Soluble Reactive P Research Articles

Drivers of excess phosphorus and stream sediments in a nested agricultural catchment during base and stormflow conditions.

A variety of landscape and hydrological characteristics influence nutrient concentrations and suspended sediments in freshwater systems, yet the combined influence of these characteristics within nested agricultural catchments is still poorly understood, particularly across varying flow states. To tease apart potential drivers at within-catchment scales, it is necessary to sample at a spatiotemporal resolution that captures how landscape drivers change with time. The overall objective of this study was to evaluate the relative influence of landscape and hydrological characteristics at sub-catchment scales in relation to total P (TP), soluble reactive P (SRP), the ratio of SRP and TP (SRP/TP), and total suspended solids (TSS) across varying flow conditions. Synoptic surveys were conducted at 13 longitudinal sampling sites under a variety of flow conditions (n=14) between 2016 and 2017 in the Innisfil Creek watershed, southern Ontario. The surveys were grouped into baseflow and stormflow conditions, and partial least squares regression (PLSR) was used to characterize the relationships between catchment characteristics, median concentrations of P, and TSS. Soil texture (i.e., clay dominated), winter wheat (Triticum aestivum L.), and constructed drain density had the largest influences on stormflow SRP and SRP/TP ratios, but measures of soil erosion, like the Bank Erosion Hazard Index and sinuosity, had the largest influence on stormflow TSS. During baseflow periods, these landscape characteristics were not informative, and they were difficult to tie to in-stream conditions. Overall, our PLSR models indicated that buried tile drainage was a major source of SRP in Innisfil Creek, whereas bank erosion was a dominant source of TSS.

Effects of the origins and stabilization of biosolids and biowastes on their phosphorous composition and extractability

Phosphorous dissolution and ensuing chemical redistribution of P in organic amendments (OA) were studied by applying a modified Hedley selective fractionation to eight water-extracted and unextracted OAs. Nine 7-day, repeated extractions were applied using a 60:1 water:dry OA (v:w) ratio at pH 8. Eight OAs were tested including five biosolids, broiler litter, dairy manure compost and municipal solid waste compost. The average PWEP9 (percent water-extractable P following nine water-extraction cycles) for the OAs was 65 ± 9% and all of the fractions, with almost no exceptions, contributed to that figure. Organic P was depleted by mineralization (in non-stabilized sludges and broiler litter) or dissolution (stabilized composts) or both (in lime-treated biosolids) and that depletion was completed within 1–2 extraction cycles. Only the organic P of the MSWC remained undepleted. Strong linear correlations were observed between the WEP9 values of the OAs (0.8–21 g P kg−1) and several more easily determined properties, including total P content (r2 = 0.84), organic N content (r2 = 0.82), the sum of Hedley’s more easily dissolved SRP (soluble reactive P) and OP (r2 = 0.95), and the total P and SRP extracted by 16 h of shaking with the bicarbonate reagent (r2 ≥ 0.90). These findings indicate that if greater P availability is desired, the stabilization of biosolids and biowastes should be minimized. These insights into the relationships between OA characteristics and P solubility may benefit the use of OAs in agricultural systems and aid assessments of the environmental significance of their use.

Interception of phosphorus release from sediments using Mg/Fe-based layered double hydroxide (MF-LDH) and MF-LDH coated magnetite as geo-engineering tools

A magnesium/iron-based layered double hydroxide (MF-LDH) and a composite of MF-LDH and magnetite (MF-LDH@Fe3O4) were synthesized, characterized and used as solid-phase phosphorus (P)-sorbents (SPPSs) to control the release of sedimentary P. The behavior and mechanism of phosphate adsorption onto MF-LDH and MF-LDH@Fe3O4 were studied. The effect of MF-LDH capping and amendment on the migration of P in sediments were comparatively investigated, and the impact of fabric-wrapped and unwrapped MF-LDH@Fe3O4 capping on P mobilization in sediments were also comparatively investigated. Results showed that both MF-LDH and MF-LDH@Fe3O4 had good phosphate adsorption performance, and the adsorption mechanisms included cation exchange, electrostatic attraction, ligand exchange and inner-sphere complex formation. Sediment capping and amendment using MF-LDH both could dramatically reduce the risk of the release of soluble reactive P (SRP) and diffusive gradient in thin-films-labile P (P-DGT) from sediments into overlying waters (OLY-Ws), and the MF-LDH capping had a better suppressing efficiency of sediment-P release into OLY-W than the MF-LDH amendment. Sediment capping with the fabric-wrapped and unwrapped MF-LDH@Fe3O4 both greatly decreased the risk of SRP and P-DGT released from sediment into OLY-W, and the efficiency of the prevention of SRP released from sediment into OLY-W by the fabric-wrapped MF-LDH@Fe3O4 capping layer (about 81–90%) was slightly lower than that by the unwrapped MF-LDH@Fe3O4 capping layer (about 94–99%). The reduction of P-DGT in the top sediment and the direct interception of the soluble P from pore water (POR-W) to OLY-W by the MF-LDH@Fe3O4 capping layer were the keys to the management of P released from sediment by the MF-LDH@Fe3O4 capping. From the standpoint of the efficiency of sedimentary P suppression, the convenience of application and the sustainability of sediment remediation, sediment capping with the fabric-wrapped MF-LDH@Fe3O4 is a promising approach to manage the release of sedimentary P into OLY-W.

Control of internal phosphorus release from sediments using magnetic lanthanum/iron-modified bentonite as active capping material

The non-magnetic capping materials are difficult to be recycled from the water bodies after their application, leading to the increase in the cost of the sediment remediation. To address this issue, a capping material, i.e., magnetic lanthanum/iron-modified bentonite (M-LaFeBT) was prepared by loading lanthanum onto a magnetic iron-modified bentonite (M-FeBT) and used to control the internal phosphorus (P) loading in this study. To determine the capping efficiency and mechanism of M-LaFeBT, the impact of M-LaFeBT and M-FeBT capping on the mobilization of P in sediments was investigated, and the stabilization of P bound by the M-LaFeBT and M-FeBT capping layers was evaluated. Results showed that M-LaFeBT possessed good magnetic property with a saturated magnetization of 14.9 emu/g, and exhibited good phosphate adsorption ability with a maximum monolayer sorption capacity (QMAX) of 14.3 mg P/g at pH 7. Moreover, M-LaFeBT capping tremendously reduced the concentration of soluble reactive P (SR-P) in the overlying water (OL-water), and the reduction efficiencies were 94.7%–97.4%. Furthermore, M-LaFeBT capping significantly decreased the concentration of SR-P in the pore water and DGT (diffusive gradient in thin films)-labile P in the profile of OL-water and sediment. Additionally, most of P bound by the M-LaFeBT capping layer (approximately 77%) was stable under natural pH and reducing conditions. The phosphate adsorption ability for M-LaFeBT was much higher than that for M-FeBT, and the QMAX value for the former was 4.86 times higher than that for the latter. M-LaFeBT capping gave rise to a higher reduction of DGT-labile concentration in the profile of OL-water and sediment than M-FeBT capping. The P adsorbed by the M-LaFeBT capping layer was more stable than that by the M-FeBT capping layer. Results of this study demonstrate that M-LaFeBT is promising for utilization as an active capping material to intercept sedimentary P release into OL-water.

Regulating Effect and Mechanism of Calcite/Chlorapatite Mixture Addition on Transformation and Transport of Phosphorus in Sediments

Understanding the effect of calcite and chlorapatite mixture (CA/ClAP) addition on the mobilization of phosphorus (P) in sediments is crucial to the application of CA/ClAP as an amendment material to control the release of P from sediments. To address this issue, batch experiments were conducted to investigate the removal performance of phosphate by CA/ClAP, and sediment incubation experiments were carried out to study the effect of CA/ClAP addition on the mobilization of P in sediments. The results showed that the removal ability of phosphate by CA/ClAP was much higher than those by calcite and chlorapatite, and the kinetics data of phosphate removal by CA/ClAP followed a pseudo-second-order kinetics model. Increasing calcite and chlorapatite dosages would be favorable for the removal of phosphate by CA/ClAP, and coexisting Ca2+ enhanced the phosphate removal. CA/ClAP addition not only reduced the concentration of soluble reactive P (SRP) in the overlying water, but also decreased the concentration of SRP in the pore water. The addition of CA/ClAP in sediments caused an increase in the content of P in the sediments, but the increased P mainly existed in the form of calcium-bound P (HCl-P), which was difficult to be re-released into the water column under anoxic and common pH (5-9) conditions. The reduction of SRP in the pore water after the addition of CA/ClAP played an important role in the prevention of sedimentary P liberation into the overlying water by the CA/ClAP amendment. The results of this work indicate that CA/ClAP can be used as an amendment material for interception of the release of P from sediments into overlying water.

Combined use of calcium nitrate, zeolite, and anion exchange resin for controlling phosphorus and nitrogen release from sediment and for overcoming disadvantage of calcium nitrate addition technology.

Ca(NO3)2 addition has proved to have a high potential to immobilize internal phosphorus (P) in sediments; however, it cannot effectively stop the release of ammonium-nitrogen (NH4+-N) from sediments into overlying waters (OL-waters). Additionally, the addition of Ca(NO3)2 alone has high risk of nitrate-nitrogen (NO3--N) releasing into OL-waters. To overcome the shortcoming of the Ca(NO3)2 addition method, we reported an integrated method, i.e., a combined method based on Ca(NO3)2 injection, zeolite capping, and anion exchange resin (AERN)-contained floating system suspending (Ca(NO3)2/zeolite/AERN). The effectiveness and mechanism of the Ca(NO3)2/zeolite/AERN method for simultaneously controlling the release of soluble reactive P (SRP) and NH4+-N were investigated, and the NO3--N releasing risk of this method was evaluated. It was found that the joint use of Ca(NO3)2 injection, zeolite capping, and AERN-contained floating system suspending not only could effectively suppress the release of SRP and NH4+-N from sediments into OL-waters simultaneously, but also had much less risk of NO3--N releasing into OL-waters as compared to the single Ca(NO3)2 injection method and the combined Ca(NO3)2/zeolite method. The inhibition of the reductive dissolution of the P-bound Fe(III) oxides/hydroxides by the presence of nitrate and the adsorption of ammonium on the zeolite played very important roles in the interception of SRP and NH4+-N releasing into OL-waters by the Ca(NO3)2/zeolite/AERN method. After the sediment remediation using the Ca(NO3)2/zeolite/AERN approach, the increase in the content of residual P in the sediment layer of 0-50mm, the decrease of mobile P in the sediment layer of 0-10mm, and the increased NH4+-N adsorption capacity for the sediment layer of 0-10mm would be conductive to the interception of SRP and NH4+-N liberation in the long run. Results of this research suggest a promising application potential of the Ca(NO3)2/zeolite/AERN method in the simultaneous control of the release of SRP and NH4+-N from sediments.

Novel, recyclable active capping systems using fabric-wrapped zirconium-modified magnetite/bentonite composite for sedimentary phosphorus release control

A zirconium-modified magnetite/bentonite composite (M-ZrFeBT) was synthesized, characterized and combined with water-permeable fabric to construct novel, recyclable active capping systems for sedimentary phosphorus (P) release control. Three fabric-wrapped M-ZrFeBT capping devices with different shapes were designed, i.e., CAP-1, CAP-2 and CAP-3, and they are disc-shaped, cuboid-shaped and spindle-shaped capping devices, respectively. The behavior and mechanism for phosphate adsorption onto M-ZrFeBT was studied. The impact of CAP-1, CAP-2 and CAP-3 capping on the mobilization of P in sediments was investigated. The results showed that M-ZrFeBT possessed good phosphate adsorption ability, with a largest monolayer adsorption capacity of 8.02 mg P/g. The replacement of Fe/Zr bound hydroxyl groups with phosphate through ligand-exchange reactions to generate the inner-sphere Fe-O-P and Zr-O-P bonding played a key part in the uptake of phosphate from water by M-ZrFeBT. Sediment capping with fabric-wrapped M-ZrFeBT not only brought about a significant decline in the concentrations of soluble reactive P (SRP) and DGT (diffusive gradient in thin films)-labile P (LPDGT) in the overlying water, but also gave rise to the diminished SRP and LPDGT concentrations in the upper sediment. Most (96.5%-98.2%) of P bound by the M-ZrFeBT in the capping layers was in the form of NaOH extractable inorganic P, HCl-extractable P and residual P, which were considered to be hard to be released back into the water column under common pH and oxygen-deficient conditions. The reduction of pore water SRP and LPDGT in the upper sediment layer induced by the adsorption of SRP on the M-ZrFeBT-based capping layer played a key part in the interception of SRP liberation from the sediment solid into the overlying water. Results indicate that fabric-wrapped M-ZrFeBT capping is promising for controlling the internal P loading from sediments in shallow freshwater bodies.

Experimental study on phosphorus release from sediment with fresh-water snail (Bellamya aeruginosa) bioturbation in eutrophic lakes

To study the effects of snail (Bellamya aeruginosa) bioturbation on phosphorus (P) mobility in sediment and the relevant mechanisms behind them. A microcosm simulation experiment was carried out at millimeter resolution with an 89 days culture of homogeneous sediments. High-resolution dialysis (HR-Peeper) and diffusive gradient in thin films (DGT) techniques were utilized to simultaneously acquire the vertical distribution of soluble and labile P/iron (Fe) in bioturbation sediments. Snail bioturbation significantly enhanced soluble reactive P (SRP) and labile P concentrations, with the maximum increment to 306% and 1133% of the control sediments within the influence depths from 0 to − 30 mm and to − 20 mm, respectively. The P fluxes released from the sediment–water interfaces in snail treatments were correspondingly enhanced with the maximum increments to 218% of the control. Also, snail bioturbation significantly enhanced the soluble Fe(II) and labile Fe concentrations by maximum increment of 218% and 199% of the control within 0 to −24 mm and − 19 mm, respectively. The high correlation between soluble/labile P and Fe suggested that P release from the sediment under snail bioturbation was due to the reduction and dissolution of Fe(III) oxyhydroxide.

Internal phosphorus loading in a small shallow Lake: Response after sediment removal

Mankind is taking advantage of numerous services by small shallow lakes such as drinking water supply, irrigation, and recreational function; however, many of these lakes suffer from eutrophication. Given the key role of phosphorus (P) in eutrophication process, one of the effective restoration methods especially for small shallow lakes is removal of sediments enriched with nutrients. In our study, we used interannual, seasonal, and spatial data to examine the changes in sediment P mobility after removal of sediments in 2016 from a 1-ha highly eutrophic lake. We measured the sediment redox potential, analyzed soluble reactive P (SRP) in the pore water and P fractional composition of the surface sediments, and calculated the diffusive flux of P in three locations in two continuous years (2017 and 2018) after the excavation. Similar measurements were done before sediment removal at central site of the lake in 2015. Removing nutrient-rich sediment also removed 6400 kg of P, and thus the potential for release of P from sediments decreased on a long-term scale. However, a large pool of releasable P was rebuilt soon after the sediment removal due to high external P loading, resulting in extensive anoxia of sediment surface and associated internal P loading as high as 1450 mg m−2 summer−1. Moreover, the Fe-P and labile P fractions were the most important sources of P release, as evidenced by their considerable seasonal and interannual changes after the sediment removal. The sediment total Fe negatively correlated with diffusive flux of P, pore water SRP, and near-bottom water total P and SRP concentrations which indicated a strong linkage between sediment P dynamics and Fe after the restoration. Sediment removal could be a beneficial restoration approach, but the effects on lake water quality remain only short-term unless there is an adequate control on external loading to the lake.

Use of Iron-modified Calcite as an Active Capping Material to Control Phosphorus Release from Sediments in Surface Water Bodies

The use of calcite (CA) as an active capping material has high potential for controlling the release of phosphorus (P) from sediments, but its efficiency still needs to be enhanced. To address this issue, an iron-modified CA (Fe-CA) was prepared, the removal performance of phosphate from aqueous solution by Fe-CA was studied, and the efficiency of the use of Fe-CA as an active capping material to prevent the liberation of P from sediments was investigated. The results showed that Fe-CA exhibited much higher phosphate removal ability than CA. The phosphate removal efficiency of Fe-CA increased with an increase in the Fe-CA dosage. Increasing the initial phosphate concentration gave rise to an increase in the amount of phosphate removed by Fe-CA, and the maximum amount of phosphate removed by Fe-CZ reached 3.09 mg·g-1. Sediment capping with Fe-CA could effectively control the release of soluble reactive P (SRP) from the sediment into the overlying water, leading to a very low concentration of SRP in the overlying water. Additionally, the Fe-CA capping also resulted in the transformation of a small amount of redox-sensitive P (BD-P) and metal-oxide-bound P (NaOH-rP) in sediments to residual P (Res-P), leading to a slight increase in the stability of P in the sediment. The overwhelming majority (90.8%) of P bound by the Fe-CA capping layer existed in the form of NaOH-rP, calcium-bound P (HCl-P), and Res-P, which are relatively very stable. Furthermore, the percentage of bioavailable P (BAP) as a proportion of total extractable P in the P-bound Fe-CA capping layer was very low, and the bound P was re-released with difficulty into the water column for algae growth. Compared to CA capping, the efficiency for the control of sedimentary P release into the overlying water by Fe-CA capping was much higher, and the stability of P bound by the Fe-CA capping layer was also higher. The results of this work indicate that Fe-CA is a very promising active capping material for the interception of the release of P from sediments into the overlying water.

Saltwater intrusion affects nutrient concentrations in soil porewater and surface waters of coastal habitats

AbstractCoastal ecosystems are undergoing major biogeochemical shifts due to climate change and sea level rise. At the same time, agricultural fertilizer applications are increasing coastal nutrient inputs. In this study, we examine the potential impact of saltwater intrusion (SWI) on nitrogen (N) and phosphorus (P) concentrations in soil porewater and surface water of different habitats within the Chesapeake Bay estuary. Study sites are located along Maryland's Lower Eastern Shore and were monitored over three summers from 2016 to 2018. These sites encompass various habitats on the land–sea interface, consisting of healthy forests, intruded forests, abandoned fields, intruded fields, agricultural ditches, tidal creeks, and tidal salt marshes. Intruded fields were being actively farmed at the time of the study. Soil porewater and surface water grab samples were collected from the habitats and analyzed for electrical conductivity, pH, dissolved organic P (DOP), dissolved inorganic P in the form of soluble reactive P (SRP), dissolved inorganic N as ammonium‐N (NH4‐N), and total dissolved iron (TDFe). Electrical conductivity was greatest in the marshes (16.58 mS/cm averaged across all years) and did not significantly differ among intruded forests, intruded fields, and agricultural ditches. As a legacy of heavy fertilizer use, DOP concentrations exceeded 0.45 mg P/L in all habitats. Concentrations of inorganic N and P differed significantly by habitat. Concentrations of NH4‐N were significantly higher in salt marsh soil porewater than in any other habitat measured. Overall, SRP concentrations were the highest in the soil porewater of intruded fields and marshes and in the surface water of agricultural ditches (0.30, 0.29, and 0.33 mg P/L averaged across all years, respectively). These concentrations greatly exceeded recommended U.S. Environmental Protection Agency nutrient pollution thresholds for the region. In its oxidized state, dissolved iron can bind to SRP and prevent it from becoming bioavailable. However, TDFe concentrations in the ditches, tidal creeks, and marshes were too low to adequately buffer against SRP loss to downstream areas. As SWI moves salts inland and increases hydrologic connectivity across coastal landscapes, it is important to consider the mechanisms through which nutrients may be released from coastal soils and their potential to impact downstream water quality.

Cyclical patterns and (im)mobilization mechanisms of phosphorus in sediments from a small creek estuary: Evidence from in situ monthly sampling and indoor experiments

Internal phosphorus (P) mobility is crucially important to overlying water ecosystems, while its spatiotemporal variations and mechanisms remain to be studied, especially in dynamic estuarine sediments. In this study, in situ monthly field sampling and indoor experiments were combined to measure the soluble reactive P (SRP), soluble Fe and diffusive gradients in thin films (DGT)-labile P/S in the overlying water, sediment and porewater in the Jiuxi River Estuary by employing high-resolution dialysis (HR-Peeper), the DGT technique and a MicroRhizon sampler. The consistent tendency between DGT-labile S and P in most seasons indicates that P mobilization was dominated by intense dissimilatory sulfate reduction (DSR), causing high SRP concentrations and active exchange with the overlying water. The circannual cyclical pattern of P is summarized, where in addition to temperature, monthly changes in runoff and tidal range are crucial external factors to control long-term P cycling via changed redox environments and terrigenous materials inputs. The mobile P, Fe and S present higher values during flood tides and lower values during ebb tides in tidal simulation experiments, demonstrating that the short-term cycling of P, Fe and S in intertidal surface sediments is highly redox-sensitive and controlled by tidal processes. The results also reveal that DSR greatly facilitates P mobility and release, while sediment oxidation and the induced enhancement in DIR and Fe cycling can effectively control P immobilization.

Recent Patterns in Lake Erie Phosphorus and Chlorophyll a Concentrations in Response to Changing Loads.

Despite the initial success of extensive efforts to reduce phosphorus (P) loading to Lake Erie as a part of the Great Lakes Water Quality Agreement, Lake Erie appears to be undergoing a re-eutrophication and it is plagued by harmful algal blooms. To offer insights into potential lake responses under differing Maumee River loads and reveal recent changes with time, we explored patterns in phosphorus and chlorophyll a data from 2008 to 2018 collected in western Lake Erie near the mouth of the Maumee River. We found high, but relatively stable Maumee River and lake concentrations of total P (TP) and soluble reactive P (SRP) with no discernable annual or seasonal patterns. Maumee spring TP load was not strongly related to lake TP, and lake SRP concentrations were positively but weakly related to SRP loads. Lake TP was a strong predictor of chlorophyll a, but the relationship was weaker at sites closer to the Maumee. These results highlight spatial differences both in P concentration and the relationship between TP and chlorophyll a, and these indicate that spring phosphorus loads are a weak algal biomass predictor in the portion of the western basin of Lake Erie represented by these sampling stations.

Utilization of zirconium-modified granular zeolite as capping and mixing materials to suppress phosphorus release from sediment in landscape water body

A zirconium-modified granular zeolite (ZMGZ) was created and investigated for the suppression of the internal P loading in shallow landscape water bodies in this study. Results showed that the as-synthesized ZMGZ possessed high phosphate sorption capacity with a Langmuir maximum monolayer uptake capacity of 6.87 mg PO43−/g. Capping of landscape water body sediments with ZMGZ reduced the releasing rate of soluble reactive P (SRP) from the sediments into the overlying water, and it significantly reduced the SRP releasing rate under anoxic condition. Furthermore, the sediment capping decreased the pore water SRP concentration, and it was difficult for most of P trapped by the ZMGZ covering layer to be re-released into the water column. When the ZMGZ in the covering layer was mixed with the sediment, the ZMGZ became a sediment amendment. Mixing of landscape water body sediments with ZMGZ also resulted in the reduction of the SRP concentrations in the overlying and pore waters under the conditions of low and high dissolved oxygen, and it was also hard for most of P adsorbed by the added ZMGZ to be released back into the water column. Additionally, the addition of sediments with ZMGZ led to the decrease of the amounts of mobile and bioavailable P in the sediments. To sum up, we conclude that the capping/mixing combination method, which involves sediment capping with ZMGZ at first and then gradual mixing of ZMGZ with sediment via various natural disturbances (e.g., physical disturbance and bioturbation), is a very promising strategy for inhibiting the release of SRP from shallow landscape water body sediments into the overlying water.

Assessment of iron-modified calcite/zeolite mixture as a capping material to control sedimentary phosphorus and nitrogen liberation.

Calcite/zeolite mixture (CZ) can be used to construct a capping layer for the simultaneous management of phosphorus (P) and nitrogen (N) liberation from sediments into the overlying water (OVER-water). However, its control efficiency of sedimentary P release still needs to be improved. To address this issue, an iron-modified CZ (Fe-CZ) was synthesized, characterized, and employed as a capping material to simultaneously prevent P and N release from sediments into OVER-water. Batch and microcosm incubation experiments were performed to study the efficiency and mechanism for the control of P and N release from sediments by capping Fe-CZ. Results showed that sediment capping with Fe-CZ resulted in the significant reduction of soluble reactive P (SRP) and ammonium-N (NH3-N) in OVER-water, with reduction rates of 77.8-99.7% and 54.0-96.7%, respectively. Furthermore, the Fe-CZ capping layer decreased the SRP concentration in the pore water (PORE-water) at depth of 0-30mm and reduced the concentration of PORE-water NH3-N at depth of 0-50mm. Moreover, the Fe-CZ capping layer gave rise to the great decrement of the concentration of the labile P measured by DGT (diffusive gradient in thin films) technology (P-DGT) in the profile of OVER-water and sediment. Additionally, the Fe-CZ capping resulted in the reduction of redox-sensitive P (P-BD) in the 0-50mm sediment and caused the transformation of P-BD to calcium-bound P (P-HCl) and residual P (P-RES) in the 0-10mm sediment as well as to P-RES in the 10-20mm sediment. Results of this work indicate that the Fe-CZ capping has a high potential for the simultaneous management of P and N release from sediments, and the decrease of the contents of sediment P-DGT, sediment P-BD, PORE-water SRP and PORE-water NH3-N as well as the conversion of mobile P to more stable P in the top sediment should have a significant role in the simultaneous interception of sedimentary P and N liberation into OVER-water by the Fe-CZ capping.

Comparison of the Control of Sedimentary Phosphorus Release Using Zirconium-, Lanthanum-, and Lanthanum/Zirconium-Modified Zeolites as Sediment Amendments

In this study, the adsorption characteristics of phosphate on zirconium-modified zeolite (ZrMZ), lanthanum-modified zeolite (LaMZ) and lanthanum/zirconium-modified zeolite (LaZrMZ) were comparatively investigated, and the effect of ZrMZ, LaMZ, and LaZrMZ addition on the mobilization of phosphorus (P) in sediments was comparatively studied. Results showed that the phosphate adsorption capacity decreased in the order of LaZrMZ > LaMZ > ZrMZ. The addition of LaZrMZ, LaMZ, and ZrMZ in sediments all could effectively reduce the concentrations of soluble reactive P (SRP) in the overlying and pore waters. Furthermore, the addition of LaZrMZ, LaMZ and ZrMZ all could decrease the amount of mobile P in sediments, and the reduction rate decreased in the order of LaMZ > LaZrMZ > ZrMZ. The amendment of sediments with LaZrMZ, LaMZ, and ZrMZ all could lead to a decrease in the amount of water-soluble P (WSP), algal available P (AA-P) and iron oxide-filter paper extractable P (FeO-P) in the sediments. The reduction rate of WSP and AA-P decreased in the order of LaMZ > LaZrMZ > ZrMZ, and the reduction rate of FeO-P decreased in the order of LaZrMZ > ZrMZ > LaMZ. The addition of LaZrMZ and LaMZ both could reduce the content of readily desorbed P (RDP), and the reduction rate decreased in the order of LaMZ>LaZrMZ. The LaZrMZ and ZrMZ amendments both could decrease the concentration of NaHCO3 extractable P (Olsen-P) in sediments, and the reduction rate decreased in the order of LaZrMZ>ZrMZ. The results of this study demonstrate that the addition of ZrMZ, LaMZ, and LaZrMZ in sediments all could effectively intercept the upward mobilization of sedimentary P to the overlying water, and LaZrMZ is a very promising amendment for the control of the internal P loading from the point of view of both phosphate adsorption ability and bioavailable P immobilization efficiency.

Fine-scale remobilization of phosphorus by rooted macrophytes (Phragmites australis) growth in lake sediments: evidence from a holistic growth period simulation study

Aquatic vegetation serves an important structuring function in shallow freshwater ecosystems. Although increasing evidence indicates that sediment-associated phosphorus (P) is mobilized by aquatic macrophytes under P-deficient conditions, the influence of the holistic growth period of rooted macrophytes on transfer mechanism and bioavailability of sediment P around rhizosphere at millimeter scale remains unclear. In this present study, a 120-day batch intact sediment microcosm simulation was implemented to explore the effect of the whole Phragmites australis growth period on the stability and exchange of sediment P across critical micro-interfaces in lacustrine ecosystems. High-resolution dialysis peeper (HR-Peeper) was used to investigate the variations of pore water P in sediments around the P. australis rhizosphere and Zr-oxide diffusive gradients in thin-film (DGT) sampler was used to capture changes in the two-dimensional (2D) images of labile P over the whole growth period. Phosphorus fractionation showed a general decrease of total phosphorus (TP) and calcium-bound P (Ca-P), whereas an increase in iron-adsorbed P (Fe-P), loosely bound P (LS-P), and organic P (Org-P) was observed on day 120 compared to the values on day 0. Notably, the Ca-P content decreased by approximately 77%, while the Fe-P content increased by approximately 400%. Highly synchronous rises in P release flux and in the morphological characteristics of P. australis were the exponential function of incubation time. High-resolution data demonstrated that concentrations of soluble reactive P (SRP) and labile P concentrations in pore water were prominently enhanced by P. australis growth. Meanwhile, a top-down root-shaped patchy distribution pattern of labile P in the pore water was obviously stimulated over time. The reason for this phenomenon could be ascribed to remobilization of sediment mineral P by root organic exudates, as well as Fe-coupled accumulation of labile P due to oxygenation of Fe2+ followed by the formation of Fe plaques on the root surface within the more oxic rhizosphere. The annual growth period of P. australis could persistently enhance the mobility of sediment P, converting it from a more inert status to a redox-sensitive species. Our findings highlight that remobilization of sediment-associated P by P. australis accounts for a significant portion of the P cycle in eutrophic lake ecosystems.

Impact of application mode on the control of phosphorus release from sediments using zirconium-modified bentonite as geo-engineering material

In this study, the influence of zirconium-modified bentonite (ZMBT) addition, capping, and addition/capping on the transport and transformation of phosphorus (P) in sediments were comparatively investigated using incubation experiments to determine the effect of ZMBT application mode on the controlling efficiency. Results showed that the release of soluble reactive P (SRP) from sediment to the overlying water was effectively intercepted by all the ZMBT treatments. The inactivation of pore-water SRP, diffusive gradients in thin films-labile P (DGT-LP) and mobile P (Mob-IP) in sediment played a pivotal role in the regulation of SRP liberation from the sediment to the overlying water by ZMBT. An application mode change from capping and addition/capping to addition resulted in a decline of the reduction efficiency of overlying water SRP by the ZMBT treatment to some extent. The variation in the reduction efficiency of pore-water SRP and DGT-LP in the uppermost sediment were responsible for the change of the reduction efficiency of overlying water SRP by the ZMBT treatment. A change in application mode from capping to addition/capping and addition caused an obvious increase in the immobilization efficiency of pore-water SRP, DGT-LP and Mob-IP in the lower sediment by the ZMBT treatment. Results of this work indicate that, when the ZMBT capping layer on the top of sediment was completely mixed with the sediment, although the stability of P in the lower sediment obviously increases, the controlling efficiency of SRP liberating from the sediment to the overlying water decreases to some extent. Thus, the repeated addition of ZMBT to form a covering layer on the ZMBT-amended sediment is very necessary for the effective control of sediment-P release to the overlying water.

Phosphorus immobilization by the surface sediments under the capping with new calcium peroxide material

Abstract Capping material plays an important role in P (phosphorus) control. However, P immobilization on surface sediments due to the remediation by the capping materials has been neglected. Herein, we conducted an experiment to research the mechanism of surface sediment on P immobilization under new calcium peroxide material (NCPM) capping at different ratios of the sedimentary mobile phosphorus (Pmobile, the sum of NH4Cl–P, BD-P and NaOH-nrP) to NCPM (1:1–1:4). The concentrations of soluble reactive P (SRP), ammonium (NH4+) and ferrous (Fe(Ⅱ)) in water and sedimentary P fractions were analyzed. The study also presents the results of Langmuir isotherm model for surface sediments and sedimentary P release under anaerobic conditions. The results show that the surface sediments under NCPM capping improved the SRP reduction both from overlying and pore water. This was attributed to Ca2+ precipitation and sedimentary micro-environment change from anaerobic conditions to aerobic conditions which resulted from the NH4+ and Fe(Ⅱ) concentration reductions. This remediation increased maximum adsorption capacity (Smax) and decreased equilibrium concentration of P adsorption and desorption (EPC0) in those surface sediments under NCPM capping, which was favourable for P adsorption and immobilization. The X-ray diffraction and energy-dispersive X-ray spectroscopy analyses results confirmed aforementioned results. The conversion of P fractions from Pmobile to inert P in the surface sediments was more obvious than control. The inert P increased from 50.15% (1:1) to 69.88% (1:4), resulting in lower P release from the surface sediments under NCPM capping, compared with the control. The results show that NCPM capping improved P removal and immobilization of the surface sediments, beneficial for controlling sedimentary P release.

A study of bioavailable phosphorus in the inflowing rivers of Lake Taihu, China

Phosphorus (P) is a vital nutrient for algal growth. Aside from soluble reactive P (SRP), organic P (OP) is used by algae via alkaline phosphatase (AP) hydrolysis, which can play an important role in supplying P. Enzymatically-hydrolysable OP (EHP) can potentially be used as an indicator of bioavailability of P other than SRP in natural waters. We investigated the ecological significance of alkaline phosphatase activity (APA), EHP concentration and P turnover time in the inflowing rivers of Lake Taihu (Taihu) during three hydrologic periods. Results indicated high SRP concentration and low SRP demand by algal suppressed APA in the inflowing rivers, the highest proportion of OP mineralization rate (v) to the maximum reaction velocity of AP (Vmax) is only 14.7%. P turnover time of the inflowing rivers was generally from 3 to 7 days and in exceptional cases, it could exceed 10 days. The high EHP reserve and the sufficient AP for OP mineralization render the rivers a significant source of utilizable OP, further exacerbating eutrophication of Taihu.