Adsorption Performance And Mechanism Of Zinc-based ZIFs Materials For Phosphate Removal In Wastewater

Phosphorus（P）is an essential element for living systems,but eutrophication of water bodies can occur when the P concentration in water exceeds 0.02 mg·L^-1.Thus,it is necessary to deeply reduce inorganic phosphate,which is the main component of P in wastewater.Adsorption has been widely applied in phosphate removal of wastewater due to its advantages of regenerable,strongly-flexible and highly-efficient.However,the presence of many co-existence matters such as Cl^-and SO₄^2-(>100 mg·L^-1)will seriously inhibit the highly-selective removal of phosphate in real wastewater.Considering that the adsorption capacity and adsorption kinetic can be used as two key factors for evaluating the performance of adsorbents,therefore,the construction of highly-selective adsorbents with high adsorption capacity and fast adsorption kinetic has become the hotspot of recent research.Metal-organic frameworks（MOFs）have been widely used in adsorption field because of the controllable structure and metal-rich active sites.Among them,a Zn-based MOFs material（ZIFs）is gradually used for phosphate removal,mainly ascribed to the strong affinity between Zn-defect sites and phosphate（p Ksp（Zn₃（PO₄）₂）=32.04）.Since the Zn-defect active sites on material surface can significantly affect the overall P-adsorbed performance of as-prepared sorbents,based on this,this paper exploited three defect-rich ZIFs-based adsorbents including two-dimensional（2D）hydrophilic ZIF-L,solvothermal modified Zn-ZIF and ZIF-L@graphene oxide（ZIF-L@GO）composite materials.Moreover,a series of characterization techniques were carried out to systematically investigate the adsorption capacity and physicochemical properties,further exploring the phosphate adsorption mechanism in wastewater by three ZIFs-based sorbents.The mainly obtained results of this paper are as follows:（1）A representative ZIFs material（ZIF-8）showed a lower adsorption capacity of54.82±4.28 mg·g^-1and adsorption kinetic of 0.01 g·mg^-1·min^-1due to its hydrophobic and three-dimensional structure.Thus,a two-dimensional hydrophilic leaf-like ZIF-L adsorbent was prepared via a simple co-precipitation method in chapter 2.The adsorption capacity and adsorption kinetic were increased to 75.18±2.65 mg·g^-1 and 0.04 g·mg^-1·min^-1,mainly attributed to the fully exposed Zn-defect active sites on the material surface.In addition,～100%of phosphate was selectively removed by 2D ZIF-L from the real influent(1.61 mg·P·L^-1)and effluent(0.12 mg·P·L^-1)of wastewater treatment plant.Such an excellent adsorption performance was attributed to:1)The interaction of abundant surface Zn-OH function groups and phosphate via ligand exchange;2)Electrostatic attraction between the positive charge on2D ZIF-L surface and the negatively-charged phosphate species.Meanwhile,the material could be regenerated for several cycles by using 0.01 M Na OH ethanol solution as the regeneration solution,suggesting that 2D ZIF-L could apply as the excellent adsorbent for phosphate removal from real wastewater.（2）Due to the lack of pore structure,some Zn-defect active sites of 2D ZIF-L material obtained in chapter 2 could be buried in the interlayer.Therefore,in chaper 3,we focused on how to expose the buried Zn-defect active sites in material layer by purposely modulating the pore structure,and further improved the adsorption capacity of ZIFs materials.Using ZIF-L as the precursor,the porous Zn-defect-rich Zn-ZIF-X（X refered to the reaction time）sorbents could be successfully prepared via a simple solvothermal method.The Zn-ZIF-72 material showed the highest adsorption capacity of 102.36±2.13 mg·g^-1 but only 75.18±2.65 mg·g^-1 of ZIF-L,mainly ascribed to the higher positive charge on surface and abundant active Zn-defect sites via developed pores.In addition,the mass transfer resistance caused by the newly-formed pores also led to a slightly slow adsorption rate of 0.01 g·mg^-1·min^-1 for Zn-ZIF-72.The P-adsorption efficiency of Zn-ZIF-72 was still higher than 90%even under the interference of100 mg·L^-1 of five common coexisting substances(such as CO₃^2-and SO₄^2-)in wastewater,mainly due to the specific inner-sphere complexation between Zn-defect sites and phosphate species on surface.Moreover,the result of column filtration experiment showed that when the phosphate concentration of effluent was 0.50 mg·L^-1 as the breakthrough point,Zn-ZIF-72adsorbent could treat about 885 bed volume（BV）of the real wastewater,much higher than～710 BV of ZIF-L.（3）The Zn-ZIF-72 material obtained in chapter 3 exhibited a slower adsorption rate due to its porous structure.Thus,in chaper 4,we focused on how to simultaneously improve the phosphate adsorption capacity and adsorption kinetic of ZIFs material.The Zn-defect-rich ZIF-L@GO-X（X refered to the amount of zinc nitrate hexahydrate）sorbent with few pores could be successfully prepared via a facile in-situ growth method by using ZIF-L as the precursor.The phosphate adsorption capacity of ZIF-L@GO-25 was as high as 116.25±2.14 mg·g^-1,mainly due to the isomorphic growth of ZIF-L on GO surface to expose more active sites and the dispersion of Zn-defect sites by the carrier GO.Moreover,the adsorption equilibrium time was only～100 min when the initial phosphate concentration set as 50 mg·L^-1,showing an excellent adsorption performance.The P-adsorption efficiency of ZIF-L@GO-25 was still higher than 80%even under the interference of 100 times concentration of five common coexisting substances(e.g.,HA and SO₄^2-)in wastewater,exhibiting the excellent anti-interference performance.In addition,0.01 M Na OH ethanol solution was able to multiply regenerate the saturated sorbent and pure hydroxyapatite could be obtained after a simple treatment for recycled P-containing solution.The phosphate adsorption mechanism of ZIF-L@GO-25 sorbent was the same as the previous two chapters,mainly attributed to:1)The ligand exchange between surface Zn-OH defects and phosphate;2)The electrostatic attraction between ZIF-L@GO-25 material and phosphorus species.