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

Combining lanthanum-modified bentonite and calcium peroxide to enhance phosphorus removal from lake sediments

Lanthanum-modified bentonite (LMB) and calcium peroxide (CP) are known for their effective removal phosphorus (P) capacities. The present study aims to investigate the effects of the combined use of LMB and CP（LMB + CP）on the sediment P, dissolved organic matter (DOM) and iron (Fe) concentrations through a 90-day incubation experiment. The combined treatment showed strong removal effects on sediment P and DOM. Indeed, the SRP and DOM concentrations in the 0-10 cm sediment layer decreased following the combined application of LMB and CP by 40.67 and 28.95%, respectively, compared to those of the control group (CK). In contrast, the HCl-P in the 0-5 cm sediment layer increased following the combined treatment by 13.28%. In addition, compared with the single application of LMB, the LMB + CP treatment significantly reduced the soluble Fe (Ⅱ) in the sediment pore water and promoted the oxidation of Fe. Therefore, LMB + CP can enhance the removal of internal P from sediments. The DOM removal and Fe oxidation in sediment pore waters are beneficial for enhancing the adsorption of P by LMB. On the other hand, the single and combined applications of LMB and CP increased the richness of the sediment microbial communities while exhibiting slight effects on their diversity. According to the results of this study, the combined use of LMB and oxidizing materials represents a novel method for treating lakes with high internal phosphorus and DOM loads in sediments.

Effect of controlling nitrogen and phosphorus release from sediment using a biological aluminum-based P-inactivation agent (BA-PIA)

A biological aluminum-based P-inactivation agent (BA-PIA) has been developed and demonstrated to effectively remove nitrogen and phosphorus; however, whether it can control the release of nitrogen and phosphorus in sediment still needs study. This study aimed to examine the effect of BA-PIA on controlling sediment nitrogen and phosphorus release. BA-PIA was prepared by artificial aeration. The use of BA-PIA in controlling nitrogen and phosphorus release was studied using water and sediment from a landscape lake in static simulation experiments. The sediment microbial community was analyzed using high-throughput sequencing. Static simulation showed that the reduction rates of total nitrogen (TN) and total phosphorus (TP) by BA-PIA were 66.8 ± 1.46% and 96.0 ± 0.98%, respectively. In addition, capping of BA-PIA promotes the conversion of easily released nitrogen (free nitrogen) in the sediment to stable nitrogen (acid-hydrolyzable nitrogen). The content of weakly adsorbed phosphorus and iron-adsorbed phosphorus in the sediment was reduced. The relative abundance of nitrifying bacteria, denitrifying bacteria, and microorganisms carrying phosphatase genes (such as Actinobacteria) in the sediment increased by 109.78%. The capping of BA-PIA not only effectively removed the nitrogen and phosphorus in water but greatly reduced the risk of nitrogen and phosphorus release from sediment. BA-PIA was able to make up for the deficiency of the aluminum-based phosphorus-locking agent (Al-PIA) that only removes phosphorus, giving it improved application prospects.

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.

Removal efficacy and mechanism of nitrogen and phosphorus by biological aluminum-based P-inactivation agent (BA-PIA)

In this study, aluminum-based P-inactivation agent (Al-PIA) was used as a high-efficiency microbial carrier, and the biological Al-PIA (BA-PIA) was prepared by artificial aeration. Laboratory static experiments were conducted to study the effect of BA-PIA on reducing nitrogen and phosphorus contents in water. Physicochemical characterization and isotope tracing method were applied to analyze the removal mechanism of nitrogen and phosphorus. High-throughput techniques were used to analyze the characteristic bacterial genus in the BA-PIA system. The nitrogen and phosphorus removal experiment was conducted for 30 days, and the removal rates of NH₄⁺-N, TN and TP by BA-PIA were 81.87%, 66.08% and 87.97%, respectively. The nitrogen removal pathways of BA-PIA were as follows: the nitrification reaction accounted for 59.0% (of which denitrification reaction accounted for 56.4%), microbial assimilation accounted for 18.1%, and the unreacted part accounted for 22.9%. The characteristic bacteria in the BA-PIA system were Streptomyces, Nocardioides, Saccharopolyspora, Nitrosomonas, and Marinobacter. The loading of microorganisms only changed the surface physical properties of Al-PIA (such as specific surface area, pore volume and pore size), without changing its surface chemical properties. The removal mechanism of nitrogen by BA-PIA is the conversion of NH₄⁺-N into NO₂^--N and NO₃^--N by nitrifying bacteria, which are then reduced to nitrogen-containing gas by aerobic denitrifying bacteria. The phosphorus removal mechanism is that metal compounds (such as Al) on the surface of BA-PIA fix phosphorus through chemisorption processes, such as ligand exchange. Therefore, BA-PIA overcomes the deficiency of Al-PIA with only phosphorus removal ability, and has better application prospects.

Three kinds of active thin-layer capping materials for reducing the phosphorus load in eutrophic water body: comparison in dynamic experiment

In this article, dynamic simulation experiments have studied the effects of three capping materials, quartz sand (QS), aluminum-based phosphorus-locking agent (Al-PIA), and lanthanum-modified bentonite (LMB) in reducing phosphorus load in eutrophic water bodies. The changes of various forms of phosphorus in Al-PIA and sediment before and after the test were analyzed, and the mechanism of phosphorus migration and transformation in different capping systems was described. The dynamic simulation test lasted 95 days. The results showed that when the initial concentration of total phosphorus (TP) was 3.55 mg/L, the capping strength was 2 kg/m² and the hydraulic retention time of water circulation was 0.5 days, indicating that the average reduction rates of TP by LMB, Al-PIA and QS systems were 74.66%, 69.54%, and 3.64%, respectively, compared with the control system. The analysis of variance showed that there were significant differences (P < 0.05) in the TP concentration of the overlying water between the LMB, Al-PIA capping system, and the control system. Lanthanum ions in LMB can fix phosphorus. Al-PIA reduces the phosphorus concentration in water by means of ion exchange, adsorption, complexation, etc. LMB and Al-PIA promoted the migration of phosphorus in sediment. Among them, the phosphorus fixed by Al-PIA was mainly in the form of non-apatite inorganic phosphorus (NAIP) in inorganic phosphorus (IP), which can be seen; Al-PIA can effectively reduce the phosphorus load of eutrophic water.

Effects of nFe3O4 capping on phosphorus release from sediments in a eutrophic lake

This study applied the techniques of high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) to explore the effects and the behind mechanism for inhibition phosphorus (P) releasing from sediments by nFe₃O₄ capping. The highest decreasing rates of SRP and labile P (i.e., 49% and 47%, respectively) and the decreased flux of SRP showed that nFe₃O₄ capping can successfully control sediment internal P release. Adsorption by Fe(III) hydroxides with the oxidation of Fe(II) was one of the reasons for the decrease of P concentrations in nFe₃O₄ capping sediments. This was supported by the increase of Eh and significant negative correlation between Eh with Fe(II) (soluble and labile Fe(II)) and P (SRP and labile P) and significant positive correlation between Fe(II) and P in sediments by nFe₃O₄ capping. An outer-sphere complex between positively charged nFe₃O₄ surface groups and P formation was the other reason to decrease the concentrations of P in the nFe₃O₄ capping sediments. This was supported by the decrease of pH value in sediments by the capping of nFe₃O₄. This study shows that nFe₃O₄, when used as capping agent, can effectively control the sediment internal P release, which is expected to be used as a potential material for repairing lake eutrophication.

The effect of secondary capping on the control of phosphorus release from sediment by activated thin-layer capping with Al-PIA

It is well-known that the activated thin-layer capping covering by secondary capping of contaminated sediment poses a threat to the inactivation of activated material. In this study, the static simulation experiment was conducted to study the effect of secondary capping thickness by sediment on the control of TP release from the sediment by aluminum-based P-inactivation agent (Al-PIA), and to propose the phosphorus adsorption pathway of Al-PIA. The results showed that Al-PIA could effectively reduce the release of phosphorus pollutants from the sediment at the capping intensity of 2 kg/m². When the secondary capping thickness of sediment were 0, 2, 4, 7, 10, and 15 mm, the average removal rates of TP were 87.57%, 76.39%, 61.22%, 51.32%, 41.93%, and 32.11%, respectively, indicating that the removal efficiency of phosphorus decreased with the increase of the secondary capping thickness of the sediment. The adsorbed phosphorus by Al-PIA was mainly non-apatite inorganic phosphorus (NAIP) in inorganic phosphorus. With the increase of the secondary capping thickness of sediment, the NAIP proportion of phosphorus adsorbed by Al-PIA increased. Meanwhile, the removal rate of phosphorus in the activated capping system showed a first increase and then decrease trend, and the removal rates of total phosphorus (TP), inorganic phosphorus (IP), and organic phosphorus (OP) were obvious except for that of organic phosphorus (OP).