Study On Phosphate Removal From Water By Iron And Cerium Modified Biochar And Phosphorus Release In Soil

Phosphorus（P）plays a crucial role in plant growth.However,excessive phosphate can lead to eutrophication of water bodies and even pose a threat to human health.Therefore,it is necessary to adopt feasible adsorption techniques to remove phosphate from water bodies.Due to the limited functional groups and negative charges on the surface of biochar（BC）,its removal effect on phosphate in water is limited to some extent.In addition,BC contains a certain amount of essential nutrients for plant growth,which can be used to improve soil properties.However,BC itself has a relatively low phosphate content and has certain limitations in its use as a soil phosphate fertilizer.Therefore,adopting various modification technologies to enhance the removal effect of BC on phosphate in water bodies and using the BC after absorbing phosphorus for soil nutrients improvement is of great significance.In this study,hydrophilic porous BC modified with magnetic Fe₂O₃and Ce CO₃OH（Fe/Ce@HPBC）was synthesized using a one-pot hydrothermal method.The adsorption performance of Fe/Ce@HPBC on phosphate was investigated through batch adsorption experiments,and its repeated regeneration performance was analyzed.In addition,the removal mechanism of phosphate was analyzed.Pot experiments were conducted to investigate the growth promotion effect of Fe/Ce@HPBC adsorbed phosphate on maize seedlings in soil.The main research contents and results are as follows:（1）The surface functional groups and morphological characteristics of Fe/Ce@HPBC were analyzed by such characterization techniques as FTIR and SEM.The results showed the successful loading of Fe₂O₃and Ce CO₃OH,and the successful adsorption of phosphate.Moreover,the adsorption performance of Fe/Ce@HPBC for phosphate in water was investigated through aqueous phase adsorption experiments.The results showed that in the kinetic test,the Avrami model was more suitable for fitting the adsorption process for phosphate,which occurred simultaneously in both physical and chemical adsorption processes;In the isotherm test,the Langmuir and Sips models had a higher degree of fitting,indicating that the adsorption process of phosphate on the material exhibited a single molecular layer adsorption with active sites uniformly distributing on the material surface,and the theoretical maximum adsorption capacity of Fe/Ce@HPBC for phosphate given by the Langmuir model was 203.88 mg/g.（2）The practical application potential and recycling performance of Fe/Ce@HPBC were explored through coexisting ions and adsorption and desorption experiments.The results showed that the effect of anions and phosphates competing for active site on the surface of materials was relatively small(HCO₃^->CO₃^2->SO₄^2->Cl^->NO₃^-)when the concentration was at 0.01 and 0.1 M.The removal rate of phosphate by Fe/Ce@HPBC was higher than 80%,which indicated that Fe/Ce@HPBC had a strong affinity for phosphate,and was suitable for practical applications.In addition,after five consecutive cycles of adsorption and desorption,the adsorption rate of Fe/Ce@HPBC for phosphate was still above 67%,and the desorption efficiency was still above89%.It indicated that Fe/Ce@HPBC was a high-performance regenerative adsorbent with excellent recyclability and applicability.（3）Pot experiments were conducted to investigate the slow release effect of Fe/Ce@HPBC adsorbed phosphate in soil and its impact on maize growth.The results showed that the soil p H values in the treatment groups of Fe@HPBC-P,Ce@HPBC-P,and Fe/Ce@HPBC-P increased first and then decreased with time,which was due to the dissolution of alkaline substances in BC and the production of root exudates from maize seedlings.Moreover,under the condition of a material dosage ratio of 1.5%,the soil available phosphorus content of the material increased from 4.21 and5.92 mg/kg to 31.29 and 41.16 mg/kg,respectively,in the group of Ce@HPBC-P and Fe/Ce@HPBC-P.As time goes on,the growth of corn requires more phosphorus to be absorbed,and the phosphorus content absorbed by the material is limited,resulting in a decrease in the available phosphorus content to 3.53 and 3.05 mg/kg,respectively;In addition,the activities of urease,acid phosphatase,and invertase,and the content of soil available nitrogen were measured,verifying that the addition of adsorbents containing phosphate helped to improve soil enzyme activity and available nutrient content.The promotion effect of the addition of adsorbents containing phosphate on root development and promoting effect of maize seedlings was further verified through their plant height,root length,fresh weight content,root vitality,photosynthetic pigment content,and total phosphorus and nitrogen content in the seedlings.（4）Through XPS analysis,the mechanisms of phosphate removal by Fe/Ce@HPBC were investigated in detail.The results showed that the adsorption of phosphate by the materials involved physical and chemical process.Among them,the physical action mainly referred to pore filling between phosphate and Fe/Ce@HPBC.The chemical process was mainly divided into the formation of the inner sphere complexation of Ce-O-P and Fe-O-P,the electrostatic attraction between Fe/Ce@HPBC and negatively charged phosphate,and the ligand exchange between C-OH,and phosphate.