Adsorption and desorption of 2,4,6-trichlorophenol onto and from ash as affected by Ag+, Zn2+, and Al3+

Metal cations and organic pollutants mostly co-exist in the natural environment. However, their interactions in adsorption processes have yet to be adequately addressed. In the current study, the effect of inorganic cations with different charges (Ag+, Zn2+, and Al3+) on the adsorption and desorption of 2,4,6-trichlorophenol (TCP) onto and from processed ash derived from wheat (Triticum aestivum L.) straw was investigated. The adsorption and desorption of TCP were both nonlinear; the isotherm and kinetics curves fitted well using the Freundlich equation and a pseudo-second-order model, respectively. The presence of Ag+ promoted TCP adsorption, while Zn2+ and Al3+ reduced TCP adsorption onto ash. The desorption of TCP from ash showed obvious hysteresis, and the presence of Ag+, Zn2+, and Al3+ caused the desorption to be less hysteretic. The suppression of TCP adsorption by Zn2+ and Al3+ was ascribed to the partial overlapping of adsorption groups between TCP and metal ions. Al3+ had a stronger inhibition effect than that of Zn2+ due to its higher binding capacity and larger hydrated ionic radius than those of Zn2+. Enhanced adsorption of TCP onto ash by Ag+ was ascribed to its ability to reduce the competitive adsorption of water molecules on ash surface by replacing the original ions, such as Na+ and Ca2+, and compressing the hydrated ionic radius of these metal ions. In addition, Ag+ was able to bind with the aromatic organic compounds containing π-electrons, which resulted in a further increase of TCP adsorption by ash.