Natural zeolites are crystalline hydrated aluminosilicate minerals with abundant pores filled with alkali, alkaline earth cations and water and have valuable physicochemical properties like unique selective sorption, ion exchange and thermal stability. Natural zeolites have been used broadly for wastewater treatment [1–7].
Previous studies have mainly focused on the application of natural, modified, and synthetic zeolites for the removal of low-concentration (< 200 mg-N/L) and trace ammonium in municipal sewage, groundwater, or tap water [8–11]. However, high-concentration ammonium removal using natural zeolite are rarely researched. Generally, air stripping method is adopted for high ammonium removal [12, 13]. However, air stripping usually requires an elevated alkalinity and temperature to increase the proportion of gaseous NH3 and continuously adds acid to reduce the alkalinity before discharge, which pollutes the atmosphere seriously and increases the operating cost. Compared to other techniques, adsorption technique has many favorable adsorption characteristics, like high affinity towards ammonium, low-cost, convenience for engineering configurations as well as environmental friendliness. In addition, natural zeolites are available in abundance. These advantages make it competitive to remove of high-concentration ammonium using natural zeolite environmentally.
Numerous literatures investigated the adsorption mechanisms of ammonium removal, but almost all of them focused on the low-concentration ammonium removal. The adsorption mechanisms were studied primarily on the basis of the adsorption kinetics, isotherm and thermodynamics analysis, and the final conclusions were drawn from the following aspects. Generally, the rate-limiting step in the adsorption process from the adsorption kinetics analysis was determined from the hypothesis and the representative significance of each parameter of the best-fitted one from the pseudo-first-order, pseudo-second-order, intraparticle diffusion and/or Elovich model or others which had the higher value of the coefficient of determination (R2)[8, 9, 15, 16]. Then, the adsorption form can be deduced from the results of the adsorption isotherm, such as monolayer is considered if Langmuir model fitted the experimental data, and multilayer if the Freundlich fitted better[17, 18]. Undoubtedly, other complicated models with more parameters were also chosen to explore the absorptive mechanism more accurately. Further, the type of interaction force, e. g. physical adsorption or chemical adsorption, was established synthetically based on the results of adsorption kinetics, equilibrium and the thermodynamic investigation[9, 19]. The different interaction implied different adsorption strength between adsorbent and adsorbate. In conclusion, the adsorption mechanism was depicted as the ion exchange, electrostatic attraction, chemisorption, or formation of complexes[17–20].
The adsorption efficiency of a specific adsorption process is influenced by many factors. Accordingly, researches on the adsorption mechanism should not only limit to the establishments of the adsorbate’s mass-transferring process and their interaction forces, which are undoubtedly vital and indispensable in the adsorption process. Fundamentally, the characteristics of mass-transferring and adsorption process are greatly affected by the physicochemical properties of adsorbents and adsorbates together with the nonnegligible operating conditions. The effects of zeolites’ framework on ammonium adsorption have been reported largely in previous studies, and the most focused on the influences of the structural features such as the surface area, particle size and surface chemistry of the adsorbents on the removal of cationic substances[18, 21, 22]. However, the effects of these factors on high-concentration ammonium were less reported. In 2013, L. Lin et al found that ion exchange was the dominant ammonium adsorption mechanism, but the order of exchange selectivity was different in different ammonium solutions (10 ~ 4000 mg-N/L) with Na+ as the predominant ions for ammonium at low concentrations while Ca2+ exceeding Na+ at high ammonium levels (༞1000mg-N/L). This shows that the adsorption behaviors of zeolites in different-level ammonium solutions are different, however, the adsorption mechanism at high concentrations was not explored further in this study. M. J. Manto et al investigated the ammonium recovery in 1000mg-N/L aqueous solution using ZSM-5 as sorbent, and observed that the uptake of ammonium was much higher than the cation exchangeable capacity (CEC) on the ZSM-5. The formation of hydrogen bonding between ammonium and the framework oxygen might cause excess capacity than CEC . Both Lin Ye et al and O'Connor et al. groups revealed the multiple hydrogen bonding between the guest ammonium and oxygen atoms were formed in the Molecular Dynamics (MD) simulation of zeolites ammonium capture [24, 25]. These results mean great potentialities of the hybrid multiple interactions and has necessity to further investigate the complex ammonium adsorption mechanism for high-concentration ammonium by zeolite.
Based on the above considerations, the major purpose of the current study was to investigate of the adsorption mechanism of high-concentration ammonium (1000 ~ 4000 mg-N/L) using Chinese natural zeolite with the strategy of experimental research and Molecular Dynamics simulation. The research contents were in the following aspects: 1) to systematically investigate adsorption characteristics including the effects of various testing parameters such as natural zeolite dosage, operational temperature, and initial ammonium concentration on ammonium removal in natural zeolite adsorption process; 2) to explore the adsorption mechanism of high-concentration ammonium onto natural zeolite from the adsorption kinetic and equilibrium studies; 3) to identify the interaction forces between ammonium and zeolite framework through MD simulation; 4) to propose the possible mechanism for the enhancement of ammonium removal performance over natural zeolite; 5) to put forward our future outlook. This investigation enriched the adsorption mechanism for ammonium removal, and provided a novel insight for designing a feasible approach for rapid reduction of high concentration ammonium using low-cost abundant minerals and alleviating subsequent water decontamination processes.