Impact of steel slag on the ammonium adsorption by zeolite and a new configuration of zeolite-steel slag substrate for constructed wetlands

The CaO dissolution from slag, as well as the effects of influencing parameters (i.e. pH and Ca²⁺ concentration) on the ammonium adsorption onto zeolite, was systematically studied in this paper. Modeling results of Ca²⁺ and OH⁻ release from slag indicated that pseudo-second-order reaction had a better fitness than pseudo-first-order reaction. Changing pH value from 7 to 12 resulted in a drastic reduction of the ammonium adsorption capacity on zeolite, from the peak adsorption capacity at pH 7. High Ca²⁺ concentration in solution also inhibited the adsorption of ammonium onto zeolite. There are two proposed mechanisms for steel slag inhibiting the ammonium adsorption capacity of zeolite. On the one hand, OH⁻ released from steel slag can react with ammonium ions to produce the molecular form of ammonia (NH₃·H₂O), which would cause the dissociation of NH₄⁺ from zeolite. On the other hand, Ca²⁺ could replace the NH₄⁺ ions to adhere onto the surface of zeolite. An innovative substrate filling configuration with zeolite placed upstream of the steel slag was then proposed to eliminate the disadvantageous effects of steel slag. Experimental results showed that this novel filling configuration was superior to two other filling configurations in terms of ammonium removal.