Adsorption properties of Fe2+ and Mn2+ from groundwater in cold regions by carbonated rice husk

In order to synchronously purify iron and manganese in groundwater at low temperature, further ensure the safety of the drinking water in the cold villages and towns, carbonated rice husk at 600℃ (CRH600) was selected and its adsorption mechanism was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer-Emmett-Teller nitrogen sorption (BET-N2), Fourier transform infrared spectroscopy (FTIR) and Boehm. Optimum adsorbent dosage and pH value were determined by single factor experiment. For further understanding the adsorption property of CRH600, the adsorption isotherm, kinetics and thermodynamics during adsorption process were investigated. The pseudo-first-order kinetics model, pseudo-second-order kinetics model, Elovich equation and pore diffusion equation were used to fit the adsorption kinetics process. Four isothermal adsorption models (Langmuir, Freundlich, Temkin and Langmuir-Freundlich) were used to analyze the adsorption kinetics properties. The regeneration ability of CRH600 at low temperature was studied. XRD analysis showed that the main component of CRH600 was amorphous silicon dioxide (SiO2). SEM analysis showed that nanoscale SiO2 particles in CRH600 were stuck loosely with each other, and a large number of honeycomb micro-pores and nano-pores (5-25 μm) were formed, which made CRH600 have a larger surface area and total pore volume and made the masked SiO2 active site and skeleton exposed obviously, and then the adsorption properties of CRH600 for Fe2+ and Mn2+ were enhanced. FTIR and Boehm analysis showed the surface functional group contents of CRH600 increased significantly, and hydroxyl (-OH) played the most important role for Fe2+ and Mn2+ removal based on ion-exchange and surface-complexation. There was no competition for CRH600 adsorbing Fe2+ and Mn2+ in mixed solution. When the temperature was controlled within 10 ℃, and the initial concentration of Fe2+ and Mn2+ was at 20 mg/L, the optimum pH value was 5 and 6, respectively, and the optimum adsorbent dosage was 6 and 10 g/L, respectively. When the equilibrium concentration was less than 2 mg/L, the equilibrium adsorption capacity of CRH600 for Fe2+ and Mn2+ increased sharply with the equilibrium concentration increasing. When the equilibrium concentration was greater than 2 mg/L, the equilibrium adsorption capacity increased slowly, and finally tended to balance. The lower the temperature, the greater the equilibrium adsorption capacity. The adsorption equilibrium could be described by Langmuir isotherm (R²>0.9960). The maximum adsorption capacity of Fe2+ and Mn2+ at 10 ℃ was 5.85 and 2.83 mg/g, respectively. Compared with other agricultural waste materials, mineral and other modified adsorbents, CRH600 had the advantage of adsorbing Fe2+ and Mn2+ from groundwater at low temperature. Results of kinetic analysis showed the Fe2+ and Mn2+ adsorption process of CRH600 was in accordance with pseudo-second-order model, as well as controlled by membrane diffusion and intra-particle diffusion. According to the changes of enthalpy, free energy and entropy, it was suggested that the Fe2+ and Mn2+ adsorption process were exothermic, spontaneous and entropy-decreasing. It was beneficial to adsorption at low temperature, and both physical and chemical adsorption existed. H2SO4 was selected as the optimal desorption agent of Fe2+ and Mn2+ for CRH600. The maximum adsorption-desorption cycle was 5 and 3, respectively. The equilibrium adsorption capacity of Fe2+ and Mn2+ onto regenerated CRH600 could reach 80% and 90% of equilibrium adsorption capacity before desorption. The research results provide sufficient basic data and theoretical support for the application of removing iron and manganese in groundwater at low temperature.