Effects of common dissolved anions on the efficiency of Fe0-based remediation systems.

Quiescent batch experiments were conducted to evaluate the influences of Cl-, F-, HCO3-, HPO42-, and SO42- on the reactivity of metallic iron (Fe0) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe0 and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0-45 d), (ii) 4 different types of Fe0, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO3- enhanced the extent of MB discoloration, while Cl-, F-, HPO42-, and SO42- inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO3- inhibited O-II discoloration. The increased MB discoloration in the HCO3- system was justified by considering the availability of FeCPs as contaminant scavengers, pH increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe0-based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.

Modeling porosity loss in Fe0-based permeable reactive barriers with Faraday's law.

Solid iron corrosion products (FeCPs), continuously generated from iron corrosion in Fe0-based permeable reactive barriers (PRB) at pH > 4.5, can lead to significant porosity loss and possibility of system's failure. To avoid such failure and to estimate the long-term performance of PRBs, reliable models are required. In this study, a mathematical model is presented to describe the porosity change of a hypothetical Fe0-based PRB through-flowed by deionized water. The porosity loss is solely caused by iron corrosion process. The new model is based on Faraday's Law and considers the iron surface passivation. Experimental results from literature were used to calibrate the parameters of the model. The derived iron corrosion rates (2.60 mmol/(kg day), 2.07 mmol/(kg day) and 1.77 mmol/(kg day)) are significantly larger than the corrosion rate used in previous modeling studies (0.4 mmol/(kg day)). This suggests that the previous models have underestimated the impact of in-situ generated FeCPs on the porosity loss. The model results show that the assumptions for the iron corrosion rates on basis of a first-order dependency on iron surface area are only valid when no iron surface passivation is considered. The simulations demonstrate that volume-expansion by Fe0 corrosion products alone can cause a great extent of porosity loss and suggests careful evaluation of the iron corrosion process in individual Fe0-based PRB.

Paddy soil geochemistry, uptake of trace elements by rice grains (Oryza sativa) and resulting health risks in the Mekong River Delta, Vietnam.

Soil geochemistry and phytoavailable trace elements were investigated in 80 paddy soil samples and corresponding rice grains from the Mekong River Delta in Vietnam. Soil parameters like Fe-, Al-, and Mn-phases, organic matter, and pH-value determine element concentrations in soil and affect their transfer into rice grains. Arsenic exceeded the allowed limit for Vietnamese agricultural soils in 11% of the samples, presumably caused by natural processes. Lead surpassed the limit in one soil sample. Other toxic elements were close to their natural concentrations and far below allowable limits for agricultural soil. There was no clear correlation of trace element concentrations in soils with those in corresponding grains, even if the different soil parameters and the large pH-range between 3.7 and 6.8 were considered. To assess health risks of critical elements in rice, the thresholds of tolerable upper intake level for total food and drinking water (UL) and of permissible maximum concentration (MC) for rice grains were evaluated. Surprisingly, rice grains grown on non- or low-polluted soils can surpass the upper limits. According to the UL concept, 12% of the grains exceeded the UL of As, 29% that of Cd, and 27% that of Pb for each gender. According to the MC concept, 5% of the rice grains exceeded the MC of inorganic As for adults and 38% that for young children. 24% of the grains surpassed the MC of Pb, while Cd in all grains was below the MC. The differing results of the UL and MC approaches show an urgent need for revision and harmonization concerning As, Cd, and Pb limits, especially regarding countries with high rice consumption.

Harmful and nutrient elements in paddy soils and their transfer into rice grains (Oryza sativa) along two river systems in northern and central Vietnam.

Thirty soil samples and 24 corresponding unpolished rice samples along the Red and Huong Rivers in northern and central Vietnam respectively, were analyzed in order to evaluate (a) soil geochemistry, (b) factors that determine the transfer of harmful and nutrient elements from soils into rice grains, (c) health risk to the local population through rice consumption. The concentrations of As, Bi, and U in the soils of this area are higher relative to those of average shale probably due to natural redox-related processes. Also, Zn, Ce, Th, La, Sn, Pb, and Cd are accumulated in some soils because of mining activities or industrial wastewater application. Arsenic concentrations exceed the Vietnamese allowable limit of 15 mg kg-1 in 80% of the tested soils. Twelve percent of the unpolished rice grains surpass the permissible maximum concentration of 0.2 mg Cd kg-1 grain dry matter by FAO/WHO and European Union, and all samples are below the Pb limit. The daily intake of As is within the range of the tolerable intake levels proposed by the European Food Safety Authority. Influences of soil parameters such as pH value, contents of soil organic matter, oxides/hydroxides of Al, Fe, and Mn cause a broad spread of transfer factors from soil to grains. Positive trends exist between the transfer factors within the groups (a) As, Sb, and U, (b) Co, Cu, Ni, and Zn, (c) Cd and Mn which indicate similar influences of soil parameters on their uptake. We propose that the allowable Cd maximum concentration for rice should be set to less than 0.2 mg kg-1. The analysis of As and Cd concentrations in soils and corresponding rice grains as well as the soil pH value should be made obligatory in order to prevent intoxication. In addition, critical elements from nonferrous metal mining and industrial areas should also be evaluated.

Alteration of trace element concentrations in plants by adhering particles - Methods of correction.

Trace element concentrations in plants may be influenced by airborne dust or adhering soil particles. Neglecting adhering particles in plant tissue leads to misinterpretation of trace element concentrations in research fields such as phytomining, phytoremediation, bio-monitoring, uptake of micronutrients and provenance studies. In case washing or brushing the samples prior to analysis is insufficient or impossible due to fragile or pre-processed samples mathematical correction should be applied. In this study three methods are presented allowing to subtract the influence of adhering particles in order to obtain the element concentrations in plants resulting only from uptake. All mathematical models are based on trace elements with negligible soil to plant transfer. A prerequisite for the correction methods is trace element analytics with good accuracy and high precision, e.g. through complete acid digestion. In a data set of 1040 plant samples grown in open field and pot trials most plants show a small but detectable amount of adhering particles. While concentrations of nutrients are nearly unaffected trace element concentrations such as Al, Cd, Co, Cr, Fe, Mn, Ni, Pb, REEs, Ti and U may be significantly altered. Different sampling techniques like cutting height can also significantly alter the concentrations measured in the samples.

Application of PAH concentration profiles in lake sediments as indicators for smelting activity.

The ability of lake sediment cores to store long-term anthropogenic pollution establishes them as natural archives. In this study, we focus on the influence of copper shale mining and smelting in the Mansfeld area of Germany, using the depth profiles of two sediment cores from Lake Süßer See. The sediment cores provide a detailed chronological deposition history of polycyclic aromatic hydrocarbons (PAHs) and heavy metals in the studied area. Theisen sludge, a fine-grained residue from copper shale smelting, reaches the lake via deflation by wind or through riverine input; it is assumed to be the main source of pollution. To achieve the comparability of absolute contaminant concentrations, we calculated the influx of contaminants based on the sedimentation rate. Compared to the natural background concentrations, PAHs are significantly more enriched than heavy metals. They are therefore more sensitive and selective for source apportionment. We suggest two diagnostic ratios of PAHs to distinguish between Theisen sludge and its leachate: the ratio fluoranthene to pyrene ~2 and the ratio of PAH with logKOW<5.7 to PAH with a logKOW>5.7 converging to an even lower value than 2.3 (the characteristic of Theisen sludge) to identify the particulate input in lake environments.

Influence of competing inorganic cations on the ion exchange equilibrium of the monovalent organic cation metoprolol on natural sediment.

The aim of this study was to systematically investigate the influence of the mono- and divalent inorganic ions Na(+) and Ca(2+) on the sorption behavior of the monovalent organic cation metoprolol on a natural sandy sediment at pH=7. Isotherms for the beta-blocker metoprolol were obtained by sediment-water batch tests over a wide concentration range (1-100000 µg L(-1)). Concentrations of the competing inorganic ions were varied within freshwater relevant ranges. Data fitted well with the Freundlich sorption model and resulted in very similar Freundlich exponents (n=0.9), indicating slightly non-linear behavior. Results show that the influence of Ca(2+) compared to Na(+) is more pronounced. A logarithmic correlation between the Freundlich coefficient K(Fr) and the concentration or activity of the competing inorganic ions was found allowing the prediction of metoprolol sorption on the investigated sediment at different electrolyte concentrations. Additionally, the organic carbon of the sediment was completely removed for investigating the influence of organic matter on the sorption of metoprolol. The comparison between the experiments with and without organic carbon removal revealed no significant contribution of the organic carbon fraction (0.1%) to the sorption of metoprolol on the in this study investigated sediment. Results of this study will contribute to the development of predictive models for the transport of organic cations in the subsurface.