Resuspension and settlement characteristics of lake sediments amended by phosphorus inactivating materials: Implications for environmental remediation

Phosphorus migration from sediment to phosphorus-inactivating material: A key process neglected by common phosphorus immobilization assessments for lake geoengineering

Various phosphorus (P)-inactivating materials with a strong capability of immobilizing P in sediment have been developed for lake geoengineering purposes to control internal P pollution. However, unsatisfactory applications have raised concerns about the reliability of the method. This study hypothesized that P migration from sediment to material is a key process regulating the immobilization, which is often neglected by common assessment procedures that assume that the material is closely in contact with sediment (e.g., as mixtures). To verify this hypothesis, 90-day incubation tests were conducted using drinking water treatment residue (DWTR). The results showed that the soluble P in the overlying water of sediment-DWTR mixtures and the mobile P in the mixtures were substantially reduced from the initial period and remained low during the whole incubation tests. However, assessment based on separated samples indicated a gradual P migration from sediment to DWTR for immobilization. Even after 90 days of incubation, mobile P still accounted for ∼5.33% of total P in the separated sediment. Further analysis suggested that using mixtures of sediment with DWTR accelerated P migration during the assessment, leading to a faster P immobilization assessment. Considering the relatively low levels of mobile P in the separated DWTR during incubation, the gradual decrease in mobile P in the separated sediment indicates that sediment P release regulates P immobilization efficiency. Therefore, designing a proper strategy to ensure sufficient time for the material to remain in close contact with the target sediment is critical to reducing uncertainties in lake geoengineering.

Comparison of magnetite, hematite and goethite amendment and capping in control of phosphorus release from sediment

The characteristics and mechanism of phosphate adsorption onto magnetite, hematite and goethite were comparatively studied, and the effects of magnetite, hematite and goethite amendment and capping on endogenous phosphorus (P) liberation from sediment into overlying water (OW) were comparatively investigated. The adsorption of phosphate onto magnetite, hematite and goethite mainly obeyed the inner-sphere complexation mechanism, and the phosphate adsorption capacity decreased in the order of magnetite > goethite > hematite. The magnetite, hematite and goethite amendment all can decrease the risk of endogenous Prelease into OW under anoxic conditions, and the inactivation of diffusion gradients in thin films-labile P in sediment made a great contribution to the restraint of endogenous P release into OW by the magnetite, hematite and goethite amendment. The efficiency of endogenous P release restraint by the iron oxide addition decreased in the order of magnetite > goethite > hematite. The magnetite, hematite and goethite capping all can be effective for the suppression of endogenous P release from sediment into OW under anoxic conditions, and most of P immobilized by the magnetite, hematite and goethite capping layers is relatively or very stable. The results obtained from this work suggest that magnetite is more suitably used a capping/amendment material to prevent P release from sediment than hematite and goethite, and magnetite capping is a promising approach for hindering sedimentary P release into OW.

A model for predicting reduction in mobile phosphorus of lake sediment by aluminum drinking water treatment residuals

Drinking water treatment residual (DWTR) derived from flocculation and sedimentation of raw water using aluminum coagulants is a valuable environmental remediation byproduct capable of inactivating phosphorus (P). However, no generalizable model exists in the literature to describe reduction of releasable (mobile) P in lake sediment as a result of DWTR addition. The reduction of mobile P (sum of labile P and reductant soluble P) was investigated in over 100 sub-samples using five sediment samples from two lakes and three DWTRs from different water treatment plants. A consistent relationship was determined across a range of mobile P contents (0.23 g/m²/cm to 0.92 g/m²/cm, or 15.8 to 186.1 µg/g DW) and DWTRs. The relationship was best described as a function of the mobile P content of the sediment and the oxalate-extractable aluminum content of the DWTR. An empirical model was developed to predict the immediate reduction in mobile P following the addition of DWTR containing aluminum. This model was validated using two additional lake sediments and one additional DWTR (R² = 0.995). Thus, the immediate inactivation of P in lake sediment following DWTR addition can be predicted with this model, which can be used with internal P loading or other water quality goals to determine an appropriate DWTR dose. Further recommendations were made about dosing DWTRs for lake restoration, allowing practitioners to use DWTR to inactivate P in lake sediment without conducting individual sorption experiments.

Study on the management efficiency of lanthanum/iron co-modified attapulgite on sediment phosphorus load

Attapulgite co-modified by lanthanum-iron (MT-LHMT) was used to study its effectiveness and mechanism in controlling phosphorus release from sediments. MT-LHMT has high adsorption capacity for phosphate and the maximum adsorption capacity of MT-LHMT to phosphate can reach 75.79 mg/g. The mechanism mainly involved electrostatic action, surface precipitation and ligand exchange between MT-LHMT bonded hydroxyl and phosphate to form La-O-P and Fe-O-P inner-sphere complexes. MT-LHMT has excellent adsorption performance in the pH range of 3-8. In addition to HCO₃^-, CO₃^2- and HA^- had a negative effect on the phosphorus removal of MT-LHMT, while NO₃^-, Cl^-, SO₄^2-, K⁺, Ca²⁺ and Mg²⁺ had a positive or no effect on phosphorus removal. MT-LHMT significantly reduced the risk of phosphorus release from overlying water in different dose effects and covering methods, as well as the unstable inactivation of flowing phosphorus, sediment dissolved reactive phosphorus (DRP) and available phosphorus with medium diffusion gradient in thin film in the sediment-water interface (Labile-P_DGT). The MT-LHMT capping wrapped with fabric can reduce the risk of nitrogen release from sediment to overlying water more than only MT-LHMT capping. The results of this study showed that the MT-LHMT capping wrapped with fabric has high potential and can be used as an active capping material to manage the nitrogen and phosphorus load in surface water.

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.

Particle size-related vertical redistribution of phosphorus (P)-inactivating materials induced by resuspension shaped P immobilization in lake sediment profile

Lake geoengineering with phosphorus (P)-inactivating materials to reduce sediment P loading is often used for eutrophication control. The redistribution of materials in sediment, especially those induced by resuspension, is reportedly a common phenomenon during practical applications, which may interfere with the pollution control. Notably, a recent study by the authors initially found that the heterogeneous properties of materials and sediments varied the P immobilization in different sized sediments which exhibited diverse movement characteristics. Therefore, this study hypothesizes a particle size-related vertical redistribution of materials in the sediment profile induced by resuspension, which shapes sediment P immobilization at different depths. Based on two differently sized materials, lanthanum (La)-modified bentonite clay (Phoslock) and drinking water treatment residue (DWTR), this study found a weakened reduction of mobile P and bioavailable P pool by both DWTR and Phoslock in surface sediment after resuspension. As the depth decreased from >12 to surface 0-1 cm, the remaining mobile P increased from 7.11%-10.8% to 11.0%-17.8% of the total P in the sediment with Phoslock and from 1.66%-4.73% to 9.70%-20.7% of the total P in the sediment with DWTR; meanwhile, bioavailable P pool reduction proportions decreased from 48.6%-72.3% to 3.23%-45.1% for Phoslock and from 51.5%-71.4% to 4.94%-25.2% for DWTR. Further analysis verified the hypothesis of this study; importantly, the redistributions of the potential target P (including mobile and bioavailable P) for immobilization were regulated by relatively small sediments (e.g., <8 μm fraction), which tended to become enriched in surface sediment after resuspension, while relatively large materials (e.g., >63 μm fraction) regulated their redistributions and were more likely to be buried at the bottom of the sediments. Accordingly, to design appropriate strategies for lake geoengineering, relatively small materials (e.g., <8 μm) targeting to immobilize both mobile and bioavailable P are typically recommended to be developed for restoration of lakes with frequent sediment resuspension.