Activated Carbon Dosage Research Articles

Biodynamic Modeling of PCB Uptake by Macoma balthica and Corbicula fluminea from Sediment Amended with Activated Carbon

Activated carbon amendment was assessed in the laboratory as a remediation strategy for freshwater sediment contaminated with polychlorinated biphenyls (PCBs) from the Grasse River (near Massena, NY). Three end points were evaluated: aqueous equilibrium PCB concentration, uptake into semipermeable membrane devices (SPMDs), and 28-day bioaccumulation in the clam Corbicula fluminea. PCB uptake by water, SPMDs, and clams followed similar trends, with reductions increasing as a function of carbon dose. Average percent reductions in clam tissue PCBs were 67, 86, and 95% for activated carbon doses of 0.7, 1.3, and 2.5% dry wt, respectively. A biodynamic model that incorporates sediment geochemistry and dietary and aqueous uptake routes was found to agree well with observed uptake by C. fluminea in our laboratory test systems. Results from this study were compared to 28-day bioaccumulation experiments involving PCB-contaminated sediment from Hunters Point Naval Shipyard (San Francisco Bay, CA) and the clam Macoma balthica. Due to differences in feeding strategy, M. balthica deposit-feeds whereas C. fluminea filter-feeds, the relative importance of the aqueous uptake route is predicted to be much higher for C. fluminea than for M. balthica. Whereas M. balthica takes up approximately 90% of its body burden through sediment ingestion, C. fluminea only accumulates approximately 45% via this route. In both cases, results strongly suggest that it is the mass transfer of PCBs from native sediment to added carbon particles, not merely reductions in aqueous PCB concentrations, that effectively reduces PCB bioavailability and uptake by sediment-dwelling organisms.

Removal of organic impurities with activated carbons for ultra-pure hydrogen peroxide preparation

Abstract The aim of this research was to select suitable activated carbon (AC) for effective removal of organic impurities from industrial aqueous hydrogen peroxide solution to produce ultra-pure hydrogen peroxide. The textural parameters and surface chemistry of four kinds of AC samples were measured and analyzed. Static and dynamic equilibrium adsorption experiments were carried out to compare the effect of AC on organic impurities adsorption and hydrogen peroxide decomposition. The effects of AC pore structure and surface chemistry on the adsorption of organic impurities were investigated. The fitting operation conditions, i.e., operating temperature and AC dosage, were also examined. The results showed that AC adsorption capacity on organic impurities from industrial aqueous hydrogen peroxide solution was mainly influenced by micropore structure of AC, as well as decomposition of hydrogen peroxide. The pore size of 1–3 nm is most effective for adsorption of organic impurities. It was found that the organic impurities in industrial hydrogen peroxide solution could be reduced effectively to meet the standard of ultra-pure hydrogen peroxide of SEMI-C8 level with the proposed AC and adsorption techniques.

Selective adsorption of phenanthrene dissolved in surfactant solution using activated carbon

Selective adsorption of a hazardous hydrophobic organic compound (HOC) by activated carbon as a means of recovering surfactants after a soil washing process was investigated. As a model system, phenanthrene was selected as a representative HOC and Triton X-100 as a nonionic surfactant. Three activated carbons that differed in size (Darco 20–40 (D20), 12–20 (D12) and 4–12 (D4) mesh sizes) were used in adsorption experiments. Adsorption of surfactant onto activated carbon showed a constant maximum above the critical micelle concentration, which were 0.30, 0.23, 0.15 g g −1 for D20, D12, and D4, respectively. Selectivity for phenanthrene to Triton X-100 was much higher than 1 over a wide range of activated carbon doses (0–6 g l −1) and initial phenanthrene concentrations (10–110 mg l −1). Selectivity generally increased with decreasing particle size, increasing activated carbon dose, and decreasing initial concentration of phenanthrene. The highest selectivity was 74.9, 57.3, and 38.3 for D20, D12, and D4, respectively, at the initial conditions of 10 mg l −1 phenanthrene, 5 g l −1 Triton X-100 and 1 g l −1 activated carbon. In the case of D20 at the same conditions, 86.5% of the initial phenanthrene was removed by sorption and 93.6% of the initial Triton X-100 remained in the solution following the selective adsorption process. The results suggest that the selective adsorption by activated carbon is a good alternative for surfactant recovery in a soil washing process.

Color Formation in Wheat Starch Based Glucose Syrups and Use of Commercially Available and Laboratory‐Prepared Agricultural Waste‐based Activated Carbons for Decolorization

Glucose syrups were produced from wheat starch using α‐amylase at 97°C for 45–90 min liquefaction times, followed by saccharification with amyloglucosidase at 60°C for 18 h to study the color formation. This was followed by decolorization studies using 0.25 to 1.00 g activated carbon per g of syrup of the commercially available NORIT and several activated carbons prepared from apricot stones and hazelnut shells and husks on laboratory scale. Increase in liquefaction time resulted in higher extents of hydrolysis in both maltodextrins and glucose syrups. In maltodextrins, 9–21% and in glucose syrups 72–98% of the linkages were hydrolyzed at 45–90 min liquefaction times. Color levels of glucose syrups increased with increased liquefaction time. The color levels were between 657–1424 ICUMSA units for glucose syrups obtained at 45–90 min liquefaction times. The dosage of activated carbon necessary to decolorize the syrups to the color level lower than 100 ICUMSA units, however, became lower and lower as the liquefaction time (and the level of starting color), was increased; and this behavior was the same for all types of activated carbon studied. Decolorization performances of NORIT, apricot stone (AS), hazelnut husk (HH) and hazelnut shell (HS) based activated carbons were compared by adjusting the activated carbon dosage to be the same as that of NORIT required for 100 ICUMSA residual color. HH was the best giving practically same residual color as NORIT for the decolorization of 90 min liquefied glucose syrups. The HS and AS had similar performances, reducing the color to less than 180 ICUMSA units for syrups liquefied for 45 and 90 min.

Deep desulfurization of diesel fuels by catalytic oxidation

Reaction feed was prepared by dissolving dibenzothiophene (DBT), which was selected as a model organosulfur compound in diesel fuels, in n-octane. The oxidant was a 30 wt-% aqueous solution of hydrogen peroxide. Catalytic performance of the activated carbons with saturation adsorption of DBT was investigated in the presence of formic acid. In addition, the effects of activated carbon dosage, formic acid concentration, initial concentration of hydrogen peroxide, initial concentration of DBT and reaction temperature on the oxidation of DBT were investigated. Experimental results indicated that performic acid and the hydroxyl radicals produced are coupled to oxidize DBT with a conversion ratio of 100%. Catalytic performance of the combination of activated carbon and formic acid is higher than that of only formic acid. The concentration of formic acid, activated carbon dosage, initial concentration of hydrogen peroxide and reaction temperature affect the oxidative removal of DBT. The higher the initial concentration of DBT in the n-octane solution, the more difficult the deep desulfurization by oxidation is.

Activated carbon adsorption of PAHs from vegetable oil used in soil remediation

Vegetable oil has been proven to be advantageous as a non-toxic, cost-effective and biodegradable solvent to extract polycyclic aromatic hydrocarbons (PAHs) from contaminated soils for remediation purposes. The resulting vegetable oil contained PAHs and therefore required a method for subsequent removal of extracted PAHs and reuse of the oil in remediation processes. In this paper, activated carbon adsorption of PAHs from vegetable oil used in soil remediation was assessed to ascertain PAH contaminated oil regeneration. Vegetable oils, originating from lab scale remediation, with different PAH concentrations were examined to study the adsorption of PAHs on activated carbon. Batch adsorption tests were performed by shaking oil-activated carbon mixtures in flasks. Equilibrium data were fitted with the Langmuir and Freundlich isothermal models. Studies were also carried out using columns packed with activated carbon. In addition, the effects of initial PAH concentration and activated carbon dosage on sorption capacities were investigated. Results clearly revealed the effectiveness of using activated carbon as an adsorbent to remove PAHs from the vegetable oil. Adsorption equilibrium of PAHs on activated carbon from the vegetable oil was successfully evaluated by the Langmuir and Freundlich isotherms. The initial PAH concentrations and carbon dosage affected adsorption significantly. The results indicate that the reuse of vegetable oil was feasible.

Generation of hydroxyl radical in aqueous solution by microwave energy using activated carbon as catalyst and its potential in removal of persistent organic substances

Salicylic acid (SA, hydroxybenzoic acid) was used as molecular probe to determine hydroxyl radical ( OH) in aqueous solutions, where microwave (MW) radiation was involved and activated carbon (AC) existed. Rapid reaction of OH with SA produced a stable fluorescence product, 2,3-dihydroxybenzoic acid (2,3-DHBA) as well as its homologue, 2,5-DHBA, which were determined by HPLC. Dose of AC, air supply, and MW power proved to be key factors influencing OH generation under MW radiation. The rate of OH generation reached 0.036 μmol s −1, and the OH yield reached 3.2 μmol in 3 min under a given condition. OH is an active species with high oxidative activity and can initiate free radical reactions. Another experiment was carried out to confirm that organic substance may degrade in aqueous solution by the MW assisted catalytic process. Pentachlorophenol (PCP) was used as target. Results showed that nearly 72–100% degradation (corresponding to 40% and 82% TOC removal) was observed in 60 min for two levels of PCP concentrations (500 and 2000 mg L −1), which implies that continuous production of OH during MW radiation supports free radical reactions that benefit PCP degradation.

Heterogeneous and homogeneous catalytic ozonation of benzothiazole promoted by activated carbon: Kinetic approach

Ozone oxidation combined with activated carbon adsorption (O 3/AC) has recently started to be developed as a single process for water and wastewater treatment. While a number of aspects of aqueous ozone decomposition are well understood, the importance and relationship between aqueous ozone decomposition and organic contaminant degradation in the presence of activated carbon is still not clear. This study focuses on determining the contribution of homogeneous and heterogeneous reactions to organic contaminants removal in O 3/AC system. Benzothiazole (BT) was selected as a target organic pollutant due to its environmental concern. A reactor system based on a differential circular flow reactor composed by a 19 cm 3 activated carbon fixed bed column and 1 dm 3 storage tank was used. Ozone was produced from pure and dry oxygen using an Ozocav ozone generator rated at 5 g O 3 h −1. Experimental results show that BT removal rate was proportional to activated carbon dosage. Activated carbon surface contribution to BT oxidation reactions with ozone, increased with pH in absence of radical scavengers. The radical reaction contribution within the pH range 2–11 accounted for 67–83% for BT removal in O 3/AC simultaneous treatment. Results suggest that at pH higher than the pH of the point of zero charge of the activated carbon dissociated acid groups such as carboxylic acid anhydrides and carboxylic acids present on activated carbon surface could be responsible for the observed increase in the ozone decomposition reaction rate. A simplified mechanism and a kinetic scheme representing the contribution of homogeneous and heterogeneous reactions on BT ozonation in the presence of activated carbon is proposed.

Modeling Polychlorinated Biphenyl Mass Transfer after Amendment of Contaminated Sediment with Activated Carbon

The sorption kinetics and concentration of polychlorinated biphenyls (PCBs) in historically polluted sediment is modeled to assess a remediation strategy based on in situ PCB sequestration by mixing with activated carbon (AC). We extend our evaluation of a model based on intraparticle diffusion by including a biomimetic semipermeable membrane device (SPMD) and a first-order degradation rate for the aqueous phase. The model predictions are compared with the previously reported experimental PCB concentrations in the bulk water phase and in SPMDs. The simulated scenarios comprise a marine and a freshwater sediment, four PCB congeners, two AC grain sizes, four doses of AC, and comparison with laboratory experiments for up to 540 days of AC amendment slowly mixed with sediment. The model qualitatively reproduces the observed shifts in the PCB distribution during repartitioning after AC amendment but systematically overestimates the overall effect of the treatment in reducing aqueous and SPMD concentrations of PCBs by a factor of 2-6. For our AC application in sediment, competitive sorption of the various solutes apparently requires a reduction by a factor of 16 of the literature values for the AC-water partitioning coefficient measured in pure aqueous systems. With this correction, model results and measurements agree within a factor of 3. We also discuss the impact of the nonlinearity of the AC sorption isotherm and first-order degradation in the aqueous phase. Regular mixing of the sediment accelerates the benefit of the proposed amendment substantially. But according to our scenario, after AC amendment is homogeneously mixed into the sediment and then left undisturbed, aqueous PCB concentrations tend toward the same reduction after approximately 5 or more years.

Effect of powdered activated carbon dosage on retarding membrane fouling in MBR

Three parallel membrane bioreactors (MBR) were operated to verify the effect of powdered activated carbon (PAC) dosage on membrane fouling for domestic wastewater treatment at PAC dosage of 0, 0.75 and 1.5 g/L, respectively. The results showed that the application of PAC was effective in decreasing the content of extracellular polymeric substances (EPS) inside the microbial floc at PAC dosage of 0.75 g/L. And the EPS deposited on the membrane was reduced with addition of PAC. This suggests that the permeable PAC particulate layer filter out microbial cells and colloids, and prevent them from reaching the membrane surface. The cake resistance decreased with the increase of PAC dosage. However, the irreversible fouling resistance was reduced effectively at PAC dosage of 0.75 g/L, and this effect weakened significantly at a higher dosage of 1.5 g/L. The degree of irreversible fouling was equal at PAC dosage of 0 and 1.5 g/L. Detailed studies on the effect of PAC dosage indicated that a PAC dosage of 0.75 g/L was the optimum in terms of organic removal and filtration flux.

2004 National Atrazine Occurrence Monitoring Program Using the Abraxis ELISA Method

The goal of this project was to gain a better understanding of atrazine occurrence in the United States by surveying drinking water utilities' sources and finished water for atrazine on a weekly basis for seven months. Atrazine is a contaminant of interest because the United States Environmental Protection Agency (USEPA) has found short-term atrazine exposure above the drinking water maximum contaminant level (MCL) to potentially cause heart, lung, and kidney congestion, low blood pressure, muscle spasms, weight loss, and damage to the adrenal glands. Long-term exposure to atrazine concentrations above the drinking water MCL has been linked to weight loss, cardiovascular damage, retinal and muscle degeneration, and cancer. This survey effort improved upon previously conducted atrazine surveys through intensive, high frequency sampling (participating plants sampled their raw and finished water on a weekly basis for approximately seven months). Such an intensive effort allowed the authors to gain a better understanding of short-term atrazine occurrence and its variability in drinking water sources. This information can benefit the drinking water industry by facilitating (1) better atrazine occurrence management (i.e., awareness when plants may be more susceptible to atrazine), (2) more efficient atrazine control (e.g., effective treatment alternatives and more effective response to atrazine occurrence), and (3) treatment cost reduction (e.g., efficient atrazine control can result in substantial cost savings). Forty-seven drinking watertreatment plants located primarily in the Midwestern United States participated in the survey and sampled their raw and finished water on a weekly basis from March through October. Samples were analyzed using the Abraxis enzyme-linked immunosorbent assay (ELISA) test kit. Confirmation samples for quality assurance/quality control (QA/QC) purposes were analyzed using solid-phase extraction (SPE) followed by gas chromatography mass spectrophotometry (GC/MS). Several important conclusions can be drawn from this study including (1) surface waters were confirmed to be more vulnerable to atrazine contamination than groundwater sources, (2) peak atrazine concentrations corresponded well to precipitation/runoff events, and (3) atrazine occurrence tended to be uniform geographically when compared by river drainage basins. In addition, this project confirmed that the Abraxis atrazine ELISA test kit tended to have a positive bias (i.e., the measured ELISA concentration was higher than the actual concentration) in most measured samples. Finished samples tended to have more of a positive bias than raw water samples. Therefore, this bias may limit the effectiveness for ELISA for regulatory monitoring. There are many other applications for ELISA, however, including frequent monitoring for early detections of atrazine concentration changes that might trigger conventional analysis by GC/MS or be used for activated carbon dosing or other treatment operating controls.

Penurunan COD (Chemical Oxygen Demand) dalam Limbah Cair Laboratorium Menggunakan Filter Karbon Aktif Arang Tempurung Kelapa

This research use activated carbon from coconut shell as a media to removal of COD in laboratory wastewater. The research is done at UII laboratory with use batch and continuous reactor. At the batch reactor use variation of pH : 7, 8, 9,10 and initial concentration of COD 85,8 mg/L, 81,3 mg/L, 61,2 mg/L, 53,8 mg/L. Doses of activated carbon is 1, 2, 3, 4 grams each 200 ml of wastewater. Rapid mixing at the batch reactor is 150 rpm as long as 2 hours. For continuous reactor is make with series, that is two reactor column. In this system is used wastewater with initial concentration of COD 97,68 mg/L and pH 8. The sampling is taken each one hours after reactor have operation. The result of sample test show that activated carbon from coconut shell be able to removal of COD in the laboratory wastewater. The biggest efficiency of removal is 68 % at the activated carbon doses 4 mg and pH 8. Models of sorption with isotherm Freundlich is gotten equation x/m = 0,0009*C0,3281. On the continuous reactor, the biggest efficiency of removal are 61,99 % at the first column and 75 % at the second column. Based on the result of this research, so that activated carbon form coconut shell possible to applied as a alternative treatment to laboratory wastewater, specially wastewater on the UII laboratory. keyword : Activated carbon, Laboratory wastewater, Removal of COD

Effects of dose and particle size on activated carbon treatment to sequester polychlorinated biphenyls and polycyclic aromatic hydrocarbons in marine sediments

Recent laboratory studies show that mixing activated carbon with contaminated sediment reduces the chemical and biological availability of hydrophobic organic contaminants. In this study, we test the effects of varying the activated carbon dose and particle size in reducing the aqueous availability of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) and the uptake of PCBs by two benthic organisms. We mixed PCB- and PAH-contaminated sediment from Hunters Point Naval Shipyard, San Francisco Bay (CA, USA), for one month with activated carbon, at doses of 0.34, 1.7, and 3.4% dry mass basis. We found that increasing the carbon dose increased the effectiveness in reducing PCB bioaccumulation. In 56-d uptake tests with the benthic organisms Neanthes arenaceodentata and Leptocheirus plumulosus, PCB bioaccumulation was reduced by 93 and 90%, respectively, with 3.4% carbon. Increasing the dose also increased the effectiveness in reducing PCB and PAH aqueous concentrations and uptake by semipermeable membrane devices and quiescent flux of PCBs to overlying water. Decreasing activated carbon particle size increased treatment effectiveness in reducing PCB aqueous concentration, and larger-sized activated carbon (400-1,700 microm) was ineffective with a contact period of one month. We invoke a numerical model based on intraparticle diffusion in sediment and activated carbon particles to help interpret our experimental results. This model was useful in explaining the trends for the effect of activated carbon dose and particle size on PCB aqueous concentrations in well-mixed systems.

The sequestration of PCBs in Lake Hartwell sediment with activated carbon

Recent laboratory studies with a marine sediment from a tidal mudflat in San Francisco Bay demonstrated that a low dose of activated carbon enhances the sequestration of PCBs and PAHs under well-mixed conditions. Here we compare activated carbon treatment for a freshwater sediment from a rural site, using PCB-contaminated sediment from Lake Hartwell, SC. An activated carbon dose of 2% of the dry sediment mass lowered total aqueous PCB concentrations by more than 95% after 1 month of treatment and more than 98% after 6 months. Aqueous PCB concentrations remained below detection limits following 18 months of sediment–carbon contact in slowly mixed systems. Uptake of PCBs into semipermeable membrane devices was reduced by 78%, 91% and 97% over 1, 6 and 18 months, respectively. These tests show that PCB sequestration with activated carbon improves with contact time and is not diminished by prolonged mixing with sediment. Desorption studies confirmed the sequestration, in which 74% of the total PCB mass could be desorbed from untreated Lake Hartwell sediment within 30 days as compared to only 7% after activated carbon treatment for 6 months. We compare these observations with marine sediment from San Francisco Bay and propose a conceptual model to rationalize how sediment properties relate to the reduction in aqueous PCB concentrations during activated carbon treatment.

Removal of some reactive dyes from synthetic wastewater by combined Al(III) coagulation/carbon adsorption process

This study was designed to investigate the removal of reactive dyes, C.I. Reactive Red 45 and C.I. Reactive Green 8, from wastewater using a two-step, Al(III) coagulation/activated carbon adsorption method. The effect of pH and coagulant dosage as well as the effects of contact time and a powdered activated carbon dosage on dye removal have been studied. The process was optimized with reasonable consumption of coagulant and quantity of obtained sludge. Coagulation as a main treatment process followed by adsorption achieved almost the total elimination of both dyes from wastewater with significant reduction (90%) of chemical oxygen demand (COD), total organic carbon (TOC) and adsorbable organic halide (AOX). Besides high efficiency of dye removal, the combined treatment process offers many advantages for potential application such as coagulant savings, minimal amount of sludge formation and also a economic feasibility since it does not require high costs for chemicals and equipment.

Influences on the removal of tastes and odours by PAC

Geosmin and 2 methylisoborneol (MIB) are earthy/musty odour compounds produced as secondary metabolites by some cyanobacteria and actinomycetes. At levels as low as 5–10 ng l −1 in drinking water they can result in consumer complaints, and consequently their removal from potable water is a priority for many suppliers. In water treatment plants where taste and odour episodes are common, the homogenous surface diffusion model (HSDM) can be used to estimate powdered activated carbon dose requirements under a range of conditions including inlet concentration and plant flow rates (controlling PAC contact time). Water quality, in particular dissolved organic carbon concentration and character, can affect the required doses of PAC, as can the addition of chemicals such as alum and chlorine. There is significant scope for the more cost-effective utilisation of powdered activated carbon for the mitigation of taste and odours, as well as a wider range of other micro-contaminants, using tools such as the HSDM, in conjunction with knowledge of the effects of water quality and water treatment processes.

Pretreatment Effects on Activated Carbon Adsorption of Humic Substances: Distributed Fictive Component Analysis

Diluted solutions of a peat water and a biological wastewater effluent were subjected to coagulation, ozonation, and chlorination. The effects of these pretreatments on the removal of humic substances by activated carbon adsorption were tested. Batch adsorption isotherms were analyzed using a distributed fictive component method, which assumed a logarithmic normal distribution of Freundlich K and a non-adsorbable fraction in the Ideal Adsorption Solution Theory for multicomponent adsorption. Coagulation treatment not only increased adsorbabilities but also decreased its heterogeneity in terms of multicomponent adsorbates. Ozonation decreased adsorbabilities and increased heterogeneity in adsorbabilities by producing weakly adsorbing compounds. The average Freundlich K decreased, while the Freundlich exponent, 1/n, and non-adsorbable fraction was increased with ozone consumption. Chlorination showed the same effects as ozonation. However, after ozonation, equilibrium capacity at low activated carbon doses was increased because of the reduced dissolved organic carbon concentration by ozonation.

A Simplified Method to Determine The Powdered Activated Carbon Dose Required to Remove Methylisoborneol

Equilibrium data obtained from a natural water with several different initial concentrations of 2-methylisoborneol (MIB) plot as a single line on a percent remaining, Ce/Co × 100%, versus carbon dose, Cc, plot. This indicates that the percent removal of MIB is independent of its initial concentration in natural water for a given PAC dose. The relationship is specific for each type of PAC, and it is not valid at very high MIB concentrations, however. These data show that, predicting the minimum amount of carbon necessary to effectively mitigate any MIB episode, may be accomplished by analyzing a single bottle-point isotherm. The robustness of this approach was shown through the use of four water sources, fourteen different carbons, and MIB concentrations ranging from 45 ng/l to 178 μg/l.

Real-Time Water Treatment Process Control with Artificial Neural Networks

The coagulation, flocculation, and sedimentation processes involve many complex physical and chemical phenomena and thus are difficult to model for process control with traditional methods. Proposed is the use of a neural network process control system for the coagulation, flocculation, and sedimentation processes. Presented is a review of influential control parameters and control requirements for these processes followed by the development of a feed forward neural network control scheme. A neural network process model was built based on nearly 2,000 sets of process control data. This model formed the major component of a software controller and was found to consistently predict the optimum alum and power activated carbon doses for different control actions. With minor modifications, the approach illustrated can be used for building control models for other water treatment processes.

A simplified method to determine the powdered activated carbon dose required to remove methylisoborneol

Equilibrium data obtained from a natural water with several different initial concentrations of 2-methylisoborneol (MIB) plot as a single line on a percent remaining, Ce/Co × 100%, versus carbon dose, Cc, plot. This indicates that the percent removal of MIB is independent of its initial concentration in natural water for a given PAC dose. The relationship is specific for each type of PAC, and it is not valid at very high MIB concentrations, however. These data show that, predicting the minimum amount of carbon necessary to effectively mitigate any MIB episode, may be accomplished by analyzing a single bottle-point isotherm. The robustness of this approach was shown through the use of four water sources, fourteen different carbons, and MIB concentrations ranging from 45 ng/l to 178 μg/l.