Preparation of mercapto modified lignite and its adsorption characteristics for Fe²⁺, Mn²⁺

In order to solve the problems of high Fe²⁺ and Mn²⁺ content and limited adsorption capacity of lignite in acid mine wastewater (AMD), lignite was selected as the base material and silane coupling agent 3-mercaptopropyl trimethoxy silane (MPTMS) was selected as the modification reagent to modify the lignite. The influence of the change of each influencing factor on the response value was investigated by single factor test.and the interaction of each influence factor was further determined by response surface methodology (RSM). The composites were characterized by energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and Fourier transform infrared absorption spectrometer (FTIR). The results show that the optimal preparation conditions of sulfhydryl modified lignite were determined by single factor test as follows: lignite particle size 0.18 mm (80 mesh), mass ratio of lignite to methanol 1∶3, reaction temperature of 40 ℃. The Design-Expert software is used to optimize the ratio, after comprehensively considering the removal rate and cost, the optimal ratio of mercapto-modified lignite in this experiment is proposed as follows: the particle size of lignite is 0.18 mm (80 mesh), the mass ratio of lignite to methanol is 1∶4, and the reaction temperature is 40 ℃, the Fe²⁺ and Mn²⁺ removal rates are 84.2% and 88.9%, respectively. The saturated adsorption capacity is 6.715mg/g and 2.295 mg/g, respectively. Compared with the original lignite, the Mn²⁺ removal rate, Fe²⁺ adsorption capacity and Mn²⁺ adsorption capacity increase by 44.9%, 27.42% and 29.01%, respectively. The isothermal adsorption lines of Fe²⁺ and Mn²⁺ adsorbed by modified lignite all conform to Langmuir model, and the adsorption conforms to monolayer adsorption process. It can be seen from the characterization results that the surface structure of lignite after modification with sulfhydryl group is destroyed and a large number of pore structures appear, and the specific surface area is greatly increased. At the same time, the molecular structure of MPYMS contains silicon, oxygen and sulphur, silica base, which are easy to hydrolyze with the carrier materials containing mercapto groups to form a C—Si—O bond, so that the mercapto groups are grafted onto the surface of the carrier materials, and the combination of heavy metal ions and adsorbent surface group could promote its skeleton deformation or fracture, generate molecular fragments, expose more hydroxyl, increase the adsorption sites, and improve the adsorption capacity of lignite.