Biomedical subjects

Benito J Mariñas

Publications and source records attributed to Benito J Mariñas.

At least 19 recordsLinked to original sources

PubMed2006

Simplification of the IAST for activated carbon adsorption of trace organic compounds from natural water.

Recent studies have shown that the ideal adsorbed solution theory (IAST) coupled with the concept of equivalent background compound (EBC) can be simplified for describing trace organic compound adsorption from natural water, provided that the adsorbent surface loading is dominated by competing natural organic matter. The resulting simplified IAST has been used to reduce the complexity of kinetic models for various dynamic adsorption processes. In order to be correctly applied, however, the simplified IAST requires some additional clarification and a quantitative evaluation of the deviation caused by the simplifying assumption. In this study, we derive a simple equation that relates the relative deviation of the simplified IAST directly to the molar ratio of EBC and trace organic compound surface loadings and their Freundlich isotherm exponents. We then verify the simplified IAST using the original IAST and experimental isotherm data from the literature for trace organic compounds at various initial concentrations in natural water. By further assuming that the adsorbed amount of the EBC is substantially greater than what remains in solution, a new pseudo single-solute isotherm equation is derived and a simple relation is subsequently established between the carbon dose and the remaining trace compound concentration. The results show that the adsorption capacity and relative removal of a trace organic compound at any carbon dose can be estimated directly with the simple equations developed here and data from a single isotherm experiment for the target compound conducted in the natural water of interest.

Adsorption↗

PubMed2006

Modeling Cryptosporidium parvum oocyst inactivation and bromate in a flow-through ozone contactor treating natural water.

A reactive transport model was developed to simultaneously predict Cryptosporidium parvum oocyst inactivation and bromate formation during ozonation of natural water. A mechanistic model previously established to predict bromate formation in organic-free synthetic waters was coupled with an empirical ozone decay model and a one-dimensional axial dispersion reactor (ADR) model to represent the performance of a lab-scale flow-through ozone bubble-diffuser contactor. Dissolved ozone concentration, bromate concentration (in flow-through experiments only), hydroxyl radical exposure and C. parvum oocyst survival were measured in batch and flow-through experiments performed with filtered Ohio River water. The model successfully represented ozone concentration and C. parvum oocyst survival ratio in the flow-through reactor using parameters independently determined from batch and semi-batch experiments. Discrepancies between model prediction and experimental data for hydroxyl radical concentration and bromate formation were attributed to unaccounted for reactions, particularly those involving natural organic matter, hydrogen peroxide and carbonate radicals. Model simulations including some of these reactions resulted in closer agreement between predictions and experimental observations for bromate formation.

Animals↗

PubMed2006

Three-component competitive adsorption model for fixed-bed and moving-bed granular activated carbon adsorbers. Part I. Model development.

Heterogeneous natural organic matter (NOM) present in all natural waters impedes trace organic contaminant adsorption, and predictive modeling of granular activated carbon (GAC) adsorber performance is often compromised by inadequate accounting forthese competitive effects. Thus, a 3-component adsorption model, COMPSORB-GAC, is developed that separately tracks NOM adsorption and its competitive effects as a function of NOM surface loading. In this model, NOM is simplified into two fictive fractions with distinct competitive effects on trace compound adsorption: a smaller, strongly competing fraction that reduces equilibrium capacity and a larger pore-blocking fraction that reduces adsorption kinetics (both external film mass transfer and surface diffusion). COMPSORB-GAC tracks these two NOM fractions, along with the trace compound, and changes adsorption parameters according to the local surface loading of the two NOM fractions. Model parameters are allowed to vary both temporally and spatially to reflect differences in the NOM preloading conditions that occur in GAC columns. This dual-resistance model is based on homogeneous surface diffusion with external film mass-transfer limitations. The governing equations are expressed in a moving-grid finite-difference formulation to accommodate the modeling of spatially varying parameters and moving-bed reactors with counter-current adsorbent flow. A series of short-term adsorption tests with fresh and preloaded GAC is proposed to determine the necessary model input parameters. The accompanying manuscript demonstrates the parameterization procedure and verifies the model with experimental data.

Adsorption↗

PubMed2006

Three-component competitive adsorption model for fixed-bed and moving-bed granular activated carbon adsorbers. Part II. Model parameterization and verification.

COMPSORB-GAC is a 3-component competitive adsorption kinetic model for granular activated carbon (GAC) adsorbers that was developed in Part I of this study, including a proposed procedure for determining model parameters in natural water applications with background natural organic matter (NOM). Part II of this study demonstrates the proposed parameterization procedure and validates the modeling approach by comparing predictions with experimental breakthrough curves at multiple empty-bed contact times for both fixed-bed and moving-bed reactors. The parameterization procedure consists of a set of independent, short-term experimental tests with fresh and batch preloaded adsorbents and then data fitting using both classic and recently developed theoretical expressions. The model and parameterization procedure simplifies NOM into two fictive fractions (pore-blocking and strongly competing) and incorporates three competitive effects that vary both temporally and axially in a GAC column (direct competition for sites, intraparticle pore blockage, and external surface pore blockage). With all three competitive mechanisms accounted for, the model could accurately predict breakthrough profiles for column lengths and durations that were much longer than those used for model parameterization. Model predictions that ignored one or more of the competitive mechanisms showed that each mechanism was important for different regions of the breakthrough curve. The external surface pore-blockage effect was predominant for the prediction of early breakthrough data, whereas direct competition for sites and intraparticle pore blockage were prevalent when predicting higher breakthrough levels and data later in the column run.

Adsorption↗

PubMed2006

Cyanogen bromide formation from the reactions of monobromamine and dibromamine with cyanide ion.

Cyanide ion (CN-) was found to reactwith monobromamine (NH2Br) and dibromamine (NHBr2) according to the reactions NH2Br + CN- + H20 --> NH3 + BrCN + OH- and NHBr2 + CN- + H20 --> NH2Br + BrCN + OH- with respective reaction rate constants of 2.63 x 10(4) M9-10 s(-1) and 1.31 x 10(8) M(-1) s(-1). These values were found to be 10(5)-10(6) times greater than those for the corresponding reactions between chloramine species and CN-. As a result, bromamines, even if present at relatively low concentrations, would tend to outcompete chloramines in reacting with CN-, and thus, the formation of BrCN would predominate that of ClCN through these reaction mechanisms. The NH2Br reaction was found to be general-acid-catalyzed. The third-order catalysis rate constants for H+, H2PO4-, HPO4(2-), H3BO3, and NH4+ correlated linearly with their corresponding acid dissociation constants, consistent with the Brønsted-Pedersen relationship. The NHBr2 reaction did not undergo catalysis. A model was developed to predict the concentrations of bromamines over time on the basis ofthe above two reactions with CN- and bromamine formation/decomposition reactions previously reported.

Bromides↗

PubMed2006

Competitive effects of natural organic matter: parametrization and verification of the three-component adsorption model COMPSORB.

Natural organic matter (NOM) hinders adsorption of trace organic compounds on powdered activated carbon (PAC) via two dominant mechanisms: direct site competition and pore blockage. COMPSORB, a three-component model that incorporates these two competitive mechanisms, was developed in a previous study to describe the removal of trace contaminants in continuous-flow hybrid PAC adsorption/membrane filtration systems. Synthetic solutions containing two model compounds as surrogates for NOM were used in the original study to elucidate competitive effects and to verify the model. In the present study, a quantitative method to characterize the components of NOM that are responsible for competitive adsorption effects in natural water was developed to extend the application of COMPSORB to natural water systems. Using batch adsorption data, NOM was differentiated into two fictive fractions, representing the strongly competing and pore blocking components, and each was treated as a single compound. The equilibrium and kinetic parameters for these fictive compounds were calculated using simplified adsorption models. This parametrization procedure was carried out on two different natural waters, and the model was verified with experimental data obtained for atrazine removal from natural water in a PAC/membrane system. The model predicted the system performance reasonably well and highlighted the importance of considering both direct site competition and pore blockage effects of NOM in modeling these systems.

Adsorption↗

PubMed2005

Modeling Cryptosporidium parvum oocyst inactivation and bromate formation in a full-scale ozone contactor.

The inactivation of Cryptosporidium parvum oocysts and the formation of bromate were assessed simultaneously by performing experiments with a full-scale ozone bubble-diffuser contactor used for drinking water disinfection. Fluorescence-dyed polystyrene microspheres were used as surrogates for C. parvum oocysts. Semi-batch ozonation experiments were performed to determine the fluorescence-intensity decay of individual microspheres, which was measured by flow cytometry. The results obtained with the microspheres were correlated to the inactivation kinetics of C. parvum oocysts by choosing an appropriate threshold fluorescence intensity below which microspheres were considered to be equivalent to nonviable oocysts. A mathematical model was then used to predict the inactivation efficiency and bromate formation. The contactor hydrodynamics were characterized by running tracer tests, and the kinetic parameters for ozone decomposition and bromate formation were obtained by performing batch experiments. Model predictions were in good agreement with full-scale experimental results. Additional model simulations revealed that ozone contactors should be designed with the lowest possible backmixing so that the target inactivation efficiency can be achieved with the lowest possible formation of bromate.

Animals↗

PubMed2005

Microbial passage in low pressure membrane elements with compromised integrity.

The main objective of this study was to evaluate microbial passage through membrane elements with various levels of compromised integrity while installed within full-scale UF/MF systems. Challenge tests were performed for this purpose with the microbial contaminant surrogates Bacillus subtilis spores at two drinking water treatment plants. The experimental units used were one of several parallel racks part of each plant in which one of the elements was installed with various levels of compromised integrity achieved by precutting several hollow fibers and inserting removable pin-plugs atthe corresponding ends of the broken fiber segments. The UF rack was operated with 38 elements online (the rack has 50 elements) and was designed for an inside-out operation, and the MF rack included 50 elements and was operated in an outside-in mode (with a permeate outlet at one fiber end only). Spore removals observed for both the UF and MF racks with all precut fibers plugged were equal to or greater than 99.9992%, and as expected the removal efficiency deteriorated with an increasing number of unplugged fibers. Predictions made with a model based on the use of the Hagen-Poisseuille equation for laminar flow and the Darcy-Weisbach expression for turbulent flow inside broken fibers were found to provide an adequate conservative representation of experimental results. Additional simulations performed with the verified model revealed the occurrence of a greater microbial passage for an inside-out configuration compared to an outside-in mode. A lower microbial passage was predicted for the outside-in element configuration with one permeate outlet as compared to an element with permeate outlets at both fiber ends. The model offers a useful tool that together with other considerations such as membrane fouling, cleaning, and durability would assist in the selection of low-pressure membrane element configuration.

Bacillus subtilis↗

PubMed2005

Development of a Ct equation taking into consideration the effect of lot variability on the inactivation of Cryptosporidium parvum oocysts with ozone.

Cryptosporidium parvum oocysts are prevalent in surface water and ground water under the influence of surface water, and are difficult to inactivate using free chlorine, the most common disinfectant currently used for treating drinking water. In contrast, it has been shown that ozone is a more effective disinfectant than chlorine. US EPA is currently evaluating a treatment rule, which addresses the control of C. parvum oocysts in drinking water. The use of Ct (average disinfectant concentration multiplied by characteristic contact time) values is being considered as one of the options for demonstrating adequate control of this microbial contaminant. The purpose of this study is to incorporate the variability in inactivation kinetics among different lots of oocysts and to develop a statistical model for Ct based on first-order delayed Chick--Watson inactivation kinetics. A Bayesian approach is used to estimate the delayed Chick--Watson kinetic parameters. A log-linear regression analysis is then used to represent the effect of temperature on the resulting kinetic parameters. The overall model developed in this study provides an approach for water utilities and regulatory agencies to decide on the level of safety needed when developing treatment requirements for the inactivation of C. parvum oocysts with ozone as part of broader risk assessment considerations.

Animals↗

PubMed2004

Bromamine decomposition kinetics in aqueous solutions.

The objectives of this study are to investigate the kinetics of bromamine decomposition and to identify the corresponding relevant reactions. Experiments were performed with a stopped-flow spectrophotometer system. Experimental variables investigated included pH (6.5-9.5), bromamines concentration (0.15-0.50 mM), ammonia to bromine ratio (5-100), and phosphate and carbonate buffers concentration (5-40 mM). The experimental results were consistent with a reaction scheme that involved the reversible disproportionation of monobromamine into dibromamine and ammonia (2NH2Br (k1)<=>(k(-1)) NHBr2 + NH3), followed by irreversible decomposition of monobromamine and dibromamine into products (2NHBr2 (k2) --> products and NH2Br + NHBr2 (k3) --> products). The monobromamine disproportionation reaction was found to undergo general acid catalysis, and the two subsequent decomposition reactions were found to experience base catalysis. Experimental results were analyzed for the determination of catalysis terms corresponding to H+, NH4+, H2PO4-, HCO3-, and H2O for rate constants k1 and k(-1); HPO4(2-) and H2O for k2; and OH-, CO3(2-), and H2O for k3. These constants were fitted with the Brønsted relationship, and the resulting fitting expressions were used to calculate any relevant catalysis rate constants that could not be determined at the range of experimental conditions used.

Bromides↗

PubMed2004

Simultaneous prediction of Cryptosporidium parvum oocyst inactivation and bromate formation during ozonation of synthetic waters.

A model was developed to simultaneously assess Cryptosporidium parvum oocyst inactivation and bromate formation during ozonation of synthetic solutions in batch and flow-through reactors. The model incorporated 65 elementary chemical reactions involved in the decomposition of ozone and the oxidation of bromine species and their corresponding rate or equilibrium constants reported in the literature. Ozonation experiments were performed with a laboratory-scale batch reactor to evaluate the model with respect to the rate of ozone decomposition and bromate formation. The model was found to provide a good representation of experimental results when the ozone decomposition initiation reaction with hydroxide ion was assumed to produce superoxide radical instead of the alternatively proposed product hydrogen peroxide. The model was further developed to simulate the performance of a flow-through bubble-diffuser reactor with an external recirculation line. Each compartment of the reactor (bubble column and recirculation line) was assumed to behave as a plug flow reactor as supported by tracer test results, and an empirical correlation was used to represent the rate of ozone gas transfer in the bubble column. Model predictions of the performance of the flow-through ozone bubble-diffuser contactor were in good agreement with experimental results obtained for bromate formation and C. parvum oocyst inactivation under all conditions investigated. Additional model simulations revealed that hydrodynamic conditions had a more pronounced effect on C. parvum oocyst inactivation than on bromate formation. In contrast, pH had a strong effect on bromate formation without affecting the inactivation efficiency of C. parvum oocysts for a given level of exposure to ozone. These findings suggested that bromate formation could be minimized while achieving target inactivation levels for C. parvum oocysts by designing ozone reactors with hydrodynamic conditions approaching that of an ideal plug flow reactor and by lowering the pH of the target water.

Animals↗

PubMed2004

Removal of biological and non-biological viral surrogates by spiral-wound reverse osmosis membrane elements with intact and compromised integrity.

The removal of bacteriophage MS2 and fluorescent-dyed polystyrene microspheres with intact and purposely compromised spiral-wound RO membrane elements was investigated. MS2 rejection with intact membrane elements was >99.9995%. A model developed for data evaluation revealed that the advective passage of MS2 through imperfections of intact membrane elements was <2 x 10(-5)% of the overall product water flow produced. The advective passage of MS2 and microspheres through a pinhole induced in one of the elements was 0.05-0.1% of the overall product water flow. Prolonged testing of both intact and compromised elements resulted in increased MS2 rejection corresponding to advective MS2 passage through membrane imperfections of <3 x 10(-7)% of the overall product water flow. The permeate flow rate obtained with an element with a larger pinhole was 5-13% greater than that of the intact element, and the corresponding rejection of MS2 and microspheres was similar to that observed for sodium chloride. The use of a cracked o-ring in the connection of the permeate tube to the element vessel end-cup resulted in advective passage of MS2 through the crack of <0.0001% of the overall permeate flow.

Fluorescent Dyes↗

PubMed2003

Three-component competitive adsorption model for flow-through PAC systems. 1. Model development and verification with a PAC/membrane system.

Natural organic matter (NOM) interferes with the adsorption of trace organic compounds on porous adsorbents such as powdered activated carbon (PAC) by pore blockage and direct competition for adsorption sites. The competitive effect of NOM in flow-through systems in which the retention time of the PAC is greater than the hydraulic retention time of the system can be magnified because NOM from the influent water can continue to adsorb on the PAC retained in the system. As a result, the adsorption capacity and the diffusion coefficient of trace compounds can decrease as NOM from the influent water accumulates. In this study, a dynamic three-component adsorption model was developed to quantitatively describe the removal of a trace compound from water in flow-through PAC processes. The system was simplified by using p-dichlorobenzene (p-DCB) to represent the NOM fraction that competes directly with the target trace organic atrazine for adsorption sites and by using poly(styrene sulfonate) (PSS-1.8k) to represent large, pore-blocking NOM. The model was based on the homogeneous surface diffusion assumption with the adsorption capacity of atrazine being gradually adjusted using a simplified version of the ideal adsorbed solution theory model developed in this study. The surface diffusion coefficients of atrazine and p-DCB were modeled as a function of the surface concentration of the pore-blocking compound, PSS-1.8k. The model was verified experimentally with a PAC/microfiltration (MF) system. The use of single-solute adsorption parameters obtained from batch isotherm and kinetic tests resulted in good model predictions for the adsorption of atrazine and the two model compounds under operating conditions typical of PAC/MF systems. The model will be applied to study various operating conditions and other system parameters of PAC/membrane systems in part 2 of this study.

Adsorption↗

PubMed2003

Three-component competitive adsorption model for flow-through PAC systems. 2. Model application to a PAC/membrane system.

A three-component competitive adsorption kinetic model, developed and validated in part 1 of this study, was applied to a continuous-flow PAC/membrane system to study the effects of various system and operating parameters on organic removal. The model quantitatively describes the two competitive adsorption mechanisms that occur during adsorption of trace organic compounds by powdered activated carbon (PAC) in flow-through systems where the PAC is retained in the system: pore blockage and direct competition for adsorption sites. Model simulations were conducted to investigate the effects of influent water composition, membrane cleaning water quality, PAC pore size distribution, and system operation conditions such as hydraulic retention time, membrane cleaning interval, and PAC dosing method on treatment efficiency. Effects of these factors on adsorption capacity as well as surface diffusion rate and consequent removal of the trace organic compound were discussed. It was found that optimal operating conditions for maximum trace organic compound removal must be determined on the basis of the adsorption properties and concentrations of the competing compounds in the influent. For the conditions investigated in this study, the small strongly competing compound, p-DCB, had greater impact on atrazine removal than the large pore-blocking compound, PSS-1.8k. Various process design and operating parameters had complex and interrelated effects on the impact of competitive adsorption and corresponding trace contaminant removal efficiency in hybrid PAC/membrane systems.

Adsorption↗

PubMed2003

Pore blockage effect of NOM on atrazine adsorption kinetics of PAC: the roles of PAC pore size distribution and NOM molecular weight.

Natural organic matter (NOM) in natural water has been found to have negative effects on the adsorption of various trace organic compounds by activated carbon through two major mechanisms: direct competition for sites and pore blockage. In this study, the pore blockage effect of NOM on atrazine adsorption kinetics was investigated. Two types of powdered activated carbon (PAC) and three natural waters were tested to determine the roles of PAC pore size distribution and NOM molecular weight distribution in the pore blockage mechanism. When PAC was preloaded with natural water, the pore blockage effect of the NOM was found to cause a reduction of up to more than two orders of magnitude in the surface diffusion rate of atrazine compared to simultaneous adsorption of atrazine and NOM with fresh PAC. The surface diffusion coefficient of atrazine for preloaded PAC decreased with a decrease in PAC dose or an increase in NOM surface concentration. Because of the pore blockage effect of NOM, a 30% drop in atrazine removal was observed in a continuous flow PAC/microfiltration (MF) system after 7 days of contact compared to the removal predicted from the batch isotherm test. Large micropores and mesopores were found to play an important role in alleviating the effect of pore blockage. A PAC with a relatively large fraction of large micropore and mesopores was shown to suffer much less from the pore blockage effect compared with a PAC that had a much smaller fraction of large pores. Natural waters with different NOM molecular weight distribution caused different extent of pore blockage. The NOM molecules with molecular weight between 200 and 700 Dalton appeared to be responsible for the pore blockage effect.

Adsorption↗

PubMed2003

Inactivation of Bacillus subtilis spores with ozone and monochloramine.

The inactivation kinetics of Bacillus subtilis spores with ozone and monochloramine was characterized by a lag phase followed by a pseudo-first-order rate of inactivation. The lag phase decreased and the post-lag phase rate constant increased with increasing temperature within the range investigated (1-30 degrees C for ozone, 1-20 degrees C for monochloramine). The corresponding activation energies were 46820 J/mol for ozone and 79640 J/mol for monochloramine. The CT concept was found to be valid within the concentration range investigated of 0.44-4.8 mg/l for ozone, and 3.8-7.7 mg/l as Cl(2) for monochloramine. The inactivation kinetics of B. subtilis spores with both ozone and monochloramine varied with pH within the range of pH 6-10 investigated. The fastest ozone and monochloramine inactivation rates were observed at pH 10 and 6, respectively. Different stocks of the same strain of B. subtilis spores had different resistance to ozone and monochloramine mainly because of discrepancies in the extent of the lag phase. B. subtilis spores might not be conservative surrogates for C. parvum oocysts for ozone disinfection at relatively low temperature mainly due to the spores having a lower activation energy compared to that for the oocysts. In contrast, the activation energy for monochloramine was comparable for both microorganisms but differences in the extent of the lag phase might result in the spores being overly conservative surrogates for the oocysts at relatively low temperature.

Bacillus subtilis↗

PubMed2003

A Bayesian method of estimating kinetic parameters for the inactivation of Cryptosporidium parvum oocysts with chlorine dioxide and ozone.

The main objective of this paper is to use Bayesian methods to estimate the kinetic parameters for the inactivation kinetics of Cryptosporidium parvum oocysts with chlorine dioxide or ozone which are characterized by the delayed Chick-Watson model, i.e., a lag phase or shoulder followed by pseudo-first-order rate of inactivation. As the length of the lag phase (CT(lag)) is not known, Bayesian statistics provides a more accurate approach than traditional statistical methods to fitting the delayed Chick-Watson kinetics. Markov Chain Monte Carlo method is used to estimate CT(lag) and first-order rate constant values. This method is also used to estimate the minimum CT requirement (with safety factor) for 99% inactivation of C. parvum oocysts.

Animals↗

PubMed2002

Displacement effect of NOM on atrazine adsorption by PACs with different pore size distributions.

This study investigated displacement of atrazine by the strongly competing fraction of natural organic matter (NOM) in batch and continuous-flow powdered activated carbon/PAC) adsorption systems. Due to the displacement effect, atrazine adsorption capacity in a continuous flow PAC/microfiltration (MF) system, where the carbon retention time is greater than the hydraulic retention time, decreased with time or NOM throughput. The capacity was lower than that measured in a batch reactor or predicted by the equivalent background compound-ideal adsorbed solution theory (EBC-IAST) method. A mathematical model previously developed to simulate the adsorption process in the PAC/MF system was modified to take into account the displacement effect. Two types of PACs were tested using a range of influent atrazine concentrations and carbon doses. The extent of atrazine displacement by NOM was found to depend on the type of PAC, while the rate of displacement was a function of PAC type as well as carbon dose. The PAC lost its adsorption capacity for atrazine faster at a lower carbon dose. PAC B, which has a higher percentage of mesopores, lost more atrazine adsorption capacity but at a slower rate than PAC A.

Adsorption↗