AbstractNatural zeolitic tuff from Brus (Serbia) consisting mostly of clinoptilolite (about 90%) has been investigated for the reduction of the Mg concentration in spring water. The sorption capacity of the zeolite is relatively low (about 2.5 mg Mg g-1for the initial concentration of 100 mg Mg dm-3). The zeolitic tuff removes Mg from water solutions by ion exchange, which has been demonstrated by energy dispersive X-ray analysis (EDS). The extent of ion exchange was influenced by the pH and the initial Mg concentration. Kinetic studies revealed that Lagergen's pseudo-second order model was followed. Intra-particle diffusion of Mg2+influenced the ion exchange, but it is not the rate-limiting step. Rather than having to dispose of the Mg-loaded (waste) zeolite, a possible application was tested. Addition to a wastewater with a low concentration of Mg showed that it could successfully make up for the lack of Mg micronutrient and, accordingly, enabled the growth of phosphate-accumulating bacteriaA. Junii, increasing the amount of phosphate removed from the wastewater.