Chromium removal from water by ion exchange using zeolites and solidification of the resulting sludge in a cement matrix

A method is presented to remove and segregate Cr 3+ present in some industrial wastewaters using zeolites, on the basis of two important properties jointly exhibited by them: cation exchange and pozzolanic activity. Zeolites, namely synthetic species A and X, chosen because of their marked selectivity towards multivalent cations, were beforehand used as cation exchangers in order to remove the pollutant cation from water. Zeolitic sludge containing Cr 3+ was afterwards solidified in a cement matrix, taking advantage of its excellent reactivity towards lime arising from cement hydration. Various amounts of zeolites A and X were explored in reactions with simulated and real Cr 3+-containing wastewater, demonstrating that 3g of zeolite X or 4g of zeolite A were able to bring Cr 3+ concentration from 45 mg/liter to values lower than the law limit (2 mg/1) in reasonable time. Subsequent safe stabilization of Cr 3+-bearing sludges was obtained, provided that the amount of Portland clinker in the blend was not lower than about 25%. In fact the resulting materials showed mechanical strengths much higher than those recommended for landfilling (0.44 MPa) and very satisfying leaching properties.