Development Of LDHs Adsorption Membrane Reactor Based On Kinetics Advantages And Its Deep Removal Performance For Phosphate

Under the situation that the total phosphorus standard of wastewater treatment is becoming increasingly strict,the research and development of deep-level phosphorus removal technology have received great attention.In traditional deep-level phosphorus removal technology,dissolved phosphate usually needs to be transformed into particulate state and then removed by solid-water separation.Flocculation-filtration is a typical technology,which is highly controllable.One problem is that the flocs are easy to adhere to the membrane surface and cause irreversible pollution.Theoretically,the adsorption membrane filtration can bind soluble phosphate through the reaction mechanism.The difficulty is that the wastewater retention time in the membrane is noticeably short,which requires that the membrane forming material is advantageous in adsorption rate.However,at present,the adsorption kinetics methods cannot be used to compare the adsorption rate,and it is mandatory to build up a new method for rate comparison.To solve the above problem,the layered double hydroxides(LDHs)nanosheet was made into membrane devices in this study based on the kinetics advantages of LDHs in phosphate adsorption,and the novel deep-level phosphorus removal process depending on the composite LDHs membrane was proposed.A comparison method for the adsorption rate was established,and the improvement effect of the preparation process on LDHs phosphorus removal ability was investigated.The deep-level removal performance and mechanism of the composed LDHs membrane for the soluble phosphate were investigated,and its technical advantages were demonstrated.The problem of membrane fouling caused by floc is essentially solved and new ideas and technical support for the development of deep-level phosphorus removal were provided by this study.The main research contents and innovations of this dissertation can be summarized into four aspects.(1)A new intrinsic kinetics model was established according to the adsorption process.The adsorption rate coefficients of different materials were cross-compared,and the advantage of selecting LDHs as membrane-forming materials was proved theoretically.Based on the collision theory,an intrinsic kinetics model was established under consideration of the adsorbate concentration and the available adsorption site density.The model parameter?_ini can be used to quantitatively describe the initial adsorption conditions,and its value indicates the trade-off relationship between adsorbate and adsorption site.Experimental results showed that the intrinsic rate coefficient could be maintained at a constant level under certain?_ini conditions,while the traditional model rate coefficient showed a range of up to 30.3%?146.6%under the same test conditions.The stability of the intrinsic rate coefficient provides a premise for the comparison of the adsorption rate between materials.After comparison,it can be confirmed that LDHs had a significant adsorption rate advantage among 60 kinds of phosphorus adsorption materials,which was more than one order of magnitude higher than that of carbon materials and more than two orders of magnitude higher than that of industrial by-products.In addition,the intrinsic kinetics model also showed certain advantages in the adsorption process prediction.The sample deviation between the predicted phosphate adsorption data and the experimental results is only 8.66%in a wide?_ini range.(2)Physical and chemical characteristics of LDHs and their phosphorus adsorption performance were explored and optimized from the aging and drying steps in the preparation process.The effect of co-precipitation and hydrothermal aging methods,as well as oven-drying and vacuum freeze-drying methods was compared and analyzed.It was found that the drying method had a significant effect on the sample size and morphology.Compared with freeze-dried samples,the size and specific surface area of the LDHs nanosheet obtained from oven-drying were smaller,and the edge of the hexagonal crystal was not as clear and sharp as the freeze-dried samples.However,the aging and drying method did not exhibit a significant effect on LDHs crystal structure characteristic,chemical group composition,surface potential distribution,etc.For adsorption capacity,the saturated phosphorus adsorption capacity of the four LDHs is between 58.76?63.77 mg/g,and the capacity of LDHs prepared by co-precipitation and oven-drying is 63.77±1.06 mg/g.For kinetics difference,the pseudo first-order,pseudo second-order and intrinsic kinetics model all indicated that the oven-dried LDHs adsorption rate coefficient is greater than that of freeze-dried samples.Therefore,under the condition of the same metal ion and alkali type,the“co-precipitation aging and oven-drying”method can be used to synthesize the LDHs sample with the best phosphorus removal performance.(3)A suction filtration assembly method was used to construct a composite LDHs membrane reactor.The characteristics of water flux,relations between filtration condition and retention efficiency were investigated.The regeneration performance of LDHs membrane were investigated under different alkali-salt systems.The breakthrough process and retention mechanism of dissolved phosphate in LDHs membrane were revealed.The composite membrane structure consists of three parts:protective layer,LDHs functional layer and support layer.The protective layer can intercept the particulate phosphorus in the sewage.The LDHs functional layer is used to absorb the dissolved phosphorus and the support layer is the base of the suction filtration.Results showed that the LDHs membrane synthesized in this study had a net water flux of 9000?35000 L/(m²·h·bar),and the flux decreased linearly with the increase of the LDHs layer thickness.Moreover,the LDHs membrane was featured with a relatively strong stability.As metal elements of the laminate,Mg²⁺lost in the effluent was less than1%of the total amount of LDHs,and Al³⁺loss is lower than 1‰.The experiment was carried without flocculation pretreatment,the filtration flow rate was set at 3.0 m L/min,the initial soluble phosphate concentration was 2.0 mg/L,p H=6.79,and the effective filtration area was 3.14 cm².At this experimental condition,the corresponding treatment volume was 211 m L when taking the total phosphorus concentration in the effluent<0.1mg/L as the standard.The volume of treatable volume is affected by filtration conditions,and it can be significantly increased by properly slowing down the filtration rate or increasing the LDHs content in the membrane.Besides,the composite LDHs membrane after adsorption breakthrough showed excellent desorption and regeneration ability.The combined desorption system of 1.0 mol/L Na OH and 1.25 mol/L Na Cl can make the regeneration efficiency reach more than 70%.As a result,the treatable volume after two regenerations decreased sequentially by 23.0%and 26.7%compared with the fresh LDHs membrane.In terms of the breakthrough process and retention mechanism of soluble phosphate,the active sites in the functional layer decreased with the continuous adsorption,and the intrinsic adsorption rate coefficient between LDHs and soluble phosphate decreased,resulting in the decrease of the adsorbed phosphate within the specific residence time,and the membrane breakthrough appeared.Characterization and calculation indicated that the dissolved phosphate ions were mainly adsorbed and trapped by LDHs membranes through ion exchange,surface complexation and electrostatic attraction,among which ion exchange contributes more than 50%.(4)The deep-level phosphorus removal performance of the composite LDHs membrane was explored using actual wastewater.The technical advantage of LDHs membrane was evaluated by comparison with flocculation filtration method.The total phosphorus concentration of the actual wastewater used in the experiment was 1.33mg/L,of which particulate phosphorus accounted for 30.8%,and the soluble phosphate concentration was 0.92 mg/L.The common 0.45?m microfiltration membrane in the control group could only remove particulate phosphorus,and the dissolved phosphorus passed nearly 100%.After flocculation pretreatment,the phosphate concentration in the effluent was stable below 0.1 mg/L.However,compared with the control group,the transmembrane pressure after flocculation increased faster,and the pressure value began to exceed 0.1 bar when the filtration time was 36 min,indicating that membrane fouling was rapidly formed.In addition,the effluent after flocculation showed obvious color due to the presence of Fe³⁺ions.In contrast,the flocculation was not necessary for the composite LDHs membrane.Under the condition that filtration flow rate was 2.0 m L/min,the effluent phosphate concentration within the first 43 minutes was kept below 0.1 mg/L,and the filtrate was clear and transparent.Compared with the control group,the working time when the transmembrane pressure reached 0.5 bar was only reduced by 5.8%for the composite LDHs membrane,while the time for flocculation-filtration process was shortened by 22.5%.Therefore,the composite LDHs membrane can effectively alleviate the increasing process of the transmembrane pressure,and avoid the problem of excessive metal ion discharge.