Method Development for Determining the Removal of Metals from the Water Column under Transformation/Dissolution Conditions for Chronic Hazard Classification

An extension of the transformation/dissolution protocol (T/DP) was developed and evaluated as a tool to measure the removal of metals from the water column for chronic aquatic hazard classification. The T/DP extension (T/DP‐E) consists of 2 parts: T/DP‐E part 1, to measure metal removal from the water column via binding of metals to a substrate and subsequent settling, and T/DP‐E part 2, to assess the potential for remobilization of metals following resuspension. The T/DP‐E methodology (672‐h [28‐d] removal period, 1‐h resuspension event, and 96‐h resettling period) was tested using Cu, Co, and Sr solutions in the presence of a substrate. The metal removal rates varied from rapid removal for Cu to slower rates of removal for Co and Sr. The resuspension event did not trigger any increase in dissolved Cu, Co, or Sr. Additional 96‐h experiments were conducted using dissolved Ni, Pb, Zn, and Ag and supported the conclusion that the T/DP‐E is sufficiently robust to distinguish removal rates between metals with a wide range of reactivities. The proposed method provides a means to quantify the rate of metal removal from the water column and evaluate remobilization potential in a standardized and reliable way. Environ Toxicol Chem 2019;38:2032–2042. © 2019 SETAC.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151361/1/etc4471.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151361/2/etc4471_am.pd

Phosphorus sequestration by oxidizing iron in groundwater fed catchments

Losses of phosphorus (P) from agricultural land to surface water may cause impaired water quality. In lowlands with shallow aquifers, the main pathway of P transfer is by leaching to the groundwater. When groundwater surfaces and feeds the streams, it becomes oxic, and the P may interact with redox-active elements such as iron (Fe). We addressed how the fate of P at the groundwater-surface water boundary is affected by that of Fe. We used four lowland catchments fed by naturally Fe-rich groundwater (average: 20 mg/L Fe, 0.4 mg/L P) as study sites. The mobility of P was studied along the trajectory of draining groundwater: from the subsurface through sediments into open drainage ditches. In the reduced groundwater, the Fe occurs as Fe2+ which is highly soluble. The Fe concentrations gradually decrease as the water flows upward into the drainage ditch: in the ditchwater, which is exposed to atmospheric oxygen, the Fe2+ is oxidized to Fe3+ according to a kinetic process, and the Fe3+ readily precipitates as oxyhydroxides. The P is removed much faster than Fe. The groundwater feeding the ditches contains up to 3 mg/L P, but this drops to less than 0.1 mg/L in the upper layer of the ditchwater. The P is immobilized by oxidizing Fe, either through adsorption on freshly formed Fe oxyhydroxides or through formation of ferric phosphate minerals. In summary, the oxidative precipitation of Fe in drainage ditches is a highly efficient P sink. In order to quantify these processes at the catchment scale, the composition of groundwater and surface water was monitored at 60 locations throughout the selected catchments. The oxidation of Fe2+ proceeds as groundwater surfaces and flows through the catchment into increasingly larger streams. The gradual removal of Fe from streams is quantitatively explained by hydrological and chemical processes: the travel time of the water in the streams, and the kinetic oxidation rate of Fe2+. The removal of P occurs much faster than that of Fe. The average P concentration in streams (42 µg/L) is one order of magnitude below that in the groundwater feeding the streams (393 µg/L), due to sequestration by the oxidizing Fe. The average P concentration in groundwater largely exceeds the local environmental limit for freshwater (140 µg/L), but in streams, it is below the limit. It is concluded that naturally occurring Fe in groundwater alleviates the environmental risk associated with P in the receiving streams.status: publishe

Larger Metal Affinity of Freshwater Dissolved Organic Matter in Wastewater Affected Rivers

In freshwater, complexation of trace metals by dissolved organic matter (DOM) alleviates metal toxicity. The toxic effect of metals is modelled by the biotic ligand model (BLM). Usually it is assumed that different aquatic DOM samples have equal metal affinity. However, our results showed a 4-fold (Cu) to 10-fold (Cd, Ni, Zn) variability in metal affinity. Metal affinity of DOM from water bodies that receive important wastewater inputs is higher than that of natural DOM. WHAM6 underestimates metal complexation in these waters by an average factor of 3. The BLM assumption of equal DOM quality may be violated in such water bodies, and the BLM is likely to set over-protective water quality criteria.status: publishe

Filtration procedures influence environmental monitoring of orthophosphate

Phosphate is becoming a main water quality determining factor in regions with intensive animal husbandry. Environmental quality limits to control eutrophication in freshwater are commonly based on orthophosphate, which is measured by a colorimetric assay as molybdate reactive P (MRP) following ISO protocols. Surprisingly, sample filtration is not specified in these protocols, and filtration procedures vary among certified laboratories using these protocols. It is well established that MRP includes both free orthophosphate and phosphate associated with colloidal material such as Fe and Al oxyhydroxides, while only a minor part of the organic P is included. The aims of this study were to identify the effect of filtration on MRP in environmental samples, and to determine how filtration procedures in different certified laboratories affect reported data. In a round-robin test, ten waters were collected from streams in Flanders (Belgium) and sent to certified laboratories. The coefficients of variation in MRP results among the certified laboratory ranged 13-115 %. The same waters were subjected to size fractionation in our laboratory using filtration (paper filter, 0.45 μm, 0.1 μm) and dialysis (12-14 kDa). The MRP concentrations decreased with decreasing size fractionation cut-off. The MRP concentrations in dialysates (14 kDa), which approximated the free orthophosphate, were 3 – 80 % (mean 38%) of the corresponding values in unprocessed samples. This percentage decreased with increasing Fe concentrations, suggesting that the non-dialyzable P was mostly bound to Fe-rich particles and colloids. The MRP concentrations in the 0.10 μm membrane filtered solution only marginally exceeded the free orthophosphate concentrations, indicating that this filtration may be a pragmatic choice to identify the free orthophosphate in freshwater. In summary, the current protocols for environmental monitoring of orthophosphate yield widely variable results due to P bound to Fe and Al oxyhydroxide particles and colloids. This highlights the importance of more stringent protocols for environmental monitoring of orthophosphate.status: publishe

Toxicity of vanadium to micro-organisms and higher plants in soil

This report describes soil ecotoxicity assays conducted as a part of the environmental risk assessment of vanadium. Vanadium toxicity to plants (barley and tomato) and to soil dwelling micro-organisms was evaluated, and multiple endpoints were used. Phytotoxicity was evaluated using a root elongation assay (ISO 11269-1) and a plant growth assay (ISO 11269-2). Microbial toxicity was evaluated using a nitrification assay (ISO 14238) and a respiration assay (OECD 217). The toxicant used was sodium metavanadate (NaVO3). The tests were conducted in five different European soils with varying properties (pH 5.2—7.8; total V 15—58 mg V/kg). In addition, the effect of ageing after spiking was evaluated in three soils; i.e. the respective toxicity of freshly spiked and aged soils was compared (1 week vs. 6—12 months-old V additions). Phytotoxicity thresholds, expressed as V added to the soil, varied widely for the different soils and depending on the endpoint differed by a factor ranging from 10—19 for the different soils tested. Tomato growth was the most sensitive phytotoxic endpoint considered. Phytotoxicity was observed at EC10 (concentrations at which the response variable is reduced by 10%) of 10 mg added V/kg dry soil. Differing V concentrations in soil solution explained differences observed in V phytotoxicity reasonably well. Thus, phytotoxicity thresholds, expressed as dissolved V in soil solutions differed by a factor ranging from 2.6—5.8. Phytotoxic thresholds in long-term contaminated soils were, on average, twofold higher than those in corresponding freshly spiked soils. Furthermore, V concentrations in soil solutions of long-term contaminated soils were approximately twofold lower than those in corresponding freshly spiked soils. This suggests that, over a period of 6—12 months, V phytotoxicity is approximately reduced twofold due to immobilization of V during ageing. Microbial toxicity thresholds differed by a factor ranging from 30 to 440 depending on the respective endpoint, when expressed as V added to the soil. Toxicity results of the respiration assay were variable, i.e., toxic thresholds have wide statistical confidence intervals. Ageing did not consistently decrease V toxicity in all soils, contrary to the phytotoxicity tests. The V concentration in soil solution does not explain the observed differences in V toxicity to micro-organisms in different soils. The reason for this discrepancy between phytotoxicity and microbial toxicity is not clear. However, different microbial communities in varying soils may respond differently to soil ageing treatments, which may partly explain the lack of coherence with soil chemical data. Soil solution analysis indicates a higher solubility of the added vanadate than the vanadium due to geogenic background in soils. An added risk approach seems therefore most appropriate and is a pragmatic choice since toxicity thresholds when expressed as added V overlap with observed background concentrations of V range in soils.Report for compliance with European legislative (REACH)nrpages: 83status: publishe

Freshwater phosphate limits are based on a poorly standardized molybdate reactive P method

Phosphate is becoming a main water quality determining factor in regions with intensive animal husbandry. The freshwater limits in Flanders are based on phosphate determined by the molybdate reactive P (MRP) method and the certified laboratories have to adhere to ISO protocols (ISO 15681-2:2003 or ISO 15923-1:2013). Surprisingly, filtration is not specified in these guidelines and sample preparation procedures range from unfiltered to 0.45 µm filtration in these certified laboratories. It is well established that MRP includes both free orthophosphate and phosphate associated with colloidal material such as Fe and Al oxyhydroxides while organic P is only weakly included. The aim of this study was to compare analytical results among certified laboratories and to identify filtration effects on the analytical results. In a round-robin test, ten waters were collected from streams in Flanders and sent to certified laboratories. In addition, the same waters were tested in our laboratory assessing effects of filtration (paper filter, 0.45 µm, 0.1 µm) and dialysis (12-14 kDa) on MRP. The MRP concentrations in the water samples decreased gradually by filtration over progressively smaller membrane pore sizes and after dialysis. Filtration over a 0.45 µm membrane filter reduced MRP concentrations to 74-84% of MRP in unfiltered waters with low Fe (2.5 mg Fe/L). In the round-robin test, the coefficient of variation of MRP among certified laboratories ranged from 4-71%. We postulate that this variation can be reduced by more stringent laboratory guidelines.status: publishe

Authigenic iron-rich sediment in the Kleine Nete basin, Belgium

In the Kleine Nete basin (Belgium), redox processes involving iron (Fe) strongly affect suspended sediment dynamics and P geochemistry. The basin is characterised by a flat topography, highly permeable aquifers containing Fe minerals, and low and fluctuating groundwater tables. The Kleine Nete river and its tributaries are predominantly groundwater-fed. These groundwaters (pH 6.2—7.2) contain elevated Fe(II) concentrations that vary widely in space and time, roughly between 1 and 100 mg Fe(II)/L. Where such Fe-containing groundwaters surface, the Fe(II) is oxidised to Fe(III) which readily precipitates and may be transported to the receiving streams. The goal of this study was to better understand the geochemistry, the relative importance, and the implications of such authigenic Fe-rich material in the water column. Iron was the most important constituent of the suspended sediment in the major rivers (18—25%) and in a small brook (37—40%). Other important constituents were C, Si, and P. EXAFS spectroscopy (Figure 1) showed that the Fe speciation in suspended sediment samples was much like that in ferrihydrite. The mean particle diameter was between 10 and 20 um. In oxidation experiments, nearly all Fe(II) in pre-filtered groundwaters was oxidised within 2 days, and the rate of oxidation could be reasonably well predicted by an existing model. The authigenic material produced in the lab had a smaller mean particle size (around 5 um) and was less hydrolysed than the samples from the field. The particulates produced in the lab contained, on average, 44% Fe. The freshly produced authigenic material was identified as hydrous ferric oxides, slightly less hydrolysed than ferrihydrite. As a rough estimate, between 30 and 50% of the isolated suspended material was of authigenic origin. The authigenic material strongly affects P chemistry in the Kleine Nete basin: groundwaters are rich in P but as these groundwaters surface, the P is trapped by the freshly formed authigenic material. This mechanism keeps dissolved P concentrations below the eutrophication limit. The authigenic material has also implications for waterway management due to an increased amount of dredged material. In a follow-up study, a model for authigenic sediment formation and transport will be refined based on the geochemical knowledge gained in this study.status: publishe

The bioavailability of colloidal phosphorus to freshwater algae

The eutrophication of freshwaters is a major environmental concern in developed countries and is often attributed to excessive P fertilizer application. However, the eutrophication risk depends strongly on P bioavailability, which in turn depends on P speciation. Colloidal P species, e.g. P associated with colloidal Fe and Al oxyhydroxides, are included in routine colorimetric measurements of the available P fraction as “molybdate reactive P”, but the availability of this colloidal P fraction remains questionable. The aim of this study was to address the bioavailability of colloidal P in a well-defined model system. Growth and P uptake by a freshwater green alga (Raphidocelis subcapitata) were measured in synthetic solutions with or without colloidal Fe oxyhydroxides. Short term (1 hour) uptake experiments using radiotracers show that algal P uptake decreases with increasing colloidal P fraction. The P uptake rate is related to the free orthophosphate fraction (quantified by 10 kDa ultrafiltration), i.e. colloidal P does not contribute to the actual P uptake. Growth experiments on the longer term (up to 14 days) under P-limited conditions reveal that colloidal P contributes partially, but not completely, to algal growth. This is likely a result of desorption when free orthophosphate is taken up and becomes depleted. This potential P bioavailability correlates to the “labile P pool”, which is quantified by dialysis of the test solution against ferrihydrite as infinite P sink. It is concluded that colloidal P is only partially bioavailable, and that the eutrophication risk in freshwaters may be overestimated if P is measured as “molybdate reactive P”.status: publishe

Iron colloids reduce the bioavailability of phosphorus to the green alga Raphidocelis subcapitata

Phosphorus (P) is a limiting nutrient in many aquatic systems. The bioavailability of P in natural waters strongly depends on its speciation. In this study, structural properties of iron colloids were determined and related to their effect on P sorption and P bioavailability. The freshwater green alga Raphidocelis subcapitata was exposed to media spiked with radiolabelled 33-PO4, and the uptake of 33-P was monitored for 1 h. The media contained various concentrations of synthetic iron colloids with a size between 10 kDa and 0.45 μm. The iron colloids were stabilised by natural organic matter. EXAFS spectroscopy showed that these colloids predominantly consisted of ferrihydrite with small amounts of organically complexed Fe. In colloid-free treatments, the P uptake flux by the algae obeyed Michaelis-Menten kinetics. In the presence of iron colloids at 9 or 90 μM Fe, corresponding to molar P:Fe ratios between 0.02 and 0.17, the truly dissolved P (<10 kDa) was between 4 and 60% of the total dissolved P (<0.45 μm). These colloids reduced the P uptake flux by R. subcapitata compared to colloid-free treatments at the same total dissolved P concentration. However, the P uptake flux from colloid containing solutions equalled that from colloid-free ones when expressed as truly dissolved P. This demonstrates that colloidal P did not contribute to the P uptake flux. It is concluded that, on the short term, phosphate adsorbed to ferrihydrite colloids is not available to the green alga R. subcapitata.publisher: Elsevier articletitle: Iron colloids reduce the bioavailability of phosphorus to the green alga Raphidocelis subcapitata journaltitle: Water Research articlelink: http://dx.doi.org/10.1016/j.watres.2014.04.010 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved.status: publishe