Microbial and environmental medium-driven responses to phosphorus fraction changes in the sediments of different lake types during the freezing period.

The comparative study of the transformation among sediment phosphorus (P) fractions in different lake types is a global issue in lake ecosystems. However, interactions between sediment P fractions, environmental factors, and microorganisms vary with the nutrient status of lakes. In this study, we combine sequential extraction and metagenomics sequencing to assess the characteristics of P fractions and transformation in sediments from different lake types in the Inner Mongolian section of the Yellow River Basin. We then further explore the response of relevant microbial and environmental drivers to P fraction transformation and bioavailability in sediments. The sediments of all three lakes exhibited strong exogenous pollution input characteristics, and higher nutritional conditions led to enhanced sediment P fraction transformation ability. The transformation capacity of the sediment P fractions also differed among the different lake types at the same latitudes, which is affected by many factors such as lake environmental factors and microorganisms. Different drivers reflected the mutual control of weakly adsorbed phosphorus (WA-P), potential active phosphorus (PA-P), Fe/Al-bound phosphorus (NaOH-P), and Ca-bound phosphorus (HCl-P) with the bio-directly available phosphorus (Bio-P). The transformation of NaOH-P in reducing environments can improve P bioavailability, while HCl-P is not easily bioavailable in weakly alkaline environments. There were significant differences in the bacterial community diversity and composition between the different lake types at the same latitude (p < 0.05), and the role of P fractions was stronger in the sediments of lakes with rich biodiversity than in poor biodiversity. Lake eutrophication recovery was somewhat hindered by the microbial interactions of P cycling and P fractions within the sediment. This study provides data and theoretical support for exploring the commonalities and differences among different lake types in the Inner Mongolian section of the Yellow River Basin. Besides, it is representative and typical for promoting the optimization of ecological security patterns in ecologically fragile watersheds.

Enhanced adsorption of hexavalent chromium and the microbial effect on quartz sand modified with Al-layered double hydroxides.

To enhance the hexavalent chromium (Cr(VI)) removal performance of simulated constructed rapid infiltration systems (CRIS) with quartz sand (QS) substrate, QS coated with Al-layered double hydroxides (Al-LDHs@QS) was prepared by the co-precipitation method under alkaline conditions. A scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) were used to characterize QS before and after modification. The result showed that the Al-LDHs were successfully coated on the surface of the QS. The isotherm adsorption experiment indicated that compared with the original QS, the adsorption property of the modified QS changed from monolayered chemical adsorption to multilayered physical adsorption, perhaps because of different types of adsorption forces. Moreover, the adsorption capacity of modified QS was significantly enhanced and ZnAl-LDHs@QS had a maximum adsorption capacity (1428.57 mg·kg-1) nearly 6 times greater than that of the original QS (232.56 mg·kg-1). Adsorption experiments at different pH showed that the adsorption capacity of ZnAl-LDHs@QS gradually increased as acidity decreased. High-throughput sequencing revealed that the relative abundance of chrome-tolerant microorganisms at the phylum and family levels were increased in modified QS compared with original QS. Hemocytometer counting revealed enhanced microbial quantity on the surface of QS after modification. The content of extracellular polymeric substances (EPS) and the enzymatic activity of the microorganisms adhered to the surface of modified and original QS were detected, results showed that Al-LDHs had an obvious influence on the promotion of EPS secretion and enhanced the enzymatic activity of microorganisms. These changes indicated that the modified QS created better conditions for microorganism growth, and the improved microbial effect caused strong biosorption, resulting in greatly enhanced Cr(VI) removal. Thus, ZnAl-LDHs@QS is a better choice for CRIS to remove Cr(VI).

Efficiency and mechanisms of cadmium removal via core-shell zeolite/Zn-layer double hydroxides.

To investigate the removal mechanisms of cadmium (Cd) by Zn-layer double hydroxides-modified zeolites substrates in constructed rapid infiltration systems (CRIS), the ZnAl-LDHs and ZnFe-LDHs were synthesized and in-situ coated on the original zeolites through co-precipitation method. The prepared Zn-LDHs-modified and original zeolites were characterized by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) methods, whose results provided the evidences that the Zn-LDHs were successfully coated on the original zeolites. From the results of purification experiments, the average Cd removal rates of ZnAl-LDHs-modified, ZnFe-LDHs-modified and original zeolites were 88.40, 86.00 and 32.52%, respectively; demonstrating that the removal rates of zeolites could significantly improve. Additionally, the modification of Zn-LDHS could enhance the theoretical adsorption ability. According to the results of isothermal adsorption and desorption tests, the desorption rates of Zn-LDHs-modified zeolites were higher than that of original zeolites. Cd adsorption capacity of ZnFe-LDHs-modified zeolites was 1428.57 mg kg-1 and original zeolites was 434.783 mg kg-1. In the adsorption kinetic studies, the pseudo-second-order models were used to well describe the experimental results of Zn-LDHs-modified zeolites, indicating that their adsorption types were attributed to be more stable chemisorption. Besides, the relevant microbial tests also confirmed that microbial enzymatic activity and extracellular polymeric substances (EPS) were significantly promoted on surface of Zn-LDHs-modified zeolites. The contents of EPS on the surface of zeolites were as following: ZnAl-LDHs-modified zeolites (78.58128 µg/g) > ZnFe-LDHs-modified zeolites (71.85445 µg/g) > original zeolites (68.69904 µg/g). Meanwhile, the results of high-throughput sequencing showed that modification by Zn-LDHs improved microbial diversity and relative abundance. The Proteobacteria was the dominant phylum and the Acidobacteria was conducive to Cd removal. Overall, it could be concluded that ZnAl-LDHs-modified zeolites might be applied as an efficient substrate for Cd removal in CRIS.

Hexavalent chromium removal from aqueous solution by adsorption on modified zeolites coated with Mg-layered double hydroxides.

In this study, MgAl-LDHs and MgFe-LDHs were synthesized via co-precipitation method and in situ coated on pre-washed zeolites through dipping process in beaker. The obtained modified zeolites and original zeolites were utilized as substrates of constructed rapid infiltration systems (CRIS) to remove hexavalent chromium (Cr(VI)) in wastewater. Micro-morphology features and chemical composition of zeolites before and after modification were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence spectrometer (XRF). The SEM, XRD, and XRF results demonstrated the feasibility of LDHs coated on the surface of the original zeolites. Purification experiments in simulated CRIS showed that the Cr(VI) removal rates of zeolites/MgAl-LDHs increased by 110.03% on average every concentration (0.5-16 mg L-1) compared with the original zeolites under 24-h HRT. The adsorption capacity of zeolites/MgAl-LDHs reached 66.98 mg kg-1 at 32 mg L-1 initial Cr(VI) concentration, which is nearly twice that of the original zeolites (33.24 mg kg-1) and 1.5 times higher than that of zeolites/MgFe-LDHs (42.01 mg kg-1). Isothermal adsorption tests showed that the Freundlich isotherm equations gave better fitting to the adsorption process. And zeolites/MgAl-LDHs showed a best fit with pseudo-second-order model which meant that the adsorption of Cr(VI) by zeolites/MgAl-LDHs was dominated by chemisorption. Thermodynamic parameters showed that the process of adsorption for the three substrates was spontaneous and endothermic intrinsically. Zeolites/MgAl-LDHs also displayed nearly 60% desorption rate with low concentration eluent (0.01 mol L-1 NaCl). Therefore, zeolites/MgAl-LDHs were chosen out as an optimal substrate for removing Cr(VI) from wastewater in CRIS. Graphical Abstract.

Microbial action and mechanisms for Cr(VI) removal performance by layered double hydroxide modified zeolite and quartz sand in constructed wetlands.

This study was conducted to evaluate the performance underlying the removal of hexavalent chromium Cr(VI) associated with Zn-layered double hydroxides (Zn-LDHs)-modified substrates utilized in simulated constructed wetlands (CWs) from a microbial perspective. To accomplish this, Zn-LDHs-modified substrates (zeolite and quartz sand (QS)) were synthesized at various Zn2+/Al3+ and Fe3+ molar ratios by co-precipitation under alkaline conditions. The experimental group was then compared with a control group to determine the microbial action responsible for Cr(VI) removal during the Cr(VI) removal experiments. The removal experiment revealed that the average Cr(VI) removal rates of the Zn-LDHs-modified substrates were superior to those of natural substrates. Subsequent evaluation of the microbial structure by Illumina high-throughput sequencing revealed that the relative abundance of Novosphingobium, Brevundimonas, Methylophilus, and Acidovorax related to Cr(VI) removal was relatively high in Zn-LDHs-modified substrates. Moreover, the extracellular polymeric substance (EPS) content was significantly influenced by the Zn-LDHs coating according to the relative microbial experiments. Similar trends were observed in enzyme activity. Taken together, these findings illustrated that the Zn-LDHs coating had a significant impact on microbial action, and the Cr(VI) removal efficiency of the Zn-LDHs-modified QS (zeolite) substrate was better than that of the natural substrate because of intracellular and extracellular removal mechanisms. Briefly, the microbial action of Zn-LDHs-modified QS played an important role in Cr(VI) removal, since the EPS content possessed the appropriate concentrations. Moreover, the microbial activity of ZnAl-LDHs-modified QS (zeolite) may have been higher than that of ZnFe-LDHs-modified QS (zeolite) because Al had a stronger promoting effect on Cr(VI) bio-removal than Fe. Therefore, the microbial Cr(VI) removal supported by ZnAl-LDHs-modified QS is a better choice for CWs.

Removal of Cd(II) by modified maifanite coated with Mg-layered double hydroxides in constructed rapid infiltration systems.

To improve the adsorption performance of Cd(II) by maifanite in constructed rapid infiltration systems (CRIS), Mg-layered double hydroxides (MgAl-LDHs, MgFe-LDHs) are prepared by a co-precipitation method and in-situ coated on the surface of original maifanite. Characterization of the successful LDHs-coating modification is realized by the following: scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Brunauer Emmett Teller (BET). In the purification experiments, the average removal rates of Cd(II) were 97.66% for maifanite/MgAl-LDHs and 97.54% for maifanite/MgFe-LDHs, approximately 11% greater than for the original maifanite. Isothermal adsorption experiments and adsorption kinetic experiments were conducted to explore the Cd(II) adsorption mechanism. The modified maifanite demonstrated a higher Langmuir adsorption capacity and stronger surface bond energies compared to the original maifanite. The adsorption type of Cd(II) by maifanite/Mg-LDHs and original maifanite was monolayer adsorption based mainly on chemical adsorption. Furthermore, the extracellular polymeric substances and dehydrogenase activities of the microorganisms were measured and analyzed to study the effect of microorganisms on the removal of Cd(II) in the test columns. High-throughput sequencing technology was also applied to analyze the composition and diversity of bacterial communities. Based on a simple estimation, the synthesis cost of maifanite/MgAl-LDHs was only ¥ 0.33/Kg. In brief, maifanite/Mg-LDHs is an efficient and economical substrate for a CRIS for Cd(II) removal.

Phosphorus removal and mechanisms by Zn-layered double hydroxide (Zn-LDHs)-modified zeolite substrates in a constructed rapid infiltration system.

This work presents novel materials, ZnFe-LDHs-modified (Zn2+ : Fe3+ molar ratio of 2 : 1 and 3 : 1) and ZnAl-LDHs-modified (Zn2+ : Al3+ molar ratio of 2 : 1 and 3 : 1) zeolites, which were synthesized under alkaline conditions via a co-precipitation method and coated in situ on original zeolites. The as-prepared LDHs-modified zeolites were used as substrates for a constructed rapid infiltration system (CRIS) to conduct purification experiments to investigate the phosphorus removal performance of all types of zeolites. The experimental results showed that the phosphorus removal rates of the Zn-LDHs-modified zeolites reached over 80%, which are superior to that of the original zeolites. Furthermore, isothermal adsorption and adsorption kinetic experiments were conducted to explore the adsorption mechanisms. The theoretical maximumadsorption capacities were efficiently enhanced owing to the Zn-LDHs coating strategy. Especially, that of the ZnFe-LDHs-modified (3 : 1) zeolites reached 434.78 mg kg-1, which is much higher than that of the original zeolites. Meanwhile, according to the fitting results of the adsorption kinetics experiments, it was found that the predominant adsorption type of the original zeolites was converted from intrinsically weak physical adsorption into more stable chemical adsorption by the Zn-LDHs coating. Furthermore, high-throughput sequencing was also exerted to uncover the phosphorus removal mechanism by microorganisms. The obtained results indicate that the relative abundance of Pseudomonas and Dechloromonas, which are closely related to phosphorus removal, effectively increased. Overall, the Zn-LDHs-modified zeolites improved the phosphorus removal performance efficiently and sustainably when applied in CRIS.

Removal of hexavalent chromium ions by core-shell sand/Mg-layer double hydroxides (LDHs) in constructed rapid infiltration system.

This study aimed to determine the efficiencies and mechanisms of hexavalent chromium (Cr(VI)) removal with modified sand coated by Mg-LDHs in constructed rapid infiltration system (CRIS). Mg-LDHs (MgAl-LDHs, MgFe-LDHs) were prepared by co-precipitation method and in situ coated onto the surface of original sand. Scanning Electron Microscope (SEM) and X-Ray Fluorescence Spectrometer (XRFS) were used to analyze physicochemical properties of sand/Mg-LDHs composites. Results obtained confirmed the successful LDHs-coating modification. During the purification experiments, the average removal rates of Cr(VI) were 47.62% for sand/MgAl-LDHs, 34.15% for sand/MgFe-LDHs and 11.61% for original sand, respectively. The sand/Mg-LDHs had a higher Langmuir adsorption capacity and desorption capability for Cr(VI) compared to original sand. The adsorption kinetic data of different adsorbents were better described by pseudo-second-order model. Intra-particle diffusion model was also used to elucidate the adsorption mechanism. Moreover, extracellular polymeric substances, biomass and enzymatic activity of microbes on the modified and original sand were testified and analyzed to study microbial effect on Cr(VI) removal in test columns. Through a rough economic estimation, the reagent cost of sand/MgAl-LDHs synthesis was only RMB ¥ 0.24/Kg. It could be concluded that MgAl-LDHs modified sand is an efficient and economical substrate of CRIS for Cr(VI) removal.

Enhanced removal performance of Cr(VI) by the core-shell zeolites/layered double hydroxides (LDHs) synthesized from different metal compounds in constructed rapid infiltration systems.

Nine kinds of LDHs were synthesized by the co-precipitation method under alkaline conditions with different combinations of trivalent metal compounds (FeCl3, AlCl3, CoCl3) and divalent metal compounds (CaCl2, MgCl2, ZnCl2), which were then coated in situ on the surface of zeolites to synthesize core-shell zeolites/LDHs composites. The zeolites before and after modification were characterized by SEM and X-ray fluorescence spectrometry. Using the different core-shell zeolites/LDHs and original zeolite substrates, the constructed rapid infiltration systems (CRIS) simulated test columns were set to treat the municipal sewage containing hexavalent chromium, Cr(VI). Isothermal adsorption tests were subsequently performed. The average removal efficiencies of the small-sized zeolites were much higher than those of the large-sized zeolites. For the small-sized zeolites, the Cr(VI) removal performances of the Mg-LDHs- and Al-LDHs-modified zeolite substrates were efficiently enhanced in particular, which could reach over 90%. And the removal rate of core-shell zeolites/ZnAl-LDHs reached 94.5%. Meanwhile, the maximum adsorption capacity of ZnAl-LDHs-modified zeolites could reach 51.0 mg/kg, indicating that the adsorption properties could be enhanced by ZnAl-LDHs coating. During the purification experiments, most of the LDHs-modified zeolites maintained their predominant chemical adsorption ability for the removal of Cr(VI). Therefore, the small-sized core-shell zeolites/ZnAl-LDHs composites could be used as potential substrates for the efficient removal of Cr(VI) in CRIS.