Assessing the viability of recovered phosphorus from eutrophicated aquatic ecosystems as a liquid fertilizer

One of the most important worldwide environmental challenges is the alteration of the biogeochemical cycle of phosphorus (P). P is globally exported from terrestrial to aquatic ecosystems, causing the eutrophication of the receiving waters. In this context, magnetic microparticles (MPs) have been recently proposed for trapping P in natural eutrophicated ecosystems, as well as in treated wastewaters. The main advantage of using MPs is that both P and MPs can be recovered from the treated water. Thus, the working hypothesis of the present study is that P can be desorbed from P-loaded MPs and recovered P can be later used as a fertilizer. To test this hypothesis, the best working conditions for desorbing P from P-loaded MPs were identified; then, an experiment with different plant nutrient solutions (neutralized solutions containing recovered P and an unfertilized control) was carried out with three different plant species: Ocimum basilicum L., Cucumis sativus L. and Cucumis melo L. Finally, germination, height, root and shoot biomass and P concentration in root and shoot were compared among treatments. Our results show that the best conditions for P desorption from P-loaded MPs occurred when using 0.1 M NH₄OH and using H₃PO₄ for neutralizing pH. The greenhouse fertirrigation pot experiment showed that the neutralized solution containing desorbed P from P-loaded MPs can be used as a liquid fertilizer, since its combination with macro and microelements significantly increased plant height, growth rate, shoot and root biomass and shoot and root P concentration. As a result, MPs can be proposed to be used for counteracting the widespread and coupled problems of the exhaustion of the P reserves and the eutrophication of aquatic ecosystems.

Evaluating the effect of CFH-12® and Phoslock® on phosphorus dynamics during anoxia and resuspension in shallow eutrophic lakes

Laboratory experiments with intact sediment cores from a hypertrophic very windy exposed shallow lake were conducted to assess the combined effect of anoxia and sediment resuspension on phosphorus (P) dynamics after adding different P adsorbents (CFH-12® and Phoslock®). In this study we hypothesize that the addition of geoengineering materials will increase P retention in the sediment even at the worst physic-chemical conditions such as anoxia and sediment resuspension. Both adsorbents significantly reduced the P release from the sediments after a 54 days-anoxic incubation period (CFH-12® by 85% and Phoslock® by 98%) and even after resuspension events (CFH-12® by 84% and Phoslock® by 88%), indicating that both adsorbents are suitable P inactivating agents for restoring shallow eutrophicated lakes under such circumstances. CFH-12® did not release dissolved Fe to the water column neither after the anoxic period nor after resuspension events compared to Control (no adsorbents addition). The La concentration was significantly higher in Phoslock® (3.5-5.7 μg L^-1) than in Control at all sampling days but it was not affected by resuspension. The high efficiency in P removal under anoxia and resuspension, the low risk of toxicity and the high maximum adsorption capacity makes CFH-12® a promising adsorbent for lake restoration. Nevertheless, further research about the influence of other factors (i.e. pH, alkalinity, interfering substances or strict anoxia) on the performance of CFH-12® is needed.

Ecotoxicity screening of novel phosphorus adsorbents used for lake restoration

Short-term standardized laboratory tests were carried out for evaluating acute and chronic toxicological effects of novel phosphorus (P) adsorbents on Raphidocelis subcapitata (algal growth rate inhibition) and on Daphnia magna (immobilization, with direct and indirect exposure to adsorbents, and uptake-depuration tests). Four P adsorbents were tested: two magnetic (HQ and Fe₃O₄) and two non magnetic (CFH-12^® and Phoslock^®). For the case of the algal growth inhibition test, the EC₅₀ was 1.5 and 0.42 g L^-1 for HQ and CFH-12^®, respectively, and no inhibition patterns were observed neither for Fe₃O₄ nor for Phoslock^®. When organisms were exposed to a direct contact, in the D. magna immobilization test, no statistically significant differences were found in the EC₅₀ values among the four studied adsorbents. The huge difference between direct and indirect contact experiments suggests that toxicity is mainly physically mediated. The uptake-depuration test evidenced a much faster uptake and depuration rates for Phoslock^®, which was precisely the adsorbent with the highest particle size. In a realistic worst-case scenario using data from Honda lake (Almería, Spain), where lake restoration is carried out by a adding a single large dose to bind surplus P in the lake, the predicted environmental concentrations for all adsorbents were lower than EC₅₀ for all adsorbents and they were found to exceed a provisional limit value for ecotoxicity after a short-term exposure. All in all, since neither accumulation nor longer term effects of P adsorbents in the pelagic phase is expected, this risk may however, on a case-to-case basis, be acceptable.

Do magnetic phosphorus adsorbents used for lake restoration impact on zooplankton community?

Magnetic microparticles (MPs) have been recently proposed as innovative and promising dissolved inorganic phosphorus (DIP) adsorbents. However, before using them in a whole-lake restoration project, it is essential to assess their toxicological effects (direct and indirect) on aquatic biota. In the present study we hypothesized that zooplankton community is affected by MPs used for lake restoration. To test our hypothesis we designed a microcosms experiment (n = 15) containing lake water and surface sediment from a hypertrophic lake. Temporal changes (70 days) on physico-chemical conditions and on zooplankton structure (rotifers, copepods and branchiopods) were monitored under different scenarios. In particular, three different treatments were considered: no addition of MPs (control) and MPs addition (1.4 g MPs L^-1) on the surface water layer (T-W) and on the sediment (T-S). After 24 h of contact time, MPs were removed with a magnetic rake. A total of 15 zooplankton species (12 rotifers, 1 branchiopod and 2 copepods) were recorded and a high abundance of zooplankton was registered during the experiment for all treatments. No significant differences (RM-ANOVA test; p > 0.05) in total abundance, species richness and species diversity among treatments were found. The absence of any effect of MPs on zooplankton can be explained because MPs did not significantly alter any of its physico-chemical (e.g. temperature, pH, O₂) or biological (e.g. food quantity and quality) drivers. These results confirm the suitability of MPs as a promising tool for removing DIP in eutrophic aquatic ecosystems.

Determining major factors controlling phosphorus removal by promising adsorbents used for lake restoration: A linear mixed model approach

Phosphorus (P) removal from lake/drainage waters by novel adsorbents may be affected by competitive substances naturally present in the aqueous media. Up to date, the effect of interfering substances has been studied basically on simple matrices (single-factor effects) or by applying basic statistical approaches when using natural lake water. In this study, we determined major factors controlling P removal efficiency in 20 aquatic ecosystems in the southeast Spain by using linear mixed models (LMMs). Two non-magnetic -CFH-12^® and Phoslock^®- and two magnetic materials -hydrous lanthanum oxide loaded silica-coated magnetite (Fe-Si-La) and commercial zero-valent iron particles (FeHQ)- were tested to remove P at two adsorbent dosages. Results showed that the type of adsorbent, the adsorbent dosage and color of water (indicative of humic substances) are major factors controlling P removal efficiency. Differences in physico-chemical properties (i.e. surface charge or specific surface), composition and structure explain differences in maximum P adsorption capacity and performance of the adsorbents when competitive ions are present. The highest P removal efficiency, independently on whether the adsorbent dosage was low or high, were 85-100% for Phoslock and CFH-12^®, 70-100% for Fe-Si-La and 0-15% for FeHQ. The low dosage of FeHQ, compared to previous studies, explained its low P removal efficiency. Although non-magnetic materials were the most efficient, magnetic adsorbents (especially Fe-Si-La) could be proposed for P removal as they can be recovered along with P and be reused, potentially making them more profitable in a long-term period.

Assessment of toxic effects of magnetic particles used for lake restoration on Chlorella sp. and on Brachionus calyciflorus

Laboratory tests, by following standardized Organization for Economic Co-operation and Development (OECD) protocols, were run for evaluating the acute effects of iron magnetic microparticles (MPs), recently proposed for lake restoration, on Chlorella sp. (algal growth) and on the rotifer B. calyciflorus (mortality). In addition, the MPs potential indirect effects on rotifer egg bank were assessed by performing hatching rate test with B. calyciflorus cysts in contact with dissolved iron (Tot-Fe_dis). In the algal growth test, no inhibition occurred at the two lowest MPs concentrations (0.01 and 0.05 g l^-1) which would correspond, considering the adsorption efficiency ratio (Phosphorus: MPs), to P concentrations lower than 0.94 mg P l^-1, much higher than typical concentrations found in natural waters. For higher MPs dose (EC₅₀ for Chlorella sp. was 0.15 g l^-1), no nutrient limitations but high turbidity and Tot-Fe_dis values cause negative effects on algal growth. For the case of B. calyciflorus, LC₅₀ was 1.63 g MPs l^-1 (corresponding to 30.7 mg P l^-1). When analyzing Tot-Fe_dis effect, the hatching rate of B. calyciflorus cysts was 100% for all treatments. To sum up our results for B. calyciflorus acute and chronic toxicity tests, it is extremely unlikely the mortality of adult organisms in contact with MPs as well as an affectation of the rotifer egg bank. In conclusion, it is expected that MPs addition in a real whole-lake application cause minor lethal and sublethal effects on both Chlorella sp. and B. calyciflorus.

Synthesis and characterization of magnetic chitosan microspheres as low-density and low-biotoxicity adsorbents for lake restoration

We propose a novel magnetic adsorbent for optimal Phosphorus (P) removal from the upper sediment layers. For this aim, magnetic chitosan microparticles were prepared using a reverse-phase suspension cross-linking technique. The resulting particles and suspensions were characterized using scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, magnetometry, thermogravimetric analysis, electrophoretic mobility and turbidity measurements. The hybrids are multicore particles consisting of well dispersed magnetite nanoparticles (approx. 10% w/w) homogeneously distributed within the biopolymer matrix. These microparticles can be easily separated from the water column and sediment using magnetic field gradients. Their P adsorption capacity is evaluated in batch conditions resulting in a maximum P adsorption capacity of M_L = 4.84 mg g^-1 at pH = 7. We demonstrate that these particles are excellent candidates to remove P from water column and also P mobile from the upper sediment layers due to two main reasons: they sediment slower and present lower potential toxicity (due to a their larger size) than conventional iron/iron oxide microparticles previously proposed for lake restoration.

A microcosm experiment to determine the consequences of magnetic microparticles application on water quality and sediment phosphorus pools

This study used microcosms to evaluate the effects of adding iron (Fe) magnetic microparticles (MPs) on water quality, focusing on P concentrations in the water column and sediment. Two treatments were considered for a constant 85:1 MP:P_Mobile molar ratio: T-W, applying MPs on the surface water layer; and T-S, applying MPs on the sediment. MP addition reduced P concentrations in lake water and sediment, with both treatments producing a mean reduction of 68±6% in dissolved inorganic P concentration (DIP) over a 70-day oxic period and reductions of 80±8% (T-W) and 80±4% (T-S) over a 5-day anoxic period. MPs also decreased reactive silicate (Si) concentrations by around 50% in both periods, but dissolved organic carbon (DOC) was reduced by only 15% at 24h after MP addition. Despite the marked decrease in DIP concentration due to MP addition, there was no reduction in chlorophyll a (Chla), because post-treatment total P concentrations (>200μgL^-1vs. >700μgL^-1 before treatments) remained higher than required for changes in the biological community (0.05-0.1mgL^-1). With T-S treatment, there was a reduction of 15% in P bound to Al oxides, clay minerals, and humic substances (P_→NaOH) and of 12% in labile organic P (Org-P_Labile) versus controls. P bound to humic substances (P_{→NaOH, Humic}) was reduced by 11-22% in both treatments. Finally, T-W rather than T-S treatments are recommended for future whole-lake applications to achieve more effective P removal from water and sediment and a higher percentage MP recovery.

Acute and chronic effects of magnetic microparticles potentially used in lake restoration on Daphnia magna and Chironomus sp

Magnetic microparticles (MPs) have been recently proposed as a new and promising tool for restoring eutrophicated waters. In this study, we analyzed the acute (immobilization) and chronic effects of iron (Fe) MPs on Daphnia magna and on the benthic macroinvertebrate Chironomus sp. In the chronic toxicity tests the offspring production (male and female) in D. magna and the mortality of larvae and pupae, and adult emergence in Chironomus sp. experiments were used as the endpoints. The concentration of MPs that caused 50% of immobilized individuals (EC₅₀) in the acute toxicity test was much higher in D. magna (0.913g MPs l^-1) than in Chironomus sp. (0.445g MPs l^-1). The results of chronic toxicity tests in D. magna showed that in presence of dissolved Fe (dFe), parthenogenetic reproduction was significantly affected, while no significant effect on mortality of larvae and pupae and on adult emergence was detected in Chironomus sp. test. Taking into account both that long-term exposure is not likely to occur and the regular dose of MPs potentially used in a restoration plan, we conclude that MPs is a riskless (no toxic effect on planktonic and benthic organisms) and efficient (high P adsorption capacity) tool for lake restoration.

Magnetic microparticles as a new tool for lake restoration: A microcosm experiment for evaluating the impact on phosphorus fluxes and sedimentary phosphorus pools

In the last decades, magnetic particles (MPs) as adsorbents have gained special attention due to their high adsorption capacity and the possibility of recovering them by applying a magnetic separation gradient. For the first time MPs have been tested as P adsorbents in a microcosm experiment in a context of lake restoration. MPs were added to sediment cores from a hypertrophic lake, at Fe:PMobile molar ratio of 285:1 and 560:1 under both, oxic and anoxic conditions. We have found that, under anoxic conditions (anoxic), MPs are able to reduce P release rate from the sediment to the overlying water and to reduce sedimentary PMobile concentration (a 22-25% reduction within 0-4 cm depth compared to controls). Under oxic conditions, the addition of MPs do not affect P fluxes across the sediment and water interface since the lake sediment is naturally rich in iron oxides. However a measured reduction in sedimentary PMobile concentration (12-16% reduction in 0-10 cm depth) contributes to a potential reduction in long-term P efflux.

The influence of pH on manganese removal by magnetic microparticles in solution

An extensive experimental work is reported that aims to assess the efficiency in manganese (Mn) removal from aqueous solution by carbonyl iron microparticles using magnetic separation techniques. A set of batch experiments are performed to explore the effect of pH, adsorbent concentration, surface coating and contact time for achieving the highest Mn removal efficiency. Mn removal efficiency is extremely high (>98%) for pH values larger than 9 as a result of the chemisorption of Mn oxides onto magnetic microparticles. In contrast, Mn removal efficiency for pH < 9 was significantly reduced as Mn remains as a soluble cation. In this manuscript we demonstrate that the efficiency clearly increases when increasing the adsorbent concentration and when using MnOx(s) coated magnetic particles instead of bare particles. Desorption rates from Mn-loaded magnetic particles at different pHs were always lower than 15%. Furthermore, Mn removal efficiency remained at a very high value (>95%) when reused particles were employed in the adsorption process.

Chemical interferences when using high gradient magnetic separation for phosphate removal: consequences for lake restoration

A promising method for lake restoration is the treatment of lake inlets through the specific adsorption of phosphate (P) on strongly magnetizable particles (Fe) and their subsequent removal using in-flow high gradient magnetic separation (HGMS) techniques. In this work, we report an extensive investigation on the chemical interferences affecting P removal efficiencies in natural waters from 20 Mediterranean ponds and reservoirs. A set of three treatments were considered based on different Fe particles/P concentration ratios. High P removal efficiencies (>80%) were found in freshwater lakes (conductivities<600 μ S cm(-1)). However, a significant reduction in P removal was observed for extremely high mineralized waters. Correlation analysis showed that major cations (Mg(2+), Na(+) and K(+)) and anions (SO(4)(2-) and Cl(-)) played an essential role in P removal efficiency. Comparison between different treatments have shown that when increasing P and Fe concentrations at the same rate or when increasing Fe concentrations for a fixed P concentration, there exist systematic reductions in the slope of the regression lines relating P removal efficiency and the concentration of different chemical variables. These results evidence a general reduction in the chemical competition between P and other ions for adsorption sites on Fe particles. Additional analyses also revealed a reduction in water color, dissolved organic carbon (DOC) and reactive silicate (Si) concentrations with the addition of Fe microparticles.

Setting up High Gradient Magnetic Separation for combating eutrophication of inland waters

To find new approaches to devise technologies for handling with eutrophication of inland waters is a global challenge. Separation of the P from water under conditions of continuous flow is proposed as an alternative and effective method. This work is based on using highly magnetic particles as the seeding adsorbent material and their later removal from solution by High Gradient Magnetic Separation (HGMS). Contrast to other methods based on batch conditions, large volumes of water can be easily handled by HGMS because of decreasing retention times. This study identifies the best working conditions for removing P from solution by investigating the effects of a set of four different experimental variables: sonication time, flow rate (as it determines the retention time of particles in the magnetic field), magnetic field strength and the iron (Fe) particles/P concentration ratio. Additionally, the change of P removal efficiency with time (build up effect) and the possibility of reusing magnetic particles were also studied. Our results evidenced that while flow rate does not significantly affect P removal efficiency in the range 0.08-0.36 mL s(-1), sonication time, magnetic field strength and the Fe particles/P concentration ratio are the main factors controlling magnetic separation process.

Factors affecting phosphate adsorption to aluminum in lake water: implications for lake restoration

Treatment of lake inlets or lake sediments with aluminum (Al) is being increasingly used for lake restoration but only few studies exist concerning competitive substances that might influence phosphate (PO(4)(3-)) removal from lake water. Therefore, chemical interferences of several ions (magnesium, silicate, chloride and humic acid) on PO(4)(3-) adsorption to Al(OH)(3) were studied. Interference of each ion was studied in artificial lake water, and the complex interactions occurring in natural water were studied in water from 30 Danish lakes at pH 7 in both cases. In the artificial lake water Al:P ratio was high as sediment P-pools were the targets while in the natural lake water Al addition was generally lower as only P present in the water was targeted (i.e. inlet water). The single-ion experiments evidenced that silicate (>200 microM) and humic acids significantly decreased the effectiveness of PO(4)(3-) adsorption to Al(OH)(3) by 10-13% at 450 microM Si and 17% at 1 mM C, respectively. NaCl did not influence adsorption of PO(4)(3-) to Al(OH)(3), however, PO(4)(3-) removal was slightly reduced in seawater, mainly due to the presence of Mg(2+). The studies on interferences in natural lake water showed that as long as the PO(4)(3-) concentration was low (<5 microM), silicate competed with PO(4)(3-) for adsorption sites on Al(OH)(3) but at higher PO(4)(3-) concentrations, color and DOC (as indicators of HA) were the main variables decreasing PO(4)(3-) removal from lake water. Inhibition of PO(4)(3-) precipitation in natural lake water appeared complex and did not allow for a simple calculation of Al dose from the concentration of potentially competitive ions. Recommendation for lake management is therefore still that precipitation assays should be carried out for any type of inlet or lake water prior to Al application.