affinity and low affinity, and mononuclear bidentate surface complexes. A constrained set of parameters was found for each metal (log K(L) = -6.63 and log K(H) = -2.45 for Ni, log K(L) = -3.92 and log K(H) = 2.14 for Zn) that successfully described adsorption over a large range of experimental conditions, covering 6 to 7 orders of magnitude in concentration, ionic strength from 10(-3) to 10(-2), and environmentally relevant pH range between 4 and 6.5. Adsorption competition was predicted using the bidentate surface species with parameters calibrated using single adsorbate data.

A multi-scale assessment of Pb(II) sorption on dolomite.

Macroscopic sorption studies indicated that Pb sorption capacity was independent of pH over the pH range 5-7, while sorption as a function of reaction time up to two weeks for systems with no bulk precipitate phases showed continuous Pb uptake on dolomite. This could be due to diffusion of Pb into the micropores of dolomite as well as an increase in surface sites caused by particle size reduction during suspension mixing. Normalized XANES spectra for systems undersaturated with respect to Pb carbonate precipitates resembled the spectrum of Pb4(OH)4(4+), suggesting that Pb is mainly coordinated to dolomite as an inner-sphere surface complex. On the other hand, the XANES spectrum for 10(-3) M Pb at 1 atm CO2(g) in a 2 M Mg(NO3)2 background electrolyte solution resembled that of cerussite, while a sample at 5 x 10(-4) M Pb in equilibrium with air and 2 M Mg(NO3)2 resembled that of hydrocerussite. EXAFS analyses of sorption samples in chloride solutions showed that there were only first-shell contributions under 1 atm CO2(g), while higher shell contributions from Ca/Mg were seen at 10(-3.42) atm CO2(g). On the other hand, EXAFS samples prepared in nitrate solutions showed noticeable differences in speciation under different reaction conditions-from outer-sphere surface complexes at low Pb concentrations and pH, to inner-sphere surface complexes at moderate Pb concentrations and neutral pH, to the formation of Pb carbonate precipitates at the highest Pb loadings.

Surface complexation modeling of zinc sorption onto ferrihydrite.

A previous study involving lead(II) [Pb(II)] sorption onto ferrihydrite over a wide range of conditions highlighted the advantages of combining molecular- and macroscopic-scale investigations with surface complexation modeling to predict Pb(II) speciation and partitioning in aqueous systems. In this work, an extensive collection of new macroscopic and spectroscopic data was used to assess the ability of the modified triple-layer model (TLM) to predict single-solute zinc(II) [Zn(II)] sorption onto 2-line ferrihydrite in NaNO(3) solutions as a function of pH, ionic strength, and concentration. Regression of constant-pH isotherm data, together with potentiometric titration and pH edge data, was a much more rigorous test of the modified TLM than fitting pH edge data alone. When coupled with valuable input from spectroscopic analyses, good fits of the isotherm data were obtained with a one-species, one-Zn-sorption-site model using the bidentate-mononuclear surface complex, (triple bond FeO)(2)Zn; however, surprisingly, both the density of Zn(II) sorption sites and the value of the best-fit equilibrium "constant" for the bidentate-mononuclear complex had to be adjusted with pH to adequately fit the isotherm data. Although spectroscopy provided some evidence for multinuclear surface complex formation at surface loadings approaching site saturation at pH >/=6.5, the assumption of a bidentate-mononuclear surface complex provided acceptable fits of the sorption data over the entire range of conditions studied. Regressing edge data in the absence of isotherm and spectroscopic data resulted in a fair number of surface-species/site-type combinations that provided acceptable fits of the edge data, but unacceptable fits of the isotherm data. A linear relationship between logK((triple bond FeO)2Zn) and pH was found, given by logK((triple bond FeO)2Znat1g/l)=2.058 (pH)-6.131. In addition, a surface activity coefficient term was introduced to the model to reduce the ionic strength dependence of sorption. The results of this research and previous work with Pb(II) indicate that the existing thermodynamic framework for the modified TLM is able to reproduce the metal sorption data only over a limited range of conditions. For this reason, much work still needs to be done in fine-tuning the thermodynamic framework and databases for the TLM.

Treatment of zinc-contaminated water using a multistage ferrihydrite sorption system.

Previous studies demonstrated the environmental and economic benefits of treating lead(II)-contaminated water streams with ferrihydrite in multiple equilibrium sorption stages. In this work, multistage ferrihydrite sorption systems were evaluated for their effectiveness in reducing single-solute zinc(II) (Zn(II)) concentrations in contaminated water streams to very low levels. As for lead(II) (Pb(II)), experimental data and modeling results indicate that a multistage sorption system can significantly reduce Zn(II) effluent concentrations for the same total amount of sorbent or, alternatively, dramatically lower total sorbent consumption for the same effluent Zn(II) concentration. Compared to Pb(II), however, Zn(II) removal requires on the order of 10 times more sorbent to achieve the same target effluent concentration for the same pH and number of stages. Model predictions were made using a steady-state, multistage, equilibrium adsorber model that was previously developed for and integrated into OLI Systems' Environmental Simulation Program (ESP). The modified triple-layer model was used to simulate Zn(II) surface-liquid equilibria within the adsorber model. Engineering screening evaluations again indicate that a 2- to 3-stage sorption process can provide significant economic savings when compared to a 1-stage process operating with the same target effluent Zn(II) concentration. Additional equilibrium stages beyond 2 or 3 provide diminishing economic returns. The major economic driver for multiple contacting stages is reduced capital investment and operating costs for sludge handling, dewatering, and disposal.

Mechanistic and thermodynamic interpretations of zinc sorption onto ferrihydrite.

Elucidating the reaction mechanisms and estimating the associated transport and thermodynamic parameters are important for an accurate description of the fate of toxic metal pollutants, such as Zn(II), in soils and aquatic ecosystems rich in iron oxides. Consequently, sorption of Zn(II) ions onto ferrihydrite was investigated with macroscopic and spectroscopic studies as a function of pH (4.0-8.0), ionic strength (10(-3)-10(-1) M NaNO(3)), aqueous Zn(II) concentration (10(-8)-10(-2) M), and temperature (4-25 degrees C). Present findings suggest that, for a given set of pH and temperature conditions, Zn sorption onto ferrihydrite can best be described by one average reaction mechanism below the saturation limits. Thermodynamic analyses reveal that the Zn(II) ions sorb onto the ferrihydrite surfaces via strong endothermic chemical reactions. Consistently, X-ray absorption spectroscopic (XAS) analyses confirm that, at pH < 6.5, for all Zn loadings, Zn(II) ions form corner-sharing, mononuclear, bidentate inner-sphere complexes with ferrihydrite, where R(Zn-O) approximately 1.97 A and R(Zn-Fe) approximately 3.48 A. For pH >/=6.5, similar sorption complexes were observed at lower sorption densities. Then again, for pH >/=6.5 and at higher sorption densities, Zn(II) ions may begin to form zinc-hydroxide-like polynuclear sorption complexes on the surfaces of the ferrihydrite, where R(Zn-Zn) approximately 3.53 A. Surprisingly, small changes in temperature had a significant impact on the affinity of zinc for the ferrihydrite surface; equilibrium sorption capacity decreased by 3-4 orders of magnitude as temperature fell from 25 to 4 degrees C for all pH. Zinc sorption onto ferrihydrite, therefore, is governed by pH as well as by temperature and sorbate/sorbent ratio.

Lead sorption onto ferrihydrite. 1. A macroscopic and spectroscopic assessment.

In this research, traditional macroscopic studies were complemented with XAS analyses to elucidate the mechanisms controlling Pb(II) sorption onto ferrihydrite as a function of pH, ionic strength, and adsorbate concentrations. Analyses of XANES and XAFS studies demonstrate that Pb(II) ions predominantly sorb onto ferrihydrite via inner-sphere complexation, not retaining their primary hydration shell upon sorption. At higher pH values (pH > or = 5.0), edge-sharing bidentate complexes are mainly formed on the oxide surface with two Fe atoms located at approximately 3.34 A. In contrast, XAS studies on Pb(II) sorption onto ferrihydrite, at pH 4.5, reveal two distinct Pb-Fe bond average radial distances of 3.34 and 3.89 A, suggestive of a mixture of monodentate and bidentate sorption complexes present at the oxide surface. Interestingly, at constant pH, the configuration of the sorption complex is independent of the adsorbate concentration. Hence, Pb(II) sorption to a highly disordered adsorbent such as ferrihydrite can be described by one average type of mechanism. Overall, this information will aid scientists and engineers in improving the current models that predict and manage the fate of toxic metals, such as Pb(II), in the aquatic and soil environments.

Lead sorpion onto ferrihydrite. 2. Surface complexation modeling.

Few studies have combined molecular- and macroscopic-scale investigations with surface complexation model (SCM) development to predict trace metal speciation and partitioning in aqueous systems over a broad range of conditions. In this work, an extensive collection of new macroscopic and spectroscopic data was used to assess the ability of the modified triple-layer model (TLM) to predict single-solute lead(II [Pb(II)] sorption onto 2-line ferrihydrite in NaNO3 solutions as a function of pH, ionic strength, and concentration. Regression of constant-pH isotherm data together with potentiometric titration and pH edge data was a much more rigorous test of the TLM than fitting pH edge data alone. When combined with spectroscopic data, the choices of feasible surface species/site types were limited to a few. In agreement with the spectroscopic data, very good fits of the isotherm data were obtained with a two-species, one-site model using the bidentate-mononuclear/monodentate-mononuclear species pairs, ((is equivalent)FeO)2Pb/(is equivalent)FeOHPb2+ and ((is equivalent)FeO)2Pb/ (is equivalent)FeOPb+-NO3-. Regressing edge data in the absence of isotherm and spectroscopic data resulted in a fair number of surface-species/site-type combinations that provided acceptable fits of the edge data but unacceptable fits of the isotherm data. Surprisingly, best-fit equilibrium "constants" for the Pb(II) surface complexes required adjustment outside the pH range of 4.5-5.5 in order to fit the isotherm data. In addition, a surface activity term was needed to reduce the ionic strength dependence of sorption forthe species pair, ((is equivalent)FeO)2Pb/(is equivalent)FeOHPb2+. In light of this, the ability of existing SCMs to predict Pb(II) sorption onto 2-line ferrihydrite over a wide range of conditions seems questionable. While many advances have been made over the past decade, much work still needs to be done in fine-tuning the thermodynamic framework and databases for the SCMs.

Lead sorption onto ferrihydrite. 3. Multistage contacting.

Few studies have demonstrated the practical application of surface complexation models, calibrated with fundamental macroscopic and spectroscopic metal sorption data, in helping to solve industrial trace metal emissions problems. In this work, multistage ferrihydrite sorption systems are evaluated for their effectiveness in reducing single-solute lead(II) [Pb(II)] concentrations in contaminated water streams to very low levels. Experimental data and modeling results indicate that a multistage sorption system can significantly reduce Pb(II) effluent concentrations for the same total amount of sorbent or, alternatively, dramatically lower total sorbent consumption for the same effluent Pb(II) concentration. Model predictions were generated using a steady-state, multistage, equilibrium adsorber model that was specifically developed for and integrated into 0LI Systems' Environmental Simulation Program. The modified triple-layer model was used to simulate Pb(II) surface-liquid equilibria within the adsorber model. Engineering screening evaluations indicate that a 2-3-stage sorption process can provide significant economic savings when compared to a 1-stage process operating with the same target effluent Pb(II) concentration. Additional equilibrium stages beyond 2 or 3 provide diminishing economic returns. The major economic driver for multiple contacting stages is reduced capital investment and operating costs for sludge handling, dewatering, and disposal.

Nuevos enfoques sobre el manejo de residuos industriales

En este artículo se analizan los costos adicionales de tratamiento que traen aparejados las nuevas instalaciones y cómo se los puede utilizar como paso intermedio para llegar a diseños integrados u holísticos. La inversión es muy significativa: U$S 3.000/gpm y U$S 6.000/(lb/h) para desechos acuosos y U$S 40 a U$S 75/scfn para las emisiones atmosféricas. Asimismo la inversión en el caso de los desechos sólidos oscila entre U$S 0,12 y U$S 1,00 por libra. Por último se describen las características de un abordaje equilibrado que, de aplicarse en todos sus pasos, aumentará en gran medida las oportunidades de éxito