Microplastics and associated chemicals in drinking water: A review of their occurrence and human health implications.

Microplastics (MPs) have entered drinking water (DW) via various pathways, raising concerns about their potential health impacts. This study provides a comprehensive review of MP-associated chemicals, such as oligomers, plasticizers, stabilizers, and ultraviolet (UV) filters that can be leached out during DW treatment and distribution. The leaching of these chemicals is influenced by various environmental and operating factors, with three major ones identified: MP concentration and polymer type, pH, and contact time. The leaching process is substantially enhanced during the disinfection step of DW treatment, due to ultraviolet light and/or disinfectant-triggered reactions. The study also reviewed human exposure to MPs and associated chemicals in DW, as well as their health impacts on the human nervous, digestive, reproductive, and hepatic systems, especially the neuroendocrine toxicity of endocrine-disrupting chemicals. An overview of MPs in DW, including tap water and bottled water, was also presented to enable a background understanding of MPs-associated chemicals. In short, certain chemicals leached from MPs in DW can have significant implications for human health and demand further research on their long-term health impacts, mitigation strategies, and interactions with other pollutants such as disinfection byproducts (DBPs) and per- and polyfluoroalkyl substances (PFASs). This study is anticipated to facilitate the research and management of MPs in DW and beverages.

Solar photocatalytic degradation of total organic halogen in water using TiO2 catalyst.

Disinfection byproducts (DBPs) in treated wastewater effluents pose environmental and health risks during water reuse. Solar-TiO2 photocatalysis is a promising technology for degrading organic pollutants in treated effluents. In this study, total organic halogen (TOX) was used as an analytical tool to determine the efficiency of solar-TiO2 photocatalytic process for the dehalogenation of DBPs in water. Natural solar photocatalytic experiments using TiO2 particles were conducted to evaluate dehalogenation kinetics of different TOX groups formed by fulvic acid including total organic chlorine (TOCl), bromine (TOBr) and iodine (TOI). The results showed that the mixed phase TiO2 (Aeroxide P25) was much more effective at TOX removal than the anatase (Hombikat UV-100) and rutile (TiOxide) TiO2 particles. The TOX photocatalytic degradation rates of different halogen substituents ranked as TOI > TOCl (NH2Cl) > TOBr > TOCl (Cl2). The TOX removal followed first-order kinetics with half-lives of 42.8, 11.0, 5.0 and 2.7 min for TOCl (Cl2), TOBr, TOCl (NH2Cl), and TOI, respectively, at the 100 mg L-1 TiO2 dose. The TOX dehalogenation was enhanced at pH 9 compared to pH 5, and the addition of hydrogen peroxide had limited improvement in the TOX removal. Hydrophobic and molecular weight (MW) > 1 kDa fractions of TOCl (Cl2) were more susceptible to the solar photocatalytic process than the hydrophilic and MW < 1 kDa fractions. The solar-TiO2 photocatalytic process also effectively removed TOX in chlorinated and chloraminated wastewater samples. The results of this study suggest that the solar-TiO2 photocatalysis is an effective treatment technology for TOX removal in water reuse.

Effects of different pairing configurations of woodchips and steel chips in dual media treatment systems on nutrient removal and organics and iron leaching.

Nitrogen and phosphorus are two primary nutrients that can promote eutrophication in aquatic ecosystems. Recycled steel chips have been proposed to be used in conjunction with woodchips in dual-media treatment systems to remove nutrients from water, but the effects of different pairing configurations of woodchips and steel chips on nutrient removal have not been fully understood. The use of woodchips and steel chips for water treatment can result in leaching of organic carbon and iron. However, little is known about the impact of different media configurations on organics and iron leaching. In this study, laboratory column reactors using woodchips and steel chips (volumetric ratio of 11:1) were constructed based on three pairing configurations: woodchips/steel chips, steel chips/woodchips, and mixture of the two media. The column reactors were operated to evaluate nitrate and phosphate removal efficiencies and organic carbon and iron leaching from different media pairing arrangements. The results showed that the three media pairing configurations achieved similar overall nitrate and phosphate removal efficiencies but resulted in substantially different organic carbon and iron concentrations in reactor effluents. Steel chips, when placed downstream of woodchips reduced reactor organic carbon leaching, whereas woodchips, when placed downstream of steel chips reduced reactor iron leaching. The mixed media reactor was able to effectively control both organic carbon and iron leaching. The results of flow and temperature variation experiments showed that phosphate removal efficiencies by the steel chip filter were much less affected by flow and temperature changes than nitrate removal efficiencies by the woodchip bioreactor.

Determination of nitrate removal kinetics model parameters in woodchip bioreactors.

Woodchip bioreactors have emerged as a viable water management tool to reduce nitrate contamination from agricultural subsurface drainage, wastewater, and stormwater. Understanding of denitrification kinetics is critical to the design and application of field woodchip bioreactors. The denitrification process in woodchip bioreactors generally obeys a model of Michaelis-Menten type enzyme kinetics. The objective of this study was to determine Michaelis-Menten model parameters for nitrate removal in laboratory bioreactors using the fresh, composted and aged woodchips. The results showed that the maximum nitrate removal rates (Vmax) were 2.09, 0.88 and 0.30 mg N/L/h, and the half saturation constants (Km) were 2.60, 2.16 and 2.01 mg N/L for the composted, fresh and aged woodchip bioreactors at 22 °C. The Vmax values decreased to 0.26 and 0.05 mg N/L/h, and the Km values decreased to 1.74 and 1.19 mg N/L when the composted and fresh woodchip bioreactors were operated at 5 °C. Denitrification in woodchip bioreactors can be operationally defined as a zero-order reaction when treating contaminated water with nitrate much higher than the Km values. The nitrate removal efficiency of the bioreactors followed the order of composted woodchips > fresh woodchips > aged woodchips. The average nitrate load reduction rates were 8.81-21.0, 7.36-9.78, and 2.46-3.54 g N/m3/d for the composted, fresh, and aged woodchip bioreactors at influent nitrate concentrations of 10-50 mg N/L and 22 °C. Woodchip composting before bioreactor installation can be used as a practical strategy to enhance denitrification performance of bioreactors.

Fixed bed column evaluation of phosphate adsorption and recovery from aqueous solutions using recycled steel byproducts.

Excessive phosphorus loading from anthropogenic sources is a major cause of eutrophication of natural waters. Phosphorus is also a non-renewable natural resource that cannot be substituted with other sources. The objective of this study was to determine the feasibility of using recycled steel byproducts to remove and recover phosphate from aqueous solutions. Laboratory fixed bed column experiments were conducted with recycled steel chips of different sizes to evaluate phosphate adsorption characteristics and phosphate recovery efficiencies using alkaline solutions. The results showed that phosphate adsorption onto steel chip filters was characterized by an initial fast breakthrough followed by a stable removal phase. The cumulative phosphate adsorption capacities of the steel chips were 8.43-10.4 mg P/g following 4800 empty bed volumes with a 3 min contact time and an initial concentration of 10 mg P/L. The phosphate adsorption onto steel chips was favored at low flow rates, low pH values, and low organic carbon concentrations. Sodium hydroxide solutions effectively desorbed phosphate from the steel chips. The total phosphate desorption percentages were 58.9%, 64.2%, and 83.4% after 120 empty bed volumes using 0.05 M, 0.10 M, and 0.20 M NaOH solutions, respectively. Steel chips also exhibited high phosphate adsorption and desorption capacities when treating agricultural subsurface drainage water, municipal wastewater, and stormwater runoff. Overall, the results of this study suggest that recycled steel byproducts are efficient and promising low-cost phosphate capturing materials for sustainable phosphorus management.

Evaluation of industrial by-products and natural minerals for phosphate adsorption from subsurface drainage.

Agricultural subsurface drainage has been recognized as an important pathway for phosphorus transport from soils to surface waters. Reactive permeable filters are a promising technology to remove phosphate from subsurface drainage. Three natural minerals (limestone, zeolite, and calcite) and five industrial by-products (steel slag, iron filings, and three recycled steel by-products) were evaluated for phosphate removal from subsurface drainage using batch adsorption experiments. Phosphate adsorption onto these materials was characterized by Langmuir isotherm and second-order kinetic models. The adsorption capacities increased by factors of 1.2-2.5 when temperature was increased from 5°C to 30°C. Industrial by-products exhibited phosphate adsorption capacities that were one order of magnitude higher than natural minerals. Medium-sized steel chips exhibited high phosphate adsorption capacities (1.64-3.38 mg/g) across different temperatures, pH values, organic matter concentrations, and real drainage water matrixes. The strong chemical bonds between phosphate and steel by-products prevented the release of adsorbed phosphate back to the solution. The steel by-product filter can be paired with a woodchip bioreactor for nitrate and phosphate removal. It is suggested that the phosphate filter be connected to a woodchip bioreactor after the startup phase to minimize the impact of dissolved organic matter on phosphate adsorption. The results of this study suggest that the low-cost steel by-products examined could be used as effective adsorption media for phosphate removal from subsurface drainage.

Effect of temperature and pH on dehalogenation of total organic chlorine, bromine and iodine in drinking water.

Disinfection byproduct (DBP) concentrations in drinking water distribution systems and indoor water uses depend on competitive formation and degradation reactions. This study investigated the dehalogenation kinetics of total organic chlorine (TOCl), bromine (TOBr) and iodine (TOI) produced by fulvic acid under different pH and temperature conditions, and total organic halogen (TOX) variations in a treated drinking water under simulated distribution system and heating scenarios. TOX dehalogenation rates were generally in the order of TOI â‰… TOCl(NH2Cl) > TOBr > TOCl(Cl2). The half-lives of different groups of TOX compounds formed by fulvic acid varied between 27 and 139 days during incubation at 20 °C and 0.98-2.17 days during heating at 55 °C. Base-catalyzed reactions played a major role in TOX degradation as evidenced by enhanced dehalogenation under high pH conditions. The results of heating of a treated water in the presence of residuals showed that TOX concentrations of chlorinated samples increased rapidly when chlorine residuals were present and then gradually decreased after chlorine residuals were exhausted. The final TOX concentrations of chlorinated samples after heating showed moderate decreases with increasing ambient water ages. Chloraminated samples with different ambient water ages exhibited similar final TOX concentrations during simulated distribution system and heating experiments. This study reinforces the importance of understanding DBP variations in indoor water uses as wells as in distribution systems to provide more accurate DBP information for exposure assessment and regulatory determination.

Characterization of dissolved organic carbon leached from a woodchip bioreactor.

Woodchip bioreactors are increasingly being applied to remove nitrate from agricultural subsurface drainage. However, dissolved organic carbon (DOC) released from woodchips may negatively affect the aquatic ecosystems and drinking water supplies. The objective of this study was to evaluate the leaching characteristics, disinfection byproduct (DBP) formation potentials, and treatability of DOC derived from a laboratory woodchip bioreactor. Initial flush of woodchips resulted in the release of high organic content from woodchips. The DOC concentration in the bioreactor effluent decreased rapidly from 71.8 to 20.7 mg/L during the first week of operation, and then gradually decreased to 3.0 mg/L after 240 days of operation under a hydraulic retention time of 24 h. A recycled steel chip filter removed an average of 44.2% of the DOC in the bioreactor effluent. Hydrophobic carbons and organic compounds with molecular weight of 10-100 KDa were the most abundant organic fractions in the DOC released from woodchips. These two DOC fractions were also the most important precursors to the formation of total organic halogen (TOX) during chlorination and chloramination. The TOX yields of woodchip DOC were similar to those of Suwannee River Fulvic Acid, suggesting that organic compounds released from woodchips have great potentials for DBP formation. Alum and polyaluminium chloride were more effective at removing woodchip DOC than ferric chloride during coagulation. Drinking water treatment plants may need to adjust coagulant types and doses in order to remove woodchip DOC in the source water to reduce the DBP formation potential.

Photolytic dehalogenation of disinfection byproducts in water by natural sunlight irradiation.

The aqueous photolysis of halogenated disinfection byproducts (DBPs) by natural sunlight irradiation was studied to determine their photolytic dehalogenation kinetics. Total organic halogen analysis was used to quantify the dehalogenation extents of DBPs during outdoor photolysis experiments. Dichloroacetamide, chloral hydrate, chloroform, dichloroacetonitrile, monochloro-, monobromo-, dichloro-, dibromo-, and trichloroacetic acids were generally resistant to photolytic dehalogenation and showed less than 10% reduction after 6 h sunlight irradiation. Monoiodoacetic acid, tribromoacetic acid, bromoform, dibromoacetonitrile, and trichloronitromethane showed moderate to high dehalogenation degrees with half-lives of 4.0-19.3 h. Diiodoacetic acid, triiodoacetic acid, and iodoform degraded rapidly under the sunlight irradiation and exhibited half-lives of 5.3-10.2 min. In general, the photosensitive cleavage of carbon-halogen bonds of DBPs increased with increasing number of halogens (tri- > di- > mono-halogenated) and size of the substituted halogens (I > Br > Cl). Nitrate, nitrite, and pH had little impact on the photodehalogenation of DBPs under typical levels in surface waters. The presence of natural organic matter (NOM) inhibited the photodehalogenation of DBPs by light screening. The NOM inhibiting effects were more pronounced for the fast degrading iodinated DBPs. The results of this study improve our understanding about the photolytic dehalogenation of wastewater-derived DBPs in surface waters during water reuse.

Nitrate and phosphate removal from agricultural subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters.

Woodchip bioreactors have been increasingly used as an edge-of-field treatment technology to reduce the nitrate loadings to surface waters from agricultural subsurface drainage. Recent studies have shown that subsurface drainage can also contribute substantially to the loss of phosphate from agricultural soils. The objective of this study was to investigate nitrate and phosphate removal in subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters. The woodchip bioreactor demonstrated average nitrate removal efficiencies of 53.5-100% and removal rates of 10.1-21.6 g N/m(3)/d for an influent concentration of 20 mg N/L and hydraulic retention times (HRTs) of 6-24 h. When the influent nitrate concentration increased to 50 mg N/L, the bioreactor nitrate removal efficiency and rate averaged 75% and 18.9 g N/m(3)/d at an HRT of 24 h. Nitrate removal by the woodchips followed zero-order kinetics with rate constants of 1.42-1.80 mg N/L/h when nitrate was non-limiting. The steel byproduct filter effectively removed phosphate in the bioreactor effluent and the total phosphate adsorption capacity was 3.70 mg P/g under continuous flow conditions. Nitrite accumulation occurred in the woodchip bioreactor and the effluent nitrite concentrations increased with decreasing HRTs and increasing influent nitrate concentrations. The steel byproduct filter efficiently reduced the level of nitrite in the bioreactor effluent. Overall, the results of this study suggest that woodchip denitrification followed by steel byproduct filtration is an effective treatment technology for nitrate and phosphate removal in subsurface drainage.

Natural solar photolysis of total organic chlorine, bromine and iodine in water.

Municipal wastewater has been increasingly used to augment drinking water supplies due to the growing water scarcity. Wastewater-derived disinfection byproducts (DBPs) may negatively affect the aquatic ecosystems and human health of downstream communities during water reuse. The objective of this research was to determine the degradation kinetics of total organic chlorine (TOCl), bromine (TOBr) and iodine (TOI) in water by natural sunlight irradiation. Outdoor solar photolysis experiments were performed to investigate photolytic degradation of the total organic halogen (TOX) formed by fulvic acid and real water and wastewater samples. The results showed that TOX degradation by sunlight irradiation followed the first-order kinetics with half-lives in the range of 2.6-10.7 h for different TOX compounds produced by fulvic acid. The TOX degradation rates were generally in the order of TOI > TOBr â‰… TOCl(NH2Cl) > TOCl(Cl2). High molecular weight TOX was more susceptible to solar photolysis than corresponding low molecular weight halogenated compounds. The nitrate and sulfite induced indirect TOX photolysis rates were less than 50% of the direct photolysis rates under the conditions of this study. Fulvic acid and turbidity in water reduced TOX photodegradation. These results contribute to a better understanding of the fate of chlorinated, brominated and iodinated DBPs in surface waters.

Correlation between SUVA and DBP formation during chlorination and chloramination of NOM fractions from different sources.

Natural organic matter (NOM) is the major precursor to the formation of disinfection byproducts (DBPs) during drinking water treatment. Specific ultraviolet absorbance (SUVA) is a widely used surrogate parameter to characterize NOM and predict its DBP formation potential. The objective of this study was to determine the relationships between SUVA and different classes of DBPs formed by NOM fractions from different sources. Three natural waters with a wide SUVA range were fractionated into differing hydrophobicity and molecular weight groups using XAD-4 and XAD-8 resins and ultrafiltration membranes. Each NOM fraction was treated with chlorine and monochloramine under controlled laboratory conditions. Different classes of DBPs showed different relationships with SUVA. SUVA correlated strongly with trihaloacetic acids (THAAs) and unknown total organic halogen (UTOX) yields whereas weak correlations were observed between SUVA and trihalomethane (THM) and dihaloacetic acid (DHAA) yields during chlorination. These results reinforce the hypothesis that DHAAs and THAAs form through different precursors and reaction pathways. Strong correlation between SUVA and UTOX was also observed during chloramination. However, no significant relationship was observed between SUVA and chloramination THMs and DHAAs. Overall, SUVA is a good indicator for the formation of unknown DBPs. This indicates that UV absorbing compounds and aromatic carbon within NOM are the primary sources of precursors for unknown DBPs.

Disinfection byproduct formation from lignin precursors.

Lignin is the most abundant aromatic plant component in terrestrial ecosystems. This study was conducted to determine the contribution of lignin residues in natural water to the formation of disinfection byproducts (DBPs) in drinking water. We investigated the formation of different classes of DBPs from lignin model compounds, lignin polymers, and humic substances using two common disinfection techniques, chlorination and chloramination. The contributions of lignin to the overall formation of DBPs from these organic products were determined based on the observed abundances of individual lignin phenols and their DBP yields. Model lignin phenols generally produced higher trichloroacetic acid (TCAA) yields than chloroform and dichloroacetic acid (DCAA) during chlorination. Lignin phenols generally produced higher DBP yields but lower percentages of unknown total organic halogen compared to bulk humic substances and lignin polymers. The relative significance of lignin phenols as chlorination DBP precursors generally follows the order of TCAA > DCAA&chloroform. The relative significance of lignin phenols to DBP formation by chloramination follows the order: TCAA > DCAA&DCAN > chloroform. Overall, lignin phenols are more important as TCAA precursors than as chloroform and DCAA precursors.

Effect of pre-ozonation on the formation and speciation of DBPs.

The objective of this study was to quantitatively evaluate the effect of pre-ozonation on the formation and speciation of disinfection byproducts (DBPs) from subsequent chlorination and chloramination. Laboratory experiments were conducted on six diverse natural waters with low to medium bromide concentrations. Four groups of DBPs were investigated in this study: trihalomethanes (THMs), trihaloacetic acids (THAAs), dihaloacetic acids (DHAAs), and dihaloacetonitriles (DHANs). The results showed that the relative destructions of chlorination DBP precursors by ozone generally follow the order of DHANs > THMs & THAAs > DHAAs. Pre-ozonation substantially increased the DHAA precursors in the waters with low specific ultraviolet absorbance values. Pre-ozonation shifted the formation of DBPs to more brominated species. The bromine substitution factors (BSF) of different chlorination DBPs typically increased by 1-8 percentage points after ozonation. Pre-ozonation reduced the yields of chloramination DHAAs and THMs and increased the BSFs of chloramination DHAAs by 1-6 percentage points.

Evaluation of bromine substitution factors of DBPs during chlorination and chloramination.

Bromine substitution factor (BSF) was used to quantify the effects of disinfectant dose, reaction time, pH, and temperature on the bromine substitution of disinfection byproducts (DBPs) during chlorination and chloramination. The BSF is defined as the ratio of the bromine incorporated into a given class of DBPs to the total concentration of chlorine and bromine in that class. Four classes of DBPs were evaluated: trihalomethanes (THMs), dihaloacetonitriles (DHANs), dihaloacetic acids (DHAAs) and trihaloacetic acids (THAAs). The results showed that the BSFs of the four classes of DBPs generally decreased with increasing reaction time and temperature during chlorination at neutral pH. The BSFs peaked at a low chlorine dose (1 mg/L) and decreased when the chlorine dose further increased. The BSFs of chlorination DBPs at neutral pH are in the order of DHAN > THM & DHAA > THAA. DHAAs formed by chloramines exhibited distinctly different bromine substitution patterns compared to chlorination DHAAs. Brominated DBP formation was generally less affected by the pH change compared to chlorinated DBP formation.

Characterization of disinfection byproduct precursors based on hydrophobicity and molecular size.

Natural organic matter (NOM) from five water sources was fractionated using XAD resins and ultrafiltration membranes into different groups based on hydrophobicity and molecular weight (MW), respectively. The disinfection byproduct formation from each fraction during chlorination and chloramination was studied. In tests using chlorination, hydrophobic and high MW (e.g., >0.5 kDa) precursors produced more unknown total organic halogen (UTOX) than corresponding hydrophilic and low MW (e.g., <0.5 kDa) precursors. Trihaloacetic acid (THAA) precursors were more hydrophobic than trihalomethane (THM) precursors. The formation of THM and THAA was similar among different fractions for a water with low humic content. Hydrophilic and low MW (<0.5 kDa) NOM fractions gave the highest dihaloacetic acid (DHAA) yields. No significant difference was found for DHAA formation among different NOM fractions during chloramination. Increasing pH from 6 to 9 led to lower TOX formation for hydrophobic and high MW NOM fractions but had little impact on TOX yields from hydrophilic and low MW fractions. Bromine and iodine were more reactive with hydrophilic and low MW precursors as measured by THM or HAA formation than their corresponding hydrophobic and high MW precursors. However, hydrophobic and high MW precursors produced more UTOX when reacting with bromine and iodine.

Comparison of disinfection byproduct formation from chlorine and alternative disinfectants.

Seven diverse natural waters were collected and treated in the laboratory under five oxidation scenarios (chlorine, chloramine, both with and without preozonation, and chlorine dioxide). The impact of these disinfectants on the formation of disinfection byproducts was investigated. Results showed that preozonation decreased the formation of trihalomethanes (THMs), haloacetic acids (HAAs) and total organic halogen (TOX) for most waters during postchlorination. A net increase in THMs, HAAs and TOX was observed for a water of low humic content. Either decreases or increases were observed in dihaloacetic acids and unknown TOX (UTOX) as a result of preozonation when used with chloramination. Chloramines and chlorine dioxide produced a higher percentage of UTOX than free chlorine. They also formed more iodoform and total organic iodine (TOI) than free chlorine in the presence of iodide. Free chlorine produced a much higher level of total organic chlorine (TOCl) and bromine (TOBr) than chloramines and chlorine dioxide in the presence of bromide.

Effect of bromide and iodide ions on the formation and speciation of disinfection byproducts during chlorination.

Two natural waters were fortified with various levels of bromide or iodide ions (0-30 microM) and chlorinated in the laboratory to study the impact of bromide and iodide ions on the formation and speciation of disinfection byproducts. Trihalomethanes (THMs), haloacetic acids (HAAs), total organic halogen (TOX), and its halogen-specific fractions total organic chlorine (TOCl), bromine (TOBr), and iodine (TOI), were measured in this work. The molar yields of THMs and HAAs increased as the initial bromide concentration increased. No significant change in TOX concentration was found for varying bromide concentrations. However, TOX concentrations decreased substantially with increasing initial iodide concentrations. At higher levels of bromide, there was a decreasing level of unknown TOX and unknown TOCl but an increasing level of unknown TOBr. The extent of iodine substitution was much lower than that of bromine substitution when comparing identical initial concentrations because a substantial amount of iodide was oxidized to iodate by chlorine. The tendency toward iodate formation resulted in the unusual situation where higher chlorine doses actually caused reduced levels of iodinated organic byproducts. Quantitative assessment of the results of this study showed a good agreement with kinetic data in the literature.

Determination of TOCl, TOBr and TOI in drinking water by pyrolysis and off-line ion chromatography.

The objective of this research was to determine the optimum total organic halogen (TOX) protocol for use with ion chromatographic (IC) detection to analyze total organic chlorine (TOCl), bromine (TOBr), and iodine (TOI) in drinking water simultaneously. Two commercial analyzers (one using a pure O2 carrier and one using O2/CO2 mixture) and three commercially available activated carbons (two coconut-based and one bituminous coal-based) were examined in this study. Results showed that the pyrolytic analyzer using pure O2 and off-line IC combined with a standard TOX carbon (coconut-based) achieved the most complete recovery of TOCl, TOBr and TOI for both model compounds and real samples. There was no obvious difference between the two analyzers when used in microcoulometric detection mode. The TOX method is moderately sensitive to nitrate rinse volume. The monohaloacetic acids were partly washed out during sample preparation. This problem was solved by a modified nitrate rinsing solution.