Construction Of Hierarchically Porous La-based Phosphorus Adsorbents Derived From Cyanometallates Frameworks And Their Adsorption Mechanism

Total phosphorus pollution is an important water environment problem in key river basins and coastal waters in China.The highly selective adsorption removal of phosphate in water bodies using La-based functional materials is considered as an ideal deep dephosphorization technology with low energy consumption and high efficiency,following the current trend of"pollution reduction and carbon reduction".However,previous La-based（hydr）oxide materials often suffer from defects such as easy agglomeration and poor structural stability,resulting adsorption sites exposed incomplete and ecological risks,respectively.The key to improving phosphorus removal lies in the construction of La-based functional materials with stable porous structures and abundant exposure of La sites.In this study,La-based cyanometallates were selected as precursors,and a series of hierarchically porous La-based（hydr）oxides were produced through a self-template transformation strategy to regulate the porosity opening process and La site exposure based on the thermal cracking temperature（La Fe CM-500,La Fe CM-700）,precursor metal ratios（La Fe15T,La Fe10T,La Fe05T）and the degree of alkaline treatment（La H,La L）.The physical and chemical properties of the materials were analyzed via SEM,XRD,XPS,BET and other structural characterization.The ion solubilization characteristics of the hierarchically porous materials derived from La-based cyanometallates frameworks were identified.The adsorption performance on phosphorus were systematically evaluated to illustrate the mechanism and the corresponding structure-activity relationship are investigated.The effects of typical environmental parameters such as p H and coexisting ions on adsorption and the cyclic adsorption process was explored.The study provides neoteric technical support for the development of functional materials for phosphorus removal,and its main conclusions are drawn as follows:（1）Structural optimisation at the template preparation stage was demonstrated to be an effective strategy for designing hierarchically porous La-based phosphorus adsorbents derived from cyanometallates frameworks.A self-templating method based on high-temperature cracking was used to fabricate hierarchically porous La Fe bimetal oxides,in which the oxidative thermal decomposition process formed a gas flow outward to achieve the pore structure,and the high supply of La ^III/Fe ^IIratio yielded more adequate coordination polymerisation and high coordinated water content,which in turn dominated its oxidation products with more mesopores.A hierarchically porous La（OH）₃material was successful developed via a novel self-templating method based on ion exchange,the OH^-from the alkali conversion reagent replaced the[Fe（CN）₆]^4-cluster in the precursor of the La Fe cyanometallates,simultaneously completed hydroxide conversion and pore-creating.The obtained La-based cyanometallates derivatives all intactly inherited the macro framework structure of the precursors,with abundant hierarchically pores and stable structure（ion solubilization is negligible）,showing outstanding selectivity for phosphorus removal in water.Ligand exchange is the main mechanism for phosphorus removal of La-based cyanometallates derivatives,and La site is the key to the formation of stable La PO₄during ligand exchange.（2）The morphological structure and phosphorus removal performance of La Fe bimetal oxides constructed by high-temperature cleavage self-templating method are closely related to the cleavage temperature and the ratio of precursor metals.Compared with La Fe CM-700,La Fe CM-500 has a richer pore structure（SA=15.9 m²/g,106%）and exhibits more outstanding phosphorus removal efficiency,with a higher outer particle diffusion rate of phosphate（184%）and adsorption capacity（45.0 mg P/g,89%）.The increased La/Fe ratio of the precursor facilitates the acquisition of the hierarchically porous La Fe bimetal oxides with more exposed active sites,which in turn improves the phosphorus removal capacity（Qo=61.4 mg P/g,at a La/Fe ratio of 1.5:1）.The La-based metal oxides are suitable for the treatment of acidic phosphorus containing wastewater at the p H of 3.4-3.8,and show excellent alkali buffering properties.La Fe15T,a La Fe oxide with an optimum La/Fe ratio（1.5:1）,exhibited the best phosphate adsorption ability,i.e,95 mg P/g at the optimum p H.（3）The highly transformed hierarchically porous La（OH）₃microsphere La H constructed via the alkali conversion self-templating method exhibited a richer mesoporous structure（8-20 nm）and higher specific surface area（113.9 m²/g）.The La（OH）₃microspheres with fully exposed active sites both inside and outside,exhibited outstanding phosphate removal effciency（maximum adsorption capacity of 163.6 mg P/g）.The structure of the material is stable,that the primary microsphere structure and secondary nanoneedle morphology could be well preserved after several application cycles.The hierarchically porous La（OH）₃microspheres have good alkali buffering properties and are suitable for the treatment of medium and alkaline phosphate wastewater.The superior adsorption capacity of hierarchically porous La（OH）₃remained relatively stable over a wide p H range（La H:6.2-10.4）.La H showed high selectivity and environmental adaptability for phosphate removal,which can rapidly reduce dissolved inorganic phosphorus levels to below 0.05 mg P/L over a wide range of salinities and temperatures,showing prominent application potential for the treatment of mariculture tailwater.（4）All three structural regulation strategies can obtain hierarchically porous La-based phosphorus adsorbents derived from cyanometallates frameworks with stable structure,fully exposed active sites and more outstanding phosphate removal effciency than Phoslock（?）.Among them,La Fe15T,a phosphate adsorbent constructed by high-temperature cleavage self-templating method suitable for acidic conditions,and La H constructed by alkali conversion self-templating method adapted to medium and alkaline conditions,were selected.The unit cost of La H for phosphorus removal is only 1.28（?）/g P,demonstrating significant economic benefits.