Effects Of Amino Acid Salts On Removal Of Cadmium Contaminated Soil

Soil contaminated by cadmium is the most among heavy metals,so it has become urgent that these soils be decontaminated.Soil washing is currently one of the most practical methods to permanently remove metals from soils.However,most common washing agents are harmful to the soil.For example,strong acids and oxidising agents destroy the soil.Synthetic chelating agents and surfactants are often resistant to biodegradation,toxic or cause secondary pollution.In this study,the Cd contaminated soil from a farmland in Xiangtan City,Hunan Province was washed by amino acid salts.15different amino acid salts were tested and the washing protocol was studied,including the concentration of the washing agents,the contact time,the liquid-soil ratio and the treatment of the wastewater.Then the influence of washing on the soil properties was analyzed.Finally,the mechanism of Cd removal was discussed.The project studies the feasibility of using amino acid salts as washing agents to remove Cd from soil.The main conclusions are as follows:?1?Screening and technology research of amino acid salts.Through comparative experiments and single factor analysis methods,the research compares the removal efficiency of Cd from 15 amino acid salts and the optimal experimental conditions.The results showed that among the selected amino acid salts,[Gly][Cl]and[Gly][NO₃]had the highest removal efficiency of Cd.The metal washing protocol used optimized washing conditions,with washing agents at a concentration of 300 mmol/L,r_{solution/soil}=4:1?w/w?and a mixing time of 120 min.The removal efficiency of Cd could reach 84.30%and78.44%,respectively.Adding NaOH to the wastewater can effectively remove Cd.When the pH of the waste liquid was adjusted to 13,the Cd concentration in the water was lower than the detection limit?5?g/L?.?2?The effect of amino acid salts on the removal of various forms of Cd and the effect on the soil properties.After washing with[Gly][Cl]and[Gly][NO3],the exchangeable Cd was removed by 76.77%and 68.39%.Cd bound to organic matter was removed by 80.33%and 85.25%.Cd bound to Fe and Mn oxides was removed by 97.17%and 95.84%.Cd bound to organic matter was removed by 89.19%and 72.97%.The residue state was removed by 44.27%and 33.83%.The soil pH decreased and the soil particle size became smaller.The agglomeration structure of soil did not change.The contents of organic matter,alkali-decomposed nitrogen,available phosphorus,and total nitrogen increased,while the contents of available potassium,total potassium,total phosphorus,and exchangeable salt decreased.The germination rate of rice seeds increased,improved growth.?3?Preliminary study on the mechanism of amino acid salts washing.The kinetics of washing was best fitted by the Elovich equation,indicating that the kinetic process of leaching desorption was a heterogeneous diffusion process.The washing method does not affect the kinetic of washing.The washing mechanism may include:1)the dissolvation or partially dissolvation of the oxides of manganese,iron,calcium,and magnesium;2)the carboxyl and amino groups on the cation complex with Cd;3)The cation exchange reaction between H⁺and Cd²⁺;4)The formation of stable Cd-Cl complexes and promotion of Cd²⁺desorption from soil colloid.