In lake restoration, internal phosphorus (P) loading is often obstructing the improvement of water quality even years after decreasing external P loading. Here, we report on a field study in the Lake Bergse Voorplas (The Netherlands), in which treatment with Al hydroxides was combined with installing a sand layer on top of the sediment. Besides surface water monitoring, sediment and porewater samples were taken and analysed before, in the year after, and ten years after treatment. Porewater profiles indicate that the treatment was successful leading to a decrease in average total P and chlorophyll-a concentrations by about 30 % and 50 %, respectively. At sites with the highest Al dose, porewater P concentrations at the interface between capping layer and sediment decreased from values above 6 mg P/L to concentrations close to detection limit, one year after treatment. The sand layer ensured the physical integrity of the Al-rich layer, which was still effective in reducing the flux of legacy P from the covered sediment to the overlying water, 10 years after treatment. Adsorption experiments, however, indicate that the added Al hydroxide approached P saturation. Additionally, indications were obtained that about half of the Al hydroxides were converted into clay minerals being the most important factor affecting the long-term capacity of the added Al hydroxides to adsorb P. This study demonstrates that stabilizing the active Al-rich layer with a sand layer provides a promising approach to reduce internal P loading in shallow lakes over the time-scale of years.

A disposal facility can be constructed in a stable geological formation to emplace packages containing radioactive waste. After closure of such a facility, the waste is isolated and the radionuclides are contained for the far future by a multibarrier system composed of engineered and natural barriers. Favourable Dutch geological formations may include Paleogene clays due to their presence in the subsurface. Radionuclides from the waste may enter the clay host rock if they have not decayed and are released from the engineered barriers. The low permeability physically retains them but there are also chemical retainment processes in the clay host rock that prevent radionuclides from entering the biosphere. Non-radioactive counterparts of radionuclides in the waste and actinides are present in trace amounts in Paleogene clays. Their concentrations can be similar to those of long-lived radionuclides in the waste forms. Their variation in concentration in the sediments is statistically highly correlated with the variation in total clay content. Ion exchange has been hypothesized to be the main mechanism to chemically retain most of these trace concentrations for the following groups: alkali metals, rare-earth elements, some actinides, transition metals and perhaps even non-metals.

Access to safe drinking water is essential for public health, making pathogen removal a key objective in water treatment. Slow sand filtration (SSF) is a widely used water treatment method due to its operational simplicity and cost-effectiveness. While biological activity is considered a key factor in SSF performance, the roles of accumulated organic and inorganic matter in pathogen removal remain unclear. This study investigated their contributions to Escherichia coli (E. coli) retention using sand columns from three mature SSF systems in the Netherlands. Baseline E. coli removal was assessed in all filters. In two filters, selective extractions were applied using: (i) sodium hypochlorite to eliminate organic matter, and (ii) sodium dithionite–citrate and acid ammonium oxalate to remove accumulated inorganic matter (e.g., metal (oxyhydr)oxide coatings and calcites). Results show that both organic and inorganic matter contribute to E. coli retention, with varying importance across filters. In the sand columns where organic or inorganic coatings were selectively removed, log removal of E. coli decreased, indicating their mechanistic roles in retention. In a filter with high biological activity and low metal (oxyhydr)oxide content, mechanisms like predation and biofilm formation appeared to dominate retention. Conversely, in a filter with low biological activity, retention was primarily linked to calcium carbonate and iron and manganese (oxyhydr)oxide coatings. Variations in organic and inorganic matter content reflect both the characteristics of the original sand and subsequent accumulation, particularly in the filter’s top layer (the schmutzdecke). These accumulations are shaped by source water and upstream treatment, highlighting the need to consider the full treatment train when optimizing SSF performance.

Slow sand filtration (SSF) is a widely used biofiltration method for drinking water treatment, yet quantitative measures of biological activity, specifically in terms of carbon uptake and mineralization, are not well established. This study assessed biological activity in mature SSF systems operated by three Dutch drinking water companies using carbon concentration measurements, isotopic signature analyses, and 13C-labeled glucose tracer experiments. Our results revealed measurable differences in carbon concentrations and isotopic signatures between influent and effluent waters. Specifically, DOC concentrations decreased by up to 0.13 mmol L- 1, while DIC concentrations increased by up to 0.84 mmol L- 1. Additionally, delta 13C-DOC and delta 13C-DIC values exhibited shifts of up to +1.9 %o and 2.9 %o, respectively, indicative of carbon uptake and mineralization. Biological activity varied across filters, as reflected in the time required for complete removal of glucose, which ranged from 5 to over 48 h and correlated with assimilable organic carbon (AOC) removal rates. AOC loading appeared to be the primary driver of bioactivity, with the lowest activity found in a filter fed with dune-infiltrated water and the highest in a filter receiving ozonated influent water. These findings highlight the importance of considering source water characteristics and the preceding treatment chain to understand their potential impact on biological activity in SSF.

Long start-up times to achieve manganese removal in rapid sand filters can pose challenges for drinking water companies. This study assessed the start-up dynamics of manganese removal in two full-scale dual-media rapid sand filters treating groundwater containing iron, ammonium, and manganese. After inoculation with 20% biologically active coated sand, ammonium and manganese removal efficiencies of ∼60-70% and ∼30-50% were achieved, respectively. Complete removal of ammonium occurred after ∼8 weeks, but ∼17 and ∼25 weeks were required for manganese removal in the two filters. Full manganese removal, accompanied by manganese oxide formation on new grains, was achieved when ∼50% of the ammonium was removed within the anthracite layer. X-ray spectroscopy of manganese oxides in the mineral coatings indicated a dominance of biologically produced manganese oxide with a structure similar to that of δ-MnO2, suggesting continuous microbial manganese oxidation in inoculated rapid sand filters. Concomitant changes in 16S rRNA gene profiles combined with qPCR and solute profiles suggest a key role for Nitrospira in both nitrification and manganese oxidation. We show that inoculation with biologically active filter medium enhances the efficiency of ammonium and manganese removal during filter start-up, offering a promising improvement strategy for rapid sand filters.

Peatlands in the Netherlands contain high amounts of sulfur (S). Drainage of these peatlands has led to oxidation of the peat, more recently enhanced through extended drought periods from the result of climate change. Oxidation of peat leads to the mobilization of S and elevated sulfate levels in surface waters. High sulfate concentrations are considered a water quality problem and can enhance eutrophication. In this study, the S content and S speciation in Dutch peats were investigated and their relation to paleoenvironment and current land-use.Peat samples from eight locations in an east-west section, varying over paleoenvironment, peat type, proximity to the River Rhine and the North Sea, and current land‑use were analyzed. Sequential sulfur extraction was performed to fractionate iron-monosulfide, pyrite and organic-bound sulfur. Porewater was analyzed for sulfate, iron and nitrate concentrations to investigate their influence on the S speciation.The analytical results could be split into 3 groups. The first group consisted of peats of marine paleoenvironment which had the highest total S content. Due to limited availability of iron (Fe), sulfur was predominantly present as organic-S and

Slow sand filtration (SSF) is one of the oldest biofiltration methods for reducing pathogens and organic matter (OM) in water. Due to its efficiency, affordability, and operational simplicity, SSF remains a widely used approach for producing biologically stable drinking water. Although biological activity plays a role in the removal of OM during SSF, its contribution is poorly constrained. Here, we explored the utility of stable isotopes for investigating this role quantitatively on the scale of an operational filter. First, by combining measurements of concentrations and natural isotopic composition in relevant carbon pools (dissolved and solid, organic and inorganic), we found evidence for OM removal through both retention and subsequent mineralization. However, their relative contributions could not be constrained due to insufficient precision and continuity of available data and incomplete knowledge about the relevant isotope fractionation factors. In the other approach, we therefore used laboratory incubations of SSF cores with 13C-labeled glucose over 14 days and found rapid removal of the tracer by the biological community, exceeding the assimilable organic carbon loading rate of the operational filter by 18 times. The glucose removal was not limited to the upper part of the sand column, the schmutzdecke, but occurred throughout the entire sand column. Furthermore, the removal was dominated by bacterial uptake over mineralization, with a substantial part likely retained as carbon reserves. The residence time of the tracer exceeded the duration of the experiment, hampering our ability to estimate the rate of OM mineralization. Analysis of the meiofauna indicated that grazing and/or predation constitutes only a minor sink for the bacterial biomass in the studied filter. Overall, this study illustrates the potential of stable isotopes for studying biological processes in SSF systems, including OM removal under diverse conditions, maturation of new or recently cleaned filters, or interactions within the endogenous biological community. To fully utilize this potential, future work should employ isotope labeling experiments with a longer duration, and consider more systematic and precise monitoring of the concentrations and isotopic composition in the relevant carbon pools.

Internal phosphorus (P) loading is a main cause for persistent eutrophication of shallow freshwater systems and can delay restoration for decades. Iron (Fe) amendment is often used to enhance P binding in the sediment and reduce benthic P fluxes. However, sufficient dosing using Fe salts is challenging due to acidification. Fe-rich water treatment residuals (Fe-WTR) are an attractive alternative, but their behavior in aquatic sediments is poorly studied. In this field study, a ditch in a peat polder was treated with 2.5 kg Fe/m2 using Fe-WTR. Sediment porewater and solid phase analyses, including sequential Fe extraction, showed that the added Fe-WTR significantly increased the reactive Fe reservoir of the surface sediment. Sediment incubation experiments and surface water monitoring for one year indicated an efficient reduction of internal P loading. Redox cycling was found to redistribute the added Fe both laterally across the ditch and vertically towards the sediment surface. Reactive Fe phases were thus continuously replenished in the surface sediment and available for P retention via co-precipitation and adsorption, potentially increasing the longevity of the treatment. Loss of the added Fe to sulfidation was limited due to the large excess of available Fe. However, the initial P-content of the Fe-WTR also increased the sediment P reservoir by 10

Internal phosphorus (P) loading is a key water quality challenge for shallow lakes. Addition of iron (Fe) salts has been used to enhance P retention in lake sediments. However, its effects on sediment geochemistry are poorly studied, albeit pivotal for remediation success. Here, we assess the factors controlling the retention of P and longterm effects following application of FeCl3 (0.5-1 mol Fe/m2, 2010) in the eutrophic, shallow peat lake Terra Nova (the Netherlands). Treatment reduced P levels in the lake for two years, but afterwards summer release of P intensified, resulting in higher surface water P concentrations than before treatment. Porewater and sediment analyses indicate that the majority of the added Fe is still undergoing redox cycling within the top 10 cm of sediment accounting for the binding of up to 70 % of sedimentary P. Sequential extractions further suggest that organic matter (OM) plays a key role in the resulting P and Fe dynamics: While reduction of P binding Fe(III) phases results in P release to porewaters, the produced Fe2+ remains bound to the solid phase presumably stabilized by OM. This causes P release from the sediments in excess to Fe during temporary low oxygen conditions in summer months, as confirmed by whole core flux incubation experiments. Quantitative coprecipitation of P with Fe upon reoxygenation of the water body is then impossible, leading to a gradual increase in surface water P. This first long-term study on a shallow peat lake underpins the role of OM for Fe cycling and the need to carefully consider the sediment properties and diagenetic pathways in the planning of Fe-amendments.

In shallow lakes, mobilization of legacy phosphorus (P) from the sediments can be the main cause for persisting eutrophication after reduction of external P input. In-lake remediation measures can be applied to reduce internal P loading and to achieve ecosystem recovery. The eutrophic shallow peat lake Terra Nova (The Netherlands) was treated with iron (Fe) to enhance P retention in the sediment. This treatment, however, intensified seasonal internal P loading. An earlier study suggested that Fe addition led to increased P binding by easily-reducible Fe(III) associated with organic matter (OM), which readily releases P when bottom waters turn hypoxic. In this complementary study, bulk and micro Fe K-edge and P K-edge X-ray absorption spectroscopy and micro-focused X-ray fluorescence spectroscopy were applied to characterize the P hosting Fe(III) pool. Combined with sequential extraction data, the synchrotron X-ray analyses revealed that a continuum of co-precipitates of Fe(III) with calcium, phosphate, manganese and organic carbon within the OM matrix constitutes the reducible Fe(III) pool. The complementary analyses also shed new light on the interpretation of sequential extraction results, demonstrating that pyrite was not quantitatively extracted by nitric acid (HNO3) and that most of the Fe(II) extracted by hydrochloric acid (HCl) originated from phyllosilicate minerals. Formation of an amorphous inorganic-organic co-precipitate upon Fe addition constitutes an effective P sink in the studied peaty sediments. However, the high intrinsic reactivity of this nanoscale co-precipitate and its fine distribution in the OM matrix makes it very susceptible to reductive dissolution, leading to P remobilization under reducing conditions.

Aeration followed by rapid sand filtration is a common method in drinking water treatment to remove iron (Fe) and manganese (Mn) from anoxic groundwater. To ensure the successful removal of Fe and Mn within a single filter, several factors such as raw water characteristics, backwash procedures and chemical and microbial interactions with the filter medium need to be considered. Here, we assess the characteristics of a single medium rapid sand filter with highly efficient removal of Fe and Mn. Using synchrotron X-ray spectroscopy, we show that formation of ferrihydrite-type Fe oxides in the top of the filter (0-50 cm) accounts for >95 % of the removal of dissolved Fe2+ in the filter. Birnessite-type Mn- oxides, which are thought to be biogenic, form over a wider depth interval (0-110 cm). Results of 16S rRNA gene amplicon sequencing indicate a corresponding distinct vertical stratification of the microbial community, with potential iron-oxidizing Gallionella, Leptothrix and Sideroxydans dominating in the upper part of the filter, and nitrifiers being more prevalent deeper in the filter. Besides Fe and Mn-oxide, Fe-flocs and bacteriological hollow sheets form in the upper part of the filter. Both the Fe-flocs, hollow Fe-sheets and part of the Fe and Mn coatings are removed through backwashing, thereby reducing the pressure difference measured over the filter medium linked to clogging of pores (from 14 kPa to 1.5 kPa) and ensuring continued water flow. Backwashing removes part of the Gallionella, but this does not negatively impact the filter performance. Strikingly, SEM imaging with EDS mapping revealed alternating layers of Fe and Mn-oxides on the coated grains throughout the filter. This indicates slow mixing of the filter medium between the upper 30 cm and the rest of the filter during backwashing. Slow mixing likely contributes to continued success of the filter by ensuring homogeneous filter bed growth, while still allowing for stratification of the microbial community.

<p>In 2019 the EU Marie Sklodowska-Curie Training Network <em>P-TRAP</em> has been launched and is now approaching its end. The project has been targeting the diffuse flux of phosphate (P) into surface waters, i.e. the problems of understanding and controlling environmental P fluxes. <em>P-TRAP</em> has been aiming to develop new methods and approaches to trap P in drained agricultural areas and in the sediments of eutrophic lakes. The <em>P-TRAP</em> technologies have in common that they rely on the naturally strong connection between the biogeochemical cycling of P and iron (Fe). Trapping of P involved the application of Fe-containing by-products from drinking water treatment. <em>P-TRAP</em> aspired the ideas of a circular economy and aimed at recovering the retained P in agricultural systems and to convert it into valuable products for agricultural applications. In order to direct and support the development of the technologies, process-orientated investigations on the behaviour of P during the transformation of Fe minerals have been conducted. The poster will highlight some results from the project and will present conclusions, which can be drawn based on the current achievements.</p>

Globally, surface water quality and ecosystem functioning are challenged by anthropogenic P inputs. While sterner legislation has led to lower external P loading, internal loading fed by legacy P accumulated in the sediment has become the controlling factor of surface water P concentrations in many European freshwater systems. Fe amendment is a treatment method to control internal P loading, but is not always successful on the long term. In Lake Terra Nova, a polymictic shallow peat lake in the Netherlands, treatment with FeCl3 only led to a temporary decrease in sedimentary P release. Two years after treatment seasonal peaks in surface water P concentrations started to appear and have been increasing in intensity for the past 8 years. Depth-resolved solid phase analysis by sequential Fe and P extractions was combined with bulk X-ray absorption spectroscopy (XAS) at the Fe K-edge and high-resolution micro-X-ray fluorescence spectrometry (µ-XRF) and µ-XAS. At spots with distinctively high Fe contents, pyrite and silicate-bound Fe are identified by microscopic and spectroscopic analyses. The spectroscopic data, however, also point to a finely dispersed Fe species in the sediment matrix which most likely corresponds to Fe complexed by OM in the surface sediment. The correlation of the distribution of P and Fe suggests that P is bound to these Fe-OM complexes. This interpretation is supported by the sequential extraction results which showed that the Fe treatment induced a shift in the dominant P pool from Ca-bound P to Fe- and OM-bound P. Overall, the results indicate that FeCl3 application caused a change in sediment P dynamics towards a highly redox sensitive system in which P bound to Fe-OM is released to the surface water during seasonally low bottom water oxygen concentrations. The results of this study therefore indicate that FeCl3 may not be the ideal additive for the remediation of internal P loading in peaty water bodies due to the high affinity of Fe to OM.

Abstract Siderite (FeCO3) is an important ferrous iron carbonate in the geochemical cycling of iron, as it is a sink for iron under reducing conditions. However, its detection is not straightforward with classical analytical approaches because in natural samples it is often fine‐grained and/or occurs in low concentrations. In this study, we explore the analytical potential of low‐temperature magnetometry. Synthetic siderites with a limited amount of associated ferric iron of up to 5 mol% and some natural siderites were subjected to investigation. Maxima in the cooling curves in a 5 T magnetic field shows that the Néel temperature of siderite is at 37 K in agreement with literature data. Those maxima appear at a higher temperature in the synthetic siderites with associated/sorbed ferric iron; it is 45 K for the 5 mol% Fe3+ synthesis. With the increasing amount of ferric iron, the synthetic siderites show an increasingly prominent remanence tail beyond the nominal Néel temperature in field‐cooled (FC) and zero‐field‐cooled (ZFC) warming curves of the remanent magnetization acquired in 5 T at 5 K. Fine‐grained siderite alters in air on laboratory time scales which is manifested by more pronounced remanence tails up to higher temperatures. Siderite's presence is best diagnosed by evaluating a combination of FC warming curves and a FC/ZFC remanence ratio >3 at 5 K. Standard addition experiments of FC warming curves enable the determination of siderite down to 0.1 wt%.

Iron-coated sand (ICS) is a by-product from drinking water treatment made of sand coated with ferric iron (hydr) oxides. It is considered a suitable material for large-scale measures for phosphate removal from natural and agricultural waters to prevent eutrophication. Previous studies demonstrated that the residence time of water must be very long to reach equilibrium partitioning between phosphate and ICS but specifics for application are missing. First, SEM-EDX images were used to support the conceptual assumption that P adsorption inside the coating is a transport-limited process. Second, a conceptual model of phosphate adsorption was proposed considering two types of sites: one type with fast adsorption kinetics and reaching equilibrium with the perco-lating solution, and another type for which adsorption is also reversible but described by pseudo-first-order kinetics. The latter is conceived to account for transport-limited adsorption in the interior of the coating while the former fraction of sites is assumed to be easily accessible and located close to the grain surface. Third, the kinetics of phosphate adsorption on ICS were quantitatively determined to describe and predict phosphate retention in filters under various flow conditions. The model was calibrated and validated with long-term column experiments, which lasted for 3500 h to approach equilibrium on the slowly reacting sites. The model reproduced the outflowing phosphate concentrations: the pronounced increase after a few pore volumes and the slow in-crease over the remaining part of the experiment. The parameterized model was also able to predict the time evolution of phosphate concentrations in the outflow of column experiments with different flow velocities, flow interruption, and in desorption experiments. The equilibrium partition coefficient for the experimental condi-tions was identified as 28.1 L/g-Fe at pH 6.8 and a phosphate concentration of 1.7 mg-P / L. The optimized first -order mass transfer coefficient for the slow adsorption process was 1.56 10-4 h-1, implying that the slow adsorption process has a time scale of several months. However, based on the parameterized model, the slow adsorption process accounted for 95.5% of the equilibrium adsorption capacity, emphasizing the potential relevance of this process for practical applications. The implications for the design, operation, and lifespan of ICS filters are exemplarily illustrated for different scenarios.