Morphology,Composition,Structure And Mechanism Of Ferrous Ion Precipitates And Phosphorus Removal

At present,with the acceleration of industrialization,the total amount of phosphate discharged into the water increases,leading to a series of problems such as rapid proliferation of algae,increase of turbidity of water,decrease of dissolved oxygen content,etc.In serious cases,it also leads to phenomena such as bloom and red tide.Meanwhile,the discharge standard of sewage is increasingly strict,so we should actively seek"efficient and low-consumption"phosphorus removal methods.This has important scientific and practical significance for water quality improvement.In this thesis,low price siderite was selected to investigate the differences in phosphorus removal efficiency of siderite under the oxidation of H₂O₂ and KMn O₄,and the underlying mechanism was explored in depth.In natural siderite system,oxidation rate and organic matter containing carboxyl have great influence on iron conversion and phosphorus removal efficiency.Therefore,in order to further study the influence of Fe（Ⅱ）oxidation path and final precipitates on phosphorus removal efficiency,the relationship between Fe SO₄ oxidation path and phosphorus removal efficiency under different concentrations of DO and Asp was explored.The experimental results show that when Fe（Ⅱ）is rapidly oxidized by H₂O₂ to Fe（Ⅲ）,the corresponding Fe（Ⅲ）-（hydrogen）oxide is amorphous Fe（OH）₃.In addition,amorphous Fe（OH）₃ coats the exposed Mn（Ⅱ）surface to form a dense passivation film,resulting in the termination of oxidation.However,KMn O₄ can oxidize Mn（Ⅱ）,and the products of Mn O₂ act as ion channels to allow further oxidation and hydrolysis of Fe（Ⅱ）within siderite.At the same time,the ion channel of Mn O₂ provides a slow oxidation pathway,which allows Fe（Ⅱ）to hydrolyze first to form crystalline Fe（OH）₂,and then to oxidize further to form crystalline2L-ferrihydrite（δ-Keggin）.As a transition precursor,Fe（OH）₂ can greatly reduce the nucleation barrier of 2L-ferrihydrite,and the different oxidation and hydrolysis pathways of iron-bearing minerals lead to the generation of iron（hydrogen）oxides with different phosphorus adsorption activities,and different anions have different effects on the adsorption of phosphate.In addition,experiments show that Asp affects the phase state,crystallinity and phosphorus removal efficiency of precipitates by affecting the hydrolysis of metals and the properties of nanoparticles.At p H=7,the presence of aspartic acid delayed the initial growth time of floc.The main composition of the floc is the mixture of lepidocrocite and akaganeite.The crystallinity of lepidocrocite usually weakened with the increase of DO,and the appropriate amount of Asp promotes the growth of the floc.At low DO,Asp promoted the growth of lepidocrocite due to the presence of precursor Fe（OH）₂,and When the concentration of DO rise,the existence of Asp induced more conversion of ferrous ions into akaganeite.However,the hollow structure of akaganeite will hinder the thickness of the cotton-like shell,while the hollow structure of akaganeite would hinder the thickness of the flocs lamellae and to some extent increase its ductility,which in turn increases the adsorption site and settlement performance of the flocs.In addition,it also provides valuable guidance for the future application of iron flocculant in water treatment.