Rare earth elements (REE) are in high demand due to their applications in the medical, green energy and high-tech industries. As a result, new REE mines are being developed with the potential of mobilizing REE and increasing their concentrations in the environment, including in landfills, wastewaters and natural waters. Unfortunately, the concentrations of trivalent REE cations are generally poorly predictive of biological effects, especially when attempting to understand the role of REE complexation within bioavailability models, such as the Biotic Ligand Model (BLM). We investigated the bioavailability of a representative REE, lanthanum (La), by measuring biouptake fluxes for Chlamydomonas reinhardtii. In the presence of a small amount (0.5 mg C L-1) of natural organic matter (NOM), for a calculated reduction in La3+ of >90%, La biouptake decreased between 43 and 87%. Furthermore, the concentration of dissolved organic carbon was not sufficient to predict the effect of NOM on La biouptake, with the effect varying greatly as a function of NOM origin. Eleven NOM samples were extensively characterised in order to relate NOM quality to its effect on biouptake. The most aromatic and allochthonous NOM had the greatest effect on reducing La biouptake. For example, for an NOM concentration of 10 mg C L-1, the most allochthonous NOM tested (Suwannee River Humic Acid, fluorescence index = 1.2) reduced La biouptake by 95% as compared to a reduction of 67% for the most autochthonous NOM (Cameroun River, fluorescence index = 1.9). The study is useful for improving our understanding of the important factors affecting REE bioavailability in natural systems.

Dissolved organic carbon (DOC) has an important, although not fully understood, ecological role in aquatic ecosystems. It influences both abiotic and biotic factors including ionoregulatory processes of the freshwater fish gill. A mechanistic understanding of the effects of DOC on these processes is crucial for monitoring aquatic health; however, this mechanistic understanding is challenging due to the complexity and heterogeneity of natural DOCs. To determine the key chemical moieties of DOC contributing to changes in ionoregulation, twenty model compounds were chosen based on the criteria that they structurally resemble and/or functionally behave like certain components of typical humic and fulvic acids found in natural waters. Each model compound was characterized using the physicochemical indices that are often used to characterize natural DOCs - e.g. specific absorbance coefficient (SAC340), octanol water partition coefficient (log Kow), molecular weight index (MWI), fluorescence index (FI), and molecular charge. Additionally, their effects on transepithelial potential (TEP), an indicator of ionoregulatory status, were tested at pH 7 and pH 4 in freshwater rainbow trout. Most characterization techniques based on absorbance and fluorescence signatures proved to be inappropriate for synthetic model compounds. However, indices based on titration properties including lipophilicity (i.e. log Kow), and charge may be used as predictors of TEP response, with careful consideration of pH. Our results demonstrate that some model compounds may be used as predictors of TEP. Specifically, phenolic groups and long aliphatic chains may be contributing to the TEP response to natural allochthonous DOCs making tannic acid (TA) and sodium dodecyl sulphate (SDS) appropriate model compounds for this type of DOC. Amino acids and peptides may contribute to the effect of autochthonous DOCs on TEP, making bovine serum albumin (BSA) a possible model compound for DOC of this origin.

Aqueous speciation can be used to explain and even predict the effects of metal contaminants on aquatic organisms. Yet, speciation is difficult to estimate in the presence of natural organic matter (NOM): a mixture of heterogeneous polymeric ligands. Partial ultrafiltration is a fast and affordable emerging technique which allows the experimental determination of metal complexation by NOM. Its applicability is based on the premise that metal-NOM complexes do not pass the low molecular weight cutoff membrane. Their retention was evaluated for samples containing different NOMs (10 mg C L-1) and a mixture of Ga, La, and Pt (360 nM) using high performance size exclusion chromatography coupled to an inductively coupled plasma mass spectrometer. The results show a satisfying separation performance, with better discrimination by the 2 kDa membrane compared to the 3 kDa membrane. The proportions of metal-NOM complexes passing into the ultrafiltrates were 0.0 +/- 0.0, 0.7 +/- 1.4, and 14.1 +/- 11.9% for Ga, La, and Pt, respectively. Furthermore, experimentally determined molecular weight cutoffs for NOM were below the nominal values: 0.4 and 0.5 kDa instead of 2 and 3 kDa, respectively. Overall, the results support the applicability of partial ultrafiltration and show its great potential for speciation measurements in natural waters.

Dissolved organic carbon (DOC) is an important contributor to both carbon cycling and other biogeochemical processes in aquatic ecosystems. The biodegradable fraction of DOC can be microbially degraded over time, producing carbon dioxide (CO2), a greenhouse gas. In addition, microbial degradation-resistant DOC can accumulate in water bodies, causing chemical and physical changes to aquatic systems. Although biodegradable DOC (BDOC) is widely studied, there is no agreed-upon standard method for assessing its biodegradability. Here, we aimed to develop and evaluate a new method for determining BDOC in freshwater samples. Our method includes filtering water samples to below 0.22 µm, to remove existing microbial cells, prior to inoculating the samples with a concentrated microbial inoculum produced by stepwise isolation of microbial cells from a peat sample. In addition, we added solutions containing nitrogen and phosphorus (in the forms of NH4NO3 and K2HPO4, respectively) to ensure that the microbes were not nutrient-limited. The samples were then capped with foam stoppers and incubated in the dark at 25⁰C on a shaker for 28 days to allow constant aeration during BDOC degradation. When applied to five freshwater samples collected from rivers, stormwater ponds, and a lake, and a glucose control, we observed that the amount of BDOC in the natural samples ranged from 15% to 53% and was 90% in the glucose control. Rates of BDOC degradation were calculated from DOC measurements at six sampling time points between days 0 and 28. We found that the DOC trends with time were best explained by two successive phases for BDOC degradation in all of the samples: an initial, fast, phase of BDOC degradation followed by a second, slower, phase of BDOC degradation where the rate constant for the second phase was between 5.57 and 565 times slower than for the initial phase. Changes in chemical characteristics of DOC measured using absorbance and fluorescence parameters including specific ultraviolet absorbance at 254 nm (SUVA254), humification index (HIX), and parallel factor analysis (PARAFAC) at each sampling time revealed that the initial, fast, phase of BDOC degradation often represents the utilization of small, non-aromatic compounds while the later, slower, phase of BDOC degradation often represents the utilization of more complex, aromatic compounds. The presented method provides a new approach to measure and characterize BDOC degradability and degradation kinetics that can be applied to future studies on biogeochemical processes in aquatic ecosystems.

Optical characterization of dissolved organic carbon (DOC) freshly collected from the circumneutral "white water" of the Rio Solimoes revealed that it had lower aromaticity, lower molecular weight, and a greater autochthonous content than DOC from the acidic "black water" of the Rio Negro. The tambaqui (Colossoma macropomum), a characid member of the Serrasalmidae, is a model neotropical fish that migrates annually between the two rivers. We analysed ionoregulatory responses of the tambaqui over 24 h in ion-poor water at pH 7.0 and pH 4.0 in the absence and presence of 10 mg L-1 Rio Solimoes DOC ("SOL DOC"). Measured parameters included transepithelial potential (TEP) across the gills, net flux rates, and plasma concentrations of Na+, Cl-, ammonia, and urea, and branchial Na+, K+ -ATPase, H+, ATPase, and carbonic anhydrase activities. Results were compared to our earlier study using similar protocols to examine the ionoregulatory effects of 10 mg L-1 DOC ("SGC DOC") collected from black water at S & atilde;o Gabriel da Cachoeira (SGC) in the upper Rio Negro. At pH 7.0, SOL DOC had no effect on the negative TEP across the gills. Exposure to pH 4.0 caused a marked depolarization of the TEP to positive values that was not ameliorated by the presence of SOL DOC. This contrasts with SGC DOC that drove TEP more negative at pH 7.0 and fully protected against the depolarization at pH 4.0. However, SOL DOC promoted positive balance of Na+ and Cl- at pH 7.0 and helped ameliorate the negative balance of these ions seen at pH 4.0. This again contrasts with SGC DOC that exacerbated ion losses at pH 4.0. The protective effects of SOL DOC on ion balance maybe related to increased v-type H+ ATPase activity in the gills, and unrelated to TEP. The very different responses to the two DOCs are discussed with respect to their optical properties, the time that they were in storage prior to testing (SOL <1 month, SGC 2 years), and the life history of the tambaqui in the two rivers.

Environmental context The on-site and real-time detection of metal ions is important for environmental monitoring and risk assessment. For appropriate management decisions, it is necessary to specifically sense the labile fraction of metal rather than total metal. This study provides a proof-of-principle that the DNAzyme GR5 can be used to sense labile lead in natural waters containing dissolved organic matter. Rationale DNAzyme-based sensors are a promising technology for possible labile metal monitoring that have not yet been fully tested in real waters. In clean, buffered, laboratory waters specific DNAzymes interact with specific metal ions and produce a signal (e.g. fluorescence). In more complex natural solutions the free ion concentration is reduced by complexation (e.g. to dissolved organic matter, DOM) and the signal would not be proportional to total metal, but hypothetically proportional to the labile fraction of total metal; i.e. the fraction of metal available to interact with the DNA. Methodology Here, an existing metal specific RNA-cleaving DNAzyme for Pb2+ (GR5) is used to test waters representative of natural solutions. Samples were prepared with ionic strengths from 25 to 100 mM using sodium acetate, sodium chloride and sodium bicarbonate. In addition, pH values of 6.5, 7.5 and 8.5 were tested for the different electrolytes, with and without added dissolved organic carbon, at 2, 5 and 8 mg C L–1. Lead additions were performed at toxicologically relevant levels (less than or equal to 1 µM of added lead). Results and discussion The GR5 response was found to be dependent on ionic strength, including identity of the background electrolyte, where high ionic strength slowed the reaction and chloride media increased reaction speed. Reproducible responses of GR5 are possible at conditions similar to natural waters, except responses were too fast for high pH (8.5 or higher), low DOC (less than 2 mg C L–1) and low ionic strength (25 mM). It is found that GR5 responds to three lead species, PbOH+, PbCl+ and Pb2+, with relative sensitivities in the same order. GR5 does not respond to lead complexed with acetate, carbonate or DOM. It is possible to use the measured first-order rate constant for lead induced fluorescence of GR5 to calculate ionic lead that agrees within a factor of two with respect to Windermere Humic Aqueous Model predictions. Thus, GR5 may represent a labile lead probe, although further work is necessary to test this.

Most Environmental Impact Assessments (EIAs) fail to generate effective monitoring and forecast triggers because there is a lack of appropriate baseline data and forecasting, especially for biotic endpoints. Herein, we provide an example of how to develop monitoring and forecast triggers with biotic data, specifically fish populations, to assess impacts of a planned refurbishment of the Mactaquac Hydroelectric Generating Station, a large hydroelectric facility. We recommend strategies for developing interim monitoring triggers until sufficient biological data is collected, including default critical effect sizes or data percentiles when there are only a few years of data. When there is sufficient data the monitoring trigger can be based on the predicted normal range, i.e., 2x standard deviation of the means. We generated forecast triggers with the general linear model, partial least squares regression, and elastic net regression. We demonstrate that interannual variability of fish population characteristics sampled consecutively for 4 years was insufficient for meaningful monitoring and forecast trigger development. Collecting sufficient baseline data for new projects in an undeveloped area will be challenging due to costs and regulatory and economic time frames as current practice is generally 1 or 2 years. Changes to existing projects, such as in this study, or new projects near existing development should have existing baseline data - if forethought is given as to effective endpoints. The alignment of monitoring requirements between developments within a watershed will improve monitoring, modelling, and prediction over the long term and for consideration of future developments.

The acid–base properties of soil organic matter are crucial in regulating plant nutrient availability in agricultural soils. This study examined the effect of long‐term application of different forms of manure: liquid swine manure (LSM), solid swine manure (SSM), and swine manure compost (SMC), applied biennially over 17 years, on the intrinsic charge characteristics of humic acid (HA) extracted from soils. Potentiometric titration and a continuous distribution p Ka model assessed buffer intensity, surface charge excess, p Ka distribution, and proton binding site capacities. The HA formed in soils amended with SMC (SMC‐HA) showed higher proton neutralization, particularly in the pH range critical for plant nutrient availability, correlating with phenol content. Variations in p Ka distribution highlighted stronger acidic sites in SMC‐HA, attributed to phenols and sulfur content, while LSM‐HA (HA formed in soils amended with LSM) and SSM‐HA (HA formed in soils amended with SSM) displayed weaker acid strength due to lower phenol content and molecular configurations. Acidic/basic site ratios revealed dominance of acidic functional groups in LSM‐HA and SSM‐HA, whereas SMC‐HA exhibited relatively higher basic site content and a lower acid/basic ratio. These findings underscore the heterogeneity of binding sites and differences in binding strengths among HA samples from various manure forms. The enhanced buffering capacity and distinct charge characteristics of SMC‐HA suggest a greater potential to improve nutrient availability and overall soil health. SMC amendment produces HAs with properties conducive to improved nutrient management in agricultural soils.

Environmental context Platinum is a metal of emerging concern in ecotoxicology, mainly due to its dissemination in the environment through the abrasion of car catalysts. However, its aqueous speciation in natural waters remains poorly known, and its binding to organic matter may modify its effects on organisms. Experimental measurements revealed the complex chemistry of Pt, low complexation and very slow kinetics. The use of PtII would be recommended for chronic exposure experiments.Rationale It is a common consensus that the free ion concentration of a metal in an aqueous medium can be a good indicator of its toxicity. In this context, the ability to determine metal speciation is of paramount importance to evaluate possible effect on aquatic ecosystems. Although speciation can be predicted for well-studied metals, the task is difficult when little information on thermodynamic constants is available, as is the case for platinum. It is then necessary to turn to experimental methods.Methodology For this purpose, a high-performance size exclusion liquid chromatography method coupled with inductively coupled plasma-mass spectrometry for online metal detection was used. In a synthetic freshwater medium, we first explored the inorganic speciation of platinum (added as PtIV) at pH 5 and 6, and using three equilibration periods (4 min, 48 h and 1 week). We then tested two different conditions with freshwater natural organic matter (NOM) from four different origins, both expected to provide different levels of complexation: low (pH = 5; [NOM] = 3 mg C L-1) and high (pH = 6; [NOM] = 10 mg C L-1).Results Several inorganic forms of platinum (PtII and PtIV) were identified, with good separation and repeatability. In the absence of NOM, dissociation of [PtIVCl6]2- complexes was clearly more important at pH 5 than at pH 6. Also, [PtIVCl6]2-persisted over time, even after 1 week, even though this redox form is believed to be unstable in these conditions, suggesting that thermodynamic equilibrium was not reached.Discussion Weak complexation by NOM was observed, and the initial form of [PtIVCl6]2- persisted. However, the presence of NOM resulted in the formation of additional Pt inorganic species. These unidentified peaks, which were relatively more abundant at high NOM levels, were interpreted as intermediate species between [PtIVCl6]2- and Pt-MON complexes.

Dissolved organic carbon (DOC) is a complex mixture of molecules that varies in composition based on origin as well as spatial and temporal factors. DOC is an important water quality parameter as it regulates many biological processes in freshwater systems, including the physiological function of the gills in fish. These effects are often beneficial, especially at low pH where DOCs mitigate ion loss and protect active ion uptake. DOCs of different compositions and quality have varied ionoregulatory effects. The molecular variability of DOCs can be characterized using optical and chemical indices, but how these indices relate to the physiological effects exerted by DOCs is not well understood. We tested the effects of five naturally sourced DOCs, at both pH 7 and pH 4, on transepithelial potential (TEP) (a diffusion potential between the blood plasma and the external water) in rainbow trout. The five chosen DOCs have been well characterized and span large differences in physicochemical characteristics. Each of the DOCs significantly influenced TEP, although in a unique manner or magnitude which was likely due to their physicochemical characteristics. These TEP responses were also a function of pH. With the goal of determining which physicochemical indices are predictive of changes in TEP, we evaluated correlations between various indices and TEP at pH 7 and pH 4. The indices included: specific absorbance coefficient at 340 nm, molecular weight index, fluorescence index, octanol–water partition coefficient, molecular charge, proton binding index,

This study evaluated the influence of organic matter (OM) constituents on the potential for recovery of P from wastewaters when FeCl3 treatment is employed for P removal. The presence of OM constituents did not influence P release from Fe-P sludges when alkaline and ascorbic acid treatments were employed. However, the overall recovery of P from wastewater was impacted by the presence of selected OM constituents through the reduction of P uptake during coagulation. The presence of protein and humic matter showed remarkably low P removal values (3.0 ± 0.4% and 23 ± 1% respectively) when compared to an inorganic control recipe (62 ± 2%). Elevated soluble Fe (SFe) residuals in the presence of proteins (87 ± 5%) and humics (51 ± 1%) indicated interactions between Fe(III) cations and negatively charged functional groups like hydroxyl, carboxyl, and phenolic groups available in these organics. Significant negative correlations between P removal and residual SFe were observed suggesting Fe solubilization by OM constituents was the mechanism responsible for reduced P removal. The findings of this study identify, for the first time, the impact of OM constituents on overall P recovery when Fe(III) salts are employed and provide insights into recoveries that can be expected when Fe is added to primary, secondary treated, and industrial wastewaters. PRACTITIONER POINTS: Low P removal values were observed for protein and humic dominated wastewater recipes. Iron(III) solubilization counted for P removal reduction by proteins and humic acids. There is no effect of OM on P release from Fe-P sludge at pH 10 and ascorbic acid treatments. OM and agent employed to release P from sludges affected overall recovery of P.

Remote sensing can provide an alternative solution to quantify Dissolved Organic Carbon (DOC) in inland waters. Sensors embedded on Unmanned Aerial Vehicles (UAV) and satellites that can capture the DOC have already shown good relationships between DOC and the Colored Dissolved Organic Matter absorption (aCDOM.) coefficients in specific spectral regions. However, since the signal recorded by the sensors is reflectance-based, DOC estimates accuracy decreases when inverting the aCDOM. coefficients to reflectance. Thus, the main objective is to study the potential of a UAV-borne hyperspectral camera to retrieve the DOC in inland waters and to develop reflectance-based models using UAV and satellite (Landsat-8 OLI and Sentinel-2 MSI) data. Ensemble based systems (EBS) were favored in this study. The EBSUAV calibration results showed that six spectral regions (543.5, 564.5, 580.5, 609.5, 660, and 684 nm) are sensitive to DOC in waters. The EBSUAV test results showed a good concordance between measured and estimated DOC with an R-2 = Nash-criterion (NASH) = 0.86, and RMSE (Root Mean Squares Error) = 0.68 mg C/L. The EBSSAT test results also showed a strong concordance between measured and estimated DOC with R-2 = NASH = 0.92 and RMSE = 0.74 mg C/L. The spatial distribution of DOC estimates showed no dependency to other optically active elements. Nevertheless, estimates were sensitive to haze and sun glint.

Many flatfish species are partially euryhaline, such as the Pacific sanddab which spawn and feed in highly dynamic estuaries ranging from seawater to near freshwater. With the rapid increase in saltwater invasion of freshwater habitats, it is very likely that in these estuaries, flatfish will be exposed to increasing levels of dissolved organic carbon (DOC) of freshwater origin at a range of salinities. As salinity fluctuations often coincide with changes in DOC concentration, two natural freshwater DOCs [Luther Marsh (LM, allochthonous) and Lake Ontario (LO, autochthonous) were investigated at salinities of 30 and 7.5 ppt. Optical characterization of the two natural DOC sources indicate salinity-dependent differences in their physicochemistry. LO and LM DOCs, as well as three model compounds [tannic acid (TA), sodium dodecyl sulfate (SDS) and bovine serum albumin (BSA)] representing key chemical moieties of DOC, were used to evaluate physiological effects on sanddabs. In the absence of added DOC, an acute decrease in salinity resulted in an increase in diffusive water flux (a proxy for transcellular water permeability), ammonia excretion and a change in TEP from positive (inside) to negative (inside). The effects of DOC (10 mg C L-1) were salinity and source-dependent, with generally more pronounced effects at 30 than 7.5 ppt, and greater potency of LM relative to LO. Both LM DOC and SDS increased diffusive water flux at 30 ppt but only SDS had an effect at 7.5 ppt. TA decreased ammonia excretion at 7.5 ppt. LO DOC decreased urea-N excretion at both salinities whereas the stimulatory effect of BSA occurred only at 30 ppt. Likewise, the effects of LM DOC and BSA to reduce TEP were present at 30 ppt but not 7.5 ppt. None of the treatments affected oxygen consumption rates. Our results demonstrate that DOCs and salinity interact to alter key physiological processes in marine flatfish, reflecting changes in both gill function and the physicochemistry of DOCs between 30 and 7.5 ppt.

Critical metals such as gallium, lanthanum and platinum are considered essential in a modern economy and for the required energy transition. Their relatively recent and increasing use in new technologies have led to an increase in their environmental mobility. As they reach aquatic systems, these metals can interact with organic ligands and especially Natural Organic Matter (NOM). The formation of organic complexes would be expected to reduce metal bioavailability and uptake by living cells, according to the Biotic Ligand Model (BLM). However, exceptions to this model have been determined for several critical metals in the past. The present work compared internalisation kinetics of Ga, La and Pt in the green alga Chlamydomonas reinhardtii in the presence of NOMs from different origins: humic and fulvic acids from Suwannee River as well as NOMs from Ontario (Bannister Lake and Luther Marsh). Complexation was determined using a partial ultrafiltration method allowing for a normalization of data based on speciation to compare all conditions based on the concentration of the metal that was not bound to NOM. While internalization metal fluxes varied greatly from one NOM source to the other, uptake was almost always significantly higher than expected based on metal speciation. Quite often, metal internalization fluxes were even significantly increased in the presence of NOM, for the same total metal exposure concentration. For instance, Pt internalization was twice greater in the presence of Bannister Lake NOM than it was in the absence of NOM. The assumption that such exceptions could be explained by NOM characteristics was contradicted by the variable results from one metal to another. To further explore this phenomenon, internalization mechanisms for these individual metals need to be elucidated. This is a necessary step to accurately estimate the risk posed by the presence of these metals in humic aquatic systems.

Environmental context Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health.Rationale Dissolved organic carbon (DOC) is both ubiquitous and heterogeneous in freshwater. Freshwaters are browning universally and pH values are decreasing with climate change. DOCs influence water pH, whereas changes in water pH potentially alter the conformation and function of DOCs. The physicochemical properties of DOCs can be characterised by optical and chemical indices, but the inter-relationships among them, and the effects of low pH, are not well understood.Methodology We characterised five naturally sourced DOCs, spanning large differences in origin and composition, at pH 7 and 4, using multiple indices: specific absorbance coefficient at 340 nm, molecular weight index, fluorescence index, octanol-water partition coefficient, molecular charge, proton binding index, size-fractionation, and percentage humic-acid-like, percentage fulvic-acid-like and percentage protein-like components by fluorescence-based parallel factor analysis.Results Many of the indices changed between pH 7 and 4 as reflected in the corresponding absorbance and fluorescence profiles. Generally, apparent aromaticity, apparent molecular weight and molecular charge all decreased with low pH, while lipophilicity increased. Key positive correlations occurred between aromaticity and apparent molecular weight, chemical reactivity and apparent molecular weight, and aromaticity and chemical reactivity, and a negative correlation between lipophilicity and molecular charge. These relationships were pH dependent.Discussion Our results highlight that physicochemical indices used to characterise DOCs from distinct sources should consider pH and be interpreted carefully. The pH-dependent changes in many of the indices are likely alterations in the conformation, reflected in the optical signatures, rather than changes in the composition of DOCs. In contrast, increased lipophilicity and reduced charge at lower pH are due to actual changes in DOC molecules, resulting from proton binding. The ecological functions of DOCs are dependent on source and will likely change with natural acidification events such as increasing atmospheric CO2.

Soil organic matter (SOM) has a critical role in regulating soil phosphorus (P) dynamics and producing phytoavailable P. However, soil P dynamics are often explained mainly by the effects of soil pH, clay contents, and elemental compositions, such as calcium, iron, and aluminum. Therefore, a better understanding of the mechanisms of how SOM influences phytoavailable P in soils is required for establishing effective agricultural management for soil health and enhancement of soil fertility, especially P-use efficiency. In this review, the following abiotic and biotic mechanisms are discussed; (1) competitive sorption between SOM with P for positively charged adsorption sites of clays and metal oxides (abiotic reaction), (2) competitive complexations between SOM with P for cations (abiotic reaction), (3) competitive complexations between incorporation of P by binary complexations of SOM and bridging cations with the formation of stable P minerals (abiotic reaction), (4) enhanced activities of enzymes, which affects soil P dynamics (biotic reaction), (5) mineralization/immobilization of P during the decay of SOM (biotic reaction), and (6) solubilization of inorganic P mediated by organic acids released by microbes (biotic reaction). Graphical Abstract

Phosphorous recovery from chemical phosphorus removal (CPR) plants is limited. CPR plants that use iron salts for phosphorous removal generate iron phosphate (Fe-P) rich sludge. This study investigates the influence of pH and competing anions (chloride), on phosphorus and soluble iron release from lab simulated Fe-P sludge. Factors influencing recovery, such as sludge age and Fe/P molar ratio are also investigated. Results showed that alkaline treatment of Fe-P sludge was not effective in releasing Fe (<3%) but effective in releasing P, particularly at controlled pH value of 10 where the %P release was (90 & PLUSMN; 2%). Sludge ageing time did not affect P release from Fe-P sludge at ages less than 5 days. However, a remarkable decrease in %P release (i.e., 50% reduction) was observed at ages of 9 and 11 days for pH values of 9-10. Arsenic extraction of exchangeable surface bound P from Fe-P sludge and surface area determination of hydrous ferric oxide (HFO) under different aging times supported qualitatively the internalization of surface bound P during aging, which is a proposed mechanism responsible for the decrease in P release for older sludge. Chloride effect in releasing P from Fe-P sludge was negligible. The reduction in P release by aging was best described by a zero-order kinetic model. Modeling with the PHREEQC geochemical software did not always agree with the measured release of phosphorus from the Fe-P sludge. PHREEQC assumes all surface sites for phosphorus are exchangeable and has no mechanism to represent phosphorus trapped within HFO particles.

Chemically mediated recovery of phosphorous (P) as vivianite from the sludges generated by chemical phosphorus removal (CPR) is a potential means of enhancing sustainability of wastewater treatment. This study marks an initial attempt to explore direct P release and recovery from lab synthetic Fe–P sludge via reductive dissolution using ascorbic acid (AA) under acidic conditions. The effects of AA/Fe molar ratio, age of Fe–P sludge and pH were examined to find the optimum conditions for Fe–P reductive solubilization and vivianite precipitation. The performance of the reductive, chelating, and acidic effects of AA toward Fe–P sludge were evaluated by comparison with hydroxylamine (reducing agent), oxalic acid (chelating agent), and inorganic acids (pH effect) including HNO3, HCl, and H2SO4. Full solubilization of Fe–P sludge and reduction of Fe3+ were observed at pH values 3 and 4 for two Fe/AA molar ratios of 1:2 and 1:4. Sludge age (up to 11 days) did not affect the reductive solubilization of Fe–P with AA addition. The reductive dissolution of Fe–P sludge with hydroxylamine was negligible, while both P (95 ± 2%) and Fe3+ (90 ± 1%) were solubilized through non-reductive dissolution by oxalic acid treatment at an Fe/oxalic acid molar ratio 1:2 and a pH 3. With sludge treatment with inorganic acids at pH 3, P and Fe release was very low (<10%) compared to AA and oxalic acid treatment. After full solubilization of Fe–P sludge by AA treatment at pH 3 it was possible to recover the phosphorus and iron as vivianite by simple pH adjustment to pH 7; P and Fe recoveries of 88 ± 2% and 90 ± 1% respectively were achieved in this manner. XRD analysis, Fe/P molar ratio measurements, and magnetic attraction confirmed vivianite formation. PHREEQC modeling showed a reasonable agreement with the measured release of P and Fe from Fe–P sludge and vivianite formation.

The Conditional Affinity Spectrum (CAS) is essential for describing the binding site heterogeneity of natural organic matter. A numerical method called ‘Fully Optimized ContinUouS’ (FOCUS) is described and employed to determine the CAS of H and Pb binding to Suwannee River Humic Acid (SRHA). The FOCUS-CAS is compared with the analytical solution of CAS based on the fitting of the experimental data to the Non-Ideal Competitive Adsorption (NICA) model. Although both methods give satisfactory results, their CAS distribution features are different, indicating that different CAS can reproduce the binding data at similar level of goodness-of-fit. Within a comparable, but not identical, affinity range, the FOCUS-CAS shows multimodal distributions, whereas the NICA-CAS displays bimodal distributions. These differences are due to the fact that the NICA-CAS assumes an a priori functionality (i.e., carboxylic and phenolic), while the FOCUS-CAS does not make this assumption. Without knowing the chemical reality of the binding site explicitly, both methods are viable. The NICA-CAS is theoretically superior to the FOCUS-CAS because it provides informative descriptions of competition between the H- and Pb-binding. Nonetheless, the FOCUS-CAS is practically more convenient because it does not require proton binding data. Additionally, the FOCUS-CAS can describe some strong metal-binding sites (e.g., thiol) that cannot be described by the NICA-CAS and, thus, constitutes a suitable alternative to NICA-CAS.

The demand for rare earth elements (REEs) is growing and as a result, environmental exposure is a concern. The objective of this research was to evaluate the acute toxicity of Tm to Hyalella azteca and to understand the potential for toxicity modification by dissolved organic matter (DOM) and the cations Ca2+, Mg2+ and Na+. Standard methods were followed for 96 h static exposures in a medium with a hardness of 60 mg CaCO3/L, pH of 7.3 at 23 °C. H. azteca neonates (2–9 d of age) were used and in unmodified media the LC50 concentration was 3.4 µM [95% CI 2.9–3.9 µM; 573 µg/L (482–663)] based on measured dissolved concentrations at the end of the test. Tests done with different concentrations of Ca (0.25, 0.5 and 1.5 mM) did not show consistent trends and there was no clear evidence of a protective effect from Ca. Variations in Na (0.26, 0.5 and 1.6 mM) resulted in no significant changes in toxicity. Similarly, Mg (0.07, 0.14 and 0.4 mM) did not result in significant changes in LC50 values, except for a reduction in toxicity for measured total Tm at the lowest Mg concentration. Our results indicate that Tm toxicity is not influenced by cationic competition (Ca, Na and Mg). Dissolved organic matter (sourced from Luther Marsh ON) offered significant protection against Tm toxicity. Addition of 9 mg DOC/L resulted in significantly increased LC50 values. This study contributes toward understanding the toxicity of Tm and the importance of considering dissolved organic matter in estimating the potential for environmental risk of REEs.