| Ionic rare earth minerals are unique and important mineral resources in China.However,due to the large amount of low concentration rare earth wastewater generated during the mining and smelting process of ionic rare earth minerals,if the government does not effectively manage this industrial wastewater and allows it to be lost,it will not only lead to the loss of a large amount of rare earth resources,but also cause serious impacts on the environmental ecology and people’s health in nearby areas.Therefore,it is necessary to effectively recover and treat low concentration ionic rare earth wastewater,and precipitation method is widely used in the extraction process of ionic rare earth.In this paper,two kinds of sludge humic acids were extracted from urban surplus sludge from wastewater treatment plants using sodium pyrophosphate method and alkali solution acid precipitation method.Two kinds of sludge humic acid solutions were used as precipitators to precipitate low concentration rare earth solutions,and relevant experimental studies were carried out.Compare and analyze the physical and chemical characteristics of the two sludge humic acids and the hydrochemical behavior characteristics of the aqueous solution;The content of oxygen containing functional groups in two kinds of sludge humic acids was investigated;The effects of single factors on the precipitation rate of low concentration rare earth ions from two types of sludge humic acids were investigated.The reaction conditions were optimized through response surface analysis experiments;The humic acid rare earth precipitate was prepared under optimal conditions,and the physicochemical properties of the humic acid rare earth precipitate were analyzed.Kinetic and isothermal equation fitting and thermodynamic analysis studies were conducted during the precipitation process.The molecular mechanism of sludge humic acid precipitation of low concentration rare earth ions was preliminarily inferred.The main content of the paper is summarized as follows:(1)Two kinds of sludge humic acids were extracted from urban surplus sludge by sodium pyrophosphate method and alkali-soluble acid extraction method.Physical and chemical characteristics of the two kinds of sludge humic acids were analyzed.The results showed that the two kinds of sludge humic acids were mainly composed of carbon,oxygen,hydrogen,nitrogen and sulfur,and the content of oxygen and carbon elements was higher.The two kinds of sludge humic acid had more oxygen-containing functional groups and more chain structure,and the degree of aromatization was higher,but the degree of humification of SHA-1 extracted by sodium pyrophosphate method was higher,and the degree of humification of SHA-2extracted by alkali-soluble acid extraction was lower.The surface of the two sludge humic acids contains a large number of phenolic hydroxyl,alcohol hydroxyl and carboxylic functional groups.SHA-1 has a rough surface and an obvious granular structure,with a large number of dense and fine points and a small number of irregular holes distributed on the particle surface.SHA-2 has a smooth surface and a layered accumulation structure,loose surface and an obvious distribution of irregular holes.(2)Based on the analysis of the hydrochemical behavior of two sludge humic acid solutions,it was found that both SHAs contained aromatic C=C bonds and other chromophores,and SHA-1 had a lower aromatic degree;UV254 detection found a good linear relationship between the concentrations of humic acids in the two sludge types and the UV254 value,with R2 being 0.99996 and 0.99997,respectively;The E4/E6 values of the two SHAs were 4.130 and 3.075,respectively,indicating that SHA-1 had a smaller molecular weight and a lower aromatic degree;SHA-1 contains more humic acids than SHA-2;The content of total acidic group,carboxyl group,and phenolic hydroxyl group in SHA-1 is higher than that in SHA-2.(3)An experimental study was conducted on the optimal precipitation conditions of two SHA solutions for precipitation of low concentration rare earth ions.The results showed that the optimal conditions for precipitation of low concentration rare earth ions with two SHA solutions were: reaction time 60 min,SHA dosage 0.25 g/L,mixed solution p H 6,stirring rate100 r/min,reaction temperature 40 ℃,rare earth ion precipitation rates 98.65%,94.33%,respectively;The optimal reaction conditions obtained from the response surface optimization experiment were as follows: reaction time 56.26 min,mixed solution p H 5.88,stirring rate106.35 r/min,reaction temperature 40.16 ℃,predicted rare earth ion precipitation rate 98.74%,and measured average 98.78%.(4)The rare earth sediments were analyzed by means of elemental analysis,scanning electron microscopy and energy spectrum analysis,X-ray diffraction analysis,X-ray photoelectron spectroscopy and infrared spectroscopy,and the kinetics and isothermal equation fitting and thermodynamic analysis of low-concentration rare earth ions precipitated by sludge humic acid were carried out.The results showed that: A variety of rare earth elements appeared in the rare earth precipitate,among which Y,Nd and La elements were in the range of 1%-6%by weight.No characteristic diffraction peak of rare earth is found in XRD pattern.Humic acid molecules and rare earth precipitates are amorphous amorphous mixtures.The rare earth precipitates are irregular cube particles with rough surface and cluster structure.The particle size ranges from 5 to 40 μm,and there are no obvious micro-pores.There are Nd,Yb,La and other elements in the rare earth precipitate,whose average mass percentage is 3.80%,2.34%,4.11%,respectively.The carboxylic acids,phenolic hydroxyl groups and amino groups in SHA structure provide a good basis for precipitation of rare earth ions.The REDOX reaction of rare earth ions did not occur in the precipitation process.The precipitation kinetics of rare earth ions fitted by SHA was consistent with the quasi-second-order kinetic model,and the precipitation equilibrium time was 60 min.The isotherm conforms to Langmuir model,and the precipitation process is a spontaneous exothermic reaction. |
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