Effects Of Different Forms Of Phosphorus On The Transformation Of Green Rust And Their Speciation

Green rust is a special bimetallic hydroxide with specific redox characteristics that can interact with oxygen-containing anions in the environment and affect its conversion.Phosphorus is an essential mineral element for plant growth and development.phosphorus in soil is mainly divided into organic phosphorus and inorganic phosphorus.Ophosphate,pyrophosphate and tripolyphosphate are the common inorganic phosphorus containing different phosphate groups,and the molecular weight of the three increases successively.Glycerophosphate（GP）,adenosine triphosphate（ATP）and inositol hexakisphosphate（IHP）are the common organic phosphorus in the soil,and the number of phosphate groups and molecular weight of the three groups also increase successively.These phosphorus can interact strongly with green rust,and the strong adsorption performance of green rust makes phosphorus fixed on its surface,forming the surface complex/phosphorus-iron（hydrogen）oxide precipitation,thus affecting the stability of green rust and the evolution of iron oxide.At the same time,the distribution,migration and attribution of phosphorus in the environment are also closely related to these iron（hydrogen）oxides.Therefore,studying the interaction between phosphorus and green rust is beneficial to further understand the environmental geochemical behavior between iron and phosphorus.This experiment through air oxidation method,solution chemical analysis,X-ray diffraction（XRD）,in situ attenuation total reflection Fourier transform infrared spectroscopy（ATR-FTIR）,scanning and transmission electron microscope technology（SEM and TEM）,the effects of molecular changes of several organic/inorganic phosphorus on the transformation mechanism and transformation products of carbonate green rust(GR1(CO₃^2-))were studied.Compared three kinds of green rust,That is,the adsorption performance of GR1(CO₃^2-),sulfate(GR2(SO₄^2-)),chloride ion（GR1（Cl^-））on IHP,and the speciation and redistribution behavior of phosphorus on the surface of green rust and its transformation products in the presence of IHP,,The following results are mainly obtained.1.When the green rust is synthesized by air oxidation method,with the continuous aeration（air）at the reaction interface,the green rust is dissolved and converted into other iron oxides,while the transformation products of layers anion green rusts have different.In the absence of phosphorus,GR1(CO₃^2-)is converted into goethite,GR2(SO₄^2-)into a large amount of lepidocrocite and goethite,and the final products of GR1（Cl^-）is a large amount of magnetite and a small amount of goethite.Comparing the adsorption of IHP by three kinds of green rust,it is found that the adsorption rate disparity of IHP between different layers anions.The strongest adsorption rate of GR1（Cl^-）reached more than 98%,followed by GR2(SO₄^2-),and GR1(CO₃^2-)was the worst.The smaller the charge density of the layers anion(Cl^-<SO₄^2-<CO₃^2-),the stronger the mineral adsorption capacity of IHP(GR1（Cl^-）>GR2(SO₄^2-)>GR1(CO₃^2-)).2.After the synthesis of green rust mineral,dissolved oxidation occurs in the presence of dissolved oxygen.The anions(Cl^-,SO₄^2-,CO₃^2-)and Fe（Ⅱ）ions in the structure are released,and the Fe（Ⅱ）and phosphorus in the suspension will adsorb and fix on the surface of green rust and its transformation products to form a ternary complex.According to electron microscopy and ATR-FTIR analysis,phosphorus is preferentially adsorbed on the edge of green rust mineral,and a large amount of phosphorus exists in the form of the main inner-sphere complex or iron phosphate precipitation on the surface of green rust and its transformation products.With the increase of phosphorus concentration and molecular weight,the occurrence of phosphorus on the mineral surface becomes more complex.3.Phosphorus inhibit the generation/growth of green rust transformation and its transformation products.Compared with the transformation of pure green rust,the green rust phase still exists in the green rust transformation within 3-4 h of the phosphorus addition system,and the retained hexagonal morphology can also be seen in the electron microscope images.At the same time,the high concentration of phosphorus makes the absorption peak of the transformation product weaken and disappear by XRD analysis,that is,the grain of the transformation product decreases and the crystal phase is weakened.When the same concentration of different molecular weight of phosphorus was added,it was found that the larger the molecular weight of phosphorus（GP<ATP<IHP,ophosphate<pyrophosphate<triphosphate）,the stronger the inhibition effect on the formation of GR1(CO₃^2-)end product（goethite）,and the greater the effect on delaying the conversion of GR1(CO₃^2-).Moreover,the greater the concentration of phosphorus,the stronger the inhibition of GR1(CO₃^2-)conversion.4.The presence of phosphorus changes the transformation mechanism of green rust.As derived from the XRD with ATR-FTIR,At 0.2-5.0 m M GP,0.2-1.0 m M ATP,0.2-0.5 m M IHP,1.0 m M ophosphate acid or 1.0 m M pyrophosphate,GR1(CO₃^2-)end product is goethite,which is converted by the dissolution-oxidation-precipitation（DOP）mechanism;While at other concentrations of phosphorus conditions,GR1(CO₃^2-)end product is mixed phase of green rust（EX-GR）and goethite,This is because the high concentration of phosphate groups adsorbed on the surface thus inhibiting the dissolution of green rust,So that the Fe（Ⅱ）in the structure is directly oxidized to Fe（Ⅲ）,that is,the green rust is converted into a structurally identical EX-GR.At this time,the conversion mode of GR1(CO₃^2-)is dominated by the solid-state oxidation（SSO）mechanism.In the IHP-GR2(SO₄^2-)transformation system,analysis by XRD and SEM showed that EX-GR was generated in the IHP concentration of 5.0 m M,indicating that the high concentration of IHP changed the GR2(SO₄^2-)transformation mechanism.However,no EX-GR phase is found in the transformation products of the GR1（Cl^-）system,and the GR1（Cl^-）transformation mechanism is a DOP mechanism.It is possible that the concentration of IHP is too small to inhibit the dissolved oxidation of GR1（Cl^-）.