Mobile phosphorus stratification in sediments by aluminum immobilization

Contrasting effects of a traditional material of polyaluminum chloride and an emerging material of lanthanum carbonate capping on sediment internal phosphorus immobilization

Polyaluminum chloride (PAC) is a traditional material used for immobilizing sediment internal phosphorus (P) in field-scale experiment. Lanthanum carbonate (LC) is an emerging material which have been used in immobilizing sediment internal P in laboratory. To promote LC in practice, the premise is that it does have advantages over traditional material when used. Herein, a 90-day incubation experiment was conducted comparing the effectiveness and mechanism of LC and PAC capping in controlling sediment internal P. The results of isotherm experiment and XPS analysis indicated that the adsorption mechanism of P onto LC and PAC involved ligand exchange and formation of inner-sphere La/Al-O-P complexes. The incubation experiment revealed that PAC capping was more effective in reducing pore water soluble reactive phosphorus (SRP), exhibiting a reduction of up to 81.32 % but showed a decrease trend. However, LC capping resulted in a reduction of pore water SRP up to 52.84 % and maintained stability. On average, LC and PAC capping reduced SRP flux by 0.27 and 0.32 μg·m-2d-1, respectively relative to the control sediment. Moreover, LC capping facilitated the formation of Fe(III)/Mn(IV) oxyhydroxides, leading to an increased P adsorption, whereas PAC capping facilitated the reduction of Fe(III)/Mn(IV) minerals with P release. Additionally, LC capping resulted in the reduction of a higher ratio of mobile P/TP to stable P forms than PAC capping, as compared to the control. In contrast to PAC capping which converted mobile P to stable NaOH-rP, LC capping transformed mobile P and NaOH-rP into more stable HCl-P and ResP. Both LC and PAC capping caused variations in sediment bacterial communities. Nevertheless, PAC capping heightened the risk of Co, Ni, Cu, and Pb releases in sediment compared to LC capping. In summary, this study suggested that LC capping surpassed PAC capping in immobilizing sediment internal P.

Principles, applications, and limitations of diffusive gradients in thin films induced fluxed in soils and sediments

The diffusive gradients in thin films (DGT) technique serves as a passive sampling method, inducing analyte transport and concentration. Its application is widespread in assessing labile components of metals, organic matter, and nutrients across various environmental media such as water, sediments, and saturated soils. The DGT devices effectively reduce the porewater concentration through irreversible binding of solutes, consequently promoting the release of labile species from the soil/sediment solid phase. However, the precise quantification of simultaneous adsorption and desorption of labile species using DGT devices alone remains a challenge. To address this challenge, the DGT-Induced Fluxes in Soils and Sediments (DIFS) model was developed. This model simulates analyte kinetics in solid phases, solutions, and binding resins by incorporating factors such as soil properties, resupply parameters, and kinetic principles. While the DIFS model has been iteratively improved to increase its accuracy in portraying kinetic behavior in soil/sediment, researchers' incomplete comprehension of it still results in unrealistic fitting outcomes and an oversight of the profound implications posed by kinetic parameters during implementation. This review provides a comprehensive overview of the optimization and utilization of DIFS models, encompassing fundamental concepts behind DGT devices and DIFS models, the kinetic interpretation of DIFS parameters, and instances where the model has been applied to study soils and sediments. It also highlights preexisting limitations of the DIFS model and offers suggestions for more precise modeling in real-world environments.

Effect of capping mode on control of phosphorus release from sediment by lanthanum hydroxide

The use of in situ active capping to control phosphorus release from sediment has attracted more and more attentions in recent years. It is important to identify the effect of capping mode on the control of phosphorus release from sediment by the in situ active capping method. In this study, the impact of capping mode on the restraint of phosphorus migration from sediment into overlying water (OW) by lanthanum hydroxide (LH) was studied. Under no suspended particulate matter (SPM) deposition condition, LH capping effectively restrained the liberation of endogenous phosphorus into OW during anoxia, and the inactivation of diffusive gradient in thin film-unstable phosphorus (UP_DGT) and mobile phosphorus (P_Mobile) in the topmost sediment served as a significant role in the restraint of endogenous phosphorus migration into OW by LH capping. Under no SPM deposition, although the transformation of capping mode from the single high dose capping to the multiple smaller doses capping had a certain negative impact on the restraint efficiency of endogenous phosphorus liberation to OW by LH in the early period of application, it increased the stability of phosphorus in the static layer in the later period of application. Under SPM deposition condition, LH capping had the capability to mitigate the risk of endogenous phosphorus liberation into OW under anoxia conditions, and the inactivation of UP_DGT and P_Mobile in the topmost sediment was a significant mechanism for the control of sediment phosphorus liberation into OW by LH capping. Under SPM deposition condition, the change in the covering mode from the one-time high dose covering to the multiple smaller doses covering decreased the efficiency of LH to limit the endogenous phosphorus transport into OW in the early period of application, but it increased the performance of LH to restrain the sedimentary P liberation during the later period of application. The results of this work suggest that the multiple LH capping is a promising approach for controlling the internal phosphorus loading in freshwater bodies where SPM deposition often occurs in the long run.

The evolution of a typical plateau lake from macrophyte to algae leads to the imbalance of nutrient retention

Long-term anthropogenic nitrogen (N) and phosphorus (P) inputs have led to lake eutrophication and decreased environmental quality. However, the imbalance in nutrient cycling caused by ecosystem transformation during lake eutrophication is still unclear. The N, P, organic matter (OM) and their extractable forms in the sediment core of Dianchi Lake were investigated. Combining ecological data and geochronological techniques, a coupling relationship between the evolution of lake ecosystems and nutrient retention was established. The results show that the evolution of lake ecosystems promotes the accumulation and mobilization of N and P in sediments, leading to an imbalance in nutrient cycling in the lake system. From the "macrophyte-dominated" period to the "algae-dominated" period, the accumulation rates of potential mobile N and P (PMN, PMP) in sediments have significantly increased, and the retention efficiency of total N and P (TN, TP) has decreased. The increased TN/TP ratio (5.38 ± 1.52 ‒ 10.19 ± 2.94) and PMN/PMP ratio (4.34 ± 0.41 ‒ 8.85 ± 4.16), as well as the reduced humic-like/protein-like ratio (H/P, 11.18 ± 4.43 ‒ 5.97 ± 3.67), indicated an imbalance in nutrient retention during sedimentary diagenesis. Our results show that eutrophication has resulted in the potential mobilization of N in sediments exceeding P, providing new insights for further understanding the nutrient cycle in the lake system and strengthening lake management.

Control of phosphorus release from sediment by hydrous zirconium oxide combined with calcite, bentonite and zeolite

This study investigated the effectiveness and mechanism for the control of internal phosphorus (P) liberation from sediment by hydrous zirconium oxide (HZrO₂) combined with calcite, bentonite and zeolite. The results suggested that coexisting calcite, calcium-modified bentonite (CaBT) and calcium-modified zeolite (CaZ) all had the ability to promote the adsorption of phosphate (PO₄^3-) onto HZrO₂. The mechanisms of PO₄^3- elimination by HZrO₂/calcite mixture involved the adsorption of PO₄^3- on calcite, the precipitation of PO₄^3- with Ca²⁺, and the inner-sphere complexation of PO₄^3- with HZrO₂. The amendment of sediment with HZrO₂/calcite, HZrO₂/CaBT or HZrO₂/CaZ mixture can effectively prevent the sedimentary P release, and the immobilization of mobile P in the sediment and the uptake of dissolved reactive P (DRP) from the interstitial water by the amendment material played a key role in the control of P release from sediment by the combined amendment. Capping sediment with HZrO₂/calcite, HZrO₂/CaBT or HZrO₂/CaZ mixture also can effectively intercept sediment P release, and the formation of P static layer attributed to the uptake of interstitial water DRP and DGT (diffusive gradient in thin-films)-unstable P in the upper sediment by the capping material was a key to the inhibition of sedimentary P migration into the overlying water by the combined capping. The great majority of P immobilized by the HZrO₂/calcite, HZrO₂/CaBT or HZrO₂/CaZ combined covering layer is stable P and it has a low re-releasing risk under dissolved oxygen-deficit and pH 5-9 condition. The stability of P bound by the combined covering layer was larger than that by the single HZrO₂ covering layer. The results of this research show that the combined use of HZrO₂ and calcite, HZrO₂ and CaBT, or HZrO₂ and CaZ as a capping material has great potential in the reduction of sediment P loading.

Effects of calcination on the environmental behavior of sediments by phosphorus speciation and interface characterization

Dredged sediments derived from eutrophicated lakes poses hardness of sludge disposal and ecological risks. The proper pretreatment and utilization of dredged sediments presented a challenge. In this study, Dianchi Lake sediments were dredged, thermally treated and utilized as particle capping material in batch experiments. The effects of calcination on phosphorus speciation and sediment-water interface environment as well as P immobility mechanism were predominantly explored. The microstructures and chemical compositions of calcined sediments were investigated, indicating the porosity and mineralization components were greatly enhanced. The fractional analysis of phosphorus revealed that the calcination process reduced the percentage of unsteady phosphorus, transforming into stable inert phosphorus fractions (Al-P, Ca-P and Res-P), respectively, thereby minimized its mobility and eutrophication risk. Interestingly, calcination temperatures of 700 °C and 800 °C resulted in smaller releasing potentials and equilibrium phosphorus concentrations, despite having lower adsorption capacities than 550 °C. Furthermore, the results of redox potential monitoring showed that the thermally treated Dianchi Lake sediments could enhance the redox potential and dissolved oxygen in the surface sediment, indicating the amelioration of interfacial environment. The practical monitoring experiments confirmed the capping depressed the DTP to 0.031 mg L^-1. The investigation of this study provided explicit evidence of Ca coupled P and aerobic Fe bound P strengthened the immobilization effects, and the development of sediment calcination demonstrates a promising strategy for alleviating the burden of endogenous pollution and improving aerobic environment, which are of great significance for lake ecological remediation.

The adsorption-release behavior of sediment phosphorus in a typical "grass-algae" coexisting lake and its influence mechanism during the transition sensitive period

In the early stage of eutrophication, the coexistence of "grass and algae" in lakes is obvious. Understanding the P sorption-desorption behavior in natural sediments during the ecologically sensitive transition period has important scientific value for predicting the deterioration of lake ecosystems and formulating restoration measures, but the related mechanisms are still unclear. In this study, the analysis results of sedimentary dissolved organic matter (DOM) fractions, extractable Fe (hydr)oxide fractions and P adsorption experiments showed that sedimentary DOM fractions, especially the tyrosine-like protein fractions and microbial humic-like fractions, played a part in determining the EPC₀ and K_d values of sediments in the plateau lake environment. The compound effect of amorphous Fe (hydr)oxides and sedimentary OM affected the increase of sedimentary P adsorption. Interestingly, these phenomena were strongly correlated with water depth. Furthermore, the distribution of water depth to aquatic plants indirectly regulated the values of sedimentary EPC₀ and K_d. Meanwhile, the ability of submerged plants to control the sedimentary EPC₀andK_d values will be forced to shift shallowly, thereby forcing a significant reduction of areas with low EPC₀ and high K_d values. This not only enhanced the risk of endogenous P release in lakes, but also accelerated the further deterioration of aquatic ecosystems. Therefore, studying the long-term scale changes of sedimentary EPC₀ and K_d values can help to understand the duration of the lake ecological transition period and prevent the transitional deterioration of ecosystem.

Effect of sediment burial depth on the control of sedimentary phosphorus release by iron/aluminum co-modified calcite and strategy for overcoming the negative effect of sediment burial

After placing an active capping material on surface sediments, the capping layer will be buried by the newly formed sediment. In this research, the influence of sediment burial depth on the performance of iron/aluminum co-modified calcite (FeAlCAL) to suppress sedimentary phosphorus (P) release into overlaying water (OL-water) was studied. Furthermore, in order to find out the strategy for overcoming the negative effect of sediment burial, the efficiencies and mechanisms of three different FeAlCAL treatments (one-time FeAlCAL capping with 3 cm sediment burial, multiple FeAlCAL capping with 1 cm sediment burial, and amendment of top 3 cm sediment with FeAlCAL) in the inhibition of sediment P release were contrastively studied. The results showed that with the increase of sediment burial depth, the efficiency of FeAlCAL to block the release of sediment P into OL-water gradually decreased until the FeAlCAL lost the ability to hinder sediment-P release. In contrast to the one-time FeAlCAL capping in the presence of 3 cm sediment burial, the multiple FeAlCAL capping in the presence of 1 cm sediment burial and amendment of top 3 cm sediment with FeAlCAL both effectively prevented the release of P from sediment into OL-water. All results of this work suggest that although sediment burial can negatively affect the ability of FeAlCAL in the inhibition of sediment P release into OL-water and the negative effect becomes stronger as the sediment burial depth increases, the transformation of the application mode of FeAlCAL from one-time capping to multiple capping or from capping to amendment can overcome the negative influence of sediment burial.

Water depth determines spatial and temporal phosphorus retention by controlling ecosystem transition and P-binding metal elements

Shallow lakes are more susceptible to eutrophication than deep lakes. The geochemical and biogeochemical mechanisms controlling the vulnerability to eutrophication for deep lakes and shallow lakes remain unknown. Therefore, we investigated the combined Phosphorus (P) retention mechanism with P fractions, water depth, distribution of P-binding metal elements, and macrophytes coverage in a degrading ecosystem of Erhai Lake. We concluded that different mechanisms control the P retention in deep-water areas and shallow-water areas. In shallow areas covered by macrophytes, the biogeochemical process manipulates the P retention by changing the total organic carbon (TOC), calcium (Ca) distributions and turbulence. In deep areas without macrophyte coverage, the aluminum (Al) and iron (Fe) distributions control the P retention by a physicochemical process. Manganese (Mn) was found to be a potential proxy in tracking the kinetic release and readsorb of redox-sensitive P (BD-P) in deep areas. The historical record and core sample indicate that the hydrological engineering induced water depth variation is a vital factor changing the ecosystem of Erhai Lake by forming a large area of intermediate area where macrophytes could only survive at low water level. The uplift of water level in the 1990s gradually changed the ecosystem of Erhai Lake from macrophyte-dominated to algal-macrophyte concomitant that reduced the accumulation of stable P fractions and their binding metals. Macrophytes were capable to preserve P in biomass in the macrophyte-dominated ecosystem, which released 150% and 72% of more labile organic P (NaOH25-nrP) and BD-P in the sediment after the deterioration than before, respectively. Therefore, water depth is a prerequisite to restoring the P preservation capacity of sediment and the macrophyte ecosystem. Further hydraulic engineering projects should consider the effect of water-level-variation-induced ecosystem transition.

Behavior of iron and other heavy metals in passivated sediments and the coupling effect on phosphorus

In situ passivation, which is easy to operate and affordable, is one of the most commonly used methods for sediment phosphorus (P) remediation. Understanding the behavior of iron and other heavy metals in passivated sediments is important for alleviating lake eutrophication and for ensuring drinking water safety. In this study, we investigated the behavior of P, Fe, Mn, Cd, Co, and Pb in lanthanum modified bentonite (LMB, Phoslock®) and polyaluminum chloride (PAC)-passivated sediments using intact sediment cores. Rhizon sampler and diffusive gradients in thin films technology (DGT) were respectively used to collect soluble and labile substances in sediment; a modified sequential selective extraction method was used to characterize metal forms. Results showed that LMB reduced soluble reactive phosphorus (SRP) at sediment depths of 0 ~ -15 mm and DGT-labile P flux at 0 ~ -50 mm. Correlation between DGT-labile P and Fe (R² = 0.71) indicated that P mobility in the LMB group was affected by the behavior of Fe. PAC decreased SRP at sediment depths of 0, -5, -10, -15, -20, -25, and -50 mm with removal rates of 100%, 90%, 45%, 35%, 81%, 89%, and 100%, respectively. DGT-labile P flux was decreased by PAC at 0 ~ -10 mm and -50 ~ -110 mm, but increased at -10 ~ -50 mm; this is a result of synthetical effect by Al flocs adsorption and Fe(III) reductive dissolution. LMB decreased Cd, Co, and Pb in LMB layer in carbonate, reducible, and oxidizable forms. PAC decreased Cd mobility but caused the transformation of Co and Pb from reducible to other forms because of Fe(III) reductive dissolution. Those results indicate that sedimentary Fe plays an important role in in situ passivation. We suggest modifying passivators to Fe(II) adsorbents and increasing DO permeability of sediment to promote the formation of an Fe(III) passivation layer and hence the effectiveness of P control.

Effectiveness and mechanism of aluminum/iron co-modified calcite capping and amendment for controlling phosphorus release from sediments

The effectiveness and mechanism of aluminum/iron co-modified calcite (Al/Fe-CA) for the control of phosphorus (P) liberation from sediments was investigated. The results showed that Al/Fe-CA possessed good sorption performance for phosphate, and the maximum phosphate sorption capacity for Al/Fe-CA could reach 27.0 mg/g. The major mechanisms involved the surface adsorption of phosphate on calcite, the precipitation between phosphate and Ca²⁺ leached from calcite, and the ligand exchange between Al/Fe-bound hydroxyl groups and phosphate to form the Al-O-P and Fe-O-P inner-sphere complexes. The re-releasing risk of Al/Fe-CA-bound P under the circumstances of normal pH (5-9) and reducing environment was very low. Al/Fe-CA addition could significantly reduce the risk of P releasing from sediment to overlying water (OL-water), and the inactivation of mobile P, reactive soluble P (SRP) and diffusive gradient in thin-films (DGT)-labile P in sediment by Al/Fe-CA had a great part in the suppression of sediment-P liberation to OL-water by the Al/Fe-CA amendment. Al/Fe-CA capping and fabric-wrapped Al/Fe-CA capping both could greatly reduce the risk of P releasing from sediment into OL-water, and the formation of a static layer with low concentrations of SRP and DGT-labile P in the upper sediment was the key to sustaining a high P controlling efficiency. When the applied mode of Al/Fe-CA varied from capping to amendment, although the inactivation efficiency of DGT-labile P in the overlying water and upper sediment by Al/Fe-CA would decrease to a certain degree, the inactivation efficiency of DGT-labile P in the lower sediment by Al/Fe-CA would increase. Results of this study suggest that Al/Fe-CA has the high potential to be used as an active capping or amendment material for the management of internal P loading in surface water bodies.

Historical changes of sedimentary P-binding forms and their ecological driving mechanism in a typical "grass-algae" eutrophic lake

With the transformation of lake ecosystem from "clear water" to "turbid water", the residual phosphorus (P) accumulated in sediments may slow down the process of aquatic ecological restoration, and the related mechanisms are complex and need to be better understood. In this study, high-resolution systematic investigation and analysis of P-binding forms in the sediments showed that Lake Dianchi, the largest plateau lake in Southwest China, was enriched with NaOH-rP, HCl-P and Res-P, but depleted in NH₄Cl-P, BD-P and NaOH-nrP. The BD-P, NaOH-nrP and NaOH-rP were the main contributors to potential P release from sediments, while the release potential of NH₄Cl-P was relatively weak (<1%). When the external P loading gradually decreased, the internal P loading of Lake Dianchi was estimated to be 522 mg P/(m²•a) in the past 30 years. The succession of "grass-algae" type in Lake Dianchi coincided with reduced absorption and transformation of potential mobile P and decreased accumulation of stable P, especially the Res-P. Meanwhile, the temporal variation of potential mobile P was a good predictor of ecological degradation and reduced ecosystem sustainability in Lake Dianchi.

Coupled influence of pH and dissolved organic carbon on the immobilization of phosphorus by lanthanum-modified zeolite

Wind-driven waves and currents in shallow lakes frequently trigger the resuspension of sediments in the photic layer, which is characterized with a high pH and high dissolved organic carbon (DOC) concentration. The mechanism of phosphorus-inactivating agents (PIAs) immobilizing phosphorus under the coupled influence of pH and DOC is not clarified, and the applicability of PIAs in eutrophic shallow lakes is thus still doubtful. We found that, under the coupled influence of pH and DOC, the uptake of phosphate by LMZ was affected mainly by pH at low DOC concentrations and by DOC at high DOC concentrations. A high pH (9.3) and high DOC concentration (24.7 mg/L) greatly increased the release of phosphorus from sediment to water. However, the addition of LMZ substantially reduced the P concentrations in water, mainly via capture of dissolved inorganic phosphorus. The results of the reversibility of the adsorption of phosphates and DOC showed that phosphate had much higher affinity than DOC towards LMZ. The phosphate once adsorbed on LMZ was resistant to release when exposed to conditions of either a high pH (9.5), high DOC concentration (250 mg/L) or both; i.e., only <5% of the adsorbed phosphate is releasable. Therefore, we proposed that, to avoid the coupled influence of pH and DOC in the photic layer of eutrophic shallow lakes, LMZ could be applied in multiple low doses in the season when the growth of algae is minimal (a low pH and low DOC concentration).

Magnetic Fe3O4@MgAl-LDH@La(OH)3 composites with a hierarchical core-shell structure for phosphate removal from wastewater and inhibition of labile sedimentary phosphorus release

In order to facilitate recovery and enhance phosphate adsorption capacity of lanthanum (La)-based materials, magnetic Fe₃O₄@MgAl-LDH@La(OH)₃ (MMAL) composites with a hierarchical core-shell structure were synthesized. In the preparation process, citric acid played a vital role in the morphology control of La(OH)₃, deciding the La content and phosphate adsorption capacity of materials. MMAL composites with a citric acid-to-La molar ratio of 0.375 (MMAL-0.375) exhibited a high adsorption capacity of 66.5 mg P/g, fast adsorption kinetics of 30 min, widely applicable pH range of 4.0-10.0, outstanding selective adsorption performance, and superior reusability in batch adsorption experiments. Moreover, the phosphate in the desorption solution could be concentrated by repeated use of desorption solution and recovered by using CaCl₂. When the obtained composites were used for the sedimentary phosphorus sequestration and recovery, the results showed that the addition of MMAL-0.375 effectively reduced the concentration of soluble reactive phosphorus (SRP) in the overlying water. Accompanied by an evident increase in HCl-extractable phosphorus (HCl-P), mobile phosphorus (P_mob) in sediments was effectively reduced. This work indicates that the MMAL-0.375 composites can serve as an effective tool for the removal of phosphate from wastewater and the control of sedimentary phosphorus.

New application of lanthanum-modified bentonite (Phoslock®) for immobilization of arsenic in sediments

Lanthanum-modified bentonite (LMB, Phoslock^®) is a well-known capping agent for phosphorus immobilization in sediments. Herein, LMB was used to immobilize As in sediments. Batch capacity experiments for arsenate and arsenite adsorption were carried out to obtain adsorption isotherms and kinetics using the Langmuir and Freundlich model calculations. High-resolution (HR) diffusive gradients in thin films (DGT) were applied to monitor the changes of weakly bound As fraction near sediment-water interface (SWI). The interaction of As(III) and As(V) with LMB was influenced by pH and initial mineral composition. As(V) was more obviously adsorbed than As(III) at pH 4 to 9, with mean adsorption of 3.89 mg g^-1 and 0.04 mg g^-1, respectively, while at pH > 9 As(III) was preferentially adsorbed. After LMB amendment for 2 months, the maximum As removal efficiency in the pore and overlying water reached 84.5% and 99.3%, respectively. The capping agent remained stable in the top sediments, while the maximum DGT labile As content decreased to 0.89 and 0.51 μg L^-1 in dosage-and time-treatments. The As concentration inflection point moved down to a deeper layer. As species changed from labile exchangeable-As to Fe-oxide-bound and residual As. The proportion of mobile As finally decreased to 10.5% of the total As in the upper 20-mm layer sediment. The increase of K_d (the distribution coefficient at SWI) and k₁ (adsorption rate constant) and the decrease of T_c (response time of (de)sorption) in the DGT-induced fluxes model (DIFS) indicated the time-dependent impediment of As release from the sediment due to LMB immobilization.

Response of Vallisneria natans to aluminum phytotoxicity and their synergistic effect on nitrogen, phosphorus change in sediments

Increasing aluminum (Al) use and its effects on aquatic systems have been a global issue, however the Al impacts on submerged plants and their ecological functions were poorly understood. Aquatic simulation experiments were performed to study Al-toxicity on the germination and seedling morphological and physiological characteristics of Vallisneria natans, and investigate their synergistic effect on nitrogen (N), phosphorus (P) change and microbial community in sediment. The seeds germination characteristics, growth and physiological parameters of seedlings, including root activity, were significantly affected by alum treatments and the inhibition levels increased with Al³⁺ concentration. The Al accumulation in roots and leaves were significantly different. Al³⁺ concentration above 0.3 mg/L showed toxic to V. natans. TN, TP, IP, Fe/Al-P contents in sediments varied markedly under co-existence of Al and V. natans. Additionally, the relative abundance of sediment microbial community related to N, P cycle was effected. Results concluded that the increasing aquatic Al-concentration inhibits growth and propagation of submerged plants and the ecological restoration effect, and exerts synergistic effect with submerged plants on N, P components in sediments. Such findings were helpful for Al ecological evaluation, and were instructive for the submerged plants restoration in shallow eutrophic lakes with Al input.

Application of DGT/DIFS combined with BCR to assess the mobility and release risk of heavy metals in the sediments of Nansi Lake, China

The heavy metal contamination of the aquatic ecosystem is still prevalent even after reduction of the external anthropogenic inputs of the metals. The release of labile heavy metals from the sediments into the water is a potential risk, responsible for the contamination of the aquatic system. Herein, samples of sedimentary column cores were collected in Nansi Lake, and the distribution profiles of the labile and soluble metals (Cd, Cu, Ni, Pb, and Zn) were obtained by the diffusive gradient in thin films (DGT) and the high-resolution dialysis (HR-peeper) technique. Furthermore, the mobility, bioavailability and release risk of the heavy metals were assessed using the results of geochemical sequential extraction, DGT as well as the DGT-induced fluxes in sediments (DIFS) model. The results showed that the profile characteristics of the DGT-labile and soluble heavy metals showed irregular distribution in the sediment cores and Cd, Pb, Zn had an obvious positive correlation with Fe/Mn (p < 0.05). Ni, Cu, and Zn existed primarily in the residual fraction (accounting for 58-76%), while Cd and Pb existed in the reducible fraction (accounting for 50-67%). The Cd and Ni (0.027-0.185) had higher mobility coefficients compared with Pb, Cu, and Zn (0-0.011), and positive diffusive fluxes also proved that Cd and Ni were easy to be released from the sediments. In addition, the R values of five metals (0.18-0.85) ranged between R_diff to 0.95, indicating that all the metals had partially sustained case from the sediments solid phase. Based on the DIFS model, the five metals had weak mobility from the sediment to pore water, but the release risks in the Nansi Lake should also be of concern, especially for the highly mobile Cd and Ni in the Dushan Lake.

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 (LP_DGT) in the overlying water, but also gave rise to the diminished SRP and LP_DGT 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 LP_DGT 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.

The effect of microenvironment in the sediment on phosphorus immobilization under capping with ACPM and Phoslock®

Currently, in situ capping is a typical popular geoengineering method for eutrophication control. It is crucial to better understand the effect of microenvironment change due to capping, such as amended calcium peroxide material (ACPM) and Phoslock®, on phosphorus (P) adsorption and immobilization under the addition of external P. The microenvironment in sediment was presented by the concentration of O₂, NH₄⁺, and Fe²⁺ and microbial activity. The P removal and immobilization were also analyzed. The results show that the stronger oxidation in the microenvironment under the capping with ACPM was due to the higher reduction of NH₄⁺ and Fe²⁺ and the higher increase of microbial activity, compared to Phoslock®. Although, under the capping of ACPM, less amount of external P was removed and there was a faster release of sedimentary P, compared to Phoslock®, ACPM improved the transformation of P from mobile P fractions to inert P fractions. In addition, sedimentary P under the capping of ACPM presents less release than that under the capping of Phoslock® during the anaerobic incubation. However, the settlement of suspended solids decreased the function of capping. All these results indicated that the mechanism of P removal and immobilization was different under the capping of ACPM and Phoslock®.

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.

Enrichment of bioavailable phosphorus in fine particles when sediment resuspension hinders the ecological restoration of shallow eutrophic lakes

Sediment resuspension is one of the main factors impacting the ecological restoration of shallow eutrophic lakes, but the mechanisms connecting suspended particles and algal growth have not been clarified. Our research presents an innovative approach based on P reallocation among particles with various sizes, considering the changes in redox and pH conditions from the sediments to the overlying water during resuspension. A lab-scale experiment was conducted to simulate P reallocation in particles during sediment resuspension by periodically dosing the system with P and/or organic carbon. The sediments were sampled and sieved into five particle size groups, namely, 50-150 μm, 30-50 μm, 10-30 μm, 5-10 μm and <5 μm, and their P fractions during the operation were analyzed. The bioavailable P associated with aluminum (Al) and iron (Fe) (hydr)oxides showed exponential enrichment as the median grain size of particles decreased, with 54% of the added P adsorbed by fine particles of <10 μm (5-10 μm and <5 μm). Furthermore, a bioassay of algae growth potential (Microcystis aeruginosa sp.), along with P adsorption isotherms, was conducted to test the ability of the different size-resolved particles to supply P for algae growth. The fine particles of <10 μm supplied more P to algae under elevated pH values than did the coarse particles (>10 μm). The restoration of shallow eutrophic lakes faces great challenges due to the connection mechanisms between sediments and algae, as revealed by this research.

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.

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 0-30 mm and reduced the concentration of PORE-water NH₃-N at depth of 0-50 mm. 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 0-50 mm sediment and caused the transformation of P-BD to calcium-bound P (P-HCl) and residual P (P-RES) in the 0-10 mm sediment as well as to P-RES in the 10-20 mm 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.

Simultaneous control of nitrogen and phosphorus release from sediments using iron-modified zeolite as capping and amendment materials

The use of zeolite as a geo-engineering tool has a high potential to control nitrogen (N) release from sediments, but its efficiency for controlling sedimentary phosphorus (P) release still need to be further increased. To address this issue, this work synthesized an iron-modified zeolite (IM-Z) by coating iron onto the surface of natural zeolite (NAT-Z) and then the as-obtained IM-Z was utilized as a geo-engineering material to block the upward mobilization of N and P from sediments to the overlying water. The efficiencies of IM-Z covering and amendment to prevent the liberation of N and P from sediments were evaluated, and the controlling mechanism was explored. Capping and amendment with IM-Z not only resulted in the tremendous reduction of the levels of ammonium-N (NH₄⁺-N) and reactive soluble P (RSP) in the overlying water, but also led to the decrease of the contents of NH₄⁺-N and RSP in the pore water. More importantly, sediment capping and amendment with IM-Z resulted in the formation of a static layer in the upper sediment directly below the sediment-water interface, with very low concentration of RSP in the pore water. In addition, IM-Z capping and addition effectively immobilized the diffusive gradients in thin films (DGT)-labile P in the overlying water and sediment. Furthermore, the decrease of the DGT-labile Fe concentrations in the overlying water as well as the top sediment were also observed after IM-Z capping and addition. Nearly 70% of P bound by IM-Z is stable and difficult to be released back into the overlying water under common pH and anoxic conditions. The adsorption of pore water NH₄⁺-N on IM-Z, the immobilization of pore water RSP and DGT-labile P by IM-Z and the uptake of DGT-labile Fe on IM-Z played a significant role in the simultaneous control of NH₄⁺-N and RSP liberation. Compared to NAT-Z, the efficiency of IM-Z to block the liberation of sedimentary P was higher. Results of this study demonstrate that IM-Z is suitable for use in the simultaneous interception of the upward transportation of NH₄⁺-N and RSP from sediments into the overlying water.

Combined use of calcium nitrate addition and anion exchange resin capping to control sedimentary phosphorus release and its nitrate‑nitrogen releasing risk

Calcium nitrate (Ca(NO₃)₂) addition can be used to control the release of phosphorus from sediments, however it can also cause an increase in the concentration of nitrate‑nitrogen (NO₃^--N) in the water column. The risk of NO₃^--N release from the Ca(NO₃)₂-injected sediments may be reduced by the placement of the anion exchange resin (AER) capping layer. In this study, the effectiveness of the combined use of Ca(NO₃)₂ addition and AER capping to prevent the liberation of phosphorus from sediments was investigated, and the reduction of the risk of NO₃^--N released from the Ca(NO₃)₂-injected sediment by the AER capping was also evaluated. The combined application of Ca(NO₃)₂ addition and AER capping could tremendously reduce the amount of soluble reactive phosphorus (SR-P) in the overlying water, with SR-P reduction rates of 75.9-98.7%. Furthermore, it could cut down the contents of high-resolution diffusive gradients in thin films (DGT)-labile phosphorus in the sediments, resulting in the formation of phosphorus static layer in the upper sediments. The combined treatment using Ca(NO₃)₂ and AER had a relatively small effect on the contents of mobile phosphorus in the sediments, but it could greatly increase the amount of residual phosphorus in the top 30mm sediments (increased by 27.7-42.9%). The amount of NO₃^--N in the overlying water under the action of the combined treatment method using Ca(NO₃)₂ and AER was much lower than that under the action of the single Ca(NO₃)₂ treatment during the early stage of sediment remediation. In conclusion, the combined use of Ca(NO₃)₂ addition and AER capping is a more promising strategy for the control of sedimentary phosphorus release than the single use of Ca(NO₃)₂ addition from the point of view of both the control efficiency of P release from sediments and the releasing risk of the added nitrate.

Magnetite-modified activated carbon based capping and mixing technology for sedimentary phosphorus release control

In this study, magnetite-modified activated carbon (MAC) was synthesized, characterized and used as capping and amendment materials to control sedimentary phosphorus (P) release. Batch experiments were applied to determine the behavior of phosphate adsorption and desorption on/from MAC. Sediment incubation experiments were utilized to evaluate the impact of MAC capping and addition on the mobilization of P in sediments. Sediment capping and amendment with MAC both can greatly reduce the amount of reactive soluble P (RS-P) in the overlying water (OLY-water), with a reduction efficiency of higher than 83%. MAC capping and amendment both can significantly reduce the concentrations of labile P measured by diffusive gradient in thin-films (DGT) in the upper sediment, which gives rise to in the formation of the static layer of P (P-S-Layer) in the upper sediment. The forms of P bound by MAC were mainly redox-sensitive P (P_RS), NaOH extractable inorganic P (IP_NaOH) and HCl extractable P (P_HCl), which accounted for 47.2, 18.5 and 32.9% of the total adsorbed P, respectively. Almost half of P adsorbed by MAC existed in the form of P_RS, which is easy to be released under anoxic condition, and the retrieval of MAC from the waterbody after its application is very necessary. The concentrations of RS-P in OLY-water and mean DGT-labile P in P-S-Layer under capping condition were much less than those under amendment condition. The reduction of the apparent diffusion efflux of P across the interface between OLY-water and sediment by the MAC capping was much larger than that by the MAC amendment. Results of this work suggest that MAC capping and amendment are very promising methods for blocking the liberation of P from sediments into OLY-water, and MAC capping can achieve a higher efficiency of sedimentary P release control compared to MAC amendment.

Aluminum distribution heterogeneity and relationship with nitrogen, phosphorus and humic acid content in the eutrophic lake sediment

Increasing amount of aluminum (Al) gets into aquatic ecosystem through anthropogenic activity, but the knowledge about Al migration and relationships with sediments possessing different physico-chemical properties in eutrophic lakes is limited. Here, the Al migration rule and relationships with sediment nutritions in the Hangzhou West Lake, China was investigated, where a certain amount of residual Al-salts can enter because of the pre-treatment of the Qiantang River diversion project every day. Results revealed the obvious spatial distribution heterogeneity of Al in sediment vertical direction and horizontal direction following water flow. The Al content in sediment ranged 0.463-1.154 g kg^-1 in Maojiabu Lake, and ranged 9.862-40.442 g kg^-1 in Xiaonanhu Lake. Higher Al content distributed in upper layer sediment in lake with more disturbance. Total nitrogen (TN) contents were higher 0.917-3.387 mg g^-1 and 0.627-0.786 mg g^-1 in upper layer sediment than that in lower layer in Maojiabu Lake and Xiaonanhu Lake, respectively. Total phosphorus (TP) content ranged 0.779-2.580 mg g^-1, in which IP and Fe/Al-P contributed 24.9-80.8% and 17.0-51.6%, respectively. Correlations between Al content with nutrition, humic acid (HA) etc. of sediment regionally varied in Maojiabu and Xiaonanhu Lake. Spatial distribution of Al-salt in eutrophic lakes closely related with the physico-chemical characteristics of nutrients, humus, human disturbance and water division parameters. Results provides new insight into Al-salts migration and references for Al-risk evaluating in eutrophic lakes.

The effect of hydrodynamic forces of drying/wetting cycles on the release of soluble reactive phosphorus from sediment

Soluble reactive phosphorus (SRP) that is released from sediment plays an important role in contributing to a lake's eutrophication. Much of the work that has studied sediment release has been conducted in the submerged bottom sediment of lakes. Less attention has paid to the littoral zones near land boundaries where the hydrodynamic disturbance of drying/wetting cycles dominates. To date, the release mechanism under drying/wetting cycles has not been revealed quantitatively. In this study, we conducted a series of laboratory experiments to evaluate the effect of varied frequencies of drying/wetting cycles to the efflux of SRP from sediment. We tested SRP, Fe²⁺, pH, and redox condition (pE) in overlying water under three frequencies of 24, 9, and 2.77 day^-1 (F1, F2, and F3, respectively). SRP concentrations of F1, F2, and F3 experimental conditions were 3.46, 1.73, and 1.38 times that of a static experimental condition, respectively, showing a significant difference (p < 0.05) among the conditions. The overlying water under drying/wetting cycles varied in weak-base and low-redox status, which facilitated ion release. The SRP concentration of the porewater varied with the different frequencies of drying/wetting cycles. These results suggested that the variation of SRP in the porewater was strongly correlated with SRP release (R² = 0.809). Drying/wetting cycles enhanced the mobilization and release of SRP from the sediment to the overlying water through porewater exchange. The evaluation model emphasized that porewater exchange made the greatest contribution to SRP release and a higher frequency of drying/wetting cycles may have promoted this exchange of porewater between the sediment and overlying water, thus facilitating the release of SRP.

Assessment of sediment capping with zirconium-modified bentonite to intercept phosphorus release from sediments

Three different types of zirconium-modified bentonites (ZrMBs) including zirconium-modified original bentonite (ZrMOB), zirconium-modified magnesium-pretreated bentonite (ZrMMgB), and zirconium-modified calcium-pretreated bentonite (ZrMCaB) were synthesized and used as active covering materials to suppress the release of phosphorus (P) from sediments. To assess the covering efficiency of ZrMBs to inhibit P release from sediments, we examined the impact of ZrMB covering layer on P mobilization in sediments at different depths as well as the release of P through the interface between sediment and overlying water (SWI) by use of simulating P release control experiments and diffusive gradients in thin films (DGT) technology. The results showed that the amount of soluble reactive P (SRP) in the overlying water greatly decreased after covering with ZrMBs. Moreover, both pore water SRP and DGT-liable P (DGT-P) in the top sediments decreased after capping with ZrMBs. An obvious stratification of DGT-P was observed along the vertical direction after covering with ZrMBs, and static and active layers were found in the top sediment and in the lower sediment directly below the static layer, respectively. Furthermore, ZrMB covering led to the change of P species from easily released P to relatively or very stable P, making P in the top sediment more stable compared to that without ZrMB covering. Besides, an overwhelming majority of P immobilized by ZrMBs is hard to be re-released into the water column in a common environment. Overall, the above results demonstrate that sediment covering with ZrMBs could effectively prevent the transport of SRP from sediments into the overlying water through the SWI, and the control of P transport into the overlying water by ZrMB covering could be mostly due to the immobilization of pore water SRP, DGT-P, and mobile P in the top sediment by ZrMBs.

Effect of zirconium-modified zeolite addition on phosphorus mobilization in sediments

There is generally a significant heterogeneity in the vertical distribution of mobile phosphorus (P) in sediments, but the previous studies concerning the effect of zirconium-modified zeolite (ZrMZ) addition on the mobilization of P in sediments neglected this feature. In this study, microcosm experiments were conducted to investigate the effect of ZrMZ addition on the mobilization of P in river surface sediments at different depths. A high-resolution diffusive gradients in thin films technology (DGT) was used to measure the concentration of labile P in the overlying water-sediment profiles at a submillimeter vertical resolution. Results showed that the ZrMZ amendment not only could reduce the concentration of soluble reactive P (SRP) in the overlying water, but also could decrease the concentrations of SRP in the pore water at different depths. Furthermore, the ZrMZ amendment resulted in the reduction of both the releasing flux of SRP from sediments to the overlying water and the diffusion flux of SRP from the pore water to the overlying water. After the addition of ZrMZ into the top sediment, the static layer with low DGT-liable P (DGT-P) concentration was observed in the upper sediment. The addition of ZrMZ into the upper sediment resulted in the reduction of mobile P (P_m) in the upper and lower sediments via the transformation of P_m to more stable NaOH-extractable P (NaOH-rP) and residual P (Res-P). In addition, the contents of bioavailable P (BAP) including water-soluble P (WSP), readily desorbable P (RDP) and iron oxide paper extractable P (FeO-P) in the upper sediment were greatly reduced by the ZrMZ addition. Results of this study show that the immobilization of pore water SRP, DGT-P, sediment P_m and sediment BAP by ZrMZ played a very important role in the control of P release from sediments to the overlying water by the ZrMZ amendment.

Assessment on the effects of aluminum-modified clay in inactivating internal phosphorus in deep eutrophic reservoirs

Aluminum-salt inactivating agents are extensively applied to the restoration of lakes polluted by internal phosphorus (hereinafter referred to as "P"). However, there is a lack of micromechanism information regarding the sediment P cycle and its interactions with aluminum salts, which has restricted the engineering applications of aluminum salts. In this study, a sediment core incubation system was used to simulate the influence of aerobic and anaerobic conditions on the effectiveness and stability of aluminum-modified clay (AMC). This study also investigated the millimeter-scale dynamics of P across the sediment-water interface (SWI) using the HR-Peeper and DGT techniques. According to the results, sediment P release mainly occurred under anaerobic conditions. When the incubation system was in an anaerobic state, AMC effectively reduced the internal-P loading. In pore water, there was a positive correlation between soluble Fe and SRP, suggesting that the reductive dissolution of Fe-P constituted the main mechanism of sediment P release. After with dosing AMC, the concentrations of SRP and labile P in the capping layer both dropped abruptly to low levels and the content of Al-P in surface sediments rose, suggesting that AMC had strongly adsorbed phosphates, formed inert Al-P and blocked the phosphate exchange between pore water and overlying water. This study elaborated on the micromechanism of the control of sediment internal P input by AMC and revealed that Al-P precipitation constituted the main mechanism of the inhibition of sediment P release by aluminum-salt inactivating agents. The research findings have a great significance for guiding field applications of aluminum-salt inactivating agents.

Effect of pre-treatment of bentonite with sodium and calcium ions on phosphate adsorption onto zirconium-modified bentonite

To understand the influence of the pre-treatment of bentonite with Na⁺ and Ca²⁺ on the adsorption of phosphate on zirconium-modified bentonite, three kinds of adsorbent materials including zirconium-modified raw, Na⁺-pretreated and Ca²⁺-pretreated bentonites were synthesized and characterized firstly, and afterward their adsorption performance and mechanism for phosphate were studied comparatively. The phosphate adsorption ability for zirconium-modified bentonite decreased after the pre-treatment of bentonite with Na⁺, but it increased after the pre-treatment of bentonite with Ca²⁺. The maximum phosphate adsorption capacity calculated from the Langmuir isotherm model for zirconium-modified Ca²⁺-pretreated bentonites (13.4 mg P/g) was much higher than that for the zirconium-modified raw bentonite (9.06 mg P/g). The pre-treatment of bentonite with Na⁺ and Ca²⁺ did not change the interaction type between zirconium-modified bentonite and phosphate, i.e., the coordination of phosphate to zirconium. The decreased phosphate adsorption capacity for zirconium-modified bentonite induced by the Na⁺ pre-treatment could be mainly attributed to the decrease of the specific surface area and the content of exchangeable Ca. The increased phosphate adsorption capacity for zirconium-modified bentonite induced by the Ca²⁺ pre-treatment could be mainly due to the increase in the amount of exchangeable Ca. Results of this work suggest that the zirconium-modified Ca²⁺-pretreated bentonite is more suitably used as an adsorbent for the removal of phosphate from wastewater than the zirconium-modified raw and Na⁺-pretreated bentonites.

Successful control of internal phosphorus loading after sediment dredging for 6years: A field assessment using high-resolution sampling techniques

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6years, due to their geographical proximity (a line distance of approximately 9km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353mg/m²/d to 0.174, 1.58 and 0.048mg/m²/d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom.

Submillimeter-scale heterogeneity of labile phosphorus in sediments characterized by diffusive gradients in thin films and spatial analysis

Sediments have a heterogeneous distribution of labile redox-sensitive elements due to a drastic downward transition from oxic to anoxic condition as a result of organic matter degradation. Characterization of the heterogeneous nature of sediments is vital for understanding of small-scale biogeochemical processes. However, there are limited reports on the related specialized methodology. In this study, the monthly distributions of labile phosphorus (P), a redox-sensitive limiting nutrient, were measured in the eutrophic Lake Taihu by Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) on a two-dimensional (2D) submillimeter level. Geographical information system (GIS) techniques were used to visualize the labile P distribution at such a micro-scale, showing that the DGT-labile P was low in winter and high in summer. Spatial analysis methods, including semivariogram and Moran's I, were used to quantify the spatial variation of DGT-labile P. The distribution of DGT-labile P had clear submillimeter-scale spatial patterns with significant spatial autocorrelation during the whole year and displayed seasonal changes. High values of labile P with strong spatial variation were observed in summer, while low values of labile P with relatively uniform spatial patterns were detected in winter, demonstrating the strong influences of temperature on the mobility and spatial distribution of P in sediment profiles.