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Under alkaline conditions, the hydrothermal coprecipitation method was used to form different layered double hydroxides (LDHs) by combining different bivalent and trivalent metal compounds, such as ZnCl 2 , MgCl 2 , AlCl 3 , and FeCl 3 , and then LDHs were coated on the surface of the original maifanite. The effect of LDHs coating-modified maifanite on the improvement of Cr(Ⅵ) adsorption in water was studied using isothermal adsorption, desorption, non-isothermal adsorption, adsorption kinetics, pH, and competitive adsorption tests, respectively. The results show that the maximum theoretical adsorption capacity of modified maifanite for Cr(Ⅵ) is close to ten times that of original maifanite at 15℃. The adsorption effect of ZnAl-LDHs coating-loaded modified maifanite is better than that of other LDHs-coating modified maifanites. In contrast, the results of the desorption experiments show that maifanite coated with LDHs enhances the reuse of substrates. The thermodynamic parameters of the substrate for Cr(Ⅵ) adsorption in the experiment were △ G θ < 0, △ H θ < 0, △ S θ > 0, indicating that the substrate adsorption process of Cr(Ⅵ) is spontaneous and exothermic. Based on the adsorption kinetics study, the adsorption process of Cr(Ⅵ) by maifanite could be fitted using a pseudo-secondary reaction process. The adsorption type was mainly chemisorption. By selecting the suitable metal ion combination method to prepare different LDHs-coating modified maifanites, the Cr(Ⅵ) performance can effectively be improved.

Under different pH conditions, the hydrothermal and co-precipitation method was used to synthesize layered double hydroxides (LDHs) coated on bio-ceramic substrates with three different Zn 2+ /Al 3+ molar ratios. Applying the original and six kinds of modified bio-ceramic substrates coated with ZnAl-LDHs (bio-ceramic/ZnAl-LDHs) in simulated vertical-flow constructed wetlands, experiments for phosphorus removal and isothermal adsorption were conducted to analyze the mechanism and effect of each synthesis factor. The results showed that ZnAl-LDHs (pH=11) had a more obvious effect on phosphorus removal, especially for bio-ceramic/ZnAl-LDHs (pH=11, 1:1), whose average removal rates of TP, TDP and SRP were enhanced over 70%. Its maximum adsorption capacity for phosphorus was three times higher than that of the original bio-ceramic. Both pH and Zn 2+ /Al 3+ molar ratio affected the configuration and coating properties of bio-ceramic/ZnAl-LDHs at the time of synthesis, and pH was the main synthesis factor for phosphorus removal efficiency of bio-ceramic/ZnAl-LDHs. Through reasonable regulation of pH and Zn 2+ /Al 3+ molar ratio when bio-ceramic/ZnAl-LDHs was synthesized, the phosphorus removal efficiency could be improved effectively.

As an effective technology for wastewater treatment, bio-filter has been widely used. Nevertheless, there is still a lack of systematic report on purification efficiency and influencing factors of combined bio-filters. To this end, a novel combined system that consisted of aerated vertical-flow filter (AVF) followed by baffled horizontal-flow filter (BHF) was designed. After setting a series of hydraulic loading rates (131, 94 and 60 mm·d -1 ) and diversion ratios (8:2、6:4、4:6), we comprehensively assessed the impact of running condition adjustment on treatment performance by multiple statistical analyses. The results showed that, the average removal rates of organic matter, ammonia nitrogen and dissolved nitrogen in AVF were all above 80%, while the average removal rates of ammonia nitrogen, total nitrogen and dissolved nitrogen in BHF were all below 40%. Different running conditions had a significant ( P <0.05) impact on treatment performance. Meanwhile, there were significant differences in purification efficiency between the two different kinds of filters. Oxidative degradation was one of the main ways to remove organic matter in the two kinds of filters. There were obvious nitrification and denitrification processes within the two kinds of filters. Nitrification followed by denitrification was the main way to remove total nitrogen since ammonium occupied the most portion of total nitrogen in the synthetic wastewater. Meanwhile, the intensity of nitrification and denitrification in AVF was obviously higher than that in BHF. Phosphorus removal was mainly controlled by hydraulic loading rate, temperature, dissolved oxygen, organic matter, etc. This might indicate that microbial absorption was one of the main ways to remove phosphorus for the two filters. Compared to the sole AVF, the removal of total organic matter and total phosphorus in the combined system was increased by 4.4% and 23.2%, respectively, but the removal of total nitrogen was reduced by 12.1%. Reducing the diversion ratio was helpful to improve the denitrification intensity in BHF. However, due to the introduction of excessive ammonia from the raw wastewater, as well as the limited nitrification capacity in BHF, the removal rate of total nitrogen for the combined system was decreased. Therefore, according to the composition of treated raw wastewater, the control of appropriate diversion ratio, residence time and redox conditions inside the filter bed was the key to enhance the overall performance of the combined system.

The reuse of tailwater as recycled water requires an advanced nitrogen removal treatment, during which carbon source is added based on the tailwater quality. This study conducted an advanced nitrogen removal model test in constructed wetland to evaluate the effects of influent NO 3 - -N loading on denitrification and carbon releasing. In the constructed wetland, bark was used as filler and carbon source for nitrogen removal. The results showed that nitrogen removal was steady in the constructed wetland filled with bark. Denitrification velocity followed the Monod equation, and it increased as the influent NO 3 - -N loading increased. The saturation constant K S was 19.10 mg·L -1 . NO 3 - -N removal rate decreased as the influent NO 3 - -N loading increased. During the early periods of operating the constructed wetland filled with bark, the carbon releasing amount and velocity increased as the influent NO 3 - -N loading increased. Both factors were positively linearly correlated with the influent NO 3 - -N loading. The static carbon releasing velocity was 0.2 mg·(g·d) -1 . Compared with the hollow and loose plant carbons, such as rotten wood, bark had a better performance and a longer cycle of carbon releasing, which made bark a good slow-release carbon source.

Zn-Layered double hydroxides (LDHs) were selected to carry out the experiment in present work based on the previous research results. According to the co-precipitation method, three kinds of different Zn-LDHs (FeZn-LDHs, CoZn-LDHs and AlZn-LDHs) were synthesized by ZnCl 2 , FeCl 3 , AlCl 3 , and CoCl 3 solution in alkaline conditions and Zn-LDHs were in-situ coated on the surface of natural zeolites. With the filling of the natural and three kinds of Zn-LDHs modified zeolites in the columns, test experiments were conducted to study the removal performance of phosphorus in vertical-flow constructed wetlands. The results showed that: compared with the natural zeolites, the removal rates of phosphorus by the three kinds of Zn-modified zeolites were greatly improved, especially for FeZn-LDHs. Moreover, the maximum adsorption capacity and the desorbed performance of phosphate were enhanced by the Zn-LDHs coated modification. The adsorption type of phosphate was converted from physical adsorption to chemical adsorption. Through reasonable selection of the type of zeolite and the method of modification, Zn-LDHs modified zeolites could be used to enhance the removal of nitrogen and phosphorus for the eutrophic water.

Six kinds of metal compounds which were CaCl2 , ZnCl2, MgCl2, FeCl3, AlCl3, and CoCl3 were formed nine kinds of different combinations in the alkaline conditions to synthesized LDHs (Layered Double Hydroxides), which were in-situ coated on the surface of zeolites. With the filling of the original and nine kinds of modified zeolites in the columns to simulate a laboratory-scale vertical-flow constructed wetland system, the experiments of purified phosphorus were conducted. Combined removal efficiency with adsorption isotherm data of the ten kinds of zeolites, mechanism for strengthening the removal rates of the phosphorus by the modified zeolites was studied. The results showed that compared with the original zeolites, the removal rates of the phosphorus by nine kinds of modified zeolites were enhanced with various degrees. In the cases of Zn involved in the modified zeolites, the removal efficiencies of phosphorus reached a high quality. Especially, the ZnFe-LDHs had the average removal rates of the total phosphorus, the dissolved phosphorus and the phosphate were over 90%, and its maximum adsorption capacity of the phosphorus was three times higher than that of the original zeolites. Therefore, by means of increasing the adsorption capacity and improving the chemical adsorption ability of phosphorus, the modification to coated LDHs on the zeolites reached the aim of strengthening the purification of the phosphorus.

As one kind of vertical-flow constructed wetlands substrates, anthracite was selected in this experiment. LDHs (layered double hydroxides) were synthesized in alkaline conditions by co-precipitation of different kinds of metal compounds, such as CaCl2, ZnCl2, MgCl2, FeCl3, AlCl3, CoCl3. The synthesized LDHs were in-situ coated onto the surface of anthracite substrate to achieve the aim of modification. Simulated test columns were constructed to study the nitrogen removal efficiency of the urban sewage using the original anthracite substrates and 9 kinds of modified anthracite substrates. The results showed that: LDHs synthesized by all the 9 different kinds of methods could effectively modify the anthracite substrate by in-situ coating. With Mg2+ involved in the synthesis of modified substrates, good TN and ammonia nitrogen removal efficiencies were observed. The modified anthracite substrates coated with MgCo-LDHs had the optimal performance with average TN and ammonia nitrogen removal efficiencies of higher than 80% and 85%, respectively. The ammonia nitrogen removal efficiencies by the modified anthracite substrates coated by LDHs reacted with Mg2+ and Fe3+ were also high. The ammonia nitrogen removal efficiencies by modified anthracite substrates coated with CaFe-LDHs and MgFe-LDHs were higher than 85%.

Bio-ceramic, anthracite, zeolite, steel slag and vermiculite were used as substrate according to different kinds of gradation to treat wastewater in vertical-flow constructed wetlands simulation systems. The results show that the removal ability of COD by graded substrates according to particle size are better than single substrates, and average removal efficiency by graded bio-ceramic is up to 72.91%. The removal rate of TN by graded zeolite, which reaches 91.23%, is higher than single zeolite. No significant difference (p < 0.05) is detected with regard to organic matter and nitrogen removal between single and combined use of bio-ceramic and zeolite. The pH values in effluents of all columns filled with steel slag and anthracite are within normal limits, but phosphorus removal of all columns filled with steel slag and anthracite are lower than that filled with single substrates, except for the column filled with anthracite, vermiculite and steel slag from up to down. No difference between planted and unplanted systems can be observed. The present results probably provide a basis for vertical-flow constructed wetland design, among which based on the characteristic of wastewater proper selection of high-efficiency graded substrates, e.g., graded bio-ceramic, graded zeolite, graded anthracite, combined use of bio-ceramic, zeolite and anthracite, is a guarantee of better performance at a high hydraulic loading rate.