Study On Mechanism And Application Of Saline-Alkali Soil Improvrment By In Situ Super-Stable Mineralization

China has a vast area of saline alkali land,most of which are distributed in arid and semi-arid areas.It is an important cultivated land resource to be developed.Saline alkali soil has high pH,high free salt content and serious hardening,which hinders the normal growth of crops to a great extent.In situ super-stable mineralization has the characteristics of no pollution,simple operation,significant improvement effect and low cost.It can be applied to the field of soil improvement on a large scale.The core of this technology is that the applied mineralizer can fix free OH^-and CO₃^2-in the stable LDH structure and reduce the alkalinity of saline alkali soil during the formation of layered bimetallic hydroxide（LDH）in saline alkali soil environment.Attapulgite with permanent negative surface charge and cation exchange properties can adsorb water-soluble Na⁺in saline alkali soil,couple mineralizers to participate in the process of metastable mineralization and reduce soil salinity.The in situ super-stable mineralization method finally improves the saline alkali soil into high-quality farmland,which verifies the feasibility of this method in the practice of large-scale saline alkali soil improvement.（1）CaCl₂and FeCl₃formed CaFe-LDH structure in alkaline solution after aging for 21 days under normal temperature and natural conditions,which provided a feasible basis for in situ mineralization experiment in soil.In the in situ mineralization experiment of soda saline alkali soil,the effects of the proportion,amount and mineralization time of acid metal salts on the reduction of soil alkalinity were investigated.The results showed that the best mineralizer for in situ super-stable mineralization method is Ca SO₄and Fe₂（SO₄）₃with a molar ratio of 2:1,the application amount is 0.16%of the soil quality.After 150 days of improvement,the pH of soil decreased from 10.5 to7.3,and the content of CO₃^2-decreased from 20.1 g/kg to 6.8 g/kg,indicating that the soil alkalinity has decreased to the normal level.X-ray diffraction（XRD）,inductively coupled plasma emission spectroscopy（ICP-OES）,Fourier transform infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and high resolution transmission electron microscopy（HRTEM）and DFT calculation were used to explore the specific mechanism of alkalinity reduction of soda saline-alkali soil.Ca²⁺and Fe³⁺in mineralizer form stable Ca Fe-LDH structure with OH^-and CO₃^2-in soil(ΔG=-66.92 e V,K_sp=1.512×10^-61).During the formation of Ca Fe-LDH,OH^-and CO₃^2-are fixed between their layers,which leads to the transformation of the existing forms of OH^-and CO₃^2-in the soil from free state to stable state,and reduces the soil pH and CO₃^2-concentration.（2）The adsorption performance of attapulgite on water-soluble Na⁺in soil increases with the increase of attapulgite application amount and improvement time.When attapulgite was applied alone,the content of water-soluble Na⁺in saline alkali soil decreased from 0.45 g/kg to 0.39 g/kg,which was caused by the electrostatic attraction of the permanent negative charge on the surface of attapulgite.When attapulgite was applied together with high valence metal salts Ca SO₄and Fe₂（SO₄）₃,the concentration of water-soluble Na⁺in saline alkali soil was significantly reduced from 0.45g/kg to 0.29 g/kg.The adsorption mechanism is that the acidity of high valence metal salt partially dissociates attapulgite and dissolves Mg,Al and Fe ions,which expands the cation exchange capacity of attapulgite and can adsorb more water-soluble Na⁺,and the acidity of high valence metal salt expands the micropores of attapulgite into mesopores and macropores,increases the total pore size and pore volume,and provides more adsorption sites for water-soluble Na⁺in soil.（3）Application of in situ super-stable mineralization in large scale saline alkali land improvement.After five months of improvement,the pH of saline-alkali land was reduced from 10.5 to 7.6,the concentration of CO₃^2-was reduced from 20.1 to 7.1 g/kg,and the electrical conductivity（EC）was reduced from 736.5 to 295.0μs/cm,the concentration of soluble Na⁺decreased from 0.45 to 0.25 g/kg,and the salinity of soil decreased significantly.At the same time,the soil bulk density decreased from 1.82 to1.38 g/cm³,the degree of soil hardening decreased,and the soil structure changed from dense and barren to breathable and water retaining.The emergence rate of Rumex patientia L.increased from 0%to 98.3%,and the growth condition was good.The saline alkali land was improved into high-quality farmland.