Adsorption of Ammonium by Coal and Coal Fly Ash Derived from Hawthorn Tree from Aquatic Systems

Excessive release of ammonium (NH4+) into aquatic ecosystems can promote eutrophication. In this study, the natural adsorbents, coal (C) prepared from Hawthorn (Acacia caven) and coal fly ash obtained from C, were used to remove NH4+ from aqueous systems through batch adsorption–desorption studies. Both adsorbents were physically and chemically characterized, while Fourier-transform infrared spectroscopy and zeta potential were used to understand the surface functional groups and surface charge characteristics. CFA showed a higher pH, BET specific surface area, electrical conductivity and higher % values for CaO and MgO than C. Kinetic studies of NH4+ adsorption at pH = 4.5 for both materials fitted the pseudo-second-order model giving the r2 of 0.970–0.983 and the χ2 of 0.008–0.005 and at pH = 6.5 only for C with the r2 of 0.986 and the χ2 of 0.013. Meanwhile, the adsorption isotherm data at pH = 4.5 for both materials and 6.5 for CFA complied with the Freundlich model (r2 > 0.965 and χ2 < 0.012), suggesting that NH4+ adsorption onto both adsorbents at those pH values occurred through the formation of a multilayer adsorption on heterogeneous surfaces. This indicates that the dominant adsorption of both adsorbents was physisorption with no site-specific interaction. Based on these results, CFA is proposed as a promising and economical material for the removal of NH4+ from aqueous systems.