Vladana N. Rajaković-Ognjanović

Determination of inorganic arsenic species in natural waters--benefits of separation and preconcentration on ion exchange and hybrid resins.

A simple method for the separation and determination of inorganic arsenic (iAs) species in natural and drinking water was developed. Procedures for sample preparation, separation of As(III) and As(V) species and preconcentration of the total iAs on fixed bed columns were defined. Two resins, a strong base anion exchange (SBAE) resin and a hybrid (HY) resin were utilized. The inductively-coupled plasma-mass spectrometry method was applied as the analytical method for the determination of the arsenic concentration in water. The governing factors for the ion exchange/sorption of arsenic on resins in a batch and a fixed bed flow system were analyzed and compared. Acidity of the water, which plays an important role in the control of the ionic or molecular forms of arsenic species, was beneficial for the separation; by adjusting the pH values to less than 8.00, the SBAE resin separated As(V) from As(III) in water by retaining As(V) and allowing As(III) to pass through. The sorption activity of the hydrated iron oxide particles integrated into the HY resin was beneficial for bonding of all iAs species over a wide range of pH values from 5.00 to 11.00. The resin capacities were calculated according to the breakthrough points in a fixed bed flow system. At pH 7.50, the SBAE resin bound more than 370 microg g(-1) of As(V) while the HY resin bound more than 4150 microg g(-1) of As(III) and more than 3500 microg g(-1) of As(V). The high capacities and selectivity of the resins were considered as advantageous for the development and application of two procedures, one for the separation and determination of As(III) (with SBAE) and the other for the preconcentration and determination of the total arsenic (with HY resin). Methods were established through basic analytical procedures (with external standards, certified reference materials and the standard addition method) and by the parallel analysis of some samples using the atomic absorption spectrometry-hydride generation technique. The analytical properties of both procedures were similar: the limit of detection was 0.24 microg L(-1), the limit of quantification was 0.80 microg L(-1) and the relative standard deviations for samples with a content of arsenic from 10.00 to 300.0 microg L(-1) ranged from 1.1 to 5.8%. The interference effects of anions commonly found in water and some organic species which can be present in water were found to be negligible. Verification with certified reference materials proved that the experimental concentrations found for model solutions and real samples were in agreement with the certified values.

Separation and determination of arsenic species in water by selective exchange and hybrid resins.

A simple and efficient method for separation and determination of inorganic arsenic (iAs) and organic arsenic (oAs) in drinking, natural and wastewater was developed. If arsenic is present in water prevailing forms are inorganic acids of As(III) and As(V). oAs can be found in traces as monomethylarsenic acid, MMA(V), and dimethylarsenic acid, DMAs(V). Three types of resins: a strong base anion exchange (SBAE) and two hybrid (HY) resins: HY-Fe and HY-AgCl, based on the activity of hydrated iron oxides and a silver chloride were investigated. It was found that the sorption processes (ion exchange, adsorption and chemisorptions) of arsenic species on SBAE (ion exchange) and HY resins depend on pH values of water. The quantitative separation of molecular and ionic forms of iAs and oAs was achieved by SBAE and pH adjustment, the molecular form of As(III) that exists in the water at pH <8.0 was not bonded with SBAE, which was convenient for direct determination of As(III) concentration in the effluent. HY-Fe resin retained all arsenic species except DMAs(V), which makes possible direct measurements of this specie in the effluent. HY-AgCl resin retained all iAs which was convenient for direct determination of oAs species concentration in the effluent. The selective bonding of arsenic species on three types of resins makes possible the development of the procedure for measuring and calculation of all arsenic species in water. In order to determine capacity of resins the preliminary investigations were performed in batch system and fixed bed flow system. Resin capacities were calculated according to breakthrough points in a fixed bed flow system which is the first step in designing of solid phase extraction (SPE) module for arsenic speciation separation and determination. Arsenic adsorption behavior in the presence of impurities showed tolerance with the respect to potential interference of anionic compounds commonly found in natural water. Proposed method was established performing standard procedures: with external standard, certified reference material and standard addition method. Two analytical techniques: the inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy-hydride generation (AAS-GH) were comparatively applied for the determination of arsenic in all arsenic species in water. ICP-MS detection limit was 0.2 μg L(-1) and relative standard deviation (RSD) of all arsenic species investigated was between 3.5 and 5.1%.

Review: the approaches for estimation of limit of detection for ICP-MS trace analysis of arsenic.

The analytical properties of an analytical method must be evaluated through validation protocols. Beside specificity and/or selectivity, linearity of calibration, repeatability and accuracy, the most important parameters are: LOD (limit of detection) and LOQ (limit of quantification). Through these limits, it is possible to define the smallest concentration of analyte that can be reliably detected and quantified. To establish these limits, an analyst should apply several estimation methods and test a large number of sample replicates. It is difficult to make a compromise between complex statistical programs and the simple analytical demand to have reliable analytical parameters. The differences and equivalency of estimation methods and approaches for analytical limits could be overcome by an experimental comparison. In this paper, the focus is the LOD of inductively coupled plasma-mass spectrometry (ICP-MS) measurements employed for the determination of arsenic. The current approaches for the calculation of the LOD are summarized and critically discussed.

Further insight into the mechanism of heavy metals partitioning in stormwater runoff.

Various particles and materials, including pollutants, deposited on urban surfaces are washed off by stormwater runoff during rain events. The interactions between the solid and dissolved compounds in stormwater runoff are phenomena of importance for the selection and improvement of optimal stormwater management practices aimed at minimizing pollutant input to receiving waters. The objective of this research was to further investigate the mechanisms responsible for the partitioning of heavy metals (HM) between the solid and liquid phases in urban stormwater runoff. The research involved the collection of samples from urban asphalt surfaces, chemical characterization of the bulk liquid samples, solids separation, particle size distribution fractionation and chemical and physico-chemical characterization of the solid phase particles. The results revealed that a negligible fraction of HM was present in the liquid phase (less than 3% by weight), while there was a strong correlation between the total content of heavy metals and total suspended solids. Examinations of surface morphology and mineralogy revealed that the solid phase particles consist predominantly of natural macroporous materials: alpha quartz (80%), magnetite (11.4%) and silicon diphosphate (8.9%). These materials have a low surface area and do not have significant adsorptive capacity. These materials have a low surface area and do not have significant adsorptive capacity. The presence of HM on the surface of solid particles was not confirmed by scanning electron microscopy and energy dispersive X-ray microanalyses. These findings, along with the results of the liquid phase sample characterization, indicate that the partitioning of HM between the liquid and solid phases in the analyzed samples may be attributed to precipitation processes.

Enhanced Arsenic Removal from Water by Activated Red Mud Based on Hydrated Iron(III) and Titan(IV) Oxides

In this study, acid heat–activated neutralized red mud (AB) was promoted as an efficient sorbent for the removal of inorganic arsenic from water. Three sorbents: neutralized red mud (Bauxsol), acid-treated Bauxsol (aBauxsol), and AB were investigated for arsenic removal. The sorption of arsenic on these adsorbents was studied as a function of contact time, initial arsenic concentration, and adsorbent dosage. The Langmuir isotherm was the best-fit adsorption isotherm model for the experimental data. The adsorption capacity of the activated red mud for As(V) was found to be 1.49 mg g−1 at a pH of water of 7.0. It was observed that AB at a dose of 6.0 g L−1 could effectively remove As(III) and As(V) from water. Anion interference on a molar basis was found to follow the order: phosphate interference was higher than sulfate interference, which was higher than bicarbonate interference.

Arsenic Removal from Water Using Industrial By-Products

In this study, removal of arsenic ions using two industrial by-products as adsorbents is represented. Removal of As(III) and As(V) from water was carried out with industrial by-products: residual from the groundwater treatment process, iron-manganese oxide coated sand (IMOCS), and blast furnace slag from steel production (BFS), both inexpensive and locally available. In addition, the BFS was modified in order to minimise its deteriorating impact on the initial water quality. Kinetic and equilibrium studies were carried out using batch and fixed-bed column adsorption techniques under the conditions that are likely to occur in real water treatment systems. To evaluate the application for real groundwater treatment, the capacities of the selected materials were further compared to those exhibited by commercial sorbents, which were examined under the same experimental conditions. IMOCS was found to be a good and inexpensive sorbent for arsenic, while BFS and modified slag showed the highest affinity towards arsenic. All examined waste materials exhibited better sorption performances for As(V). The maximum sorption capacity in the batch reactor was obtained for blast furnace slag, 4040 μgAs(V)/g.

A New Approach in Regression Analysis for Modeling Adsorption Isotherms

Numerous regression approaches to isotherm parameters estimation appear in the literature. The real insight into the proper modeling pattern can be achieved only by testing methods on a very big number of cases. Experimentally, it cannot be done in a reasonable time, so the Monte Carlo simulation method was applied. The objective of this paper is to introduce and compare numerical approaches that involve different levels of knowledge about the noise structure of the analytical method used for initial and equilibrium concentration determination. Six levels of homoscedastic noise and five types of heteroscedastic noise precision models were considered. Performance of the methods was statistically evaluated based on median percentage error and mean absolute relative error in parameter estimates. The present study showed a clear distinction between two cases. When equilibrium experiments are performed only once, for the homoscedastic case, the winning error function is ordinary least squares, while for the case of heteroscedastic noise the use of orthogonal distance regression or Margarts percent standard deviation is suggested. It was found that in case when experiments are repeated three times the simple method of weighted least squares performed as well as more complicated orthogonal distance regression method.

Spatial variations in the distribution of trace ionic impurities in the water-steam cycle in a thermal power plant based on a multivariate statistical approach

In this study, a multivariate statistical approach was used to identify the key variables responsible for process water quality in a power plant. The ion species that could cause corrosion in one of the major thermal power plants (TPP) in Serbia were monitored. A suppressed ion chromatographic (IC) method for the determination of the target anions and cations at trace levels was applied. In addition, some metals important for corrosion, i.e., copper and iron, were also analysed by the graphite furnace atomic absorption spectrophotometric (GFAAS) method. The control parameters, i.e., pH, dissolved oxygen and silica, were measured on-line. The analysis of a series of representative samples from the TPP Nikola Tesla, collected in different plant operation modes, was performed. Every day laboratory and on-line analysis provides a large number of data in relation to the quality of water in the water-steam cycle (WSC) which should be evaluated and processed. The goal of this investigation was to apply multivariate statistical techniques and choose the most applicable technique for this case. Factor analysis (FA), especially principal component analysis (PCA) and cluster analysis (CA) were investigated. These methods were applied for the evaluation of the spatial/temporal variations of process water and for the estimation of 13 quality parameters which were monitored at 11 locations in the WSC in different working conditions during a twelve month period. It was concluded that PCA was the most useful method for identifying functional relations between the elements. After data reduction, four main factors controlling the variability were identified. Hierarchical cluster analysis (HCA) was applied for sample differentiation according to the sample location and working mode of the TPP. On the basis of this research, the new design of an optimal monitoring strategy for future analysis was proposed with a reduced number of measured parameters and with reduced frequency of their measurements.Graphical abstract

Corroded scale analysis from water distribution pipes

The subject of this study was the steel pipes that are part of Belgrades drinking water supply network. In order to investigate the mutual effects of corrosion and water quality, the corrosion scales on the pipes were analyzed. The idea was to improve control of corrosion processes and prevent impact of corrosion on water quality degradation. The instrumental methods for corrosion scales characterization used were: scanning electron microscopy (SEM), for the investigation of corrosion scales of the analyzed samples surfaces, X-ray diffraction (XRD), for the analysis of the presence of solid forms inside scales, scanning electron microscopy (SEM), for the microstructural analysis of the corroded scales, and BET adsorption isotherm for the surface area determination. Depending on the composition of water next to the pipe surface, corrosion of iron results in the formation of different compounds and solid phases. The composition and structure of the iron scales in the drinking water distribution pipes depends on the type of the metal and the composition of the aqueous phase. Their formation is probably governed by several factors that include water quality parameters such as pH, alkalinity, buffer intensity, natural organic matter (NOM) concentration, and dissolved oxygen (DO) concentration. Factors such as water flow patterns, seasonal fluctuations in temperature, and microbiological activity as well as water treatment practices such as application of corrosion inhibitors can also influence corrosion scale formation and growth. Therefore, the corrosion scales found in iron and steel pipes are expected to have unique features for each site. Compounds that are found in iron corrosion scales often include goethite, lepidocrocite, magnetite, hematite, ferrous oxide, siderite, ferrous hydroxide, ferric hydroxide, ferrihydrite, calcium carbonate and green rusts. Iron scales have characteristic features that include: corroded floor, porous core that contains both fluid and solid, relatively dense shell-like layer that covers the porous core and provides structural integrity to the scale, and surface layer that is present on top of the shell-like layer at scale-water interface and loosely attached to the shell-like layer. Iron(II) deposits are formed under reducing conditions. The presence of relatively soluble Fe(II) deposits such as siderite and ferrous hydroxide was confirmed by XRD and SEM analysis. In the presence of carbonic species, siderite (FeCO3) is prevailing ferrous deposit. Further studies are needed for obtaining greater knowledge on the mechanism of iron release from corroded pipes and the influence of water quality to iron corrosion.

Build-up and characterisation of pollutants on urban impervious surfaces

This paper presents the results of the investigation of pollutant build-up on impervious surfaces of a parking lot in Belgrade, Serbia during the summer months. Contaminant build-up was found to be greater on asphalt surfaces directly exposed to vehicular traffic than on concrete walkways. The difference in the amounts of accumulated pollutants between asphalt and concrete were significant: for total solids (TS), total suspended solids (TSS), chemical oxygen demand (COD), heavy metals and total phosphorus (TP) accumulations were two to three times higher, while only 30% higher for total nitrogen (TN) and anions. Build-up of most of the measured parameters was best described by power functions. The highest surface loads were found for solids, COD, iron and zinc. A strong correlation was found between turbidity, TS, TSS, COD, heavy metals and phosphorus, while conductivity, nitrates and nitrites were weakly correlated to other parameters.